This is true, and the congestion mentioned here was subtle and not called out - typically flows are handled in a stateless manner by load balancers that hash on

This is true, and the congestion mentioned here was subtle and not called out - typically flows are handled in a stateless manner by load balancers that hash on some set of MAC/IP/PORT features of the packet. This is where congestion occurs and the paper mentions it here: All that is needed for congestion is for two large flows to hash to the same intermediate link; this hot spot will persist for the life of the flows and cause delays for any other messages that also pass over the affected link. It makes logical sense, but I'd love to see the evidence for this.