Understanding Networks and TCP/IP
To examine the issues associated with security in a converged environment, we need to establish a common knowledge base of components and building blocks. To properly build, maintain, and secure a network you first have to know what a network really is, and how information travels along through a network. Although you will find this chapter to be a useful, high-level overview, if you have a Cisco Certified Internetwork Expert certification (CCIE) or years of networking background, you may want to skim this chapter.
What Is a Network?
Well, that depends on whom you ask. A person with a strong telecommunications background will most likely tell you a network consists of PBXs, tie trunks, adjunct systems, PRI lines, T1s, handsets, and miles of fiber and copper cable. A person with a strong data background will probably tell you a network is made up of workstations, servers, routers, WAN connections, hubs, switches, and miles of fiber and copper cable. While both parties can agree that cabling is an integral part of the network, that's where the agreement will most likely end. In essence, both the telco and data people are right; however, for the purposes of this discussion, this chapter concentrates on data networks.
In its simplest form, a data network is a means to connect two or more computer systems for the purposes of sharing information. Networks come in all shapes and sizes: from two home PCs connected with a single cable to the colossal Internet, spanning the globe and connecting millions of distributed systems. Despite the extreme differences between various network installations, you can generally define a given network in terms of its architecture, topology, and protocol.
Traditionally, network architectures are divided into two types: local area networks (LANs) and wide area networks (WANs). LANs consist of two or more systems typically in the same geographic area connected with fiber or copper cabling and a network infrastructure consisting of switches and hubs. All personal networks and the vast majority of office networks fall into the LAN category. WANs consist of two or more systems in geographically separated areas connected with any of a variety of methods such as leased lines, radio waves, satellite relays, microwaves, or even dial-up connections. The Internet is by far the largest WAN in existence. With the advent of wireless networking, optical, and cellular technology, the lines between LANs and WANs sometimes blur, merging seamlessly into a single network entity. Recently, more terms have been added to better classify and describe network architectures:
CAN (campus area network, also referred to as a campus wide area network)—A network connecting a number of buildings such as a university or office complex.
Intranet—A private network belonging to an organization that is only accessible to authorized users (typically members of that organization). Most large corporations operate an intranet to promote information sharing and increase productivity.
Internet—The network connecting hundreds of millions of systems and users on a global scale.
MAN (metropolitan area network)—A network designed for a town or a city. Typically larger than a LAN, but usually smaller than a WAN.
SAN (storage area network)—A high-speed network connecting a variety of storage-related devices such as RAID arrays, tape systems, file servers, and so on.
VLAN (virtual local area network)—A network allowing systems on separate physical networks to communicate as if they were connected to the same physical network.
Client-Server—A network where powerful, dedicated systems called servers provide resources, files, devices, and even processing power to individual workstations or clients.
Peer-to-peer—A network where each system has equal capabilities or responsibilities, such as home networks.
Closely related to the architecture of a network is the topology of the network: how the network is physically arranged. Traditionally, networks are described as being one of three major types:
Star—Network components are connected to a central point (as seen in Figure 3.1).
Bus—Network components are connected to the same cable, often called the bus or the backbone (as seen in Figure 3.2).
Ring—Network components are connected to each other in a closed loop with each device directly connected to two other devices (as seen in Figure 3.3).
Most networks use one or more of these topologies. For example, an office complex may have a series of star topology LANs in each building connected to a larger bus network that is connected to other buildings using a ring topology, like that shown in Figure 3.4.
Figure 3.1 Star topology.
Figure 3.2 Bus topology.
Figure 3.3 Ring topology.
Figure 3.4 Mixed topology.
Network Protocols and the OSI Model
Another critical component of a network is the protocol, which is an agreed upon format for exchanging or transmitting data between systems. A protocol defines a number of agreed upon parameters such as the type of error checking to use, the data compression method, and mechanisms for systems to signal when they have finished either receiving or transmitting data. There are a wide variety of protocols, each designed with certain benefits and uses in mind. Some of the more common protocols are
AppleTalk—The communications protocol developed by Apple to connect Macintosh computers and printers.
Asynchronous Transfer Mode (ATM)—A protocol based on transferring data in fixed-size packets. Using fixed packet sizes allows for high speed, equitable transfer of data, video, and voice over the same network.
DECnet—The protocol developed by Digital Equipment Corporation used to connect PDP and VAX systems.
Ethernet—The LAN protocol developed jointly by Xerox, DEC, and Intel. The most widely implemented LAN standard.
Fiber Distributed Data Interface (FDDI)—The protocol for sending digital data over fiber optic cabling.
Internet Protocols (IP)—The protocols for managing and transmitting data between packet-switched computer networks originally developed for the Department of Defense. E-mail, File Transfer Protocol (FTP), Telnet, and Hypertext Transfer Protocol (HTTP) are all Internet Protocols.
Internetwork Packet Exchange (IPX)—The networking protocol used by Novell NetWare operating systems.
NetWare—The LAN protocol developed by Novell Corporation.
Signaling System 7 (SS7)—The telecommunications protocol developed by the International Telecommunication Union. This protocol is used between PBXs to handle tasks such as call setup, routing, and teardown.
Systems Network Architecture (SNA)—A set of network protocols developed by IBM, originally used to connect IBM's mainframe systems.
Token Ring—A LAN protocol developed by IBM where systems must possess the network token before transmitting data.
Transmission Control Protocol/Internet Protocol (TCP/IP)—The collection of communications protocols used to connect hosts on the Internet. TCP/IP is by far the most commonly used network protocol. The TCP and IP protocols are both part of TCP/IP.
X.25—A protocol developed by the Comité Consultatif International Téléphonique et Télégraphique (CCITT) for use in packet-switched networks. The CCITT was a subgroup within the ITU before the CCITT was disbanded in 1992.
In general, communications protocols adhere to the Open System Interconnection (OSI) model. The OSI model, or OSI Reference Model, is an ISO standard for worldwide communications, defining a framework for implementing protocols in seven distinct layers. Within the seven-layer OSI model, control is passed from one layer to another (top down) before it exits one system and enters another system where control is passed bottom up to complete the communications cycle. Although most protocols loosely follow the OSI model, several protocols combine one or more layers into a single function. One of they key benefits of the layered model is that each layer hides the detailed functions it performs from the other layers. Additionally, each layer only has to know how to communicate with the layer above and below it. With that in mind, you can see from Table 3.1 that the application layer does not need to know how to communicate directly with the physical layer. The application layer only needs to communicate with the presentation layer.
Table 3.1 The OSI Model
Deals with program-level communication.
Performs data conversion functions when needed.
Establishes and maintains communications channels.
Handles end-to-end transmission and integrity of transmitted data.
Routes data from one system to another.
Handles the physical passing of data from one system to another.
Manages the transmission and reception of data on the network media.