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Introduction to the Internet

Introduction to the Internet. Internet service concepts ... Introduction to information services Finger-, Inetd-, and Telnet-based services FTP WAIS Gopher World-Wide Web Email services Security Future topics and research opportunities. Internet service concepts ….

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Introduction to the Internet

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  1. Introduction to the Internet • Internet service concepts ... • Introduction to information services • Finger-, Inetd-, and Telnet-based services • FTP • WAIS • Gopher • World-Wide Web • Email services • Security • Future topics and research opportunities

  2. Internet service concepts … • What is the Internet? ... • Internet services structure ... • TCP/IP ... • Info-clients info-servers comm.

  3. What is the Internet? … • Federation of computer networks ... • Three roles ... • What does it mean to be on the Internet? ... • What is on the Internet? ...

  4. Federation of computer networks ... • Speak same protocols • Computer network protocols • TCP/IP • Connected with high-speed telephone circuits

  5. Three roles ... • Information providers • Users (customers) • Connection providers

  6. What does it mean to be on the Internet? ... • “Really” on the Internet • On-demand; no Internet protocols • Hybrid

  7. Full-time IP connection ... • IP protocol

  8. Dial-up access • terminal emulation

  9. Hybrid ... • SLIP: Serial Line Internet Protocol • PPP: Point-to-Point Protocol • ISDN: Integrated Services Digital Network ... • Running Internet protocols over dial-up ...

  10. ISDN: Integrated Services Digital Network ... • Voice, data, both • On-demand, full-time • Speed comparable to leased-line connection

  11. Running Internet protocols over dial-up ... • Dial-up hybrid

  12. What is on the Internet? ... • Computers: all kinds • People: large number, variety • Places

  13. Internet services structure … • Client/server architecture ... • Information service client/server ... • Any computer can be a service provider • Compare: commercial networks ...

  14. Client/server architecture ... • Client: interact with user • Server: perform tasks as directed by user • Same/different computers

  15. Information service client/server … • Client ... • Server ...

  16. Client ... • Accept queries/other input from users • Send queries/data to server • Manage display of results

  17. Server ... • Receive queries/other data from clients • Process • Send results/status

  18. Compare: commercial networks ... • E.g., CompuServe • All services from computers run by network • No information by users (except e-mail)

  19. TCP/IP ... • Packets: data; acknowledge data received • Layered protocols ... • inetd superserver ... • Stand-alone servers ... • Naming ...

  20. Layered protocols … • IP ... • TCP (connection-oriented) ... • UDP (connectionless) ...

  21. IP ... • Internet Protocol • IP packet • Addresses: 32-bit numbers ... • No guaranteed delivery • No guaranteed serialization

  22. Addresses: 32-bit numbers ... • 0 to ~4.3 billion • Dotted-octet notation ... • Net, subnet hierarchy

  23. Dotted-octet notation ... • 4 sets of 8 bits • Convert to decimal • Concatenate, separated by dots

  24. TCP (connection-oriented) ... • Transmission Control Protocol • Two services IP doesn't • Guaranteed delivery • Serialization of data • How? ... • ==> Mimic stream over dedicated line • Also, port numbers ...

  25. How? ... • Sequence numbers for IP packets • Check arriving packets for completeness, order • Get missing packets resent

  26. Also, port numbers ... • Layer below IP address (computer) • Identify services within computer • 16-bit • “Well known,” e.g., ...

  27. “Well known,” e.g., ... • 25: Simple Mail Transfer Protocol (SMTP) mailer • 23: Telnet • “Privileged” ports (UNIX) • Numbers < 1024 • Only root can start • Security • Prevent “listening”

  28. UDP (connectionless) ... • User Datagram Protocol • Two additional features to IP • Port #s (16-bit, not same as TCP) • Checksumming (optional) • Detect modifications • Avoid TCP for 1-packet q's / responses

  29. inetd superserver ... • Many servers, idle periods ==> waste • inetd: Internet Superserver • Listen to ports • Start requested server • Server talks directly to client • inetd.conf ...

  30. inetd.conf ... • Determine servers at start-up • E.g.,: ...

  31. E.g., ...

  32. To find ... • % grep telnet /etc/services • telnet 23/tcp • Connection request ==> transaction in syslog

  33. Stand-alone servers ... • inetd restarts servers whenever needed • Auto handle of config change • Inefficient for busy service • Takes time for inetd to start server • ==> Run server stand-alone

  34. Daemon ... • Listen to assigned port • Spawn copy of self to handle ea. request • + Fast • – Crash unnoticed; explicit reread of config files when changed

  35. Naming ... • Domain names ... • Aliases ...

  36. Domain names ... • cs.uml.edu (from specific to general) • .edu, .com, .gov, .org, .mil • 2-letter country codes (ISO 3166) • Servers: Domain Name Server (DNS) • Clients: resolvers • Send queries to DNS for IP addr. <--> name

  37. Aliases ... • Pointers to other domain names • Easier to remember • E.g., ftp.uml.edu, www.cs.uml.edu

  38. Info-clients info-servers comm. • Gopher ... • Web ... • FTP ...

  39. Gopher ... • User request resource on Gopher server • Client query ... • Gopher server response ... • Client stores file • TCP connection torn down • TCP up and down for each request

  40. Client query ... • Accept input & translate into query • Send query to Gopher server • Look in /etc/services to find port number • gopher 70/tcp • Query --> TCP packet(s) --> server (port 70)

  41. Gopher server response ... • Listen on port 70 / inetd listen • inetd check destination • /etc/services to find port number • Map to service name • Gopher • Start server, gopherd • Gopher interpret query • Open, read file, send to client

  42. Web ... • Similar to Gopher • Ea. doc. new TCP connection • Ea. inline graphics ==> separate TCP connection • With approp. server where graphics is

  43. FTP ... • Different from Gopher, Web • 2 connections server <--> client • Control: up through time user logged in • Data: up when needed

  44. Introduction to information services • Overview of Internet information services ... • What to do with Internet Services, e.g., ... • System and network requirements ... • Human requirements ...

  45. Overview of Internet information services … • Mail-based ... • FTP: file transfer protocol ... • Telnet & finger ... • Gopher ... • WAIS: Wide Area Information Servers ... • WWW: The World-Wide Web ...

  46. Mail-based ... • Reach largest population • Get quickest response • Grab attention • Compare: Gopher, WWW • User has to check • Two types ...

  47. Two types … • Mailing lists ... • Archive servers ...

  48. Mailing lists ... • List of e-mail addresses • Reach all with one address • E-mail list server programs • Majordomo • Listserv • listproc, e.g., ...

  49. listproc, e.g., ... • To: listproc@online.ora.com • -------- • subscribe ora-news John Q. User of Some Organization

  50. Archive servers ... • Send out files by e-mail • Response to users' e-mail requests • Also on-the-fly • First index, then file name • Archive: encodes, splits to pieces, sends • User: reassembles, decodes • ftpmail: xfer files from other hosts & deliver by e-mail

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