File size: 1,328 Bytes
7df9186 | 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 | // Subtracts 0x800 from an ICMP checksum
// Sounds easy, right? But actually pretty tricky to correctly
// handle 16-bit one's-complement arithmetic on data flying by 8-bits
// at a time. Tricky enough that Du got it wrong in icmp_rx.v;
// that code gets the right checksum only 31/32 of the time.
// Quoting RFC 792:
// The checksum is the 16-bit ones's [sic] complement of the one's
// complement sum of the ICMP message starting with the ICMP Type.
// Output is delayed two cycles from input.
// kick needs to be asserted on the second of the two cycles
// with the checksum at the input.
module hack_icmp_cksum(
input clk,
input kick,
input [7:0] idat,
output [7:0] odat
);
// Precompute the all-ones condition
wire ones_i = &idat;
reg ones_r=0;
always @(posedge clk) ones_r <= ones_i;
wire all_ones = ones_i & ones_r;
// Insert modified checksum into stream
wire [15:0] x3;
reg [7:0] dat1=0, dat2=0;
always @(posedge clk) begin
dat1 <= kick ? x3[7:0] : idat;
dat2 <= kick ? x3[15:8] : dat1;
end
// 16-bit checksum arithmetic
// Because of the extra inversion, we really add 0x800.
wire [15:0] x0 = {dat1, idat};
wire [15:0] x1 = {x0[10:0], x0[15:11]}; // rotate right 11 bits
wire [15:0] x2 = x1 + 1 + all_ones;
assign x3 = {x2[4:0], x2[15:5]}; // rotate left 11 bits
assign odat = dat2;
endmodule
|