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
- The special value ''clear" indicates that the origin requests all alternatives for that origin to be invalidated.
- The ALPN protocol identifier. Examples include h2 for HTTP/2 and h3-25 for draft 25 of the HTTP/3 protocol.
- The quoted string specifying the alternative authority which consists of an optional host override, a colon, and a mandatory port number.
- 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
Ageheader). If the cached entry expires, the client can no longer use this alternative service for new connections.
- 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
|HTTP Alternative Services |
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- Alternative Services (article about
Alt-Svcby HTTP Working Group chair Mark Nottingham)