Cisco Networking: OSI Model Layer 4 - Transport
The transport layer of the Open System Interconnection (OSI) model is closely related to the network layer, but adds functionality to it. You can think of this layer as a traffic cop or a supervisor for the mindless mob of packets that is the network layer. The transport layer takes responsibility for verifying data delivery.
The network layer may have a high success rate in getting data to the destination, but the transport layer is specifically told to ensure delivery of data. Either way, you are going to use transport layer delivery mechanisms, but you (or the application you are using) will need to make a decision as to whether you will be using verified data delivery or unverified data delivery.
In both cases, you are still using a transport layer component or protocol.
The actual mechanisms that are used to guarantee data delivery are dependent on the networking protocol that is in use, whereas the concept of delivery reliability is more universal. When working with the TCP/IP protocol suite, verified delivery is performed using the Transmission Control Protocol (TCP), while unverified delivery is performed using User Datagram Protocol (UDP).
For example, compare a text message to a phone conversation. Even though a large number of text messages get through, there is no guarantee that any given message to a person has been received, whereas an answered phone call and the ensuing interchange with the other person gives you the immediate feedback that the words are received by the recipient. Similarly, voicemail is equally as useless as a text message in terms of feedback.
Now you may wonder why you would want to use unverified delivery. Well, in the case of person-to-person interaction, some people prefer text messages to voice calls, because they can get their message to someone without getting into a lengthy conversation. Confirmation that the message got through occurs when they receive a reply. If that reply does not occur within a reasonable time frame, the message can be resent.
The same is true of network communications, where the cost of verified delivery is additional overhead in establishing a delivery channel, verifying the data is received after you have sent the message, and closing down the delivery channel. If the data you plan to send is small or not very important, then this overhead can be tedious and slow the communication process down; but if you eliminate the overhead, you also eliminate the verification.
This is not a problem that is of concern to the transport layer, the request to send that data came from the session layer.
If the session layer has marked the data as no verification required, then the session layer is responsible for performing its own check or verification of delivery. Based on the data, the session layer may choose not to worry about delivery or the session layer may have its own rules of delivery verification, such as it expects a response from the destination computer within five seconds, after which the session layer will resend the data.