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IP Multicast

In early IP networks, a packet could be sent to either a single device (unicast) or to all devices (broadcast). A single transmission destined for a group of devices was not possible. However, during the past few years, a new set of applications has emerged. These applications use multicast transmissions to enable efficient communication between groups of devices. Data is transmitted to a single multicast IP address and received by any device that needs to obtain the
transmission.

Multicast addressing

Multicast devices use Class D IP addresses to communicate. These addresses are contained in the range encompassing 224.0.0.0 through 239.255.255.255. For each multicast address, there exists a set of zero or more hosts that listen for packets transmitted to the address. This set of devices is called a host group. A host that sends packets to a specific group does not need to be a member of the group. The host might not even know the current members in the group. There are two types of host groups:

Permanent: Applications that are part of this type of group have an IP address permanently assigned by the IANA. Membership in this type of host group is not permanent; a host can join or leave the group as required. A permanent group continues to exist even if it has no members. The list of IP addresses assigned to permanent host groups is included in RFC 3232. These reserved addresses include:

– 224.0.0.0: Reserved base address

– 224.0.0.1: All systems on this subnet

– 224.0.0.2: All routers on this subnet

– 224.0.0.9: All RIP2 routers

Other address examples include those reserved for OSPF (refer to 5.6, “Open Shortest Path First (OSPF)” on page 196). They include:

– 224.0.0.5: All OSPF routers

– 224.0.0.6: OSPF designated routers

Additionally, IGMPv3 (defined in RFC 3376), reserves the following address:

– 224.0.0.22: All IGMPv3-capable multicast routers

An application can use DNS to obtain the IP address assigned to a permanent host group using the domain mcast.net. It can determine the permanent group from an address by using a pointer query in the domain 224.in-addr.arpa.

Transient: Any group that is not permanent is transient. The group is available for dynamic assignment as needed. Transient groups cease to exist when the number of members drops to zero.