[lwip-devel] [bug #27783] Silly window avoidance for small window sizes

[David Empson]

URL:
  <http://savannah.nongnu.org/bugs/?27783>

                 Summary: Silly window avoidance for small window sizes
                 Project: lwIP - A Lightweight TCP/IP stack
            Submitted by: dempson
            Submitted on: Thu 22 Oct 2009 21:19:50 GMT
                Category: TCP
                Severity: 3 - Normal
              Item Group: Faulty Behaviour
                  Status: None
                 Privacy: Public
             Assigned to: None
             Open/Closed: Open
         Discussion Lock: Any
         Planned Release: 
            lwIP version: 1.3.1

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Details:

Kieran Mansley wrote:
> There were two changes in 1.3.1 [in the area of receive window
management]:
> - 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.

As it stands, the code has a degenerate case when WND = MSS, which in (2)
results in the window closing all the way to zero before opening up to full
again. It isn't quite as bad for MSS < WND < MSS*2, but still results in the
window getting smaller than TCP says it should.




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