Networking Self-Teaching Guide: OSI, TCP/IP, LANs, MANs, WANs, Implementation, Management, and Maintenance

Chapter 9. The Transport Layer

	Transport of the mails, transport of the human voice, transport of flickering pictures—in this century as in others our highest accomplishments still have the single aim of bringing men together.
	--Antoine de Saint-Exupéry

The last chapter talked about the upper layers of the OSI reference model. You learned the specific purpose of each layer and how the layers interact with each other. This chapter covers the Transport layer, Layer 4 of the OSI reference model.

The Transport layer is the highest layer of the lower layers of the OSI reference model. The Transport layer sits on top of the Network layer and below the Session layer. This layer is responsible for the end-to-end connection and datagram delivery, as well as congestion control and flow control. The two main protocols that operate at this layer are UDP and TCP, which were discussed in Chapter 5.

The purpose of the Transport layer is to set up connections, maintain connections, shut down connections, and perform error checking.^[304] The protocols that operate at this layer are considered either connection-oriented (i.e., TCP) or connectionless (i.e., UDP). Remember that connection-oriented means that the connection must be set up before data can be transmitted, and connectionless means that data can flow without the connection being established first.

So far this book has explained what the Transport layer is and the services and protocols it provides. This chapter takes a little deeper look ...