Networking Protocols

 Intro

Information technology has affected virtually every sector of the indus- try—from steel to automobiles and pharmaceuticals to telecom. Today, we cannot think of a corporate office that does not use computers to improve the overall efficiency and productivity.
Internet and all other computer applications require computers to be networked. Whether we want to share hardware resources, Internet connections, or data, networking (both LAN and WAN) provides a solution to all these requirements. It is one of the fastest growing segments of the IT industry.

What is a protocol

In a network, computers are inter-connected to each other both physically by cables and logically  connection refer to the protocols
Protocols are basically similar to a language, which computers use to communicate with each other. These are sets of rules or technical procedures designed to enable computers to connect with one another and peripheral devices to exchange information with as little error as possible. Protocols ease the complex process of enabling computers of different makes and models to communicate.
In a network, there are many protocols. While each protocol allows basic communication, all the protocols have different purposes and accomplish different tasks. Each protocol has its own advantages and restrictions.
Some protocols work at various OSI layers. The layer at which a protocol works describes its functions. Several protocols may work together in what is known as a ‘protocol stack’. Taken together, the protocols describe the entire stack’s functions and capabilities. Protocols which do not support routing are called nonroutable protocols; for example, NetBeUI. These protocols cannot connect two different LAN segments.
Protocols that support multipath LAN-to-LAN communication are known as routable protocols. These protocols are gaining importance because the industry is moving towards wide-area networking (WAN) wherein several LANs are connected together.

How protocols work

Once a client has connected to a service on a particular port, it accesses the service using a specific protocol. The protocol is the pre-defined way that someone who wants to use a service talks with that service. The "someone" could be a person, but more often it is a computer program like a Web browser. Protocols are often text, and simply describe how the client and server will have their conversation.
Perhaps the simplest protocol is the daytime protocol. If you connect to port 13 on a machine that supports a daytime server, the server will send you its impression of the current date and time and then close the connection. The protocol is, "If you connect to me, I will send you the date and time and then disconnect."
 for better understanding :
The entire technical operation of transmitting data over the network is broken down into discrete systematic steps. At each step, a certain action takes place, which cannot take place at any other step. Steps must be carried out in a consistent order that is same on every computer in the network. 
In The entire technical operation of transmitting data over the network is broken down into discrete systematic steps. At each step, a certain action takes place, which cannot take place at any other step. Steps must be carried out in a consistent order that is same on every computer in the network. In the sending computer, these steps must be carried out from top to down. But in the receiving machine, these must be carried out vice versa.
At the sending computer, a protocol breaks the data into smaller sections called packets, and adds addressing information to these packets so that the destination computer on the network can know that the data belongs to it. It then prepares the data for actual transmission through the network adaptor card and out onto the network.
At the receiving computer, a protocol takes data packets off the cable and brings these into the computer through the network adaptor card. It then strips data packets of all the transmitting information added by the sending computer, and passes the reassembled data to the application in usable form.

Protocol stack

A protocol stack (sometimes communications stack) is a particular software implementation of a computer networking protocol suite. The terms are often used interchangeably. Strictly speaking, the suite is the definition of the protocols, and the stack is the software implementation of them
Individual protocols within a suite are often designed with a single purpose in mind. This modularization makes design and evaluation easier. Because each protocol module usually communicates with two others, they are commonly imagined as layers in a stack of protocols. The lowest protocol always deals with "low-level", physical interaction of the hardware. Every higher layer adds more features. User applications usually deal only with the topmost layers

In practical implementation

protocol stacks are often divided into three major sections:
1)media, 2) transport, and  3)applications.
A particular operating system or platform will often have two well-defined software interfaces: one between the media and transport layers, and one between the transport layers and applications.
The media-to-transport interface defines how transport protocol software makes use of particular media and hardware types .The application-to-transport interface defines how application programs make use of the transport layers.  

General protocol suite description

T ~ ~ ~ T  [A]     [B]_____[C]

Imagine three computers: A, B, and C. A and B both have radio equipment, and can communicate via the airwaves using a suitable network protocol B and C are connected via a cable, using it to exchange data . However, neither of these two protocols will be able to transport information from A to C, because these computers are conceptually on different networks. One, therefore, needs an inter-network protocol to "connect" them.
One could combine the two protocols to form a powerful third, mastering both cable and wireless transmission, but a different super-protocol would be needed for each possible combination of protocols. It is easier to leave the base protocols alone, and design a protocol that can work on top of any of them . This will make two stacks of two protocols each. The inter-network protocol will communicate with each of the base protocol in their simpler language; the base protocols will not talk directly to each other.
A request on computer A to send a chunk of data to C is taken by the upper protocol, which knows that C is reachable through B. It, therefore, instructs the wireless protocol to transmit the data packet to B. On this computer, the lower layer handlers will pass the packet up to the inter-network protocol, which, on recognizing that B is not the final destination, will again invoke lower-level functions. This time, the cable protocol is used to send the data to C. There, the received packet is again passed to the upper protocol, which  will pass it on to a higher protocol or application on C. Often an even higher-level protocol will sit on top, and incur further processing.

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