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TCP maximum segment size and related topics (RFC0879)

IP.com Disclosure Number: IPCOM000003928D
Original Publication Date: 1983-Nov-01
Included in the Prior Art Database: 2000-Sep-13
Document File: 9 page(s) / 21K

Publishing Venue

Internet Society Requests For Comment (RFCs)

Related People

J. Postel: AUTHOR

Abstract

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 text was extracted from a ASCII document.
This is the abbreviated version, containing approximately 13% of the total text.

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.

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...