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Re: [lwip-users] TCP problem

From: Kieran Mansley
Subject: Re: [lwip-users] TCP problem
Date: Wed, 14 Oct 2009 15:53:57 +0100

On Wed, 2009-10-14 at 16:21 +0200, Jan Wester wrote:
> Hi
> I’m using lwip 1.3.0 and updated to 1.3.1 and found problem with
> window size
> I receive small blocks of data (30 bytes) every 500 ms and on each I
> call tcp_recved()
> On version 1.3.0 the window size on tcp ack is 512 (what is my
> maximum), 
> but for version 1.3.1 the window size Is decreased with the received
> 30 until the size is less and it wait short time,
> after that the window size is 512 again and it continue. This is
> repeated the hole time (I’m using the same application)

Could you illustrate this with a packet capture?

It sounds like you've got TCP_WND set to 512.  It would be interested to
know what your MTU and MSS is configured at, as having a window smaller
than the MSS is not optimal.  For example, this bug illustrates how a
window that is always smaller than the MSS can cause problems:


NB. I don't think that is your problem, but may be of interest.

The window advertisement code was re-worked in 1.3.1 to only send an
explicit update when the change in window is greater than 
TCP_WND_UPDATE_THRESHOLD.  This defaults to (TCP_WND / 4). You could
define this to be zero to get the old behaviour, but the new behaviour
should be much better.  I would be interested to know what the problem
you encounter as a result of the less-frequent window updates is.  Note
that any ACKs for data that get returned should have the up-to-date
window information in them - it is only the sending of explicit window
update ACKs that is restricted.


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