computer network introduction. psc notes . Assisant professor in cse.
- 1. COMPUTER NETWORKS Class 27 Mrs.Parvathy Gopakumar
- 3. Architecture of W W W ● The WWW today is a distributed client-server service, in which a client using a browser can access a service using a server. However, the service provided is distributed over many locations called sites. ● Each site holds one or more documents, referred to as Web pages. Each Web page, however, can contain some links to other Web pages in the same or other sites. Each Web page is a file with a name and address. ● A Web page can be simple or composite. ○ A simple Web page has no link to other Web pages ○ A composite Web page has one or more links to other Web pages.
- 4. Simple webpage Documents of the webpage are in the single file .It can be retrieved using one single request/ response transaction
- 5. Composite webpage The main document and the contents are stored in two separate files in the same site (file A and file B); the referenced text file is stored in another site (file C). Since we are dealing with three different files, we need three transactions if we want to see the whole document.
- 6. Hypertext and Hypermedia ● Hypertext are documents that refer to other documents. another document, a defined as document. ● Hypermedia In a hypertext part of text can be a link to is a term applied to document that contains links to other textual document or documents containing graphics, video, or audio.
- 7. WEB CLIENT VS WEB SERVER
- 8. WEB CLIENT (Browser) ● A browsers interpret and display a Web document, and all of the ventors use nearly the same architecture. ● Each browser usually consists of three parts: ○ A controller ○ Client protocol ○ Interpreters
- 9. The controller receives input from the keyboard or the mouse and uses the client programs to access the document. After the document has been accessed, the controller uses one of the interpreters to display the document on the screen. The client protocol can be FTP, or TELNET, or HTTP .The interpreter can be HTML, Java, or JavaScript,depending on the type of document.Some commercial browsers include Internet Explorer, Netscape Navigator, and Firefox
- 10. WebServer ● The Web page is stored at the server. ● Each time a client request arrives, the corresponding document is sent to the client. To improve efficiency, servers normally store requested files in a cache in memory; it is faster to access than disk. ● A server can also become more efficient through multithreading or multiprocessing. In this case, a server can answer more than one request at a time. ● Some popular Web servers include Apache and Microsoft Internet Information Server.
- 11. Uniform Resource Locator (URL) ● A client that wants to access a Web page needs the file name and the address. ● The uniform resource locator (URL) is a standard locator for specifying any kind of information on the Internet. The URL defines four things: protocol, host computer, port, and path
- 12. ● The documents in the WWW can be grouped into three broad categories: ○ Static ○ Dynamic ○ Active ● The category is based on the time the contents of the document are determined. WEB DOCUMENTS
- 13. ● Static documents are fixed-content documents that are created and stored in a server.The client can get a copy of the document only. ● Static documents are prepared using one of the several languages: Hypertext Markup Language (HTML), Extensible Markup Language (XML), Extensible Style Language (XSL), and Extended Hypertext Markup Language (XHTML) STATIC DOCUMENTS
- 14. ● A dynamic document is created by a Web server whenever a browser requests the document. ● When a request arrives, the Web server runs an application program or a script that creates the dynamic document. The server returns the output of the program or script as a response to the browser that requested the document. ● Because a fresh document is created for each request, the contents of a dynamic document may vary from one request to another. ● eg:retrieval of the time and date from a server. Time and date are kinds of information that are dynamic in that they change from moment to moment. DYNAMIC DOCUMENTS
- 15. ● The Common Gateway Interface (CGI) is a technology that creates and handles dynamic documents. CGI is a set of standards that defines how a dynamic document is written, how data are input to the program, and how the output result is used. ● CGI is not a new language; instead, it allows programmers to use any of several languages such as C, C++, Bourne Shell, Korn Shell, C Shell, Tcl, or Perl. The only thing that CGI defines is a set of rules and terms that the programmer must follow. ● If we use CGI, the program must create an entire document each time a request is made
- 16. Scripting Technologies for Dynamic Documents : ● The problem with CGI technology is the inefficiency that results if part of the dynamic document that is to be created is fixed and not changing from request to request. If we use CGI, the program must create an entire document each time a request is made. ● The solution is to create a file containing the fixed part of the document using HTML and embed a script, a source code, that can be run by the server to provide the varying data section ● Eg: Hypertext Preprocessor (PHP),Java Server Pages (JSP),Active Server Pages (ASP),ColdFusion
- 17. ● For many applications, we need a program or a script to be run at the client site. These are called active documents. ● For example, suppose we want to run a program that creates animated graphics on the screen or a program that interacts with the user. When a browser requests an active document, the server sends a copy of the document or a script. The document is then run at the client (browser) site. ● Eg:java applet ,java script Active Documents
- 18. HTTP -Hyper TextTransfer Protocol
- 19. ● The Hypertext Transfer Protocol (HTTP) is a Application layer protocol used mainly to access data on the World Wide Web. ● The data transferred between the client and the server look like SMTP messages.The format of the messages is controlled by MIME-like headers. ● Unlike SMTP, the HTTP messages are not destined to be read by humans; they are read and interpreted by the HTTP server and HTTP client (browser).
- 20. ● SMTP messages are stored and forwarded, but HTTP messages are delivered immediately. ● The commands from the client to the server are embedded in a request message.The contents of the requested file or other information are embedded in a response message. ● HTTP uses the services of TCP on well-known port 80.
- 21. HTTP Transaction:AlthoughHTTP uses the services of TCP, HTTP itself is a stateless protocol, which means that the server does not keep information about the client. The client initializes the transaction by sending a request. The server replies by sending a response
- 22. Request Message A request message consists of 1. Request line 2. Header 3. Sometimes a body.
- 23. 1. Request Line : ● The first line in a request message is called a request line. There are three fields in this line separated by some character delimiter . ● The fields are called ○ Methods ○ URL ○ Version. ● These three should be separated by a space character. At the end two characters, a carriage return followed by a line feed terminate the line.
- 24. a. HTTP METHODS : HTTP request methods specify the action to perform through the request. Put some content in the specified URL
- 25. b. The requested URL The URL typically functions as a name for the resource being requested, together with an optional query string containing parameters that the client is passing to that resource. c. The HTTP version being used The only HTTP versions in common use on the Internet are 1.0 and 1.1, and most browsers use version 1.1 by default.
- 26. 2. Header Line : ● After the request line, we can have zero or more request header lines. ● Each header line sends additional information from the client to the server. ● For example, the client can request that the document be sent in a special format.Using the request header, the client can send additional information to the server about the request as well as the client itself.
- 27. There can be either zero or more headers in the request :
- 28. 3.Body InRequest Message : The body can be present in a request message. Usually, it contains the comment to be sent.
- 29. Response Message A response message consists of 1. Status line 2. Header lines 3. Sometimes a body
- 30. 1.The Status line ● Status Line of every HTTP response consists of three items, separated by spaces: ○ The HTTP version being used. ○ A numeric status code indicating the result of the request. ■ 200 is the most common status code; it means that the request was successful and that the requested resource is being returned. ○ A textual “reason phrase” further describing the status of the response. This can have any value and is not used for any purpose by current browsers.
- 33. 2. Header Lines InResponse Message:After the status line, we can have zero or more response header lines. Each header line sends additional information from the server to the client
- 34. 3. Body ● The body contains the document to be sent from the server to the client. The body is present unless the response is an error message. ● Consists requested of the resource data by the client. eg: we requested the book’s data, and the response body consists of the different books present in the database along with their information.
- 37. Persistence HTTP, prior to version 1.1, specified a nonpersistent connection, while a persistent connection is the default in version 1.1. a. Nonpersistent Connection: In a nonpersistentconnection,one TCP connectionis made for each request/response. The following lists the steps in this strategy: 1. The client opens a TCP connection and sends a request. 2. The server sends the response and closes the connection. 3. The client reads the data until it encounters an end-of-file marker; it then closes the connection.
- 38. b. Persistent Connection ● HTTP version 1.1 specifies a persistent connection by default. In a persistent connection, the server leaves the connection open for more requests after sending a response. ● The server can close the connection at the request of a client or if a time-out has been reached.
- 39. Cookies: ● The World Wide Web was originally designed as a stateless entity. A client sends a request; a server responds. Their relationship is over. ● Cookies are text files with small pieces of data that are used to identify your computer as you use a computer network. ○ When a server receives a request from a client, it stores information about the client in a file or a string. ○ The server includes the cookie in the response that it sends to the client. ○ When the client receives the response, the browser stores the cookie in the cookie directory, which is sorted by the domain server name.
- 40. WebCaching: Proxy Server ● A proxy server is a computer that keeps copies of responses to recent requests. ● The HTTP client sends a request to the proxy server. The proxy server checks its cache. If the response is not stored in the cache, the proxy server sends the request to the corresponding server. ● Incoming responses are sent to the proxy server and stored for future requests from other clients.
- 41. Proxy Server Location The proxy servers are normally located at the client site. This means that we can have a hierarchy of proxy servers as shown below: 1. A client computer can also be used as a proxy server in a small capacity that stores responses to requests often invoked by the client. 2. In a company, a proxy server may be installed on the computer LAN to reduce the load going out of and coming into the LAN. 3. An ISP with many customers can install a proxy server to reduce the load going out of and coming into the ISP network.
- 42. HTTP SECURITY: ● The HTTP does not offer security.HTTP can be run over the Secure Socket Layer (SSL) is referred to as HTTPS. ● HTTPS provides confidentiality, client and server authentication, and data integrity ● SSL is the standard security technology for establishing an encrypted link between the two systems. These can be browser to server, server to server or client to server. Basically, SSL ensures that the data transfer between the two systems remains encrypted and private.