CS120: Lecture 8

1
CS120 Lecture 8

• MP Johnson
• Hunter
• mpjohnson_at_gmail.com

2
Agenda

• networks
• Basics, def
• Internet/web
• Internet Applications
• FTP
• telnet
• HTTP/HTML
• Internet protocols/arch
• TCP/IP
• HTML language

3
Internet Arch

• Each gp/company/school has domain
• microsoft.com
• cuny.edu
• Names registered with ICANN
• Internet Corp for Assigned Names Netws
• Domain connected to Internet cloud w/ gateway

4
Network topologies

• Ethernet bus-based
• To send msg, machine broadcasts to all
• 2 machines cannot send a msg at once
• If 2 try at once, both stop,
• Wait random amount of time
• Like conversation

5
Internet organization

• Phone system circuit-switched
• Create connection between endpts for each call
• Each node connects to only 1 other node at a time
  (usually)
• Too many calls -- circuits overloaded
• But usually ok
• Computer networks different
• Each node connects to many other nodes at once
• Want near-instant access to every other node (web
  browsing)
• But circuit-switched -- conns grows
  quadratically

6
Internet TCP/IP

• Internets TCP/IP is packet-switched
• Protocols for communication on the internet
• Dont have direct conns
• -- msg must hop from node to node - routing
• In practice each node knows next hop for msg
• To send msg
• Msg broken up into many small packets
• Packets sent individually - maybe with diff paths
• Packets reassembled after received
• Different packets may take different routes
• Cold War motivation. no partic node essential

7
C-S v. P2P

• Most apps client-server
• Many machines requests info/service
• One central machine responds
• Websites, print server, file server
• Peer-to-peer
• Machines comm. as peers
• Grokster, Kazaa
• Napster (but with central directory)

8
Internet apps mail, telnet

• Email
• Each domain has a mail server
• Receives/stores mail for users in domain
• matt_at_mail.ccny.cuny.edu
• Or mapped to matt_at_ccny.cuny.edu
• Telnet telnet//addr
• Access one machine from another
• After login, get a command prompt
• Run programs, etc.
• Secure version ssh

9
Internet apps FTP

• FTP file transfer protocol ftp//addr
• Transfer file between machines on network
• Text v. binary
• On old teletype machines
• End-of-line needed LF (10) and CR (13)
• Displays arent physical, dont need both
• Unix only LF, Mac only CR, Win both
• So for text transfer, must translate EOLs
• But not for binary!

10
Internet apps HTML

• Html hypertext transfer protocol
• Transmit webpages, etc
• Written in HTML
• URL/URI http//www-cs.ccny.cuny.edu/mjohnson/100
  /
• Invented by Tim Berners-Lee, CERN/W3C
• Originally just text, Lynx, 1989-1991
• Marc Andreesen, NCSA, Illinois, 1993 Mosiac
• ? Netscape, IE, Firefox

11
HTML

• Mark-up language
• Consists of data in tags
• Tags contain tags
• Has text, but also format/display info
• Html called source
• Describes what browser displays
• Also pictures
• Pages link to one another
• Use CSS to enforce look

12
Dynamic web

• Regular webpages static
• textfiles
• Many apps require dynamic info
• Web-based email
• Search engines
• Shopping
• Many solutions
• Srv-side progs CGI, PHP, ASP, servlets
• Client-side progs Java applets, ActiveX comps,
  Flash
• Client-side scripting JavaScript, DHTML

13
Security issues

• Access
• Privacy of communication
• Public-key encryption
• Integrity of machine exposed to internet
• Attacks viruses and worms
• Defense firewall

14
Security issues

• Viruses/Trojan horse
• Worms
• DoS
• Mitnick worm ? DoS

15
html

• Stephenson handout on HTML

16
Html

• Nice reference here
• http//hotwired.lycos.com/webmonkey/reference/html
  _cheatsheet/
• Common tags
• Html, body, title, b, I, u, font, p, br, ol,li,
  ul
• More complicated form, table

17
Future

• future lab session/hw2
• Goal create and post a webpage
• Also written portion (done independently)