Cloud data centers now provide a plethora of rich online applications such as web search, social networking, and cloud computing. A key challenge for such applications, however, is to meet soft real-time constraints. Due to the deadline-agnostic congestion control in Transmission Control Protocol (TCP), many deadline-sensitive flows cannot finish transmission before their deadlines. In this paper, we propose an SDN-based Explicit-Deadline-aware TCP (SED) for cloud Data Center Networks (DCN). SED assigns a base rate for non-deadline flows first and gives spare bandwidth to the deadline flows as much as possible. Subsequently, a Retransmission-enhanced SED (RSED) is introduced to solve the packet-loss timeout problem. Through our experiments, we show that SED can make flows meet deadlines effectively, and that it significantly outperforms previous protocols in the cloud data center environment.