RDM Discovery packets

[head_up]

Thanks again, ericthegeek! Your answer is realy helpful and i need it realy much! I like DMX and RDM - they look to be simple and effective realization of almost full lighting control.
May be the question is stupid or come too late when i'm already in the protocol development level, but ... there is something in the timing that i can't understand:
How to detect the correctly packages with length less than 513 bytes (possible RDM, but not only!) if i can't explore it "on the fly"?
The interslot delay in RDM systems must be no more than 2.1 ms, ok - if i must answer to the controller no later than 2 ms it is in fact impossible - i'm waiting 2.1 ms after the last rx byte and if there is no other byte for this time i try to explore the packet - if it is correct RDM packet, i will answer to the controller. Now this method work fine - the controller is accepting my answers. But is this correct - to handle one packet as not-full DMX512A (possible RDM or other unknown package) if there is time-out over 2.1 ms for interslot delay?
How u do this in the real RDM systems?
If i use this time-out as reference for non-DMX package - this may be will not work with old DMX system where the timing for interslot delay can be 1s?!
This is something strange for me and i found it like possible problem. If you can explain me how to do this correctly it will be great!