ContextQMD
Back to Content

Downlink header

files/en-us/web/http/reference/headers/downlink/index.md

latest2.3 KB
Original Source

{{SeeCompatTable}}

The HTTP Downlink {{Glossary("request header")}} is used in Client Hints to provide the approximate bandwidth in Mbps of the client's connection to the server.

The hint allows a server to choose what information is sent based on the network bandwidth. For example, a server might choose to send smaller versions of images and other resources on low bandwidth networks.

[!NOTE] The {{HTTPHeader("Vary")}} header is used in responses to indicate that a different resource is sent for every different value of the header (see HTTP Caching Vary). Even if Downlink is used to configure what resources are sent, consider omitting it in the {{HTTPHeader("Vary")}} header — it is likely to change often, which effectively makes the resource uncacheable.

<table class="properties"> <tbody> <tr> <th scope="row">Header type</th> <td> {{Glossary("Request header")}}, <a href="/en-US/docs/Web/HTTP/Guides/Client_hints">Client hint</a> </td> </tr> <tr> <th scope="row">{{Glossary("Forbidden request header")}}</th> <td>No</td> </tr> </tbody> </table>

Syntax

http
Downlink: <number>

Directives

  • <number>
    • : The downlink rate in Mbps, rounded to the nearest 25 kilobits. The downlink rate may be used as a {{glossary("fingerprinting")}} variable, so values for the header are intentionally coarse to reduce the potential for its misuse.

Examples

A server first needs to opt in to receive the Downlink header by sending the {{HTTPHeader("Accept-CH")}} response header containing Downlink.

http
Accept-CH: Downlink

Then on subsequent requests the client might send a Downlink header back:

http
Downlink: 1.7

Specifications

{{Specifications}}

Browser compatibility

{{Compat}}

See also