To send a multicast datagram, specify an IP multicast address in the range ff00::0/8 as the destination address in a sendto(3SOCKET) call.
By default, IP multicast datagrams are sent with a hop limit of 1, which prevents them from being forwarded beyond a single subnetwork. The socket option IPV6_MULTICAST_HOPS allows the hoplimit for subsequent multicast datagrams to be set to any value from 0 to 255, to control the scope of the multicasts:
uint_l; setsockopt(sock, IPPROTO_IPV6, IPV6_MULTICAST_HOPS, &hops,sizeof(hops))
Multicast datagrams with a hoplimit of 0 are not transmitted on any subnet, but can be delivered locally if the sending host belongs to the destination group and if multicast loopback has not been disabled on the sending socket (see below). Multicast datagrams with hoplimit greater than one can be delivered to more than one subnet if one or more multicast routers are attached to the first-hop subnet. The IPv6 multicast addresses, unlike their IPv4 counterparts, contain explicit scope information encoded in the first part of the address. The defined scopes are (where X is unspecified):
Node-local scope -- restricted to the same node
An application can, separately from the scope of the multicast address, use different hoplimit values. For example, an application might perform an "expanding-ring search" for a network resource by sending a multicast query, first with a hoplimit of 0, and then with larger and larger hoplimits, until a reply is received, using (for example) the hoplimit sequence 0, 1, 2, 4, 8, 16, 32.
Each multicast transmission is sent from a single network interface, even if the host has more than one multicast-capable interface. (If the host is also a multicast router and the hoplimit is greater than 1, a multicast can be forwarded to interfaces other than originating interface.) A socket option is available to override the default for subsequent transmissions from a given socket:
uint_t ifindex; ifindex = if_nametoindex )"hme3"); setsockopt(sock, IPPROTO_IPV6, IPV6_MULTICAST_IF, &ifindex, sizeof(ifindex))where ifindex is the interface index for the desired outgoing interface. Revert to the default interface by specifying the value 0.
If a multicast datagram is sent to a group to which the sending host itself belongs (on the outgoing interface), a copy of the datagram is, by default, looped back by the IP layer for local delivery. Another socket option gives the sender explicit control over whether or not subsequent datagrams are looped back:
uint_t loop; setsockopt(sock, IPPROTO_IPV6, IPV6_MULTICAST_LOOP, &loop, sizeof(loop))where loop is 0 to disable loopback, and 1 to enable loopback. This option provides a performance benefit for applications that have only one instance on a single host (such as a router or a mail demon), by eliminating the overhead of receiving their own transmissions. It should not normally be used by applications that can have more than one instance on a single host (such as a conferencing program) or for which the sender does not belong to the destination group (such as a time querying program).
If the sending host belongs to the destination group on another interface, a multicast datagram sent with an initial hoplimit greater than 1 can be delivered to the sending host on the other interface. The loopback control option has no effect on such delivery.