Host Congestion Control

Abstract

The conventional wisdom in systems and networking communities is that congestion happens primarily within the network fabric. However, adoption of high-bandwidth access links and relatively stagnant technology trends for resources within hosts have led to emergence of host congestion---that is, congestion within the host network that enables data exchange between NIC and CPU/memory. Such host congestion alters the many assumptions entrenched within decades of research and practice of congestion control. We present hostCC, a congestion control architecture to handle both host and network fabric congestion. hostCC embodies three key ideas. First, in addition to congestion signals that originate within the network fabric, hostCC collects host congestion signals that capture the precise time, location, and reason for host congestion. Second, hostCC introduces a sub-RTT granularity host-local congestion response that uses congestion signals to allocate host resources between network traffic and host-local traffic. Finally, hostCC uses both host and network congestion signals to allocate network resources at an RTT granularity. We realize hostCC within the Linux network stack. Our hostCC implementation requires no modifications in applications, host hardware, and/or network hardware; moreover, it can be integrated with existing congestion control protocols to handle both host and network fabric congestion. Evaluation of Linux DCTCP with and without hostCC suggests that, in the presence of host congestion, hostCC significantly reduces queueing and packet drops at the host, resulting in improved performance of networked applications in terms of throughput and tail latency.