Lecture3

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Internet Applications for IT1 Course

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Lecture3

  1. 1. Internet Applications IT1 Course Slide Instructor: Majid Taghiloo
  2. 2. Internet Applications• Domain Name System• Electronic mail• Remote login• File transfer• World Wide Web• All use client‐server model2
  3. 3. Names• Internet communication requires IP addresses• Humans prefer to use computer names• Automated system available to translate names to addresses dd• Known as Domain Name System (DNS) y ( )3
  4. 4. DNS Functionality• Given – Name of a computer p• Returns –C Computer’s internet address ’ i dd• Method – Distributed lookup – Client contacts server(s) as necessary4
  5. 5. Domain Name Syntax• Alphanumeric segments separated by dots• Examples www.netbook.cs.purdue.edu www.eg.bucknell.edu b k ll d• Most significant part on right g p g5
  6. 6. Obtaining a Domain Name• Organization i i – Chooses a desired name – Must be unique – Registers with central authority g y – Placed under one top‐level domain• Names subject to international law for – Trademarks –CCopyright i ht6
  7. 7. Top‐Level Top Level Domains• Meaning assigned to each7
  8. 8. Within Organization• Subdivision possible• Arbitrary levels possible• Not standardized• Controlled locally by organization8
  9. 9. Example Name Structure• First level is .com• Second level is company name• Third level is division within company• Fourth level either – Company subdivision – Individual computer9
  10. 10. An Example• Assume – Company is Foobar p y – Has two divisions • Soap division • Candy division – Candy division has subdivisions – Soap Division has no subdivisions p10
  11. 11. An Example (continued)• Names in soap division have form computer.soap.foobar.com• Names in candy division have form computer.subdivision.candy.foobar.com11
  12. 12. Illustration of Foobar Naming Hierarchy12
  13. 13. The Point About Names The number of segments in a domain name  corresponds to the naming hierarchy.  There is no  corresponds to the naming hierarchy. There is no universal standard; each organization can choose  how to structure names in its hierarchy.   y Furthermore, names within an organization do not  need to follow a uniform pattern; individual groups  within the organization can choose a hierarchical  structure that is appropriate for the group.13
  14. 14. DNS Client Server Interaction Client‐Server• Client known as resolver• Multiple DNS servers used• Arranged in hierarchy• Each server corresponds to contiguous part of naming hierarchy g y14
  15. 15. Two Possible DNS Hierarchies• Choice made by organization15
  16. 16. Inter‐Server Inter Server LinksAll domain name servers are linked together to form a unified system.  Each t th t f ifi d t E hserver knows how to reach a root server and how to reach servers that are  dh t h th tauthorities for names further down the hierarchy.hi h16
  17. 17. In Practice• DNS uses backup server(s)• ISPs and others – Offer DNS service to subscribers• Small organizations and individuals – Only need domain names for computers running y p g servers – Contract with an ISP for domain service17
  18. 18. DNS Lookup • Application – Becomes DNS client – Sends request to local DNS server • Local server – If answer known, returns response – If answer unknown • Starts at top‐level server top level • Follows links • Returns response – Called name resolution18
  19. 19. Caching in DNS• Server always caches answers• Host can cache answers• Caching – Improves efficiency – Eliminates unnecessary search y – Works well because high locality of reference19
  20. 20. DNS Types• E h entry i server consists of Each in i f – Domain name – DNS type for name – Value to which name corresponds• During lookup, client must supply – Name – Type• Server – Matches both name and type20
  21. 21. The Point About Types The domain name system stores a type  with each entry.  When a resolver looks  with each entry When a resolver looks up a name, the resolver must specify the  type that is desired; a DNS server returns  type that is desired; a DNS server returns only entries that match the specified  type. type21
  22. 22. Example DNS Types• Type A ( dd (Address) ) – Value is IP address for named computer• Type MX (Mail eXchanger) – Value is IP address of computer with mail server for name• Type CNAME (Computer NAME) – Value is another domain name – U d t establish alias ( Used to t bli h li (www) )22
  23. 23. Domain Name Abbreviation• DNS lookup uses full names• Users desire abbreviations• Technique – Configure resolver with list of suffixes – Try suffixes one at a time y23
  24. 24. Example of DNS Abbreviations• Suffixes are ffi – foobar.com – candy.foobar.com• User enters name walnut• Resolver tries – walnut l t – walnut.foobar.com – walnut.candy.foobar.com24
  25. 25. Other Internet Applications• Invoked directly by user – E‐mail – Remote login – File transfer – Web browsing25
  26. 26. Electronic Mail• Originally i i ll – Memo sent from one user to another• Now – Memo sent to one or more mailboxes• Mailbox – D ti ti point f messages Destination i t for – Can be storage or program – Given unique address26
  27. 27. E‐mail E mail Address• T string Text i• Specifies mail destination• General f l form mailbox@computer• computer – Domain name of computer – Actually type MX• mailbox – Destination on the computer27
  28. 28. Use of E‐mail Address E mailEach electronic mailbox has a unique address, which is divided into two parts: the address which is divided into two parts: thefirst identifies a user’s mailbox, and the second identifies a computer on which the second identifies a computer on which themailbox resides.  E‐mail software on the sender s computer uses the second part to sender’s computer uses the second part toselect a destination; e‐mail software on the recipient s computer uses the first part to recipient’s computer uses the first part toselect a particular mailbox.28
  29. 29. Mail Message Format• Header – Identifies sender, recipient(s), memo contents , p ( ), – Lines of form keyword:information• Blank line• Body – Contains text of message29
  30. 30. Example E mail Header Fields E‐mail• Most header lines optional30
  31. 31. Extending E mail E‐mail• Original e‐mail i i l il – SMTP ‐ message restricted to ASCII text• Users desire to send – Image files – Audio clips – Compiled (binary) programs• Solution – Multi‐purpose Internet Mail Extensions (MIME)31
  32. 32. MIME• Allows transmission of – Binary data y – Multimedia files (video/audio clips) – Multiple types in single message – Mixed formats• Backward compatible32
  33. 33. MIME Encoding• Sender d – Inserts additional header lines – Encodes binary data in (printable) ASCII• Sent like standard message• Receiver –I t Interprets header li t h d lines – Extracts and decodes parts• Separate standards for content and encoding33
  34. 34. Example of MIME• Header lines addedMIME-Version: 1.0Content-Type: Multipart/Mixed; Boundary=Mime_sep• Specifies – Using MIME version 1.0 – Line Mime_sep appears before each message part34
  35. 35. MIMEAlthough Internet e‐mail only transfers text, MIME can be used to transport binary data MIME can be used to transport binary databy encoding it in printed characters.  A MIME mail message includes additional MIME mail message includes additionalinformation that a receiving application uses to decode the message.35
  36. 36. Mail Transfer• P Protocol i Si l M il T l is Simple Mail Transfer P f Protocol (SMTP) l• Runs over TCP• Used b d between – Mail transfer program on sender’s computer – Mail server on recipient’s computer recipient s• Specifies how – Client interacts with server – Recipients specified – Message is transferred36
  37. 37. Illustration of Mail Transfer• Server – Required to receive mail – Places message in user’s mailbox37
  38. 38. Terminology • Mail exploder – Program – Accepts incoming message – Delivers to multiple recipients p p • Mailing list – Database – Used by exploder • Mail gateway – Connects two mail systems38
  39. 39. Illustration of a Mailing List• Separate permissions for – M ili to li Mailing list – Adding/deleting members • Public – anyone can join • Private – access restricted by owner39
  40. 40. Illustration of a Mail Gateway • Can connect two – Heterogeneous systems – Internet to non‐Internet40
  41. 41. Automated Mailing Lists• Automated program to handle routine chores of maintaining mailing list: list manager g g g• Used in conjunction with exploder• E Example expected command: l d d add mailbox to list41
  42. 42. Computers Without Mail Servers• Typically – Small, personal computer ,p p – Not continuously connected to Internet• T receive e‐mail, user must To i il t – Establish mailbox on large computer – Access mailbox as necessary• Post Office Protocol (POP) used42
  43. 43. Illustration of POP• Current version named POP343
  44. 44. Remote Login• Provide interactive access to computer from remote site• Standard protocol is TELNET44
  45. 45. TELNET• Text‐oriented i i d interface f• User – Invokes client – Specifies remote computer• Client –FForms TCP connection t server ti to – Passes keystrokes over connection – Displays output on screen45
  46. 46. File Transfer• Complete file copy• Major protocol is File Transfer Protocol (FTP) – Uses TCP –SSupports bi binary or text transfers f – Large set of commands – Until 1995 was major source of packets in Internet46
  47. 47. FTP Paradigm • Command‐line interface • User – Forms TCP connection to server (called control connection) ) – Logs in – Enters commands to list directories, transfer files • Server – Established new TCP connection for each transfer47
  48. 48. Transfer ModesFTP has two basic transfer modes: one used for text files and the other for all non text for text files and the other for all non‐textfiles.  Although binary mode produces an exact copy of the bits, the resulting copy exact copy of the bits the resulting copymay be meaningless because does not convert values to the local representations.48
  49. 49. Illustration of TCP Connections During an FTP File Transfer• Two TCP connections used49
  50. 50. TFTP• Second file transfer service in TCP/IP: Trivial File Transfer Protocol (TFTP) ( )• Uses UDP instead of TCP• O l supports fil transfer Only file f• Useful for bootstrapping a hardware device pp g that has no disk for system software50
  51. 51. Network File System• File transfer not needed f all d il f d d for ll data transfers f• File access service allows remote clients to copy or change small parts of file• File access mechanism used with TCP/IP is Network File System (NSF) – Allows client to copy or change pieces of file – Allows shared file access –I t Integrated i t computer’s fil system t d into t ’ file t51
  52. 52. Summary• Applications use client‐server paradigm for interaction• Client – A bit Arbitrary application li ti – Actively initiates communication – Must know server’s • IP address • Protocol port number52
  53. 53. Summary (continued)• Server – Specialized p g p program – Runs forever – Usually offers one service – Passively waits for clients – Can handle multiple clients simultaneously53
  54. 54. Summary (continued)• Domain Name System – Maps name to IP address p – Uses on‐line servers – Uses caching for efficiency• Two e‐mail transfer protocols – SMTP – POP354
  55. 55. Summary (continued)• Remote login – Remote, interactive use , – Protocol is TELNET• Fil t File transfer f – Copy of entire file – Protocol is FTP55

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