The Alt-Svc HTTP response header is used to advertise alternative services through which the same resource can be reached. An alternative service is defined by a protocol/host/port combination.


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 Age header). 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="", h2=":443"
Alt-Svc: h3-25=":443"; ma=3600, h2=":443"; ma=3600


Specification Status Comment
RFC 7838 IETF RFC Initial definition.

Browser compatibility

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See also