wdiff rfc6940.original rfc6940.txt

P2PSIP
Internet Engineering Task Force (IETF) C. Jennings
Internet-Draft
Request for Comments: 6940 Cisco
Intended status:
Category: Standards Track B. Lowekamp, Ed.
Expires: August 28, 2013
ISSN: 2070-1721 Skype
E. Rescorla
RTFM, Inc.
S. Baset
H. Schulzrinne
Columbia University
February 24,
December 2013

REsource LOcation And Discovery (RELOAD) Base Protocol
draft-ietf-p2psip-base-26

Abstract

This specification defines REsource LOcation And Discovery (RELOAD),
a peer-to-peer (P2P) signaling protocol for use on the Internet. A
P2P signaling protocol provides its clients with an abstract storage
and messaging service between a set of cooperating peers that form
the overlay network. RELOAD is designed to support a P2P Session
Initiation Protocol (P2PSIP) network, but can be utilized by other
applications with similar requirements by defining new usages that
specify the kinds Kinds of data that needs need to be stored for a particular
application. RELOAD defines a security model based on a certificate
enrollment service that provides unique identities. NAT traversal is
a fundamental service of the protocol. RELOAD also allows access
from "client" nodes that do not need to route traffic or store data
for others.

Status of This Memo

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This Internet-Draft will expire on August 28, 2013.
http://www.rfc-editor.org/info/rfc6940.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 7
1.1. Basic Setting . . . . . . . . . . . . . . . . . . . . . 8
1.2. Architecture . . . . . . . . . . . . . . . . . . . . . . 10
1.2.1. Usage Layer . . . . . . . . . . . . . . . . . . . . 13 12
1.2.2. Message Transport . . . . . . . . . . . . . . . . . 13
1.2.3. Storage . . . . . . . . . . . . . . . . . . . . . . 14
1.2.4. Topology Plugin Plug-in . . . . . . . . . . . . . . . . . . 15
1.2.5. Forwarding and Link Management Layer . . . . . . . . 15
1.3. Security . . . . . . . . . . . . . . . . . . . . . . . . 16
1.4. Structure of This Document . . . . . . . . . . . . . . . 17
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 17
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 18
3.
4. Overlay Management Overview . . . . . . . . . . . . . . . . . 22
3.1. 21
4.1. Security and Identification . . . . . . . . . . . . . . 22
3.1.1. 21
4.1.1. Shared-Key Security . . . . . . . . . . . . . . . . 24
3.2. 23
4.2. Clients . . . . . . . . . . . . . . . . . . . . . . . . 24
3.2.1. 23
4.2.1. Client Routing . . . . . . . . . . . . . . . . . . . 25
3.2.2. 24
4.2.2. Minimum Functionality Requirements for Clients . . . 26
3.3. 24
4.3. Routing . . . . . . . . . . . . . . . . . . . . . . . . 26
3.4. 25
4.4. Connectivity Management . . . . . . . . . . . . . . . . 30
3.5. 28
4.5. Overlay Algorithm Support . . . . . . . . . . . . . . . 31
3.5.1. 29
4.5.1. Support for Pluggable Overlay Algorithms . . . . . . 31
3.5.2. 30
4.5.2. Joining, Leaving, and Maintenance Overview . . . . . 31
3.6. 30
4.6. First-Time Setup . . . . . . . . . . . . . . . . . . . . 33
3.6.1. 31
4.6.1. Initial Configuration . . . . . . . . . . . . . . . 33
3.6.2. 32
4.6.2. Enrollment . . . . . . . . . . . . . . . . . . . . . 33
3.6.3. 32
4.6.3. Diagnostics . . . . . . . . . . . . . . . . . . . . 34
4. 32
5. Application Support Overview . . . . . . . . . . . . . . . . 34
4.1. 32
5.1. Data Storage . . . . . . . . . . . . . . . . . . . . . . 34
4.1.1. 33
5.1.1. Storage Permissions . . . . . . . . . . . . . . . . 35
4.1.2. 34
5.1.2. Replication . . . . . . . . . . . . . . . . . . . . 36
4.2. 35
5.2. Usages . . . . . . . . . . . . . . . . . . . . . . . . . 37
4.3. 35
5.3. Service Discovery . . . . . . . . . . . . . . . . . . . 37
4.4. 36
5.4. Application Connectivity . . . . . . . . . . . . . . . . 38
5. RFC 2119 Terminology . . . . . . . . . . . . . . . . . . . . 38 36
6. Overlay Management Protocol . . . . . . . . . . . . . . . . . 38 36
6.1. Message Receipt and Forwarding . . . . . . . . . . . . . 38 37
6.1.1. Responsible ID . . . . . . . . . . . . . . . . . . . 39 37
6.1.2. Other ID . . . . . . . . . . . . . . . . . . . . . . 40 38
6.1.3. Opaque ID . . . . . . . . . . . . . . . . . . . . . 42 40
6.2. Symmetric Recursive Routing . . . . . . . . . . . . . . 42 40
6.2.1. Request Origination . . . . . . . . . . . . . . . . 42 41
6.2.2. Response Origination . . . . . . . . . . . . . . . . 43 42
6.3. Message Structure . . . . . . . . . . . . . . . . . . . 43 42
6.3.1. Presentation Language . . . . . . . . . . . . . . . 44 43
6.3.1.1. Common Definitions . . . . . . . . . . . . . . . 45 44
6.3.2. Forwarding Header . . . . . . . . . . . . . . . . . 48 46
6.3.2.1. Processing Configuration Sequence Numbers . . . . 51 48
6.3.2.2. Destination and Via Lists . . . . . . . . . . . . 51 49
6.3.2.3. Forwarding Option . . . . . . . . . . . . . . . . 54 51
6.3.3. Message Contents Format . . . . . . . . . . . . . . 55 52
6.3.3.1. Response Codes and Response Errors . . . . . . . 57 54
6.3.4. Security Block . . . . . . . . . . . . . . . . . . . 60 56
6.4. Overlay Topology . . . . . . . . . . . . . . . . . . . . 64 60
6.4.1. Topology Plugin Plug-in Requirements . . . . . . . . . . . . 64 60
6.4.2. Methods and types Types for use Use by topology plugins Topology Plug-ins . . . 65 61
6.4.2.1. Join . . . . . . . . . . . . . . . . . . . . . . 65 61
6.4.2.2. Leave . . . . . . . . . . . . . . . . . . . . . . 66 62
6.4.2.3. Update . . . . . . . . . . . . . . . . . . . . . 67 62
6.4.2.4. RouteQuery . . . . . . . . . . . . . . . . . . . 67 63
6.4.2.5. Probe . . . . . . . . . . . . . . . . . . . . . . 68 64
6.5. Forwarding and Link Management Layer . . . . . . . . . . 70 66
6.5.1. Attach . . . . . . . . . . . . . . . . . . . . . . . 71 67
6.5.1.1. Request Definition . . . . . . . . . . . . . . . 72 67
6.5.1.2. Response Definition . . . . . . . . . . . . . . . 74 70
6.5.1.3. Using ICE With with RELOAD . . . . . . . . . . . . . . 75 71
6.5.1.4. Collecting STUN Servers . . . . . . . . . . . . . 76 71
6.5.1.5. Gathering Candidates . . . . . . . . . . . . . . 76 72
6.5.1.6. Prioritizing Candidates . . . . . . . . . . . . . 77 72
6.5.1.7. Encoding the Attach Message . . . . . . . . . . . 78 73
6.5.1.8. Verifying ICE Support . . . . . . . . . . . . . . 78 74
6.5.1.9. Role Determination . . . . . . . . . . . . . . . 78 74
6.5.1.10. Full ICE . . . . . . . . . . . . . . . . . . . . 79 74
6.5.1.11. No-ICE . . . . . . . . . . . . . . . . . . . . . 79 75
6.5.1.12. Subsequent Offers and Answers . . . . . . . . . . 79 75
6.5.1.13. Sending Media . . . . . . . . . . . . . . . . . . 79 75
6.5.1.14. Receiving Media . . . . . . . . . . . . . . . . . 80 75
6.5.2. AppAttach . . . . . . . . . . . . . . . . . . . . . 80 75
6.5.2.1. Request Definition . . . . . . . . . . . . . . . 80 76
6.5.2.2. Response Definition . . . . . . . . . . . . . . . 81 77
6.5.3. Ping . . . . . . . . . . . . . . . . . . . . . . . . 82 77
6.5.3.1. Request Definition . . . . . . . . . . . . . . . 82 77
6.5.3.2. Response Definition . . . . . . . . . . . . . . . 82 77
6.5.4. ConfigUpdate . . . . . . . . . . . . . . . . . . . . 83 78
6.5.4.1. Request Definition . . . . . . . . . . . . . . . 83 78
6.5.4.2. Response Definition . . . . . . . . . . . . . . . 84 79
6.6. Overlay Link Layer . . . . . . . . . . . . . . . . . . . 85 80
6.6.1. Future Overlay Link Protocols . . . . . . . . . . . 86 81
6.6.1.1. HIP . . . . . . . . . . . . . . . . . . . . . . . 86 82
6.6.1.2. ICE-TCP . . . . . . . . . . . . . . . . . . . . . 87 82
6.6.1.3. Message-oriented Message-Oriented Transports . . . . . . . . . . . 87 82
6.6.1.4. Tunneled Transports . . . . . . . . . . . . . . . 87 82
6.6.2. Framing Header . . . . . . . . . . . . . . . . . . . 87 83
6.6.3. Simple Reliability . . . . . . . . . . . . . . . . . 89 84
6.6.3.1. Stop and Wait Sender Algorithm . . . . . . . . . 90 85
6.6.4. DTLS/UDP with SR . . . . . . . . . . . . . . . . . . 91 86
6.6.5. TLS/TCP with FH, No-ICE . . . . . . . . . . . . . . 91 86
6.6.6. DTLS/UDP with SR, No-ICE . . . . . . . . . . . . . . 92 87
6.7. Fragmentation and Reassembly . . . . . . . . . . . . . . 92 87
7. Data Storage Protocol . . . . . . . . . . . . . . . . . . . . 93 88
7.1. Data Signature Computation . . . . . . . . . . . . . . . 95 90
7.2. Data Models . . . . . . . . . . . . . . . . . . . . . . 96 90
7.2.1. Single Value . . . . . . . . . . . . . . . . . . . . 97 91
7.2.2. Array . . . . . . . . . . . . . . . . . . . . . . . 97 92
7.2.3. Dictionary . . . . . . . . . . . . . . . . . . . . . 98 92
7.3. Access Control Policies . . . . . . . . . . . . . . . . 99 93
7.3.1. USER-MATCH . . . . . . . . . . . . . . . . . . . . . 99 93
7.3.2. NODE-MATCH . . . . . . . . . . . . . . . . . . . . . 99 93
7.3.3. USER-NODE-MATCH . . . . . . . . . . . . . . . . . . 100 93
7.3.4. NODE-MULTIPLE . . . . . . . . . . . . . . . . . . . 100 94
7.4. Data Storage Methods . . . . . . . . . . . . . . . . . . 100 94
7.4.1. Store . . . . . . . . . . . . . . . . . . . . . . . 100 94
7.4.1.1. Request Definition . . . . . . . . . . . . . . . 100 94
7.4.1.2. Response Definition . . . . . . . . . . . . . . . 105 99
7.4.1.3. Removing Values . . . . . . . . . . . . . . . . . 107 100
7.4.2. Fetch . . . . . . . . . . . . . . . . . . . . . . . 108 101
7.4.2.1. Request Definition . . . . . . . . . . . . . . . 108 101
7.4.2.2. Response Definition . . . . . . . . . . . . . . . 110 103
7.4.3. Stat . . . . . . . . . . . . . . . . . . . . . . . . 111 105
7.4.3.1. Request Definition . . . . . . . . . . . . . . . 112 105
7.4.3.2. Response Definition . . . . . . . . . . . . . . . 112 105
7.4.4. Find . . . . . . . . . . . . . . . . . . . . . . . . 114 107
7.4.4.1. Request Definition . . . . . . . . . . . . . . . 115 107
7.4.4.2. Response Definition . . . . . . . . . . . . . . . 115 108
7.4.5. Defining New Kinds . . . . . . . . . . . . . . . . . 116 109
8. Certificate Store Usage . . . . . . . . . . . . . . . . . . . 117 109
9. TURN Server Usage . . . . . . . . . . . . . . . . . . . . . . 118 110
10. Chord Algorithm . . . . . . . . . . . . . . . . . . . . . . . 119 112
10.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 120 113
10.2. Hash Function . . . . . . . . . . . . . . . . . . . . . 121 113
10.3. Routing . . . . . . . . . . . . . . . . . . . . . . . . 121 114
10.4. Redundancy . . . . . . . . . . . . . . . . . . . . . . . 122 114
10.5. Joining . . . . . . . . . . . . . . . . . . . . . . . . 122 114
10.6. Routing Attaches . . . . . . . . . . . . . . . . . . . . 124 116
10.7. Updates . . . . . . . . . . . . . . . . . . . . . . . . 124 116
10.7.1. Handling Neighbor Failures . . . . . . . . . . . . . 126 118
10.7.2. Handling Finger Table Entry Failure . . . . . . . . 126 118
10.7.3. Receiving Updates . . . . . . . . . . . . . . . . . 127 118
10.7.4. Stabilization . . . . . . . . . . . . . . . . . . . 128 119
10.7.4.1. Updating the Neighbor Table . . . . . . . . . . . . . 128 120
10.7.4.2. Refreshing the Finger Table . . . . . . . . . . . . . 128 120
10.7.4.3. Adjusting Finger Table size Size . . . . . . . . . . . 129 121
10.7.4.4. Detecting partitioning Partitioning . . . . . . . . . . . . . 130 121
10.8. Route query Query . . . . . . . . . . . . . . . . . . . . . . 130 122
10.9. Leaving . . . . . . . . . . . . . . . . . . . . . . . . 130 122
11. Enrollment and Bootstrap . . . . . . . . . . . . . . . . . . 131 123
11.1. Overlay Configuration . . . . . . . . . . . . . . . . . 132 123
11.1.1. RELAX NG Grammar . . . . . . . . . . . . . . . . . . 140 131
11.2. Discovery Through through Configuration Server . . . . . . . . . 142 133
11.3. Credentials . . . . . . . . . . . . . . . . . . . . . . 143 134
11.3.1. Self-Generated Credentials . . . . . . . . . . . . . 145 136
11.4. Contacting a Bootstrap Node . . . . . . . . . . . . . . 146 137
12. Message Flow Example . . . . . . . . . . . . . . . . . . . . 146 137
13. Security Considerations . . . . . . . . . . . . . . . . . . . 152 143
13.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 152 143
13.2. Attacks on P2P Overlays . . . . . . . . . . . . . . . . 153 144
13.3. Certificate-based Certificate-Based Security . . . . . . . . . . . . . . . 153 144
13.4. Shared-Secret Security . . . . . . . . . . . . . . . . . 154 145
13.5. Storage Security . . . . . . . . . . . . . . . . . . . . 155 146
13.5.1. Authorization . . . . . . . . . . . . . . . . . . . 155 146
13.5.2. Distributed Quota . . . . . . . . . . . . . . . . . 156 147
13.5.3. Correctness . . . . . . . . . . . . . . . . . . . . 156 147
13.5.4. Residual Attacks . . . . . . . . . . . . . . . . . . 156 147
13.6. Routing Security . . . . . . . . . . . . . . . . . . . . 157 148
13.6.1. Background . . . . . . . . . . . . . . . . . . . . . 157 148
13.6.2. Admissions Control . . . . . . . . . . . . . . . . . 158 149
13.6.3. Peer Identification and Authentication . . . . . . . 158 149
13.6.4. Protecting the Signaling . . . . . . . . . . . . . . 159 150
13.6.5. Routing Loops and Dos DoS Attacks . . . . . . . . . . . 159 150
13.6.6. Residual Attacks . . . . . . . . . . . . . . . . . . 160 151
14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 160 151
14.1. Well-Known URI Registration . . . . . . . . . . . . . . 160 151
14.2. Port Registrations . . . . . . . . . . . . . . . . . . . 160 151
14.3. Overlay Algorithm Types . . . . . . . . . . . . . . . . 161 152
14.4. Access Control Policies . . . . . . . . . . . . . . . . 161 152
14.5. Application-ID . . . . . . . . . . . . . . . . . . . . . 162 153
14.6. Data Kind-ID . . . . . . . . . . . . . . . . . . . . . . 162 153
14.7. Data Model . . . . . . . . . . . . . . . . . . . . . . . 163 154
14.8. Message Codes . . . . . . . . . . . . . . . . . . . . . 163 154
14.9. Error Codes . . . . . . . . . . . . . . . . . . . . . . 165 156
14.10. Overlay Link Types . . . . . . . . . . . . . . . . . . . 165 156
14.11. Overlay Link Protocols . . . . . . . . . . . . . . . . . 166 157
14.12. Forwarding Options . . . . . . . . . . . . . . . . . . . 166 157
14.13. Probe Information Types . . . . . . . . . . . . . . . . 167 158
14.14. Message Extensions . . . . . . . . . . . . . . . . . . . 167 158
14.15. reload Reload URI Scheme . . . . . . . . . . . . . . . . . . . 168 159
14.15.1. URI Registration . . . . . . . . . . . . . . . . . . 169 160
14.16. Media Type Registration . . . . . . . . . . . . . . . . 170 160
14.17. XML Name Space Namespace Registration . . . . . . . . . . . . . . 171 . 161
14.17.1. Config URL . . . . . . . . . . . . . . . . . . . . . 171 161
14.17.2. Config Chord URL . . . . . . . . . . . . . . . . . . 171 162
15. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 171 162
16. References . . . . . . . . . . . . . . . . . . . . . . . . . 172 162
16.1. Normative References . . . . . . . . . . . . . . . . . . 172 162
16.2. Informative References . . . . . . . . . . . . . . . . . 176 165
Appendix A. Routing Alternatives . . . . . . . . . . . . . . . . 180 168
A.1. Iterative vs vs. Recursive . . . . . . . . . . . . . . . . . 181 168
A.2. Symmetric vs vs. Forward response Response . . . . . . . . . . . . . 181 169
A.3. Direct Response . . . . . . . . . . . . . . . . . . . . 181 169
A.4. Relay Peers . . . . . . . . . . . . . . . . . . . . . . 183 170
A.5. Symmetric Route Stability . . . . . . . . . . . . . . . 183 171
Appendix B. Why Clients? . . . . . . . . . . . . . . . . . . . . 184 172
B.1. Why Not Only Peers? . . . . . . . . . . . . . . . . . . 184 172
B.2. Clients as Application-Level Agents . . . . . . . . . . 184 172

1. Introduction

This document defines REsource LOcation And Discovery (RELOAD), a
peer-to-peer (P2P) signaling protocol for use on the Internet. It
RELOAD provides a generic, self-organizing overlay network service,
allowing nodes to route messages to other nodes and to store and
retrieve data in the overlay. RELOAD provides several features that
are critical for a successful P2P protocol for the Internet:

Security Framework: A P2P network will often be established among a
set of peers that do not trust each other. RELOAD leverages a
central enrollment server to provide credentials for each peer peer,
which can then be used to authenticate each operation. This
greatly reduces the possible attack surface.

Usage Model: RELOAD is designed to support a variety of
applications, including P2P multimedia communications with the
Session Initiation Protocol [I-D.ietf-p2psip-sip]. (SIP) [SIP-RELOAD]. RELOAD allows the
definition of new application usages, each of which can define its
own data types, along with the rules for their use. This allows
RELOAD to be used with new applications through a simple
documentation process that supplies the details for each
application.

NAT Traversal: RELOAD is designed to function in environments where
many
many, if not most most, of the nodes are behind NATs or firewalls.
Operations for NAT traversal are part of the base design,
including using Interactive Connectivity Establishment (ICE)
[RFC5245] to establish new RELOAD or application protocol
connections.

Optimized Routing: The very nature of overlay algorithms introduces
a requirement that peers participating in the P2P network route
requests on behalf of other peers in the network. This introduces
a load on those other peers, peers in the form of bandwidth and
processing power. RELOAD has been defined with a simple,
lightweight forwarding header, thus minimizing the amount of
effort for intermediate peers.

Pluggable Overlay Algorithms: RELOAD has been designed with an
abstract interface to the overlay layer to simplify implementing a
variety of structured (e.g., distributed hash tables) tables (DHTs)) and
unstructured overlay algorithms. The idea here is that RELOAD
provides a generic structure that can fit most types of overlay
topologies (ring, hyperspace, etc.). To instantiate an actual
network, you combine RELOAD with a specific overlay algorithm,
which defines how to construct the overlay topology and route
messages efficiently within it. This specification also defines
how RELOAD is used with the Chord Chord-based [Chord] based DHT algorithm,
which is mandatory to implement. Specifying a default "mandatory
to implement" "mandatory-
to-implement" overlay algorithm promotes interoperability, while
extensibility allows selection of overlay algorithms optimized for
a particular application.

Support for Clients: RELOAD clients differ from RELOAD peers
primarily in that they do not store information on behalf of other
nodes in the overlay, but only overlay. Rather, they use the overlay only to locate
users and
resources resources, as well as to store information. information and to
contact other nodes.

These properties were designed specifically to meet the requirements
for a P2P protocol to support SIP. This document defines the base
protocol for the distributed storage and location service, as well as
critical usage for NAT traversal. The SIP Usage itself is described
separately in [I-D.ietf-p2psip-sip]. [SIP-RELOAD]. RELOAD is not limited to usage by SIP
and could serve as a tool for supporting other P2P applications with
similar needs.

1.1. Basic Setting

In this section, we provide a brief overview of the operational
setting for RELOAD. A RELOAD Overlay Instance consists of a set of
nodes arranged in a partly connected graph. Each node in the overlay
is assigned a numeric Node-ID for the lifetime of the node node, which,
together with the specific overlay algorithm in use, determines its
position in the graph and the set of nodes it connects to. The
Node-ID is also tightly coupled to the certificate (see
Section 13.3). The figure below shows a trivial example which isn't
drawn from any particular overlay algorithm, but was chosen for
convenience of representation.

+--------+ +--------+ +--------+
| Node 10|--------------| Node 20|--------------| Node 30|
+--------+ +--------+ +--------+
| | |
| | |
+--------+ +--------+ +--------+
| Node 40|--------------| Node 50|--------------| Node 60|
+--------+ +--------+ +--------+
| | |
| | |
+--------+ +--------+ +--------+
| Node 70|--------------| Node 80|--------------| Node 90|
+--------+ +--------+ +--------+
|
|
+--------+
| Node 85|
|(Client)|
+--------+

Because the graph is not fully connected, when a node wants to send a
message to another node, it may need to route it through the network.
For instance, Node 10 can talk directly to nodes 20 and 40, but not
to Node 70. In order to send a message to Node 70, it would first
send it to Node 40 40, with instructions to pass it along to Node 70.
Different overlay algorithms will have different connectivity graphs,
but the general idea behind all of them is to allow any node in the
graph to efficiently reach every other node within a small number of
hops.

The RELOAD network is not only a messaging network. It is also a
storage network, albeit one designed for small-scale transient
storage rather than for bulk storage of large objects. Records are
stored under numeric addresses, called Resource-IDs, which occupy the
same space as node identifiers. Peers are responsible for storing
the data associated with some set of addresses addresses, as determined by
their Node-ID. For instance, we might say that every peer is
responsible for storing any data value which has an address less than
or equal to its own Node-ID, but greater than the next lowest
Node-ID. Thus,
Node-20 Node 20 would be responsible for storing values
11-20.

RELOAD also supports clients. These are nodes which have Node-IDs
but do not participate in routing or storage. For instance, in the
figure above above, Node 85 is a client. It can route to the rest of the
RELOAD network via Node 80, but no other node will route through it it,
and Node 90 is still responsible for all addresses between 81-90. in the range [81..90].
We refer to non-client nodes as peers.

Other applications (for instance, SIP) can be defined on top of
RELOAD and can use these two basic RELOAD services to provide their
own services.

1.2. Architecture

RELOAD is fundamentally an overlay network. The following figure
shows the layered RELOAD architecture.

Application

+-------+ +-------+
| SIP | | XMPP | ...
| Usage | | Usage |
+-------+ +-------+
------------------------------------ Messaging Service Boundary
+------------------+ +---------+
| Message |<--->| Storage |
| Transport | +---------+
+------------------+ ^
^ ^ |
| v v
| +-------------------+
| | Topology |
| | Plugin Plug-in |
| +-------------------+
| ^
v v
+------------------+
| Forwarding & |
| Link Management |
+------------------+
------------------------------------ Overlay Link Service Boundary
+-------+ +-------+
|TLS | |DTLS | ...
|Overlay| |Overlay|
|Link | |Link |
+-------+ +-------+

The major components of RELOAD are:

Usage Layer: Each application defines a RELOAD usage; Usage, which is a set
of data Kinds and behaviors which describe how to use the services
provided by RELOAD. These usages all talk to RELOAD through a
common Message Transport Service.

Message Transport: Handles end-to-end reliability, manages request
state for the usages, and forwards Store and Fetch operations to
the Storage component. Delivers It delivers message responses to the
component initiating the request.

Storage: The Storage component is responsible for processing
messages relating to the storage and retrieval of data. It talks
directly to the Topology Plugin Plug-in to manage data replication and
migration, and it talks to the Message Transport component to send
and receive messages.

Topology Plugin: Plug-in: The Topology Plugin Plug-in is responsible for
implementing the specific overlay algorithm being used. It uses
the Message Transport component to send and receive overlay
management messages, the Storage component to manage data
replication, and the Forwarding Layer to control hop-by-hop
message forwarding. This component superficially parallels
conventional routing algorithms, but is more tightly coupled to
the Forwarding Layer Layer, because there is no single "routing table" "Routing Table"
equivalent used by all overlay algorithms. The topology plugin Topology Plug-in
has two functions, functions: constructing the local forwarding instructions, instructions
and selecting the operational topology (i.e., creating links by
sending overlay management messages).

Forwarding and Link Management Layer: Stores and implements the
Routing Table by providing packet forwarding services between
nodes. It also handles establishing new links between nodes,
including setting up connections for overlay links across NATs
using ICE.

Overlay Link Layer: Responsible for actually transporting traffic
directly between nodes. TLS Transport Layer Security (TLS) [RFC5246]
and DTLS Datagram Transport Layer Security (DTLS) [RFC6347] are the
currently defined "overlay link layer" protocols used by RELOAD
for hop-by-hop communication. Each such protocol includes the
appropriate provisions for per-hop framing or and hop-by-hop ACKs
needed by unreliable underlying transports. New protocols can be
defined, as described in Section Sections 6.6.1 and Section 11.1. As this
document defines only TLS and DTLS, we use those terms throughout
the remainder of the document with the understanding that some
future specification may add new overlay link layers.

To further clarify the roles of the various layers, this the following
figure parallels the architecture with each layer's role from an
overlay perspective and implementation layer in the internet: Internet:

Internet | Internet Model |
Model | Equivalent | Reload
| in Overlay | Architecture
-------------+-----------------+------------------------------------
| | +-------+ +-------+
| Application | | SIP | | XMPP | ...
| | | Usage | | Usage |
| | +-------+ +-------+
| | ----------------------------------
| |+------------------+ +---------+
| Transport || Message |<--->| Storage |
| || Transport | +---------+
| |+------------------+ ^
| | ^ ^ |
| | | v v
Application | | | +-------------------+
| (Routing) | | | Topology |
| | | | Plugin Plug-in |
| | | +-------------------+
| | | ^
| | v v
| Network | +------------------+
| | | Forwarding & |
| | | Link Management |
| | +------------------+
| | ----------------------------------
Transport | Link | +-------+ +------+
| | |TLS | |DTLS | ...
| | +-------+ +------+
-------------+-----------------+------------------------------------
Network |
|
Link |

In addition to the above components, nodes may communicate with a
central provisioning infrastructure (not shown) to get configuration
information, authentication credentials, and the initial set of nodes
to communicate with to join the overlay.

1.2.1. Usage Layer

The top layer, called the Usage Layer, has application usages, such
as the SIP Registration Usage [I-D.ietf-p2psip-sip], [SIP-RELOAD], that use the abstract
Message Transport Service provided by RELOAD. The goal of this layer
is to implement application-specific usages of the generic overlay
services provided by RELOAD. The usage Usage defines how a specific
application maps its data into something that can be stored in the
overlay, where to store the data, how to secure the data, and finally
how applications can retrieve and use the data.

The architecture diagram shows both a SIP usage Usage and an XMPP usage. Usage. A
single application may require multiple usages; for example example, a
voicemail feature in a softphone application that stores links to the
messages in the overlay would require a different usage than the type
of rendezvous service of XMPP or SIP. A usage may define multiple
Kinds of data that are stored in the overlay and may also rely on
Kinds originally defined by other usages.

Because the security and storage policies for each Kind are dictated
by the usage defining the Kind, the usages may be coupled with the
Storage component to provide security policy enforcement and to
implement appropriate storage strategies according to the needs of
the usage. The exact implementation of such an interface is outside
the scope of this specification.

1.2.2. Message Transport

The Message Transport component provides a generic message routing
service for the overlay. The Message Transport layer is responsible
for end-to-end message transactions. Each peer is identified by its
location in the overlay overlay, as determined by its Node-ID. A component
that is a client of the Message Transport can perform two basic
functions:

o Send a message to a given peer specified by Node-ID or to the peer
responsible for a particular Resource-ID.

o Receive messages that other peers sent to a Node-ID or Resource-ID
for which the receiving peer is responsible.

All usages rely on the Message Transport component to send and
receive messages from peers. For instance, when a usage wants to
store data, it does so by sending Store requests. Note that the
Storage component and the Topology Plugin Plug-in are themselves clients of
the Message Transport, because they need to send and receive messages
from other peers.

The Message Transport Service is responsible for end-to-end
reliability, which is accomplished by timer-based retransmissions.
Unlike the Internet transport layer, however, this layer does not
provide congestion control. RELOAD is a request-response protocol,
with no more than two pairs of request-response messages used in
typical transactions between pairs of nodes, therefore nodes; therefore, there are no
opportunities to observe and react to end-to-end congestion. As with
all Internet applications, implementers are strongly discouraged from
writing applications that react to loss by immediately retrying the
transaction.

The Message Transport Service is similar to those described as
providing "Key based "key-based routing" (KBR)[wikiKBR], (KBR) [wikiKBR], although as RELOAD
supports different overlay algorithms (including non-DHT overlay
algorithms) that calculate keys (storage indices, not encryption
keys) in different ways, the actual interface needs to accept
Resource Names rather than actual keys.

Stability

The Forwarding and Link Management layers are responsible for
maintaining the overlay in the face of changes in the available nodes
and underlying network supporting the overlay (the
Internet) and Internet). They
also handle congestion control between overlay neighbors, which and
exchange routing updates and data replicas in addition to forwarding
end-to-end messages, is handled by the Forwarding and Link Management
layer described below. messages.

Real-world experience has shown that a fixed timeout for the end-to-
end retransmission timer is sufficient for practical overlay
networks. This timer is adjustable via the overlay configuration.
As the overlay configuration can be rapidly updated, this value could
be dynamically adjusted at coarse time scales, although algorithms
for determining how to accomplish this are beyond the scope of this
specification. In many cases, however, more appropriate other means of improving
network performance, such as having the Topology Plugin removing Plug-in remove lossy
links from use in overlay routing or reducing the overall hop- hop count
of end-to-end paths paths, will be more effective than simply increasing
the retransmission timer.

1.2.3. Storage

One of the major functions of RELOAD is to allow storage of data, that is,
allowing nodes to store data in the overlay and to retrieve data
stored by other nodes or by themselves. The Storage component is
responsible for processing data storage and retrieval messages. For
instance, the Storage component might receive a Store request for a
given resource from the Message Transport. It would then query the
appropriate usage before storing the data value(s) in its local data
store and sending a response to the Message Transport for delivery to
the requesting node. Typically, these messages will come from other
nodes, but depending on the overlay topology, a node might be
responsible for storing data for itself as well, especially if the
overlay is small.

A peer's Node-ID determines the set of resources that it will be
responsible for storing. However, the exact mapping between these is
determined by the overlay algorithm in use. The Storage component
will only receive a Store request from the Message Transport if this
peer is responsible for that Resource-ID. The Storage component is
notified by the Topology Plugin Plug-in when the Resource-IDs for which it
is responsible change, and the Storage component is then responsible
for migrating resources to other peers.

1.2.4. Topology Plugin Plug-in

RELOAD is explicitly designed to work with a variety of overlay
algorithms. In order to facilitate this, the overlay algorithm
implementation is provided by a Topology Plugin Plug-in so that each overlay
can select an appropriate overlay algorithm that relies on the common
RELOAD core protocols and code.

The Topology Plugin Plug-in is responsible for maintaining the overlay
algorithm Routing Table, which is consulted by the Forwarding and
Link Management Layer before routing a message. When connections are
made or broken, the Forwarding and Link Management Layer notifies the
Topology Plugin, Plug-in, which adjusts the Routing Table as appropriate.
The Topology Plugin Plug-in will also instruct the Forwarding and Link
Management Layer to form new connections as dictated by the
requirements of the overlay algorithm Topology. The Topology Plugin Plug-in
issues periodic update requests through Message Transport to maintain
and update its Routing Table.

As peers enter and leave, resources may be stored on different peers,
so the Topology Plugin Plug-in also keeps track of which peers are
responsible for which resources. As peers join and leave, the
Topology Plugin Plug-in instructs the Storage component to issue resource
migration requests as appropriate, in order to ensure that other
peers have whatever resources they are now responsible for. The
Topology Plugin Plug-in is also responsible for providing for redundant data
storage to protect against loss of information in the event of a peer
failure and to protect against compromised or subversive peers.

1.2.5. Forwarding and Link Management Layer

The Forwarding and Link Management Layer is responsible for getting a
message to the next peer, as determined by the Topology Plugin. Plug-in.
This
Layer layer establishes and maintains the network connections as
needed by the Topology Plugin. Plug-in. This layer is also responsible for
setting up connections to other peers through NATs and firewalls
using ICE, and it can elect to forward traffic using relays for NAT
and firewall traversal.

Congestion control is implemented at this layer to protect the
Internet paths used to form the link in the overlay. Additionally,
retransmission is performed to improve the reliability of end-to-end
transactions. The relation of this layer to the Message Transport
Layer can be likened to the relation of the link-level congestion
control and retransmission in modern wireless networks ` to Internet
transport protocols.

This layer provides a generic interface that allows the topology
plugin Topology
Plug-in to control the overlay and resource operations and messages.
Since
Because each overlay algorithm is defined and functions differently,
we generically refer to the table of other peers that the overlay
algorithm maintains and uses to route requests (neighbors) as a Routing Table.
The Topology Plugin Plug-in actually owns the Routing Table, and forwarding
decisions are made by querying the Topology Plugin Plug-in for the next hop
for a particular Node-ID or Resource-ID. If this node is the
destination of the message, the message is delivered to the Message
Transport.

This layer also utilizes a framing header to encapsulate messages as
they are forwarded along each hop. This header aids reliability
congestion control, flow control, etc. It has meaning only in the
context of that individual link.

The Forwarding and Link Management Layer sits on top of the Overlay
Link Layer protocols that carry the actual traffic. This
specification defines how to use DTLS and TLS protocols to carry
RELOAD messages.

1.3. Security

RELOAD's security model is based on each node having one or more
public key certificates. In general, these certificates will be
assigned by a central server server, which also assigns Node-IDs, although
self-signed certificates can be used in closed networks. These
credentials can be leveraged to provide communications security for
RELOAD messages. RELOAD provides communications security at three
levels:

Connection Level: level: Connections between nodes are secured with TLS,
DTLS, or potentially some to be defined to-be-defined future protocol.

Message Level: level: Each RELOAD message is signed.

Object Level: Stored objects are signed by the creating node.

These three levels of security work together to allow nodes to verify
the origin and correctness of data they receive from other nodes,
even in the face of malicious activity by other nodes in the overlay.
RELOAD also provides access control built on top of these
communications security features. Because the peer responsible for
storing a piece of data can validate the signature on the data being
stored, the responsible peer it can determine whether or not a given operation is permitted or not.
permitted.

RELOAD also provides an optional shared secret based shared-secret-based admission
control feature using shared secrets and TLS-PSK/TLS-SRP. TLS pre-shared keys (PSK) or
TLS Secure Remote Password (SRP). In order to form a TLS connection
to any node in the overlay, a new node needs to know the shared
overlay key, thus restricting access to authorized users only. This
feature is used together with certificate-based access control, not
as a replacement for it. It is typically used when self-signed
certificates are being used but would generally not be used when the
certificates were all signed by an enrollment server.

1.4. Structure of This Document

The remainder of this document is structured as follows.

o Section 2 3 provides definitions of terms used in this document.

o Section 3 4 provides an overview of the mechanisms used to establish
and maintain the overlay.

o Section 4 5 provides an overview of the mechanism RELOAD provides to
support other applications.

o Section 6 defines the protocol messages that RELOAD uses to
establish and maintain the overlay.

o Section 7 defines the protocol messages that are used to store and
retrieve data using RELOAD.

o Section 8 defines the Certificate Store Usages.

o Section 9 defines the TURN Server Usage needed to locate TURN
(Traversal Using Relays around NAT) servers for NAT traversal.

o Section 10 defines a specific Topology Plugin Plug-in using Chord based a Chord-based
algorithm.

o Section 11 defines the mechanisms that new RELOAD nodes use to
join the overlay for the first time.

o Section 12 provides an extended example.

2. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].

3. Terminology

Terms in this document are defined inline in-line when used and are also
defined below for reference. The definitions in this section use
terminology and concepts that are not explained until later in the
specification.

Admitting Peer: A Peer (AP): A peer in the Overlay overlay which helps the Joining
Node join the Overlay.

Bootstrap Node: A network node used by Joining Nodes to help locate
the Admitting Peer.

Client: A host that is able to store data in and retrieve data from
the overlay overlay, but which is does not participating participate in routing or data storage
for the overlay.

Configuration Document: An XML document containing all the Overlay
Parameters for one overlay instance.

Connection Table: Contains connection information for the set of
nodes to which a node is directly connected, which include nodes
that are not yet available for routing.

Destination List: A list of Node-IDs, Resource-ID Resource-IDs, and Opaque IDs
through which a message is to be routed, in strict order. A
single Node-ID, Resource-ID Resource-ID, or Opaque ID is a trivial form of
destination list.
Destination List. When multiple Node-IDs are specified, a
Destination List is a loose source route. The list is reduced
hop-by-hop, hop
by hop, and does not include the source but includes does include the
destination.

DHT: A distributed hash table. A DHT is an abstract hash table storage service
realized by storing the contents of the hash table across a set of
peers.

ID: A generic term for any kind of identifiers in an Overlay. This
document specifies an ID as being a an Application-ID, Kind-ID , a Kind-ID, a
Node-ID, Transaction a transaction ID, a component ID, a response ID, a
Resource-ID, or an Opaque ID.

Joining Node: Node (JN): A node that is attempting to become a Peer peer in a
particular Overlay.

Kind: A Kind defines a particular type of data that can be stored in
the overlay. Applications define new Kinds to store the data they
use. Each Kind is identified with a unique integer called a
Kind-ID.

Kind-ID: A unique 32 bit 32-bit value identifying a Kind. Kind-IDs are
either private or allocated by IANA (see Section 14.6).

Maximum Request Lifetime: The maximum time a request will wait for a
response. This value is equal to the overlay-reliability-timer value defined of the overlay
reliability value (defined in Section 11.1 11.1) multiplied by the
number of
transmissions, as defined transmissions (defined in Section 6.2.1, 6.2.1), and so
defaults to 15 seconds.

Node: The term "Node" is used to refer "node" refers to a host that may be either a Peer peer or
a Client. client. Because RELOAD uses the same protocol for both clients
and peers, much of the text applies equally to both.
Therefore Therefore,
we use "Node" "node" when the text applies to both Clients clients and peers, and
Peers
and we use the more specific term (i.e., client "client" or peer) "peer") when
the text applies only to Clients clients or only to Peers. peers.

Node-ID: A value of fixed but configurable length that uniquely
identifies a node. Node-IDs of all 0s and all 1s are reserved; a reserved. A
value of zero 0 is not used in the wire protocol protocol, but can be used to
indicate an invalid node in implementations and APIs; the APIs. The Node-ID
of all 1s is used on the wire protocol as a wildcard.

Overlay Algorithm: An overlay algorithm defines the rules for
determining which peers in an overlay store a particular piece of
data and for determining a topology of interconnections amongst
peers in order to find a piece of data.

Overlay Instance: A specific overlay algorithm and the collection of
peers that are collaborating to provide read and write access to
it. There can be any Any number of overlay instances can be running in an IP
network at a time, and each operates in isolation of the others.

Overlay Parameters: A set of values that are shared between among all nodes
in an overlay. The overlay parameters are distributed in an XML
document called the Configuration Document.

Peer: A host that is participating in the overlay. Peers are
responsible for holding some portion of the data that has been
stored in the overlay overlay, and also route they are responsible for routing
messages on behalf of other hosts as needed by the Overlay
Algorithm.

Peer Admission: The act of admitting a node (the "Joining Node") Joining Node) into
an Overlay. After the admission process is over, the joining
node Joining Node
is a fully-functional fully functional peer of the overlay. During the admission
process, the joining node Joining Node may need to present credentials to prove
that it has sufficient authority to join the overlay.

Resource: An object or group of objects stored in a P2P network.

Resource-ID: A value that identifies some resources and which is
used as a key for storing and retrieving the resource. Often this
is not human friendly/readable. One way to generate a Resource-ID
is by applying a mapping function to some other unique name (e.g.,
user name or service name) for the resource. The Resource-ID is
used by the distributed database algorithm to determine the peer
or peers that are responsible for storing the data for the
overlay. In structured P2P networks, Resource-IDs are generally
fixed length and are formed by hashing the resource name. Resource Name. In
unstructured networks, resource names Resource Names may be used directly as
Resource-IDs and may be of variable lengths. length.

Resource Name: The name by which a resource is identified. In
unstructured P2P networks, the resource name Resource Name is sometimes used
directly as a Resource-ID. In structured P2P networks networks, the
resource name
Resource Name is typically mapped into a Resource-ID by using the
string as the input to hash function. Structured and unstructured
P2P networks are described in [RFC5694]. A SIP resource, for
example, is often identified by its AOR (address-of-record), which
is an example of a Resource Name.

Responsible Peer: The peer that is responsible for a specific
resource, as defined by the topology plugin Topology Plug-in algorithm.

Routing Table: The set of directly connected peers which a node can
use to forward overlay messages. In normal operation, these peers
will all be on in the Connection Table Table, but not vice versa, because
some peers may not yet be available for routing. Peers may send
messages directly to peers that are in their Connection Tables Tables,
but may only forward messages to peers that are not in their Connection
Table only through peers that are in the Routing Table.

Successor Replacement Hold-Down Time: The amount of time to wait
before starting replication when a new successor is found; it
defaults to 30 seconds.

Transaction ID: A randomly chosen identifier selected by the
originator of a request and that is used to correlate requests and
responses.

Usage: A usage is the The definition of a set of data structures (data Kinds) that
an application wants to store in the overlay. A usage may also
define a set of network protocols (application (Application IDs) that can be
tunneled over TLS or DTLS direct connections between nodes.
E.g., For
example, the SIP usage Usage defines a SIP registration data Kind that Kind, which
contains information on how to reach a SIP endpoint endpoint, and two
application
Application IDs corresponding to the SIP and SIPS protocols.

User: A user is a physical person identified by the certificates assigned to
them.

User Name: A name identifying a user of the overlay, typically used
as a Resource Name, Name or as a label on a Resource resource that identifies the
user owning the resource.

4. Overlay Management Overview

The most basic function of RELOAD is as a generic overlay network.
Nodes need to be able to join the overlay, form connections to other
nodes, and route messages through the overlay to nodes to which they
are not directly connected. This section provides an overview of the
mechanisms that perform these functions.

3.1.

4.1. Security and Identification

The overlay parameters are specified in a configuration document. Configuration Document.
Because the parameters include security critical information security-critical information, such as
the certificate signing trust anchors, the configuration document Configuration Document
needs to be retrieved securely. The initial configuration document Configuration Document
is either initially fetched over HTTPS or manually provisioned;
subsequent configuration document provisioned.
Subsequent Configuration Document updates are received either by as a
result of being refreshed periodically refreshing from by the configuration server,
or, more commonly, by being flood filled flood-filled through the overlay, which
allows for fast propagation once an update is pushed. In the latter
case, updates are via digital signatures tracing that trace back to the
initial
configuration document. Configuration Document.

Every node in the RELOAD overlay is identified by a Node-ID. The
Node-ID is used for three major purposes:

o To address the node itself.

o To determine its the node's position in the overlay topology (if the
overlay is structured; overlays do not need to be structured).

o To determine the set of resources for which the node is
responsible.

Each node has a certificate [RFC5280] containing this its Node-ID in a
subjectAltName extension, which is unique within an overlay instance.

The certificate serves multiple purposes:

o It entitles the user to store data at specific locations in the
Overlay Instance. Each data Kind defines the specific rules for
determining which certificates can access each Resource-ID/Kind-ID
pair. For instance, some Kinds might allow anyone to write at a
given location, whereas others might restrict writes to the owner
of a single certificate.

o It entitles the user to operate a node that has a Node-ID found in
the certificate. When the node forms a connection to another
peer, it uses this certificate so that a node connecting to it
knows it is connected to the correct node (technically: node. (Technically, a (D)TLS TLS or
DTLS association with client authentication is formed.) In
addition, the node can sign messages, thus providing integrity and
authentication for messages which are sent from the node.

o It entitles the user to use the user name found in the
certificate.

If a user has more than one device, typically they would get one
certificate for each device. This allows each device to act as a
separate peer.

RELOAD supports multiple certificate issuance models. The first is
based on a central enrollment process process, which allocates a unique name
and Node-ID and puts them in a certificate for the user. All peers
in a particular Overlay Instance have the enrollment server as a
trust anchor and so can verify any other peer's certificate.

In some

The second model is useful in settings, when a group of users want to
set up an overlay network but are not concerned about attack by other
users in the network. For instance, users on a LAN might want to set
up a short term short-term ad hoc network without going to the trouble of
setting up an enrollment server. RELOAD supports the use of self-generated, self-
generated, self-signed certificates. When self-signed certificates
are used, the node also generates its own Node-ID and user name. The
Node-ID is computed as a digest of the public key, to prevent Node-ID
theft. Note that the relevant cryptographic property for the digest
is partial preimage resistance. Collision-resistance Collision resistance is not needed since needed,
because an attacker who can create two nodes with the same Node-ID
but a different public key obtains no advantage. This model is still
subject to a number of known attacks (most notably notably, Sybil attacks
[Sybil]) and can only be safely used only in closed networks where users
are mutually trusting. Another drawback of this approach is that the
user's data is then tied to their keys, key, so if a key is changed changed, any
data stored under their Node-ID needs to be re-stored. This is not
an issue for centrally- centrally issued Node-IDs provided that the CA re-issues
Certification Authority (CA) reissues the same Node-ID when a new
certificate is generated.

The general principle here is that the security mechanisms (TLS or
DTLS at the data link layer and message signatures at the message
transport layer) are always used, even if the certificates are self-
signed. This allows for a single set of code paths in the systems systems,
with the only difference being whether certificate verification is
used to chain to a single root of trust.

3.1.1.

4.1.1. Shared-Key Security

RELOAD also provides an admission control system based on shared
keys. In this model, the peers all share a single key which is used
to authenticate the peer-to-peer connections via TLS-PSK [RFC4279] or
TLS-SRP [RFC5054].

3.2.

4.2. Clients

RELOAD defines a single protocol that is used both as the peer
protocol and as the client protocol for the overlay. This Having a single
protocol simplifies implementation, particularly for devices that may
act in either role, and allows clients to inject messages directly
into the overlay.

We use the term "peer" to identify a node in the overlay that routes
messages for nodes other than those to which it is directly
connected. Peers also have storage responsibilities. We use the
term "client" to refer to nodes that do not have routing or storage
responsibilities. When text applies to both peers and clients, we
will simply refer to such devices as "nodes." "nodes".

RELOAD's client support allows nodes that are not participating in
the overlay as peers to utilize the same implementation and to
benefit from the same security mechanisms as the peers. Clients
possess and use certificates that authorize the user to store data at
certain locations in the overlay. The Node-ID in the certificate is
used to identify the particular client as a member of the overlay and
to authenticate its messages.

In RELOAD, unlike some other designs, clients are not a first-class
entity.
entities. From the perspective of a peer, a client is a node that
has connected to the overlay, but that has not yet taken steps to
insert itself into the overlay topology. It might never do so (if
it's a
client) client), or it might eventually do so (if it's just a node that's
that is taking a long time to join). The routing and storage rules
for RELOAD provide for correct behavior by peers regardless of
whether other nodes attached to them are clients or peers. Of
course, a client implementation needs to know that it intends to be a
client, but this localizes complexity only to that node.

For more discussion of about the motivation for RELOAD's client support,
see Appendix B.

3.2.1.

4.2.1. Client Routing

Clients may insert themselves in the overlay in two ways:

o Establish a connection to the peer responsible for the client's
Node-ID in the overlay. Then Then, requests may be sent from/to the
client using its Node-ID in the same manner as if it were a peer,
because the responsible peer in the overlay will handle the final
step of routing to the client. This may require a TURN [RFC5766]
relay in cases where NATs or firewalls prevent a client from
forming a direct connection with its responsible peer. Note that
clients that choose this option need to process Update
(Section 6.4.2.3) messages
from the peer. Those peer (Section 6.4.2.3). These updates can indicate that
the peer is no longer is responsible for the Client's client's Node-ID. The
client would then need to form a connection to the appropriate
peer. Failure to do so will result in the client no longer
receiving messages.

o Establish a connection with an arbitrary peer in the overlay
(perhaps based on network proximity or an inability to establish a
direct connection with the responsible peer). In this case, the
client will rely on RELOAD's Destination List feature
(Section 6.3.2.2)
feature to ensure reachability. The client can initiate
requests, and any node in the overlay that knows the Destination
List to its current location can reach it, but the client is not
directly reachable using only its Node-ID. If the client is to
receive incoming requests from other members of the overlay, the
Destination List needed to reach the client needs to be learnable
via other mechanisms, such as being stored in the overlay by a
usage. A client connected this way using a certificate with only
a single Node-ID can proceed to use the connection without
performing an Attach (Section 6.5.1). A client wishing to connect
using this mechanism with a certificate with multiple Node-IDs can
use a Ping (Section 6.5.3) to probe the Node-ID of the node to
which it is connected before doing performing the Attach.

3.2.2.

4.2.2. Minimum Functionality Requirements for Clients

A node may act as a client simply because it does not have the
capacity,
capacity or even an implementation of the topology plugin defined in
Section 6.4.1, needed need to act as a peer in the overlay. overlay, or because it does
not even have an implementation of the Topology Plug-in defined in
Section 6.4.1, needed to act as a peer in the overlay. In order to
exchange RELOAD messages with a peer, a client needs to meet a
minimum level of functionality. Such a client will:

o Implement RELOAD's connection-management operations that are used
to establish the connection with the peer.

o Implement RELOAD's data retrieval methods (with client
functionality).

o Be able to calculate Resource-IDs used by the overlay.

o Possess security credentials needed by the overlay that it is
implementing.

A client speaks the same protocol as the peers, knows how to
calculate Resource-IDs, and signs its requests in the same manner as
peers. While a client does not necessarily require a full
implementation of the overlay algorithm, calculating the Resource-ID
requires an implementation of an appropriate algorithm for the
overlay.

3.3.

4.3. Routing

This section discusses the capabilities of RELOAD's routing layer, layer and
the protocol features used to implement them, the capabilities, and
provides a brief overview of how they are used. Appendix A discusses
some alternative designs and the tradeoffs trade-offs that would be necessary
to support them.

RELOAD's routing provides the following capabilities:

Resource-based routing: Routing: RELOAD supports routing messages based
solely on the name of the resource. Such messages are delivered
to a node that is responsible for that resource. Both structured
and unstructured overlays are supported, so the route may not be
deterministic for all Topology Plugins. Plug-ins.

Node-based routing: Routing: RELOAD supports routing messages to a specific
node in the overlay.

Clients: RELOAD supports requests from and to clients that do not
participate in overlay routing, routing. The clients are located via
either of the mechanisms described above.

NAT Traversal: RELOAD supports establishing and using connections
between nodes separated by one or more NATs, including locating
peers behind NATs for those overlays allowing/requiring it.

Low state: State: RELOAD's routing algorithms do not require significant
state (i.e., state linear or greater in the number of outstanding
messages that have passed through it) to be stored on intermediate
peers.

Routability in unstable topologies: Unstable Topologies: Overlay topology changes
constantly in an overlay of moderate size due to the failure of
individual nodes and links in the system. RELOAD's routing allows
peers to re-route reroute messages when a failure is detected, and replies
can be returned to the requesting node as long as the peers that
originally forwarded the successful request do not fail before the
response is returned.

RELOAD's routing utilizes three basic mechanisms:

Destination Lists: While While, in principle principle, it is possible to just
inject a message into the overlay with a single Node-ID as the
destination, RELOAD provides a source routing source-routing capability in the
form of "Destination Lists". A Destination List provides a list
of the nodes through which a message flows in order (i.e., it is
loose source routed). The minimal destination list Destination List contains just
a single value.

Via Lists: In order to allow responses to follow the same path as
requests, each message also contains a "Via List", which is
appended to by each node a message traverses. This via list Via List can
then be inverted and used as a destination list Destination List for the response.

RouteQuery: The RouteQuery method allows a node to query a peer for
the next hop it will use to route a message. This method is
useful for diagnostics and for iterative routing (see
Section 6.4.2.4).

The basic routing mechanism used by that RELOAD uses is Symmetric Recursive. symmetric recursive.
We will first describe symmetric recursive routing and then discuss
its advantages in terms of the requirements discussed above.

Symmetric recursive routing requires that a request message follow a
path through the overlay to the destination: each peer forwards the
message closer to its destination. The return path of the response
is then
goes through the same path followed in reverse. nodes as the request (though it may also go
through some new intermediate nodes due to topology changes). Note
that a failure on the reverse path caused by a topology change after
the request was sent will be handled by the end-to-end retransmission
of the response as described in Section 6.2.1. For example, the
following figure shows a message following a route from A to Z
through B and X:

A B X Z
-------------------------------

---------->
Dest=Z
---------->
Via=A
Dest=Z
---------->
Via=A,B
Dest=Z

<----------
Dest=X,B,A
<----------
Dest=B,A
<----------
Dest=A

Note that the preceding Figure this figure does not indicate whether A is a client or peer:
peer. A forwards its request to B B, and the response is returned to A
in the same manner regardless of A's role in the overlay.

This figure shows use of full via lists Via Lists by intermediate peers B and
X. However, if B and/or X are willing to store state, then they may
elect to truncate the lists, lists and save that the truncated information
internally (keyed
by using the transaction ID as a key to allow it to be
retrieved later. Later, when the response message arrives, the
transaction ID), ID would be used to recover the truncated information and
return the response message along the path from which it was received when the response is received. request
arrived. This option requires a greater amount of state to be stored
on intermediate peers peers, but saves a small amount of bandwidth and
reduces the need for modifying the message en route. Selection of
this mode of operation is a choice for the individual peer; the
techniques are interoperable even on a single message. The figure
below shows B using full via lists Via Lists, but X truncating them to X1 and
saving the state internally.

A B X Z
-------------------------------

---------->
Dest=Z
---------->
Via=A
Dest=Z
---------->
Via=X1
Dest=Z

<----------
Dest=X,X1
<----------
Dest=B,A
<----------
Dest=A

As before, when B receives the message, B creates a via list Via List
consisting of [A]. However, instead of sending [A, B], X creates an
opaque ID X1 which maps internally to [A, B] (perhaps by being an
encryption of [A, B]) and then forwards to Z with only X1 as the via list. Via
List. When the response arrives at X, it maps X1 back to [A, B] and B],
then inverts it to produce the new destination list Destination List [B, A] A], and
finally routes it to B.

RELOAD also supports a basic Iterative iterative "routing" mode (where mode, in which the
intermediate peers merely return a response indicating the next hop,
but do not actually forward the message to that next hop themselves). themselves.
Iterative "routing" routing is implemented using the RouteQuery method (see
Section 6.4.2.4), which requests this behavior. Note that iterative
"routing"
routing is selected only by the initiating node.

3.4.

4.4. Connectivity Management

In order to provide efficient routing, a peer needs to maintain a set
of direct connections to other peers in the Overlay Instance. Due to
the presence of NATs, these connections often cannot be formed
directly. Instead, we use the Attach request to establish a
connection. Attach uses Interactive Connectivity Establishment (ICE)
[RFC5245] to establish the connection. It is assumed that the reader
is familiar with ICE.

Say that peer A wishes to form a direct connection to peer B, either
to join the overlay or to add more connections in its Routing Table.
It gathers ICE candidates and packages them up in an Attach request request,
which it sends to B through usual overlay routing procedures. B does
its own candidate gathering and sends back a response with its
candidates. A and B then do ICE connectivity checks on the candidate
pairs. The result is a connection between A and B. At this point, A
and B MAY send messages directly between themselves without going
through other overlay peers. In other words, A and B are on in each
other's Connection Tables. They MAY then execute an Update process,
resulting in additions to each other's Routing Tables, and may then
become able to route messages through each other to other overlay nodes
nodes.

There are two cases where Attach is not used. The first is when a
peer is joining the overlay and is not connected to any peers. In
order to support this case, some a small number of "bootstrap nodes" bootstrap nodes
typically need to be publicly accessible so that new peers can
directly connect to them. Section 11 contains more detail on this.
The second case is when a client connects to a peer at an arbitrary
IP address, rather than to its responsible peer, as described in the
second bullet point of Section 3.2.1. 4.2.1.

In general, a peer needs to maintain connections to all of the peers
near it in the Overlay Instance and to enough other peers to have
efficient routing (the details, e.g., details on what "enough" or and "near"
means, mean
depend on the specific overlay). If a peer cannot form a connection
to some other peer, this is not necessarily a disaster; overlays can
route correctly even without fully connected links. However, a peer
needs to try to maintain the specified Routing Table defined by the topology plugin
Topology Plug-in algorithm and needs to form new connections if it
detects that it has fewer direct connections than specified by the
algorithm. This also implies that peers, in accordance with the topology plugin
Topology Plug-in algorithm, need to periodically verify that the
connected peers are still alive and and, if not not, need to try to
reform re-form
the connection connections or form an alternate one. ones. See Section 10.7.4.3 for an
example on how a specific overlay algorithm implements these
constraints.

3.5.

4.5. Overlay Algorithm Support

The Topology Plugin Plug-in allows RELOAD to support a variety of overlay
algorithms. This specification defines a DHT based on Chord, which
is mandatory to implement, but the base RELOAD protocol is designed
to support a variety of overlay algorithms. The information needed
to implement this DHT is fully contained in this specification specification, but
it is easier to understand if you are familiar with Chord Chord-based
[Chord] based DHTs. A nice tutorial can be found at [wikiChord].

3.5.1.

4.5.1. Support for Pluggable Overlay Algorithms

RELOAD defines three methods for overlay maintenance: Join, Update,
and Leave. However, the contents of those these messages, when they are
sent, and their precise semantics are specified by the actual overlay
algorithm, which is specified by configuration for all nodes in the
overlay,
overlay and thus is known to nodes prior to their attempting before they attempt to join the
overlay. RELOAD merely provides a framework of commonly-needed commonly needed
methods that provides provide uniformity of notation (and ease of debugging)
for a variety of overlay algorithms.

3.5.2.

4.5.2. Joining, Leaving, and Maintenance Overview

When a new peer wishes to join the Overlay Instance, it will need a
Node-ID that it is allowed to use and a set of credentials which
match that Node-ID. When an enrollment server is used, the Node-ID
used is the Node-ID one found in the certificate received from the enrollment
server. The details of the joining procedure are defined by the
overlay algorithm, but the general steps for joining an Overlay
Instance are:

o Forming Form connections to some other peers.

o Acquiring Acquire the data values this peer is responsible for storing.

o Informing Inform the other peers which were previously responsible for that
data that this peer has taken over responsibility.

The first thing the peer needs to do is to form a connection to some
"bootstrap node".
bootstrap node. Because this is the first connection the peer makes,
these nodes will need public IP addresses so that they can be
connected to directly. Once a peer has connected to one or more
bootstrap nodes, it can form connections in the usual way way, by routing
Attach messages through the overlay to other nodes. Once After a peer has
connected to the overlay for the first time, it can cache the set of
past adjacencies which have public IP address addresses and can attempt to
use them as future bootstrap nodes. Note that this requires some
notion of which addresses are likely to be public as discussed in
Section 9.

Once

After a peer has connected to a bootstrap node, it then needs to take
up its appropriate place in the overlay. This requires two major
operations:

o Forming Form connections to other peers in the overlay to populate its
Routing Table.

o Getting Get a copy of the data it is now responsible for storing storing, and
assuming
assume responsibility for that data.

The second operation is performed by contacting the Admitting Peer
(AP), the node which is currently responsible for that the relevant
section of the overlay.

The details of this operation depend mostly on the overlay algorithm
involved, but a typical case would be:

1. JN (Joining Node) sends a Join request to AP (Admitting Peer) announcing its intention to join.

2. AP sends a Join response.

3. AP does a sequence of Stores to JN to give it the data it will
need.

4. AP does Updates to JN and to other peers to tell it them about its
own Routing Table. At this point, both JN and AP consider JN
responsible for some section of the Overlay Instance.

5. JN makes its own connections to the appropriate peers in the
Overlay Instance.

After this process is completed, completes, JN is a full member of the Overlay
Instance and can process Store/Fetch requests.

Note that the first node is a special case. When ordinary nodes
cannot form connections to the bootstrap nodes, then they are not
part of the overlay. However, the first node in the overlay can
obviously not connect to other nodes. In order to support this case,
potential first nodes (which can also initially serve as bootstrap nodes
initially)
nodes) need to somehow be instructed that they are the entire
overlay, rather than not part of it. an existing overlay (e.g., by comparing
their IP address to the bootstrap IP addresses in the configuration file)
file).

Note that clients do not perform either of these operations.

3.6.

4.6. First-Time Setup

Previous sections addressed how RELOAD works once after a node has
connected. This section provides an overview of how users get
connected to the overlay for the first time. RELOAD is designed so
that users can start with the name of the overlay they wish to join
and perhaps an account name and password, and can leverage that these into
having a working peer with minimal user intervention. This helps
avoid the problems that have been experienced with conventional SIP
clients where in which users need to manually configure a large number of
settings.

3.6.1.

4.6.1. Initial Configuration

In the first phase of the setup process, the user starts out with the
name of the overlay and uses this it to download an initial set of overlay
configuration parameters. The node does a DNS SRV [RFC2782] lookup
on the overlay name to get the address of a configuration server. It
can then connect to this server with HTTPS [RFC2818] to download a
configuration document
Configuration Document which contains the basic overlay configuration
parameters as well as a set of bootstrap nodes which can be used to
join the overlay. The details of the relations relationships between names in
the HTTPS certificates, certificates and the overlay names are described in
Section 11.2.

If a node already has the valid configuration document Configuration Document that it
received by some out of band an out-of-band method, this step can be skipped. Note
that that out of band this out-of-band method needs to provide authentication and
integrity, because the configuration document Configuration Document contains the trust
anchors used by the overlay.

3.6.2.

4.6.2. Enrollment

If the overlay is using centralized enrollment, then a user needs to
acquire a certificate before joining the overlay. The certificate
attests both to the user's name within the overlay and to the Node-
IDs
Node-IDs which they are permitted to operate. In that this case, the
configuration document
Configuration Document will contain the address of an enrollment
server which can be used to obtain such a certificate, certificate and will also
contain the trust anchor, so this document must be retrieved securely
(see Section 11.2). The enrollment server may (and probably will)
require some sort of account name for the user and a password before
issuing the certificate. The enrollment server's ability to ensure
attackers can not cannot get a large number of certificates for the overlay
is one of the cornerstones of RELOAD's security.

3.6.3.

4.6.3. Diagnostics

Significant advice around managing a RELOAD overlay and extensions
for diagnostics are described in [I-D.ietf-p2psip-diagnostics].

4. [P2P-DIAGNOSTICS].

5. Application Support Overview

RELOAD is not intended to be used alone, but rather as a substrate
for other applications. These applications can use RELOAD for a
variety of purposes:

o To store data in the overlay and to retrieve data stored by other
nodes.

o As a discovery mechanism for services such as TURN.

o To form direct connections which can be used to transmit
application-level messages without using the overlay.

This section provides an overview of these services.

4.1.

5.1. Data Storage

RELOAD provides operations to Store and Fetch data. Each location in
the Overlay Instance is referenced by a Resource-ID. However, each
location may contain data elements corresponding to multiple Kinds
(e.g., certificate, certificate and SIP registration). Similarly, there may be
multiple elements of a given Kind, as shown below:

+--------------------------------+
| Resource-ID |
| |
| +------------+ +------------+ |
| | Kind 1 | | Kind 2 | |
| | | | | |
| | +--------+ | | +--------+ | |
| | | Value | | | | Value | | |
| | +--------+ | | +--------+ | |
| | | | | |
| | +--------+ | | +--------+ | |
| | | Value | | | | Value | | |
| | +--------+ | | +--------+ | |
| | | +------------+ |
| | +--------+ | |
| | | Value | | |
| | +--------+ | |
| +------------+ |
+--------------------------------+

Each Kind is identified by a Kind-ID, which is a code point either
assigned by IANA or allocated out of a private range. As part of the
Kind definition, protocol designers may define constraints, such constraints (such as
limits on size, size) on the values which may be stored. For many Kinds,
the set may be restricted to a single value; value, while some sets may be
allowed to contain multiple identical items while items, and others may only have only
unique items. Note that a Kind may be employed by multiple usages usages,
and new usages are encouraged to use previously defined Kinds where
possible. We define the following data models in this document, though
although other usages can define their own structures:

single value: There can be at most one item in the set set, and any
value overwrites the previous item.

array: Many values can be stored and addressed by a numeric index.

dictionary: The values stored are indexed by a key. Often Often, this key
is one of the values from the certificate of the peer sending the
Store request.

In order to protect stored data from tampering by other nodes, each
stored value is individually digitally signed by the node which
created it. When a value is retrieved, the digital signature can be
verified to detect tampering. If the certificate used to verify the
stored value signature expires, the value can no longer be retrieved
(though
(although it may not be immediately garbage collected by the storing node)
node), and the creating node will need to store the value again if it
desires that the stored value to continue to be available.

4.1.1.

5.1.1. Storage Permissions

A major issue in peer-to-peer storage networks is minimizing the
burden of becoming a peer, and peer and, in particular particular, minimizing the amount
of data which any peer needs to to store for other nodes. RELOAD
addresses this issue by only allowing any given node to store data only at
a small number of locations in the overlay, with those locations
being determined by the node's certificate. When a peer uses a Store
request to place data at a location authorized by its certificate, it
signs that data with the private key that corresponds to its
certificate. Then the peer responsible for storing the data is able
to verify that the peer issuing the request is authorized to make
that request. Each data Kind defines the exact rules for determining
what certificate is appropriate.

The most natural rule is that a certificate authorizes a user to
store data keyed with their user name X. Thus, only a user with a
certificate for "alice@example.org" could write to that location in
the overlay (see Section 11.3). However, other usages can define any
rules they choose, including publicly writable values.

The digital signature over the data serves two purposes. First, it
allows the peer responsible for storing the data to verify that this
Store is authorized. Second, it provides integrity for the data.
The signature is saved along with the data value (or values) so that
any reader can verify the integrity of the data. Of course, the
responsible peer can "lose" the value value, but it cannot undetectably
modify it.

The size requirements of the data being stored in the overlay are
variable. For instance, a SIP AOR and voicemail differ widely in the
storage size. RELOAD leaves it to the Usage usage and overlay
configuration to limit size imbalance imbalances of various Kinds.

4.1.2.

5.1.2. Replication

Replication in P2P overlays can be used to provide:

persistence: if the responsible peer crashes and/or if the storing
peer leaves the overlay

security: to guard against DoS attacks by the responsible peer or
routing attacks to that responsible peer

load balancing: to balance the load of queries for popular
resources. resources

A variety of schemes are used in P2P overlays to achieve some of
these goals. Common techniques include replicating on neighbors of
the responsible peer, randomly locating replicas around the overlay,
or
and replicating along the path to the responsible peer.

The core RELOAD specification does not specify a particular
replication strategy. Instead, the first level of replication
strategies are is determined by the overlay algorithm, which can base the
replication strategy on its particular topology. For example, Chord
places replicas on successor peers, which will take over
responsibility if the responsible peer fails [Chord].

If additional replication is needed, for example example, if data persistence
is particularly important for a particular usage, then that usage may
specify additional replication, such as implementing random
replications by inserting a different well known well-known constant into the
Resource Name used to store each replicated copy of the resource.
Such replication strategies can be added independent independently of the
underlying algorithm, and their usage can be determined based on the
needs of the particular usage.

4.2.

5.2. Usages

By itself, the distributed storage layer just provides only the
infrastructure on which applications are built. In order to do
anything useful, a usage needs to be defined. Each Usage usage needs to
specify several things:

o Register Kind-ID code points for any Kinds that the Usage usage defines
(Section 14.6).

o Defines Define the data structure for each of the Kinds (the value member
in Section 7.2). If the data structure contains character string,
strings, conversion rules between characters and the binary
storage need to be specified.

o Define access control rules for each of the Kinds (Section 7.3).

o Define how the Resource Name is used to form the Resource-ID where
each Kind is stored.

o Describe how values will be merged when a network partition is
being healed.

The Kinds defined by a usage may also be applied to other usages.
However, a need for different parameters, such as a different access
control model, would imply the need to create a new Kind.

4.3.

5.3. Service Discovery

RELOAD does not currently define a generic service discovery
algorithm as part of the base protocol, although a simplistic TURN-
specific discovery mechanism is provided. A variety of service
discovery algorithms can be implemented as extensions to the base
protocol, such as the service discovery algorithm ReDIR
[opendht-sigcomm05] or [I-D.ietf-p2psip-service-discovery].

4.4. and [REDIR-RELOAD].

5.4. Application Connectivity

There is no requirement that a RELOAD usage Usage needs to use RELOAD's
primitives for establishing its own communication if it already
possesses its own means of establishing connections. For example,
one could design a RELOAD-based resource discovery protocol which
used HTTP to retrieve the actual data.

For more common situations, however, it is the overlay itself - --
rather than an external authority such as DNS - -- which is used to
establish a connection. RELOAD provides connectivity to applications
using the AppAttach method. For example, if a P2PSIP node wishes to
establish a SIP dialog with another P2PSIP node, it will use
AppAttach to establish a direct connection with the other node. This
new connection is separate from the peer protocol connection. It is
a dedicated DTLS or TLS flow used only for the SIP dialog.

5. RFC 2119 Terminology

6. Overlay Management Protocol

This section defines the basic protocols used to create, maintain,
and use the RELOAD overlay network. We start by defining the basic
concept of how message destinations are interpreted when routing
messages. We then describe the symmetric recursive routing model,
which is RELOAD's default routing algorithm. We then Finally, we define the
message structure and then finally define the messages used to join and maintain the
overlay.

6.1. Message Receipt and Forwarding

When a node receives a message, it first examines the overlay,
version, and other header fields to determine whether the message is
one it can process. If any of these are incorrect, as defined in
Section 6.3.2, it is an error and the message MUST be discarded. The
peer SHOULD generate an appropriate error error, but local policy can
override this and cause the messages message to be silently dropped.

Once the peer has determined that the message is correctly formatted
(note that this does not include signature checking signature-checking on intermediate
nodes as the message may be fragmented) fragmented), it examines the first entry
on the destination list. Destination List. There are three possible cases here:

o The first entry on the destination list Destination List is an ID for which the
peer is responsible. A peer is always responsible for the
wildcard Node-ID. Handling of this case is described in
Section 6.1.1.

o The first entry on the destination list Destination List is an ID for which another
peer is responsible. Handling of this case is described in
Section 6.1.2.

o The first entry on the destination list Destination List is an opaque ID that is
being used for destination list Destination List compression. Handling of this
case is described in Section 6.1.3. Note that opaque IDs can be
distinguished from Node-IDs and Resource-IDs on the wire as
described in Section 6.3.2.2.

These cases are handled as discussed below.

6.1.1. Responsible ID

If the first entry on the destination list Destination List is an ID for which the
peer is responsible, there are several (mutually exclusive) sub-cases subcases
to consider.

o If the entry is a Resource-ID, then it MUST be the only entry on
the destination list. Destination List. If there are other entries, the message
MUST be silently dropped. Otherwise, the message is destined for
this node node, so it the node MUST verify the signature as described in
Section 7.1 and MUST pass it up to the upper layers. "Upper layers"
is used here to mean the components above the "Overlay Link
Service Boundary" line in the figure in Section 1.2.

o If the entry is a Node-ID which equals this node's Node-ID, then
the message is destined for this node. If this it is the only entry on
the destination list, Destination List, the message is destined for this node and so
the node passes it up to the upper layers. Otherwise Otherwise, the node
removes the entry from the destination list Destination List and repeats the
routing process with the next entry on the destination list. Destination List. If
the message is a response and list compression was used, then the
node first modifies the destination list Destination List to reinsert the saved
state, e.g., by unpacking any opaque IDs.

o If the entry is the wildcard Node-ID (all "1"s), the message is
destined for this node node, and it the node passes it up the message to the
upper layers. A message with a wildcard Node-ID as its first
entry is never forwarded
and forwarded; it is consumed locally.

o If the entry is a Node-ID which is not equal to this node, then
the node MUST drop the message silently unless the Node-ID
corresponds to a node which is directly connected to this node
(i.e., a client). In the latter case, it the node MUST attempt to
forward the message to the destination node as described in the
next section.

Note that section (though this implies may fail for connectivity reasons,
because the TTL has expired, or because of some other error.)

Note that this process implies that in order to address a message to
"the peer that controls region X", a sender sends to Resource-ID X,
not Node-ID X.

6.1.2. Other ID

If the first entry in on the destination list Destination List is neither an opaque ID
nor an ID the peer is responsible for, then the peer MUST forward the
message towards this that entry. This means that it MUST select one of
the peers to which it is connected and which is most likely to be
responsible (according to the topology plugin) Topology Plug-in) for the first entry
on the destination list. Destination List. For the CHORD-RELOAD topology, the routing
to the most likely responsible node is explained in Section 10.3. If
the first entry on the destination list Destination List is in the peer's Connection
Table, then it the peer MUST forward the message to that peer directly.
Otherwise, the peer consults the Routing Table to forward the
message.

Any intermediate peer which forwards a RELOAD request MUST ensure
that if it receives a response to that message message, the response can be
routed back through the set of nodes through which the request
passed. The peer selects one of these approaches:

o The peer can add an entry to the via list Via List in the forwarding header
that will enable it to determine the correct node. This is done
by appending to the via list Via List the Node-ID of the node that sent from which
the request to this node. was received.

o The peer can keep per-transaction state which will allow it to
determine the correct node.

As an example of the first strategy, consider an example with nodes
A, B, C, D D, and E. If node D receives a message from node C with via
list Via
List [A, B], then D would forward to the next node E with via list Via List
[A, B, C]. Now, if E wants to respond to the message, it reverses
the via list Via List to produce the destination list, Destination List, resulting in
[D, C, B, A]. When D forwards the response to C, the destination list Destination
List will contain [C, B, A].

As an example of the second strategy, if node D receives a message
from node C with transaction ID X (as assigned by A) and via list Via List
[A, B], it could store [X, C] in its state database and forward the
message with the via list Via List unchanged. When D receives the response,
it consults its state database for transaction ID X, determines that
the request came from C, and forwards the response to C.

Intermediate peers which modify the via list Via List are not required to
simply add entries. The only requirement is that the peer MUST be
able to reconstruct the correct destination list Destination List on the return route.
RELOAD provides explicit support for this functionality in the form
of opaque IDs, which can replace any number of via list Via List entries.

For instance, in the above example, Node D might send E a via list Via List
containing only the opaque ID I. E would then use the destination
list Destination
List [D, I] to send its return message. When D processes this
destination list,
Destination List, it would detect that I is an opaque ID, recover the
via list
Via List [A, B, C], and reverse that to produce the correct
destination list
Destination List [C, B, A] before sending it to C. This feature is
called List Compression. "list compression". Possibilities for an opaque ID include a
compressed and/or encrypted version of the original via list or Via List and an
index into a state database containing the original via list, Via List, but the
details are a local matter.

No matter what mechanism for storing via list Via List state is used, if an
intermediate peer exits the overlay, then on the return trip the
message cannot be forwarded and will be dropped. The ordinary
timeout and retransmission mechanisms provide stability over this
type of failure.

Note that if an intermediate peer retains per-transaction state
instead of modifying the via list, Via List, it needs some mechanism for timing
out that state, otherwise state; otherwise, its state database will grow without
bound. Whatever algorithm is used, unless a FORWARD_CRITICAL
forwarding option (Section 6.3.2.3) or an overlay configuration
option explicitly indicates this state is not needed, the state MUST
be maintained for at least the value of the overlay-reliability-timer
configuration parameter and MAY be kept longer. Future extension, extensions,
such as
[I-D.ietf-p2psip-rpr], [P2PSIP-RELAY], may define mechanisms for determining when
this state does not need to be retained.

There is no requirement to ensure that a request issued after the
receipt of a response follows the same path as the response. As a
consequence, there is no requirement to use either of the mechanisms
described above (via list (Via List or state retention) when processing a
response message.

An intermediate

A node receiving a request from another node MUST
return a ensure that any
response to this that request exits that node with a destination list Destination List
equal to the concatenation of the Node-ID of the node that sent from which the
request was received with the via list Via List in the original request. The
intermediate node normally learns the Node-ID that the other node is
using via an Attach, but a node using a certificate with a single
Node-ID MAY elect to not to send an Attach (see Section 3.2.1 4.2.1, bullet
2). If a node with a certificate with multiple Node-IDs attempts to
route a message other than a Ping or Attach through a node without
performing an Attach, the receiving node MUST reject the request with
an Error_Forbidden error. The node MUST implement support for
returning responses to a Ping or Attach request made by a joining node Joining
Node Attaching to its responsible peer.

6.1.3. Opaque ID

If the first entry in on the destination list Destination List is an opaque ID (e.g., a
compressed via list), Via List), the peer MUST replace that the entry with the
original via list Via List that it replaced and then re-examine the
destination list
Destination List to determine which of the three cases in Section 6.1
now applies.

6.2. Symmetric Recursive Routing

This Section section defines RELOAD's Symmetric Recursive Routing (SRR) algorithm,
which is the default algorithm used by nodes to route messages
through the overlay. All implementations MUST implement this routing
algorithm. An overlay MAY be configured to use alternative routing
algorithms, and alternative routing algorithms MAY be selected on a
per-message basis. I.e., That is, a node in an overlay which supports SRR
Symmetric Recursive Routing and some other routing algorithm called
XXX might use SRR Symmetric Recursive Routing some of the time and XXX some of the time. at
other times.

6.2.1. Request Origination

In order to originate a message to a given Node-ID or Resource-ID, a
node MUST construct an appropriate destination list. Destination List. The simplest
such destination list Destination List is a single entry containing the Node-ID or
Resource-ID. The resulting message MUST use be forwarded to its
destination via the normal overlay routing mechanisms to forward the message to that destination. mechanisms. The node MAY
also construct a more complicated destination list Destination List for source
routing.

Once the message is constructed, the node sends the message to some an
adjacent peer. If the first entry on the destination list Destination List is
directly connected, then the message MUST be routed down that
connection. Otherwise, the topology plugin Topology Plug-in MUST be consulted to
determine the appropriate next hop.

Parallel requests for a resource are a common solution to improve
reliability in the face of churn or of subversive peers. Parallel
searches for usage-specified replicas are managed by the usage layer,
for instance instance, by having the usage store data at multiple Resource-IDs
Resource-IDs, with the requesting node sending requests to each of
those Resource-
IDs. Resource-IDs. However, a single request MAY also be routed
through multiple adjacent peers, even when they are known to be sub-optimal,
suboptimal, to improve reliability [vulnerabilities-acsac04]. Such
parallel searches MAY be specified by the topology plugin, Topology Plug-in, in which
case it would return multiple next hops and the request would be
routed to all of them.

Because messages can be lost in transit through the overlay, RELOAD
incorporates an end-to-end reliability mechanism. When an
originating node transmits a request request, it MUST set a timer to the
current overlay-reliability-timer. If a response has not been
received when the timer fires, the request MUST be retransmitted with
the same transaction identifier. The request MAY be retransmitted up
to 4 times (for times, for a total of 5 messages). messages. After the timer for the fifth
transmission fires, the message MUST be considered to have failed.
Although the originating node will be doing both end-to-end and hop-by-hop hop-
by-hop retransmissions, the end-by-end retransmission procedure is
not followed by intermediate nodes. They follow the hop-by-hop
reliability procedure described in Section 6.6.3.

The above algorithm can result in multiple requests being delivered
to a node. Receiving nodes MUST generate semantically equivalent
responses to retransmissions of the same request (this can be
determined by the transaction ID) if the request is received within
the maximum request lifetime (15 seconds). For some requests (e.g.,
Fetch)
Fetch), this can be accomplished merely by processing the request
again. For other requests, requests (e.g., Store) Store), it may be necessary to
maintain state for the duration of the request lifetime.

6.2.2. Response Origination

When a peer sends a response to a request using this routing
algorithm, it MUST construct the destination list Destination List by reversing the
order of the entries on the via list. Via List. This has the result that the
response traverses the same peers as the request traversed, except in
reverse order (symmetric routing) and possibly with extra nodes
(loose routing).

6.3. Message Structure

RELOAD is a message-oriented request/response protocol. The messages
are encoded using binary fields. All integers are represented in
network byte order. The general philosophy behind the design was to
use Type, Length, Value (TLV) fields to allow for extensibility.
However, for the parts of a structure that were required in all
messages, we just define these in a fixed position, as adding a type
and length for them is unnecessary and would simply only increase bandwidth
and
introduces introduce new potential for interoperability issues.

Each message has three parts, concatenated which are concatenated, as shown below:

+-------------------------+
| Forwarding Header |
+-------------------------+
| Message Contents |
+-------------------------+
| Security Block |
+-------------------------+

The contents of these parts are as follows:

Forwarding Header: Each message has a generic header which is used
to forward the message between peers and to its final destination.
This header is the only information that an intermediate peer
(i.e., one that is not the target of a message) needs to examine.
Section 6.3.2 describes the format of this part.

Message Contents: The message being delivered between the peers.
From the perspective of the forwarding layer, the contents are
opaque,
opaque; however, they are interpreted by the higher layers.
Section 6.3.3 describes the format of this part.

Security Block: A security block containing certificates and a
digital signature over the "Message Contents" section. Note that
this signature can be computed without parsing the message
contents. All messages MUST be signed by their originator.
Section 6.3.4 describes the format of this part.

6.3.1. Presentation Language

The structures defined in this document are defined using a C-like
syntax based on the presentation language used to define TLS
[RFC5246]. Advantages of this style include:

o It is familiar enough looking that most readers can grasp it quickly.

o The ability to define nested structures allows a separation
between high-level and low-level message structures.

o It has a straightforward wire encoding that allows quick
implementation, but the structures can be comprehended without
knowing the encoding.

o The ability It is possible to mechanically compile encoders and decoders.

Several idiosyncrasies of this language are worth noting. noting:

o All lengths are denoted in bytes, not objects.

o Variable length Variable-length values are denoted like arrays arrays, with angle
brackets.

o "select" is used to indicate variant structures.

For instance, "uint16 array<0..2^8-2>;" represents up to 254 bytes bytes,
which corresponds to up to 127 values of two bytes (16 bits) each.

A repetitive structure member shares a common notation with a member
containing a variable length variable-length block of data. The latter always starts
with "opaque" "opaque", whereas the former does not. For instance instance, the
following denotes a variable block of data:

opaque data<0..2^32-1>;

whereas the following denotes a list of 0, 1 1, or more instances of
the Name element:

Name names<0..2^32-1>;

6.3.1.1. Common Definitions

This section provides an introduction to the presentation language
used throughout RELOAD.

An enum represents an enumerated type. The values associated with
each possibility are represented in parentheses parentheses, and the maximum
value is represented as a nameless value, for purposes of describing
the width of the containing integral type. For instance, Boolean
represents a true or false:

enum { false(0), true(1), (255) } Boolean;

A boolean value is either a 1 or a 0. The max value of 255 indicates
that this is represented as a single byte on the wire.

The NodeId, shown below, represents a single Node-ID.

typedef opaque NodeId[NodeIdLength];

A NodeId is a fixed-length structure represented as a series of
bytes, with the most significant byte first. The length is set on a
per-overlay basis within the range of 16-20 bytes (128 to 160 bits).
(See Section 11.1 for how NodeIdLength is set.) Note: Note that the use of
"typedef" here is an extension to the TLS language, but its meaning
should be relatively obvious. Note Also note that the [ size ] syntax
defines a
fixed length fixed-length element that does not include the length of
the element in the on the wire on-the-wire encoding.

A ResourceId, shown below, represents a single Resource-ID.

typedef opaque ResourceId<0..2^8-1>;

Like a NodeId, a ResourceId is an opaque string of bytes, but unlike
NodeIds, ResourceIds are variable length, up to 254 bytes (2040 bits)
in length. On the wire, each ResourceId is preceded by a single
length byte (allowing lengths up to 255). 255 bytes). Thus, the 3-byte
value "FOO" would be encoded as: 03 46 4f 4f. Note the < range >
syntax defines a variable length element that does include includes the length of
the element in the on the wire on-the-wire encoding. The number of bytes to
encode the length on the wire is derived by range; i.e., it is the
minimum number of bytes which can encode the largest range value.

A more complicated example is IpAddressPort, which represents a
network address and can be used to carry either an IPv6 or IPv4
address:

enum { invalidAddressType(0), ipv4_address(1), ipv6_address(2),
(255) } AddressType;

struct {
uint32 addr;
uint16 port;
} IPv4AddrPort;

struct {
uint128 addr;
uint16 port;
} IPv6AddrPort;

struct {
AddressType type;
uint8 length;

select (type) {
case ipv4_address:
IPv4AddrPort v4addr_port;

case ipv6_address:
IPv6AddrPort v6addr_port;

/* This structure can be extended */
};
} IpAddressPort;

The first two fields in the structure are the same no matter what
kind of address is being represented:

type: the The type of address (v4 (IPv4 or v6). IPv6).

length: the The length of the rest of the structure.

By having the type and the length appear at the beginning of the
structure regardless of the kind of address being represented, an
implementation which does not understand new address type X can still
parse the IpAddressPort field and then discard it if it is not
needed.

The rest of the IpAddressPort structure is either an IPv4AddrPort or
an IPv6AddrPort. Both of these simply consist of an address
represented as an integer and a 16-bit port. As an example, here is
the wire representation of the IPv4 address "192.0.2.1" with port
"6084".

01 ; type = IPv4
06 ; length = 6
c0 00 02 01 ; address = 192.0.2.1
17 c4 ; port = 6084

Unless a given structure that uses a select explicitly allows for
unknown types in the select, any unknown type SHOULD be treated as an a
parsing error error, and the whole message SHOULD be discarded with no
response.

6.3.2. Forwarding Header

The forwarding header is defined as a ForwardingHeader structure, as
shown below.

struct {
uint32 relo_token;
uint32 overlay;
uint16 configuration_sequence;
uint8 version;
uint8 ttl;
uint32 fragment;
uint32 length;
uint64 transaction_id;
uint32 max_response_length;
uint16 via_list_length;
uint16 destination_list_length;
uint16 options_length;
Destination via_list[via_list_length];
Destination destination_list
[destination_list_length];
ForwardingOption options[options_length];
} ForwardingHeader;

The contents of the structure are:

relo_token: The first four bytes identify this message as a RELOAD
message. This field MUST contain the value 0xd2454c4f (the string
'RELO'
"RELO" with the high bit of the first byte set).

overlay: The 32 bit 32-bit checksum/hash of the overlay being used. This
MUST be formed by taking the lower 32 bits of the SHA-1 [RFC3174]
hash of the overlay name. The purpose of this field is to allow
nodes to participate in multiple overlays and to detect accidental
misconfiguration. This is not a security critical security-critical function. The
overlay name MUST consist of a sequence of characters what that would
be allowable as a DNS name. Specifically, as it is used in a DNS
lookup, it will need to be compliant with the grammar for the
domain as specified in section Section 2.3.1 of [RFC1035] . [RFC1035].

configuration_sequence: The sequence number of the configuration
file. See Section 6.3.2.1 for details details.

version: The version of the RELOAD protocol being used. This is a
fixed point used times 10.
RELOAD version numbers are fixed-point decimal numbers between
fixed-point integer between 0.1 and 25.4. This document describes
version 1.0, with a value of 0x0a. [Note: Pre-RFC (Note that versions used prior
to the publication of this RFC used version number 0.1]. 0.1.) Nodes
MUST reject messages with other versions.

ttl: An 8 bit 8-bit field indicating the number of iterations, or hops, a
message can experience before it is discarded. The TTL (time-to-
live) value MUST be decremented by one at every hop along the
route the message traverses just before transmission. If a
received message has a TTL of 0, 0 and the message is not destined
for the receiving node, then the message MUST NOT be propagated further
further, and a
"Error_TTL_Exceeded" an Error_TTL_Exceeded error should be generated. The
initial value of the TTL SHOULD be 100 and MUST NOT exceed 100
unless defined otherwise by the overlay configuration.
Implementations which receive message messages with a TTL greater than the
current value of initial-ttl (or the 100 default) default of 100) MUST discard
the message and send an "Error_TTL_Exceeded" Error_TTL_Exceeded error.

fragment: This field is used to handle fragmentation. The high bit
(0x80000000) MUST be set for historical reasons. If the next bit
(0x40000000) is set to 1, it indicates that this is the last (or
only) fragment. The next six bits (0x20000000 to through 0x01000000)
are reserved and SHOULD be set to zero. The remainder of the
field is used to indicate the fragment offset; see Section 6.7. 6.7 for
details.

length: The count in bytes of the size of the message message, including the
header, after the eventual fragmentation.

transaction_id: A unique 64 bit 64-bit number that identifies this
transaction and also allows receivers to disambiguate transactions
which are otherwise identical. In order to provide a high
probability that transaction IDs are unique, they MUST be randomly
generated. Responses use the same transaction ID as the request
to which they correspond to. correspond. Transaction IDs are also used for
fragment reassembly. See Section 6.7 for details.

max_response_length: The maximum size in bytes of a response. Used This
is used by requesting nodes to avoid receiving (unexpected) very
large responses. If this value is non-zero, responding peers MUST
check that any response would not exceed it and if so generate an
"Error_Incompatible_with_Overlay"
Error_Incompatible_with_Overlay value. This value SHOULD be set
to zero for responses.

via_list_length: The length of the via list Via List in bytes. Note that in
this field and the following two length fields fields, we depart from the
usual variable-length convention of having the length immediately
precede the value value, in order to make it easier for hardware
decoding engines to quickly determine the length of the header.

destination_list_length: The length of the destination list Destination List in
bytes.

options_length: The length of the header options in bytes.

via_list: The via_list contains the sequence of destinations through
which the message has passed. The via_list starts out empty and
grows as the message traverses each peer. In stateless cases, the
previous hop that the message is from is appended to the via list Via List
as specified in Section 6.1.2.

destination_list: The destination_list contains a sequence of
destinations through which the message should pass through. pass. The
destination list
Destination List is constructed by the message originator. The
first element in on the destination list Destination List is where the message goes
next. The Generally, the list shrinks as the message traverses each
listed peer. peer, though if list compression is used, this may not be
true.

options: Contains a series of ForwardingOption entries. See
Section 6.3.2.3.

6.3.2.1. Processing Configuration Sequence Numbers

In order to be part of the overlay, a node MUST have a copy of the
overlay configuration document. Configuration Document. In order to allow for configuration
document changes, each version of the configuration document Configuration Document MUST
contain a sequence number which MUST be monotonically increasing mod
65535. Because the sequence number may may, in principle principle, wrap, greater
than or less than are interpreted by modulo arithmetic as in TCP.

When a destination node receives a request, it MUST check that the
configuration_sequence field is equal to its own configuration
sequence number. If they do not match, it the node MUST generate an
error, either Error_Config_Too_Old or Error_Config_Too_New. In
addition, if the configuration file in the request is too old, it the
node MUST generate a ConfigUpdate message to update the requesting
node. This allows new
configuration documents Configuration Documents to propagate quickly
throughout the system. The one exception to this rule is that if the
configuration_sequence field is equal to 65535, 65535 and the message type
is ConfigUpdate, then the message MUST be accepted regardless of the
receiving node's configuration sequence number. Since 65535 is a
special value, peers sending a new configuration when the
configuration sequence is currently 65534 MUST set the configuration
sequence number to 0 when they send out a new configuration.

6.3.2.2. Destination and Via Lists

The destination list Destination List and via list Via List are sequences of Destination
values:

enum { invalidDestinationType(0), node(1), resource(2),
opaque_id_type(3), /* 128-255 not allowed */ (255) }
DestinationType;

select (destination_type) {
case node:
NodeId node_id;

case resource:
ResourceId resource_id;

case opaque_id_type:
opaque opaque_id<0..2^8-1>;

/* This structure may be extended with new types */
} DestinationData;

struct {
DestinationType type;
uint8 length;
DestinationData destination_data;
} Destination;

struct {
uint16 opaque_id; /* top Top bit MUST be 1 */
} Destination;

If the destination structure is a 16 bit 16-bit integer, then the first bit
MUST be set to 1 1, and it MUST be treated as if it were a full
structure with a DestinationType of opaque_id_type and an opaque_id
that was 2 bytes long with the value of the 16 bit 16-bit integer. If the
destination structure is starting starts with DestinationType, then the first bit
MUST be set to 0 0, and it is using the destination structure must use a TLV
structure with the following contents:

type
The type of the DestinationData Payload Data Unit (PDU). This It may
be one of "node", "resource", or "opaque_id_type".

length
The length of the destination_data.

destination_data
The destination value itself, which is an encoded DestinationData
structure, depending
structure that depends on the value of "type".

Note: This

Note that the destination structure encodes a type, length, value. Type, Length, Value.
The length Length field specifies the length of the DestinationData values,
which allows the addition of new DestinationTypes. This It also allows an
implementation which does not understand a given DestinationType to
skip over it.

A DestinationData can be one of three types:

node
A Node-ID.

opaque
A compressed list of Node-IDs and an eventual Resource-ID.
Because this value was has been compressed by one of the peers, it is only
meaningful only to that peer and cannot be decoded by other peers.
Thus, it is represented as an opaque string.

resource
The Resource-ID of the resource which is desired. This type MUST
only
appear only in the final location of a destination list Destination List and MUST
NOT appear in a via list. Via List. It is meaningless to try to route
through a resource.

One possible encoding of the 16 bit 16-bit integer version as an opaque
identifier is to encode an index into a Connection Table. To avoid
misrouting responses in the event a response is delayed and the
Connection Table entry has changed, the identifier SHOULD be split
between an index and a generation counter for that index. At
startup, When a
Node first joins the overlay, the generation counters SHOULD be
initialized to random values. An implementation MAY use 12 bits for
the Connection Table index and 3 bits for the generation counter.
(Note that this does not suggest a 4096 entry 4096-entry Connection Table for
every peer, only the ability to encode for a larger Connection
Table.) When a Connection Table slot is used for a new connection,
the generation counter is incremented (with wrapping). Connection
Table slots are used on a rotating basis to maximize the time
interval between uses of the same slot for different connections.

When routing a message to an entry in the destination list Destination List encoding a
Connection Table entry, the peer MUST confirm that the generation
counter matches the current generation counter of that index before
forwarding the message. If it does not match, the message MUST be
silently dropped.

6.3.2.3. Forwarding Option

The Forwarding header can be extended with forwarding header options,
which are a series of ForwardingOption structures:

enum { invalidForwardingOptionType(0), (255) }
ForwardingOptionType;

struct {
ForwardingOptionType type;
uint8 flags;
uint16 length;
select (type) {
/* This type may be extended */
};
} ForwardingOption;

Each ForwardingOption consists of the following values:

type
The type of the option. This structure allows for unknown options
types.

flags
Three flags are defined defined: FORWARD_CRITICAL(0x01),
DESTINATION_CRITICAL(0x02), and RESPONSE_COPY(0x04). These flags
MUST NOT be set in a response. If the FORWARD_CRITICAL flag is
set, any peer that would forward the message but does not
understand this options option MUST reject the request with an
Error_Unsupported_Forwarding_Option error response. If the
DESTINATION_CRITICAL flag is set, any node that generates a
response to the message but does not understand the forwarding
option MUST reject the request with an
Error_Unsupported_Forwarding_Option error response. If the
RESPONSE_COPY flag is set, any node generating a response MUST
copy the option from the request to the response except that the
RESPONSE_COPY, FORWARD_CRITICAL FORWARD_CRITICAL, and DESTINATION_CRITICAL flags
MUST be cleared.

length
The length of the rest of the structure. Note that a 0 length may
be reasonable if the mere presence of the option is meaningful and
no value is required.

option
The option value.

6.3.3. Message Contents Format

The second major part of a RELOAD message is the contents part, which
is defined by MessageContents:

enum { invalidMessageExtensionType(0),
(2^16-1) } MessageExtensionType;

struct {
MessageExtensionType type;
Boolean critical;
opaque extension_contents<0..2^32-1>;
} MessageExtension;

struct {
uint16 message_code;
opaque message_body<0..2^32-1>;
MessageExtension extensions<0..2^32-1>;
} MessageContents;

The contents of this structure are as follows:

message_code
This indicates the message that is being sent. The code space is
broken up as follows.

0 Reserved

1 follows:

0x0 Invalid Message Code. This code will never be assigned.

0x1 .. 0x7fff 0x7FFF Requests and responses. These code points are
always paired, with requests being an odd value and the
corresponding response being the request code plus 1. Thus,
"probe_request" (the Probe request) has the value 1 and
"probe_answer" (the Probe response) has the value 2

0x8000 .. 0xfffe 0xFFFE Reserved

0xffff
0xFFFF Error

The message codes are defined in Section 14.8 14.8.

message_body
The message body itself, represented as a variable-length string
of bytes. The bytes themselves are dependent on the code value.
See the sections describing the various RELOAD methods (Join,
Update, Attach, Store, Fetch, etc.) for the definitions of the
payload contents.

extensions
Extensions to the message. Currently no extensions are defined,
but new extensions can be defined by the process described in
Section 14.14.

All extensions have the following form:

type
The extension type.

critical
Whether this extension needs to be understood in order to process
the message. If critical = True and the recipient does not
understand the message, it MUST generate an
Error_Unknown_Extension error. If critical = False, the recipient
MAY choose to process the message even if it does not understand
the extension.

extension_contents
The contents of the extension (extension-dependent). (which are extension dependent).

The subsections in Section 6.4.2, Section 6.5 6.5, and Section 7 describe structures that are
inserted inside the message_body member, depending on the value of
the message_code value. For example example, a message_code value of
join_req means that the structure named JoinReq is inserted inside
message_body. This document does not contain a mapping between
message_code values and structure names names, as the conversion between
the two is obvious.

Similarly

Similarly, this document uses the name of the structure without the
"Req" or "Ans" suffix to mean the execution of a transaction
comprised
consisting of the matching request and answer. For example example, when the
text says "perform an Attach", it must be understood as performing a
transaction composed of an AttachReq and an AttachAns.

6.3.3.1. Response Codes and Response Errors

A node processing a request MUST return its status in the
message_code field. If the request was a success, then the message
code MUST be set to the response code that matches the request (i.e.,
the next code up). The response payload is then as defined in the
request/response descriptions.

If the request has failed, then the message code MUST be set to
0xffff (error) and the payload MUST be an error_response message, as
shown below.

When the message code is 0xffff, 0xFFFF, the payload MUST be an
ErrorResponse.
ErrorResponse:

public struct {
uint16 error_code;
opaque error_info<0..2^16-1>;
} ErrorResponse;

The contents of this structure are as follows:

error_code
A numeric error code indicating the error that occurred.

error_info
An optional arbitrary byte string. Unless otherwise specified,
this will be a UTF-8 text string providing that provides further information
about what went wrong. Developers are encouraged to put include
enough diagnostic information to be useful in error_info. The
specific text to be used and any relevant language or encoding
thereof is left to the implementation.

The following error code values are defined. The numeric values for
these are defined in Section 14.9.

Error_Forbidden:

Error_Forbidden
The requesting node does not have permission to make this request.

Error_Not_Found:

Error_Not_Found
The resource or node cannot be found or does not exist.

Error_Request_Timeout:

Error_Request_Timeout
A response to the request has not been received in a suitable
amount of time. The requesting node MAY resend the request at a
later time.

Error_Data_Too_Old:

Error_Data_Too_Old
A store cannot be completed because the storage_time precedes the
existing value.

Error_Data_Too_Large:

Error_Data_Too_Large
A store cannot be completed because the requested object exceeds
the size limits for that Kind.

Error_Generation_Counter_Too_Low:

Error_Generation_Counter_Too_Low
A store cannot be completed because the generation counter
precedes the existing value.

Error_Incompatible_with_Overlay:

Error_Incompatible_with_Overlay
A peer receiving the request is using a different overlay, overlay
algorithm, or hash algorithm, or some other parameter that is
inconsistent with the overlay configuration.

Error_Unsupported_Forwarding_Option:

Error_Unsupported_Forwarding_Option
A node receiving received the request with a forwarding options flagged as critical
critical, but the node does not support this option. See section
Section 6.3.2.3.

Error_TTL_Exceeded:

Error_TTL_Exceeded
A peer receiving received the request where in which the TTL got was decremented to
zero. See section Section 6.3.2.

Error_Message_Too_Large:

Error_Message_Too_Large
A peer receiving the received a request that was too large. See section Section 6.6.

Error_Response_Too_Large:

Error_Response_Too_Large
A node would have generated a response that is too large per the
max_response_length field.

Error_Config_Too_Old:

Error_Config_Too_Old
A destination node received a request with a configuration
sequence that's that is too old. See Section 6.3.2.1.

Error_Config_Too_New:

Error_Config_Too_New
A destination node received a request with a configuration
sequence that's that is too new. See Section 6.3.2.1.

Error_Unknown_Kind:

Error_Unknown_Kind
A destination peer received a request with an unknown Kind-ID.
See Section 7.4.1.2.

Error_In_Progress:

Error_In_Progress
An Attach to this peer is already in progress to this peer. progress. See
Section 6.5.1.2.

Error_Unknown_Extension:

Error_Unknown_Extension
A destination node received a request with an unknown extension.

Error_Invalid_Message:

Error_Invalid_Message
Something about this message is invalid invalid, but it doesn't does not fit the
other error codes. When this message is sent, implementations
SHOULD provide some meaningful description in error_info to aid in
debugging.

Error_Exp_A:

Error_Exp_A
For the purposes of experimentation. Not It is not meant for
vendor vendor-
specific use of any sort and MUST NOT be used for operational
deployments.

Error_Exp_B:

Error_Exp_B
For the purposes of experimentation. Not It is not meant for
vendor vendor-
specific use of any sort and MUST NOT be used for operational
deployments.

6.3.4. Security Block

The third part of a RELOAD message is the security block. The
security block is represented by a SecurityBlock structure:

struct {
CertificateType type; // From RFC 6091
opaque certificate<0..2^16-1>;
} GenericCertificate;

struct {
GenericCertificate certificates<0..2^16-1>;
Signature signature;
} SecurityBlock;

The contents of this structure are:

certificates
A bucket of certificates.

signature
A signature.

The certificates bucket SHOULD contain all the certificates necessary
to verify every signature in both the message and the internal
message objects, except for those certificates in a root-cert element
of the current configuration file. This is the only location in the
message which contains certificates, thus allowing for only a single copy
of each certificate to be sent. In systems that have an alternative
certificate distribution mechanism, some certificates MAY be omitted.
However, unless an alternative mechanism for immediately generating
certificates, such as shared secret security (Section 13.4) is used, implementors
implementers MUST include all referenced certificates.

NOTE TO IMPLEMENTERS: This requirement implies that a peer storing
data is obligated to retain certificates for the data that it holds.

Each certificate is represented by a GenericCertificate structure,
which has the following contents:

type
The type of the certificate, as defined in [RFC6091]. Only the
use of X.509 certificates is defined in this document.

certificate
The encoded version of the certificate. For X.509 certificates,
it is the DER Distinguished Encoding Rules (DER) form.

The signature is computed over the payload and parts of the
forwarding header. In case of a Store Store, the payload MUST contain an
additional signature computed as described in Section 7.1. All
signatures MUST be formatted using the Signature element. This
element is also used in other contexts where signatures are needed.
The input structure to the signature computation MAY vary depending
on the data element being signed.

enum { invalidSignerIdentityType(0),
cert_hash(1), cert_hash_node_id(2),
none(3)
(255) } SignerIdentityType;

struct {
select (identity_type) {

case cert_hash;
HashAlgorithm hash_alg; // From TLS
opaque certificate_hash<0..2^8-1>;

case cert_hash_node_id:
HashAlgorithm hash_alg; // From TLS
opaque certificate_node_id_hash<0..2^8-1>;

case none:
/* empty */
/* This structure may be extended with new types if necessary*/
};
} SignerIdentityValue;

struct {
SignerIdentityType identity_type;
uint16 length;
SignerIdentityValue identity[SignerIdentity.length];
} SignerIdentity;

struct {
SignatureAndHashAlgorithm algorithm; // From TLS
SignerIdentity identity;
opaque signature_value<0..2^16-1>;
} Signature;

The Signature construct contains the following values:

algorithm
The signature algorithm in use. The algorithm definitions are
found in the IANA TLS SignatureAlgorithm and HashAlgorithm
Registries.
registries. All implementations MUST support RSASSA-PKCS1-v1_5
[RFC3447] signatures with SHA-256 hashes. hashes [RFC6234].

identity
The identity, as defined in the two paragraphs following this
list, used to form the signature.

signature_value
The value of the signature.

Note that storage operations allow for special values of algorithm
and identity. See the Store Request Definition definition (Section 7.4.1.1)
and the Fetch Response Definition definition (Section 7.4.2.2).

There are two permitted identity formats, one for a certificate with
only one Node-ID and one for a certificate with multiple Node-IDs.
In the first case, the cert_hash type MUST be used. The hash_alg
field is used to indicate the algorithm used to produce the hash.
The certificate_hash contains the hash of the certificate object
(i.e., the DER-encoded certificate).

In the second case, the cert_hash_node_id type MUST be used. The
hash_alg is as in cert_hash cert_hash, but the cert_hash_node_id is computed
over the NodeId used to sign concatenated with the certificate.
I.e., certificate; i.e.,
H(NodeId || certificate). The NodeId is represented without any
framing or length fields, as simple raw bytes. This is safe because
NodeIds are fixed-length a fixed length for a given overlay.

For signatures over messages messages, the input to the signature is computed
over:

overlay || transaction_id || MessageContents || SignerIdentity

where overlay and transaction_id come from the forwarding header and
|| indicates concatenation.

The input to signatures over data values is different, different and is
described in Section 7.1.

All RELOAD messages MUST be signed. Intermediate nodes do not verify
signatures. Upon receipt (and fragment reassembly reassembly, if needed) needed), the
destination node MUST verify the signature and the authorizing
certificate. If the signature fails, the implementation SHOULD
simply drop the message and MUST NOT process it. This check provides
a minimal level of assurance that the sending node is a valid part of
the overlay as well as overlay, and it provides cryptographic authentication of the
sending node. In addition, responses MUST be checked as follows by
the requesting node:

1. The response to a message sent to a Node-ID MUST have been sent
by that Node-ID, Node-ID unless it the response has being been sent to the wildcard
Node-ID.

2. The response to a message sent to a Resource-ID MUST have been
sent by a Node-ID which is at least as close to or closer to the target
Resource-ID than as any node in the requesting node's Neighbor Table.

The second condition serves as a primitive check for responses from
wildly wrong nodes but is not a complete check. Note that in periods
of churn, it is possible for the requesting node to obtain a closer
neighbor while the request is outstanding. This will cause the
response to be rejected and the request to be retransmitted.

In addition, some methods (especially Store) have additional
authentication requirements, which are described in the sections
covering those methods.

6.4. Overlay Topology

As discussed in previous sections sections, RELOAD defines a default overlay
topology (CHORD-RELOAD) but allows for other topologies through the
use of Topology Plugins. Plug-ins. This section describes the requirements
for new topology plugins Topology Plug-ins and the methods that RELOAD provides for
overlay topology maintenance.

6.4.1. Topology Plugin Plug-in Requirements

When specifying a new overlay algorithm, at least the following MUST
be described:

o Joining procedures, including the contents of the Join message.

o Stabilization procedures, including the contents of the Update
message, the frequency of topology probes and keepalives, and the
mechanism used to detect when peers have disconnected.

o Exit procedures, including the contents of the Leave message.

o The length of the Resource-IDs. For DHTs, Resource-IDs and for DHTs the hash algorithm to
compute the hash of an identifier.

o The procedures that peers use to route messages.

o The replication strategy used to ensure data redundancy.

All overlay algorithms MUST specify maintenance procedures that send
Updates to clients and peers that have established connections to the
peer responsible for a particular ID when the responsibility for that
ID changes. Because tracking this information is difficult, overlay
algorithms MAY simply specify that an Update is sent to all members
of the Connection Table whenever the range of IDs for which the peer
is responsible changes.

6.4.2. Methods and types Types for use Use by topology plugins Topology Plug-ins

This section describes the methods that topology plugins Topology Plug-ins use to
join, leave, and maintain the overlay.

6.4.2.1. Join

A new peer (but one that (which already has credentials) uses the JoinReq message
to join the overlay. The JoinReq is sent to the responsible peer
depending on the routing mechanism described in the topology
plugin. Topology Plug-in.
This message notifies the responsible peer that the new peer is
taking over some of the overlay and that it needs to synchronize its
state.

struct {
NodeId joining_peer_id;
opaque overlay_specific_data<0..2^16-1>;
} JoinReq;

The minimal JoinReq contains only the Node-ID which the sending peer
wishes to assume. Overlay algorithms MAY specify other data to
appear in this request. Receivers of the JoinReq MUST verify that
the joining_peer_id field matches the Node-ID used to sign the
message and and, if not MUST reject not, the message MUST be rejected with an
Error_Forbidden error.

Because joins may only be executed only between nodes which are directly
adjacent, receiving peers MUST verify that any JoinReq they receive
arrives from a transport channel that is bound to the Node-ID to be
assumed by the joining node. This also prevents replay attacks
provided that Joining Node. Implementations MUST use DTLS
anti-replay is used. mechanisms, thus preventing replay attacks.

If the request succeeds, the responding peer responds with a JoinAns
message, as defined below:

struct {
opaque overlay_specific_data<0..2^16-1>;
} JoinAns;

If the request succeeds, the responding peer MUST follow up by
executing the right sequence of Stores and Updates to transfer the
appropriate section of the overlay space to the joining node. Joining Node. In
addition, overlay algorithms MAY define data to appear in the
response payload that provides additional info. information.

Joining nodes Nodes MUST verify that the signature on the JoinAns message
matches the expected target (i.e., the adjacency over which they are
joining.)
joining). If not, they MUST discard the message.

In general, nodes which cannot form connections SHOULD report an
error to the user. However, implementations MUST provide some
mechanism whereby nodes can determine that they are potentially the
first node and can take responsibility for the overlay (the overlay. (The idea is
to avoid having ordinary nodes try to become responsible for the
entire overlay during a partition.) This specification does not
mandate any particular mechanism, but a configuration flag or setting
seems appropriate.

6.4.2.2. Leave

The LeaveReq message is used to indicate that a node is exiting the
overlay. A node SHOULD send this message to each peer with which it
is directly connected prior to exiting the overlay.

struct {
NodeId leaving_peer_id;
opaque overlay_specific_data<0..2^16-1>;
} LeaveReq;

LeaveReq contains only the Node-ID of the leaving peer. Overlay
algorithms MAY specify other data to appear in this request.
Receivers of the LeaveReq MUST verify that the leaving_peer_id field
matches the Node-ID used to sign the message and and, if not MUST reject not, the message
MUST be rejected with an Error_Forbidden error.

Because leaves may only be executed only between nodes which are directly
adjacent, receiving peers MUST verify that any LeaveReq they receive
arrives from a transport channel that is bound to the Node-ID to be
assumed by the leaving peer. This also prevents replay attacks attacks,
provided that DTLS anti-replay is used.

Upon receiving a Leave request, a peer MUST update its own Routing
Table,
Table and send the appropriate Store/Update sequences to re-
stabilize re-stabilize
the overlay.

LeaveAns is an empty message.

6.4.2.3. Update

Update is the primary overlay-specific maintenance message. It is
used by the sender to notify the recipient of the sender's view of
the current state of the overlay (its (that is, its routing state), and it
is up to the recipient to take whatever actions are appropriate to
deal with the state change. In general, peers send Update messages
to all their adjacencies whenever they detect a topology shift.

When a peer receives an Attach request with the send_update flag set
to True (Section 6.4.2.4.1), it MUST send an Update message back to
the sender of the Attach request after the completion of the
corresponding ICE check and TLS connection. Note that the sender of
a
such an Attach request may not have joined the overlay yet.

When a peer detects through an Update that it is no longer
responsible for any data value it is storing, it MUST attempt to
Store a copy to the correct node unless it knows the newly
responsible node already has a copy of the data. This prevents data
loss during large-scale topology shifts shifts, such as the merging of
partitioned overlays.

The contents of the UpdateReq message are completely overlay- overlay
specific. The UpdateAns response is expected to be either success or
an error.

6.4.2.4. RouteQuery

The RouteQuery request allows the sender to ask a peer where they
would route a message directed to a given destination. In other
words, a RouteQuery for a destination X requests the Node-ID for the
node that the receiving peer would next route to in order to get to
X. A RouteQuery can also request that the receiving peer initiates initiate an
Update request to transfer the receiving peer's Routing Table.

One important use of the RouteQuery request is to support iterative
routing. The sender selects one of the peers in its Routing Table
and sends it a RouteQuery message with the destination field set to
the Node-ID or Resource-ID to which it wishes to route to. route. The
receiving peer responds with information about the peers to which the
request would be routed. The sending peer MAY then use the Attach
method to attach to that peer(s), peer(s) and repeat the RouteQuery.
Eventually, the sender gets a response from a peer that is closest to
the identifier in the destination field as determined by the topology plugin. Topology
Plug-in. At that point, the sender can send messages directly to
that peer.

6.4.2.4.1. Request Definition

A RouteQueryReq message indicates the peer or resource that the
requesting node is interested in. It also contains a "send_update"
option allowing that allows the requesting node to request a full copy of the
other peer's Routing Table.

struct {
Boolean send_update;
Destination destination;
opaque overlay_specific_data<0..2^16-1>;
} RouteQueryReq;

The contents of the RouteQueryReq message are as follows:

send_update
A single byte. This may be set to True to indicate that the
requester wishes the responder to initiate an Update request
immediately. Otherwise, this value MUST be set to False.

destination
The destination which the requester is interested in. This may be
any valid destination object, including a Node-ID, opaque ID, or
Resource-ID.
Note: If implementations are using opaque IDs for privacy
purposes, answering RouteQueryReqs for opaque IDs will allow the
requester to translate an opaque ID. Implementations MAY wish to
consider limiting the use of RouteQuery for opaque IDs in such
cases.

overlay_specific_data
Other data as appropriate for the overlay.

6.4.2.4.2. Response Definition

A response to a successful RouteQueryReq request is a RouteQueryAns
message. This message is completely overlay specific.

6.4.2.5. Probe

Probe provides primitive "exploration" services: it allows a node to
determine which resources another node is responsible for. A probe
can be addressed to a specific Node-ID, Node-ID or to the peer controlling a
given location (by using a Resource-ID). In either case, the target
node responds with a simple response containing some status
information.

6.4.2.5.1. Request Definition

The ProbeReq message contains a list (potentially empty) of the
pieces of status information that the requester would like the
responder to provide.

enum { invalidProbeInformationType(0), responsible_set(1),
num_resources(2), uptime(3), (255) }
ProbeInformationType;

struct {
ProbeInformationType requested_info<0..2^8-1>;
} ProbeReq;

The currently defined values for ProbeInformationType are:

responsible_set
indicates
Indicates that the peer should Respond with the fraction of the
overlay for which the responding peer is responsible.

num_resources
indicates
Indicates that the peer should Respond with the number of
resources currently being stored by the peer. Note that multiple
values under the same Resource-ID are counted only once.

uptime
indicates
Indicates that the peer should Respond with how long the peer has
been up up, in seconds.

6.4.2.5.2. Response Definition

A successful ProbeAns response contains the information elements
requested by the peer.

struct {
select (type) {
case responsible_set:
uint32 responsible_ppb;

case num_resources:
uint32 num_resources;

case uptime:
uint32 uptime;

/* This type may be extended */
};
} ProbeInformationData;

struct {
ProbeInformationType type;
uint8 length;
ProbeInformationData value;
} ProbeInformation;

struct {
ProbeInformation probe_info<0..2^16-1>;
} ProbeAns;

A ProbeAns message contains a sequence of ProbeInformation
structures. Each has a "length" indicating the length of the
following value field. This structure allows for unknown option
types.

Each of the current possible Probe information types is a 32-bit
unsigned integer. For type "responsible_ppb", it is the fraction of
the overlay for which the peer is responsible responsible, in parts per billion.
For type "num_resources", it is the number of resources the peer is
storing. For the type "uptime" "uptime", it is the number of seconds the peer
has been up.

The responding peer SHOULD include any values that the requesting
node requested and that it recognizes. They SHOULD be returned in
the requested order. Any other values MUST NOT be returned.

6.5. Forwarding and Link Management Layer

Each node maintains connections to a set of other nodes defined by
the topology plugin. Topology Plug-in. This section defines the methods RELOAD uses
to form and maintain connections between nodes in the overlay. Three
methods are defined:

Attach: used

Attach
Used to form RELOAD connections between nodes using ICE for NAT
traversal. When node A wants to connect to node B, it sends an
Attach message to node B through the overlay. The Attach contains
A's ICE parameters. B responds with its ICE parameters parameters, and the
two nodes perform ICE to form connection. Attach also allows two
nodes to connect via No-ICE instead of full ICE.

AppAttach: used

AppAttach
Used to form application layer application-layer connections between nodes.

Ping: is a

Ping
A simple request/response which is used to verify connectivity of
the target peer.

6.5.1. Attach

A node sends an Attach request when it wishes to establish a direct
Overlay Link connection to another node for the purpose of sending
RELOAD messages. A client that can establish a connection directly
need not send an Attach Attach, as described in the second bullet of
Section 3.2.1 4.2.1.

As described in Section 6.1, an Attach may be routed to either a
Node-ID or to a Resource-ID. An Attach routed to a specific Node-ID
will fail if that node is not reached. An Attach routed to a
Resource-ID will establish a connection with the peer currently
responsible for that Resource-ID, which may be useful in establishing
a direct connection to the responsible peer for use with frequent or
large resource updates.

An Attach Attach, in and of itself itself, does not result in updating the Routing
Table of either node. That function is performed by Updates. If
node A has Attached to node B, but has not received any Updates from
B, it MAY route messages which are directly addressed to B through
that
channel channel, but it MUST NOT route messages through B to other peers
via that channel. The process of Attaching is separate from the
process of becoming a peer (using Join and Update), to prevent half-open half-
open states where a node has started to form connections but is not
really ready to act as a peer. Thus, clients (unlike peers) can
simply Attach without sending Join or Update.

6.5.1.1. Request Definition

An Attach request message contains the requesting node ICE connection
parameters formatted into a binary structure.

enum { invalidOverlayLinkType(0), DTLS-UDP-SR(1),
DTLS-UDP-SR-NO-ICE(3), TLS-TCP-FH-NO-ICE(4),
(255) } OverlayLinkType;

enum { invalidCandType(0),
host(1), srflx(2), prflx(3), /* RESERVED(3), */ relay(4),
(255) } CandType;

struct {
opaque name<0..2^16-1>;
opaque value<0..2^16-1>;
} IceExtension;

struct {
IpAddressPort addr_port;
OverlayLinkType overlay_link;
opaque foundation<0..255>;
uint32 priority;
CandType type;
select (type) {
case host:
; /* Empty */
case srflx:
case prflx:
case relay:
IpAddressPort rel_addr_port;
};
IceExtension extensions<0..2^16-1>;
} IceCandidate;

struct {
opaque ufrag<0..2^8-1>;
opaque password<0..2^8-1>;
opaque role<0..2^8-1>;
IceCandidate candidates<0..2^16-1>;
Boolean send_update;
} AttachReqAns;

The values contained in AttachReqAns are:

ufrag
The username fragment (from ICE).

password
The ICE password.

role
An active/passive/actpass attribute from RFC 4145 [RFC4145]. This
value MUST be 'passive' "passive" for the offerer (the peer sending the
Attach request) and 'active' "active" for the answerer (the peer sending
the Attach response).

candidates
One or more ICE candidate values, as described below.

send_update
Has the same meaning as the send_update field in RouteQueryReq.

Each ICE candidate is represented as an IceCandidate structure, which
is a direct translation of the information from the ICE string
structures, with the exception of the component ID. Since there is
only one component, it is always 1, and thus left out of the
structure. The remaining values are specified as follows:

addr_port
corresponds
Corresponds to the ICE connection-address and port productions.

overlay_link
corresponds
Corresponds to the ICE transport production, production. Overlay Link
protocols used with No-ICE MUST specify "No-ICE" in their
description. Future overlay link values can be added by defining
new OverlayLinkType values in the IANA registry as described in
Section 14.10. Future extensions to the encapsulation or framing, framing
that provide for backward compatibility with the previously
specified encapsulation or framing, framing values MUST use that the same
OverlayLinkType value that was previously defined.
OverlayLinkType protocols are defined in Section 6.6

A single AttachReqAns MUST NOT include both candidates whose
OverlayLinkType protocols use ICE (the default) and candidates
that specify "No-ICE".

foundation
corresponds
Corresponds to the ICE foundation production.

priority
corresponds
Corresponds to the ICE priority production.

type
corresponds
Corresponds to the ICE cand-type production.

rel_addr_port
corresponds
Corresponds to the ICE rel-addr and rel-port productions. Only It is
present only for types "relay", "srflx" "prfix", and "prflx". "srflx".

extensions
ICE extensions. The name and value fields correspond to binary
translations of the equivalent fields in the ICE extensions.

These values should be generated using the procedures described in
Section 6.5.1.3.

6.5.1.2. Response Definition

If a peer receives an Attach request, it MUST determine how to
process the request as follows:

o If it the peer has not initiated an Attach request to the originating
peer of this Attach request, it MUST process this request and
SHOULD generate its own response with an AttachReqAns. It should
then begin ICE checks.

o If it the peer has already sent an Attach request to and received the
response from the originating peer of this Attach request, and request and, as
a result, an ICE check and TLS connection is are in progress, then it
SHOULD generate an Error_In_Progress error instead of an
AttachReqAns.

o If it the peer has already sent an Attach request to but not yet
received the response from the originating peer of this Attach
request, it SHOULD apply the following tie-breaker heuristic to
determine how to handle this Attach request and the incomplete
Attach request it has sent out:

* If the peer's own Node-ID is smaller when compared as big-
endian unsigned integers, it MUST cancel retransmission of its
own incomplete Attach request. It MUST then process this
Attach request, generate an AttachReqAns response, and proceed
with the corresponding ICE check.

* If the peer's own Node-ID is larger when compared as big-endian
unsigned integers, it MUST generate an Error_In_Progress error
to this Attach request, and then proceed to wait for and
complete the Attach and the corresponding ICE check it has
originated.

o If the peer is overloaded or detects some other kind of error, it
MAY generate an error instead of an AttachReqAns.

When a peer receives an Attach response, it SHOULD parse the response
and begin its own ICE checks.

6.5.1.3. Using ICE With with RELOAD

This section describes the profile of ICE that is used with RELOAD.
RELOAD implementations MUST implement full ICE.

In ICE ICE, as defined by [RFC5245], SDP the Session Description Protocol
(SDP) is used to carry the ICE parameters. In RELOAD, this function
is performed by a binary encoding in the Attach method. This
encoding is more restricted than the SDP encoding because the RELOAD
environment is simpler:

o Only a single media stream is supported.

o In this case, the "stream" refers not to RTP or other types of
media, but rather to a connection for RELOAD itself or other
application-layer protocols protocols, such as SIP.

o RELOAD only allows only for a single offer/answer exchange. Unlike the
usage of ICE within SIP, there is never a need to send a
subsequent offer to update the default candidates to match the
ones selected by ICE.

An agent follows the ICE specification as described in [RFC5245] with
the changes and additional procedures described in the subsections
below.

6.5.1.4. Collecting STUN Servers

ICE relies on the node having one or more STUN Session Traversal Utilities
for NAT (STUN) servers to use. In conventional ICE, it is assumed
that nodes are configured with one or more STUN servers through some out of band
out-of-band mechanism. This is still possible in RELOAD RELOAD, but RELOAD
also learns STUN servers as it connects to other peers. Because all RELOAD peers implement ICE and use STUN
keepalives, every peer is a capable of responding to STUN Binding
requests [RFC5389]. Accordingly, any peer that a node knows about
can be used like a STUN server -- though of course it may be behind a
NAT.

A peer on a well-provisioned wide-area overlay will be configured
with one or more bootstrap nodes. These nodes make an initial list
of STUN servers. However, as the peer forms connections with
additional peers, it builds more peers that it can use like STUN
servers.

Because complicated NAT topologies are possible, a peer may need more
than one STUN server. Specifically, a peer that is behind a single
NAT will typically observe only two IP addresses in its STUN checks:
its local address and its server reflexive address from a STUN server
outside its NAT. However, if there are more NATs are involved, it a peer may
learn additional server reflexive addresses (which vary based on
where in the topology the STUN server is). To maximize the chance of
achieving a direct connection, a peer SHOULD group other peers by the
peer-reflexive addresses it discovers through them. It SHOULD then
select one peer from each group to use as a STUN server for future
connections.

Only peers to which the peer currently has connections may be used.
If the connection to that host is lost, it MUST be removed from the
list of STUN servers servers, and a new server from the same group MUST be
selected unless there are no others servers in the group group, in which
case some other peer MAY be used.

6.5.1.5. Gathering Candidates

When a node wishes to establish a connection for the purposes of
RELOAD signaling or application signaling, it follows the process of
gathering candidates as described in Section 4 of ICE [RFC5245].
RELOAD utilizes a single component. Consequently, gathering for
these "streams" requires a single component. In the case where a
node has not yet found a TURN server, the agent would not include a
relayed candidate.

The ICE specification assumes that an ICE agent is configured with,
or somehow knows of, TURN and STUN servers. RELOAD provides a way
for an agent to learn these by querying the overlay, as described in
Section
Sections 6.5.1.4 and Section 9.

The default candidate selection described in Section 4.1.4 of ICE is
ignored; defaults are not signaled or utilized by RELOAD.

An alternative to using the full ICE supported by the Attach request
is to use the No-ICE mechanism by providing candidates with "No-ICE"
Overlay Link protocols. Configuration for the overlay indicates
whether or not these Overlay Link protocols can be used. An overlay
MUST be either all ICE or all No-ICE.

No-ICE will not work in all of the scenarios where ICE would work, but
in some cases, particularly those with no NATs or firewalls, it will
work.

6.5.1.6. Prioritizing Candidates

However, standardization

Standardization of additional protocols for use with ICE is expected,
including TCP [RFC6544] and protocols such as SCTP the Stream Control
Transmission Protocol (SCTP) [RFC4960] and DCCP Datagram Congestion
Control Protocol (DCCP) [RFC4340]. UDP encapsulations for SCTP and
DCCP would expand the available Overlay Link protocols available for RELOAD.

When additional protocols are available, the following prioritization
is RECOMMENDED:

o Highest priority is assigned to protocols that offer well-
understood congestion and flow control without head of line
blocking. For head-of-line
blocking, for example, SCTP without message ordering, DCCP, or and
those protocols encapsulated using UDP.

o Second highest priority is assigned to protocols that offer well-
understood congestion and flow control control, but that have head of line
blocking head-of-line
blocking, such as TCP.

o Lowest priority is assigned to protocols encapsulated over UDP
that do not implement well-established congestion control
algorithms. The DTLS/UDP with SR Simple Reliability (SR) overlay
link protocol is an example of such a protocol.

Head of line

Head-of-line blocking is undesirable in an Overlay Link protocol protocol,
because the messages carried on a RELOAD link are independent, rather
than stream-oriented. Therefore, if message N on a link is lost,
delaying message N+1 on that same link until N is successfully
retransmitted does nothing other than increase the latency for the
transaction of message N+1 N+1, as they are unrelated to each other.
Therefore, while the high quality, performance, and availability of
modern TCP implementations makes them very attractive, their
performance as an Overlay Link protocol protocols is not optimal.

Note that none of the protocols defined in this document meets these
conditions, but it is expected that new Overlay link Link protocols
defined in the future will fill this gap.

6.5.1.7. Encoding the Attach Message

Section 4.3 of ICE describes procedures for encoding the SDP for
conveying RELOAD candidates. Instead of actually encoding an SDP
message, the candidate information (IP address and port and transport
protocol, priority, foundation, type type, and related address) is carried
within the attributes of the Attach request or its response.
Similarly, the username fragment and password are carried in the
Attach message or its response. Section 6.5.1 describes the detailed
attribute encoding for Attach. The Attach request and its response
do not contain any default candidates or the ice-lite attribute, as
these features of ICE are not used by RELOAD.

Since the Attach request contains the candidate information and short
term credentials, it is considered as an offer for a single media
stream that happens to be encoded in a format different than SDP, but
is otherwise considered a valid offer for the purposes of following
the ICE specification. Similarly, the Attach response is considered
a valid answer for the purposes of following the ICE specification.

6.5.1.8. Verifying ICE Support

An agent MUST skip the verification procedures in Section Sections 5.1 and
6.1 of ICE. Since RELOAD requires full ICE from all agents, this
check is not required.

6.5.1.9. Role Determination

The roles of controlling and controlled controlled, as described in Section 5.2
of ICE ICE, are still utilized with RELOAD. However, the offerer (the
entity sending the Attach request) will always be controlling, and
the answerer (the entity sending the Attach response) will always be
controlled. The connectivity checks MUST still contain the ICE-
CONTROLLED and ICE-CONTROLLING attributes, however, even though the
role reversal capability for which they are defined will never be
needed with RELOAD. This is to allow for a common codebase between
ICE for RELOAD and ICE for SDP.

6.5.1.10. Full ICE

When the overlay uses ICE, connectivity checks and nominations are
used as in regular ICE.

6.5.1.10.1. Connectivity Checks

The processes of forming check lists in Section 5.7 of ICE,
scheduling checks in Section 5.8, and checking connectivity checks in
Section 7 are used with RELOAD without change.

6.5.1.10.2. Concluding ICE

The procedures in Section 8 of ICE are followed to conclude ICE, with
the following exceptions:

o The controlling agent MUST NOT attempt to send an updated offer
once the state of its single media stream reaches Completed.

o Once the state of ICE reaches Completed, the agent can immediately
free all unused candidates. This is because RELOAD does not have
the concept of forking, and thus the three second three-second delay in Section
8.3 of ICE does not apply.

6.5.1.10.3. Media Keepalives

STUN MUST be utilized for the keepalives described in Section 10 of
ICE.

6.5.1.11. No-ICE

No-ICE is selected when either side has provided "no ICE" Overlay
Link candidates. STUN is not used for connectivity checks when doing
No-ICE; instead instead, the DTLS or TLS handshake (or similar security layer
of future overlay link protocols) forms the connectivity check. The
certificate exchanged during the (D)TLS TLS or DTLS handshake MUST match the
node
that which sent the AttachReqAns AttachReqAns, and if it does not, the connection
MUST be closed.

6.5.1.12. Subsequent Offers and Answers

An agent MUST NOT send a subsequent offer or answer. Thus, the
procedures in Section 9 of ICE MUST be ignored.

6.5.1.13. Sending Media

The procedures of Section 11 of ICE apply to RELOAD as well.
However, in this case, the "media" takes the form of application application-
layer protocols (e.g., RELOAD) over TLS or DTLS. Consequently, once
ICE processing completes, the agent will begin TLS or DTLS procedures
to establish a secure connection. The node which that sent the Attach
request MUST be the TLS server. The other node MUST be the TLS
client. The server MUST request TLS client authentication. The
nodes MUST verify that the certificate presented in the handshake
matches the identity of the other peer as found in the Attach
message. Once the TLS or DTLS signaling is complete, the application
protocol is free to use the connection.

The concept of a previous selected pair for a component does not
apply to RELOAD, since ICE restarts are not possible with RELOAD.

6.5.1.14. Receiving Media

An agent MUST be prepared to receive packets for the application
protocol (TLS or DTLS carrying RELOAD) at any time. The jitter and
RTP considerations in Section 11 of ICE do not apply to RELOAD.

6.5.2. AppAttach

A node sends an AppAttach request when it wishes to establish a
direct connection to another node for the purposes of sending
application layer
application-layer messages. AppAttach is nearly identical to Attach,
except for the purpose of the connection: it is used to transport
non-RELOAD "media". A separate request is used to avoid implementor implementer
confusion between the two methods (this was found to be a real
problem with initial implementations). The AppAttach request and its
response contain an application attribute, which indicates what
protocol is to be run over the connection.

6.5.2.1. Request Definition

An AppAttachReq message contains the requesting node's ICE connection
parameters formatted into a binary structure.

struct {
opaque ufrag<0..2^8-1>;
opaque password<0..2^8-1>;
uint16 application;
opaque role<0..2^8-1>;
IceCandidate candidates<0..2^16-1>;
} AppAttachReq;

The values contained in AppAttachReq and AppAttachAns are:

ufrag
The username fragment (from ICE) ICE).

password
The ICE password.

application
A 16-bit application-id Application-ID, as defined in the Section 14.5. This
number represents the IANA registered IANA-registered application that is going to
send data on this connection.

role
An active/passive/actpass attribute from RFC 4145 [RFC4145].

candidates
One or more ICE candidate values values.

The application using the connection that is set up with this request
is responsible for providing sufficiently frequent keep traffic for of sufficient frequency to keep
the NAT and Firewall keep alive and for deciding when binding alive. Applications will often send
traffic every 25 seconds to close the
connection. ensure this.

6.5.2.2. Response Definition

If a peer receives an AppAttach request, it SHOULD process the
request and generate its own response with a AppAttachAns. It should
then begin ICE checks. When a peer receives an AppAttach response,
it SHOULD parse the response and begin its own ICE checks. If the
application
Application ID is not supported, the peer MUST reply with an
Error_Not_Found error.

struct {
opaque ufrag<0..2^8-1>;
opaque password<0..2^8-1>;
uint16 application;
opaque role<0..2^8-1>;
IceCandidate candidates<0..2^16-1>;
} AppAttachAns;

The meaning of the fields is the same as in the AppAttachReq.

6.5.3. Ping

Ping is used to test connectivity along a path. A ping can be
addressed to a specific Node-ID, to the peer controlling a given
location (by using a Resource-ID) Resource-ID), or to the wildcard Node-ID.

6.5.3.1. Request Definition

The PingReq structure is used to make a Ping request.

struct {
opaque<0..2^16-1> padding;
} PingReq;

The Ping request is empty of meaningful contents. However, it may
contain up to 65535 bytes of padding to facilitate the discovery of
overlay maximum packet sizes.

6.5.3.2. Response Definition

A successful PingAns response contains the information elements
requested by the peer.

struct {
uint64 response_id;
uint64 time;
} PingAns;
A PingAns message contains the following elements:

response_id
A randomly generated 64-bit response ID. This is used to
distinguish Ping responses.

time
The time when the Ping response was created created, represented in the
same way as storage_time storage_time, defined in Section 7.

6.5.4. ConfigUpdate

The ConfigUpdate method is used to push updated configuration data
across the overlay. Whenever a node detects that another node has
old configuration data, it MUST generate a ConfigUpdate request. The
ConfigUpdate request allows updating of two kinds of data: the
configuration data (Section 6.3.2.1) and the Kind information
(Section 7.4.1.1).

6.5.4.1. Request Definition

The ConfigUpdateReq structure is used to provide updated
configuration information.

enum { invalidConfigUpdateType(0), config(1), kind(2), (255) }
ConfigUpdateType;

typedef uint32 KindId;
typedef opaque KindDescription<0..2^16-1>;

struct {
ConfigUpdateType type;
uint32 length;

select (type) {
case config:
opaque config_data<0..2^24-1>;

case kind:
KindDescription kinds<0..2^24-1>;

/* This structure may be extended with new types*/ types */
};
} ConfigUpdateReq;

The ConfigUpdateReq message contains the following elements:

type
The type of the contents of the message. This structure allows
for unknown content types.

length
The length of the remainder of the message. This is included to
preserve backward compatibility and is 32 bits instead of 24 to
facilitate easy conversion between network and host byte order.

config_data (type==config)
The contents of the configuration document. Configuration Document.

kinds (type==kind)
One or more XML kind-block productions (see Section 11.1). These
MUST be encoded with UTF-8 and assume a default namespace of
"urn:ietf:params:xml:ns:p2p:config-base".

6.5.4.2. Response Definition

The ConfigUpdateAns structure is used to respond to a ConfigUpdateReq
request.

struct {
} ConfigUpdateAns;

If the ConfigUpdateReq is of type "config" "config", it MUST only be processed only
if all the following are true:

o The sequence number in the document is greater than the current
configuration sequence number.

o The configuration document Configuration Document is correctly digitally signed (see
Section 11 for details on signatures.)

Otherwise signatures).

Otherwise, appropriate errors MUST be generated.

If the ConfigUpdateReq is of type "kind" "kind", it MUST only be processed only
if it is correctly digitally signed by an acceptable Kind signer
(i.e., one listed in the current configuration file). Details on kind-
signer the
kind-signer field in the configuration file are described in
Section 11.1. In addition, if the Kind update conflicts with an
existing known Kind (i.e., it is signed by a different signer), then
it should be rejected with "Error_Forbidden". an Error_Forbidden error. This should not
happen in correctly functioning overlays.

If the update is acceptable, then the node MUST reconfigure itself to
match the new information. This may include adding permissions for
new Kinds, deleting old Kinds, or even, in extreme circumstances,
exiting and reentering re-entering the overlay, if, for instance, the DHT
algorithm has changed.

If an implementation misses enough ConfigUpdates which that include key
changes, it is possible that it will no longer be able to verify new
valid ConfigUpdates. In that this case, the only available recovery
mechanism is to attempt to retrieve a new configuration document, Configuration Document,
typically by the mechanisms it would use used for initial bootstrapping. It is up
to implementors implementers whether or how to decide to employ this sort of
recovery mechanism.

The response for ConfigUpdate is empty.

6.6. Overlay Link Layer

RELOAD can use multiple Overlay Link protocols to send its messages.
Because ICE is used to establish connections (see Section 6.5.1.3),
RELOAD nodes are able to detect which Overlay Link protocols are
offered by other nodes and establish connections between them. Any
link protocol needs to be able to establish a secure, authenticated
connection and to provide data origin authentication and message
integrity for individual data elements. RELOAD currently supports
three Overlay Link protocols:

o DTLS [RFC6347] over UDP with Simple Reliability (SR)
(OverlayLinkType=DTLS-UDP-SR)

o TLS [RFC5246] over TCP with Framing Header, No-ICE
(OverlayLinkType=TLS-TCP-FH-NO-ICE)

o DTLS [RFC6347] over UDP with SR, No-ICE (OverlayLinkType=DTLS-UDP-
SR-NO-ICE)
(OverlayLinkType=DTLS-UDP-SR-NO-ICE)

Note that although UDP does not properly have "connections", both TLS
and DTLS have a handshake which that establishes a similar, stateful
association, and we simply
association. We refer to these as "connections" for the purposes of
this document.

If a peer receives a message that is larger than the value of max-
message-size defined in the overlay configuration, the peer SHOULD
send an Error_Message_Too_Large error and then close the TLS or DTLS
session from which the message was received. Note that this error
can be sent and the session closed before receiving the peer receives the
complete message. If the forwarding header is larger than the max-message-
size, max-
message-size, the receiver SHOULD close the TLS or DTLS session
without sending an error.

The RELOAD mechanism requires that failed links are be quickly removed
from the routing table Routing Table so end-to-end retransmission can handle lost
messages. Overlay link Link protocols MUST be designed with a mechanism
that quickly signals a likely failure failure, and implementations SHOULD
quickly act to remove it a failed link from the routing table Routing Table when
receiving this signal. The entry can be restored if it proves to
resume functioning, or it can be replaced at some point in the future
if necessary. Section 10.7.2 contains more details specific to the
CHORD-RELOAD
topology plugin. Topology Plug-in.

The Framing Header (FH) is used to frame messages and provide timing
when used on a reliable stream-based transport protocol. Simple
Reliability (SR) makes use of uses the FH to provide congestion control and semi-reliability
partial reliability when using unreliable message-oriented transport
protocols. We will first define each of these algorithms in
Section Sections
6.6.2 and Section 6.6.3, and then define overlay link Overlay Link protocols that use them
in Section Sections 6.6.4, Section 6.6.5 6.6.5, and Section 6.6.6.

Note: We expect future Overlay Link protocols to define replacements
for all components of these protocols, including the framing header.
These Framing Header.
The three protocols that we will discuss have been chosen for
simplicity of implementation and reasonable performance.

6.6.1. Future Overlay Link Protocols

It is possible to define new link-layer protocols and apply them to a
new overlay using the "overlay-link-protocol" configuration directive
(see Section 11.1.). However, any new protocols MUST meet the
following requirements. requirements:

Endpoint authentication authentication: When a node forms an association with
another endpoint, it MUST be possible to cryptographically verify
that the endpoint has a given Node-ID.

Traffic origin authentication and integrity integrity: When a node receives
traffic from another endpoint, it MUST be possible to
cryptographically verify that the traffic came from a given
association and that it has not been modified in transit from the
other endpoint in the association. The overlay link protocol MUST
also provide replay prevention/detection.

Traffic confidentiality confidentiality: When a node sends traffic to another
endpoint, it MUST NOT be possible for a third party that is not
involved in the association to determine the contents of that
traffic.

Any new overlay protocol MUST be defined via RFC 5226 Standards
Action; see Action
[RFC5226]. See Section 14.11.

6.6.1.1. HIP

In a Host Identity Protocol Based Overlay Networking Environment (HIP
BONE) [RFC6079] [RFC6079], HIP [RFC5201] provides connection management (e.g.,
NAT traversal and mobility) and security for the overlay network.
The P2PSIP Working Group has expressed interest in supporting a HIP-
based link protocol. Such support would require specifying such
details as:

o How to issue certificates which provided provide identities meaningful to
the HIP base exchange. We anticipate that this would require a
mapping between ORCHIDs Overlay Routable Cryptographic Hash Identifiers
(ORCHIDs) and NodeIds.

o How to carry the HIP I1 and I2 messages.

o How to carry RELOAD messages over HIP.

[I-D.ietf-hip-reload-instance]

[HIP-RELOAD] documents work in progress on using RELOAD with the HIP
BONE.

6.6.1.2. ICE-TCP

The ICE-TCP RFC [RFC6544] allows TCP to be supported as an Overlay
Link protocol that can be added using ICE.

6.6.1.3. Message-oriented Message-Oriented Transports

Modern message-oriented transports offer high performance, performance and good
congestion control, and they avoid head of line head-of-line blocking in case of
lost data. These characteristics make them preferable as underlying
transport protocols for RELOAD links. SCTP without message ordering
and DCCP are two examples of such protocols. However, currently they
are not well-supported by commonly available NATs, and specifications
for ICE session establishment are not available.

6.6.1.4. Tunneled Transports

As of the time of this writing, there is significant interest in the
IETF community in tunneling other transports over UDP, which is
motivated by the situation that UDP is well-supported by modern NAT hardware,
hardware and
similar by the fact that performance can be achieved similar to a native implementation.
Currently
implementation can be achieved. Currently, SCTP, DCCP, and a generic
tunneling extension are being proposed for message-oriented
protocols. Once ICE traversal has been specified for these tunneled
protocols, they should be straightforward to support as overlay link
protocols.

6.6.2. Framing Header

In order to support unreliable links and to allow for quick detection
of link failures when using reliable end-to-end transports, each
message is wrapped in a very simple framing layer (FramedMessage) (FramedMessage),
which is only used only for each hop. This layer contains a sequence
number which can then be used for ACKs. The same header is used for
both reliable and unreliable transports for simplicity of
implementation.

The definition of FramedMessage is:

enum { data(128), ack(129), (255) } FramedMessageType;

struct {
FramedMessageType type;

select (type) {
case data:
uint32 sequence;
opaque message<0..2^24-1>;

case ack:
uint32 ack_sequence;
uint32 received;
};
} FramedMessage;

The type field of the PDU is set to indicate whether the message is
data or an acknowledgement.

If the message is of type "data", then the remainder of the PDU is as
follows:

sequence
the
The sequence number. This increments by 1 one for each framed
message sent over this transport session.

message
the
The message that is being transmitted.

Each connection has it own sequence number space. Initially Initially, the
value is zero zero, and it increments by exactly one for each message sent
over that connection.

When the receiver receives a message, it SHOULD immediately send an
ACK message. The receiver MUST keep track of the 32 most recent
sequence numbers received on this association in order to generate
the appropriate ack. ACK.

If the PDU is of type "ack", the contents are as follows:

ack_sequence
The sequence number of the message being acknowledged.

received
A bitmask indicating if each of the previous 32 sequence numbers
before this packet has been among the 32 packets most recently
received on this connection. When a packet is received with a
sequence number N, the receiver looks at the sequence number of
the previously 32 packets previously received packets on this connection. Call We call
the previously received packet number M. For each of the previous
32 packets, if the sequence number M is less than N but greater
than N-32, the N-M bit of the received bitmask is set to one; otherwise
otherwise, it is set to zero. Note that a bit being set to one
indicates positively that a particular packet was received, but a
bit being set to zero means only that it is unknown whether or not
the packet has been received, because it might have been received
before the 32 most recently received packets.

The received field bits in the ACK provide a high degree of
redundancy so that the sender can figure out which packets the
receiver has received and can then estimate packet loss rates. If
the sender also keeps track of the time at which recent sequence
numbers have been sent, the RTT (round-trip time) can be estimated.

Note that because retransmissions receive new sequence numbers,
multiple ACKs may be received for the same message. This approach
provides more information than traditional TCP sequence numbers, but
care must be taken when applying algorithms designed based on TCP's
stream-oriented sequence number.

6.6.3. Simple Reliability

When RELOAD is carried over DTLS or another unreliable link protocol,
it needs to be used with a reliability and congestion control
mechanism, which is provided on a hop-by-hop basis. The basic
principle is that each message, regardless of whether or not it
carries a request or response, will get an ACK and be reliably
retransmitted. The receiver's job is very simple, and is limited to
just sending ACKs. All the complexity is at the sender side. This
allows the sending implementation to trade off performance versus
implementation complexity without affecting the wire protocol.

Because the receiver's role is limited to providing packet
acknowledgements, a wide variety of congestion control algorithms can
be implemented on the sender side while using the same basic wire
protocol. The sender algorithm used MUST meet the requirements of
[RFC5405].

6.6.3.1. Stop and Wait Sender Algorithm

This section describes one possible implementation of a sender
algorithm for Simple Reliability. It is adequate for overlays
running on underlying networks with low latency and loss (LANs) or
low-traffic overlays on the Internet.

A node MUST NOT have more than one unacknowledged message on the DTLS
connection at a time. Note that because retransmissions of the same
message are given new sequence numbers, there may be multiple
unacknowledged sequence numbers in use.

The RTO ("Retransmission TimeOut") (Retransmission TimeOut) is based on an estimate of the
round-trip time (RTT). RTT.
The value for RTO is calculated separately for each DTLS session.
Implementations can use a static value for RTO or a dynamic estimate estimate,
which will result in better performance. For implementations that
use a static value, the default value for RTO is 500 ms. Nodes MAY
use smaller values of RTO if it is known that all nodes are within
the local network. The default RTO MAY be
chosen larger, and this set to a larger value,
which is RECOMMENDED if it is known in advance (such as on high high-
latency access links) that the round-trip time RTT is larger.

Implementations that use a dynamic estimate to compute the RTO MUST
use the algorithm described in RFC 6298[RFC6298], 6298 [RFC6298], with the exception
that the value of RTO SHOULD NOT be rounded up to the nearest second second,
but instead rounded up to the nearest millisecond. The RTT of a
successful STUN transaction from the ICE stage is used as the initial
measurement for formula 2.2 of RFC 6298. The sender keeps track of
the time each message was sent for all recently sent messages. Any
time an ACK is received, the sender can compute the RTT for that
message by looking at the time the ACK was received and the time when
the message was sent. This is used as a subsequent RTT measurement
for formula 2.3 of RFC 6298 to update the RTO estimate. (Note that
because retransmissions receive new sequence numbers, all received
ACKs are used.)

An initiating node SHOULD retransmit a message if it has not received
an ACK after an interval of RTO (transit nodes do not retransmit at
this layer). The node MUST double the time to wait after each
retransmission. For each retransmission, the sequence number MUST be
incremented.

Retransmissions continue until a response is received, or until a total
of 5 requests have been sent or sent, until there has been a hard ICMP error [RFC1122]
[RFC1122], or until a TLS alert. alert indicating the end of the connection
has been sent or received. The sender knows a response was received
when it receives an ACK with a sequence number that indicates it is a
response to one of the transmissions of this
messages. message. For example,
assuming an RTO of 500 ms, requests would be sent at times 0 ms, 500
ms, 1500 ms, 3500 ms, and 7500 ms. If all retransmissions for a
message fail, then the sending node SHOULD close the connection
routing the message.

To determine when a link might be failing without waiting for the
final timeout, observe when no ACKs have been received for an entire
RTO interval, and then wait for three retransmissions to occur beyond
that point. If no ACKs have been received by the time the third
retransmission occurs, it is RECOMMENDED that the link be removed
from the Routing Table. The link MAY be restored to the Routing
Table if ACKs resume before the connection is closed, as described
above.

A sender MUST wait 10ms 10 ms between receipt of an ACK and transmission
of the next message.

6.6.4. DTLS/UDP with SR

This overlay link protocol consists of DTLS over UDP while
implementing the Simple Reliability SR protocol. STUN Connectivity connectivity checks and
keepalives are used. Any compliant sender algorithm may be used.

6.6.5. TLS/TCP with FH, No-ICE

This overlay link protocol consists of TLS over TCP with the framing
header. Because ICE is not used, STUN connectivity checks are not
used upon establishing the TCP connection, nor are they used for
keepalives.

Because the TCP layer's application-level timeout is too slow to be
useful for overlay routing, the Overlay Link implementation MUST use
the framing header to measure the RTT of the connection and calculate
an RTO as specified in Section 2 of [RFC6298]. The resulting RTO is
not used for retransmissions, but rather as a timeout to indicate
when the link SHOULD be removed from the Routing Table. It is
RECOMMENDED that such a connection be retained for 30s 30 seconds to
determine if the failure was transient before concluding the link has
failed permanently.

When sending candidates for TLS/TCP with FH, No-ICE, a passive
candidate MUST be provided.

6.6.6. DTLS/UDP with SR, No-ICE

This overlay link protocol consists of DTLS over UDP while
implementing the Simple Reliability protocol. Because ICE is not
used, no STUN connectivity checks or keepalives are used.

6.7. Fragmentation and Reassembly

In order to allow transmission over datagram protocols such as DTLS,
RELOAD messages may be fragmented.

Any node along the path can fragment the message message, but only the final
destination reassembles the fragments. When a node takes a packet
and fragments it, each fragment has a full copy of the Forwarding
Header forwarding
header, but the data after the Forwarding Header forwarding header is broken up in
appropriate into
appropriately sized chunks. The size of the payload chunks needs to
take into account space to allow the via Via and destination lists Destination Lists to
grow. Each fragment MUST contain a full copy of the via list,
destination list, Via List,
Destination List, and ForwardingOptions and MUST contain at least 256
bytes of the message body. If these elements cannot fit within the
MTU of the underlying datagram protocol, RELOAD fragmentation is not
performed
performed, and IP-layer fragmentation is allowed to occur. The
length field MUST contain the size of the message after
fragmentation. When a message MUST be fragmented, it SHOULD be split
into equal-sized fragments that are no larger than the PMTU Path MTU
(PMTU) of the next overlay link minus 32 bytes. This is to allow the via list
Via List to grow before further fragmentation is required.

Note that this fragmentation is not optimal for the end-to-end
path - -- a message may be refragmented multiple times as it traverses
the
overlay overlay, but it is only assembled only at the final destination. This
option has been chosen as it is far easier to implement than e2e end-to-
end (e2e) PMTU discovery across an ever-changing overlay, overlay and it
effectively addresses the reliability issues of relying on IP-layer
fragmentation. However, Ping can be used to allow e2e PMTU discovery
to be implemented if desired.

Upon receipt of a fragmented message by the intended peer, the peer
holds the fragments in a holding buffer until the entire message has
been received. The message is then reassembled into a single message
and processed. In order to mitigate denial of service denial-of-service (DoS) attacks,
receivers SHOULD time out incomplete fragments after the maximum
request lifetime (15 seconds). Note this This time was derived from looking at
the end-to-end retransmission time and saving fragments long enough
for the full end-to-end retransmissions to take place. Ideally Ideally, the
receiver would have enough buffer space to deal with as many
fragments as can arrive in the maximum request lifetime. However, if
the receiver runs out of buffer space to reassemble the messages a message, it
MUST drop the message.

The fragment field of the forwarding header is used to encode
fragmentation information. The offset is the number of bytes between
the end of the forwarding header and the start of the data. The
first fragment therefore has an offset of 0. The last fragment
indicator MUST be appropriately set. If the message is not
fragmented, it is simply treated as if it is the only fragment: the
last fragment bit is set and the offset is 0 0, resulting in a fragment
value of 0xC0000000.

Note: the The reason for this definition of the fragment field is that
originally
originally, the high bit was defined in part of the specification as
"is fragmented" and fragmented", so there was some specification ambiguity about how
to encode messages with only one fragment. This ambiguity was
resolved in favor of always encoding as the "last" fragment with
offset 0, thus simplifying the receiver code path, but resulting in
the high bit being redundant. Because messages MUST be set with the
high bit set to 1, implementations SHOULD discard any message with it
set to 0. Implementations (presumably legacy ones) which choose to
accept such messages MUST either ignore the remaining bits or ensure
that they are 0. They MUST NOT try to interpret as fragmented
messages with the high bit set low.

7. Data Storage Protocol

RELOAD provides a set of generic mechanisms for storing and
retrieving data in the Overlay Instance. These mechanisms can be
used for new applications simply by defining new code points and a
small set of rules. No new protocol mechanisms are required.

The basic unit of stored data is a single StoredData structure:

struct {
uint32 length;
uint64 storage_time;
uint32 lifetime;
StoredDataValue value;
Signature signature;
} StoredData;

The contents of this structure are as follows:

length
The size of the StoredData structure structure, in bytes bytes, excluding the size
of length itself.

storage_time
The time when the data was stored stored, represented as the number of
milliseconds elapsed since midnight Jan 1, 1970 UTC UTC, not counting
leap seconds. This will have the same values for seconds as
standard UNIX time or POSIX time. More information can be found at
[UnixTime]. Any attempt to store a data value with a storage time
before that of a value already stored at this location MUST
generate a an Error_Data_Too_Old error. This prevents rollback
attacks. The node SHOULD make a best-effort attempt to use a
correct clock to determine this number, however, number. However, the protocol
does not require synchronized clocks: the receiving peer uses the
storage time in the previous store, not its own clock. Clock
values are used so that when clocks are generally synchronized,
data may be stored in a single transaction, rather than querying
for the value of a counter before the actual store.

If a node attempting to store new data in response to a user
request (rather than as an overlay maintenance operation such as
occurs when healing the overlay from a partition) is rejected with
an Error_Data_Too_Old error, the node MAY elect to perform its
store using a storage_time that increments the value used with the
previous store. store (this may be obtained by doing a Fetch). This
situation may occur when the clocks of nodes storing to this
location are not properly synchronized.

lifetime
The validity period for the data, in seconds, starting from the
time the peer receives the StoreReq.

value
The data value itself, as described in Section 7.2.

signature
A signature signature, as defined in Section 7.1.

Each Resource-ID specifies a single location in the Overlay Instance.
However, each location may contain multiple StoredData values values,
distinguished by Kind-ID. The definition of a Kind describes both
the data values which may be stored and the data model of the data.
Some data models allow multiple values to be stored under the same
Kind-ID. Section 7.2 describes the available data models. Thus, for
instance, a given Resource-ID might contain a single-value element
stored under Kind-ID X and an array containing multiple values stored
under Kind-ID Y.

7.1. Data Signature Computation

Each StoredData element is individually signed. However, the
signature also must be self-contained and must cover the Kind-ID and
Resource-ID
Resource-ID, even though they are not present in the StoredData
structure. The input to the signature algorithm is:

resource_id || kind || storage_time || StoredDataValue ||
SignerIdentity

Where

where || indicates concatenation.

Where concatenation and where these values are:

resource_id
The Resource-ID where this data is stored.

kind
The Kind-ID for this data.

storage_time
The contents of the storage_time data value.

StoredDataValue
The contents of the stored data value, as described in the
previous sections.

SignerIdentity
The signer identity identity, as defined in Section 6.3.4.

Once the signature has been computed, the signature is represented
using a signature element, as described in Section 6.3.4.

Note that there is no necessary relationship between the validity
window of a certificate and the expiry of the data it is
authenticating. When signatures are verified, the current time MUST
be compared to the certificate validity period. Stored data MAY be
set to expire after the signing certificate's validity period. Such
signatures are not considered valid after the signing certificate
expires. Implementations may garbage collect "garbage collect" such data at their
convenience, either by purging it automatically (perhaps by setting
the upper bound on data storage to the lifetime of the signing
certificate) or by simply leaving it in-place in place until it expires
naturally and relying on users of that data to notice the expired
signing certificate.

7.2. Data Models

The protocol currently defines the following data models:

o single value

o array

o dictionary

These are represented with the StoredDataValue structure. The actual
data model is known from the Kind being stored.

struct {
Boolean exists;
opaque value<0..2^32-1>;
} DataValue;

struct {
select (DataModel) {
case single_value:
DataValue single_value_entry;

case array:
ArrayEntry array_entry;

case dictionary:
DictionaryEntry dictionary_entry;

/* This structure may be extended */
};
} StoredDataValue;

We now

The following sections discuss the properties of each data model in turn: model.

7.2.1. Single Value

A single-value element is a simple sequence of bytes. There may be
only one single-value element for each Resource-ID, Kind-ID pair.

A single value element is represented as a DataValue, which contains
the following two elements:

exists
This value indicates whether the value exists at all. If it is
set to False, it means that no value is present. If it is True,
that
this means that a value is present. This gives the protocol a
mechanism for indicating nonexistence as opposed to emptiness.

value
The stored data.

7.2.2. Array

An array is a set of opaque values addressed by an integer index.
Arrays are zero based. Note that arrays can be sparse. For
instance, a Store of "X" at index 2 in an empty array produces an
array with the values [ NA, NA, "X"]. Future attempts to fetch
elements at index 0 or 1 will return values with "exists" set to
False.

An array element is represented as an ArrayEntry:

struct {
uint32 index;
DataValue value;
} ArrayEntry;

The contents of this structure are:

index
The index of the data element in the array.

value
The stored data.

7.2.3. Dictionary

A dictionary is a set of opaque values indexed by an opaque key key, with
one value for each key. A single dictionary entry is represented as
follows:

A dictionary element is represented as
a DictionaryEntry:

typedef opaque DictionaryKey<0..2^16-1>;

struct {
DictionaryKey key;
DataValue value;
} DictionaryEntry;

The contents of this structure are:

key
The dictionary key for this value.

value
The stored data.

7.3. Access Control Policies

Every Kind which is storable in an overlay MUST be associated with an
access control policy. This policy defines whether a request from a
given node to operate on a given value should succeed or fail. It is
anticipated that only a small number of generic access control
policies are required. To that end, this section describes a small
set of such policies policies, and Section 14.4 establishes a registry for new
policies
policies, if required. Each policy has a short string identifier
which is used to reference it in the configuration document. Configuration Document.

In the following policies, the term "signer" refers to the signer of
the StoredValue object and, in the case of non-replica stores, to the
signer of the StoreReq message. I.e., That is, in a non-replica store,
both the signer of the StoredValue and the signer of the StoreReq
MUST conform to the policy. In the case of a replica store, the
signer of the StoredValue MUST conform to the policy policy, and the
StoreReq itself MUST be checked as described in Section 7.4.1.1.

7.3.1. USER-MATCH

In the USER-MATCH policy, a given value MUST be written (or
overwritten) if and only if the signer's certificate has a user name
which hashes (using the hash function for the overlay) to the
Resource-ID for the resource. Recall that the certificate may,
depending on the overlay configuration, be self-signed.

7.3.2. NODE-MATCH

In the NODE-MATCH policy, a given value MUST be written (or
overwritten) if and only if the signer's certificate has a specified
Node-ID which hashes (using the hash function for the overlay) to the
Resource-ID for the resource and that Node-ID is the one indicated in
the SignerIdentity value cert_hash.

7.3.3. USER-NODE-MATCH

The USER-NODE-MATCH policy may only be used only with dictionary types.
In the USER-NODE-MATCH policy, a given value MUST be written (or
overwritten) if and only if the signer's certificate has a user name
which hashes (using the hash function for the overlay) to the
Resource-ID for the resource. In addition, the dictionary key MUST
be equal to the Node-ID in the certificate certificate, and that Node-ID MUST be
the one indicated in the SignerIdentity value cert_hash.

7.3.4. NODE-MULTIPLE

In the NODE-MULTIPLE policy, a given value MUST be written (or
overwritten) if and only if the signer's certificate contains a
Node-ID such that H(Node-ID || i) is equal to the Resource-ID for
some small integer value of i and that Node-ID is the one indicated
in the SignerIdentity value cert_hash. When this policy is in use,
the maximum value of i MUST be specified in the Kind definition.

Note that as because i is not carried on the wire, the verifier MUST
iterate through potential i values values, up to the maximum value in order value, to
determine whether a store is acceptable.

7.4. Data Storage Methods

RELOAD provides several methods for storing and retrieving data:

o Store values in the overlay overlay.

o Fetch values from the overlay overlay.

o Stat: get Get metadata about values in the overlay overlay.

o Find the values stored at an individual peer peer.

These methods are each described in the following sections.

7.4.1. Store

The Store method is used to store data in the overlay. The format of
the Store request depends on the data model model, which is determined by
the Kind.

7.4.1.1. Request Definition

A StoreReq message is a sequence of StoreKindData values, each of
which represents a sequence of stored values for a given Kind. The
same Kind-ID MUST NOT be used twice in a given store request. Each
value is then processed in turn. These operations MUST be atomic.
If any operation fails, the state MUST be rolled back to what it was
before the request was received.

The store request is defined by the StoreReq structure:

struct {
KindId kind;
uint64 generation_counter;
StoredData values<0..2^32-1>;
} StoreKindData;

struct {
ResourceId resource;
uint8 replica_number;
StoreKindData kind_data<0..2^32-1>;
} StoreReq;

A single Store request stores data of a number of Kinds to a single
resource location. The contents of the structure are:

resource
The resource at which to store at. store.

replica_number
The number of this replica. When a storing peer saves replicas to
other peers peers, each peer is assigned a replica number number, starting from 1
and
1, that is sent in the Store message. This field is set to 0 when
a node is storing its own data. This allows peers to distinguish
replica writes from original writes. Different topologies may
choose to allocate or interpret the replica number differently
(see Section 10.4).

kind_data
A series of elements, one for each Kind of data to be stored.

If the replica number is zero, then the

The peer MUST check that it is responsible for the resource and, if not, reject the request. If the
replica number is nonzero, then zero; if it is not, the peer must reject the
request. The peer MUST check that it expects to be a replica for the
resource and that the request sender is consistent with being the
responsible node (i.e., that the receiving peer does not know of a
better node) and, if not, the replica number is nonzero; if the request sender
is not consistent, it should reject the request.

Each StoreKindData element represents the data to be stored for a
single Kind-ID. The contents of the element are:

kind
The Kind-ID. Implementations MUST reject requests corresponding
to unknown Kinds.

generation_counter
The expected current state of the generation counter
(approximately the number of times that this object has been
written; see below for details).

values
The value or values to be stored. This may contain one or more
stored_data values values, depending on the data model associated with
each Kind.

The peer MUST perform the following checks:

o The Kind-ID is known and supported.

o The signatures over each individual data element (if any) element, if any, are
valid. If this check fails, the request MUST be rejected with an
Error_Forbidden error.

o Each element is signed by a credential which is authorized to
write this Kind at this Resource-ID. If this check fails, the
request MUST be rejected with an Error_Forbidden error.

o For original (non-replica) stores, the StoreReq is signed by a
credential which is authorized to write this Kind at this
Resource-ID. If this check fails, the request MUST be rejected
with an Error_Forbidden error.

o For replica stores, the StoreReq is signed by a Node-ID which is a
plausible node to either have originally stored the value or have
been in the replica set. What this means is overlay specific, but
in the case of the Chord based Chord-based DHT defined in this specification,
replica StoreReqs MUST come from nodes which are either in the
known replica set for a given resource or which are closer than
some node in the replica set. If this check fails, the request
MUST be rejected with an Error_Forbidden error.

o For original (non-replica) stores, the peer MUST check that if the
generation counter is non-zero, nonzero, it equals the current value of the
generation counter for this Kind. This feature allows the
generation counter to be used in a way similar to the HTTP Etag ETag
feature.

o For replica Stores, the peer MUST set the generation counter to
match the generation counter in the message, message and MUST NOT check the
generation counter against the current value. Replica Stores MUST
NOT use a generation counter of 0.

o The storage time values are greater than that of any value values which
would be replaced by this Store.

o The size and number of the stored values is are consistent with the
limits specified in the overlay configuration.

o If the data is signed with identity_type set to "none" and/or
SignatureAndHashAlgorithm values set to {0, 0} ("anonymous" and
"none"), the StoreReq MUST be rejected with an Error_forbidden
error. Only synthesized data returned by the storage can use
these values (see Section 7.4.2.2)

If all these checks succeed, the peer MUST attempt to store the data
values. For non-replica stores, if the store succeeds and the data
is changed, then the peer MUST increase the generation counter by at
least one. 1. If there are multiple stored values in a single
StoreKindData, it is permissible for the peer to increase the
generation counter by only 1 for the entire Kind-ID, Kind-ID or by 1 or more
than one 1 for each value. Accordingly, all stored data values MUST have
a generation counter of 1 or greater. 0 is used in the Store request
to indicate that the generation counter should be ignored for
processing this request; however request. However, the responsible peer should
increase the stored generation counter and should return the correct
generation counter in the response.

When a peer stores data previously stored by another node (e.g., for
replicas or topology shifts) shifts), it MUST adjust the lifetime value
downward to reflect the amount of time the value was stored at the
peer. The adjustment SHOULD be implemented by an algorithm
equivalent to the following: at the time the peer initially receives
the StoreReq StoreReq, it notes the local time T. When it then attempts to do
a StoreReq to another node node, it should decrement the lifetime value by
the difference between the current local time and T.

Unless otherwise specified by the usage, if a peer attempts to store
data previously stored by another node (e.g., for replicas or
topology shifts) and that store fails with either an
Error_Generation_Counter_Too_Low or an Error_Data_Too_Old error, the
peer MUST fetch the newer data from the peer generating the error and
use that to replace its own copy. This rule allows resynchronization
after partitions heal.

When a network partition is being healed and unless otherwise
specified, the default merging rule is to act as if all the values
that need to be merged were stored and as if the order they were
stored in corresponds to the stored time values associated with (and
carried in) their values. Because the stored time values are those
associated with the peer which did the writing, clock skew is
generally not an issue. If two nodes are on different partitions,
write to the same location, and have clock skew, this can create
merge conflicts. However However, because RELOAD deliberately segregates
storage so that data from different users and peers is stored in
different locations, and a single peer will typically only be in a
single network partition, this case will generally not arise.

The properties of stores for each data model are as follows:

Single-value:

single-value: A store of a new single-value element creates the
element if it does not exist and overwrites any existing value
with the new value.

Array:

array: A store of an array entry replaces (or inserts) the given
value at the location specified by the index. Because arrays are
sparse, a store past the end of the array extends it with
nonexistent values (exists = False) as required. A store at index
0xffffffff places the new value at the end of the array array,
regardless of the length of the array. The resulting StoredData
has the correct index value when it is subsequently fetched.

Dictionary:

dictionary: A store of a dictionary entry replaces (or inserts) the
given value at the location specified by the dictionary key.

The following figure shows the relationship between these structures
for an example store which stores the following values at resource
"1234"
"1234":

o The value "abc" is in the single value single-value location for Kind X X.

o The value "foo" at index 0 is in the array for Kind Y Y.

o The value "bar" at index 1 is in the array for Kind Y Y.

Store
resource=1234
replica_number = 0
/ \
/ \
StoreKindData StoreKindData
kind=X (Single-Value) kind=Y (Array)
generation_counter = 99 generation_counter = 107
| /\
| / \
StoredData / \
storage_time = xxxxxxx / \
lifetime = 86400 / \
signature = XXXX / \
| | |
| StoredData StoredData
| storage_time = storage_time =
| yyyyyyyy zzzzzzz
| lifetime = 86400 lifetime = 33200
| signature = YYYY signature = ZZZZ
| | |
StoredDataValue | |
value="abc" | |
| |
StoredDataValue StoredDataValue
index=0 index=1
value="foo" value="bar"

7.4.1.2. Response Definition

In response to a successful Store request request, the peer MUST return a
StoreAns message containing a series of StoreKindResponse elements
containing elements,
which contains the current value of the generation counter for each
Kind-ID, as well as a list of the peers where the data will be
replicated by the node processing the request.

struct {
KindId kind;
uint64 generation_counter;
NodeId replicas<0..2^16-1>;
} StoreKindResponse;

struct {
StoreKindResponse kind_responses<0..2^16-1>;
} StoreAns;

The contents of each StoreKindResponse are:

kind
The Kind-ID being represented.

generation_counter
The current value of the generation counter for that Kind-ID.

replicas
The list of other peers at which the data was/will be replicated.
In overlays and applications where the responsible peer is
intended to store redundant copies, this allows the storing node
to independently verify that the replicas have in fact been
stored. It does this verification by using the Stat method (see
Section 7.4.3). Note that the storing node is not required to
perform this verification.

The response itself is just StoreKindResponse values packed end-to- end to
end.

If any of the generation counters in the request precede the
corresponding stored generation counter, then the peer MUST fail the
entire request and respond with an Error_Generation_Counter_Too_Low
error. The error_info in the ErrorResponse MUST be a StoreAns
response containing the correct generation counter for each Kind and
the replica list, which will be empty. For original (non-replica)
stores, a node which receives such an error SHOULD attempt to fetch
the data and, if the storage_time value is newer, replace its own
data with that newer data. This rule improves data consistency in
the case of partitions and merges.

If the data being stored is too large for the allowed limit by the
given usage, then the peer MUST fail the request and generate an
Error_Data_Too_Large error.

If any type of request tries to access a data Kind that the peer does
not know about, the peer MUST fail the request and generate an
Error_Unknown_Kind MUST be generated. error. The error_info in the Error_Response is:

KindId unknown_kinds<0..2^8-1>;

which lists all the Kinds that were unrecognized. A node which
receives this error MUST generate a ConfigUpdate message which
contains the appropriate Kind definition (assuming that which, in fact fact, a
Kind was used which was defined in the configuration document). document was used).

7.4.1.3. Removing Values

RELOAD does not have an explicit Remove operation. Rather, values
are Removed by storing "nonexistent" values in their place. Each
DataValue contains a boolean value called "exists" which indicates
whether a value is present at that location. In order to effectively
remove a value, the owner stores a new DataValue with "exists" set to
False:

exists = False

value = {} (0 length)

The owner SHOULD use a lifetime for the nonexistent value that is at
least as long as the remainder of the lifetime of the value it is replacing;
otherwise
replacing. Otherwise, it is possible for the original value to be
accidentally or maliciously re-stored after the storing node has
expired it. Note that there is still a window of vulnerability for replay attack
still exists after the original lifetime has expired (as with any
store). This attack can be mitigated by doing a nonexistent store
with a very long lifetime.

Storing nodes MUST treat these nonexistent values the same way they
treat any other stored value, including overwriting the existing
value, replicating them, and aging them out as necessary when the
lifetime expires. When a stored nonexistent value's lifetime
expires, it is simply removed from the storing node like node, as happens when
any other stored value expiration. expires.

Note that in the case of arrays and dictionaries, expiration may
create an implicit, unsigned "nonexistent" value to represent a gap
in the data structure, as might happen when any value is aged out.
However, this value isn't persistent persistent, nor is it replicated. It is
simply synthesized by the storing node.

7.4.2. Fetch

The Fetch request retrieves one or more data elements stored at a
given Resource-ID. A single Fetch request can retrieve multiple
different Kinds.

7.4.2.1. Request Definition

Fetch requests are defined by the FetchReq structure:

struct {
int32 first;
int32 last;
} ArrayRange;

struct {
KindId kind;
uint64 generation;
uint16 length;

select (DataModel) {
case single_value: ; /* Empty */

case array:
ArrayRange indices<0..2^16-1>;

case dictionary:
DictionaryKey keys<0..2^16-1>;

/* This structure may be extended */

} model_specifer; model_specifier;
} StoredDataSpecifier;

struct {
ResourceId resource;
StoredDataSpecifier specifiers<0..2^16-1>;
} FetchReq;

The contents of the Fetch requests are as follows:

resource
The Resource-ID to fetch from.

specifiers
A sequence of StoredDataSpecifier values, each specifying some of
the data values to retrieve.

Each StoredDataSpecifier specifies a single Kind of data to retrieve
and (if appropriate)
and, if appropriate, the subset of values that are to be retrieved.
The contents of the StoredDataSpecifier structure are as follows:

kind
The Kind-ID of the data being fetched. Implementations SHOULD
reject requests corresponding to unknown Kinds unless specifically
configured otherwise.

DataModel
The data model of the data. This is not transmitted on the wire wire,
but comes from the definition of the Kind.

generation
The last generation counter that the requesting node saw. This
may be used to avoid unnecessary fetches fetches, or it may be set to
zero.

length
The length of the rest of the structure, thus allowing
extensibility.

model_specifier
A reference to the data value being requested within the data
model specified for the Kind. For instance, if the data model is
"array", it might specify some subset of the values.

The model_specifier is as follows:

o If the data model is single value, the specifier is empty.

o If the data model is array, the specifier contains a list of
ArrayRange elements, each of which contains two integers. The
first integer is the beginning of the range range, and the second is the
end of the range. 0 is used to indicate the first element element, and
0xffffffff is used to indicate the final element. The first
integer MUST be less than or equal to the second. While multiple
ranges MAY be specified, they MUST NOT overlap.

o If the data model is dictionary dictionary, then the specifier contains a
list of the dictionary keys being requested. If no keys are
specified,
than then this is a wildcard fetch and all key-value pairs
are returned.

The generation counter is used to indicate the requester's expected
state of the storing peer. If the generation counter in the request
matches the stored counter, then the storing peer returns a response
with no StoredData values.

7.4.2.2. Response Definition

The response to a successful Fetch request is a FetchAns message
containing the data requested by the requester.

struct {
KindId kind;
uint64 generation;
StoredData values<0..2^32-1>;
} FetchKindResponse;

struct {
FetchKindResponse kind_responses<0..2^32-1>;
} FetchAns;

The FetchAns structure contains a series of FetchKindResponse
structures. There MUST be one FetchKindResponse element for each
Kind-ID in the request.

The contents of the FetchKindResponse structure are as follows:

kind
the
The Kind that this structure is for.

generation
the
The generation counter for this Kind.

values
the
The relevant values. If the generation counter in the request
matches the generation counter in the stored data, then no
StoredData values are returned. Otherwise, all relevant data
values MUST be returned. A nonexistent value (i.e., one which the
node has no knowledge of) is represented by a synthetic value with
"exists" set to False and has an empty signature. Specifically,
the identity_type is set to "none", the SignatureAndHashAlgorithm
values are set to {0, 0} ("anonymous" and "none" "none", respectively),
and the signature value is of zero length. This removes the need
for the responding node to do signatures for values which do not
exist. These signatures are unnecessary unnecessary, as the entire response
is signed by that node. Note that entries which have been removed
by the procedure of given in Section 7.4.1.3 and which have not yet
expired also have exists = False False, but have valid signatures from
the node which did the store.

Upon receipt of a FetchAns message, nodes MUST verify the signatures
on all the received values. Any values with invalid signatures
(including expired certificates) MUST be discarded. Note that this
implies that implementations which wish to store data for long
periods of time must have certificates with appropriate expiry expiration
dates or must re-store periodically. Implementations MAY return the
subset of values with valid signatures, but in that case case, they SHOULD
somehow signal to the application that a partial response was
received.

There is one subtle point about signature computation on arrays. If
the storing node uses the append feature (where the
index=0xffffffff), then the index in the StoredData that is returned
will not match that used by the storing node, which would break the
signature. In order to avoid this issue, the index value in the
array is set to zero before the signature is computed. This implies
that malicious storing nodes can reorder array entries without being
detected.

7.4.3. Stat

The Stat request is used to get metadata (length, generation counter,
digest, etc.) for a stored element without retrieving the element
itself. The name is from the UNIX stat(2) system call call, which
performs a similar function for files in a file system. It also
allows the requesting node to get a list of matching elements without
requesting the entire element.

7.4.3.1. Request Definition

The Stat request is identical to the Fetch request. It simply
specifies the elements to get metadata about.

struct {
ResourceId resource;
StoredDataSpecifier specifiers<0..2^16-1>;
} StatReq;

7.4.3.2. Response Definition

The Stat response contains the same sort of entries that a Fetch
response would contain; however, contain. However, instead of containing the element
data
data, it contains metadata.

struct {
Boolean exists;
uint32 value_length;
HashAlgorithm hash_algorithm;
opaque hash_value<0..255>;
} MetaData;

struct {
uint32 index;
MetaData value;
} ArrayEntryMeta;

struct {
DictionaryKey key;
MetaData value;
} DictionaryEntryMeta;

struct {
select (DataModel) {
case single_value:
MetaData single_value_entry;

case array:
ArrayEntryMeta array_entry;

case dictionary:
DictionaryEntryMeta dictionary_entry;

/* This structure may be extended */
};
} MetaDataValue;

struct {
uint32 value_length;
uint64 storage_time;
uint32 lifetime;
MetaDataValue metadata;
} StoredMetaData;

struct {
KindId kind;
uint64 generation;
StoredMetaData values<0..2^32-1>;
} StatKindResponse;

struct {
StatKindResponse kind_responses<0..2^32-1>;
} StatAns;
The structures used in StatAns parallel those used in FetchAns: a
response consists of multiple StatKindResponse values, one for each
Kind that was in the request. The contents of the StatKindResponse
are the same as those in the FetchKindResponse, except that the
values list contains StoredMetaData entries instead of StoredData
entries.

The contents of the StoredMetaData structure are the same as the
corresponding fields in StoredData StoredData, except that there is no signature
field and the value is a MetaDataValue rather than a StoredDataValue.

A MetaDataValue is a variant structure, like a StoredDataValue,
except for the types of each arm, which replace DataValue with
MetaData.

The only really new structure is MetaData, which has the following contents:

exists
Same as in DataValue DataValue.

value_length
The length of the stored value.

hash_algorithm
The hash algorithm used to perform the digest of the value.

hash_value
A digest using hash_algorithm on the value field of the DataValue DataValue,
including its 4 leading length bytes.

7.4.4. Find

The Find request can be used to explore the Overlay Instance. A Find
request for a Resource-ID R and a Kind-ID T retrieves the Resource-ID
(if any)
Resource-ID, if any, of the resource of Kind T known to the target
peer which is closest to R. This method can be used to walk the
Overlay Instance by iteratively fetching R_n+1=nearest(1 + R_n).

7.4.4.1. Request Definition

The FindReq message contains a Resource-ID and a series of Kind-IDs
identifying the resource the peer is interested in.

struct {
ResourceId resource;
KindId kinds<0..2^8-1>;
} FindReq;
The request contains a list of Kind-IDs which the Find is for, as
indicated below:

resource
The desired Resource-ID Resource-ID.

kinds
The desired Kind-IDs. Each value MUST only appear once, and if
not only once.
Otherwise, the request MUST be rejected with an error.

7.4.4.2. Response Definition

A response to a successful Find request is a FindAns message
containing the closest Resource-ID on the peer for each Kind
specified in the request.

struct {
KindId kind;
ResourceId closest;
} FindKindData;

struct {
FindKindData results<0..2^16-1>;
} FindAns;

If the processing peer is not responsible for the specified
Resource-ID, it SHOULD return an Error_Not_Found error code.

For each Kind-ID in the request request, the response MUST contain a
FindKindData indicating the closest Resource-ID for that Kind-ID,
unless the Kind is not allowed to be used with Find Find, in which case a
FindKindData for that Kind-ID MUST NOT be included in the response.
If a Kind-ID is not known, then the corresponding Resource-ID MUST be
0. Note that different Kind-IDs may have different closest Resource-
IDs.
Resource-IDs.

The response is simply a series of FindKindData elements, one per
Kind, concatenated end-to-end. end to end. The contents of each element are:

kind
The Kind-ID.

closest
The closest Resource-ID to the specified Resource-ID. This It is 0 if
no Resource-ID is known.

Note that the response does not contain the contents of the data
stored at these Resource-IDs. If the requester wants this, it must
retrieve it using Fetch.

7.4.5. Defining New Kinds

There are two ways to define a new Kind. The first is by writing a
document and registering the Kind-ID with IANA. This is the
preferred method for Kinds which may be widely used and reused. The
second method is to simply define the Kind and its parameters in the
configuration document
Configuration Document using the section of Kind-ID space set aside
for private use. This method MAY be used to define ad hoc Kinds in
new overlays.

However a Kind is defined, the definition MUST include:

o The meaning of the data to be stored (in some textual form).

o The Kind-ID.

o The data model (single value, array, dictionary, etc). etc.).

o The access control model.

In addition, when Kinds are registered with IANA, each Kind is
assigned a short string name which is used to refer to it in
configuration documents.
Configuration Documents.

While each Kind needs to define what data model is used for its data,
that
this does not mean that it must define new data models. Where
practical, Kinds should use the existing data models. The intention
is that the basic data model set be sufficient for most applications/
usages.

8. Certificate Store Usage

The Certificate Store usage Usage allows a node to store its certificate in
the overlay.

A user/node MUST store its certificate at Resource-IDs derived from
two Resource Names:

o The user name in the certificate.

o The Node-ID in the certificate.

Note that in the second case case, the certificate for a peer is not
stored at its Node-ID but rather at a hash of its Node-ID. The
intention here (as is common throughout RELOAD) is to avoid making a
peer responsible for its own data.

New certificates are stored at the end of the list. This structure
allows users to store an old and a new certificate that both have the
same Node-ID, which allows for migration of certificates when they
are renewed.

This usage defines the following Kinds:

Name: CERTIFICATE_BY_NODE

Data Model: The data model for CERTIFICATE_BY_NODE data is array.

Access Control: NODE-MATCH. NODE-MATCH

Name: CERTIFICATE_BY_USER

Data Model: The data model for CERTIFICATE_BY_USER data is array.

Access Control: USER-MATCH. USER-MATCH

9. TURN Server Usage

The TURN server usage Server Usage allows a RELOAD peer to advertise that it is
prepared to be a TURN server server, as defined in [RFC5766]. When a node
starts up, it joins the overlay network and forms several connections
in the process. If the ICE stage in any of these connections returns
a reflexive address that is not the same as the peer's perceived
address, then the peer is behind a NAT and SHOULD NOT be a candidate
for a TURN server. Additionally, if the peer's IP address is in the
private address space range as defined by [RFC1918], then it is also
SHOULD NOT be a candidate for a TURN server. Otherwise, the peer
SHOULD assume that it is a potential TURN server and follow the
procedures below.

If the node is a candidate for a TURN server server, it will insert some
pointers in the overlay so that other peers can find it. The overlay
configuration file specifies a turn-density parameter that indicates
how many times each TURN server SHOULD record itself in the overlay.
Typically
Typically, this should be set to the reciprocal of the estimate of
what percentage of peers will act as TURN servers. If the turn-
density is not set to zero, for each value, called d, between 1 and
turn-density, the peer forms a Resource Name by concatenating its
Node-ID and the value d. This Resource Name is hashed to form a
Resource-ID. The address of the peer is stored at that Resource-ID
using type TURN-SERVICE and the TurnServer object:

struct {
uint8 iteration;
IpAddressPort server_address;
} TurnServer;

The contents of this structure are as follows:

iteration
the
The d value value.

server_address
the
The address at which the TURN server can be contacted.

Note: Correct functioning of this algorithm depends on having turn-
density be an a reasonable estimate of the reciprocal of the
proportion of nodes in the overlay that can act as TURN servers.
If the turn-density value in the configuration file is too low,
then
the process of finding TURN servers becomes more expensive expensive, as
multiple candidate Resource-IDs must be probed to find a TURN
server.

Peers that provide this service need to support the TURN extensions
to STUN for media relay relay, as defined in [RFC5766].

This usage defines the following Kind to indicate that a peer is
willing to act as a TURN server:

Name

Name: TURN-SERVICE

Data Model Model: The TURN-SERVICE Kind stores a single value for each
Resource-ID.

Access Control Control: NODE-MULTIPLE, with a maximum iteration of counter
20.

Peers MAY find other servers by selecting a random Resource-ID and
then doing a Find request for the appropriate Kind-ID with that
Resource-ID. The Find request gets routed to a random peer based on
the Resource-ID. If that peer knows of any servers, they will be
returned. The returned response may be empty if the peer does not
know of any servers, in which case the process gets repeated with
some other random Resource-ID. As long as the ratio of servers
relative to peers is not too low, this approach will result in
finding a server relatively quickly.

Note to implementers: The certificates used by TurnServer entries
need to be retained retained, as described in Section 6.3.4.

10. Chord Algorithm

This algorithm is assigned the name CHORD-RELOAD to indicate that it
is an adaptation of the basic Chord based Chord-based DHT algorithm.

This algorithm differs from the originally presented Chord algorithm that was originally
presented in [Chord]. It has been updated based on more recent
research results and implementation experiences, and to adapt it to
the RELOAD protocol. A Here is a short list of differences:

o The original Chord algorithm specified that a single predecessor
and a successor list be stored. The CHORD-RELOAD algorithm
attempts to have more than one predecessor and successor. The
predecessor sets help other neighbors learn their successor list.

o The original Chord specification and analysis called for iterative
routing. RELOAD specifies recursive routing. In addition to the
performance implications, the cost of NAT traversal dictates
recursive routing.

o Finger Table entries are indexed in the opposite order. Original
Chord specifies finger[0] as the immediate successor of the peer.
CHORD-RELOAD specifies finger[0] as the peer 180 degrees around
the ring from the peer. This change was made to simplify
discussion and implementation of variable sized variable-sized Finger Tables.
However, with either approach approach, no more than O(log N) entries
should typically be stored in a Finger Table.

o The stabilize() and fix_fingers() algorithms in the original Chord
algorithm are merged into a single periodic process.
Stabilization is implemented slightly differently because of the
larger neighborhood, and fix_fingers is not as aggressive to
reduce load, nor does it search for optimal matches of the Finger
Table entries.

o RELOAD allows for a 128 bit 128-bit hash instead of a 160 bit 160-bit hash, as
RELOAD is not designed to be used in networks with close to or
more than 2^128 nodes or objects (and it is hard to see how one
would assemble such a network).

o RELOAD uses randomized finger entries entries, as described in
Section 10.7.4.2.

o This The CHORD-RELOAD algorithm allows the use of either reactive or
periodic recovery. The original Chord paper used periodic
recovery. Reactive recovery provides better performance in small
overlays, but is believed to be unstable in large (>1000) overlays
(greater than 1000) with high levels of churn
[handling-churn-usenix04]. The overlay configuration file
specifies a "chord-reactive" element that indicates whether
reactive recovery should be used.

10.1. Overview

The algorithm described here, CHORD-RELOAD, is a modified version of
the Chord algorithm. In Chord (and in the algorithm described here),
nodes are arranged in a ring ring, with node n being adjacent to nodes n-1
and n+1, n+1 and with all arithmetic being done modulo 2^{k}, where k is
the length of the Node-ID in bits, so that node 2^{k} - 1 is directly
before node 0.

Each peer keeps track of a Finger Table and a Neighbor Table. The
Neighbor Table contains at least the three peers before and after
this peer in the DHT ring. There may not be three entries in all
cases
cases, such as small rings or while the ring topology is changing.
The first entry in the Finger Table contains the peer half-way halfway around
the ring from this peer; peer, the second entry contains the peer that is
1/4
1/4th of the way around; around, the third entry contains the peer that is
1/8th of the way around, and so on. Fundamentally, the Chord DHT can
be thought of as a doubly-linked doubly linked list formed by knowing the
successors and predecessor peers in the Neighbor Table, sorted by the
Node-ID. As long as the successor peers are correct, the DHT will
return the correct result. The pointers to the prior peers are kept
to enable the insertion of new peers into the list structure.
Keeping multiple predecessor and successor pointers makes it possible
to maintain the integrity of the data structure even when consecutive
peers simultaneously fail. The Finger Table forms a skip
list[wikiSkiplist], list
[wikiSkiplist] so that entries in the linked list can be found in
O(log(N)) time instead of the typical O(N) time that a linked list
would provide provide, where N represents the number of nodes in the DHT.

The Neighbor Table and Finger Table entries contain logical Node-IDs
as values values, but the actual mapping of an IP level addressing
information to reach that Node-ID is kept in the Connection Table.

A peer, x, is responsible for a particular Resource-ID k Resource-ID, k, if k is
less than or equal to x and k is greater than p, where p is the
Node-ID of the previous peer in the Neighbor Table. Care must be
taken when computing to note that all math is modulo 2^128.

10.2. Hash Function

For this Chord based topology plugin, Chord-based Topology Plug-in, the size of the Resource-ID is
128 bits. The hash of a Resource-ID MUST be computed using SHA-1
[RFC3174]
[RFC3174], and then the SHA-1 result MUST be truncated to the most
significant 128 bits.

10.3. Routing

The Routing Table is conceptually the union of the Neighbor Table and
the Finger Table.

If a peer is not responsible for a Resource-ID k, but is directly
connected to a node with Node-ID k, then it MUST route the message to
that node. Otherwise, it MUST route the request to the peer in the
Routing Table that has the largest Node-ID that is in the interval
between the peer and k. If no such node is found, it the peer finds the
smallest Node-ID that is greater than k and MUST route the message to
that node.

10.4. Redundancy

When a peer receives a Store request for Resource-ID k, k and it is
responsible for Resource-ID k, it MUST store the data and returns return a
success response. It MUST then send a Store request to its successor
in the Neighbor Table and to that peer's successor, incrementing the
replica number for each successor. Note that these Store requests
are addressed to those specific peers, even though the Resource-ID
they are being asked to store is outside the range that they are
responsible for. The peers receiving these SHOULD check that they
came from an appropriate predecessor in their Neighbor Table and that
they are in a range that this predecessor is responsible for, and then for. Then,
they MUST store the data. They do not themselves perform further
Stores
Stores, because they can determine that they are not responsible for
the Resource-ID.

Note that this topology plugin do Topology Plug-in does not use the replica number for
purposes other purpose than knowing the difference between a replica and a
non-replica.

Managing replicas as the overlay changes is described in
Section 10.7.3.

The sequential replicas used in this overlay algorithm protect
against peer failure but not against malicious peers. Additional
replication from the Usage is required to protect resources from such
attacks, as discussed in Section 13.5.4.

10.5. Joining

The join process for a Joining Node (JN) with Node-ID n is as
follows.
follows:

1. JN MUST connect to its chosen bootstrap node node, as specified in
Section 11.4.

2. JN SHOULD send an Attach request to the admitting peer Admitting Peer (AP) for
Resource-ID n+1. The "send_update" flag can be used to acquire
the routing table Routing Table of AP.

3. JN SHOULD send Attach requests to initiate connections to each of
the peers in the Neighbor Table as well as to the desired peers
in the Finger
Table entries. Table. Note that this does not populate their
Routing Tables, but only their Connection Tables, so JN will not
get messages that it is expected to route to other nodes.

4. JN MUST enter into its Routing Table all the peers that it has
successfully contacted into
its Routing Table. contacted.

5. JN MUST send a Join to AP. The AP MUST send the response to the
Join.

6. AP MUST do a series of Store requests to JN to store the data
that JN will be responsible for.

7. AP MUST send JN an Update explicitly labeling JN as its
predecessor. At this point, JN is part of the ring and is
responsible for a section of the overlay. AP MAY now forget any
data which is assigned to JN and not AP. AP SHOULD NOT forget
any data where AP is the replica set for the data.

8. The AP MUST send an Update to all of its neighbors (including JN)
with the new values of its neighbor set (including JN).

9. The JN MUST send Updates to all of the peers in its Neighbor Table.

If JN sends an Attach to AP with send_update, it immediately knows
most of its expected neighbors from AP's Routing Table update and MAY
directly connect to them. This is the RECOMMENDED procedure.

If for some reason JN does not get AP's Routing Table, it MAY still
populate its Neighbor Table incrementally. It SHOULD send a Ping
directed at Resource-ID n+1 (directly after its own Resource-ID).
This allows it JN to discover its own successor. Call that node p0. It JN
then SHOULD send a ping Ping to p0+1 to discover its successor (p1). This
process MAY be repeated to discover as many successors as desired.
The values for the two peers before p will be found at a later stage stage,
when n receives an Update. An alternate procedure is to send
Attaches to those nodes rather than pings, Pings, which forms form the connections immediately
immediately, but may be slower if the nodes need to collect ICE candidates, thus reducing parallelism.
candidates.

In order to set up its i'th Finger Table entry, JN MUST send an
Attach to peer n+2^(128-i). This will be routed to a peer in
approximately the right location around the ring. (Note that the
first entry in the Finger Table has i=1 and not i=0 in this formulation).
formulation.)

The joining node Joining Node MUST NOT send any Update message placing itself in
the overlay until it has successfully completed an Attach with each
peer that should be in its Neighbor Table.

10.6. Routing Attaches

When a peer needs to Attach to a new peer in its Neighbor Table, it
MUST source-route the Attach request through the peer from which it
learned the new peer's Node-ID. Source-routing these requests allows
the overlay to recover from instability.

All other Attach requests, such as those for new Finger Table
entries, are routed conventionally through the overlay.

10.7. Updates

An Update for this DHT is defined as as:

enum { invalidChordUpdateType(0),
peer_ready(1), neighbors(2), full(3), (255) }
ChordUpdateType;

struct {
uint32 uptime;
ChordUpdateType type;
select (type){
case peer_ready: /* Empty */
;

case neighbors:
NodeId predecessors<0..2^16-1>;
NodeId successors<0..2^16-1>;

case full:
NodeId predecessors<0..2^16-1>;
NodeId successors<0..2^16-1>;
NodeId fingers<0..2^16-1>;
};
} ChordUpdate;

The "uptime" field contains the time this peer has been up in
seconds.

The "type" field contains the type of the update, which depends on
the reason the update was sent.

peer_ready: this

peer_ready
This peer is ready to receive messages. This message is used to
indicate that a node which has Attached is a peer and can be
routed through. It is also used as a connectivity check to
non-neighbor non-
neighbor peers.

neighbors: this

neighbors
This version is sent to members of the Chord Neighbor Table.

full: this

full
This version is sent to peers which request an Update with a
RouteQueryReq.

If the message is of type "neighbors", then the contents of the
message will be:

predecessors
The predecessor set of the Updating peer.

successors
The successor set of the Updating peer.

If the message is of type "full", then the contents of the message
will be:

predecessors
The predecessor set of the Updating peer.

successors
The successor set of the Updating peer.

fingers
The Finger Table of the Updating peer, in numerically ascending
order.

A peer MUST maintain an association (via Attach) to every member of
its neighbor set. A peer MUST attempt to maintain at least three
predecessors and three successors, even though this will not be
possible if the ring is very small. It is RECOMMENDED that O(log(N))
predecessors and successors be maintained in the neighbor set. There
are many ways to estimate N, some of which are discussed in
[I-D.ietf-p2psip-self-tuning].
[DHT-RELOAD].

10.7.1. Handling Neighbor Failures

Every time a connection to a peer in the Neighbor Table is lost (as
determined by connectivity pings or the failure of some request), the
peer MUST remove the entry from its Neighbor Table and replace it
with the best match it has from the other peers in its Routing Table.
If using reactive recovery, it the peer MUST send an immediate Update to
all nodes in its Neighbor Table. The update will contain all the Node-
IDs
Node-IDs of the current entries of the table (after the failed one
has been removed). Note that when replacing a successor successor, the peer
SHOULD delay the creation of new replicas for the successor
replacement hold-
down hold-down time (30 seconds) after removing the failed
entry from its Neighbor Table in order to allow a triggered update to
inform it of a better match for its Neighbor Table.

If the neighbor failure affects the peer's range of responsible IDs,
then the Update MUST be sent to all nodes in its Connection Table.

A peer MAY attempt to reestablish connectivity with a lost neighbor
either by waiting additional time to see if connectivity returns or
by actively routing a new Attach to the lost peer. Details for these
procedures are beyond the scope of this document. In the case of an
attempt to reestablish connectivity with a lost neighbor, the peer
MUST be removed from the Neighbor Table. Such a peer is returned to
the Neighbor Table once connectivity is reestablished.

If connectivity is lost to all successor peers in the Neighbor Table,
then this peer SHOULD behave as if it is joining the network and MUST
use Pings to find a peer and send it a Join. If connectivity is lost
to all the peers in the Finger Table, this peer SHOULD assume that it
has been disconnected from the rest of the network, and it SHOULD
periodically try to join the DHT.

10.7.2. Handling Finger Table Entry Failure

If a Finger Table entry is found to have failed (as determined by
connectivity pings or the failure of some request), all references to
the failed peer MUST be removed from the Finger Table and replaced
with the closest preceding peer from the Finger Table or Neighbor
Table.

If using reactive recovery, the peer MUST initiate a search for a new
Finger Table entry entry, as described below.

10.7.3. Receiving Updates

When a peer, x, peer x receives an Update request, it examines the Node-IDs in
the UpdateReq and at its Neighbor Table and decides if this UpdateReq
would change its Neighbor Table. This is done by taking the set of
peers currently in the Neighbor Table and comparing them to the peers
in the update Update request. There are two major cases:

o The UpdateReq contains peers that match x's Neighbor Table, so no
change is needed to the neighbor set.

o The UpdateReq contains peers that x does not know about that
should be in x's Neighbor Table, Table; i.e., they are closer than
entries in the Neighbor Table.

In the first case, no change is needed.

In the second case, x MUST attempt to Attach to the new peers peers, and if
it is successful successful, it MUST adjust its neighbor set accordingly. Note
that it x can maintain the now inferior peers as neighbors, but it MUST
remember the closer ones.

After any Pings and Attaches are done, if the Neighbor Table changes
and the peer is using reactive recovery, the peer MUST send an Update
request to each member of its Connection Table. These Update
requests are what end up filling in the predecessor/successor tables
of peers that this peer is a neighbor to. A peer MUST NOT enter
itself in its successor or predecessor table and instead should leave
the entries empty.

If peer x is responsible for a Resource-ID R, R and x discovers that the
replica set for R (the next two nodes in its successor set) has
changed, it MUST send a Store for any data associated with R to any
new node in the replica set. It SHOULD NOT delete data from peers
which have left the replica set.

When a peer x detects that it is no longer in the replica set for a
resource R (i.e., there are three predecessors between x and R), it
SHOULD delete all data associated with R from its local store.

When a peer discovers that its range of responsible IDs have has changed,
it MUST send an Update to all entries in its Connection Table.

10.7.4. Stabilization

There are four components to stabilization:

1. exchange Exchange Updates with all peers in its Neighbor Table to exchange
state.

2. search Search for better peers to place in its Finger Table.

3. search Search to determine if the current Finger Table size is
sufficiently large.

4. search Search to determine if the overlay has partitioned and needs to
recover.

10.7.4.1. Updating the Neighbor Table

A peer MUST periodically send an Update request to every peer in its
Neighbor Table. The purpose of this is to keep the predecessor and
successor lists up to date and to detect failed peers. The default
time is about every ten minutes, but the configuration server SHOULD
set this in the configuration document Configuration Document using the "chord-update-
interval" element (denominated in seconds.) seconds). A peer SHOULD randomly
offset these Update requests so they do not occur all at once.

10.7.4.2. Refreshing the Finger Table

A peer MUST periodically search for new peers to replace invalid
entries in the Finger Table. For peer x, the i'th Finger Table entry
is valid if it is in the range [ x+2^( 128-i ),
x+2^( 128-(i-1) )-1 ]. Invalid entries occur in the Finger Table
when a previous Finger Table entry has failed or when no peer has
been found in that range.

Two possible methods for searching for new peers for the Finger Table
entries are presented:

Alternative 1: A peer selects one entry in the Finger Table from
among the invalid entries. It pings for a new peer for that Finger
Table entry. The selection SHOULD be exponentially weighted to
attempt to replace earlier (lower i) entries in the Finger Table. A
simple way to implement this selection is to search through the
Finger Table entries from i=1 i=1, and each time an invalid entry is
encountered, send a Ping to replace that entry with probability 0.5.

Alternative 2: A peer monitors the Update messages received from its
connections to observe when an Update indicates a peer that would be
used to replace in an invalid Finger Table entry, i, and flags that
entry in the Finger Table. Every "chord-ping-interval" seconds, the
peer selects from among those flagged candidates using an
exponentially weighted probability probability, as above.

When searching for a better entry, the peer SHOULD send the Ping to a
Node-ID selected randomly from that range. Random selection is
preferred over a search for strictly spaced entries to minimize the
effect of churn on overlay routing [minimizing-churn-sigcomm06]. An
implementation or subsequent specification MAY choose a method for
selecting Finger Table entries other than choosing randomly within
the range. Any such alternate methods SHOULD be employed only on
Finger Table stabilization and not for the selection of initial
Finger Table entries unless the alternative method is faster and
imposes less overhead on the overlay.

A peer SHOULD NOT send Ping requests looking for new finger table
entries more often than the configuration element "chord-ping-
interval", which defaults to 3600 seconds (one per hour).

A peer MAY choose to keep connections to multiple peers that can act
for a given Finger Table entry.

10.7.4.3. Adjusting Finger Table size Size

If the Finger Table has less fewer than 16 entries, the node SHOULD
attempt to discover more fingers to grow the size of the table to 16.
The value 16 was chosen to ensure high odds of a node maintaining
connectivity to the overlay even with strange network partitions.

For many overlays, 16 Finger Table entries will be enough, but as an
overlay grows very large, more than 16 entries may be required in the
Finger Table for efficient routing. An implementation SHOULD be
capable of increasing the number of entries in the Finger Table to
128 entries.

Although log(N) entries are all that are required for optimal
performance, careful implementation of stabilization will result in
no additional traffic being generated when maintaining a Finger Table
larger than log(N) entries. Implementers are encouraged to make use
of RouteQuery and algorithms for determining where new Finger Table
entries may be found. Complete details of possible implementations
are outside the scope of this specification.

A simple approach to sizing the Finger Table is to ensure that the
Finger Table is large enough to contain at least the final successor
in the peer's Neighbor Table.

10.7.4.4. Detecting partitioning Partitioning

To detect that a partitioning has occurred and to heal the overlay, a
peer P MUST periodically repeat the discovery process used in the
initial join for the overlay to locate an appropriate bootstrap node,
B. P SHOULD then send a Ping for its own Node-ID routed through B.
If a response is received from a peer S', which is not P's successor,
then the overlay is partitioned and P SHOULD send an Attach to S'
routed through B, followed by an Update sent to S'. (Note that S'
may not be in P's Neighbor Table once the overlay is healed, but the
connection will allow S' to discover appropriate neighbor entries for
itself via its own stabilization.)

Future specifications may describe alternative mechanisms for
determining when to repeat the discovery process.

10.8. Route query Query

For CHORD-RELOAD, the RouteQueryReq contains no additional
information. The RouteQueryAns contains the single Node-ID of the
next peer to which the responding peer would have routed the request
message in recursive routing:

struct {
NodeId next_peer;
} ChordRouteQueryAns;

The contents of this structure are as follows:

next_peer
The peer to which the responding peer would route the message in
order to deliver it to the destination listed in the request.

If the requester has set the send_update flag, the responder SHOULD
initiate an Update immediately after sending the RouteQueryAns.

10.9. Leaving

To support extensions, such as [I-D.ietf-p2psip-self-tuning], Peers [DHT-RELOAD], peers SHOULD send a
Leave request to all members of their Neighbor Table
prior to before exiting
the Overlay Instance. The overlay_specific_data field MUST contain
the ChordLeaveData structure structure, defined below:

enum { invalidChordLeaveType(0),
from_succ(1), from_pred(2), (255) }
ChordLeaveType;

struct {
ChordLeaveType type;

select (type) {
case from_succ:
NodeId successors<0..2^16-1>;

case from_pred:
NodeId predecessors<0..2^16-1>;
};
} ChordLeaveData;
The 'type' "type" field indicates whether the Leave request was sent by a
predecessor or a successor of the recipient:

from_succ
The Leave request was sent by a successor.

from_pred
The Leave request was sent by a predecessor.

If the type of the request is 'from_succ', "from_succ", the contents will be:

successors
The sender's successor list.

If the type of the request is 'from_pred', "from_pred", the contents will be:

predecessors
The sender's predecessor list.

Any peer which receives a Leave for a peer n in its neighbor set MUST
follow procedures as if it had detected a peer failure as described
in Section 10.7.1.

11. Enrollment and Bootstrap

The section defines the format of the configuration data as well the
process to join a new overlay.

11.1. Overlay Configuration

This specification defines a new content type "application/
p2p-overlay+xml" for an a MIME entity that contains overlay information.
An example document is shown below. below:

<?xml version="1.0" encoding="UTF-8"?>
<overlay xmlns="urn:ietf:params:xml:ns:p2p:config-base"
xmlns:ext="urn:ietf:params:xml:ns:p2p:config-ext1"
xmlns:chord="urn:ietf:params:xml:ns:p2p:config-chord">
<configuration instance-name="overlay.example.org" sequence="22"
expiration="2002-10-10T07:00:00Z" ext:ext-example="stuff" >
<topology-plugin> CHORD-RELOAD </topology-plugin>
<node-id-length>16</node-id-length>
<root-cert>
MIIDJDCCAo2gAwIBAgIBADANBgkqhkiG9w0BAQUFADBwMQswCQYDVQQGEwJVUzET
MBEGA1UECBMKQ2FsaWZvcm5pYTERMA8GA1UEBxMIU2FuIEpvc2UxDjAMBgNVBAoT
BXNpcGl0MSkwJwYDVQQLEyBTaXBpdCBUZXN0IENlcnRpZmljYXRlIEF1dGhvcml0
eTAeFw0wMzA3MTgxMjIxNTJaFw0xMzA3MTUxMjIxNTJaMHAxCzAJBgNVBAYTAlVT
MRMwEQYDVQQIEwpDYWxpZm9ybmlhMREwDwYDVQQHEwhTYW4gSm9zZTEOMAwGA1UE
ChMFc2lwaXQxKTAnBgNVBAsTIFNpcGl0IFRlc3QgQ2VydGlmaWNhdGUgQXV0aG9y
aXR5MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDDIh6DkcUDLDyK9BEUxkud
+nJ4xrCVGKfgjHm6XaSuHiEtnfELHM+9WymzkBNzZpJu30yzsxwfKoIKugdNUrD4
N3viCicwcN35LgP/KnbN34cavXHr4ZlqxH+OdKB3hQTpQa38A7YXdaoz6goW2ft5
Mi74z03GNKP/G9BoKOGd5QIDAQABo4HNMIHKMB0GA1UdDgQWBBRrRhcU6pR2JYBU
bhNU2qHjVBShtjCBmgYDVR0jBIGSMIGPgBRrRhcU6pR2JYBUbhNU2qHjVBShtqF0
pHIwcDELMAkGA1UEBhMCVVMxEzARBgNVBAgTCkNhbGlmb3JuaWExETAPBgNVBAcT
CFNhbiBKb3NlMQ4wDAYDVQQKEwVzaXBpdDEpMCcGA1UECxMgU2lwaXQgVGVzdCBD
ZXJ0aWZpY2F0ZSBBdXRob3JpdHmCAQAwDAYDVR0TBAUwAwEB/zANBgkqhkiG9w0B
AQUFAAOBgQCWbRvv1ZGTRXxbH8/EqkdSCzSoUPrs+rQqR0xdQac9wNY/nlZbkR3O
qAezG6Sfmklvf+DOg5RxQq/+Y6I03LRepc7KeVDpaplMFGnpfKsibETMipwzayNQ
QgUf4cKBiF+65Ue7hZuDJa2EMv8qW4twEhGDYclpFU9YozyS1OhvUg==
</root-cert>
<root-cert> YmFkIGNlcnQK </root-cert>
<enrollment-server>https://example.org</enrollment-server>
<enrollment-server>https://example.net</enrollment-server>
<self-signed-permitted
digest="sha1">false</self-signed-permitted>
<bootstrap-node address="192.0.0.1" port="6084" />
<bootstrap-node address="192.0.2.2" port="6084" />
<bootstrap-node address="2001:DB8::1" port="6084" />
<turn-density> 20 </turn-density>
<clients-permitted> false </clients-permitted>
<no-ice> false </no-ice>
<chord:chord-update-interval>
400</chord:chord-update-interval>
<chord:chord-ping-interval>30</chord:chord-ping-interval>
<chord:chord-reactive> true </chord:chord-reactive>
<shared-secret> password </shared-secret>
<max-message-size>4000</max-message-size>
<initial-ttl> 30 </initial-ttl>
<overlay-reliability-timer> 3000 </overlay-reliability-timer>
<overlay-link-protocol>TLS</overlay-link-protocol>
<configuration-signer>47112162e84c69ba</configuration-signer>
<kind-signer> 47112162e84c69ba </kind-signer>
<kind-signer> 6eba45d31a900c06 </kind-signer>
<bad-node> 6ebc45d31a900c06 </bad-node>
<bad-node> 6ebc45d31a900ca6 </bad-node>

<ext:example-extension> foo </ext:example-extension>

<mandatory-extension>
urn:ietf:params:xml:ns:p2p:config-ext1
</mandatory-extension>

<required-kinds>
<kind-block>
<kind name="SIP-REGISTRATION">
<data-model>SINGLE</data-model>
<access-control>USER-MATCH</access-control>
<max-count>1</max-count>
<max-size>100</max-size>
</kind>
<kind-signature>
VGhpcyBpcyBub3QgcmlnaHQhCg==
</kind-signature>
</kind-block>
<kind-block>
<kind id="2000">
<data-model>ARRAY</data-model>
<access-control>NODE-MULTIPLE</access-control>
<max-node-multiple>3</max-node-multiple>
<max-count>22</max-count>
<max-size>4</max-size>
<ext:example-kind-extension> 1
</ext:example-kind-extension>
</kind>
<kind-signature>
VGhpcyBpcyBub3QgcmlnaHQhCg==
</kind-signature>
</kind-block>
</required-kinds>
</configuration>
<signature> VGhpcyBpcyBub3QgcmlnaHQhCg== </signature>

<configuration instance-name="other.example.net">
</configuration>
<signature> VGhpcyBpcyBub3QgcmlnaHQhCg== </signature>

</overlay>

The file MUST be a well formed well-formed XML document document, and it SHOULD contain an
encoding declaration in the XML declaration. The file MUST use the
UTF-8 character encoding. The namespace namespaces for the elements defined in
this specification is are urn:ietf:params:xml:ns:p2p:config-base and
urn:ietf:params:xml:ns:p2p:config-chord".

Note that elements or attributes that are defined as type xsd:boolean
in the RELAX NG schema (Section 11.1.1) have two lexical
representations, "1" or "true" for the concept true true, and "0" or
"false" for the concept false. Whitespace and case processing
follows the rules of [OASIS.relax_ng] and XML Schema Datatypes
[W3C.REC-xmlschema-2-20041028] .
[W3C.REC-xmlschema-2-20041028].

The file MAY contain multiple "configuration" elements elements, where each
one contains the configuration information for a different overlay.

Each configuration element MAY be followed by signature elements that
provides
provide a signature over the preceding configuration element. Each
configuration element has the following attributes:

instance-name: the

instance-name
The name of the overlay (referred to as "overlay name" in this
specification)

expiration: time

expiration
Time in the future at which this overlay configuration is no
longer valid. The node SHOULD retrieve a new copy of the
configuration at a randomly selected time that is before the
expiration time. Note that if the certificates expire before a
new configuration is retried, the node will not be able to
validate the configuration file. All times MUST conform to the
Internet Date/Time Format date/time format defined in [RFC3339] and be specified
using Coordinated Universal Time (UTC).

sequence: a UTC.

sequence
A monotonically increasing sequence number between 0 and
2^16-2 2^16-2.

Inside each overlay element, the following elements can occur:

topology-plugin

topology-plug-in
This element defines the overlay algorithm being used. If missing
missing, the default is "CHORD-RELOAD".

node-id-length
This element contains the length of a NodeId
(NodeIdLength) (NodeIdLength), in
bytes. This value MUST be between 16 (128 bits) and 20 (160
bits). If this element is not present, the default of 16 is used.

root-cert
This element contains a base-64 encoded base-64-encoded X.509v3 certificate that
is a root trust anchor used to sign all certificates in this
overlay. There can be more than one root-
cert root-cert element.

enrollment-server
This element contains the URL at which the enrollment server can
be reached in a "url" element. This URL MUST be of type "https:".
More than one enrollment-server element MAY be present. Note that
there is no necessary relationship between the overlay name/configuration name/
configuration server name and the enrollment server name.

self-signed-permitted
This element indicates whether self-signed certificates are
permitted. If it is set to "true", then self-
signed self-signed certificates
are allowed, in which case the enrollment-
server enrollment-server and root-cert
elements MAY be absent. Otherwise, it SHOULD be absent, but MAY
be set to "false". This element also contains an attribute "digest"
"digest", which indicates the digest to be used to compute the
Node-ID. Valid values for this parameter are "sha1" and "sha256" "sha256",
representing SHA-1 [RFC3174] and SHA-256 [RFC6234] [RFC6234], respectively.
Implementations MUST support both of these algorithms.

bootstrap-node
This element represents the address of one of the bootstrap nodes.
It has an attribute called "address" that represents the IP
address (either IPv4 or IPv6, since they can be distinguished) and
an optional attribute called "port" that represents the port and
defaults to 6084. The IPv6 address is in typical hexadecimal form
using standard period and colon separators as specified in
[RFC5952]. More than one bootstrap-
node bootstrap-node element MAY be present.

turn-density
This element is a positive integer that represents the approximate
reciprocal of density of nodes that can act as TURN servers. For
example, if 5% of the nodes can act as TURN servers, this element
would be set to 20. If it is not present, the default value is 1.
If there are no TURN servers in the overlay, it is set to zero.

clients-permitted
This element represents whether clients are permitted or whether
all nodes must be peers. If clients are permitted, the element
MUST be set to "true" or be absent. If the nodes are not allowed
to remain clients after the initial join, the element MUST be set
to "false". There is currently no way for the overlay to enforce
this.

no-ice
This element represents whether nodes are REQUIRED to use the "No-ICE" "No-
ICE" Overlay Link protocols in this overlay. If it is absent, it
is treated as if it were set to "false".

chord-update-interval
The update frequency for the CHORD-RELOAD
topology plugin Topology Plug-in (see
Section 10).

chord-ping-interval
The ping Ping frequency for the CHORD-RELOAD
topology plugin Topology Plug-in (see
Section 10).

chord-reactive
Whether reactive recovery SHOULD be used for this overlay. Set It is
set to "true" or "false". Default if missing If missing, the default is "true". "true" (see
Section 10).

shared-secret
If shared secret mode is used, this element contains the shared
secret. The security guarantee here is that any agent which is
able to access the configuration document Configuration Document (presumably protected by
some sort of HTTP access control or network topology) is able to
recover the shared secret and hence join the overlay.

max-message-size
Maximum size size, in bytes bytes, of any message in the overlay. If this
value is not present, the default is 5000.

initial-ttl
Initial default TTL (time to live, see Section 6.3.2) for messages. messages (see Section 6.3.2). If this
value is not present, the default is 100.

overlay-reliability-timer
Default value for the end-to-end retransmission timer for
messages, in milliseconds. If not present, the default value is
3000. The value MUST be at least 200 milliseconds, which means
the minimum time delay before dropping a link is 1000
milliseconds.

overlay-link-protocol
Indicates a permissible overlay link protocol (see Section 6.6.1
for requirements for such protocols). An arbitrary number of
these elements may appear. If none appear, then this implies the
default value, "TLS", which refers to the use of TLS and DTLS. If
one or more elements appear, then no default value applies.

kind-signer
This contains a single Node-ID in hexadecimal and indicates that
the certificate with this Node-ID is allowed to sign Kinds.
Identifying kind-signer by Node-ID instead of certificate allows
the use of short lived short-lived certificates without constantly having to
provide an updated configuration file.

configuration-signer
This contains a single Node-ID in hexadecimal and indicates that
the certificate with this Node-ID is allowed to sign
configurations for this instance-name. Identifying the signer by
Node-ID instead of certificate allows the use of short
lived short-lived
certificates without constantly having to provide an updated
configuration file.

bad-node
This contains a single Node-ID in hexadecimal and indicates that
the certificate with this Node-ID MUST NOT be considered valid.
This allows certificate revocation. An arbitrary number of these
elements can be provided. Note that because certificates may
expire, bad-node entries need only be present only for the lifetime of
the certificate. Technically speaking, bad Node-IDs may be reused once
after their certificates have
expired, the expired. The requirement for
Node-IDs to be pseudo randomly pseudorandomly generated gives this event a
vanishing probability.

mandatory-extension
This element contains the name of an XML namespace that a node
joining the overlay MUST support. The presence of a mandatory-extension mandatory-
extension element does not require the extension to be used in the
current configuration file, but can indicate that it may be used
in the future. Note that the namespace is case-sensitive, as
specified in [w3c-xml-namespaces] Section 2.3. 2.3 of [w3c-xml-namespaces]. More than one
mandatory-extension element MAY be present.

Inside each configuration element, the required-kinds element MAY
also occur. This element indicates the Kinds that members MUST
support and contains multiple kind-block elements that each define a
single Kind that MUST be supported by nodes in the overlay. Each
kind-block consists of a single kind element and a kind-signature.
The kind element defines the Kind. The kind-signature is the
signature computed over the kind element.

Each kind element has either an id attribute or a name attribute.
The name attribute is a string representing the Kind (the name
registered to IANA) IANA), while the id is an integer Kind-ID allocated out
of private space.

In addition, the kind element MUST contain the following elements:

max-count: the

max-count
The maximum number of values which members of the overlay must
support.

data-model: the

data-model
The data model to be used.

max-size: the

max-size
The maximum size of individual values.

access-control: the

access-control
The access control model to be used.

The kind element MAY also contain the following element:

max-node-multiple: if

max-node-multiple
If the access control is NODE-MULTIPLE, this element MUST be
included. This indicates the maximum value for the i counter. It
MUST be an integer greater than 0.

All of the non optional non-optional values MUST be provided. If the Kind is
registered with IANA, the data-model and access-control elements MUST
match those in the Kind registration, and clients MUST ignore them in
favor of the IANA versions. Multiple kind-block elements MAY be
present.

The kind-block element also MUST contain a "kind-signature" element.
This signature is computed across the kind element from the beginning
of the first < of the kind element to the end of the last > of the
kind element in the same way as the signature element described later
in this section. kind-block elements MUST be signed by a node listed
in the kind-signers block of the current configuration. Receivers
MUST verify the signature prior to accepting a kind-block.

The configuration element MUST be treated as a binary blob that
cannot be changed - -- including any whitespace changes - -- or the
signature will break. The signature MUST be computed by taking each
configuration element and starting from, and including, the first <
at the start of <configuration> up to and including the > in
</configuration> </
configuration> and treating this as a binary blob that MUST be signed
using the standard SecurityBlock defined in Section 6.3.4. The
SecurityBlock MUST be base 64 base-64 encoded using the base64 alphabet from
[RFC4648] and MUST be put in the signature element following the
configuration object in the configuration file. Any configuration
file MUST be signed by one of the configuration-signer elements from
the previous extant configuration. Recipients MUST verify the
signature prior to accepting the configuration file.

When a node receives a new configuration file, it MUST change its
configuration to meet the new requirements. This may require the
node to exit the DHT and re-join. rejoin. If a node is not capable of
supporting the new requirements, it MUST exit the overlay. If some
information about a particular Kind changes from what the node
previously knew about the Kind (for example example, the max size), the new
information in the configuration files overrides any previously
learned information. If any Kind data was signed by a node that is
no longer allowed to sign Kinds, that Kind MUST be discarded along
with any stored information of that Kind. Note that forcing an
avalanche restart of the overlay with a configuration change that
requires re-joining rejoining the overlay may result in serious performance
problems, including total collapse of the network if configuration
parameters are not properly considered. Such an event may be
necessary in case of a compromised CA or similar problem, but for
large overlays overlays, it should be avoided in almost all circumstances.

11.1.1. RELAX NG Grammar

The grammar for the configuration data is:

namespace chord = "urn:ietf:params:xml:ns:p2p:config-chord"
namespace local = ""
default namespace p2pcf = "urn:ietf:params:xml:ns:p2p:config-base"
namespace rng = "http://relaxng.org/ns/structure/1.0"

anything =
(element * { anything }
| attribute * { text }
| text)*

foreign-elements = element * - (p2pcf:* | local:* | chord:*)
{ anything }*
foreign-attributes = attribute * - (p2pcf:*|local:*|chord:*)
{ text }*
foreign-nodes = (foreign-attributes | foreign-elements)*

start = element p2pcf:overlay {
overlay-element
}

overlay-element &= element configuration {
attribute instance-name { xsd:string },
attribute expiration { xsd:dateTime }?,
attribute sequence { xsd:long }?,
foreign-attributes*,
parameter
}+
overlay-element &= element signature {
attribute algorithm { signature-algorithm-type }?,
xsd:base64Binary
}*

signature-algorithm-type |= "rsa-sha1"
signature-algorithm-type |= xsd:string # signature alg extensions

parameter &= element topology-plugin { topology-plugin-type }?
topology-plugin-type |= xsd:string # topo plugin extensions
parameter &= element max-message-size { xsd:unsignedInt }?
parameter &= element initial-ttl { xsd:int }?
parameter &= element root-cert { xsd:base64Binary }*
parameter &= element required-kinds { kind-block* }?
parameter &= element enrollment-server { xsd:anyURI }*
parameter &= element kind-signer { xsd:string }*
parameter &= element configuration-signer { xsd:string }*
parameter &= element bad-node { xsd:string }*
parameter &= element no-ice { xsd:boolean }?
parameter &= element shared-secret { xsd:string }?
parameter &= element overlay-link-protocol { xsd:string }*
parameter &= element clients-permitted { xsd:boolean }?
parameter &= element turn-density { xsd:unsignedByte }?
parameter &= element node-id-length { xsd:int }?
parameter &= element mandatory-extension { xsd:string }*
parameter &= foreign-elements*

parameter &=
element self-signed-permitted {
attribute digest { self-signed-digest-type },
xsd:boolean
}?
self-signed-digest-type |= "sha1"
self-signed-digest-type |= xsd:string # signature digest extensions

parameter &= element bootstrap-node {
attribute address { xsd:string },
attribute port { xsd:int }?
}*

kind-block = element kind-block {
element kind {
( attribute name { kind-names }
| attribute id { xsd:unsignedInt } ),
kind-parameter
} &
element kind-signature {
attribute algorithm { signature-algorithm-type }?,
xsd:base64Binary
}?
}

kind-parameter &= element max-count { xsd:int }
kind-parameter &= element max-size { xsd:int }
kind-parameter &= element max-node-multiple { xsd:int }?

kind-parameter &= element data-model { data-model-type }
data-model-type |= "SINGLE"
data-model-type |= "ARRAY"
data-model-type |= "DICTIONARY"
data-model-type |= xsd:string # data model extensions

kind-parameter &= element access-control { access-control-type }
access-control-type |= "USER-MATCH"
access-control-type |= "NODE-MATCH"
access-control-type |= "USER-NODE-MATCH"
access-control-type |= "NODE-MULTIPLE"
access-control-type |= xsd:string # access control extensions

kind-parameter &= foreign-elements*

kind-names |= "TURN-SERVICE"
kind-names |= "CERTIFICATE_BY_NODE"
kind-names |= "CERTIFICATE_BY_USER"
kind-names |= xsd:string # kind extensions

# Chord specific parameters
topology-plugin-type |= "CHORD-RELOAD"
parameter &= element chord:chord-ping-interval { xsd:int }?
parameter &= element chord:chord-update-interval { xsd:int }?
parameter &= element chord:chord-reactive { xsd:boolean }?

11.2. Discovery Through through Configuration Server

When a node first enrolls in a new overlay, it starts with a
discovery process to find a configuration server.

The node MAY start by determining the overlay name. This value MUST
be provided by the user or some other out of band out-of-band provisioning
mechanism. The out of band mechanisms out-of-band mechanism MAY also provide an optional
URL for the configuration server. If a URL for the configuration
server is not provided, the node MUST do a DNS SRV query using a
Service name of "reload-config" and a protocol of TCP to find a
configuration server and form the URL by appending a path of "/.well-
known/reload-config" to the overlay name. This uses the "well known "well-known
URI" framework defined in [RFC5785]. For example, if the overlay
name was example.com, the URL would be
"https://example.com/.well-known/reload-config".

Once an address and URL for the configuration server is are determined,
the peer MUST form an HTTPS connection to that IP address. If an
optional URL for the configuration server was provided, the
certificate MUST match the domain name from the URL as described in
[RFC2818]; otherwise otherwise, the certificate MUST match the overlay name as
described in [RFC2818]. If the HTTPS certificates passes pass the name
matching, the node MUST fetch a new copy of the configuration file.
To do this, the peer performs a GET to the URL. The result of the
HTTP GET is an XML configuration file described above. If the XML is
not valid, valid or the instance-name attribute of the overlay-element in
the XML does not match the overlay name, this configurations file
SHOULD be discarded. Otherwise, the new configuration MUST replace
any previously learned configuration file for this overlay.

For overlays that do not use a configuration server, nodes MUST
obtain the configuration information needed to join the overlay
through some out of band approach out-of-band approach, such as an XML configuration file
sent over email.

11.3. Credentials

If the configuration document Configuration Document contains a an enrollment-server element,
credentials are REQUIRED to join the Overlay Instance. A peer which
does not yet have credentials MUST contact the enrollment server to
acquire them.

RELOAD defines its own trivial certificate request protocol. We
would have liked to have used an existing protocol protocol, but were
concerned about the implementation burden of even the simplest of
those protocols, such as [RFC5272] and [RFC5273]. The objective was
to have a protocol which could be easily implemented in a Web server
which the operator did not control (e.g., in a hosted service) and
which was compatible with the existing certificate handling certificate-handling tooling
as used with the Web certificate infrastructure. This means
accepting bare PKCS#10 requests and returning a single bare X.509
certificate. Although the MIME types for these objects are defined,
none of the existing protocols support exactly this model.

The certificate request protocol MUST be performed over HTTPS. The
server certificate MUST match the overlay name as described in
[RFC2818]. The request MUST be an HTTP POST with the parameters
encoded as described in [RFC2388] and with the following properties:

o If authentication is required, there MUST be form parameters of
"password" and "username" containing the user's account name and
password in the clear (hence the need for HTTPS). The username
and password strings MUST be UTF-8 strings compared as binary
objects. Applications using RELOAD SHOULD define any needed
string preparation as per [RFC4013] or its successor documents.

o If more than one Node-ID is required, there MUST be a form
parameter of "nodeids" containing the number of Node-IDs required.

o There MUST be a form parameter of "csr" with a content type of
"application/pkcs10", as defined in [RFC2311] [RFC2311], that contains the
certificate signing request (CSR).

o The Accept header MUST contain the type "application/pkix-cert",
indicating the type that is expected in the response.

The enrollment server MUST authenticate the request using the
provided account name and password. The reason for using the RFC
2388 "multipart/form-data" encoding is so that the password parameter
will not be encoded in the URL URL, to reduce the chance of accidental
leakage of the password. If the authentication succeeds and the
requested user name in the CSR is acceptable, the server MUST
generate and return a certificate for the CSR in the "csr" parameter
of the request. The SubjectAltName field in the certificate MUST
contain the following values:

o One or more Node-IDs which MUST be cryptographically random
[RFC4086]. Each MUST be chosen by the enrollment server in such a
way that they are it is unpredictable to the requesting user. E.g., For example,
the user MUST NOT be informed of potential (random) Node-IDs prior
to authenticating. Each is placed in the subjectAltName using the
uniformResourceIdentifier type and type, each MUST contain RELOAD URIs URI, as
described in Section 14.15 14.15, and each MUST contain a Destination list
List with a single entry of type "node_id". The enrollment server
SHOULD maintain a mapping of users to Node-IDs and if the same
user returns (e.g., to have their certificate re-issued) re-issued), the
enrollment server should return the same Node-IDs, thus avoiding
the need for implementations to re-store all their data when their
certificates expire.

o A single name (the "user name") that this user is allowed to use
in the overlay, using type rfc822Name. Enrollment servers SHOULD
take care to only allow only legal characters in the name (e.g., no
embedded NULs), rather than simply accepting any name provided by
the user. In some usages, the right-hand-side right side of the user name will
match the overlay name, but there is no requirement for this match
in this specification. Applications using this specification MAY
define such a requirement, requirement or MAY otherwise limit the allowed range
of allowed user names.

The SubjectAltName field in the certificate MUST NOT contain any
other
identities other than those listed above. The subject distinguished
name in the certificate MUST be empty.

The certificate MUST be returned as type "application/pkix-cert" "application/pkix-cert", as
defined in [RFC2585], with an HTTP status code of 200 OK.

Certificate processing errors SHOULD result in a an HTTP return code of
403 "Forbidden" Forbidden, along with a body of type "text/plain" and body that
consists of one of the tokens defined in the following list:

failed_authentication
The account name and password combination used in the HTTPS
request was not valid.

username_not_available
The requested user name in the CSR was not acceptable.

Node-IDs_not_available
The number of Node-IDs requested was not acceptable.

bad_CSR
There was some other problem with the CSR.

If the client receives an unknown token in the body, it SHOULD treat
it as a failure for an unknown reason.

The client MUST check that the certificate returned certificate chains back to
one of the certificates received in the "root-cert" list of the
overlay configuration data (including PKIX BasicConstraints checks.) checks).
The node then reads the certificate to find the Node-ID it can use.

11.3.1. Self-Generated Credentials

If the "self-signed-permitted" element is present in the
configuration and is set to "true", then a node MUST generate its own
self-signed certificate to join the overlay. The self-signed
certificate MAY contain any user name of the users user's choice.

For self-signed certificate certificates containing only one Node-ID, the Node-ID
MUST be computed by applying the digest specified in the self-signed-
permitted element to the DER representation of the user's public key
(more specifically specifically, the subjectPublicKeyInfo) and taking the high high-
order bits. For self-signed certificates containing multiple Node-
IDs,
Node-IDs, the index of the Node-ID (from 1 to the number of Node-IDs
needed) must be prepended as a 4 bytes big endian 4-byte big-endian integer to the DER
representation of the user's public key and taking the high order high-order
bits. When accepting a self-signed certificate, nodes MUST check
that the Node-ID and public keys match. This prevents Node-ID theft.

Once the node has constructed a self-signed certificate, it MAY join
the overlay. It MUST store its certificate in the overlay
(Section 8) 8), but SHOULD look to see if the user name is already taken
before and
and, if so so, choose another user name. Note that this only provides
protection only against accidental name collisions. Name theft is
still possible. If protection against name theft is desired, then
the enrollment service MUST be used.

11.4. Contacting a Bootstrap Node

In order to join the overlay, the joining node Joining Node MUST contact a node in
the overlay. Typically this means contacting the bootstrap nodes,
since they are reachable by the local peer or have public IP
addresses. If the joining node Joining Node has cached a list of peers that it
has previously been connected with in this overlay, as an
optimization it MAY attempt to use one or more of them as bootstrap
nodes before falling back to the bootstrap nodes listed in the
configuration file.

When contacting a bootstrap node, the joining node Joining Node MUST first form
the DTLS or TLS connection to the bootstrap node and then send an
Attach request over this connection with the destination Resource-ID
set to the joining node's Joining Node's Node-ID plus 1.

When the requester node finally does receive a response from some
responding node, it MUST use the Node-ID in the response to start
sending requests to join the Overlay Instance as described in
Section 6.4.

After a node has successfully joined the overlay network, it will
have direct connections to several peers. Some MAY be added to the
cached bootstrap nodes list and used in future boots. Peers that are
not directly connected MUST NOT be cached. The suggested number of
peers to cache is 10. Algorithms for determining which peers to
cache are beyond the scope of this specification.

12. Message Flow Example

The following abbreviations are used in the message flow diagrams:
JN = joining node, Joining Node, AP = admitting peer, Admitting Peer, NP = next peer after the AP,
NNP = next next peer which is the peer after NP, PP = previous peer
before the AP, PPP = previous previous peer which is the peer before
the PP, BP = bootstrap peer. node.

In the following example, we assume that JN has formed a connection
to one of the bootstrap nodes. JN then sends an Attach through that
peer to a Resource-ID of itself plus 1 (JN+1). It gets routed to the
admitting peer (AP)
AP, because JN is not yet part of the overlay. When AP responds, JN
and the AP use ICE to set up a connection and then set up DTLS. Once
AP has connected to JN, AP sends to JN an Update to populate its
Routing Table. The following example shows the Update happening
after the DTLS connection is formed formed, but it could also happen before before,
in which case the Update would often be routed through other nodes.

JN PPP PP AP NP NNP BP
| | | | | | |
| | | | | | |
| | | | | | |
|AttachReq Dest=JN+1| | | | |
|---------------------------------------------------------->|
| | | | | | |
| | | | | | |
| | | |AttachReq Dest=JN+1| |
| | | |<----------------------------|
| | | | | | |
| | | | | | |
| | | |AttachAns | |
| | | |---------------------------->|
| | | | | | |
| | | | | | |
|AttachAns | | | | |
|<----------------------------------------------------------|
| | | | | | |
|ICE | | | | | |
|<===========================>| | | |
| | | | | | |
|TLS | | | | | |
|<...........................>| | | |
| | | | | | |
| | | | | | |
| | | | | | |
|UpdateReq| | | | | |
|<----------------------------| | | |
| | | | | | |
| | | | | | |
|UpdateAns| | | | | |
|---------------------------->| | | |
| | | | | | |
| | | | | | |
| | | | | | |

Figure 1

The JN then forms connections to the appropriate neighbors, such as
NP, by sending an Attach which gets routed via other nodes. When NP
responds, JN and NP use ICE and DTLS to set up a connection.

JN PPP PP AP NP NNP BP
| | | | | | |
| | | | | | |
| | | | | | |
|AttachReq NP | | | | |
|---------------------------->| | | |
| | | | | | |
| | | | | | |
| | | |AttachReq NP | |
| | | |-------->| | |
| | | | | | |
| | | | | | |
| | | |AttachAns| | |
| | | |<--------| | |
| | | | | | |
| | | | | | |
|AttachAns| | | | | |
|<----------------------------| | | |
| | | | | | |
| | | | | | |
|ICE | | | | | |
|<=====================================>| | |
| | | | | | |
| | | | | | |
|TLS | | | | | |
|<.....................................>| | |
| | | | | | |
| | | | | | |
| | | | | | |
| | | | | | |

Figure 2

The JN also needs to populate its Finger Table (for the Chord based Chord-based
DHT). It issues an Attach to a variety of locations around the
overlay. The diagram below shows it JN sending an Attach halfway around
the Chord ring to the JN + 2^127.

JN NP XX TP
| | | |
| | | |
| | | |
|AttachReq JN+2<<126| |
|-------->| | |
| | | |
| | | |
| |AttachReq JN+2<<126|
| |-------->| |
| | | |
| | | |
| | |AttachReq JN+2<<126
| | |-------->|
| | | |
| | | |
| | |AttachAns|
| | |<--------|
| | | |
| | | |
| |AttachAns| |
| |<--------| |
| | | |
| | | |
|AttachAns| | |
|<--------| | |
| | | |
|ICE | | |
|<===========================>|
| | | |
|TLS | | |
|<...........................>|
| | | |
| | | |

Figure 3

Once JN has a reasonable set of connections, it is ready to take its
place in the DHT. It does this by sending a Join to AP. AP does sends a
series of Store requests to JN to store the data that JN will be
responsible for. AP then sends JN an Update that explicitly labeling labels
JN as its predecessor. At this point, JN is part of the ring and is
responsible for a section of the overlay. AP can now forget any data
which is assigned to JN and not to AP.

JN PPP PP AP NP NNP BP
| | | | | | |
| | | | | | |
| | | | | | |
|JoinReq | | | | | |
|---------------------------->| | | |
| | | | | | |
| | | | | | |
|JoinAns | | | | | |
|<----------------------------| | | |
| | | | | | |
| | | | | | |
|StoreReq Data A | | | | |
|<----------------------------| | | |
| | | | | | |
| | | | | | |
|StoreAns | | | | | |
|---------------------------->| | | |
| | | | | | |
| | | | | | |
|StoreReq Data B | | | | |
|<----------------------------| | | |
| | | | | | |
| | | | | | |
|StoreAns | | | | | |
|---------------------------->| | | |
| | | | | | |
| | | | | | |
|UpdateReq| | | | | |
|<----------------------------| | | |
| | | | | | |
| | | | | | |
|UpdateAns| | | | | |
|---------------------------->| | | |
| | | | | | |
| | | | | | |
| | | | | | |
| | | | | | |

Figure 4

In Chord, JN's Neighbor Table needs to contain its own predecessors.
It couldn't connect to them previously previously, because it did not yet know
their addresses. However, now that it has received an Update from
AP, as in the previous diagram, it has AP's predecessors, which are
also its own, so it sends Attaches to them. Below Below, it is shown
connecting only to AP's closest predecessor, PP.

JN PPP PP AP NP NNP BP
| | | | | | |
| | | | | | |
| | | | | | |
|AttachReq Dest=PP | | | | |
|---------------------------->| | | |
| | | | | | |
| | | | | | |
| | |AttachReq Dest=PP | | |
| | |<--------| | | |
| | | | | | |
| | | | | | |
| | |AttachAns| | | |
| | |-------->| | | |
| | | | | | |
| | | | | | |
|AttachAns| | | | | |
|<----------------------------| | | |
| | | | | | |
| | | | | | |
|TLS | | | | | |
|...................| | | | |
| | | | | | |
| | | | | | |
|UpdateReq| | | | | |
|------------------>| | | | |
| | | | | | |
| | | | | | |
|UpdateAns| | | | | |
|<------------------| | | | |
| | | | | | |
| | | | | | |
|UpdateReq| | | | | |
|---------------------------->| | | |
| | | | | | |
| | | | | | |
|UpdateAns| | | | | |
|<----------------------------| | | |
| | | | | | |
| | | | | | |
|UpdateReq| | | | | |
|-------------------------------------->| | |
| | | | | | |
| | | | | | |
|UpdateAns| | | | | |
|<--------------------------------------| | |
| | | | | | |
| | | | | | |
Figure 5

Finally, now that JN has a copy of all the data and is ready to route
messages and receive requests, it sends Updates to everyone in its
Routing Table to tell them it is ready to go. Below, it is shown
sending such an update to TP.

JN NP XX TP
| | | |
| | | |
| | | |
|UpdateReq| | |
|---------------------------->|
| | | |
| | | |
|UpdateAns| | |
|<----------------------------|
| | | |
| | | |
| | | |
| | | |

Figure 6

13. Security Considerations

13.1. Overview

RELOAD provides a generic storage service, albeit one designed to be
useful for P2PSIP. In this section section, we discuss security issues that
are likely to be relevant to any usage of RELOAD. More background
information can be found in [RFC5765].

In any Overlay Instance, any given user depends on a number of peers
with which they have no well-defined relationship except that they
are fellow members of the Overlay Instance. In practice, these other
nodes may be friendly, lazy, curious, or outright malicious. No
security system can provide complete protection in an environment
where most nodes are malicious. The goal of security in RELOAD is to
provide strong security guarantees of some properties even in the
face of a large number of malicious nodes and to allow the overlay to
function correctly in the face of a modest number of malicious nodes.

P2PSIP deployments require the ability to authenticate both peers and
resources (users) without the active presence of a trusted entity in
the system. We describe two mechanisms. The first mechanism is
based on public key certificates and is suitable for general
deployments. The second is an admission control mechanism based on
an overlay-wide shared symmetric key.

13.2. Attacks on P2P Overlays

The two basic functions provided by overlay nodes are storage and
routing: some peer is responsible for storing a node's data and for
allowing a third node to fetch this stored data. Other data, while other peers
are responsible for routing messages to and from the storing nodes.
Each of these issues is covered in the following sections.

P2P overlays are subject to attacks by subversive nodes that may
attempt to disrupt routing, corrupt or remove user registrations, or
eavesdrop on signaling. The certificate-based security algorithms we
describe in this specification are intended to protect overlay
routing and user registration information in RELOAD messages.

To protect the signaling from attackers pretending to be valid nodes
(or nodes other than themselves), the first requirement is to ensure
that all messages are received from authorized members of the
overlay. For this reason, RELOAD MUST transport all messages over a
secure channel (TLS and DTLS are defined in this document) which
provides message integrity and authentication of the directly
communicating peer. In addition, messages and data MUST be digitally
signed with the sender's private key, providing end-to-end security
for communications.

13.3. Certificate-based Certificate-Based Security

This specification stores users' registrations and possibly other
data in an overlay network. This requires a solution both to
securing this data as well as and to securing, as well as possible, the routing
in the overlay. Both types of security are based on requiring that
every entity in the system (whether user or peer) authenticate
cryptographically using an asymmetric key pair tied to a certificate.

When a user enrolls in the Overlay Instance, they request or are
assigned a unique name, such as "alice@dht.example.net". These names
MUST be unique and are meant to be chosen and used by humans much
like a SIP Address of Record address-of-record (AOR) or an email address. The user
MUST also be assigned one or more Node-IDs by the central enrollment
authority. Both the name and the Node-IDs are placed in the
certificate, along with the user's public key.

Each certificate enables an entity to act in two sorts of roles:

o As a user, storing data at specific Resource-IDs in the Overlay
Instance corresponding to the user name.

o As a overlay peer with the Node-ID(s) Node-IDs listed in the certificate.

Note that since only users of this Overlay Instance need to validate
a certificate, this usage does not require a global PKI. Public Key
Infrastructure (PKI). Instead, certificates MUST be signed by a
central enrollment authority which acts as the certificate authority
for the Overlay Instance. This authority signs each node's
certificate. Because each node possesses the CA's certificate (which
they receive on enrollment) upon enrollment), they can verify the certificates of
the other entities in the overlay without further communication.
Because the certificates contain the user/
node's user's/node's public key,
communications from the user/node can be verified can, in turn. turn, be verified.

If self-signed certificates are used, then the security provided is
significantly decreased, since attackers can mount Sybil attacks. In
addition, attackers cannot trust the user names in certificates
(though
(although they can trust the Node-IDs Node-IDs, because they are
cryptographically verifiable). This scheme may be appropriate for
some small deployments, such as a small office or an ad hoc overlay
set up among participants in a meeting where all hosts on the network
are trusted. Some additional security can be provided by using the
shared secret admission control scheme as well.

Because all stored data is signed by the owner of the data data, the
storing node can verify that the storer is authorized to perform a
store at that Resource-ID and also can allow any consumer of the data
to verify the provenance and integrity of the data when it retrieves
it.

Note that RELOAD does not itself provide a revocation/status
mechanism (though (although certificates may may, of course course, include OCSP Online
Certificate Status Protocol [OCSP] responder information). Thus,
certificate lifetimes SHOULD be chosen to balance the compromise
window versus the cost of certificate renewal. Because RELOAD is
already designed to operate in the face of some fraction of malicious
nodes, this form of compromise is not fatal.

All implementations MUST implement certificate-based security.

13.4. Shared-Secret Security

RELOAD also supports a shared secret admission control scheme that
relies on a single key that is shared among all members of the
overlay. It is appropriate for small groups that wish to form a
private network without complexity. In shared secret mode, all the
peers MUST share a single symmetric key which is used to key TLS-PSK
or TLS-SRP mode. A peer which does not know the key cannot form TLS
connections with any other peer and therefore cannot join the
overlay.

One natural approach to a shared-secret scheme is to use a user-
entered password as the key. The difficulty with this is that in
TLS-PSK mode, such keys are very susceptible to dictionary attacks.
If passwords are used as the source of shared-keys, shared keys, then TLS-SRP is a
superior choice choice, because it is not subject to dictionary attacks.

13.5. Storage Security

When certificate-based security is used in RELOAD, any given
Resource-ID/Kind-ID pair is bound to some small set of certificates.
In order to write data, the writer must prove possession of the
private key for one of those certificates. Moreover, all data is
stored, signed with the same private key that was used to authorize
the storage. This set of rules makes questions of authorization and
data integrity - integrity, which have historically been thorny for overlays - overlays,
relatively simple.

13.5.1. Authorization

When a node wants to store some value, it MUST first digitally sign
the value with its own private key. It then sends a Store request
that contains both the value and the signature towards the storing
peer (which is defined by the Resource Name construction algorithm
for that particular Kind of value).

When the storing peer receives the request, it MUST determine whether
the storing node is authorized to store at this Resource-ID/Kind-ID
pair. Determining this requires comparing the user's identity to the
requirements of the access control model (see Section 7.3). If it
satisfies those requirements requirements, the user is authorized to write,
pending quota checks checks, as described in the next section.

For example, consider the a certificate with the following properties:

User name: alice@dht.example.com
Node-ID: 013456789abcdef
Serial: 1234

If Alice wishes to Store a value of the "SIP Location" Kind, the
Resource Name will be the SIP AOR "sip:alice@dht.example.com". The
Resource-ID will be determined by hashing the Resource Name. Because
SIP Location uses the USER-NODE-MATCH policy, it first verifies that
the user name in the certificate hashes to the requested Resource-ID.

It then verifies that the Node-ID in the certificate matches the
dictionary key being used for the store. If both of these checks
succeed, the Store is authorized. Note that because the access
control model is different for different Kinds, the exact set of
checks will vary.

13.5.2. Distributed Quota

Being a peer in an Overlay Instance carries with it the
responsibility to store data for a given region of the Overlay
Instance. However, allowing nodes to store unlimited amounts of data
would create unacceptable burdens on peers and would also enable
trivial denial of service denial-of-service (DoS) attacks. RELOAD addresses this issue
by requiring configurations to define maximum sizes for each Kind of
stored data. Attempts to store values exceeding this size MUST be
rejected (if
rejected. (If peers are inconsistent about this, then strange
artifacts will happen when the zone of responsibility shifts and a
different peer becomes responsible for overlarge data). data.) Because each
Resource-ID/Kind-ID pair is bound to a small set of certificates,
these size restrictions also create a distributed quota mechanism,
with the quotas administered by the central configuration server.

Allowing different Kinds of data to have different size restrictions
allows new usages the flexibility to define limits that fit their
needs without requiring all usages to have expansive limits.

13.5.3. Correctness

Because each stored value is signed, it is trivial for any retrieving
node to verify the integrity of the stored value. Some more More care needs to
be taken to prevent version rollback attacks. Rollback attacks on
storage are prevented by the use of store times and lifetime values
in each store. A lifetime represents the latest time at which the
data is valid and thus limits (though (although does not completely prevent)
the ability of the storing node to perform a rollback attack on
retrievers. In order to prevent a rollback attack at the time of the
Store request, it is REQUIRED that storage times be monotonically
increasing. Storing peers MUST reject Store requests with storage
times smaller than or equal to those that they are currently storing.
In addition, a fetching node which receives a data value with a
storage time older than the result of the previous fetch knows that a
rollback has occurred.

13.5.4. Residual Attacks

The mechanisms described here provides provide a high degree of security, but
some attacks remain possible. Most simply, it is possible for
storing peers to refuse to store a value (i.e., they reject any
request). In addition, a storing peer can deny knowledge of values
which it has previously accepted. To some extent extent, these attacks can
be ameliorated by attempting to store to/retrieve to and retrieve from replicas,
but a retrieving node does not know whether or not it should try this or not, since
this, as there is a cost to doing so.

The certificate-based authentication scheme prevents a single peer
from being able to forge data owned by other peers. Furthermore,
although a subversive peer can refuse to return data resources for
which it is responsible, it cannot return forged data data, because it
cannot provide authentication for such registrations. Therefore Therefore,
parallel searches for redundant registrations can mitigate most of
the effects of a compromised peer. The ultimate reliability of such
an overlay is a statistical question based on the replication factor
and the percentage of compromised peers.

In addition, when a Kind is multivalued (e.g., an array data model),
the storing peer can return only some subset of the values, thus
biasing its responses. This can be countered by using single values
rather than sets, but that makes coordination between multiple
storing agents much more difficult. This is a trade off trade-off that must be
made when designing any usage.

13.6. Routing Security

Because the storage security system guarantees (within limits) the
integrity of the stored data, routing security focuses on stopping
the attacker from performing a DOS DoS attack that misroutes requests in
the overlay. There are a few obvious observations to make about
this. First, it is easy to ensure that an attacker is at least a
valid node in the Overlay Instance. Second, this is a DOS DoS attack
only. Third, if a large percentage of the nodes on the Overlay
Instance are controlled by the attacker, it is probably impossible to
perfectly secure against this.

13.6.1. Background

In general, attacks on DHT routing are mounted by the attacker
arranging to route traffic through one or two nodes that it controls.
In the Eclipse attack [Eclipse] [Eclipse], the attacker tampers with messages
to and from nodes for which it is on-path with respect to a given
victim node. This allows it to pretend to be all the nodes that are
reachable through it. In the Sybil attack [Sybil], the attacker
registers a large number of nodes and is therefore able to capture a
large amount of the traffic through the DHT.

Both the Eclipse and Sybil attacks require the attacker to be able to
exercise control over her Node-IDs. The Sybil attack requires the
creation of a large number of peers. The Eclipse attack requires
that the attacker be able to impersonate specific peers. In both
cases, RELOAD attempts to mitigate these attacks are limited by the use of
centralized, certificate-based admission control.

13.6.2. Admissions Control

Admission to a RELOAD Overlay Instance is controlled by requiring
that each peer have a certificate containing its Node-ID. The
requirement to have a certificate is enforced by using certificate-
based mutual authentication on each connection. (Note: the following only
applies only when self-signed certificates are not used.) Whenever a
peer connects to another peer, each side automatically checks that
the other has a suitable certificate. These Node-IDs MUST be
randomly assigned by the central enrollment server. This has two
benefits:

o It allows the enrollment server to limit the number of Node-IDs
issued to any individual user.

o It prevents the attacker from choosing specific Node-IDs.

The first property allows protection against Sybil attacks (provided
that the enrollment server uses strict rate limiting rate-limiting policies). The
second property deters but does not completely prevent Eclipse
attacks. Because an Eclipse attacker must impersonate peers on the
other side of the attacker, the attacker must have a certificate for
suitable Node-IDs, which requires him to repeatedly query the
enrollment server for new certificates, which will match only by
chance. From the attacker's perspective, the difficulty is that if
the attacker only has only a small number of certificates, the region of
the Overlay Instance he is impersonating appears to be very sparsely
populated by comparison to the victim's local region.

13.6.3. Peer Identification and Authentication

In general, whenever a peer engages in overlay activity that might
affect the Routing Table Table, it must establish its identity. This
happens in two ways. First, whenever a peer establishes a direct
connection to another peer peer, it authenticates via certificate-based
mutual authentication. All messages between peers are sent over this
protected channel channel, and therefore the peers can verify the data origin
of the last hop last-hop peer for requests and responses without further
cryptography.

In some situations, however, it is desirable to be able to establish
the identity of a peer with whom one is not directly connected. The
most natural case is when a peer Updates its state. At this point,
other peers may need to update their view of the overlay structure,
but they need to verify that the Update message came from the actual
peer rather than from an attacker. To prevent this, having a peer accept
Update messages from an attacker, all overlay routing messages are
signed by the peer that generated them.

Replay is typically prevented for

For messages that impact the topology of the overlay overlay, replay is
typically prevented by having the information come directly, directly from, or
be verified by, the nodes that claimed to have generated the update.
Data storage replay detection is done by signing the time of the node
that generated the signature on the store request Store request, thus providing a
time based
time-based replay protection protection, but the time synchronization is only needed
only between peers that can write to the same location.

13.6.4. Protecting the Signaling

The goal here is to stop an attacker from knowing who is signaling
what to whom. An attacker is unlikely to be able to observe the
activities of a specific individual individual, given the randomization of IDs
and routing based on the present peers discussed above. Furthermore,
because messages can be routed using only the header information, the
actual body of the RELOAD message can be encrypted during
transmission.

There are two lines of defense here. The first is the use of TLS or
DTLS for each communications link between peers. This provides
protection against attackers who are not members of the overlay. The
second line of defense is to digitally sign each message. This
prevents adversarial peers from modifying messages in flight, even if
they are on the routing path.

13.6.5. Routing Loops and Dos DoS Attacks

Source routing

Source-routing mechanisms are known to create the possibility for DoS
amplification, especially by the induction of routing loops
[RFC5095]. In order to limit amplification, the initial-ttl value in
the configuration file SHOULD be set to a value slightly larger than
the longest expected path through the network. For Chord, experience
has shown that log(2) of the number of nodes in the network + 5 is a
safe bound. Because nodes are required to enforce the initial-ttl as
the maximum value, an attacker cannot achieve an amplification factor
greater than initial-ttl, thus limiting the additional capabilities
provided by source routing.

In order to prevent the use of loops for targeted implementation
attacks, implementations SHOULD check the destination list Destination List for
duplicate entries and discard such records with an
"Error_Invalid_Message" error. This does not completely prevent
loops
loops, but it does require that at least one attacker node be part of
the loop.

13.6.6. Residual Attacks

The routing security mechanisms in RELOAD are designed to contain
rather than eliminate attacks on routing. It is still possible for
an attacker to mount a variety of attacks. In particular, if an
attacker is able to take up a position on the overlay routing between
A and B B, it can make it appear as if B does not exist or is
disconnected. It can also advertise false network metrics in an
attempt to reroute traffic. However, these are primarily DOS DoS
attacks.

The certificate-based security scheme secures the namespace, but if
an individual peer is compromised or if an attacker obtains a
certificate from the CA, then a number of subversive peers can still
appear in the overlay. While these peers cannot falsify responses to
resource queries, they can respond with error messages, effecting a
DoS attack on the resource registration. They can also subvert
routing to other compromised peers. To defend against such attacks,
a resource search must still consist of parallel searches for
replicated registrations.

14. IANA Considerations

This section contains the new code points registered by this
document. [NOTE TO IANA/RFC-EDITOR: Please replace RFC-to-be with
the RFC number for this specification in the following list.]

14.1. Well-Known URI Registration

IANA SHALL make the following "Well Known has registered a "well-known URI" registration as described in [RFC5785]:

[[Note to RFC Editor - this paragraph can be removed before
publication. ]] A review request was sent to
wellknown-uri-review@ietf.org on October 12, 2010.

14.2. Port Registrations

[[Note to RFC Editor - this paragraph can be removed before
publication. ]]

IANA has already allocated a TCP port for the main
peer to peer peer-to-peer
protocol. This port has had the name p2psip-enroll and the port number
of 6084. Per this document, IANA needs to update has updated this registration to
change the service name to reload-config and to define it for UDP as
well as TCP. reload-config.

IANA SHALL make has made the following port registration:

14.3. Overlay Algorithm Types

IANA SHALL create has created a "RELOAD Overlay Algorithm Type" Types" Registry.
Entries in this registry are strings denoting the names of overlay
algorithms
algorithms, as described in Section 11.1 of [RFC-to-be]. [RFC6940]. The
registration policy for this registry is RFC 5226 IETF Review. "IETF Review" [RFC522]. The
initial contents of this registry are:

The value EXP-OVERLAY has been made available for the purposes of
experimentation. This value is not meant for vendor specific vendor-specific use of
any sort sort, and it MUST NOT be used for operational deployments.

14.4. Access Control Policies

IANA SHALL create has created a "RELOAD Access Control Policy" Policies" Registry.
Entries in this registry are strings denoting access control
policies, as described in Section 7.3 of [RFC-to-be]. [RFC6940]. New entries in
this registry SHALL be registered via RFC 5226 Standards Action. Action [RFC5226].
The initial contents of this registry are:

14.5. Application-ID

IANA SHALL create has created a "RELOAD Application-ID" Registry. Entries in this
registry are 16-bit integers denoting application-ids Application-IDs, as described
in Section 6.5.2 of [RFC-to-be]. [RFC6940]. Code points in the range
0x0001 1 to 0x7fff 32767
SHALL be registered via RFC 5226 Standards Action. Action [RFC5226]. Code points in
the range 0x8000 32768 to 0xf000 61440 SHALL be registered via RFC
5226 Expert Review. Review
[RFC5226]. Code points in the range 0xf001 61441 to 0xfffe 65534 are reserved for
private use. The initial contents of this registry are:

+-------------+----------------+-------------------------------+
| Application | Application-ID | Specification |
+-------------+----------------+-------------------------------+
| INVALID | 0 | RFC-to-be RFC 6940 |
| SIP | 5060 | Reserved for use by SIP Usage |
| SIP | 5061 | Reserved for use by SIP Usage |
| Reserved | 0xffff 65535 | RFC-to-be RFC 6940 |
+-------------+----------------+-------------------------------+

14.6. Data Kind-ID

IANA SHALL create has created a "RELOAD Data Kind-ID" Registry. registry. Entries in this
registry are 32-bit integers denoting data Kinds, as described in
Section 4.2 5.2 of [RFC-to-be]. [RFC6940]. Code points in the range 0x00000001 to
0x7fffffff
0x7FFFFFFF SHALL be registered via RFC 5226 Standards Action. Action [RFC5226]. Code
points in the range 0x8000000 to 0xf0000000 0xF0000000 SHALL be registered via
RFC 5226
Expert Review. Review [RFC5226]. Code points in the range 0xf0000001 0xF0000001 to
0xfffffffe
0xFFFFFFFE are reserved for private use via the Kind description
mechanism described in Section 11 of [RFC-to-be]. [RFC6940]. The initial contents
of this registry are:

14.7. Data Model

IANA SHALL create has created a "RELOAD Data Model" Registry. registry. Entries in this
registry are strings denoting data models, as described in
Section 7.2 of [RFC-to-be]. [RFC6940]. New entries in this registry SHALL be
registered via RFC 5226 Standards Action. Action [RFC5226]. The initial contents of
this registry are:

The value EXP-DATA has been made available for the purposes of
experimentation. This value is not meant for vendor specific vendor-specific use of
any sort sort, and it MUST NOT be used for operational deployments.

14.8. Message Codes

IANA SHALL create has created a "RELOAD Message Code" Registry. Codes" registry. Entries in this
registry are 16-bit integers denoting method codes codes, as described in
Section 6.3.3 of [RFC-to-be]. [RFC6940]. These codes SHALL be registered via
RFC 5226
Standards Action. Action [RFC5226]. The initial contents of this registry
are:

+---------------------------------+----------------+-----------+

The values exp_a_req, exp_a_ans, exp_b_req, and exp_b_ans have been
made available for the purposes of experimentation. These values are
not meant for vendor specific vendor-specific use of any sort sort, and they MUST NOT be
used for operational deployments.

14.9. Error Codes

IANA SHALL create has created a "RELOAD Error Code" Registry. registry. Entries in this
registry are 16-bit integers denoting error codes codes, as described in
Section 6.3.3.1 of [RFC-to-be]. [RFC6940]. New entries SHALL be defined via RFC
5226
Standards Action. Action [RFC5226]. The initial contents of this registry
are:

The values Error_Exp_A and Error_Exp_B have been made available for
the purposes of experimentation. These values are not meant for
vendor specific
vendor-specific use of any sort sort, and they MUST NOT be used for
operational deployments.

14.10. Overlay Link Types

IANA SHALL create has created a "RELOAD Overlay Link Registry". Entries in this
registry are 8 bit integers 8-bit integers, as described in Section 6.5.1.1 of [RFC-
to-be].
[RFC6940]. For more information on the link types defined here, see
Section 6.6 of [RFC-to-be]. [RFC6940]. New entries SHALL be defined via RFC
5226 Standards Action.
Action [RFC5226]. This registry SHALL be has been initially populated with
the following values:

+--------------------+------+---------------+

The value EXP-LINK has been made available for the purposes of
experimentation. This value is not meant for vendor specific vendor-specific use of
any sort sort, and it MUST NOT be used for operational deployments.

14.11. Overlay Link Protocols

IANA SHALL create an "Overlay has created a "RELOAD Overlay Link Protocol Registry". Entries
in this registry are strings denoting protocols as described in
Section 11.1 of [RFC-to-be] this document and SHALL be defined via RFC 5226 Standards Action.
Action [RFC5226]. This registry SHALL be has been initially populated with
the following values:

+---------------+---------------+

+---------------+-----------+
| Link Protocol | Specification Reference |
+---------------+---------------+
+---------------+-----------+
| TLS | RFC-to-be RFC 6940 |
| EXP-PROTOCOL | RFC-to-be RFC 6940 |
+---------------+---------------+
+---------------+-----------+

The value EXP-PROTOCOL has been made available for the purposes of
experimentation. This value is not meant for vendor specific vendor-specific use of
any sort sort, and it MUST NOT be used for operational deployments.

14.12. Forwarding Options

IANA SHALL create has created a "Forwarding "RELOAD Forwarding Option Registry". Entries in
this registry are 8-bit integers denoting options options, as described in -
Section 6.3.2.3 of [RFC-to-be]. [RFC6940]. Values between 1 and 127 SHALL be
defined via RFC 5226 Standards Action. Action [RFC5226]. Entries in this registry
between 128 and 254 SHALL be defined via RFC 5226 Specification
Required. Required
[RFC5226]. This registry SHALL be has been initially populated with the
following values:

+-------------------------+------+---------------+

The value exp-forward has been made available for the purposes of
experimentation. This value is not meant for vendor specific vendor-specific use of
any sort sort, and it MUST NOT be used for operational deployments.

14.13. Probe Information Types

IANA SHALL create has created a "RELOAD Probe Information Type Registry". Entries
are 8-bit integers denoting types as described in Section 6.4.2.5.1
of [RFC-to-be] [RFC6940] and SHALL be defined via RFC 5226 Standards Action. Action [RFC5226].
This registry SHALL be has been initially populated with the following values:

+--------------------+------+---------------+

The value exp-probe has been made available for the purposes of
experimentation. This value is not meant for vendor specific vendor-specific use of
any sort sort, and it MUST NOT be used for operational deployments.

14.14. Message Extensions

IANA SHALL create has created a "RELOAD Extensions Registry". Entries in this
registry are 8-bit integers denoting extensions as described in
Section 6.3.3 of [RFC-to-be] [RFC6940] and SHALL be defined via RFC 5226 Specification Required.
Required [RFC5226]. This registry SHALL be has been initially populated with
the following values:

+-----------------------------+--------+---------------+

The value exp-ext has been made available for the purposes of
experimentation. This value is not meant for vendor specific vendor-specific use of
any sort sort, and it MUST NOT be used for operational deployments.

14.15. reload Reload URI Scheme

This section describes the scheme for a reload URI, which can be used
to refer to either:

o A peer, e.g., as used in a certificate (see Section 11.3 of [RFC-
to-be]).
[RFC6940]).

o A resource inside a peer.

The reload URI is defined using a subset of the URI schema specified
in Appendix A of RFC 3986 [RFC3986] and the associated URI Guidelines
[RFC4395] per the following ABNF syntax:

RELOAD-URI = "reload://" destination "@" overlay "/"
[specifier]
destination = 1 * HEXDIG 1*HEXDIG
overlay = reg-name
specifier = 1*HEXDIG

The definitions of these productions are as follows:

destination: a hex-encoded

destination
A hexadecimal-encoded Destination List object (i.e., multiple
concatenated Destination objects with no length prefix prior to
the object as a whole.)

overlay: the whole).

overlay
The name of the overlay.

specifier : a hex-encoded
A hexadecimal-encoded StoredDataSpecifier indicating the data
element.

If no specifier is present then present, this URI addresses the peer which can be
reached via the indicated destination list Destination List at the indicated overlay
name. If a specifier is present, then the URI addresses the data value.

14.15.1. URI Registration

[[ Note to RFC Editor - please remove this paragraph before
publication. ]] A review request was sent to uri-review@ietf.org on
Oct 7, 2010.

The following summarizes the information necessary to register the
reload URI.

URI Scheme Name: reload

Status: permanent

URI Scheme Syntax: see Section 14.15 of [RFC-to-be] RFC 6940

URI Scheme Semantics: The reload URI is intended to be used as a
reference to a RELOAD peer or resource.

Encoding Considerations: The reload URI is not intended to be human-
readable text, so it is encoded entirely in US-ASCII.

Applications/protocols that use Use this URI scheme: Scheme: The RELOAD protocol
described in RFC-to-be. RFC 6940.

Interoperability considerations: Considerations: See RFC-to-be. RFC 6940.

Security considerations: Considerations: See RFC-to-be RFC 6940

Contact: Cullen Jennings <fluffy@cisco.com>

Author/Change controller: Controller: IESG

References: RFC-to-be RFC 6940

14.16. Media Type Registration

[[ Note to RFC Editor - please remove this paragraph before
publication. ]] A review request was sent to ietf-types@iana.org on
May 27, 2011.

Type name: Name: application

Subtype name: Name: p2p-overlay+xml

Required parameters: Parameters: none

Optional parameters: Parameters: none

Encoding considerations: Considerations: Must be binary encoded.

Security considerations: Considerations: This media type is typically not used to
transport information that needs to be kept confidential, however confidential. However,
there are cases where it is integrity of the information is
important. For these cases cases, using a digital signature is
RECOMMENDED. One way of doing this is specified in RFC-to-be. RFC 6940. In the
case when the media includes a "shared-secret" shared-secret element, then the contents of
the file MUST be kept confidential or else anyone that who can see the
shared-secret and effect
shared secret can affect the RELOAD overlay network.

Interoperability considerations: Considerations: No known interoperability
consideration beyond those identified for application/xml in
[RFC3023].

Published specification: RFC-to-be Specification: RFC 6940

Applications that use Use this media type: Media Type: The type is used to configure
the peer to peer peer-to-peer overlay networks defined in RFC-to-be. RFC 6940.

Additional information: Information: The syntax for this media type is specified
in Section 11.1 of [RFC-to-be]. [RFC6940]. The contents MUST be valid XML that is
compliant with the RELAX NG grammar specified in RFC-to-be RFC 6940 and that
use the UTF-8[RFC3629] character encoding.

Magic number(s): Number(s): none

File extension(s): Extension(s): relo

Macintosh file type code(s): File Type Code(s): none

Person & email address Email Address to contact Contact for further information: Further Information: Cullen
Jennings <fluffy@cisco.com>

Intended usage: Usage: COMMON

Restrictions on usage: Usage: None

Author: Cullen Jennings <fluffy@cisco.com>

Change controller: Controller: IESG

14.17. XML Name Space Namespace Registration

This document registers two URIs for the config and config-chord XML
namespaces in the IETF XML registry defined in [RFC3688].

14.17.1. Config URL

URI: urn:ietf:params:xml:ns:p2p:config-base

Registrant Contact: The IESG.

XML: N/A, the requested URIs are XML namespaces

14.17.2. Config Chord URL

URI: urn:ietf:params:xml:ns:p2p:config-chord

Registrant Contact: The IESG.

XML: N/A, the requested URIs are XML namespaces

15. Acknowledgments

This specification is a merge of the "REsource LOcation And Discovery
(RELOAD)" draft document by David A. Bryan, Marcia Zangrilli Zangrilli, and Bruce B.
Lowekamp,
Lowekamp; the "Address Settlement by Peer to Peer" draft document by Cullen
Jennings, Jonathan Rosenberg, and Eric Rescorla, Rescorla; the "Security
Extensions for RELOAD" draft document by Bruce B. Lowekamp and James Deverick,
Deverick; the "A Chord-based DHT for Resource Lookup in P2PSIP" by
Marcia Zangrilli and David A. Bryan, Bryan; and the Peer-to-Peer Protocol
(P2PP)
draft document by Salman A. Baset, Henning Schulzrinne, and Marcin
Matuszewski. Thanks to the authors of RFC 5389 [RFC5389] for text included
from that. that document. Vidya Narayanan provided many comments and
improvements.

The ideas and text for the Chord specific Chord-specific extension data to the Leave
mechanisms was were provided by Jouni Maenpaa, Gonzalo Camarillo, and
Jani Hautakorpi.

Thanks to the many people who contributed contributed, including Ted Hardie,
Michael Chen, Dan York, Das Saumitra, Lyndsay Campbell, Brian Rosen,
David Bryan, Dave Craig, and Julian Cain. Extensive last call
comments were provided by: by Jouni Maenpaa, Roni Even, Gonzalo
Camarillo, Ari Keranen, John Buford, Michael Chen, Frederic-Philippe
Met, Mary Barnes, Roland Bless, David Bryan Bryan, and Polina Goltsman.
Special thanks to Marc Petit-Huguenin Petit-Huguenin, who provided an amazing amount
of detailed review.

Dean Willis and Marc Petit-Huguenin helped resolve and provided text
to fix many comments received during the IESG review.

16. References

16.1. Normative References

[OASIS.relax_ng]
Bray, T. and M. Murata, "RELAX NG Specification",
December 2001.

[RFC1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
E. Lear, "Address Allocation for Private Internets",
BCP 5, RFC 1918, February 1996.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC2388] Masinter, L., "Returning Values from Forms: multipart/
form-data", RFC 2388, August 1998.

[RFC2585] Housley, R. and P. Hoffman, "Internet X.509 Public Key
Infrastructure Operational Protocols: FTP and HTTP",
RFC 2585, May 1999.

[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.

[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.

[RFC3023] Murata, M., St. Laurent, S., and D. Kohn, "XML Media
Types", RFC 3023, January 2001.

[RFC3174] Eastlake, D. and P. Jones, "US Secure Hash Algorithm 1
(SHA1)", RFC 3174, September 2001.

[RFC3339] Klyne, G., Ed. and C. Newman, "Date and Time on the
Internet: Timestamps", RFC 3339, July 2002.

[RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography
Standards (PKCS) #1: RSA Cryptography Specifications
Version 2.1", RFC 3447, February 2003.

[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.

[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.

[RFC4279] Eronen, P. and H. Tschofenig, "Pre-Shared Key Ciphersuites
for Transport Layer Security (TLS)", RFC 4279,
December 2005.

[RFC4395] Hansen, T., Hardie, T., and L. Masinter, "Guidelines and
Registration Procedures for New URI Schemes", BCP 35,
RFC 4395, February 2006.

[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, October 2006.

[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.

[RFC5245] Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", RFC 5245,
April 2010.

[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.

[RFC5272] Schaad, J. and M. Myers, "Certificate Management over CMS
(CMC)", RFC 5272, June 2008.

[RFC5273] Schaad, J. and M. Myers, "Certificate Management over CMS
(CMC): Transport Protocols", RFC 5273, June 2008.

[RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
"Session Traversal Utilities for NAT (STUN)", RFC 5389,
October 2008.

[RFC5405] Eggert, L. and G. Fairhurst, "Unicast UDP Usage Guidelines
for Application Designers", BCP 145, RFC 5405,
November 2008.

[RFC5766] Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using
Relays around NAT (TURN): Relay Extensions to Session
Traversal Utilities for NAT (STUN)", RFC 5766, April 2010.

[RFC5952] Kawamura, S. and M. Kawashima, "A Recommendation for IPv6
Address Text Representation", RFC 5952, August 2010.

[RFC6091] Mavrogiannopoulos, N. and D. Gillmor, "Using OpenPGP Keys
for Transport Layer Security (TLS) Authentication",
RFC 6091, February 2011.

[RFC6234] Eastlake, D. and T. Hansen, "US Secure Hash Algorithms
(SHA and SHA-based HMAC and HKDF)", RFC 6234, May 2011.

[RFC6298] Paxson, V., Allman, M., Chu, J., and M. Sargent,
"Computing TCP's Retransmission Timer", RFC 6298,
June 2011.

[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, January 2012.

[W3C.REC-xmlschema-2-20041028]
Malhotra, A. and P. Biron, "XML Schema Part 2: Datatypes
Second Edition", World Wide Web Consortium
Recommendation REC-
xmlschema-2-20041028, REC-xmlschema-2-20041028, October 2004, <http://
www.w3.org/TR/2004/
REC-xmlschema-2-20041028>.
<http://www.w3.org/TR/2004/REC-xmlschema-2-20041028>.

[w3c-xml-namespaces]
Bray, T., Hollander, D., Layman, A., Tobin, R., and Henry
S. , "Namespaces in XML 1.0 (Third Edition)",
December 2008.

16.2. Informative References

[Chord] Stoica, I., Morris, R., Liben-
Nowell, Liben-Nowell, D., Karger, D.,
Kaashoek, M., Dabek, F., and H. Balakrishnan, "Chord: A
Scalable Peer-to-peer Lookup Protocol for Internet
Applications", IEEE/ACM Transactions on Networking Volume
11, Issue 1, 17-32, Feb 2003, 2001.

[DHT-RELOAD]
Maenpaa, J. and G. Camarillo, "A Self-tuning Distributed
Hash Table (DHT) for REsource LOcation And Discovery
(RELOAD)", Work in Progress, August 2013.

[Eclipse] Singh, A., Ngan, T., Druschel, T., and D. Wallach,
"Eclipse Attacks on Overlay Networks: Threats and
Defenses", INFOCOM 2006, April 2006.

[I-D.ietf-hip-reload-instance]

[HIP-RELOAD]
Keranen, A., Camarillo, G., and J. Maenpaa, "Host Identity
Protocol-Based Overlay Networking Environment (HIP BONE)
Instance Specification for REsource LOcation And Discovery
(RELOAD)", draft-ietf-hip-
reload-instance-06 (work Work in
progress), November 2012.

[I-D.ietf-p2psip-diagnostics] Progress, October 2013.

[P2P-DIAGNOSTICS]
Song, H., Jiang, X., Even, R., and D. Bryan, "P2PSIP "P2P Overlay
Diagnostics",
draft-ietf-p2psip-diagnostics-09
(work Work in progress), Progress, August 2012.

[I-D.ietf-p2psip-rpr] 2013.

[P2PSIP-RELAY]
Zong, N., Jiang, X., Even, R., and Y. Zhang, "An extension
to RELOAD to support Relay Peer Routing",
draft-ietf-p2psip-rpr-03 (work Work
in progress), Progress, October 2012.

[I-D.ietf-p2psip-self-tuning] Maenpaa, J., Camarillo, G., and
J. Hautakorpi, "A Self-tuning
Distributed Hash Table (DHT) for
REsource LOcation And Discovery
(RELOAD)",
draft-ietf-p2psip-self-tuning-06
(work in progress), July 2012.

[I-D.ietf-p2psip-service-discovery] 2013.

[REDIR-RELOAD]
Maenpaa, J. and G. Camarillo, "Service Discovery Usage for
REsource LOcation And Discovery (RELOAD)", draft-ietf-p2psip-
service-discovery-06 (work in
progress), October 2012.

[I-D.ietf-p2psip-sip] Jennings, C., Lowekamp, B.,
Rescorla, E., Baset, S.,
Schulzrinne, H., and T. Schmidt,
"A SIP Usage for RELOAD",
draft-ietf-p2psip-sip-08 (work Work
in progress), December 2012. Progress, August 2013.

[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.

[RFC1122] Braden, R., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122, October 1989.

[RFC2311] Dusse, S., Hoffman, P., Ramsdell, B., Lundblade, L., and
L. Repka, "S/MIME Version 2 Message Specification",
RFC 2311, March 1998.

[RFC3688] Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
January 2004.

[RFC4013] Zeilenga, K., "SASLprep: Stringprep Profile for User Names
and Passwords", RFC 4013, February 2005.

[RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness
Requirements for Security", BCP 106, RFC 4086, June 2005.

[RFC4145] Yon, D. and G. Camarillo, "TCP-
Based "TCP-Based Media Transport in
the Session Description Protocol (SDP)", RFC 4145,
September 2005.

[RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram
Congestion Control Protocol (DCCP)", RFC 4340, March 2006.

[RFC4787] Audet, F. and C. Jennings, "Network Address Translation
(NAT) Behavioral Requirements for Unicast UDP", BCP 127,
RFC 4787, January 2007.

[RFC4960] Stewart, R., "Stream Control Transmission Protocol",
RFC 4960, September 2007.

[RFC5054] Taylor, D., Wu, T., Mavrogiannopoulos, N., and T. Perrin,
"Using the Secure Remote Password (SRP) Protocol for TLS
Authentication", RFC 5054, November 2007.

[RFC5095] Abley, J., Savola, P., and G. Neville-Neil, "Deprecation
of Type 0 Routing Headers in IPv6", RFC 5095,
December 2007.

[RFC5201] Moskowitz, R., Nikander, P., Jokela, P., and T. Henderson,
"Host Identity Protocol", RFC 5201, April 2008.

[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, May 2008.

[RFC5694] Camarillo, G. and IAB, "Peer-to-
Peer "Peer-to-Peer (P2P) Architecture:
Definition, Taxonomies, Examples, and Applicability",
RFC 5694, November 2009.

[RFC5765] Schulzrinne, H., Marocco, E., and E. Ivov, "Security
Issues and Solutions in Peer-to-Peer Systems for Realtime
Communications", RFC 5765, February 2010.

[RFC5785] Nottingham, M. and E. Hammer-
Lahav, Hammer-Lahav, "Defining Well-Known
Uniform Resource Identifiers (URIs)", RFC 5785,
April 2010.

[RFC6079] Camarillo, G., Nikander, P., Hautakorpi, J., Keranen, A.,
and A. Johnston, "HIP BONE: Host Identity Protocol (HIP)
Based Overlay Networking Environment (BONE)", RFC 6079,
January 2011.

[RFC6544] Rosenberg, J., Keranen, A., Lowekamp, B., and A. Roach,
"TCP Candidates with Interactive Connectivity
Establishment (ICE)", RFC 6544, March 2012.

[SIP-RELOAD]
Jennings, C., Lowekamp, B., Rescorla, E., Baset, S.,
Schulzrinne, H., and T. Schmidt, "A SIP Usage for RELOAD",
Work in Progress, July 2013.

[Sybil] Douceur, J., "The Sybil Attack", IPTPS 02, March 2002.

[UnixTime]
Wikipedia, "Unix Time", 2013, <h
ttp:/wikipedia.org/wiki/
Unix_time>. <http://en.wikipedia.org/w/
index.php?title=Unix_time&oldid=551527446>.

[bryan-design-hotp2p08]
Bryan, D., Lowekamp, B., and M. Zangrilli, "The Design of
a Versatile, Secure P2PSIP Communications Architecture for
the Public Internet", Hot-
P2P'08, Hot-P2P'08, 2008.

[handling-churn-usenix04]
Rhea, S., Geels, D., Roscoe, T., and J. Kubiatowicz,
"Handling Churn in a DHT", In Proc. of the USENIX Annual
Technical Conference June 2004 USENIX 2004, 2004.

[lookups-churn-p2p06]
Wu, D., Tian, Y., and K. Ng, "Analytical Study on
Improving DHT Lookup Performance under Churn", IEEE
P2P'06, 2006.

[minimizing-churn-sigcomm06]
Godfrey, P., Shenker, S., and I. Stoica, "Minimizing Churn
in Distributed Systems", SIGCOMM 2006, 2006.

[non-transitive-dhts-worlds05]
Freedman, M., Lakshminarayanan, K., Rhea, S., and I.
Stoica, "Non-Transitive Connectivity and DHTs",
WORLDS'05, 2005.

[opendht-sigcomm05]
Rhea, S., Godfrey, B., Karp, B., Kubiatowicz, J.,
Ratnasamy, S., Shenker, S., Stoica, I., and H. Yu,
"OpenDHT: A Public DHT and its Uses", SIGCOMM'05, 2005.

[vulnerabilities-acsac04]
Srivatsa, M. and L. Liu, "Vulnerabilities and Security
Threats in Structured Peer-to-
Peer Peer-to-Peer Systems: A Quantitative
Analysis", ACSAC 2004, 2004.

[wikiChord]
Wikipedia, "Chord (peer-to-
peer)", (peer-to-peer)", 2013, <http://
en.wikipedia.org/wiki/
Chord_(peer-to-peer)>.
en.wikipedia.org/w/
index.php?title=Chord_%28peer-to-peer%29&oldid=549516287>.

[wikiKBR] Wikipedia, "Key-based routing", 2013, <http://en.wikipedia.org/
wiki/Chord_(peer-to-peer)>. <en.wikipedia.org/w/
index.php?title=Key-based_routing&oldid=543850833>.

[wikiSkiplist]
Wikipedia, "Skip list", 2013, <h
ttp://en.wikipedia.org/wiki/
Skip_list>. <http://en.wikipedia.org/w/
index.php?title=Skip_list&oldid=551304213>.

Appendix A. Routing Alternatives

Significant discussion has been focused on the selection of a routing
algorithm for P2PSIP. This section discusses the motivations for
selecting symmetric recursive routing for RELOAD and describes the
extensions that would be required to support additional routing
algorithms.

A.1. Iterative vs vs. Recursive

Iterative routing has a number of advantages. It is easier to debug,
consumes fewer resources on intermediate peers, and allows the
querying peer to identify and route around misbehaving peers
[non-transitive-dhts-worlds05]. However, in the presence of NATs,
iterative routing is intolerably expensive expensive, because a new connection
must be established for each hop (using ICE) [bryan-design-hotp2p08].

Iterative routing is supported through the RouteQuery mechanism and
is primarily intended for debugging. It also allows the querying
peer to evaluate the routing decisions made by the peers at each hop,
consider alternatives, and perhaps detect at what point the
forwarding path fails.

A.2. Symmetric vs vs. Forward response Response

An alternative to the symmetric recursive routing method used by
RELOAD is Forward-Only forward-only routing, where the response is routed to the
requester as if it were a new message initiated by the responder (in responder.
(In the previous example, Z sends the response to A as if it were
sending a request). request.) Forward-only routing requires no state in either
the message or intermediate peers.

The drawback of forward-only routing is that it does not work when
the overlay is unstable. For example, if A is in the process of
joining the overlay and is sending a Join request to Z, it is not yet
reachable via forward forward-only routing. Even if it is established in the
overlay, if network failures produce temporary instability, A may not
be reachable (and may be trying to stabilize its network connectivity
via Attach messages).

Furthermore, forward-only responses are less likely to reach the
querying peer than symmetric recursive ones are, because the forward
path is more likely to have a failed peer than is the request path
(which was just tested to route the request)
[non-transitive-dhts-worlds05].

An extension to RELOAD that supports forward-only routing but relies
on symmetric responses as a fallback would be possible, but due to
the complexities of determining when to use forward-only routing and
when to fallback to symmetric, symmetric routing, we have chosen not to include
it as an option at this point.

A.3. Direct Response

Another routing option is Direct Response direct response routing, in which the
response is returned directly to the querying node. In the previous
example, if A encodes its IP address in the request, then Z can
simply deliver the response directly to A. In the absence of NATs or
other connectivity issues, this is the optimal routing technique.

The challenge of implementing direct response routing is the presence
of NATs. There are a number of complexities that must be addressed.
In this discussion, we will continue our assumption that A issued the
request and Z is generating the response.

o The IP address listed by A may be unreachable, either due to NAT
or firewall rules. Therefore, a direct response technique must
fallback to symmetric response [non-transitive-dhts-worlds05].
The hop-by-hop ACKs used by RELOAD allow Z to determine when A has
received the message (and the TLS negotiation will provide earlier
confirmation that A is reachable), but this fallback requires a
timeout that will increase the response latency whenever A is not
reachable from Z.

o Whenever A is behind a NAT it it, will have multiple candidate IP
addresses, each of which must be advertised to ensure
connectivity; therefore
connectivity. Therefore, Z will need to attempt multiple
connections to deliver the response.

o One (or all) of A's candidate addresses may route from Z to a
different device on the Internet. In the worst case case, these nodes
may actually be running RELOAD on the same port. Therefore, it is
absolutely necessary to establish a secure connection to
authenticate A before delivering the response. This step
diminishes the efficiency of direct response routing, because
multiple
roundtrips round-trips are required before the message can be
delivered.

o If A is behind a NAT and does not have a connection already
established with Z, there are only two ways the direct response
will work. The first is that A and Z must both be behind the same
NAT, in which case the NAT is not involved. In the more common
case, when Z is outside A's NAT, the response will only be received
only if A's NAT implements endpoint-independent filtering. As the
choice of filtering mode conflates application transparency with
security
[RFC4787], [RFC4787] and no clear recommendation is available, the
prevalence of this feature in future devices remains unclear.

An extension to RELOAD that supports direct response routing but
relies on symmetric responses as a fallback would be possible, but
due to the complexities of determining when to use direct response
routing and when to fallback to symmetric, symmetric routing, and the reduced
performance for responses to peers behind restrictive NATs, we have
chosen not to include it as an option at this point.

A.4. Relay Peers

[I-D.ietf-p2psip-rpr]

[P2PSIP-RELAY] has proposed implementing a form of direct response by
having A identify a peer, Q, that will be directly reachable by any
other peer. A uses Attach to establish a connection with Q and
advertises Q's IP address in the request sent to Z. Z sends the
response to Q, which relays it to A. This then reduces the latency to
two hops, plus and Z is negotiating a secure connection to Q.

This technique relies on the relative population of nodes such as A
that require relay peers and peers such as Q that are capable of
serving as a relay peer. It also requires nodes to be able to
identify which category they are in. This identification problem has
turned out to be hard to solve and is still an open area of
exploration.

An extension to RELOAD that supports relay peers is possible, but due
to the complexities of implementing such an alternative, we have not
added such a feature to RELOAD at this point.

A concept similar to relay peers, essentially choosing a relay peer
at random, has previously been suggested to solve problems of
pairwise pair-
wise non-transitivity [non-transitive-dhts-worlds05], but
deterministic filtering provided by NATs makes random relay peers no
more likely to work than the responding peer.

A.5. Symmetric Route Stability

A common concern about symmetric recursive routing has been that one
or more peers along the request path may fail before the response is
received. The significance of this problem essentially depends on
the response latency of the overlay. An overlay that produces slow
responses will be vulnerable to churn, whereas responses that are
delivered very quickly are vulnerable only to failures that occur
over that small interval.

The other aspect of this issue is whether the request itself can be
successfully delivered. Assuming typical connection maintenance
intervals, the time period between the last maintenance and the
request being sent will be orders of magnitude greater than the delay
between the request being forwarded and the response being received.
Therefore, if the path was stable enough to be available to route the
request, it is almost certainly going to remain available to route
the response.

An overlay that is unstable enough to suffer this type of failure
frequently is unlikely to be able to support reliable functionality
regardless of the routing mechanism. However, regardless of the
stability of the return path, studies show that in the event of high
churn, iterative routing is a better solution to ensure request
completion [lookups-churn-p2p06] [non-transitive-dhts-worlds05]

Finally, because RELOAD retries the end-to-end request, that retry
will address the issues of churn that remain.

Appendix B. Why Clients?

There are a wide variety of reasons a node may act as a client rather
than as a peer. This section outlines some of those scenarios and
how the client's behavior changes based on its capabilities.

B.1. Why Not Only Peers?

For a number of reasons, a particular node may be forced to act as a
client even though it is willing to act as a peer. These include:

o The node does not have appropriate network connectivity, typically
because it has a low-bandwidth network connection.

o The node may not have sufficient resources, such as computing
power, storage space, or battery power.

o The overlay algorithm may dictate specific requirements for peer
selection. These may include participating in the overlay to
determine trustworthiness; trustworthiness, controlling the number of peers in the
overlay to reduce overly-long overly long routing paths; or paths, and ensuring minimum
application uptime before a node can join as a peer.

The ultimate criteria for a node to become a peer are determined by
the overlay algorithm and specific deployment. A node acting as a
client that has a full implementation of RELOAD and the appropriate
overlay algorithm is capable of locating its responsible peer in the
overlay and using Attach to establish a direct connection to that
peer. In that way, it may elect to be reachable under either of the
routing approaches listed above. Particularly for overlay algorithms
that elect nodes to serve as peers based on trustworthiness or
population, the overlay algorithm may require such a client to locate
itself at a particular place in the overlay.

B.2. Clients as Application-Level Agents

SIP defines an extensive protocol for registration and security
between a client and its registrar/proxy server(s). Any SIP device
can act as a client of a RELOAD-based P2PSIP overlay if it contacts a
peer that implements the server-side functionality required by the
SIP protocol. In this case, the peer would be acting as if it were
the user's peer, peer and would need the appropriate credentials for that
user.

Application-level support for clients is defined by a usage. A usage
offering support for application-level clients should specify how the
security of the system is maintained when the data is moved between
the application and RELOAD layers.

Authors' Addresses

Cullen Jennings
Cisco
400 3rd Avenue SW, Suite 350
Calgary
Canada

EMail: fluffy@cisco.com

Bruce B. Lowekamp (editor)
Skype
Palo Alto, CA
USA

EMail: bbl@lowekamp.net

Eric Rescorla
RTFM, Inc.
2064 Edgewood Drive
Palo Alto, CA 94303
USA

Phone: +1 650 678 2350
EMail: ekr@rtfm.com

Salman A. Baset
Columbia University
1214 Amsterdam Avenue
New York, NY
USA

EMail: salman@cs.columbia.edu

Henning Schulzrinne
Columbia University
1214 Amsterdam Avenue
New York, NY
USA

EMail: hgs@cs.columbia.edu