Transcript Chapter 17 - Networking Essentials

A+ Guide to Managing and
Maintaining Your PC, 7e
Chapter 17
Networking Essentials
Objectives
• Identify the 7 OSI layers and its different functions
• Understand TCP/IP protocol and how it relates to
OSI layers
• Identify different hardware and application
associated with OSI layers
• Understand different important protocols used under
TCP/IP
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The OSI Reference Model
• Networks rely upon standards
• Open Systems Interconnection (OSI) reference model
– Fundamental network communications model
• OSI model product of two standards organizations
– International Organization for Standardization (ISO)
– American National Standards Institute (ANSI)
• OSI is theoretical, not specific hardware or software
• OSI guidelines analogized to a grammar
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The OSI Reference Model (continued)
• Accomplishments of the OSI model
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Enabling communications among LANs, MANs, WANs
Standardizing network equipment
Enabling backward compatibility to protect investments
Enabling development of software and hardware with
common interfaces
– Making worldwide networks possible; e.g., the Internet
• OSI model consists of seven distinct layers
– Physical, Data Link, Network, Transport, Session,
Presentation, and Application
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Guide for Memorization :
Top to Bottom: All People Seem To Need Data Processing
Bottom to Top : Programmers Do Not Throw Sausage Pizza Away
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The OSI Reference Model (continued)
• Set of layers in OSI model is called a stack
• Layers called by actual name or placement in stack
• Layers also divided into three groups
– Bottom: handles physical communications
– Middle: coordinates communication between nodes
– Top: involves data presentation
• Contact between two network devices
– Communications traverse layered stack in each device
– Each layer handles specific tasks
– Each layer communicates with next layer using protocol
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Physical Layer
• Layer purpose: transmit and receive signals with data
• Responsibilities of the Physical layer (Layer 1)
– All data transfer mediums
• wire cable, fiber optics, radio waves, and microwaves
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Network connectors
The network topology
Signaling and encoding methods
Data transmission devices
Network interfaces
Detection of signaling errors
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Physical Layer (continued)
• Network signals are either analog or digital
• Analog signal
– Wave pattern with positive and negative voltages
– Examples: ordinary telephone or radio signal
– Used in WANs that employ analog modems
• Digital signal generates binary 1s or 0s
– Most common signaling method on LANs and highspeed WANs
– Example 1: +5 volts produces 1, 0 volts produce 0
– Example 2: +5 volts produces 1, -5 volts produce 0
– Example 3 (Fiber-optics): presence of light is 1, else 0
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Physical Layer (continued)
• Physical network problems affect physical layer
– Example 1: broken cable
– Example 2: electrical or magnetic interference
• Electromagnetic interference (EMI)
– Caused by magnetic force fields
– Generated by certain electrical devices
• Fans, electric motors, portable heaters, air-conditioners
• Radio frequency interference (RFI)
– Caused by electrical devices emitting radio waves
• Radio and television stations, radio operators, cable TV
– Problem when frequency matches network signal
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Data Link Layer
• Layer purpose: format bits into frames
• Frame: discrete unit of information
– Contains control and address information
– Does not contain routing information
• Steps required to activate data link
– Two nodes establish physical connection
– Data Link layers connected logically through protocols
– Data Link layer decodes signal into individual frames
• Cyclic redundancy check (CRC): monitor duplication
– Calculates size of information fields in frame
– Data Link layer at sender inserts value at end of frame
– Receiving Data Link layer checks value in frame
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Data Link Layer (continued)
• Logical link control sublayer (LLC)
– Initiates communication link between two nodes
– Guards against interruptions to link
– Link to Network layer may be connection-oriented
• Media access control sublayer (MAC)
– Examines physical (device or MAC) address in frame
– Frame discarded if address does not match workstation
– Regulates communication sharing
• MAC address burned into chip on network interface
– Coded as a hexadecimal number; e.g., 0004AC8428DE
• First half refers to vendor, second half unique to device
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Data Link Layer (continued)
The Data Link layer and its sublayers
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Data Link Layer (continued)
Figure 2-6 A NIC’s Mac address
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Network Layer
• Layer purpose: control passage of packets on network
– Physical routes: cable and wireless paths
– Logical routes: software paths
• Packet: discrete unit of information (like a frame)
– Formatted for transmission as signal over network
– Composed of data bits in fields of information
– Corresponds to network information sent at Network
layer of OSI model
• Specific tasks of Network layer
– Optimize physical and logical routes
– Permit routers to move packets between networks
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Network Layer (continued)
• Discovery: process of information gathering
– Obtain metrics about location of networks and nodes
• Virtual circuits: logical communication paths
– Send and receive data
– Known only to Network layers between nodes
– Benefit: manage parallel data paths
• Extra duties using virtual circuits
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Checks (and corrects) packet sequence
Addresses packets
Resizes packets to match receiving network protocol
Synchronizes flow of data between Network layers
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Transport Layer
• Layer purpose: reliable data transmission
– Ensures data sent and received in same order
– Receiving node sends acknowledgement ("ack")
• Transport layer support of virtual circuits
– Tracks unique identification value assigned to circuit
• Value called a port or socket
• Port assigned by Session layer
– Establishes level of packet checking
• Five reliability measures used by protocols
• Transport layer mediates between different protocols
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Transport Layer cont’d
• MTU (maximum transmission unit)
– Largest data unit network will carry
– Ethernet default: 1500 bytes
– Discovery routine used to determine MTU
• Reassembly
– Process of reconstructing segmented data units
• Sequencing
– Method of identifying segments belonging to the
same group of subdivided data
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Transport Layer cont’d
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Session Layer
• Multiple goals
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Establish and maintain link between two nodes
Provide for orderly transmission between nodes
Determine how long node can transmit
Determine how to recover from transmission errors
Link unique address to each node (like a zip code)
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Presentation Layer
• Primary purpose: manages data formatting
– Acts like a syntax checker
– Ensures data is readable to receiving Presentation layer
• Translates between distinct character codes
– EBCDIC (Extended Binary Coded Decimal Interchange
Code)
• 8-bit coding method for 256-character set
• Used mainly by IBM computers
– ASCII (American Standard Code for Information
Interchange)
• 8-bit character coding method for 128 characters
• Used by workstations running Windows XP, Fedora, Linux
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Presentation Layer (continued)
• Two additional responsibilities
– Encryption: scrambling data to foil unauthorized users
• Example 1: account password encrypted on LAN
• Example 2: credit card encrypted on a LAN
• Encryption tool: Secure Sockets Layer (SSL)
– Data compression: compact data to conserve space
• Presentation layer at receiving node decompresses data
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Application Layer
• Services managed by Application layer
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File transfer, file management, remote access to files
Remote access to printers
Message handling for electronic mail
Terminal emulation
• Connecting workstations to network services
– Link application into electronic mail
– Providing database access over the network
• Microsoft Windows redirector
– Makes computer visible to another for network access
– Example: access shared folder using redirector
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Network Hardware associated in OSI
Routers & Intelligent Switches
Switches
Hubs
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TCP/IP Protocol
• it provides protocol or rules of behavior and conventions when
sending messages between two hosts of different hardware.
• it provides the “rule of the road” for internets and the Internet
• an acronym of its two most important protocols, namely,
Transmission Control Protocol (TCP), Internet Protocol (IP).
But it is actually a suite of many protocols that’s why it is
called a suite in the first place.
• It is the universal language for computer communication thru
an internetwork and it is the glue that binds the Internet and
the World Wide Web together.
• It is above all a stacked of protocols
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OSI vs TCP/IP
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TCP/IP Protocol Layers
Application
Transport
Internetwork
Network
Interface
Physical
Figure 8-4 TCP/IP protocol suite
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Role of the Physical Layer
• Specifies the physical (connectors, plugs,
adapters) and electrical (voltage and currents)
interface between data communication device
and the network.
• It corresponds to the layer 1 of OSI reference
model.
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Role of the Network Access Layer
• Provides a physical (MAC) address for the network
interface
• Verifies that incoming frames have the correct
destination MAC address
• Defines and follows media access rules
• Receives packets from the Internetwork layer and
encapsulates them to create frames
• De-encapsulates received frames and sends the
resulting packets to the Internetwork layer
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Role of the Network Access Layer
• Provides frame error detection in the form of a CRC
code
• Transmits and receives bit signals
• Defines the signaling needed to transmit bits,
whether electrical, light pulses, or radio waves
• Defines the media and connectors needed to make
a physical network connection
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Role of the Internetwork Layer
 The Internetwork layer is where administrators usually
do the most network configuration
 This is where the IP protocol operates and is the heart
of the TCP/IP protocol suite
 Responsible for four main tasks:
 Defines and verifies IP addresses
 Routes packets through an internetwork
 Resolves MAC addresses from IP addresses
 Delivers packets efficiently
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Defines and Verifies IP Addresses
• An IP address is assigned to every computer and
network device using TCP/IP for communications
• IP addresses are used for two main purposes
– To identify a network device at the Internetwork layer
– To identify the network on which a device resides
• When a device receives an IP packet, it compares
the destination IP address with its own
– If it matches or is a broadcast, the packet is
processed
– It is does not match then it is discarded
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Routes Packets Through an
Internetwork
• The Internetwork layer determines the best way to
get a packet from network to network until it reaches
its destination
• Most large internetworks, such as the Internet, have
multiple paths for getting from one network to
another
• Routers work at the Internetwork layer and it is their
job to select the best path to the destination
– Routers use the network identifier portion of IP
addresses along with their routing tables to determine
the best path
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Resolves MAC Addresses from IP
Addresses
• Every frame contains source and destination physical
MAC and logical IP addresses
• When a packet is ready to be sent to the Network access
layer, the destination device’s MAC address must be
retrieved before the frame header can be constructed
• TCP/IP uses Address Resolution Protocol (ARP) to find
MAC addresses
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Protocols at the Internetwork Layer
• Address Resolution Protocol (cont.)
– To avoid sending an ARP request every time an IP
packet is sent, PCs and other devices store learned
IP address/MAC address pairs in an ARP cache,
which is a temporary location in RAM
– If the destination computer is on another network, the
computer uses ARP to retrieve the MAC address of
the router configured as its default gateway
• The packet is delivered to the router and the router determines
where the packet should go next to get to its destination
• When the packet gets to the destination network, the router on the
destination network uses ARP to get the destination computer’s
MAC address
• Simulation 9 – The changing frame header
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RARP (Reverse Address Resolution
Protocol)
• Problem: cannot use ARP
– If device does not know its own IP address
• Solution: RARP
– Client sends broadcast message with MAC address
• Receives IP address in reply
• RARP server maintains table
– Contains MAC addresses, associated IP addresses
• RARP originally developed diskless workstations
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Protocols at the Internetwork Layer
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Protocols at the Internetwork Layer
• Internet Control Message Protocol (ICMP)
– Used to send error and control messages between
systems or devices
– Specialized IP packet with its own header
– Ping program uses ICMP Echo packets to request a
response from another computer or to verify whether
it is available for communication
– An ICMP Reply indicates whether the host is
reachable and how long the message’s round trip
from sender to receiver took
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Delivers Packets Efficiently
• Internetwork-layer protocols are primarily focused on
efficient delivery of packets
– Internetwork-layer protocols don’t include features
such as flow control, delivery confirmation, or
message assembly
– These features require overhead to ensure reliable
delivery
– Rely on the protocols in the Transport and Application
layers to provide these reliability features
– Considered a connectionless protocol – relies on
upper-layer protocols to ensure the packet’s safe
journey
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Role of the Transport Layer
• Transport layer provides reliability needed to handle the
unpredictable nature of the Internet
• Two protocols:
– Transmission Control Protocol (TCP):
• Connection-oriented and designed for reliable transfer of information in
complex internetworks
– User Datagram Protocol (UDP):
• Connectionless and designed for efficient communication of generally small
amounts of data
– Both:
• Work with segments
• Provide a means to identify the source and destination applications involved
in a communication
• Protect data in the segment with a checksum
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TCP (Transmission Control Protocol)
• Transport layer protocol
• Provides reliable data delivery services
– Connection-oriented subprotocol
• Establish connection before transmitting
– Sequencing and checksums
– Flow control
• Data does not flood node
• TCP segment format
– Encapsulated by IP datagram in Network layer
• Becomes IP datagram’s “data”
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TCP (cont’d.)
Figure 4-1 A TCP segment
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TCP (cont’d.)
Figure 4-2 TCP segment data
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TCP (cont’d.)
• Three segments establish connection
• Computer A issues message to Computer B
– Sends segment
• SYN field: Random synchronize sequence number
• Computer B receives message
– Sends segment
• ACK field: sequence number Computer A sent plus 1
• SYN field: Computer B random number
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TCP (cont’d.)
• Computer A responds
– Sends segment
• ACK field: sequence number Computer B sent plus 1
• SYN field: Computer B random number
• FIN flag indicates transmission end
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Figure 4-3 Establishing a TCP connection
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UDP (User Datagram Protocol)
• Transport layer protocol
• Provides unreliable data delivery services
– Connectionless transport service
• No assurance packets received in correct sequence
• No guarantee packets received at all
• No error checking, sequencing
– Lacks sophistication
• More efficient than TCP
• Useful situations
– Great volume of data transferred quickly
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UDP (cont’d.)
Figure 4-4 A UDP segment
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Role of the Application Layer
• The Application layer provides network services to
user applications that access network resources
• With most Application layer protocols, both a client
and a server version exist
• The Application layer provides these functions:
– Access by applications to network services
– Client/server data access
– Name resolution
– Dynamic address assignment
– Authentication/user logon
– Data formatting and translation
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HTTP:Protocol of the World Wide Web
• Originally, its main purpose was to transfer static
web pages written in HTML
• Now, it is also used for general file transfer and
downloading/displaying multimedia files
• Uses TCP as its Transport-layer protocol
• Default TCP port number is 80
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POP3, IMAP, and SMTP: E-mail
Protocols
• Post Office Protocol version 3 (POP3) is used to
download incoming messages from e-mail servers
to local desktops (uses TCP port 110)
• Internet Message Access Protocol (IMAP) is used to
manage email messages locally, yet stores them on
a server (uses TCP port 143)
• Simple Mail Transfer Protocol (SMTP) is the
standard protocol for sending email over the Internet
(uses TCP port 25)
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DHCP (Dynamic Host Configuration
Protocol)
• Assigns network device unique IP address
– Automatically
• Application layer protocol
• Developed by IETF (BOOTP replacement)
• Operation
– Similar to BOOTP
– Lower administrative burden
• Administrator does not maintain table
– Requires DHCP service on DHCP server
• Many reasons to use
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DHCP Leasing Process
• Device borrows (leases) IP address
– Devices use IP address temporarily
• Specified time limit
• Lease time
– Determine when client obtains IP address at log on
– User may force lease termination
• DHCP service configuration
– Specify leased address range
– Configure lease duration
• Several steps to negotiate client’s first lease
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DHCP Leasing Process (cont’d.)
Figure 4-11 The DHCP leasing process
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Terminating a DHCP Lease
• Lease expiration
– Automatic
• Established in server configuration
– Manually terminated at any time
• Client’s TCP/IP configuration
• Server’s DHCP configuration
• Circumstances requiring lease termination
– DHCP server fails and replaced
• Windows: release of TCP/IP settings
• DHCP services run on several server types
– Installation and configurations vary
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Domain Name System
• DNS is a name-to-address resolution protocol that keeps
a list of computer names and their IP addresses
• With a DNS, a user can use a computer’s name instead
of its IP address
• Example:
– When you enter www.course.com in your Web browser, the browser
contacts the DNS server specified in your OS’s IP configuration and
requests that the url be resolved to an IP address
– Once the IP address for the website is returned, your computer can
contact Web server to request a Web page
• DNS uses UDP because DNS messages usually consist
of a single packet of data
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DNS (Domain Name System)
• DNS redundancy
– Many computers across globe related in hierarchical
manner
– Root servers
• 13 computers (ultimate authorities)
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Figure 4-14 Domain name resolution
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DNS (cont’d.)
• Three components
– Resolvers
• Any hosts on Internet needing to look up domain name
information
– Name servers (DNS servers)
• Databases of associated names, IP addresses
• Provide information to resolvers on request
– Namespace
• Abstract database of Internet IP addresses, associated
names
• Describes how name servers of the world share DNS
information
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Summary
• A network is a system interconnecting two or more
PCs
– Basic network types
• PAN, LAN, Wireless LAN, MAN, or WAN
• There are many broadband technologies
– Cable and DSL popular
• Major issue for wireless networks is security
• Types of hardware
– Adapters, routers, hubs, switches
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Summary (cont’d.)
• Three layers of communication
– Hardware, operating system, application
• An IP address is a 32-bit address identifying network
node
• TCP/IP protocol suite uses protocols at the
application level
• Connecting to a wired network
• Connecting to a wireless networks
– Public, unsecured hotspots or private, secured
hotspots
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