Subject: anycast in DNS queries

From: Jun-ichiro itojun Hagino <[email-masked]>
Date: Thu, 3 Aug 2000 22:52:04 +0900 (JST)

in IETF48 dnsext session, i made a comment to ohta-san about use of
anycast for DNS root servers.
i checked ohta-san's draft.  the word "anycast" is used differently from
RFC2460.  i assuemd "anycast" in RFC2460 sense.  sorry if i was not clear
enough.

RFC2460 anycast cannot be used in UDP source address.
if you send UDP DNS query to RFC2460 anycast address, source address for
the reply will use different address from the destination address of query.
	query: X -> anycast
	reply: unicast -> X	(anycast != anycast)
therefore, normal resolvers will choke.

itojun

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From: Francis Dupont <[email-masked]>
Date: Thu, 03 Aug 2000 17:36:50 +0200

In your previous mail you wrote:

   	reply: unicast -> X	(unicast != anycast)
   
=> Aboba & all proposal has the same problem, it uses multicast and:
        query: X -> multicast
        reply: unicast -> multicast or X (*)
        unicast != multicast
*: the I-D is not very clear, it says "the response is multicast to
the LINKLOCAL address" (ie. there is at least a spelling error?).
But for the argument this doesn't matter...

[email-masked]

PS: for IPv6 there is a tradeoff between multicast/anycast (as
query destination) and unicast/multicast (as response destination).
In all cases source addresses MUST be unicast (I don't consider yet
the strange idea to use an unspecified source: I think this will never
be useful).