his, every RTT the window size is increased by only 1 segment per RTT, placing the sender in "congestion avoidance" mode instead "slow start." As the congestion window and hence the number of segments in flight starts to grow, the load on the link does as well. Hence, at a certain point in time, we can expect that the network will become over- loaded. This is the congestion phase. What happens is that somewhere in the network (the first overloaded router), a packet will get dropped. This means that the sent segment will not get acknowledged within the expected time. The transmitting TCP node will react to this by slowing down the congestion window to a lower value (originally, back to 1 segment size, but newer TCP versions such as TCP Reno are less defensive), effectively lowering the transmission speed. Chapter 2 System components, requirements, and interfaces The point of explaining this is to show that TCP protocol behavior is that a single lost packet is enough an indication of a congested network to drastically lower its transmission speed. This means that we have no reason to drop multiple packets belonging to one con- nection if a router gets congested. Another behavior is that when a router carrying multiple TCP connection starts tail-dropping packets, all TCP connections carried by the router will see lost packets, hence all lowering their transmission speed. This causes a phenomenon called known as "global synchronization," where each connection goes through the slow- start mechanism at the same time, causing very ineffective use of the available bandwidth. Both these issues with TCP as the most used higher layer transport protocol (between the UE/client and the server on the other side of the core network) tell us that we need a smarter mechanism at the routers to deal with a congested link. As said before, RED is an example of this. RED assumes that we can calculate the average length of the output queues in a router. When the average length of a queue increases above a certain level, RED does not sim