Re: [gentoo-user] {OT} ISP requires MTU below 1500?

[Grant]

> Leaving your MTU at the default ethernet size of 1500 on your PC/server should
> not cause a problem for most day to day operations, because modern end-point
> OS and network devices use Path MTU Detection.  Problems will arise when you
> come across a misconfigured router/firewall/server (internet black hole) which
> drops  ICMP Fragmentation Needed (Type 3, Code 4) packets and won't adjust its
> MTU to make sure you can receive packets of the appropriate size.


And I believe that's exactly what I have as far as my AT&T
modem/router which seems to drop all icmp packets.  I think that's why
it's important for me to set an MTU for my network which is not
greater than the MTU of the modem/router which appears to be 1492.


> I have no idea if PMTUD is in any way relevant to the TCP queue spikes you
> have observed, but they are caused by TCP buffers overflowing.  Some detective
> work at the time these overflows take place would show what the server is doing
> at the time.


Any idea which tool to use?  I could start keeping an eye on output
when things are good and then again when things are bad so I can
compare the two states.

- Grant