The TCP implementations in lwIP and uIP are driven by incoming packets and timer events. IP calls TCP when a TCP packet arrives and the main control loop calls TCP periodically.
Incoming packets are parsed by TCP and if the packet contains data that is to be delivered to the application, the application is invoked by the means of a function call. If the incoming packet acknowledges previously sent data, the connection state is updated and the application is informed, allowing it to send out new data.