The OSI (Open Systems Interconnect) Reference Model (ISO 7498) defines a seven-layer model of data communication with physical transport at the lower layer and application protocols at the upper layers. This model, shown in the figure below, is widely accepted as a basis for the understanding of how a network protocol stack should operate and as a reference tool for comparing network stack implementation.

Each layer provides a set of functions to the layer above and, in turn, relies on the functions provided by the layer below. Although messages can only pass vertically through the stack from layer to layer, from a logical point of view, each layer communicates directly with its peer layer on other nodes.
The seven layers are:

• Application Network applications such as terminal emulation and file transfer
• Presentation Formatting of data and encryption
• Session Establishment and maintenance of sessions
• Transport Provision of reliable and unreliable end-to-end delivery
• Network Packet delivery, including routing
• Data Link Framing of units of information and error checking
• Physical Transmission of bits on the physical hardware

In contrast to TCP/IP, the OSI approach started from a clean slate and defined standards, adhering tightly to their own model, using a formal committee process without requiring implementations. Internet protocols use a less formal engineering approach, where anybody can propose and comment on RFCs, and implementations are required to verify feasibility.