The SBC as a P-CSCF supports S-CSCF restoration procedures as defined in 3GPP TS 23.380.  If the SBC as a P-CSCF does not receive any response to a SIP request, sent to the IMS core or if the next hop is blacklisted by the SBC’s Address Reachability Service (ARS), it rejects the request with a 504 (Server Time-out) response. The SBC uses its path header as P-Asserted-Id header in the response message. The error response also carries MIME body type "application/3gpp-ims+xml" with the following content:

<ims-3gpp> root element with version set to 1
 
    <alternate-service> child element with parameters set as below:
        <type> child element set to "restoration"
        <reason> child element set to "IMS CN not reachable"
        <action>  child element set to "Initial Registration"
The SBC supports IMS restoration procedures using the IP Signaling Profile flag Support S-CSCF Restoration Procedures on an external PSX. The flag is disabled by default. When enabled, the SBC acting as P-CSCF rejects a request with 504 (Server Time-out) response carrying MIME body of type application/3gpp-ims+xml if the SBC does not receive a response to the request from S-CSCF network, or if the next hop address is blacklisted as part of ARS (Address Reachability Service) service.

For SBC configuration details, refer to:

For PSX-related information, refer to the PSX documentation.

 

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