  This memo explains that deploying active queue management (AQM) to counter
bufferbloat will not prevent TCP from overriding the AQM and building large
queues in a range of not uncommon scenarios. This is a brief paper study to
explain this effect which was observed in a number of low latency testbed
experiments.
  To keep its queue short, an AQM drops (or marks) packets to make the TCP
flow(s) traversing it reduce their packet rate. Nearly all TCP implementations
will not run at less than two packets per round trip time (RTT). 2pkt / RTT
need not imply low bit-rate if the RTT is small. For instance, it represents
2Mb/s over a 6ms round trip. When a few TCP flows share a link, in certain
scenarios, including regular broadband and data centres, no matter how much the
AQM signals to the flows to keep the queue short, they will not obey, because
it is impossible for them to run below this floor. The memo proposes the
necessary modification to the TCP standard.
