Florin Dinu, Detecting Congestion Patterns using ECN Marking

Detecting the existence and location of congestion in the network is an important problem. It is essential for network management, it can help improve the performance of overlays and can be used to implement cooperative congestion control. Additional information, such as the level of congestion can also help decide the severity of the response measures. Simple solutions that involve regular polling of the router state are not sufficient. They can only provide coarse grained information and are not suitable for real-time applications. The previous research work on this topic can be grouped into active and passive measurements. Active probing solutions gather delay information between hosts and subsequently use the delay distributions to infer potential congestion. These solutions add traffic overhead and may also artificially increase the severity of the congestion. Moreover, they require the assistance of end-users. Few passive detection methods have been proposed and they are very sensitive to common network conditions like background traffic.

In this work we consider using explicit congestion information (ECN) to infer congestion. Typically ECN is used to increase the performance of reliable transport protocols like TCP. We propose using ECN information to infer network characteristics. Our method is completely passive. It only uses the ECN bits that are already present in packet headers and puts little extra burden on the routers. We present our approach using an intra-domain scenario. We also discuss a possible application of our method for guiding the route selection process in fast re-route algorithms. These algorithms strive to provide connectivity even in the face of failures by discovering backup routes. Such routes are either precomputed or generated on-demand. However, none of these routing algorithms consider congestion when selecting backup routes. This could easily lead to packet loss and poor connectivity during convergence, precisely the negative effects that fast-reroute is hoping to solve.