Re: [lwip-users] TCP Checksum Errors - NOT.

[address@hidden]

Nick Thomas wrote:
> Does anyone on this list also use the STMAC110 with OSPlus? If so, how did
> you get around this problem? I am not too happy about copying all of the
> pbufs in the chain to a buffer prior to sending!
Sending like that will certainly the main bottleneck in your lwIP setup. 
The ports in contrib that do it like that are not speed-optimized as 
they don't use real hardware (in the win32 port, for example, it is a 
limitation of the pcap library).
> For one thing I don't know
> how big to make the buffer in advance? What if there are a lot of pbufs in
> the chain?
You always know how much data your hardware can send at maximum. I.e. 
the MTU will be 1500 for most ethernet networks. Adding the ethernet 
header, VNET support and some room for aligment and so on, you'll be on 
the safe side with 1536 bytes (which is used quite often since it works 
with most cache line sizes / data alignment requirements). This will be 
different for non-standard MTUs, of course (e.g. jumbo frames).
> I notice that some examples in the contrib sections use stack
> variable buffer like 'unsigned char buffer[1600];' but then don't do a
> length check on the memcpy(s) as it builds up the buffer with data!
>   
I'll check that. Although this is taken care of by setting the MTU (and 
relying on the rest of the stack to not exceeding this MTU), I think it 
should be checked! Thanks for the info.

Unfortunately, I can't help with your hardware / OS problem. :-(

Simon