Expanding the Internet example above may make these points clearer. The Internet protocol suite contains a protocol that plays the role of a spanning layer, IP. Consider how IP supports the forwarding of simple text mail. The format of mail is defined by the standard RFC-822,1 the required common agreement at the application layer. This protocol in turn depends for its delivery on the Internet's TCP. And finally, TCP uses the services of IP, which provides a uniform interface to whatever network technology is involved.
How does the IP spanning layer achieve its purpose? It defines a basic set of services, which were carefully designed so that they could be constructed from a wide range of underlying network technologies. Software, as a part of the Internet layer, translates what each of these lower-layer technologies offers into the common service of the Internet layer. The claim is that as long as two computers running RFC-822 mail are connected to networks over which Internet is defined to run, RFC-822 mail can interoperate.
This example illustrates the role of the spanning layer as the foundation on which interoperation sits. To determine whether two implementations of RFC-822 mail can interoperate, it is not necessary to look at the implementation of Internet over any specific network technologies. The details of how IP is implemented over Ethernet or over ATM are not relevant to the interoperation of mail. Instead, one looks at the extent, in practice, to which IP has succeeded in spanning a number of network technologies. And the practical conclusion, as reflected in the marketplace, is that IP defines a successful spanning layer. The functions and semantics of the IP layer are well defined, as shown by the fact that many companies have become successful by selling routers, the devices that implement the translation required by IP.
As a part of its proposed open data network (ODN) architecture for the national information infrastructure (NII), the report Realizing the Information Future (RTIF 1R) proposes a specific spanning layer, a module it calls the ODN bearer service. This is illustrated on p. 53 of the report, in the "hourglass picture," which is reproduced here in Figure 2. It illustrates a collection of applications at the top (presumably desirous of interoperation), and at the bottom a collection of network technologies, which support the interoperation.
In the middle of this picture, at the narrow point in the hourglass, is the ODN bearer service. This layer is the key to the approach that RTIF takes to interoperation. The bearer service provides a set of capabilities sufficient to support the range of applications illustrated above it. It implements these capabilities by building on the more basic capabilities of the various network technologies below. The bearer service would thus span the broad range of network technologies illustrated below it, hide the detailed differences among these various technologies, and present a uniform service interface to the applications above. The bearer service is thus an example of a spanning layer, with specific features and capabilities.