The Clack graphical router: visualizing network software

We present Clack, a graphical environment for teaching students how Internet routers work and other core networking concepts. Clack is a router written as a Java Applet, and routes live network traffic in real-time. Students can look inside the router to see how packets are processed, and watch the dynamics of the queues. They can modify and enhance the router, making it handle packets as they wish. Clack provides multiple views of the operational router including the full network topology, the router's software components, and the packet-level view of traffic as it passes through the router. Clack's detailed visual interface to the software internals of a functioning router, as well as its ability to modify and observe live Internet traffic, provide a unique environment to aid in networking education.Over the last two years, Clack has been used in the classroom at six universities. Feedback from the students through anonymous, formal evaluations has been positive. In this paper we describe the goals and design of Clack as well as our experiences using it in the classroom.