[Docs] [txt|pdf] [Tracker] [Email] [Nits]

Versions: 00

INTERNET-DRAFT                             J. Ott/C. Perkins/D. Kutscher
Expires: December 1999       Universitaet Bremen/UCL/Universitaet Bremen
                                                               June 1999


                The Message Bus: Messages and Procedures
                 draft-ott-mmusic-mbus-semantics-00.txt


Status of this memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet- Drafts as reference
   material or to cite them other than as "work in progress."

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

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

Abstract


   In a variety of conferencing scenarios, a local communication channel
   is desirable for conference-related information exchange between co-
   located but otherwise independent application entities, for example
   those taking part in application sessions that belong to the same
   conference.  In loosely coupled conferences such a mechanism allows
   for coordination of applications entities to e.g. implement
   synchronization between media streams or to configure entities
   without user interaction. It can also be used to implement tightly
   coupled conferences enabling a conference controller to enforce
   conference wide control within a end system.

   The local conference Message Bus (Mbus) provides a means to achieve
   the necessary amount of coordination between co-located conferencing
   applications for virtually any type of conference.  The Message Bus
   comprises two logically distinct parts: a message transport and
   addressing infrastructure and a set of common as well as media tool
   specific messages.  This document defines protocol procedures for the
   Message Bus operation and the syntax and semantics of several sets of
   Mbus messages: common messages understood by all application entities
   as well as messages specific to particular classes of applications.

Ott/Perkins/Kutscher                                            [Page 1]


INTERNET-DRAFT  The Message Bus: Messages and Procedures       June 1999

   The underlying message passing and addressing mechanisms for the Mbus
   is defined in a companion Internet draft [3].

   This document is a contribution to the Multiparty Multimedia Session
   Control (MMUSIC) working group of the Internet Engineering Task
   Force.  Comments are solicited and should be addressed to the working
   group's mailing list at confctrl@isi.edu and/or the authors.


1.  Introduction


1.1.  Background


   The requirement specification as defined in the companion
   requirements specification [2] provides a set of scenario
   descriptions for the usage of a local coordination infrastructure.
   The Message Bus defined in this and a companion document provides a
   suitable means for local communication that serves all of the
   purposes mentioned in the requirement draft.

1.2.  Scope of this Document


   Two components constitute the Message Bus: the (lower level) message
   passing mechanisms and the (higher level) messages and their
   semantics.  While the basic transport mechanisms for the Mbus are
   defined in a companion Internet Draft [3], the purpose of this
   document is to define common as well as application-specific Mbus
   messages and their semantics.  This includes

   o    definition of a simple naming scheme to allow for unambiguous
        command names as well as easy and conflict-free extensibility of
        the Mbus command set.

   o    definition of a set of mandatory Mbus management messages and
        associated procedures that enable the Mbus to function in
        virtually all kinds of conference settings;

   o    definition of a set of conditionally mandatory messages for
        conference control; and

   o    definition of several sets of optional messages for specific
        media types and/or functions.

   The main body of this document addresses the first three bullet items
   thereby providing the foundation for the operation of the Mbus.  The
   Mbus messages specific to particular protocols, media, and functions
   are contained in independent appendices to this document.




Ott/Perkins/Kutscher                                            [Page 2]


INTERNET-DRAFT  The Message Bus: Messages and Procedures       June 1999

2.  Command Naming Scheme


   The general command syntax is described in the companion Mbus
   protocol Internet draft[3]. Command names SHALL be constructed using
   hierarchical names to group conceptually related commands under a
   common hierarchy. The delimiter between names in the hierarchy is
   ``.''  (dot).

   The Mbus addressing scheme defined in [3] provides for specifying
   incomplete addresses by omitting certain elements of an address
   element list, enabling entities to send commands to a group of Mbus
   entities. Therefore all command names must be unambiguous in a way
   that it is possible to interpret or neglect them without considering
   the message's address.

   A set of commands within a certain hierarchy that MUST be understood
   by every entity is defined in section 4 ("Entity Control"). Table 1
   lists the pre-defined command prefixes:

        +---------------+------------------------------------------+
        |Command prefix | Description of command class             |
        +---------------+------------------------------------------+
        |mbus.          | General Basic Mbus commands              |
        |conf.          | Commands related to conference control   |
        |rtp.           | RTP-related commands                     |
        |audio.         | Commands specific to audio tools/engines |
        |video.         | Commands specific to video tools/engines |
        |security.      | Security-related commands                |
        |status.        | Commands to communicate status           |
        |               | information, error conditions etc.       |
        +---------------+------------------------------------------+
               Table 1: Naming conventions for Mbus commands


   In addition, tool specific commands have to be defined as well thus
   allowing each Mbus entity to define and use a number of private
   commands. All such commands must begin with the sequence

        tool.<tool-name>,

   for example tool.rat.

   The following sections define the mentioned command classes.

3.  Basic Mbus Operation and Management


3.1.  Requirements


   Before components of a conferencing system can communicate with one
   another using the Mbus, they need to mutually find out about their

Ott/Perkins/Kutscher                                            [Page 3]


INTERNET-DRAFT  The Message Bus: Messages and Procedures       June 1999

   existence.  After this bootstrap procedure that each Mbus entity goes
   through all other entities listening to the same Mbus know about the
   newcomer and the newcomer has learned about all the other entities.

   In order to minimize the dependencies of applications on one another
   and on the environment they are operating in, a bootstrap procedure
   must take into account that

   o    Mbus entities may be started in an arbitrary order

        -    manually by the user from a command line interpreter
             (possibly with provision a set of command line parameters),

        -    manually by the user from a window manager menu (i.e. in
             contrast to the former without explicit parameterization),

        -    automatically from a conference-aware tool (such as SDR),
             or

        -    initially at system startup (and thus may be listening in
             the background).[1]

   o    some Mbus entities may depend on continued existence of other
        Mbus entities or need to synchronize with them before being able
        to perform their functions properly; and

   o    a local coordination entity -- if present -- may take over
        control of the tools, but cooperation via the Mbus works as well
        if no such controller is present.

3.2.  Basic Mechanisms


   From the aforementioned requirements, the following mechanisms are
   devised as being necessary:

   1.   Self-announcement messages

        Any Mbus entity is supposed to announce its presence (on the
        Mbus) after starting up.  This is to be done repeatedly
        throughout its lifetime to address the issues of startup
        sequence.

   2.   Alive messages

_________________________
  [1] Note that the main distinction between these various ways of
starting  application  entities are a) the amount of configuration
information passed to the application as command line options  and
b)  whether applications are single-session or multi-session capa-
ble (i.e. whether or not a single application process is  able  to
act as multiple instances on the Mbus or not).


Ott/Perkins/Kutscher                                            [Page 4]


INTERNET-DRAFT  The Message Bus: Messages and Procedures       June 1999

        Any Mbus entity should frequently indicate that it is still
        alive.  This mechanism may be combined with the aforementioned
        self-announcement.

   3.   Synchronization messages

        An Mbus entity should be able to indicate that it is waiting for
        a certain event to happen (similar to a P() operation on a
        semaphore but without creating external state somewhere).  In
        conjunction with this, an Mbus entity should be capable of
        indicating to another entity that this condition is now
        satisfied (similar to a semaphore's V() operation).

   An appropriate set of commands that implements this conceptual
   specification is presented in the following section.

3.3.  Mbus Management Protocol


3.3.1.  Mbus Message Syntax and Procedures


   The following Mbus messages are defined to implement the mechanisms
   described above:

HELLO


   Syntax:     mbus.hello ()

   Parameters: - none -

   Each Mbus entity MUST send HELLO messages after startup to the global
   Mbus channel.  After transmission of the HELLO message, it shall
   start a timer after the expiration of which the next HELLO message
   shall be transmitted.  The timer shall be set to a random value
   t_hello <= t <= t_hello + t_dither to avoid synchronization of HELLO
   messages.  Transmission of HELLO messages MUST NOT be stopped unless
   the entity detaches from the Mbus.

   HELLO messages are sent unreliably to all Mbus entities.

   Each Mbus entity learns about other Mbus entities by observing their
   HELLO messages and tracking the sender address of each message.

   The HELLO message is also used to track the liveness of any Mbus
   entity.  Whenever an Mbus entity has not heard for a time span of
   n_dead*(t_hello+t_dither) from another Mbus entity it may consider
   this entity to have failed (or have quit silently).  Note that no
   need for any action is necessarily implied from this observation.




Ott/Perkins/Kutscher                                            [Page 5]


INTERNET-DRAFT  The Message Bus: Messages and Procedures       June 1999

BYE


   Syntax:     mbus.bye ()

   Parameters: - none -

   An Mbus entity that is about to terminate (or ``detach'' from the
   Mbus) announces this by transmitting a BYE message.

   The BYE message is sent unreliably to all receivers.

WAITING


   Syntax:     mbus.waiting (condition)

   Parameters: symbol condition
               The condition parameter is used to indicate that the
               entity transmitting this message is waiting for a
               particular event to occur.

   The WAITING messages may be broadcast to all Mbus entities, multicast
   an arbitrary subgroup, or unicast to a particular peer.  Transmission
   of the WAITING message MUST be unreliable and hence has to be
   repeated at an application-defined interval (until the condition is
   satisfied).

   If an application wants to indicate that it is waiting for several
   conditions to be met, several WAITING messages are sent (possibly
   included in the same Mbus payload).  Note that HELLO and WAITING
   messages may also be transmitted in a single Mbus payload.

   Appendix D presents a tool configuration scheme that allow tools to
   be parameterized on the command line in order to start them in
   ``waiting mode''.

GO


   Syntax:     mbus.go (condition)

   Parameters: symbol condition
               This parameter specifies which condition is met.

   The GO message is sent by an Mbus entity to ``unblock'' another Mbus
   entity -- the latter of which has indicated that it is waiting for a
   certain condition to be met.  Only a single condition can be
   specified per GO message.  If several conditions are satisfied
   simultaneously multiple GO messages MAY be combined in a single Mbus
   payload.

   The GO message MUST be sent reliably via unicast to the Mbus entity

Ott/Perkins/Kutscher                                            [Page 6]


INTERNET-DRAFT  The Message Bus: Messages and Procedures       June 1999

   to unblock.

QUERY


   Syntax:     mbus.query (variable)

   Parameters: symbol variable
               This parameter specifies the variable name.

   The QUERY message is a general mechanism for obtaining arbitrary
   information from Mbus entities. The semantics and the variable names
   are application specific. QUERY messages are answered with INFO
   messages (see below).

   The QUERY message can be multicast or sent reliably via unicast to a
   single Mbus entity or a group of entities.

INFO


   Syntax:     mbus.info (variable value)

   Parameters: symbol variable
               This parameter specifies the variable name.

               string value
               The variable parameter specifies the variable name
               requested in a QUERY messages and the value parameter,
               that can be of an arbitrary type, provides the requested
               information.

   The INFO message is a general mechanism for delivering arbitrary
   information to Mbus entities. The semantics and the variable names
   are application specific.

   The INFO message can be multicast or sent reliably via unicast to a
   single Mbus entity or a group of entities.

POLL


   Syntax:     mbus.poll (question alt_c choice_c (alternatives)
               context)

   Parameters: symbol question
               This parameter question specifies the question topic a
               vote is requested for.

               Integer alt_c

               Integer choice_c


Ott/Perkins/Kutscher                                            [Page 7]


INTERNET-DRAFT  The Message Bus: Messages and Procedures       June 1999

               List (alternatives)
               A sequence of parameters specifying how to process and
               answer the question in the following format: (n k (c1 ...
               cn))

                 +------------+--------------------------------------+
                 |n           | Number of alternatives               |
                 |k           | Number of allowed choices            |
                 |(c1 ... cn) | A List of alternatives (type string) |
                 +------------+--------------------------------------+
                  Table 2: Specification parameters of a POLL message

               list context
               The context parameter contains application specific
               context information required for deciding on the
               question.

   The POLL message is a general mechanism for requesting decisions on
   arbitrary questions from Mbus entities. The semantics, the question
   names and the alternatives are application specific.

   The POLL message can be multicast or sent reliably via unicast to a
   single Mbus entity or a group of entities.

   It is up to the inquiring application to ensure that

        a) all desired entities are inquired (and reached) and

        b) all responses are collected

VOTE


   Syntax:     mbus.vote (question (choice-list))

   Parameters: symbol question
               This parameter question specifies the question topic this
               vote refers to.

               List choice-list
               A list of selected choices that must conform to the
               specification that was received in a POLL message before.
               The list elements are of type Integer and represent
               indices to the alternatives-list of the POLL message
               starting from 0.

   The VOTE message is a general mechanism for answering POLL messages.
   A entity can have a group of other entities decide on a question,
   collect the results as VOTE messages and evaluate these to infer
   further actions. See appendix H for a sample POLL/VOTE conversation
   scenario.

   The VOTE message can be multicast or sent reliably via unicast to a

Ott/Perkins/Kutscher                                            [Page 8]


INTERNET-DRAFT  The Message Bus: Messages and Procedures       June 1999

   single Mbus entity or a group of entities.

3.3.2.  Timers and Counters


   The following values for timers and counters used in this document
   shall apply.

                   +----------------+------------------+
                   |Timer / Counter | Value            |
                   +----------------+------------------+
                   |t_hello         | 1 second         |
                   |t_dither        | 100 milliseconds |
                   |n_dead          | 5                |
                   +----------------+------------------+
                     Table 2: Timer and counter values

   As the Mbus is designed for a local system architecture it is not
   considered necessary to provide dynamic adaptation of these timers
   and counters to the number of Mbus entities.

4.  Conference Control


   The conference control part of the Mbus messages is intended to

   a)   provide a means for obtaining information about capabilities
        from other local application entities -- for both using this
        information for capability negotiation with other end systems
        and determining which commands are understood by another
        application entity --;

   b)   allow dynamic (re-)configuration of application entities with
        respect to various session parameters;

   c)   forward conference state changes (potentially negotiated by a
        local conference controller through a horizontal control
        protocol) to all other application entities; and

   d)   provide means for controlling call control entities, such as SIP
        or H.323 engines; and

   e)   allow application entities (in tightly controlled conferences)
        to request invocation of conference control services from a
        conference controller (within whatever system and conference
        policy constraints apply).

   Commands of the ``Conference Control'' class SHALL be prefixed with
   ``conf.''.  Table 3 lists the currently defined prefixes under the
   conf hierarchy.




Ott/Perkins/Kutscher                                            [Page 9]


INTERNET-DRAFT  The Message Bus: Messages and Procedures       June 1999

       +-------------------+----------------------------------------+
       |Command prefix     | Description of command class           |
       +-------------------+----------------------------------------+
       |conf.cap.          | Capability commands                    |
       |conf.transport.    | Media transport configuration commands |
       |conf.call-control. | Call control commands                  |
       +-------------------+----------------------------------------+
     Table 3: Naming conventions for Mbus conference control commands


4.1.  Capabilities


   In order to enable tightly controlled conferences a conference
   controller -- potentially the local coordination entity -- needs to
   determine not only which application entities are present but also
   which capabilities they have, in which application sessions they
   participate, and so forth.  This leads to the following:

   Each Mbus entity should support a capability query and respond by
   providing the requested information to the querier.  The query may be
   asking for all capabilities of the queried entity or for a particular
   subset specified in the query.  Upon receipt of such a query the
   inquired Mbus entity provides successively all the desired capability
   information, possibly after recursive queries from the querier.

   Capability commands are to be defined at a later time.

4.2.  Media transport configuration


   The hierarchy conf.transport.  contains commands for configuring
   media transport parameters of entities such as IP addresses, port
   numbers, ttl. These commands SHOULD generally not be sent to entity
   groups because each entity will require a unique parameter set.

4.2.1.  address


   Syntax:     conf.transport.address (ipaddr port [cport])

   Parameters: string ipaddr

               list of integer port-list

               integer ttl

               The IP unicast or multicast address and associated port
               number(s) to be used for information transmission (and,
               in case of multicast) reception.  For application
               entities using RTP, the port for RTCP addresses may be
               specified as the second element of the port list. TTL
               values can be specified with the ttl parameter.

Ott/Perkins/Kutscher                                           [Page 10]


INTERNET-DRAFT  The Message Bus: Messages and Procedures       June 1999

4.3.  Miscellaneous Conference Control


   A conference controller that cannot map or translate every conference
   control related state transition in a tightly coupled conference to a
   entity specific remote control Mbus message will want to ``forward''
   this information to the group of all Mbus entities in order to allow
   each entity to interpret this information independently. The
   objective is to define a set of generic Mbus commands that allows to
   represent the change of conference global variables without
   necessarily using these commands to remote-control entities
   explicitly.

   Conference control related commands are prefixed with ``conf.''.
   Table 4 lists the prefixes under the conf hierarchy.

       +------------------+-----------------------------------------+
       |Command prefix    | Description of command class            |
       +------------------+-----------------------------------------+
       |conf.name         | Set conference/session name             |
       |conf.call-control | Commands for signaling invitations etc. |
       |conf.floor        | Floor control commands/indications      |
       |                  |                                         |
       |conf.member       | Membership lists                        |
       |conf.chair        | Conference chair indications            |
       +------------------+-----------------------------------------+
        Table 5: Naming conventions for Mbus control.conf commands

4.3.1.  Conference parameters


   A set of commands is defined to communicate conference parameters
   like conference/session names etc.

4.3.1.1.  Name


   Syntax:     conf.name (conference-name [session-name])

   Parameters: conference-name
               session-name
               Sets the conference (and session) name for a conference.

4.4.  Call Control Messages


   Call control messages are intended for interaction with call control
   and invitation protocols such as H.323 and SIP.  They are designed to
   constitute the union of the call control messaging needed by
   endpoints, gateways, proxies, multipoint controllers, and
   gatekeepers.  This allows the use of the Message Bus as a gluing
   mechanisms to create any type of system from roughly the same
   building blocks.

Ott/Perkins/Kutscher                                           [Page 11]


INTERNET-DRAFT  The Message Bus: Messages and Procedures       June 1999

   Mbus call control messages are based on a common basic message set
   defined in the following that shall be supported by any kind of call
   control protocol entity.  The basic message set may be augmented by
   protocol-specific extensions required for protocol specific
   interactions between a local controller and/or local applications on
   one side and the respective protocol engine on the other.

   The following prefixing conventions apply for call control messages:

   +---------------------------------------------------------------------+
   |Command prefix                              Description of command class
   +---------------------------------------------------------------------+
   |conf.call-control.                             basic call control    |
   |                                               message set           |
   |conf.call-control.h323.                        extensions for        |
   |                                               H.323-specific call   |
   |                                               control messages      |
   |conf.call-control.sip.                         extensions for SIP-   |
   |                                               specific call control |
   |                                               messages              |
   +---------------------------------------------------------------------+
                  Table 6: Call control message prefixes


4.4.1.  The Basic Call Control Message Set


   The basic set of messages is defined to provide the core
   functionality of initiating a call on one side, accepting or refusing
   it on the other, and providing progress information as well as
   allowing termination of the call on either side.  The basic call
   control message set MUST be supported by any call control engine.

   These messages are exchanged using unicast addressing between some
   local controlling entity and a call control engine implementing a
   call control or initiation protocol such as H.323 or SIP:

   o    Outgoing calls may be initiated by any local entity; the call
        control engine has to keep track of the initiator of a
        particular call and return all responses or events relating to
        this call to this entity -- which may be different on a per call
        basis.  If the call control engine notices that the controlling
        entity for a particular call has gone (e.g. because the Mbus
        reliability mechanism indicates non-delivery of a call control
        message or a BYE message was seen from this entity), these
        messages are forwarded to the local controller.  If no local
        controller is available, the call SHOULD be terminated.

   o    Indications about incoming calls are always forwarded to the
        local controller.  If no local controller is present incoming
        calls SHOULD automatically be rejected by the call control
        engine.


Ott/Perkins/Kutscher                                           [Page 12]


INTERNET-DRAFT  The Message Bus: Messages and Procedures       June 1999

   All messages of the basic call control message set are sent reliably
   via unicast to the call control engine.

   In this first draft, the definition of the basic call control message
   set is deliberately kept very much restricted:

   o    In this revision of the document, only the messages for
        establishing a simple (point-to-point) call are specified, along
        with very few messages dealing with supplementary services.
        Further Mbus messages supporting supplementary services are TBD.

   o    No explicit consideration is given so far for devices other than
        end systems -- although a simple gateway could probably be built
        based upon the present subset.

   The following messages are specified so far:

CALL


   The CALL message is sent to the call control engine to make the
   engine initiate a call to another endpoint using the parameters
   specified as part of the CALL message.



   Syntax:   conf.call-control.call (ref upi address-list call-type gw-
             proxy-list media-list status)


   Parameters: string ref
               A unique identifier for the call.  This reference MUST be
               used for all further interactions relating to this
               between the call control engine and the initiating
               entity. Every newly created call identifier MUST be
               composed of the Mbus address of the creating entity and a
               second entity specific part in order to ensure
               uniqueness.

               string upi
               A universal personal identifier for the callee.

               list of string address-list
               An ordered list of transport addresses, alias names,
               UCIs, or phone numbers -- as indicated by the prefix
               preceding each address to be called.  It is assumed that
               all these addresses refer to the same user, and only a
               single call will be established.  The order in which the
               addresses are specified indicates a preference and
               contacting the target SHOULD be tried in that order.

               symbol call-type
               Indicates the intention of the call: join a conference

Ott/Perkins/Kutscher                                           [Page 13]


INTERNET-DRAFT  The Message Bus: Messages and Procedures       June 1999

               (or an n-way call), invite another user into a conference
               or an n-way call, or create a new call or conference.

               list of string gw-proxy-list
               An ordered list of (ordered) lists identifying proxies or
               gateways to be used for call setup if they are known.
               The n-th element in the list is a list of alternative
               gateways/proxies to be used in the n-th step in the call
               setup process.  The gw-proxy-list may be empty.

               list of string media-list
               A list of media along with the preferred capability
               descriptions to be used for this particular call.  If the
               same media type (e.g. audio) occurs repeatedly in the
               list (e.g. with different codecs) the relative order of
               the media descriptions indicates a preference (e.g. of
               one codec over the other).

               Currently, media-list are SDP specifiations, but a new
               general format will be specified in the next revision of
               this draft.

               symbol status
               A parameter used to indicate certain attributes of a call
               process. This list is a subset of the following list of
               symbols:

               +-------------------+------------------------------------+
               |    Symbol         |       Description                  |
               +-------------------+------------------------------------+
               |    complete       |       Means that this call         |
               |                   |       command contains all         |
               |                   |       required information and     |
               |                   |       that it can be processed     |
               |                   |       immediately by a             |
               |                   |       receiving entity.            |
               +-------------------+------------------------------------+
               |    partial        |       This call command is         |
               |                   |       part of a series of call     |
               |                   |       commands related to a        |
               |                   |       single call.                 |
               +-------------------+------------------------------------+
               |    final          |       This is the last call        |
               |                   |       command in a series of       |
               |                   |       call commands related to     |
               |                   |       a single call.               |
               +-------------------+------------------------------------+
               Table 7: Status symbols in a conf.call-control.call command

               The symbols complete, partial, and final SHALL be used
               exclusively.



Ott/Perkins/Kutscher                                           [Page 14]


INTERNET-DRAFT  The Message Bus: Messages and Procedures       June 1999

DISCONNECT


   The DISCONNECT command is sent by the local controller to the call
   control engine to indicate that the specified call is to be
   disconnected. It can also be used by the local controller to inform
   the call control engine that a call has already been terminated by
   out-of-band communication, e.g. a horizontal conference control
   protocol. In this case a special (yet to be defined) reason code has
   to be passed with the command.

   Syntax:   conf.call-control.disconnect (ref reason)


   Parameters: string ref
               Identifies the call the DISCONNECT command refers to.

               string reason
               Indicates why the call was terminated.  The reason will
               be set to user-initiated if the user simply ''hung up``
               the phone.  Other reason codes will be imported from
               H.323 and SIP.  They are to be defined.

RINGING


   The RINGING message is sent by the call control engine to the entity
   it received the corresponding CALL message from.  RINGING indicates
   that one or more addresses at the far end were contacted and are now
   alerting the user.

   Syntax:   conf.call-control.ringing (ref (address-list))


   Parameters: string ref
               Identifies the call the RINGING message refers to.

               list of string address-list
               An ordered list of transport addresses, alias names,
               UCIs, or phone numbers -- as indicated by the prefix
               preceding each address to be called.  It is assumed that
               all these addresses refer to the same user, and only a
               single call will be established.  The order in which the
               addresses are specified indicates a preference and
               contacting the target SHOULD be tried in that order.

CONNECTED


   The CONNECTED message is sent by the call control engine to the
   entity that initiated the call (on the calling side) and to the local
   controller (on the called side) to indicate that the call was
   successfully established.

Ott/Perkins/Kutscher                                           [Page 15]


INTERNET-DRAFT  The Message Bus: Messages and Procedures       June 1999

   Syntax:   conf.call-control.connected (ref peer-address (media-list))


   Parameters: string ref
               Identifies the call the CONNECTED message refers to.

               string peer-address
               Indicates the (transport/alias/UCI/etc.) address of the
               peer endpoint (or a proxy/gateway/gatekeeper hiding its
               actual identity) that the call was finally established
               with.

               list of string media-list
               A list of media along with the capability descriptions
               that were initially negotiated for this particular call.

REJECTED


   The REJECTED message is sent by the call control engine to the entity
   that initiated the call (on the calling side) and to the local
   controller (on the called side) to indicate that the call was
   rejected.

   Syntax:   conf.call-control.rejected (ref ((address reason)-list))


   Parameters: string ref
               Identifies the call the REJECTED message refers to.

               list of list of string ((address reason)-list)
               The (address reason) pair specifies which target address
               has rejected the call for which reason.  As several
               addresses may have been tried explicitly, a list of
               addresses is returned, each paired with its particular
               rejection reason and possibly associated parameters.
               The details of the reason codes are to be defined; they
               are to be derived from H.323 and SIP and will include
               e.g.  busy no-answer user-reject no-resources and
               authentication-failure among many others.

DISCONNECTED


   The DISCONNECTED message is sent by the call control engine to the
   entity that initiated the call (on the calling side) and to the local
   controller (on the called side) to indicate that the call was
   disconnected.

   Syntax:   conf.call-control.disconnected (ref reason)


   Parameters: string ref

Ott/Perkins/Kutscher                                           [Page 16]


INTERNET-DRAFT  The Message Bus: Messages and Procedures       June 1999

               Identifies the call the DISCONNECTED message refers to.

               symbol reason
               Indicates why the call was terminated.  The reason will
               be set to user-initiated if the user simply ''hung up``
               the phone.  Other reason codes will be imported from
               H.323 and SIP.  They are to be defined.

INCOMING CALL


   The INCOMING CALL messages is sent by the call control engine to the
   local controller to indicate a call request from another endpoint.

   Syntax:
        conf.call-control.incoming-call (ref src-address (address-list)
        call-type (gw-proxy-list) (media-list))


   Parameters: string ref
               A unique identifier for the call. (See the notes
               concerning unique addresses at the description of the
               CALL command.) This reference MUST be used for all
               further interactions relating to this between the call
               control engine and the local controller entity.

               string src-address
               The address (transport address, alias name, UCI, phone
               number, etc.)  of the endpoint initiating the call.

               list of string address-list
               An ordered list of transport addresses, alias names,
               UCIs, or phone numbers as sent by the calling endpoint
               with semantics similar to the address-list in the CALL
               message.

               symbol call-type
               Indicates the intention of the call; again, similar to
               the CALL message.

               list of string gw-proxy-list
               An ordered list of (ordered) lists identifying proxies or
               gateways to be used for call setup if they are known.
               Similar to the CALL message.

               list of string media-list
               A list of media along with the preferred capability
               descriptions proposed by the calling endpoint to be used
               for this particular call.  If the same media type (e.g.
               audio) occurs repeatedly in the list (e.g. with different
               codecs) the relative order of the media descriptions
               indicates a preference (e.g. of one codec over the
               other).

Ott/Perkins/Kutscher                                           [Page 17]


INTERNET-DRAFT  The Message Bus: Messages and Procedures       June 1999

ACCEPT


   An ACCEPT message is sent by the local controller to the call control
   engine that has indicated an INCOMING CALL to indicate acceptance of
   the call.

   Syntax:         conf.call-control.accept (ref (media-list))


   Parameters: string ref
               Identifies the call the ACCEPT message refers to.

               list of string media-list
               A list of media along with the preferred capability
               descriptions selected by the local controller.  This
               SHOULD be a strict subset of the media descriptions the
               calling endpoint has proposed for this particular call.

REJECT


   A REJECT message is sent by the local controller to the call control
   engine that has indicated an INCOMING CALL to indicate rejection of
   the call.

   Syntax:         conf.call-control.reject (ref reason [params])


   Parameters: string ref
               Identifies the call the RINGING message refers to.

               symbol reason
               The reason code indicates why the call attempt was
               refused by the callee.
               The details of the reason codes are to be defined; they
               are to be derived from H.323 and SIP and will include
               e.g.  busy no-answer user-reject no-resources and
               authentication-failure among many others.

   params          Additional parameters may be provided along with the
                   reason code.  TBD.

REDIRECT


   The REDIRECT command is sent by the local controller to the call
   control engine to indicate that the specified call is to be
   redirected to another specified address.

   Syntax:   conf.call-control.redirect (ref upi address-list attr)



Ott/Perkins/Kutscher                                           [Page 18]


INTERNET-DRAFT  The Message Bus: Messages and Procedures       June 1999

   Parameters: string ref
               Identifies the call the REDIRECT command refers to.

               string upi
               A universal personal identifier for the callee.

               list of string address-list
               List of addresses where the call should be redirected to.

               symbol attr
               A symbol with the value ``temporarily'' or
               ``permanently'', signaling whether the redirection is
               temporarily or not.

REDIRECTED


   The REDIRECTED command is sent by a call control engine to the local
   controller to indicate that the specified call has been redirected to
   the specified address. The default semantics in this case are that
   the call control engine can purge any state related to that call and
   the local controller has to decide on further actions.  In case the
   redirection should be obeyed the local controller can initiate a new
   call by sending the CALL command with the address parameter obtained
   from the REDIRECTED command.

   Call control engines that can decide themselves what to do after the
   reception of a protocol specific redirection can signal this by
   setting the status parameter to ``ACTIVE''. The semantics in this
   case are that the call control engine performs any required protocol
   specific action autonomously and that it will send the usual call
   setup related commands (CONNECTED, REJECTED etc.) during the course
   of the call setup. The local controller can terminate the call at any
   time with a DISCONNECT command. This behaviour would have to be
   configured by out of band means; the default behaviour is that the
   local controller is responsible for any reaction on REDIRECTED
   commands (signalled by setting status to PASSIVE).

   Syntax:   conf.call-control.redirected (ref upi addr-list attr
             status)


   Parameters: string ref
               Identifies the call the REDIRECT command refers to.

               string upi
               A universal personal identifier for the callee.

               list of string addr-list
               Address where the call should be redirected to.

               symbol attr
               A symbol with the value ``temporarily'' or

Ott/Perkins/Kutscher                                           [Page 19]


INTERNET-DRAFT  The Message Bus: Messages and Procedures       June 1999

               ``permanently'', signaling whether the redirection is
               temporarily or not.

               symbol status
               One of ACTIVE and PASSIVE. Used to signal whether a call
               control engine performs the redirection iself or not.

FORWARD


   The FORWARD command is sent by the local controller to the call
   control engine to indicate that the specified incoming call is to be
   forwarded to another (optionally specified) address. The second
   parameter is a list of strings that are to be interpreted as
   addresses. This list can be empty. The presence of elements in the
   address list denotes that the call control engine should use the
   specified address instead of determining it independently. If no
   addresses are provided the call control engine is requested to
   determine an address independently.

   The FORWARD command can be used instead of REDIRECT when the end
   system acts as a application layer proxy that decides which calls are
   allowed to be forwarded. The forwarding can either happen with the
   call control protocol's implicit semantics (e.g. SIP forwarding) or
   the controller can explicitely specify the forwarding address.

   The call control engine will send CONNECTED or REJECTED responses to
   inform the local controller of the result of the forwarding process.

   Syntax:   conf.call-control.forward (ref addr-list)


   Parameters: string ref
               Identifies the call the FORWARD command refers to.

               list of string addr-list
               List of (optional) address specifying where the call
               should be forwarded to.

FORWARDED


   The FORWARDED command is sent by the call control engine to the local
   controller to indicate that the specified call has been forwarded to
   the specified address. The local controller can decide whether the
   call setup should continue or be interrupted (by sending a DISCONNECT
   command).

   Syntax:   conf.call-control.forwarded (ref addr-list)


   Parameters: string ref
               Identifies the call the FORWARD command refers to.

Ott/Perkins/Kutscher                                           [Page 20]


INTERNET-DRAFT  The Message Bus: Messages and Procedures       June 1999

               list of string addr-list
               List of (optional) address specifying where the call has
               been forwarded to.

RELAYED


   The RELAYED command is sent by the local controller to the call
   control engine to indicate that the specified incoming call is being
   forwarded to the specified address via another call control engine
   (e.g. from another protocol of administrative domain).

   One scenario would be a incoming call that has been signalled by a
   SIP call control engine with a INCOMING-CALL command and that is then
   relayed to the H.323 call control engine. The local controller would
   inform the SIP engine by sending the RELAYED command.

   A notation for address specifications (h323:bla) needs to be defined.

   Syntax:   conf.call-control.relayed (ref addr)


   Parameters: string ref
               Identifies the call this command refers to.

               string addr-list
               The address specifying where the call has been relayed
               to.




   Further messages deemed necessary for the basic call control message
   set include the following

   o    PROGRESS

   The details of these as well as further messages are to be defined.

4.4.2.  Running a conference


   Floor control, chairperson, policy, membership lists, etc.

4.4.2.1.  Floor control


owner


   Syntax:     conf.floor.owner (cname)

   Parameters: cname

Ott/Perkins/Kutscher                                           [Page 21]


INTERNET-DRAFT  The Message Bus: Messages and Procedures       June 1999

               The cname parameter designates the participant who the
               floor is currently assigned to.

need


   Syntax:     conf.floor.need ()

   Parameters: --
               Used to indicate that an application entity is requesting
               the floor for its media session.

release


   Syntax:     conf.floor.release ()

   Parameters: --
               Used to indicate that an application entity is releasing
               the floor it formerly requested for its media session.

4.4.2.2.  Membership Lists


   Commands starting with conf.member are intented for general messages
   on membership information that are not bound to specific transport
   protocols such as RTP[4].

   Syntax:     conf.member.list ([cname]*)

   Parameters: cname
               The cname parameter designates the current list of
               members.


   Syntax:     conf.member.add (cname)

   Parameters: cname
               The cname parameter designates a member that has joined
               the session.


   Syntax:     conf.member.remove (cname)

   Parameters: cname
               The cname parameter designates a member that has left the
               session.

4.4.2.3.  Conference Chairs


   Syntax:     conf.chair (cname)


Ott/Perkins/Kutscher                                           [Page 22]


INTERNET-DRAFT  The Message Bus: Messages and Procedures       June 1999

   Parameters: cname
               The cname parameter designates the current conference
               chair.

4.5.  Status commands


   The following table lists a few commands for generic status and error
   reports:

               +-------------+------------------------------+
               |Command      | Description of command class |
               +-------------+------------------------------+
               |status.error | Error messages               |
               |status.warn  | Warnings                     |
               |status.info  | Informational messages       |
               +-------------+------------------------------+
                       Table 8: Mbus Status commands

   All status commands MUST be used with at least one string parameter
   that contains information on the status reported. Status commands MAY
   be used with arbitrary command prefixes. This allows classification
   of status commands where appropriate. E.G. status commands that are
   related to call-control MAY be prefixed with conf.call-control
   allowing an receiving entity to treat the command as a call-control
   specific status command. By convention status commands may have
   additional application specific parameters that are only useful if
   the concrete application context is known from the command prefix.

   All status messages SHOULD be sent unreliably to all entities.


5.  Authors' Addresses


   Joerg Ott <jo@tzi.org>
   Universitaet Bremen, TZI, MZH 5180
   Bibliothekstr. 1
   D-28359 Bremen
   Germany
   voice +49 421 201-7028
   fax +49 421 218-7000


   Colin Perkins <c.perkins@cs.ucl.ac.uk>
   Department of Computer Science
   University College London
   Gower Street
   London WC1E 6BT
   United Kingdom




Ott/Perkins/Kutscher                                           [Page 23]


INTERNET-DRAFT  The Message Bus: Messages and Procedures       June 1999

   Dirk Kutscher <dku@tzi.org>
   Universitaet Bremen, TZI, MZH 5160
   Bibliothekstr. 1
   D-28359 Bremen
   Germany
   voice +49 421 218-7595
   fax +49 421 218-7000


6.  References


   [1]  S. Bradner, ``Key words for use in RFCs to Indicate Requirement
        Levels'' RFC 2119, March 1997

   [2]  J. Ott, C. Perkins, and D. Kutscher, ``Requirements for Local
        Conference Control'', Internet Draft draft-ott-mmusic-mbus-
        req-00.txt, Work in Progress, June 1999.

   [3]  J. Ott, C. Perkins, and D. Kutscher, ``A Message Bus for
        Conferencing Systems'', Internet Draft draf-ott-mmusic-mbus-
        transport-00.txt, Work in Progress, June 1999.

   [4]  H. Schulzrinne, S. Casner, R. Frederick, V. Jacobson, ``RTP: A
        Transport Protocol for Real-Time Applications,'' RFC 1889,
        January 1996.




























Ott/Perkins/Kutscher                                           [Page 24]


INTERNET-DRAFT  The Message Bus: Messages and Procedures       June 1999

Appendix A: RTP specific commands


   The following commands are used to provide information about an
   RTP[4] media source. Each source in media sessions is identified by
   its SSRC (not by the CNAME, since this would not be unique).
   Correlation to CNAMEs for cross-media references (eg: for lip-
   synchronization) has to be done by receiving entities.


   rtp.ssrc (ssrc)
        Sent to inform an entity of the SSRC it is to use for the
        remainder of the session.

   rtp.source.exists (ssrc validityTime)
        The rtp.source.exists command is sent by a media engine to
        assert that a particular source is present in a session. The
        validityTime parameter is the time for which that source should
        be considered valid, in seconds.  If another rtp.source.exists
        command has not been received for that source within this time
        period, the source is implicitly timed out. The validityTime
        SHOULD be three times the RTCP reporting interval for that
        session.

   rtp.source.remove (ssrc)
        The rtp.source.remove command is used to indicate that a source
        has left the session.

   rtp.source.name  (ssrc name)
   rtp.source.email (ssrc email)
   rtp.source.phone (ssrc phone)
   rtp.source.loc   (ssrc loc)
   rtp.source.tool  (ssrc tool)
   rtp.source.note  (ssrc note)
   rtp.source.cname (ssrc cname)

        These commands are used to pass RTCP SDES information from a
        media engine to a user interface. If sent to a media engine,
        they have no effect unless the ssrc field is the SSRC of that
        engine, in which case the are used to change the SDES
        information being transmitted by that media engine.

   rtp.source.reception (ssrc packetsRecv packetsLost packetsMisordered
        jitter validityTime)
        This command is used to pass RTCP RR information from a media
        engine to a user interface. The total number of packets
        received, lost and misordered are sent, together with the
        network timing jitter in milliseconds and a validity time for
        this report in seconds.


   rtp.source.packet.loss (dest_ssrc src_ssrc loss% validityTime)
        Sent by a media engine to indicate the instantaneous packet loss

Ott/Perkins/Kutscher                                           [Page 25]


INTERNET-DRAFT  The Message Bus: Messages and Procedures       June 1999

        observed between two sources. The validityTime for this report
        is in milliseconds.

   rtp.source.active   (ssrc validityTime)
   rtp.source.inactive (ssrc)

        The rtp.source.active command indicates that a source is
        transmitting data into the session. The validityTime field
        indicates the period for which this source should be considered
        active, in milliseconds.  The source.inactive command indicates
        that the source has stopped transmitting.

   rtp.source.mute (ssrc muteState)
        The rtp.source.mute command indicates that a source is to be
        muted/ unmuted. The value of the muteState parameter is 0 to
        indicate unmuted, and 1 to indicate muted.

   rtp.source.packet.duration (ssrc packetDuration)
        Sent by a media engine to indicate the duration, in
        milliseconds, of packets received from a source. This may be
        used to control the duration of packets sent by a media engine,
        if sent to that engine with the cname of the engine.

   rtp.source.codec (ssrc codec)
        Sent by a media engine to indicate the codec in use by a source.

   rtp.source.playout (ssrc playoutDelay)
        Sent by a media engine to indicate the playout delay, in
        milliseconds, for a source (that is, end-to-end time from
        capture to playout).  This allows for lip-synchronization
        between audio and video streams.























Ott/Perkins/Kutscher                                           [Page 26]


INTERNET-DRAFT  The Message Bus: Messages and Procedures       June 1999

Appendix B: Media Engine Control Commands -- Audio


   The following commands are used to control the playout of data by the
   media engine. If sent to a media engine they change output settings,
   if sent by a media engine they inform other entities of the state of
   the output.

   It is expected that a similar set of commands will be defined to
   control the operation of other media engines (eg: video, shared-
   workspace).

   audio.output.gain (gain)
        Select gain (volume) of the output device.

   audio.output.port (port)
        Select port to be used. The allowable values for the port are
        ``speaker'', ``line'' and ``headset''.

   audio.output.mute (muteState)
        Mute (if muteState is 1) or unute (if muteState is 0) the
        output.

   audio.output.powermeter (level%)
        Sent from media engine to user interface to control display of
        audio power meters. Level is specified as a percentage.

   audio.output.agc (state)
        Enable/disable automatic gain control for output.  state is 1 to
        enable, 0 to disable.

   audio.output.synchronize (state entity)
        Enable synchronization with the Mbus entity specified. This uses
        the rtp.source.playout messages to achieve synchronization (eg:
        lip-sync between audio and video). The value of state is 0 to
        disable synchronization, 1 to enable. Note that this option may
        conflict with the audio.output.playout.delay.*  commands.

   audio.input.gain (gain)
   audio.input.port (port)
   audio.input.mute (muteState)
   audio.input.powermeter (level)
   audio.input.agc (agcState)

        These commands function in a manner analogous to the
        audio.output.*  commands, but affect input of data by the media
        engine.

   audio.codec.describe (codec name clock-rate channels)
        Describe a codec. The codec parameter is an opaque codec
        identifier. The name parameter provides a human readable
        description of the codec, clock-rate and channels are self
        explanatory.

Ott/Perkins/Kutscher                                           [Page 27]


INTERNET-DRAFT  The Message Bus: Messages and Procedures       June 1999

        For an RTP-based media tool, the codec parameter will be an RTP
        payload type number.




   audio.codec.supported.tx (codec ...)
   audio.codec.supported.rx (codec ...)

        Indicates that the specified codec(s) are supported by this
        media engine. This list includes basic codecs only, repair
        schemes such as redundancy, FEC and interleaving are not
        included in the response.

        The audio.codec.supported.tx indicates codecs which this media
        engine can transmit. The audio.codec.supported.rx indicates
        codecs which are supported for reception.

   audio.codec.supported.tx.request
   audio.codec.supported.rx.request

        Request that the recipient responds with a codec.supported
        message.

   audio.codec (codec)
        If sent to a media engine, select the codec to be used when
        transmitting. If sent by a media engine, indicates the codec
        being used when transmitting.

   audio.suppress.silence (state)
        Enable/disable silence suppression on the input signal. State is
        1 to enable, 0 to disable.

   audio.channel.coding (coding parameters ...)
        Indicate the channel coding scheme to be used. The following
        values are currently defined for coding

        none            - No special channel coding.

        redundant       - RFC2198 redundancy

        interleaved     - Interleaving.

        Others may be defined in future.

        The parameters section is specific to a particular encoding. For
        redundant coding, parameters are a series of codec, offset
        pairs.  For interleaved coding, the number of blocks and block
        separation are parameters.

   audio.channel.repair (repair-scheme)
        Indicate the loss-repair scheme to be used at the receiver. The
        following repair schemes are currently defined:

Ott/Perkins/Kutscher                                           [Page 28]


INTERNET-DRAFT  The Message Bus: Messages and Procedures       June 1999

        -    repetition

        -    pattern-match

        Others may be defined in future.

















































Ott/Perkins/Kutscher                                           [Page 29]


INTERNET-DRAFT  The Message Bus: Messages and Procedures       June 1999

Appendix C: Security commands


   security.encryption.algorithm (algorithm)
   security.encryption.key (key)

        Specify the encryption algorithm and key to be used.  The
        algorithm defaults to ``DES'' if not specified.

   security.encryption (state)
        Enable/disable encryption. Enable if state is 1, disable if
        state is 0. A security.encryption.key command MUST be sent
        before this command.









































Ott/Perkins/Kutscher                                           [Page 30]


INTERNET-DRAFT  The Message Bus: Messages and Procedures       June 1999

Appendix D: Tool configuration


   IETF conferencing tools so far make use of a variety of to some
   degree standardized command line options, including (but not limited
   to) the following:

              tool -C conference -N name ... mc-addr/port/ttl

   In order to accommodate the aforementioned boot phase procedures, the
   following parameters with the associated behavior are needed:

   -wait [condition]Wait for a particular event to happen; if no
                   condition is specified on the command line, an
                   internally pre-defined condition is used.  The
                   application entity is supposed to start emitting
                   self-announcements to the Mbus but should defer any
                   further action (with respect to self-configuration
                   and media exchange) until the condition is met.

                   Besides waiting for the indicated condition to occur,
                   an application entity is required to react to Mbus
                   configuration requests directed to it, since their
                   execution may be a prerequisite for it eventually
                   receiving the ``GO'' message.

   -nowait         Start immediately.

   Examples

   rat                 Start and go ahead with whatever you want to do.

   rat -wait ...       Takes default condition: Wait for the own user
                       interface to show up and configure the engine.

   rat -wait controllerWait for a local conference controller to show up
                       and make the application continue.

















Ott/Perkins/Kutscher                                           [Page 31]


Html markup produced by rfcmarkup 1.129c, available from https://tools.ietf.org/tools/rfcmarkup/