A repair server polls multiple transmit servers to accumulate as many of {HEAD ~ the packets} missing from {TAIL ~ the multicast session} as possible 
A repair server polls multiple transmit servers to accumulate as many of { HEAD ~ the packets } missing from { TAIL ~ the multicast session } as possible
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH