An intermediary node on a data communications network implements a flow control algorithm to control network congestion. The intermediary node receives messages destined for servers or other upstream nodes from clients or other downstream nodes and determines whether to forward the messages based on the flow control algorithm, which may be adaptive. The algorithm may consider explicit back-off messages received from upstream nodes, failure of upstream nodes to respond to messages forwarded by the intermediary node, and/or other criteria. When a back-off condition applies, the intermediary node refrains from forwarding messages it receives and sends back-off messages to nodes initiating the messages. Back-off messages may include a back-of time period and/or other back-off criteria.