1 HTTP/2 with curl 2 ================ 3 4 [HTTP/2 Spec](https://www.rfc-editor.org/rfc/rfc7540.txt) 5 [http2 explained](http://daniel.haxx.se/http2/) 6 7 Build prerequisites 8 ------------------- 9 - nghttp2 10 - OpenSSL, NSS, GnutTLS or PolarSSL with a new enough version 11 12 [nghttp2](https://nghttp2.org/) 13 ------------------------------- 14 15 libcurl uses this 3rd party library for the low level protocol handling 16 parts. The reason for this is that HTTP/2 is much more complex at that layer 17 than HTTP/1.1 (which we implement on our own) and that nghttp2 is an already 18 existing and well functional library. 19 20 We require at least version 1.0.0. 21 22 Over an http:// URL 23 ------------------- 24 25 If `CURLOPT_HTTP_VERSION` is set to `CURL_HTTP_VERSION_2_0`, libcurl will 26 include an upgrade header in the initial request to the host to allow 27 upgrading to HTTP/2. 28 29 Possibly we can later introduce an option that will cause libcurl to fail if 30 not possible to upgrade. Possibly we introduce an option that makes libcurl 31 use HTTP/2 at once over http:// 32 33 Over an https:// URL 34 -------------------- 35 36 If `CURLOPT_HTTP_VERSION` is set to `CURL_HTTP_VERSION_2_0`, libcurl will use 37 ALPN (or NPN) to negotiate which protocol to continue with. Possibly introduce 38 an option that will cause libcurl to fail if not possible to use HTTP/2. 39 Consider options to explicitly disable ALPN and/or NPN. 40 41 ALPN is the TLS extension that HTTP/2 is expected to use. The NPN extension is 42 for a similar purpose, was made prior to ALPN and is used for SPDY so early 43 HTTP/2 servers are implemented using NPN before ALPN support is widespread. 44 45 SSL libs 46 -------- 47 48 The challenge is the ALPN and NPN support and all our different SSL 49 backends. You may need a fairly updated SSL library version for it to 50 provide the necessary TLS features. Right now we support: 51 52 - OpenSSL: ALPN and NPN 53 - NSS: ALPN and NPN 54 - GnuTLS: ALPN 55 - PolarSSL: ALPN 56 57 Multiplexing 58 ------------ 59 60 Starting in 7.43.0, libcurl fully supports HTTP/2 multiplexing, which is the 61 term for doing multiple independent transfers over the same physical TCP 62 connection. 63 64 To take advantage of multiplexing, you need to use the multi interface and set 65 `CURLMOPT_PIPELINING` to `CURLPIPE_MULTIPLEX`. With that bit set, libcurl will 66 attempt to re-use existing HTTP/2 connections and just add a new stream over 67 that when doing subsequent parallel requests. 68 69 While libcurl sets up a connection to a HTTP server there is a period during 70 which it doesn't know if it can pipeline or do multiplexing and if you add new 71 transfers in that period, libcurl will default to start new connections for 72 those transfers. With the new option `CURLOPT_PIPEWAIT` (added in 7.43.0), you 73 can ask that a transfer should rather wait and see in case there's a 74 connection for the same host in progress that might end up being possible to 75 multiplex on. It favours keeping the number of connections low to the cost of 76 slightly longer time to first byte transferred. 77 78 Applications 79 ------------ 80 81 We hide HTTP/2's binary nature and convert received HTTP/2 traffic to headers 82 in HTTP 1.1 style. This allows applications to work unmodified. 83 84 curl tool 85 --------- 86 87 curl offers the `--http2` command line option to enable use of HTTP/2 88 89 HTTP Alternative Services 90 ------------------------- 91 92 Alt-Svc is a suggested extension with a corresponding frame (ALTSVC) in HTTP/2 93 that tells the client about an alternative "route" to the same content for the 94 same origin server that you get the response from. A browser or long-living 95 client can use that hint to create a new connection asynchronously. For 96 libcurl, we may introduce a way to bring such clues to the applicaton and/or 97 let a subsequent request use the alternate route 98 automatically. [Spec](https://tools.ietf.org/html/draft-ietf-httpbis-alt-svc-05) 99 100 TODO 101 ---- 102 103 - Provide API to set priorities / dependencies of individual streams 104 105 - Implement "prior-knowledge" HTTP/2 connecitons over clear text so that 106 curl can connect with HTTP/2 at once without 1.1+Upgrade. 107 108
