INTERNET-DRAFT                                Carsten Bormann, Joerg Ott
Expires: December 1999                               Universitaet Bremen
                                                            Nils Seifert
                                                               June 1999

                   MTP/SO: Self-Organizing Multicast

Status of this memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC 2026.

   Internet-Drafts are working documents of the Internet Engineering
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   MTP/SO is a reliable multicast protocol based on the earlier
   protocols MTP (RFC1301) and MTP-2, simplifying the protocol
   considerably while adding self-organization of the members of the
   group into a hierarchy of local regions, local retransmissions, local
   NAK damping, and both global and local forward error correction.
   MTP/SO retains the coordinated many-to-many multicast model of MTP-2
   while improving scalability.

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1.  Introduction

   Multiparty cooperative applications have received much attention over
   the past years, as has the multicasting of datagrams in the Internet.
   The Internet datagram multicasting mechanism is not reliable, often
   requiring a higher level protocol to achieve the level of reliability
   required for an application.

   Much of the early work on reliable multicast protocols has assumed
   relatively stable groups that need to ensure that all messages are
   eventually received by all members of this well-defined group.
   Recently, work on loosely coupled teleconferencing has directed
   attention to a class of multicast applications that scale up to an
   extent where this assumption is no longer practical.  Many other
   applications in the area of synchronous groupware also do not need
   the strong property of reliability, but can nonetheless benefit from
   a multicast protocol providing some weaker form of reliable

   An interesting multicast transport protocol with a somewhat relaxed
   view of reliability is defined in RFC 1301 [1].  MTP can be used with
   unreliable and not necessarily sequence preserving underlying
   multicast (or broadcast) network protocols such as IP multicast.  MTP
   provides globally ordered, receiver reliable, rate controlled and
   atomic transfer of messages to multiple recipients.

   A revised version of MTP, the Multicast Transport Protocol MTP-2, has
   been used for a number of applications for some time [2].  MTP-2 has
   been designed to avoid some of the practical problems experienced in
   using MTP and introduces a number of additional facilities that
   increase its utility.  In particular, MTP-2 no longer has a single
   point of failure.

   This document defines Self-Organizing Multicast, MTP/SO.  MTP/SO uses
   MTP-2 as a basis and adds spontaneous self-organization of the
   members of the group into a hierarchy of local regions.  Scalability
   is increased by providing passive group joining and local
   retransmission of lost packets, as well as forward error correction

2.  Requirements

   Even more so than for unicast protocols, there are difficult trade-
   offs in designing a multicast protocol.  It is unlikely that a single
   reliable multicast protocol can be applicable to all kinds of
   multicast applications, from a small set of replicated database
   systems synchronizing their updates, to distributed interactive
   simulation systems with hundreds of thousands of processes joining
   and leaving large numbers of groups with high frequency.

   Any design of a protocol that aims to cover a part of the ground must
   therefore be explicit about the specific requirements the designers
   had in mind.  Concentrating on any single objective is unlikely to

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   yield a generally applicable protocol.  In this section, we list what
   we perceive to be the main requirements that went into the design of
   MTP/SO, in order of importance.

 o Scalability

   While the actual usage pattern of synchronous group communication
   software is not yet known, it is clear that groups of wildly
   different sizes will need to be accommodated.  A protocol that is not
   scalable to large groups with a significant rate of membership change
   will not be a viable multicast platform.

   Many existing protocols that focus on reliability require a positive
   acknowledgment from each recipient to the sender of each message.
   This does not scale to large groups without elaborate aggregation
   schemes.  Also, group management algorithms that require an
   acknowledgment from each member to accept a new member are not
   acceptable in large groups (in particular, building a group creates
   an n-square problem).

   As a first level of attack, this scaling problem can be circumvented
   by using negative acknowledgements (NAKs).  Unfortunately, this also
   conflicts with a strict reliability requirement: Not every failure
   will be immediately detected, since the normal behavior of a
   recipient, i.e. being silent, cannot be distinguished from a silent
   failure.  There is a trade-off between scalability and the kind of
   reliability that can be realized.

 o Efficiency

   A reliable multicast protocol should be comparable in performance to
   special protocols specifically designed for an application.  Just as
   TCP generally is slightly less efficient than a specially designed
   protocol would be, some more packets and additional per-packet
   overhead as well as some additional processing time will be
   tolerable.  However, the protocol needs to be in the same class of
   overhead to be applicable to an application.

 o Robustness and Reliability

   A reliable multicast protocol should obviously be ``reliable'' in
   some sense.  Given the conflict with scalability, we define
   reliability to mean: A recipient can (within bounded time) find out
   when it is failing or being partitioned from active senders.  A
   sender is assured (with sufficient probability) that all its messages
   reach within bounded time all recipients that are not failing or
   being partitioned.

   Obviously, this strict definition of reliability needs to be
   complemented by some measure of robustness: A protocol that declares
   failure or creates significant delays in the face of trivial errors
   may meet this definition but is not useful.  In a teleconferencing
   environment, a desirable robustness property is the ability to

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   continue operating within partitions should the group become
   partitioned.  Ultimately, the applications that use the multicast
   transport platform should be the ones to decide when the situation
   has deteriorated to a point where continuing is meaningless.

 o Ordering

   Many applications are simplified considerably when (at least a
   certain subset of all) messages exchanged in the group arrive in the
   same order at all recipients, even if originated at different
   senders.  This requirement distinguishes MTP/SO from other multiple
   sender multicast protocols such as SRM [5] that work best when the
   shared state of the multicast group is the (commutative and
   associative) sum of the independent contributions of all

3.  Overview

   This section gives an overview over the protocol functions of MTP/SO.
   (Note to readers that have seen MTP or MTP-2: This overview is given
   in terms that are more generic than those used in older protocol
   definitions.  In particular, the terms group, coordinator, sender,
   and receiver have been substituted for the traditional terms web,
   master, producer, and consumer.)

   In MTP/SO there are three different roles of members in a group:
   coordinator, sender and receiver.  The coordinator provides the
   message ordering for all members in a group and oversees the global
   rate management.  Senders send data in messages (each sent as a
   sequence of one or more data packets) after obtaining a token from
   the designated coordinator.  Receivers receive these messages and
   request the retransmission of packets that did not arrive.

   In MTP/SO, many actions such as retransmitting control packets or
   requesting retransmissions depend on a time interval that is a
   parameter to the whole group.  This interval is called heartbeat and
   is measured in microseconds.

3.1.  Global ordering

   The coordinator assigns a global sequence number to each message.  In
   the simplest mode of transmission, before a sender is allowed to
   start sending a new message, it has to obtain a token from the
   coordinator.  This can be done by transmitting a special request
   packet to the coordinator or by sending the request along with data
   packets belonging to other messages.  The coordinator answers with a
   confirm packet, which contains the sequence number for the new
   message.  Senders will then send this sequence number in every data
   packet belonging to the message.  It is the responsibility of the
   receivers to deliver messages in the correct order to the
   applications, if sequenced delivery has been specified for a message.

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   This results in an ordering class called ``global ordering'', which
   means that even when there are many senders simultaneously sending
   messages, every receiver will receive the messages in the same order
   (which comes close to the order in which the tokens were requested).

   As the sequencing will quite often result in an additional delay (for
   example when a short message is preceded by a very long one),
   applications can assign messages to different streams.  A message is
   delivered irrespective of messages belonging to other streams, even
   if these carry lower sequence numbers.  By using streams,
   applications can avoid unnecessary delays, simply by assigning
   independent messages to different streams.

   A message that can be processed independent of the ones preceding it
   can be marked with a sequencing_off bit.  Messages so marked can be
   immediately delivered to the application by receivers, even if the
   stream numbers of preceding messages are still unknown.

   Normally the coordinator grants the tokens in the same order the
   token request packets are received.  If there is a need to transmit
   some messages with a higher priority, applications can assign a
   priority to every message.  This priority is only considered while
   granting a token (hence only when there are many tokens requested at
   the same time) and has no effect on the transmission rate of the
   message once a token has been assigned.  As a result, when a sender
   sends messages with different priorities, it is no longer guaranteed
   that these are received in the same order they were queued for
   sending -- if they are in the same stream, they are, however,
   received in the same order by all receivers (including the sender).

3.2.  Rate and Load management

   Rate management is overseen by the coordinator.  A parameter global
   to the group defines the maximum bandwidth to be used by the group.
   The coordinator dynamically adjusts a per-message parameter called
   window to divide up the total rate into the number of tokens
   currently granted, controlling the inter-packet-interval at which
   senders pace data packets belonging to one message.  So the
   coordinator can ensure that the maximum throughput defined for the
   group is not exceeded.

   An argument often heard against using a central coordinator is that
   it might limit scalability by becoming a bottleneck.  First, it needs
   to be noted that in the worst case (all messages are one packet long)
   the coordinator handles three times as many packets as each other
   group member that does not send: one (small) token request, one
   (small) token confirm, and the actual reception of the data packet.
   There is no scalability problem involved, except the general problem
   that many active senders will generate many packets (independent of
   whether coordinated or not).

   There remains one problem, however.  If more members desire to send
   than can be granted a token at any one time, a distributed queue

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   needs to be formed.  To be able to sustain large queues of senders,
   the coordinator maintains a global damping factor d for token
   requests.  A new value for d is distributed every heartbeat by the
   coordinator (as a rounded-down base 2 logarithm).  In normal
   operation, d is 1.  When requesting a token or retransmitting this
   request, senders use the current value of 1/d in each heartbeat as
   the probability for actually sending the request in this heartbeat.
   Senders echo the damping factor used in each token request actually
   sent.  The coordinator weighs the token requests by their damping
   factor to allocate tokens.  Piggy-backed token requests are
   considered to have a damping factor of one (no damping is applied to
   piggy-backed token requests).

   The coordinator computes d as:

           max(1, w / (max(12,k)*2) - 1)

   where w is the sum of the echoed damping factors received in token
   requests during the last heartbeat and k is an exponentially weighted
   moving average of the number of token requests granted in recent

3.3.  Atomicity

   Atomicity (arrival of a message at all members or at none of the
   members) is a desirable property of a group communication protocol.
   Unfortunately, full atomicity requires collecting positive
   acknowledgements from all group members until a message can be acted
   upon, too heavy-weight for the goals of MTP/SO.  Instead, MTP/SO
   defines a lighter-weight form of atomicity that is still useful for
   many applications.

   At any point in time, each message is assigned a state by the
   coordinator: pending, accepted, or rejected.

   The state of a message is set to accepted when the coordinator did
   receive the complete message.  As soon as a sender notices one of its
   messages to be accepted, it sends an acknowledgement of successful
   transmission to its application.  Such an acknowledgement does not
   mean that every receiver received the message.  It only guarantees
   that at least the coordinator was able to receive it correctly.  (It
   also provides the sequence number assigned to the message so that the
   application can order its own messages with respect to other messages
   it may have received).

   A message marked as rejected was not completely received (even after
   requesting retransmissions) by the coordinator.  Normally, every
   receiver will drop such a message and the sender of the message will
   indicate an unsuccessful-transmission error to its application.

   Receivers do not deliver pending or rejected messages to the
   application.  If a specific receiver does not completely receive a
   message (even after requesting retransmissions) that is finally

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   marked by the coordinator as accepted, it will signal this as an
   unsuccessful-reception error to its application.

   In summary, it is guaranteed that a message was either delivered
   correctly to every receiver, that it was delivered to no receiver and
   the sender is signalled an error, or that any receiver that did not
   receive the message is signalled an error.  (Of course, the protocol
   works hard to minimize the number of such errors, but the above
   statements are guarantees of the protocol.)

   Atomicity increases the message latency: applications need to wait
   for the accepted state propagating from the coordinator before they
   can act on a message.  In order to allow every member to quickly
   learn about the state of messages, every packet contains a copy of
   the most recent information available about the state of the most
   recent messages.  If application semantics do not require atomicity,
   unnecessary delay can be avoided by marking a message such that it is
   delivered to applications even before accepted by the coordinator

3.4.  Retransmission

   Receivers request retransmissions of data packets when there is a gap
   in the sequence numbers of data packets received for a message or if
   no further data packet has arrived for more than one heartbeat while
   the message is still incomplete.  In case all data packets for a
   message have been lost, this will be recognized from the message
   state of packets from following messages or when the coordinator
   propagates the state of the most recent messages.  In any case, the
   request for retransmission can be generated at the latest after two
   full heartbeats.

   Retransmission requests, or NAKs (negative acknowledgements) for
   short, are multicast to the group to reduce the implosion problem.
   Receivers dither the time at which they send NAKs and postpone
   sending a NAK when they have recently received one or more NAKs that
   together cover the same set of packets.

   In order to answer NAKs, senders keep a copy of every data packet
   they sent.  To limit the number of packets stored, senders are
   allowed to discard these copies after a defined period of time which
   is measured in heartbeats and depending on a special factor called
   retention.  After retention+4 heartbeats the copies are no longer
   available and requests for retransmissions received after that period
   are ignored.  This makes sure packets are available for at least
   retention retransmissions.

   Nonetheless there is a nonzero probability that all retransmissions
   (or retransmission requests) related to a packet are lost and some
   receivers do not receive the message correctly.  For example a
   network partitioning that lasts longer than heartbeat*retention will
   result in lost messages.

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   This sounds undesirable, but it is similar to the retry limit used in
   positively acknowledged protocols, only that the normally relatively
   small value of heartbeat*retention puts a limit to the length of an
   outage that can be tolerated.  We assume that the application
   protocol will have a way to handle receivers that experience such a
   long gap in reception, because it already needs a way to treat new
   members that appear late in the group.  Note that for applications
   where this is undesirable, MTP/SO could be augmented by the
   equivalent of log servers [3].  In any case, MTP/SO guarantees that
   when a message was not completely received by every receiver, either
   the affected receivers or the responsible sender will indicate the
   error to the application.

3.5.  Self-organization and Repetitors

   Once MTP/SO groups get large, even the handling of NAK-based
   retransmission traffic becomes a scalability problem.  As with many
   scaling problems, the obvious solution is to introduce some form of
   hierarchy into the group.  This allows at least some of the NAKs and
   resulting retransmissions to be handled locally within trunks and
   branches of that hierarchy.  As MTP/SO is a many-to-many protocol, it
   does not make much sense to base the hierarchy on the multicast tree
   from any specific sender (including the coordinator, which generally
   is not the sole sender and which may transfer its role to another
   member during the activity of the group).

   Instead, MTP/SO introduces the concept of a regional repetitor*.
   Receivers multicast NAKs locally before multicasting them to the
   entire group.  Repetitors that have previously received the requested
   data, retransmit locally after receiving a local NAK.  Repetitors
   that don't have the data simply send a NAK to the next higher level
   of hierarchy, up to the whole group (where, finally, the sender
   replies with another copy of the data).

   A prerequisite to this mechanism is a way to do a local multicast (of
   a NAK as well as of a retransmission).  In current IP multicast
   implementations, one way to define such regions is with TTL threshold
   scoping; together with an appropriate protocol; administrative
   scoping provides a similar method [7].  The algorithms described in
   the rest of this section work best when such a scoping mechanism is
   in effect; leaks or other imperfections in the scoping boundaries do
   not cause catastrophic failures, though.  The following discussion
   assumes three levels of local scopes, e.g., site, country, and
   continent; the exact choice of number and extent of scopes is a
   global parameter of the group.

   With three local scopes and one global scope, each group member is by
  * This  was  called  ``repeater''  in  earlier  presentations of
MTP/SO [6].  We are now avoiding this term as it is sometimes used
as  an  alternative  term for a transport layer ``bridge'' between
disconnected multicast domains.

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   definition in four scopes, where each local scope is contained by the
   next higher scope in the hierarchy.  Any member that takes on a
   receiver role can also decide to be a potential repetitor for any of
   the local scopes (e.g. depending on the cost structure of the
   Internet service or on the availability of local memory space).

   For scopes that contain only one member, it does not matter whether a
   member considers itself to be a repetitor for that scope or not.  For
   scopes that contain more than one member, a protocol is needed that
   makes this fact known and selects one scope member as the repetitor.
   This protocol needs not necessarily ensure that there is exactly one
   repetitor for each scope at any time, as the retransmission protocol
   still works without a repetitor or with more than one repetitor per
   scope, albeit less efficient.

   Repetitor selection should favor the ``best'' member in the scope,
   i.e. a member that has particularly good reception from the senders,
   as it is most likely that this member will have received the data to
   be able to perform a local retransmission.  Each potential repetitor
   therefore maintains a reception quality parameter that, on a first
   level of approximation, tallies the quotient of the number of
   recently correctly received packets to the number of packets that
   should have been received.

   Members that consider themselves repetitor for a scope periodically
   multicast a repetitor announcement message within the scope,
   containing the current value of the reception quality parameter.
   Potential repetitors observe these messages.  If, within the most
   local scope, a potential repetitor has a considerably better
   reception quality parameter than the current repetitor, it sends a
   repetitor announcement at the start of its next heartbeat interval
   and assumes the role of the repetitor.  Only the repetitors of the
   most local scope compete for the repetitor role of the next higher
   scope, and so on.  (A new repetitor that displaces a member that was
   repetitor at higher level scopes also announces itself as repetitor
   at these higher level scopes.)

   To better cope with repetitor failure, receivers that are not
   repetitors send NAKs at the most local scope first and escalate them
   up the hierarchy if neither a retransmission nor a more global NAK
   follows within one heartbeat.  Repetitors for a set of scopes begin
   sending NAKs within the next higher scope and then escalate them the
   same way.  Retransmissions always occur at the highest level of scope
   that the NAKs leading to that retransmission carried (NAKs have a
   scope field for this purpose).

   A repetitor that leaves a group simply sends a repetitor announcement
   with reception quality zero.  A repetitor that crashes stops sending
   repetitor announcements, causing potential repetitors to start
   sending repetitor announcements after a time interval that is
   inversely related to their reception quality parameter.

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3.6.  Coordinator function

   As it is responsible for assigning tokens and updating the message
   state, the coordinator plays a central role in MTP/SO.  If the member
   carrying the coordinator function leaves the group, the coordinator
   function will be passed to one of the remaining members

   To avoid the coordinator being a single point of failure, MTP/SO
   provides a coordinator recovery function.  This allows the group to
   elect a new coordinator when the old one crashes or becomes
   unreachable.  The new coordinator will then collect all information
   needed from the group members so that no information is lost.  (This
   protocol should be, but is not yet, integrated with the repetitor

   In order to enhance the performance of MTP/SO it may be useful to
   actively influence which member performs the coordinator function.
   For example if only one member will send messages for a longer period
   of time, the group can migrate the coordinator function to that
   member, thereby avoiding the overhead caused by requesting and
   obtaining tokens (between one and two packets for every message).
   MTP/SO allows either to request the coordinator function for oneself
   or the coordinator to pass the coordinator function to another

3.7.  Membership classes

   Not all members of the group will be in a position to take over the
   functions of a coordinator or of a repetitor.  We therefore
   distinguish several ``classes'' of members:
        class |  description
          1   |  normal member, potential coordinator and repetitor
          2   |  normal member, potential repetitor
          3   |  normal member
          4   |  unreliable receiver, normal sender
          5   |  unreliable member

   Most members of an MTP/SO group will be class 1 members, i.e. they
   are prepared to take over the coordinator role if this is required in
   a coordinator recovery*.  Class 2 members do not want to take on this
   role (for application reasons or for reasons of limited resources),
   but compete for the repetitor function.  Class 3 members take over
   neither special function, but take part as normal members in the
   group; in particular, they are allowed to send NAKs.
  * Instead,  a  single member can be designated fixed coordinator
by assuming class 0.  This means that the multicast  group  shares
its fate with the class 0 coordinator.

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   Class 4 members never send NAKs.  Their reception of messages in the
   group is therefore unreliable.  Nonetheless, they can originate
   messages that are reliably received by the class 3 or higher members
   of the group.

   Class 5 members listen only; the only packet type they can send to
   the group is unreliable multicast datagrams (not yet described in
   this version of the draft).  When a minimum quality of
   transmission/reception is defined for the group (see group[info]
   packets below), members may have to downgrade themselves to class 5
   when they find out their own quality has dropped below the acceptable

   To aid class 4 and class 5 members, and as a general optimization,
   the multicast group can be configured to immediately add a percentage
   of redundancy packets to the data packets sent.  This allows the
   receivers to reconstruct missing data packets by interpreting these
   redundancy packets.  Redundancy packets also can be independently
   added by repetitors based on the local NAK rate.

4.  Protocol Definition

4.1.  Notational Conventions

   For convenience, the datagrams transmitted by MTP/SO group members
   are called packets in this document.

   MTP/SO packet types are written major[minor], where major is the
   major type of the packet and minor is the subtype within the major
   type.  E.g., there are data[data] packets as well as data[eom]

4.2.  Protocol Functions and Packet Types

 o Heartbeat

   All members operate on a time line that is divided into heartbeats.
   The nominal length of a heartbeat is a global parameter of the group.
   The actual heartbeat boundaries (or heartbeats for short) are
   dithered around the nominal value.  Most protocol actions are
   performed at the start of a new heartbeat interval.  An exception is
   the actual transmission of data packets, which is evenly distributed
   over the heartbeat interval to which the data packets are allocated.
   Also, token requests (and token request cancellations) can be unicast
   to the coordinator and be responded to by the coordinator at any

 o Global Ordering

   A sender that wants to send a message applies for a token by
   unicasting a token[request] packet to the coordinator.

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   Alternatively, the sender can include a token request field in a data
   packet that is sent under a previously obtained token.

   As soon as a token becomes available, the coordinator replies with a
   token[confirm] containing a new global sequence number, under
   consideration of the queue of token requests and the priority of the
   token request.  The sender uses this global sequence number as the
   message number in every data packet pertaining to this message.

 o Message Acceptance

   The coordinator maintains the message acceptance state for recent
   messages.  For the 12 most recent messages, the message acceptance
   state is disseminated in every packet.  Packets sent by the
   coordinator contain the current message acceptance state; packets
   sent by other members contain a copy of the most recent message
   acceptance state available to that sender (for data packets, this is
   often the state obtained via the token[confirm] packet).  As the
   field that is used to disseminate that state only has 12 entries, the
   number of messages that can be pending at any point in time is

   To ensure that the most recent message acceptance state is always
   disseminated, the coordinator sends a group[info] packet at least* in
   every heartbeat in which no other member is scheduled to send packets
   based on tokens sent out.

 o Retransmissions

   At each heartbeat, receivers that are missing packets of a message
   multicast nak[request] packets (see also the discussion of self-
   organization and repetitors above).  A nak[request] contains a list
   of ranges of sequence numbers for one or more messages.  Ranges can
   be open, i.e. implicitly include all further packets when the ending
   packet number is not known.  A nak[request] that is received by a
   receiver postpones sending a nak[request] for the set of packets
   listed in the nak[request].  Empty nak[request] packets are never

4.3.  Addresses

   A MTP/SO group has one group address and as many member addresses as
   there are members.

   The member address is the combination of a 128-bit IPv6 host address
   (possibly in IPv4 compatibility format, i.e. with 96 bits of leading
   zeroes) and a 16-bit UDP port number.

  * In  periods  of  continuous  activity,  additional group[info]
packets are sent at a reduced rate to allow  unreliable  receivers
to join.

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   The group address is the pair of a 128-bit IPv6 multicast address
   (again, possibly IPv4 compatible) and a group-ID.  The group-ID
   simply is the member address of the current coordinator.

   MTP/SO multicasts always use the UDP destination port number 47112
   (to be assigned) and the UDP source port number from the member
   address.  MTP/SO unicasts use UDP source and destination port numbers
   in the range 47112+1 to 49152-1 (note that the number 49152 marks the
   end of the medium priority port number space in some current IP
   multicast router implementations).

4.4.  Packet Formats

                    Figure 1: Standard packet header
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
|   Version     | Type  |  Mod  |         (Port part)           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-                         -+-+
|                         (Address part)                        |
+-                                                             -+
|                                                               |
+-            For multicast packets: Group ID                  -+
|                                                               |
+-                                                             -+
|                                                               |
|  Heartbeat    |     Coordinator State Sequence Number         |
|  Retention    |     Message Acceptance Sequence Number        |
|T| Number|Prio |   |Mes|sag|e A|cce|pta|nce| St|ate| Ar|ray|   |
|             Window            |

   The standard packet header contains the following fields:

 o Version

   For the current version of MTP/SO, version is always 3.

 o Type, Mod

   Packet type and type modifier (subtype).

 o Group ID

   For multicast packets, this field gives the member address of the
   current coordinator.  For unicast packets, this field is not used.

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 o Coordinator State Sequence Number

   A sequence number for the version of the coordinator state that is
   disseminated with this message.

 o Message Acceptance Sequence Number, Message Acceptance State Array

   Let n be the message acceptance sequence number, then message
   acceptance state array contains the most recent message acceptance
   states known for messages n-1 to n-12:

   0   pending
   1   accepted
   2   rejected
   3   (reserved)

 o T, Number, Prio

   If the T bit is set, Number gives the serial number and Prio the
   priority of a token request piggybacked in this packet.

 o Heartbeat, Retention, Window

   Current values for these three global parameters of the group.  These
   parameters are given as pseudo-floating-point numbers:

   parameter   bits   mantissa (msb)   exponent (lsb)               unit
   heartbeat     8           3               5             microseconds (0 to 7*2^31)
   retention     8           4               4                       1 (0 to 15*2^15)
   window       16          11               5          microseconds (0 to 2047*2^31)

                        Figure 2: token[request]
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
                                |1| Number|Prio |1| Number|Prio |
|1| Number|Prio | . . . . . . . . . . . . . . . |0 0 0|  dlb    |
+-+-+-+-+-+-+-+-+-                              +-+-+-+-+-+-+-+-+

   A token[request] packet is unicast from a member to the coordinator
   to apply for one or more tokens.  Each of these requests for a token
   contains a serial number of that request plus a request priority.
   The first token request is carried in the token request part of the
   standard header; additional token requests can be sent in the packet
   type specific part following the standard header.  This part ends
   with a byte echoing the base 2 logarithm of the damping factor used;

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   this byte can be left off if dlb is zero.

                Figure 3: token[confirm], token[cancel]
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
                                |    New  Message   Sequence    |
|   Number      |

   A token[confirm] is unicast from the coordinator to the member that
   requested the token.  A token[cancel] can be used by the token
   holding member to return the token to the coordinator.

               Figure 4: data packets (except data[eom])
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
                                |        stream number          |
|S|A|R|0 0|O| L |         Message Sequence Number               |
|                    Packet Sequence Number                     |
|                           Data                                |
:                                                               :

   The S bit, if set, indicates that ordered delivery is not required
   for this message (``sequencing_off'').  The A bit, if set, indicates
   that atomic delivery is not required for this message
   (``atomicity_off'').  The R bit, if set, indicates that this message
   is not transmitted reliably, i.e., the producer is not going to
   answer any nak[request]s.  Consumers are expected to wait for any
   missing packet of this message for one heartbeat and then mark the
   message as not received.  The O bit (``original'') is set only for
   the first transmission of the data packet by the original sender.  It
   is reset for any kind of retransmission (regardless whether performed
   by the original sender or not).  L (``level'') is a binary number
   ranging from 0 to 3.  Level 0 indicates a global transmission; levels
   1 to 3 indicate transmission of the packets at the second most global
   to most local level scope, resp.  (For a retransmission, the
   transmission level indicates the scope in which this data packet was
   sent; lower level repetitors can use this information to decide
   whether they can defer their own retransmissions.)

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                          Figure 4a: data[eom]
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
                                |        stream number          |
|S|A|R|0 0|O| L |         Message Sequence Number               |
|                    Packet Sequence Number                     |
|             0 (AL)            |                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-                             -+
|                                                               |
+-                                                             -+
|                                                               |
+-             original sender's member address                -+
|                                                               |
+-                                                             -+
|                                                               |
:              authentication information (optional)            :
|                           Data                                |
:                                                               :

   To ensure that the original sender of a message becomes known even if
   the only packets a receiver has received from this message were
   repetitor retransmissions, the data[eom] packet differs from the
   other data packets in that it contains a copy of the original
   sender's member address.  (Note that this information is redundant
   for packets that have the O-bit set; it is retained in favor of a
   common packet format for all cases.)  With an optional authentication
   protocol (not specified in this version of the document),
   authentication information can be given with this last packet of the
   message; the length in 32-bit words is in AL.

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                          Figure 4b: data[fec]
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
                                |                0              |
|   FEC Type    | FEC Parameter |         FEC Data Size         |
|                                                               |
+            FEC Data (including modified header)               +
:                                                               :
|       0       |            Message Sequence Number            |
|                   Packet Sequence Number                      |
|       0       |            Message Sequence Number            |
|                   Packet Sequence Number                      |
:                                                               :

   Senders and repetitors can send FEC packets in addition to (or
   instead of) data packets.  A data[fec] packet contains forward error
   correction information computed out of one or more original data
   packets, including their headers, where the port part of the group
   address is replaced by a two-byte original packet length field and
   the rest of the group address is left out; these data packets are
   each identified by their message sequence number and packet sequence
   number.  FEC data size is the total size of this information.  The
   resulting information (starting at a two-byte boundary) is padded to
   a four-byte boundary.  Only data packets with the same group address
   can be combined; they are sent with a copy of this group address in
   the header of the data[fec] packet.  FEC type and parameter define
   the exact FEC code used; FEC type 1 is defined as XOR.

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                         Figure 5: nak[request]
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
                                |             0             | L |
|F|     0       |         Message Sequence Number               |
|               Packet Sequence Number (Low)                    |
|               Packet Sequence Number (High)                   |
|F|     0       |         Message Sequence Number               |
|               Packet Sequence Number (Low)                    |
|               Packet Sequence Number (High)                   |
:                                                               :

   The F bit, if set, indicates that, starting at the packet sequence
   number (low), all packets from the given message are missing.  As
   with data packets, L gives the scope level at which this NAK is being
   multicast.  NAK request packets inhibit the transmission of further
   such packets from other potential transmitters (for one heartbeat)
   only at the level of scope given.  A retransmission that is a
   response to a NAK request should be sent at the level of scope given.

                Figure 6: status[request], status[deny]
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
                                |             0             | L |
|       0       |         Message Sequence Number               |
|       0       |         Message Sequence Number               |
:                                                               :

   A status request packet can be multicast by a member to request
   status for messages that already have scrolled off the message
   acceptance state array in the standard header.  A status deny
   response indicates that the retention time for keeping information
   about the status of the messages has passed.

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                         Figure 7: status[info]
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
                                |               0               |
|U| S |    0    |         Message Sequence Number               |
|U| S |    0    |         Message Sequence Number               |
:                                                               :

   Responding to status requests, a repetitor (for local scopes) or the
   coordinator can multicast status info.  The U bit, if set, indicates
   that the status of the given message is unknown.  The S field gives
   the message acceptance state as in the message acceptance state

                         Figure 8: group[seek]
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
                                | Scope         |     0     |C|K|
| Group Name . . .

   The K-bit, if set, indicates that reliable receiver status
   (membership class 1 to 3) is intended, i.e., that an explicit
   acknowledgement for this member has to be given within a group[info].
   The C-bit, if set, indicates that the transmitter is a potential
   coordinator (membership class 1); it causes other potential
   coordinators with a higher member address to back off.  The scope
   field gives the actual scope in which this packet was transmitted
   (this cannot just be given as a scope level number as the actual
   scope levels used in this group may not yet be known to the

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                         Figure 9: group[info]
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
                                |            Quality            |
|           Activity            |       0       |   dlb   |E| L |
| TTL Scope 0   | TTL Scope 1   | TTL Scope 2   | TTL Scope 3   |
|                    Network Packet Size                        |
|     min. Receive Quality      |       min. Send Quality       |
|        Group Name Length      | Group Name ...                :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
:                           (zero-padded to 4 byte alignment)   |
:     type      |     length    |           extension           :
:-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               :
:                                                               :
:     type      |     length    |           extension           :
:-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               :
:                                                               :

   The group[info] packet is periodically transmitted by the coordinator
   and by each repetitor to ensure that all group members are aware of
   the global parameters of the group and of the quality of the current

   Three parameters give dynamic information about the transmitter and
   about the group: Quality is the (0,16 bit fixed point) product of
   reception and transmission quality of the transmitter.  Activity is a
   measure for the recent activity of this group (useful for merging
   decisions by applications).  The dlb field gives the base-2 logarithm
   of the current token request damping factor d (i.e., dlb is normally
   zero unless damping is required).

   The other fields of the packet give global group parameters that
   usually are constant: The E-Bit (``elect'') is set for group[info]
   packets originated by the coordinator in case it is willing to
   transfer the coordinator function to a higher quality member; it
   requests other potential coordinators to announce their quality (if
   better) via group[info].  L gives the scope level, and, indirectly,
   the source of the group[info]: level 0 packets are originated by the
   coordinator or by other potential coordinators (the latter if the
   source address is not equal to the coordinator part of the group
   address), level 1 to 3 packets are originated by repetitors of the
   respective level.  Analogously, the TTL fields provide the TTL scopes
   of the levels: TTL 0 is the scope of the entire group, TTL 1 to TTL 3

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   give the scopes of the most global to most local repetitor levels.
   Setting the scope for a level to zero indicates that this level is
   not in use.  The fields minimal send quality and minimal receive
   quality give minimum levels of quality for a member that wants to
   send reliable messages or that wants to request retransmissions
   (reliable reception); if not met, they cause the member to assume a
   lower membership class.

   At the end of the fixed part of group[info] packets, extensions can
   be added.  Their type is identified by a one-byte type code their
   length given by a one-byte length field, giving the number of 32-bit
   words beyond the initial one in this extension.

            Figure 9a: group[info] extension for member acks
:       1       |       4       |         (Port part)           :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-                         -+-+
:                         (Address part)                        :
+-                                                             -+
:                          Acknowledged                         :
+-                        Member-Address                       -+
:                                                               :
+-                                                             -+
:                                                               :

   Type 1 group[info] extensions are used to carry an acknowledgement
   for a group[seek] requests by a member that needs to achieve reliable
   reception status quickly (K-bit in group[seek] set).

4.5.  Summary of packet types

 packet type          type[code]   multi/uni   sent by          see Figure
 data[data]           0[0]          m           C,R,s           4
 data[eom]            0[1]          m           C,R,s           4a
 data[ceom]           0[3]          m           C,R,s           4*)
 data[fec]            0[4]          m           C,R,s           4b
 nak[request]         1[0]          mu          r               5
 group[info]          2[0]          m           C,R             9
 group[seek]          2[1]          m           C,R,s,r         8
 token[request]       4[0]          u           s               2
 token[confirm]       4[1]          u           C               3
 token[cancel]        4[2]          u           s               3
 status[request]      5[0]          m           C,R,s,r         6
 status[info]         5[1]          m           C,R             7
 coord[suspected]     6[0]          m           R,s,r           *)
 coord[inforeq]       6[4]          u           p               *)
 coord[info]          6[5]          u           C               *)

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 coord[statusreq]     6[6]          m           p               *)
 coord[status]        6[7]          m           R,s,r           *)
 coord[give]          6[8]          u           C               *)
 coord[accept]        6[9]          u           p               *)

   multi/uni: m is multicast, u is unicast.

   sent by: C is coordinator (p is potential coordinator), R is
   repetitor, s is sender, r is receiver.

   *) Not yet described in the present version of the document.

5.  References

   [1]  S. Armstrong, A. Freier, K. Marzullo: ``Multicast Transport
        Protocol'', RFC 1301, February 1992.

   [2]  C. Bormann, J. Ott, H.-C. Gehrcke, T. Kerschat and N. Seifert:
        ``MTP-2: Towards Achieving the S.E.R.O. Properties for Multicast
        Transport'', International Conference on Computer Communications
        and Networks (ICCCN 94), 1994 (available from ftp://ftp.cs.tu-

   [3]  Holbrook, H.W., Singhal, S.K., and Cheriton, D.R., Log-based
        Receiver-Reliable Multicast for Distributed Interactive
        Simulation.  SIGCOMM '95, Cambridge, MA, August, 1995.

   [4]  N. Seifert, C. Bormann, J. Ott: MTP/SO: Self-Organizing
        Multicast, First Multicast-Workshop, GI/TU Braunschweig, May

   [5]  S. Floyd, V. Jacobson, S. McCanne: A Reliable Multicast
        Framework for Light-weight Sessions and Application Level
        Framing, SIGCOMM '95, Cambridge, MA, August, 1995.

   [6]  C. Bormann, J. Ott, N. Seifert: MTP/SO: Receiver-Reliable
        Coordinated Many-to-Many Multicast, Presentation at the SIGCOMM
        96 Workshop on Matters Mbone (``SIG-Bone''), Palo Alto;, 27-August-1996.

   [7]  R. Kermode: Scoped Address Discovery Protocol (SADP), November
        1998, Internet-draft draft-kermode-sadp-00.txt.

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6.  Authors' addresses

   Carsten Bormann, Joerg Ott
   Universitaet Bremen FB3 TZI
   Postfach 330440
   D-28334 Bremen, GERMANY
   phone +49.421.218-7024, 201-7028
   fax +49.421.218-7000

   Nils Seifert, Joerg Ott
   Tellique GmbH
   Gustav-Meyer-Allee 25, Haus 12
   D-13355 Berlin, GERMANY
   phone +49.30.46307-551, -550
   fax +49.30.46307-579

Bormann, Ott, Seifert                                          [Page 23]