(Manual) maximum segment size(MSS)_the maximum segment size-优快云博客

2014-02-11 wcdj

The maximum segment size (MSS) is the largest amount of data, specified in bytes, that acomputer or communications device can handle in a single, unfragmented piece. For optimumcommunications, the number of bytes in the data segment and the header must add up toless than the number of bytes in the maximum transmission unit (MTU).

The MSS is an important consideration in Internet connections, particularly Web browsing. Whenthe Internet's Transmission Control Protocol (TCP) isused to achieve an Internet connection, the computers being connected must each agree on, and thenset, the MTU size acceptable to both. Typical MTU size in TCP for a home computer Internetconnection is either 576 or 1500 bytes. Headers are 40 bytes long; the MSS is equal to thedifference, either 536 or 1460 bytes. In some instances the MTU size is less than 576 bytes, andthe 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 datasegment size is too large for any of the routers through which the data passes, the oversizesegment(s) are fragmented. This slows down the connection speed as seen by the computer user. Insome cases the slowdown is dramatic. The likelihood of such fragmentation can be minimized bykeeping the MSS as small as reasonably possible. For most computer users, the MSS is setautomatically by the operating system.