[Discuss-gnuradio] in-band lib, the original intention of 'tag'

[George Nychis]

What was the original intention of the 'tag' field in the in-band USB 
packet header?

As I play more and more the in-band stuff and MAC designs, I'm beginning 
to realize we are missing one major thing: notification of packet 
transmission.

I think this is a missing piece.  It would be extremely useful to the 
host if it could determine that its packet was not transmitted.  For 
example, the packet was sent with an expired timestamp and was thrown 
away.  The host has no way of knowing the packet was not transmitted.

I'm not sure if this was the original intention of the tag, but it would 
be extremely useful.  My guess is that the tag was meant to be sort of 
like the invocation handle.  I think this would be sufficient enough for 
determining success.

Our queue is big enough for 4 packets (off the top of my head), 
therefore the tag would only need a history of size 4.  The tag is 4 
bits, the bottom 3 bits could be used as a relative packet number, and 
the top bit notifies success or failure.

This is an extremely important feature for our MAC enhancements, where 
we implement time-critical functionality like carrier sense on the 
board.  Aside from timestamps expiring, if our carrier sense decides to 
throw away a packet, it is important for the host to know.  Therefore, 
we need some feedback mechanism.  I'm going to implement some feedback 
mechanism period, I'm just trying to coordinate with the larger 
framework so it's useful for more than just myself :)

- George