Measuring bottleneck link speed in packet-switched networks

Abstract

The quality of available network connections can have a large impact on the performance of distributed applications. For example, for document transfer applications such as FTP, Gopher and the World Wide Web, document transfer time is often directly related to the available bandwidth of the connection. Available bandwidth depends on two things: 1) the underlying capacity of the path between client and server, which is limited by the bottleneck link; and 2) the amount of other traffic competing for links on the path. If measurements of these quantities were available to the application, the current utilization of paths could be calculated. Network utilization could then be used as a basis for selection from a set of alternative paths or servers, providing reduced response time. Such a dynamic server selection scheme would be especially useful in a mobile computing environment in which the set of available servers is frequently changing. In order to provide these measurements at the application level, we introduce two tools: bprobe, which provides an estimate of the uncongested bandwidth of a path; and Cprobe, which gives an estimate of the current congestion along a path. These two measures may be used in combination to provide the application with an estimate of available bandwidth between server and client, thereby enabling application-level congestion avoidance. In this paper we discuss the design and implementation of the probe tools, specifically illustrating the techniques used to achieve accuracy and robustness. We present validation studies for both tools which demonstrate their accuracy in the face of actual Internet conditions. Finally, we give results of a survey of available bandwidth to a random set of WWW servers as a sample application of our probe technique.