ght choose to send fewer
resources.

**Note:** The [ ` Vary ` ](/en-US/docs/Web/HTTP/Headers/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 ](/en-
US/docs/Web/HTTP/Caching#vary) ). Even if [ ` RTT ` ](/en-
US/docs/Web/HTTP/Headers/RTT) is used to configure what resources are sent
consider omitting it in the [ ` Vary ` ](/en-US/docs/Web/HTTP/Headers/Vary)
header â it is likely to change often, which effectively makes the resource
uncacheable.

##  Syntax

http

    
    
    RTT: <number>
    

##  Directives

<number>

    

The approximate round trip time in milliseconds, rounded to the nearest 25
milliseconds.

##  Examples

A server first needs to opt in to receive the ` RTT ` header by sending the [
` Accept-CH ` ](/en-US/docs/Web/HTTP/Headers/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

Specification  
---  
[ Network Information API  
#  rtt-request-header-field  ](https://wicg.github.io/netinfo/#rtt-request-
header-field)  
  
##  Browser compatibility

BCD tables only load in the browser  with JavaScript enabled. Enable
JavaScript to view data.

##  See also

  * [ Improving user privacy and developer experience with User-Agent Client Hints ](https://developer.chrome.com/docs/privacy-security/user-agent-client-hints) (developer.chrome.com) 
  * Network client hints 
    * [ ` Downlink ` ](/en-US/docs/Web/HTTP/Headers/Downlink)
    * [ ` ECT ` ](/en-US/docs/Web/HTTP/Headers/ECT)
    * [ ` Save-Data ` ](/en-US/docs/Web/HTTP/Headers/Save-Data)
  * [ ` Accept-CH ` ](/en-US/docs/Web/HTTP/Headers/Accept-CH)
  * [ HTTP Caching > Vary ](/en-US/docs/Web/HTTP/Caching#vary) and [ ` Vary ` ](/en-US/docs/Web/HTTP/Headers/Vary)
  * [ ` NetworkInformation.effectiveType ` ](/en-US/docs/Web/API/NetworkInformation/effectiveType)

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