Examples: Sending and receiving a multicast datagram

IP multicasting provides the capability for an application to send a single IP datagram that a group of hosts in a network can receive. The hosts that are in the group may reside on a single subnet or may be on different subnets that connect multicast-capable routers. Hosts may join and leave groups at any time. There are no restrictions on the location or number of members in a host group. A class D Internet address in the range 224.0.0.1 to 239.255.255.255 identifies a host group.

An application program can send or receive multicast datagrams by using the socket() API and connectionless SOCK_DGRAM type sockets. Multicasting is a one-to-many transmission method. You cannot use connection-oriented sockets of type SOCK_STREAM for multicasting. When a socket of type SOCK_DGRAM is created, an application can use the setsockopt() function to control the multicast characteristics associated with that socket. The setsockopt() function accepts the following IPPROTO_IP level flags:

• IP_ADD_MEMBERSHIP: Joins the multicast group specified.
• IP_DROP_MEMBERSHIP: Leaves the multicast group specified.
• IP_MULTICAST_IF: Sets the interface over which outgoing multicast datagrams are sent.
• IP_MULTICAST_TTL: Sets the Time To Live (TTL) in the IP header for outgoing multicast datagrams.
• IP_MULTICAST_LOOP: Specifies whether or not a copy of an outgoing multicast datagram is delivered to the sending host as long as it is a member of the multicast group.

The following examples enable a socket to send and receive multicast datagrams. The steps needed to differ from the steps needed to .

Example: Sending a multicast datagram

The following example enables a socket to perform the steps listed below and to send multicast datagrams:

1. Create an AF_INET, SOCK_DGRAM type socket.
2. Initialize a sockaddr_in structure with the destination group IP address and port number.
3. Set the IP_MULTICAST_LOOP socket option according to whether the sending system should receive a copy of the multicast datagrams that are transmitted.
4. Set the IP_MULTICAST_IF socket option to define the local interface over which you want to send the multicast datagrams.
5. Send the datagram.

#include 
#include 
#include 
#include 
#include 
#include 
 
 
struct in_addr        localInterface;
struct sockaddr_in    groupSock;
int                   sd;
int                   datalen;
char                  databuf[1024];
 
int main (int argc, char *argv[])
{
 
  /* ------------------------------------------------------------*/
  /*                                                             */
  /* Send Multicast Datagram code example.                       */
  /*                                                             */
  /* ------------------------------------------------------------*/
 
  /*
   * Create a datagram socket on which to send.
   */
  sd = socket(AF_INET, SOCK_DGRAM, 0);
  if (sd < 0) {
    perror("opening datagram socket");
    exit(1);
  }
 
  /*
   * Initialize the group sockaddr structure with a
   * group address of 225.1.1.1 and port 5555.
   */
  memset((char *) &groupSock, 0, sizeof(groupSock));
  groupSock.sin_family = AF_INET;
  groupSock.sin_addr.s_addr = inet_addr("225.1.1.1");
  groupSock.sin_port = htons(5555);
 
  /*
   * Disable loopback so you do not receive your own datagrams.
   */
  {
    char loopch=0;
 
    if (setsockopt(sd, IPPROTO_IP, IP_MULTICAST_LOOP,
                   (char *)&loopch, sizeof(loopch)) < 0) {
      perror("setting IP_MULTICAST_LOOP:");
      close(sd);
      exit(1);
    }
  }
 
  /*
   * Set local interface for outbound multicast datagrams.
   * The IP address specified must be associated with a local,
   * multicast-capable interface.
   */
  localInterface.s_addr = inet_addr("9.5.1.1");
  if (setsockopt(sd, IPPROTO_IP, IP_MULTICAST_IF,
                 (char *)&localInterface,
                 sizeof(localInterface)) < 0) {
    perror("setting local interface");
    exit(1);
  }
 
 
  /*
   * Send a message to the multicast group specified by the
   * groupSock sockaddr structure.
   */
  datalen = 10;
  if (sendto(sd, databuf, datalen, 0,
             (struct sockaddr*)&groupSock,
             sizeof(groupSock)) < 0)
  {
    perror("sending datagram message");
  }
}