The UE decides to initiate de-registration. To de-register, the UE sends a new REGISTER request with an expiration value of zero seconds. The UE sends the REGISTER information flow to the proxy (Public User Identity, Private User Identity, home network domain name, UE IP address).
Upon receipt of the register information flow, it shall examine the "home domain name" to discover the entry point to the home network (i.e. the I-CSCF). The proxy does not use the entry point cached from prior registrations. The proxy shall send the Register information flow to the I-CSCF (P-CSCF address/name, Public User Identity, Private User Identity, P-CSCF network identifier, UE IP address). A name-address resolution mechanism is utilised in order to determine the address of the home network from the home domain name. The P-CSCF network identifier is a string that identifies at the home network, the network where the P-CSCF is located (e.g., the P-CSCF network identifier may be the domain name of the P-CSCF network).
The I-CSCF shall send the Cx-Query information flow to the HSS (Public User Identity, Private User Identity, P-CSCF network identifier).
The HSS shall determine that the Public User Identity is currently registered. The Cx-Query Resp (indication of entry point, e.g. S-CSCF) is sent from the HSS to the I-CSCF.
The I-CSCF, using the name of the S-CSCF, shall determine the address of the S-CSCF through a name-address resolution mechanism and then shall send the de-register information flow (P-CSCF address/name, Public User Identity, Private User Identity, UE IP address) to the S-CSCF.
Based on the filter criteria, the S-CSCF shall send de-registration information to the service control platform and perform whatever service control procedures are appropriate. Service control platform removes all subscription information related to this specific Public User Identity.
Based on operator choice the S-CSCF can send either Cx-Put (Public User Identity, Private User Identity, clear S-CSCF name) or Cx-Put (Public User Identity, Private User Identity, keep S-CSCF name), and the Public User Identity is no longer considered registered in the S-CSCF. If the user has (originating - see 5.6.5, or terminating - see 5.12) services related to unregistered state, the S-CSCF sends Cx-Put (Public User Identity, Private User Identity, keep S-CSCF name) in order to keep the S-CSCF name in the HSS for these services.
The HSS then either clears or keeps the S-CSCF name for that Public User Identity according to the Cx-Put request. If the S-CSCF name is kept, then the HSS shall be able to clear the serving S-CSCF name at any time.
The HSS shall send Cx-Put Resp to the S-CSCF to acknowledge the sending of Cx-Put.
The S-CSCF shall return the 200 OK information flow to the I-CSCF. The S-CSCF may release all registration information regarding this specific registration of the Public User Identity after sending information flow 200 OK.
The I-CSCF shall send information flow 200 OK to the P-CSCF.
The P-CSCF shall send information flow 200 OK to the UE. The P-CSCF releases all registration information regarding this specific registration of the Public User Identity after sending information flow 200 OK. If the P-CSCF has an active subscription to notifications of the status of the IMS Signalling connectivity, the P-CSCF shall cancel the subscription (see TS 23.203
and TS 23.503
for more details).