HIP Working Group                                                M. Komu
Internet-Draft                                                      HIIT
Intended status: Experimental                               T. Henderson
Expires: May 4, 2009                                  The Boeing Company
                                                             P. Matthews
                                                           H. Tschofenig
                                                  Nokia Siemens Networks
                                                         A. Keranen, Ed.
                                            Ericsson Research Nomadiclab
                                                        October 31, 2008

   Basic HIP Extensions for Traversal of Network Address Translators

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
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   This Internet-Draft will expire on May 4, 2009.


   This document specifies extensions to the Host Identity Protocol
   (HIP) to facilitate Network Address Translator (NAT) traversal.  The
   extensions are based on the use of the Interactive Connectivity
   Establishment (ICE) methodology to discover a working path between
   two end-hosts, and on standard techniques for encapsulating

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   Encapsulating Security Payload (ESP) packets within the User Datagram
   Protocol (UDP).  This document also defines elements of procedure for
   NAT traversal, including the optional use of a HIP relay server.
   With these extensions HIP is able to work in environments that have
   NATs and provides a generic NAT traversal solution to higher-layer
   networking applications.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  6
   3.  Overview of Operation  . . . . . . . . . . . . . . . . . . . .  7
   4.  Protocol Description . . . . . . . . . . . . . . . . . . . . .  8
     4.1.  Relay Registration . . . . . . . . . . . . . . . . . . . .  8
     4.2.  ICE Candidate Gathering  . . . . . . . . . . . . . . . . . 10
     4.3.  NAT Traversal Mode Negotiation . . . . . . . . . . . . . . 10
     4.4.  Connectivity Check Pacing Negotiation  . . . . . . . . . . 12
     4.5.  Base Exchange via HIP Relay Server . . . . . . . . . . . . 12
     4.6.  ICE Connectivity Checks  . . . . . . . . . . . . . . . . . 14
     4.7.  NAT Keepalives . . . . . . . . . . . . . . . . . . . . . . 15
     4.8.  Base Exchange without ICE Connectivity Checks  . . . . . . 16
     4.9.  Simultaneous Base Exchange with and without UDP
           Encapsulation  . . . . . . . . . . . . . . . . . . . . . . 16
     4.10. Sending Control Messages after the Base Exchange . . . . . 17
   5.  Packet Formats . . . . . . . . . . . . . . . . . . . . . . . . 17
     5.1.  HIP Control Packets  . . . . . . . . . . . . . . . . . . . 18
     5.2.  Connectivity Checks  . . . . . . . . . . . . . . . . . . . 18
     5.3.  Keepalives . . . . . . . . . . . . . . . . . . . . . . . . 19
     5.4.  NAT Traversal Mode Parameter . . . . . . . . . . . . . . . 20
     5.5.  Connectivity Check Transaction Pacing Parameter  . . . . . 20
     5.6.  Relay and Registration Parameters  . . . . . . . . . . . . 21
     5.7.  LOCATOR Parameter  . . . . . . . . . . . . . . . . . . . . 22
     5.8.  RELAY_HMAC Parameter . . . . . . . . . . . . . . . . . . . 23
     5.9.  Registration Types . . . . . . . . . . . . . . . . . . . . 23
     5.10. ESP Data Packets . . . . . . . . . . . . . . . . . . . . . 24
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 24
     6.1.  Privacy Considerations . . . . . . . . . . . . . . . . . . 24
     6.2.  Opportunistic Mode . . . . . . . . . . . . . . . . . . . . 25
     6.3.  Base Exchange Replay Protection for HIP Relay Server . . . 25
     6.4.  Demuxing Different HIP Associations  . . . . . . . . . . . 25
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 25
   8.  Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 26
   9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 26
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 26
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 26
     10.2. Informative References . . . . . . . . . . . . . . . . . . 27
   Appendix A.  Selecting a Value for Check Pacing  . . . . . . . . . 28
   Appendix B.  IPv4-IPv6 Interoperability  . . . . . . . . . . . . . 29
   Appendix C.  Base Exchange through a Rendezvous Server . . . . . . 29
   Appendix D.  Document Revision History . . . . . . . . . . . . . . 29
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 30
   Intellectual Property and Copyright Statements . . . . . . . . . . 32

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1.  Introduction

   HIP [RFC5201] is defined as a protocol that runs directly over IPv4
   or IPv6, and HIP coordinates the setup of ESP security associations
   [RFC5202] that are also specified to run over IPv4 or IPv6.  This
   approach is known to have problems traversing NATs and other
   middleboxes [RFC5207].  This document defines 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 these types of middleboxes.

   Currently deployed NAT devices do not operate consistently even
   though a recommended behavior is described in [RFC4787].  The HIP
   protocol extensions in this document make as few assumptions as
   possible about the behavior of the NAT devices so that NAT traversal
   will work even with legacy NAT devices.  The purpose of these
   extensions is to allow two HIP-enabled hosts to communicate with each
   other even if one or both of the communicating hosts are in a network
   that is behind one or more NATs.

   Using the extensions defined in this document, HIP end-hosts use
   techniques drawn from the Interactive Connectivity Establishment
   (ICE) methodology [I-D.ietf-mmusic-ice] to find operational paths for
   the HIP control protocol and for ESP encapsulated data traffic.  The
   hosts test connectivity between different locators and try to
   discover a direct end-to-end path between them.  However, with some
   legacy NATs, utilizing the shortest path between two end-hosts
   located behind NATs is not possible without relaying the traffic
   through a relay, such as a TURN server [RFC5128].  Because relaying
   traffic increases the roundtrip delay and consumes resources from the
   relay, with the extensions described in this document, hosts try to
   avoid using the TURN server whenever possible.

   HIP has defined a Rendezvous Server [RFC5204] to allow for mobile HIP
   hosts to establish a stable point-of-contact in the Internet.  This
   document defines extensions to the Rendezvous Server that solve the
   same problems but for both NATed and non-NATed networks.  The
   extended Rendezvous Server, called a "HIP relay server," forwards all
   HIP control packets between an Initiator and Responder, allowing
   Responders to be located behind NATs.  This behavior is in contrast
   to the HIP rendezvous service that forwards only the initial I1
   packet of the base exchange, which is less likely to work in a NATed
   environment [RFC5128].  Therefore, when using relays to traverse
   NATs, HIP uses a HIP relay server for the control traffic and a TURN
   server for the data traffic.

   The basis for the connectivity checks is ICE [I-D.ietf-mmusic-ice].
   [I-D.ietf-mmusic-ice] describes ICE as follows:

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      "The Interactive Connectivity Establishment (ICE) methodology is a
      technique for NAT traversal for UDP-based media streams (though
      ICE can be extended to handle other transport protocols, such as
      TCP) established by the offer/answer model.  ICE is an extension
      to the offer/answer model, and works by including a multiplicity
      of IP addresses and ports in SDP offers and answers, which are
      then tested for connectivity by peer-to-peer connectivity checks.
      The IP addresses and ports included in the SDP and the
      connectivity checks are performed using the revised STUN
      specification [RFC5389], now renamed to Session Traversal
      Utilities for NAT."

   The standard ICE [I-D.ietf-mmusic-ice] is specified with SIP in mind
   and it has some features that are not necessary or suitable as such
   for other protocols.  [I-D.rosenberg-mmusic-ice-nonsip] gives
   instructions and recommendations on how ICE can be used for other
   protocols and this document follows those guidelines.

   Two HIP hosts that implement this specification communicate their
   locators to each other in the HIP base exchange.  The locators are
   then paired with the locators of the other endpoint and prioritized
   according to recommended and local policies.  These locator pairs are
   then tested sequentially by both of the end hosts.  The tests may
   result in multiple operational pairs but ICE procedures determine a
   single preferred address pair to be used for subsequent

   In summary, the extensions in this document define:

   o  UDP encapsulation of HIP packets

   o  UDP encapsulation of IPsec ESP packets

   o  registration extensions for HIP relay services

   o  how the ICE "offer" and "answer" are carried in the base exchange

   o  interaction with ICE connectivity check messages

   o  backwards compatibility issues with rendezvous servers

   o  a number of optimizations (such as when the ICE connectivity tests
      can be omitted)

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2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

   This document borrows terminology from [RFC5201], [RFC5206],
   [RFC4423], [I-D.ietf-mmusic-ice], and [RFC5389].  Additionally, the
   following terms are used:

   Rendezvous server:
      A host that forwards I1 packets to the Responder.

   HIP relay server:
      A host that forwards all HIP control packets between the Initiator
      and the Responder.

   TURN server:
      A server that forwards data traffic between two end-hosts as
      defined in [I-D.ietf-behave-turn].

      As defined in [RFC5206]: "A name that controls how the packet is
      routed through the network and demultiplexed by the end-host.  It
      may include a concatenation of traditional network addresses such
      as an IPv6 address and end-to-end identifiers such as an ESP SPI.
      It may also include transport port numbers or IPv6 Flow Labels as
      demultiplexing context, or it may simply be a network address."
      It should noted that "address" is used in this document as a
      synonym for locator.

   LOCATOR (written in capital letters):
      Denotes a HIP control message parameter that bundles multiple
      locators together.

   ICE offer:
      The Initiator's LOCATOR parameter in a HIP I2 control message.

   ICE answer:
      The Responder's LOCATOR parameter in a HIP R2 control message.

   Transport address:
      Transport layer port and the corresponding IPv4/v6 address.

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

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

   Server reflexive candidate:
      A translated transport address of a host as observed by a HIP
      relay server or a STUN/TURN server.

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

   Relayed candidate:
      A transport address that exists on a TURN server.  Packets that
      arrive at this address are relayed towards the TURN client.

3.  Overview of Operation

                                 | HIP   |
              +--------+         | Relay |         +--------+
              | TURN   |         +-------+         | STUN   |
              | Server |        /         \        | Server |
              +--------+       /           \       +--------+
                              /             \
                             /               \
                            /                 \
                           /  <- Signaling ->  \
                          /                     \
                    +-------+                +-------+
                    |  NAT  |                |  NAT  |
                    +-------+                +-------+
                     /                              \
                    /                                \
               +-------+                           +-------+
               | Init- |                           | Resp- |
               | iator |                           | onder |
               +-------+                           +-------+

                  Figure 1: Example network configuration

   In an example configuration depicted in Figure 1, both Initiator and
   Responder are behind one or more NATs, and both private networks are
   connected to the public Internet.  To be contacted from behind a NAT,
   the Responder must be registered with a HIP relay server reachable on
   the public Internet, and we assume as a starting point that the
   Initiator knows both the Responder's HIT and the address of one of

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   its relay servers (how the Initiator learns of the Responder's relay
   server is outside of the scope of this document, but may be through
   DNS or another name service).

   The first steps are for both the Initiator and Responder to register
   with a relay server (need not be the same one) and gather a set of
   address candidates.  Next, the HIP base exchange is carried out by
   encapsulating the HIP control packets in UDP datagrams and sending
   them through the Responder's relay server.  As part of the base
   exchange, each HIP host learns of the peer's candidate addresses
   through the ICE offer/answer procedure embedded in the base exchange.

   Once the base exchange is completed, HIP has established a working
   communication session (for signaling) via a relay server, but the
   hosts still work to find a better path, preferably without a relay,
   for the ESP data flow.  For this, ICE connectivity checks are carried
   out until a working pair of addresses is discovered.  At the end of
   the procedure, if successful, the hosts will have enabled a UDP-based
   flow that traverses both NATs, with the data flowing directly from
   NAT to NAT or via a TURN server.  Further HIP signaling can be sent
   over the same address/port pair and is demultiplexed from data
   traffic via a marker in the payload.  Finally, NAT keepalives will be
   sent as needed.

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

4.  Protocol Description

   This section describes the normative behavior of the protocol
   extension.  Examples of packet exchanges are provided for
   illustration purposes.

4.1.  Relay Registration

   HIP rendezvous servers operate in non-NATed environments and their
   use is described in [RFC5204].  This section specifies a new
   middlebox extension, called the HIP relay server, for operating in
   NATed environments.  A HIP relay server forwards all HIP control
   packets between the Initiator and the Responder.

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   End-hosts cannot use the HIP relay service for forwarding the ESP
   data plane.  Instead, they use TURN servers [I-D.ietf-behave-turn]
   for that.

   A HIP relay server MUST silently drop packets to a HIP relay client
   that has not previously registered with the HIP relay.  The
   registration process follows the generic registration extensions
   defined in [RFC5203] and is illustrated in Figure 2.

      HIP                                                      HIP
      Relay                                                    Relay
      Client                                                   Server
        |   1. UDP(I1)                                           |
        |                                                        |
        |   2. UDP(R1(REG_INFO(RELAY_UDP_HIP)))                  |
        |                                                        |
        |   3. UDP(I2(REG_REQ(RELAY_UDP_HIP)))                   |
        |                                                        |
        |   4. UDP(R2(REG_RES(RELAY_UDP_HIP), REG_FROM))         |

               Figure 2: Example Registration to a HIP Relay

   In step 1, the relay client (Initiator) starts the registration
   procedure by sending an I1 packet over UDP.  It is RECOMMENDED that
   the Initiator selects a random port number from the ephemeral port
   range 49152-65535 for initiating a base exchange.  However, the
   allocated port MUST be maintained until all of the corresponding HIP
   Associations are closed.  Alternatively, a host MAY also use a single
   fixed port for initiating all outgoing connections.  The HIP relay
   server MUST listen to incoming connections at UDP port HIPPORT.

   In step 2, the HIP relay server (Responder) lists the services that
   it supports in the R1 packet.  The support for HIP-over-UDP relaying
   is denoted by the RELAY_UDP_HIP value.

   In step 3, the Initiator selects the services it registers for and
   lists them in the REG_REQ parameter.  The Initiator registers for HIP
   relay service by listing the RELAY_UDP_HIP value in the request

   In step 4, the Responder concludes the registration procedure with an
   R2 packet and acknowledges the registered services in the REG_RES
   parameter.  The Responder denotes unsuccessful registrations (if any)

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   in the REG_FAILED parameter of R2.  The Responder also includes a
   REG_FROM parameter that contains the transport address of the client
   as observed by the relay (Server Reflexive candidate).  After the
   registration, the client sends NAT keepalives periodically to the
   relay to keep possible NAT bindings between the client and the relay

4.2.  ICE Candidate Gathering

   If a host is going to use ICE, it needs to gather a set of address
   candidates.  The candidate gathering SHOULD be done as defined in
   Section 4.1 of [I-D.ietf-mmusic-ice].  Candidates need to be gathered
   for only one media stream and component.  Component ID 1 should be
   used for ICE processing, where needed.  Initiator takes the role of
   the ICE controlling agent.

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

4.3.  NAT Traversal Mode Negotiation

   This section describes the usage of a new non-critical parameter
   type.  The presence of the parameter in a HIP base exchange means
   that the end-host supports NAT traversal extensions described in this
   document.  As the parameter is non-critical, it can be ignored by an
   end-host which means that the host does not support or is not willing
   to use these extensions.

   The NAT traversal mode parameter applies to a base exchange between
   end-hosts, but currently does not apply to a registration with a HIP
   relay server.  The NAT traversal mode negotiation in base exchange is
   illustrated in Figure 3.

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     Initiator                                                Responder
       | 1. UDP(I1)                                                   |
       |                                                              |
       | 2. UDP(R1(.., NAT_TRAVERSAL_MODE(list of modes), ..))        |
       |                                                              |
       | 3. UDP(I2(.., NAT_TRAVERSAL_MODE(selected mode), LOCATOR..)) |
       |                                                              |
       | 4. UDP(R2(.., LOCATOR, ..))                                  |
       |                   ....                                       |

                Figure 3: Negotiation of NAT Traversal Mode

   In step 1, the Initiator sends an I1 to the Responder.  In step 2,
   the Responder responds with an R1.  The R1 contains a list of NAT
   traversal modes the Responder supports in the NAT_TRAVERSAL_MODE
   parameter as shown in Table 1.

   | Type              | Purpose                                       |
   | RESERVED          | Reserved for future use                       |
   | UDP-ENCAPSULATION | Use only UDP encapsulation of the HIP         |
   |                   | signaling traffic and ESP (no ICE             |
   |                   | connectivity checks)                          |
   | ICE-STUN-UDP      | UDP encapsulated control and data traffic     |
   |                   | with ICE-based connectivity checks using STUN |
   |                   | messages                                      |

                       Table 1: NAT Traversal Modes

   In step 3, the Initiator sends an I2 that includes a
   NAT_TRAVERSAL_MODE parameter.  It contains the mode selected by the
   Initiator from the list of modes offered by the Responder.  The I2
   also includes the locators of the Initiator in a LOCATOR parameter.
   The locator parameter in I2 is the "ICE offer".

   In step 4, the Responder concludes the base exchange with an R2
   packet.  The Responder includes a LOCATOR parameter in the R2 packet.
   The locator parameter in R2 is the "ICE answer".

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4.4.  Connectivity Check Pacing Negotiation

   As explained in [I-D.ietf-mmusic-ice], when a NAT traversal mode with
   connectivity checks is used, new transactions should not be started
   too fast to avoid congestion and overwhelming the NATs.

   For this purpose, during the base exchange, hosts can negotiate a
   transaction pacing value, Ta, using a TRANSACTION_PACING parameter in
   I2 and R2 messages.  The parameter contains the minimum time
   (expressed in milliseconds) the host would wait between two NAT
   traversal transactions, such as starting a new connectivity check or
   retrying a previous check.  If a host does not include this parameter
   in the base exchange, a Ta value of 500ms MUST be used as that host's
   minimum value.  The value that is used by both of the hosts is the
   higher out of the two offered values.

   Hosts SHOULD NOT use values smaller than 20ms for the minimum Ta,
   since such values may not work well with some NATs, as explained in

   The minimum Ta value SHOULD be configurable.  Guidelines for
   selecting a Ta value are given in Appendix A.  Currently this feature
   applies only to the ICE-STUN-UDP NAT traversal mode.

4.5.  Base Exchange via HIP Relay Server

   This section describes how Initiator and Responder perform a base
   exchange through a HIP relay server.  The NAT traversal mode
   negotiation (denoted as NAT_TM in the example) was described in the
   previous section and shall not be repeated here.  If a relay receives
   an R1 or I2 packet without the NAT traversal mode parameter, it drops
   it and sends a NOTIFY error message to the sender of the R1/I2.

   It is RECOMMENDED that the Initiator sends an I1 packet encapsulated
   in UDP when it is destined to an IPv4 address of the Responder.
   Respectively, the Responder MUST respond to such an I1 packet with an
   R1 packet over the transport layer and using the same transport
   protocol.  The rest of the base exchange, I2 and R2, MUST also use
   the same transport protocol.

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

              Figure 4: Base Exchange via a HIP Relay Server

   In step 1 of Figure 4, the Initiator sends an I1 packet over the
   transport layer to the HIT of the Responder (and IP address of the
   relay).  The source address is one of the locators of the Initiator.

   In step 2, the HIP relay server 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 silently.  The relay appends a RELAY_FROM parameter to the I1
   packet which contains the transport source address and port of the I1
   as observed by the relay.  The relay protects the I1 packet with
   RELAY_HMAC as described in [RFC5204], except that the parameter type
   is different (see Section 5.8).  The relay changes the source and
   destination ports and IP addresses 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 forward the packet to the Responder.

   In step 3, the Responder receives the I1 packet.  The Responder
   processes it according to the rules in [RFC5201].  In addition, the
   Responder validates the RELAY_HMAC according to [RFC5204] and
   silently drops the packet if the validation fails.  The Responder
   replies with an R1 packet to which it includes a RELAY_TO parameter.
   The RELAY_TO parameter MUST contain same information as the
   RELAY_FROM parameter, i.e., the Initiator's transport address, but
   the type of the parameter is different.  The RELAY_TO parameter is
   not integrity protected by the signature of the R1 to allow pre-

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   created R1 packets at the Responder.

   In step 4, the relay receives the R1 packet.  The relay drops the
   packet silently if the source HIT belongs to an unregistered host.
   The relay MAY verify the signature of the R1 packet and drop it if
   the signature is invalid.  Otherwise, the relay rewrites the source
   address and port, and changes the destination address and port to
   match RELAY_TO information.  Finally, the relay recalculates
   transport checksum and forwards the packet.

   In step 5, the Initiator receives the R1 packet and processes it
   according to [RFC5201].  It replies with an I2 packet that uses the
   destination transport address of R1 as the source address and port.
   The I2 contains a LOCATOR parameter that lists all the ICE candidates
   (ICE offer) of the Initiator.  The candidates are encoded using the
   format defined in Section 5.7.  The I2 packet MUST also contain the
   NAT traversal mode parameter with ICE-STUN-UDP or some other selected

   In step 6, the relay receives the I2 packet.  The relay appends a
   RELAY_FROM and a RELAY_HMAC to the I2 packet as explained in step 2.

   In step 7, the Responder receives the I2 packet and processes it
   according to [RFC5201].  It replies with an R2 packet and includes a
   RELAY_TO parameter as explained in step 3.  The R2 packet includes a
   LOCATOR parameter that lists all the ICE candidates (ICE answer) of
   the Responder.  The RELAY_TO parameter is protected by the HMAC.

   In step 8, the relay processes the R2 as described in step 4.  The
   relay forwards the packet to the Initiator.

   Hosts MAY include the address of their HIP relay server in the
   LOCATOR parameter in I2/R2.  The traffic type of this address MUST be
   "HIP signaling" and it MUST NOT be used as an ICE candidate.  This
   address MAY be used for HIP signaling also after the base exchange.
   If the HIP relay server locator is not included in I2/R2 LOCATOR
   parameters, it SHOULD NOT be used after the base exchange, but the
   HIP signaling SHOULD use the same path as the data traffic.

4.6.  ICE Connectivity Checks

   If a HIP relay server was used, the Responder completes the base
   exchange with the R2 packet through the relay.  When the Initiator
   successfully receives and processes the R2, both hosts have
   transitioned to ESTABLISHED state.  However, the destination address
   the Initiator and Responder used for delivering base exchange packets
   belonged to the HIP relay server.  Therefore, the address of the
   relay MUST NOT be used for sending ESP traffic.  Instead, if a NAT

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   traversal mode with ICE connectivity checks was selected, the
   Initiator and Responder MUST start the connectivity checks.

   Creating the check list for the ICE connectivity checks should be
   performed as described in Section 5.7 of [I-D.ietf-mmusic-ice]
   bearing in mind that only one media stream and component is needed
   (so there will be only a single checklist and all candidates should
   have the same component ID value).  The actual connectivity checks
   MUST be performed as described in Section 7 of [I-D.ietf-mmusic-ice].
   Regular mode SHOULD be used for the candidate nomination.
   Section 5.2 defines the details of the STUN control packets.  As a
   result of the ICE connectivity checks, ICE nominates a single
   transport address pair to be used if an operational address pair was
   found.  The end-hosts MUST use this address pair for the ESP traffic.

   The connectivity check messages MUST be paced by the value negotiated
   during the base exchange as described in Section 4.4.  If neither one
   of the hosts announced a minimum pacing value, value of 500ms MUST be

   For retransmissions, the RTO value should be calculated as follows:

      RTO = MAX (500ms, Ta * P)

   In the RTO formula, Ta is the value used for the connectivity check
   pacing and P is the number of pairs in the checklist when the
   connectivity checks begin.  This is identical to the formula in
   [I-D.ietf-mmusic-ice] if there is only one checklist.

4.7.  NAT Keepalives

   To prevent NAT states from expiring, communicating hosts send
   periodically keepalives to each other.  HIP relay servers MAY refrain
   from sending keepalives if it's known that they are not behind a
   middlebox that requires keepalives.  An end-host MUST send keepalives
   every 15 seconds to refresh the UDP port mapping at the NAT(s) when
   the control or data channel is idle.  To implement failure tolerance,
   an end-host SHOULD have shorter keepalive period.

   The keepalives are STUN Binding Indications if the hosts have agreed
   on ICE-STUN-UDP NAT traversal mode during the base exchange.
   Otherwise, HIP NOTIFY messages MAY be used.  A HIP relay server MUST
   NOT forward the NOTIFY messages.

   The communicating hosts MUST send keepalives to each other using the
   transport locators they agreed to use for data and signaling when
   they are in ESTABLISHED state.  Also, the Initiator MUST send a
   NOTIFY message to the relay to keep the NAT states alive on the path

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   between the Initiator and relay when the Initiator has not received
   any response to its I1 or I2 from the Responder in 15 seconds.  The
   relay MUST NOT forward the NOTIFY messages.

4.8.  Base Exchange without ICE Connectivity Checks

   In certain network environments, the ICE connectivity checks can be
   omitted to reduce initial connection set up latency because base
   exchange acts as an implicit connectivity test itself.  There are
   three assumptions about such as environments.  First, the Responder
   should have a long-term, fixed locator in the network.  Second, the
   Responder should not have a HIP relay server configured for itself.
   Third, the Initiator can reach the Responder by simply UDP
   encapsulating HIP and ESP packets to the host.  Detecting and
   configuring this particular scenario is prone to administrative
   failure unless carefully planned.

   In such a scenario, the Responder MAY include only the UDP-
   ENCAPSULATION NAT traversal mode in the R1 message.  Likewise, if the
   Initiator knows that it can receive ESP and HIP signaling traffic by
   using simply UDP encapsulation, it can choose the UDP-ENCAPSULATION
   mode in the I2 message, if the Responder listed it in the supported
   modes.  In both of these cases the locators from I2 and R2 packets
   will be used also for the UDP encapsulated ESP.

   When no ICE connectivity checks are used, locator exchange and return
   routability tests for mobility and multihoming are done as specified
   in [RFC5206] with the exception that UDP encapsulation is used.

4.9.  Simultaneous Base Exchange with and without UDP Encapsulation

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

   The I1 packet without UDP encapsulation may arrive directly at the
   Responder.  When the recipient is the Responder, the procedures in
   [RFC5201] are followed for the rest of the base exchange.  The
   Initiator may receive multiple R1 messages, with and without UDP
   encapsulation, from the Responder.  However, after receiving a valid
   R1 and answering to it with an I2, further R1 messages that are not
   retransmits of the original R1 MUST be ignored.

   The I1 packet without UDP encapsulation may also arrive at a HIP-

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   capable middlebox.  When the middlebox is a HIP rendezvous server and
   the Responder has successfully registered to the rendezvous service,
   the middlebox follows rendezvous procedures in [RFC5204].

   If the Initiator receives a NAT traversal mode parameter in R1
   without UDP encapsulation, the Initiator MAY ignore this parameter
   and send an I2 without UDP encapsulation and without any selected NAT
   traversal mode.  When the Responder receives the I2 without UDP
   encapsulation and without NAT traversal mode, it will assume that no
   NAT traversal mechanism is needed.  The packet processing will be
   done as described in [RFC5201].  The Initiator MAY store the NAT
   traversal modes for future use e.g., to be used in case of mobility
   or multihoming event which causes NAT traversal to be taken in to use
   during the lifetime of the HIP association.

4.10.  Sending Control Messages after the Base Exchange

   After the base exchange, the end-hosts MAY send HIP control messages
   directly to each other using the transport address pair established
   for data channel without sending the control packets through the HIP
   relay server.  When a host does not get acknowledgments, e.g., to an
   UPDATE or CLOSE message after a timeout based on local policies, the
   host SHOULD resend the packet through the relay, if it was listed in
   the LOCATOR parameter in the base exchange.

   If control messages are sent through a HIP relay server, the sender
   MUST include a RELAY_TO parameter to them.  Also the HIP relay server
   MUST add a RELAY_FROM parameter to the control messages it relays.

5.  Packet Formats

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

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5.1.  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            |
     |                       32 bits of zeroes                       |
     |                                                               |
     ~                    HIP Header and Parameters                  ~
     |                                                               |

         Figure 5: Format of UDP-encapsulated HIP Control Packets

   HIP control packets are encapsulated in UDP packets as defined in
   Section 2.2 of [RFC3948], "rules for encapsulating IKE messages",
   except a different port number is used.  Figure 5 illustrates the
   encapsulation.  The UDP header is followed by 32 zero bits that can
   be used to differentiate HIP control packets from ESP packets.  The
   HIP header and parameters follow the conventions of [RFC5201] 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
   [RFC5201] includes the IP addresses in the checksum calculation.  The
   NATs unaware of HIP cannot recompute the HIP checksum after changing
   IP addresses.

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

5.2.  Connectivity Checks

   The connectivity checks are performed using STUN Binding Requests as
   defined in [I-D.ietf-mmusic-ice].  This section describes the details
   of the parameters in the STUN messages.

   The Binding Requests MUST use STUN short term credentials with HITs
   of the Initiator and Responder as the username fragments.  The
   username is formed from the username fragments as defined in Section of [I-D.ietf-mmusic-ice] with the Initiator being the
   "offerer" and the Responder being the "answerer".  The HITs are used
   as usernames by expressing them in IPv6 hexadecimal ASCII format
   [RFC1884], using lowercase letters, each 16 bit HIT fragment
   separated by a one byte colon (hex 0x3a).  The leading zeroes MUST

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   NOT be omitted so that the username's size is fixed.

   The STUN password is drawn from the DH keying material.  Drawing of
   HIP keys is defined in [RFC5201] Section 6.5 and drawing of ESP keys
   in [RFC5202] Section 7.  Correspondingly, the hosts MUST draw
   symmetric keys for STUN according to [RFC5201] Section 6.5.  The
   hosts draw the STUN key after HIP keys, or after ESP keys if ESP
   transform was successfully negotiated in the base exchange.  Both
   hosts draw a 128 bit key from the DH keying material, express that in
   hexadecimal ASCII format using only lowercase letters (resulting in
   32 numbers or lowercase letters), and use that as both the local and
   peer password.  [RFC5389] describes how hosts use the password for
   message integrity of STUN messages.

   Both the username and password are expressed in ASCII hexadecimal
   format to prevent the need to run them through SASLPrep as defined in

   The connectivity checks MUST contain PRIORITY attribute.  They MAY
   contain USE-CANDIDATE attribute as defined in Section of

   The Initiator is always in the controller role during a base
   exchange.  Hence, the ICE-CONTROLLED and ICE-CONTROLLING attributes
   are not needed and SHOULD NOT be used.  When two hosts are initiating
   a connection to each other simultaneously, HIP state machine detects
   it and assigns the host with the larger HIT as the Responder as
   explained in Sections 4.4.2 and 6.7 in [RFC5201].

5.3.  Keepalives

   The keepalives for HIP associations that are created with ICE are
   STUN Binding Indications, as defined in [RFC5389].  In contrast to
   the UDP encapsulated HIP header, the non-ESP-marker between the UDP
   header and the STUN header is excluded.  Keepalives MUST contain the
   FINGERPRINT STUN attribute but SHOULD NOT contain any other STUN
   attributes and SHOULD NOT utilize any authentication mechanism.  STUN
   messages are demultiplexed from ESP and HIP control messages using
   the STUN markers, such as the magic cookie value and the FINGERPRINT

   Keepalives for HIP associations created without ICE are HIP control
   messages that have NOTIFY as the packet type.  The NOTIFY messages do
   not contain any parameters.

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5.4.  NAT Traversal Mode Parameter

   Format of the NAT_TRAVERSAL_MODE parameter is similar to the format
   of the ESP_TRANSFORM parameter in [RFC5202] and is shown in the
   Figure 6.  This specification defines traversal mode identifiers UDP-
   ENCAPSULATION and ICE-STUN-UDP.  The identifier RESERVED is reserved
   for future use.  Future specifications may define more traversal

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |             Type              |             Length            |
     |           Reserved            |            Mode ID #1         |
     |           Mode ID #2          |            Mode ID #3         |
     |           Mode ID #n          |             Padding           |

     Type       [ TBD by IANA: 608 ]
     Length     length in octets, excluding Type, Length, and padding
     Reserved   zero when sent, ignored when received
     Mode ID    defines the NAT traversal mode to be used

     The following NAT traversal mode IDs are defined:

         ID                 Value
         RESERVED             0
         ICE-STUN-UDP         2

           Figure 6: Format of the NAT_TRAVERSAL_MODE parameter

   The sender of a NAT_TRAVERSAL_MODE parameter MUST make sure that
   there are no more than six (6) Mode IDs in one NAT_TRAVERSAL_MODE
   parameter.  The limited number of Mode IDs sets the maximum size of
   the NAT_TRAVERSAL_MODE parameter.

5.5.  Connectivity Check Transaction Pacing Parameter

   The TRANSACTION_PACING parameter shown in Figure 7 contains only the
   connectivity check pacing value, expressed in milliseconds, as 32 bit
   unsigned integer.

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      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |             Type              |             Length            |
     |                            Min Ta                             |

     Type     [ TBD by IANA: 610 ]
     Length   length in octets, excluding Type and Length
     Min Ta   the minimum connectivity check transaction pacing
              value the host would use

           Figure 7: Format of the TRANSACTION_PACING parameter

5.6.  Relay and Registration Parameters

   Format of the REG_FROM, RELAY_FROM and RELAY_TO parameters is shown
   in Figure 8.  All parameters are identical except for the type.
   REG_FROM is the only parameter covered with the signature.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |             Type              |             Length            |
     |             Port              |    Protocol   |     Reserved  |
     |                                                               |
     |                            Address                            |
     |                                                               |
     |                                                               |

     Type       [ TBD by IANA:
                REG_FROM:   950
                RELAY_FROM: 63998 (2^16 - 2^11 + 2^9 - 2)
                RELAY_TO:   64002 (2^16 - 2^11 + 2^9 + 2) ]
     Length     20
     Port       transport port number; zero when plain IP is used
     Protocol   IANA assigned, Internet Protocol number.
                17 for UDP, 0 for plain IP.
     Reserved   reserved for future use; zero when sent, ignored
                when received
     Address    an IPv6 address or an IPv4 address in "IPv4-Mapped
                IPv6 address" format

   Figure 8: Format of the REG_FROM, RELAY_FROM and  RELAY_TO parameters

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   REG_FROM contains the transport address and protocol where the HIP
   relay server sees the registration coming from.  RELAY_FROM contains
   the address where the relayed packet was received from by the relay
   server and the protocol that was used.  The RELAY_TO contains same
   information about the address where a packet should be forwarded to.

5.7.  LOCATOR Parameter

   The generic LOCATOR parameter format is the same as in [RFC5206].
   However, presenting ICE candidates requires a new locator type.  The
   generic and NAT traversal specific locator parameters are illustrated
   in Figure 9.

      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 9: LOCATOR parameter

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

   | Field     | Value(s) | Purpose                                    |
   | Type      | 193      | Parameter type                             |
   | Length    | Variable | Length in octets, excluding Type and       |
   |           |          | Length fields and padding                  |
   | Traffic   | 0-2      | Is the locator for HIP signaling (1), for  |
   | Type      |          | ESP (2), or for both (0)                   |
   | Locator   | 2        | "Transport address" locator type           |
   | Type      |          |                                            |
   | Locator   | 7        | Length of the fields after Locator         |
   | Length    |          | Lifetime in 4-octet units                  |
   | Reserved  | 0        | Reserved for future extensions             |
   | Preferred | 0        | Not used for transport address locators;   |
   | (P) bit   |          | MUST be ignored by the receiver.           |
   | Locator   | Variable | Locator lifetime in seconds                |
   | Lifetime  |          |                                            |
   | Transport | Variable | Transport layer port number                |
   | Port      |          |                                            |
   | Transport | Variable | IANA Assigned, transport layer Internet    |
   | Protocol  |          | Protocol number. Currently only UDP (17)   |
   |           |          | is supported.                              |
   | Kind      | Variable | 0 for host, 1 for server reflexive, 2 for  |
   |           |          | peer reflexive or 3 for relayed address    |
   | Priority  | Variable | Locator's priority as described in         |
   |           |          | [I-D.ietf-mmusic-ice]                      |
   | SPI       | Variable | SPI value which the host expects to see in |
   |           |          | incoming ESP packets that use this locator |
   | Locator   | Variable | IPv6 address or an "IPv4-Mapped IPv6       |
   |           |          | address" format IPv4 address [RFC3513]     |

                 Table 2: Fields of the LOCATOR parameter

5.8.  RELAY_HMAC Parameter

   The RELAY_HMAC parameter value has the TLV type 65520 (2^16 - 2^5 +
   2^4).  It has the same semantics as RVS_HMAC [RFC5204].

5.9.  Registration Types

   The REG_INFO, REG_REQ, REG_RESP and REG_FAILED parameters contain
   values for HIP relay server registration.  The value for
   RELAY_UDP_HIP is 2.

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5.10.  ESP Data Packets

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

   During the HIP base exchange, the two peers exchange parameters that
   enable them to define a pair of IPsec ESP security associations (SAs)
   as described in [RFC5202].  When two peers perform a UDP-encapsulated
   base exchange, they MUST define a pair of IPsec SAs that produces
   UDP-encapsulated ESP data traffic.

   The management of encryption/authentication protocols and SPIs is
   defined in [RFC5202].  The UDP encapsulation format and processing of
   HIP ESP traffic is described in Section 6.1 of [RFC5202].

6.  Security Considerations

6.1.  Privacy Considerations

   The locators are in plain text format in favor of inspection at HIP-
   aware middleboxes in the future.  The current draft does not specify
   encrypted versions of LOCATORs even though it could be beneficial for
   privacy reasons.

   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 host addresses.  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 [RFC5128].  With such a legacy NAT, the only option left
   would be to use a relayed transport address from a TURN server.

   As a consequence, a host may support locator-based privacy by leaving
   out the reflexive candidates.  However, the trade-off in using only
   host candidates can produce suboptimal paths that can congest the
   TURN server.

   The use of HIP relay servers or TURN relays can be also useful for
   protection against Denial-of-Service attacks.  If a Responder reveals
   only its HIP relay server addresses and Relayed candidates to

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   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.

6.2.  Opportunistic Mode

   A HIP relay server should have one address per relay client when a
   HIP relay is serving more than one relay clients and supports
   opportunistic mode.  Otherwise, it cannot be guaranteed that the HIP
   relay server can deliver the I1 packet to the intended recipient.

6.3.  Base Exchange Replay Protection for HIP Relay Server

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

   The relay can protect itself against replay attacks by involving in
   the base exchange by introducing nonces that the end-hosts (Initiator
   and Responder) have to sign.  One way to do this is to add
   ECHO_REQUEST_M parameters to the R1 and I2 messages as described in
   [I-D.heer-hip-middle-auth] and drop the I2 or R2 messages if the
   corresponding ECHO_RESPONSE_M parameters are not present.

6.4.  Demuxing Different HIP Associations

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

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

7.  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

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   change all occurrences of port HIPPORT to the port IANA has
   registered.  The HIPPORT number 50500 should be used for initial

   This document updates the IANA Registry for HIP Parameter Types by
   assigning new HIP Parameter Type values for the new HIP Parameters:
   RELAY_FROM, RELAY_TO and REG_FROM (defined in Section 5.6),
   RELAY_HMAC (defined in Section 5.8), TRANSACTION_PACING (defined in
   Section 5.5), and NAT_TRAVERSAL_MODE (defined in Section 5.4).

8.  Contributors

   This draft is a product of a design team which also included Marcelo
   Bagnulo and Jan Melen who both have made major contributions to this

9.  Acknowledgments

   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 Andrei Gurtov, Simon Schuetz, Martin Stiemerling, Lars Eggert,
   Vivien Schmitt, Abhinav Pathak for their contributions and Tobias
   Heer, Teemu Koponen, Juhana Mattila, Jeffrey M. Ahrenholz, Kristian
   Slavov, Janne Lindqvist, Pekka Nikander, Lauri Silvennoinen, Jukka
   Ylitalo, Juha Heinanen, Joakim Koskela, Samu Varjonen, Dan Wing and
   Jani Hautakorpi for their comments on this document.

   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
   Forces, and Ericsson and Birdstep.

10.  References

10.1.  Normative References

              Rosenberg, J., Mahy, R., and P. Matthews, "Traversal Using
              Relays around NAT (TURN): Relay Extensions to Session
              Traversal Utilities for NAT (STUN)",
              draft-ietf-behave-turn-11 (work in progress),
              October 2008.

              Rosenberg, J., "Interactive Connectivity Establishment

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              (ICE): A Protocol for Network Address  Translator (NAT)
              Traversal for Offer/Answer Protocols",
              draft-ietf-mmusic-ice-19 (work in progress), October 2007.

   [RFC1884]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 1884, December 1995.

   [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.

   [RFC3513]  Hinden, R. and S. Deering, "Internet Protocol Version 6
              (IPv6) Addressing Architecture", RFC 3513, April 2003.

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

   [RFC5201]  Moskowitz, R., Nikander, P., Jokela, P., and T. Henderson,
              "Host Identity Protocol", RFC 5201, April 2008.

   [RFC5202]  Jokela, P., Moskowitz, R., and P. Nikander, "Using the
              Encapsulating Security Payload (ESP) Transport Format with
              the Host Identity Protocol (HIP)", RFC 5202, April 2008.

   [RFC5203]  Laganier, J., Koponen, T., and L. Eggert, "Host Identity
              Protocol (HIP) Registration Extension", RFC 5203,
              April 2008.

   [RFC5204]  Laganier, J. and L. Eggert, "Host Identity Protocol (HIP)
              Rendezvous Extension", RFC 5204, April 2008.

   [RFC5206]  Nikander, P., Henderson, T., Vogt, C., and J. Arkko, "End-
              Host Mobility and Multihoming with the Host Identity
              Protocol", RFC 5206, April 2008.

   [RFC5389]  Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
              "Session Traversal Utilities for NAT (STUN)", RFC 5389,
              October 2008.

10.2.  Informative References

              Heer, T., Wehrle, K., and M. Komu, "End-Host
              Authentication for HIP Middleboxes",
              draft-heer-hip-middle-auth-01 (work in progress),

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              July 2008.

              Rosenberg, J., "Guidelines for Usage of Interactive
              Connectivity Establishment (ICE) by non  Session
              Initiation Protocol (SIP) Protocols",
              draft-rosenberg-mmusic-ice-nonsip-01 (work in progress),
              July 2008.

   [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.

   [RFC5128]  Srisuresh, P., Ford, B., and D. Kegel, "State of Peer-to-
              Peer (P2P) Communication across Network Address
              Translators (NATs)", RFC 5128, March 2008.

   [RFC5207]  Stiemerling, M., Quittek, J., and L. Eggert, "NAT and
              Firewall Traversal Issues of Host Identity Protocol (HIP)
              Communication", RFC 5207, April 2008.

Appendix A.  Selecting a Value for Check Pacing

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

   The Ta value may be configured by the user in environments where the
   network characteristics are known beforehand.  However, if the
   characteristics are not know, it is recommended that the value is
   adjusted dynamically.  In this case it's recommended that the hosts
   estimate the RTT between them and set the minimum Ta value so that
   only two connectivity check messages are sent on every RTT.

   One way to estimate the RTT is to use the time it takes for the HIP
   relay server registration exchange to complete; this would give an
   estimate on the registering host's access link's RTT.  Also the I1/R1
   exchange could be used for estimating the RTT, but since the R1 can
   be cached in the network, or the relaying service can increase the
   delay notably, it is not recommended.

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Appendix B.  IPv4-IPv6 Interoperability

   Currently relay client and server do not have to run any ICE
   connectivity tests as described in Section 4.8.  However, it could be
   useful for IPv4-IPv6 interoperability when the HIP relay server
   actually includes both the NAT traversal mode parameter and multiple
   locators in R2.  The interoperability benefit is that the relay could
   support IPv4-based Initiators and IPv6-based Responders by converting
   the network headers and recalculating UDP checksums.

   Such an approach is underspecified in this document currently.  It is
   not yet recommended because it may consume resources at the relay and
   requires also similar conversion support at the TURN relay for data

Appendix C.  Base Exchange through a Rendezvous Server

   When the Initiator looks up the information of the Responder from
   DNS, it's possible that it discovers an RVS record [RFC5204].  In
   this case, if the Initiator uses NAT traversal methods described in
   this document, it uses its own HIP relay server to forward HIP
   traffic to the Rendezvous server.  The Initiator will send the I1
   message using its HIP relay server which will then forward it to the
   RVS server of the Responder.  In this case, the value of the protocol
   field in the RELAY_TO parameter MUST be IP since RVS does not support
   UDP encapsulated base exchange packets.  The Responder will send the
   R1 packet directly to the Initiator's HIP relay server and the
   following I2 and R2 packets are also sent directly using the relay.

   In case the Initiator is not able to distinguish which records are
   RVS address records and which are Responder's address records (e.g.,
   if the DNS server did not support HIP extensions), the Initiator
   SHOULD first try to contact the Responder directly, without using a
   HIP relay server.  If none of the addresses is reachable, it MAY try
   out them using its own HIP relay server as described above.

Appendix D.  Document Revision History

   To be removed upon publication

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   | Revision                    | Comments                            |
   | draft-ietf-nat-traversal-00 | Initial version.                    |
   | draft-ietf-nat-traversal-01 | Draft based on RVS.                 |
   | draft-ietf-nat-traversal-02 | Draft based on Relay proxies and    |
   |                             | ICE concepts.                       |
   | draft-ietf-nat-traversal-03 | Draft based on STUN/ICE formats.    |
   | draft-ietf-nat-traversal-04 | Issues 25-27,29-36                  |
   | draft-ietf-nat-traversal-05 | Issues 28,40-43,47,49,51            |

Authors' Addresses

   Miika Komu
   Helsinki Institute for Information Technology
   Metsanneidonkuja 4

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

   Thomas Henderson
   The Boeing Company
   P.O. Box 3707
   Seattle, WA

   Email: thomas.r.henderson@boeing.com

   Philip Matthews

   Email: philip_matthews@magma.ca

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   Hannes Tschofenig
   Nokia Siemens Networks
   Linnoitustie 6
   Espoo  02600

   Phone: +358 (50) 4871445
   Email: Hannes.Tschofenig@gmx.net
   URI:   http://www.tschofenig.com

   Ari Keranen (editor)
   Ericsson Research Nomadiclab
   Hirsalantie 11
   02420 Jorvas

   Phone: +358 9 2991
   Email: ari.keranen@ericsson.com

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