Explain how to adapt the algorithm for reliable multicast over IP multicast to eliminate the hold- back queue – so that a received message that is not a duplicate can be delivered immediately, but without any ordering guarantees. Hint: use sets of sequence numbers to represent the messages that have been delivered so far.

What will be an ideal response?

Messages in the hold-back queue serve as markers indicating missing messages. In the original prootocol, missing messages are retrieved and once a message in the hold-back queue has a sequence number 1 greater than the last message R-delivered, it too is R-delivered and removed from the queue. However it is not necessary to hold such messages back. Instead, they can be R-delivered immediately and a record (a set) kept, for each group, of the sequence numbers of all missing messages. Once a missing message has been retrieved, it too is immediately R-delivered and its sequence number removed from the set.