ALPN (Application-Layer Protocol Negotiation) is a TLS extension defined in RFC 7301, which allows a client and a server to agree within the TLS handshake on which application-layer protocol (e.g. HTTP/1.1, HTTP/2, HTTP/3) they will use over the encrypted connection, without additional communication rounds.

In effect, ALPN streamlines setup: the client sends a list of supported protocols in its ClientHello message, and the server picks one and responds in ServerHello, saving extra negotiation steps.

How it Works in Practice

• During the TLS handshake, the ClientHello includes an ALPN extension listing the application protocols the client supports (for example: “h2” for HTTP/2, “http/1.1”, maybe “h3” for HTTP/3) in order of preference.
• The server checks which protocols it also supports, picks the most suitable one, and sends that choice in the ServerHello (via its own ALPN extension).
• After that point, both sides know exactly which application protocol to use (HTTP/2, HTTP/1.1, etc.), and proceed accordingly.

Because this negotiation happens within the TLS handshake, ALPN avoids extra round trips or separate upgrades – making the setup faster and more efficient.

Why ALPN Matters

• Performance & latency: Because the choice of HTTP/2 or other protocols is made during the handshake, there's no extra waiting or protocol upgrade. This helps pages load faster and reduces overhead.
• Protocol evolution support: ALPN enables modern protocols (HTTP/2, HTTP/3) to be negotiated over the same TLS connections without needing separate ports or extra negotiation logic.
• Security / consistency: The server can even select different certificates or settings depending on which protocol was negotiated.