Network Working Group                                       J. Rosenberg
Request for Comments: 3581                                   dynamicsoft
Category: Standards Track                                 H. Schulzrinne
                                                     Columbia University
                                                             August 2003


       An Extension to the Session Initiation Protocol (SIP) for
                       Symmetric Response Routing

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2003).  All Rights Reserved.

Abstract

   The Session Initiation Protocol (SIP) operates over UDP and TCP,
   among others.  When used with UDP, responses to requests are returned
   to the source address the request came from, and to the port written
   into the topmost Via header field value of the request.  This
   behavior is not desirable in many cases, most notably, when the
   client is behind a Network Address Translator (NAT).  This extension
   defines a new parameter for the Via header field, called "rport",
   that allows a client to request that the server send the response
   back to the source IP address and port from which the request
   originated.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Client Behavior  . . . . . . . . . . . . . . . . . . . . . . .  3
   4.  Server Behavior  . . . . . . . . . . . . . . . . . . . . . . .  4
   5.  Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . .  5
   6.  Example  . . . . . . . . . . . . . . . . . . . . . . . . . . .  5
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . .  6
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  6
   9.  IAB Considerations . . . . . . . . . . . . . . . . . . . . . .  6
       9.1.  Problem Definition . . . . . . . . . . . . . . . . . . .  8
       9.2.  Exit Strategy  . . . . . . . . . . . . . . . . . . . . .  8
       9.3.  Brittleness Introduced by this Specification . . . . . .  9
       9.4.  Requirements for a Long Term Solution  . . . . . . . . . 10
       9.5.  Issues with Existing NAPT Boxes  . . . . . . . . . . . . 10
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
       11.1. Normative References . . . . . . . . . . . . . . . . . . 11
       11.2. Informative References . . . . . . . . . . . . . . . . . 11
   12. Intellectual Property and Copyright Statements . . . . . . . . 11
   13. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 12
   14. Full Copyright Statement . . . . . . . . . . . . . . . . . . . 13

1.  Introduction

   The Session Initiation Protocol (SIP) [1] operates over UDP and TCP.
   When used with UDP, responses to requests are returned to the source
   address the request came from, and to the port written into the
   topmost Via header field value of the request.  This results in a
   "hybrid" way of computing the destination of the response.  Half of
   the information (specifically, the IP address) is taken from the IP
   packet headers, and the other half (specifically, the port) from the
   SIP message headers.  SIP operates in this manner so that a server
   can listen for all messages, both requests and responses, on a single
   IP address and port.  This helps improve scalability.  However, this
   behavior is not desirable in many cases, most notably, when the
   client is behind a NAT.  In that case, the response will not properly
   traverse the NAT, since it will not match the binding established
   with the request.

   Furthermore, there is currently no way for a client to examine a
   response and determine the source port that the server saw in the
   corresponding request.  Currently, SIP provides the client with the
   source IP address that the server saw in the request, but not the
   port.  The source IP address is conveyed in the "received" parameter
   in the topmost Via header field value of the response.  This
   information has proved useful for basic NAT traversal, debugging

   purposes, and support of multi-homed hosts.  However, it is
   incomplete without the port information.

   This extension defines a new parameter for the Via header field,
   called "rport", that allows a client to request that the server send
   the response back to the source IP address and port where the request
   came from.  The "rport" parameter is analogous to the "received"
   parameter, except "rport" contains a port number, not the IP address.

2.  Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
   and "OPTIONAL" are to be interpreted as described in BCP 14, RFC 2119
   [2] and indicate requirement levels for compliant implementations.

3.  Client Behavior

   The client behavior specified here affects the transport processing
   defined in Section 18.1 of SIP (RFC 3261) [1].

   A client, compliant to this specification (clients include UACs and
   proxies), MAY include an "rport" parameter in the top Via header
   field value of requests it generates.  This parameter MUST have no
   value; it serves as a flag to indicate to the server that this
   extension is supported and requested for the transaction.

   When the client sends the request, if the request is sent using UDP,
   the client MUST be prepared to receive the response on the same IP
   address and port it used to populate the source IP address and source
   port of the request.  For backwards compatibility, the client MUST
   still be prepared to receive a response on the port indicated in the
   sent-by field of the topmost Via header field value, as specified in
   Section 18.1.1 of SIP [1].

   When there is a NAT between the client and server, the request will
   create (or refresh) a binding in the NAT.  This binding must remain
   in existence for the duration of the transaction in order for the
   client to receive the response.  Most UDP NAT bindings appear to have
   a timeout of about one minute.  This exceeds the duration of non-
   INVITE transactions.  Therefore, responses to a non-INVITE request
   will be received while the binding is still in existence.  INVITE
   transactions can take an arbitrarily long amount of time to complete.
   As a result, the binding may expire before a final response is
   received.  To keep the binding fresh, the client SHOULD retransmit
   its INVITE every 20 seconds or so.  These retransmissions will need
   to take place even after receiving a provisional response.

   A UA MAY execute the binding lifetime discovery algorithm in Section
   10.2 of RFC 3489 [4] to determine the actual binding lifetime in the
   NAT.  If it is longer than 1 minute, the client SHOULD increase the
   interval for request retransmissions up to half of the discovered
   lifetime.  If it is shorter than one minute, it SHOULD decrease the
   interval for request retransmissions to half of the discovered
   lifetime.  Note that discovery of binding lifetimes can be
   unreliable.  See Section 14.3 of RFC 3489 [4].

4.  Server Behavior

   The server behavior specified here affects the transport processing
   defined in Section 18.2 of SIP [1].

   When a server compliant to this specification (which can be a proxy
   or UAS) receives a request, it examines the topmost Via header field
   value.  If this Via header field value contains an "rport" parameter
   with no value, it MUST set the value of the parameter to the source
   port of the request.  This is analogous to the way in which a server
   will insert the "received" parameter into the topmost Via header
   field value.  In fact, the server MUST insert a "received" parameter
   containing the source IP address that the request came from, even if
   it is identical to the value of the "sent-by" component.  Note that
   this processing takes place independent of the transport protocol.

   When a server attempts to send a response, it examines the topmost
   Via header field value of that response.  If the "sent-protocol"
   component indicates an unreliable unicast transport protocol, such as
   UDP, and there is no "maddr" parameter, but there is both a
   "received" parameter and an "rport" parameter, the response MUST be
   sent to the IP address listed in the "received" parameter, and the
   port in the "rport" parameter.  The response MUST be sent from the
   same address and port that the corresponding request was received on.
   This effectively adds a new processing step between bullets two and
   three in Section 18.2.2 of SIP [1].

   The response must be sent from the same address and port that the
   request was received on in order to traverse symmetric NATs.  When a
   server is listening for requests on multiple ports or interfaces, it
   will need to remember the one on which the request was received.  For
   a stateful proxy, storing this information for the duration of the
   transaction is not an issue.  However, a stateless proxy does not
   store state between a request and its response, and therefore cannot
   remember the address and port on which a request was received.  To
   properly implement this specification, a stateless proxy can encode
   the destination address and port of a request into the Via header
   field value that it inserts.  When the response arrives, it can
   extract this information and use it to forward the response.

5.  Syntax

   The syntax for the "rport" parameter is:

   response-port = "rport" [EQUAL 1*DIGIT]

   This extends the existing definition of the Via header field
   parameters, so that its BNF now looks like:

   via-params        =  via-ttl / via-maddr
                        / via-received / via-branch
                        / response-port / via-extension

6.  Example

   A client sends an INVITE to a proxy server which looks like, in part:

   INVITE sip:user@example.com SIP/2.0
   Via: SIP/2.0/UDP 10.1.1.1:4540;rport;branch=z9hG4bKkjshdyff

   This INVITE is sent with a source port of 4540 and a source IP
   address of 10.1.1.1.  The proxy is at 192.0.2.2 (proxy.example.com),
   listening on both port 5060 and 5070.  The client sends the request
   to port 5060.  The request passes through a NAT on the way to the
   proxy, so that the source IP address appears as 192.0.2.1 and the
   source port as 9988.  The proxy forwards the request, but not before
   appending a value to the "rport" parameter in the proxied request:

   INVITE sip:user@example.com SIP/2.0
   Via: SIP/2.0/UDP proxy.example.com;branch=z9hG4bKkjsh77
   Via: SIP/2.0/UDP 10.1.1.1:4540;received=192.0.2.1;rport=9988
    ;branch=z9hG4bKkjshdyff

   This request generates a response which arrives at the proxy:

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP proxy.example.com;branch=z9hG4bKkjsh77
   Via: SIP/2.0/UDP 10.1.1.1:4540;received=192.0.2.1;rport=9988
    ;branch=z9hG4bKkjshdyff

   The proxy strips its top Via header field value, and then examines
   the next one.  It contains both a "received" parameter and an "rport"
   parameter.  The server follows the rules specified in Section 4 and
   sends the response to IP address 192.0.2.1, port 9988, and sends it
   from port 5060 on 192.0.2.2:

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP 10.1.1.1:4540;received=192.0.2.1;rport=9988
    ;branch=z9hG4bKkjshdyff

   This packet matches the binding created by the initial request.
   Therefore, the NAT rewrites the destination address of this packet
   back to 10.1.1.1, and the destination port back to 4540.  It forwards
   this response to the client, which is listening for the response on
   that address and port.  The client properly receives the response.

7.  Security Considerations

   When a server uses this specification, responses that it sends will
   now include the source port where the request came from.  In some
   instances, the source address and port of a request are sensitive
   information.  If they are sensitive, requests SHOULD be protected by
   using SIP over TLS [1].  In such a case, this specification does not
   provide any response routing functions (as these only work with TCP);
   it merely provides the client with information about the source port
   as seen by the server.

   It is possible that an attacker might try to disrupt service to a
   client by acting as a man-in-the-middle, modifying the "rport"
   parameter in a Via header in a request sent by a client.  Removal of
   this parameter will prevent clients from behind NATs from receiving
   service.  The addition of the parameter will generally have no
   impact.  Of course, if an attacker is capable of launching a man-in-
   the-middle attack, there are many other ways of denying service, such
   as merely discarding the request.  Therefore, this attack does not
   seem significant.

8.  IANA Considerations

   There are no IANA considerations associated with this specification.

9.  IAB Considerations

   The IAB has studied a class of protocols referred to as Unilateral
   Self Address Fixing (UNSAF) protocols [5].  These protocols allow a
   client behind a NAT to learn the IP address and port that a NAT will
   allocate for a particular request, in order to use this information
   in application layer protocols.  An example of an UNSAF protocol is
   the Simple Traversal of UDP Through NATs (STUN) [4].

   Any protocol is an UNSAF protocol if it reveals, to a client, the
   source IP address and port of a packet sent through that NAT.
   Although not designed for that purpose, this specification can be
   used as an UNSAF protocol.  Using the "rport" parameter (defined
   here) and the "received" parameter (defined in RFC 3261 [1]) in the
   topmost Via header field value of a response, a client sending a
   request can learn its address as it was seen by the server which sent
   the response.

   There are two uses of this information.  The first is for
   registrations.  Consider a client behind a NAT wishing to register
   with a proxy/registrar on the other side of the NAT.  The client must
   provide, in its registration, the address at which it should receive
   incoming SIP requests from the proxy.  However, since the client is
   located behind a NAT, none of the addresses on any of its interfaces
   will be reachable from the proxy.  If the client can provide the
   proxy with an address that the proxy can reach, the client can
   receive incoming requests.  Using this specification, a client behind
   a NAT can learn its address and port as seen by the proxy which
   receives a REGISTER request.  The client can then perform an
   additional registration, using this address in a Contact header.
   This would allow a client to receive incoming requests, such as
   INVITE, on the IP address and port it used to populate the source IP
   address and port of the registration it sent.  This approach will
   only work when servers send requests to a UA from the same address
   and port on which the REGISTER itself was received.

   In many cases, the server to whom the registration is sent won't be
   the registrar itself, but rather a proxy which then sends the request
   to the registrar.  In such a case, any incoming requests for the
   client must traverse the proxy to whom the registration was directly
   sent.  The Path header extension to SIP [3] allows the proxy to
   indicate that it must be on the path of such requests.

   The second usage is for record routing, to address the same problem
   as above, but between two proxies.  A proxy behind a NAT which
   forwards a request to a server can use OPTIONS, for example, to learn
   its address as seen by that server.  This address can be placed into
   the Record-Route header field of requests sent to that server.  This
   would allow the proxy to receive requests from that server on the
   same IP address and port it used to populate the source IP address
   and port of the OPTIONS request.

   Because of this potential usage, this document must consider the
   issues raised in [5].

9.1.  Problem Definition

   From [5], any UNSAF proposal must provide:

      Precise definition of a specific, limited-scope problem that is to
      be solved with the UNSAF proposal.  A short term fix should not be
      generalized to solve other problems; this is why "short term fixes
      usually aren't".

   This specification is primarily aimed at allowing SIP responses to be
   received when a request is sent through a NAT.  In this primary
   application, this specification is not an UNSAF proposal.  However,
   as a side effect of this capability, this specification can be used
   as an UNSAF protocol.  In that usage, it would address two issues:

   o  Provide a client with an address that it could use in the Contact
      header of a REGISTER request when it is behind a NAT.

   o  Provide a proxy with an address that it could use in a Record-
      Route header in a request, when it is behind a NAT.

9.2.  Exit Strategy

   From [5], any UNSAF proposal must provide:

      Description of an exit strategy/transition plan.  The better short
      term fixes are the ones that will naturally see less and less use
      as the appropriate technology is deployed.

   The SIP working group has recognized that the usage of this
   specification to support registrations and record-routing through
   NATs is not appropriate.  It has a number of known problems which are
   documented below.  The way to eliminate potential usage of this
   specification for address fixing is to provide a proper solution to
   the problems that might motivate the usage of this specification for
   address fixing.  Specifically, appropriate solutions for
   registrations and record-routing in the presence of NATs need to be
   developed.  These solutions would not rely on address fixing.

   Requirements for such solutions are already under development [6].

   Implementors of this specification are encouraged to follow this work
   for better solutions for registrations and record-routing through
   NAT.

9.3.  Brittleness Introduced by this Specification

   From [5], any UNSAF proposal must provide:

      Discussion of specific issues that may render systems more
      "brittle".  For example, approaches that involve using data at
      multiple network layers create more dependencies, increase
      debugging challenges, and make it harder to transition.

   This specification, if used for address fixing, introduces several
   points of brittleness into a SIP system:

   o  If used for UDP registrations, a client will need to frequently
      re-register in order to keep the NAT bindings fresh.  In many
      cases, these registrations will need to take place nearly one
      hundred times more frequently than the typical refresh interval of
      a registration.  This introduces load into the system and hampers
      scalability.

   o  A client cannot accurately determine the binding lifetime of a NAT
      it is registering (or record-routing) through.  Therefore, there
      may be periods of unreachability that occur between the time a
      binding expires and the next registration or OPTIONS refresh is
      sent.  This may result in missed calls, messages, or other
      information.

   o  If the NAT is of the symmetric variety [4], a client will only be
      able to use its address to receive requests from the server it has
      sent the request to.  If that server is one of many servers in a
      cluster, the client may not be able to receive requests from other
      servers in the cluster.  This may result in missed calls,
      messages, or other information.

   o  If the NAT is of the symmetric variety [4], a client will only be
      able to use its address to receive requests if the server sends
      requests to the client from the same address and port the server
      received the registrations on.  This server behavior is not
      mandated by RFC 3261 [1], although it appears to be common in
      practice.

   o  If the registrar and the server to whom the client sent its
      REGISTER request are not the same, the approach will only work if
      the server uses the Path header field [3].  There is not an easy
      and reliable way for the server to determine that the Path header
      should be used for a registration.  Using Path when the address in
      the topmost Via header field is a private address will usually
      work, but may result in usage of Path when it is not actually
      needed.

9.4.  Requirements for a Long Term Solution

   From [5], any UNSAF proposal must provide:

      Identify requirements for longer term, sound technical solutions
      -- contribute to the process of finding the right longer term
      solution.

   The brittleness described in Section 9.3 has led us to the following
   requirements for a long term solution:

   The client should not need to specify its address.  Registrations and
      record routing require the client to specify the address at which
      it should receive requests.  A sound technical solution should
      allow a client to explicitly specify that it wants to receive
      incoming requests on the connection over which the outgoing
      request was sent.  In this way, the client does not need to
      specify its address.

   The solution must deal with clusters of servers.  In many
      commercially deployed SIP systems, there will be multiple servers,
      each at different addresses and ports, handling incoming requests
      for a client.  The solution must explicitly consider this case.

   The solution must not require increases in network load.  There
      cannot be a penalty for a sound technical solution.

9.5.  Issues with Existing NAPT Boxes

   From [5], any UNSAF proposal must provide:

      Discussion of the impact of the noted practical issues with
      existing, deployed NA[P]Ts and experience reports.

   To our knowledge, at the time of writing, there is only very limited
   usage of this specification for address fixing.  Therefore, no
   specific practical issues have been raised.

10.  Acknowledgements

   The authors would like to thank Rohan Mahy and Allison Mankin for
   their comments and contributions to this work.

11.  References

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

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

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

11.2.  Informative References

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

   [6] Mahy, R., "Requirements for Connection Reuse in the Session
       Initiation Protocol (SIP)", Work in Progress.

12.  Intellectual Property Statement

   The IETF takes no position regarding the validity or scope of any
   intellectual property or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; neither does it represent that it
   has made any effort to identify any such rights.  Information on the
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   The IETF invites any interested party to bring to its attention any
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13.  Authors' Addresses

   Jonathan Rosenberg
   dynamicsoft
   600 Lanidex Plaza
   Parsippany, NJ  07054
   US

   Phone: +1 973 952-5000
   EMail: jdrosen@dynamicsoft.com
   URI:   http://www.jdrosen.net


   Henning Schulzrinne
   Columbia University
   M/S 0401
   1214 Amsterdam Ave.
   New York, NY  10027
   US

   EMail: schulzrinne@cs.columbia.edu
   URI:   http://www.cs.columbia.edu/~hgs

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