# 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.


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://http3-explained.haxx.se/en/h3/h3-altsvc.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.