Transcript Document 7277031

EMTM 553: E-commerce Systems
Lecture 2: The Internet and the Web
Insup Lee
Department of Computer and Information Science
University of Pennsylvania
[email protected]
www.cis.upenn.edu/~lee
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Computer Networks
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Development of the Internet
• A network of networks, or an “inter-network”
• ARPA Net in 1960s
• 1980s, NSF Net to connect universities and
research labs
• 1991, NSF allowed commercial traffic onto the
Internet
• 1995, Internet Service Providers (ISP): companies
that provide and charge a fee for connections to
the Internet
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Uses of the Internet
• E-mail
– to send messages to one or many across the Internet
• File Transfer Protocol (FTP)
– to transfer data files from one computer to another
• Telnet
– to remotely logon to another computer
• World Wide Web (WWW)
– to access information using a common interface
• Videoconferencing
– to use video across the Internet for conferencing purposes
• Multimedia
– to use video, audio, and animations across the Internet
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Design Principles of the Internet
• Interoperability
– Independent implementations of Internet protocols actually
work together.
• Layering and Simplicity
– IP itself is simple.
– Below IP, IP hides the complexity of many different kinds of
network hardware.
– Above IP, higher-level protocols offer services abstractions
that are easy to use and understand for application programs.
• Uniform naming and addressing
– IP address: 32-bit (e.g., 123.45.678.0 in dotted quad form)
– 128-bit address for replacement
• End-to-end protocols
– The network needs to know only the destination address for
delivering a packet.
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Layering of Internet Protocols
Application (Email, Web Browser)
End-to-End Protocol (TCP/UDP)
Host-to-Host Protocol (IP)
Physical Layer
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Internet protocols
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Host computers and routers
Computers form networks
Routers connect networks
Each host has a unique address
Internet protocol (IP)
– IP addressing
– IP datagram
– Ports: (IP address, Port number)
• Transmission Control Protocol (TCP)
• Unreliable Datagram Protocol (UDP)
• Domain Name System (DNS)
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Physical Layer
• No single technology:
– Ethernet, token ring, FDDI (Fiber Distributed Data
Interconnect), ATM (Asynchronous Transfer Mode), etc.
• ARP (Address Resolution Protocol) to translated
IP addresses (32-bit) into Ethernet addresses
(48-bit)
• Routing protocols
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Internet Protocol (IP)
• Protocol that supports the interconnection of
multiple networking technologies into a single,
logical inter-network.
• IP specifies the format of packet or datagrams.
• IP provides the addressing scheme: global, unique,
hierarchical. (ex. 158.130.64.176)
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Packet-Switched Networks
• The Internet uses Packet switching
– Files and messages are broken down into packets, which
are electronically labeled with their origin and
destination
– The destination computer collects the packets and
reassembles the data from the pieces in each packet
– Each computer the packet encounters decides the best
route towards its destination
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Packet-Switched Network
and Message Packets
Source: Schneider and Perry
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TCP: Transmission Control Protocol
• End-to-End protocol on top of IP
• TCP provides a reliable, in-order delivery of packets using
acknowledgements, checksums and sequence number.
• Flow control, congestion control
• Suitable for file transfer, email
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UDP: Unreliable Datagram Protocol
• End-to-End protocol on top of IP.
• It does multiplexing/demultiplexing and ensures the
correctness by using checksum.
• UDP is used for small/real-time packet delivery such as
voice data.
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Domain Name System (DNS)
• IP addresses are difficult for human to remember.
• Mapping IP to readable host names.
• For example: red.seas.upenn.edu (158.130.64.176)
edu
cmu
upenn
cis
clapton
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rice
washington
seas
red
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Other Internet Protocols
• Hypertext Transfer Protocol (HTTP)
– Responsible for transferring and displaying Web pages
• Simple Mail Transfer Protocol (SMTP)
– Specifies the exact format of a mail message
• Post Office Protocol (POP)
– Responsible for retrieving e-mail from a mail server
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Other Internet Protocols
• Interactive Mail Access Protocol (IMAP)
– Latest protocol, may replace POP
– Defines how a client program asks a mail server to
present available mail
o Download only selected messages, instead of all
messages
o View headers only
o Create and manipulate mailboxes on the server
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Other Internet Protocols
• File Transfer Protocol (FTP)
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Transfers files between TCP/IP-connected computers
Uses client/server model
Transfers both binary and ASCII text
Displays and manipulates remote and local computer file
directories
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The World Wide Web
• In 1992, Tim Berners-Lee at CERN released the first
implementation of the WWW.
• A global hypertext network of Web servers and Web
browsers connected by HTTP (Hypertext Transfer Protocol).
• The Web is a collection of “pages” located on “servers” all
over the world.
• Servers store HTML (Hypertext Markup Language) files and
respond to request.
• A browser provides a point-and-click user interface to access
pages in HTML.
• Each page is assigned a URL (Uniform Resource Locator),
which is the page’s worldwide name.
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Advantages of the Web
• A global information sharing architecture that
integrates online content and information servers
in an easy-to-use manner.
– Ease of navigation and use
– Ease of publishing content
• New distribution channel (of digital goods) such as
software, documents, music, video, etc.
• Enable a network-centric computing paradigm.
• New business applications (e.g., auction of surplus
capacity)
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Markup Languages and the Web
• Standard Generalized Markup Language (SGML)
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Regulated ISO standard since 1986
Nonproprietary
Supports user-defined tags
Costly to set up
Expensive compared to HTML
Steep learning curve
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Markup Languages and the Web
• Hypertext Markup Language (HTML)
– Based on SGML
– Easier to learn and support
– Supports commonly used text markup features
o Headings, title bars, bullets, lines, lists
o Precise graphic positioning, tables, and frames
– Standard language for Web pages
• Extensible Markup Language (XML)
– Descendant of SGML
– Defines which data to display, instead of how a page is
displayed
– Describes a page’s actual content, unlike HTML
– Data-tracking capability
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Traditional vs. Hyperlinked Document Pages
Source: Schneider and Perry
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HTML
• HTML tags
– <tagname properties>Displayed information affected by
tag</tagname>
o <B>best</B> - Bolds the word “best”
o <P align=“right”> - Aligns text to the right
• HTML code defines the formatting of the page,
but a page may look different on two different
browsers
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HTML Codes to Format Memo Page
Source: Schneider and Perry
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Internet Explorer Display of Memo Page
Source: Schneider and Perry
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More about HTML
• HTML Links
– Anchor tags used to link to text within the same
document, or on a distant computer
o <A HREF=“address”>Visible link text</A>
o <A HREF=http://www.upenn.edu>University of
Pennsylvania</A>
o <A HREF=“#references”>References are found
here</A>
– Text between the anchors appears as a hyperlink
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Hyperlink Structures
Source: Schneider and Perry
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HTML Version History
• Version 1.0 appeared in the summer of 1991
• Version 2.0 was released in September 1995
– Internet Explorer 2.0 and Netscape Navigator 2.0 appeared
• Version 3.2 was released in 1997
– Provided support for tables, complex numbers, and text flow
around images
• Version 4.0 was released in December 1997
– Support for OBJECT tag and Cascading Style Sheets (CSS)
– Internationalization for various languages
– Accessibility features
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HTML Editors
• Used to generate the HTML code
– Simple text editors offer limited flexibility
– Any word processor can be used
– Web site builders offer more control
o Microsoft FrontPage
o Dreamweaver
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Web Clients and Servers
• Client computers typically request services,
including printing, information retrieval, and
database access
• Servers are responsible for processing the clients’
requests
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Client/Server Structure of the WWW
Source: Schneider and Perry
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Web Browser
• Implements HTTP (HyperText Transfer Protocol)
– Interact with servers
– Displays web pages
– Caching, freshness control
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Page rendering, font mapping
Compression, decompression
Handles multimedia, supports plug-ins
Interprets scripts
Executes Java applets
Maintains cache, history
Manipulates cookies
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Web Browser
HTTP: HyperText
Transport Protocol
URL: Uniform
Resource Locator
Request/Reply pages
Ex. www.google.com
HTML: HyperText
Markup Language
Ex. Graphic,colorful page
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Web Server Software
• Capabilities/Features
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Support HTTP protocol
Support Secure Sockets Layer (SSL)
File Transfer Protocol (FTP)
Search engines and indexing
Data Analysis
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Uniform Resource Locators (URL)
• http://www.w3.org/example/index.html
• Protocol designator (http:)
• Server name (servername.domain)
– DNS name
– The browser use the DNS to translate the name to an IP
network address
• Pathname (/path/name/of/object.html)
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Message Flow Between Web Client
and Server
Source: Schneider and Perry
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Hypertext Transfer Protocol (HTTP)
• The standard Web transfer protocol.
• With HTTP, a client opens a TCP connection to the
web server.
• Two types of messages
– Requests from browsers to servers; e.g.,
o GET to retrieve document
o PUT to upload files to the server
o POST to send the results of a form filled out by the
user
– Responses from servers to browsers
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Basic Web application architecture
Database
Web Server
requests
Programs
downloads
Web Browser
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Two-Tier Client/Server
Tier 1
Tier 2
Network
DMS
Client
Server
• User Interface
• Web Server
• Web Applications
• Database Server
• Application Server
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Three-Tier Client/Server
Tier 1
Tier 2
Network
Tier 3
Network
DMS
Client
Server
Database Server
• User Interface
• Web Server
• Web Applications
• Application Server
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N-Tier Client/Server
Tier 1
Tier 2
Tier 3
Tier N
DMS
Client
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Web Server
Application
Server
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Server
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Q&A
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