example of tcp dsack undo

Hi,

While designing some testing examples for the undo mechanisms, I found that
undo_dsack is only used when the state is Disorder. However, I assume that
dsack happens when there are retransmissions, which means Recovery or Loss.

I can think of an example of receiving dsack following a successful
recovery. But
the dsack ACKs old data and can not bring the state into Disorder.
Can anybody give me an example when *valid* dsack is received while in Disorder
state.


Thank you!

rl