ALPN

Application-Layer Protocol Negotiation (ALPN) is a TLS extension 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 0x31 0x2E 0x31 (“http/1.1”)
HTTP/2	0x68 0x32 (“h2”)
HTTP/2 over cleartext TCP	0x68 0x32 0x63 (“h2c”)
HTTP/3	0x68 0x33 (“h3”)

• https://developer.mozilla.org/en-US/docs/Glossary/ALPN
• https://www.iana.org/assignments/tls-extensiontype-values/tls-extensiontype-values.xhtml