RTT header

Limited availability

This feature is not Baseline because it does not work in some of the most widely-used browsers.

Experimental:This is anexperimental technology
Check theBrowser compatibility table carefully before using this in production.

The HTTPRTTrequest header is anetwork 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 preventfingerprinting, 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:TheVary header is used in responses to indicate that a different resource is sent for every different value of the header (seeHTTP Caching Vary). Even ifRTT is used to configure what resources are sent consider omitting it in theVary header — it is likely to change often, which effectively makes the resource uncacheable.