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MCITP MCSE : Security Specialist Frame Components   A frame (the term for a data packet in the network interface layer) consists of three components: the header, the data, and the trailer.   Header The header includes: An alert signal to indicate that the packet is being transmitted. The source address. The destination address.   Data This is the actual information sent by the application. This component of the packet varies in size, depending on the size limits set by the network. The data section on most networks varies from 0.5 kilobytes (KB) to 4 KB. With Ethernet, the size of the data is approximately 1.5 KB. Because most original data strings are much longer than 4 KB, data must be broken into pieces small enough to be put into packets. It takes many packets to complete the transmission of a large file.   Trailer The exact content of the trailer varies depending on the network interface layer protocol. However, the trailer usually contains an error-checking component called a cyclical redundancy check (CRC). The CRC is a number produced by a mathematical calculation on the packet at its source. When the packet arrives at its destination, the calculation is made again. If the results of both calculations are the same, this indicates that the data in the packet has remained stable. If the calculation at the destination differs from the calculation at the source, this means that the data has changed during transmission. In that case, the source computer retransmits the data.   Data Flow   The packets of data being transmitted from one computer to another travel  through the layers of the TCP/IP protocol stack. As the data packets pass through each layer, the protocols in that layer attach specific information to the header. The information added by every protocol includes error-checking information, known as the checksum. The checksum is used to verify whether the header information added by the protocol arrived intact at the destination protocol, compared with the CRC, which verifies the whole packet. The information added by the protocols in one layer is encapsulated as data by the protocols in the layer below. When the packet is received at the destination, the corresponding layer strips off a header and treats the remaining packet as data. The packet is then passed up the protocol stack to the appropriate protocol.                        Application Layer The data transmission process begins at the application layer of the TCP/IP protocol stack. An application, such as the Ftp utility, initiates the process at the source computer by preparing the data in a format that the application at the destination computer recognizes. The application at the source computer controls the entire process.     Transport Layer From the application layer, the data moves to the transport layer. This layer contains the TCP and UDP protocols. The application initiating the transmission request selects which protocol to use—TCP or UDP—and the checksum is added for both TCP and UDP.   If selected, TCP: Assigns a sequence number to each segment to be transmitted. Adds acknowledgement information for a connection-oriented transmission. Adds the TCP port number for the source and destination applications.   If selected, UDP: Adds the UDP port number for the source and destination applications.