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Re: [lwip-users] lwIP out of sync


From: Kieran Mansley
Subject: Re: [lwip-users] lwIP out of sync
Date: Fri, 17 Aug 2007 08:51:22 +0100

On Thu, 2007-08-16 at 13:05 -0700, Steve W wrote:
> Hello,
> 
> I have found that when I pause for a few seconds before reading the input
> buffers in my MAC, lwIP gets out of sync and slows way down.  Once this
> occurs, I can send a packet but will not get a response until I send it
> again or the network sends it again.  This problem goes away if I use a
> single RX buffer design but that has its drawbacks.   I currently have 16
> input buffers and I know they are not full when this happens.  My
> application does not resend the data if it does not get an ACK but the
> network does.  Any ideas?

I'm not quite sure what you describing.  Let me say what I think is
happening, but please correct me if I misunderstand:

Normally you would read data into lwIP from your MAC RX buffers quickly,
but sometimes you pause for a few seconds before doing so.  When this
happens, the network retransmits some of the outstanding packets (I
presume you're using TCP), and the data rate slows down.  Is that right?

If that is what is happening, the reason for the slow down is probably
the retransmissions: TCP assumes that if it has to retransmit it must be
because packets were lost, and that if packets were lost it must be due
to network overload, and so it cuts back its transmission rate to give
the network some slack.  This isn't always the case as you can see, but
that's the way TCP works.  There are some bits of TCP that try to notice
when retransmissions were not due to loss but I can't remember off hand
if lwIP is sophisticated enough to include these.

If I'm wrong with the above interpretation, then it could of course be
something more serious.

Kieran 





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