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Seminar 37-310 / Summer Semester 2000

Internet Connectivity

Christian A. Plattner, 25.04.2000

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Internet Connectivity

• On Power-Law Relationships of the Internet Topology

• End-to-End Routing Behaviour in the Internet

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Power-Laws of the Internet

• What does the Internet look like? • Are there any properties that don’t

change in time? • How will it look like a year from now? • How can I generate Internet-like graphs

for my simulations?

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Benefits from Understanding the Topology of the Internet

• Protocol design• More accurate artificial models• Estimates for topological parameters

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Internet Topology

• Two different views:router level vs. interdomain level

• Partitioning the network into AS‘s• Intra- vs. Inter-AS routing

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Router- vs. Interdomain-level

Domain 2

Domain 3

Domain 1

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AS Example: UUnet

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Power-Laws of the Internet

• Power-Law 1: dv rvR

• Power-Law 2: fd dO

• Hop-plot exponent: P(h) hH

• Edges: E = N/(2R + 2) (1-1/NR+1)• Diameter : = (N2 / N + 2E) 1/ H

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Practical Uses of the Power-Laws

• Describing graphs • Protocol performance• Graph generation and selection• Predictions and extrapolations

Year 1999 2000 2001 2002

Nodes 4389 5763 7137 8511

Edges 8256 12639 15301 18394

d 4.26 4.39 4.61 4.78

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End-to-End Routing Behaviour in the Internet

• What sorts of pathologies and failures occur in the Internet?

• Do routes remain stable over time or change frequently?

• Do routes from A to B tend to be symmetric as routes from B to A?

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But we know the protocols very well...

• Distinction between protocols and behaviour is important!

• It’s not obvious how the backbone dynamics translate into the routing dynamics seen by an end user

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Methodology

• Measurement of a large sample of Internet routes

• Measurements were done in two different periods

• Measuring instrument: traceroute

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The Design of the Experiment

• Network probe daemon • Npd_control• Key property: N2 scaling

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How representative can the Observations be ?

• About 30 Internet hosts participated• 1995: 6.6 million Internet hosts!• About 1000 active AS’s in 1995

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The Experiment

• D1: November 8 through December 24, 1994• D2: November 3 through December 21, 1995

• Main difference: D2 consists of paired traceroutes

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Routing Pathologies

• Routing loops• Erroneous routing• Connectivity altered mid-stream• Fluttering• Infrastructure failures• Unreachable due to many hops • Temporary outages

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Summary of Routing Pathologies

Pathology Probability Trend

Persistent loops 0.13-0.16%

Erroneous routing 0.004-0.004%

Mid-stream change 0.16% 0.44% worse

Infrastructure failure 0.21% 0.48% worse

Outage >= 30 secs 0.96% 2.2% worse

Total pathologies 1.5% 3.3% worse

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End-to-End Routing Stability

• Two definitions of stability:

· Routing prevalence

· Routing persistence

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Routing (A)symmetry

• One-way propagation time• Anticipatory flow state• Network troubleshooting

• Causes of routing asymmetries?

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Summary

• Overwiew of the Internet topology• Power-Laws of the Internet and their

practical use• End-to-end routing behaviour