Packet marking strategies for Explicit Congestion Notification (ECN)

The Transmission Control Protocol (TCP) has congestion control mechanisms built into the protocol which reacts to congestion within the network by appropriately adjusting the sending rate of the TCP source. The congestion control mechanisms use packet drops as a means to detect congestion occurring in the network. Unnecessary packet drops lead to poor performance for low-bandwidth delay-sensitive applications like telnet. Explicit Congestion Notification (ECN) is a proposed mechanism that provides feedback to the sources about impending congestion in the routers without needing to drop packets at the congested routers. This requires support at the router to mark the ECN bit of the IP packet based on mechanisms to identify congestion, like Random Early Detection (RED). We examine three different marking strategies, viz., mark-tail, mark-front and mark-random, to mark a packet in the presence of impending congestion. We examine the throughput performance of ECN flows and the unfairness among the ECN flows. We show the throughput performance of new TCP flows with ECN and also study the interaction between ECN and non-ECN flows.