The protocols that make up the TCP/IP protocol suite can be described in terms of the OSI reference model. In the OSI model, the network access layer and the application layer of the TCP/IP model are further divided to describe discrete functions that must occur at these layers.
At the network access layer, the TCP/IP protocol suite does not specify which protocols to use when transmitting over a physical medium; it only describes the handoff from the internet layer to the physical network protocols. OSI Layers 1 and 2 discuss the necessary procedures to access the media and the physical means to send data over a network.
As shown in the figure, the critical parallels between the two network models occur at the OSI Layers 3 and 4. OSI Layer 3, the network layer, is almost universally used to describe the range of processes that occur in all data networks to address and route messages through an internetwork. IP is the TCP/IP suite protocol that includes the functionality described at OSI Layer 3.
Layer 4, the transport layer of the OSI model, describes general services and functions that provide ordered and reliable delivery of data between source and destination hosts. These functions include acknowledgement, error recovery, and sequencing. At this layer, the TCP/IP protocols TCP and User Datagram Protocol (UDP) provide the necessary functionality.
The TCP/IP application layer includes a number of protocols that provide specific functionality to a variety of end user applications. The OSI model Layers 5, 6, and 7 are used as references for application software developers and vendors to produce products that operate on networks.