MTU (Maximum Transmission Unit) is the maximum size of an IP packet that can cross a link in one piece. Standard Ethernet uses 1500 bytes; the smallest MTU along a path determines the path MTU.
Why it matters
If a packet is larger than a link's MTU it must be fragmented or dropped. When the Don't Fragment bit is set and ICMP messages are blocked, large packets vanish while small ones work, so connections hang after the handshake. Tunnels such as VPNs add headers and reduce the usable MTU, so settings like 1400 or 1420 are common there.
Example
$ ping -M do -s 1472 example.comA 1472-byte payload plus 28 bytes of IP and ICMP headers equals 1500. If this fails with "message too long" but a smaller size works, the path MTU is lower than 1500.
Common mistakes
- Blocking ICMP fragmentation-needed messages, which breaks path MTU discovery.
- Leaving MTU at 1500 inside a VPN tunnel and getting stalled large transfers.
- Raising MTU to jumbo frames on only part of a network.
Related
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