Network and Transport Layers (Data Communications and Networking)

The Network layer and transport layer are responsible for moving messages from end to end in a network. They are so closely tied together that they are usually discussed together. The transport layer (layer 4) performs three functions: linking the application layer to the network, segmenting (breaking long messages into smaller packets for transmission), and session management (establishing an end-to-end connection between the sender and receiver). The network layer (layer 3) performs two functions: routing (determining the next computer to which the message should be sent to reach the final destination) and addressing (finding the address of that next computer). There are several standard transport and network layer protocols that specify how packets are to be organized, in the same way that there are standards for data link layer packets. However, only one protocol is in widespread use today: TCP/IP, the protocol used on the Internet. This topic takes a detailed look at how TCP/IP works.


■ Be aware of the TCP/IP protocols

■ Be familiar with linking to the application layer, segmenting, and session management

■ Be familiar with addressing

■ Be familiar with routing

■ Understand how TCP/IP works


The transport and network layers are so closely tied together that they are almost always discussed together. For this reason, we discuss them in the same topic. There are several different protocols that can be used at the transport and network layers, in the same way there are several different data link layer protocols. TCP/IP is the most commonly used set of protocols and is well on its way to eliminating the other protocols. Therefore, this topic focuses exclusively on TCP/IP.

The transport layer links the application software in the application layer with the network and is responsible for the end-to-end delivery of the message. The transport layer accepts outgoing messages from the application layer and segments them for transmission. Figure 5.1 shows the application layer software producing an SMTP packet that is split into two smaller TCP segments by the transport layer.

Message transmission using layers. HTTP = Hypertext Transfer Protocol; IP = Internet Protocol; TCP = Transmission Control Protocol

Figure 5.1 Message transmission using layers. HTTP = Hypertext Transfer Protocol; IP = Internet Protocol; TCP = Transmission Control Protocol

The Protocol Data Unit (PDU) at the transport layer is called a segment. The network layer takes the messages from the transport layer and routes them through the network by selecting the best path from computer to computer through the network (and adds an IP packet). The data link layer adds an Ethernet frame and instructs the physical layer hardware when to transmit. As we saw in next topic, each layer in the network has its own set of protocols that are used to hold the data generated by higher layers, much like a set of Matryoshka (nested Russian dolls).

The network and transport layers also accept incoming messages from the data link layer and organize them into coherent messages that are passed to the application layer. For example, as in Figure 5.1 a large e-mail message might require several data link layer frames to transmit. The transport layer at the sender would break the message into several smaller segments and give them to the network layer to route, which in turn gives them to the data link layer to transmit. The network layer at the receiver would receive the individual packets from the data link layer, process them, and pass them to the transport layer, which would reassemble them into the one e-mail message before giving it to the application layer.

In this topic, we provide a brief look at the transport and network layer protocols, before turning our attention to how TCP/IP works. We first examine the transport layer functions. Addressing and routing are performed by the transport layer and network layers working together, so we will discuss them together rather than separate them according to which part is performed by the transport layer and which by the network layer.

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