RED (Random Early Detection) Definition
This is a congestion control technique in which the router doubts the forth coming congestion and start discarding packet right before it is too late. Routers maintain an average of their queue lengths. When the queue length exceeds that value, it starts discarding forth coming packets.
When the source does not get any acknowledgement, it realizes that the packet is lost due to congestion and hence decreases its transmission rate and retransmits the discarded packets. One problem with this technique is that packets may also be missing due to other factors even though there is no congestion. Even then the source decreases its transmission rate unnecessarily.