[ih] Why was DVMRP put into IGMP?

[John Kristoff]

I'm curious why DVMRP was put into IGMP, as opposed to being put on top
of UDP or perhaps even as an extension to RIP, which DVMRP's close
cousin used.

Maybe the use of IGMP by DVMRP was just to keep IP multicast protocols
essentially together, or was there some other rationale?

No other IP multicast routing protocol that I'm aware went this route
and only a small handful of little used measurement features ended up
going into IGMP, performing something beyond its core functionality of
group membership maintenance.

Perhaps Steve Deering or something who was involved in this early work
can shed some light on the original ideas?  Thank you,

John