Vertical Interface Terminology in the OSI Model

In addition to defining an idealized network architecture and the network functions allocated to each layer, the OSI model also defines a standard set of rules that govern the interfaces between layers.

The active protocol elements in each layer are called entities, typically implemented by means of a software process. Entities in the same layer on different computers are called peer entities . For example, the TCP/IP protocol suite contains two entities within its transport layer: Transmission Control Protocol (TCP) and User Datagram Protocol (UDP).

Layer n-1 , the layer directly below the entities of layer n , implements services that are used by layer n .

For data transfer services, OSI defines the terminology for the discrete data components passed across the interface and between peer entities. Figure A.3 illustrates vertical interface entities.

Figure A.3 Vertical interface entities

• The layer- n entity passes an interface data unit (IDU) to the layer-( n – 1) entity.

• The IDU consists of a protocol data unit (PDU) and some interface control information (ICI). The ICI is information, such as the length of the SDU, and the addressing information that the layer below needs to perform its function.

• The PDU is the data that the layer- n entity wishes to pass across the network to its peer entity. It consists of the layer- n header and the data that layer n received from layer (n+1) .

• The layer- n PDU becomes the layer-( n – 1) service data unit (SDU), because it is the data unit that will be serviced by layer n.

• When layer n – 1 receives the layer- n IDU, it strips off and "considers" the ICI, adds the header information for its peer entity across the network, adds ICI for the layer below, and passes the resulting IDU to the layer n – 2 entity.

Problems can occur in the data path between two network stations, including errant, restricted, or even halted communication.