Re: [lwip-users] Timing problem on UDP sending

[address@hidden]

I'm sorry to say that this is a known issue and we have an open task for 
this: http://savannah.nongnu.org/task/?7896
The two possible workaraounds for now are.
a) block until the packet is sent
b) do a memcpy into non-volatile memory before enqueueing in the MAC

This problem currently only affects UDP (with ALL APIs - raw API as 
well) since TCP must preserve data much longer.
A solution for this is not yet found :-(

Simon



Muhamad Ikhwan Ismail wrote:
> Hi guys,
>
> I have observed the following :
>
> 1.I do send() on a UDP socket, which then post the reference to data 
> into the socket layer.
> The application thread then blocks on the semaphore op_completed.
>
> 2.The TCPIP thread then checks the mailbox and starts preparing the 
> packet and queue the reference in
> the Ethernet buffer descriptor ring for transmission.Since the ring 
> buffer descriptor is rather full the packet
> cannot be transmitted at that exact moment by the MAC.
>
> 3. The TCPIP thread returns to the point where the semaphore is 
> triggered. It then blocks and there
> is a task change and the application thread is now running.
>
> 4. The application thread now rewrites the same data which was not yet 
> sent. Hence the previous data packet are corrupted.
>
> This only happens when we have high frequency sending and only with 
> UDP since TCP copies all the data into the TCPIP stack.
> I dont know if this qualifies as a bug, I just wanted to point it out 
> to you guys that this could happen. The workaround can easily
> be done on the application layer of course.
>
> My implementation is lwip1.3.0 on PowerPC processor
>
> M Ikhwan Ismail
>
>
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