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| Session Initiation: Non Hiding |
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| Session Initiation: Hiding |
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This signalling flow assumes that:
1. the same PDP Context allocated during the initial registration scenario is still used for deregistration
2. the DHCP procedure employed for P-CSCF discovery is not needed
3. the S-CSCF selection procedure invoked by the I-CSCF is not needed.
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UE1 intends to de-register itself. It does so by sending a new REGISTER request. This request looks
similar as in reregister case, but the Expires header contains zero. This request is sent to the same P-CSCF
with which UE1 initially registered.
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P-Access-Network-Info:
UE1 provides the access-type and access-info, related to the serving access network.
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From:
This indicates the public user identity originating the REGISTER request. The public user identity
may be obtained from the USIM.
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To:
This indicates public user identity. This is the identity by which other parties know this subscriber.
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Contact:
This indicates the point-of-presence for the subscriber - the IP address of UE1. This is the
temporary identifier for the subscriber that is being de-registered. The 0 value of the expires
parameter indicates the registration is being cancelled.
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Authorization:
It carries authentication information. The private user identity is carried in the usernamed field of
the Digest AKA protocol. The deregistration process also includes the cached information (realm,
nonce, algorithm, uri, response).
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Request-URI:
The Request-URI (the URI that follows the method name, "REGISTER", in the first line) indicates
the destination domain of this REGISTER request. The rules for routing a SIP request describe
how to use DNS to resolve this domain name ("home1.net") into an address or entry point into the
home operator's network (the I-CSCF). This information is stored in the USIM.
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Security-Verify:
Contains the security agreement as represented by the received Security-Server header.
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Supported:
This header is included to indicate to the recipient that the UE supports the Path header.
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Upon receiving this request the P-CSCF will reset the SIP registration timer for this UE to 0.
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Based on the user's URI, P-CSCF1 determines that UE1 is registering from a visiting domain and performs
a DNS query 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. The DNS provides P-CSCF1 with an address of the I-CSCF in the home
network. P-CSCF1 must not use the I-CSCF address cached as a result of the previous registration.
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P-CSCF1 removes the Security-Client header and associated "sec-agree" option-tags prior to
forwarding the request. As the Proxy-Require header is empty, it removes this header completely.
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Path:
This is P-CSCF1 URI and is included to inform S-CSCF1 where to route terminating requests.
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Require:
This header is included to ensure that the recipient correctly handles the Path header. If the
recipient does not support the path header, a response will be received with a status code of
420 and an Unsupported header indicating "path". Such a response indicates a
misconfiguration of the routing tables and the request has been routed outside the IM CN
subsystem.
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P-Visited-Network-ID:
It contains the identifier of P-CSCF1 network at the home network.
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I-CSCF1 (THIG) adds itself to the Path header.
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Upon receiving this request the S-CSCF will reset the SIP registration timer for this UE to 0.
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S-CSCF1 informs the HSS that the user is no longer registered and S-CSCF1 either notifies the HSS to
clear or requests to keep its location information for that subscriber. The HSS then either clears or keeps the
S-CSCF name for that subscriber according to request. In both cases the state of the subscriber identity is
stored as unregistered in the HSS and the S-CSCF. The HSS acknowledges the request.
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