PCSI LI Support

Overview

The lawful intercept (LI) network solution design requires the LI Intercept Access Point (IAP) vendors to implement support for multiple (two instances) regionally deployed SS8 Delivery Function systems. The SBC acts as an interconnect SBC to provide peering between an IMS network and other peering networks (CDMA, business, and other service providers). The SBC interfaces with two XCIPIO mediation servers. Any of these mediation servers can provision the network elements deployed in the network. Network elements expose provisioning, call data and/or call content interface towards these mediation servers for lawful interception. This feature supports the following types of media which can be intercepted:

• Audio
• Video
• MSRP
• FECC
• BFCP-TCP

The lawful Interception solution has three interfaces between the network element and the mediation server. These are used to provide provisioning, call data (signaling), and call content (media) information. These interfaces are created after the connection is established between the mediation server (DF – delivery function) and the network element (AF – access function). The interface from the mediation server to the lawful interception agency is standardized. The interfaces between AF and DF are defined as:

• X1 or INI-1 interface for provisioning targets
• X2 or INI-2 interface for providing signaling information for the subject
• X3 or INI-3 interface for providing media or call content for the subject

Where, X interfaces are specified by 3GPP standards while INI interfaces are specified by ETSi standards.

The P-Com.Session-Info (PCSI) Lawful Interception (LI) solution requires the Ribbon network elements (EMS, PSX and SBC 7000) to provide lawful interception by supporting the X1 and X3 interfaces only. In the Ribbon LI solution the X1 interface is supported by the EMS and the storage of the target table is done by the PSX. The X3 interface is supported by SBC7000 (hereafter referred to as SBC).

The supported LI elements (SBC, PSX and EMS) with SS8 as the mediation server are required to supported X1 and X3 interfaces only. The X2 interface is not implemented by the SBC. 

Call Flow

1. Provision the intercept server X1 transport address and TLS certificate that EMS has to use to initiate a transport connection over the X1 interface. These are individually specified for the mediation servers in the network.
2. Provision the SBC using the RibbonEMS X3 intercept transport address and required IPsec details. This address is used to send X3 messages. These are individually specified for the mediation servers in the network.
   
3. The EMS establishes a connection with the SS8 intercept server and executes the necessary authentication procedures to bring up the transport connection.
4. The SS8 Intercept server, using the X1 interface, provisions the target URI which is sent to the PSX over the PIPE interface by the EMS. This is a fully specified SIP/SIPS or TEL URI. This completes the provisioning of a target using the X1 interface. Any call received by this target is intercepted.
5. The IMS network sends a SIP INVITE/18x/200 OK or re-INVITE/200 OK that contains a P-Com-Session-Info header. This header contains a correlation ID and one of more prospective target URI, which is looked into the target table maintained in the PSX.
6. The SBC receives a P-Com-Session-Info header in any of the above listed SIP messages and sends a policy request with the target URI to PSX. PSX uses this to perform a lookup in the target table created using the X1 provisioning interface. Assuming that a target URI matches, PSX provides the X3 transport address that is stored along with the target URI. This information is received by the SBC in the policy response.

7. The SBC finds the LI information in the policy response and understands that the call has to be intercepted. It uses the correlation ID received in SIP messages as well as X3 transport address received from the PSX to fork the media stream to the appropriate mediation server using the X3 interface. This interface forks the media stream over TCP that is established over a secured IPsec tunnel.

Figure 1: PCSI Call Flow

EMS Enhancement

The SS8 mediation server is represented as DF (Delivery Function). The EMS and PSX together are represented as AF (Access Function). The LI database operations are performed on the PSX and X1 provisioning interface is implemented on the EMS. For any X1 operation which has impact on PSX LI database operation, EMS uses PIPE interface with the PSX.

This transport connection is with or without TLS depending upon the configuration. The EMS sends the Authentication Request towards DF with the following IEs when the transport connection is successfully established:

• Generic Message Header: This header uses an IE Access Function Identifier set to zero in Authentication Request.
• Transaction Identity: EMS generates a unique transaction identifier and saves a copy of this.
• Interface Type: Is set to 0x01 – INI1.
• Serial Number: EMS maps it from system configuration.
• MAC Address: EMS maps it from system configuration.
• Model Number: EMS maps it from system configuration.
• Software Revision: EMS maps it from system configuration.
• Provisioning State:
  
  • Set to 0x00 when PSX LI DB is not initialized.
  • Set to 0xDC when PSX LI DB is initialized.
• Hash: NA

The EMS accepts Global Info Set Request coming from DF with the following IE:

• Link Polling Frequency
• Link Inactivity Interval
• AF Database Clean-up Interval

It checks that the Access Function Identifier IE of Generic Message Header in Global Info Set Request has non zero value. If Access Function Identifier is a non-zero value, EMS stores this value and uses it in subsequent EMS generated messages. EMS sends the Transaction Identifier that it receives in the Global Info Set Request message. It also accepts the Service Provider Identity.

Link Options IE is enumerated type with following sub fields, which EMS saves and uses during session maintenance and release procedures:

• Link Polling Frequency
• Link Inactivity Interval
• AF Database Clean-up Interval

The EMS checks that data type for all these sub fields is integer and stores their respective values. Timestamp data type for this IE is in time format. If the AF (PSX) Provisioning State database is empty, it accepts the zero value of this IE.  Any IE received with incorrect data type is rejected with message Operation Result with IEs.

If Access Function Identifier is set to zero then the EMS rejects using Operation Result message with following IEs.

The EMS supports session maintenance, session release, and database clean up procedures using the timers mentioned below:

• Link Polling Frequency (LPF) or Keepalive Interval
• Link Inactivity Interval (LII)
• AF Database Clean-up Interval (DCI)
  

For configuring the EMS for this feature, refer to Managing Mediation Servers.

PSX Enhancement

The EMS configures the PSX LI DB over the PIPE interface. It creates entries for the Target Service List and other parameters. The LI DB performs validation of data type for each parameter.

The PSX queries the LI DB based on the key Target Service List received in a Warrant Info Get Request and returns all parameters. If the Target Service List is missing in the Warrant Info Get Request, the query is based on the key DF1 ID next Target Service List in LI DB.

The PSX provides the following LI information in POL-RES:

• IP address type
• IP address
• Transport protocol type
• Port
• Encryption 
  

SBC Enhancement

The SBC as an interconnect SBC accepts P-Com.Session-Info headers received in the following message types received from the S-CSCF:

• Initial INVITE
• 18x SIP response
• 200 OK response to the initial INVITE
• re-INVITE and the 200 OK response to the re-INVITE for scenarios such as call transfer.

Where the P-Com.Session-Info format is:

P-Com.Session-info = "P-Com.Session-Info" HCOLON= 
corrID *( SEMI involved-party )
corrID = "corrID" EQUAL word [ "@" word ]
involved-party = LAQUOT involved-uri RAQUOT
involved-uri = SIP-URI / TEL-URI / SIPS-URI

 SIP-URI / TEL-URI / SIPS-URI is specified and can be in a different format as mentioned in the following table:

The SBC sends the URI received in P-Com.Session-Info to the PSX in a POL-REQ message. This header is received in any of the SIP message (Invite/re-INVITE . 200 OK, 18X) from IMS network. Presence of P-Com.Session-Info in SIP response messages like 18x/200OK or in SIP RE_INVITE request triggers a light weight policy dip. The SBC truncates the Correlation ID (corrID), copied from P-Com.Session-Info, to 16 byte ASCII character (if it is more than 16 byte). If the correlation ID size is less than 16 bytes, the corrID is added with zero. The SBC replies with 4xx response when Correlation ID (corrID) validations fails.

The SBC accepts the LI information from the PSX in POL-RES message. The absence of LI information in POL-RES from the PSX to the SBC means interception is disabled for target URI.  The connection towards X3 connection is created when the mediation server state is enabled from the CLI. The checks listed here are to trigger interception of media. Apart from the checks listed here, the IP address of the mediation server returned in POL-RSP is matched with the address of the mediation server address configured in the CDC configurations. Also, the matched mediation server is ensured to be admin enabled before an interception is triggered.

• Transport Type = TCP
• Encryption = None/IPsec
• IP Address Type = IPv4 or IPv6

The Mediation Server creates a connection with IPsec over TCP connection towards the IP and port received in the LI information of POL-RES and meets the following IPsec requirements:

• Transport mode and Authentication algorithm pre-shared key. 
  

The Mediation Server IP, port and pre-shared keys for X3 connection are already configured on the SBC. The SBC verifies that the configured X3 connection details (IP, type and port) are same under CDC as received in the LI information of POL-RES message.

If there is a change in mediation server through X1 interface for a target URI between the INVITE and Re-INVITE message, the SBC forks the media streams over the X3 connection for the new mediation server.