[c-nsp] Anycast services

[Paul Cairney]

On Thu, May 18, 2006 at 11:19:37AM -0400, Tim Durack wrote:
> 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.

Using Anycast for stateful applications is not advisable as IP makes no provision to route packets for the same destination via the same route. The clasic corner case against using TCP over Anycast is that per-packet load balancing may lead to packets of a stateful connection being routed to different Anycast instances.
If you have control over the appplication, you may be able to engineer it so the client connects to the anycast host which then establishes a new TCP connection back from its Unicast address.

Prolexic claim to have "the worlds first TCP over Anycast network", however I am unsure how they get arround this unless it is only anycasted within their network(s).

The more common way to leverage the benifits of Anycast is to use it to direct your clients to the nearest(in BGP terms anyway) DNS server, which resolves the requested record to the  IP address of a suitable (nearest/healthy/load blanced etc) service host such as web server or proxy.


Paul