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Internet Draft P. Cameron
Xylogics, International Ltd.
D. Crocker
Silicon Graphics, Inc.
August 1993
Transport Multiplexing Protocol (TMux)
Status of this Memo
This document is an Internet Draft. Internet Drafts are working
documents of the Internet Engineering Task Force (IETF), its
Areas, and its Working Groups. Note that other groups may also
distribute working documents as Internet Drafts. Internet Drafts
are draft documents valid for a maximum of six months.
Internet Drafts may be updated, replaced, or obsoleted by other
documents at any time. It is not appropriate to use Internet
Drafts as reference material or to cite them other than as a
"working draft" or "work in progress." Please check the I-D
abstract listing contained in each Internet Draft directory to
learn the current status of this or any other Internet Draft.
It is intended that this document will be submitted to the IESG
for consideration as a standards document. Distribution of this
document is unlimited.
Abstract
One of the problems with the use of terminal servers is the
large number of small packets they can generate. Frequently,
most of these packets are destined for only one or two hosts.
TMux is a protocol which allows multiple short transport
segments, independent of application type, to be combined
between a server and host pair.
Acknowledgments
This specification is the result of several discussions and
related initiatives through IETF working groups.
(We have heard that Danny Cohen, of USC's ISI, suggested a
scheme like TMux about 8 years ago, but have not yet located the
reference.)
1. Introduction
When network designers consider which protocols generate the
most load, they naturally tend to consider protocols which
transfer large blocks of data (e.g. FTP, NFS). What is often
not considered is the load generated by Telnet and Rlogin
because of the assumption that users type slowly and the packets
are very small. This is a grave underestimation of the load on
networks and hosts which have many Telnet and Rlogin ports on
multiple terminal servers.
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The problem stems from the fact that the work a host must do to
process a 1-byte packet is very nearly as much as the work it
must do to process a 1500-byte packet. That is, it is the
overhead of processing a packet which consumes a hosts
resources, not the processing of the data.
If one assumes that most users connected to a terminal server
will be connecting to only a few hosts, then it should be
obvious that the network and host load could be greatly reduced
if traffic from multiple users, destined for the same host,
could be sent in the same packet.
TMux is designed to improve network utilization and reduce the
interrupt load on hosts which conduct multiple sessions
involving many short packets. It does this by multiplexing
transport traffic onto a single IP datagram, thereby resulting
in fewer, larger packets. TMux is highly constrained in its
method of accomplishing this task, seeking simplicity rather
than sophistication.
2. Protocol Design and Subconnection Messages
IP hosts may engage in the use of TMux transparently, and may
even switch back and forth between use of TMux and carriage of
transport segments in the usual, independent IP datagrams.
TMux operates by placing a set of transport segments into the
same IP datagram. Each segment has a TMux mini-header which
specifies the segment length and the actual segment transport
protocol. The receiving host demultiplexes the individual
transport segments and presents it to the transport layer as if
it had been received in the usual IP/transport packaging. The
transport layer is, therefore, unaware of the special
encapsulation which was used.
Hence, a TMux datagram appears as:
| IP hdr | TM hdr | Tport segment | TM hdr | Tport segment| ...|
Where:
TM hdr is a TMux mini-header and specifies the following
Tport segment.
Tport segment refers to the entire transport segment, including
transport headers.
2.1. IP Protocol field value
TMux is indicated in an IP datagram by the Protocol (ID) value
of XXXXX.
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2.2. Header Format
Each 4 octet TMux mini-header header has the following general
format:
| 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
+-------------------------------+
| Length |
+-------------------------------+
| Protocol ID |
+-------------------------------+
| Length check |
+-------------------------------+
| ID check |
+-------------------------------+
| Transport segment |
| ... |
| ... |
The LENGTH field specifies the byte count for the transport seg-
ment, from 0-255 octets. For segments that are longer, individ-
ual datagrams should be sent.
The Protocol ID field contains the value that would normally
have been placed in the IP header Protocol field.
The two check fields are the 1's complement of the first two
fields (as a simple and quick to do checksum). Thus if the
first two octets are treated as a single 16 bit value, the sec-
ond two octets treated as a 16 bit value will also be the 1's
complement of the first. The transport segment associated with
the header is not checksummed, it is the responsibility of the
upper level protocol to check this.
To ensure that TCP, UDP etc. segments keep their 32 bit align-
ment, where the segments being multiplexed is not a multiple of
32 bits long, extra octets will be added to re-align the end of
the segment, and hence the next segment. These octets will be
zeroed on output and ignored on input. This padding will not
effect the LENGTH field, it will still contain the real length
of the segment.
2.3. Sending Data
Host endpoints may choose to use TMux at any time and in either
(or both) directions. They also may switch between use of TMux
packaging and the usual individual packets for individual trans-
port associations. The only barrier to the use of TMux is for
the sender to know whether TMux is supported by the receiver,
which is important, since early use of TMux is likely to be lim-
ited.
The easiest way to achieve this, is to only send TMux messages
to a host that has already sent you a TMux message. This then
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leaves the problem of one host starting the TMux connection.
This is most easily accomplished by the host sending an IP data-
gram with no data, but with a Protocol field value for TMux.
This is referred to as a TMux ENQ message. The host receiving
this message then knows that the originator supports TMux, and
can start to send TMux messages. This will in turn cause the
originator of the ENQ message to start to use TMux. If for any
reason, the receiver does not intend to send TMux messages to
the originator, but is prepared to accept them, then it can
reply with another ENQ message.
If an ENQ message does not get a response, then it is reasonable
to resend the ENQ a while later in case the message was lost.
If this again is lost, the ENQ may be repeated as often as
needed, but the time between requests should increase exponen-
tially. Suitable times between ENQs would be 15 seconds, 30
seconds, 60 seconds, 120 seconds etc.
Note that this checking process does not need to impede any of
the transport (user) data, which may be sent as convenient,
albeit in its less-efficient small-packet form.
The only problem with this scheme, is that any host advertising
that it supports TMux, then stopping supporting it will cause
any other hosts a problem. The solution to this, is to put a
Time To Live (TTL) value on a record of a host sending a TMux
packet, and expire them after a suitable time, eg. 1 hour.
3. Protocol Behavior
3.1. Transport Flow Control
TMux operates as an extension to the IP datagram protocol.
Hence, it has no impact on most flow control mechanisms, since
they operate at the transport layer -- above TMux.
3.2. Connection Management
The concept of a connection pertains to certain transport proto-
cols, but not to IP or to TMux. Hence, when connection manage-
ment is required by a transport protocol using TMux, it occurs
in the same fashion as it does for IP. In fact, the transport
protocol is not to be aware that TMux is being used.
3.3 Multiplexed Message Construction
When a transport provider (eg. TCP or UDP) sends a packet, TMux
prepends that packet with a header to create a TMux message,
then appends the message to the Multiplexed Message under con-
struction.
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When the first message to be transmitted is placed into the Mul-
tiplexed Message under construction, a timer is started. When
the timer expires, all outstanding message are placed into the
Multiplexed Message and it is transmitted. This ensures that all
messages constructed before the timer expires are sent in a sin-
gle Multiplexed Message. If during construction of the Multi-
plexed Message the buffer holding the packet fills, the Multi-
plexed Message is transmitted immediately.
The delay time should be user configurable; a reasonable time is
20 to 30 milliseconds. The time period wants to be large enough
to give a reasonable probability of sending multiple packets but
not so large that the echo response time becomes a problem.
This suggests that the upper limit for the timer is probably
1/10th second. As the cost of using timeouts on many systems is
quite large it is recommended that a single timer is used and
that all connections are serviced on each expiry period.
Additionally, configuration options may limit the number of
included data packets or the maximum size of the Multiplexed
Message before it is transmitted. It is also suggested that
larger packets (eg those over 30 octets) should be sent as stan-
dard IP packets, and not multiplexed. This is to ensure that
the delay used, does not put a delay on those packets for which
it is inadvisable.
4. Security Considerations
Because TMux is effectively an extension to IP, it does not have
any more impact on site security than does IP. Security should
be dealt with by upper layer protocols.
5. Author's Addresses
P. Cameron
Xylogics International, Ltd.
Featherstone Rd., Wolverton Mill
Milton Keynes MK12 5RD
United Kingdom
Telephone: +44 908 222112
Fax: +44 908 222115
Email: cameron@xylint.co.uk
D. Crocker
Silicon Graphics, Inc.
2011 N. Shoreline Blvd.
P.O. Box 7311
Mountain View, CA 94039-7311
USA
Telephone: +1 415 390 1804
Fax: +1 415 962 8404
Email: dcrocker@sgi.com
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Internet Draft TMux August 1993
6. Discussion List
There is a discussion list for this protocol, which for histori-
cal reasons is called:
cmp-id@xylint.co.uk
Request to join the list should be sent to:
cmp-id-request@xylint.co.uk
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