Re: [lwip-users] PPP: IPCP bad fcs issue

[Sylvain Rochet]

Hi,

On Wed, Aug 15, 2018 at 05:46:21PM -0400, Patrick Klos wrote:
> On 8/15/2018 4:19 PM, Sylvain Rochet wrote:
> >But what is very strange is that checksum including checksum itself for
> >comparison purpose for "0x01 0x01 0x16 0x03 0x6 0x81 0x6 0x83 0x06 0x6e
> >0xdb" is 0xeb 0xbe, which is not 0x4114 (nor 0x1441 with byte swap) as
> >displayed in your "Dropping bad fcs" debug print, how is that even
> >possible ?
> 
> For what it's worth, 0x4114 is the complement of the byte swap 0xebbe (and
> that's how the FCS is added to a PPP packet).  I didn't look at the actual
> code to see at what stage and which value the message prints, but it's close
> enough to say it's probably right.

Oh dear, I actually checked that, but looks like I failed somewhere :-) 

Indeed, the PPP_GOODFCS value is the checksum value of the full frame 
including the checksum but without the final XOR.

Sylvain

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