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I think it should be clarified that "destination" refers to an IP address, not an individual host. My understanding is that anycasting means a single address c
by chousuke 9y ago
I think it should be clarified that "destination" refers to an IP address, not an individual host. My understanding is that anycasting means a single address corresponds to multiple hosts achieved by simply advertising it from several sources with BGP, and you will often still have multiple redundant routes to any of the individual hosts behind the anycast IP because most locations will have redundant internet links.
Depending on how the routing is set up, it doesn't matter if the route changes so long as you end up on the same host consistently (or one that can at least pretend it's the same host if you do some kind of fancy session mirroring, perhaps)
- tpetry 9y agoGoogle Cloud Global Loadbalancer seems to do this fancy session mirroring because you only have one IP for the http load balancing. I am very often impressed by the GCP products.
- chousuke 9y agoThere are other techniques that Google's routers will most likely use to load-balance traffic transparently to multiple hosts. A relatively simple way is to hash the (source, destination) address pair of the IP packet to determine which host to forward the packet to, so it doesn't necessarily require mirroring or any state. Only seamless failover when the host fails requires the fancy tricks.