ALPN

Application-Layer Protocol Negotiation (ALPN) is a TLS extension which indicates what application layer protocol is negotiating the encrypted connection without requiring additional round trips.

Important protocol identifiers:
Protocol Identification sequence
HTTP/1.1 0x68 0x74 0x74 0x70 0x2F 0x31 0x2E 0x31 ("http/1.1")
HTTP/2 0x68 0x32 ("h2")
HTTP/2 over cleartext TCP 0x68 0x32 0x63 ("h2c")

Specifications

Specification Status Notes
RFC 7301 IETF RFC Initial definition.

See also