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From a security perspective, encryption is important because it provides the ability to keep information confidential.Connection Versus ConnectionlessAt the transport layer are the two protocols known as TCP and UDP; these protocols areknown as connection and connectionless respectively. Connection-oriented protocols operate byacknowledging or confirming every connection request or transmission, much like getting a returnreceipt for a letter. Connectionless protocols are those that do not require an acknowledgementand in fact do not ask for nor get one. The difference between these two is the overhead that isinvolved. Due to connection-oriented protocol’s need for acknowledgements, the overhead is moreand the performance is less, while connectionless is faster due to its lack of this requirement.NOTEExamples of these formats include American Standard Code for Information Interchange (ASCII) and Extended Binary Coded Decimal Interchange Code (EBCDIC).
30PART 1|Hacker Techniques and Tools Layer 7: Application LayerCapping off the OSI reference model is the application layer (Layer 7). The application layer hosts several application services that are used by applications and other services running on the system. For example, Web browsers that would be classified as a user-level application run on a system and access the network by “plugging” into the services at this layer to use the network. This layer includes network monitoring, management, file sharing, RPC, and other services used by applications. The application layer is one that most users are familiar with because it is the home of e-mail programs, file transfer protocol (FTP), Telnet, Web browsers, office productivity suites, and many other applications. It is also the home of many malicious programs such as viruses, worms, Trojan horse programs, and other malevolent applications. The Role of EncapsulationIn the OSI framework, the concept of encapsulationis the process of “packaging” infor-mation prior to transmitting it from one location to another. When transmitted across the network, it moves down from the application layer to the physical layer and then through the physical medium. As the data moves from the application layer down, the information is packaged and manipulated along the way until it becomes a collection of bits that race down the wire to the receiving station, where the process is reversed as the data moves back up the model. DataApplicationTransportInternetLinkUDPDataIP DataUDPHeaderFrameHeaderFrameFooterIPHeaderFrame DataFIGURE 2-3Encapsulation.Be sure that when thinking of the name “application layer,” you take care not to think ofsoftware applications. Software applications are those items that a user of a system interactswith directly, such as e-mail applications and Web browsers. The application layer is the pointat which software applications access network services as needed. Think of the softwareapplications as a microwave oven in your home and the application layer as the electrical outletthat the microwave plugs into to get power.