GPRS Tunnelling Protocol User Plane Message

Description

GTP-U (GPRS Tunnelling Protocol User Plane) messages are a subset of GTP Protocol Data Units (GTP-PDUs) that operate in the user plane. They are exchanged between GTP-U entities, which include network nodes such as RNC, SGSN, GGSN, eNodeB, SGW, ePDG, gNB, N3IWF, UPF, PGW, TWAN, and MME. GTP-U messages serve two primary purposes: carrying user data packets (known as G-PDUs) and conveying signalling messages for path management and tunnel management.

User plane messages, or G-PDUs, encapsulate the actual payload from the user equipment (UE). When a UE sends data, the packet is encapsulated with a GTP-U header and sent through a GTP tunnel to the appropriate gateway. The GTP-U header includes a Tunnel Endpoint Identifier (TEID) that identifies the tunnel, allowing the receiving entity to decapsulate and forward the payload correctly. This tunnelling mechanism enables mobility and session continuity as the UE moves across different cells and network nodes.

Signalling messages in GTP-U are used for path management and tunnel management. Path management messages, such as Echo Request and Echo Response, are used to check the reachability of a peer and to detect path failures. Tunnel management messages, such as End Marker, are used to indicate the end of a data burst on a tunnel, facilitating handover procedures. These signalling messages do not carry user data but are essential for maintaining the integrity and performance of the user plane.

The GTP-U protocol operates over UDP/IP, typically using UDP port 2152. The GTP-U header contains fields such as version, protocol type (1 for GTP-U), message type, length, and TEID. The message type field distinguishes between user data (G-PDU) and various signalling messages. For example, message type 255 is used for G-PDU, while type 1 is Echo Request, type 2 is Echo Response, and type 254 is End Marker.

GTP-U entities are nodes that implement at least one side of the GTP user plane protocol. The list of entities has expanded over releases to include new nodes introduced in 5G, such as gNB, N3IWF, and UPF. This reflects the evolution of mobile networks from 2G/3G to LTE and 5G, where GTP-U remains the primary user plane tunnelling protocol. In 5G, GTP-U is used on the N3 interface between the gNB and UPF, and on the N9 interface between UPFs.

The GTP-U message concept is fundamental to the operation of mobile packet core networks. It provides a standardized way to transport user data and manage tunnels, ensuring interoperability between multi-vendor network elements. Understanding GTP-U messages is essential for engineers involved in core network design, testing, and troubleshooting.

Purpose & Motivation

GTP-U messages exist to provide a standardized mechanism for tunnelling user data and managing user plane paths in mobile core networks. Before GTP-U, earlier GPRS protocols had limited user plane tunnelling capabilities. GTP-U was introduced to support the separation of control and user planes, allowing scalable and efficient data transport. It solves the problem of mobility by enabling tunnels to be established, modified, and torn down as UEs move, ensuring session continuity. Additionally, GTP-U signalling messages provide path management, which is critical for detecting failures and maintaining reliable communication. The evolution of GTP-U to support new radio technologies (LTE, 5G) demonstrates its flexibility and enduring importance. It addresses the need for a unified user plane protocol that can operate across different access technologies and core network architectures, facilitating interworking and seamless service delivery.