Three ways to move a packet

Switching policy decides when a router forwards. Store-and-forward waits for the whole packet; virtual cut-through forwards after the header when the full packet fits downstream; wormhole forwards flit by flit.

Credits keep buffers safe

A credit counts a free downstream buffer slot. The sender transmits only with credit in hand, and the receiver returns one credit per drained flit after the round-trip delay.

Depth follows Little's law: at 1 flit per cycle with a 6-cycle round trip, depth 6 sustains full rate. The sender below also reuses a credit that arrives in the same cycle, matching real bypass behavior.

Head-of-line blocking

A blocked wormhole header parks its flits across several routers. Every packet queued behind it waits, even when its own path is free; that is head-of-line blocking.

Partitioning buffers into virtual channels lets one class pass around another. The signature of credit starvation is local queue growth with delayed credit returns while neighboring links sit idle.

Testbench

The model below runs two credit lanes for 60 cycles against a 6-cycle return pipeline: depth 6 must never stall, depth 2 must deliver exactly 2 flits per round trip, and each credit count must stay inside its depth.

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module credit_sender #(parameter int DEPTH = 6) (
  input  logic clk,
  input  logic rst_n,
  input  logic want,
  input  logic ret_credit,
  output logic send,
  output logic [31:0] credit_o
);
  logic [31:0] credit;
  assign send = want && ((credit > 0) || ret_credit);
  assign credit_o = credit;
  always_ff @(posedge clk or negedge rst_n) begin
    if (!rst_n) credit <= DEPTH;
    else if (send && !ret_credit) credit <= credit - 1;
    else if (!send && ret_credit) credit <= credit + 1;
  end
endmodule

Related topics

• Arbitration and QoS

• Valid/Ready Handshakes

• XY routing and deadlock