ccna 640-802 s3-5/13@20/791

QUESTION NO: 5
Which layer of the OSI reference model is responsible for ensuring reliable
end-to-end delivery of data?
A. Application
B. Presentation
C. Session
D. Transport
E. Network
F. Data-Link

Answer: D
Explanation:
A key function of the transport layer is to provide connection services for the protocols
and applications that run at the levels above it. These can be categorized as either
connection-oriented services or connectionless services. Some protocol suites, such as
TCP/IP, provide both a connection-oriented and a connectionless transport layer protocol,
to suit the needs of different applications.
The transport layer is also the place in the layer stack where functions are normally
included to add features to end-to-end data transport. Where network layer protocols are
normally concerned with just "best effort" communications, where delivery is not
guaranteed. Transport layer protocols are given intelligence in the form of algorithms that
ensure that reliable and efficient communication between devices takes place. This
encompasses several related jobs, including lost transmission detection and handling, and
managing the rate at which data is sent to ensure that the receiving device is not
overwhelmed.
Transmission quality, meaning ensuring that transmissions are received as sent, is so
important that some networking references define the transport layer on the basis of
reliability and flow-control functions. However, not all transport layer protocols provide
these services. Just as a protocol suite may have a connection-oriented and a
connectionless transport layer protocol, it may also have one that provides reliability and
data management services, and one that does not. Again, this is the case with TCP/IP:
there is one main transport layer protocol; TCP, that includes reliability and flow control
features, and a second, UDP, that doesn't.