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HTTP/2 with curl

HTTP/2 Spec http2 explained

Build prerequisites


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 1.12.0.

Over an http:// URL

If CURLOPT_HTTP_VERSION is set to CURL_HTTP_VERSION_2_0, libcurl includes an upgrade header in the initial request to the host to allow upgrading to HTTP/2.

Possibly we can later introduce an option that causes libcurl to fail if it is 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 uses ALPN to negotiate which protocol to continue with. Possibly introduce an option that causes libcurl to fail if not possible to use HTTP/2.

CURL_HTTP_VERSION_2TLS was added in 7.47.0 as a way to ask libcurl to prefer HTTP/2 for HTTPS but stick to 1.1 by default for plain old HTTP connections.

ALPN is the TLS extension that HTTP/2 is expected to use.

CURLOPT_SSL_ENABLE_ALPN is offered to allow applications to explicitly disable ALPN.


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 attempts to reuse existing HTTP/2 connections and just add a new stream over that when doing subsequent parallel requests.

While libcurl sets up a connection to an HTTP server there is a period during which it does not know if it can pipeline or do multiplexing and if you add new transfers in that period, libcurl defaults to starting 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 is a connection for the same host in progress that might end up being possible to multiplex on. It favors keeping the number of connections low to the cost of slightly longer time to first byte transferred.


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.

curl offers the --http2-prior-knowledge command line option to enable use of HTTP/2 without HTTP/1.1 Upgrade.

Since 7.47.0, the curl tool enables HTTP/2 by default for HTTPS connections.

curl tool limitations

The command line tool does not support HTTP/2 server push. It supports multiplexing when the parallel transfer option is used.

HTTP Alternative Services

Alt-Svc is an 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 application and/or let a subsequent request use the alternate route automatically.

Detailed in RFC 7838