HP-UX C SIP Stack Programmer's Guide (Novembery 2007)

Session Initiation Protocol (SIP) 11
Entity Interaction
CALL FLOW
1. First a SIP INVITE message is sent to [e-mail], but
finds the Redirect server sip.acme.com along the signaling
path.
2. The Redirect server looks up Bob’s current location in a
Location Service using a non-SIP protocol (for example,
LDAP).
3. The Location Service returns Bob’s current location: SIP
address [e-mail].
4. The Redirect Server returns this information to the calling UA
using a 302 (Moved Temporarily) response. In the response
message it enters a contact header and sets the value to Bob’s
current location, [e-mail].
5. The calling UA acknowledges the response by sending an
ACK message.
6. The calling UAC then continues by sending a new INVITE
directly to gw.telco.com.
7. gw.telco.com is able to notify Bob’s terminal of the call and
Bob “picks up” the call. A 200 (OK) response is sent back to
the calling UA.
8. The calling UA acknowledges with an ACK message.