INTERNET-DRAFT                             J. Ott/C. Perkins/D. Kutscher
Expires: February 1999       Universitaet Bremen/UCL/Universitaet Bremen
                                                             August 1998

                 A Message Bus for Conferencing Systems

Status of this memo

   This document is an Internet-Draft.  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
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   To learn the current status of any Internet-Draft, please check the
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   Distribution of this document is unlimited.


   In a variety of 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.
   Such a mechanism allows for coordination of applications entities to
   e.g. implement synchronization between media streams or realize
   tightly coupled conferences.  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 documents 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.  The
   common procedures for Mbus operation as well as the common set of
   application/media specific messages are introduced in a companion
   Internet draft[9].

   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

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   working group's mailing list at and/or the authors.

1.  Introduction

1.1.  Background

   In the Mbone community a model has arisen whereby a set of loosely
   coupled tools are used to participate in a conference.  A typical
   scenario is that audio, video and shared workspace functionality is
   provided by three separate tools (although some combined tools
   exist).  This maps well onto the underlying RTP [5] (as well as
   other) media streams, which are also transmitted separately.  Given
   such an architecture, it is useful to be able to perform some
   coordination of the separate media tools.  For example, it may be
   desirable to communicate playout-point information between audio and
   video tools, in order to implement lip-synchronisation, to arbitrate
   the use of shared resources (such as input devices), etc.

   A refinement of this architecture relies on the presence of a number
   of media engines which perform protocol functions as well as
   capturing and playout of media.  In addition, one (or more)
   (separate) user interface agents exist that interact with and control
   their media engine(s).  Such an approach allows flexibility in the
   user-interface design and implementation, but obviously requires some
   means by which the various involved agents may communicate with one
   another.  This is particularly desirable to enable a coherent
   response to a user's conference-related actions (such as joining or

   Although current practice in the Mbone community is to work with a
   loosely coupled conference control model, situations arise where this
   is not appropriate and a more tightly coupled wide-area conference
   control protocol must be employed (e.g. for IP telephony).  In such
   cases, it is highly desirable to be able to re-use the existing tools
   (media engines) available for loosely coupled conferences and
   integrate them with a system component implementing the tight
   conference control model.  One appropriate means to achieve this
   integration is a communication channel that allows a dedicated
   conference control entity to ``remotely'' control the media engines
   in addition to or instead of their respective user interfaces.

   The Message Bus defined in this and a companion document provides a
   suitable means for local communication that serves all of the above

1.2.  Purpose

   Two components constitute the Message Bus: the (lower level) message
   passing mechanisms and the (higher level) messages and their

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   The purpose of this document is to define the characteristics of the
   basic 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;

   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); and

   o    the details of the Mbus message syntax.

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.

1.4.  Definition of terms

   o    Conference

        The relationship between a set of human beings that are
        communicating together.  In this document, the term is used for
        both tightly and loosely coupled [4] computer based conferences.

   o    Participant

        A (typically human) being that takes part in a conference.

   o    Member

        The system, including all software and hardware components, that
        is used in a teleconference to represent a single participant.

   o    End system

        A host or a set of locally interconnected hosts[1] that is used
  [1] In this document, we use the term ``end system'' as  a  syn-
onym  for  ``host''  in  the simplest case.  We do not want to ex-
clude, however, that the local system that serves one  participant
may be composed of several ``hosts'' in the Internet sense.

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        as an interface to a teleconference by a single participant.
        The end system runs all the required conferencing software (e.g.
        media agents, session directory, and a controlling entity).  End
        system and software together constitute a member in each of the
        conferences a user participates in.

   o    Conference controller

        A dedicated application running on an end system that implements
        a horizontal conference control protocol through which it
        interacts with conference controllers on other end systems to
        implement (typically tight) conference control mechanisms and
        conference policies.  The conference controller constitutes the
        electronic representation of its (human) user and her actions
        with respect to conference(s) as a whole (rather than with
        respect to individual media sessions).

   o    UCI

        A universal communication identifier of a person.  This may be
        the e-mail address of an individual (or some other globally
        unique identifier) that is part of the information to identify
        her within a conference but can also be used to invite her via
        the Session Initiation Protocol (SIP) [6] protocol.

   o    Presence

        A presence corresponds to a person (identified by a UCI) being
        ``logged in'' at an end system and available for conferencing,
        i.e. a presence may be identified by the pair of a user's UCI
        and the respective end system's identification (such as a host
        name).  A presence of a user may appear in many conferences (see

   o    Appearance

        An instantiation of a user's presence actually participating
        (i.e. appearing) in a conference is referred to as an
        appearance.  There is a one-to-one correspondence between
        appearances and members.

   o    Conference context

        All state information kept about a conference at each member of
        this conference.

   o    Application session (AS), Session

        The set of media agents/applications that act as peers to each
        other within a conference.  For real-time data, this generally
        will be an RTP session [5]; for other application protocols,
        other horizontal protocols may define their own type of session
        concept.  Possible synonyms are ``application group'' or ``media

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        agent group''.

   o    Application instance, application entity, media agent

        A program instance taking part in an application session for a
        conference participant. There can be more than one instance of
        the same program in one session, there can also be more than one
        instance in different sessions.

2.  Requirements and Concepts

   The Mbus is supposed to operate in a variety of scenarios as outlined
   in the introduction.  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 may need to be destined for all local application entities,
   such as membership information, floor control notifications,
   dissemination conference state changes, etc.  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 3.4) and by providing a flexible
   addressing scheme (defined in section 3.2).

   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.

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   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
   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 3.2.

   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
   numbers 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 "TERMINATE
   CONFERENCE" command).  To eliminate the possibility of receiving
   bogus Mbus messages, the Mbus protocol therefore 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 appendix C
   (Mbus configuration).

3.  Message Bus Specification

3.1.  Message Format

   A conference coordination 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

   o    The MsgDigest is a Base64-encoded[3] calculated hash value of
        the entire message (starting from the ProtocolID field) as
        described in appendices A (Algorithms) and C (Mbus

   o    A fixed ProtocolID field identifies the version of the message
        bus protocol used. The protocol defined in this document is

   o    A sequence number SeqNum is contained in each message. The first

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

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

   o    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.

   o    The SrcAddr field identifies the sender of a message. This MUST
        be a full address, with no wildcards present. The addressing
        scheme is described in section 3.2.

   o    The DestAddr field identifies the intended recipient(s) of the
        message. This field MAY contain wildcards and hence address any
        number (including zero) of application entities. The addressing
        scheme is described in section 3.2.

   o    The AckList field comprises a list of SeqNums for which this
        message is an acknowledgment. See section 3.3 for details.

   The header is followed by the message body which contains one or more
   messages to be delivered to the destination entity. The syntax for a
   complete message is given in section ``syntax''.

3.2.  Addressing

   Each entity on the message bus SHOULD respond to messages sent to one
   (or more) addresses. Addresses are quad-tuples written as:
                (MediaType ModuleType AppName AppInstance)
   where one or more fields MAY be wildcarded (with `*') in some cases.
   All fields in an address are case sensitive.

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

            audio        An RTP audio stream
            video        An RTP video stream

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

   The AppInstance 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 AppInstance
   MUST NOT be the process-id, unless e.g.. the host name or its IP
   address are included as well. The companion draft "The Message Bus:
   Messages and Procedures"[9] defines a bootstrap procedure ensuring
   that entities can track the abandoning and restarting of application
   instances as long as unique AppInstance values are being used.

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

   (audio ui rat 124)   The user interface of the rat application with instance-id 124
   (workspace ui * *)   The user interfaces of all workspace applications
   (audio * * *)        All audio applications
   (* * rat *)          All instances of the rat application

3.3.  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, without the use of wildcards.[2]
  [2] Disallowing  reliable  message delivery for messages sent to
multiple destinations is motivated by simplicity of the  implemen-
tation  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

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   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 SHALL retransmit the
   message (with the same message sequence number), increase the
   timeout, and restart the timer.  Messages SHALL be retransmitted a
   small number of times 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 taken.

   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 other commands.

   The precise procedures are as follows:

        A sender A of a reliable message M to receiver B SHALL transmit
        the message via multicast or via unicast, keep a copy of M,
        initialize a retransmission counter N to '1', and start a
        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 SHALL
        BE retransmitted, the counter N SHALL BE incremented by one, and
        the timer SHALL 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 SHALL BE discarded, and the sending
        application SHALL BE notified.

        A receiver B of a reliable message from a sender A SHALL
        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 optimizing: gathering and piggy-backing ACKs
        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 shall start an
        acknowledgment timer TA (initialized to T_c).  When the timer
        expires, B shall create a dedicated acknowledgment message and
        send it to A.  If B is to transmit another Mbus message
each subgroup which is deemed too much effort.

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        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.

        Suggested values are T_r=100ms, N_r=3, T_c=70ms,

3.4.  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 Appendix ``Mbus
        configuration'') to a fixed, yet to be assigned link-local
        address of the administratively scoped multicast space as
        described in RFC 2365 [8]. There is a base port for each
        presence conducting conferences using the Mbus.  This port SHALL
        be used for communication between application entities not
        associated with a particular conference.  For each conference
        that a person participates in, a dedicated port is used for
        conference-specific communication.  Messages of interest for all
        conferences a presence is involved in SHALL be sent to the base
        port.  Messages intended for a specific conference (i.e.
        messages relating to an appearance only) SHALL be sent to the
        port of the respective conference.  Message intended for several
        but not all conferences SHALL be sent individually to the
        specific ports of these conference (one by one). The concrete
        port numbers are taken from a reserved set of ports from a
        defined PORTBASE to PORTBASE+#ports. Appendix B (Port
        Allocation) defines procedures for port allocation.

   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 use in 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.

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

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   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.

3.5.  Message Syntax

3.5.1.  Message Encoding

   All messages SHALL 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 appendix A (Algorithms).

3.5.2.  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 appendix A 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:

           mbus/1.0 <SeqNum> <TimeStamp> <MessageType> <SrcAddr> <DestAddr> \

   The header fields are defined in section 3.1.

3.5.3.  Command Syntax

   The header is followed by zero, or more, messages 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 ... )

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

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   |DataType | Syntax             | Description                     |
   |Integer  | "-"[0-9]+          |                                 |
   |Float    | "-"[0-9]+"."[0-9]+ |                                 |
   |String   | """..."""          | See below for escape characters |
   |         |                    |                                 |
   |List     | (DataType DataType |                                 |
   |         | ...)               |                                 |
   |Symbol   | [A-Za-z0-9_-.]+    | A predefined protocol value     |
   |Data     | "<"data">"         | 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:

   Opaque data is represented as Base64-encoded [3] character strings
   surrounded by "<" and ">"

   |Escape Sequence |  Meaning  |
   |      \\        |    \      |
   |      \''       |    ''     |
   |      \n        | <newline> |

3.6.  Messages

   The specific messages applications will send using the Mbus are not
   defined in this document. Currently a companion document[9] is
   produced defining classes of messages which are of use in certain
   application areas. Additional documents are expected to follow.

4.  Author's Addresses

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

   l.  Colin Perkins <> Department of Computer
   Science University College London Gower Street London WC1E 6BT United

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

5.  References

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

   [2]  H. Krawczyk, M. Bellare, R. Canetti, ``HMAC: Keyed-Hashing for
        Message Authentication'', RFC 2104, February 1997

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

   [4]  Mark Handley, Jon Crowcroft, Carsten Bormann, ``The Internet
        Multimedia Conferencing Architecture,'' Internet Draft draft-
        ietf-mmusic-confarch-00.txt, Work in Progress, February 1996.

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

   [6]  Mark Handley, Henning Schulzrinne, Eve Schooler, Jonathan
        Rosennberg, ``SIP: Session Initiation Protocol'', Internet Draft
        draft-ietf-mmusic-sip-07.txt, Work in Progress, July 16, 1998

   [7]  M. Handley, V. Jacobson, ``SDP: Session Description Protocol'',
        RFC 2327, April 1998

   [8]  D. Meyer ``Administratively Scoped IP Multicast'', RFC 2365,
        July 1998

   [9]  J. Ott, C. Perkins, and D. Kutscher, ``The Message Bus: Messages
        and Procedures'', Internet Draft draft-ietf-mmusic-mbus-
        semantics-00.txt, Work in Progress, August 1998.

Appendix A: Algorithms

   Message Authentication
        Either MD5 or SHA-1 SHALL 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 [2] MUST be applied with hash values truncated to
        80 bits. The resulting hash values SHALL be Base64 encoded (16
        characters). The HMAC algorithm works with both, MD5 and SHA-1.

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

        Hash keys SHALL have a length of 96 bit, that are 20
        Base64-encoded characters.

        Either DES, 3DES (triple DES) or IDEA SHALL 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 electronic codebook (ECB) mode. Chaining modes are not
        appropriate due to (possible) unreliable message transport. For
        algorithms requiring en/decryption data to be padded to certain
        boundaries ASCII code 32 SHALL be used for padding characters.
        IDEA uses 128-bit keys (24 Base64-encoded characters). DES SHALL
        be used with 56-bit keys (12 Base64-encoded characters).

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

   See appendix C for a specification of notations for Base64-strings.

Appendix B: Port allocation

   The reserved Mbus port numbers are in the range from PORTBASE to
   PORTBASE+(n*(m+1)) (n=number of base ports, m=reasonable maximum
   number of conferences per presence). The first n ports are reserved
   for base ports. The set of conference specific ports starts at offset
   n and has a cardinality of n*m.

   Implementations SHALL use the presence-id (see below) to calculate a
   valid offset to the set of base port numbers for a person's presence.
   Offsets to conference specific port numbers SHALL be obtained by
   using the conference name. The conference name is a SDP session
   name[7] and MUST be known in advance of port allocation.

   Base port number calculation SHALL rely on the following algorithm:
   All UTF-8 octets of the session name are considered for building a
   sum of their key codes. The offset to the base port number is the
   result of the modulo division of the sum by n (number of base ports).

   Offsets for per-conference port numbers SHALL be calculated
   analogously: The key codes of the presence-id's characters are summed
   up and the the offset is obtained by adding the result of modulo
   dividing the sum by m (number of conference ports per presence). The
   actual port number is obtained by adding the result to
   PORTBASE+(n*(baseport offset+1)).


Ott/Perkins/Kutscher                                           [Page 14]

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   PORTBASE  = 2000
   nr of base ports n= 10
   nr of conference ports m= 6

   session name= abc

   baseport offset= (97+98+99) % 10
             = 4
   baseport  = PORTBASE + 4
             = 2004

   conference port offset= (97+64+99+46+111+114+103) % 6
             = 4
   conference port= PORTBASE + (6* (baseport offset+1))
               + conference port offset
             = 2034

Appendix C: Mbus configuration

   An implementation MUST be configurable by the following parameters:

   Encryption key   The secret key used for message encryption.
   Hash key         The hash key used for message authentication.
   Presence ID      The UCI of the person participating in a conference.
   Scope            The Internet scope to be used for sent messages.

   The logical structure of the specified parameters is as follows:[3]

   hashkey      ::= algo-id expiration key
   secretkey    ::= algo-id expiration key

   presence     ::= uci

   expiration   ::= digits

   algo-id      ::= ``NOENCR'' | ``DES'' | ``3DES'' | ``IDEA'' | ``HMAC-MD5-80'' | ``HMAC-SHA1-80''

   scope        ::= ``HOSTLOCAL'' | ``LINKLOCAL''

   key          ::= base64string
   uci          ::= alpha

   A Base64-String consists of the characters defined in the Base64
   char-set [3] including all eventual padding characters, i.e. the
   length of Base64-string is always a multiple of 4.

  [3] syntactical definitions follow below

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Appendix D: Parameter storage

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

   File based parameter storage:

   The file name for a Mbus configuration file is ``.mbus'' in the
   user's home-directory which MAY be overridden by an environment
   variable called MBUS.  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 SHALL be UTF-8 encoded and SHALL be structured as follows:


   A key entry MUST be in this notation:

           ``(''algo-id``,'' expiration``,''base64string``)''

   algo-id is one of the character strings specified above and
   expiration is a decimal number representing the date that the key
   invalidates at, notated in seconds counting from 00:00:00, UTC,
   January 1, 1970.

   The presence-id is a universal communication identifier (UCI) for a
   conference participant. This can be a canonical email address like
   ``''.  In case the same UCI is actually used to represent
   different presences, e.g. to express different affiliations of a
   person or to let different person use a single-user end-system
   concurrently, the presence-id MAY be constituted of a UCI and a
   presence ``modifier'' like ``'', ``'' and
   so on. Presence-ids MUST be in the US-ASCII subset of

   An example Mbus-configuration file:

Ott/Perkins/Kutscher                                           [Page 16]

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   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-registry) SHALL be used.
   The hierarchy for Mbus related registry entries is as follows:[4]

   HKEY_CURRENT_USER\Software\Mbone Applications\Mbus

   The entries in this hierarchy section are

   |Name          | Type   |
   |HASHKEY       | String |
   |ENCRYPTIONKEY | String |
   |PRESENCE      | String |
   |SCOPE         | String |
   The same syntax for key values as for the file based configuration
   facility MUST be used.

  [4] complies with vat's registry hierarchy

Ott/Perkins/Kutscher                                           [Page 17]