Practical Issues in Internet Measurement
adapted fromMark Crovella and Balachander Krishnamurthy
Where Can Measurements Be Made?
IXP
Where Can Measurements Be Made?
• Local Area Network– Local testbeds– Complete control, precise measurements
Where Can Measurements Be Made?
• Inside a backbone– Routine measurements• ensure availability• scan for outages or attacks• topology changes• compliance with SLAs• traffic trends
– Inra-AS measurements• SNMP: Simple Network Management Protocol• Provisioning
– macro: traffic shifts– micro: setting parameters of gateways etc.
Where Can Measurements Be Made?
• Entry points into a network– Gateway routers• NetFlow or sFlow
– Peering routers• BGP• Public or Private
– IXP to direct links• might trigger policy changes• improve paths
– Access routers• residential customers to commercial servers• performance monitoring• SLA
Where Can Measurements Be Made?
• Internet Exchange Point (IXP)– Free or Fee-based– traffic patterns– ensure locality
Where Can Measurements Be Made?
• Wide Area Network– various places in the network– multi-site measurements• simultaneous or over a period of time• PlanetLab, M-Lab, RIPE Atlas, Ark, …
– Representativeness• Keynote
Role of Time in Measurements
• Capturing time accurately– True time t– Reported time C(t)– Offset: θ(t) = C(t) - t– Rate: First derivative of apparent time to true time• ϒ(t) = dC(t)/dt
– Skew: Difference between its rate and correct time• skew = ϒ - 1
– Resolution: Smallest amount by which C(t) can change
– Accuracy is more stringent requirement than zero skew and is harder to obtain
– Resolution requirements diminish higher in protocol stack
Role of Time in Measurements
• Sources of Time Information– External Time Sources• Radio clocks, GPS, CDMA
– PC based clocks • Hardware clock and Software clock• Time Stamp Counter
• Synchronized Time– Synchronized clocks• Network Time Protocol
– Synchronizing measured times after the fact• relative skew and offset
Role of Internet Directories and Databases
• Address registries• Domain Name System (DNS)
• Internet Address and Routing Registries– Internet Assigned Numbers Authority (IANA)
– Internet Routing Registry• Clearinghouse for AS number mapping
– Regional Internet Registries (RIR)
Role of Internet Directories and Databases
Role of Internet Directories and Databases
• Domain Name System
• Measurement related issues in dealing with databases– Out-of-date– Old cache
Measurement Across Various Layers
• Issues in capturing data– Lower-level protocol data– Gathering packet traces and flows– Application level data gathering
• Changes to Infrastructure/Instrumentation
• Local vs Remote vs Distributed data gathering
• Measurement on Overlays
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