files/en-us/web/http/reference/headers/ect/index.md
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The HTTP ECT {{Glossary("request header")}} is used in Client Hints to indicate the {{Glossary("effective connection type")}}: slow-2g, 2g, 3g, or 4g.
The value represents the "network profile" that best matches the connection's latency and bandwidth, rather than the actual mechanisms used for transferring the data.
For example, 2g might be used to represent a slow Wi-Fi connection with high latency and low bandwidth, while 4g might represent a fast fibre-based broadband network.
The hint allows a server to choose what information is sent based on the broad characteristics of the network. For example, a server might choose to send smaller versions of images and other resources on less capable connections. The value might also be used as a starting point for determining what information is sent, which is further refined using information in {{HTTPHeader("RTT")}} and {{HTTPHeader("Downlink")}} hints.
<table class="properties"> <tbody> <tr> <th scope="row">Header type</th> <td> {{Glossary("Request header")}}, <a href="/en-US/docs/Web/HTTP/Guides/Client_hints">Client hint</a> </td> </tr> <tr> <th scope="row">{{Glossary("Forbidden request header")}}</th> <td>No</td> </tr> </tbody> </table>[!NOTE] A server that specifies
ECTin {{HTTPHeader("Accept-CH")}} may also specify it in {{HTTPHeader("Vary")}} to indicate that responses should be cached for different ECT values.
ECT: <value>
<value>
slow-2g, 2g, 3g, or 4g.A server first needs to opt in to receive the ECT header by sending the {{HTTPHeader("Accept-CH")}} response header containing ECT.
Accept-CH: ECT
Then on subsequent requests the client might send an ECT header back:
ECT: 2g
{{Specifications}}
{{Compat}}
Improving user privacy and developer experience with User-Agent Client Hints (developer.chrome.com)
Network client hints
{{HTTPHeader("Accept-CH")}}
HTTP Caching > Vary and {{HTTPHeader("Vary")}}
{{domxref("NetworkInformation.effectiveType")}}