  On the Internet, sub-millisecond queueing delay and capacity-seeking have
traditionally been considered mutually exclusive. We introduce a service that
offers both: Low Latency Low Loss Scalable throughput (L4S). When tested under
a wide range of conditions emulated on a testbed using real residential
broadband equipment, queue delay remained both low (median 100--300 $\mu$s) and
consistent (99th percentile below 2 ms even under highly dynamic workloads),
without compromising other metrics (zero congestion loss and close to full
utilization). L4S exploits the properties of `Scalable' congestion controls
(e.g., DCTCP, TCP Prague). Flows using such congestion control are however very
aggressive, which causes a deployment challenge as L4S has to coexist with
so-called `Classic' flows (e.g., Reno, CUBIC). This paper introduces an
architectural solution: `Dual Queue Coupled Active Queue Management', which
enables balance between Scalable and Classic flows. It counterbalances the more
aggressive response of Scalable flows with more aggressive marking, without
having to inspect flow identifiers. The Dual Queue structure has been
implemented as a Linux queuing discipline. It acts like a semi-permeable
membrane, isolating the latency of Scalable and `Classic' traffic, but coupling
their capacity into a single bandwidth pool. This paper justifies the design
and implementation choices, and visualizes a representative selection of
hundreds of thousands of experiment runs to test our claims.
