Re: [lwip-users] TCP Checksum errors.

[address@hidden]

Nick Thomas wrote:
> Hi, I am using the netconn_X functions. In stepping through
> inet_chksum_pseudo() routine I notice that checksum errors are observed when
> there is more than one pbuf in the chain.
Did you use the netconn API for UDP also?
inet_chksum_pseudo() is used both for rx and tx, at which point did you 
step inside? From what you said, it's not clear whether you see checksum 
errors on packets sent by lwIP or received by it!
> I never seem to get more than 2
> pbufs in a chain.
That depends on the amount of data passed to netconn_write and the speed 
of the ACKs returning from the remote side.
> But, anyway, when the chain only has one pbuf then the
> checksum appears to be calculated OK (at least wireshark doesn't complain
> about it.).
>   
What most people do wrong when seeing this error is in the netif driver 
(in netif->linkoutput, I think), they only copy the first pbuf in a 
chain, instead of copying ever p and p->next and stop at the last pbuf 
of a packet (which has p->len == p->tot_len; NOT p->next == NULL). If 
you have a DMA enabled MAC, it could be that you pass p->payload and 
p->tot_len of the first pbuf in the chain to the DMA hardware.
You should check whether the data sent by lwIP (as seen in debugger or 
as expected to send) matches the data received in wireshark (except for 
the checksum, of course). That way you can see whether it is a 
data-copy-problem or really a checksum-problem.


Simon