1. 1 CS101 Introduction to Computing Lecture 30 Internet Services

2. 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. 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. 4 The Internet is unlike any previous human invention. It is a world-wide resource, accessible to all of the humankind.

5. 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. 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. 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. 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. 9 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

10. 10 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

11. 11 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

12. 12 1990's 1993-1994 - Web browsers Mosaic & Netscape Navigator are introduced Their GUI makes WWW & Internet more appealing to the general public

13. 13 Today’s 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

14. 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. 15 203.215.177.33 www.vu.edu.pk IP address DNS address

16. 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: –203.215.177.33 (IP address of the VU Web server)

17. 17 ? client server

18. 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. 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: 203.215.177.33www.vu.edu.pk 216.239.33.101www.google.com

20. 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. 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. 22 Internet Services There are many, but we will look at only the following: FTP Telnet Web eMail Instant messaging VoIP

23. 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. 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. 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. http://www.vu.edu.pk/cs/index.html Before, going any further, let us dissect this URL

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

27. 27 How does the Web work?

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

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

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

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

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

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

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

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

36. 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. 37 How does an eMail system work?

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

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

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

41. 41 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

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

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

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

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

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

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

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

49. 49 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

50. 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. 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. 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. 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. 54 How instant messaging works?

55. 55 User launches the IM client My Computer IM Client Internet

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

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

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

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

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

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

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

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

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

65. 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. 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. 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. 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. 69 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. 70 Today’s 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

71. 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