Loopback routeing

Description

Loopback routeing is a technique used in IP Multimedia Subsystem (IMS) networks to route Session Initiation Protocol (SIP) requests back to the visited network for local breakout. It is defined in the context of the Roaming Architecture for Voice over IMS with Local Breakout (RAVEL), as specified in 3GPP TS 23.228. This architecture allows a roaming user to have their voice calls anchored in the visited network rather than being routed all the way to the home network, which can reduce latency and optimize resource usage. Loopback routeing is a key enabler of this architecture.

In a typical roaming scenario, when a user makes a call, the SIP request is initially sent to the home network's IMS. However, with local breakout, the goal is to keep the media and signalling within the visited network as much as possible. Loopback routeing achieves this by routing the SIP request from the home network back to the visited network. This is done by manipulating the SIP routing headers, such as the Route header, to direct the request to a specific node in the visited network, often the P-CSCF (Proxy Call Session Control Function) or a Transit Function. The request then proceeds through the visited network's IMS to establish the session.

The mechanism involves several steps. When the UE initiates a call, it sends a SIP INVITE to the P-CSCF in the visited network. The P-CSCF forwards it to the home network's S-CSCF (Serving CSCF) based on the user's profile. The S-CSCF, applying the RAVEL architecture, determines that local breakout is required and modifies the SIP request to include a Route header pointing back to a node in the visited network. This node, often called the Loopback Function or Transit Function, receives the request and routes it to the appropriate destination within the visited network, such as the IMS core or the PSTN gateway. The response follows the reverse path.

Loopback routeing is specified in 3GPP TS 24.229 and TS 29.079. TS 24.229 defines the SIP protocol procedures for IMS, including the handling of Route headers for loopback. TS 29.079 specifies the signalling flows and procedures for the RAVEL architecture, including the loopback function. The term was introduced in Release 11, which is when the RAVEL architecture was standardized. Since then, the definition has remained largely unchanged, although the reference to the specification was updated in Release 17 from TS 23.228 to TS 24.229, reflecting a shift in the primary specification for the definition.

Loopback routeing is essential for enabling efficient VoLTE roaming. Without it, all SIP signalling and media would be tromboned to the home network, leading to increased latency and inefficient use of international links. By keeping the traffic local, operators can offer better quality of service and reduce costs. The technique is transparent to the end user and is implemented within the IMS core network.

Purpose & Motivation

Loopback routeing was created to address the inefficiencies of traditional roaming architectures for voice over IMS. In early IMS roaming, all SIP signalling and media were routed through the home network, even when both the calling and called parties were in the same visited network. This 'tromboning' resulted in unnecessary latency, increased bandwidth consumption on international links, and higher costs for operators. The RAVEL architecture was developed to enable local breakout, where the media and signalling could be kept within the visited network. Loopback routeing is the mechanism that makes this possible by allowing the home network to redirect SIP requests back to the visited network.

The problem it solves is particularly relevant for VoLTE roaming, where quality of service and latency are critical. By routing the SIP request back to the visited network, the call setup can be completed locally, and the media path can be optimized. This reduces the round-trip time and improves the user experience. It also allows the visited network to apply local policies and services, such as emergency calls and lawful interception, more effectively.

Loopback routeing was introduced in Release 11 as part of the RAVEL work item. Prior to this, there was no standardized way to achieve local breakout for voice over IMS, leading to vendor-specific implementations and interoperability issues. The standardization of loopback routeing provided a common framework that operators and vendors could rely on, facilitating multi-vendor deployments and roaming agreements. The concept has since been adopted in later releases and is a fundamental part of VoLTE roaming.