The main objective of the network layer is to allow endsystems, connected to different networks, to exchange information through intermediate systems called router. The unit of information in the network layer is called a packet.
Before explaining the network layer in detail, it is useful to begin by analysing the service provided by the datalink layer. There are many variants of the datalink layer. Some provide a connection-oriented service while others provide a connectionless service. In this section, we focus on connectionless datalink layer services as they are the most widely used. Using a connection-oriented datalink layer causes some problems that are beyond the scope of this chapter. See RFC 3819 for a discussion on this topic. There are three main types of datalink layers. The simplest datalink layer is when there are only two communicating systems that are directly connected through the physical layer. Such a datalink layer is used when there is a point-to-point link between the two communicating systems. The two systems can be endsystems or routers. PPP (Point-to-Point Protocol), defined in RFC 1661, is an example of such a point-to-point datalink layer. Datalink layers exchange frames and a datalink frame sent by a datalink layer entity on the left is transmitted through the physical layer, so that it can reach the datalink layer entity on the right. Point-to-point datalink layers can either provide an unreliable service (frames can be corrupted or lost) or a reliable service (in this case, the datalink layer includes retransmission mechanisms similar to the ones used in the transport layer). The unreliable service is frequently used above physical layers (e.g. optical fiber, twisted pairs) having a low bit error ratio while reliability mechanisms are often used in wireless networks to recover locally from transmission errors.
The second type of datalink layer is the one used in Local Area Networks (LAN). Conceptually, a LAN is a set of communicating devices such that any two devices can directly exchange frames through the datalink layer. Both endsystems and routers can be connected to a LAN. Some LANs only connect a few devices, but there are LANs that can connect hundreds or even thousands of devices.
In the next chapter, we describe the organisation and the operation of Local Area Networks. An important difference between the point-to-point datalink layers and the datalink layers used in LANs is that in a LAN, each communicating device is identified by a unique datalink layer address. This address is usually embedded in the hardware of the device and different types of LANs use different types of datalink layer addresses. A communicating device attached to a LAN can send a datalink frame to any other communicating device that is attached to the same LAN. Most LANs also support special broadcast and multicast datalink layer addresses. A frame sent to the broadcast address of the LAN is delivered to all communicating devices that are attached to the LAN. The multicast addresses are used to identify groups of communicating devices. When a frame is sent towards a multicast datalink layer address, it is delivered by the LAN to all communicating devices that belong to the corresponding group.
The third type of datalink layers are used in Non-Broadcast Multi-Access (NBMA) networks. These networks are used to interconnect devices like a LAN. All devices attached to an NBMA network are identified by a unique datalink layer address. However, and this is the main difference between an NBMA network and a traditional LAN, the NBMA service only supports unicast. The datalink layer service provided by an NBMA network supports neither broadcast nor multicast.
Unfortunately no datalink layer is able to send frames of unlimited side. Each datalink layer is characterised by a maximum frame size. There are more than a dozen different datalink layers and unfortunately most of them use a different maximum frame size. The network layer must cope with the heterogeneity of the datalink layer.
The network layer itself relies on the following principles :
- Each network layer entity is identified by a network layer address. This address is independent of the datalink layer addresses that it may use.
- The service provided by the network layer does not depend on the service or the internal organisation of the underlying datalink layers.
- The network layer is conceptually divided into two planes : the data plane and the control plane. The data plane contains the protocols and mechanisms that allow hosts and routers to exchange packets carrying user data. The control plane contains the protocols and mechanisms that enable routers to efficiently learn how to forward packets towards their final destination.
The independence of the network layer from the underlying datalink layer is a key principle of the network layer. It ensures that the network layer can be used to allow hosts attached to different types of datalink layers to exchange packets through intermediate routers. Furthermore, this allows the datalink layers and the network layer to evolve independently from each other. This enables the network layer to be easily adapted to a new datalink layer every time a new datalink layer is invented.
There are two types of service that can be provided by the network layer :
- an unreliable connectionless service
- a connection-oriented, reliable or unreliable, service
Connection-oriented services have been popular with technologies such as X.25 and ATM or frame-relay, but nowadays most networks use an unreliable connectionless service. This is our main focus in this chapter.
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