HTTP/3 support is still in Alpha, and should be used with caution. Outstanding issues required for HTTP/3 to go GA can be found here For example QUIC does not currently support in-place filter chain updates, so users requiring dynamic config reload for QUIC should wait until #13115 has been addressed.
For general feature requests beyond production readiness, you can track the area-quic tag.
Downstream Envoy HTTP/3 support can be turned up via adding quic_options, ensuring the downstream transport socket is a QuicDownstreamTransport, and setting the codec to HTTP/3.
See example downstream HTTP/3 configuration for example configuration.
Note that the example configuration includes both a TCP and a UDP listener, and the TCP
listener is advertising http/3 support via an
alt-svc header. Advertising HTTP/3 is not necessary for
in-house deployments where HTTP/3 is explicitly configured, but is needed for internet facing deployments
where TCP is the default, and clients such as Chrome will only attempt HTTP/3 if it is explicitly advertised.
By default the example configuration uses kernel UDP support, but for production performance use of BPF is strongly advised if Envoy is running with multiple worker threads. Envoy will attempt to use BPF on Linux by default if multiple worker threads are configured, but may require root, or at least sudo-with-permissions (e.g. sudo setcap cap_bpf+ep). If multiple worker threads are configured, Envoy will log a warning on start-up if BPF is unsupported on the platform, or is attempted and fails.
HTTP/3 upstream support is still in Alpha, and should be used with caution. Outstanding issues required for HTTP/3 to go GA can be found here
Envoy HTTP/3 support can be turned up by turning up HTTP/3 support in http_protocol_options, Either configuring HTTP/3 explicitly on, or using the auto_http option to use HTTP/3 if it is supported.
See here for more information about HTTP/3 connection pooling, including detailed information of where QUIC will be used, and how it fails over to TCP when QUIC use is configured to be optional.
An example upstream HTTP/3 configuration file can be found here.