In one embodiment, a first networking device sends, to a second networking device, a training frame of a transmitter training signal (TTS) that includes a request for the second networking device to enable a protocol on the second networking device. In addition, the first networking device receives, from the second networking device, an indication that the second networking device has enabled the protocol on the second networking device. The first networking device may also send, using the protocol, first flow-control information indicating a number of data packets sent by the first networking device to the second networking device and a number of acknowledgments received from the second networking device, in addition to receiving second flow-control information indicating a number of data packets received at the second networking device from the first networking device and a number of acknowledgments sent by the second networking device to the first networking device.