Re: SV: [lwip-users] TCP payload is doubled

[David Empson]

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
> TCP_WND_UPDATE_THRESHOLD.
> - 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 4.2.3.3) 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.