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Versions: 00 01 02 03 04 05                                             
Internet Engineering Task Force                                R. Sparks
Internet-Draft                                               dynamicsoft
Expires: August 27, 2001                               February 26, 2001


                      SIP Call Control - Transfer
                     draft-ietf-sip-cc-transfer-04

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 groups may also distribute working documents as
   Internet-Drafts.

   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
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on August 27, 2001.

Copyright Notice

   Copyright (C) The Internet Society (2001). All Rights Reserved.

Abstract

   This document defines a SIP extension, REFER, and demonstrates its
   use to provide Call Transfer capabilities. This work is part of the
   Call Control Framework.












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Table of Contents

   1.      Overview . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.      Changes from draft-sparks-sip-cc-transfer-03 . . . . . . .  3
   3.      The REFER Method . . . . . . . . . . . . . . . . . . . . .  4
   3.1     The Refer-To Header  . . . . . . . . . . . . . . . . . . .  4
   3.1.1   Examples . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.2     The Referred-By Header . . . . . . . . . . . . . . . . . .  5
   3.2.1   A PGP based signature-scheme . . . . . . . . . . . . . . .  5
   3.2.2   Examples . . . . . . . . . . . . . . . . . . . . . . . . .  6
   3.3     Header Field Support for the REFER Method  . . . . . . . .  6
   3.4     Message Body Inclusion . . . . . . . . . . . . . . . . . .  7
   3.5     Responses within the REFER transaction . . . . . . . . . .  7
   3.6     Behavior of SIP User Agents  . . . . . . . . . . . . . . .  8
   3.6.1   Accessing the referred-to resource . . . . . . . . . . . .  8
   3.6.2   Reporting on the results of the reference  . . . . . . . .  8
   3.6.2.1 Using NOTIFY . . . . . . . . . . . . . . . . . . . . . . .  8
   3.6.2.2 The body of the NOTIFY . . . . . . . . . . . . . . . . . .  8
   3.7     Behavior of SIP Registrars/Redirect Servers  . . . . . . .  9
   3.8     Behavior of SIP Proxies  . . . . . . . . . . . . . . . . .  9
   3.9     Prototypical REFER callflow  . . . . . . . . . . . . . . . 10
   3.10    Security Considerations  . . . . . . . . . . . . . . . . . 11
   3.10.1  Circumventing privacy  . . . . . . . . . . . . . . . . . . 11
   3.10.2  Circumventing security . . . . . . . . . . . . . . . . . . 12
   3.10.3  Limiting the breach  . . . . . . . . . . . . . . . . . . . 12
   4.      Call Transfer  . . . . . . . . . . . . . . . . . . . . . . 13
   4.1     Actors and Roles . . . . . . . . . . . . . . . . . . . . . 13
   4.2     Requirements . . . . . . . . . . . . . . . . . . . . . . . 13
   4.3     Using REFER to achieve Call Transfer . . . . . . . . . . . 14
   4.4     Unattended Transfer  . . . . . . . . . . . . . . . . . . . 14
   4.4.1   Successful Unattended Transfer . . . . . . . . . . . . . . 15
   4.4.2   Failed Unattended Transfer . . . . . . . . . . . . . . . . 16
   4.5     Unattended Transfer with Consultation Hold . . . . . . . . 16
   4.5.1   Variation 1 : Exposes transfer target  . . . . . . . . . . 17
   4.5.2   Variation 2 : Protects transfer target . . . . . . . . . . 18
   4.5.3   Consultation Hold in the presence of forking proxies . . . 18
   4.6     Attended Transfer  . . . . . . . . . . . . . . . . . . . . 19
   4.7     Transfer with multiple parties . . . . . . . . . . . . . . 19
   5.      Open Issues  . . . . . . . . . . . . . . . . . . . . . . . 20
   5.1     REFER is now dependent on sip-events . . . . . . . . . . . 20
   5.2     Registering the events with IANA . . . . . . . . . . . . . 20
   6.      Acknowledgments  . . . . . . . . . . . . . . . . . . . . . 20
           References . . . . . . . . . . . . . . . . . . . . . . . . 20
           Author's Address . . . . . . . . . . . . . . . . . . . . . 21
           Full Copyright Statement . . . . . . . . . . . . . . . . . 22






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1. Overview

   This document defines a SIP[1] extension, REFER, and details its use
   to provide Call Transfer capabilities. This is part of a family of
   Call Control extensions described in the Call Control Framework[2]
   document.

   Editor's note: Per consensus at the February interim WG meeting, the
   two parts of this document, the definition of REFER and its use to
   achieve transfer, will be separated into separate documents. That
   separation will take place with the next revision of this document.

   The mechanisms discussed here are most closely related to
   traditional unattended and consultation hold transfers. Discussion
   of attended transfer (where all parties are briefly in a conference)
   is deferred until the conferencing features in this framework are
   addressed.

   This work has roots in draft-ietf-sip-cc-01[4] but some basic
   semantics are different. In particular, transfers are achieved
   through a new method that does not terminate the original signaling
   relationship. By disassociating transfers from the processing of
   BYE, these changes facilitate recovery of failed transfers and
   clarify state management in the participating entities.

2. Changes from draft-sparks-sip-cc-transfer-03

   Editor's note: The changes for this revision focused on the changes
   to the NOTIFY response mechanism discussed at February's interim
   meeting, and clarification to the REFER method itself. As mentioned
   above, the intent is to separate this document into two. The split
   was postponed a version to ensure the edits to REFER/NOTIFY would be
   reflected in the ID repository in time for discussion at IETF 50.

   o  Modified contents of NOTIFYs to reflect the consensus at the
      interim meeting. One event type, with an application/sip body
      representing the information to be returned.
   o  Added message detail to the prototypical flow
   o  More explicitly stated that REFER MAY occur outside an existing
      call-leg
   o  Reinforced that REFER can be record-routed
   o  Made the presence of a Contact header mandatory
   o  Changed the positive response to a REFER request to 202 Accepted
      instead of 200 OK
   o  Corrected editing errors in examples and message diagrams






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3. The REFER Method

   REFER is a SIP method as defined by RFC2543[1]. The REFER method
   indicates that the recipient (identified by the Request-URI) should
   contact a third party using the contact information provided in the
   method. A success response indicates that the recipient was able to
   contact the third party.

   Unless stated otherwise, the protocol for emitting and responding to
   a REFER request are identical to those for a BYE request in [1]. The
   behavior of SIP entities not implementing the REFER (or any other
   unknown) method is explicitly defined in [1].

   A REFER request MAY be placed outside the scope of a call-leg
   created with an INVITE. REFER MAY be Record-Routed, hence MUST
   contain a single Contact header. REFERs occurring inside an existing
   call-leg MUST follow the Route/Record-Route logic of that call-leg.
   REFERs occurring outside an existing call-leg effectively create a
   new call-leg following the behavior of SUBSCRIBE specified [3].

3.1 The Refer-To Header

   Refer-To is a request-header as defined by [1]. It may only appear
   in a REFER request.

      Refer-To = ("Refer-To" | "r") ":" URL

   A REFER method MUST contain exactly one Refer-To header.

   The Refer-To header MAY be encrypted as part of end-end encryption.

        The Contact header is an important part of the Route/Record-Route
        mechanism and is not available for this task.

3.1.1 Examples

         Refer-To: sip:alice@atlanta.com

         Refer-To: sip:bob@biloxi.com?Accept-Contact=sip:bobsdesk.
                   biloxi.com&Call-ID=55432@alicepc.atlanta.com

         Refer-To: sip:carol@cleveland.com;method=SUBSCRIBE

         Refer-To: http://www.ietf.org

   Long headers are line-wrapped here for clarity only.





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3.2 The Referred-By Header

   Referred-By is a request-header as defined by [1]. It can appear in
   any request. It conveys the identity of the original REFERrer to the
   referred-to party, optionally proving the identity and that the
   REFERrer actually issued this reference.


        Referred-By      = ("Referred-By" | "b") ":" referrer-url ";"
                              (     referenced-url
                                | ( referenced-url ";" ref-signature )
                                | ( ref-signature ";" referenced-url )
                              )
        referrer-url     = ( name-addr | addr-spec )
        referenced-url   = "ref" "=" "<" URL ">"
        ref-signature    = signature-scheme *( ";" sig-scheme-params )
        signature-scheme = "scheme" "=" token
        sig-scheme-parms = token "=" ( token | quoted-string )

   The referrer-url contains the SIP URL of the party sending the REFER
   request. The referenced-url contains a copy of the URL placed in the
   Refer-To: header. Any occurrences of < or > in the referenced-url
   MUST be escaped. The ref-signature contains a signature over the
   concatenation of referrer-url and referenced-url.  An example
   signature scheme is given in section 3.1.2.

   A REFER request MUST contain exactly one Referred-By header.

   The Referred-By header SHOULD be signed to help detection of REFERs
   from unauthorized third parties. A signed Referred-By header SHOULD
   include a Date header in the referrer-url to facilitate detection of
   replay attacks.

   A UA MAY reject a request containing an unsigned Referred-By header.
   A UA SHOULD verify the signature on any Referred-By header it
   receives.

   The Referred-By header MAY be encrypted as part of end-end
   encryption.

3.2.1 A PGP based signature-scheme

   One signature-scheme for Referred-By headers uses PGP as follows:

        signature-scheme = "scheme" "=" "pgp"
        sig-scheme-parms = pgp-version | signed-by | pgp-signature

   pgp-version, signed-by and pgp-signature are defined in section 15.1
   of RFC2543, with the modification that the signature is computed


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   across the concatenation of the referrer-url and the referenced-url.

3.2.2 Examples


        Referred-By: sip:alice@atlanta.com;ref=<http:www.ietf.org>

        Referred-By: "Bob" <sip:bob@biloxi.com>;
           ref=<sip:alice@atlanta.com>;scheme=pgp;
           pgp-version="5.0";signature="the signature"


   (Note that in the last example, the signature would be over the
   string "sip:bob@biloxi.comsip:alice@atlanta.com")

3.3 Header Field Support for the REFER Method

   This table adds a column to tables 4 and 5 in [1], describing header
   presence in a REFER method. See [1] for a key for the symbols used.
   A row for the Refer-To: and Referred-By request-header should be
   inferred, each mandatory for REFER. Refer-To is not applicable for
   all other methods. Referred-By is a general Request header. The enc
   and e-e columns in [1] apply to the REFER method unmodified.

            Header                    Where  REFER
            Accept                      R       -
            Accept-Encoding             R       -
            Accept-Language             R       o
            Allow                       R       -
            Allow                      405      m
            Authorization               R       o
            Call-ID                    gc       m
            Contact                     R       m
            Contact                    1xx      -
            Contact                   2-6xx     o
            Content-Encoding            e       -
            Content-Length              e       o
            Content-Type                e       -
            CSeq                       gc       m
            Date                        g       o
            Encryption                  g       o
            Expires                     R       o
            From                       gc       m
            Hide                        R       o
            Max-Forwards                R       o
            Organization                g       o
            Priority                    R       -
            Proxy-Authenticate         407      o
            Proxy-Authorization         R       o


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            Proxy-Require               R       o
            Require                     R       o
            Retry-After                 R       -
            Retry-After            404,480,486  o
            Retry-After                503      o
            Retry-After              600,603    o
            Response-Key                R       o
            Record-Route                R       o
            Record-Route               2xx      o
            Route                       R       o
            Server                      r       o
            Subject                     R       -
            Timestamp                   g       o
            To                        gc(1)     m
            Unsupported                420      o
            User-Agent                  g       o
            Via                       gc(2)     m
            Warning                     r       o
            WWW-Authenticate           401      o

3.4 Message Body Inclusion

   A REFER method may contain a body which SHOULD be processed
   according to its Content-Type.

3.5 Responses within the REFER transaction

   An agent responding to a REFER Method MUST return a 400 Bad Request
   if the request contained zero or more than one Refer-To headers. An
   agent responding to a REFER Method MUST return a 400 Bad Request if
   the request contained zero or more than one Referred-By headers. An
   agent (including proxies generating local responses) MAY return a
   100 Trying or any appropriate 400-600 class response as prescribed
   by [1]. If the recipient's agent decides to contact the resource in
   the Refer-To header, a 202 Accepted response MUST be returned before
   the REFER transaction expires.

   Editor's note - earlier versions of this draft required the agent
   responding to REFER to wait until the referred action completed
   before sending a final response to the REFER. That final response
   reflected the success or failure of the referred action. This was
   infeasible due to the transaction timeout rules defined for
   non-INVITE requests in [1]. A REFER must always receive an immediate
   (within the lifetime of a non-INVITE transaction) final response.







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3.6 Behavior of SIP User Agents

3.6.1 Accessing the referred-to resource

   A UA receiving a well-formed REFER request SHOULD request approval
   from the user to proceed (this request could be interactive or
   through configuration). Upon receiving approval from the user, the
   UA MUST contact the resource identified by the URL in the Refer-To:
   header. Note that if the URL is a SIP URL, it could contain header
   fields such as Call-Id that will be used to form the resulting
   request. If the URL is a SIP URL, the Referred-By header in the
   REFER request should be copied into the request sent to the
   referred-to resource.

3.6.2 Reporting on the results of the reference

3.6.2.1 Using NOTIFY

   Once it is known whether the reference succeeded or failed, the UA
   receiving the REFER SHOULD notify the agent sending the refer using
   the NOTIFY mechanism defined in Event Notification in SIP[3] as if
   the the REFER had established a subscription. In particular:

   o  Each NOTIFY should reflect the To:, From:, and Call-ID headers
      from the REFER as if they had arrived in a SUBSCRIBE.

   o  Each NOTIFY MUST contain an event header of Event: refer

   o  Each NOTIFY MUST contain a body of type "application/sip". The
      contents of this body are detailed in Section 3.6.2.2

   o  Analogous to the case for SUBSCRIBE described in [3], the agent
      that issued the REFER MUST be prepared to receive a NOTIFY before
      the REFER transaction completes.

   Open Issue: This makes REFER dependent on sip-events (Section 5.1)

   Open Issue: The refer event will need to be registered with IANA
   (Section 5.2)

3.6.2.2 The body of the NOTIFY

   Each NOTIFY MUST contain a body of type "application/sip". This body
   MUST begin with a SIP Response Status-Line as defined in [1]. The
   response class in this status line indicates the success of the
   referred action. The body MAY contain other SIP headers to provide
   information about the outcome of the referenced action.




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   A minimal, but complete, implementation can respond with a single
   NOTIFY containing either the body:

      SIP/2.0 200 OK

   if the reference was successful or the body:

      SIP/2.0 503 Service Unavailable

   if the reference failed.

   An implementation MAY include more of a SIP message in that body to
   convey more information. Warning headers received in responses to
   the referred action are good candidates. In fact, if the reference
   was to a SIP URL, entire response to the referenced action could be
   returned. However, doing so could have grave security repercussions
   (see Section 3.10). Implementers must carefully consider what they
   choose to include.

   Note that if the reference was to a non-SIP URL, status in any
   NOTIFYs to the referrer must still be in the form of SIP Response
   Status-Lines. The minimal implementation discussed above is
   sufficient to provide a basic indication of success or failure. For
   example, if a client receives a REFER to a HTTP URL, and is
   successful in accessing the resource, its NOTIFY to the referrer can
   contain the application/sip body of "SIP/2.0 200 OK"

3.7 Behavior of SIP Registrars/Redirect Servers

   Registrars and Redirect Servers SHOULD return a 603 to a REFER
   request, unless they are also playing some other SIP role.

3.8 Behavior of SIP Proxies

   SIP Proxies do not require modification to support the REFER method.
   Specifically, as required by [1], a proxy should process a REFER
   request the same way it processes an OPTIONS request.














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3.9 Prototypical REFER callflow

          Agent A                  Agent B
             |                        |
             |   REFER                |
             |----------------------->|
             |        202 Accepted    |
             |<-----------------------|
             |                        |
             |                        |------->
             |                        |  (whatever)
             |                        |<------
             |                        |
             |            NOTIFY      |
             |<-----------------------|
             |   200 OK               |
             |----------------------->|
             |                        |
             |                        |

   Here are examples of what the four messages between Agent A and
   Agent B might look like if the reference to (whatever) that Agent B
   makes is successful:
      Message One

       REFER sip:b@agentland SIP/2.0
       Via: SIP/2.0/UDP agenta.agentland
       To: sip:b@agentland
       From: sip:a@agentland;tag=193402342
       Call-ID: 898234234@agenta.agentland
       CSeq: 93809823 REFER
       Refer-To: (whatever URL)
       Referred-By: <sip:a@agentland>;ref=<whatever URL>;
          scheme=pgp;pgp-version="5.0";signature="signature goes here"
       Contact: sip:a@agentland
       Content-Length: 0

      Message Two

       SIP/2.0 202 Accepted
       Via: SIP/2.0/UDP agenta.agentland
       To: sip:b@agentland;tag=4992881234
       From: sip:a@agentland;tag=193402342
       Call-ID: 898234234@agenta.agentland
       CSeq: 93809823 REFER
       Contact: sip:b@agentland
       Content-Length: 0

      Message Three


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       NOTIFY sip:a@agentland SIP/2.0
       Via: SIP/2.0/UDP agentb.agentland
       To: sip:a@agentland;tag=193402342
       From: sip:b@agentland;tag=4992881234
       Call-ID: 898234234@agenta.agentland
       CSeq: 1993402 NOTIFY
       Event: refer
       Contact: sip:b@agentland
       Content-Type: application/sip
       Content-Length: 16

       SIP/2.0 200 OK


      Message Four

       SIP/2.0 200 OK
       Via: SIP/2.0/UDP agentb.agentland
       To: sip:a@agentland;tag=193402342
       From: sip:b@agentland;tag=4992881234
       Call-ID: 898234234@agenta.agentland
       CSeq: 1993402 NOTIFY
       Contact: sip:a@agentland
       Content-Length: 0

3.10 Security Considerations

   The security requirements of [1] apply to the REFER method.

   This mechanism relies on providing contact information for the
   referred-to resource to the party being referred. Care should be
   taken to provide a suitably restricted URI if the referred to
   resource should be protected.

   Care should be taken when implementing the logic that determines
   whether or not to accept the REFER request. A UA not capable of
   accessing non-SIP URLs SHOULD NOT accept REFER requests to them.

   Using application/sip bodies to return the progress and results of a
   REFER request is extremely powerful. Careless use of that capability
   will compromise security and privacy. Here are a couple of simple,
   somewhat contrived, examples to demonstrate the potential for harm.

3.10.1 Circumventing privacy

   Suppose Alice has a user-agent that accepts REFER requests to SIP
   INVITE URLs, and NOTIFYs the referrer of the progress of the INVITE
   by copying each response to the INVITE into the body of a NOTIFY.



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   Suppose further that Carol has a reason to avoid Mallory and has
   configured her system at her proxy to only accept calls from a
   certain set of people she trusts (including Alice), so that Mallory
   doesn't learn when she's around, or what user agent she's actually
   using.

   Mallory can send a REFER to Alice, with a Refer-To: indicating
   Carol. If Alice can reach Carol, the 200 OK Carol sends gets
   returned to Mallory in a NOTIFY, letting him know not only that
   Carol is around, but also the IP address of the agent she's using.

3.10.2 Circumventing security

   Suppose Alice, with the same user agent as above, is working at a
   company that is working on the greatest SIP device ever invented -
   the SIP FOO. The company has been working for months building the
   device and the marketing materials, carefully keeping the idea, even
   the name of the idea secret (since a FOO is one of those things that
   anybody could do if they'd just had the idea first). FOO is up and
   running, and anyone at the company can use it, but it's not
   available outside the company firewall.

   Mallory has heard rumor that Alice's company is onto something big,
   and has even managed to get his hands on a URL that he suspects
   might have something to do with it. He sends a REFER to ALICE with
   the mysterious URL and as Alice connects to the FOO, Mallory gets
   NOTIFYs with bodies containing

      Server: FOO/v0.9.7

3.10.3 Limiting the breach

   For each of these cases, and in general, returning a carefully
   selected subset of the information available about the progress of
   the reference through the NOTIFYs mitigates risk. The minimal
   implementation described in Section 3.6.2.2 exposes the least
   information about what the agent operating on the REFER request has
   done, and is least likely to be a useful tool for malicious users.













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4. Call Transfer

4.1 Actors and Roles

   There are three actors in a given transfer event, each playing one
   of the following roles:

        Transferee -      the party being transferred to the Transfer
                          Target.

        Transferor -      the party initiating the transfer

        Transfer Target - the new party being introduced into a call with
                          the Transferee.

   The following roles are used to describe transfer requirements and
   scenarios:

        Originator -  wishes to place a call to the Recipient. This actor
                      is the source of the first INVITE in a session, to
                      either a Facilitator or a Screener.

        Facilitator - receives a call or out-of-band request from the
                      Originator, establishes a call to the Recipient
                      through the Screener, and connects the Originator to
                      the Recipient.

        Screener -    receives a call ultimately intended for the Recipient
                      and transfers the calling party to the Recipient if
                      appropriate.

        Recipient -   the party the Originator is ultimately connected to.

4.2 Requirements
   1.  Any party in a SIP session MUST be able to transfer any other
       party in that session at any point in that session.
   2.  The Transferor and the Transferee MUST NOT be removed from a
       session as part of a transfer transaction.

             At first glance, requirement 2 may seem to indicate
             that the user experience in a transfer must be
             significantly different from what a current PBX or
             Centrex user expects. As the call-flows in this
             document show, this is not the case. A client MAY
             preserve the current experience. In fact, without
             this requirement, some forms of the current
             experience (ringback on unattended transfer failure
             for instance) will be lost.



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   3.  The Transferor MUST know whether or not the transfer was
       successful (this is significantly different from the
       requirements of draft-ietf-sip-cc-01).

4.3 Using REFER to achieve Call Transfer

   A REFER can be issued by the Transferor to cause the Transferee to
   issue an INVITE to the Transfer-Target. Note that a successful REFER
   transaction does not terminate the session between the Transferor
   and the Transferee. If those parties wish to terminate their
   session, they must do so with a subsequent BYE request. The media
   negotiated between the transferee and the transfer target is not
   affected by the media that had been negotiated between the
   transferor and the transferee. In particular, the INVITE issued by
   the Transferee will have the same SDP body it would have if he
   Transferee had initiated that INVITE on its own. Further, the
   disposition of the media streams between the Transferor and the
   Transferee is not altered by the REFER method. Agents may alter a
   session's media through additional signaling. For example, they may
   make use of the SIP hold re-INVITE [1] or the conferencing
   extensions provided by this framework.

4.4 Unattended Transfer

   Unattended Transfer consists of the Transferor providing the
   Transfer Target's contact to the Transferee. The Transferee attempts
   to establish a session using that contact and reports the results of
   that attempt to the Transferor. The signaling relationship between
   the Transferor and Transferee is not terminated, so the call is
   recoverable if the Transfer Target cannot be reached. Note that the
   Transfer Target's contact information has been exposed to the
   Transferee. The provided contact can be used to make new calls in
   the future. The diagrams below show indicate the first line of each
   message. All messages in a particular diagram share the same
   Call-ID. In these diagrams, media is managed through reINVITE holds,
   but other mechanisms (mixing multiple media streams at the UA or
   using the conferencing extensions for example) are valid.














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4.4.1 Successful Unattended Transfer

              Transferor           Transferee             Transfer
                   |                    |                  Target
                   |            INVITE  |                    |
                   |<-------------------|                    |
                   |            200 OK  |                    |
                   |------------------->|                    |
                   |            ACK     |                    |
                   |<-------------------|                    |
                   |  INVITE (hold)     |                    |
                   |------------------->|                    |
                   |  200 OK            |                    |
                   |<-------------------|                    |
                   |  ACK               |                    |
                   |------------------->|                    |
                   |  REFER             |                    |
                   |------------------->|                    |
                   |  202 Accepted      |                    |
                   |<-------------------|                    |
                   |                    |  INVITE            |
                   |                    |------------------->|
                   |                    |  200 OK            |
                   |                    |<-------------------|
                   |                    |  ACK               |
                   |                    |------------------->|
                   |  NOTIFY (200 OK)   |                    |
                   |<-------------------|                    |
                   |            200 OK  |                    |
                   |------------------->|                    |
                   |  BYE               |                    |
                   |------------------->|                    |
                   |  200 OK            |                    |
                   |<-------------------|                    |
                   |                    |             BYE    |
                   |                    |<-------------------|
                   |                    |             200 OK |
                   |                    |------------------->|













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4.4.2 Failed Unattended Transfer

              Transferor           Transferee             Transfer
                   |                    |                  Target
                   |                    |                    |
                   |            INVITE  |                    |
                   |<-------------------|                    |
                   |            200 OK  |                    |
                   |------------------->|                    |
                   |            ACK     |                    |
                   |<-------------------|                    |
                   |  INVITE (hold)     |                    |
                   |------------------->|                    |
                   |  200 OK            |                    |
                   |<-------------------|                    |
                   |  ACK               |                    |
                   |------------------->|                    |
                   |  REFER             |                    |
                   |------------------->|                    |
                   |  202 Accepted      |                    |
                   |<-------------------|                    |
                   |                    |  INVITE            |
                   |                    |------------------->|
                   |                    |  486 Busy Here     |
                   |                    |<-------------------|
                   |                    |  ACK               |
                   |                    |------------------->|
                   |    NOTIFY (503 Service Unavailable)     |
                   | or NOTIFY (486 Busy Here)               |
                   |<-------------------|                    |
                   |            200 OK  |                    |
                   |------------------->|                    |
                   |  INVITE (unhold)   |                    |
                   |------------------->|                    |
                   |  200 OK            |                    |
                   |<-------------------|                    |
                   |  ACK               |                    |
                   |------------------->|                    |
                   |  BYE               |                    |
                   |------------------->|                    |
                   |  200 OK            |                    |
                   |<-------------------|                    |

4.5 Unattended Transfer with Consultation Hold

   Transfer with Consultation Hold involves a session between the
   transferor and the transfer target before the transfer actually
   takes place. This is implemented with SIP Hold and Unattended
   Transfer as described above.


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4.5.1 Variation 1 : Exposes transfer target

   The transferor places the transferee on hold, establishes a call
   with the transfer target to alert them to the impending transfer,
   terminates the connection with the transfer target, then proceeds
   with unattended transfer as above. This variation can be used to
   provide an experience similar to that expected by current PBX and
   Centrex users.

   To (hopefully) improve clarity, non-REFER transactions have been
   collapsed into one indicator with the arrow showing the direction of
   the request.

              Transferor           Transferee             Transfer
                   |                    |                  Target
                   |                    |                    |
         Call-ID:1 | INVITE/200 OK/ACK  |                    |
                   |<-------------------|                    |
         Call-ID:1 | INVITE (hold)/200 OK/ACK                |
                   |------------------->|                    |
         Call-ID:2 | INVITE/200 OK/ACK  |                    |
                   |---------------------------------------->|
         Call-ID:2 | BYE/200 OK         |                    |
                   |---------------------------------------->|
         Call-ID:1 | REFER              |                    |
                   |------------------->|                    |
                   | 202 Accepted       |                    |
                   |<-------------------|                    |
         Call-ID:1 |                    |  INVITE/200 OK/ACK |
                   |                    |------------------->|
         Call-ID:1 | NOTIFY (200 OK)    |                    |
                   |<-------------------|                    |
                   |            200 OK  |                    |
                   |------------------->|                    |
         Call-ID:1 | BYE/200 OK         |                    |
                   |------------------->|                    |
         Call-ID:1 |                    |         BYE/200 OK |
                   |                    |<-------------------|













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4.5.2 Variation 2 : Protects transfer target

   The transferor places the transferee on hold, establishes a call
   with the transfer target and then reverses their roles, transferring
   the original transfer target to the original transferee. This has
   the advantage of hiding information about the original transfer
   target from the original transferee. On the other hand, the
   Transferee's experience is different that in current systems. The
   Transferee is effectively "called back" by the Transfer Target.

              Transferor           Transferee             Transfer
                   |                    |                  Target
                   |                    |                    |
         Call-ID:1 | INVITE/200 OK/ACK  |                    |
                   |<-------------------|                    |
         Call-ID:1 | INVITE (hold)/200 OK/ACK                |
                   |------------------->|                    |
         Call-ID:2 | INVITE/200 OK/ACK  |                    |
                   |---------------------------------------->|
         Call-ID:2 | INVITE (hold)/200 OK/ACK                |
                   |---------------------------------------->|
         Call-ID:2 | REFER              |                    |
                   |---------------------------------------->|
                   | 202 Accepted       |                    |
                   |<----------------------------------------|
         Call-ID:2 |                    |  INVITE/200 OK/ACK |
                   |                    |<-------------------|
         Call-ID:2 | NOTIFY (200 OK)    |                    |
                   |<----------------------------------------|
                   |                    |            200 OK  |
                   |---------------------------------------->|
         Call-ID:1 | BYE/200 OK         |                    |
                   |------------------->|                    |
         Call-ID:2 | BYE/200 OK         |                    |
                   |---------------------------------------->|
         Call-ID:2 |                    |  BYE/200 OK        |
                   |                    |------------------->|

4.5.3 Consultation Hold in the presence of forking proxies

   It is worth noting that the examples given above abstract away any
   proxies that might be between the three parties. In 4.5.1 for
   example, the URL used to reach the Transfer Target may go through a
   forking proxy. There is no guarantee that the Transferee's and
   Transferor's invitations to the Transfer Target will reach the same
   endpoint. If the proxy forked in parallel, both invitations could
   cause multiple endpoints to ring. To increase the probability of the
   desired behavior of having the referred invite reach and ring only
   the same endpoint as the consultation invite, the Transferor SHOULD


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   issue the REFER request with the Refer-To: header containing the
   Contact the Transfer Target provided in its 200 OK to the
   Transferor's INVITE. If that REFER fails, the Transferor SHOULD
   issue another REFER with the Refer-To: header containing the URL it
   used to reach the Transfer Target, augmented with an Accept-Contact
   header containing the Contact the Transfer Target provided.

4.6 Attended Transfer

   In an attended transfer, the three actors participate in an ad-hoc
   conference as part of the event. Discussion of the implementation of
   attended transfer is thus deferred until the conferencing portion of
   the Call Control framework has been addressed.

4.7 Transfer with multiple parties

   In this example the Originator places call to the Facilitator who
   reaches the Recipient through the Screener. The Recipient's contact
   information is exposed to the Facilitator and the Originator. This
   example is provided for clarification of the semantics of the REFER
   method only and should not be used as the design of an
   implementation.

            Originator   Facilitator   Screener   Recipient
        Call-ID  |            |            |          |
            1    |INVITE/200 OK/ACK        |          |"Get Fred for me!"
                 |----------->|            |          |     "Right away!"
            1    |INVITE (hold)/200 OK/ACK |          |
                 |<-----------|            |          |
            2    |            |INVITE/200 OK/ACK      |"I have a call
                 |            |----------->|          |from Mary for Fred"
            2    |            |INVITE (hold)/200 OK/ACK   "Hold please"
                 |            |<-----------|          |
            3    |            |            |INVITE/200 OK/ACK
                 |            |            |--------->|"You have a call
                 |            |            |          |from Mary"
                 |            |            |          |  "Put her through"
            3    |            |            |INVITE (hold)/200 OK/ACK
                 |            |            |--------->|
            2    |            |REFER       |          |
                 |            |<-----------|          |
                 |            |202 Accepted|          |
                 |            |----------->|          |
            2    |            |INVITE/200 OK/ACK      |
                 |            |---------------------->|"This is Fred"
            2    |            |NOTIFY (200 OK)        |  "Please hold for
                 |            |----------->|          |              Mary"
                 |            |200 OK      |          |
                 |            |<-----------|          |


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            2    |            |BYE/200 OK  |          |
                 |            |<-----------|          |
            3    |            |            |BYE/200 OK|
                 |            |            |--------->|
            2    |            |INVITE (hold)/200 OK/ACK
                 |            |---------------------->|
            1    |REFER       |            |          |
                 |<-----------|            |          |
                 |202 Accepted|            |          |
                 |----------->|            |          |
            1    |INVITE/200 OK/ACK        |          |
                 |----------------------------------->| "Hey Fred"
            1    |NOTIFY (200 OK)          |          |    "Hello Mary"
                 |----------->|            |          |
                 |200 OK      |            |          |
                 |<-----------|            |          |
            1    |BYE/200 OK  |            |          |
                 |<-----------|            |          |
            2    |            |BYE/200 OK  |          |
                 |            |---------------------->|
            1    |BYE/200 OK  |            |          |
                 |<-----------------------------------| "See you later"

5. Open Issues

5.1 REFER is now dependent on sip-events

   By introducing NOTIFY, this work is prevented from moving to RFC
   until the sip-events draft moves to that level (that work is
   currently an individual submission). What needs to happen to our
   deliverable schedule to allow for completing the sip-events work?

5.2 Registering the events with IANA

   When we near the end of the process, the refer event will need to be
   registered with IANA per [3].

6. Acknowledgments

   This draft is a collaborative product of the SIP working group. The
   editor thanks the following for their early contributions to this
   work:  Ben Campbell, Chris Cunningham, Steve Donovan, Alan Johnston,
   Kevin Summers and Dean Willis.

References

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



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   [2]  Campbell, B., "Framework for SIP Call Control Extensions",
        draft-ietf-sip-cc-framework-00 (work in progress), March 2000.

   [3]  Roach, A., "Event Notification in SIP",
        draft-roach-sip-subscibe-notify-03 (work in progress), February
        2001.

   [4]  Schulzrinne, H. and J. Rosenberg, "SIP Call Control Services",
        draft-ietf-sip-cc-01 (work in progress - expired), June 1999.


Author's Address

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

   email: rsparks@dynamicsoft.com































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Full Copyright Statement

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Acknowledgement

   Funding for the RFC editor function is currently provided by the
   Internet Society.



















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