APST interfaces in LINCS

APST is an acronym for the four highest of the seven layers of the LINCS hierarchy of communication protocols: (from high to low) Application, Presentation, Session, and Transport. Routines in each but the lowest of these APST layers can utilize the facilities of any lower APST layer (normally, but not necessarily, the immediately next lower layer) by invoking various primitives (macros that in most cases are subroutine calls) defining the upper interface of the lower layer. So there are three APST interfaces: Presentation layer, used by the Application layer; Session layer, normally used by the Presentation layer; and Transport layer, normally used by the Session layer. Logically, each end of a stream (unidirectional sequence of transmitted information) is handled by three modules, one module each for the Presentation, Session, and Transport layers, and each of these modules deals with only that one end of that one stream. The internal workings of the layers, particularly the Transport layer, do not necessarily exhibit this same modularization; for example, the two oppositely directed streams between the same two ends (constituting an association) may interact within a layer. However, such interaction is an implementational detail of no direct interest to those utilizing the layer.more » The present document does not describe implementation, nor does it discuss in any detail how the modules employ packet headings and data formats to communicate with their partner modules at the other end of a stream. There being one logical module per end of stream is a characteristic only of the Presentation, Session, and Transport layers. An Application layer module usually manages several streams, orchestrating them to achieve some desired purpose. The modules of the layers (Network, Link, and Physical) below the APST layers each handle many streams, multiplexing them through the nodes and channels of the network to transmit them from their origins to their destinations.« less