RFC 879 (rfc879) - Page 2 of 11


TCP maximum segment size and related topics



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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).










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