Chapter 2 An Introduction to Networking

Objectives After reading this chapter and completing the exercises, you will be able to:

• Describe the basic elements of computer-based data communication • Know the key entities and organizations behind current networking standards, as well as the purpose of and

intent behind the more widely used standards • Explain the nature and intent of the OSI reference model, and list and describe each of the model’s seven

layers • Describe the nature of the Internet and the relationship between the TCP/IP protocol and the Internet

Teaching Tips

Networking Fundamentals In a fundamental exchange of information between two parties, one party—the sender—communicates a message to a second party—the receiver—over some medium. Any communications medium may be subject to various types of interference, which is commonly called noise. Noise occurs in a variety of forms:

• Attenuation • Crosstalk • Distortion • Echo • Impulse noise • White noise

Reasons to Network Data communications and networking go hand in hand. Data communications is the exchange of messages across a medium, and networking is the interconnection of groups or systems with the purpose of exchanging information. There are a number of reasons to build a network:

• To exchange information • To share scarce or expensive resources • To allow distributed organizations to act as if centrally located

Types of Networks Networks can be categorized by components, size, layout or topology, or media. Networks by Components Networks categorized by components include peer-to-peer (P2P) networks, server-based networks, and distributed multi-server networks. In peer-to-peer networks, the individual users or clients directly interact and share resources, without the benefit of a central repository or server.

Networks by Size One of the most common methods of categorizing networks is by size.

• Local area network (LAN) • Metropolitan area network (MAN) • Wide area network (WAN)

Networks by Topology The topology can be physical or logical. Physical topology describes how the network is cabled, while logical topology describes how the network functions. The dominant physical topologies are ring, bus, star, hierarchy, mesh, and hybrid. Networks by Media Networks are also categorized by media type. Media typically fall into two categories—guided and unguided, also known as wired and wireless.

Network Standards Protocols that are widely accepted become standards. Some standards are formal, or de jure; these formal standards have been reviewed by a group of experts and endorsed by a standards body. Other standards are informal, or de facto, and have simply been widely adopted without having been formally reviewed. The Internet standards such as TCP/IP are de facto standards, while Ethernet standards (IEEE 802.3) are de jure standards. Internet Society (ISOC) The Internet Society seeks to formalize many of the standards used to facilitate the development of compatible technologies.

• Internet Engineering Task Force (IETF) • Internet Research Task Force (IRTF) • Internet Engineering Steering Group (IESG) • Internet Architecture Board (IAB)

Internet Assigned Numbers Authority (IANA) Originally part of the ISOC, the IANA is responsible for the coordination of domain names, IP addresses and protocols, and port assignments. American National Standards Institute (ANSI) The American National Standards Institute (ANSI) serves to reinforce the position of the U.S. government and industry while helping to ensure the safety and health of consumers and ensuring environmental protection. The institute supervises the creation and use of thousands of standards and directives, which directly affect companies and government agencies in almost every sector of the economy. International Telecommunication (ITU) The ITU is the principal agency of the United Nations for communication and information technologies. Acting as a focus for government interests and the private sector, ITU serves to help the world communicate in three critical sectors: radio communication, calibration, and development.

Institute of Electrical and Electronics Engineers (IEEE) A nonprofit organization, IEEE is the world’s principal trade association for the advancement of technology. IEEE is active in many industrial sectors including computers and telecommunications, electric power, and electronics. Telecommunications Industry Association (TIA) TIA is a partnership among information, communications, and entertainment companies. TIA is active in the development of standards and has a role in the development of both domestic and international policy. International Organization for Standardization (ISO) ISO is the global leader in developing and publishing international standards. It is a network of the national standards bodies from among 157 countries with one member per country.

Quick Quiz 1. A _______ is the geometric association of components of a network in relation to each other. ANSWER: topology 2. ________ can be categorized by components, size, layout or topology, or media. ANSWER: Networks

OSI Reference Model and Security In 1982, the ISO and the International Telecommunication Union Standardization Sector (ITU-T) began working to develop a vendor-neutral, nonproprietary set of network standards in an effort to establish a method of creating networking components that used common protocols The Physical Layer The primary function of the Physical layer is to place the transmission signal carrying the message onto the communications media—that is, to put “bits on a wire.” The functions of the Physical layer are:

• Establish and terminate the physical and logical connection to the media • Manage the flow and communication on the media • Embed the message onto the signal carried across the physical media

Network Media Network media can be guided—that is, wire and cables—or unguided—that is, wireless, microwave, and infrared. Embedding the Message The method used to embed the message on the signal depends on the type of message and type of signal. There are two types of messages (or information). Analog information, such as voice communications, is a continuously varying source, while digital information, such as computer communications is discrete between a few values—in this case, between 1s and 0s

Managing Communication Bit (or signal) flow down the media can be conducted in a number of ways. Simplex transmissions flow one way through a medium—this is how broadcast television and radio work. Half-duplex transmissions can flow either way, but in only one direction at a time, similar to a walkie-talkie radio. Full-duplex transmissions can flow both ways at the same time, like the telephone. Serial transmissions flow one bit at a time down a single communications channel, while parallel transmissions flow multiple bits at a time down multiple channels. Data Link Layer The Data Link layer (DLL) is the primary networking support layer. It is sometimes referred to as the first “subnet” layer because it provides addressing, packetizing, media access control, error control, and some flow control for the local network. DLL Protocols The dominant protocol for local area networking is Ethernet for wired networks and Wi-Fi for wireless networks. Other DLL LAN protocols include token ring, FDDI, PPP, PPTP, and L2TP. WANs typically use ATM and frame relay. Forming Packets The first responsibility of the DLL is converting the Network layer packet into a DLL frame. Unlike higher levels, the DLL adds not only a header component but also a trailer. When necessary, the packet is fragmented into one or more frames, with corresponding information embedded into the frame header Addressing Addressing at the Data Link layer is accomplished with a number embedded in the network interface card (NIC) by the manufacturer. Media Access Control One of the primary functions of the DLL is the control of the flow of traffic—that is, determining which station is allowed to transmit when. There are two general approaches to this task, control (or deterministic) and contention (stochastic). Switches and Bridges To connect networks at the Data Link layer, specific technologies are employed. While the hub connects networks at the Physical layer, connecting two networks with a hub results in one large network (or collision domain). Connecting them with a Layer 2 switch, which is capable of bridging, maintains separate collision domains. Bridging is the process of connecting networks with the same DLL protocols while maintaining the integrity of each network and only passing messages that need to be transmitted between the two. Network Layer The Network layer is the primary layer for communications between networks. This layer has three key functions: packetizing, addressing, and routing.

Packetizing The Network layer takes the segments sent from the transport layer and organizes them into one or more packets for transmission across a network. Addressing The Network layer uses a network-layer address to uniquely identify a destination across multiple networks. A typical address consists of multiple components: the network ID and the host ID. Routing Routing is the process of moving a Network layer packet across multiple networks. The devices that connect networks are called routers. Routers work at the Network layer to receive packets and direct them toward their ultimate destination. The transmission links between routers work at the Data Link layer Transport Layer The primary function of the Transport layer is to provide reliable end-to-end transfer of data between user applications. While the lower layers focus on networking and connectivity, the upper layers, beginning with the Transport layer, focus on application-specific services. The Transport layer can also provide support for a virtual circuit, which is an extension of the connection-oriented model. Session Layer The Session layer is responsible for establishing, maintaining, and terminating communication sessions between two systems. It also regulates whether communications are preformed in a simplex (one way only), half-duplex (one way at a time), or full-duplex (bidirectional) mode. Presentation Layer The Presentation layer is responsible for data translation and encryption functions. Application Layer At the Application layer, the user is provided with a number of services, perhaps most aptly called application protocols. The TCP/IP protocol suite includes applications such as e-mail (SMTP and POP), the World Wide Web (HTTP and HTTPS), file transfer (FTP and SFTP), and others. Table 2-5 provides a more comprehensive list of Application layer protocols and their uses.

Quick Quiz 1. The ___________ layer is the primary networking support layer. ANSWER: Data Link 2. ________ is the process of moving a Network layer packet across multiple networks. ANSWER: Routing 3. The _________ layer is responsible for data translation and encryption functions. ANSWER: Presentation

4. A typical address consists of multiple components: __________. ANSWER: network ID and host ID

The Internet and TCP/IP The Internet incorporates millions of small, independent networks, connected by most of the major common carriers (AT&T, ITT, MCI, Sprint, etc.). Most of the services we associate with the Internet are based on Application layer protocols like e-mail, the Web, FTP, and instant messaging (IM). Because the subject of the Internet and the World Wide Web is so vast, this section will only provide a brief overview of the Internet and its primary protocols, TCP and IP. The World Wide Web It is important to distinguish the Internet from the World Wide Web (WWW). The Internet is a physical set of networks, while the Web is a set of applications that runs on top of the Internet. Specifically, the Web is a series of hyperlinked documents that allow simple creation and retrieval of information from a number of locations using Domain Name based Uniform Resource Identifiers (URIs). The ubiquitous Uniform Resource Locator, or URL, is the best-known type of URI. The Web was created in 1989 by Sir Tim Berners-Lee while working at CERN in Geneva, Switzerland. Berners-Lee has since actively guided the development of many Web standards, including markup languages like HTML and XML. TCP/IP TCP/IP is actually a suite of protocols used to facilitate communications across the Internet. Developed before the OSI reference model, the TCP/IP suite is similar in concept, but different in detail. Application Layer The TCP/IP Application layer consists of the utility protocols that provide value to the end user. Data from the users and use of these utilities are passed down to the Transport layer for processing. Transport Layer The Transport layer is responsible for the basic capacity of transferring messages, including resolution of errors, managing necessary fragmentation, and the control of message flow, regardless of the underlying network. At this layer, two basic message approaches are used: (1) a connection-oriented approach such as that implemented in the Transmission Control Protocol (TCP) or (2) a connectionless-oriented approach such as the one used in the User Datagram Protocol (UDP). Internetwork Layer The Internetwork layer addresses the problem of moving packets in a single network. Examples of such protocols are X.25 and the ARPANET’s Host/IMP Protocol. The Internet Protocol (IP) performs the basic task of moving packets of data from a source host to a destination host. IP carries data for many different upper-layer protocols. Subnet Layer The TCP/IP Subnet layers include the Data Link and Physical layers. TCP/IP relies on whatever native network subnet layers are present. If the user’s network is Ethernet, then the IP packets are encapsulated into Ethernet frames. As such, TCP/IP provides no specification for the Data Link layer or Physical layer.

Class Discussion Topics

1. What are the primary security features that a firewall performs?

2. Ask students to do some research on low-cost firewalls suited for home and small businesses. Identify which of these have NAT functions.

3. Discuss the kinds of tactics that hackers might use to gain access to a site.

Additional Case Projects

1. Provide definitions for the following terms: • NAT • VPN • DNS • OPSEC

2. Identify some of the limitations of packet filtering.

Further Readings or Resources

1. For a summary of how firewalls work, see: www.howstuffworks.com/firewall.htm. For product comparisons, see: http://products.datamation.com/security//firewalls/.