Mechanism to throttle a network consumer when downstream broker is slow

Issue

We have a requirement to throttle a network consumer when the downstream broker is slow. The downstream broker is outside of our business and we have not influence over its behavior.

The scenario is Broker A -> sends to Broker B via a network consumer. Our network consumer is fast and always ACK's messages quickly. The peer broker accumulates messages and affects other destinations on the peer broker. When this condition occurs, we would like to throttle the downstream broker at the source.