Routing Principles, Routing and Overview, DVR and LSR – Part 1/5

Routing Principles, Routing and Overview, DVR and LSR - Part 1

The network layer

The transport layer enables the applications to efficiently and reliably exchange data. Transport layer entities expect to be able to send a segment to any destination without having to understand anything about the underlying subnetwork technologies. Many subnetwork technologies exist. Most of them differ in subtle details (frame size, addressing, …). ( Routing Principles, Routing and Overview, DVR and LSR )The network layer is the glue between these sub-networks and the transport layer. It hides to the transport layer all the complexity of the underlying subnetworks and ensures that information can be exchanged between hosts connected to different types of subnetworks.

In this chapter, we first explain the principles of the network layer. These principles include the datagram and virtual circuit modes, the separation between the data plane and the control plane and the algorithms used by routing protocols. Then, we explain, in more detail, the network layer on the Internet, starting with IPv4 and IPv6 and then moving to the routing protocols (RIP, OSPF, and BGP).


Routing Principles, Routing and Overview, DVR and LSR

The main objective of the network layer is to allow end systems, connected to different networks, to exchange information through intermediate systems called router. The unit of information in the network layer is called a packet.

The network layer in the reference model

Routing Principles, Routing and Overview, DVR and LSR

Before explaining the network layer in detail, it is useful to begin by analyzing the service provided by the data link layer. There are many variants of the data link layer. Some provide a connection-oriented service while others provide a connectionless service. In this section, we focus on connectionless data link layer services as they are the most widely used. Using a connection-oriented data link layer causes some problems that are beyond the scope of this chapter.

The point-to-point data link layer

Routing Principles, Routing and Overview, DVR and LSR

There are three main types of datalink layers. The simplest datalink layer is when there are only two communicating systems that are directly connected to the physical layer. Such a data link layer is used when there is a point-to-point link between the two communicating systems. The two systems can be end systems or routers. PPP, defined in RFC 1661, is an example of such a point-to-point data link layer. Datalink layers exchange frames and a datalink frame sent by a data link layer entity on the left is transmitted through the physical layer so that it can reach the data link 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 data link 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 data link 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 data link layer. Both end systems 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.

A local area network

Routing Principles, Routing and Overview, DVR and LSR

In the next chapter, we describe the organization and the operation of Local Area Networks. An important difference between the point-to-point data link layers and the data link layers used in LANs is that in a LAN, each communicating device is identified by a unique data link layer address. This address is usually embedded in the hardware of the device and different types of LANs use different types of data link layer addresses. A communicating device attached to a LAN can send a data link frame to any other communicating device that is attached to the same LAN. Most LANs also support special broadcast and multicast data link 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 data link layer address, it is delivered by the LAN to all communicating devices that belong to the corresponding group.

The third type of datalink layers is 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 data link layer address. However, and this is the main difference between an NBMA network and a traditional LAN, the NBMA service only supports unicast. The data link layer service provided by an NBMA network supports neither broadcast nor multicast.

Unfortunately, no data link layer is able to send frames of the unlimited side. Each data link layer is characterized by a maximum frame size. There are more than a dozen different data link layers and unfortunately, most of them use a different maximum frame size. The network layer must cope with the heterogeneity of the data link layer.

The network layer itself relies on the following principles:

  1. Each network layer entity is identified by a network layer address. This address is independent of the data link layer addresses that it may use.
  2. The service provided by the network layer does not depend on the service or the internal organization of the underlying datalink layers.
  3. 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.

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