Thank you for sharing this, definitely looks like the blksize isn't variable here.  Next would be to see if AIX open boot supports the http interface for receiving the boot image.
I will try modifying the /etc/bootptab to define the tftp filename as a URL and see if that works.  Let me know if at the surface if the below bootp config should cause Open Firmware to retrieve the file over http, if IBM Open Firmware supports http...

eg. (using data from here)
# modify /etc/bootptab to insert ""

Hope that works tomorrow, otherwise time to build out a new NIM service.


On Thu, Jun 9, 2016 at 7:37 PM, James Cameron <> wrote:
I don't have the source for AIX open boot, but in Open Firmware the
TFTP implementation is without block size support.



2. "blksize"

On Thu, Jun 09, 2016 at 06:20:13PM -0700, Alec wrote:
> I am attempting to do an IBM NIM install across a 20ms latency. 
> Is there an ability to kick off the network boot on AIX open boot with a block
> size option for tftp from the ok? Prompt?
> Forgive me if this is not an appropriate place for this.
> Alec

James Cameron