Some slides are in courtesy of J. Kurose and K. RossReview of Previous Lecture• Electronic Mail: SMTP, POP3, IMAP•DNS• Socket programming with TCPAnnouncement• Homework 1 due Wed. midnight• Should have completed at least part I of project 1• Recitation tomorrow on homework 1 and project 1Outline• Socket programming with TCP• Socket programming with UDP• I/O multiplexing• Web cachingsocket()bind()listen()accept()read()write()read()close()Socket()connect()write()read()close()TCP ClientTCP ServerWell-known portblocks until connection from clientprocess requestConnection establishmentData(request)Data(reply)End-of-file notificationint connect_ socket( char *hostname, int port) {int sock;struct sockaddr_in sin;struct hostent *host;sock = socket( AF_ INET, SOCK_ STREAM, 0);if (sock == -1)return sock;host = gethostbyname( hostname);if (host == NULL) {close( sock);return -1;}memset (& sin, 0, sizeof( sin));sin. sin_ family = AF_ INET;sin. sin_ port = htons( port);sin. sin_ addr. s_ addr = *( unsigned long *) host-> h_ addr_ list[ 0];if (connect( sock, (struct sockaddr *) &sin, sizeof( sin)) != 0) {close (sock);return -1;}return sock;}Resolve the hoststruct hostent *gethostbyname( const char *hostname);/*Return nonnull pointer if OK, NULL on error */Setup up the structunit16_t htons(unit16_t host16bitvaule)/*Change the port number from host byte order to network byte order */Connectconnect(int socketfd, const struct sockaddr * servaddr, socket_t addrlen)/*Perform the TCP three way handshaking*/Hostent structurestruct hostent{char * h_name /*official name of host*/char ** h_aliases; /* pointer ot array of\pointers to alias name*/int h_addrtype /* host address type*/int h_length /* length of address */char ** h_addr_list /*prt to array of ptrs with \IPv4 or IPv6 address*/}Ipv4 socket address structurestruct socketaddr_in{uint8_t sin_len; /*length of the structure (16)*/sa_falimily_t sin_family /* AF_INT*/in_port_t sin_port /* 16 bit TCP or UDP port number*/struct in_addr sin_addr /* 32 bit Ipv4 address */char sin_zero(8)/* unused*/}Make the socketSocket(int family , int type, in t protocol); return nonnegative value for OK, -1 for errorServer – high level viewCreate a socketBind the socketListen for connectionsAccept new client connectionsRead/write to client connectionsShutdown connectionAssigning an address to a socket•The bind() system call is used to assign an address to an existing socket.int bind( int sockfd, const struct sockaddr *myaddr, int addrlen);• bind returns 0 if successful or -1 on error.bind()• calling bind() assigns the address specified by the sockaddr structure to the socket descriptor.•You can give bind() a sockaddr_instructure:bind( mysock, (struct sockaddr*) &myaddr,sizeof(myaddr) );bind() Exampleint mysock,err;struct sockaddr_in myaddr;mysock = socket(PF_INET,SOCK_STREAM,0);myaddr.sin_family = AF_INET;myaddr.sin_port = htons( portnum );myaddr.sin_addr = htonl( ipaddress);err=bind(mysock, (sockaddr *) &myaddr, sizeof(myaddr));Make listen socket (TCP)int make_ listen_ socket( int port) {struct sockaddr_ in sin;int sock;sock = socket( AF_ INET, SOCK_ STREAM, 0);if (sock < 0)return -1;memset(& sin, 0, sizeof( sin));sin. sin_ family = AF_ INET;sin. sin_ addr. s_ addr = htonl( INADDR_ ANY);sin. sin_ port = htons( port);if (bind( sock, (struct sockaddr *) &sin, sizeof( sin)) < 0)return -1;return sock;}Make the socketSetup up the structBindbind(int sockfd, const struct sockaddr * myaddr, socklen_t addrlen);/* return 0 if OK, -1 on errorassigns a local protocol adress to a socket*/listen()int listen( int sockfd, int backlog);sockfd is the TCP socket (already bound to an address)backlog is the number of incoming connections the kernel should be able to keep track of (queue for us).listen() returns -1 on error (otherwise 0).Accepting an incoming connection.• Once we call listen(), the O.S. will queue incoming connections– Handles the 3-way handshake– Queues up multiple connections.• When our application is ready to handle a new connection, we need to ask the O.S. for the next connection.accept()int accept( int sockfd,struct sockaddr* cliaddr,socklen_t *addrlen);sockfd is the passive mode TCP socket.cliaddr is a pointer to allocatedspace.addrlen is a value-resultargument– must be set to the size of cliaddr– on return, will be set to be the number of used bytes in cliaddr.accept() return valueaccept() returns a new socket descriptor (small positive integer) or -1 on error.After accept returns a new socket descriptor, I/O can be done using the read() and write()system calls.read() and write() operate a little differently on sockets (vs. file operation)!Accepting a client connection (TCP)int get_ client_ socket( int listen_ socket) {struct sockaddr_ in sin;int sock;int sin_ len;memset(& sin, 0, sizeof( sin));sin_ len = sizeof( sin);sock = accept( listen_ socket, (struct sockaddr *) &sin, &sin_ len);return sock;}Setup up the structAccept the client connectionaccept(int sockefd, struct sockaddr * claddr, socklen_t * addrlen)/* return nonnegative descriptor if OK, -1 on errorreturn the next completed connection from the front of thecompleted connection queue.if the queue is empty, the process is put to sleep(assuming blocking socket)*/Reading from/writing to a TCP socketint read( int fd, char *buf, int max);int write( int fd, char *buf, int num);•By default read() will block until data is available.• reading from a TCP socket may return less than max bytes (whatever is available).• You must be prepared to read data 1 byte at a time! • write might not be able to write all num bytes (on a nonblocking socket).Terminating a TCP connection• When finished using a socket, the socket should be closed:• status = close(s);– status: 0 if successful, -1 if error– s: the file descriptor (socket being closed)• Closing a socket– closes a connection (for SOCK_STREAM)– frees up the port used by the socketsocket()bind()listen()accept()read()write()read()close()Socket()connect()write()read()close()TCP ClientTCP ServerWell-known portblocks until connection from clientprocess requestConnection establishmentData(request)Data(reply)End-of-file notificationOutline• Socket programming with TCP• Socket programming with UDP• I/O multiplexing• Web cachingSocket programming with UDPUDP: no “connection” between client and server•no handshaking• sender explicitly attaches IP address and port of destination to each