PFC
Days AGO per-priority pause state inside switch-egress logical processes.
Scope: Days AGO. The legacy frame and PFC actor pipeline is described in Legacy L2 and PFC.
Days AGO implements Priority-based Flow Control as state inside each switch-egress LP, not as separate link actors.
PFC state
PfcIngressStatetracks byte occupancy, XON/XOFF thresholds, capacity, and asserted state for one monitored incoming link.PfcQueueState::paused_by_controllerrecords every downstream controller that currently pauses each priority.
Admission increases monitored occupancy. Service selection decreases it. Crossing XOFF emits a 64-byte PacketKind::Pfc pause on the reverse-link channel; crossing XON emits a resume. The control packet arrives as an ordinary RemoteArrival, and the egress queue remains paused until every asserting controller resumes it.
The mechanism is edge-triggered. Nonzero refresh and drain timers are rejected during lowering. A zero XOFF or pause-quanta entry disables that priority. PFC can coexist with lowered propagation delay because the reverse control channel carries the exact 64-byte serialization and propagation delay.
Backends
Scalar, CPU, Metal, and CUDA execute PFC, and every device run produces the same complete state as Scalar.
See L2 and PFC configuration for thresholds and validation rules.