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Network Interface Guide

Using Multicast

IP multicasting is only supported on AF_INET6 and AF_INETsockets of type SOCK_DGRAM and SOCK_RAW, and only on subnetworks for which the interface driver supports multicasting.

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.

Receiving IPv4 Multicast Datagrams

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

    struct ip_mreq mreq;
    setsockopt(sock, IPPROTO_IP, IP_ADD_MEMBERSHIP, &mreq, sizeof(mreq))
where mreq is the structure 
    struct ip_mreq {
        struct in_addr imr_multiaddr;   /* multicast group to join */
        struct in_addr imr_interface;   /* interface to join on */
    }
Each membership is associated with a single interface, and it is possible to join the same group on more than one interface. Specify imr_interface to be in6addr_any to choose the default multicast interface, or one of the host's local addresses to choose a particular (multicast-capable) interface.

To drop a membership, use 

    struct ip_mreq mreq;
    setsockopt(sock, IPPROTO_IP, IP_DROP_MEMBERSHIP, &mreq, sizeof(mreq))
where mreq contains the same values used to add the membership. The memberships associated with a socket are also dropped when the socket is closed or the process holding the socket is killed. More than one socket can claim a membership in a particular group, and the host remains a member of that group until the last claim is dropped.

Incoming multicast packets are accepted by the kernel IP layer if any socket has claimed a membership in the destination group of the datagram. Delivery of a multicast datagram to a particular socket is based on the destination port and the memberships associated with the socket (or protocol type, for raw sockets), just as with unicast datagrams. 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, every incoming multicast or broadcast UDP datagram destined to the shared port is delivered to all sockets bound to the port. For backwards compatibility reasons, this 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.

Sending IPv6 Multicast Datagrams

To send a 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 1, which prevents them from being forwarded beyond a single subnetwork. The socket option IPV6_MULTICAST_HOPS allows the hoplimit for subsequent multicast datagrams to be set to any value from 0 to 255, to control the scope of the multicasts: 

    uint_l;
    setsockopt(sock, IPPROTO_IPV6, IPV6_MULTICAST_HOPS, &hops,sizeof(hops))

Multicast datagrams with a hoplimit 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 hoplimit greater than one can be delivered to more than one subnet if one or more multicast routers are attached to the first-hop subnet. The IPv6 multicast addresses, unlike their IPv4 counterparts, contain explicit scope information encoded in the first part of the address. The defined scopes are (where X is unspecified):

ffX1::0/16

Node-local scope -- restricted to the same node

ffX2::0/16

Link-local scope

ffX5::0/16

Site-local scope

ffX8::0/16

Organization-local scope

ffXe::0/16

Global scope

An application can, separately from the scope of the multicast address, use different hoplimit values. For example, an application might perform an "expanding-ring search" for a network resource by sending a multicast query, first with a hoplimit of 0, and then with larger and larger hoplimits, until a reply is received, using (for example) the hoplimit sequence 0, 1, 2, 4, 8, 16, 32.

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 hoplimit 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: 

    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 (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: 

    uint_t loop;
    setsockopt(sock, IPPROTO_IPV6, IPV6_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 hoplimit greater than 1 can be delivered to the sending host on the other interface. The loopback control option has no effect on such delivery.

Receiving IPv6 Multicast Datagrams

Before a host can receive IP multicast datagrams, it must become a member of one, or more, IP multicast group. 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, and it is possible to join the same group on more than one interface. Specify ipv6_interface to be 0 to choose the default multicast interface, or 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 memberships associated with a socket are also dropped when the socket is closed or the process holding the socket is killed. More than one socket can claim a membership in a particular group, and the host remains a member of that group until the last claim is dropped.

Incoming multicast packets are accepted by the kernel IP layer if any socket has claimed a membership in the destination group of the datagram. Delivery of a multicast datagram to a particular socket is based on the destination port and the memberships associated with the socket (or protocol type, for raw sockets), just as with unicast datagrams. 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, every incoming multicast UDP datagram destined to the shared port is delivered to all sockets bound to the port. For backwards compatibility reasons, this 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.