Maximum segment size

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. It does not count the TCP header or the IP header (unlike, for example, the MTU for IP datagrams). 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 in the final, reconstructed TCP segment. To avoid fragmentation in the 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. Therefore, IPv4 hosts are required to be able to handle an MSS of 536 octets (= 576 - 20 - 20) and IPv6 hosts are required to be able to handle an MSS of 1220 octets (= 1280 - 40 - 20). Small MSS values will reduce or eliminate IP fragmentation but will result in higher overhead. 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. The default TCP Maximum Segment Size is 536. 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. In order to notify MSS to the other end, an inter-layer communication is done as follows: The Network Driver (ND) or interface should know the Maximum Transmission Unit (MTU) of the directly attached network. The IP should ask the Network Driver for the Maximum Transmission Unit. The TCP should ask the IP for the Maximum Datagram Data Size (MDDS).