http3

 http3  is the third version of HTTP. Unlike previous versions, it uses QUIC to multiplex connections over a UDP connection with the goal of reducing latency and avoiding congestion.

More information is available at QUICWG and the following standards:


 * RFC 8999 - Version-Independent Properties of QUIC
 * RFC 9000 - QUIC: A UDP-Based Multiplexed and Secure Transport
 * RFC 9001 - Using TLS to Secure QUIC
 * RFC 9002 - QUIC Loss Detection and Congestion Control

http3 can be used (but is not necessary enabled by default) by most modern browsers and some command line tools (e.g. CURL)

http3 is used within and supported by networking infrastructure including iCloud Private Relay, Cloudflare, and Google.

Some notes:


 * Libraries are available for both client and server support.


 * On the client side, when a connection is established over https an `alt-svc` header (per 7838 RFC 7838; see also mdn web docs) can signal support for http3 (protocol "h3"), and the next connection can attempt to use http3.


 * There are extensions which provide browsers with an indicator of the http transport in use:


 * for Firefox: "HTTP Version Indicator"


 * for Chrome: "HTTP Indicator"