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CS120: Lecture 8

MP Johnson

Hunter

mpjohnson@gmail.com

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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– matt@mail.ccny.cuny.edu– Or mapped to matt@ccny.cuny.edu

• 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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Security issues

• Viruses/Trojan horse

• Worms

• DoS

• Mitnick: worm DoS

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html

• Stephenson handout on HTML…

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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

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Future

• future: lab session/hw2– Goal: create and post a webpage

– Also written portion (done independently)