Maximum segment size
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The maximum segment size (MSS) is the largest amount of data, specified in bytes, that a computer or communications device can handle in a single, unfragmented piece. For optimum communications, the number of bytes in the data segment and the headers must not add up to more than the number of bytes in the maximum transmission unit (MTU).
The MSS is an important consideration in Internet connections. TCP and IPv4 headers are 20 bytes long each, whereas an IPv6 Header is 40 bytes long, so the MSS is equal to MTU - 40 when using IPv4, and MTU - 60 when using IPv6.
As data is routed over the Internet, it must pass through multiple gateway routers. Ideally, each data segment can pass through every router without being fragmented. If the data segment size is too large for any of the routers through which the data passes, the oversized segments are fragmented. This slows down the connection speed as seen by the computer user. In some cases the slowdown is dramatic. The likelihood of such fragmentation can be minimized by keeping the MSS as small as reasonably possible. For most computer users, the MSS is set automatically by the operating system.
