L9 - Measurement II: Strategies, Pitfalls, Platforms Nick...

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Measurement II: Strategies, Pitfalls, Platforms Nick Feamster CS 7260 February 12, 2007
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2 Internet Measurement Process of collecting data that measure certain phenomena about the network Should be a science Today: closer to an art form Key goal: Reproducibility “Bread and butter” of networking research Deceptively complex Probably one of the most difficult things to do correctly
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3 The Importance of Context: Case Studies with Routing Data
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4 Context Pitfall: AS-Level Topologies Question: What is the Internet’s AS-level topology? Strawman approach Routeviews routing table dumps Adjacency for each pair of ASes in the AS path Problems with the approach? Completeness: Many edges could be missing. Why? Single-path routing Policy: ranking and filtering Limited vantage points Accuracy Coarseness
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5 Context Pitfall: Routing Instability Question: Does worm propagation cause routing instability? Strawman approach: Observe routing data collected at RIPE RIRs Correlate routing update traffic in logs with time of worm spread Finding: Lots of routing updates at the time of the worm sprreading! (Bogus) conclusion: Worm spreading causes route instability Missing/Ignored Context: Instability + eBGP multihop … Cowie et al., “ Global Routing Instabilities Triggered by Code Red II and Nimda Worm Attacks”
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6 Strategy: Examine the Zeroth-Order Paxson calls this “looking at spikes and outliers” More general: Look at the data, not just aggregate statistics Tempting/dangerous to blindly compute aggregates Timeseries plots are telling (gaps, spikes, etc.) Basics Are the raw trace files empty? Need not be 0-byte files ( e.g., BGP update logs have state messages but no updates) Metadata/context: Did weird things happen during collection (machine crash, disk full, etc.)
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7 Strategy: Cross-Validation Paxson breaks cross validation into two aspects Self-consistency checks (and sanity checks) Independent observations Looking at same phenomenon in multiple ways What are some other examples of each of these?
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8 Example Sanity Checks Is time moving backwards? PS1’s IGP packet trace Paxson’s probing example Typical cause: Clock synchronization issues Has the the speed of light increased? E.g. , 10ms cross-country latencies Do values make sense? IP addresses that look like 0.0.1.2 indicate bug
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BGP Routing Updates: Example Accuracy issue: Old versions of Zebra would not process updates during a table dump…buggy timestamps. TIME: 07/06/06 19:49:52 TYPE: BGP4MP/STATE_CHANGE PEER: 18.31.0.51 AS65533 STATE: Active/Connect TIME: 07/06/06 19:49:52 TYPE: BGP4MP/STATE_CHANGE PEER: 18.31.0.51 AS65533 STATE: Connect/Opensent TIME: 07/06/06 19:49:52 TYPE: BGP4MP/STATE_CHANGE PEER: 18.31.0.51 AS65533 STATE: Opensent/Active TIME: 07/06/06 19:49:55 TYPE: BGP4MP/MESSAGE/Update FROM: 18.168.0.27 AS3 TO: 18.7.14.168 AS3
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L9 - Measurement II: Strategies, Pitfalls, Platforms Nick...

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