Alternative Selection Protocols: Anycast & HTTP
đź’ˇ The Core Idea
Aside from DNS, CDNs can use network-layer or application-layer protocols to route users to the correct server.
đź§ Comparison
IP Anycast (Network Layer)
- Mechanism: Assign the same IP address to multiple servers in different locations. BGP routes the user to the “topologically closest” server.
- Use Case: DNS Root Servers, Google Public DNS (8.8.8.8).
- Cons: BGP doesn’t know about server load or link congestion, only hop count.
HTTP Redirection (Application Layer)
- Mechanism: The server receives a GET request and replies with
3xx Redirectpointing to a new server. - Use Case: Load balancing (e.g., YouTube). If a cluster is overloaded, redirect the user to a neighbor.
- Cons: Slow. Requires an initial TCP connection and HTTP exchange just to be told to go elsewhere (extra RTT).