On the Congestion Responsiveness of Aggregate Internet Traffic: Open-loop vs Closed-loop Session Arrivals

Abstract

A traffic aggregate is congestion responsive if it reacts to network congestion by reducing its rate. The congestion responsiveness of Internet traffic has been largely attributed to TCP's congestion control. In this paper, we argue that congestion control for individual transfers is not sufficient to produce responsive aggregate traffic. The offered load at a network link is generated from users/applications that generate finite-length flows or groups of flows (sessions). We examine two session generation models. First, a closed-loop model where each user from a certain population can generate a new session only after the completion of her previous session. Second, an open-loop model where sessions arrive independently of previous sessions. These two models produce traffic with very different congestion responsiveness, even if each flow is controlled by TCP. We introduce two metrics to quantify the congestion responsiveness of a traffic aggregate, the throughput responsiveness and the flow rate responsiveness, and show that the closed-loop model results in congestion responsive traffic, while the open-loop model can lead to persistent overload and congestion collapse. We then measure the congestion responsiveness of the traffic at a university access link. These experiments show that both responsiveness metrics are close to zero, which explains why that link is often under persistent overload. We also present an estimation methodology to classify the traffic at a link as open-loop or closed-loop. Our measurements at a dozen of access and core links show that more than 70% of the traffic we analyzed follows the closed-loop model. This implies that a major reason for the congestion responsiveness of Internet traffic may be that most traffic reacts to congestion at the session generation layer.