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Enhancing Link Capacity Estimates with CapProbe: A Study of Passive and Active Approaches

Investigating the accuracy of link capacity estimates using CapProbe in conditions of varying loads and cross-traffic interference. Analyzing the effects of dispersion expansion and compression on capacity estimation through packet pair techniques. Comparing active and passive CapProbe methods and discussing the implications for future improvements in capacity estimation.

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Enhancing Link Capacity Estimates with CapProbe: A Study of Passive and Active Approaches

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  1. Accuracy of Link Capacity Estimates using Passive and Active Approaches with CapProbe Rohit Kapoor, Ling-Jyh Chen, M. Y. Sanadidi, Mario Gerla Dept. of Computer Science, University of California at Los Angeles

  2. Ideal Case: Packet pair Techniques ISCC 2004

  3. Packet Pair and Train Dispersion ISCC 2004

  4. Packet Pairs Bandwidth Histogram Packet-pair estimates: multimodality with cross traffic: • Light load conditions(20%) (b) heavy load conditions(80%) • SCDR is caused by dispersion expansion • PNCM is caused by dispersion compression ISCC 2004

  5. Packet Train Bandwidth Histogram • As trains get longer, get “Asymptotic Dispersion Rate” or ADR • ADR is not equal to Residual (available) Capacity • We found and proved a physical interpretation (to be published): ADR is the flow share, when merges are proportional to arrival rates at each link • Dovrolis’ results obtained for non-responsive cross traffic flows ISCC 2004

  6. CapProbe: The Main Idea • Observation: Both expansion and compression of dispersion involve queuing due to cross traffic: • Dispersion expansion => second packet queued more • Dispersion compression => first packet queued more • Packet pair with minimal end-to-end delay sum, is likely to be dispersed corresponding to narrow link capacity • Looking for packet pair with minimal delay sum is inexpensive • CapProbe appears accurate in most of our experiments, simulations and measurements • CapProbe fails under heavy (~>75%) utilization by non-responsive (UDP) traffic ISCC 2004

  7. CapProbe • Ideal Case: no cross traffic • Real Case: dispersion may be compressed or expanded by the cross traffic Under-estimation due to expansion Over-estimation due to compression ISCC 2004

  8. CapProbe • Both expansion and compression are due to queuing • A Packet-Pair sample with Minimal Delay Sum can be used for Capacity Estimation ISCC 2004

  9. Wireless Measurements • Bad channelretransmissionlarger dispersionslower estimated capacity Results for Bluetooth-interfered 802.11b, TCP cross-traffic ISCC 2004

  10. Comparison to Earlier Tools ISCC 2004

  11. Implementation Issues • User vs. Kernel Mode generation of probes and measurements • End systems processing speed • Probe packet size ISCC 2004

  12. Testbed • User mode and kernel mode implementations • Slow system: Pentium II 500MHz CPU; Fast system: Pentium IV 1.8 GHz CPU • Probe packet sizes varied from 500 Bytes to 5K Bytes ISCC 2004

  13. Measurement Experiments on Internet Unit: Mbps ISCC 2004

  14. Discussion • For high speed networks, either a high time resolution machine or a large probing packet size is needed for accuracy • Fine resolution may not be possible in user mode • A large packet size increases the chances expansion of dispersion • Required time resolution T = pksize / C: Required time resolution for accurate estimation ISCC 2004

  15. Passive CapProbe • CapProbe is an active approach and using ICMP packets. • Passive approach is less intrusive, thus more scalable • Passive CapProbing within TCP requires back to back TCP packet transmission • Simulation => 15~20% of TCP data packets are sent back-to-back ISCC 2004

  16. Simulation • The network topology used in our simulations consists of a six-hop path with capacities {10, 7.5, 5.5, 4, 6 and 8} Mbps. • DelACK is disabled in the simulation. • Different cross traffic are used, with packet size 1000 bytes and 200 bytes. ISCC 2004

  17. Active CapProbe Passive CapProbe ISCC 2004

  18. Conclusion • Either a high time resolution machine or a large probing packet size is necessary for accurate capacity estimation • Passive CapProbing within TCP is feasible, minor TCP sender modification helps a lot (future work) • Other Future work: • Experiments at speeds higher than 100 Mbps • Passive CapProbing in TFRC, other applications • Use of capacity estimates in TCPW and overlays construction ISCC 2004

  19. Improving Wireless Link Throughput via Interleaved FEC T h a n k s ISCC 2004

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