IPv6 provides enhanced support for multicast applications, including a more granular scope for multicast addressing and socket options that enable an application to use this support. Table 1 lists IPv4 multicast setsockopt() and getsockopt() options, the equivalent IPv6 multicast options, and protocol-independent multicast options.

In addition to the changes in the setsockopt() and getsockopt() options, the input and output parameters specified for these options are also changed when compared to IPv4. For example, selecting an outgoing interface for sending multicast IPv6 datagram involves passing an interface index that identifies the interface versus passing the IP address of the interface. For a detailed description of the IPv6 multicast options see Options to control sending of multicast packets.

An important consideration in updating your multicast application for IPv6 is how these changes are provided to the other partner applications participating in these multicast operations. For example, if a partner application in the network that is receiving these multicast packets is not updated, then the application sending the multicast datagrams might need to send them twice, once to an IPv4 multicast address and once to an IPv6 multicast address. Also, in order to perform this type of processing the application needs to create two separate sockets, an AF_INET socket and a AF_INET6 socket. There is no support equivalent to IPv4-mapped IPv6 addresses that would allow an AF_INET6 socket to be used in sending IPv4 multicast packets. As an alternative solution, first enable all the receiver applications for IPv6 and then enable the sender applications.