UNP Chapter 4: Elementary TCP SocketsIntroductionTypical TCP Client-Server Scenariosocket()socket() parameterssocket() parameterssocket() parametersconnect()connect() parametersbind()bind() parameterslisten()accept()Concurrent Serversclose()getsockname() and getpeername()UNP Chapter 4: Elementary TCP SocketsCMPS 105: Systems ProgrammingProf. Scott BrandtT Th 2-3:45Soc Sci 2, Rm. 167Introduction First: Elementary socket functions Next: TCP client-server example Concurrent services Common Unix server technique Allows server to simultaneously serve multiple clients Server forks a separate process to serve each clientTypical TCP Client-Server Scenario Figure 4.1 (p. 96) Server starts up Client starts up and connects to server Client contacts server Server processes client request Server responds to client Client closes connection Server goes back to waiting for clients to connectsocket() To perform network I/O, a process first calls socket() Called by both client and server #include <sys/socket.h> int socket(intfamily, inttype, intprotocol); returns a socket descriptorsocket() parametersfamilyspecifies the protocol family or domain AF_INET: IPv4 protocols AF_INET6: IPv6 protocols AF_LOCAL: Unix domain protocols AF_ROUTE: Routing sockets AF_KEY: Key socket Usually: AF_INET AF_LOCAL might work on CATS machinessocket() parameterstypespecifies type of communication SOCK_STREAM: stream socket SOCK_DGRAM: datagram socket SOCK_SEQPACKET: sequenced packet socket SOCK_RAW: raw socket TCP supports only SOCK_STREAM UDP supports SOCK_DGRAMsocket() parametersprotocolspecifies transport protocol IPPROTO_TCP: TCP transport protocol IPPROTO_UDP: UDP transport protocol IPPROTO_SCTP: SCTP transport protocol TCP provides reliable, in-order streaming communication Resends lost packets, guarantees order UDP provides possibly unreliable, possibly out-of-order message delivery Doesn’t guarantee anything, but usually works fineconnect() Use by a TCP client to establish a connection with a TCP server #include <sys/socket.h> int connect(intsockfd, const structsockaddr *servaddr, socklen_taddrlen); Returns when the connection has been established Possible errors: timeout, host not found, connection refusedconnect() parameterssockfdis the socket descriptor returned by socket()servaddris the socket address Contains the IP address and port number of the serveraddrlenis the size of the address structurebind() Used by server to bind a socket to an IP address/port pair #include <sys/socket.h> int bind(intsockfd, const structsockaddr *myaddr, socklen_taddrlen); Common error: EADDRINUSEbind() parameterssockfdis the socket descriptor returned by socket()myaddris the address of this server Note: addresses are protocol-specific Usually well-known addresses so that clients can find the serveraddrlenis the length of the addresslisten() Called by a TCP server to: Set the socket up to receive connections Specify the maximum number of connections the kernel should queue up for this socket #include <sys/socket.h> int listen(intsockfd, intbacklog); Normally called after socket() and bind() and before accept()sockfdis the socket descriptor from socket()backlogshould be non-zeroaccept() Called by a TCP server to return the next completed connection from a client Blocks if no connection is available #include <sys/socket.h> int accept(intsockfd, struct sockaddr *cliaddr, socklen_t *addrlen);sockfdis the socket descriptor from socket()cliaddris the address of the client that connected to the server Returns a new socket descriptor for the connection to the clientint main(int argc, char **argv) {int listenfd, connfd;struct sockaddr_in servaddr;char buff[maxline];time_t ticks;listenfd = socket(AF_INET, SOCK_STREAM, 0);bzero(&servaddr, sizeof(servaddr);servaddr.sin_family = AF_INET;servaddr.sin_addr.s_addr = htonl(INADDR_ANY);servaddr.sin_port = htons(13); // daytime severbind(listenfd, (SA *) &servaddr, sizeof(servaddr));listen(listenfd, LISTENQ);while(1) {connfd = accept(listenfd, (SA *)NULL, NULL);ticks = time(NULL);snprintf(buff, sizeof(buff), “%.24s\r\n”, ctime(&ticks));write(connfd, buff, strlen(buff));close(connfd);}}Concurrent Servers The previous example is an iterative server It sequentially receives connections and processes each request in turn A concurrent serverforks a child process to handle each request Multiple requests can be handled in parallel by multiple concurrently executing child processespid_t pid;int listenfd, connfd;listenfd = socket(…);bind(listenfd, …);listen(listenfd, LISTENQ);while(1) {connfd = accept(listenfd, …);if( (pid = fork()) == 0) {close(listenfd);doit(connfd); // service the requestclose(connfd);exit(0);}close(connfd);}close() Normal close() is used to close sockets #include <unistd.h> int close(intsockfd);getsockname() and getpeername() getsockname() returns the local protocol address associate with a socket Used to get socket address when responding to a wildcard connection getpeername() returns the foreign protocol address associated with a socket Used to get client address in an