World Wide Web

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World Wide Web

• Basics

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What is an internet anyway?

• 2 or more networks that can communicate

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Historical View Internet

• 1969 - Telnet
• 1970 - 4 computers
• Stanford, UCLA, UC Santa Barbara, U Utah
• 1971 - FTP
• 1983 - 562 computers on the internet
• 1993 - 1.2 million computers on the internet
• 1999 - ?? Kazillions

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What is the World Wide Web ?

• Hypertext connectivity of documents

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Size of the Internet
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(No Transcript)
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The web

• The Web is protocol that uses the internet as the
  communication structure
• links documents stored in computers communicating
  with the internet
• main authority W3 consortium
• www.w3.com

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Historical View WWW

• 1989 - Berners Lee - web doc proposal
• 1990 - Berners Lee - text browser (physicists)
• 1992 - public access to web docs at CERN
• 1993 - 60 web servers Mosaic (graphics)
• (500 servers by year end)
• 1995 - more Internet email than US post
• 1999 - x million docs y million servers

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http//www.lexis-nexis.com
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Basics

• Web server - machine that services internet
  request
• Web client - machine that initiates internet
  request
• Browser - software to interact with internet data
  at the client
• TCP/IP - internet data protocol
• FTP - internet file transfer protocol
• HTTP - hypertext transfer protocol
• HTML - hypertext markup language

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Client-Server Model
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Looking in the Cloud

• /opt/sbin/traceroute

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CAnet2 layer 2 links
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CAnet3 Physical Links
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1. Client-Server Web

• Cloud model
• TCP/IP
• HTTP and MIME types
• FTP
• protocol stacks

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Servers and Clients

• Servers - computer systems at the end of a
  network that store files and provide other
  services
• Clients - computer systems that are end points
  for users of the data

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Network Architectures

• ISOs OSI model
• 1970s
• International Organization for Standards
• Open Systems Interconnection reference Model
• 7 layer architecture

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ISO - OSI Model

• Application layer
• presentation layer
• session layer
• transport layer
• network layer
• data link layer
• physical layer

• Ftp, telnet, etc
• data compression, format
• set up connections
• end-to-end trans of packets
• guide packets along links
• send packet between nodes
• deliver bits between nodes

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ISO OSI model
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INTERNET MODEL

• 4 layers
• Application layer
• communication services (ftp, telnet, email)
• transport layer
• transmission of messages end-to-end
• network layer
• transmission of messages sequence of links
• link layer
• transmission of packet across one link

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Internet layers
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Application Layer

• FTP
• HTTP
• SMTP
• telnet
• etc

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TCP/IP

• Suite of protocols made the standard for the
  Internet
• facilitates communication between heterogeneous
  and similar networks that are connected together
• reliable, connection oriented, byte stream
  protocol

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Transport layer TCP and UDP

• TCP
• transmission control protocol
• full duplex byte stream
• virtual path (connected)
• error free
• uses acknowledgements
• 16 bit address of ports

• UDP
• user datagram protocol
• connectionless
• no acknowledgements
• no flow control
• no resending of erroroneous packets
• some error detection
• 16 bit port addresses

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TCP/IPTransport Control ProtocolInternet
Protocol
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TCP and IP
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Network Layer IP

• Delivers packets up to 64kbytes, 1 at a time
• Each packet has a header
• sending host and intended host network addresses
• 32 bit addresses
• IP layer (like UDP)
• unreliable
• connectionless

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Link Layer links

• Connect computer to Internet
• SLIP
• serial line IP (asynchronous, 1 char at a time)
• move IP packets to common link (phone line)
• PPP
• point-to-point protocol
• also synchronous transfer for packets

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Data encapsulation using TCP on Ethernet
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TCP/IP apps

• TCP/IP software usually includes
• remote terminal client using TELNET protocol for
  remote login
• electronic mail client using SMTP protocol to
  transfer e-mail to remote system
• file transfer client using FTP protocol to
  transfer files between 2 machines

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HTTPHyperText Transport Protocol

• Native protocol for WWW
• sits on top of internets TCP/IP protocol
• HTTP is a 4 step process per transaction
• uses a predefined set of document formats from
  MIME

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MIME

• MIME - multipurpose internet mail extensions
• defines file formats (images, video, text, etc)
• e.g. Content-type text/html
• Data type/subtype
• text/html
• text/plain
• image/gif
• video/mpeg
• application/msword
• etc!!!

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HTTP Connection

• 1. Client
• makes an HTTP request for a web page
• makes a TCP/IP connection
• 2. Server accepts request
• sends page as HTTP
• 3. Client downloads page
• 4. Server breaks the connection

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HTTP is Stateless!!!!

• Each operation or transaction makes a new
  connection
• each operation is unaware of any other connection
• each click is a new connection
• So how do they do those shopping carts??

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What does it look like?

• Header object file
• Header
• plain text
• info about the object (MIME etc)
• methods allowed
• etc
• browser sends a header to server each time you
  ask for information
• server sends a header and possibly content

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HTTP Transaction Example

• GET /catalog/ip/ip.htm HTTP 1.0
• Accept text/plain
• Accept text/html
• Referer http//www.june.com/catalog.html
• User-Agent Mozilla/2.0
• ltCR/LFgt

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HTTP REQUEST PROTOCOL

• Request Simple Full
• Simple GET ltURIgt CRLF
• Full Method URI ProtVersion CRLF
  ltHTRQ Headergt CRLF ltdatagt
• Method GET POST HEAD .
• ltHTRQ Headergt ltFieldnamegtltValuegtCRLF
• ltdatagt MIME conforming message
• www.w3.org/Protocols/HTTP/

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HTTP Header fields

• General-header fields
• used for both requests and responses
• Request-header fields
• used for responses
• extra client information for use by server
• optional

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General-header fields

• Date mon,11, Jan 1999 081432 GMT
• MIME-version 1.0
• Pragma no cache
• directives

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Request-header fields

• acceptable MIME types for response
• Accepttext/html
• Accept.
• 401 response from client
• Authorization Basic abcdef (uuencoded username
  and password)
• Fromclient-email-addr

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More Request-header fields

• If-Modified-Sincedate
• conditional get
• source of current requested Url
• RefererURL
• robot/browser identification
• User-AgentMozilla/2.0

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Looking at the HTTP Header Values

• In Perl
• ENVFrom
• In Netscape
• www.cs.dal.ca/watters/cgi-bin/webcourse/env.html

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HTTP Methods

• Client requests either
• simple request
• full request
• Request-line method Request-URI HTTP-version
  CRLF
• GET /catalog/ip.html
  HTTP/1.0

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Simple requests

• Only for HTTP 0.9
• only uses Get method
• causes the server to locate and transfer the
  object specified
• client responsible for handling the object
• GET lturigt CRLF

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Full Request

• Uses HTTP version and more methods
• method tells server what to do to the resource
  requested
• Methods
• GET
• POST
• HEAD

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GET Method

• Request server to retrieve object specified
• conditional GET
• request message includes
• If-Modified-Since in header

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HEAD Method

• Like GET but does not return the object
• returns a header about the resource requested
  (metainformation)
• good way to test link validity

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POST Method

• Include an object in the request
• server should use that object in processing the
  request
• must include a Content-Length in header

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HTTP Response Message

• HTTP protocol version
• 3 digit status code
• reason phrase
• CRLF
• optional header fields
• CRLF

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HTTP Response Header Fields

• Additional information about the server
• such as
• LOCATION exact URI address
• SERVER server software (CERN/3.0)
• WWW-AUTHENTICATE
• status 401 responses (unauthorized request)
• server challenges client
• client may use to send authorization info to
  server

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Understanding STATUS Codes

• 1xx - not yet in use
• 2xx - action successful
• 3xx - further action needed
• 4xx - client request error
• 5xx - server error

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HTTP Transaction

• 1. Client and server establish a connection
• 2. Client makes a request
• 3. Server makes a response
• 4. Server terminates connection

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• Step 1 establish connection
• TCP/IP connection set up
• uses a port number as application reference
• usually port 80
• ports lt 1024 are privileges (gt1024 are open)
• Step 2 client request
• Http message sent with a request line
• request-line method URL HTTP version

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Web Port Assignments

• 21 FTP
• 23 Telnet
• 25 smtp (mail)
• 70 gopher
• 79 finger
• 80 HTTP

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• Step 3 Server response
• server sends Http message and optionally
  requested data
• resp-message HTTP version statuscode
  reason-phrase optional stuff
• Step 4 connection terminated
• usually the server
• sometimes the client stops it
• anything else, whoever notices terminates