CF-Changes Workers - New L4 transport telemetry fields in Workers

[ENF News Bot]

Three new properties are now available on request.cf in Workers that expose Layer 4 transport telemetry from the client connection. These properties let your Worker make decisions based on real-time connection quality signals — such as round-trip time and data delivery rate — without requiring any client-side changes.

Previously, this telemetry was only available via the Server-Timing: cfL4 response header. These new properties surface the same data directly in the Workers runtime, so you can use it for routing, logging, or response customization.

New properties​

https://developers.cloudflare.com/changelog/rss/#new-properties

Property	Type	Description
clientTcpRtt	number | undefined	The smoothed TCP round-trip time (RTT) between Cloudflare and the client in milliseconds. Only present for TCP connections (HTTP/1, HTTP/2). For example, 22.
clientQuicRtt	number | undefined	The smoothed QUIC round-trip time (RTT) between Cloudflare and the client in milliseconds. Only present for QUIC connections (HTTP/3). For example, 42.
edgeL4	Object | undefined	Layer 4 transport statistics. Contains deliveryRate (number) — the most recent data delivery rate estimate for the connection, in bytes per second. For example, 123456.

Example: Log connection quality metrics​

https://developers.cloudflare.com/changelog/rss/#example-log-connection-quality-metrics

export default {
  async fetch(request) {
    const cf = request.cf;

    const rtt = cf.clientTcpRtt ?? cf.clientQuicRtt ?? 0;
    const deliveryRate = cf.edgeL4?.deliveryRate ?? 0;
    const transport = cf.clientTcpRtt ? "TCP" : "QUIC";

    console.log(`Transport: ${transport}, RTT: ${rtt}ms, Delivery rate: ${deliveryRate} B/s`);

    const headers = new Headers(request.headers);
    headers.set("X-Client-RTT", String(rtt));
    headers.set("X-Delivery-Rate", String(deliveryRate));

    return fetch(new Request(request, { headers }));
  },
};

For more information, refer to Workers Runtime APIs: Request.

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