[dns-operations] PMTUD of .org servers

Mark Andrews marka at isc.org
Sat Jun 20 12:38:36 UTC 2009

In message <871vpfdyq1.fsf at mid.deneb.enyo.de>, Florian Weimer writes:
> * Paul Vixie:
> >> What Solaris needs is hooks to allow this to be done on a per socket
> >> basis or to only apply it to TCP traffic.
> >
> > seems more likely that they'll leave DF on by default.  EDNS speakers
> > should probably open the ICMP socket and look for evidence of DF damage.
> On the client side, a raw IP socket (perhaps bound to the UDP protocol
> if the stack allows it) and looking for tail fragments (which are
> likely to get through because they don't exhaust the path MTU) could
> provide some data.  It might be easier to run fragment assembly
> entirely in user space instead of matching this data to the UDP
> sockets you receive. 8-/

	Path MTU discover is just a waste of resources for DNS/UDP.
	There is no benefit to be gained from it.  We want the
	client to be doing reassembly because we know the packet
	is too big to get through w/ fragmentation.

	Path MTU discover helps for TCP because it removes re-assembly
	and results in efficient re-sends when packets are lost.
	It also leaves the packets of the fast paths in routers.

	If we were trying to optimise for non-fragmented traffic then
	there would be some benefit but we are not doing that.

Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742                 INTERNET: marka at isc.org

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