The TCP Maximum Segment Size and Related Topics
RFC 879
Document | Type |
RFC - Historic
(November 1983; Errata)
Obsoleted by RFC 7805
Updated by RFC 6691
|
|
---|---|---|---|
Authors | |||
Last updated | 2020-07-29 | ||
Stream | Legacy | ||
Formats | plain text html pdf htmlized with errata bibtex | ||
Stream | Legacy state | (None) | |
Consensus Boilerplate | Unknown | ||
RFC Editor Note | (None) | ||
IESG | IESG state | RFC 879 (Historic) | |
Telechat date | |||
Responsible AD | (None) | ||
Send notices to | (None) |
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,Show full document text