RFC 879 (rfc879) - Page 1 of 11

TCP maximum segment size and related topics

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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
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