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Versions: 00 01 02 draft-ietf-hip-nat-traversal

HIP Working Group                                             V. Schmitt
Internet-Draft                                                       NEC
Expires: April 20, 2007                                        A. Pathak
                                                              IIT Kanpur
                                                                 M. Komu
                                                                    HIIT
                                                               L. Eggert
                                                          M. Stiemerling
                                                                     NEC
                                                        October 17, 2006


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

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Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract



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   This document specifies extensions to Host Identity Protocol (HIP) to
   support traversal of Network Address Translator (NAT) middleboxes.
   The traversal mechanism tunnels HIP control and data traffic over UDP
   and enables HIP initiators which MAY be behind NATs to contact HIP
   responders which MAY be behind another NAT.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Detecting NATs . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  HIP Across NATs  . . . . . . . . . . . . . . . . . . . . . . .  4
     3.1.  Packet Formats . . . . . . . . . . . . . . . . . . . . . .  5
       3.1.1.  Control Traffic  . . . . . . . . . . . . . . . . . . .  5
       3.1.2.  Control Channel Keep-Alives  . . . . . . . . . . . . .  5
       3.1.3.  Data Traffic . . . . . . . . . . . . . . . . . . . . .  6
       3.1.4.  FROM_NAT Parameter . . . . . . . . . . . . . . . . . .  6
       3.1.5.  VIA_RVS_NAT Parameter  . . . . . . . . . . . . . . . .  7
     3.2.  UDP Encapsulation/Decapsulation of IPsec BEET-Mode ESP . .  7
       3.2.1.  UDP Encapsulation of IPsec BEET-Mode ESP . . . . . . .  7
       3.2.2.  UDP Decapsulation of IPsec BEET-Mode ESP . . . . . . .  8
     3.3.  Initiator Behind NAT . . . . . . . . . . . . . . . . . . .  8
       3.3.1.  NAT Traversal of HIP Control Traffic . . . . . . . . .  9
       3.3.2.  NAT Traversal of HIP Data Traffic  . . . . . . . . . . 12
       3.3.3.  Use of the Rendezvous Service when only the
               Initiator Is Behind NAT  . . . . . . . . . . . . . . . 14
     3.4.  Responder Behind NAT . . . . . . . . . . . . . . . . . . . 15
       3.4.1.  Rendezvous Client Registration From Behind NAT . . . . 15
       3.4.2.  NAT Traversal of HIP Control Traffic . . . . . . . . . 17
       3.4.3.  NAT Traversal of HIP Data Traffic  . . . . . . . . . . 19
     3.5.  Both Hosts Behind NAT  . . . . . . . . . . . . . . . . . . 21
       3.5.1.  NAT Traversal of HIP Control Traffic . . . . . . . . . 21
       3.5.2.  NAT Traversal of HIP Data Traffic  . . . . . . . . . . 23
     3.6.  NAT Keep-Alives  . . . . . . . . . . . . . . . . . . . . . 25
     3.7.  HIP Mobility . . . . . . . . . . . . . . . . . . . . . . . 26
     3.8.  HIP Multihoming  . . . . . . . . . . . . . . . . . . . . . 27
     3.9.  Firewall Traversal . . . . . . . . . . . . . . . . . . . . 27
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . . 28
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 29
   6.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 29
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 29
     7.1.  Normative References . . . . . . . . . . . . . . . . . . . 29
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 30
   Appendix A.  Document Revision History . . . . . . . . . . . . . . 31
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 31
   Intellectual Property and Copyright Statements . . . . . . . . . . 33





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1.  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 e.g.
   mobility and multihoming in the Internet architecture.

   The HIP protocol [I-D.ietf-hip-base] cannot operate across Network
   Address Translator (NAT) middleboxes, as described in
   [I-D.irtf-hiprg-nat].  Several different flavors 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.  It generally uses the term NAT to
   refer to both types of middleboxes, unless it needs to distinguish
   between the two types.

   Three basic cases 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 host is assumed to be in the
   public Internet in both cases.  In the third case, both parties are
   located behind (different) NATs.  This document describes extensions
   for the first case in Section 3.3, for the second case in Section 3.4
   and in Section 3.5 for the third case.

   The mechanisms described here also cover use of rendezvous server
   from NATted environments.  The use rendezvous server MUST be used
   when the responder is behind a NAT and the rendezvous MUST be located
   in a public network.  Chaining of NAT enabled rendezvous servers is
   not possible, altough there may be other kind of rendezvous servers
   on the path.  The limitation of the described rendezvous mechanisms
   is that it requires NAT boxes supporting both endpoint independent
   mapping [I-D.srisuresh-behave-p2p-state].

   The mechanisms described in this document are based on encapsulating
   both the control and data traffic in UDP in order to traverse NAT(s).
   The data traffic is assumed to be ESP.  Other types of data traffic
   are out of scope.

   The mobility and multihoming mechanisms of HIP [I-D.ietf-hip-mm],
   allow HIP hosts to change network location during the lifetime of a
   HIP association.  Consequently, hosts need to start using the
   proposed NAT traversal mechanisms after a mobility event relocates
   one or both peers behind a NAT.  They may also stop using the
   proposed mechanisms if they both relocate to the public Internet.

   Finally, the key words "MUST", "MUST NOT", "REQUIRED", "SHALL",



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


2.  Detecting NATs

   In order to know whether to use the NAT traversal mechanisms, HIP
   hosts need to detect presence of NAT middleboxes between them.  This
   document does not describe any NAT detection mechanism but rather
   assumes the NAT is detected using some external mechanism.  Hence, no
   special HIP parameters are required in HIP control messages to detect
   NATs.  The NAT detection MUST occur prior to base exchange, or after
   node movement, prior to sending UPDATE messages.

   For example, STUN [RFC3489] offers a generic mechanism using which a
   host behind NAT can detect the presence of NAT and type of NAT
   present.  In STUN, the host contacts a STUN server which is located
   always in public network and the STUN server replies back letting the
   host know whether the host is behind NAT or in public network.  STUN
   can be used to detect NATs in all but one case where both of the host
   are behind the same NAT.  This is commonly referred as the Hairpin
   translation [I-D.srisuresh-behave-p2p-state] .  The hairpin
   translation poses an unnecessary overhead in terms of UDP processing
   of packets and routing of packets through the NAT despite the hosts
   being located within the same network.

   As a solution to the hairpin problem, an implementation MAY choose
   first to send I1 packets without UDP encapsulation and wait for the
   response for an implementation specific time.  If the initiator does
   not get a reply from the responder, it then can start retransmitting
   I1 packets UDP encapsulated.  This approach solves the hairpin
   problem, but incurs extra latency for the HIP connection.


3.  HIP Across NATs

   HIP based communications between two hosts consists effectively of
   HIP control traffic and ESP encrypted data traffic.  Before ESP data
   traffic can be sent, the hosts send HIP control messages to negotiate
   algorithms and exchange keys.  After this, the hosts can start
   sending encrypted ESP data traffic.

   The HIP based communications defined in [I-D.ietf-hip-base] works
   well in public networks.  However, this does not work with some
   legacy NATs which just drop HIP control traffic and ESP data traffic.
   As a solution for this, we propose UDP encapsulation of control and
   data traffic using a specific scheme described in this document.  The



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   scheme also allows hosts behind NATs to act as servers.

   [RFC3948] describes UDP encapsulation of IPsec ESP transport and
   tunnel mode.  This document only describes the changes required for
   UDP encapsulation of BEET mode [I-D.nikander-esp-beet-mode].

3.1.  Packet Formats

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

3.1.1.  Control Traffic

      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 1: Format for UDP-encapsulated HIP control traffic.

   Figure 1 shows how HIP control packets are encapsulated within UDP.
   A minimal UDP packet carries a complete HIP packet in its payload.
   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 headers checksum is not used because it is
   redundant and requires the use of inner addresses (extra complexity
   for UDP-NAT transformations).

3.1.2.  Control Channel Keep-Alives

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







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3.1.3.  Data Traffic

      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 2: Format for UDP-encapsulated IPsec ESP BEET-mode traffic.

   Figure 2 shows how IPsec ESP BEET-mode packets are encapsulated
   within UDP.  Again, a minimal UDP packet carries the ESP packet in
   its payload.  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].

3.1.4.  FROM_NAT Parameter

     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                           |
    |                                                               |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         UDP Port              |       Padding                 |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    Type        [ TBD by IANA (63998 = 2^16 - 2^11 + 2^9 - 2) ]
    Length      18
    Address     An IPv6 address or an IPv4-in-IPv6 format IPv4 address.
    UDP Port    A UDP port number

                  Figure 3: Format for FROM_NAT Parameter

   Figure 3 shows FROM_NAT parameter.  The use of this parameter is
   described in later sections.





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3.1.5.  VIA_RVS_NAT Parameter

      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                            |
     |                                                               |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          UDP Port             |            Padding            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Type        [ TBD by IANA (64002 = 2^16 - 2^11 + 2^9 + 2) ]
     Length      16
     Address     An IPv6 address or an IPv4-in-IPv6 format IPv4 address
     UDP Port    A UDP port

                Figure 4: Format for VIA_RVS_NAT Parameter

   Figure 4 shows VIA_RVS_NAT parameter.  The parameter is used for
   diagnostic purposes, similarly as VIA_RVS parameter in
   [I-D.ietf-hip-rvs].  The exact use of this parameter is described in
   later sections.

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

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

3.2.1.  UDP Encapsulation of IPsec BEET-Mode ESP

   In BEET IPsec mode, any present transport-layer checksums in the
   payload data are consequently 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].

   The resulting BEET-mode packet is then 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 as defined in later
   sections.  The UDP checksum MUST be calculated based on an IP header
   that contains the outer addresses of the SA.  The other fields of the
   UDP header are computed as described in [RFC0768].

   The resulting UDP packet MUST then undergo BEET IP header processing



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   as defined in Section 5.4 of [I-D.nikander-esp-beet-mode].

   Figure 5 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 5: UDP Encapsulation of an IPsec BEET-mode ESP packet
                         containing a TCP segment.

3.2.2.  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].

   The NAT traversal methods described in this section are based on
   connection reversal and UDP hole punching similar to
   [I-D.ietf-behave-nat-udp].  However, the methods in this section are
   adapted for HIP purposes, especially the rendezvous server in mind.

3.3.  Initiator Behind NAT

   This section discusses mechanisms to reach a HIP responder located in
   publicly addressable network by a HIP initiator that is located



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   behind a NAT.  The case where the responder is using a rendezvous
   service is also described.

   Table 1 lists some short-hand notations used in this section.  For
   simplicity, the ports mangled by NAT are presented as example port
   numbers (11111 and 22222) instead of symbolic ones.  In the examples,
   we assume that the NAT(s) timeout after I1-R1 exchange for
   illustration purposes, hence there different port numbers for I2-R2
   exchange.

   +------------------+------------------------------------------------+
   | Notation         | Explanation                                    |
   +------------------+------------------------------------------------+
   | HIT-I            | Initiator's HIT                                |
   | HIT-R            | Responder's HIT                                |
   | IP-I             | Initiator's IP address                         |
   | IP-R             | Responder's IP address                         |
   | IP-RVS           | IP address of the responder's rendezvous       |
   |                  | server                                         |
   | IP-NAT-I         | Public IP of the NAT of the initiator          |
   | IP-NAT-R         | Public IP of the NAT of the responder          |
   | UDP(50500,11111) | UDP packet with source port 50500 and          |
   |                  | destination port 11111                         |
   | UDP(11111,22222) | Example port numbers mangled by a NAT          |
   | UDP(44444,22222) | Port 44444 is used throughout the examples to  |
   |                  | denote the NAT mangled source port of I2 as    |
   |                  | received by the rendezvous server during the   |
   |                  | registration                                   |
   +------------------+------------------------------------------------+

                  Table 1: Notations Used in This Section

3.3.1.  NAT Traversal of HIP Control Traffic

   This section describes the details of enabling NAT traversal for HIP
   control traffic for the base exchange [I-D.ietf-hip-base] through UDP
   encapsulation for the case when initiator of the association is
   located behind a NAT and responder is located in publicly addressable
   network.  UDP-encapsulated HIP control traffic MUST use the packet
   formats described in Section 3.1.  When sending UDP-encapsulated HIP
   control traffic, a HIP implementation MUST zero the HIP header
   checksum before calculating the UDP checksum.  The receiver MUST only
   verify the correctness of the UDP checksum and MUST NOT verify the
   checksum of the HIP header.

   The initiator of a UDP-encapsulated HIP base exchange MUST use the
   UDP destination port 50500 for all control packets it sends.  It is
   RECOMMENDED to use 50500 as the source port as well, but an



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   implementation MAY use a (randomly selected) unoccupied source port.
   If it uses a random source port, it MUST listen for and accept
   arriving HIP control/ESP Data packets on this port until the
   corresponding HIP association is torn down.  The random source port
   is RECOMMENDED to be in the range of the dynamic and private ports
   (49152-65535).  Using a random source port instead of a fixed one
   makes it possible to have multiple clients behind a NAT middlebox
   that does only address translation but no port translation.  This is
   referred to as port overloading in [I-D.ietf-behave-nat-udp].

   The responder of a UDP-encapsulated HIP base exchange MUST use 50500
   as the source port for all UDP-encapsulated control packets it sends.
   The source address for all the packets that the responder sends MUST
   be the same as the IP address on which responder receives packets
   from initiator.  The responder MUST NOT respond to any arriving UDP-
   encapsulated control message with an decapsulated reply.  HIP
   implementations that implement the NAT traversal mechanisms MUST
   process UDP-encapsulated base exchange messages equivalently to
   decapsulated messages, i.e., according to [I-D.ietf-hip-base].

   The remainder of this section clarifies this process through an
   example which is illustrated in Figure 6.  It shows an initiator with
   the private IP address I behind a NAT.  The NAT has the public IP
   address as NAT.  The responder is located in the public Internet at
   the IP address R.

        +---+           +---+                                 +---+
        |   |----(1)--->|   |---------------(2)-------------->|   |
        |   |           | N |                                 |   |
        |   |<---(4)----| A |<--------------(3)---------------|   |
        | I |           | T |                                 | R |
        |   |----(5)--->| - |---------------(6)-------------->|   |
        |   |           | I |                                 |   |
        |   |<---(8)----|   |<--------------(7)---------------|   |
        +---+           +---+                                 +---+

        1. IP(IP-I, IP-R)      UDP(50500, 50500)  I1(HIT-I, HIT-R)
        2. IP(IP-NAT-I, IP-R)  UDP(11111, 50500)  I1(HIT-I, HIT-R)
        3. IP(IP-R, IP-NAT-I)  UDP(50500, 11111)  R1(HIT-R, HIT-I)
        4. IP(IP-R, IP-I)      UDP(50500, 50500)  R1(HIT-R, HIT-I)
        5. IP(IP-I, IP-R)      UDP(50500, 50500)  I2(HIT-I, HIT-R)
        6. IP(IP-NAT-I, IP-R)  UDP(22222, 50500)  I2(HIT-I, HIT-R)
        7. IP(IP-R, IP-NAT-I)  UDP(50500, 22222)  R2(HIT-R, HIT-I)
        8. IP(IP-R, IP-I)      UDP(50500, 50500)  R2(HIT-R, HIT-I)

   Figure 6: Example of a UDP-encapsulated HIP base exchange (initiator
             behind a NAT, responder on the public Internet).




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   Before beginning the base exchange, the initiator detects that it is
   behind a NAT.  The initiator starts the base exchange by sending a
   UDP-encapsulated I1 packet to the responder.  According to the rules
   specified above, the source IP address of this I1 packet is IP-I and
   its source UDP port is 50500.  It is addressed to IP-R on port 50500.
   The NAT in Figure 6 forwards the I1 but substitutes the source
   address IP-I with its own public address IP-NAT-I and the source UDP
   port 50500 with 11111.

   When the responder in Figure 6 receives the UDP-encapsulated I1
   packet on UDP port 50500, it processes it according to
   [I-D.ietf-hip-base].  The responder replies back with an R1 using the
   addresses and port information of I1.  Thus, the R1 packet is
   destined to the source IP address and UDP port of the I1, i.e., IP
   address IP-NAT-I and port 11111.  The NAT receives the I1 and
   substitutes the destination of this packet with the initiator address
   (IP-I) and port information (50500).

   The initiator receives a UDP-encapsulated R1 packet from the
   responder and processes it according to [I-D.ietf-hip-base].  When it
   responds with a UDP-encapsulated I2 packet, it uses the same IP
   source and destination addresses and UDP source and destination ports
   that it used for sending the corresponding I1 packet, i.e., the
   packet is addressed from IP-I port 50500 to IP-R port 50500.  The NAT
   again substitutes the source information.  To illustrate timeout, the
   NAT chooses a different source port (22222) for the I2 than for the
   I1 (11111) in this case.

   When a responder receives a UDP-encapsulated I2 packet destined to
   UDP port 50500, it MUST use the UDP source port contained in this
   packet for further HIP communications with the initiator.  It then
   processes the I2 packet according to [I-D.ietf-hip-base].  When it
   responds with an R2 message, it UDP-encapsulates the message, using
   the UDP source port of the I2 packet as the destination UDP port, and
   sends it to the source IP address of the I2 packet, i.e., it sends
   the R2 packet from IP-R port 50500 to IP-NAT-I port 22222.  The NAT
   again replaces the destination information in the R2 with IP-I port
   50500

   Usually, the I1-R1 and I2-R2 exchanges occur fast enough for the NAT
   state to persist.  This means that the NAT uses the same port for the
   I1-R1 exchange to translate as the I2-R2 exchange.  However, an
   implementation MUST handle even the case where the NAT state times
   out between the two exchanges and the I1 and I2 arrive from different
   UDP source ports and/or IP addresses, as shown in Figure 6.






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3.3.2.  NAT Traversal of HIP Data Traffic

   This section describes the details of enabling NAT traversal of HIP
   data traffic.  As described in Section 3, HIP data traffic is carried
   in UDP-encapsulated IPsec BEET-mode ESP packets.

3.3.2.1.  IPsec BEET-Mode Security Associations

   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].  As mentioned in
   Section 3.3.1, when two peers perform a UDP-encapsulated base
   exchange, they MUST define a pair of IPsec SAs that result in UDP-
   encapsulated BEET-mode ESP data traffic.

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

   The initiator MUST use UDP destination port 50500 for all UDP-
   encapsulated ESP packets it sends.  It MAY also use port 50500 as
   source port or it MAY use a random source port.  If it uses a random
   source port, it MUST listen for and accept arriving UDP-encapsulated
   ESP packets on this port until the corresponding HIP association is
   torn down.

   The responder of a UDP-encapsulated IPsec BEET-mode ESP exchange MUST
   use 50500 as the source port for all UDP-encapsulated ESP packets it
   sends.  The destination port is the port from which the responder is
   receiving UDP encapsulated ESP data from the initiator.

   Both initiator and responder of a HIP association that uses the NAT
   traversal mechanism as described in this draft MUST define BEET mode
   with UDP encapsulation as IPsec mode for SA after a successful base
   exchange.  The inner source address MUST be local HIT used during
   base exchange and inner destination address MUST be HIT of the
   respective peer.  The other parts of the SA are described in
   individual sections.

3.3.2.1.1.  Security Associations at the Initiator

   The initiator of a UDP-encapsulated base exchange defines its
   outbound SA as shown in Table 2





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   +--------------+----------------------------------------------------+
   | Field        | Value                                              |
   +--------------+----------------------------------------------------+
   | Outer src    | Same local IP address from which the base exchange |
   | address      | packets were transmitted                           |
   | Outer dst    | Same peer IP address to which base exchange        |
   | address      | packets were transmitted                           |
   | UDP src port | Same port number as chosen for I2 packet in base   |
   |              | exchange                                           |
   | UDP dst port | Port 50500                                         |
   +--------------+----------------------------------------------------+

                     Table 2: Outbound SA at initiator

   The initiator of a UDP-encapsulated base exchange defines its inbound
   SA as shown in Table 3

   +--------------+----------------------------------------------------+
   | Field        | Value                                              |
   +--------------+----------------------------------------------------+
   | Outer src    | Same peer IP address to which base exchange        |
   | address      | packets were transmitted                           |
   | Outer dst    | Same local IP address from which the base exchange |
   | address      | packets were transmitted                           |
   | UDP src port | Port 50500                                         |
   | UDP dst port | Initiator MUST use the UDP source port it uses in  |
   |              | the outbound SA here                               |
   +--------------+----------------------------------------------------+

                     Table 3: Inbound SA at initiator

3.3.2.1.2.  Security Associations at the Responder

   The responder of a UDP-encapsulated base exchange defines its
   outbound SA shown in Table 4.

   +-------------+-----------------------------------------------------+
   | Field       | Value                                               |
   +-------------+-----------------------------------------------------+
   | Outer src   | Same local IP address from which the base exchange  |
   | address     | packets were transmitted                            |
   | Outer dst   | Peer IP address of the I2 packet received during    |
   | address     | the base exchange                                   |
   | UDP src     | Port 50500                                          |
   | port        |                                                     |
   | UDP dst     | Source UDP port of the I2 packet received from the  |
   | port        | initiator during base exchange                      |
   +-------------+-----------------------------------------------------+



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                     Table 4: Outbound SA at Responder

   Similarly, the responder of a UDP-encapsulated base exchange defines
   its inbound SA as shown in Table 5

   +-------------+-----------------------------------------------------+
   | Field       | Value                                               |
   +-------------+-----------------------------------------------------+
   | Outer src   | Source IP address of the I2 packet received from    |
   | address     | the initiator during base exchange                  |
   | Outer dst   | Same local IP address from which the base exchange  |
   | address     | packets were transmitted                            |
   | UDP src     | Source UDP port of the I2 packet received from the  |
   | port        | initiator during base exchange                      |
   | UDP dst     | Port 50500                                          |
   | port        |                                                     |
   +-------------+-----------------------------------------------------+

                     Table 5: Inbound SA at responder

3.3.3.  Use of the Rendezvous Service when only the Initiator Is Behind
        NAT

   The rendezvous extensions for HIP without NAT traversal have been
   defined in [rvs].  This section addresses only the scenario where a
   NATted HIP node uses rendezvous service to contact another HIP node
   in a publicly addressable network.  Figure 7 illustrates the
   mechanism described in this section.

   A rendezvous server MUST listen on UDP port number 50500 for incoming
   UDP encapsulated I1 packets.  However, in this specific case with
   only initiator behind NAT, the rendezvous server MUST not relay the
   I1 packets at all because the UDP hole punching does not work.
   Instead, the rendezvous server replies to the initiator with a NOTIFY
   message that includes the responder's locator in VIA_RVS parameter.

   Upon receiving the NOTIFY with the locators of the responder through
   the NAT, the initiator MUST send an I1 to the responder.  However, it
   MUST continue retransmissions using the RVS location.  This is
   mandatory because NOTIFY messages are not protected with signatures
   and can be forged by a rogue host.

   When the initiator receives an R1 through the NAT, the responder
   verifies the integrity of the packet and replies with an I2.  The
   responder should be aware that the I2 may arrive from a different
   port than the I1.  In such a case, the responder should send the R2
   to the source port of I2.




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     +---+           +---+                     +-------+   +---+
     |   |----(1)--->|   |---------------(2)-->|       |   |   |
     |   |           |   |                     | RVS R |   |   |
     |   |<---(4)----|   |<--------------(3)---|       |   |   |
     |   |           |   |                     +-------+   |   |
     |   |           | N |                                 |   |
     |   |----(5)--->| A |---------------(6)-------------->|   |
     | I |           | T |                                 | R |
     |   |<---(8)----| - |<--------------(7)---------------|   |
     |   |           | T |                                 |   |
     |   |----(9)--->|   |---------------(10)------------->|   |
     |   |           |   |                                 |   |
     |   |<---(11)---|   |<--------------(12)--------------|   |
     +---+           +---+                                 +---+

     1.  IP(IP-I, IP-RVS)      UDP(50500, 50500)   I1(HIT-I, HIT-R)
     2.  IP(IP-NAT-I, IP-RVS)  UDP(11111, 50500)   I1(HIT-I, HIT-R)
     3.  IP(IP-RVS, IP-NAT-I)  UDP(50500, 11111)
                                   NOTIFY(HIT-I, HIT-R, VIA_RVS(IP-R))
     4.  IP(IP-I, IP-NAT-I)    UDP(50500, 50500)
                                   NOTIFY(HIT-I, HIT-R, VIA_RVS(IP-R))
     5.  IP(IP-I, IP-R)        UDP(50500, 50500)   I1(HIT-I, HIT-R)
     6.  IP(IP-NAT-I, IP-R)    UDP(22222, 50500)   I1(HIT-I, HIT-R)
     7.  IP(IP-R, IP-NAT-I)    UDP(50500, 22222)   R1(HIT-R, HIT-I)
     8.  IP(IP-R, IP-I)        UDP(50500, 50500)   R1(HIT-R, HIT-I)
     9.  IP(IP-I, IP-R)        UDP(50500, 50500)   I2(HIT-I, HIT-R)
     10. IP(IP-NAT-I, IP-R)    UDP(33333, 50500)   I2(HIT-I, HIT-R)
     11. IP(IP-R, IP-NAT-I)    UDP(50500, 33333)   R2(HIT-R, HIT-I)
     12. IP(IP-R, IP-I)        UDP(50500, 50500)   R2(HIT-R, HIT-I)

     Figure 7: Example of a UDP-encapsulated HIP base exchange via RVS
    (initiator behind a NAT, responder and RVS on the public Internet).

3.4.  Responder Behind NAT

   This section discusses mechanisms to reach a HIP responder that is
   located behind a NAT.  This section assumes that the initiator is
   located on publicly addressable network.  The initiator contacts the
   responder through an RVS server.

3.4.1.  Rendezvous Client Registration From Behind NAT

   The rendezvous client registration [rvs] describes the case when
   rendezvous client is present in publicly addressable network.  This
   section defines an extension to the rendezvous client registration
   for the case when the rendezvous client has detected that it is
   behind a NAT.  The process in the NAT case is identical to the case
   without NAT, except that UDP encapsulation is used.  The registration



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   is illustrated in Figure 8.

   A node behind a NAT MUST first register to the RVS when it is going
   to act as a responder for some other nodes.  The node (i.e.
   rendezvous client) performs a base exchange with the RVS over UDP as
   described in Section 3.3 by sending I1 UDP encapsulated and 50500 as
   destination port number.  RVS sends REG_INFO parameter in R1 to which
   rendezvous client replies with REG_REQ in I2.  Both I1 and R1 are
   sent using UDP.  If RVS grants service to the rendezvous client, it
   MUST store the source IP address and source port number of the I2 UDP
   packet that it had received from the rendezvous client during base
   exchange.  The source IP address belongs to the NAT and the source
   port number is the NAT mangled port.  RVS then replies with REG_RESP
   in R2 over UDP.  If the registration process results in a successful
   REG_RESP, the rendezvous client MUST send NAT keepalives
   (Section 3.1.2) to keep the mapping in the NAT with the RVS open.
   The NAT keepalives sent from rendezvous client to the RVS MUST have
   the same source port as the I2 packet.

   When the RVS receives an I1 packet from a HIP node to be relayed to
   the successfully registered rendezvous client behind NAT, RVS MUST
   relay the I1 over UDP with the destination port as the one stored
   during registration.  The RVS also zeroes the checksum of the I1.
   This process is explained in Section 3.4.2.



























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        +---+           +---+                                 +---+
        |   |----(1)--->|   |---------------(2)-------------->|   |
        |   |           | N |                                 |   |
        |   |<---(4)----| A |<--------------(3)---------------|   |
        | I |           | T |                                 | R |
        |   |----(5)--->| - |---------------(6)-------------->|   |
        |   |           | I |                                 |   |
        |   |<---(8)----|   |<--------------(7)---------------|   |
        +---+           +---+                                 +---+

        Initiator = Rendezvous client, Responder = Rendezvous server

        1. IP(IP-I, IP-R)     UDP(50500, 50500) I1(HIT-I, HIT-R)
        2. IP(IP-NAT-I, IP-R) UDP(33333, 50500) I1(HIT-I, HIT-R)
        3. IP(IP-R, IP-NAT-I) UDP(50500, 33333)
              R1(HIT-R, HIT-I, REG_INFO)
        4. IP(IP-R, IP-I)     UDP(50500, 50500)
              R1(HIT-R, HIT-I, REG_INFO)
        5. IP(IP-I, IP-R)     UDP(50500, 50500)
              I2(HIT-I, HIT-R, REG_REQ)
        6. IP(IP-NAT-I, IP-R) UDP(44444, 50500)
              I2(HIT-I, HIT-R, REG_REQ)
        7. IP(IP-R, IP-NAT-I) UDP(50500, 44444)
              R2(HIT-R, HIT-I, REG_RES)
        8. IP(IP-R, IP-I)     UDP(50500, 50500)
              R2(HIT-R, HIT-I, REG_RES)

               Figure 8: Rendezvous NAT Client Registration

3.4.2.  NAT Traversal of HIP Control Traffic

   This section describes the details of enabling NAT traversal for base
   exchange packets [I-D.ietf-hip-base] through UDP encapsulation, for
   the case when the HIP initiator is on publicly addressable network
   and the HIP responder is behind NAT.  The process is illustrated in
   Figure 9.

   Before the HIP base exchange starts, the responder of the HIP base
   exchange MUST have completed a successful rendezvous client
   registration using the scheme defined in Section 3.4.1.

   The initiator of the HIP base exchange sends a plain I1 packet
   (without UDP encapsulation) to the RVS as described in [rvs].  The
   RVS relays the inbound I1 packet to the registered rendezvous client.
   In this case, the incoming I1 is not UDP encapsulated, but the
   rendezvous client has registered using UDP.

   To relay the I1 packet, RVS SHOULD zero the HIP header checksum from



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   the I1 packet.  RVS must add a FROM parameter, as described in [rvs],
   which contains the IP address of HIP initiator.  The FROM parameter
   is integrity protected by a RVS_HMAC as described in [rvs].  RVS
   replaces the destination IP address in the IP header of the packet
   with IP that it had stored during the rendezvous client registration
   (which is the IP address of the outermost NAT behind which rendezvous
   client is located).  It MUST then encapsulate the I1 packet within
   UDP.  The source port in the UDP header MUST be 50500 and the
   destination port MUST be the same as the source port number (44444)
   of the I2 packet which it had stored during the registration process.
   RVS then recomputes the IP header checksum and sends the packet.

                        +-------+
                        |       |
                 +----->|  RVS  +-----+           +----+
   +---+         |      |       |     |           |    |          +---+
   |   |---(1)---+      +-------+     +----(2)--->|    |---(3)--->|   |
   |   |                                          | N  |          |   |
   |   |<------------------(5)--------------------| A  |<--(4)----|   |
   | I |                                          | T  |          | R |
   |   |-------------------(6)------------------->| -  |---(7)--->|   |
   |   |                                          | R  |          |   |
   |   |<------------------(9)--------------------|    |<--(8)----|   |
   +---+                                          +----+          +---+

   1. IP(IP-I, IP-RVS)     I1(HIT-I, HIT-R)
   2. IP(IP-RVS, IP-NAT-R) UDP(50500, 44444)
         I1(HIT-I, HIT-R, FROM:IP-I, RVS_HMAC)
   3. IP(IP-RVS, IP-R)     UDP(50500, 50500)
         I1(HIT-I, HIT-R, FROM:IP-I, RVS_HMAC)
   4. IP(IP-R, IP-I)
         UDP(50500, 50500) R1(HIT-R, HIT-I, VIA_RVS_NAT(RVS-IP, 50500))
   5. IP(IP-NAT-R, IP-I)
         UDP(44444,   50500) R1(HIT-R, HIT-I, VIA_RVS_NAT(RVS-IP, 50500)
   6. IP(IP-I, IP-NAT-R)   UDP(50500, 44444)   I2(HIT-I, HIT-R)
   7. IP(IP-I, IP-R)       UDP(50500, 50500) I2(HIT-I, HIT-R)
   8. IP(IP-R, IP-I)       UDP(50500, 50500) R2(HIT-R, HIT-I)
   9. IP(IP-NAT-R, IP-I)   UDP(44444, 50500) R2(HIT-R, HIT-I)

     Figure 9: UDP-encapsulated HIP base exchange (initiator on public
                    Internet, responder behind a NAT).

   The relayed I1 packet travels from RVS to the NAT.  The NAT changes
   the destination IP address of the UDP encapsulated I1 packet, and the
   destination port number in the UDP header.  The responder accepts the
   packet from the RVS and processes it according to [rvs].  The
   resulting R1 must be encapsulated within UDP.  The responder MAY
   append a VIA_RVS_NAT parameter to the message, which contains the IP



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   address of the rendezvous and the port the rendezvous used for
   relaying the R1.  The RECOMMENDED source port is 50500 and the
   destination port number MUST be 50500.  The destination address in
   the IP header MUST be the same as the one specified in the FROM
   parameter of the relayed I1 packet.

   The initiator MUST listen on port 50500 and it receives the UDP
   encapsulated R1.  After verifying the HIP packet, it concludes that
   the responder is behind a NAT because the packet was UDP
   encapsulated.  The initiator processes the R1 control packet
   according to [I-D.ietf-hip-base] and replies using I2 that is UDP
   encapsulated.  The addresses and ports are derived from the received
   R1.

   The NAT translates and forwards the UDP encapsulated I2 packet to the
   responder.  The resulting R2 packet is also UDP encapsulated using
   the address and port information from the received I2 packet.

3.4.3.  NAT Traversal of HIP Data Traffic

   After a successful base exchange, both of the HIP nodes have all the
   parameters with them needed to establish UDP BEET mode Security
   Association.  The following section describe inbound and outbound
   security associations at initiator and responder.

3.4.3.1.  Security Associations at the Initiator

   The initiator of a base exchange defines its outbound SA as shown in
   Table 6

   +--------------+----------------------------------------------------+
   | Field        | Value                                              |
   +--------------+----------------------------------------------------+
   | Outer src    | Same local IP address from which the base exchange |
   | address      | packets were transmitted                           |
   | Outer dst    | Same peer IP address from which R2 packet was      |
   | address      | received during base exchange                      |
   | UDP src port | Port 50500                                         |
   | UDP dst port | Source port of incoming R2 packet during base      |
   |              | exchange                                           |
   +--------------+----------------------------------------------------+

                     Table 6: Outbound SA at initiator

   The initiator of a base exchange defines its inbound SA as shown in
   Table 7





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   +--------------+----------------------------------------------------+
   | Field        | Value                                              |
   +--------------+----------------------------------------------------+
   | Outer src    | Same peer IP address from which R2 packet was      |
   | address      | received during base exchange                      |
   | Outer dst    | Same local IP address from which the base exchange |
   | address      | packets were transmitted                           |
   | UDP src port | Source port of incoming R2 packet during base      |
   |              | exchange                                           |
   | UDP dst port | Port 50500                                         |
   +--------------+----------------------------------------------------+

                     Table 7: Inbound SA at initiator

3.4.3.2.  Security Associations at the Responder

   The responder of a UDP-encapsulated base exchange defines its
   outbound SA shown in Table 8.

   +--------------+----------------------------------------------------+
   | Field        | Value                                              |
   +--------------+----------------------------------------------------+
   | Outer src    | Same local IP address from which the base exchange |
   | address      | packets were transmitted                           |
   | Outer dst    | Same peer IP as that used during base exchange     |
   | address      |                                                    |
   | UDP src port | Same as source port chosen during base exchange    |
   | UDP dst port | Port 50500                                         |
   +--------------+----------------------------------------------------+

                     Table 8: Outbound SA at Responder

   Similarly, the responder of a UDP-encapsulated base exchange defines
   its inbound SA as shown in Table 9

   +--------------+----------------------------------------------------+
   | Field        | Value                                              |
   +--------------+----------------------------------------------------+
   | Outer src    | Source peer IP address as used in base exchange    |
   | address      |                                                    |
   | Outer dst    | Same local IP address from which the base exchange |
   | address      | packets were transmitted                           |
   | UDP src port | Port 50500                                         |
   | UDP dst port | Same as source port chosen during base exchange    |
   +--------------+----------------------------------------------------+

                     Table 9: Inbound SA at responder




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3.5.  Both Hosts Behind NAT

   This section describes the details of enabling NAT traversal for HIP
   control and ESP data traffic, such as the base exchange
   [I-D.ietf-hip-base], through UDP encapsulation, for the case when the
   HIP initiator and the HIP responder are both behind two separate
   NATs.  The described mechanism applies also when the hosts are behind
   the same NAT but may result in inefficient routing paths, unless the
   countermeasures described in section Section 2 are followed.  The
   limitation of this approach is that it requires that the NAT boxes
   support endpoint independent mapping
   [I-D.srisuresh-behave-p2p-state].

   The registration and rendezvous relay are handled similarly as
   described in Section 3.3.3 and Section 3.4.1.  Now that both hosts
   are behind NATs, both the initiator (Section 3.3) and responder
   (Section 3.4) mechanisms are combined here.

3.5.1.  NAT Traversal of HIP Control Traffic

   This section describes traversal mechanism for HIP control traffic in
   the situation when both the initiator and the responder are behind
   NATs.  Both hosts MUST first detect using external mechanism that
   they are located behind NAT.  The RVS MUST be located on publicly
   addressable network.  Before initiator begins the base exchange, the
   responder MUST have completed a successful rendezvous client
   registration with the RVS using the mechanism described in
   Section 3.4.1.

   Initiator of the HIP base exchange starts the base exchange by
   sending an UDP encapsulated I1 packet to RVS.  The UDP packet MUST
   have destination port number 50500 and initiator is RECOMMENDED to
   use 50500 as source port number.  RVS MUST listen on UDP port 50500.
   RVS MUST accept the packet as described in Section 3.3.3.  As there
   has been a successful rendezvous client registration between the
   responder and the RVS as described in Section 3.4.1, the RVS knows
   the port number which it can use to communicate with the responder
   through the NAT.  RVS MUST add a FROM_NAT parameter to the I1 packet.
   The FROM_NAT parameter contains the source address of the I1 packet,
   which is effectively the address of the outermost NAT of the
   initiator.  The RVS copies the source port of the UDP encapsulated I1
   packet into the port number field of the FROM_NAT parameter.  The
   FROM_NAT parameter is integrity protected by an RVS_HMAC as described
   in [rvs].  It MUST replace the destination IP address of the I1
   packet by the one it had stored earlier during rendezvous client
   registration.  It MUST replace source IP address of I1 packet with
   its own address.  UDP source port of the relayed I1 packet MUST be
   50500 and destination port MUST be the same as one it had stored



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   during the client rendezvous registration.  It MUST recompute the IP
   header checksum.

   In this case, in which the I1 was UDP encapsulated and the rendezvous
   client is also behind a NAT, the rendezvous server sends two packets.
   First, it MUST relay the I1 packet to the responder (rendezvous
   client) using UDP.  Second, it MUST send the locator and port (as
   observed by the rendezvous) of the responder in a VIA_RVS_NAT
   parameter in a NOTIFY packet to the inititiator.  However, this will
   actually launch two parallel base exchanges.  In the first case, the
   initiator receives the NOTIFY message, and acts on it as described in
   section Section 3.3.3, i.e., it sends an I1 directly to the address
   in the VIA_RVS_NAT parameter and continues to retransmit packet
   through the RVS.  In the second case, the responder will receive the
   I1 relayed by the rendezvous.  The responder acts as described in
   section Section 3.4.2 by replying with an R1.

   This scheme launches two parallel exchanges, one of which is phased
   later than the other.  Although this kind of operation is not usually
   very desirable, it is essential to guarantee successful NAT hole
   punching.  The base exchange has been designed to handle simultaneous
   base exchanges and the race between the two parallel base exchange
   eventually terminates after initiator is in established state.




























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    +---+        +----+         +-------+            +----+        +---+
    |   |--(1)-->|    |---(2)-->+       |            |    |        |   |
    |   |        |    |         | RVS R |            |    |        |   |
    |   |        |    |<--(3a)--+       |---(3b)---->|    |        |   |
    |   |        | N  |         +-------+            | N  |        |   |
    |   |<-(4a)--| A  |                              | A  |--(4b)->|   |
    | I |        | T  |                              | T  |        | R |
    |   |--(5a)->| -  |                              | -  |<-(5b)--|   |
    |   |        | I  |<-(6b)------------------(6a)->| R  |        |   |
    |   |        |    |                              |    |        |   |
    ....................................................................
    +---+        +----+                              +----+        +---+

    1.   IP(IP-I, IP-RVS)       UDP(50500, 50500)  I1(HIT-I, HIT-R)
    2.   IP(IP-NAT-I, IP-RVS)   UDP(11111, 50500)  I1(HIT-I, HIT-R)

    3a.  IP(IP-RVS, IP-NAT-I)   UDP(50500, 11111)
            NOTIFY(HIT-R, HIT-I, VIA_RVS_NAT(IP-NAT-R, 44444)
    3b.  IP(IP-RVS, IP-NAT-R)   UDP(50500, 44444)
            I1(HIT-I, HIT-R, FROM_NAT:[IP-NAT-I,11111], RVS_HMAC)

    4a.  IP(IP-RVS-R, IP-I)     UDP(50500, 50500)
            NOTIFY(HIT-R, HIT-I, VIA_RVS_NAT(IP-NAT-R, 44444)
    4b.  IP(IP-RVS, IP-R)       UDP(50500, 50500)
            I1(HIT-I, HIT-R, FROM_NAT:[NAT-I,11111], RVS_HMAC)

    5a.  IP(IP-I, IP-NAT-R)     UDP(50500, 44444) I1(HIT-I, HIT-R)
    5b.  IP(IP-R, IP-NAT-I)     UDP(50500, 11111)
            R1(HIT-R, HIT-I, VIA_RVS_NAT(RVS-IP, 50500))

    6a.  IP(IP-NAT-I, IP-NAT-R) UDP(11111, 44444) I1(HIT-I, HIT-R)
    6b.  IP(IP-NAT-R, IP-NAT-I) UDP(44444, 11111)
            R1(HIT-R, HIT-I, VIA_RVS_NAT(RVS-IP, 50500))

       Figure 10: UDP-encapsulated HIP base exchange (initiator and
                responder behind a NAT, RVS on public IP).

3.5.2.  NAT Traversal of HIP Data Traffic

   After a successful base exchange, both the HIP nodes have all the
   parameters with them to establish UDP BEET mode Security Association.
   The following section describes inbound and outbound security
   associations at initiator and responder.

3.5.2.1.  Security Associations at the Initiator

   The initiator of a base exchange defines its outbound SA as shown in
   Table 10



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   +--------------+----------------------------------------------------+
   | Field        | Value                                              |
   +--------------+----------------------------------------------------+
   | Outer src    | Same local IP address from which the base exchange |
   | address      | packets were transmitted                           |
   | Outer dst    | Same peer IP address from which R2 packet was      |
   | address      | received during base exchange                      |
   | UDP src port | Same as the port number chosen to send I2 during   |
   |              | base exchange                                      |
   | UDP dst port | Source port of incoming R2 packet during base      |
   |              | exchange                                           |
   +--------------+----------------------------------------------------+

                    Table 10: Outbound SA at initiator

   The initiator of a base exchange defines its inbound SA as shown in
   Table 11

   +--------------+----------------------------------------------------+
   | Field        | Value                                              |
   +--------------+----------------------------------------------------+
   | Outer src    | Same peer IP address from which R2 packet was      |
   | address      | received during base exchange                      |
   | Outer dst    | Same local IP address from which the base exchange |
   | address      | packets were transmitted                           |
   | UDP src port | Source port of incoming R2 packet during base      |
   |              | exchange                                           |
   | UDP dst port | Same as the port number chosen to send I2 during   |
   |              | base exchange                                      |
   +--------------+----------------------------------------------------+

                     Table 11: Inbound SA at initiator

3.5.2.2.  Security Associations at the Responder

   The responder of a UDP-encapsulated base exchange defines its
   outbound SA shown in Table 12.

   +--------------+----------------------------------------------------+
   | Field        | Value                                              |
   +--------------+----------------------------------------------------+
   | Outer src    | Same local IP address from which the base exchange |
   | address      | packets were transmitted                           |
   | Outer dst    | Same peer IP as that used during base exchange     |
   | address      |                                                    |
   | UDP src port | Same as source port chosen send R2 during base     |
   |              | exchange                                           |




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   | UDP dst port | Same as source port number of I2 packet during     |
   |              | base exchange                                      |
   +--------------+----------------------------------------------------+

                    Table 12: Outbound SA at Responder

   Similarly, the responder of a UDP-encapsulated base exchange defines
   its inbound SA as shown in Table 13

   +--------------+----------------------------------------------------+
   | Field        | Value                                              |
   +--------------+----------------------------------------------------+
   | Outer src    | Source peer IP address as used in base exchange    |
   | address      |                                                    |
   | Outer dst    | Same local IP address from which the base exchange |
   | address      | packets were transmitted                           |
   | UDP src port | Same as source Port received from I2 during base   |
   |              | exchange                                           |
   | UDP dst port | Same as source port used to send R2 during base    |
   |              | exchange                                           |
   +--------------+----------------------------------------------------+

                     Table 13: Inbound SA at responder

3.6.  NAT Keep-Alives

   Typically, NATs cache an established binding and time it out if they
   have not used it to relay traffic for a given period of time.  This
   timeout is different for different NAT implementations.  The BEHAVE
   working group is discussing recommendations for standardized timeout
   values.  To prevent NAT bindings that support the traversal of UDP-
   encapsulated HIP traffic from timing out during times when there is
   no control or data traffic, HIP hosts SHOULD send periodic keep-alive
   messages.

   Typically, only outgoing traffic acts refreshes the NAT port state
   for security reasons.  Consequently, both hosts SHOULD send periodic
   keep-alives for the UDP channel of all their established HIP
   associations if the channel has been idle for a specific period of
   time.

   For the UDP channel, keep-alives MUST be UDP-encapsulated HIP UPDATE
   packets as defined in Section 3.1.2.  The packets MUST use the same
   source and destination ports and IP addresses as the corresponding
   UDP tunnel.  The default keep-alive interval for control channels
   MUST be 20 seconds.  The responder of the HIP association should just
   discard the keep-alives.




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3.7.  HIP Mobility

   After a successful base exchange, either host can change its network
   location using the mechanisms defined in [I-D.ietf-hip-mm].  This
   section describes such mobility mechanisms in the presence of NATs.
   However, double jump scenario, where both hosts move simultaneously,
   is excluded.

   The mobile node can change its location as described in Table 14.

   +----+---------------------------+----------------------------------+
   | No | From network              | To network                       |
   +----+---------------------------+----------------------------------+
   | 1  | Behind NAT                | Publicly Addressable Network     |
   | 2  | Publicly Addressable      | Behind NAT                       |
   |    | Network                   |                                  |
   | 3  | Behind NAT-A              | Stays behind NAT-A, but          |
   |    |                           | different IP                     |
   | 4  | Behind NAT-A              | Behind NAT-B                     |
   | 5  | Publicly Addressable      | Publicly Addressable Network     |
   |    | Network                   |                                  |
   +----+---------------------------+----------------------------------+

                   Table 14: End host mobility scenarios

   The corresponding peer node can be located as follows Table 15

             +----+------------------------------------------+
             | No | Peer Node network                        |
             +----+------------------------------------------+
             | A  | Publicly Addressable Network With RVS    |
             | B  | Publicly Addressable Network Without RVS |
             | C  | Behind NAT With RVS                      |
             | D  | Behind NAT Without RVS                   |
             +----+------------------------------------------+

                   Table 15: Peer host Network Scenarios

   The NAT traversal mechanisms may not work when the corresponding node
   is behind a NAT without RVS (case D), except when the mobile node
   stays behind the same cone NAT (case 3D).

   When a host changes its location, it SHOULD detect the presence of
   NATs along the new paths to its peers using some external mechanism
   before sending any UPDATE messages.  Alternatively, it MAY use some
   heuristics to conclude that it is behind a NAT rather than incur the
   latency of running NAT detection first.




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   The mobile node MUST send the UPDATE packet through the corresponding
   node's RVS if it has one, in addition to sending it to the
   corresponding node directly.  The mobile node encapsulates the UPDATE
   packet within UDP only when it is behind a NAT.  The corresponding
   node MUST reply using UDP when the packet was encapsulated within
   UDP, or without UDP when the UDP header was not present in the UPDATE
   packet.

   The rendezvous server UPDATE relaying process is similar to I1.  The
   rendezvous server MUST add FROM parameter when it gets a UPDATE
   packet without UDP encapsulation, or a FROM_NAT parameter when the
   UPDATE packet it receives is UDP encapsulated and MUST protect the
   packet with HMACs.  Upon replying to the UPDATE, the corresponding
   node MUST add a VIA_RVS (or VIA_RVS_NAT) parameter to the reply.

   When the UDP encapsulation for NAT traversal is used, private IP
   addresses should be filtered out from the LOCATOR parameter in the
   HIP control packets.  Exposing private addresses may impose privacy
   related problems.

3.8.  HIP Multihoming

   Multiple security associations can exists between the same hosts.
   They may be connected through several paths, some of which may
   include a NAT and others may not.  Implementations that support
   multihoming MUST support concurrent HIP associations between the same
   host pair in a way that allows some of them to use UDP encapsulation
   while others use basic HIP.  Implementations MAY distinguish HIP
   associations based on the SPI instead of a HIT pair for this purpose.

3.9.  Firewall Traversal

   When the initiator or the responder of a HIP association is behind a
   firewall, additional issues arise.

   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 destination port 50500 from arbitrary source
   ports and to receive UDP response traffic from that port to the
   chosen source port.

   Most firewall implementations support "UDP connection tracking",
   i.e., after a host behind a firewall has initiated a 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-



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   alive interval used is less than the refresh interval of the
   firewall.

   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 port 50500 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 receive UDP traffic
   on port 50500 from arbitrary source IP addresses and ports.

   The NAT traversal mechanisms described in Section 3 require that the
   firewall - stateful or not - allow inbound UDP traffic to port 50500
   and allow outbound UDP traffic to arbitrary UDP ports.  If necessary
   for firewall traversal, ports reserved for IKE MAY be used for
   initiating new connections, but the implementation MUST be able to
   listen for UDP packets from port 50500.


4.  Security Considerations

   Section 5.1 of [RFC3948] describes a security issue for the UDP
   encapsulation of 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 the 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 the HITs to
   distinguish between different communication instances.

   The rendezvous usage in this draft has been designed to follow the
   design of the RVS draft [I-D.ietf-hip-rvs] and only I1 relayed.
   However, as NAT networking presents some additional challenges, it is
   not possible two follow the RVS design exactly.  Particularly, the
   mechanisms described in Figure 7 and Section 3.5.1 require that the
   rendezvous server replies back to the initiator with a message which
   includes the address and port of the responder NAT.  Another design
   choice would have been to relay also the R1 (and I2 in case of both
   hosts behind NAT) through the rendezvous server to delay the exposure
   of the responder NAT address and port related information for
   additional DoS protection.  However, this choice was not selected to
   reduce round trip time.  As a consequence, the renzvous client must
   be accept the risk of lowered privacy protection when it registers to
   the RVS over UDP as defined in section Figure 8.



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5.  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" range, i.e., 50500.  Upon publication of this document, IANA is
   requested to register two UDP ports and the RFC editor is requested
   to change all occurrences of port 50500 to the port IANA has
   registered.


6.  Acknowledgements

   The authors would like to thank Tobias Heer, Teemu Koponen, Juhana
   Mattila, Jeffrey M. Ahrenholz, Thomas Henderson, Kristian Slavov,
   Janne Lindqvist, Pekka Nikander and Lauri Silvennoinen for their
   comments on this document.

   [I-D.nikander-hip-path] presented some initial ideas for NAT
   traversal of HIP communication.  This document describes
   significantly different mechanisms that, among other differences, use
   external NAT discovery and do not require encapsulation servers.

   Lars Eggert 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 for InfraHIP research group at Helsinki
   Institute for Information Technology (HIIT).  The InfraHIP project is
   funded by Tekes, Elisa, Nokia, The Finnish Defence Forces and
   Ericsson.


7.  References

7.1.  Normative References

   [I-D.ietf-hip-base]
              Moskowitz, R., "Host Identity Protocol",
              draft-ietf-hip-base-05 (work in progress), March 2006.

   [I-D.ietf-hip-esp]
              Jokela, P., "Using ESP transport format with HIP",
              draft-ietf-hip-esp-02 (work in progress), March 2006.



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   [I-D.ietf-hip-mm]
              Nikander, P., "End-Host Mobility and Multihoming with the
              Host Identity Protocol", draft-ietf-hip-mm-03 (work in
              progress), March 2006.

   [I-D.ietf-hip-rvs]
              Laganier, J. and L. Eggert, "Host Identity Protocol (HIP)
              Rendezvous Extension", draft-ietf-hip-rvs-04 (work in
              progress), October 2005.

   [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-05
              (work in progress), February 2006.

   [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.

   [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.

   [rvs]      Laganier, J. and L. Eggert, "Host Identity Protocol (HIP)
              Rendezvous Extension".

7.2.  Informative References

   [I-D.ietf-behave-nat-udp]
              Audet, F. and C. Jennings, "NAT Behavioral Requirements
              for Unicast UDP", draft-ietf-behave-nat-udp-07 (work in
              progress), June 2006.

   [I-D.irtf-hiprg-nat]
              Stiemerling, M., "NAT and Firewall Traversal Issues of
              Host Identity Protocol (HIP)  Communication",
              draft-irtf-hiprg-nat-02 (work in progress), May 2006.

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




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   [I-D.srisuresh-behave-p2p-state]
              Srisuresh, P., "State of Peer-to-Peer(P2P) Communication
              Across Network Address  Translators(NATs)",
              draft-srisuresh-behave-p2p-state-03 (work in progress),
              June 2006.

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

   [RFC3489]  Rosenberg, J., Weinberger, J., Huitema, C., and R. Mahy,
              "STUN - Simple Traversal of User Datagram Protocol (UDP)
              Through Network Address Translators (NATs)", RFC 3489,
              March 2003.

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


Appendix A.  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                                            |
   +------------+------------------------------------------------------+


Authors' Addresses

   Vivien Schmitt
   NEC Network Laboratories
   Kurfuerstenanlage 36
   Heidelberg  69115
   Germany

   Phone: +49 6221 90511 0
   Fax:   +49 6221 90511 55
   Email: schmitt@netlab.nec.de
   URI:   http://www.netlab.nec.de/






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   Abhinav Pathak
   IIT Kanpur
   B204, Hall - 1, IIT Kanpur
   Kanpur  208016
   India

   Phone: +91 9336 20 1002
   Email: abhinav.pathak@hiit.fi
   URI:   http://www.iitk.ac.in/


   Miika Komu
   Helsinki Institute for Information Technology
   Tammasaarenkatu 3
   Helsinki
   Finland

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


   Lars Eggert
   NEC Network Laboratories
   Kurfuerstenanlage 36
   Heidelberg  69115
   Germany

   Phone: +49 6221 90511 43
   Fax:   +49 6221 90511 55
   Email: lars.eggert@netlab.nec.de
   URI:   http://www.netlab.nec.de/


   Martin Stiemerling
   NEC Network Laboratories
   Kurfuerstenanlage 36
   Heidelberg  69115
   Germany

   Phone: +49 6221 90511 13
   Fax:   +49 6221 90511 55
   Email: stiemerling@netlab.nec.de
   URI:   http://www.netlab.nec.de/






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Full Copyright Statement

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