Downlink
The HTTP Downlink request header is a client hint reporting the approximate downstream bandwidth of the connection in megabits per second (Mbps).
Usage
The Downlink header is part of the Network Information API. Servers receive this header when they opt in using the Accept-CH response header. The header provides an estimate of the current network bandwidth, allowing servers to adapt content based on connection speed.
The value is calculated from recently observed application
layer throughput across active connections. The browser
rounds the value to the nearest multiple of 25 kilobits per
second to limit fingerprinting surface. A value of 1.5
represents approximately 1.5 Mbps. A value of 10
represents 10 Mbps.
Servers combine Downlink with other network client hints like ECT, RTT, and Save-Data to make informed decisions about image quality, video bitrate, resource prioritization, and page structure. The header appears in requests after the server signals acceptance through Accept-CH and the browser measures connection performance.
Values
Bandwidth estimate
The value is a decimal number representing bandwidth in
Mbps, rounded to the nearest 25 Kbps. Common values include
0.5 (500 Kbps), 1.5 (1.5 Mbps), 5 (5 Mbps), and 10
(10 Mbps).
Example
A browser on a moderate mobile connection reports an estimated bandwidth of 2.5 Mbps. The server uses this information to serve medium-resolution images instead of full-resolution assets.
Downlink: 2.5
A high-speed connection reports 10 Mbps. The server responds with high-quality video streams and preloads additional resources.
Downlink: 10
Takeaway
The Downlink header provides servers with bandwidth estimates so they respond with content adapted to the current network speed.