ALPN

Application-Layer Protocol Negotiation (ALPN) is a {{Glossary("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
{{Glossary("HTTP", "HTTP/1.1")}}	`0x68 0x74 0x74 0x70 0x2F 0x31 0x2E 0x31` ("http/1.1")
{{Glossary("HTTP 2", "HTTP/2")}}	`0x68 0x32` ("h2")
HTTP/2 over cleartext {{Glossary("TCP")}}	`0x68 0x32 0x63` ("h2c")
{{Glossary("HTTP 3", "HTTP/3")}}	`0x68 0x33` ("h3")

See also