I am using tcpdump to measure latency on a linux router, i.e. the delay experienced by packets as they pass through the router (made of Ubuntu 10). For this purpose, I captured packets in the IN and Out interfaces and subtracted their log timestamp to know the latency inside the router. I was expecting some sort of flat latency; but, in constrast, the latency increased linearly with time. I used iperf to send about 800,000 packets (of size 1500Byte) through the router (having two Gibabit links).
The surprising point is that although the latency on the router was increasing from about 20 microseconds to 7 milliseconds, the average throughput measured every second was almost flat.
Would you please let me know why the latency has increased? There might be some problem in properly recording the time of capturing OUT interface packets.