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CS120: Lecture 8. MP Johnson Hunter [email protected]. Agenda. networks Basics, def Internet/web Internet Applications FTP telnet HTTP/HTML Internet protocols/arch TCP/IP HTML language. Internet Arch. Each gp/company/school has domain microsoft.com cuny.edu - PowerPoint PPT Presentation
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Agenda• networks
– Basics, def– Internet/web– Internet Applications
• FTP• telnet• HTTP/HTML
– Internet protocols/arch• TCP/IP
• HTML language
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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
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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
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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
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Internet & TCP/IP
• Internet’s TCP/IP is packet-switched– Protocols for communication on the internet– Don’t 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
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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)
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Internet apps: mail, telnet
• Email– Each domain has a mail server– Receives/stores mail for users in domain– [email protected]– Or mapped to [email protected]
• Telnet: telnet://addr– Access one machine from another– After login, get a command prompt– Run programs, etc.– Secure version: ssh
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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 aren’t physical, don’t need both• Unix: only LF, Mac: only CR, Win: both
• So for text transfer, must translate EOLs• But not for binary!
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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
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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
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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
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Security issues
• Access:
• Privacy of communication– Public-key encryption
• Integrity of machine exposed to internet– Attacks: viruses and worms– Defense: firewall
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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