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Slide 1: Socket Programming connecting processes Jignesh Patel Palanivel Rathinam

Slide 2: Overview • Introduction to Sockets • A generic Client-Server application • Programming Client-Server in C • Programming Client-Server in Java • References

Slide 3: Introduction to Sockets

Slide 4: Introduction to Sockets • Why Sockets? – Used for Interprocess communication. • The Client-Server model – Most interprocess communication uses client-server model – Client & Server are two processes that wants to communicate with each other – The Client process connects to the Server process, to make a request for information/services own by the Server. – Once the connection is established between Client process and Server process, they can start sending / receiving information. • What are Sockets? Socket – End-point of interprocess communication. – An interface through which processes can send / receive information

Slide 5: Introduction to Sockets • What exactly creates a Socket? – tuple • What makes a connection? – {Source , Destination } i.e. source socket – destination socket pair uniquely identifies a connection. • Example Client 1343 192.168.0.2 80 1343 Server Client 192.168.0.3 192.168.0.1 Client 5488 192.168.0.2

Slide 6: Introduction to Sockets • Socket Types – STREAM – uses TCP which is reliable, stream oriented protocol – DATAGRAM – uses UDP which is unreliable, message oriented protocol – RAW – provides RAW data transfer directly over IP protocol (no transport layer) • Sockets can use – “unicast” ( for a particular IP address destination) – “multicast” ( a set of destinations – 224.x.x.x) – “broadcast” (direct and limited) – “Loopback” address i.e. 127.x.x.x

Slide 7: A generic Client-Server application

Slide 8: A generic TCP application • algorithm for TCP client – Find the IP address and port number of server – Create a TCP socket – Connect the socket to server (Server must be up and listening for new requests) – Send/ receive data with server using the socket – Close the connection • algorithm for TCP server – Find the IP address and port number of server – Create a TCP server socket – Bind the server socket to server IP and Port number (this is the port to which clients will connect) – Accept a new connection from client • returns a client socket that represents the client which is connected – Send/ receive data with client using the client socket – Close the connection with client

Slide 9: A generic UDP application • algorithm for UDP client – Find the IP address and port number of server – Create a UDP socket – Send/ receive data with server using the socket – Close the connection • algorithm for UDP server – Find the IP address and port number of server – Create a UDP server socket – Bind the server socket to server IP and Port number (this is the port to which clients will send) – Send/ receive data with client using the client socket – Close the connection with client

Slide 10: Programming Client-Server in C

Slide 11: Programming Client-Server in C • The steps involved in establishing a socket on the client side are as follows: – Create a socket with the socket() system call – Connect the socket to the address of the server using the connect() system call – Send and receive data using send() and recv() system calls. • The steps involved in establishing a socket on the server side are as follows: – Create a socket with the socket() system call – Bind the socket to an address using the bind() system call. For a server socket on the Internet, an address consists of a port number on the host machine. – Listen for connections with the listen() system call – Accept a connection with the accept() system call. This call typically blocks until a client connects with the server. – Send and receive data

Slide 12: Programming TCP Client in C Client.c Socket System contain an internet address /* a structure to Call – create an end point for defined in the include file */ communication struct sockaddr_in { #include #include /* should be AF_INET */ short sin_family; #include #include u_short sin_port; #include struct in_addr sin_addr; #include int socket(int domain, int/* not used, must be zero char sin_zero[8]; type, int protocol); #include */ }; Returns a descriptor domain: selects protocol family void error(char *msg){ perror(msg); exit(0);} struct in_addrPF_IPX, PF_X25, PF_APPLETALK e.g. { type: specifies communication semantics unsigned long s_addr; int main(int argc, char *argv[]){ }; e.g. SOCK_DGRAM, SOCK_RAW int sockfd, portno, n; protocol: specifies a particular protocol to be used struct sockaddr_in serv_addr; e.g. IPPROTO_UDP, IPPROTO_ICMP struct hostent *server; char buffer[256]; if (argc < 3) { fprintf(stderr,"usage %s hostname port ", argv[0]); exit(0); } portno = atoi(argv[2]); sockfd = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP); if (sockfd < 0) error("ERROR opening socket");

Slide 13: Programming TCP Client in C Client.c server = gethostbyname(argv[1]); if (server == NULL) { fprintf(stderr,"ERROR, no such host "); exit(0); } bzero((char *) &serv_addr, sizeof(serv_addr)); serv_addr.sin_family = AF_INET; bcopy((char *)server->h_addr, (char *)&serv_addr.sin_addr.s_addr , server->h_length); serv_addr.sin_port = htons(portno); if (connect(sockfd,&serv_addr,sizeof(serv_addr)) < 0) error("ERROR connecting"); Close Recv System Call – send socket descriptor Send System Call – receive a message a socket Connect System Callcloseaamessage to from a a – initiates a connection on printf("Please enter the message: "); socket bzero(buffer,256); fgets(buffer,255,stdin); #include n = send(sockfd,buffer,strlen(buffer),0); #include if (n < 0) error("ERROR writing to socket"); int close( int s); #include int send( int s, const void *msg, size_t len, bzero(buffer,256); int connect(s, int flags); *buff, size_t len, recv( int int sockfd, Returns 0 on success const void n = recv(sockfd,buffer,255,0); s: descriptor to be closed sockaddr *serv_addr, int flags); const struct if (n < 0) socklen_t addrlen); Returns number of characters sent on success error("ERROR reading from socket"); s: descriptor that of bytes received on success Returns number must refer to a socket in printf("%s ",buffer); s: descriptor success Returns 0 onthat must refer to a socket in connected state close(sockfd); sockfd: descriptor that must refer to a socket msg: data that we want to send connected state serv_addr:thatdatawantwhich we want to connect len: length of we to to receive buff: data address return 0; addrlen: length of 0. MSG_OOB, MSG_DONTWAIT flags: use default serv_addr len: length of data } flags: use default 0. MSG_OOB, MSG_DONTWAIT

Slide 14: Programming TCP Server in C Server.c #include #include #include #include void error(char *msg){ perror(msg); exit(0);} int main(int argc, char *argv[]){ int sockfd, newsockfd, portno, clilen; char buffer[256]; struct sockaddr_in serv_addr, cli_addr; int n; if (argc < 2) { fprintf(stderr,"ERROR, no port provided "); exit(1); } sockfd = socket(AF_INET, SOCK_STREAM, 0); if (sockfd < 0) error("ERROR opening socket"); bzero((char *) &serv_addr, sizeof(serv_addr)); portno = atoi(argv[1]); serv_addr.sin_family = AF_INET; serv_addr.sin_addr.s_addr = INADDR_ANY; serv_addr.sin_port = htons(portno);

Slide 15: Programming TCP Server in C Server.c if (bind(sockfd, (struct sockaddr *) &serv_addr, sizeof(serv_addr)) < 0) error("ERROR on binding"); listen(sockfd,5); clilen = sizeof(cli_addr); newsockfd = accept(sockfd, (struct sockaddr *) &cli_addr, &clilen); if (newsockfd < 0) error("ERROR on accept"); bzero(buffer,256); n = recv(newsockfd,buffer,255,0); if (n < 0) error("ERROR reading from socket"); printf("Here is the message: %s ",buffer); n = send(newsockfd,"I got your message",18,0); Accept System Call accepts a to a socket Listen System Call bind a for connections on a Bind System Call – ––listenname connectionon a if (n < 0) error("ERROR writing to socket"); socket close(newsockfd); #include close(sockfd); #include return 0; #include int bind( int sockfd, } int listen( int s,sockfd, int backlog); accept( int const struct sockaddr *serv_addr, socklen_t addrlen); *addr, const struct sockaddr Returns 0 on socklen_t addrlen); success s: descriptor success Returns 0 onthat must refer to a socket backlog: a non-negative descriptor on success sockfd: descriptor that must refer to afor completely Returns maximum length the queue socket sockfd: descriptor that must we want socket serv_addr: address to whichto be to a to connect established sockets waiting refer accepted addr: filled with address of connecting entity addrlen: length of serv_addr addrlen: length of addr

Slide 16: Programming UDP Client in C • The client code for a datagram socket client is the same as that for a stream socket with the following differences. – the socket system call has SOCK_DGRAM instead of SOCK_STREAM as its second argument & IPPROTO_UDP instead of IPPROTO_TCP as its third argument. – there is no connect() system call – instead of send() and recv(), the client uses sendto() and recvfrom() sock = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP); len = sizeof(struct sockaddr_in); while (1) { /* write */ n = sendto(sock,“Got your message ",17, 0,(struct sockaddr *) &server, len); f (n < 0) error("sendto"); /* read */ n = recvfrom(sock,buf,1024,0,(struct sockaddr *)&from, len); if (n < 0) error("recvfrom"); }

Slide 17: Programming UDP Server in C • Server code with a datagram socket is similar to the stream socket code with following differences. – Servers using datagram sockets do not use the listen() or the accept() system calls. – After a socket has been bound to an address, the program calls recvfrom() to read a message or sendto() to send a message. sock = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP); len = sizeof(struct sockaddr_in); while (1) { /* read */ n = recvfrom(sock,buf,1024,0,(struct sockaddr *)&from, len); if (n < 0) error("recvfrom"); /* write */ n = sendto(sock,"Got your message ",17, 0,(struct sockaddr *)&from, len); f (n < 0) error("sendto"); }

Slide 18: Programming Client-Server in C • In case of Windows Everything in the code is same as described previously except the following differences – You have to tell your compiler to link in the Winsock library, usually called wsock32.lib or winsock32.lib – On Visual C++, this can be done through the Project menu, under Settings.... Click the Link tab, and look for the box titled "Object/library modules". Add "wsock32.lib" to that list. – On Visual Studio .NET, add “wsock32.lib” under Project menu, Properties -> Linker -> Input -> Additional Dependencies #include ….. void main(int argc,char *argv[]){ WSADATA wsda; // if this doesn’t work // WSAData wsda; // then try this WSAStartup(0x0101,&wsda); ….. WSACleanup(); closesocket(sockfd); }

Slide 19: Programming Client-Server in Java

Slide 20: Programming TCP Client-Server in Java • All the classes related to sockets are in the java.net package, so make sure to import that package when you program sockets. • All the input/output stream classes are in the java.io package, include this also • How to open a socket? – If you are programming a client, then you would create an object of Socket class – Machine name is the machine you are trying to open a connection to, – PortNumber is the port (a number) on which the server you are trying to connect to is running. select one that is greater than 1,023! Why?? Socket MyClient; try { MyClient = new Socket("Machine name", PortNumber); } catch (IOException e) { System.out.println(e); }

Slide 21: Programming TCP Client-Server in Java • If you are programming a server, then this is how you open a socket: ServerSocket MyService; try { MyServerice = new ServerSocket(PortNumber); } catch (IOException e) { System.out.println(e); } • When implementing a server you also need to create a socket object from the ServerSocket in order to listen for and accept connections from clients. Socket clientSocket = null; try { clientSocket = MyService.accept(); } catch (IOException e) { System.out.println(e); }

Slide 22: Programming TCP Client-Server in Java • How to create an input stream? – On the client side, you can use the DataInputStream class to create an input stream to receive response from the server: DataInputStream input; try { input = new DataInputStream(MyClient.getInputStream()); } catch (IOException e) { System.out.println(e); } – The class DataInputStream allows you to read lines of text and Java primitive data types in a portable way. It has methods such as read, readChar, readInt, readDouble, and readLine,. – On the server side, you can use DataInputStream to receive input from the client: DataInputStream input; try { input = new DataInputStream(clientSocket.getInputStream()); } catch (IOException e) { System.out.println(e); }

Slide 23: Programming TCP Client-Server in Java • How to create an output stream? – On the client side, you can create an output stream to send information to the server socket using the class PrintStream or DataOutputStream of java.io: PrintStream output; try { output = new PrintStream(MyClient.getOutputStream()); } catch (IOException e) { System.out.println(e); } – The class PrintStream has methods for displaying textual representation of Java primitive data types. Its write and println methods are important. Also, you may want to use the DataOutputStream: DataOutputStream output; try { output = new DataOutputStream(MyClient.getOutputStream()); } catch (IOException e) { System.out.println(e); } – Many of its methods write a single Java primitive type to the output stream. The method writeBytes is a useful one.

Slide 24: Programming TCP Client-Server in Java • On the server side – you can use the class PrintStream to send information to the client. PrintStream output; try { output = new PrintStream(clientSocket.getOutputStream()); } catch (IOException e) { System.out.println(e); } • Note: You can use the class DataOutputStream as mentioned previously.

Slide 25: Programming TCP Client-Server in Java • How to close sockets? – You should always close the output and input stream before you close the socket. – On the client side: try { output.close(); input.close(); MyClient.close(); } catch (IOException e) { System.out.println(e); } – On the server side: try { output.close(); input.close(); clientSocket.close(); MyService.close(); } catch (IOException e) { System.out.println(e); }

Slide 26: Programming UDP Client-Server in Java • How to open a datagram socket? – If you are programming a client, then you would create an object of DatagramSocket class try { DatagramSocket socket = new DatagramSocket(); } catch (IOException e) { System.out.println(e); } • If you are programming a server, then this is how you open a socket: DatagramSocket socket = null; try { socket = new DatagramSocket(4445); } catch (IOException e) { System.out.println(e); }

Slide 27: Programming UDP Client-Server in Java • How to send/receive on Datagram sockets? – On the client side, you can use the DatagramPacket class – To send data byte[] buf = new byte[256]; InetAddress address = InetAddress.getByName(args[0]); DatagramPacket packet = new DatagramPacket(buf, buf.length, address, 4445); socket.send(packet); – To receive data packet = new DatagramPacket(buf, buf.length); socket.receive(packet); String received = new String(packet.getData()); System.out.println(“Received from server: " + received);

Slide 28: Programming UDP Client-Server in Java • How to send/receive on Datagram sockets? – On the Server side, you can use the DatagramPacket class – To receive data byte[] buf = new byte[256]; DatagramPacket packet = new DatagramPacket(buf, buf.length); socket.receive(packet); • To send data InetAddress address = packet.getAddress(); int port = packet.getPort(); packet = new DatagramPacket(buf, buf.length, address, port); socket.send(packet); • How to close a Datagram socket? socket.close();

Slide 29: References • Man pages in Linux Accesssible through following command – man 2 – E.g. man 2 socket • “Unix network programming” by Richard Stevens • Beej’s guide to Network Programming http://beej.us/guide/bgnet/ • The Java Tutorial – Custom Networking http://java.sun.com/docs/books/tutorial/networking/ • Lecture notes of cs423 from Dr. Bob Cotter http://www.sce.umkc.edu/~cotterr/cs423_fs05/cs423_fs05_lectures.html