1. When will an IP process drop a datagram? 2. When will an IP process fragment a datagram? 3. When will a TCP process drop a segment?

Transcription

1 Questions 1. When will an IP process drop a datagram? 2. When will an IP process fragment a datagram? 3. When will a TCP process drop a segment? 4. When will a TCP process resend a segment? CP476 Internet Computing Lecture 1 Introduction 1

2 Lecture 4. WWW and HTTP World Wide Web (WWW) is an application of the Internet computing It is used to navigate a collection of documents of information, distributed over the internet, and connected by hyperlinks It uses browser to navigate and download HTML documents stored on web servers It grows to involve many existing applications, such as and file transfer HyperText Transfer Protocol (HTTP) is the core protocol of the WWW used by brower (client) and web server which was defined ed in HTTP1.0 (RFC:1945) and currently use HTTP1.1 RFC:2616 2

3 Tim Berners-Lee at CERN invented World Wide Web application. He created HTTP, HTML, the first browser, and the first web server. He put all these as an open source for free usage. Watch talk of TBL at TED 3

4 HTTP is based on TCP/IP HTTP is implemented by two programs Client program, Web browser, executing on a client machine Sever program, Web server, running on a server machine They talk each other by exchanging HTTP massages HTML (Hypertext Markup Language) is a coding language create hypertext. A Web document (or Web page) is an object HTML file, a JPEG image, GIF image, Java applet,.. A Web page is stored in a Web server machine, and has an location, and is accessible by URL 4

5 Uniform Resource Locator (URL) is the global address of documents and other resources on the World Wide Web Example: URL contains three parts: The first part indicates what protocol to use. The second part specifies the IP address or the domain name of the server host. Third part is the path of the object in the server directory. 5

6 HTTP functions like the FTP and SMTP It does file transfer from server to client. But it uses only one connection, i.e., it uses the services of TCP on port 80. Like SMTP, a client sends a request which looks like a mail. The server responds like send a mail to the client The server is stateless, i.e., it does not keep records of previous requests. HTTP1.0 is nonpersistent, i.e., the connection is closed after a request is done HTTP/1.1 is persistent, i.e., the connection will keep alive for a short period time, another request coming within the time can use the same connection. 6

7 Connection establishement PC running a browser Server running a Server program Initiate TCP connection Request and response 7

8 Request line A request massage Request line request type + URL+HTTP version General header Request header Entity header Blank line Body Request types HTTP/1.0 : GET, POST, HEAD HTTP/1.1: GET, POST, HEAD, PUT, DELETE URL scheme://host:port/path scheme : http, ftp, telnet, file, mailto, news. 8

9 General header cache control, connection, date, MIME-version, upgrade Request header: accept, accept-charset, acceptencoding, accept-language, authorization, from, host, referrer, user-agent Entity header: allow, content-encoding, content- language, content-range, content-type, type expires, last-modified, location 9

10 Example of HTTP request GET /somedir/page.html HTTP/1.1 Host: User-agent: Mozilla/4.0 Connection: close Accept-language:fr 10

11 Response massage Status line General header Response header Entity header Blank line Body 11

12 Status line: HTTP version+status code+status phase 3 digit status code 100 = continues, 101=switching, 200=OK, 201=created, 202=accepted, 204=no content 301=multiple choices, 302=moved permanently 304=moved temporarily, 400=bad request, 401=unauthorized, 403=forbidden, 404=not found 405=method not allowed, 406=not acceptable 500=internal service error, 501=not implemented 503=service unavailable General header Response header: accept-range, age, public, retry-after, server 12

13 Example HTTP/ OK Date: Mon, 27 Jun :22:47 GMT Server: Apache/ (Unix) (Red-Hat/Linux) Last-modified: Wed, 26 Jun :12:29 GMT Etag: "841fb-4b-3d1a0179" Accept-ranges: bytes Content-length: 75 Connection: close Content-type: text/html data data data data data... 13

14 Proxy server A proxy server is one that receives requests intended for another server and that acts on the behalf of the client (as the client proxy) to obtain the requested service. HTTP client sends a request to the proxy server. The proxy server sends the response back Proxy server can keeps copies of responses to recent requests for future requests from the other clients. A proxy server is often used when the client and the server are incompatible for direct connection. It may also be used for screening purposes to enable the administrator to control access to undesirable sites 14

15 Cookies Stateless in server site. Cookies are used on client side to store access information to a page Three components 1. cookie header line in the HTTP response message 2. cookie header line in HTTP request message 3. cookie file kept on user s host and managed by user s s browser 15

16 Web Browsers A client software program that is used to connect web server to get web document on the internet Well-known browser TBL s text browser, Mosaic, Netscape Microsoft Internet Explorer Mozilla, Firefox Apple Safari Opera Google Chrome 16

17 How a browser work? 1. Given a URL, a browser intimates a request to a DNS server to resolve the IP address of the host specified in URL. 2. Combining the IP address and the document path of the URL, the browser initiates a HTTP request for the web document to the server, and then wait for the server responses to the request 3. Further process the received web document. If it is an HTML web document, it parse it and then render the web page in the content display window If it is other types of document, it call related local program process it. A browser provides a graphic interface for a rich. repository of rich text, hypertext, images, audio, video, and other document types 17

18 Basic components of a browser 1. GUI 2. Domain name resolution module 3. Requesting module 4. Response processing module 5. A Web document parser module 6. A document display (rendering) engine. 18

19 Case study: Mosaic Mosaic is the first graphical browser for the WWW that supports hypermedia, developed by NCSA which quickly gained popular and became the industry standard. HTTP is the primary protocol used by Mosaic to distribute documents from the HTTPD information management services to Mosaic clients. Check the source code of Mosaic for detail 19

20 Case study: Chrome Chrome is the most recent browser developed by Google, The most popular browser, about 20%. Very active development, Chromium project Open source, seem more Use many modules such as Webkit layout engine Support HTML5 20

21 Web Servers A computer that provides World Wide Web services on the Internet. It includes hardware, operating system, Web server software, TCP/IP software, and Web pages and many programs. Web server software is an application program that t serves Web pages to Web browsers using the HTTP protocol. Provide responses to browser s requests for either existing documents or dynamically built documents Examples Apache, 59% as of 2010 Internet Information Server (IIS) by Microsoft Tomcat, based on Java technology 21

22 How a Web server work? The primary task of a Web server is to monitor a communication port on its host machine, accept HTTP commands through that port, and perform the operations specified by the commands. Example of HTTP commands: GET, PUT, POST, HEAD, and DELET. All commands include a URL When a Web server starts, it tell its OS it is ready to accept communications through a specific port, usually 80 When the URL is received, it is translated into either a filename (in which case the file is returned to the requesting client), or a program name (which case the program is run and its output is sent to requesting client) 22

23 Contemporary web servers provide many services Virtual hosts - multiple sites on the same system Proxy servers - to serve documents from the document roots of other sites Besides HTTP, support for FTP, News, Support for database access, etc 23

24 Case study: Apache Apache began with the NCSA server httpd working together with Mosaic. Apache is now an open source Web server originally formed by taking all the "patches" (fixes) to the NCSA Web server and making a new server out of it, as a project organized by Apache Software Foundation Apache has a long list of services beyond the basic process of serving documents to clients. Services can be configured Three configuration files: httpd.conf, srm.conf, and access.concon More than 150 different directives can be specified When Apache starts execution, it reads the three configuration files and sets its parameters accordingly 24

25 The file structure of Apache Apache has two separate directories: document root, server root The document root is the root directory of all servable documents Example: An Apache server runs on hopper Server Name: Domain name: IP address: Document root: Server root: The server can have virtual document trees. Sometimes a different disk, possibly on a different machine, is used after the original disk is filled 25

26 The server root is the root directory for all of the code that implements the server The server root usually has four files: One is the code for the server itself. Three others are subdirectories conf - for configuration information logs - to store what has happened cgi-bin - for executable scripts The configuration file is named httpd.conf The directives in the configuration file control the operation of the server Configuration file format: Non-blank lines that do not begin with # must begin with a directive name, which may take parameters, separated by white space When Apache begins, it reads the configuration files and sets its parameters according to what it reads 26

27 Alias - to specify a virtual document tree ScriptAlias - creates a virtual directory for cgi files ScriptAlias /cgi-bin/ /usr/local/etc/httpd/cgi-bin/ Redirect DirectoryIndex index.html contents.html index.cgi UserDir public_htm (UserDir disabled) Log file 27