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Versions: (draft-schmitt-hip-nat-traversal) 00 01 02 03 04 05 06 07 08 09 RFC 5770

HIP Working Group                                           M. Komu, Ed.
Internet-Draft                                                      HIIT
Intended status: Experimental                                 S. Schuetz
Expires: January 7, 2008                                  M. Stiemerling
                                                                     NEC
                                                            July 6, 2007


    HIP Extensions for the Traversal of Network Address Translators
                    draft-ietf-hip-nat-traversal-02

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
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   This Internet-Draft will expire on January 7, 2008.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   The Host Identity Protocol (HIP) provides a new namespace that can be
   used for uniquely identifying hosts in public and also in private
   address realms.  Usually, HIP control and data traffic cannot
   traverse Network Address Translators (NATs), that hinders general
   deployment.  This document specifies NAT traversal extensions for
   HIP.  As HIP is located between network and transport layer, the



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   extensions also provide general-purpose NAT traversal support for all
   high-layer networking applications that run over HIP.  The basic
   design concepts for these extensions have been adopted from the
   Interactive Connectivity Establishment (ICE) protocol to HIP.  Using
   the specified extensions, two HIP-capable hosts are able to
   communicate with each other even when they are in different private
   address realms.


Table of Contents

   1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  HIP Across NATs  . . . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  Port Number Selection  . . . . . . . . . . . . . . . . . .  6
     3.2.  Relay Registration and NAT Detection . . . . . . . . . . .  6
     3.3.  Base Exchange via Relay  . . . . . . . . . . . . . . . . .  8
     3.4.  Base Exchange without a Relay  . . . . . . . . . . . . . . 10
     3.5.  Connectivity Tests . . . . . . . . . . . . . . . . . . . . 11
     3.6.  Selecting an Address Pair  . . . . . . . . . . . . . . . . 13
     3.7.  Mobility . . . . . . . . . . . . . . . . . . . . . . . . . 14
     3.8.  NAT Keepalives . . . . . . . . . . . . . . . . . . . . . . 15
     3.9.  Closing of HIP Associations  . . . . . . . . . . . . . . . 16
     3.10. Communication with HIP Hosts without NAT Traversal
           Support  . . . . . . . . . . . . . . . . . . . . . . . . . 16
   4.  Packet Formats . . . . . . . . . . . . . . . . . . . . . . . . 17
     4.1.  HIP Control Packets  . . . . . . . . . . . . . . . . . . . 17
     4.2.  Control Channel Keep-Alives  . . . . . . . . . . . . . . . 18
     4.3.  RELAY_FROM, RELAY_TO and RELAY_VIA Parameters  . . . . . . 18
     4.4.  LOCATOR Parameter  . . . . . . . . . . . . . . . . . . . . 19
     4.5.  RELAY_HMAC . . . . . . . . . . . . . . . . . . . . . . . . 20
     4.6.  Registration Types . . . . . . . . . . . . . . . . . . . . 20
     4.7.  ESP Data Packets . . . . . . . . . . . . . . . . . . . . . 21
     4.8.  UDP Encapsulation/Decapsulation of IPsec BEET-Mode ESP . . 21
   5.  Firewall Traversal . . . . . . . . . . . . . . . . . . . . . . 23
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 23
     6.1.  A Difference to RFC3948  . . . . . . . . . . . . . . . . . 23
     6.2.  Privacy Considerations . . . . . . . . . . . . . . . . . . 24
     6.3.  Opportunistic Mode . . . . . . . . . . . . . . . . . . . . 24
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 25
   8.  Acknowlegements  . . . . . . . . . . . . . . . . . . . . . . . 25
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 26
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 26
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 27
   Appendix A.  Differences to ICE  . . . . . . . . . . . . . . . . . 28
   Appendix B.  Document Revision History . . . . . . . . . . . . . . 29
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 29
   Intellectual Property and Copyright Statements . . . . . . . . . . 31



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1.  Terminology

   In general, this document borrows the terminology from
   [I-D.ietf-hip-base] and [RFC4423].  Additional terms are defined in
   the table below."  These draft e.g. define "Initiator" and
   "Responder"

   +---------------------+---------------------------------------------+
   | Term                | Explanation                                 |
   +---------------------+---------------------------------------------+
   | Rendezvous server   | A host that forwards I1 packets to the      |
   |                     | Responder                                   |
   | HIP Relay           | A host that forwards all HIP control        |
   |                     | packets between an Initiator and Responder  |
   | ESP Relay           | A host that forwards ESP traffic between    |
   |                     | two HIP-enabled hosts                       |
   | Locator             | A routable IPv4 or IPv6 address             |
   | Transport locator   | Transport layer port and the corresponding  |
   |                     | IPv4/v6 address                             |
   | Unreflexive locator | An IPv4 or IPv6 address of a network        |
   |                     | interface of a host                         |
   | Relay reflexive     | A translated transport locator of a host as |
   | transport locator   | observed by a relay                         |
   | Peer reflexive      | A translated transport locator of a host as |
   | transport locator   | observed by its peer                        |
   | Leased transport    | Transport locator of an ESP relay           |
   | locator             |                                             |
   +---------------------+---------------------------------------------+

                           Table 1: Terminology


2.  Introduction

   The Host Identity Protocol (HIP) describes a new communication
   mechanism for Internet hosts [RFC4423].  It introduces a new
   namespace and protocol layer between the network and transport layers
   that decouples the identifier and locator roles to support mobility
   and multihoming in the Internet architecture.  HIP also secures
   application layer communications using IPsec ESP [I-D.ietf-hip-esp].

   The HIP protocol [I-D.ietf-hip-base] cannot operate across legacy NAT
   middleboxes as described in [I-D.irtf-hiprg-nat].  This document
   specifies mechanisms that allow HIP to traverse through such NAT
   middleboxes that are neither HIP-aware nor ESP-aware, without manual
   configuration of the NAT middleboxes.

   HIP introduces a new namespace for hosts that decouples the identity



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   of a host from its location [RFC4423].  The namespace consists of
   Host Identifiers which are public keys.  The hosts create the
   corresponding private keys by themselves which makes identity theft
   more difficult.

   The new namespace of HIP has some additional benefits when the
   extensions defined in this document are used.  First, it is possible
   to address hosts behind a single NAT middlebox in a relatively simple
   way.  The NAT middlebox translates the locators, but the Host
   Identifiers remain the same and can be used for uniquely identifying
   a host inside the private address realm.  Second, multiple services
   on different hosts can share the same transport layer port number
   behind a single legacy NAT.  There is no multiplexing issue as long
   as these hosts have different Host Identifiers and UDP encapsulation
   is used for traversing the legacy NAT.

   Several different types of NATs exist [RFC2663].  This document
   describes HIP extensions for the traversal of both Network Address
   Translator (NAT) and Network Address and Port Translator (NAPT)
   middleboxes.  The document generally uses the term NAT to refer to
   both types of middleboxes, unless it needs to distinguish between the
   two types.

   Three basic scenarios exist for NAT traversal.  In the first case,
   only the Initiator of a HIP base exchange is located behind a NAT.
   In the second case, only the Responder of a HIP base exchange is
   located behind a NAT.  The respective peer is assumed to be located
   at a publicly reachable address in both cases.  In the third case,
   both peers are located behind (possible different) NATs.  All of the
   use cases are addressed in the draft in a unified method that has
   been adopted from Interactive Connectivity Establishment (ICE)
   protocol [I-D.ietf-mmusic-ice] and adapted to HIP.

   Legacy NAT devices do not operate consistently although the behavior
   for new NAT devices has been unified in [RFC4787].  The HIP protocol
   extensions in this document make as little assumptions as possible of
   the behavior of the NAT devices so that NAT traversal will work even
   with legacy NAT devices in the most general sense.  The purpose of
   the extensions is to allow two HIP-enabled hosts to communicate with
   each other even if one or both communicating hosts are in private
   address realms.  With some legacy NAT devices, connecting two hosts
   behind different address realms is impossible without relaying all
   traffic through a third party host [I-D.ietf-behave-p2p-state].  As a
   consequence, the relay host introduces additional hops between the
   hosts and can become a point of network congestion.  In the
   extensions described in this document, the peers try to avoid the use
   of a relay for data traffic and only make use of it when necessary.




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   Hosts that always get a public addresses can use the rendezvous
   services as described in [I-D.ietf-hip-rvs].  Hosts that can be
   located in private-address realms may use a transport-layer based
   relay service as defined in this document.  Both rendezvous and relay
   services forward HIP control packets, but the main difference is that
   the rendezvous service forwards only the initial I1 packet of the
   base exchange while all other HIP control packets are sent directly
   between the communicating hosts.  In contrast, the relay service
   relays all HIP control packets because p2p-unfriendly NAT devices
   drop the packets otherwise [I-D.ietf-behave-p2p-state].  The peers
   use the control channel to communicate their current locators to each
   other to find a direct path for carrying ESP encapsulated data
   traffic.  A direct path between the hosts enables efficient delivery
   of data traffic without relaying of ESP packets through an
   intermediary ESP relay.  The direct path is searched using
   connectivity tests.

   The basis for the connectivity tests is ICE [I-D.ietf-mmusic-ice].
   Two hosts communicate their transport locator (a port and an IP
   address) to each other in a base exchange.  The local locators are
   paired with peer locators and the pairs are prioritized according to
   their proximity.  The locator pairs are tested sequentially in
   priority order using return routability tests [I-D.ietf-hip-mm].
   Both sides participate in the connectivity tests.  The tests also
   determine whether transport layer encapsulation is required or not.
   As a result, the hosts either detect that no transport locator pairs
   are working, or establish a number of working locator pairs and
   select a single pair to be used for communication.

   The same connectivity tests are also used in situations when a mobile
   host moves to a different network.  The mobile host communicates its
   new location to the corresponding node through the relay server of
   its peer and starts the connectivity tests.

   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].


3.  HIP Across NATs

   This section describes NAT traversal between two HIP end-hosts.  A
   successful NAT traversal requires at least the Responder located in a
   private address realm to register to a relay server.  The use of the
   relay is optional when the Responder is located in a public address
   realm without rendezvous server.

   The base exchange is relayed through the relay server.  Next, the



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   hosts test the reachability between the different locators to
   construct a direct route.  When a direct route is not possible, the
   hosts resort to ESP relays.  When locators of a host change, the
   hosts test reachability of locators again and select the "optimal"
   locator.  End-hosts can tear down HIP associations using the CLOSE
   mechanism through the relay.

3.1.  Port Number Selection

   This document defines only UDP encapsulation for HIP and ESP packets.
   Further extensions may define bindings for other transport protocols.
   The RECOMMENDED transport protocol is UDP.

   It is RECOMMENDED that an Initiator selects a random port number
   between the ephemeral port ranged 49152-65535 for initiating a base
   exchange even for registration.  However, the allocated port MUST be
   maintained until all of the corresponding Host Associations are
   closed.  Alternatively, a host MAY also use a single fixed port for
   initiating all outgoing connections.

   A relay or a Responder without a relay MUST listen at transport port
   HIPPORT for incoming UDP-encapsulated HIP control packets.

3.2.  Relay Registration and NAT Detection

   HIP rendezvous servers are used in non-NATted environments and its
   use is described in [I-D.ietf-hip-rvs].  This section defines the
   another types middleboxes, called HIP and ESP Relays, which are used
   in NATted environments.

   A HIP relay forwards UDP-encapsulated traffic, and in future
   extensions, a relay may also forward TCP-encapsulated traffic.  A
   single relay may forward only HIP control packets, ESP traffic or
   both.  A host acting as a Responder in a private address realm SHOULD
   use a HIP relay for NAT traversal.  It is RECOMMENDED that the
   Responder uses also an ESP relay to guarantee successful NAT
   traversal with p2p-unfriendly NAT devices.

   A relay MUST NOT forward any packets to a host that has not
   successfully registered to the relay.  The registration process
   follows the generic registration extensions defined in
   [I-D.ietf-hip-registration].  The registration process is illustrated
   in Figure 1.








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      Relay                                                    Relay
      Client                                                   Server
        |   1. I1                                                |
        +------------------------------------------------------->|
        |                                                        |
        |  2. R1(LOCATOR,REG_INFO(RELAY_UDP_HIP,RELAY_UDP_ESP))  |
        |<-------------------------------------------------------+
        |                                                        |
        |   3. I2(LOCATOR,REG_REQ(RELAY_UDP_HIP,RELAY_UDP_ESP))  |
        +------------------------------------------------------->|
        |                                                        |
        |   4. R2(REG_RES(RELAY_UDP_HIP,RELAY_UDP_ESP),REG_FROM) |
        |<-------------------------------------------------------|
        |                                                        |
        |               5. Connectivity tests                    |
        |<------------------------------------------------------>|

                 Figure 1: Example registration to a relay

   In the above figure, the end-host is referred to as a relay client
   and the relay middlebox as a relay server.  The registration is
   piggybacked to a base exchange, but it can be done also using HIP
   UPDATE control packets as described in [I-D.ietf-hip-registration].

   In step 1, the relay client starts the registration procedure by
   sending an I1 packet over the transport layer.  The port selection
   was explained in section Section 3.1.

   In step 2, the Responder lists the services that it supports in the
   R1 packet.  The support for HIP-over-UDP relaying is denoted by
   RELAY_UDP_HIP value and the support for ESP-over-UDP relaying is
   denoted by a RELAY_UDP_ESP value in the REG_INFO parameter.

   In step 3, the Initiator selects the services it registers to and
   lists them in the REG_REQ parameter.  In this example, the Initiator
   registers both to HIP and ESP relay services.

   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 may also denote unsuccessful registrations in
   the REG_FAILED parameter in R2.  After the registration, the hosts
   MUST send periodically NAT keepalive packets to each other as defined
   later in this document.

   In step 5, the client and server handle connectivity tests.  The
   procedure is described in a later section.

   When the ESP relay registration was successful, the relay server uses



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   the source IP address and port of the R2 packet (HIPPORT) to relay
   ESP traffic with the client.  This address-port pair of the relay is
   referred to as a "leased transport locator" in this document.  As the
   port number may be shared by multiple clients, the ESP relay MUST
   multiplex the ESP traffic based on SPIs and not the just the port
   number.

   The R2 packet also includes an REG_FROM parameter that indicates the
   transport locator of the client as observed by the server.  The
   transport locator may be translated by a number of NAT middleboxes
   between the client and the server.  This locator is referred to as
   the "relay reflexive transport locator" later in this document.

   A single server can provide multiple HIP middlebox services or the
   services can be distributed among multiple servers.  The difference
   between a HIP rendezvous server [I-D.ietf-hip-rvs] and a HIP relay
   server depends on the registration.  The rendezvous server processing
   rules apply when the Responder has registered to a middlebox with the
   RVS registration type.  Correspondingly, the middlebox applies the
   relay extensions defined in this document when the Responder has
   registered using the relay registration types.  When a single server
   provides both rendezvous and relay services, they are multiplexed
   depending on the absence or presence of transport layer
   encapsulation.

   The Relay Client MUST include a LOCATOR parameter in I2 which lists
   all of the locators of the Initiator.  The Relay Server MUST include
   a LOCATOR parameter in R1, but it is RECOMMENDED that the LOCATOR
   parameter includes only the source transport LOCATOR of R1 as the
   only locator.  The case when the Relay Server includes more locators
   may require IP header conversion between IPv4 and IPv6, insertion, or
   removal of, UDP header and fragmentation handling.  Multiple locators
   in R1 is left for further experimentation.

3.3.  Base Exchange via Relay

   It is RECOMMENDED that the Initiator sends an I1 packet over the
   transport layer when it is destined to an IPv4 address of the
   Responder.  Respectively, the Responder MUST respond to a such I1
   packet with an R1 packet over the transport layer and using the same
   transport protocol.  The rest of the base exchange, I2 and R2, MUST
   also be sent over the transport layer.  However, the transport layer
   encapsulation can be unnecessary when there are no NATs between the
   Initiator and Responder.  This will be detected in the connectivity
   tests described in the next section.

   When the Initiator has an IPv6 address and it has discovered only an
   IPv6 address for the peer, it MUST send it directly over IP.  In such



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   a case, the Initiator MUST follow the procedures described in
   [I-D.ietf-hip-base].  Otherwise, it is RECOMMENDED that the Initiator
   proceeds as shown in Figure 2.

      I                              Relay                          R
      | 1. I1                          |                            |
      +------------------------------->| 2. I1(RELAY_FROM)          |
      |                                +--------------------------->|
      |                                |                            |
      |                                |    3. R1(LOCATOR,RELAY_TO) |
      |        4. R1(LOCATOR,RELAY_TO) |<---------------------------+
      |<-------------------------------+                            |
      |                                |                            |
      | 5. I2(LOCATOR)                 |                            |
      +------------------------------->|                            |
      |                                | 6. I2(LOCATOR,RELAY_FROM)  |
      |                                +--------------------------->|
      |                                |                            |
      |                                |            7. R2(RELAY_TO) |
      |                8. R2(RELAY_TO) |<---------------------------+
      |<-------------------------------+                            |
      |                                |                            |

                    Figure 2: Base Exchange via a relay

   In step 1 of the figure, the Initiator discovers the HIT of the
   Responder and the IPv4 address of the relay of the Responder.  The
   Initiator sends an I1 packet over the transport layer to the HIT of
   the Responder.  The port selection was explained in Section 3.1.  The
   source address is one of the routable addresses of the host is called
   "unreflexive locators" in this document.

   In step 2, the relay receives the I1 packet at port HIPPORT.  If the
   destination HIT belongs to a registered Responder, the relay
   processes the packet.  Otherwise, the relay MUST drop the packet.
   The relay MUST append a RELAY_FROM parameter to the I1 packet which
   preserves the transport source address and port of the Initiator.
   The relay protects the I1 packet with RELAY_HMAC as described in
   [I-D.ietf-hip-rvs], except that the parameter type is different.  The
   relay MUST change the transport source to and destination of the
   packet to match the values the Responder used when registering to the
   relay, i.e., the reverse of the R2 used in the registration.  The
   relay MUST recalculate the transport checksum and forwards the packet
   to the Responder.

   In step 3, the Responder receives the I1 packet at the transport
   layer.  The Responder MUST process it according to the rules in
   [I-D.ietf-hip-base].  In addition, the Responder MUST validate the



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   RELAY_HMAC according to [I-D.ietf-hip-rvs] and drop the packet if the
   validation fails.  The Responder replies with an R1 packet that MUST
   contain a LOCATOR parameter that lists the locators of the Responder.
   The locator list consists of unreflexive, reflexive and leased
   transport locators of the Responder.  The R1 packet also contains a
   RELAY_TO parameter.  The RELAY_TO parameter contains same information
   as the RELAY_FROM parameter, i.e., Initiator transport locator, 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 relay receives the R1 packet.  The relay MUST drop the
   packet if the source HIT belongs to an unregistered host.  The relay
   MAY verify the signature of the R1 packet and drop it when the
   signature is invalid.  Otherwise, the relay changes the destination
   transport header to match RELAY_TO information, recalculates
   transport checksum and forwards the packet.

   In step 5, the Initiator receives the R1 packet and processes it
   accordingly to [I-D.ietf-hip-base].  It replies with an I2 packet
   that has the same transport locator as R1, but the source and
   destination ports are swapped.  The I2 contains a LOCATOR parameter
   containing the listing unreflexive, reflexive and leased transport
   locators of the Initiator

   In step 6, the relay receives the I2 packet.  The relay appends a
   RELAY_FROM and a RELAY_HMAC to the I2 packet as in the second step.

   In step 7, the Responder receives the I2 packet and processes it
   according to [I-D.ietf-hip-base].  It replies with an R2 packet and
   includes a RELAY_TO parameter as in step three.  The RELAY_TO
   parameter is protected by the HMAC.

   In step 8, the relay processes the R2 as described in step four.  The
   relay forwards the packet to the Responder.

3.4.  Base Exchange without a Relay

   A host that has a publicly addressable, fixed IP address MAY exclude
   registration to a Relay.  As the Relay is not present, the host MUST
   listen at HIPPORT for transport-encapsulated HIP and ESP packets.  An
   UDP-encapsulated base exchange with such an host does not have the
   RELAY_TO and RELAY_FROM parameters present.  Connectivity tests MUST
   be handled as defined in the following section before any ESP traffic
   is allowed.






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3.5.  Connectivity Tests

   The base exchange is completed with an R2 packet.  Then, the state of
   the HIP associations at both peers is ESTABLISHED, but the peers MUST
   NOT allow any ESP traffic until the connectivity tests described in
   the next section are performed successfully.  All of the locators,
   except the relay address, are in UNVERIFIED state.  In the
   connectivity tests, the hosts test connectivity between different
   locator pairs in order to find a working one.  The connectivity tests
   are illustrated in Figure 3.  In this example, both hosts are behind
   NATs.

     I                              Relay                            R
     |        2. R2(RELAY_TO)        |        1. R2(RELAY_TO)        |
     +<------------------------------+-------------------------------+
     |                                                               |
     |                3. UPDATE(ECHO_REQUEST,FROM_PEER)    NAT-R:DROP|
     +------------------------------------------------------------->X|
     |                                                               |
     |                4. UPDATE(ECHO_REQUEST,FROM_PEER)              |
     |<--------------------------------------------------------------+
     |                                                               |
     |                5. UPDATE(ECHO_RESP,TO_PEER)                   |
     +-------------------------------------------------------------->+
     |                                                               |
     |                6. UPDATE(ECHO_REQUEST,FROM_PEER)              |
     +-------------------------------------------------------------->|
     |                                                               |
     |                7. UPDATE(ECHO_RESP,TO_PEER)                   |
     |<--------------------------------------------------------------+
     |                                                               |

                       Figure 3: Connectivity tests

   The connectivity tests are handled as the mobility extensions defined
   in [I-D.ietf-hip-mm] and are therefore subject to the same processing
   rules.  The packets include ESP_INFO, SEQ, ACK, HMAC, SIGNATURE
   parameters that are omitted in this section for simplicity.  The
   differences to the mobility extensions are described in this section.

   In steps 1 and 2, the R2 packet is relayed from the Responder through
   the Relay to the Responder.  After this, both hosts start
   connectivity tests using the return routability tests defined in
   [I-D.ietf-hip-mm].  The return routability tests are used to probe
   for connectivity between each locator pair obtained from the local
   and peer locators obtained during base exchange.  The return
   routability tests are also used as a UDP hole punching mechanism.
   The tests are carried in certain order which determined by the



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   priorization algorithm defined in the next section.

   As an example, let's consider the case where hosts are testing each
   others outermost NAT addresses, i.e., relay reflexive transport
   locators.  In step 3, host I sends an UPDATE message containing an
   ECHO_REQUEST to the R. This will punch a hole the NAT of I, but the
   NAT of R drops the message because the NAT of R has no state with I.

   In step 4, R starts also reachability detection by sending an UPDATE
   with ECHO_REQUEST.  This traverses the NAT of I successfully because
   Initiator had already punched an hole into its NAT in step 3.  The
   Responder replies using ECHO_RESPONSE in step 5.  Upon receiving the
   ECHO_RESPONSE, the Responder transitions the address pair to VERIFIED
   state.

   In step 6, host I starts a new return routability test either due to
   a retransmission timer or as a reaction to UPDATE with ECHO_REQUEST
   received from R. In step 7, host R receives and sends a response to
   I. Upon receiving the response, host R transitions the locator pair
   being tested to VERIFIED state.

   All locators in UNVERIFIED state MUST be retransmitted RTIME times.
   The retransmission packets MUST be paced Ta ms apart as defined in
   [I-D.ietf-mmusic-ice].  The retransmission are ordered in a sequence
   determined by the priority of the transport locator pairs, as
   described in the next section.

   The source address of the UPDATE messages containing ECHO_REQUEST
   parameter is always an unreflexive IPv4 locator of the host.  The
   destination locator is the peer's unreflexive, reflexive or leased
   transport locator, depending on which address is being tested for
   reachability.  Implementations may add RTT measurement information to
   the ECHO_REQUEST parameter in addition to a nonce.

   The UPDATE messages carrying ECHO_REQUEST include a FROM_PEER
   parameter.  The sender of the UPDATE MUST copy the source address of
   the UPDATE to the FROM_PEER parameter.  When the peer receives the
   UPDATE, it responds with an UPDATE containing and a ECHO_REQUEST and
   TO_PEER parameters.  The TO_PEER parameter MUST contain the source
   address of the UPDATE redundantly.  The reason from the FROM_PEER and
   TO_PEER parameters is that it is possible to learn new addresses
   using them.  When there is p2p-unfriendly NAT between the peers, it
   may cause translate port number of the UPDATE packets to something
   that has not been communicated through the relay before.  Such an
   addresses are called "peer reflexive transport locators" in this
   document.  The FROM_PEER and TO_PEER parameters can be used for
   detecting peer reflexive locators.  The learned locators are added to
   the connectivity tests.



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   UPDATE packets destined to the unreflexive locators are sent directly
   over IP.  UPDATE packets destined for reflexive peer, relay and
   leased locators are sent transport layer encapsulated.

   Hosts proceed sequentially through the locator pairs in the order
   described in the next section.  A host MUST transition the state of
   transport locator pairs verified by the return routability tests to
   the ACTIVE state.  Keepalive mechanisms described in later sections
   MUST be applied to refresh the port state in NAT devices for locators
   in the ACTIVE state.  A host MUST also set up the Security
   Associations for the inbound ESP traffic for such locators.  The
   selection of a default outbound SA is defined in the next section.

3.6.  Selecting an Address Pair

   This section describes priority ordering of connectivity tests and
   locators pair selection based on ICE [I-D.ietf-mmusic-ice].  As part
   of the priority calculation, each locator has a preference based on
   its type.  The values for these preferences are shown in Table 2.

            +-----------------------------------+------------+
            | Locator Type                      | Preference |
            +-----------------------------------+------------+
            | The preferred locator             | 127        |
            | Unreflexive locator               | 126        |
            | Peer reflexive transport locator  | 120        |
            | Relay reflexive transport locator | 100        |
            | Leased transport locator          | 0          |
            +-----------------------------------+------------+

                     Table 2: Locator Type Preferences

   In addition to the "type" priority, the priority of a locator is also
   affected by the "local" priority.  A (multihoming) host may have
   multiple locators of same type and SHOULD assign a unique local
   priority for each locator.  Hosts preferring IPv6 communication can
   assign higher local preferences for IPv6 locators than for
   unreflexive IPv4 locators.  ECHO_REQUEST parameters may include RTT
   calculation information that an implementation may use to increase
   the local priority.  A host SHOULD calculate locator priority based
   on the local and type priorities as shown in Figure 4.  The locator
   priority MUST always be included in the type 3 locator fields in
   LOCATOR parameters as described in section Section 4.4.

              Locator priority = (2^24) * (type preference) +
                                 (2^8) * (local preference)

                        Figure 4: Locator priority



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   A host SHOULD calculate a priority for each locator pair as shown in
   Figure 5.  I and R denote the priorities of locators of Initiator and
   Responder.  The use of the same formula at both ends gives more
   guarantees that the peers prefer shortest paths between them.  It
   also converges the selection of the locator pair towards a symmetric
   pair instead of an asymmetric pair even though it is not completely
   guaranteed.  The reasoning for the formula is described in
   [I-D.ietf-mmusic-ice].

      Pair priority = 2^32 * MIN(I,R) + 2 * MAX(I,R) + (I > R ? 1 : 0)

                          Figure 5: Pair priority

   After reachability tests, both hosts SHOULD assign the transport
   address pair with the highest pair priority as their default outgoing
   SA for ESP.

3.7.  Mobility

   When one of the hosts changes its locators, it has to notify its
   peers of the address change.  This is handled as described in the
   connectivity tests in Section 3.5 with the exception that the UPDATE
   with parameter LOCATOR is used as the trigger to start connectivity
   tests instead of the R2.  The UPDATE packet contains a LOCATOR
   parameter listing unreflexive, reflexive and leased transport
   locators of the Initiator.  This is illustrated in Figure 6.

























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     Mobile                         Relay                Corresponding
     Node                            |                            Node
     |                               |                               |
     |     1. UPDATE(LOCATOR)        |  2. UPDATE(LOCATOR,RELAY_TO)  |
     +-------------------------------+------------------------------>|
     |                                                               |
     |                3. UPDATE(ECHO_REQUEST,FROM_PEER)     NAT: DROP|
     +------------------------------------------------------------->X|
     |                                                               |
     |                4. UPDATE(ECHO_REQUEST,FROM_PEER)              |
     |<--------------------------------------------------------------+
     |                                                               |
     |                5. UPDATE(ECHO_RESP,TO_PEER)                   |
     |-------------------------------------------------------------->|
     |                                                               |
     |                6. UPDATE(ECHO_REQUEST,FROM_PEER)              |
     |<--------------------------------------------------------------|
     |                                                               |
     |                7. UPDATE(ECHO_RESP,TO_PEER)                   |
     |-------------------------------------------------------------->|
     |                                                               |

                            Figure 6: Handover

   When a mobile host moves from a private address realm to another, it
   can obtain the same locator on both networks.  To denote that the new
   locator requires reachability detection, the mobile host MUST use a
   new SPI for the new locator.

   A host can also use the UPDATE mechanism can also be used for
   switching to a more optimal path after connectivity tests.  In the
   connectivity tests, the host may implement RTT measurements within
   ECHO_REQUEST and ECHO_RESPONSE messages.  In some cases the result of
   the RTT measurements may indicate that another locator pair is more
   optimal than the locator pair resulting from the connectivity and
   priority tests.  In such a case, the host MAY send UPDATE with
   LOCATOR parameter with the optimal locator with the preferred bit on.
   This gives the highest priority for the most optimal locator and will
   be used if the connectivity tests succeed.

3.8.  NAT Keepalives

   A NAT can delete the mapping state after a timeout when there is no
   traffic refreshing the state.  For this reason, both hosts MUST send
   keep-alives to each other for all locators pairs that are in the
   ACTIVE state.  Keepalives MUST be sent every 20 seconds for UDP.  The
   keepalive is a NOTIFY packet without parameters.




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   The keep-alives MAY also be used to implement failure detection
   between end-hosts as in [I-D.oliva-hiprg-reap4hip] (XX FIXME: this
   needs still more details).  The basic idea is to keep track of HIP
   control and ESP packets received over a transport port.  When there
   is no HIP or ESP traffic (not even keep-alives) arriving during a
   certain time period, the host switches to an alternative locator
   pair.  The host transitions the default locator pair to the
   UNVERIFIED state and replaces the currently default SA to correspond
   to the ACTIVE locator pair with the highest priority.  The host may
   also try to send an UPDATE packet with the LOCATOR parameter after a
   certain time period if connectivity is still broken.

   End-host may also used the keep-alives to detect loss of connectivity
   with relay server.  When this occurs, the end-host can register to a
   new relay and replace the IP address of the old relay server with a
   new one in DNS or DHT.

3.9.  Closing of HIP Associations

   A host closes a HIP association as described in [I-D.ietf-hip-base]
   except that the CLOSE and CLOSE_ACK packets are sent over transport
   layer and through the relay as illustrated in Figure 7.  Hosts MUST
   transition the corresponding locator pairs to the DEPRECATED state
   after a successful CLOSE-CLOSE_ACK exchange.  The corresponding
   inbound and outbound SAs must be deleted on such occasion.

     I                            Relay                              R
     | 1. CLOSE                    |                                 |
     +---------------------------->| 2. CLOSE                        |
     |                             +-------------------------------->|
     |                             |                                 |
     |                             |                    3. CLOSE_ACK |
     |                4. CLOSE_ACK |<--------------------------------+
     |<----------------------------+                                 |
     |                             |                                 |

                  Figure 7: Closing of a HIP association

   The hosts may also use the CLOSE mechanism to remove redundant SAs
   remaining from the connectivity tests.  However, the removal can
   prolong the recovery in the event of connectivity failures.

3.10.  Communication with HIP Hosts without NAT Traversal Support

   The UDP encapsulation of HIP and ESP control packets has not been
   defined in any other IETF document and legacy hosts drop all UDP
   encapsulated HIP and ESP traffic.  Processing of unknown locator
   types terminates the base exchange or UPDATE.  As such, the



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   extensions defined in this document are not completely backwards
   compatible and require a minimal support in implementations.

   A minimal implementation MUST provide UDP encapsulation of HIP and
   ESP packets.  In such a case, the minimal NAT traversal
   implementation MUST silently discard the processing of type 3
   locators to allow communication with implementations supporting NAT
   traversal defined in this document.  The minimal implementation MUST
   support UDP keepalives to refresh state of the NAT(s).

   Hosts that conform to [I-D.ietf-hip-mm] respond to UPDATE messages
   containing an ECHO_REQUEST with an UPDATE message containing an
   ECHO_RESPONSE.  This completes the connectivity tests for the host
   supporting the extensions defined in this document.  As long as the
   implementation supports UDP encapsulation of HIP control packets,
   this requires no changes.

   The Relay extensions defined in this document do not work with
   minimalistic implementations.  When there is a Relay between the
   hosts, both the Initiator and Responder MUST support the extensions
   defined in this document.  The presence of RELAY_TO and RELAY_FROM
   parameters denotes the precence of a relay.


4.  Packet Formats

   This section defines an UDP-encapsulation packet format for HIP base
   exchange and control traffic, IPsec ESP BEET-mode traffic and NAT
   keep-alive packets.

4.1.  HIP Control Packets

      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            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     ~                    HIP Header and Parameters                  ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

         Figure 8: Format for UDP-encapsulated HIP control packets

   Figure 8 shows how HIP control packets are encapsulated within UDP.
   A minimal UDP packet carries a complete HIP packet in its payload.



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   Contents of the UDP source and destination ports are described below.
   The UDP length and checksum field MUST be computed as described in
   [RFC0768].  The HIP header and parameter follow the conventions
   [I-D.ietf-hip-base] with the exception that the HIP header checksum
   MUST be zero.  The HIP header checksum is zero for two reasons.
   First, the UDP header contains already a checksum.  Second, the
   checksum definition in [I-D.ietf-hip-base] includes the IP addresses
   in the checksum calculation.  The NATs unaware of HIP cannot
   recompute the HIP checksum after changing IP addresses.

4.2.  Control Channel Keep-Alives

   The keep-alive for control channel are UDP encapsulated NOTIFY
   packets [I-D.ietf-hip-base].  The NOTIFY packets MAY contain HIP
   parameters.  The NAT traversal mechanisms encapsulate these NOTIFY
   packets within the payload of UDP packets.

4.3.  RELAY_FROM, RELAY_TO and RELAY_VIA Parameters

      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            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |                             Address                           |
     |                                                               |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |             Port              |             Padding           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Type        [ TBD by IANA:
                   RELAY_FROM: (63998 = 2^16 - 2^11 + 2^9 - 2)
                   RELAY_TO:   (64002 = 2^16 - 2^11 + 2^9 + 2)
                   RELAY_VIA:  (64006 = 2^16 - 2^11 + 2^9 + 6) ]
     <!-- AG: those are not described?
                   TO_PEER:    (64010 = 2^16 - 2^11 + 2^9 + 10)
                   REG_FROM:   (64010 = 2^16 - 2^11 + 2^9 + 12) ]
     -->
     Length      18
     Address     An IPv6 address or an IPv4 address in IPv4-in-IPv6
                 format.
     Port        Transport port number

       Figure 9: Format for the RELAY_FROM,  RELAY_TO and RELAY_VIA
                                parameters




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   Figure 9 shows the format of RELAY_FROM, RELAY_TO and RELAY_VIA
   parameters.

4.4.  LOCATOR Parameter

   The generic LOCATOR parameter format is the same as in
   [I-D.ietf-hip-mm].  However, presenting transport locators requires a
   new locator type.  The generic and NAT specific locator parameters
   are illustrated in Figure 10.

      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                              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                            Locator                            |
     |                                                               |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 10: Locator parameter

   The individual fields in the LOCATOR parameter are described in
   Table 3.





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   +------------+----------+-------------------------------------------+
   | Field      | Value(s) | Purpose                                   |
   +------------+----------+-------------------------------------------+
   | Type       | 193      | Parameter type                            |
   | Length     | Variable | Length in octets, excluding Type and      |
   |            |          | Length fields, and excluding padding.     |
   | Traffic    | 0-2      | 2 for unreflexive and leased, 1 for relay |
   | Type       |          | reflexive                                 |
   | Locator    | 3        | Transport locator                         |
   | Type       |          |                                           |
   | Locator    | 19       | Length of the Locator field in 4-octet    |
   | Length     |          | units                                     |
   | Reserved   | 0        | Reserved for future extensions            |
   | Preferred  | 0        | Usually zero for type 3 locators          |
   | (P) bit    |          |                                           |
   | Locator    | Variable | Locator lifetime in seconds               |
   | Lifetime   |          |                                           |
   | Transport  | Variable | Zero for unreflexive and greater than     |
   | Port       |          | zero otherwise                            |
   | Transport  | 0        | Zero for UDP                              |
   | Protocol   |          |                                           |
   | Kind       | Variable | 0 for unreflexive, 1 for relay reflexive, |
   |            |          | 2 for leased                              |
   | Priority   | Variable | Locator preference, see Section 3.6       |
   | SPI        | Variable | 0 for relay reflexive, otherwise greater  |
   |            |          | than zero                                 |
   | Locator    | Variable | An IPv6 address or an IPv4-in-IPv6 format |
   |            |          | IPv4 address[RFC2373]                     |
   +------------+----------+-------------------------------------------+

                 Table 3: Fields of the locator parameter

4.5.  RELAY_HMAC

   The RELAY_HMAC parameter value has the TLV type 65520 (2^16 - 2^5 +
   2^4).  It has the same semantics as RVS_HMAC [I-D.ietf-hip-rvs].

4.6.  Registration Types

   The REG_INFO, REQ_REQ, REG_RESP and REG_FAILED parameters contains
   values for relay registration.  The value for RELAY_UDP_HIP is 2.
   The value for RELAY_UDP_ESP is 3.









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4.7.  ESP Data Packets

      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            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     ~                          ESP Header                           ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    Figure 11: Format for UDP-encapsulated IPsec ESP BEET-mode traffic

   Figure 11 shows how IPsec ESP BEET-mode packets are encapsulated
   within UDP.  Again, a minimal UDP packet carries the ESP packet in
   its payload.  The contents of the UDP source and destination ports
   are described in later sections.  The UDP length and checksum field
   MUST be computed as described in [RFC0768].

4.8.  UDP Encapsulation/Decapsulation of IPsec BEET-Mode ESP

   [RFC3948] describes UDP encapsulation of the IPsec ESP transport and
   tunnel mode.  This section describes the UDP encapsulation of the
   BEET mode.

4.8.1.  UDP Encapsulation of IPsec BEET-Mode ESP

   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 [I-D.ietf-hip-esp].  When two peers perform a
   UDP-encapsulated base exchange, they MUST define a pair of IPsec SAs
   that produces UDP-encapsulated BEET-mode ESP data traffic.

   The management of encryption/authentication protocols and security
   parameter indices (SPIs) is defined in [I-D.ietf-hip-esp].
   Additional SA parameters, such as IP addresses and UDP ports, MUST be
   defined according to this section.  Two SAs MUST be defined on each
   host for one HIP association; one for outgoing data and another one
   for incoming data.

   The BEET mode provides limited tunnel mode semantics without the
   regular tunnel mode overhead [I-D.nikander-esp-beet-mode].  In the
   BEET mode, transport-layer checksums in the payload data are based on
   the HITs.  The packet MUST then undergo BEET-mode ESP cryptographic
   processing as defined in Section 5.3 of [I-D.nikander-esp-beet-mode].



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   Next, the resulting BEET-mode packet is UDP encapsulated.  For this
   purpose, a UDP header MUST be inserted between the IP and ESP header.
   The source and destination ports are filled in.  The UDP checksum
   MUST be calculated based on the outer addresses (locators) of the
   IPsec security association.  The other fields of the UDP header are
   computed as described in [RFC0768].

   The resulting UDP packet MUST then undergo BEET IP header processing
   as defined in Section 5.4 of [I-D.nikander-esp-beet-mode].

   Figure 12 illustrates the BEET-mode UDP encapsulation procedure for a
   TCP packet.

     ORIGINAL TCP PACKET:
        +------------------------------------------+
        | inner IPv6 hdr |  ext hdrs  |     |      |
        |   with HITs    | if present | TCP | Data |
        +------------------------------------------+

     PACKET AFTER BEET-MODE ESP PROCESSING:
        +----------------------------------------------------------+
        | inner IPv6 hdr | ESP | dest |     |      |  ESP    | ESP |
        |   with HITs    | hdr | opts.| TCP | Data | Trailer | ICV |
        +----------------------------------------------------------+
                               |<------- encryption -------->|
                         |<----------- integrity ----------->|

     FINAL PACKET AFTER BEET_MODE IP HEADER PROCESSING:
        +------------------------------------------------------------+
        | outer IPv4 | UDP | ESP | dest |     |      |  ESP    | ESP |
        |    hdr     | hdr | hdr | opts.| TCP | Data | Trailer | ICV |
        +------------------------------------------------------------+
                                 |<------- encryption -------->|
                           |<----------- integrity ----------->|

      Figure 12: UDP encapsulation of an IPsec  BEET-mode ESP packet
                         containing a TCP segment

4.8.2.  UDP Decapsulation of IPsec BEET-Mode ESP

   An incoming UDP-encapsulated IPsec BEET-mode ESP packet is
   decapsulated as follows.  First, if the UDP checksum is invalid, then
   the packet MUST be dropped.  Then, the packet MUST be verified as
   defined in [I-D.nikander-esp-beet-mode].  If verified, the ESP data
   contained in the payload of the UDP packet MUST be decrypted as
   described in [I-D.nikander-esp-beet-mode].





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5.  Firewall Traversal

   This section describes firewall traversal issues separately from NAT
   issues.  When the Initiator or the Responder of a HIP association is
   behind a firewall, additional issues arise.

   The NAT traversal mechanisms described in Section 3 require that the
   firewall - stateful or not - allows UDP traffic.  At the minimum,
   successful firewall control packet traversal requires that the host
   behind the firewall is allowed to communicate packets with a HIP
   relay (or a Responder without Relay) that is listening on UDP port
   HIPPORT.  Successful ESP data packet traversal requires the same for
   the ESP relay.  For unrelayed traffic, the destination port HIPPORT
   should be open at the firewall to all hosts behind the firewall.

   Most firewall implementations support "UDP connection tracking",
   i.e., after a host behind a firewall has initiated UDP communication
   to the public Internet, the firewall relays UDP response traffic in
   the return direction.  If no such return traffic arrives for a
   specific period of time, the firewall stops relaying the given IP
   address and port pair.  The mechanisms described in Section 3 already
   enable traversal of such firewalls, if the keep-alive interval used
   is less than the refresh interval of the firewall.

   When the Initiator is behind a firewall, the NAT traversal mechanisms
   described in Section 3 depend on the ability to initiate
   communication via UDP to the destination port HIPPORT from arbitrary
   source ports and to receive UDP response traffic from that port to
   the chosen source port.  If the Initiator is behind a firewall that
   does not support "UDP connection tracking", the NAT traversal
   mechanisms described in Section 3 can still be supported, if the
   firewall allows permanently inbound UDP traffic from the port HIPPORT
   and destined to arbitrary source IP addresses and UDP ports.

   When the Responder is behind a firewall, the NAT traversal mechanisms
   described in Section 3 depend on the ability to send and receive UDP
   traffic originating from HIPPORT of the HIP and ESP relays.  When
   unrelayed traffic is preferred, arbitrary source IP addresses and
   ports are required.


6.  Security Considerations

6.1.  A Difference to RFC3948

   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



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   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 IPsec BEET
   mode as described in Section 3, because the Responder use HITs to
   distinguish between different communication instances.

6.2.  Privacy Considerations

   The LOCATORs are sent in plain text.  Alternatively, they could be
   encrypted.  This option was not chosen to allow packet inspection by
   middleboxes.  Plain text locators may be useful for HIP-aware
   middleboxes in the future.

   It is possible that an Initiator or Responder may not want to reveal
   all of its locators to its peer.  For example, a host may not want to
   reveal the internal topology of the private address realm and it
   discards unreflexive locators.  Such behavior creates non-optimal
   paths when the hosts are located behind the same NAT.  Especially,
   this could be a problem 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 [I-D.ietf-behave-p2p-state].  With such a legacy NAT,
   the only option left would be to use a leased transport locator from
   a relay.  As a consequence, a host may support locator-based privacy
   by leaving out the reflexive locators.  Using only unreflexive
   locators can produce suboptimal paths possibly causing congestion.

   The use of relays can be useful for protection against Denial-of-
   Service attacks.  If a Responder reveals only its HIP and ESP relay
   addresses to malign Initiators, the Initiators can only attack the
   relays that does not prevent the Responder from initiating new
   outgoing connections if a path around the relay exists.

6.3.  Opportunistic Mode

   The use of opportunistic HIP is NOT RECOMMENDED and its use is not
   defined in this document.  In opportunistic HIP, the Initiator sends
   the I1 message with null destination HIT.  Private address realms do
   not have unique addresses by definition.  Therefore, opportunistic
   mode is subject to failure even when there are no attackers present.
   In a normal HIP base exchange, a well-behaving Responder drops the I1
   packet when the destination HIT does not belong to it.  An attacker
   could respond to the I1, but the base exchange would eventually fail
   as the attacker would fail to prove its ownership of the destination
   HIT of the I1.




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7.  IANA Considerations

   This section is to be interpreted according to [RFC2434].

   This draft currently uses a UDP port in the "Dynamic and/or Private
   Port" and HIPPORT.  Upon publication of this document, IANA is
   requested to register a UDP port and the RFC editor is requested to
   change all occurrences of port HIPPORT to the port IANA has
   registered.  The HIPPORT number 50500 should be used for initial
   experimentation.

   This document updates the IANA Registry for HIP Parameters Types by
   assigning new HIP Parameter Types values for the new HIP Parameters
   defined in Section 4: o RELAY_FROM (defined in Section 4.3) o
   RELAY_TO (defined in Section 4.3) o RELAY_VIA (defined in Section
   4.3) o RELAY_HMAC (defined in Section 4.5)


8.  Acknowlegements

   The authors would like to thank Lars Eggert, Vivien Schmitt, Abhinav
   Pathak and Andrei Gurtov for their contributions to previous versions
   of this draft.  Thanks for Philip Matthews on introducing ICE
   concepts to the authors and for proposing the initial design.  Thanks
   for Jonathan Rosenberg and the rest of the MMUSIC WG folks for the
   excellent work on ICE.  In addition, the authors would like to thank
   Tobias Heer, Teemu Koponen, Juhana Mattila, Jeffrey M. Ahrenholz,
   Thomas Henderson, Kristian Slavov, Janne Lindqvist, Pekka Nikander,
   Lauri Silvennoinen, Jukka Ylitalo, Juha Heinanen, Joakim Koskela,
   Samu Varjonen, Dan Wing, Hannes Tschofenig and Jani Hautakorpi for
   their comments on this document.

   [I-D.nikander-hip-path] presented some initial ideas for NAT
   traversal of HIP communication.  The idea was based on NAT detection
   using extra parameters in the base exchange.  This document takes a
   different approach based on ICE.

   Simon Schuetz and Martin Stiemerling are partly funded by Ambient
   Networks, a research project supported by the European Commission
   under its Sixth Framework Program.  The views and conclusions
   contained herein are those of the authors and should not be
   interpreted as necessarily representing the official policies or
   endorsements, either expressed or implied, of the Ambient Networks
   project or the European Commission.

   Miika Komu is working in the Networking Research group at Helsinki
   Institute for Information Technology (HIIT).  The InfraHIP project
   was funded by Tekes, Telia-Sonera, Elisa, Nokia, the Finnish Defence



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   Forces and Ericsson.  Miika Komu wrote draft-ietf-hip-nat-02 version
   from scratch based on ICE-related comments from Philip Matthews.


9.  References

9.1.  Normative References

   [I-D.ietf-hip-base]
              Moskowitz, R., "Host Identity Protocol",
              draft-ietf-hip-base-08 (work in progress), June 2007.

   [I-D.ietf-hip-esp]
              Jokela, P., "Using ESP transport format with HIP",
              draft-ietf-hip-esp-06 (work in progress), June 2007.

   [I-D.ietf-hip-mm]
              Henderson, T., "End-Host Mobility and Multihoming with the
              Host Identity Protocol", draft-ietf-hip-mm-05 (work in
              progress), March 2007.

   [I-D.ietf-hip-registration]
              Laganier, J., "Host Identity Protocol (HIP) Registration
              Extension", draft-ietf-hip-registration-02 (work in
              progress), June 2006.

   [I-D.ietf-hip-rvs]
              Laganier, J. and L. Eggert, "Host Identity Protocol (HIP)
              Rendezvous Extension", draft-ietf-hip-rvs-05 (work in
              progress), June 2006.

   [I-D.ietf-mmusic-ice]
              Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address  Translator (NAT)
              Traversal for Offer/Answer Protocols",
              draft-ietf-mmusic-ice-16 (work in progress), June 2007.

   [I-D.nikander-esp-beet-mode]
              Melen, J. and P. Nikander, "A Bound End-to-End Tunnel
              (BEET) mode for ESP", draft-nikander-esp-beet-mode-07
              (work in progress), February 2007.

   [RFC0768]  Postel, J., "User Datagram Protocol", STD 6, RFC 768,
              August 1980.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.




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   [RFC2373]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 2373, July 1998.

   [RFC2434]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 2434,
              October 1998.

   [RFC4423]  Moskowitz, R. and P. Nikander, "Host Identity Protocol
              (HIP) Architecture", RFC 4423, May 2006.

9.2.  Informative References

   [I-D.ietf-behave-p2p-state]
              Srisuresh, P., "State of Peer-to-Peer(P2P) Communication
              Across Network Address  Translators(NATs)",
              draft-ietf-behave-p2p-state-03 (work in progress),
              July 2007.

   [I-D.irtf-hiprg-nat]
              Stiemerling, M., "NAT and Firewall Traversal Issues of
              Host Identity Protocol (HIP)  Communication",
              draft-irtf-hiprg-nat-04 (work in progress), March 2007.

   [I-D.nikander-hip-path]
              Nikander, P., "Preferred Alternatives for Tunnelling HIP
              (PATH)", draft-nikander-hip-path-01 (work in progress),
              March 2006.

   [I-D.oliva-hiprg-reap4hip]
              Oliva, A. and M. Bagnulo, "Fault tolerance configurations
              for HIP multihoming", draft-oliva-hiprg-reap4hip-00 (work
              in progress), July 2007.

   [RFC2663]  Srisuresh, P. and M. Holdrege, "IP Network Address
              Translator (NAT) Terminology and Considerations",
              RFC 2663, August 1999.

   [RFC3948]  Huttunen, A., Swander, B., Volpe, V., DiBurro, L., and M.
              Stenberg, "UDP Encapsulation of IPsec ESP Packets",
              RFC 3948, January 2005.

   [RFC4787]  Audet, F. and C. Jennings, "Network Address Translation
              (NAT) Behavioral Requirements for Unicast UDP", BCP 127,
              RFC 4787, January 2007.







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Appendix A.  Differences to ICE

   The protocol extensions defined in this draft are based on ICE.  The
   extensions are a rough translation of ICE concepts to HIP protocol.
   The translation preserved certain concepts as they are, but there are
   subtle differences.  This section tries to explain how ICE concepts
   were mapped to HIP protocol and what are the differences.

   The terminology for this draft is a hybrid of ICE and HIP
   terminology.  "Agent" was translated to "host" in favour of HIP
   terminology.  Transport address was changed to transport locator.
   Similarly, address pair is denoted as locator pair.  This document
   does not really talk about "candidate addresses", but just "locators"
   which may or may not be verified using the return routability tests,
   in favour of mobility terminology in [I-D.ietf-hip-mm].  Host
   candidate of ICE became unreflexive locator, server reflexive
   candidate was mapped to relay reflexive transport locator, peer
   reflexive candidate was mapped to peer reflexive locator and relayed
   candidate became leased transport locator.

   The component, base and foundation terms are not used in the document
   as there is only a single "media stream" for all (ESP) traffic
   between two hosts.

   There is no "lite" version ICE in this document, just full, as the
   full version is the preferred one also for ICE.  One specific
   scenario defined in this document has some resemblance to the lite
   ICE.  When a Responder is a publicly accessible server with fixed
   address, it may exclude the use of the relay.  In that case, it does
   not have to handle the RELAY parameters but still has to respond to
   the connectivity checks.

   A connectivity check is not a STUN Binding Request.  Instead, it is
   return routability check as defined in [I-D.ietf-hip-mm].  "Triggered
   check" occurs when a host receives a UPDATE with ECHO_REQUEST and it
   responds using a ECHO_RESPONSE and sends its own ECHO_REQUEST.  A
   "check list" is effectively a LOCATOR parameter as defined in
   [I-D.ietf-hip-mm].  The term "ordinary check" is not really used in
   this document as it HIP packets are retransmitted periodically when
   the LOCATORs are in UNVERIFIED state.  "Valid list" corresponds to
   locator pairs that have been verified successfully by the return
   routability tests.

   The peers trigger the connectivity checks after the base exchange or
   after a base exchange.  The conclusion of the connectivity checks,
   i.e., selection of the final address pair, differs the most as a
   result of fitting the ICE nomination algorithm to HIP mobility
   mechanisms.  There is no "controlling agent" and the end-hosts make a



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   local decision on which locator pair to choose.  This could lead to
   asymmetric address pairs, but the priority algorithm guarantees that
   the address pairs converge.  Also, there is are no aggressive and
   regular nomination modes as a consequence of the lack of controlling
   agent.

   ICE uses TLS, usernames and passwords as security mechanisms.  HIP
   has built-in security mechanisms that preferred over the ones that
   are used in ICE.


Appendix B.  Document Revision History

   To be removed upon publication

   +------------+------------------------------------------------------+
   | Revision   | Comments                                             |
   +------------+------------------------------------------------------+
   | schmitt-00 | Initial version.                                     |
   | ietf-00    | Officially adopted as WG item. Solved issues         |
   |            | 1-9,11,12                                            |
   | ietf-01    | Solved remaining issues except that relaying ESP and |
   |            | mobility were still incomplete.                      |
   | ietf-02    | Miika rewrote almost from scratch based on ICE.      |
   |            | Editorial corrections from Simon and Andrei.         |
   +------------+------------------------------------------------------+


Authors' Addresses

   Miika Komu (editor)
   Helsinki Institute for Information Technology
   Metsanneidonkuja 4
   Espoo
   Finland

   Phone: +358503841531
   Fax:   +35896949768
   Email: miika@iki.fi
   URI:   http://www.hiit.fi/











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   Simon Schuetz
   NEC Network Laboratories
   Kurfuerstenanlage 36
   Heidelberg  69115
   Germany

   Phone: +49 6221 4342 165
   Fax:   +49 6221 4342 155
   Email: simon.schuetz@netlab.nec.de
   URI:   http://www.netlab.nec.de/


   Martin Stiemerling
   NEC Network Laboratories
   Kurfuerstenanlage 36
   Heidelberg  69115
   Germany

   Phone: +49 6221 4342 113
   Fax:   +49 6221 4342 155
   Email: stiemerling@netlab.nec.de
   URI:   http://www.netlab.nec.de/





























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Full Copyright Statement

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