    Network Working Group                                            R. Mahy
    Request for Comments: 3891                           Cisco Systems, Inc.
    Category: Standards Track                                       B. Biggs
                                                                     R. Dean
                                                              September 2004

            The Session Initiation Protocol (SIP) "Replaces" Header

    Status of this Memo

       This document specifies an Internet standards track protocol for the
       Internet community, and requests discussion and suggestions for
       improvements.  Please refer to the current edition of the "Internet
       Official Protocol Standards" (STD 1) for the standardization state
       and status of this protocol.  Distribution of this memo is unlimited.

    Copyright Notice

       Copyright (C) The Internet Society (2004).

    Abstract

       This document defines a new header for use with Session Initiation
       Protocol (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 the definition of these example features is non-
       normative.

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    RFC 3891               The SIP "Replaces" Header          September 2004

    Table of Contents

       1.  Overview. . . . . . . . . . . . . . . . . . . . . . . . . . .   2
       2.  Conventions . . . . . . . . . . . . . . . . . . . . . . . . .   4
       3.  User Agent Server Behavior: Receiving a Replaces Header . . .   4
       4.  User Agent Client Behavior: Sending a Replaces Header . . . .   6
       5.  Proxy Behavior. . . . . . . . . . . . . . . . . . . . . . . .   7
       6.  Syntax. . . . . . . . . . . . . . . . . . . . . . . . . . . .   7
           6.1.  The Replaces Header . . . . . . . . . . . . . . . . . .   7
           6.2.  New Option Tag for Require and Supported Headers. . . .   8
       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. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  13
       11. References. . . . . . . . . . . . . . . . . . . . . . . . . .  13
           11.1. Normative References. . . . . . . . . . . . . . . . . .  13
           11.2. Informative References. . . . . . . . . . . . . . . . .  14
       12. Authors' Addresses. . . . . . . . . . . . . . . . . . . . . .  15
       13. Full Copyright Statement. . . . . . . . . . . . . . . . . . .  16

    1.  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.

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    RFC 3891               The SIP "Replaces" Header          September 2004

       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 a
       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
       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 mechanism.  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       |
            |<--NOTIFY/200 (trying)-----------|--INVITE/200/ACK-->|
            |<--NOTIFY/200 (success)----------|<=================>|
            |------------BYE/200------------->|                   |
            |               |                 |                   |
            |               |                 |                   |
            |  Alice later retrieves call from another phone      |
            |               |                 |                   |
            |            *3 |-INV w/Replaces->|                   |
            |               |<--200-----------|                   |
            |               |---ACK---------->|----BYE/200------->|
            |               |<===============>|                   |
            |               |                 |                   |

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    RFC 3891               The SIP "Replaces" Header          September 2004

       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>

       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

    2.  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 BCP 14, RFC 2119 [2].

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

    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 User Agent (UA) attempts to match this
       information with a confirmed or early dialog.  The User Agent Server
       (UAS) matches the to-tag and from-tag parameters as if they were tags

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    RFC 3891               The SIP "Replaces" Header          September 2004

       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 matched invitation was just terminated, the
       replacement request should fail as well.  Declining the request with
       a 600-class response prevents an irritating race-condition where the
       UA rings or alerts for a replacement call which is not wanted.)

       If the Replaces header field matches an active dialog, the UA MUST
       verify that the initiator of the new INVITE is authorized to replace
       the matched dialog.  If the initiator of the new INVITE has been
       successfully authenticated as equivalent to the user who is being
       replaced, then the replacement is authorized.  For example, if the
       user being replaced and the initiator of the replacement dialog share
       the same credentials for Digest authentication [6], or they sign the
       replacement request with S/MIME [7] with the same private key and
       present the (same) corresponding certificate used in the original
       dialog, then the replacement is authorized.

       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 that 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].

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    RFC 3891               The SIP "Replaces" Header          September 2004

       The UA MAY apply other local policy to authorize the remainder of the
       request.  In other words, the UAS may apply a 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 described 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.

       (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
       User Agent Client (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

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    RFC 3891               The SIP "Replaces" Header          September 2004

       the Replaces header field.  A UAC MUST NOT send an INVITE with a
       Replaces header field that attempts to replace an early dialog which
       was not originated by the target of the INVITE with a 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
       Bob 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 other 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
       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].

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    RFC 3891               The SIP "Replaces" Header          September 2004

          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].  The syntax below relies on
       a number of productions from SIP [1].

          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 RFC 2543 [9] compliant UAs, a
       tag of zero matches both tags of zero and null.  A Replaces header
       field MAY contain the early-flag.

       Examples:

          Replaces: 98732@sip.example.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

    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

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    RFC 3891               The SIP "Replaces" Header          September 2004

    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 SIP
       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 of taking 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------------->|                        |
            |                       |                        |
            |                       |                        |

       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>

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    RFC 3891               The SIP "Replaces" Header          September 2004

       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

       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>

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    RFC 3891               The SIP "Replaces" Header          September 2004

       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 MUST 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.  All SIP implementations are already required
       to support Digest Authentication.  In addition, implementations which
       support the Replaces header SHOULD also implement the Referred-By
       mechanism.

       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 an 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
       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

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    RFC 3891               The SIP "Replaces" Header          September 2004

       other expressing this relationship.  Likewise, in a call center
       environment, each call center agent could possess credentials to
       which supervisors also have access.

       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 1, 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 S/MIME 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 is another way to authorize that 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
       notifications 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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    RFC 3891               The SIP "Replaces" Header          September 2004

    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.  Acknowledgments

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

    11.  References

    11.1.  Normative References

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

       [2]  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 Session Initiation Protocol (SIP) Referred-By
            Mechanism", RFC 3892, September 2004.

       [5]  Peterson, J., "The Session Initiation Protocol (SIP)
            Authenticated Identity Body (AIB) Format", RFC 3893, September
            2004.

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    RFC 3891               The SIP "Replaces" Header          September 2004

       [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., "Secure/Multipurpose Internet Mail Extensions
            (S/MIME) Version 3.1 Message Specification", RFC 3851, July
            2004.

    11.2.  Informative 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)", Work in Progress, March
            2003.

       [11] Rosenberg, J., Peterson, J., Schulzrinne, H., and G. Camarillo,
            "Best Current Practices for Third Party Call Control (3pcc) in
            the Session Initiation Protocol (SIP)", BCP 85, RFC 3725, April
            2004.

       [12] Sparks, R. and A. Johnston, "Session Initiation Protocol Call
            Control - Transfer", Work in Progress, February 2003.

       [13] Rosenberg, J. and H. Schulzrinne, "An INVITE Initiated Dialog
            Event Package for the Session Initiation Protocol (SIP)", Work
            in Progress, March 2003.

       [14] Johnston, A. and S. Donovan, "Session Initiation Protocol
            Service Examples", 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.

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    RFC 3891               The SIP "Replaces" Header          September 2004

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

       [20] Campbell, B., "SIMPLE Presence Publication Mechanism", Work in
            Progress, February 2003.

    12.  Authors' Addresses

       Rohan Mahy
       Cisco Systems, Inc.
       5617 Scotts Valley Dr
       Scotts Valley, CA  95066
       USA

       EMail: rohan@cisco.com

       Billy Biggs

       EMail: bbiggs@dumbterm.net

       Rick Dean

       EMail: rfc@fdd.com

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    RFC 3891               The SIP "Replaces" Header          September 2004

    13.  Full Copyright Statement

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