Purpose of MSS in TCP

Introduction

Laymen explanation

If you have seen house construction, you might have noticed how the stone/brick is moved from big lot to the construction room so that meson can use it. Sometimes, two people stands and one person passes the stone. Receiving person keep stone in floor for meson use. Notice that receiving person has limitation on the weight he can hold in his hand. So, he will tell not to give very big size stone. MSS does the same thing.

Technical explanation

The maximum segment size (MSS) is a parameter of the options field of the TCP header that specifies the largest amount of data, specified in bytes, that a computer or communications device can receive in a single TCP segment.

Useful observations

• • MSS should be calculated carefully since large MSS can result in IP fragmentaion. Small MSS values will reduce or eliminate IP fragmentation, but will result in higher overhead.

• IPv4 hosts are required to be able to handle an MSS of 536 octets (= 576[3] - 20 - 20) and IPv6 hosts are required to be able to handle an MSS of 1220 octets (= 1280[4] - 40 - 20).

  • For most computer users, the MSS option is established by the operating system. Where a host wishes to set the maximum segment size to a value other than the default, the maximum segment size is specified as a TCP option, initially in the TCP SYN packet during the TCP handshake. The value cannot be changed after the connection is established

MSS and PMTU

MSS is a concept used by TCP in the Transport layer and it specifies the largest amount of data that a computer or communications device can receive in a single TCP segment. While PMTU is used by the IP layer and it specifies the largest packet size that can be sent over this path without suffering fragmentation.

Reference

https://en.wikipedia.org/wiki/Maximum_segment_size

http://www.tcpipguide.com/free/t_TCPMaximumSegmentSizeMSSandRelationshiptoIPDatagra.htm