Overview of the Transport layer

The Internet is based on the TCP/IP protocol suite. In general, data packets handled at the transport layer are called segments and the data packets handled at the network layer are called data grams. However, in some literature, the data packets handled by the TCP protocol are called segments whereas data packets of the UDP protocol are called data grams. Unfortunately,there is some confusion in the terms used by different documents. For simplicity’s sake assume that a segment refers to a transport layer data packet while a data gram refers to a network layer data packet.

The network layer of the Internet is based on the Internet Protocol or IP. IP is a best‐effort delivery
service which means that it will try its best to deliver data packets but makes no guarantees about the delivery. IP does not guarantee the delivery of segments (transport data packets), does not guarantee the order of the delivered segments and does not guarantee the integrity of the delivered segments (that the segments delivered are the same ones transmitted by the sender).

As we have seen previously, there are two major distinct protocols commonly used in the
Transport layer: Transmission Control Protocol and the User Data gram Protocol. Both protocols provides additional services to those provided by the Internet Protocol to ensure that two applications can communication with each other. Both contain checksum to ensure that segments are delivered error free. However, the UDP is an unreliable service while the TCP is a reliable service. The two main services provided by the UDP are the process‐to‐process data delivery and the error checking.

On the other hand, the TCP provides a reliable end‐to‐end delivery services. It guarantees that all segments transmitted are delivered to the destination, delivered segments are error free and the segments are delivered in the same order in which they were transmitted. It also handles congestion control. Congestion control refers to the regulating of the data flows between two applications so that a fast transmitter does not flood a slow receiver.