Correlation Identifier

Description

The Correlation ID is a parameter used in 3GPP networks to support Local IP Access (LIPA) for a PDN connection. It enables a direct user plane path between the Home eNodeB (HeNB) and the Local Gateway (L-GW), bypassing the core network for local traffic. This is particularly important for femtocell deployments where traffic destined for the local network should not be routed through the mobile core, reducing latency and backhaul costs. The Correlation ID is exchanged during the establishment of a LIPA PDN connection and is used by the HeNB and L-GW to correlate the user plane bearers.

In a LIPA scenario, the HeNB and L-GW are typically co-located or connected via a local network. When a UE requests a PDN connection for LIPA, the MME selects an L-GW and includes the Correlation ID in the session management messages. The HeNB receives this ID and uses it to establish a direct tunnel to the L-GW. The Correlation ID ensures that the correct L-GW is paired with the correct HeNB and PDN connection, preventing misrouting of traffic. It is a key element in the architecture defined in 3GPP TS 23.401 and TS 23.060 for LIPA and SIPTO (Selected IP Traffic Offload).

The Correlation ID is typically a binary string or integer that is unique within the scope of the HeNB and L-GW. It is generated by the L-GW or the MME and is included in the GTP-C messages (e.g., Create Session Request/Response) and in the S1-AP messages between the HeNB and MME. The HeNB uses the Correlation ID to identify the L-GW's user plane address and to set up the direct user plane path. This allows the HeNB to forward uplink packets directly to the L-GW and receive downlink packets from it, without traversing the S-GW and P-GW.

The introduction of the Correlation ID in Release 10 was part of the broader effort to support femtocells and local offloading. It addresses the need for efficient local routing in home and enterprise environments, where a significant portion of traffic may be local (e.g., accessing a home media server or corporate intranet). By enabling a direct path, it reduces the load on the core network and improves user experience. The Correlation ID is also used in SIPTO scenarios, where traffic is offloaded at the local network.

From a standardization perspective, the Correlation ID is defined in TS 23.060 (GPRS) and TS 23.401 (EPS). Its definition has remained stable since Release 10, with only minor editorial changes. It is a critical parameter for LIPA and SIPTO, and its proper handling is essential for the correct operation of these features. The Correlation ID is not user-visible and is used solely within the network infrastructure.

Purpose & Motivation

The Correlation ID was introduced to enable efficient local IP access (LIPA) and selected IP traffic offload (SIPTO) in femtocell deployments. Before its introduction, all traffic from a HeNB, including local traffic, had to be routed through the core network, which was inefficient and costly. The Correlation ID solves this by allowing the HeNB and L-GW to establish a direct user plane path, bypassing the core network for local traffic.

This is particularly important for home and enterprise environments, where users often access local resources such as printers, media servers, or intranet servers. Routing such traffic through the core network would introduce unnecessary latency and consume backhaul capacity. The Correlation ID enables the HeNB to identify the correct L-GW and set up a direct tunnel, ensuring that local traffic stays local. It also supports the offloading of internet traffic at the local network, reducing the load on the core.

The Correlation ID was standardized in Release 10 as part of the LIPA and SIPTO work items. It addresses the limitations of previous approaches, which did not support local routing. By providing a unique identifier for the L-GW, it ensures that the correct gateway is used for each PDN connection. This is essential for multi-vendor deployments, where the HeNB and L-GW may come from different vendors. The Correlation ID thus enables a scalable and interoperable solution for local offloading.