> For the complete documentation index, see [llms.txt](https://http3-explained.haxx.se/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://http3-explained.haxx.se/en/h3/h3-altsvc.md).

# Bootstrap with Alt-svc

The alternate service (Alt-svc:) header and its corresponding `ALT-SVC` HTTP/2 frame are not specifically created for QUIC or HTTP/3. They are part of an already designed and created mechanism for a server to tell a client: *"look, I run the same service on THIS HOST using THIS PROTOCOL on THIS PORT"*. See details in [RFC 7838](https://tools.ietf.org/html/rfc7838).

A client that receives such an Alt-svc response is then advised to, if it supports and wants to, connect to that given other host in parallel in the background - using the specified protocol - and if it is successful switch its operations over to that instead of the initial connection.

If the initial connection uses HTTP/2 or even HTTP/1, the server can respond and tell the client that it can connect back and try HTTP/3. It could be to the same host or to another one that knows how to serve that origin. The information given in such an Alt-svc response has an expiry timer, making clients able to direct subsequent connections and requests directly to the alternative host using the suggested alternative protocol, for a certain period of time.

## Example

An HTTP server includes an `Alt-Svc:` header in its response:

```
Alt-Svc: h3=":50781"
```

This indicates that HTTP/3 is available on UDP port 50781 at the same host name that was used to get this response.

A client can then attempt to setup a QUIC connection to that destination and if successful, continue communicating with the origin like that instead of the initial HTTP version.