HIP Working Group                                                M. Komu
Internet-Draft                                                      HIIT
Intended status: Experimental                               T. Henderson
Expires: January 16, 2009                             The Boeing Company
                                                             P. Matthews
                                                           H. Tschofenig
                                                  Nokia Siemens Networks
                                                        A. Keraenen, Ed.
                                            Ericsson Research Nomadiclab
                                                           July 15, 2008

   Basic HIP Extensions for Traversal of Network Address Translators

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
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   This Internet-Draft will expire on January 16, 2009.


   The Host Identity Protocol (HIP) provides a new namespace that can be
   used for uniquely identifying hosts.  Existing HIP experimental
   specifications do not specify protocol operations across Network
   Address Translators (NATs).

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   This document specifies basic NAT traversal extensions for HIP.  The
   HIP shim layer is located between the network and transport layer,
   the extensions can also provide a more general-purpose NAT traversal
   support for higher-layer networking applications.  The extensions are
   based on the use of the The Interactive Connectivity Establishment
   (ICE) methodology to discover a working path between two end-hosts.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Protocol Description . . . . . . . . . . . . . . . . . . . . .  6
     3.1.  Relay Registration . . . . . . . . . . . . . . . . . . . .  6
     3.2.  NAT Transformation Negotiation . . . . . . . . . . . . . .  8
     3.3.  Base Exchange via HIP Relay  . . . . . . . . . . . . . . .  9
     3.4.  ICE Connectivity Checks  . . . . . . . . . . . . . . . . . 11
     3.5.  NAT Keepalives . . . . . . . . . . . . . . . . . . . . . . 12
     3.6.  Base Exchange without ICE Connectivity Checks  . . . . . . 12
     3.7.  Base Exchange without UDP Encapsulation  . . . . . . . . . 13
     3.8.  Sending Control Messages using the Data Plane  . . . . . . 13
   4.  Packet Formats . . . . . . . . . . . . . . . . . . . . . . . . 13
     4.1.  HIP Control Packets  . . . . . . . . . . . . . . . . . . . 14
     4.2.  Connectivity Checks  . . . . . . . . . . . . . . . . . . . 14
     4.3.  Keepalives . . . . . . . . . . . . . . . . . . . . . . . . 15
     4.4.  Relay and Registration Parameters  . . . . . . . . . . . . 16
     4.5.  LOCATOR Parameter  . . . . . . . . . . . . . . . . . . . . 17
     4.6.  RELAY_HMAC . . . . . . . . . . . . . . . . . . . . . . . . 19
     4.7.  Registration Types . . . . . . . . . . . . . . . . . . . . 19
     4.8.  ESP Data Packets . . . . . . . . . . . . . . . . . . . . . 20
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 20
     5.1.  Privacy Considerations . . . . . . . . . . . . . . . . . . 20
     5.2.  Opportunistic Mode . . . . . . . . . . . . . . . . . . . . 21
     5.3.  Base Exchange Replay Protection for HIP Relay Server . . . 21
     5.4.  Demuxing Different HIP Associations  . . . . . . . . . . . 21
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 21
   7.  Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 22
   8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 22
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 22
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 24
   Appendix A.  IPv4-IPv6 Interoperability  . . . . . . . . . . . . . 24
   Appendix B.  Base Exchange through a Rendezvous Server . . . . . . 24
   Appendix C.  Document Revision History . . . . . . . . . . . . . . 25
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 25
   Intellectual Property and Copyright Statements . . . . . . . . . . 27

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1.  Introduction

   HIP [RFC5201] is defined as a protocol that runs directly over IPv4
   or IPv6.  This approach is known to have problems traversing NATs.  A
   detailed description of HIP problems with traversing legacy
   middleboxes is documented in [I-D.irtf-hiprg-nat].  This document
   describes HIP extensions for the traversal of both Network Address
   Translator (NAT) and Network Address and Port Translator (NAPT)
   middleboxes.  The document generally uses the term NAT to refer to
   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 communicating hosts are in private address

   Using the extensions defined in this draft, HIP end-hosts use the HIP
   control channel to communicate their current locators to each other
   to find a operational path for the ESP encapsulated data traffic.
   The hosts test connectivity between different locators and try to
   discover direct end-to-end path between the end-hosts.  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].  As a consequence,
   the TURN server increases the roundtrip delay and may become a point
   of network congestion.  With the extensions described in this
   document, hosts try to avoid the use the TURN server when possible.

   This document defines new middlebox extensions to allow NAT traversal
   for HIP control plane.  The HIP Relay extensions define mechanisms to
   forward HIP control plane.  A distinction must be made here between a
   HIP rendezvous services [RFC5204]) and HIP Relay services.  HIP
   rendezvous servers solve initial contact and mobility related
   problems in networks without NATs.  HIP Relay servers solve the same
   problems, in addition to NAT traversal problems.  HIP Relay servers
   can be used both in NATed and non-NATed networks.

   Both rendezvous and relay services forward HIP control packets, but
   the main difference is that the rendezvous service forwards only the
   initial I1 packet of the base exchange while all other HIP control
   packets are sent directly between the communicating hosts.  In
   contrast, the relay service relays all HIP control packets because
   NATs can drop the packets otherwise [RFC5128].

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

   [I-D.ietf-mmusic-ice] describes ICE as follows:

      "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 [I-D.ietf-behave-rfc3489bis], now renamed to Session
      Traversal Utilities for NAT."

   ICE for SIP is specified in [I-D.ietf-mmusic-ice] and ICE for non-SIP
   protocols is specified in [I-D.rosenberg-mmusic-ice-nonsip].

   Two hosts communicate their locators to each other in the HIP base
   exchange.  They are then paired with the locally operational address
   of the other endpoint and prioritized according local and recommended
   policies.  These address sets are then tested sequentially based on
   the procedures specified in ICE.  Both sides participate in the
   connectivity checks.  The tests may also discover multiple
   operational address pairs but determine a single preferred address
   pair to be used for subsequent communication.

   In a nutshell, the extensions in this document defines:

   o  encapsulatation of HIP packets in UDP

   o  UDP encapsulation of IPsec ESP packets

   o  registration extensions for HIP Relay services

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

   o  interaction with ICE connectivity messages

   o  backwards compatibility issues with rendezvous servers

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

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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 [I-D.ietf-behave-rfc3489bis].
   Additionally, the following terms are used:

    Rendezvous server:

      A host that forwards I1 packets to the Responder

   HIP Relay:

      A host that forwards all HIP control packets between an Initiator
      and Responder

   TURN server:

      A server that forwards data traffic between two end-hosts


      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 noticed that "address" is used in this document as a
      synonym for locator.

   HIP Relay:

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

   ICE Offer:

      The Initiator's LOCATOR parameter in HIP I2 control message.

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   ICE Answer:

      The Responder's LOCATOR parameter in HIP R2 control message

   Transport address:

      Transport layer port and the corresponding IPv4/v6 address


      A transport address that has not been verified yet for
      reachability using ICE

   Host candidate:

      An IPv4 or IPv6 address of a network interface of a host

   Server reflexive transport candidate:

      A translated transport address of a host as observed by a HIP
      Relay or a STUN server

   Peer reflexive transport candidate:

      A translated transport address of a host as observed by its peer

   Relayed transport candidate:

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

3.  Protocol Description

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

3.1.  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, to perate in NATted

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   environments.  HIP Relay servers forward all HIP control packets
   between the Initiator and the Responder over UDP.

   End-hosts cannot use the HIP Relay service for forward ESP data
   plane.  Instead, they use TURN servers [I-D.ietf-behave-turn] for
   relaying the ESP traffic.  A HIP end-host SHOULD register to a TURN
   server before registering to a HIP Relay to guarantee that the host
   can accept ESP traffic immediately after HIP Relay registration.

   A HIP Relay 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 1.

      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 1: 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.

   In step 2, the 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.  The R1 SHOULD not contain any
   NAT transform parameter.

   In step 3, the Initiator selects the services it registers for and
   lists them in the REG_REQ parameter.  In this example, the Initiator
   registers for HIP Relay service.

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   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 in the
   REG_FAILED parameter in 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 Initiator sends periodically NAT keepalives.

   There are different ways for an Initiator to learn it's publicly
   visible IP address and port that are referred to as the "server
   reflexive transport candidate" in this document.  It is a local
   decision on how the end-host learnds the candidate, but either of the
   following methods is RECOMMENDED:

   o  The Relay client may use STUN servers to detect the server
      reflexive locator, as described in [RFC5128].

   o  Alternatively, the Relay Client can learn it from the REG_FROM
      parameter when registering to a Relay.

3.2.  NAT Transformation Negotiation

   This section describes the usage of a new non-critical transform
   parameter type.  The presence of the parameter in HIP base exchange
   means that the end-host supports ICE connectivity checks.  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 ICE
   connectivity checks.

   The NAT transform parameter applies to a base exchange between end-
   hosts, but currently does not apply to with a registration with a HIP
   Relay server.  The NAT transform applies only to a base exchange with
   transport layer encapsulation and MUST not be included without
   transport layer encapsulation.  The NAT transform negotiation in base
   exchange is illustrated in Figure 2.

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     Initiator                                                Responder
       | 1. UDP(I1)                                                   |
       |                                                              |
       | 2. UDP(R1(.., NAT_TRANSFORM(list of transforms), ..))        |
       |                                                              |
       | 3. UDP(I2(.., NAT_TRANSFORM(selected transform), LOCATOR..)) |
       |                                                              |
       | 4. UDP(R2(.., LOCATOR, ..))                                  |
       |                   ....                                       |

                  Figure 2: Negotiation of NAT Transforms

   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
   transforms the Responder supports in NAT_TRANSFORM parameter as shown
   in Table 1.

   | Transform    | Purpose                                            |
   | Type         |                                                    |
   | RESERVED     | Reserved for future use                            |
   | ICE-STUN-UDP | UDP encapsulated control and data traffic with     |
   |              | ICE-based connectivity checks using STUN messages  |

                     Table 1: Locator Transformations

   In step 3, the Initiator sends an I2 that includes a NAT_TRANSFORM
   parameter.  It contains the transform type selected by the Initiator
   from the list of transforms 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 locators parameter in R2 is the "ICE answer".

3.3.  Base Exchange via HIP Relay

   This section describes how Initiator and Responder establish a base
   exchange through a HIP Relay.  The NAT transform negotiation (denoted
   as NAT_TFM in the example) was described in previous section and
   shall not be repeated here.  When a Relay receives an R1 or I2 packet

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   without the NAT transform packet, it drops it and sends a NOTIFY
   error message to the originator.

   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 layer.

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

                  Figure 3: Base Exchange via a HIP Relay

   In step 1 of Figure 3, the Initiator sends an I1 packet over the
   transport layer to the HIT of the Responder.  The source address is
   one of the locators of the host.  The locators belonging to the end-
   hosts are referred as "host candidates" in this document.

   In step 2, the HIP Relay receives the I1 packet at port HIPPORT.  If
   the destination HIT belongs to a registered Responder, the Relay
   processes the packet.  Otherwise, the Relay MUST drop the packet
   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.  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.

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   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 and the
   nonce, but the type of the parameter is different.  The RELAY_TO
   parameter is not integrity protected by the signature of the R1 to
   allow pre-created R1 packets at the Responder.

   In step 4, the Relay receives the R1 packet.  The Relay 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 4.5.  The I2 packet MUST also contain the
   NAT transform parameter with ICE-STUN-UDP or some other transform
   selected because otherwise the Relay may drop the I2 packet.

   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 the
   second step.

   In step 7, the Responder receives the I2 packet and processes it
   according to [RFC5201].  It replies with a R2 packet and includes a
   RELAY_TO parameter as explained in step three.  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

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

3.4.  ICE Connectivity Checks

   The Responder completes the base exchange with the R2 packet through
   the Relay.  When 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

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   base exchange packets belonged to the Relay as indicated by the
   RELAY_FROM and RELAY_TO parameters.  Therefore, the address of the
   Relay MUST not be used for sending ESP traffic unless it was listed
   as a TURN server in the offer/answer.  Instead, the Initiator and
   Responder MUST start ICE connectivity tests after they have
   transitioned to ESTABLISHED state after the base exchange when they
   do not have valid locator pair for ESP traffic and the NAT transform
   parameter was negotiated successfully.

   The ICE connectivity checks are defined in [I-D.ietf-mmusic-ice].
   Section Section 4.2 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 could be
   found.  The end-hosts MUST use this address pair for the ESP traffic.

3.5.  NAT Keepalives

   To prevent NAT state from expiring, communicating end-hosts hosts
   send periodically keepalives to each other.  NAT Relays MUST not send
   any 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 NAT_TRANSFORM during the base exchange, or HIP NOTIFY messages
   otherwise.  A HIP Relay MUST not forward NOTIFY messages.

   The communicating hosts MUST send keepalives to each other using the
   transport locators exchanged in the base exchange when they are in
   ESTABLISHED state.  Also, the Initiator MUST send a NOTIFY message to
   the Relay to refresh the NAT state alive on the path 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.

3.6.  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 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 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.

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   In such a scenario, the Initiator sends an I1 packet over UDP to the
   Responder.  The Responder replies with a R1 packet that does not
   contain the NAT transform parameter.  The Initiator receives the R1
   packet and determines from the absence of the NAT transform and
   RELAY_TO parameters that ICE connectivity checks can be omitted.
   Finally, the hosts can start to use the locators from the concluding
   I2 and R2 packets of the base exchange for ESP without ICE
   connectivity checks.

3.7.  Base Exchange without UDP Encapsulation

   The Initiator MAY also try to establish a base exchange with the
   Responder without UDP encapsulation.  In such a case, the Initiator
   sends first an I1 packet without UDP encapsulation to the Responder.
   After 100 ms, the Initiator MUST then send an UDP-encapsulated I1
   packet.  For retransmissions, the procedure is repeated.

   The I1 packet may arrive directly at the Responder.  When the
   recipient is the Responder, the proceduces in [RFC5201] are followed
   for the rest of the base exchange.  The Initiator may receive
   multiple R1 messages from the Responder, but upon receiving a valid
   R1 without UDP encapsulation, the Initiator MUST ignore further R1
   messages with UDP encapsulation encapsulation.  The end-hosts do not
   trigger ICE connectivity checks after the base exchange since the UDP
   encapsulation was excluded.

   The packet may also arrive at a HIP-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 middlebox is a
   HIP Relay server, it drops the I1 packet silently.

3.8.  Sending Control Messages using the Data Plane

   The end-hosts MAY send control messages directly to each other using
   the transport address pair established for data channel without
   sending the control packets through the Relay.  When a host does not
   get acknowledgements e.g. to an UPDATE or CLOSE message after a
   timeout based on local policies, the host SHOULD resend the packet
   through the Relay.  This optimization requires further

4.  Packet Formats

   The following subsections define the parameter and packet encodings.
   All values MUST be in network byte order.

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4.1.  HIP Control Packets

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |        Source Port            |      Destination Port         |
     |           Length              |           Checksum            |
     |                       32 bits of zeroes                       |
     |                                                               |
     ~                    HIP Header and Parameters                  ~
     |                                                               |

         Figure 4: Format for 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 for a different port number.  Figure 4 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 or a Responder without a relay MUST listen at transport
   port HIPPORT for incoming UDP-encapsulated HIP control packets.

4.2.  Connectivity Checks

   The connectivity checks are performed using STUN Binding Requests.
   This section describes the details of the parameters in the STUN

   The username is formed from the username fragments as defined in
   section of [I-D.ietf-mmusic-ice].  The requests MUST use STUN
   short term credentials with HITs of the Initiator and Responder
   concatenated as a username fragment.  The HITs are concatenated
   according to the Sort(HIT-I, HIT-R) algorithm defined in [RFC5201]
   section 6.5.  The HIT username fragment MUST contain a UTF-8
   [RFC3629] encoded sequence and MUST have been processed using
   SASLPrep [RFC4013] as defined section 15.3 of

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   [I-D.ietf-behave-rfc3489bis].  The concatenated HIT pair MUST have a
   fixed size that is accomplished by including the leading zeroes for
   the HITs.

   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 keys after HIP keys, or after ESP keys if ESP
   transform was successfully negotiated in the base exchange.  The
   hosts draw two keys which they MUST use to generate the STUN
   password.  As the STUN password is the same at both ends, the two
   drawn keys MUST be concatenated with the key for the greater HIT
   first.  Section 15.4 of [I-D.ietf-behave-rfc3489bis] describes how
   hosts use the password for message integrity of STUN messages.

   The connectivity checks MUST contain PRIORITY attribute.  They MAY
   contain USE-CANDIDATE attributes 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
   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 and 6.7 in [RFC5201].

4.3.  Keepalives

   The keepalives for HIP associations agreed that are NAT_TRANSFORM
   capable are STUN Binding Indications, as defined in
   [I-D.ietf-behave-rfc3489bis].  The source and destination ports are
   in the UDP header are the same as used for HIP (50500).  However, in
   contrast to the UDP encapsulated HIP header, there 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 attribute.

   Keepalives for aHIP associations that are not NAT_TRANSFORM capable
   are HIP control messages that have NOTIFY as the packet type.  The
   NOTIFY messages do not contain any parameters.

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4.4.  Relay and Registration Parameters

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

     Type        [ TBD by IANA:
                   REG_FROM: (64010 = 2^16 - 2^11 + 2^9 + 10) ]

     Length      20
     Address     An IPv6 address or an IPv4 address in "IPv4-compatible
                 IPv6 address" format
     Port        Transport port number; zero when plain IP is used
     Transport   Transport protocol type; zero for UDP

                  Figure 5: Format for REG_FROM parameter

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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            |
     |                                                               |
     |                            Address                            |
     |                                                               |
     |                                                               |
     |             Port              |           Transport           |
     |                             Nonce                             |

     Type        [ TBD by IANA:
                   Critical parameters:
                   RELAY_FROM: (63998 = 2^16 - 2^11 + 2^9 - 2)
                   RELAY_TO:   (64002 = 2^16 - 2^11 + 2^9 + 2)

     Length      24
     Address     An IPv6 address or an IPv4 address in "IPv4-compatible
                 IPv6 address" format
     Port        Transport port number; zero when plain IP is used
     Transport   Transport protocol type; zero for UDP
     Nonce       A nonce assigned by the Relay server.

        Figure 6: Format for the RELAY_FROM and RELAY_TO parameters

   Format for the REG_FROM parameter is shown in Figure 5, and
   RELAY_FROM and RELAY_TO in Figure 6.  Parameters are identical except
   for the type and nonce fields.

   The nonce field is an experimental field for the RELAY_FROM and
   RELAY_TO parameters.  It allows the Relay to have constant state
   towards the Initiators without allowing the Responder to send R1 or
   R2 packets to arbitrary hosts through the Relay.

4.5.  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 7.

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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             |
     | 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 7: LOCATOR parameter

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

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   | 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 Locator field in 4-octet     |
   | Length    |          | 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 | 0        | 0 for UDP                                  |
   | Protocol  |          |                                            |
   | 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-compatible IPv6   |
   |           |          | address" format IPv4 address [RFC3513],    |
   |           |          | obfuscated by XORing it with the owner's   |
   |           |          | HIT                                        |

                 Table 2: Fields of the LOCATOR 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].

4.7.  Registration Types

   The REG_INFO, REQ_REQ, REG_RESP and REG_FAILED parameters contain
   values for HIP Relay registration.  The value for RELAY_UDP_HIP is 2.
   The value for RELAY_UDP_ESP is 3.

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4.8.  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].

5.  Security Considerations

5.1.  Privacy Considerations

   The locators are in XORed format in plain text 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 Relays or TURN Relays can be useful for
   protection against Denial-of-Service attacks.  If a Responder reveals
   only its HIP Relay addresses and Relayed transport candidates to
   Initiators, the Initiators can only attack the relays that does not
   prevent the Responder from initiating new outgoing connections if a

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   path around the relay exists.

5.2.  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
   Relay can deliver the I1 packet to the intended recipient.

5.3.  Base Exchange Replay Protection for HIP Relay Server

   On certain scenarios, it is possible that an attacker, or two
   attackers, can replay an earlier base exchange through a 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, Responder has to be disconnected from
   the Relay in order to masquarade successfully as the Responder.

   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.  The Relay MAY add ECHO_REQUEST_M
   parameters to the R1 and I2 messages as described in
   [I-D.heer-hip-middle-auth] and drops the I2 or R2 messages if the
   corresponding ECHO_RESPONSE_M parameters are not present.

5.4.  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 use HITs to distinguish
   between different Initiators.

6.  IANA Considerations

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

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

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   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 4.4) and
   RELAY_HMAC (defined in Section 4.6).  NAT_TRANSFORM is also a new

7.  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

8.  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, Thomas
   Henderson, 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

   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.

9.  References

9.1.  Normative References

              Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
              "Session Traversal Utilities for (NAT) (STUN)",
              draft-ietf-behave-rfc3489bis-16 (work in progress),
              July 2008.

              Rosenberg, J., Mahy, R., and P. Matthews, "Traversal Using
              Relays around NAT (TURN): Relay Extensions to Session
              Traversal Utilities for NAT (STUN)",

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              draft-ietf-behave-turn-08 (work in progress), June 2008.

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

              Rosenberg, J., "NICE: Non Session Initiation Protocol
              (SIP) usage of Interactive  Connectivity Establishment
              (ICE)", draft-rosenberg-mmusic-ice-nonsip-00 (work in
              progress), February 2008.

   [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.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, November 2003.

   [RFC4013]  Zeilenga, K., "SASLprep: Stringprep Profile for User Names
              and Passwords", RFC 4013, February 2005.

   [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-

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              Host Mobility and Multihoming with the Host Identity
              Protocol", RFC 5206, April 2008.

9.2.  Informative References

              Heer, T., Wehrle, K., and M. Komu, "End-Host
              Authentication for HIP Middleboxes",
              draft-heer-hip-middle-auth-01 (work in progress),
              July 2008.

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

   [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.

Appendix A.  IPv4-IPv6 Interoperability

   Currently Relay Client and Server do not have to run any ICE
   connectivity tests as described in Section 3.6.  However, it could be
   useful for IPv4-IPv6 interoperability when the Relay Server actually
   includes both the NAT transform 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 B.  Base Exchange through a Rendezvous Server

   This section describes handling for a scenario where Initiator looks

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   up the information of the Responder from DNS and discovers a RVS
   record [RFC5204].  In such a case, the Initiator uses its own HIP
   Relay to forward HIP traffic to the Rendezvous server.  The Initiator
   will send the I1 message using the its HIP Relay server which will
   then forward it to the RVS server of the responder.  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

   In case the Initiator is not able to distinguish which records are
   RVS address records and which are Responders address records, then
   the Initiator SHOULD first try to contact the Responder directly and
   if none of the addresses is reachable it MAY try out them using its
   own HIP Relay as described in the above.

Appendix C.  Document Revision History

   To be removed upon publication

   | 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                  |

Authors' Addresses

   Miika Komu
   Helsinki Institute for Information Technology
   Metsanneidonkuja 4

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

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

   Email: thomas.r.henderson@boeing.com

   Philip Matthews

   Email: philip_matthews@magma.ca

   Hannes Tschofenig
   Nokia Siemens Networks
   Linnoitustie 6
   Espoo  02600

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

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

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

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Full Copyright Statement

   Copyright (C) The IETF Trust (2008).

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