Re: [lwip-users] losing input packets and repeated acks

[Jonathan Larmour]

Carl D. Blake wrote:
> Thank you for your help.  I've listed the things I've tried below.
>
> On Fri, 2007-01-12 at 19:53, Jonathan Larmour wrote:
> > Carl D. Blake wrote:
> > > I've attached my lwipopts.h file and a tcpdump trace.
> > I just noticed a few things in your config too...
> >
> > > /* #define TCP_TMR_INTERVAL     250 */
> > > #define TCP_TMR_INTERVAL        10
> > That is very very small. You will probably be too busy processing the TCP 
> > timer to do any real work. The default should be adequate.
> I set it back to 250.  I noticed that I was getting some 250 ms delays
> between packets, so I reduced this to try and improve throughput.

The delays are symptoms rather than cause.

> > > #define PBUF_LINK_HLEN          16
> > Are you sure you mean 16 and not 14?
>
> I definitely mean 16 here.  I tried 14, but some combination of lwip,
> the GNU C compiler V3.4.4, MEM_ALIGNMENT set to 4, and my processor
> (MPC8271) makes the whole thing not work if HLEN is 14.  16 seems to
> work.

Odd. I've used it with GCC 3.4.4, although not PowerPC yet.

> I made the changes you suggested and things are a little better.  I'm
> still getting the huge delay periodically, but the multiple acks of the
> same packet and the same window size are gone.  I still get multiple
> acks of the same packet, but the system seems to be advertising a larger
> window size for each ack.  I seem to be getting some corrupt data, but
> it's not nearly as bad as it was before.

Missing data is one thing, but corrupt data implies to me that there is 
something more problematic going on at a lower level, possibly hardware, 
more likely your software ethernet device driver.

> I enabled LWIP_STATS and printed out the stats before and after the
> transfer.  TCP stats is reporting several checksum errors.  PBUF stats
> is reporting several errors.  I enabled debug messages for pbufs and I'm
> getting the message "pbuf_alloc: Out of pbufs in pool."  This happens
> quite a bit during the transfer of 1 Mbyte.  I hadn't expected this to
> be a problem since my pbuf memory is 45Kbytes and my TCP window is
> 16Kbytes.  Any other suggestions?

Indeed the remote side should not send more data than the receive window, 
although it may use up a few more pbufs with things like acks. I guess you 
could try and find out where the pbufs are going - it's possible your 
driver might be leaking pbufs. When the stack goes back to quiescent, do 
the stats return to the starting state? i.e. record stats, exchange lots of 
data for a bit, stop exchanging data, record stats again.

Jifl
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