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Part 2 - Part 2 Tunneling and its Protocols Encapsulation...

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Part 2 Tunneling and its Protocols: Encapsulation: Encapsulation is the process of taking data from one protocol and  translating it into another protocol so the data can continue across a  network.  encapsulation is a method of designing modular communication protocols in which logically separate functions in the network are abstracted from their underlying structures by inclusion or information hiding within higher level objects.The physical layer is responsible for physical transmission of the data. Link encapsulation allows local area networking and IP provides global addressing of individual computers; UDP adds application or process selection, i.e., the portspecifies the service such as a web or TFTP server. In discussions of encapsulation, the more abstract layer is often called the upper layer protocol while the more specific layer is called the lower layer protocol. Sometimes, however, the terms upper layer protocols and lower layer protocolsare used to describe the layers above and below IP, respectively. Encapsulation is a characteristic feature of most networking models, including the OSI model, and TCP/IP.
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In the chapter describing the OSI Reference Model , I looked at several ways that protocols at various layers in a networking protocol stack interact with each other. One of the most important concepts in inter-protocol operation is that of encapsulation . Most data originates within the higher layers of the OSI model. The protocols at these layers pass the data down to lower layers for transmission, usually in the form of discrete messages. Upon receipt, each lower- level protocol takes the entire contents of the message received and encapsulates it into its own message format, adding a header and possibly a footer that contain important control information . Encapsulation is explained in general terms in a separate topic . A good analogy for how encapsulation works is a comparison to sending a letter enclosed in an envelope. You might write a letter and put it in a white envelope with a name and address, but if you gave it to a courier for overnight delivery, they would take that envelope and put it in a larger delivery envelope. ( I actually have written a complete description of this sort of analogy , if you are interested.) Due to the prominence of TCP/IP, the Internet Protocol is one of the most important places where data encapsulation occurs on a modern network. Data is passed to IP typically from one of the two main transport layer protocols: TCP or UDP. This data is already in the form of a TCP or UDP message with TCP or UDP headers. This is then encapsulated into the body of an IP message, usually called an IP datagram or IP packet . Encapsulation and formatting of an IP datagram is also sometimes called packaging —again, the implied comparison to an envelope is obvious. The process is shown in Figure 85 .
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