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Versions: 00 01

Network Working Group                                          R. Sparks
Internet-Draft                                               dynamicsoft
Expires: December 31, 2004                                  July 2, 2004


       Multiple Dialog Usages in the Session Initiation Protocol
                  draft-sparks-sipping-dialogusage-00

Status of this Memo

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   patent or other IPR claims of which I am aware have been disclosed,
   and any of which I become aware will be disclosed, in accordance with
   RFC 3668.

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   This Internet-Draft will expire on December 31, 2004.

Copyright Notice

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

Abstract

   Several methods in the Session Initiation Protocol can create an
   association between endpoints known as a dialog.  Some of these
   methods can also create a new association within an existing dialog.
   These multiple associations, or dialog usages, require carefully
   coordinated processing as they have independent life-cycles, but
   share common dialog state.

   This memo argues that multiple dialog usages should be avoided.  It
   discusses alternatives to their use and clarifies essential behavior
   for elements that cannot currently avoid them.



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   This is an informative document and makes no normative statements of
   any kind.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Examples of Multiple Usages  . . . . . . . . . . . . . . . . .  3
     2.1   Transfer . . . . . . . . . . . . . . . . . . . . . . . . .  3
     2.2   Reciprocal Subscription  . . . . . . . . . . . . . . . . .  5
   3.  Proper Handling of Multiple Usages . . . . . . . . . . . . . .  7
     3.1   A survey of the effect of failure responses on usages
           and dialogs  . . . . . . . . . . . . . . . . . . . . . . .  8
     3.2   Transaction timeouts . . . . . . . . . . . . . . . . . . . 14
     3.3   Matching requests to usages  . . . . . . . . . . . . . . . 14
     3.4   Target refresh requests  . . . . . . . . . . . . . . . . . 15
     3.5   Refreshing and Terminating Usages  . . . . . . . . . . . . 15
     3.6   Refusing new usages  . . . . . . . . . . . . . . . . . . . 16
     3.7   Replacing usages . . . . . . . . . . . . . . . . . . . . . 16
   4.  Avoiding Multiple Usages . . . . . . . . . . . . . . . . . . . 16
   5.  Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 21
       Author's Address . . . . . . . . . . . . . . . . . . . . . . . 22
   6.  Informative References . . . . . . . . . . . . . . . . . . . . 21
   A.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 22
       Intellectual Property and Copyright Statements . . . . . . . . 23



























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1.  Introduction

   Several methods in SIP can establish a dialog.  When they do so, they
   also establish an association between the endpoints within that
   dialog.  This assocation has been known for some time as a "dialog
   usage" in the developer community.  A dialog initiated with an INVITE
   request has an invite usage.  A dialog initiated with a SUBSCRIBE
   request has a subscribe usage.

   Dialogs with multiple usages arise from actions like a REFER or
   SUBSCRIBE issued inside a dialog established with an INVITE request.
   Multiple REFERs within a dialog create multiple subscriptions, each
   of which is a new dialog usage sharing common dialog state.  This
   state includes:
   o  the Call-ID
   o  the local Tag
   o  the remote Tag
   o  the local CSeq
   o  the remote CSeq
   o  the Route-set
   o  the local contact
   o  the remote target

   A dialog comes into existence with the creation of the first usage,
   and continues to exist until the last usage is terminated (reference
   counting).  Unfortunately, many of the usage management aspects of
   SIP, such as authentication, were originally designed with the
   implicit assumption that there was one usage per dialog.  The
   resulting mechanisms have mixed effects, some influencing the usage,
   and some influencing the entire dialog.

   The current specifications define two usages, invite and subscribe.
   A dialog can share up to one invite usage and arbitrarily many
   subscribe usages.  The pseudo-dialog behavior of REGISTER could be
   considered a third usage.  Fortunately, no existing implementations
   have attempted to mix a registration usage with any other usage.

2.  Examples of Multiple Usages

2.1  Transfer

   In Figure 1, Alice transfers a call she received from Bob to Carol.
   A dialog (and an invite dialog usage) between Alice and Bob came into
   being with the 200 OK labeled F1.  A second usage (a subscription to
   event refer) springs into being with the NOTIFY labeled F2.  This
   second usage ends when the subscription is terminated by the NOTIFY
   transaction labeled F3.  The dialog still has one usage (the invite
   usage), which lasts until the BYE transaction labeled F4.  At this



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   point, the dialog has no remaining usages, so it ceases to exist.

                                Alice              Bob         Carol
                                  |    INVITE       |            |
                                  |<----------------|            |
    Dialog 1  Usage 1             |    200 OK (F1)  |            |
    -start-   -start- ----------->|---------------->|            |
       |         |                |    ACK          |            |
       |         |                |<----------------|            |
       |         |                | reINVITE/200/ACK|            |
       |         |                |   (hold)        |            |
       |         |                |---------------->|            |
       |         |                |   REFER         |            |
       |         |     Dialog 1   |---------------->|            |
       |         |     Usage 2    |   NOTIFY (F2)   |            |
       |         |     -start- -->|<----------------| INVITE     |
       |         |        |       |   200 NOTIFY    |----------->|
       |         |        |       |---------------->| 200 OK     |
       |         |        |       |   200 REFER     |<-----------|
       |         |        |       |<----------------| ACK        |
       |         |        |       |   NOTIFY (F3)   |----------->|
       |         |        |       |<----------------|            |
       |         |        |       |   200           |     .      |
       |         |      -end-  -->|---------------->|     .      |
       |         |                |   BYE (F4)      |  Dialog 2  |
       |         |                |<----------------|  proceeds  |
       |         |                |   200           |     .      |
     -end-     -end- ------------>|---------------->|     .      |

     Message Details (abridged to show only dialog or usage details)
     F1
       SIP/2.0 200 OK
       Call-ID: dialog1@bob.example.com
       CSeq: 100 INVITE
       To: <sip:Alice@alice.example.com>;tag=alicetag1
       From: <sip:Bob@bob.example.com>;tag=bobtag1
       Contact: <sip:aliceinstance@alice.example.com>

     F2
       NOTIFY sip:aliceinstance@alice.example.com SIP/2.0
       Event: refer
       Call-ID: dialog1@bob.example.com
       CSeq: 101 NOTIFY
       To: <sip:Alice@alice.example.com>;tag=alicetag1
       From: <sip:Bob@bob.example.com>;tag=bobtag1
       Contact: <sip:bobinstance@bob.example.com>

     F3



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       NOTIFY sip:aliceinstance@alice.example.com SIP/2.0
       Event: refer
       Subscription-State: terminated;reason=noresource
       Call-ID: dialog1@bob.example.com
       CSeq: 102 NOTIFY
       To: <sip:Alice@alice.example.com>;tag=alicetag1
       From: <sip:Bob@bob.example.com>;tag=bobtag1
       Contact: <sip:bobinstance@bob.example.com>
       Content-Type: message/sipfrag

       SIP/2.0 200 OK

     F4

       BYE sip:aliceinstance@alice.example.com SIP/2.0
       Call-ID: dialog1@bob.example.com
       CSeq: 103 BYE
       To: <sip:Alice@alice.example.com>;tag=alicetag1
       From: <sip:Bob@bob.example.com>;tag=bobtag1
       Contact: <sip:bobinstance@bob.example.com>




                                Figure 1


2.2  Reciprocal Subscription

   In Figure 2, Alice subscribes to Bob's presence.  For simplicity,
   assume Bob and Alice are both serving their presence from their
   endpoints instead of a presence server.  For space, the figure leaves
   out any rendezvous signaling through which Alice discovers Bob's
   endpoint.

   Bob is interested in Alice's presence too, so he subscribes to Alice
   (in most deployed presence/IM systems, people watch each other).  He
   decides skip the rendezvous step since he's already in a dialog with
   Alice, and sends his SUBSCRIBE inside that dialog (a few early SIMPLE
   clients behaved exactly this way).

   The dialog and its first usage comes into being at F1, which
   establishes Alice's subscription to Bob.  Its second usage begins at
   F2, which establishes Bob's subscription to Alice.  These two
   subscriptions are independent - they have distinct and different
   expirations, but they share all the dialog state.

   The first usage ends when Alice decides to unsubscribe at F3.  Bob's



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   subscription to Alice, and thus the dialog, continues to exist.
   Alice's UA must maintain this dialog state even though the
   subscription that caused it to exist in the first place is now over.
   The second usage ends when Alice decides to terminate Bob's
   subscription at F4 (she's probably going to reject any attempt on
   Bob's part to resubscribe until she's ready to subscribe to Bob
   again).  Since this was the last usage, the dialog also terminates.

                               Alice                 Bob
                                 |                    |
                                 | SUBSCRIBE          |
                                 |------------------->|
    Dialog    Usage 1            | NOTIFY (F1)        |
    -start-   -start-  --------->|<-------------------|
       |         |               | 200 SUBSCRIBE      |
       |         |               |<-------------------|
       |         |               | 200 NOTIFY         |
       |         |               |------------------->|
       |         |               | SUBSCRIBE          |
       |         |               |<-------------------|
       |         |    Usage 2    | NOTIFY (F2)        |
       |         |    -start- -->|------------------->|
       |         |       |       | 200 SUBSCRIBE
       |         |       |       |------------------->|
       |         |       |       | 200 NOTIFY         |
       |         |       |       |<-------------------|
       |         |       |       |         :          |
       |         |       |       |         :          |
       |         |       |       | (un)SUBSCRIBE (F3) |
       |         |       |       |------------------->|
       |         |       |       | 200                |
       |       -end-  ---------->|<-------------------|
       |                 |       | NOTIFY             |
       |                 |       |<-------------------|
       |                 |       | 200                |
       |                 |       |------------------->|
       |                 |       |         :          |
       |                 |       |         :          |
       |                 |       | NOTIFY        (F4) |
       |                 |       | (Terminated)       |
       |                 |       |------------------->|
       |                 |       | 200                |
     -end-             -end-  -->|<-------------------|
                                 |                    |

     Message Details (abridged to show only dialog or usage details)
     F1
       NOTIFY sip:aliceinstance@alice.example.com SIP/2.0



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       Event: presence
       Subscription-State: active;expires=600
       Call-ID: alicecallid1@alice.example.com
       From: <sip:Bob@bob.example.com>;tag=bobtag2
       To: <sip:Alice@alice.example.com>;tag=alicetag2
       CSeq: 100 NOTIFY
       Contact: <sip:bobinstance@bob.example.com>

     F2
       NOTIFY sip:bobinstance@bob.example.com SIP/2.0
       Event: presence
       Subscription-State: active;expires=1200
       Call-ID: alicecallid1@alice.example.com
       To: <sip:Bob@bob.example.com>;tag=bobtag2
       From: <sip:Alice@alice.example.com>;tag=alicetag2
       CSeq: 500 NOTIFY
       Contact: <sip:aliceinstance@alice.example.com>

     F3
       SUBSCRIBE sip:bobinstance@bob.example.com SIP/2.0
       Event: presence
       Expires: 0
       Call-ID: alicecallid1@alice.example.com
       To: <sip:Bob@bob.example.com>;tag=bobtag2
       From: <sip:Alice@alice.example.com>;tag=alicetag2
       CSeq: 501 SUBSCRIBE
       Contact: <sip:aliceinstance@alice.example.com>

     F4
       NOTIFY sip:bobinstance@bob.example.com SIP/2.0
       Event: presence
       Subscription-State: terminated;reason=deactivated
       Call-ID: alicecallid1@alice.example.com
       To: <sip:Bob@bob.example.com>;tag=bobtag2
       From: <sip:Alice@alice.example.com>;tag=alicetag2
       CSeq: 502 NOTIFY
       Contact: <sip:aliceinstance@alice.example.com>



                                Figure 2


3.  Proper Handling of Multiple Usages

   The examples in Section 2 show straightforward cases where it is
   fairly obvious when the dialog begins and ends.  Unfortunately, there
   are many scenarios where such clarity is not present.  For instance,



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   in Figure 1, what would it mean if the response to the NOTIFY (F2)
   were a 481? Does that simply terminate the refer subscription, or
   does it destroy the entire dialog? This section explores the problem
   spots with multiple usages that have been identified to date.

3.1  A survey of the effect of failure responses on usages and dialogs

   For this survey, consider a subscribe usage inside a dialog
   established with an invite usage.  Unless stated otherwise, we'll
   discuss the effect on each usage and the dialog when a client issuing
   a NOTIFY inside the subscribe usage receives a failure response (such
   as a transferee issuing a NOTIFY to event refer).

   This survey is written from the perspective of a client receiving the
   error response.  The effect on dialogs and usages at the server
   issuing the response is the same.

   3xx responses: Redirection mid-dialog is not well understood in SIP,
      but whatever effect it has impacts the entire dialog and all of
      its usages equally.  In our example scenario, both the
      subscription and the invite usage would be redirected by this
      single response.

   400 and unrecognized 4xx responses: These responses affect only the
      NOTIFY transaction, not the subscription, the dialog it resides in
      (beyond affecting the local CSeq), or any other usage of that
      dialog.  In general, the response is a complaint about this
      transaction, not the usage or dialog the transaction occurs in.

   401 Unauthorized ,407 Proxy Authentication Required: This request,
      not the subscription or dialog, is being challenged.  The usages
      and dialog are not terminated.

   402 Payment Required: This is a reserved response code.  If
      encountered, it should be treated as an unrecognized 4xx.

   403 Forbidden: This response terminates the subscription, but has no
      effect on any other usages of the dialog.  In our example
      scenario, the invite usage continues to exist.  Similarly, if the
      403 came in response to a reINVITE, the invite usage would be
      terminated, but not the subscription.

   404 Not Found: This response destroys the dialog and all usages
      sharing it.  The Request-URI that is being 404ed is the remote
      target set by the Contact provided by the peer.  Getting this
      response means something has gone fundamentally wrong with the
      dialog state.




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   405 Method Not Allowed: In our example scenario, this response
      destroys the subscription, but not the invite usage or the dialog.
      It's an aberrant case for NOTIFYs to receive a 405 since they only
      come as a result to something that creates subscription.  In
      general, a 405 within a given usage affects only that usage, but
      does not affect other usages of the dialog.

   406 Not Acceptable: These responses concern details of the message in
      the transaction.  Subsequent requests in this same usage may
      succeed.  Neither the usage nor dialog is terminated, other usages
      sharing this dialog are unaffected.

   408 Request Timeout: OPEN ISSUE.  3261 explicitly says this
      terminates dialogs.  Does it really mean it terminates the invite
      usage? Should it truly tear down all usages on a dialog where it
      occurs? More on this in Section 3.2.

   410 Gone: This response destroys the dialog and all usages sharing
      it.  The Request-URI that is being rejected is the remote target
      set by the Contact provided by the peer.  Similar to 404, getting
      this response means something has gone fundamentally wrong with
      the dialog state, its slightly less aberrant in that the other
      endpoint recognizes that this was once a valid URI that it isn't
      willing to respond to anymore.

   412 Conditional Request Failed:
   413 Request Entity Too Large:
   414 Request-URI Too Long:
   415 Unsupported Media Type: These responses concern details of the
      message in the transaction.  Subsequent requests in this same
      usage may succeed.  Neither the usage nor dialog is terminated,
      other usages sharing this dialog are unaffected.

   416 Unsupported URI Scheme: Similar to 404 and 410, this response
      came to a request whose Request-URI was provided by the peer in a
      Contact header field.  Something has gone fundamentally wrong, and
      the dialog and all of its usages are destroyed.

   420 Bad Extension, 421 Extension Required: These responses are
      objecting to the request, not the usage.  The usage is not
      affected.  The dialog is only affected by a change in its local
      CSeq.  No other usages of the dialog are affected.

   423 Interval Too Brief: This response won't happen in our example
      scenario, but if it came in response to a reSUBSCRIBE, the
      subscribe usage is not destroyed (or otherwise affected).  No
      other usages of the dialog are affected.



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   429 Provide Referrer Identity: This response won't be returned to a
      NOTIFY as in our example scenario, but when it is returned to a
      REFER, it is objecting to the REFER request itself, not any usage
      the REFER occurs within.  The usage is unaffected.  Any other
      usages sharing this dialog are unaffected.  The dialog is only
      affected by a change in its local CSeq.

   480 Temporarily Unavailable: OPEN ISSUE: Similar to 404,410 this
      response is to a R-URI that was provided by the peer in a Contact.
      Is it reasonable for a request to such a URI to return a 480? For
      instance, if someone places a call on hold and activates
      Do-not-disturb, would 480 be a reasonable response to decline
      reINVITEs? We need more clarity around what a mid-usage 480 means.
      I propose we declare it an error and that this section has an
      answer like 404s.

   481 Call/Transaction Does Not Exist: This response indicates that the
      peer has lost its copy of the dialog state.  The dialog and any
      usages sharing it are destroyed.

   482 Loop Detected: This response is aberrant mid-dialog.  It will
      only occur if the Record-Route header field was improperly
      constructed by the proxies involved in setting up the dialog's
      initial usage, or if a mid-dialog request forks and merges (which
      should never happen).  Future requests using this dialog state
      will also fail.  The dialog and any usages sharing it are
      destroyed.  OPEN ISSUE: This response may have been triggered by
      method (and perhaps usage) specific handling.  Is destroying the
      entire dialog too severe?

   483 Too Many Hops: Similar to 482, receiving this mid-dialog is
      aberrant.  Unlike 482, recovery may be possible by increasing
      Max-Forwards (assuming that the requester did something strange
      like using a smaller value for Max-Forwards in mid-dialog requests
      than it used for an initial request).  If the request isn't tried
      with an increased Max-Forwards, then the agent should attempt to
      gracefully terminate this usage and all other usages that share
      its dialog.  OPEN ISSUE: Is this the right behavior, or should we
      just declare the dialog terminated?

   484 Address Incomplete, 485 Ambiguous: Similar to 404 and 410, these
      responses came to a request whose Request-URI was provided by the
      peer in a Contact header field.  Something has gone fundamentally
      wrong, and the dialog and all of its usages are destroyed.






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   486 Busy Here: This response is non-sensical in our example scenario,
      or in any scenario where this response comes inside an established
      usage.  If it occurs in that context, it should be treated as an
      unknown 4xx response.  The usage, and any other usages sharing its
      dialog are unaffected.  The dialog is only affected by the change
      in its local CSeq.  If this response is to a request that is
      attempting to establish a new usage within an existing dialog
      (such as an INVITE sent within a dialog established by a
      subscription), the request fails, no new usage is created, and no
      other usages are affected.

   487 Request Terminated: This response speaks to the disposition of a
      particular request (transaction).  The usage in which that request
      occurs is not affected by this response (it may be affected by
      another associated request within that usage).  No other usages
      sharing this dialog are affected.

   488 Not Acceptable Here: This response is objecting to the request,
      not the usage.  The usage is not affected.  The dialog is only
      affected by a change in its local CSeq.  No other usages of the
      dialog are affected.

   489 Bad Event: In our example scenario, [3] declares that the
      subscription usage in which the NOTIFY is sent is terminated.  The
      invite usage is unaffected and the dialog continues to exist.
      This response is only valid in the context of SUBSCRIBE and
      NOTIFY.  UAC behavior for receiving this response to other methods
      is not specified, but treating it as an unknown 4xx is a
      reasonable practice.

   491 Request Pending: This response addresses in-dialog request glare.
      Its affect is scoped to the request.  The usage in which the
      request occurs is not affected.  The dialog is only affected by
      the change in its local CSeq.  No other usages sharing this dialog
      are affected.

   493 Undecipherable: This response objects to the request, not the
      usage.  The usage is not affected.  The dialog is only affected by
      a change in its local CSeq.  No other usages of the dialog are
      affected.

   494 Security Agreement Required: This response is objecting to the
      request, not the usage.  The usage is not affected.  The dialog is
      only affected by a change in its local CSeq.  No other usages of
      the dialog are affected.





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   500 and 5xx unrecognized responses: These responses are complaints
      against the request (transaction), not the usage.  If the response
      contains a Retry-After header field value, the server thinks the
      condition is temporary and the request can be retried after the
      indicated interval.  This usage, and any other usages sharing the
      dialog are unaffected.  If the response does not contain a
      Retry-After header field value, the UA may decide to retry after
      an interval of its choosing or attempt to gracefully terminate the
      usage.  Whether or not to terminate other usages depends on the
      application.  If the UA receives a 500 (or unrecognized 5xx) in
      response to an attempt to gracefully terminate this usage, it can
      treat this usage as terminated.  If this is the last usage sharing
      the dialog, the dialog is also terminated.

   501 Not Implemented: This would be a degenerate response in our
      example scenario since the NOTIFY is being sent as part of an
      established subscribe usage.  In this case, the UA knows the
      condition is unrecoverable and should stop attempting to send
      NOTIFYs on this usage.  (It may or may not destroy the usage.  If
      it remembers the bad behavior, it can reject any refresh
      subscription).  In general, this response may or may not affect
      the usage (a 501 to an unknown method or an INFO will not end an
      invite usage).  It will never affect other usages sharing this
      usage's dialog.

   502 Bad Gateway: OPEN ISSUE: I think this is similar to "Loop
      Detected".

   503 Service Unavailable: As per [2], the logic handling locating SIP
      servers for transactions may handle 503 requests (effectively
      sequentially forking at the endpoint based on DNS results).  If
      this process does not yield a better response, a 503 may be
      returned to the transaction user.  Like a 500 response, the error
      is a complaint about this transaction, not the usage.  Because
      this response occurred in the context of an established usage
      (hence an existing dialog), the route-set has already been formed
      and any opportunity to try alternate servers (as recommended in
      [1] has been exhausted by the RFC3263 logic.  The response should
      be handled as described for 500 earlier in this memo.

   504 Server Time-out: It is not obvious under what circumstances this
      response would be returned to a request in an existing dialog.  If
      it occurs it should have the same affect on the dialog and its
      usages as described for unknown 5xx responses.






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   505 Version Not Supported, 513 Message Too Large: These responses are
      objecting to the request, not the usage.  The usage is not
      affected.  The dialog is only affected by a change in its local
      CSeq.  No other usages of the dialog are affected.

   580 Precondition Failure: This response is objecting to the request,
      not the usage.  The usage is not affected.  The dialog is only
      affected by a change in its local CSeq.  No other usages of the
      dialog are affected.

   600 and 6xx unrecognized responses: Unlike 400 Bad Request, a 600
      response code says something about the recipient user, not the
      request that was made.  This end user is stating an unwillingness
      to communicate.  OPEN ISSUE: It is not clear what this means in
      response to a mid-dialog request.  I propose this behavior if you
      receive a 600 or unknown 6xx: If the response contains a
      Retry-After header field value, the user is indicating willingness
      to communicate later and the request can be retried after the
      indicated interval.  This usage, and any other usages sharing the
      dialog are unaffected.  If the response does not contain a
      Retry-After header field value, the UA may decide to retry after
      an interval of its choosing or attempt to gracefully terminate the
      usage.  Whether or not to terminate other usages depends on the
      application.  If the UA receives a 600 (or unrecognized 6xx) in
      response to an attempt to gracefully terminate this usage, it can
      treat this usage as terminated.  If this is the last usage sharing
      the dialog, the dialog is also terminated.

   603 Decline: This response declines the action indicated by the
      associated request.  It can be used, for example, to decline a
      hold or transfer attempt.  Receiving this response does NOT
      terminate the usage it occurs in.  Other usages sharing the dialog
      are unaffected.

   604 Does Not Exist Anywhere: Like 404, this response destroys the
      dialog and all usages sharing it.  The Request-URI that is being
      604ed is the remote target set by the Contact provided by the
      peer.  Getting this response means something has gone
      fundamentally wrong with the dialog state.

   606 Not Acceptable: This response is objecting to aspects of the
      associated request, not the usage the request appears in.  The
      usage is unaffected.  Any other usages sharing the dialog are
      unaffected.  The only affect on the dialog is the change in the
      local CSeq.





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3.2  Transaction timeouts

   [1] states that a UAC should terminate a dialog (by sending a BYE) if
   no response is received for a request sent within a dialog.  This
   recommendation should have been limited to the invite usage instead
   of the whole dialog.  [3] states that a timeout for a NOTIFY removes
   a subscription, but a SUBSCRIBE that fails with anything other than a
   481 does not.  Given these statements, it is unclear whether a
   refresh SUBSCRIBE issued in a dialog shared with an invite usage
   destroys either usage or the dialog if it times out.

   Generally, a transaction timeout should affect only the usage in
   which the transaction occurred.  Other uses sharing the dialog should
   not be affected.  In the worst case of timeout due to total transport
   failure, it may require multiple failed messages to remove all usages
   from a dialog (at least one per usage).

   There are some mid-dialog messages that never belong to any usage.
   If they timeout, they will have no effect on the dialog or its
   usages.

3.3  Matching requests to usages

   For many mid-dialog requests, identifying the usage they belong to is
   obvious.  A dialog can have at most one invite usage, so any INVITE,
   UPDATE, PRACK, ACK, CANCEL, BYE, or INFO requests belong to it.  The
   usage (i.e.  the particular subscription) SUBSCRIBE, NOTIFY, and
   REFER requests belong to can be determined from the Event header
   field of the request.  REGISTER requests within a (pseudo)-dialog
   belong to the registration usage.  (As mentioned before,
   implementations aren't mixing registration usages with other usages,
   so this document isn't exploring the consequences of that bad
   behavior).

   According to [1], "an OPTIONS request received within a dialog
   generates a 200 OK response that is identical to one constructed
   outside a dialog and does not have any impact on that dialog".  Thus
   OPTIONS does not belong to any usage.  Only those failures discussed
   in Section 3.1 and Section 3.2 that destroy entire dialogs will have
   any effect on the usages sharing the dialog with a failed OPTIONS
   request.

   MESSAGE requests are not currently allowed inside a dialog (though
   some implementations use it that way, against the standard
   recommendation).  As it is not meant to be part of any given dialog,
   it cannot be part of any given usage.  A failed MESSAGE request
   should have similar effects on a dialog and its usages as a failed
   OPTIONS request.



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   Mid-dialog requests with unknown methods cannot be matched with a
   usage.  Servers will return a failure response (likely a 501).  The
   effect on the dialog and its usages at either the client or the
   server should be similar to that of a failed OPTIONS request.

3.4  Target refresh requests

   Target refresh requests update the remote target of a dialog when
   they are successfully processed.  The currently defined target
   refresh requests are INVITE, UPDATE, SUBSCRIBE and NOTIFY (clarified
   in a bug against RFC3565).  REFER could also be a target refresh
   request since it can establish a new usage (and even a new dialog).
   (OPEN ISSUE: Is REFER a target refresh request?)

   The remote target is part of the dialog state.  When a target refresh
   request affects it, it affects it for ALL usages sharing that dialog.
   If a subscription and invite usage are sharing a dialog, sending a
   refresh SUBSCRIBE with a different contact will cause reINVITEs from
   the peer to go to that different contact.

   A UAS will only update the remote target if it sends a 200 class
   response to a target refresh request.  A UAC will only update the
   remote target if it receives a 200 class response to a target refresh
   request.  Again, any update to a dialog's remote target affects all
   usages of that dialog.

3.5  Refreshing and Terminating Usages

   Subscription and registration usages expire over time and must be
   refreshed (with a refresh SUBSCRIBE for example).  This expiration is
   usage state, not dialog state.  If several subscriptions share a
   dialog, refreshing one of them has no effect on the expiration of the
   others.

   Normal termination of a usage has no effect on other usages sharing
   the same dialog.  For instance terminating a subscription with a
   NOTIFY/Subscription-State: terminated will not terminate an invite
   usage sharing its dialog.  Likewise, ending an invite usage with a
   BYE does not terminate any active Event: refer subscriptions
   established on that dialog.

   Abnormal termination can effect all usages on a dialog.  Rejecting a
   NOTIFY with a 481 (incorrectly recommended in the past as an
   inexpensive way to terminate a REFER subscription) destroys the
   dialog and all of its usages.






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3.6  Refusing new usages

   As the survey of the effect of failure responses shows, care must be
   taken when refusing a new usage inside an existing dialog.  Choosing
   the wrong response code will terminate the dialog and all of its
   usages.  Generally, returning a 603 Decline is the safest way to
   refuse a new usage.

3.7  Replacing usages

   [5] defines a mechanism through which one usage can replace another.
   It can be used, for example, to associate the two dialogs a transfer
   target is involved in during an attended transfer.  It is written
   using the term "dialog", but its intent was to only affect the invite
   usage of the dialog it targets.  Any other usages inside that dialog
   are unaffected.  For some applications, the other usages may no
   longer make sense, and the application may terminate them as well.

   However, the interactions between Replaces and multiple dialog usages
   have not been well explored.  More discussion of this topic is
   needed.  Implementers should avoid this scenario completely.

4.  Avoiding Multiple Usages

   Processing multiple usages correctly is not completely understood.
   What is understood is difficult to implement and is very likely to
   lead to interoperability problems.  The best way to avoid the trouble
   that comes with such complexity is to avoid it altogether.

   When designing new applications that use SIP dialogs, do not
   construct multiple usages.  If a peer attempts to create a second
   usage inside a dialog, refuse it.

   Unfortunately, there are existing applications, like transfer, that
   currently entail multiple usages, so the simple solution of "don't do
   it" will require some transitional work.  This section will look at
   the pressures that led to these existing multiple usages and suggest
   alternatives.

   When executing a transfer, the transferor and transferee currently
   share an invite usage and a subscription usage within the dialog
   between them.  This is a result of sending the REFER request within
   the dialog established by the invite usage.  Implementations were led
   to this behavior by two primary pressures:
   1.  There was no way to ensure that a REFER on a new dialog would
       reach the particular endpoint involved in a transfer.  Many
       factors, including details of implementations and changes in
       proxy routing between an INVITE and a REFER could cause the REFER



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       to be sent to the wrong place.  Sending the REFER down the
       existing dialog ensured it got to the endpoint we were already
       talking to.
   2.  It was unclear how to associate an existing invite usage with a
       REFER arriving on a new dialog, where it was completely obvious
       what the association was when the REFER came on the invite
       usage's dialog.
   3.  There were concerns with authorizing out-of-dialog REFERs.  The
       authorization policy for REFER in most implementations piggybacks
       on the authorization policy for INVITE (which is, in most cases,
       based simply on "I placed or answered this call").

   GRUUs ([6]) have been defined specifically to address problem 1.
   Problem 2 can be addressed using a GRUU's grid parameter.  In the
   immediate term, this solution to problem 2 allows the existing REFER
   authorization policy to be reused.  Figure 3 shows a transfer where
   any given dialog has exactly one usage.


         Each message in this flow passes through a server at
         example.com, which forwards messages to the endpoints
         based on the AOR or GRUU in the Request-URI. This hop
         through the server has been removed from the diagram
         to make it easier to read. An "S" appears in the middle
         of each arrow as a reminder of the visit to this intermediary.

    Alice                             Bob                           Carol
      |                                |                              |
      | F1 INVITE (Bob's AOR)          |                              |
      |    Call-ID: (call-id one)      |                              |
      |    Contact: (Alice-GRUU-grid1) |                              |
      |-------------S----------------->|                              |
      | F2 200 OK                      |                              |
      |    Contact: (Bob-GRUU-grid1)   |                              |
      |<------------S------------------|                              |
      |    ACK                         |                              |
      |-------------S----------------->|                              |
      |             :                  |                              |
      |  (Bob places Alice on hold)    |                              |
      |             :                  | F3 INVITE (Carol's AOR)      |
      |                                |    Call-ID: (call-id two)    |
      |                                |    Contact: (Bob-GRUU-grid2) |
      |                                |-------------S--------------->|
      |                                | F4 200 OK                    |
      |                                |    Contact: (Carol-GRUU-grid1)
      |                                |<------------S----------------|
      |                                |    ACK                       |
      |                                |-------------S--------------->|



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      |                                |            :                 |
      |                                |  (Bob places Carol on hold)  |
      | F5 REFER (Alice-GRUU-grid1)    |            :                 |
      |    Call-ID: (call-id three)    |                              |
      |    Refer-To: (Carol-GRUU-grid1)|                              |
      |    Contact: (Bob-GRUU-grid1)   |                              |
      |<-----------S-------------------|                              |
      |    202 Accepted                |                              |
      |------------S------------------>|                              |
      |    NOTIFY (Bob-GRUU-grid1)     |                              |
      |------------S------------------>|                              |
      |    200 OK                      |                              |
      |<-----------S-------------------|                              |
      |                                |                              |
      |                                |                              |
      |                                |                              |
      |                                |                              |
      |                                |                              |
      |                  F6 INVITE (Carol-GRUU-grid1)                 |
      |                     Call-ID: (call-id four)                   |
      |                     Contact: (Alice-GRUU-grid2)               |
      |-----------------------------S-------------------------------->|
      |                  F7 200 OK                                    |
      |                     Contact: (Carol-GRUU-grid2)               |
      |<----------------------------S---------------------------------|
      |                     ACK                                       |
      |-----------------------------S-------------------------------->|
      |                                |                              |
      | F8 NOTIFY (Bob-GRUU-grid1)     |                              |
      |-------------S----------------->|                              |
      |    200 OK                      |                              |
      |<------------S------------------|                              |
      |    BYE (Alice-GRUU-grid1)      |                              |
      |<------------S------------------|   BYE (Carol-GRUU-grid1)     |
      |    200 OK                      |-------------S--------------->|
      |-------------S----------------->|   200 OK                     |
      |                                |<------------S----------------|
      |                                |                              |


                Figure 3: Transfer without dialog reuse

   In message F1, Alice invites Bob indicating support for GRUUs (and
   offering a GRUU for herself):







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      Message F1 (abridged, detailing pertinent fields)

         INVITE sip:bob@example.com SIP/2.0
         Call-ID: 13jfdwer230jsdw@alice.example.com
         Supported: gruu
         Contact: <sip:aanewmr203raswdf@example.com;grid=au9a3e>


   Message F2 lets Alice know that Bob understands GRUUs.  If Bob did
   not indicate this support, the original multi-usage approach to
   transfer would have to be used.

      Message F2 (abridged, detailing pertinent fields)

         SIP/2.0 200 OK
         Supported: gruu
         Contact: <sip:boaiidfjjereis@example.com;grid=baierac>


   Bob decides to try to transfer Alice to Carol, so he puts Alice on
   hold and sends message F3 to Carol.  Notice that Bob has provided a
   different grid to Carol than he provided to Alice.  This is a
   significant part of the solution to problem 2 - if Alice or Carol
   were to beat Bob to a REFER, this will let Bob know which invite
   usage (the one with Alice or the one with Carol) to affect.

      Message F3 (abridged, detailing pertinent fields)

         INVITE sip:carol@example.com SIP/2.0
         Call-ID: 23rasdnfoa39i4jnasdf@bob.example.com
         Supported: gruu
         Contact: <sip:boaiidfjjereis@example.com;grid=bc923a3d>


   Carol indicates her own support of GRUU and provides her GRUU for
   this dialog in message F4:

      Message F4 (abridged, detailing pertinent fields)

         SIP/2.0 200 OK
         Supported: gruu
         Call-ID: 23rasdnfoa39i4jnasdf@bob.example.com
         To: <sip:carol@example.com>;tag=foiew3n
         From: <sip:bob@example.com>;tag=baeih23n
         Contact: <sip:c239fniuweorw9sdfn@example.com;grid=cbfnei2>


   After consulting Carol, Bob places her on hold and refers Alice to



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   her using message F5.  Notice that the Refer-To URI is Carol's GRUU,
   and that this is on a different Call-ID than message F1.  (The URI in
   the Refer-To header is line-broken for readability in this draft, it
   would not be valid to break the URI this way in a real message)

      Message F5 (abridged, detailing pertinent fields)

         REFER sip:aanewmr203raswdf@example.com;grid=au9a3e SIP/2.0
         Call-ID: 39fa99r0329493asdsf3n@bob.example.com
         Refer-To: <sip:c239fniuweorw9sdfn@example.com;grid=cbfnei2
                    ?Replaces=23rasdnfoa39i4jnasdf@bob.example.com;
                     to-tag=foiew3n;from-tag=baeih23n>
         Supported: gruu
         Contact: <sip:boaiidfjjereis@example.com;grid=baierac>

   Alice accepts this REFER, sends Bob the obligatory immediate NOTIFY,
   and proceeds to INVITE Carol with message F6.  Notice that Alice
   gives Carol a GRUU with a different grid than she gave Bob.  If Bob
   decided not to terminate his dialog with Alice (possibly sending her
   another REFER) and/or Carol decided to transfer Alice again, this
   becomes an important part of associating Bob or Carol's REFER with
   the correct invite usage.

      Message F6 (abridged, detailing pertinent fields)

         INVITE sip:c239fniuweorw9sdfn@example.com;grid=cbfnei2 SIP/2.0
         Call-ID: 4zsd9f234jasdfasn3jsad@alice.example.com
         Replaces: 23rasdnfoa39i4jnasdf@bob.example.com;
                   to-tag=foiew3n;from-tag=baeih23n
         Supported: gruu
         Contact: <sip:aanewmr203raswdf@example.com;grid=ac99asn>


   Carol accepts Alice's invitation to replace her dialog (invite usage)
   with Bob with F7.  For the same reasons listed above, Carol hands
   Alice a different grid than the one she handed Bob (which was the one
   Alice used to reach her in F6).

      Message F7 (abridged, detailing pertinent fields)

         SIP/2.0 200 OK
         Supported: gruu
         Contact: <sip:c239fniuweorw9sdfn@example.com;grid=canasdi>


   Alice notifies Bob that the REFERenced INVITE succeeded with F8:





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      Message F8 (abridged, detailing pertinent fields)

         NOTIFY sip:boaiidfjjereis@example.com;grid=baierac SIP/2.0
         Subscription-State: terminated;reason=noresource
         Contact: <sip:aanewmr203raswdf@example.com;grid=au9a3e>
         Content-Type: message/sipfrag

         SIP/2.0 200 OK


   Bob then ends his invite usages with both Alice and Carol using BYEs.

   Generalizing what was said several times during that flow: Using a
   different grid for each usage between two endpoints lets endpoints
   involved in more than one dialog figure out which dialog is related
   to a new out-of-dialog request.  Having that association can affect
   whether this new out-of-dialog request is accepted.

   One potential application for REFER that has been discussed at
   several working group meetings is using an out-of-dialog REFER to ask
   an endpoint to join a conference.  An endpoint receiving such a REFER
   would have to authorize it (by prompting its user for permission
   perhaps).  If this REFER arrived while the user was in another call,
   the lack of a grid parameter matching the call that is ongoing lets
   the UA know that this REFER is not a transfer of the existing call.

5.  Conclusion

   Handling multiple usages within a single dialog is complex and
   introduces scenarios where the right thing to do is not clear.
   Implementations should avoid entering into multiple usages whenever
   possible.  New applications should be designed to never introduce
   multiple usages.

   There are some accepted SIP practices, including transfer, that
   currently require multiple usages.  Recent work, most notably GRUU,
   makes those practices unnecessary.  The standardization of those
   practices and the implementations should be revised as soon as
   possible to use only single-usage dialogs.

6  Informative References

   [1]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
        Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP:
        Session Initiation Protocol", RFC 3261, June 2002.

   [2]  Rosenberg, J. and H. Schulzrinne, "Session Initiation Protocol
        (SIP): Locating SIP Servers", RFC 3263, June 2002.



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   [3]  Roach, A., "Session Initiation Protocol (SIP)-Specific Event
        Notification", RFC 3265, June 2002.

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

   [5]  Biggs, B., Dean, R. and R. Mahy, "The Session Inititation
        Protocol (SIP) 'Replaces' Header", draft-ietf-sip-replaces-04
        (work in progress), August 2003.

   [6]  Rosenberg, J., "Obtaining and Using Globally Routable User Agent
        (UA) URIs (GRUU) in the  Session Initiation Protocol (SIP)",
        draft-ietf-sip-gruu-01 (work in progress), February 2004.


Author's Address

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

   EMail: rsparks@dynamicsoft.com

Appendix A.  Acknowledgments

   The ideas in this draft have been refined over several IETF meetings
   with many participants.  Significant contribution was provided by
   Adam Roach, Alan Johnston, Ben Campbell, Cullen Jennings, Jonathan
   Rosenberg and Rohan Mahy.  Members of the reSIProcate project (http:/
   /www.sipfoundry.org/reSIProcate) also shared their difficulties and
   discoveries while implementing multiple-usage dialog handlers.


















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