ALPN (Application-Layer Protocol Negotiation)
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A TLS extension (RFC 7301) that allows the client and server to negotiate which application protocol they will use during the TLS handshake, before any application data is sent. The client includes a list of supported protocols (e.g., h2, http/1.1, h3) in the ClientHello, and the server responds with the selected protocol in the ServerHello. ALPN eliminates the need for an extra round trip to negotiate the protocol after TLS is established, enabling HTTP/2 and gRPC to upgrade directly without an additional handshake.