Overview
Maximum Segment Size (MSS) is a parameter in the TCP header that is negotiated during the three-way handshake. Unlike MTU, which applies to the entire packet (including headers), MSS only counts the payload (the actual data).
Relationship to MTU
Typically, MSS = MTU - (IP Header Size) - (TCP Header Size). For a standard 1500-byte MTU, the MSS is usually 1460 bytes.