Downlink header

Limited availability

This feature is not Baseline because it does not work in some of the most widely-used browsers.

See full compatibility

Experimental: This is an experimental technology
Check the Browser compatibility table carefully before using this in production.

The HTTP Downlink 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 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.

Header type	Request header, Client hint
Forbidden request header	No

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 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 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

Specification
Network Information API # downlink-request-header-field

Browser compatibility

	desktop					mobile
	Chrome	Edge	Firefox	Opera	Safari	Chrome Android	Firefox for Android	Opera Android	Safari on iOS	Samsung Internet	WebView Android	WebView on iOS
`Downlink` Experimental	Chrome – Full support Chrome 67 (Release date: 2018-05-29) footnote footnote The value is never greater than 10 Mbps, as a non-standard anti-fingerprinting measure.	Edge – Full support Edge 79 (Release date: 2020-01-15) footnote footnote The value is never greater than 10 Mbps, as a non-standard anti-fingerprinting measure.	Firefox – No support Firefox footnote No support	Opera – Full support Opera 54 (Release date: 2018-06-28) footnote footnote The value is never greater than 10 Mbps, as a non-standard anti-fingerprinting measure.	Safari – No support Safari footnote No support	Chrome Android – Full support Chrome Android 67 (Release date: 2018-05-31) footnote footnote The value is never greater than 10 Mbps, as a non-standard anti-fingerprinting measure.	Firefox for Android – No support Firefox for Android footnote No support	Opera Android – Full support Opera Android 48 (Release date: 2018-11-08) footnote footnote The value is never greater than 10 Mbps, as a non-standard anti-fingerprinting measure.	Safari on iOS – No support Safari on iOS footnote No support	Samsung Internet – Full support Samsung Internet 9 (Release date: 2018-09-15) footnote footnote The value is never greater than 10 Mbps, as a non-standard anti-fingerprinting measure.	WebView Android – Full support WebView Android 67 (Release date: 2018-05-31) footnote footnote The value is never greater than 10 Mbps, as a non-standard anti-fingerprinting measure.	WebView on iOS – No support WebView on iOS footnote No support

Tip: you can click/tap on a cell for more information.

Full support: Full support
No support: No support
: Experimental. Expect behavior to change in the future.
: See implementation notes.

Downlink header

Syntax

Directives

Examples

Specifications

Browser compatibility

Legend

See also