ALPN is the in-band signal during the TLS handshake that says “I support h2 and http/1.1”; the server picks one. No second round-trip needed.
ALPN
Application-Layer Protocol Negotiation — TLS extension that picks HTTP/1 vs 2 vs 3.
Application-Layer Protocol Negotiation — TLS extension that picks HTTP/1 vs 2 vs 3.
ALPN is the in-band signal during the TLS handshake that says “I support h2 and http/1.1”; the server picks one. No second round-trip needed.