INTERNET-DRAFT Carsten Bormann,
Expires: May 1998 Joerg Ott
Universitaet Bremen
Nils Seifert
TU Berlin
November 1997
MTP/SO: Self-Organizing Multicast
draft-bormann-mtp-so-01.txt
Status of this memo
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Abstract
Multiparty cooperative applications have recently received much
attention, 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 extensive work on reliable multicast protocols has
assumed relatively stable groups that need to ensure that all
messages are 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.
An interesting multicast transport protocol is defined in RFC 1301.
MTP provides globally ordered, receiver reliable, rate controlled and
atomic transfer of messages to multiple recipients. A revised, more
practical version of MTP, the Multicast Transport Protocol MTP-2 has
been in use for some time.
Self-Organizing Multicast, MTP/SO, uses MTP-2 as a basis and adds
spontaneous self-organization of the members of the group into local
regions. Scalability is increased by providing passive group joining
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and local retransmission of lost packets.
This version of the document is not yet complete but contains most of
the vital parts.
1. Introduction
Multiparty cooperative applications have recently received much
attention, as has the multicasting of datagrams in the internet. The
internet datagram multicasting mechanism is inherently unreliable,
often requiring a higher level protocol to achieve the level of
reliability required for an application. Just as TCP has proven to
be a useful basis for many applications that could in theory motivate
the design of application specific transport protocols, it is likely
that generally available reliable multicast protocols would relieve
many multiparty applications from the details of efficiently coping
with unreliable delivery in their application protocol designs.
Much of the extensive 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
transport.
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.
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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
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
acknowledgement from each recipient to the sender of each message.
This obviously does not scale to large groups. Also, group
management algorithms that require an acknowledgement 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
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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
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 all (or 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.
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 rate
control. 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 like 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
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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.
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 corresponds to the order in which they were sent.
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 control
Rate control is overseen by the coordinator. A parameter global to
the group defines the maximum throughput of the group. The
coordinator dynamically adjusts a per-message parameter called window
to the number of tokens granted (up to 11). Senders are not allowed
to send data packets belonging to one message at an interval smaller
than window (measured in microseconds). So the coordinator can
ensure that the maximum throughput for the group is not exceeded.
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3.3. Atomicity
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
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 with
atomicity_off.
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
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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 denied with a special control packet. 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.
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 log servers
as in [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 Repeaters
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).
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Instead, MTP/SO introduces the concept of a regional repeater.
Receivers multicast NAKs locally before multicasting them to the
entire group. Repeaters that have previously received the requested
data retransmit locally after receiving a local NAK. Repeaters that
don't have the data just relay the 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 multicasting
(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; with IPv6, administrative scoping will provide a similar
method. 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
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 and one global scope, each group member is by
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 in principle also be a repeater for any of the
local scopes (each member can decide whether it wants to be a
potential repeater or not, e.g. depending in 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 repeater 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 member as the repeater. This
protocol needs not necessarily ensure that there is exactly one
repeater for each scope at any time, as the retransmission protocol
still works without a repeater or with more than one repeater per
scope, albeit less efficient.
Repeater 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 repeater
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 repeater for a scope periodically
multicast a repeater announcement message within the scope,
containing the current value of the reception quality parameter.
Potential repeaters observe these messages. If, within the most
local scope, a potential repeater has a considerably better reception
quality parameter than the current repeater, it sends a repeater
announcement at the start of its next heartbeat interval and assumes
the role of the repeater. Only the repeaters of the most local scope
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compete for the repeater role of the next higher scope, and so on.
(A new repeater that displaces a member that was repeater at higher
level scopes also announces itself as repeater at these higher level
scopes.)
To better cope with repeater failure, receivers that are not
repeaters 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. Repeaters 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 repeater that leaves a group simply sends a repeater announcement
with reception quality zero. A repeater that crashes stops sending
repeater announcements, causing potential repeaters to start sending
repeater announcements after a time interval that is inversely
related to their reception quality parameter.
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
automatically.
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 absolutely no information is
lost. (This protocol should be, but is not yet, integrated with the
repeater function.)
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
member.
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 repeater. We therefore
distinguish several ``classes'' of members:
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|
class | description
------+----------------------------------------------------
1 | normal member, potential coordinator and repeater
2 | normal member, potential repeater
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 repeater 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.
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. One way to join an MTP/SO group is to start as a class
4 member, send a message at an appropriate time, and upgrade to a
higher class when the message has been accepted by the coordinator.
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
level.
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]
packets.
4.2. Protocol Functions and Packet Types
o Heartbeat
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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. The only
exception is the actual transmission of data packets, which is evenly
distributed over the heartbeat interval to which the data packets are
allocated.
o Global Ordering
A sender that wants to send a message applies for a token by
unicasting a token[request] packet to the coordinator.
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
limited.
To ensure that the most recent message acceptance state is always
disseminated, the coordinator sends an empty[info] packet 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 repeaters 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
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sent.
If a sender no longer has a copy of the data that needs to be
retransmitted, it multicasts a nak[deny] packet.
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.
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
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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.
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
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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*2^32)
retention 8 4 4 1 (0 to 15*2^16)
window 16 11 5 microseconds (0 to 2047*2^32)
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 | . . .
+-+-+-+-+-+-+-+-+-
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.
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
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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 reliable which means that 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 repeaters can
use this information to decide whether they can defer their own
retransmissions.)
Bormann, Ott, Seifert [Page 15]
INTERNET-DRAFT MTP/SO: Self-Organizing Multicast November 1997
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|0 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
repeater 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.
Bormann, Ott, Seifert [Page 16]
INTERNET-DRAFT MTP/SO: Self-Organizing Multicast November 1997
Figure 5: nak[request], nak[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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|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/replied to. NAK request and deny 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| scope | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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.
Bormann, Ott, Seifert [Page 17]
INTERNET-DRAFT MTP/SO: Self-Organizing Multicast November 1997
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 repeater (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
array.
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
transmitter).
Bormann, Ott, Seifert [Page 18]
INTERNET-DRAFT MTP/SO: Self-Organizing Multicast November 1997
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 |U|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 ... :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
: (zeros) 4 byte alignment |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: type | length | extension :
:-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ :
: :
:-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-:
: type | length | extension :
:-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ :
: :
The group[info] packet is periodically transmitted by the coordinator
and by each repeater to ensure that all group members are aware of
the global parameters of the group and of the quality of the current
repeater.
Two 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 other fields of the packet give global group parameters that
usually are constant: The U-Bit (``unreliable''), when set, indicates
that this group operates entirely without NAKs and retransmissions.
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 repeaters 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 give the scopes of
Bormann, Ott, Seifert [Page 19]
INTERNET-DRAFT MTP/SO: Self-Organizing Multicast November 1997
the most global to most local repeater 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[dally] 0[2] m C,R,s 4*)
data[ceom] 0[3] m C,R,s 4*)
nak[request] 1[0] m r 5
nak[deny] 1[1] m C,s 5
group[info] 2[0] m C,R 9
group[seek] 2[1] m C,R,s,r 8
quit[order] 3[0] u C,R *)
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[deny] 5[1] m C 6
status[info] 5[2] m C,R 7
Bormann, Ott, Seifert [Page 20]
INTERNET-DRAFT MTP/SO: Self-Organizing Multicast November 1997
coord[suspected] 6[0] m R,s,r *)
coord[established] 6[1] m C *)
coord[seek] 6[2] m C *)
multi/uni: m is multicast, u is unicast.
sent by: C is coordinator, R is repeater, 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-
berlin.de/pub/local/kbs/mtp/doc/sero.ps).
[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.
6. Authors' addresses
Carsten Bormann, Joerg Ott
Universitaet Bremen FB3 TZI
Postfach 330440
D-28334 Bremen, GERMANY
cabo, jo@tzi.org
phone +49.421.218-7024
Nils Seifert,
Technische Universitaet Berlin FR6-3
Franklinstrasse 28/29
D-10587 Berlin, GERMANY
nilss@cs.tu-berlin.de
phone +49.30.314-73389
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