Anycast networks are a pretty interesting way to fix quite a few issues with networked services that involve needing global spread. One of the interesting things is that a computer cannot really tell (unless it has a full routing table of many providers in more than one geographic location maybe) that a IP is using anycast.
Before I start this, I just want to go though what a anycast actually is.
Most programmers who have done networking have heard the comparison between a IP address and a telephone number, if you want to explain the idea of a anycast IP address to someone, I believe that the most simple way is to explain that if your computer is
126.96.36.199 (not my IP, not even routable) and lets say the direct translation to a phone number was
0121 441 526 and a anycast IP was
188.8.131.52. Then its phone number would be like that of
999 (For Americans
911, Other EU places
This kind of number is the “anycast” of telephone systems, because that phone number will be directed to the most preferable (hopefully the closest) call center in your region, this is why if I was to dial
999 in London, I would most likely be sent to a call center in London to take my emergency call, rather to one in Scotland.
Networks do the same thing basically.
Here is a small drawing of a network setup:
Here you see a group of routers, all have the same IP address, Lets pretend that my router does know about all 3 of them, Because how most routing tables are setup, the packets will be sent to the fastest route possible (though, that does mean the fastest route it has, that may not in all cases mean the closest, “fastest” can in some cases also mean “cheapest”) this is achieved by the “metric” on the route. Assuming the route metrics have been setup correctly. Packets will flow to the right place.
Since there is nothing special about anycast addresses, other than that they are addresses that are “advertised” in the internet routing table in more than one place. There is no way to look at an address (other than what I mentioned above about seeing the global routing in many places) and know its anycasted. One way for a human to check that an address is anycasted it to just trace route it from more than one location, Here is an example with 184.108.40.206 (an anycasted DNS server) Server on West Coast USA:
[email protected]:~$ mtr -rwc 15 220.127.116.11 HOST: storm Loss% Snt Last Avg Best Wrst StDev 1. 18.104.22.168 0.0% 15 0.6 6.3 0.6 59.1 15.0 2. 10.1.1.5 0.0% 15 0.6 12.3 0.5 90.0 23.7 3. any2ix.coresite.com 0.0% 15 8.0 8.4 7.9 12.3 1.1 4. 22.214.171.124 0.0% 15 8.2 11.5 8.1 42.3 9.1 5. google-public-dns-a.google.com 0.0% 15 8.3 9.7 7.9 18.7 3.5
Server in Amsterdam:
[email protected]:~$ mtr -rwc 15 126.96.36.199 HOST: Spitfire Loss% Snt Last Avg Best Wrst StDev 1. 188.8.131.52 0.0% 15 1.6 8.3 0.5 68.9 17.2 2. 80ge.cr0-br2-br3.smartdc.rtd.i3d.net 0.0% 15 1.3 4.4 0.5 13.8 4.5 3. 30ge.ar0-cr0.nikhef.ams.i3d.net 0.0% 15 1.5 17.9 1.5 56.5 17.8 4. core1.ams.net.google.com 0.0% 15 2.6 2.9 2.0 4.5 0.8 5. 184.108.40.206 0.0% 15 3.8 4.1 2.2 14.2 3.1 6. google-public-dns-a.google.com 0.0% 15 2.4 3.0 2.0 5.7 0.9
Notice on both of those servers, the round trip time is less than 10ms on each? Now unless routers have invented electron teleportation (the bare minimum time in a single direction to get from those two servers is 42ms) this is an anycast address
and here is it testing against a set of unicast destinations: University of Sydney:
StackExchange (They are based in NY)
and finally the BBC:
Want to try the tool out for yourself? You can find mine here: https://anycatch.benjojo.co.uk Or if you want to build your own in the case that you have a anycast network infra of your own, You can find the code here: https://github.com/benjojo/AnyCatch If you are using it, or find anything interesting with my own tool, Do let me know!