|  |  | Overview
As you know, a router has the ability to make intelligent decisions regarding the best path for delivery of data over a network. This is based on a Layer 3 or network layer addressing scheme. The router uses this information to make forwarding decisions. Once data packets go through the network layer, the transport layer, Layer 4, assumes that it can use the network as a "cloud" to send data packets from the source to the destination. The cloud resolves issues such as "Which of several paths is best for a given route?" In this chapter, you will start to see the role that routers perform in this process. In addition, you will learn how the transport layer regulates the flow of information from source to destination reliably and accurately. This chapter explains the primary functions that occur at the transport layer. This includes end-to-end control provided by sliding windows and the reliability in sequencing numbers and acknowledgements. In addition, this chapter describes how the transport-layer data stream is a logical connection between the endpoints of a network. Keeping this in mind, you will learn how the transport-layer data stream provides transport services from the host to the destination, often referred to as end-to-end services. In addition, you will learn about TCP and UDP and how they use port numbers to keep track of different conversations that cross the network at the same time, to pass information to the upper layers.