TCP/UDP cksum offload on hme(4)

[Kevin Day]

On Jun 15, 2004, at 10:45 PM, Pyun YongHyeon wrote:
>
>  1. UDP TX cksum offload has an issue. The hardware doesn't flip the
>     cksum bits when the computed cksum is 0x0000. I have no idea this
>     is the reason why STP2002QFP says it supports only TCP RX/TX cksum.
>     (pp. 29, pp. 40, pp. 42)
>
>

I'm not sure if you're aware of this or not, but:

> If the computed  checksum  is zero,  it is transmitted  as all ones 
> (the
> equivalent  in one's complement  arithmetic).   An all zero  
> transmitted
> checksum  value means that the transmitter  generated  no checksum  
> (for
> debugging or for higher level protocols that don't care)


So, if a UDP packet has an all zero checksum, it's supposed to mean 
there was no checksum performed. If you legitimately came up with 
0x0000 for a checksum, you're supposed to set the header field to 
0xffff.