Communications Programming Concepts

IP Multicasts

The use of Internet Protocol (IP) multicasting enables a message to be transmitted to a group of hosts, instead of having to address and send the message to each group member individually. Internet addressing provides for Class D addressing that is used for multicasting.

When a datagram socket is defined, the setsockopt subroutine can be modified. To join or leave a multicast group, use the setsockopt subroutine with the IP_ADD_MEMBERSHIP or IP_DROP_MEMBERSHIP flags. The interface that is used and the group used are specified in an ip_mreq structure that contains the following fields:

struct ip_mreq{
   struct in_addr imr.imr_interface.s_addr;
   struct in_addr imr.imr_multiaddr.s_addr;
}

The in_addr structure is defined as:

struct in_addr{
  ulong s_addr;
}

In order to send to a multicasting group it is not necessary to join the groups. For receiving transmissions sent to a multicasting group, membership is required. For multicast sending, use an IP_MULTICAST_IF flag with the setsockopt subroutine. This specifies the interface to be used. It may be necessary to call the setsockopt subroutine with the IP_MULTICAST_LOOP flag in order to control the loopback of multicast packets. By default, packets are delivered to all members of the multicast group including the sender, if it is a member. However, this can be disabled with the setsockopt subroutine using the IP_MULTICAST_LOOP flag.

The setsockopt subroutine flags that are required for multicast communication and used with the IPPROTO_IP protocol level follow:

IP_ADD_MEMBERSHIP	Joins a multicast group as specified in the OptionValue parameter of type struct ip_mreq. A maximum of 20 groups may be joined per socket.
IP_DROP_MEMBERSHIP	Leaves a multicast group as specified in the OptionValue parameter of type struct ip_mreq. Only allowable for processes with a user ID (UID) value of zero.
IP_MULTICAST_IF	Permits sending of multicast messages on an interface as specified in the OptionValue parameter of type struct ip_addr. An address of INADDR_ANY (0x000000000) removes the previous selection of an interface in the multicast options. If no interface is specified then the interface leading to the default route is used.
IP_MULTICAST_LOOP	Sets multicast loopback, determining whether or not transmitted messages are delivered to the sending host. An OptionValue parameter of type char is used to control loopback being on or off.
IP_MULTICAST_TTL	Sets the time-to-live (TTL) for multicast packets. An OptionValue parameter of type char is used to set this value between 0 and 255.

The following examples demonstrate the use of the setsockopt function with the protocol level set to Internet Protocol (IPPROTO_IP).

To mark a socket for sending to a multicast group on a particular interface:

struct ip_mreq imr;
setsockopt(s, IPPROTO_IP, IP_MULTICAST_IF, &imr.imr_interface.s_addr, sizeof(struct in_addr));

To disable the loopback on a socket:

char loop = 0;
setsockopt(s, IPPROTO_IP, IP_MULTICAST_LOOP, &loop, sizeof(char));

To allow address reuse for binding multiple multicast applications to the same IP group address:

int on = 1;
setsockopt(s, SOL_SOCKET, SO_REUSEADDR, &on, sizeof(int));

To join a multicast group for receiving:

struct ip_mreq imr;
setsockopt(s, IPPROTO_IP, IP_ADD_MEMBERSHIP, &imr, sizeof(struct ip_mreq));

To leave a multicast group:

struct ip_mreq imr;
setsockopt(s, IPPROTO_IP, IP_DROP_MEMBERSHIP, &imr, sizeof(struct ip_mreq));

The getsockopt function can also be used with the multicast flags to obtain information about a particular socket.

IP_MULTICAST_IF	Retrieves the interface's IP address.
IP_MULTICAST_LOOP	Retrieves the specified looping mode from the multicast options.
IP_MULTICAST_TTL	Retrieves the time-to-live in the multicast options.