3GPP TS 24.228 -- IMS Signalling flows for Register
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| Session Initiation: Non Hiding |
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| Session Initiation: Hiding |
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This example describes the subscription procedure for the network initiated deregistration event, whereby the P-CSCF
requests to be notified by the S-CSCF when the event has occurred.
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It is assumed that the user has registered prior to initiating subscription of an event.
For this example the trigger point at the P-CSCF for sending out the SUBSCRIBE
request is the 200 (OK) response of the user's registration.
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P-CSCF1 performs the DNS queries to locate the I-CSCF in the home network. The look up in the
DNS is based on the address specified in the Request URI.
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From:
This header is populated with the SIP URI that identifies the P-CSCF.
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Contact:
This is where the NOTIFY requests for this subscription will be sent.
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Event:
This field is set to the value 'reg' to specify the use of the reg event package.
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I-CSCF1 sends a query to the HSS to find out the S-CSCF of the called user. The HSS responds with
the address of the current S-CSCF for the terminating subscriber.
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S-CSCF1 first authorizes the subscription. As S-CSCF1 can trust the content of the P-Asserted-Identity
header and <sip:pcscf1.visited1.net> is on the list of the authorized users for the "reg" event
package stored by S-CSCF1, therefore S-CSCF1 sends an acknowledgement towards I-CSCF1
indicating that the subscription was successful.
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Expires:
If the value of the Expires header in SUBSCRIBE request is different from the one received in
REGISTER method, then the value of Expires header in the 200 (OK) response is set to match the
value of Expires header in REGISTER method.
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S-CSCF1 sends a first NOTIFY request towards P-CSCF1 in order to inform P-CSCF1 about the
registration status of the monitored user.
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From:
The tag of this field matches that of the "To" field in the received 200 (OK) response for the
SUBSCRIBE request.
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Content-Type:
Set to the value of the Accept header received in the SUBSCRIBE request or
"application/reginfo+xml" if the Accept header was not present in the SUBSCRIBE request.
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