When a sender starts and there's no receiver already attached then the first 250 messages the sender sends get released. After that the router waits for the a receiver to attach before issuing more credit to the sender. The proton c++ simple_send and simple receive clients expose this problem.
1. Start router with attached config file
2. Start sender
simple_send -a 127.0.0.1:5672/multicast/q1 -m 500
3. Start receiver
simple_recv -a 127.0.0.1:5672/multicast/q1 -m 500
The sender competes with 'all messages confirmed'.
The receiver is waiting for the second 250 messages.