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SIPPING Working Group                                    C. Boulton, Ed.
Internet-Draft                             Ubiquity Software Corporation
Expires: December 28, 2006                                  J. Rosenberg
                                                           Cisco Systems
                                                            G. Camarillo
                                                                Ericsson
                                                           June 26, 2006


            Best Current Practices for NAT Traversal for SIP
                  draft-ietf-sipping-nat-scenarios-05

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Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   Traversal of the Session Initiation Protocol (SIP) and the sessions
   it establishes through Network Address Translators (NAT) is a complex
   problem.  Currently there are many deployment scenarios and traversal
   mechanisms for media traffic.  This document aims to provide concrete
   recommendations and a unified method for NAT traversal as well as



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   documenting corresponding call flows.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Problem Statement  . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Solution Technology Outline Description  . . . . . . . . . . .  6
     3.1.  SIP Signaling  . . . . . . . . . . . . . . . . . . . . . .  7
       3.1.1.  Symmetric Response . . . . . . . . . . . . . . . . . .  7
       3.1.2.  Connection Re-use  . . . . . . . . . . . . . . . . . .  8
     3.2.  Media Traversal  . . . . . . . . . . . . . . . . . . . . .  8
       3.2.1.  Symmetric RTP  . . . . . . . . . . . . . . . . . . . .  8
       3.2.2.  STUN . . . . . . . . . . . . . . . . . . . . . . . . .  9
       3.2.3.  TURN . . . . . . . . . . . . . . . . . . . . . . . . . 10
       3.2.4.  ICE  . . . . . . . . . . . . . . . . . . . . . . . . . 10
       3.2.5.  Solution Profiles  . . . . . . . . . . . . . . . . . . 10
   4.  NAT Traversal Scenarios  . . . . . . . . . . . . . . . . . . . 11
     4.1.  Basic NAT SIP Signaling Traversal  . . . . . . . . . . . . 11
       4.1.1.  Registration (Registrar/Proxy Co-Located)  . . . . . . 11
       4.1.2.  Registration(Registrar/Proxy not Co-Located) . . . . . 15
       4.1.3.  Initiating a Session . . . . . . . . . . . . . . . . . 18
       4.1.4.  Receiving an Invitation to a Session . . . . . . . . . 20
     4.2.  Basic NAT Media Traversal  . . . . . . . . . . . . . . . . 23
       4.2.1.  Endpoint independent NAT . . . . . . . . . . . . . . . 24
       4.2.2.  Port and Address Dependant NAT . . . . . . . . . . . . 40
     4.3.  Address independent Port Restricted NAT --> Address
           independent Port Restricted NAT traversal  . . . . . . . . 46
     4.4.  Internal TURN Usage (Enterprise Deployment)  . . . . . . . 46
   5.  Intercepting Intermediary (B2BUA)  . . . . . . . . . . . . . . 46
   6.  IPv4-IPv6 Transition . . . . . . . . . . . . . . . . . . . . . 47
     6.1.  IPv4-IPv6 Transition for SIP Signalling  . . . . . . . . . 47
     6.2.  IPv4-IPv6 Transition for Media . . . . . . . . . . . . . . 47
   7.  ICE with RTP/TCP . . . . . . . . . . . . . . . . . . . . . . . 50
   8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 50
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 50
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 50
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 52
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 53
   Intellectual Property and Copyright Statements . . . . . . . . . . 54











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1.  Introduction

   NAT (Network Address Translators) traversal has long been identified
   as a large problem when considered in the context of the Session
   Initiation Protocol (SIP)[1] and it's associated media such as Real
   Time Protocol (RTP)[2].  The problem is further confused by the
   variety of NATs that are available in the market place today and the
   large number of potential deployment scenarios.  Detail of different
   NAT behaviors can be found in 'NAT Behavioral Requirements for
   Unicast UDP' [11].

   The IETF has produced many specifications for the traversal of NAT,
   including STUN, ICE, rport, symmetric RTP, TURN, SIP Outbound, SDP
   attribute for RTCP, and others.  These each represent a part of the
   solution, but none of them gives the overall context for how the NAT
   traversal problem is decomposed and solved through this collection of
   specifications.  This document serves to meet that need.

   This document attempts to provide a definitive set of 'Best Common
   Practices' to demonstrate the traversal of SIP and its associated
   media through NAT devices.  The document does not propose any new
   functionality but does draw on existing solutions for both core SIP
   signaling and media traversal (as defined in Section 3).

   The draft will be split into distinct sections as follows:
   1.  A clear definition of the problem statement
   2.  Description of proposed solutions for both SIP protocol signaling
       and media signaling
   3.  A set of basic and advanced call flow scenarios


2.  Problem Statement

   The traversal of SIP through NAT can be split into two categories
   that both require attention - The core SIP signaling and associated
   media traversal.

   The core SIP signaling has a number of issues when traversing through
   NATs.

   Firstly, the default operation for SIP response generation using
   unreliable protocols such as the Unicast Datagram Protocol (UDP)
   results in responses generated at the User Agent Server (UAS) being
   sent to the source address, as specified in either the SIP 'Via'
   header or the 'received' parameter (as defined in RFC 3261 [1]).  The
   port is extracted from the SIP 'Via' header to complete the IP
   address/port combination for returning the SIP response.  While the
   destination is correct, the port contained in the SIP 'Via' header



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   represents the listening port of the originating client and not the
   port representing the open pin hole on the NAT.  This results in
   responses being sent back to the NAT but to a port that is likely not
   open for SIP traffic.  The SIP response will then be dropped at the
   NAT.  This is illustrated in Figure 1 which depicts a SIP response
   being returned to port 5060.


     Private                       NAT                         Public
     Network                        |                          Network
                                    |
                                    |
     --------     SIP Request       |open port 5650            --------
    |        |-------------------->--->-----------------------|        |
    |        |                      |                         |        |
    | Client |                      |port 5060   SIP Response | Proxy  |
    |        |                      x<------------------------|        |
    |        |                      |                         |        |
     --------                       |                          --------
                                    |
                                    |
                                    |


   Figure 1

   Secondly, when using a reliable, connection orientated transport
   protocol such as TCP, SIP has an inherent mechanism that results in
   SIP responses reusing the connection that was created/used for the
   corresponding transactional request.  The SIP protocol does not
   provide a mechanism that allows new requests generated in the reverse
   direction of the originating client to use the existing TCP
   connection created between the client and the server during
   registration.  This results in the registered contact address not
   being bound to the "connection" in the case of TCP.  Requests are
   then blocked at the NAT, as illustrated in Figure 2.  This problem
   also exists for unreliable transport protocols such as UDP where
   external NAT mappings need to be re-used to reach a SIP entity on the
   private side of the network.












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     Private                       NAT                         Public
     Network                        |                          Network
                                    |
                                    |
     -------- (UAC 8023)    REGISTER/Response       (UAS 5060) --------
    |        |-------------------->---<-----------------------|        |
    |        |                      |                         |        |
    | Client |                      |5060  INVITE   (UAC 8015)| Proxy  |
    |        |                      x<------------------------|        |
    |        |                      |                         |        |
     --------                       |                          --------
                                    |
                                    |
                                    |

   Figure 2

   In Figure 2 the original REGISTER request is sent from the client on
   port 8023 and received on port 5060, establishing a reliable
   connection and opening a pin-hole in the NAT.  The generation of a
   new request from the proxy results in a request destined for the
   registered entity (Contact IP address) which is not reachable from
   the public network.  This results in the new SIP request attempting
   to create a connection to a private network address.  This problem
   would be solved if the original connection was re-used.  While this
   problem has been discussed in the context of connection orientated
   protocols such as TCP, the problem exists for SIP signaling using any
   transport protocol.  The solution proposed for this problem in
   section 3 of this document is relevant for all SIP signaling,
   regardless of the transport protocol.

   NAT policy can dictate that connections should be closed after a
   period of inactivity.  This period of inactivity can range
   drastically from a number seconds to hours.  Pure SIP signaling can
   not be relied upon to keep alive connections for a number of reasons.
   Firstly, SIP entities can sometimes have no signaling traffic for
   long periods of time which has the potential to exceed the inactivity
   timer, and this can lead to problems where endpoints are not
   available to receive incoming requests as the connection has been
   closed.  Secondly, if a low inactivity timer is specified, SIP
   signaling is not appropriate as a keep-alive mechanism as it has the
   potential to add a large amount of traffic to the network which uses
   up valuable resource and also requires processing at a SIP stack,
   which is also a waste of processing resources.

   Media associated with SIP calls also has problems traversing NAT.
   RTP [2]] is one of the most common media transport types used in SIP
   signaling.  Negotiation of RTP occurs with a SIP session



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   establishment using the Session Description Protocol(SDP) [3] and a
   SIP offer/answer exchange[5].  During a SIP offer/answer exchange an
   IP address and port combination are specified by each client in a
   session as a means of receiving media such as RTP.  The problem
   arises when a client advertises its address to receive media and it
   exists in a private network that is not accessible from outside the
   NAT.  Figure 3 illustrates this problem.


                 NAT             Public Network           NAT
                  |                                        |
                  |                                        |
                  |                                        |
     --------     |            SIP Signaling Session       |   --------
    |        |----------------------->---<--------------------|        |
    |        |    |                                        |  |        |
    | Client |    |                                        |  | Client |
    |   A    |>=====>RTP>==Unknown Address==>X             |  |   B    |
    |        |    |             X<==Unknown Address==<RTP<===<|        |
     --------     |                                        |   --------
                  |                                        |
                  |                                        |
                  |                                        |


   Figure 3

   The connection address representing both clients are not available on
   the public internet and traffic can be sent from both clients through
   their NATs.  The problem occurs when the traffic reaches the public
   internet and is not resolvable.  The media traffic fails.  The
   connection address extracted from the SDP payload is that of an
   internal address, and so not resolvable from the public side of the
   NAT.  To complicate the problem further, a number of different NAT
   topologies with different default behaviors increase the difficulty
   of proposing a single solution.


3.  Solution Technology Outline Description

   When analyzing issues associated with traversal of SIP through
   existing NAT, it has been identified that the problem can be split
   into two clear solution areas as defined in section 2 of this
   document.  The traversal of the core protocol signaling and the
   traversal of the associated media as specified in the Session
   Description Payload (SDP) of a SIP offer/answer exchange[5].  The
   following sub-sections outline solutions that enable core SIP
   signaling and its associated media to traverse NATs.



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3.1.  SIP Signaling

   SIP signaling has two areas that result in transactional failure when
   traversing through NAT, as described in section 2 of this document.
   The remaining sub-sections describe appropriate solutions that result
   in SIP signalling traversal through NAT, regardless of transport
   protocol.  IT is RECOMMEDED that SIP compliant entities follow the
   guidelines presented in this section to enable traversal of SIP
   signaling through NATs.

3.1.1.  Symmetric Response

   As described in section 2 of this document, when using an unreliable
   transport protocol such as UDP, SIP responses are sent to the IP
   address and port combination contained in the SIP 'Via' header field
   (or default port for the appropriate transport protocol if not
   present).  This can result in responses being blocked at a NAT.  In
   such circumstances, SIP signaling requires a mechanism that will
   allow entities to override the basic response generation mechanism in
   RFC 3261 [1].  Once the SIP response is constructed, the destination
   is still derived using the mechanisms described in RFC 3261 [1].  The
   port (to which the response will be sent), however, will not equal
   that specified in the SIP 'Via' header field but will be the port
   from which the original request was sent.  This results in the pin-
   hole opened for the requests traversal of the NAT being reused, in a
   similar manner to that of reliable connection orientated transport
   protocols such as TCP.  Figure 4 illustrates the response traversal
   through the open pin hole using this method.


     Private                        NAT                       Public
     Network                         |                        Network
                                     |
                                     |
     --------                        |                        --------
    |        |                       |                       |        |
    |        |send/receive           |           send/receive|        |
    | Client |port 5060-----<<->>---------<<->>-----port 5060| Client |
    |   A    |                       |                       |   B    |
    |        |                       |                       |        |
     --------                        |                        --------
                                     |
                                     |
                                     |

   Figure 4

   The exact functionality for this method of response traversal is



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   called 'Symmetric Response' and the details are documented in RFC
   3581 [6].  Additional requirements are imposed on SIP entities in
   this specification such as listening and sending SIP requests/
   responses from the same port.

3.1.2.  Connection Re-use

   The second problem with sip signaling, as defined in Section 2 and
   illustrated in Figure 2, is to allow incoming requests to be properly
   routed.

   Guidelines for devices such as User Agents that can only generate
   outbound connections through a NAT are documented in 'SIP Conventions
   for UAs with Outbound Only Connections'[13].  The document provides
   techniques that use a unique User Agent instance identifier
   (instance-id) in association with a flow identifier (Reg-id).  The
   combination of the two identifiers provides a key to a particular
   connections (both UDP and TCP) that are stored in association with
   registration bindings.  On receiving an incoming request to a SIP
   Address-Of-Record (AOR), a proxy routes to the associated flow
   created by the registration and thus a route through a NAT.  It also
   provides a keepalive mechanism for clients to keep NAT bindings
   alive.  This is achieved using peer-to-peer STUN multiplexed over the
   SIP signaling connection.  Usage of this specification is
   RECOMMENDED.  This mechanism is not transport specific and should be
   used for any transport protocol.

   Even if the SIP Outbound draft is not used, clients generating SIP
   requests SHOULD use the same IP address and port (i.e., socket) for
   both transmission and receipt of SIP messages.  Doing so allows for
   the vast majority of industry provided solutions to properly
   function.

3.2.  Media Traversal

   This document has already provided guidelines that recommend using
   extensions to the core SIP protocol to enable traversal of NATs.
   While ultimately not desirable, the additions are relatively straight
   forward and provide a simple, universal solution for varying types of
   NAT deployment.  The issues of media traversal through NATs is not
   straight forward and requires the combination of a number of
   traversal methodologies.  The technologies outlined in the remainder
   of this section provide the required solution set.

3.2.1.  Symmetric RTP

   The primary problem identified in section 2 of this document is that
   internal IP address/port combinations can not be reached from the



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   public side of a NAT.  In the case of media such as RTP, this will
   result in no audio traversing a NAT(as illustrated in Figure 3).  To
   overcome this problem, a technique called 'Symmetric' RTP can be
   used.  This involves an SIP endpoint both sending and receiving RTP
   traffic from the same IP Address/Port combination.  This technique
   also requires intelligence by a client on the public internet as it
   identifies that incoming media for a particular session does not
   match the information that was conveyed in the SDP.  In this case the
   client will ignore the SDP address/port combination and return RTP to
   the IP address/port combination identified as the source of the
   incoming media.  This technique is known as 'Symmetric RTP' and is
   documented in [15].  'Symmetric RTP' SHOULD only be used for
   traversal of RTP through NAT when one of the participants in a media
   session definitively knows that it is on the public network.

3.2.1.1.  RTCP Attribute

   Normal practice when selecting a port for defining Real Time Control
   Protocol(RTCP) [2] is for consecutive order numbering (i.e select an
   incremented port for RTCP from that used for RTP).  This assumption
   causes RTCP traffic to break when traversing many NATs due to blocked
   ports.  To combat this problem a specific address and port need to be
   specified in the SDP rather than relying on such assumptions.  RFC
   3605 [6] defines an SDP attribute that is included to explicitly
   specify transport connection information for RTCP.  The address
   details can be obtained using any appropriate method including those
   detailed previously in this section (e.g.  STUN, TURN).

3.2.2.  STUN

   Simple Traversal of User Datagram Protocol (UDP) through Network
   Address Translators(NAT) or STUN is defined in RFC 3489bis [10].
   STUN is a lightweight tool kit and protocol that provides details of
   the external IP address/port combination used by the NAT device to
   represent the internal entity on the public facing side of a NAT.  On
   learning of such an external representation, a client can use it
   accordingly as the connection address in SDP to provide NAT
   traversal.  Using terminology defined in the draft 'NAT Behavioral
   Requirements for Unicast UDP' [11], STUN does work with 'Endpoint
   Independent Mapping' but does not work with either 'Address Dependent
   Mapping' or 'Address Dependent and Port Mapping' type NATs.  Using
   STUN with either of the previous two NAT mappings to probe for the
   external IP address/port representation will provide a different
   result to that required for traversal by an alternative SIP entity.
   The IP address/port combination deduced for the STUN server would be
   blocked for incoming packets from an alterative SIP entity.

   As mentioned in Section 3.1.2, STUN is also used as a peer-to-peer



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   keep-alive mechanism.

3.2.3.  TURN

   As described in the previous section, the STUN protocol does not work
   for UDP traversal through certain identified NAT mappings.
   'Obtaining Relay Addresses from Simple Traversal of UDP Through NAT
   (known as TURN)' is a usage of the STUN protocol for deriving (from a
   STUN server) an address that will be used to relay packet towards a
   client.  TURN provides an external address (globally routable) at a
   STUN server that will act as a media relay which guarantees traffic
   will reach the associated internal address.  The full details of the
   TURN specification are defined in [12].  A TURN service will almost
   always provide media traffic to a SIP entity but it is RECOMMENDED
   that this method only be used as a last resort and not as a general
   mechanism for NAT traversal.  This is because using TURN has high
   performance costs when relaying media traffic and can lead to
   unwanted latency.

3.2.4.  ICE

   Interactive Connectivity Establishment (ICE) is the RECOMMENDED
   method for traversal of existing NAT if Symmetric RTP is not
   appropriate.  ICE is a methodology for using existing technologies
   such as STUN, TURN and any other UNSAF[9] compliant protocol to
   provide a unified solution.  This is achieved by obtaining as many
   representative IP address/port combinations as possible using
   technologies such as STUN/TURN etc.  Once the addresses are
   accumulated, they are all included in the SDP exchange in a new media
   attribute called 'candidate'.  Each 'candidate' SDP attribute entry
   has detailed connection information including a media addresses
   (including optional RTCP information), priority, username, password
   and a unique session ID.  The appropriate IP address/port
   combinations are used in the correct order depending on the specified
   priority.  A client compliant to the ICE specification will then
   locally run instances of STUN servers on all addresses being
   advertised using ICE.  Each instance will undertake connectivity
   checks to ensure that a client can successfully receive media on the
   advertised address.  Only connections that pass the relevant
   connectivity checks are used for media exchange.  The full details of
   the ICE methodology are contained in [16].

3.2.5.  Solution Profiles

   This draft has documented a number of technology solutions for the
   traversal of media through differing NAT deployments.  A number of
   'profiles' will now be defined that categorize varying levels of
   support for the technologies described.



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3.2.5.1.  Primary Profile

   A client falling into the 'Primary' profile supports ICE in
   conjunction with STUN, TURN and RFC 3605 [6] for RTCP.  ICE is used
   in all cases and falls back to standard operation when dealing with
   non-ICE clients.  A client which falls into the 'Primary' profile
   will be maximally interoperable and function in a rich variety of
   environments including enterprise, consumer and behind all varieties
   of NAT.

3.2.5.2.  Consumer Profile

   A client falling into the 'Consumer' profile supports STUN and RFC
   3605 [6] for RTCP.  It uses STUN to allocate bindings, and can also
   detect when it is in the unfortunate situation of being behind a
   'Symmetric' NAT, although it simply cannot function in this case.
   These clients will only work in deployment situations where the
   access is sufficiently controlled to know definitively that there
   won't be Symmetric NAT.  This is hard to guarantee as users can
   always pick up their client and connect via a different access
   network.

3.2.5.3.  Minimal Profile

   A client falling into the 'Minimal' profile will send/receive RTP
   form the same IP/port combination.  This client requires proprietary
   network based solutions to function in any NAT traversal scenario.

   All clients SHOULD support the 'Primary Profile', MUST support the
   'Minimal Profile' and MAY support the 'Consumer Profile'.


4.  NAT Traversal Scenarios

   This section of the document includes detailed NAT traversal
   scenarios for both SIP signaling and the associated media.

4.1.  Basic NAT SIP Signaling Traversal

   The following sub-sections concentrate on SIP signaling traversal of
   NAT.  The scenarios include traversal for both reliable and un-
   reliable transport protocols.

4.1.1.  Registration (Registrar/Proxy Co-Located)

   The set of scenarios in this section document basic signaling
   traversal of a SIP REGISTER method through a NAT.




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4.1.1.1.  UDP



           Client              NAT               Proxy
             |                  |                  |
             |(1) REGISTER      |                  |
             |----------------->|                  |
             |                  |                  |
             |                  |(1) REGISTER      |
             |                  |----------------->|
             |                  |                  |
             |                  |(2) 401 Unauth    |
             |                  |<-----------------|
             |                  |                  |
             |(2) 401 Unauth    |                  |
             |<-----------------|                  |
             |                  |                  |
             |(3) REGISTER      |                  |
             |----------------->|                  |
             |                  |                  |
             |                  |(3) REGISTER      |
             |                  |----------------->|
             |                  |                  |
             |*************************************|
             |    Create Connection Re-use Tuple   |
             |*************************************|
             |                  |                  |
             |                  |(4) 200 OK        |
             |                  |<-----------------|
             |                  |                  |
             |(4) 200 OK        |                  |
             |<-----------------|                  |
             |                  |                  |


   Figure 5

   In this example the client sends a SIP REGISTER request through a NAT
   which is challenged using the Digest authentication scheme.  The
   client will include an 'rport' parameter as described in section
   3.1.1 of this document for allowing traversal of UDP responses.  The
   original request as illustrated in (1) in Figure 5 is a standard
   REGISTER message:







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    REGISTER sip:proxy.example.com SIP/2.0
    Via: SIP/2.0/UDP client.example.com:5060;rport;branch=z9hG4bK
    Max-Forwards: 70
    Supported: path,gruu
    From: Client <sip:client@example.com>;tag=djks8732
    To: Client <sip:client@example.com>
    Call-ID: 763hdc73y7dkb37@example.com
    CSeq: 1 REGISTER
    Contact: <sip:client@client.example.com>;reg-id=1
         ;+sip.instance="<urn:uuid:00000000-0000-0000-0000-00A95A0E120>"
    Content-Length: 0

   This proxy now generates a SIP 401 response to challenge for
   authentication, as depicted in (2) from Figure 5:

    SIP/2.0 401 Unauthorized
    Via: SIP/2.0/UDP client.example.com:5060
       ;rport=8050;branch=z9hG4bK;received=192.0.1.2
    From: Client <sip:client@example.com>;tag=djks8732
    To: Client <sip:client@example.com>;tag=876877
    Call-ID: 763hdc73y7dkb37@example.com
    CSeq: 1 REGISTER
    WWW-Authenticate: [not shown]
    Content-Length: 0

   The response will be sent to the address appearing in the 'received'
   parameter of the SIP 'Via' header (address 192.0.1.2).  The response
   will not be sent to the port deduced from the SIP 'Via' header, as
   per standard SIP operation but will be sent to the value that has
   been stamped in the 'rport' parameter of the SIP 'Via' header (port
   8050).  For the response to successfully traverse the NAT, all of the
   conventions defined in RFC 3581 [6] MUST be obeyed.  Make note of the
   both the 'connectionID' and 'sip.instance' contact header parameters.
   They are used to establish a connection re-use tuple as defined in
   [13].  The connection tuple creation is clearly shown in Figure 5.
   This ensures that any inbound request that causes a registration
   lookup will result in the re-use of the connection path established
   by the registration.  This exonerates the need to manipulate contact
   header URI's to represent a globally routable address as perceived on
   the public side of a NAT.  The subsequent messages defined in (3) and
   (4) from Figure 5 use the same mechanics for NAT traversal.

   [Editors note: Will provide more details on heartbeat mechanism in
   next revision]

   [Editors note: Can complete full flows if required on heartbeat
   inclusion]




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4.1.1.2.  Reliable Transport



           Client              NAT             Registrar
             |                  |                  |
             |(1) REGISTER      |                  |
             |----------------->|                  |
             |                  |                  |
             |                  |(1) REGISTER      |
             |                  |----------------->|
             |                  |                  |
             |                  |(2) 401 Unauth    |
             |                  |<-----------------|
             |                  |                  |
             |(2) 401 Unauth    |                  |
             |<-----------------|                  |
             |                  |                  |
             |(3) REGISTER      |                  |
             |----------------->|                  |
             |                  |                  |
             |                  |(3) REGISTER      |
             |                  |----------------->|
             |                  |                  |
             |*************************************|
             |    Create Connection Re-use Tuple   |
             |*************************************|
             |                  |                  |
             |                  |(4) 200 OK        |
             |                  |<-----------------|
             |                  |                  |
             |(4) 200 OK        |                  |
             |<-----------------|                  |
             |                  |                  |

     Figure 6.

   Traversal of SIP REGISTER requests/responses using a reliable,
   connection orientated protocol such as TCP does not require any
   additional core SIP signaling extensions.  SIP responses will re-use
   the connection created for the initial REGISTER request, (1) from
   Figure 6:









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    REGISTER sip:proxy.example.com SIP/2.0
    Via: SIP/2.0/TCP client.example.com:5060;branch=z9hG4bKyilassjdshfu
    Max-Forwards: 70
    Supported: path,gruu
    From: Client <sip:client@example.com>;tag=djks809834
    To: Client <sip:client@example.com>
    Call-ID: 763hdc783hcnam73@example.com
    CSeq: 1 REGISTER
    Contact: <sip:client@client.example.com;transport=tcp>;reg-id=1
         ;+sip.instance="<urn:uuid:00000000-0000-0000-0000-00A95A0E121>"
    Content-Length: 0

   This example was included to show the inclusion of the connection re-
   use Contact header parameters as defined in the Connection Re-use
   draft [13].  This creates an association tuple as described in the
   previous example for future inbound requests directed at the newly
   created registration binding with the only difference that the
   association is with a TCP connection, not a UDP pin hole binding.

   [Editors note: Will provide more details on heartbeat mechanism in
   next revision]

   [Editors note: Can complete full flows on inclusion of heartbeat
   mechanism]

4.1.2.  Registration(Registrar/Proxy not Co-Located)

   This section demonstrates traversal mechanisms when the Registrar
   component is not co-located with the edge proxy element.  The
   procedures described in this section are identical, regardless of
   transport protocol and so only one example will be documented in the
   form of TCP.



















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     Client              NAT               Proxy            Registrar
       |                  |                  |                  |
       |(1) REGISTER      |                  |                  |
       |----------------->|                  |                  |
       |                  |                  |                  |
       |                  |(1) REGISTER      |                  |
       |                  |----------------->|                  |
       |                  |                  |                  |
       |                  |                  |(2) REGISTER      |
       |                  |                  |----------------->|
       |                  |                  |                  |
       |                  |                  |(3) 401 Unauth    |
       |                  |                  |<-----------------|
       |                  |                  |                  |
       |                  |(4) 401 Unauth    |                  |
       |                  |<-----------------|                  |
       |                  |                  |                  |
       |(4)401 Unauth     |                  |                  |
       |<-----------------|                  |                  |
       |                  |                  |                  |
       |(5)REGISTER       |                  |                  |
       |----------------->|                  |                  |
       |                  |                  |                  |
       |                  |(5)REGISTER       |                  |
       |                  |----------------->|                  |
       |                  |                  |                  |
       |                  |                  |(6)REGISTER       |
       |                  |                  |----------------->|
       |                  |                  |                  |
       |                  |                  |(7)200 OK         |
       |                  |                  |<-----------------|
       |                  |                  |                  |
       |********************************************************|
       |             Create Connection Re-use Tuple             |
       |********************************************************|
       |                  |                  |                  |
       |                  |(8)200 OK         |                  |
       |                  |<-----------------|                  |
       |                  |                  |                  |
       |(8)200 OK         |                  |                  |
       |<-----------------|                  |                  |
       |                  |                  |                  |

     Figure 7.

   This scenario builds on the previous example contained in
   Section 4.1.1.2.  The primary difference being that the REGISTER
   request is routed onwards from a Proxy Server to a separated



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   Registrar.  The important message to note is (6) in Figure 7.  The
   Edge proxy, on receiving a REGISTER request that contains a
   'sip.instance' media feature tag, forms a unique flow identifier
   token as discussed in [13] .  At this point, the proxy server routes
   the SIP REGISTER message to the Registrar.  The proxy will create the
   connection tuple as described in SIP Outbound at the same moment as
   the co-located example, but for subsequent messages to arrive at the
   Proxy, the element needs to request to remain in the signaling path.
   To achieve this the proxy inserts to REGISTER message (5) a SIP PATH
   extension header, as defined in RFC 3327 [7].  The previously created
   flow token is inserted in a position within the Path header where it
   can easily be retrieved at a later point when receiving messages to
   be routed to the registration binding.  REGISTER message (5) would
   look as follows:

    REGISTER sip:registrar.example.com SIP/2.0
    Via: SIP/2.0/TCP proxy.example.com:5060;branch=z9hG4njkca8398hadjaa
    Via: SIP/2.0/TCP client.example.com:5060;branch=z9hG4bKyilassjdshfu
    Max-Forwards: 70
    Supported: path,gruu
    From: Client <sip:client@example.com>;tag=djks809834
    To: Client <sip:client@example.com>
    Call-ID: 763hdc783hcnam73@example.com
    CSeq: 1 REGISTER
    Path: <sip:3HS28o8HAKJSH&&U@proxy.example.com;lr>
    Contact: <sip:client@client.example.com;transport=tcp>;
         ;+sip.instance="<urn:uuid:00000000-0000-0000-0000-00A95A0E121>";reg-id=1
    Content-Length: 0


   This REGISTER request results in the Path header being stored along
   with the AOR and it's associated binding at the Registrar.  The URI
   contained in the Path header will be inserted as a pre-loaded SIP
   'Route' header into any request that arrives at the Registrar and is
   directed towards the associated binding.  This guarantees that all
   requests for the new Registration will be forwarded to the Edge
   Proxy.  In our example, the user part of the SIP 'Path' header URI
   that was inserted by the Edge Proxy contains the unique token
   identifying the flow to the client.  On receiving subsequent
   requests, the edge proxy will examine the user part of the pre-loaded
   SIP 'route' header and extract the unique flow token for use in its
   connection tuple comparison, as defined in the SIP Outbound
   specification [13].  An example which builds on this scenario
   (showing an inbound request to the AOR) is detailed in section
   4.1.4.2 of this document.






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4.1.3.  Initiating a Session

   This section covers basic SIP signaling when initiating a call from
   behind a NAT.

4.1.3.1.  UDP

   Initiating a call using UDP.



      Client              NAT               Proxy              [..]
        |                  |                  |
        |(1) INVITE        |                  |                 |
        |----------------->|                  |                 |
        |                  |                  |                 |
        |                  |(1) INVITE        |                 |
        |                  |----------------->|                 |
        |                  |                  |                 |
        |                  |(2) 407 Unauth    |                 |
        |                  |<-----------------|                 |
        |                  |                  |                 |
        |(2) 407 Unauth    |                  |                 |
        |<-----------------|                  |                 |
        |                  |                  |                 |
        |(3) INVITE        |                  |                 |
        |----------------->|                  |                 |
        |                  |                  |                 |
        |                  |(3) INVITE        |                 |
        |                  |----------------->|                 |
        |                  |                  |                 |
        |                  |                  |(4) INVITE       |
        |                  |                  |---------------->|
        |                  |                  |                 |
        |                  |                  |(5)180 RINGING   |
        |                  |                  |<----------------|
        |                  |                  |                 |
        |                  |(6)180 RINGING    |                 |
        |                  |<-----------------|                 |
        |                  |                  |                 |
        |(6)180 RINGING    |                  |                 |
        |<-----------------|                  |                 |
        |                  |                  |                 |
        |                  |                  |(7)200 OK        |
        |                  |                  |<----------------|
        |                  |                  |                 |
        |                  |(8)200 OK         |                 |
        |                  |<-----------------|                 |



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        |                  |                  |                 |
        |(8)200 OK         |                  |                 |
        |<-----------------|                  |                 |
        |                  |                  |                 |
        |(9)ACK            |                  |                 |
        |----------------->|                  |                 |
        |                  |                  |                 |
        |                  |(9)ACK            |                 |
        |                  |----------------->|                 |
        |                  |                  |                 |
        |                  |                  |(10) ACK         |
        |                  |                  |---------------->|
        |                  |                  |                 |

     Figure 8.

   The initiating client generates an INVITE request that is to be sent
   through the NAT to a Proxy server.  The INVITE message is represented
   in Figure 8 by (1) and is as follows:



   INVITE sip:clientB@example.com SIP/2.0
   Via: SIP/2.0/UDP client.example.com:5060;rport;branch=z9hG4bK74husdHG
   Max-Forwards: 70
   Route: <sip:proxy.example.com;lr>
   From: clientA <sip:clientA@example.com>;tag=7skjdf38l
   To: clientB <sip:clientB@example.com>
   Call-ID: 8327468763423@example.com
   CSeq: 1 INVITE
   Contact:<sip:clientA@example.com;gruu
      ;opaque=urn:uuid:ijed7ush-4jan-53120-aee5-e0aecwee6wef;grid=45a>
   Content-Type: application/sdp
   Content-Length: ..

   [SDP not shown]

   There are a number of points to note with this message:
   1.  Firstly, as with the registration example in Section 4.1.1.1,
       responses to this request will not automatically pass back
       through a NAT and so the SIP 'Via' header 'rport' is included as
       described in the 'Symmetric response' Section 3.1.1 and defined
       in RFC 3581 [6].
   2.  Secondly, the contact inserted contains the GRUU previously
       obtained from the SIP 200 OK response to the registration.  Use
       of the GRUU ensures that any SIP requests within the dialog that
       in the opposite direction will be able to traverse the NAT.  This
       occurs using the mechanisms defined in the SIP Outbound



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       specification[13].  A request arriving at the entity which
       resolves to the GRUU is then able to determine a previously
       registered connection that will allow the request to traverse the
       NAT and reach the intended endpoint.

4.1.3.2.  Reliable Transport

   When using a reliable transport such as TCP the call flow and
   procedures for traversing a NAT are almost identical to those
   described in Section 4.1.3.1.  The primary difference when using
   reliable transport protocols is that Symmetric response[6] are not
   required for SIP responses to traverse a NAT.  RFC 3261[1] defines
   procedures for SIP response messages to be sent back on the same
   connection on which the request arrived.

4.1.4.  Receiving an Invitation to a Session

   This section details scenarios where a client behind a NAT receives
   an inbound request through a NAT.  These scenarios build on the
   previous registration scenario from Section 4.1.1 and Section 4.1.2
   in this document.

4.1.4.1.  Registrar/Proxy Co-located

   The core SIP signaling associated with this call flow is not impacted
   directly by the transport protocol and so only one example scenario
   is necessary.  The example uses UDP and follows on from the
   registration installed in the example from Section 4.1.1.1.























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       Client              NAT         Registrar/Proxy       SIP Entity
         |                  |                  |                 |
         |*******************************************************|
         |           Registration Binding Installed in           |
         |                    section 4.1.1.1                    |
         |*******************************************************|
         |                  |                  |                 |
         |                  |                  |(1)INVITE        |
         |                  |                  |<----------------|
         |                  |                  |                 |
         |                  |(2)INVITE         |                 |
         |                  |<-----------------|                 |
         |                  |                  |                 |
         |(2)INVITE         |                  |                 |
         |<-----------------|                  |                 |
         |                  |                  |                 |
         |                  |                  |                 |


     Figure 9.

   An INVITE request arrives at the Registrar with a destination
   pointing to the AOR of that inserted in section 4.1.1.1.  The message
   is illustrated by (1) in Figure 9 and looks as follows:


   INVITE sip:client@example.com SIP/2.0
   Via: SIP/2.0/UDP external.example.com;branch=z9hG4bK74huHJ37d
   Max-Forwards: 70
   From: External <sip:External@external.example.com>;tag=7893hd
   To: client <sip:client@example.com>
   Call-ID: 8793478934897@external.example.com
   CSeq: 1 INVITE
   Contact: <sip:external@192.0.1.4>
   Content-Type: application/sdp
   Content-Length: ..

   [SDP not shown]

   The INVITE request matches the registration binding previously
   installed at the Registrar and the INVITE request-URI is re-written
   to the selected onward address.  The proxy then examines the request
   URI of the INVITE and compares with its list of current open flows.
   It uses the incoming AOR to commence the check for associated open
   connections/mappings.  Once matched, the proxy checks to see if the
   unique instance identifier (+sip.instance) associated with the
   binding equals the same instance identifier associated with the flow.
   The request is then dispatched on the appropriate flow.  This is



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   message (2) from Figure 9 and is as follows:


   INVITE sip:sip:client@client.example.com SIP/2.0
   Via: SIP/2.0/UDP proxy.example.com;branch=z9hG4kmlds893jhsd
   Via: SIP/2.0/UDP external.example.com;branch=z9hG4bK74huHJ37d
   Max-Forwards: 70
   From: External <sip:External@external.example.com>;tag=7893hd
   To: client <sip:client@example.com>
   Call-ID: 8793478934897@external.example.com
   CSeq: 1 INVITE
   Contact: <sip:external@192.0.1.4>
   Content-Type: application/sdp
   Content-Length: ..

   [SDP not shown]

   It is a standard SIP INVITE request with no additional functionality.
   The major difference being that this request will not follow the
   address specified in the Request-URI, as standard SIP rules would
   enforce but will be sent on the flow associated with the registration
   binding (look-up procedures in RFC 3263 [6] are overridden).  This
   then allows the original connection/mapping from the initial
   registration process to be re-used.

4.1.4.2.  Registrar/Proxy Not Co-located

























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   Client          NAT           Proxy        Registrar       SIP Entity
     |              |              |              |              |
     |***********************************************************|
     |            Registration Binding Installed in              |
     |                      section 4.1.2                        |
     |***********************************************************|
     |              |              |              |              |
     |              |              |              |(1)INVITE     |
     |              |              |              |<-------------|
     |              |              |              |              |
     |              |              |(2)INVITE     |              |
     |              |              |<-------------|              |
     |              |              |              |              |
     |              |(3)INVITE     |              |              |
     |              |<-------------|              |              |
     |              |              |              |              |
     |(3)INVITE     |              |              |              |
     |<-------------|              |              |              |
     |              |              |              |              |
     |              |              |              |              |


     Figure 9.

4.2.  Basic NAT Media Traversal

   This section provides example scenarios to demonstrate basic media
   traversal using the techniques outlined earlier in this document.

   In the flow diagrams STUN messages have been annotated for simplicity
   as follows:
   o  The "Src" attribute represents the source transport address of the
      message.
   o  The "Dest" attribute represents the destination transport address
      of the message.
   o  The "Map" attribute represents the reflexive transport address.
   o  The "Rel" attribute represents the relayed transport address.

   The meaning of each STUN attribute is extensively explained in the
   core STUN[10] and TURN usage[12] drafts.

   The examples also contain a mechanism for representing transport
   addresses.  It would be confusing to include representations of
   network addresses in the call flows and make them hard to follow.
   For this reason network addresses will be represented using the
   following annotation.  The first component will contain the a
   representation of the client responsible for the address.  For
   example in the majority of the examples "L" (left client), "R" (right



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   client), NAT-PUB" (NAT public), PRIV (Private), and "STUN-PUB" (STUN
   Public) are used.  To allow for multiple addresses from the same
   network element, each representation can also be followed by a
   number.  These can also be used in combination.  For example "L-NAT-
   PUB-1" would represent a public network address on the left hand side
   NAT while "R-NAT-PUB-1" would represent a public network address in
   the right hand side of the NAT.  "L-PRIV-1" would represent a private
   network address on the left hand side of the NAT while "R-PRIV-1"
   represents a private address on the right hand side of the NAT.

   It should also be noted that during the examples it might be
   appropriate to signify an explicit part of a transport address.  This
   is achieved by adding either the '.address' or '.port' tag on the end
   of the representation.  For example, 'L-PRIV-1.address' and 'L-PRIV-
   1.port'.

4.2.1.  Endpoint independent NAT

   This section demonstrates an example of a client both initiating and
   receiving calls behind an 'Endpoint independent' NAT.  An example is
   included for both STUN and ICE with ICE being the RECOMMENDED
   mechanism for media traversal.

4.2.1.1.  STUN Solution

   It is possible to traverse media through an 'Endpoint Independent NAT
   using STUN.  The remainder of this section provides a simplified
   examples of the 'Binding Discovery' STUN usage as defined in [10].
   The STUN messages have been simplified and do not include 'Shared
   Secret' requests used to obtain the temporary username and password.

   [Editors Note: Expand to show full flow in including Auth?.]

4.2.1.1.1.  Initiating Session

   The following example demonstrates media traversal through a NAT
   demonstrating 'Address Independent' NAT behavior using STUN.  It is
   assumed in this example that the STUN client and SIP Client are co-
   located on the same machine.  Note that some SIP signalling messages
   have been left out for simplicity.



     Client              NAT               STUN                [..]
                                          Server
       |                  |                  |                  |
       |(1) STUN Req      |                  |                  |
       |Src=L-PRIV-1      |                  |                  |



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       |Dest=STUN-PUB     |                  |                  |
       |----------------->|                  |                  |
       |                  |                  |                  |
       |                  |(2) STUN Req      |                  |
       |                  |Src=NAT-PUB-1     |                  |
       |                  |Dest=STUN-PUB     |                  |
       |                  |----------------->|                  |
       |                  |                  |                  |
       |                  |(3) STUN Resp     |                  |
       |                  |<-----------------|                  |
       |                  |Src=STUN-PUB      |                  |
       |                  |Dest=NAT-PUB-1    |                  |
       |                  |Map=NAT-PUB-1     |                  |
       |                  |                  |                  |
       |(4) STUN Resp     |                  |                  |
       |<-----------------|                  |                  |
       |Src=STUN-PUB      |                  |                  |
       |Dest=L-PRIV-1     |                  |                  |
       |Map=NAT-PUB-1     |                  |                  |
       |                  |                  |                  |
       |(5) STUN Req      |                  |                  |
       |Src=L-PRIV-2      |                  |                  |
       |Dest=STUN-PUB     |                  |                  |
       |----------------->|                  |                  |
       |                  |                  |                  |
       |                  |(6) STUN Req      |                  |
       |                  |Src=NAT-PUB-2     |                  |
       |                  |Dest=STUN-PUB     |                  |
       |                  |----------------->|                  |
       |                  |                  |                  |
       |                  |(7) STUN Resp     |                  |
       |                  |<-----------------|                  |
       |                  |Src=STUN-PUB      |                  |
       |                  |Dest=NAT-PUB-2    |                  |
       |                  |Map=NAT-PUB-2     |                  |
       |                  |                  |                  |
       |(8) STUN Resp     |                  |                  |
       |<-----------------|                  |                  |
       |Src=STUN-PUB      |                  |                  |
       |Dest=L-PRIV-2     |                  |                  |
       |Map=NAT-PUB-2     |                  |                  |
       |                  |                  |                  |
       |(9)SIP INVITE     |                  |                  |
       |----------------->|                  |                  |
       |                  |                  |                  |
       |                  |(10)SIP INVITE    |                  |
       |                  |------------------------------------>|
       |                  |                  |                  |



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       |                  |                  |(11)SIP 200 OK    |
       |                  |<------------------------------------|
       |                  |                  |                  |
       |(12)SIP 200 OK    |                  |                  |
       |<-----------------|                  |                  |
       |                  |                  |                  |
       |========================================================|
       |>>>>>>>>>>>>Outgoing Media sent from L-PRIV-1>>>>>>>>>>>|
       |========================================================|
       |                                                        |
       |========================================================|
       |<<<<<<<<<<<<Incoming Media sent to NAT-PUB-1<<<<<<<<<<<<|
       |========================================================|
       |                                                        |
       |========================================================|
       |>>>>>>>>>>>>Outgoing RTCP sent from L-PRIV-2>>>>>>>>>>>>|
       |========================================================|
       |                                                        |
       |========================================================|
       |<<<<<<<<<<<<Incoming RTCP sent to NAT-PUB-2<<<<<<<<<<<<<|
       |========================================================|
       |                  |                  |                  |
       |(13)SIP ACK       |                  |                  |
       |----------------->|                  |                  |
       |                  |                  |                  |
       |                  |(14) SIP ACK      |                  |
       |                  |------------------------------------>|
       |                  |                  |                  |


   Figure 18: Address and Port Dependant NAT with STUN - Initiating

   o  On deciding to initiate a SIP voice session the client starts a
      local STUN client on the interface and port that is to be used for
      media (send/receive).  The STUN client generates a standard STUN
      request as indicated in (1) from Figure 18 which also highlights
      the source address and port for which the client device wishes to
      obtain a mapping.  The STUN request is sent through the NAT
      towards the public internet and STUN server.
   o  STUN message (2) traverses the NAT and breaks out onto the public
      internet towards the public STUN server.  Note that the source
      address of the STUN requests now represents the public address and
      port from the public side of the NAT.
   o  The STUN server receives the request and processes it
      appropriately.  This results in a successful STUN response being
      generated and returned (3).  The message contains details of the
      mapped public address (contained in the STUN MAPPED-ADDRESS
      attribute) which is to be used by the originating client to



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      receive media (see 'Map=NAT-PUB-1' from (3)).
   o  The STUN response traverses back through the NAT using the binding
      created by the STUN request and presents the new mapped address to
      the client (4).  At this point the process is repeated to obtain a
      second mapped address (as shown in (5)-(8)) for an alternative
      local address (Address has changed from "L-PRIV-1" to "L-PRIV-2").
   o  The client now constructs a SIP INVITE message(9).  Note that
      traversal of SIP is not covered in this example and is discussed
      in earlier sections of the document.  The INVITE request will use
      the addresses it has obtained in the previous STUN transactions to
      populate the SDP of the SIP INVITE as shown below:

      v=0
      o=test 2890844526 2890842807 IN IP4 $L-PRIV-1.address
      c=IN IP4 $NAT-PUB-1.address
      t=0 0
      m=audio $NAT-PUB-1.port RTP/AVP 0
      a=rtcp:$NAT-PUB-2.port


   o  Note that the mapped address obtained from the STUN transactions
      are inserted as the connection address for the SDP (c=NAT-PUB-
      1.address).  The Primary port for RTP is also inserted in the SDP
      (m=audio NAT-PUB-1.port RTP/AVP 0).  Finally, the port gained from
      the additional STUN binding is placed in the RTCP attribute (as
      discussed in Section 3.2.1.1) for traversal of RTCP (a=rtcp:NAT-
      PUB-2.port).
   o  The SIP signalling then traverses the NAT and sets up the SIP
      session (10-12).  Note that the client transmits media as soon as
      the 200 OK to the INVITE arrives at the client (12).  Up until
      this point the incoming media and RTCP will not pass through the
      NAT as no outbound association has been created with the far end
      client.  Two way media communication has now been established.

4.2.1.1.2.  Receiving Session Invitation

   Receiving a session for an 'Address and Port dependant' NAT using
   STUN is very similar to the example outlined in Section 4.2.1.1.1.
   Figure 20 illustrates the associated flow of messages.



     Client              NAT               STUN                [..]
                                          Server
       |                  |                  | (1)SIP INVITE    |
       |                  |<-----------------|------------------|
       |                  |                  |                  |
       |(2) SIP INVITE    |                  |                  |



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       |<-----------------|                  |                  |
       |                  |                  |                  |
       |(3) STUN Req      |                  |                  |
       |Src=L-PRIV-1      |                  |                  |
       |Dest=STUN-PUB     |                  |                  |
       |----------------->|                  |                  |
       |                  |                  |                  |
       |                  |(4) STUN Req      |                  |
       |                  |Src=NAT-PUB-1     |                  |
       |                  |Dest=STUN-PUB     |                  |
       |                  |----------------->|                  |
       |                  |                  |                  |
       |                  |(5) STUN Resp     |                  |
       |                  |<-----------------|                  |
       |                  |Src=STUN-PUB      |                  |
       |                  |Dest=NAT-PUB-1    |                  |
       |                  |Map=NAT-PUB-1     |                  |
       |                  |                  |                  |
       |(6) STUN Resp     |                  |                  |
       |<-----------------|                  |                  |
       |Src=STUN-PUB      |                  |                  |
       |Dest=L-PRIV-1     |                  |                  |
       |Map=NAT-PUB-1     |                  |                  |
       |                  |                  |                  |
       |(7) STUN Req      |                  |                  |
       |Src=L-PRIV-2      |                  |                  |
       |Dest=STUN-PUB     |                  |                  |
       |----------------->|                  |                  |
       |                  |                  |                  |
       |                  |(8) STUN Req      |                  |
       |                  |Src=NAT-PUB-2     |                  |
       |                  |Dest=STUN-PUB     |                  |
       |                  |----------------->|                  |
       |                  |                  |                  |
       |                  |(9) STUN Resp     |                  |
       |                  |<-----------------|                  |
       |                  |Src=STUN-PUB      |                  |
       |                  |Dest=NAT-PUB-2    |                  |
       |                  |Map=NAT-PUB-2     |                  |
       |                  |                  |                  |
       |(10) STUN Resp    |                  |                  |
       |<-----------------|                  |                  |
       |Src=STUN-PUB      |                  |                  |
       |Dest=L-PRIV-2     |                  |                  |
       |Map=NAT-PUB-2     |                  |                  |
       |                  |                  |                  |
       |(11)SIP 200 OK    |                  |                  |
       |----------------->|                  |                  |



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       |                  |(12)SIP 200 OK    |                  |
       |                  |------------------------------------>|
       |                  |                  |                  |
       |========================================================|
       |>>>>>>>>>>>>Outgoing Media sent from L-PRIV-1>>>>>>>>>>>|
       |========================================================|
       |                  |                  |                  |
       |========================================================|
       |<<<<<<<<>><<<Incoming Media sent to L-PRIV-1<<<<<<<<<<<<|
       |========================================================|
       |                  |                  |                  |
       |========================================================|
       |>>>>>>>>>>>>Outgoing RTCP sent from L-PRIV-2>>>>>>>>>>>>|
       |========================================================|
       |                  |                  |                  |
       |========================================================|
       |<<<<<<<<<<<<<Incoming RTCP sent to L-PRIV-2<<<<<<<<<<<<<|
       |========================================================|
       |                  |                  |                  |
       |                  |                  |(13)SIP ACK       |
       |                  |<------------------------------------|
       |                  |                  |                  |
       |(14)SIP ACK       |                  |                  |
       |<-----------------|                  |                  |
       |                  |                  |                  |


   Figure 20: Restricted NAT with STUN - Receiving

   o  On receiving an invitation to a SIP voice session (SIP INVITE
      request) the User Agent starts a local STUN client on the
      appropriate port on which it is to receive media.  The STUN client
      generates a standard STUN request as indicated in (3) from
      Figure 20 which also highlights the source address and port for
      which the client device wishes to obtain a mapping.  The STUN
      request is sent through the NAT towards the public internet and
      STUN Server.
   o  STUN message (4) traverses the NAT and breaks out onto the public
      internet towards the public STUN server.  Note that the source
      address of the STUN requests now represents the public address and
      port from the public side of the NAT.
   o  The STUN server receives the request and processes it
      appropriately.  This results in a successful STUN response being
      generated and returned (5).  The message contains details of the
      mapped public address (contained in the STUN MAPPED-ADDRESS
      attribute) which is to be used by the originating client to
      receive media (see 'Map=NAT-PUB-1' from (5)).




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   o  The STUN response traverses back through the NAT using the binding
      created by the outgoing STUN request and presents the new mapped
      address to the client (6).  At this point the process is repeated
      to obtain a second mapped address (as shown in (7)-(10)) for an
      alternative local address (local port has now changed and is
      represented by L-PRIV-2 in (7)).
   o  The client now constructs a SIP 200 OK message (11) in response to
      the original SIP INVITE requests.  Note that traversal of SIP is
      not covered in this example and is discussed in earlier sections
      of the document.  SIP Provisional responses are also left out for
      simplicity.  The 200 OK response will use the addresses it has
      obtained in the previous STUN transactions to populate the SDP of
      the SIP 200 OK as shown below:

      v=0
      o=test 2890844526 2890842807 IN IP4 $L-PRIV-1.address
      c=IN IP4 $NAT-PUB-1.address
      t=0 0
      m=audio $NAT-PUB-1.port RTP/AVP 0
      a=rtcp:$NAT-PUB-2.port


   o  Note that the mapped address obtained from the initial STUN
      transaction is inserted as the connection address for the SDP
      (c=NAT-PUB-1.address).  The Primary port for RTP is also inserted
      in the SDP (m=audio NAT-PUB-1.port RTP/AVP 0).  Finally, the port
      gained from the additional binding is placed in the RTCP attribute
      (as discussed in Section 3.2.1.1) for traversal of RTCP (a=rtcp:
      NAT-PUB-2.port).
   o  The SIP signalling then traverses the NAT and sets up the SIP
      session (11-14).  Note that the client transmits media as soon as
      the 200 OK to the INVITE is sent to the UAC(11).  Up until this
      point the incoming media will not pass through the NAT as no
      outbound association has been created with the far end client.
      Two way media communication has now been established.

4.2.1.2.  ICE Solution

   The preferred solution for media traversal of NAT is using ICE, as
   described in Section 3.2.4, regardless of the NAT type.  The
   following examples illustrate the traversal of an 'Endpoint
   independent' NAT for both an initiating.  The example only covers ICE
   in association with STUN and TURN usage.

4.2.1.2.1.  Initiating Session

   The following example demonstrates an initiating traversal through an
   'Endpoint independent' NAT using ICE.



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   [Editors Note: Example needs to be expanded to include more ICE
   detail e.g. timers etc.]



   L               NAT             STUN              NAT               R
                                   Server
   |                |                |                |                |
   |(1) Alloc Req   |                |                |                |
   |Src=L-PRIV-1    |                |                |                |
   |Dest=STUN-PUB-1 |                |                |                |
   |--------------->|                |                |                |
   |                |                |                |                |
   |                |(2) Alloc Req   |                |                |
   |                |Src=L-NAT-PUB-1 |                |                |
   |                |Des=STUN-PUB-1  |                |                |
   |                |--------------->|                |                |
   |                |                |                |                |
   |                |(3) Alloc Resp  |                |                |
   |                |<---------------|                |                |
   |                |Src=STUN-PUB-1  |                |                |
   |                |Dest=L-NAT-PUB-1|                |                |
   |                |Map=L-NAT-PUB-1 |                |                |
   |                |Rel=STUN-PUB-2  |                |                |
   |                |                |                |                |
   |(4) Alloc Resp  |                |                |                |
   |<---------------|                |                |                |
   |Src=STUN-PUB-1  |                |                |                |
   |Dest=L-PRIV-1   |                |                |                |
   |Map=L-NAT-PUB-1 |                |                |                |
   |Rel=STUN-PUB-2  |                |                |                |
   |                |                |                |                |
   |(5) STUN Req    |                |                |                |
   |Src=L-PRIV-2    |                |                |                |
   |Dest=STUN-PUB-1 |                |                |                |
   |--------------->|                |                |                |
   |                |                |                |                |
   |                |(6) Alloc Req   |                |                |
   |                |Src=L-NAT-PUB-2 |                |                |
   |                |Dest=STUN-PUB-1 |                |                |
   |                |--------------->|                |                |
   |                |                |                |                |
   |                |(7) Alloc Resp  |                |                |
   |                |<---------------|                |                |
   |                |Src=STUN-PUB-1  |                |                |
   |                |Dest=NAT-PUB-2  |                |                |
   |                |Map=NAT-PUB-2   |                |                |
   |                |Rel=STUN-PUB-3  |                |                |



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   |                |                |                |                |
   |(8) Alloc Resp  |                |                |                |
   |<---------------|                |                |                |
   |Src=STUN-PUB-1  |                |                |                |
   |Dest=L-PRIV-2   |                |                |                |
   |Map=L-NAT-PUB-2 |                |                |                |
   |Rel=STUN-PUB-3  |                |                |                |
   |                |                |                |                |
   |(9) SIP INVITE  |                |                |                |
   |------------------------------------------------->|                |
   |                |                |                |                |
   |                |                |                |(10) SIP INVITE |
   |                |                |                |--------------->|
   |                |                |                |                |
   |                |                |                |(11) Alloc Req  |
   |                |                |                |<---------------|
   |                |                |                |Src=R-PRIV-1    |
   |                |                |                |Dest=STUN-PUB-1 |
   |                |                |                |                |
   |                |                |(12) Alloc Req  |                |
   |                |                |<---------------|                |
   |                |                |Src=R-NAT-PUB-1 |                |
   |                |                |Dest=STUN-PUB-1 |                |
   |                |                |                |                |
   |                |                |(13) Alloc Res  |                |
   |                |                |--------------->|                |
   |                |                |Src=STUN-PUB-1  |                |
   |                |                |Dest=R-NAT-PUB-1|                |
   |                |                |Map=R-NAT-PUB-1 |                |
   |                |                |Rel=STUN-PUB-4  |                |
   |                |                |                |                |
   |                |                |                |(14) Alloc Res  |
   |                |                |                |--------------->|
   |                |                |                |Src=STUN-PUB-1  |
   |                |                |                |Dest=R-PRIV-1   |
   |                |                |                |Map=R-NAT-PUB-1 |
   |                |                |                |Rel=STUN-PUB-4  |
   |                |                |                |                |
   |                |                |                |(15) Alloc Req  |
   |                |                |                |<---------------|
   |                |                |                |Src=R-PRIV-2    |
   |                |                |                |Dest=STUN-PUB-1 |
   |                |                |                |                |
   |                |                |(16) Alloc Req  |                |
   |                |                |<---------------|                |
   |                |                |Src=R-NAT-PUB-2 |                |
   |                |                |Dest=STUN-PUB-1 |                |
   |                |                |                |                |



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   |                |                |(17) Alloc Res  |                |
   |                |                |--------------->|                |
   |                |                |Src=STUN-PUB-1  |                |
   |                |                |Dest=R-NAT-PUB-2|                |
   |                |                |Map=R-NAT-PUB-2 |                |
   |                |                |Rel=STUN-PUB-5  |                |
   |                |                |                |                |
   |                |                |                |(18) Alloc Res  |
   |                |                |                |--------------->|
   |                |                |                |Src=STUN-PUB-1  |
   |                |                |                |Dest=R-PRIV-2   |
   |                |                |                |Map=R-NAT-PUB-2 |
   |                |                |                |Rel=STUN-PUB-5  |
   |                |                |                |                |
   |                |                |                |(19) SIP 200 OK |
   |                |<-------------------------------------------------|
   |                |                |                |                |
   |(20) SIP 200 OK |                |                |                |
   |<---------------|                |                |                |
   |                |                |                |                |
   |(21) SIP ACK    |                |                |                |
   |------------------------------------------------->|                |
   |                |                |                |                |
   |                |                |                |(22) SIP ACK    |
   |                |                |                |--------------->|
   |                |                |                |                |
   |(23) Bind Req   |                |                |                |
   |------------------------>x       |                |                |
   |Src=L-PRIV-1    |                |                |                |
   |Dest=R-PRIV-1   |                |                |                |
   |                |                |                |                |
   |(24) Bind Req   |                |                |                |
   |--------------->|                |                |                |
   |Src=L-PRIV-1    |                |                |                |
   |Dest=R-NAT-PUB-1|                |                |                |
   |                |                |                |                |
   |                |(25) Bind Req   |                |                |
   |                |-------------------------------->|                |
   |                |Src=L-NAT-PUB-1 |                |                |
   |                |Dest=R-NAT-PUB-1|                |                |
   |                |                |                |                |
   |                |                |                |(26) Bind Req   |
   |                |                |                |--------------->|
   |                |                |                |Src=L-NAT-PUB-1 |
   |                |                |                |Dest=R-PRIV-1   |
   |                |                |                |                |
   |                |                |                |(27) Bind Res   |
   |                |                |                |<---------------|



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   |                |                |                |Src=R-PRIV-1    |
   |                |                |                |Dest=L-NAT-PUB-1|
   |                |                |                |Map=L-NAT-PUB-1 |
   |                |                |                |                |
   |                |                |(28) Bind Res   |                |
   |                |<--------------------------------|                |
   |                |                |Src=R-NAT-PUB-1 |                |
   |                |                |Dest=L-NAT-PUB-1|                |
   |                |                |Map=L-NAT-PUB-1 |                |
   |                |                |                |                |
   |(29) Bind Res   |                |                |                |
   |<---------------|                |                |                |
   |Src=R-NAT-PUB-1 |                |                |                |
   |Dest=L-PRIV-1   |                |                |                |
   |Map=L-NAT-PUB-1 |                |                |                |
   |                |                |                |                |
   |===================================================================|
   |>>>>>>>>>>>>>>>>>>>>>Outgoing RTP sent from >>>>>>>>>>>>>>>>>>>>>>>|
   |===================================================================|
   |                |                |                |                |
   |                |                |                |(30) Bind Req   |
   |                |                |        x<-----------------------|
   |                |                |                |Src=R-PRIV-1    |
   |                |                |                |Dest=L-PRIV-1   |
   |                |                |                |                |
   |                |                |                |(31) Bind Req   |
   |                |                |                |<---------------|
   |                |                |                |Src=R-PRIV-1    |
   |                |                |                |Dest=L-NAT-PUB-1|
   |                |                |                |                |
   |                |                |(32) Bind Req   |                |
   |                |<--------------------------------|                |
   |                |                |Src=R-NAT-PUB-1 |                |
   |                |                |Dest=L-NAT-PUB-1|                |
   |                |                |                |                |
   |(33) Bind Req   |                |                |                |
   |<---------------|                |                |                |
   |Src=R-NAT-PUB-1 |                |                |                |
   |Dest=L-PRIV-1   |                |                |                |
   |                |                |                |                |
   |(34) Bind Res   |                |                |                |
   |--------------->|                |                |                |
   |Src=L-PRIV-1    |                |                |                |
   |Dest=R-NAT-PUB-1|                |                |                |
   |Map=R-NAT-PUB-1 |                |                |                |
   |                |                |                |                |
   |                |(35) Bind Res   |                |                |
   |                |-------------------------------->|                |



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   |                |Src=L-NAT-PUB-1 |                |                |
   |                |Dest=R-NAT-PUB-1|                |                |
   |                |Map=R-NAT-PUB-1 |                |                |
   |                |                |                |                |
   |                |                |                |(36) Bind Res   |
   |                |                |                |--------------->|
   |                |                |                |Src=L-NAT-PUB-1 |
   |                |                |                |Dest=R-PRIV-1   |
   |                |                |                |Map=R-NAT-PUB-1 |
   |                |                |                |                |
   |===================================================================|
   |<<<<<<<<<<<<<<<<<<<<<Outgoing RTP sent from <<<<<<<<<<<<<<<<<<<<<<<|
   |===================================================================|
   |                |                |                |                |
   |(37) Bind Req   |                |                |                |
   |--------------->|                |                |                |
   |Src=L-PRIV-2    |                |                |                |
   |Dest=R-NAT-PUB-2|                |                |                |
   |                |                |                |                |
   |                |(38) Bind Req   |                |                |
   |                |-------------------------------->|                |
   |                |Src=L-NAT-PUB-2 |                |                |
   |                |Dest=R-NAT-PUB-2|                |                |
   |                |                |                |                |
   |                |                |                |(39) Bind Req   |
   |                |                |                |--------------->|
   |                |                |                |Src=L-NAT-PUB-2 |
   |                |                |                |Dest=R-PRIV-2   |
   |                |                |                |                |
   |                |                |                |(40) Bind Res   |
   |                |                |                |<---------------|
   |                |                |                |Src=R-PRIV-2    |
   |                |                |                |Dest=L-NAT-PUB-2|
   |                |                |                |Map=L-NAT-PUB-2 |
   |                |                |                |                |
   |                |                |(41) Bind Res   |                |
   |                |<--------------------------------|                |
   |                |                |Src=R-NAT-PUB-2 |                |
   |                |                |Dest=L-NAT-PUB-2|                |
   |                |                |Map=L-NAT-PUB-2 |                |
   |                |                |                |                |
   |(42) Bind Res   |                |                |                |
   |<---------------|                |                |                |
   |Src=R-NAT-PUB-2 |                |                |                |
   |Dest=L-PRIV-2   |                |                |                |
   |Map=L-NAT-PUB-2 |                |                |                |
   |                |                |                |                |
   |===================================================================|



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   |>>>>>>>>>>>>>>>>>>>>>Outgoing RTCP sent from >>>>>>>>>>>>>>>>>>>>>>|
   |===================================================================|
   |                |                |                |                |
   |                |                |                |(43) Bind Req   |
   |                |                |                |<---------------|
   |                |                |                |Src=R-PRIV-2    |
   |                |                |                |Dest=L-NAT-PUB-2|
   |                |                |                |                |
   |                |                |(44) Bind Req   |                |
   |                |<--------------------------------|                |
   |                |                |Src=R-NAT-PUB-2 |                |
   |                |                |Dest=L-NAT-PUB-2|                |
   |                |                |                |                |
   |(45) Bind Req   |                |                |                |
   |<---------------|                |                |                |
   |Src=R-NAT-PUB-2 |                |                |                |
   |Dest=L-PRIV-2   |                |                |                |
   |                |                |                |                |
   |(46) Bind Res   |                |                |                |
   |--------------->|                |                |                |
   |Src=L-PRIV-2    |                |                |                |
   |Dest=R-NAT-PUB-2|                |                |                |
   |Map=R-NAT-PUB-2 |                |                |                |
   |                |                |                |                |
   |                |(47) Bind Res   |                |                |
   |                |-------------------------------->|                |
   |                |Src=L-NAT-PUB-2 |                |                |
   |                |Dest=R-NAT-PUB-2|                |                |
   |                |Map=R-NAT-PUB-2 |                |                |
   |                |                |                |                |
   |                |                |                |(48) Bind Res   |
   |                |                |                |--------------->|
   |                |                |                |Src=L-NAT-PUB-2 |
   |                |                |                |Dest=R-PRIV-2   |
   |                |                |                |Map=R-NAT-PUB-2 |
   |                |                |                |                |
   |===================================================================|
   |<<<<<<<<<<<<<<<<<<<<<Outgoing RTCP sent from <<<<<<<<<<<<<<<<<<<<<<|
   |===================================================================|
   |                |                |                |                |
   |(49) SIP INVITE |                |                |                |
   |------------------------------------------------->|                |
   |                |                |                |                |
   |                |                |                |(50) SIP INVITE |
   |                |                |                |--------------->|
   |                |                |                |                |
   |                |                |                |(51) SIP 200 OK |
   |                |<-------------------------------------------------|



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   |                |                |                |                |
   |(52) SIP 200 OK |                |                |                |
   |<---------------|                |                |                |
   |                |                |                |                |
   |(53) SIP ACK    |                |                |                |
   |------------------------------------------------->|                |
   |                |                |                |                |
   |                |                |                |(54) SIP ACK    |
   |                |                |                |--------------->|
   |                |                |                |                |


   Figure 22: Endpoint Independent NAT with ICE

   o  On deciding to initiate a SIP voice session the SIP client 'L'
      starts a local STUN client.  The STUN client generates a standard
      Allocate request as indicated in (1) from Figure 22 which also
      highlights the source address and port combination for which the
      client device wishes to obtain a mapping.  The STUN request is
      sent through the NAT towards the public internet.
   o  Allocate message (2) traverses the NAT and breaks out onto the
      public internet towards the public STUN server.  Note that the
      source address of the Allocate request now represents the public
      address and port from the public side of the NAT (L-NAT-PUB-1).
   o  The STUN server receives the Allocate request and processes
      appropriately.  This results in a successful Allocate response
      being generated and returned (3).  The message contains details of
      the reflexive address which is to be used by the originating
      client to receive media (see 'Map=L-NAT-PUB-1') from (3)).  It
      also contains an appropriate relay address that can be used at the
      STUN server (see 'Rel=STUN-PUB-2').
   o  The STUN response traverses back through the NAT using the binding
      created by the initial Allocate request and presents the new
      mapped address to the client (4).  The process is repeated and a
      second STUN derived set of address' are obtained, as illustrated
      in (5)-(8) in Figure 22.  At this point the User Agent behind the
      NAT has pairs of derived external reflexive and relayed
      representations.  The client would be free to gather any number of
      external representations using any UNSAF[9] compliant protocol.
   o  The client now constructs a SIP INVITE message (9).  The INVITE
      request will use the addresses it has obtained in the previous
      STUN/TURN interactions to populate the SDP of the SIP INVITE.
      This should be carried out in accordance with the semantics
      defined in the ICE specification[16], as shown below in Figure 23
      (*note - /* signifies line continuation):






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      v=0
      o=test 2890844526 2890842807 IN IP4 $L-PRIV-1
      c=IN IP4 $L-PRIV-1.address
      t=0 0
      a=ice-pwd:$LPASS
      m=audio $L-PRIV-1.port RTP/AVP 0
      a=rtpmap:0 PCMU/8000
      a=rtcp:$L-PRIV-2.port
      a=candidate:$L1 1 UDP 1.0 $L-PRIV-1.address $L-PRIV-1.port
      a=candidate:$L1 2 UDP 1.0 $L-PRIV-2.address $L-PRIV-2.port
      a=candidate:$L2 1 UDP 0.7 $L-NAT-PUB-1.address $L-NAT-PUB-1.port
      a=candidate:$L2 2 UDP 0.7 $L-NAT-PUB-2.address $L-NAT-PUB-2.port
      a=candidate:$L3 1 UDP 0.3 $STUN-PUB-2.address $STUN-PUB-2.port
      a=candidate:$L3 2 UDP 0.3 $STUN-PUB-3.address $STUN-PUB-3.port


      Figure 23: ICE SDP Offer

   o  The SDP has been constructed to include all the available
      candidate pairs that have been assembled.  The first candidate
      pair (as identified by $L1) contain two local addresses that have
      the highest priority (1.0).  They are also encoded into the
      connection (c=) and media (m=) lines of the SDP.  The second
      'candidate' address pair, as identified by the component-id,
      contains the two NAT reflexive addresses obtained from the STUN
      server for both RTP and RTCP traffic (identified by candidate-id
      $L2).  This entry has been given a priority (0.7) by the client.
      The third and final candidate pair represents the relayed
      addresses (as identified by $L3) obtained from the STUN server.
      This pair has the lowest priority (0.3) and will be used as a last
      resort.
   o  The SIP signalling then traverses the NAT and sets up the SIP
      session (9)-(10).  On advertising a candidate address, the client
      should have a local STUN server running on each advertised
      candidate address.  This is for the purpose of responding to
      incoming connectivity checks.
   o  On receiving the SIP INVITE request (10) client 'R' also starts
      local STUN servers on appropriate address/port combinations and
      gathers potential candidate addresses to be encoded into the SDP.
      Steps (11-18) involve client 'R' carrying out the same steps as
      client 'L'.  This involves obtaining local, reflexive and relayed
      addresses.  Client 'R' is now ready to generate an appropriate
      answer in the SIP 200 OK message (19).  The example answer follows
      in Figure 23 (*note - /* signifies line continuation):







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      v=0
      o=test 3890844516 3890842803 IN IP4 $R-PRIV-1
      c=IN IP4 $R-PRIV-1.address
      t=0 0
      a=ice-pwd:$RPASS
      m=audio $R-PRIV-1.port RTP/AVP 0
      a=rtpmap:0 PCMU/8000
      a=rtcp:$R-PRIV-2.port
      a=candidate:$L1 1 UDP 1.0 $R-PRIV-1.address $R-PRIV-1.port
      a=candidate:$L1 2 UDP 1.0 $R-PRIV-2.address $R-PRIV-2.port
      a=candidate:$L2 1 UDP 0.7 $R-NAT-PUB-1.address $R-NAT-PUB-1.port
      a=candidate:$L2 2 UDP 0.7 $R-NAT-PUB-2.address $R-NAT-PUB-2.port
      a=candidate:$L3 1 UDP 0.3 $STUN-PUB-2.address $STUN-PUB-4.port
      a=candidate:$L3 2 UDP 0.3 $STUN-PUB-3.address $STUN-PUB-5.port


      Figure 24: ICE SDP Answer

   o  The two clients will now form candidate pairs and the transport
      address check list as specified in ICE.  Both 'L' and 'R' will
      start the check list with the currently active component pair
      (contained in the 'c=' and 'm=' of the SDP).  As illustrated in
      (23), client 'L' constructs a STUN Bind request in an attempt to
      validate the connection address received in the SDP of the 200 OK
      (20).  As a private address was specified in the active address in
      the SDP, the Stun Bind request fails to reach its destination
      causing a bind failure.  Client 'L' now attempts a STUN Bind
      request for the first candidate pair in the returned SDP with the
      highest priority (24).  As can be seen from messages (24) to (29),
      the STUN Bind request is successful and returns a positive outcome
      for the connectivity check.  Client 'L' is now free to steam media
      to the candidate pair.  Client 'R' also carries out the same set
      of binding requests.  It firstly (in parallel) tries to contact
      the active address contained in the SDP (30).  Client 'R' now
      attempts a STUN Bind request for the first candidate pair in the
      returned SDP with the highest priority (31).  As can be seen from
      messages (31) to (36), the STUN bind request is successful and
      returns a positive outcome for the connectivity check.  The
      previously described check confirm on both sides that connectivity
      can be achieved through appropriate candidates.  As part of the
      process in this example, both 'L' and 'R' will now complete the
      same connectivity checks for part 2 of the component named for
      each candidate for use with RTCP.  The connectivity checks for
      part '2' of the candidate component are shown in 'L'(37-42) and
      'R'(43-48).
   o  The candidates have now been fully verified (Valid status) and as
      they are the highest priority, an updated offer (49-50) is now
      sent from the offerer (client 'L') to the answerer (client 'R'



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      representing the new active candidates.  The new offer would look
      as follows:

      v=0
      o=test 2890844526 2890842808 IN IP4 $L-PRIV-1
      c=IN IP4 $L-NAT-PUB-1.address
      t=0 0
      a=ice-pwd:$LPASS
      m=audio $L-NAT-PUB-1.port RTP/AVP 0
      a=rtpmap:0 PCMU/8000
      a=rtcp:$L-NAT-PUB-2.port
      a=candidate:$L2 1 UDP 0.7 $L-NAT-PUB-1.address $L-NAT-PUB-1.port
      a=candidate:$L2 2 UDP 0.7 $L-NAT-PUB-2.address $L-NAT-PUB-2.port



      Figure 25: ICE SDP Updated Offer

   o  The resulting answer (51-52) for 'R' would look as follows:

      v=0
      o=test 3890844516 3890842803 IN IP4 $R-PRIV-1
      c=IN IP4 $R-PRIV-1.address
      t=0 0
      a=ice-pwd:$RPASS
      m=audio $R-PRIV-1.port RTP/AVP 0
      a=rtpmap:0 PCMU/8000
      a=rtcp:$R-PRIV-2.port
      a=candidate:$L2 1 UDP 0.7 $R-NAT-PUB-1.address $R-NAT-PUB-1.port
      a=candidate:$L2 2 UDP 0.7 $R-NAT-PUB-2.address $R-NAT-PUB-2.port


      Figure 26: ICE SDP Updated Answer

4.2.2.  Port and Address Dependant NAT

4.2.2.1.  STUN Failure

   This section highlights that while STUN is the preferred mechanism
   for traversal of NAT, it does not solve every case.  The use of basic
   STUN on its own will not guarantee traversal through every NAT type,
   hence the recommendation that ICE is the preferred option.









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     Client     PORT/ADDRESS Dependant     STUN                [..]
                         NAT              Server
       |                  |                  |                  |
       |(1) STUN Req      |                  |                  |
       |Src=L-PRIV-1      |                  |                  |
       |Dest=STUN-PUB     |                  |                  |
       |----------------->|                  |                  |
       |                  |                  |                  |
       |                  |(2) STUN Req      |                  |
       |                  |Src=NAT-PUB-1     |                  |
       |                  |Dest=STUN-PUB     |                  |
       |                  |----------------->|                  |
       |                  |                  |                  |
       |                  |(3) STUN Resp     |                  |
       |                  |<-----------------|                  |
       |                  |Src=STUN-PUB      |                  |
       |                  |Dest=NAT-PUB-1    |                  |
       |                  |Map=NAT-PUB-1     |                  |
       |                  |                  |                  |
       |(4) STUN Resp     |                  |                  |
       |<-----------------|                  |                  |
       |Src=STUN-PUB      |                  |                  |
       |Dest=L-PRIV-1     |                  |                  |
       |Map=NAT-PUB-1     |                  |                  |
       |                  |                  |                  |
       |(5)SIP INVITE     |                  |                  |
       |------------------------------------------------------->|
       |                  |                  |                  |
       |                  |                  |(6)SIP 200 OK     |
       |                  |<------------------------------------|
       |                  |                  |                  |
       |(7)SIP 200 OK     |                  |                  |
       |<-----------------|                  |                  |
       |                  |                  |                  |
       |========================================================|
       |>>>>>>>>>>>>>>Outgoing Media sent from L-PRIV-1>>>>>>>>>|
       |========================================================|
       |                  |                  |                  |
       |                  x=====================================|
       |                  xIncoming Media sent to L-PRIV-1<<<<<<|
       |                  x=====================================|
       |                  |                  |                  |
       |(8)SIP ACK        |                  |                  |
       |----------------->|                  |                  |
       |                  |(9) SIP ACK       |                  |
       |                  |------------------------------------>|
       |                  |                  |                  |




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   Figure 27: Port/Address Dependant NAT with STUN - Failure

   The example in Figure 27 is conveyed in the context of a client
   behind the 'Port/Address Dependant' NAT initiating a call.  It should
   be noted that the same problem applies when a client receives a SIP
   invitation and is behind a Port/Address Dependant NAT.
   o  In Figure 27 the client behind the NAT obtains an external
      representation using standard STUN mechanisms (1)-(4) that have
      been used in previous examples in this document (e.g
      Section 4.2.1.1.1).
   o  The external mapped address (reflexive) obtained is also used in
      the outgoing SDP contained in the SIP INVITE request(5).
   o  In this example the client is still able to send media to the
      external client.  The problem occurs when the client outside the
      NAT tries to use the reflexive address supplied in the outgoing
      INVITE request to traverse media back through the 'Port/Address
      Dependent' NAT.
   o  A 'Port/Address Dependant' NAT has differing rules from the
      'Endpoint Independent' type of NAT (as defined in [11]).  For any
      internal IP address and port combination, data sent to a different
      external destination does not provide the same public mapping at
      the NAT.  In Figure 27 the STUN query produced a valid external
      mapping or receiving media.  This mapping, however, can only be
      used in the context of the original STUN request that was sent to
      the STUN server.  Any packets that attempt to use the mapped
      address, that does not come from the STUN server IP address and
      optionally port, will be dropped at the NAT.  Figure 27 shows the
      media being dropped at the NAT after (7) and before (8).  This
      then leads to one way audio.

4.2.2.2.  TURN Usage Solution

   As identified in Section 4.2.2.1, STUN provides a useful tool kit for
   the traversal of the majority of NATs but fails with Port/Address
   Dependant NAT.  This led to the development of the TURN usage
   solution [12] which uses the STUN toolkit.  It allows a client to
   request a relayed address at the STUN server rather than a reflexive
   representation.  This then introduces a media relay in the path for
   NAT traversal (as described in Section 3.2.3).  The following example
   explains how the TURN usage solves the previous failure when using
   STUN to traverse a 'Port/Address Dependant' type NAT.



       L        Port/Address Dependant     STUN                [..]
                         NAT              Server
       |                  |                  |                  |
       |(1) Alloc Req     |                  |                  |



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       |Src=L-PRIV-1      |                  |                  |
       |Dest=STUN-PUB-1   |                  |                  |
       |----------------->|                  |                  |
       |                  |                  |                  |
       |                  |(2) Alloc Req     |                  |
       |                  |Src=NAT-PUB-1     |                  |
       |                  |Dest=STUN-PUB-1   |                  |
       |                  |----------------->|                  |
       |                  |                  |                  |
       |                  |(3) Alloc Resp    |                  |
       |                  |<-----------------|                  |
       |                  |Src=STUN-PUB-1    |                  |
       |                  |Dest=NAT-PUB-1    |                  |
       |                  |Map=NAT-PUB-1     |                  |
       |                  |Rel=STUN-PUB-2    |                  |
       |                  |                  |                  |
       |(4) Alloc Resp    |                  |                  |
       |<-----------------|                  |                  |
       |Src=STUN-PUB-1    |                  |                  |
       |Dest=L-PRIV-1     |                  |                  |
       |Map=NAT-PUB-1     |                  |                  |
       |Rel=STUN-PUB-2    |                  |                  |
       |                  |                  |                  |
       |(5) Alloc Req     |                  |                  |
       |Src=L-PRIV-2      |                  |                  |
       |Dest=STUN-PUB-1   |                  |                  |
       |----------------->|                  |                  |
       |                  |                  |                  |
       |                  |(6) Alloc Req     |                  |
       |                  |Src=NAT-PUB-2     |                  |
       |                  |Dest=STUN-PUB-1   |                  |
       |                  |----------------->|                  |
       |                  |                  |                  |
       |                  |(7) Alloc Resp    |                  |
       |                  |<-----------------|                  |
       |                  |Src=STUN-PUB-1    |                  |
       |                  |Dest=NAT-PUB-2    |                  |
       |                  |Map=NAT-PUB-2     |                  |
       |                  |Rel=STUN-PUB-3    |                  |
       |                  |                  |                  |
       |(8) Alloc Resp    |                  |                  |
       |<-----------------|                  |                  |
       |Src=STUN-PUB-1    |                  |                  |
       |Dest=L-PRIV-2     |                  |                  |
       |Map=NAT-PUB-2     |                  |                  |
       |Rel=STUN-PUB-3    |                  |                  |
       |                  |                  |                  |
       |(9)SIP INVITE     |                  |                  |



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       |----------------->|                  |                  |
       |                  |                  |                  |
       |                  |(10)SIP INVITE    |                  |
       |                  |------------------------------------>|
       |                  |                  |                  |
       |                  |                  |(11)SIP 200 OK    |
       |                  |<------------------------------------|
       |                  |                  |                  |
       |(12)SIP 200 OK    |                  |                  |
       |<-----------------|                  |                  |
       |                  |                  |                  |
       |========================================================|
       |>>>>>>>>>>>>>Outgoing Media sent from L-PRIV-1>>>>>>>>>>|
       |========================================================|
       |                  |                  |                  |
       |                  |                  |==================|
       |                  |                  |<<<Media Sent to<<|
       |                  |                  |<<<<STUN-PUB-2<<<<|
       |                  |                  |==================|
       |                  |                  |                  |
       |=====================================|                  |
       |<Incoming Media Relayed to L-PRIV-1<<|                  |
       |=====================================|                  |
       |                  |                  |                  |
       |                  |                  |==================|
       |                  |                  |<<<RTCP Sent to<<>|
       |                  |                  |<<<<STUN-PUB-3<<<<|
       |                  |                  |==================|
       |                  |                  |                  |
       |=====================================|                  |
       |<<Incoming RTCP Relayed to L-PRIV-2<<|                  |
       |=====================================|                  |
       |                  |                  |                  |
       |(13)SIP ACK       |                  |                  |
       |----------------->|                  |                  |
       |                  |                  |                  |
       |                  |(14) SIP ACK      |                  |
       |                  |------------------------------------>|
       |                  |                  |                  |


   Figure 28: Port/Address Dependant NAT with TURN Usage - Success

   o  In this example, client 'L' issues a TURN allocate request(1) to
      obtained a relay address at the STUN server.  The request
      traverses through the 'Port/Address Dependant' NAT and reaches the
      STUN server (2).  The STUN server generates an Allocate response
      (3) that contains both a reflexive address (Map=NAT-PUB-1) of the



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      client and also a relayed address (Rel=STUN-PUB-2).  The relayed
      address maps to an address mapping on the STUN server which is
      bound to the public pin hole that has been opened on the NAT by
      the Allocate request.  This results in any traffic sent to the
      STUN server relayed address (Rel=STUN-PUB-2) being forwarded to
      the external representation of the pin hole created by the
      Allocate request(NAT-PUB-1).
   o  The TURN derived address (STUN-PUB-2) arrives back at the
      originating client(4) in an Allocate response.  This address can
      then be used in the SDP for the outgoing SIP INVITE request as
      shown in the following example (note that the example also
      includes client 'L' obtaining a second relay address for use in
      the RTCP attribute (5-8)):
   o

      v=0
      o=test 2890844342 2890842164 IN IP4 $L-PRIV-1
      c=IN IP4 $STUN-PUB-2.address
      t=0 0
      m=audio $STUN-PUB-2.port RTP/AVP 0
      a=rtcp:$STUN-PUB-3.port


   o  On receiving the INVITE request, the UAS is able to stream media
      and RTCP to the relay address (STUN-PUB-2 and STUN-PUB-3) at the
      STUN server.  As shown in Figure 28 (between messages (12) and
      (13), the media from the UAS is directed to the relayed address at
      the STUN server.  The STUN server then forwards the traffic to the
      open pin holes in the Port/Address Dependant NAT (NAT-PUB-1 and
      NAT-PUB-2).  The media traffic is then able to traverse the 'Port/
      Address Dependant' NAT and arrives back at client 'L'.
   o  The TURN usage of STUN on its own will work for 'Port/Address
      Dependent' and other types of NAT mentioned in this specification
      but should only be used as a last resort.  The relaying of media
      through an external entity is not an efficient mechanism for NAT
      traversal and comes at a high processing cost.

4.2.2.3.  ICE Solution

   The previous two examples have highlighted the problem with using
   core STUN usage for all forms of NAT traversal and a solution using
   TURN usage for the Port/Address Dependant NAT case.  As mentioned
   previously in this document, the RECOMMENDED mechanism for traversing
   all varieties of NAT is using ICE, as detailed in Section 3.2.4.  ICE
   makes use of core STUN, TURN usage and any other UNSAF[9] compliant
   protocol to provide a list of prioritised addresses that can be used
   for media traffic.  Detailed examples of ICE can be found in
   Section 4.2.1.2.1.  These examples are associated with an 'Endpoint



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   Independent' type NAT but can be applied to any NAT type variation,
   including 'Port/Address Dependant' type NAT.  The procedures are the
   same and of the list of candidate addresses, a client will choose
   where to send media dependant on the results of the STUN connectivity
   checks on each candidate address and the associated priority (highest
   priority wins).  For more information see the core ICE
   specification[16]

   [Editors Note: TODO - a detailed example will be included here which
   includes promotion of a TURN relayed address to the active candidate
   to traverse a 'Port/Address Dependent' type NAT.]

4.3.  Address independent Port Restricted NAT --> Address independent
      Port Restricted NAT traversal

   [Editors Note: TODO - a detailed example will be included where User
   A and B are both behind Address Independent NATs that have Port
   restricted properties.  This means that the stun-derived addresses
   will work, but each side must send a 'suicide' or 'primer' STUN
   packet that creates a permission in the NAT.  So, the main thing to
   show here is how the first packet from B to A will create a
   permission in B's NAT but gets dropped at A. When A gets the answer
   it starts its STUN checks and the packet from A to B creates a
   permission in A's NAT and gets through B's NAT because of the
   previously installed permission.  This now triggers B to resend its
   stun request which now works.]

4.4.  Internal TURN Usage (Enterprise Deployment)

   [Editors Note: TODO - a detailed example will be included for User A
   and User B. User A is in an enterprise, which has a address and port
   restricted NAT.  User B is on the public internet.  There is a TURN+
   STUN server deployed INSIDE the enterprise NAT.  The NAT has a static
   set of ports forwarded to the internal TURN server (say, 100 ports).
   The TURN server is configured with those ports.  So, when user A
   talks to the TURN server it gets an address and port on the *public*
   side of the NAT, with a preconfigured port forwarding rule.  Indeed,
   the client is configured with two TURN servers.  Both are physically
   the same TURN server.  However, when talking to one instance the
   client gets the public address.  When talking to the other instance
   it gets a private address inside the NAT.  The ice process ends up
   selecting the public address given out by the TURN server usage.]


5.  Intercepting Intermediary (B2BUA)

   [Editors Note: TODO - a detailed example demonstrating how a B2BUA
   can obtain STUN/TURN addresses for the purpose of allocating to



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   Clients.  This example shows how intermediaries can control the flow
   of media without having to directly access SDP on the signalling
   plane.


6.  IPv4-IPv6 Transition

   This section describes how IPv6-only SIP user agents can communicate
   with IPv4-only SIP user agents.

6.1.  IPv4-IPv6 Transition for SIP Signalling

   IPv4-IPv6 translations at the SIP level usually take place at dual-
   stack proxies that have both IPv4 and IPv6 DNS entries.  Since this
   translations do not involve NATs that are placed in the middle of two
   SIP entities, they fall outside the scope of this document.  A
   detailed description of this type of translation can be found in [19]

6.2.  IPv4-IPv6 Transition for Media

   Figure 30 shows a network of IPv6 SIP user agents that has a relay
   with a pool of public IPv4 addresses.  The IPv6 SIP user agents of
   this IPv6 network need to communicate with users on the IPv4
   Internet.  To do so, the IPv6 SIP user agents use TURN to obtain a
   public IPv4 address from the relay.  The mechanism that an IPv6 SIP
   user agent follows to obtain a public IPv4 address from a relay using
   TURN is the same as the one followed by a user agent with a private
   IPv4 address to obtain a public IPv4 address.  The example in
   Figure 31 explains how to use TURN to obtain an IPv4 address and how
   to use the ANAT semantics [17] of the SDP grouping framework [8] to
   provide both IPv4 and IPv6 addresses for a particular media stream.




















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                             +----------+
                             |   /  \   |
                                /SIP \
                               /Phone \
                              /        \
                             ------------

           IPv4 Network
                             192.0.2.0/8
                             +---------+
                             |         |
       ----------------------|   NAT   |--------------------------
                             |         |
                             +---------+
           IPv6 Network

                                     ++
                                     ||
                               +-----++
                               | IPv6 |
                               | SIP  |
                               | user |
                               | agent|
                               +------+


   Figure 30: IPv6-IPv4 transition scenario
























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    IPv6 SIP                               TURN              IPv4 SIP
   User Agent                             Server            User Agent
       |                                     |                  |
       |         (1) TURN Allocate           |                  |
       |         src=[2001:DB8::1]:30000     |                  |
       |------------------------------------>|                  |
       |         (2) TURN Resp               |                  |
       |         map=192.0.2.2:25000         |                  |
       |         dest=[2001:DB8::1]:30000    |                  |
       |<------------------------------------|                  |
       |         (3) SIP INVITE              |                  |
       |------------------------------------------------------->|
       |         (4) SIP 200 OK              |                  |
       |<-------------------------------------------------------|
       |                                     |                  |
       |=====================================|                  |
       |>>>>>>>>>> Outgoing Media >>>>>>>>>>>|                  |
       |=====================================|                  |
       |                                     |==================|
       |                                     |>>>>>> Media >>>>>|
       |                                     |==================|
       |                                     |                  |
       |                                     |==================|
       |                                     |<<<<<< Media <<<<<|
       |                                     |==================|
       |=====================================|                  |
       |<<<<<<<<<< Outgoing Media <<<<<<<<<<<|                  |
       |=====================================|                  |
       |                                     |                  |
       |         (5) SIP ACK                 |                  |
       |------------------------------------------------------->|
       |                                     |                  |


   Figure 31: IPv6-IPv4 translation with TURN

   o  The IPv6 SIP user agent obtains a TURN-derived IPv4 address by
      issuing a TURN allocate request (1).  The TURN server generates a
      response that contains the public IPv4 address.  This IPv4 address
      maps to the IPv6 source address of the TURN allocate request,
      which the IPv6 address of the SIP user agent.  This results in any
      traffic being sent to the IPv4 address provided by TURN server
      (192.0.2.2:25000) will be redirected to the IPv6 address of the
      SIP user agent ([2001:DB8::1]:30000).
   o  The TURN-derived address (192.0.2.2:25000) arrives back at the
      originating user agent (2).  This address can then be used in the
      SDP for the outgoing SIP INVITE request.  The user agent builds
      two media lines, one with its IPv6 address and the other with the



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      IPv4 address that was just obtained.  The user agent groups both
      media lines using the ANAT semantics as shown below (note that the
      RTCP attribute in the IPv4 media line would have been obtained by
      another TURN-derived address which is not shown in the call flow
      for simplicity).


      v=0
      o=test 2890844342 2890842164 IN IP6 2001:DB8::1
      t=0 0
      a=group:ANAT 1 2
      m=audio 20000 RTP/AVP 0
      c=IN IP6 2001:DB8::1
      a=mid:1
      m=audio 25000 RTP/AVP 0
      c=IN IP4 192.0.2.2
      a=rtcp:25001
      a=mid:2

   o  On receiving the INVITE request, the user agent server rejects the
      IPv6 media line by setting its port to zero in the answer and
      starts sending media to the IPv4 address in the offer.  The IPv6
      user agent sends media through the relay as well, as shown in
      Figure 31.


7.  ICE with RTP/TCP

   [Editors Note: TODO - a detailed example will be included on using
   ICE with RTP/TCP - as define in [18]


8.  Acknowledgments

   The authors would like to thank the members of the IETF SIPPING WG
   for their comments and suggestions.  Detailed comments were provided
   by Francois Audet, kaiduan xie and Hans Persson.


9.  References

9.1.  Normative References

   [1]   Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
         Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
         Session Initiation Protocol", RFC 3261, June 2002.

   [2]   Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,



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         "RTP: A Transport Protocol for Real-Time Applications",
         RFC 1889, January 1996.

   [3]   Handley, M. and V. Jacobson, "SDP: Session Description
         Protocol", RFC 2327, April 1998.

   [4]   Tsirtsis, G. and P. Srisuresh, "Network Address Translation -
         Protocol Translation (NAT-PT)", RFC 2766, February 2000.

   [5]   Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
         Session Description Protocol (SDP)", RFC 3264, June 2002.

   [6]   Rosenberg, J. and H. Schulzrinne, "An Extension to the Session
         Initiation Protocol (SIP) for Symmetric Response Routing",
         RFC 3581, August 2003.

   [7]   Willis, D. and B. Hoeneisen, "Session Initiation Protocol (SIP)
         Extension Header Field for Registering Non-Adjacent Contacts",
         RFC 3327, December 2002.

   [8]   Camarillo, G., Eriksson, G., Holler, J., and H. Schulzrinne,
         "Grouping of Media Lines in the Session Description Protocol
         (SDP)", RFC 3388, December 2002.

   [9]   Daigle, L. and IAB, "IAB Considerations for UNilateral Self-
         Address Fixing (UNSAF) Across Network Address Translation",
         RFC 3424, November 2002.

   [10]  Rosenberg, J., "Simple Traversal of UDP Through Network Address
         Translators (NAT) (STUN)", draft-ietf-behave-rfc3489bis-03
         (work in progress), March 2006.

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

   [12]  Rosenberg, J., "Obtaining Relay Addresses from Simple Traversal
         of UDP Through NAT (STUN)", draft-ietf-behave-turn-00 (work in
         progress), March 2006.

   [13]  Jennings, C. and A. Hawrylyshen, "SIP Conventions for UAs with
         Outbound Only Connections", draft-jennings-sipping-outbound-01
         (work in progress), February 2005.

   [14]  Rosenberg, J., "Obtaining and Using Globally Routable User
         Agent (UA) URIs (GRUU) in the  Session Initiation Protocol
         (SIP)", draft-ietf-sip-gruu-09 (work in progress), June 2006.




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   [15]  Wing, D., "Symmetric RTP and RTCP Considered Helpful",
         draft-wing-mmusic-symmetric-rtprtcp-01 (work in progress),
         October 2004.

   [16]  Rosenberg, J., "Interactive Connectivity Establishment (ICE): A
         Methodology for Network  Address Translator (NAT) Traversal for
         Offer/Answer Protocols", draft-ietf-mmusic-ice-08 (work in
         progress), March 2006.

   [17]  Camarillo, G., "The Alternative Network Address Types Semantics
         (ANAT) for theSession  Description Protocol (SDP) Grouping
         Framework", draft-ietf-mmusic-anat-02 (work in progress),
         October 2004.

   [18]  Rosenberg, J., "TCP Candidates with Interactive Connectivity
         Establishment (ICE)", draft-ietf-mmusic-ice-tcp-00 (work in
         progress), March 2006.

9.2.  Informative References

   [19]  Camarillo, G., "IPv6 Transcition in the Session Initiation
         Protocol (SIP)", draft-camarillo-sipping-v6-transition-00 (work
         in progress), February 2005.




























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Authors' Addresses

   Chris Boulton
   Ubiquity Software Corporation
   Eastern Business Park
   St Mellons
   Cardiff, South Wales  CF3 5EA

   Email: cboulton@ubiquitysoftware.com


   Jonathan Rosenberg
   Cisco Systems
   600 Lanidex Plaza
   Parsippany, NJ  07054

   Email: jdrosen@cisco.com


   Gonzalo Camarillo
   Ericsson
   Hirsalantie 11
   Jorvas  02420
   Finland

   Email: Gonzalo.Camarillo@ericsson.com

























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