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RTT header

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The HTTP RTT {{Glossary("request header")}} is a network client hint which provides the approximate round trip time on the application layer, in milliseconds. The RTT hint includes server processing time, unlike transport layer RTT.

The RTT value is rounded to the nearest 25 milliseconds to prevent fingerprinting, although there are many other mechanisms an attacker might use to obtain similar round-trip information.

The hint allows a server to choose what information is sent based on the network responsiveness/latency. For example, it might choose to send fewer resources.

[!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 Vary). Even if RTT 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.

<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>

Syntax

http
RTT: <number>

Directives

  • <number>
    • : The approximate round trip time in milliseconds, rounded to the nearest 25 milliseconds.

Examples

Using RTT client hints

A server first needs to opt in to receive the RTT header by sending the {{HTTPHeader("Accept-CH")}} response header containing RTT.

http
Accept-CH: RTT

Then on subsequent requests the client might send an RTT header back:

http
RTT: 125

Specifications

{{Specifications}}

Browser compatibility

{{Compat}}

See also