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

[Sylvain Rochet]

Hi,

On Wed, Aug 15, 2018 at 03:40:22PM -0400, Patrick Klos wrote:
> On 8/15/2018 11:25 AM, Axel Lin wrote:
> >Hi list,
> >
> >I'm testing a LTE module with lwIP and found always got bad fcs as below.
> >I dump the received data below:
> >Is there anything wrong in the received data?
> >
> >sent [IPCP ConfReq id=0x1 <addr 0.0.0.0> <ms-dns1 0.0.0.0> <ms-dns2 
> >0.0.0.0>]␍␊
> >pppos_write[0]: len=26␍␊
> >pppos_input[0]: got 17 bytes␍␊
> >7e ff 3 80 21 1 1 16 3 6 81 6 83 6 6e db 7e ␍␍␊
> >pppos_input[0]: Dropping bad fcs 0x4114 proto=0x8021␍␊
> 
> That doesn't look like a good PPP packet.
> 
>    ff 3 80 21 1 1 16 3 6 81 6 83 6 6e db
> 
> ff 03 = address and control field
> 80 21 = IPCP protocol ID
> 01 = Configure request
> 01 = Identifier
> 16 03 = SHOULD BE PACKET LENGTH

Added to that, there is something very wrong here, HDLC checksum 
(polynomial 0x1021, init 0xffff, xor final 0xffff, reflected input, 
reflected result, network byte order result) for "0xff 0x03 0x80 0x21 
0x01 0x01 0x16 0x03 0x6 0x81 0x6 0x83 0x06" is 0x19 0x17 which is 
obviously not 0x6e 0xdb.

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 ?

I just tested on good packets and the debug print actually print 0xf0b8 
for FCS which is the PPP_GOODFCS value.

Sylvain

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