1. TCP Probe: A TCP with Built-in Path Capacity Estimation Anders Persson, Cesar Marcondes, Ling-Jyh Chen, Li Lao, M. Y. Sanadidi, Mario Gerla Computer Science Department University of California, Los Angeles

2. March 18, 2005 Global Internet 2005 2 Motivation Knowledge of narrow link capacity is important for: Optimizing network utilization via better congestion control & adaptive streaming Tracking dynamic changes in capacity due to Vertical Handoffs

3. March 18, 2005 Global Internet 2005 3 Bandwidth Estimation Techniques Active Send out-of-band packets into the network CapProbe [SIGCOMM04] and Pathrate [INFOCOM01] Passive Use ongoing data packets without additional overhead to the network TCP Probe – similar to CapProbe technique but uses only actual data packets

4. March 18, 2005 Global Internet 2005 4 CapProbe Recap Packet pair technique Compression & expansion due to cross traffic distorts T b Filter out distorted samples by using samples with minimum end-to-end delay sum Fast and accurate estimation of bottleneck capacity

5. March 18, 2005 Global Internet 2005 5 TCP Probe Idea: Use CapProbe technique but passively within TCP Rely on dispersion of ACK pairs and filter distorted dispersion based on end-to-end delay sum Have to ensure that data packet pairs & corresponding ACK pairs produce accurate capacity estimates

6. March 18, 2005 Global Internet 2005 6 Challenge TCP implementations try to reduce network overhead by sending one ACK for two data packets, but: We want an ACK for each data packet!

7. March 18, 2005 Global Internet 2005 7 Solution Sender sends inverted data packets Receiver replies a duplicate ACK and an incremental ACK

8. March 18, 2005 Global Internet 2005 8 Packet Size Issue Probing packet size: forward direction: TCP data 1500 bytes reverse direction: TCP ACK 40 bytes The newly developed AsymProbe [Networking05] has shown: If, TCP Probe estimates the capacity of the forward direction link If, TCP Probe estimates the capacity of the reverse direction link

9. March 18, 2005 Global Internet 2005 9 Implementation Implemented in NS-2 and Linux 2.4 Network Stack Approach: Periodically mark data packet pairs Sender timestamps the data packets and the corresponding ACKs ( sec granularity)

10. March 18, 2005 Global Internet 2005 10 Simulation Scenario NS-2 simulation Compare TCP Probe capacity estimation with TCPW BE (bandwidth estimation) TCPW BE is based on dispersion of ACK packets without filtering samples

11. March 18, 2005 Global Internet 2005 11 TCP Probe vs. TCPW Capacity Estimation (Mbps) TCP ProbeTCPW BE 20 flows of the same kind TCP Probe: All flows estimate C=10 Mbps TCPW: Flows estimate BE between 0 to 7 Mbps

12. March 18, 2005 Global Internet 2005 12 TCP Probe vs. TCPW 1 TCP Probe and 5 TCP New Reno flows 1 TCPW and 5 TCP New Reno flows TCP Probe TCPW BE Capacity Estimation (Mbps)

13. March 18, 2005 Global Internet 2005 13 Internet Measurement Three different paths: Los Angeles: 3 Mbps (cable modem) China: 45 Mbps Alabama: 100 Mbps

14. March 18, 2005 Global Internet 2005 14 Cable Modem (3 Mbps)

15. March 18, 2005 Global Internet 2005 15 China (45 Mbps)

16. March 18, 2005 Global Internet 2005 16 Alabama (100 Mbps)

17. March 18, 2005 Global Internet 2005 17 Statistical Analysis 30 measurements to each destination

18. March 18, 2005 Global Internet 2005 18 Applications TCP Probe in Vertical Handoff Scenario A vertical handoff involves two different network interfaces Usually represent different technologies and thus result in a drastic change in link capacity How to get advantage of the extra capacity when we are in congestion avoidance? Revised TCP Probe Re-initiate the estimation every n samples (n=50) Detect a huge increase in the capacity estimation Trigger a new slow start phase (i.e. fast rate adaptation)

19. March 18, 2005 Global Internet 2005 19 Simulation Scenario TCP Probe: 1 6 Pareto flows: 7 10, 8 9, 11 14, 12 13 Capacity between node 1 and 2 increases from 10Mbps to 100Mbps at 80 second

20. March 18, 2005 Global Internet 2005 20 Fast Rate Adaptation

21. March 18, 2005 Global Internet 2005 21 Conclusions TCP Probe passively provides to a TCP flow sender an accurate estimate of path capacity Estimated capacity can be used as an upper bound on TCP sender rate Vertical Handoff applications can benefit from detecting capacity change Work in progress TCP Probe provides improved performance under high loss rate (5%) Astart [INFOCOM04] can benefit from accurate capacity estimation

22. March 18, 2005 Global Internet 2005 22 Adaptive ERE in TCPW Sender calculates approximate sending rate ERE (Eligible Rate Estimate) BE (bandwidth estimation) is aggressive RE (rate estimation) is conservative RE ERE BE depending on congestion level dkdk tktk BE: