Alternative Selection Protocols: Anycast & HTTP

💡 The Core Idea

Aside from DNS, CDNs can use network-layer or application-layer protocols to route users to the correct server.

🧠 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 Redirect pointing 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).

🔗 Connections

• Source: Source - CDNs and Overlay Networks