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This paper performs controlled experiments with two popular virtualization techniques, Linux-VServer and Xen, to examine the effects of virtualization on packet sending and receiving delays. Using a controlled setting allows us to independently investigate the influence on delay measurements when competing virtual machines (VMs) perform tasks that consume CPU, memory, I/O, hard disk, and network bandwidth. Our results indicate that heavy network usage from competing VMs can introduce delays as high as 100 ms to round-trip times. Furthermore, virtualization adds most of this delay when sending packets, whereas packet reception introduces little extra delay. Based on our findings, we discuss guidelines and propose a feedback mechanism to avoid measurement bias under virtualization.