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Networking Fundamentals · Intermediate

Maximum Segment Size (MSS)

The largest amount of TCP payload data a host will accept in one segment, advertised in the SYN.

Definition

MSS excludes the IP and TCP headers, so on a 1500-byte MTU IPv4 link it is typically 1460 bytes (1500 minus 20 IP minus 20 TCP). Each side advertises its MSS in the TCP options of its SYN or SYN-ACK. Routers can clamp MSS to avoid fragmentation across tunnels.

Example

On a 1500-byte MTU, IPv4 MSS is 1460 bytes and IPv6 MSS is 1440 bytes.

Exam tip

MSS = MTU minus IP header minus TCP header; do not confuse it with MTU itself.

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