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Re: [dnsop] DNS Anycast revisited (fwd)

  • From: Nicholas Suan
  • Date: Tue May 03 22:34:42 2005

Dean Anderson wrote:
On Tue, 3 May 2005, Paul G wrote:


i'm terribly sorry, but i'm unable to extract any meaning at all from these
statements. when i parse them, they make no sense at all (not in terms of
being wrong, just not understandable). could you rephrase them?

coherency and consistency are well-defined terms in systems engineering. we
are talking about dns queries and hence coherency of zone data (the shared
resource). i fail to see how this is open to any interpretation at all.

Sorry, The original statement Vixie made is nonsense. Here is the original
statement again:

Vixe writes:
lest anyone be confused, ultradns's anycast for .ORG is completely
coherent and doesn't admit the possibility of giving out different
responses from different anycast nodes for policy reasons or any other
reason, and so it's an example of "good" anycast the way i count such
things.

Vixie seems to be responding to concern raised for Ultradns' pervasive use
of anycasting. This was the only issue raised involving Ultradns.

During the anycast discussion on DNSOP, the subject of zone coherency (as
normally used) was not an issue. So there is no question of zone
coherency for Ultradns' servers. We assumed (and did not dispute) that
zone updates were unaffected by anycast. Zone updates happen over private
secure channels on non-anycasted IP addreses. They ought to be as coherent
as DNS gets. They ought not be affected by anycast.

Vixie ends by saying essentially, that because of Ultradns' coherency, it
is an example of "good anycast". But the two issues (coherency and
anycast) have no relationship. There is no reason to conclude that
coherency means anycast is either good or bad. Hence, his statement is
nonsense.
Context helps.

In the previous paragraph Vixie said:

while i'm on the subject, i also remain convinced that using anycast to do
distributed load balancing for applications like WWW, on the assumption
that the path you heard a dns query on is instructive as to what content
would be best to answer with, is silly, and will more often do harm or do
nothing than do good.  (and i've told akamai and speedera this many times.)
("but it makes for great marketing slideware.")

In other words this is a bad idea:

[[email protected] FT]$ dig a248.e.akamai.net @69.45.79.10

;; ANSWER SECTION:
a248.e.akamai.net. 20 IN A 80.67.72.214
a248.e.akamai.net. 20 IN A 80.67.72.201

[email protected]:~$ dig a248.e.akamai.net @69.45.79.10

;; ANSWER SECTION:
a248.e.akamai.net. 20 IN A 69.45.79.15
a248.e.akamai.net. 20 IN A 69.45.79.16

While I'm not a mind reader, It seems he's saying that, since Ultradns doesn't use anycast to do this, it is an example of 'good anycast.'