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This feature is marked experimental as of this time and needs to be explicitly enabled in the build to function.
HTTP/3 is the HTTP version that is designed to communicate over QUIC. QUIC can for most particular purposes be considered a TCP+TLS replacement.
All transfers that use HTTP/3 will therefore not use TCP. They will use QUIC. QUIC is a reliable transport protocol built over UDP. HTTP/3 implies use of QUIC.
HTTP/3 is performed over QUIC which is always using TLS, so HTTP/3 is by definition always encrypted and secure. Therefore, curl only uses HTTP/3 for
As a shortcut straight to HTTP/3, to make curl attempt a QUIC connect directly to the given host name and port number, use
--http3. Like this:
curl --http3 https://example.com/
Normally, without the
HTTPS://URL implies that a client needs to connect to it using TCP (and TLS).
Note that you need that feature built-in and that it does not switch to HTTP/3 for the current request unless the alt-svc cache is already populated, but it will rather store the info for use in the next request to the host.
A certain amount of QUIC connection attempts will fail, partly because many networks and hosts block or throttle the traffic.
--http3is used, curl will start a second transfer attempt a few hundred milliseconds after the QUIC connection is initiated which is using HTTP/2 or HTTP/1, so that if the connection attempt over QUIC fails or turns out to be unbearably slow, the connection using an older HTTP version can still succeed and perform the transfer. This allows users to use
--http3with some amount of confidence that the operation will work.
--http3-onlyis provided to explicitly not try any older version in parallel, but will thus make the transfer fail immediately if no QUIC connection can be established.