Internet Core Protocols: The Definitive Guide

Name

Maximum Segment Size

Synopsis

Used by both endpoints to publish their local MTU or MRU values during the circuit-setup process.

Type

2

Size

Thirty-two bits total (sixteen bits are used for the option-data field).

Option Fields

Table 7.5 lists the format of the Maximum Segment Size option.

Table 7.5. The Format of the Maximum Segment Size Option

Field	Size (Bytes)	Purpose
Option Type	1	Identifies this option as the Maximum Segment Size option
Option Size	1	Identifies the total length of the option (including all of the fields and data)
MRU	2	Used to publish the MRU (or more often, the MTU) of the local network

Defined In

RFC 793.

Status

Standard.

Notes

Before two endpoints can begin exchanging data, they must first understand the segment sizing restrictions that will be imposed upon them by the other end of the connection. Part of this process involves discovering the remote system’s Maximum Transfer Unit or Maximum Receive Unit size. The Maximum Segment Size option provides this service, allowing the two endpoints to publish their local MTU/MRU sizes during the synchronization process.

When an application issues an “open” request to TCP, TCP will issue a startup segment with the Synchronize flag set, and with the local network’s MRU or MTU information placed inside of the Maximum Segment Size option data field. The remote system will respond with a segment that also has the Synchronize flag set, and with its MRU or MTU size stored in the Maximum Segment Size option data field of the response segment. ...