Transcript One Decoding Step - University of Wisconsin

A Signal Analysis of Network
Traffic Anomalies
Paul Barford
with Jeffery Kline, David Plonka, Amos Ron
University of Wisconsin – Madison
Summer, 2002
Motivation
• Traffic anomalies are a fact of life in computer networks
– Outages, attacks, etc…
• Anomaly detection and identification is challenging
– Operators typically monitor by eye using SNMP or IP flows
• Obviously, this does not scale!
– Simple thresholding is ineffective
– Some anomalies are obvious, other are not
• Characteristics of anomalous behavior in IP traffic are
not well understood
– Do same types of anomalies have same characteristics?
– Can characteristics be effectively used in detection systems?
2
Introduction
• Objective: Improve our understanding network traffic
anomalies
• Approach: Wavelet analysis of data set that includes IP
flow data, SNMP data and a catalog of observed
anomalies
• Method: Integrated Measurement Analysis Platform for
Internet Traffic (IMAPIT)
• Results: We demonstrate how anomalies can be
exposed using wavelets and develop new method for
exposing short-lived events
3
Related Work
• Network traffic characterization
– Eg. Caceres89, Leland93, Paxson97, Zhang01
• Focus on typical behavior
– Abry98 use wavelets to analyze LRD traffic
• Fault and anomaly detection techniques
– Eg. Feather93, Brutlag00
• Focus on thresholds and time series models
– Eg. Paxson99
• Rule based tool for intrusion detection
– Eg. Moore01
• Backscatter technique can be used to identify DoS attacks
– Eg. Huang01
• Wavelet-based approach to detecting network performance problems
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Simple Network Management Protocol
• SNMP is the standard protocol for monitoring/managing
networked systems
• SNMP defines a set of MIB (management information
base) data exported from routers
– RFC2863
• We sample High Capacity Interface using MRTG (MultiRouter Traffic Grapher) at 5 minute intervals
– Archive byte and packet traffic in each direction
– 64-bit counters on each of 15 WAN links
• SNMP count precision is yet to be determined…
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IP Flows
• An IP Flow is defined as a unidirectional series of
packets between source/dest IP/port pair over a period of
time
{SRC_IP/Port,DST_IP/Port,Pkts,Bytes,Start/End Time,TCP Flags,IP Prot …}
– Exported by Lightweight Flow Accounting Protocol (LFAP)
enabled routers (Cisco’s NetFlow, Juniper cflowd flow export)
• We use FlowScan [Plonka00] to collect and post-process
IP flow data collected at 5 minute intervals
– Combines flow collection engine, database, visulaization tool
– Provides a near real-time visualization of network traffic
– Breaks down traffic into well known service or application
6
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Our Approach to Data Gathering
• Consider anomalies in IP flow and SNMP data
– Collected at UW border router (Juniper M10)
– Archive of ~6 months worth of data (packets, bytes, flows)
– Includes catalog of anomalies (after-the-fact analysis)
• Group observed anomalies into four categories
– Network anomalies (41)
• Steep drop offs in service followed by quick return to normal behavior
– Flash crowd anomalies (4)
• Steep increase in service followed by slow return to normal behavior
– Attack anomalies (46)
• Steep increase in flows in one direction followed by quick return to normal
behavior
– Measurement anomalies (18)
• Short-lived anomalies which are not network anomalies or attacks
8
Our Approach to Analysis
• Wavelets provide a means for describing time series
data that considers both frequency and time
– Particularly useful for characterizing data with sharp
spikes and discontinuities
• More robust than Fourier analysis which only shows what
frequencies exist in a signal
– Tricky to determine which wavelets provide best
resolution of signals in data
• We use tools developed at UW which together make
up IMAPIT
– FlowScan software
– The IDR Framenet software
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Our Wavelet System
• After evaluating different candidates we selected a
wavelet system called Pseudo Splines(4,1) Type 2.
– A framelet system developed by Daubechies et al. ‘00
– Very good frequency localization properties
• Three output signals are extracted from input
– Low Frequency (L): synthesis of all wavelet coefficients
from level 9 and up
– Mid Frequency (M): synthesis of wavelet coefficients 6, 7, 8
– High Frequency (H): synthesis of wavelet coefficients 1 to 5
• Thresholding (set to zero all coefficients whose absolute value is below
a threshold) is used on these coefficients
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Ambient IP Flow Traffic
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Ambient SNMP Traffic
12
Byte Traffic for Flash Crowd
13
Average Packet Size for Flash Crowd
14
Flow Traffic During DoS Attacks
15
Byte Traffic During Measurement Anomalies
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Anomaly Detection via Deviation Score
•
We develop an automated means for identifying shortlived anomalies based on variability in H and M signals
1. Compute local variability (using specified window) of H and
M parts of signal
2. Combine local variability of H and M signals (using a
weighted sum) and normalize by total variability to get
deviation score V
3. Apply threshold to V then measure peaks
•
Our analysis shows that V peaks over 2.0 indicate
short-lived anomalies with high confidence
–
We threshold at V = 1.25 and set window size to ~3 hours
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Deviation Score for Three Anomalies
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Deviation Score for Network Outage
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Anomalies in Aggregate Signals
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Hidden Anomalies in Low Frequency
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Deviation Score Evaluation
• How effective is deviation score at detecting anomalies?
– Compare versus set of 39 anomalies
• Set is unlikely to be complete so we don’t treat false-positives
– Compare versus Holt-Winters Forecasting
• Sophisticated time series technique
• Requires some configuration
• Holt-Winters reported many more positives and sometimes
oscillated between values
Total
Candidate
Anomalies
Candidates
detected by
Deviation
Score
Candidates
detected by
Holt-Winters
39
38
37
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Conclusion and Next Steps
• We present an evaluation of signal characteristics of network
traffic anomalies
– Using IP flow and SNMP data collected at UW border router
• 106 anomalies have been grouped into four categories
– IMAPIT developed to apply wavelet analysis to data
– Deviation score developed to automate anomaly detection
• Results
– Characteristics of anomalies exposed using different filters
and data
– Deviation score is effective detection method
• Future
– Development of anomaly classification methods
– Application of results in (distributed) detection systems
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