wdiff draft-ietf-ppsp-peer-protocol-12.original draft-ietf-ppsp-peer-protocol-12.txt

PPSP
Internet Engineering Task Force (IETF) A. Bakker
Internet-Draft
Request for Comments: XXXX Vrije Universiteit Amsterdam
Intended status:
Category: Standards Track R. Petrocco
Expires: June 1, 2015
ISSN: 2070-1721 V. Grishchenko
Technische Universiteit Delft
November 28, 2014
May 2015

Peer-to-Peer Streaming Peer Protocol (PPSPP)
draft-ietf-ppsp-peer-protocol-12

Abstract

The Peer-to-Peer Streaming Peer Protocol (PPSPP) is a protocol for
disseminating the same content to a group of interested parties in a
streaming fashion. PPSPP supports streaming of both pre-recorded
(on-demand) and live audio/video content. It is based on the peer-
to-peer paradigm, where clients consuming the content are put on
equal footing with the servers initially providing the content, to
create a system where everyone can potentially provide upload
bandwidth. It has been designed to provide short time-till-playback
for the end user, and to prevent disruption of the streams by
malicious peers. PPSPP has also been designed to be flexible and
extensible. It can use different mechanisms to optimize peer
uploading, prevent freeriding, and work with different peer discovery
schemes (centralized trackers or Distributed Hash Tables). It
supports multiple methods for content integrity protection and chunk
addressing. Designed as a generic protocol that can run on top of
various transport protocols, it currently runs on top of UDP using
LEDBAT for congestion control.

Status of This Memo

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provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on June 1, 2015.
http://www.rfc-editor.org/info/rfcXXXX.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1. Purpose . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2. Requirements Language . . . . . . . . . . . . . . . . . . 6
1.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6
2. Overall Operation . . . . . . . . . . . . . . . . . . . . . . 9
2.1. Example: Joining a Swarm . . . . . . . . . . . . . . . . 9
2.2. Example: Exchanging Chunks . . . . . . . . . . . . . . . 10
2.3. Example: Leaving a Swarm . . . . . . . . . . . . . . . . 10
3. Messages . . . . . . . . . . . . . . . . . . . . . . . . . . 11
3.1. HANDSHAKE . . . . . . . . . . . . . . . . . . . . . . . . 11
3.1.1. Handshake Procedure . . . . . . . . . . . . . . . . . 12
3.2. HAVE . . . . . . . . . . . . . . . . . . . . . . . . . . 14
3.3. DATA . . . . . . . . . . . . . . . . . . . . . . . . . . 15
3.4. ACK . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
3.5. INTEGRITY . . . . . . . . . . . . . . . . . . . . . . . . 15
3.6. SIGNED_INTEGRITY . . . . . . . . . . . . . . . . . . . . 16
3.7. REQUEST . . . . . . . . . . . . . . . . . . . . . . . . . 16
3.8. CANCEL . . . . . . . . . . . . . . . . . . . . . . . . . 16
3.9. CHOKE and UNCHOKE . . . . . . . . . . . . . . . . . . . . 17
3.10. Peer Address Exchange . . . . . . . . . . . . . . . . . . 17
3.10.1. PEX_REQ and PEX_RES Messages . . . . . . . . . . . . 17
3.11. Channels . . . . . . . . . . . . . . . . . . . . . . . . 18
3.12. Keep Alive Signalling . . . . . . . . . . . . . . . . . . 19
4. Chunk Addressing Schemes . . . . . . . . . . . . . . . . . . 20
4.1. Start-End Ranges . . . . . . . . . . . . . . . . . . . . 20
4.1.1. Chunk Ranges . . . . . . . . . . . . . . . . . . . . 20
4.1.2. Byte Ranges . . . . . . . . . . . . . . . . . . . . . 21
4.2. Bin Numbers . . . . . . . . . . . . . . . . . . . . . . . 21
4.3. In Messages . . . . . . . . . . . . . . . . . . . . . . . 23
4.3.1. In HAVE Messages . . . . . . . . . . . . . . . . . . 23
4.3.2. In ACK Messages . . . . . . . . . . . . . . . . . . . 23
5. Content Integrity Protection . . . . . . . . . . . . . . . . 23
5.1. Merkle Hash Tree Scheme . . . . . . . . . . . . . . . . . 24
5.2. Content Integrity Verification . . . . . . . . . . . . . 25
5.3. The Atomic Datagram Principle . . . . . . . . . . . . . . 26
5.4. INTEGRITY Messages . . . . . . . . . . . . . . . . . . . 27
5.5. Discussion and Overhead . . . . . . . . . . . . . . . . . 27
5.6. Automatic Detection of Content Size . . . . . . . . . . . 28
5.6.1. Peak Hashes . . . . . . . . . . . . . . . . . . . . . 28
5.6.2. Procedure . . . . . . . . . . . . . . . . . . . . . . 30
6. Live Streaming . . . . . . . . . . . . . . . . . . . . . . . 31
6.1. Content Authentication . . . . . . . . . . . . . . . . . 31
6.1.1. Sign All . . . . . . . . . . . . . . . . . . . . . . 32
6.1.2. Unified Merkle Tree . . . . . . . . . . . . . . . . . 32
6.1.2.1. Signed Munro Hashes . . . . . . . . . . . . . . . 33
6.1.2.2. Munro Signature Calculation . . . . . . . . . . . 35
6.1.2.3. Procedure . . . . . . . . . . . . . . . . . . . . 36
6.1.2.4. Secure Tune In . . . . . . . . . . . . . . . . . 36
6.2. Forgetting Chunks . . . . . . . . . . . . . . . . . . . . 37
7. Protocol Options . . . . . . . . . . . . . . . . . . . . . . 37
7.1. End Option . . . . . . . . . . . . . . . . . . . . . . . 38
7.2. Version . . . . . . . . . . . . . . . . . . . . . . . . . 38
7.3. Minimum Version . . . . . . . . . . . . . . . . . . . . . 39
7.4. Swarm Identifier . . . . . . . . . . . . . . . . . . . . 39
7.5. Content Integrity Protection Method . . . . . . . . . . . 40
7.6. Merkle Tree Hash Function . . . . . . . . . . . . . . . . 40
7.7. Live Signature Algorithm . . . . . . . . . . . . . . . . 41
7.8. Chunk Addressing Method . . . . . . . . . . . . . . . . . 41
7.9. Live Discard Window . . . . . . . . . . . . . . . . . . . 42
7.10. Supported Messages . . . . . . . . . . . . . . . . . . . 43
7.11. Chunk Size . . . . . . . . . . . . . . . . . . . . . . . 43
8. UDP Encapsulation . . . . . . . . . . . . . . . . . . . . . . 44
8.1. Chunk Size . . . . . . . . . . . . . . . . . . . . . . . 44
8.2. Datagrams and Messages . . . . . . . . . . . . . . . . . 45
8.3. Channels . . . . . . . . . . . . . . . . . . . . . . . . 46
8.4. HANDSHAKE . . . . . . . . . . . . . . . . . . . . . . . . 46
8.5. HAVE . . . . . . . . . . . . . . . . . . . . . . . . . . 47
8.6. DATA . . . . . . . . . . . . . . . . . . . . . . . . . . 47
8.7. ACK . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
8.8. INTEGRITY . . . . . . . . . . . . . . . . . . . . . . . . 49
8.9. SIGNED_INTEGRITY . . . . . . . . . . . . . . . . . . . . 50
8.10. REQUEST . . . . . . . . . . . . . . . . . . . . . . . . . 51
8.11. CANCEL . . . . . . . . . . . . . . . . . . . . . . . . . 51
8.12. CHOKE and UNCHOKE . . . . . . . . . . . . . . . . . . . . 52
8.13. PEX_REQ, PEX_RESv4, PEX_RESv6 and PEX_REScert . . . . . . 52
8.14. KEEPALIVE . . . . . . . . . . . . . . . . . . . . . . . . 54
8.15. Flow and Congestion Control . . . . . . . . . . . . . . . 54
8.16. Example of Operation . . . . . . . . . . . . . . . . . . 56
9. Extensibility . . . . . . . . . . . . . . . . . . . . . . . . 60
9.1. Chunk Picking Algorithms . . . . . . . . . . . . . . . . 60
9.2. Reciprocity Algorithms . . . . . . . . . . . . . . . . . 60
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 60
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 61
11.1. PPSP Peer Protocol Message Type Registry . . . . . . . . 61
11.2. PPSP Peer Protocol Option Registry . . . . . . . . . . . 61
11.3. PPSP Peer Protocol Version Number Registry . . . . . . . 61
11.4. PPSP Peer Protocol Content Integrity Protection Method
Registry . . . . . . . . . . . . . . . . . . . . . . . . 61
11.5. PPSP Peer Protocol Merkle Hash Tree Function Registry . 61
11.6. PPSP Peer Protocol Chunk Addressing Method Registry . . 62
12. Manageability Considerations . . . . . . . . . . . . . . . . 62
12.1. Operations . . . . . . . . . . . . . . . . . . . . . . . 62
12.1.1. Installation and Initial Setup . . . . . . . . . . . 62
12.1.2. Requirements on Other Protocols and Functional
Components . . . . . . . . . . . . . . . . . . . . . 63
12.1.3. Migration Path . . . . . . . . . . . . . . . . . . . 63
12.1.4. Impact on Network Operation . . . . . . . . . . . . 63
12.1.5. Verifying Correct Operation . . . . . . . . . . . . 63
12.1.6. Configuration . . . . . . . . . . . . . . . . . . . 64
12.2. Management Considerations . . . . . . . . . . . . . . . 64
12.2.1. Management Interoperability and Information . . . . 65
12.2.2. Fault Management . . . . . . . . . . . . . . . . . . 65
12.2.3. Configuration Management . . . . . . . . . . . . . . 65
12.2.4. Accounting Management . . . . . . . . . . . . . . . 66
12.2.5. Performance Management . . . . . . . . . . . . . . . 66
12.2.6. Security Management . . . . . . . . . . . . . . . . 66
13. Security Considerations . . . . . . . . . . . . . . . . . . . 66
13.1. Security of the Handshake Procedure . . . . . . . . . . 66
13.1.1. Protection Against Attack 1 . . . . . . . . . . . . 67
13.1.2. Protection Against Attack 2 . . . . . . . . . . . . 68
13.1.3. Protection Against Attack 3 . . . . . . . . . . . . 68
13.2. Secure Peer Address Exchange . . . . . . . . . . . . . . 69
13.2.1. Protection against the Amplification Attack . . . . 69
13.2.2. Example: Tracker as Certification Authority . . . . 70
13.2.3. Protection Against Eclipse Attacks . . . . . . . . . 71
13.3. Support for Closed Swarms ([RFC6972] PPSP.SEC.REQ-1) . . 71
13.4. Confidentiality of Streamed Content ([RFC6972]
PPSP.SEC.REQ-1) . . . . . . . . . . . . . . . . . . . . 71
13.5. Strength of the Hash Function for Merkle Hash Trees . . 72
13.6. Limit Potential Damage and Resource Exhaustion by Bad or
Broken Peers ([RFC6972] PPSP.SEC.REQ-2) . . . . . . . . 72
13.6.1. HANDSHAKE . . . . . . . . . . . . . . . . . . . . . 72
13.6.2. HAVE . . . . . . . . . . . . . . . . . . . . . . . . 73
13.6.3. DATA . . . . . . . . . . . . . . . . . . . . . . . . 73
13.6.4. ACK . . . . . . . . . . . . . . . . . . . . . . . . 73
13.6.5. INTEGRITY and SIGNED_INTEGRITY . . . . . . . . . . . 74
13.6.6. REQUEST . . . . . . . . . . . . . . . . . . . . . . 74
13.6.7. CANCEL . . . . . . . . . . . . . . . . . . . . . . . 74
13.6.8. CHOKE . . . . . . . . . . . . . . . . . . . . . . . 74
13.6.9. UNCHOKE . . . . . . . . . . . . . . . . . . . . . . 75
13.6.10. PEX_RES . . . . . . . . . . . . . . . . . . . . . . 75
13.6.11. Unsolicited Messages in General . . . . . . . . . . 75
13.7. Exclude Bad or Broken Peers ([RFC6972] PPSP.SEC.REQ-2) . 75
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 75
14.1. Normative References . . . . . . . . . . . . . . . . . . 75
14.2. Informative References . . . . . . . . . . . . . . . . . 77
Appendix A. Revision History . . . . . . . . . . . . . . . . . . 81
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 111 82

1. Introduction

1.1. Purpose

This document describes the Peer-to-Peer Streaming Peer Protocol
(PPSPP), designed for disseminating the same content to a group of
interested parties in a streaming fashion. PPSPP supports streaming
of both pre-recorded (on-demand) and live audio/video content. It is
based on the peer-to-peer paradigm where clients consuming the
content are put on equal footing with the servers initially providing
the content, to create a system where everyone can potentially
provide upload bandwidth.

PPSPP has been designed to provide short time-till-playback for the
end user, and to prevent disruption of the streams by malicious
peers. Central in this design is a simple method of identifying
content based on self-certification. In particular, content in PPSPP
is identified by a single cryptographic hash that is the root hash in
a Merkle hash tree calculated recursively from the content
[MERKLE][ABMRKL]. This self-certifying hash tree allows every peer
to directly detect when a malicious peer tries to distribute fake
content. The tree can be used for both static and live content.
Moreover, it ensures only a small amount of information is needed to
start a download and to verify incoming chunks of content, thus
ensuring short start-up times.

PPSPP has also been designed to be extensible for different
transports and use cases. Hence, PPSPP is a generic protocol which
can run directly on top of UDP, TCP, or other protocols. As such,
PPSPP defines a common set of messages that make up the protocol,
which can have different representations on the wire depending on the
lower-level protocol used. When the lower-level transport allows,
PPSPP can also use different congestion control algorithms.

At present, PPSPP is set to run on top of UDP using LEDBAT for
congestion control [RFC6817]. Using LEDBAT enables PPSPP to serve
the content after playback (seeding) without disrupting the user who
may have moved to different tasks that use its network connection.

PPSPP is also flexible and extensible in the mechanisms it uses to
promote client contribution and prevent freeriding, that is, how to
deal with peers that only download content but never upload to
others. It also allows different schemes for chunk addressing and
content integrity protection, if the defaults are not fit for a
particular use case. In addition, it can work with different peer
discovery schemes, such as centralized trackers or fast Distributed
Hash Tables [JIM11]. Finally, in this default setup, PPSPP maintains
only a small amount of state per peer. A reference implementation of
PPSPP over UDP is available [SWIFTIMPL].

The protocol defined in this document assumes that a peer has already
discovered a list of (initial) peers using, for example, a
centralized tracker [I-D.ietf-ppsp-base-tracker-protocol]. Once a
peer has this list of peers, PPSPP allows the peer to connect to
other peers, request chunks of content, and discover other peers
disseminating the same content.

The design of PPSPP is based on our research into making BitTorrent
[BITTORRENT] suitable for streaming content [P2PWIKI]. Most PPSPP
messages have corresponding BitTorrent messages and vice versa.
However, PPSPP is specifically targeted towards streaming audio/video
content and optimizes time-till-playback. It was also designed to be
more flexible and extensible.

1.2. 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].

1.3. Terminology

message
The basic unit of PPSPP communication. A message will have
different representations on the wire depending on the transport
protocol used. Messages are typically multiplexed into a
datagram for transmission.

datagram
A sequence of messages that is offered as a unit to the
underlying transport protocol (UDP, etc.). The datagram is
PPSPP's Protocol Data Unit (PDU).

content
Either a live transmission or a pre-recorded multimedia file.

chunk
The basic unit in which the content is divided. E.g. a block of
N kilobyte. A chunk may be of variable size.

chunk ID
Unique identifier for a chunk of content (e.g. an integer). Its
type depends on the chunk addressing scheme used.

chunk specification
An expression that denotes one or more chunk IDs.

chunk addressing scheme
Scheme for identifying chunks and expressing the chunk
availability map of a peer in a compact fashion.

chunk availability map
The set of chunks a peer has successfully downloaded and checked
the integrity of.

bin
A number denoting a specific binary interval of the content
(i.e., one or more consecutive chunks) in the bin numbers chunk
addressing scheme (see Section 4).

content integrity protection scheme
Scheme for protecting the integrity of the content while it is
being distributed via the peer-to-peer network. That is, methods
for receiving peers to detect whether a requested chunk has been
modified, either maliciously by the sending peer or accidentally
in transit.

hash
The result of applying a cryptographic hash function, more
specifically a modification detection code (MDC) [HAC01], such as
SHA-256 [FIPS180-4], to a piece of data.

Merkle hash tree
A tree of hashes whose base is formed by the hashes of the chunks
of content, and its higher nodes are calculated by recursively
computing the hash of the concatenation of the two child hashes
(see Section 5.1).

root hash
The root in a Merkle hash tree calculated recursively from the
content (see Section 5.1).

munro hash
The hash of a subtree that is the unit of signing in the Unified
Merkle Tree content authentication scheme for live streaming (see
Section 6.1.2.1).

swarm
A group of peers participating in the distribution of the same
content.

swarm ID
Unique identifier for a swarm of peers, in PPSPP a sequence of
bytes. For video-on-demand with content integrity protection
enabled, the identifier is the so-called root hash of a Merkle
hash tree over the content. For live streaming, the swarm ID is
a public key.

tracker
An entity that records the addresses of peers participating in a
swarm, usually for a set of swarms, and makes this membership
information available to other peers on request.

choking
When a peer A is choking peer B it means that A is currently not
willing to accept requests for content from B.

seeding
Peer A is said to be seeding when A has downloaded a static
content file completely and is now offering it for others to
download.

leeching
Peer A is said to be leeching when A has not completely
downloaded a static content file yet or is not offering to upload
it to others.

channel
A logical connection between two peers. The channel concept
allows peers to use the same transport address for communicating
with different peers.

channel ID
Unique, randomly chosen identifier for a channel, local to each
peer. So the two peers logically connected by a channel each
have a different channel ID for the channel.

heavy payload
A datagram has a heavy payload when it contains DATA messages,
SIGNED_INTEGRITY messages, or a large number of smaller messages.

In this document the prefixes kilo, mega, etc. denote base 1024.

2. Overall Operation

The basic unit of communication in PPSPP is the message. Multiple
messages are multiplexed into a single datagram for transmission. A
datagram (and hence the messages it contains) will have different
representations on the wire depending on the transport protocol used
(see Section 8).

The overall operation of PPSPP is illustrated in the following
examples. The examples assume that the content distributed is
static, UDP is used for transport, the Merkle Hash Tree scheme is
used for content integrity protection, and that a specific policy is
used for selecting which chunks to download.

2.1. Example: Joining a Swarm

Consider a user who wants to watch a video. To play the video, the
user clicks on the play button of a HTML5 <video> element shown in
his PPSPP-enabled browser. Imagine this element has a PPSPP URL (to
be defined elsewhere) identifying the video as its source. The
browser passes this URL to its PPSP protocol handler. Let's call
this protocol handler peer A. Peer A parses the URL to retrieve the
transport address of a PPSP tracker and swarm metadata of the
content. The tracker address may be optional in the presence of a
decentralized tracking mechanism. The mechanisms for tracking peers
are outside of the scope of this document.

Peer A now registers with the tracker following the PPSP tracker
protocol [I-D.ietf-ppsp-base-tracker-protocol] and receives the IP
address and port of peers already in the swarm, say B, C, and D. At
this point the PPSPP peer protocol starts operating. Peer A now
sends a datagram containing a PPSPP HANDSHAKE message to B, C, and D.
This message conveys protocol options. In particular, peer A
includes the ID of the swarm (part of the swarm metadata) as a
protocol option, because the destination peers can listen for
multiple swarms on the same transport address.

Peer B and C respond with datagrams containing a PPSPP HANDSHAKE
message and one or more HAVE messages. A HAVE message conveys (part
of) the chunk availability of a peer and thus contains a chunk
specification that denotes what chunks of the content peer B,
respectively C have. Peer D sends a datagram with a HANDSHAKE and
HAVE messages, but also with a CHOKE message. The latter indicates
that D is not willing to upload chunks to A at present.

2.2. Example: Exchanging Chunks

In response to B and C, A sends new datagrams to B and C containing
REQUEST messages. A REQUEST message indicates the chunks that a peer
wants to download, and thus contains a chunk specification. The
REQUEST messages to B and C refer to disjunct disjoint sets of chunks. B and
C respond with datagrams containing HAVE, DATA and, in this example,
INTEGRITY messages. In the Merkle hash tree content protection
scheme (see Section 5.1), the INTEGRITY messages contain all
cryptographic hashes that peer A needs to verify the integrity of the
content chunk sent in the DATA message. Using these hashes peer A
verifies that the chunks received from B and C are correct against
the trusted swarm ID. Peer A also updates the chunk availability of
B and C using the information in the received HAVE messages. In
addition, it passes the chunks of video to the user's browser for
rendering.

After processing, A sends a datagram containing HAVE messages for the
chunks it just received to all its peers. In the datagram to B and C
it includes an ACK message acknowledging the receipt of the chunks,
and adds REQUEST messages for new chunks. ACK messages are not used
when a reliable transport protocol is used. When e.g. C finds that
A obtained a chunk (from B) that C did not yet have, C's next
datagram includes a REQUEST for that chunk.

Peer D also sends HAVE messages to A when it downloads chunks from
other peers. When D is willing to accept REQUESTs from A, D sends a
datagram with an UNCHOKE message to inform A. If B or C decide to
choke A they send a CHOKE message and A should then re-request from
other peers. B and C may continue to send HAVE, REQUEST, or periodic
keep-alive messages such that A keeps sending them HAVE messages.

Once peer A has received all content (video-on-demand use case) it
stops sending messages to all other peers that have all content
(a.k.a. seeders). Peer A can also contact the tracker or another
source again to obtain more peer addresses.

2.3. Example: Leaving a Swarm

To leave a swarm in a graceful way, peer A sends a specific HANDSHAKE
message to all its peers (see Section 8.4) and deregisters from the
tracker following the (PPSP) tracker protocol. Peers receiving the
datagram should remove A from their current peer list. If A crashes
ungracefully, peers should remove A from their peer list when they
detect it no longer sends messages (see Section 3.12).

3. Messages

No error codes or responses are used in the protocol; absence of any
response indicates an error. Invalid messages are discarded, and
further communication with the peer SHOULD be stopped. The rationale
is that it is sufficient to classify peers as either good or bad and
only use the good ones. A good peer is a peer that responds with
chunks; a peer that does not respond, or does not respond in time is
classified as bad. The idea is that in PPSPP the content is
available from multiple sources (unlike HTTP), so a peer should not
invest too much effort in trying to obtain it from a particular
source. This classification in good or bad allows a peer to deal
with slow, crashed and (silent) malicious peers.

Multiple messages MUST be multiplexed into a single datagram for
transmission. Messages in a single datagram MUST be processed in the
strict order in which they appear in the datagram. If an invalid
message is found in a datagram, the remaining messages MUST be
discarded.

For the sake of simplicity, one swarm of peers deals with one content
file or stream only. There is a single division of the content into
chunks that all peers in the swarm adhere to, determined by the
content publisher. Distribution of a collection of files can be done
either by using multiple swarms or by using an external storage
mapping from the linear byte space of a single swarm to different
files, transparent to the protocol. In other words, the audio/video
container format used is outside the scope of this document.

3.1. HANDSHAKE

For a peer P to establish communication with a peer Q in swarm S the
peers must first exchange HANDSHAKE messages by means of a handshake
procedure. The initiating peer P needs to know the metadata of swarm
S, which consists of:

(a) the swarm ID of the content (see Section 5.1 and Section 6),

(b) the chunk size used,

(d) the content integrity protection method used, and

(e) the Merkle hash tree function used (if applicable).

(f) If automatic content size detection (see Section 5.6) is not
used, the content length is also part of the metadata (for
static content.)

This document assumes the swarm metadata is obtained from a trusted
source. In addition, peer P needs to know a transport address for
peer Q, obtained from a peer discovery/tracking protocol.

The payload of the HANDSHAKE message contains a sequence of protocol
options. The protocol options encode the swarm metadata just
described to enable an end-to-end check whether the peers are in the
right swarm, and a number of per-peer configuration parameters. The
complete set of protocol options are specified in Section 7. The
HANDSHAKE message also contains a channel ID, for multiplexing
communication and security, see Section 3.11 and Section 13.1. A
HANDSHAKE message MUST always be the first message in a datagram.

3.1.1. Handshake Procedure

The handshake procedure for a peer P to start communication with
another peer Q in swarm S is now as follows.

1. The first datagram the initiating peer P sends to peer Q MUST
start with a HANDSHAKE message. This HANDSHAKE message MUST
contain:

* A channel ID, chanP, randomly chosen as specified in
Section 13.1.

* The metadata of swarm S, encoded as protocol options, as
specified in Section 7. In particular, the initiating peer P
MUST include the swarm ID.

* The capabilities of peer P, in particular, its supported
protocol versions, "Live Discard Window" (in case of a live
swarm) and "Supported Messages", encoded as protocol options.

This first datagram MUST be prefixed with the (destination)
channel ID 0, see Section 3.11. Hence, the datagram contains two
channel IDs: the destination channel ID prefixed to the datagram,
and the channel ID chanP included in the HANDSHAKE message inside
the datagram. This datagram MAY also contain some minor
additional payload, e.g. HAVE messages to indicate P's current
progress, but MUST NOT include any heavy payload (defined in
Section 1.3), such as a DATA message Allowing minor payload
minimizes the number of initialization round-trips, thus
improving time-till-playback. Forbidding heavy payload prevents
an amplification attack (see Section 13.1.)
2. The receiving peer Q checks the HANDSHAKE message from peer P.
If any check by Q fails, or if P and Q are not in the same swarm,
Q MUST NOT send a HANDSHAKE (or any other) message back, as the
message from P may have been spoofed (see Section 13.1). Only
Otherwise, if P and Q are in the same swarm, and Q is interested in communicating with P, Q MUST
send a datagram to P that starts with a HANDSHAKE message. This
reply HANDSHAKE MUST contain:

* A channel ID, chanQ, randomly chosen as specified in
Section 13.1.

* The metadata of swarm S, encoded as protocol options, as
specified in Section 7. In particular, the responding peer Q
MAY include the swarm ID.

* The capabilities of peer Q, in particular, its supported
protocol versions, its "Live Discard Window" (in case of a
live swarm) and "Supported Messages", encoded as protocol
options.

This reply datagram MUST be prefixed with the channel ID chanP
sent by P in the first HANDSHAKE message (see Section 3.11).
This reply datagram MAY also contain some minor additional
payload, e.g. HAVE messages to indicate Q's current progress, or
REQUEST messages (see Section 3.7), but MUST NOT include any
heavy payload.

3. The initiating peer P checks the reply datagram from Q. If the
reply datagram is not prefixed with (destination) channel ID
chanP, peer P MUST discard the datagram. P SHOULD continue to
process datagrams from Q that do meet this requirement. This
check prevents interference by spoofing, see Section 13.1. If
P's channel ID is echoed correctly, the initiator P knows that
the addressed peer Q really responds.

4. Next, peer P checks the HANDSHAKE message in the datagram from Q.
If any check by P fails, or P is no longer interested in
communicating with Q, P MAY send a HANDSHAKE message to inform Q
it will cease communication. This closing HANDSHAKE message MUST
contain an all 0-zeros channel ID and a list of protocol options.
The list MUST be either empty or contain the maximum version
number peer P supports, following the Min/max versioning scheme
defined in [RFC6709], Section 4.1. The datagram containing this
closing HANDSHAKE message MUST be prefixed with (destination)
channel ID chanQ. Peer P MAY also simply cease communication.

5. If addressed peer Q does not respond to initiating peer P's first
datagram, peer P MAY resend that datagram, until peer Q is
considered dead, according to the rules specified in
Section 3.12.

6. If the reply datagram by Q does pass the checks by peer P and P
wants to continue interacting with peer Q, P can now send
REQUEST, PEX_REQ and other messages to Q. Datagrams carrying
these messages MUST be prefixed with the channel ID chanQ sent by
Q. More specifically, because P knows that Q really responds, P
MAY start sending Q messages with heavy payload. That means that
P MAY start responding to any REQUEST messages that Q may have
sent in this first reply datagram with DATA messages. Hence,
transfer of chunks can start soon in PPSPP.

7. If peer Q receives any datagram (apparently) from P that does not
contain channel ID chanQ, Q MUST discard the datagram, but SHOULD
continue to process datagrams from P that do meet this
requirement. Once Q receives a datagram from P that does contain
the channel ID chanQ, Q knows that P really received its reply
datagram, and the three-way handshake and channel establishment
is complete. Q MAY now also start sending messages with heavy
payload to P.

8. If peer P decides it no longer wants to communicate with Q, or
vice versa, the peer SHOULD send a closing HANDSHAKE message to
the other, as described above.

3.2. HAVE

The HAVE message is used to convey which chunks a peer has available
for download. The set of chunks it has available may be expressed
using different chunk addressing and availability map compression
schemes, described in Section 4. HAVE messages can be used both for
sending a complete overview of a peer's chunk availability as well as
for updates to that set.

In particular, whenever a receiving peer P has successfully checked
the integrity of a chunk, or interval of chunks, it MUST send a HAVE
message to all peers Q1..Qn it wants to allow to download those
chunk(s). A policy in peer P determines when the HAVE is sent. P
may sent it directly, or peer P may wait until either it has other
data to sent to Qi, or until it has received and checked multiple
chunks. The policy will depend on how urgent it is to distribute
this information to the other peers. This urgency is generally
determined in turn by the chunk picking policy (see Section 9.1). In
general, the HAVE messages can be piggybacked onto other messages.
Peers that do not receive HAVE messages are effectively prevented
from downloading the newly available chunks, hence the HAVE message
can be used as a method of choking.

The HAVE message MUST contain the chunk specification of the received
and verified chunks. A receiving peer MUST NOT send a HAVE message
to peers for which the handshake procedure is still incomplete, see
Section 13.1. A peer SHOULD NOT send a HAVE message to peers that
have the complete content already (e.g. in video-on-demand
scenarios).

3.3. DATA

The DATA message is used to transfer chunks of content. The DATA
message MUST contain the chunk ID of the chunk and chunk itself. A
peer MAY send the DATA messages for multiple chunks in the same
datagram. The DATA message MAY contain additional information if
needed by the specific congestion control mechanism used. At present
PPSPP uses LEDBAT [RFC6817] for congestion control, which requires
the current system time to be sent along with the DATA message, so
the current system time MUST be included.

3.4. ACK

ACK messages MUST be sent to acknowledge received chunks if PPSPP is
run over an unreliable transport protocol. ACK messages MAY be sent
if a reliable transport protocol is used. In the former case, a
receiving peer that has successfully checked the integrity of a
chunk, or interval of chunks C MUST send an ACK message containing a
chunk specification for C. As LEDBAT is used, an ACK message MUST
contain the one-way delay, computed from the peer's current system
time received in the DATA message. A peer MAY delay sending ACK
messages as defined in the LEDBAT specification.

3.5. INTEGRITY

The INTEGRITY message carries information required by the receiver to
verify the integrity of a chunk. Its payload depends on the content
integrity protection scheme used. When the Merkle Hash Tree scheme
is used, an INTEGRITY message MUST contain a cryptographic hash of a
subtree of the Merkle hash tree and the chunk specification that
identifies the subtree.

As a typical example, when a peer wants to send a chunk and Merkle
hash trees are used, it creates a datagram that consists of several
INTEGRITY messages containing the hashes the receiver needs to verify
the chunk and the actual chunk itself encoded in a DATA message.
What are the necessary hashes and the exact rules for encoding them
into datagrams is specified in Section 5.3, and Section 5.4,
respectively.

3.6. SIGNED_INTEGRITY

The SIGNED_INTEGRITY message carries digitally signed information
required by the receiver to verify the integrity of a chunk in live
streaming. It logically contains a chunk specification, a timestamp
and a digital signature. Its exact payload depends on the live
content integrity protection scheme used, see Section 6.1.

3.7. REQUEST

While bulk download protocols normally do explicit requests for
certain ranges of data (i.e., use a pull model, for example,
BitTorrent [BITTORRENT]), live streaming protocols quite often use a
request-less push model to save round trips. PPSPP supports both
models of operation.

The REQUEST message is used to request one or more chunks from
another peer. A REQUEST message MUST contain the specification of
the chunks the requester wants to download. A peer receiving a
REQUEST message MAY send out the requested chunks (by means of DATA
messages). When peer Q receives multiple REQUESTs from the same peer
P, peer Q SHOULD process the REQUESTs in the order received.
Multiple REQUEST messages MAY be sent in one datagram, for example,
when a peer wants to request several rare chunks at once.

When live streaming via a push model, a peer receiving REQUESTs also
MAY send some other chunks in case it runs out of requests or for
some other reason. In that case the only purpose of REQUEST messages
is to provide hints and coordinate peers to avoid unnecessary data
retransmission.

3.8. CANCEL

When downloading on demand or live streaming content, a peer can
request urgent data from multiple peers to increase the probability
of it being delivered on time. In particular, when the specific
chunk picking algorithm (see Section 9.1), detects that a request for
urgent data might not be served on time, a request for the same data
can be sent to a different peer. When a peer P decides to request
urgent data from a peer Q, peer P SHOULD send a CANCEL message to all
the peers to which the data has been previously requested. The
CANCEL message contains the specification of the chunks P no longer
wants to request. In addition, when peer Q receives a HAVE message
for the urgent data from peer P, peer Q MUST also cancel the previous
REQUEST(s) from P. In other words, the HAVE message acts as an
implicit CANCEL.

3.9. CHOKE and UNCHOKE

Peer A can send a CHOKE message to peer B to signal it will no longer
be responding to REQUEST messages from B, for example, because A's
upload capacity is exhausted. Peer A MAY send a subsequent UNCHOKE
message to signal that it will respond to new REQUESTs from B again
(A SHOULD discard old requests). When peer B receives a CHOKE
message from A it MUST NOT send new REQUEST messages and it cannot
expect answers to any outstanding ones, as the transfer of chunks is
choked. When peer B is choked but receives a HAVE message from A it
is not automatically unchoked and MUST NOT send any new REQUEST
messages. The CHOKE and UNCHOKE messages are informational as
responding to REQUESTs is OPTIONAL, see Section 3.7.

3.10. Peer Address Exchange

3.10.1. PEX_REQ and PEX_RES Messages

Peer address exchange messages (or PEX messages for short) are common
in many peer-to-peer protocols. They allow peers to exchange the
transport addresses of the peers they are currently interacting with,
thereby reducing the need to contact a central tracker (or
Distributed Hash Table) to discovery new peers. The strength of this
mechanism is therefore that it enables decentralized peer discovery:
after an initial bootstrap no central tracker is needed anymore. Its
weakness is that it enables a number of attacks, so it should not be
used on the Internet unless extra security measures are in place.

PPSPP supports peer-address exchange on the Internet and in benign
private networks, as an OPTIONAL feature (not mandatory to implement)
under certain conditions. The general mechanism works as follows.
To obtain some peer addresses a peer A MAY send a PEX_REQ message to
peer B. Peer B MAY respond with one or more PEX_REScert messages.
Logically, a PEX_REScert reply message contains the address of a
single peer Ci. The address in the PEX_REScert message MUST be of a
peer B has exchanged messages with in the last 60 seconds to
guarantee liveliness. Upon receipt, peer A may contact any or none
of the returned peers Ci. Alternatively, peers MAY ignore PEX_REQ
and PEX_REScert messages if uninterested in obtaining new peers or
because of security considerations (rate limiting) or any other
reason. The PEX messages can be used to construct a dedicated
tracker peer.

To use PEX in PPSPP on the Internet, two conditions must be met:

1. Peer transport addresses must be relatively stable.

2. A peer must not obtain all its peer addresses through PEX.

The full security analysis for PEX messages can be found in
Section 13.2. Physically, a PEX_REScert message carries a swarm-
membership certificate rather than an IP address and port. A
membership certificate for peer C states that peer C at address
(ipC,portC) is part of swarm S at time T and is cryptographically
signed by an issuer. The receiver A can check the certificate for a
valid signature by a trusted issuer, the right swarm and liveliness
and only then consider contacting C. These swarm-membership
certificates correspond to signed node descriptors in secure
decentralized peer sampling services [SPS].

Several designs are possible for the security environment for these
membership certificates. That is, there are different designs
possible for who signs the membership certificates and how public
keys are distributed. Section 13.2.2 describes an example where a
central tracker acts as the Certification Authority.

In a hostile environment, such as the Internet, peers must also
ensure that they do not end up interacting only with malicious peers
when using the peer-address exchange feature. To this extent, peers
MUST ensure that part of their connections are to peers whose
addresses came from a trusted and secured tracker (see
Section 13.2.3).

In addition to the PEX_REScert, there are two other PEX reply
messages. The PEX_RESv4 message contains a single IPv4 address and
port. The PEX_RESv6 contains a single IPv6 address and port. They
MUST only be used in a benign environment, such as a private network,
as they provide no guarantees that the host addressed actually
participates in a PPSPP swarm.

Once a PPSPP implementation has obtained a list of peers (either via
PEX, from a central tracker or via a DHT), it has to determine which
peers to actually contact. In this process, a PPSPP implementation
can benefit from information by network or content providers to help
improve network usage and boost PPSPP performance. How a P2P system
like PPSPP can perform these optimizations using the ALTO protocol is
described in detail in [I-D.ietf-alto-protocol], Section 7.

3.11. Channels

It is increasingly complex for peers to enable communication between
each other due to NATs and firewalls. Therefore, PPSPP uses a
multiplexing scheme, called channels, to allow multiple swarms to use
the same transport address. Channels loosely correspond to TCP
connections and each channel belongs to a single swarm, as
illustrated in Figure 1. As with TCP connections, a channel is
identified by a unique identifier local to the peer at each end of
the connection (cf. TCP port), which MUST be randomly chosen. In
other words, the two peers connected by a channel use different IDs
to denote the same channel. The IDs are different and random for
security reasons, see Section 13.1.

In the PPSP-over-UDP encapsulation (Section 8.3), when a channel C
has been established between peer A and peer B, the datagrams
containing messages from A to B are prefixed with the four byte
channel ID allocated by peer B, and vice versa for datagrams from B
to A. The channel IDs used are exchanged as part of the handshake
procedure, see Section 8.4. In that procedure, the channel ID with
value 0 is used for the datagram that initiates the handshake. PPSPP
can be used in combination with STUN [RFC5389].

_________ _________ _________
| | | | | |
| Swarm | | Swarm | | Swarm |
| Mgr | | A | | B |
|_______| |_______| |_______|
| | / \
| | / \
____|____ ____|____ ______/__ _\_______
| | | | | | | |
| Chan | | Chan | | Chan | | Chan |
| 0 | | 481 | | 836 | | 372 |
|_______| |_______| |_______| |_______|
| | | |
| | | |
____|____________|____________|____________|____
| |
| UDP |
| port 6778 |
|______________________________________________|

Network stack of a PPSPP peer that is reachable on UDP port 6778 and
is connected via channel 481 to one peer in swarm A and two peers in
swarm B via channels 836 and 372, respectively. Channel ID 0 is
special and is used for handshaking.

Figure 1

3.12. Keep Alive Signalling

A peer SHOULD send a "keep alive" message periodically to each peer
it is interested in, but has no other messages to send to them at
present. The goal of the keep alives is to keep a signaling channel
open to peers that are of interest. Which peers those are is
determined by a policy that decides which peers are of interest now
and in the near future. This document does not prescribe a policy,
but examples of interesting peers are: (a) peers that have chunks on
offer that this client needs, or (b) peers that currently do not have
interesting chunks on offer (because they are still downloading
themselves, or in live streaming), but gave good performance in the
past. When these peers have new chunks to offer, the peer that kept
a signaling channel open can use them again. Periodically sending
"keep alive" messages prevents other peers declaring the peer dead.
A guideline for declaring a peer dead when using UDP consists of a 3
minute delay since that last packet has been received from that peer,
and at least 3 datagrams were sent to that peer during the same
period. When a peer is declared dead, the channel to it is closed,
no more messages will be sent to that peer and the local
administration about the peer is discarded. Busy servers can force
idle clients to disconnect by not sending keep alives. PPSPP does
not define an explicit message type for "keep alive" messages. In
the PPSP-over-UDP encapsulation they are implemented as simple
datagrams consisting of a 4-byte channel ID only, see Section 8.3 and
Section 8.4.

4. Chunk Addressing Schemes

PPSPP can use different methods of chunk addressing, that is, support
different ways of identifying chunks and different ways of expressing
the chunk availability map of a peer in a compact fashion.

All peers in a swarm MUST use the same chunk addressing method.

4.1. Start-End Ranges

A chunk specification consists of a single (start specification,end
specification) pair that identifies a range of chunks (end
inclusive). The start and end specifications can use one of multiple
addressing schemes. Two schemes are currently defined, chunk ranges
and byte ranges.

4.1.1. Chunk Ranges

The start and end specification are both chunk identifiers. Chunk
identifiers are 32-bit or 64-bit unsigned integers. A PPSPP peer
MUST support this scheme.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Start chunk (32 or 64) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ End chunk (32 or 64) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

4.1.2. Byte Ranges

The start and end specification are 64-bit byte offsets in the
content. The support for this scheme is OPTIONAL.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Start byte offset (64) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| End byte offset (64) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

4.2. Bin Numbers

PPSPP introduces a novel method of addressing chunks of content
called "bin numbers" (or "bins" for short). Bin numbers allow the
addressing of a binary interval of data using a single integer. This
reduces the amount of state that needs to be recorded per peer and
the space needed to denote intervals on the wire, making the protocol
light-weight. In general, this numbering system allows PPSPP to work
with simpler data structures, e.g. to use arrays instead of binary
trees, thus reducing complexity. The support for this scheme is
OPTIONAL.

In bin addressing, the smallest binary interval is a single chunk
(e.g. a block of bytes which may be of variable size), the largest
interval is a complete range of 2**63 chunks. In a novel addition to
the classical scheme, these intervals are numbered in a way which
lays them out into a vector nicely, which is called bin numbering, as
follows. Consider an chunk interval of width W. To derive the bin
numbers of the complete interval and the subintervals, a minimal
balanced binary tree is built that is at least W chunks wide at the
base. The leaves from left-to-right correspond to the chunks 0..W-1
in the interval, and have bin number I*2 where I is the index of the
chunk (counting beyond W-1 to balance the tree). The bin number of
higher level nodes P in the tree is calculated as follows:

binP = (binL + binR) / 2

where binL is the bin of node P's left-hand child and binR is the bin
of node P's right-hand child. Given that each node in the tree
represents a subinterval of the original interval, each such
subinterval now is addressable by a bin number, a single integer.
The bin number tree of an interval of width W=8 looks like this:

7
/ \
/ \
/ \
/ \
3 11
/ \ / \
/ \ / \
/ \ / \
1 5 9 13
/ \ / \ / \ / \
0 2 4 6 8 10 12 14

C0 C1 C2 C3 C4 C5 C6 C7

The bin number tree of an interval of width W=8

Figure 2

So bin 7 represents the complete interval, bin 3 represents the
interval of chunk C0..C3, bin 1 represents the interval of chunks C0
and C1, and bin 2 represents chunk C1. The special numbers
0xFFFFFFFF (32-bit) or 0xFFFFFFFFFFFFFFFF (64-bit) stands for an
empty interval, and 0x7FFF...FFF stands for "everything".

When bin numbering is used, the ID of a chunk is its corresponding
(leaf) bin number in the tree and the chunk specification in HAVE and
ACK messages is equal to a single bin number (32-bit or 64-bit), as
follows.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Bin number (32 or 64) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

4.3. In Messages

4.3.1. In HAVE Messages

When a receiving peer has successfully checked the integrity of a
chunk or interval of chunks it MUST send a HAVE message to all peers
it wants to allow download of those chunk(s) from. The ability to
withhold HAVE messages allows them to be used as a method of choking.
The HAVE message MUST contain the chunk specification of the biggest
complete interval of all chunks the receiver has received and checked
so far that fully includes the interval of chunks just received. So
the chunk specification MUST denote at least the interval received,
but the receiver is supposed to aggregate and acknowledge bigger
intervals, when possible.

As a result, every single chunk is acknowledged a logarithmic number
of times. That provides some necessary redundancy of acknowledgments
and sufficiently compensates for unreliable transport protocols.

Implementation note:

To record which chunks a peer has in the state that an
implementation keeps for each peer, an implementation MAY use the
efficient "binmap" data structure, which is a hybrid of a bitmap
and a binary tree, discussed in detail in [BINMAP].

4.3.2. In ACK Messages

PPSPP peers MUST use ACK messages to acknowledge received chunks if
an unreliable transport protocol is used. When a receiving peer has
successfully checked the integrity of a chunk or interval of chunks C
it MUST send a ACK message containing the chunk specification of its
biggest, complete interval covering C to the sending peer (see HAVE).

5. Content Integrity Protection

PPSPP can use different methods for protecting the integrity of the
content while it is being distributed via the peer-to-peer network.
More specifically, PPSPP can use different methods for receiving
peers to detect whether a requested chunk has been maliciously
modified by the sending peer. In benign environments, content
integrity protection can be disabled.

For static content, PPSPP currently defines one method for protecting
integrity, called the Merkle Hash Tree scheme. If PPSPP operates
over the Internet, this scheme MUST be used. If PPSPP operates in a
benign environment this scheme MAY be used. So the scheme is
mandatory-to-implement, to satisfy the requirement of strong security
for an IETF protocol [RFC3365]. An extended version of the scheme is
used to efficiently protect dynamically generated content (live
streams), as explained below and in Section 6.1.

The Merkle Hash Tree scheme can work with different chunk addressing
schemes. All it requires is the ability to address a range of
chunks. In the following description abstract node IDs are used to
identify nodes in the tree. On the wire these are translated to the
corresponding range of chunks in the chosen chunk addressing scheme.

5.1. Merkle Hash Tree Scheme

PPSPP uses a method of naming content based on self-certification.
In particular, content in PPSPP is identified by a single
cryptographic hash that is the root hash in a Merkle hash tree
calculated recursively from the content [ABMRKL]. This self-
certifying hash tree allows every peer to directly detect when a
malicious peer tries to distribute fake content. It also ensures
only a small the amount of information is needed to start a download
(the root hash and some peer addresses). For live streaming a
dynamic tree and a public key are used, see below.

The Merkle hash tree of a content file that is divided into N chunks
is constructed as follows. Note the construction does not assume
chunks of content to be fixed size. Given a cryptographic hash
function, more specifically a modification detection code (MDC)
[HAC01] , such as SHA-256, the hashes of all the chunks of the
content are calculated. Next, a binary tree of sufficient height is
created. Sufficient height means that the lowest level in the tree
has enough nodes to hold all chunk hashes in the set, as with bin
numbering. The figure below shows the tree for a content file
consisting of 7 chunks. As before with the content addressing
scheme, the leaves of the tree correspond to a chunk and in this case
are assigned the hash of that chunk, starting at the left-most leaf.
As the base of the tree may be wider than the number of chunks, any
remaining leaves in the tree are assigned an empty hash value of all
zeros. Finally, the hash values of the higher levels in the tree are
calculated, by concatenating the hash values of the two children
(again left to right) and computing the hash of that aggregate. If
the two children are empty hashes, the parent is an empty all zeros
hash as well (to save computation). This process ends in a hash
value for the root node, which is called the "root hash". Note the
root hash only depends on the content and any modification of the
content will result in a different root hash.

7 = root hash
/ \
/ \
/ \
/ \
3* 11
/ \ / \
/ \ / \
/ \ / \
1 5 9 13* = uncle hash
/ \ / \ / \ / \
0 2 4 6 8 10* 12 14

C0 C1 C2 C3 C4 C5 C6 E
=chunk index ^^ = empty hash

The Merkle hash tree of a content file with N=7 chunks

Figure 3

5.2. Content Integrity Verification

Assuming a peer receives the root hash of the content it wants to
download from a trusted source, it can check the integrity of any
chunk of that content it receives as follows. It first calculates
the hash of the chunk it received, for example chunk C4 in the
previous figure. Along with this chunk it MUST receive the hashes
required to check the integrity of that chunk. In principle, these
are the hash of the chunk's sibling (C5) and that of its "uncles". A
chunk's uncles are the sibling Y of its parent X, and the uncle of
that Y, recursively until the root is reached. For chunk C4 its
uncles are nodes 13 and 3 and its sibling is 10; all marked with a *
in the figure. Using this information the peer recalculates the root
hash of the tree, and compares it to the root hash it received from
the trusted source. If they match the chunk of content has been
positively verified to be the requested part of the content.
Otherwise, the sending peer either sent the wrong content or the
wrong sibling or uncle hashes. For simplicity, the set of sibling
and uncles hashes is collectively referred to as the "uncle hashes".

In the case of live streaming the tree of chunks grows dynamically
and the root hash is undefined or, more precisely, transient, as long
as new data is generated by the live source. Section 6.1.2 defines a
method for content integrity verification for live streams that works
with such a dynamic tree. Although the tree is dynamic, content
verification works the same for both live and predefined content,
resulting in a unified method for both types of streaming.

5.3. The Atomic Datagram Principle

As explained above, a datagram consists of a sequence of messages.
Ideally, every datagram sent must be independent of other datagrams,
so each datagram SHOULD be processed separately and a loss of one
datagram must not disrupt the flow of datagrams between two peers.
Thus, as a datagram carries zero or more messages, both messages and
message interdependencies SHOULD NOT span over multiple datagrams.

This principle implies that as any chunk is verified using its uncle
hashes the necessary hashes SHOULD be put into the same datagram as
the chunk's data. If this is not possible because of a limitation on
datagram size, the necessary hashes MUST be sent first in one or more
datagrams. As a general rule, if some additional data is still
missing to process a message within a datagram, the message SHOULD be
dropped.

The hashes necessary to verify a chunk are in principle its sibling's
hash and all its uncle hashes, but the set of hashes to send can be
optimized. Before sending a packet of data to the receiver, the
sender inspects the receiver's previous acknowledgments (HAVE or ACK)
to derive which hashes the receiver already has for sure. Suppose,
the receiver had acknowledged chunks C0 and C1 (first two chunks of
the file), then it must already have uncle hashes 5, 11 and so on.
That is because those hashes are necessary to check C0 and C1 against
the root hash. Then, hashes 3, 7 and so on must be also known as
they are calculated in the process of checking the uncle hash chain.
Hence, to send chunk C7, the sender needs to include just the hashes
for nodes 14 and 9, which let the data be checked against hash 11
which is already known to the receiver.

The sender MAY optimistically skip hashes which were sent out in
previous, still unacknowledged datagrams. It is an optimization
trade-off between redundant hash transmission and possibility of
collateral data loss in the case some necessary hashes were lost in
the network so some delivered data cannot be verified and thus has to
be dropped. In either case, the receiver builds the Merkle tree on-
demand, incrementally, starting from the root hash, and uses it for
data validation.

In short, the sender MUST put into the datagram the hashes he
believes are necessary for the receiver to verify the chunk. The
receiver MUST remember all the hashes it needs to verify missing
chunks that it still wants to download. Note that the latter implies
that a hardware-limited receiver MAY forget some hashes if it does
not plan to announce possession of these chunks to others (i.e., does
not plan to send HAVE messages.)

5.4. INTEGRITY Messages

Concretely, a peer that wants to send a chunk of content creates a
datagram that MUST consist of a list of INTEGRITY messages followed
by a DATA message. If the INTEGRITY messages and DATA message cannot
be put into a single datagram because of a limitation on datagram
size, the INTEGRITY messages MUST be sent first in one or more
datagrams. The list of INTEGRITY messages sent MUST contain a
INTEGRITY message for each hash the receiver misses for integrity
checking. A INTEGRITY message for a hash MUST contain the chunk
specification corresponding to the node ID of the hash and the hash
data itself. The chunk specification corresponding to a node ID is
defined as the range of chunks formed by the leaves of the subtree
rooted at the node. For example, node 3 in Figure 3 denotes chunks
0,2,4,6, so the chunk specification should denote that interval. The
list of INTEGRITY messages MUST be sorted in order of the tree height
of the nodes, descending (the leaves are at height 0). The DATA
message MUST contain the chunk specification of the chunk and chunk
itself. A peer MAY send the required messages for multiple chunks in
the same datagram, depending on the encapsulation.

5.5. Discussion and Overhead

The current method for protecting content integrity in BitTorrent
[BITTORRENT] is not suited for streaming. It involves providing
clients with the hashes of the content's chunks before the download
commences by means of metadata files (called .torrent files in
BitTorrent.) However, when chunks are small as in the current UDP
encapsulation of PPSPP this implies having to download a large number
of hashes before content download can begin. This, in turn,
increases time-till-playback for end users, making this method
unsuited for streaming.

The overhead of using Merkle hash trees is limited. The size of the
hash tree expressed as the total number of nodes depends on the
number of chunks the content is divided (and hence the size of
chunks) following this formula:

nnodes = math.pow(2,math.log(nchunks,2)+1)

In principle, the hash values of all these nodes will have to be sent
to a peer once for it to verify all chunks. Hence the maximum on-
the-wire overhead is hashsize * nnodes. However, the actual number
of hashes transmitted can be optimized as described in Section 5.3.

To see a peer can verify all chunks whilst receiving not all hashes,
consider the example tree in Section 5.1. In case of a simple
progressive download, of chunks 0,2,4,6, etc. the sending peer will
send the following hashes:

+-------+---------------------------------------------+
| Chunk | Node IDs of hashes sent |
+-------+---------------------------------------------+
| 0 | 2,5,11 |
| 2 | - (receiver already knows all) |
| 4 | 6 |
| 6 | - |
| 8 | 10,13 (hash 3 can be calculated from 0,2,5) |
| 10 | - |
| 12 | 14 |
| 14 | - |
| Total | # hashes 7 |
+-------+---------------------------------------------+

Table 1: Overhead for the example tree

So the number of hashes sent in total (7) is less than the total
number of hashes in the tree (16), as a peer does not need to send
hashes that are calculated and verified as part of earlier chunks.

5.6. Automatic Detection of Content Size

In PPSPP, the size of a static content file, such as a video file,
can be reliably and automatically derived from information received
from the network when fixed sized chunks are used. As a result, it
is not necessary to include the size of the content file as the
metadata of the content, for such files. Implementations of PPSPP
MAY use this automatic detection feature. Note this feature is the
only feature of PPSPP that requires that a fixed-sized chunk is used.
This feature builds on the Merkle hash tree and the trusted root hash
as swarm ID as follows.

5.6.1. Peak Hashes

The ability for a newcomer peer to detect the size of the content
depends heavily on the concept of peak hashes. The concept of peak
hashes depends on the concepts of filled and incomplete nodes.
Recall that when constructing the binary trees for content
verification and addressing the base of the tree may have more leaves
than the number of chunks in the content. In the Merkle hash tree
these leaves were assigned empty all-zero hashes to be able to
calculate the higher level hashes. A filled node is now defined as a
node that corresponds to an interval of leaves that consists only of
hashes of content chunks, not empty hashes. Reversely, an incomplete
(not filled) node corresponds to an interval that contains also empty
hashes, typically an interval that extends past the end of the file.
In the following figure nodes 7, 11, 13 and 14 are incomplete the
rest is filled.

Formally, a peak hash is the hash of a filled node in the Merkle
tree, whose sibling is an incomplete node. Practically, suppose a
file is 7162 bytes long and a chunk is 1 kilobyte. That file fits
into 7 chunks, the tail chunk being 1018 bytes long. The Merkle tree
for that file is shown in Figure 4. Following the definition the
peak hashes of this file are in nodes 3, 9 and 12, denoted with a *.
E denotes an empty hash.

7
/ \
/ \
/ \
/ \
3* 11
/ \ / \
/ \ / \
/ \ / \
1 5 9* 13
/ \ / \ / \ / \
0 2 4 6 8 10 12* 14

C0 C1 C2 C3 C4 C5 C6 E
= 1018 bytes

Peak hashes in a Merkle hash tree.

Figure 4

Peak hashes can be explained by the binary representation of the
number of chunks the file occupies. The binary representation for 7
is 111. Every "1" in binary representation of the file's packet
length corresponds to a peak hash. For this particular file there
are indeed three peaks, nodes 3, 9, 12. The number of peak hashes
for a file is therefore also at most logarithmic with its size.

A peer knowing which nodes contain the peak hashes for the file can
therefore calculate the number of chunks it consists of, and thus get
an estimate of the file size (given all chunks but the last are fixed
size). Which nodes are the peaks can be securely communicated from
one (untrusted) peer A to another B by letting A send the peak hashes
and their node IDs to B. It can be shown that the root hash that B
obtained from a trusted source is sufficient to verify that these are
indeed the right peak hashes, as follows.

Lemma: Peak hashes can be checked against the root hash.

Proof: (a) Any peak hash is always the left sibling. Otherwise, be
it the right sibling, its left neighbor/sibling must also be a filled
node, because of the way chunks are laid out in the leaves,
contradiction. (b) For the rightmost peak hash, its right sibling is
zero. (c) For any peak hash, its right sibling might be calculated
using peak hashes to the left and zeros for empty nodes. (d) Once the
right sibling of the leftmost peak hash is calculated, its parent
might be calculated. (e) Once that parent is calculated, we might
trivially get to the root hash by concatenating the hash with zeros
and hashing it repeatedly.

Informally, the Lemma might be expressed as follows: peak hashes
cover all data, so the remaining hashes are either trivial (zeros) or
might be calculated from peak hashes and zero hashes.

Finally, once peer B has obtained the number of chunks in the content
it can determine the exact file size as follows. Given that all
chunks except the last are fixed size B just needs to know the size
of the last chunk. Knowing the number of chunks B can calculate the
node ID of the last chunk and download it. As always B verifies the
integrity of this chunk against the trusted root hash. As there is
only one chunk of data that leads to a successful verification the
size of this chunk must be correct. B can then determine the exact
file size as

(number of chunks -1) * fixed chunk size + size of last chunk

5.6.2. Procedure

A PPSPP implementation that wants to use automatic size detection
MUST operate as follows. When a peer A sends a DATA message for the
first time to a peer B, A MUST first send all the peak hashes for the
content, in INTEGRITY messages, unless B has already signalled
earlier in the exchange that it knows the peak hashes by having
acknowledged any chunk. If they are needed, the peak hashes MUST be
sent as an extra list of uncle hashes for the chunk, before the list
of actual uncle hashes of the chunk as described in Section 5.3. The
receiver B MUST check the peak hashes against the root hash to
determine the approximate content size. To obtain the definite
content size peer B MUST download the last chunk of the content from
any peer that offers it.

As an example, let's consider a 7162 bytes long file, which fits in 7
chunks of 1 kilobyte, distributed by a peer A. Figure 4 shows the
relevant Merkle hash tree. A peer B which only knows the root hash
of the file, after successfully connecting to A, requests the first
chunk of data, C0 in Figure 4. Peer A replies to B by including in
the datagram the following messages in this specific order. First
the three peak hashes of this particular file, the hashes of nodes 3,
9 and 12. Second, the uncle hashes of C0, followed by the DATA
message containing the actual content of C0. Upon receiving the peak
hashes, peer B checks them against the root hash determining that the
file is 7 chunks long. To establish the exact size of the file, peer
B needs to request and retrieve the last chunk containing data, C6 in
Figure 4. Once the last chunk has been retrieved and verified, peer
B concludes that it is 1018 bytes long, hence determining that the
file is exactly 7162 bytes long.

6. Live Streaming

The set of messages defined above can be used for live streaming as
well. In a pull-based model, a live streaming injector can announce
the chunks it generates via HAVE messages, and peers can retrieve
them via REQUEST messages. Areas that need special attention are
content authentication and chunk addressing (to achieve an infinite
stream of chunks).

6.1. Content Authentication

For live streaming, PPSPP supports two methods for a peer to
authenticate the content it receives from another peer, called "Sign
All" and "Unified Merkle Tree".

In the "Sign All" method, the live injector signs each chunk of
content using a private key and peers, upon receiving the chunk,
check the signature using the corresponding public key obtained from
a trusted source. Support for this method is OPTIONAL.

In the "Unified Merkle Tree" method, PPSPP combines the Merkle Hash
Tree scheme for static content with signatures to unify the video-on-
demand and live streaming scenarios. The use of Merkle hash trees
reduces the number of signing and verification operations, hence
providing a similar signature amortization to the approach described
in [SIGMCAST]. If PPSPP operates over the Internet, the "Unified
Merkle Tree" method MUST be used. If the protocol operates in a
benign environment the "Unified Merkle Tree" method MAY be used. So
this method is mandatory-to-implement.

In both methods the swarm ID consists of a public key encoded as in a
DNSSEC DNSKEY resource record without BASE-64 encoding [RFC4034]. In
particular, the swarm ID consists of a 1 byte Algorithm field that
identifies the public key's cryptographic algorithm and determines
the format of the Public Key field that follows. The value of this
Algorithm field is one of the Domain Name System Security (DNSSEC)
Algorithm Numbers [IANADNSSECALGNUM]. The RSASHA1 [RFC4034],
RSASHA256 [RFC5702], and ECDSAP256SHA256 and ECDSAP384SHA384
[RFC6605] algorithms are MANDATORY to implement.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Algo Number(8)| ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ DNSSEC Public Key (variable) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

6.1.1. Sign All

A peer that wants to send a chunk of content creates a datagram that
MUST contain a SIGNED_INTEGRITY message with the chunk's signature,
followed by a DATA message with the actual chunk. If the
SIGNED_INTEGRITY message and DATA message cannot be contained into a
single datagram, because of a limitation on datagram size, the
SIGNED_INTEGRITY message MUST be sent first in a separate datagram.
The SIGNED_INTEGRITY message consists of the chunk specification, the
timestamp, and the digital signature.

The digital signature algorithm which is used, is determined by the
Live Signature Algorithm protocol option, see Section 7.7. The
signature is computed over a concatenation of the on-the-wire
representation of the chunk specification, a 64-bit timestamp in NTP
Timestamp format [RFC5905], and the chunk, in that order. The
timestamp is the time signature that was made at the injector in UTC.

6.1.2. Unified Merkle Tree

In this method, the chunks of content are used as the basis for a
Merkle hash tree as for static content. However, because chunks are
continuously generated, this tree is not static, but dynamic. As a
result, the tree does not have a root hash, or more precisely has a
transient root hash. A public key therefore serves as swarm ID of
the content. It is used to digitally sign updates to the tree,
allowing peers to expand it based on trusted information using the
following process.

6.1.2.1. Signed Munro Hashes

The live injector generates a number of chunks, denoted
NCHUNKS_PER_SIG, corresponding to fixed power of 2
(NCHUNKS_PER_SIG>=2), which are added as new leaves to the existing
hash tree. As a result of this expansion the hash tree contains a
new subtree, that is NCHUNKS_PER_SIG chunks wide at the base. The
root of this new subtree is referred to as the munro of that subtree,
and its hash as the munro hash of the subtree, illustrated in
Figure 5. In this figure, node 5 is the new munro, labeled with a $
sign.

3
/ \
/ \
/ \
1 5$
/ \ / \
0 2 4 6

Expanded live tree. With NCHUNKS_PER_SIG=2, node 5 is the munro for
the new subtree spanning 4 and 6. Node 1 is the munro for the
subtree spanning chunks 0 and 2, created in the previous iteration.

Figure 5

Informally, the process now proceeds as follows. The injector now
signs only the munro hash of the new subtree using its private key.
Next, the injector announces the existence of the new subtree to its
peers using HAVE messages. When a peer, in response to the HAVE
messages, requests a chunk from the new subtree, the injector first
sends the signed munro hash corresponding to the requested chunk.
Afterwards, similar to static content, the injector sends the uncle
hashes necessary to verify that chunk, as in Section 5.1. In
particular, the injector sends the uncle hashes necessary to verify
the requested chunk against the munro hash. This differs from static
content, where the verification takes places against the root hash.
Finally, the injector sends the actual chunk.

The receiving peer verifies the signature on the signed munro using
the swarm ID (a public key), and updates its hash tree. As the peer
now knows the munro hash is trusted, it can verify all chunks in the
subtree against this munro hash, using the accompanying uncle hashes
as in Section 5.1.

To illustrate this procedure, lets consider the next iteration in the
process. The injector has generated the current tree shown in
Figure 5 and it is connected to several peers that currently have the
same tree and all posses chunks 0, 2, 4 and 6. When the injector
generates two new chunks, NCHUNKS_PER_SIG=2, the hash tree expands as
shown in Figure 6. The two new chunks, 8 and 10, extend the tree on
the right side, and to accommodate them a new root is created, node
7. As this tree is wider at the base than the actual number of
chunks, there are currently two empty leaves. The munro node for the
new subtree is 9, labeled with a $ sign.

7
/ \
/ \
/ \
/ \
3 11
/ \ / \
/ \ / \
/ \ / \
1 5 9$ 13
/ \ / \ / \ / \
0 2 4 6 8 10 E E

Expanded live tree. With NCHUNKS_PER_SIG=2, node 9 is the munro of
the newly added subtree spanning chunks 8 and 10.

Figure 6

The injector now needs to inform its peers of the updated tree,
communicating the addition of the new munro hash 9. Hence, it sends
a HAVE message with a chunk specification for nodes 8+10 to its
peers. As a response, a peer P requests the newly created chunk,
e.g. chunk 8, from the injector by sending a REQUEST message. In
reply, the injector sends the signed munro hash of node 9 as an
INTEGRITY message with the hash of node 9, and a SIGNED_INTEGRITY
message with the signature of the hash of node 9. These messages are
followed by an INTEGRITY message with the hash of node 10, and a DATA
message with chunk 8.

Upon receipt, peer P verifies the signature of the munro and expands
its view of the tree. Next, the peer computes the hash of chunk 8
and combines it with the received hash of node 10, computing the
expected hash of node 9. He can then verify the content of chunk 8
by comparing the computed hash of node 9 with the munro hash of the
same node he just received, hence P has successfully verified the
integrity of chunk 8.

This procedure requires just one signing operation for every
NCHUNKS_PER_SIG chunks created, and one verification operation for
every NCHUNKS_PER_SIG received, making it much cheaper than "Sign
All". A receiving peer does additionally need to check one or more
hashes per chunk via the Merkle Tree scheme, but this has less
hardware requirements than a signature verification for every chunk.
This approach is similar to signature amortization via Merkle Tree
Chaining [SIGMCAST]. The downside of scheme is in an increased
latency. A peer cannot download the new chunks until the injector
has computed the signature and announced the subtree. A peer MUST
check the signature before forwarding the chunks to other peers
[POLLIVE].

The number of chunks per signature NCHUNKS_PER_SIG MUST be a fixed
power of 2 for simplicity. NCHUNKS_PER_SIG MUST be larger than 1 for
performance reasons. There are two related factors to consider when
choosing a value for NCHUNKS_PER_SIG. First, the allowed CPU load on
clients due to signature verifications, given the expected bitrate of
the stream. To achieve a low CPU load in a high bitrate stream,
NCHUNKS_PER_SIG should be high. Second, the effect on latency, which
increases when NCHUNKS_PER_SIG gets higher, as just discussed. Note
how the procedure does not preclude the use of variable-sized chunks.

This method of integrity verification provides an additional benefit.
If the system includes some peers that saved the complete broadcast,
as soon as the broadcast ends, the content is available as a video-
on-demand download using the now stabilized tree and the final root
hash as swarm identifier. Peers which saved all the chunks, can now
announce the root hash to the tracking infrastructure and instantly
seed the content.

6.1.2.2. Munro Signature Calculation

The digital signature algorithm used is determined by the Live
Signature Algorithm protocol option, see Section 7.7. The signature
is computed over a concatenation of the on-the-wire representation of
the chunk specification of the munro node (see Section 6.1.2.1), a
timestamp in 64-bit NTP Timestamp format [RFC5905], and the hash
associated with the munro node, in that order. The timestamp is the
time signature that was made at the injector in UTC.

6.1.2.3. Procedure

Formally, the injector MUST NOT send a HAVE message for chunks in the
new subtree until it has computed the signed munro hash for that
subtree.

When peer B requests a chunk C from peer A (either the injector or
another peer), and peer A decides to reply, it must do so as follows.
First, peer A MUST send an INTEGRITY message with the chunk
specification for the munro of chunk C and the munro's hash, followed
by a SIGNED_INTEGRITY message with the chunk specification for the
munro, timestamp and its signature, in a single datagram, unless B
indicated earlier in the exchange that it already possess a chunk
with the same corresponding munro (by means of HAVE or ACK messages).
Following these two messages (if any), peer A MUST send the necessary
missing uncles hashes needed for verifying the chunk against its
munro hash, and the chunk itself, as described in Section 5.4,
sharing datagrams if possible.

6.1.2.4. Secure Tune In

When a peer tunes into a live stream it has to determine what is the
last chunk the injector has generated. To facilitate this process in
the Unified Merkle Tree scheme, each peer shares its knowledge about
the injector's chunks with the others by exchanging their latest
signed munro hashes, as follows.

Recall that in PPSPP, when peer A initiates a channel with peer B,
peer A sends a first datagram with a HANDSHAKE message, and B
responds with a second datagram also containing a HANDSHAKE message
(see Section 3.1). When A sends a third datagram to B, and it is
received by B both peers know that the other is listening on its
stated transport address. B is then allowed to send heavy payload
like DATA messages in the fourth datagram. Peer A can already safely
do that in the third datagram.

In the Unified Merkle Tree scheme, peer A MUST send its right-most
signed munro hash to B in the third datagram, and in any subsequent
datagrams to B, until B indicates that it possess a chunk with the
same corresponding munro or a more recent munro (by means of a HAVE
or ACK message). B may already have indicated this fact by means of
HAVE messages in the second datagram. Conversely, when B sends the
fourth datagram or any subsequent datagram to A, B MUST send its
right-most signed munro hash, unless A indicated knowledge of it or
more recent munros. The right-most signed munro hash of a peer is
defined as the munro hash signed by the injector of the right-most
subtree of width NCHUNKS_PER_SIG chunks in the peer's Merkle hash
tree. Peer A and B MUST NOT send the signed munro hash in the first,
respectively, second datagram as it is considered heavy payload.

When a peer receives a SIGNED_INTEGRITY message with a signed munro
hash but the timestamp is too old, the peer MUST discard the message.
Otherwise it SHOULD use the signed munro to update its hash tree and
pick a tune-in point in the live stream. A peer may use the
information from multiple peers to pick the tune-in point.

6.2. Forgetting Chunks

As a live broadcast progresses a peer may want to discard the chunks
that it already played out. Ideally, other peers should be aware of
this fact such that they will not try to request these chunks from
this peer. This could happen in scenarios where live streams may be
paused by viewers, or viewers are allowed to start late in a live
broadcast (e.g., start watching a broadcast at 20:35 whereas it began
at 20:30).

PPSPP provides a simple solution for peers to stay up-to-date with
the chunk availability of a discarding peer. A discarding peer in a
live stream MUST enable the Live Discard Window protocol option,
specifying how many chunks/bytes it caches before the last chunk/byte
it advertised as being available (see Section 7.9). Its peers SHOULD
apply this number as a sliding window filter over the peer's chunk
availability as conveyed via its HAVE messages.

Three factors are important when deciding for an appropriate value
for this option: the desired amount of playback buffer for peers, the
bitrate of the stream and the available resources of the peer.
Consider the case of a fresh peer joining the stream. The size of
the discard window of the peers it connects to influences how much
data it can directly download to establish its prebuffer. If the
window is smaller than the desired buffer, the fresh peer has to wait
until the peers downloaded more of the stream before it can start
playback. As media buffers are generally specified in terms of a
number of seconds, the size of the discard window is also related to
the (average) bitrate of the stream. Finally, if a peer has little
resources to store chunks and metadata it should chose a small
discard window.

7. Protocol Options

The HANDSHAKE message in PPSPP can contain the following protocol
options. Unless stated otherwise, a protocol option consists of an
8-bit code followed by an 8-bit value. Larger values are all encoded
big-endian. Each protocol option is explained in the following
subsections. The list of protocol options MUST be sorted on code
value (ascending) in a HANDSHAKE message.

+--------+-------------------------------------+
| Code | Description |
+--------+-------------------------------------+
| 0 | Version |
| 1 | Minimum Version |
| 2 | Swarm Identifier |
| 3 | Content Integrity Protection Method |
| 4 | Merkle Hash Tree Function |
| 5 | Live Signature Algorithm |
| 6 | Chunk Addressing Method |
| 7 | Live Discard Window |
| 8 | Supported Messages |
| 9 | Chunk Size |
| 10-254 | Unassigned |
| 255 | End Option |
+--------+-------------------------------------+

Table 2: PPSP Peer Protocol Options

7.1. End Option

A peer MUST conclude the list of protocol options with the end
option. Subsequent octets should be considered protocol messages.
The code for the end option is 255, and unlike others it has no value
octet, so the option's length is 1 octet.

0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|1 1 1 1 1 1 1 1|
+-+-+-+-+-+-+-+-+

7.2. Version

A peer MUST include the maximum version of the PPSPP protocol it
supports as the first protocol option in the list. The code for this
option is 0. Defined values are listed in Table 3.

+---------+----------------------------------------+
| Version | Description |
+---------+----------------------------------------+
| 0 | Reserved |
| 1 | Protocol as described in this document |
| 2-255 | Unassigned |
+---------+----------------------------------------+

Table 3: PPSP Peer Protocol Version Numbers

0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 0| Version (8) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

7.3. Minimum Version

When a peer initiates the handshake it MUST include the minimum
version of the PPSPP protocol it supports in the list of protocol
options, following the Min/max versioning scheme defined in
[RFC6709], Section 4.1, strategy 5. The code for this option is 1.
Defined values are listed in Table 3.

0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 1| Min. Ver. (8) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

7.4. Swarm Identifier

When a peer initiates the handshake it MUST include a single swarm
identifier option. If the peer is not the initiator, it MAY include
a swarm identifier option, as an end-to-end check. This option has
the following structure:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 1 0| Swarm ID Length (16) | ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Swarm Identifier (variable) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Swarm ID Length field contains the length of the single Swarm
Identifier that follows in bytes. The Length field is 16 bits wide
to allow for large public keys as identifiers in live streaming.

Each PPSPP peer knows the IDs of the swarms it joins so this
information can be immediately verified upon receipt.

7.5. Content Integrity Protection Method

A peer MUST include the content integrity method used by a swarm.
The code for this option is 3. Defined values are listed in Table 4.

+--------+-------------------------+
| Method | Description |
+--------+-------------------------+
| 0 | No integrity protection |
| 1 | Merkle Hash Tree |
| 2 | Sign All |
| 3 | Unified Merkle Tree |
| 4-255 | Unassigned |
+--------+-------------------------+

Table 4: PPSP Peer Content Integrity Protection Methods

The "Merkle Hash Tree" method is the default for static content, see
Section 5.1. "Sign All", and "Unified Merkle Tree" are for live
content, see Section 6.1, with "Unified Merkle Tree" being the
default.

0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 1 1| CIPM (8) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

7.6. Merkle Tree Hash Function

When the content integrity protection method is "Merkle Hash Tree"
this option defining which hash function is used for the tree MUST be
included. The code for this option is 4. Defined values are listed
in Table 5 (see [FIPS180-4] for the function semantics).

+----------+-------------+
| Function | Description |
+----------+-------------+
| 0 | SHA-1 |
| 1 | SHA-224 |
| 2 | SHA-256 |
| 3 | SHA-384 |
| 4 | SHA-512 |
| 5-255 | Unassigned |
+----------+-------------+

Table 5: PPSP Peer Protocol Merkle Hash Functions

Implementations MUST support SHA-1 (see Section 13.5) and SHA-256.
SHA-256 is the default.

0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 1 0 0| MHF (8) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

7.7. Live Signature Algorithm

When the content integrity protection method is "Sign All" or
"Unified Merkle Tree" this option MUST be defined. The code for this
option is 5. The 8-bit value of this option is one of the Domain
Name System Security (DNSSEC) Algorithm Numbers [IANADNSSECALGNUM].
The RSASHA1 [RFC4034], RSASHA256 [RFC5702], ECDSAP256SHA256 and
ECDSAP384SHA384 [RFC6605] algorithms are MANDATORY to implement.
Default is ECDSAP256SHA256.

0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 1 0 1| LSA (8) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

7.8. Chunk Addressing Method

A peer MUST include the chunk addressing method it uses. The code
for this option is 6. Defined values are listed in Table 6.

+--------+---------------------+
| Method | Description |
+--------+---------------------+
| 0 | 32-bit bins |
| 1 | 64-bit byte ranges |
| 2 | 32-bit chunk ranges |
| 3 | 64-bit bins |
| 4 | 64-bit chunk ranges |
| 5-255 | Unassigned |
+--------+---------------------+

Table 6: PPSP Peer Chunk Addressing Methods

Implementations MUST support "32-bit chunk ranges" and "64-bit chunk
ranges". Default is "32-bit chunk ranges".

0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 1 1 0| CAM (8) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

7.9. Live Discard Window

A peer in a live swarm MUST include the discard window it uses. The
code for this option is 7. The unit of the discard window depends on
the chunk addressing method used, see Table 6. For bins and chunk
ranges it is a number of chunks, for byte ranges it is a number of
bytes. Its data type is the same as for a bin, or one value in a
range specification. In other words, its value is a 32-bit or 64-bit
integer in big endian format. If this option is used, the Chunk
Addressing Method MUST appear before it in the list. This option has
the following structure:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 1 1 1| Live Discard Window (32 or 64) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

A peer that does not, under normal circumstances, discard chunks MUST
set this option to the special value 0xFFFFFFFF (32-bit) or
0xFFFFFFFFFFFFFFFF (64-bit). For example, peers that record a
complete broadcast to offer it directly as a static file after the
broadcast ends use these values (see Section 6.1.2). Section 6.2
explains how to determine a value for this option.

7.10. Supported Messages

Peers may support just a subset of the PPSPP messages. For example,
peers running over TCP may not accept ACK messages, or peers used
with a centralized tracking infrastructure may not accept PEX
messages. For these reasons, peers who support only a proper subset
of the PPSPP messages MUST signal which subset they support by means
of this protocol option. The code for this option is 8. The value
of this option is a length octet (SupMsgLen) indicating the length in
bytes of the compressed bitmap that follows.

The set of messages supported can be derived from the compressed
bitmap by padding it with bytes of value 0 until it is 256 bits in
length. Then a 1 bit in the resulting bitmap at position X
(numbering left to right) corresponds to support for message type X,
see Table 7. In other words, to construct the compressed bitmap,
create a bitmap with a 1 for each message type supported and a 0 for
a message type that is not, store it as an array of bytes and
truncate it to the last non-zero byte. An example of the first 16
bits of the compressed bitmap for a peer supporting every message
except ACKs and PEXs is: 11011001 11110000.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 1 0 0 0| SupMsgLen (8) | ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Supported Messages Bitmap (variable, max 256) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

7.11. Chunk Size

A peer in a swarm MUST include the chunk size the swarm uses. The
code for this option is 9. Its value is a 32-bit integer denoting
the size of the chunks in bytes in big endian format. When variable
chunk sizes are used, this option MUST be set to the special value
0xFFFFFFFF. Section 8.1 explains how content publishers can
determine a value for this option.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 1 0 0 1| Chunk Size (32) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ |
+-+-+-+-+-+-+-+-+

8. UDP Encapsulation

PPSPP implementations MUST use UDP as transport protocol and MUST use
LEDBAT for congestion control [RFC6817]. Using LEDBAT enables PPSPP
to serve the content after playback (seeding) without disrupting the
user who may have moved to different tasks that use its network
connection. Future PPSPP versions can also run over other transport
protocols, or use different congestion control algorithms.

8.1. Chunk Size

In general, an UDP datagram containing PPSPP messages SHOULD fit
inside a single IP packet, so its maximum size depends on the MTU of
the network. If the UDP datagram does not fit, its chance of getting
lost in the network increases as the loss of a single fragment of the
datagram causes the loss of the complete datagram.

The largest message in a PPSPP datagram is the DATA message carrying
a chunk of content. So the (maximum) size of a chunk to choose for a
particular swarm depends primarily on the expected MTU. The chunk
size should be chosen such that a chunk and its required INTEGRITY
messages can generally be carried inside a single datagram, following
the Atomic Datagram Principle (Section 5.3). Other considerations
are the hardware capabilities of the peers. Having large chunks and
therefore less chunks per megabyte of content reduces processing
costs. The chunk addressing schemes can all work with different
chunk sizes, see Section 4.

The RECOMMENDED approach is to use fixed-sized chunks of 1024 bytes,
as this size has a high likelihood of travelling end-to-end across
the Internet without any fragmentation. In particular, with this
size a UDP datagram with a DATA message can be transmitted as a
single IP packet over an Ethernet network with 1500-byte frames.

A PPSPP implementation MAY use a variant of the Packetization Layer
Path MTU Discovery (PLPMTUD), described in [RFC4821], for discovering
the optimal MTU between sender and destination. As in PLPMTUD,
progressively larger probing packets are used to detect the optimal
MTU among a link. However, in PPSPP, probe packets SHOULD contain
actual messages, in particular, multiple DATA messages. By using
actual DATA messages as probe packets, the returning ACK messages
will confirm the probe delivery, effectively updating the MTU
estimate on both ends of the link. To be able to scale up probe
packets with sensible increments, a minimum chunk size of 512 bytes
SHOULD be used. Smaller chunk sizes lead to an inefficient protocol.
An implication is that PPSP supports datagrams over IPv4 of 576 bytes
or more only. This variant is not mandatory to implement.

The chunk size used for a particular swarm, or that fact that it is
variable MUST be part of the swarm's metadata (which then minimally
consists of the swarm ID and the chunk nature and size).

8.2. Datagrams and Messages

When using UDP, the abstract datagram described above corresponds
directly to a UDP datagram. Most messages within a datagram have a
fixed length, which generally depends on the type of the message.
The first byte of a message denotes its type. The currently defined
types are:

+----------+------------------+
| Msg Type | Description |
+----------+------------------+
| 0 | HANDSHAKE |
| 1 | DATA |
| 2 | ACK |
| 3 | HAVE |
| 4 | INTEGRITY |
| 5 | PEX_RESv4 |
| 6 | PEX_REQ |
| 7 | SIGNED_INTEGRITY |
| 8 | REQUEST |
| 9 | CANCEL |
| 10 | CHOKE |
| 11 | UNCHOKE |
| 12 | PEX_RESv6 |
| 13 | PEX_REScert |
| 14-254 | Unassigned |
| 255 | Reserved |
+----------+------------------+

Table 7: PPSP Peer Protocol Message Types

Furthermore, integers are serialized in the network (big-endian) byte
order. So consider the example of a HAVE message (Section 3.2) using
bin chunk addressing. It has message type of 0x03 and a payload of a
bin number, a four-byte integer (say, 1); hence, its on the wire
representation for UDP can be written in hex as: "0300000001".

All messages are idempotent or recognizable as duplicates.
Idempotent means that processing a message more than once does not
lead to a different state from if it was processed just once. In
particular, a peer MAY resend DATA, ACK, HAVE, INTEGRITY, PEX_*,
SIGNED_INTEGRITY, REQUEST, CANCEL, CHOKE and UNCHOKE messages without
problems when loss is suspected. When a peer resends a HANDSHAKE
message it can be recognized as duplicate by the receiver, because it
already recorded the first connection attempt, and be dealt with.

8.3. Channels

As described in Section 3.11 PPSPP uses a multiplexing scheme, called
channels, to allow multiple swarms to use the same UDP port. In the
UDP encapsulation, each datagram from peer A to peer B is prefixed
with the channel ID allocated by peer B. The peers learn about each
other's channel ID during the handshake as explained in a moment.
Section 3.1.1. A channel ID consists of 4 bytes and MUST be
generated following the requirements in [RFC4960] (Sec. 5.1.3).

8.4. HANDSHAKE

A channel is established with a handshake. To start a handshake, the
initiating peer needs to know the swarm metadata, defined in
Section 3.1 and the IP address and UDP port of a peer. A datagram
containing a HANDSHAKE message then looks as follows:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination Channel ID (32) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 0| Source Channel ID (32) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Protocol Options ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

where:

Destination Channel ID:

If the message datagram is sent by the initiating peer than it MUST be
an all 0-zeros channel ID.

If the message datagram sent by the responding peer than it MUST
consist of the Source Channel ID from the sender's HANDSHAKE
message

The octet 0x00: The HANDSHAKE message: 0x00

The Source Channel ID: A locally unused channel ID
Protocol Options: A list of protocol options encoding the swarm's
metadata, as defined in Section 7.

A peer SHOULD explicitly close a channel by sending a HANDSHAKE
message that MUST contain an all 0-zeros Source Channel ID and a list
of protocol options. The list MUST be either empty or contain the
maximum version number the sender supports, following the Min/max
versioning scheme defined in [RFC6709], Section 4.1.

8.5. HAVE

A HAVE message (type 0x03) consists of a single chunk specification
that states that the sending peer has those chunks and successfully
checked their integrity. The single chunk specification represents a
consecutive range of verified chunks. A bin consists of a single
integer, and a chunk or byte range of two integers, of the width
specified by the Chunk Addressing protocol options, encoded big
endian.

A HAVE message using 32-bit chunk ranges as Chunk Addressing method:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 1 1| Start chunk (32) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | End chunk (32) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+

where the first octet is the HAVE message (0x03), followed by the
start chunk and the end chunk describing the chunk range. Note this
diagram shows a message and not a datagram, so it is not prefixed by
the destination Channel ID. This holds for all subsequent message
diagrams.

8.6. DATA

A DATA message (type 0x01) consists of a chunk specification, a
timestamp and the actual chunk. In case a datagram contains one DATA
message, a sender MUST always put the DATA message in the tail of the
datagram. A datagram MAY contain multiple DATA messages when the
chunk size is fixed and when none of DATA messages carry the last
chunk if that is smaller than the chunk size. As the LEDBAT
congestion control is used, a sender MUST include a timestamp, in
particular, a 64-bit integer representing the current system time
with microsecond accuracy. The timestamp MUST be included between
chunk specification and the actual chunk.

A DATA message using 32-bit chunk ranges as Chunk Addressing method:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 1| Start chunk (32) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | End chunk (32) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Timestamp (64) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Data ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

where the first octet is the DATA message (0x01), followed by the
start chunk and the end chunk describing the single chunk, the
timestamp and the actual data.

8.7. ACK

An ACK message (type 0x02) acknowledges data that was received from
its addressee; to comply with the LEDBAT delay-based congestion
control an ACK message consists of a chunk specification and a
timestamp representing an one-way delay sample. The one-way delay
sample is a 64-bit integer with microsecond accuracy, and is computed
from the timestamp received from the previous DATA message containing
the chunk being acknowledged following the LEDBAT specification.

An ACK message using 32-bit chunk ranges as Chunk Addressing method:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 1 0| Start chunk (32) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | End chunk (32) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| One-way delay sample (64) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+

where the first octet is the ACK message (0x02), followed by the
start chunk and the end chunk describing the chunk range, and the
one-way delay sample.

8.8. INTEGRITY

An INTEGRITY message (type 0x04) consists of a chunk specification
and the cryptographic hash for the specified chunk or node. The type
and format of the hash depends on the protocol options.

An INTEGRITY message using 32-bit chunk ranges as Chunk Addressing
method and a SHA-256 hash:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 1 0 0| Start chunk (32) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | End chunk (32) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Hash (256) ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+
where the first octet is the INTEGRITY message (0x04), followed by
the start chunk and the end chunk describing the chunk range, and the
hash.

8.9. SIGNED_INTEGRITY

A SIGNED_INTEGRITY message (type 0x07) consists of a chunk
specification, a 64-bit timestamp in NTP Timestamp format [RFC5905]
and a digital signature encoded as a Signature field would be in a
RRSIG record in DNSSEC without the BASE-64 encoding [RFC4034]. The
signature algorithm is defined by the Live Signature Algorithm
protocol option, see Section 7.7. The plaintext over which the
signature is taken depends on the content integrity protection method
used, see Section 6.1.

A SIGNED_INTEGRITY message using 32-bit chunk ranges as Chunk
Addressing method:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 1 1 1| Start chunk (32) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | End chunk (32) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Timestamp (64) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Signature ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

where the first octet is the SIGNED_INTEGRITY message (0x07),
followed by the start chunk and the end chunk describing the chunk
range, the timestamp, and the Signature.

The length of the digital signature can be derived from the Live
Signature Algorithm protocol option and the swarm ID as follows. The
first MANDATORY algorithms are RSASHA1 and RSASHA256. For those
algorithms, the swarm ID consists of a 1-byte Algorithm field
followed by a RSA public key stored as a tuple (exponent
length,exponent,modulus) [RFC3110]. Given the exponent length and
the length of the public key tuple in the swarm ID, the length of the
modulus in bytes can be calculated. This yields the length of the
signature as in RSA this is the length of the modulus [HAC01]. The
other MANDATORY algorithms are ECDSAP256SHA256 and ECDSAP384SHA384
[RFC6605]. For these algorithms the length of the digital signature
is 64 and 96 bytes, respectively.

8.10. REQUEST

A REQUEST message (type 0x08) consists of a chunk specification for
the chunks the requester wants to download.

A REQUEST message using 32-bit chunk ranges as Chunk Addressing
method:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 1 0 0 0| Start chunk (32) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | End chunk (32) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+

where the first octet is the REQUEST message (0x08), followed by the
start chunk and the end chunk describing the chunk range.

8.11. CANCEL

A CANCEL message (type 0x09) consists of a chunk specification for
the chunks the requester no longer is interested in.

A CANCEL message using 32-bit chunk ranges as Chunk Addressing
method:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 1 0 0 1| Start chunk (32) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | End chunk (32) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+

where the first octet is the CANCEL message (0x09), followed by the
start chunk and the end chunk describing the chunk range.

8.12. CHOKE and UNCHOKE

Both CHOKE and UNCHOKE messages (types 0x0a and 0x0b, respectively)
carry no payload.

A CHOKE message:

0
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|0 0 0 0 1 0 1 0|
+-+-+-+-+-+-+-+-+

where the first octet is the CHOKE message (0x0a).

An UNCHOKE message:

0
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|0 0 0 0 1 0 1 1|
+-+-+-+-+-+-+-+-+

where the first octet is the UNCHOKE message (0x0b).

8.13. PEX_REQ, PEX_RESv4, PEX_RESv6 and PEX_REScert

A PEX_REQ (0x06) message has no payload. A PEX_RESv4 (0x05) message
consists of an IPv4 address in big endian format followed by a UDP
port number in big endian format. A PEX_RESv6 (0x0c) message
contains a 128-bit IPv6 address instead of an IPv4 one. If a PEX_REQ
message does not originate from a private, unique-local, link-local
or multicast address [RFC1918][RFC4193][RFC4291], then the PEX_RES*
messages sent in reply MUST NOT contain such addresses. This is to
prevent leaking of internal addresses to external peers.

A PEX_REQ message:

0
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|0 0 0 0 0 1 1 0|
+-+-+-+-+-+-+-+-+

where the first octet is the PEX_REQ message (0x06).

A PEX_RESv4 message:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 1 0 1| IPv4 Address (32) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | Port (16) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

where the first octet is the PEX_RESv4 message (0x05), followed by
the IPv4 address and the port number.

A PEX_RESv6 message:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 1 1 0 0| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv6 Address (128) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | Port (16) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

where the first octet is the PEX_RESv6 message (0x0c), followed by
the IPv6 address and the port number.

A PEX_REScert (0x0d) message consists of a 16-bit integer in big
endian specifying the size of the membership certificate that
follows, see Section 13.2.1. This membership certificate states that
peer P at time T is a member of swarm S and is a X.509v3 certificate
[RFC5280] that is encoded using the ASN.1 distinguished encoding
rules (DER) [CCITT.X208.1988]. The certificate MUST contain a
"Subject Alternative Name" extension, marked as critical, of type
uniformResourceIdentifier.

A PEX_REScert message:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 1 1 0 1| Size of Memb. Cert. (16) | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Membership Certificate ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

where the first octet is the PEX_REScert message (0x0d), followed by
the size of the membership certificate, and the membership
certificate.

The URL contained in the name extension MUST follow the generic
syntax for URLs [RFC3986], where its scheme component is "file", the
host in the authority component is the DNS name or IP address of peer
P, the port in the authority component is the port of peer P, and the
path contains the swarm identifier for swarm S, in hexadecimal form.
In particular, the preferred form of the swarm identifier is
xxyyzz..., where the 'x's, 'y's and 'z's are 2 hexadecimal digits of
the 8-bit pieces of the identifier. The validity time of the
certificate is set with notBefore UTCTime set to T and notAfter
UTCTime set to T plus some expiry time defined by the issuer. An
example URL:

file://192.0.2.0:6778/e5a12c7ad2d8fab33c699d1e198d66f79fa610c3

8.14. KEEPALIVE

Keepalives do not have a message type on UDP. They are just simple
datagrams consisting of the 4-byte channel ID of the destination
only.

A keepalive datagram:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Channel ID (32) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

8.15. Flow and Congestion Control

Explicit flow control is not required for PPSPP-over-UDP. In the
case of video-on-demand, the receiver explicitly requests the content
from peers, and is therefore in control of how much data is coming
towards it. In the case of live streaming, where a push-model may be
used, the amount of data incoming is limited to the stream bitrate,
which the receiver must be able to process for a continuous playback.
Should, for any reason, the receiver get saturated with data, the
congestion control at the sender side will detect the situation and
adjust the sending rate accordingly.

PPSPP-over-UDP can support different congestion control algorithms.
At present, it uses the LEDBAT congestion control algorithm
[RFC6817]. LEDBAT is a delay-based congestion control algorithm that
is used everyday by millions of users as part of the uTP transmission
protocol of BitTorrent [LBT],[LCOMPL] and is suitable for P2P
streaming [PPSPPERF].

LEDBAT monitors the delay of the packets on the data path. It uses
the one-way delay variations to react early and limit the congestion
that the stream may induce in the network [RFC6817]. Using LEDBAT
enables PPSPP to serve the content to other interested peers after
the playback has finished (seeding), without disrupting the user.
After the playback, the user might move to different tasks that use
its network link, which are prioritized over PPSPP traffic. Hence
the user does not notice the background PPSPP traffic, which in turn
increases the chances of seeding the content for a longer period of
time.

The property of reacting early is not a problem in a peer-to-peer
system where multiple sources offer the content. Considering the
case of congestion near the sender, LEDBAT's early reaction impacts
the transmission of chunks to the receiver. However, for the
receiver it is actually beneficial to learn early that the
transmission from a particular source is impacted. The receiver can
then choose to download time-critical chunks from other sources
during its chunk picking phase.

If the bottleneck is near the receiver, the receiver is indeed
unlucky that transmissions from any source that runs through this
bottleneck will back off quite fast due to LEDBAT. For the rest of
the network (and the network operator), this is, however, beneficial
as the video streaming system will back off early enough and not
contribute too much to the congestion.

The power of LEDBAT is that its behaviour can be configured. In the
case of live streaming, a PPSPP deployer may want a more aggressive
behaviour to ensure quality of service. In that case, LEDBAT can be
configured to be more aggressive. In particular, LEDBAT's queuing
target delay value (TARGET in [RFC6817]) and other parameters can be
adjusted such that it acts as aggressive as TCP (or even more).
Hence LEDBAT is an algorithm that works for many scenarios in a peer-
to-peer context.

8.16. Example of Operation

We present a small example of communication between a leecher and a
seeder. The example presents the transmission of the file "Hello
World!", which fits within a 1024 byte chunk. For an easy
understanding we use the message description names, as listed in
Table 7, and the protocol option names as listed in Table 2, rather
than the actual binary value.

To do the handshake the initiating peer sends a datagram that MUST
start with an all 0-zeros channel ID (0x00000000), followed by a
HANDSHAKE message, whose payload is a locally unused, random channel
ID (in this case 0x00000001) and a list of protocol options. Channel
IDs MUST be randomly chosen, as described in Section 13.1.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| HANDSHAKE |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 1| Version |0 0 0 0 0 0 0 1| Min Version |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 1| Swarm ID |0 0 0 0 0 0 0 0 0 0 0 1 0 1 0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 1 0 0 0 1 1 1 1 0 1 0 0 0 0 0 0 0 0 1 0 0 1 1 1 1 1 0 0 1 1 0|
~ ..... ~
|1 0 0 0 0 1 1 0 1 0 1 0 1 0 1 0 1 0 1 1 0 0 0 0 0 0 1 1 1 0 1 1|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cont. Int. |0 0 0 0 0 0 0 1| Mer.H.Tree F. |0 0 0 0 0 0 1 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Chunk Add. |0 0 0 0 0 0 1 0| Chunk Size |0 0 0 0 0 0 0 0~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0| End |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The protocol options are:

Version: 1

Minimum supported Version: 1

Swarm Identifier: A 32-byte root hash (47a0...b03b) identifying
the content.

Content Integrity Protection Method: Merkle Hash Tree.

Merkle Tree Hash Function: SHA-256.

Chunk Addressing Method: 32-bit chunk ranges.

Chunk Size: 1024.

The receiving peer MAY respond, in which case the returned datagram
MUST consist of the channel ID from the sender's HANDSHAKE message
(0x00000001), a HANDSHAKE message, whose payload is a locally unused,
random channel ID (0x00000008) and a list of protocol options,
followed by any other messages it wants to send.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| HANDSHAKE |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 1 0 0 0| Version |0 0 0 0 0 0 0 1| Cont. Int. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 1| Mer.H.Tree F. |0 0 0 0 0 0 1 0| Chunk Add. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 1 0| Chunk Size |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0| End | HAVE |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|0 0 0 0 0 0 0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|0 0 0 0 0 0 0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

With the protocol options the receiving peer agrees on speaking
protocol version 1, on using the Merkle Hash Tree as Content
Integrity Protection Method, SHA-256 hash as Merkle Tree Hash
Function, 32-bit chunk ranges as Chunk Addressing Method, and Chunk
Size 1024. Furthermore, it sends a HAVE message within the same
datagram, announcing that it has locally available the first chunk of
content.

At this point, the initiator knows that the peer really responds; for
that purpose channel IDs MUST be random enough to prevent easy
guessing. So, the third datagram of a handshake MAY already contain
some heavy payload. To minimize the number of initialization round
trips, the first two datagrams MAY also contain some minor payload,
e.g. the HAVE message.

The initiating peer MAY send a request for the chunks of content it
wants to retrieve from the receiving peer, e.g. the first chunk
announced during the handshake. It always precedes the message with
the channel ID of the peer it is communicating with (e.g. 0x00000008
in our example), as described in Section 3.11. Furthermore, it MAY
add additional messages such as a PEX_REQ.

When receiving the third datagram, both peers have the proof they
really talk to each other; the three-way handshake is complete. The
receiving peer responds to the request by sending a DATA message
containing the requested content.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DATA |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 0|0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 0|0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 1 1 1 0 1 0 0 1|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 1 0 0 0 0 0 1 1 0 0 0 0 0 0 0 1 0 1 1 0 1 1 1 1 1 0 1 1 0 1 1|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 1 0 0 0 1 0 0|0 1 0 0 1 0 0 0 0 1 1 0 0 1 0 1 0 1 1 0 1 1 0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ..... ~
|0 1 1 0 1 1 0 0 0 1 1 0 0 1 0 0 0 0 1 0 0 0 0 1 0 0 0 0 1 0 1 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The DATA message consists of:

The 32-bit chunk range: 0,0 (the first chunk).

The timestamp value: 0004e94180b7db44

The Data message: 48656c6c6f20776f726c6421 (the "Hello world!"
file)

Note that the above datagram does not include the INTEGRITY message,
as the entire content can fit into a single message, hence the
initiating peer is able to verify it against the root hash. Also, in
this example the peer does not respond to the PEX_REQ as it does not
know any third peer participating in the swarm.

Upon receiving the requested data, the initiating peer responds with
an acknowledgement message for the first chunk, containing a one way
delay sample (100ms). Furthermore it also adds a HAVE message for
the chunk.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ACK |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 0|0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 0|0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 1 1 0 0 1 0 0| HAVE |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

At this point the initiating peer has successfully retrieved the
entire file. It then explicitly closes the connection by sending a
HANDSHAKE message that contains an all 0-zeros Source Channel ID.

9. Extensibility

9.1. Chunk Picking Algorithms

Chunk (or piece) picking entirely depends on the receiving peer. The
sender peer is made aware of preferred chunks by the means of REQUEST
messages. In some (live) scenarios it may be beneficial to allow the
sender to ignore those hints and send unrequested data.

The chunk picking algorithm is external to the PPSPP protocol and
will generally be a pluggable policy that uses the mechanisms
provided by PPSPP. The algorithm will handle the choices made by the
user consuming the content, such as seeking, switching audio tracks
or subtitles. Example policies for P2P streaming can be found in
[BITOS], and [EPLIVEPERF].

9.2. Reciprocity Algorithms

The role of reciprocity algorithms in peer-to-peer systems is to
promote client contribution and prevent freeriding. A peer is said
to be freeriding if it only downloads content but never uploads to
others. Examples of reciprocity algorithms are tit-for-tat as used
in BitTorrent [TIT4TAT] and Give-to-Get [GIVE2GET]. In PPSPP,
reciprocity enforcement is the sole responsibility of the sender
peer.

10. Acknowledgements

Arno Bakker, Riccardo Petrocco and Victor Grishchenko are partially
supported by the P2P-Next project (http://www.p2p-next.org/), a
research project supported by the European Community under its 7th
Framework Programme (grant agreement no. 216217). The views and
conclusions contained herein are those of the authors and should not
be interpreted as necessarily representing the official policies or
endorsements, either expressed or implied, of the P2P-Next project or
the European Commission.

The PPSPP protocol was designed by Victor Grishchenko at Technische
Universiteit Delft. Delft under supervision of Johan Pouwelse. The authors
would like to thank the following people for their contributions to
this draft: the chairs (Martin Stiemerling, Yunfei Zhang, Stefano
Previdi, Ning Zong) and members of the IETF PPSP working group, and
Mihai Capota, Raul Jimenez, Flutra Osmani, Johan Pouwelse, and Raynor Vliegendhart.

11. IANA Considerations

IANA is to create a new top-level registry called "Peer-to-Peer
Streaming Peer Protocol (PPSPP)", which will host the six new sub-
registries defined below for the extensibility of the protocol. For
all registries, assignments consist of a name and its associated
value. Also for all registries, the "Unassigned" ranges designated
are governed by the policy 'IETF Review' as described in [RFC5226].

11.1. PPSP Peer Protocol Message Type Registry

Registry name is "PPSP Peer Protocol Message Type Registry". Values
are integers in the range 0-255, with initial assignments and
reservations given in Table 7.

11.2. PPSP Peer Protocol Option Registry

Registry name is "PPSP Peer Protocol Option Registry". Values are
integers in the range 0-255, with initial assignments and
reservations given in Table 2.

11.3. PPSP Peer Protocol Version Number Registry

Registry name is "PPSP Peer Protocol Version Number Registry".
Values are integers in the range 0-255, with initial assignments and
reservations given in Table 3.

11.4. PPSP Peer Protocol Content Integrity Protection Method Registry

Registry name is "PPSP Peer Protocol Content Integrity Protection
Method Registry". Values are integers in the range 0-255, with
initial assignments and reservations given in Table 4.

11.5. PPSP Peer Protocol Merkle Hash Tree Function Registry

Registry name is "PPSP Peer Protocol Merkle Hash Tree Function
Registry". Values are integers in the range 0-255, with initial
assignments and reservations given in Table 5.

11.6. PPSP Peer Protocol Chunk Addressing Method Registry

Registry name is "PPSP Peer Protocol Chunk Addressing Method
Registry". Values are integers in the range 0-255, with initial
assignments and reservations given in Table 6.

12. Manageability Considerations

This section presents operations and management considerations
following the checklist in [RFC5706], Appendix A.

In this section "PPSPP client" is defined as a PPSPP peer acting on
behalf of an end user which may not yet have a copy of the content,
and "PPSPP server" as a PPSPP peer that provides the initial copies
of the content to the swarm on behalf of a content provider.

12.1. Operations

12.1.1. Installation and Initial Setup

A content provider wishing to use PPSPP to distribute content should
set up at least one PPSPP server. PPSPP servers need to have access
to either some static content or to some live audio/video sources.
To provide flexibility for implementors, this configuration process
is not standardized. The output of this process will be a list of
metadata records, one for each swarm. A metadata record consists of
the swarm ID, the chunk size used, the chunk addressing method used,
the content integrity protection method used, and the Merkle hash
tree function used (if applicable). If automatic content size
detection (see Section 5.6) is not used, the content length is also
part of the metadata record for static content. Note the swarm ID
already contains the Live Signature Algorithm used, in case of a live
stream.

In addition, a content provider should set up a tracking facility for
the content by configuring, for example, a PPSP tracker
[I-D.ietf-ppsp-base-tracker-protocol] or a Distributed Hash Table.
The output of the latter process is a list of transport addresses for
the tracking facility.

The list of metadata records of available content, and transport
address for the tracking facility, can be distributed to users in
various ways. Typically, they will be published on a Web site as
links. When a user clicks such a link the PPSPP client is launched,
either as a standalone application or by invoking the browser's
internal PPSPP protocol handler, as exemplified in Section 2. The
clients use the tracking facility to obtain the transport address of
the PPSPP server(s) and other peers from the swarm, executing the
peer protocol to retrieve and redistribute the content. The format
of the PPSPP URLs should be defined in an extension document. The
default protocol options should be exploited to keep the URLs small.

The minimal information a tracking facility must return when queried
for a list of peers for a swarm is as follows. Assuming the
communication between tracking facility and requester is protected,
the facility must at least return for each peer in the list its IP
address, transport protocol identifier (i.e., UDP), and transport
protocol port number.

12.1.2. Requirements on Other Protocols and Functional Components

When using the PPSP tracker protocol, PPSPP requires a specific
behavior from this protocol for security reasons, as detailed in
Section 13.2.

12.1.3. Migration Path

This document does not detail a migration path since there is no
previous standard protocol providing similar functionality.

12.1.4. Impact on Network Operation

PPSPP is a peer-to-peer protocol that takes advantage of the fact
that content is available from multiple sources to improve
robustness, scalability and performance. At the same time, poor
choices in determining which exact sources to use can lead to bad
experience for the end user and high costs for network operators.
Hence, PPSPP can benefit from the ALTO protocol to steer peer
selection, as described in Section 3.10.1.

12.1.5. Verifying Correct Operation

PPSPP is operating correctly when all peers obtain the desired
content on time. Therefore the PPSPP client is the ideal location to
verify the protocol's correct operation. However, it is not feasible
to mandate logging the behavior of PPSPP peers in all implementations
and deployments, for example, due to privacy reasons. There are two
alternative options:

o Monitoring the PPSPP servers initially providing the content,
using standard metrics such as bandwidth usage, peer connections
and activity, can help identify trouble, see next section and
[RFC2564].

o The PPSP tracker protocol may be used to gather information about
all peers in a swarm, to obtain a global view of operation,
according to [RFC6972] (requirement PPSP.OAM.REQ-3).

Basic operation of the protocol can be easily verified when a tracker
and swarm metadata are known by starting a PPSPP download. Deep
packet inspection for DATA and ACK messages help to establish that
actual content transfer is happening and that the chunk availability
signaling and integrity checking are working.

12.1.6. Configuration

Table 8 shows the PPSPP parameters, their defaults and where the
parameter is defined. For parameters that have no default, the table
row contains the word "var" and refers to the section discussing the
considerations to make when choosing a value.

Table 8: PPSPP Defaults

12.2. Management Considerations

The management considerations for PPSPP are very similar to other
protocols that are used for large-scale content distribution, in
particular HTTP. How does one manage large numbers of servers? How
does one push new content out to a server farm and allows staged
releases? How to detect faults and how to measure servers and end-
user performance? As standard solutions to these challenges are
still being developed, this section cannot provide a definitive
recommendation on how PPSPP should be managed. Hence, it describes
the standard solutions available at this time, and assumes a future
extension document will provide more complete guidelines.

12.2.1. Management Interoperability and Information

As just stated, PPSPP servers providing initial copies of the content
are akin to WWW and FTP servers. They can also be deployed in large
numbers and thus can benefit from standard management facilities.
PPSPP servers may therefore implement an SNMP management interface
based on the APPLICATION-MIB [RFC2564], where the file object can be
used to report on swarms.

What is missing is the ability to remove or rate limit specific PPSPP
swarms on a server. This corresponds to removing or limit specific
virtual servers on a Web server. In other words, as multiple pieces
of content (swarms, virtual WWW servers) are multiplexed onto a
single server process, more fine-grained management of that process
is required. This functionality is currently missing.

Logging is an important functionality for PPSPP servers and,
depending on the deployment, PPSPP clients. Logging should be done
via syslog [RFC5424].

12.2.2. Fault Management

The facilities for verifying correct operation and server management
(just discussed) appear sufficient for PPSPP fault monitoring. This
can be supplemented with host resource [RFC2790] and UDP/IP network
monitoring [RFC4113], as PPSPP server failures can generally be
attributed directly to conditions on the host or network.

Since PPSPP has been designed to work in a hostile environment, many
benign faults will be handled by the mechanisms used for managing
attacks. For example, when a malfunctioning peer starts sending the
wrong chunks, this is detected by the content integrity protection
mechanism and another source is sought.

12.2.3. Configuration Management

Large-scale deployments may benefit from a standard way of
replicating a new piece of content on a set of initial PPSPP servers.
This functionality may need to include controlled releasing, such
that content becomes available only at a specific point in time (e.g.
the release of a movie trailer). This functionality could be
provided via NETCONF [RFC6241], to enable atomic configuration
updates over a set of servers. Uploading the new content could be
one configuration change, making the content available for download
by the public another.

12.2.4. Accounting Management

Content providers may offer PPSPP hosting for different customers and
will want to bill these customers, for example, based on bandwidth
usage. This situation is a common accounting scenario, similar to
billing per virtual server for Web servers. PPSPP can therefore
benefit from general standardization efforts in this area [RFC2975]
when they come to fruition.

12.2.5. Performance Management

Depending on the deployment scenarios, the application performance
measurement facilities of [RFC3729] and associated [RFC4150] can be
used with PPSPP.

In addition, when the PPSPP tracker protocol is used, it provides a
built-in, application-level, performance measurement infrastructure
for different metrics. See [RFC6972] (requirement PPSP.OAM.REQ-3).

12.2.6. Security Management

Malicious peers should ideally be locked out long-term. This is
primarily for performance reasons, as the protocol is robust against
attacks (see next section). Section 13.7 describes a procedure for
long-term exclusion.

13. Security Considerations

As any other network protocol, the PPSPP faces a common set of
security challenges. An implementation must consider the possibility
of buffer overruns, DoS attacks and manipulation (i.e. reflection
attacks). Any guarantee of privacy seems unlikely, as the user is
exposing its IP address to the peers. A probable exception is the
case of the user being hidden behind a public NAT or proxy. This
section discusses the protocol's security considerations in detail.

13.1. Security of the Handshake Procedure

Borrowing from the analysis in [RFC5971], the PPSP peer protocol may
be attacked with 3 types of denial-of-service attacks:

1. DOS amplification attack: attackers try to use a PPSPP peer to
generate more traffic to a victim.

2. DOS flood attack: attackers try to deny service to other peers by
allocating lots of state at a PPSPP peer.

3. Disrupt service to an individual peer: attackers send bogus e.g.
REQUEST and HAVE messages appearing to come from victim peer A to
the peers B1..Bn serving that peer. This causes A to receive
chunks it did not request or to not receive the chunks it
requested.

The basic scheme to protect against these attacks is the use of a
secure handshake procedure. In the UDP encapsulation the handshake
procedure is secured by the use of randomly chosen channel IDs as
follows. The channel IDs must be generated following the
requirements in [RFC4960] (Sec. 5.1.3).

When UDP is used, all datagrams carrying PPSPP messages are prefixed
with a 4-byte channel ID. These channel IDs are random numbers,
established during the handshake phase as follows. Peer A initiates
an exchange with peer B by sending a datagram containing a HANDSHAKE
message prefixed with the channel ID consisting of all 0s. Peer A's
HANDSHAKE contains a randomly chosen channel ID, chanA:

A->B: chan0 + HANDSHAKE(chanA) + ...

When peer B receives this datagram, it creates some state for peer A,
that at least contains the channel ID chanA. Next, peer B sends a
response to A, consisting of a datagram containing a HANDSHAKE
message prefixed with the chanA channel ID. Peer B's HANDSHAKE
contains a randomly chosen channel ID, chanB.

B->A: chanA + HANDSHAKE(chanB) + ...

Peer A now knows that peer B really responds, as it echoed chanA. So
the next datagram that A sends may already contain heavy payload,
i.e., a chunk. This next datagram to B will be prefixed with the
chanB channel ID. When B receives this datagram, both peers have the
proof they are really talking to each other, the three-way handshake
is complete. In other words, the randomly chosen channel IDs act as
tags (cf. [RFC4960] (Sec. 5.1)).

A->B: chanB + HAVE + DATA + ...

13.1.1. Protection Against Attack 1

In short, PPSPP does a so-called return routability check before
heavy payload is sent. This means that attack 1 is fended off: PPSPP
does not send back much more data than it received, unless it knows
it is talking to a live peer. Attackers sending a spoofed HANDSHAKE
to B pretending to be A now need to intercept the message from B to A
to get B to send heavy payload, and ensure that that heavy payload
goes to the victim, something assumed too hard to be a practical
attack.

Note the rule is that no heavy payload may be sent until the third
datagram. This has implications for PPSPP implementations that use
chunk addressing schemes that are verbose. If a PPSPP implementation
uses large bitmaps to convey chunk availability these may not be sent
by peer B in the second datagram.

13.1.2. Protection Against Attack 2

On receiving the first datagram peer B will record some state about
peer A. At present this state consists of the chanA channel ID, and
the results of processing the other messages in the first datagram.
In particular, if A included some HAVE messages, B may add a chunk
availability map to A's state. In addition, B may request some
chunks from A in the second datagram, and B will maintain state about
these outgoing requests.

So presently, PPSPP is somewhat vulnerable to attack 2. An attacker
could send many datagrams with HANDSHAKEs and HAVEs and thus allocate
state at the PPSPP peer. Therefore peer A MUST respond immediately
to the second datagram, if it is still interested in peer B.

The reason for using this slightly vulnerable three-way handshake
instead of the safer handshake procedure of SCTP [RFC4960] (Sec. 5.1)
is quicker response time for the user. In the SCTP procedure, peer A
and B cannot request chunks until datagrams 3 and 4 respectively, as
opposed to 2 and 1 in the proposed procedure. This means that the
user has to wait shorter in PPSPP between starting the video stream
and seeing the first images.

13.1.3. Protection Against Attack 3

In general, channel IDs serve to authenticate a peer. Hence, to
attack, a malicious peer T would need to be able to eavesdrop on
conversations between victim A and a benign peer B to obtain the
channel ID B assigned to A, chanB. Furthermore, attacker T would
need to be able to spoof e.g. REQUEST and HAVE messages from A to
cause B to send heavy DATA messages to A, or prevent B from sending
them, respectively.

The capability to eavesdrop is not common, so the protection afforded
by channel IDs will be sufficient in most cases. If not, point-to-
point encryption of traffic should be used, see below.

13.2. Secure Peer Address Exchange

As described in Section 3.10, a peer A can send Peer-Exchange
messages PEX_RES to a peer B, which contain the IP address and port
of other peers that are supposedly also in the current swarm. The
strength of this mechanism is that it allows decentralized tracking:
after an initial bootstrap no central tracker is needed anymore. The
vulnerability of this mechanism (and DHTs) is that malicious peers
can use it for an Amplification attack.

In particular, a malicious peer T could send PEX_RES messages to
well-behaved peer A with addresses of peers B1,B2,...,BN and on
receipt, peer A could send a HANDSHAKE to all these peers. So in the
worst case, a single datagram results in N datagrams. The actual
damage depends on A's behavior. E.g. when A already has sufficient
connections it may not connect to the offered ones at all, but if it
is a fresh peer it may connect to all directly.

In addition, PEX can be used in Eclipse attacks [ECLIPSE] where
malicious peers try to isolate a particular peer such that it only
interacts with malicious peers. Let us distinguish two specific
attacks:

E1. Malicious peers try to eclipse the single injector in live
streaming.

E2. Malicious peers try to eclipse a specific consumer peer.

Attack E1 has the most impact on the system as it would disrupt all
peers.

13.2.1. Protection against the Amplification Attack

If peer addresses are relatively stable, strong protection against
the attack can be provided by using public key cryptography and
certification. In particular, a PEX_REScert message will carry
swarm-membership certificates rather than IP address and port. A
membership certificate for peer B states that peer B at address
(ipB,portB) is part of swarm S at time T and is cryptographically
signed. The receiver A can check the certificate for a valid
signature, the right swarm and liveliness and only then consider
contacting B. These swarm-membership certificates correspond to
signed node descriptors in secure decentralized peer sampling
services [SPS].

Several designs are possible for the security environment for these
membership certificates. That is, there are different designs
possible for who signs the membership certificates and how public
keys are distributed. As an example, we describe a design where the
PPSP tracker acts as certification authority.

13.2.2. Example: Tracker as Certification Authority

A peer A wanting to join swarm S sends a certificate request message
to a tracker X for that swarm. Upon receipt, the tracker creates a
membership certificate from the request with swarm ID S, a timestamp
T and the external IP and port it received the message from, signed
with the tracker's private key. This certificate is returned to A.

Peer A then includes this certificate when it sends a PEX_REScert to
peer B. Receiver B verifies it against the tracker public key. This
tracker public key should be part of the swarm's metadata, which B
received from a trusted source. Subsequently, peer B can send the
member certificate of A to other peers in PEX_REScert messages.

Peer A can send the certification request when it first contacts the
tracker, or at a later time. Furthermore, the responses the tracker
sends could contain membership certificates instead of plain
addresses, such that they can be gossiped securely as well.

We assume the tracker is protected against attacks and does a return
routability check. The latter ensures that malicious peers cannot
obtain a certificate for a random host, just for hosts where they can
eavesdrop on incoming traffic.

The load generated on the tracker depends on churn and the lifetime
of a certificate. Certificates can be fairly long lived, given that
the main goal of the membership certificates is to prevent that
malicious peer T can cause good peer A to contact *random* hosts.
The freshness of the timestamp just adds extra protection in addition
to achieving that goal. It protects against malicious hosts causing
a good peer A to contact hosts that previously participated in the
swarm.

The membership certificate mechanism itself can be used for a kind of
amplification attack against good peers. Malicious peer T can cause
peer A to spend some CPU to verify the signatures on the membership
certificates that T sends. To counter this, A SHOULD check a few of
the certificates sent and discard the rest if they are defective.

The same membership certificates described above can be registered in
a Distributed Hash Table that has been secured against the well-known
DHT specific attacks [SECDHTS].

Note that this scheme does not work for peers behind a symmetric
Network Address Translator, but neither does normal tracker
registration.

13.2.3. Protection Against Eclipse Attacks

Before we can discuss Eclipse attacks we first need to establish the
security properties of the central tracker. A tracker is vulnerable
to Amplification attacks too. A malicious peer T could register a
victim B with the tracker, and many peers joining the swarm will
contact B. Trackers can also be used in Eclipse attacks. If many
malicious peers register themselves at the tracker, the percentage of
bad peers in the returned address list may become high. Leaving the
protection of the tracker to the PPSP tracker protocol specification,
we assume for the following discussion that it returns a true random
sample of the actual swarm membership (achieved via Sybil attack
protection). This means that if 50% of the peers is bad, you'll
still get 50% good addresses from the tracker.

Attack E1 on PEX can be fended off by letting live injectors disable
PEX. Or at least, let live injectors ensure that part of their
connections are to peers whose addresses came from the trusted
tracker.

The same measures defend against attack E2 on PEX. They can also be
employed dynamically. When the current set of peers B that peer A is
connected to doesn't provide good quality of service, A can contact
the tracker to find new candidates.

13.3. Support for Closed Swarms ([RFC6972] PPSP.SEC.REQ-1)

The Closed Swarms [CLOSED] and Enhanced Closed Swarms [ECS]
mechanisms provide swarm-level access control. The basic idea is
that a peer cannot download from another peer unless it shows a
Proof-of-Access. Enhanced Closed Swarms improve on the original
Closed Swarms by adding on-the-wire encryption against man-in-the-
middle attacks and more flexible access control rules.

The exact mapping of ECS to PPSPP is defined in
[I-D.gabrijelcic-ppsp-ecs].

13.4. Confidentiality of Streamed Content ([RFC6972] PPSP.SEC.REQ-1)

No extra mechanism is needed to support confidentiality in PPSPP. A
content publisher wishing confidentiality should just distribute
content in cyphertext / DRM-ed format. In that case it is assumed a
higher layer handles key management out-of-band. Alternatively, pure
point-to-point encryption of content and traffic can be provided by
the proposed Closed Swarms access control mechanism, or by DTLS
[RFC6347] or IPsec [RFC4301].

When transmitting over DTLS, PPSPP can obtain the PMTU estimate
maintained by the IP layer to determine how much payload can be put
in a single datagram without fragmentation ([RFC6347], Sec. 4.1.1.1).
If PMTU changes and the chunk size becomes too large to fit into a
single datagram, PPSPP can choose to allow fragmentation by clearing
the DF-bit. Alternatively, the content publisher can decide to use
smaller chunks and transmit multiple in the same datagram when the
MTU allows.

13.5. Strength of the Hash Function for Merkle Hash Trees

Implementations MUST support SHA-1 as the hash function for content
integrity protection via Merkle Hash trees. SHA-1 may be preferred
over stronger hash functions by content providers because it reduces
on-the-wire overhead. As such it presents a trade-off between
performance and security. The security considerations for SHA-1 are
discussed in [RFC6194].

In general, note that the hash function is used in a hash tree, which
makes it more complex to create collisions. In particular, if
attackers manage to find a collision for a hash it can replace just
one chunk, so the impact is limited. If fixed sized chunks are used,
the collision even has to be of the same size as the original chunk.
For hashes higher up in the hash tree, a collision must be a
concatenation of two hashes. In sum, finding collisions that fit
with the hash tree are generally harder to find than regular
collisions.

13.6. Limit Potential Damage and Resource Exhaustion by Bad or Broken
Peers ([RFC6972] PPSP.SEC.REQ-2)

In this section an analysis is given of the potential damage a
malicious peer can do with each message in the protocol, and how it
is prevented by the protocol (implementation).

13.6.1. HANDSHAKE

o Secured against DoS amplification attacks as described in
Section 13.1.

o Threat HS.1: An Eclipse attack where peers T1..Tn fill all
connection slots of A by initiating the connection to A.

Solution: Peer A must not let other peers fill all its available
connection slots, i.e., A must initiate connections itself too, to
prevent isolation.

13.6.2. HAVE

o Threat HAVE.1: Malicious peer T can claim to have content which it
hasn't. Subsequently T won't respond to requests.

Solution: peer A will consider T to be a slow peer and not ask it
again.

o Threat HAVE.2: Malicious peer T can claim not to have content.
Hence it won't contribute.

Solution: Peer and chunk selection algorithms external to the
protocol will implement fairness and provide sharing incentives.

13.6.3. DATA

o Threat DATA.1: peer T sending bogus chunks.

Solution: The content integrity protection schemes defend against
this.

o Threat DATA.2: peer T sends peer A unrequested chunks.

To protect against this threat we need network-level DoS
prevention.

13.6.4. ACK

o Threat ACK.1: peer T acknowledges wrong chunks.

Solution: peer A will detect inconsistencies with the data it sent
to T.

o Threat ACK.2: peer T modifies timestamp in ACK to peer A used for
time-based congestion control.

Solution: In theory, by decreasing the timestamp peer T could fake
there is no congestion when in fact there is, causing A to send
more data than it should. [RFC6817] does not list this as a
security consideration. Possibly this attack can be detected by
the large resulting asymmetry between round-trip time and measured
one-way delay.

13.6.5. INTEGRITY and SIGNED_INTEGRITY

o Threat INTEGRITY.1: An amplification attack where peer T sends
bogus INTEGRITY or SIGNED_INTEGRITY messages, causing peer A to
checks hashes or signatures, thus spending CPU unnecessarily.

Solution: If the hashes/signatures don't check out A will stop
asking T because of the atomic datagram principle and the content
integrity protection. Subsequent unsolicited traffic from T will
be ignored.

o Threat INTEGRITY.2: An attack where peer T sends old
SIGNED_INTEGRITY messages in the Unified Merkle Tree scheme,
trying to make peer A tune in at a past point in the live stream.

Solution: The timestamp in the SIGNED_INTEGRITY message protects
against such replays. Subsequent traffic from T will be ignored.

13.6.6. REQUEST

o Threat REQUEST.1: peer T could request lots from A, leaving A
without resources for others.

Solution: A limit is imposed on the upload capacity a single peer
can consume, for example, by using an upload bandwidth scheduler
that takes into account the need of multiple peers. A natural
upper limit of this upload quotum is the bitrate of the content,
taking into account that this may be variable.

13.6.7. CANCEL

o Threat CANCEL.1: peer T sends CANCEL messages for content it never
requested to peer A.

Solution: peer A will detect the inconsistency of the messages and
ignore them. Note that CANCEL messages may be received
unexpectedly when a transport is used where REQUEST messages may
be lost or reordered with respect to the subsequent CANCELs.

13.6.8. CHOKE

o Threat CHOKE.1: peer T sends REQUEST messages after peer A sent B
a CHOKE message.

Solution: peer A will just discard the unwanted REQUESTs and
resend the CHOKE, assuming it got lost.

13.6.9. UNCHOKE

o Threat UNCHOKE.1: peer T sends an UNCHOKE message to peer A
without having sent a CHOKE message before.

Solution: peer A can easily detect this violation of protocol
state, and ignore it. Note this can also happen due to loss of a
CHOKE message sent by a benign peer.

o Threat UNCHOKE.2: peer T sends an UNCHOKE message to peer A, but
subsequently does not respond to its REQUESTs.

Solution: peer A will consider T to be a slow peer and not ask it
again.

13.6.10. PEX_RES

o Secured against amplification and Eclipse attacks as described in
Section 13.2.

13.6.11. Unsolicited Messages in General

o Threat: peer T could send a spoofed PEX_REQ or REQUEST from peer B
to peer A, causing A to send a PEX_RES/DATA to B.

Solution: the message from peer T won't be accepted unless T does
a handshake first, in which case the reply goes to T, not victim
B.

13.7. Exclude Bad or Broken Peers ([RFC6972] PPSP.SEC.REQ-2)

A receiving peer can detect malicious or faulty senders as just
described, which it can then subsequently ignore. However, excluding
such a bad peer from the system completely is complex. Random
monitoring by trusted peers that would blacklist bad peers as
described in [DETMAL] is one option. This mechanism does require
extra capacity to run such trusted peers, which must be
indistinguishable from regular peers, and requires a solution for the
timely distribution of this blacklist to peers in a scalable manner.

14. References

14.1. Normative References
[CCITT.X208.1988]
International International Telephone and Telegraph
Consultative Committee, "Specification of Abstract Syntax
Notation One (ASN.1)", CCITT Recommendation X.208,
November 1988.

[FIPS180-4]
Information Technology Laboratory, National Institute of
Standards and Technology, "Federal Information Processing
Standards: Secure Hash Standard (SHS)", Publication 180-4,
Mar 2012.

[IANADNSSECALGNUM]
IANA, "Domain Name System Security (DNSSEC) Algorithm
Numbers", Mar 2014,
<http://www.iana.org/assignments/dns-sec-alg-numbers>.

[RFC1918] Rekhter, Y., Moskowitz, R., B., Karrenberg, D., J. de Groot,
G., and E. Lear, "Address Allocation for Private
Internets", BCP 5, RFC 1918, February 1996. 1996,
<http://www.rfc-editor.org/info/rfc1918>.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. 1997,
<http://www.rfc-editor.org/info/rfc2119>.

[RFC3110] Eastlake, Eastlake 3rd, D., "RSA/SHA-1 SIGs and RSA KEYs in the
Domain Name System (DNS)", RFC 3110, May 2001. 2001,
<http://www.rfc-editor.org/info/rfc3110>.

[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66, RFC
3986, January 2005. 2005,
<http://www.rfc-editor.org/info/rfc3986>.

[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Resource Records for the DNS Security Extensions",
RFC 4034, March 2005. 2005,
<http://www.rfc-editor.org/info/rfc4034>.

[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006. 2006,
<http://www.rfc-editor.org/info/rfc4291>.

[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. 2008,
<http://www.rfc-editor.org/info/rfc5280>.

[RFC5702] Jansen, J., "Use of SHA-2 Algorithms with RSA in DNSKEY
and RRSIG Resource Records for DNSSEC", RFC 5702, October
2009.
2009, <http://www.rfc-editor.org/info/rfc5702>.

[RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
"Network Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, June 2010. 2010,
<http://www.rfc-editor.org/info/rfc5905>.

[RFC6605] Hoffman, P. and W. Wijngaards, "Elliptic Curve Digital
Signature Algorithm (DSA) for DNSSEC", RFC 6605, April
2012.
2012, <http://www.rfc-editor.org/info/rfc6605>.

[RFC6817] Shalunov, S., Hazel, G., Iyengar, J., and M. Kuehlewind,
"Low Extra Delay Background Transport (LEDBAT)", RFC 6817,
December 2012. 2012, <http://www.rfc-editor.org/info/rfc6817>.

14.2. Informative References

[ABMRKL] Bakker, A., "Merkle hash torrent extension", BitTorrent
Enhancement Proposal 30, Mar 2009,
<http://bittorrent.org/beps/bep_0030.html>.

[BINMAP] Grishchenko, V. and J. Pouwelse, "Binmaps: hybridizing
bitmaps and binary trees", Technical Report PDS-2011-005,
Parallel and Distributed Systems Group, Fac. of Electrical
Engineering, Mathematics, and Computer Science, Delft
University of Technology, The Netherlands, Apr 2009.

[BITOS] Vlavianos, A., Iliofotou, M., Mathieu, F., and M.
Faloutsos, "BiToS: Enhancing BitTorrent for Supporting
Streaming Applications", IEEE INFOCOM Global Internet
Symposium Barcelona, Spain, Apr 2006.

[BITTORRENT]
Cohen, B., "The BitTorrent Protocol Specification",
BitTorrent Enhancement Proposal 3, Feb 2008,
<http://bittorrent.org/beps/bep_0003.html>.

[CLOSED] Borch, N., Mitchell, K., Arntzen, I., and D. Gabrijelcic,
"Access Control to BitTorrent Swarms Using Closed Swarms",
ACM workshop on Advanced Video Streaming Techniques for
Peer-to-Peer Networks and Social Networking (AVSTP2P '10),
Florence, Italy, Oct 2010,
<http://doi.acm.org/10.1145/1877891.1877898>.

[DETMAL] Shetty, S., Galdames, P., Tavanapong, W., and Ying. Cai,
"Detecting Malicious Peers in Overlay Multicast
Streaming", IEEE Conference on Local Computer Networks
(LCN'06). Tampa, FL, USA, Nov 2006.

[ECLIPSE] Sit, E. and R. Morris, "Security Considerations for Peer-
to-Peer Distributed Hash Tables", IPTPS '01: Revised
Papers from the First International Workshop on Peer-to-
Peer Systems pp. 261-269, Springer-Verlag, 2002.

[ECS] Jovanovikj, V., Gabrijelcic, D., and T. Klobucar, "Access
Control in BitTorrent P2P Networks Using the Enhanced
Closed Swarms Protocol", International Conference on
Emerging Security Information, Systems and Technologies
(SECURWARE 2011), pp. 97-102, Nice, France, Aug 2011.

[EPLIVEPERF]
Bonald, T., Massoulie, L., Mathieu, F., Perino, D., and A.
Twigg, "Epidemic Live Streaming: Optimal Performance
Trade-offs", Proceedings of the 2008 ACM SIGMETRICS
International Conference on Measurement and Modeling of
Computer Systems Annapolis, MD, USA, Jun 2008.

[GIVE2GET]
Mol, J., Pouwelse, J., Meulpolder, M., Epema, D., and H.
Sips, "Give-to-Get: Free-riding Resilient Video-on-demand
in P2P Systems", Proceedings Multimedia Computing and
Networking conference (Proceedings of SPIE Vol. 6818) San
Jose, California, USA, Jan 2008.

[HAC01] Menezes, A., van Oorschot, P., and S. Vanstone, "Handbook
of Applied Cryptography", CRC Press, (Fifth Printing,
August 2001), Oct 1996.

[I-D.gabrijelcic-ppsp-ecs]
Gabrijelcic, D., "Enhanced Closed Swarm protocol", draft-
ppsp-gabrijelcic-ecs (work in progress), November 2012.

[I-D.ietf-alto-protocol]
Alimi, R., Penno, R., and Y. Yang, "ALTO Protocol", draft-
ietf-alto-protocol-27 (work in progress), March 2014.

[I-D.ietf-ppsp-base-tracker-protocol]
Cruz, R., Nunes, M., Yingjie, G., Xia, J., Taveira, J.,
and D. Lingli, "PPSP Tracker Protocol-Base Protocol (PPSP-
TP/1.0)", draft-ietf-ppsp-base-tracker-protocol-06 (work
in progress), October 2014.

[JIM11] Jimenez, R., Osmani, F., and B. Knutsson, "Sub-Second
Lookups on a Large-Scale Kademlia-Based Overlay", IEEE
International Conference on Peer-to-Peer Computing
(P2P'11), Kyoto, Japan, Aug 2011.

[LBT] Rossi, D., Testa, C., Valenti, S., and L. Muscariello,
"LEDBAT: the new BitTorrent congestion control protocol",
Computer Communications and Networks (ICCCN), Zurich,
Switzerland, Aug 2010.

[LCOMPL] Testa, C. and D. Rossi, "On the impact of uTP on
BitTorrent completion time", IEEE International Conference
on Peer-to-Peer Computing (P2P'11), Kyoto, Japan, Aug
2011.

[MERKLE] Merkle, R., "Secrecy, Authentication, and Public Key
Systems", Ph.D. thesis Dept. of Electrical Engineering,
Stanford University, CA, USA, pp 40-45, 1979.

[P2PWIKI] Bakker, A., Petrocco, R., Dale, M., Gerber, J.,
Grishchenko, V., Rabaioli, D., and J. Pouwelse, "Online
video using BitTorrent and HTML5 applied to Wikipedia",
IEEE International Conference on Peer-to-Peer Computing
(P2P'10), Delft, The Netherlands, Aug 2010.

[POLLIVE] Dhungel, P., Hei, Xiaojun., Ross, K., and N. Saxena,
"Pollution in P2P Live Video Streaming", International
Journal of Computer Networks & Communications (IJCNC)
Vol.1, No.2, Jul 2009.

[PPSPPERF]
Petrocco, R., Pouwelse, J., and D. Epema, "Performance
analysis of the Libswift P2P streaming protocol", IEEE
International Conference on Peer-to-Peer Computing
(P2P'12), Tarragona, Spain, Sep 2012.

[RFC2564] Kalbfleisch, C., Krupczak, C., Presuhn, R., and J.
Saperia, "Application Management MIB", RFC 2564, May 1999. 1999,
<http://www.rfc-editor.org/info/rfc2564>.

[RFC2790] Waldbusser, S. and P. Grillo, "Host Resources MIB", RFC
2790, March 2000. 2000,
<http://www.rfc-editor.org/info/rfc2790>.

[RFC2975] Aboba, B., Arkko, J., and D. Harrington, "Introduction to
Accounting Management", RFC 2975, October 2000. 2000,
<http://www.rfc-editor.org/info/rfc2975>.

[RFC3365] Schiller, J., "Strong Security Requirements for Internet
Engineering Task Force Standard Protocols", BCP 61, RFC
3365, August 2002. 2002,
<http://www.rfc-editor.org/info/rfc3365>.

[RFC3729] Waldbusser, S., "Application Performance Measurement MIB",
RFC 3729, March 2004. 2004,
<http://www.rfc-editor.org/info/rfc3729>.

[RFC4113] Fenner, B. and J. Flick, "Management Information Base for
the User Datagram Protocol (UDP)", RFC 4113, June 2005. 2005,
<http://www.rfc-editor.org/info/rfc4113>.

[RFC4150] Dietz, R. and R. Cole, "Transport Performance Metrics
MIB", RFC 4150, August 2005. 2005,
<http://www.rfc-editor.org/info/rfc4150>.

[RFC4193] Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
Addresses", RFC 4193, October 2005. 2005,
<http://www.rfc-editor.org/info/rfc4193>.

[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005. 2005,
<http://www.rfc-editor.org/info/rfc4301>.

[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821, March 2007. 2007,
<http://www.rfc-editor.org/info/rfc4821>.

[RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol",
RFC 4960, September 2007. 2007,
<http://www.rfc-editor.org/info/rfc4960>.

[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008. 2008, <http://www.rfc-editor.org/info/rfc5226>.

[RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
"Session Traversal Utilities for NAT (STUN)", RFC 5389,
October 2008. 2008, <http://www.rfc-editor.org/info/rfc5389>.

[RFC5424] Gerhards, R., "The Syslog Protocol", RFC 5424, March 2009. 2009,
<http://www.rfc-editor.org/info/rfc5424>.

[RFC5706] Harrington, D., "Guidelines for Considering Operations and
Management of New Protocols and Protocol Extensions", RFC
5706, November 2009. 2009,
<http://www.rfc-editor.org/info/rfc5706>.

[RFC5971] Schulzrinne, H. and R. Hancock, "GIST: General Internet
Signalling Transport", RFC 5971, October 2010. 2010,
<http://www.rfc-editor.org/info/rfc5971>.

[RFC6194] Polk, T., Chen, L., Turner, S., and P. Hoffman, "Security
Considerations for the SHA-0 and SHA-1 Message-Digest
Algorithms", RFC 6194, March 2011. 2011,
<http://www.rfc-editor.org/info/rfc6194>.

[RFC6241] Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
and A. Bierman, Ed., "Network Configuration Protocol
(NETCONF)", RFC 6241, June 2011. 2011,
<http://www.rfc-editor.org/info/rfc6241>.

[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, January 2012. 2012,
<http://www.rfc-editor.org/info/rfc6347>.

[RFC6709] Carpenter, B., Aboba, B., Ed., and S. Cheshire, "Design
Considerations for Protocol Extensions", RFC 6709,
September 2012. 2012, <http://www.rfc-editor.org/info/rfc6709>.

[RFC6972] Zhang, Y. and N. Zong, "Problem Statement and Requirements
of the Peer-to-Peer Streaming Protocol (PPSP)", RFC 6972,
July 2013. 2013, <http://www.rfc-editor.org/info/rfc6972>.

[SECDHTS] Urdaneta, G., Pierre, G., and M. van Steen, "A Survey of
DHT Security Techniques", ACM Computing Surveys vol.
43(2), Jun 2011.

[SIGMCAST]
Wong, C. and S. Lam, "Digital Signatures for Flows and
Multicasts", IEEE/ACM Transactions on Networking 7(4), pp.
502-513, 1999.

[SPS] Jesi, G., Montresor, A., and M. van Steen, "Secure Peer
Sampling", Computer Networks vol. 54(12), pp. 2086-2098,
Elsevier, Aug 2010.

[SWIFTIMPL]
Grishchenko, V., Paananen, J., Pronchenkov, A., Bakker,
A., and R. Petrocco, "Swift reference implementation",
2014,
2015, <https://github.com/libswift/libswift>.

[TIT4TAT] Cohen, B., "Incentives Build Robustness in BitTorrent",
1st Workshop on Economics of Peer-to-Peer Systems,
Berkeley, CA, USA, Jun 2003.

Appendix A. Revision History

-00 2011-12-19 Initial version.

-01 2012-01-30 Minor text revision:

* Changed heading to "A. Bakker"

* Changed title to *Peer* Protocol, and abbreviation PPSPP.

* Replaced swift with PPSPP.

* Removed Sec. 6.4. "HTTP (as PPSP)".

* Renamed Sec. 8.4. to "Chunk Picking Algorithms".

* Resolved Ticket #3: Removed sentence about random set of
peers.

* Resolved Ticket #6: Added clarification to "Chunk Picking
Algorithms" section.

* Resolved Ticket #11: Added Sec. 3.12 on Storage Independence

* Resolved Ticket #14: Added clarification to "Automatic Size
Detection" section.

* Resolved Ticket #15: Operation section now states it shows
example behaviour for a specific set of policies and schemes.

* Resolved Ticket #30: Explained why multiple REQUESTs in one
datagram.

* Resolved Ticket #31: Renamed PEX_ADD message to PEX_RES.

* Resolved Ticket #32: Renamed Sec 3.8. to "Keep Alive
Signaling", and updated explanation.

* Resolved Ticket #33: Explained NAT hole punching via only
PPSPP messages.

* Resolved Ticket #34: Added section about limited overhead of
the Merkle hash tree scheme.

-02 2012-04-17 Major revision

* Allow different chunk addressing and content integrity
protection schemes (ticket #13):

* Added chunk ID, chunk specification, chunk addressing scheme,
etc. to terminology.

* Created new Sections 4 and 5 discussing chunk addressing and
content integrity protection schemes, respectively and moved
relevant sections on bin numbering and Merkle hash trees
there.

* Renamed Section 4 to "Merkle Hash Trees and The Automatic
Detection of Content Size".

* Reformulated automatic size detection in terms of nodes, not
bins.

* Extended HANDSHAKE message to carry protocol options and
created Section 8 on Protocol options. VERSION and
MSGTYPE_RCVD messages replaced with protocol options.

* Renamed HASH message to INTEGRITY.

* Renamed HINT to REQUEST.

* Added description of chunk addressing via (start,end) ranges.

* Resolved Ticket #26: Extended "Security Considerations" with
section on the handshake procedure.

* Resolved Ticket #17: Defined recently as "in last 60 seconds"
in PEX.

* Resolved Ticket #20: Extended "Security Considerations" with
design to make Peer Address Exchange more secure.

* Resolved Ticket #38+39 / PPSP.SEC.REQ-2+3: Extended "Security
Considerations" with a section on confidentiality of content.

* Resolved Ticket #40+42 / PPSP.SEC.REQ-4+6: Extended "Security
Considerations" with a per-message analysis of threats and how
PPSPP is protected from them.

* Progressed Ticket #41 / PPSP.SEC.REQ-5: Extended "Security
Considerations" with a section on possible ways of excluding
bad or broken peers from the system.

* Moved Rationale to Appendix.

* Resolved Ticket #43: Updated Live Streaming section to include
"Sign All" content authentication, and reference to [SIGMCAST]
following discussion with Fabio Picconi.

* Resolved Ticket #12: Added a CANCEL message to cancel REQUESTs
for the same data that were sent to multiple peers at the same
time in time-critical situations.

-03 2012-10-22 Major revision

* Updated Abstract and Introduction, removing download case.

* Resolved Ticket #4: Added explicit CHOKE/UNCHOKE messages.

* Removed directory lists unused in streaming.

* Resolved Ticket #22, #23, #28: Failure behaviour, error codes
and dealing with peer crashes.

* Resolved Ticket #13: Chunk ranges are the default chunk
addressing scheme that all peers MUST support.

* Added a section on compatibility between chunk addressing
schemes.

* Expanded the explanation of Unified Merkle Trees as a method
for content integrity protection for live streams.

* Added a section on forgetting chunks in live streaming.

* Added "End" option to protocol options and corrected bugs in
UDP encapsulation, following Karl Knutsson's comments.

* Added SHA-2 support for Merkle Hash functions.

* Added content integrity protection methods for live streaming
to the relevant protocol option.

* Added a Live Signature Algorithm protocol option.

* Resolved Ticket #24+27: The choice for UDP + LEDBAT as
transport has now been reflected in the draft. TCP and RTP
encapsulations have been removed.

* Superfluous parts of Section 10 on extensibility have been
removed.

* Removed appendix with Rationale.

* Resolved Ticket #21+25: PPSPP currently uses LEDBAT and the
DATA and ACK messages now contain the time fields it requires.
Should other congestion control algorithms be supported in the
future, a protocol option will be added.

-04 2012-11-07 Minor revision

* Corrected typos.

* Added empty protocol option list when HANDSHAKE is used for
explicitly closing a channel in the UDP encapsulation.

* Corrected definition of a range chunk specification to be a
single (start,end) pair. To send multiple disjunct ranges
multiple messages should be used.

* Clarified that in a range chunk specification the end is
inclusive. I.e., [start,end] not [start,end)
* Added PEX_REScert message to carry a membership certificate.
Renamed PEX_RES to PEX_RESv4.

* Added a guideline about private and link-local addresses in
PEX_RES messages.

* Defined the format of the public key that is used as swarm ID
in live streaming.

* Clarified that a HANDSHAKE message must be the first message
in a datagram.

* Clarified sending INTEGRITY messages ahead in a separate
datagram if not all necessary hashes that still need to be
sent and the chunk fit into a single datagram. Defined an
order for the INTEGRITY messages.

* Clarified rare case of sending multiple DATA messages in one
datagram.

* Clarified UDP datagrams carrying PPSPP should adhere to the
network's MTU to avoid IP fragmentation.

* Defined value for version protocol option.

* Added small clarifications and corrected typos.

* Extended versioning scheme to Min/max versioning scheme
defined in [RFC6709], Section 4.1, following Riccardo
Bernardini's suggestion.

* Processed comments on unclear phrasing from Riccardo
Bernardini.

* Added a guideline on when to declare a peer dead.

* Made sure all essential references are listed as Normative
references following RFC3967.

-05 2013-01-23 Minor revision

* Corrected category to Standards Track.

* Clarified that swarm identifier is a required protocol option
in an initiating HANDSHAKE in the UDP encapsulation.

* Added IANA considerations and tablised name spaces for
registry definition.

-06 2013-02-11 Minor revision

* Updated "Overall Operation" to have more context (HTML5
video).

* Clarified wording on PEX_REQ.

* Clarified wording on SIGNED_INTEGRITY.

* Added a reference on how ALTO can be used with PPSPP.

* Added Manageability Consideration section following RFC5706.

* Clarified that implementations SHOULD implement the "Unified
Merkle Tree" content integrity protection method for live, and
MAY implement "Sign All".

* Made SHA1 hash function mandatory-to-implement as Merkle Tree
Hash function and explained the security considerations.

* Made RSA/SHA1 mandatory-to-implement as Live Signature
Algorithm for integrity protection while live streaming.

* Clarified that implementations MUST implement addressing via
32-bit chunk ranges.

* Made LEDBAT an Informational reference to prevent a so-called
"down ref".

* Updated reference to PPSP problem statement and requirements
document.

* Used kibibyte unit in formal sections.

-07 2013-06-19 Revision following AD Review

Quoting the AD review by Martin Stiemerling: ***High-level
issues:

1) Merkle Hash Trees I have found the document very confusing
on whether Merkle Hash Trees (MHTs) and the for the MHT
required bin numbering scheme are now optional or mandatory.
Parts of the draft make the impression that either of them or
both or optional (mainly in the beginning of the document),
while Section 5 and later Sections are relying heavily on
MHTs. My naive reading of the current draft is that you could
rely on start-end ranges for chunk addressing and MHTs for
content protection. However, I do know that this combination
is not working. If MHTs are really optional, including the
bin numbering, the document should really state this and make
clear what the operations of the protocol are with the
mandatory to implement (MTI) mechanisms. The MHT, bins, and
all the protocol handling should go in an appendix. There is
a call to make for the WG: I do know that MHTs were considered
by some as burden and they have called for a leaner way, i.e.,
the start-end ranges. The call for the leaner way has been
implemented in the document but not fully.

+ The text now states that MHTs SHOULD be used unless in
benign environments and are mandatory-to-implement. It
also states that only start-end chunk range is mandatory-
to-implement, and bins are optional.

2) LEDBAT as congestion control vs. PPSPP The PPSP peer
protocol is intended for the Standards Track and relies in a
normative manner on LEDBAT (RFC 6817). LEDBAT as such is an
**experimental** delay-based congestion control algorithm. A
Standards Track protocol cannot normatively rely on an
Experimental congestion control mechanism (or RFC in general).
There are ways out of this situation: i) Do not use ledbat:
this would call for another congestion control mechanism to be
described in the PPSPP draft. ii) Work on an 'upgrade' of the
LEDBAT specification to Standards Track: Possible, but a very
long way. iii) Agree on having PPSPP also as Experimental
protocol. I'm currently leaning towards option iii), but this
is my pure personal opinion as an individual in the IETF.

+ A new paragraph has been added to Section 8.15 describing
the widespread use of LEDBAT in current P2P systems.
Hence, aim is a DOWNREF procedure.

3) No formal protocol message definition Section 7 and more
specific Section 8 describe the protocol syntax of the
protocol options and the messages, though Section 8 is talking
about UDP encapsulation. Section 7 is hard to digest if
someone should implement the options, see also later, but
Section 8 is almost impossible to understand by somebody who
has not been involved in the PPSP working group. See also
further down for a more detailed review of the sections. To
give an example out of Section 8.4: This section describes the
HANDSHAKE message and gives examples how such a HANDSHAKE
message could look like. But no formal definition of the
message is given leaving a number of thins unclear, such as
what the local channel number and what's the remote channel
number is. This is implicitly defined, but that is not a good
way of writing Standards Track drafts.

+ We added the usual bit-based ASCII art representations.

4) Implicit use of default values There are a number of places
all over the draft where default values are defined. Many of
those default values are used when there are no values
explicitly signaled, e.g., the default chunk size of 1 Kbyte
in Section 8.4 or Section Section 7.5. with the default for
the Content Integrity Protection Method. I have the feeling
that the protocol and the surroundings (e.g., what comes in
via the 'tracker') are over-optimized, e.g., always providing
the Content Integrity Protection Method as part of the
Protocol options will not waste more than 2 bytes in a
HANDSHAKE message. Further, I do not see the need to define a
default chunk size in the base protocol specification, as this
default can look very different, depending on who is deploying
the protocol and in what context. This calls for a more
dynamic way of handling the system chunk size, either as part
of an external mechanisms (e.g. via the tracker) or in the
HANDSHAKE message.

+ Removed implicit defaults from protocol options. Chunk
size is part of the content's metadata and thus
configurable. The default 1KiB has been turned into a
recommendation.

5) Concept of channels The concept of channels is good but it
is introduced too late in the draft, namely in Section 8.3,
and it is introduced with very few words. Why isn't this
introduced as part of Section 2 or Section 3, also in the
relationship to the used transport protocol? I.e., the
intention is to keep only one transport 'connection' between
two distinct peers and to allow to run multiple swarm
instances at the same time over the same transport. And how
do swarms and channels correlate?

+ Concept now introduced in Section 3 with a figure.

***Technicals:

- Section 2.1, 2nd paragraph, about the tracker: I haven't
seen a single place where the interaction with a tracker is
discussed or where the tracker less operation is discussed in
contrast. It is further unclear what type of information is
really required from a tracker. A tracker (or a resource
directory) would need to provide more then IP address & port,
e.g., the used transport protocol for the protocol exchange
(given that other transports are allowed), used chunk size,
chunk addressing scheme, etc
+ Interaction with tracking facilities in general is
discussed in the Operations and Management section,
Section 12.1.1. This also discusses swarm metadata and
information required from tracking facility. Decentralized
tracking in PPSPP is discussed in Section 3.10.1.

- Section 2.3, the 1st paragraph, 'close-channel': This has
been the first time where I stumbled over the channel without
knowing the concept.

+ Rephrased.

- Section 3.1: ordering of messages The 1st sentence implies
that ordering of messages in a datagram matters a lot. This
is outlined later in the document, but I would add this as
part of 3., i.e., the messages are processed in the strict
order or something along this line.

+ Phrase added.

- Section 3.1, 1st paragraph, options to include I would not
say anything about 'SHOULD include options' here, as this is
anyhow described in Section 8.

+ Phrase removed.

- Section 3.1, 2nd paragraph: "Datagrams exchanged MAY also
contain some minor payload, e.g. HAVE messages to indicate
the current progress of a peer or a REQUEST (see
Section 3.7)." to be added, just to make it clear IMHO: ", but
MUST NOT include any DATA message".

+ Added.

- Section 3.2, 2nd paragraph: "In particular, whenever a
receiving peer has successfully checked the integrity of a
chunk or interval of chunks it MUST send a HAVE message to all
peers it wants to interact with in the near future." This
looks like a place where a lot of traffic can be send out of a
peer, i.e., whenever a chunk arrives a HAVE message must be
sent. I don't believe that this should be mandated by the
protocol specification, but there should guidance on when to
send this, e.g., peers might be also able to wait for a short
period of time to gather more chunks to be reported in HAVE.
Or should in this case a single UDP datagram contain multiple
HAVEs?
+ Clarified that this is indeed controlled by a policy
outside the peer protocol that can decide to piggyback onto
other traffic or wait till multiple chunks are verified.

- Section 3.4 on ACKs This section looks pretty weak, as ACKs
may be sent but on the other hand MUST be sent if ledbat is
used. I would simply say: - ACK MUST be sent if an unreliable
transport protocol is used - ACK MAY be sent if a reliable
transport protocol is used - keep clarification about ledbat.

+ DONE.

- Section 3.5: Give text where INTEGERITY is described at
least for the MTI scheme.

+ DONE.

- Section 3.7, 2nd paragraph - all 'MAY' are actually not
right here. Please remove or replace them with lower letters
if appropriate. - It is not clear what the 'sequentially'
means exactly. Is it in the received order?

+ Rephrased MAYs. "Sequentially" replaced with "received
order".

- Section 3.8: Please replace 'MAY' by can, as those are not
normative behaviors but more the fact that peers can, for
instance, request urgent data.

+ DONE.

- Section 3.9 Same comment as for the Section 3.8 just above
this comment.

+ DONE.

- Section 3.9 waiting for responses OLD " When peer B receives
a CHOKE message from A it MUST NOT send new REQUEST messages
and SHOULD NOT expect answers to any outstanding ones." NEW "
When peer B receives a CHOKE message from A it MUST NOT send
new REQUEST messages and it cannot expect answers to any
outstanding ones, as the transfer of chunks is choked."

+ DONE.

- Section 3.10.2 This whole section about PEX hole punching
reads very, very experimental. The STUN method is ok, but PEX
isn't. First of all, the safe behavior for a peer when it
receives unsolicited PEX messages, is to discard those
messages. Second, this unsolicited PEX messages trigger some
behavior which may open an attack vector. The best way, but
this needs more discussion, is to include to some token in the
messages that are exchanged in order to make avoid any blind
attacks here. However, this will need more and detailed
discussions of the purpose of this.

+ We moved parts of the security analysis of PEX up, such
that all mechanisms are explained in the main text, and the
analysis of what attacks there are and how these mechanisms
prevent them is in the Sec. Considerations section.

+ The section about hole punching was removed, lacking a
reference to the experiments we conducted with this exact
variant of the mechanism.

- Section 3.11 I don't see the 'MUST send keep-alive' as a
mandatory requirement, as peers might have good reasons not to
send any keep alive. Why not saying 'A peer can send a keep-
alive' and it 'MUST use the simple datagram...' as already
described. Though there is also no really need to say MUST.

+ Now Section 3.12. Rephrased and clarified the reason and
consequences of sending keep-alive msgs.

- Section 4 The syntax definition for each of the chunk
addressing schemes is missing. This is not suitable for any
specification that aims at interoperable implementations.

+ We added the usual bit-based ASCII art representations.

- Section 4.3.2 PPSPP peers MUST use the ACK message if an
unreliable transport protocol is used.

+ DONE.

- Section 4.4 Has been tested in an implementation? I would
like to understand the need for such a section, as in my
understanding a peer implementation should chose one scheme
and support this and there shouldn't be the need to convert
between the different schemes.

+ Yes, the reference implementation translates from chunk
ranges on the wire to bins internally. However, for
simplicity we now state that all peers in a swarm MUST use
the same method and the compatibility section has been
removed.

- Section 5 This reads that MHTs are mandatory to implement
while the document makes the impression that MHTs are
optional.

+ Rephrased, see High-level issues.

- Section 5.3 " so each datagram SHOULD be processed
separately and a loss of one datagram MUST NOT disrupt the
flow" The MUST NOT is not a protocol specification
requirement, but more an informative part saying that a lost
message shouldn't impact the protocol machinery, but it can
impact the overall operation. What is the flow here in that
sentence?

+ Rephrased.

- Section 5.6.2. An illustrative example explaining how the
automatic size detection works is required here.

+ Added a paragraph with an example that follows the figure
used during the explanation. A state diagram could also be
added, but might be a bit redundant.

- Section 6.1, 4th paragraph: Where do I find the 1 byte
algorithm field in the swarm ID? The swarm ID is not really
defined in a single place.

+ Expanded. Added a formal definition.

- Section 7.3 The described min/max versioning relies on the
fact that there are major and minor version numbers. I cannot
find any major and minor version number scheme in the draft.

+ Actually, it does not. There is a single unstructured
version number.

- Section 7.4, Length field It is not clear what the 'Length'
field is referring to. Further, it is not clear of the swam
IDs are concatenated in one swarm ID option, of each swarm ID
must be placed in a separate swam ID option.

+ Clarified.

- Section 7.6 MHTs are mandatory to support though MHTs are
optional?

+ Clarified.

- Section 7.7 'key size ... derived from the swarm ID'. This
relates to my high level comment no 4. on the use of implicit
information. Either it is clearly specified how this
information is derived or there is a protocol field/
information about the size.

+ Key size derivation procedure added to description of
SIGNED_INTEGRITY in UDP encapsulation.

- Section 7.8 I would recommend to say that the default MUST
be supported, but the peer must always signal what method it
is supporting or at least using.

+ Corrected, see High-level issues 4.)

- Section 7.10 I have not understood how the 'Lenght' field
relates to the message bitmap and how long the message bitmap
can grow. The figure looks like a maximum of 16 bits?

+ Clarified.

- Section 8 I do not see the value of the text in the preface
of Section 8. I would say that this text should say what is
mandatory and what's not, i.e., MUST use UDP and MUST use
LEDBAT. Potentially saying that future protocol versions can
also run over other transport protocols.

+ Adjusted.

- Section 8.1 about Maximum Transfer Unit (MTU) The text is
discussing that a Ethernet can carry 1500 bytes. This is
true, but the Ethernet payload is not the normative MTU across
all of the Internet. For IPv6 the min MTU is 1280 bytes and
for IPv4 it is 576 bytes, though for IPv4 it can be
theoretically much lower at 64 bytes. It would move the
definition of the default chunk size to a recommendation with
text saying that this size has a high likelihood to travel
end-to-end in the Internet without any fragmentation.
Fragmentation might increase the loss of complete chunks, as
one lost fragment will cause the loss of a complete chunk.
One way of getting an informed decision on whether chunks can
travel in their size is to use the Don't Fragment (DF) bit in
IPv4 and also to watch for ICMP error messages. However, ICMP
error messages are not a reliable indication, but they can be
some indication.

+ 1 KiB chunk size has been made a recommendation.

+ Added a small paragraph discussing the optional integration
of MTU path discovery.

- Section 8.1 Definition of the default chunk size There is no
need to define a default chunk size, if the chunk size would
be always signaled per swarm. This is another default/
implicit value places that is unnecessary.

+ The chunk size is always part of the content's metadata.

- Section 8.3: see also my comment no 3. The concept of
channels is introduced very late and with few words. A figure
to explain the concept will help a lot and also more formal
text on what a channel is and how they are identified. Also
what the init channel is.

+ Concept now introduced in Section 3.11.

- Section 8 in general: There is no formal definition of the
messages, just bit pattern examples.

+ We added the usual bit-based ASCII art representations.

- Section 8.4 (as example for the other Sections in 8.x): i)
What is the '(CHANNEL' paramter? Is it actually a parameter?
ii) it is implicit that the first channel no (0000000) is the
remote peer's channel and that the second channel no
(00000011) is the local peer's channel, right? This isn't
clear from the text, but my guess.

+ We added the usual bit-based ASCII art representations.

- Section 8.5 Can HAVE messages multiple bin specs in one
message or do I have to make a HAVE message for each bin?

+ Clarified.

- Section 8.6 What is the formal defintion of a DATA message?
That's completely missing or I have not understood it.

+ We added the usual bit-based ASCII art representations.

- Section 8.7 looks just underspecified, especially as this is
the link to LEDBAT.

+ Implementors will unfortunately need to read the full
LEDBAT specification.

- Section 8.11 How are the chunks specified here? The formal
syntax definition or reference to one is missing.

We added the usual bit-based ASCII art representations.

- Section 8.13 I'm lost on this section, as I haven't fully
understood the concept of the PEX in this document.
Especially not why there is the PEX_REScert.

+ We moved parts of the security analysis of PEX up into
3.10, such that all mechanisms are explained in the main
text, and the analysis of what attacks there are and how
these mechanisms prevent them is in the Sec. Considerations
section.

- Section 11 The RFC required for protocol extensions of a
standards track protocol looks odd. This must be at least
IETF Review or Standards Action.

+ Policy changed to "IETF Review" and the section was
extended with information about data types and required
information.

***Editorials:

- Abstract (and probably also other places), 1st sentence of,
PPSPP is not a transport protocol, just a protocol

+ DONE.

- Section 1.1, 4th paragraph: I would remove the reference to
rmcat, as it is not yet clear what the outcome of the rmcat wg
will be

+ DONE.

- Section 1.3, on page 8, about seeding/leeching: I would
break it in to sub-bullets.

+ DONE.

- Section 2.1 and following: These are examples, isn'it? If
so, this should be mentioned or clarified.

+ DONE. All subsections now labeled "Example:".

- Section 2.1: What is the PPSP Url?
+ Reformulated in terms of "Imagine there is a PPSP URL".

- Section 2.3, the 1st paragraph, detection of dead peers: It
would be good to say where this detection is described in the
remainder of the draft. Just for completeness.

+ DONE. Dead peer detection is now a separate section and
referenced here.

- Section 2.2, the very last paragraph, 'Peer A MAY also':
This 'MAY' is not useful here. I would just write 'Peer A can
also', as there is nothing normative described here.

+ DONE.

- Section 3.2, last paragraph: What is the latter confinement?
This is not clear to me.

+ Rephrased.

- Section 3.9, last sentence I am not sure to what the
reference to Section 3.7 is pointing in this respect.

+ Rephrased.

- Section 3.10.1 about PEX messages The text says 'PPSPP
optionally features...'. I have not understood if this
optionally refers to mandatory to implement but optionally to
use, or if the PEX messages are optionally to implement.

+ Made it clear that is OPTIONAL and not mandatory-to-
implement.

- Section 3.12 I'm not sure what this section is telling
exactly. Isn't just saying that PPSPP as such does not care
how chunks are stored locally, as this is implementation
dependent?

+ Yes. Removed.

- Section 4.2, page 15, 1st paragraph: OLD 'A PPSPP peer MAY
support' NEW 'The support for this scheme is OPTIONAL'

+ DONE, for byte ranges as well.

- Section 6.1.1 This section is not describing sign-all, but
rather a justification why it may still work. This doesn't
help at all.

- Section 7, 1st paragraph Why is there a reference to RFC
2132?

+ Removed, just similarity in format.

- Section 7 in general i) It is common to give bit positions
in the figures where the syntax of options is described. This
allows to count how many bits are used for a protocol field
more easily and also way more reliable. ii) Please add also
Figure labels to the syntax definitions of the options. This
makes it easier to reference them later on if needed.

- Section 8.1 1 kibibyte is 1 kbyte?

+ Mentioned base 1024 in Terminology. Changed to 1024 bytes
where appropriate.

- Section 8.2, last paragraph i ) "All messages are
idempotent" in what respect? ii) "or recognizable as
duplicates" but how are the recognized as duplicates?

+ Idempotent means that processing a message twice does not
lead to a different state than processing them once.
Resent handshakes can be recognized as duplicates because a
peer already recorded the first connection attempt in its
state. Updated text.

- Section 8.5, last sentence in brackets: What is this last
sentence about?

+ Was explanation of the on-the-wire bytes shown.

- Section 8.13 " If sender of the PEX_REQ message does not
have a private or link-local address, then the PEX_RES*
messages MUST NOT contain such addresses [RFC1918][RFC4291]."
What is this text saying? Do not include what you do not have
anyway?

+ Rephrased. It tries to say that internal addresses must
not be leaked to external peers.

- Section 8.14 There is no single place where all the
constants are collected and also documented what the default
values or the recommended values. For instance in this
Section 8.14 where the dead peer time out is set to 3 minutes
and also the number of datagrams that should have sent. I
would make a section or subsection to discuss dead peers and
how they are detected and just link to the keep-alive
mechanism in Section 8.14.

+ The Section 12.1.6 section was rewritten for this in the
Ops & Mgmt part.

- Section 11 This section needs to be overhauled once the
document is ready for the IESG. The section is not wrong but
can be improved to help IANA.

+ The section was extended with information about data types
and required information.

-08 2013-08-8 Continued Revision following AD Review

Please see the -07 entry for our responses to the comments.

Added ECDSAP256SHA256 and ECDSAP384SHA384 as mandatory-to-
implement live signature algorithms, as they provide small
swarm IDs.

Added line that a peer SHOULD NOT send HAVEs to peers that
already have the complete content (e.g. in video-on-demand
scenarios).

In response to a remark at WG meeting at IETF 87 we added a
paragraph on OPTIONAL MTU discovery using PPSPP messages to
Section 8.1.

-09 2014-04-4 Nits fixed

Nits about e.g. newer references fixed.

-10 2014-06-17 DOWNREF restored

Reference to LEDBAT was not in Normative references as it
should have been.

-11 2014-06-18 IANA not OK

In the 2nd Last Call IANA posed two questions:

" QUESTION: Are the authors intended to have one single top-
level registry to host these six new registries defined in
this draft? Please see http://www.iana.org/assignments/ancp
as an example, that the ANCP registry hosts multiple sub-
registries."
+ Yes. We updated the IANA Considerations section with IANA's
Pearl Liang proposed text to request this.

"QUESTION: Section 11.3 specifies that the values are integers
in the range 0-255. However value 0 is not included in the
above table. Section 7.2 (Version) does not clearly explain
value 0."

+ 0 defined as reserved.

Text cleanup

+ Terminology: states chunks may be variable size explicitly
also here.

+ Figure 3 label improved.

+ 5.4 Explicitly state that leaves have tree height 0.

+ 5.5 Repaired split paragraph.

+ 5.6 Rephrased start to be consistent with minimally
required metadata.

+ 5.6.1 Removed remark about peak hashes role in static/live
download unification as that is no longer the case.

+ 5.6.2 Explicitly stated that peak hashes are transported in
INTEGRITY messages.

+ 6.1.2.1 Changed "computing the computed" to "comparing the
computed"

+ 6.2 Changed sentence to "*is* related to bitrate"

+ 8.4 Explicitly stated that the *Source* Channel ID is all
0-zeros in closing HANDSHAKE.

+ 8.5 Removed spurious line.

+ 8.6 Explicitly stated that the chunk specification in a
DATA message denotes a single chunk.

+ 8.8 Changed copy+paste from ACK to INTEGRITY message.

+ 8.12 Renamed PEX_RES to PEX_RESv4 where needed.

+ 8.17 Explicitly stated that the *Source* Channel ID is all
0-zeros in closing HANDSHAKE.

+ 12.1.5 Changed to read that a swarm's operation can easily
verified when swarm metadata and tracker info is available.

+ 13.2.1 "cert" -> "certificate"

+ Added refs to RFC6972 where required.

==============================================================
=========== IESG telechat DISCUSSES ==========================
===============================================

**Alissa Cooper:**

"I'm a little surprised about the choice of LEDBAT for
congestion control of live streams. It seems like LEDBAT is
not what the receiver would want the sender to use for live-
streamed content, because if a bottleneck is encountered on
the path, the live stream will yield early, and the
recipient's perception of quality will degrade. If the
bottleneck is near the recipient, then every sender sending
chunks will yield early, and there may be no senders available
to stream at an acceptable level of quality. I'm assuming the
WG discussed this -- it would be helpful to understand why a
more aggressive congestion control was not selected for live
streaming."

+ Updated 8.16 to discuss why LEDBAT is good in a peer-to-
peer context (can be friendly to network as a whole and has
configurable aggressiveness)

--------------------------------------------------------------

**Stephen Farrell:**

"(1) 3.10: What is a "benign" environment? I actually do
understand what is meant, but how could a program evaluate
that in order to decicde whether or not to send a PES_RESv4?
You then refer to a "potentially hostile environment" which
could presumably be anywhere, so are you really saying that
PES_REScert is the "right thing" to do, but you know it won't
be done so these are weasel words around that awkward fact?
(Apologies if I'm wrong on that, but that's the impression I
got when reading this, but maybe that's just my paranoia:-)"
+ Rephrased to show PEX_REScert is the only option on the
Internet.

"(2) 6.1.2.2: What exactly are the "munro" bytes that are the
first input to the signature? Where are those defined?
(Sorry if I missed/skipped over that;-)"

+ Added to Terminology and added an explicit reference in
this section.

"(3) 7.6 and 13.5: SHA1 as the MTI is wrong. Why is that ok,
given the collision resistance is less that designed for? 7.7
also calls for SHA256 being implemented in any case. The run-
time argument in 13.5 does not convince me. Attacks only ever
get worse, so the collision resistance property which this
protocol needs ought lead to selection of an as-far-as-known
good hash function. Today that means SHA256 and not SHA1."

+ SHA-256 is now the default. SHA-1 is still MTI to give
content providers a trade-off between performance and
security, as the on-the-wire overhead is 37.5% smaller.

"(4) 7.7: Why RSASHA1 and not RSA with SHA256?"

+ RSA256 is now also MTI, but RSASHA1 is also required, as
argued in the previous point.

"(5) 7.10: The message number is wrong in the figure."

+ Fixed.

"(6) 8.4: I don't see the swarm's metadata record in the ascii
art diagram and you just say "look at section 7" so two
questions: a) where is the "chunk size used" option in section
7? and b) do all the swarm metadata options have to be sent
each time with no limit on ordering except as given in section
7 (which had one such order sensitive limit I think)?"

+ (a) We once envisioned that a peer could start with just a
swarm ID+chunk size as metadata and obtain all protocol
options (chunk addressing, integrity protection, etc.) from
a peer. As this turned out to be too complex to secure
(peers may lie about the options), we decided to make the
options all part of the swarm metadata after the AD review.
This renders the protocol options in the HANDSHAKE to an
end-to-end test really. Chunk size was never part of that
negotiation because writing code that would handle bad
input on that parameter was definitely too complicated.
Chunk size has now been added as a protocol option.

+ (b) The HANDSHAKE message and hence protocol options are
sent only in the first datagram. After that this
information is part of the context of the channel that has
been established. We added a limit on ordering (sort on
code value, ascending) as a simplification.

"(7) 8.13: Don't you need to register the ppsp URI scheme? In
case its useful, which I doubt, if you have code: RFC6920 URIs
could be used for this if you wanted and would save you adding
ppsp to the IANA URI scheme registry (and having to deal with
the URI police:-)"

+ Not sure. The problem is we need to denote a peer in a
swarm here. This means we need to encode the swarm ID and
the peer address. IMHO, this means we cannot use the ni:
RFC6920 scheme here, because only the hash determines
identity. If we would encode the swarm ID there (SHA-256
hash of the swarm ID), we need a place for the peer address
and the authority part does not make the URL unique. The
ni: URL will still only identify the swarm. Encoding the
peer address in OtherNames instead of
uniformResourceIdentifier is troublesome too. We could
find no single object type to denote a transport address
(IP+port) that supports both IPv4 and IPv6 (udpDomain is
IPv4 only). Using SAN ipAddress for the address and a
separate OtherName for the port number (e.g.
udpEndpointLocalPort from [RFC4113]) is not ideal, as the
port number by itself is not a name for the subject.
Hence, we replaced the ppsp: scheme with the file: scheme,
which has an authority part where we can naturally encode
the peer address in.

"(8) 13.4: Wouldn't DTLS change the chunk size considerations
and also influence how messages map to datagrams? Isn't more
specification needed to say how to really use DTLS here? Just
saying "use DTLS or IPsec or higher layer crypto" doesn't
really seem sufficient. And doing the DTLS bits right
shouldn't be very hard either."

+ According to RFC6347, for "DTLS over UDP, the upper layer
protocol SHOULD be allowed to obtain the PMTU estimate
maintained in the IP layer" (Sec. 4.1.1.1). So we know
beforehand how much payload we can send in a datagram
without fragmentation. If PMTU changes and the chunk size
becomes too large, we can choose to allow fragmentation
("the upper layer protocol SHOULD be allowed to set the
state of the DF bit (in IPv4) or prohibit local
fragmentation (in IPv6)."). Alternatively, the content
publisher can decide to use smaller chunks and transmit
multiple in the same datagram when the MTU allows. We
added an explanatory paragraph to Section 13.4.

**Other DISCUSSes: TODO.**

-12 2014-11-16 Other DISCUSSes and reviews

==============================================================
=========== IESG telechat DISCUSSES (continued) ==============
===========================================================

**Richard Barnes:**

"My DISCUSS here is based mainly on the readability of the
document, which seems bad enough to be an impediment to
interoperability. As far as I can tell, this document does
not define a protocol, in the sense of a set of actions
required to achieve a given objective. Instead, it presents a
pile of piece parts with a couple of combinations, and notes
that these combinations could be used to achieve, e.g., live
streaming. (In the language of patents, it has not been
"reduced to practice".) What are the steps an implementation
follows to join a swarm? To connect to a new peer and request
chunks? The pieces seem to be here, but the big picture is
completely absent."

+ Clarified the DISCUSS via email. In response, we made the
distinction between tracker protocol and peer protocol more
clear in the Introduction and Section 2.

+ We also rewrote the section on the HANDSHAKE message to
include an explicit handshake procedure in the format
suggested.

--------------------------------------------------------------

**Kathleen Moriarty:**

"I am still reading this draft, but don't see any response to
the SecDir review that raised some very important points for
discussion: http://www.ietf.org/mail-
archive/web/secdir/current/msg04879.html I'll amend this when
I get further into my review and would appreciate a response
to the SecDir review."
+ Please see our responses to the SecDir review via email:
http://www.ietf.org/mail-archive/web/secdir/current/
msg04914.html and below.

--------------------------------------------------------------

**OpsDir review** by Tina TSOU (replied to by email, July 9th,
2014):

"You probably want to mention SHA-256 rater than SHA-1 [...]"

+ SHA-256 made default. See reply to Farrell.

"Section 8.13: You should also include ULAs"

+ Added.

"Section 8.16: This doesn't seem like a good justification for
not having flow control. Could you please elaborate on why
flow control is not needed for this case?"

+ Explained in email reply.

"Section 8.17, page 53: The channel ID values employed might
give the reader the impression that they are non-random."

+ Stated explicitly they must be random with a reference to
why.

Nits

+ Processed

--------------------------------------------------------------

**GenArt review** by Christer Holmberg (replied to by email,
July 9th, 2014)

"Q1: The sending of keep alives is a SHOULD, and there are no
procedures on how to act if keep alives are not received.
There isn't even a mechanism to negotiate the sending of keep
alives.

+ Rewrote Section 3.12 to explain what happens when no keep
alives are received. In particular, given certain
conditions the peer is declared dead and no more messages
are sent to it, and the local administration about that
peer is discarded. The exact conditions were specified in
Sec. 8.15 but are now defined in 3.12 at Christer's
suggestion in his follow-up to our reply. Section 8.15
removed.

"Q2: As the sending of keep alives is a SHOULD, are there
example cases when keep alives would NOT be sent?"

+ Added example of busy server garbage collecting idle
clients by not sending keep alives.

"Q3: The text saying "to each peer it wants to interact with
in the future" sounds a little strange to me. How does a peer
know with whom it wants to interact in the future? Perhaps
the text instead should talk about peers with whom one wants
to maintain a signaling channel, or something like that?"

+ Rephrased in Sec. 3.12 to "interesting peers" and explain
there is a policy that determines which peers are of
interest. E.g. peers that have chunks that the downloader
is still missing.

--------------------------------------------------------------

**SecDir review** by David Harrington (replied to by email,
July 10th, 2014)

Editorials:

+ Incorporated.

"6) tech: I feel uncomfortable with section 2 containing
examples that describe the overall flow. Examples are non-
normative text, usually contained in a non-normative appendix.
These examples describe the order of messages, and it is "

+ Please see our actions taken on Richard Barnes' DISCUSS.

"7) in example 2.2, the integrity hash is provided by the peer
that is providing the (potentially maliciously modified)
content. Isn't that like asking the fox to verify that the
henhouse is safe?"

+ Added that the hashes can be verified against the trusted
swarm ID using the Merkle tree content integrity protection
scheme, defined later in the document.

"9) in 3, paragraph 1, it says "this behavior", but I'm not
sure which behavior it is referencing. It is unclear whether
not sending error messages, or discarding messages, or
stopping communication, or classifying peers is the behavior
that allows a peer to deal with slow, crashed, or silent
peers. I don't understand HOW any of the behaviors mentioned
would allow a peer to deal with slow, crashed, or silent
peers. I do not understand on what basis peers are judged
"good" or "bad"."

+ Added explanation how in a peer-to-peer system with
multiple sources to obtain the content from, the
classification in good and bad can be used to deal with
malfunctioning peers.

"11) in 3, paragraph 3, the second sentence seems to
contradict the first sentence, and since neither is written
using RFC2119 keywords, it seems to really leave the whole
question open to implementer interpretation."

+ Added that the video container format used is outside the
scope of this document.

``"A SIGNED_INTEGRITY message (type 0x07) consists of a chunk
specification, a 64-bit NTP timestamp [RFC5905] and a digital
signature encoded as a Signature field in a RRSIG record in
DNSSEC without the BASE-64 encoding [RFC4034]." Can this work
in an implementation with no NTP support?''

+ Yes. It is sufficient for the injector's and receivers'
clocks to be roughly synchronized. Rephrased to "64-bit
timestamp in NTP Timestamp format".

"8.14 describes a keep alive message format, but no processing
instructions."

+ Please see our actions following the GenArt review.

"Multiple messages are multiplexed in a datagram. How are the
messages delimited? If there is any corruption in one
message, how does the receiver find the end of the message and
the start of the next message? If I understand correctly,
invalid messages are discarded and no error code is sent. If
one of the messages are found to be invalid, are all messages
in that datagram discarded? or are all subsequent messages in
that datagram discarded? or is it valid to process the
remaining messages in the datagram after an invalid message is
detected? If so, would that conflict with the rule that all
messages must be processed in order?"
+ Messages are fixed size, or contain size fields. Made it
more clear in Section 3 that when an invalid message is
encountered in a datagram, the remaining messages MUST be
discarded.

--------------------------------------------------------------

** COMMENTS by Spencer Dawkins, July 8th 2014**

"3. Messages In general, no error codes or responses are used
in the protocol; absence of any response indicates an error.
Is there accurate qualifier more narrow than "in general" that
you could substitute?"

+ Qualifier removed.

"3.1. HANDSHAKE [heavy/minor confusing]"

+ Heavy payload has been explicitly defined in Terminology.
Please see our response to Richard Barnes' DISCUSS for a
rephrased definition of the handshake procedure.

"3.2. HAVE In particular, whenever a receiving peer P has
successfully checked the integrity of a chunk, or interval of
chunks, it SHOULD send a ^^^^^^ HAVE message to all peers
Q1..Qn it wants to interact with in the near future. A policy
in peer P determines when the HAVE is sent. P may sent it
directly, or peer P may wait until either it has other data to
sent to Qi, or until it has received and checked multiple
chunks. This wasn't clear to me. I'm not understanding why a
SHOULD is appropriate, but I suspect I shouldn't be askig a
2119 question, because this is tangled between "send a HAVE to
the peers you want to interact with in the near future" and
"if you don't want to interact with a specific peer in the
near future, you can wait to send a HAVE". Is that even
close? "

+ Yes. Changed to a MUST and rephrased that the HAVE is sent
only to the peers the sender wants to allow download of
those chunks from.

"3.4. ACK [unreliable/reliable discussion in WG]"

+ The swift protocol on which this draft is based had been
designed from the start to be transport-agnostic. We tried
to preserve that as much as possible.

"5.3. The Atomic Datagram Principle [...] With that many
SHOULDs, I'd be worried that implementations using PPSPP can't
count on much. If I receive a message that spans multiple
datagrams (even though it shouldn't), that don't include the
necessary hashes (even though it should), and I don't drop a
message with missing data (even though I should), is that all
fine?"

+ Yes. Unfortunately, there are some exceptional cases that
have to be dealt with. E.g. because of reordering you may
want to hang on to a datagram with a DATA message that
cannot yet be verified because the datagram with the
required hashes was delayed. In general, there will be
multiple sources to obtain the content from, so a peer/
implementor may choose to get the chunks from a different
source with a better path.

"5.4. INTEGRITY Messages Concretely, a peer that wants to
send a chunk of content creates a datagram that MUST consist
of a list of INTEGRITY messages followed by a DATA message.
If the INTEGRITY messages and DATA message cannot be put into
a single datagram because of a limitation on datagram size,
the INTEGRITY messages MUST be sent first in one or more
datagrams. Is this assuming that the path between peers will
never reorder packets?"

+ No. Hence, the many SHOULDs in the previous Section. This
facilitates processing in the normal case.

--------------------------------------------------------------

**COMMENT by Jari Arkko, July 8th 2014**

+ Please see responses to GenArt review.

--------------------------------------------------------------

**COMMENTs by Barry Leiba, July 9th 2014**

"General question on the chunking: Is it the case that a given
piece of content is chunked in a specific way, with known
chunk IDs, such that every peer that's serving that content up
(at least in the same swarm) uses the same chunks with the
same chunk IDs? One can guess that from the way things work,
but shouldn't the document say that? Or does it, and I missed
it?"

+ Explicitly stated this in Section 3.

"-- Section 3.7 -- When peer Q receives multiple REQUESTs from
the same peer P, peer Q SHOULD process the REQUESTs in the
order received. What happens if it doesn't? Is there an
interoperability issue here? A performance issue? Or what?
(That is, why is this a 2119 SHOULD?)"

+ Consider the case where peer P is operating near the
playback deadline, i.e., the last chunk it has needs to be
given to the video player very soon or it will stall (i.e.,
P has nearly run out of buffer). In that case it is
important that the chunks P requests arrive in order (and
it will be requesting a range of chunks at a time to get a
pipeline going). So the SHOULD is there to help prevent a
performance problem.

"-- Section 5.3 -- Thus, as a datagram carries zero or more
messages, neither messages nor message interdependencies
SHOULD span over multiple datagrams. The negatives in this
sentence really make the SHOULD a hidden SHOULD NOT, and its
meaning is unclear. I think it would be clearer if it were
worded that way:"

+ Replaced with suggested wording.

--------------------------------------------------------------

**COMMENTs by Alia Atlas, July 9th 2014**

+ Requested modifications made.

"Sec 8.1: The paragraph on PLPMTUD is a bit confusing.
Presumably this is between two peers - but the chunk sizes
used by the swarm would be specified by the initial seeder.
Thus I can see the PLPMTUD variant being useful to decide upon
the PPSPP datagram size, but not the chunk size. Could you
please clarify either what I'm missing?"

+ This section discussed considerations for choosing a chunk
size. Deployments can use PLPMTUD, but in that case they
should partition the content using a small chunk size, such
that the datagram can be scaled up or down depending on the
actual network properties.

--------------------------------------------------------------

**COMMENTs by Stephen Farrell, July 10th 2014**.

For DISCUSSes, please see above at revision -11.

"- The elephant is in the room, but not the intro:-) Surely
some comparison with BT is needed in the intro?"

+ Paragraph added.

"- 1.1: I really dislike the term self-certification as its
quite misleading."

+ AFAIK the term was quite common for this type of mechanism,
see e.g. http://en.wikipedia.org/wiki/Self-
certifying_File_System

"- 1.3, 'content': s/asset/file/ would be better I think and
less capitalist;-)"

+ Done. We had commercial partners in our research project
;-)

"- 3: I don't get what is meant by this "an external storage
mapping from the linear byte space of a single swarm to
different files" I can sorta see what's meant, but am not
sure. Maybe try clarify?"

+ In other words, we do not prescribe the video container
format. Added to the paragraph.

"- 5.3, last para: Is the 1st MUST there really implementable
in general? I think the MUST might be to include those hashes
that the sender thinks the receiver needs."

+ Clarified

"- 6.1 - this defines two methods yet says "If the protocol
operates in a benign environment the method MAY be used."
Which is meant here?"

+ Clarified.

"- 6.1.2.1: what if different folks think NCHUNKS_PER_SIG has
different values? How do we all agree on a value? (BTW, the
last sentence of this section is a cool thing.)"

+ This value is set by the content publisher, and is then
derived from the chunk specification of the signed munro
hash by all peers.

"- 7.4: "In other cases a peer MAY include a swarm identifier
option, as an end-to-end check." That's not clear to me, what
other cases?"

+ Rephrased.

"- 7.8: The width of the figure seems wrong."

+ Corrected.

"- 7.10: An example compressed encoding would be useful."

+ Added a small example.

"- 8.16: "perfectly detected" - huh? what does that mean?"

+ Rephrased in revision -11.

--------------------------------------------------------------

Authors' Addresses

Arno Bakker
Vrije Universiteit Amsterdam
De Boelelaan 1081
Amsterdam 1081HV
The Netherlands

Email:

EMail: arno@cs.vu.nl

Riccardo Petrocco
Technische Universiteit Delft
Mekelweg 4
Delft 2628CD
The Netherlands

Email:

EMail: r.petrocco@gmail.com

Victor Grishchenko
Technische Universiteit Delft
Mekelweg 4
Delft 2628CD
The Netherlands

Email:

EMail: victor.grishchenko@gmail.com