3.6.8 Application Layer Protocol Negotiation (ALPN)

The TLS protocol was extended in draft-ietf-tls-applayerprotoneg-00 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 you can use these functions with protocols that are registered in the Application Layer Protocol Negotiation IANA registry. If you wish to use them for other custom protocols then prefix them with the 3 characters ‘exp’ (lower-case).

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