One document matched: draft-polk-sip-rph-in-responses-01.txt
Differences from draft-polk-sip-rph-in-responses-00.txt
SIP WG James Polk
Internet-Draft Cisco Systems
Intended status: Standards Track (PS) February 24, 2008
Expires: Aug 24, 2008
Updates: RFC 4412 (if published)
Allowing SIP Resource-Priority Header in SIP Responses
draft-polk-sip-rph-in-responses-01
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Copyright Notice
Copyright (C) The IETF Trust (2008).
Abstract
The Session Initiation Protocol (SIP) Resource-Priority Header is
ignored in SIP responses, according to RFC 4412. 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 (UA) 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
[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 (UA) to process the
Resource-Priority Header in a response.
There are inconsistencies within RFC 4412 as to whether or not a SIP
entity can process a Resource-Priority header in a response; Section
3.3 of [RFC4412] states (with a table) a Resource-Priority cannot be
looked for in a response, whereas section 4.7.3 of [RFC4412]
discusses how SIP entities deal with a Resource-Priority in a
response. Here is a more thorough examination of what RFC 4412 says
in both sections.
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 [RFC3261], 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 a Resource-Priority value
can be found in a response, or what a SIP element is to do with it
once one is received.
Even though Tables 2 and 3 of RFC 3261 are not normative, this is
frequently a discussion topic in and out of IETF meetings, and in
other SDOs - resulting in industry confusion.
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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 Resource-Priority
value within a transaction from the request message, maintaining
state of the value for the response.
Yet, Section 4.7.3 of RFC 4412 states this
"If a stateful proxy has authorized a particular Resource-Priority
level, and if it offers differentiated treatment to responses
containing Resource-Priority levels, the proxy SHOULD ignore any
higher value contained in responses, to prevent colluding user
agents from artificially raising the priority level."
The above quote from RFC 4412 was concerning stateful proxies, and
there is a need now to have stateless SIP servers have the
Resource-Priority header in responses in some environments,
typically when surrounded by stateful proxy servers more towards the
edge of the network. This is a design choice several vendors want
to have, and they want SIP specifications to state what they want is
not illegal, according to RFC(s).
This document clarifies what was inconsistent in RFC 4412, by
allowing a proxy to "amdr" an Resource-Priority value in a response,
though this should only occur in certain network environments.
There was a proposal to use SIP Resource-Priority in a SIP response,
when that transaction's SIP request is received by a certain type of
authorization server, to establish the namespace and priority-value
for a dialog (as the signaling continued to set-up the call). This
was loosely named "use-case#2" to establish how and why
Resource-Priority is necessary in SIP responses. That user-case has
been abandoned. What remains here is what was called "use-case#1"
for how and why this update to RFC 4412 is necessary.
This document updates RFC 4412, but requests no IANA changes.
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
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The difference is in the "where" column, in which the "R" is
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,
wherever this table is kept intact (knowing this table is not
normative anywhere within current SIP RFCs, but is often used as a
reference by readers where a header is to be used, and what the
expectations are within SIP Methods).
3. Use-Case #1 and SIP Resource-Priority in Responses
The usage for SIP Resource-Priority in Responses has been described
as "use-case#1". Use-Case#1 involves large networks that will no
longer have to maintain stateful proxies throughout their networks
in order to comply with RFC 4412. With this update to RFC 4412,
large networks can now have transaction or dialog stateful servers
at the perimeter of their network, but now can have the faster and
more scalable stateless servers in the core of their networks -
knowing no SIP requests or responses will be received by these
stateless servers without first being processed by the stateful
servers (i.e., at least providing the necessary
authentication/authorization on the usage of Resource-Priority
values in the messages).
What was described briefly in the Intro section of this document as
"use-case#2" (using a SIP response to carry an authorized new
Resource-Priority header value to a server that will continue the
transaction towards the UAS with this Resource-Priority in the
request) MUST NOT be done. There are more appropriate protocols to
do this function than a SIP response message. A SIP SUB/NOT
transaction MAY be used for that function, but the scoping and
defining of that operation is outside the scope of this document -
which focuses exclusively on use-case#1, described above.
4. SIP Element Behaviors for Resource-Priority in Responses
4.1 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,
it is RECOMMENDED local policy determine which bi-direction
priority-value to process the messages within this transaction on,
which will likely be at the same priority-value as was in the SIP
request.
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4.2 UAS Behavior
The UAS MAY include the Resource-Priority header in responses.
It is RECOMMENDED the Resource-Priority header value 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.
4.3 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 Resource-Priority value(s) in a response 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. This Resource-Priority
header value MAY be the same or different between request and
response, depending on local policy downstream of a proxy (or UAS).
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 Resource-Priority enabled SIP entities
that are configured to remain aware of the Resource-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.
5. IANA Considerations
There are no IANA considerations in this document.
[NOTE: If this document is to be published as an RFC, this section
can be removed.]
6. 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 Resource-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
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marking could grant inappropriate treatment to SIP messages that do
not deserve it.
7. Acknowledgements
Thanks to Janet Gunn, Keith Drage, Dean Willis, Tim Dwight and
Martin Dolly for the helpful comments.
8. References
8.1 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 Polk
3913 Treemont Circle
Colleyville, Texas 76034
USA
Phone: +1-817-271-3552
Fax: none
Email: jmpolk@cisco.com
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