# QUIC RTT is now visible in rules! Route by connection quality

Hi everyone, it's Shiichan! Today I found a small but genuinely useful update that lets you peek a bit deeper into your network connections.

## What was announced?

On the Cloudflare Changelog, two new fields were added to rule expressions. They surface Layer 4 (transport layer) telemetry from the client connection, right inside your rules.

- `cf.timings.client_quic_rtt_msec` (Integer): the smoothed QUIC round-trip time (RTT) between Cloudflare and the client, in milliseconds. Only populated for QUIC (HTTP/3) connections; returns 0 for TCP connections
- `cf.edge.l4.delivery_rate` (Integer): the most recent data delivery rate estimate for the client connection, in bytes per second. Returns 0 when L4 statistics aren't available for the request

Combined with the existing `cf.timings.client_tcp_rtt_msec` field, you get a complete picture of connection quality for both TCP and QUIC traffic.

## The story so far

This QUIC RTT and delivery rate data already existed, but it was only available via the `Server-Timing: cfL4` response header. You couldn't use it directly as a condition inside a rule expression.

## What changes

The same data is now available as rule fields, so you can build transport-aware rules in Transform Rules, WAF Custom Rules, and any other phase that supports dynamic fields, all without requiring any client-side changes.

## Dive Deep

The changelog post shares two practical examples right away.

**Route high-latency connections to a lighter origin**

Use a request header transform rule to tag requests coming from high-latency connections, so your origin can serve a lighter page variant.

Rule expression:
```
cf.timings.client_tcp_rtt_msec > 200 or cf.timings.client_quic_rtt_msec > 200
```

Header modification: set `X-High-Latency` to `true`

**Match low-bandwidth connections**

```
cf.edge.l4.delivery_rate > 0 and cf.edge.l4.delivery_rate < 100000
```

You can use a delivery-rate threshold like this to identify low-bandwidth connections too.

## Wrap-up

- New rule fields `cf.timings.client_quic_rtt_msec` (QUIC RTT) and `cf.edge.l4.delivery_rate` (delivery rate) are now available
- Data that used to require the `Server-Timing: cfL4` header can now be used directly in Transform Rules, WAF Custom Rules, and more
- You can tag high-latency requests or detect low-bandwidth connections and route by connection quality, with no client-side changes needed

This one's for anyone who wants to fine-tune edge routing based on real connection quality!