Load Balancing Protocols
A load balancer is a middlebox with a backend list. At layer 4, it rewrites destination addresses in NAT and forwards connections; at layer 7, it terminates the connection, reads the request, and opens a new one to a chosen backend. We trace both paths and the health checks that keep backends in or out of the pool.
Topics covered:
- L4 NAT: rewriting destination IPs and ports, connection tracking
- L7 proxying: termination, request inspection, and re-connection
- Health checks: TCP probes, HTTP status probes, passive failure detection
- Algorithms: round robin, least connections, and consistent hashing
- How retries and timeouts are the balancer's real failure handling
- Why L7 costs more: full termination means double the connections
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