D. Transport Layer
Transport Layer is responsible for encapsulating application
data blocks into data units (datagrams, segments) suitable for
transmission to the destination host. The protocols of the
Transport Layer are TransmissionControl Protocol (TCP) and
User Datagram Protocol (UDP).
TCP provides the QOS for the applications and UDP only
provides the best efforts which can’t assure the QOS for the
communication.
E. Application Layer
Application Layer provides services for an application
program to ensure that effective communication with another
application program in a network is possible.