rfc879.txt

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Network Working Group                                          J. Postel
Request for Comments: 879                                            ISI
                                                           November 1983



                      The TCP Maximum Segment Size
                           and Related Topics

This memo discusses the TCP Maximum Segment Size Option and related
topics.  The purposes is to clarify some aspects of TCP and its
interaction with IP.  This memo is a clarification to the TCP
specification, and contains information that may be considered as
"advice to implementers".

1.  Introduction

   This memo discusses the TCP Maximum Segment Size and its relation to
   the IP Maximum Datagram Size.  TCP is specified in reference [1].  IP
   is specified in references [2,3].

   This discussion is necessary because the current specification of
   this TCP option is ambiguous.

   Much of the difficulty with understanding these sizes and their
   relationship has been due to the variable size of the IP and TCP
   headers.

   There have been some assumptions made about using other than the
   default size for datagrams with some unfortunate results.

      HOSTS MUST NOT SEND DATAGRAMS LARGER THAN 576 OCTETS UNLESS THEY
      HAVE SPECIFIC KNOWLEDGE THAT THE DESTINATION HOST IS PREPARED TO
      ACCEPT LARGER DATAGRAMS.

         This is a long established rule.

   To resolve the ambiguity in the TCP Maximum Segment Size option
   definition the following rule is established:

      THE TCP MAXIMUM SEGMENT SIZE IS THE IP MAXIMUM DATAGRAM SIZE MINUS
      FORTY.

         The default IP Maximum Datagram Size is 576.
         The default TCP Maximum Segment Size is 536.









Postel                                                          [Page 1]



RFC 879                                                    November 1983
TCP Maximum Segment Size                                                


2.  The IP Maximum Datagram Size

   Hosts are not required to reassemble infinitely large IP datagrams.
   The maximum size datagram that all hosts are required to accept or
   reassemble from fragments is 576 octets.  The maximum size reassembly
   buffer every host must have is 576 octets.  Hosts are allowed to
   accept larger datagrams and assemble fragments into larger datagrams,
   hosts may have buffers as large as they please.

   Hosts must not send datagrams larger than 576 octets unless they have
   specific knowledge that the destination host is prepared to accept
   larger datagrams.

3.  The TCP Maximum Segment Size Option

   TCP provides an option that may be used at the time a connection is
   established (only) to indicate the maximum size TCP segment that can
   be accepted on that connection.  This Maximum Segment Size (MSS)
   announcement (often mistakenly called a negotiation) is sent from the
   data receiver to the data sender and says "I can accept TCP segments
   up to size X". The size (X) may be larger or smaller than the
   default.  The MSS can be used completely independently in each
   direction of data flow.  The result may be quite different maximum
   sizes in the two directions.

   The MSS counts only data octets in the segment, it does not count the
   TCP header or the IP header.

   A footnote:  The MSS value counts only data octets, thus it does not
   count the TCP SYN and FIN control bits even though SYN and FIN do
   consume TCP sequence numbers.

4.  The Relationship of TCP Segments and IP Datagrams

   TCP segment are transmitted as the data in IP datagrams.  The
   correspondence between TCP segments and IP datagrams must be one to
   one.  This is because TCP expects to find exactly one complete TCP
   segment in each block of data turned over to it by IP, and IP must
   turn over a block of data for each datagram received (or completely
   reassembled).










Postel                                                          [Page 2]



RFC 879                                                    November 1983
TCP Maximum Segment Size                                                


5.  Layering and Modularity

   TCP is an end to end reliable data stream protocol with error
   control, flow control, etc.  TCP remembers many things about the
   state of a connection.

   IP is a one shot datagram protocol.  IP has no memory of the
   datagrams transmitted.  It is not appropriate for IP to keep any
   information about the maximum datagram size a particular destination
   host might be capable of accepting.

   TCP and IP are distinct layers in the protocol architecture, and are
   often implemented in distinct program modules.

   Some people seem to think that there must be no communication between
   protocol layers or program modules.  There must be communication
   between layers and modules, but it should be carefully specified and
   controlled.  One problem in understanding the correct view of
   communication between protocol layers or program modules in general,
   or between TCP and IP in particular is that the documents on
   protocols are not very clear about it.  This is often because the
   documents are about the protocol exchanges between machines, not the
   program architecture within a machine, and the desire to allow many
   program architectures with different organization of tasks into
   modules.

6.  IP Information Requirements

   There is no general requirement that IP keep information on a per
   host basis.

   IP must make a decision about which directly attached network address
   to send each datagram to.  This is simply mapping an IP address into
   a directly attached network address.

   There are two cases to consider:  the destination is on the same
   network, and the destination is on a different network.

      Same Network

         For some networks the the directly attached network address can
         be computed from the IP address for destination hosts on the
         directly attached network.

         For other networks the mapping must be done by table look up
         (however the table is initialized and maintained, for
         example, [4]).



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RFC 879                                                    November 1983
TCP Maximum Segment Size                                                


      Different Network

         The IP address must be mapped to the directly attached network
         address of a gateway.  For networks with one gateway to the
         rest of the Internet the host need only determine and remember
         the gateway address and use it for sending all datagrams to
         other networks.

         For networks with multiple gateways to the rest of the
         Internet, the host must decide which gateway to use for each
         datagram sent.  It need only check the destination network of
         the IP address and keep information on which gateway to use for
         each network.

   The IP does, in some cases, keep per host routing information for
   other hosts on the directly attached network.  The IP does, in some
   cases, keep per network routing information.

   A Special Case

      There are two ICMP messages that convey information about
      particular hosts.  These are subtypes of the Destination
      Unreachable and the Redirect ICMP messages.  These messages are
      expected only in very unusual circumstances.  To make effective
      use of these messages the receiving host would have to keep
      information about the specific hosts reported on.  Because these
      messages are quite rare it is strongly recommended that this be
      done through an exception mechanism rather than having the IP keep
      per host tables for all hosts.

7.  The Relationship between IP Datagram and TCP Segment Sizes

   The relationship between the value of the maximum IP datagram size
   and the maximum TCP segment size is obscure.  The problem is that
   both the IP header and the TCP header may vary in length.  The TCP
   Maximum Segment Size option (MSS) is defined to specify the maximum
   number of data octets in a TCP segment exclusive of TCP (or IP)
   header.

   To notify the data sender of the largest TCP segment it is possible
   to receive the calculation of the MSS value to send is:

      MSS = MTU - sizeof(TCPHDR) - sizeof(IPHDR)

   On receipt of the MSS option the calculation of the size of segment
   that can be sent is:

      SndMaxSegSiz = MIN((MTU - sizeof(TCPHDR) - sizeof(IPHDR)), MSS)


Postel                                                          [Page 4]



RFC 879                                                    November 1983
TCP Maximum Segment Size                                                


   where MSS is the value in the option, and MTU is the Maximum
   Transmission Unit (or the maximum packet size) allowed on the
   directly attached network.

   This begs the question, though.  What value should be used for the
   "sizeof(TCPHDR)" and for the "sizeof(IPHDR)"?

   There are three reasonable positions to take: the conservative, the
   moderate, and the liberal.

   The conservative or pessimistic position assumes the worst -- that
   both the IP header and the TCP header are maximum size, that is, 60
   octets each.

      MSS = MTU - 60 - 60 = MTU - 120

      If MTU is 576 then MSS = 456

   The moderate position assumes the that the IP is maximum size (60
   octets) and the TCP header is minimum size (20 octets), because there
   are no TCP header options currently defined that would normally be
   sent at the same time as data segments.

      MSS = MTU - 60 - 20 = MTU - 80

      If MTU is 576 then MSS = 496

   The liberal or optimistic position assumes the best -- that both the
   IP header and the TCP header are minimum size, that is, 20 octets
   each.

      MSS = MTU - 20 - 20 = MTU - 40

      If MTU is 576 then MSS = 536

      If nothing is said about MSS, the data sender may cram as much as
      possible into a 576 octet datagram, and if the datagram has
      minimum headers (which is most likely), the result will be 536
      data octets in the TCP segment.  The rule relating MSS to the
      maximum datagram size ought to be consistent with this.

   A practical point is raised in favor of the liberal position too.
   Since the use of minimum IP and TCP headers is very likely in the
   very large percentage of cases, it seems wasteful to limit the TCP
   segment data to so much less than could be transmitted at once,
   especially since it is less that 512 octets.




Postel                                                          [Page 5]



RFC 879                                                    November 1983
TCP Maximum Segment Size                                                


      For comparison:  536/576 is 93% data, 496/576 is 86% data, 456/576
      is 79% data.

8.  Maximum Packet Size

   Each network has some maximum packet size, or maximum transmission
   unit (MTU).  Ultimately there is some limit imposed by the
   technology, but often the limit is an engineering choice or even an
   administrative choice.  Different installations of the same network
   product do not have to use the same maximum packet size.  Even within
   one installation not all host must use the same packet size (this way
   lies madness, though).

   Some IP implementers have assumed that all hosts on the directly
   attached network will be the same or at least run the same
   implementation.  This is a dangerous assumption.  It has often
   developed that after a small homogeneous set of host have become
   operational additional hosts of different types are introduced into
   the environment.  And it has often developed that it is desired to
   use a copy of the implementation in a different inhomogeneous
   environment.

   Designers of gateways should be prepared for the fact that successful
   gateways will be copied and used in other situation and