Network Working Group                                          R. Sparks
Request for Comments: 4321                              Estacado Systems
Category: Informational                                     January 2006

                Problems Identified Associated with the
       Session Initiation Protocol's (SIP) Non-INVITE Transaction

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2006).


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

Table of Contents

   1. Problems under the Current Specifications .......................2
      1.1. NITs must complete immediately or risk losing a race .......2
      1.2. Provisional responses can delay recovery from lost
           final responses ............................................3
      1.3. Delayed responses will temporarily blacklist an element ....4
      1.4. 408 for non-INVITE is not useful ...........................6
      1.5. Non-INVITE timeouts doom forking proxies ...................7
      1.6. Mismatched timer values make winning the race harder .......7
   2. Security Considerations .........................................8
   3. Acknowledgements ................................................8
   4. Informative References ..........................................9

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RFC 4321                SIP Non-INVITE Problems             January 2006

1.  Problems under the Current Specifications

   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 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: Non-Invite Race Condition

   The User Agent Server (UAS) in this figure believes it has responded
   to the request in time, and that the request succeeded.  The User
   Agent Client (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 that the UAS
   and UAC have conflicting views of the outcome of the transaction.

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RFC 4321                SIP Non-INVITE Problems             January 2006

   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

   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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RFC 4321                SIP Non-INVITE Problems             January 2006

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

                 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

1.3.  Delayed responses will temporarily blacklist an element

   A SIP element's use of DNS Service Record Resource Records [3] is
   specified in RFC 3263 [2].  That specification discusses how SIP
   ensures high availability by having upstream elements detect failure
   of downstream elements.  It proceeds to define several types of
   failure detection and instructions for failover.  Two of the
   behaviors it describes are important to this document:

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RFC 4321                SIP Non-INVITE Problems             January 2006

   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

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RFC 4321                SIP Non-INVITE Problems             January 2006

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
   non-INVITE request 64*T1 after receiving the request if they have not
   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.  Therefore, 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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RFC 4321                SIP Non-INVITE Problems             January 2006

                     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.  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

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 that
   they will never directly communicate with elements with default

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RFC 4321                SIP Non-INVITE Problems             January 2006

2.  Security Considerations

   This document describes some problems in the core SIP specification
   [1] related to the SIP non-INVITE requests, the messages other than
   INVITE that begin transactions.  A few of the problems lead to
   flooding or forgery risk, and could possibly be exploited by an
   adversary in a denial of service attack.  Solutions are defined in
   the companion document [4].

   One solution there prohibits proxies and User Agents from sending 408
   responses to non-INVITE transactions.  Without this change, proxies
   automatically generate a storm of useless responses.  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 solution 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).

3.  Acknowledgements

   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.

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RFC 4321                SIP Non-INVITE Problems             January 2006

4. Informative 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.

   [4]  Sparks, R., "Actions Addressing Identified Issues with the
        Session Initiation Protocol's (SIP) Non-INVITE Transaction", RFC
        4320, January 2006.

Author's Address

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


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