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This commit adds support for RFC7301, application-layer protocol
negotiation. ALPN is the standard based approach to the NPN
extension, and is required for HTTP/2.
ALPN lives side by side with NPN and provides an equivalent
feature but in this case it is the server that decides what
protocol to use, not the client.
When both ALPN and NPN are sent by a client, and the server is
configured with both ALPN and NPN options, ALPN will always
take precedence. This behavior can also be found in the OpenSSL
implementation of ALPN.
ALPN and NPN share the ssl:negotiated_protocol/1 function for
retrieving the negotiated protocol. The previously existing
function ssl:negotiated_next_protocol/1 still exists, but has
been deprecated and removed from the documentation.
The tests against OpenSSL require OpenSSL version 1.0.2+.
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