> 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
