Nokia Bell Labs Nokia Bell Labs Independent This specification defines the Prague congestion control scheme, which is derived from DCTCP and adapted for Internet traffic by implementing the Prague L4S requirements. Over paths with L4S support at the bottleneck, it adapts the DCTCP mechanisms to achieve consistently low latency and full throughput. It is defined independently of any particular transport protocol or operating system, but notes are added that highlight issues specific to certain transports and OSs. It is mainly based on the current default options of the reference Linux implementation of TCP Prague, but it includes experience from other implementations where available. It separately describes non-default and optional parts, as well as future plans. The implementation does not satisfy all the Prague requirements (yet) and the IETF might decide that certain requirements need to be relaxed as an outcome of the process of trying to satisfy them all. In two cases, research code is replaced by placeholders until full evaluation is complete.