MSSMaximum Segment Size

The largest TCP payload that fits in one packet — typically MTU minus 40 bytes of TCP/IP overhead.

TCP peers negotiate MSS during the three-way handshake. For Ethernet with MTU 1500, MSS is usually 1460 (1500 − 20 IP − 20 TCP).

Middleboxes sometimes "clamp" MSS to a smaller value to fit downstream tunnels that have lower MTU — a technique called MSS clamping.