The large-scale behavior of routing in the Internet has gone virtually without any formal study, the exceptions being Chinoy's analysis of the dynamics of Internet routing information [Ch93], and recent work, similar in spirit, by Labovitz, Malan and Jahanian [LMJ97]. We report on an analysis of 40,000 end-to-end route measurements conducted using repeated “traceroutes ” between 37 Internet sites. We analyze the routing behavior for pathological conditions, routing stability, and routing symmetry. For pathologies, we characterize the prevalence of routing loops, erroneous routing, infrastructure failures, and temporary outages. We find that the likelihood of encountering a major routing pathology more than doubled between the end of 1994 and the end of 1995, rising from 1.5 % to 3.3%. For routing stability, we define two separate types of stability, “prevalence, ” meaning the overall likelihood that a particular route is encountered, and “persistence, ” the likelihood that a route remains unchanged over a long period of time. We find that Internet paths are heavily dominated by a single prevalent route, but that the time periods over which routes persist show wide variation, ranging from seconds up to days. About 2/3's of the Internet paths had routes persisting for either days or weeks. For routing symmetry, we look at the likelihood that a path through the Internet visits at least one different city in the two directions. At the end of 1995, this was the case half the time, and at least one different autonomous system was visited 30 % of the time.

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Packet filters are rules for classifying packets based on their header fields. A filter specifies a pattern for each of the key header fields, and an action that is applied to the packet matching this filter. Packet classification is essential to routers supporting services such as Quality of Service (QoS), Virtual Private Networks (VPNs), and firewalls. A filter conflict occurs when two or more filters overlap, creating an ambiguity in packet classification. Current techniques for resolving filter conflicts are based on prioritizing conflicting filters, and choosing the higher priority filter. We show that prioritizing does not always work. Instead, we propose a new scheme for conflict resolution, which is based on the idea of adding resolve filters. Our main results are a geometric framework for studying filters, an algorithm for detecting conflicts in a filter database, and an algorithm for resolving conflicts. In the special case of 2-dimensional (Source-Destination) filt...