MTU Calculator

MTU is the largest IP packet a link accepts without fragmentation. TCP MSS is the maximum TCP payload advertised for a connection. With minimum headers, MSS is MTU minus 40 bytes for IPv4 or 60 bytes for IPv6. IP or TCP options and encapsulation can reduce the available payload further.

PPPoE adds 8 bytes inside a standard 1500-byte Ethernet payload, leaving 1492 bytes for the IP packet unless the network supports a larger frame. Some broadband providers use PPPoE and others do not. Use the MTU specified by your provider or measured on the actual path.

UDP has a smaller transport header than TCP, but an OpenVPN tunnel's usable payload also depends on IP version, cipher, authentication, options, and outer-path MTU. There is no universal 1462 or 1438 setting. Follow the VPN configuration and test the deployed path; avoid changing tunnel MTU from this estimate alone.

For IPv4, a do-not-fragment ICMP echo can test candidate packet sizes. A 1472-byte echo payload plus the minimum 20-byte IPv4 and 8-byte ICMP headers makes a 1500-byte packet. Results are inconclusive when a firewall filters ICMP, and IPv6 or tunneled paths use different overhead. Use platform documentation and packet capture when the simple probe disagrees with application traffic.

Jumbo frames can reduce packet-processing overhead on controlled storage or cluster networks, but the benefit depends on hardware and workload. Every interface and path segment must support a compatible MTU. Measure before enabling them, and do not assume a private jumbo-frame setting works across the public internet.