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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
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.
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.
- Spring '14