Sending IPv4 Multicast Datagrams

To send a multicast datagram, specify an IP multicast address in the range 224.0.0.0 to 239.255.255.255 as the destination address in a sendto(3SOCKET) call.

By default, IP multicast datagrams are sent with a time-to-live (TTL) of 1, which prevents them from being forwarded beyond a single subnetwork. The socket option IP_MULTICAST_TTL allows the TTL for subsequent multicast datagrams to be set to any value from 0 to 255, to control the scope of the multicasts:

    u_char ttl;
    setsockopt(sock, IPPROTO_IP, IP_MULTICAST_TTL, &ttl,sizeof(ttl))

Multicast datagrams with a TTL 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 TTL greater than one can be delivered to more than one subnet if one or more multicast routers are attached to the first-hop subnet. To provide meaningful scope control, the multicast routers support the notion of TTL "thresholds", which prevent datagrams with less than a certain TTL from traversing certain subnets. The thresholds enforce the following convention that multicast datagrams with initial TTL:

0	Are restricted to the same host
1	Are restricted to the same subnet
32	Are restricted to the same site
64	Are restricted to the same region
128	Are restricted to the same continent
255	Are unrestricted in scope

"Sites" and "regions" are not strictly defined, and sites can be subdivided into smaller administrative units, as a local matter.

An application can choose an initial TTL other than the ones listed above. For example, an application might perform an "expanding-ring search" for a network resource by sending a multicast query, first with a TTL of 0, and then with larger and larger TTLs, until a reply is received, using (for example) the TTL sequence 0, 1, 2, 4, 8, 16, 32.

The multicast router refuses to forward any multicast datagram with a destination address between 224.0.0.0 and 224.0.0.255, inclusive, regardless of its TTL. This range of addresses is reserved for the use of routing protocols and other low-level topology discovery or maintenance protocols, such as gateway discovery and group membership reporting.

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 TTL 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:

    struct in_addr addr;
    setsockopt(sock, IPPROTO_IP, IP_MULTICAST_IF, &addr, sizeof(addr))

where addr is the local IP address of the outgoing interface you want. Revert to the default interface by specifying the address INADDR_ANY. The local IP address of an interface is obtained with the SIOCGIFCONF ioctl. To determine if an interface supports multicasting, fetch the interface flags with the SIOCGIFFLAGS ioctl and test if the IFF_MULTICAST flag is set. (This option is intended primarily for multicast routers and other system services specifically concerned with internet topology.)

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:

    u_char loop;
    setsockopt(sock, IPPROTO_IP, IP_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 TTL greater than 1 can be delivered to the sending host on the other interface. The loopback control option has no effect on such delivery.