Usually all TCP/IP traffic is unicast. This means that communication is performed between two hosts in the network. The sender puts the IP address of the destination into the IP header of each package and sends it on the network where it is propagated to the destination.
In the era of multimedia transmission a one-to-one communication is no longer appropriate. Transmitting video streams from one source to a group of clients would be an extreme waste of bandwith (just think about an internet TV broadcast station which wants to send a TV show to hundreds of clients in the internet).
Thus an addressing scheme for groups of hosts had to be established. The IANA saved the address space between 22.214.171.124 and 126.96.36.199 for multicast addressing. The multicast IP address defines a group of hosts participating on the communication. All messages sent to this group address are forwarded to all members of that group. Unfortunately routing packages through the net becomes more difficult, as routers and gateways have to keep track, in which network segments they have to to propagate the groups messages. The IGMP (Internet Group Management Protocol) informs routers and gateways about the status and group membership of hosts within the networks segment.
The use of multicasting is one of the main drawbacks using ACN. Although current versions of operating systems like Windows and Unix already support multicast transmission, there are almost no applications available using this new technology. Therefore multicasting is almost untested and proper configuration of hosts and routers might impose new challenges to ACN users.
How to obtain a valid Multicast Address ?
In order to establish a new multicast group, the initiator has to obtain a valid multicast IP address. Unlike DHCP for unicast addreses, MADCAP servers (UDP port 2535) provide a centralized solution for hosts to obtain a unique multicast address from a set of usable multicast IP addresses. In many smaller systems setting up a MADCAP server for the distribution of some few multicast addresses is not acceptable. Most systems will even require only one single multicast address to send messages from one single controller to many fixtures. E.17 EPI 10 proposes a mechanism to automatically generate a valid multicast address from the CID (component identificator) and parts of the components unicast address. This does not guarantee, that the found address is unique. It may happen that another group of hosts may already use the multicast address and gets flooded by addtional network packages of the new initator. Fortunately this type of collisions will be rare and the SDT layer of the ACN protocol is able to filter unwanted traffic from other groups.
IGMP (Internet Group Management Protocol)
RFC 2236, Internet Engineering Task Force (IETF)
MADCAP (Multicast Address Dynamic Client Allocation Protocol)
RFC 2730, Internet Engineering Task Force (IETF)