The Design and Evaluation of an Off-Host Communications Protocol Architecture

The University of Virginia Computer Networks Laboratory has developed an implementation of the SAFENET lightweight suite of communication protocols using an off-host implementation of the Xpress Transfer Protocol (XTP). We used an attached protocol processor to test the hypothesis that such an architecture could simultaneously optimize protocol performance (by giving it a dedicated coprocessor) and user application performance (by freeing the host from the burden of protocol processing). Our experience indicates that the choice of implementation architecture has a profound impact upon the overall system performance. This work surveys the design issues inherent to off-host communications architectures and discusses the design choices made in our own architecture. Also presented are our performance results and the many lessons learned from the analysis of our implementation architecture. We also provide a survey of related work performed by other researchers. To assist the designers of future off-host architectures and to evaluate the degree to which we exploited our particular architecture, we develop a simple analytic model to predict the performance of an off-host architecture using readily-obtainable input parameters. The predictions of the model as applied to our system are compared with the system's observed performance. ii Acknowledgments

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