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Advanced Technology LaboratoriesAdvanced Technology Laboratories

Network Performance Monitoring at Small Time Scales

Dina Papagiannaki, Rene Cruz, Christophe Diot

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Motivation

• Network management for large-scale networks almost exclusively relies on SNMP.

• SNMP reports on aggregate link activity for the duration of the polling interval (5 mins).

• Operators provision their network around these values according to provider-specific “acceptable” utilization levels.

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Questions

• Can one infer delay “degradation” from SNMP link counters?

• Can one infer delay performance from output link utilization?– At what time scale should these measurements be

taken?

• How do we summarize such high-resolution measurements in a 5-minute counter?

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Terminology

A micro-congestion episode is a short-lived episode in the lifetime of a link when packets face

increased delays due to crosstraffic.

Metrics: Amplitude, Duration, Frequency

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Measurement Data: Sampling the Output Queue

• Collect packet traces from links attached to the same router (set 1: OC-3, set 2: OC-12)

• Compute single-hop delay using GPS accurate timestamps for arrival and departure

OC-3

OC-3

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Methodology

• Compute link throughput for non-overlapping time intervals of (1ms, 10ms, 100ms, 1s) duration

• Collect all delay samples for each interval• Associate throughput level with delay distribution

d1d2

OutputLink

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Delay performance (OC-3)

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Instantaneous link utilization and delay

• Instantaneous link utilization may be high even when packets do not experience congestion!

NO QUEUEINGDELAY

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5 minutes too long to capture micro-congestion

5 minutes

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Inference of Duration and Frequency

• If a micro-congestion episode persists in time, it should be visible across time scales

• For each time scale τ we count the number of intervals exceeding θ throughput level

• Measure fraction of overloaded intervals within each 5-minute interval

Output link

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Inference of Duration (cntd)

• If the fraction of “congested” intervals exceeding θ at time scale τ+1 is greater than the fraction of “congested” intervals at time scale τ, then significant fluctuations at time scale τ.

Reporting interval

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Duration/Frequency

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Summary

• 5 minute average utilization measurements can hide micro-congestion episodes.

• There is no unique time scale that captures micro-congestion.

• Impact needs to be studied at multiple time scales simultaneously.

• New metric to address network performance monitoring at small time scales.

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Ongoing Work

• We need to identify the impact of– Link capacity

– Traffic arrival pattern

• We have instrumented an entire router and analyze busy periods.