The alternate service (Alt-svc:) header and its corresponding
frame are not specifically created for QUIC or HTTP/3. They are part of an
already designed and created mechanism for a server to tell a client: "look,
I run the same service on THIS HOST using THIS PROTOCOL on THIS PORT". See
details in RFC 7838.
A client that receives such an Alt-svc response is then advised to, if it supports and wants to, connect to that given other host in parallel in the background - using the specified protocol - and if it is successful switch its operations over to that instead of the initial connection.
If the initial connection uses HTTP/2 or even HTTP/1, the server can respond and tell the client that it can connect back and try HTTP/3. It could be to the same host or to another one that knows how to serve that origin. The information given in such an Alt-svc response has an expiry timer, making clients able to direct subsequent connections and requests directly to the alternative host using the suggested alternative protocol, for a certain period of time.
An HTTP server includes an
Alt-Svc: header in its response:
This indicates that HTTP/3 is available on UDP port 50781 at the same host name that was used to get this response.
A client can then attempt to setup a QUIC connection to that destination and if successful, continue communicating with the origin like that instead of the initial HTTP version.