Unicast vs Multicast
Definition
Unicast and multicast are two ways of delivering a stream. In unicast, the sender transmits a separate copy to each receiver, so the load on the sender and on the network grows with the number of receivers. In multicast, the sender transmits one copy to a group address, and the network duplicates packets only where the paths to the receivers divide. Multicast suits one-to-many distribution such as sACN, NDI multicast and AES67. ANSI E1.31 (sACN) sends each universe to its own multicast address in the 239.255.0.0/16 range and also allows unicast. Without IGMP snooping on the switches, multicast is forwarded to every port like broadcast.
In the field
Sending sACN in unicast to 40 nodes multiplies the traffic leaving the console by 40, whereas multicast sends one stream that the switches replicate. Without IGMP snooping, the same multicast reaches every port, so snooping and a querier are configured before a system is moved to multicast.
Related lessons
Sources
- ANSI E1.31-2018, sACN : univers en multicast (239.255.x.y), unicast autorisé · tsp.esta.org
- RFC 1112 : multicast IP, remise à un groupe d'hôtes · rfc-editor.org
- RFC 4541 : sans snooping, le multicast est relayé sur toutes les interfaces · rfc-editor.org
- RFC 2365 (BCP 23) : 239.0.0.0/8 est l'espace multicast administrativement délimité ; 239.255.0.0/16 en est la portée locale (IPv4 Local Scope) · rfc-editor.org