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Versions: 00 01 02 RFC 4321

Network Working Group                                          R. Sparks
Internet-Draft                                               dynamicsoft
Expires: January 13, 2005                                  July 15, 2004



 Problems identified associated with the Session Initiation Protocol's
                         non-INVITE Transaction
                    draft-sparks-sip-nit-problems-01


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Copyright Notice


   Copyright (C) The Internet Society (2004).  All Rights Reserved.


Abstract


   This draft describes several problems that have been identified with
   the Session Initiation Protocol's non-INVITE transaction.











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Table of Contents


   1.  Problems under the current specifications  . . . . . . . . . .  3
     1.1   NITs must complete immediately or risk losing a race . . .  3
     1.2   Provisional responses can delay recovery from lost
           final responses  . . . . . . . . . . . . . . . . . . . . .  4
     1.3   Delayed responses will temporarily blacklist an element  .  5
     1.4   408 for non-INVITE is not useful . . . . . . . . . . . . .  6
     1.5   Non-INVITE timeouts doom forking proxies . . . . . . . . .  8
     1.6   Mismatched timer values make winning the race harder . . .  8
   2.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . .  9
   3.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
       Author's Address . . . . . . . . . . . . . . . . . . . . . . .  9
       Intellectual Property and Copyright Statements . . . . . . . . 10






































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1.  Problems under the current specifications


   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 as currently
   defined.


1.1  NITs must complete immediately or risk losing a race


   The non-INVITE transaction defined in RFC 3261 [1] is designed to
   have a fixed and finite duration (dependent on T1).  A consequence of
   this design is that participants must strive to complete the
   transaction as quickly as possible.  Consider the race condition
   shown in Figure 1.




                      UAC           UAS
                       |   request   |
                  ---  |---.         |
                   ^   |    `---.    |
                   |   |         `-->|  ---
                   |   |             |   ^
                   |   |             |   |
                 64*T1 |             |   |
                   |   |             |   |
                   |   |             | 64*T1
                   |   |             |   |
                   |   |             |   |
                   v   |             |   |
     timeout <=== ---  |   200 OK    |   |
                       |         .---|   v
                       |    .---'    |  ---
                       |<--'         |



                      Figure 1: NI Race Condition


   The UAS in this figure believes it has responded to the request in
   time, and that the request succeeded.  The UAC, on the other hand,
   believes the request has timed-out, hence failed.  No longer having a
   matching client transaction, the UAC core will ignore what it
   believes to be a spurious response.  As far as the UAC is concerned,
   it received no response at all to its request.  The ultimate result
   is the UAS and UAC have conflicting views of the outcome of the
   transaction.





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   Therefore, a UAS cannot wait until the last possible moment to send a
   final response within a NIT.  It must, instead, send its response so
   that it will arrive at the UAC before that UAC times out.
   Unfortunately, the UAS has no way to accurately measure the
   propagation time of the request or predict the propagation time of
   the response.  The uncertainty it faces is compounded by each proxy
   that participates in the transaction.  Thus, the UAS's only choice is
   to send its final response as soon as it possibly can and hope for
   the best.


   This result constrains the set of problems that can be solved with a
   single NIT.  Any delay introduced during processing of a request
   increases the probability of losing the race.  If the timing
   characteristics of that processing are not predictable and
   controllable, a single NIT is an inappropriate model for handling the
   request.  One viable alternative is to accept the request with a 202
   and send the ultimate results in a new request in the reciprocal
   direction.


   In specialized networks, a UAS might have some reliable knowledge of
   inter-hop latency and could use that knowledge to determine if it has
   time to delay its final response in order to perform some processing
   such as a database lookup while mitigating its risk of losing the
   race in Figure 1.  Establishing this knowledge across arbitrary
   networks (perhaps using resource reservation techniques and
   deterministic transports) is not currently feasible.


1.2  Provisional responses can delay recovery from lost final responses


   The non-INVITE client transaction state machine provides reliability
   for NITs over unreliable transports (UDP) through retransmission of
   the request message.  Timer E is set to T1 when a request is
   initially transmitted.  As long as the machine remains in the Trying
   state, each time Timer E fires, it will be reset to twice its
   previous value (capping at T2) and the request is retransmitted.


   If the non-INVITE client transaction state machine sees a provisional
   response, it transitions to the Proceeding state, where
   retransmission continues, but the algorithm for resetting Timer E is
   simply to use T2 instead of doubling at each firing.  (Note that
   Timer E is not altered during the transition to Proceeding).


   Making the transition to the Proceeding state before Timer E is reset
   to T2 can cause recovery from a lost final response to take extra
   time.  Figure 2 shows recovery from a lost final response with and
   without a provisional message during this window.  Recovery occurs
   within 2*T1 in the case without the provisional.  With the
   provisional, recovery is delayed until T2, which by default is 8*T1.




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   In practical terms, a provisional response to a NIT in currently
   deployed networks can delay transaction completion by up to 3.5
   seconds.



              UAC       UAS               UAC        UAS
               |         |                 |          |
         ---   |----.    |            ---  |----.     |
          ^    |     `-->|             ^   |     `--->|
      E = T1   |         |         E = T1  |    .-----|(provisional)
          v    |         |             v   |<--'      |
         ---   |----.    |            ---  |----.     |
          ^    |     `-->|             ^   |     `--->|
          |    |   X<----|(lost final) |   |   X<-----|(lost final)
          |    |         |             |   |          |
      E = 2*T1 |         |             |   |          |
          |    |         |             |   |          |
          |    |         |             |   |          |
          v    |         |             |   |          |
         ---   |----.    |             |   |          |
               |     `-->|             |   |          |
               |   .-----|(final)      |   |          |
               |<-'      |             |   |          |
               |         |             |   |          |
              \/\       /\/           /\/ /\/        /\/
                                   E = T2
              \/\       /\/           /\/ /\/        /\/
               |         |             |   |          |
               |         |             v   |          |
               |         |            ---  |----.     |
               |         |                 |     `--->|
               |         |                 |    .-----|(final)
               |         |                 |<--'      |
               |         |                 |          |



                Figure 2: Provisionals can harm recovery


   No additional delay is introduced if the first provisional response
   is received after Timer E has reached its maximum reset interval of
   T2.


1.3  Delayed responses will temporarily blacklist an element


   A SIP element's use of SRV is specified in RFC 3263 [2].  That
   specification discusses how SIP assures high availability by having
   upstream elements detect failure of downstream elements.  It proceeds
   to define several types of failure detection and instructions for




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   failover.  Two of the behaviors it describes are important to this
   document:
   o  Within a transaction, transport failure is detected either through
      an explicit report from the transport layer or through timeout.
      Note specifically that timeout will indicates transport failure
      regardless of the transport in use.  When transport failure is
      detected, the request is retried at the next element from the
      sorted results of the SRV query.
   o  Between transactions, locations reporting temporary failure
      (through 503/Retry-After for example) are not used until their
      requested black-out period expires.
   The specification notes the benefit of caching locations that are
   successfully contacted, but does not discuss how such a cache is
   maintained.  It is unclear whether an element should stop using
   (temporarily blacklist) a location returned in the SRV query that
   results in a transport error.  If it does, when should such a
   location be removed from the blacklist?


   Without such a blacklist (or equivalent mechanism), the intended
   availability mechanism fails miserably.  Consider traffic between two
   domains.  Proxy pA in domain A needs to forward a sequence of
   non-INVITE requests to domain B.  Through DNS SRV, pA discovers pB1
   and pB2, and the ordering rules of [2] and [3] indicate it should use
   pB1 first.  The first request to pB1 times out.  Since pA is a proxy
   and a NIT has a fixed duration, pA has no opportunity to retry the
   request at pB2.  If pA does not remember pB1's failure, the second
   request (and all subsequent non-INVITE requests until pB1 recovers)
   are doomed to the same failure.  Caching would allow the subsequent
   requests to be tried at pB2.


   Since miserable failure is not acceptable in deployed networks, we
   should anticipate that elements will, in fact, cache timeout failures
   between transactions.  Then the race in Figure 1 becomes important.
   If an element fails to respond "soon enough", it has effectively not
   responded at all, and will be blacklisted at its peer for some period
   of time.


   (Note that even with caching, the first request timeout results in a
   timeout failure all the way back to the original submitter.  The
   failover mechanisms in [2] work well to increase the resiliency of a
   given INVITE transaction, but do nothing for a given non-INVITE
   transaction.)


1.4  408 for non-INVITE is not useful


   Consider the race condition in Figure 1 when the final response is
   408 instead of 200.  Under the current specification, the race is
   guaranteed to be lost.  Most existing endpoints will emit a 408 for a




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   non-INVITE request 64*T1 after receiving the request if they haven't
   emitted an earlier final response.  Such a 408 is guaranteed to
   arrive at the next upstream element too late to be useful.  In fact,
   in the presence of proxies, these messages are even harmful.  When
   the 408 arrives, each proxy will have already terminated its
   associated client transaction due to timeout.  So, each proxy must
   forward the 408 upstream statelessly.  This, in turn, is guaranteed
   to arrive too late.  As Figure 3 shows, this can  ultimately result
   in bombarding the original requester with spurious 408s.  (Note that
   the proxy's client transaction state machine never enters the
   Completed state, so Timer K does not enter into play).




                  UAC        P1         P2         P3         UAS
                   |          |          |          |          |
             ---  ===---.     |          |          |          |
              ^    |     `-->===---.     |          |          |
              |    |          |     `-->===---.     |          |
              |    |          |          |     `-->===---.     |
            64*T1  |          |          |          |     `-->===
              |    |          |          |          |          |
              |    |          |          |          |          |
              v    |          |          |          |          |
   (timeout) ---  ===         |          |          |          |
                   |    .-408===         |          |          |
                   |<--'      |    .-408===         |          |
                   |    .-408-|<--'      |    .-408===         |
                   |<--'      |    .-408-|<--'      |    .-408===
                   |    .-408-|<--'      |    .-408-|<--'      |
                   |<--'      |    .-408-|<--'      |          |
                   |    .-408-|<--'      |          |          |
                   |<--'      |          |          |          |
                   |          |          |          |          |




                  Figure 3: late 408s to non-INVITEs


   This response bombardment is not limited to the 408 response, though
   it only exists when participating client transaction state machines
   are timing out.  Figure 4 generalizes Figure 1 to include multiple
   hops.  Note that even though the UAS responds "in time" to P3, the
   response is too late for P2, P1 and the UAC.








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                  UAC        P1         P2         P3         UAS
                   |          |          |          |          |
             ---  ===---.     |          |          |          |
              ^    |     `-->===---.     |          |          |
              |    |          |     `-->===---.     |          |
              |    |          |          |     `-->===---.     |
            64*T1  |          |          |          |     `-->===
              |    |          |          |          |          |
              |    |          |          |          |          |
              v    |          |          |          |          |
   (timeout) ---  ===         |          |          |          |
                   |    .-408===         |          |    .-200-|
                   |<--'      |    .-408===   .-200-|<--'      |
                   |    .-408-|<--'.-200-|<--'     ===         |
                   |<--'.-200-|<--'      |          |         ===
                   |<--'      |          |          |          |
                   |          |          |          |          |



               Figure 4: Additional timeout related error



1.5  Non-INVITE timeouts doom forking proxies


   A single branch with a delayed or missing final response will
   dominate the processing at proxy that receives no 2xx responses to a
   forked non-INVITE request.  Since this proxy is required to allow all
   of its client transactions to terminate before choosing a "best
   response".  This forces the proxy's server transaction to lose the
   race in Figure 1.  Any response it ultimately forwards (a 401 for
   example) will arrive at the upstream elements too late to be used.
   Thus, if no element among the branches would return a 2xx response,
   failure of a single element (or its transport) dooms the proxy to
   failure.


1.6  Mismatched timer values make winning the race harder


   There are many failure scenarios due to misconfiguration or
   misbehavior that the SIP specification does not discuss.  One is
   placing two elements with different configured values for T1 and T2
   on the same network.  Review of Figure 1 illustrates that the race
   failure is only made more likely in this misconfigured state (it may
   appear that shortening T1 at the element behaving as a UAS improves
   this particular situation, but remember that these elements may trade
   roles on the next request).  Since the protocol provides no mechanism
   for discovering/negotiating a peer's timer values, exceptional care
   must be taken when deploying systems with non-defaults to ensure they
   will _never_ directly communicate with elements with default values.




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2.  Acknowledgments


   This document captures many conversations about non-INVITE issues.
   Significant contributers include Ben Campbell, Gonzalo Camarillo,
   Steve Donovan, Rohan Mahy, Dan Petrie, Adam Roach, Jonathan
   Rosenberg, and Dean Willis.


3  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]  Gulbrandsen, A., Vixie, P. and L. Esibov, "A DNS RR for
        specifying the location of services (DNS SRV)", RFC 2782,
        February 2000.



Author's Address


   Robert J. Sparks
   dynamicsoft
   5100 Tennyson Parkway
   Suite 1200
   Plano, TX  75024


   EMail: rsparks@dynamicsoft.com






















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