HTTP2 with libcurl

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

 Build prerequisites
  - nghttp2
  - OpenSSL or NSS or GnutTLS with a new enough version

 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

  We require at least version 0.3.0

 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.

  ALPN is the TLS extension that http2 is expected to use. The NPN extension
  is for a similar purpose, was made prior to ALPN and is used for SPDY so
  early http2 servers are implemented using NPN before ALPN support is
  widespread.

SSL libs

  The challenge is the ALPN and NPN support and all our different SSL
  backends. You may need a fairly updated SSL library version for it to
  provide the necessary TLS features. Right now we support:

    OpenSSL: ALPN and NPN
    NSS:     ALPN and NPN
    GnuTLS:  ALPN

Alt-Svc

  Alt-Svc is a suggested new header with a corresponding frame (ALTSVC) in
  http2 that tells the client about an alternative "route" to the same content
  for the same origin server that you get the response from. A browser or
  long-living client can use that hint to create a new connection
  asynchronously.  For libcurl, we may introduce a way to bring such clues to
  the applicaton and/or let a subsequent request use the alternate route
  automatically.

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?