Editing Maximum segment size (section)

{{Short description|Transmission Control Protocol parameter}}
The '''maximum segment size''' ('''MSS''')  is a parameter of the ''Options'' field of the [[Transmission Control Protocol|TCP]] header that specifies the largest amount of data, specified in [[bytes]], that a computer or communications device can receive in a single [[Transmission Control Protocol#TCP segment structure|TCP segment]]. It does not count the [[Transmission Control Protocol#TCP segment structure|TCP header]] or the [[IP header]] (unlike, for example, the [[Maximum transmission unit|MTU]] for IP datagrams).{{Ref RFC|9293|rsection=3.7.1}} The [[IP datagram]] containing a TCP segment may be self-contained within a single packet, or it may be reconstructed from several [[IP fragmentation|fragmented]] pieces; either way, the MSS limit applies to the total amount of data contained in the final, reconstructed TCP segment.

To avoid fragmentation in the [[Internet layer|IP layer]], a host must specify the maximum segment size as equal to the largest IP datagram that the host can handle minus the IP and TCP header sizes. Though there is no minimum required MSS defined in [[Request for Comments|IETF RFCs]], there is a minimum MTU, and so a default MSS is calculated by subtracting the minimum IP and TCP header sizes from the MTU. For example, [[IPv4]] hosts typically default to an MSS of 536 octets (= 576  − 20 − 20) and [[IPv6]] hosts generally are able to handle an MSS of 1220 octets (= 1280 − 40 − 20).{{Ref RFC|9293|rsection=3.7.1}}

Small MSS values will reduce or eliminate IP fragmentation but will result in higher [[Protocol overhead|overhead]].<ref>[http://www.tcpipguide.com/free/t_TCPMaximumSegmentSizeMSSandRelationshiptoIPDatagra.htm The TCP/IP Guide], TCP Maximum Segment Size (MSS) and Relationship to IP Datagram Size</ref>

Each direction of data flow can use a different MSS.

For most computer users, the MSS option is established by the [[operating system]].

TCP options size (Variable 0–320 bits, in units of 32 bits) must be deducted from MSS size if TCP options are enabled. For example, TCP Time Stamps are enabled by default on Linux platforms.