R. Mahy / Cisco Internet Draft D. Petrie / Pingtel Document: draft-mahy-sipping-join-and-fork-00.txt November 2001 Expires: May 2002 The SIP Join and Fork Headers Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026 [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. 1. Abstract This document proposes a new headers for use in SIP within the SIP call control framework. The Join and Fork headers are used for peer-to-peer call control. 2. Conventions used in this document 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 [RFC2119]. 3. Overview This document describes [SIP] extensions for distributed call control as part of the SIP call control [framework]. These extensions enable endpoints to request that they be added as a participant in a conversation. The mechanics of any mixing, conferencing, or media combining required to enable this request are abstracted from the requester. New headers are proposed in this document. These are intentionally similar to the syntax of the [Replaces] header which is also used for distributed call control. Mahy, Petrie Expires: May 2002 1 SIP Join and Fork Headers To better understand the purpose of these new headers let us first define some terminology: 3.1. Definitions Conference bridge: A SIP UA which terminates two or more dialogs and provides a mutually exclusive media mix across the dialogs. That is, the parties on the dialogs receive a media stream that is the mix of the contributed media of all parties excluding its own. The media for all of the dialogs is joined by the conference bridge. The bridge itself may also contribute an audio stream (for example, a phone that is acting as a bridge for a 3-way call). Example: The bridge B has dialogs with A and C. A receives a mix of B & C. B receives a mix of A & C. C receives a mix of A & B. /B+C =(dialog)=> A B <=(local)= A+C \A+B =(dialog)=> C Anchor: A SIP UA which terminates two or more dialogs and provides a media topology in which only the anchor receives a mix of the media from all UAs and the other UAs receive only the anchor’s media. The anchor’s media is forked to all the dialogs. Example: B receives notifications from portal P while communicating with A. /B =(dialog)=> A B <=(local)= A+P \B =(dialog)=> P [Issue: This does not provide the functionality for barge in. If Supervisor A barges in to CSR B’s call with customer C, the supervisor cannot hear the customer C.] The anchor or bridge may be a phone, gateway, media server or a dedicated device for the purpose of connecting dialogs in the desired media topology. It is also assumed that some UAs may be capable of dynamically configuring the media topology. We propose here two new call control operations that are indicated as part of an INVITE in setting up a new dialog. A join operation by a UAC indicates to the UAS that it is to assume a conference bridge role (or find a server to fulfill the media handling role) between the new dialog and a pre-existing dialog. A fork operation by a UAC indicates to the UAS that it is to assume an anchor role (or find a server to fulfill the media handling role) between the new dialog and a pre- existing dialog.3.2. Philosophy Mahy, Petrie Expires: May 2002 2 SIP Join and Fork Headers INVITEs are requests. A User Agent that accepts a request with one of the call-control headers agrees to take responsibility for creating the appropriate media relationships. Of course, like other requests, a User Agent can also reject or decline such a request. For example, a UA that receives an INVITE with a Join header MAY provide media "mixing" locally, rearrange existing call-legs (using 3pcc or REFER for example) so that all the participants are using some centralized mixing resource, or reject the request. Consider the following two scenarios. In scenario 1 (local control), a SIP UAC is operating as either a conference bridge or an anchor, and is initiating a new dialog. Since the SIP UAC knows and controls the desired media topology, it is easy for it to add the new dialog and control the media topology appropriately. In scenario 2 (remote control), a SIP UAC is initiating a dialog to a UAS that the UAC would like to use as an anchor or conference bridge. How does the UAC communicate the desired topology to the UAS? In the parlance of the SIP Call Control Model document [cc-models], the "Join" header is used to perform an "Insert" into a conversation space. (The UAC asks the UAS to "Add" it to the conversation space). The "Fork" header is used to perform a "Far fork" into a conversation space. (The UAC asks the UAS to become the anchor of the fork). Note that if a UA already in a "call" wants to Add a new party, this does not requirethe use of the Join or Fork header. Note that such functionality is already available using 3rd party call control [3pcc] 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. The Join header enables services such as barge-in, call center monitoring, and call recording in a distributed peer-to-peer way. The Fork header enables services such as hotword detection and "whispered" in-call notifications ("You have an appointment in 5 minutes"). This list of services is not exhaustive. 4. Behavior of SIP User Agents The Replaces, Join, and Fork headers have contradictory call control semantics. If more than one of these headers is present in a request, the request MUST be rejected with a 400 "Bad Request" response. Like the Replaces header, the Join and Fork headers contain information used to match an existing call-leg (call-id, to-tag, and from-tag). Upon receiving a new INVITE with one of these headers, the UA MUST attempt to match this information with an established or Mahy, Petrie Expires: May 2002 3 SIP Join and Fork Headers early dialog. When trying to find a matching dialog, the to-tag and from-tag parameters in the Join or Fork header are matched as if they were 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. Join or Fork headers in a reINVITE SHOULD be ignored. [OPEN ISSUE: are there legitimate uses for these headers in a reINVITE?] If the Join or Fork header matches more than one dialog, the UA MAY use other headers if present (ex: the Referred-By header) to attempt to match a single dialog. If a single matching dialog is not found, the UA MUST act as if no match is found. If no match is found, the UAS MUST ignore the header and process the INVITE normally. OPEN ISSUE: If no match is found, should the UAS ignore the header and process normally, or return a 481? If the call control header matches a dialog which was not created with an INVITE, the UAS MUST reject the request with an appropriate resonse (488 Not Acceptable Here?). Once a matching call-leg is found, the UAS MAY authenticate the INVITE request. If the request is successfully authenticated or already preauthorized, the UAS SHOULD proceed with processing. The UAS MAY prompt the user to accept or reject unauthenticated requests. The UAS MAY reject the request with any appropriate response (for example: 603 "Decline", 403 "Forbidden", or 486 "Busy Here"). 4.1. Semantics of Join and Fork A User Agent that accepts a Join header MUST attempt to setup call- legs such that the requesting UAC is logically added to the conversation space associated with the matched call-leg. Any call- legs which are already logically associated with the matched call- leg in the same conversation space are include as well. All the participants in a conversation space should have access to all the media/content sent in the context of that conversation space. That a participant does not negotiate a specific type of media does not mean that it is not otherwise a full participant. For a detailed description of various conferencing mechanisms that could be used to handle a Join, please consult [Conf]. Note that if a UA already in a conference wants to Add a new party, this does not require the use of the Join or Fork header. A User Agent that accepts a Fork header MUST attempt to setup a new conversation space that the user participates in at the same time as the conversation space associated with the matched call-leg. 4.2. Handling the Request Mahy, Petrie Expires: May 2002 4 SIP Join and Fork Headers If the UAS has sufficient resources to locally handle the Join or Fork request, the UAS SHOULD accept the call and perform the appropriate media mixing or combining. The UAS MAY rearrange appropriate call legs instead as described below, based on some local policy. If the UAS does not have sufficient resources locally to handle the request, but is aware of other resources which could be used to satisfy the request (ex: a centralized mixer), the UA SHOULD redirect the requestor to this resource and ask other participants in the same conversation space to use this resource. The UA MAY use any appropriate mechanism to transition participants to the new resource (ex: 3xx repsonse, [3pcc] style reINVITE, [REFER], reINVITE to a multicast group). The UA SHOULD only use mechanisms which are expected to be acceptable to the participants. For example, the UA SHOULD NOT attempt to transition the participants to a multicast group unless the UA can reasonably expect all the particpants to support multicast. If the UAS is incapable of satisfying the Join or Fork request, it MUST return a 488 "Not Acceptable Here" response. 5. Formal Syntax This document defines two new SIP headers: "Join" and "Fork". The following syntax specification uses the augmented Backus-Naur Form (BNF) as described in RFC-2234 [BNF]. cc-header is appended to the list of request headers cc-header = ( join | fork | token ) ":" SP call-info join = "Join" fork = "Fork" call-info = callid *( ";" header-params ) header-params = ( to-tag | from-tag | extension ) callid = token ["@" token] to-tag = "to-tag=" UUID from-tag = "from-tag=" UUID extension = param [ "=" val ] param = token val = token | quoted-string 6. Examples of usage Mahy, Petrie Expires: May 2002 5 SIP Join and Fork Headers The examples which follow are not intended to enumerate all the possibilities for the usage of these extensions, but rather to provide examples and ideas for some of the possibilities. 6.1. Join accepted for local mixing A B C | callid: 4@A | callid: 7@c | | | | | |<============>| | | | |INVITE------>| | |Join: 7@c | | | | | |<----200-----| | |-----ACK---->| | | | | | .. begins mixing .. | | | | |<===========>|<============>| |<::::::::::::::::::::::::::>| After accepting a Join from A, the conversation space which contained B and C, now has added A as well: [ B , C ] --> [ A , B , C ] 6.2. Join accepted and transitioned to central mixer A B C mixer | callid: 4@A | callid: 7@c | | | | | | | |<============>| | | | | | |INVITE------>| | | |Join: 7@c |--INVITE-------------------->| | |<----200---------------------| | |-----ACK-------------------->| |<----300-----| | |INVITE------------------------------------>| |<--200-------------------------------------| |---ACK------------------------------------>| | |--REFER------>| | | |<---200-------|--INVITE----->| | | |<----200------| | |<--NOTIFY-----|-----ACK----->| | |------200---->| | | |---BYE------->| | | |<--200--------| | Mahy, Petrie Expires: May 2002 6 SIP Join and Fork Headers | | | | |<=========================================>| mixes the | |<===========================>| three sessions | | |<============>| together The conversation space now looks identical to the previous example. Details of how the Join are implemented are transparent to A. B could have also used 3rd party call control to move the necessary sessions. [ B , C ] --> [ A , B , C ] 6.3. Join rejected A B C | callid: 4@A | callid: 7@c | | | | | |<============>| | | | |INVITE------>| | |Join: 7@c | | | | | |<----486-----| | |-----ACK---->| | | | | In this example B is Busy (does not want to be disturbed), and therefore does not wish to add A. 6.4. Fork accepted for local forking A B C | callid: 4@A | callid: 7@c | | | | | |<============>| | | | |INVITE------>| | |Fork: 7@c | | | | | |<----200-----| | |-----ACK---->| | | | | | .. begins forking .. | | | | |<===========>|<============>| | | | Mahy, Petrie Expires: May 2002 7 SIP Join and Fork Headers After this fork request, B is the anchor of two related conversation spaces, and participates on both at the same time. +--------+ | | +----------------+ | +------|---------+ | | | | | | | B C | ====> | | B | C | +----------------+ | +------|---------+ | | | | | | | | | A | | | +--------+ 6.5. Fork rejected A B C | callid: 4@A | callid: 7@c | | | | | |<============>| | | | |INVITE------>| | |Fork: 7@c | | | | | |<----603-----| | |-----ACK---->| | | | | Here B declines the forking session. 7. Security Considerations This extension can be used to monitor or insert content during a multimedia conversation. As such, invitations with these headers SHOULD only be accepted from authenticated or trusted parties. This extension was designed to take advantage of future signature parameters or authorization tokens defined by the SIP Working Group. In general, call control features would benefit considerably from such work. 8. References [SIP] M. Handley, E. Schooler, and H. Schulzrinne, "SIP: Session Initiation Protocol", RFC2543, Internet Engineering Task Force, Mahy, Petrie Expires: May 2002 8 SIP Join and Fork Headers Nov 1998. [cc-models] R. Mahy, "A Call Control Model for SIP", Internet Draft , IETF; July 2001. Work in progress. [cc-framework] B. Campbell, "SIP Call Control - Framework ", Internet Draft , IETF, Mar 2001. Work in progress. [Replaces] B. Biggs, R. Dean, "The SIP Replaces Header", Internet Draft , IETF; July 2001. Work in progress. [Conf] J. Rosenberg, H. Schulzrinne, "Models for Multi Party Conferencing in SIP", Internet Draft , IETF; Nov 2001. Work in progress. [REFER] R. Sparks, "The REFER Method", Internet Draft , IETF; Oct 2001. Work in progress. [3pcc] J. Rosenberg, J. Peterson, H. Schulzrinne, "Third Party Call Control in SIP", Internet Draft , IETF; Nov. 2000. Work in progress [RFC2026] S Bradner, "The Internet Standards Process -- Revision 3", RFC2026 (BCP), IETF, October 1996. [RFC2119] S. Bradner, "Key words for use in RFCs to indicate requirement levels," Request for Comments (Best Current Practice) 2119, Internet Engineering Task Force, Mar. 1997. [BNF] D Crocker, P Overell, "Augmented BNF for Syntax Specifications: ABNF", RFC2234, IETF, Nov 1997. 10. Acknowledgments The authors would like extend thanks to Billy Biggs and Rick Dean for their work on the Replaces header upon which this document is based. Thanks to Robert Sparks, Alan Johnston, and Ben Campbell and many other members of the SIP WG for their continued support of the cause of distributed call control in SIP. 11. Author's Addresses Rohan Mahy Cisco Systems 170 West Tasman Dr, MS: SJC-21/3 Phone: +1 408 526 8570 Email: rohan@cisco.com Mahy, Petrie Expires: May 2002 9 SIP Join and Fork Headers Dan Petrie Pingtel Corp. 400 West Cummings Park, Suite 2200 Woburn, MA, 01801 USA Email: dpetrie@pingtel.com Full Copyright Statement "Copyright (C) The Internet Society (date). All Rights Reserved. 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