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What is TCP/IPTCP/IP is a suite of network protocols based on a standard architectural model called the OSI reference model (also called the OSI stack). This model consists of seven layers, each layer communicating with each of its neighbor layers only thereby separating specific tasks and/or protocols. The OSI Model
The Application layer is the layer at which applications access network services. This layer represents the services that directly support applications such as software for file transfers, database access, email, and network games. Presentation Session Transport Network Data Link Physical How a Protocol Stack WorksAs the reference model indicates, protocols (which compose the various layers) are like a pile of building blocks stacked one upon another. Because of this structure, groups of related protocols are often called stacks or protocol stacks.Data is passed down the stack from one layer to the next, until it is transmitted over the network by the network access layer protocols. At the remote end, the data is passed up the stack to the receiving application. The individual layers do not need to know how the layers above or below them function; they only need to know how to pass data to them. Each layer in the stack adds control information (such as destination address, routing controls, and checksum) to ensure proper delivery. This control information is called a header and/or a trailer because it is placed in front of or behind the data to be transmitted. Each layer treats all of the information that it receives from the layer above it as data, and it places its own header and/or trailer around that information. These wrapped messages are then passed into the layer below along
with additional control information, some of which may be forwarded
or derived from the higher layer. By the time a message exits the
system on a physical link (such as a wire), the original message
is enveloped in multiple, nested wrappers—one for each layer
of protocol through which the data passed. When a protocol uses
headers or trailers to package the data from another protocol, the
process is called encapsulation. Each layer has specific responsibilities and specific rules for carrying out those responsibilities, and it knows nothing about the procedures that the other layers follow. A layer carries out its tasks and delivers the message to the next layer in the protocol stack. An address mechanism is the common element that allows data to be routed through the various layers until it reaches its destination. Each layer also has its own independent data structures. Conceptually, a layer is unaware of the data structures used by the layers above and below it. In reality, the data structures of a layer are designed to be compatible with the structures used by the surrounding layers for the sake of more efficient data transmission. Still, each layer has its own data structures and its own terminology to describe those structures. Higher Layer Application ProtocolsProtocols are often packaged together with TCP/IP as a "suite." Higher layer application protocols in the TCP/IP suite include the Hypertext Transfer Protocol (HTTP), the File Transfer Protocol (FTP), Telnet (Telnet) which lets you logon to remote computers, Post Office Protocol (POP) and the Simple Mail Transfer Protocol (SMTP). Telnet is a user command and an underlying TCP/IP protocol for accessing remote computers. Through Telnet, an administrator or another user can access someone else's computer remotely. On the Web, HTTP and FTP protocols allow you to request specific files from remote computers, but not to actually be logged on as a user of that computer. With Telnet, you log on as a regular user with whatever privileges you may have been granted to the specific application and data on that computer. A Telnet command request looks like this (the computer name is made-up):
The result of this request would be an invitation to log on with a userid and a prompt for a password. If accepted, you would be logged on like any user who used this computer every day. Telnet is most likely to be used by program developers and anyone who has a need to use specific applications or data located at a particular host computer. All clients can use applications to access data at remote servers or other clients by opening a listening port. These ports are reserved by design and documented to anyone writing applications that utilize the TCP/IP suite of protocols. For example, telnet uses port 23. When an application uses telnet to access port 23 on another client or server, they have to "speak" telnet. The syntax of each protocol is documented in its RFC. Another example of using telnet is to telnet to a different port - say POP 110 - to retrieve your mail, or 20 to retrieve a file. In each case, the application of the user must "speak" in the POP language if connection to 110 or in the FTP language if connecting to port 20. Below is a list of what is called "well known" ports (the top 4 most common are highlighted):
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