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 header must add up to less than the number of bytes in the maximum transmission unit (MTU).
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The MSS is an important consideration in Internet connections, particularly Web browsing. When the Internet's Transmission Control Protocol (TCP) is used to achieve an Internet connection, the computers being connected must each agree on, and then set, the MTU size acceptable to both. Typical MTU size in TCP for a home computer Internet connection is either 576 or 1500 bytes. Headers are 40 bytes long; the MSS is equal to the difference, either 536 or 1460 bytes. In some instances the MTU size is less than 576 bytes, and the data segments must therefore be smaller than 536 bytes.
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 oversize segment(s) 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.