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Security in Networks

Defend your network and follow this in-depth guide through networks, threats against them, preventative measures, and more.
This chapter is from the book

This chapter is from the book

In this chapter:

  • How networks differ from and are similar to single, stand-alone applications and environments

  • Threats against networked applications, including denial of service, web site defacements, malicious mobile code, and protocol attacks

  • Controls against network attacks: physical security, policies and procedures, and a range of technical controls

  • Firewalls: design, capabilities, limitations

  • Intrusion detection systems

  • Private e-mail: PGP and S/MIME

Networks—their design, development, and usage—are critical to computing, at least for the next few years. We interact with networks daily, if not more frequently, when we perform banking transactions, make telephone calls, or ride trains and planes. The utility companies use networks to track electricity or water usage and bill for it. When we pay for groceries or gasoline, networks enable our credit or debit card transactions and billing. Life without networks would be considerably less convenient, and many activities would be impossible. Not surprisingly, then, computing networks are attackers' present and future targets of choice. Because of their actual and potential impact, network attacks attract the attention of journalists, managers, auditors, and the general public. For example, when you read the daily newspapers, you are likely to find a story about a network-based attack at least every month. The coverage itself evokes a sense of evil, using terms such as hijacking, distributed denial of service, and our familiar friends viruses, worms, and Trojan horses. Because any large-scale attack is likely to put thousands of computing systems at risk, with potential losses well into the millions of dollars, network attacks make good copy.

The media coverage is more than hype; network attacks are critical problems. Fortunately, your bank, your utility company, and even your Internet service provider take network security very seriously. Because they do, they are vigilant about applying the most current and most effective controls to their systems. Of equal importance, these organizations continually assess their risks and learn about the latest attack types and defense mechanisms, so that they can maintain the protection of their networks.

In this chapter we describe what makes a network similar to and different from an application program or an operating system, which you have studied in earlier chapters. In investigating networks, you will learn how the concepts of confidentiality, integrity, and availability apply in networked settings. At the same time, you will see that the basic notions of identification and authentication, access control, accountability, and assurance are the basis for network security, just as they have been in other settings.

Networking is growing and changing perhaps even faster than other computing disciplines. Consequently, this chapter is unlikely to present you with the most current technology, the latest attack, or the newest defense mechanism; you can read about those in daily newspapers and at web sites. But the novelty and change build on what we know today: the fundamental concepts, threats, and controls for networks. By developing an understanding of the basics, you can absorb the most current news quickly and easily. More importantly, your understanding can assist you in building, protecting, and using networks.

Network Concepts

To study network threats and controls, we first must review some of the relevant networking terms and concepts. This review does not attempt to provide the depth of a classic networking reference, such as [GAL99] or [TAN03]. In earlier chapters, our study of security focused on the individual pieces of a computing system, such as a single application, an operating system, or a database. Networks involve not only the pieces but also—importantly—the connections among them.

Networks are both fragile and strong. To see why, think about the power, cable television, telephone, or water network that serves your home. If a falling tree branch breaks the power line to your home, you are without electricity until that line is repaired; you are vulnerable to what is called a single point of failure, because one cut to the network destroys electrical functionality for your entire home. Similarly, there may be one telephone trunk line or water main that serves your home and those nearby; a failure can leave your building, street, or neighborhood without service. But we have ways to keep the entire network from failing. If we trace back through the network from your home to the source of what flows through it, we are likely to see that several main distribution lines support an entire city or campus. That is, there is more than one way to get from the source to your neighborhood, enabling engineers to redirect the flow along alternative paths. Redundancy makes it uncommon for an entire city to lose service from a single failure. For this reason, we say that such a network has resilience or fault tolerance.

Complex routing algorithms reroute the flow not just around failures but also around overloaded segments. The routing is usually done automatically; the control program is often supplemented by human supervision or intervention. Many types of networks have very high reliability by design, not by accident. But because there often is less redundancy near a network's endpoints than there is elsewhere, we say that the network has great strength in the middle and fragility at the perimeter.

From the user's perspective, a network is sometimes designed so that it looks like two endpoints with a single connection in the middle. For example, the municipal water supply may appear to be little more than a reservoir (the source), the pipes (the transmission or communication medium), and the water faucet (the destination). Although this simplistic view is functionally correct, it ignores the complex design, implementation, and management of the "pipes." In a similar way, we describe computer networks in this chapter in ways that focus on the security concepts but present the networks themselves in a simplistic way, to highlight the role of security and prevent the complexity of the networks from distracting our attention. Please keep in mind that our network descriptions are often abstractions of a more complex actuality.

The Network

Figure 7-1 shows a network in its simplest form, as two devices connected across some medium by hardware and software that enable the communication. In some cases, one device is a computer (sometimes called a "server") and the other is a simpler device (sometimes called a "client") enabled only with some means of input (such as a keyboard) and some means of output (such as a screen). For example, a powerful computer can be a server, but a handheld personal digital assistant (PDA) or a cell phone might be a network client.

Figure 1Figure 7-1 Simple View of Network.

Although this model defines a basic network, the actual situation is frequently significantly more complicated.

  • The simpler client device, employed for user-to-computer communication, is often a PC or workstation, so the client has considerable storage and processing capability.

  • A network can be configured as just a single client connected to a single server. But more typically, many clients interact with many servers.

  • The network's services are often provided by many computers. As a single user's communication travels back and forth from client to server, it may merely pass through some computers but pause at others for significant interactions.

  • The end user is usually unaware of many of the communications and computations taking place in the network on the user's behalf.

Most real-world situations are more like Figure 7-2. In this second view, the user at one of the lettered client machines may send a message to System 3, unaware that communication is actually passing through the active Systems 1 and 2. In fact, the user may be unaware that System 3 sometimes passes work to System 4.

A single computing system in a network is often called a node, and its processor (computer) is called a host. A connection between two hosts is known as a link. Network computing consists of users, communications media, visible hosts, and systems not generally visible to end users. In Figure 7-2, Systems 1 through 4 are nodes. In our figure the users are at the lettered client machines, perhaps interacting with Server F.

Figure 7-2Figure 7-2 More Complex but More Typical View of Networks.

Users communicate with networked systems by interacting directly with terminals, workstations, and computers. A workstation is an end-user computing device, usually designed for a single user at a time. Workstations often have powerful processors and good-sized memory and storage so that they can do sophisticated data manipulation (such as converting coded data to a graphical format and displaying the picture). A system is a collection of processors, perhaps including a mixture of workstations and independent processors, typically with more processing power and more storage capacity than a workstation.

Environment of Use

The biggest difference between a network and a stand-alone device is the environment in which each operates. Although some networks are located in protected spaces (for example, a local area network in a single laboratory or office), at least some portion of most networks is exposed, often to total strangers. The relatively simple network in Figure 7-2 is a good example. Systems 2, 3, and 4 are remote from System 1, and they may be under different ownership or control.

Networks can be described by several typical characteristics:

  • Anonymity. A cartoon image shows a dog typing at a workstation, and saying to another dog, "On the Internet, nobody knows you're a dog." A network removes most of the clues, such as appearance, voice, or context, by which we recognize acquaintances.

  • Automation. In some networks, one or both endpoints, as well as all intermediate points, involved in a given communication may be machines with only minimal human supervision.

  • Distance. Many networks connect endpoints that are physically far apart. Although not all network connections involve distance, the speed of communication is fast enough that humans usually cannot tell whether a remote site is near or far.

  • Opaqueness. Because the dimension of distance is hidden, users cannot tell whether a remote host is in the room next door or in a different country. In the same way, users cannot distinguish whether they are connected to a node in an office, school, home, or warehouse, or whether the node's computing system is large or small, modest or powerful. In fact, users cannot tell if the current communication involves the same host with which they communicated the last time.

  • Routing diversity. To maintain or improve reliability and performance, routings between two endpoints are usually dynamic. That is, the same interaction may follow one path through the network the first time and a very different path the second time. In fact, a query may take a different path from the response that follows a few seconds later.

Shape and Size

The way a network is configured, in terms of nodes and connections, is called the network topology. You can think of the topology as the shape of the network. The topology ranges from very simple, such as two hosts connected by one path, to very complex, such as the Internet. These two extremes highlight three dimensions of networks that have particular bearing on a network's security.

  • Boundary. The boundary distinguishes an element of the network from an element outside it. For a simple network, we can easily list all the components and draw an imaginary line around it to separate what is in the network from what is outside. But listing all the hosts connected to the Internet is practically impossible. For example, a line surrounding the Internet would have to surround the entire globe today, and Internet connections also pass through satellites in orbit around the earth. Moreover, as people and organizations choose to be connected or not, the number and type of hosts change almost second by second, with the number generally increasing over time.

  • Ownership. It is often difficult to know who owns each host in a network. The network administrator's organization may own the network infrastructure, including the cable and network devices. However, certain hosts may be connected to a network for convenience, not necessarily implying ownership.

  • Control. Finally, if ownership is uncertain, control must be, too. To see how, pick an arbitrary host. Is it part of network A? If yes, is it under the control of network A's administrator? Does that administrator establish access control policies for the network, or determine when its software must be upgraded and to what version? Indeed, does the administrator even know what version of software that host runs?

The truth is that, for many networks, it is difficult and at times impossible to tell which hosts are part of that network, who owns the hosts, and who controls them. Even for networks significantly smaller than the Internet, major corporate, university, or government networks are hard to understand and are not even well known by their system administrators. Although it seems contrary to common sense, many corporations today have no accurate picture of how their networks are configured. To understand why, consider a network of automated teller machines for a multinational bank. The bank may have agreements with other banks to enable customers to withdraw money anywhere in the world. The multinational bank may understand its own bank's network, but it may have no conception of how the connecting banks' networks are configured; there is no "big picture" of how the combined networks look or operate. Similarly, a given host may be part of more than one network. In such a situation, suppose a host has two network interfaces. Whose rules does that host (and that host's administrator) have to follow?

Depicting, configuring, and administering networks are not easy tasks.

Mode of Communication

A computer network implements communication between two endpoints. Data are communicated either in digital format (in which data items are expressed as discrete binary values) or analog (in which data items are expressed as points in a continuous range, using a medium like sound or electrical voltage). Computers typically store and process digital data, but some telephone and similar cable communications are in analog form (because telephones were originally designed to transmit voice). When the transmission medium expects to transfer analog data, the digital signals must be converted to analog for transmission and then back to digital for computation at the receiving end. Some mostly analog networks may even have some digital segments, so the analog signals are digitized more than once. These conversions are performed by a modem (the term is derived from modulator-demodulator), which converts a digital data stream to tones and back again.


Communication is enabled using several kinds of media. We can choose among several types, such as along copper wires or optical fiber, or through the air, as with cellular phones. Let us look at each type in turn.


Because much of our computer communication has historically been done over telephone lines, the most common network communication medium today is wire. Inside our homes and offices, we use a pair of insulated copper wires, called a twisted pair or unshielded twister pair (UTP). Copper has good transmission properties at a relatively low cost. The bandwidth of UTP is limited to under 10 megabits per second (Mbps),1 so engineers cannot transmit a large number of communications simultaneously on a single line. Moreover, the signal strength degrades as it travels through the copper wire, and it cannot travel long distances without a boost. Thus, for many networks, line lengths are limited to approximately 300 feet. Single twisted pair service is most often used locally, within a building or up to a local communications drop (that is, the point where the home or office service is connected to the larger network, such as the commercial telephone system). Although regular copper wire can transmit signals, the twisting reduces crossover (interference and signal transfer) between adjacent wires.

Another choice for network communication is coaxial (coax) cable, the kind used for cable television. Coax cable is constructed with a single wire surrounded by an insulation jacket. The jacket is itself surrounded by a braided or spiral-wound wire. The inner wire carries the signal, and the outer braid acts as a ground. The most widely used computer communication coax cable is Ethernet, carrying up to 100 Mbps over distances of up to 1500 feet.

Coax cable also suffers from degradation of signal quality over distance. Repeaters (for digital signals) or amplifiers (for analog signals) can be spaced periodically along the cable to pick up the signal, amplify it, remove spurious signals called "noise," and retransmit it.

Optical Fiber

A newer form of cable is made of very thin strands of glass. Instead of carrying electrical energy, these fibers carry pulses of light. The bandwidth of optical fiber is up to 1000 Mbps, and the signal degrades less over fiber than over wire or coax; the fiber is good for a run of approximately 2.5 miles. Optical fiber involves less interference, less crossover between adjacent media, lower cost, and less weight than copper. Thus, optical fiber is generally a much better transmission medium than copper. Consequently, as copper ages, it is being replaced by optical fiber in most communication systems. In particular, most long distance communication lines are now fiber.


Radio signals can also carry communications. Similar to pagers, wireless microphones, garage door openers, and portable telephones, wireless radio can be used in networks, following a protocol developed for short-range telecommunications, designated the 802.11 family of standards. The wireless medium is used for short distances; it is especially useful for networks in which the nodes are physically close together, such as in an office building or at home. Many 802.11 devices are becoming available for home and office wireless networks.


Microwave is a form of radio transmission especially well suited for outdoor communication. Microwave has a channel capacity similar to coax cable; that is, it carries similar amounts of data. Its principal advantage is that the signal is strong from point of transmission to point of receipt. Therefore, microwave signals do not need to be regenerated with repeaters, as do signals on cable.

However, a microwave signal travels in a straight line, presenting a problem because the earth curves. Microwave signals travel by line of sight: The transmitter and receiver must be in a straight line with one another, with no intervening obstacles, such as mountains. As shown in Figure 7-3, a straight microwave signal transmitted between towers of reasonable height can travel a distance of only about 30 miles because of the earth's curvature. Thus, microwave signals are "bounced" from receiver to receiver, spaced less than 30 miles apart, to cover a longer distance.

Figure 7-3Figure 7-3 Microwave Transmission.


Infrared communication carries signals for short distances (up to 9 miles) and also requires a clear line of sight. Because it does not require cabling, it is convenient for portable objects, such as laptop computers and connections to peripherals. An infrared signal is difficult to intercept because it is a point-to-point signal. However, it is subject to "in the middle" attacks in which the interceptor functions like a repeater, receiving the signal, extracting any desired data, and retransmitting to the original destination the original signal or a modified version. Because of line-of-sight requirements and limited distance, infrared is typically used in a protected space, such as an office, in which in-the-middle attacks would be difficult to conceal.


Many communications, such as international telephone calls, must travel around the earth. In the early days of telephone technology, telephone companies ran huge cables along the ocean's bottom, enabling calls to travel from one continent to another. Today, we have other alternatives. The communication companies place satellites in orbits that are synchronized with the rotation of the earth (called geosynchronous orbits), so the satellite appears to hover in a fixed position 22,300 miles above the earth. Although the satellite can be expensive to launch, once in space it is essentially maintenance free. Furthermore, the quality of a satellite communication link is often better than an earth-bound wire cable.

Satellites act as naïve transponders: Whatever they receive they broadcast out again. Thus, satellites are really sophisticated receivers, in that their sole function is to receive and repeat signals. From the user's point of view, the signal essentially "bounces" off the satellite and back to earth. For example, a signal from North America travels 22,300 miles into the sky and the same distance back to a point in Europe. The process of bouncing a signal off a satellite is shown in Figure 7-4.

Figure 7-4Figure 7-4 Satellite Communication.

We can project a signal to a satellite with reasonable accuracy, but the satellite is not expected to have the same level of accuracy when it sends the signal back to earth. Thus, to reduce complexity and eliminate beam focusing, satellites typically spread their transmissions over a very wide area. A rather narrow angle of dispersion from the satellite's transmitter produces a fairly broad pattern (called the footprint) on the surface of the earth because of the 22,300-mile distance from the satellite to earth. Thus, a typical satellite transmission can be received over a path several hundred miles wide; some cover the width of the entire continental United States in a single transmission. For some applications, such as satellite television, a broad footprint is desirable. But for secure communications, the smaller the footprint, the less the risk of interception.


When we use a network, the communication media are usually transparent to us. That is, most of us do not know whether our communication is carried over copper wire, optical fiber, satellite, microwave, or some combination. In fact, the communication medium may change from one transmission to the next. This ambiguity is actually a positive feature of a network: its independence. That is, the communication is separated from the actual medium of communication. Independence is possible because we have defined protocols that allow a user to view the network at a high, abstract level of communication (viewing it in terms of user and data); the details of how the communication is accomplished are hidden within software and hardware at both ends. The software and hardware enable us to implement a network according to a protocol stack, a layered architecture for communications. Each layer in the stack is much like a language for communicating information relevant at that layer.

Two popular protocol stacks are used frequently for implementing networks: the Open Systems Interconnection (OSI) and the Transmission Control Protocol and Internet Protocol (TCP/IP) architecture. We examine each one in turn.

ISO OSI Reference Model

The International Standards Organization (ISO) Open Systems Interconnection (OSI) model consists of layers by which a network communication occurs. The OSI reference model contains the seven layers listed in Table 7-1.

Table 7-1 OSI Protocol Layer Levels.






User-level data



Standardized data appearance, blocking, text compression



Sessions or logical connections between parts of an application; message sequencing, recovery



Flow control, end-to-end error detection and correction, priority service



Routing, message blocking into uniformly sized packets


Data Link

Reliable data delivery over physical medium; transmission error recovery, separating packets into

uniformly sized frames



Actual communication across physical medium; individual bit transmission

How communication works across the different layers is depicted in Figure 7-5. We can think of the layers as creating an assembly line, in which each layer adds its own service to the communication. In concert, the layers represent the different activities that must be performed for actual transmission of a message. Separately, each layer serves a purpose; equivalent layers perform similar functions for the sender and receiver. For example, the sender's layer four affixes a header to a message, designating the sender, the receiver, and relevant sequence information. On the receiving end, layer four reads the header to verify that the message is for the intended recipient, and then removes this header.

Figure 7-5Figure 7-5 ISO OSI Network Model.

Each layer passes data in three directions: above with a layer communicating more abstractly, parallel or across to the same layer in another host, and below with a layer handling less abstract (that is, more fundamental) data items. The communications above and below are actual interactions, while the parallel one is a virtual communication path. Parallel layers are called "peers."

Let us look at a simple example of protocol transmission. Suppose that, to send e-mail to a friend, you run an application such as Eudora, Outlook, or Unix mail. You type a message, using the application's editor, and the application formats the message into two parts: a header that shows to whom the message is intended (as well as other things, such as sender and time sent), and a body that contains the text of your message. The application reformats your message into a standard format so that even if you and your friend use different mail applications, you can still exchange e-mail. This transformation is shown in Figure 7-6.

Figure 7-6Figure 7-6 Transformation.

However, the message is not transmitted exactly as you typed it, as raw text. Raw text is a very inefficient coding, because an alphabet uses relatively few of the 255 possible characters for an 8-bit byte. Instead, the presentation layer is likely to change the raw text into something else. It may do compression, character conversions, and even some cryptography. An e-mail message is a one-way transfer (from sender to receiver), so it is not initiating a session in which data fly back and forth between the two endpoints. Because the notion of a communication session is not directly relevant in this scenario, we ignore the session layer for now. Occasionally, spurious signals intrude in a communication channel, as when static rustles a telephone line or interference intrudes on a radio or television signal. To address this, the transport layer adds error detection and correction coding to filter out these spurious signals.


Suppose your message is addressed to yourfriend@somewhere.net. This notation means that "somewhere.net" is the name of a destination host (or more accurately, a destination network). At the network layer, a hardware device called a router will actually send the message from your network to a router on the network somewhere.net. The network layer adds two headers to show your computer's address as the source and somewhere.net's address as the destination. Logically, your message is prepared to move from your machine to your router to your friend's router to your friend's computer. (In fact, between the two routers there may be many other routers in a path through the networks from you to your friend.) Together, the network layer structured with destination address, source address, and data is called a packet. The basic network layer protocol transformation is shown in Figure 7-7.

Figure 7-7Figure 7-7 Network Layer Transformation.

The message must travel from your computer to your router. Every computer connected to a network has a network interface card (NIC) with a unique physical address, called a MAC address (for Media Access Control). At the data link level, two more headers are added, one for your computer's NIC address (the source MAC) and one for your router's NIC address. A data link layer structure with destination MAC, source MAC, and data is called a frame. Every NIC selects from the network those frames with its own address as a destination address. As shown in Figure 7-8, the data link layer adds the structure necessary for data to get from your computer to another computer (a router is just a dedicated computer) on your network.

Figure 7-8Figure 7-8 Data Link Layer Transformation.

Finally, the message is ready to be sent out as a string of bits. We noted earlier that analog transmissions communicate bits by using voltage or tone changes, and digital transmissions communicate them as discrete pulses. The physics and electronics of how bits are actually sent are handled at the physical layer.

On the receiving (destination) side, this process is exercised in reverse: Analog or digital signals are converted to digital data. The NIC card receives frames destined for it. The recipient network layer checks that the packet is really addressed to it. Packets may not arrive in the order in which they were sent (because of network delays or differences in paths through the network), so the session layer may have to reorder packets. The presentation layer removes compression and sets the appearance appropriate for the destination computer. Finally, the application layer formats and delivers the data as an e-mail message to your friend.

The layering and coordinating are a lot of work, and each protocol layer does its own part. But the work is worth the effort because the different layers are what enable Outlook running on an IBM PC on an Ethernet network in Washington D.C. to communicate with a user running Eudora on an Apple computer via a dial-up connection in Prague. Moreover, the separation by layers helps the network staff troubleshoot when something goes awry.


Each layer reformats the transmissions and exchanges information with its peer layer. Let us summarize what each layer contributes. Figure 7-9 shows a typical message that has been acted upon by the seven layers in preparation for transmission. Layer 6 breaks the original message data into blocks. At the session layer (5), a session header is added to show the sender, the receiver, and some sequencing information. Layer 4 adds information concerning the logical connection between the sender and receiver. The network layer (3) adds routing information and divides the message into units called packets, the standard units of communication in a network. The data link layer (2) adds both a header and a trailer to ensure correct sequencing of the message blocks and to detect and correct transmission errors. The individual bits of the message and the control information are transmitted on the physical medium by level 1. All additions to the message are checked and removed by the corresponding layer on the receiving side.

Figure 7-9Figure 7-9 Message Prepared for Transmission.

The OSI model is one of several transmission models. Different network designers implement network activities in slightly different combinations, although there is always a clear delineation of responsibility. Some designers argue that the OSI model is overly complex—it has too many levels—and so other models are typically shorter.


The OSI model is a conceptual one; it shows the different activities required for sending a communication. However, full implementation of a seven-layer transmission carries too much overhead for megabit-per-second communications; the OSI protocol slows things down to unacceptable levels. For this reason, TCP/IP (Transmission Control Protocol/ Internet Protocol) is the protocol stack used for most wide area network communications. TCP/IP was invented for what became the Internet. TCP/IP is defined by protocols, not layers, but we can think of it in terms of four layers: application, host-to-host (end-to-end) transport, Internet, and physical. In particular, an application program deals only with abstract data items meaningful to the application user. Although TCP/IP is often used as a single acronym, it really denotes two different protocols: TCP implements a connected communications session on top of the more basic IP transport protocol. In fact, a third protocol, UDP (user datagram protocol) is also an essential part of the suite.

The transport layer receives variable-length messages from the application layer; the transport layer breaks them down into units of manageable size, transferred in packets. The Internet layer transmits application layer packets in datagrams, passing them to different physical connections based on the data's destination (provided in an address accompanying the data). The physical layer consists of device drivers to perform the actual bit-by-bit data communication. Table 7-2 shows how each layer contributes to the complete interaction.

Table 7-2 Internet Communication Layers.





Prepare messages from user interactions

User interaction, addressing


Convert messages to packets

Sequencing, reliability (integrity), error correction


Convert packets to datagrams

Flow control, routing


Transmit datagrams as individual bits

Data communication

The TCP protocol must ensure the correct sequencing of packets as well as the integrity (correct transmission) of data within packets. The protocol will put out-of-sequence packets in proper order, call for retransmitting a missing packet, and obtain a fresh copy of a damaged packet. In this way, TCP hands a stream of correct data in proper order to the invoking application. But this service comes at a price. Recording and checking sequence numbers, verifying integrity checks, and requesting and waiting for retransmissions of faulty or missing packets take time and induce overhead. Most applications expect a flawless stream of bits, but some applications can tolerate a less accurate stream of data if speed or efficiency is critical.

A TCP packet is a data structure that includes a sequence number, an acknowledgment number for connecting the packets of a communication session, flags, and source and destination port numbers. A port is a number designating a particular application running on a computer. For example, if Jose and Walter begin a communication, they establish a unique channel number by which their computers can route their respective packets to each of them. The channel number is called a port. Each service uses a well-known port, such as port 80 for HTTP (web pages), 23 for Telnet (remote terminal connection), 25 for SMTP (e-mail), or 161 for SNMP (network management). More precisely, each of these services has a waiting process that monitors the specified port number and tries to perform its service on any data passed to the port.

The UDP protocol does not provide the error-checking and correcting features of TCP, but it is a much smaller, faster protocol. For instance, a UDP datagram adds 8 bytes for control information, whereas the more complex TCP packet adds at least 24 bytes.

Most applications do not interact directly in TCP or UDP themselves. Instead, they operate on data structured by an application-level protocol applied on top of TCP or UDP. Some of the more common Internet protocols are shown in Table 7-3.

Table 7-3 Internet Services.


TCP Protocols

UDP Protocols

Application Protocol

SMTP (Simple Mail Transfer Protocol): used for communicating e-mail HTTP (Hypertext Transfer Protocol): used for communicating web pages FTP (File Transfer Protocol): used for receiving or sending files Telnet (Terminal Emulation Protocol): used for performing remote operations as if directly connected to the host from a terminal and others

SNMP (Simple Network Monitoring Protocol): used for controlling network devices Syslog (System Audit Log): used for entering records in the system log Time: used for communicating and synchronizing time among network devices and others








Data communication

Data communication

Whatever the model, a layer will typically subdivide data it receives from a higher layer and then add header and/or trailer information to the data before passing it to a lower layer. Each layer encapsulates the higher layer, so that higher layer headers and trailers are seen simply as part of the data to be transmitted.

Addressing Scheme

For communication to occur, the bits have to be directed to somewhere. All networks use an addressing scheme so that data can be directed to the expected recipient. Because it is the most common, we use the Internet addressing scheme known as IP addresses in our examples, since it is the addressing handled by the IP protocol.

All network models implement an addressing scheme. An address is a unique identifier for a single point in the network. For obvious reasons, addressing in shared, wide area networks follows established rules, while addressing in local area networks is less constrained.

Starting at the local area network, each node has a unique address, defined in hardware on the network connector device (such as a network interface card) or its software driver. A network administrator may choose network addresses to be easy to work with, such as 1001, 1002, 1003 for nodes on one LAN, and 2001, 2002, and so forth on another.

A host on a TCP/IP wide area network has a 32-bit address,2 called an IP address. An IP address is expressed as four 8-bit groups in decimal notation, separated by periods, such as People prefer speaking in words or pseudowords, so network addresses are also known by names, such as ATT.COM or CAM.AC.UK. Addressing tables convert these acronyms to numeric format.

An IP address is parsed from right to left. The rightmost portion, such as .COM, .EDU, .NET, .ORG, or .GOV, or one of the two-letter country specific codes, such as .UK, .FR, .JP, or .DE, is called a top-level domain. A small set of organizations called the Internet Registrars controls these top-level domains; the registrars also control the registration of second-level domains, such as ATT in ATT.COM. Essentially, the registrars publish addresses of hosts that maintain tables of the second-level domains contained in the top-level domain. A host connected to the Internet queries one of these tables to find the numeric IP address of ATT in the .COM domain. AT&T, the company owning the ATT Internet site, must maintain its own host to resolve addresses within its own domain, such as MAIL.ATT.COM. You may find that the first time you try to resolve a fully qualified domain name to its IP address, your system performs a lookup starting at the top; for subsequent attempts, your system maintains a cache of domain name records that lets it resolve addresses locally. Finally, a domain name is translated into a 32-bit, four-octet address, and that address is included in the IP packets destined for that address. (We return to name resolution later in this chapter because it can be used in network attacks.)

Routing Concepts

A host needs to know how to direct a packet from its own IP address. Each host knows to what other hosts it is directly connected, and hosts communicate their connections to their neighbors. For the example network of Figure 7-2, System 1 would inform System 2 that it was one hop away from Clients A, B, and C. In turn, System 2 would inform its other neighbor, System 3, that it (System 2) was two hops away from Clients A, B, and C. From System 3, System 2 would learn that System 3 was one hop away from Clients D and E, Server F, and System 4, which System 2 would then pass to System 1 as being a distance of two hops. The routing protocols are actually more complex than this description, but the concepts are the same; hosts advertise to their neighbors to describe to which hosts (addresses) they can route traffic and at what cost (number of hops). Each host routes traffic to a neighbor that offers a path at the cheapest cost.

Types of Networks

A network is a collection of communicating hosts. But to understand the network and how it works, we have several key questions to ask, such as How many hosts? Communicating by what means? To answer these questions, we are helped by an understanding of several types of subclasses of networks, since they commonly combine into larger networks. The subclasses are general notions, not definitive distinctions. But since the terms are commonly used, we present several common network subclasses that have significant security properties.

Local Area Networks

As the name implies, a local area network (or LAN) covers a small distance, typically within a single building. Usually a LAN connects several small computers, such as personal computers, as well as printers and perhaps some dedicated file storage devices. Figure 7-10 shows the arrangement of a typical LAN. The primary advantage of a LAN is the opportunity for its users to share data and programs and to share access to devices such as printers.

Figure 7-10Figure 7-10 Typical LAN.

Most LANs have the following characteristics.

  • Small. Typically, fewer than 100 users share a single LAN, within a distance less than 3 kilometers, or 2 miles. More commonly, a LAN is much smaller, stretching less than 1 kilometer inside a single building.

  • Locally controlled. The equipment is owned and managed by a single organization. The users all are affiliated with a single organization, such as a company, a department, a workgroup, or a physical proximity.

  • Physically protected. The LAN is on the premises of a company or other organization, so malicious outsiders usually cannot readily get to the LAN equipment.

  • Limited scope. Many LANs support a single group, department, floor, activity, or other geographical or administrative unit. As a result, each has a narrowly scoped set of function it performs.

Wide Area Networks

A wide area network, or WAN, differs from a local area network in terms of both size or distance (as its name implies, it covers a wider geographic area than does a LAN) and control or ownership (it is more likely not to be owned or controlled by a single body). Still, there tends to be some unifying principle to a WAN. The hosts on a WAN may all belong to a company with many offices, perhaps even in different cities or countries, or they may be a cluster of independent organizations within a few miles of each other, who share the cost of networking hardware. These examples also show how WANs themselves differ. Some are under close control and maintain a high degree of logical and physical isolation (typically, these are WANs controlled by one organization), while others are only marriages of convenience. Typical characteristics of WANs are these.

  • Single control. Typically, a single organization is responsible for and controls a wide area network. Even if a network is shared by several unrelated subscribers, one organization usually determines who may join the network.

  • Covers a significant distance. A WAN generally serves a distance greater than a LAN can cover, typically from a few miles to the entire globe.

  • Physically exposed (often, but not always). Most wide area networks use publicly available communications media, which are relatively exposed. However, the fact that many subscribers share those media helps protect the privacy of any one subscriber.

Other network types include campus area networks (CANs) and metropolitan area networks (MANs). A CAN is usually under the control of a single organization, such as a university or company, and covers the adjacent buildings of one site of that organization. A MAN often covers a city, with the communication offering of one provider in that area. CANs, MANs, and WANs cover a wide range of possibilities; they loosely characterize everything between LANs and Internets, the two extremes of the networking spectrum.

Internetworks (Internets)

Networks of networks, or internetwork networks, are sometimes called internets. An internet is a connection of two or more separate networks, in that they are separately managed and controlled. The most significant internetwork is known as the Internet, because it connects so many of the other public networks.

The Internet is, in fact, a federation of networks, loosely controlled by the Internet Society. The Internet Society enforces certain minimal rules of fair play to ensure that all users are treated equitably, and it supports standard protocols so that users can communicate. These are the characteristics of the Internet.

  • Federation. Almost no general statements can be made about Internet users or even network service providers. Some may access the network through businesses or government organizations whose memberships are very restrictive, while others may obtain access simply by paying a small monthly fee.

  • Enormous. No one really knows how large the Internet is. Our knowledge is incomplete in part because new hosts are added daily, in part because one Internet access point can support hundreds or thousands of machines connected through that single access point, and in part because nobody has laid the basis for an accurate census. The Internet connects many thousands of networks. In 2002, there were almost 200 million Internet hosts and well over 700 million users.3 Based on past history, we can expect the size of the Internet to double each year. Sidebar 7-1 describes the large number of outside accesses just to one site at the University of Illinois.

Sidebar 7-1 Traffic at a Typical Web Site

The University of Illinois at Urbana-Champaign tracks the usage of its web site over time. On one summer day (26 August 2001) browsers were used by 6,817 hosts in 57 countries making 75,062 accesses to the university's Engineering Workstations World Wide Web server. This volume and diversity are usual for a site that is not a popular public site like Google or Yahoo. The latest statistics are available at http://www.cen.uiuc.edu/bstats/latest.html.

The network administrators look not only at where the accesses come from but also at what the users are trying to do. Most of the accesses to this server were for viewing the home pages of engineering students who had posted information at the site. Many sites gather statistics on local and remote accesses for performance and security.

But these statistics count all traffic, not just the security-relevant activity. The security company ISS (Internet Security Systems) tracks the status of actual Internet security risk. Its four-point scale goes from 1 (normal risk from random malicious attacks experienced by all site administrators) to 4 (actual or potential catastrophic security event requiring immediate defense). During a period from April to June 2002, ISS reported 56 days at level 1, 22 at level 2, and 7 at level 3 [ISS02].

  • Heterogeneous. Probably at least one of every kind of commercially available hardware and software is connected to the Internet. Unix is popular as the operating system at the Internet connection point, although most other multiuser operating systems could support access.

  • Physically and logically exposed. Since there is no global access control, practically any attacker can access the Internet and, because of its complex connectivity, reach practically any resource on the net.


The topology of a network can affect its security. Three basic patterns come from LANs, but the structures describe wider networks, or parts of wider networks, as well. These three patterns are depicted in Figure 7-11.

Figure 7-11Figure 7-11 Network Topologies.

Common Bus

Conceptually, a common bus is a single wire to which each node of a LAN is connected. Timing signals on the bus help the nodes communicate. This medium is especially convenient for LANs, since users and machines are frequently rearranged and new connections can be added easily.

Nodes must continually monitor the bus to retrieve communications addressed for them. In that respect, every communication is accessible to every node, not just the designated addressee. Each host acts cooperatively but autonomously.

Star or Hub

In a star or hub network, each node is connected to a central "traffic controller" node. All transmissions flow from the source node to the traffic controller and then from the traffic controller to the destination node. Such a central node can monitor and control traffic to defeat covert channels.

Each message is read only by the traffic controller (presumably for address only) and the intended recipient. There is a unique path between any two nodes, and this path is inaccessible to any others.


In a ring topology, each node receives many messages, scans each, removes ones designated for it, adds any more it wants to transmit, and sends the pack of messages to the next node.

As with the bus, there is no central control. In this topology, however, each node has greater responsibility to the others because a single node's failure to pass along all the messages it has received would deny data to other nodes.

Distributed Systems

Distributed systems are related to networks. A distributed system is one in which computation is spread across two or more computers. From a security point of view, we are most interested in the type of distributed system in which one computer invokes a process on another computer without the direct participation of, or necessarily even the knowledge of, the user. For example, a user task on one machine might require data or specialized processing from another machine, such as a central machine on which a database is maintained. Alternatively, several machines may share tasks, depending on the current workload to optimize performance for all users.

For our purposes, the significant characteristic of a distributed system is its use of multiple, independent, and physically separated computers. The computers may be directly connected to one another, nodes on a LAN, or connected to a wider network.

In a client server architecture one host, the client, requests services from another, the server. A peer-to-peer system is a collection of equals, in which no one host consistently seeks or provides services from or to any other; all hosts could be called servers to the others. A distinction often inferred between clients and servers is that servers are expected to be more trustworthy than clients, meaning that a server must protect itself from any faulty data or processing requests from its client. On the other hand, the client, often a networking end user's workstation, does little to protect itself from potential rogue servers. Clearly, this distinction is flawed; as we will see, a client should not assume that all servers are trustworthy and should apply prudent protection measures.


Applications Programming Interfaces or APIs are definitions of interfaces to modules or systems. More and more frequently, software systems are not written from scratch. Instead, they are composed of components, some of which are new but many of which are purchased or modified from other applications. For example, developers may purchase graph-drawing routines, statistics packages, or sorts and searches, rather than write their own. In many cases, the services provided by the components are not embedded in a given application but are invoked from software resident on another machine. That is, the software is in a sense networked among several places, so we need standard, controlled ways to invoke services and routines. An API is the specification of what parameters in what forms must be passed to a routine, as well as what results that routine will provide. With an API, a developer need know only enough about a routine to be able to invoke it, without needing to understand how it operates or is structured internally.

GSSAPI or Generic Security Services API [LIN97] is a template for the many kinds of security services that a routine could provide. The template is independent of the mechanisms or structures that actually implement the security services. It is based on the notion that callers have credentials denoting their identities or authorizations to view and manipulate data. With these credentials, callers establish contexts or environments with security permissions. A caller with credentials operating in a particular context can invoke security services to implement confidentiality or integrity. GSSAPI defines calls to manage credentials, establish and destroy contexts, and obtain security services.

CAPI or Cryptographic API [COL96] is a Microsoft API for cryptographic services. Such APIs are useful because they let us separate the actual cryptographic implementation from a routine that needs cryptographic service; in this way, a user can invoke cryptographic algorithms of different strengths as they are needed. CAPI is a procedure that calls for generic cryptographic services, without specifying implementation or particular algorithm.

Advantages of Computing Networks

Computer networks offer several advantages over single-processor systems.

  • Resource sharing. Network users can access a variety of resources through the network, rather than having them at hand locally. For instance, sharing databases, data and program files, and other resources reduces maintenance and storage costs while providing each user with improved access. For a single individual, usage may be too low to justify buying a specialized or expensive device. However, being able to share the device with many network users may justify its purchase.

  • Distributing the workload. A single system's usage varies as users join and leave a system. The degree of workload fluctuation for a single system can be moderated in a network, shifting the workload from a heavily loaded system to an underutilized one.

  • Increased reliability. Since a computing network consists of more than one computing system, the failure of one system or of just one component need not necessarily block users from continuing to compute. If similar systems exist, users can move their computing tasks to other systems when one system fails.

  • Expandability. Network systems can be expanded easily by addition of new nodes. This expansion of the user base can occur without the manager of any single system having to take special action.

In earlier chapters, we considered computing systems as self-contained entities. A single security policy is associated with each computing system, addressing integrity of data, secrecy of data, and availability of service. A single operating system enforces the security policy; hardware controls assist the operating system; and some users augment the controls from the operating system with security features in individual applications programs. In general, users trust the operating system to provide a certain level of protection. In particular, the operating system can protect resources because it exercises complete control over those resources.

Computing networks have similar characteristics. The network must ensure integrity of data, secrecy of data, and availability of service. Each user accesses the network through a single operating system, which also includes network interface responsibilities. Users still expect the operating systems to enforce the security policies of the network. However, in a network the operating systems at the two ends of the communication, as well as the operating systems of all computers in between, must cooperate to enforce security.

We cannot always protect the whole network because its distant points are not under our control. However, we can consider our computer's or system's relationship to the rest of the network and focus on local users' accesses to the network, data received from and sent to the network, and possible accesses by other more distant users. In the next section we analyze the security ramifications of these network aspects.

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