HTTP/2 with curl
================

[HTTP/2 Spec](https://www.rfc-editor.org/rfc/rfc7540.txt)
[http2 explained](https://daniel.haxx.se/http2/)

Build prerequisites
-------------------
  - nghttp2
  - OpenSSL, libressl, BoringSSL, GnuTLS, mbedTLS, wolfSSL or Schannel
    with a new enough version.

[nghttp2](https://nghttp2.org/)
-------------------------------

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 includes
an upgrade header in the initial request to the host to allow upgrading to
HTTP/2.

Possibly we can later introduce an option that causes libcurl to fail if it is
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 uses ALPN
to negotiate which protocol to continue with. Possibly introduce an option
that causes 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
attempts to reuse 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 defaults to starting 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 is a
connection for the same host in progress that might end up being possible to
multiplex on. It favors 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.

[Detailed in RFC 7838](https://datatracker.ietf.org/doc/html/rfc7838)