Analysis of RED Goal: impact of RED on loss and delay of bursty (TCP) and less bursty or smooth (UDP) traffic RED eliminates loss bias against bursty traffic by increasing loss of less bursty traffic RED results in higher number of consecutive packet drops, which may lead to global TCP synchronization RED reduces average delay, but increases delay jitter of less bursty flows Thus, applications such as audio may suffer more with RED than Tail Drop Also, loss rate seen by a flow proportional to flow intensity is only true for Poisson arrivals

Bias Against Bursty Traffic Assume bursts (batches) of B packets arriving as Poisson Lower bound on difference between smooth traffic and more bursty (e.g. LRD) traffic Use PASTA (Poisson Arrival See Time Averages) Compute drop probability for TD Assume instantaneous queue size measurements (weight = 1) Approximation 1: same drop probability used on all packets in same burst Lower bound on drop rate Accurate for small min_thresh, high maxP, B << buffer size K Drop probability is always higher with RED than TD For high load, loss probability for RED almost equals that of M/M/1/K Tail Drop

RED with Bursty and Smooth Traffic Bias against bursty traffic with TD RED distributes drops among both types of traffic Both drop rates are equal to average TD drop rate for high load

With Queue Size Averaging With small weight to instantaneous measurement (as recommended), average varies slowly ==> makes approximation 1 valid Drop rate for bursty traffic decreases only when it is a small fraction For a large fraction of bursty traffic (the case in practice), loss rate for non-bursty traffic increases significantly Without PASTA, drop probability for Pareto traffic is different from that for smooth traffic, even for RED

Synchronization of TCP Flows Mainly observed in simulation! Assume Poisson traffic Compute distribution of number of consecutive losses Approximation 2: consecutively dropped packets are dropped with same probability Upper bound on number of consecutive drops Accurate for smooth drop function and high load Mean and variance of number of consecutive drops are lower with RED than TD With queue size averaging, small weight, approximation 2 valid No of consecutive drops is infinite with positive probability RED results in higher mean and variance, which may lead to global synchronization

Queueing Delay For small weight and high offered load, estimated average queue size slowly oscillate around max_thresh RED reduces mean delay, but also add jitter Audio quality will suffer

Simulation Loss probability for TCP does not change between RED and TD Loss rate for UDP increases significantly when going from TD to RED Total TCP throughput does not increase with RED With 100 TCP flows and different RTTs, no global synchronization is observed Average queue with RED stays close to max_thresh Instantaneous queue size varies heavily with time compared to TD Observations hold independent of number of flows TD even has higher throughput when number of flows is small