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I've been searching for the reasons why IP multicast isn't widely supported on the Public Internet, and a commonly-cited reason is the difficulty ISPs have in tracking Multicast usage for later billing. Given this difficulty, since ISPs control the routers and they're not forced to support Multicast (as per IPv4), they just disable it. I couldn't find what this difficulty was though.

Since an ISP has full control of any inbound and outbound traffic, be it Unicast or Multicast, what's the difficulty in tracking and billing the latter that does not exist in the former?

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I can't see that being the case.

Most ISPs will poll a customer's interface using SNMP and record the deltas between polling runs for the input and output byte count (inOctets OID, outOctets OID, etc.)

On most, if not all network equipment, multicast traffic increments these counters, so the ISP would simply bill you for the additional usage on the port with no problem at all.

I'd imagine the real reasons are some combination between the challenges of establishing cross-ISP multicast connectivity and making sure the existing edge/border routers can accommodate multicast traffic without choking (there's a limit to the number of IGMP groups a device can effectively be subscribed to).

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