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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 RFC 4575

Internet Engineering Task Force                               SIPPING WG
Internet Draft                                              J. Rosenberg
                                                          H. Schulzrinne
                                                             Columbia U.
June 24, 2002
Expires: December 2002

 A Session Initiation Protocol (SIP) Event Package for Conference State


   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-

   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

   To view the list Internet-Draft Shadow Directories, see


   This document defines a conference event package for the SIP Events
   architecture, along with a data format used in notifications for this
   package. The conference package allows users to subscribe to a SIP
   URI that is associated with a conference. Notifications are sent
   about changes in the membership of this conference, changes in active
   speaker, and media mixing information.

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                           Table of Contents

   1          Introduction ........................................    4
   2          Terminology .........................................    4
   3          Conference Event Package ............................    4
   3.1        Event Package Name ..................................    5
   3.2        Event Package Parameters ............................    5
   3.3        SUBSCRIBE Bodies ....................................    5
   3.4        Subscription Duration ...............................    6
   3.5        NOTIFY Bodies .......................................    6
   3.6        Notifier Processing of SUBSCRIBE Requests ...........    6
   3.7        Notifier Generation of NOTIFY Requests ..............    6
   3.8        Subscriber Processing of NOTIFY Requests ............    6
   3.9        Handling of Forked Requests .........................    7
   3.10       Rate of Notifications ...............................    7
   3.11       State Agents ........................................    7
   4          Conference Data Format ..............................    7
   4.1        Structute of the Format .............................    8
   4.1.1      User Element ........................................    8
   4.1.2      Status ..............................................    9
   4.1.3      Dialog ..............................................    9
   4.1.4      Media Status ........................................    9
   4.2        Constructing Coherent State .........................    9
   4.3        Schema ..............................................   10
   4.4        Example .............................................   13
   5          Security Considerations .............................   13
   6          IANA Considerations .................................   13
   6.1        application/conference-info+xml MIME Registration
   ................................................................   13
   6.2        URN Sub-Namespace Registration for
   urn:ietf:params:xml:ns:conference-info .........................   14
   7          Acknowledgements ....................................   15
   8          Authors Addresses ...................................   15
   9          Normative References ................................   15
   10         Informative References ..............................   16

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1 Introduction

   The SIP Events framework [1] defines general mechanisms for
   subscription to, and notification of, events within SIP networks. It
   introduces the notion of a package, which is a specific
   "instantiation" of the events mechanism for a well-defined set of
   events. Packages have been defined for user presence [9], watcher
   information [10], and message waiting indicators [11], amongst
   others. Here, we define an event package for SIP conferences. This
   package will work for any conference in which there is a central
   signaling element [12].

   The conferencing package allows a subscriber to learn about the
   identities of participants and their status in the conference, the
   dialogs that are used by each user, the media makeup of the
   conference, and the mixing matrix (i.e., who is receiving whose
   audio). This information is vital to building conferencing
   applications using SIP.

2 Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   and "OPTIONAL" are to be interpreted as described in RFC 2119 [2] and
   indicate requirement levels for compliant implementations.

3 Conference Event Package

   The conference event package allows a user to subscribe to a
   conference. In SIP, conferences are represented by URIs. These URIs
   route to a SIP user agent that is responsible for ensuring that all
   users in the conference can communicate with each other. Detailed
   information on conferencing models can be found in [12]. This package
   can work with any of the tightly-coupled models described in that
   specification. However, the conference server may not have complete
   information on the media makeup of the conference. This is the case
   for multicast-based conferences, where the server cannot know the
   mixing matrix for the conference. In this case, that information
   would simply not be reported through this package.

   The specific information conveyed in notifications in this package

        o The SIP URI identifying the user.

        o The dialog state associated with that users attachment to the

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        o Their status in the conference (active, declined, departed).

        o Their status in terms of receiving media in the conference.

   This section provides the details for defining a SIP Events package,
   as specified by [1].

3.1 Event Package Name

   The name of this event package is "conference". This package name is
   carried in the Event and Allow-Events header, as defined in [1].

3.2 Event Package Parameters

   A single parameter, "recurse" is defined for this package. When
   present, it informs the server that it should check whether any
   participants in the conference are themselves conferences, and if so,
   generate a subscription to their conference state with the "recurse"
   attribute. The users reported in notifications from this recursed
   subscription are reported to the original subscriber. The result is
   that the list of users reported to the subscriber represents the
   complete user list even when cascaded conferences are used.


   Event: conference;recurse

3.3 SUBSCRIBE Bodies

   A SUBSCRIBE for a dialog package MAY contain a body. This body
   defines a filter to apply to the subscription.

   A SUBSCRIBE for a conference package MAY be sent without a body. This
   implies the default subscription filtering policy. The default policy

        o Notifications are generated every time there is any change in
          the set of users participating in the conference, or a change
          their state (dialog state, media mixing state, etc.)

        o Notifications do not normally contain full state; rather, they
          only indicate the state of the participant whose state has
          changed. The exception is a NOTIFY sent in response to a
          SUBSCRIBE. These NOTIFYs contain the complete view of
          conference state.

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        o For a given user, the notifications contain the identity
          information and status.

3.4 Subscription Duration

   The default expiration time for a subscription to a conference is one
   hour. Of course, once the conference ends, all subscriptions to that
   particular conference are terminated, with a reason of "noresource"

3.5 NOTIFY Bodies

   The body of the notification contains a conference information
   document. All implementations MUST support the type
   "application/conference-info+xml", defined in Section 4. Other
   formats MAY be supported.

   When a client generates a SUBSCRIBE request, and that request
   contains an Accept header, the "application/conference-info+xml"
   format MUST be included in the header. Other formats MAY be listed.
   The default value for the Accept header when it is not present in a
   request is "application/conference-info+xml".

   Of course, the notifications generated by the server MUST be in one
   of the formats specified in the Accept header in the SUBSCRIBE

3.6 Notifier Processing of SUBSCRIBE Requests

   The conference information contains very sensitive information.
   Therefore, all subscriptions SHOULD be authenticated and then
   authorized before approval. Authorization policy is at the discretion
   of the administrator, as always. However, a few recommendations can
   be made.

   It is RECOMMENDED that all users in the conference be allowed to
   subscribe to the conference.

3.7 Notifier Generation of NOTIFY Requests

   Notifications SHOULD be generated for the conference whenever a new
   participant joins, a participant leaves, and a dial-out attempt
   succeeds or fails. Notifications MAY be generated for the conference
   whenever the media mixing status of a user changes.

3.8 Subscriber Processing of NOTIFY Requests

   The SIP Events framework expects packages to specify how a subscriber

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   processes NOTIFY requests in any package specific ways, and in
   particular, how it uses the NOTIFY requests to contruct a coherent
   view of the state of the subscribed resource.

   Typically, the NOTIFY for the conference package will only contain
   information about those users whose state in the conference has
   changed. To construct a coherent view of the total state of all
   users, a subscriber to the dialog package will need to combine
   NOTIFYs received over time.

   Notifications within this package can convey partial information;
   that is, they can indicate information about a subset of the state
   associated with the subscription. This means that an explicit
   algorithm needs to be defined in order to construct coherent and
   consistent state. The details of this mechanism are specific to the
   particular document type. See Section 4.2 for information on
   constructing coherent information from an application/conference-
   info+xml document.

3.9 Handling of Forked Requests

   By their nature, the conferences supported by this package are
   centralized. Therefore, SUBSCRIBE requests for a conference should
   not generally fork. Users of this package MUST NOT install more than
   a single subscription as a result of a single SUBSCRIBE request.

3.10 Rate of Notifications

   For reasons of congestion control, it is important that the rate of
   notifications not become excessive. As a result, it is RECOMMENDED
   that the server not generate notifications for a single subscriber at
   a rate faster than once every 5 seconds.

3.11 State Agents

   Conference state is ideally maintained in the element in which the
   conference resides. Therefore, the elements that maintain the
   conference are the ones best suited to handle subscriptions to it.
   Therefore, the usage of state agents is NOT RECOMMENDED for this

4 Conference Data Format

   Conference information is an XML document [3] that MUST be well-
   formed and SHOULD be valid. Dialog information documents MUST be
   based on XML 1.0 and MUST be encoded using UTF-8. This specification
   makes use of XML namespaces for identifying dialog information
   documents and document fragments. The namespace URI for elements

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   defined by this specification is a URN [4], using the namespace
   identifier 'ietf' defined by [5] and extended by [6]. This URN is:


   A conference information document begins with the root element tag

4.1 Structute of the Format

   Conference information begins with the top level element
   "conference-info".  This element has three mandatory attributes:

        version: This attribute allows the recipient of conference
             information documents to properly order them. Versions
             start at 0, and increment by one for each new document sent
             to a subscriber. Versions are scoped within a subscription.
             Versions MUST be representable using a 32 bit integer.

        state: This attribute indicates whether the document contains
             the full conference information, or whether it contains
             only information on those users whose state have changed
             since the previous document (partial).

        entity: This attribute contains a URI that identifies the
             conference whose information is reported in the remainder
             of the document.

   The "conference-info" element has zero or more "user" sub-elements
   which contain information on the users in the conference.

4.1.1 User Element

   Each user element has zero or one "status" elements, indicating their
   status in the conference, zero or one "dialog" elements, indicating
   their dialog information, and zero or one "media-state" elements,
   indicating their media reception information.

   The user element has one mandatory attribute, "uri" that indicates
   the URI for the user in the conference. This is a logical identifier,
   not a machine specific one (i.e., its taken from the To/From, not the
   Contact). The optional attribute "display-name" contains a display
   name for the user. The standard "xml:lang" language attribute can
   also be present to indicate the language of the display name.

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4.1.2 Status

   The status element contains the status of the user in the conference.
   The following statuses are defined:

        active: The user is in an active dialog with the conference

        departed: The user sent a BYE, thus leaving the conference.

        booted: The user was sent a BYE by the conference host, booting
             them out of the conference.

        failed: The server tried to bring the user into the conference,
             but its attempt to contact the specific user resulted in a
             non-200 class final response.

4.1.3 Dialog

   The dialog element is defined in [7].

4.1.4 Media Status

   The "media-status" element indicates the types of media that the user
   is currently receiving in the conference. It consists of a series of
   "media-stream" sub-elements, each of one describes a particular media
   stream. Each "media-stream" element has a mandatory "media-type"
   attribute that indicates the MIME media type for the stream. There is
   also an optional "send-status" and "recv-status" attribute which
   attribute which indicates the status of the media to/from that user.

   If the "send-status" is "received-by-all", it means that the media
   for that stream that is being generated by the user is being mixed by
   the server and sent to all recipients. "muted" means that no one is
   receiving their media. If it is "unknown", the server does not know
   the status, perhaps because the media is being distributed via
   multicast or multi-unicast.

   If the "recv-status" is "receiving-all" it means that the user is
   hearing all other participants. If it is "anchor-only", the user is
   hearing media from just a single participant. If it is "unknown", the
   server does not know the status, perhaps because the media is being
   distributed via multicast or multi-unicast.

4.2 Constructing Coherent State

   The conference information subscriber maintains a table for the list
   of users in the conference. The table contains a row for each user.

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   Each row is indexed by a URI, present in the "uri" attribute of the
   "user" element. The contents of each row contain the state of that
   user as conveyed in the document. The table is also associated with a
   version number. The version number MUST be initialized with the value
   of the "version" attribute from the "conference-info" element in the
   first document received. Each time a new document is received, the
   value of the local version number, and the "version" attribute in the
   new document, are compared. If the value in the new document is one
   higher than the local version number, the local version number is
   increased by one, and the document is processed. If the value in the
   document is more than one higher than the local version number, the
   local version number is set to the value in the new document, the
   document is processed, and the subscriber SHOULD generate a refresh
   request to trigger a full state notification. If the value in the
   document is less than the local version, the document is discarded
   without processing.

   The processing of the conference information document depends on
   whether it contains full or partial state. If it contains full state,
   indicated by the value of the "state" attribute in the "conference-
   info" element, the contents of the table are flushed. They are
   repopulated from the document. A new row in the table is created for
   each "user" element. If the document contains partial state, as
   indicated by the value of the "state" attribute in the "conference-
   info" element, the document is used to update the table. For each
   "user" element in the document, the subscriber checks to see whether
   a row exists for that user. This check is done by comparing the URI
   in the "uri" attribute of the "user" element with the URI associated
   with the row. If the user doesn't exist in the table, a row is added,
   and its state is set to the information from that "user" element. If
   the user does exist, its state is updated to be the information from
   that "user" element. If a row is updated or created, such that its
   state is now booted, failed or departed, that entry MAY be removed
   from the table at any time.

4.3 Schema

   The following is the schema for the application/conference-info+xml

   <?xml version="1.0" encoding="UTF-8"?>
   <xs:schema targetNamespace="urn:ietf:params:xml:ns:conference-info"

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     <!-- This import brings in the XML language attribute xml:lang-->
     <xs:import namespace="http://www.w3.org/XML/1998/namespace"

     <!-- This import brings in the dialog-info element dialog-->
     <xs:import namespace="urn:ietf:params:xml:ns:dialog-info"

     <xs:element name="conference-info">
       <xs:complexType name="c-info-type">
           <xs:element name="user" type="tns:user-type"
                       minOccurs="0" maxOccurs="unbounded"/>
           <xs:any namespace="##other" processContents="lax"
                       minOccurs="0" maxOccurs="unbounded"/>
         <xs:attribute name="version" type="xs:nonNegativeInteger"
         <xs:attribute name="state" use="required">
             <xs:restriction base="xs:string">
               <xs:enumeration value="full"/>
               <xs:enumeration value="partial"/>
         <xs:attribute name="entity" type="xs:anyURI" use="required"/>
     <xs:complexType name="user-type">
         <xs:element name="status" type="tns:status-type" minOccurs="0"/>
         <xs:element ref="di:dialog" minOccurs="0"/>
         <xs:element name="media-status" minOccurs="0">
           <xs:complexType name="media-status-type">
               <xs:element name="media-stream" type="tns:media-stream-type"
                           minOccurs="0" maxOccurs="unbounded"/>
               <xs:any namespace="##other" processContents="lax"
                       minOccurs="0" maxOccurs="unbounded"/>
       <xs:attribute name="uri" type="xs:anyURI" use="required"/>
       <xs:attribute name="display-name" type="xs:string" use="optional"/>
       <xs:attribute ref="xml:lang"/>

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     <xs:complexType name="status-type">
         <xs:restriction base="xs:string">
           <xs:enumeration value="active"/>
           <xs:enumeration value="departed"/>
           <xs:enumeration value="booted"/>
           <xs:enumeration value="failed"/>
     <xs:complexType name="media-stream-type">
       <xs:attribute name="media-type" type="tns:mimetypes" use="required"/>
       <xs:attribute name="send-status" type="tns:send-status-type"
       <xs:attribute name="recv-status" type="tns:recv-status-type"
     <xs:simpleType name="mimetypes">
           <xs:restriction base="xs:string">
             <xs:enumeration value="audio"/>
             <xs:enumeration value="video"/>
             <xs:enumeration value="message"/>
             <xs:enumeration value="application"/>
     <xs:simpleType name="send-status-type">
       <xs:restriction base="xs:string">
       <xs:enumeration value="received-by-all"/>
       <xs:enumeration value="muted"/>
       <xs:enumeration value="unknown"/>
     <xs:simpleType name="recv-status-type">
       <xs:restriction base="xs:string">
        <xs:enumeration value="receiving-all"/>
        <xs:enumeration value="anchor-only"/>
         <xs:enumeration value="unknown"/>

        OPEN ISSUE: We need to register the schema for the dialog
        info package in order to have a stable reference for it in
        the conference info schema above.

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4.4 Example

   The following is an example conference information document:

   <conference-info version="0" state="full" entity="conf233@example.com">
     <user uri="sip:jdrosen@dynamicsoft.com" display-name="Jonathan Rosenberg">
         <media-stream media-type="audio"/>
     <user uri="sip:hgs@cs.columbia.edu" display-name="Henning Schulzrinne">

   This document describes a conference with two users, both of which
   are active.

5 Security Considerations

   Subscriptions to conference state can reveal very sensitive
   information. For this reason, the document recommends authentication
   and authorization, and provides guidelines on sensible authorization

   Since the data in notifications is sensitive as well, end-to-end SIP
   encryption mechanisms using S/MIME SHOULD be used to protect it.

6 IANA Considerations

   This document registers a new MIME type, application/conference-
   info+xml and registers a new XML namespace.

6.1 application/conference-info+xml MIME Registration

        MIME media type name: application

        MIME subtype name: conference-info+xml

        Mandatory parameters: none

        Optional parameters: Same as charset parameter application/xml
             as specified in RFC 3023 [8].

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        Encoding considerations: Same as encoding considerations of
             application/xml as specified in RFC 3023 [8].

        Security considerations: See Section 10 of RFC 3023 [8] and
             Section 5 of this specification.

        Interoperability considerations: none.

        Published specification: This document.

        Applications which use this media type: This document type has
             been used to support SIP conferencing applications.

        Additional Information:

             Magic Number: None

             File Extension: .cif or .xml

             Macintosh file type code: "TEXT"

        Personal and email address for further information: Jonathan
             Rosenberg, <jdrosen@jdrosen.net>

        Intended usage: COMMON

        Author/Change controller: The IETF.

6.2 URN Sub-Namespace Registration for

   This section registers a new XML namespace, as per the guidelines in

        URI: The URI for this namespace is

        Registrant Contact: IETF, SIMPLE working group,
             <simple@mailman.dynamicsoft.com>, Jonathan Rosenberg


             <?xml version="1.0"?>
             <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML Basic 1.0//EN"
             <html xmlns="http://www.w3.org/1999/xhtml">

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               <meta http-equiv="content-type"
               <title>Dialog Information Namespace</title>
               <h1>Namespace for Dialog Information</h1>
               <p>See <a href="[[[URL of published RFC]]]">RFCXXXX</a>.</p>

7 Acknowledgements

   The authors would like to thank Dan Petrie ans Sean Olson for their

8 Authors Addresses

   Jonathan Rosenberg
   72 Eagle Rock Avenue
   First Floor
   East Hanover, NJ 07936
   email: jdrosen@dynamicsoft.com

   Henning Schulzrinne
   Columbia University
   M/S 0401
   1214 Amsterdam Ave.
   New York, NY 10027-7003
   email: schulzrinne@cs.columbia.edu

9 Normative References

   [1] A. Roach, "SIP-specific event notification," Internet Draft,
   Internet Engineering Task Force, Mar. 2002.  Work in progress.

   [2] S. Bradner, "Key words for use in RFCs to indicate requirement
   levels," RFC 2119, Internet Engineering Task Force, Mar. 1997.

   [3] W. W. W. C. (W3C), "Extensible markup language (xml) 1.0." The

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   XML 1.0 spec can be found at http://www.w3.org/TR/1998/REC-xml-

   [4] R. Moats, "URN syntax," RFC 2141, Internet Engineering Task
   Force, May 1997.

   [5] R. Moats, "A URN namespace for IETF documents," RFC 2648,
   Internet Engineering Task Force, Aug. 1999.

   [6] M. Mealling, "The IANA XML registry," Internet Draft, Internet
   Engineering Task Force, Nov. 2001.  Work in progress.

   [7] J. Rosenberg and H. Schulzrinne, "A session initiation protocol
   (SIP) event package for dialog state," Internet Draft, Internet
   Engineering Task Force, June 2002.  Work in progress.

   [8] M. Murata, S. S. Laurent, and D. Kohn, "XML media types," RFC
   3023, Internet Engineering Task Force, Jan. 2001.

10 Informative References

   [9] J. Rosenberg, "Session initiation protocol (SIP) extensions for
   presence," Internet Draft, Internet Engineering Task Force, May 2002.
   Work in progress.

   [10] J. Rosenberg, "A session initiation protocol (SIP)event
   template-package for watcher information," Internet Draft, Internet
   Engineering Task Force, May 2002.  Work in progress.

   [11] R. Mahy, "A message summary and message waiting indication event
   package for the session initiation protocol (SIP)," Internet Draft,
   Internet Engineering Task Force, June 2002.  Work in progress.

   [12] J. Rosenberg and H. Schulzrinne, "Models for multi party
   conferencing in SIP," Internet Draft, Internet Engineering Task
   Force, Nov. 2001.  Work in progress.

   Full Copyright Statement

   Copyright (c) The Internet Society (2002). All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works. However, this

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   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
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   developing Internet standards in which case the procedures for
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   The limited permissions granted above are perpetual and will not be
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   This document and the information contained herein is provided on an

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