Envoy 1.22: Configuring HTTP/3 Fallback Policy for Reliable Connections

We want to enable stable HTTP/3 support in Envoy 1.22 for outbound traffic using the h3 cluster type and the enable_http3: true flag. The configuration also requires a QUIC transport socket with the QUIC library compiled in. However, automatic downgrade to HTTP/2 on a QUIC handshake failure is not enabled by default; users must set the fallback_policy explicitly.

The current uncertainty lies in which fallback_policy value should be selected to guarantee that requests are not dropped when QUIC fails, and how that choice impacts metrics and logs. Additionally, the h3_datagram feature remains experimental and its default state is unclear for deployments that do not yet support datagram traffic.

Questions:

• What fallback_policy options are available in Envoy 1.22, and what behavior does each trigger on QUIC handshake failure?
• How can we verify that the chosen fallback_policy is correctly applied without dropping requests?
• Is it advisable to enable the h3_datagram field by default, or should it remain disabled until downstream services confirm support?