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Re: [dnsop] DNS Anycast revisited (fwd)

  • From: Edward B. Dreger
  • Date: Tue May 03 22:28:13 2005

PWG> Date: Tue, 3 May 2005 21:58:37 -0400
PWG> From: Patrick W. Gilmore

PWG> Just to make life fun, there is the whole "anycast a bunch of name
PWG> servers, each with different zone files pointing at local HTTP
PWG> servers".  Since the "anycast" portion is over UDP, it avoids a lot
PWG> of the problems (real or otherwise) mentioned here, and the HTTP is
PWG> still unicast but distributed and can be made resilient to failure.

Actually, it's not because the anycast portion is over UDP _per se_.
Someone hits an anycasted [Akamai] DNS server, which then transfers the
session to a unicast IP address c/o DNS.

Once the data is flowing via unicast, there's no further chance of
transferring the session to another anycast box.  DNS --> <whatever> is
a quick-and-dirty way of accomplishing the session transfer once and for
all without requiring additional protocol intelligence.  It's also
visible to the world, as opposed to behind-the-scenes session transfer.


PWG> Of course, the DNS backend is then .. uh .. "de-coherent"? :-)  But

Comparing anycasted DNS service with CDNs is about as valid as comparing
TCP with BGP.  They serve different purposes at different levels, and
one relies on the other.


PWG> it works, and works well, in many currently operational configurations.

Indeed.  One could move beyond CDNs and cite many corporate WANs as
examples of working configurations that lack coherent DNS.  Unicast
nameservers can lack coherency.  As several others have said, anycast
and coherency are orthogonal.


PWG> Does PPLB (or anything else) break this?  I'm certain I could find
PWG> things that would break this if I looked hard enough.  But as I've

The trick is finding something that breaks your config without affecting
"typical" ones.


PWG> said many times, reality trumps NANOG posts.  Since this is a
PWG> _working_ configuration today, I would say that disproves any claims
PWG> that it cannot or will not work.

I'd not go so far as to say "disproves", but it shifts the burden of
proof.


Eddy
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