reassembles it into a stream of data on the receiving
device. This layer specifies several mechanisms so that
data is reliably transmitted between devices. Error
detection and recovery mechanisms and information
flow control ensure reliable service. This is also where
the delineation between the application and the network
is usually done. The transport layer and the network,
data link, and physical layers make up the data transport
layers. The main purpose of these layers is to reliably
transfer the data from the sending device to the receiving
device. The layers on top of the transport layer—the
session, presentation, and application layers—as you will
soon see, are concerned more with application issues and
making sure data is formatted the right way. Two types
of protocols are defined at this layer: connection-
oriented and connectionless protocols.
A connection-oriented protocol establishes an end-to-
end connection between the sender and the receiver,
keeps track of all the segments that are being
transmitted, and has a retransmission mechanism in
place. Retransmissions happen in cases in which some of
the segments that are sent are lost on the way and don’t
get to their destination or become corrupted and cannot
be recovered at the receiver. A system of
acknowledgments is in place to signal successfully
receiving the data and availability to receive additional
data. A sliding window mechanism dynamically adjusts
the amount of data that is transmitted on the wire before
the sender is waiting for an acknowledgment from the
receiver. Large data transfers, SSH terminal sessions,
and World Wide Web and email sessions use connection-
oriented protocols at the transport layer. Although not
defined with the OSI specification, Transmission Control
Protocol (TCP) is the most common connection-oriented
protocol at the transport layer.
A connectionless protocol does not establish an end-to-
end connection between the sender and the receiver.