[Serusers] what makes a transaction?

I'm trying to understand stateful vs. stateless transactions. A stateless 'transaction' simply lasts as long as it takes to process the current sip request. It is then forgotten. However, a stateful transaction I'm having a little problem grasping. For instance, if I t_relay() an INVITE, logically the transaction is started....so when the response comes it isn't delivered to the main route {} block, but instead it is delivered to the awaiting stateful t_relay(), probably keyed by callid. So, for instance: INVITE <- 401 Unauthorized -> Is this entire sequence a transaction? Right after this the UA sends an ACK, this is a transaction unto itself? ACK <- The UA then sends : INVITE <- 100 Trying -> 180 Ringing -> 200 OK -> All of these messages are a 'transaction' handled by t_relay()? The next transaction is: ACK <- Finally, the last transaction would be (as an example): BYE <- 200 OK -> So the ser route{} block would need to recognize and handle INVITE ACK BYE in this context. Is that correct? If I was running 'statelessly' I would need to handle and send every single message listed? ---greg