We use HTTP Alternative Services (RFC 7838). This appears as a Alt-Svc response header that advertises the availability of HTTP/3 to a client. The client uses

We use HTTP Alternative Services (RFC 7838). This appears as a Alt-Svc response header that advertises the availability of HTTP/3 to a client. The client uses its local knowledge (and maybe other heuristics) to decide if it wants to change protocols.