Measurements of Path Characteristics in PlanetLab
Michael Olbrich, Felix Nadolni, Filip Idzikowski, Hagen Woesner · 2009
We discuss findings from Internet measurements conducted within the global PlanetLab overlay network. Our study comprises results from two traceroute round-trip measurements, covering more than 250 nodes, as well as one-way packet dynamics measurements for another 20 selected nodes worldwide. For that purpose we devised a PCAP-based active probing framework with packet replay capabilites, using fixed-sized, Poisson sampled UDP probe packets. We combine our results with geotargeting information, and from that derive distance and Hop-Count dependent path characteristics, such as Delay, Delay Variation and Packet Loss. Based on assumptions provided by ITU G.114, packet delays are found to be averagely induced by one third within router queues and by two third due to signal propagation times. Spatial properties and routing behaviour within PlanetLab, including observed routing anomalies are treated as well. In order to assess measuring accuracy on PlanetLab nodes, we then examine the Linux timer system, show the impact of clock ”noise” on the Poisson sampling process, and point out simple enhancements of the Linux kernel, that significantly improve timing accuracy. We show the impact of clock skew and provide theoretical background to encounter such uncertainties. Then real-world application performance is also adressed by utilizing Voice over IP measurements on particular Internet2 paths, finding jitter not a limiting factor on voice quality.