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Versions: (draft-biggs-sip-replaces) 00 01 02 03 04 05 RFC 3891

SIP WG                                                           R. Mahy
Internet-Draft                                       Cisco Systems, Inc.
Expires: December 24, 2003                                      B. Biggs
                                                                 R. Dean
                                                           June 25, 2003


        The Session Inititation Protocol (SIP) "Replaces" Header
                     draft-ietf-sip-replaces-04.txt

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that other
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   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time. It is inappropriate to use Internet-Drafts as reference
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   The list of current Internet-Drafts can be accessed at http://
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   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on December 24, 2003.

Copyright Notice

   Copyright (C) The Internet Society (2003). All Rights Reserved.

Abstract

   This document defines a new header for use with SIP multi-party
   applications and call control. The Replaces header is used to
   logically replace an existing SIP dialog with a new SIP dialog.  This
   primitive can be used to enable a variety of features, for example:
   "Attended Transfer" and "Call Pickup". Note that definition of these
   example features is non-normative.








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Table of Contents

   1.   Conventions  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.   Overview . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.   User Agent Server Behavior: Receiving a Replaces Header  . .   5
   4.   User Agent Client Behavior: Sending a Replaces header  . . .   7
   5.   Proxy behavior . . . . . . . . . . . . . . . . . . . . . . .   7
   6.   Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . .   7
   6.1  The Replaces Header  . . . . . . . . . . . . . . . . . . . .   7
   6.2  New option tag for Require and Supported headers . . . . . .   9
   7.   Usage Examples . . . . . . . . . . . . . . . . . . . . . . .   9
   7.1  Replacing an Early Dialog at the originator  . . . . . . . .   9
   8.   Security Considerations  . . . . . . . . . . . . . . . . . .  11
   9.   IANA Considerations  . . . . . . . . . . . . . . . . . . . .  13
   9.1  Registration of "Replaces" SIP header  . . . . . . . . . . .  13
   9.2  Registration of "replaces" SIP Option-tag  . . . . . . . . .  13
   10.  Changes  . . . . . . . . . . . . . . . . . . . . . . . . . .  13
   10.1 Changes Since -03  . . . . . . . . . . . . . . . . . . . . .  13
   10.2 Changes Since -02  . . . . . . . . . . . . . . . . . . . . .  13
   10.3 Changes Since -01  . . . . . . . . . . . . . . . . . . . . .  14
   10.4 Changes Since -00  . . . . . . . . . . . . . . . . . . . . .  14
   11.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . .  14
        Normative References . . . . . . . . . . . . . . . . . . . .  15
        Informational References . . . . . . . . . . . . . . . . . .  15
        Authors' Addresses . . . . . . . . . . . . . . . . . . . . .  16
        Intellectual Property and Copyright Statements . . . . . . .  17

























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1. Conventions

   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 RFC-2119 [2].

   This document refers frequently to the terms "confirmed dialog" and
   "early dialog". These are defined in Section 12 of SIP [1].

2. Overview

   This document describes a SIP [1] extension header field as part of
   the SIP multiparty applications architecture framework [10]. The
   Replaces header is used to logically replace an existing SIP dialog
   with a new SIP dialog. This is especially useful in peer-to-peer call
   control environments.

   One use of the "Replaces" header is to replace one participant with
   another in a multimedia conversation.  While this functionality is
   already available using 3rd party call control [11] style call
   control, the 3pcc model requires a central point of control which may
   not be desirable in many environments. As such, a method of
   performing these same call control primitives in a distributed,
   peer-to-peer fashion is very desirable.

   Use of a new INVITE with a new header for dialog matching was chosen
   over making implicit associations in an incoming INVITE based on
   call-id or other fields for the following reasons:

   o  An INVITE already has the correct semantics for a new call

   o  Using an explicit Replaces header in a new request makes the
      intent of the request obvious.

   o  A unique call-id may be given to the replacement call.  This
      avoids dialog matching problems in any of the related User Agents.

   o  There are no adverse effects if the header is unsupported.

   The Replaces header enables services such as attended call transfer,
   retrieve from park, and transition from locally mixed conferences to
   two party calls in a distributed peer-to-peer way. This list of
   services is not exhaustive.  Although the Replaces header is
   frequently used in combination with the REFER [8] method as used in
   cc-transfer [12], they may be used independently.

   For example, Alice is talking to Bob from phone1.  She transfers Bob
   to a Parking Place while she goes to the lab.  When she gets there



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   she retrieves the "parked" call from phone2 by sending an INVITE with
   a Replaces header field to Bob with the dialog information Bob shared
   with the Parking Place.  Alice got this information using some out of
   band mechansim. Perhaps she subscribed to this information from the
   Parking Place (using the session dialog package [13]), or went to a
   website and clicked on a URI.  A short call flow for this example
   follows. (Via and Max-Forwards headers are omitted for clarity.)

        Alice          Alice                             Parking
        phone1         phone2            Bob               Place
        |               |                 |                   |
        |<===============================>|                   |
        |               |                 |                   |
        |        Alice transfers Bob to Parking Place         |
        |               |                 |                   |
        |------------REFER/200----------->|    *1    *2       |
        |               |                 |--INVITE/200/ACK-->|
        |<-----------NOTIFY/200-----------|<=================>|
        |------------BYE/200------------->|                   |
        |               |                 |                   |
        |               |                 |                   |
        |  Alice later retrieves call from another phone      |
        |               |                 |                   |
        |            *3 |-INV w/Replaces->|                   |
        |               |<--200-----------|                   |
        |               |---ACK---------->|----BYE/200------->|
        |               |<===============>|                   |
        |               |                 |                   |


   Message *1: Bob-> Parking Place

   INVITE sip:parkingplace@example.org SIP/2.0
   To: <sip:parkingplace@example.org>
   From: <sip:bob@example.org>;tag=7743
   Call-ID: 425928@bobster.example.org
   CSeq: 1 INVITE
   Contact: <sip:bob@bobster.example.org>
   Referred-By: <sip:alice@phone1.example.org>

   Message *2: Parking Place -> Bob

   SIP/2.0 200 OK
   To: <sip:parkingplace@example.org>;tag=6472
   From: <sip:bob@example.org>;tag=7743
   Call-ID: 425928@bobster.example.org
   CSeq: 1 INVITE
   Contact: <sip:parkplace@monopoly.example.org>



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   Message *3: Alice@phone2 -> Bob

   INVITE sip:bob@bobster.example.org
   To: <sip:bob@example.org>
   From: <sip:alice@phone2.example.org>;tag=8983
   Call-ID: 09870@phone2.example.org
   CSeq: 1 INVITE
   Contact: <sip:alice@phone2.example.org>
   Require: replaces
   Replaces: 425928@bobster.example.org;to-tag=7743;from-tag=6472


3. User Agent Server Behavior: Receiving a Replaces Header

   The Replaces header contains information used to match an existing
   SIP dialog (call-id, to-tag, and from-tag).  Upon receiving an INVITE
   with a Replaces header, the UA attempts to match this information
   with a confirmed or early dialog.  The to-tag and from-tag parameters
   are matched as if they were tags present in an incoming request.  In
   other words the to-tag parameter is compared to the local tag, and
   the from-tag parameter is compared to the remote tag.

   If more than one Replaces header field is present in an INVITE, or if
   a Replaces header field is present in a request other than INVITE,
   the UAS MUST reject the request with a 400 Bad Request response.

   The Replaces header has specific call control semantics.  If both a
   Replaces header field and another header field with contradictory
   semantics are present in a request, the request MUST be rejected with
   a 400 "Bad Request" response.

   If the Replaces header field matches more than one dialog, the UA
   MUST act as if no match is found.

   If no match is found, the UAS rejects the INVITE and returns a 481
   Call/Transaction Does Not Exist response. Likewise, if the Replaces
   header field matches a dialog which was not created with an INVITE,
   the UAS MUST reject the request with a 481 response.

   If the Replaces header field matches a dialog which has already
   terminated, the UA SHOULD decline the request with a 603 Declined
   response.

   If the Replaces header field matches an active dialog, the UA SHOULD
   verify that the initiator of the new INVITE is authorized to replace
   the matched dialog.  If the initiator of the new INVITE has
   authenticated successfully as equivalent to the user who is being
   replaced, then the replacement is authorized.  For example, the user



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   being replaced and the initator of the replacement dialog could share
   credentials for Digest authentication [6], or could sign the
   replacement request with S/MIME [7] with the same private key and
   present the corresponding same certificate used in the original
   dialog.

   Alternatively, the Referred-By mechanism [4] defines a mechanism that
   the UAS can use to verify that a replacement request was sent on
   behalf of the other participant in the matched dialog (in this case,
   triggered by a REFER request). If the replacement request contains a
   Referred-By header which corresponds to the user being replaced, the
   UA SHOULD treat the replacement as if the replacement was authorized
   by the replaced party. The Referred-By header SHOULD reference a
   corresponding, valid Refererred-By Authenticated Identity Body [5].
   The UA MAY apply other local policy to authorize the remainder of the
   request.  In other words the UAS may apply different policy to the
   replacement dialog than was applied to the replaced dialog.

   In addition, the UA MAY use other authorization mechanisms defined
   for this purpose in standards track extensions.  Extensions could
   define other mechanisms for transitively asserting authorization of a
   replacement.

   If authorization is successful, the UA attempts to accept the new
   INVITE, reassign the user interface and other resources of the
   matched dialog to the new INVITE, and shut down the replaced dialog.
   If the UA cannot accept the new INVITE (for example: it cannot
   establish required QoS or keying, or it has incompatible media), the
   UA MUST return an appropriate error response and MUST leave the
   matched dialog unchanged.

   If the Replaces header field matches a confirmed dialog, it checks
   for the presence of the "early-only" flag in the Replaces header
   field. (This flag allows the UAC to prevent a potentially undesirable
   race condition desribed in Section 7.1.) If the flag is present, the
   UA rejects the request with a 486 Busy response. Otherwise it accepts
   the new INVITE by sending a 200-class response, and shuts down the
   replaced dialog by sending a BYE. If the Replaces header field
   matches an early dialog that was initiated by the UA, it accepts the
   new INVITE by sending a 200-class response, and shuts down the
   replaced dialog by sending a CANCEL.

   If the Replaces header field matches an early dialog that was not
   initiated by this UA, it returns a 481 (Call/Transaction Does Not
   Exist) response to the new INVITE, and leaves the matched dialog
   unchanged. Note that since Replaces matches only a single dialog, the
   replacement dialog will not be retargeted according to the same
   forking logic as the original request which created the early dialog.



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   (Currently no use cases have been identified for replacing just a
   single dialog in this circumstance.)

4. User Agent Client Behavior: Sending a Replaces header

   A User Agent that wishes to replace a single existing early or
   confirmed dialog with a new dialog of its own, MAY send the target
   User Agent an INVITE request containing a Replaces header field.  The
   UAC places the Call-ID, to-tag, and from-tag information for the
   target dialog in a single Replaces header field and sends the new
   INVITE to the target.  If the user agent only wishes to replace an
   early dialog (as in the Call Pickup example in Section 7.1), the UAC
   MAY also include the "early-only" parameter in the Replaces header
   field. A UAC MUST NOT send an INVITE with Replaces header field which
   attempts to replace an early dialog which was not originated by the
   target of the INVITE with Replaces header field.

   Note that use of this mechanism does not provide a way to match
   multiple dialogs, nor does it provide a way to match an entire call,
   an entire transaction, or to follow a chain of proxy forking logic.
   For example, if Alice replaces Cathy in an early dialog with Bob, but
   he does not answer, Alice's replacement request will not match other
   dialogs to which Bob's UA redirects, nor other branches to which his
   proxy forwards. Although this specification takes reasonable
   precautions to prevent unexpected behavior in the face of forking,
   implementations SHOULD only address replacement requests (i.e. set
   the Request-URI of the replacement request) to the SIP Contact URI of
   the target.

5. Proxy behavior

   Proxy Servers do not require any new behavior to support this
   extension. They simply pass the Replaces header field transparently
   as described in the SIP specification.

   Note that it is possible for a proxy (especially when forking based
   on some application layer logic, such as caller screening or
   time-of-day routing) to forward an INVITE request containing a
   Replaces header field to a completely orthogonal set of Contacts than
   the original request it was intended to replace.  In this case, the
   INVITE request with the Replaces header field will fail.

6. Syntax

6.1 The Replaces Header

   The Replaces header field indicates that a single dialog identified
   by the header field is to be shut down and logically replaced by the



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   incoming INVITE in which it is contained.  It is a request header
   only, and defined only for INVITE requests.  The Replaces header
   field MAY be encrypted as part of end-to-end encryption. Only a
   single Replaces header field value may be present in a SIP request

   This document adds the following entry to Table 2 of [1]. Additions
   to this table are also provided for extension methods defined at the
   time of publication of this document.  This is provided as a courtesy
   to the reader and is not normative in any way. MESSAGE, SUBSCRIBE and
   NOTIFY, REFER, INFO, UPDATE, PRACK, and PUBLISH are defined
   respectively in [15], [16], [8], [17], [18], [19], and [20].

      Header field    where   proxy   ACK  BYE  CAN  INV  OPT  REG  MSG
      ------------    -----   -----   ---  ---  ---  ---  ---  ---  ---
      Replaces          R              -    -    -    o    -    -    -


                                      SUB  NOT  REF  INF  UPD  PRA  PUB
                                      ---  ---  ---  ---  ---  ---  ---
      Replaces          R              -    -    -    -    -    -    -

   The following syntax specification uses the augmented Backus-Naur
   Form (BNF) as described in RFC-2234 [3].

      Replaces        = "Replaces" HCOLON callid *(SEMI replaces-param)
      replaces-param  = to-tag / from-tag / early-flag / generic-param
      to-tag          = "to-tag" EQUAL token
      from-tag        = "from-tag" EQUAL token
      early-flag      = "early-only"


   A Replaces header field MUST contain exactly one to-tag and exactly
   one from-tag, as they are required for unique dialog matching.  For
   compatibility with dialogs initiated by RFC2543 [9] compliant UAs, a
   tag of zero matches both tags of zero and null tags. A Replaces
   header field MAY contain the early-flag.

   Examples:

      Replaces: 98732@sip.billybiggs.com
                ;from-tag=r33th4x0r
                ;to-tag=ff87ff

      Replaces: 12adf2f34456gs5;to-tag=12345;from-tag=54321;early-only

      Replaces: 87134@171.161.34.23;to-tag=24796;from-tag=0





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6.2 New option tag for Require and Supported headers

   This specification defines a new Require/Supported header option tag
   "replaces". UAs which support the Replaces header MUST include the
   "replaces" option tag in a Supported header field.  UAs that want
   explicit failure notification if Replaces is not supported MAY
   include the "replaces" option in a Require header field.

   Example:

      Require: replaces, 100rel


7. Usage Examples

   The following non-normative examples are not intended to enumerate
   all the possibilities for the usage of this extension, but rather to
   provide examples or ideas only.  For more examples, please see
   service-examples [14]. Via and Max-Forwards headers are omitted for
   clarity and brevity.

7.1 Replacing an Early Dialog at the originator

   In this example, Bob just arrived in the lab and hasn't registered
   there yet. He hears his desk phone ring.  He quickly logs into a
   software UA on a nearby computer. Among other things, the software UA
   has access to the dialog state of his desk phone. When it notices
   that his phone is ringing it offers him the choice to take the call
   there. The software UA sends an INVITE with Replaces to Alice.  When
   Alice's UA receives this new INVITE, it CANCELs her original INVITE
   and connects Alice to Bob.

                              Bob                      Bob
       Alice                  desk                     lab
        |                       |                        |
    *1  |-----INVITE----------->|                        |
    *2  |<----180---------------|  Bob hears desk phone  |
        |                       |  ringing from lab but  |
        |                       |  isn't REGISTERed yet  |
        |                       |                        |
        |                       |<--fetch dialog state --|
        |                       |---response ----------->|
   *3/4 |<-----INVITE with Replaces/200/ACK--------------|
   *5/6 |------CANCEL/200------>|                        |
   *7   |<-----487--------------|                        |
        |------ACK------------->|                        |
        |                       |                        |
        |                       |                        |



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   Message *1: Alice -> Bob's desk phone

   INVITE sip:bob@example.org SIP/2.0
   To: <sip:bob@example.org>
   From: <sip:alice@example.org>;tag=7743
   Call-ID: 425928@phone.example.org
   CSeq: 1 INVITE
   Contact: <sip:alice@phone.example.org>

   Message *2: Bob's desk phone -> Alice

   SIP/2.0 180 Ringing
   To: <sip:bob@example.org>;tag=6472
   From: <sip:alice@example.org>;tag=7743
   Call-ID: 425928@phone.example.org
   CSeq: 1 INVITE
   Contact: <sip:bob@bobster.example.org>

   Message *3: Bob in lab -> Alice

   INVITE sip:alice@phone.example.org
   To: <sip:alice@example.org>
   From: <sip:bob@example.org>;tag=8983
   Call-ID: 09870@labpc.example.org
   CSeq: 1 INVITE
   Contact: <sip:bob@labpc.example.org>
   Replaces: 425928@phone.example.org
    ;to-tag=7743;from-tag=6472;early-only

   Message *4: Alice -> Bob in lab

   SIP/2.0 200 OK
   To: <sip:alice@example.org>;tag=9232
   From: <sip:bob@example.org>;tag=8983
   Call-ID: 09870@labpc.example.org
   CSeq: 1 INVITE
   Contact: <sip:alice@phone.example.org>

   Message *5: Alice -> Bob's desk

   CANCEL sip:bob@example.org SIP/2.0
   To: <sip:bob@example.org>
   From: <sip:alice@example.org>;tag=7743
   Call-ID: 425928@phone.example.org
   CSeq: 1 CANCEL
   Contact: <sip:alice@phone.example.org>

   Message *6: Bob's desk -> Alice



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   SIP/2.0 200 OK
   To: <sip:bob@example.org>
   From: <sip:alice@example.org>;tag=7743
   Call-ID: 425928@phone.example.org
   CSeq: 1 CANCEL
   Contact: <sip:bob@bobster.example.org>

   Message *7: Bob's desk -> Alice

   SIP/2.0 487 Request Terminated
   To: <sip:bob@example.org>;tag=6472
   From: <sip:alice@example.org>;tag=7743
   Call-ID: 425928@phone.example.org
   CSeq: 1 INVITE


8. Security Considerations

   The extension specified in this document significantly changes the
   relative security of SIP devices.  Currently in SIP, even if an
   eavesdropper learns the Call-ID, To, and From headers of a dialog,
   they cannot easily modify or destroy that dialog if Digest
   authentication or end-to-end message integrity are used.

   This extension can be used to disconnect participants or replace
   participants in a multimedia conversation.  As such, invitations with
   the Replaces header SHOULD only be accepted if the peer requesting
   replacement has been properly authenticated using a standard SIP
   mechanism (Digest or S/MIME), and authorized to request a replacement
   of the target dialog.

   How a User Agent determines which requests are legitimately
   authorized to make dialog replacements is non-trivial and depends on
   a considerable amount of local policy configuration.  In general,
   there are four cases when a authorization for a replacement is
   reasonable or warranted.

   1.  Replacement made by a party considered equivalent to the replaced
       party

   2.  Replacement made on behalf of the replaced party (perhaps
       transitively)

   3.  Replacement made by a former participant

   4.  Replacement made by a specifically authorized party

   Starting with #1 for example, if an executive and an assistant both



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   receive requests for a shared address-of-record, if so configured,
   either should be able to replace dialogs of the other for the shared
   identity. Both could even share the same keying material (Digest or
   S/MIME), or one could hold an authorization document signed by the
   other expressing this relationship.  Likewise in a call center
   environment, each call center agent could possess credentials which
   supervisors also have access to.

   The most common use case of a replacement is on the request of the
   replaced participant (who no longer wants to be involved). This is
   the case in many features such as completing an Attended Transfer and
   converting a 3-way call to a point-to-point call.  Such replacements
   are typically triggered by a REFER [8] request from the replaced
   participant.  The Referred-By [4] mechanism defines one way to
   identify the apparent original requester and can point to a SIP
   Authenticated Identity Body [5] (an S/MIME-based signed assertion) to
   secure this information.

   In the example in section 2, Alice sends an INVITE with Replaces to
   Bob.  Alice was a former participant in the conversation and had a
   previous dialog relationship with Bob.  Alice can use the same Digest
   or SMIME credentials she used to authenticate with Bob during the
   original call to prove that she was a former participant.  Note that
   this justification  for replacing calls is more dangerous than the
   others, and in most cases another way to authorize the replacing
   participant is available.  Implementations SHOULD NOT rely on this
   method as an authorization mechanism.

   The last scenario is the easiest to secure but the least likely to be
   useful in practice.  It is unlikely that an arbitrary host in the
   Internet is aware of any special authorization relationship between
   the replaced and the replacing parties.  However, this use case may
   be useful in some environments.  Since this usage does not
   effectively degrade the security of the solution it is still allowed.

   Some mechanisms for obtaining the dialog information needed by the
   Replaces header (Call-ID, to-tag, and from-tag) include URIs on a web
   page, subscriptions to an appropriate event package, and notifcations
   after a REFER request. Since manipulating this dialog information
   could cause User Agents to replace the wrong dialog, use of message
   integrity protection for this information is STRONGLY RECOMMENDED,
   Use of end-to-end security mechanisms to encrypt this information is
   also RECOMMENDED.

   This extension was designed to take advantage of future signature or
   authorization schemes defined in standards track extensions.  In
   general, call control features benefit considerably from such work.




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9. IANA Considerations

9.1 Registration of "Replaces" SIP header

   Name of Header:          Replaces

   Short form:              none

   Normative description:   section 6.1 of this document


9.2 Registration of "replaces" SIP Option-tag

   Name of option:          replaces

   Description:             Support for the SIP Replaces header

   SIP headers defined:     Replaces

   Normative description:   This document



10. Changes

   *** [Note to RFC editor. Please remove this entire section when this
   draft is published as an RFC.] ***

10.1 Changes Since -03

   o  Added the "early-only" parameter to prevent a race condition
      during Call Pickup and related features.

   o  Made Referred-By and Auth-ID Body normative references, and
      generally improved the strength of the authorization section based
      on comments during AD review.

   o  Updated references and added reference for the PUBLISH method.


10.2 Changes Since -02

   o  Removed the ability to match an early dialog at the receiver of
      the matching dialog, since all the use cases apparently needing
      this feature actually need to match an entire set of targets in a
      chain of proxy forking logic.  Also removed all references to the
      687 response code which was only used for this purpose.




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   o  Added more detail in section 3 and section 8 about how to
      authorize replacements.


10.3 Changes Since -01

   o  Removed the to-tag=* matching mechanism, and related proxy
      requirements and examples based on WG consensus at interim meeting
      and on the mailing list.

   o  Reorganized motivational overview material

   o  Moved extra examples to service-flows

   o  Added authorization language in UAS behavior section

   o  Removed allowance to match on one of multiple matching dialogs
      with no tags

   o  Updated references


10.4 Changes Since -00

   o  When no dialog matches the Call-ID and tags in a Replaces header,
      the UAS now returns a 481 instead of silently accepting the
      INVITE.

   o  Changed the BNF to match the explicit whitespace BNF now used by
      SIP.

   o  Added the to-tag=* matching mechanism.

   o  Added requirements for forking proxies and a discussion of the
      consequences if forking proxies do not support Replaces.

   o  Added last two examples.

   o  Split normative and non-normative references


11. Acknowledgments

   Thanks to Robert Sparks, Alan Johnston, Dan Petrie, and Ben Campbell
   and many other members of the SIP WG for their continued support of
   the cause of distributed call control in SIP.

Normative References



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

   [3]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
        Specifications: ABNF", RFC 2234, November 1997.

   [4]  Sparks, R., "The SIP Referred-By Mechanism",
        draft-ietf-sip-referredby-01 (work in progress), February 2003.

   [5]  Peterson, J., "SIP Authenticated Identity Body (AIB) Format",
        draft-ietf-sip-authid-body-01 (work in progress), March 2003.

   [6]  Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
        Leach, P., Luotonen, A. and L. Stewart, "HTTP Authentication:
        Basic and Digest Access Authentication", RFC 2617, June 1999.

   [7]  Ramsdell, B., "S/MIME Version 3 Message Specification", RFC
        2633, June 1999.

Informational References

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

   [9]   Handley, M., Schulzrinne, H., Schooler, E. and J. Rosenberg,
         "SIP: Session Initiation Protocol", RFC 2543, March 1999.

   [10]  Mahy, R., "A Call Control and Multi-party usage framework for
         the Session  Initiation Protocol (SIP)",
         draft-ietf-sipping-cc-framework-02 (work in progress), March
         2003.

   [11]  Rosenberg, J., Schulzrinne, H., Camarillo, G. and J. Peterson,
         "Best Current Practices for Third Party Call Control in the
         Session  Initiation Protocol", draft-ietf-sipping-3pcc-03 (work
         in progress), March 2003.

   [12]  Sparks, R. and A. Johnston, "Session Initiation Protocol Call
         Control - Transfer", draft-ietf-sipping-cc-transfer-01 (work in
         progress), February 2003.

   [13]  Rosenberg, J. and H. Schulzrinne, "An INVITE Inititiated Dialog
         Event Package for the Session Initiation  Protocol (SIP",
         draft-ietf-sipping-dialog-package-01 (work in progress), March



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         2003.

   [14]  Johnston, A. and S. Donovan, "Session Initiation Protocol
         Service Examples", draft-ietf-sipping-service-examples-04 (work
         in progress), March 2003.

   [15]  Campbell, B., Rosenberg, J., Schulzrinne, H., Huitema, C. and
         D. Gurle, "Session Initiation Protocol (SIP) Extension for
         Instant Messaging", RFC 3428, December 2002.

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

   [17]  Donovan, S., "The SIP INFO Method", RFC 2976, October 2000.

   [18]  Rosenberg, J., "The Session Initiation Protocol (SIP) UPDATE
         Method", RFC 3311, October 2002.

   [19]  jdrosen@dynamicsoft.com and schulzrinne@cs.columbia.edu,
         "Reliability of Provisional Responses in Session Initiation
         Protocol (SIP)", RFC 3262, June 2002.

   [20]  Campbell, B., "SIMPLE Presence Publication Mechanism",
         draft-ietf-simple-publish-00 (work in progress), February 2003.


Authors' Addresses

   Rohan Mahy
   Cisco Systems, Inc.
   101 Cooper St
   Santa Cruz, CA  95060
   USA

   EMail: rohan@cisco.com


   Billy Biggs

   EMail: bbiggs@dumbterm.net


   Rick Dean

   EMail: rfc@fdd.com






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