Monitoring Persistently Congested Internet Links

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Monitoring Persistently CongestedInternet Links

• Leiwen (Karl) Deng
• Aleksandar Kuzmanovic
• Northwestern University

http//networks.cs.northwestern.edu
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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

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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 of one or more minutes.

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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

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Coordinated Probing
Probe
S
D
f (forward) probe
b (backward) probe
,
,
s (source) probe
d (destination) probe
,
A Simplified Case Symmetric Path
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Coordinated Probing
Probe
?f
?d
S
D
?s
?b
?fs
?fd
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Infer Underlying Path Topology Conditions
Probe
?f
?d
S
D
?s
?b
Condition ?f ?b ?s ?d
Path Pattern 4-p probing scenario
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Infer Underlying Path Topology Conditions
Probing technique
Condition
?fs
?fd
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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
?f ?b ?s ?d ?f ?s ?d ?s ?f ?b unconditional
(Last resort)
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Locating Congested Links
Perform coordinated probing for all intermediate
nodes
S
D
Probe all nodes simultaneously
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Locating Congested Links
Switch Point Approach
S
D
Correlate probes to neighboring nodes
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Tracing Congestion Status
S
D
Link C (Identified congested link)
Congestion Status
Link C
Time
Use fast rate end-to-end probing
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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.
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Emulab Experiment Example
At the Beginning
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Emulab Experiment Example
After adding backward bottlenecks
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Emulab Experiment Example
After adding two more forward bottlenecks
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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

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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.

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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

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Thank you!
Questions?