Programming Interfaces Guide

Receiving IPv6 Multicast Datagrams

Before a host can receive IP multicast datagrams, the host must become a member of one, or more IP multicast groups. A process can ask the host to join a multicast group by using the following socket option:

    struct ipv6_mreq mreq;
    setsockopt(sock, IPPROTO_IPV6, IPV6_JOIN_GROUP, &mreq, sizeof(mreq)) 

where mreq is the structure:

    struct ipv6_mreq {
        struct in6_addr ipv6mr_multiaddr;   /* IPv6 multicast addr */
        unsigned int    ipv6mr_interface;   /* interface index */

Each membership is associated with a single interface. You can join the same group on more than one interface. Specify ipv6_interface to be 0 to choose the default multicast interface. Specify an interface index for one of the host's interfaces to choose that multicast-capable interface.

To leave a group, use:

    struct ipv6_mreq mreq;
    setsockopt(sock, IPPROTO_IPV6, IP_LEAVE_GROUP, &mreq, sizeof(mreq))  

where mreq contains the same values used to add the membership. The socket drops associated memberships when the socket is closed, or when the process that holds the socket is killed. More than one socket can claim a membership in a particular group. The host remains a member of that group until the last claim is dropped.

The kernel IP layer accepts incoming multicast packets if any socket has claimed a membership in the destination group of the datagram. Delivery of a multicast datagram to a particular socket is determined by the destination port and the memberships associated with the socket, or by the protocol type for raw sockets. To receive multicast datagrams sent to a particular port, bind to the local port, leaving the local address unspecified, such as INADDR_ANY.

More than one process can bind to the same SOCK_DGRAM UDP port if the bind(3SOCKET) is preceded by:

    int one = 1;
    setsockopt(sock, SOL_SOCKET, SO_REUSEADDR, &one, sizeof(one))  

In this case, all sockets that are bound to the port receive every incoming multicast UDP datagram destined to the shared port. For backward compatibility reasons, this delivery does not apply to incoming unicast datagrams. Unicast datagrams are never delivered to more than one socket, regardless of how many sockets are bound to the datagram's destination port. SOCK_RAW sockets do not require the SO_REUSEADDR option to share a single IP protocol type.

The definitions required for the new, multicast-related socket options are found in <netinet/in.h>. All IP addresses are passed in network byte-order.