RFC 7838 defines an HTTP header
which lets a server tell a client that there is one or more alternatives for
that server at another place with the use of the Alt-Svc: response header.
The alternatives the server suggests can include a server running on another port on the same host, on another completely different hostname and it can even perhaps offer the service over another protocol.
To make curl consider offered alternatives, tell curl to use a specific alt-svc cache file like this:
curl --alt-svc altcache.txt https://example.com/
then curl loads existing alternative service entries from the file at start-up
and consider those when doing HTTP requests, and if the servers sends new or
updated Alt-Svc: headers, curl stores those in the cache at exit.
The alt-svc cache is similar to a cookie jar. It is a text based file that stores one alternative per line and each entry also has an expiry time for which duration that particular alternative is valid.
Alt-Svc: is only trusted and parsed from servers when connected to over
HTTPS.
The use of Alt-Svc: headers is as of August 2019 the only defined way to
bootstrap a client and server into using HTTP/3. The server then hints to the
client over HTTP/1 or HTTP/2 that it also is available over HTTP/3 and then
curl can connect to it using HTTP/3 in the subsequent request if the alt-svc
cache says so.