I believe that these sizes are actually negotiated with each appliance in the 
path. All it would take is one device configured such and all along that path 
would follow suit. 

I'd try a trace route on each path and compare the results. Then have the 
network folks check the routers/firewalls in the z/os path but not in the PC 
path.     



-----Original Message-----
From: IBM Mainframe Discussion List [mailto:IBM-MAIN@bama.ua.edu] On Behalf Of 
Mauri Kanter
Sent: Wednesday, June 06, 2012 9:05 AM
To: IBM-MAIN@bama.ua.edu
Subject: Question on TCP window size for FTP

Good morning list:

I'm FTPing files from one z/OS mainframe at my site to another mainframe. 

On a packet trace I can clearly see that the FTP window is small (about 20K) 
while if I FTP from my PC to my mainframe I see that the window is scaled and 
it is 256K.

My side uses

TCPSENDBFRSIZE 256K
TCPRCVBUFRSIZE 256K  

and I understood the target side uses the same values.

What are the knobs that control the window size? Is there any parameter in the 
z/OS FTP client or z/OS FTP server side to control this? 

Thanks in advance for your help.

Mauri.

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