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
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 ondemand measuring
– A building block of a large scale triggered
monitoring system for Internet congestion
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Karl Deng
Monitoring Persistently Congested Internet Links
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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Karl Deng
Monitoring Persistently Congested Internet Links
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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Karl Deng
Monitoring Persistently Congested Internet Links
Coordinated Probing
Probe
f
S
s
D
d
b
f (“forward”) probe , b (“backward”) probe ,
s (“source”) probe , d (“destination”) probe
A Simplified Case – Symmetric Path
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Karl Deng
Monitoring Persistently Congested Internet Links
Coordinated Probing
Probe
f
S
Δd
s
b
Δb
Δfd
Half-path queuing delay
D
d
Δs
Δfs
Δf
Locating
Tracing
Congested
Congestion
Links
Status
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Karl Deng
Monitoring Persistently Congested Internet Links
Infer Underlying Path Topology Conditions
Probe
f
S
Δf
Δd
s
D
d
Δs
b
Δb
Condition: Δf +Δb ≈Δs +Δd
Path Pattern: 4-p probing scenario
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Karl Deng
Monitoring Persistently Congested Internet Links
Infer Underlying Path Topology Conditions
Probe
f
s
Congestion
Probe
Pair up
Congestion
D
S
d
b
Paired d probe
D
d
Observed by b probe only b
Paired d probe
Fsd probing
Probing technique
Condition
4-p probing
Δf +Δb ≈Δs +Δd
Fsd probingΔfd Δf ≈Δs +Δd
Fsb probing
Pair up
S
4-p probing
Δfs
f
s
Δs ≈Δf +Δb
Estimates of half-path
2-p probing
unconditional
queuing delay
Probe
Congestion
f
s
D
S
b
No suitable d probes to pair up with this s probe
Fsb probing
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Karl Deng
Monitoring Persistently Congested Internet Links
Select Probing Techniques
Adjust probing technique online based on
quality of measurability (QoM)
Definition of QoM
4-p probing
Δf +Δb ≈Δs +Δd
QoM4p = 1 −
|(Δf +Δb) − (Δs +Δd)|
max(Δf +Δb, Δs +Δd)
Δf ≈Δs +Δd
QoMfsd = 1 −
|(Δf − (Δs +Δd)|
max(Δf, Δs +Δd)
Δs ≈Δf +Δb
QoMfsb = 1 −
|(Δs − (Δf +Δb)|
max(Δs, Δf +Δb)
Promote
Condition
Fsd probing
Fsb probing
2-p probing
Demote
Probing technique
unconditional
(Last resort)
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Karl Deng
Monitoring Persistently Congested Internet Links
Locating Congested Links
Perform coordinated probing for all
intermediate nodes
Probe Probe Probe Probe Probe
S
D
Δfs Δfs Δfs Δfs Δfs
Δfd Δfd Δfd Δfd Δfd
Probe all nodes simultaneously
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Karl Deng
Monitoring Persistently Congested Internet Links
Locating Congested Links
Switch Point Approach
Congestion
Probe Probe
S
D
Correlate probes to neighboring nodes
Congested link is identified
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Karl Deng
Monitoring Persistently Congested Internet Links
Tracing Congestion Status
Congestion
f
S
D
Link C (Identified congested link)
Congestion Status
Link C
Time
Use fast rate end-to-end probing
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Karl Deng
Monitoring Persistently Congested Internet Links
Emulab Experiment Example
1
1
2
2
3
3
4
4
0.37s on/off
0.47s on/off
5
5
6
6
7
7
8
8
0.71s on/off
9
9
10
10
11
11
12
0.53s on/off
0.83s on/off
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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Karl Deng
Monitoring Persistently Congested Internet Links
Emulab Experiment Example
1
1
2
0.37s on/off
2
3
3
4
4
5
5
6
6
7
7
8
8
0.71s on/off
9
9
10
10
11
11
12
0.53s on/off
At the Beginning
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Karl Deng
Monitoring Persistently Congested Internet Links
Emulab Experiment Example
1
1
2
2
3
0.37s on/off
0.47s on/off
3
4
4
5
5
6
6
7
7
8
8
0.71s on/off
9
9
10
10
11
11
12
0.53s on/off
0.83s on/off
After adding backward bottlenecks
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Karl Deng
Monitoring Persistently Congested Internet Links
Emulab Experiment Example
1
1
2
0.37s on/off
2
3
3
4
4
5
5
6
6
7
7
8
8
0.71s on/off
9
9
10
10
11
11
12
0.53s on/off
0.29s on/off
0.63s on/off
After adding two more forward bottlenecks
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Karl Deng
Monitoring Persistently Congested Internet Links
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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Karl Deng
Monitoring Persistently Congested Internet Links
Quantify Measurement Quality
Help select vantage points
Link measurability score
1. Probing technique and quality of measurability
2. Queuing delay threshold quality
3. 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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Karl Deng
Monitoring Persistently Congested Internet Links
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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Karl Deng
Monitoring Persistently Congested Internet Links
Thank you!
Questions?
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Karl Deng
Monitoring Persistently Congested Internet Links