Subject: Seeking Anycast ROUTING materials

From: "Gu huaping" <[email-masked]>
Date: Sat, 23 Oct 2004 15:40:03 +0800

Hi all:
I have read RFC2373 where the authors give definition for Anycast Address. But I still cannot find the detail ROUTING materials. Can some one give me some hints to find some material relating to Anycast?

3x
Gu huaping

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From: Fred Baker <[email-masked]>
Date: Fri, 27 Aug 2004 07:41:47 -0700

At 12:40 PM 10/23/04 +0800, Gu huaping wrote:
>I have read RFC2373 where the authors give definition for Anycast Address. 
>But I still cannot find the detail ROUTING materials. Can some one give me 
>some hints to find some material relating to Anycast?

An anycast address is simply an address, and is routed the same way any 
other address is routed.

What makes an anycast address interesting is the fact that several servers 
respond to it. For example, many of the DNS roots distribute their load by 
deploying instances of their service in a variety of places. Standard 
routing will take a given request to one or another of them, which will 
reply. This offloads a central device and the communication paths leading 
to it, and provides faster service to the initial requester.

There are two fundamental problems with anycast. One is that if one of 
those devices fails and routing is not changed to remove it from the 
picture, the service is effectively black-holed in the region. To solve 
this, folks implement routing processes on the server itself - it 
advertises to the network that it has routing connectivity to the address, 
not that it *is* the address. Should it fail, the putative router fails, 
and routing is recalculated without it. Voila, the service is now offered 
via only the remaining servers.

The other problem is that routing can change, with the result that a 
session to an anycast server migrates in mid-communication from one server 
to another. For this reason, anycast is useful today for simple 
applications like DNS, but isn't very useful for more general services like 
www. There has been a proposal in the past, which never got traction I 
guess, to use Mobile IP with optimized routing. One treats the anycast 
address as the home address, the client of the service as the correspondent 
node, and the address of the individual server as the care-of-address of 
the mobile node. When the anycast server replies to the initial SYN, the 
client starts talking to the actual server's address, meaning that the 
session doesn't migrate even if routing changes. It would be good, I think, 
if this could be standardized and deployed, as it would then allow us to 
use distributed services in an improved way.

I should say that we can implement distributed services today, using DNS - 
we list several server addresses in the a record. The problem is that the 
use of these is random, rather than being distributed rationally using 
routing. The Mobile IP based scenario I just described enables us to do so 
in a way that predictably improves things.