Interaction Between OSI Model Layers

Like many ISO standards, much of its formal theory does not make it into the real world of actual implementation, but the powerful concepts that the OSI model present are a key element in most modern network system designs.  Anyone who has worked with data networking or security has likely heard the terms “layer three” or “layer two” or “application layer.”  This terminology stems directly from the ISO model and how it is applied to practical solutions. The model concepts are conventionally used to design and troubleshoot networks, and the seven-layer model is standard fare on any network engineering certification exam or interview.  Careful study of the model can show us
support for concepts we have learned from more conventional forms of information security theory, and understanding and applying the model to information security scenarios can also help us assess and address information security threats in a network environment, allowing us to organize efforts to make security assessments and perform forensic analysis of compromised systems and threats presented in theory and found in the wild.

A given layer in the OSI model generally communicates with three other OSI layers: the layer directly above it, the layer directly below it, and its peer layer in other networked computer systems. The data link layer in System A, for example, communicates with the network layer of System A, the physical layer of System A, and the data link layer in System B.

One OSI layer communicates with another layer to make use of the services provided by the second layer. The services provided by adjacent layers help a given OSI layer communicate with its peer layer in other computer systems. Three basic elements are involved in layer services: the service user, the service provider, and the service access point (SAP).

The service user is the OSI layer that requests services from an adjacent OSI layer. The service provider is the OSI layer that provides services to service users. OSI layers can provide services to multiple service users. The SAP is a conceptual location at which one OSI layer can request the services of another OSI layer.