A host application is an application that lives on an end-node and communicates with PLAN active nodes via PLAN ports. PLAN ports can be opened from a host application by opening a TCP connection on the port that the PLAN router is listening on. Also, a PLAN port may be opened from an active router using the openPort service. Once the TCP connection is established, it is assumed that all messages going to an active node are marshalled objects of class ActivePacket. Messages coming from the active node are service dependent. For example, the print service sends strings, while the deliver service sends marshalled objects of class Value. Marshalling is done via the Marhsalling class. All of these classes are present in the PLAN basis package.