Basic ProtocolsSocketsProtocolsTCP/IPHostnames and DNS serversDHCPURLsShowURL.javaAbout the ShowURL.java appletRunning the appletApplet resultsTryURL.java, ITryURL.java, IIThe EndJan 14, 2019Basic Protocols2SocketsSockets, or ports, are a very low level software construct that allows computers to talk to one anotherWhen you send information from one computer to another, you send it to a port on the receiving computerIf the computer is “listening” on that port, it receives the informationIn order for the computer to “make sense” of the information, it must know what protocol is being usedCommon port numbers are 80 (for web pages), 23 (for telnet) and 25 and 110 (for mail)Port numbers above 1024 are available for other kinds of communication between our programs3ProtocolsIn order for computers to communicate with one another, they must agree on a set of rules for who says what, when they say it, and what format they say it inThis set of rules is a protocolDifferent programs can use different protocolsProtocols may be in ASCII (characters) or in binarySome common protocols are HTTP (for web pages), FTP (for file transfer), and SMTP (Simple Mail Transfer Protocol)4TCP/IPThe Internet (and most other computer networks) are connected through TCP/IP networksTCP/IP is actually a combination of two protocols:IP, Internet Protocol, is used to move packets (chunks) of data from one place to anotherPlaces are specified by IP addresses: four single-byte (0..255) numbers separated by periodsExample: 192.168.1.1TCP, Transmission Control Protocol, ensures that all necessary packets are present, and puts them together in the correct orderTCP/IP forms a “wrapper” around data of any kindThe data uses its own protocol, for example, FTP5Hostnames and DNS serversThe “real” name of a computer on the internet is its four-byte IP addressPeople, however, don’t like to remember numbers, so we use hostnames insteadFor example, the hostname www.cis.upenn.edu is 158.130.12.9A DNS (Domain Name Server) is a computer that translates hostnames into IP addressesThink of it as like a phone book--names to useful numbersOf course, you have to know the IP address of the DNS in order to use it!You usually get two DNS numbers from your Internet Service Provider (ISP)6DHCPIf you have a web site, it must be hosted on a computer that is “permanently” on the WebThis computer must have a permanent IP addressThere aren’t enough IP addresses for the number of computers there are these daysIf you have no permanent web site, you can be given a temporary (dynamically allocated) IP address each time you connect to the WebSimilarly, if you have a home or office network, only one computer needs a permanent IP addressThe rest of the computers can be assigned internal, permanent IP addresses (not known to the rest of the world)They can also be assigned internal IP addresses dynamicallyDHCP (Dynamic Host Configuration Protocol) is a way of assigning temporary IP addresses as needed7URLsA URL, Uniform Resource Locater, defines a location on the WebA URL has up to five parts: http://www.xyz.com:80/ad/index.html#specialsProtocol -- http is used for Web pagesHostnamePort -- 80 is default for http requestsPath to a given pageAnchor -- a location within the page8ShowURL.javaimport java.net.*; // Gittleman, Example 2.2, pp. 67-68import java.applet.Applet;public class ShowURL extends Applet { public void init() { try { URL url = new URL(getParameter("url")); getAppletContext().showDocument(url); } catch(MalformedURLException e) { e.printStackTrace(); } }}9About the ShowURL.java appletimport java.net.*;This is the package that defines sockets, URLs, etc.URL url = new URL(getParameter("url"));Constructs a URL object from a text stringgetAppletContext()An AppletContext describes the document containing this applet and the other applets in the same documentshowDocument(url)Replaces the Web page currently being viewed with the given URLcatch(MalformedURLException e)This exception is thrown if the given String cannot be parsed by newURL(S tring)10Running the applet12311Applet resultsIf the applet is run using appletviewer, you get an applet, but it’s blankUnless, that is, the page you go to has a applet on itIf the applet is run using a browser,First, a web page appears, with a gray rectangle for the applet (which is just starting up)Then the initial web page is replaced by the web page specified by the URLYou are now in your regular browser, just as if you had typed the URL into itThis applet does not work for me in Firefox!Security issue?12TryURL.java, IShowURL.java is an applet; it runs in a browser, gets a web page, and displays it in that browserTryURL.java is an application; it runs standalone, gets a web page, and displays the HTML that it gotBasic structure:import java.net.*;import java.io.*;public class TryURL { public static void main(String[] args) { try { ...important code goes here.... } catch (Exception e) { ... } }}13TryURL.java, IIBufferedReader input;String line;URL url = new URL( "http://www.cis.upenn.edu/~matuszek/cit597-2004");input = new BufferedReader( new InputStreamReader(url.openStream()));line = input.readLine();while (line != null) { System.out.println(line); line = input.readLine();}input.close();14The
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