[c-nsp] Anycast services

[Sam Stickland]

Imagine that you announcing the Anycast address 10.10.10.10 into the routing
table from a three separate points. Which of these points your packets get
routed to depends on which way the routing protocols sends your packets.
Different bits of the network will see 10.10.10.10 in multiple places, and
which one they pick will depend on many aspects, some of them transient,
such as interface/route metrics, route-dampening etc.

Say you start a TCP/IP session with 10.10.10.10. There is nothing to
guarantee that your packets will keep going to the server you first opened
the session to. At any point they could go to another 10.10.10.10 server
that the routing protocols have now decided is a better bet and your session
would break.

In fact, the sudden switch of servers in response to network changes is the
whole point of Anycast. ;)

S

> -----Original Message-----
> From: Tim Durack [mailto:tdurack at gmail.com]
> Sent: 18 May 2006 16:57
> To: Sam Stickland
> Cc: cisco-nsp at puck.nether.net
> Subject: Re: [c-nsp] Anycast services
> 
> Okay, assuming I'm not doing per-packet load balancing (because that
> breaks lots of things.)
> 
> I don't see why I can't run something simple like a web proxy anycast.
> Even if I load balance per session, and have multiple equal cost paths
> to the anycast address, I don't see how this would break.
> 
> But I have also never tried it...
> 
> Tim:>
> 
> On 5/18/06, Sam Stickland <sam_mailinglists at spacething.org> wrote:
> > Hi,
> >
> > Comments inline.
> >
> > > -----Original Message-----
> > > From: cisco-nsp-bounces at puck.nether.net [mailto:cisco-nsp-
> > > bounces at puck.nether.net] On Behalf Of Tim Durack
> > > Sent: 18 May 2006 16:20
> > > To: cisco-nsp at puck.nether.net
> > > Subject: [c-nsp] Anycast services
> > >
> > > The subject of Anycast DNS has raised my curiosity. Anyone tried
> > > anycast of other services, such as a web proxy? Or is there a better
> > > way to do this?
> > >
> > > I'm interested in this from a geographical redundancy perspective, as
> > > opposed to purely clustering for load-balancing/redundancy.
> >
> > You can't ensure that packets routed to an Anycast address will always
> > arrive at the same end point. They might arrive at a different node.
> >
> > Hence, you can't use Anycast for any kind of stateful service (e.g.
> TCP/IP).
> > The correct approach to using it here would be to Anycast your DNS
> servers,
> > and only have your DNS return entries for servers/cluster-groups that
> are
> > known to be up.
> >
> > Sam
> >
> >