Restoration Procedures

When functioning as a P-CSCF within an IMS network, an SBC is required to conform to Restoration Procedures as defined in 3GPP TS 23.380, IMS Restoration Procedures, and 3GPP TS 24.229, IP Multimedia Call Control Protocol Based on Session Initiation Protocol (SIP) and Session Description Protocol (SDP).

TS 23.380 defines a set of standardized procedures for automatic restoration of connectivity after node or link failure. Section 4.4.3 of TS 23.380 imposes a specific injunction requiring that if the UE initiates an originating service request different from SIP REGISTER and the P-CSCF is unable to contact the S-CSCF in the Route, the P-CSCF shall return a specific error response to the UE to trigger a new registration. Section 5.2.6.3.2A of the related 3GPP TS 24.229 defines the structure and content of the required error response.

In the event of a P-CSCF failure to forward a request to the next hop device (whatever the reason), TS 24.229 mandates that the P-CSCF shall reject the call by returning a 504 (Server Time-out) response to the user endpoint. The 504 response must contain:

• a Content-Type header field with the value "application/3gpp-ims+xml"
• a P-Asserted-Identity header field set to the value of the SIP URI of the P-CSCF included in the Path header field during registration of the user whose UE sent the request causing this response
• an attached XML node-set (specified in Section 4.4.1 of 3GPP TS 24.615, Communication Waiting (CW) using IP Multimedia (IM) Core Network (CN) subsystem, containing a child element that conveys the reason for requiring a new registration

After issuing the 504 Server Time-out, the P-CSCF must reject any messages from the user endpoint except for registration requests.