Downlink Client hint request header field provides the approximate bandwidth of the client's connection to the server, in Mbps.
|Header type||Request header, Client hint|
|Forbidden header name||no|
Downlink value is given in Mbps and rounded to the nearest 25 kilobits per second to prevent fingerprinting; There are many other mechanisms an attacker might use to obtain similar information.
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.
Vary header is used in responses to indicate that a different resource is sent for every different value of the header (see HTTP Caching > Varying responses). Even if
Downlink is used to configure what resources are sent, consider omitting it in the
Vary header — it is likely to change often, which effectively makes the resource uncachable.
The downlink rate in Mbps, rounded to the nearest 25 kilobits.
A server first needs to opt in to receive the
Downlink header by sending the
Accept-CH response header containing
Then on subsequent requests the client might send a
Downlink header back:
|Network Information API |
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