L2 (Frames + PFC)

Scope: Legacy Days. For the Days AGO PFC state model, see PFC.

Legacy PFC requires the l2_pfc Cargo feature and link.mode = "Pfc". The topology inserts three actors on each directed fabric edge:

Scheduler
  -> PfcEgressGate
  -> Link
  -> PfcIngressPort
  -> downstream PacketSwitch

The gate and ingress exchange PfcFrame values; data uses LinkFrame::Data(Packet). The Link type can serialize either LinkFrame variant at the configured bit rate. The installed topology sends forward data through that actor, while the ingress sends pause and resume frames directly to the upstream gate through pfc_output.

PfcIngressPort owns eight byte queues and tracks occupancy per 802.1Q priority. It asserts XOFF at or above xoff, clears it at or below xon, enforces optional ingress capacities, and can refresh pause frames or retry a blocked drain. Its can_forward closure reads the downstream scheduler's shared QueueState to avoid forwarding into a full queue.

PfcEgressGate owns per-priority queues and paused_until deadlines derived from pause quanta. It starts service only for an unpaused priority.

Legacy rejects configured propagation delay with PFC because this actor pipeline does not combine the two mechanisms.