My ISP sometimes drops IP Fragments. This manifests itself in SSH thus: I can login fine and run commands that output short strings, but running something like "ps fax" causes the link to lock up.
The PMTU is 1500. So it seems that SSH is trying to transmit packets bigger than this that are then getting fragmented and therefore lost. I can't easily reduce the remote MTU, and I am reluctant to do so, since the MTU is correct (1500).
What is going on here? I thought TCP sets its MSS to the correct value to avoid fragmentation. How am I getting fragments?
EDIT: The remote machine is Centos 6.4. By "sometimes" I meant that the ISP has bad days where their network breaks and it really does drop just fragments. Sniffing both ends of the link I see the Packet and fragments from a large ping leave my system, but only the "main" packet arrives at the target.
The problem manifested itself as above and also a black screen when logging into an RDP session. The problem is fixed now, so I can't do any more tests until it happens again.
A bit more testing on another system shows that SSH sets the DF bit. So now I really don't understand what is going on.
I wasn't able to test if Fragmentation needed ICMP messages appear (they shouldn't) as network accepts packets up to Ethernet size (1500) so my own router would issue the frag needed message.
I complained to the ISP, but they tried to convince me that this was by design and that their network would never allow a ping like "ping -s 5000". Not true of course. Especially given they fixed it.