Broken PMTUD for . + TLD servers, was: Re: Smallest Transit MTU

Mon Jan 10 23:08:37 UTC 2005

On 10-jan-05, at 17:15, Stephen J. Wilcox wrote:

>>> Windows appears to always set DF, is there a reason why they did 
>>> that?

>> Of course I wanted to see this for myself. I used Quicktime to 
>> generate
>> some UDP, but no DFs, either on Win98 or XP.

> ah i was meaning tcp, afaik it sets DF on at least win2k

All OSes that I know of do this in order to do path MTU discovery. The 
PMTUD RFC encourages implementers to detect changes in the path MTU as 
fast as possible, which they took to mean "set the DF bit on ALL 
packets". Which is unfortunate, because in cases where PMTUD doesn't 
work, no communication is possible. Setting the DF bit on 50, 10, or 2 
% would accomplish PMTUD fine too, but it would also allow the session 
to survive brokennes.

> do any of those tiny resourced internet bootstrapping hosts exist? 
> perhaps we
> can deprecate DF?!

I've said I'd like to see this happen in the past. The trouble is 
without DF, current PMTUD doesn't work anymore, and router CPUs will be 
spending unnecessary CPU cycles on fragmentation. So some of the 
packets should trigger an ICMP too big, but others should just be 
fragmented. Figuring out the right mix probably requires some 
experimentation.