Allows a server to direct a client to load a resource from a different server, usually to upgrade to HTTP/2 or HTTP/3..

Fastly writes this header into responses. It is defined by an external standard.

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. The most common use case for this is to indicate that a client currently connected over HTTP/1.1 should reconnect using a newer version of HTTP.

While it is possible to set the value of this header manually using edge code, it is usually unnecessary. Fastly services will automatically support and negotiate HTTP/2 at the protocol level if your chosen TLS configuration supports it.

Our support for HTTP/3 is currently in limited availability. HTTP/3 requires an Alt-Svc header to upgrade from H1 or H2, because HTTP/3 uses UDP, while HTTP 1 and 2 use TCP. In VCL services it is advisable to use the h3.alt_svc function to achieve this, because it will always generate an offer matching Fastly's current support for the HTTP/3 standard as it evolves. For more information about supporting HTTP/3, see routing traffic to Fastly.