HSS-based P-CSCF restoration

As an IMS core, Project Clearwater’s I- and S-CSCF can be used within a VoLTE Solution, providing voice service to 4G (LTE) mobile subscribers. It is then possible for a P-CSCF to fail independently of the S-CSCF. Since UE’s do not monitor the P-CSCF, the UE will be unaware of the P-CSCF failure until it tries to re-register due to registration timer expiry, which could be a long time depending on the operator’s choice for registration timer. Meanwhile, all MT (Mobile Terminated) calls to every subscriber on the failed P-CSCF will fail. MO (Mobile Originated) calls would detect the failure and trigger the device to re-register with the 2nd P-CSCF received in the PCO from the PGW during the original IMS attach request. 

3GPP TS 23.380 provides several possible restoration mechanisms for recovery from P-CSCF failure. One such mechanism is “HSS-based P-CSCF restoration for 3gpp access” as defined in section 5.4.  Figure from 3GPP TS 23.380 

How does it work? 

The feature works like this for a Public User Identity (IMPU) registered with only one Private User Identity (IMPI): 

  • The S-CSCF receives an MT call (1) and forwards to the UE’s terminating P-CSCF (2).
  • The S-CSCF identifies that the P-CSCF has failed (3). This could be due to no response, a transport error or certain other error codes (e.g. the UE’s registration data is not present).  
  • Having identified the failure, the S-CSCF checks the registration status of the IMPU and determines that there is a single IMPI registered. A Cx SAR is sent to the HSS that contains a P-CSCF restoration indication with the intention that this IMPI is un-registered (4). 
  • HSS attempts to implement the restoration process in conjunction with various EPC functions resulting in the UE registering via another P-CSCF so that future MT calls will succeed.
  • The HSS determines whether the MME / SGSN serving nodes support the P-CSCF restoration for that UE. If this is supported, then the HSS sets the registration state to UN-REGISTERED and sends a successful Cx SAA response to the S-CSCF (5a). The S-CSCF then sends a P-CSCF restoration indication using either S6a/S6d IDR/IDA or Gr ISD request/answer (5b).  
  • The MME/SGSN will prompt the UE to remove it’s IMS PND connection (7), at which point the UE will activate a new IMS PDN connection and perform a new initial registration (8). 

 What if HSS-based P-CSCF restoration is not supported? 

If either the HSS or MME/SGSN do not support the feature, then the registration state of the UE is not updated. It makes more sense to keep the existing registration lest the P-CSCF failure is temporary one. In this case, the HSS provides an error response in the Cx SAA to the S-CSCF. 

What happens if there are multiple registrations? 

As well as the single registration case, we may have multiple IMPI/IMPU pairs registered, only one of which is attached to a failed P-CSCF. P-CSCF restoration can still take place as above, but rather than changing the state to UN-REGISTERED, the S-CSCF instead requests that the relevant IMPU/IMPU pair alone is DEREGISTERED. In this way, the remaining pairs are left intact.  

Enhancing Project Clearwater to implement this feature could be an interesting project. 

Ben Laing

Ben Laing is a Software Engineer at Metaswitch Networks. Prior to Project Clearwater, he worked on Metaswitch’s Unified Communication products. When not working, he enjoys running, playing the accordion and follows Middlesbrough FC.

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