Editing Transmission Control Protocol (section)

===Maximum segment size===
The [[maximum segment size]] (MSS) is the largest amount of data, specified in bytes, that TCP is willing to receive in a single segment. For best performance, the MSS should be set small enough to avoid [[IP fragmentation]], which can lead to packet loss and excessive retransmissions. To accomplish this, typically the MSS is announced by each side using the MSS option when the TCP connection is established. The option value is derived from the [[MTU (networking)|maximum transmission unit]] (MTU) size of the data link layer of the networks to which the sender and receiver are directly attached. TCP senders can use [[path MTU discovery]] to infer the minimum MTU along the network path between the sender and receiver, and use this to dynamically adjust the MSS to avoid IP fragmentation within the network.

MSS announcement may also be called ''MSS negotiation'' but, strictly speaking, the MSS is not ''negotiated''. Two completely independent values of MSS are permitted for the two directions of data flow in a TCP connection,{{sfn|RFC 1122}}{{sfn|RFC 9293}} so there is no need to agree on a common MSS configuration for a bidirectional connection.