Description
The Contributing Source (CSRC) is a critical reference point within the 3GPP Evolved Packet Core (EPC) architecture, formally defined in technical specification 29.414. It represents the logical interface and associated protocol stack between the Serving Gateway (S-GW) and the Packet Data Network Gateway (P-GW) when these network functions are deployed in a separated manner. The CSRP interface is the physical realization based on this CSRC reference point. The interface supports both the S5 interface (used when the S-GW and P-GW are within the same PLMN for non-roaming scenarios or home-routed traffic) and the S8 interface (used for roaming scenarios where the S-GW is in the Visited PLMN and the P-GW is in the Home PLMN).
Architecturally, the CSRC interface utilizes the GPRS Tunneling Protocol (GTP) as its core protocol. It operates over a UDP/IP transport layer. For the control plane, it uses GTP version 2 (GTPv2-C), defined in 3GPP TS 29.274, which handles session management procedures such as Create Session, Modify Bearer, Delete Session, and Bearer Resource Command. These procedures are essential for establishing, modifying, and tearing down GTP tunnels (bearers) that carry user data. For the user plane, it uses GTP version 1 (GTPv1-U), defined in 3GPP TS 29.281, which encapsulates and tunnels the actual user data packets (IP packets) between the S-GW and P-GW. Each bearer is associated with a unique Tunnel Endpoint Identifier (TEID) at both ends.
The operation of the CSRC interface is central to UE mobility and session continuity. When a UE attaches to the network, the MME selects an S-GW and a P-GW. The MME then triggers the creation of a default bearer by sending a Create Session Request to the S-GW, which forwards it to the P-GW over the CSRC interface. This establishes the first GTP tunnel. During handovers (e.g., S1-based or inter-RAT), the S-GW may remain anchored while the base station changes, requiring bearer modification procedures over CSRC to update the downlink path. The P-GW acts as the IP anchor point, providing UE IP address allocation and policy enforcement, while the S-GW handles local mobility anchoring and lawful interception.
Key components involved in the CSRC interface include the GTP-C and GTP-U protocol entities within the S-GW and P-GW, the underlying IP transport network (which may utilize different QoS classes for control and user plane), and the Diameter-based interfaces (Gx, Gy) that the P-GW uses to interact with the Policy and Charging Rules Function (PCRF) and Online Charging System (OCS) to apply policies determined during CSRC signaling. The interface's design allows for the S-GW and P-GW to be collocated (in which case the S5 interface is internal) or separated across different geographical locations, enabling scalable and flexible network deployments, such as centralized P-GW pools and distributed S-GWs.
Purpose & Motivation
The CSRC interface was created to fulfill a fundamental architectural requirement of the 3GPP System Architecture Evolution (SAE) and the Evolved Packet Core (EPC) introduced with LTE in Release 8. Previous 3GPP packet core architectures, like GPRS, had more monolithic gateway nodes (GGSN). The EPC design principle mandated a clear separation between the mobility anchoring function (S-GW) and the external IP connectivity function (P-GW). This separation required a standardized, robust, and scalable interface to facilitate communication between these now-distinct network functions.
The primary problem CSRC solves is enabling flexible and efficient network deployment models. By providing a standardized interface, it allows mobile operators to deploy S-GWs and P-GWs independently. Operators can place S-GWs closer to the radio network edge to optimize user plane latency and handover performance, while centralizing P-GWs in fewer locations to simplify policy enforcement, charging, and interconnection with external packet data networks (like the internet or IMS). This separation is crucial for supporting both roaming and non-roaming scenarios efficiently, as the S8 interface (a specific application of CSRC for roaming) allows a visited network's S-GW to connect seamlessly to a home network's P-GW.
Furthermore, CSRC, through its use of GTP, provides a proven tunneling mechanism for bearer management. It solves the problem of maintaining IP session continuity for mobile users as they move. The GTP tunnels established over CSRC hide the user's mobility from the external IP network, making the UE appear stationary from an IP routing perspective. This design addresses the limitations of earlier mobile IP approaches by integrating mobility management directly into the core network protocol, offering better scalability and integration with existing 3GPP authentication and billing systems. The interface's support for multiple dedicated bearers per default bearer also enables sophisticated QoS handling and service-aware charging, which were growing requirements for new multimedia services.
Key Features
- Supports both GTPv2-C for control plane signaling and GTPv1-U for user plane tunneling
- Enables the S5 interface (non-roaming/home-routed) and S8 interface (roaming) between S-GW and P-GW
- Provides bearer management procedures including creation, modification, and deletion of GTP tunnels
- Facilitates UE IP address preservation and session continuity during mobility events
- Allows decoupled deployment of S-GW (edge) and P-GW (central) functions for network flexibility
- Integrates with policy control via triggers to the P-GW's Gx interface during session establishment
Evolution Across Releases
Initial introduction as part of the Evolved Packet Core (EPC) for LTE. Defined the CSRC reference point and the GTP-based S5/S8 interfaces between the separated S-GW and P-GW. Established core procedures for default and dedicated bearer establishment, mobility anchoring, and roaming support, forming the foundation for all subsequent LTE/EPC deployments.
Defining Specifications
| Specification | Title |
|---|---|
| TS 29.414 | 3GPP TS 29.414 |