1. CS101 Introduction to Computing Lecture 30 Internet Services

2. During the last lecture … (Introduction to the Internet)
We looked at the role Internet plays in today’s computing
We reviewed some of the history and evolution of the Internet

3. 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

4. The Internet is unlike any previous human invention
The Internet is unlike any previous human invention. It is a world-wide resource, accessible to all of the humankind.

5. 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

6. 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

7. Internet: Network of Networks
A large number of networks, interconnected physically
Capable of communicating and sharing data with each other
From the user’s point view, Internet – a collection of interconnected networks – looks like a single, unified network

8. 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

9. 1960's
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

10. 1980's
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

11. 1990's
CERN releases WWW, developed by Tim Berners-Lee
It uses HTTP and hypertext, revolutionizing the way info is presented & accessed on Internet

12. 1990's
Web browsers Mosaic & Netscape Navigator are introduced
Their GUI makes WWW & Internet more appealing to the general public

13. Today’s Goal: Internet Services
To look at several services provided by the Internet
FTP
Telnet
Web
Instant messaging
VoIP
But first, we need to find out about the addressing scheme used on the Internet

14. 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

15.
DNS address
IP address

16. 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:
(IP address of the VU Web server)

17. ?
server
client

18. 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

19. 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:

20. 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

21. 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

22. Internet Services
There are many, but we will look at only the following:
FTP
Telnet
Web
Instant messaging
VoIP

23. 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 developer’s PC to the Web server

24. Telnet Protocol
Using Telnet, a user can remotely log on to a computer (connected to the user’s 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

25. 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.
Before, going any further, let us dissect this URL

26. http://www.vu.edu.pk/cs/index.html Protocol Identifier Server Address
Directory & File Name

27. How does the Web work?

28. User launches the browser on his/her computer
User’s
Computer
Browser

29. User types in the URL into the browser
User’s
Computer

30. The browser breaks down the URL
User’s
Computer
cs/index.html
Directory &
File Name
Server’s Name
http
Protocol
Identifier

31. Browser sends server’s name to the DNS server
Domain Name
User’s
Computer
DNS
Server
IP Address

32. Browser establishes a connection with the server
User’s
Computer
Internet
Web
Server

33. Browser sends a ‘GET’ request for cs/index.html
User’s
Computer
Web
Server

34. Server sends the requested file to the browser
User’s
Computer
Web
Server

35. Browser displays index.html
User’s
Computer X

36. 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

37. How does an eMail system work?

38. But first, the components:
client
SMTP server
POP3 server

39. Clients
Programs used for writing, sending, receiving, and displaying messages
Examples: Outlook, Communicator, Hotmail, YahooMail

40. SMTP: Simple Mail Transfer Protocol
A protocol used to send and receive messages over a TCP/IP network

41. POP3: Post Office Protocol
A protocol used for receiving messages
A POP3 server maintains text files (one file per user account) containing all messages received by a user
client interacts with the POP3 server for discovering and downloading new messages

42. The message is prepared using the eMail client
Sender’s
Computer
Client

43. The eMail client sends it to the SMTP server
Sender’s
Computer
SMTP
Server

44. If the receiver is local, it goes to the POP3 server
Sender’s
Computer
POP3
Server
SMTP
Server

45. The receiver picks it at his/her convenience
Sender’s
Computer
POP3
Server
SMTP
Server
Receiver's
Computer

46. Otherwise, it is sent to receiver's SMTP server
Sender’s
Computer
SMTP
Server
Internet
SMTP
Server

47. Which forwards it to the local POP3 server
Sender’s
Computer
SMTP
Server
POP3
Server
SMTP
Server

48. The receiver picks it at his/her convenience
Sender’s
Computer
SMTP
Server
Receiver's
Computer
POP3
Server
SMTP
Server

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

50. 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

51. 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

52. Using Instant Messaging (2)
The selected contact will receive that message almost immediately after you press ‘Enter’
When the contact’s IM client receives the message, it alerts the contact with a blinking message and by playing a sound

53. 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

54. How instant messaging works?

55. User launches the IM client
Internet
My Computer

56. IM client finds the IM server & logs in
My Computer
IM Server

57. It sends communication info (IP address, etc) to the IM server
Temporary File
My Computer
IM Server

58. IM server finds user’s contacts & sends him/her the communication info for the ones online
My Computer
IM Server

59. IM server also tells the contacts that the user is online; sends his/her communication info to them
Computer
My Computer
IM Server

60. The IM server doesn’t play any part in this P2P communication
Now the user’s & the contact’s IM clients are ready to communicate directly (P2P)
Contact’s
Computer
My Computer
IM Server
The IM server doesn’t play any part in this P2P communication

61. As new contact’s come online, IM server informs them about the user being online & vice versa
A’s Computer
My Computer
IM Server
Contact
B’s Computer

62. Multiple, simultaneous conversations are possible
Contact
A’s Computer
My Computer
IM Server
Contact
B’s Computer

63. When the user logs-off, his/her IM client informs the IM server
Contact
A’s Computer
My Computer
IM Server
Contact
B’s Computer

64. IM server erases the temporary file and informs the user’s contact’s about his/her ‘offline’ status
A’s Computer
My Computer
IM Server
Contact
B’s Computer

65. Key Point
Once the IM server provides the communication info to the user and his/her contact’s IM client, the two are able to communicate with each other without the IM server’s assistance
This server-less connection is termed as a P2P connection

66. Question Why do we require the server in the first place?
Why doesn’t my IM client look for the user’s contact’s IM client without the IM server’s help?

67. 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

68. 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)

69. Much cheaper than traditional phone service
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

70. Today’s Goal: Internet Services
We looked at several services provided by the Internet
FTP
Telnet
Web
Instant messaging
VoIP
We also found out about the addressing scheme used on the Internet

71. 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