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IETF RFC 3581
An Extension to the Session Initiation Protocol (SIP) for Symmetric Response Routing
Last modified on Friday, August 22nd, 2003
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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 © 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.
Rosenberg & Schulzrinne Standards Track PAGE 1 
RFC 3581 Symmetric Response Routing August 2003
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
Rosenberg & Schulzrinne Standards Track PAGE 2
RFC 3581 Symmetric Response Routing August 2003
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.
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RFC 3581 Symmetric Response Routing August 2003
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.
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RFC 3581 Symmetric Response Routing August 2003
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:
Rosenberg & Schulzrinne Standards Track PAGE 5
RFC 3581 Symmetric Response Routing August 2003
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].
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RFC 3581 Symmetric Response Routing August 2003
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].
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RFC 3581 Symmetric Response Routing August 2003
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.
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RFC 3581 Symmetric Response Routing August 2003
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.
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RFC 3581 Symmetric Response Routing August 2003
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.
Rosenberg & Schulzrinne Standards Track PAGE 10
RFC 3581 Symmetric Response Routing August 2003
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
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances of
licenses to be made available, or the result of an attempt made to
obtain a general license or permission for the use of such
proprietary rights by implementors or users of this specification can
be obtained from the IETF Secretariat.
Rosenberg & Schulzrinne Standards Track PAGE 11
RFC 3581 Symmetric Response Routing August 2003
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.
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
Rosenberg & Schulzrinne Standards Track PAGE 12
RFC 3581 Symmetric Response Routing August 2003
14. Full Copyright Statement
Copyright © The Internet Society (2003). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assignees.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Rosenberg & Schulzrinne Standards Track PAGE 13
An Extension to the Session Initiation Protocol (SIP) for Symmetric Response Routing
RFC TOTAL SIZE: 29121 bytes
PUBLICATION DATE: Friday, August 22nd, 2003
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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