Re: [lwip-users] lwIP + PPP

[David Empson]

"Nicolas Dioli" <address@hidden> wrote:
> Hello, i am using lwip 1.3.2 + PPP with FreeRTOS.
> I have a question about pppInProc.
> When i receive a ppp packet which start by 0x7E, when the function is 
> testing the fcs: he found an invalid fcs and drop the packet.
> I don't understand why ( i have already read rfc but i'm lost).
>
> perhaps, the packet is wrong: my sio_read return only the packet between 2 
> flags 0x7E.

Is the FCS error happening on the first PPP frame received, or after a few 
frames have been received?

In order to determine what is going wrong, you probably need to look at the 
raw data as received at your serial port, and compare it to the data 
delivered to pppInProc. The bytes from the opening 0x7E to the closing 0x7E 
must be identical (and you must deliver the 0x7E bytes to pppInProc). If 
there is any difference in the receive data (including insertion, deletion, 
transformation or reordering of bytes) then have a close look at your serial 
driver.

If the received data and data delivered to pppInProc are identical, and you 
are still getting an FCS error, then either the FCS calculation has a bug 
(seems unlikely), or your modem is not delivering the same data that the 
transmitting PPP device sent. In this case you probably need to know exactly 
what data was transmitted to be able to get an idea about what happened to 
it.

> An didn't return "CONNECT 115200" from the modem GPRS.

That doesn't matter. It isn't part of PPP and would be ignored by pppInProc 
if it was received before the first PPP frame.