Abstract Stream Socket Service

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1 COM 362 Computer Networks I Lec 2: Applicatin Layer: TCP/UDP Socket Programming 1. Principles of Networks Applications 2. TCP Socket Programming 3. UDP Socket Programming Prof. Dr. Halûk Gümüşkaya Computing Engineering Department Sunday, February 24, Operations of a Socket Abstract Stream Socket Service A socket can perform 7 basic operations: Connect to a remote machine (i.e. prepare to send or receive data) Send data Receive data Close a connection Java s Socket Class (used by both clients and servers) Asymmetric set-up, circuit abstraction Server is passive, waits for connections Client initiates the connections Bi-directional, continuous byte stream TCP is free to break up and reorder the data however it likes as long as the user sees an ordered byte stream But often doesn t Bind to a port Listen for incoming data Accept connections from remote machines on the bound port. ServerSocket Class (needed only by servers) 3 Stream jargon: A stream is a sequence of characters that flow into or out of a process. An input stream is attached to some input source for the process, eg, keyboard or socket. An output stream is attached to an output source, eg, monitor or socket. 4

2 Java Socket Class (java.net package) The Socket class represents one end of a two-way connection between your Java program and another program on the network. It implements the client and data side of the two-way link. If you are writing server software, the ServerSocket class implements the server side of the two-way link. Socket Constructors Commonly used constructors: public Socket(String host, int port) throws UnknownHostException, IOException public Socket(InetAddress host, int port) throws IOException Example: Socket fatih = new Socket( fatih.edu.tr", 80); catch (UnknownHostException e) { 5 6 Stream Sockets in Java Useful methods Socket and ServerSocket, classes Both are TCP communication objects Abstract asymmetry of client/server communication Contain the stream objects once socket is connected InPutStream, OutPutStream classes get and receive bytes from a socket InetAddress class Object for containing an using IP addresses Methods for viewing and changing IP addresses and symbolic names inetaddress socket.getlocaladdress() get the machine s local address socket.setsotimeout(int milliseconds) block only for int milliseconds before returning socket.tostring get the IP address and port number in a string 7 8

3 Reading From a Socket To read from a Socket, after you create a socket to a given address, you open an input stream on the socket. The getinputstream() method returns an input stream that can read data from the socket into a program. You usually chain this InputStream to a filter stream or reader that offers more functionality DataInputStream or InputStreamReader, for example before reading input. It's also extremely helpful to buffer the input by chaining it to a BufferedInputStream or a BufferedReader for performance reasons. Writing to a Socket To write to a Socket, after you create a socket to a given address, you open an output stream on the socket. The getoutputstream( ) method returns a raw OutputStream for writing data from your application to the other end of the socket. You usually chain this stream to a more convenient class like DataOutputStream or OutputStreamWriter before using it. For performance reasons, it's a good idea to buffer it as well Server Side Socket: ServerSocket Java provides a ServerSocket class to allow programmers to write servers. Basically, a server socket's job is to sit by the phone and wait for incoming calls. More technically, a ServerSocket runs on the server and listens for incoming TCP connections. Each ServerSocket listens on a particular port on the server machine. When a client Socket on a remote host attempts to connect to that port, the server wakes up, negotiates the connection between the client and the server, and opens a regular Socket between the two hosts. Server sockets wait for connections while client sockets initiate connections. 11 Constructors There are 4 public ServerSocket constructors: public ServerSocket(int port) throws IOException public ServerSocket(int port, int backlog) throws IOException public ServerSocket(int port, int backlog, InetAddress bindaddress) throws IOException public ServerSocket() throws IOException These constructors let you specify the port, the length of the queue used to hold incoming connection requests, and the local network interface to bind to. They pretty much all do the same thing, though some use default values for the queue length and the address to bind to. 12

4 Life Cycle of a Server 1. A new ServerSocket is created on a particular port using a ServerSocket( ) constructor. 2. The ServerSocket listens for incoming connection attempts on that port using its accept( ) method. accept( ) blocks until a client attempts to make a connection, at which point accept( ) returns a Socket object connecting the client and the server. 3. Depending on the type of server, either the Socket's getinputstream( ) method, getoutputstream( ) method, or both are called to get input and output streams that communicate with the client. 4. The server and the client interact according to an agreed-upon protocol until it is time to close the connection. 5. The server, the client, or both close the connection. 6. The server returns to step 2 and waits for the next connection. Iterative Connection-Oriented TCP Server ServerSocket server; // bağlantı istekleri için Socket connection; // sunucu veri haberleşme kanalı için DataOutputStream output; // I/O stream ler DataInputStream input; // Adım 1: Bir ServerSocket i oluştur. server = new ServerSocket( 5000 ); while ( true ) { // Adım 2: Bir bağlantı için bekle. connection = server.accept(); // Adım 3: Giriş/çıkış stream leri hazırla. input = new DataInputStream(connection.getInputStream()); output = new DataOutputStream(connection.getOutputStream()); // Adım 4: İşlem bağlantısı: sunucu-istemci iletişimi... // Adım 5: bağlantıyı kapa. connection.close(); // Adım 6: Adım 2 ye dön. catch ( IOException e ) { TCP Client Iterative and Concurrent TCP Servers Socket client; // istemci veri haberleşme kanalı için DataInputStream input; // I/O stream ler DataOutputStream output; String hostname = // Sunucu makinenin DNS adresi // Adım 1: Bağlantı yapmak için bir Socket oluştur. client = new Socket(hostname, 5000); // Adım 2: Giriş/çıkış stream leri hazırla. input = new DataInputStream( client.getinputstream() ); output = new DataOutputStream( client.getoutputstream() ); // Adım 3: İşlem bağlantısı. //... sunucu-istemci iletişimi... // Adım 4: Bağlantıyı kapa.. client.close(); catch ( IOException e ) { 15 16

5 Example 2: Port Scanner Java Ağ Programcılığı, page: 368 public class PortScanner { public static void main(string[] args) { if (args.length!= 1) { System.out.println("kullanim: java PortScanner server"); System.exit(0); InetAddress biladd = null; biladd = InetAddress.getByName(args[0]); catch (UnknownHostException e) { for (int i = 0; i < 100; i++) { System.out.print(i + " "); InetSocketAddress socketaddress = new InetSocketAddress(bilAdd, i); Socket s = new Socket(); s.connect(socketaddress, 500); System.out.println("\nPort " + i + " calismaktadir."); s.close(); // Belirtilen portta hizmet bulunmamaktatir. 17 Port Scanner >java PortScanner Port 7 calismaktadir. 8 9 Port 9 calismaktadir Port 13 calismaktadir Port 19 calismaktadir Port 21 calismaktadir Port 23 calismaktadir Port 37 calismaktadir Port 79 calismaktadir. 80 Port 80 calismaktadir Port Scanner Example 3: Getting Connection Information Java Ağ Programcılığı, page: 371 >java PortScanner Port 21 calismaktadir Port 80 calismaktadir >java PortScanner What s wrong? If you increase timeout, we will also get response from the microsoft from some ports like 80. Your results will vary, depending on which ports are occupied. As a rule, more ports will be occupied on a Unix workstation than on a PC or a Mac. 19 public class ConnInfo { public static void main(string[] args) { if (args.length!= 1) { System.out.println("kullanim: java ConnInfo hostname"); System.exit(0); InetAddress host = null; host = InetAddress.getByName(args[0]); catch (UnknownHostException e) { System.exit(0); Socket thesocket = new Socket(host, 80); System.out.println("\ngetInetAddress() " + thesocket.getinetaddress()); System.out.println("getPort() " + thesocket.getport()); System.out.println("getLocalAddress() " + thesocket.getlocaladdress()); System.out.println("getLocalPort() " + thesocket.getlocalport()); System.out.println("getRemoteSocketAddress() " + thesocket.getremotesocketaddress()); System.out.println("getLocalSocketAddress() " + thesocket.getlocalsocketaddress()); thesocket.close(); 20

6 Two Different Runs >java ConnInfo getinetaddress() getport() 80 getlocaladdress() / getlocalport() 1236 getremotesocketaddress() getlocalsocketaddress() / :1236 >java ConnInfo getinetaddress() getport() 80 getlocaladdress() / getlocalport() 1237 getremotesocketaddress() getlocalsocketaddress() / :1237 Example 4: Daytime Protocol Client One of the simplest protocols is called "daytime", and is defined in RFC 867. The client opens a socket to port 13 on the daytime server. In response, the server sends the time in a human-readable format and closes the connection. You can test the daytime server with Telnet like this: telnet 13 Conneting To :10: UTC(NIST) * Let s write Daytime Client in Java: Daytime Protocol Client Java Ağ Programcılığı, page: 373 public class DayTimeClient { public static void main(string[] args) { if (args.length!= 1) { System.out.println("kullanim: java DayTimeClient hostname"); System.exit(0); InetAddress host = null; host = InetAddress.getByName(args[0]); catch (UnknownHostException e) { System.exit(0); Socket thesocket = new Socket(host, 13); InputStream is = thesocket.getinputstream(); InputStreamReader isr = new InputStreamReader(is); BufferedReader br = new BufferedReader(isr); String data = br.readline(); while (data!= null) { System.out.println(data); data = br.readline(); thesocket.close(); >java DayTimeClient :10: UTC(NIST) * >java DayTimeClient Tue Sep 24 10:09: Sockets and Ports in DaytimeClient 24

7 Iterative Connection-Oriented Server: DayTimeServer Example 5: Java Ağ Programcılığı, page: 385 import java.util.date; public class DayTimeServer { public static void main(string[] args) { ServerSocket srvrsocket = null; Socket connsocket; OutputStream os; srvrsocket = new ServerSocket(13); System.out.println("Daytime Server is listening on port 13"); Syste.exit(0); while (true) { connsocket = srvrsocket.accept(); System.out.println("New User from " + connsocket.getinetaddress()); os = connsocket.getoutputstream(); Date now = new Date(); String currentdate = now.tostring() + "\r\n"; os.write(currentdate.getbytes()); os.close(); connsocket.close(); // end of while // end of main 25 DayTimeServer and 2 Different Clients >java DayTimeServer Daytime Server is listening on port 13 New User from / New User from / TELNET CLIENT: tuna:/home1/csstu/oboyac$telnet Trying Connected to Escape character is '^]'. Wed Sep 26 00:30:54 EEST 2001 Connection closed by foreign host. tuna:/home1/csstu/oboyac$ Web Browser Client 26 Example 6: A Simple Client/Server Application Java Ağ Programcılığı, page: 388 Server public class Server extends JFrame { private JTextArea display; private JScrollPane sp; public Server() { super("server"); display = new JTextArea(); sp = new JScrollPane(display); getcontentpane().add(sp, BorderLayout.CENTER); setsize(350, 250); setvisible(true); public static void main(string[] args) { Server s = new Server(); s.setdefaultcloseoperation(exit_on_close); s.runserver(); public void runserver() { ServerSocket server; Socket connection; DataOutputStream output; DataInputStream input; String line; int counter = 1;

8 Server (cont.) Server (cont.) // Adim 1: Bir ServerSocket olustur. server = new ServerSocket(6000, 100); while (true) { // Adim 2: Bir baglanti bekle... display.append("server is waiting for a client connection...\n"); connection = server.accept(); display.append("connection " + counter + " received from: " + connection.getinetaddress().gethostname()); // Adim 3: Giris ve cikis streamleri olustur input = new DataInputStream(connection.getInputStream()); output = new DataOutputStream(connection.getOutputStream()); // Adim 4: Istemci sunucu islemleri line = input.readutf(); if (line.compareto("selam") == 0) { output.writeutf("ok"); display.append("\nsending message \"Connection successful\"\n"); output.writeutf("connection successful"); display.append("client message: " + input.readutf()); else { output.writeutf("sorry, your password is wrong..."); System.out.println(e);.. // Adim 5: Baglantiyi kapat. display.append("\ntransmission complete. " + "Closing socket.\n\n"); connection.close(); ++counter; // Adim 6: 2. Adima geri don. e.printstacktrace(); Client Client (cont.) public class Client extends JFrame { private JTextArea display; private JScrollPane sp; public Client() { super("client"); display = new JTextArea(); sp = new JScrollPane(display); getcontentpane().add(sp, BorderLayout.CENTER); setsize(350, 250); setvisible(true); public void runclient() { Socket client; DataInputStream input; DataOutputStream output; String line; // Adim 1: Server'a bir Socket ile baglanmaya calis client = new Socket(InetAddress.getLocalHost(), 6000); display.append("connected to: " + client.getinetaddress().gethostname()); // Adim 2: Baglanti yapildi, I/O stream'leri olustur. input = new DataInputStream(client.getInputStream()); output = new DataOutputStream(client.getOutputStream()); // Adim 3: Server ile etkilesim... //output.writeutf("merhaba"); output.writeutf("selam"); line = input.readutf();. 31 if (line.compareto("ok") == 0) { display.append("\nserver message: " + input.readutf()); display.append("\nsending message \"Tesekkur ederim.\"\n"); output.writeutf("tesekkur ederim."); else { display.append("\nserver message: " + line); System.out.println(e); display.append("\nerror\n"+e); // Adim 4: Baglantiyi kapat. display.append("\ntransmission complete. " + "Closing connection.\n"); client.close(); e.printstacktrace(); display.append("\nerror\n"+e); public static void main(string[] args) { Client c = new Client(); c.setdefaultcloseoperation(exit_on_close); c.runclient(); 32

9 More Examples: A Concurrent Client/Server Application Java Ağ Programcılığı, page: 399 A Chat Program: A Client/Server Application Java Ağ Programcılığı, page: A Chat Program: A Client/Server Application Java Ağ Programcılığı, page: Principles of Application Layer Protocols 2. TCP Socket Programming 3. UDP Socket Programming 35 36

10 Abstract Datagram Socket Service No circuit abstraction, just send when ready Server is passive, waits for datagrams Client initiates the send Discrete packets (up to 64Kb long for UDP) UDP/IP maintains packet integrity All data in packet arrives or doesn t (e.g. no half-packet) Even if lower layers fragment No data corruption (e.g. bit errors) Best effort Does not retransmit lost packets 1 lost fragment -> whole packet is lost 37 Constructors: DatagramPacket(byte[] buf, int length) DatagramPacket Class DatagramPacket(byte[] buf, int offset, int length) DatagramPacket(byte[] buf, int length, InetAddress address, int port) DatagramPacket(byte[] buf, int offset, int length, InetAddress address, int port) DatagramPacket(byte[] buf, int offset, int length, SocketAddress address) DatagramPacket(byte[] buf, int length, SocketAddress address) The first two constructors are used when a DatagramPackect will be received from the network. The others are used when a DatagramPacket will be sent. 38 DatagramPacket Preparation To create a DatagramPacket object to receive a data packet from net: byte buffer = new byte[512]; DatagramPacket dp = new DatagramPacket(buffer, buffer.length); DatagramSocket Class To send or receive a DatagramPacket, you need to open a datagram socket. In Java, a datagram socket is created and accessed through the DatagramSocket class: public class DatagramSocket extends Object To send a DatagramPacket to the network: InetAddress addr = new InetAddess("www.doaminname.com"); int port = 3000; String s = "UDP veri paketim: Selam sunucu" byte[] b = s.getbytes(); DatagramPacket dp = new DatagramPacket(b, b.length, addr, port); catch (UnknownHostException e) { Constructors: public DatagramSocket() throws SocketException public DatagramSocket(int port) throws SocketException public DatagramSocket(int port, InetAddress laddr) throws SocketException public DatagramSocket(SocketAddress bindaddr) throws SocketException 39 40

11 Sending a Datagram InetAddress addr = new InetAddess("wwww.server.com"); int port = 3000; String s = "UDP veri paketim: Selam sunucu" byte[] b = s.getbytes(); DatagramPacket dp = new DatagramPacket(b, b.length, addr, port); catch (UnknownHostException e) { DatagramSocket sender = new DatagramSocket(); sender.send(dp); Receiving a Datagram byte buffer = new byte[65536]; DatagramPacket incoming = new DatagramPacket(buffer, buffer.length); DatagramSocket ds = new DatagramSocket(2000); ds.receive(dp); // wait for a UDP datagram byte[] data = dp.getdata(); String s = new String(data, 0, data.getlength()); System.out.println("Port " + dp.getport() + " on " + dp.getaddress() + " sent this message:"); System.out.println(s); Example 8: UDPCalc Client/Server Application Java Ağ Programcılığı, page: 445 UDPCalcClient (1) class UDPCalcClient { public static void main(string args[]) throws Exception { String host = "localhost"; if (args.length > 0) { host = args[0]; System.out.print("Lutfen iki adet sayi giriniz:"); BufferedReader readnumbers = new BufferedReader(new InputStreamReader(System.in)); String sentence = readnumbers.readline(); byte[] senddata = sentence.getbytes(); InetAddress IPAddress = InetAddress.getByName(host); DatagramPacket sendpacket = new DatagramPacket(sendData, senddata.length, IPAddress, 2468); DatagramSocket calcclientsocket = new DatagramSocket(); calcclientsocket.setsotimeout(5000); int counter = 1; byte[] receivedata = new byte[65508]; DatagramPacket resultpacket = new DatagramPacket(receiveData, receivedata.length); 43 44

12 UDPCalcClient (2) UDPCalcServer while (counter < 4) { calcclientsocket.send(sendpacket); System.out.print("Paket " + counter + ".kez Gonderildi. ); System.out.println( Cevap Bekleniyor..."); calcclientsocket.receive(resultpacket); String result = new String(resultPacket.getData(), 0, resultpacket.getlength()); System.out.print("Paket alindi. Cevap: " + result); break; catch (Exception e) { counter++; continue; // while son if (counter == 4) System.out.println("Cevap alinamadi."); //main() son // sınıf son 45 import java.util.stringtokenizer; class UDPCalcServer { public static void main(string args[]) throws Exception { DatagramSocket calcserversocket = new DatagramSocket(2468); byte[] receivedata = new byte[65508]; byte[] senddata = new byte[1024]; while (true) { DatagramPacket receivepacket = new DatagramPacket(receiveData, receivedata.length); calcserversocket.receive(receivepacket); System.out.print("Packet received from " + receivepacket.getaddress()); System.out.print(" port " + receivepacket.getport()); String sentence = new String(receivePacket.getData(), 0, receivepacket.getlength()); System.out.println(" Message:" + sentence); String newsentence; StringTokenizer st = new StringTokenizer(sentence, " x*"); int d1 = Integer.parseInt(st.nextToken()); int d2 = Integer.parseInt(st.nextToken()); newsentence = d1 + "*" + d2 + "=" + (d1 * d2) + "\n"; catch (Exception e) { newsentence = "Lutfen bosluk ile ayrilmis iki sayi gonderiniz.\n"; senddata = newsentence.getbytes(); InetAddress IPAddress = receivepacket.getaddress(); int port = receivepacket.getport(); DatagramPacket sendpacket = new DatagramPacket(sendData, senddata.length,ipaddress, port); calcserversocket.send(sendpacket); 46 Program Run Example 9: DatagramBomber Application Java Ağ Programcılığı, page: 451 >java UDPCalcClient (wrong server IP number) Lutfen iki adet sayi giriniz:24x10 Paket 1.kez Gonderildi.Cevap Bekleniyor... Paket 2.kez Gonderildi.Cevap Bekleniyor... Paket 3.kez Gonderildi.Cevap Bekleniyor... Cevap alinamadi. >java UDPCalcClient Lutfen iki adet sayi giriniz:24x10 Paket 1.kez Gonderildi.Cevap Bekleniyor... Paket alindi. Cevap: 24*10=240 > 47 48

13 DatagramBomberServer class DatagramBomberServer { public static void main(string args[]) throws Exception { DatagramSocket serversocket = new DatagramSocket(13579); byte[] receivedata = new byte[1024]; while (true) { DatagramPacket receivepacket = new DatagramPacket(receiveData, receivedata.length); serversocket.receive(receivepacket); System.out.print("Packet received from " + receivepacket.getaddress()); System.out.print(" port " + receivepacket.getport()); String sentence = new String(receivePacket.getData(), 0, receivepacket.getlength()); System.out.println( + sentence); String newsentence; InetAddress IPAddress = receivepacket.getaddress(); int port = receivepacket.getport(); for (int x = 0; x < ; x++) { newsentence = "" + x; byte[] senddata = newsentence.getbytes(); DatagramPacket sendpacket = new DatagramPacket(sendData, senddata.length, IPAddress, port); serversocket.send(sendpacket); //for() son // while() son //main() son //sınıf son DatagramBomberClient (1) class DatagramBomberClient { public static void main(string args[]) throws Exception { DatagramSocket clientsocket = new DatagramSocket(); String host = "localhost"; if (args.length > 0) { host = args[0]; InetAddress IPAddress = InetAddress.getByName(host); System.out.print("Lutfen isteginizi yaziniz:"); BufferedReader infromuser = new BufferedReader( new InputStreamReader(System.in)); String sentence = infromuser.readline(); byte[] senddata = sentence.getbytes(); DatagramPacket sendpacket = new DatagramPacket(sendData, senddata.length, IPAddress, 13579); clientsocket.send(sendpacket); System.out.println("Packet Sent."); int counter = 0; byte[] receivedata = new byte[65508]; DatagramBomberClient (2).. while (true) { DatagramPacket receivepacket = new DatagramPacket(receiveData, receivedata.length); clientsocket.receive(receivepacket); String recstr = new String(receivePacket.getData(), 0, receivepacket.getlength()); if (!recstr.equalsignorecase(new String("" + counter))) { System.out.print("Sirasi bozulmus paket. Gelen:" + recstr); System.out.println(" Beklenen:" + counter); else { System.out.print(recStr + " "); counter++; // while() son // main() son // sınıf son Output (1) > java DatagramBomberClient Lutfen isteginizi yaziniz:test Packet Sent. Sirasi bozulmus paket. Gelen:1 Beklenen:0 Sirasi bozulmus paket. Gelen:0 Beklenen:1 Sirasi bozulmus paket. Gelen:6 Beklenen:2 Sirasi bozulmus paket. Gelen:7 Beklenen:3 Sirasi bozulmus paket. Gelen:2 Beklenen:4 Sirasi bozulmus paket. Gelen:3 Beklenen:5 Sirasi bozulmus paket. Gelen:8 Beklenen:6 Sirasi bozulmus paket. Gelen:9 Beklenen:7 Sirasi bozulmus paket. Gelen:4 Beklenen:8 Sirasi bozulmus paket. Gelen:10 Beklenen:9 Sirasi bozulmus paket. Gelen:11 Beklenen:10 Sirasi bozulmus paket. Gelen:5 Beklenen: Sirasi bozulmus paket. Gelen:579 Beklenen:213 Sirasi bozulmus paket. Gelen:1008 Beklenen:214 Sirasi bozulmus paket. Gelen:1257 Beklenen:215 Sirasi bozulmus paket. Gelen:1513 Beklenen:216 Sirasi bozulmus paket. Gelen:1743 Beklenen: Sirasi bozulmus paket. Gelen: Beklenen:753 Sirasi bozulmus paket. Gelen: Beklenen:754 Sirasi bozulmus paket. Gelen: Beklenen:755 Sirasi bozulmus paket. Gelen: Beklenen:756 Sirasi bozulmus paket. Gelen: Beklenen:757 Sirasi bozulmus paket. Gelen: Beklenen:

14 UDP and TCP Comparison We begin by listing the main functions of the TCP protocol: Packet ordering: TCP delivers IP packets in the order transmitted, through sequencing. Reliable transmission: Lost packets are detected and recovered through retransmission. Streaming abstraction: The abstraction of a continuous bidirectional communications byte-stream between two entries (data packets are delivered reliably and in the order transmitted) Flow control: The receiver can slow down the server if data is received at a rate that exceeds the receiver s processing capacity. Congestion control: TCP implements a distributed congestion control algorithm that slows down individual stream transmission if packet loss (network traffic) is detected. The algorithm is tuned to provide fair sharing of network resources among TCP streams. 53 UDP vs. TCP In summary, the TCP provides a useful service to applications that need to exchange relatively large amounts of data in a reliable manner and without real-time delivery constraints. The UDP protocol does not have all these features. The UDP protocol provides a less reliable, packet based service without support for flow or congestion control. It is therefore suitable for applications fitting different criteria. 54 Java UDP Support UDP Based Communication The DatagramPacket, DatagramSocket and MulticastSocket classes in the java.net package implement system-independent datagram communication using UDP. An application can send and receive DatagramPackets through a DatagramSocket. In addition, DatagramPackets can be broadcast to multiple recipients all listening to a MulticastSocket. (Chapter 12 in our book). server/ client DatagramPacket DatagramSocket socket at a well-known port used for all communication server application process operating system RTP and JMF RTP (Real Time Protocol) is an unreliable end-to-end application layer protocol based on UDP datagram protocol providing sequencing, time stamping, and data-source information. The Java Media Framework (JMF) extension provides support for the RTP and other multimedia protocols. 55 The process (thread) structure of a UDP server or UDP client iterative, connectionless There is no notion of a server socket. The same socket, DatagramSocket, is used to send data and to listen for incoming connections. 56

15 Individual Datagram Packets vs TCP Stream Based Network Connection Although TCP ultimately packetizes the data you send, TCP sockets allow you to treat a network connection as a stream; you send and receive with input and output streams that you get from the socket. UDP doesn't allow this; you always work with individual datagram packets. All the data you stuff into a single datagram is sent as a single packet, and is lost as a group. Connectionless Communication No Connection between two hosts and Data from Many Hosts UDP doesn't have any concept of a connection between two hosts; it only knows about individual datagrams. Figuring out who sent what data is the application's responsibility. A single DatagramSocket can receive data from many independent hosts. The socket isn't dedicated to a single connection, as it is in TCP References 1. Computer Networking: A Top-Down Approach Featuring the Internet, 6th Edition, J. F. Kurose, K. W. Ross, Addison Wesley, Java Ağ Programcılığı, (Chapters: 10-11), H. Gümüşkaya, Ö. Boyacı, ALFA, Java Network Programming (2nd Edition), E. R. Harold, 731 pages, O Reilly, Beginning Java Networking, Chad Darby,, 900 pages, Wrox Pres, All About Sockets (Sun tutorial), /jw-12-sockets.html 6. Socket Programming in Java: a tutorial, 7. C-language tutorial (audio/slides): Unix Network Programming (J. Kurose) 59

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