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Versions: 00 02 03 RFC 4320
Network Working Group R. Sparks
Internet-Draft Xten
Expires: July 2, 2005 Jan 2005
Actions addressing identified issues with the Session Initiation
Protocol's non-INVITE Transaction
draft-sparks-sip-nit-actions-03
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Copyright Notice
Copyright (C) The Internet Society (2005).
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.
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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
4. Normative Updates to RFC 3261 . . . . . . . . . . . . . . . . 5
4.1 Action 1 . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.2 Action 2 . . . . . . . . . . . . . . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
7. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 6
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6
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 (NIT) 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] 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
Since non-INVITE transactions must complete rapidly ([3]), any
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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 helps with the first two goals, dramatically improving an
element's ability to distinguish between failure and delayed response
from the next downstream element. Some 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] for that case.
Achieving the third goal requires more agressive changes to the
protocol. As noted in [3], future non-invite transactions are likely
to fail again unless the implementation takes steps beyond what is
defined in [1] and [2] to remember non-responsive destinations
between transactions. Standardizing these extra steps is left to
future work.
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 non-INVITE 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.
A SIP element MAY respond to a non-INVITE request with a Status-Code
of 100 over a reliable transport at any time.
Without regard to transport, a SIP element MUST respond to a
non-INVITE request with a Status-Code of 100 if it has not otherwise
responed after the amount of time it takes a client transaction's
Timer E to be reset to T2.
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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. Security Considerations
This document makes a number of small changes to the core SIP
specification [1] to improve the robustness of SIP non-INVITE
transactions. Many of these actions also prevent flooding and
denial-of-service attacks.
One change prohibits proxies and User Agents from sending 408
responses to non-INVITE transactions. Without this change, proxies
automatically generate a storm of useless responses as described in
[3]. An attacker could capitalize on this by enticing User Agents to
send non-INVITE requests to a black hole (through social engineering
or DNS poisoning) or by selectively dropping responses.
Another change prohibits proxies from forwarding late responses.
Without this change, an attacker could easily forge messages which
appear to be late responses. All proxies compliant with RFC 3261 are
required to forward these responses, wasting bandwidth and CPU and
potentially overwhelming target User Agents (especially those with
low speed connections).
The remainder of these changes do not affect the security of the SIP
protocol.
6. IANA Considerations
This document requires no action by IANA.
7. Contributors
Rohan Mahy provided the Security Considerations section.
8 References
[1] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP:
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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
Xten
5100 Tennyson Parkway
Suite 1000
Plano, TX 75024
EMail: RjS@xten.com
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