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Ports. Port - A 16-bit number that identifies the application process that receives an incoming message. Reserved ports or well-known ports ( 0 to 1023 ) Standard ports for well-known applications. Telnet (23), ftp(21), http (80).
  • Port - A 16-bit number that identifies the application process that receives an incoming message.
  • Reserved ports or well-known ports (0 to 1023)
  • Standard ports for well-known applications. Telnet (23), ftp(21), http (80). See /etc/services file on any UNIX machine for listing of services on reserved ports.
  • Ephemeral ports (1024-65535)
  • For ordinary user-developed programs.Associations
  • A socket address is the triple:
  • {protocol, local-IP, local-port}For example,{tcp,, 23}
  • An association is the 5-tuple that completely specifies the two end-points that comprise a connection:
  • {protocol, local-IP, local-port, remote-IP, remote-port}For example: {tcp,, 23,, 1024} Associations….
  • A half-association is
  • {protocol, local-IP, local-port}or{protocol, remote-IP, remote-port} which specify each half of the connection.
  • The half-association is also called a socket or a transport address.
  • UDP Daytime Client and Server
  • Server waits for requests on a known port.
  • Client sends a UDP request to server.
  • Server responds with daytime info.
  • No connection establishment!
  • No reliability!
  • UDP Client#include "unp.h"int main(int argc, char **argv){ int sockfd, n, servlen; char req[10], recvline[MAXLINE + 1]; struct sockaddr_in servaddr; if( argc != 2 )err_quit(“usage : gettime <IP address>”); /* Create a UDP socket */ sockfd = Socket(AF_INET, SOCK_DGRAM, 0); /* Specify server’s IP address and port */bzero(&servaddr, sizeof(servaddr)); servaddr.sin_family = AF_INET; servaddr.sin_port = htons(13); /* daytime server port */Inet_pton(AF_INET, argv[1], &servaddr.sin_addr);/* Send message to server requesting date/time */strcpy(req, “GET_TIME”); Sendto(sockfd, req, strlen(req), 0, (struct sockaddr *)&servaddr, sizeof(servaddr)); /* Read date/time from the socket */ servlen = sizeof(servaddr); n= Recvfrom(sockfd, recvline, MAXLINE, 0, (struct sockaddr *)&servaddr, &servlen); recvlen[n] = 0; printf(“%s”, recvlen); close(sockfd);}UDP Server#include "unp.h"#include <time.h>int main(int argc, char **argv){ int sockfd, clilen; struct sockaddr_in servaddr, cliaddr; char buff[MAXLINE], req[REQ_LEN]; time_t ticks;/* Create a socket */ sockfd = Socket(AF_INET, SOCK_DGRAM, 0);/* Initialize server’s address and well-known port */bzero(&servaddr, sizeof(servaddr)); servaddr.sin_family = AF_INET; servaddr.sin_addr.s_addr = htonl(INADDR_ANY); servaddr.sin_port = htons(13); /* daytime server *//* Bind server’s address and port to the socket */Bind(sockfd, (struct sockaddr *) &servaddr, sizeof(servaddr)); for ( ; ; ) {/* Wait for client request */len = sizeof(cliaddr); n = Recvfrom( sockfd, req, REQ_LEN, 0, &cliaddr, &clilen); /* Retrieve the system time */ ticks = time(NULL); snprintf(buff, sizeof(buff), "%.24s\r\n", ctime(&ticks)); /* Send to client*/Sendto(sockfd, buff, strlen(buff), 0, &cliaddr, clilen); }}TCP Connection SequenceServersocket()Clientbind()socket()listen()3-way handshakeconnect()accept()write()dataread()datawrite()read()EOFclose()read()close()UDP Connection SequenceServerClientsocket()socket()bind()sendto()datarecvfrom()datasendto()recvfrom()close()Socket API summary
  • int socket( int family, int type, int protocol)
  • Creates a network plug point that enables the client/server to communicate
  • int connect( int sockfd, const struct sockaddr *servaddr, socklen_t addrlen)
  • Enables a client to connect to a server.
  • int bind( int sockfd, const struct sockaddr *myaddr, socklen_t addrlen)
  • Allows a server to specify the IP address/port_number associated with a socketSocket API summary…
  • int listen(int sockfd, int backlog)
  • Allows the server to specify that a socket can be used to accept connections.
  • int accept(int sockfd, struct sockaddr *client_addr, socklen_t *addrlen)
  • Allows a server to wait till a new connection request arrives.
  • int close(int sockfd)
  • Terminates any connection associated with a socket and releases the socket descriptor.Socket API summary…
  • int read(int sockfd, void *buf, int count);
  • Read data from a TCP connection.
  • int write(int sockfd, void *buf, int count)
  • Write data to a TCP connection.
  • int sendto(int sockfd, void *msg, … )
  • Send a UDP message on a socket to specified destination
  • int recvfrom(int sockfd, void *buf, … )
  • Recv a UDP message on a socket along with address of sending source.
  • Similarly check man pages for sendmsg(), recvmsg().
  • Converting Between Address formats
  • From ASCII to numeric
  • “”  32-bit network byte ordered value
  • inet_aton(…) with IPv4
  • inet_pton(…) with IPv4 and IPv6
  • From numeric to ASCII
  • 32-bit value  “”
  • inet_ntoa(…) with IPv4
  • inet_ntop(…) with IPv4 and IPv6
  • Note – inet_addr(…) obsolete
  • cannot handle broadcast address “” (0xFFFFFFFF)
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