What kind of traffic iperf UDP creates?

Description: Iperf is used to generate UDP cross traffic. Constant UDP flow helps us to tell whether AB_estimations are correct for rapid flows. So we desire that the udp flow is constant-bit-rate. But is UDP generated by iperf really CBR flow?
Experiments: One UDP flow from smithfd-rapid6 to smithfd-rapid3. Dag trace is taken between the even and odd switch. The specified throughput given to iperf is 300Mbps and 600Mbps.

1. 300Mbps UDP cross traffic

When throughput is calculated every 1s/100ms/10ms:

When throughput is calculated every 1ms:

When throughput is calculated every 100us:

Zoom in a little bit:
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The CDF of throughput every 100us:
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2. 600Mbps UDP cross traffic

When throughput is calculated every 1s/100ms/10ms:

When throughput is calculated every 1ms:

When throughput is calculated every 100us:

Zoom in a little bit:
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The CDF of throughput every 100us:
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So in large time scale, the throughput is stable. But in small time scale, the udp generated by iperf is not constant bit rate. But each probe stream deals with cross traffic in micro-seconds time scale. So the AB_est might not be a constant value. But most of time AB_est = 1G - udp +/- 50Mbps. So I argue that the udp cbr traffic is reliable and insightful to examine how AB_est reacts to constant cross traffic.