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HTTP/2 with curl
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================
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[HTTP/2 Spec](https://www.rfc-editor.org/rfc/rfc7540.txt)
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[http2 explained](https://daniel.haxx.se/http2/)
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Build prerequisites
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-------------------
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- nghttp2
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- OpenSSL, libressl, BoringSSL, NSS, GnutTLS, mbedTLS, wolfSSL or Schannel
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with a new enough version.
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[nghttp2](https://nghttp2.org/)
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-------------------------------
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libcurl uses this 3rd party library for the low level protocol handling
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parts. The reason for this is that HTTP/2 is much more complex at that layer
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than HTTP/1.1 (which we implement on our own) and that nghttp2 is an already
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existing and well functional library.
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We require at least version 1.0.0.
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Over an http:// URL
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-------------------
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If `CURLOPT_HTTP_VERSION` is set to `CURL_HTTP_VERSION_2_0`, libcurl will
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include an upgrade header in the initial request to the host to allow
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upgrading to HTTP/2.
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Possibly we can later introduce an option that will cause libcurl to fail if
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not possible to upgrade. Possibly we introduce an option that makes libcurl
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use HTTP/2 at once over http://
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Over an https:// URL
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--------------------
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If `CURLOPT_HTTP_VERSION` is set to `CURL_HTTP_VERSION_2_0`, libcurl will use
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ALPN (or NPN) to negotiate which protocol to continue with. Possibly introduce
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an option that will cause libcurl to fail if not possible to use HTTP/2.
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`CURL_HTTP_VERSION_2TLS` was added in 7.47.0 as a way to ask libcurl to prefer
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HTTP/2 for HTTPS but stick to 1.1 by default for plain old HTTP connections.
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ALPN is the TLS extension that HTTP/2 is expected to use. The NPN extension is
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for a similar purpose, was made prior to ALPN and is used for SPDY so early
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HTTP/2 servers are implemented using NPN before ALPN support is widespread.
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`CURLOPT_SSL_ENABLE_ALPN` and `CURLOPT_SSL_ENABLE_NPN` are offered to allow
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applications to explicitly disable ALPN or NPN.
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SSL libs
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--------
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The challenge is the ALPN and NPN support and all our different SSL
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backends. You may need a fairly updated SSL library version for it to provide
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the necessary TLS features. Right now we support:
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- OpenSSL: ALPN and NPN
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- libressl: ALPN and NPN
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- BoringSSL: ALPN and NPN
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- NSS: ALPN and NPN
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- GnuTLS: ALPN
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- mbedTLS: ALPN
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- Schannel: ALPN
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- wolfSSL: ALPN
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- Secure Transport: ALPN
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Multiplexing
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------------
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Starting in 7.43.0, libcurl fully supports HTTP/2 multiplexing, which is the
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term for doing multiple independent transfers over the same physical TCP
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connection.
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To take advantage of multiplexing, you need to use the multi interface and set
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`CURLMOPT_PIPELINING` to `CURLPIPE_MULTIPLEX`. With that bit set, libcurl will
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attempt to re-use existing HTTP/2 connections and just add a new stream over
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that when doing subsequent parallel requests.
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While libcurl sets up a connection to a HTTP server there is a period during
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which it doesn't know if it can pipeline or do multiplexing and if you add new
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transfers in that period, libcurl will default to start new connections for
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those transfers. With the new option `CURLOPT_PIPEWAIT` (added in 7.43.0), you
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can ask that a transfer should rather wait and see in case there's a
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connection for the same host in progress that might end up being possible to
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multiplex on. It favours keeping the number of connections low to the cost of
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slightly longer time to first byte transferred.
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Applications
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------------
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We hide HTTP/2's binary nature and convert received HTTP/2 traffic to headers
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in HTTP 1.1 style. This allows applications to work unmodified.
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curl tool
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---------
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curl offers the `--http2` command line option to enable use of HTTP/2.
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curl offers the `--http2-prior-knowledge` command line option to enable use of
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HTTP/2 without HTTP/1.1 Upgrade.
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Since 7.47.0, the curl tool enables HTTP/2 by default for HTTPS connections.
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curl tool limitations
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---------------------
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The command line tool won't do any HTTP/2 multiplexing even though libcurl
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supports it, simply because the curl tool is not written to take advantage of
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the libcurl API that's necessary for this (the multi interface). We have an
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outstanding TODO item for this and **you** can help us make it happen.
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The command line tool also doesn't support HTTP/2 server push for the same
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reason it doesn't do multiplexing: it needs to use the multi interface for
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that so that multiplexing is supported.
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HTTP Alternative Services
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-------------------------
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Alt-Svc is an extension with a corresponding frame (ALTSVC) in HTTP/2 that
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tells the client about an alternative "route" to the same content for the same
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origin server that you get the response from. A browser or long-living client
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can use that hint to create a new connection asynchronously. For libcurl, we
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may introduce a way to bring such clues to the application and/or let a
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subsequent request use the alternate route automatically.
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[Detailed in RFC 7838](https://tools.ietf.org/html/rfc7838)
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