gnutls.info: Application Layer Protocol Negotiation (ALPN)

TLS Extensions

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3.6.9 Application Layer Protocol Negotiation (ALPN)
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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 Note: gnutls_alpn_set_protocols (gnutls_session_t SESSION, const gnutls_datum_t * PROTOCOLS, unsigned PROTOCOLS_SIZE, unsigned int FLAGS)'
'INT Note: 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.