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Queueing Problem

to next router. (drop-tail) queue. server. packet losses. Queueing Problem. The performance of network systems rely on different delays. Propagation/processing/transmission/queueing delays Which delay is affected most by network congestion? Queueing delay.

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Queueing Problem

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  1. to next router (drop-tail) queue server packet losses Queueing Problem • The performance of network systems rely on different delays. • Propagation/processing/transmission/queueing delays • Which delay is affected most by network congestion? • Queueing delay • How can we control this queue behavior? • How can we control the arrival rate and service rate? • Do we know anything about queueing behavior? • Can we analyze its behavior more rigorously using probability?

  2. packet arrivals time Preliminaries • How do we define the arrival process? • How do we define the service process? • Our assumptions • The arrival process follows a Poisson process. • The service process follows a Poisson process. • In other words, the interarrival times follow an exponential distribution. • Our ultimate interests • Average queue length • Average waiting time

  3. Our Goals • How do we compute the average queue length? • Suppose that the queue has 5 packets with 0.5 probability, 10 packets with 0.3 probability, and 20 packets with 0.2 probability. • What is the average queue length? • Can you generalize this process? • How do we compute the average wait time? • Assume the same probability distribution as the above. • What is the total wait time for a packet arriving with 4 previous packets in the queue? • Can you generalize this process?

  4. to next router (drop-tail) queue server packet losses Queue Mangement • Without proper queue management, we may have the following problems: • Packet drops • Global synchronization • Bias against bursty traffic • How do we manage queue behavior to mitigate these problems?

  5. Pdrop No dropping or marking Drop with P=1 1 Randomly drop some Drop all Pmax Mark with P Linearly increasing From 0 to Pmax Qavg 0 Thmin Thmax Drop Probability P Average Queue Length Random Early Detection (RED) • Try to detect incipient congestion early to reduce the number of packet drops. • Use randomness to mitigate global synchronization and bias against bursty traffic.

  6. Active Queue Management • Why RED good? • Simple • Not require source cooperation. • Not require per-connection state management. • Can identify connections using a large share of bandwidth (misbehaving flows). • Is RED perfect? Of course not. It raised many related questions • Do we count bytes or packets in RED? • RED with drop preference? • Any implementation issues in RED? • Marking or dropping, and TCP? • How about packet loss rate and link utilization? • Is RED effective for Web traffic?

  7. ECN marked Router Source Dest ACKs With ECN Explicit Congestion Notification (ECN) • Problems with non-ECN-compatible equipment: • 2,151 of 24,030 web servers were not accessible to ECN-capable • clients (tests in December 2000 using TBIT[2]).

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