Which layer of the OSI model controls the reliability of communications between network devices
using flow control, sequencing and acknowledgments?

A.
Physical

B.
Data-link

C.
Transport

D.
Network

Explanation:
There are many services that can be optionally provided by a transport-layer protocol, and
different protocols may or may not implement them.
Connection-oriented communication: It is normally easier for an application to interpret a
connection as a data stream rather than having to deal with the underlying connection-less
models, such as the datagram model of the User Datagram Protocol (UDP) and of the Internet
Protocol (IP).
Byte orientation: Rather than processing the messages in the underlying communication system
format, it is often easier for an application to process the data stream as a sequence of bytes. This
simplification helps applications work with various underlying message formats.
Same order delivery: The network layer doesn’t generally guarantee that packets of data will arrive
in the same order that they were sent, but often this is a desirable feature. This is usually done
through the use of segment numbering, with the receiver passing them to the application in order.
This can cause head-of-line blocking.
Reliability: Packets may be lost during transport due to network congestion and errors. By means
of an error detection code, such as a checksum, the transport protocol may check that the data is

not corrupted, and verify correct receipt by sending an ACK or NACK message to the sender.
Automatic repeat request schemes may be used to retransmit lost or corrupted data.
Flow control: The rate of data transmission between two nodes must sometimes be managed to
prevent a fast sender from transmitting more data than can be supported by the receiving data
buffer, causing a buffer overrun. This can also be used to improve efficiency by reducing buffer
underrun.
Congestion avoidance: Congestion control can control traffic entry into a telecommunications
network, so as to avoid congestive collapse by attempting to avoid oversubscription of any of the
processing or link capabilities of the intermediate nodes and networks and taking resource
reducing steps, such as reducing the rate of sending packets. For example, automatic repeat
requests may keep the network in a congested state; this situation can be avoided by adding
congestion avoidance to the flow control, including slow-start. This keeps the bandwidth
consumption at a low level in the beginning of the transmission, or after packet retransmission.
Multiplexing: Ports can provide multiple endpoints on a single node. For example, the name on a
postal address is a kind of multiplexing, and distinguishes between different recipients of the same
location. Computer applications will each listen for information on their own ports, which enables
the use of more than one network service at the same time. It is part of the transport layer in the
TCP/IP model, but of the session layer in the OSI model.