Re: [lwip-users] Losing packets

[Leon Woestenberg]

Hello Chris,

Chris WIlliams wrote:
> I am losing packets when transmitting:
>
> I am sending packets to my ethernet interface using the
> low_level_output() routine in ethenetif.c. I am sending the data very
> quickly, and at some point I fill up the buffer in the ethernet chip and
> I can't store any more data. I therefore drop the packet, and return
> ERR_IF.
>
> I assumed that at this point the TCP/IP would stop, and retry again in
> the future. However, the returned value from low_level_output() is never
> examined and the packet is lost. The next packet is transmitted OK, and
> then it takes time for the system to recover.
Exactly what TCP is supposed to do. IP does not guarantee delivery.

> Does anybody have an comments or a fix for this?
The plan is to enhance the etharp queueing code to have the driver tell
whether the packet could be transmit on the wire, and only then remove 
it from the queue.

Please note that etharp queuing seems to be misbehaving for some people, 
as reported on the mailing list. This might be a bug, but is still under
investigation.

Please note however, that the queue may grow and the system will starve
in that case, unless other measures are taken. Also in such cases, no
guarantees can be made about delivery.

If you burst packets, the queue approach might be the right approach.
The queue will smoothen out the packet transmission in time.

Regards,

Leon.

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