HTTP2 with libcurl

 Spec: http://tools.ietf.org/html/draft-ietf-httpbis-http2

 nghttp2 (https://github.com/tatsuhiro-t/nghttp2)

  libcurl uses this 3rd party library for the low level protocol handling
  parts. The reason for this is that HTTP2 is much more complex at that layer
  than HTTP1.1 (which we implement on our own) and that nghttp2 is an already
  existing and well functional library.

  Right now, nghttp2 implements http2 draft-09

 Over an http:// URL

  If CURLOPT_HTTP_VERSION is set to CURL_HTTP_VERSION_2, libcurl will include
  an upgrade header in the initial request to the host to allow upgrading to
  http2.

  Possibly we can later introduce an option that will cause libcurl to fail if
  not possible to upgrade. Possibly we introduce an option that makes libcurl
  use http2 at once over http://

 Over an https:// URL

  If CURLOPT_HTTP_VERSION is set to CURL_HTTP_VERSION_2, libcurl will use ALPN
  (or NPN) to negotiate which protocol to continue with. Possibly introduce an
  option that will cause libcurl to fail if not possible to use http2.
  Consider options to explicitly disable ALPN and/or NPN.

SSL libs

  The challange is the ALPN and NPN support and all our different SSL
  backends.  Right now we support ALPN and VPN with OpenSSL and ALPN with
  GnuTLS.

Applications

  We hide http2's binary nature and convert received http2 traffic to headers
  in HTTP 1.1 style. This allows applications to work unmodified.

curl tool

  curl offers the --http2 command line option to enable use of http2

To consider:

  - How to tell libcurl when using the multi interface that all or some of the
    handles are allowed to re-use the same physical connection. Can we just
    re-use existing pipelining logic?