1. One More Bit Is Enough Yong Xia, RPI Lakshminarayanan Subramanian, UCB Ion Stoica, UCB Shivkumar Kalyanaraman, RPI SIGCOMM’05, August 22-26, 2005, Philadelphia, Pennsylvania, USA

2. VCP Goal 2 Achieve fair bandwidth allocation and high utilization, and minimize packet loss in high bandwidth-delay product (BDP) network  TCP ?  TCP + AQM / ECN?  XCP ?

3. VCP3 Why TCP does not scale?  TCP uses duplicate ACK or timeout: no degree of congestion and instability time congestion window Multiplicative Decrease (MD) Additive Increase (AI) congestion slow start congestion avoidance  AI with a fixed step-size can be very slow for large bandwidth slow!

4. VCP4 TCP does not perform well in high b/w bottleneck utilization round-trip delay (ms) TCP 50% 100% bandwidth (Mbps) bottleneck utilization TCP 70% 100%

5. VCP5 XCP scales x senderreceiverrouter C spare bandwidth DATA ACK  rate  But, XCP needs multiple bits (128 bits in its current IETF draft) to carry the congestion-related information from/to network

6. VCP Goal 6 Design a TCP-like scheme that:  requires a small amount of congestion information (e.g., 2 bits)  scales across a wide range of network scenarios

7. General idea degree of explicit information TCP: no explicit feedback, relay on duplicate ACK and timeout ECN: one bit, underload and overload VCP: two bits, low-load, high-load and overload XCP: multiple bits, report spare bandwidth

8. VCP8 Variable-structure congestion Control Protocol (VCP) senderreceiver x router  Routers signal the level of congestion  End-hosts adapt the control algorithm accordingly traffic ratelink capacity (11) (10) (01) code load factor region low-load high-load overload control Multiplicative Decrease (MD) Additive Increase (AI) Multiplicative Increase (MI) range of interest ACK 2-bit ECN 0 1 scale-free 2-bit ECN

9. VCP9 VCP vs. ECN senderreceiver x router code load factor region overload control Multiplicative Decrease (MD) 0 1  ECN doesn’t differentiate between low-load and high-load regions TCPAQM / ECN (11) (10) (01) low-load high-load Additive Increase (AI) Multiplicative Increase (MI) ACK 2-bit ECN VCP (1) (0) underloadAdditive Increase (AI) ACK 1-bit ECN

10. VCP10 VCP vs. ECN TCP+RED/ECN cwnd utilization time (sec) VCP utilization cwnd time (sec)

11. VCP11 VCP key ideas and properties  Achieve high efficiency, low loss, and small queue  Fairness model is similar to TCP:  Long flows get lower bandwidth than in XCP (proportional vs. max-min fairness)  Fairness convergence much slower than XCP  Use network link load factor as the congestion signal  Decouple efficiency and fairness controls in different load regions overload high-load low-load router fairness control efficiency controlMI AIMD end-host

12. VCP12 Major design issues  At the router  How to measure and encode the load factor?  At the end-host  When to switch from MI to AI?  What MI / AI / MD parameters to use?  How to handle heterogeneous RTTs? control Multiplicative Decrease (MD) Additive Increase (AI) Multiplicative Increase (MI) (11) (10) (01) codeloadregion low-load high-load overload

13. VCP13 Design issue #1: measuring and encoding load factor  Calculate the link load factor  demand load_ factor capacity = link_bandwidth * t  arrival_traffic + queue_size = t  = 200ms  most rtt  The load factor is quantized and encoded into the two ECN bits

14. VCP14 Design issue #2: setting MI / AI / MD parameters ( , ,  ) overload high-load low-load MI AIMD

15. VCP Design issue #2: setting MI / AI / MD parameters ( , ,  ) load factor 100% 80% safety margin 0% 0.875 15 TCP :  = 1.0 VCP :  = 0.06 STCP :  = 0.01 MI AI MD k (1   * ) /  * where k = 0.25 ( for stability) 80% 20% 1.0  = =  = =  = * =* =

16. VCP Design issue #3: Handling RTT heterogeneity for MI/AI 16

17. VCP VCP scales across b/w, rtt, num flows  Evaluation using extensive ns2 simulations 17  150Mbps, 80ms, 50 forward flows and 50 reverse flows

18. Impact of bottleneck capacity vary link capacity from 100Kbps to 5Gbps high utilization, gap comparing to XCP at most 7% at extremely low capacities alpha = 1 is too large for such low capacity

19. Impact of feedback delay fix bottleneck capacity at 150Mbps, vary round-trip propagation delay from 1ms to 1500ms utilization > 90% average queue < 5% maximal queue < 15% no packet loss lower utilization due to comparable small measurement interval, i.e. t p = 200ms << RTT

20. Impact of number of long-lived flows increase the # forward FTP flows small queue length, even outperform XCP high utilization zero packet drop

21. Impact of short-lived traffic arrive according to Poisson process, avg. arrival rate varying from 1/s to 1k/s, transfer size obeys Pareto distribution with avg. 30 packets high utilization small queue length zero packet drop

22. Multiple bottlenecks typical parking-lot topology as good as single-bottleneck scenarios zero packet drop < 0.2% buffer size average queue length

23. Fairness (a)same RTT (b)small RTT difference (c)huge RTT difference even distribution among all flows in case (a) and (b) fairness discrepancy due to high value of MI and AI, whose value is bound in implementation to prevent burst

24. Convergence behavior introduce 5 flows one after another high utilization during the whole period use router buffer size to scale queue length axis; low queue length during the whole period

25. Sudden demand change 150 flows join at 80s and leave at 160s high utilization remain much lower than full size

26. VCP Conclusions 26  With a few minor changes over TCP + AQM / ECN, VCP is able to approximate the performance of XCP  High efficiency  Low persistent bottleneck queue  Negligible congestion-caused packet loss  Reasonable (i.e., TCP-like) fairness

27. VCP VCP comparisons 27  Compared to TCP+AQM/ECN  Same architecture (end-hosts control, routers signal)  Router congestion detection: queue-based  load-based  Router congestion signaling: 1-bit  2-bit ECN  End-host adapts (MI/AI/MD) according to the ECN feedback  End-host scales its MI/AI parameters with its RTT  Compared to XCP  Decouple efficiency/fairness control across load regions  Functionality primarily placed at end-hosts, not in routers

28. VCP28 THE END

29. VCP Parameter Setting

30. Modified from www.ecse.rpi.edu/Homepages/ shivkuma/research/papers/vcp05.ppt Remark