תקשורת באינטרנט Tutorial 8

2 n Socket programming u What is socket ? u Sockets architecture u Types of Sockets u The Socket system calls u Data Transfer u Service functions for sockets programming u Examples for Client - Server communication F STREAM Communication (TCP) F DATAGRAM Communication (UDP) u Port reservation u Example Contents

3 What is Sockets ? n First appeared in 4.1 BSD UNIX n Sockets: u is an abstraction used as communication endpoints. u is implemented as file descriptor with some state information stored in the socket library.

4 The Client Server model request reply

5 Example: UDP Echo n The simplest form of client-server interaction uses unreliable datagram delivery to convey messages from client to server and back. clientserver Waits for a datagram to arrive the echo port Reserve the source and destination addresses (IP addresses and ports) Return the datagram to its original sender Request sent to a well-known port The program become a UDP echo client when it allocates an unused UDP protocol port sends a UDP message to the UDP echo server Waits for the replay Response sent to the client’s port

6 Example: UDP Echo (Cont.) n The example illustrates two important points that are generally true about client-server interaction: u The difference between the lifetime of servers and clients. u The use of reserved and non-reserved port identifiers.

7 Sockets architecture Kernel User Application TCP/UDP layer IP (network) layer Socket API MAC layer

8 Type of Sockets n Socket has an associated type that determine the semantic of the communication : u SOCK_STREAM: connection oriented byte stream - TCP. u SOCK_DGRAM: unreliable, connectionless communication - UDP u and more

9 Protocol family for sockets functions n The family specifies the address family: group of protocols with the same address format n Example of address family constants: u AF_INET - IPv4 protocols u AF_INET6 - IPv6 protocols u AF_LOCAL (AF_UNIX) - Unix domain protocols

10 Data Structures struct sockaddr_in { short int sin_family; /* we use AF_INET */ unsigned short int sin_port; /* port number */ struct in_addr sin_addr; /* comp. address */ unsigned char sin_zero[8]; /* filled with 0s */ }; struct in_addr { u_long s_addr; /* unsigned long */ };

11 The Socket system calls n The Socket system calls: n int socket (int family, int type, int protocol) allocate a socket return: a socket descriptor or -1 family: communication domain type: type of socket- SOCK_STREAM, SOCK_DGRAM, SOCK_RAW protocol: particular protocol to use - 0 n int bind (int sd, struct sockaddr *addr, size_t addrlen) Bind a name to a socket return: 0 on success or -1 sd: socket descriptor (from the socket() system call) addr: pointer to the socket address addrlen: socket address length n int connect (int sd, const struct sockaddr *addr, size_t addrlen) Try to connect to a socket (done by the client) return: 0 on success or -1 sd: socket descriptor (from the socket() system call) addr: pointer to the socket address addrlen: socket address length

12 The Socket system calls (cont) n int listen (int sd, int backlog) a queue for incoming connections is activated using listen() return: 0 on success or -1 sd: socket descriptor (from the socket() system call) backlog : Maximum queue length of processes waiting for connection. n int accept (int sd, struct sockaddr *addr, size_t *addrlen); Wait for a connection request (done by the server) return: new socket descriptor or -1 in case of failure sd: socket descriptor (from the socket() system call) addr: pointer to the socket address addrlen: socket address length. n int close (int sd) Close the socket or the connection with the remote host. sd: socket descriptor (from the socket() system call)

13 Data Transfer - send u int send (int sd, char *buf, int buf_len) transmit a message to another socket. send() can be used only when the socket is in a connected state. return: the number of bytes that has been delivered or -1. sd: socket descriptor (from the socket() system call) buf: the message you send. buf_len: the data length of buf u int sendto (int sd, char *buf, int buf_len, const struct sockaddr *to_addr, size_t to_len) used to transmitting data to remote host return: the number of bytes that has been delivered or -1. sd: socket descriptor (from the socket() system call) buf: a pointer to the data location buf_len: the data length to_addr: the socket address of the destination. to_len: the “to_addr” structure length.

14 Data Transfer - receive u recv (int sd, char *buf, int bub_len) Obtain data from the socket. Used only when the socket is in a connected state. return: the number of bytes at the obtain input or -1. sd: socket descriptor (from the socket() system call) buf: the buffer address (in memory) for holding the received data. buf_len: the buffer length u int recvfrom (int sd, char *buf, int bub_len, struct sockaddr *from_addr, int from_len) Obtain data from the socket. return: the number of bytes at the obtain input or -1. sd: socket descriptor (from the socket() system call) buf: the buffer address (in memory) for holding the received data. buf_len: the buffer length from_addr: the socket address of the transmitter from_len: the “from_addr” structure length

15 Service functions for sockets programming n Byte ordering u short htons(short) - convert 16-bit value from host to network u long htonl(long) - convert 32-bit value from host to network u short ntohs(short) - convert 16-bit value from network to host u long ntohl(long) - convert 32-bit value from network to host n Deal with IP address u ina.sin_addr.s_addr = inet_addr(" ") u returns the address in Network Byte Order already. n DNS u int gethostname(char* name, int namelen) return local host name u struct hostent *gethostbyname(char* name) use to get computer address by the name. u struct hostent *gethostbyaddr(char *addrp, int len, int type)

16 From Beej's Guide to Network Programming n socket()--Get the File Descriptor! n bind()--What port am I on? n connect()--Hey, you! n listen()--Will somebody please call me? n accept()--"Thank you for calling port 3490." n send() and recv()--Talk to me, baby! n sendto() and recvfrom()--Talk to me, DGRAM-style n close() and shutdown()--Get outta my face!

17 Stream Sockets TCP ClientTCP Server sd = socket(…) bind(sd,port) connect(sd,dest) send(sd,…) recv(sd,…) close(sd) sd = socket(…) bind(sd,port) listen(sd) new_sd = accept(sd) recv(new_sd,…) send(new_sd,…) close(new_sd,...)

18 Datagram Sockets UDP Client UDP Server sd = socket(…) sendto(sd,…) recvfrom(sd,…) close(sd) sd = socket(…) bind(sd,port) recvfrom(sd,…) sendto(sd,…)

19 Port reservation n Ports 0 through 1023 are reserved. n Other ports are used by system for assignment to clients.

20 Example (UDP Client) // This program sends UDP packets to the given address #include #define SERVER_ADDR " " #define SERVER_PORT 5555 void error(char *str) { printf("\n%s", str); exit(0); } int main(int argc, char *argv[]) { char message[100], message2[10]; int sockfd, res; struct sockaddr_in client_addr, server_addr; int i, mesNum; printf("\nClient is running..."); if (argc < 2) error("\nYou should supply parameter: the number of messages to send");

21 Example (UDP Client) // Opening socket if ((sockfd = socket (AF_INET, SOCK_DGRAM, 0)) < 0) error("Could not open socket"); // Sending a message to the server bzero((char*) &server_addr, sizeof(server_addr)); server_addr.sin_family = AF_INET; server_addr.sin_addr.s_addr = inet_addr(SERVER_ADDR); server_addr.sin_port = htons(SERVER_PORT); mesNum = atoi(argv[1]); if (mesNum == 0) error("\nIllegal parameter"); for (i=0; i<mesNum; i++) { strcpy(message, "Test message: "); sprintf(message2, "%d", i+mesNum); strcat(message, message2); res = sendto (sockfd, message, strlen(message)+1, 0, (struct sockaddr*)&server_addr, sizeof(server_addr)); printf("\nClient sent %d bytes", res); }

22 Example (UDP Server) // This program receives UDP packets #include #define SERVER_PORT 5555 #define MAX_MESSAGE_SIZE 100 void error(char *str) { printf("\n%s", str); exit(0); } int main() { char message[MAX_MESSAGE_SIZE]; int sockfd, res; struct sockaddr_in client_addr, server_addr; int addr_len; printf("\nServer is running..."); fflush(stdout);

23 Example (UDP Server) // Opening socket if ((sockfd = socket (AF_INET, SOCK_DGRAM, 0)) < 0) error("Could not open socket"); // Bind local ip and process addresses bzero((char*) &server_addr, sizeof(server_addr)); server_addr.sin_family = AF_INET; server_addr.sin_addr.s_addr = htonl(INADDR_ANY); server_addr.sin_port = htons(SERVER_PORT); if ( bind (sockfd, (struct sockaddr*)&server_addr, sizeof(server_addr)) < 0) error("Could not bind to the socket"); while (1) { // Receiving a message from the client addr_len = sizeof(client_addr); res = recvfrom (sockfd, message, MAX_MESSAGE_SIZE, 0, (struct sockaddr*)&client_addr, &addr_len); printf("\nServer received %d bytes", res); printf("\n%s", message); fflush(stdout); }