Data Center Transport Mechanisms: Congestion Control Theory and IEEE Standardization

Data Center Networks present a novel, unique and rich environment for algorithm development and deployment. Projects are underway in the IEEE 802.1 standards body, especially in the Data Center Bridging Task Group, to define new switched Ethernet functions for data center use. One such project is IEEE 802.1Qau, the Congestion Notification project, whose aim is to develop an Ethernet congestion control algorithm for hardware implementation. A major contribution of this paper is the description and analysis of the congestion control algorithm-QCN, for Quantized Congestion Notification-which has been developed for this purpose. A second contribution of the paper is an articulation of the Averaging Principle: a simple method for making congestion control loops stable in the face of increasing lags. This contrasts with two well-known methods of stabilizing control loops as lags increase; namely, (i) increasing the order of the system by sensing and feeding back higher-order derivatives of the state, and (ii) determining the lag and then choosing appropriate loop gains. Both methods have been applied in the congestion control literature to obtain stable 'algorithms for high bandwidth-delay product paths in the Internet. However, these methods are either undesirable or infeasible in the Ethernet context. The Averaging Principle provides a simple alternative, one which we are able to theoretically characterize.