Analysis for unfairness of TCP outcast problem in data center networks

Abstract

There is a form of unfairness phenomenon found in data center networks,which was identified by P. Prakash. This phenomenon shows that the throughput of a flow with smaller RTT is less than the throughput of a flow with large RTT. This is completely contrary to the classic TCP protocol. P. Prakash gives an explanation based on port blackout theory. However, after careful analysis, we have found another reason for this unusual phenomenon, which may be more important. This is because of an unfair distribution of the flows with different RTT on the physical link and the differences in congestion window size when finishing the current block transmission. In this paper, we develop an analytical model for the goodput in the outcast behavior. Then, we design a window notification based protocol to address this phenomenon. By using the mean value of the congestion window measurement, we unified the congestion window size for flows with different RTT, which improves the goodput of the flows with small RTT. By designing several experiments using the ns-2 simulator, we demonstrate that our explanation of outcast phenomenon is correct. Furthermore, we verify the effectiveness of the proposed algorithms.