ALPN
Application-Layer Protocol Negotiation: a TLS extension that lets client and server agree on a protocol (like HTTP/2 or HTTP/3) during the handshake.
ALPN (Application-Layer Protocol Negotiation) is a TLS extension that allows a client and server to negotiate which application protocol they will use as part of the TLS handshake, without requiring an extra round trip. The client sends a list of supported protocols (e.g., h2, http/1.1) inside the TLS ClientHello, and the server picks one. ALPN is how browsers and servers agree to speak HTTP/2 or HTTP/3 instead of falling back to HTTP/1.1. Without ALPN, modern web performance optimisations like multiplexed connections would not be practical over TLS.