[dns-operations] PMTUD of .org servers

[Mark Andrews]

In message <87ws794cj7.fsf at mid.deneb.enyo.de>, Florian Weimer writes:
> * Michael Graff:
> 
> > Florian Weimer wrote:
> >> Is it just me, or do some of the .org servers (notably
> >> 199.19.56.1/a0.org.afilias-nst.info) ignore ICMP "fragmentation
> >> needed, but DF bit set" message, even though they set the DF bit in
> >> the IP header of the response packet?
> >
> > Is PMTU even defined on UDP?
> 
> Yes, it is.  RFC 1191 is somewhat protocol agnostic.
> 
> > 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?
> 
> EDNS0 requires PMTUD for DNS, see section 4.5.2 of RFC 2671.  It is
> debatable if this requirement is desirable.

4.5.2. Both requestors and responders are advised to take account of the
       path's discovered MTU (if already known) when considering message
       sizes.

This does not come even close to requiring PMTUD for DNS.

Mark

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Mark Andrews, ISC
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