Document Type

Technical Report

Publication Date



Computer input-output equipment -- Technological advances, Network processors -- Programming, FreeBSD


Advances in hardware technology has fueled the proliferation of dynamically configurable network interface cards. This empowers mobile laptop users to select the most appropriate interface for their current environment. Unfortunately, the majority of system software remains "customized" for a particular network configuration, and assumes many network characteristics remain invariant over the runtime of the software. Physical Media Independence (PMI) is the concept of making assumptions about a particular device explicit, detecting events which invalidate these assumptions, and recovering once events are detected. This paper presents a model supporting PMI. Based on device availablilty, the model identifies implicit device-related assumptions made by contemporary network stacks, describes a methodology for making them explicit, and outlines what adaptation should occur when they are invalidated. The model is used to construct a new kernel entity, called the Interface Management Module (IMM), that supports PMI in the FreeBSD operating system. The benefits and limitationsof the system are illustrated using a variety of network applications. The results show that transparency is difficult to maintain for all applications because they cache information such as IP addresses and network bandwidth characteristics. We conclude that while low level support for PMI is important, the IMM needs to provide an interface for mapping application-level semantics down to low-level policy decisions.


Technical Report CSE-97-001, Department of Computer Science and Engineering, Oregon Graduate Institute of Science and Technology.

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