Alt-Svc

The Alt-Svc ( Alternative Service ) a header in HTTP defines that, when an origin has resources that are accessible through a different protocol/host/port combination, it is said to have an alternative service available.

Description

An alternative service can be used to interact with the resources on an origin server at a separate location on the network, possibly using a different protocol configuration. Alternative services are considered authoritative for an origin's resources. like ( response data etc )
but you have to over TLS ( Transport Layer Security ) based connection to access these resources and the same certificate can be used for Alternative Services.

Note:- h2c protocol never be used as an Alternative Service because h2c never used over TLS based connection.

For example as client request for the resource in alternative services [uri-host]

   ("http", "www.example.com", "80")
resources are also accessible at the alternative service:
("h2", "new.example.com", "81")

Syntax

Alt-Svc: clear
Alt-Svc: <service-list>; ma=<max-age>
Alt-Svc: <service-list>; ma=<max-age>; persist=1

clear

A field value the special value ''clear" indicates that the origin requests all alternatives for that origin to be invalidated (including those specified in the same response. ( clear as string only )

<service-list>

A comma-separated list of service definitions in the format of <service-name>=​"<host-name>:<port-number>", where <service-name> is a valid ALPN identifier. ( ex: h2="www.example.com"; , h2="h2:port"; )

note: h2=": port" means alternative service on the same origin with different port

<max-age>

The delta -seconds values indicate the number of seconds since the response was generated for which the alternative service is considered fresh. (ex: ma=60 )

persistOptional

Optional parameter specifying that the alternative service can be remembered between sessions and network changes.( ex= persist=1 )

Example

Alt-Svc: h2=":443"; ma=2592000;
Alt-Svc: h2=":443"; ma=2592000; persist=1

Specifications

Browser compatibility

See also

• Identifying resources on the Web