For this the presentation layer may perform one or

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Unformatted text preview: k-independent transport service. Using this service, all the details of the communication subnet are sealed and one subnet can be replaced with another without disturbing the layers above the transport layer. In particular, the transport layer accepts message of arbitrary length from the session layer, segments them into packets, submits them to the network layer for transmission, and finally reassembles the packets at the destination. Some packets may be lost on the way from the sender to the receiver, and depending on the routing algorithms used in the network layer, packets may arrive at the destination in a sequence that is different from the order in which they are sent. The transport layer protocols include mechanisms for handling lost and out-of-sequence packets. For this, the transport layer records a sequence number in each packet and uses the sequence numbers for detecting lost packets and for ensuring that messages are reconstructed in the correct sequence. The two most popular transport layer protocols are the Transport Control Protocol (TCP) and the User Datagram Protocol (UDP). TCP is a connection-oriented transport protocol. It uses end-to-end mechanisms to ensure reliable, ordered delivery of data over a logical connection. These goals are basically achieved by using packet sequence numbers and positive acknowledgements with timeout and retransmission. The UDP is a connectionless transport protocol. It is an unreliable protocol because, when it is used, message packets can be lost, duplicated, or arrive out of order. Therefore, only those applications that do not need reliable communication should use UDP. Session Layer The purpose of the session layer is to provide the means by which presentation entities can organize and synchronize their dialog and manage their data exchange. It allows the two parties to authenticate each other before establishing a dialog session between them. It also specifies dialog type - one way, two way alternate, or two way simultaneous - and initiates a...
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This document was uploaded on 04/07/2014.

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