CSIT560 Internet Infrastructure: Switches and Routers

1. CSIT560Internet Infrastructure:Switches and Routers Active Queue Management Presented By: Gary Po, Henry Hui and Kenny Chong

2. Agenda • Why AQM? • In Considerations • AQM Algorithms • Commercial Effort • Conclusions

3. Why AQM? • Two Classes of Router Algorithms for Congestion Control • What is Congestion? • Congestion avoidance in TCP, is it good enough? • Our Choice : Active Queue Management

4. What do we consider when implementing AQM? • QoS • Keep Average queue size small • Bounded Delay • Link Utilization • Avoid Global Synchronization • Absorbs bursts without dropping packets • Fairness • Punishes misbehaving flows • Prevent bias against bursty connections • Implementation • Ease of Configurations • Buffer Size Requirement (Large or Small) • Per-flow State Information • Computational Overhead

5. AQM Algorithms

6. FIFO + Drop Tail • Problems: • No isolation • No policing • Large queues for high utilizations • Synchronization problem • Lock-out problem

7. Min thresh Max thresh Average queue length RED (Random Early Detection) • FIFO scheduling Define Two Threshold Values Make Use of Average Queue Length Case 1: Average Queue Length < Min. Thresh Value Admit the New Packet

8. p p 1-p 1-p RED (Cont’d) Min thresh Max thresh Average queue length Case 2: Average Queue Length between Min. and Max. Threshold Value Admit the New Packet With Probability p… Or Drop the New Packet With Probability 1-p

9. Average queue length RED (Cont’d) Min thresh Max thresh Case 3: Avg. Queue Length > Max. Threshold Value New Packet will be dropped As no new packets can be admitted, the average queue length decreases. Until the average queue length drops below the max threshold value New packet could be admitted with a probability p… or being dropped with a probability 1-p…

10. RED Flow Diagram

11. Delay is bounded RED (Cont’d) • Queue Size versus Time RED: Queue Size Global Synchronization solved

12. Unfairness of RED An unresponsive flow occupies over 95% of bandwidth Unresponsive Flow (such as UDP) 32 TCP Flows 1 UDP Flow

13. CHOKe(CHOose and Keep) • Based on RED • Simple • Designed for fairness • Penalize the unresponsive flow

14. CHOKe (Cont’d) • Mechanism

15. Min thresh Max thresh Average queue length CHOKe (Cont’d) Case 1: Average Queue Length < Min. Thresh Value Admit the New Packet

16. p 1-p Average queue length CHOKe (Cont’d) Min thresh Max thresh Case 2: Avg. Queue Length is between Min. and Max. Threshold Values A packet is randomly chosen from the queue to compare with the new arrival packet If they are from different flows, the same logic in RED applies If they are from the same flow, both packets will be dropped

17. Average queue length CHOKe (Cont’d) Min thresh Max thresh If they are from different flows, the new packet will be dropped Case 3: Avg. Queue Length > Max. Threshold Value A random packet will be chosen for comparison If they are from the same flow, both packets will be dropped

18. Evaluate CHOKe’s performance using NS-2

19. Simulation Scenario source destination 10Mbps 10Mbps UDP UDP 1Mbps router router TCP TCP • Topology: Dumb-bell • Metrics: throughput and queue size

20. Performance of CHOKe Unresponsive Flow (UDP) Fair Share Level Bandwidth is evenly shared 32 TCP Flows 1 UDP Flow

21. Parameters • Number of responsive/unresponsive flows • Transfer rate of different flows • Number of random candidates chosen for comparison

22. CHOKe Simulation • Different Parameters, different performance CHOKe-1 32 TCPs 1 UDP CHOKe-2 32 TCPs, 3 UDPs of different rate CHOKe-2 32 TCPs, 3 UDPs of same rate CHOKe-2 32 TCPs, 1 UDP of high rate

23. Evolutions of AQM Algorithms • RED • Merits • Early congestion detection • No bias against bursty traffic • No global synchronization • Drawbacks • Difficulty in parameter setting • Insensitivity to traffic load and drain rates • SRED • Merits • Stabilized queue occupancy • Protection from misbehaving flows • Drawbacks • Some per-flow state (zombie list) • RED disadvantages • FRED • Merits • Good protection from misbehaving flows • Drawbacks • Per-flow state • RED disadvantages • BLUE • Merits • Simplicity • High throughput • Drawbacks • No early congestion detection (Pdrop updated only on queue overflow or link idle events) • Slow response and dependence on history • REM • Merits • Low delay and small queues • Independence of the number of users • Drawbacks • Some complexity due to parameters • Low throughput for Web traffic • Inconsistency with TCP sender mechanism; works best with ECN • LDC • Merits • Sensitivity to traffic load and drain rate • Low delay • Target delay achieved • Intuitive parameters, meaningful to users (target delay) • Drawbacks • Some complexity due to parameters • Low throughput in some cases REM, AVQ, PI Controller FIFO+ DropTail BLUE SFB CHOKe SAC RED SRED FRED

24. Commercial Efforts & Conclusion

25. Commercial Efforts & Conclusion (Cont’d) • “Applying AQM over 3G wireless network”– a paper supported by Motorola Canada Ltd. (Mar. 2003) • 3G network, real-time applications have hard time deadlines for packet delivery at the receiver. • Use AQM to avoid long queuing delay and prevent expiring packets.

26. Commercial Efforts & Conclusion (Cont’d) • AQM improves overall system performance by increasing throughput and reducing end-to-end delay.

27. Commercial Efforts & Conclusion (Cont’d) • “Effect of AQM on Web Performance”– a paper supported by Cisco Systems and IBM. (Aug. 2003) • Proportional Integrator (PI) controller • Random Exponential Marking (REM) controller • Adaptive Random Early Detection (ARED). • IETF proposed standard : Explicit Congestion Notification (ECN)

28. Commercial Efforts & Conclusion (Cont’d) • ECN has significant impact with AQM scheme in web performance. • Many researches and efforts are going on in the field of AQM. • Simple and Easy to implement