RTT
The HTTP RTT request header reports the estimated round-trip time of the client's network connection at the time the request is made.
Usage
The RTT header is part of the Network Information
Client Hints infrastructure defined by the WICG. The
header allows servers to receive information about the client's network
conditions and adapt responses based on connection quality. A server
opts into receiving this header by sending Accept-CH with
RTT in the response.
The value represents effective round-trip time in milliseconds, based on recently observed application-layer RTT measurements across recently active connections. The browser rounds the value to the nearest multiple of 25 milliseconds to prevent fingerprinting. A connection with 142ms measured RTT rounds to 150ms in the header.
Servers use this information to optimize content delivery. High RTT values indicate slower connections where smaller payloads, simplified layouts, or reduced image quality improve the experience. Low RTT values signal fast connections where full-quality assets deliver without perceived delay.
The RTT header works alongside other network client hints including Downlink and ECT to provide a complete picture of connection quality.
Note
The RTT header is supported by Chromium-based browsers (Chrome, Edge, Opera). The header requires a secure context (HTTPS).
Vary caveat
Including RTT in Vary headers is discouraged due to high value cardinality, which fragments cache entries and increases fingerprinting surface.
Values
Round-trip time in milliseconds
The value is an integer representing the estimated round-trip time in
milliseconds, rounded to the nearest 25ms. Common values range from 0
(extremely fast local connections) to 3000 (slow satellite or heavily
congested connections).
Example
A client on a typical broadband connection sends an estimated RTT of 50 milliseconds. The server receives this hint and decides to send full-resolution images.
RTT: 50
A client on a slower mobile connection reports 300 milliseconds. The server adapts by reducing image sizes and deferring non-critical resources.
RTT: 300
A server requests network quality hints to enable adaptive content delivery. The Accept-CH header lists RTT alongside downlink speed and effective connection type.
Accept-CH: RTT, Downlink, ECT
Note
Vary: RTT is shown in some examples but is discouraged in
practice. High cardinality of RTT values fragments caches and
exposes fingerprinting surface.
Takeaway
The RTT header provides servers with round-trip time estimates rounded to 25ms intervals, enabling adaptive content delivery based on measured network latency.