One document matched: draft-polk-sip-rph-in-responses-00.txt
SIP WG James Polk
Internet-Draft Cisco Systems
Intended status: Standards Track June 13th, 2007
Expires: December 13th, 2007
Updates: RFC 4412 (if published)
Allowing SIP Resource Priority Header in SIP Responses
draft-polk-sip-rph-in-responses-00
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Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
The Session Initiation Protocol (SIP) Resource Priority Header
(RPH), in its current form, is ignored in SIP responses. This was a
design choice during RFC 4412's development. This is now considered
a bad design choice in certain scenarios. This document corrects
RFC 4412's communications model by optionally allowing a SIP server
or user agent client to process the Resource-Priority Header in a
response.
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The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
1. Introduction
The Session Initiation Protocol (SIP) Resource Priority Header
(RPH) [RFC4412], in its current form, is ignored by SIP entities if
in SIP responses. It was a design choice during RFC 4412's
development that only stateful servers would grant SIP messages
preferential treatment. This is now considered a bad design choice
in certain scenarios, such as those entities within trusted
networks, and where stateless servers are surrounded by more
stateful servers. This document corrects RFC 4412's communications
model by allowing a SIP server or user agent client to process the
Resource-Priority Header in a response.
RFC 4412 defines the SIP Resource-Priority header, and is a
standards track extension to SIP [RFC3261]. Section 3.3 of RFC 4412
has the following table 2 entry:
Header field where proxy INV ACK CAN BYE REG OPT PRA
----------------------------------------------------------------
Resource-Priority R amdr o o o o o o o
Header field where proxy SUB NOT UPD MSG REF INF PUB
----------------------------------------------------------------
Resource-Priority R amdr o o o o o o o
According to RFC 3261, the 'R' in the "where" column states a
particular header is found in requests, and ignored in responses.
Table 2 is a quick reference of usage of a header, but alone, is
insufficient to define the expected behavior of a SIP header,
relying instead on what the header description text says in the RFC
that creates the header. RFC 4412 fails to provide clear normative
text indicating whether or not an RPH can be found in a response, or
what a SIP element is to do with it once received.
The assumption at the time of RFC 4412 was that the
Resource-Priority header would only be used in managed IP networks
where all SIP servers were statefully aware of the RPH value within
a transaction from the request message, maintaining state of the
value for the response.
There is a need now to have stateless SIP servers have the
Resource-Priority header in responses in some environments.
This document IANA registers the Resource-Priority header for usage
in responses.
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This document updates RFC 4412.
2. Adding Resource-Priority Header in SIP Responses
The following the correction of the table 2 entry for the
Resource-Priority header:
Header field where proxy INV ACK CAN BYE REG OPT PRA
----------------------------------------------------------------
Resource-Priority amdr o o o o o o o
Header field where proxy SUB NOT UPD MSG REF INF PUB
----------------------------------------------------------------
Resource-Priority amdr o o o o o o o
The difference is in the "where" column, in which the "R" was
removed. The specific behaviors resulting from this are explained
in the next 3 sub-sections.
The above is to replace what is currently stated by RFC 4412, and
what is IANA registered.
2.1 UAC Behavior
The UAC MAY process SIP responses containing the Resource-Priority
header according to the local policy of the network or UAC. If the
response header value is different than the original request value,
local policy SHOULD determine which to process the message based on,
but will likely be at the same priority-value as was in the request
the UAC send to the UAS.
2.2 UAS Behavior
The UAS MAY include the Resource-Priority header in responses.
Typically the Resource-Priority header value will be the same in the
response as it was in the request. The UAS MAY change the
Resource-Priority header value, depending on local policy. Reasons
for this are outside the scope of this document.
2.3 Proxy Behavior
SIP Proxies MAY process the Resource-Priority header in responses;
meaning, in certain environments, the choice of whether or not to
process the RPH will not be in doubt. This configuration choice
could be on a per transaction basis, on a per server basis, or under
some other parameter choice, all based on local policy of the proxy.
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This Resource-Priority header value MAY be the same or different
between request and response, depending on local policy downstream
of this proxy. SIP Proxies MAY add or modify the Resource-Priority
header value in responses with this update. SIP Proxies MAY, but
SHOULD NOT delete Resource-Priority header value in responses, as a
Resource-Priority header value MAY have use other than at this
particular proxy. Local policy will determine this configuration.
SIP Proxies SHOULD be able to ignore the header by configuration, in
such environments that have RPH enabled SIP entities that are
configured to remain aware of the RPH priority-value in a request
part of the transaction, or do not trust the possibility of a
priority mark up, from what was in the request message.
3. Acknowledgements
Your name here, or, if you write a fair piece of text, you can
become a co-author...
4. IANA Considerations
The following is replace what is registered in the sip-parameters
section of IANA for the Resource-Priority header:
Header field where proxy INV ACK CAN BYE REG OPT PRA
----------------------------------------------------------------
Resource-Priority amdr o o o o o o o
Header field where proxy SUB NOT UPD MSG REF INF PUB
----------------------------------------------------------------
Resource-Priority amdr o o o o o o o
[Editor's NOTE: since this registration replaces an existing
registration, but does not offer supporting text for what was not
changed, does IANA give references to both this doc and RFC 4412
once this doc is RFC'd?]
5. Security Considerations
The Security considerations that apply to RFC 4412 [RFC4412] apply
here. The only new security threat this document introduces
relative to RFC 4412 is in SIP entities that grant unconditional,
stateless, preferential treatment based on the RPH priority-value.
This is a configuration issue, and not a implementation issue, and
operators should avoided allowing the configuration of blind SIP
entities to process according to a priority marking without having a
means of checking if the marking is valid. Invalid marking could
grant inappropriate treatment to SIP messages that do not deserve
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it.
6. References
6.1 Normative References
[RFC2119] S. Bradner, "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997
[RFC4412] Schulzrinne, H., Polk, J., "Communications Resource
Priority for the Session Initiation Protocol (SIP)", RFC
4411, Feb 2006
[RFC3261] J. Rosenberg, H. Schulzrinne, G. Camarillo, A. Johnston, J.
Peterson, R. Sparks, M. Handley, and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, May 2002.
Author's Addresses
James M. Polk
3913 Treemont Circle
Colleyville, Texas 76034
USA
Phone: +1-817-271-3552
Fax: none
Email: jmpolk@cisco.com
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