1. The document outlines the steps in establishing and terminating a SIP session between two endpoints. It begins with an INVITE request sent to initiate a call, which is routed through S-CSCF and I-CSCF nodes to the terminating S-CSCF. SDP offers and responses are then exchanged to negotiate media. 2. Upon acceptance, a 200 OK response is returned to complete call setup and ringing occurs at the terminating endpoint. The call remains established until one endpoint sends a BYE request to terminate the session, triggering ACK responses to be sent in the reverse direction.

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2. 1. The SIP INVITE request is sent from the UE to S-CSCF#1 by the procedures of the
originating flow. This message should contain the initial media description offer in
the SDP.
2. S-CSCF#1 invokes whatever service logic is appropriate for this session setup attempt
3. S-CSCF#1 performs an analysis of the destination address, and determines the
network operator to whom the subscriber belongs. Since it is local, the request is
passed to a local I-CSCF.
4. I-CSCF shall query the HSS for current location information.
5. HSS responds with the address of the current Serving-CSCF for the terminating user.
6. I-CSCF forwards the INVITE request to the S-CSCF (S-CSCF#2) that will handle the
session termination.
7. S-CSCF#2 invokes whatever service logic is appropriate for this session setup attempt
8. The sequence continues with the message flows determined by the termination
procedure.
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3. 9-12. The terminating end point responds with an answer to the offered SDP and this
message is passed along the established session path.
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4. 13-16. The originator decides on the offered set of media streams, confirms receipt of
the Offer Response with a Response Confirmation, and forwards this information to
S-CSCF#1 by the origination procedures. This message is forwarded via the established
session path to the terminating end point. The Response Confirmation may also contain
SDP. This may be the same SDP as in the Offer Response received in Step 12 or a subset.
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5. 17-20. Terminating end point responds to the offered SDP and the response if forwarded
to the originating end point via the established session path.
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6. 21-24. Originating end point sends successful resource reservation information towards
the terminating end point via the established session path.
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7. 25-28. Terminating end point sends successful resource reservation acknowledgement
towards the originating end point via the established session path
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8. 29-32. Terminating end point sends ringing message toward the originating end point via
the established session path.
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9. 33-36. The SIP final response, 200-OK, is sent by the terminating endpoint over the
signalling path. This is typically generated when the user has accepted the incoming
session setup attempt. The message is sent to S-CSCF#2 per the termination procedure.
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10. 37-40. The originating endpoint sends the final acknowledgement to S-CSCF#1 by the
origination procedures and it is then sent over the signalling path to the terminating end
point.
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11. 1. One party hangs up, which generates a message (Bye message in SIP) from the UE to
the P-CSCF.
2. Depending on the bearer establishment mode selected for the IP-CAN session,
release resource(s) shall be initiated either by the UE or by the IP-CAN itself.
3. The P-CSCF/PCRF removes the authorization for resources that had previously been
issued for this endpoint for this session. This step will also result in a release
indication to the IP-CAN to confirm that the IP bearers associated with the session
have been deleted.
4. The P-CSCF sends a Hangup to the S-CSCF of the releasing party.
5. The S-CSCF invokes whatever service logic procedures are appropriate for this ending
session.
6. The S-CSCF of the releasing party forwards the Hangup to the S-CSCF of the other
party.
7. The S-CSCF invokes whatever service logic procedures are appropriate for this ending
session.
8. The S-CSCF of the other party forwards the Hangup on to the P-CSCF.
9. The P-CSCF/PCRF removes the authorization for resources that had previously been
issued for this endpoint for this session. This step also results in a release indication
to the IP-CAN to confirm that the IP bearers associated with the UE#2 session have
been deleted.
10. The P-CSCF forwards the Hangup on to the UE.
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12. 11. The terminal responds with an acknowledgement, the SIP OK message (number
200), that is sent back to the P-CSCF.
12. Depending on the bearer establishment mode selected for the IP-CAN session,
release resource(s) shall be initiated either by the UE or by the IP-CAN itself. The UE
initiates the release procedures for the resources used for this session as shown in
Figure 5.22. Otherwise, the IP-CAN initiates the release of used resources after
step 11.
13. The SIP OK message is sent to the S-CSCF.
14. The S-CSCF of the other party forwards the OK to the S-CSCF of the releasing.
15. The S-CSCF of the releasing party forwards the OK to the P-CSCF of the releasing.
16. The P-CSCF of the releasing party forwards the OK to the UE.
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