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Re: [lwip-users] TCP problem

From: Simon Goldschmidt
Subject: Re: [lwip-users] TCP problem
Date: Thu, 22 Oct 2009 12:33:04 +0200

Kieran Mansley wrote:
> On Wed, 2009-10-21 at 17:04 +0200, address@hidden wrote:
> > But is that really what we want (an bug free)? If so, we would really 
> > need a compile-time check that TCP_WND is at least 2*TCP_MSS (or only 
> > greater than TCP_MSS?) to prevent problems:
> I think a compile time check would be a good idea.

Unless we fix it.

> >  If, in Jan's case, the 
> > remote side was lwIP, too, the connection would stall as the window 
> > would never reopen because the remote side wouldn't be able to send a 
> > 2-byte-segment. I know we can fix this in lwIP, too, but there might
> > be 
> > other embedded stacks having a problem with this?
> I don't understand this new problem.

Looking at Jan's 1.3.1 trace, lwIP shrinks the window down to 2. At that moment 
(after a timeout, I guess), the remote side sends a packet of length 2. This 
results in the window getting 0 (not advertised) and our algorithm opening the 
window by MSS (resulting in a window of MSS).

*If* the remote side would be lwIP too, and it would have pre-splitted segments 
on the unsent queue that are bigger than 2 bytes, it would stall because lwIP 
currently cannot split segments that are already created and thus wouldn't be 
able to send a 2-byte-segment that would be necessary to reopen the window.

I don't know if this is still a problem after we change the algorithm, we'll 
have to see that.

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