Network Working Group                                                Ott
Internet-Draft                                  TZI, Universitaet Bremen
Expires: May 21, August 8, 2001                                          Perkins
                                      USC Information Sciences Institute
                                                TZI, Universitaet Bremen
                                                       November 20, 2000
                                                        February 7, 2001

                  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
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   This Internet-Draft will expire on May 21, August 8, 2001.

Copyright Notice

   Copyright (C) The Internet Society (2000). (2001). All Rights Reserved.


   The local Message Bus (Mbus) is a simple message-oriented
   coordination infrastructure for group communication within groups of
   co-located application entities. The Message Bus comprises three
   logically distinct parts: a message transport infrastructure, a
   structured message hierarchy, and a general purpose addressing
   scheme. This document deals with specifies message addressing, transport, and
   security issues procedures 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 documents.

   This document is a product of 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 and/or the authors.

Table of Contents

   1.    Introduction . . . . . . . . . . . . . . . . . . . . . . . .  4
   1.1   Application Scenarios  . . . . . . . . . . . . . . . . . . .  4
   1.2   Purpose  . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   1.3   Terminology for requirement specifications . . . . . . . . .  4
   2.    General Outline  . . . . . . . . . . . . . . . . . . . . . .  6
   3.    Common Formal Syntax Rules . . . . . . . . . . . . . . . . .  8
   4.    Message Format . . . . . . . . . . . . . . . . . . . . . . . 10
   5.    Addressing . . . . . . . . . . . . . . . . . . . . . . . . . 12 11
   5.1   Mandatory Address Elements . . . . . . . . . . . . . . . . . 13 12
   6.    Message Syntax . . . . . . . . . . . . . . . . . . . . . . . 14 13
   6.1   Message Encoding . . . . . . . . . . . . . . . . . . . . . . 14 13
   6.2   Message Header . . . . . . . . . . . . . . . . . . . . . . . 14 13
   6.3   Command Syntax . . . . . . . . . . . . . . . . . . . . . . . 15 13
   7.    Transport  . . . . . . . . . . . . . . . . . . . . . . . . . 17 16
   7.1   Local Multicast/Broadcast  . . . . . . . . . . . . . . . . . 16
   7.1.1 Mbus multicast groups for IPv4 . . . . . . . . . . . . . . . 17
   7.1.2 Mbus multicast groups for IPv6 . . . . . . . . . . . . . . . 17
   7.1.3 Use of Broadcast . . . . . . . . . . . . . . . . . . . . . . 18
   7.1.4 Mbus Port Number . . . . . . . . . . . . . . . . . . . . . . 18
   7.2   Directed Unicast . . . . . . . . . . . . . . . . . . . . . . 18
   8.    Reliability  . . . . . . . . . . . . . . . . . . . . . . . . 20 21
   9.    Awareness of other Entities  . . . . . . . . . . . . . . . . 22 23
   9.1   Hello Message Transmission Interval  . . . . . . . . . . . . 22 23
   9.1.1 Calculating the Interval for Hello Messages  . . . . . . . . 23 24
   9.1.2 Initialization of Values . . . . . . . . . . . . . . . . . . 24 25
   9.1.3 Adjusting the Hello Message Interval when the Number of
         Entities increases . . . . . . . . . . . . . . . . . . . . . 24 25
   9.1.4 Adjusting the Hello Message Interval when the Number of
         Entities decreases . . . . . . . . . . . . . . . . . . . . . 24 25
   9.1.5 Expiration of hello timers . . . . . . . . . . . . . . . . . 25 26
   9.2   Calculating the Timeout for Hello Messages . . . . . . . . . 25 26
   10.   Messages . . . . . . . . . . . . . . . . . . . . . . . . . . 26 27
   10.1  mbus.hello . . . . . . . . . . . . . . . . . . . . . . . . . 26 27
   10.2  mbus.bye . . . . . . . . . . . . . . . . . . . . . . . . . . 26 27
   10.3  . . . . . . . . . . . . . . . . . . . . . . . . . 27 28
   10.4  mbus.quit  . . . . . . . . . . . . . . . . . . . . . . . . . 27 28
   10.5  mbus.waiting . . . . . . . . . . . . . . . . . . . . . . . . 27 28
   10.6  mbus.go  . . . . . . . . . . . . . . . . . . . . . . . . . . 28 29
   11.   Constants  . . . . . . . . . . . . . . . . . . . . . . . . . 29 30
   12.   Mbus Security  . . . . . . . . . . . . . . . . . . . . . . . 30 31
   12.1  Security Model . . . . . . . . . . . . . . . . . . . . . . . 30 31
   12.2  Message Authentication  Encryption . . . . . . . . . . . . . . . . . . . 30 . . . . . . 31
   12.3  Encryption  Message Authentication . . . . . . . . . . . . . . . . . . . 32
   12.4  Procedures for Senders and Receivers . . . . . . 31 . . . . . . 32
   13.   Mbus Configuration . . . . . . . . . . . . . . . . . . . . . 32 34
   13.1  File based parameter storage . . . . . . . . . . . . . . . . 34 36
   13.2  Registry based parameter storage . . . . . . . . . . . . . . 35 37
   14.   Security Considerations  . . . . . . . . . . . . . . . . . . 36 38
   15.   IANA Considerations  . . . . . . . . . . . . . . . . . . . . 37 39
         References . . . . . . . . . . . . . . . . . . . . . . . . . 38 40
         Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 39 41
   A.    About References . . . . . . . . . . . . . . . . . . . . . . 43
   B.    Limitations and Future Work  . . . . . . . . . . . . . . . . 44
         Full Copyright Statement . . . . . . . . . . . . . . . . . . 41 45

1. Introduction

1.1 Application Scenarios

   The implementation of multiparty multimedia conferencing systems is
   one example where a simple coordination infrastructure can be
   useful: In a variety of conferencing scenarios, a local
   communication channel can provide conference-related information
   exchange between co- located 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 an end system.

1.2 Purpose

   Three components constitute the message bus: the low level message
   passing mechanisms, a command syntax and naming hierarchy, and the
   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
      concrete addressing schemes are to be defined by application
      specific profiles);

   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.

2. General Outline

   Local coordination involves a widely varying number of entities:
   some messages (such as membership information, floor control
   notifications, dissemination of 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 7) and by providing a flexible
   addressing scheme (defined in Section 5).

   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 specified in this document 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 5.

   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 host for
   conferencing or are using end systems spread across the same network
   link: in either case, the used Mbus multicast address and the port
   number may be identical 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. To eliminate the
   possibility of receiving unwanted 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 13.

3. Common Formal Syntax Rules

   This section contains some definitions of common ABNF [14] [12] syntax
   elements that are later referenced by other definitions in this

   	      base64          = base64_terminal /
                                   ( 1*(4base64_CHAR) [base64_terminal] )

   	      base64_char     = UPALPHA / LOALPHA / DIGIT / "+" / "/"
                                   ;; Case-sensitive

   	      base64_terminal = (2base64_char "==") / (3base64_char "=")

   	      UPALPHA         = %x41-5A            ;; Uppercase: A-Z

   	      LOALPHA         = %x61-7A            ;; Lowercase: a-z

   	      ALPHA           =  %x41-5A / %x61-7A   ; A-Z / a-z

   	      CHAR            =  %x01-7F
                                    ; any 7-bit US-ASCII character,
                                     excluding NUL

   	      OCTET           =  %x00-FF
                                    ; 8 bits of data

   	      CR              =  %x0D
                                    ; carriage return

   	      CRLF            =  CR LF
                                    ; Internet standard newline

   	      DIGIT           =  %x30-39
                                    ; 0-9

   	      DQUOTE          =  %x22
                                    ; " (Double Quote)

   	      HTAB            =  %x09
                                    ; horizontal tab

   	      LF              =  %x0A
                                    ; linefeed

   	      LWSP            =  *(WSP / CRLF WSP)
                                    ; linear white space (past newline)
   	      SP              =  %x20
                                    ; space

   	      WSP             =  SP / HTAB
                                    ; white space

    Taken from RFC 2234 [14] [12] and RFC 2554 [15]. [13].

4. 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[6]) calculated
      hash value of the entire message (starting from the ProtocolID
      field) as described in Section 12 and Section 13.

      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 milliseconds since 00:00:00, UTC, January 1,

      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 5.

      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 5.

      The AckList field comprises a list of SeqNums for which this
      message is an acknowledgment. See Section 8 for details.

   The header is followed by the message body which contains zero or
   more commands to be delivered to the destination entity. The syntax
   for a complete message is given in Section 6.

   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.

5. Addressing

   Each entity on the message bus SHOULD respond has a unique Mbus address that is used to
   identify the entity. Senders and receivers of messages sent to
   one (or more) are
   identified by their Mbus addresses. Addresses Mbus 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

   Note that this and other ABNF definitions in this document use the
   core rules defined in Section 3.

   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.

5.1 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] RFC 2373[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 a
      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 7). 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:


6. Message Syntax

6.1 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 12.

6.2 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 12 followed by CRLF.

   The other fields in the header are separated by white space characters,
   and followed by CRLF. The format of the header is as follows:

   msg_header = MsgDigest CRLF "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 4). Here
   are the ABNF syntax definitions for the header fields:

   	      MsgDigest   = base64

   	      SeqNum      = 1*10DIGIT

   	      TimeStamp   = 1*10DIGIT

   	      MessageType = "R" / "U"

   	      ScrAddr     = mbus_address

   	      DestAddr    = mbus_address

   	      AckList     = "(" *(1*DIGIT)) ")"

    See Section 5 for a definition of "mbus_address".

   The syntax definition of a complete message is as follows:

   	      mbus_message = msg_header *1(CRLF msg_payload)

   	      msg_payload  = mbus_command *(CRLF mbus_command)

    The definition of production rules for Mbus command is given below.

6.3 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 ... )

   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:

   	      val             = Integer / Float / String / List / Symbol / Data

   	      Integer         = *1"-" 1*DIGIT

   	      Float           = *1"-" 1*DIGIT "." 1*DIGIT

   	      String          = DQUOTE *CHAR DQUOTE
   	                        ; see below for escape characters

   	      List            = "(" *(val *(1*WSP val)) ")"

   	      Symbol          = ALPHA *(ALPHA / DIGIT / "_" / "-" / ".")

   	      Data            = "<" *base64 ">"

   Boolean values are encoded as an integer, with the value of zero
   representing false, and non-zero representing true.

   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[6])
   character strings surrounded by "< " and "> "

   The ABNF syntax definition for Mbus commands is as follows:

   	      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). Applications Application
   profiles MUST NOT define commands starting with "mbus.".

   The Mbus addressing scheme defined in Section 5 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 10).

7. Transport

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

   1.  Local multicast/broadcast:
       This transport class MUST be used for all messages that are not
       sent to a fully qualified target address. It MAY also be used
       for messages that are sent to a fully qualified target address.
       It MUST be provided by conforming implementations. See Section
       7.1 for details.

   2.  Directed unicast:
       This transport class MAY be used for messages that are sent to a
       fully qualified destination address. It is OPTIONAL and does not
       have to be provided by conforming implementations.

   Note that "unicast", "multicast" and "broadcast" mean IP-Unicast,
   IP-Multicast and IP-Broadcast respectively. It is possible to send a
   Mbus message that is addressed to a single entity using

   Since messages

   Messages are transmitted in UDP datagrams, a maximum message 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.

   Note that "unicast", "multicast" and "broadcast" mean IP-Unicast,
   IP-Multicast and IP-Broadcast respectively. It is possible to send
   an Mbus message that is addressed to a single entity using
   IP-multicast. This specification deals with both Mbus over UDP/IPv4
   and Mbus over UDP/IPv6.

7.1 Local Multicast/Broadcast

   Local multicast (host-local or link-local, see

   In general, the Mbus configuration
   (Section 13)), uses multicast with a limited scope relative for message
   transport. Two different Mbus multicast address scopes are defined:

   1.  host-local

   2.  link-local

   Participants of an Mbus session have to know the
   administratively scoped multicast space as described address
   in RFC
   2365[11]. This scope relativ advance -- it cannot be negotiated during the session since it is
   already needed for any communication between the participants. I can
   also not be allocated prior to an Mbus session because there would
   be no mechanism to announce the allocated address to all potential
   Mbus participants. Therefore the multicast address cannot be
   allocated dynamically, e.g. using multicast address allocation
   protocols, but has yet to be assigned statically. This document defines
   the use of statically assigned addresses and also provides a
   specification of how an Mbus session can be configured to use
   non-standard addresses (see Section 15). Implementations MUST NOT 13).

   An Mbus session can be configured to use either one of the mentioned
   scopes. The following sections specify the use of multicast
   addresses for IPv4 and IPv6.

7.1.1 Mbus multicast groups for IPv4

   For IPv4, there are two address ranges for "local scope" multicast:

   The IPv4 Local Scope -- is the minimal
      enclosing scope for administratively scoped multicast (as defined
      by RFC 2365[10]) and not further divisible -- its exact extent is
      site dependent. Allocating a statically assigned address in this
      scope would require to allocate a scope relative multicast
      address (the high order /24 in every scoped region is reserved
      for relative assignments), because the main address space is to
      be assigned dynamically, e.g. by using address allocation

   The IPv4 statically assigned link-local scope -- is the address range for statically assigned
      multicast address for link-local multicast. Multicast routers
      should not forward any multicast datagram with destination
      addresses in this range, regardless of its TTL.

   Because of the unexact extent of scopes
   larger and the fact
   that the only way to allocate a static address is the use of an
   assigned scope relative address the Mbus uses an multicast address
   from the statically assigned link-local scope (

   Host-local Mbus scope in an IPv4 environment MUST be implemented by
   using an IPv4 link-local address and an IP-Multicast-TTL of zero.
   Link-local Mbus scope in an IPv4 environment MUST be implemented by
   using an IPv4 link-ocal Scope address and an IP-Multicast-TTL
   greater than zero.

   The IPv4 link-local multicast address has yet to be assigned (see
   Section 15).

7.1.2 Mbus multicast groups for IPv6

   IPv6 has different address ranges for different multicast scopes and
   distinguishes node local and link local scopes, that are implemented
   as a set of address prefixes for the different address ranges (RFC
   2373[18]). The link-local scope.

   There prefix is FF02, the node-local prefix is
   FF01. A permanently assigned multicast address will also be a fixed, registered port number used for Mbus
   multicast communication, i.e. an address that is independent of the
   scope value and that can be used for all Mbus
   entities scopes. Implementations for
   IPv6 MUST use. Until use the scope independent address and port number are assigned the values given in appropriate
   prefix for the selected scope. For host-local Mbus communication the
   IPv6 node-local scope prefix MUST be used, for link-local Mbus
   communication the IPv6 link-local scope prefix MUST be used.

   The permanent IPv6 multicast addresses has yet to be assigned (see
   Section 15 15).

   If a single application system is distributed across several
   co-located hosts, link local scope SHOULD be used. 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. See
   Section 13 for specifications of Mbus configuration mechanisms.

7.1.3 Use of Broadcast

   In situations where multicast is not available, broadcast MAY be
   used instead. In these cases an IP broadcast address for the
   connected network SHOULD be used for sending. The node-local
   broadcast address for IPv6 is FF01:0:0:0:0:0:0:1, the link-local
   broadcast address for IPv6 is FF02:0:0:0:0:0:0:1. For IPv4, the
   generic broadcast address (for link-local broadcast) is It is RECOMMENDED that IPv4-implementations use the
   generic broadcast address and a TTL of zero for host-local

   Broadcast MUST NOT be used in situations where multicast is
   available and supported by all systems participating in
   an Mbus session.

   If a single application system is distributed across several
   co-located hosts, link local scope SHOULD be used by all systems participating in an Mbus

   See Section 13 for multicasting specifications of how to configure the use of

7.1.4 Mbus messages Port Number

   There will also be a fixed, registered port number that potentially have recipients on the other hosts.
   The all Mbus protocol is not intended (and hence deliberately not
   designed) for communication between hosts not on
   entities MUST use. Until the same link. address and port number are assigned
   the values given in Section 15 SHOULD be used.

7.2 Directed Unicast

   Directed unicast (via UDP) to the port of a specific application is
   an alternative transport class. Directed unicast is an OPTIONAL
   optimization and MAY 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. Note that the DestAddr field of such
   messages MUST still be filled in properly. 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 15).

   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

   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 an Mbus address is unique considering the current set of
   Mbus entities:

   		  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 5. (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 is equal to the first address
      element. An address element is considered equal to another
      address element if it provides the same values for both of the
      two address element fields (key and value).

8. 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 10.1)). A sending entity
   MUST NOT send a message reliably if the target address is not
   unique. (See Transport (Section 7) 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 exptected to be rare, achieving reliability for
   such messages would require full knowledge of the membership for
   each subgroup which is deemed too much effort. The desired effect
   can be achieved by application layers by sending individual reliable
   messages to each fully qualified destination address, if the
   membership information for the Mbus session is available.

   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 transmission or 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 actions
   (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 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 IP-multicast or via IP-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 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 reception 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:





9. 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 10.1) and mbus.bye (Section
   10.2). 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 9.2) 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 an
   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 9.1 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 9.2.
   Section 10 specifies the command synopsis for the corresponding Mbus

9.1 Hello Message Transmission Interval

   Since Mbus sessions may vary in size concerning the number of
   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 an 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.

9.1.1 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

   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 9
   provides a specification of how to obtain the number of currently
   known entities. Section 11 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 11).

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

9.1.2 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 9.1.1. The next hello message is
   scheduled for transmission at hello_n.

9.1.3 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 9.1.5).

9.1.4 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
   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.

9.1.5 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 9.1.1 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.

9.2 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 9.1.1 specifies how to obtain hello_d. Section 11 defines
   values for the constants c_hello_dead and c_hello_dither_max.

10. 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.

10.1 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 9.

   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 11).

10.2 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 entities.



      Parameters: - none - can be used to solicit other entities to signal their
   existence by replying with a mbus.hello message. Each protocol
   implementation MUST understand 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 11).

   As specified in Section 10.1 hello messages MUST be sent unreliably
   to all Mbus entities. This is also the case for replies to ping
   messages. An entity that replies to 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 9.1.1 this can be achieved by setting hello_p to hello_now.) 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.

10.4 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.

10.5 mbus.waiting



         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 to 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.

10.6 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.

11. 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                      |     -        |

12. Mbus Security

12.1 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 12.2). 12.3). 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 ([20]) message
   authentication MUST be provided by conforming implementations.

   Privacy of Mbus message transport can be achieved by optionally
   using symmetric encryption methods (Section 12.3). 12.2). 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 13 defines given Mbus
   configuration entity. Section 13 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 13 is applicable alternative methods of
   configuring Mbus protocol entities MAY be deployed.

   The algorithms and procedures for applying encryption and
   authentication techniques are specified in the following sections.

12.2 Encryption

   Encryption of messages is OPTIONAL, that means, an Mbus MAY be
   configured not to use encryption.

   Implementations can choose between different encryption algorithms.
   Either AES [17], DES [15], 3DES (triple DES) [15] or IDEA [21]
   SHOULD be used for encryption. Implementations MUST at least provide
   AES and it is RECOMMENDED that they support the other algorithms as

   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.

   The length of the encryption keys is determined by the currently
   used encryption algorithm. This means, the mandatory and optional
   parameters configured encryption key
   MUST NOT be shorter than the native key length for the currently
   configured algorithm.

   DES implementations MUST use the DES Cipher Block Chaining (CBC)
   mode. DES keys (56 bits) MUST be encoded as well 8 octets as storage procedures for different platforms.
   Only described in cases where none of the options for configuration entities
   RFC1423[11], resulting in Section 13 is applicable alternative methods of
   configuring 12 Base64-encoded characters. IDEA uses
   128-bit keys (24 Base64-encoded characters). AES can use either
   128-bit, 192-bit or 256-bit keys. For Mbus protocol entities MAY encryption only 128-bit
   keys (24 Base64-encoded characters) MUST be deployed.

12.2 used.

12.3 Message Authentication

   Either MD5 [16] or SHA-1 [17] SHOULD be used for

   For authentication of messages, hashed message authentication codes (MACs). An implementation MAY provide SHA-1,
   whereas MD5
   (HMACs) as described in RFC2104[4] are deployed. In general,
   implementations can choose between a number of digest algorithms.
   For Mbus authentication, the HMAC algorithm MUST be implemented. To generate applied in the
   following way:

      The keyed hash values value is calculated using the HMAC algorithm described
      specified in RFC2104[4] RFC2104[4]. The concrete hash algorithm and the
      secret hash key MUST be applied with obtained from the Mbus configuration (see
      Section 13).

      The keyed hash values (see RFC2104[4]) MUST be truncated to 96
      bits (12 bytes). The octets).

      Subsequently, the resulting hash values 12 octets 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.

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

12.3 Encryption characters (see RFC1521[6]).

   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 MD5 [14] or encryption laws would SHA-1 [16] SHOULD be
   offended otherwise. However, the used for message
   authentication codes (MACs). An implementation MAY provide MD5,
   whereas SHA-1 MUST be implemented.

   The length of DES the hash keys is
   RECOMMENDED as a baseline. DES implementations determined by the selected hashing
   algorithm. This means, the configured hash key MUST use NOT be shorter
   than the DES
   Cipher Block Chaining (CBC) mode. For native key length for the currently configured algorithm.

12.4 Procedures for Senders and Receivers

   The mandatory subset of algorithms requiring
   en/decryption data to that MUST be padded to certain boundaries octets with provided by
   implementations is AES and SHA-1.

   See Section 13 for a
   value specification of 0 SHOULD be used notations for padding characters. The padding
   characters Base64-strings.

   A sender MUST apply the following operations to a message that is to
   be appended after calculating sent:

   1.  If encryption is enabled, the message digest
   when encoding and MUST be erased before recalculating encrypted using
       the message
   digest when decoding. IDEA uses 128-bit keys (24 Base64-encoded
   characters). DES keys (56 bits) configured algorithm and the configured encryption key.
       Padding (adding extra-characters) for block-ciphers MUST be encoded as 8 octets
       applied as
   described specified in RFC1423[13], Section 12.2. If encryption is not
       enabled, the message is left unchanged.

   2.  Subsequently, a message authentication code (MAC) for the
       encrypted message MUST be calculated using the configured
       HMAC-algorithm and the configured hash key.

   3.  The MAC MUST then be converted to Base64 encoding, resulting in
       12 Base64-encoded characters. Base64-charcters as specified in Section 12.3.

   4.  At last, the sender MUST construct the final message by placing
       the encrypted message after the base64-encoded MAC and a CRLF.
       The mandatory subset of algorithms ABNF definition for the final message is as follows:

   final_msg = MsgDigest CRLF encr_msg

   MsgDigest = base64

   encr_msg  = *OCTET

   A receiver MUST apply the following operations to a message that it
   has received:

   1.  Separate the base64-encoded MAC from the encypted message and
       decode the MAC.

   2.  Re-calculate the MAC for the message using the configured
       HMAC-algorithm and the configured hash key.

   3.  Compare the original MAC with re-calculated MAC. If they differ,
       the message MUST NOT be provided by
   implementations decrypted and parsed further.

   4.  If encryption is DES enabled, the message MUST be decrypted using
       the confiured algorithm and MD5.

   See Section 13 for the configured encryption key.
       Trailing octets with a specification value of notations for Base64-strings. 0 MUST be deleted.

13. 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 multicast 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 7)):


         The non-standard multicast/broadcast multicast address to use for message

      Use of Broadcast

         It can be specified whether broadcast should be used. If
         broadcast has been configured implementations SHOULD use the
         network broadcast address (as specified in Section 7.1.3)
         instead of the standard multicast address.

      Port Number

         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. For other systems it is RECOMMENDED that the
   file-based configuration mechanism is 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[14]) RFC2234[12]) productions that are later
   referenced for the definitions for the configuration file syntax and
   registry entries:

   algo-id                 =    "NOENCR" / "AES" / "DES" / "3DES" / "IDEA" /
                                "HMAC-MD5-96" / "HMAC-SHA1-96"

   scope                   =    "HOSTLOCAL" / "LINKLOCAL"

   key                     =    base64

   version_number          =    1*10DIGIT

   key_value               =    "(" algo-id "," key ")"

   address                 =    IPv4address / IPv6address / "BROADCAST"

   port                    =    1*5DIGIT

   Given the definition above, a key entry MUST be specified using this


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

   A Base64 string consists of the characters defined in the Base64
   char-set (see RFC1521[6]) including all eventual padding characters,
   i.e. the length of a Base64-string is always a multiple of 4.

   The scope parameter is used to configure an IP-Multicast scope and
   may be set to either "HOSTLOCAL" or "LINKLOCAL". Implementations
   SHOULD choose an appropriate IP-Multicast scope depending on the
   value of this parameter and construct an effective IP-Address using
   considering the Mbus scope relative address (see specifications of Section 15). Implementations
   MUST NOT 7.1.

   The use scopes larger than link-local scope. Note that the IPv4
   local scope of broadcast is Host-local scope in an IPv4
   environment MUST be implemented configured by using a local scope providing the value
   "BROADCAST" for the address and
   an IP-Multicast-TTL of zero. Link-local scope in an IPv4 environment
   MUST be implemented by using a local scope field. If broadcast has been configured,
   implementations SHOULD use the network broadcast address and an
   IP-Multicast-TTL of one. For IPv6 for the link-local prefix is FF02,
   used IP version instead of the
   node-local prefix is FF01. standard multicast address.

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

13.1 File based parameter storage

   The file name for a an 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:

   	      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=" address

    Please refer to [3] for productions of IPv4address.

   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:


13.2 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:

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

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

14. Security Considerations

   The Mbus security mechanisms are specified in Section 12.1.

   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 intended to be the
   cryptographically stronger function ([18]).

   Similar remarks can be made on the encryption functions. The base
   specification requires DES, an algorithm provide reasonable
   security by mandating algorithms and key lengths that has shown are currently
   considered to be
   vulnerable cryptographically strong enough.

   However, in order to brute-force attacks ([18], [19]).

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

   o  The problem of receiving unauthenticated messages allow for efficiency it is considered allowable to be the main security threat use
   cryptographically weaker algorithms, for Mbus communication. We believe
      that example HMAC-MD5 instead of
   HMAC-SHA1. Furthermore, encryption can be turned off completely if
   privacy is sufficiently secure as provided by other means or not considered important for a baseline algorithm. For
      application requiring special security concerning authentication
   certain application.

   Users 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 should therefore be aware of Mbus communication this is feasible for most use
      cases. In case the base DES encryption is not considered
      sufficient there is still selected security
   configuration and should check if it meets the possibility security demands for
   a given application. Since every implementation MUST provide the
   cryptographically strong algorithm it should always be possible to use implementations
   configure an Mbus in a way that implement 3DES or IDEA.

   However, secure communication with
   authentication and privacy is ensured.

   In any way, 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. When using of
   administratively scoped multicast users cannot always assume the
   presence of correctly configured boundary routers. In these cases
   the use of encryption SHOULD be considered if privacy is desired.

15. IANA Considerations

   The IANA is requested to assign a port number link-local IPv4 multicast address
   from the address space, an IPv6 permanent multicast
   address and a scope relative
   multicast address. port number. For the time being the tentative IPv4
   multicast address and the port number 47000
   (decimal) SHOULD be used.


   [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.

        MESSAGES", August 1982.

   [6]  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

   [7]  Handley, M., Crowcroft, J., Bormann, C. and J. Ott, "The
        Internet Multimedia Conferencing Architecture", Internet Draft
        draft-ietf-mmusic-confarch-03.txt, July 2000.

   [8]  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.

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


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


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


   [13]  Myers, J., "SMTP Service Extension for Authentication", RFC
         2554, March 1999.


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


   [15]  U.S. DEPARTMENT OF COMMERCE/National Institute of Standards
         and Technology, "Data Encryption Standard (DES)", FIPS PUB
         46-3, Category Computer Security, Subcategory Cryptography,
         October 1999.

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

   [17]  Daemen, J.D. and V.R. Rijmen, "AES Proposal: Rijndael", March

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

   [19]  Handley, M., Crowcroft, J., Bormann, C. and J. Ott, "The
         Internet Multimedia Conferencing Architecture", Internet Draft
         draft-ietf-mmusic-confarch-03.txt, status: non-normative, July

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

   [21]  Schneier, B., "Applied Cryptography", Edition 2, Publisher
         John Wiley & Sons, Inc., status: non-normative, 1996.


   [22], "Project DES", WWW, status: non-normative, 1999.

   [20]  Microsoft, "BUG: Winsock Sends IP Packets with TTL 0", WWW, March 1999

Authors' Addresses

   Joerg Ott
   TZI, Universitaet Bremen
   Bibliothekstr. 1
   Bremen  28359

   Phone: +49.421.201-7028
   Fax:   +49.421.218-7000
   Colin Perkins
   USC Information Sciences Institute
   4350 N. Fairfax Drive #620
   Arlington VA 22203


   Dirk Kutscher
   TZI, Universitaet Bremen
   Bibliothekstr. 1
   Bremen  28359

   Phone: +49.421.218-7595
   Fax:   +49.421.218-7000

Appendix A. About References

   Please note that the list of references contains normative as well
   as non-normative references. Each Non-normative references is marked
   as "status: non-normative". All unmarked references are normative.

Appendix B. Limitations and Future Work

   The Mbus is a light-weight local coordination mechanism and
   deliberately not designed for larger scope coordination. It is
   expected to be used on a single node or -- at most -- on a single
   network link.

   Therefore the Mbus protocol does not contain features that would be
   required to qualify it for the use over the global Internet:

      There are no mechanisms to provide congestion control. The issue
      of congestion control is a general problem for multicast
      protocols. The Mbus allows for un-acknowledged messages that are
      sent unreliably, for example as event notifications, from one
      entity to another. Since negative acknowledgements are not
      defined there is no way the sender could realize that it is
      flooding another entity or congesting a low bandwidth network

      The reliability mechanism, i.e. the retransmission timers, are
      designed to provide effective, responsive message transport on
      local links but are not suited to cope with larger delays that
      could be introduced from router queues etc.

   Some experiments are currently underway to test the applicability of
   bridges between different distributed Mbus domains without changing
   the basic protocol semantics. Since the use of such bridges should
   be orthogonal to the basic Mbus protocol definitions and since these
   experiments are still work in progress there is no mention of this
   concept in this specification.

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