Transport layer protocols can provide an application with one or more security service. For example, a transport protocol in the sending host can encrypt and transmit all of its data by the sending process. In the receiving host, the transport protocol can decrypt off of its data before it is sent to the receiving process. One or more services run by these services proved confidentiality between these two processes. This protocol can also provide other security services in addition to confidentiality. Other services which are provided include data integrity and end-point authentication. This is an interesting layer, because it represents the center of the model and provides an important transition point between all of the associated layers. Protocols running at this layer must provide several important services for security reasons. Transport layer protocols are import and are strongly tied to the network later protocols.
The following services are provided to support Transport Layer Protocols:

TCP Service: connection-oriented . o Connection-oriented: Meaning a setup is required between a client and server. o Reliable Transport: It is a reliable transport between sending and receiving process. o Flow Control: Means the sender won’t overwhelm the receiver. o Congestion Control: Means to throttle the sender when network is overloaded. o Does not provide: It does not provide timing and minimum bandwidth guarantees.

UDP Service: is connectionless. o Unreliable: It has an unreliable data transfer between sending and receiving processes. o Does not provide: It does not provide connection setup, reliability, flow control, o congestion control, timing, or minimum bandwidth guarantees
Examples

Useful Links http://www.cs.tut.fi/kurssit/TLT-2756/lect06.pdf http://www.highteck.net/EN/Transport/OSI_Transport_Layer.html" http://www.cs.odu.edu/~cboyle/cs312/cs312_04_ClientServer_Fall2k12.pdf" http://dtn.dsg.cs.tcd.ie/sft/thesis.pdf
http://www.javvin.com/protocolISOTP.html