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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.
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