Experimental analysis of the impact of RTT differences on the throughput of two competing TCP flows The network phase effect explored Maarten Burghout Individual Research Assignment, June 2008 Committee Dr. ir. Pieter-Tjerk de Boer Dr. ir. Geert Heijenk Abstract Two TCP flows sharing a common bottleneck link, obtain a fraction of the available throughput, based on the (inverse) ratio of their roundtrip times (RTT): the higher the RTT, the less throughput. This report investigates how a higher RTT can deliver a higher throughput, in contrast with normal TCP calculation rules. Underlying cause for this deviation of regular throughput predictions is the fact that modern TCP traffic shows a strong periodicity at bottleneck links, as a result of most packets carrying maximum payload. Traffic sources that tune their RTT to be in phase with the transmissions at that bottleneck are known to obtain a higher throughput at that link. This phenomenon is called the Network Phase Effect. In this report, results of a series of experiments that address this phenomenon are discussed. A physical implementation of a simple network was constructed to further investigate preliminary simulation results. A theoretical analysis is also given, with conditions for the phase effect to occur derived and tested against physical experiments. These experiments confirm the possibility of a higher throughput on a bottleneck link, as a result of RTT tuning according to phase effect conditions. 2 Table of contents 1 INTRODUCTION ....................................................................................................................................... 5 1.1 SCOPE ...................................................................................................................................................... 5 2 PROBLEM DEFINITION ............................................................................................................................. 6 2.1 SIMULATION SETUP .................................................................................................................................... 6 2.2 NS2 SIMULATION RESULTS ........................................................................................................................... 7 3 ANALYSIS ................................................................................................................................................. 8 3.1 NEUTRAL SITUATION ................................................................................................................................. 10 3.2 CONGESTION WINDOW INCREASE ................................................................................................................ 10 3.3 DELAY .................................................................................................................................................... 10 3.4 THE NETWORK PHASE EFFECT ...................................................................................................................... 11 3.5 NS2 SETUP RESULTS EXPLAINED ................................................................................................................... 11 4 EXPERIMENTS ........................................................................................................................................ 13 4.1 THE ASSIGNMENT : TESTS TO BE CONDUCTED ................................................................................................. 13 4.2 TEST ENVIRONMENT ................................................................................................................................. 13 4.3 MEASUREMENT METHOD ........................................................................................................................... 13 4.4 COMPARISON TO NS2 SETUP AND PREDICTIONS ............................................................................................. 14 4.5 DIFFERENT TCP FLAVORS ........................................................................................................................... 14 5 RESULTS ................................................................................................................................................ 16 5.1 DATA PROCESSING .................................................................................................................................... 16 5.2 PHASE EFFECT VISIBLE ............................................................................................................................... 16 5.3 MEASUREMENT ABNORMALITIES ................................................................................................................. 17 5.3.1 Small payload packets .................................................................................................................... 17 5.3.2 Terminated measurements ............................................................................................................. 17 5.4 PHASE EFFECT FOR DIFFERENT TCP FLAVORS .................................................................................................. 18 5.4.1 Equal tripping points ....................................................................................................................... 19 5.4.2 Bandwidth sharing ratio ................................................................................................................. 19 5.4.3 Low delay and Scalable ................................................................................................................... 20 5.5 COMPARISON OF DIFFERENT FLAVORS .......................................................................................................... 20 5.6 EQUAL DELAY SETTINGS ............................................................................................................................. 21 6 CONCLUSIONS ....................................................................................................................................... 24 6.1 PHASE EFFECT POSSIBLE IN LAB ENVIRONMENT ............................................................................................... 24 6.2 DIFFERENT TCP FLAVORS AGAINST EACH OTHER ............................................................................................. 24 6.3 PREDICTIONS ON (SEMI -)PUBLIC NETWORKS .................................................................................................. 24 APPENDIX A - DEFINING GOODPUT................................................................................................................ 25 APPENDIX B – NS2 GIVES FAULTY/UNEXPECTED BEHAVIOR IN A SPECIFIC CASE ............................................ 26 WHY ................................................................................................................................................................... 26 NETWORK SETUP ................................................................................................................................................... 26 NETWORK PROPERTIES ........................................................................................................................................... 26 DESCRIPTION OF THE PROBLEM ................................................................................................................................ 26 TEST RESULTS ....................................................................................................................................................... 27 Both flows start at 0 ..................................................................................................................................... 27 3 Starting both flows separate ........................................................................................................................ 28 Conclusion .................................................................................................................................................... 29 LITERATURE ................................................................................................................................................... 30 4 1 Introduction In today’s network topology, a not-uncommon scenario has two or more TCP flows sharing a common bottleneck link, e.g. multiple users on a LAN, sharing a DSL link. When competing for bandwidth on that bottleneck, the round-trip time (RTT) is a determining factor for the throughput obtained by those flows. In general, a higher RTT means a lower throughput. Under general conditions, flows share the available bandwidth by the inverse of their RTT ratio. In this project, we will investigate how tuning the RTT for a flow can help in obtaining a higher throughput. Simulations in the Ns2 network simulator suggest that by opting for a higher RTT, a flow can actually get more throughput than its competitors (which do not tune their RTT). Main focus for this project is a physical implementation of such a setup and parameter strategy. Measurements will be done to see whether a higher RTT will give a flow a greater share of the available bandwidth, or even make it the sole user on the bottleneck, as some simulations suggest. 1.1 Contents The cause for this assignment was a set of simulation results on the Ns2 network simulator, which was believed to be erroneous. These simulation results are discussed in chapter 2, with a theoretical analysis for this (correct!) behavior given in chapter 3. Based on this analysis, a series of experiments were drawn up, as is discussed