ISD110: Introduction To The Internet and World Wide Web

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ISD110 Introduction To The Internet and World
Wide Web
Introductory to Basic Network Features and
Configuration Looking at some of
terms/terminology used Reference OSI Model and
its components OSI and TCP/IP Simple Network
Topology
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Introduction

• WHAT IS A NETWORK?

Interconnected group or system such as
Roads Business Telecommunication Postal
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Computer Network

• Definition is more appropriate is this case

As a computer network is simply a
system of interconnected computers.
Network is built based on a reference MODEL
(discuss later)
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Basic Features
Some very important functions of a network maybe
The ability to address computers independently of
physical/network cards The ability to
Internetwork Routing messages through
network Provide Error control Provide
connectivity, integrity, reliability, access to
information and distributed data
processing Provide useful applications to users
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The Internet
The Internet is a network of networks, linking
computers to computers sharing and providing
information for access and viewing. The Internet
is the transport vehicle for the information
stored in files or documents on another computer.
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Network built based on a Reference OSI 7 Layer
Model

• Most Network today follows this model

Application Presentation Session Transport Net
work Data Link Physical
Defines 7 layers of communication types and
Interfaces
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Layer Functions

• Each layer depends on the services provided by
  the layer below.

User applications - E-mail, File Transfer,
Database access
protocol conversation, data translation,
compression, encryption, character set
conversion, and graphical command interpretation
between the computer and the network.
Performs the setup functions to create the
communication sessions between computers
To ensure that packets are send error-free to the
receiving computer in proper sequence with no
loss of data or duplication
Responsible for making routing decisions and
forwards packets that are farther then one link
away
Responsible for the flow of data over the network
from one device to another
Deals with the physical, electrical, and
connector types involved with making a network
connection.
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Pictorial Layer Functions
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Protocols and Information Packets

• So how do computers in network communicate?

Protocol and Information Packets
What is a Protocol?
Protocols are agreements between processes
(computer programs) about which may do what to
whom and when
In a nutshell it is a set of rules for
communication between processes, giving a means
to control the orderly communications of
information between stations on a data link
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Protocols and Information Packets

• Internet today has evolved from the traditional
  X-25 protocol.
• Looking at how it operates will give a better
  understanding of
• the TCP/IP

Data Communication Packets
Flag
Flag
Address
Control
Data.
FCS
Opening Flag, 8 bits 01111110, 7E
hex Address, 8 bits could be more Control,
8 bits, or 16 bits Data Payload, Information
fields, Multiple of 8 bits FCS (Frame Check
Sequence), 16 bits, or 32 bits Closing Flag, 8
bits 01111110, 7E hex
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X-25 and HDLC

• FlagDelimits the beginning and end of the
  frame. Bit stuffing is used to ensure that the
  flag pattern does not occur within the body of
  the frame.
• AddressSource or Destination address.
• ControlQualifies command and response frames
  and indicates whether the frame is an I-frame, an
  S-frame, or a U-frame. In addition, this field
  contains the frame's sequence number and its
  function (for example, whether receiver-ready or
  disconnect).
• DataIs the actual information being
  transmitted. It can contain Control Information
  for handshaking, or actual Data used by
  applications .
• FCSHandles error checking and ensures the
  integrity of the transmitted data.

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Connection

• Normal operation

Are you awake ?
Station A
Station B
Yes I am, Ready to receive
This is an Information Frame
Received with No Errors
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TCP/IP

• TCP/IP - Transmission Control Protocol / Internet
• Protocol is a layered protocol

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TCP/IP

• TCP/IP - Transmission Control Protocol / Internet
  Protocol

Application (http, telnet, snmp, smtp, nfs, ftp)
Application layer
Transport (TCP, UDP)
Network layer
Internet (IPv4/IPv6)
Data Link layer
Network Access
(HDLC)
Physical layer
Physical layer
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TCP/IP

• TCP/IP - Transmission Control Protocol / Internet
  Protocol

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TCP/IP

• TCP/IP - Transmission Control Protocol / Internet
  Protocol

Sender
Receiver
Application Layer
Application Layer
HTTP
Request
HTTP
Request
Transport Layer
Transport Layer
HTTP
TCP
Request
HTTP
TCP
Request
Network Layer
Network Layer
HTTP
TCP
IP
Request
HTTP
TCP
IP
Request
Data Link Layer
Data Link Layer
HTTP
TCP
IP
Ethernet
Request
HTTP
TCP
IP
Ethernet
Request
Physical Layer
Physical Layer
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INSIDE TCP/IP

• Residing within the TCP/IP standard there are a
  number of protocols for handling data
  communication
• TCP (Transmission Control Protocol) communication
  between applications
• UDP (User Datagram Protocol) simple communication
  between applications
• IP (Internet Protocol) communication between
  computers
• ICMP (Internet Control Message Protocol) for
  errors and statistics
• DHCP (Dynamic Host Configuration Protocol) for
  dynamic addressing

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INSIDE TCP/IP

• TCP/IP is TCP and IP working together.
• TCP takes care of the communication between your
  application software (i.e. your browser) and your
  network software.
• IP takes care of the communication with other
  computers.
• TCP is responsible for breaking data down into IP
  packets before they are sent, and for assembling
  the packets when they arrive.
• IP is responsible for sending the packets to the
  receiver.

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Packet Encapsulation

• IP Encapsulation

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IP Packet Encapsulation
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IP Packet Encapsulation
VERSION (4 bits) The version field is set to the
value '4' in decimal or '0100' in binary. The
value indicates the version of IP (4 or 6, there
is no version 5). IHL (4 bits) The Internet
Header Length (IHL) describes how wide the header
is in 32-bit words. For instance, the minimum
value is 5, as that is the minimum size of an IP
header that contains all the correct fields is
160 bits, or 20 bytes. This allows the receiver
to know exactly where the payload data
begins. TOS (8 bits) Type of service allows the
intermediate receiving stations (the routers) to
have some notion of the quality of service
desired. This allows the network to make
adaptations for delay, throughput, or
reliability. TOTAL LENGTH (16 bits) This informs
the receiver of the datagram where the end of the
data in this datagram is. This is the length of
the entire datagram in octets, including the
header. This is why an IP datagram can be up to
65,535 bytes long, as that is the maximum value
of this 16-bit field.
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IP Packet Encapsulation
IDENTIFICATION (16 bits) Sometimes, a device in
the the middle of the network path cannot handle
the datagram at the size it was originally
transmitted, and must break it into fragments. If
an intermediate system needs to break up the
datagram, it uses this field to aid in
identifying the fragments. FLAGS (3 bits) The
flags field contains single-bit flags that
indicate whether the datagram is a fragment,
whether it is permitted to be fragmented, and
whether the datagram is the last fragment, or
there are more fragments. The first bit in this
field is always zero. FRAGMENT OFFSET (13
bits) When a datagram is fragmented, it is
necessary to reassemble the fragments in the
correct order. The fragment offset numbers the
fragments in such a way that they can be
reassembled correctly.
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IP Packet Encapsulation
TIME TO LIVE (8 bits) This field determines how
long a datagram will exist. At each hop along a
network path, the datagram is opened and it's
time to live field is decremented by one (or more
than one in some cases). When the time to live
field reaches zero, the datagram is said to have
'expired' and is discarded. This prevents
congestion on the network that is created when a
datagram cannot be forwarded to it's destination.
Most applications set the time to live field to
30 or 32 by default. PROTOCOL (8 bits) This
indicates what type of protocol is encapsulated
within the IP datagram. Some of the common values
seen in this field include Protocol Number(Decima
l) ICMP 1 IGMP 2 TCP 6 UDP 17
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IP Packet Encapsulation
HEADER CHECKSUM (16 bits) The checksum allows IP
to detect datagrams with corrupted headers and
discard them. Since the time to live field
changes at each hop, the checksum must be
re-calculated at each hop. In some cases, this is
replaced with a cyclic redundancy check
algorithm. SOURCE ADDRESS (32 bits) This is the
IP address of the sender of the IP
datagram. DESTINATION ADDRESS (32 bits) This is
the IP address of the intended receiver(s) of the
datagram. If the host portion of this address is
set to all 1's, the datagram is an 'all hosts'
broadcast. OPTIONS PADDING (variable) Various
options can be included in the header by a
particular vendor's implementation of IP. If
options are included, the header must be padded
with zeroes to fill in any unused octets so that
the header is a multiple of 32 bits, and matches
the count of bytes in the Internet Header Length
(IHL) field.
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World Wide Web
WWW is an application layer protocol that is a
large network of servers providing Hypertext (and
other services) to Web Clients running
applications such as a Browser
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Network Topology

• Star

Computers are Connected to a Central Hub and
every transaction is through the Hub. What are
the disadvantages?
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Network Topology

• Ring

Computers are in a close loop and one computer is
connected to 2 others
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Network Topology

• Bus

All computers are connected to a central spine
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Summary

• Today we have look at the Basic Components of
  involved in
• Networking.
• Review these slides as they should give you a
  fair idea what
• we will discus next week on other issue such as
  
• MAC(Media Access Control), HTTP, HTML, URL
• and IP address