Selected IP Traffic Offload

Description

Selected IP Traffic Offload (SIPTO) is a 3GPP standard for optimizing data traffic routing in mobile networks by offloading specific IP data flows from the central core network to a more local breakout point. The primary objective is to avoid tunneling all user plane traffic through the central Packet Data Network Gateway (P-GW) in the Evolved Packet Core (EPC) or the User Plane Function (UPF) in the 5G Core (5GC), especially for traffic destined for the internet or a local service network. SIPTO enables the establishment of a user plane connection from the Radio Access Network (RAN) directly to a Local Gateway (L-GW) collocated with, for example, a macro eNodeB, a Home eNodeB gateway, or a centralized RAN aggregation point. For 5G, this concept extends to local UPF selection.

Architecturally, SIPTO involves network-based traffic detection and routing decisions. In EPC, when a UE activates a Packet Data Network (PDN) connection, the network (specifically the MME in consultation with the HSS) can select a P-GW that is topologically close to the UE's current point of attachment. For even more granular offload, the 'SIPTO at the Local Network' feature allows a Local IP Access (LIPA) enabled HeNB to use a collocated L-GW to offload traffic directly. The control signaling (GTP-C) still traverses the core network (MME, S-GW), but the user plane data (GTP-U) takes a shortcut. In 5GC, the Session Management Function (SMF) selects an appropriate UPF based on the UE's location and traffic requirements to serve as a local breakout point, guided by Network Repository Function (NRF) discovery and local routing policies.

The process works by leveraging APN (in EPC) or DNN (in 5GC) configurations and subscriber profiles. An operator can define that traffic matching a certain APN/DNN (e.g., 'internet') is a candidate for SIPTO. When the UE requests a connection for that APN/DNN, the core network node (MME/AMF/SMF) evaluates the UE's location, the capabilities of nearby gateways, and network policies. If conditions are met, it selects a local gateway/UPF and establishes the bearer/PDU session accordingly. The key components are the local gateway (L-GW in EPC, local UPF in 5GC), the control plane nodes that make the selection, and the policy framework that determines which traffic is eligible for offload. SIPTO is transparent to the end-user device, requiring no specific UE support for basic network-based offload, making it a powerful tool for network efficiency.

Purpose & Motivation

SIPTO was created to address the explosive growth of mobile data traffic, particularly bandwidth-intensive, internet-destined traffic like video streaming and web browsing, which was causing congestion and scalability challenges in the mobile core network and expensive backhaul links. The traditional model of routing all data through a centralized P-GW/GGSN created inefficient traffic tromboning, where data destined for a local internet exchange point would first travel hundreds of miles to the core only to be sent back, increasing latency and transport costs.

The historical context is the evolution from 3G to 4G LTE, where data volumes began to strain network economics. Previous approaches lacked a standardized method for intelligent local breakout. Operators resorted to proprietary solutions or features like LIPA, which was limited to femtocell environments. SIPTO, introduced in Release 10, provided a standardized, scalable, and network-controlled mechanism to offload selected traffic closer to the edge. It was motivated by the need to reduce latency for a better user experience, minimize capital and operational expenses by saving backhaul and core network resources, and prepare networks for the data deluge expected from smartphones and, later, IoT devices. It laid the groundwork for key 5G principles like distributed user plane and edge computing.

Key Features

• Network-based selection of a local gateway (L-GW/UPF) topologically close to the UE's RAN node for user plane breakout
• Reduces latency and load on the core network and backhaul links by optimizing routing paths for selected IP traffic (e.g., internet)
• Traffic selection is based on APN (EPC) or DNN (5GC) and can be governed by subscriber profiles and operator policies
• Can operate above the RAN (e.g., at a RAN aggregation node) or at the RAN node itself (e.g., collocated with a HeNB)
• Transparent to the UE for basic offload, requiring no client modifications
• Foundational concept enabling later technologies like Mobile Edge Computing (MEC) and Local Area Data Network (LADN) in 5G

Evolution Across Releases

Defining Specifications