Defending against denial of service attacks in scout

We describe a two-dimensional architecture for defending against denial of service attacks. In one dimension, the architecture accounts for all resources consumed by each I/O path in the system; this accounting mechanism is implemented as an extension to the path object in the Scout operating system. In the second dimension, the various modules that define each path can be configured in separate protection domains; we implement hardware enforced protection domains, although other implementations are possible. The resulting system-which we call Escort-is the first example of a system that simultaneously does end-to-end resource accounting (thereby protecting against resource based denial of service attacks where principals can be identified) and supports multiple protection domains (thereby allowing untrusted modules to be isolated from each other). The paper describes the Escort architecture and its implementation in Scout, and reports a collection of experiments that measure the costs and benefits of using Escort to protect a web server from denial of service attacks.