1. 2440: 141 Web Site Administration Web Application Resources Instructor: Enoch E. Damson

2. 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 Web Application Resources2

3. 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 Web Application Resources3

4. 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 (25), http (80) Web Application Resources4

5. 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) The National Center for Supercomputing Applications (NCSA) also created an HTTP server which quickly became popular between 1993 and 1995 The Apache server (based on the NCSA implementation) has become very popular Web Application Resources5

6. 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 There are several browsers on the market now Web Application Resources6

7. 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, compressed ASCII text, etc Web Application Resources7

8. 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 Web Application Resources8

9. 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) Web Application Resources9

10. 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 Web Application Resources10

11. 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 Web Application Resources11

12. 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 13:56:05 GMT Last-Modified: Wed, 06 Sept 2006 13:50:23 GMT Server: Apache/2.0.52 Connection: close Content-Length: 25 Content-Type: text/html Hello world! Web Application Resources12

13. 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) Web Application Resources13

14. HTTP Request Methods The HTTP command of the Web browser’s request line The Web server uses the request method to determine what to do with the browser’s 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 Web Application Resources14

15. HTTP Request Methods… Other http methods include: PUT DELETE TRACE TRACK OPTIONS CONNECT Web Application Resources15

16. HTTP Server Responses Process client requests The server generates an error if the resource requested cannot be located The response line of the server response has 3 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 client’s request was incomplete or incorrect Server error – the request was not fulfilled due to a server problem Description phrase Web Application Resources16

17. Common Response Codes Informational 1xx 100 Continue – the initial part of the request has been received and the client should continue 200 OK – the client’s request was successful and the server’s 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 Web Application Resources17

18. 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 Web Application Resources18

19. 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 Web Application Resources19

20. Common Request Headers Accept – specifies the client’s 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 Web Application Resources20

21. 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 Web Application Resources21

22. 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 Web Application Resources22

23. 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) Web Application Resources23

24. 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 Web Application Resources24

25. 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 Web Application Resources25

26. 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 Web Application Resources26

27. Server-Side Programming Server-side programs provide dynamic content and allows interaction with users using: Perl Active Server Pages (ASP) ASP.NET Java Server Pages (JSP) and Java Servlets PHP: Hypertext Preprocessor (PHP) Coldfusion Web Application Resources27

28. Databases Database – a collection of data and metadata about related entities A large corporate database should typically be installed on its own dedicated server and not on a Web server machine Metadata – data about other data Database management system (DBMS) – software that is used to manipulate a database Structured Query Language (SQL) – language built in relational DBMSs to create and manipulate databases E.g. of relational DBMSs include Oracle, MySQL, Microsoft SQL Server, Microsoft Access etc Web Application Resources28

29. 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 Web Application Resources29