North American Network Operators Group
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Re: Broken PMTUD for . + TLD servers, was: Re: Smallest Transit MTU

  • From: Iljitsch van Beijnum
  • Date: Mon Jan 10 18:09:51 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.