Headers: Downlink

Downlink header

Limited availability

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

Learn more
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

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.

Accept-CH: Downlink

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

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`	67 The value is never greater than 10 Mbps, as a non-standard anti-fingerprinting measure.	79 The value is never greater than 10 Mbps, as a non-standard anti-fingerprinting measure.	No	54 The value is never greater than 10 Mbps, as a non-standard anti-fingerprinting measure.	No	67 The value is never greater than 10 Mbps, as a non-standard anti-fingerprinting measure.	No	48 The value is never greater than 10 Mbps, as a non-standard anti-fingerprinting measure.	No	9.0 The value is never greater than 10 Mbps, as a non-standard anti-fingerprinting measure.	67 The value is never greater than 10 Mbps, as a non-standard anti-fingerprinting measure.	No