Content for  TS 33.127  Word version:  18.6.0

The capabilities defined in this clause apply to the interception of IMS-based services. The target of interception can be a subscriber of the CSP, an inbound roamer or a non-local ID. The network function involved in providing the interception of IMS-based services are determined based on the deployment option, the network configuration, LI service scope and the IMS session including the roaming scenarios. The IRI-POI functions are provided by the network functions that handle the SIP messages (those network functions are referred to as IMS Signalling Functions) and the triggered CC-POI functions are provided by the network functions that handle the media (these network functions are referred to as IMS Media Functions). The CC-TF functions are also provided by the network functions that handle the SIP messages (referred to as IMS Signalling Functions) and manage the IMS Media Functions. The network functions that provide the CC-TF functions can be different from the network functions that provide the IRI-POI functions. An architecture depicting the LI for IMS is depicted in Figure 7.4-2 below.

The LICF present in the ADMF receives the warrant from an LEA, derives the intercept information from the warrant and provides it to the LIPF. The LIPF present in the ADMF provisions IRI-POI, CC-TF, MDF2 and MDF3 over the LI_X1 interfaces. The CC-TF sends the CC intercept trigger to the CC-POI over LI_T3 interface. The IRI-POI generates the xIRI and delivers the same to the MDF2 over LI_X2 interface. The CC-POI generates the xCC and delivers the same to the MDF3 over LI_X3 interface. The MDF2 generates IRI messages from the received xIRI and delivers those IRI messages to the LEMF over LI_HI2 interface. The MDF3 generates the CC from the received xCC and delivers that CC to the LEMF over LI_HI3 interface. The network configuration and IMS service scenarios including the roaming scenarios determine the network functions that provide the IRI-POI, CC-TF and CC-POI functions. The network function that provides the IRI-POI or CC-TF is referred to as IMS Signalling Function in Figure 7.4-2 and the network function that provides the CC-POI functions is referred to as IMS Media Function in Figure 7.4-1.

The LIPF provisions the intercept related information associated with the following target identities to the IRI-POI and CC-TF present in the IMS Signalling Functions:

• IMPU.
• IMPI.
• PEI (IMEI only).
• IMEI.

The IRI-POI and CC-TF shall also support interception of non-local identities in any of the IMPU formats (SIP URI, TEL URI as well as the E.164 number in a SIP URI or TEL URI). In case a PBX is connected to a PLMN, the interception of targets that are served by PBX may be treated as non-local identities in the connected PLMN.

Depending on the session direction, different SIP parameters are used to identify the target subscriber. Further details on the use of SIP parameters in identifying a target is described in TS 33.128.

The IRI-POI detects the SIP messages that are related to a target subscriber and then generates and delivers the related xIRI to the MDF2 over LI_X2. The following IMS Network Functions (i.e. IMS Signalling Functions) that handle SIP signalling for IMS sessions may provide the IRI-POI functions:

• S-CSCF.
• E-CSCF.
• P-CSCF.
• IBCF.
• MGCF.
• Conference AS/MRFC.
• Telephony AS.
• PTC server.

Clause 7.4.6 gives more information from network topology/session perspective how different IMS Network Functions are to be used in providing the IRI-POI functions. The Telephony AS is one of the IMS Network Functions that provides the IRI-POI for STIR/SHAKEN and RCD/eCNAM (see clause 7.14.2).

The IRI-POI present in the IMS Signalling Function generates the following xIRI:

• Encapsulated SIP message.
• CC unavailable in serving PLMN.
• Start of interception with an established IMS session.

The encapsulated SIP message xIRI is generated and delivered to the MDF2 when the IRI-POI in the IMS Signalling Function detects that a SIP message is received from, or sent to, a target or processed on behalf of a target at the IMS Signalling Function. The CC unavailable in PLMN xIRI is generated and delivered to the MDF2 for the session scenarios where access to the target media is not available to the CSP (see clause 7.4.7.1). The start of interception with an established IMS session xIRI is generated when an interception is activated on an established IMS session. To support the possibility of generating such an xIRI, the IMS Signalling Function shall store and maintain the session related information including the media information for the life of all IMS sessions.

The list of parameters in each xIRI are defined in TS 33.128. Each xIRI shall include at the minimum the following information:

• Target identity.
• Additional identities associated with the target as observed by the IRI-POI.
• Time stamp.
• Correlation information.

The parameters in each xIRI are defined in TS 33.128.

The CC-TF detects the SIP messages that are related a target and then generates and sends a trigger to the CC-POI over the LI_T3 reference point. The CC-POI based on the trigger detects the media to be intercepted, generates the xCC and delivers the same to the MDF3. The following IMS Network Functions (i.e. IMS Media Functions and IMS Signalling Functions) may provide the CC-POI and CC-TF functions:

• IMS-AGW with CC-TF in P-CSCF.
• TrGW with CC-TF in IBCF.
• IM-MGW with CC-TF in MGCF.
• MRFP with CC-TF in AS/MRFC (see NOTE 3).
• MRFP with CC-TF in Conference AS/MRFC (see NOTE 2).
• PTC Server with CC-TF in PTC Server (see NOTE 1).

Clause 7.4.6 gives more information from network topology/session perspective how different IMS Network Functions are to be used in providing the CC-TF/CC-POI functions.

The CC-TF shall send CC intercept trigger to the CC-POI over LI_T3. The CC intercept trigger, at the minimum, shall consist of the following:

• Correlation Identifier.
• Media Identifier (e.g. SDP information).

The Correlation Identifier is used to correlate the xCC with the corresponding xIRI and is delivered from the CC-POI over the LI_X3 interface to the MDF3. The Media Identifier is used to identify the media packets that have to be intercepted.

For the delivery of intercepted media packets, the following information shall be passed from the CC-POI to the MDF3 in addition to the intercepted media packets:

• Target identity.
• Correlation identifier
• Time stamp.
• Direction (indicates media is from or to the target).

The IMS Network Functions that provide the IRI-POI, CC-TF and CC-POI functions can vary based on the network topology and session scenarios such as:

• Network topologies:
  • Non-roaming.
  • Roaming case with Local Break Out (LBO), VPLMN.
  • Roaming case with LBO, HPLMN.
  • Roaming case with Home Routed (HR), VPLMN.
  • Roaming case with HR, in HPLMN.
• Session types:
  • Normal sessions.
  • Emergency sessions.
  • Redirected sessions.
  • IMS specific services such as conferencing, PTC, music, announcements.
  • SMS over IMS.
• Target type:
  • Non-local ID target.

A deployment option within the CSP may also have a role in selection of the Network Functions. In the case of roaming case, the interception performed in the VPLMN and HPLMN are based on separate independent warrants. More detailed description of these scenarios is given in TS 33.128.

The IMS Network Functions that handle the target side of the session provide the IRI-POI functions except when the alternate option is used for the non-local ID target. When the alternate option is used for the non-local ID target, the IMS network function that handles the session-leg of the local served user connected directly to the non-local ID target. Table 7.4.6.2-1 below identifies the IMS Network Functions in providing the IRI-POI functions in a non-roaming case for various session scenarios.

Table 7.4.6.2-2 below identifies the IMS Network Functions in providing the IRI-POI functions in a roaming case for various session scenarios.

The interception capabilities for normal sessions as defined in Table 7.4.6.2-1 (non-roaming) and Table 7.4.6.2-2 (roaming) shall be used for the cases where the Conf-AS and the PTSC-Server are not under the control of CSP serving the warrant.

Table 7.4.6.2-3 shows the IMS Network Functions that provide the IRI-POI functions in the HPLMN for redirected sessions in a roaming case when the alternate option is used to provide the IRI-POI functions for the normal case. The IMS Network Functions that provide the IRI-POI for STIR/SHAKEN and RCD/eCNAM are listed in clause 7.14.2.

The IMS Network Functions that handle the target side (including the non-local ID target) of the session provide the CC-TF and CC-POI functions. For redirected scenarios, the IMS Network Functions that handle the redirected-to-user side of the session provide the CC-TF and CC-POI functions. Table 7.4.6.3-1 provides the IMS Network Functions that provide the CC-TF functions when the CC-POI functions are provided by the IMS Media Functions as indicated (also see clause 7.4.4.1).

Table 7.4.6.3-2 below identifies the IMS Media Functions that provide the CC-POI functions in a non-roaming case for session scenarios (PGW and PGW-U based options are not shown in the Table).

Table 7.4.6.3-3 below identifies the IMS Media Functions that provide the CC-POI functions in a roaming case for various session scenarios (PGW and PGW-U based options are not shown in the Table).