Main control loop

The lwIP and uIP stacks can be run either as a task in a multitasking system, or as the main program in a singletasking system. In both cases, the main control loop (Figure 3) does two things repeatedly:

1. Check if a packet has arrived from the network.
2. Check if a periodic timeout has occurred.

If a packet has arrived, the input handler of the TCP/IP stack is invoked. The input handler function will never block, but will return at once. When it returns, the stack or the application for which the incoming packet was intended may have produced one or more reply packets which should be sent out. If so, the network device driver is called to send out these packets.

Periodic timeouts are used to drive TCP mechanisms that depend on timers, such as delayed acknowledgments, retransmissions and round-trip time estimations. When the main control loop infers that the periodic timer should fire, it invokes the timer handler of the TCP/IP stack. Because the TCP/IP stack may perform retransmissions when dealing with a timer event, the network device driver is called to send out the packets that may have been produced.

This is similar to how the BSD implementations drive the TCP/IP stack, but BSD uses software interrupts and a task scheduler to initiate input handlers and timers. In our limited system, we do not depend on such mechanisms being available.