In 2016, is IP multicast still a thing not to rely on with IPv4?

Is the only alternative still to duplicate packets?


This question is related to P2Peers (plural) topology, with one sender and many receivers over the Internet (broadcast like).

Lot of the multicast information is dated now (wikipedia -> 2010, others 2006), but many say that it is problematic and reserved to research.

The goal is to avoid sending the same data multiple times to different IP addresses.

2 Answers 2


Multicast is often used within an ISP network to distribute tv and radio channels. But it's limited to the ISP. Usually there are no multicast interconnects between ISPs so global multicast is still not really usable.


The real problem is that the Internet is made up of many different ISPs which are connected together. Each ISP is independent and negotiates any connections with other ISPs. Multicast comes in two basic flavors: DM (Dense Mode) and SM (Sparse Mode).

In DM (and at the start of, and periodically during, SM), multicast is flooded across every router and router port. This just isn't practical to do on the Internet, and ISPs which have nothing to do with the source or ultimate destinations of the multicast traffic would be forced to carry it, using their precious bandwidth for no reason.

SM initially, and periodically, floods the way DM does, but it will eventually back off of sending multicast traffic everywhere. That requires an RP (Rendezvous Point) for each multicast group. Someone would need to purchase and maintain these RPs.

Also, there are a limited number of multicast groups. The groups would need to be registered and assigned to companies the way IP addresses are. In a global multicast, only one assignee in the world could use a multicast group at any given time.

  • PIM ASM does not flood traffic initially. The multicast source sends packets which the first hop router (FHR) will send in unicast register messages, which contain the encapsulated multicast. These are sent towards the RP. If there are no receivers, the RP will send a Register Stop. This is not the same as flooding in dense mode though. Maybe you were referring to the RPT and SPT?
    – Daniel Dib
    Commented Mar 6, 2016 at 18:27

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