Editing Protocol stack (section)

==Spanning layer==
An important feature of many communities of interoperability based on a common protocol stack is a '''spanning layer''', a term coined by [[David D. Clark|David Clark]]<ref>{{cite book |title=Interoperation, Open Interfaces, and Protocol Architecture |author=David Clark |publisher=National Research Council |isbn=9780309060363 |date=1997 |work=The Unpredictable Certainty: White Papers}}</ref>
<blockquote>Certain protocols are designed with the specific purpose of bridging differences at the lower layers, so that common agreements are not required there. Instead, the layer provides the definitions that permit translation to occur between a range of services or technologies used below. Thus, in somewhat abstract terms, at and above such a layer common standards contribute to interoperation, while below the layer translation is used. Such a layer is called a ''spanning layer'' in this paper. As a practical matter, real interoperation is achieved by the definition and use of effective spanning layers. But there are many different ways that a spanning layer can be crafted.</blockquote>

In the  Internet protocol stack, the [[Internet Protocol Suite]] constitutes a spanning layer that defines a [[best-effort service]] for global routing of [[datagrams]] at [[Layer 3]]. The [[Internet]] is the community of [[interoperation]] based on this spanning layer.