MTU of the Internet?

Marc Slemko writes...

First, I _really_ doubt the web server set the DF bit. It is almost
certainly the OS, probably trying to do path MTU discovery.

If there is a smaller MTU in the middle, and someone is filtering ICMP
can't fragment errors then yes, you will have trouble with PTMU discovery.
The fix is to not blindly filter ICMP. Increasing the client MTU won't
fix this.

Agreed that one should not blindly filter ICMP. However not all filter
tests don't support discriminating individual ICMP types. Ascend is one

If the client lowers their MTU, there is no problem because then they
advertise an appropriate MSS and no stack should try sending packets that
it knows will need to be fragmented given the client MSS.

That's one reason I went with lower MTU until I could replace SLIP with PPP.
But then I discovered other things work better, such as interactive telnet
during multiple concurrent downloads. So I leave MTU low until I get that
OC-12 into my apartment.

Why not? You have the options to use IP filters which are fairly limited,
or generic filters where you can specify more details of the filter.