The current maximum segment size (MSS), in bytes, used for transmitting packets in the current TCP connection to the client. Its initial value is the minimum of the MSS option received from the client during connection handshake and the historical MSS available on this path.
Changes detected in path maximum transmission unit (PMTU), even by other connections to the same peer, can result in the MSS changing during the lifetime of the connection. Larger values are more efficient as the per-packet overhead is reduced, but smaller values transit the internet more reliably. MSS values are slightly smaller than PMTU to allow room for TCP and IP overhead.