A simple solution to the problem of packet loss in reliable multicast is by having recipients acknowledge data packets through positive acknowledgements sent to the sender of the data packet. Because it is the sender who is responsible for tracking whether packets need to be resent, the term sender initiated protocol is sometimes used.

In sender initiated protocols, the sender must keep track of all the recipients. This might pose a scalability problem in large networks.

Another scalability problem becomes evident when a large number of recipients sends acknowledgements to the sender of a data packet, the network close to the sender can become congested by all the acknowledgements. This is denoted as ACK implosion problem.