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3.6.9 Application Layer Protocol Negotiation (ALPN)

The TLS protocol was extended in RFC7301 to provide the application layer a method of negotiating the application protocol version. This allows for negotiation of the application protocol during the TLS handshake, thus reducing round-trips. The application protocol is described by an opaque string. To enable, use the following functions.

int gnutls_alpn_set_protocols (gnutls_session_t session, const gnutls_datum_t * protocols, unsigned protocols_size, unsigned int flags)
int gnutls_alpn_get_selected_protocol (gnutls_session_t session, gnutls_datum_t * protocol)

Note that these functions are intended to be used with protocols that are registered in the Application Layer Protocol Negotiation IANA registry. While you can use them for other protocols (at the risk of collisions), it is preferable to register them.