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+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.
+