Communications between broker

When WM brokers communicate each other there is acknowledgement of receiving the message between the brokers.My question is that when a broker receives a message and it sends an acknowledgement, there is network failure and acknowledgement does not reach the message sender broker(1st broker who is expecting the acknowledgement).In this case what happens.Will 1st broker keep on sending the same message, will 1st broker keep on sending other messages, whether acknowledgement sending is a synchronous/asynchronous call etc.

You can also suggest me some documents to refer on communication between brokers.