Application-Layer Protocol Negotiation (ALPN) is a TLS extension (defined in RFC 7301) for identifying what application-layer protocol is negotiating the encrypted connection, without requiring additional round trips to do so....Protocol Identification sequence HTTP/1.1 0x68 0x74 0x74 0x70 0x2F...0x2E 0x31 ("http/1.1") HTTP/2 0x68 0x32 ("h2") HTTP/2 over cleartext...