Network Working Group                                         P. Cameron
Request for Comments: 1692                  Xylogics, International Ltd.
Category: Standards Track                                     D. Crocker
                                                  Silicon Graphics, Inc.
                                                                D. Cohen
                                                               J. Postel
                                                             August 1994

                 Transport Multiplexing Protocol (TMux)

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.


   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.


   This specification is the result of the merger of two documents: the
   original TMux proposal which was the result of several discussions
   and related initiatives through IETF working groups; and IEN 90 [1]
   originally proposed by Danny Cohen and Jon Postel in May 1979.

Applicability Statement

   The TMux protocol is intended to optimize the transmission of large
   numbers of small data packets that are generated in situations where
   many interactive Telnet and Rlogin sessions are connected to a few
   hosts on the network.  In these situations, TMux can improve both
   network and host performance.  TMux is not intended for multiplexing
   long streams composed of large blocks of data that are typically
   transmitted by such applications as FTP.

   The TMux protocol may be applicable to other situations where small
   packets are generated, but this was not considered in the design.

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RFC 1692                          TMux                       August 1994

   The use of the TMux protocol in any other situation may require some

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.

   The problem stems from the fact that the work a host must do to
   process a 1-octet packet is very nearly as much as the work it must
   do to process a 1500-octet packet.  That is, it is the overhead of
   processing a packet which consumes a host's resources, not the
   processing of the data.

   In particular, communication load is not measured only in bits per
   seconds but also in packets per seconds, and in many situation the
   latter is the true performance limit, not the former.  The proposed
   multiplexing is aimed at alleviating this situation.

   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

   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 [2], 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

   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 is preceded by 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 them to the transport layer as if they had been

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RFC 1692                          TMux                       August 1994

   received in the usual IP/transport packaging.  The transport layer
   is, therefore, unaware of the special encapsulation which was used.

   Hence, a TMux message appears as:

     | IP hdr | TM hdr | Tport segment | TM hdr | Tport segment| ...|


   TM hdr         is a TMux mini-header and specifies the following
                  Tport segment.

   Tport segment  refers to the entire transport segment, including
                  transport headers.

   The TMux Protocol is defined to allow the combining of transmission
   units of different higher level protocols in one transmission unit of
   a lower level protocol. Only segments with the same Internet Protocol
   (IP) header, (with the possible exception of the protocol and check-
   sum fields) may be combined. For example, the segment (H1, B1) and
   the segment (H2, B2), where Hi and Bi are the headers and the bodies
   of the segment, respectively, may be combined (multiplexed) only if
   H=H1=H2. The combined TMux message is either (H, B1, B2) or (H, B2,

   The receiver of this combined message should treat it as if the two
   original segments, (H,B1), and (H,B2), arrived separately.  It is
   recommended, though not a requirement, that the segments in the TMux
   message should be processed in the same order that they are in the
   TMux message.

   The multiplexing is achieved by combining the individual segments,
   (H,B1) through (H,Bn), into a single message.  This single message
   has an IP header which is equal to H, but having in the PROTOCOL
   field the value 18 which is the protocol number of the TMux protocol.
   This IP header is followed by all the segments, B1 through Bn.  Each
   segment, Bi, is preceded by a 4 octet TMux mini header. This contains
   the number of the protocol to which this segment is addressed. It
   also contains the total length of this segment, including this mini
   header. Since this mini header is not otherwise protected by a check-
   sum, it also includes a checksum field which just covers this mini

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RFC 1692                          TMux                       August 1994

2.1. IP Protocol field value

   TMux is indicated in an IP datagram by the Protocol (ID) value of 18
   (22 octal), see [3].

2.2. Header Format

   Each 4 octet TMux mini-header has the following general format:

                     |         Length high           |
                     |          Length low           |
                     |         Protocol ID           |
                     |          Checksum             |
                     |      Transport segment        |
                     |       ...                     |
                     |       ...                     |

   The LENGTH field specifies the octet count for this mini header and
   the following transport segment, from 0-65535 octets.  Hence, the
   length field has a minimum value of 4.  For segments that are larger
   than the maximum allowed for TMux (see section 5.1), individual IP
   datagrams should be sent.

   The Protocol ID field contains the value that would normally have
   been placed in the IP header Protocol field.

   The 'Checksum' field is the XOR of the first 3 octets.

   To ensure that TCP, UDP and other segments keep their 32 bit
   alignment, where the segments being multiplexed are 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 ignored on
   input.  This padding will not affect 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 back and forth between use of
   TMux packaging and the usual individual IP datagrams for individual
   transport associations.  The only barrier to the use of TMux is for
   the sender to know whether TMux is supported by the receiver.  This
   is important, since early use of TMux is likely to be limited.

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RFC 1692                          TMux                       August 1994

   The easiest way to detect TMUX support is to only send TMux messages
   to hosts from which a valid TMux message has already been received.
   This then leaves the problem of one host starting the TMux
   connection.  This is most easily accomplished by the host sending an
   IP datagram with no data (i.e., with the IP total length field of
   20), but with an IP Protocol field value of 18 for TMux.  This is
   referred to as a TMux ENQ (enquiry) 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 original ENQ message was
   lost.  If this again is lost, the ENQ may be repeated as often as
   needed, but the time between requests should increase exponentially
   up to a limit of about 1 hour.  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 IP datagram form.

   The only problem with this scheme is that a host which supports TMux
   may stop supporting it, as might happen when the host is re-booted.
   Other hosts need to learn of this change.  The solution to this is to
   maintain a Time To Live (TTL) value for hosts from which TMux
   messages have been received.  This TTL is a timed TTL, rather than a
   count as used in the IP TTL field, and this time stamp is updated
   every time a TMux message is received.  This can then be used to
   expire the information held by TMux on the host after a suitable
   time, e.g., 1 minute.

   This TTL time stamp is used as follows. When TMux is passed a segment
   to be sent to a host, a check is made to see if the time to live has
   expired.  If the TTL has not expired, the segment is sent in a TMux
   message as normal.  If the TTL has expired, the host is marked as
   being unable to TMux, but the segment is STILL sent as a TMux message
   (i.e., with the normal delay to allow other segments to be
   multiplexed).  If the host is really unable to TMux anymore (a rare
   occurrence) then this segment will be timed out and retried by the
   transport provider i.e., TCP.  Because the host was marked as not
   able to TMux, the retry will be sent as a normal IP datagram.  If the
   remote host is still able to TMux then it should send back TMux
   traffic (even if it has been rebooted), typically a TCP window
   update, and the local host will mark it as able to TMux again. This
   way of operating removes any performance problem caused by

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RFC 1692                          TMux                       August 1994

   continually dropping out of TMuxing and having to send probe
   messages.  If the IP datagram to be sent is from UDP, then the remote
   host may not send anything in reply. So for UDP this scheme will not
   be any better than just stopping sending TMux messages to the host,
   but it is also no worse.

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 protocols,
   but not to IP or to TMux.  Hence, when connection management 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 (e.g., TCP or UDP) sends a segment, TMux
   first removes the IP header (if present) and adds a TMux mini-header
   and the segment to the Multiplexed Message under construction for the
   host specified by the destination address of the segment.

   When the first message to be transmitted is placed into the
   Multiplexed Message under construction, a timer is started.  When the
   timer expires, the Multiplexed Message under construction is
   transmitted. This ensures that all segments available for sending
   before the timer expires are sent in a single Multiplexed Message.
   If, during construction of the Multiplexed Message, the buffer
   holding the message fills, the Multiplexed Message is transmitted

   The delay time should be user configurable; a reasonable time is 20
   to 30 milliseconds.  The time period should be large enough to give a
   reasonable probability of sending multiple segments 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 be used and that all TMux messages under
   construction are sent when the timer expires.

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RFC 1692                          TMux                       August 1994

   Additionally, configuration options may limit the number of included
   data segments or the maximum size of the Multiplexed Message before
   it is transmitted.  It is also suggested that larger segments (e.g.,
   those over 700 octets) should be sent as standard IP datagrams, and
   not multiplexed.  This is to ensure that the delay caused by the TMux
   timer does not put a delay on those segments for which it is
   inadvisable.  The size of the largest segments to be multiplexed
   should (if possible) be configurable.

4. Protocol Example

   This example shows a TMux message consisting of three multiplexed

   A TCP segment consisting of a 20 octet TCP header, 5 octets of data
   and 3 octets of padding.  Thus the length field is

             Mini header + TCP header + data
           =     4       +     20     +  5
           =     29

   The padding is NOT included in the length.

   A TCP segment consisting of a 20 octet TCP header, 4 octets of data.
   This segment does not require padding.

   A UDP segment consisting of a 4 octet UDP header, 41 octets of data
   and 3 octets of padding.

                     |         Length = 29           |
                     |         (2 octets)            |
                     |     Protocol ID = 6 (TCP)     |
                     |          Checksum             |
                     |         TCP Header            |
                     |        (20 octets)            |
                     |          TCP data             |
                     |         (5 octets)            |
                     |          Padding              |
                     |         (3 octets)            |
                     |         Length = 28           |
                     |         (2 octets)            |

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RFC 1692                          TMux                       August 1994

                     |     Protocol ID = 6 (TCP)     |
                     |          Checksum             |
                     |         TCP Header            |
                     |        (20 octets)            |
                     |          TCP data             |
                     |         (4 octets)            |
                     |         Length = 49           |
                     |         (2 octets)            |
                     |    Protocol ID = 17 (UDP)     |
                     |          Checksum             |
                     |         UDP Header            |
                     |         (4 octets)            |
                     |          UDP data             |
                     |         (41 octets)           |
                     |          Padding              |
                     |         (3 octets)            |

5. Implementation Suggestion

5.1 Maximum TMux Message Size

   In section 3.3, a note is made about sending messages immediately if
   the limit on TMux message size is reached.  On systems where Path MTU
   Discovery (as per RFC 1191 [4]) has been implemented this should be
   used to discover the maximum message size that can be transmitted,
   and this should be used as the maximum TMux message size.

5.2 Deciding Which Segments to Multiplex

   It is the responsibility of the sender to decide which segments
   should be TMux'd and which should not.  For example, segments sent by
   FTP should not normally be multiplexed.  In many situations, it may
   be sensible to restrict the sessions that can be multiplexed to just
   those involved in interactive traffic (Telnet and Rlogin) by
   examining the source and destination TCP port numbers.  However, if a
   segment that would not normally be multiplexed is to be sent and a
   TMux message is already under construction, then the extra segment

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RFC 1692                          TMux                       August 1994

   can be added to the TMux message under construction, and this
   complete message should be sent immediately, rather than waiting for
   the timer to expire.

6. Implementation notes

   The following notes are the result of experience gained during the
   testing of early implementations of TMux.  Whilst they do not form
   part of the actual standard, they should be followed if possible to
   ensure compatibility with other implementations.

   Because the TMux mini-header does not contain a TOS field, only
   segments with the same IP TOS field should be contained in a single
   TMux message.  As most systems do not use the TOS feature, this is
   not a major restriction.  Where the TOS field is used, it may be
   desirable to hold several messages under construction for a host, one
   for each TOS value.

   Segments containing IP options should not be multiplexed.

   Only unicast addresses should be considered for multiplexing.

   Segments addressed to the loopback address ( are not
   candidates for multiplexing.

   Only segments with a source or destination port that is for an
   interactive session (i.e., Telnet and Rlogin) should be considered
   for multiplexing using TMux.

   If an error is discovered in a checksum of a TMux header, the rest of
   the message, starting there, is ignored.  If an unknown PROTOCOL
   field is discovered in any TMux header, this segment, and only this
   one, is ignored.

   If the TMux implementation is continually sending TMux messages
   containing exactly one segment (because is there is little traffic to
   multiplex), then TMux may be turned off.  This implies that TMux may
   be switched off when there is no congestion.

   To prevent intermediate nodes from fragmenting and reconstructing
   TMux frames, implementations may want to set the "do not fragment"
   flag in the IP datagram of TMux messages.

   If host B receives a TMux ENQ message from host A, but does not have
   any data for host A, then it may also send back an ENQ message.
   However, host A may send another ENQ message in response to this, so
   causing B to respond and so on.  Thus if this facility is used, code
   must be included to prevent this looping behavior happening.  Sending

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RFC 1692                          TMux                       August 1994

   an ENQ in response to an ENQ is not recommended, except in special

   It is recommended that the following aspects of the TMux protocol be
   user configurable:

      The maximum size of a segment that can be multiplexed by TMux.

      The delay between the first segment being placed into the message
      under construction and the message being sent.

7. 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.

   Because some routers filter packets on the TCP port numbers, any
   segments sent using TMux will not be subject to this filtering as it
   will obscure the TCP port number However, larger segments for the
   same TCP connection will still be sent as IP datagrams, and so will
   be subject to filtering, thus giving rise to a potential problem.
   For this reason, any routers that do not support TMux, but which do
   support this type of filtering should not allow TMux messages through
   (in either direction).  This will cause both hosts to think the other
   does not support TMux, so all segments will be sent as IP datagrams,
   thus eliminating this problem.

   A better solution to this problem, is for routers to understand the
   TMux protocol, and to inspect each of the multiplexed segments and
   remove those segments that fail the filtering.

8. References

   [1] Cohen, D., and Postel, J., "Multiplexing Protocol", IEN 90,
       USC/Information Sciences Institute,, May 1979.

   [2] Postel, J., "Internet Protocol", STD 5, RFC 791, USC/Information
       Sciences Institute, September 1981.

   [3] Reynolds, J. and J. Postel, "Assigned Numbers", STD 2, RFC 1340,
       USC/Information Sciences Institute, March 1990.

   [4] Mogul, J., and S. Deering, "Path MTU discovery", RFC 1191, DECWRL
       and Stanford University, November 1990.

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RFC 1692                          TMux                       August 1994

9. Authors' Addresses

       Peter Cameron
       Xylogics International, Ltd.
       Featherstone Rd.
       Wolverton Mill
       Milton Keynes
       MK12 5RD
       United Kingdom

       Phone: +44  908 222112
       Fax:   +44  908 222115

       David Crocker
       Silicon Graphics, Inc.
       2011 N. Shoreline Blvd.
       P.O. Box 7311
       Mountain View, CA 94039-7311

       Phone: +1 415 390 1804
       Fax:   +1 415 962 8404

       Danny Cohen
       325 N. Santa Anita Ave.
       Arcadia, CA 91006

       Phone: +1 818 821 5555

       Jon Postel
       USC/Information Sciences Institute
       4676 Admiralty Way
       Marina del Rey, CA  90292-6695

       Phone: +1 310 822 1511
       Fax:   +1 310 823 6714
       EMail: Postel@ISI.EDU

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RFC 1692                          TMux                       August 1994

10. Discussion List

       There is a discussion list for this protocol, which for
       historical reasons is called:


   Requests to join the list should be sent to:


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