Outline the design of a scheme that uses message retransmissions with IP multicast to overcome the problem of dropped messages. Your scheme should take the following points into account:

Question

i) there may be multiple senders;
ii) generally only a small proportion of messages are dropped;
iii) unlike the request-reply protocol, recipients may not necessarily send a message within any particular time limit.
Assume that messages that are not dropped arrive in sender ordering.

Answer 1

To allow for point (i) senders must attach a sequence number to each message. Recipients record last sequence number from each sender and check sequence numbers on each message received.
For point (ii) a negative acknowledgement scheme is preferred (recipient requests missing messages, rather than acknowledging all messages). When they notice a missing message, they send a message to the sender to ask for it. To make this work, the sender must store all recently sent messages for retransmission. The sender re-transmits the messages as a unicast datagram.
Point (iii) - refers to the fact that we can’t rely on a reply as an acknowledgement. Without acknowledgements, the sender will be left holding all sent messages in its store indefinitely. Possible solutions: a) senders discards stored messages after a time limit b) occasional acknowledgements from recipients which may be piggy backed on messages that are sent.
Note requests for missing messages and acknowledgments are simple - they just contain the sequence numbers of a range of lost messages.