Maximum segment size
Encyclopedia
The maximum segment size (MSS) is a parameter of the TCP protocol
that specifies the largest amount of data, specified in octets
, that a computer or communications device can receive in a single TCP segment, and therefore in a single IP datagram. It does not count the TCP header or the IP
header. The IP datagram containing a TCP segment may be self-contained within a single packet, or it may be reconstructed from several fragmented
pieces; either way, the MSS limit applies to the total amount of data contained within the final reconstructed TCP segment.
Therefore: Headers + MSS ≤ MTU
The Minimum MSS = Maximum datagram size - IP header size - TCP header size
So every IPv4 host is required to be able to handle an MSS of at least 536 octets (= 576 - 20 - 20)
and every IPv6 host is required to be able to handle an MSS of at least 1220 octets (= 1280 - 40 - 20).
For most computer users, the MSS option is established by operating system
on the SYN packet during the TCP handshake. Each direction of data flow can use a different MSS.
Transmission Control Protocol
The Transmission Control Protocol is one of the core protocols of the Internet Protocol Suite. TCP is one of the two original components of the suite, complementing the Internet Protocol , and therefore the entire suite is commonly referred to as TCP/IP...
that specifies the largest amount of data, specified in octets
Octet (computing)
An octet is a unit of digital information in computing and telecommunications that consists of eight bits. The term is often used when the term byte might be ambiguous, as there is no standard for the size of the byte.-Overview:...
, that a computer or communications device can receive in a single TCP segment, and therefore in a single IP datagram. It does not count the TCP header or the IP
Internet Protocol
The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
header. The IP datagram containing a TCP segment may be self-contained within a single packet, or it may be reconstructed from several fragmented
IP fragmentation
The Internet Protocol implements datagram fragmentation, so that packets may be formed that can pass through a link with a smaller maximum transmission unit than the original datagram size....
pieces; either way, the MSS limit applies to the total amount of data contained within the final reconstructed TCP segment.
Therefore: Headers + MSS ≤ MTU
Maximum transmission unit
In computer networking, the maximum transmission unit of a communications protocol of a layer is the size of the largest protocol data unit that the layer can pass onwards. MTU parameters usually appear in association with a communications interface...
The Minimum MSS = Maximum datagram size - IP header size - TCP header size
So every IPv4 host is required to be able to handle an MSS of at least 536 octets (= 576 - 20 - 20)
and every IPv6 host is required to be able to handle an MSS of at least 1220 octets (= 1280 - 40 - 20).
For most computer users, the MSS option is established by operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
on the SYN packet during the TCP handshake. Each direction of data flow can use a different MSS.