Practical Issues in Internet Measurement adapted from Mark 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

Domain Name System Measurement related issues in dealing with databases – Out-of-date – Old cache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