• As per the 3GPP standard specification, if S6b re-authorization request is used for P-CSCF Restoration for WLAN, then for extended P-CSCF Restoration the gateway may send authorization request with only mandatory AVPs. However, in the current implementation, ReAuth used for extended P-CSCF Restoration is a common authorization request of normal ReAuth. It will contain all the AVP of ReAuthorization AAR.

• For P-CSCF Restoration extension mechanism during P-CSCF ReDiscovery with P-CSCF FQDN, local DNS cache will be queried first. If FQDN is already present in local DNS cache, then DNS query will not be sent out to the DNS server and P-GW will immediately get the DNS response.

  Since local DNS cache flush will not be done, operator should configure the cache accordingly.

Use of P-CSCF Restoration requires that a valid license key be installed. Contact your local Sales or Support representative for information on how to obtain a license.

PCRF Based P-CSCF Restoration for LTE (S5/S8)

The PCRF-based P-CSCF Restoration makes use of the path through an alternative P-CSCF and PCRF to inform the P-GW regarding P-CSCF Restoration.

There are two mechanisms to handle the P-CSCF Restoration support:

1. Basic Restoration Support: UE does not support the P-CSCF Re-selection. P-GW informs UE to release the PDN connection. P-GW initiates bearer deactivation procedure for the default bearer with cause "reactivation requested".

2. Extended Restoration Support: P-GW sends the UBR with PCO having list of alternate P-CSCF addresses after the P-CSCF rediscovery. The optional extension avoids the PDN deactivation and re-activation and is based on the P-GW identifying whether or not the UE supports "Update PDP context/bearer at P-CSCF failure". The UE indicates this capability to the P-GW at the activation of the PDN connection in a PCO parameter.

Figure 1. PCRF Based P-CSCF Restoration - EPC

P-CSCF Restoration for WLAN (S2a/S2b)

This section describes solutions to support P-CSCF Restoration for UEs with WLAN access.

There are two mechanisms to handle the P-CSCF Restoration support:

1. Basic Restoration Support: The basic mechanism for the HSS-based solution and for the PCRF-based solution relies on the release of the PDN connection, followed by its re-establishment to trigger a new IMS registration by the UE.

2. Extended Restoration Support: The extension mechanism for trusted WLAN and untrusted WLAN access avoids the release of the PDN connection and triggers a new IMS registration by the UE over the existing PDN connection.

   The extension mechanism for trusted WLAN is supported only for multi-connection mode (MCM).

In the basic P-CSCF Restoration for TWAN access or untrusted WLAN access, the P-GW sets the cause "Reactivation requested" when tearing down the PDN connection.

If the re-authorization request is used for the purpose of the P-CSCF Restoration for WLAN, only the P-CSCF Restoration Request bit shall be set in the RAR Flags.

If the P-CSCF Restoration Request bit in the RAR Flags is set in ReAuthorization request (S6b):

• When P-GW triggers the extended P-CSCF restoration mechanism, the P-GW may send the authorization request.

• When P-GW triggers the basic P-CSCF restoration mechanism, the P-GW shall send a Session Termination Request to the 3GPP AAA Server.

For Trusted WLAN Access

• The TWAN shall advertise the support of the WLCP PDN connection modification request procedure over S2a at establishment (or handover) of the PDN connection. This allows the P-GW to use the P-CSCF Restoration extension on this TWAN.

• UE capability (UE support of the P-CSCF Restoration extension for the TWAN access) to the P-GW at the establishment (or handover) of the PDN connection over the WLAN is transferred via PCO IE.

• Upon receipt of a P-CSCF Restoration Indication, the P-GW may invoke this P-CSCF Restoration extension procedure if:

  • The UE is accessing the EPC via a TWAN in the multi-connection mode

  • The UE indicated support of this extension for the TWAN access via PCO, and

  • If the TWAN indicated support of the WLCP PDN connection modification procedure.

  • If UE requested P-CSCF address at establishment or handover.

  Otherwise, the basic restoration procedure is executed.

• For a trusted WLAN with the single connection mode or the transparent single connection mode, only the basic P-CSCF Restoration mechanism may apply.

• In the P-CSCF Restoration extension procedure for TWAN access, the P-GW shall send the updated list of the addresses of available P-CSCFs toward the UE via the TWAN using the PCO IE.

Figure 2. PCRF Based P-CSCF Restoration for Trusted WLAN Access

Figure 3. HSS Based P-CSCF Restoration for Trusted WLAN Access

For Un-Trusted WLAN Access

An ePDG which supports the P-CSCF Restoration extension for untrusted WLAN shall forward the UE capability (UE support of the P-CSCF restoration extension) in the APCO information element to the P-GW over the S2b interface at the PDN connection establishment (or handover) over S2b.

In the P-CSCF Restoration extension procedure for untrusted WLAN access, the P-GW shall send the updated list of the addresses of available P-CSCFs toward the UE via the ePDG using the APCO IE.

Figure 4. PCRF Based P-CSCF Restoration for Un-Trusted WLAN Access

Figure 5. HSS Based P-CSCF Restoration for Un-Trusted WLAN Access

• Release 13 3GPP TS 23.380: IMS Restoration Procedures

• Release 13 3GPP TS 24.008: Mobile radio interface Layer 3 specification; Core network protocols

• Release 13 3GPP TS 29.212: Policy and Charging Control (PCC); Reference points

• Release 13 3GPP TS 29.273: 3GPP EPS AAA Interfaces

• Release 13 3GPP TS 29.274: 3GPP Evolved Packet System (EPS); Evolved General Packet Radio Service (GPRS) Tunnelling Protocol for Control plane (GTPv2-C); Stage 3

Use the following configuration commands for encoding Supported-Features AVP in the AAR message to AAA server via S6b interface.

configure
   context context_name
      aaa group group_name
         diameter authentication encode-supported-features pcscf-restoration-indication
         end

Use the following configuration for P-CSCF Restoration supported feature.

configure
   context context_name
      ims-auth-service service_name
         policy-control
            diameter encode-supported-features pcscf-restoration-ind
            end

Use the following configuration to enable P-CSCF Restoration for Emergency PDNs.

configure
   context context_name
      pgw-service service_name
         pcscf-restoration { hss-solution | custom-hss-solution }
         pcscf-restoration emergency-pdn
         end

Use the following configuration to enable Re-Auth after S6b triggered P-CSCF Restoration of WLAN.

configure
   context context_name
      pgw-service service_name
         pcscf-restoration s6b-reauth
         end

show aaa group all

This show command displays pcscf-restoration-ind as part of Supported-Features if this feature is configured under AAA group.

show aaa group all
 Group name:    default
 Context:   local
 
 Diameter config:
  Authentication:
....
Supported-Features:   pcscf-restoration-ind
....

show ims-authorization sessions full all

This command generates a display that indicates the negotiation status of this feature.

The following sample display is only a portion of the output which shows pcscf-restoration-ind among the Negotiated Supported Features.

show ims-authorization sessions full all
   
 CallId: 00004e22            Service Name:  imsa-Gx
   IMSI: 123456789012341
   ....
 Negotiated Supported Features:
   3gpp-r8
   pcscf-restoration-ind
 ....

show pgw-service name <pgw_service>

This command generates a display that indicates the configuration status of this feature.

The following sample display is only a portion of the output.

P-GW service output is enhanced to clearly specify HSS-based solution of MME-Triggered; this avoids confusion with the HSS-based S6b Triggered solution. In addition, it displays whether P-CSCF Restoration supported for Emergency PDNs and/or Re-Auth After s6b Triggered P-CSCF Restoration is enabled.

show pgw-service name <pgw_service>
   
    Service name : pgw_service
    Restoration solution : HSS-based MME-Triggered (Rel12)
    P-CSCF Restoration supported for Emergency PDNs : Yes/No
    Re-Auth After s6b Triggered P-CSCF Restoration : Enabled / Disabled
 ....

show aaa group all

The Supported Features field in this show command output displays whether or not the P-CSCF Restoration feature is configured as part of the Supported-Features AVP.

This supported feature is displayed only when the feature license is configured.

show ims-authorization sessions full all

The Negotiated Supported Features field in this show command output displays whether or not the P-CSCF Restoration feature is negotiated with PCRF.

This supported feature is displayed only when the feature license is configured.

show license information

If the license to enable the P-CSCF Restoration feature is configured, then the show license information command displays the associated license information.

show pgw-service name <pgw_service>

P-GW service output is enhanced to clearly specify HSS-based solution of MME-Triggered; this avoids confusion with the HSS-based S6b Triggered solution. In addition, it displays whether P-CSCF Restoration supported for Emergency PDNs and/or Re-Auth After s6b Triggered P-CSCF Restoration is enabled.

show pgw-service name <pgw_service>
   
    Service name : pgw_service
    Restoration solution : HSS-based MME-Triggered (Rel12)
    P-CSCF Restoration supported for Emergency PDNs : Yes/No
    Re-Auth After s6b Triggered P-CSCF Restoration : Enabled / Disabled
 ....

show pgw-service statistics all

This command provides statistics on the number of P-CSCF Restorations.

The MME received P-CSCF Restoration count has moved from " P-CSCF Restoration Indications received:" to "MME triggered Restoration". Now, " P-CSCF Restoration Indications received:" shows total number of P-CSCF Restoration indications received (HSS Triggered and PCRF Triggered). Bulkstat counter "sessstat-pcscf-recovery-count" will continue to display the MME received P-CSCF Restoration only.

The total number of triggers received on any interface (MME/PCRF/S6b) = Basic + Extended + Ignored (ignored for reasons such as restoration already in progress, license not present, validation check fails, or call not connected).

PDNs Released By Reason:
  Network initiated release:           0    MME initiated release:             0
    Admin disconnect:                  0    S4 SGSN initiated release:         0
    GTP-U error ind:                   0
    SGW path failure:                  0
    Local fallback timeout:            0
    UE P-CSCF Reselect not supported:  0
…

 S2bGTP-to-eHRPD handover:               eHRPD-to-S2bGTP handover:
        Attempted:                       0      Attempted:                      0
        Succeeded:                       0      Succeeded:                      0
        Failed:                          0      Failed                          0
 …

     P-CSCF Restoration Indications received:              <total_count at service level>
      HSS Triggered Restoration:
        MME Triggered Restoration:                      <>
            Basic Restoration Performed:                <> 
            Extension Restoration Performed:            <>
        S6b Triggered Restoration:                      <>
            Basic Restoration Performed:                <> 
            Extension Restoration Performed:            <>
      PCRF Triggered Restoration:                       <>
          Basic Restoration Performed:                  <>
          Extension Restoration Performed:              <>

      Data Statistics Per Interface:
…

show srp checkpoint statistics active verbose

This command provides the following P-CSCF Restoration micro checkpoint information:

.
.
     Total pgw ubr_mbr micro-chkpnt sent:                 0
     Total pcscf update micro-chkpnt sent:                 0

show srp checkpoint statistics standby verbose

This command provides the following P-CSCF Restoration micro checkpoint information:

.
.
 PGW ubr_mbr session microchkpt rcvd:           0
   
PCSCF info update microchkpt rcvd:             0

This section provides information on how to monitor the logs that are generated relating to the HSS/PCRF-based P-CSCF Restoration feature.

Gx Diameter Protocol Logs

<<<<OUTBOUND  13:52:06:117 Eventid:92820(5)
....
 [V] [M] Supported-Features: 
   [M] Vendor-Id: 10415 
   [V] Feature-List-ID: 1 
   [V] Feature-List: 16777217 
....

INBOUND>>>>>  13:52:06:118 Eventid:92821(5)
....
 [V] [M] Supported-Features: 
   [M] Vendor-Id: 10415 
   [V] Feature-List-ID: 1 
   [V] Feature-List: 16777216
....

INBOUND>>>>>  13:52:26:119 Eventid:92821(5)
....
[M] Re-Auth-Request-Type: AUTHORIZE_ONLY (0)
[V] PCSCF-Restoration-Indication: 0
....

S6b Diameter Protocol Logs

<<<<OUTBOUND  15:37:23:561 Eventid:92870(5)
....
 [V] [M] Supported-Features: 
   [M] Vendor-Id: 10415 
   [V] Feature-List-ID: 1 
   [V] Feature-List: 1 
....

INBOUND>>>>>  15:37:23:562 Eventid:92871(5)
....
 [V] [M] Supported-Features: 
   [M] Vendor-Id: 10415 
   [V] Feature-List-ID: 1 
   [V] Feature-List: 1 
....

INBOUND>>>>>  15:37:43:562 Eventid:92871(5)
....
[M] Re-Auth-Request-Type: AUTHORIZE_ONLY (0)
[V] RAR-Flags: 2
....

P-GW Schema

The following counters are specific to pcscf-recovery:

• sessstat-pcscf-recovery-count

• sessstat-pcscf-recovery-basic-count

• sessstat-pcscf-recovery-extension-count

• sessstat-s6b-pcscf-recovery-count

• sessstat-s6b-pcscf-recovery-basic-count

• sessstat-s6b-pcscf-recovery-extension-count

• sessstat-pcrf-pcscf-recovery-count

• sessstat-pcrf-pcscf-recovery-basic-count

• sessstat-pcrf-pcscf-recovery-extension-count

SAEGW Schema

The following counters are specific to pcscf-recovery:

• pgw-sessstat-pcscf-recovery-count

• pgw-sessstat-pcscf-recovery-basic-count

• pgw-sessstat-pcscf-recovery-extension-count

• pgw-sessstat-s6b-pcscf-recovery-count

• pgw-sessstat-s6b-pcscf-recovery-basic-count

• pgw-sessstat-s6b-pcscf-recovery-extension-count

• pgw-sessstat-pcrf-pcscf-recovery-count

• pgw-sessstat-pcrf-pcscf-recovery-basic-count

• pgw-sessstat-pcrf-pcscf-recovery-extension-count