Alt-Svc

The Alt-Svc HTTP header allows a server to indicate that a particular resource should be loaded from a different server — while still appearing to the user as if were loaded from the same server.

Alt-Svc: clear
Alt-Svc: <protocol-id>=<alt-authority>; ma=<max-age>
Alt-Svc: <protocol-id>=<alt-authority>; ma=<max-age>; persist=1

clear

The special value ''clear" indicates that the origin requests all alternatives for that origin to be invalidated.

<protocol-id>

The ALPN protocol identifier. Examples include h2 for HTTP/2 and h3-25 for draft 25 of the HTTP/3 protocol.

<alt-authority>

The quoted string specifying the alternative authority which consists of an optional host override, a colon, and a mandatory port number.

ma=<max-age>Optional

The number of seconds for which the alternative service is considered fresh. If omitted, it defaults to 24 hours. Alternative service entries can be cached for up to <max-age> seconds, minus the age of the response (from the Age header). If the cached entry expires, the client can no longer use this alternative service for new connections.

persist=1Optional

Usually cached alternative service entries are cleared on network configuration changes. Use of the persist=1 parameter ensures that the entry is not deleted through such changes.

Multiple entries can be specified in a single Alt-Svc header using comma as separator. In that case, early entries are considered more preferable.

Alt-Svc: h2=":443"; ma=2592000;
Alt-Svc: h2=":443"; ma=2592000; persist=1
Alt-Svc: h2="alt.example.com:443", h2=":443"
Alt-Svc: h3-25=":443"; ma=3600, h2=":443"; ma=3600

• Alternative Services (article about Alt-Svc by HTTP Working Group chair Mark Nottingham)