RE: [lwip-users] tcp_write() errors on snd_queuelen

[Kieran Mansley]

On Mon, 2011-03-14 at 12:27 +0000, Tim Lambrix wrote:
> 
> I do not see the send queue ever return to zero even when the load is
> removed. For example, I see it start at 0 and I blast a little data
> where it may climb to 50 and then I stop the volume of data.  Usually
> it returns to 0 within a couple seconds.  I blast a little more data
> and then it climbs again to 40 and I stop again.  This time, the value
> doesn't drop below 6 even after 10 minutes.  I blast a little more
> data and it climbs to 100 and stop again.  This time the value won't
> drop below 63.  It never returns to 0.  If this doesn't return, does
> that number represent data not received by the destination?  It is
> hard for me to verify right now as the tcp_write fail doesn't send
> data.
> 
> So it seems the lwip believes there is data someplace and so I am
> trying to discover the underlying problem.  I could just set the value
> to 0 but I don't know what that would break or why lwip thinks it
> can't go back down.  I was wondering if it was due to a problem with
> my configuration file with the buffers?  

I wouldn't suggest manually setting the value to zero - it will likely
confuse lwIP greatly.

Perhaps a good way to see what is going on would be to make sure that
your calls to tcp_write() have identifiable (human readable) data, e.g.
first write is all zeros, second write all ones, and so on.  Then get a
wireshark packet capture to coincide with your tests as described above.
This should show us what is going on the wire, and by deduction what is
queued but not sent.  The packet capture may also reveal why lwIP can't
send any more.  

Kieran