Keep-Alive
A mechanism that lets HTTP or TCP connections stay open across multiple requests instead of opening a new connection each time.
Keep-alive (persistent connection) lets a single TCP or HTTP connection serve multiple requests, avoiding the latency cost of repeated three-way handshakes and TLS negotiations. HTTP/1.1 made keep-alive the default, controlled by the `Connection` header. TCP itself has a separate `SO_KEEPALIVE` option that sends periodic probes to detect dead peers on idle connections. HTTP/2 and HTTP/3 multiplex many streams over one long-lived connection, making keep-alive even more important. Reverse proxies and load balancers expose their own keep-alive timeouts that must be tuned together with upstream timeouts to avoid mid-request connection resets.