At 10:04 AM -0400 4/30/02, Rob Austein wrote:
>Perhaps this week's version of the proposed solution for this is something
>that walks and quacks like an anycast route but is called something else
>so that you can use it as a source address.
Rob,
No, "this week's" version of the proposed solution is to use three well-
known, site-local unicast addresses. The difference between a unicast
address and an anycast address is that an anycast address is shared by
more than one node (or, more accurately, more than one interface),
while a unicast address is not. The prohibition against using an
anycast address as a source address is due to the fact that multiple
components of the IP architecture (v4 or v6) assume that a source
address belongs, for a non-negligible length of time, to exactly one
node, and those components may break in circumstances where that
assumption doesn't hold. This is not a new rule -- it has always
been forbidden to use multicast addresses as source addresses, for
the same reason that they cannot be trusted to belong to only one node.
>I don't really care either way, it's still an anycast route in all
>but name.
No, it's a host route. Host routes are supported by all IP unicast
routing protocols (I think), and have been used for nigh-on 20 years,
long before the term anycast was coined. To label all use of host
routes as "anycast" is more likely to obfuscate than clarify what's
what's going on.
At 11:28 PM -0400 4/29/02, Rob Austein wrote:
>[*] Or two, or three, or whatever small fixed number of anycast
^^^^^^^
unicast
>addresses you choose to allocate, and to date I haven't seen an
>explanation of how servers anycast::1, anycast::2, and anycast::3
>are supposed to sort out which one gets which anycast address without
>some kind of coordination mechanism that either involves yet another
>new protocol or something that quacks like manual configuration.
Manual configuration is exactly what is being proposed, at least to
start with. Instead of manually configuring a database in an
intermediary with the addresses of three DNS servers, you configure
the three DNS servers themselves, thus eliminating an unnecessary
intermediary.
Eventually, it would be nice to fix the DNS service to not insist on
using the destination address of a query as the source address of a
reply, at which point any of the three addresses could also be an
anycast, to lift the arbitrary limit of three DNS servers available
for use by plug-and-play clients within a site.
Steve
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