Tools such as pathchar, clink, and pchar attempt to measure the capacity of every layer-3 (L3) hop in a network path. These tools use the same underlying measurement methodology, which we refer to as Variable Packet Size (VPS) probing. The key assumption in VPS is that each L3 hop along a path increases the delay of a packet by a "serialization latency", which is the ratio of the packet size over that hop's capacity. Unfortunately, the capacity estimates of VPS tools are sometimes wrong. In this paper, we investigate the source of these errors, and show that the presence of layer-2 (L2) store-and-forward devices, such as Ethernet switches, have a detrimental effect on the accuracy of VPS tools. Specifically, each L2 store-and-forward device introduces an additional serialization latency in a packet's delay, which results in consistent underestimation of that L3 hop's capacity. We analyze this negative effect, deriving the measured capacity of an L3 hop as a function of the L2 link capacities at that hop. Experimental results in local, campus, and ISP networks verify the model, illustrating that L2 devices should be expected in networks of diverse type and size. Finally, we characterize some other sources of error in VPS tools, such as queueing delays, limited clock resolution, variation in ICMP generation delays, and error propagation along the measured path.

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A common shortcoming of many network measurement programs is that their metrics are not expressed in terms directly applicable to applications and users. To address this problem, we have developed the IP Benchmark (IPB), a benchmark program that synthetically generates traffic to simulate the network activity of common Internet applications. It measures the performance experienced by the simulated applications, and expresses that performance in terms of application-specific metrics. After presenting same details of the design and implementation of IPB, we show how we use it to measure HTTP performance along various long-delay, lossy ATM links, such as might be found in satellite or wireless ATM environments. These links themselves are simulated by using a BBN Long Link Emulator which adds delays and errors to local-area OC-3 links. Finally, we present some preliminary results.

The workload of the global Internet is dominated by the Hypertext Transfer Protocol (HTTP), an application protocol used by World Wide Web clients and servers. Simulation studies of IP networks require a model of the traffic patterns of the World Wide Web, in order to investigate the effects of this increasingly popular application. We have developed an empirical model of network traffic produced by HTTP. Instead of relying on server or client logs, our approach is based on packet traces of HTTP conversations. Through traffic analysis, we have determined statistics and distributions for higher-level quantities such as the size of HTTP files, the number of files per "Web page", and user browsing behavior. These quantities form a model can then be used by simulations to mimic World Wide Web network applications.

Many future applications will require guarantees on network performance, such as bounds on throughput, delay, delay jitter, and reliability. To address this need, the authors have designed, simulated, and implemented a suite of network protocols to support real-time channels (network connections with mathematically provable performance guarantees). The protocols, which constitute the prototype Tenet real-time protocol suite (Suite 1), run on a packet-switching internetwork and can coexist with the popular Internet protocol suite. The authors rely on the use of connection-oriented communication, per-channel admission control, channel rate control, and priority scheduling. This protocol suite is the first set of transport and network-layer communication protocols that can transfer real-time streams with guaranteed quality in packet-switching internetworks. The authors have performed a number of experiments and demonstrations on multiple platforms using continuous-media loads (particularly video). The results show that the approach is both feasible and practical to build, and that it can successfully provide performance guarantees to real-time applications. The paper describes the design and implementation of, the suite, the experiments performed, and some of the lessons learned.