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Next Protocol Negotiation is a TLS extension that was created and used for agreeing to use the SPDY protocol (the precursor to HTTP/2) for HTTPS. In the early days of HTTP/2, before the spec was finalized and shipped, the protocol could be enabled using this extension with some servers. curl supports the NPN extension with some TLS backends since then, with a command line option `--npn` and in libcurl with `CURLOPT_SSL_ENABLE_NPN`. HTTP/2 proper is made to use the ALPN (Application-Layer Protocol Negotiation) extension and the NPN extension has no purposes anymore. The HTTP/2 spec was published in May 2015. Today, use of NPN in the wild should be extremely rare and most likely totally extinct. Chrome removed NPN support in Chrome 51, shipped in June 2016. Removed in Firefox 53, April 2017. Closes #9307
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HTTP/2 with curl
Build prerequisites
- nghttp2
- OpenSSL, libressl, BoringSSL, NSS, GnuTLS, mbedTLS, wolfSSL or Schannel with a new enough version.
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 1.12.0.
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 HTTP/2.
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 HTTP/2 at once over http://
Over an https:// URL
If CURLOPT_HTTP_VERSION is set to CURL_HTTP_VERSION_2_0, libcurl will use
ALPN to negotiate which protocol to continue with. Possibly introduce an
option that will cause libcurl to fail if not possible to use HTTP/2.
CURL_HTTP_VERSION_2TLS was added in 7.47.0 as a way to ask libcurl to prefer
HTTP/2 for HTTPS but stick to 1.1 by default for plain old HTTP connections.
ALPN is the TLS extension that HTTP/2 is expected to use.
CURLOPT_SSL_ENABLE_ALPN is offered to allow applications to explicitly
disable 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 an HTTP server there is a period during
which it does not 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 transferred.
Applications
We hide HTTP/2's binary nature and convert received HTTP/2 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 HTTP/2.
curl offers the --http2-prior-knowledge command line option to enable use of
HTTP/2 without HTTP/1.1 Upgrade.
Since 7.47.0, the curl tool enables HTTP/2 by default for HTTPS connections.
curl tool limitations
The command line tool does not support HTTP/2 server push. It supports multiplexing when the parallel transfer option is used.
HTTP Alternative Services
Alt-Svc is an extension with a corresponding frame (ALTSVC) in HTTP/2 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 application and/or let a subsequent request use the alternate route automatically.