congestion notification message (DCB) (CNM)

In Data Center Bridging( DCB), the bridges represent the congestion points that continuously monitor the queues at the output.

If the queue grows above a predefined congestion value, the bridges send a Congestion Notification Message (CNM) back to the sending station. In the case of minor congestion, an overload is indicated after about one hundred data packets; in the case of high congestion, after about 10 data packets. The CNM message contains various data fields from which the sender can identify the reason for the data congestion. The CNM message can be transmitted via the Quantized Congestion Notification( QCN).

After the sender has received the CNM message, it lowers the transmission rate in order to solve the congestion situation. If no more congestion situation is detected after a few data packets, the sending station switches back to the previous data rate.