wdiff rfc9028v1.txt rfc9028.txt

Internet Engineering Task Force (IETF) A. Keränen
Request for Comments: 9028 J. Melén
Category: Experimental M. Komu, Ed.
ISSN: 2070-1721 Ericsson
May
June 2021

Native NAT Traversal Mode for the Host Identity Protocol

Abstract

This document specifies a new Network Address Translator (NAT)
traversal mode for the Host Identity Protocol (HIP). The new mode is
based on the Interactive Connectivity Establishment (ICE) methodology
and UDP encapsulation of data and signaling traffic. The main
difference from the previously specified modes is the use of HIP
messages instead of ICE for all NAT traversal procedures due to the
kernel-space dependencies of HIP.

Status of This Memo

This document is not an Internet Standards Track specification; it is
published for examination, experimental implementation, and
evaluation.

This document defines an Experimental Protocol for the Internet
community. This document is a product of the Internet Engineering
Task Force (IETF). It represents the consensus of the IETF
community. It has received public review and has been approved for
publication by the Internet Engineering Steering Group (IESG). Not
all documents approved by the IESG are candidates for any level of
Internet Standard; see Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9028.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

Table of Contents

1. Introduction
2. Terminology
3. Overview of Operation
4. Protocol Description
4.1. Relay Registration
4.2. Transport Address Candidate Gathering at the Relay Client
4.3. NAT Traversal Mode Negotiation
4.4. Connectivity Check Pacing Negotiation
4.5. Base Exchange via Control Relay Server
4.6. Connectivity Checks
4.6.1. Connectivity Check Procedure
4.6.2. Rules for Connectivity Checks
4.6.3. Rules for Concluding Connectivity Checks
4.7. NAT Traversal Optimizations
4.7.1. Minimal NAT Traversal Support
4.7.2. Base Exchange without Connectivity Checks
4.7.3. Initiating a Base Exchange Both with and without UDP
Encapsulation
4.8. Sending Control Packets after the Base Exchange
4.9. Mobility Handover Procedure
4.10. NAT Keepalives
4.11. Closing Procedure
4.12. Relaying Considerations
4.12.1. Forwarding Rules and Permissions
4.12.2. HIP Data Relay and Relaying of Control Packets
4.12.3. Handling Conflicting SPI Values
5. Packet Formats
5.1. HIP Control Packets
5.2. Connectivity Checks
5.3. Keepalives
5.4. NAT Traversal Mode Parameter
5.5. Connectivity Check Transaction Pacing Parameter
5.6. Relay and Registration Parameters
5.7. LOCATOR_SET Parameter
5.8. RELAY_HMAC Parameter
5.9. Registration Types
5.10. Notify Packet Types
5.11. ESP Data Packets
5.12. RELAYED_ADDRESS and MAPPED_ADDRESS Parameters
5.13. PEER_PERMISSION Parameter
5.14. HIP Connectivity Check Packets
5.15. NOMINATE Parameter
6. IAB Considerations
7. Security Considerations
6.1.
7.1. Privacy Considerations
6.2.
7.2. Opportunistic Mode
6.3.
7.3. Base Exchange Replay Protection for Control Relay Server
6.4.
7.4. Demultiplexing Different HIP Associations
6.5.
7.5. Reuse of Ports at the Data Relay Server
6.6.
7.6. Amplification Attacks
6.7.
7.7. Attacks against Connectivity Checks and Candidate Gathering
6.8.
7.8. Cross-Protocol Attacks
7.
8. IANA Considerations
8.
9. References
8.1.
9.1. Normative References
8.2.
9.2. Informative References
Appendix A. Selecting a Value for Check Pacing
Appendix B. Differences with Respect to ICE
Appendix C. Differences to Base Exchange and UPDATE Procedures
Appendix D. Multihoming Considerations
Appendix E. DNS Considerations
Acknowledgments
Contributors
Authors' Addresses

1. Introduction

The Host Identity Protocol (HIP) [RFC7401] is specified to run
directly on top of IPv4 or IPv6. However, many middleboxes found in
the Internet, such as NATs and firewalls, often allow only UDP or TCP
traffic to pass [RFC5207]. Also, NATs usually require the host
behind a NAT to create a forwarding state in the NAT before other
hosts outside of the NAT can contact the host behind the NAT. To
overcome this problem, different methods, commonly referred to as NAT
traversal techniques, have been developed.

As one solution, the HIP experiment report [RFC6538] mentions Teredo-
based NAT traversal for HIP and related Encapsulating Security
Payload (ESP) traffic (with double tunneling overhead). Another
solution is specified in [RFC5770], which will be referred to as
"Legacy ICE-HIP" in this document. The experimental Legacy ICE-HIP
specification combines the Interactive Connectivity Establishment
(ICE) protocol (originally [RFC5245]) with HIP so that basically, ICE
is responsible for NAT traversal and connectivity testing, while HIP
is responsible for end-host authentication and IPsec key management.
The resulting protocol uses HIP, Session Traversal Utilities for NAT
(STUN), and ESP messages tunneled over a single UDP flow. The
benefit of using ICE and its STUN / Traversal Using Relays around NAT
(TURN) messaging formats is that one can reuse the NAT traversal
infrastructure already available in the Internet, such as STUN and
TURN servers. Also, some middleboxes may be STUN aware and may be
able to do something "smart" when they see STUN being used for NAT
traversal.

HIP poses a unique challenge to using standard ICE, not only due to
kernel-space dependencies of HIP, but also due to its close
integration with kernel-space IPsec; and, while [RFC5770] provides a
technically workable path, HIP incurs unacceptable performance
drawbacks for kernel-space implementations. Also, implementing and
integrating a full ICE/STUN/TURN protocol stack as specified in
Legacy ICE-HIP results in a considerable amount of effort and code,
which could be avoided by reusing and extending HIP messages and
state machines for the same purpose. Thus, this document specifies
an alternative NAT traversal mode referred to as "Native ICE-HIP"
that employs the HIP messaging format instead of STUN or TURN for the
connectivity checks, keepalives, and data relaying. Native ICE-HIP
also specifies how mobility management works in the context of NAT
traversal, which is missing from the Legacy ICE-HIP specification.
The native specification is also based on HIPv2, whereas the legacy
specification is based on HIPv1. The differences to Legacy ICE-HIP
are further elaborated in Appendix B.

Similar to Legacy ICE-HIP, this specification builds on the HIP
registration extensions [RFC8003] and the base exchange procedure
[RFC7401] and its closing procedures; therefore, the reader is
recommended to get familiar with the relevant specifications. In a
nutshell, the registration extensions allow a HIP Initiator (usually
a "client" host) to ask for specific services from a HIP Responder
(usually a "server" host). The registration parameters are included
in a base exchange, which is essentially a four-way Diffie-Hellman
key exchange authenticated using the public keys of the end hosts.
When the hosts negotiate support for ESP [RFC7402] during the base
exchange, they can deliver ESP-protected application payload to each
other. When either of the hosts moves and changes its IP address,
the two hosts re-establish connectivity using the mobility extensions
[RFC8046]. The reader is also recommended to get familiar with the
mobility extensions; basically, the process is a three-way procedure
where the mobile host first announces its new location to the peer,
then peer;
then, the peer tests for connectivity (the so-called return
routability check), check); and then then, the mobile hosts host must respond to the
announcement in order to activate its new location. This
specification builds on the mobility procedures, but modifies them to
be compatible with ICE. The differences in the mobility extensions
are specified in Appendix C. It is worth noting that multihoming
support as specified in [RFC8047] is left for further study.

This specification builds heavily on the ICE methodology, so it is
recommended that the reader is familiar with the ICE specification
[RFC8445] (especially the overview). However, Native ICE-HIP does
not implement all the features in ICE; hence, the different features
of ICE are cross referenced using [RFC2119] terminology for clarity.
Appendix B explains the differences to ICE, and it is recommended
that the reader read this section in addition to the ICE
specification.

2. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

This document borrows terminology from [RFC5770], [RFC7401],
[RFC8046], [HIP], [RFC9068], [RFC8445], and [RFC8489]. The following terms
recur in the text:

ICE:
Interactive Connectivity Establishment (ICE) protocol as specified
in [RFC8445].

Legacy ICE-HIP:
Refers to the "Basic Host Identity Protocol (HIP) Extensions for
Traversal of Network Address Translators" as specified in
[RFC5770]. The protocol specified in this document offers an
alternative to Legacy ICE-HIP.

Native ICE-HIP:
The protocol specified in this document (Native NAT Traversal Mode
for HIP).

Initiator:
The host that initiates the base exchange using I1 message
[RFC7401].

Responder:
The host that receives the I1 packet from the Initiator [RFC7401].

Control Relay Server
A registrar host that forwards any kind of HIP control plane
packets between the Initiator and the Responder. This host is
critical because it relays the locators between the Initiator and
the Responder so that they can try to establish a direct
communication path with each other. This host is used to replace
HIP rendezvous servers Rendezvous Servers [RFC8004] for hosts operating in private
address realms. In the Legacy ICE-HIP specification [RFC5770],
this host is denoted as "HIP Relay Server".

Control Relay Client:
A requester host that registers to a Control Relay Server
requesting it to forward control plane traffic (i.e., HIP control
messages). In the Legacy ICE-HIP specification [RFC5770], this is
denoted as "HIP Relay Client".

Data Relay Server:
A new entity introduced in this document; a registrar host that
forwards HIP related data plane packets, such as Encapsulating
Security Payload (ESP) [RFC7402], between two hosts. This host
implements similar functionality as TURN servers.

Data Relay Client:
A requester host that registers to a Data Relay Server requesting
it to forward data plane traffic (e.g. ESP traffic). This
functionality is a new and introduced in this document.

Locator:
As defined in [RFC8046]: "A name that controls how the packet is
routed through the network and demultiplexed by the end host. It
may include a concatenation of traditional network addresses such
as an IPv6 address and end-to-end identifiers such as an ESP SPI.
It may also include transport port numbers or IPv6 Flow Labels as
demultiplexing context, or it may simply be a network address."

LOCATOR_SET (written in capital letters):
Denotes a HIP control packet parameter that bundles multiple
locators together [RFC8046].

HIP offer:
Before two end hosts can establish a communication channel using
the NAT traversal procedures defined in this document, they need
to exchange their locators (i.e., candidates) with each other. In
ICE, this procedure is called Candidate Exchange; it does not
specify how the candidates are exchanged, but Session Description
Protocol (SDP) "offer/answer" is mentioned as an example. In
contrast, the Candidate Exchange in HIP is the base exchange
itself or a subsequent UPDATE procedure occurring after a
handover. Following [RFC5770] and SDP-related naming conventions
[RFC3264], "HIP offer" is the Initiator's LOCATOR_SET parameter in
a HIP I2 or in an UPDATE control packet.

HIP answer:
The Responder's LOCATOR_SET parameter in a HIP R2 or UPDATE
control packet. The HIP answer corresponds to the SDP answer
parameter [RFC3264] but is HIP specific. Please refer also to the
longer description of the "HIP offer" term above.

HIP connectivity checks:
In order to obtain a direct end-to-end communication path (without
employing a Data Relay Server), two communicating HIP hosts try to
"punch holes" through their NAT boxes using this mechanism. It is
similar to the ICE connectivity checks but implemented using HIP
return routability checks.

Controlling host:
The controlling host [RFC8445] is always the Initiator in the
context of this specification. It nominates the candidate pair to
be used with the controlled host.

Controlled host:
The controlled host [RFC8445] is always the Responder in the
context of this specification. It waits for the controlling host
to nominate an address candidate pair.

Checklist:
A list of address candidate pairs that need to be tested for
connectivity (same as in [RFC8445]).

Transport address:
Transport-layer port and the corresponding IPv4/v6 address (same
as in [RFC8445]).

Candidate:
A transport address that is a potential point of contact for
receiving data (same as in [RFC8445]).

Host candidate:
A candidate obtained by binding to a specific port from an IP
address on the host (same as in [RFC8445]).

Server-reflexive candidate:
A translated transport address of a host as observed by a Control
or Data Relay Server (same as in [RFC8445]).

Peer-reflexive candidate:
A translated transport address of a host as observed by its peer
(same as in [RFC8445]).

Relayed candidate:
A transport address that exists on a Data Relay Server. Packets
that arrive at this address are relayed towards the Data Relay
Client. The concept is the same as in [RFC8445], but a Data Relay
Server is used instead of a TURN server.

Permission:
In the context of Data Relay Server, permission refers to a
concept similar to TURN's [RFC8656] channels. Before a host can
use a relayed candidate to forward traffic through a Data Relay
Server, the host must activate the relayed candidate with a
specific peer host.

Base:
Similar to that described in [RFC8445], the base of a candidate is
the local source address a host uses to send packets for the
associated candidate. For example, the base of a server-reflexive
address is the local address the host used for registering itself
to the associated Control or Data Relay Server. The base of a
host candidate is equal to the host candidate itself.

3. Overview of Operation

+--------------+
| Control |
+--------+ | Relay Server | +--------+
| Data | +----+-----+---+ | Data |
| Relay | / \ | Relay |
| Server | / \ | Server |
+--------+ / \ +--------+
/ \
/ \
/ \
/ <- Signaling -> \
/ \
+-------+ +-------+
| NAT | | NAT |
+-------+ +-------+
/ \
/ \
+-------+ +-------+
| Init- | | Resp- |
| iator | | onder |
+-------+ +-------+

Figure 1: Example Network Configuration

In the example configuration depicted in Figure 1, both Initiator and
Responder are behind one or more NATs, and both private networks are
connected to the public Internet. To be contacted from behind a NAT,
at least the Responder must be registered with a Control Relay Server
reachable on the public Internet. The Responder may have also
registered to a Data Relay Server that can forward the data plane in
case NAT traversal fails. While, strictly speaking, the Initiator
does not need a Data Relay Server, it may act in the other role with
other hosts; connectivity with the Data Relay Server of the Responder
may fail, so the Initiator may also need to register to a Control
and/or Data Relay Server. It is worth noting that a Control and Data
Relay does not forge the source address of a passing packet but
always translates the source address and source port of a packet to
be forwarded (to its own).

We assume, as a starting point, that the Initiator knows both the
Responder's Host Identity Tag (HIT) and the address(es) of the
Responder's Control Relay Server(s) (how the Initiator learns of the
Responder's Control Relay Server is outside of the scope of this
document, but it may be learned through DNS or another name service).
The first steps are for both the Initiator and Responder to register
with a Control Relay Server (need not be the same one) and gather a
set of address candidates. The hosts use either Control Relay
Servers or Data Relay Servers for gathering the candidates. Next,
the HIP base exchange is carried out by encapsulating the HIP control
packets in UDP datagrams and sending them through the Responder's
Control Relay Server. As part of the base exchange, each HIP host
learns of the peer's candidate addresses through the HIP offer/answer
procedure embedded in the base exchange.

Once the base exchange is completed, two HIP hosts have established a
working communication session (for signaling) via a Control Relay
Server, but the hosts still have to find a better path, preferably
without a Data Relay Server, for the ESP data flow. For this,
connectivity checks are carried out until a working pair of addresses
is discovered. At the end of the procedure, if successful, the hosts
will have established a UDP-based tunnel that traverses both NATs
with the data flowing directly from NAT to NAT or via a Data Relay
Server. At this point, the HIP signaling can also be sent over the
same address/port pair, and is demultiplexed (or, in other words,
separated) from IPsec as described in the UDP encapsulation standard
for IPsec [RFC3948]. Finally, the two hosts send NAT keepalives as
needed in order keep their UDP-tunnel state active in the associated
NAT boxes.

If either one of the hosts knows that it is not behind a NAT, hosts
can negotiate during the base exchange a different mode of NAT
traversal that does not use HIP connectivity checks, but only UDP
encapsulation of HIP and ESP. Also, it is possible for the Initiator
to simultaneously try a base exchange with and without UDP
encapsulation. If a base exchange without UDP encapsulation
succeeds, no HIP connectivity checks or UDP encapsulation of ESP are
needed.

4. Protocol Description

This section describes the normative behavior of the "Native ICE-HIP"
protocol extension. Most of the procedures are similar to what is
defined in [RFC5770] but with different, or additional, parameter
types and values. In addition, a new type of relaying server, Data
Relay Server, is specified. Also, it should be noted that HIP
version 2 [RFC7401] MUST be used instead of HIPv1 with this NAT
traversal mode.

4.1. Relay Registration

In order for two hosts to communicate over NATed environments, they
need a reliable way to exchange information. To achieve this, "HIP
Relay Server" is defined in [RFC5770]. It supports the relaying of
HIP control plane traffic over UDP in NATed environments and forwards
HIP control packets between the Initiator and the Responder. In this
document, the HIP Relay Server is denoted as "Control Relay Server"
for better alignment with the rest of the terminology. The
registration to the Control Relay Server can be achieved using the
RELAY_UDP_HIP parameter as explained later in this section.

To also guarantee data plane delivery over varying types of NAT
devices, a host MAY also register for UDP-encapsulated ESP relaying
using Registration Type RELAY_UDP_ESP (value 3). This service may be
coupled with the Control Relay Server or offered separately on
another server. If the server supports relaying of UDP-encapsulated
ESP, the host is allowed to register for a data-relaying service
using the registration extensions in Section 3.3 of [RFC8003]. If
the server has sufficient relaying resources (free port numbers,
bandwidth, etc.) available, it opens a UDP port on one of its
addresses and signals the address and port to the registering host
using the RELAYED_ADDRESS parameter (as defined in Section 5.12 in
this document). If the Data Relay Server would accept the data-
relaying request but does not currently have enough resources to
provide data-relaying service, it MUST reject the request with
Failure Type "Insufficient resources" [RFC8003].

The registration process follows the generic registration extensions
defined in [RFC8003]. The HIP control plane relaying registration
follows [RFC5770], but the data plane registration is different. It
is worth noting that if the HIP control and data plane relay services
reside on different hosts, the client has to register separately to
each of them. In the example shown in Figure 2, the two services are
coupled on a single host. The text uses "Relay Client" and "Relay
Server" as a shorthand when the procedures apply both to control and
data cases.

Control/Data Control/Data
Relay Client (Initiator) Relay Server (Responder)
| 1. UDP(I1) |
+---------------------------------------------------------------->|
| |
| 2. UDP(R1(REG_INFO(RELAY_UDP_HIP,[RELAY_UDP_ESP]))) |
|<----------------------------------------------------------------+
| |
| 3. UDP(I2(REG_REQ(RELAY_UDP_HIP),[RELAY_UDP_ESP])) |
+---------------------------------------------------------------->|
| |
| 4. UDP(R2(REG_RES(RELAY_UDP_HIP,[RELAY_UDP_ESP]), REG_FROM, |
| [RELAYED_ADDRESS])) |
|<----------------------------------------------------------------+
| |

Figure 2: Example Registration with a HIP Relay

In step 1, the Relay Client (Initiator) starts the registration
procedure by sending an I1 packet over UDP to the Relay Server. It
is RECOMMENDED that the Relay Client select a random source port
number from the ephemeral port range 49152-65535 for initiating a
base exchange. Alternatively, a host MAY also use a single fixed
port for initiating all outgoing connections. However, the allocated
port MUST be maintained until all of the corresponding HIP
associations are closed. It is RECOMMENDED that the Relay Server
listen to incoming connections at UDP port 10500. If some other port
number is used, it needs to be known by potential Relay Clients.

In step 2, the Relay Server (Responder) lists the services that it
supports in the R1 packet. The support for HIP control plane over
UDP relaying is denoted by the Registration Type value RELAY_UDP_HIP
(see Section 5.9). If the server also supports the relaying of ESP
traffic over UDP, it also includes the Registration type Type value
RELAY_UDP_ESP.

In step 3, the Relay Client selects the services for which it
registers and lists them in the REG_REQ parameter. The Relay Client
registers for the Control Relay service by listing the RELAY_UDP_HIP
value in the request parameter. If the Relay Client also requires
ESP relaying over UDP, it lists also RELAY_UDP_ESP.

In step 4, the Relay Server concludes the registration procedure with
an R2 packet and acknowledges the registered services in the REG_RES
parameter. The Relay Server denotes unsuccessful registrations (if
any) in the REG_FAILED parameter of R2. The Relay Server also
includes a REG_FROM parameter that contains the transport address of
the Relay Client as observed by the Relay Server (server-reflexive
candidate). If the Relay Client registered to ESP-relaying service,
the Relay Server includes a RELAYED_ADDRESS parameter that describes
the UDP port allocated to the Relay Client for ESP relaying. It is
worth noting that the Data Relay Client must first activate this UDP
port by sending an UPDATE message to the Data Relay Server that
includes a PEER_PERMISSION parameter as described in Section 4.12.1
both after base exchange and handover procedures. Also, the Data
Relay Server should follow the port allocation recommendations in
Section 6.5. 7.5.

After the registration, the Relay Client periodically sends NAT
keepalives to the Relay Server in order to keep the NAT bindings
between the Relay Client and the relay alive. The keepalive
extensions are described in Section 4.10.

The Data Relay Client MUST maintain an active HIP association with
the Data Relay Server as long as it requires the data-relaying
service. When the HIP association is closed (or times out), or the
registration lifetime passes without the Data Relay Client refreshing
the registration, the Data Relay Server MUST stop relaying packets
for that host and close the corresponding UDP port (unless other Data
Relay Clients are still using it).

The Data Relay Server SHOULD offer a different relayed address and
port for each Data Relay Client because not doing so can cause
problems with stateful firewalls (see Section 6.5). 7.5).

When a Control Relay Client sends an UPDATE (e.g., due to host
movement or to renew service registration), the Control Relay Server
MUST follow the general guidelines defined in [RFC8003], with the
difference that all UPDATE messages are delivered on top of UDP. In
addition to this, the Control Relay Server MUST include the REG_FROM
parameter in all UPDATE responses sent to the Control Relay Client.
This applies to both renewals of service registration and to host
movement. It is especially important for the case of host movement,
as this is the mechanism that allows the Control Relay Client to
learn its new server-reflexive address candidate.

A Data Relay Client can request multiple relayed candidates from the
Data Relay Server (e.g., for the reasons described in
Section 4.12.3). After the base exchange with registration, the Data
Relay Client can request additional relayed candidates similarly as
during the base exchange. The Data Relay Client sends an UPDATE
message REG_REQ parameter requesting for the RELAY_UDP_ESP service.
The UPDATE message MUST also include a SEQ and an ECHO_REQUEST_SIGNED
parameter. The Data Relay Server MUST respond with an UPDATE message
that includes the corresponding response parameters: REG_RES, ACK and
ECHO_REQUEST_SIGNED .
ECHO_REQUEST_SIGNED. In case the Data Relay Server allocated a new
relayed UDP port for the Data Relay Client, the REG_RES parameter
MUST list RELAY_UDP_ESP as a service and the UPDATE message MUST also
include a RELAYED_ADDRESS parameter describing the relayed UDP port.
The Data Relay Server MUST also include the server-reflexive
candidate in a REG_FROM parameter. It is worth mentioning that the
Data Relay Client MUST activate the UDP port as described in
Section 4.12.1 before it can be used for any ESP relaying.

A Data Relay Client may unregister a relayed candidate in two ways.
It can wait for its lifetime to expire or it can explicitly request
it with zero lifetime using the UPDATE mechanism. The Data Relay
Client can send a REG_REQ parameter with zero lifetime to the Data
Relay Server in order to expire all relayed candidates. To expire a
specific relayed candidate, the Data Relay Client MUST also include a
RELAYED_ADDRESS parameter as sent by the server in the UPDATE
message. Upon closing the HIP association (CLOSE-CLOSE-ACK procedure
initiated by either party), the Data Relay Server MUST also expire
all relayed candidates.

Please also refer to Section 6.8 7.8 for protection against cross-
protocol attacks for both Control Relay Client and Server.

4.2. Transport Address Candidate Gathering at the Relay Client

An Initiator needs to gather a set of address candidates before
contacting a (non-relay) Responder. The candidates are needed for
connectivity checks that allow two hosts to discover a direct, non-
relayed path for communicating with each other. One server-reflexive
candidate can be discovered during the registration with the Control
Relay Server from the REG_FROM parameter (and another from Data Relay
Server if one is employed).

The candidate gathering can be done at any time, but it needs to be
done before sending an I2 or R2 in the base exchange if ICE-HIP-UDP
mode is to be used for the connectivity checks. It is RECOMMENDED
that all three types of candidates (host, server reflexive, and
relayed) are gathered to maximize the probability of successful NAT
traversal. However, if no Data Relay Server is used, and the host
has only a single local IP address to use, the host MAY use the local
address as the only host candidate and the address from the REG_FROM
parameter discovered during the Control Relay Server registration as
a server-reflexive candidate. In this case, no further candidate
gathering is needed.

A Data Relay Client MAY register only a single relayed candidate that
it uses with multiple other peers. However, it is RECOMMENDED that a
Data Relay Client registers a new server relayed candidate for each
of its peers for the reasons described in Section 4.12.3. The
procedures for registering multiple relayed candidates are described
in Section 4.1.

If a Relay Client has more than one network interface, it can
discover additional server-reflexive candidates by sending UPDATE
messages from each of its interfaces to the Relay Server. Each such
UPDATE message MUST include the following parameters: the
registration request (REG_REQ) parameter with Registration Type
CANDIDATE_DISCOVERY (value 4) and the ECHO_REQUEST_SIGNED parameter.
When a Control Relay Server receives an UPDATE message with
registration request containing a CANDIDATE_DISCOVERY type, it MUST
include a REG_FROM parameter, containing the same information as if
this were a Control Relay Server registration, to the response (in
addition to the mandatory ECHO_RESPONSE_SIGNED parameter). This
request type SHOULD NOT create any state at the Control Relay Server.

The rules in Section 5.1.1 of [RFC8445] for candidate gathering are
followed here. A number of host candidates (loopback, anycast and
others) should be excluded as described in the ICE specification
(Section 5.1.1.1 of [RFC8445]). Relayed candidates SHOULD be
gathered in order to guarantee successful NAT traversal, and
implementations SHOULD support this functionality even if it will not
be used in deployments in order to enable it by software
configuration update if needed at some point. Similarly, as
explained in the ICE specification (Section 5.1.1.2 of [RFC8445]), if
an IPv6-only host is in a network that utilizes NAT64 [RFC6146] and
DNS64 [RFC6147] technologies, it may also gather IPv4 server-
reflexive and/or relayed candidates from IPv4-only Control or Data
Relay Servers. IPv6-only hosts SHOULD also utilize IPv6 prefix
discovery [RFC7050] to discover the IPv6 prefix used by NAT64 (if
any) and generate server-reflexive candidates for each IPv6-only
interface, accordingly. The NAT64 server-reflexive candidates are
prioritized like IPv4 server-reflexive candidates.

HIP-based connectivity can be utilized by IPv4 applications using
Local Scope Identifiers (LSIs) and by IPv6-based applications using
HITs. The LSIs and HITs of the local virtual interfaces MUST be
excluded in the candidate gathering phase as well to avoid creating
unnecessary loopback connectivity tests.

Gathering of candidates MAY also be performed by other means than
described in this section. For example, the candidates could be
gathered as specified in Section 4.2 of [RFC5770] if STUN servers are
available, or if the host has just a single interface and no STUN or
Data Relay Server are available.

Each local address candidate MUST be assigned a priority. The
following recommended formula (as described in [RFC8445]) SHOULD be
used:

priority = (2^24)*(type preference) + (2^8)*(local preference) +
(2^0)*(256 - component ID)

In the formula, the type preference follows the ICE specification (as
defined in Section 5.1.2.1 of [RFC8445]): the RECOMMENDED values are
126 for host candidates, 100 for server-reflexive candidates, 110 for
peer-reflexive candidates, and 0 for relayed candidates. The highest
value is 126 (the most preferred) and lowest is 0 (last resort). For
all candidates of the same type, the preference type value MUST be
identical, and, correspondingly, the value MUST be different for
different types. For peer-reflexive values, the type preference
value MUST be higher than for server-reflexive types. It should be
noted that peer-reflexive values are learned later during
connectivity checks, so a host cannot employ it during candidate
gathering stage yet. checks.

Following the ICE specification, the local preference MUST be an
integer from 0 (lowest preference) to 65535 (highest preference)
inclusive. In the case the host has only a single address candidate,
the value SHOULD be 65535. In the case of multiple candidates, each
local preference value MUST be unique. Dual-stack considerations for
IPv6 also apply here as defined in Section 5.1.2.2 of [RFC8445].

Unlike with SDP used in conjunction with ICE, this protocol only
creates a single UDP flow between the two communicating hosts, so
only a single component exists. Hence, the component ID value MUST
always be set to 1.

As defined in Section 14.3 of [RFC8445], the retransmission timeout
(RTO) for address gathering from a Control/Data Relay Server SHOULD
be calculated as follows:

RTO = MAX (1000 ms, Ta * (Num-Of-Cands))

where Ta is the value used for the connectivity check pacing and Num-
Of-Cands is the number of server-reflexive and relay candidates. A
smaller value than 1000 ms for the RTO MUST NOT be used.

4.3. NAT Traversal Mode Negotiation

This section describes the usage of a non-critical parameter type
called NAT_TRAVERSAL_MODE with a new mode called ICE-HIP-UDP. The
presence of the new mode in the NAT_TRAVERSAL_MODE parameter in a HIP
base exchange means that the end host supports NAT traversal
extensions described in this document. As the parameter is non-
critical (as defined in Section 5.2.1 of [RFC7401]), it can be
ignored by an end host, which means that the host is not required to
support it or may decline to use it.

With registration with a Control/Data Relay Server, it is usually
sufficient to use the UDP-ENCAPSULATION mode of NAT traversal since
the Relay Server is assumed to be in public address space. Thus, the
Relay Server SHOULD propose the UDP-ENCAPSULATION mode as the
preferred or only mode. The NAT traversal mode negotiation in a HIP
base exchange is illustrated in Figure 3. It is worth noting that
the Relay Server could be located between the hosts, but is omitted
here for simplicity.

Initiator Responder
| 1. UDP(I1) |
+----------------------------------------------------------------->|
| |
| 2. UDP(R1(.., NAT_TRAVERSAL_MODE(ICE-HIP-UDP), ..)) |
|<-----------------------------------------------------------------+
| |
| 3. UDP(I2(.., NAT_TRAVERSAL_MODE(ICE-HIP-UDP), ENC(LOC_SET), ..))|
+----------------------------------------------------------------->|
| |
| 4. UDP(R2(.., ENC(LOC_SET), ..)) |
|<-----------------------------------------------------------------+
| |

Figure 3: Negotiation of NAT Traversal Mode

In step 1, the Initiator sends an I1 to the Responder.

In step 2, the Responder responds with an R1. As specified in
[RFC5770], the NAT_TRAVERSAL_MODE parameter in R1 contains a list of
NAT traversal modes the Responder supports. The mode specified in
this document is ICE-HIP-UDP (value 3).

In step 3, the Initiator sends an I2 that includes a
NAT_TRAVERSAL_MODE parameter. It contains the mode selected by the
Initiator from the list of modes offered by the Responder. If ICE-
HIP-UDP mode was selected, the I2 also includes the "Transport
address" locators (as defined in Section 5.7) of the Initiator in a
LOCATOR_SET parameter (denoted here with LOC_SET). With ICE-HIP-UDP
mode, the LOCATOR_SET parameter MUST be encapsulated within an
ENCRYPTED parameter (denoted here with ENC) according to the
procedures in Sections 5.2.18 and 6.5 in [RFC7401]. The locators in
I2 are the "HIP offer".

In step 4, the Responder concludes the base exchange with an R2
packet. If the Initiator chose ICE-HIP-UDP traversal mode, the
Responder includes a LOCATOR_SET parameter in the R2 packet. With
ICE-HIP-UDP mode, the LOCATOR_SET parameter MUST be encapsulated
within an ENCRYPTED parameter according to the procedures in Sections
5.2.18 and 6.5 in [RFC7401]. The locators in R2, encoded like the
locators in I2, are the "ICE answer". If the NAT traversal mode
selected by the Initiator is not supported by the Responder, the
Responder SHOULD reply with a NOTIFY packet with type
NO_VALID_NAT_TRAVERSAL_MODE_PARAMETER and abort the base exchange.

4.4. Connectivity Check Pacing Negotiation

As explained in Legacy ICE-HIP [RFC5770], when a NAT traversal mode
with connectivity checks is used, new transactions should not be
started too fast to avoid congestion and overwhelming the NATs. For
this purpose, during the base exchange, hosts can negotiate a
transaction pacing value, Ta, using a TRANSACTION_PACING parameter in
R1 and I2 packets. The parameter contains the minimum time
(expressed in milliseconds) the host would wait between two NAT
traversal transactions, such as starting a new connectivity check or
retrying a previous check. The value that is used by both of the
hosts is the higher of the two offered values.

The minimum Ta value SHOULD be configurable, and if no value is
configured, a value of 50 ms MUST be used. Guidelines for selecting
a Ta value are given in Appendix A. Hosts MUST NOT use values
smaller than 5 ms for the minimum Ta, since such values may not work
well with some NATs (as explained in [RFC8445]). The Initiator MUST
NOT propose a smaller value than what the Responder offered. If a
host does not include the TRANSACTION_PACING parameter in the base
exchange, a Ta value of 50 ms MUST be used as that host's minimum
value.

4.5. Base Exchange via Control Relay Server

This section describes how the Initiator and Responder perform a base
exchange through a Control Relay Server. Connectivity pacing
(denoted as TA_P here) was described in Section 4.4 and is not
repeated here. Similarly, the NAT traversal mode negotiation process
(denoted as NAT_TM in the example) was described in Section 4.3 and
is also not repeated here. If a Control Relay Server receives an R1
or I2 packet without the NAT traversal mode parameter, it MUST drop
it and SHOULD send a NOTIFY error packet with type
NO_VALID_NAT_TRAVERSAL_MODE_PARAMETER to the sender of the R1 or I2.

It is RECOMMENDED that the Initiator send an I1 packet encapsulated
in UDP when it is destined to an IP address of the Responder.
Respectively, the Responder MUST respond to such an I1 packet with a
UDP-encapsulated R1 packet, and also the rest of the communication
related to the HIP association MUST also use UDP encapsulation.

Figure 4 illustrates a base exchange via a Control Relay Server. We
assume that the Responder (i.e., a Control Relay Client) has already
registered to the Control Relay Server. The Initiator may have also
registered to another (or the same Control Relay Server), but the
base exchange will traverse always through the Control Relay Server
of the Responder.

Initiator Control Relay Server Responder
| 1. UDP(I1) | |
+--------------------------------->| 2. UDP(I1(RELAY_FROM)) |
| +------------------------------->|
| | |
| | 3. UDP(R1(RELAY_TO, NAT_TM, |
| | TA_P)) |
| 4. UDP(R1(RELAY_TO, NAT_TM, |<-------------------------------+
| TA_P)) | |
|<---------------------------------+ |
| | |
| 5. UDP(I2(ENC(LOC_SET)), | |
| NAT_TM, TA_P)) | |
+--------------------------------->| 6. UDP(I2(ENC(LOC_SET), |
| | RELAY_FROM, NAT_TM, TA_P))|
| +------------------------------->|
| | |
| | 7. UDP(R2(ENC(LOC_SET), |
| 8. UDP(R2(ENC(LOC_SET), | RELAY_TO)) |
| RELAY_TO)) |<-------------------------------+
|<---------------------------------+ |
| | |

Figure 4: Base Exchange via a HIP Relay Server

In step 1 of Figure 4, the Initiator sends an I1 packet over UDP via
the Control Relay Server to the Responder. In the HIP header, the
source HIT belongs to the Initiator and the destination HIT to the
Responder. The Initiator sends the I1 packet from its IP address to
the IP address of the Control Relay Server over UDP.

In step 2, the Control Relay Server receives the I1 packet. If the
destination HIT belongs to a successfully registered Control Relay
Client (i.e., the host marked "Responder" in Figure 4), the Control
Relay Server processes the packet. Otherwise, the Control Relay
Server MUST drop the packet silently. The Control Relay Server
appends a RELAY_FROM parameter to the I1 packet, which contains the
transport source address and port of the I1 as observed by the
Control Relay Server. The Control Relay Server protects the I1
packet with RELAY_HMAC, except that the parameter type is different
as described in Section 5.8. The Control Relay Server changes the
source and destination ports and IP addresses of the packet to match
the values the Responder used when registering to the Control Relay
Server, i.e., the reverse of the R2 used in the registration. The
Control Relay Server MUST recalculate the transport checksum and
forward the packet to the Responder.

In step 3, the Responder receives the I1 packet. The Responder
processes it according to the rules in [RFC7401]. In addition, the
Responder validates the RELAY_HMAC according to Section 5.8 and
silently drops the packet if the validation fails. The Responder
replies with an R1 packet to which it includes RELAY_TO and NAT
traversal mode parameters. The Responder MUST include ICE-HIP-UDP in
the NAT traversal modes. The RELAY_TO parameter MUST contain the
same information as the RELAY_FROM parameter, i.e., the Initiator's
transport address, but the type of the parameter is different. The
RELAY_TO parameter is not integrity protected by the signature of the
R1 to allow pre-created R1 packets at the Responder.

In step 4, the Control Relay Server receives the R1 packet. The
Control Relay Server drops the packet silently if the source HIT
belongs to a Control Relay Client that has not successfully
registered. The Control Relay Server MAY verify the signature of the
R1 packet and drop it if the signature is invalid. Otherwise, the
Control Relay Server rewrites the source address and port, and
changes the destination address and port to match RELAY_TO
information. Finally, the Control Relay Server recalculates the
transport checksum and forwards the packet.

In step 5, the Initiator receives the R1 packet and processes it
according to [RFC7401]. The Initiator MAY use the address in the
RELAY_TO parameter as a local peer-reflexive candidate for this HIP
association if it is different from all known local candidates. The
Initiator replies with an I2 packet that uses the destination
transport address of R1 as the source address and port. The I2
packet contains a LOCATOR_SET parameter inside an ENCRYPTED parameter
that lists all the HIP candidates (HIP offer) of the Initiator. The
candidates are encoded using the format defined in Section 5.7. The
I2 packet MUST also contain a NAT traversal mode parameter that
includes ICE-HIP-UDP mode. The ENCRYPTED parameter along with its
key material generation is described in detail in Sections 5.2.18 and
6.5 in [RFC7401].

In step 6, the Control Relay Server receives the I2 packet. The
Control Relay Server appends a RELAY_FROM and a RELAY_HMAC to the I2
packet similar to that explained in step 2, and forwards the packet
to the Responder.

In step 7, the Responder receives the I2 packet and processes it
according to [RFC7401]. The Responder validates the RELAY_HMAC
according to Section 5.8 and silently drops the packet if the
validation fails. It replies with an R2 packet and includes a
RELAY_TO parameter as explained in step 3. The R2 packet includes a
LOCATOR_SET parameter inside an ENCRYPTED parameter that lists all
the HIP candidates (ICE answer) of the Responder. The RELAY_TO
parameter is protected by the Hashed Message Authentication Code
(HMAC). The ENCRYPTED parameter along with its key material
generation is described in detail in Sections 5.2.18 and 6.5 in
[RFC7401].

In step 8, the Control Relay Server processes the R2 as described in
step 4. The Control Relay Server forwards the packet to the
Initiator. After the Initiator has received the R2 and processed it
successfully, the base exchange is completed.

Hosts MUST include the address of one or more Control Relay Servers
(including the one that is being used for the initial signaling) in
the LOCATOR_SET parameter in I2 and R2 messages if they intend to use
such servers for relaying HIP signaling immediately after the base
exchange completes. The traffic type of these addresses MUST be "HIP
signaling" (see Section 5.7) and they MUST NOT be used for the
connectivity tests described in Section 4.6. If the Control Relay
Server locator used for relaying the base exchange is not included in
I2 or R2 LOCATOR_SET parameters, it SHOULD NOT be used after the base
exchange. Instead, further HIP signaling SHOULD use the same path as
the data traffic. It is RECOMMENDED to use the same Control Relay
Server throughout the lifetime of the host association that was used
for forwarding the base exchange if the Responder includes it in the
locator parameter of the R2 message.

4.6. Connectivity Checks

When the Initiator and Responder complete the base exchange through
the Control Relay Server, both of them employ the IP address of the
Control Relay Server as the destination address for the packets. The
address of the Control Relay Server MUST NOT be used as a destination
for data plane traffic unless the server also supports Data Relay
Server functionality, and the Client has successfully registered to
use it. When NAT traversal mode with ICE-HIP-UDP was successfully
negotiated and selected, the Initiator and Responder MUST start the
connectivity checks in order to attempt to obtain direct end-to-end
connectivity through NAT devices. It is worth noting that the
connectivity checks MUST be completed even though no ESP_TRANSFORM
would be negotiated and selected.

The connectivity checks follow the ICE methodology [ICE-NONSIP], but
UDP-encapsulated HIP control messages are used instead of ICE
messages. As stated in the ICE specification, the basic procedure
for connectivity checks has three phases: sorting the candidate pairs
according to their priority, sending checks in the prioritized order,
and acknowledging the checks from the peer host.

The Initiator MUST take the role of controlling host, and the
Responder acts as the controlled host. The roles MUST persist
throughout the HIP associate lifetime (to be reused in the possibly even during
mobility UPDATE procedures). In the case in which both communicating
nodes are initiating communication to each other using an I1 packet,
the conflict is resolved as defined in Section 6.7 of [RFC7401]; the
host with the "larger" HIT changes its role to Responder. In such a
case, the host changing its role to Responder MUST also switch to the
controlled role.

The protocol follows standard HIP UPDATE sending and processing rules
as defined in Sections 6.11 and 6.12 in [RFC7401], but some new
parameters are introduced (CANDIDATE_PRIORITY, MAPPED_ADDRESS,
NOMINATE, PEER_PERMISSION, and
NOMINATE). RELAYED_ADDRESS).

4.6.1. Connectivity Check Procedure

Figure 5 illustrates connectivity checks in a simplified scenario
where the Initiator and Responder have only a single candidate pair
to check. Typically, NATs drop messages until both sides have sent
messages using the same port pair. In this scenario, the Responder
sends a connectivity check first but the NAT of the Initiator drops
it. However, the connectivity check from the Initiator reaches the
Responder because it uses the same port pair as the first message.
It is worth noting that the message flow in this section is
idealistic, and, in practice, more messages would be dropped,
especially in the beginning. For instance, connectivity tests always
start with the candidates with the highest priority, which would be
host candidates (which would not reach the recipient in this
scenario).

Initiator NAT1 NAT2 Responder
| | 1. UDP(UPDATE(SEQ, CAND_PRIO, | |
| | ECHO_REQ_SIGN)) | |
| X<-----------------------------------+----------------+
| | | |
| 2. UDP(UPDATE(SEQ, ECHO_REQ_SIGN, CAND_PRIO)) | |
+-------------+------------------------------------+--------------->|
| | | |
| 3. UDP(UPDATE(ACK, ECHO_RESP_SIGN, MAPPED_ADDR)) | |
|<------------+------------------------------------+----------------+
| | | |
| 4. UDP(UPDATE(SEQ, ECHO_REQ_SIGN, CAND_PRIO)) | |
|<------------+------------------------------------+----------------+
| | | |
| 5. UDP(UPDATE(ACK, ECHO_RESP_SIGN, MAPPED_ADDR)) | |
+-------------+------------------------------------+--------------->|
| | | |
| 6. Other connectivity checks using UPDATE over UDP |
|<------------+------------------------------------+---------------->
| | | |
| 7. UDP(UPDATE(SEQ, ECHO_REQ_SIGN, CAND_PRIO, NOMINATE)) |
+-------------+------------------------------------+--------------->|
| | | |
| 8. UDP(UPDATE(SEQ, ACK, ECHO_REQ_SIGN, ECHO_RESP_SIGN, |
| NOMINATE)) | |
|<------------+------------------------------------+----------------+
| | | |
| 9. UDP(UPDATE(ACK, ECHO_RESP_SIGN)) | |
+-------------+------------------------------------+--------------->+
| | | |
| 10. ESP data traffic over UDP | |
+<------------+------------------------------------+--------------->+
| | | |

Figure 5: Connectivity Checks

In step 1, the Responder sends a connectivity check to the Initiator
that the NAT of the Initiator drops. The message includes a number
of parameters. As specified in [RFC7401], the SEQ parameter includes
a running sequence identifier for the connectivity check. The
candidate priority (denoted CAND_PRIO in the figure) describes the
priority of the address candidate being tested. The
ECHO_REQUEST_SIGNED (denoted ECHO_REQ_SIGN in the figure) includes a
nonce that the recipient must sign and echo back as it is.

In step 2, the Initiator sends a connectivity check, using the same
address pair candidate as in the previous step, and the message
successfully traverses the NAT boxes. The message includes the same
parameters as in the previous step. It should be noted that the
sequence identifier is locally assigned by the Initiator, so it can
be different than in the previous step.

In step 3, the Responder has successfully received the previous
connectivity check from the Initiator and starts to build a response
message. Since the message from the Initiator included a SEQ, the
Responder must acknowledge it using an ACK parameter. Also, the
nonce contained in the echo request must be echoed back in an
ECHO_RESPONSE_SIGNED (denoted ECHO_RESP_SIGN) parameter. The
Responder also includes a MAPPED_ADDRESS parameter (denoted
MAPPED_ADDR in the figure) that contains the transport address of the
Initiator as observed by the Responder (i.e., peer-reflexive
candidate). This message is successfully delivered to the Initiator;
upon reception, the Initiator marks the candidate pair as valid.

In step 4, the Responder retransmits the connectivity check sent in
the first step, since it was not acknowledged yet.

In step 5, the Initiator responds to the previous connectivity check
message from the Responder. The Initiator acknowledges the SEQ
parameter from the previous message using an ACK parameter and the
ECHO_REQUEST_SIGNED parameter with ECHO_RESPONSE_SIGNED. In
addition, it includes the MAPPED_ADDR parameter that includes the
peer-reflexive candidate. This response message is successfully
delivered to the Responder; upon reception, the Initiator marks the
candidate pair as valid.

In step 6, despite the two hosts now having valid address candidates,
the hosts still test the remaining address candidates in a similar
way as in the previous steps. It should be noted that each
connectivity check has a unique sequence number in the SEQ parameter.

In step 7, the Initiator has completed testing all address candidates
and nominates one address candidate to be used. It sends an UPDATE
message using the selected address candidates that includes a number
of parameters: SEQ, ECHO_REQUEST_SIGNED, CANDIDATE_PRIORITY, and the
NOMINATE parameter.

In step 8, the Responder receives the message with the NOMINATE
parameter from the Initiator. It sends a response that includes the
NOMINATE parameter in addition to a number of other parameters. The
ACK and ECHO_RESPONSE_SIGNED parameters acknowledge the SEQ and
ECHO_REQUEST_SIGNED parameters from the previous message from the
Initiator. The Responder includes SEQ and ECHO_REQUEST_SIGNED
parameters in order to receive an acknowledgment from the Responder.

In step 9, the Initiator completes the candidate nomination process
by confirming the message reception to the Responder. In the
confirmation message, the ACK and ECHO_RESPONSE_SIGNED parameters
correspond to the SEQ and ECHO_REQUEST_SIGNED parameters in the
message sent by the Responder in the previous step.

In step 10, the Initiator and Responder can start sending application
payload over the successfully nominated address candidates.

It is worth noting that if either host has registered a relayed
address candidate from a Data Relay Server, the host MUST activate
the address before connectivity checks by sending an UPDATE message
containing the PEER_PERMISSION parameter as described in
Section 4.12.1. Otherwise, the Data Relay Server drops ESP packets
using the relayed address.

It should be noted that in the case in which both the Initiator and
Responder are advertising their own relayed address candidates, it is
possible that the two hosts choose the two relayed addresses as a
result of the ICE nomination algorithm. While this is possible (and
even could be desirable for privacy reasons), it can be unlikely due
to low priority assigned for the relayed address candidates. In such
an event, the nominated address pair is always symmetric; the
nomination algorithm prevents asymmetric address pairs (i.e., each
side choosing different pair) such as a Data Relay Client using its
own Data Relay Server to send data directly to its peer while
receiving data from the Data Relay Server of its peer.

4.6.2. Rules for Connectivity Checks

The HITs of the two communicating hosts MUST be used as credentials
in this protocol (in contrast to ICE, which employs username-password
fragments). A HIT pair uniquely identifies the corresponding HIT
association, and a SEQ number in an UPDATE message identifies a
particular connectivity check.

All of the connectivity check messages MUST be protected with
HIP_HMAC and signatures (even though the illustrations in this
specification omit them for simplicity) according to [RFC7401]. Each
connectivity check sent by a host MUST include a SEQ parameter and
ECHO_REQUEST_SIGNED parameter; correspondingly, the peer MUST respond
to these using ACK and ECHO_RESPONSE_SIGNED according to the rules
specified in [RFC7401].

The host sending a connectivity check MUST validate that the response
uses the same pair of UDP ports, and drop the packet if this is not
the case.

A host may receive a connectivity check before it has received the
candidates from its peer. In such a case, the host MUST immediately
queue a response by placing it in the triggered-check queue and then
continue waiting for the candidates. A host MUST NOT select a
candidate pair until it has verified the pair using a connectivity
check as defined in Section 4.6.1.

Section 5.3.5 of [RFC7401] states that UPDATE packets have to include
either a SEQ or ACK parameter (but can include both). In the
connectivity check procedure specified in Section 4.6.1, each SEQ
parameter should be acknowledged separately. In the context of NATs,
this means that some of the SEQ parameters sent in connectivity
checks will be lost or arrive out of order. From the viewpoint of
the recipient, this is not a problem since the recipient will just
"blindly" acknowledge the SEQ. However, the sender needs to be
prepared for lost sequence identifiers and ACK parameters that arrive
out of order.

As specified in [RFC7401], an ACK parameter may acknowledge multiple
sequence identifiers. While the examples in the previous sections do
not illustrate such functionality, it is also permitted when
employing ICE-HIP-UDP mode.

In ICE-HIP-UDP mode, a retransmission of a connectivity check SHOULD
be sent with the same sequence identifier in the SEQ parameter. Some
tested address candidates will never produce a working address pair
and may thus cause retransmissions. Upon successful nomination of an
address pair, a host SHOULD immediately stop sending such
retransmissions.

Full ICE procedures for prioritizing candidates, eliminating
redundant candidates, forming checklists (including pruning) pruning), and
triggered-check queues MUST be followed as specified in Section 6.1
of [RFC8445], with the exception being that the foundation, frozen
candidates, and default candidates are not used. From the viewpoint
of the ICE specification [RFC8445], the protocol specified in this
document operates using a component ID of 1 on all candidates, and
the foundation of all candidates is unique. This specification
defines only "full ICE" mode, and the "lite ICE" is not supported.
The reasoning behind the missing features is described in Appendix B.

The connectivity check messages MUST be paced by the Ta value
negotiated during the base exchange as described in Section 4.4. If
neither one of the hosts announced a minimum pacing value, a value of
50 ms MUST be used.

Both hosts MUST form a priority ordered checklist and begin to check
transactions every Ta milliseconds as long as the checks are running
and there are candidate pairs whose tests have not started. The
retransmission timeout (RTO) for the connectivity check UPDATE
packets SHOULD be calculated as follows:

RTO = MAX (1000 ms, Ta * (Num-Waiting + Num-In-Progress))

In the RTO formula, Ta is the value used for the connectivity check
pacing, Num-Waiting is the number of pairs in the checklist in the
"Waiting" state, and Num-In-Progress is the number of pairs in the
"In-Progress" state. This is identical to the formula in [RFC8445]
when there is only one checklist. A smaller value than 1000 ms for
the RTO MUST NOT be used.

Each connectivity check request packet MUST contain a
CANDIDATE_PRIORITY parameter (see Section 5.14) with the priority
value that would be assigned to a peer-reflexive candidate if one was
learned from the corresponding check. An UPDATE packet that
acknowledges a connectivity check request MUST be sent from the same
address that received the check and delivered to the same address
where the check was received from. Each acknowledgment UPDATE packet
MUST contain a MAPPED_ADDRESS parameter with the port, protocol, and
IP address of the address where the connectivity check request was
received from.

Following the ICE guidelines [RFC8445], it is RECOMMENDED to restrict
the total number of connectivity checks to 100 for each host
association. This can be achieved by limiting the connectivity
checks to the 100 candidate pairs with the highest priority.

4.6.3. Rules for Concluding Connectivity Checks

The controlling agent may find multiple working candidate pairs. To
conclude the connectivity checks, it SHOULD nominate the pair with
the highest priority. The controlling agent MUST nominate a
candidate pair essentially by repeating a connectivity check using an
UPDATE message that contains a SEQ parameter (with a new sequence
number), an ECHO_REQUEST_SIGNED parameter, the priority of the
candidate in a CANDIDATE_PRIORITY parameter, and a NOMINATE parameter
to signify conclusion of the connectivity checks. Since the
nominated address pair has already been tested for reachability, the
controlled host should be able to receive the message. Upon
reception, the controlled host SHOULD select the nominated address
pair. The response message MUST include a SEQ parameter with a new
sequence identifier, acknowledgment of the sequence from the
controlling host in an ACK parameter, a new ECHO_REQUEST_SIGNED
parameter, an ECHO_RESPONSE_SIGNED parameter corresponding to the
ECHO_REQUEST_SIGNED parameter from the controlling host, and the
NOMINATE parameter. After sending this packet, the controlled host
can create IPsec security associations using the nominated address
candidate for delivering application payload to the controlling host.
Since the message from the controlled host included a new sequence
identifier echo request for the signature, the controlling host MUST
acknowledge this with a new UPDATE message that includes an ACK and
ECHO_RESPONSE_SIGNED parameters. After this final concluding
message, the controlling host also can create IPsec security
associations for delivering application payload to the controlled
host.

It is possible that packets are delayed by the network. Both hosts
MUST continue to respond to any connectivity checks despite an
address pair having been nominated.

If all the connectivity checks have failed, the hosts MUST NOT send
ESP traffic to each other but MAY continue communicating using HIP
packets and the locators used for the base exchange. Also, the hosts
SHOULD notify each other about the failure with a
CONNECTIVITY_CHECKS_FAILED NOTIFY packet (see Section 5.10).

4.7. NAT Traversal Optimizations

4.7.1. Minimal NAT Traversal Support

If the Responder has a fixed and publicly reachable IPv4 address and
does not employ a Control Relay Server, the explicit NAT traversal
mode negotiation MAY be omitted; thus, even the UDP-ENCAPSULATION
mode does not have to be negotiated. In such a scenario, the
Initiator sends an I1 message over UDP and the Responder responds
with an R1 message over UDP without including any NAT traversal mode
parameter. The rest of the base exchange follows the procedures
defined in [RFC7401], except that the control and data plane use UDP
encapsulation. Here, the use of UDP for NAT traversal is agreed upon
implicitly. This way of operation is still subject to NAT timeouts,
and the hosts MUST employ NAT keepalives as defined in Section 4.10.

When UDP-ENCAPSULATION mode is chosen either explicitly or
implicitly, the connectivity checks as defined in this document MUST
NOT be used. When hosts lose connectivity, they MUST instead utilize
[RFC8046] or [RFC8047] procedures, but with the difference being that
UDP-based tunneling MUST be employed for the entire lifetime of the
corresponding Host Association. HIP association.

4.7.2. Base Exchange without Connectivity Checks

It is possible to run a base exchange without any connectivity checks
as defined in Legacy ICE-HIP (Section 4.8 of [RFC5770]). The
procedure is also applicable in the context of this specification, so
it is repeated here for completeness.

In certain network environments, the connectivity checks can be
omitted to reduce initial connection setup latency because a base
exchange acts as an implicit connectivity test itself. For this to
work, the Initiator MUST be able to reach the Responder by simply UDP
encapsulating HIP and ESP packets sent to the Responder's address.
Detecting and configuring this particular scenario is prone to
failure unless carefully planned.

In such a scenario, the Responder MAY include UDP-ENCAPSULATION NAT
traversal mode as one of the supported modes in the R1 packet. If
the Responder has registered to a Control Relay Server in order to
discover its address candidates, it MUST also include a LOCATOR_SET
parameter encapsulated inside an ENCRYPTED parameter in an R1 message
that contains a preferred address where the Responder is able to
receive UDP-encapsulated ESP and HIP packets. This locator MUST be
of type "Transport address", its Traffic type MUST be "both", and it
MUST have the "Preferred bit" set (see Table 2). If there is no such
locator in R1, the Initiator MUST use the source address of the R1 as
the Responder's preferred address.

The Initiator MAY choose the UDP-ENCAPSULATION mode if the Responder
listed it in the supported modes and the Initiator does not wish to
use the connectivity checks defined in this document for searching
for a more optimal path. In this case, the Initiator sends the I2
with UDP-ENCAPSULATION mode in the NAT traversal mode parameter
directly to the Responder's preferred address (i.e., to the preferred
locator in R1 or to the address where R1 was received from if there
was no preferred locator in R1). The Initiator MAY include locators
in I2 but they MUST NOT be taken as address candidates, since
connectivity checks defined in this document will not be used for
connections with UDP-ENCAPSULATION NAT traversal mode. Instead, if
R2 and I2 are received and processed successfully, a security
association can be created and UDP-encapsulated ESP can be exchanged
between the hosts after the base exchange completes according to the
rules in Section 4.4 of [RFC7401].

The Control Relay Server can be used for discovering address
candidates but it is not intended to be used for relaying end-host
packets using the UDP-ENCAPSULATION NAT mode. Since an I2 packet
with UDP-ENCAPSULATION NAT traversal mode selected MUST NOT be sent
via a Control Relay Server, the Responder SHOULD reject such I2
packets and reply with a NO_VALID_NAT_TRAVERSAL_MODE_PARAMETER NOTIFY
packet (see Section 5.10).

If there is no answer for the I2 packet sent directly to the
Responder's preferred address, the Initiator MAY send another I2 via
the Control Relay Server, but it MUST NOT choose UDP-ENCAPSULATION
NAT traversal mode for that I2.

4.7.3. Initiating a Base Exchange Both with and without UDP
Encapsulation

It is possible to run a base exchange in parallel both with and
without UDP encapsulation as defined in Legacy ICE-HIP (Section 4.9
of [RFC5770]). The procedure is also applicable in the context of
this specification, so it is repeated here for completeness.

The Initiator MAY also try to simultaneously perform a base exchange
with the Responder without UDP encapsulation. In such a case, the
Initiator sends two I1 packets, one without and one with UDP
encapsulation, to the Responder. The Initiator MAY wait for a while
before sending the other I1. How long to wait and in which order to
send the I1 packets can be decided based on local policy. For
retransmissions, the procedure is repeated.

The I1 packet without UDP encapsulation may arrive directly, without
passing a Control Relay Server, at the Responder. When this happens,
the procedures in [RFC7401] are followed for the rest of the base
exchange. The Initiator may receive multiple R1 packets, with and
without UDP encapsulation, from the Responder. However, after
receiving a valid R1 and answering it with an I2, further R1 packets
that are not retransmissions of the R1 message received first MUST be
ignored.

The I1 packet without UDP encapsulation may also arrive at a HIP-
capable middlebox. When the middlebox is a HIP rendezvous server Rendezvous Server and
the Responder has successfully registered with the rendezvous
service, the middlebox follows rendezvous procedures in [RFC8004].

If the Initiator receives a NAT traversal mode parameter in R1
without UDP encapsulation, the Initiator MAY ignore this parameter
and send an I2 without UDP encapsulation and without any selected NAT
traversal mode. When the Responder receives the I2 without UDP
encapsulation and without NAT traversal mode, it will assume that no
NAT traversal mechanism is needed. The packet processing will be
done as described in [RFC7401]. The Initiator MAY store the NAT
traversal modes for future use, e.g., in case of a mobility or
multihoming event that causes NAT traversal to be used during the
lifetime of the HIP association.

4.8. Sending Control Packets after the Base Exchange

The same considerations with regard to sending control packets after
the base exchange as described in Legacy ICE-HIP (Section 5.10 of
[RFC5770]) also apply here, so they are repeated here for
completeness.

After the base exchange, the two end hosts MAY send HIP control
packets directly to each other using the transport address pair
established for a data channel without sending the control packets
through any Control Relay Servers . Servers. When a host does not receive
acknowledgments, e.g., to an UPDATE or CLOSE packet after a timeout
based on local policies, a host SHOULD resend the packet through the
associated Data Relay Server of the peer (if the peer listed it in
its LOCATOR_SET parameter in the base exchange according to the rules
specified in Section 4.4.2 of [RFC7401]).

If a Control Relay Client sends a packet through a Control Relay
Server, the Control Relay Client MUST always utilize the RELAY_TO
parameter. The Control Relay Server SHOULD forward HIP control
packets originating from a Control Relay Client to the address
denoted in the RELAY_TO parameter. In the other direction, the
Control Relay Server SHOULD forward HIP control packets to the
Control Relay Clients and MUST add a RELAY_FROM parameter to the
control packets it relays to the Control Relay Clients.

If the Control Relay Server is not willing or able to relay a HIP
packet, it MAY notify the sender of the packet with a
MESSAGE_NOT_RELAYED error notification (see Section 5.10).

4.9. Mobility Handover Procedure

A host may move after base exchange and connectivity checks.
Mobility extensions for HIP [RFC8046] define handover procedures
without NATs. In this section, we define how two hosts interact with
handover procedures in scenarios involving NATs. The specified
extensions define only simple mobility using a pair of security
associations, and multihoming extensions are left to be defined in
later specifications. The procedures in this section offer the same
functionality as "ICE restart" specified in [RFC8445]. The example
described in this section shows only a Control Relay Server for the
peer host for the sake of simplicity, but the mobile host may also
have a Control Relay Server.

The assumption here is that the two hosts have successfully
negotiated and chosen the ICE-HIP-UDP mode during the base exchange
as defined in Section 4.3. The Initiator of the base exchange MUST
store information that it was the controlling host during the base
exchange. Similarly, the Responder MUST store information that it
was the controlled host during the base exchange.

Prior to starting the handover procedures with all peer hosts, the
mobile host SHOULD first send its locators in UPDATE messages to its
Control and Data Relay Servers if it has registered to such. It
SHOULD wait for all of them to respond for a configurable time, by
default two minutes, and then continue with the handover procedure
without information from the Relay Server that did not respond. As
defined in Section 4.1, a response message from a Control Relay
Server includes a REG_FROM parameter that describes the server-
reflexive candidate of the mobile host to be used in the candidate
exchange during the handover. Similarly, an UPDATE to a Data Relay
Server is necessary to make sure the Data Relay Server can forward
data to the correct IP address after a handoff. handover.

The mobility extensions for NAT traversal are illustrated in
Figure 6. The mobile host is the host that has changed its locators,
and the peer host is the host it has a host association with. The
mobile host may have multiple peers, and it repeats the process with
all of its peers. In the figure, the Control Relay Server belongs to
the peer host, i.e., the peer host is a Control Relay Client for the
Control Relay Server. Note that the figure corresponds to figure 3
in [RFC8046], but the difference is that the main UPDATE procedure is
carried over the relay and the connectivity is tested separately.
Next, we describe the procedure of that figure in detail.

Figure 6: HIP UPDATE Procedure

In step 1, the mobile host has changed location and sends a location
update to its peer through the Control Relay Server of the peer. It
sends an UPDATE packet with the source HIT belonging to itself and
destination HIT belonging to the peer host. In the packet, the
source IP address belongs to the mobile host and the destination to
the Control Relay Server. The packet contains an ESP_INFO parameter
where, in this case, the OLD SPI and NEW SPI parameters both contain
the pre-existing incoming SPI. The packet also contains the locators
of the mobile host in a LOCATOR_SET parameter, encapsulated inside an
ENCRYPTED parameter (see Sections 5.2.18 and 6.5 in [RFC7401] for
details on the ENCRYPTED parameter). The packet also contains a SEQ
number to be acknowledged by the peer. As specified in [RFC8046],
the packet may also include a HOST_ID (for middlebox inspection) and
DIFFIE_HELLMAN parameter for rekeying.

In step 2, the Control Relay Server receives the UPDATE packet and
forwards it to the peer host (i.e., Control Relay Client). The
Control Relay Server rewrites the destination IP address and appends
a RELAY_FROM parameter to the message.

In step 3, the peer host receives the UPDATE packet, processes it,
and responds with another UPDATE message. The message is destined to
the HIT of the mobile host and to the IP address of the Control Relay
Server. The message includes an ESP_INFO parameter where, in this
case, the OLD SPI and NEW SPI parameters both contain the pre-
existing incoming SPI. The peer includes a new SEQ and
ECHO_REQUEST_SIGNED parameter to be acknowledged by the mobile host.
The message acknowledges the SEQ parameter of the earlier message
with an ACK parameter. The RELAY_TO parameter specifies the address
of the mobile host where the Control Relay Server should forward the
message.

In step 4, the Control Relay Server receives the message, rewrites
the destination IP address and UDP port according to the RELAY_TO
parameter, and then forwards the modified message to the mobile host.

In step 5, the mobile host receives the UPDATE packet and processes
it. The mobile host concludes the handover procedure by
acknowledging the received SEQ parameter with an ACK parameter and
the ECHO_REQUEST_SIGNED parameter with an ECHO_RESPONSE_SIGNED
parameter. The mobile host delivers sends the packet to the HIT of the peer
and to the address of the HIP relay. The mobile host can start
connectivity checks after this packet.

In step 6, the HIP relay receives the UPDATE packet and forwards it
to the peer host (i.e., Relay Client). The HIP relay rewrites the
destination IP address and port, and then appends a RELAY_FROM
parameter to the message. When the peer host receives this
concluding UPDATE packet, it can initiate the connectivity checks.

In step 7, the two hosts test for connectivity across NATs according
to procedures described in Section 4.6. The original Initiator of
the communications is the controlling host and the original Responder
is the controlled host.

In step 8, the connectivity checks are successfully completed and the
controlling host has nominated one address pair to be used. The
hosts set up security associations to deliver the application
payload.

It is worth noting that the Control and Data Relay Client do not have
to reregister for the related services after a handoff. handover. However, if
a Data Relay Client has registered a relayed address candidate from a
Data Relay Server, the Data Relay Client MUST reactivate the address
before the connectivity checks by sending an UPDATE message
containing the PEER_PERMISSION parameter as described in
Section 4.12.1. Otherwise, the Data Relay Server drops ESP packets
sent to the relayed address.

In the so-called "double jump" or simultaneous mobility scenario,
both peers change their location simultaneously. In such a case,
both peers trigger the procedure described earlier in this section at
the same time. In other words, both of the communicating hosts send
an UPDATE packet carrying locators at the same time or with some
delay. When the locators are exchanged almost simultaneously
(reliably via Control Relay Servers), the two hosts can continue with
connectivity checks after both have completed separately the steps in
Figure 6. The problematic case occurs when one of the hosts
(referred to here as host "M") moves later during the connectivity
checks. In such a case, host M sends a locator to the peer, which is
in the middle of connectivity checks. Upon receiving the UPDATE
message, the peer responds with an UPDATE with ECHO_REQ_SIGN as
described in step 3 in Figure 6. Upon receiving the valid response
from host M as described in step 6, the peer host MUST restart the
connectivity checks with host M. This way, both hosts start the
connectivity checks roughly in a synchronized way. It is also
important that the peer host does not restart the connectivity checks
until step 6 is successfully completed, because the UPDATE message
carrying locators in step 1 could be replayed by an attacker.

4.10. NAT Keepalives

To prevent NAT states from expiring, communicating hosts MUST send
periodic keepalives to other hosts with which they have established a
Host Association
HIP association every 15 seconds (the so-called Tr value in ICE).
Other values MAY be used, but a Tr value smaller than 15 seconds MUST
NOT be used. Both a Control/Data Relay Client and Control/Data Relay
Server, as well as two peers employing UDP-ENCAPSULATION or ICE-HIP-
UDP mode, SHOULD send HIP NOTIFY packets unless they have exchanged
some other traffic over the used UDP ports. However, the Data Relay
Client and Data Relay Server MUST employ only HIP NOTIFY packets in
order to keep the server-reflexive candidates alive. The keepalive
message encoding format is defined in Section 5.3. If the base
exchange or mobility handover procedure occurs during an extremely
slow path, a host (with a Host Association HIP association with the peer) MAY also
send HIP NOTIFY packets every 15 seconds to keep the path active with
the recipient.

4.11. Closing Procedure

The two-way procedure for closing a HIP association and the related
security associations is defined in [RFC7401]. One host initiates
the procedure by sending a CLOSE message and the recipient confirms
it with CLOSE_ACK. All packets are protected using HMACs and
signatures, and the CLOSE messages include an ECHO_REQUEST_SIGNED
parameter to protect against replay attacks.

The same procedure for closing HIP associations also applies here,
but the messaging occurs using the UDP-encapsulated tunnel that the
two hosts employ. A host sending the CLOSE message SHOULD first send
the message over a direct link. After a number of retransmissions,
it MUST send over a Control Relay Server of the recipient if one
exists. The host receiving the CLOSE message directly without a
Control Relay Server SHOULD respond directly. If the CLOSE message
came via a Control Relay Server, the host SHOULD respond using the
same Control Relay Server.

4.12. Relaying Considerations

4.12.1. Forwarding Rules and Permissions

The Data Relay Server uses a similar permission model as a TURN
server: before the Data Relay Server forwards any ESP data packets
from a peer to a Data Relay Client (or the other direction), the
client MUST set a permission for the peer's address. The permissions
also install a forwarding rule for each direction, similar to TURN's
channels, based on the Security Parameter Index (SPI) values in the
ESP packets.

Permissions are not required for HIP control packets. However, if a
relayed address (as conveyed in the RELAYED_ADDRESS parameter from
the Data Relay Server) is selected to be used for data, the Control
Relay Client MUST send an UPDATE message to the Data Relay Server
containing a PEER_PERMISSION parameter (see Section 5.13) with the
following information: the UDP port and address for the server-
reflexive address, the UDP port and address of the peer, and the
inbound and outbound SPIs used for ESP. The packet MUST be sent to
the same UDP tunnel the Client employed in the base exchange to
contact the Server (i.e., not to the port occupied by the server-
reflexive candidate). To avoid packet dropping of ESP packets, the
Control Relay Client SHOULD send the PEER_PERMISSION parameter before
connectivity checks both in the case of base exchange and a mobility
handover. It is worth noting that the UPDATE message includes a SEQ
parameter (as specified in [RFC7401]) that the Data Relay Server must
acknowledge, so that the Control Relay Client can resend the message
with the PEER_PERMISSION parameter if it gets lost.

When a Data Relay Server receives an UPDATE with a PEER_PERMISSION
parameter, it MUST check if the sender of the UPDATE is registered
for data-relaying service, and drop the UPDATE if the host was not
registered. If the host was registered, the Data Relay Server checks
if there is a permission with matching information (protocol,
addresses, ports, and SPI values). If there is no such permission, a
new permission MUST be created and its lifetime MUST be set to 5
minutes. If an identical permission already existed, it MUST be
refreshed by setting the lifetime to 5 minutes. A Data Relay Client
SHOULD refresh permissions 1 minute before the expiration when the
permission is still needed.

When a Data Relay Server receives an UPDATE from a registered client
but without a PEER_PERMISSION parameter and with a new locator set,
the Data Relay Server can assume that the mobile host has changed its
location and is thus not reachable in its previous location. In such
an event, the Data Relay Server SHOULD deactivate the permission and
stop relaying data plane traffic to the client.

The relayed address MUST be activated with the PEER_PERMISSION
parameter both after a base exchange and after a handover procedure
with another ICE-HIP-UDP-capable host. Unless activated, the Data
Relay Server MUST drop all ESP packets. It is worth noting that a
Data Relay Client does not have to renew its registration upon a
change of location UPDATE, but only when the lifetime of the
registration is close to end.

4.12.2. HIP Data Relay and Relaying of Control Packets

When a Data Relay Server accepts to relay UDP-encapsulated ESP
between a Data Relay Client and its peer, the Data Relay Server opens
a UDP port (relayed address) for this purpose as described in
Section 4.1. This port can be used for also delivering control
packets because connectivity checks also cover the path through the
Data Relay Server. If the Data Relay Server receives a UDP-
encapsulated HIP control packet on that port, it MUST forward the
packet to the Data Relay Client and add a RELAY_FROM parameter to the
packet as if the Data Relay Server were acting as a Control Relay
Server. When the Data Relay Client replies to a control packet with
a RELAY_FROM parameter via its Data Relay Server, the Data Relay
Client MUST add a RELAY_TO parameter containing the peer's address
and use the address of its Data Relay Server as the destination
address. Further, the Data Relay Server MUST send this packet to the
peer's address from the relayed address.

If the Data Relay Server receives a UDP packet that is not a HIP
control packet to the relayed address, it MUST check if it has a
permission set for the peer the packet is arriving from (i.e., the
sender's address and SPI value matches to an installed permission).
If permissions are set, the Data Relay Server MUST forward the packet
to the Data Relay Client that created the permission. The Data Relay
Server MUST also implement the similar checks for the reverse
direction (i.e., ESP packets from the Data Relay Client to the peer).
Packets without a permission MUST be dropped silently.

4.12.3. Handling Conflicting SPI Values

From the viewpoint of a host, its remote peers can have overlapping
inbound SPI numbers because the IPsec also uses the destination IP
address to index the remote peer host. However, a Data Relay Server
can represent multiple remote peers, thus masquerading the actual
destination. Since a Data Relay Server may have to deal with a
multitude of Relay Clients and their peers, a Data Relay Server may
experience collisions in the SPI namespace, thus being unable to
forward datagrams to the correct destination. Since the SPI space is
32 bits and the SPI values should be random, the probability for a
conflicting SPI value is fairly small but could occur on a busy Data
Relay Server. The two problematic cases are described in this
section.

In the first scenario, the SPI collision problem occurs if two hosts
have registered to the same Data Relay Server and a third host
initiates base exchange with both of them. Here, the two Responders
(i.e., Data Relay Clients) claim the same inbound SPI number with the
same Initiator (peer). However, in this case, the Data Relay Server
has allocated separate UDP ports for the two Data Relay Clients
acting now as Responders (as recommended in Section 6.5). 7.5). When the
third host sends an ESP packet, the Data Relay Server is able to
forward the packet to the correct Data Relay Client because the
destination UDP port is different for each of the clients.

In the second scenario, an SPI collision may occur when two
Initiators run a base exchange to the same Responder (i.e., Data
Relay Client), and both of the Initiators claim the same inbound SPI
at the Data Relay Server using the PEER_PERMISSION parameter. In
this case, the Data Relay Server cannot disambiguate the correct
destination of an ESP packet originating from the Data Relay Client
because the SPI could belong to either of the peers (and the
destination IP and UDP port belonging to the Data Relay Server are
not unique either). The recommended way and a contingency plan to
solve this issue are described below.

The recommend way to mitigate the problem is as follows. For each
new Host Association, HIP association, a Data Relay Client acting as a Responder SHOULD
register a new server-reflexive candidate as described in
Section 4.2. Similarly, the Data Relay Server SHOULD NOT reuse the
port numbers as described in Section 6.5. 7.5. This way, each server-
reflexive candidate for the Data Relay Client has a separate UDP port
that the Data Relay Server can use to disambiguate packet
destinations in case of SPI collisions.

When the Data Relay Client is not registering or failed to register a
new relay candidate for a new peer, the Data Relay Client MUST follow
a contingency plan as follows. Upon receiving an I2 with a colliding
SPI, the Data Relay Client acting as the Responder MUST NOT include
the relayed address candidate in the R2 message because the Data
Relay Server would not be able to demultiplex the related ESP packet
to the correct Initiator. The same also applies to the handover
procedures; the Data Relay Client MUST NOT include the relayed
address candidate when sending its new locator set in an UPDATE to
its peer if it would cause an SPI conflict with another peer.

5. Packet Formats

The following subsections define the parameter and packet encodings
for the HIP and ESP packets. All values MUST be in network byte
order.

It is worth noting that all of the parameters are shown for the sake
of completeness even though they are specified already in Legacy ICE-
HIP [RFC5770]. New parameters are explicitly described as new.

5.1. HIP Control Packets

Figure 7 illustrates the packet format for UDP-encapsulated HIP. The
format is identical to Legacy ICE-HIP [RFC5770].

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Port | Destination Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 32 bits of zeroes |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ HIP Header and Parameters ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 7: Format of UDP-Encapsulated HIP Control Packets

HIP control packets are encapsulated in UDP packets as defined in
Section 2.2 of [RFC3948], "IKE Header Format for Port 4500", except
that a different port number is used. Figure 7 illustrates the
encapsulation. The UDP header is followed by 32 zero bits that can
be used to differentiate HIP control packets from ESP packets. The
HIP header and parameters follow the conventions of [RFC7401] with
the exception that the HIP header checksum MUST be zero. The HIP
header checksum is zero for two reasons. First, the UDP header
already contains a checksum. Second, the checksum definition in
[RFC7401] includes the IP addresses in the checksum calculation. The
NATs that are unaware of HIP cannot recompute the HIP checksum after
changing IP addresses.

A Control/Data Relay Server or a non-relay Responder SHOULD listen at
UDP port 10500 for incoming UDP-encapsulated HIP control packets. If
some other port number is used, it needs to be known by potential
Initiators.

UDP encapsulation of HIP packets reduces the Maximum Transfer Transmission
Unit (MTU) size of the control plane by 12 bytes (8-byte UDP header
plus 4-byte zero SPI marker), and the data plane by 8 bytes.
Additional HIP relay parameters, such as RELAY_HMAC, RELAY_UDP_HIP,
RELAY_UDP_ESP, etc., further increase the size of certain HIP
packets. In regard to MTU, the following aspects need to be
considered in an implementation:

* A HIP host SHOULD implement ICMP message handling to support Path
MTU Discovery (PMTUD) as described in [RFC1191] and [RFC8201].

* Reliance on IP fragmentation is unlikely to be a viable strategy
through NATs. If ICMP MTU discovery is not working, MTU-related
path black holes may occur.

* A mitigation strategy is to constrain the MTU, especially for
virtual interfaces, to expected safe MTU values, e.g., 1400 bytes
for the underlying interfaces that support 1500 bytes MTU.

* Further extensions to this specification may define a HIP-based
mechanism to find a working path MTU without unnecessary
constraining that size using Packetization Layer Path MTU
Discovery for Datagram Transports [RFC8899]. For instance, such a
mechanism could be implemented between a HIP Relay Client and HIP
Relay Server.

* It is worth noting that further HIP extensions can trim off 8
bytes in the ESP header by negotiating implicit initialization
vector (IV) support in the ESP_TRANSFORM parameter as described in
[RFC8750].

5.2. Connectivity Checks

HIP connectivity checks are HIP UPDATE packets. The format is
specified in [RFC7401].

5.3. Keepalives

The RECOMMENDED encoding format for keepalives is HIP NOTIFY packets
as specified in [RFC7401] with the Notify message type field set to
NAT_KEEPALIVE (16385) and with an empty Notification data field. It
is worth noting that the sending of such a HIP NOTIFY message SHOULD
be omitted if the host is actively (or passively) sending some other traffic (HIP or ESP) to
the peer host over the related UDP tunnel during the Tr period. For
instance, the host MAY actively send ICMPv6 requests (or respond with
an ICMPv6 response) inside the ESP tunnel to test the health of the
associated IPsec security association. Alternatively, the host MAY
use UPDATE packets as a substitute. A minimal UPDATE packet would
consist of a SEQ and a single ECHO_REQ_SIGN parameters, parameter, and a more
complex one would involve rekeying procedures as specified in
Section 6.8 of [RFC7402]. It is worth noting that a host actively
sending periodic UPDATE packets to a busy server may increase the
computational load of the server since it has to verify HMACs and
signatures in UPDATE messages.

5.4. NAT Traversal Mode Parameter

The format of the NAT traversal mode parameter is defined in Legacy
ICE-HIP [RFC5770] but repeated here for completeness. The format of
the NAT_TRAVERSAL_MODE parameter is similar to the format of the
ESP_TRANSFORM parameter in [RFC7402] and is shown in Figure 8. The
Native ICE-HIP extension specified in this document defines the new
NAT traversal mode identifier for ICE-HIP-UDP and reuses the UDP-
ENCAPSULATION mode from Legacy ICE-HIP [RFC5770]. The identifier
named RESERVED is reserved for future use. Future specifications may
define more traversal modes.

Figure 8: Format of the NAT_TRAVERSAL_MODE Parameter

Type: 608

Length: Length in octets, excluding Type, Length, and padding Padding

Reserved: Zero when sent, ignored when received

Mode ID: Defines the proposed or selected NAT traversal mode(s)

The following NAT traversal mode IDs are defined:

+===================+=======+
| ID name | Value |
+===================+=======+
| RESERVED | 0 |
+-------------------+-------+
| UDP-ENCAPSULATION | 1 |
+-------------------+-------+
| ICE-STUN-UDP | 2 |
+-------------------+-------+
| ICE-HIP-UDP | 3 |
+-------------------+-------+

Table 1: NAT Traversal
Mode IDs

The sender of a NAT_TRAVERSAL_MODE parameter MUST make sure that
there are no more than six (6) Mode IDs in one NAT_TRAVERSAL_MODE
parameter. Conversely, a recipient MUST be prepared to handle
received NAT traversal mode parameters that contain more than six
Mode IDs by accepting the first six Mode IDs and dropping the rest.
The limited number of Mode IDs sets the maximum size of the
NAT_TRAVERSAL_MODE parameter. The modes MUST be in preference order,
most preferred mode(s) first.

Implementations conforming to this specification MUST implement UDP-
ENCAPSULATION and SHOULD implement ICE-HIP-UDP modes.

5.5. Connectivity Check Transaction Pacing Parameter

The TRANSACTION_PACING parameter is defined in [RFC5770] but repeated
in Figure 9 for completeness. It contains only the connectivity
check pacing value, expressed in milliseconds, as a 32-bit unsigned
integer.

Figure 9: Format of the TRANSACTION_PACING Parameter

Type: 610

Length: 4

Min Ta: The minimum connectivity check transaction pacing value
the host would use (in milliseconds)

5.6. Relay and Registration Parameters

The format of the REG_FROM, RELAY_FROM, and RELAY_TO parameters is
shown in Figure 10. All parameters are identical except for the
type. Of the three, only REG_FROM is covered by the signature.

Figure 10: Format of the REG_FROM, RELAY_FROM, and RELAY_TO
Parameters

Type: REG_FROM: 950
RELAY_FROM: 63998
RELAY_TO: 64002

Length: 20

Port: Transport port number; zero when plain IP is used

Protocol: IANA-assigned, Internet Protocol number. 17 for UDP; 0
for plain IP

Reserved: Reserved for future use; zero when sent, ignored when
received

Address: An IPv6 address or an IPv4 address in "IPv4-mapped IPv6
address" format

REG_FROM contains the transport address and protocol from which the
Control Relay Server sees the registration coming. RELAY_FROM
contains the address from which the relayed packet was received by
the Control Relay Server and the protocol that was used. RELAY_TO
contains the same information about the address to which a packet
should be forwarded.

5.7. LOCATOR_SET Parameter

This specification reuses the format for UDP-based locators as
specified in Legacy ICE-HIP [RFC5770] to be used for communicating
the address candidates between two hosts. The generic and NAT-
traversal-specific locator parameters are illustrated in Figure 11.

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Traffic Type | Locator Type | Locator Length| Reserved |P|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Locator Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Locator |
| |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Traffic Type | Loc Type = 2 | Locator Length| Reserved |P|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Locator Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Transport Port | Transp. Proto| Kind |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Priority |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SPI |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address |
| |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 11: LOCATOR_SET Parameter

The individual fields in the LOCATOR_SET parameter are described in
Table 2.

+===========+==========+============================================+

Table 2: Fields of the LOCATOR_SET Parameter

The LOCATOR parameter MUST be encapsulated inside an ENCRYPTED
parameter.

5.8. RELAY_HMAC Parameter

As specified in Legacy ICE-HIP [RFC5770], the RELAY_HMAC parameter
value has the TLV type 65520. It has the same semantics as RVS_HMAC
as specified in Section 4.2.1 of [RFC8004]. Similar to RVS_HMAC,
RELAY_HMAC is also keyed with the HIP integrity key (HIP-lg or HIP-gl
as specified in Section 6.5 of [RFC7401]), established during the
relay registration procedure as described in Section 4.1.

5.9. Registration Types

The REG_INFO, REG_REQ, REG_RESP, and REG_FAILED parameters contain
Registration Type [RFC8003] values for Control Relay Server
registration. The value for RELAY_UDP_HIP is 2 as specified in
Legacy ICE-HIP [RFC5770]. The value for RELAY_UDP_ESP is 3.

5.10. Notify Packet Types

A Control/Data Relay Server and end hosts can use NOTIFY packets to
signal different error conditions. The NOTIFY packet types are the
same as in Legacy ICE-HIP [RFC5770] except for the two last ones,
which are new.

The Notify Packet Types [RFC7401] are shown below. The Notification
Data field for the error notifications SHOULD contain the HIP header
of the rejected packet and SHOULD be empty for the
CONNECTIVITY_CHECKS_FAILED type.

Table 3: Notify Packet Types

5.11. ESP Data Packets

The format for ESP data packets is identical to Legacy ICE-HIP
[RFC5770].

[RFC3948] describes the UDP encapsulation of the IPsec ESP transport
and tunnel mode. On the wire, the HIP ESP packets do not differ from
the transport mode ESP; thus, the encapsulation of the HIP ESP
packets is same as the UDP encapsulation transport mode ESP.
However, the (semantic) difference to Bound End-to-End Tunnel (BEET)
mode ESP packets used by HIP is that the IP header is not used in
BEET integrity protection calculation.

During the HIP base exchange, the two peers exchange parameters that
enable them to define a pair of IPsec ESP security associations (SAs)
as described in [RFC7402]. When two peers perform a UDP-encapsulated
base exchange, they MUST define a pair of IPsec SAs that produces
UDP-encapsulated ESP data traffic.

The management of encryption/authentication protocols and SPIs is
defined in [RFC7402]. The UDP encapsulation format and processing of
HIP ESP traffic is described in Section 6.1 of [RFC7402].

5.12. RELAYED_ADDRESS and MAPPED_ADDRESS Parameters

While the type values are new, the format of the RELAYED_ADDRESS and
MAPPED_ADDRESS parameters (Figure 12) is identical to REG_FROM,
RELAY_FROM, and RELAY_TO parameters. This document specifies only
the use of UDP relaying; thus, only protocol 17 is allowed. However,
future documents may specify support for other protocols.

Figure 12: Format of the RELAYED_ADDRESS and MAPPED_ADDRESS
Parameters

Type: RELAYED_ADDRESS: 4650
MAPPED_ADDRESS: 4660

Length: 20

Port: The UDP port number

Protocol: IANA-assigned, Internet Protocol number (17 for UDP)

Reserved: Reserved for future use; zero when sent, ignored when
received

Address: An IPv6 address or an IPv4 address in "IPv4-mapped IPv6
address" format

5.13. PEER_PERMISSION Parameter

The format of the new PEER_PERMISSION parameter is shown in
Figure 13. The parameter is used for setting up and refreshing
forwarding rules and the permissions for data packets at the Data
Relay Server. The parameter contains one or more sets of Port,
Protocol, Address, Outbound SPI (OSPI), and Inbound SPI (ISPI)
values. One set defines a rule for one peer address.

Figure 13: Format of the PEER_PERMISSION Parameter

Type: 4680

Length: 48

RPort: The transport-layer (UDP) port at the Data Relay Server
(i.e., the port of the server-reflexive candidate)

PPort: The transport-layer (UDP) port number of the peer

Protocol: IANA-assigned, Internet Protocol number (17 for UDP)

Reserved: Reserved for future use; zero when sent, ignored when
received

RAddress: An IPv6 address, or an IPv4 address in "IPv4-mapped IPv6
address" format, of the server-reflexive candidate

PAddress: An IPv6 address, or an IPv4 address in "IPv4-mapped IPv6
address" format, of the peer

OSPI: The outbound SPI value the Data Relay Client is using for
the peer

ISPI: The inbound SPI value the Data Relay Client is using for
the peer

5.14. HIP Connectivity Check Packets

The connectivity request messages are HIP UPDATE packets containing a
new CANDIDATE_PRIORITY parameter (Figure 14). Response UPDATE
packets contain a MAPPED_ADDRESS parameter (Figure 12).

Figure 14: Format of the CANDIDATE_PRIORITY Parameter

Type: 4700

Length: 4

Priority: The priority of a (potential) peer-reflexive candidate

5.15. NOMINATE Parameter

Figure 15 shows the NOMINATE parameter that is used to conclude the
candidate nomination process.

Figure 15: Format of the NOMINATE Parameter

Type: 4710

Length: 4

Reserved: Reserved for future extension purposes

6. IAB Considerations

The ICE specification [RFC8445] discusses "Unilateral Self-Address
Fixing" in Section 18. This protocol is based on ICE; thus, the same
considerations also apply here.

7. Security Considerations

Since the control plane protocol and Control Relay Server are
essentially the same (with some minor differences) in this document
as in Legacy ICE-HIP [RFC5770], the same security considerations (in
Sections 6.1, 6.2, 6.3, 7.1, 7.2, 7.3, and 6.4) 7.4) are still valid, but are repeated
here for the sake of completeness. New security considerations
related to the new Data Relay Server are discussed in Section 6.5, 7.5,
and considerations related to the new connectivity check protocol are
discussed in Sections 6.6 7.6 and 6.7.

6.1. 7.7.

7.1. Privacy Considerations

It is also possible that end users may not want to reveal all
locators to each other. For example, tracking the physical location
of a multihoming end host may become easier if it reveals all
locators to its peer during a base exchange. Also, revealing host
addresses exposes information about the local topology that may not
be allowed in all corporate environments. For these two local policy
reasons, it might be tempting to exclude certain host addresses from
the LOCATOR_SET parameter of an end host, but this is NOT
RECOMMENDED. For instance, such behavior creates non-optimal paths
when the hosts are located behind the same NAT. Especially, this
could be problematic with a legacy NAT that does not support routing
from the private address realm back to itself through the outer
address of the NAT. This scenario is referred to as the hairpin
problem [RFC5128]. With such a legacy NAT, the only option left
would be to use a relayed transport address from a Data Relay Server.

The use of Control and Data Relay Servers can also be useful for
privacy purposes. For example, a privacy-concerned Responder may
reveal only its Control Relay Server and Relayed candidates to
Initiators. This partially protects the Responder against Denial-of-
Service (DoS) attacks by allowing the Responder to initiate new
connections even if its relays would be unavailable due to a DoS
attack.

6.2.

7.2. Opportunistic Mode

In opportunistic HIP mode (cf. Section 4.1.8 of [RFC7401]), an
Initiator sends an I1 without setting the destination HIT of the
Responder (i.e., the Control Relay Client). A Control Relay Server
SHOULD have a unique IP address per the Control Relay Client when the
Control Relay Server is serving more than one Control Relay Client
and supports opportunistic mode. Otherwise, the Control Relay Server
cannot guarantee to deliver the I1 packet to the intended recipient.
Future extensions of this document may allow opportunistic mode to be
used with non-unique IP addresses to be utilized either as a HIP-
level anycast or multicast mechanism. Both of the mentioned cases
would require separate registration parameters that the Control Relay
Server proposes and the Control Client Server accepts during
registration.

6.3.

7.3. Base Exchange Replay Protection for Control Relay Server

In certain scenarios, it is possible that an attacker, or two
attackers, can replay an earlier base exchange through a Control
Relay Server by masquerading as the original Initiator and Responder.
The attack does not require the attacker(s) to compromise the private
key(s) of the attacked host(s). However, for this attack to succeed,
the legitimate Responder has to be disconnected from the Control
Relay Server.

The Control Relay Server can protect itself against replay attacks by
becoming involved in the base exchange by introducing nonces that the
end hosts (Initiator and Responder) are required to sign. One way to
do this is to add ECHO_REQUEST_M parameters to the R1 and I2 packets
as described in [HIP-MIDDLEBOXES] and drop the I2 or R2 packets if
the corresponding ECHO_RESPONSE_M parameters are not present.

6.4.

7.4. Demultiplexing Different HIP Associations

Section 5.1 of [RFC3948] describes a security issue for the UDP
encapsulation in the standard IP tunnel mode when two hosts behind
different NATs have the same private IP address and initiate
communication to the same Responder in the public Internet. The
Responder cannot distinguish between two hosts because security
associations are based on the same inner IP addresses.

This issue does not exist with the UDP encapsulation of HIP ESP
transport format because the Responder uses HITs to distinguish
between different Initiators.

6.5.

7.5. Reuse of Ports at the Data Relay Server

If the Data Relay Server uses the same relayed address and port (as
conveyed in the RELAYED_ADDRESS parameter) for multiple Data Relay
Clients, it appears to all the peers, and their firewalls, that all
the Data Relay Clients are at the same address. Thus, a stateful
firewall may allow packets to pass from hosts that would not normally
be able to send packets to a peer behind the firewall. Therefore, a
Data Relay Server SHOULD NOT reuse the port numbers. If port numbers
need to be reused, the Data Relay Server SHOULD have a sufficiently
large pool of port numbers and randomly select ports from the pool to
decrease the chances of a Data Relay Client obtaining the same
address that another host behind the same firewall is using.

6.6.

7.6. Amplification Attacks

A malicious host may send an invalid list of candidates to its peer
that are used for targeting a victim host by flooding it with
connectivity checks. To mitigate the attack, this protocol adopts
the ICE mechanism to cap the total amount of connectivity checks as
defined in Section 4.7.

6.7.

7.7. Attacks against Connectivity Checks and Candidate Gathering

Section 19.2 of [RFC8445] describes attacks against ICE connectivity
checks. HIP bases its control plane security on Diffie-Hellman key
exchange, public keys, and Hashed Message Authentication codes,
meaning that the mentioned security concerns do not apply to HIP
either. The mentioned section also discusses man-in-the-middle
replay attacks that are difficult to prevent. The connectivity
checks in this protocol are effectively immune against replay attacks
because a connectivity request includes a random nonce that the
recipient must sign and send back as a response.

Section 19.3 of [RFC8445] describes attacks on server-reflexive
address gathering. Similarly here, if the DNS, a Control Relay
Server, or a Data Relay Server has been compromised, not much can be
done. However, the case where attackers can inject fake messages
(located on a shared network segment like Wi-Fi) does not apply here.
HIP messages are integrity and replay protected, so it is not
possible to inject fake server-reflexive address candidates.

Section 19.4 of [RFC8445] describes attacks on relayed candidate
gathering. Similarly to ICE TURN servers, a Data Relay Server
requires an authenticated base exchange that protects relayed address
gathering against fake requests and responses. Further, replay
attacks are not possible because the HIP base exchange (and also
UPDATE procedure) is protected against replay attacks.

6.8.

7.8. Cross-Protocol Attacks

Section 4.1 explains how a Control Relay Client registers for the
RELAY_UDP_HIP service from a Control Relay Server. However, the same
server may also offer Rendezvous functionality; thus, a client can
register both to a RELAY_UDP_HIP and a RENDEZVOUS (see [RFC8004])
service from the same server. Potentially, this introduces a cross-
protocol attack (or actually a "cross-message" attack) because the
key material is the same for the Control Relay Service and Rendezvous
HMACs. While the problem could be avoided by deriving different keys
for the Control Relay Service, a more simple measure was chosen
because the exact attack scenario was unclear. Consequently, this
section defines a mandatory mitigation mechanism against the cross-
protocol attack that works by preventing the simultaneous use of
Rendezvous and Control Relay Service in the context of a single HIP
Association.

The registration involves three parameters typically delivered
sequentially in R1 (REG_INFO parameter), I2 (REG_REQUEST), and R2
(REG_RESPONSE) messages but can also be delivered in UPDATE messages
as described in [RFC8003]. The parameters and the modifications to
their processing are described below:

REG_INFO: The Control Relay Server advertises its available services
using this parameter. RELAY_UDP_HIP and RENDEZVOUS services MAY
be included in the first advertisement for the HIP association,
but subsequent ones MUST include only one of them as agreed in
earlier registrations (see steps 2 and 3).

REG_REQUEST: The Control Relay Client chooses the services it
requires using this parameter. If the Control Relay Server
offered both RENDEZVOUS or RELAY_UDP_HIP, the Control Relay Client
MUST choose only one of them in the REG_REQUEST parameter. Upon
choosing one of the two, it persists throughout the lifetime of
the HIP association, and the Control Relay Client MUST NOT
register the other remaining one in a subsequent UPDATE message.

REG_RESPONSE: The Control Relay Server verifies the services
requested by the Control Relay Client using this parameter. If
the Control Relay Server offered both RENDEZVOUS and RELAY_UDP_HIP
service, and the Control Relay Client requested for both of them,
the Control Relay Client MUST offer only RELAY_UDP_HIP service in
the REG_RESPONSE parameter and include a REG_FAILED parameter in
the same message, with RENDEZVOUS as the Registration Type and 9
as the Failure Type.

As a further measure against cross-protocol attacks, the Control
Relay Client MUST drop any HIP message that includes an RVS_HMAC
parameter when it originates from a successfully registered Control
Relay Server. Upon such an (unintended) event, the Control Relay
Client MUST send a NOTIFY message with RVS_HMAC_PROHIBITED_WITH_RELAY
as the Notify Message Type to the Control Relay Server.

8. IANA Considerations

This section is to be interpreted according to [RFC8126].

This document reuses the same default UDP port number 10500 as
specified by Legacy ICE-HIP [RFC5770] for tunneling both HIP control
and data plane traffic. The port was registered separately for
[RFC5770] to coauthor Ari Keränen originally, but it has been
reassigned for IESG control. With the permission of Ari Keränen, the
new assignee is the IESG and the contact is <chair@ietf.org>. In
addition, IANA has added a reference to this document in the entry
for UDP port 10500 in the "Service Name and Transport Protocol Port
Number Registry". The selection between Legacy ICE-HIP and Native
ICE-HIP mode is negotiated using the NAT_TRAVERSAL_MODE parameter
during the base exchange. By default, hosts listen to this port for
incoming UDP datagrams and can also use it for sending UDP datagrams.
Other ephemeral port numbers are negotiated and utilized dynamically.

IANA has assigned the following values in the HIP "Parameter Types"
registry [RFC7401]: 4650 for RELAYED_ADDRESS (length 20), 4660 for
MAPPED_ADDRESS (length 20; defined in Section 5.12), 4680 for
PEER_PERMISSION (length 48; defined in Section 5.13), 4700 for
CANDIDATE_PRIORITY (length 4; defined in Section 5.14), and 4710 for
NOMINATE (length 4; defined in Section 5.15).

IANA has assigned the following value in the "HIP NAT Traversal
Modes" registry specified in Legacy ICE-HIP [RFC5770]: 3 for ICE-HIP-
UDP (defined in Section 5.4).

IANA has assigned the following values in the HIP "Notify Message
Types" registry: 16385 for NAT_KEEPALIVE in Section 5.3, 63 for
SERVER_REFLEXIVE_CANDIDATE_ALLOCATION_FAILED in Section 5.10, and 64
for RVS_HMAC_PROHIBITED_WITH_RELAY in Section 5.10.

IANA has assigned the following values in the "Registration Types"
registry for the HIP Registration Extension [RFC8003]: 3 for
RELAY_UDP_ESP (defined in Section 5.9) for allowing registration with
a Data Relay Server for ESP-relaying service, and 4 for
CANDIDATE_DISCOVERY (defined in Section 4.2) for performing server-
reflexive candidate discovery.

IANA has assigned one value in the "Registration Failure Types"
registry as defined in Section 6.8. 7.8. The value is 9, and the
Registration Failure Type is "Simultaneous Rendezvous and Control
Relay Service usage prohibited".

The ICE specification [RFC8445] discusses "Unilateral Self-Address
Fixing" in Section 18. This protocol is based on ICE; thus, the same
considerations also apply here.

9. References

8.1.

9.1. Normative References

[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
DOI 10.17487/RFC1191, November 1990,
<https://www.rfc-editor.org/info/rfc1191>.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14,

[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 8174, 4291, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[RFC7401] Moskowitz, R., Ed., Heer, 10.17487/RFC4291, February
2006, <https://www.rfc-editor.org/info/rfc4291>.

[RFC5770] Komu, M., Henderson, T., Tschofenig, H., Melen, J., and A.
Keranen, Ed., "Basic Host Identity Protocol (HIP)
Extensions for Traversal of Network Address Translators",
RFC 5770, DOI 10.17487/RFC5770, April 2010,
<https://www.rfc-editor.org/info/rfc5770>.

[RFC7050] Savolainen, T., Korhonen, J., and D. Wing, "Discovery of
the IPv6 Prefix Used for IPv6 Address Synthesis",
RFC 7050, DOI 10.17487/RFC7050, November 2013,
<https://www.rfc-editor.org/info/rfc7050>.

[RFC7401] Moskowitz, R., Ed., Heer, T., Jokela, P., and T.
Henderson, "Host Identity Protocol Version 2 (HIPv2)",
RFC 7401, DOI 10.17487/RFC7401, April 2015,
<https://www.rfc-editor.org/info/rfc7401>.

[RFC7402] Jokela, P., Moskowitz, R., and J. Melen, "Using the
Encapsulating Security Payload (ESP) Transport Format with
the Host Identity Protocol (HIP)", RFC 7402,
DOI 10.17487/RFC7402, April 2015,
<https://www.rfc-editor.org/info/rfc7402>.

[RFC8003] Laganier, J. and L. Eggert, "Host Identity Protocol (HIP)
Registration Extension", RFC 8003, DOI 10.17487/RFC8003,
October 2016, <https://www.rfc-editor.org/info/rfc8003>.

[RFC8004] Laganier, J. and L. Eggert, "Host Identity Protocol (HIP)
Rendezvous Extension", RFC 8004, DOI 10.17487/RFC8004,
October 2016, <https://www.rfc-editor.org/info/rfc8004>.

[RFC8005] Laganier, J., "Host Identity Protocol (HIP) Domain Name
System (DNS) Extension", RFC 8005, DOI 10.17487/RFC8005,
October 2016, <https://www.rfc-editor.org/info/rfc8005>.

[RFC8046] Henderson, T., Ed., Vogt, C., and J. Arkko, "Host Mobility
with the Host Identity Protocol", RFC 8046,
DOI 10.17487/RFC8046, February 2017,
<https://www.rfc-editor.org/info/rfc8046>.

[RFC8047] Henderson, T., Ed., Vogt, C., and J. Arkko, "Host
Multihoming with the Host Identity Protocol", RFC 8047,
DOI 10.17487/RFC8047, February 2017,
<https://www.rfc-editor.org/info/rfc8047>.

[RFC8489] Petit-Huguenin, M., Salgueiro, G., Rosenberg, J., Wing,
D., Mahy, R., and P. Matthews, "Session Traversal
Utilities for NAT (STUN)", RFC 8489, DOI 10.17487/RFC8489,
February 2020, <https://www.rfc-editor.org/info/rfc8489>.

[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, DOI 10.17487/RFC4291, February
2006, <https://www.rfc-editor.org/info/rfc4291>.

[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[RFC8201] McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed.,
"Path MTU Discovery for IP version 6", STD 87, RFC 8201,
DOI 10.17487/RFC8201, July 2017,
<https://www.rfc-editor.org/info/rfc8201>.

[RFC8445] Keranen, A., Holmberg, C., and J. Rosenberg, "Interactive
Connectivity Establishment (ICE): A Protocol for Network
Address Translator (NAT) Traversal", RFC 8445,
DOI 10.17487/RFC8445, July 2018,
<https://www.rfc-editor.org/info/rfc8445>.

[RFC7050] Savolainen, T., Korhonen,

[RFC8489] Petit-Huguenin, M., Salgueiro, G., Rosenberg, J., and D. Wing, "Discovery of
the IPv6 Prefix Used
D., Mahy, R., and P. Matthews, "Session Traversal
Utilities for IPv6 Address Synthesis",
RFC 7050, DOI 10.17487/RFC7050, November 2013,
<https://www.rfc-editor.org/info/rfc7050>.

[RFC8005] Laganier, J., "Host Identity Protocol (HIP) Domain Name
System (DNS) Extension", RFC 8005, DOI 10.17487/RFC8005,
October 2016, <https://www.rfc-editor.org/info/rfc8005>.

[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
DOI 10.17487/RFC1191, November 1990,
<https://www.rfc-editor.org/info/rfc1191>.

[RFC8201] McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed.,
"Path MTU Discovery for IP version 6", STD 87, RFC 8201,
DOI 10.17487/RFC8201, July 2017,
<https://www.rfc-editor.org/info/rfc8201>.

[RFC5770] Komu, M., Henderson, T., Tschofenig, H., Melen, J., and A.
Keranen, Ed., "Basic Host Identity Protocol (HIP)
Extensions for Traversal of Network Address Translators", NAT (STUN)", RFC 5770, 8489, DOI 10.17487/RFC5770, April 2010,
<https://www.rfc-editor.org/info/rfc5770>. 10.17487/RFC8489,
February 2020, <https://www.rfc-editor.org/info/rfc8489>.

[RFC8961] Allman, M., "Requirements for Time-Based Loss Detection",
BCP 233, RFC 8961, DOI 10.17487/RFC8961, November 2020,
<https://www.rfc-editor.org/info/rfc8961>.

8.2.

9.2. Informative References

[HIP] Moskowitz, R.

[HIP-MIDDLEBOXES]
Heer, T., Hummen, R., Wehrle, K., and M. Komu, "Host Identity Protocol
Architecture", "End-Host
Authentication for HIP Middleboxes", Work in Progress,
Internet-Draft, draft-
ietf-hip-rfc4423-bis-20, 14 February 2019,
<https://tools.ietf.org/html/draft-ietf-hip-rfc4423-bis-
20>.

[RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
and W. Weiss, "An Architecture for Differentiated
Services", RFC 2475, DOI 10.17487/RFC2475, December 1998,
<https://www.rfc-editor.org/info/rfc2475>.

[RFC5207] Stiemerling, M., Quittek, J., and L. Eggert, "NAT and
Firewall Traversal Issues of Host Identity Protocol (HIP)
Communication", RFC 5207, DOI 10.17487/RFC5207, April
2008, <https://www.rfc-editor.org/info/rfc5207>.

[RFC5245] Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", RFC 5245,
DOI 10.17487/RFC5245, April 2010,
<https://www.rfc-editor.org/info/rfc5245>. draft-heer-hip-middle-auth-04, 31 October
2011, <https://tools.ietf.org/html/draft-heer-hip-middle-
auth-04>.

[ICE-NONSIP]
Rosenberg, J., "Guidelines for Usage of Interactive
Connectivity Establishment (ICE) by non Session Initiation
Protocol (SIP) Protocols", Work in Progress, Internet-
Draft, draft-rosenberg-mmusic-ice-nonsip-01, 14 July 2008,
<https://tools.ietf.org/html/draft-rosenberg-mmusic-ice-
nonsip-01>.

[RFC6538] Henderson, T.

[RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
and A. Gurtov, "The Host Identity Protocol
(HIP) Experiment Report", W. Weiss, "An Architecture for Differentiated
Services", RFC 6538, 2475, DOI 10.17487/RFC6538,
March 2012, <https://www.rfc-editor.org/info/rfc6538>.

[RFC5128] Srisuresh, P., Ford, B., 10.17487/RFC2475, December 1998,
<https://www.rfc-editor.org/info/rfc2475>.

[RFC3264] Rosenberg, J. and D. Kegel, "State of Peer-to-
Peer (P2P) Communication across Network Address
Translators (NATs)", H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 5128, 3264,
DOI 10.17487/RFC5128, March
2008, <https://www.rfc-editor.org/info/rfc5128>.

[HIP-MIDDLEBOXES]
Heer, T., Hummen, R., Wehrle, K., and M. Komu, "End-Host
Authentication for HIP Middleboxes", Work in Progress,
Internet-Draft, draft-heer-hip-middle-auth-04, 31 October
2011, <https://tools.ietf.org/html/draft-heer-hip-middle-
auth-04>. 10.17487/RFC3264, June 2002,
<https://www.rfc-editor.org/info/rfc3264>.

[RFC3948] Huttunen, A., Swander, B., Volpe, V., DiBurro, L., and M.
Stenberg, "UDP Encapsulation of IPsec ESP Packets",
RFC 3948, DOI 10.17487/RFC3948, January 2005,
<https://www.rfc-editor.org/info/rfc3948>.

[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
DOI 10.17487/RFC3264, June 2002,
<https://www.rfc-editor.org/info/rfc3264>.

[RFC8750] Migault, D., Guggemos, T.,

[RFC5128] Srisuresh, P., Ford, B., and Y. Nir, "Implicit
Initialization Vector (IV) for Counter-Based Ciphers in
Encapsulating Security Payload (ESP)", D. Kegel, "State of Peer-to-
Peer (P2P) Communication across Network Address
Translators (NATs)", RFC 8750, 5128, DOI 10.17487/RFC8750, 10.17487/RFC5128, March 2020,
<https://www.rfc-editor.org/info/rfc8750>.

[RFC8899] Fairhurst, G., Jones, T., Tüxen,
2008, <https://www.rfc-editor.org/info/rfc5128>.

[RFC5207] Stiemerling, M., Rüngeler, I., Quittek, J., and T.
Völker, "Packetization Layer Path MTU Discovery for
Datagram Transports", L. Eggert, "NAT and
Firewall Traversal Issues of Host Identity Protocol (HIP)
Communication", RFC 8899, 5207, DOI 10.17487/RFC8899,
September 2020, <https://www.rfc-editor.org/info/rfc8899>.

[RFC8656] Reddy, T., Ed., Johnston, A., Ed., Matthews, P., and J. 10.17487/RFC5207, April
2008, <https://www.rfc-editor.org/info/rfc5207>.

[RFC5245] Rosenberg, "Traversal Using Relays around NAT (TURN):
Relay Extensions to Session J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal Utilities for NAT
(STUN)", Offer/Answer Protocols", RFC 8656, 5245,
DOI 10.17487/RFC8656, February 2020,
<https://www.rfc-editor.org/info/rfc8656>. 10.17487/RFC5245, April 2010,
<https://www.rfc-editor.org/info/rfc5245>.

[RFC6146] Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
NAT64: Network Address and Protocol Translation from IPv6
Clients to IPv4 Servers", RFC 6146, DOI 10.17487/RFC6146,
April 2011, <https://www.rfc-editor.org/info/rfc6146>.

[RFC6147] Bagnulo, M., Sullivan, A., Matthews, P., and I. van
Beijnum, "DNS64: DNS Extensions for Network Address
Translation from IPv6 Clients to IPv4 Servers", RFC 6147,
DOI 10.17487/RFC6147, April 2011,
<https://www.rfc-editor.org/info/rfc6147>.

[RFC6538] Henderson, T. and A. Gurtov, "The Host Identity Protocol
(HIP) Experiment Report", RFC 6538, DOI 10.17487/RFC6538,
March 2012, <https://www.rfc-editor.org/info/rfc6538>.

[RFC8656] Reddy, T., Ed., Johnston, A., Ed., Matthews, P., and J.
Rosenberg, "Traversal Using Relays around NAT (TURN):
Relay Extensions to Session Traversal Utilities for NAT
(STUN)", RFC 8656, DOI 10.17487/RFC8656, February 2020,
<https://www.rfc-editor.org/info/rfc8656>.

[RFC8750] Migault, D., Guggemos, T., and Y. Nir, "Implicit
Initialization Vector (IV) for Counter-Based Ciphers in
Encapsulating Security Payload (ESP)", RFC 8750,
DOI 10.17487/RFC8750, March 2020,
<https://www.rfc-editor.org/info/rfc8750>.

[RFC8899] Fairhurst, G., Jones, T., Tüxen, M., Rüngeler, I., and T.
Völker, "Packetization Layer Path MTU Discovery for
Datagram Transports", RFC 8899, DOI 10.17487/RFC8899,
September 2020, <https://www.rfc-editor.org/info/rfc8899>.

[RFC9068] Moskowitz, R., Ed. and M. Komu, "Host Identity Protocol
Architecture", RFC 9063, DOI 10.17487/RFC9068, June 2021,
<https://www.rfc-editor.org/info/rfc9068>.

Appendix A. Selecting a Value for Check Pacing

Selecting a suitable value for the connectivity check transaction
pacing is essential for the performance of connectivity check-based
NAT traversal. The value should not be so small that the checks
cause network congestion or overwhelm the NATs. On the other hand, a
pacing value that is too high makes the checks last for a long time,
thus increasing the connection setup delay.

The Ta value may be configured by the user in environments where the
network characteristics are known beforehand. However, if the
characteristics are not known, it is recommended that the value is
adjusted dynamically. In this case, it is recommended that the hosts
estimate the round-trip time (RTT) between them, and they SHOULD set
the minimum Ta value so that at most a single connectivity check
message is sent on every RTT.

One way to estimate the RTT is to use the time that it takes for the
Control Relay Server registration exchange to complete; this would
give an estimate on the registering host's access link's RTT. Also,
the I1/R1 exchange could be used for estimating the RTT, but since
the R1 can be cached in the network, or the relaying service can
increase the delay notably, this is not recommended. In general,
estimating RTT can be difficult and error prone; thus, the guidelines
for choosing a Ta value in Section 4.4 MUST be followed.

Appendix B. Differences with Respect to ICE

Legacy ICE-HIP reuses the ICE/STUN/TURN protocol stack as it is. The
benefits of such as an approach include the reuse of STUN/TURN
infrastructure and possibly the reuse of existing software libraries,
but there are also drawbacks with the approach. For example, ICE is
meant for application-layer protocols, whereas HIP operates at layer
3.5 between transport and network layers. This is particularly
problematic because the implementations employ kernel-space IPsec ESP
as their data plane: demultiplexing of incoming ESP, HIP, and TURN
messages required the capturing of all UDP packets destined to port
10500 to the userspace (due to different, incompatible markers in ESP
and STUN), thus causing additional software complexity and an
unnecessary latency/throughput bottleneck for the dataplane
performance. It is also worth noting that the demultiplexing of STUN
packets in the kernel would also incur a performance impact (albeit
smaller than with userspace demultiplexing), and secure verification
of STUN messages would require communication between the kernel-space
STUN detector and HIP daemon typically residing in the userspace
(thus again increasing the performance overhead).

Legacy ICE-HIP also involves some other complexities when compared to
the approach taken in this document. The relaying of ESP packets via
TURN relays was not considered that simple because TURN relays
require adding and removing extra TURN framing for the relayed
packets. Finally, the developers of the two Legacy ICE-HIP
implementations concluded that

| effort needed for integrating an ICE
library into a HIP
| implementation turned out to be quite a bit higher that
than initially
| estimated. Also, the amount of extra code (some 10
kLoC) needed
| for all the new parsers, state machines, etc., is was quite
high and
| by re-using reusing the HIP code code, one should be able to do with much
less.
| This should result in smaller binary size, less bugs, and
easier
| debugging.

Consequently, the HIP working group decided to follow ICE methodology
but reuse HIP messaging format to achieve the same functionality as
ICE; the result of that is this document, which specifies the Native
ICE-HIP protocol.

The Native ICE-HIP protocol specified in this document follows the
semantics of ICE as close as possible, and most of the differences
are syntactical due to the use of a different protocol. In this
section, we describe the differences to the ICE protocol.

* ICE operates at the application layer, whereas this protocol
operates between transport and network layers, thus hiding the
protocol details from the application.

* The STUN protocol is not employed. Instead, Native ICE-HIP reuses
the HIP control plane format in order to simplify the
demultiplexing of different protocols. For example, the STUN
binding response is replaced with a HIP UPDATE message containing
an ECHO_REQUEST_SIGNED parameter and the STUN binding response
with a HIP UPDATE message containing an ECHO_RESPONSE_SIGNED
parameter as defined in Section 4.6. It is worth noting that a
drawback of not employing STUN is that discovery of the address
candidates requires creating (using HIP base exchange) and
maintaining (using HIP UPDATE procedures) state at the Control
Relay Client and Control Relay Server. Future extensions to this
document may define a stateless, HIP-specific mechanism for an end
host to discover its address candidates.

* The TURN protocol is not utilized. Instead, Native ICE-HIP reuses
Control Relay Servers for the same purpose.

* ICMP errors may be used in ICE to signal failure. In the Native
ICE-HIP protocol, HIP NOTIFY messages are used instead.

* Instead of the ICE username fragment and password mechanism for
credentials, Native ICE-HIP uses the HIT, derived from a public
key, for the same purpose. The username fragments are "transient
host identifiers, bound to a particular session established as
part of the candidate exchange" [RFC8445]. Generally in HIP, a
local public key and the derived HIT are considered long-term
identifiers and invariant across different host associations and
different transport-layer flows.

* In ICE, the conflict when two communicating endpoints take the
same controlling role is solved using random values (a so-called
tie-breaker value). In the Native ICE-HIP protocol, the conflict
is solved by the standard HIP base exchange procedure, where the
host with the "larger" HIT switches to the Responder role, thus
also changing to the controlled role.

* The ICE-CONTROLLED and ICE-CONTROLLING attributes are not included
in the connectivity checks.

* The foundation concept is unnecessary in Native ICE-HIP because
only a single UDP flow for the IPsec tunnel will be negotiated.

* Frozen candidates are omitted for the same reason the foundation
concept is excluded.

* Components are omitted for the same reason the foundation concept
is excluded.

* Native ICE-HIP supports only "full ICE" where the two
communicating hosts participate actively to the connectivity
checks, and the "lite" mode is not supported. This design
decision follows the guidelines of ICE, which recommends full ICE
implementations. However, it should be noted that a publicly
reachable Responder may refuse to negotiate the ICE mode as
described in Section 4.7.2. This would result in a [RFC7401]
based HIP base
exchange (as per [RFC7401]) tunneled over UDP UDP, followed by ESP
traffic over the same tunnel, without the connectivity check
procedures defined in this document (in some sense, this mode
corresponds to the case where two ICE lite implementations connect
since no connectivity checks are sent).

* As the "ICE lite" is not adopted here and both sides are capable
of ICE-HIP-UDP mode (negotiated during the base exchange), default
candidates are not employed in Native ICE-HIP.

* If the agent is using Diffserv Codepoint markings [RFC2475] in its
media packets, it SHOULD apply those same markings to its
connectivity checks.

* Unlike in ICE, the addresses are not XORed in the Native ICE-HIP
protocol but rather encrypted to avoid middlebox tampering.

* The ICE defines Related Address and Port attributes used for
diagnostic/SIP purposes, but the Native ICE-HIP protocol does not
employ the ICE-related
address and related port attributes (that are used for diagnostic
or SIP purposes). these attributes.

* The minimum RTO is 500 ms in ICE but 1000 ms in the Native ICE-HIP
protocol in favor of [RFC8961].

Appendix C. Differences to Base Exchange and UPDATE Procedures

This section gives some design guidance for implementers on how the
extensions in this protocol extend and differ from [RFC7401] and
[RFC8046].

* Both the control and data plane are operated on top of UDP, not
directly on IP.

* A minimal implementation would conform only to Sections 4.7.1 or
4.7.2, thus merely tunneling HIP control and data traffic over
UDP. The drawback here is that it works only in the limited cases
where the Responder has a public address.

* It is worth noting that while a rendezvous server Rendezvous Server [RFC8004] has
not been designed to be used in NATed scenarios because it just
relays the first I1 packet and does not employ UDP encapsulation,
the Control Relay Server forwards all control traffic and, hence,
is more suitable in NATed environments. Further, the Data Relay
Server guarantees forwarding of data plane traffic also in cases
where the NAT traversal procedures fail.

* Registration procedures with a Control/Data Relay Server are
similar as with a rendezvous server. Rendezvous Server. However, a Control/Data
Relay Server has different registration parameters than a
rendezvous server
Rendezvous Server because it offers a different service. Also,
the Control/Data Relay Server also includes a REG_FROM parameter
that informs the Control/Data Relay Client about its server-
reflexive address. A Data Relay Server also includes a
RELAYED_ADDRESS containing the relayed address for the Data Relay
Client.

* In [RFC7401], the Initiator and Responder can start to exchange
application payload immediately after the base exchange. While
exchanging data immediately after a base exchange via a Data
Control Relay would also be possible here, we follow the ICE
methodology to establish a direct path between two hosts using
connectivity checks. This means that there will be some
additional delay after the base exchange before application
payload can be transmitted. The same applies for the UPDATE
procedure as the connectivity checks introduce some additional
delay.

* In HIP without any NAT traversal support, the base exchange acts
as an implicit connectivity check, and the mobility and
multihoming extensions support explicit connectivity checks.
After a base exchange or UPDATE-based connectivity checks, a host
can use the associated address pair for transmitting application
payload. In this Native ICE-HIP extension, we follow the ICE
methodology where one endpoint acting in the controlled role
chooses the used address pair also on behalf of the other endpoint
acting in the controlled role, which is different from HIP without
NAT traversal support. Another difference is that the process of
choosing an address pair is explicitly signaled using the
nomination packets. The nomination process in this protocol
supports only a single address pair, and multihoming extensions
are left for further study.

* The UPDATE procedure resembles the mobility extensions defined in
[RFC8046]. The first UPDATE message from the mobile host is
exactly the same as in the mobility extensions. The second UPDATE
message from the peer host and third from the mobile host are
different in the sense that they merely acknowledge and conclude
the reception of the candidates through the Control Relay Server.
In other words, they do not yet test for connectivity (besides
reachability through the Control Relay Server) unlike in the
mobility extensions. The idea is that the connectivity check
procedure follows the ICE specification, which is somewhat
different from the HIP mobility extensions.

* The connectivity checks as defined in the mobility extensions
[RFC8046] are triggered only by the peer of the mobile host.
Since successful NAT traversal requires that both endpoints test
connectivity, both the mobile host and its peer host have to test
for connectivity. In addition, this protocol also validates the
UDP ports; the ports in the connectivity check must match with the
response, as required by ICE.

* In HIP mobility extensions [RFC8046], an outbound locator has some
associated state: UNVERIFIED means that the locator has not been
tested for reachability, ACTIVE means that the address has been
verified for reachability and is being used actively, and
DEPRECATED means that the locator lifetime has expired. In the
subset of ICE specifications used by this protocol, an individual
address candidate has only two properties: type and priority.
Instead, the actual state in ICE is associated with candidate
pairs rather than individual addresses. The subset of ICE
specifications utilized by this protocol require the following
attributes for a candidate pair: valid bit, nominated bit, base,
and the state of the connectivity check. The connectivity checks
have the following states: Waiting, In-progress, Succeeded, and
Failed. Handling of this state attribute requires some additional
logic when compared to the mobility extensions, since the state is
associated with a local-remote address pair rather than just a
remote address; thus, the mobility and ICE states do not have an
unambiguous one-to-one mapping.

* Credit-based authorization as defined in [RFC8046] could be used
before candidate nomination has been concluded upon discovering
working candidate pairs. However, this may result in the use of
asymmetric paths for a short time period in the beginning of
communications. Thus, support of credit-based authorization is
left for further study.

Appendix D. Multihoming Considerations

This document allows a host to collect address candidates from
multiple interfaces but does not support activation and the
simultaneous use of multiple address candidates. While multihoming
extensions to support functionality similar to that found in
[RFC8047] are left for further study and experimentation, we envision
here some potential compatibility improvements to support
multihoming:

Data Relay Registration: a Data Relay Client acting as an Initiator
with another peer host should register a new server-reflexive
candidate for each local transport address candidate. A Data
Relay Client acting as a Responder should register a new server-
reflexive candidate for each {local transport address candidate,
new peer host} pair for the reasons described in Section 4.12.3.
In both cases, the Data Relay Client should request the additional
server-reflexive candidates by sending UPDATE messages originating
from each of the local address candidates as described in
Section 4.1. As the UPDATE messages are originating from an
unknown location from the viewpoint of the Data Relay Server, it
must also include an ECHO_REQUEST_SIGNED in the response in order
to test for return routability.

Data Relay unregistration: This follows the procedure in Section 4,
but the Data Relay Client should unregister using the particular
transport address to be unregistered. All transport address pair
registrations can be unregistered when no RELAYED_ADDRESS
parameter is included.

PEER_PERMISSION parameter: This needs to be extended or an
additional parameter is needed to declare the specific local
candidate of the Data Relay Client. Alternatively, the use of the
PEER_PERMISSION could be used as a wild card to open permissions
for a specific peer to all of the candidates of the Data Relay
Client.

Connectivity checks: The controlling host should be able to nominate
multiple candidates (by repeating step 7 in Figure 5 in
Section 4.6 using the additional candidate pairs).

Keepalives: These should be sent for all the nominated candidate
pairs. Similarly, the Control/Data Relay Client should send
keepalives from its local candidates to its Control/Data Relay
Server transport addresses.

Appendix E. DNS Considerations

This section updates Appendix B of [RFC5770], which will be replaced
with the mechanism described in this section.

[RFC5770] did not specify how an end host can look up another end
host via DNS and initiate a UDP-based HIP base exchange with it, so
this section makes an attempt to fill this gap.

[RFC8005] specifies how a HIP end host and its Rendezvous server Server is
registered to DNS. Essentially, the public key of the end host is
stored as a HI record and its Rendezvous Server as an A or AAAA
record. This way, the Rendezvous Server can act as an intermediary
for the end host and forward packets to it based on the DNS
configuration. The Control Relay Server offers similar functionality
to the Rendezvous Server, with the difference being that the Control
Relay Server forwards all control messages, not just the first I1
message.

Prior to this document, the A and AAAA records in the DNS refer
either to the HIP end host itself or a Rendezvous Server [RFC8005],
and control and data plane communication with the associated host has
been assumed to occur directly over IPv4 or IPv6. However, this
specification extends the records to be used for UDP-based
communications.

Let us consider the case of a HIP Initiator with the default policy
to employ UDP encapsulation and the extensions defined in this
document. The Initiator looks up the Fully Qualified Domain Name
(FQDN) of a Responder, and retrieves its HI, A, and AAAA records.
Since the default policy is to use UDP encapsulation, the Initiator
MUST send the I1 message over UDP to destination port 10500 (either
over IPv4 in the case of an A record or over IPv6 in the case of an
AAAA record). It MAY send an I1 message both with and without UDP
encapsulation in parallel. In the case in which the Initiator
receives R1 messages both with and without UDP encapsulation from the
Responder, the Initiator SHOULD ignore the R1 messages without UDP
encapsulation.

The UDP-encapsulated I1 packet could be received by four different
types of hosts:

HIP Control Relay Server: In this case, the A/AAAA records refer to
a Control Relay Server, which will forward the packet to the
corresponding Control Relay Client based on the destination HIT in
the I1 packet.

HIP Responder supporting UDP encapsulation: In this case, the A/AAAA
records refer to the end host. Assuming the destination HIT
belongs to the Responder, it the Responder receives and processes it the
I1 packet according to the negotiated NAT traversal mechanism.
The support for the protocol defined in this document vs [RFC5770] document, as opposed
to the support defined in [RFC5770], is dynamically negotiated
during the base exchange. The details are specified in
Section 4.3.

HIP Rendezvous Server: This entity is not listening to UDP port
10500, so it will drop the I1 message.

HIP Responder not supporting UDP encapsulation: The targeted end
host is not listening to UDP port 10500, so it will drop the I1
message.

The A/AAAA record MUST NOT be configured to refer to a Data Relay
Server unless the host in question also supports Control Relay Server
functionality.

It is also worth noting that SRV records are not employed in this
specification. While they could be used for more flexible UDP port
selection, they are not suitable for end-host discovery but rather
would be more suitable for the discovery of HIP-specific
infrastructure. Further extensions to this document may define SRV
records for Control and Data Relay Server discovery within a DNS
domain.

Acknowledgments

Thanks to Jonathan Rosenberg, Christer Holmberg, and the rest of the
MMUSIC WG folks for the excellent work on ICE. The authors would
also like to thank Andrei Gurtov, Simon Schuetz, Martin Stiemerling,
Lars Eggert, Vivien Schmitt, and Abhinav Pathak for their
contributions, and Tobias Heer, Teemu Koponen, Juhana Mattila,
Jeffrey M. Ahrenholz, Kristian Slavov, Janne Lindqvist, Pekka
Nikander, Lauri Silvennoinen, Jukka Ylitalo, Juha Heinanen, Joakim
Koskela, Samu Varjonen, Dan Wing, Tom Henderson, Alex Elsayed, Jani
Hautakorpi, Tero Kauppinen, and Timo Simanainen for their comments to
[RFC5770] and this document. Thanks to Éric Vyncke, Alvaro Retana,
Adam Roach, Ben Campbell, Eric Rescorla, Mirja Kühlewind, Spencer
Dawkins, Derek Fawcus, Tianran Zhou, Amanda Barber, Colin Perkins,
Roni Even, Alissa Cooper, Carl Wallace, Martin Duke, and Benjamin
Kaduk for reviewing this document.

This work has been partially funded by the Cyber Trust Program by
Digile/Tekes in Finland.

Contributors

Marcelo Bagnulo, Philip Matthews, and Hannes Tschofenig have
contributed to [RFC5770]. This document leans heavily on the work in
that RFC.

Authors' Addresses

Ari Keränen
Ericsson
Hirsalantie 11
FI-02420 Jorvas
Finland

Email: ari.keranen@ericsson.com

Jan Melén
Ericsson
Hirsalantie 11
FI-02420 Jorvas
Finland

Email: jan.melen@ericsson.com

Miika Komu (editor)
Ericsson
Hirsalantie 11
FI-02420 Jorvas
Finland

Email: miika.komu@ericsson.com