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Versions: (draft-hasebe-sipping-race-examples) 00 01 02 03 04 05 06 RFC 5407

Internet Engineering Task Force                                M. HASEBE
Internet-Draft                                                J. KOSHIKO
Intended status: Best Current Practice                         Y. SUZUKI
Expires: Dec 30, 2007                                       T. YOSHIKAWA
                                                                NTT-East
                                                              P. Kyzivat
                                                     Cisco Systems, Inc.
                                                          Jun 30th, 2007


  Examples call flow in race condition on Session Initiation Protocol
               draft-ietf-sipping-race-examples-02.txt

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Copyright Notice

   Copyright (C) The IETF Trust (2007).


Abstract

   This document gives examples of the Session Initiation Protocol (SIP)
   call flows in race condition.  Call flows in race condition are
   confusing, and this document shows the best practice to handle them.
   The elements in these call flows include SIP User Agents and SIP
   Proxies.  Call flow diagrams and message details are shown.




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Table of Contents

   1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . .2
      1.1 General Assumptions. . . . . . . . . . . . . . . . . . . . .3
      1.2 Legend for Message Flows . . . . . . . . . . . . . . . . . .3
      1.3 SIP Protocol Assumptions . . . . . . . . . . . . . . . . . .3
   2. The Dialog State Machine for INVITE dialog usage . . . . . . . .4
   3. Race Conditions. . . . . . . . . . . . . . . . . . . . . . . . .9
      3.1 Receiving message in the Moratorium State. . . . . . . . . .9
        3.1.1 Receiving Initial INVITE retransmission. . . . . . . . .10
        3.1.2 Receiving CANCEL(Early state). . . . . . . . . . . . . .11
        3.1.3 Receiving BYE (Early state). . . . . . . . . . . . . . .13
        3.1.4 Receiving re-INVITE (Established state)(case 1). . . . .15
        3.1.5 Receiving re-INVITE (Established state)(case 2). . . . .19
        3.1.6 Receiving BYE (Established state). . . . . . . . . . . .22
      3.2 Receiving message in the Mortal State. . . . . . . . . . . .24
        3.2.1 Receiving BYE(Established state) . . . . . . . . . . . .24
        3.2.2 Receiving re-INVITE(Established state) . . . . . . . . .26
        3.2.3 Receiving 200 OK for re-INVITE(Established state). . . .28
        3.2.4 Receiving ACK (Moratorium state) . . . . . . . . . . . .31
      3.3 Other race conditions. . . . . . . . . . . . . . . . . . . .32
        3.3.1 Re-INVITE crossover. . . . . . . . . . . . . . . . . . .32
        3.3.2 UPDATE and re-INVITE crossover . . . . . . . . . . . . .36
        3.3.3 Receiving REFER(Established state) . . . . . . . . . . .40
   4. IANA Considerations. . . . . . . . . . . . . . . . . . . . . . .42
   5. Security Considerations. . . . . . . . . . . . . . . . . . . . .42
   6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . .42
   7. References . . . . . . . . . . . . . . . . . . . . . . . . . . .42
      7.1 Normative References . . . . . . . . . . . . . . . . . . . .42
      7.2 Informative References . . . . . . . . . . . . . . . . . . .42
   Appendix A. BYE on the Early Dialog . . . . . . . . . . . . . . . .43
   Appendix B. BYE request overlapped on re-INVITE . . . . . . . . . .44
   Appendix C. UA's behavior for CANCEL. . . . . . . . . . . . . . . .46
   Appendix D. Notes on the request in Mortal state. . . . . . . . . .48
   Appendix E. Forking and receiving new To tags . . . . . . . . . . .49
   Author's Addresses. . . . . . . . . . . . . . . . . . . . . . . . .53
   Intellectual Property and Copyright Statements. . . . . . . . . . .54


1.  Overview

   The call flows shown in this document were developed in the design of
   a SIP IP communications network.  These examples are of race
   condition, which stems from the dialog state transition mainly
   established by INVITE.

   When implementing SIP, various complex situations may arise.
   Therefore, it will be helpful to provide implementors of the protocol
   with examples of recommended terminal and server behavior.



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   This document clarifies SIP UA behaviors when messages cross each
   other as race conditions.  By clarifying operation under race
   conditions, inconsistent interpretations between implementations are
   avoided and interoperability is expected to be promoted.

   It is the hope of the authors that this document will be useful for
   SIP implementors, designers, and protocol researchers and will help
   them achieve the goal of a standard implementation of RFC 3261 [1].

   These call flows are based on the version 2.0 of SIP defined in RFC
   3261 [1] with SDP usage described in RFC 3264 [2].

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in BCP 14, RFC 2119 [3].


1.1 General Assumptions

   A number of architecture, network, and protocol assumptions underlie
   the call flows in this document.  Note that these assumptions are not
   requirements.  They are outlined in this section so that they may be
   taken into consideration and help understanding the call flow
   examples.

   These flows do not assume specific underlying transport protocols
   such as TCP, TLS, and UDP.  See the discussion in RFC 3261 [1] for
   details on the transport issues for SIP.


1.2 Legend for Message Flows

   Dashed lines (---) and slash lines (/, \) represent signaling
   messages that are mandatory to the call scenario.  (X) represents
   crossover of signaling messages.  (->x, x<-) indicate that the packet
   is lost.  The arrow indicates the direction of message flow.  Double
   dashed lines (===) represent media paths between network elements.

   Messages are identified in the figures as F1, F2, etc.  These numbers
   are used for references to the message details that follow the
   Figure.
   Comments in the message details are shown in the following form:

    /* Comments.  */


1.3 SIP Protocol Assumptions

   This document does not prescribe the flows precisely as they are
   shown, but rather illustrates the principles for best practice.


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   They are best practice usages (orderings, syntax, selection of
   features for the purpose, or handling of error) of SIP methods,
   headers and parameters.  Note: The flows in this document must not
   be copied as they are by implementors because additional
   characteristics were incorporated into the document for ease of
   explanation.  To sum up, the procedures described in this document
   represent well-reviewed examples of SIP usage, which are best common
   practice according to IETF consensus.

   For simplicity in reading and editing the document, there are a
   number of differences between some of the examples and actual SIP
   messages.  For instance, Call-IDs are often repeated, CSeq often
   begins at 1, header fields are usually shown in the same order,
   usually only the minimum required header field set is shown, and
   other headers which would usually be included such as Accept, Allow,
   etc. are not shown.

   Actors:

   Element     Display Name  URI                            IP Address
   -------     ------------  ---                            ----------

   User Agent  Alice         sip:alice@atlanta.example.com  192.0.2.101
   User Agent  Bob           sip:bob@biloxi.example.com     192.0.2.201
   User Agent  Carol         sip:carol@chicago.example.com  192.0.2.202
   Proxy Server              ss.atlanta.example.com         192.0.2.111


2.  The Dialog State Machine for INVITE dialog usage

   Race conditions are generated when the dialog state of the receiving
   side differs from that of the sending side.

   For instance, a race condition occurs when UAC (User Agent Client)
   sends a CANCEL in the Early state while UAS (User Agent Server) is
   transiting from the Early state to the Confirmed state by sending a
   200 OK to initial INVITE (indicated as "ini-INVITE" hereafter).
   The DSM (dialog state machine) for the INVITE dialog usage is
   presented as follows to help understanding UA's behavior in race
   conditions.

   The DSM clarifies UA's behavior by subdividing the dialog state shown
   in RFC 3261 [1] into some internal states.  We call the state before
   a dialog establishment the Preparative state.  The Confirmed state is
   subdivided into two substates, the Moratorium and Established states,
   and the Terminated state is subdivided into the Mortal and Morgue
   states. Messages which are the triggers of the state transitions
   between these states are indicated with arrows.  In this figure,
   messages which are not related to state transition are omitted.



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   Below are the DSMs for UAC and UAS respectively.


   INV +-----------------------------------------------+
   --->|                 Preparative                   |
       +-----------------------------------------------+
         |                    |                      |
         | 3xx-6xx            | 1xx-tag              | 2xx
         |                    |                      |
         |                    |        1xx-tag       |
         |                    V        w/new tag     |
         |         +-----------------+  [new DSM]    |
         | 3xx-6xx |                 |   | (new DSM  |
         +<--------|      Early      |   |  instance |
         |         |                 |<--+  created) |
         |         +-----------------+               |
         |            |             |                |  2xx w/new tag
         |            | BYE         | 2xx            |   [new DSM]
         |            |             +------------>+<-+      | (new DSM
         |            |                           |         |  instance
   +-----C------------C-----+         +-----------C------+  |  created)
   |     | Terminated |     |         | Confirmed |      |  |
   |     |            +<----C---------|           |      |  |
   |     |            |     | BYE(sr) |           |      |  |
   |     |            V     |         |           V      |  |
   | 2xx |  +-----------+   |         |   +-----------+  |  |
   | +---C--|           |---C-+       |   |           |  |  |
   | |   |  |   Mortal  |   | | BYE(r)|   | Moratorium|<-C--+
   | +---C->|           |<--C-+       |   |           |  |
   | ACK |  +-----------+   |         |   +-----------+  |
   |     |    |             |         |         |        |
   |     |    | Timeout     |         |         | ACK    |
   |     |    |             |         |         |        |
   |     V    V             |         |         V        |
   |   +---------------+    |         |   +-----------+  |
   |   |               |    |         |   |           |--C-+
   |   |     Morgue    |    |         |   |Established|  | | 2xx,ACK
   |   |               |    |         |   |           |<-C-+
   |   +---------------+    |         |   +-----------+  |
   |                        |         |                  |
   +------------------------+         +------------------+

   (r): indicates only reception is allowed.
        Where (r) is not indicated, response means receive, request
        means send.

      Figure 1.  DSM for INVITE dialog usage (Caller)

   Figure 1 represents the caller's DSM for the INVITE dialog usage.
   Caller MAY send a BYE in the Early state, even though this behavior


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   is NOT RECOMMENDED.  The BYE sent in the Early state terminates the
   early dialog with a specific To tag.  That is, when a proxy is
   performing forking, the BYE is only able to terminate the early
   dialog with a particular UA.  If caller wants to terminate all early
   dialogs instead of that with a particular UA, it needs to send
   CANCEL, not BYE.  However, it is not illegal to send BYE in the Early
   state to terminate a specific early dialog according to the caller's
   intent.  Moreover, until caller receives a final response and
   terminates the INVITE transaction, the caller MUST be prepared to
   establish a dialog by receiving a new response to the INVITE even
   though it had sent a CANCEL or BYE and terminated the dialog (see
   Appendix A).








































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   INV +-----------------------------------------------+
   --->|                 Preparative                   |
       +-----------------------------------------------+
         |                         |                 |
         | 3xx-6xx                 | 1xx-tag         | 2xx
         |                         |                 |
         |                         V                 |
         |         +------------------+              |
         | 3xx-6xx |                  |              |
         +<--------|      Early       |              |
         |         |                  |              |
         |         +------------------+              |
         |            |             |                |
         |            | BYE         | 2xx            |
         |            |             +------------>+<-+
         |            |                           |
   +-----C------------C-----+         +-----------C------+
   |     | Terminated |     |         | Confirmed |      |
   |     |            +<----C---------|           |      |
   |     |            |     | BYE(sr) |           |      |
   |     |            V     |         |           V      |
   |     | +------------+   |         |   +-----------+  |
   |     | |            |---C-+       |   |           |--C-+
   |     | |   Mortal   |   | | BYE   |   | Moratorium|  | | 2xx
   |     | |            |<--C-+       |   |           |<-C-+ if ACK not
   |     | +------------+   |         |   +-----------+  |   received
   |     |   |              |         |         |        |
   |     |   | Timeout      |         |         | ACK    |
   |     |   |              |         |         |        |
   |     V   V              |         |         V        |
   |   +---------------+    |         |   +-----------+  |
   |   |               |    |         |   |           |  |
   |   |     Morgue    |    |         |   |Established|  |
   |   |               |    |         |   |           |  |
   |   +---------------+    |         |   +-----------+  |
   |                        |         |                  |
   +------------------------+         +------------------+

    (sr): indicates that both sending and reception are allowed.
         Where (sr) is not indicated, response means send,
         request means receive.

      Figure 2.  DSM for INVITE dialog usage (Callee)

   Figure 2 represents callee's DSM for the INVITE dialog usage.
   The figure does not illustrate the state transition related to
   CANCEL request.  CANCEL request does not cause a dialog state
   transition.  However, the callee terminates the dialog and triggers
   the dialog transition by sending 487 immediately after the reception
   of the CANCEL.  Considering this, the behavior upon the reception of


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   the CANCEL request is further explained in Appendix C.

   Following are UA's behaviors in each state.

      Preparative (Pre): The Preparative state is a state until the
         early dialog is established by sending or receiving a
         provisional response with To tag after an ini-INVITE is sent or
         received.  The dialog has not existed yet in the Preparative
         state.  If UA sends or receives a 2xx response, the dialog
         state transit from the Preparative to the Moratorium state
         which is a substate of the Confirmed state.
         In addition, if UA sends or receives a 3xx-6xx response the
         dialog state transit to the Morgue state which is a substate of
         the Terminated state.  Sending an ACK for a 3xx-6xx response
         and retransmissions of 3xx-6xx are not expressed on the DSMs
         because they are sent by the INVITE transaction.

      Early (Ear): The early dialog is established by sending or
         receiving a provisional response with To tag.  The early dialog
         exists though the dialog does not exist in this state yet.
         The dialog state transits from the Early to Moratorium state, a
         substate of the Confirmed state, by sending or receiving a 2xx
         response.  In addition, the dialog state transits to the Morgue
         state, a substate of the Terminated state, by sending or
         receiving a 3xx-6xx response.  Sending an ACK for a 3xx-6xx
         response and retransmissions of 3xx-6xx are not expressed on
         this DSM because they are automatically processed on
         transaction layer and don't influence the dialog state.  UAC
         may send CANCEL in the Early state.  UAC may send BYE
         (although it is not recommended).  UAS may send a 1xx-6xx
         response.  Sending or receiving of a CANCEL request does not
         have direct influences on dialog state.  The UA's behavior upon
         the reception of the CANCEL request is further explained in
         Appendix C.

      Confirmed (Con): Sending or receiving of a 2xx final response
         establishes a dialog.  Dialog exists in this state.  The
         Confirmed state transits to the Mortal state, a substate of the
         Terminated state, by sending or receiving a BYE request.  The
         Confirmed state has two substates, the Moratorium and
         Established state, which are different in messages UAs are
         allowed to send.

      Moratorium (Mora): The Moratorium state is a substate of the
         Confirmed state and inherits the behavior of the superstate.
         The Moratorium state transits to the Established state by
         sending or receiving an ACK request.  UAC may send ACK and UAS
         may send a 2xx final response.

      Established (Est): The Established state is a substate of the


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         Confirmed state and inherits the behavior of superstate.  Both
         caller and callee may send various messages which influence a
         dialog.  Caller supports the transmission of ACK in response to
         retransmission of a 2xx response to an ini-INVITE.

      Terminated (Ter): The Terminated state is divided into two
         substates, the Mortal and Morgue states, to cover the behavior
         when a dialog is being terminated.  In this state, UAs hold
         information about the dialog which is being terminated.

      Mortal (Mort): Caller and callee enter the Mortal state by sending
         or receiving a BYE.  UA MUST NOT send any new requests within
         the dialog because there is no dialog.  (Here the new requests
         do not include ACK for 2xx and BYE for 401 or 407 as further
         explained in Appendix D below.)
         In this state, only BYE or its response can be handled, and no
         other messages can be received. This addresses the case where
         BYE is sent by both a caller and a callee to exchange reports
         about the session when it is being terminated. Therefore the
         UA possesses dialog information for internal processing but the
         dialog shouldn't be externally visible.  The UA stops managing
         its dialog state and changes it to the Morgue state, when the
         BYE transaction is terminated.

      Morgue (Morg): The dialog no longer exists in this state.
         Sending or receiving of signaling which influences a dialog is
         not performed.  (A dialog is literally terminated.)
         Caller and callee enter the Morgue state via the termination of
         the BYE or INVITE transaction.


3.  Race conditions

   This section details race condition between two SIP UAs, Alice and
   Bob.  Alice (sip:alice@atlanta.example.com) and Bob
   (sip:bob@biloxi.example.com) are assumed to be SIP phones or
   SIP-enabled devices.  Only significant signaling is illustrated.
   Dialog state transitions caused by sending or receiving of SIP
   messages as well as '*race*', which indicates race condition, are
   shown.  (For abbreviations for the dialog state transitions, refer to
   Section 2.)
   '*race*' indicates the moment when a race condition occurs.

   Examples of race conditions are shown below.


3.1 Receiving message in the Moratorium State

   This section shows some examples of call flow in race condition when
   receiving the message from other states in the Moratorium state.


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3.1.1 Receiving Initial INVITE retransmission (Preparative state)
      in Moratorium state

  State  Alice                               Bob  State
         |                                     |
         |            ini-INVITE F1            |
         |------------------------------------>|
    Pre  |         180 F2(Packet loss)         |  Pre
         |            x<-----------------------|
         |                                     |  Ear
         | ini-INVITE F4(=F1)           200 F3 |
         |------------------     --------------|
         |                   \ /               |  Mora
         |                    X                |
         |                   / \               |
         |<-----------------     ------------->|  *race*
   Mora  |                ACK F5               |
         |------------------------------------>|
    Est  |                                     |  Est
         |                                     |


   This scenario illustrates the race condition which occurs when UAS
   receives a Preparative message in the Moratorium state.  All
   provisional responses to the initial INVITE (ini-INVITE F1) are lost,
   and UAC retransmits an ini-INVITE (F4).  At the same time as
   retransmission, UAS generates a 200 OK (F3) to the ini-INVITE and it
   terminates an INVITE server transaction, according to Section
   13.3.1.4 of RFC 3261 [1].
   However, it is reported that terminating an INVITE server transaction
   by 200 OK is a SIP bug.  (http://bugs.sipit.net/, #769)
   Therefore, the INVITE server transaction is not terminated at F3, and
   the F4 MUST be properly handled as a retransmission.
   (UAs that do not deal with this bug still need to recognize the
   dialog relying on its From tag and Call-ID, and the retransmitted
   request relying on the CSeq header field value even though it does
   not match the transaction.)

   In RFC 3261 [1], it is not specified whether UAS retransmits 200 to
   the retransmission of ini-INVITE.  Considering the retransmission of
   200 triggered by timer (TU keeps retransmitting 200 based on T1 and
   T2 until it receives an ACK), according to Section 13.3.1.4 of RFC
   3261 [1], it seems unnecessary to retransmit 200 when the UAS
   receives the retransmission of ini-INVITE.  (For implementation, it
   does not matter if the UAS sends the retransmission of 200, since the
   200 does not cause any problem.)






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   Message Details

   F1 INVITE Alice -> Bob

   F2 180 Ringing Bob -> Alice

   /* 180 response is lost and does not reach Alice.  */

   F3 200 OK Bob -> Alice

   /* According to 13.3.1.4 of RFC 3261, an INVITE server transaction is
      terminated at this point.  However, this has been reported as a
      SIP bug, and the UAS MUST correctly recognize the ini-INVITE (F4)
      as a retransmission.  */

   F4 INVITE (retransmission) Alice -> Bob

   /* F4 is a retransmission of F1.  They are exactly the same INVITE
      request.  For UAs do not deal with the bug reported in #769 (an
      INVITE server transaction is terminated by 200 to INVITE), this
      request does not match the transaction as well as the dialog
      since it does not have a To tag.
      However, Bob have to recognize the retransmitted INVITE correctly,
      without treating it as a new INVITE.  */

   F5 ACK Alice -> Bob


3.1.2 Receiving CANCEL (Early state)
      in Moratorium state

  State  Alice                        Bob  State
         |                              |
         |          INVITE F1           |
         |----------------------------->|
    Pre  |       180 Ringing F2         |  Pre
         |<-----------------------------|
    Ear  |                              |  Ear
         |CANCEL F3       200(INVITE) F4|
         |------------     -------------|
         |             \ /              |  Mora
         |              X               |
         |             / \              |
         |<-----------     ------------>|  *race*
   Mora  |                              |
         | ACK F6         200(CANCEL) F5|
         |------------     -------------|
    Est  |             \ /              |
         |              X               |
         |             / \              |


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         |<-----------     ------------>|
         |                              |  Est
         |       One Way RTP Media      |
         | (Two Way RTP Media possible) |
         |<=============================|
         |            BYE F7            |
         |----------------------------->|
   Mort  |            200 F8            |  Mort
         |<-----------------------------|
         | ^                          ^ |
         | | Timer K                  | |
         | V                          | |
   Morg  |                    Timer J | |
         |                            V |
         |                              |  Morg
         |                              |


   This scenario illustrates the race condition which occurs when UAS
   receives an Early message, CANCEL, in the Moratorium state.  Alice
   sends a CANCEL and Bob sends a 200 OK response to the initial INVITE
   message at the same time.  As described in the previous section,
   according to RFC 3261, an INVITE server transaction is supposed to be
   terminated by a 200 response, but this has been reported as a bug
   #769.

   This section describes a case in which an INVITE server transaction
   is not terminated by a 200 response to the INVITE request.  In this
   case, there is an INVITE transaction which the CANCEL request
   matches, so a 200 response is sent to the request.  This 200 response
   simply means that the next hop received the CANCEL request
   (Successful CANCEL (200) does not mean an INVITE failure).  When UAS
   does not deal with #769, UAC MAY receive a 481 response for CANCEL
   since there is no transaction which the CANCEL request matches.  This
   481 simply means that there is no matching INVITE server transaction
   and CANCEL is not sent to the next hop.
   Regardless of the success/failure of the CANCEL, Alice checks the
   final response to INVITE, and if she receives 200 to the INVITE
   request she immediately sends a BYE and terminates the dialog.
   (Section 15, RFC 3261 [1])

   From the time F1 is received by Bob until the time that F8 is sent by
   Bob, media may be flowing one way from Bob to Alice.  From the time
   that an answer is received by Alice from Bob there is the possibility
   that media may flow from Alice to Bob as well.  However, once Alice
   has decided to cancel the call, she presumably will not send media,
   so practically speaking the media stream will remain one way.





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   Message Details

   F1 INVITE Alice -> Bob

   F2 180 Ringing Bob -> Alice

   F3 CANCEL Alice -> Bob

   /* Alice sends a CANCEL in the Early state.  */

   F4 200 OK (INVITE) Bob -> Alice

   /* Alice receives a 200 to INVITE (F1) in the Moratorium state.
      Alice has the potential to send as well as receive media, but in
      practice will not send because there is an intent to end the
      call.  */

   F5 200 OK (CANCEL) Bob -> Alice

   /* 200 to CANCEL simply means that the CANCEL was received.
      The 200 response is sent, since this document deals with the bug
      reported in #769.  When an INVITE server transaction is terminated
      as the procedure stated in RFC 3261, UAC MAY receive 481 response
      instead of 200.  */

   F6 ACK Alice -> Bob

   /* INVITE is successful, and the CANCEL becomes invalid.  Bob
      establishes RTP streams.
      However, the next BYE request immediately terminates the dialog
      and session.  */

   F7 BYE Alice -> Bob

   F8 200 OK Bob -> Alice

3.1.3 Receiving BYE (Early state)
      in Moratorium state


  State  Alice                          Bob  State
         |                                |
         |         ini-INVITE F1          |
         |------------------------------->|
    Pre  |            180 F2              |  Pre
         |<-------------------------------|
    Ear  |                                |  Ear
         |    BYE F4        200(INVITE) F3|
         |-------------     --------------|
   Mort  |              \ /               |  Mora


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         |               X                |
         |              / \               |
         |<------------     ------------->|  *race*
         |                                |  Mort
         |    ACK F5         200(BYE) F6  |
         |-------------     --------------|
         |              \ /            ^  |
         |               X             |  |
         |              / \            |  |
         |<------------     ------------->|
         | ^                           |  |
         | | Timer K                   |  |
         | V                           |  |
   Morg  |                     Timer J |  |
         |                             V  |
         |                                |  Morg
         |                                |


   This scenario illustrates the race condition which occurs when UAS
   receives an Early message, BYE, in the Moratorium state.  Alice sends
   a BYE in the Early state and Bob sends a 200 OK response to the
   initial INVITE request at the same time.  Bob receives the BYE in the
   Confirmed dialog state though Alice sent the request in the Early
   state (As explained in Section 2 and Appendix A, this behavior is NOT
   RECOMMENDED).  When a proxy is performing forking, the BYE is only
   able to terminate the early dialog with a particular UA.  If caller
   wants to terminate all early dialogs instead of that with a
   particular UA, it needs to send CANCEL, not BYE.  However, it is not
   illegal to send BYE in the Early state to terminate a specific early
   dialog according to the caller's intent.

   The BYE functions normally even if it is received after the INVITE
   transaction termination because BYE differs from CANCEL, and is sent
   not to the request but to the dialog.  Alice gets into the Mortal
   state on sending the BYE request, and remains Mortal until the
   Timer K timeout occurs.  In the Mortal state, UAC does not establish
   a session, even though it receives a 200 response to INVITE.  Even
   so, the UAC sends an ACK to 200 for the completion of INVITE
   transaction.  The ACK is always sent to complete the three-way
   handshake of INVITE transaction (Further explained in Appendix D
   below).


   Message Details

   F1 INVITE Alice -> Bob

   F2 180 Ringing Bob -> Alice



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   F3 200 OK (ini-INVITE) Bob -> Alice

   F4 BYE Alice -> Bob

   /* Alice transits to the Mortal state upon sending BYE.
      Therefore, after this, she does not begin a session even though
      she receives a 200 response with an answer.  */

   F5 ACK Alice -> Bob

   F6 200 OK (BYE) Bob -> Alice


3.1.4 Receiving re-INVITE (Established state)
      in Moratorium state (case 1)


  State  Alice                          Bob  State
         |                                |
         |    ini-INVITE w/offer1 F1      |
         |------------------------------->|
    Pre  |             180 F2             |  Pre
         |<-------------------------------|
    Ear  |                                |  Ear
         |   200(ini-INV) w/answer1 F3    |
         |<-------------------------------|
   Mora  |       ACK F4(packet loss)      |  Mora
         |-------------------->x          |
    Est  |                                |
         | re-INVITE F6      200 F5(=F3)  |
         |   w/offer2         w/answer1   |
         |-------------     --------------|
         |              \ /               |
         |               X                |
         |              / \               |
         |<------------     ------------->|  *race*
         |                  200(re-INV) F8|
         | ACK F7(=F4)        w/answer2   |
         |-------------     --------------|
         |              \ /               |
         |               X                |
         |              / \               |
         |<------------     ------------->|
         |         ACK (re-INV) F9        |  Est
         |------------------------------->|
         |                                |
         |                                |


   This scenario illustrates the race condition which occurs when UAS


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   receives re-INVITE request sent from the Established state, in the
   Moratorium state.

   UAS receives a re-INVITE (w/offer2) before receiving an ACK for
   ini-INVITE (w/offer1).  UAS sends a 200 OK (w/answer2) to the
   re-INVITE (F8) because it has sent a 200 OK (w/answer1) to the
   ini-INVITE (F3, F5) and the dialog has already been established.
   (Because F5 is a retransmission of F3, SDP negotiation is not
   performed here.)

   If a 200 OK to the ini-INVITE has an offer and the answer is in the
   ACK, it is recommended that UA return a 491 to the re-INVITE (refer
   to 3.1.5).  (Note: 500 with Retry-After header may be returned, if
   the 491 response is understood to indicate request collision.
   However, 491 is recommended here because 500 applies to so many cases
   that it is difficult to determine what the real problem was.)
   As it can be seen in Section 3.3.2 below, the 491 response seems to
   be closely related to session establishment, even in cases other than
   INVITE cross-over.  This example recommends 200 be sent instead of
   491 because it does not have influence on session.  However, a 491
   response can also lead to the same outcome, so the either response
   can be used.

   Moreover, if UAS doesn't receive an ACK for a long time, it should
   send a BYE and terminate the dialog.  Note that ACK F7 has the same
   CSeq number as ini-INVITE F1 (See Section 13.2.2.4 of RFC 3261).
   The UA should not reject or drop the ACK on grounds of CSeq number.

   Note: There is a hint for implementation to avoid the race conditions
   of this type.  That is for the caller to delay sending re-INVITE F6
   for some period of time (2 seconds, perhaps), from which the caller
   is reasonably able to assume that its ACK has been received.
   Implementors can decouple the actions of the user (e.g. pressing the
   hold button) from the actions of the protocol (the sending of
   re-INVITE F6), so the UA can behave as such.  In this case, it is
   dependent on the implementor's choice how long it waits.  In most
   cases, such implementation may be useful to prevent a type of race
   condition shown in this section.


   Message Details

   F1 INVITE Alice -> Bob

   INVITE sip:bob@biloxi.example.com SIP/2.0
   Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
   Max-Forwards: 70
   From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
   To: Bob <sip:bob@biloxi.example.com>
   Call-ID: 3848276298220188511@atlanta.example.com


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   CSeq: 1 INVITE
   Contact: <sip:alice@client.atlanta.example.com;transport=udp>
   Content-Type: application/sdp
   Content-Length: 137

   v=0
   o=alice 2890844526 2890844526 IN IP4 client.atlanta.example.com
   s=-
   c=IN IP4 192.0.2.101
   t=0 0
   m=audio 49172 RTP/AVP 0
   a=rtpmap:0 PCMU/8000

   /* Detailed messages are shown for the sequence to illustrate offer
      and answer examples.  */


   F2 180 Ringing Bob -> Alice

   SIP/2.0 180 Ringing
   Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
    ;received=192.0.2.101
   From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
   To: Bob <sip:bob@biloxi.example.com>;tag=8321234356
   Call-ID: 3848276298220188511@atlanta.example.com
   CSeq: 1 INVITE
   Contact: <sip:bob@client.biloxi.example.com;transport=udp>
   Content-Length: 0


   F3 200 OK Bob -> Alice

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
    ;received=192.0.2.101
   From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
   To: Bob <sip:bob@biloxi.example.com>;tag=8321234356
   Call-ID: 3848276298220188511@atlanta.example.com
   CSeq: 1 INVITE
   Contact: <sip:bob@client.biloxi.example.com;transport=udp>
   Content-Type: application/sdp
   Content-Length: 133

   v=0
   o=bob 2890844527 2890844527 IN IP4 client.biloxi.example.com
   s=-
   c=IN IP4 192.0.2.201
   t=0 0
   m=audio 3456 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


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   F4 ACK Alice -> Bob

   ACK sip:bob@client.biloxi.example.com SIP/2.0
   Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bKnashd8
   Max-Forwards: 70
   From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
   To: Bob <sip:bob@biloxi.example.com>;tag=8321234356
   Call-ID: 3848276298220188511@atlanta.example.com
   CSeq: 1 ACK
   Content-Length: 0

   /* ACK request is lost.  */


   F5(=F3) 200 OK Bob -> Alice (retransmission)

   /* UAS retransmits a 200 OK to the ini-INVITE since it has not
      received an ACK.  */


   F6 re-INVITE Alice -> Bob

   INVITE sip:sip:bob@client.biloxi.example.com SIP/2.0
   Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf91
   Max-Forwards: 70
   From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
   To: Bob <sip:bob@biloxi.example.com>;tag=8321234356
   Call-ID: 3848276298220188511@atlanta.example.com
   CSeq: 2 INVITE
   Content-Length: 147

   v=0
   o=alice 2890844526 2890844527 IN IP4 client.atlanta.example.com
   s=-
   c=IN IP4 192.0.2.101
   t=0 0
   m=audio 49172 RTP/AVP 0
   a=rtpmap:0 PCMU/8000
   a=sendonly


   F7(=F4) ACK Alice -> Bob (retransmission)

   F8 200 OK (re-INVITE) Bob -> Alice

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf91
   Max-Forwards: 70
   From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
   To: Bob <sip:bob@biloxi.example.com>;tag=8321234356


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   Call-ID: 3848276298220188511@atlanta.example.com
   CSeq: 2 INVITE
   Content-Length: 143

   v=0
   o=bob 2890844527 2890844528 IN IP4 client.biloxi.example.com
   s=-
   c=IN IP4 192.0.2.201
   t=0 0
   m=audio 3456 RTP/AVP 0
   a=rtpmap:0 PCMU/8000
   a=recvonly


   F9 ACK (re-INVITE) Alice -> Bob

   ACK sip:sip:bob@client.biloxi.example.com SIP/2.0
   Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK230f21
   Max-Forwards: 70
   From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
   To: Bob <sip:bob@biloxi.example.com>;tag=8321234356
   Call-ID: 3848276298220188511@atlanta.example.com
   CSeq: 2 ACK
   Content-Length: 0


3.1.5 Receiving re-INVITE (Established state)
      in Moratorium state (case 2)

  State  Alice                          Bob  State
         |                                |
         |    ini-INVITE (no offer) F1    |
         |------------------------------->|
    Pre  |             180 F2             |  Pre
         |<-------------------------------|
    Ear  |                                |  Ear
         |    200(ini-INV) w/offer1 F3    |
         |<-------------------------------|
   Mora  |  ACK w/answer1 F4(packet loss) |  Mora
         |-------------------->x          |
    Est  |                                |
         | re-INVITE F6      200 F5(=F3)  |
         |   w/offer2         w/offer1    |
         |-------------     --------------|
         |              \ /               |
         |               X                |
         |              / \               |
         |<------------     ------------->|
         | ACK F7(=F4)      491(re-INV) F8|
         |-------------     --------------|


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         |              \ /               |
         |               X                |
         |              / \               |
         |<------------     ------------->|
         |        ACK (re-INV) F9         |  Est
         |------------------------------->|
         |                                |
         |                                |


   This scenario is basically the same as that of Section 3.1.4, but
   differs in sending an offer in 200 and an answer in ACK.  Different
   to the previous case, the offer in the 200 (F3) and the offer in the
   re-INVITE (F6) collide with each other.

   Bob sends 491 to re-INVITE (F6) since he is not able to properly
   handle a new request until he receives an answer.  (Note: 500 with
   Retry-After header may be returned, if the 491 response is understood
   to indicate request collision.  However, 491 is recommended here
   because 500 applies to so many cases that it is difficult to
   determine what the real problem was.)
   Even if F6 is UPDATE with offer, it will reach the same result.

   Note: As noted in Section 3.1.4, it may be useful for the caller to
   delay sending re-INVITE F6 for some period of time (2 seconds,
   perhaps), from which the caller is reasonably able to assume that its
   ACK has been received, to prevent a type of race condition shown in
   this section.


   Message Details

   F1 INVITE Alice -> Bob

   INVITE sip:bob@biloxi.example.com SIP/2.0
   Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
   Max-Forwards: 70
   From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
   To: Bob <sip:bob@biloxi.example.com>
   Call-ID: 3848276298220188511@atlanta.example.com
   CSeq: 1 INVITE
   Contact: <sip:alice@client.atlanta.example.com;transport=udp>
   Content-Length: 0

   /* The request does not contain an offer.  Detailed messages are
      shown for the sequence to illustrate offer and answer
      examples.  */


   F2 180 Ringing Bob -> Alice


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   F3 200 OK Bob -> Alice

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
    ;received=192.0.2.101
   From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
   To: Bob <sip:bob@biloxi.example.com>;tag=8321234356
   Call-ID: 3848276298220188511@atlanta.example.com
   CSeq: 1 INVITE
   Contact: <sip:bob@client.biloxi.example.com;transport=udp>
   Content-Type: application/sdp
   Content-Length: 133

   v=0
   o=bob 2890844527 2890844527 IN IP4 client.biloxi.example.com
   s=-
   c=IN IP4 192.0.2.201
   t=0 0
   m=audio 3456 RTP/AVP 0
   a=rtpmap:0 PCMU/8000

   /* An offer is made in 200.  */


   F4 ACK Alice -> Bob

   ACK sip:bob@client.biloxi.example.com SIP/2.0
   Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bKnashd8
   Max-Forwards: 70
   From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
   To: Bob <sip:bob@biloxi.example.com>;tag=8321234356
   Call-ID: 3848276298220188511@atlanta.example.com
   CSeq: 1 ACK
   Content-Type: application/sdp
   Content-Length: 137

   v=0
   o=alice 2890844526 2890844526 IN IP4 client.atlanta.example.com
   s=-
   c=IN IP4 192.0.2.101
   t=0 0
   m=audio 49172 RTP/AVP 0
   a=rtpmap:0 PCMU/8000

   /* The request contains an answer, but the request is lost.  */


   F5(=F3) 200 OK Bob -> Alice (retransmission)

   /* UAS retransmits a 200 OK to the ini-INVITE since it has not


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      received an ACK.  */


   F6 re-INVITE Alice -> Bob

   INVITE sip:sip:bob@client.biloxi.example.com SIP/2.0

   Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf91
   Max-Forwards: 70
   From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
   To: Bob <sip:bob@biloxi.example.com>;tag=8321234356
   Call-ID: 3848276298220188511@atlanta.example.com
   CSeq: 2 INVITE
   Content-Length: 147

   v=0
   o=alice 2890844526 2890844527 IN IP4 client.atlanta.example.com
   s=-
   c=IN IP4 192.0.2.101
   t=0 0
   m=audio 49172 RTP/AVP 0
   a=rtpmap:0 PCMU/8000
   a=sendonly

   /* The request contains an offer.  */


   F7(=F4) ACK Alice -> Bob (retransmission)

   /* A retransmission triggered by the reception of a retransmitted
      200.  */


   F8 491 (re-INVITE) Bob -> Alice

   /* Bob sends 491 (Request Pending), since Bob has a pending
      offer.  */

   F9 ACK (re-INVITE) Alice -> Bob


3.1.6 Receiving BYE (Established state)
      in Moratorium state

  State  Alice                     Bob  State
         |                           |
         |         INVITE F1         |
         |-------------------------->|
    Pre  |      180 Ringing F2       |  Pre
         |<--------------------------|


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    Ear  |                           |  Ear
         |         200 OK F3         |
         |<--------------------------|
   Mora  |    ACK F4(packet loss)    |  Mora
         |--------------->x          |
    Est  |   Both Way RTP Media      |
         |<=========================>|
         |   BYE F6       200 F5(=F3)|
         |-----------     -----------|
   Mort  |            \ /            |
         |             X             |
         |            / \            |
         |<----------     ---------->|  *race*
         |ACK F7(=F4)     200(BYE) F8|  Mort
         |-----------     -----------|
         |            \ /            |
         |             X             |
         |            / \            |
         |<----------     ---------->|
         | ^                       ^ |
         | | Timer K               | |
         | V                       | |
   Morg  |                 Timer J | |
         |                         V |
         |                           |  Morg
         |                           |


   This scenario illustrates the race condition which occurs when UAS
   receives an Established message, BYE, in the Moratorium state.
   An ACK request for a 200 OK response is lost (or delayed).
   Immediately after Bob retransmits the 200 OK to ini-INVITE, and Alice
   sends a BYE request at the same time.  Depending on the
   implementation of a SIP UA, Alice may start a session again by the
   reception of the retransmitted 200 OK with SDP since she has already
   terminated a session by sending a BYE.  In that case, when UAC
   receives a retransmitted 200 OK after sending a BYE, a session should
   not be started again since the session which is not associated with
   dialog still remains.  Moreover, in the case where UAS sends an offer
   in a 200 OK , UAS should not start a session again for the same
   reason if UAS receives a retransmitted ACK after receiving a BYE.

   Note: As noted in Section 3.1.4, there is a hint for implementation
   to avoid the race conditions of this type.  That is for the caller to
   delay sending BYE F6 for some period of time (2 seconds, perhaps),
   from which the caller is reasonably able to assume that its ACK has
   been received.  Implementors can decouple the actions of the user
   (e.g. hanging up) from the actions of the protocol (the sending of
   BYE F6), so the UA can behave as such.  In this case, it is dependent
   on the implementor's choice how long it waits.  In most cases, such


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   implementation may be useful to prevent a type of race condition
   shown in this section.


   Message Details

   F1 INVITE Alice -> Bob

   F2 180 Ringing Bob -> Alice

   F3 200 OK Bob -> Alice

   F4 ACK Alice -> Bob

   /* ACK request is lost.  */


   F5(=F3) 200 OK Bob -> Alice

   /* UAS retransmits a 200 OK to the ini-INVITE since it has not
      received an ACK.  */


   F6 BYE Alice -> Bob

   /* Bob retransmits a 200 OK and Alice sends a BYE at the same time.
      Alice transits to the Mortal state, so she does not begin a
      session after this even though she receives a 200 response to
      the re-INVITE.  */

   F7(=F4) ACK Alice -> Bob

   F8 200 OK (BYE) Bob -> Alice

   /* Bob sends a 200 OK to the BYE.  */


3.2 Receiving message in the Mortal State

   This section shows some examples of call flow in race condition
   when receiving the message from other states in the Mortal state.


3.2.1 Receiving BYE (Establish state)
      in Mortal state

  State  Alice                  Bob  State
         |                        |
         |       INVITE F1        |
         |----------------------->|


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    Pre  |    180 Ringing F2      |  Pre
         |<-----------------------|
    Ear  |                        |  Ear
         |       200 OK F3        |
         |<-----------------------|
   Mora  |         ACK F4         |  Mora
         |----------------------->|
    Est  |   Both Way RTP Media   |  Est
         |<======================>|
         |                        |
         | BYE F5         BYE F6  |
         |---------     ----------|
   Mort  |          \ /           |  Mort
         |           X            |
         |          / \           |
         |<--------     --------->|  *race*
         |                        |
         | 200 F8         200 F7  |
         |---------     ----------|
         |          \ /           |
         |           X            |
         |          / \           |
         |<--------     --------->|
         | ^                    ^ |
         | | Timer K            | |
         | V                    | |
   Morg  |              Timer J | |
         |                      V |
         |                        |  Morg
         |                        |


   This scenario illustrates the race condition which occurs when UAS
   receives an Established message, BYE, in the Mortal state.
   Alice and Bob send a BYE at the same time.  A dialog and session are
   ended shortly after a BYE request is passed to a client transaction.
   As shown in Section 2, UA remains in the Mortal state.

   UAs in the Mortal state return error responses to the requests that
   operate dialog or session, such as re-INVITE, UPDATE, or REFER.
   However, UA shall return 200 OK to the BYE taking the use case into
   consideration where BYE request is sent by both a caller and a callee
   to exchange reports about the session when it is being terminated.
   (Since the dialogue and the session both terminate when a BYE is
   sent, the choice of sending 200 or an error response upon receiving
   BYE in the Mortal state does not affect the resulting termination.
   Therefore, even though this example uses a 200 response, other
   responses can also be used.)




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   Message Details

   F1 INVITE Alice -> Bob

   F2 180 Ringing Bob -> Alice

   F3 200 OK Bob -> Alice

   F4 ACK Alice -> Bob

   F5 BYE Alice -> Bob

   /* The session is terminated at the moment Alice sends a BYE.
      The dialog still exists then, but it is certain to be terminated
      in a short period of time.  The dialog is completely
      terminated when the timeout of the BYE request occurs.  */


   F6 BYE Bob -> Alice

   /* Bob has also transmitted a BYE simultaneously with Alice.
      Bob terminates the session and the dialog.  */


   F7 200 OK Bob -> Alice

   /* Since the dialog is in the Moratorium state, Bob responds with
      a 200 to the BYE request.  */

   F8 200 OK Alice -> Bob

   /* Since Alice has transited from the Established state to the Mortal
      state by sending BYE, Alice responds with a 200 to the BYE
      request.  */


3.2.2 Receiving re-INVITE (Establish state)
      in Mortal state

  State  Alice                  Bob  State
         |                        |
         |       INVITE F1        |
         |----------------------->|
    Pre  |    180 Ringing F2      |  Pre
         |<-----------------------|
    Ear  |                        |  Ear
         |       200 OK F3        |
         |<-----------------------|
   Mora  |         ACK F4         |  Mora
         |----------------------->|


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    Est  |   Both Way RTP Media   |  Est
         |<======================>|
         |                        |
         | BYE F5     re-INVITE F6|
         |---------     ----------|
   Mort  |          \ /           |
         |           X            |
         |          / \           |
 *race*  |<--------     --------->|
         |                        |  Mort
         | 481 F8         200 F7  |
         | (re-INV)       (BYE)   |
         |---------     ----------|
         |          \ /           |^
         |           X            ||
         |          / \           ||Timer J
         |<--------     --------->||
        ^|    ACK (re-INV) F9     ||
        ||<-----------------------||
 Timer K||                        ||
        V|                        ||
   Morg  |                        |V
         |                        |  Morg
         |                        |


   This scenario illustrates the race condition which occurs when UAS
   receives an Established message, re-INVITE, in the Mortal state.
   Bob sends a re-INVITE, and Alice sends a BYE at the same time.
   The re-INVITE is responded by a 481, since the TU of Alice has
   transited from the Established state to the Mortal state by sending
   BYE.  Bob sends ACK for the 481 response, because the ACK for error
   responses is handled by the transaction layer and at the point of
   receiving the 481 the INVITE client transaction still remains (even
   though the dialog has been terminated).


   Message Details

   F1 INVITE Alice -> Bob

   F2 180 Ringing Bob -> Alice

   F3 200 OK Bob -> Alice

   F4 ACK Alice -> Bob

   F5 BYE Alice -> Bob

   /* Alice sends a BYE and terminates the session, and transits from


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      the Established state to the Mortal state.  */

   F6 re-INVITE Bob -> Alice

   /* Alice sends a BYE, and Bob sends a re-INVITE at the same time.
      The dialog state transits to the Mortal state at the moment
      Alice sends the BYE, but Bob does not know it until he receives
      the BYE.  Therefore, the dialog is in the Terminated state from
      Alice's point of view, but it is the Confirmed state
      from Bob's point of view.  A race condition occurs.  */


   F7 200 OK (BYE) Bob -> Alice

   F8 481 Call/Transaction Does Not Exist (re-INVITE) Alice -> Bob

   /* Since Alice is in the Mortal state, she responds with a 481 to the
      re-INVITE.  */


   F9 ACK (re-INVITE) Bob -> Alice

   /* ACK for an error response is handled by Bob's INVITE client
      transaction.  */


3.2.3 Receiving 200OK for re-INVITE (Established state)
      in Mortal state

  State  Alice                  Bob  State
         |                        |
         |       INVITE F1        |
         |----------------------->|
    Pre  |    180 Ringing F2      |  Pre
         |<-----------------------|
    Ear  |                        |  Ear
         |       200 OK F3        |
         |<-----------------------|
   Mora  |         ACK F4         |  Mora
         |----------------------->|
    Est  |   Both Way RTP Media   |  Est
         |<======================>|
         |                        |
         |      re-INVITE F5      |
         |<-----------------------|
         | 200 F7         BYE F6  |
         |---------     ----------|
         |          \ /           |  Mort
         |           X            |
         |          / \           |


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         |<--------     --------->|  *race*
   Mort  | 200 F8         ACK F9  |
         | (BYE)         (re-INV) |
         |---------     ----------|
         | ^        \ /           |
         | |         X            |
         | |        / \           |
         |<--------     --------->|
         | |                    ^ |
         | |            Timer K | |
         | |                    V |
         | | Timer J              |  Morg
         | V                      |
   Morg  |                        |
         |                        |

   This scenario illustrates the race condition which occurs when UAS
   receives an Established message, 200 to re-INVITE, in the Mortal
   state.  Bob sends a BYE immediately after sending a re-INVITE.
   (A user is not conscious that refresher sends re-INVITE
   automatically.  For example, in the case of a telephone application,
   it is possible that a user places a receiver immediately after
   refresher.)
   Bob sends ACK for a 200 response to INVITE in the Mortal state, so
   that he completes the INVITE transaction.

   Note: As noted in Section 3.1.4, there is a hint for implementation
   to avoid the race conditions of this type.  That is for the UAC to
   delay sending BYE F6 until re-INVITE F5 transaction completes.
   Implementors can decouple the actions of the user (e.g. hanging up)
   from the actions of the protocol (the sending of BYE F6), so the UA
   can behave as such.  In this case, it is dependent on the
   implementor's choice how long it waits.  In most cases, such
   implementation may be useful to prevent a type of race condition
   shown in this section.


   Message Details

   F1 INVITE Alice -> Bob

   F2 180 Ringing Bob -> Alice

   F3 200 OK Bob -> Alice

   F4 ACK Alice -> Bob

   F5 re-INVITE Bob -> Alice

   INVITE sip:alice@client.atlanta.example.com SIP/2.0


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   Via: SIP/2.0/UDP client.biloxi.example.com:5060;branch=z9hG4bKnashd7
   Session-Expires: 300;refresher=uac
   Supported: timer
   Max-Forwards: 70
   From: Bob <sip:bob@biloxi.example.com>;tag=8321234356
   To: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
   Call-ID: 3848276298220188511@atlanta.example.com
   CSeq: 1 INVITE
   Content-Length: 0

   /* Some detailed messages are shown for the sequence to illustrate
      that the re-INVITE is handled in the usual manner in the Mortal
      state.  */


   F6 BYE Bob -> Alice

   /* Bob sends BYE immediately after sending the re-INVITE.
      Bob terminates the session and transits from the Established
      state to the Mortal state.  */


   F7 200 OK (re-INVITE) Alice -> Bob

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bKnashd7
    ;received=192.0.2.201
   Require: timer
   Session-Expires: 300;refresher=uac
   From: Bob <sip:bob@biloxi.example.com>;tag=8321234356
   To: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
   Call-ID: 3848276298220188511@atlanta.example.com
   CSeq: 1 INVITE
   Content-Length: 0

   /* Bob sends BYE, and Alice responds with a 200 OK to the re-INVITE.
      A race condition occurs.  */


   F8 200 OK (BYE) Alice -> Bob

   F9 ACK (re-INVITE) Bob -> Alice

   ACK sip:alice@client.atlanta.example.com SIP/2.0
   Via: SIP/2.0/UDP client.biloxi.example.com:5060;branch=z9hG4bK74b44
   Max-Forwards: 70
   From: Bob <sip:bob@biloxi.example.com>;tag=8321234356
   To: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
   Call-ID: 3848276298220188511@atlanta.example.com
   CSeq: 2 ACK


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   Content-Length: 0

   /* Bob sends ACK in the Mortal state to complete the three-way
      handshake of the INVITE transaction.  */


3.2.4 Receiving ACK (Moratorium state)
      in Mortal state

  State  Alice                          Bob  State
         |                                |
         |         ini-INVITE F1          |
         |------------------------------->|
    Pre  |            180 F2              |  Pre
         |<-------------------------------|
    Ear  |            200 F3              |  Ear
         |<-------------------------------|
   Mora  |                                |  Mora
         |    ACK F4            BYE F5    |
         |-------------     --------------|
    Est  |              \ /               |  Mort
         |               X                |
         |              / \               |
         |<------------     ------------->|  *race*
   Mort  |            200 F6              |
         |------------------------------->|
         | ^                            ^ |
         | |                    Timer K | |
         | |                            V |
         | | Timer J                      |  Morg
         | V                              |
   Morg  |                                |
         |                                |


   This scenario illustrates the race condition which occurs when UAS
   receives an Established message, ACK to 200, in the Mortal state.
   Alice sends an ACK and Bob sends a BYE at the same time.  When the
   offer is in a 2xx, and the answer is in an ACK, this example is in
   a race condition.  The session is not started by receiving the ACK
   because Bob has already terminated the session by sending the BYE.
   The answer in the ACK request is just ignored.

   Note: As noted in Section 3.1.4, there is a hint for implementation
   to avoid the race conditions of this type.  Implementors can decouple
   the actions of the user (e.g. hanging up) from the actions of the
   protocol (the sending of BYE F5), so the UA can behave as such.  In
   this case, it is dependent on the implementor's choice how long it
   waits.  In most cases, such implementation may be useful to prevent a
   type of race condition shown in this section.


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   Message Details

   F1 INVITE Alice -> Bob

   F2 180 Ringing Bob -> Alice

   F3 200 OK Bob -> Alice

   F4 ACK Alice -> Bob

   /* RTP streams are established between Alice and Bob */

   F5 BYE Alice -> Bob

   F6 200 OK Bob -> Alice

   /* Alice sends a BYE and terminates the session and dialog.  */


3.3 Other race conditions

   This section shows the examples in race condition that are not
   directly related to the dialog state transition.  In SIP, processing
   functions are deployed in three layers, dialog, session, and
   transaction.  There are related each other, but have to be treated
   separately.  Section 17 of RFC 3261 [1] details the processing on
   transactions.  This draft has tried so far to clarify the processing
   on dialogs.  This section explains race conditions which are related
   to sessions established by SIP.


3.3.1 Re-INVITE crossover

   Alice                         Bob
     |                            |
     |         INVITE F1          |
     |--------------------------->|
     |      180 Ringing F2        |
     |<---------------------------|
     |          200 OK F3         |
     |<---------------------------|
     |           ACK F4           |
     |--------------------------->|
     |     Both Way RTP Media     |
     |<==========================>|
     |                            |
     |re-INVITE F5   re-INVITE F6 |
     |------------   -------------|
     |            \ /             |
     |             X              |


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     |            / \             |
     |<-----------   ------------>|
     |   491 F8        491 F7     |
     |------------   -------------|
     |            \ /             |
     |             X              |
     |            / \             |
     |<-----------   ------------>|
     |  ^ ACK F9         ^ ACK F10|
     |--|---------   ----|--------|
     |  |          \ /   |        |
     |  |           X    |        |
     |  |          / \   |        |
     |<-|----------   ---|------->|
     |  |                |        |
     |  |0-2.0 sec       |        |
     |  |                |        |
     |  v  re-INVITE F11(=F6)     |
     |<------------------|--------|
     |     200 OK F12    |        |
     |-------------------|------->|
     |       ACK F13     |        |
     |<------------------|--------|
     |                   |        |
     |                   |2.1-4.0 sec
     |                   |        |
     |re-INVITE F14(=F5) v        |
     |--------------------------->|
     |         200 OK F15         |
     |<---------------------------|
     |          ACK F16           |
     |--------------------------->|
     |                            |
     |                            |


   In this scenario, Alice and Bob send re-INVITE at the same time.
   When two re-INVITEs cross in the same dialog, they resend re-INVITEs
   after different interval for each, according to Section 14.1, of
   RFC 3261 [1].  When Alice sends the re-INVITE and it crosses, the
   re-INVITE will be sent again after 2.1-4.0 seconds because she owns
   the Call-ID (she generated it).  Bob will send an INVITE again after
   0.0-2.0 seconds, because Bob isn't the owner of the Call-ID.
   Therefore, each user agent must remember whether it has generated the
   Call-ID of the dialog or not, in case an INVITE may cross with
   another INVITE.

   In this example, Alice's re-INVITE is for session modification and
   Bob's re-INVITE is for session refresh.  In this case, after the 491
   responses, Bob retransmits the re-INVITE for session refresh earlier


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   than Alice.  If Alice was to retransmit her re-INVITE (that is, if
   she was not the owner of Call-ID), the request would refresh and
   modify the session at the same time.  Then Bob would know that he
   would not need to retransmit his re-INVITE to refresh the session.

   In another instance where two re-INVITEs for session modification,
   cross over, retransmitting the same re-INVITE again after 491 by the
   Call-ID owner (the UA which retransmits its re-INVITE after the other
   UA) may result in a behavior different from what the user originally
   intended to, so the UA needs to decide if the retransmission of the
   re-INVITE is necessary.
   (For example, when a call hold and an addition of video media cross
   over, mere retransmission of the re-INVITE at the firing of the timer
   may result in the situation where the video is transmitted
   immediately after the holding of the audio.  This behavior is
   probably not intended by the users.)


   Message Details

   F1 INVITE Alice -> Bob

   F2 180 Ringing Bob -> Alice

   F3 200 OK Bob -> Alice

   F4 ACK Alice -> Bob

   F5 re-INVITE Alice -> Bob

   INVITE sip:sip:bob@client.biloxi.example.com SIP/2.0
   Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
   Max-Forwards: 70
   From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
   To: Bob <sip:bob@biloxi.example.com>;tag=8321234356
   Call-ID: 3848276298220188511@atlanta.example.com
   CSeq: 2 INVITE
   Content-Length: 147

   v=0
   o=alice 2890844526 2890844527 IN IP4 client.atlanta.example.com
   s=-
   c=IN IP4 192.0.2.101
   t=0 0
   m=audio 49172 RTP/AVP 0
   a=rtpmap:0 PCMU/8000
   a=sendonly

   /* Some detailed messages are shown for the sequence to illustrate
      what sort of INVITE requests crossed over each other.  */


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   F6 re-INVITE Bob -> Alice

   INVITE sip:alice@client.atlanta.example.com SIP/2.0
   Via: SIP/2.0/UDP client.biloxi.example.com:5060;branch=z9hG4bKnashd7
   Session-Expires: 300;refresher=uac
   Supported: timer
   Max-Forwards: 70
   From: Bob <sip:bob@biloxi.example.com>;tag=8321234356
   To: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
   Call-ID: 3848276298220188511@atlanta.example.com
   CSeq: 1 INVITE
   Content-Length: 0

   /* A re-INVITE request for a session refresh and that for a call hold
      are sent at the same time.  */


   F7 491 Request Pending Bob -> Alice

   /* Since a re-INVITE is in progress, a 491 response is returned.  */

   F8 491 Request Pending Alice -> Bob

   F9 ACK (INVITE) Alice -> Bob

   F10 ACK (INVITE) Bob -> Alice

   F11 re-INVITE Bob -> Alice

   INVITE sip:alice@client.atlanta.example.com SIP/2.0
   Via: SIP/2.0/UDP client.biloxi.example.com:5060;branch=z9hG4bKnashd71
   Session-Expires: 300;refresher=uac
   Supported: timer
   Max-Forwards: 70
   From: Bob <sip:bob@biloxi.example.com>;tag=8321234356
   To: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
   Call-ID: 3848276298220188511@atlanta.example.com
   CSeq: 2 INVITE
   Content-Type: application/sdp
   Content-Length: 133

   v=0
   o=bob 2890844527 2890844527 IN IP4 client.biloxi.example.com
   s=-
   c=IN IP4 192.0.2.201
   t=0 0
   m=audio 3456 RTP/AVP 0
   a=rtpmap:0 PCMU/8000

   /* Since Bob is not the owner of the Call-ID, he sends a re-INVITE


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      again after 0.0-2.0 seconds.  */


   F12 200 OK Alice -> Bob

   F13 ACK Bob -> Alice

   F14 re-INVITE Alice -> Bob

   INVITE sip:sip:bob@client.biloxi.example.com SIP/2.0
   Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf91
   Max-Forwards: 70
   From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
   To: Bob <sip:bob@biloxi.example.com>;tag=8321234356
   Call-ID: 3848276298220188511@atlanta.example.com
   CSeq: 3 INVITE
   Content-Length: 147

   v=0
   o=alice 2890844526 2890844527 IN IP4 client.atlanta.example.com
   s=-
   c=IN IP4 192.0.2.101
   t=0 0
   m=audio 49172 RTP/AVP 0
   a=rtpmap:0 PCMU/8000
   a=sendonly

   /* Since Alice is the owner of the Call-ID, Alice sends a re-INVITE
      again after 2.1-4.0 seconds.  */


   F15 200 OK Bob -> Alice

   F16 ACK Alice -> Bob


3.3.2 UPDATE and re-INVITE crossover

   Alice                         Bob
     |                            |
     |         INVITE F1          |
     |--------------------------->|
     |      180 Ringing F2        |
     |<---------------------------|
     |                            |
     |          200 OK F3         |
     |<---------------------------|
     |           ACK F4           |
     |--------------------------->|
     |     Both Way RTP Media     |


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     |<==========================>|
     |                            |
     |  UPDATE F5    re-INVITE F6 |
     |------------   -------------|
     |            \ /             |
     |             X              |
     |            / \             |
     |<-----------   ------------>|
     |   491 F8        491 F7     |
     |   (re-INVITE)   (UPDATE)   |
     |------------   -------------|
     |            \ /             |
     |             X              |
     |            / \             |
     |<-----------   ------------>|
     |  ^       ACK F9   ^        |
     |<-|----------------|--------|
     |  |                |        |
     |  |0-2.0 sec       |        |
     |  |                |        |
     |  v  re-INVITE F10 |        |
     |<------------------|--------|
     |     200 OK F11    |        |
     |-------------------|------->|
     |       ACK F12     |        |
     |<------------------|--------|
     |                   |        |
     |                   |2.1-4.0 sec
     |                   |        |
     |      UPDATE F13   v        |
     |--------------------------->|
     |         200 OK F14         |
     |<---------------------------|
     |                            |
     |                            |


   In this scenario, the UPDATE contains a SDP offer, therefore the
   UPDATE and re-INVITE are responded with 491 as in the case of
   "re-INVITE crossover".  When an UPDATE for refresher which doesn't
   contain a session description and a re-INVITE crossed each other,
   both requests succeed with 200 (491 means that UA have a pending
   request).  Moreover, the same is equally true for UPDATE crossover.
   In the former case where either UPDATE contains a session description
   the requests fail with 491, and in the latter cases succeed with 200.

Editor's Note:
 A 491 response is sent because SDP offer is pending, and 491 is an
 error which is related to matters that behave on the session
 established by SIP.


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   Message Details

   F1 INVITE Alice -> Bob

   F2 180 Ringing Bob -> Alice

   F3 200 OK Bob -> Alice

   F4 ACK Alice -> Bob

   F5 UPDATE Alice -> Bob

   UPDATE sip:sip:bob@client.biloxi.example.com SIP/2.0
   Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
   Max-Forwards: 70
   From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
   To: Bob <sip:bob@biloxi.example.com>;tag=8321234356
   Call-ID: 3848276298220188511@atlanta.example.com
   CSeq: 2 UPDATE
   Content-Length: 147

   v=0
   o=alice 2890844526 2890844527 IN IP4 client.atlanta.example.com
   s=-
   c=IN IP4 192.0.2.101
   t=0 0
   m=audio 49172 RTP/AVP 0
   a=rtpmap:0 PCMU/8000
   a=sendonly

   /* Some detailed messages are shown for the sequence to illustrate
      the example messages crossed over each other.  */


   F6 re-INVITE Bob -> Alice

   INVITE sip:alice@client.atlanta.example.com SIP/2.0
   Via: SIP/2.0/UDP client.biloxi.example.com:5060;branch=z9hG4bKnashd7
   Session-Expires: 300;refresher=uac
   Supported: timer
   Max-Forwards: 70
   From: Bob <sip:bob@biloxi.example.com>;tag=8321234356
   To: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
   Call-ID: 3848276298220188511@atlanta.example.com
   CSeq: 1 INVITE
   Content-Length: 0

   /* A case where a re-INVITE for a session refresh and a UPDATE for a
      call hold are sent at the same time.  */



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   F7 491 Request Pending (UPDATE) Bob -> Alice

   /* Since a re-INVITE is in process, a 491 response are returned.  */

   F8 491 Request Pending (re-INVITE) Alice -> Bob

   F9 ACK (re-INVITE) Alice -> Bob

   F10 re-INVITE Bob -> Alice

   INVITE sip:alice@client.atlanta.example.com SIP/2.0
   Via: SIP/2.0/UDP client.biloxi.example.com:5060;branch=z9hG4bKnashd71
   Session-Expires: 300;refresher=uac
   Supported: timer
   Max-Forwards: 70
   From: Bob <sip:bob@biloxi.example.com>;tag=8321234356
   To: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
   Call-ID: 3848276298220188511@atlanta.example.com
   CSeq: 2 INVITE
   Content-Type: application/sdp
   Content-Length: 133

   v=0
   o=bob 2890844527 2890844527 IN IP4 client.biloxi.example.com
   s=-
   c=IN IP4 192.0.2.201
   t=0 0
   m=audio 3456 RTP/AVP 0
   a=rtpmap:0 PCMU/8000

   /* Since Bob is not the owner of Call-ID, Bob sends an INVITE again
      after 0.0-2.0 seconds.  */


   F11 200 OK Alice -> Bob

   F12 ACK Bob -> Alice

   F13 UPDATE Alice -> Bob

   UPDATE sip:sip:bob@client.biloxi.example.com SIP/2.0
   Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK74bf91
   Max-Forwards: 70
   From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
   To: Bob <sip:bob@biloxi.example.com>;tag=8321234356
   Call-ID: 3848276298220188511@atlanta.example.com
   CSeq: 3 UPDATE
   Content-Length: 147

   v=0


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   o=alice 2890844526 2890844527 IN IP4 client.atlanta.example.com
   s=-
   c=IN IP4 192.0.2.101
   t=0 0
   m=audio 49172 RTP/AVP 0
   a=rtpmap:0 PCMU/8000
   a=sendonly

   /* Since Alice is the owner of the Call-ID, Alice sends the UPDATE
      again after 2.1-4.0 seconds.  */

   F14 200 OK Bob -> Alice


3.3.3 Receiving REFER (Establish state)
      in Mortal state

  State  Alice                  Bob  State
         |                        |
         |       INVITE F1        |
         |----------------------->|
    Pre  |    180 Ringing F2      |  Pre
         |<-----------------------|
    Ear  |                        |  Ear
         |       200 OK F3        |
         |<-----------------------|
   Mora  |         ACK F4         |  Mora
         |----------------------->|
    Est  |   Both Way RTP Media   |  Est
         |<======================>|
         |                        |
         | BYE F5        REFER F6 |
         |---------     ----------|
   Mort  |          \ /           |
         |           X            |
         |          / \           |
 *race*  |<--------     --------->|
         |                        |  Mort
         | 481 F8         200 F7  |
         | (REFER)        (BYE)   |
         |---------     ----------|
         |          \ /         ^ |
         |           X          | |
         |          / \         | |
         |<--------     --------->|
         | ^                    | |
         | | Timer K            | |
         | V            Timer J | |
   Morg  |                      V |
         |                        |  Morg


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         |                        |


   This scenario illustrates the race condition which occurs when UAS
   receives an Established message, REFER, in the Mortal state.
   Bob sends a REFER, and Alice sends a BYE at the same time.  Bob send
   the REFER in the same dialog.  Alice's dialog state moves to the
   Mortal state at the point of sending BYE.  In the Mortal state, UA
   possesses dialog information for internal process but dialog
   shouldn't exist outwardly.  Therefore, UA sends an error response to
   a REFER which is transmitted as a mid-dialog request.  So, Alice in
   the Mortal state sends an error response to the REFER.
   However, Bob has already started the SUBSCRIBE usage with REFER, so
   the dialog continues until the SUBSCRIBE usage terminates, even
   though the INVITE dialog usage terminates by receiving BYE.  Bob's
   behavior in this case needs to follow the procedure in the dialog
   usage draft [6].  (For handling of dialogs with multiple usages see
   the dialog usage draft [6].)


   Message Details

   F1 INVITE Alice -> Bob

   F2 180 Ringing Bob -> Alice

   F3 200 OK Bob -> Alice

   F4 ACK Alice -> Bob

   F5 BYE Alice -> Bob

   /* Alice sends a BYE request and terminates the session, and transits
      from the Confirmed state to Terminated state.  */

   F6 REFER Bob -> Alice

   /* Alice sends a BYE, and Bob sends a REFER at the same time.  Bob
      sends the REFER on the INVITE dialog.  The dialog state transits
      to the Mortal state at the moment Alice sends the BYE, but Bob
      doesn't know it until he receives the BYE.  A race condition
      occurs.  */


   F7 200 OK (BYE) Bob -> Alice

   F8 481 Call/Transaction Does Not Exist (REFER) Alice -> Bob

   /* Alice in the Mortal state sends a 481 to the REFER.  */



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4.  IANA Considerations

   This document has no actions for IANA.


5.  Security Considerations

   This document contains clarifications of behavior specified in RFCs
   3261 [1], 3264 [2], and 3515 [4].  The security considerations of
   those documents continue to apply after the application of these
   clarifications.


6.  Acknowledgements

   The authors would like to thank Robert Sparks, Dean Willis,
   Cullen Jennings, James M. Polk, Gonzalo Camarillo, Kenichi Ogami,
   Akihiro Shimizu, Mayumi Munakata, Yasunori Inagaki,
   Tadaatsu Kidokoro, Kenichi Hiragi, Dale Worley, Vijay K. Gurbani
   and Anders Kristensen for their comments on this document.


7.  References

7.1 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, "An Offer/Answer Model with
       the Session Description Protocol (SDP)", RFC 3264, June 2002.

   [3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
       Levels", BCP 14, RFC 2119, March 1997.

   [4] Sparks, R., "The Session Initiation Protocol (SIP) Refer
       Method", RFC 3515, April 2003.

   [5] Rosenberg, J. and H. Schulzrinne, "Reliability of Provisional
       Responses in the Session Initiation Protocol (SIP)", RFC 3262,
       June 2002.

7.2 Informative References

   [6] Sparks, R., "Multiple Dialog Usages in the Session Initiation
       Protocol", draft-ietf-sipping-dialogusage-06 (work in progress),
       February 16, 2007.




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Appendix A - BYE on the Early Dialog

   This section, related to Section 3.1.3, explains why BYE is not
   recommended in the Early state, illustrating the case in which BYE in
   the early dialog triggers confusion.

   Alice            Proxy               Bob   Carol
     |                |                  |      |
     |   INVITE F1    |                  |      |
     |--------------->|    INVITE F2     |      |
     |     100 F3     |----------------->|      |
     |<---------------| 180(To tag=A) F4 |      |
     |    180(A) F5   |<-----------------|      |
     |<---------------|                  |      |
     |                |       INVITE(Fork) F6   |
     |                |------------------------>|
     |                |                100 F7   |
     |    BYE(A) F8   |<------------------------|
     |--------------->|    BYE(A) F9     |      |
     |                |----------------->|      |
     |                |  200(A,BYE) F10  |      |
     | 200(A,BYE) F11 |<-----------------|      |
     |<---------------|  487(A,INV) F12  |      |
     |                |<-----------------|      |
     |                |    ACK(A) F13    |      |
     |                |----------------->|      |
     |                |                  |      |
     |                |                         |
     |                |     200(To tag=B) F13   |
     |   200(B) F14   |<------------------------|
     |<---------------|                         |
     |   ACK(B) F15   |                         |
     |--------------->|            ACK(B) F16   |
     |                |------------------------>|
     |   BYE(B) F17   |                         |
     |--------------->|            BYE(B) F18   |
     |                |------------------------>|
     |                |            200(B) F19   |
     |   200(B) F20   |<------------------------|
     |<---------------|                         |
     |                |                         |
     |                |                         |

   Care is advised in sending BYE in the Early state when forking by a
   proxy is expected.  In this example, the BYE request progresses
   normally, and it succeeds in correctly terminating the dialog with
   Bob.  After Bob terminates the dialog by receiving the BYE, he sends
   487 to the ini-INVITE.  According to Section 15.1.2 of RFC 3261 [1],
   it is RECOMMENDED for UAS to generate 487 to any pending requests
   after receiving BYE.  In the example, Bob sends 487 to ini-INVITE


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   since he receives BYE while the ini-INVITE is in pending state.

   However, Alice receives a final response to INVITE (a 200 from Carol)
   even though she has successfully terminated the dialog with Bob.
   This means that, regardless of the success/failure of the BYE in the
   Early state, Alice MUST be prepared for the establishment of a new
   dialog until receiving the final response for the INVITE and
   terminating the INVITE transaction.

   It is not illegal to send BYE in the Early state to terminate a
   specific early dialog according to the caller's intent.  However, the
   choice of BYE or CANCEL in the Early state must be made carefully.
   CANCEL is appropriate when the goal is to abandon the call attempt
   entirely.  BYE is appropriate when the goal is to abandon a
   particular early dialog while allowing the call to be completed with
   other destinations.  When using either BYE or CANCEL the UAC must be
   prepared for the possibility that a call may still be established to
   one (or more) destinations.


Appendix B - BYE request overlapped on re-INVITE

   UAC                    UAS
    |                      |
 The session has been already established
   ==========================
    |   re-INVITE F1       |
    |--------------------->|
    |   BYE F2             |
    |--------------------->|
    |   200(BYE) F3        |
    |<---------------------|
    |   INVITE F4(=F1)     |
    |--------------------->|
    |                      |
    |                      |

   This case could look similar to the one in Section 3.2.3.  However,
   it is not a race condition.  This case describes the behavior where
   there is no response for INVITE for some reasons.  The appendix
   explains the behavior in such case and its rationale behind, since
   this case is likely to cause confusion.

   First of all, it is important not to confuse the behavior of the
   transaction layer and that of the dialog layer.  RFC 3261 [1] details
   the transaction layer behavior.  The dialog layer behavior is
   explained in this document.  It has to be noted that these behaviors
   are independent of each other, even though the both layers change
   their states triggered by sending or receiving of the same SIP
   messages (A dialog can be terminated even though a transaction still


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   remain, and vice versa).

   In the sequence above, there is no response for F1, and F2 (BYE) is
   sent immediately after F1 (F1 is a mid-dialog request.  If F1 was
   ini-INVITE, BYE could not be sent before UAC received a provisional
   response to the request with To tag).

   Below is a figure which illustrates UAC's dialog state and
   transaction state.

  BYE   INV  dialog UAC                    UAS
               :     |                      |
               :     |                      |
               |     |   re-INVITE F1       |
         o     |     |--------------------->|
         |     |     |   BYE F2             |
   o     |  (Mortal) |--------------------->|
   |     |     |     |   200(BYE) F3        |
   |     |     |     |<---------------------|
   |     |     |     |   INVITE F4(=F1)     |
   |     |     |     |--------------------->|
   |     |     |     |   481(INV) F5        |
   |     |     |     |<---------------------|
   |     |     |     |   ACK(INV) F6        |
   |     |     |     |--------------------->|
   |     |     |     |                      |
   o     |     o     |                      |
         |           |                      |
         o           |                      |
                     |                      |

   For UAC, the INVITE client transaction begins at the point F1 is
   sent.  The UAC sends BYE (F2) immediately after F1.  This is a
   legitimate behavior.  (Usually the usage of each SIP method is
   independent, for BYE and others.  However, it should be noted that it
   is prohibited to send a request with a SDP offer while the previous
   offer is in progress.)

   After that, F2 triggers the BYE client transaction.  At the same
   time, the dialog state transits to the Mortal state and then only a
   BYE or its response can be handled.

   It is permitted to send F4 (a retransmission of INVITE) in the Mortal
   state, because the retransmission of F1 is handled by the transaction
   layer, and the INVITE transaction has not yet transited to the
   Terminated state.  As it is mentioned above, the dialog and the
   transaction behave independently each other.
   Therefore the transaction handling has to be continued even though
   the dialog moved to the Terminated state.



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   Next, UAS's state is shown below.

  UAC                    UAS dialog  INV   BYE
   |                      |     :
   |                      |     :
   |   re-INVITE F1       |     |
   |-------------->x      |     |
   |   BYE F2             |     |
   |--------------------->|     |           o
   |   200(BYE) F3        |  (Mortal)       |
   |<---------------------|     |           |<-Start TimerJ
   |   INVITE F4(=F1)     |     |           |
   |--------------------->|     |     o     |
   |   4xx/5xx(INV) F5    |     o     |     o
   |<---------------------|           |
   |   ACK(INV) F6        |           |
   |--------------------->|           |<-Start TimerI
   |                      |           |
   |                      |           |
   |                      |           o
   |                      |

   For UAS, it can be regarded that F1 packet is lost or delayed (Here
   the behavior is explained for the case UAS receives F2 BYE before F1
   INVITE).  Therefore, F2 triggers the BYE transaction for UAS, and
   simultaneously the dialog moves to the Mortal state.
   Then, upon the reception of F4 the INVITE server transaction begins.
   (It is allowed to start the INVITE server transaction in the Mortal
   state.  The INVITE server transaction begins to handle received SIP
   request regardless of the dialog state.)
   UAS's TU sends an appropriate error response for F4 INVITE, either
   481 (because the TU knows that the dialog which matches to the
   INVITE is in the Terminated state) or 500 (because the re-sent F4 has
   an out-of-order CSeq).
   (It is mentioned above that F4 (and F1) INVITE is a mid-dialog
   request.  Mid-dialog requests have a To tag.  It should be noted that
   UAS's TU does not begin a new dialog upon the reception of INVITE
   with a To tag.)


Appendix C - UA's behavior for CANCEL

   This section explains the CANCEL behaviors which indirectly involve
   in the dialog state transition in the Early state.  CANCEL does not
   have any influence on UAC's dialog state.  However, the request has
   indirect influence on the dialog state transition because it has a
   significant effect on ini-INVITE.  For UAS the CANCEL request has
   more direct effects on the dialog than the sending of CANCEL by UAC,
   because they can be a trigger to send the 487 response.  Figure 3
   explains UAS's behavior in the Early state.  This flow diagram is


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   only an explanatory figure, and the actual dialog state transition is
   as illustrated in Figure 1 and 2.

   In the flow, full lines are related to dialog state transition, and
   dotted lines are involved with CANCEL.  (r) represents the reception
   of signaling, and (s) means sending.  There is no dialog state for
   CANCEL, but here the Cancelled state is virtually handled just for
   the ease of understanding of the UA's behavior when it sends and
   receives CANCEL.

   Below, UAS's flow is explained.

   +-------------+
   | Preparative |---+
   +-------------+   |
     :   | 1xx(s)    |
     :   V           |
     : +-------+     | 2xx(s)
     : | Early |-----+------+
     : +-------+            |
     :     :                V
     :     :           +-----------+
     :     :           | Confirmed |<...
     :.....:           +-----------+   :
        :                   |  :       :
        :             BYE(r)|  :       :
        : CANCEL(r)         |  :.......:
        V                   |    CANCEL(r)
    .............           |
    : Cancelled :           |
    :...........:           |
       | 487(s)             |
       |                    |
       +--------------------+
                  |
                  V
            +------------+
            | Terminated |
            +------------+

 Figure 3.  CANCEL flow diagram for UAS

   There are two behaviors for UAS depending on the state when it
   receives CANCEL.

   One is when UAS receives CANCEL in the Early states.  In this case
   the UAS immediately sends 487 for the INVITE, and the dialog transits
   to the Terminated state.

   The other is the case in which UAS receives CANCEL in the Confirmed


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   state.  In this case the dialog state transition does not occur
   because UAS has already sent a final response to the INVITE to which
   the CANCEL is targeted.
   (Note that, because of UAC's behavior, a UAS that receives a CANCEL
   in the Confirmed state can expect to receive a BYE immediately and
   move to the Terminated state.  However, the UAS's state does not
   transit until it actually receives BYE.)


Appendix D - Notes on the request in Mortal state

   This section describes UA's behavior in the Mortal state which needs
   careful attention.  Note that every transaction completes independent
   of others, following the principle of RFC 3261 [1].

   In the Mortal state, BYE can be accepted, and the other messages in
   the INVITE dialog usage are responded with an error.  However,
   sending of ACK and the authentication procedure for BYE are conducted
   in this state.  (The handling of messages concerning multiple dialog
   usages is out of the scope of this document.  Refer to [6] for
   further information.)

   ACK for error responses is handled by the transaction layer, so the
   handling is not related to the dialog state.  Unlike the ACK for
   error responses, ACK for 2xx responses is a request newly generated
   by TU.  However, the ACK for 2xx and the ACK for error responses are
   both a part of the INVITE transaction, even though their handling
   differs (Section 17.1.1.1, RFC 3261 [1]).
   Therefore, the INVITE transaction is completed by the three-way
   handshake, which includes ACK, even in the Mortal state.

   Considering actual implementation, UA needs to keep the INVITE dialog
   usage until the Mortal state finishes, so that it is able to ACK for
   a 2xx response in the Mortal state.
   If a 2xx to INVITE is received in the Mortal state, the duration of
   the INVITE dialog usage will be extended to 64*T1 seconds after the
   receiving of the 2xx, to cope with the possible 2xx retransmission.
   (The duration of the 2xx retransmission is 64*T1, so the UA need to
   be prepared to handle the retransmission for this duration.)
   However, the UA shall send error response to other requests, since
   the INVITE dialog usage in the Mortal state is kept only for the
   sending of ACK for 2xx.

   BYE authentication procedure shall be processed in the Mortal state.
   When authentication is requested by 401 or 407 response, UAC resends
   BYE with an appropriate credentials.  Also UAS handles the
   retransmission of the BYE which it requested authentication itself.





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Appendix E - Forking and receiving new To tags

   This section details the behavior of TU when it receives multiple
   responses with a different To tag to ini-INVITE.

   When an INVITE is forked inside a SIP network, there is a possibility
   that the TU receives multiple responses with a different To tag to
   ini-INVITE (See Section 12.1, 13.1, 13.2.2.4, 16.7, 19.3, etc. of RFC
   3261).  If the TU receives multiple 1xx responses with a different
   To tag, the original DSM forks and a new DSM instance is created.  As
   a consequence multiple early dialogs are generated.

   If one of the multiple early dialogs receives a 2xx response, it
   naturally transits to the Confirmed state.  No DSM state transition
   occurs for the other early dialogs, and their sessions (early media)
   terminate.  The TU of the UAC terminates the INVITE transaction after
   64*T1 seconds starting at the point of receiving the first 2xx
   response.  Moreover, all mortal early dialogs which do not transit to
   the Established state are terminated (See Section 13.2.2.4 of
   RFC 3261).  By "mortal early dialog" we mean any early dialog that
   the UA will terminate when another early dialog is confirmed.

   Below is an example sequence in which two 180 responses with a
   different To tag are received, and then a 200 response for one of the
   early dialogs (dialog A) is received.  Dot lines (..) in sequences
   are auxiliary lines to represent the influence on dialog B.

                                 UAC
                  dialog(A)       |    INVITE F1
                   Pre o          |------------------------->
                       |          |    100 F2
                       |          |<-------------------------
                       |          |    180(To tag=A) F3
                   Ear |          |<-------------------------
        dialog(B)      |          |
    forked new DSM     |          |    180(To tag=B) F4
        Ear o..........|..........|<-------------------------
            |          |          |
            |          |          |    200(A) F5
 terminate->|.....Mora |..........|<-------------------------
   early    |          | ^        |    ACK(A) F6
    media   |      Est | |        |------------------------->
            |          | |        |
            |          | |64*T1   |
            |          | |(13.2.2.4 of RFC 3261)
            |          | |        |
            |          | |        |
            |          | V        |
            o..........|.(terminate INVITE transaction)
        terminated     |          |


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         dialog(B)     |          |
                       |          |

 Figure 4. Receiving 1xx responses with different To tags

   The figure above shows the DSM inside a SIP TU.  Triggered by the
   reception of a provisional response with a different To tag
   (F4 180(To tag=B)), DSM forks and the early dialog B is generated.
   After 64*T1 seconds after the dialog A receives 200 OK response, the
   dialog B, which does not transit to the Established state,
   terminates.

   Next, the behavior of a TU which receives multiple 2xx responses with
   a different To tag is explained.  When a mortal early dialog, which
   did not match the first 2xx response the TU received, receives
   another 2xx response which matches its To tag before 64*T1 INVITE
   transaction timeout, its DSM state transits to the Confirmed state.
   However, the session on the mortal early dialog is terminated when
   the TU receives the first 2xx to establish a dialog, so no session is
   established for the mortal early dialog.  Therefore, when the mortal
   early dialog receives a 2xx response, the TU send an ACK and,
   immediately after, the TU usually sends a BYE to terminate DSM.  (In
   special cases, e.g. a UA intends to establish multiple dialogs, the
   TU may not send the BYE.)

   The handling of the second early dialog after receiving the 200 for
   the first dialog is quite appropriate for a typical device, such as a
   phone.  It is important to note that what is being shown is a
   typically useful action and not the only valid one.  Some devices
   might want to handle things differently.  For instance, a conference
   focus that has sent out an INVITE that forks may want to accept and
   mix all the dialogs it gets.  In that case, no early dialog is
   treated as mortal.

   Below is an example sequence in which two 180 responses with a
   different To tag are received and then a 200 response for each of the
   early dialogs is received.

                                 UAC
                  dialog(A)       |    INVITE F1
                   Pre o          |----------------------->
                       |          |    100 F2
                       |          |<-----------------------
                       |          |    180(To tag=A) F3
       dialog(B)   Ear |          |<-----------------------
   forked new DSM      |          |    180(To tag=B) F4
        Ear o..........|..........|<-----------------------
            |          |          |
            |          |          |    200(A) F5
 terminate->|.....Mora |..........|<-----------------------


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   early    |          | ^        |    ACK(A) F6
    media   |      Est | |        |----------------------->
            |          | |64*T1   |
            |          | |        |    200(B) F7
       Mora |..........|.|........|<-----------------------
            |          | |        |    ACK(B) F8
        Est |..........|.|........|----------------------->
            |          | |        |    BYE(B) F9
       Mort |..........|.|........|----------------------->
        ^   |          | |        |    200(B) F10
        |   |          | |        |<-----------------------
        |Timer K       | |        |
        |   |          | V        |
        |   |          | (terminate INVITE transaction)
        V   |          |          |
       Morg o          |          |
                       |          |

 Figure 5. Receiving 1xx and 2xx responses with different To tags

   Below is an example sequence when a TU receives multiple 200
   responses with a different To tag before 64*T1 timeout of the INVITE
   transaction, even though a TU does not receive provisional response,
   the TU needs to respond to the 2xx responses (See Section 13.2.2.4 of
   RFC 3261).  In that case, the DSM state is forked at the Confirmed
   state, and then the TU sends an ACK for the 2xx response and,
   immediately after, the TU usually sends a BYE.  (In special cases,
   e.g. a UA intends to establish multiple dialogs, the TU may not send
   the BYE.)

                                 UAC
                  dialog(A)       |    INVITE F1
                   Pre o          |----------------------->
                       |          |    100 F2
                       |          |<-----------------------
                       |          |    180(To tag=A) F3
                   Ear |          |<-----------------------
                       |          |
                       |          |    200(A) F4
                  Mora |..........|<-----------------------
                       | ^        |    ACK(A) F5
                   Est | |        |----------------------->
                       | |        |
       dialog(B)       | |64*T1   |
   forked new DSM      | |        |    200(To tag=B) F6
       Mora o..........|.|........|<-----------------------
            |          | |        |    ACK(B) F7
        Est |..........|.|........|----------------------->
            |          | |        |    BYE(B) F8
       Mort |..........|.|........|----------------------->


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        ^   |          | |        |    200(B) F9
        |   |          | |        |<-----------------------
        |   |          | V        |
        |Timer K       | (terminate INVITE transaction)
        |   |          |          |
        V   |          |          |
       Morg o          |          |
                       |          |

 Figure 6. Receiving 2xx responses with different To tags

   Below is an example sequence in which the option tag 100rel (RFC 3262
   [5]) is required by a 180.

   If a forking proxy supports 100rel, it transparently transmits to the
   UAC a provisional response which contains a Require header with the
   value of 100rel.  Upon receiving a provisional response with 100rel,
   the UAC establishes the early dialog (B) and send PRACK.  (Here, also
   every transaction completes independent of others".)

   As Figure. 4, the early dialog (B) terminates at the same time the
   INVITE transaction terminates.  In the case where a proxy does not
   support 100rel, the provisional response will be handled in the usual
   way (a provisional response with 100rel is discarded by the proxy,
   not to be transmitted to the UAC).

                                 UAC
                  dialog(A)       |    INVITE F1
                   Pre o          |------------------------->
                       |          |    100 F2
                       |          |<-------------------------
                       |          |    180(To tag=A) F3
                   Ear |          |<-------------------------
                       |          |    200(A) F4
                  Mora |..........|<-------------------------
                       | ^        |    ACK(A) F5
                   Est | |        |------------------------->
        dialog(B)      | |        |
    forked new DSM     | |        |    180(To tag=B) w/100rel F6
        Ear o..........|.|........|<-------------------------
            |          | |        |    PRACK(B) F7
            |          | |        |------------------------->
            |          | |        |    200(B,PRACK) F8
            |          | |        |<-------------------------
            |          | |64*T1   |
            |          | |(13.2.2.4 of RFC 3261)
            |          | |        |
            |          | |        |
            |          | |        |
            |          | V        |


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            o..........|.(terminate INVITE transaction)
        terminated     |          |
         dialog(B)     |          |
                       |          |

 Figure 7. Receiving 1xx responses with different To tags (with 100rel)


Author's Addresses

   All listed authors actively contributed large amounts of text to this
   document.

   Miki Hasebe
   NTT-east Corporation
   19-2 Nishi-shinjuku 3-chome Shinjuku-ku Tokyo 163-8019 Japan

   EMail: hasebe.miki@east.ntt.co.jp


   Jun Koshiko
   NTT-east Corporation
   19-2 Nishi-shinjuku 3-chome Shinjuku-ku Tokyo 163-8019 Japan

   EMail: j.koshiko@east.ntt.co.jp


   Yasushi Suzuki
   NTT-east Corporation
   19-2 Nishi-shinjuku 3-chome Shinjuku-ku Tokyo 163-8019 Japan

   EMail: suzuki.yasushi@east.ntt.co.jp


   Tomoyuki Yoshikawa
   NTT-east Corporation
   19-2 Nishi-shinjuku 3-chome Shinjuku-ku Tokyo 163-8019 Japan

   EMail: tomoyuki.yoshikawa@east.ntt.co.jp


   Paul H. Kyzivat
   Cisco Systems, Inc.
   1414 Massachusetts Avenue
   Boxborough, MA  01719
   USA

   Email: pkyzivat@cisco.com




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