Hi Dan:
	I strongly suspect that what you're seeing is an artifact of the
way that FTP is doing the speed measurement.  It doesn't give you a
windowed average but an overall average.  When the FTP application first
starts transferring data to the stack, the internal buffers between the
app and stack get filled up and this counts as data being "sent" even
though it's never actually left the box.  The FTP app is then blocked
waiting for buffer space as the data starts getting transferred out over
the wire at the real transfer rate (which is considerably slower than
the artificially high initial rate).  Given that it's an overall
average, you then see the average slowly decrease to the real transfer
rate over time.  Of course, this rate will be affected by other
applications in the system taking time away from the FTP app and stack,
but there's nothing in there in terms of intent from the OS point of
view to throttle things back.

	The overall "real" rate of the connection is based upon the TCP
protocol which includes some pretty complicated mechanisms of feedback,
windowing and acknowledgements to control the rate of transfer given
possible network congestion / unreliability and end-point capability of
dealing with the data.  


	Hope that helps,
		Robert.


-----Original Message-----
From: Daniel Rodrick [mailto:daniel.rodrick@gmail.com] 
Sent: Wednesday, July 02, 2008 7:22 AM
To: drivers-networking
Subject: Speed drop while ftp

Hi list,

I see that when I to download an huge file (using standard qnx ftp
client) on my QNX machine, the download speed (as displayed by ftp
client) gradually decreases. 

It starts from around 7MB/s and the speed decreases by about 0.1 MB/s
per second. So at the end of roughly 10-12 secs, I see a speed of around
6 MB/S.

I am thinking if this is by design ?(Is there any scheduling decision
that lowers the priority of ftp client dynamically because it is doing
too much I/O?). The trend is very clear as I am able to reproduce this
everytime.

TIA,

Dan

_______________________________________________
Networking Drivers
http://community.qnx.com/sf/go/post9899