CS101 Introduction to Computing Lecture 30 Internet Services

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CS101 Introduction to ComputingLecture
30Internet Services
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During the last lecture (Introduction to the
Internet)

• We looked at the role Internet plays in todays
  computing
• We reviewed some of the history and evolution of
  the Internet

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Internet The Enabler

• Enables attractively-priced workers located in
  Pakistan to provide services to overseas clients
• Enables users to easily share information with
  others located all over the world
• Enables users to easily, inexpensively
  communicate with others remote users
• Enables the users to operate and run programs on
  computers located all over the world

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The Internet is unlike any previous human
invention. It is a world-wide resource,
accessible to all of the humankind.
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Key Characteristics (1)

• Geographic Distribution
• Global - reaches around the world
• Robust Architecture
• Adapts to damage and error
• Speed
• Data can travels at near c on copper, fiber,
  airwaves

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Key Characteristics (2)

• Universal Access
• Same functionality to everyone
• Growth Rate
• The fastest growing technology ever
• Freedom of Speech
• Promotes freedom of speech
• The Digital Advantage
• Is digital can correct errors

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• Internet Network of Networks

• A large number of networks, interconnected
  physically
• Capable of communicating and sharing data with
  each other
• From the users point view, Internet a
  collection of interconnected networks looks
  like a single, unified network

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TCP/IP (2)Transmission Control Protocol/Internet
Protocol

• TCP breaks down the message to be sent over the
  Internet into packets
• IP routes these packets through the Internet to
  get them to their destination
• When the packets reach the destination computer,
  TCP reassembles them into the original message

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• 1960's
• 1969 - DoD-ARPA creates an experimental network
  ARPANET as a test-bed for emerging networking
  technologies
• ARPANET originally connected 4 universities
  enabled scientists to share info resources
  across long distances

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• 1980's
• 1983 - The TCP/IP protocols becomes the only set
  of protocols used on the ARPANET
• This sets a standard for all networks, and
  generates the use of the term Internet as the net
  of nets

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• 1990's
• 1993 - CERN releases WWW, developed by Tim
  Berners-Lee
• It uses HTTP and hypertext, revolutionizing the
  way info is presented accessed on Internet

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• 1990's
• 1993-1994 - Web browsers Mosaic Netscape
  Navigator are introduced
• Their GUI makes WWW Internet more appealing to
  the general public

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Todays Goal Internet Services

• To look at several services provided by the
  Internet
• FTP
• Telnet
• Web
• eMail
• Instant messaging
• VoIP
• But first, we need to find out about the
  addressing scheme used on the Internet

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Internet Addressing

• Regular post cannot be delivered unless we write
  a destination address on the envelope
• Same is true for the Internet
• Regular post can be delivered at the intended
  address even if the given address is not precise.
  That is not the case for Internet addressing

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203.215.177.33www.vu.edu.pk
IP address
DNS address
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IP Address (1)

• A unique identifier for a computer on a TCP/IP
  network
• Format four 8-bit numbers separated by periods.
  Each 8-bit number can be 0 to 255
• Example
• 203.215.177.33 (IP address of the VU Web server)

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server
?
client
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IP Address (2)

• Networks using TCP/IP route messages based on the
  IP address of the destination
• Any IP addresses (as long as they are unique) can
  be assigned within a PN
• However, connecting a PN to the Internet requires
  using unique, registered IP addresses

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Domain Names

• IP addresses are fine for computers, but
  difficult to recognize and remember for humans
• A domain name is a meaningful, easy-to-remember
  label for an IP address
• Examples
• 203.215.177.33 www.vu.edu.pk
• 216.239.33.101 www.google.com

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DNS Domain Name System (1)

• DNS is the way that Internet domain names are
  located translated into IP addresses
• Maintaining a single, central table of domain
  name/IP address relationships is impractical
• Billions of DNS-IP translations take place every
  day
• The DNS-IP tables get updated continuously

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DNS Domain Name System (2)

• Tables of DNs IP addresses are distributed
  throughout the Internet on numerous servers
• There is a DNS server at most ISPs. It converts
  the domain names in our Internet requests to
  actual IP addresses
• In case it does not have a particular domain name
  in its table, it makes a request to another DNS
  server on the Internet

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Internet Services

• There are many, but we will look at only the
  following
• FTP
• Telnet
• Web
• eMail
• Instant messaging
• VoIP

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FTP File Transfer Protocol

• Used to transfer files between computers on a
  TCP/IP network (e.g Internet)
• Simple commands allow the user to
• List, change, create folders on a remote computer
• Upload and download files
• Typical use Transferring Web content from the
  developers PC to the Web server

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Telnet Protocol

• Using Telnet, a user can remotely log on to a
  computer (connected to the users through a
  TCP/IP network, e.g. Internet) have control
  over it like a local user, including control over
  running various programs
• In contrast, FTP allows file operations only
• Typical use Configuring and testing of a remote
  Web server

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The Web

• The greatest, shared resource of information
  created by humankind
• A user may access any item on the Web through a
  URL, e.g.
• http//www.vu.edu.pk/cs/index.html
• Before, going any further, let us dissect this
  URL

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• http//www.vu.edu.pk/cs/index.html

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How does the Web work?
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User launches the browser on his/her computer
Users Computer
Browser
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User types in the URL into the browser
Users Computer
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The browser breaks down the URL
Users Computer
http//www.vu.edu.pk/cs/index.html
cs/index.html Directory File Name
www.vu.edu.pk Servers Name
http Protocol Identifier
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Browser sends servers name to the DNS server
Domain Name
Users Computer
DNS Server
IP Address
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Browser establishes a connection with the server
Users Computer
Internet
Web Server
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Browser sends a GET request for cs/index.html
Users Computer
Web Server
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Server sends the requested file to the browser
Users Computer
Web Server
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Browser displays index.html
Users Computer
X
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eMail

• Computer-to-computer messaging
• Inexpensive, and quite quick, but not instant!
• The most popular service on the Internet, even
  more than surfing, but soon to be overtaken by
  instant messaging
• Billions are sent every day

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How does an eMail system work?
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But first, the components

• eMail client
• SMTP server
• POP3 server

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eMail Clients

• Programs used for writing, sending, receiving,
  and displaying eMail messages
• Examples Outlook, Communicator, Hotmail,
  YahooMail

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SMTP Simple Mail Transfer Protocol

• A protocol used to send and receive eMail
  messages over a TCP/IP network

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POP3 Post Office Protocol

• A protocol used for receiving eMail messages
• A POP3 server maintains text files (one file per
  user account) containing all messages received by
  a user
• eMail client interacts with the POP3 server for
  discovering and downloading new eMail messages

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The message is prepared using the eMail client
Senders Computer
eMail Client
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The eMail client sends it to the SMTP server
Senders Computer
SMTP Server
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If the receiver is local, it goes to the POP3
server
Senders Computer
POP3 Server
SMTP Server
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The receiver picks it at his/her convenience
Senders Computer
POP3 Server
SMTP Server
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Otherwise, it is sent to receiver's SMTP server
Senders Computer
SMTP Server
Internet
SMTP Server
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Which forwards it to the local POP3 server
Senders Computer
SMTP Server
POP3 Server
SMTP Server
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The receiver picks it at his/her convenience
Senders Computer
SMTP Server
POP3 Server
SMTP Server
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The Trouble with eMail

• Slow response times
• No way of knowing if the person we are sending
  eMail to is there to read it
• The process of having a conversation through
  eMail by exchanging several short messages is too
  cumbersome
• Instant messaging (IM) solves these problems

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Instant Messaging

• The IM services available on the Internet (e.g.
  ICQ, AIM, MSN Messenger, Yahoo! Messenger) allow
  us to maintain a list of people (contacts) that
  we interact with regularly
• We can send an instant messages to any of the
  contacts in our list as long as that contact is
  online

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Using Instant Messaging (1)

• Whenever a contact in our list comes online, the
  IM client informs us through an alert message and
  by playing a sound
• To send an instant message to a contact, just
  click on the contact in the IM client, and start
  typing the message

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Using Instant Messaging (2)

• The selected contact will receive that message
  almost immediately after you press Enter
• When the contacts IM client receives the
  message, it alerts the contact with a blinking
  message and by playing a sound

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Using Instant Messaging (3)

• That contact then can type a response to the
  received message, and send it instantly
• Several such conversations can be carried out
  simultaneously, each occupying a separate IM
  windows

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How instant messaging works?
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User launches the IM client
IM Client
Internet
My Computer
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IM client finds the IM server logs in
My Computer
IM Server
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It sends communication info (IP address, etc) to
the IM server
Temporary File
My Computer
IM Server
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IM server finds users contacts sends him/her
the communication info for the ones online
My Computer
IM Server
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IM server also tells the contacts that the user
is online sends his/her communication info to
them
My Computer
IM Server
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Now the users the contacts IM clients are
ready to communicate directly (P2P)
Contacts Computer
My Computer
IM Server
The IM server doesnt play any part in this P2P
communication
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As new contacts come online, IM server informs
them about the user being online vice versa
Contact As Computer
My Computer
IM Server
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Multiple, simultaneous conversations are possible
Contact As Computer
My Computer
IM Server
Contact Bs Computer
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When the user logs-off, his/her IM client informs
the IM server
Contact As Computer
My Computer
IM Server
Contact Bs Computer
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IM server erases the temporary file and informs
the users contacts about his/her offline
status
Contact As Computer
My Computer
IM Server
Contact Bs Computer
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Key Point

• Once the IM server provides the communication
  info to the user and his/her contacts IM client,
  the two are able to communicate with each other
  without the IM servers assistance
• This server-less connection is termed as a P2P
  connection

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Question

• Why do we require the server in the first place?
• Why doesnt my IM client look for the users
  contacts IM client without the IM servers help?

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Answer

• Many users (including almost all home users) do
  not have permanent IP addresses. They are
  assigned temporary IP addresses by their ISP each
  time they connect to the Internet
• The server-based IM scheme removes the need of
  having permanent IP numbers
• It also gives IM users true mobility, allowing
  them the use of IM from any Internet-connected
  computer

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VoIP Voice over IP

• Voice delivered from one device to another using
  the Internet Protocol
• Voice is first converted into a digital form, is
  broken down into packets, and then transmitted
  over a TCP/IP network (e.g. Internet)
• Four modes
• C2C
• C2T
• T2C
• T2T (with a TCP/IP net somewhere in between)

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Pro

• Much cheaper than traditional phone service

Con

• Noticeably poor quality of voice as compared with
  land-line phone service, but not much worse than
  cell phone service

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Todays Goal Internet Services

• We looked at several services provided by the
  Internet
• FTP
• Telnet
• Web
• eMail
• Instant messaging
• VoIP
• We also found out about the addressing scheme
  used on the Internet

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Next Lecture

• Next lecture (Lecture 31) - the third one in the
  four-lecture productivity SW sequence - will be
  on developing presentations
• However, during lecture 33, we will become
  familiar with the role that graphics and
  animations play in computing