A system for controlling data flow across a computer network comprising a first performance enhancing proxy device, PEP1, on a client-facing segment of the network and a second performance enhancing proxy device, PEP2, on a server-facing segment of the network, the system comprising: the PEP2, the PEP2 including: a receiver for receiving flow control information from across a control channel between the PEP 1 and the PEP2, the flow control information including round-trip packet latency measurement, Li, on the client-facing segment and transmit buffer congestion window information of the PEP1, wherein the flow control information is for each TCP connection on the client-facing segment; a determiner for determining whether each TCP connection of the client-facing segment has a congested TCP packet flow; and a congestion reliever for relieving the congestion of each TCP connection of the client-facing segment having a congested TCP packet flow, the congestion reliever comprising: a synchronizer for synchronizing a receive buffer receive window size of the PEP2 to be the same as a transmit buffer congestion window size of the PEP1, characterized in that the congestion reliever further comprises: a delayer for delaying acknowledgments, ACKs, received by the PEP2 from the PEP1 by a time Tack, wherein Tack a) is based on Li when Li is less than a preset threshold level Tlim, and b) is based on a preset threshold level, Tlim, level when Li is greater than or equal to the preset threshold level Tlim. the PEP2, the PEP2 including: a receiver for receiving flow control information from across a control channel between the PEP 1 and the PEP2, the flow control information including round-trip packet latency measurement, Li, on the client-facing segment and transmit buffer congestion window information of the PEP1, wherein the flow control information is for each TCP connection on the client-facing segment; a determiner for determining whether each TCP connection of the client-facing segment has a congested TCP packet flow; and a congestion reliever for relieving the congestion of each TCP connection of the client-facing segment having a congested TCP packet flow, the congestion reliever comprising: a synchronizer for synchronizing a receive buffer receive window size of the PEP2 to be the same as a transmit buffer congestion window size of the PEP1, characterized in that the congestion reliever further comprises: a delayer for delaying acknowledgments, ACKs, received by the PEP2 from the PEP1 by a time Tack, wherein Tack a) is based on Li when Li is less than a preset threshold level Tlim, and b) is based on a preset threshold level, Tlim, level when Li is greater than or equal to the preset threshold level Tlim. a receiver for receiving flow control information from across a control channel between the PEP 1 and the PEP2, the flow control information including round-trip packet latency measurement, Li, on the client-facing segment and transmit buffer congestion window information of the PEP1, wherein the flow control information is for each TCP connection on the client-facing segment; a determiner for determining whether each TCP connection of the client-facing segment has a congested TCP packet flow; and a congestion reliever for relieving the congestion of each TCP connection of the client-facing segment having a congested TCP packet flow, the congestion reliever comprising: a synchronizer for synchronizing a receive buffer receive window size of the PEP2 to be the same as a transmit buffer congestion window size of the PEP1, characterized in that the congestion reliever further comprises: a delayer for delaying acknowledgments, ACKs, received by the PEP2 from the PEP1 by a time Tack, wherein Tack a) is based on Li when Li is less than a preset threshold level Tlim, and b) is based on a preset threshold level, Tlim, level when Li is greater than or equal to the preset threshold level Tlim. a synchronizer for synchronizing a receive buffer receive window size of the PEP2 to be the same as a transmit buffer congestion window size of the PEP1, characterized in that the congestion reliever further comprises: a delayer for delaying acknowledgments, ACKs, received by the PEP2 from the PEP1 by a time Tack, wherein Tack a) is based on Li when Li is less than a preset threshold level Tlim, and b) is based on a preset threshold level, Tlim, level when Li is greater than or equal to the preset threshold level Tlim.