Embedded Systems November 2000 Vol13_12

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A variety of less time-critical tasks fall outside the core processing or forwarding requirements of a network device. These are called control plane tasks. FIGURE 1 Fast path, slow path • Open TCP connection with web server (three-way handshake) • Send buffe red request (TCP / IP protocol) Note that, fo r a given bandwidth, the web switch processing requi re- ments are much higher, and require much more state than the rou ter pro- cessing requi rements. The difference arises because a router processes packets, but a web switch processes connections. FIGURI2 Genenc network processor CPU Interface Network processing requirements, part 2 The previous description of the core operations of a router and a web switch were not complete. A maj or piece was missing. What was it? Device management. How do you configu re and control this device? A variety of less time-critical tasks as they make their way across the Internet. He re is a simplified version of the IP routing algorithm: • Remove the link layer header • Find the destination IP address in the IP header • Do a table lookup to detennine the IP address of the next hop • Determine link layer address of the next hop • Add link layer header to packet • Queue packet for sending • Send or drop packet (if link is con- gested ) Web switches, by contrast, are a new type of network device. They address the problem of trying to increase the responsiveness of a pop- ular Web site by u ing more than one web server. A web switch can direct incoming HTTP requests to diffe rent servers based on a variety of network- ing parameters, including the URL itself. For instance, all secure HTTP requests could be forwarded to a spe- cial web server with cryptographic h ardware to accele t·a te those requests. Here is a simplified web switch algorithm: • Accept incoming TCP connection (three-way handshake) • Buffer incoming TCP data stream (TCP / IP protocol) • Parse the stream to find the URL being requested • Do a table lookup to determine where to forward the request 46 NOVEMBER 2000 Embedded Systems Programming fall outside the core processing or for- warding requirements of a network device. These are called contml plane tasks. For a router, these tasks include routing protocols like OSPF and BGP, and management interfaces li ke serial ports, telnet, and SNMP. For a web switch, these tasks include receivi ng upda tes abou t the status of web servers and providing a web interface for configuration and management. For both devices, error handling and logging are important control plane tasks. Another way to distinguish data plane tasks from control plan e tasks is to look at each packet's path. Packets handled by data plane tasks usually travel through the device, while pack- ets handled by control plane tasks usu- ally originate or terminate at the device. Data plane vs. control plane tasks Network engineers have noticed an in teresting relationship between data plane tasks and con trol plane tasks. Data plane tasks require a small amount of code, but a large amount of processing power. In contrast, control

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