A protocol is a set of rules governing the way data will be transmitted and received over data communication networks. Protocols are then the rules that determine everything about the way a network operates. Protocols must provide reliable, error-free communication of user data as well as a network management function.
Therefore, protocols govern how applications access the network, the way that data from an application is divided into packets for transmission through cable, and which electrical signals represent data on a network cable. The OSI model, defined by a seven-layer architecture, is partitioned into a vertical set of layers.
The OSI model is based on open systems and peerto- peer communications. Each layer performs a related subset of the functions required to communicate with another system. Each system contains seven layers. If a user or application entity A wishes to send a message to another user or application entity B, it invokes the application layer (layer 7).
Layer 7 (corresponding to application A) establishes a peer relationship with layer 7 of the target machine (application B), using a layer 7 protocol. In an effort to standardise a way of looking at network protocols, the TCP/IP four-layer model is created with reference to the seven-layer OSI model.
The protocol suite is designed in distinct layers to make it easier to substitute one protocol for another. The protocol suite governs how data is exchanged above and below each protocol layer. When protocols are designed, specifications set out how a protocol exchanges data with a protocol layered above or below it.
Both the OSI model and the TCP/IP layered model are based on many similarities, but there are philosophical and practical differences between the two models. However, they both deal with communications among heterogeneous computers.
Since TCP was developed before the OSI model, the layers in the TCP/IP protocol model do not exactly match those in the OSI model. The important fact is the hierarchical ordering of protocols. The TCP/IP model is made up of four layers:
- Network Access Layer
The network access layer contains protocols that provide access to a communication network. At this layer, systems are interfaced to a variety of networks. One function of this layer is to route data between hosts attached to the same network. The services to be provided are flow control and error control between hosts.
The network access layer is invoked either by the Internet layer or the application layer. This layer provides the device drivers that support interactions with communications hardware such as the token ring or Ethernet.
The IEEE token ring, referred to as the Newhall ring, is probably the oldest ring control technique and has become the most popular ring access technique in the USA. The Fiber Distributed Data Interface (FDDI) is a standard for a high-speed ring LAN. Like the IEEE 802 standard, FDDI employs the token ring algorithm.
- Internet Layer
The Internet layer provides a routing function. Therefore, this layer consists of the procedures required within hosts and gateways to allow data to traverse multiple networks. A gateway connecting two networks relays data between networks using an internetwork protocol. This layer consists of the Internet Protocol (IP) and the Internet Control Message Protocol (ICMP).
- Transport Layer
The transport layer delivers data between two processes on different host computers. A protocol entity at this level provides a logical connection between higher-level entities. Possible services include error and flow controls and the ability to deal with control signals not associated with a logical data connection. This layer contains the Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP).
- Application Layer
This layer contains protocols for resource sharing and remote access. The application layer actually represents the higher-level protocols that are used to provide a direct interface with users or applications.
Some of the important application protocols are File Transfer Protocol (FTP) for file transfers, HyperText Transfer Protocol (HTTP) for the World Wide Web, and Simple Network Management Protocol (SNMP) for controlling network devices.
The Domain Naming Service (DNS) is also useful because it is responsible for converting numeric IP addresses into names that can be more easily remembered by users. Many other protocols dealing with the finer details of applications are included in this application layer.
These include Simple Mail Transport Protocol (SMTP), Post Office Protocol (POP), Internet Mail Access Protocol (IMAP), Internet Control Message Protocol (ICMP) for email, Privacy Enhanced Mail (PEM), Pretty Good Privacy (PGP) and Secure Multimedia Internet Mail Extensions (S/MIME) for e-mail security.