No ACK is sent - an ACK is only sent in response to a response to an INVITE request
Registration is another common operation in SIP. Registration is one
way that the biloxi.com server can learn the current location of Bob.
Upon initialization, and at periodic intervals, Bob's SIP phone sends
REGISTER messages to a server in the biloxi.com domain known as a SIP
registrar. The REGISTER messages associate Bob's SIP or SIPS URI
(sip:[email protected]) with the machine into which he is currently
logged (conveyed as a SIP or SIPS URI in the Contact header field).
The registrar writes this association, also called a binding, to a
database, called the location service, where it can be used by the
proxy in the biloxi.com domain. Often, a registrar server for a
domain is co-located with the proxy for that domain. It is an
important concept that the distinction between types of SIP servers
is logical, not physical.