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diff --git a/docs/HTTP2.md b/docs/HTTP2.md new file mode 100644 index 00000000..b4e2983b --- /dev/null +++ b/docs/HTTP2.md @@ -0,0 +1,107 @@ +HTTP/2 with curl +================ + +[HTTP/2 Spec](https://www.rfc-editor.org/rfc/rfc7540.txt) +[http2 explained](http://daniel.haxx.se/http2/) + +Build prerequisites +------------------- + - nghttp2 + - OpenSSL, NSS, GnutTLS or PolarSSL 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 1.0.0. + +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 (or NPN) to negotiate which protocol to continue with. Possibly introduce +an option that will cause libcurl to fail if not possible to use HTTP/2. +Consider options to explicitly disable ALPN and/or NPN. + +ALPN is the TLS extension that HTTP/2 is expected to use. The NPN extension is +for a similar purpose, was made prior to ALPN and is used for SPDY so early +HTTP/2 servers are implemented using NPN before ALPN support is widespread. + +SSL libs +-------- + +The challenge is the ALPN and NPN support and all our different SSL +backends. You may need a fairly updated SSL library version for it to +provide the necessary TLS features. Right now we support: + + - OpenSSL: ALPN and NPN + - NSS: ALPN and NPN + - GnuTLS: ALPN + - PolarSSL: 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 a HTTP server there is a period during +which it doesn't 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 + +HTTP Alternative Services +------------------------- + +Alt-Svc is a suggested 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 applicaton and/or +let a subsequent request use the alternate route +automatically. [Spec](https://tools.ietf.org/html/draft-ietf-httpbis-alt-svc-05) + +TODO +---- + + - Provide API to set priorities / dependencies of individual streams + + - Implement "prior-knowledge" HTTP/2 connecitons over clear text so that + curl can connect with HTTP/2 at once without 1.1+Upgrade. + |