RFC 879 (rfc879) - Page 2 of 11
TCP maximum segment size and related topics
Alternative Format: Original Text Document
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
