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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
Expires: Jun 11th, 2007                                        Y. SUZUKI
                                                            T. YOSHIKAWA
                                                                NTT-East
                                                              P. Kyzivat
                                                     Cisco Systems, Inc.
                                                          Dec 11th, 2006


  Examples call flow in race condition on Session Initiation Protocol
               draft-ietf-sipping-race-examples-00.txt

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Copyright Notice

   Copyright (C) The Internet Society (2006).


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 condition . . . . . . . . . . . . . . . . . . . . . . . . .9
      3.1 Receiving message in the Moratorium State. . . . . . . . . .9
        3.1.1 Receiving Initial INVITE retransmission(Trying state). .9
        3.1.2 Receiving CANCEL(Proceeding or Early state). . . . . . .11
        3.1.3 Receiving BYE (Early state). . . . . . . . . . . . . . .12
        3.1.4 Receiving re-INVITE (Established state)(case 1). . . . .14
        3.1.5 Receiving re-INVITE (Established state)(case 2). . . . .18
        3.1.6 Receiving BYE (Established state). . . . . . . . . . . .21
      3.2 Receiving message in the Mortal State. . . . . . . . . . . .23
        3.2.1 Receiving BYE(Established state) . . . . . . . . . . . .23
        3.2.2 Receiving re-INVITE(Established state) . . . . . . . . .26
        3.2.3 Receiving 200OK for re-INVITE(Established state) . . . .29
        3.2.4 Receiving ACK (Moratorium state) . . . . . . . . . . . .31
      3.3 other race condition . . . . . . . . . . . . . . . . . . . .33
        3.3.1 re-INVITE crossover. . . . . . . . . . . . . . . . . . .33
        3.3.2 UPDATE and re-INVITE crossover . . . . . . . . . . . . .37
        3.3.3 Receiving REFER(Established state) . . . . . . . . . . .41
   Appendix A. BYE on the Early Dialog . . . . . . . . . . . . . . . .43
   Appendix B. BYE request overlapped on re-INVITE . . . . . . . . . .44
   Appendix C. UA's behaviour for CANCEL . . . . . . . . . . . . . . .46
   Appendix D. Notes on the request in Mortal state. . . . . . . . . .48
   References  . . . . . . . . . . . . . . . . . . . . . . . . . . . .49
   Author's Addresses. . . . . . . . . . . . . . . . . . . . . . . . .49
   Intellectual Property Statement . . . . . . . . . . . . . . . . . .50
   Disclaimer of Validity. . . . . . . . . . . . . . . . . . . . . . .51
   Copyright Statement . . . . . . . . . . . . . . . . . . . . . . . .51
   Acknowledgment. . . . . . . . . . . . . . . . . . . . . . . . . . .51


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.

   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


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   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 [4].


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 indicate 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.
   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


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   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 ini-INVITE.

   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 some internal states
   showed in the FSM (Finite State Machine) for dialog state of the
   dialog-package [7], without changing the states of the dialog,
   "early", "confirmed", and "terminated" shown in RFC3261 [1].
   The Preparative state is put before the Early state, which includes
   the Trying and Proceeding states.  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.

   Below are the DSMs for UAC and UAS respectively.


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       +-----------------------------------------------+
       |                 Preparative                   |
       |   +----------+           +--------------+     |
       |   |          |    100    |              |-----C-+
       |   |  Trying  |---------->|  Proceeding  |     | | 100
       |   |          |           |              |<----C-+
       |   +----------+           +--------------+     |
       |                                               |
       +-----------------------------------------------+
         |                         |                 |
         | 3xx-6xx                 | 1xx-tag         | 2xx
         |                         |                 |
         |                         V                 |
         |         +------------------+              |
         | 3xx-6xx |                  |--+ 1xx-tag   |
         +<--------|      Early       |  | w/new tag |
         |         |                  |<-+ (new DSM  |
         |         +------------------+     instance |
         |            |             |       created) |
         |            | BYE         | 2xx            |
         |            |             +------------>+<-+
         |            |                           |
   +-----C------------C-----+         +-----------C------+
   |     | Terminated |     |         | Confirmed |      |
   |     |            +<----C---------|           |      |
   |     |            |     |   BYE   |           |      |
   |     |            V     |         |           V      |
   |     | +------------+   |         |   +-----------+  |
   |     | |            |---C-+       |   |           |--C-+ 2xx
   |     | |   Mortal   |   | | BYE(r)|   | Moratorium|  | | w/new tag
   |     | |            |<--C-+       |   |           |<-C-+ (new DSM
   |     | +------------+   |         |   +-----------+  |    instance
   |     |   |              |         |         |        |    created)
   |     |   | Timeout      |         |         | ACK    |
   |     |   | (Timer K)    |         |         |        |
   |     V   V              |         |         V        |
   |   +---------------+    |         |   +-----------+  |
   |   |               |    |         |   |           |  |
   |   |     Morgue    |    |         |   |Established|  |
   |   |               |    |         |   |           |  |
   |   +---------------+    |         |   +-----------+  |
   |                        |         |                  |
   +------------------------+         +------------------+

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

      Figure 1.  DSM for INVITE dialog usage (UAC)


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   Figure 1 represents the UAC's DSM for the INVITE dialog usage.
   UAC MAY send a BYE in the Early state, even though this behavior 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 between a particular UA.  If UAC wants to terminate all Early
   dialogs instead of that with a particular UA, it needs to send
   CANCEL, not BYE.  Moreover, until UAC receives a final response and
   terminates the INVITE transaction, the UAC MUST be prepared to
   establish a dialog by receiving a new response to the INVITE even
   though it had sent a BYE and terminated the dialog (see Appendix A).


       +-----------------------------------------------+
       |                 Preparative                   |
       |   +----------+           +--------------+     |
       |   |          |    100    |              |-----C-+
       |   |  Trying  |---------->|  Proceeding  |     | | 100
       |   |          |           |              |<----C-+
       |   +----------+           +--------------+     |
       |                                               |
       +-----------------------------------------------+
         |                         |                 |
         | 3xx-6xx                 | 1xx-tag         | 2xx
         |                         |                 |
         |                         V                 |
         |         +------------------+              |
         | 3xx-6xx |                  |--+           |
         +<--------|      Early       |  | 1xx-tag   |
         |         |                  |<-+           |
         |         +------------------+              |
         |            |             |                |
         |            | BYE         | 2xx            |
         |            |             +------------>+<-+
         |            |                           |
   +-----C------------C-----+         +-----------C------+
   |     | Terminated |     |         | Confirmed |      |
   |     |            +<----C---------|           |      |
   |     |            |     | BYE(sr) |           |      |
   |     |            V     |         |           V      |
   |     | +------------+   |         |   +-----------+  |
   |     | |            |---C-+       |   |           |--C-+
   |     | |   Mortal   |   | | BYE   |   | Moratorium|  | | 2xx
   |     | |            |<--C-+       |   |           |<-C-+
   |     | +------------+   |         |   +-----------+  |
   |     |   |              |         |         |        |
   |     |   | Timeout      |         |         | ACK    |
   |     |   | (Timer J)    |         |         |        |


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   |     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 (UAS)


   Figure 2 represents UAS'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 UAS 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 the
   CANCEL request is further explained in the 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 and receiving a
         provisional response with To-tag after an ini-INVITE is sent
         and received.  The dialog has not existed yet in the
         Preparative state.  The dialog state transits from the
         Preparative to the Early by sending or receiving a provisional
         response with To-tag.  Moreover, if UA sends or receives a 2xx
         response, the dialog state transit 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 this DSM
         because they are sent by the INVITE transaction.

      Trying (Try): The Trying state is a substate of the Preparative
         state and inherits the behavior of the superstate.  The Trying
         state is started by sending and receiving of an ini-INVITE.
         It transits to the Proceeding state by sending or receiving a
         1xx (usually 100 trying) without To-tag.  UAC may retransmit an
         INVITE on transaction layer and must not send a CANCEL request.
         UAS may send a 1xx-6xx response.

      Proceeding (Pro): The Proceeding state is a substate of the


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         Preparative state and inherits the behavior of the superstate.
         Dialog becomes the Proceeding state if a dialog in the Trying
         state sends or receives a 1xx without To-tag (usually 100
         trying).  UAC may send CANCEL, and UAS may send a 1xx-6xx
         response in the Proceeding state.

      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 existed 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 and
         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 the
         Appendix C.

      Confirmed (Con): Sending or receiving of a 2xx final response
         establishes a dialog.  Dialog exists in this state.  The BYE
         request the changes state from the Confirmed to Mortal state,
         a substate of the Terminated state.  The Confirmed 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
         Confirmed state and inherits the behavior of superstate.  Both
         caller and callee may send various messages which influences a
         dialog.  Caller supports the transmission of ACK for a
         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.  The
         Confirmed state transits to the Mortal state, a substate of the
         Terminated state, by sending or receiving a BYE request.



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      Mortal (Mort): Caller and callee becomes Mortal state by sending
         or receiving a BYE.  UA MUST NOT send any new requests since
         there is no dialog.  (Here the new requests do not include ACK
         for 2xx and BYE for 401 or 407 as further explained in the
         Appendix D below.)
         In this state, only BYE or its response can be handled, and no
         other messages can be received.  This is because the use case
         is taken into consideration that BYE is sent by both a caller
         and a callee to exchange reports about the session when it is
         being terminated.  Therefore, UA possesses dialog information
         for internal process but dialog shouldn't exist outwardly.  The
         UA stops managing its dialog state and changes it to the Morgue
         state, when the BYE transaction is finished by timer
         (Timer F or Timer K for UAC.  Timer J for UAS).

      Morgue (Morg): Dialog does not exist any more in this state.
         Sending or receiving of a signal which influences a dialog is
         not performed.  (A dialog is literally terminated.)


3.  Race condition

   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 signals are illustrated.  Dialog state transitions
   caused by sending and receiving of SIP messages as well as '*race*',
   which indicates race condition are shown.  (For abbreviations for
   the dialog state transitions, refer to Chapter 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.


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

  State  Alice                               Bob  State
         |                                     |
         |            ini-INVITE F1            |
    Pre  |------------------------------------>|  Pre
         |         180 F2(Packet loss)         |
         |            x<-----------------------|  Ear


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         |                                     |
         | ini-INVITE(=F1) F4           200 F3 |
         |------------------     --------------|  Mora
         |                   \ /               |
         |                    X                |
         |                   / \               |
   Mora  |<-----------------     ------------->|  *race*
         |                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
   terminate an INVITE server transaction, according to Section 13.3.1.4
   of RFC3261 [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 RFC3261 [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
   RFC3261 [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.)


   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 RFC3261, 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.  */


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   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 (Proceeding or Early state)
      in Moratorium state

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


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         |                              |


   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 RFC3261, 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 response to the CANCEL 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 a dialog.
   (Section 15, RFC3261 [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
   than 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.


   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


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      bug reported in #769.  When an INVITE server transaction is
      terminated as the procedure stated in RFC3261, 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  |------------------------------->|  Pre
         |            180 F2              |
    Ear  |<-------------------------------|  Ear
         |                                |
         |    BYE F4        200(INVITE) F3|
   Mort  |-------------     --------------|  Mora
         |              \ /               |
         |               X                |
         |              / \               |
         |<------------     ------------->|  Mort & *race*
         |                                |
         |    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


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   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, this behavior is NOT RECOMMENDED).
   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 receiving the BYE response, 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 the Appendix D
   below).


   Message Details

   F1 INVITE Alice -> Bob

   F2 180 Ringing Bob -> Alice

   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 F1          |
    Pre  |------------------------------->|  Pre
         |             180 F2             |
    Ear  |<-------------------------------|  Ear
         |                                |
         |         200(ini-INV) F3        |
   Mora  |<-------------------------------|  Mora
         |       ACK F4(packet loss)      |
    Est  |-------------------->x          |
         |                                |


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         | re-INVITE F6      200(=F3) F5  |
         |-------------     --------------|
         |              \ /               |
         |               X                |
         |              / \               |
         |<------------     ------------->|  *race*
         | ACK(=F4) F7      200(re-INV) F8|
         |-------------     --------------|
         |              \ /               |
         |               X                |
         |              / \               |
         |<------------     ------------->|  Est
         |             ACK F9             |
         |------------------------------->|
         |                                |
         |                                |


   This scenario illustrates the race condition which occurs when UAS
   receives re-INVITE request sent from the Established state, in the
   Moratorium state.
   UAS receives a re-INVITE before receiving an ACK for ini-INVITE.  UAS
   sends a 200 OK to the re-INVITE (F8) because it has sent a 200 OK 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, UA should return by a 491 to the
   re-INVITE (refer to 3.1.5).  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.


   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-Type: application/sdp


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   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

   /* ini-INVITE contains an offer.  */


   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


   F4 ACK Alice -> Bob


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   ACK sip:bob@client.biloxi.example.com SIP/2.0
   Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bKnashds8
   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 200 OK (=F3) 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=z9hG4bK74bf9.1
   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 ACK (=F4) 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=z9hG4bK74bf9.1
   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


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   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 Alice -> Bob

   ACK sip:sip:bob@client.biloxi.example.com SIP/2.0
   Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bK230f2.1
   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 F1          |
    Pre  |------------------------------->|  Pre
         |             180 F2             |
    Ear  |<-------------------------------|  Ear
         |                                |
         |        200(ini-INV) F3         |
   Mora  |<-------------------------------|  Mora
         |       ACK F4(packet loss)      |
    Est  |-------------------->x          |
         |                                |
         | re-INVITE F6      200(=F3) F5  |
         |-------------     --------------|
         |              \ /               |
         |               X                |
         |              / \               |
         |<------------     ------------->|
         | ACK(=F4) F7      491(re-INV) F8|
         |-------------     --------------|
         |              \ /               |
         |               X                |
         |              / \               |


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         |<------------     ------------->|  Est
         |             ACK F9             |
         |------------------------------->|
         |                                |
         |                                |


   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 since he is not able to properly handle a
   new request until he receives an answer.


   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.  */


   F2 180 Ringing Bob -> Alice

   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=-


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   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=z9hG4bKnashds8
   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 200 OK (=F3) 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=z9hG4bK74bf9.1
   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


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   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 ACK (=F4) 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 Alice -> Bob


3.1.6 Receiving BYE (Established state)
      in Moratorium state

  State  Alice                     Bob  State
         |                           |
         |         INVITE F1         |
    Pre  |-------------------------->|  Pre
         |      180 Ringing F2       |
    Ear  |<--------------------------|  Ear
         |                           |
         |         200 OK F3         |
   Mora  |<--------------------------|  Mora
         |    ACK F4(packet loss)    |
    Est  |--------------->x          |
         |   Both Way RTP Media      |
         |<=========================>|
         |   BYE F6       200(=F3) F5|
   Mort  |-----------     -----------|
         |            \ /            |
         |             X             |
         |            / \            |
         |<----------     ---------->|  Mort & *race*
         |ACK(=F4) F7     200(BYE) F8|
         |-----------     -----------|
         |            \ /            |
         |             X             |
         |            / \            |
         |<----------     ---------->|


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         | ^                       ^ |
         | | 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 want to 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.


   Message Details

   F1 INVITE Alice -> Bob

   F2 180 Ringing Bob -> Alice

   F3 200 OK Bob -> Alice

   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=z9hG4bKnashds8
   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 200 OK (retransmission) Bob -> Alice

   SIP/2.0 200 OK


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   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

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


   F6 BYE Alice -> Bob

   BYE sip:bob@client.biloxi.example.com SIP/2.0
   Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bKnashds9
   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 BYE
   Content-Length: 0

   /* 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 ACK(=F4) Alice -> Bob

   F8 200 OK (BYE) Bob -> Alice

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bKnashds9
    ;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: 2 BYE
   Content-Length: 0


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   /* 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
         |                        |
         |       INVITE F1        |
    Pre  |----------------------->|  Pre
         |    180 Ringing F2      |
    Ear  |<-----------------------|  Ear
         |                        |
         |       200 OK F3        |
   Mora  |<-----------------------|  Mora
         |         ACK F4         |
    Est  |----------------------->|  Est
         |   Both Way RTP Media   |
         |<======================>|
         |                        |
         | 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


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   receives an Established message, BYE, in the Mortal state.
   Alice and Bob send a BYE at the same time.  A dialog and session is
   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.)


   Message Details

   F1 INVITE Alice -> Bob

   F2 180 Ringing Bob -> Alice

   F3 200 OK Bob -> Alice

   F4 ACK Alice -> Bob

   F5 BYE Alice -> Bob

   BYE sip:bob@client.biloxi.example.com SIP/2.0
   Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bKnashds8
   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 BYE
   Content-Length: 0

   /* 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

   BYE sip:alice@client.atlanta.example.com SIP/2.0
   Via: SIP/2.0/UDP client.biloxi.example.com:5060;branch=z9hG4bKnashds7
   Max-Forwards: 70
   From: Bob <sip:bob@biloxi.example.com>;tag=8321234356
   To: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl


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   Call-ID: 3848276298220188511@atlanta.example.com
   CSeq: 1 BYE
   Content-Length: 0

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


   F7 200 OK Bob -> Alice

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bKnashds8
    ;received=192.0.2.201
   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 BYE
   Content-Length: 0

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


   F8 200 OK Alice -> Bob

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP client.biloxi.example.com:5060;branch=z9hG4bKnashds7
    ;received=192.0.2.201
   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 BYE
   Content-Length: 0

   /* 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
         |                        |
         |       INVITE F1        |
    Pre  |----------------------->|  Pre
         |    180 Ringing F2      |
    Ear  |<-----------------------|  Ear
         |                        |
         |       200 OK F3        |


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   Mora  |<-----------------------|  Mora
         |         ACK F4         |
    Est  |----------------------->|  Est
         |   Both Way RTP Media   |
         |<======================>|
         |                        |
         | BYE F5     re-INVITE F6|
   Mort  |---------     ----------|
         |          \ /           |
         |           X            |
         |          / \           |
 *race*  |<--------     --------->|  Mort
         |                        |
         | 481 F8         200 F7  |
         |---------     ----------|
         |          \ /           |^
         |           X            ||
         |          / \           ||Timer J
         |<--------     --------->||
        ^|         ACK 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



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   F5 BYE Alice -> Bob

   BYE sip:bob@client.biloxi.example.com SIP/2.0
   Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bKnashds8
   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 BYE
   Content-Length: 0

   /* Alice sends a BYE and terminates the session, and transits from
      the Established state to the Mortal state.  */


   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=z9hG4bKnashds7
   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

   /* 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 Bob -> Alice

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bKnashds8
    ;received=192.0.2.201
   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 BYE
   Content-Length: 0


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


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   SIP/2.0 481 Call/Transaction Does Not Exist
   Via: SIP/2.0/UDP client.biloxi.example.com:5060;branch=z9hG4bKnashds7
    ;received=192.0.2.201
   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

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


   F9 ACK 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
         |                        |
         |       INVITE F1        |
    Pre  |----------------------->|  Pre
         |    180 Ringing F2      |
    Ear  |<-----------------------|  Ear
         |                        |
         |       200 OK F3        |
   Mora  |<-----------------------|  Mora
         |         ACK F4         |
    Est  |----------------------->|  Est
         |   Both Way RTP Media   |
         |<======================>|
         |                        |
         |      re-INVITE F5      |
         |<-----------------------|
         | 200 F7         BYE F6  |
         |---------     ----------|  Mort
         |          \ /           |
         |           X            |
         |          / \           |
   Mort  |<--------     --------->|  *race*
         | 200 F8         ACK F9  |
         |---------     ----------|
         | ^        \ /           |
         | |         X            |
         | |        / \           |


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         |<--------     --------->|
         | |                    ^ |
         | |            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.


   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
   Via: SIP/2.0/UDP client.biloxi.example.com:5060;branch=z9hG4bKnashds7
   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


   F6 BYE Bob -> Alice

   BYE sip:alice@client.atlanta.example.com SIP/2.0
   Via: SIP/2.0/UDP client.biloxi.example.com:5060;branch=z9hG4bKnashds8
   Max-Forwards: 70


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   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 BYE
   Content-Length: 0

   /* 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=z9hG4bKnashds7
    ;received=192.0.2.201
   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

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP client.biloxi.example.com:5060;branch=z9hG4bKnashds8
    ;received=192.0.2.201
   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 BYE
   Content-Length: 0


   F9 ACK Bob -> Alice

   /* 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          |


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    Pre  |------------------------------->|  Pre
         |            180 F2              |
    Ear  |<-------------------------------|  Ear
         |            200 F3              |
   Mora  |<-------------------------------|  Mora
         |                                |
         |    ACK F4            BYE F5    |
    Est  |-------------     --------------|  Mort
         |              \ /               |
         |               X                |
         |              / \               |
   Mort  |<------------     ------------->|  *race*
         |            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.


   F1 INVITE Alice -> Bob

   F2 180 Ringing Bob -> Alice

   F3 200 OK Bob -> Alice

   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=z9hG4bK74bd5
   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

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


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   F5 BYE Alice -> Bob

   BYE sip:bob@client.biloxi.example.com SIP/2.0
   Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bKnashds8
   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 BYE
   Content-Length: 0

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


   F6 200 OK Bob -> Alice

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bKnashds8
    ;received=192.0.2.201
   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 BYE
   Content-Length: 0


3.3 Other race condition

   This section shows the examples in race condition that are not
   directly related to the dialog state transition.  Here explains
   the way to handle the race condition generated when UAs treat
   "What is established by SIP", which has some relation to dialog.


3.3.1 re-INVITE crossover

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


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     |<==========================>|
     |                            |
     |re-INVITE F5   re-INVITE F6 |
     |------------   -------------|
     |            \ /             |
     |             X              |
     |            / \             |
     |<-----------   ------------>|
     |   491 F8        491 F7     |
     |------------   -------------|
     |            \ /             |
     |             X              |
     |            / \             |
     |<-----------   ------------>|
     |  ^ ACK F9         ^ ACK F10|
     |--|---------   ----|--------|
     |  |          \ /   |        |
     |  |           X    |        |
     |  |          / \   |        |
     |<-|----------   ---|------->|
     |  |                |        |
     |  |0-2.0 sec       |        |
     |  |                |        |
     |  v  re-INVITE(=F6) F11     |
     |<------------------|--------|
     |     200 OK F12    |        |
     |-------------------|------->|
     |       ACK F13     |        |
     |<------------------|--------|
     |                   |        |
     |                   |2.1-4.0 sec
     |                   |        |
     |re-INVITE(=F5) F14 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
   RFC3261 [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


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   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 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


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   a=sendonly

   /* re-INVITE for session modification (a=sendrecv -> sendonly).  */


   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=z9hG4bKnashds7
   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=z9hG4bKnashds7.1
   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


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   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
      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=z9hG4bK74bf9.1
   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         |


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     |<---------------------------|
     |           ACK F4           |
     |--------------------------->|
     |     Both Way RTP Media     |
     |<==========================>|
     |                            |
     |  UPDATE F5    re-INVITE F6 |
     |------------   -------------|
     |            \ /             |
     |             X              |
     |            / \             |
     |<-----------   ------------>|
     |   491 F8        491 F7     |
     |------------   -------------|
     |            \ /             |
     |             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:


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 A 491 response is considered as a result that UA judged the
 effectiveness of request to "What is established by SIP".
 Therefore, it is considered that 491 will be used in all the
 requests that demand operation to "What is established by SIP".


   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


   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=z9hG4bKnashds7
   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



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


   F7 491 Request Pending Bob -> Alice
   /* Since a re-INVITE is in process, a 491 response are returned.  */

   F8 491 Request Pending 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=z9hG4bKnashds7.1
   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=z9hG4bK74bf9.1
   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


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   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 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  |----------------------->|  Pre
         |    180 Ringing F2      |
    Ear  |<-----------------------|  Ear
         |                        |
         |       200 OK F3        |
   Mora  |<-----------------------|  Mora
         |         ACK F4         |
    Est  |----------------------->|  Est
         |   Both Way RTP Media   |
         |<======================>|
         |                        |
         | BYE F5        REFER F6 |
   Mort  |---------     ----------|
         |          \ /           |
         |           X            |
         |          / \           |
 *race*  |<--------     --------->|  Mort
         |                        |
         | 481 F8         200 F7  |
         |---------     ----------|
         |          \ /         ^ |
         |           X          | |
         |          / \         | |
         |<--------     --------->|
         | ^                    | |
         | | Timer K            | |


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         | V                    | |
   Morg  |              Timer J | |
         |                      V |
         |                        |  Morg
         |                        |


   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 sends an error response
   to the requests which operates the session, such as REFER, because
   by sending the BYE, Alice had terminated the session which would
   have corresponded to the REFER.  For handling of dialogs with
   multiple usages, as can be seen in the use of REFER method,
   see the draft on dialog usage [8].


   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

   REFER sip:alice@client.atlanta.example.com SIP/2.0
   Via: SIP/2.0/UDP client.biloxi.example.com:5060;branch=z9hG4bKnashds7
   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
   Refer-To: sip:carol@cleveland.example.org
   Contact: <sip:bob@client.biloxi.example.com;transport=udp>
   CSeq: 1 REFER
   Content-Length: 0

   /* 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.  */


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   F7 200 OK Bob -> Alice

   F8 481 Call/Transaction Does Not Exist Alice -> Bob
   /* Since Alice has terminated the session, she responds with a 481
      to the REFER.  */


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=1) F4 |      |
     |    180(1) F5   |<-----------------|      |
     |<---------------|                  |      |
     |                |       INVITE(Fork) F6   |
     |                |------------------------>|
     |                |                100 F7   |
     |    BYE(1) F8   |<------------------------|
     |--------------->|    BYE(1) F9     |      |
     |                |----------------->|      |
     |                |    200(1) F10    |      |
     |   200(1) F11   |<-----------------|      |
     |<---------------|    487(1) F12    |      |
     |                |<-----------------|      |
     |                |    ACK(1) F13    |      |
     |                |----------------->|      |
     |                |                  |      |
     |                |                         |
     |                |     200(To-tag=2) F13   |
     |   200(2) F14   |<------------------------|
     |<---------------|                         |
     |   ACK(2) F15   |                         |
     |--------------->|            ACK(2) F16   |
     |                |------------------------>|
     |   BYE(2) F17   |                         |
     |--------------->|            BYE(2) F18   |
     |                |------------------------>|
     |                |            200(2) F19   |
     |   200(2) F20   |<------------------------|
     |<---------------|                         |


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     |                |                         |
     |                |                         |


   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 RFC3261 [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
   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.

   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
   ==========================
    |  F1 re-INVITE        |
    |--------------------->|
    |  F2 BYE              |
    |--------------------->|
    |  F3 200(BYE)         |
    |<---------------------|
    |  F4 INVITE(=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


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   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.  RFC3261 [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 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
               :     |                      |
               :     |                      |
               |     |  F1 re-INVITE        |
         o     |     |--------------------->|
         |     |     |  F2 BYE              |
   o     |  (Mortal) |--------------------->|
   |     |     |     |  F3 200(BYE)         |
   |     |     |     |<---------------------|
   |     |     |     |  F4 INVITE(=F1)      |
   |     |     |     |--------------------->|
   |     |     |     |  F5 481(INV)         |
   |     |     |     |<---------------------|
   |     |     |     |  F6 ACK(INV)         |
   |     |     |     |--------------------->|
   |     |     |     |                      |
   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


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   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.

   Next, UAS's state is shown below.



  UAC                    UAS dialog  INV   BYE
   |                      |     :
   |                      |     :
   |  F1 re-INVITE        |     |
   |-------------->x      |     |
   |  F2 BYE              |     |
   |--------------------->|  (Mortal)       o
   |  F3 200(BYE)         |     |           |
   |<---------------------|     |           |<-Start TimerJ
   |  F4 INVITE(=F1)      |     |           |
   |--------------------->|     |     o     |
   |  F5 4xx(INV)         |     o     |     o
   |<---------------------|           |
   |  F6 ACK(INV)         |           |
   |--------------------->|           |<-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 (probably 481) for F4
   INVITE, because the TU knows that the dialog which matches to the
   INVITE is in the Terminated state.
   (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 behaviour for CANCEL


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   This section explains the CANCEL and the Expires header 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.
   Similarly, the Expires header does not have direct influence on the
   dialog state transition, but it indirectly affect the state
   transition because its expiration triggers the sending of CANCEL.
   For UAS the CANCEL request and the Expires header timeout have 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
   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 signals, 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.


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


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            +------------+
            | 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 Proceeding and 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
   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, from the point of UAC's behavior, it can be expected
   that UAS receives BYE immediately after the reception of CANCEL and
   moves 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 need
   careful attention.

   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 [8] 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 one for error responses are
   both a part of the INVITE transaction, even though their hadlings
   differ (Section 17.1.1.1, RFC3261 [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.)


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   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.


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
       SDP", RFC 3264, April 2002.

   [3] Johnston, A., Donovan, S., Sparks, R., Cunningham, C. and K.
       Summers, "Session Initiation Protocol (SIP) Basic Call Flow
       Examples", BCP 75, RFC 3665, December 2003.

   [4] Johnston, A., Donovan, S., Sparks, R., Cunningham, C. and K.
       Summers, "Session Initiation Protocol (SIP) Public Switched
       Telephone Network (PSTN) Call Flows", BCP 76, RFC 3666, December
       2003.

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

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

   [7] Rosenberg, J., Schulzrinne, H., Mahy, R., "An INVITE-Initiated
       Dialog Event Package for the Session Initiation Protocol (SIP)",
       RFC 4235, November 2005.

   [8] Sparks, R., "Multiple Dialog Usages in the Session Initiation
       Protocol", draft-ietf-sipping-dialogusage-05 (work in progress),
       November 9, 2006.


Author's Addresses

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

   Miki Hasebe


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   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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   such proprietary rights by implementors or users of this
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   http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
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Disclaimer of Validity

   This document and the information contained herein are provided on an
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   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
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   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.


Copyright Statement

   Copyright (C) The Internet Society (2006).  This document is subject
   to the rights, licenses and restrictions contained in BCP 78, and
   except as set forth therein, the authors retain all their rights.


Acknowledgment

   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 and Kenichi Hiragi for
   the comments on this document.
















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