RFC 4320

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 Notice

   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 ....................................................2
   2. Improving the Situation When Responses Are Only Delayed .........2
      2.1. Action 1: Make the best use of provisional responses .......2
      2.2. Action 2: Remove the useless late-response storm ...........3
   3. Improving the Situation When an Element Is Not Going to
      Respond .........................................................4
   4. Normative Updates to RFC 3261 ...................................4
      4.1. Action 1 ...................................................4
      4.2. Action 2 ...................................................5
   5. Security Considerations .........................................5
   6. Contributors ....................................................5
   7. Normative References ............................................6

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RFC 4320                 SIP Non-INVITE Actions             January 2006

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 that
   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 document 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

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RFC 4320                 SIP Non-INVITE Actions             January 2006

   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

   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
   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 ([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 cannot do anything with this knowledge.  Thus,
   the 408 is 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

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RFC 4320                 SIP Non-INVITE Actions             January 2006

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 that the peer is not available and follow the existing
   requirements in [1] and [2] for that case.

   Achieving the third goal requires more aggressive 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

   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.

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RFC 4320                 SIP Non-INVITE Actions             January 2006

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 [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 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
   low-speed connections).

   The remainder of these changes do not affect the security of the SIP

6.  Contributors

   Rohan Mahy provided the Security Considerations section.

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RFC 4320                 SIP Non-INVITE Actions             January 2006

7.  Normative 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 (SIP) Non-INVITE Transaction", RFC 4321,
        January 2006.

Author's Address

   Robert J. Sparks
   Estacado Systems
   17210 Campbell Road
   Suite 250
   Dallas, TX 75252-4203

   EMail: rjsparks@estacado.net

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RFC 4320                 SIP Non-INVITE Actions             January 2006

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