Native NAT Traversal Mode for the Host Identity Protocol
RFC 9028

Document Type RFC - Experimental (July 2021; No errata)
Authors Ari Keränen  , Jan Melen  , Miika Komu 
Last updated 2021-07-15
Replaces draft-keranen-hip-native-nat-traversal
Stream Internet Engineering Task Force (IETF)
Formats plain text html xml pdf htmlized bibtex
Reviews
Stream WG state Submitted to IESG for Publication
Document shepherd Gonzalo Camarillo
Shepherd write-up Show (last changed 2018-02-02)
IESG IESG state RFC 9028 (Experimental)
Action Holders
(None)
Consensus Boilerplate Yes
Telechat date
Responsible AD Éric Vyncke
Send notices to Gonzalo Camarillo <gonzalo.camarillo@ericsson.com>
IANA IANA review state Version Changed - Review Needed
IANA action state RFC-Ed-Ack


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

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   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
     7.1.  Privacy Considerations
     7.2.  Opportunistic Mode
     7.3.  Base Exchange Replay Protection for Control Relay Server
     7.4.  Demultiplexing Different HIP Associations
     7.5.  Reuse of Ports at the Data Relay Server
     7.6.  Amplification Attacks
     7.7.  Attacks against Connectivity Checks and Candidate Gathering
     7.8.  Cross-Protocol Attacks
   8.  IANA Considerations
   9.  References
     9.1.  Normative References
     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, the peer tests for connectivity (the so-called return
   routability check); and then, the mobile 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], [RFC9063], [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 [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 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 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 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.  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 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.

   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 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 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), 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 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 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.  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 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.

   Mobile Host               Control Relay Server              Peer Host
   | 1. UDP(UPDATE(ESP_INFO,          |                                |
   |          ENC(LOC_SET), SEQ))     |                                |
   +--------------------------------->| 2. UDP(UPDATE(ESP_INFO,        |
   |                                  |          ENC(LOC_SET), SEQ,    |
   |                                  |          RELAY_FROM))          |
   |                                  +------------------------------->|
   |                                  |                                |
   |                                  | 3. UDP(UPDATE(ESP_INFO, SEQ,   |
   |                                  |          ACK, ECHO_REQ_SIGN,   |
   |                                  |          RELAY_TO))            |
   | 4. UDP(UPDATE(ESP_INFO, SEQ,     |<-------------------------------+
   |          ACK, ECHO_REQ_SIGN,     |                                |
   |          RELAY_TO))              |                                |
   |<---------------------------------+                                |
   |                                  |                                |
   | 5. UDP(UPDATE(ACK,               |                                |
   |          ECHO_RESP_SIGNED))      |                                |
   +--------------------------------->| 6. UDP(UPDATE(ACK,             |
   |                                  |          ECHO_RESP_SIGNED,     |
   |                                  |          RELAY_FROM))          |
   |                                  +------------------------------->|
   |                                  |                                |
   |                   7. connectivity checks over UDP                 |
   +<----------------------------------------------------------------->+
   |                                  |                                |
   |                      8. ESP data over UDP                         |
   +<----------------------------------------------------------------->+
   |                                  |                                |

                       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 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 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
   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 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 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 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 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 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 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 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.

      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            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |           Reserved            |            Mode ID #1         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |           Mode ID #2          |            Mode ID #3         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |           Mode ID #n          |             Padding           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            Figure 8: Format of the NAT_TRAVERSAL_MODE Parameter

   Type:       608

   Length:     Length in octets, excluding Type, Length, and 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.

    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            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            Min Ta                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            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.

      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            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |             Port              |    Protocol   |     Reserved  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |                            Address                            |
     |                                                               |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

        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.

    +===========+==========+=========================================+
    | Field     | Value(s) | Purpose                                 |
    +===========+==========+=========================================+
    | Type      | 193      | Parameter type                          |
    +-----------+----------+-----------------------------------------+
    | Length    | Variable | Length in octets, excluding Type and    |
    |           |          | Length fields and padding               |
    +-----------+----------+-----------------------------------------+
    | Traffic   | 0-2      | The locator for either HIP signaling    |
    | Type      |          | (1) or ESP (2), or for both (0)         |
    +-----------+----------+-----------------------------------------+
    | Locator   | 2        | "Transport address" locator type        |
    | Type      |          |                                         |
    +-----------+----------+-----------------------------------------+
    | Locator   | 7        | Length of the fields after Locator      |
    | Length    |          | Lifetime in 4-octet units               |
    +-----------+----------+-----------------------------------------+
    | Reserved  | 0        | Reserved for future extensions          |
    +-----------+----------+-----------------------------------------+
    | Preferred | 0 or 1   | Set to 1 for a Locator in R1 if the     |
    | (P) bit   |          | Responder can use it for the rest of    |
    |           |          | the base exchange, otherwise set to     |
    |           |          | zero                                    |
    +-----------+----------+-----------------------------------------+
    | Locator   | Variable | Locator lifetime in seconds, see        |
    | Lifetime  |          | Section 4 of [RFC8046]                  |
    +-----------+----------+-----------------------------------------+
    | Transport | Variable | Transport-layer port number             |
    | Port      |          |                                         |
    +-----------+----------+-----------------------------------------+
    | Transport | Variable | IANA-assigned, transport-layer Internet |
    | Protocol  |          | Protocol number.  Currently, only UDP   |
    |           |          | (17) is supported.                      |
    +-----------+----------+-----------------------------------------+
    | Kind      | Variable | 0 for host, 1 for server reflexive, 2   |
    |           |          | for peer reflexive (currently unused),  |
    |           |          | or 3 for relayed address                |
    +-----------+----------+-----------------------------------------+
    | Priority  | Variable | Locator's priority as described in      |
    |           |          | [RFC8445].  It is worth noting that     |
    |           |          | while the priority of a single locator  |
    |           |          | candidate is 32 bits, an implementation |
    |           |          | should a 64-bit integer to calculate    |
    |           |          | the priority of a candidate pair for    |
    |           |          | the ICE priority algorithm.             |
    +-----------+----------+-----------------------------------------+
    | SPI       | Variable | Security Parameter Index (SPI) value    |
    |           |          | that the host expects to see in         |
    |           |          | incoming ESP packets that use this      |
    |           |          | locator                                 |
    +-----------+----------+-----------------------------------------+
    | Address   | Variable | IPv6 address or an "IPv4-mapped IPv6    |
    |           |          | address" format IPv4 address [RFC4291]  |
    +-----------+----------+-----------------------------------------+

               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.

      +=====================================================+=======+
      | NOTIFICATION PARAMETER - ERROR TYPES                | Value |
      +=====================================================+=======+
      | NO_VALID_NAT_TRAVERSAL_MODE_PARAMETER               | 60    |
      |                                                     |       |
      | If a Control Relay Server does not forward a base   |       |
      | exchange packet due to a missing NAT traversal mode |       |
      | parameter, or the Initiator selects a NAT traversal |       |
      | mode that the (non-relay) Responder did not expect, |       |
      | the Control Relay Server or the Responder may send  |       |
      | back a NOTIFY error packet with this type.          |       |
      +-----------------------------------------------------+-------+
      | CONNECTIVITY_CHECKS_FAILED                          | 61    |
      |                                                     |       |
      | Used by the end hosts to signal that NAT traversal  |       |
      | connectivity checks failed and did not produce a    |       |
      | working path.                                       |       |
      +-----------------------------------------------------+-------+
      | MESSAGE_NOT_RELAYED                                 | 62    |
      |                                                     |       |
      | Used by a Control Relay Server to signal that it    |       |
      | was not able or willing to relay a HIP packet.      |       |
      +-----------------------------------------------------+-------+
      | SERVER_REFLEXIVE_CANDIDATE_ALLOCATION_FAILED        | 63    |
      |                                                     |       |
      | Used by a Data Relay Server to signal that it was   |       |
      | not able or willing to allocate a new server-       |       |
      | reflexive candidate for the Data Relay Client.      |       |
      +-----------------------------------------------------+-------+
      | RVS_HMAC_PROHIBITED_WITH_RELAY                      | 64    |
      |                                                     |       |
      | In the unintended event that a Control Relay Server |       |
      | sends any HIP message with an RVS_HMAC parameter,   |       |
      | the Control Relay Client drops the received HIP     |       |
      | message and sends a notify message back to the      |       |
      | Control Relay Server using this notify 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.

      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            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |             Port              |    Protocol   |    Reserved   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |                            Address                            |
     |                                                               |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

        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.

      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            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |            RPort              |             PPort             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Protocol    |          Reserved                             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |                            RAddress                           |
     |                                                               |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |                            PAddress                           |
     |                                                               |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                              OSPI                             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                              ISPI                             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

             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).

      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            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                            Priority                           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

           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.

      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            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           Reserved                            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                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 7.1, 7.2, 7.3, and 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 7.5,
   and considerations related to the new connectivity check protocol are
   discussed in Sections 7.6 and 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.

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.

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.

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.

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.

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.

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.

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 7.8.  The value is 9, and the
   Registration Failure Type is "Simultaneous Rendezvous and Control
   Relay Service usage prohibited".

9.  References

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>.

   [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>.

   [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>.

   [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>.

   [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>.

   [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>.

9.2.  Informative References

   [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://datatracker.ietf.org/doc/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://datatracker.ietf.org/doc/html/draft-rosenberg-
              mmusic-ice-nonsip-01>.

   [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>.

   [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>.

   [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>.

   [RFC5128]  Srisuresh, P., Ford, B., and D. Kegel, "State of Peer-to-
              Peer (P2P) Communication across Network Address
              Translators (NATs)", RFC 5128, DOI 10.17487/RFC5128, March
              2008, <https://www.rfc-editor.org/info/rfc5128>.

   [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>.

   [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>.

   [RFC9063]  Moskowitz, R., Ed. and M. Komu, "Host Identity Protocol
              Architecture", RFC 9063, DOI 10.17487/RFC9063, July 2021,
              <https://www.rfc-editor.org/info/rfc9063>.

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
   than initially estimated.  Also, the amount of extra code (some 10
   kLoC) needed for all the new parsers, state machines, etc., was quite
   high and by reusing the HIP 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 HIP base
      exchange (as per [RFC7401]) tunneled over 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.

   *  ICE defines Related Address and Port attributes used for
      diagnostic/SIP purposes, but the Native ICE-HIP protocol does not
      employ 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 [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.  However, a Control/Data
      Relay Server has different registration parameters than a
      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 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, the Responder receives and processes the
      I1 packet according to the negotiated NAT traversal mechanism.
      The support for the protocol defined in this 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