I'm developing an emulator for a non-Intel architecture for a client. The client uses the emulator under Linux so I'm developing using several different Linux VM's on my Mac, running Fusion 6.0.4 under OS X 10.9.4. (In particular, RHEL 5.8, RHEL 6.5, Ubuntu 12.04, and Mint 17.) All VM's are configured with NAT Ethernet interfaces.
The emulator is working quite well but, at the customer's request, I'm now updating the 30 year old TCP stack of the OS running in the emulator to add support for TCP window scaling. I can see that window scaling is enabled when I'm communicating between the emulator and the Linux VM.
When I try to test with a remote system, window scaling is disabled. As an experiment, I ran the emulator directly on my Mac under OS X. Lo and behold, window scaling is enabled when communicating from the emulator to that remote system.
So, it appears that the Fusion Ethernet drivers disable window scaling. Is there anything I can do to change this?