At an application executing in conjunction with a vSwitch in a host system, a CWND value is computed corresponding to a flow from a VM using a period measured by a timer and a number of packets of the flow received and acknowledged in response packets, the number being counted by a counter, the timer being associated with a packet of the flow. The CWND value is stored in a field in a response packet received from a receiver of the flow, the field being designated for carrying a RWND value, the response packet corresponding to a packet in the flow. The storing forms a modified response packet.