An Introduction to the OSI and TCPIP Layers

An Introduction to the OSI and TCP/IP Layers

We discuss two networking models in this chapter: OSI and TCP/IP. The OSI networking model is not the only interconnect model out there; however, it is the ISO's adopted model. This model defines networking as consisting of seven layers. In addition to the OSI model, there is the TCP/IP model. This is the model that is most commonly referenced in today's IT world, and it should be mentioned that the TCP/IP model was around before the OSI model. The TCP/IP model originally was named the DOD model after its creator, the Department of Defense. It consists of the same initial layers that make up the OSI model; however, it combines the last four layers.

A comparison of these layers is shown in Table 13-1.

Table 13-1. OSI Layers, Descriptions, and Corresponding TCP/IP Layers

OSI Layers	Description	TCP/IP Layers
Application	This is where the user process resides: NIS, NFS, DNS, TELNET, mail, and other network applications.
Presentation	In the TCP/IP model, the representation of the data is defined by the application. However, in new TCP/IP protocols such as External Data Representation (XDR), this data representation is handled by the protocol at this layer.
Session	RPC is one example of a session protocol. An application can be built on top of TCP or UDP, but the management of cooperating applications is controlled by the RPC. NFS is a program that uses RPC for its session protocol, while the underlying layer is UDP. The latest versions of NFS can use either UDP or TCP. For a list of applications that use RPC, run this command: cat /etc/rpc.	Application Programming Interface
Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) are two
Transport (TCP and UDP)	transport protocols that sit at this layer. TCP requires a "handshake" socket connection, which allows for keeping track of packet delivery and packets that need to be resent. UDP uses less of this overhead for better performance at the cost of error checking (which is done within the application).	Transport (TCP and UDP)
Network (IP)	The Internet Protocol (IP), which routes packets from one location to another. This layer also handles the breaking up of large datagrams into smaller units if the datagrams exceed the Maximum Transmission Unit (MTU) size defined in the lower level. Commonly referred to as the TCP/IP network layer.	Internetwork layer (IP, AppleTalk, IPX)
Data link	Handles the transmission and reception of data and passes the packets to the network interface (Ethernet, FDDI, and so on).
	Addresses the physical interface for the network. An Ethernet interface has a unique 48-bit MAC address.	Physical network access layer
	Additionally, this layer includes the MTU, checksums, source address, and destination address.	For example, Ethernet, 802.11 (wireless), Token Ring, and so on.
Physical	The physical medium: cables, switches, pin locations, and converters, which convert one media type to another.