HTTP headers | Alt-Svc

Last Updated : 10 May, 2020

The HTTP headers Alt-Svc header is a response-type header it has been used to advertise alternative service. Which services can be defined by a protocol/host/port combination.

Syntax:

Alt-Svc: clear

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

Directives: This header accept five directives as mentioned above and described below:

clear: This directive define that all alternative services for that origin to be invalidated.
<protocol-id>: This directive is the ALPN protocol identifier.
<alt-authority>: This directive defines an alternative authority which consists of an optional host override, a colon, and a mandatory port number.
ma=<max-age>: It holds the number that defines seconds for which the alternative service is considered fresh if it is omitted then the default value will be 86400. It is an optional directive.
persist=1: This directive holds the alternative service which cleared on network configuration changes. Use of the persist=1 ensures that the entry is not deleted through such changes.

Example:

Alt-Svc: h2=":425"; ma=2585900; persist=1

```
Alt-Svc: h2=":485"; ma=592000; 
```

To check this Alt-Svc in action go to Inspect Element -> Network check the request header for Alt-Svc like below, Alt-Svc is highlighted you can see.