The hourglass model of layered systems architecture is a visual and conceptual representation of an approach to design that seeks to support a great diversity of applications and allow implementation using a great diversity of supporting services. At the center of the hourglass model is a distinguished layer in a stack of abstractions that is chosen as the sole means of accessing the lower-level resources of the system. This distinguished layer can be implemented using services that are considered as lying below it in the stack as well as other services and applications that are considered as lying above it. However, the components that lie above the distinguished layer cannot directly access the services that lie below it.
David Clark called the distinguished layer the "spanning layer" because in the Internet architecture it bridges the multiple local area network implementations that lie below it in the stack (see Figure 1). Clark defined the function of the spanning layer by its ability to "... hide the detailed differences among these various technologies, and present a uniform service interface to the applications above" and identified the Internet Protocol as the spanning layer of the Internet (see Figure 2).5 Arguably the spanning layer also includes other elements of the Internet Protocol Suite that access lower-layer services (such as ARP and DHCP).
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