Thread: The basic operation of a network processor

The basic operation of a network processor, as expected, begins with a stream of arriving packets. The packets are processed by the network processor, which may include modification, addition and/or removal of data to/ from packets. When this processing is finished, the packets are forwarded on by the network processor. A networking device has four functional partitions: physical layer processing, packet processing, host processing and switching.

The physical-layer processing function converts the analogue Signal transmitted over a physical medium into a digital bitstream with some type of frame format. Packet processing performs all the necessary operations on network traffic at wire rates as it passes through the device. These operations are also known as 'fast path' or 'data path' operations. At the same time, host processing handles a number of functions that lie outside the fast path. These functions range from certain
'slow path' operations performed on a small percentage of packets to control operations such as device configuration switching handles the forwarding of data traffic between the ingress and egress ports of the bus, backplane or other switch fabric of the router.



The figure shows where the fast-path and the slow-path, operations take place in a networking device. The fast path and slow path (for control functions) in a networking device are often referred to as the data plane and control plane, respectively. A networking application typically operates on both of these planes. The data plane processes and forwards packets and is thus responsible for computation and actual data movement through the system. Data plane operations include such functions as packet classification, modification, queuing and encryption. As the data plane operates on packets moving around the system, it faces real-time performance constraints.

The control plane handles a number of operations dealing with specific packets that do not need to be processed at wire speed and is also responsible for control and management of network devices. Packets that are separated out of the main data path and passed to the control plane typically have complex processing requirements. Thus, if these packets were to be processed by the data plane, they would significantly decrease packet throughput.

Control plane operations involving these packets are less time-critical and include execution of routing protocols, management of routing tables and so on. In general, the control plane has more diversity in terms of functionality than the data plane. A general-purpose processor can be used to execute control plane operations, while the time-eonstrained data plane operations are normally.accelerated by executing them on a number of dedicated packet processing engines. Early network processors. generally used embedded on-chip control-processors to run software for both the data and control planes.