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A simple protocol stack,
organized as layers.
The Internet Protocols as an
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.
A Proposal for a Spanning Layer
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
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.