Computer Networking : Principles, Protocols and Practice

Preface

This textbook came from a frustration of its main author. Many authors chose to write a textbook because there are no textbooks in their field or because they are not satisfied with the existing textbooks. This frustration has produced several excellent textbooks in the networking community. At a time when networking textbooks were mainly theoretical, Douglas Comer chose to write a textbook entirely focused on the TCP/IP protocol suite [Comer1988], a difficult choice at that time. He later extended his textbook by describing a complete TCP/IP implementation, adding practical considerations to the theoretical descriptions in [Comer1988]. Richard Stevens approached the Internet like an explorer and explained the operation of protocols by looking at all the packets that were exchanged on the wire [Stevens1994]. Jim Kurose and Keith Ross reinvented the networking textbooks by starting from the applications that the students use and later explained the Internet protocols by removing one layer after the other [KuroseRoss09].

 The frustrations that motivated this book are different. When I started to teach networking in the late 1990s, students were already Internet users, but their usage was limited. Students were still using reference textbooks and spent time in the library. Today’s students are completely different. They are avid and experimented web users who find lots of information on the web. This is a positive attitude since they are probably more curious than their predecessors. Thanks to the information that is available on the Internet, they can check or obtain additional information about the topics explained by their teachers. This abundant information creates several challenges for a teacher. Until the end of the nineteenth century, a teacher was by definition more knowledgeable than his students and it was very difficult for the students to verify the lessons given by their teachers. Today, given the amount of information available at the fingertips of each student through the Internet, verifying a lesson or getting more information about a given topic is sometimes only a few clicks away. Websites such as wikipedia provide lots of information on various topics and students often consult them. Unfortunately, the organisation of the information on these websites is not well suited to allow students to learn from them. Furthermore, there are huge differences in the quality and depth of the information that is available for different topics.

The second reason is that the computer networking community is a strong participant in the open-source movement. Today, there are high-quality and widely used open-source implementations for most networking protocols. This includes the TCP/IP implementations that are part of linux, freebsd or the uIP stack running on 8bits controllers, but also servers such as bind, unbound, apache or sendmail and implementations of routing protocols such as xorp or quagga . Furthermore, the documents that define almost all of the Internet protocols have been developed within the Internet Engineering Task Force (IETF) using an open process. The IETF publishes its protocol specifications in the publicly available RFC and new proposals are described in Internet drafts.

This open textbook aims to fill the gap between the open-source implementations and the open-source network specifications by providing a detailed but pedagogical description of the key principles that guide the operation of the Internet. The book is released under a creative commons licence. Such an open-source license is motivated by two reasons. The first is that we hope that this will allow many students to use the book to learn computer networks. The second is that I hope that other teachers will reuse, adapt and improve it. Time will tell if it is possible to build a community of contributors to improve and develop the book further. As a starting point, the first release contains all the material for a one-semester first upper undergraduate or a graduate networking course.

As of this writing, most of the text has been written by Olivier Bonaventure. Laurent Vanbever, Virginie Van den 3 Saylor URL:  Schriek, Damien Saucez and Mickael Hoerdt have contributed to exercises. Pierre Reinbold designed the icons used to represent switches and Nipaul Long has redrawn many figures in the SVG format. Stephane Bortzmeyer sent many suggestions and corrections to the text. Additional information about the textbook is available at http://inl.info.ucl.ac.be/CNP3.

Introduction

When the first computers were built during the second world war, they were expensive and isolated. However, after about twenty years, as their prices gradually decreased, the first experiments began to connect computers together. In the early 1960s, researchers including Paul Baran, Donald Davies or Joseph Licklider independently published the first papers describing the idea of building computer networks [Baran] [Licklider1963] . Given the cost of computers, sharing them over a long distance was an interesting idea. In the US, the ARPANET started in 1969 and continued until the mid 1980s [LCCD09]. In France, Louis Pouzin developed the Cyclades network [Pouzin1975]. Many other research networks were built during the 1970s [Moore]. At the same time, the telecommunication and computer industries became interested in computer networks. The telecommunication industry bet on the X25. The computer industry took a completely different approach by designing Local Area Networks (LAN). Many LAN technologies such as Ethernet or Token Ring were designed at that time. During the 1980s, the need to interconnect more and more computers led most computer vendors to develop their own suite of networking protocols. Xerox developed [XNS] , DEC chose DECNet [Malamud1991] , IBM developed SNA [McFadyen1976] , Microsoft introduced NetBIOS [Winston2003] , Apple bet on Appletalk [SAO1990] . In the research community, ARPANET was decommissioned and replaced by TCP/IP [LCCD09] and the reference implementation was developed inside BSD Unix [McKusick1999]. Universities who were already running Unix could thus adopt TCP/IP easily and vendors of Unix workstations such as Sun or Silicon Graphics included TCP/IP in their variant of Unix. In parallel, the ISO, with support from the governments, worked on developing an open 1 Suite of networking protocols. In the end, TCP/IP became the de facto standard that is not only used within the research community. During the 1990s and the early 2000s, the growth of the usage of TCP/IP continued, and today proprietary protocols are seldom used. As shown by the figure below, that provides the estimation of the number of hosts attached to the Internet, the Internet has sustained large growth throughout the last 20+ years.

Recent estimations of the number of hosts attached to the Internet show a continuing growth since 20+ years. However, although the number of hosts attached to the Internet is high, it should be compared to the number of mobile phones that are in use today. More and more of these mobile phones will be connected to the Internet. Furthermore, thanks to the availability of TCP/IP implementations requiring limited resources such as uIP [Dunkels2003], we can expect to see a growth of TCP/IP enabled embedded devices.

Before looking at the services provided by computer networks, it is useful to agree on some terminology that is widely used in networking literature. First of all, computer networks are often classified in function of the geographical area that they cover • LAN : a local area network typically interconnects hosts that are up to a few or maybe a few tens of kilometers apart. • MAN : a metropolitan area network typically interconnects devices that are up to a few hundred kilometers apart • WAN : a wide area network interconnect hosts that can be located anywhere on Earth 2 Another classification of computer networks is based on their physical topology. In the following figures, physical links are represented as lines while boxes show computers or other types of networking equipment. Computer networks are used to allow several hosts to exchange information between themselves. To allow any host to send messages to any other host in the network, the easiest solution is to organise them as a full-mesh, with a direct and dedicated link between each pair of hosts. Such a physical topology is sometimes used, especially when high performance and high redundancy is required for a small number of hosts. However, it has two major drawbacks :

 The second possible physical organisation, which is also used inside computers to connect different extension cards, is the bus. In a bus network, all hosts are attached to a shared medium, usually a cable through a single interface. When one host sends an electrical signal on the bus, the signal is received by all hosts attached to the bus. A drawback of bus-based networks is that if the bus is physically cut, then the network is split into two isolated networks. For this reason, bus-based networks are sometimes considered to be difficult to operate and maintain, especially when the cable is long and there are many places where it can break. Such a bus-based topology was used in early Ethernet networks.

A third organisation of a computer network is a star topology. In such topologies, hosts have a single physical interface and there is one physical link between each host and the center of the star. The node at the center of the star can be either a piece of equipment that amplifies an electrical signal, or an active device, such as a piece of equipment that understands the format of the messages exchanged through the network. Of course, the failure of the central node implies the failure of the network. However, if one physical link fails (e.g. because the cable has been cut), then only one node is disconnected from the network. In practice, star-shaped networks are easier to operate and maintain than bus-shaped networks. Many network administrators also appreciate the fact that they can control the network from a central point. Administered from a Web interface, or through a console-like connection, the center of the star is a useful point of control (enabling or disabling devices) and an excellent observation point (usage statistics).

A fourth physical organisation of a network is the Ring topology. Like the bus organisation, each host has a single physical interface connecting it to the ring. Any signal sent by a host on the ring will be received by all hosts attached to the ring. From a redundancy point of view, a single ring is not the best solution, as the signal only travels in one direction on the ring; thus if one of the links composing the ring is cut, the entire network fails. In practice, such rings have been used in local area networks, but are now often replaced by star-shaped networks. In metropolitan networks, rings are often used to interconnect multiple locations. In this case, two parallel links, composed of different cables, are often used for redundancy. With such a dual ring, when one ring fails all the traffic can be quickly switched to the other ring. 

A fifth physical organisation of a network is the tree. Such networks are typically used when a large number of customers must be connected in a very cost-effective manner. Cable TV networks are often organised as trees.

In practice, most real networks combine part of these topologies. For example, a campus network can be organised as a ring between the key buildings, while smaller buildings are attached as a tree or a star to important buildings