ECT
The HTTP ECT request header is an experimental client hint reporting the effective connection type of the network. The header is part of the Network Information API, supported only by Chromium-based browsers. The header is not registered in the IANA HTTP Field Name Registry.
Usage
The ECT header (Effective Connection Type) is part of the Network Information API. Servers receive this header after opting in through the Accept-CH response header. The header classifies the current network connection into one of four categories based on observed latency and bandwidth characteristics.
The browser determines the effective connection type by
measuring round-trip time and throughput. The value
reflects real network performance rather than the nominal
connection type. A device connected to a congested 4G
network might report 3g or 2g if observed performance
matches those profiles.
Servers use ECT alongside other network client hints
like Downlink, RTT, and
Save-Data to tailor responses. A server might
deliver simplified layouts and compressed images for 2g
connections, while serving rich media and interactive
features for 4g connections.
Values
slow-2g
The slow-2g value indicates a severely constrained
connection with high latency and minimal bandwidth, typical
of early 2G networks or failing connections.
2g
The 2g value represents limited bandwidth and elevated
latency, similar to a functional 2G mobile connection.
3g
The 3g value signals moderate bandwidth and latency,
comparable to typical 3G mobile networks.
4g
The 4g value indicates high bandwidth and low latency,
matching the performance profile of modern 4G LTE or
better connections.
Example
A browser on a reliable broadband connection reports the connection as equivalent to 4G performance. The server responds with full-resolution assets and prefetches additional resources.
ECT: 4g
A mobile device experiencing poor network conditions
reports 2g even though the device is connected to a 4G
tower. The server delivers a lightweight version of the
page.
ECT: 2g
Takeaway
The ECT header classifies network performance into one of four connection types, enabling servers to adapt content based on observed bandwidth and latency.