4.14 Border Control concepts

23.2283GPPIP Multimedia Subsystem (IMS)Release 18Stage 2TS

Based on operator preference, border control functions may be applied between two IM CN subsystem networks or between an IM CN subsystem network and other SIP based multimedia network. These functions are provided by the IBCF and include:

– Controlling transport plane functions;

– Supporting functions to allow establishing communication between disparate address realms’ SIP applications;

– Supporting functions to allow establishing communication between IM CN subsystems using different media codecs based on the interworking agreement and session information;

– Providing network configuration hiding to restrict the following information from being passed outside of an operator’s network: exact number of S‑CSCFs, capabilities of S‑CSCFs, or capacity of the network, etc;

NOTE 1: Network configuration hiding was not intended to be invoked in IMS roaming scenarios when the P‑CSCF and IBCF are both located in the visited network as information available in certain SIP headers may be used by the home network for further processing of signalling messages.

– Screening SIP signalling information based on source/destination and operator policy (e.g. remove information that is of local significance to an operator) and optionally, for an IBCF located in the home network, policing the IMS Communication Service ID;

– Generation of CDRs;

– Invoking an IWF when interworking between different SIP profiles or different protocols (e.g., SIP and H.323) is necessary; in this case the IWF acts as an entry point for the IMS network;

NOTE 2: IWF and IBCF may be co-located. The IWF is not specified within this release of the specification.

– Selecting the appropriate signalling interconnect.

– Indicating whether an incoming SIP request is to be handled as an originating request by subsequent nodes in the IMS network.

– For an originating session leaving an IBCF, the IBCF of the originating network, if configured through operator policies, invokes an AS for the signing of attestation and identity information, if available in the incoming request. The IBCF includes the signed information in the outgoing request.

– For an originating session leaving an IBCF, the IBCF of the originating network, if configured through operator policies, invokes an AS for the signing of Resource-Priority related information, if available in the incoming request. The IBCF includes the signed Resource-Priority related information in the outgoing request.

– For a terminating session entering the IBCF without identity attestation information, the IBCF adds, if configured through policies, gateway attestation information based on the network from which the request was received.

– For a terminating session entering the IBCF with signed identity attestation information, the IBCF, if configured through policies, invokes an AS for signature verification.

– For a terminating session entering the IBCF with signed Resource-Priority information, the IBCF, if configured through policies, invokes an AS for signature verification.

If border control concepts are to be applied in an IMS network, the IBCF acts as an entry point for this network (instead of the I‑CSCF), and also acts as an exit point for this network.

NOTE 3: In this case the IBCF and I‑CSCF may be co-located as a single physical node.

Based on local configuration, the IBCF may perform transit routing functions (see clause 5.19).

More detailed description of these functions is provided in Annex I.