Physical Media Independence: System Support for Dynamically Available Network Interfaces
Jim Binkley, Jon Inouye, Jonathan Walpole · 1997
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.