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Network Working Group                                           A. Knauf
Internet-Draft                                                   G. Hege
Intended status: Standards Track                            T C. Schmidt
Expires: November 9, 2012                                    HAW Hamburg
                                                            M. Waehlisch
                                                    link-lab & FU Berlin
                                                             May 8, 2012

       A RELOAD Usage for Distributed Conference Control (DisCo)


   This document defines a RELOAD Usage for Distributed Conference
   Control (DisCo) with SIP.  DisCo preserves conference addressing
   through a single SIP URI by splitting its semantic of identifier and
   locator using a new Kind data structure.  Conference members are
   enabled to select conference controllers based on proximity awareness
   and to recover from failures of individual resource instances.  DisCo
   proposes call delegation to balance the load at focus peers.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Drafts is at http://datatracker.ietf.org/drafts/current/.

   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."

   This Internet-Draft will expire on November 9, 2012.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents

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   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Overview of DisCo  . . . . . . . . . . . . . . . . . . . . . .  6
     3.1.  Reference Scenario . . . . . . . . . . . . . . . . . . . .  6
     3.2.  Initiating a Distributed Conference  . . . . . . . . . . .  7
     3.3.  Joining a Conference . . . . . . . . . . . . . . . . . . .  8
     3.4.  Conference State Synchronization . . . . . . . . . . . . .  9
     3.5.  Call delegation  . . . . . . . . . . . . . . . . . . . . . 10
     3.6.  Resilience . . . . . . . . . . . . . . . . . . . . . . . . 10
     3.7.  Topology Awareness . . . . . . . . . . . . . . . . . . . . 10
   4.  RELOAD Usage for Distributed Conference Control  . . . . . . . 11
     4.1.  Shared Resource DisCo-Registration . . . . . . . . . . . . 11
     4.2.  Kind Data Structure  . . . . . . . . . . . . . . . . . . . 11
     4.3.  Variable Conference Identifier . . . . . . . . . . . . . . 12
     4.4.  Conference Creation  . . . . . . . . . . . . . . . . . . . 12
     4.5.  Advertising Focus Ability  . . . . . . . . . . . . . . . . 13
     4.6.  Determining Coordinates  . . . . . . . . . . . . . . . . . 14
     4.7.  Proximity-aware Conference Participation . . . . . . . . . 14
     4.8.  Configuration Document Extension . . . . . . . . . . . . . 16
   5.  Conference State Synchronization . . . . . . . . . . . . . . . 18
     5.1.  Event Package Overview . . . . . . . . . . . . . . . . . . 18
     5.2.  <distributed-conference> . . . . . . . . . . . . . . . . . 20
     5.3.  <version-vector>/<version> . . . . . . . . . . . . . . . . 20
     5.4.  <conference-description> . . . . . . . . . . . . . . . . . 21
     5.5.  <focus>  . . . . . . . . . . . . . . . . . . . . . . . . . 22
       5.5.1.  <focus-state>  . . . . . . . . . . . . . . . . . . . . 23
       5.5.2.  <users>/<user> . . . . . . . . . . . . . . . . . . . . 23
       5.5.3.  <relations>/<relation> . . . . . . . . . . . . . . . . 24
     5.6.  Distribution of Change Events  . . . . . . . . . . . . . . 24
     5.7.  Translation to Conference-Info Event Package . . . . . . . 25
       5.7.1.  <conference-info>  . . . . . . . . . . . . . . . . . . 26
       5.7.2.  <conference-description> . . . . . . . . . . . . . . . 26
       5.7.3.  <host-info>  . . . . . . . . . . . . . . . . . . . . . 26
       5.7.4.  <conference-state> . . . . . . . . . . . . . . . . . . 26
       5.7.5.  <users>/<user> . . . . . . . . . . . . . . . . . . . . 27
       5.7.6.  <sidebars-by-ref>/<sidebars-by-value>  . . . . . . . . 27
   6.  Distributed Conference Control with SIP  . . . . . . . . . . . 28
     6.1.  Call delegation  . . . . . . . . . . . . . . . . . . . . . 28
     6.2.  Conference Access  . . . . . . . . . . . . . . . . . . . . 29

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     6.3.  Media Negotiation and Distribution . . . . . . . . . . . . 30
       6.3.1.  Offer/Answer . . . . . . . . . . . . . . . . . . . . . 30
       6.3.2.  New Peers Joining  . . . . . . . . . . . . . . . . . . 31
     6.4.  Restructuring a Conference . . . . . . . . . . . . . . . . 31
       6.4.1.  On Graceful Leave  . . . . . . . . . . . . . . . . . . 31
       6.4.2.  On Unexpected Leave  . . . . . . . . . . . . . . . . . 32
   7.  DisCo Kind Definition  . . . . . . . . . . . . . . . . . . . . 33
   8.  XML Schema . . . . . . . . . . . . . . . . . . . . . . . . . . 34
   9.  Relax NG Grammar . . . . . . . . . . . . . . . . . . . . . . . 38
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 39
     10.1. Trust Aspects  . . . . . . . . . . . . . . . . . . . . . . 39
   11. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 40
   12. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 41
   13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 42
     13.1. Normative References . . . . . . . . . . . . . . . . . . . 42
     13.2. Informative References . . . . . . . . . . . . . . . . . . 43
   Appendix A.  Change Log  . . . . . . . . . . . . . . . . . . . . . 44
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 46

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1.  Introduction

   This document describes a RELOAD Usage for distributed conference
   control (DisCo) in a tightly coupled model with SIP [RFC3261].  The
   Usage provides self-organizing and scalable signaling that allows
   RELOAD peers, clients and plain SIP user agents to participate in a
   managed P2P conference.  DisCo defines the following functions:

   o  A SIP protocol scheme for distributed conference control

   o  RELOAD Usage and definition of conferencing Kind

   o  Mechanisms for conference synchronization and call delegation

   o  Mechanism for proximity-aware routing within a conference

   o  An XML event package for distributed conferences

   In this document, the term distributed conferencing refers to a
   multiparty conversation in a tightly coupled model in which the point
   of control (i.e., the focus) is identified by a unique URI, but the
   focus service is located at many independent entities.  Multiple SIP
   [RFC3261] user agents uniformly control and manage a multiparty
   session.  This document defines a new Usage for RELOAD, including an
   additional Kind code point with a corresponding data structure that
   complies with the demands for distributed conferences.  The 'DisCo'
   data structure stores the mapping of a single conference URI to
   multiple conference controllers and thereby separates the conference
   identifier from focus instantiations.

   Authorized controllers of a conference are permitted to register
   their mapping in the DisCo data structure autonomously.  Thus, the
   DisCo data structure represents a co-managed Resource in RELOAD.  To
   provide trusted and secure access to a co-managed Resource, this
   document uses the definitions for Shared Resources (ShaRe)

   Delay and jitter are critical issues in multimedia communications.
   The proposed conferencing scheme supports mechanisms to build an
   optimized interconnecting graph between conference participants and
   their responsible conference controllers.  Conference members will be
   enabled to select the closest focus with respect to delay or jitter.

   DisCo extends conference control mechanisms to provide a consistent
   and reliable conferencing environment.  Controlling peers maintain a
   consistent view of the entire conference state.  The multiparty
   system can be re-structured based on call delegation operations.

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2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

   We use the terminology and definitions from der RELOAD base
   draft[I-D.ietf-p2psip-base], the peer-to-peer SIP concepts draft
   [I-D.ietf-p2psip-concepts], the usage for shared resources draft
   [I-D.knauf-p2psip-share],and the terminology formed by the framework
   for conferencing with SIP [RFC4353].  Additionally the following
   terms are used:

   Coordinate Value:  An opaque string that describes a host's relative
      position in the network topology.

   Focus peer:  A RELOAD peer that provides SIP conferencing functions
      and implements the Usage for distributed conferencing.  It is
      called 'active' if already involved in signaling relation to
      conference participants.  Otherwise, if only registered in a
      distributed conference data structure, it is referred to as a
      'potential' focus peer.

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3.  Overview of DisCo

3.1.  Reference Scenario

   The reference scenario for the Distributed Conference Control (DisCo)
   is shown in Figure 1.  Peers are connected via a RELOAD
   [I-D.ietf-p2psip-base] instance, in which peers A and B are managing
   a single multiparty conference.  The conference is identified by a
   unique conference URI, but located at peers A and B fulfilling the
   role of the focus.  The mapping of the conference URI to one or more
   responsible focus peers is stored in a new RELOAD Resource for
   distributed conferencing within a data structure denoted as DisCo-
   Registration.  The storing peer SP of the distributed conference
   resource holds this data.

   The focus peers A and B maintain SIP signaling relations to
   conference participants, which may have different conference protocol
   capabilities.  In this example, peer A is the focus for the RELOAD
   peer C and the plain SIP user agent E whereas peer B serves as a
   focus for RELOAD peer D and the RELOAD client F.

   RELOAD peers and clients obtain the contact information for the
   conference from the storing peer.  In contrast to this, the user
   agent E receives the conference URI not by RELOAD mechanisms, but
   resolves the ID and joins the conference by plain SIP negotiation.

   Focus peers maintain a SIP signaling relation among each other used
   for notification messages that synchronize the conference focus
   peers' knowledge about the entire conference state.  Additionally,
   focus peers can transfer calls to each other by a call delegation

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                  +-------------------+  +------------------+
                  |Access Control List|  |DisCo-Registration|
                  +-------------------+  +------------------+
                                    \   /
              # # # # # # # # # # | Peer  | # # # # # # # # # #
             #                    |  SP   |                    #
            #                     +-------+                     #
           #                                                     #
          #                                                       #
         #                                                         #
      +----+                                                     +----+
      |Peer| \               RELOAD Instance                     |Peer|
      | C  |  \                                                  | D  |
      +----+   \                                                 +----+
         #     SIP                                                 #
          #      \                                                #
           #      \                                              #
            #     +-------+                       +-------+     #(
             #    | Focus |                       | Focus |    #  )
              # # | Peer  | # # # # # # # # # # # | Peer  | # #  (
                  |   A   | <===Conf.Events/====> |   B   |       )
                  +-------+    Call delegation    +-------+    Overlay
                 /                                        \     Comm.
                /                                          \     (
              SIP                                          SIP    )
              /                                              \   (
             /                                                \   )
        +----------+                                      +--------+
        |User Agent|                                      | Client |
        |    E     |                                      |   F    |
        +----------+                                      +--------+

   Figure 1: Reference Scenario: Focus peers A,B maintain a distributed

3.2.  Initiating a Distributed Conference

   To create a conference the initiating user agent announces itself as
   a focus for the conference.  It stores its own contact information
   (Node-ID) as a DisCo-Registration Kind (cf. Figure 2) in the RELOAD
   overlay.  The hashed conference URI is used as the Resource-ID.  This
   Resource will later contain the contact IDs of all potential focus
   peers including optional topological descriptors.

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3.3.  Joining a Conference

   A RELOAD-aware node (cf. Bob in Figure 2) intending to join an
   existing conference requests the list of potential focus peers stored
   in the DisCo-Registration under the conference's Resource-ID.  The
   node selects any of the focus peers (e.g., Alice) and establishes a
   connection using AppAttach/ICE [RFC5245].  This transport is then
   used to send an INVITE to the conference applying the chosen focus as
   the contact.  The selection of the focus peer can optionally be based
   on proximity information if available.

   A conference member proposes itself as a focus for subsequent
   participants by adding its Node-ID to the DisCo-Registration stored
   under the conference URI in the RELOAD overlay.  The decision whether
   a peer announces as a focus incorporates bandwidth, power, and other
   constraints, but details are beyond the scope of this document.

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    Alice                          RELOAD                            Bob
    (initiating peer)                                     (joining peer)
      |                               |                               |
      |      Alice stores her mapping to register a conference        |
      | Store mapping(ConfURI, Alice) |                               |
      |------------------------------>|                               |
      |      Bob requests the list of potential focus peers           |
      |                               |        Lookup ConfURI         |
      |                               |<------------------------------|
      |                               |   Result list of conf. focus  |
      |                               |------------------------------>|
      |                               |                               |
      |       Bob establishes transport connection to Alice           |
      |                           AppAttach                           |
      |                           AppAttach                           |
      |                            INVITE                             |
      |                              OK                               |
      |                              ACK                              |
      |                             Media                             |
      |                               |                               |
      |       Bob stores his mapping to become a focus peer too       |
      |                               |  Store mapping(ConfURI, Bob)  |
      |                               |<------------------------------|
      |                               |                               |

                  Figure 2: DisCo Usage generic Call Flow

3.4.  Conference State Synchronization

   Each focus of a conference maintains signaling connections to its
   related participants independently from other conference controllers.
   This distributed conference design effects that the entire SIP
   conference state is jointly held by all focus peers.  In DisCo, state
   synchronization is based on a SIP specific event notifications
   mechanism [RFC3265].

   Each focus peer maintains its view of the entire conference state by
   subscribing to the other focus peers' XML event package for
   distributed conferences.  This is based on the event package for
   conference state (cf. [RFC4575]).  Details are defined in this
   document in Section 5.  Receivers of event notifications update their

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   local conference state document to represent a valid view of current
   total conference state.

   The event notification package for distributed conferences enables
   focus peers to synchronize the entire conference state.  The event
   package defines additional XML elements and complex types (see
   Section 8 for more details), which describe the responsibilities of
   any focus peer in the conference.  By providing a global view each
   focus peer is enabled to perform additional load balancing operations
   and enhances the robustness against departures of focus peers.

3.5.  Call delegation

   Call delegation (see Section 6.1) is a feature used to transfer an
   incoming participation request to another focus peer.  It can be
   applied to prevent overloading of focus peers.  Call delegation is
   realized through SIP REFER requests, which carry signaling and
   session description information of the caller to be transferred.
   This feature is achieved transparently for the transferred user agent
   by using a source routing mechanism at SIP dialog establishment.
   Descriptions of overload detection are beyond the scope of this

3.6.  Resilience

   A focus peer can decide to leave the conference or may ungracefully
   fail.  In a traditional conferencing scenario, loss of the conference
   controller or the media distributor would cause a complete failure of
   the multiparty conversation.  Distributed conferencing uses the
   redundancy provided by multiple focus peers to reconfigure a current
   multiparty conversation.  Participants that loose their entry point
   to the conference re-invite themselves via the remaining focus peers
   or will be re-invited by the latter.  This option is based on the
   conference state and call delegation functions.

3.7.  Topology Awareness

   DisCo supports the optimization of the conference topology in respect
   of the underlying network using topological descriptors.  An
   extension for the RELOAD XML configuration document is defined in
   Section 4.8 to support landmarking approaches.  Each peer intending
   to create or participate in a distributed conference MAY determine a
   topological descriptor that describes its relative position in the
   network.  Focus peers store these coordinate values in an additional
   data field in the DisCo-Registration data structure.  This enables
   peers joining the conference to select the closest focus with respect
   to its coordinate values.

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4.  RELOAD Usage for Distributed Conference Control

4.1.  Shared Resource DisCo-Registration

   A distributed conference is a cooperative service of several
   individual devices that use a common identifier.  To enable a mapping
   from one conference identifier to multiple focus peers, this document
   defines the new Kind data structure DisCo-Registration.  The DisCo
   Kind uses the definitions for Shared Resources
   [I-D.knauf-p2psip-share] to allow a jointly maintenance by multiple
   focus peers.  Hence, write permission to a DisCo-registration is
   shared by the conference creator with all authorized focus peers.

   DisCo complies with the following requirements for Shared Resources:

   Isolated Data Storage:  DisCo uses the dictionary data model.  Each
      dictionary key is the Node-ID of the certificate that will be used
      to sign the stored data

   ResourceNameExtension field:  A DisCo-Registration can contain the
      ResourceNameExtension structure an initial field in the Kind data
      structure.  It contains the conference URI of the registered

4.2.  Kind Data Structure

   Each DisCo-Registration data structure stores a mapping of a
   conference identifier to one or multiple focus peers that
   cooperatively control the conference.  Additionally, each DisCo-
   Registration provides the coordinate value, which indicates the
   relative network position of the focus peers.

   The data structure uses the RELOAD dictionary type.  The dictionary
   key MUST be the Node-ID of the focus peer that is associated with the
   dictionary entry.  This allows a focus peer to update only its own
   mapping.  The DisCo data structure of type DisCoRegistration is
   constructed as follows:

     struct {
       /* This field is optional, see documentation */
       ResourceNameExtension res_name_ext;
       opaque coordinate<0..2^16-1>;
       NodeId node_id;
     } DisCoRegistration;

   The DisCoRegistration structure is composed of the following values:

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   res_name_ext:  This field can contain the conference URI.  It meets
      the requirement for the USER-CHAIN-ACL access policy defined in
      [I-D.knauf-p2psip-share] to enable variable resource names.

   coordinate:  This field contains a topological descriptor that
      indicates the relative position of the peer in the network.  To
      support different algorithms the coordinate field is represented
      as an opaque string.

   node_id:  This field contains the Node-ID of the peer storing the
      DisCoRegistration and is used to contact the peer when utilizing
      its service as a focus.

4.3.  Variable Conference Identifier

   DisCo-Registrations can be stored based on a variable Resource Name.
   However, a conference identifier set by a user MUST follow the
   requirements for Kinds using variable Resource Names as defined in
   the ShaRe Usage [I-D.knauf-p2psip-share].

4.4.  Conference Creation

   The registration of a distributed conference includes the storage of
   the following two Kinds (see Figure 3).

   DisCo-Registration Kind:  contains the conference identifier and the
      optional coordinate value.  If used, the coordinate value MAY be
      determined previously to the conference registration.  The
      Resource Name and hence the Resource-ID is defined by the hash
      over the desired conference identifier (using the hash algorithm
      of the overlay).

   Access Control List Kind:  contains the conference participants that
      are allowed to register as focus peers for a conference (see
      [I-D.knauf-p2psip-share]).  Its Resource Name/ID is equal to those
      of the corresponding DisCo-Registration.

   Preliminary to storage of DisCo-Registration and Access Control List
   (ACL) Kinds the conference creator SHOULD perform a RELOAD StatReq to
   verify that no former conference is present.  If neither a DisCo-
   Registration nor an associated ACL exist, the conference creator
   stores a DisCo-Registration with a valid conference identifier (see
   Section 4.3) and an ACL referring to the DisCo-Registration Kind-ID.

   If DisCo registrations and ACL Kinds from previous conferences are
   still existing there are two options.  First, if conference creator
   is aware of the indexes from previous ACL Kinds, it refreshes the
   root item of this ACL and stores its registration as focus peer as

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   DisCo-Registration Kind.  Second, If the creator is unaware of
   indexes, it fetches all Access List Kinds to determine the index of
   the root item.

        Alice        Peer1       Overlay       PeerN     Storing Peer
          |   StatReq Res:Conf-URI                |            |
          |             StatAns      |            |            |
          |   StoreReq Res:Conf-URI Kinds:DisCo, Access-List   |
          |           StoreAns       |            |            |
          |             |            |            |            |

          Figure 3: Initial creation of a Distributed Conference

   Optionally the conference initiator (or any active focus) MAY store
   an additional RELOAD SIP-Registration in the overlay
   [I-D.ietf-p2psip-sip] or even at a standard SIP registrar [RFC3261]
   under a URI for which it has write permission.  This allows DisCo-
   unaware or even legacy SIP user agents to participate in the
   conference.  Those registrations SHOULD always point to a currently
   active focus, who is prepared to accept legacy user agents.  The user
   agent who initially performed the registrations is responsible for
   updating them appropriately.  How authorization has been obtained to
   perform those registration is out of scope of this document.

   The lifetime of a distributed conference is not necessarily limited
   by the participation time of its creator.  As long as the root item
   of an Access Control List to a DisCo-Registration is not expired,
   Authorized Peers are allowed to further provide a conferencing
   service and even store new focus registrations.

4.5.  Advertising Focus Ability

   All participants of a distributed conference MAY potentially become a
   focus peer for their conference.  This depends on its capacities such
   as sufficient processing power (CPU, Memory) for the desired media
   type, the quality of the network connectivity, and the conference
   policy.  Information about network connectivity with respect to NAT
   or restricted firewalls can be obtained via ICE [RFC5245]
   connectivity checks.  If a peer is behind a NAT, it SHOULD allow for
   incoming connections via AppAttach/ICE.  Peers that can only accept
   connections with the support of TURN should not act as a focus.

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   Nevertheless, under special circumstances it might be reasonable to
   locate a focus peer behind such a NAT (e.g., within a companies
   network infrastructure).

   If a participant is a candidate to become a focus for the conference,
   it stores its Node-ID and optional coordinate value into the DisCo
   data structure.  An entry in the corresponding ACL
   [I-D.knauf-p2psip-share] must be present to allow this peer to write
   the DisCo-Registration resource.  By storing the mapping into the
   data structure a participant becomes a potential focus.

4.6.  Determining Coordinates

   Each RELOAD peer within the context of a distributed conference MAY
   be aware of its relative position in the network topology.  To
   constuct a topology-aware conference, the DisCo Usage provides the
   coordinate value within the DisCo-Registration data structure.  Focus
   peers store their relative network position together with the
   announcement as conference focus.  Joining peers that are able to
   determine their coordinates may then select a focus peer whose
   relative position is in its vicinity (see Section 4.7).

   Some algorithms determine topology information by measuring Round-
   Trip Times (RTT) towards a set of hosts serving as landmarks (e.g.,
   [landmarks-infocomm02]).  To support such algorithms this document
   describes an extension to the RELOAD XML configuration document that
   allows to configure the set of landmark hosts peer must use for
   position estimation (see Section 4.8).  Once a focus peer has
   registered its mapping in the DisCo data structure, it also stores
   the according coordinates in the same mapping.  These <Node-
   ID,coordinates> vectors are used by peers joining the conference to
   select the focus peer that is relatively closest to itself.

   Because topology-awareness can be obtained by many different
   approaches a concrete algorithms is out of scope of this document.

4.7.  Proximity-aware Conference Participation

   The participation procedure to a distributed conference is composed
   of three operation.

   1.  Resolution of the conference identifier.

   2.  Establishment of of transport connection.

   3.  SIP signaling to join a conference.

   Figure 4 and the following specifications give a more detailed view

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   on the joining procedure.

         Bob      Peer1     Overlay     PeerN    Storing Peer  Alice
          |   StatReq Res:Conf-URI         |          |          |
          |--------->|--------->|--------->|--------->|          |
          |          |StatAns   |          |          |          |
          |<---------|<---------|<---------|<---------|          |
          |   FetchReq Res:Conf-URI Kind:DisCo,ACL    |          |
          |--------->|--------->|--------->|--------->|          |
          |          |FetchAns  |          |          |          |
          |<---------|<---------|<---------|<---------|          |
          |          |          |          |          |          |
          |  Bob calculates Alice as closest Focus    |          |
          |          |          |          |          |          |
          |          |AppAttach application:5060      |          |
          |          |AppAttach application:5060      |          |
          |          |          |          |          |          |
          |<-------------------ICE Checks----------------------->|
          |          |          |          |          |          |
          |          |         INVITE sip:Alice       |          |
          |          |         200 OK      |          |          |
          |          |         ACK         |          |          |
          |          |          |          |          |          |

            Figure 4: Participation of a Distributed Conference

   1.  The joining peer MAY determine its own coordinate value (if

   2.  The joining peer sends a StatReq message to obtain all indexes of
       the Access Control List (ACL) Kinds stored.

   3.  The joining peer sends a FetchReq message for the DisCo and ACL
       Kind to the Resource-ID of the conference URI.  The FetchReq
       SHOULD NOT include any specific dictionary keys, but SHOULD fetch
       for those array ranges previously determined the StatReq.  With
       the ACL items, the joining peer is able to verify whether DisCo-
       Registrations are stored by authorized focus peers (see

   4.  Using the retrieved coordinates values of the DisCo-
       Registrations, the joining peer MAY calculate which of than

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       opaque <0..2^16-1> initial field in the Kind data structure focus
       peers is the relatively closest to itself.

   5.  The joining peer then establishes a transport using RELOADs
       AppAttach, respectively, ICE procedures to create a transport to
       the chosen focus peer.

   6.  Finally, the established transport is used to create a SIP dialog
       from the joining peer to the focus peers.

   The SIP INVITE request MAY contain the joining peer's topological
   descriptor as a URI-parameter called 'coord' in the contact-header in
   base64 encoded form [RFC4648].  An example contact URI is "sip:alice@
   example.com;coord=PEknbSBhIHRvcG9sb2dpY2FsIGRlc2NyaXB0b3I+".  When
   the called focus is full and needs to delegate the call it uses the
   contents of the 'coord'-parameter.  It determines the next available
   focus closest to the calling peer (Section 4.6) using the received
   descriptor and the other focuses' descriptors from the conference
   state synchronization document and delegates the call accordingly
   (see Section 6.1).

   A conference focus MAY allow the joining peer to also become a focus
   (depending on the conference policy see Section 6.2).  Therefore, it
   stores a new ACL Kind that delegates write permission for the DisCo-
   Registration to the new participant.  It sets the 'user_name' field
   in the ACL Kind to its own user name and sets the 'to_user' field to
   the user name of the joining peer.  If no other conference policy is
   defined, the focus peer MAY set the allow_delegation flag to true.
   For further details about the trust delegation using the ACL Kind see

4.8.  Configuration Document Extension

   This section defines an additional parameter for the <configuration>
   element that extends the RELOAD XML configuration document.  The
   proposed <landmarks> element allows RELOAD provider to publish a set
   of accessible and reliable hosts that SHOULD be used if RELOAD peers
   use landmarking algorithms to determine relative position in the
   network topology.

   The <landmarks> element serves as container for the <landmark-host>
   sub-elements, each representing a single host that serves as a
   landmark.  The <landmark-host> uses the following attributes:

   address:  The IP address (IPv4 or IPv6) of the landmark host.

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   port:  The port on which the landmark host responds for distance

   More than one landmark hosts SHOULD be present in the configuration

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5.  Conference State Synchronization

   The global knowledge about signaling and media relations among the
   conference participants and focus peers defines the global state of a
   distributed conference.  It is composed of the union of every local
   state at the focus peers.  To enable focus peers to provide
   conference control operations that modify and/or require the global
   state of a conference, this document defines a mechanism for inter-
   focus state synchronization.  It is based on mutual subscriptions for
   an Event Package for Distributed Conferences and allows to preserve a
   coherent knowledge of the global conference state.  This XML based
   event package named 'distributed-conference' MUST be supported by
   each RELOAD peer that is registered with a DisCo-Registration.
   Participants of a distributed conference MAY support it.  To provide
   backward compatibility to conference members that do not support the
   distributed-conference event package, this document defines a
   translation to the Event Package for Conference State [RFC4575].

5.1.  Event Package Overview

   The 'distributed-conference' event package is designed to convey
   information about roles and relations of the conference participants.
   Conference controllers obtain the global state of the conference and
   use this information for load balancing or conference restructuring
   mechanisms in case of a focus failure.  Figure Figure 5 gives a
   general overview of the document hierarchy.

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     |-- version-vector
     |    |-- version
     |    |-- version
     |-- conference-description
     |-- focus
     |    |-- focus-state
     |    |    |-- user-count
     |    |    |-- coordinate
     |    |    |-- maximum-user-count
     |    |    |-- active
     |    |    |-- locked
     |    |    |-- conf-uris
     |    |    |-- available-media
     |    |
     |    |-- users
     |    |    |-- user
     |    |    |    |-- endpoint
     |    |    |    |    |-- media
     |    |    |    |    |-- call-info
     |    |
     |    |-- relations
     |    |    |-- relation
     |-- focus
     |    |-- ...

    Figure 5: Overview of the event package for distributed conferences

   The document structure is designed to allow concurrent change events
   at several focus peers.  To prevent race conditions each focus peer
   has exclusive writing permission to the 'focus' sub element that
   describes itself.  It is achieved by a unique mapping from a focus
   peer to its XML element using the 'Element Keys' mechanisms for
   partial notification [RFC4575](sections 4.4-5.).  A focus peer is
   only allowed to update or change that <focus> sub element, whose
   'entity' Element Key contains its RELOAD user name.  This restriction
   also applies to the child elements of the 'version-vector' element.
   Each 'version' number belongs to a specific focus peer maintaining
   the version number.

   The local state of a focus peer is described within a 'focus'
   element.  It provides general information about a focus peer and its
   signaling and media relations to participants and focus peers.  The
   Conference participants are aggregated within 'users' elements,
   respectively, 'user' sub elements.

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   General information about the conference as a whole, is provided
   within a 'conference-description' element.  In contrast to the
   'focus' and 'version-vector' elements, conference description is not
   meant for concurrent updating.  Instead, it provides static
   conference descriptions that rarely change during a multiparty

   More detailed descriptions about the event package and its elements
   are provided in the following sections.  The complete XML schema
   definition is given in Section 8.

5.2.  <distributed-conference>

   The <distributed-conference> element is the root of a distributed
   conference XML document.  It uses the following attributes:

   entity:  This attribute contains the conference URI that identifies
      the distributed conference.  A SIP SUBSCRIBE request addressed to
      this URI initiates an subscribe/notify relation between
      participants and their related focus peer.

   state:  This attribute indicates whether the content of a distributed
      conference document is a 'full', 'partial' or 'deleted'
      information.  It is in accordance with [RFC4575] to enable the
      partial notification mechanism.

   The <distributed-conference> child elements are <vector-version>,
   <conference-description> and the <focus> elements.  An event
   notification of state 'full' MUST include all these elements.  An
   event notification of state 'partial' MUST contain at least <version-
   vector> and its sub elements.

5.3.  <version-vector>/<version>

   The Event Package for Distributed Conferences uses the <version-
   vector> and its <version> sub elements to enable a coherent
   versioning scheme.  It is based on vector clocks as defined in
   [timestamps-acsc88].  Each <version> element contains a unsigned
   integer that describes the state of a specific focus peer and
   contains the following attributes:

   entity:  This attribute contains the user name of the focus peer
      whose local version number is described by this element.

   node-id:  This attribute contains the Node-ID of the focus peer.

   Whenever the local status of a focus peer changes (e.g. a new
   participant arrived) the version number of the corresponding

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   <version> element MUST be incremented by one.  Each change in the
   local state also triggers a new event notification containing the
   entire <version-vector> and the changes contained in a <focus>

   The recipient of a change event needs to update it local XML
   document.  If a received <version> number is higher that the local,
   it updates the <version> element and its associated <focus> element
   with the retrieved elements.  All other elements remain constant.

   If the length or contents of the retrieved <version-vector> is
   different to the local copy it indicates a incoherent knowledge about
   the entire conference state.  If the retrieved <version-vector>
   contains any unknown focus peers and any local version numbers for
   the known focus peers is lower, the receiver SHOULD request a 'full'
   XML notification.

   If any local <version> number is retarded more than one, the receiver
   SHOULD request a 'full' event notification from the sender.  The full
   state notification updates all <focus> elements whose corresponding
   <version> element is out of date.

5.4.  <conference-description>

   The <conference-description> element provides general information and
   links to auxiliary services for the conference.  The following sub
   elements provide human-readable text descriptions about the

   <display-text>:  Contains a short textual description about the

   <subject>:  Contains the subject of the conference

   <free-text>:  Contains a longer textual description about the

   <keywords>:  Contains a list of keywords that match the conference
      topic.  The XML schema definition and semantic is imported from
      the 'conference-info' event package [RFC4575].

   The <service-uris> sub element enables focus peers to advertise
   auxiliary services for the conference.  The XML schema definition and
   semantic is imported from the 'conference-info' event package

   The <conference-description> element uses the 'state' Element Key to
   enable the partial notification mechanism.

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5.5.  <focus>

   Each <focus> element describes a focus peer actively controlling the
   conference.  It provides general information about a focus peer
   (e.g., display-text, languages, etc.), contains conference specific
   information about the state of a focus peer (user-count, available
   media types, etc.) and announces signaling and media information
   about the maintained participants.  Additionally, it describes
   signaling or media relations to further focus peers.

   The <focus> element uses the following attributes:

   entity:  This attribute contains the user name of the RELOAD peer
      acting as focus peer.  It uniquely identifies the focus peer that
      is allowed to update or change all sub elements of <focus>.  All
      other focus peers SHOULD NOT update or change sub elements of this
      <focus> element.  A SUBSCRIBE request addressed to the user name
      initiates a conference state synchronization with the focus peer.

   Node-ID  This attributed contains the Node-ID of the peer acting as
      conference focus

   state:  In accordance to [RFC4575], this attribute indicates whether
      the content of the <focus> element is a 'full', 'partial' or
      'deleted' information.  A 'partial' notification contains at
      maximum a single <focus> element.

   The following sub elements of <focus> provide general information
   about a focus peer:

   <display-text>:  Contains a short text description of the user acting
      as focus peer.

   <associated-aors>:  This element contains additional URIs that are
      associated with this user.

   <roles:>  This element MAY contain human-readable text descriptions
      about the roles of the user in the conference.

   <languages>:  This element contains a list of tokens, each describing
      a language understood by the user.

   The XML schema definition and semantic for <associated-aors>, <roles>
   and <languages> are imported from the 'conference-info' event package

   Following, a detailed description of the remaining sub elements.

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5.5.1.  <focus-state>

   The <focus-state> element aggregates a set of conference specific
   information about the RELOAD user acting as focus peer.  The
   following attribute is defined for the <focus-state> element:

   status:  This attribute indicates whether the content of the <focus-
      state> element is a 'full', 'partial' or 'deleted' information.

   The <focus-state> element has the following sub elements:

   <user-count>:  This element contains the number of participants that
      are connected to the conference via this focus peer at a certain

   <coordinate>  This element contains the coordinate value Section 4.2
      of the focus peer

   <maximum-user-count>:  This number indicates a threshold of
      participants a focus peer is able to serve.  This value might
      change during a conference, depending on the focus peers current

   <conf-uris>:  This element MAY contain other conference URIs in order
      to access the conference via different signaling means.  The XML
      schema definition and semantic is imported from [RFC4575].

   <available-media>:  This element is imports the <conference-media-
      type> type XML scheme definitions from [RFC4575].  It allows a
      focus peer to list its available media streams.

   <active>:  This boolean element indicates whether a focus peer is
      currently active.  Conference participation requests or a call
      delegation request SHOULD succeed.

   <locked>:  In contrast to <active>, this element indicates that a
      focus peer is not willing to accept anymore participation or call
      delegation request.

5.5.2.  <users>/<user>

   The <users>, respectively, each <user> element describes a single
   participant that is maintained by the focus peer described by the
   parent <focus> element.  The <users> element XML schema definition
   and its semantic is imported from the 'conference-info' event package

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5.5.3.  <relations>/<relation>

   The <relations> element serves as container for <relation> elements,
   each describing a specific connection to another focus peer.  The
   parent element <relations> uses the 'state' attribute to enable the
   partial notification mechanism.  For the <relation> element the
   following attributes are defined:

   entity:  This attribute contains the user name of the remote focus.

   node-id  This attribute contains the Node-ID of the remote focus

   The content of each <relation> is a comma separated string that
   TYPE is a string token describing the type of connection (e.g. media,
   signaling, etc.) and the IDENTIFIER contains a variable connection
   identifier.  It is a generic method to announce any kind of
   connection to a remote focus.  This specification defines following

   media:<label>:  This tuple identifies a single media stream.  The
      'label' variable contains the SDP "label" attribute. (see

   sync:<call-id>:  This tuple indicates a subscription for the Event
      Package for Distributed Conferences.  The 'call-id' variable
      contains the call-id of the SIP subscription dialog.

5.6.  Distribution of Change Events

   Each focus peer in a distributed conference must be able to retrieve
   all change events from all other focus peers.  A simple approach
   would be to inter-connect each focus with all other focus in a full
   meshed topology.  To avoid a full mesh, this document describes a
   'mutual' subscription scheme that constructs a spanning tree topology
   among the focus peers.

   A conference participant that becomes a focus peer MUST send a SIP
   SUBSCRIBE to request the Event Package for Distributed Conferences to
   its own focus peer.  The subscription request is addressed to user
   name of the active focus peer.  The latter interprets this
   subscription as a request for conference state synchronization.  The
   corresponding NOTIFY message contains a 'full' distributed-conference
   state XML document (see section Section Section 5.1).

   The subscribed focus peer in turn subscribes the upcoming focus peer
   for the distributed conference event package.  The corresponding

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   NOTIFY message carries a 'partial' conference state XML document.  It
   contains the received <version-vector> including a new <version>
   element for itself and a new <focus> element that describes its local
   state (see Section 5.5).

   Resulting by this subscription scheme, each focus peer has at least
   one subscription to obtain updates for the conference state and is a
   notifier for change events originated itself.  In a incrementally
   increasing conference, the 1st and 2nd focus peer have a mutual
   subscription for conference change events.  A 3rd focus could have a
   mutual subscription with the 1st focus, a 4th focus to the 2nd focus
   and so forth.  The result is a spanning tree topology among the focus
   peers in which each focus peer is a possible root for distribution
   tree for conference change events.

   However, the fact that event notifications need to traverse one or
   more intermediate focus peers until conference-wide delivery, demands
   a forwarding mechanism for change events.  On receiving a change
   event, a notified focus validates based on the <version-vector>
   whether the incoming state event is not a duplicate to previews
   notifications.  If its not a duplicate, the received change event
   triggers a new event notification at the receiver of the change
   event.  It notifies all its subscribers with excepting the sender of
   the event notification.  In this way, the change event will be
   'flooded' among the focus peer of a conference.

5.7.  Translation to Conference-Info Event Package

   The Event Package for Distributed Conferences imports several XML
   element definitions of the Event Package for Conference State
   [RFC4575].  This is caused by two reasons.  First, the semantic of
   these elements are fitting the demands to describe the global state
   of a distributed conference and, second, it facilitates a re-
   translation to [RFC4575] to enable a backward compatibility to DisCo-
   unaware clients.  Therefore, each focus peer MAY provide a separate
   [RFC4575] conform event notification service to its connected

   The following sections describe the translation to the Event Package
   for Conference State [RFC4575] by defining translation rules for the
   root element and its direct sub elements.  For a better
   understanding, the following sections use a notation ci.<ELEMENT> to
   refer to a sub element of the conference-info element, and
   disco.<ELEMENT> to refer to an element of the distributed-conference
   event package.

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5.7.1.  <conference-info>

   The root element of Event Package for Conference State uses the
   attributes 'entity', 'state' and 'version' and is the counterpart of
   the <distributed-conference> root element in the DisCo Event Package.
   The former two attributes 'entity' and 'state' are equal in both root
   elements and can be seamlessly translated.

   According to [RFC4575], the 'version' attribute SHOULD be incremented
   by one at any time a new notification being sent to a subscriber.
   Hence, in DisCo the 'version' attributed increments with each change
   event that originated by focus peer and each reception of a change
   events of remote focus peer.

5.7.2.  <conference-description>

   The <conference-description> element exists in both event packages,
   conference-info and distributed-conference.  Thus, the following
   elements are seamlessly translatable: <keywords>, <display-text>,
   <subject>, <free-text> and <service-uris>.

   The sub elements <conf-uris>, <maximum-user-count> and <available-
   media> in conference-info have there counterparts below the
   \focus\focus-state element of the distributed-conference event
   package.  Each describes a local state of a focus peer in the
   conference.  Hence, the intersection of every disco.<conf-uris>,
   disco.<available-media> and the sum over each disco.<maximum-user-
   count> element of each disco.<focus> element in distributed-
   conference, specifies the content of the corresponding conference-
   info elements.

5.7.3.  <host-info>

   According to [RFC4575] the ci.<host-info> element contains
   information about the entity hosting the conference.  For
   participants in a distributed conference, the hosting entity is their
   focus peer.  Thus, the ci.<host-info> element contains information
   about a focus peer.

5.7.4.  <conference-state>

   The ci.<conference-state> element allows subscribers obtain
   information about overall state of a conference.  Its sub elements
   ci.<user-count>, ci.<active> and ci.<locked> are reused as sub
   elements of \focus\focus-state to describe the local state of a focus
   peer in a distributed conference.  The translation rules from the
   distributed-conference to the conference-info event package are the

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   <user-count>:  The sum over each value of the disco.<user-count>
      element defines the corresponding ci.<user-count>.

   <active>:  The boolean ci.<active> element is the logical
      concatenation over all disco.<active> elements by an OR-operator.

   <locked>  The boolean ci.<locked> element is the logical
      concatenation over all disco.<locked> elements by an AND-operator.

5.7.5.  <users>/<user>

   The distributed-conference event package imports the definitions of
   the ci.<users> and ci.<user> elements under a parent disco.<focus>
   element for each focus peer in a conference.  Thus, the aggregation
   over all disco.<users> elements specifies the content of the
   corresponding ci.<users> element.

5.7.6.  <sidebars-by-ref>/<sidebars-by-value>

   In accordance to [RFC4575], if a participant is connected to a
   sidebar, its responsible focus peer creates a new <user> by
   referencing to the corresponding sidebar conference.

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6.  Distributed Conference Control with SIP

   Distributed conference control with SIP defined in this document
   refers to multiparty conversation in a tightly coupled model that is
   controlled by several independent entities.  It enables a resilient
   conferencing service for P2P scenarios and provides mechanisms for
   load-balancing among the focus peers.  This section describes
   additional control operations for distributed conferences with SIP.

6.1.  Call delegation

   Distributed conference control enables load-balancing by a mechanism
   for call delegation.  Call delegations are performed by focus peers
   that are running out of capacities to serve more participants.
   Incoming participation requests are then transferred to other
   established focus peer or conference participants that are registered
   as potential focus peers in the overlay.  Call delegations use SIP
   REFER requests [RFC3515] that contain additional session information
   and are achieved transparently to the transferred party.

   A focus peer initiates a call delegation by sending SIP REFER request
   containing the URI of the participant in the Refer-To header field.
   Additionally, the focus peer appends the following parameter to the
   URI of the participant:

   call-id:  Contains the call-ID of the initial SIP dialog between the
      referred participant and the referring focus peer.

   sess-id:  Contains the 'session identifier' value of the original SDP
      'o=' field of the original offer/answer process between referred
      participant and referring focus peer.

   If the recipient accepts the REFER request, it generates a re-INVITE
   towards the referred party and sets the SIP call-id header and the
   SDP 'session-identifier' field in the SDP offer, according to the URI
   parameter values of the initial REFER request.  The From header field
   and contact header are set to the conference URI with setting the
   'isfocus' tag to contact header.  This identifies the peer as a focus
   to the conference and identifies this re-INVITE as a request of the
   SIP dialog between the party and the conference.  To ensure that
   further signaling messages will be routed correctly, the new focus
   adds a Record-Route header field that contains its contact
   information (URI, IP-address,..).

   An example call flow for call delegation is shown in Figure 6.

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   Participant    Referring Focus                         Remote Focus
        |    Dialog    |                                     |
        |<============>|                                     |
        |              |                                     |
        |  Delegating a participant to remote focus          |
        |              |                                     |
        |              |(1) REFER refer-to:<uri>?call-id=123&sess-id=456
        |              |------------------------------------>|
        |              |(2) 200 OK                           |
        |              |<------------------------------------|
        |              |(3) Notify: pending                  |
        |              |<------------------------------------|
        |              |(4) 200 OK                           |
        |              |------------------------------------>|
        |              |                                     |
        |  Remote focus adds RR-header that carries its URI  |
        |              |                                     |
        |  (5) INVITE sip:<uri> record-route:<rem.focus>     |
        |  (6) 200 OK  |                                     |
        |  (7) ACK     |                                     |
        |              |(8) Notify: active                   |
        |              |<------------------------------------|
        |              |(9) 200 OK                           |
        |              |------------------------------------>|

             Figure 6: Delegating a participant with SIP REFER

   Note, subscriptions for the event packages 'distributed-conference'
   and 'conference-info' are in scope of a specific focus peer and its
   connected participants.  Hence, after a successful call delegation,
   the referring focus peer SHOULD terminate any subscription to the
   referred participant.  The notifier SHOULD include a reason parameter
   "deactivated" to indicate a migration of the subscription as defined
   in [RFC3265].  The new SUBSCRIBE request by the party MUST be sent
   via the SIP dialog to the conference.

6.2.  Conference Access

   A conference policy defines who is allowed to participate in a
   multimedia conference.  In many cases, a group conversation can be an
   open discussion free to participate, while in other occasions a
   closed privacy of a multiparty session is demanded.  In distributed
   conferences, it is also an issue which of the conference participants
   is allowed to become a controller of the multiparty session.

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   Thus it must be decided whether:

   o  A peer is allowed to participate in a conference

   o  A peer is allowed to become a focus of the conference

   Standard SIP authentication mechanisms can be used to authenticate
   and accordingly authorize joining participants.

6.3.  Media Negotiation and Distribution

   This section describes a basic scheme for media negotiation and
   distribution, which is done in an ad-hoc fashion.

   In an established DisCo conference, each participant is attached to
   one focus (possibly itself), and all focus peers maintain mutual
   signaling relationships.  Each focus peer receives media streams from
   two groups, its locally attached members, as well as the neighboring
   focus peers.  It has two options when re-distributing media streams
   to the conference.  It either mixes streams from each group and thus
   reduces the number of media sessions, or propagates the streams in
   individual media sessions.

   The basic media distribution naturally follows the SIP signaling
   paths.  Each focus peer forwards all media streams it receives from
   the conference (possibly mixed) to all connected peers it is
   responsible for, and similarly all streams from its peers to its
   responsible focus.  Implementations can choose more sophisticated
   schemes for media distribution, e.g., some form of overlay multicast,
   but MUST take measures to prevent loops in media routing.

6.3.1.  Offer/Answer

   A peer joining a conference negotiates media types and media
   parameters with its designated focus using the standard SDP offer/
   answer protocol [RFC3264].  The focus SHOULD offer all existing media
   streams that it receives from the conference.

   A new participant does not necessarily know about all media streams
   present in a conference beforehand, and thus some of the media
   streams might not be included in the initial SDP offer sent by the
   joining peer.  An SDP answer sent by the corresponding focus though
   cannot offer additional media types that do not match an offer (cf.
   [RFC3264] Section 6).  A joining peer, which is aware of a
   heterogeneous conference, can invert the offer/answer dialog by
   omitting an SDP offer in the initial INVITE message.  Instead it MAY
   send an empty INVITE to which the focus replies with an OK,
   containing the SDP offer.  This prevents the need for a second offer/

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   answer-dialog to modify the session.  But for compatibility the
   normal behavior with the INVITE containing the offer MUST be

   For new media streams (e.g. those sent by the new participant), the
   focus SHOULD only offer media types and codecs which are already used
   in the conference and which will probably be accepted when forwarded
   to neighboring peers, unless the focus is prepared to do transcoding
   and/or mixing of the received streams.

6.3.2.  New Peers Joining

   When a new peer has been attached to a focus, new media streams may
   be available to the focus that need to be forwarded to the
   conference.  To accomplish this, the new media streams need to be
   signalled to the other participants.  This is commonly done by
   sending a SIP re-INVITE [RFC4353] for modifying the media sessions,
   adding the new streams to the SDP.  This renegotiation can be costly
   since it needs to be propagated throughout the entire conference.  In
   addition, distributing all media streams separately to all
   participants can be quite bandwidth intensive.  Both problems can
   partially be mitigated by focus peers performing mixing of media
   streams, thus trading bandwidth and signalling overheads for
   computational load on focus peers.

6.4.  Restructuring a Conference

   Distributed conference control provides the possibility to delegate
   calls to remote focus peers.  This feature is used to restructure a
   conference in case of departure of a focus peer.  Following, this
   section presents restructuring schemes for graceful and unexpected
   leaves of conference focus peers.

6.4.1.  On Graceful Leave

   In a graceful case, the leaving focus peer (LP) accomplishes the
   following procedure:

   o  LP deletes its mapping in the DisCo-Registration by storing the
      "non-existing" value as described in the RELOAD base document
      [I-D.ietf-p2psip-base].  Afterwards, it fetches the lasted version
      of the DisCo-Registration to obtain all potential focus peers.

   o  LP calculates for all its participants the closest focus among all
      active and potential focus peer using the algorithm described in
      Section 4.6.  LP then delegates all participants to those focus

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   o  LP then announces its leave by sending a NOTIFY to all its
      subscribers for the extended conference event package, setting its
      <focus> state to 'deleted'.  Thereafter, it ends its own SIP
      conference dialog by sending by to its related focus peer.

   Since the state synchronization topology in a distributed conference
   is commonly arranged in a spanning tree, a leave of a focus peer
   effects a gab in the tree structure.  Those focus peers which had the
   leaving focus peer as their parent, are supposed to reconnect to the
   synchronization graph by subscribing the parent focus of the leaving

6.4.2.  On Unexpected Leave

   If an unexpected leave is detected by a participant (e.g. missing
   signaling and/or media packets) it MUST repeat the joining procedure
   as described in Section 4.7.

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7.  DisCo Kind Definition

   This section formally defines the DisCo kind.


    Kind IDs
        The Resource name DISCO-REGISTRATION Kind-ID is the AoR of the
        conference. The data stored is the DisCoRegistrationData, that
        contains the Node-ID of a peer acting as a focus for the
        conference and optionally a coordinates value describing a
        peer's relative network position.

    Data Model
        The data model for the DISCO-REGISTRATION Kind-ID is dictionary.
        The dictionary key is the Node-ID of the peer action as focus.

    Access Control

   The data stored for the Kind-ID DISCO-REGISTRATION is of type
   DisCoRegistration.  It contains a "coordinates" value, that describes
   the peers relative network position and the Node-ID of the registered
   focus peer.

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8.  XML Schema

   The XML schema for the event package for distributed conferences is:

   <?xml version="1.0" encoding="UTF-8"?>
   <xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
       This imports the definitions in conference-info
   <xs:import namespace="urn:ietf:params:xml:ns:conference-info"
   <xs:import namespace="http://www.w3.org/XML/1998/namespace"
   <xs:element name="distributed-conference"
     <xs:complexType name="distributed-conference-type">
         <xs:element name="version-vector"
               type="version-vector-type" minOccurs="1"/>
         <xs:element name="conference-description"
         minOccurs="0" maxOccurs="1"/>
         <xs:element name="focus"
          <xs:any namespace="##other" processContents="lax"/>
        <xs:attribute name="state" type="ci:state-type"/>
        <xs:attribute name="entity" type="xs:anyURI"/>
        <xs:anyAttribute namespace="##other" processContents="lax"/>
     <xs:complexType name="version-vector-type">

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         <xs:element name="version"
         <xs:any namespace="##other" processContents="lax"/>
       <xs:anyAttribute namespace="##other" processContents="lax"/>
     <xs:complexType name="conference-description-type">
         <xs:element name="display-text"
                     type="xs:string" minOccurs="0"/>
         <xs:element name="subject" type="xs:string" minOccurs="0"/>
         <xs:element name="free" type="xs:string" minOccurs="0"/>
         <xs:element name="keywords"
                     type="ci:keywords-type" minOccurs="0"/>
         <xs:element name="service-uris"
                     type="ci:uris-type" minOccurs="0"/>
         <xs:any namespace="##other" processContents="lax"/>
       <xs:attribute name="state" type="ci:state-type"/>
       <xs:anyAttribute namespace="##other" processContents="lax"/>
     <xs:complexType name="focus-type">
         <xs:element name="display-text"
                     type="xs:string" minOccurs="0"/>
         <xs:element name="associated-aors"
                     type="ci:uris-type" minOccurs="0"/>
         <xs:element name="roles"
                     type="ci:user-roles-type" minOccurs="0"/>
         <xs:element name="languages"
                     type="ci:user-languages-type" minOccurs="0"/>
         <xs:element name="focus-state"
                     type="focus-state-type" minOccurs="0"/>
         <xs:element name="users"
                     type="ci:users-type" minOccurs="0"/>
         <xs:element name="relations"
                     type="relations-type" minOccurs="0"/>
         <xs:any namespace="#other" processContents="lax"/>

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       <xs:attribute name="entity" type="xs:anyURI"/>
       <xs:attribute name="node-id" type="xs:string"/>
       <xs:attribute name="state" type="ci:state-type"/>
       <xs:anyAttribute namespace="##other" processContents="lax"/>
     <xs:complexType name="version-type">
         <xs:extension base="xs:unsignedInt">
           <xs:attribute name="entity" type="xs:anyURI"/>
           <xs:attribute name="node-id" type="xs:string"/>
           <xs:anyAttribute namespace="##other" processContents="lax"/>
     <xs:complexType name="focus-state-type">
         <xs:element name="user-count"
                     type="xs:unsignedInt" minOccurs="0"/>
         <xs:element name="coordinate"
                     type="xs:string" minOccurs="0"/>
         <xs:element name="maximal-user-count"
                     type="xs:unsignedInt" minOccurs="0"/>
         <xs:element name="conf-uris"
                     type="ci:uris-type" minOccurs="0"/>
         <xs:element name="available-media"
                     type="ci:conference-media-type" minOccurs="0"/>
         <xs:element name="active" type="xs:boolean" minOccurs="0"/>
         <xs:element name="locked" type="xs:boolean" minOccurs="0"/>
         <xs:any namespace="##other" processContents="lax"/>
       <xs:attribute name="state" type="ci:state-type"/>
       <xs:anyAttribute namespace="##other" processContents="lax"/>
     <xs:complexType name="relations-type">
         <xs:element name="relation"
                     minOccurs="0" maxOccurs="unbounded"/>
         <xs:any namespace="##other" processContents="lax"/>

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       <xs:attribute name="state" type="ci:state-type"/>
       <xs:anyAttribute namespace="##other" processContents="lax"/>
     <xs:complexType name="relation-type">
         <xs:extension base="xs:string">
           <xs:attribute name="entity" type="xs:anyURI"/>
           <xs:anyAttribute namespace="##other" processContents="lax"/>

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9.  Relax NG Grammar

   The grammar for the Landmark configuration document extension is:

   parameter &= element landmarks {
       attribute version { xsd:int }
       element landmark-host {
           attribute address { xsd:string },
           attribute port { xsd:int }

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10.  Security Considerations

10.1.  Trust Aspects


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11.  IANA Considerations

   TODO: register Kind-ID code point at the IANA

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12.  Acknowledgments

   This work was stimulated by fruitful discussions in the P2PSIP
   working group and SAM research group.  We would like to thank all
   active members for constructive thoughts and feedback.  In
   particular, the authors would like to thank (in alphabetical order)
   David Bryan, Toerless Eckert, Lothar Grimm, Cullen Jennings, Peter
   Musgrave, Joerg Ott, Peter Pogrzeba, Brian Rosen, and Jan Seedorf.

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13.  References

13.1.  Normative References

              Jennings, C., Lowekamp, B., Rescorla, E., Baset, S., and
              H. Schulzrinne, "REsource LOcation And Discovery (RELOAD)
              Base Protocol", draft-ietf-p2psip-base-21 (work in
              progress), March 2012.

              Knauf, A., Hege, G., Schmidt, T., and M. Waehlisch, "A
              Usage for Shared Resources in RELOAD (ShaRe)",
              draft-knauf-p2psip-share-03 (work in progress),
              April 2012.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3261]  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.

   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
              with Session Description Protocol (SDP)", RFC 3264,
              June 2002.

   [RFC3265]  Roach, A., "Session Initiation Protocol (SIP)-Specific
              Event Notification", RFC 3265, June 2002.

   [RFC3515]  Sparks, R., "The Session Initiation Protocol (SIP) Refer
              Method", RFC 3515, April 2003.

   [RFC4574]  Levin, O. and G. Camarillo, "The Session Description
              Protocol (SDP) Label Attribute", RFC 4574, August 2006.

   [RFC4575]  Rosenberg, J., Schulzrinne, H., and O. Levin, "A Session
              Initiation Protocol (SIP) Event Package for Conference
              State", RFC 4575, August 2006.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, October 2006.

   [RFC5245]  Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address Translator (NAT)
              Traversal for Offer/Answer Protocols", RFC 5245,
              April 2010.

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13.2.  Informative References

              Bryan, D., Willis, D., Shim, E., Matthews, P., and S.
              Dawkins, "Concepts and Terminology for Peer to Peer SIP",
              draft-ietf-p2psip-concepts-04 (work in progress),
              October 2011.

              Jennings, C., Lowekamp, B., Rescorla, E., Baset, S., and
              H. Schulzrinne, "A SIP Usage for RELOAD",
              draft-ietf-p2psip-sip-07 (work in progress), January 2012.

   [RFC4353]  Rosenberg, J., "A Framework for Conferencing with the
              Session Initiation Protocol (SIP)", RFC 4353,
              February 2006.

              Ratnasamy, Handley, Karp, and Shenker, "Topologically-
              Aware Overlay Construction and Server Selection", Proc. of
              21st Annual Joint Conference of the IEEE Computer and
              Communications Societies (INFOCOM '02) pp. 1190-1199,

              Fidge, C., "Timestamps in Message-Passing Systems that
              Preserve the Partial Ordering", Proceedings of 11th
              Australian Computer Science Conference, pp. 56-66,
              February 1988.

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Appendix A.  Change Log

   The following changes have been made from version

   1.  Editorial improvements.

   2.  Updated references.

   The following changes have been made from version

   1.  Adapted mechanisms for storing DisCo-Registrations to new
       requirements of Shared Resources draft [I-D.knauf-p2psip-share]

   The following changes have been made from version

   1.  DisCo-Registration uses now only the USER-CHAIN-ACL access
       control policy.

   2.  Adapted mechanisms for storing DisCo-Registrations to new
       requirements of Shared Resources draft [I-D.knauf-p2psip-share]

   The following changes have been made from version

   1.  The conference registration is now based on the Shared Resources
       draft [I-D.knauf-p2psip-share]:

       *  DisCo-Registration Kind now meets the requirements for ShaRe.

       *  Conference creation procedure now uses the ShaRe Access List.

       *  Replaced USER-CHAIN-MATCH access policy for DisCo-
          Registration.  Now uses USER-CHAIN-ACL or USER-PATTERN-MATCH.

   2.  Allow focus peers behind NAT

   3.  Added a 'node-id' attribute to the event package XML <version>

   4.  Added a 'node-id' attribute to the event package XML <focus>

   5.  Added a 'coordinate' child element to the event package XML
       <focus> element.

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   6.  Corrected typos/wording

   The following changes have been made from version

   1.  Updated references.

   2.  Corrected typos.

   3.  New Section: Conference State Synchronization

   4.  XML Event Package for Distributed Conferences

   5.  New mechanism for generating chained conference certificates

   6.  Allow shared writing of resources using Access Control Policy

   7.  Media Negotiation mechanism in a distributed conference

   8.  New Section: Distributed Conference Control with SIP

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Authors' Addresses

   Alexander Knauf
   HAW Hamburg
   Berliner Tor 7
   Hamburg  D-20099

   Phone: +4940428758067
   Email: alexanderknauf@gmail.com
   URI:   http://inet.cpt.haw-hamburg.de/members/knauf

   Gabriel Hege
   HAW Hamburg
   Berliner Tor 7
   Hamburg  D-20099

   Phone: +4940428758067
   Email: hege@fhtw-berlin.de
   URI:   http://inet.cpt.haw-hamburg.de/members/hege

   Thomas C. Schmidt
   HAW Hamburg
   Berliner Tor 7
   Hamburg  D-20099

   Email: schmidt@informatik.haw-hamburg.de
   URI:   http://inet.cpt.haw-hamburg.de/members/schmidt

   Matthias Waehlisch
   link-lab & FU Berlin
   Hoenower Str. 35
   Berlin  D-10318

   Email: mw@link-lab.net
   URI:   http://www.inf.fu-berlin.de/~waehl

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