1. Modeling of Web/TCP Transfer Latency Yujian Peter Li January 22, 2004 M. Sc. Committee: Dr. Carey Williamson Dr. Wayne Eberly Dr. Elena Braverman Department of Computer Science, University of Calgary

2. 2 Outline  TCP Overview and Related Work  The Proposed TCP Transfer Latency Model  Model Validation by Simulation  Extending the Proposed Model to CATNIP TCP  Conclusions  Motivation and Objectives

3. 3 Motivation  Web response time  Highly dominated by TCP performance  Understanding the sensitivity of TCP to network conditions helps to improve TCP performance  No work on modeling CATNIP TCP

4. 4 Objectives  To survey and compare existing TCP models  To develop an accurate model for short-lived TCP flows  To model CATNIP TCP

5. 5 SYN SYN/ACK ACK FIN FIN/ACK ACK TCP Overview Characteristics Reliable, in-order byte stream Flow control Connection-oriented Congestion Control Web Browser Web Server DATA

6. 6 TCP Overview Congestion Control When intermediate nodes (routers) become overloaded, the condition is called congestion. The mechanisms to solve the problem are called congestion control.

7. 7 TCP Overview – Congestion Control Slow Start & Congestion Avoidance  Slow start: cwnd=cwnd+1 for every received ACK  Congestion avoidance: cwnd = cwnd + 1/cwnd

8. 8 Related Work  TCP Steady State Throughput Model  [Padhye et al. 1998]  TCP Response Time Models  Cardwell-00 Model[Cardwell et al. 2000]  Padhye Model[Cardwell et al. 1998]  Cardwell-98 Model[Cardwell et al. 1998]  Sikdar Model[Sikdar et al. 2001]

9. 9 The Proposed TCP Response Time Model Assumptions  Bernoulli packet loss model, i.e., packet is independently lost with fixed probability p  Congestion avoidance algorithm ignored, i.e., cwnd always increases by one upon receiving one ACK (exponentially)  Packet loss can be via RTO or triple duplicate ACKs  The effect of delayed ACK, T delay, is added when necessary

10. 10 The Proposed Model (Cont’d) Congestion Window Evolution

11. 11 Simulation Experiments Network Topology

12. 12 FactorLevels Transfer Size1KB, 4KB, 8KB, 16KB, 32KB, 50KB, 64KB, 90KB, 110KB, 128KB, 160KB, 180KB, 200KB Packet Loss Probability1%, 3%, 5%, 8%, 10% Simulation Experiments Metric & Experimental Factors  Performance Metric: Data Transfer Time, the time from when the sender sends the first packet until the time when the sender receives the ACK of the last data packet.  Experimental factors and levels

13. 13 Simulation Results Short-lived Flows ( p=3%)(p=10%)

14. 14 CATNIP TCP C. Williamson and Q. Wu : “A Case for Context-Aware TCP/IP”. ACM Performance Evaluation Review, Vol. 29, No. 4, pp. 11-23, March 2002.  Convey application-layer context information to TCP/IP  Not all packet losses created equal IP TCP HTTP Document Size Packet Loss Priority

15. 15 CATNIP TCP v.s. Partial CATNIP TCP PacketsFirst three Last three cwnd<3 CATNIP Partial CATNIP 

16. 16 CATNIP TCP v.s. Partial CATNIP TCP p=3% p=5% p=10% PDFCDF

17. 17 Modeling Partial CATNIP TCP Short-lived Flows ( p=3% p’=0%)(p=10% p’=0%)

18. 18 Conclusions  The proposed TCP latency model fits the simulation results better than earlier models.  The differences between Partial CATNIP and CATNIP are minimal when p<10%.  Partial CATNIP TCP model matches the simulation as well.  Partial CATNIP TCP improves TCP latency compared to TCP Reno. For short-lived flows, Partial CATNIP TCP is about 10% faster than TCP Reno in most cases.  CATNIP TCP is a suitable approach to improve TCP Performance.

19. 19 TCP setup a logical end-to-end channel to transfer data HTTP TCP IP Data Link Physical HTTP TCP IP Data Link Physical Web BrowserWeb Server Internet HTTP message TCP segment Motivation Web Response –> TCP Response Time

20. 20 The Proposed Model (Cont’d) The Slow Start Phase

21. 21 The Proposed Model (Cont’d) The Packet Loss Phase If (RAND<=Q(p,w)) Tloss=pss*E[ZTO] // RTO Else Tloss = pss *RTT // fast retransmit

22. 22 Modeling Partial CATNIP TCP Long-lived Flows ( p=3% p`=0%)(p=10% p`=0%)

23. 23 Modelp=1%p=3%p=5%p=8%p=10% Sikdar0.150.410.250.511.06 Cardwell-000.170.440.150.100.15 Proposed0.030.050.080.050.08 Simulation Results Short-lived Flows  Relative error comparison of the proposed model with existing models for short-lived flows

24. 24 Simulation Results Long-lived Flows ( p=3%)(p=10%)