To send an IPv6 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 one, which prevents the datagrams from being forwarded beyond a single subnetwork. The socket option IPV6_MULTICAST_HOPS allows the hop limit for subsequent multicast datagrams to be set to any value from 0 to 255. This ability is used to control the scope of the multicasts:
uint_l; setsockopt(sock, IPPROTO_IPV6, IPV6_MULTICAST_HOPS, &hops,sizeof(hops))
You cannot transmit multicast datagrams with a hop limit of zero on any subnet, but you can deliver the datagrams locally if:
The sending host belongs to the destination group
Multicast loopback on the sending socket is enabled
You can deliver multicast datagrams with a hop limit that is greater than one to more than one subnet if the first-hop subnet attaches to one or more multicast routers. The IPv6 multicast addresses, unlike their IPv4 counterparts, contain explicit scope information that is 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 hop limit values. For example, an application might perform an expanding-ring search for a network resource by sending a multicast query, first with a hop limit of 0, and then with larger and larger hop limits, until a reply is received.
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 hop limit is greater than 1, a multicast can be forwarded to interfaces other than the 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, 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 to loop back subsequent datagrams:
uint_t loop; setsockopt(sock, IPPROTO_IPV6, IPV6_MULTICAST_LOOP, &loop, sizeof(loop))
where loop is zero to disable loopback and one 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. 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) should not use this option.
If the sending host belongs to the destination group on another interface, a multicast datagram sent with an initial hop limit greater than 1 can be delivered to the sending host on the other interface. The loopback control option has no effect on such delivery.