Selected IP Traffic Offload Correlation Identifier

Description

The SIPTO Correlation ID is a key parameter in 3GPP core network architecture that facilitates Selected IP Traffic Offload (SIPTO) at the local network. SIPTO is a feature introduced in 3GPP Release 10 that allows mobile operators to offload specific types of traffic, such as internet-bound traffic, at a point closer to the user equipment (UE) rather than routing it all the way through the core network. This reduces backhaul costs, lowers latency, and improves user experience. The SIPTO Correlation ID specifically applies to the case where SIPTO is performed at a local network, meaning the traffic is offloaded at a local gateway (L-GW) that is collocated with the radio base station, such as a Home eNodeB (HeNB) or an eNodeB (eNB).

When a UE establishes a PDN connection for SIPTO at local network, the network needs to ensure that the user plane path between the base station and the L-GW is direct, without traversing the core network. The SIPTO Correlation ID is a parameter that enables this direct path when the (H)eNB and L-GW are collocated. It is used during the PDN connection establishment procedure to correlate the request from the UE with the local gateway. The correlation ID is typically generated by the L-GW or the MME and is included in signaling messages to ensure that the correct local gateway is selected and that the user plane is established directly between the base station and the L-GW. This avoids the need to route user plane traffic through the Serving Gateway (SGW) and Packet Data Network Gateway (PGW) in the core network, thereby achieving the offload objective.

The architecture for SIPTO at local network involves several key components. The UE requests a PDN connection with a specific Access Point Name (APN) that is configured for SIPTO. The MME, during the PDN connection establishment, determines whether SIPTO at local network is allowed based on the UE's subscription and local policies. If allowed, the MME selects an L-GW that is collocated with the (H)eNB. The SIPTO Correlation ID is then used to bind the PDN connection to that specific L-GW. The (H)eNB, upon receiving the correlation ID, can establish a direct user plane path to the L-GW, often using a local interface such as the S1-U or a proprietary interface. This direct path bypasses the SGW and PGW, reducing latency and backhaul usage. The correlation ID ensures that the correct L-GW is used and that the user plane is properly anchored.

The SIPTO Correlation ID is particularly important in scenarios where multiple L-GWs are present or where the (H)eNB and L-GW are collocated but need to be logically separated. It provides a unique identifier that links the PDN connection to the specific local gateway, enabling the direct user plane path. Without this correlation ID, the network would not be able to determine which L-GW to use for the offload, and traffic might be routed through the core network unnecessarily. The correlation ID is typically a string or an integer that is unique within the scope of the local network. It is included in the PDN Connectivity Request and subsequent signaling messages, such as the Create Session Request and Create Session Response.

In terms of standardization, the SIPTO Correlation ID is defined in 3GPP TS 23.060 (General Packet Radio Service (GPRS) enhancements for Evolved Universal Terrestrial Radio Access Network (E-UTRAN) access) and TS 23.401 (General Packet Radio Service (GPRS) enhancements for Evolved Universal Terrestrial Radio Access Network (E-UTRAN) access). These specifications describe the procedures for SIPTO at local network and the role of the correlation ID. The definition has remained stable across releases from Rel-12 to Rel-19, indicating that the concept has not undergone significant changes. This stability reflects the maturity of the SIPTO feature and its continued relevance in 4G and 5G networks, where local offload remains an important tool for operators to manage traffic growth and reduce costs.

Purpose & Motivation

The SIPTO Correlation ID exists to enable efficient and direct user plane paths for Selected IP Traffic Offload (SIPTO) at the local network, specifically when the base station and local gateway are collocated. SIPTO was introduced to address the exponential growth of mobile data traffic and the resulting strain on core network resources and backhaul. By offloading traffic at the edge of the network, operators can reduce the load on the core network, lower latency, and improve the overall user experience. However, to achieve these benefits, the network must be able to establish a direct user plane path between the base station and the local gateway without routing through the core network. The SIPTO Correlation ID provides the necessary correlation to ensure that the correct local gateway is selected and that the user plane is established directly.

Before the introduction of SIPTO, all user plane traffic had to traverse the core network, including the Serving Gateway (SGW) and Packet Data Network Gateway (PGW). This centralized architecture was inefficient for traffic that could be offloaded locally, such as internet-bound traffic from users in a specific location. It also increased latency and backhaul costs. SIPTO was designed to break out such traffic at a local gateway, which could be collocated with the base station. However, without a correlation mechanism, the network could not reliably associate a PDN connection with a specific local gateway, especially in deployments with multiple local gateways. The SIPTO Correlation ID solves this problem by providing a unique identifier that links the PDN connection to the local gateway, enabling the direct user plane path.

The SIPTO Correlation ID also addresses the need for seamless mobility and session continuity. When a UE moves between base stations, the correlation ID helps ensure that the SIPTO session is maintained or correctly re-anchored. It allows the network to identify that a PDN connection is subject to SIPTO at local network and to apply the appropriate policies. This is particularly important in heterogeneous networks where macro and small cells coexist, and traffic offload is a key strategy for capacity and coverage. The correlation ID is a small but critical piece of the SIPTO architecture, enabling operators to realize the benefits of local offload without compromising network integrity or user experience.