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While datacenters are increasingly adopting VMs to provide elastic cloud services, they still rely on traditional TCP for congestion control. In this talk, I will first show that VM scheduling delays can heavily contaminate RTTs sensed by VM senders, preventing TCP from correctly learning the physical network condition. Focusing on the incast problem, which is commonly seen in large-scale distributed data processing such as MapReduce and web search, I find that the solutions that have been developed for *physical* clusters fall short in a Xen *virtual* cluster. Second, I will provide a concrete understanding of the problem, and reveal that the situations that when the sending VM is preempted versus when the receiving VM is preempted, are different. Third, I will introduce my recent attempts on paravirtualizing TCP to overcome the negative effect caused by VM scheduling delays.