Page 186 - DCAP406_DCAP_207_Computer Networks

Unit 11: Transport Layer

The transport layer is a true source-to-destination or end-to-end layer. The OSI Transport Notes
layer protocol (ISO-TP) manages end-to-end control and error checking to ensure complete
data exchange. It provides “peer to peer” communication, with the transport entity of
destination machine (remote peer).

The transport layer provides a reliable on top of the unreliable services provided by the
network layer. Option negotiation among different quality of service parameters renders
an efficient, reliable and cost effective transport services to the user applications.

Flow control manages data transmission between devices so that the transmitting device
does not send more data than the receiving device can process. Multiplexing enables data
from several applications to be transmitted onto a single physical link.
Virtual circuits are established, maintained, and terminated by the transport layer. Error
checking involves creating various mechanisms for detecting transmission errors, while
error recovery involves taking an action, such as requesting that data be retransmitted, to
resolve any errors that occur.

Transport primitives are an effective way of exchanging data on top of network layer. The
services provided at the transport layer appear to be similar to the services at the data link
layer. However, they differ in many ways where data link layer provides connection
between two routers using physical channel while the transport layer uses subnets.

TCP is the protocol in the TCP/IP suite that provides reliable transmission of data. Applications
that require the transport protocol to provide reliable data delivery use TCP because it
verifies that data is delivered across the network accurately and in the proper sequence.

UDP is a connectionless unreliable protocol that reduces load on the CPU’s of the processes.
The performance issues, which however, does not have scientific model to back them, are
supported from experiences and examples. They attempt to address performance problems
in computer networks, measuring network performance, system design for better
performance, fast TPDU processing and protocols for future high-performance networks.

11.5 Keywords

Addressing: Transport Layer deals with addressing or labeling a frame.
Connection Establishment Delay: It is the amount of time when an acknowledgement is received
from the destination machine to which a connection is requested. Obviously, lesser is the delay,
better is the service.

Connection Establishment Failure Probability: Due to congestion in the network, lack of
availability of space in table, some internal problem etc, causes the connection not to set within
the establishment delay.

Connection Establishment/Release: The transport layer for creating and releasing the connections
across the network includes naming mechanism so that a process on one machine can indicate
with whom it wishes to communicate.
Demultiplexing: When several connections are multiplexed, they call for demultiplexing at the
receiving end.
Differentiated Service: It refers to provide predictable performance in terms of delay, through
put, packet loss, etc. for a given load at a given time.

Error Control: To control errors from lost or duplicate segments the transport layer enables
unique segment sequence numbers to the different packets of message, creating virtual circuits,
allowing only one virtual circuit per session.

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