Customer is using PPC w. 6.4.0

He wants to create a simple filter to receive a packet from a specific IP addres  and 'reflect' it back as fast as 
possible. Initially it is to compare performance with other OSes, but ultimately it is to forward (modified) packets.

His current implementation is a process that receives a packet (from Windows), tweaks it and resends it - via sockets. 
It is too slow - he's hoping to do a roundtrip in under 1ms, and he's seeing just over 2ms right now, which I understand
, given it is going up and down the entire stack, copy out, copy in etc.

The question is, is there a good piece of filter reference code (with documentation!) to start from, that he can use to 
create his very modest requirement ? 

Any pointers greatly appreciated...

Dave