MSS

Problem

TCP puts its data inside an IP packet. But that packet has to fit inside the network's MTU. If: MTU = 1500 some space is already consumed by IP and TCP headers. How much TCP payload should we send?

Solution — MSS

MSS stands for Maximum Segment Size. TCP peers advertise the maximum TCP payload they can receive without exceeding the relevant packet size.

A common IPv4 example:
1500 MTU
- 20 IP
- 20 TCP
────────
1460 MSS

Core idea: MTU limits the IP packet; MSS limits the TCP payload inside it.