This type of data transmission is mostly used for real-
time traffic such as voice and video, in which it is better
to ignore the data segments that are lost or corrupted on
the way than to stop the transfer of data and ask for
retransmission. For example, in the case of a voice over
IP (VoIP) call, the end-user experience is much better if
the network just ignores a few milliseconds of lost data
that the human ear can compensate for anyway than if
the network stops the flow of data, queues the data
already received at the receiver, retransmits the missing
few milliseconds, replays the message, and then
dequeues the data that was waiting for playback at the
receiver. User Datagram Protocol (UDP) is the most
common connectionless protocol used at the transport
layer.
The session layer establishes, manages, and terminates
sessions between two communicating devices. On top of
the end-to-end communication channel that the
transport layer establishes, the session layer has the role
of keeping track of whose turn is to transmit data, and it
ensures that the same operation is not performed at the
same time by different clients or even by resuming failed
transmissions. Applications that implement the Remote
Procedure Call (RPC) framework use the session layer
extensively.
The presentation layer ensures that the information
from the application layer of the sending device is
readable and interpretable by the application layer of the
receiving device. The presentation layer is not focused on
the moving of bits between devices as much as it is
focused on how the data being transferred is organized
and the syntax and semantics used for this data. For
example, if different encodings for text-based exchange
of information are used on the sending and receiving
devices (for example, ASCII versus EBCDIC), the
presentation layer uses a common encoding format for
translating between the two encodings and makes sure