Queueing Problem The performance of network systems rely on different delays. Propagation/processing/transmission/queueing delays Which delay is affected.

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Presentation transcript:

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 server (drop-tail) queue to next router packet losses 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?

Preliminaries How do we define the arrival process? How do we define the service process? time packet arrivals 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

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?

Queue Mangement server (drop-tail) queue to next router packet losses 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?

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. Thmin Thmax Qavg Pmax Pdrop 1 Drop Probability P Randomly drop some Drop all Mark with P Linearly increasing From 0 to Pmax No dropping or marking Drop with P=1 Average Queue Length

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?

Explicit Congestion Notification (ECN) ECN marked Router Source Dest ACKs With 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]).