Application-Layer Protocol Negotiation (ALPN) is a Transport Layer Security (TLS) extension that allows the application layer to negotiate which protocol should be performed over a secure connection in a manner that avoids additional round trips and which is independent of the application-layer protocols. It is needed by secure HTTP/2 connections, which improves the compression of web pages and reduces their latency compared to HTTP/1.x. The ALPN and HTTP/2 standards emerged from development work done by Google on the now withdrawn SPDY protocol.
https://en.wikipedia.org/wiki/Application-Layer_Protocol_Negotiation
1:30 TCP Handshake
1:40 TLS
The Backend Engineering Show with Hussein Nasser