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Anycast means there are multiple routes going to the same destination. You get the route that is the shortest path via BGP to the anycast IP (least number of BG

Anycast means there are multiple routes going to the same destination. You get the route that is the shortest path via BGP to the anycast IP (least number of BGP hops). Once you have an established TCP session via one route, it will remain established through that route, as long as that route is still the “shortest” between your IP and the anycast IP. The route will not “change” unless cloudflare changes their routing, or you change your location/IP so that a shorter route exists. Once you’ve changed your IP, you’ve already interrupted any TCP sessions anyway. You might find these two blog posts from LinkedIn to be helpful: https://engineering.linkedin.com/network-performance/tcp-over-ip-anycast-pipe-dream-or-reality https://engineering.linkedin.com/network-performance/tcp-ove... https://engineering.linkedin.com/blog/2016/04/the-joy-of-anycast--in-side-the-datacenter https://engineering.linkedin.com/blog/2016/04/the-joy-of-any...

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 ago
  Google 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 ago
    There 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.
dx034 9y ago
> The route will not “change” unless cloudflare changes their routing, or you change your location/IP so that a shorter route exists. Once you’ve changed your IP, you’ve already interrupted any TCP sessions anyway. That's what I thought, too. But the article explicitly states this as a potential issue.

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 ago
Google 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 ago
  There 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.

dx034 9y ago

> The route will not “change” unless cloudflare changes their routing, or you change your location/IP so that a shorter route exists. Once you’ve changed your IP, you’ve already interrupted any TCP sessions anyway. That's what I thought, too. But the article explicitly states this as a potential issue.