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