Radon spectroscopy of inter-packet delay

We demonstrate the feasibility of Internet spectroscopy techniques for analysis of rate limiting, packet interarrival delay and passive bitrate estimation of cellor slot-based broadband connections. Working with highly diverse packet trace data, we find that delay’s quantization in microand millisecond range is ubiquitous in today’s Internet and that different providers have strong preferences for specific delay quanta in their infrastructures. The paper consists of two parts. First part presents an algorithm that evaluates interarrival delay quantum (cell time) for rate-limited cell-based links. The algorithm takes a joint 2D distribution of packet sizes and interarrival times as its input. The 2D distribution is then converted by a coarsegrained Radon transform to a family of 1D marginals. Each marginal has semantics of interand intra-packet delay (i.e. link idle time) histogram that corresponds to an assumed value of cell time. Our estimate of cell time is the value that minimizes entropy of such a marginal, i.e. makes it closest to a delta function. As an application of Radon transform technique, we determine the target cell time for the rate limiting performed by the university commodity ISP to provide 20 Mbps connection over 155 Mbps link. This allows to verify an underfulfilment of the contract. The knowledge of cell time enables us to compute the distribution of inter-packet delay. We find that it consists of two separate components overlapping in time domain: a spike with a width of two cell times that corresponds to the rate limiter’s fluctuations around target rate, and ”true” link idle time, whose integral (ccdf) closely follows a Weibull curve, while individual values are subject to a fine-grained delay quantization. We also find that the link’s high load makes the packet arrival process to be very dissimilar with Poisson. Combined with the rate limiter’s long-term memory, this deviation makes the byte counting process to be strictly non-Gaussian over a very wide range of aggregation intervals (up to 1 sec). In the second part we analyze bitrates and other properties of broadband mass-market connections. We determine interarrival times for DSL and cable modem sources by a simplified 1D version of the min-entropy Radon algorithm applied to packets of fixed size (40 or 1500 bytes). We find that delay quantization in broadband access is dependent upon providers, technologies and markets, even though the number of choices appears to be rather limited. This suggests that network spectroscopy has a potential for source recognition, if a library of interarrival quanta and interpacket delay distributions is available. Support for this work is provided by the Defense Advanced Research Project Agency (DARPA) NMS (N66001-01-1-8909) program and DISA’s National Communiations System Organization. CAIDA is a collaborative organization supporting cooperative efforts among the commercial, government and research communities aimed at promoting a scalable, robust Internet infrastructure. CAIDA is based at the University of California’s San Diego Supercomputer Center (SDSC). www.caida. org. Ryan King’s work was done during his 2002 summer internship with CAIDA, supported by Rice University’s EE department.