Bufferbloat is excessive delay due to the accumulation of packets in a router’s oversized queues. CoDel and PIE are two recent Active Queue Management (AQM) algorithms that have been proposed to address bufferbloat by reducing the queuing delay while trying to maintain a high bottleneck utilization. This paper fills a gap by outlining what are the operating ranges, that is the network characteristics (in terms of round-trip times and bottleneck capacity), for which these algorithms achieve their design goals. This new approach to the problem lets us identify deployment scenarios where both AQM schemes result in poor performance when used with default parameters. Because PIE and CoDel have been proposed with RED’s deployment issues in mind, it was essential to evaluate to what extent we can tune them to achieve various trade-offs and let them control the queuing delay outside their default operating range. We find that, by appropriate tuning (1) the amount of buffering can easily be controlled with PIE, (2) the Round Trip Time (RTT) sensitivity of CoDel can be reduced. Also, we observe there is more correlation between the congestion level, the achieved queuing delay and the targeted delay with CoDel than with PIE. This paper therefore concludes there is no single overall best AQM scheme, as each scheme proposes a specific trade-off.

A number of recently proposed Active Queue Management (AQM) mechanisms instantiate shallow buffers with burst tolerance to minimise the time that packets spend enqueued at a bottleneck. However, shallow buffering causes noticeable TCP performance degradation as a path’s underlying round trip time (RTT) heads above typical intra-country levels. Using less-aggressive multiplicative backoffs in TCP can compensate for shallow bottleneck buffering. AQM mechanisms may either drop packets or mark them using Explicit Congestion Notification (ECN), depending on whether the sender marked packets as ECN-capable. While a drop may therefore stem from any type of queue, an ECN-mark indicates that an AQM mechanism has done its job, and therefore the queue is likely to be shallow. We propose ABE: “Alternative Backoff with ECN”, which consists of enabling ECN and letting individual TCP senders back off less aggressively in reaction to ECN-marks from AQM-enabled bottlenecks. Using controlled testbed experiments with standard NewReno and CUBIC flows, we show significant performance gains in lightly-multiplexed scenarios, without losing the delay-reduction benefits of deploying AQM. ABE is a sender-side-only modification that can be deployed across networks incrementally (requiring no flag-day) and offers a compelling reason to deploy and enable ECN across the Internet.

In order to evaluate the performance of multi-path transport protocols, a straightforward initial step is to perform simulations. OMNeT++, together with the INET Framework, provide a powerful Open Source platform for running network simulations. This talk provides an overview of simulating multi-path transport with OMNeT++ and the INET Framework. Particular focus is on the Concurrent Multipath Transfer extension for the Stream Control Transmission Protocol (SCTP). Furthermore, useful additions like the NetPerfMeter application model, the extended network auto-configurator as well as the Simulation Processing Tool-Chain (SimProcTC) are explained.