MTU

networking

Stands for: Maximum Transmission Unit

The largest packet, in bytes, a link will carry without fragmenting.

Maximum Transmission Unit is the size ceiling of a network hop, classically 1500 bytes on Ethernet. Get it wrong and packets fragment or get dropped, which is why MTU and MSS are worth calculating rather than guessing.

MTU problems have a signature that makes them recognizable once you have seen one: small things work and large things hang. The connection establishes, a ping succeeds, a short request returns instantly, and then a file transfer or a large response stalls forever. Nothing is down, and every simple test passes.

The cause is usually a path that cannot carry the packet size one end has assumed, combined with something blocking the ICMP messages that would have said so. Path MTU discovery depends on routers being able to report that a packet was too big; when a firewall drops those messages as a matter of policy, the sender never learns and simply retransmits into a black hole. This is why the advice to block all ICMP is bad advice with a long tail of consequences.

Tunnels are where this bites most often, because every layer of encapsulation steals bytes from the payload. VPNs, GRE, and overlay networks all shrink the usable size, and the fix is frequently MSS clamping so TCP negotiates a segment size that survives the whole path rather than discovering the problem after the connection is already established.

Also known as: mtu, jumbo frame

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