Transport Control Protocol (TCP) is a reliable protocol of the transport layer in networks. However, TCP causes incast congestion in high bandwidth and low latency networks. This happens when multiple servers send data to the same receiver in parallel. This many-to-one traffic pattern is common in many important data center applications such as MapReduce. Data centers provide large volumes of computation and storage resources for supporting today's Internet services. Hence TCP incast congestion may severely degrade their performances, e.g., by increasing response time and reducing throughput. In this paper, the TCP incast is studied in detail.