This chapter describes the internal workings of the WAP box: how state machines are implemented, other major code modules, threads, communication with WML content servers, how WML and WMLScript are converted to a binary format, timeouts with HTTP requests and other external communications, how to deal with a broken bearer box connection, how the box is started automatically. This chapter will be written by Lars and Aarno. Each transaction and session is represented by a data structure that implements (together with the static state machine engine code) the state machine for that layer. Each new WDP packet is handled by a new thread in a WAP box. That thread finds the correct WTP state machine or creates a new one. It then feeds the WDP packet as an event to the WTP state machine. If that state machine needs to deal with a session, it calls the WSP engine code to find or create a WSP state machine, and feeds the suitable events to that state machine. This is all handled by the same thread that was originally created to handle the WDP packet. And similarly, if the WSP state machine needs to give events to a WTP state machine, it is handled by the same thread. Once all the events that are spawned by the WDP packet are handled, the thread dies. It is possible that bursts of WDP packets arrive faster than they can be processed. In that case, normal locking of data structures ensures that only one packet at a time is handled, and the rest are queued. In fact, if the newly created thread for a new WDP packet notices it can't process the packet, because the corresponding WTP state machine is still locked by the previous packet, it will put the event in a WTP state machine specific queue and terminate. The thread that has the lock will, after it has processed its packet (or other event), look at the state machine's pending event queue and process the packets there. This way, there need not be unnecessary threads. This is important when there is lots of traffic due to lots of users: it is important that the thread table of the operating system is not filled. types of events (WTP): WTP PDU's: different event for each PDU type WSP service primitives timer signals types of events (WSP): WTP service primitives pseudo-events generated by the WSP/WAP box implementation http request events? data structures. WTPEvent WTPMachine WSPSessionEvent WSPSessionMachine WSPMethodEvent WSPMethodMachine algorithm (WTP engine): read message from bearer box convert it into a WTPEvent find or create the appropriate WTPMachine feed the WTPEvent to the WTPMachine the WTPMachine handles the event according to the state tables in the WAP spec if necessary, the WTPMachine sends events to WSP engine algorithm (WSP engine): get WSPEvent from WTP engine find or create the appropriate WSPSessionMachine or WSPMethodMachine feed the WSPEvent to the state machine the state machine handles the event according to state tables in WAP spec coding strategy for WTP and WSP engines: like for messages