1. Chapter 2: An Introduction to Networking
ISA 3200 Network Security
FIREWALLS & NETWORK SECURITY with Intrusion Detection and VPNs, 2nd ed.
2 An Introduction to Networking
Chapter 2: An Introduction to Networking

2. Learning Objectives
Upon completion of this chapter, you should 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
Learning Objectives
Upon completion of this chapter you should 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
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3. Networking Fundamentals
Fundamental exchange of information: sender communicates message to receiver over some medium
Communication only occurs when recipient is able to receive, process, and comprehend message
One-way flow of information is called a channel
When recipient becomes a sender, for example by responding to original sender’s message, this two-way flow is called a circuit
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4. Networking Fundamentals (continued)
Any medium may be subject to interference, called noise, which occurs in variety of forms
Attenuation: loss of signal strength as signal moves across media
Crosstalk: occurs when one transmission “bleeds” over to another
Distortion: unintentional variation of communication over media
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5. Networking Fundamentals (continued)
Any medium may be subject to interference, called noise, which occurs in variety of forms (continued)
Echo: reflection of a signal due to equipment malfunction or poor design
Impulse: sudden, short-lived increase in signal frequency or amplitude, also known as a spike
Jitter: signal modification caused by malfunctioning equipment
White noise: unwanted noise due to signal coming across medium at multiple frequencies
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Firewalls & Network Security, 2nd ed. - Chapter 2
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6. Reasons to Network
Data communications: exchange of messages across a medium
Networking: interconnection of groups or systems with purpose of exchanging information
Some reasons to build a network:
To exchange information
To share scarce or expensive resources
To allow distributed organizations to act as if centrally located
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7. Types of Networks Networks can be categorized by:
Components: peer-to-peer (P2P), server- based, distributed multi-server
Size: local area network (LAN), metropolitan area network (MAN), wide area network (WAN)
Layout or topology: physical (ring, bus, star, hierarchy, mesh, hybrid), logical (bus, star)
Media: guided (wired), unguided (wireless)
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8. Network Standards
Among the agencies that work on data communications standards are:
Internet Society (ISOC)
Internet Assigned Numbers Authority (IANA)
American National Standards Institute (ANSI)
International Telecommunication Union (ITU)
Institute of Electrical and Electronics Engineers (IEEE)
Telecommunications Industry Association (TIA)
International Organization for Standardization (ISO)
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9. Layered Schemes
Communication is so complex that it is very helpful to modularize the systems involved
The scheme generally used is a layered scheme
Each layer in a communication ‘stack’ handles one aspect of communication over a network
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10. Logical vs. Actual A layer operates by
Receiving data from a higher layer
Sending data to a lower layer
Logically, a layer acts as if it is communicating with the associated layer on a different system
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11. Layer 1 Layer 2 Layer 3 Host A Layer 1 Layer 2 Layer 3 Host B
Logical Channels
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12. Hops Connecting one network to another
Some hosts belong to two or more networks
Communication can move from physical network to physical network
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13. OSI Reference Model and Security
OSI reference model allocates functions of network communications into seven distinct layers, each with its own functions and protocols
Premise of model is information sent from one host is translated and encoded through various layers, from Application layer to Physical layer
Physical layer initiates transmission to receiver
Receiver translates and decodes message by processing information through each layer in reverse order
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14. 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
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15. Network Media Dominant media types and standards include:
Coaxial cable
Fiber-Optic cable
Twisted-pair wire
Wireless LAN
Bluetooth
Infrared
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16. Embedding the Message
Method used to embed message on signal depends on type of message and type of signal
Two types of message (or information):
Analog information: continuously varying source (such as voice communications)
Digital information: discrete, between a few values (such as computer communications)
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17. Embedding the Message (continued)
Multiplexing combines several circuits to create high-bandwidth stream to carry multiple signals long distances
Three dominant multiplexing methods are:
Frequency division multiplexing (FDM): combines voice channels
Time division multiplexing (TDM): assigns a time block to each client
Wave division multiplexing (WDM): uses different frequencies of light so multiple signals can travel on same fiber-optic cable
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18. Managing Communication
Bit (or signal) flow conducted in several ways:
Simplex transmissions: flow one way through a medium
Half-duplex transmissions: flow either way, but in only one direction at a time
Full-duplex transmissions: can flow both ways at the same time
Serial transmissions: flow one bit at a time down a single communications channel
Parallel transmissions: flow multiple bits at a time down multiple channels
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19. Managing Communication (continued)
Asynchronous (or timing-independent)
Formulate data flow so each byte or character has its own start and stop bit
Used in older modem-based data transfers to send individual characters between systems
Synchronous (or timing-dependent)
Use computer clocking to transmit data in continuous stream between two systems
Clock synchronization makes it possible for end nodes to identify start and end of data flow
This protocol is much more efficient
ISA Summer 2010
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Firewalls & Network Security, 2nd ed. - Chapter 2
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20. Data Link Layer Primary networking support layer
Referred to as first “subnet” layer because it provides addressing, packetizing, media access control, error control, and some flow control for local network
In LANs, it handles client-to-client and client-to-server communications
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21. Data Link Layer (continued)
DLL is further divided into two sublayers:
Logical Link Control (LLC) sublayer
Primarily designed to support multiplexing and demultiplexing protocols transmitted over MAC layer
Also provides flow control and error detection and retransmission
Media Access Control (MAC) sublayer
Designed to manage access to communications media—in other words, to regulate which clients are allowed to transmit and when
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22. DLL Protocols
Dominant protocol for local area networking is Ethernet for wired networks and Wi-Fi for wireless networks
Other DLL LAN protocols include:
Token ring
Fiber Distributed Data Interface (FDDI)
Point-to-Point Protocol (PPP)
Point-to-Point Tunneling Protocol (PPTP)
Layer Two Tunneling Protocol (L2TP)
WANs typically use ATM and frame relay
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Firewalls & Network Security, 2nd ed. - Chapter 2
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23. Forming Packets and Addressing
First responsibility of DLL is converting Network layer packet into DLL frame
DLL adds not only a header but also a trailer
When necessary, packet is fragmented into frames, with corresponding information embedded into each frame header
Addressing is accomplished with a number embedded in network interface card (NIC)
This MAC address allows packets to be delivered to an endpoint; typically shown in hexadecimal format (e.g., A3-6A-B2-1A)
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24. Media Access Control
A primary function of DLL is controlling flow of traffic—that is, determining which station is allowed to transmit when
Two general approaches:
Control
Contention
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25. Media Access Control (continued)
Control (deterministic)
Well-regulated network: traffic transmitted in orderly fashion, maintaining optimal data rate
Facilitate priority system: key clients or servers can be polled more frequently than others
Contention (stochastic)
Clients listen to determine if channel is free and then transmit
Must have mechanisms to deal with collisions
Collision avoidance vs. collision detection
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Firewalls & Network Security, 2nd ed. - Chapter 2
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26. Switches and Bridges
Specific technologies used to connect networks at Data Link layer
While hub connects networks at Physical layer, connecting two networks with hub results in one large network (or collision domain)
Connection via Layer 2 switch, capable of bridging, maintains separate collision domains
Bridging: process of connecting networks with DLL protocols while maintaining integrity of each network, only passing messages that need to be transmitted between the two
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27. Network Layer and Packetizing
Network layer is primary layer for communications between networks
Three key functions:
Packetizing
Addressing
Routing
During packetizing, Network layer takes segments sent from Transport layer and organizes them into packets for transmission across a network
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28. Addressing
Network layer uses network-layer address to uniquely identify destination across multiple networks
Typical address consists of the network ID and the host ID
In TCP/IP, IP address is network-layer address
IP address contains source and destination IP address along with additional packet information
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29. Addressing (continued)
Addresses maintained and issued by Internet Assigned Numbers Authority (IANA)
In early years, addresses distributed as follows:
Class A: consists of primary octet (the netid) with three octets providing host ID portion; allows up to 16,777,214 hosts on network
Class B: consists of two octets in netid with two octets providing host IDs
Class C: consists of three octets in netid with one octet providing 254 host IDs
Class D and Class E addresses are reserved
ISA Summer 2010
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30. Addressing (continued)
This address assignment method proves inefficient
Internet moving to new version of IP, IPv6, which uses 128-bit address instead of 32-bit
Increases available addresses by factor of 2128
Network Address Translation (NAT): uses device, like a router, to segregate external Internet from internal network
Device maps organizational addresses to different addresses inside the intranet
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Firewalls & Network Security, 2nd ed. - Chapter 2
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31. Routing Moving Network layer packets across networks
Routing protocols include static and dynamic
Internal routing protocols:
Used inside autonomous system (AS)
Distance-vector routing protocols and link-state routing protocols
External routing protocols:
Communicate between autonomous systems
Translate different internal routing protocols
Border Gateway Protocol (BGP)
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32. Transport Layer
Primary function of Transport layer is to provide reliable end-to-end transfer of data between user applications
Lower layers focus on networking and connectivity while upper layers, beginning with Transport layer, focus on application-specific services
Transport layer also responsible for end- to-end error control, flow control, and several other functions
ISA Summer 2010
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Firewalls & Network Security, 2nd ed. - Chapter 2
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33. Error Control
Process of handling problems with transfer process, which may result in modified or corrupted segments
Broken into two components: error detection and error correction
Errors are typically single-bit or multiple- bit
Bit errors are most likely the result of noise interference
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34. Error Control (continued)
Errors detected using one of several schemes:
Repetition: data transmitted redundantly
Parity: “check bits” at end of each byte of data
Redundancy: parity calculated for blocks of data rather than individual byte (LRC, VRC, CRC)
Errors typically corrected by retransmission of damaged segment
Dominant error correction techniques are automatic repeat requests (ARQs)
Three most common ARQs are Stop-And- Wait, Go-Back-N, and Selective Repeat
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Firewalls & Network Security, 2nd ed. - Chapter 2
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35. Flow Control
Purpose is to prevent receiver from being overwhelmed with segments, preventing effective processing of each received segment
Some error correction techniques have built- in flow control
Dominant technique is sliding window protocol, which provides mechanism by which receiver can specify number of segments (or bytes) it can receive before sender must wait
Receiver enlarges or reduces window size as necessary
ISA Summer 2010
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Firewalls & Network Security, 2nd ed. - Chapter 2
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36. Other Functions of the Transport Layer
Assignment of ports, which identify the service requested by a user
Combination of Network layer address and port is referred to as a socket
Tunneling protocols also work at Transport layer
These protocols work with Data Link layer protocols to provide secure connections
ISA Summer 2010
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Firewalls & Network Security, 2nd ed. - Chapter 2
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37. Session Layer
Responsible for establishing, maintaining, and terminating communications sessions between two systems
Regulates whether communications are simplex (one way only), half-duplex (one way at a time), or full-duplex (bidirectional)
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38. Presentation Layer
Responsible for data translation and encryption functions
For example, if one system is using standard ASCII and another is using EBCDIC, the Presentation layer performs the translation
Encryption can also be part of operations performed at this level
Presentation layer encapsulates Application layer messages prior to passing them down to Transport layer
ISA Summer 2010
5/31 and 6/2
Firewalls & Network Security, 2nd ed. - Chapter 2
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39. Application Layer
At Application layer, user is provided with a number of services, most aptly called application protocols
TCP/IP protocol suite includes applications such as (SMTP and POP), World Wide Web (HTTP and HTTPS), file transfer (FTP and SFTP), and others
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Firewalls & Network Security, 2nd ed. - Chapter 2
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40. The Internet and TCP/IP
The Internet incorporates millions of small, independent networks, connected by most of the major common carriers
Most services we associate with the Internet are based on Application layer protocols
The Internet is a physical set of networks, while the World Wide Web (WWW) is a set of applications that run on top of the Internet
Web uses domain name-based Uniform Resource Identifiers (URIs), Uniform Resource Locator (URL) being best-known type
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Firewalls & Network Security, 2nd ed. - Chapter 2
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41. TCP/IP
TCP/IP actually suite of protocols used to facilitate communications across the Internet
Developed before OSI reference model, it is similar in concept but different in detail
TCP/IP model is less formal than OSI reference model
Each of the four layers of TCP/IP model represents a section of one or more layers of OSI model
ISA Summer 2010
5/31 and 6/2
Firewalls & Network Security, 2nd ed. - Chapter 2
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42. Application Layer
TCP/IP Application layer consists of utility protocols that provide value to end user
Data from users and utilities are passed down to Transport layer for processing
Wide variety of Application layer protocols that support Internet users: SMTP, POP for e- mail, FTP for data transfer, HTTP for Web content
Application layers on each host interact directly with corresponding applications on other hosts to provide requisite communications support
ISA Summer 2010
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Firewalls & Network Security, 2nd ed. - Chapter 2
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43. Transport Layer
Responsible for transferring of messages, including resolution of errors, managing necessary fragmentation, and control of message flow, regardless of underlying network
Connection or connectionless messages
Connects applications through use of ports
Lowest layer of TCP/IP stack to offer any form of reliability
TCP: connected, reliable protocol
UDP: connectionless, unreliable protocol
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Firewalls & Network Security, 2nd ed. - Chapter 2
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44. Internetwork Layer Handles moving packets in a single network
Examples of protocols are X.25 and ARPANET’s Host/IMP Protocol
Internet Protocol (IP) performs task of moving packets from source host to destination host
IP carries data for many different upper- layer protocols
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Firewalls & Network Security, 2nd ed. - Chapter 2
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45. Internetwork Layer (continued)
Some protocols carried by IP function on top of IP but perform other Internetwork layer functions
All routing protocols are also part of Network layer
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Firewalls & Network Security, 2nd ed. - Chapter 2
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46. Subnet Layers
TCP/IP Subnet layers include Data Link and Physical layers
TCP/IP relies on whatever native network subnet layers are present
For example, if user’s network is Ethernet then IP packets are encapsulated into Ethernet frames
No specification for Data Link layer or Physical layer
ISA Summer 2010
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Firewalls & Network Security, 2nd ed. - Chapter 2
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47. Chapter Summary
Fundamental exchange of information: sender communicates message to receiver over some medium
Communication only occurs when recipient is able to receive, process, and comprehend message
Any medium may be subject to interference: attenuation, crosstalk, distortion, echo, impulse, jitter, white noise
ISA Summer 2010
5/31 and 6/2
Firewalls & Network Security, 2nd ed. - Chapter 2
Slide 47 47

48. Chapter Summary (continued)
Some reasons to build a network:
To exchange information
To share scarce or expensive resources
To allow distributed organizations to act as if centrally located
Networks can be categorized by: components, size, layout or topology, media
OSI reference model allocates functions of network communications into seven distinct layers, each with its own functions and protocols
ISA Summer 2010
5/31 and 6/2
Firewalls & Network Security, 2nd ed. - Chapter 2
Slide 48 48

49. Chapter Summary (continued)
OSI reference model layers:
Physical: puts transmissions onto media
Data Link: primary networking support layer
Network: primary layer for communications between networks
Transport: provides reliable end-to-end transfer of data between user applications
Session: establishes, maintains, terminates communications sessions between two systems
Presentation: data translation and encryption
Application: provides application protocols
ISA Summer 2010
5/31 and 6/2
Firewalls & Network Security, 2nd ed. - Chapter 2
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50. Chapter Summary (continued)
Each of four layers of TCP/IP model represents a section of one or more layers of OSI model
Application: consists of utility protocols that provide value to end user
Transport: responsible for transferring messages, regardless of underlying network
Internetwork: handles moving packets in a single network
Subnet: includes Data Link and Physical layers, relying on whatever native network subnet layers are present for signal transmission
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Firewalls & Network Security, 2nd ed. - Chapter 2
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51. Demo ipconfig in Windows ifconfig in Linux/Unix ping nslookup
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52. Demo Setting up a shared folder on the host Accessing from Windows
//vmware-host/Shared Folders
Accessing from Linux
/mnt/hgfs
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