Network Protocol

All communication, whether face-to-face or over a network, is governed by predetermined rules called protocols. These protocols are specific to the characteristics of the conversation. In our day-to-day personal communication, the rules we use to communicate over one medium, like a telephone call, are not necessarily the same as the protocols for using another medium, such as sending a letter.

Think of how many different rules or protocols govern all the different methods of communication that exist in the world today.

Successful communication between hosts on a network requires the interaction of many different protocols. A group of inter-related protocols that are necessary to perform a communication function is called a protocol suite. These protocols are implemented in software and hardware that is loaded on each host and network device.

One of the best ways to visualize how all of the protocols interact on a particular host is to view it as a stack. A protocol stack shows how the individual protocols within the suite are implemented on the host. The protocols are viewed as a layered hierarchy, with each higher level service depending on the functionality defined by the protocols shown in the lower levels. The lower layers of the stack are concerned with moving data over the network and providing services to the upper layers, which are focused on the content of the message being sent and the user interface.

Using layers to describe face-to-face communication

For example, consider two people communicating face-to-face. As the figure shows, we can use three layers to describe this activity. At the bottom layer, the Physical layer, we have two people, each with a voice that can utter words aloud. At the second layer, the Rules layer, we have an agreement to speak in a common language. At the top layer, the Content layer, we have the words actually spoken-the content of the communication.

Were we to witness this conversation, we would not actually see "layers" floating in space. It is important to understand that the use of layers is a model and, as such, it provides a way to conveniently break a complex task into parts and describe how they work.

At the human level, some communication rules are formal and others are simply understood, or implicit, based on custom and practice. For devices to successfully communicate, a network protocol suite must describe precise requirements and interactions.

Networking protocol suites describe processes such as:

• The format or structure of the message
• The method by which networking devices share information about pathways with other networks
• How and when error and system messages are passed between devices
• The setup and termination of data transfer sessions

Individual protocols in a protocol suite may be vendor-specific and proprietary. Proprietary, in this context, means that one company or vendor controls the definition of the protocol and how it functions. Some proprietary protocols can be used by different organizations with permission from the owner. Others can only be implemented on equipment manufactured by the proprietary vendor.

Often, many of the protocols that comprise a protocol suite reference other widely utilized protocols or industry standards. A standard is a process or protocol that has been endorsed by the networking industry and ratified by a standards organization, such as the Institute of Electrical and Electronics Engineers (IEEE) or the Internet Engineering Task Force (IETF).

The use of standards in developing and implementing protocols ensures that products from different manufacturers can work together for efficient communications. If a protocol is not rigidly observed by a particular manufacturer, their equipment or software may not be able to successfully communicate with products made by other manufacturers.

In data communications, for example, if one end of a conversation is using a protocol to govern one-way communication and the other end is assuming a protocol describing two-way communication, in all probability, no information will be exchanged.

An example of the use of a protocol suite in network communications is the interaction between a web server and a web browser. This interaction uses a number of protocols and standards in the process of exchanging information between them. The different protocols work together to ensure that the messages are received and understood by both parties. Examples of these protocols are:

Application Protocol:

Hypertext Transfer Protocol (HTTP) is a common protocol that governs the way that a web server and a web client interact. HTTP defines the content and formatting of the requests and responses exchanged between the client and server. Both the client and the web server software implement HTTP as part of the application. The HTTP protocol relies on other protocols to govern how the messages are transported between client and server

Transport Protocol:

Transmission Control Protocol (TCP) is the transport protocol that manages the individual conversations between web servers and web clients. TCP divides the HTTP messages into smaller pieces, called segments, to be sent to the destination client. It is also responsible for controlling the size and rate at which messages are exchanged between the server and the client.

Internetwork Protocol:

The most common internetwork protocol is Internet Protocol (IP). IP is responsible for taking the formatted segments from TCP, encapsulating them into packets, assigning the appropriate addresses, and selecting the best path to the destination host.

Network Access Protocols:

Network access protocols describe two primary functions, data link management and the physical transmission of data on the media. Data-link management protocols take the packets from IP and format them to be transmitted over the media. The standards and protocols for the physical media govern how the signals are sent over the media and how they are interpreted by the receiving clients. Transceivers on the network interface cards implement the appropriate standards for the media that is being used.