Skip to main content

2. The Service Interface for Requesting IP Multicast Reception

2. The Service Interface for Requesting IP Multicast Reception​

Within an IP system, there is (at least conceptually) a service interface used by upper-layer protocols or application programs to ask the IP layer to enable and disable reception of packets sent to specific IP multicast addresses. In order to take full advantage of the capabilities of IGMPv3, a system's IP service interface must support the following operation:

IPMulticastListen ( socket, interface, multicast-address,
                    filter-mode, source-list )

where:

  • "socket" is an implementation-specific parameter used to distinguish among different requesting entities (e.g., programs or processes) within the system; the socket parameter of BSD Unix system calls is a specific example.

  • "interface" is a local identifier of the network interface on which reception of the specified multicast address is to be enabled or disabled. Interfaces may be physical (e.g., an Ethernet interface) or virtual (e.g., the endpoint of a Frame Relay virtual circuit or the endpoint of an IP-in-IP "tunnel"). An implementation may allow a special "unspecified" value to be passed as the interface parameter, in which case the request would apply to the "primary" or "default" interface of the system (perhaps established by system configuration). If reception of the same multicast address is desired on more than one interface, IPMulticastListen is invoked separately for each desired interface.

  • "multicast-address" is the IP multicast address, or group, to which the request pertains. If reception of more than one multicast address on a given interface is desired, IPMulticastListen is invoked separately for each desired multicast address.

  • "filter-mode" may be either INCLUDE or EXCLUDE. In INCLUDE mode, reception of packets sent to the specified multicast address is requested only from those IP source addresses listed in the source-list parameter. In EXCLUDE mode, reception of packets sent to the given multicast address is requested from all IP source addresses except those listed in the source-list parameter.

  • "source-list" is an unordered list of zero or more IP unicast addresses from which multicast reception is desired or not desired, depending on the filter mode. An implementation MAY impose a limit on the size of source lists, but that limit MUST NOT be less than 64 addresses per list. When an operation causes the source list size limit to be exceeded, the service interface MUST return an error.

For a given combination of socket, interface, and multicast address, only a single filter mode and source list can be in effect at any one time. However, either the filter mode or the source list, or both, may be changed by subsequent IPMulticastListen requests that specify the same socket, interface, and multicast address. Each subsequent request completely replaces any earlier request for the given socket, interface and multicast address.

Previous versions of IGMP did not support source filters and had a simpler service interface consisting of Join and Leave operations to enable and disable reception of a given multicast address (from all sources) on a given interface. The equivalent operations in the new service interface follow:

The Join operation is equivalent to

IPMulticastListen ( socket, interface, multicast-address,
                    EXCLUDE, {} )

and the Leave operation is equivalent to:

IPMulticastListen ( socket, interface, multicast-address,
                    INCLUDE, {} )

where {} is an empty source list.

An example of an API providing the capabilities outlined in this service interface is in [FILTER-API].

Last updated on