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Maximum Segment Size (MSS) refers to the ceiling, or largest data segment, accommodated as an unfragmented piece by a computer or communications device.
The MSS does not include the size of the TCP and IP headers in computation. However, to achieve optimum communication, the number of bytes and headers are combined but must not exceed the number of bytes specified in the Maximum Transmission Unit (MTU). A lower MSS ensures that fragmentation is not present during the process, even though there is a decrease in data and header ration.
The MSS is typically established by the operating system (OS) when a TCP handshake occurs in a SYN packet. Each data flow direction may use a different MSS. The MSS is an important Internet connection component, especially when using a Web browser. When TCP is used in an Internet connection, the connected computers must establish a specific MTU size agreeable to all connected parties.
Internet data goes through several gateway routers. Data segments must pass through all of these routers without becoming fragmented. If the data is too large, the oversized segment is broken into smaller fragments, resulting in decreased Internet speed, which may be noticeable to users. However, this cannot be avoided if the gateway routers on the path set a smaller MTU size, or if the user’s computer has set a larger MSS value.