it must be possible to build net- works by components (nodes and parts of nodes) that are supplied by different implementors, and since network systems must be open to technology changes, an implementation-oriented view, such as the one indicated, is out of the question as a productive way to create network system models. An important objective with the OSI RM is to devise a method for describing networks in a way that leaves suppliers of network products to solve all implemen- tation issues. The OSI RM, therefore, makes the concept layer known. While a software element or software system, for example, must be confined to a particular node and processing system, a layer is a function that is network-wide. This means that it consists of functional parts that are distributed to different nodes (at least two) and communicate by means of message passing. The OSI RM also defines "layer" as (potentially) a part of the total functionality of a network, by saying that a layer may rely on services that are provided by other layers over layer interfaces, thereby constructing the picture of a layered system, such as the OSI RM seven- layer structure. A layer interface connects two layer elements of different layers to each other. Layer interfaces are normally regarded as interfaces inside nodes (we will, however, modify this assumption later). 2 Layer Structures The parts of a layer exist in nodes and are called layer elements. Layer elements become fully defined by specifying the messages and rules for exchange of mes- sages (in a protocol specification) within the layer, as well as how they access the services of other layers (specified in layer-interface specifications, also called "service definition" in ITU-T standards¹. This implies that a model created by this method leaves the question of how to implement layer elements completely open. The layer element, therefore, becomes the target for suppliers' implementation efforts. This is a short recapitulation of what we trust is more or less common knowl- edge for most re