protocols September 15,

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1 Contents SCI Protocols, WWW Internet applications WWW, document technology Lennart Herlaar Original slides by Piet van Oostrum September 15, 2003 SCI X SCI X Internet applications How to make an Internet Application? Invent your own protocol Write your own server Write client programs for as many platforms as you can Try to get client and server software at the right places Examples: FTP, chat, MUD (multi-user dungeons) Simpler Solution Use the existing WWW infrastructure: Use HTTP protocol GUI (browser) as client is already present If necessary add extra software to browsers Expand the servers with specialized modules if necessary (Much) more on this later... The big problem is to get the proper software on the proper places. People have to download your client software and install it on their local (home) computers) Your server software must be installed e.g. at Internet Service Provider computers SCI X SCI X Internet communication TCP/UDP Internet connections are usually made with TCP (Transport Control Protocol) UDP (User Datagram Protocol) Each end of a connection has an Internet address (IP address) and a port number IP address is a 32-bit number (in the future 128-bit) IP address is the address of the computer (better: a network interface in the computer) Port numbers are used for a specific service or process in the computer Some port numbers are fixed (25=SMTP, 80=HTTP, 119=NNTP) TCP is connection oriented (like a telephone call) Set up a connection Send and receive messages Close the connection Messages are received in the same order as they are sent UDP is connectionless (like a letter or telegram) Every message is sent independently You are never sure that a message will arrive Messages may arrive in a different order than they are sent SCI X SCI X protocols September 15,

2 Sockets 1 Socket = mechanism that the Operating System offers to programs to make Internet connections A process that wants to accept connections from other processes listens to a specific port These are often servers E.g. an HTTP server listens on port 80 A process that wants to create a connection to another process creates a socket and connects the socket to the desired (IPaddress, port) combination This is often a client (e.g. Web browser) Sockets 2 The server notices an incoming connection and accepts it (or could choose not to accept it) After accepting there is also a socket in the server This is a new socket for each accepted connection The socket has a new port number The client side also gets a port number from the operating system Most of the time this is not interesting The client and server can communicate over the socket in both directions SCI X SCI X Connection Server waits for connection Server has accepted the connection Client-server structure With a client-server configuration the communication often has the following structure Server listens on a specific port Client opens a connection to the server (that port) Client sends a request Server sends a response This will continue a number of times SCI X SCI X Example: Netscape / Internet Explorer Example: SMTP O.S. netscape netwerk softw GET ~piet/index.html inhoud van index.html Internet O.S. netwerk softw netwerk http server 220 mail.cs.uu.nl ESMTP Postfix HELO nono.cs.uu.nl 250-mail.cs.uu.nl MAIL FROM: <lennart@cs.uu.nl> 250 Ok RCPT TO: <lherlaar@inter.nl.net> 250 Ok DATA 354 End data with <CR><LF>.<CR><LF> This is a test.. Lines starting with. escaped Hello. 250 Ok: queued as A B QUIT 221 Bye SCI X SCI X protocols September 15,

3 Internet protocol conventions Many (but not all) Internet application protocols use the following conventions Request and reply are in ASCII as much as possible Lines are separated by CR,LF (ASCII 10,13, hex 0A, 0D) Reply is often a code (number) with some explaining text The number is meant to be used by programs, the text by humans A program needs only the number to take the correct action Reply codes 1xx = informative 2xx = operation succeeded 3xx = operation needs continuation 4xx = error 5xx = error Error codes SMTP HTTP 4xx = temporary error, may go away 5xx = permanent error, do not repeat exactly the same request 4xx = client has made an error (404 not found) 5xx = server has made an unexpected error, e.g. server is too busy SCI X SCI X Peer-to-peer communication Peer-to-peer = between equal partners Client-Client: ex: Kazaa Every client functions also as a server for his peers Server-server: ex: FTP An FTP client can give a command to transfer a file from one FTP server to another FTP server The one FTP server will then do a request to the other server N.B. Sometimes a server is a client for another server: a HTTP server can be a client for a database server Sockets in Java Java has a number of classes for sockets (package java.net) Socket for normal (TCP) sockets DatagramSocket for datagrams (UDP) ServerSocket for the server-side (listening) SCI X SCI X Sockets in Java Client side import java.net.*; mysocket = new Socket(" 80); hostname, portnr "localhost" for the local machine or an InetAddress object A socket has an InputStream and an OutputStream. getinputstream() gives the InputStream getoutputstream() gives the OutputStream InputStream can be used to read bytes OutputStream can be used to write bytes Different streams Reading and writing bytes is often not the best or easiest way to communicate We want to communicate characters, not bytes In some cases you must use it, e.g. when you have to read/write a GIF file In Java we can used more advanced Streams to use higher level communication DataInputStream, DataOutputStream if you want to read/write binary data (ints, doubles, etc.) ObjectInputStream, ObjectOutputStream if you want to communicate with serialized objects. This is only useful if the other side is also written in Java BufferedReader, PrintWriter if you want to communicate with text (like most Internet protocols) SCI X SCI X protocols September 15,

4 Object Streams Output OutputStream ostream = mysocket.getoutputstream(); ObjectOutputStream os = new ObjectOutputStream(ostream); os.writeint(12345); os.writeobject("today"); os.writeobject(new Date()); os.flush(); ostream.close(); You must flush when the other side must do something, otherwise the data may be buffered in the sender Input Object Streams InputStream istream = mysocket.getinputstream(); ObjectInputStream os = new ObjectInputStream(istream); int i = os.readint(); String today = (String)os.readObject(); Date date = (Date)os.readObject(); istream.close(); Of course you can also use objects from your own classes SCI X SCI X Text If we want to communicate with text then PrintWriter and BufferedReader should be used PrintWriter out = new PrintWriter(mySocket.getOutputStream(), true); out.println("get /directory/file.html"); BufferedReader in = new BufferedReader(new InputStreamReader( mysocket.getinputstream())); instr = in.readline(); The second parameter of PrintWriter tells to flush when println is used Server side ServerSocket serversocket; serversocket = new ServerSocket(portnr); catch (IOException e) {... Socket clientsocket = null; clientsocket = serversocket.accept(); catch (IOException e) {... // Processing code here... clientsocket.close(); SCI X SCI X Multi-client server After the accept we usually want to Communicate with the client Wait for new requests If the communication with the client is simple we can just have a loop with accept and process Otherwise we must start a new Thread for the communication with the client Example 1 Socket clientsocket = null; while (notfinished) { clientsocket = serversocket.accept(); catch (IOException e) {... new ProcessingThread(clientSocket).start(); serversocket.close(); The processing Threads will run parallel with the accept loop and with each other SCI X SCI X protocols September 15,

5 Example 2 public class ProcessingThread extends Thread { private Socket socket = null; public ProcessingThread(Socket socket) { this.socket = socket; public void run() { PrintWriter out = new PrintWriter(socket.getOutputStream(), true); BufferedReader in = new BufferedReader( new InputStreamReader( socket.getinputstream()));... // Do the work... out.close(); in.close(); socket.close(); catch (IOException e) {... SCI X A simple SMTP client Here is a Java program that sends an message Suppose we have a user interface with fields for Sender (From:) Adressee(s) (To:/Cc:/Bcc:) Subject The message Communicate through a socket with the SMTP server Put the headers and the message body together Use the commands: HELO, MAIL FROM:, RCPT TO:, DATA, QUIT Check return codes (only first digit) On error: display the message and stop communicating The name of the client computer can be found with sock.getlocaladdress().gethostname() SCI X Importants points of the protocol Check the greeting of the server After each command sent to the server check the response After the message (. ) check the response After an error (code 4 or 5 ) stop the protocol and display the message In the message add headers for From, To, Cc, Subject like Subject: Hello from Internet Programming The headers must be followed by an empty line In the message text any line that starts with. must get an extra. at the beginning How to do checking Use of if statements for checking read greeting if (code OK) send HELO if (code OK) send MAIL FROM: if (code OK)... etc... SCI X SCI X Or with return Use of return statements for checking With exceptions 1 Use of exceptions for checking void sendmessage(...) read greeting if (not code OK) return send HELO if (not code OK) return send MAIL FROM: if (not code OK) return... etc... read and check greeting send HELO read and check response send MAIL FROM: read and check response... etc... catch (ProtocolException...) {... SCI X SCI X protocols September 15,

6 With exceptions 2 In method read and check response if (not code OK) throw new ProtocolException(...) You have to define your ProtocolException class ProtocolException extends Exception {... constructor... Java code: exception public class SMTPProtocolException extends Exception { public SMTPProtocolException(String mm) { super(mm); public void checkerror(string reply) throws SMTPProtocolException { char cc=reply.charat(0); if ((cc== 4 ) (cc== 5 )) throw new SMTPProtocolException("ERROR: "+reply); SCI X SCI X Java code: methods BufferedReader in; Printwriter out; public void checkresponse() throws IOException, SMTPProtocolException { String reply; reply = in.readline(); checkerror(reply); public void sendcheck(string mesg) throws IOException, SMTPProtocolException { String reply; out.println(mesg; reply = in.readline(); checkerror(reply); Java code: methods public void send(string mesg) throws IOException { out.println(mesg; public void connect(string server) { mysocket= new Socket(server,25); in = new BufferedReader(new InputStreamReader( mysocket.getinputstream())); checkresponse(); out= new PrintWriter(mySocket.getOutputStream(), true); catch (IOException e) { System.out.println("Error while creating socket"); SCI X SCI X Java code: sending String reply; connect(); String local = mysocket.getlocaladdress().gethostname(); sendcheck("helo "+local); sendcheck("mail FROM: "+sender); sendcheck("rcpt TO "+address); if (! CC.equals("")) sendcheck("rcpt TO "+CC)... etc... catch (IOException e) { System.out.println("Error while sending HELO"); Message void sendmessage(string sender, String address, String CC, String subject, String message) throws... { sendcheck("data"); send("from: "+sender); send("to: "+address); if (! CC.equals("")) send("to: "+CC); send("subject: "+subject); send(""); sendbody(message); sendcheck("."); SCI X SCI X protocols September 15,

7 Message body void sendbody(string msg) throws...{ StringTokenizer st = new StringTokenizer(msg, "\n"); while (st.hasmoretokens()) { String line = st.nexttoken(); if (line.charat(0) ==. ) line = "."+line; send(line); The WWW WWW = World Wide Web = A collection of hypertext documents hypertext = text augmented with links to other documents WWW is more than text pictures movies sound/music database retrieval SCI X SCI X The paper myth 1 Information is important (knowledge is power!) There is an information overload We drown in paper Many reports are only browsed, not properly read Decisions are based on incomplete information Coopers and Lybrand (1987): 95% of information is on paper The paper myth 2 Conclusion: the paper document is to blame Consequence: myriads of techniques, software, hardware Mainly focussed on imaging Current situation 90% of information is on paper Still information overload Paperless Office? SCI X SCI X Paper as common medium Research on paper use In USA 1 billion (1000 million) pages per day 70 80% generated by computers 60% used for data-entry The underlying problem Current information systems do not share the richness of document-based information Paper is still the most portable transport medium for information Exercise Trace the paperwork that is involved in a consumer sale Which paper work can be avoided by using electronic communication? (EDI) How much work can be avoided? What is necessary to make this possible? SCI X SCI X protocols September 15,