TCP MSS from MTU Calculator

TCP MSS, MTU & IP Overhead

The Maximum Segment Size (MSS) is the biggest chunk of TCP payload you can pack into one IP packet without it fragmenting. You get it straight from the link MTU (Maximum Transmission Unit) by peeling off the network and transport headers: MSS = MTU - IP_header(20) - TCP_header(20). Run the numbers for standard Ethernet at MTU 1500 bytes and MSS lands at 1500 - 20 - 20 = 1460 bytes per segment.

IPv6 carries a 40-byte base header instead of the 20 in IPv4, so MSS drops to 1440 bytes on that same 1500 MTU link. Move into datacenters and storage networks and jumbo frames at MTU 9000 take MSS all the way up to 8960 bytes, which slashes the per-byte CPU cost of bulk transfers. Get the MSS wrong and you invite IP fragmentation, or something nastier: black-holed packets, the kind you see when a firewall drops ICMP and breaks PMTU discovery.

Applications

MSS math shows up across a network engineer's day. It sizes VPN tunnels, where IPsec/GRE/WireGuard pile on 20-80 bytes of overhead. It drives MSS clamping on edge routers and Path MTU Discovery tuning under RFC 8201 for IPv6. And it feeds capacity planning on high-throughput links, where jumbo frames mean 6× fewer packets per gigabyte.

FAQ

Why is MSS 1460 and not 1500? Because MTU measures the whole IP packet (headers + payload), while MSS only counts the TCP payload. Knock off the 20 + 20 bytes of IPv4 and TCP headers and you're left with 1460.

What is MSS clamping? A router edits the MSS option inside the TCP SYN, forcing a smaller value so traffic doesn't fragment on tunnels where the effective MTU has shrunk, like PPPoE running at MTU 1492.

When should I use jumbo frames? Keep them inside a controlled L2 segment (datacenter, SAN, iSCSI) where every device actually handles MTU 9000. Don't reach for them on the public Internet, since intermediate hops will just fragment or drop anything oversized.