Destination ADdress

Description

The Destination ADdress (DAD) is a core addressing element used within various 3GPP protocol data units (PDUs) to identify the intended endpoint for a transmitted packet. It functions as a critical routing parameter within the control and user planes, particularly in the packet core network. The DAD is not a single, monolithic address but a conceptual field whose specific format and content depend on the protocol layer and interface in use. For instance, within the GPRS Tunneling Protocol (GTP) employed on the Gn (between SGSNs or SGSN and GGSN) and S5/S8 (between S-GW and P-GW) interfaces, the DAD is embedded in GTP headers. In this context, it typically contains a Tunnel Endpoint Identifier (TEID) and an IP address (IPv4 or IPv6) that uniquely identifies the receiving GTP entity (e.g., a GGSN or P-GW) for a specific user's bearer. This combination allows network nodes to demultiplex incoming GTP tunnels and correctly associate packets with the appropriate Packet Data Protocol (PDP) context or EPS bearer.

Architecturally, the DAD is a key component in establishing and maintaining GTP tunnels, which form the backbone for user data transport in 3GPP packet-switched systems. During procedures like PDP Context Activation or EPS Bearer Setup, the network nodes negotiate and exchange DAD information. The creating entity (e.g., SGSN or S-GW) assigns a local TEID and informs the peer entity of this TEID along with its own IP address, which together constitute the DAD for uplink traffic from the peer's perspective. This bidirectional exchange ensures that both ends of the tunnel have the necessary addressing information to correctly route packets. The DAD, therefore, enables the stateless routing of millions of simultaneous user sessions by providing a unique, session-specific destination identifier within the core network's IP transport layer.

Beyond GTP, the concept of a destination address permeates other 3GPP protocols and interfaces. In the Diameter-based interfaces (e.g., S6a, Gx), the DAD would correspond to the Destination-Host and Destination-Realm Attribute-Value Pairs (AVPs), directing signaling messages to the correct home subscriber server (HSS) or policy and charging rules function (PCRF). In the radio access network, Layer 3 radio resource control (RRC) messages also contain destination information, though it is often implicit within the cell- or UE-specific signaling connection. The reliability and unambiguous nature of the DAD are paramount; any corruption or misconfiguration can lead to misrouted packets, session drops, or security vulnerabilities. Its consistent application across releases, as noted in the specifications, underscores its role as a foundational, stable addressing primitive upon which more complex services and network functions are built.

Purpose & Motivation

The purpose of the Destination ADdress (DAD) is to provide a standardized, unambiguous method for identifying the recipient of a packet within 3GPP-defined network protocols. It solves the fundamental problem of packet routing and session multiplexing in a scalable, multi-tenant mobile core network. Prior to standardized tunneling protocols like GTP, early data services lacked a robust mechanism to efficiently separate and route traffic for millions of subscribers over a shared IP backbone. The DAD, as part of the GTP header, was created to enable the establishment of logical point-to-point tunnels (bearers) between network nodes, each uniquely identified for a specific user's data session. This abstraction allows the underlying IP transport network to route packets based on node addresses, while the core network nodes use the DAD (TEID + IP) to forward packets to the correct internal bearer context, enabling efficient support for always-on connectivity and multiple concurrent applications per user.

Historically, the introduction of GPRS and the move towards an all-IP core network in 3GPP Release 97/98 necessitated a tunneling mechanism that could separate control and user plane traffic and manage mobility transparently to the external packet data networks (PDNs). The DAD concept within GTP was motivated by the need to maintain session state and routing continuity as users moved between serving nodes. It addressed the limitation of using only the subscriber's IP address for routing, which is insufficient because a single user equipment (UE) may have multiple IP addresses (for different PDNs) and the same IP address may be reused by different UEs over time. By incorporating a dynamically assigned TEID specific to a tunnel endpoint, the DAD provides a unique session identifier that is independent of the user's IP address, ensuring precise delivery even during inter-node handovers and session modifications. This design is crucial for fulfilling the quality of service (QoS) and charging requirements mandated by 3GPP architectures.