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SIP WG                                                           R. Mahy
Internet-Draft                                       Cisco Systems, Inc.
Expires: March 31, 2004                                         O. Levin
                                                   Microsoft Corporation
                                                                Oct 2003


       Remote Call Control in SIP using the REFER method and the
                    session-oriented dialog package
                    draft-mahy-sip-remote-cc-00.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
   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 March 31, 2004.

Copyright Notice

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

Abstract

   This document describes how to use the SIP REFER method and the
   dialog package to manipulate conversations, dialogs, and sessions on
   remote User Agents.  This functionality is most  useful for
   collections of loosely coupled User Agents that wish to present a
   coordinated user experience.  It does not require a Third-Party Call
   Control controller to be involved in any of the manipulated dialogs.







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Table of Contents

   1.  Conventions  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Examples of Remote Call Control Operations . . . . . . . . . .  4
   4.  User Agent Behavior  . . . . . . . . . . . . . . . . . . . . .  8
   4.1 Organizing requests within dialogs . . . . . . . . . . . . . .  8
   4.2 Addressing the relevant parties  . . . . . . . . . . . . . . . 10
   4.3 Selecting an existing dialog context for the triggered
       request  . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
   4.4 Accessing Local Services Remotely  . . . . . . . . . . . . . . 11
   4.5 Authorizing remote call control requests . . . . . . . . . . . 12
   5.  More complex examples  . . . . . . . . . . . . . . . . . . . . 13
   6.  Handling DTMF  . . . . . . . . . . . . . . . . . . . . . . . . 15
   7.  Formal Syntax  . . . . . . . . . . . . . . . . . . . . . . . . 15
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 16
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 16
   10. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 16
       Normative References . . . . . . . . . . . . . . . . . . . . . 16
       Informational References . . . . . . . . . . . . . . . . . . . 17
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 18
       Intellectual Property and Copyright Statements . . . . . . . . 19





























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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].

   To simplify discussions related to the REFER method and its
   extensions, three new terms will be used:
      REFER-Issuer: the UA issuing the REFER request. Sometimes this
      document will also use the term "controller".
      REFER-Recipient: the UA receiving the REFER request
      REFER-Target: the UA designated in the Refer-To URI

2. Introduction

   The SIP [1] core protocol describes how User Agents originate and
   terminate sessions. The SIP call control framework [11] also
   describes how User Agents involved in these sessions can manipulate
   conversations based on the sessions to provide functionality such as
   transfer, pickup, and barge-in. Third-Party Call Control [13] goes on
   to describe how a controller can setup dialogs with a number of
   participants in order to manipulate sessions among the participants.

   Remote call control is the manipulation of conversations and
   session-oriented dialogs by a UA that is not directly involved in any
   of the relevant conversations, dialogs, or sessions. This
   manipulation generally involves sending REFER [4] requests to a UA
   which is directly involved, using information obtained via the dialog
   package [5]. (Although many are familiar with REFER only as used to
   implement call transfer [12], the authors of the REFER method never
   intended this limitation. In fact the REFER method was created when
   the SIP working group realized that a generic request to ask another
   UA to do something on your behalf was much more powerful than just
   doing transfers.)

   Unlike the Third-Party Call Control (3pcc) model which requires its
   controller to act as a B2BUA and maintain dialog state for all
   relevant dialogs, all the SIP entities involved in remote call
   control using REFER are just regular SIP User Agents. For convenience
   we can still describe the SIP entity that sends requests to
   manipulate remote sessions "the controller", but this is just a
   logical role. A UA that acts as a controller for one request can
   terminate and originate its own sessions, and even receive remote
   call control requests as other requests.

      Some readers may question if remote call control is an appropriate
      use of SIP, instead possibly something more appropriate for MGCP
      [16] or Megaco [17]. The authors believe that remote call control



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      is an appropriate and natural extension of SIP. Manipulating
      sessions and dialogs is certainly consistent with core
      functionality of SIP. This usage of SIP is much different from an
      MGCP or Megaco master/slave approach. For example, multiple UAs
      can send remote call control requests. All remote call control
      requests can be refused based on local authorization policy or if
      the request doesn't make sense. Finally, each UA is still fully
      responsible and authoritative for their own dialog and session
      state.  In other words, each UA still has the last word on its
      sessions and dialogs, even if asked to perform manipulations on
      that state by another entity.  This seems completely appropriate
      with the design of SIP. In fact these requirements and goals are
      well documented in the SIP Call Control Framework.

   Remote call control is especially useful for collections of loosely
   coupled User Agents which would like to present a coordinated user
   experience. Among other things, this allows User Agents which handle
   orthogonal media types but which would like to be present in a single
   conversation to add and remove each other from the conversation as
   needed.  This is especially appropriate when coordinating
   conversations among organizers, general purpose computers, and
   special purpose communications appliances like telephones, Internet
   televisions, in-room video systems, electronic whiteboards, and
   gaming devices.

   For example using remote call control, an Instant Messaging client
   could initiate a multiplayer gaming session and an audio session to a
   chat conversation. Likewise a telephone could add an electronic
   whiteboard session to a voice conversation. Finally, a computer or
   organizer could cause a nearby phone to dial from numbers or URIs in
   a document, email, or address book; allow users to answer or deflect
   incoming calls without removing hands from the computer keyboard;
   place calls on hold; and join other sessions on the phone or
   otherwise.

3. Examples of Remote Call Control Operations

   This entire section provide non-normative examples of functionality
   where a computer or PDA manipulates a telephone. The behavior for
   remote call control with other types of devices is similar, but
   describing similar manipulations for other media or device types
   would naturally use a different set of vocabulary.

   In the requests labeled with 1 and 2, Alice's PC or PDA sets up a
   subscription to the dialog package from Alice's phone (messages shown
   in a later section). All of the subsequent NOTIFY messages are
   notifications about changes in the dialog state at Alice's phone.  In
   message 3, Alice's PC or PDA asks her phone to "call Bob" (message



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   4), which eventually results in an early dialog (5) with one of Bob's
   Contacts.  [Note Well: Parts of the flow marked in parentheses
   (including messages 6, 7, 9, and 10) show alternative outcomes in the
   call flow.]

     Alice's              Alice's               Bob               Cathy
     PC or PDA            Phone

        | Call-ID: 123       | Call-ID: 456      | Call-ID: 789     |
        |                    |                   |                  |
        |---SUBSCRIBE/200--->| 1                 |                  |
        |<--NOTIFY/200-------| 2                 |                  |
        |                    |                   |                  |
        |                    |                   |                  |
        |---REFER/202------->| 3                 |                  |
        |<--NOTIFY/200-------|---INVITE--------->| 4                |
        |                    |<-----180----------| 5                |
        |<--NOTIFY/200-------|                   |                  |
        |                    |                   |                  |
        |                    |                   |                  |
      ( |---REFER/202------->| 6                 | )                |
      ( |                    |---CANCEL/200----->| )                |
      ( |<--NOTIFY/200-------|<-----487/ACK------| )                |
        |                    |                   |                  |
        |                    |                   |                  |
        |                    |<-----200/ACK------|                  |
        |<--NOTIFY/200-------|                   |                  |
        |                    |                   |                  |
      ( |---REFER/202------->| 7                 | )                |
      ( |                    |---BYE/200-------->| )                |
      ( |<--NOTIFY/200-------|                   | )                |
        |                    |                   |                  |
        |                    |                   |                  |
        |                    |<----------------------INVITE/180-----| 8
        |<--NOTIFY/200-------|                   |                  |
        |                    |                   |                  |
        |                    |                   |                  |
      ( |---REFER/202------->| 9                 |                  | )
      ( |<--NOTIFY/200-------|--------------------------302/ACK---->| )
        |                    |                   |                  |
        |                    |                   |                  |
      ( |---REFER/202------->| 10                |                  | )
      ( |<--NOTIFY/200-------|--------------------------486/ACK---->| )
        |                    |                   |                  |
        |                    |                   |                  |
        |---REFER/202------->| 11                |                  |
        |<--NOTIFY/200-------|--------------------------200/ACK---->|
        |                    |                   |                  |



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        |                    |                   |                  |


   Messages 3, 4, and 5 follow. The norefersub option-tag on each REFER
   suppresses the implicit subscription which would normally follow the
   REFER (the notifications in the call flow diagram are for the dialog
   package subscription in messages 1 and 2).

   Via and Max-Forward headers and session descriptions are omitted for
   brevity and clarity. In some cases, display names are added for
   simplify the task of the reader following the examples. Note that
   URIs in SIP cannot wrap lines. Due to RFC formatting conventions,
   this draft splits URIs across lines where the URI would exceed 72
   characters. A backslash character marks where this line folding has
   taken place.  Finally, some of the URIs shown here are not escaped
   properly to aid in readability. In message 9 the @ in the Refer-To
   URI should be escaped.

   Message 3:

   REFER sip:reg2@10.1.1.3 SIP/2.0
   To: "Alice's phone" <sip:reg2@10.1.1.3>;tag=def
   From: "Alice's PC or PDA" <sip:alice1@10.1.1.2>;tag=abc
   Call-ID: 123
   CSeq: 2 REFER
   Require: remotecc
   Supported: norefersub
   Refer-To: "Bob" <sip:bob@example.net>

   Message 4:

   INVITE sip:bob@example.net SIP/2.0
   To: "Bob" <sip:bob@example.net>
   From: "Alice" <sip:alice@example.com>;tag=xyz
   Call-ID: 456
   CSeq: 1 INVITE
   Contact: "Alice's Phone" <sip:reg2@10.1.1.3>
   Content-Type: application/sdp
   Content-Length: xxx

   Message 5:

   SIP/2.0 180 Ringing
   To: "Bob" <sip:bob@example.net>;tag=uvw
   From: "Alice's phone" <sip:reg2@10.1.1.3>;tag=xyz
   Call-ID: 456
   CSeq: 1 INVITE
   Contact: "Bob's Contact" <sip:line1@192.168.0.5>



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   Content-Type: application/sdp
   Content-Length: xxx

   The rest of the REFER messages in this example are identical  to the
   REFER in Message 3 except for the Refer-To header , CSeq header. and
   Via branch id (not shown). Message fragment 6 and 7 show the Refer-To
   headers which Alice's PC or PDA would send to cause Alice's phone to
   terminate the session which Message 4 attempted to originate. The
   extra parameters in the Refer-To header are used to explicitly match
   a specific dialog. They are more fully described in a later section.

   Header for Message 6:

   Refer-To: "Bob's Contact" <sip:line1@192.168.0.5;method=CANCEL>
    ;call-id=456;remote-tag=;local-tag=xyz

   Header for Message 7:

   Refer-To: "Bob's Contact" <sip:line1@192.168.0.5;method=BYE>
    ;call-id=456;remote-tag=uvw;local-tag=xyz


   Message 8 is an invitation received by Alice's phone from Cathy.
   Refer-To headers for Messages 9, 10, and 11 are shown below which
   would cause Alice's phone to redirect, reject, or accept the
   invitation. They use the response URI parameter defined in [6]. Note
   that some specialized User Agents might not be capable of accepting
   an invitation autonomously. For example, a SIP user agent which
   connect to an analog telephone cannot physically force the phone to
   go offhook.





















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   Message 8:

   INVITE sip:reg2@10.1.1.3 SIP/2.0
   To: "Alice" <sip:alice@example.com>
   From: "Cathy" <sip:cathy@example.net>;tag=ijk
   Call-ID: 789
   CSeq: 1 INVITE
   Contact: "Cathy's Contact" <sip:cathy-pc.example.net>
   Content-Type: application/sdp
   Content-Length: xxx


   Header for 9:

    Refer-To: <sip:cathy-pc.example.net;method=INVITE;response=302 \
     ?Contact=sip:doug@example.com>
     ;call-id=789;remote-tag=ijk;local-tag=lmn

   Header for 10:

    Refer-To: <sip:cathy-pc.example.net;method=INVITE;response=486>
     ;call-id=789;remote-tag=ijk;local-tag=lmn

   Header for 11:

   Refer-To: <sip:cathy-pc.example.net;method=INVITE;response=200>
     ;call-id=789;remote-tag=ijk;local-tag=lmn


4. User Agent Behavior

4.1 Organizing requests within dialogs

   REFER messages used for call transfer usually arrive within an
   existing dialog which was created with the INVITE method. In general,
   REFER messages  can be sent within an existing dialog, or they can
   start a new dialog (the dialog used by the implicit subscription they
   create). In many use cases of remote call control, receiving
   notifications about the status of a REFER request are superfluous, as
   the Refer-Issuer typically maintains a long duration subscription to
   the dialog package. This situation can be addressed by including the
   norefersub option-tag, defined in section 7 of [6]. When the
   norefersub option tag is present, a REFER request which would have
   created a new subscription and dialog becomes a standalone
   transaction instead. Each such standalone REFER transaction MUST use
   a new  (unique) Call-Id  header field value. The following three use
   cases are suggested:




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   1. In the most common usage, the controller maintains a long duration
   subscription to the dialog package, and sends REFER requests within
   that dialog. Each REFER  is  sent within the context of the dialog
   created for the subscription to the dialog package, and should
   include the norefersub option-tag in a Supported header field value.

   2. Occasionally the dialog package is only supported via a dialog
   state agent separate from the Refer-Receiver, in which case the
   controller maintains a long duration subscription to the dialog
   package to a dialog state agent, and the controller  sends these
   individual REFER requests as standalone requests each with a
   different (unique) Call-ID header field value, which could also
   include the norefersub option-tag in a Supported header field value.

   3. In some cases, the controller does not maintain a dialog package
   subscription for the Refer-Receiver. This might be the case for a
   "webdialer" or other application which associates with other UAs on
   an adhoc and intermittent basis. An initial REFER request is sent to
   start a new dialog, which is followed by notifications for the refer
   event type (the norefersub option-tag SHOULD NOT be used in this
   case).  These notifications could contain message/sipfrag or
   application/dialog-info+xml notification bodies as described in
   Section 4 of [6].
      OPEN ISSUE: Should we restrict usage to one of these three models?
      OPEN ISSUE: Are there other models possible?

   Message 1:

   SUBSCRIBE sip:reg2@10.1.1.3 SIP/2.0
   To: "Alice's phone" <sip:reg2@10.1.1.3>
   From: "Alice's PC or PDA" <sip:alice1@10.1.1.2>;tag=abc
   Call-ID: 123
   CSeq: 1 SUBSCRIBE
   Event: dialog
   Contact: <sip:alice1@10.1.1.2>

   Message 2:

   NOTIFY sip:reg2@10.1.1.3 SIP/2.0
   To: "Alice's PC or PDA" <sip:alice1@10.1.1.2>;tag=abc
   From: "Alice's phone" <sip:reg2@10.1.1.3>;tag=def
   Call-ID: 123
   CSeq: 1 NOTIFY
   Event: dialog
   Contact: <sip:reg2@10.1.1.3>
   Subscription-State: active;expires=3600
   Content-Type: application/dialog-info+xml
   Content-Length: xxx



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4.2 Addressing the relevant parties

   REFER requests contain a number of URIs which need to address the
   appropriate parties. A list of the relevant fields include the
   Request-URI, To header URI, From header URI, Contact header URI,
   Refer-To header URI, and the Referred-By header URI.  This section
   defines the semantics of each field.

   In most cases, remote call control seeks to manipulate dialogs or
   sessions on a specific UA.  For this reason, the Request URI of the
   REFER request SHOULD be a valid GRUU for a singel UA (a Contact URI).
   Contact URIs for a UA can be discovered by subscribing to the
   registration package for the relevant AORs.

   In the rare exceptions when the controller does not care which
   specifc UA it manipulates, an AOR MAY be used instead. When an AOR is
   used, the REFER request can include appropriate caller-preferences to
   encourage selection of an appropriate Contact.  The norefersub
   option-tag MUST NOT be used when the REFER Request-URI is an AOR, as
   the REFER Request could fork and cause incorrect behavior.  While,
   the controller can discourage a proxy from forking remote call
   control request by using the Request-Disposition: no-fork header
   field, insuring that no proxy forks requires the use of the
   callerpref option-tag in a Proxy-Require header field value. Because
   any proxy in the chain of this request which did not support caller
   preferences would cause the request to fail, use of Proxy-Require is
   NOT RECOMMENDED.

   For remote call control requests to operate as expected, the
   Refer-Issuer needs to be confident that the Refer-Receiver supports
   the extensions and conventions described here. Otherwise, the
   triggered request might have completely different semantics from the
   request which was indicated in the Refer-To header. (Most
   implementations ignore unknown URI and header parameters). For
   example a REFER intended to cause the Refer-Receiver to send a 486
   Busy Here response for an existing dialog, might instead trigger a
   new INVITE to the sender of the  original INVITE.  Implementations
   which send remote call control requests MUST include the remotecc
   option-tag in a Require header field value in each REFER request.
   (Note that support for this option-tag also implies support for the
   response URI parameter in a Refer-To header.)

   The To header field in the REFER request should contain the same URI
   as in the Request-URI, and the From identifies the AOR of the
   controller. The Refer-To is set to whatever URI would normally be
   inserted by a user of the Refer-Receiver (if operated autonomously).
   A REFER triggering a standalone request or dialog starting request,
   could send to either an AOR or a Contact address, but typically to an



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   AOR. A REFER request  triggering a request which is in a dialog  MUST
   always   place a Contact URI in the Refer-To header.

   When set, the Referred-By [7] header field SHOULD be the same URI as
   the URI in the Contact address of the REFER.  If included by the
   Refer-Issuer, it SHOULD be protected with a signed authenticated
   identity body [8] as recommended in the Referred-By specification.

4.3 Selecting an existing dialog context for the triggered request

   Many uses of remote call control require that the Refer-Receiver
   generate a new request or response in the context of an existing
   dialog, which was not necessarily initiated by the controller. For
   example, the controller might want the Refer-Receiver to send a BYE,
   CANCEL, or response to an INVITE in the context of a dialog created
   with INVITE. For subscriptions, the controller might want the
   Refer-Receiver to unsubscribe (send a SUBSCRIBE with an Expires
   header field of 0).

   To select the appropriate dialog from which to source the request,
   this document proposes a few new (header) parameters to the Refer-To
   header (the call-id, remote-tag, and local-tag parameters).  Explicit
   header parameters were selected because they can apply to non SIP
   URIs.  For example, the following URI, loads a "How To" website in
   the context of an existing dialog (presumably one created with an
   INVITE). When the associated dialog completes, the content may be
   hidden or dismissed with the context with which it was associated

   Refer-To: <http://support.example.com/howto.html>
    ;call-id=xyz;remote-tag=123;local-tag=456

    When describing the context of a subscription, the event and
   event-id parameters are also used. These correspond to the event type
   and the event-id parameter in the Event header (if present).

   Explicit matching of target dialogs and subscriptions was
   intentonally selected instead of including the appropriate values in
   embedded Call-ID, To, From, and Event headers. Among other benefits,
   this reduces the length of the URI portion of the Refer-To header and
   simplifies URI encoding requirements dramatically.

4.4 Accessing Local Services Remotely

   It may be desirable to have a URI convention for contacts on some UAs
   which gives autoanswer behavior. This allows for the development of
   several services if properly authorized (for example, an intercom
   service). In the context of remote call control, this URI could be
   placed in the Request-URI of a REFER requesting a new dialog (as in



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   Message 3 in the examples) instead of Alice's regular Contact address
   (at issue is if non-autoanswer behavior if desirable on devices which
   are capable of autoanswer). There are several possible syntactic
   choices for an autoanswer URI based on Alice's registration. (We
   cannot restrict ourselves to only one autoanswer URI on Alice's
   phone, since multiple registrations  may exhibit different
   authorization, alerting, and other behavior.) Three of these choices
   are listed below.

   (option1)    sip:reg2+autoanswer@10.1.1.3
   (option2)    sip:reg2;autoanswer@10.1.1.3
   (option3)    sip:reg2@10.1.1.3;autoanswer

   Hold is a very common local service on phones. Unfortunately hold has
   two semantics. Hold is often used to describe a primitive operation
   of setting all media lines in a session to inactive. (We will call
   this Simple Hold). Hold also describes a server running on a phone
   which can cause different behavior, for example, music on hold, tone
   on hold, or simple hold. It is desirable to be able to access the
   "Hold" service on Alice's phone via a URI. In the absense of any
   other convention, we will use the following URI:
   sip:service.hold@10.1.1.3 .  Using this "service URI" on the phone,
   Alice's PC or PDA can ask the phone to place a specific session on or
   off hold as shown below.

   REFER sip:reg2@10.1.1.3 SIP/2.0
   Refer-To: <sip:service.hold@10.1.1.3>
     ;call-id=789;remote-tag=ijk;local-tag=lmn

   REFER sip:reg2@10.1.1.3 SIP/2.0
   Refer-To: <sip:service.unhold@10.1.1.3>
     ;call-id=789;remote-tag=ijk;local-tag=lmn

   The SIP conferencing framework [14] and the cc-conferencing describe
   the Conference Factory as a service which assigns new conference
   URIs. The conference factory is itself accessible via a URI. In some
   cases, this conference factory is colocated with a phone or other
   single-user UA. In the absense of any other convention, we use the
   following URI to reference the conference factory service on Alice's
   phone:  sip:service.conf-factory@10.1.1.3 .
      OPEN ISSUE: How are service URIs registered?
      OPEN ISSUE: Is this a complete set of services that are needed for
      remote call control?

4.5 Authorizing remote call control requests

   To be written.




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5. More complex examples

   The following example shows how a controller can cause the
   Refer-Recevier to complete a transfer between two existing calls.

     Alice's              Alice's               Bob               Cathy
     PC or PDA            Phone

        |                    | Call-ID: 456      | Call-ID: 789     |
        |                    |<=================>|                  |
        |                    |<====================================>|
        |                    |                   |                  |
        |                    |                   |                  |
        |                    |                   |                  |
      A |---REFER/202------->|                   |                  |
        |                 B  |---REFER/202------>|                  |
        |                 C  |<--NOTIFY/200------|---INVITE-------->|
        |                    |                   |   w/Replaces     |
        |                    |                   |<--200/ACK--------|
        |                 D  |<--NOTIFY/200------|                  |
        |                    |<--BYE/200----------------------------|
      E |<--NOTIFY/200-------|                   |                  |
        |                    |---BYE/200-------->|                  |
      F |<--NOTIFY/200-------|                   |<================>|
        |                    |                   |                  |
        |                    |                   |                  |

   Message A:  (CHECK TAGS!)  (character escaping)

   Refer-To: "Bob's Contact" <sip:line1@192.168.0.5;method=REFER \
    ?Refer-To=<sip::cathy-pc.example.net?Replaces=789;to-tag=ijk;from-tag=lmn>>
    ;call-id=456;remote-tag=uvw;local-tag=xyz

   Messages C and D are notifications to the refer package.
   Messages E and F are notifications to the dialog package.



   The following example shows how a controller can cause the
   Refer-Receiver to join two existing sessions using a SIP conference
   server (labeled "Focus" in the call flow diagram). Note that <  >  ;
   and = characters in these URIs need to be escaped

     Alice's           Focus          Alice's         Bob           Cathy
     PC or PDA                        Phone

        |                |              | Call-ID: 456 | Call-ID: 789 |
        |                |              |<============>|              |



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        |                |              |<===========================>|
        |                |              |              |              |
        |                |              |              |              |
      G |---REFER/202------------------>|              |              |
        |                |<--INVITE-----|              |              |
        |<--NOTIFY/200--(100)-----------|              |              |
      H |                |---200/ACK--->|              |              |
        |<--NOTIFY/200----(200)---------|              |              |
        |                |<============>|              |              |
        |                |              |              |              |
      I |---REFER/202--->|              |              |              |
        |                |---INVITE-w/ Replaces---------------------->|
        |                |<--200/ACK----------------------------------|
        |                |              |<---BYE/200------------------|
        |<--NOTIFY/200------------------|              |              |
        |                |<==========================================>|
        |                |              |              |              |
      J |---REFER/202--->|              |              |              |
        |                |---INVITE-w/ Replaces------->|              |
        |                |<--200/ACK-------------------|              |
        |                |              |<---BYE/200---|              |
        |<--NOTIFY/200------------------|              |              |
        |                |<===========================>|              |
        |                |              |              |              |
        |                |              |              |              |

   Message G:

   Refer-To: <sip:service.conf-factory@10.1.1.3>
    ;call-id=456;remote-tag=uvw;local-tag=xyz

   Message H:

   SIP/2.0 200 OK
   To: "Conference Factory" <sip:service.conf-factory@10.1.1.3>;tag=ccc
   From: "Alice" <sip:alice@example.com>;tag=bbb
   Call-ID: aaa
   CSeq 1 INVITE
   Contact: "Conference #3" <sip:conf3@10.1.1.3>;isFocus
   Content-Type: application/sdp
   Content-Length: xxx

   Message I:

   REFER sip:conf3@10.1.1.3 SIP/2.0
   Refer-To: <sip:cathy-pc.example.net \
    ?Replaces=789;to-tag=ijk;from-tag=lmn>
   Referred-By: "Alice's PC or PDA" <sip:alice1@10.1.1.2>;cid="333@444"



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   Message J:

   REFER sip:conf3@10.1.1.3 SIP/2.0
   Refer-To: "Bob's Contact" <sip:line1@192.168.0.5 \
    ?Replaces=456;to-tag=uvw;from-tag=xyz>
   Referred-By: "Alice's PC or PDA" <sip:alice1@10.1.1.2>;cid="111@222"




6. Handling DTMF

   Occasionally it is useful for one UA to collect digits on behalf of a
   User Agent which can actually send them using RFC2833 [19]. One of
   the options is that the collecting UA send KPML [20] responses to the
   UA capable of turning these keypad events into DTMF media. The method
   used for sending markup responses is under discussion currently, but
   one proposal is a new method called FEEDBACK as part of [21]. For
   example, it is possible that the KPML response tag include new
   parameters as shown below to identify the dialog for which the keypad
   markup is intended.

   FEEDBACK sip:reg2@10.1.1.3 SIP/2.0
   To: "Alice's phone" <sip:reg2@10.1.1.3>;tag=def
   From: "Alice's PC or PDA" <sip:alice1@10.1.1.2>;tag=abc
   Call-ID: 123
   CSeq: 27 FEEDBACK
   Content-Type: application/kpml+xml
   Content-Length: xxx

   <?xml version="1.0">
   <kpml version="1.0">
     <response digits="9999#" call-id="456"
               remote-tag="uvw" local-tag="xyz"/>
   </kpml>


      OPEN ISSUE: Need further investigation of mechanism to carry DTMF.
      Can this be generalized?

7. Formal Syntax

   The following syntax specification extends the Refer-To header
   described in RFC3515 using the augmented Backus-Naur Form (BNF) as
   described in RFC-2234 [3].






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   Refer-To = ("Refer-To" / "r") HCOLON ( name-addr / addr-spec ) *
      (SEMI referto-params)

   referto-params = callid-param / rtag-param / ltag-param /
                     event-param / eventid-param / generic-param

   callid-param = "call-id" EQUAL ( token / LDQOT callid RDQUOT )
   rtag=param = "remote-tag" EQUAL token
   ltag=param = "local-tag" EQUAL token
   event-param = "event" EQUAL event-type
   eventid-param = "event-id" EQUAL token


8. Security Considerations

   The functionality described in this document allows an authorized
   party to manipulate SIP sessions and dialogs in arbitrary ways.
   Implementations need to take reasonable precautions to insure
   authenticity of remote call control request, which MUST be sent using
   either hop-by-hop TLS [10] via a SIPS URI, or individually signed
   usingSMIME [9].  Signing remote call control requests with SMIME is
   RECOMMENDED. In addition, UAs which support remote call control
   SHOULD sign Referred-By headers in remote call control requests in an
   appropriate authenticated identity body.  UAs which support remote
   call control MUST implement SIPS,  SHOULD implement SMIME signing and
   verification, and SHOULD implement separate signing of Referred-By
   headers in an appropriate authenticated identity body.

9. IANA Considerations

   Need to register the remotecc option-tag, the Refer-To header
   parameters, and the SIP URI parameters

10. Acknowledgments

   Many thanks to Sean Olson, Robert Sparks, and Alan Johnston.

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.



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

   [5]   Rosenberg, J. and H. Schulzrinne, "An INVITE Inititiated Dialog
         Event Package for the Session Initiation  Protocol (SIP",
         draft-ietf-sipping-dialog-package-02 (work in progress), July
         2003.

   [6]   Olson, S., "Extensions to the REFER mechanism for Third Party
         Call Control", draft-olson-sipping-refer-extensions-00 (work in
         progress), June 2003.

   [7]   Sparks, R., "The SIP Referred-By Mechanism",
         draft-ietf-sip-referredby-03 (work in progress), August 2003.

   [8]   Peterson, J., "SIP Authenticated Identity Body (AIB) Format",
         draft-ietf-sip-authid-body-02 (work in progress), July 2003.

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

   [10]  Dierks, T., Allen, C., Treese, W., Karlton, P., Freier, A. and
         P. Kocher, "The TLS Protocol Version 1.0", RFC 2246, January
         1999.

Informational References

   [11]  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.

   [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., Peterson, J., Schulzrinne, H. and G. Camarillo,
         "Best Current Practices for Third Party Call Control in the
         Session  Initiation Protocol", draft-ietf-sipping-3pcc-04 (work
         in progress), July 2003.

   [14]  Rosenberg, J., "A Framework for Conferencing with the Session
         Initiation Protocol",
         draft-ietf-sipping-conferencing-framework-00 (work in
         progress), May 2003.

   [15]  Rosenberg, J., Schulzrinne, H. and P. Kyzivat, "Caller
         Preferences for the Session Initiation Protocol (SIP)",



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         draft-ietf-sip-callerprefs-09 (work in progress), July 2003.

   [16]  Andreasen, F. and B. Foster, "Media Gateway Control Protocol
         (MGCP) Version 1.0", RFC 3435, January 2003.

   [17]  Groves, C., Pantaleo, M., Anderson, T. and T. Taylor, "Gateway
         Control Protocol Version 1", RFC 3525, June 2003.

   [18]  Burger, E., "Basic Network Media Services with SIP",
         draft-burger-sipping-netann-07 (work in progress), September
         2003.

   [19]  Schulzrinne, H. and S. Petrack, "RTP Payload for DTMF Digits,
         Telephony Tones and Telephony Signals", RFC 2833, May 2000.

   [20]  Burger, E., "Keypad Stimulus Protocol (KPML)",
         draft-ietf-sipping-kpml-00 (work in progress), September 2003.

   [21]  Jennings, C., "SIP Support for Application Initiation",
         draft-jennings-sip-app-info-01 (work in progress), July 2003.


Authors' Addresses

   Rohan Mahy
   Cisco Systems, Inc.
   5617 Scotts Valley Drive, Suite 200
   Scotts Valley, CA  95066
   USA

   EMail: rohan@cisco.com


   Orit Levin
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052
   USA

   EMail: oritl@microsoft.com











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   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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