IPv6 Multicasting (1) host API

Host API is defined in the basic API.
socket options
IPV6_MULTICAST_IF: specify the outgoing interface.
IPV6_MULTICAST_HOPS: specify the hoplimit of outgoing packets.
IPV6_JOIN_GROUP: join an IPv6 multicast group.
IPV6_LEAVE_GROUP: leave an IPv6 multicast group.
use ipv6_mreq{} to specify a group.
note that it takes an interface identifier, not an address.
   struct ipv6_mreq {
       struct in6_addr ipv6mr_multiaddr; /* IPv6 multicast addr */
       unsigned int    ipv6mr_interface; /* interface index */
   };

The advanced API can also be used to specify interfaces, but would be less portable than the basic one.
portability issues (as earlier mentioned)