GPRS Tunnelling Protocol Tunnel

Description

A GTP (GPRS Tunnelling Protocol) tunnel is a fundamental concept in mobile core networks, defined in 3GPP specifications 29.060 and 29.274. It provides a mechanism to encapsulate and forward user data packets (GTP-U plane) and control signalling (GTP-C plane) between network nodes such as SGSN, GGSN, SGW, PGW, eNodeB, and RNC. The tunnel is essentially a virtual connection that carries packets for a specific PDP context, MBMS service, or RAB. In the GTP-U plane, a tunnel is established for each PDP context or each MBMS service in the GSNs and/or each RAB in the RNC. In the GTP-C plane, a tunnel is defined for all PDP contexts with the same PDP address (or two IP addresses for IPv4v6) and APN, for each MBMS service, or for each MS.

Each GTP tunnel is uniquely identified within a node by a Tunnel Endpoint Identifier (TEID), an IP address, and a UDP port number. The TEID is a 32-bit field in the GTP header that allows the receiving node to demultiplex packets to the correct tunnel. The IP address and UDP port identify the endpoint of the tunnel. When a packet arrives at a node, the combination of TEID, IP address, and UDP port determines the tunnel to which the packet belongs. This identification is crucial for routing packets correctly between the external packet data network and the mobile station.

The GTP tunnel operates in two planes: the user plane (GTP-U) and the control plane (GTP-C). GTP-U is used for carrying user data packets, while GTP-C is used for signalling messages such as create, update, and delete tunnel requests. The control plane tunnel is used to manage the user plane tunnels. For example, when a UE initiates a PDP context activation, the SGSN creates a GTP-C tunnel to the GGSN to establish the PDP context, and then a GTP-U tunnel is created to carry user data. The tunnels are maintained as long as the PDP context or service is active.

GTP tunnels are essential for mobility and session management in GPRS, UMTS, and LTE networks. They allow the core network to route packets to the correct gateway and base station, even as the UE moves. When a UE hands over between base stations, the GTP tunnel may be re-established or updated to point to the new serving node. In LTE, GTP tunnels are used between the SGW and PGW (S5/S8 interfaces) and between the eNodeB and SGW (S1-U interface). In 5G, GTP-U is still used in the N3 interface between gNB and UPF, although the control plane uses PFCP instead of GTP-C.

The evolution of GTP tunnels reflects the changing needs of mobile networks. In early releases, tunnels were defined per PDP context, but with the introduction of IPv4v6 and PDN connections, the definition was updated to support multiple IP addresses and PDN connections. The tunnel concept has proven scalable and flexible, supporting the massive growth in mobile data traffic. Despite the introduction of new protocols like PFCP in 5G, GTP-U remains the de facto standard for user plane encapsulation in cellular networks.

Purpose & Motivation

GTP tunnels were created to solve the problem of transporting user data and control signalling across the GPRS core network, which consists of multiple nodes that need to communicate efficiently. Before GTP, there was no standardized way to encapsulate packets between SGSN and GGSN, leading to interoperability issues. GTP provides a uniform tunnelling mechanism that allows packets to be forwarded based on TEIDs, enabling seamless mobility and session management. It also supports multiple PDP contexts per user, allowing simultaneous connections to different external networks.

The tunnel concept addresses the need for a scalable and flexible data transport mechanism. By identifying tunnels with TEIDs, IP addresses, and UDP ports, GTP allows a single node to handle thousands of tunnels, each corresponding to a user session or service. This is essential for mobile networks where users are constantly moving and sessions are established and torn down dynamically. GTP tunnels also enable charging and policy enforcement by allowing the core network to inspect and control traffic per tunnel.

Over time, GTP tunnels have evolved to support new features such as IPv4v6 dual-stack, MBMS, and PDN connections. The introduction of PDN connections in Rel-11 simplified the tunnel definition by grouping PDP contexts with the same PDN connection. This evolution reflects the ongoing need to adapt to new services and technologies while maintaining backward compatibility. Today, GTP tunnels are a cornerstone of mobile packet core networks, and their principles are carried forward into 5G with GTP-U for user plane transport.