Modifying the guts of TCP/IP in Mac OS X

[RGould8 at aol.com]

This may be way more than Revolution was intended for, but I haven't found 
much that can't be done with it:

I've got a client (an ISP), who has determined that for their fiber-optic 
network, they need to adjust the computer's "MUT_SIZE (maximum transmission unit 
size)" setting within the TCP/IP protocol.   This needs to be done in Mac OS 
X.   

They are presently doing this in Windows by adjusting certain registry values 
(the "TCP_RECEIVE_WINDOW_SIZE TCP/IP" registry)

This is more a Mac OS X question than anything - - - is type of TCP/IP 
modification even something that can be adjusted in Mac OS?