Anycast can be run for a number of reasons, a classic example would be as follows:
Institution has a high traffic web server, that's accessed from all over the world. A.) they want to cut costs but having multiple cheapest point access to the server (national versus international bandwidth etc) B.) they want redundancy for the server C.) they want a form of load balancing There could be a number of additional reasons, but anyway... Said server then gets 2 addresses, a unicast and an anycast address. The web server will listen on the unicast address at each point, it will also use the unicast interface to synchronize with the other servers or a central point On the anycast address, the web server will run a DNS Server. Each DNS server will have a different A record for the web server dns entry, that point to the servers unicast. So: Three Servers: Server 1: Unicast 1.2.3.4 Server 2: Unicast 4.3.2.1 Server 3: Unicast 9.8.7.6 Server 4: Unicast 6.7.8.9 Anycast across all servers: 22.22.22.22 Server 1's DNS entry for www.anycastdemo.net responds with 1.2.3.4 Server 2's DNS entry for www.anycastdemo.net responds with 4.3.2.1 Server 3's DNS entry for www.anycastdemo.net responds with 9.8.7.6 Server 4's DNS entry for www.anycastdemo.net responds with 6.7.8.9 Client queries www.anycastdemo.net authorative nameserver (22.22.22.22) and gets back the closest entry, being 1.2.3.4, and starts downloading a large file from it. Routing changes occur, suddenly server 2 is closest entry. Due to the fact that the DNS is anycast, and the web server itself is TCP based unicast he doesn't drop connection, UNTIL another query is done (typically the A records here would have very low expiry times on them), at which point he would get a new A record pointing to server 2. Almost a... protected version of anycast for TCP. This would be ONE of MANY possibilities for using Anycast. Would this qualify as "critical infrastructure?" I'm not sure... Thoughts? Andrew -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Michuki Mwangi Sent: Friday, May 11, 2007 1:46 PM To: AfriNIC Resource Policy Discussion List Subject: Re: [AfriNIC-rpd] Anycast prefix's Hi Andrew, Andrew Alston wrote: > > Very basic concept: > > > > Because you cannot announce anything smaller than a /24 into the global > tables, in order to provide for anycast you need to announce an entire > /24, and in an anycast situation this is announced from multiple asn's. > AfriNIC's current v4 policy states that the minimum allocation size on > initial is a /22. To use a /22 for anycast when you potentially are > only using 3 addresses in the block is a huge waste. > If the minimal allocation is a /22 there is no written rule that one should aggregate and announce the entire /22 it can be de-aggregated to announce a /24 from the allocation for anycast purposes. > > > To my knowledge RIPE also has a policy that allows for this kind of > allocation. > Question would be "who runs anycast and why?.If they form part of the critical infrastructure there is room for them to apply additional /24 for anycasting as per the current AfriNIC IPv4 policy for critical infrastructure. -- Michuki Mwangi KENIC _______________________________________________ rpd mailing list [email protected] https://lists.afrinic.net/mailman/listinfo.cgi/rpd _______________________________________________ rpd mailing list [email protected] https://lists.afrinic.net/mailman/listinfo.cgi/rpd
