GPRS Tunnelling Protocol Protocol Data Unit

Description

The GTP Protocol Data Unit (GTP-PDU) is a fundamental concept in the GPRS Tunnelling Protocol (GTP) suite, which is used to encapsulate and tunnel user data and control signalling within mobile core networks. GTP-PDU represents the generic unit of data exchanged between GTP entities, such as SGSN, GGSN, SGW, PGW, eNodeB, gNB, and UPF. It serves as an umbrella term that covers all types of messages defined in the GTP specifications, including GTP-C messages for control plane procedures and GTP-U messages for user plane data and signalling.

In the control plane, GTP-C messages are used for session management, bearer management, and mobility management between core network nodes. These messages are themselves GTP-PDUs. In the user plane, GTP-U messages can be either user data packets (G-PDUs) carrying the actual payload from the UE, or signalling messages used for path management (e.g., Echo Request/Response) and tunnel management (e.g., End Marker). Thus, a GTP-PDU can be a control message, a user data packet, or a user plane signalling message.

The structure of a GTP-PDU depends on whether it is GTP-C or GTP-U. GTP-C messages typically consist of a GTP header, followed by information elements (IEs) that convey parameters for the procedure. GTP-U messages, particularly G-PDUs, consist of a GTP-U header, an optional extension header chain, and the payload (the original IP packet from the UE). The GTP header includes fields such as version, protocol type (GTP-C or GTP-U), message type, length, and a Tunnel Endpoint Identifier (TEID) that identifies the tunnel.

GTP-PDUs are transported over UDP/IP. For GTP-C, the UDP port is typically 2123; for GTP-U, it is 2152. The TEID in the GTP header allows multiplexing of multiple tunnels between the same pair of nodes. In the user plane, the TEID is used to identify the bearer context, enabling the receiving entity to forward the payload correctly. In the control plane, TEIDs are used to correlate request and response messages.

The concept of GTP-PDU is essential for understanding how data and signalling are encapsulated and routed in 3GPP networks. It provides a unified abstraction for all GTP traffic, facilitating the design of network nodes that handle both control and user plane functions. As networks evolve towards 5G and beyond, GTP remains a key protocol for user plane tunnelling (e.g., N3 interface between gNB and UPF), and the GTP-PDU concept continues to be relevant.

In summary, GTP-PDU is a generic term for any message or packet formatted according to GTP specifications. It encompasses GTP-C messages, GTP-U signalling messages, and G-PDUs (user data packets). Understanding GTP-PDU is crucial for engineers working with mobile core networks, as it underpins the tunnelling mechanisms that enable mobility, session management, and data transport.

Purpose & Motivation

The GTP-PDU concept exists to provide a unified terminology for all data units exchanged within the GTP protocol suite. Before the formalization of this term, engineers often referred to specific message types (e.g., 'GTP-C message', 'G-PDU') without a common abstraction. The introduction of GTP-PDU in 3GPP specifications helped standardize documentation and discussions by defining a single term that covers both control and user plane messages. This is particularly important because GTP is used across multiple interfaces (e.g., Gn/Gp, S5/S8, N3) and involves various network nodes. By having a clear definition, implementers can ensure that their parsers and state machines correctly handle all possible GTP messages. Moreover, as networks evolved from 2G/3G to LTE and 5G, the GTP protocol was extended, and the GTP-PDU term provided a stable reference point for these extensions. It solves the problem of ambiguity in protocol descriptions and facilitates interoperability testing by clearly delineating what constitutes a GTP message.