Network to network . Host to host Can be provided with software or hardware solutions. Easy to build over ATM or Frame Relay. To be used when integrity and security of data is a concern. 3-4 hands raised.
Nobody asks for it, so no softs are developed, so nobody asks for it.
Egg and chicken problem. Compatible with existing routers. IOS not older than March, though. PIM-SM deployed in Sprint Internet network . MSDP peering between intra-domain RPs .