Internet Engineering Task Force SIP WG
Internet Draft J. Rosenberg
dynamicsoft
J. Weinberger
dynamicsoft
H. Schulzrinne
Columbia U.
draft-ietf-sip-nat-02.txt
July 1, 2002
Expires: January 2003
An Extension to the Session Initiation Protocol (SIP)
for Symmetric Response Routing
STATUS OF THIS MEMO
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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Abstract
The Session Initiation Protocol (SIP) operates over UDP and TCP. When
used with UDP, responses to requests are returned to the source
address the request came from, but from the port written into the
topmost Via header of the request. This behavior is not desirable in
many cases, most notably, when the client is behind a NAT. This
extension defines a new parameter for the Via header, 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.
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Internet Draft Response Routing July 1, 2002
Table of Contents
1 Introduction ........................................ 3
2 Terminology ......................................... 3
3 Client Behavior ..................................... 3
4 Server Behavior ..................................... 4
5 Syntax .............................................. 4
6 Example ............................................. 5
7 Security Considerations ............................. 6
8 IANA Considerations ................................. 6
9 Acknowledgements .................................... 6
10 Author's Addresses .................................. 7
11 Normative References ................................ 7
12 Informative References .............................. 7
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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, but from the port written into the
topmost Via header of the request. 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.
Related to this, there is currently no way in SIP for a client to
learn, from a response to its request, the source port that the
server saw in the request. Currently, SIP does provide the client
with the source IP address that the server saw in the request. This
information is conveyed in the received parameter in the topmost Via
header 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, 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 analagous to the received
parameter, except rport contains a port number, not the IP address.
2 Terminology
In this document, the key words "MUST", "MUSTNOT", "REQUIRED",
"SHALL", "SHALLNOT", "SHOULD", "SHOULDNOT", "RECOMMENDED", "MAY", and
"OPTIONAL" are to be interpreted as described in RFC 2119 [2] and
indicate requirement levels for compliant SIP implementations.
3 Client Behavior
The client behavior specified here affects the transport processing
defined in Section 18.1 of SIP [1].
A client compliant to this specification (clients include UACs and
proxies) MAY include an rport parameter in the top Via header 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
socket the request was sent on. Specifically, it MUST be prepared to
receive the response on the same IP address and port present in the
source IP address and source port of the request. For backwards
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Internet Draft Response Routing July 1, 2002
compatibility, the client MUST still be prepared to receive a
response on the port indicated in the sent-by field of the topmost
Via, as specified in Section 18.1.1 of SIP [1].
In the case where there is a NAT between the client and server, in
order for the response to always be received, the NAT binding must
remain in existence for the duration of the transaction. Most UDP NAT
bindings appear to have a timeout of about one minute. Therefore,
non-INVITE transactions will have no problem. For INVITE
transactions, the client may need to retransmit its INVITE every 20
seconds or so, even after receiving a provisional response, in order
to keep the binding open to receive the final response.
OPEN ISSUE: This is awful. Perhaps a further sign that the
only real answer is TCP? Or, perhaps this belongs in
sipping-nat-scenarios, not here.
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. If
this Via header contains an rport parameter with no value, it MUST
insert the port the request was received from as the value of this
parameter. This is analagous to the way in which a server will insert
the receieved parameter with the source IP address the request was
received from. 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 field. Note that this
processing takes place independent of the transport protocol.
When a server attempts to send a response over an unreliable unicast
transport, such as UDP, and there is no Via maddr parameter present,
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. This effectively adds
a new processing step between bullets two and three in Section 18.2.2
of SIP [1].
5 Syntax
The syntax for the rport parameter is:
response-port = "rport" [EQUAL 1*DIGIT]
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This extends the existing definition of the Via header parameters, so
that its BNF now looks like:
via-params = via-ttl / via-maddr
/ via-received / via-branch
/ response-port / via-extension
6 Example
Consider an example. A client sends an INVITE which looks like:
INVITE sip:user@domain SIP/2.0
Via: SIP/2.0/UDP 10.1.1.1:4540;rport
This INVITE is sent with a source port of 4540 and source IP address
of 10.1.1.1. The request is natted, so that the source IP appears as
68.44.20.1 and the source port as 9988. This is received at a proxy.
The proxy forwards the request, but not before appending a value to
the rport parameter in the proxied request:
INVITE sip:user@domain2 SIP/2.0
Via: SIP/2.0/UDP proxy.domain.com
Via: SIP/2.0/UDP 10.1.1.1:4540;received=68.44.20.1;rport=9988
This request generates a response, which arrives at the proxy:
SIP/2.0 200 OK
Via: SIP/2.0/UDP proxy.domain.com
Via: SIP/2.0/UDP 10.1.1.1:4540;received=68.44.20.1;rport=9988
The proxy strips its top Via, and then examines the next one. It
contains both a received param, and an rport. The result is that the
following response is sent to IP address 68.44.20.1, port 9988:
SIP/2.0 200 OK
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Internet Draft Response Routing July 1, 2002
Via: SIP/2.0/UDP 10.1.1.1:4540;received=68.44.20.1;rport=9988
The NAT rewrites the destination address of this packet back to IP
10.1.1.1, port 4540, and is received by the client.
7 Security Considerations
Since this extension merely adds source port information to the
source address information already present in SIP, it does not appear
to add any additional security considerations.
8 IANA Considerations
There are no IANA Considerations associated with this specification.
9 Acknowledgements
The authors would like to thank Rohan Mahy for his comments and
contributions to this work.
Full Copyright Statement
Copyright (c) The Internet Society (2002). 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 assigns.
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
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Internet Draft Response Routing July 1, 2002
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
10 Author's Addresses
Jonathan Rosenberg
dynamicsoft
72 Eagle Rock Avenue
First Floor
East Hanover, NJ 07936
email: jdrosen@dynamicsoft.com
Joel Weinberger
dynamicsoft
72 Eagle Rock Avenue
First Floor
East Hanover, NJ 07936
email: jweinberger@dynamicsoft.com
Henning Schulzrinne
Columbia University
M/S 0401
1214 Amsterdam Ave.
New York, NY 10027-7003
email: schulzrinne@cs.columbia.edu
11 Normative References
[1] J. Rosenberg, H. Schulzrinne, et al. , "SIP: Session initiation
protocol," Internet Draft, Internet Engineering Task Force, Feb.
2002. Work in progress.
[2] S. Bradner, "Key words for use in RFCs to indicate requirement
levels," RFC 2119, Internet Engineering Task Force, Mar. 1997.
12 Informative References
Full Copyright Statement
Copyright (c) The Internet Society (2002). All Rights Reserved.
This document and translations of it may be copied and furnished to
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Internet Draft Response Routing July 1, 2002
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 assigns.
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.
J. Rosenberg et. al. [Page 8]
