ALPN

RU: ALPN

A TLS extension (RFC 7301) where the client lists its supported application protocols in the ClientHello and the server selects one and echoes it in the ServerHello — all within the existing TLS handshake, adding no extra round trips. Most commonly used to negotiate HTTP/2 ("h2"): the protocol is agreed during the TLS handshake with no separate upgrade step. HTTP/3 uses a different path — its "h3" identifier is negotiated inside QUIC's own embedded TLS 1.3 handshake (not TLS-over-TCP), and clients typically discover HTTP/3 support via the Alt-Svc response header first.