HTTP2 with libcurl

 Spec: http://www.rfc-editor.org/rfc/rfc7540.txt

 Document explaining it: http://daniel.haxx.se/http2/

 Build prerequisites
  - nghttp2
  - OpenSSL, NSS, GnutTLS or PolarSSL 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 HTTP/2 is much more complex at that layer
  than HTTP/1.1 (which we implement on our own) and that nghttp2 is an already
  existing and well functional library.

  We require at least version 1.0.0.

 Over an http:// URL

  If CURLOPT_HTTP_VERSION is set to CURL_HTTP_VERSION_2_0, 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_0, 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
    PolarSSL: ALPN

Multiplexing

  Starting in 7.43.0, libcurl fully supports HTTP/2 multiplexing, which is the
  term for doing multiple independent transfers over the same physical TCP
  connection.

  To take advantage of multiplexing, you need to use the multi interface and
  set CURLMOPT_PIPELINING to CURLPIPE_MULTIPLEX. With that bit set, libcurl
  will attempt to re-use existing HTTP/2 connections and just add a new stream
  over that when doing subsequent parallel requests.

  While libcurl sets up a connection to a HTTP server there is a period during
  which it doesn't know if it can pipeline or do multiplexing and if you add
  new transfers in that period, libcurl will default to start new connections
  for those transfers. With the new option CURLOPT_PIPEWAIT (added in 7.43.0),
  you can ask that a transfer should rather wait and see in case there's a
  connection for the same host in progress that might end up being possible to
  multiplex on. It favours keeping the number of connections low to the cost
  of slightly longer time to first byte transfered.

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

HTTP Alternative Services

  Alt-Svc is a suggested extension 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. Spec:
  http://tools.ietf.org/html/draft-ietf-httpbis-alt-svc-05

TODO:

  - Provide API to set priorities / dependencies of individual streams

  - Implement "prior-knowledge" HTTP/2 connecitons over clear text so that
    curl can connect with HTTP/2 at once without 1.1+Upgrade.

