NIST HELPS DEMONSTRATE MOBILE-CODE CONTROL.

On Dec. 6, 2000, researchers at NIST and a private company demonstrated a novel technique for predicting and controlling CPU use by mobile code in the Internet. Once developed further, this breakthrough might ultimately lead to significant improvements in the safety and efficiency of Internet applications, which increasingly use mobile code, such as applets, scripts, and dynamically linked libraries, to deliver new software to millions of users. Without understanding the CPU time required by dynamically downloaded software, computer operating systems cannot effectively manage system resources or control the execution of mobile code. Unfortunately, since mobile code can be downloaded and executed on a wide variety of computer systems with a vast range of capabilities, software developers cannot specify CPU requirements a priority.

While investigating this problem, NIST researchers designed and evaluated a novel technique that shows some promise. The new technique adapts CPU time allocation to account jointly for the needs of specific programs and the capabilities of various computer nodes. The researchers discovered that CPU time requirements do not depend solely on the processor speed of various computers, but rather on a complex array of hardware and software factors. As a result, they developed benchmarks to calibrate the performance of computer platforms with respect to the most significant factors, and then designed an application model that can be expressed in terms of those factors. Further, the researchers developed transformation algorithms to scale an application model between pairs of computers. By choosing one computer as a reference node, two simple transformations enable an application model to be understood by any computer within a network. Initial experiments, involving two different execution environments, four distin ct applications, and five computer systems have been successful,

Researchers at the private company incorporated the NIST CPU usage prediction models into two demonstrations presented at a recent DARPA Active Networks workshop. The first demonstration compared the effectiveness of controlling CPU usage in network nodes using three different policies: fixed CPU time per packet, predicted CPU time scaled based on relative processor speeds, and predicted CPU time scaled based on the NIST models. The demonstration relayed packets, some containing erroneous code, from a source to a sink through two network nodes, one fast and one slow. Erroneously coded packets were designed to consume as much CPU time as possible, while normally coded packets simply performed the required processing and then moved on to the next node. The demonstration revealed that assigning fixed CPU time to each packet allowed erroneously coded packets to steal significant amounts of CPU time on fast nodes and caused slow nodes to prematurely terminate normally coded packets. After scaling CPU time usage p redictions to account for relative processor speeds, the results improved; however, the NIST-developed models performed best.

In a second demonstration, the private company incorporated the NIST CPU usage prediction models into the Active Virtual Network Management Prediction (AVNMP) system, a technology that predicts future load in a network. AVNMP runs ahead in time of the real network, using network-traffic models to predict future network conditions. As real time passes various prediction points, AVNMP compares reality against prediction and, if necessary, rolls back and restarts the simulation to limit prediction error. Given a fixed goal for prediction error, AVNMP takes appropriate actions to achieve the goal. For example, higher inaccuracies in predictions lead AVNMP to make a greater number of rollbacks, resulting in a smaller look-ahead into the future. In the demonstration, AVNMP rollbacks were triggered by inaccuracies in either the predicted message load or CPU usage on each node. When predicting CPU usage based on a fixed allocation, AVNMP required as many as 12 rollbacks per prediction cycle to maintain the desired f idelity during 200 simulated seconds. In contrast, when using the NIST CPU usage model, AVNMP required a maximum of three rollbacks per prediction cycle.

While these initial results appear promising, more research remains before the ideas can be applied practically. Three issues in particular must be resolved. First, the new NIST technique assumes that all application behavior can be measured prior to injecting a model into network nodes. Unfortunately, application behaviors often reflect conditions that cannot be known before a program reaches a node. For this reason, the application model must be enhanced to account for such node-dependent conditions. Second, the models consist of fine-grained histograms, which must be exercised with Monte Carlo simulations in order to predict CPU usage. As a result, specific application models can be large and can require substantial computation to produce predictions. To some degree the space-time properties of the model can be modulated; however, the prediction error also changes accordingly. The third issue to be resolved involves characterization of error bounds. Before taking operational decisions based on predictions from the model, a node must consider the possible range of prediction error. NIST researchers have yet to characterize the error properties of their model.