L2 (Frames + PFC)

Layer-2 support is optional and is designed to have zero overhead when disabled:

• compile-time opt-in via Cargo features (l2, l2_pfc)
• runtime opt-in via config (link.mode = "Pfc")

Frame model

When L2 is compiled in, internal link pipelines use LinkFrame (src/l2/frame.rs):

• LinkFrame::Data(Packet)
• LinkFrame::Pfc(PfcFrame) (only with l2_pfc)

Frames carry timestamps and sizes so that links can serialize them at a given rate.

Link serializer

Link (src/l2/link.rs) is a small model that:

• queues LinkFrames,
• schedules a delivery event after size_bits / rate_bps,
• forwards the frame downstream via output: Output<LinkFrame>.

This lets L2 control frames and data frames share the same serialization logic.

PFC (802.1Qbb-style pause)

PFC is implemented by two per-edge actors (src/l2/pfc.rs):

• Ingress (PfcIngressPort): per-priority buffering, XON/XOFF thresholds, generates pause/resume frames upstream.
• Egress (PfcEgressGate): per-priority gating of outbound frames based on received pause frames.

Pipeline constructed by the topology builder (Topology::attach_pfc_link):

Scheduler
  -> PfcEgressGate (gates per priority)
  -> Link (serializes frames)
  -> PfcIngressPort (buffers + generates PFC)
  -> downstream PacketSwitch

PfcIngressPort.pfc_output -> PfcEgressGate.pfc_received (reverse direction)

Ingress behavior

PfcIngressPort tracks per-priority occupancy and:

• asserts pause when occupancy >= xoff[prio],
• clears pause when occupancy <= xon[prio],
• optionally refreshes pause frames and schedules drain retries when downstream is congested.

Forward progress depends on a can_forward(packet) closure, which is connected to scheduler queue occupancy (QueueState) so that ingress can avoid triggering downstream drops.

Egress behavior

PfcEgressGate maintains:

• paused_until[prio] deadlines,
• per-priority queues,
• a scheduled “resume” event at the earliest next deadline.

It only drains frames for priorities that are not paused, and time-stamps drained frames at the current simulated time.