In a layered approach to protocol architecture, protocols have a relationship to one another such that a protocol at layer (n) uses the services of the layer below it — the (n - 1) services — which, in turn, are provided by a layer (n - 1) protocol. One of the services used by a layer (n) protocol is the encapsulation of its (n) Protocol Data Units (PDUs) in a way which is transparent to it. Such encapsulation is realized by the carriage of the (n) PDUs as user data in an (n - 1) Service Data Unit (SDU).

In a limited case, the operation of a particular protocol at layer (n - 1) implies the operation, above layer (n - 1), of a single layer (n) protocol or single set of related (n) / (n + 1). . . protocols. However, in a more general case, there may be more than one protocol (or set of related protocols starting) at layer (n) that can operate above layer (n - 1) in a given environment. In such cases, there is a need for explicit identification of the protocol (or set of protocols starting) at layer (n).

There also may be a need to manipulate the (n - 1) protocol (i.e. the encapsulating protocol) in certain ways specific to the layer (n) protocol (i.e. the encapsulated protocol). Such manipulations form the basis of a set of procedures that must be specified for the layer (n) protocol.

The above observations regarding protocol identification and encapsulation are also applicable in cases where an (n) layer is further divided into sublayers.

Cases in which an (n) protocol operates for the purpose of establishing a parallel universe of protocols (regardless of the layered structure of that universe) also give rise to a need for the (n) protocol to be able to identify the protocol(s) in the parallel universe. In these cases, however, there is no encapsulating/encapsulated relationship between the (n) protocol and the parallel universe set of protocols.

The above principles lead to a need to establish a framework for protocol identification and encapsulation. These principles apply to the relationship between two protocols (recognizing that one of them may be a set of related protocols) and can be applied recursively. This Recommendation | International Standard provides a framework for explicit protocol identification and for protocol encapsulation. Implicit protocol identification (see 4.2) is beyond the scope of this Recommendation | International Standard.