Exit Point

Description

In the IP Multimedia Subsystem (IMS) architecture, an exit point is a functional entity that serves as the gateway for signaling and media traffic leaving an operator's network towards another network. It is defined in 3GPP TS 23.228 (IMS architecture) and TS 24.229 (IMS call control protocols). The exit point is realized by the Interconnection Border Control Function (IBCF), which is responsible for border control between IMS networks. When an IMS entity within a network needs to send a request to an entity in another network, it routes the request via the local exit point (IBCF). The IBCF then forwards the request to the destination network, potentially discovering the entry point (IBCF) of the other network if necessary.

The exit point plays a crucial role in inter-operator communication, ensuring security, interoperability, and policy enforcement. The IBCF performs functions such as topology hiding, session border control, protocol interworking, and media handling. It also applies operator-defined policies for border control, which may include access control, QoS marking, and charging. The exit point is typically paired with an entry point in the peer network, forming a border control layer between IMS domains.

The concept of border control in IMS was introduced to address the challenges of interconnecting IMS networks from different operators, which may have different capabilities, security policies, and addressing schemes. Without a standardized border control function, interconnection would require custom solutions, leading to complexity and interoperability issues. The IBCF, acting as the exit point, provides a standardized way to handle signaling and media at the network boundary.

In terms of operation, when a request is sent towards another network, the originating network's IBCF (exit point) receives the request from an internal IMS entity (e.g., S-CSCF). The IBCF may modify the request according to local policy, such as hiding internal network topology, and then forwards it to the peer network's IBCF (entry point). The peer IBCF then forwards the request to the appropriate entity in its network. The discovery of the entry point can be done via DNS (e.g., using ENUM or NAPTR records) or through pre-configured routes.

The exit point is also involved in media handling. For media, the IBCF may control a Media Gateway (MGW) or a Transition Gateway (TrGW) to perform functions like transcoding, media relaying, and NAT traversal. This ensures that media streams can traverse network boundaries even if the networks use different IP versions or codecs.

Over the releases, the definition of exit point has remained largely consistent, with minor editorial changes. The core concept of routing via a local IBCF has been stable, reflecting the maturity of the IMS border control architecture. The exit point continues to be a fundamental element for secure and efficient inter-IMS communication.

Purpose & Motivation

The exit point was created to solve the problem of interconnecting IMS networks securely and efficiently. In the early days of IMS, operators needed a way to exchange signaling and media with other networks without exposing their internal topology or compromising security. The exit point, implemented by the IBCF, provides a controlled boundary where policies can be enforced, protocols can be interworked, and topology can be hidden. It also simplifies routing by providing a single point of egress for all external communications. Before the exit point concept, interconnection was often ad-hoc, leading to interoperability issues and security vulnerabilities. The standardized exit point, along with the entry point, enables seamless inter-operator IMS services such as voice over LTE (VoLTE) and Rich Communication Services (RCS). It also supports regulatory requirements like lawful interception and emergency calls. The exit point remains essential as IMS evolves towards 5G, where it continues to facilitate interconnection between 5G core networks and IMS.