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.ProtocolIdentification sequenceHTTP/1.10x68 0x74 0x74 0x70 0x2F 0x31 0x2E 0x31 ("http/1.1")HTTP/20x68 0x32 ("h2")HTTP/2 over cleartext TCP0x68 0x32 0x63 ("h2c")HTTP/30x68 0x33 ("h3")
0x68 0x74 0x74 0x70 0x2F 0x31 0x2E 0x31
0x68 0x32
0x68 0x32 0x63
0x68 0x33
nextHopProtocol