Summary. Routers are more than routers. They are also firewalls, load balancers, address translators, etc. Unfortunately, implementing these additional router responsibilities is onerous; most routers are closed platforms with inflexible designs. The Click router architecture, on the other hand, permits the creation of highly modular and flexible routers with reasonable performance. Click is implemented in C++ and compiles router specifications to routers which run on general purpose machines.
Much like how Unix embraces modularity by composing simple programs using pipes, the Click architecture organizes a network of elements connected by connections. Each element represents an atomic unit of computation (e.g. counting packets) and is implemented as a C++ object that points to the other elements to which it is connected. Each element has input and output ports, can be provided arguments as configuration strings, and can expose arbitrary methods to other elements and to users.
Connections and ports are either push-based or pull-based. The source element of a push connection pushes data to the destination element. For example, when a network device receives a packet, it pushes it to other elements. Dually, the destination element of a pull connection can request to pull data from the source element or receive null if no such data is available. For example, when a network device is ready to be written to, it may pull data from other elements. Ports can be designated as pull, push, or agnostic. Pull ports must be matched with other pull ports, push ports must be matched with other push ports, and agnostic ports are labeled as pull or push ports during router initialization. Queues are packet storing elements with a push input port and pull output port; they form the interface between push and pull connections.
Some elements can process packets with purely local information. Other elements require knowledge of other elements. For example, a packet dropping element placed before a queue might integrate the length of the queue in its packet dropping policy. As a compromise between purely local information and complete global information, Click provides flow-based router context to elements allowing them to answer queries such as "what queue would a packet reach if I sent it out of my second port?".
Click routers are specified using a simple declarative configuration language. The language allows users to group elements into compound elements that behave as a single element.
The Click kernel driver is a single kernel thread that processes elements on a task queue. When an element is processed, it may in turn push data to or pull data from other elements forcing them to be processed. To avoid interrupt and device management overheads, the driver uses polling. Every input and output device polls for data whenever it is run by the driver. Router configurations are loaded and element methods are called via the /proc file system. The driver also supports hot-swapping in new router configurations which take over the state of the previous router.
The authors implement a fully compliant IP router using Click and explore various extensions to it including scheduling and dropping packets. The performance of the IP router is measured and analyzed.