Multicast

Multicast is an IP transmission method defined by the Internet Engineering Task Force (IEFT) and designed to send datagrams to a specific group of receptors. When a group of hosts is interested in receiving the same stream (for example, a video), it can be sent in an only datagrams transmission flow, which will be replicated in the closest routers to the destination to create a distribution tree. 

The use of multicast methods has many benefits when it is compared with unicast applications. A unicast transmission must send duplicated data streams to communicate with multiple hosts (in the shared links by two or more hosts). This behavior implies an unnecessary overuse of resources in traffic source and in the network. The multicast applications are much more efficient for the network since they do not waste the bandwidth and it is easier for the source to process and generate an only data stream. On the other hand, multicast allows communication between traffic sources with receptors, of which the source has no information. The network uses different multicast protocols to route the packets to the hosts, without the source knowing these devices. 

In general, the use of multicast schemes presents a benefit in forwarding and resources level, and an added complexity in the control level (multicast protocols, adjacency state, quality of service, etc.). For these reasons, it is important to analyze why the scheme will be used and which the best way to apply it is.

The main protocols used in Multicast environments are: 

  • Internet Group Management Protocol (IGMP) and Multicast Listener Discovery  IPv6 (MLD); whose application range is between hosts and routers. 
  • Protocol Independent Multicast (PIM), Distance Vector Multicast Routing Protocol (DVMRP), Multicast OSPF (MOSPF) and MultiProtocol BGP (MP-BGP); which are used in routers as multicast routing protocols to transport routing information between the source and the receptors. 
  • Multicast Source Discovery Protocol (MSDP); This protocol is used for the connection of Multicast domains. 
  • Multicast DNS (MDNS); service of names resolution defined in the RFC 6762, used, in general, in small networks to make up for the lack of DNS servers.

More information at: 

https://tools.ietf.org/html/rfc2460
https://tools.ietf.org/html/rfc4601
https://tools.ietf.org/html/rfc3973
https://tools.ietf.org/html/rfc6762

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