Imagine that there is a video streaming client that takes its content stream
from a combination of a multicast channel and a unicast channel. At a coarse
level, the client receives packets over the multicast link and requests
retransmission over the unicast.
There was some very preliminary work recently presented to the (RTP) “PAYLOAD”
working group describing a proposal to optimize a HTTP Adaptive Streaming video
infrastructure (i.e., "HTTP Live Streaming" or “DASH”) when a live video event
is carried over a multicast network. The proposal currently has lots of issues
to be fleshed out, and (AFAIK) it’s still unclear whether it'll become a
working group - but if you’re interested, see the thread here:
https://mailarchive.ietf.org/arch/search/?email_list=payload&qdr=y&gbt=1&index=N4qSH2GcGVdMwDUNbWEdDCy5bjs
Ross.