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Monitoring Persistently Congested Internet Links

Monitoring Persistently Congested Internet Links. Leiwen (Karl) Deng Aleksandar Kuzmanovic Northwestern University. http://networks.cs.northwestern.edu. Objective. New probing methods that can improve measurement observability for core congestion

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Monitoring Persistently Congested Internet Links

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  1. Monitoring PersistentlyCongestedInternet Links Leiwen (Karl) Deng Aleksandar Kuzmanovic Northwestern University http://networks.cs.northwestern.edu

  2. Objective • New probing methods that can improve measurement observability for core congestion • Pong – a tool specialized in measuring a subset of non-edge links exhibiting repetitive congestion • Can reveal systematic problems such as routing pathologies, poorly-engineered network policies, or non-cooperative inter-AS relationships • Lightweight: monitoring in addition to on-demand measuring • A building block of a large scale triggered monitoring system for Internet congestion

  3. Repetitive Congestion • We focus on locating and monitoring non-edge links that exhibit repetitive congestion • Queuing delay as congestion indicator • Queue building-up repetitively happens on time scales ofone or more minutes.

  4. Methodology Highlights • Coordinated probing • Probe from both endpoints of a path • Combine end-to-end probes with (TTL limited) probes to intermediate routers • Infer underlying path topology conditions • Implicit inference • Based on measured queuing delays on different probing paths • Use statistics over longer time scales • Quantify measurement accuracy • Link measurability score

  5. f s d b Coordinated Probing Probe S D f (“forward”) probe b (“backward”) probe , , s (“source”) probe d (“destination”) probe , A Simplified Case – Symmetric Path

  6. f s d b Locating Congested Links Tracing Congestion Status Half-path queuing delay Coordinated Probing Probe Δf Δd S D Δs Δb Δfs Δfd

  7. f s d b Infer Underlying Path Topology Conditions Probe Δf Δd S D Δs Δb Condition: Δf +Δb ≈Δs +Δd Path Pattern: 4-p probing scenario

  8. Probe Probe Probe f f Pair up Pair up s s Congestion Congestion D S D S d d Observed by b probe only b Paired d probe b Paired d probe 4-p probing Fsd probing f s Congestion 4-p probing Δf +Δb ≈Δs +Δd D S Fsd probing Δf ≈Δs +Δd Fsb probing Δs ≈Δf +Δb b No suitable d probes to pair up with this s probe 2-p probing unconditional Estimates of half-path queuing delay Fsb probing Infer Underlying Path Topology Conditions Probing technique Condition Δfs Δfd

  9. |(Δf +Δb) − (Δs +Δd)| QoM4p = 1 − max(Δf +Δb, Δs +Δd) Demote |(Δf − (Δs +Δd)| Promote QoMfsd = 1 − max(Δf, Δs +Δd) |(Δs − (Δf +Δb)| QoMfsb = 1 − max(Δs, Δf +Δb) Select Probing Techniques • Adjust probing technique online based on quality of measurability (QoM) Definition of QoM Condition Probing technique 4-p probing Fsd probing Fsb probing 2-p probing Δf +Δb ≈Δs +Δd Δf ≈Δs +Δd Δs ≈Δf +Δb unconditional (Last resort)

  10. Probe Probe Probe Probe Probe Δfd Δfs Δfs Δfd Δfs Δfd Δfd Δfs Δfd Δfs Locating Congested Links Perform coordinated probing for all intermediate nodes S D Probe all nodes simultaneously

  11. Probe Probe Congestion Congested link is identified Locating Congested Links Switch Point Approach S D Correlate probes to neighboring nodes

  12. Congestion f f f f f f f f f f f f f f f Tracing Congestion Status S D Link C (Identified congested link) Congestion Status Link C Time Use fast rate end-to-end probing

  13. 5 6 7 3 4 9 10 11 8 2 1 9 3 8 12 4 10 11 5 7 6 1 2 0.53s on/off 0.37s on/off 0.71s on/off 0.47s on/off 0.83s on/off Emulab Experiment Example Topology: 12 nodes (PCs), 11 links Link: 100Mbps, 2ms Cross traffic: Each consists of 3 parallel TCP flows, 50% time on and 50% time off. Build multiple bottlenecks: Cross traffics are added to corresponding links concurrently.

  14. 5 6 7 3 4 9 10 11 8 2 1 9 3 8 12 4 10 11 5 7 6 1 2 0.53s on/off 0.37s on/off 0.71s on/off Emulab Experiment Example At the Beginning

  15. 5 6 7 3 4 9 10 11 8 2 1 9 3 8 12 4 10 11 5 7 6 1 2 0.53s on/off 0.37s on/off 0.71s on/off 0.47s on/off 0.83s on/off Emulab Experiment Example After adding backward bottlenecks

  16. 5 6 7 3 4 9 10 11 8 2 1 9 3 8 12 4 10 11 5 7 6 1 2 0.53s on/off 0.37s on/off 0.71s on/off 0.29s on/off 0.63s on/off Emulab Experiment Example After adding two more forward bottlenecks

  17. Optimizing Pong in the Internet • Set queuing delay threshold • Based on distribution of queuing delay samples • Tune other parameters based on experiments on the PlanetLab • Minimize measurement errors • Detect and react to anomalies (clock skews, router alterations, ICMP queuing, etc) • Use instantaneous quality of measurement value as sample weight • Quantify measurement quality • Help select vantage points

  18. Quantify Measurement Quality • Help select vantage points • Link measurability score • Probing technique and quality of measurability • Queuing delay threshold quality • Observability score • Congestion observed on a less frequently congested link can be blurred by a much more frequently congested link on the same path.

  19. Conclusion • Pong – a tool specialized in measuring a subset of non-edge links exhibiting repetitive congestion • Coordinated probing • Infer underlying path topology conditions • Select probing techniques online • Quality of measurability • Quantify measurement quality • Link measurability score

  20. Thank you! Questions?

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