Part 2 book “Computer networks” has contents: The network layer, the transport layer, the application layer, network security, reading list and bibliography, authentication protocols, communication security, management of public keys, and other contents. | 5 THE NETWORK LAYER The network layer is concerned with getting packets from the source all the way to the destination. Getting to the destination may require making many hops at intermediate routers along the way. This function clearly contrasts with that of the data link layer, which has the more modest goal of just moving frames from one end of a wire to the other. Thus, the network layer is the lowest layer that deals with end-to-end transmission. To achieve its goals, the network layer must know about the topology of the network (., the set of all routers and links) and choose appropriate paths through it, even for large networks. It must also take care when choosing routes to avoid overloading some of the communication lines and routers while leaving others idle. Finally, when the source and destination are in different networks, new problems occur. It is up to the network layer to deal with them. In this chapter we will study all these issues and illustrate them, primarily using the Internet and its network layer protocol, IP. NETWORK LAYER DESIGN ISSUES In the following sections, we will give an introduction to some of the issues that the designers of the network layer must grapple with. These issues include the service provided to the transport layer and the internal design of the network. 355 356 THE NETWORK LAYER CHAP. 5 Store-and-Forward Packet Switching Before starting to explain the details of the network layer, it is worth restating the context in which the network layer protocols operate. This context can be seen in Fig. 5-1. The major components of the network are the ISP’s equipment (routers connected by transmission lines), shown inside the shaded oval, and the customers’ equipment, shown outside the oval. Host H1 is directly connected to one of the ISP’s routers, A, perhaps as a home computer that is plugged into a DSL modem. In contrast, H2 is on a LAN, which might be an office Ethernet, with a router, F, owned and operated
