Network Working Group R. Sparks
Internet-Draft dynamicsoft
Expires: August 6, 2004 February 6, 2004
Actions addressing identified issues with the Session Initiation
Protocol's non-INVITE Transaction
draft-sparks-sip-nit-actions-00
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Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This draft describes modifications to the Session Initiation Protocol
(SIP) to address problems that have been identified with the SIP
non-INVITE transaction. These modifications reduce the probability of
messages losing the race condition inherent in the non-INVITE
transaction and reduce useless network traffic. They also improve the
robustness of SIP networks when elements stop responding. These
changes update behavior defined in RFCs 3261 and 3263.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Improving the situation when responses are only delayed . . . 3
2.1 Action 1: Make the best use of provisional responses . . . . . 3
2.2 Action 2: Remove the useless late-response storm . . . . . . . 4
3. Improving the situation when an element is not going to
respond . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1 Action 3: Strengthen specification of caching success and
failures in RFC 3263 . . . . . . . . . . . . . . . . . . . . . 5
4. Normative Updates to RFC 3261 . . . . . . . . . . . . . . . . 5
4.1 Action 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.2 Action 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
5. Normative Updates to RFC 3263 . . . . . . . . . . . . . . . . 6
5.1 Action 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 7
Intellectual Property and Copyright Statements . . . . . . . . 8
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1. Introduction
There are a number of unpleasant edge conditions created by the SIP
non-INVITE transaction model's fixed duration. The negative aspects
of some of these are exacerbated by the effect provisional responses
have on the non-INVITE transaction state machines. These problems are
documented in [3]. In summary:
A non-INVITE transaction must complete immediately or risk losing
a race
Losing the race will cause the requester to stop sending traffic
to the responder (the responder will be temporarily blacklisted)
Provisional responses can delay recovery from lost final responses
The 408 response is useless for the non-INVITE transaction
As non-INVITE transactions through N proxies time-out, there can
be an O(N^2) storm of the useless 408 responses
This draft specifies updates to RFC 3261 [1] and RFC 3263 [2] to
improve the behavior of SIP elements when these edge conditions
arise.
2. Improving the situation when responses are only delayed
There are two goals to achieve when we constrain the problem to those
cases where all elements are ultimately responsive and networks
ultimately deliver messages:
o Reduce the probability of losing the race, preferably to the point
that it is negligible
o Reduce or eliminate useless messaging
2.1 Action 1: Make the best use of provisional responses
o Disallow non-100 provisionals to non-INVITE requests
o Disallow 100 Trying to non-INVITE requests before Timer E reaches
T2 (for UDP hops)
o Allow 100 Trying after Timer E reaches T2 (for UDP hops)
o Allow 100 Trying for hops over reliable transports
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Since non-INVITE transactions must complete rapidly ([3]), any
information beyond "I'm here" (which can be provided by a 100 Trying)
can be just as usefully delayed to the final response. Sending
non-100 provisionals wastes bandwidth.
As shown in [3], sending any provisional response inside a NIT before
Timer E reaches T2 damages recovery from failure of an unreliable
transport.
Without a provisional, a late final response is the same as no
response at all and will likely result in blacklisting the late
responding element ([3]), If an element is delaying its final
response at all, sending a 100 Trying after Timer E reaches T2
prevents this blacklisting without damaging recovery from unreliable
transport failure.
Blacklisting on a late response occurs even over reliable transports.
Thus, if an element processing a request received over a reliable
transport is delaying its final response at all, sending a 100 Trying
well in advance of the timeout will prevent blacklisting. Sending a
100 Trying immediately will not harm the transaction as it would over
UDP, but a policy of always sending such a message results in
unneccessary traffic. A policy of sending a 100 Trying after the
period of time in which Timer E reaches T2 had this been a UDP hop is
one reasonable compromise.
2.2 Action 2: Remove the useless late-response storm
o Disallow 408 to non-INVITE requests
o Absorb stray non-INVITE responses at proxies
A 408 to non-INVITE will always arrive too late to be useful ([3]),
The client already has full knowledge of the timeout. The only
information this message would convey is whether or not the server
believed the transaction timed out. However, with the current design
of the NIT, a client can't do anything with this knowledge. Thus the
408 simply wasting network resources and contributes to the response
bombardment illustrated in [3].
Late non-INVITE responses by definition arrive after the client
transaction's Timer F has fired and the client transaction has
entered the Terminated state. Thus, these responses cannot be
distinguished from strays. Changing the protocol behavior to prohibit
forwarding non-INVITE stray responses stops the late response storm.
It also improves the proxy's defenses against malicious users
counting on the RFC 3261 requirement to forward such strays.
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3. Improving the situation when an element is not going to respond
When we expand the scope of the problem to also deal with element or
network failure, we have more goals to achieve:
o Identifying when an element is non-responsive
o Minimizing or eliminating falsely identifying responsive elements
as non-responsive
o Avoiding non-responsive elements with future requests
Action 1 dramatically improves an elements ability to distinguish
between failure and delayed response from the next downstream
element. Ssome response, either provisional or final, will almost
certainly be received before the transaction times out. So, an
element can more safely assume that no response at all indicates the
peer is not available and follow the existing requirements in [1] and
[2] (as amended by this memo) for that case.
As [3] discusses, behavior once an element is identified as
non-responsive is currently underspecified. [2] speaks only
non-normatively about caching the addresses of servers that have
successfully been communicated with for an unspecified period of
time.
3.1 Action 3: Strengthen specification of caching success and failures
in RFC 3263
o Make the caching recommendation normative for servers successfully
reached
o Add failures due to non-responsiveness to that cache
This cache will also be used to remember servers that have issued a
503 with or without a Retry-After.
4. Normative Updates to RFC 3261
4.1 Action 1
A SIP element MUST NOT send any provisional response with a
Status-Code other than 100 to a non-INVITE request.
A SIP element MUST NOT respond to a request with a Status-Code of 100
over any unreliable transport, such as UDP, before the amount of time
it takes a client transaction's Timer E to be reset to T2.
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A SIP element MAY respond to a request with a Status-Code of 100 over
an unreliable transport after the amount of time it takes a client
transaction's Timer E to be reset to T2.
A SIP element MAY respond to a request with a Status-Code of 100 over
a reliable transport at any time.
4.2 Action 2
A transaction-stateful SIP element MUST NOT send a response with
Status-Code of 408 to a non-INVITE request. As a consequence, an
element that can not respond before the transaction expires will not
send a final response at all.
A transaction-stateful SIP proxy MUST NOT send any response to a
non-INVITE request unless it has a matching server transaction that
is not in the Terminated state. As a consequence, this proxy will not
forward any "late" non-INVITE response.
5. Normative Updates to RFC 3263
5.1 Action 3
(Note that RFC 3263 uses "client" for "any SIP element wishing to
send a request".)
Once a client identifies an available server for a domain name using
the algorithms defined in RFC 3263, it SHOULD cache the identity of
that server in an available-cache. This identity MUST be
periodically removed from the cache, and its time-to-live in that
cache SHOULD be short. If the server with that identity becomes
unavailable, the identity MUST be immediately removed from the cache
and SHOULD be placed in an unavailable-cache. The next attempt to
reach that domain name MUST invoke the algorithms in RFC 3263.
If any attempt to contact a server based on the output of the
algorithms of RFC 3263 yeilds that the server is unavailable (the
request times out or the server returns a 503 Status-Code), the
identity of that server SHOULD be placed in an unavailable-cache.
This identity MUST be periodically removed from that cache, and its
time-to-live in that cache SHOULD be short. If information about the
period of unavailability is present (such as in a Retry-After header
field in a 503 response), the time-to-live in this cache SHOULD
reflect that information.
If the algorithms of RFC 3263 yeild a server identity that is in an
unavailable-cache, that identity MUST be discarded and the algorithm
MUST be continued to search for another candidate.
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OPEN ISSUE: Can we strengthen placing identities in an
unavailable-cache to MUST? RFC 3263 failover for non-INVITE will not
work without it.
OPEN ISSUE: Is it possible to recommend a time more specific than
"short" in these requirements?
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] Rosenberg, J. and H. Schulzrinne, "Session Initiation Protocol
(SIP): Locating SIP Servers", RFC 3263, June 2002.
[3] Sparks, R., "Problems identified associated with the Session
Initiation Protocol's non-INVITE Transaction",
draft-sparks-sip-nit-problems (work in progress), February 2004.
Author's Address
Robert J. Sparks
dynamicsoft
5100 Tennyson Parkway
Suite 1200
Plano, TX 75024
EMail: rsparks@dynamicsoft.com
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