21 Nov 2022 Macbooks Networking

Find Max MTU Size on Mac OS X Using Ping (No Fragmentation)

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Finding the maximum MTU size on Mac OS X uses the ping command with the no-fragmentation flag. Run `ping -D -s `, incrementing the packet size until ping reports the packet cannot be sent. The largest successful size plus 28 bytes for IP and ICMP headers equals your true MTU ceiling.

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1.
What it is
This article explains how to use the ping command on Mac OS X to determine the maximum transmission unit (MTU) - the largest packet size that can travel between your computer and a destination without being broken apart.
2.
Why it matters
Finding the correct MTU helps diagnose network slowness and performance issues caused by packet fragmentation, which occurs when your system sends packets larger than the network path can handle.
3.
Key takeaway
Use ping with specific flags to test different packet sizes and find the optimal MTU that prevents fragmentation and improves network performance.

~1 min read

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 andrewbaker.ninja