Network processing requirements

Part 1

Not all network devices have the same processing requirements. However, a lot of similarities exist. As an example, I will roughly describe the packet processing duties of a router and a web switch. These core, time-critical duties are also called data plane tasks.

Routers are the workhorses of the Internet. A router accepts packets from one of several network interfaces, and either drops them or sends them out through one or more of its other interfaces. Packets may traverse a dozen or more routers as they make their way across the Internet. Here 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 determine 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 congested)

Web switches, by contrast, are a new type of network device. They address the problem of trying to increase the responsiveness of a popular Web site by using more than one web server. A web switch can direct incoming HTTP requests to different servers based on a variety of networking parameters, including the URL itself. For instance, all secure HTTP requests could be forwarded to a special web server with cryptographic hardware to accelerate 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
• Open TCP connection with web server (three-way handshake)
• Send buffered request (TCP/IP protocol)

Note that, for a given bandwidth, the web switch processing requirements are much higher, and require much more state than the router processing requirements. The difference arises because a router processes packets, but a web switch processes connections.

Part 2

The previous description of the core operations of a router and a web switch were not complete. A major piece was missing. What was it? Device management. How do you configure and control this device?

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. For a router, these tasks include routing protocols like OSPF and BGP, and management interfaces like serial ports, telnet, and SNMP. For a web switch, these tasks include receiving updates about 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 plane tasks is to look at each packet's path. Packets handled by data plane tasks usually travel through the device, while packets handled by control plane tasks usually originate or terminate at the device.