Network Working Group R. Sparks
Request for Comments: 4320 Estacado Systems
Updates: 3261 January 2006
Category: Standards Track
Actions Addressing Identified Issues with the
Session Initiation Protocol's (SIP) Non-INVITE Transaction
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright (C) The Internet Society (2006).
This document 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 RFC 3261.
Table of Contents
1. Introduction ....................................................22. Improving the Situation When Responses Are Only Delayed .........22.1. Action 1: Make the best use of provisional responses .......22.2. Action 2: Remove the useless late-response storm ...........33. Improving the Situation When an Element Is Not Going to
Respond .........................................................44. Normative Updates to RFC 3261 ...................................44.1. Action 1 ...................................................44.2. Action 2 ...................................................55. Security Considerations .........................................56. Contributors ....................................................57. Normative References ............................................6
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 that
provisional responses have on the non-INVITE transaction state
machines. These problems are documented in . In summary:
A non-INVITE transaction must complete immediately or risk losing
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 document specifies updates to RFC 3261  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 (), 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 , sending any provisional response inside a NIT before
Timer E reaches T2 damages recovery from failure of an unreliable
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 (). 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
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
unnecessary 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 (),
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 cannot do anything with this knowledge. Thus,
the 408 is simply wasting network resources and contributes to the
response bombardment illustrated in .
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
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
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 that the peer is not available and follow the existing
requirements in  and  for that case.
Achieving the third goal requires more aggressive changes to the
protocol. As noted in , future non-INVITE transactions are likely
to fail again unless the implementation takes steps beyond what is
defined in  and  to remember non-responsive destinations
between transactions. Standardizing these extra steps is left to
4. Normative Updates to RFC 3261
4.1. Action 1
An SIP element MUST NOT send any provisional response with a Status-
Code other than 100 to a non-INVITE request.
An 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.
An 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, an SIP element MUST respond to a non-
INVITE request with a Status-Code of 100 if it has not otherwise
responded after the amount of time it takes a client transaction's
Timer E to be reset to T2.
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 cannot 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 responses.
5. Security Considerations
This document makes a number of small changes to the core SIP
specification  to improve the robustness of SIP non-INVITE
transactions. Many of these actions also prevent flooding and
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
. 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 that
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
The remainder of these changes do not affect the security of the SIP
Rohan Mahy provided the Security Considerations section.
7. Normative References
 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.
 Rosenberg, J. and H. Schulzrinne, "Session Initiation Protocol
(SIP): Locating SIP Servers", RFC 3263, June 2002.
 Sparks, R., "Problems Identified Associated with the Session
Initiation Protocol's (SIP) Non-INVITE Transaction", RFC 4321,
Robert J. Sparks
17210 Campbell Road
Dallas, TX 75252-4203
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