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Versions: (draft-keranen-hip-native-nat-traversal) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20

HIP Working Group                                             A. Keranen
Internet-Draft                                                  J. Melen
Intended status: Standards Track                            M. Komu, Ed.
Expires: October 27, 2017                                       Ericsson
                                                          April 25, 2017


        Native NAT Traversal Mode for the Host Identity Protocol
                 draft-ietf-hip-native-nat-traversal-20

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 for all NAT traversal procedures.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on October 27, 2017.

Copyright Notice

   Copyright (c) 2017 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
   (http://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



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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Overview of Operation . . . . . . . . . . . . . . . . . . . .   7
   4.  Protocol Description  . . . . . . . . . . . . . . . . . . . .   9
     4.1.  Relay Registration  . . . . . . . . . . . . . . . . . . .   9
     4.2.  Transport Address Candidate Gathering at the Relay Client  12
     4.3.  NAT Traversal Mode Negotiation  . . . . . . . . . . . . .  14
     4.4.  Connectivity Check Pacing Negotiation . . . . . . . . . .  16
     4.5.  Base Exchange via Control Relay Server  . . . . . . . . .  16
     4.6.  Connectivity Checks . . . . . . . . . . . . . . . . . . .  19
       4.6.1.  Connectivity Check Procedure  . . . . . . . . . . . .  20
       4.6.2.  Rules for Connectivity Checks . . . . . . . . . . . .  23
       4.6.3.  Rules for Concluding Connectivity Checks  . . . . . .  25
     4.7.  NAT Traversal Optimizations . . . . . . . . . . . . . . .  26
       4.7.1.  Minimal NAT Traversal Support . . . . . . . . . . . .  26
       4.7.2.  Base Exchange without Connectivity Checks . . . . . .  26
       4.7.3.  Initiating a Base Exchange both with and without UDP
               Encapsulation . . . . . . . . . . . . . . . . . . . .  28
     4.8.  Sending Control Packets after the Base Exchange . . . . .  28
     4.9.  Mobility Handover Procedure . . . . . . . . . . . . . . .  29
     4.10. NAT Keepalives  . . . . . . . . . . . . . . . . . . . . .  32
     4.11. Closing Procedure . . . . . . . . . . . . . . . . . . . .  32
     4.12. Relaying Considerations . . . . . . . . . . . . . . . . .  32
       4.12.1.  Forwarding Rules and Permissions . . . . . . . . . .  32
       4.12.2.  HIP Data Relay and Relaying of Control Packets . . .  33
       4.12.3.  Handling Conflicting SPI Values  . . . . . . . . . .  34
   5.  Packet Formats  . . . . . . . . . . . . . . . . . . . . . . .  35
     5.1.  HIP Control Packets . . . . . . . . . . . . . . . . . . .  35
     5.2.  Connectivity Checks . . . . . . . . . . . . . . . . . . .  36
     5.3.  Keepalives  . . . . . . . . . . . . . . . . . . . . . . .  36
     5.4.  NAT Traversal Mode Parameter  . . . . . . . . . . . . . .  36
     5.5.  Connectivity Check Transaction Pacing Parameter . . . . .  37
     5.6.  Relay and Registration Parameters . . . . . . . . . . . .  38
     5.7.  LOCATOR_SET Parameter . . . . . . . . . . . . . . . . . .  39
     5.8.  RELAY_HMAC Parameter  . . . . . . . . . . . . . . . . . .  41
     5.9.  Registration Types  . . . . . . . . . . . . . . . . . . .  41
     5.10. Notify Packet Types . . . . . . . . . . . . . . . . . . .  41
     5.11. ESP Data Packets  . . . . . . . . . . . . . . . . . . . .  42
     5.12. RELAYED_ADDRESS and MAPPED_ADDRESS Parameters . . . . . .  42
     5.13. PEER_PERMISSION Parameter . . . . . . . . . . . . . . . .  43
     5.14. HIP Connectivity Check Packets  . . . . . . . . . . . . .  44
     5.15. NOMINATE parameter  . . . . . . . . . . . . . . . . . . .  45
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  45



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     6.1.  Privacy Considerations  . . . . . . . . . . . . . . . . .  45
     6.2.  Opportunistic Mode  . . . . . . . . . . . . . . . . . . .  46
     6.3.  Base Exchange Replay Protection for Control Relay Server   46
     6.4.  Demultiplexing Different HIP Associations . . . . . . . .  47
     6.5.  Reuse of Ports at the Data Relay Server . . . . . . . . .  47
     6.6.  Amplification attacks . . . . . . . . . . . . . . . . . .  47
     6.7.  Attacks against Connectivity Checks and Candidate
           Gathering . . . . . . . . . . . . . . . . . . . . . . . .  47
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  48
   8.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  48
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  49
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  49
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  49
     10.2.  Informative References . . . . . . . . . . . . . . . . .  50
   Appendix A.  Selecting a Value for Check Pacing . . . . . . . . .  51
   Appendix B.  Differences with respect to ICE  . . . . . . . . . .  52
   Appendix C.  Differences to Base Exchange and UPDATE procedures .  53
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  55

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, especially 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 that
   Teredo based NAT traversal for HIP and related ESP traffic (with
   double tunneling overhead).  Another solution is specified in
   [RFC5770], which will be referred as "Legacy ICE-HIP" in this
   document.  The experimental Legacy ICE-HIP specification combines
   Interactive Connectivity Establishment (ICE) protocol
   [I-D.ietf-ice-rfc5245bis] with HIP, so that basically ICE is
   responsible of NAT penetration and connectivity testing, while HIP is
   responsible of end-host authentication and IPsec key management.  The
   resulting protocol uses HIP, STUN and ESP messages tunneled over a
   single UDP flow.  The benefit of using ICE and its STUN/TURN
   messaging formats is that one can re-use 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.





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   Implementing 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 re-using and extending HIP messages and
   state machines for the same purpose.  Thus, this document specifies
   an alternative NAT traversal mode referred as "Native ICE-HIP" that
   employs 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 legacy
   specification is based on HIPv1.

   Similarly as Legacy ICE-HIP, also this specification builds on the
   HIP registration extensions [RFC8003] and the base exchange procedure
   [RFC7401] and its closing procedures, so 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, but basically it is a three-way procedure, where the
   mobile host first announces its new location to the peer, and then
   the peer tests for connectivity (so called return routability check),
   for which the mobile hosts must respond in order to activate its new
   location.  This specification builds on the mobility procedures, but
   modifies it to be compatible with ICE.  The differences to the
   mobility extensions specified in Appendix C.

   This specification builds heavily on the ICE methodology, so it is
   recommended that the reader is familiar with the ICE specification
   [I-D.ietf-ice-rfc5245bis] (especially the overview).  However, native
   ICE-HIP does not implement all the features in ICE, and, hence, the
   different features of ICE are cross referenced using [RFC2119]
   terminology for clarity.  Appendix B explains the differences to ICE.

2.  Terminology

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





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   This document borrows terminology from [RFC5770], [RFC7401],
   [RFC8046], [RFC4423], [I-D.ietf-ice-rfc5245bis], and [RFC5389].  The
   following terms recur in the text:

   ICE:
      Interactive Connectivity Establishment (ICE) protocol as specified
      in [I-D.ietf-ice-rfc5245bis]

   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 Initiator is the host that initiates the base exchange using
      I1 message.

   Responder:
      The Responder is the host that receives the I1 packet from the
      Initiator.

   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, 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, this is denoted
      as "HIP Relay Client".


   Data Relay Server:
      A registrar host that forwards HIP related data plane packets,
      such as Encapsulating Security Payload (ESP) [RFC7402], between




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      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).


   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.

   ICE offer:
      The Initiator's LOCATOR_SET parameter in a HIP I2 control packet.
      Corresponds to the ICE offer parameter, but is HIP specific.

   ICE answer:
      The Responder's LOCATOR_SET parameter in a HIP R2 control packet.
      Corresponds to the ICE answer parameter, but is HIP specific.

   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 is the Initiator.  It nominates the candidate
      pair to be used with the controlled host.

   Controlled host:
      The controlled host is the Responder.  It waits for the
      controlling to nominate an address candidate pair.

   Checklist:
      A list of address candidate pairs that need to be tested for
      connectivity.

   Transport address:



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      Transport layer port and the corresponding IPv4/v6 address.

   Candidate:
      A transport address that is a potential point of contact for
      receiving data.

   Host candidate:
      A candidate obtained by binding to a specific port from an IP
      address on the host.

   Server reflexive candidate:
      A translated transport address of a host as observed by a Control
      or Data Relay Server.

   Peer reflexive candidate:
      A translated transport address of a host as observed by its peer.

   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.

   Permission:
      In the context of Data Relay Server, permission refers to a
      concept similar to TURN's 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:
      The base of an 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













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                             +--------------+
                             |    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 penetration fails.  While, strictly speaking, the Initiator
   does not need any Relay Servers, it may act in the other role for
   other hosts and connectivity with the Data Relay Server of the
   Responder may fail, so it is the Initiator may also have registered
   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(es) (how the Initiator learns of the
   Responder's Control Relay Server is outside of the scope of this
   document, but may be 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 (or other infrastructure including STUN or TURN



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   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, also the HIP signaling can be sent over the
   same address/port pair, and is demultiplexed 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] (instead of [RFC5201] used in [RFC5770]) is
   expected to be used with this NAT traversal mode.

4.1.  Relay Registration

   In order for two hosts to communicate over NATted environments, they
   need a reliable way to exchange information.  To achieve this, "HIP
   relay server" is defined in [RFC5770].  It supports relaying of HIP
   control plane traffic over UDP in NATted 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"



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   for better alignment with the rest of the terminology.  The
   registration to the Control Relay Server can be achieved using
   RELAY_UDP_ESP parameter as explained later in this section.

   To guarantee also 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 [TBD by IANA: 3]).  This
   service may be coupled with the Control Relay Server 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].

   A Control Relay Server MUST silently drop packets to a Control Relay
   Client that has not previously registered with the HIP relay.  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.




















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     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 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 supports also relaying of ESP
   traffic over UDP, it includes also 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 Data Relay service by listing the
   RELAY_UDP_HIP value in the request parameter.  If the Relay Client
   requires also 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



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   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 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.

   After the registration, the Relay Client sends periodically 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 MAY use the same relayed address and port for
   multiple Data Relay Clients, but since this can cause problems with
   stateful firewalls (see Section 6.5) it is NOT RECOMMENDED.

   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 both renewals of service registration but also to host
   movement, where especially the latter requires the Control Relay
   Client to learn its new server reflexive address candidate.

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).  It should be noted discovering multiple
   address candidates in a multihoming configuration are left for
   further study.




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   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.

   ICE guidelines [I-D.ietf-ice-rfc5245bis] for candidate gathering are
   followed here.  A number of host candidates (loopback, anycast and
   others) should be excluded as described in the ICE specification
   [I-D.ietf-ice-rfc5245bis].  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.  A host SHOULD employ only a single server for
   gathering the candidates for a single HIP association; either a one
   server providing both Control and Data Relay Server functionality, or
   one Control Relay Server and also Data Relay Server if the
   functionality is offered by another server.  When the relay service
   is split between two hosts, the server reflexive candidate from the
   Control Relay Server SHOULD be used instead of the one provided by
   the Data Relay Server.  If a relayed candidate is identical to a host
   candidate, the relayed candidate must be discarded.  NAT64
   considerations in [I-D.ietf-ice-rfc5245bis] apply as well.

   HIP based connectivity can be utilized by IPv4 applications using
   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
   [I-D.ietf-ice-rfc5245bis]) SHOULD be used:

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



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

   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 in ICE apply also here.

   Unlike 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 ICE , the retransmission timeout (RTO) for address
   gathering from a Control/Data Relay Server SHOULD be calculated as
   follows:

      RTO = MAX (500ms, Ta * (Num-Of-Pairs))

   where Ta is the value used for Ta is the value used for the
   connectivity check pacing and Num-Of-Pairs is number of pairs of
   candidates with Control and Data Relay Servers (e.g. in the case of a
   single server, it would be 1).  A smaller value than 500 ms for the
   RTO MUST NOT be used.

4.3.  NAT Traversal Mode Negotiation

   This section describes the usage of a new non-critical parameter
   type.  The presence of the 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 a 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



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   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), LOC_SET, ..))   |
    +--------------------------------------------------------------->|
    |                                                                |
    | 4. UDP(R2(.., 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 [TBD by IANA: 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 LOC_SET).  The locators in I2 are
   the "ICE offer".

   In step 4, the Responder concludes the base exchange with an R2
   packet.  If the Initiator chose ICE NAT traversal mode, the Responder
   includes a LOCATOR_SET parameter in the R2 packet.  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.






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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 SHOULD NOT use values
   smaller than 5 ms for the minimum Ta, since such values may not work
   well with some NATs (as explained in [I-D.ietf-ice-rfc5245bis]).  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 neither
   repeated here.  Similarly, the NAT traversal mode negotiation process
   (denoted as NAT_TM in the example) was described in Section 4.3 and
   is neither 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 IPv4 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.




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   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(LOC_SET, NAT_TM,       |                                |
   |        TA_P))                    |                                |
   +--------------------------------->| 6. UDP(I2(LOC_SET, RELAY_FROM, |
   |                                  |           NAT_TM, TA_P))       |
   |                                  +------------------------------->|
   |                                  |                                |
   |                                  | 7. UDP(R2(LOC_SET, RELAY_TO))  |
   | 8. UDP(R2(LOC_SET, 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 registered Responder, 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 as described in [RFC8004], except that the parameter type
   is different (see 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



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   Responder validates the RELAY_HMAC according to [RFC8004] 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
   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 that lists all the HIP
   candidates (ICE 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.

   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 similarly as 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].  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 that lists all the HIP candidates (ICE answer)
   of the Responder.  The RELAY_TO parameter is protected by the HMAC.

   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.




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   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 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" and they MUST NOT be used as HIP candidates.  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.
   This address MUST NOT be used as a destination for ESP traffic (i.e.,
   the corresponding Control Relay Client cannot advertise it to its
   peer) unless the server supports also Data Relay Server
   functionality, for which the client has successfully registered to.
   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 [MMUSIC-ICE], but
   UDP encapsulated HIP control messages are used instead of ICE
   messages.  Only normal nomination MUST be used for the connectivity
   checks, i.e., aggressive nomination MUST NOT be employed.  As stated
   in the ICE specification, the basic procedure for connectivity checks
   has three phases: sorting the candidate pairs according their
   priority, sending checks in the prioritized order and acknowledging
   the checks from the peer host.

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



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

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).































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   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



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   traverses successfully 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 Responder, 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_REQUEST_SIGNED (denoted ECHO_REQUEST_SIGN) parameter.  The
   Responder includes also 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,
   and 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 ACK parameter and the
   ECHO_REQUEST_SIGN parameter with ECHO_RESPONSE_SIGNED.  In addition,
   it includes MAPPED_ADDR parameter that includes the peer reflexive
   candidate.  This response message is successfully delivered to the
   Responder, and 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 (due to the use of normal nomination).
   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_SIGN, CANDIDATE_PRIORITY and the
   NOMINATE parameter.

   In step 8, the Responder receives the message with 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_SIGN parameters from previous message from the
   Initiator.  The Responder includes SEQ and ECHO_REQUEST_SIGN
   parameters in order to receive an acknowledgment from the Responder.



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   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_SIGN 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 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 both Initiator and Responder both
   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 a
   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 packets MUST be protected with HMACs
   and signatures (even though the illustrations in this specification
   omit them for simplicity).  Each connectivity check sent by a host
   MUST include a SEQ parameter and ECHO_REQUEST_SIGN parameter, and
   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.





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   A host may receive a connectivity check before it has received the
   candidates from its peer.  In such a case, the host MUST immediately
   generate a response, 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 Section 4.6.1.

   [RFC7401] states that UPDATE packets have to include either a SEQ or
   ACK parameter (or both).  According to the RFC, 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 ACKs 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 thus may cause retransmissions.  Upon successful nomination an
   address pair, a host MAY immediately stop sending such
   retransmissions.

   ICE procedures for prioritizing candidates, eliminating redundant
   candidates and forming check lists (including pruning) must be
   followed (as specified in [I-D.ietf-ice-rfc5245bis]), with the
   exception that the foundation, frozen candidates and default
   candidates are not used.  From viewpoint of the ICE specification
   [I-D.ietf-ice-rfc5245bis], the protocol specified in this document
   operates using 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
   20 ms SHOULD 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




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   retransmission timeout (RTO) for the connectivity check UPDATE
   packets SHOULD be calculated as follows:

      RTO = MAX (500ms, 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
   [I-D.ietf-ice-rfc5245bis] when there is only one checklist.  A
   smaller value than 500 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 [I-D.ietf-ice-rfc5245bis], 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 new sequence
   number), a ECHO_REQUEST_SIGN parameter, the priority of the candidate
   in a CANDIDATE_PRIORITY parameter and 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 id,
   acknowledgment of the sequence from the controlling host in an ACK
   parameter, a new ECHO_REQUEST_SIGNED parameter, 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



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   using the nominated address candidate for delivering application
   payload to the controlling host.  Since the message from the
   controlled host included a new sequence id and echo request for
   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, and 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
   implicitly.  This way of operation is still subject to NAT timeouts,
   and the hosts MUST employ NAT keepalives as defined in Section 4.10.

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 [RFC5770].  The procedure is
   applicable also 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 set-up 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.




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   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, it MUST also
   include a LOCATOR_SET parameter in R1 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 1).  If there is no such locator in R1, the source
   address of R1 is used 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.  However, the
   Responder SHOULD NOT send any ESP to the Initiator's address before
   it has received data from the Initiator, as specified in Sections
   4.4.3. and 6.9 of [RFC7401] and in Sections 3.2.9 and 5.4 of
   [RFC8046].

   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.









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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 in
   [RFC5770].  The procedure is applicable also 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 any Control Data Relays, 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 original R1 message 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 of sending control packets after the base
   exchange described in legacy ICE-HIP section 5.10 in [RFC5770] apply
   also here, so they are repeated here for completeness.





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   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.

   If 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
   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
   [I-D.ietf-ice-rfc5245bis].  The example described in this section
   shows only a Control Relay Server for the peer host for the sake of
   simplicity, but also 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 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 two minutes and then continue with the



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   handover procedure without information from the Relay Server that did
   not respond.  As defined in section Section 4.1, a response message
   from a Control Relay Server includes a REG_FROM parameter that
   describes the server reflexive candidate of the mobile host to be
   used in the candidate exchange during the handover.  Similarly, an
   UPDATE to a Data Relay Server is necessary to make sure the Data
   Relay Server can forward data to the correct IP address after a
   handoff.

   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.  Next, we describe the procedure in the figure
   in detail.

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



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   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.  The packet contains
   also 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 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 parameters
   to be acknowledged by the mobile host.  The message acknowledges the
   SEQ parameter of the earlier message with an ACK parameter.  After
   this step, the peer host can initiate the connectivity checks.

   In step 4, the Control Relay Server receives the message, rewrites
   the destination IP address, appends an RELAY_TO parameter and
   forwards the modified message to the mobile host.  When mobile host
   has processed the message successfully, it can initiate the
   connectivity checks.

   In step 5, 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 and the original Responder is
   the controlled host.

   In step 6, 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.

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







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4.10.  NAT Keepalives

   To prevent NAT states from expiring, communicating hosts send
   periodic keepalives to other hosts with which they have established a
   host associating.  Both a registered client and Control/Data Relay
   Server SHOULD send HIP NOTIFY packets to each other every 15 seconds
   (the so called Tr value in ICE) unless they have exchange some other
   traffic over the used UDP ports.  Other values MAY be used, but a Tr
   value smaller than 15 seconds MUST NOT be used.  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 MAY also send HIP NOTIFY packet 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 includes a ECHO_REQUEST_SIGNED
   parameter to protect against replay attacks.

   The same procedure for closing HIP associations applies also 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 Data Relay SHOULD respond directly.  If CLOSE message came
   via a Control Data Relay, the host SHOULD respond using the same
   Control Data Relay.

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



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   Relay Client MUST send an UPDATE message to the Data Relay Server
   containing a PEER_PERMISSION parameter (see Section 5.13) with the
   address of the peer, and the outbound and inbound SPI values the
   Control Relay Client is using with this particular peer.  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 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 (address,
   protocol, port 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, thus, is 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 delivering also 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



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   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

   Since a Data Relay Server may have to deal with multiple Relay
   Clients and their peers, such a Relay may experience collisions in
   the SPI namespace.  Two problematic cases are described in this
   section.

   In the first scenario, an SPI collision may occur when two Initiators
   run a base exchange to the same Responder (i.e.  Data Relay Client),
   and both the Initiators claim the same inbound SPI at the Data Relay
   Server using 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 destination IP and UDP port belonging to
   the Data Relay Server are not unique either).  The problem can be
   mitigated at the Data Relay Clients (i.e.  Responder).  Upon
   receiving an I2 with a colliding SPI, the Responder MUST NOT include
   the relayed address candidate in the R2 message because the Data
   Relay Server would not be able demultiplex the related ESP packet to
   the correct Initiator.  The same applies also 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 a SPI conflict with another peer.  Since
   the SPI space is 32 bits and the SPI values should be random, the
   probability for a conflicting SPI value is fairly small.  However, a
   Data Relay Client with many peers MAY proactively decrease the odds
   of a conflict by registering to multiple Data Relay Servers.  Thus,
   the described collision scenario can be avoided if the Responder
   delivers a new relayed address candidate upon SPI collisions.  Each



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   relayed address has a separate UDP port reserved to it, so collision
   problem does not occur.

   In the second scenario, the SPI collision problems 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.  When the peer 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 for each of the
   clients.

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 most 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



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   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.

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 Notify message type field set to
   NAT_KEEPALIVE [TBD by IANA: 16385] and with an empty Notification
   data field.  It is worth noting that sending of such a HIP NOTIFY
   message MAY be omitted if the host is actively (or passively) sending
   other traffic to the peer host over the UDP tunnel associate with the
   host association (and IPsec security associations since the same port
   pair is reused) 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 associations.  Alternatively, the host MAY use UPDATE
   packets as a substitute.  A minimal UPDATE packet would consist of a
   SEQ and ECHO_REQ_SIGN parameters, and a more complex would involve
   rekeying procedures as specified in section 6.8 in [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 NAT traversal mode parameter is borrowed from Legacy
   ICE-HIP [RFC5770].  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



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   [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           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     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-HIP-UDP          3

           Figure 8: Format of the NAT_TRAVERSAL_MODE Parameter

   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 is a new parameter, and it shown in Figure 9
   contains only the connectivity check pacing value, expressed in
   milliseconds, as a 32-bit unsigned integer.




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      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                             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Type     610
     Length   4
     Min Ta   the minimum connectivity check transaction pacing
              value the host would use (in milliseconds)

           Figure 9: Format of the TRANSACTION_PACING Parameter

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.  REG_FROM is the only parameter covered with 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                            |
     |                                                               |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     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

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



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   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



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   The individual fields in the LOCATOR_SET parameter are described in
   Table 1.

   +-----------+----------+--------------------------------------------+
   | Field     | Value(s) | Purpose                                    |
   +-----------+----------+--------------------------------------------+
   | Type      | 193      | Parameter type                             |
   | Length    | Variable | Length in octets, excluding Type and       |
   |           |          | Length fields and padding                  |
   | Traffic   | 0-2      | Is the locator for HIP signaling (1), for  |
   | Type      |          | 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                |
   | Lifetime  |          |                                            |
   | 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 or 3 for relayed address    |
   | Priority  | Variable | Locator's priority as described in         |
   |           |          | [I-D.ietf-ice-rfc5245bis]. It is worth     |
   |           |          | noting that while the priority of a single |
   |           |          | locator candidate is 32-bits, but an       |
   |           |          | implementation should use 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 1: Fields of the LOCATOR_SET Parameter








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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
   [RFC8004].

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].

5.10.  Notify Packet Types

   A Control 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].

   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 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 is was not able or
      willing to relay a HIP packet.







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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, and 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 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, and, thus, only protocol 17 is allowed.
   However, future documents may specify support for other protocols.





















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      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                            |
     |                                                               |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Type      [TBD by IANA;
               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

        Figure 12: Format of the RELAYED_ADDRESS and MAPPED_ADDRESS
                                Parameters

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.
















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     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                           |
    |                                                               |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                              OSPI                             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                              ISPI                             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |                              ...                              |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    Type      [TBD by IANA; 4680]
    Length    length in octets, excluding Type and Length
    Port      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
    Address   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 with the Address and Port
    ISPI      the inbound SPI value the Data Relay Client is using for
              the peer with the Address and Port

            Figure 13: Format of the PEER_PERMISSION Parameter

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).










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      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                           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Type      [TBD by IANA; 4700]
     Length    4
     Priority  the priority of a (potential) peer reflexive candidate

           Figure 14: Format of the CANDIDATE_PRIORITY Parameter

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                            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Type      [TBD by IANA; 4710]
     Length    4
     Reserved  Reserved for future extension purposes

                Figure 15: Format of the NOMINATE Parameter

6.  Security Considerations

   The security considerations are the same as in Legacy ICE-HIP
   [RFC5770], but are repeated here for the sake of completeness.

6.1.  Privacy Considerations

   The locators are in plain text format in favor of inspection at HIP-
   aware middleboxes in the future.  The current document does not
   specify encrypted versions of LOCATOR_SETs, even though it could be
   beneficial for privacy reasons to avoid disclosing them to
   middleboxes.

   It is also possible that end-users may not want to reveal all
   locators to each other.  For example, tracking the physical location



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   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 reasons, an
   end-host may exclude certain host addresses from its LOCATOR_SET
   parameter.  However, 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 TURN server.

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

6.2.  Opportunistic Mode

   A Control Relay Server should have one address per Control Relay
   Client when the Control Relay Server is serving more than one Control
   Relay Client and supports opportunistic mode.  Otherwise, it cannot
   be guaranteed that the Control Relay Server can deliver the I1 packet
   to the intended recipient.

6.3.  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 legimitate 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-MIDDLE] and drop the I2 or R2 packets if the
   corresponding ECHO_RESPONSE_M parameters are not present.






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6.4.  Demultiplexing Different HIP Associations

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

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

6.5.  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 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 re-use the port numbers.  If port
   numbers need to be re-used, the Data Relay Server SHOULD have a
   sufficiently large pool of port numbers and select ports from the
   pool randomly to decrease the chances of a Data Relay Client
   obtaining the same address that a another host behind the same
   firewall is using.

6.6.  Amplification attacks

   A malicious host may send an invalid list of candidates for 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 Section 4.7.

6.7.  Attacks against Connectivity Checks and Candidate Gathering

   [I-D.ietf-ice-rfc5245bis] 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 discusses also of man-in-the-middle
   replay attacks that are difficult to prevent.  The connectivity
   checks in this protocol are immune against replay attacks because a
   connectivity request includes a random nonce that the recipient must
   sign and send back as a response.




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   [I-D.ietf-ice-rfc5245bis] 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 attacker can inject fake messages
   (located on a shared network segment like Wifi) does not apply here.
   HIP messages are integrity and replay protected, so it is not
   possible inject fake server reflexive address candidates.

   [I-D.ietf-ice-rfc5245bis] describes attacks on relayed candidate
   gathering.  Similarly to ICE TURN servers, Data Relay Server require
   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.  IANA Considerations

   This section is to be interpreted according to [RFC5226].

   This document updates the IANA Registry for HIP Parameter Types
   [RFC7401] by assigning new HIP Parameter Type values for the new HIP
   Parameters: RELAYED_ADDRESS, MAPPED_ADDRESS (defined in
   Section 5.12), and PEER_PERMISSION (defined in Section 5.13).

   This document updates the IANA Registry for HIP NAT traversal modes
   specified in Legacy ICE-HIP [RFC5770] by assigning value for the NAT
   traversal mode ICE-HIP-UDP (defined in Section 5.4) This
   specification introduces a new keepalive Notify message type field
   NAT_KEEPALIVE.

   This document defines additional registration types for the HIP
   Registration Extension [RFC8003] that allow registering with a Data
   Relay Server for ESP relaying service: RELAY_UDP_ESP (defined in
   Section 4.1.

   ICE specification [I-D.ietf-ice-rfc5245bis] discusses "Unilateral
   Self-Address Fixing" . This protocol is based on ICE, and thus the
   same considerations apply also here with one exception: this protocol
   does not hide binary IP addresses from application-level gateways.

8.  Contributors

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






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9.  Acknowledgments

   Thanks to Jonathan Rosenberg and the rest of the MMUSIC WG folks for
   the excellent work on ICE.  In addition, the authors would 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 and Jani Hautakorpi for their comments to
   [RFC5770], which is the basis for this document.

   This work has been partially funded by CyberTrust programme by
   Digile/Tekes in Finland.

10.  References

10.1.  Normative References

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

   [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,
              <http://www.rfc-editor.org/info/rfc7401>.

   [RFC8003]  Laganier, J. and L. Eggert, "Host Identity Protocol (HIP)
              Registration Extension", RFC 8003, DOI 10.17487/RFC8003,
              October 2016, <http://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, <http://www.rfc-editor.org/info/rfc8004>.

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

   [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,
              <http://www.rfc-editor.org/info/rfc5770>.




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   [RFC5389]  Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
              "Session Traversal Utilities for NAT (STUN)", RFC 5389,
              DOI 10.17487/RFC5389, October 2008,
              <http://www.rfc-editor.org/info/rfc5389>.

   [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,
              <http://www.rfc-editor.org/info/rfc7402>.

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

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              DOI 10.17487/RFC5226, May 2008,
              <http://www.rfc-editor.org/info/rfc5226>.

   [I-D.ietf-ice-rfc5245bis]
              Keranen, A., Holmberg, C., and J. Rosenberg, "Interactive
              Connectivity Establishment (ICE): A Protocol for Network
              Address Translator (NAT) Traversal", draft-ietf-ice-
              rfc5245bis-08 (work in progress), December 2016.

   [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,
              <http://www.rfc-editor.org/info/rfc2475>.

10.2.  Informative References

   [RFC4423]  Moskowitz, R. and P. Nikander, "Host Identity Protocol
              (HIP) Architecture", RFC 4423, DOI 10.17487/RFC4423, May
              2006, <http://www.rfc-editor.org/info/rfc4423>.

   [RFC5201]  Moskowitz, R., Nikander, P., Jokela, P., Ed., and T.
              Henderson, "Host Identity Protocol", RFC 5201,
              DOI 10.17487/RFC5201, April 2008,
              <http://www.rfc-editor.org/info/rfc5201>.

   [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, <http://www.rfc-editor.org/info/rfc5207>.





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

   [MMUSIC-ICE]
              Rosenberg, J., "Guidelines for Usage of Interactive
              Connectivity Establishment (ICE) by non Session Initiation
              Protocol (SIP) Protocols", Work in Progress, July 2008.

   [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, <http://www.rfc-editor.org/info/rfc5128>.

   [HIP-MIDDLE]
              Heer, T., Wehrle, K., and M. Komu, "End-Host
              Authentication for HIP Middleboxes", Work in Progress,
              February 2009.

   [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,
              <http://www.rfc-editor.org/info/rfc3948>.

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 set the minimum
   Ta value so that only two connectivity check messages are 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.




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Appendix B.  Differences with respect to ICE

   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.

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

   o  The STUN protocol is not employed.  Instead, native ICE-HIP reuses
      the HIP control plane format in order simplify demultiplexing of
      different protocols.  For example, the STUN binding response is
      replaced with a HIP UPDATE message containing an ECHO_REQUEST_SIGN
      parameter and the STUN binding response with a HIP UPDATE message
      containing an ECHO_RESPONSE_SIGNED parameter as defined in section
      Section 4.6.

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

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

   o  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" [I-D.ietf-ice-rfc5245bis].
      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.

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

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

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





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

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

   o  Native ICE-HIP supports only "full ICE" where the two
      communicating hosts participate actively to the connectivity
      checks, and the "lite" mode is not supported.  This design
      decision follows the guidelines of ICE which recommends full ICE
      implementations.  However, it should be noted that a publicly
      reachable Responder may refuse to negotiate the ICE mode as
      described in Section 4.7.2.  This would result in a [RFC7401]
      based HIP base exchange tunneled over UDP followed 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).

   o  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.

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

   o  Unlike in ICE, the addresses are not XOR-ed in Native ICE-HIP
      protocol in order to avoid middlebox tampering.

   o  Native ICE-HIP protocol does not employ the ICE related address
      and related port attributes (that are used for diagnostic or SIP
      purposes).

Appendix C.  Differences to Base Exchange and UPDATE procedures

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

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

   o  A minimal implementation would conform only to Section 4.7.1 or
      Section 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.




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   o  It is worth noting that while a rendezvous server [RFC8004] has
      not been designed to be used in NATted 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 NATted environments.  Further, the Data Relay
      Server guarantees forwarding of data plane traffic also in the
      cases when the NAT penetration procedures fail.

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

   o  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 be possible also 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.

   o  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 end-point acting in the controlled role
      chooses the used address pair also on behalf of the other end-
      point acting in 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 single address pair, and multihoming extensions are
      left for further study.

   o  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



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      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 connectivity check
      procedure follows the ICE specification, which is somewhat
      different from the HIP mobility extensions.

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

   o  In HIP mobility extensions [RFC8046], an outbound locator has some
      associated state: UNVERIFIED mean 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 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 just a remote
      address, and, thus, the mobility and ICE states do not have an
      unambiguous one-to-one mapping.

   o  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 (similarly as in aggressive ICE nomination).  Thus,
      support of credit-based authorization is left for further study.

Authors' Addresses









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   Ari Keranen
   Ericsson
   Hirsalantie 11
   02420 Jorvas
   Finland

   Email: ari.keranen@ericsson.com


   Jan Melen
   Ericsson
   Hirsalantie 11
   02420 Jorvas
   Finland

   Email: jan.melen@ericsson.com


   Miika Komu (editor)
   Ericsson
   Hirsalantie 11
   02420 Jorvas
   Finland

   Email: miika.komu@ericsson.com


























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