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Versions: 00 01 02 03 04 05 RFC 3259

Network Working Group                                                Ott
Internet-Draft                                  TZI, Universitaet Bremen
Expires: January 12, 2001                                        Perkins
                                      USC Information Sciences Institute
                                                TZI, Universitaet Bremen
                                                           July 14, 2000

                  A Message Bus for Local Coordination

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 are draft documents valid for a maximum of six
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     The list of current Internet-Drafts can be accessed at

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   This Internet-Draft will expire on January 12, 2001.

Copyright Notice

   Copyright (C) The Internet Society (2000). All Rights Reserved.


   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 Message Bus (Mbus) provides a means to achieve the

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   necessary amount of coordination between co-located conferencing
   applications for virtually any type of conference as postulated in a
   a companion requirement document[11]. The Message Bus comprises two
   logically distinct parts: a message transport infrastructure and a
   set of common as well as protocol/ media/tool-specific messages
   along with a conference-specific addressing scheme. This document
   deals with message addressing, transport, and security issues and
   defines the message syntax for the Mbus.  It does not define
   application oriented semantics and procedures for using the message
   bus. Application specific command sets and procedures for
   applications using the Mbus are expected to be defined in follow-up

   This document is intended for discussion in 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.

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Table of Contents

   1.    Introduction . . . . . . . . . . . . . . . . . . . . . . . .  5
   1.1   Background . . . . . . . . . . . . . . . . . . . . . . . . .  5
   1.2   Purpose  . . . . . . . . . . . . . . . . . . . . . . . . . .  5
   1.3   Terminology for requirement specifications . . . . . . . . .  5
   2.    General Outline  . . . . . . . . . . . . . . . . . . . . . .  6
   3.    Message Format . . . . . . . . . . . . . . . . . . . . . . .  8
   4.    Addressing . . . . . . . . . . . . . . . . . . . . . . . . . 10
   4.1   Mandatory Address Elements . . . . . . . . . . . . . . . . . 11
   5.    Reliability  . . . . . . . . . . . . . . . . . . . . . . . . 12
   6.    Transport  . . . . . . . . . . . . . . . . . . . . . . . . . 14
   7.    Message Syntax . . . . . . . . . . . . . . . . . . . . . . . 16
   7.1   Message Encoding . . . . . . . . . . . . . . . . . . . . . . 16
   7.2   Message Header . . . . . . . . . . . . . . . . . . . . . . . 16
   7.3   Command Syntax . . . . . . . . . . . . . . . . . . . . . . . 16
   8.    Awareness of other Entities  . . . . . . . . . . . . . . . . 19
   8.1   Hello Message Transmission Interval  . . . . . . . . . . . . 19
   8.1.1 Calculating the Interval for Hello Messages  . . . . . . . . 20
   8.1.2 Initialization of Values . . . . . . . . . . . . . . . . . . 21
   8.1.3 Adjusting the Hello Message Interval when the Number of
         Entities increases . . . . . . . . . . . . . . . . . . . . . 21
   8.1.4 Adjusting the Hello Message Interval when the Number of
         Entities decreases . . . . . . . . . . . . . . . . . . . . . 21
   8.1.5 Expiration of hello timers . . . . . . . . . . . . . . . . . 22
   8.2   Calculating the Timeout for Hello Messages . . . . . . . . . 22
   9.    Messages . . . . . . . . . . . . . . . . . . . . . . . . . . 23
   9.1   mbus.hello . . . . . . . . . . . . . . . . . . . . . . . . . 23
   9.2   mbus.bye . . . . . . . . . . . . . . . . . . . . . . . . . . 23
   9.3   mbus.ping  . . . . . . . . . . . . . . . . . . . . . . . . . 24
   9.4   mbus.quit  . . . . . . . . . . . . . . . . . . . . . . . . . 24
   9.5   mbus.waiting . . . . . . . . . . . . . . . . . . . . . . . . 24
   9.6   mbus.go  . . . . . . . . . . . . . . . . . . . . . . . . . . 25
   10.   Constants  . . . . . . . . . . . . . . . . . . . . . . . . . 26
   11.   Mbus Security  . . . . . . . . . . . . . . . . . . . . . . . 27
   11.1  Security Model . . . . . . . . . . . . . . . . . . . . . . . 27
   11.2  Message Authentication . . . . . . . . . . . . . . . . . . . 27
   11.3  Encryption . . . . . . . . . . . . . . . . . . . . . . . . . 28
   12.   Mbus Configuration . . . . . . . . . . . . . . . . . . . . . 29
   12.1  File based parameter storage . . . . . . . . . . . . . . . . 30
   12.2  Registry based parameter storage . . . . . . . . . . . . . . 31
   13.   Security Considerations  . . . . . . . . . . . . . . . . . . 33
   14.   IANA Considerations  . . . . . . . . . . . . . . . . . . . . 34
         References . . . . . . . . . . . . . . . . . . . . . . . . . 35
         Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 36
   A.    Mbus Addresses for Conferencing  . . . . . . . . . . . . . . 37
         Full Copyright Statement . . . . . . . . . . . . . . . . . . 39

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1. Introduction

1.1 Background

   The requirement specification as defined in the requirements
   draft[11] 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 document.

1.2 Purpose

   Two components constitute the Message Bus: the (lower level) message
   passing mechanisms and the (higher level) messages and their
   semantics along with their addressing scheme.

   The purpose of this document is to define the characteristics of the
   lower level Mbus message passing mechanism which is common to all
   Mbus implementations.  This includes the specification of

   o  the generic Mbus message format;

   o  the addressing concept for application entities (note that
      addressing details are defined by the application environment);

   o  the transport mechanisms to be employed for conveying messages
      between (co-located) application entities;

   o  the security concept to prevent misuse of the Message Bus (as
      taking control of another user's conferencing environment);

   o  the details of the Mbus message syntax; and

   o  a set of mandatory application independent commands that are used
      for bootstrapping Mbus sessions.

1.3 Terminology for requirement specifications

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   and "OPTIONAL" are to be interpreted as described in RFC 2119[1] and
   indicate requirement levels for compliant Mbus implementations.

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2. General Outline

   The Mbus is supposed to operate in a variety of scenarios as
   outlined in the companion requirement document[11]. From these
   scenarios, the following (minimum) requirements are derived that
   have to be met by the Mbus design to provide a suitable local
   communication infrastructure.

   Local coordination involves a widely varying number of entities:
   some messages (such as membership information, floor control
   notifications, dissemination conference state changes, etc.) may
   need to be destined for all local application entities. Messages may
   also be targeted at a certain application class (e.g. all
   whiteboards or all audio tools) or agent type (e.g. all user
   interfaces rather than all media engines).  Or there may be any
   (application- or message- specific) subgrouping defining the
   intended recipients, e.g. messages related to media synchronization.
   Finally, there will be messages that are directed to a single
   entity, for example, specific configuration settings that a
   conference controller sends to a application entity or
   query-response exchanges between any local server and its clients.

   The Mbus concept as presented here satisfies these different
   communication models by defining different message transport
   mechanisms (defined in Section 6) and by providing a flexible
   addressing scheme (defined in Section 4).

   Furthermore, Mbus messages exchanged between application entities
   may have different reliability requirements (which are typically
   derived from their semantics).  Some messages will have a rather
   informational character conveying ephemeral state information (which
   is refreshed/updated periodically), such as the volume meter level
   of an audio receiver entity to be displayed by its user interface
   agent.  Certain Mbus messages (such as queries for parameters or
   queries to local servers) may require a response from the peer(s)
   thereby providing an explicit acknowledgment at the semantic level
   on top of the Mbus.  Other messages will modify the application or
   conference state and hence it is crucial that they do not get lost.
   The latter type of message has to be delivered reliably to the
   recipient, whereas message of the first type do not require
   reliability mechanisms at the Mbus transport layer. For messages
   confirmed at the application layer it is up to the discretion of the
   application whether or not to use a reliable transport underneath.

   In some cases, application entities will want to tailor the degree
   of reliability to their needs, others will want to rely on the
   underlying transport to ensure delivery of the messages -- and this
   may be different for each Mbus message.  The Mbus message passing
   mechanism described in this paper provides a maximum of flexibility

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   by providing reliable transmission achieved through transport-layer
   acknowledgments (in case of point-to-point communications only) as
   well as unreliable message passing (for unicast, local multicast,
   and local broadcast).  We address this topic in Section 4.

   Finally, accidental or malicious disturbance of Mbus communications
   through messages originated by applications from other users needs
   to be prevented.  Accidental reception of Mbus messages from other
   users may occur if either two users share the same workstation for
   conferencing or are using end systems spread across the same
   physical network: in either case, the Mbus multicast address and the
   port number may match leading to reception of the other party's Mbus
   messages in addition to a user's own ones.  Malicious disturbance
   may happen because of applications multicasting (e.g. at a global
   scope) or unicasting Mbus messages (which could contain a
   "conf.terminated" command).  To eliminate the possibility of
   receiving bogus Mbus messages, the Mbus protocol contains message
   digests for authentication.  Furthermore, the Mbus allows for
   encryption to ensure privacy and thus enable using the Mbus for
   local key distribution and other functions potentially sensitive to
   eavesdropping.  This document defines the framework for configuring
   Mbus applications with regard to security parameters in Section 12.

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3. Message Format

   A Mbus message comprises a header and a body. The header is used to
   indicate how and where a message should be delivered, the body
   provides information and commands to the destination entity. The
   following information is included in the header:

      The MsgDigest is a Base64-encoded (see RFC1521[5]) calculated
      hash value of the entire message (starting from the ProtocolID
      field) as described in Section 11 and Section 12.

      A fixed ProtocolID field identifies the version of the message
      bus protocol used. The protocol defined in this document is
      "mbus/1.0" (case-sensitive).

      A sequence number (SeqNum) is contained in each message. The
      first message sent by a source SHOULD have SeqNum equal to zero,
      and it MUST increment by one for each message sent by that
      source. A single sequence number is used for all messages from a
      source, irrespective of the intended recipients and the
      reliability mode selected. SeqNums are decimal numbers in ASCII

      The TimeStamp field is also contained in each message and SHOULD
      contain a decimal number representing the time at message
      construction in seconds since 00:00:00, UTC, January 1, 1970.

      A MessageType field indicates the kind of message being sent.
      The value "R" indicates that the message is to be transmitted
      reliably and MUST be acknowledged by the recipient, "U" indicates
      an unreliable message which MUST NOT be acknowledged.

      The SrcAddr field identifies the sender of a message. This MUST
      be a complete address, with all address elements specified. The
      addressing scheme is described in Section 4.

      The DestAddr field identifies the intended recipient(s) of the
      message. This field MAY contain wildcards by omitting address
      element and hence address any number (including zero) of
      application entities. The addressing scheme is described in
      Section 4.

      The AckList field comprises a list of SeqNums for which this
      message is an acknowledgment. See Section 5 for details.

   The header is followed by the message body which contains one or
   more commands to be delivered to the destination entity. The syntax
   for a complete message is given in Message syntax (Section 7).

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   If multiple commands are contained within the same Mbus message
   payload, they MUST to be delivered to the Mbus application in the
   same sequence in which they appear in the message payload.

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4. Addressing

   Each entity on the message bus SHOULD respond to messages sent to
   one (or more) addresses. Addresses are sequences of address elements
   that are tag/value pairs. The tag and the value are separated by a
   colon and tag/value pairs are separated by whitespace, like this:

            (tag:value tag:value ...)

   The formal ABNF syntax definition for Mbus addresses and their
   elements is as follows:

            mbus_address    = "(" *address_element ")"
            address_element = *WSP address_tag ":" address_value *WSP
            address_tag     = 1*32(ALPHA)
            address_value   = 1*64(%x21-7F)
                              ; any 7-bit US-ASCII character
                              ; excluding white space
                              ; and control characters

    Each entity has a fixed sequence of address elements constituting
   its address and MUST only process messages sent to addresses that
   either match all elements or consist of a subset of its own address
   elements.  Each element value in this subset must match the
   correspoding value of the receiver's address element value. The
   order of address elements in an address sequence is not relevant.
   For example, an entity with an address of:

   (conf:test media:audio module:engine app:rat id:4711-1@

    will process messages sent to

   (media:audio module:engine)



    but must ignore messages sent to

   (conf:test media:audio module:engine app:rat id:123-4@ foo:bar)



   A message that should be processed by all entities requires an empty
   set of address elements.

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4.1 Mandatory Address Elements

   Each Mbus entity MUST provide one mandatory address element that
   allows to identify the entity. The element name is "id" and the
   value MUST be be composed of the following components:

   o  The IP address of the interface that is used for sending messages
      to the Mbus. For IPv4 this the address in decimal dotted
      notation. For IPv6 the interface-ID-part of an address in textual
      representation as specified in [3] MUST be used. In this
      specification, this part is called the "host-ID".

   o  An identifier ("entity-ID") that is unique within the scope of
      single host-ID. The entity comprises two parts. For systems where
      the concept of a process ID is applicable it is RECOMMENDED this
      identifier be composed using a process-ID and a per-process
      disambiguator for different Mbus entities of a process. If a
      process ID is not available, this part of the entity-ID may be
      randomly chosen (it is recommended that at least a 32 bit random
      number is chosen). Both numbers are represented in decimal
      textual form and MUST be separated by a '-' character.

   Note that the entity-ID cannot be the port number of the endpoint
   used for sending messages to the Mbus because implementations MAY
   use the common Mbus port number for sending to and receiving from
   the multicast group (as specified in Section 6). The total
   identifier has the following structure:

              id-element   = "id:" id-value
              id-value     = entity-id "@" host-id
              entity-id    = 1*10DIGIT "-" 1*5DIGIT
              host-id      = (IPv4address / IPv6address)

    Please refer to [3] for productions of IPv4address and IPv6address.

   An example for an id element:


   A set of the address elements that are to be used by conferencing
   applications is specified in "Mbus Addresses for Conferencing"
   (Appendix A).

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5. Reliability

   While most messages are expected to be sent using unreliable
   transport, it may be necessary to deliver some messages reliably.
   Reliability can be selected on a per message basis by means of the
   MessageType field.  Reliable delivery is supported for messages with
   a single recipient only; i.e., all components of the DestAddr field
   have to be specified. An entity can thus only send reliable messages
   to known addresses, i.e. it can only send reliable messages to
   entities that have announced their existence on the Mbus (e.g. by
   means of mbus.hello() messages (Section 9.1)). A sending entity MUST
   NOT send a message reliably if the target address is not unique.
   (See Transport (Section 6) for the specification of an algorithm to
   determine whether an address is unique.) A receiving entity MUST
   only process and acknowledge a reliable message if the destination
   address exactly matches its own source address (the destination
   address MUST NOT be a subset of the source address).

   Disallowing reliable message delivery for messages sent to multi-
   ple destinations is motivated by simplicity of the implementation as
   well as the protocol.  Although ACK implosions are not really an
   issue and losses are rare, achieving reliability for such messages
   would require full knowledge of the membership for each subgroup
   which is deemed too much effort.

   Each message is tagged with a message sequence number.  If the
   MessageType is "R", the sender expects an acknowledgment from the
   recipient within a short period of time.  If the acknowledgment is
   not received within this interval, the sender SHOULD retransmit the
   message (with the same message sequence number), increase the
   timeout, and restart the timer. Messages MUST be retransmitted a
   small number of times (see below) before the recipient is considered
   to have failed.  If the message is not delivered successfully, the
   sending application is notified.  In this case, it is up to this
   application to determine the specific action(s) (if any) to be

   Reliable messages are acknowledged by adding their SeqNum to the
   AckList field of a message sent to the originator of the reliable
   message.  Multiple acknowledgments MAY be sent in a single message.
   It is possible to either piggy-back the AckList onto another message
   sent to the same destination, or to send a dedicated acknowledgment
   message, with no commands in the message payload part.

   The precise procedures are as follows:

   Sender: A sender A of a reliable message M to receiver B SHOULD
      transmit the message via multicast or via unicast, keep a copy of
      M, initialize a retransmission counter N to '1', and start a

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      retransmission timer T (initialized to T_r). If an acknowledgment
      is received from B, timer T MUST BE cancelled and the copy of M
      is discarded.  If T expires, the message M SHOULD BE
      retransmitted, the counter N SHOULD BE incremented by one, and
      the timer SHOULD BE restarted (set to N*T_r).  If N exceeds the
      retransmission threshold N_r, the transmission is assumed to have
      failed, further retransmission attempts MUST NOT be undertaken,
      the copy of M SHOULD BE discarded, and the sending application
      SHOULD BE notified.

   Receiver: A receiver B of a reliable message from a sender A SHOULD
      acknowledge receipt of the message within a time period T_c <
      T_r.  This MAY be done by means of a dedicated acknowledgment
      message or by piggy-backing the acknowledgment on another message
      addressed only to A.

   Receiver optimization: In a simple implementation, B may choose to
      immediately send a dedicated acknowledgment message.  However,
      for efficiency, it could add the SeqNum of the received message
      to a sender-specific list of acknowledgments; if the added SeqNum
      is the first acknowledgment in the list, B SHOULD start an
      acknowledgment timer TA (initialized to T_c).  When the timer
      expires, B SHOULD create a dedicated acknowledgment message and
      send it to A.  If B is to transmit another Mbus message addressed
      only to A, it should piggy-back the acknowledgments onto this
      message and cancel TA.  In either case, B should store a copy of
      the acknowledgment list as a single entry in the per- sender copy
      list, keep this entry for a period T_k, and empty the
      acknowledgment list.  In case any of the messages kept in an
      entry of the copy list is received again from A, the entire
      acknowledgment list stored in this entry is scheduled for
      (re-)transmission following the above rules.

   Constants and Algorithms: The following constants and algorithms
      SHOULD be used by implementations:





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6. Transport

   All messages are transmitted as UDP messages with two ways of
   sending messages being possible:

   1.  Local multicast (host-local or link-local, see Mbus
       configuration (Section 12)) to a fixed, yet to be assigned (see
       Section 14) link-local address of the administratively scoped
       multicast space as described in RFC 2365[10]. There will also be
       a fixed, registered port number that all Mbus entities MUST use.
       Until the address and port numer are assigned,
       is used as the multicast address and 47000 (decimal) as the port

   2.  Directed unicast (via UDP) to the port of a specific
       application. This still requires the DestAddr field to be filled
       in properly.  Directed unicast is intended for situations where
       node local multicast is not available. It MAY also be used by
       Mbus implementations for delivering messages addressed at a
       single application entity only -- the address of which the Mbus
       implementation has learned from other message exchanges before.
       Every Mbus entity SHOULD use a unique endpoint address for every
       message it sends to the Mbus multicast group or to individual
       receiving entities. A unique endpoint address is a tuple
       consisting of the entity's IP address and a port number, where
       the port number is different from the standard Mbus port number
       (yet to be assigned, see Section 14). When multicast is
       available, messages MUST only be sent via unicast if the Mbus
       target address is unique and if the sending entity can verify
       that the receiving entity uses a unique endpoint address. The
       latter can be verified by considering the last message received
       from that entity. (Note that several Mbus entities, say within
       the same process, may share a common transport address; in this
       case, the contents of the destination address field is used to
       further dispatch the message. Given the definition of "unique
       endpoint address" above the use of a shared endpoint address and
       a dispatcher still allows other Mbus entities to send unicast
       messages to one of the entities that share the endpoint address.
       So this can be considered an implementation detail.) When
       multicast is not available messages can be sent via unicast but
       all messages that do not contain a unique target address MUST be
       sent to all known entities via unicast. Messages with an empty
       target address list MUST always be sent to all Mbus entities
       (via multicast if available).
       The following algorithm can be used by sending entities to
       determine whether a Mbus address is unique considering the
       current set of Mbus entities:

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                    let ta=the target address;
                    iterate through the set of all
                    currently known Mbus addresses {
                      let ti=the address in each iteration;
                      count the addresses for which
                      the predicate isSubsetOf(ta,ti) yields true;

          If the count of matching addresses is exactly 1 the address
          is unique. The following algorithm can be used for the
          predicate isSubsetOf, that checks whether the second message
          matches the first according to the rules specified in Section
          4. (A match means that a receiving entity that uses the
          second Mbus address must also process received messages with
          the first address as a target address.

                    isSubsetOf(addr a1,a2) yields true, iff
                      every address element of a1 is contained
                      in a2's address element list

          An address element is contained in an address element list if
          the list contains an element that provides same values for
          the two address element fields key and value.

   If a single application system is distributed across several
   co-located hosts, link local scope SHOULD be used for multicasting
   Mbus messages that potentially have recipients on the other hosts.
   The Mbus protocol is not intended (and hence deliberately not
   designed) for communication between hosts not on the same link.

   Since messages are transmitted in UDP datagrams, a maximum size of
   64 KBytes MUST NOT be exceeded. It is RECOMMENDED that applications
   using a non host-local scope do not exceed a message size of the
   network's MTU.

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7. Message Syntax

7.1 Message Encoding

   All messages MUST use the UTF-8 character encoding. Note that US
   ASCII is a subset of UTF-8 and requires no additional encoding, and
   that a message encoded with UTF-8 will not contain zero bytes.

   Each Message MAY be encrypted using a secret key algorithm as
   defined in Section 11.

7.2 Message Header

   A message starts with the header. The first field in the header is
   the message digest calculated using a keyed hash algorithm as
   described in Section 11 followed by a newline character. The other
   fields in the header are separated by white space characters, and
   followed by a newline. The format of the header is as follows:

   msg_header = MsgDigest LF "mbus/1.0" 1*WSP SeqNum 1*WSP TimeStamp 1*WSP
                MessageType 1*WSP SrcAddr 1*WSP DestAddr 1*WSP AckList

   The header fields are explained in Message Format (Section 3). Here
   are the ABNF syntax definitions for the header fields:

              MsgDigest   = base64
              SeqNum      = 1*DIGIT
              TimeStamp   = 1*DIGIT
              MessageType = "R" / "U"
              ScrAddr     = mbus_address
              DestAddr    = mbus_address
              AckList     = "(" *(1*DIGIT)) ")"

    The syntax definition of a complete message is as follows:

              mbus_message = msg_header *1(LF msg_payload)
              msg_payload  = mbus_command *(LF mbus_command)

    See Figure 18 for the definition a Mbus command.

7.3 Command Syntax

   The header is followed by zero, or more, commands to be delivered to
   the application(s) indicated by the DestAddr field. Each message
   comprises a command followed by a list of zero, or more, parameters,
   and is followed by a newline.

              command ( parameter parameter ... )

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   Syntactically, the command name MUST be a `symbol' as defined in the
   following table. The parameters MAY be any data type drawn from the
   following table:

   |DataType | Syntax                  | Description                    |
   |val      | (Integer / Float /      |                                |
   |         | String / List Symbol    |  a value can be of one of      |
   |         | Data)                   |  these types                   |
   |         |                         |                                |
   |Integer  | "-" 1*DIGIT             |                                |
   |Float    | "-" 1*DIGIT "." 1*DIGIT |                                |
   |String   | DQUOTE *CHAR DQUOTE     | See below for escape characters|
   |         |                         |                                |
   |List     | "(" *(val               |                                |
   |         | *(WSP val)) ")"         |                                |
   |         |                         |                                |
   |Symbol   | ALPHA *(ALPHA / DIGIT / | A predefined protocol value    |
   |         | "_" / "-" / ".")        |                                |
   |         |                         |                                |
   |Data     | "<" *base64 ">"         |  Opaque Data                   |

   Boolean values are encoded as an integer, with the value of zero
   representing false, and non-zero representing true (as in the `C'
   programming language).

   String parameters in the payload MUST be enclosed in the double
   quote ('') character. Within strings, the escape character is the
   backslash (\), and the following escape sequences are defined:

              |Escape Sequence |  Meaning  |
              |      \\        |    \      |
              |      \"        |     "     |
              |      \n        | newline   |

   List parameters do not have to be homogeneous lists, i.e. they can
   contain parameters of varying types.

   Opaque data is represented as Base64-encoded (see RFC1521[5])
   character strings surrounded by "< " and "> "

   The ABNF syntax definition for Mbus commands is as follows:

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              mbus_command = command_name arglist
              command_name = ALPHA *(ALPHA / DIGIT / "_" / ".")
              arglist      = "(" *(*WSP parameter *WSP) ")"
              parameter    = Integer / Float / String / List
                             Symbol / Data

   Command names SHOULD 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 Addressing (Section 4)
   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
   SHOULD be unambiguous in a way that it is possible to interpret or
   ignore 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 Messages (Section 9).

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8. Awareness of other Entities

   Before Mbus entities can communicate with one another, they need to
   mutually find out about their 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. Furthermore entities need
   to be able to to notice the failure (or leaving) of other entities.

   Any Mbus entity MUST 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: Entities should always
   become aware of other entities independent of the order of starting.

   Each Mbus entity MUST maintain the number of Mbus session members
   and continously update this number according to any observed
   changes. The mechanisms of how the existence and the leaving of
   other entities can be detected are dedicated Mbus messages for
   entity awareness: mbus.hello (Section 9.1) and mbus.bye (Section
   9.2). Each Mbus protocol implementation MUST periodically send
   mbus.hello messages that are used by other entities to monitor the
   existence of that entity. If an entity has not received mbus.hello
   messages for a certain time (see Section 8.2) from an entity the
   respective entity is considered to have left the Mbus and MUST be
   excluded from the set of currently known entities. Upon the
   reception of a mbus.bye messages the respective entity is considered
   to have left the Mbus as well and MUST be excluded from the set of
   currently known entities immediately.

   Each Mbus entity MUST send hello messages after startup to the Mbus.
   After transmission of the hello message, it shall start a timer
   after the expiration of which the next hello message is to be
   transmitted. Transmission of hello messages MUST NOT be stopped
   unless the entity detaches from the Mbus. The interval for sending
   hello messages is depending on the current number of entities in a
   Mbus group and can thus change dynamically in order to avoid
   congestion due to many entities sending hello messages at a constant
   high rate.

   Section 8.1 specifies the calculation of hello message intervals
   that MUST be used by protocol implementations. Using the values that
   are calculated for obtaining the current hello message timer, the
   timeout for received hello messages is calculated in Section 8.2.
   Section 9 specifies the command synopsis for the corresponding Mbus

8.1 Hello Message Transmission Interval

   Since Mbus sessions may vary in size concerning the number of

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   entities care must be taken to allow the Mbus protocol to scale well
   over different numbers of entities automatically. The average rate
   at which hello messages are received would increase linearly to the
   number of entities in a session if the sending interval was set to a
   fixed value. Given a interval of 1 second this would mean that an
   entity taking part in an Mbus session with n entities would receive
   n hello messages per second. Assuming all entities resided on one
   host this would lead to n*n messages that have to be processed per
   second -- which is obviously not a viable solution for larger
   groups. It is therefore necessary to deploy dynamically adapted
   hello message intervals taking varying numbers of entities into
   account. In the following we specify an algorithm that MUST be used
   by implementations to calculate the interval for hello messages
   considering the observed number of Mbus entities.

   The algorithm features the following characteristics:

   o  The number of hello messages that are received by a single entity
      in a certain time unit remains approximately constant as the
      number of entities changes.

   o  The effective interval that is used by a specific Mbus entity is
      randomized in order to avoid unintentional synchronization of
      hello messages within a Mbus session. The first hello message of
      an entity is also delayed by a certain random amount of time.

   o  A timer reconsideration mechanism is deployed in order to adapt
      the interval more appropriately in situations where a rapid
      change of the number of entities is observed. This is useful when
      an entity joins an Mbus sessions and is still learning of the
      existence of other entities or when a larger number of entities
      leaves the Mbus at once.

8.1.1 Calculating the Interval for Hello Messages

   The following names for values are used in the calculation specified
   below (all time values in milliseconds):

   hello_p: The last time a hello message has been sent by a Mbus

   hello_now: The current time

   hello_d: The deterministic calculated interval between hello

   hello_e: The effective (randomized) interval between hello messages.

   hello_n: The time for the next scheduled transmission of a hello

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   entities_p: The numbers of entities at the time hello_n has been
      last recomputed.

   entities: The number of currently known entities.

   The interval between hello messages MUST be calculated as follows:

   The number of currently known entities is multiplied by
   c_hello_factor, yielding the interval between hello messages in
   milliseconds. This is the deterministic calculated interval,
   denominated hello_d. The minimum value for hello_d is c_hello_min.
   Thus hello_d=max(c_hello_min,c_hello_factor * entities). Section 8
   provides a specification of how to obtain the number of currently
   known entities. Section 10 provides values for the constants
   c_hello_factor and c_hello_min.

   The effective interval hello_e that is to be used by individual
   entities is calculated by multiplying hello_d with a randomly chosen
   number between c_hello_dither_min and c_hello_dither_max (see
   Section 10).

   hello_n, the time for the next hello message in milliseconds is set
   to hello_e + hello_now.

8.1.2 Initialization of Values

   Upon joining a session a protocol implementation sets hello_p,
   hello_now to 0 and entities, entities_p to 1 (the current Mbus
   entity itself) and then calculates the time for the next hello
   message as specified in Section 8.1.1. The next hello message is
   scheduled for transmission at hello_n.

8.1.3 Adjusting the Hello Message Interval when the Number of Entities

   When the existence of a new entity is observed by a protocol
   implementation the number of currently known entities is updated. No
   further action concerning the calculation of the hello message
   interval is required. The reconsideration of the timer interval
   takes place when the current timer for the next hello message
   expires (see Section 8.1.5).

8.1.4 Adjusting the Hello Message Interval when the Number of Entities

   Upon realizing that an entity has left the Mbus the number of
   currently known entities is updated and the following algorithm

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   should be used to reconsider the timer interval for hello messages:

   1.  The value for hello_n is updated by setting hello_n to
       hello_now + (entities/entities_p)*(hello_n - hello_now)

   2.  The value for hello_p is updated by setting hello_p to
       hello_now - (entities/entities_p)*(hello_now - hello_p)

   3.  The currently active timer for the next hello messages is
       cancelled and a new timer is started for hello_n.

   4.  entities_p is set to entities.

8.1.5 Expiration of hello timers

   When the hello message timer expires, the protocol implementation
   MUST perform the following operations:

      The hello interval hello_e is computed as specified in Section


      1.  hello_e + hello_p is less than or equal to hello_now, a hello
          message is transmitted. hello_p is set to hello_now, hello_e
          is calculated again as specified in Section 8.1.1 and hello_n
          is set to hello_e + hello_now.

      2.  else if hello_e + hello_p is greater than hello_now, hello_n
          is set to hello_e + hello_p.  A new timer for the next hello
          message is started to expire at hello_n. No hello message is

      entities_p is set to entities.

8.2 Calculating the Timeout for Hello Messages

   Whenever an Mbus entity has not heard for a time span of
   c_hello_dead*(hello_d*c_hello_dither_max) milliseconds from another
   Mbus entity it may consider this entity to have failed (or have quit
   silently). The number of the currently known entities MUST be
   updated accordingly. Note that no need for any further action is
   necessarily implied from this observation.

   Section 8.1.1 specifies how to obtain hello_d. Section 10 defines
   values for the constants c_hello_dead and c_hello_dither_max.

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9. Messages

   This section defines some basic application independent messages
   that MUST be understood by all implementations. This specification
   does not contain application specific messages which are to be
   defined outside of the basic Mbus protocol specification.

   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 commend set to implement the aforementioned concepts
   is presented in the following sections.

9.1 mbus.hello


      Parameters: see below

   HELLO messages MUST be 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 and
   can thus calculate the current number of entities.

   HELLO messages MUST be sent periodically in dynamically calculated
   intervals as specified in Section 8.

   Upon startup the first HELLO message MUST be sent after a delay
   hello_delay, where hello_delay be a randomly chosen number between 0
   and c_hello_min (see Section 10).

9.2 mbus.bye


      Parameters: - none -

   An Mbus entity that is about to terminate (or "detach" from the
   Mbus) SHOULD announce this by transmitting a BYE message.

   The BYE message MUST be sent unreliably to all receivers.

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9.3 mbus.ping


      Parameters: - none -

   mbus.ping can be used to solicit other entities to signal their
   existence by replying with a mbus.hello message.  Each protocol
   implementation MUST understand mbus.ping and reply with a mbus.hello
   message. The reply hello message MUST be delayed for hello_delay
   milliseconds, where hello_delay be a randomly chosen number between
   0 and c_hello_min (see Section 10).

   As specified in Section 9.1 hello messages MUST be sent unreliably
   to all Mbus entities. An entity that replies to mbus.ping with
   mbus.hello should stop any outstanding timers for hello messages
   after sending the hello message and schedule a new timer event for
   the subsequent hello message. (Note that using the variables and the
   algorithms of Section 8.1.1 this can be achieved by setting hello_p
   to hello_now.)

   mbus.ping allows a new entity to quickly check for other entities
   without having to wait for the regular individual hello messages. By
   specifying a target address the new entity can restrict the
   solicitation for hello messages to a subset of entities it is
   interested in.

9.4 mbus.quit


      Parameters: - none -

   The QUIT message is used to request other entities to terminate
   themselves (and detach from the Mbus). Whether this request is
   honoured by receiving entities or not is up to the discretion of the

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

9.5 mbus.waiting



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

   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.

9.6 mbus.go



         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

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

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10. Constants

   The following values for timers and counters mentioned in this
   document SHOULD be used by implementations:

            |Timer / Counter    | Value                  | Unit         |
            |c_hello_factor     | 200                    |     -        |
            |c_hello_min        | 1000                   | milliseconds |
            |c_hello_dither_min | 0.9                    |     -        |
            |c_hello_dither_max | 1.1                    |     -        |
            |c_hello_dead       | 5                      |     -        |

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11. Mbus Security

11.1 Security Model

   In order to prevent accidental or malicious disturbance of Mbus
   communications through messages originated by applications from
   other users message authentication is deployed (Section 11.2). For
   each message a digest is calculated based on the value of a shared
   secret key value. Receivers of messages can check if the sender
   belongs to the same Mbus security domain by re-calculating the
   digest and comparing it to the received value. Only if both values
   are equal the messages must be processed further. In order to allow
   different simultaneous Mbus sessions at a given scope and to
   compensate defective implementations of host local multicast ([18])
   message authentication MUST be provided by conforming

   Privacy of Mbus message transport can be achieved by optionally
   using symmetric encryption methods (Section 11.3). Each message can
   be encrypted using an additional shared secret key and a symmetric
   encryption algorithm. Encryption is OPTIONAL for applications, i.e.
   it is allowed to configure an Mbus domain not to use encryption. But
   conforming implementations MUST provide the possibility to use
   message encryption (see below).

   Message authentication and encryption can be parameterized by
   certain values, e.g. by the algorithms to apply or by the keys to
   use. These parameters (amongst others) are defined in an Mbus
   configuration entity that is accessible to all Mbus entities that
   participate in an Mbus session. In order to achieve interoperability
   conforming implementations SHOULD consider the given Mbus
   configuration entity. Section 12 defines the mandatory and optional
   parameters as well as storage procedures for different platforms.
   Only in cases where none of the options for configuration entities
   mentioned in Section 12 is applicable alternative methods of
   configuring Mbus protocol entities MAY be deployed.

11.2 Message Authentication

   Either MD5 [14] or SHA-1 [15] SHOULD be used for message
   authentication codes (MACs).  An implementation MAY provide SHA-1,
   whereas MD5 MUST be implemented. To generate keyed hash values the
   algorithm described in RFC2104[4] MUST be applied with hash values
   truncated to 96 bits (12 bytes). The resulting hash values MUST be
   Base64 encoded (16 characters). The HMAC algorithm works with both,
   MD5 and SHA-1.

   HMAC values, regardless of the algorithm, MUST therefore always
   consist of 16 Base64-encoded characters.

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   Hash keys MUST have a length of 96 bit (12 bytes), that are 16
   Base64-encoded characters.

11.3 Encryption

   Either DES, 3DES (triple DES) or IDEA SHOULD be used for encryption.
   Encryption MAY be neglected for applications, e.g.  in situations
   where license regulations, export or encryption laws would be
   offended otherwise. However, the implementation of DES is
   RECOMMENDED as a baseline. DES implementations MUST use the DES
   Cipher Block Chaining (CBC) mode. For algorithms requiring
   en/decryption data to be padded to certain boundaries octets with a
   value of 0 SHOULD be used for padding characters.  The padding
   characters MUST be appended after calculating the message digest
   when encoding and MUST be erased before recalculating the message
   digest when decoding.  IDEA uses 128-bit keys (24 Base64-encoded
   characters). DES keys (56 bits) MUST be encoded as 8 octets as
   described in RFC1423[12], resulting in 12 Base64-encoded characters.

   The mandatory subset of algorithms that MUST be provided by
   implementations is DES and MD5.

   See Section 12 for a specification of notations for Base64-strings.

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12. Mbus Configuration

   An implementation MUST be configurable by the following parameters:

      Configuration version

         The version number of the given configuration entity. Version
         numbers allow implementations to check if they can process the
         entries of a given configuration entity. Version number are
         integer values. The version number for the version specified
         here is 1.

      Encryption key

         The secret key used for message encryption.

      Hash key

         The hash key used for message authentication.


         The Internet scope to be used for sent messages.

   The upper parameters are mandatory and MUST be present in every Mbus
   configuration entity.

   The following parameters are optional. When they are present they
   MUST be honoured but when they are not present implementations
   SHOULD fall back to the predefined default values (as defined in
   Transport (Section 6)):


         The non-standard multicast address to use for message


         The non-standard port number to use for message transport.

   Two distinct facilities for parameter storage are considered: For
   Unix-like systems a configuration file SHOULD be used and for
   Windows-95/98/NT/2000 systems a set of registry entries is defined
   that SHOULD be used.

   The syntax of the values for the respective parameter entries
   remains the same for both configuration facilities. The following
   defines a set of ABNF (see RFC2234[13]) productions that are later

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   referenced for the definitions for the configuration file syntax and
   registry entries:

   algo-id                 =    "NOENCR" / "DES" / "3DES" / "IDEA" /
                                "HMAC-MD5-96" / "HMAC-SHA1-96"
   scope                   =    "HOSTLOCAL" / "LINKLOCAL"
   key                     =    base64string
   version_number          =    1*10DIGIT
   base64string            =    *(ALPHA / DIGIT / "+" / "/" / "=")
   key_value               =    "(" algo-id "," key ")"
   ipv4_addr               =    ipv4_octet 3*3("." ipv4_octet)
   ipv4_octet              =    1*3DIGIT
   port                    =    1*5DIGIT

   A key entry MUST be specified using this notation:


   algo-id is one of the character strings specified above. For
   algo-id=``NOENCR'' the other fields are ignored. The de- limiting
   commas MUST always be present though.

   A Base64 string consists of the characters defined in the Base64
   char-set (see RFC1521[5]) including all eventual padding characters,
   i.e. the length of Base64-string is always a multiple of 4.

   The version_number parameter specifies a version number for the used
   configuration entity.

12.1 File based parameter storage

   The file name for a Mbus configuration file is ".mbus" in the user's
   home-directory. If an environment variable called MBUS is defined
   implementations SHOULD interpret the value of this variable as a
   fully qualified file name that is to be used for the configuration
   file. Implementations MUST ensure that this file has appropriate
   file permissions that prevent other users to read or write it.  The
   file MUST exist before a conference is initiated. Its contents MUST
   be UTF-8 encoded and MUST be structured as follows:

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              mbus-file     =    mbus-topic LF *(entry LF)
              mbus-topic    =    "[MBUS]"
              entry         =     1*(version_info / hashkey_info
                                     / encryptionkey_info / scope_info
                                     / port_info / address_info)
              version_info  =    "CONFIG_VERSION=" version_number
              hashkey_info  =    "HASHKEY=" key_value
              encrkey_info  =    "ENCRYPTIONKEY=" key_value
              scope_info    =    "SCOPE=" scope
              port_info     =    "PORT=" port
              address_info  =    "ADDRESS=" ipv4_addr

   The following entries are defined: CONFIG_VERSION, HASHKEY,

   The entries CONFIG_VERSION, HASHKEY and ENCRYPTIONKEY are mandatory,
   they MUST be present in every Mbus configuration file. The order of
   entries is not significant.

   An example Mbus configuration file:


12.2 Registry based parameter storage

   For systems lacking the concept of a user's home-directory as a
   place for configuration files the suggested database for
   configuration settings (e.g. the Windows9x-, Windows NT-, Windows
   2000-registry) SHOULD be used.  The hierarchy for Mbus related
   registry entries is as follows:

              HKEY_CURRENT_USER\Software\Mbone Applications\Mbus

   The entries in this hierarchy section are:

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              |Name           | Type   | ABNF production|
              |CONFIG_VERSION | DWORD  | version_number |
              |HASHKEY        | String | key_value      |
              |ENCRYPTIONKEY  | String | key_value      |
              |SCOPE          | String | scope          |
              |ADDRESS        | String | ipv4_addr      |
              |PORT           | DWORD  | port           |

   The same syntax for key values as for the file based configuration
   facility MUST be used.

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13. Security Considerations

   The Mbus security mechanismns are specified in Section 11.1.

   It should be noted that the Mbus transport specification defines a
   mandatory baseline set of algorithms that have to be supported by
   implementations. This baseline set does not neccessarily provide the
   best security due to the cryptographic weaknesses of the individual
   algorithms. For example, it has been stated in [4] that MD5 had been
   shown to be vulnerable to collision search attacks (although this
   was believed not to compromise the use of MD5 within HMAC
   generation). However, SHA-1 is usually considered to be the
   cryptographically stronger function ([16]).

   Similar remarks can be made on the encryption functions. The base
   specification requires DES, an algorithm that has shown to be
   vulnerable to brute-force attacks ([16], [17]).

   We do not consider the well-known weaknesses of the mentioned
   algorithms a problem:

   o  The problem of receiving unauthenticated messages is considered
      to be the main security threat for Mbus communication. We believe
      that HMAC-MD5 is sufficiently secure as a baseline algorithm. For
      application requiring special security concerning authentication
      of messages there is the option of using implementations that
      implement SHA-1.

   o  Encryption is optional anyway, i.e. users can decide to have
      their implementations sending clear text Mbus messages. Given the
      local nature of Mbus communication this is feasible for most use
      cases. In case the base DES encryption is not considered
      sufficient there is still the possibility to use implementations
      that implement 3DES or IDEA.

   However, application developers should be aware of incorrect IP
   implementations that do not conform to RFC 1122[2] and do send
   datagrams with TTL values of zero, resulting in Mbus messages sent
   to the local network link although a user might have selected host
   local scope in the Mbus configuration. In these cases the use of
   encryption SHOULD be considered if privacy is desired.

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14. IANA Considerations

   The IANA is requested to assign a port number and a multicast
   address. For the time being the tentative multicast address and the port number 47000 (decimal) SHOULD be used.

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

   [2]  Braden, R., "Requirements for Internet Hosts -- Communication
        Layers", RFC 1122, October 1989.

   [3]  Hinden, R. and S. Deering, "IP Version 6 Addressing
        Architecture", RFC 2373, July 1998.

   [4]  Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed-Hashing
        for Message Authentication", RFC 2104, February 1997.

   [5]  Borenstein, N. and N. Freed, "MIME (Multipurpose Internet Mail
        Extensions) Part One: Mechanisms for Specifying and Describing
        the Format of Internet Message Bodies", RFC 1521, September

   [6]  Handley, M., Crowcroft, J., Bormann, C. and J. Ott, "The
        Internet Multimedia Conferencing Architecture", Internet Draft
        draft-ietf-mmusic-confarch-02.txt, October 1999.

   [7]  Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobsen,
        "RTP: A Transport Protocol for Real-Time Applications", RFC
        1889, January 1996.

   [8]  Handley, M., Schulzrinne, H., Schooler, E. and J. Rosenberg,
        "SIP: Session Initiation Protocol", RFC 2543, March 1999.

   [9]  Handley, M. and V. Jacobsen, "SDP: Session Description
        Protocol", RFC 2327, April 1998.

   [10]  Meyer, D., "Administratively Scoped IP Multicast", RFC 2365,
         July 1998.

   [11]  Ott, J., Perkins, C. and D. Kutscher, "Requirements for Local
         Conference Control", Internet Draft
         draft-ietf-mmusic-mbus-req-00.txt, December 1999.

   [12]  Balenson, D., "Privacy Enhancement for Internet Electronic
         Mail: Part III: Algorithms, Modes, and Identifiers", RFC 1423,
         February 1993.

   [13]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
         Specifications: ABNF", RFC 2234, November 1997.

   [14]  Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
         April 1992.

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   [15]  U.S. DEPARTMENT OF COMMERCE/National Institute of Standards
         and Technology, "Secure Hash Standard", FIPS PUB 180-1, April

   [16]  Schneier, B., "Applied Cryptography", Edition 2, Publisher
         John Wiley & Sons, Inc., 1996.

   [17]  distributed.net, "Project DES", WWW
         http://www.distributed.net/des/, 1999.

   [18]  Microsoft, "BUG: Winsock Sends IP Packets with TTL 0", WWW
         http://support.microsoft.com/support/kb/articles/Q138/2/68.asp, March 1999

Authors' Addresses

   Joerg Ott
   TZI, Universitaet Bremen
   Bibliothekstr. 1
   Bremen  28359

   Phone: +49.421.201-7028
   Fax:   +49.421.218-7000
   EMail: jo@tzi.uni-bremen.de

   Colin Perkins
   USC Information Sciences Institute
   4350 N. Fairfax Drive #620
   Arlington VA 22203

   EMail: csp@isi.edu

   Dirk Kutscher
   TZI, Universitaet Bremen
   Bibliothekstr. 1
   Bremen  28359

   Phone: +49.421.218-7595
   Fax:   +49.421.218-7000
   EMail: dku@tzi.uni-bremen.de

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Appendix A. Mbus Addresses for Conferencing

   For conferencing application 5 address element keys are predefined
   (in addition to the mandatory element key "id"):

            conf       conference identifier
            media      media type processed by application
            module     module type of Mbus entity in a application
            app        application name

   The conf element is used to designate the name of a conference in
   order to distinguish between entities that are present in more than
   one conference. See Transport (Section 6) for further notes
   concerning multiple presences using the Mbus.

   The media element identifies the type of media processed by an
   application. Currently defined values are:

            audio        An RTP audio stream
            video        An RTP video stream
            workspace    A shared workspace
            whiteboard   A shared whiteboard
            editor       A shared text editor
            sap          A session announcement tool, using SAP
            sip          A session invitation tool, using SIP
            h323         An ITU-T H.323 conference controller
            rtsp         An RTSP session controller
            control      A local coordination entity

   Other values are likely to be defined at a later date.

   The module element defines a logical part of an application. The
   value `ui' denotes the user-interface of an application, and the
   value `engine' defines a media/protocol engine, and `transcoder'
   defines a media transcoder. Other values may be defined in future.

   The app element identifies the application being used (e.g.: rat,
   vic, etc.).

   The instance element is used to distinguish several instances of the
   same application. This is a per-instance-unique identifier, which is
   not necessarily an integer. Many Unix applications will use the
   process-id (PID) number, although this is not a requirement.  Note
   that if an end system is spread across several hosts, the instance
   MUST NOT be the process-id, unless e.g.. the host name or its IP
   address are included as well. Section 9 defines a bootstrap
   procedure ensuring that entities can track the abandoning and
   restarting of application instances as long as unique instance
   values are being used.

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   The following examples illustrate how to make use of the addresses:

   |(conf:test media:audio      | The user interface of                |
   |module:ui app:rat           | the rat application with             |
   |id:4711-99@  | the given id is taking               |
   |                            | part in conference test              |
   |(media:workspace module:ui) | The user interfaces of               |
   |                            | all workspace applications           |
   |(media:audio)               | All audio applications               |
   |(app:rat)                   | All instances of the rat application |
   |()                          | All entities                         |

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