Mac OS X: Find the maximum unfragmented packet size (MTU) to reach a host

If you have ever tried to use jumbo packets, or trace a weird slowness on the network – one of the things that frequently comes up is packet fragmentation. This is basically where a source machine is sending bigger packets than can be consumed along its pathway to a destination machine. This means the packets will need to be split and essentially causes a host of performance issues.

So how do you diagnose this? Well Ping is your friend. It allows you to flag packets to not be fragmented and specify a minimum and maximum packet size. Using this you can simply loop through test packet sizes until a packet fails and then you have your MTU.

The command below sends packets from 1350 to 1520 and increases the packet size by 10 bytes each time.

ping -g 1350 -G 1520 -h 10 -D

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