Flows

Scope: Days AGO. The legacy actor implementation of flows is described in Legacy flows.

The scenario compiler lowers supported TOML into backend-neutral flow descriptors and source-owned generator state inside the SimulationImage.

Flow descriptors

Lowering creates a FlowDescriptor with a stable FlowId, source and target NodeId values, priority, a directed data route, and a reverse feedback route. Routes contain LinkId values, not actor IDs.

Generators

The source host LP owns a FlowGeneratorState. The compiler lowers supported traffic into these generator families:

• constant open-loop traffic for PacketDistribution
• source-owned TCP Reno or CUBIC state
• the Mellanox-form DCQCN controller and credit pacer for unreliable DCQCN flows, and the same controller with Go-back-N sender state and the window for RoCE queue pairs
• resolved stages for the collectives (RingAllReduce, AllGather, ReduceScatter, AllToAll, and SendRecv), the timers of compute stages, and, on the Spectrum-X fabric, the stage notifies that carry same-server messages

Target hosts own TCP cumulative-ACK state, the DCQCN notification point of unreliable flows, and the Go-back-N receiver of queue pairs, which echoes ECN on its ACKs and NACKs. Generator status records whether the next emission is scheduled, blocked, finished, or stopped beyond the scenario endpoint.

What lowering accepts

Lowering derives flow IDs and routes from canonical semantic keys. It rejects explicit flow IDs, dependency lists, per-flow routing, and explicit paths instead of discarding them. It also rejects nondeterministic packet distributions and transport options outside the implemented mechanism set. See Flow configuration for the accepted subsets.

Backend limits

Scalar and CPU execute every generator family. Metal and CUDA execute constant, TCP, DCQCN and RoCE queue-pair generators, including the DCQCN controller, the notification point and the queue-pair window, plus the standalone Rate generator that image builders can construct through the executor API (TOML lowering produces rate state only inside DCQCN). They also execute collective and compute stages over TCP and RoCE.