1. QoS II - Adaptive Virtual Queue - Fair Queueing for Multiple Link 12 th Mar., 2002 Eun-Chan Park CSL, SoEECS, SNU

2. S. Kunniyur, R.Srikant “Analysis and Design of an Adaptive Virtual Queue (AVQ) Algorithm for Active Queue Management,” SIGCOMM 2001

3. Contents Background Congestion control Active Queue Management Related works RED, PI, REM Adaptive Virtual Queue (AVQ) AVQ algorithm Stability analysis Simulation results Conclusion

4. Congestion Control Congestion control schemes  End-to-end control - TCP-Tahoe, TCP-Reno, TCP-Vegas Router supported control - AQM: RED, REM, PI control, AVQ

5. TCP congestion control algorithm Window-based transmission control  sender limits its transmission rate by controlling window size slow-start, congestion avoidance, fast-recovery Feedback Implicit: Timeout, Duplicate ACKs Explicit Congestion Notification (ECN)

6. Active Queue Management Drop-tail queue has several problems Reduce rates only after overflow loss Results in significant packet loss Packet drop could result in a global sync. Active Queue Management Resolves the problem of drop-tail Drops or marks packets at the buffer of router

7. Random Early Detection (RED) S. Floyd and V. Jacobson, “random early detection gateways for congestion avoidance,” IEEE/ACM trans. On networking, vol. 1, pp. 397-413, 1993. Detect congestion using average queue size Intelligently drop/mark packet before buffer overflow

8. RED (cont.) Advantages Prevent global synchronization Reduce packet loss Minimize biases against bursty traffic Simple, low-overhead Disadvantages Difficulty of appropriate parameter setting Sensitive queueing delay and throughput to the traffic load and to parameters Argument in the case of small buffer size

9. Random Exponential Marking (REM) S. Athuraliya et al., “REM: Active Queue Management,” IEEE Network Magazine May/June. 2001 “Match Rate, Clear Buffer” Match aggregated input rate to network capacity Stabilize queue around a small target Sum prices Prices: Differentiated from the calculation of dropping or marking prob. End-to-End marking prob. exponentially increases with the sum of prices

10. REM (Cont.) Link price Updated periodically Depends on mismatches of rate and queue length Marking Prob.

11. Proportional-Integral (PI) Controller C.Hollot et al., “On designing improved controllers for AQM routers supporting TCP flows,” IEEE INFOCOM, 2001 Uses instantaneous queue length, while RED uses EWMA. Proportional to queue length mismatch and to its accumulation (time integral) Equivalent to the price of REM

12. AVQ Algorithm C : link capacity, : virtual capacity VQ : virtual queue VQ=B (buffer size) On a packet arrival, it enqueues VQ, if no rooms available in VQ, marks it updates on each packet arrival If input rate is below than desired rate, VC increases and less aggressive marking Otherwise, VC decreases and more aggressive marking

13. AVQ Algorithm (cont.) At a packet arrival Update VQ size: If VQ+b > B, then mark packet Else, VQ  VQ+b Update VC

14. Properties of AVQ Rate-based marking Provides early feedback Achieves input rate to the desired utilization Regulates utilization instead of queue length as RED,PI,REM Robust to short flows Two design parameters (alpha, gamma) determine robustness and stability

15. TCP/AVQ model for analysis Similar to stochastic fluid-based TCP dynamics [14] Ignores slow-start and time-out of TCP Characterize AIMD Linearize TCP/AQM model

16. Stability Analysis of AVQ (1/3) Main ideas Take Laplace Transform to the linearized TCP/AVQ model Obtain the characteristic equation Find the condition where all roots of char. Eq. is on the LPF.

17. Characteristic Eq. Stability Analysis (2/3)

18. What value of K yields? Can it guarantee the unique d?  Make the condition less strict  Find necessary condition Stability Analysis (3/3)

19. Simulation (1/3) Compare performance (loss, utilization, avg. queue length) of various AQM schemes when traffic load varies

20. Simulation (2/3) Compare responsiveness of AQM schemes when flows are dropped and then established again t= 0 s, N=140 t=100 s, N=35 t=150 s, N=140

21. Simulation (3/3) Investigate the effect of short-flow

22. Conclusion AVQ algorithm is proposed Maintains small queue length with consistent utilization and small loss Robust to short-flows Stability is analyzed relating to control parameters and feedback delay A design guideline is provided However, Simulation result showing the validity of analysis is missed Simulation results are unfair (number of packet drop) Queueing delay is not effectively regulated

23. J. M. Blanquer, B. Ozden “Fair Queueing for Aggregated Multiple Links,” SIGCOMM 2001

24. Contents Introduction & Background GPS, PGPS (WFQ) MSFQ Preliminary properties of MSFQ Bound on packet delay of MSFQ Bound on per-flow service of MSFQ Fairness and MSF 2 Q Applications Conclusion

25. Introduction Fair queueing/scheduling is required due to Increased variety of traffic diverse requirement for QoS limited network resources Fair queueing disciplines based on GPS have been studied considerably in case of single server, however, not in case of multiple server system

26. Generalized Processor Sharing (GPS) L. Kleinrock “Queueing Systems Vol 2: Computer Applications,” Wiley, 1976 GPS server serving N flows is characterized by where, is the amount of traffic for flow i served in With Leaky Bucket algorithm, it guarantees bandwidth share Also provides an end-to-end bounded delay service

27. GPS (Cont.) Idealized discipline that it can not be implemented A server can transmit only one packet at a time, not several packets simultaneously Traffic can not be divided infinitely As a solution to implementation, several realizable schemes proposed Packet-by-packet GPS (PGPS), Weighted Fair Queueing (WFQ) Virtual clock, Self-clocked fair queueing, Start-time fair queueing

28. Packet-by-packet GPS (PGPS) K. Parekh, “A Generalized Processor Sharing Approach to Flow Control in IntServ Network,” IEEE Tran. On Networking, 1993 Also known as Weighted Fair Queueing (WFQ) A. Demers, et al. “Design and Analysis of a Fair Queueing Algorithm,” SIGCOMM 1989 Provides guarantees on throughput and worst-case packet delay Packet delay compared to that of GPS is not grater than the transmission time of one maximum size packet Bits served for each flow do not fall behind corresponding GPS by more than one maximum size packet

29. GPS & PGPS Packet Arrivals of flow 1 and flow 2

30. MSFQ  Multi-server version of WFQ  multi-server version of GPS (MSFQ,N,r) (GPS,1,Nr) Compare how well (MSFQ,N,r) approximates (GPS,1,Nr) in terms of worst-case packet delay amount of traffic served for each flow

31. Preliminary properties of MSFQ: Total service Let the total # of bits serviced in by GPS, MSFQ be and, respectively, then Left ineq. implies: When GPS is busy, MSFQ is busy, too. However, the converse is not true. Right ineq. implies the need for a buffer size of

32. Preliminary properties of MSFQ: Waiting time of packet Upper bound of waiting time for packet k to be scheduled Pf: Consider the worst case: i) The previous packets have occupied all N server just before the arrival of packet k, ii) all servers finish at the same time

33. Bound on packet delay of MSFQ (1) Consider two extreme cases For GPS, best case: (2) with assumption For MSFQ, worst case: (3) (3)-(2) yields (1) Compare it with the delay in single server

34. Bound on per-flow service of MSFQ Maximum difference occurs when flow i becomes idle in GPS a packet of flow i begins transmission in MSFQ Proof done case by case (follow yourself ^^) For total service: For a single server:

35. Fairness of MSFQ The eq. is incomplete to guarantee fairness Why? The eq. does not ensure that the amount of per- flow service does not exceed arbitrary the amount under GPS i.e., there is no lower limit in the eq. To resolve this problem Introduce MSF 2 Q, which is an extended version of WF 2 Q for multi-server system

36. MSF 2 Q (1/3) Queued packets at t=0: ten packets of flow 1 one packet of each flow 2~N  GPS Scheduling  MSF 2 Q Scheduling

37. MSF 2 Q (2/3) Scheduling discipline Define # of outstanding packet of flow i at time t where, outstanding packet is a packet being transmitted of picked for transmission At time t, when a server is idle and there is a packet to serve, MSF 2 Q schedules among flows satisfying:

38. MSF 2 Q (3/3) Properties of MSF 2 Q MSF 2 Q provides the lower bound of difference of per-flow service Similar to WF 2 Q Note that MSF 2 Q is not work-conserving Future work: investigate implement of work- conserving scheduler

39. Applications Ethernet link aggregation Cost-effective and fault tolerant solution for scaling the network capacity IEEE 802.3ad: Standard for Ethernet link aggregation Access of storage I/O RAID system with multiple SCIS channels to improve I/O performance MSF 2 Q is expected to provide QoS guarantee and fair sharing of multiple I/O channels

40. Conclusion Service guarantee and Fairness and for aggregated links have been studied Extended version of PGPS for multiple server has been analyzed in terms of packet delay and per-flow service Proposed a new fair queueing in multiple servers, MSF 2 Q Future works Implementation issues Quantitative comparison to the approach of partitioning flows Extension of hierarchal GPS and servers with different rates