In grid computing environments, network bandwidth discovery and allocation is a serious issue. Before their applications are running, grid users will need to choose hosts based on available bandwidth. Running applications may need to adapt to a changing set of hosts. Hence, a tool is needed for monitoring network performance that is integral to the grid environment. To address this need, Gloperf was developed as part of the Globus grid computing toolkit. Gloperf is designed for ease of deployment and makes simple, end-to-end TCP measurements requiring no special host permissions. Scalability is addressed by a hierarchy of measurements based on group membership and by limiting overhead to a small, acceptable, fixed percentage of the available bandwidth. Since this fixed overhead may push host-pair revisit time into the tens-of-hours, we also quantitatively examine the "trajectory" of the cost-error trade-off for measurement frequency.