index.md

---

title: Downlink
slug: Web/HTTP/Headers/Downlink
status:
  - experimental
browser-compat: http.headers.downlink

---

{{HTTPSidebar}} {{SeeCompatTable}}

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

Header type	{{Glossary("Request header")}}, Client hint
{{Glossary("Forbidden header name")}}	no

The 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.

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 Varying responses). Even if {{HTTPHeader("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.

Syntax

Downlink: <number>

Directives

• <number>

  • : The downlink rate in Mbps, rounded to the nearest 25 kilobits.

Examples

A server first needs to opt in to receive the Downlink header by sending the {{HTTPHeader("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

{{Specifications}}

Browser compatibility

{{Compat}}

See also

• Improving user privacy and developer experience with User-Agent Client Hints (web.dev)

• Network client hints

  • {{HTTPHeader("RTT")}}

  • {{HTTPHeader("ECT")}}

  • {{HTTPHeader("Save-Data")}}

• {{HTTPHeader("Accept-CH")}}

• HTTP Caching > Varying responses and {{HTTPHeader("Vary")}}

• {{domxref("NetworkInformation.effectiveType")}}