FIREWALLS & NETWORK SECURITY with Intrusion Detection and VPNs, 2nd ed.
2An Introduction to
Networking
By Whitman, Mattord, & Austin © 2008 Course Technology
Firewalls & Network Security, 2nd ed. - Chapter 2 Slide 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
Firewalls & Network Security, 2nd ed. - Chapter 2 Slide 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
Firewalls & Network Security, 2nd ed. - Chapter 2 Slide 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
Firewalls & Network Security, 2nd ed. - Chapter 2 Slide 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
Firewalls & Network Security, 2nd ed. - Chapter 2 Slide 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
Firewalls & Network Security, 2nd ed. - Chapter 2 Slide 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)
Firewalls & Network Security, 2nd ed. - Chapter 2 Slide 8
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
Firewalls & Network Security, 2nd ed. - Chapter 2 Slide 9
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
Firewalls & Network Security, 2nd ed. - Chapter 2 Slide 10
Network Media
Dominant media types and standards include:– Coaxial cable
– Fiber-Optic cable
– Twisted-pair wire
– Wireless LAN
– Bluetooth
– Infrared
Firewalls & Network Security, 2nd ed. - Chapter 2 Slide 11
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)
Firewalls & Network Security, 2nd ed. - Chapter 2 Slide 12
Embedding the Message (continued)
Multiplexing combines several circuits to create high-bandwidth stream to carry multiple signals long distances
Firewalls & Network Security, 2nd ed. - Chapter 2 Slide 13
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
Firewalls & Network Security, 2nd ed. - Chapter 2 Slide 14
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
Firewalls & Network Security, 2nd ed. - Chapter 2 Slide 15
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
Firewalls & Network Security, 2nd ed. - Chapter 2 Slide 16
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
Firewalls & Network Security, 2nd ed. - Chapter 2 Slide 17
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 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., 00-00-A3-6A-B2-1A)
Firewalls & Network Security, 2nd ed. - Chapter 2 Slide 18
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
Firewalls & Network Security, 2nd ed. - Chapter 2 Slide 19
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) CSMA/CD– Clients listen to determine if channel is free and
then transmit
– Must have mechanisms to deal with collisions
– Collision avoidance vs. collision detection
Firewalls & Network Security, 2nd ed. - Chapter 2 Slide 20
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: Filter process of connecting networks with DLL protocols while maintaining integrity of each network, only passing messages that need to be transmitted between the two
Firewalls & Network Security, 2nd ed. - Chapter 2 Slide 21
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
Firewalls & Network Security, 2nd ed. - Chapter 2 Slide 22
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
Firewalls & Network Security, 2nd ed. - Chapter 2 Slide 23
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 65534 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
Firewalls & Network Security, 2nd ed. - Chapter 2 Slide 24
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
Firewalls & Network Security, 2nd ed. - Chapter 2 Slide 25
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)
Firewalls & Network Security, 2nd ed. - Chapter 2 Slide 26
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
Firewalls & Network Security, 2nd ed. - Chapter 2 Slide 27
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
Firewalls & Network Security, 2nd ed. - Chapter 2 Slide 28
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
Firewalls & Network Security, 2nd ed. - Chapter 2 Slide 29
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
Firewalls & Network Security, 2nd ed. - Chapter 2 Slide 30
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
Firewalls & Network Security, 2nd ed. - Chapter 2 Slide 31
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)
Firewalls & Network Security, 2nd ed. - Chapter 2 Slide 32
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
Firewalls & Network Security, 2nd ed. - Chapter 2 Slide 33
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 e-mail (SMTP and POP), World Wide Web (HTTP and HTTPS), file transfer (FTP and SFTP), and others
Firewalls & Network Security, 2nd ed. - Chapter 2 Slide 34
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
Firewalls & Network Security, 2nd ed. - Chapter 2 Slide 35
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
Firewalls & Network Security, 2nd ed. - Chapter 2 Slide 36
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
Firewalls & Network Security, 2nd ed. - Chapter 2 Slide 37
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
Firewalls & Network Security, 2nd ed. - Chapter 2 Slide 38
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
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
Firewalls & Network Security, 2nd ed. - Chapter 2 Slide 39
Firewalls & Network Security, 2nd ed. - Chapter 2 Slide 40
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
Firewalls & Network Security, 2nd ed. - Chapter 2 Slide 41
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
Firewalls & Network Security, 2nd ed. - Chapter 2 Slide 42
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
Firewalls & Network Security, 2nd ed. - Chapter 2 Slide 43
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
Firewalls & Network Security, 2nd ed. - Chapter 2 Slide 44
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