[dns-operations] PMTUD of .org servers

Rick Jones rick.jones2 at hp.com
Thu Jun 18 18:43:03 UTC 2009

Michael Graff wrote:
> Is PMTU even defined on UDP?  IMHO, UDP should not have DF set in the
> common case, and never for DNS.  I thought PMTU was more of a TCP thing
> anyway?

PMTU is defined in the context of IP.  Whether the information about an 
arriving datagram too big ICMP is propagated up to the UDP-using 
application is platform-specific.  The reason to tell the UDP using 
application would be to tell it its traffic was lost and it needs to 
resend.  However, since a UDP application has to deal (somehow) with 
packet loss anyway, when it does so, presumably there will be a PMTU 
route in the tables which will cause the subsequent traffic to be 
fragmented to the desired/necessary size.

However, as this can sometimes call for successive resends, IIRC many 
(most?) stacks may disable setting DF on IPv4 datagrams carrying UDP 

per the dimm wetware memory of
rick jones

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