Rationale of window adjustment code

[Simon Paradis]

Hi,

I'm using SSHD 0.6.0 on Oracle JVM 1.6.0_23-b05 on Windows 7 x64. I have a 
custom server channel subclassing AbstractServerChannel. I'm wondering if my 
handling of SSH window adjustment message is correct. I'm sending a window 
adjustment everytime the window size decreases by half. Apparenly, some OpenSSH 
implementation works this way. This code is talking to another C++ process 
built with libssh2 1.3.0.

protected void doWriteData(byte[] p_Data, int p_Offset, int p_Length) 
  throws IOException
{
  // The localWindow.consumeAndCheck() send too much window adjustments.        
  localWindow.consume(p_Length);
  int size = localWindow.getSize();
  int maxSize = localWindow.getMaxSize();
  if (size < maxSize/2) {
    localWindow.expand(maxSize - size);
    sendWindowAdjust(maxSize - size);
  }

  // Do stuff ...
}

Before that, I was calling localWindow.consumeAndCheck(p_Length) but I found 
out it sends quite a lot of these adjustment down the wire. In Window.java, we 
have this

public void consumeAndCheck(int len) throws IOException {
  synchronized (lock) {
    size -= len;
    if (log.isTraceEnabled()) {
      log.trace("Consume " + name + " by " + len + " down to " + size);
    }
    check(maxSize);
  }
}

public void check(int maxFree) throws IOException {
  synchronized (lock) {
    if ((size < maxFree) && (maxFree - size > packetSize * 3 || size < maxFree 
/ 2)) {
      if (log.isDebugEnabled()) {
        log.debug("Increase " + name + " by " + (maxFree - size) + " up to " + 
maxFree);
      }
      channel.sendWindowAdjust(maxFree - size);
      size = maxFree;
    }
  }
}

This is sending a window adjustment every 3 * packetSize, so every 96kb in my 
case since the packet size is 32kb. Is there a practical reason for doing this 
instead of sending it every half of the maximum size ? 

Thanks,

Simon Paradis