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Re: [lwip-users] TCP payload is doubled

From: Simon Goldschmidt
Subject: Re: [lwip-users] TCP payload is doubled
Date: Thu, 22 Oct 2009 08:33:24 +0200

That's all true, but David's and Jan's recent problem was that the window was 
changed although tcp_recved() was called right away. This is not an 
RFC-violating problem but a problem in lwIP (when and how it calculates the 
window changes).

However, adding a bug entry for your comments below would be best, I guess.


David Empson wrote:
> Kieran Mansley wrote:
> > I was a bit wrong there.  I've taken another look at the code and this
> > change in behaviour is intentional.  There were two changes in 1.3.1:
> > - only send an explicit window update if the change is greater than
> > - only change the window advertised (including in normal ACKs) if the
> > change is greater than 1 MSS.  This is silly window avoidance, and
> > designed to prevent the sender getting small amounts of window and then
> > sending small packets.
> The description of silly window avoidance in RFC1122 (section
> says
> that there is supposed to be a second test.
> The threshold to advertise an increase in window size should be the lesser
> of MSS and WND/2 (assuming the recommended fraction of 1/2). This would
> allow the use of window sizes smaller than MSS*2. The absolute minimum
> (but
> horribly inefficient) value of WND would be 2.
> Assuming WND is set equal to MSS:
> 1. If the application is reading data slowly a few bytes at a time with a
> sender able to supply data continuously, then the typical pattern should
> be
> to get an initial transmit burst of the full window size, followed by
> half-full segements (WND/2 = MSS/2) once the receiver has processed half
> the
> initial data.
> 2. If the sender is delivering small packets at a moderate pace and the
> receiver is able to keep up, the expected pattern should be to see the
> window size decreasing in each ACK until just after half the window is
> gone,
> then it should advertise an almost full window again.
> It seems reasonable for LWIP to recommend that users set WND to at least
> MSS*2, but unless LWIP enforces this minimum, the silly window avoidance
> code should allow for small windows by checking WND/2 as well as MSS.
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