Kurose & Ross (2010, p92) list the four classes of services that a transport protocol can provide as “reliable data transfer, throughput, timing and security”.
The reliable data transfer service is one that guarantees the delivery of data between source and destination in applications that require such a guarantee. An example of such a data transfer would be a bank transaction where data sent must be completely and reliably received in any communication and vice versa between applications with no degree of tolerance. In terms of data transfer, the Transfer Control Protocol (TCP), even though delays can occur, takes advantage of this service and is used to provide reliable data transfer. On the other hand, the User Datagram Protocol (UDP) offers data transfer with a degree of tolerance on losses and therefore cannot be classed as taking advantage of the reliable data transfer service.
The throughput service guarantees throughput (bandwidth) at a certain rate that could be requested and guaranteed by the transport protocol. An application could take advantage of this if its required throughput was known (the communication data rate) and the required bandwidth could be guaranteed for a specified period of time. Such an application that would require a throughput guarantee, termed a “bandwidth-sensitive application” Kurose & Ross (2010, p93), could be a media streaming application that requires a certain bandwidth in order to deliver the media to the destination at an acceptable rate, although some media applications use techniques to negate the use of a minimum bandwidth. Similarly, the timing service could be used to require interactive responses within a certain time limit in real-time applications (e.g. VoIP). TCP and UDP do not offer the ability to use guaranteed throughput or timing as this would impact on data transmission performance by making throughput unavailable to other requests.
The security service provides encryption at the source and decryption at the destination for any data. TCP and UDP do not take advantage of the security service and it is possible for the content of the data contained in these transmissions to be intercepted by sniffing and man-in-the-middle attacks.