[Cerowrt-devel] Multicast

Mikael Abrahamsson swmike at swm.pp.se
Wed Apr 24 07:42:42 EDT 2013

On Wed, 24 Apr 2013, Dave Taht wrote:

>> I can't find any mention of IGMP proxying in the pimd manual I found. 
>> ISPs generally do not allow residential customers to talk PIM with the 
>> network, they expect an IGMP join, which is what an IGMP proxy would 
>> do.
> Sample application?

IPTV. IGMP join is sent from customer device to network which starts 
sending the multicast stream afterwards.

This either requires the IPTV box to have direct L2 connectivity to the 
ISP network, or it requires whatever home device connects the home to the 
ISP network to do IGMP proxying from LAN port to WAN port. It then also of 
course needs to create forwarding entries so the multicast received is 
forwarded to the LAN.

PIM is used within the ISP / enterprise for multicast routing. IGMP is 
used for end devices who would like to subscribe to a multicast group. A 
NAT router in a home environment needs to act like a hybrid device, it 
needs to use IGMP join to get the IPTV stream, but it then needs to 
forward it as if it was a router.

For instance:


Mikael Abrahamsson    email: swmike at swm.pp.se

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