1HTTP/2 with curl
2================
3
4[HTTP/2 Spec](https://www.rfc-editor.org/rfc/rfc7540.txt)
5[http2 explained](https://daniel.haxx.se/http2/)
6
7Build prerequisites
8-------------------
9  - nghttp2
10  - OpenSSL, libressl, BoringSSL, NSS, GnutTLS, mbedTLS, wolfSSL or Schannel
11    with a new enough version.
12
13[nghttp2](https://nghttp2.org/)
14-------------------------------
15
16libcurl uses this 3rd party library for the low level protocol handling
17parts. The reason for this is that HTTP/2 is much more complex at that layer
18than HTTP/1.1 (which we implement on our own) and that nghttp2 is an already
19existing and well functional library.
20
21We require at least version 1.0.0.
22
23Over an http:// URL
24-------------------
25
26If `CURLOPT_HTTP_VERSION` is set to `CURL_HTTP_VERSION_2_0`, libcurl will
27include an upgrade header in the initial request to the host to allow
28upgrading to HTTP/2.
29
30Possibly we can later introduce an option that will cause libcurl to fail if
31not possible to upgrade. Possibly we introduce an option that makes libcurl
32use HTTP/2 at once over http://
33
34Over an https:// URL
35--------------------
36
37If `CURLOPT_HTTP_VERSION` is set to `CURL_HTTP_VERSION_2_0`, libcurl will use
38ALPN (or NPN) to negotiate which protocol to continue with. Possibly introduce
39an option that will cause libcurl to fail if not possible to use HTTP/2.
40
41`CURL_HTTP_VERSION_2TLS` was added in 7.47.0 as a way to ask libcurl to prefer
42HTTP/2 for HTTPS but stick to 1.1 by default for plain old HTTP connections.
43
44ALPN is the TLS extension that HTTP/2 is expected to use. The NPN extension is
45for a similar purpose, was made prior to ALPN and is used for SPDY so early
46HTTP/2 servers are implemented using NPN before ALPN support is widespread.
47
48`CURLOPT_SSL_ENABLE_ALPN` and `CURLOPT_SSL_ENABLE_NPN` are offered to allow
49applications to explicitly disable ALPN or NPN.
50
51SSL libs
52--------
53
54The challenge is the ALPN and NPN support and all our different SSL
55backends. You may need a fairly updated SSL library version for it to provide
56the necessary TLS features. Right now we support:
57
58  - OpenSSL:          ALPN and NPN
59  - libressl:         ALPN and NPN
60  - BoringSSL:        ALPN and NPN
61  - NSS:              ALPN and NPN
62  - GnuTLS:           ALPN
63  - mbedTLS:          ALPN
64  - Schannel:         ALPN
65  - wolfSSL:          ALPN
66  - Secure Transport: ALPN
67
68Multiplexing
69------------
70
71Starting in 7.43.0, libcurl fully supports HTTP/2 multiplexing, which is the
72term for doing multiple independent transfers over the same physical TCP
73connection.
74
75To take advantage of multiplexing, you need to use the multi interface and set
76`CURLMOPT_PIPELINING` to `CURLPIPE_MULTIPLEX`. With that bit set, libcurl will
77attempt to re-use existing HTTP/2 connections and just add a new stream over
78that when doing subsequent parallel requests.
79
80While libcurl sets up a connection to a HTTP server there is a period during
81which it doesn't know if it can pipeline or do multiplexing and if you add new
82transfers in that period, libcurl will default to start new connections for
83those transfers. With the new option `CURLOPT_PIPEWAIT` (added in 7.43.0), you
84can ask that a transfer should rather wait and see in case there's a
85connection for the same host in progress that might end up being possible to
86multiplex on. It favours keeping the number of connections low to the cost of
87slightly longer time to first byte transferred.
88
89Applications
90------------
91
92We hide HTTP/2's binary nature and convert received HTTP/2 traffic to headers
93in HTTP 1.1 style. This allows applications to work unmodified.
94
95curl tool
96---------
97
98curl offers the `--http2` command line option to enable use of HTTP/2.
99
100curl offers the `--http2-prior-knowledge` command line option to enable use of
101HTTP/2 without HTTP/1.1 Upgrade.
102
103Since 7.47.0, the curl tool enables HTTP/2 by default for HTTPS connections.
104
105curl tool limitations
106---------------------
107
108The command line tool won't do any HTTP/2 multiplexing even though libcurl
109supports it, simply because the curl tool is not written to take advantage of
110the libcurl API that's necessary for this (the multi interface). We have an
111outstanding TODO item for this and **you** can help us make it happen.
112
113The command line tool also doesn't support HTTP/2 server push for the same
114reason it doesn't do multiplexing: it needs to use the multi interface for
115that so that multiplexing is supported.
116
117HTTP Alternative Services
118-------------------------
119
120Alt-Svc is an extension with a corresponding frame (ALTSVC) in HTTP/2 that
121tells the client about an alternative "route" to the same content for the same
122origin server that you get the response from. A browser or long-living client
123can use that hint to create a new connection asynchronously.  For libcurl, we
124may introduce a way to bring such clues to the application and/or let a
125subsequent request use the alternate route automatically.
126
127[Detailed in RFC 7838](https://tools.ietf.org/html/rfc7838)
128