The University of Akron Summit College Business Technology Dept.

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The University of AkronSummit CollegeBusiness
Technology Dept.

• 2440 141Web Site Administration
• Web Application Resources
• Instructor Enoch E. Damson

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Client/Server Basics

• The client/server model generally has clients and
  servers, and business/application logic (rules)
• Client hardware or software used to communicate
  with a data provider (server)
• Normally, one user uses a specific client at a
  time
• Server a computer capable of providing data to
  many clients at the same time
• The term server can mean
• the physical computer or piece of hardware
• the actual server software or daemon running on
  that machine
• Daemon a program that offers a service to other
  programs, usually over a network
• Application/business logic rules written in a
  programming language to help the communication
  between clients and the server

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Client/Server Basics

• The client and the server are usually on separate
  machines connected by some kind of network but
  can also exist on the same machine
• The client/server model is ideal for distributed
  applications

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Network Connections and Ports

• Computers connected to the Internet typically
  communicate using TCP/IP (Transmission Control
  Protocol and the Internet Protocol)
• TCP/IP allows computers to communicate at a
  network-level using IP (Internet Protocol)
  addresses
• TCP (Transmission Control Protocol invented by
  Robert Kahn
• IP (Internet Protocol) created by Dr. Vinton
  Cert together with Robert Kahn

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Network Connections and Ports

• IP Address a unique identifier assigned to each
  computer on the Internet
• Port number a unique identifier that indicates
  each service running on a computer
• Most services have standard port numbers
• Some standard ports numbers are
• ftp (20,21), telnet (23), smtp(35), http (80)

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Web Servers

• Service client requests for documents
• The European Lab for Particle Physics (CERN)
  produced one of the first Web servers
• The World Wide Web Consortium (W3C) took over the
  development of the CERN HTTPD (W3C HTTPD) but no
  longer supports it
• The W3C later supported a Java-based server
  called Jigsaw
• Both CERN HTTPD and Jigsaw are reference
  implementations (not meant for large-scale use
• The National Center for Supercomputing
  Applications (NCSA) also created an HTTP server
  which quickly became a popular between 1993 and
  1995 but development has since ceased
• The Apache server (based on the NCSA
  implementation) has become very popular

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Web Browsers

• Used as a client to retrieve and display
  information from Web servers
• Earlier Web browsers were text-based
• E.g. gopher, WAIS, telnet, FTP
• NCSA Mosaic was the first real HTML browser that
  was developed in 1993
• NCSA Mosaic became popular because it was
• Free like most current Web browsers
• Available for all major platforms (UNIX,
  Macintosh, and Windows)
• Easy to create content

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Electronic Publishing

• The two main types of files in Web publishing
  are
• ASCII Text Files uses numeric values (0 127)
  to represent letters, numbers, and other
  characters
• Each byte of the file represents a specific
  character
• Binary Files contain images, sounds, and
  compressed ASCII text

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Images

• There are several file formats available for
  storing graphics and images
• Web browsers support just a few image format
• The most common type of formats are
• GIF (Graphic Interchange Format) mostly used
  for graphics (like clip arts logos, icons, etc)
  requiring fewer colors because it displays only
  256 colors
• JPEG/JPG (Joint Photographic Expert Group)
  mostly used for photographs and images covering a
  wide spectrum of color because it uses the full
  16.7 million colors
• PNG (Portable Network Graphic) offer millions
  of colors, lossless compression, and other
  features that make them a good alternative to GIF
  images in most cases

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Audio

• Embedded in Web applications to play
  automatically, or used as links to be played when
  a user clicks on a link to the sound file
• Some of the common sound files include
• WAV originated on Windows-based machines
• AIFF originated on Macintosh
• AU originated on Sun Microsystems workstations
  (UNIX)

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Multipurpose Internet Mail Extensions (MIME) Types

• Set of rules that allow multimedia documents to
  be exchanged among many different computer
  systems
• Originally designed for sending email attachments
  and incorporated into http
• Both the Web server and the Web browser must have
  a MIME types database
• Mime type database used to identify the file
  extension and determine how the file must be sent

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The Hypertext Transfer Protocol (HTTP)

• Provides a standard way of communication and
  information exchange between the Web browsers and
  Web servers
• Expects the client to initiate a request and the
  server to respond
• Each request and response has three parts
• Request/Response line contains a request
  method, the document location, and the protocol
  version
• Header section series of lines with http
  headers that are used to pass other information
  about the request and the client to the server
• Entity body contains other data to be passed to
  the server

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HTTP Request/Response Example

• Client Request
• GET /hello.html HTTP/1.1
• Host test.com
• User-Agent Morzilla/4.5

• Server Response
• HTTP/1.1 200 OK
• Date Wed, 06 Sept 2006 135605 GMT
• Last-Modified Wed, 06 Sept 2006 135023 GMT
• Server Apache/2.0.52
• Connection close
• Content-Length 25
• Content-Type text/html
• lthtmlgt
• Hello world!
• lt/htmlgt

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HTTP Request/Response Example

• Client Request
• The hello.html file was requested from the server
  test.com using HTTP version 1.1
• the Web browser used was Mozilla version 4.5

• Server Response
• The request was successful (status code 200) and
  returned 25 bytes of text
• The server indicates the date and time it thinks
  the resource was requested and last modified and
  the software it is running (Apache version 2.0.52)

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Request Methods

• The HTTP command of the Web browsers request
  line
• The Web server uses the request method to
  determine what to do with the browsers request
• There are several methods defined by the HTTP 1.1
  standard but only a few are widely supported by
  the HTTP servers
• The most widely used methods are
• GET for retrieving information from a Web
  server
• Can also be used to pass information to the Web
  server as part of a URL
• HEAD for verifying the existence of information
  on a Web server
• POST for sending (updating) information to a
  Web server
• Other methods include
• PUT for creating and publishing documents on a
  server
• DELETE for deleting information from a Web
  server

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Server Responses

• Process client requests
• If the resource requested is cannot be located or
  there is a problem with the request, the server
  generates an error
• The response (status) line of the server response
  has three parts
• Protocol version should always be HTTP with
• Status code a three-digit indicator result code
  defined by the HTTP specification
• The first digit represents the category of the
  response
• There are currently five categories
• Informational the request was received and is
  being processed
• Success the client request was successful
• Client error the clients request was
  incomplete or incorrect
• Server error the request was not fulfilled due
  to a server problem
• Description phrase

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Common Response Codes

• Informational 1xx
• 100 Continue the initial part of the request
  has been received and the client should continue
• 200 OK the clients request was successful and
  the servers response contains the resource
  requested
• 204 No Content The request was successful but
  the response is empty
• Redirection 3xx
• 301 Moved Permanently the URL requested is no
  longer valid
• 302 Found (Moved Temporarily) the URL requested
  currently resides in a different location
• 304 Not Modified the client performed a
  conditional GET and the document has not been
  modified

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Common Response Codes

• Client Error 4xx
• 400 Bad Request the server could not understand
  the request
• 403 Forbidden the client requested data that it
  did not have permission to access
• 404 Not Found the resource requested was not
  found on the server
• Server Error 5xx
• 500 Internal Server Error something unexpected
  happened on the server side

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HTTP Headers

• Used to transfer Information between the client
  and server
• Has a name and value associated with it
• There is one header per line and each line
  contains
• the header name
• a colon
• a space
• the value of the header name

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Common Request Headers

• Accept specifies the clients preferred media
  type
• Cookie contains cookie information (name/value
  pair) for the requested URL
• If-Modified-Since used for a conditional GET
  request (when a document has been modified)
• Referrer allows the client to specify the URL
  of the page from which the currently requested
  URL was obtained
• User-Agent identifies the Web browser software

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Common Response Headers

• Server identifies the server software
• Set-Cookie Allows the server to set a cookie on
  the client browser (when permitted) for the given
  URL or domain

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Common Entity Headers

• Content-Length specifies the size (in bytes) of
  the data transferred in the entity body
• Content Type specifies the MIME Type of the
  data returned in the entity body
• Expires specifies the date/time after which the
  response is considered outdated
• Last-Modified specifies the date/time the
  document was last modified

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Other Web-Related Servers

• Some of the most common servers that run along
  HTTP servers are
• Proxy servers
• Streaming Audio/Video
• File Transfer Protocol (FTP)
• Databases
• Secure Socket Layer (SSL)

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Proxy Servers

• An intermediary server between a client and
  destination server
• Allows extra processing of returned data
• Have three main uses
• Security act as a firewall by rejecting other
  protocols
• Content filtering restricts access to content
  based on file type
• Caching stores frequently accessed pages

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Streaming Audio and Video

• Allows a media player (or plug-in) to start
  playing multimedia content while data is still
  being received
• Can broadcast live audio/video feeds or serve
  prerecorded clips
• Web browsers employ a plug-in to view any type of
  streaming content
• Unlike HTTP, many streaming media formats may use
  User Data Protocol (UDP) instead of TCP/IP as a
  network protocol
• UDP is good for transmitting very small pieces of
  data quickly
• UDP will not retransmit data if there is an error
• UDP is faster than TCP/IP although TCP/IP is more
  reliable

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File Transfer Protocol (FTP)

• Used to transfer files between computers on a
  network
• Relies on client and server software, unlike HTTP
• FTP daemon (FTPD) runs on the server to allow
  clients to connect
• It provides a means of authentication for only
  authorized users to transfer files
• UNIX servers generally install an FTPD by default

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Databases

• A large corporate database should typically be
  installed on its own dedicated server and not on
  a Web server machine
• Many database packages now come with tools to
  make Web-database authoring much easier
• Products like Oracle have HTML generation tools,
  integrated with Java, and may even provide a Web
  server built into the database software

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Secure Socket Layer (SSL)

• A protocol that allows secure, encrypted
  communication over TCP/IP
• Often used with HTTP to allow information
  exchange between a Web browser and a Web server
• Most commercial Web server software includes an
  SSL server that can run alongside the HTTP daemon
  (HTTPD)
• An Internet socket is defined as a combination of
  an IP address, a port, and a protocol

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Server-Side Programming

• Server-side programs provide dynamic content and
  allows interaction with users using
• Dynamic documents
• Common Gateway Interface (CGI) and forms
• Server-Side Includes (SSI)
• Active Server Pages (ASP)
• Servlets and Java Server Pages (JSP)
• PHP Hypertext Preprocessor (PHP)