Network Working Group A. B. Roach
Internet-Draft dynamicsoft
Expires: February 3, 2004 August 5, 2003
A "Conflict" Response Code for the Session Initiation Protocol (SIP)
draft-roach-sip-409-00
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Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This document proposes an addition of a "409 Conflict" response code
for the Session Initiation protocol. This response code is required
for a number of application-specific purposes, and is expected to be
useful in future extensions to the protocol.
1. Introduction
The first published version of SIP, RFC 2543, included a "409
Conflict" response code borrowed from HTTP/1.1 [2]. When copied into
SIP, however, this response code was unfortunately defined to apply
to an extremely narrow use case. This single use case was
subsequently deprecated by RFC 3261 [1]; seeing no further use for
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the 409 response code, the authors elected to removed it at the same
time.
Operational experience has demonstrated that various specialized
applications occasionally encounter circumstances in which a request
cannot be processed because doing so would result in an inconsistent
or disallowed state. This document seeks to reinstate the 409
response code in a more general form so that such situations can be
accurately signalled to UACs.
2. "409 Conflict" Response Code
The 409 response is added to the "Client-Error" header field
definition. "409 Conflict" is used to indicate that the request
could not be completed due to a conflict with the current state of
the resource. This code is only allowed in situations where it is
expected that the user might be able to resolve the conflict and
resubmit the request.
Conflicts are most likely to occur in response to requests that
create state in the network, such as REGISTER and SUBSCRIBE. The
circumstances under which a 409 response code is returned are
expected to be highly dependent on the application that the UAS
provides.
3. Backwards Compatibility
It should be noted that section 8.1.3.2 of RFC 3261 [1] defines UAC
behavior upon receipt of an unrecognized response code. Under the
behavior specified therein, a 409 response will be treated by a
client that does not understand it as if it were a "400 Bad Request"
response. The semantics defined for that response indicate that the
UAC should not retry the same request without modification, which is
generally a reasonable course of action to take in the case of a
conflict.
4. Security Considerations
It is not beleived that the addition of the described 409 response
code has the ability to reveal sensitive information or provide any
additional avenues for attack.
5. IANA Considerations
This document defines an additional SIP response code, which is to be
added to the method and response-code sub-registry under http://
www.iana.org/assignments/sip-parameters.
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Response Code Number: 409
Default Reason Phrase: Conflict
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] Fielding, R., Gettys, J., Mogul, J., Nielsen, H., Masinter, L.,
Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol --
HTTP/1.1", RFC 2616, June 1999.
Author's Address
Adam Roach
dynamicsoft
5100 Tennyson Pkwy
Suite 1200
Plano, TX 75024
US
EMail: adam@dynamicsoft.com
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