> The even stranger case was when there was a Linux box doing forwarding that had segment offload left on. It was taking in several 1500 byte TCP packets from one side, smashing them into ~9000 byte monsters, and then tried to send those over a VPNish network interface that absolutely couldn't handle that. Even if the network in the middle bothered to generate the ICMP too big message, the source would have been thoroughly confused because it never sent anything over 1500.

This is an old Linux tcp offloading bug; large receive offload smooshes the inbound packet, then it's too big to forward.

I had to track down the other side of this. FreeBSD used to resend the whole send queue if it got a too big message, even if the size did not change. Sending all at once made it pretty likely for the broken forwarder to get packets close enough to do LRO, which resulted in large enough packet sending to show up as network problems.

I don't remember where the forwarder seemed to be, somewhere far away, IIRC.