asterisk users - Jun 2006 - question about the register/invite call flow

Hi,

Does anyone know what asterisk  do during the register & invite
process using ARA (realtime) with Nat enabled?

Say there is an UA1 send an register request to asterisk.  Asterisk
will parse the register request header (in which source file?).  It
will get the necessary information & update the user table in DB with
the field username, ipaddr & port.  At the same time, it will update
the asterisk db (astdb).  There is a key value pair like,
/SIP/Register/Sipaccount :
extIP:extport:registerttl:sipaccount:sip:sipaccount@intip:intport.

Now, UA1 wants to send a invite request to other parties.  The UA1
invite request information can be found from the invite header.   Does
asterisk will update the Database and astdb at the same time?

There is a party wants to make call to UA1, What information (from DB
or astdb) will asterisk get to make the invite request to UA1?

Anyone can tell me the flow of above process?