Systems and methods of optimizing packet flow in a network are disclosed. An internetwork has a first network and plural second networks. A respective packet-in-flight counter is established for each one of the plural second networks and a respective maximum packet-in-flight indication for each one of the plural second networks. The respective packet-in-flight counter for each one of the plural second networks when a request packet is sent from any node in the first network to any node in that one of the plural second networks, or a response packet is received from any node that one of the plural second networks. it is determined whether a second request packet can be sent from any node in the first network to any of the plurality of nodes in one of the plural second networks, based on processing a value of the respective packet-in-flight counter for that one of the plural second networks and a value of the respective maximum packet-in-flight indication for that one of the plural second networks. The respective maximum packet-in-flight indication for each one of the plural second networks is adjusted based on a statistic that is indicative of roundtrip time for request packets sent from any node of the first network to any of the plurality of nodes in that one of the plural second networks.