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5. TLS and HTTP

Available inall subroutines.

When a client connection is using a version of HTTP older than HTTP/3, invoking this function triggers Fastly to advertise HTTP/3 support, on the eventual client response, allowing the client to switch to HTTP/3 for future requests.

When the connection is over HTTP/3 already, this function has no effect.

The addition of the response header happens after the header block has been finalised in vcl_deliver. As a result, using restart or error within your VCL code (which will destroy the current response and create a new one), will not affect the addition of the Alt-Svc header. It is also not possible to read or unset the value of the header in VCL, since it has not yet been added to the response.

If resp.http.alt-svc is already set, using this function will cause it to be overwritten.