In data center networks, many congestion control algorithms have been proposed to achieve low latency, in which DX is a famous latency-based congestion control algorithm. Although the advantages of DX have been confirmed by experimental results, the theoretical analysis of DX is absent. Accordingly, some drawbacks of DX under special environments are unexplored. In this paper, we conduct fluid-flow analysis over DX, deduce sufficient conditions for the stability of DX and reveal its special stable state when with a large number of concurrent flows or the small Round Trip Time (RTT). Analytical results uncover two problems of DX: 1) it has poor throughput when either the base RTT is very large or the number of flows is relatively small; 2) it suffers from large queueing delay when either the base RTT is relatively small or the number of concurrent flows is large. The reason is that the congestion window is always required to be reduced even if it already is the smallest value 1, owing to the small RTT in data centers. In order to solve the problem that DX cannot continue to reduce the sending rate in the special stable state, we propose DX+. DX+ regulates the time interval of sending packets when the congestion window is 1 but the link is still congested. Extensive simulation results verify our analytical results and show that DX+ can further reduce queueing delay without losing throughput under special stable conditions compared to DX.

在数据中心网络中，已经提出了许多拥塞控制算法来实现低延迟，其中DX是著名的基于延迟的拥塞控制算法。尽管实验结果证实了DX的优点，但缺乏DX的理论分析。因此，在特殊环境下DX的一些缺点尚待开发。在本文中，我们对DX进行了流体流动分析，推导出了DX稳定性的充分条件，并揭示了当有大量并发流量或较小的往返时间（RTT）时DX的特殊稳定状态。分析结果揭示了DX的两个问题：1）当基本RTT很大或流量相对较小时，它的吞吐量很差。2）当基本RTT相对较小或并发流数量较大时，它会遭受较大的排队延迟。原因是由于数据中心的RTT很小，即使拥塞窗口已经是最小值1，也总是需要减小它。为了解决DX在特殊稳定状态下无法继续降低发送速率的问题，我们提出了DX +。当拥塞窗口为1但链路仍然拥塞时，DX +会调整发送数据包的时间间隔。大量的仿真结果验证了我们的分析结果，并表明DX +与DX相比，在特殊的稳定条件下，可以进一步减少排队延迟而不会损失吞吐量。由于数据中心的RTT很小。为了解决DX在特殊稳定状态下无法继续降低发送速率的问题，我们提出了DX +。当拥塞窗口为1但链路仍然拥塞时，DX +会调整发送数据包的时间间隔。大量的仿真结果验证了我们的分析结果，并表明与DX相比，DX +在特殊的稳定条件下可以进一步减少排队延迟，而不会损失吞吐量。由于数据中心的RTT很小。为了解决DX在特殊稳定状态下无法继续降低发送速率的问题，我们提出了DX +。当拥塞窗口为1但链路仍然拥塞时，DX +会调整发送数据包的时间间隔。大量的仿真结果验证了我们的分析结果，并表明与DX相比，DX +在特殊的稳定条件下可以进一步减少排队延迟，而不会损失吞吐量。