RTT
Experimental: This is an experimental technology
Check the Browser compatibility table carefully before using this in production.
The RTT
Client hint request header field provides the approximate round trip time on the application layer, in milliseconds. The RTT hint, unlike transport layer RTT, includes server processing time.
Header type | Request header, Client hint |
---|---|
Forbidden header name | no |
The RTT value is rounded to the nearest 25 milliseconds to prevent fingerprinting. 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 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 Vary
header — 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
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 |
Browser compatibility
BCD tables only load in the browser
See also
- Improving user privacy and developer experience with User-Agent Client Hints (web.dev)
- Network client hints
Accept-CH
- HTTP Caching > Vary and
Vary
NetworkInformation.effectiveType