By default, Windows machines use the algorithm NewReno. Older algorithms like NewReno are known for causing horrible buffer-bloat in comparison to algorithms li

By default, Windows machines use the algorithm NewReno. Older algorithms like NewReno are known for causing horrible buffer-bloat in comparison to algorithms like VEGAS which is used in Linux. The reason is pretty simple. The two main metrics used for determining when to send a packet in TCP are round trip delay and packet loss. Reno/NewReno slow their send rate mainly when they detect lost packets, whereas VEGAS and similar mainly detect round trip delay but also respond to loss. The problem with primarily loss-based strategies is that most routers won't drop packets until their buffers are full. In modern times these buffers can be very large (many seconds). TCP Reno will keep sending packets until downstream routers have full buffers and drop packets, which could be when they're already holding seconds worth of data. This is buffer-bloat. Any packets that makes it to these routers will spend seconds waiting to be put on the wire. VEGAS on the other hand will try to maintain a constant round trip delay. It uses TCP's ACK packets to gauge how long it takes packets to go back and forth and reduces send rate when this starts to rise. This keeps router buffers empty, delay low, and packet loss near zero. BBR is a further enhancement of the VEGAS delay sensing strategy. As mentioned in this chain, CoDel is one strategy for "fixing' loss based aggressive protocols like TCP Reno. It detects "flows" (IP/port pair combinations) that are causing the local outbound buffers to overflow and starts selectively dropping packets on them. If all internet protocols were delay based like TCP VEGAS CoDel would not be needed to keep traffic flows "fair". Without using something like CoDel on your routers to punish aggressive strategies, scheduling algorithms like NewReno will cause your routers outgoing buffers to always be full and out-compete friendlier traffic like VEGAS that cuts itself back when buffers fill.