tcp_mtu_probing on by default?

Martin Pool mbp at
Mon Feb 6 23:15:10 UTC 2012

I have helped a few people recently who were having path MTU discovery
problems, causing bulk TCP transfers to hang quasi-intermittently.
Once you know the likely cause it's fairly easy but it's a fairly
annoying problem for someone who doesn't recognize it.

There is a kernel sysctl "sudo sysctl -w net.ipv4.tcp_mtu_probing=1"
that seems fairly effective at detecting when the problem is occurring
and automatically fixing it.   This implements RFC 4821.  It is off by
default in the kernel.  I haven't seen any reports of problems caused
by turning it on, but there may be some.

I wonder if Ubuntu should turn it on in /etc/sysctl.d?


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