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Versions: 00 02 03 RFC 4320

Network Working Group                                          R. Sparks
Internet-Draft                                               dynamicsoft
Expires: January 14, 2005                                  July 16, 2004



    Actions addressing identified issues with the Session Initiation
                   Protocol's non-INVITE Transaction
                    draft-sparks-sip-nit-actions-02


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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.






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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.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  6
       Author's Address . . . . . . . . . . . . . . . . . . . . . . .  6
       Intellectual Property and Copyright Statements . . . . . . . .  7






































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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] 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  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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   Internet Society.




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