Anycast
Anycast is a network routing method in which many servers in different locations share the same IP address, and each user is automatically routed to the nearest or best-performing one. CDNs and DNS providers use anycast to send traffic to a nearby data center. If one location fails, traffic flows to the next closest one.
Why it matters
Anycast routes viewers to a nearby server without extra lookups, which speeds up connections and makes failover automatic. For streaming services, it helps keep playback fast across regions and limits the damage when one data center has problems. It also helps absorb attacks by spreading traffic across many locations.
How it works
- Many servers in different cities announce the same IP address to the internet.
- Internet routers send each user's traffic along the shortest network path to one of them.
- If a site goes offline, it stops announcing the address.
- Routers then send traffic to the next nearest site, with no change needed on the user's side.
Anycast vs DNS-based routing: what is the difference?
Anycast routes users at the network level by sharing one IP address across many sites, while DNS-based routing gives different users different IP addresses. Anycast reacts to failures as soon as network routes change. DNS routing allows more detailed control, such as by cost or performance, but depends on DNS caches that may hold old answers.
Is anycast good for video streaming?
Anycast is good for the connection setup and DNS parts of video streaming, and many CDNs use it widely. Long video sessions can occasionally be affected if network routes shift mid-stream and traffic lands on a different site. Streaming over HTTP handles this well, because each segment is a separate short request.
Where it's used
Live sports streamers choose CDNs built on anycast so viewers in every country connect to a nearby server automatically, and traffic shifts on its own if one site fails.
By the numbers
- Anycast was first formally documented by the IETF in RFC 1546, Host Anycasting Service, in November 1993. (Anycast (Wikipedia))
- RFC 4786, Operation of Anycast Services, was published in December 2006 as BCP 126. (Anycast (Wikipedia))
