Packet Data Network Address

Description

The PDN Address is a fundamental identifier in 3GPP packet-switched networks, representing the IP address allocated to a User Equipment (UE) by the Packet Data Network Gateway (PDN GW). When a UE initiates a PDN connection, it requests an IP address (IPv4, IPv6, or IPv4v6) from the network. The PDN GW, acting as the anchor point for the UE's session, assigns an address from its pool and communicates it back to the UE via the Mobility Management Entity (MME) and Serving Gateway (SGW). This address is essential for the UE to send and receive IP packets to and from external networks such as the Internet or corporate intranets. The PDN Address is included in session management messages, such as the Activate Default EPS Bearer Context Request, and is stored in the UE's context along with other parameters like the Access Point Name (APN) and Quality of Service (QoS) profile.

The assignment process involves several network elements. During the initial attach procedure, the UE includes a PDN Address Allocation (PAA) information element in the PDN Connectivity Request, indicating whether it prefers IPv4, IPv6, or both. The MME forwards this request to the SGW, which relays it to the PDN GW. The PDN GW may allocate a dynamic address using DHCP or statically provision one based on subscription data. For IPv4, a single address is assigned; for IPv6, a /64 prefix is typically delegated to the UE, allowing it to autoconfigure its interface. In dual-stack scenarios (IPv4v6), both an IPv4 address and an IPv6 prefix are provided. The PDN GW then returns the assigned address in a Create Session Response, which propagates back to the UE. The UE uses this address as its source IP for all traffic within that PDN connection.

The PDN Address is tightly coupled with the concept of a PDN Connection, which is the association between the UE and a PDN identified by an APN. Each PDN connection has at least one PDN Address, and the address remains valid for the lifetime of the connection. If the UE moves between access technologies (e.g., from E-UTRAN to UTRAN), the PDN Address is preserved to maintain session continuity, provided the PDN GW remains unchanged. This is a key feature of 3GPP's mobility management, ensuring that ongoing IP flows are not disrupted. The address can be released when the PDN connection is deactivated, either by the UE, the network, or due to inactivity.

In terms of security, the PDN Address is protected by the underlying network security mechanisms, such as IPsec for control plane signaling and ciphering for user plane data. The address itself is not encrypted over the air, but its assignment is authenticated to prevent spoofing. The PDN GW enforces policies based on the address, such as firewall rules and traffic shaping. Additionally, the PDN Address is used in charging records, allowing operators to bill subscribers based on data usage per PDN connection.

With the advent of 5G, the concept of PDN Address has evolved into the PDU Session Address, but the principles remain similar. In 5G, the Session Management Function (SMF) allocates the IP address, often in collaboration with the User Plane Function (UPF). However, for networks that still support EPS fallback or interworking with 4G, the PDN Address continues to be relevant. The term is defined in 3GPP TS 24.301 for EPS session management and referenced in other specs like TS 29.804 for interworking. Understanding PDN Address is crucial for engineers working on core network design, as it directly impacts IP addressing plans, routing, and service delivery.

Purpose & Motivation

The PDN Address exists to provide a unique, routable IP identity for a UE within a packet data network, enabling end-to-end IP communication. Without it, the UE cannot send or receive data to external networks, as there would be no destination or source address for packets. It solves the problem of address allocation in a mobile environment where UEs frequently change cells and even access technologies. By anchoring the address at the PDN GW, 3GPP ensures that the IP address remains stable during mobility, which is essential for maintaining TCP connections and real-time services.

Historically, before the introduction of the PDN Address in Release 8, earlier GPRS networks used PDP addresses within PDP contexts. The PDN Address concept was introduced with the System Architecture Evolution (SAE) and the Evolved Packet Core (EPC) to support always-on IP connectivity, higher data rates, and multiple PDN connections. It addressed the limitations of PDP addresses, which were tied to a single PDP context and did not support dual-stack or multiple simultaneous connections as efficiently. The PDN Address also facilitated the separation of control and user planes, allowing for more scalable and flexible network architectures.

Moreover, the PDN Address enables advanced features like IPv6 prefix delegation, which is critical for the proliferation of IoT devices and the transition to IPv6. It also supports policy and charging control by providing a granular identifier for each PDN connection, allowing operators to apply different QoS and charging rules per address. In summary, the PDN Address is a cornerstone of mobile broadband, providing the necessary IP layer connectivity for a myriad of services.