Reference Model Services

A Protocol Reference Model is an abstraction of the communication subsystem of a system. Thus, it is appropriate to focus on the protocol reference model when examining the issue of secure communications. In this paper, we discuss some issues in incorporating security services into a protocol reference model. The security services considered are authentication, con dentiality, integrity and access control. We adopt a functional de nition for a protocol reference model in terms of the communication services it provides at various layers. We then present two perspectives towards reasoning about the incorporation of security services into a protocol reference model: a perspective that centers on the security requirements, and another that centers on the communication services already present in the protocol reference model. Existing work focuses on the rst approach. We focus on the second approach, that is, on the issue of how well a security service slated for incorporation meshes in with the existing communication services provided at a layer in the protocol reference model. By considering communication services at a lower level of abstraction, in terms of the controls and the associated variables that are used to realize it, we present a criterion on which to base the incorporation of security services into a layer. We assume that each control is modeled as a Mealy machine and that each state is labeled with a particular set of values for the variables. We then identify variables associated with the controls of the security services that capture their functionality, but isolate them from speci c mechanisms to realize them. The criterion for incorporation of the security services into a layer requires that a security service be incorporated into a layer only if the variables associated with the security service are already present among the communication services o ered at the layer. We then show the use of the criterion on two examples: the reference models behind the TCP/IP protocol suite and Asynchronous Transfer Mode (ATM). The main contributions of this paper lie in the clari cation of the di erent levels of abstraction to the problem, a treatment of the issues at a level of abstraction not found in existing work on the topic, which gives new insights, and in a concrete criterion on which to base the incorporation of security services into a protocol reference model.