Downlink Client hint request header field provides the approximate bandwidth of the client's connection to the server, in Mbps.
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 Vary). 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 uncacheable.
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:
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