RTTRTT (Round-Trip Time) is a Network Information Client Hint request header conveying the client's estimated application-layer round-trip latency in milliseconds. Together with Downlink (bandwidth) and ECT (tier) it reports the network's latency characteristics to the server.
The value is the browser's estimate of effective round-trip time from recent connections, rounded to a multiple of 25 ms for anti-fingerprinting, e.g. 100. Even with ample bandwidth, high latency (RTT) makes request-heavy pages feel slow, so bandwidth and latency should be treated as separate signals and considered together. It is readable in JS via navigator.connection.rtt.
In use, high RTT suggests optimizing to reduce round trips (request coalescing, preloading, inlining, more server-side rendering) or adjusting expectations for real-time interaction. But it is an instantaneous estimate that fluctuates, so it is more stable to combine it with ECT, Downlink, and Save-Data than to judge on it alone.
These headers vary in support and exposure across browsers and are trending toward reduction due to privacy/fingerprinting concerns, so the value may be absent. Always keep a fallback, and if the response varies by RTT, send Vary: RTT so caches do not confuse per-latency representations.
RTT: <milliseconds>e.g. RTT: 100
<milliseconds> | Estimated round-trip time in ms, rounded to a multiple of 25 ms for anti-fingerprinting. |