Re: [PATCH] anycast support for IPv6, linux-2.5.31

On Wed, 28 Aug 2002, David Stevens wrote:

1) The API
      Although the RFC's liken anycasting to ordinary unicasting, I think
it's more appropriate to tie it closely to particular applications, so I've
chosen an API similar to multicasting. So, rather than having a permanent
anycast address associated with the machine, particular applications
that use anycasting can join or leave "anycast groups," and the machine will
recognize the anycast addresses as its own when one or more applications have
joined the group.
      So, for example, someone using anycasting for DNS high availability
can add a join to the anycast group in the server and as long as the DNS server
is running, the machine will answer to that anycast address. But the machine
will not respond to anycasts when the service that's using it isn't available,
so a broken server application that has exited won't deny that service if
there are other working members of the anycast group on other hosts.
      I don't know if that's controversial or not-- the RFC's are written
more from the external context, but seem to imply a model along the lines of
using "ifconfig" to add anycast addresses. I think that model doesn't fit the
best uses of anycasting, but I'd like to hear your thoughts on it.
      The application interface for joining and leaving anycast groups is 2
new setsockopt() calls: IPV6_JOIN_ANYCAST and IPV6_LEAVE_ANYCAST. The arguments
are the same as the corresponding multicast operations. The kernel keeps a
reference count of members; when that goes to zero, the anycast address is not
recognized as a local address. While nonzero, the host listens on the solicited
node for that address, sends advertisements in response to solicitations (with
override=0) and delivers packets sent to the anycast address to upper layers.
      There's also an in-kernel interface described below, which is used by
IPv6 mobility, for example.

Before going too much down this path, I think one should write an Internet
Draft about the proposed API (should be quite short & simple) and see what
kind of response it has in the relevant working groups.

-- 
Pekka Savola                 "Tell me of difficulties surmounted,
Netcore Oy                   not those you stumble over and fall"
Systems. Networks. Security.  -- Robert Jordan: A Crown of Swords