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Internet Engineering Task Force (IETF)
Request for Comments: 6459
Category: Informational
ISSN: 2070-1721
J. Korhonen, Ed.
Nokia Siemens Networks
J. Soininen
Renesas Mobile
B. Patil
T. Savolainen
G. Bajko
Nokia
K. Iisakkila
Renesas Mobile
January 2012

IPv6 in 3rd Generation Partnership Project (3GPP)

Evolved Packet System (EPS)

Abstract

The use of cellular broadband for accessing the Internet and other data services via smartphones, tablets, and notebook/netbook computers has increased rapidly as a result of high-speed packet data networks such as HSPA, HSPA+, and now Long-Term Evolution (LTE) being deployed. Operators that have deployed networks based on 3rd Generation Partnership Project (3GPP) network architectures are facing IPv4 address shortages at the Internet registries and are feeling pressure to migrate to IPv6. This document describes the support for IPv6 in 3GPP network architectures.

Status of This Memo

This document is not an Internet Standards Track specification; it is published for informational purposes.

This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Not all documents approved by the IESG are a candidate for any level of Internet Standard; see Section 2 of RFC 5741.

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc6459.

Copyright Notice

Copyright © 2012 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................4
   2. 3GPP Terminology and Concepts ...................................5
      2.1. Terminology ................................................5
      2.2. The Concept of APN ........................................10
   3. IP over 3GPP GPRS ..............................................11
      3.1. Introduction to 3GPP GPRS .................................11
      3.2. PDP Context ...............................................12
   4. IP over 3GPP EPS ...............................................13
      4.1. Introduction to 3GPP EPS ..................................13
      4.2. PDN Connection ............................................14
      4.3. EPS Bearer Model ..........................................15
   5. Address Management .............................................16
      5.1. IPv4 Address Configuration ................................16
      5.2. IPv6 Address Configuration ................................16
      5.3. Prefix Delegation .........................................17
      5.4. IPv6 Neighbor Discovery Considerations ....................18
   6. 3GPP Dual-Stack Approach to IPv6 ...............................18
      6.1. 3GPP Networks Prior to Release-8 ..........................18
      6.2. 3GPP Release-8 and -9 Networks ............................20
      6.3. PDN Connection Establishment Process ......................21
      6.4. Mobility of 3GPP IPv4v6 Bearers ...........................23
   7. Dual-Stack Approach to IPv6 Transition in 3GPP Networks ........24
   8. Deployment Issues ..............................................25
      8.1. Overlapping IPv4 Addresses ................................25
      8.2. IPv6 for Transport ........................................26
      8.3. Operational Aspects of Running Dual-Stack Networks ........26
      8.4. Operational Aspects of Running a Network with
           IPv6-Only Bearers .........................................27
      8.5. Restricting Outbound IPv6 Roaming .........................28
      8.6. Inter-RAT Handovers and IP Versions .......................29
      8.7. Provisioning of IPv6 Subscribers and Various
           Combinations during Initial Network Attachment ............29
   9. Security Considerations ........................................31
   10. Summary and Conclusions .......................................32
   11. Acknowledgements ..............................................32
   12. Informative References ........................................33

1. Introduction

IPv6 support has been part of the 3rd Generation Partnership Project (3GPP) standards since the first release of the specifications (Release 99). This support extends to all radio access and packet- based system variants of the 3GPP architecture family. In addition, a lot of work has been invested by the industry to investigate different transition and deployment scenarios over the years. However, IPv6 deployment in commercial networks remains low. There are many factors that can be attributed to this lack of deployment. The most relevant factor is essentially the same as the reason for IPv6 not being deployed in other networks either, i.e., the lack of business and commercial incentives for deployment.

3GPP network architectures have continued to evolve in the time since Release 99, which was finalized in early 2000. The most recent version of the 3GPP architecture, the Evolved Packet System (EPS) -- commonly referred to as System Architecture Evolution (SAE), Long- Term Evolution (LTE), or Release-8 -- is a packet-centric architecture. In addition, the number of subscribers and devices using the 3GPP networks for Internet connectivity and data services has also increased phenomenally -- the number of mobile broadband subscribers has increased exponentially over the last couple of years.

With subscriber growth projected to increase even further, and with recent depletion of available IPv4 address space by IANA, 3GPP operators and vendors are now in the process of identifying the scenarios and solutions needed to deploy IPv6.

This document describes the establishment of IP connectivity in 3GPP network architectures, specifically in the context of IP bearers for 3G General Packet Radio Service (GPRS) and for EPS. It provides an overview of how IPv6 is supported as per the current set of 3GPP specifications. Some of the issues and concerns with respect to deployment and shortage of private IPv4 addresses within a single network domain are also discussed.

The IETF has specified a set of tools and mechanisms that can be utilized for transitioning to IPv6. In addition to operating dual- stack networks during the transition from IPv4 to IPv6, the two alternative categories for the transition are encapsulation and translation. The IETF continues to specify additional solutions for enabling the transition based on the deployment scenarios and operator/ISP requirements. There is no single approach for transition to IPv6 that can meet the needs for all deployments and models. The 3GPP scenarios for transition, described in [TR.23975], can be addressed using transition mechanisms that are already available in the toolbox. The objective of transition to IPv6 in 3GPP networks is to ensure that:

  1. Legacy devices and hosts that have an IPv4-only stack will continue to be provided with IP connectivity to the Internet and services.
  1. Devices that are dual-stack can access the Internet either via IPv6 or IPv4. The choice of using IPv6 or IPv4 depends on the capability of:

A. the application on the host,

B. the support for IPv4 and IPv6 bearers by the network, and/or

C. the server(s) and other end points.

3GPP networks are capable of providing a host with IPv4 and IPv6 connectivity today, albeit in many cases with upgrades to network elements such as the Serving GPRS Support Node (SGSN) and the Gateway GPRS Support Node (GGSN).

2. 3GPP Terminology and Concepts

2.1. Terminology

   Access Point Name

The Access Point Name (APN) is a Fully Qualified Domain Name (FQDN) and resolves to a set of gateways in an operator's network. The APNs are piggybacked on the administration of the DNS namespace.

Dual Address PDN/PDP Type

The dual address Packet Data Network/Packet Data Protocol (PDN/ PDP) Type (IPv4v6) is used in 3GPP context in many cases as a synonym for dual-stack, i.e., a connection type capable of serving both IPv4 and IPv6 simultaneously. 

   Evolved Packet Core

The Evolved Packet Core (EPC) is an evolution of the 3GPP GPRS system characterized by a higher-data-rate, lower-latency, packet- optimized system. The EPC comprises subcomponents such as the Mobility Management Entity (MME), Serving Gateway (SGW), Packet Data Network Gateway (PDN-GW), and Home Subscriber Server (HSS). 

   Evolved Packet System

The Evolved Packet System (EPS) is an evolution of the 3GPP GPRS system characterized by a higher-data-rate, lower-latency, packet- optimized system that supports multiple Radio Access Technologies (RATs). The EPS comprises the EPC together with the Evolved Universal Terrestrial Radio Access (E-UTRA) and the Evolved Universal Terrestrial Radio Access Network (E-UTRAN). 

   Evolved UTRAN

The Evolved UTRAN (E-UTRAN) is a communications network, sometimes referred to as 4G, and consists of eNodeBs (4G base stations), which make up the E-UTRAN. The E-UTRAN allows connectivity between the User Equipment and the core network. 

   GPRS Tunnelling Protocol

The GPRS Tunnelling Protocol (GTP) [TS.29060] [TS.29274] [TS.29281] is a tunnelling protocol defined by 3GPP. It is a network-based mobility protocol and is similar to Proxy Mobile IPv6 (PMIPv6) [RFC5213]. However, GTP also provides functionality beyond mobility, such as in-band signaling related to Quality of Service (QoS) and charging, among others.

GSM EDGE Radio Access Network

The Global System for Mobile Communications (GSM) EDGE Radio Access Network (GERAN) is a communications network, commonly referred to as 2G or 2.5G, and consists of base stations and Base Station Controllers (BSCs), which make up the GSM EDGE radio access network. The GERAN allows connectivity between the User Equipment and the core network.

Gateway GPRS Support Node

The Gateway GPRS Support Node (GGSN) is a gateway function in the GPRS that provides connectivity to the Internet or other PDNs. The host attaches to a GGSN identified by an APN assigned to it by an operator. The GGSN also serves as the topological anchor for addresses/prefixes assigned to the User Equipment.

General Packet Radio Service

The General Packet Radio Service (GPRS) is a packet-oriented mobile data service available to users of the 2G and 3G cellular communication systems -- the GSM -- specified by 3GPP. 

   High-Speed Packet Access

The High-Speed Packet Access (HSPA) and HSPA+ are enhanced versions of the Wideband Code Division Multiple Access (WCDMA) and UTRAN, thus providing more data throughput and lower latencies. 

   Home Location Register

The Home Location Register (HLR) is a pre-Release-5 database (but is also used in Release-5 and later networks in real deployments) that contains subscriber data and information related to call routing. All subscribers of an operator, and the subscribers' enabled services, are provisioned in the HLR. 

   Home Subscriber Server

The Home Subscriber Server (HSS) is a database for a given subscriber and was introduced in 3GPP Release-5. It is the entity containing the subscription-related information to support the network entities actually handling calls/sessions. 

   Mobility Management Entity

The Mobility Management Entity (MME) is a network element that is responsible for control-plane functionalities, including authentication, authorization, bearer management, layer-2 mobility, etc. The MME is essentially the control-plane part of the SGSN in the GPRS. The user-plane traffic bypasses the MME. 

   Mobile Terminal

The Mobile Terminal (MT) is the modem and the radio part of the Mobile Station (MS).

Public Land Mobile Network

The Public Land Mobile Network (PLMN) is a network that is operated by a single administration. A PLMN (and therefore also an operator) is identified by the Mobile Country Code (MCC) and the Mobile Network Code (MNC). Each (telecommunications) operator providing mobile services has its own PLMN.

Policy and Charging Control

The Policy and Charging Control (PCC) framework is used for QoS policy and charging control. It has two main functions: flow- based charging, including online credit control; and policy control (e.g., gating control, QoS control, and QoS signaling). It is optional to 3GPP EPS but needed if dynamic policy and charging control by means of PCC rules based on user and services are desired. 

   Packet Data Network

The Packet Data Network (PDN) is a packet-based network that either belongs to the operator or is an external network such as the Internet or a corporate intranet. The user eventually accesses services in one or more PDNs. The operator's packet core networks are separated from packet data networks either by GGSNs or PDN Gateways (PDN-GWs).

Packet Data Network Gateway

The Packet Data Network Gateway (PDN-GW) is a gateway function in the Evolved Packet System (EPS), which provides connectivity to the Internet or other PDNs. The host attaches to a PDN-GW identified by an APN assigned to it by an operator. The PDN-GW also serves as the topological anchor for addresses/prefixes assigned to the User Equipment.

Packet Data Protocol Context

A Packet Data Protocol (PDP) context is the equivalent of a virtual connection between the User Equipment (UE) and a PDN using a specific gateway.

Packet Data Protocol Type

A Packet Data Protocol Type (PDP Type) identifies the used/allowed protocols within the PDP context. Examples are IPv4, IPv6, and IPv4v6 (dual-stack).

S4 Serving GPRS Support Node

The S4 Serving GPRS Support Node (S4-SGSN) is compliant with a Release-8 (and onwards) SGSN that connects 2G/3G radio access networks to the EPC via new Release-8 interfaces like S3, S4, and S6d. 

   Serving Gateway

The Serving Gateway (SGW) is a gateway function in the EPS, which terminates the interface towards the E-UTRAN. The SGW is the Mobility Anchor point for layer-2 mobility (inter-eNodeB handovers). For each UE connected with the EPS, at any given point in time, there is only one SGW. The SGW is essentially the user-plane part of the GPRS's SGSN.

Serving GPRS Support Node

The Serving GPRS Support Node (SGSN) is a network element that is located between the radio access network (RAN) and the gateway (GGSN). A per-UE point-to-point (p2p) tunnel between the GGSN and SGSN transports the packets between the UE and the gateway. 

   Terminal Equipment

The Terminal Equipment (TE) is any device/host connected to the Mobile Terminal (MT) offering services to the user. A TE may communicate to an MT, for example, over the Point to Point Protocol (PPP). 

   UE, MS, MN, and Mobile

The terms UE (User Equipment), MS (Mobile Station), MN (Mobile Node), and mobile refer to the devices that are hosts with the ability to obtain Internet connectivity via a 3GPP network. A MS is comprised of the Terminal Equipment (TE) and a Mobile Terminal (MT). The terms UE, MS, MN, and mobile are used interchangeably within this document.

UMTS Terrestrial Radio Access Network

The Universal Mobile Telecommunications System (UMTS) Terrestrial Radio Access Network (UTRAN) is a communications network, commonly referred to as 3G, and consists of NodeBs (3G base stations) and Radio Network Controllers (RNCs), which make up the UMTS radio access network. The UTRAN allows connectivity between the UE and the core network. The UTRAN is comprised of WCDMA, HSPA, and HSPA+ radio technologies. 

   User Plane

The user plane refers to data traffic and the required bearers for the data traffic. In practice, IP is the only data traffic protocol used in the user plane.

Wideband Code Division Multiple Access

The Wideband Code Division Multiple Access (WCDMA) is the radio interface used in UMTS networks. 

   eNodeB

The eNodeB is a base station entity that supports the Long-Term Evolution (LTE) air interface.

2.2. The Concept of APN

The Access Point Name (APN) essentially refers to a gateway in the 3GPP network. The 'complete' APN is expressed in a form of a Fully Qualified Domain Name (FQDN) and also piggybacked on the administration of the DNS namespace, thus effectively allowing the discovery of gateways using the DNS. The UE can choose to attach to a specific gateway in the packet core. The gateway provides connectivity to the Packet Data Network (PDN), such as the Internet. An operator may also include gateways that do not provide Internet connectivity but rather provide connectivity to a closed network providing a set of the operator's own services. A UE can be attached to one or more gateways simultaneously. The gateway in a 3GPP network is the GGSN or PDN-GW. Figure 1 illustrates the APN-based network connectivity concept. 

                                                            .--.
                                                          _(.   `)
                        .--.         +------------+     _(   PDN  `)_
                      _(Core`.       |GW1         |====(  Internet   `)
           +---+     (   NW   )------|APN=internet|   ( `  .        )  )
   [UE]~~~~|RAN|----( `  .  )  )--+  +------------+    `--(_______)---'
    ^      +---+     `--(___.-'   |
    |                             |                       .--.
    |                             |  +----------+       _(.PDN`)
    |                             +--|GW2       |     _(Operator`)_
    |                                |APN=OpServ|====(  Services   `)
   UE is attached                    +----------+   ( `  .        )  )
   to GW1 and GW2                                    `--(_______)---'
   simultaneously

Figure 1: User Equipment Attached to Multiple APNs Simultaneously

3. IP over 3GPP GPRS

3.1. Introduction to 3GPP GPRS

A simplified 2G/3G GPRS architecture is illustrated in Figure 2. This architecture basically covers the GPRS core network from R99 to Release-7, and radio access technologies such as GSM (2G), EDGE (2G, often referred to as 2.5G), WCDMA (3G), and HSPA(+) (3G, often referred to as 3.5G). The architecture shares obvious similarities with the Evolved Packet System (EPS), as will be seen in Section 4. Based on Gn/Gp interfaces, the GPRS core network functionality is logically implemented on two network nodes -- the SGSN and the GGSN. 

                     3G
                    .--.                                     .--.
             Uu   _(    `.  Iu   +----+      +----+        _(    `.
       [UE]~~|~~~(  UTRAN )--|---|SGSN|--|---|GGSN|--|----(   PDN  )
                ( `  .  )  )     +----+  Gn  +----+  Gi  ( `  .  )  )
                 `--(___.-'        / |                    `--(___.-'
                                  /  |
                     2G       Gb--   |
                    .--.       /     |
                  _(    `.    /      --Gp
       [UE]~~|~~~(   PDN  )__/       |
             Um ( `  .  )  )        .--.
                 `--(___.-'       _(.   `)
                                _( [GGSN] `)_
                               (    other    `)
                              ( `  . PLMN   )  )
                               `--(_______)---'

Figure 2: Overview of the 2G/3G GPRS Logical Architecture

   Gn/Gp:  Interfaces that provide a network-based mobility service for
   
           a UE and are used between an SGSN and a GGSN.  The Gn
           interface is used when the GGSN and SGSN are located inside
           one operator (i.e., a PLMN).  The Gp-interface is used if the
           GGSN and the SGSN are located in different operator domains
           (i.e., a different PLMN).  GTP is defined for the Gn/Gp
           interfaces (both GTP-C for the control plane and GTP-U for
           the user plane).
   
   Gb:     The Base Station System (BSS)-to-SGSN interface, which is
           used to carry information concerning packet data transmission
           and layer-2 mobility management.  The Gb-interface is based
           on either Frame Relay or IP.
   
   Iu:     The Radio Network System (RNS)-to-SGSN interface, which is
           used to carry information concerning packet data transmission
           and layer-2 mobility management.  The user-plane part of the
           Iu-interface (actually the Iu-PS) is based on GTP-U.  The
           control-plane part of the Iu-interface is based on the Radio
           Access Network Application Protocol (RANAP).
   
   Gi:     The interface between the GGSN and a PDN.  The PDN may be an
           operator's external public or private packet data network, or
           an intra-operator packet data network.
   
   Uu/Um:  2G or 3G radio interfaces between a UE and a respective radio
           access network.

The SGSN is responsible for the delivery of data packets from and to the UE within its geographical service area when a direct tunnel option is not used. If the direct tunnel is used, then the user plane goes directly between the RNC (in the RNS) and the GGSN. The control-plane traffic always goes through the SGSN. For each UE connected with the GPRS, at any given point in time, there is only one SGSN.

3.2. PDP Context

A PDP (Packet Data Protocol) context is an association between a UE represented by one IPv4 address and/or one /64 IPv6 prefix, and a PDN represented by an APN. Each PDN can be accessed via a gateway (typically a GGSN or PDN-GW). On the UE, a PDP context is equivalent to a network interface. A UE may hence be attached to one or more gateways via separate connections, i.e., PDP contexts. 3GPP GPRS supports PDP Types IPv4, IPv6, and since Release-9, PDP Type IPv4v6 (dual-stack) as well.

Each primary PDP context has its own IPv4 address and/or one /64 IPv6 prefix assigned to it by the PDN and anchored in the corresponding gateway. The GGSN or PDN-GW is the first-hop router for the UE. Applications on the UE use the appropriate network interface (PDP context) for connectivity to a specific PDN. Figure 3 represents a high-level view of what a PDP context implies in 3GPP networks. 

        Y
        |                               +---------+       .--.
        |--+ __________________________ | APNx in |     _(    `.
        |  |O______PDPc1_______________)| GGSN /  |----(Internet)
        |  |                            | PDN-GW  |   ( `  .  )  )
        |UE|                            +---------+    `--(___.-'
        |  | _______________________ +---------+          .--.
        |  |O______PDPc2____________)| APNy in |        _(Priv`.
        +--+                         | GGSN /  |-------(Network )
                                     | PDN-GW  |      ( `  .  )  )
                                     +---------+       `--(___.-'

Figure 3: PDP Contexts between the MS/UE and Gateway

In the above figure, there are two PDP contexts at the MS/UE: the 'PDPc1' PDP context, which is connected to APNx, provides Internet connectivity, and the 'PDPc2' PDP context provides connectivity to a private IP network via APNy (as an example, this network may include operator-specific services, such as the MMS (Multimedia Messaging Service)). An application on the host, such as a web browser, would use the PDP context that provides Internet connectivity for accessing services on the Internet. An application such as a MMS would use APNy in the figure above, because the service is provided through the private network.

4. IP over 3GPP EPS

4.1. Introduction to 3GPP EPS

In its most basic form, the EPS architecture consists of only two nodes on the user plane: a base station and a core network Gateway (GW). The basic EPS architecture is illustrated in Figure 4. The functional split of gateways allows operators to choose optimized topological locations of nodes within the network and enables various deployment models, including the sharing of radio networks between different operators. This also allows independent scaling, growth of traffic throughput, and control-signal processing. 

                                                              +--------+
                                                              |   IP   |
                         S1-MME  +-------+  S11               |Services|
                       +----|----|  MME  |----|----+          +--------+
                       |         |       |         |               |SGi
                       |         +-------+         |      S5/      |
    +----+ LTE-Uu +-------+ S1-U                +-------+  S8  +-------+
    |UE  |----|---|eNodeB |---|-----------------| SGW   |--|---|PDN-GW |
    |    |========|=======|=====================|=======|======|       |
    +----+        +-------+Dual-Stack EPS Bearer+-------+      +-------+

Figure 4: EPS Architecture for 3GPP Access

   S5/S8:   Provides user-plane tunnelling and tunnel management between
   
            the SGW and PDN-GW, using GTP (both GTP-U and GTP-C) or
            PMIPv6 [RFC5213] [TS.23402] as the network-based mobility
            management protocol.  The S5 interface is used when the
            PDN-GW and SGW are located inside one operator (i.e., a
            PLMN).  The S8-interface is used if the PDN-GW and the SGW
            are located in different operator domains (i.e., a different
            PLMN).
   
   S11:     Reference point for the control-plane protocol between the
            MME and SGW, based on GTP-C (GTP control plane) and used,
            for example, during the establishment or modification of the
            default bearer.
   
   S1-U:    Provides user-plane tunnelling and inter-eNodeB path
            switching during handover between the eNodeB and SGW, using
            GTP-U (GTP user plane).
   
   S1-MME:  Reference point for the control-plane protocol between the
            eNodeB and MME.
   
   SGi:     The interface between the PDN-GW and the PDN.  The PDN may
            be an operator-external public or private packet data
            network or an intra-operator packet data network.

4.2. PDN Connection

A PDN connection is an association between a UE represented by one IPv4 address and/or one /64 IPv6 prefix, and a PDN represented by an APN. The PDN connection is the EPC equivalent of the GPRS PDP context. Each PDN can be accessed via a gateway (a PDN-GW). The PDN is responsible for the IP address/prefix allocation to the UE. On the UE, a PDN connection is equivalent to a network interface. A UE may hence be attached to one or more gateways via separate 

   connections, i.e., PDN connections. 3GPP EPS supports PDN Types IPv4,
   IPv6, and IPv4v6 (dual-stack) since the beginning of EPS, i.e., since
   Release-8.

Each PDN connection has its own IP address/prefix assigned to it by the PDN and anchored in the corresponding gateway. In the case of the GTP-based S5/S8 interface, the PDN-GW is the first-hop router for the UE, and in the case of PMIPv6-based S5/S8, the SGW is the first- hop router. Applications on the UE use the appropriate network interface (PDN connection) for connectivity.

4.3. EPS Bearer Model

The logical concept of a bearer has been defined to be an aggregate of one or more IP flows related to one or more services. An EPS bearer exists between the UE and the PDN-GW and is used to provide the same level of packet-forwarding treatment to the aggregated IP flows constituting the bearer. Services with IP flows requiring different packet-forwarding treatment would therefore require more than one EPS bearer. The UE performs the binding of the uplink IP flows to the bearer, while the PDN-GW performs this function for the downlink packets.

In order to always provide low latency on connectivity, a default bearer will be provided at the time of startup, and an IPv4 address and/or IPv6 prefix gets assigned to the UE (this is different from GPRS, where UEs are not automatically connected to a PDN and therefore do not get an IPv4 address and/or IPv6 prefix assigned until they activate their first PDP context). This default bearer will be allowed to carry all traffic that is not associated with a dedicated bearer. Dedicated bearers are used to carry traffic for IP flows that have been identified to require specific packet-forwarding treatment. They may be established at the time of startup -- for example, in the case of services that require always-on connectivity and better QoS than that provided by the default bearer. The default bearer and the dedicated bearer(s) associated to it share the same IP address(es)/prefix.

An EPS bearer is referred to as a Guaranteed Bit Rate (GBR) bearer if dedicated network resources related to a GBR value that is associated with the EPS bearer are permanently allocated (e.g., by an admission control function in the eNodeB) at bearer establishment/modification. Otherwise, an EPS bearer is referred to as a non-GBR bearer. The default bearer is always non-GBR, with the resources for the IP flows not guaranteed at the eNodeB, and with no admission control. However, the dedicated bearer can be either GBR or non-GBR. A GBR bearer has a GBR and Maximum Bit Rate (MBR), while more than one non-GBR bearer belonging to the same UE shares an Aggregate MBR

(AMBR). Non-GBR bearers can suffer packet loss under congestion, while GBR bearers are immune to such losses as long as they honor the contracted bit rates.

5. Address Management

5.1. IPv4 Address Configuration

The UE's IPv4 address configuration is always performed during PDP context/EPS bearer setup procedures (on layer 2). DHCPv4-based [RFC2131] address configuration is supported by the 3GPP specifications, but is not used on a wide scale. The UE must always support address configuration as part of the bearer setup signaling, since DHCPv4 is optional for both UEs and networks.

The 3GPP standards also specify a 'deferred IPv4 address allocation' on a PMIPv6-based dual-stack IPv4v6 PDN connection at the time of connection establishment, as described in Section 4.7.1 of [TS.23402]. This has the advantage of a single PDN connection for IPv6 and IPv4, along with deferring IPv4 address allocation until an application needs it. The deferred address allocation is based on the use of DHCPv4 as well as appropriate UE-side implementation- dependent triggers to invoke the protocol.

5.2. IPv6 Address Configuration

IPv6 Stateless Address Autoconfiguration (SLAAC), as specified in [RFC4861] and [RFC4862], is the only supported address configuration mechanism. Stateful DHCPv6-based address configuration [RFC3315] is not supported by 3GPP specifications. On the other hand, stateless DHCPv6 service to obtain other configuration information is supported [RFC3736]. This implies that the M-bit is always zero and that the O-bit may be set to one in the Router Advertisement (RA) sent to the UE.

The 3GPP network allocates each default bearer a unique /64 prefix, and uses layer-2 signaling to suggest to the UE an Interface Identifier that is guaranteed not to conflict with the gateway's Interface Identifier. The UE must configure its link-local address using this Interface Identifier. The UE is allowed to use any Interface Identifier it wishes for the other addresses it configures. There is no restriction, for example, on using privacy extensions for SLAAC [RFC4941] or other similar types of mechanisms. However, there are network drivers that fail to pass the Interface Identifier to the stack and instead synthesize their own Interface Identifier (usually a Media Access Control (MAC) address equivalent). If the UE skips the Duplicate Address Detection (DAD) and also has other issues with the Neighbor Discovery protocol (see Section 5.4), then there is a small theoretical chance that the UE will configure exactly the same link-local address as the GGSN/PDN-GW. The address collision may then cause issues in IP connectivity -- for instance, the UE not being able to forward any packets to the uplink.

In the 3GPP link model, the /64 prefix assigned to the UE cannot be used for on-link determination (because the L-bit in the Prefix Information Option (PIO) in the RA must always be set to zero). If the advertised prefix is used for SLAAC, then the A-bit in the PIO must be set to one. Details of the 3GPP link-model and address configuration are provided in Section 11.2.1.3.2a of [TS.29061]. More specifically, the GGSN/PDN-GW guarantees that the /64 prefix is unique for the UE. Therefore, there is no need to perform any DAD on addresses the UE creates (i.e., the 'DupAddrDetectTransmits' variable in the UE could be zero). The GGSN/PDN-GW is not allowed to generate any globally unique IPv6 addresses for itself using the /64 prefix assigned to the UE in the RA.

The current 3GPP architecture limits the number of prefixes in each bearer to a single /64 prefix. If the UE finds more than one prefix in the RA, it only considers the first one and silently discards the others [TS.29061]. Therefore, multi-homing within a single bearer is not possible. Renumbering without closing the layer-2 connection is also not possible. The lifetime of the /64 prefix is bound to the lifetime of the layer-2 connection even if the advertised prefix lifetime is longer than the layer-2 connection lifetime.

5.3. Prefix Delegation

IPv6 prefix delegation is a part of Release-10 and is not covered by any earlier releases. However, the /64 prefix allocated for each default bearer (and to the UE) may be shared to the local area network by the UE implementing Neighbor Discovery proxy (ND proxy) [RFC4389] functionality.

The Release-10 prefix delegation uses the DHCPv6-based prefix delegation [RFC3633]. The model defined for Release-10 requires aggregatable prefixes, which means the /64 prefix allocated for the default bearer (and to the UE) must be part of the shorter delegated prefix. DHCPv6 prefix delegation has an explicit limitation, described in Section 12.1 of [RFC3633], that a prefix delegated to a requesting router cannot be used by the delegating router (i.e., the PDN-GW in this case). This implies that the shorter 'delegated prefix' cannot be given to the requesting router (i.e., the UE) as such but has to be delivered by the delegating router (i.e., the PDN-GW) in such a way that the /64 prefix allocated to the default bearer is not part of the 'delegated prefix'. An option to exclude a prefix from delegation [PD-EXCLUDE] prevents this problem.

5.4. IPv6 Neighbor Discovery Considerations

The 3GPP link between the UE and the next-hop router (e.g., the GGSN) resembles a point-to-point (p2p) link, which has no link-layer addresses [RFC3316], and this has not changed from the 2G/3G GPRS to the EPS. The UE IP stack has to take this into consideration. When the 3GPP PDP context appears as a PPP interface/link to the UE, the IP stack is usually prepared to handle the Neighbor Discovery protocol and the related Neighbor Cache state machine transitions in an appropriate way, even though Neighbor Discovery protocol messages contain no link-layer address information. However, some operating systems discard Router Advertisements on their PPP interface/link as a default setting. This causes SLAAC to fail when the 3GPP PDP context gets established, thus stalling all IPv6 traffic.

Currently, several operating systems and their network drivers can make the 3GPP PDP context appear as an IEEE 802 interface/link to the IP stack. This has a few known issues, especially when the IP stack is made to believe that the underlying link has link-layer addresses. First, the Neighbor Advertisement sent by a GGSN as a response to a Neighbor Solicitation triggered by address resolution might not contain a Target Link-Layer Address option (see Section 4.4 of [RFC4861]). It is then possible that the address resolution never completes when the UE tries to resolve the link-layer address of the GGSN, thus stalling all IPv6 traffic.

Second, the GGSN may simply discard all Neighbor Solicitation messages triggered by address resolution (as Section 2.4.1 of [RFC3316] is sometimes misinterpreted as saying that responding to address resolution and next-hop determination is not needed). As a result, the address resolution never completes when the UE tries to resolve the link-layer address of the GGSN, thus stalling all IPv6 traffic. There is little that can be done about this in the GGSN, assuming the neighbor-discovery implementation already does the right thing. But the UE stacks must be able to handle address resolution in the manner that they have chosen to represent the interface. In other words, if they emulate IEEE 802 interfaces, they also need to process Neighbor Discovery messages correctly.

6. 3GPP Dual-Stack Approach to IPv6

6.1. 3GPP Networks Prior to Release-8

3GPP standards prior to Release-8 provide IPv6 access for cellular devices with PDP contexts of type IPv6 [TS.23060]. For dual-stack access, a PDP context of type IPv6 is established in parallel to the PDP context of type IPv4, as shown in Figures 5 and 6. For IPv4-only service, connections are created over the PDP context of type IPv4, and for IPv6-only service, connections are created over the PDP context of type IPv6. The two PDP contexts of different type may use the same APN (and the gateway); however, this aspect is not explicitly defined in standards. Therefore, cellular device and gateway implementations from different vendors may have varying support for this functionality. 

           Y                                        .--.
           |                                      _(IPv4`.
           |---+              +---+    +---+     (  PDN   )
           | D |~~~~~~~//-----|   |====|   |====( `  .  )  )
           | S | IPv4 context | S |    | G |     `--(___.-'
           |   |              | G |    | G |        .--.
           | U |              | S |    | S |      _(IPv6`.
           | E | IPv6 context | N |    | N |     (  PDN   )
           |///|~~~~~~~//-----|   |====|(s)|====( `  .  )  )
           +---+              +---+    +---+     `--(___.-'

Figure 5: Dual-Stack (DS) User Equipment Connecting to Both IPv4 and

IPv6 Internet Using Parallel IPv4-Only and IPv6-Only PDP Contexts 

           Y
           |
           |---+              +---+    +---+
           | D |~~~~~~~//-----|   |====|   |        .--.
           | S | IPv4 context | S |    | G |      _( DS `.
           |   |              | G |    | G |     (  PDN   )
           | U |              | S |    | S |====( `  .  )  )
           | E | IPv6 context | N |    | N |     `--(___.-'
           |///|~~~~~~~//-----|   |====|   |
           +---+              +---+    +---+

Figure 6:

Dual-Stack User Equipment Connecting to Dual-Stack Internet Using Parallel IPv4-Only and IPv6-Only PDP Contexts

The approach of having parallel IPv4 and IPv6 types of PDP contexts open is not optimal, because two PDP contexts require double the signaling and consume more network resources than a single PDP context. In Figure 6, the IPv4 and IPv6 PDP contexts are attached to the same GGSN. While this is possible, the dual-stack MS may be attached to different GGSNs in the scenario where one GGSN supports IPv4 PDN connectivity while another GGSN provides IPv6 PDN connectivity.

6.2. 3GPP Release-8 and -9 Networks

Since 3GPP Release-8, the powerful concept of a dual-stack type of PDN connection and EPS bearer has been introduced [TS.23401]. This enables parallel use of both IPv4 and IPv6 on a single bearer (IPv4v6), as illustrated in Figure 7, and makes dual stack simpler than in earlier 3GPP releases. As of Release-9, GPRS network nodes also support dual-stack (IPv4v6) PDP contexts. 

           Y
           |
           |---+              +---+    +---+
           | D |              |   |    | P |        .--.
           | S |              |   |    | D |      _( DS `.
           |   | IPv4v6 (DS)  | S |    | N |     (  PDN   )
           | U |~~~~~~~//-----| G |====| - |====( `  .  )  )
           | E | bearer       | W |    | G |     `--(___.-'
           |///|              |   |    | W |
           +---+              +---+    +---+

Figure 7: Dual-Stack User Equipment Connecting to Dual-Stack Internet

Using a Single IPv4v6 PDN Connection

The following is a description of the various PDP contexts/PDN bearer types that are specified by 3GPP: 

   1.  For 2G/3G access to the GPRS core (SGSN/GGSN) pre-Release-9,
   
       there are two IP PDP Types: IPv4 and IPv6.  Two PDP contexts are
       needed to get dual-stack connectivity.
   
   2.  For 2G/3G access to the GPRS core (SGSN/GGSN), starting with
       Release-9, there are three IP PDP Types: IPv4, IPv6, and IPv4v6.
       A minimum of one PDP context is needed to get dual-stack
       connectivity.
   
   3.  For 2G/3G access to the EPC (PDN-GW via S4-SGSN), starting with
       Release-8, there are three IP PDP Types: IPv4, IPv6, and IPv4v6
       (which gets mapped to the PDN connection type).  A minimum of one
       PDP context is needed to get dual-stack connectivity.
  1. For LTE (E-UTRAN) access to the EPC, starting with Release-8, there are three IP PDN Types: IPv4, IPv6, and IPv4v6. A minimum of one PDN connection is needed to get dual-stack connectivity.

6.3. PDN Connection Establishment Process

The PDN connection establishment process is specified in detail in 3GPP specifications. Figure 8 illustrates the high-level process and signaling involved in the establishment of a PDN connection. 

      UE        eNodeB/      MME         SGW       PDN-GW       HSS/
      |           BS          |           |           |         AAA
      |           |           |           |           |           |
      |---------->|(1)        |           |           |           |
      |           |---------->|(1)        |           |           |
      |           |           |           |           |           |
      |/---------------------------------------------------------\|
      |             Authentication and Authorization              |(2)
      |\---------------------------------------------------------/|
      |           |           |           |           |           |
      |           |           |---------->|(3)        |           |
      |           |           |           |---------->|(3)        |
      |           |           |           |           |           |
      |           |           |           |<----------|(4)        |
      |           |           |<----------|(4)        |           |
      |           |<----------|(5)        |           |           |
      |/---------\|           |           |           |           |
      | RB setup  |(6)        |           |           |           |
      |\---------/|           |           |           |           |
      |           |---------->|(7)        |           |           |
      |---------->|(8)        |           |           |           |
      |           |---------->|(9)        |           |           |
      |           |           |           |           |           |
      |============= Uplink Data =========>==========>|(10)       |
      |           |           |           |           |           |
      |           |           |---------->|(11)       |           |
      |           |           |           |           |           |
      |           |           |<----------|(12)       |           |
      |           |           |           |           |           |
      |<============ Downlink Data =======<===========|(13)       |
      |           |           |           |           |           |

Figure 8: Simplified PDN Connection Setup Procedure in Release-8 

   1.   The UE (i.e., the MS) requires a data connection and hence
        decides to establish a PDN connection with a PDN-GW.  The UE
        sends an "Attach" request (layer-2) to the base station (BS).
        The BS forwards this Attach request to the MME.
   
   2.   Authentication of the UE with the Authentication, Authorization,
        and Accounting (AAA) server/HSS follows.  If the UE is
        authorized to establish a data connection, the process continues
        with the following steps:
   
   3.   The MME sends a "Create Session" request message to the SGW.
        The SGW forwards the Create Session request to the PDN-GW.  The
        SGW knows the address of the PDN-GW to which it forwards the
        Create Session request as a result of this information having
        been obtained by the MME during the authentication/authorization
        phase.

The UE IPv4 address and/or IPv6 prefix gets assigned during this step. If a subscribed IPv4 address and/or IPv6 prefix is statically allocated for the UE for this APN, then the MME passes this previously allocated address information to the SGW and eventually to the PDN-GW in the Create Session request message. Otherwise, the PDN-GW manages the address assignment to the UE (there is another variation to this step where IPv4 address allocation is delayed until the UE initiates a DHCPv4 exchange, but this is not discussed here). 

   4.   The PDN-GW creates a PDN connection for the UE and sends a
        Create Session response message to the SGW from which the
        session request message was received.  The SGW forwards the
        response to the corresponding MME that originated the request.
   
   5.   The MME sends the "Attach Accept/Initial Context Setup" request
        message to the eNodeB/BS.
   
   6.   The radio bearer (RB) between the UE and the eNodeB is
        reconfigured based on the parameters received from the MME.
        (See Note 1 below.)
   
   7.   The eNodeB sends an "Initial Context" response message to
        the MME.
   
   8.   The UE sends a "Direct Transfer" message, which includes the
        "Attach Complete" signal, to the eNodeB.
   
   9.   The eNodeB forwards the Attach Complete message to the MME.
  1. The UE can now start sending uplink packets to the PDN GW.
  1. The MME sends a "Modify Bearer" request message to the SGW.
  1. The SGW responds with a Modify Bearer response message. At this time, the downlink connection is also ready.
  1. The UE can now start receiving downlink packets, including possible SLAAC-related IPv6 packets.

The type of PDN connection established between the UE and the PDN-GW can be any of the types described in the previous section. The dual- stack PDN connection, i.e., the one that supports both IPv4 and IPv6 packets, is the default connection that will be established if no specific PDN connection type is specified by the UE in Release-8 networks.

Note 1: The UE receives the PDN Address Information Element [TS.24301] at the end of radio bearer setup messaging. This information element contains only the Interface Identifier of the IPv6 address. In the case of the GPRS, the PDP Address Information Element [TS.24008] would contain a complete IPv6 address. However, the UE must ignore the IPv6 prefix if it receives one in the message (see Section 11.2.1.3.2a of [TS.29061]).

6.4. Mobility of 3GPP IPv4v6 Bearers

3GPP discussed at length various approaches to support mobility between a Release-8 LTE network and a pre-Release-9 2G/3G network without an S4-SGSN for the new dual-stack bearers. The chosen approach for mobility is as follows, in short: if a UE is allowed to do handovers between a Release-8 LTE network and a pre-Release-9 2G/3G network without an S4-SGSN while having open PDN connections, only single-stack bearers are used. Essentially, this indicates the following deployment options:

  1. If a network knows a UE may do handovers between a Release-8 LTE network and a pre-Release-9 2G/3G network without an S4-SGSN, then the network is configured to provide only single-stack bearers, even if the UE requests dual-stack bearers.
  1. If the network knows the UE does handovers only between a Release-8 LTE network and a Release-9 2G/3G network or a pre-Release-9 network with an S4-SGSN, then the network is configured to provide the UE with dual-stack bearers on request. The same also applies for LTE-only deployments.

When a network operator and their roaming partners have upgraded their networks to Release-8, it is possible to use the new IPv4v6 dual-stack bearers. A Release-8 UE always requests a dual-stack bearer, but accepts what is assigned by the network.

7. Dual-Stack Approach to IPv6 Transition in 3GPP Networks

3GPP networks can natively transport IPv4 and IPv6 packets between the UE and the gateway (GGSN or PDN-GW) as a result of establishing either a dual-stack PDP context or parallel IPv4 and IPv6 PDP contexts.

Current deployments of 3GPP networks primarily support IPv4 only. These networks can be upgraded to also support IPv6 PDP contexts. By doing so, devices and applications that are IPv6 capable can start utilizing IPv6 connectivity. This will also ensure that legacy devices and applications continue to work with no impact. As newer devices start using IPv6 connectivity, the demand for actively used IPv4 connections is expected to slowly decrease, helping operators with a transition to IPv6. With a dual-stack approach, there is always the potential to fall back to IPv4. A device that may be roaming in a network wherein IPv6 is not supported by the visited network could fall back to using IPv4 PDP contexts, and hence the end user would at least get some connectivity. Unfortunately, the dual- stack approach as such does not lower the number of used IPv4 addresses. Every dual-stack bearer still needs to be given an IPv4 address, private or public. This is a major concern with dual-stack bearers concerning IPv6 transition. However, if the majority of active IP communication has moved over to IPv6, then in the case of Network Address Translation from IPv4 to IPv4 (NAT44), the number of active NAT44-translated IPv4 connections can still be expected to gradually decrease and thus give some level of relief regarding NAT44 function scalability.

As the networks evolve to support Release-8 EPS architecture and the dual-stack PDP contexts, newer devices will be able to leverage such capability and have a single bearer that supports both IPv4 and IPv6. Since IPv4 and IPv6 packets are carried as payload within GTP between the MS and the gateway (GGSN/PDN-GW), the transport-network capability in terms of whether it supports IPv4 or IPv6 on the interfaces between the eNodeB and SGW or between the SGW and PDN-GW is immaterial.

8. Deployment Issues

8.1. Overlapping IPv4 Addresses

Given the shortage of globally routable public IPv4 addresses, operators tend to assign private IPv4 addresses [RFC1918] to UEs when they establish an IPv4-only PDP context or an IPv4v6 PDN context. About 16 million UEs can be assigned a private IPv4 address that is unique within a domain. However, for many operators, the number of subscribers is greater than 16 million. The issue can be dealt with by assigning overlapping RFC 1918 IPv4 addresses to UEs. As a result, the IPv4 address assigned to a UE within the context of a single operator realm would no longer be unique. This has the obvious and known issues of NATed IP connections in the Internet. Direct UE-to-UE connectivity becomes complicated; unless the UEs are within the same private address range pool and/or anchored to the same gateway, referrals using IP addresses will have issues, and so forth. These are generic issues and not only a concern of the EPS. However, 3GPP as such does not have any mandatory language concerning NAT44 functionality in the EPC. Obvious deployment choices apply also to the EPC:

  1. Very large network deployments are partitioned, for example, based on geographical areas. This partitioning allows overlapping IPv4 address ranges to be assigned to UEs that are in different areas. Each area has its own pool of gateways that are dedicated to a certain overlapping IPv4 address range (also referred to as a zone). Standard NAT44 functionality allows for communication from the [RFC1918] private zone to the Internet. Communication between zones requires special arrangement, such as using intermediate gateways (e.g., a Back-to-Back User Agent (B2BUA) in the case of SIP).
  1. A UE attaches to a gateway as part of the Attach process. The number of UEs that a gateway supports is on the order of 1 to 10 million. Hence, all of the UEs assigned to a single gateway can be assigned private IPv4 addresses. Operators with large subscriber bases have multiple gateways, and hence the same [RFC1918] IPv4 address space can be reused across gateways. The IPv4 address assigned to a UE is unique within the scope of a single gateway.
  1. New services requiring direct connectivity between UEs should be built on IPv6. Possible existing IPv4-only services and applications requiring direct connectivity can be ported to IPv6.

8.2. IPv6 for Transport

The various reference points of the 3GPP architecture, such as S1-U, S5, and S8, are based on either GTP or PMIPv6. The underlying transport for these reference points can be IPv4 or IPv6. GTP has been able to operate over IPv6 transport (optionally) since R99, and PMIPv6 has supported IPv6 transport since its introduction in Release-8. The user-plane traffic between the UE and the gateway can use either IPv4 or IPv6. These packets are essentially treated as payload by GTP/PMIPv6 and transported accordingly, with no real attention paid (at least from a routing perspective) to the information contained in the IPv4 or IPv6 headers. The transport links between the eNodeB and the SGW, and the link between the SGW and PDN-GW, can be migrated to IPv6 without any direct implications to the architecture.

Currently, the inter-operator (for 3GPP technology) roaming networks are all IPv4 only (see Inter-PLMN Backbone Guidelines [GSMA.IR.34]). Eventually, these roaming networks will also get migrated to IPv6, if there is a business reason for that. The migration period can be prolonged considerably, because the 3GPP protocols always tunnel user-plane traffic in the core network, and as described earlier, the transport-network IP version is not in any way tied to the user-plane IP version. Furthermore, the design of the inter-operator roaming networks is such that the user-plane and transport-network IP addressing schemes are completely separated from each other. The inter-operator roaming network itself is also completely separated from the Internet. Only those core network nodes that must be connected to the inter-operator roaming networks are actually visible there, and are able to send and receive (tunneled) traffic within the inter-operator roaming networks. Obviously, in order for the roaming to work properly, the operators have to agree on supported protocol versions so that the visited network does not, for example, unnecessarily drop user-plane IPv6 traffic.

8.3. Operational Aspects of Running Dual-Stack Networks

Operating dual-stack networks does imply cost and complexity to a certain extent. However, these factors are mitigated by the assurance that legacy devices and services are unaffected, and there is always a fallback to IPv4 in case of issues with the IPv6 deployment or network elements. The model also enables operators to develop operational experience and expertise in an incremental manner.

Running dual-stack networks requires the management of multiple IP address spaces. Tracking of UEs needs to be expanded, since it can be identified by either an IPv4 address or an IPv6 prefix. Network elements will also need to be dual-stack capable in order to support the dual-stack deployment model.

Deployment and migration cases (see Section 6.1) for providing dual- stack capability may mean doubled resource usage in an operator's network. This is a major concern against providing dual-stack connectivity using techniques discussed in Section 6.1. Also, handovers between networks with different capabilities in terms of whether or not networks are capable of dual-stack service may prove difficult for users to comprehend and for applications/services to cope with. These facts may add other than just technical concerns for operators when planning to roll out dual-stack service offerings.

8.4. Operational Aspects of Running a Network with IPv6-Only Bearers

It is possible to allocate IPv6-only bearers to UEs in 3GPP networks. The IPv6-only bearer has been part of the 3GPP specification since the beginning. In 3GPP Release-8 (and later), it was defined that a dual-stack UE (or when the radio equipment has no knowledge of the UE IP stack's capabilities) must first attempt to establish a dual-stack bearer and then possibly fall back to a single-stack bearer. A Release-8 (or later) UE with an IPv6-only stack can directly attempt to establish an IPv6-only bearer. The IPv6-only behavior is up to subscription provisioning or PDN-GW configuration, and the fallback scenarios do not necessarily cause additional signaling.

Although the bullets below introduce IPv6-to-IPv4 address translation and specifically discuss NAT64 technology [RFC6144], the current 3GPP Release-8 architecture does not describe the use of address translation or NAT64. It is up to a specific deployment whether address translation is part of the network or not. The following are some operational aspects to consider for running a network with IPv6-only bearers:

  • The UE must have an IPv6-capable stack and a radio interface capable of establishing an IPv6 PDP context or PDN connection.
  • The GGSN/PDN-GW must be IPv6 capable in order to support IPv6 bearers. Furthermore, the SGSN/MME must allow the creation of a PDP Type or PDN Type of IPv6.
  • Many of the common applications are IP version agnostic and hence would work using an IPv6 bearer. However, applications that are IPv4 specific would not work.
  • Inter-operator roaming is another aspect that causes issues, at least during the ramp-up phase of the IPv6 deployment. If the visited network to which outbound roamers attach does not support PDP/PDN Type IPv6, then there needs to be a fallback option. The fallback option in this specific case is mostly up to the UE to implement. Several cases are discussed in the following sections.
  • If and when a UE using an IPv6-only bearer needs access to the IPv4 Internet/network, some type of translation from IPv6 to IPv4 has to be deployed in the network. NAT64 (or DNS64) is one solution that can be used for this purpose and works for a certain set of protocols (read TCP, UDP, and ICMP, and when applications actually use DNS for resolving names to IP addresses).

8.5. Restricting Outbound IPv6 Roaming

Roaming was briefly touched upon in Sections 8.2 and 8.4. While there is interest in offering roaming service for IPv6-enabled UEs and subscriptions, not all visited networks are prepared for IPv6 outbound roamers:

  • The visited-network SGSN does not support the IPv6 PDP context or IPv4v6 PDP context types. These should mostly concern pre-Release-9 2G/3G networks without an S4-SGSN, but there is no definitive rule, as the deployed feature sets vary depending on implementations and licenses.
  • The visited network might not be commercially ready for IPv6 outbound roamers, while everything might work technically at the user-plane level. This would lead to "revenue leakage", especially from the visited operator's point of view (note that the use of a visited-network GGSN/PDN-GW does not really exist today in commercial deployments for data roaming).

It might be in the interest of operators to prohibit roaming selectively within specific visited networks until IPv6 roaming is in place. 3GPP does not specify a mechanism whereby IPv6 roaming is prohibited without also disabling IPv4 access and other packet services. The following options for disabling IPv6 access for roaming subscribers could be available in some network deployments:

  • Policy and Charging Control (PCC) [TS.23203] functionality and its rules, for example, could be used to cause bearer authorization to fail when a desired criteria is met. In this case, that would be PDN/PDP Type IPv6/IPv4v6 and a specific visited network. The rules can be provisioned either in the home network or locally in the visited network.
  • Some Home Location Register (HLR) and Home Subscriber Server (HSS) subscriber databases allow prohibiting roaming in a specific (visited) network for a specified PDN/PDP Type.

The obvious problems are that these solutions are not mandatory, are not unified across networks, and therefore also lack a well-specified fallback mechanism from the UE's point of view.

8.6. Inter-RAT Handovers and IP Versions

It is obvious that as operators start to incrementally deploy the EPS along with the existing UTRAN/GERAN, handovers between different radio technologies (inter-RAT handovers) become inevitable. In the case of inter-RAT handovers, 3GPP supports the following IP addressing scenarios:

  • The E-UTRAN IPv4v6 bearer has to map one to one to the UTRAN/GERAN IPv4v6 bearer.
  • The E-UTRAN IPv6 bearer has to map one to one to the UTRAN/GERAN IPv6 bearer.
  • The E-UTRAN IPv4 bearer has to map one to one to the UTRAN/GERAN IPv4 bearer.

Other types of configurations are not standardized. The above rules essentially imply that the network migration has to be planned and subscriptions provisioned based on the lowest common denominator, if inter-RAT handovers are desired. For example, if some part of the UTRAN cannot serve anything but IPv4 bearers, then the E-UTRAN is also forced to provide only IPv4 bearers. Various combinations of subscriber provisioning regarding IP versions are discussed further in Section 8.7.

8.7. Provisioning of IPv6 Subscribers and Various Combinations during

Initial Network Attachment

Subscribers' provisioned PDP/PDN Types have multiple configurations. The supported PDP/PDN Type is provisioned per each APN for every subscriber. The following PDN Types are possible in the HSS for a Release-8 subscription [TS.23401]:

  • IPv4v6 PDN Type (note that the IPv4v6 PDP Type does not exist in an HLR and Mobile Application Part (MAP) [TS.29002] signaling prior to Release-9).
  • IPv6-only PDN Type.
  • IPv4-only PDN Type.
  • IPv4_or_IPv6 PDN Type (note that the IPv4_or_IPv6 PDP Type does not exist in an HLR or MAP signaling. However, an HLR may have multiple APN configurations of different PDN Types; these configurations would effectively achieve the same functionality).

A Release-8 dual-stack UE must always attempt to establish a PDP/PDN Type IPv4v6 bearer. The same also applies when the modem part of the UE does not have exact knowledge of whether the UE operating system IP stack is dual-stack capable or not. A UE that is IPv6-only capable must attempt to establish a PDP/PDN Type IPv6 bearer. Last, a UE that is IPv4-only capable must attempt to establish a PDN/PDP Type IPv4 bearer.

In a case where the PDP/PDN Type requested by a UE does not match what has been provisioned for the subscriber in the HSS (or HLR), the UE possibly falls back to a different PDP/PDN Type. The network (i.e., the MME or the S4-SGSN) is able to inform the UE during network attachment signaling as to why it did not get the requested PDP/PDN Type. These response/cause codes are documented in [TS.24008] for requested PDP Types and [TS.24301] for requested PDN Types:

  • (E)SM cause #50 "PDN/PDP type IPv4 only allowed".
  • (E)SM cause #51 "PDN/PDP type IPv6 only allowed".
  • (E)SM cause #52 "single address bearers only allowed".

The above response/cause codes apply to Release-8 and onwards. In pre-Release-8 networks, the response/cause codes that are used vary, depending on the vendor, unfortunately.

Possible fallback cases when the network deploys MMEs and/or S4-SGSNs include (as documented in [TS.23401]):

  • Requested and provisioned PDP/PDN Types match => requested.
  • Requested IPv4v6 and provisioned IPv6 => IPv6, and a UE receives an indication that an IPv6-only bearer is allowed.
  • Requested IPv4v6 and provisioned IPv4 => IPv4, and the UE receives an indication that an IPv4-only bearer is allowed.
  • Requested IPv4v6 and provisioned IPv4_or_IPv6 => IPv4 or IPv6 is selected by the MME/S4-SGSN based on an unspecified criteria. The UE may then attempt to establish, based on the UE implementation, a parallel bearer of a different PDP/PDN Type.
  • Other combinations cause the bearer establishment to fail.

In addition to PDP/PDN Types provisioned in the HSS, it is also possible for a PDN-GW (and an MME/S4-SGSN) to affect the final selected PDP/PDN Type:

  • Requested IPv4v6 and configured IPv4 or IPv6 in the PDN-GW => IPv4 or IPv6. If the MME operator had included the "Dual Address Bearer" flag in the bearer establishment signaling, then the UE would have received an indication that an IPv6-only or IPv4-only bearer is allowed.
  • Requested IPv4v6 and configured IPv4 or IPv6 in the PDN-GW => IPv4 or IPv6. If the MME operator had not included the "Dual Address Bearer" flag in the bearer establishment signaling, then the UE may have attempted to establish, based on the UE implementation, a parallel bearer of a different PDP/PDN Type.

An SGSN that does not understand the requested PDP Type is supposed to handle the requested PDP Type as IPv4. If for some reason an MME does not understand the requested PDN Type, then the PDN Type is handled as IPv6.

9. Security Considerations

This document does not introduce any security-related concerns. Section 5 of [RFC3316] already contains an in-depth discussion of IPv6-related security considerations in 3GPP networks prior to Release-8. This section discusses a few additional security concerns to take into consideration.

In 3GPP access, the UE and the network always perform a mutual authentication during the network attachment [TS.33102] [TS.33401]. Furthermore, each time a PDP context/PDN connection gets created, a new connection, a modification of an existing connection, and an assignment of an IPv6 prefix or an IP address can be authorized against the PCC infrastructure [TS.23203] and/or PDN's AAA server.

The wireless part of the 3GPP link between the UE and the (e)NodeB as well as the signaling messages between the UE and the MME/SGSN can be protected, depending on the regional regulation and the operator's deployment policy. User-plane traffic can be confidentiality protected. The control plane is always at least integrity and replay protected, and may also be confidentiality protected. The protection within the transmission part of the network depends on the operator's deployment policy [TS.33401].

Several of the on-link and neighbor-discovery-related attacks can be mitigated due to the nature of the 3GPP point-to-point link model, and the fact that the UE and the first-hop router (PDN-GW/GGSN or SGW) are the only nodes on the link. For off-link IPv6 attacks, the 3GPP EPS is as vulnerable as any IPv6 system.

There have also been concerns that the UE IP stack might use permanent subscriber identities, such as an International Mobile Subscriber Identity (IMSI), as the source for the IPv6 address Interface Identifier. This would be a privacy threat and would allow tracking of subscribers. Therefore, the use of an IMSI (or any identity defined by [TS.23003]) as the Interface Identifier is prohibited [TS.23401]. However, there is no standardized method to block such misbehaving UEs.

10. Summary and Conclusions

The 3GPP network architecture and specifications enable the establishment of IPv4 and IPv6 connections through the use of appropriate PDP context types. The current generation of deployed networks can support dual-stack connectivity if the packet core network elements, such as the SGSN and GGSN, have that capability. With Release-8, 3GPP has specified a more optimal PDP context type that enables the transport of IPv4 and IPv6 packets within a single PDP context between the UE and the gateway.

As devices and applications are upgraded to support IPv6, they can start leveraging the IPv6 connectivity provided by the networks while maintaining the ability to fall back to IPv4. Enabling IPv6 connectivity in the 3GPP networks by itself will provide some degree of relief to the IPv4 address space, as many of the applications and services can start to work over IPv6. However, without comprehensive testing of current widely used applications and solutions for their ability to operate over IPv6 PDN connections, an IPv6-only access would cause disruptions.

11. Acknowledgements

The authors thank Shabnam Sultana, Sri Gundavelli, Hui Deng, Zhenqiang Li, Mikael Abrahamsson, James Woodyatt, Wes George, Martin Thomson, Russ Mundy, Cameron Byrne, Ales Vizdal, Frank Brockners, Adrian Farrel, Stephen Farrell, Paco Cortes, and Jari Arkko for their reviews and comments on this document.

12. Informative References

   [GSMA.IR.34]  GSMA, "Inter-PLMN Backbone Guidelines", GSMA
                 PRD IR.34.4.9, March 2010.
   
   [PD-EXCLUDE]  Korhonen, J., Ed., Savolainen, T., Krishnan, S., and O.
                 Troan, "Prefix Exclude Option for DHCPv6-based Prefix
                 Delegation", Work in Progress, December 2011.
   
   [RFC1918]     Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot,
                 G., and E. Lear, "Address Allocation for Private
                 Internets", BCP 5, RFC 1918, February 1996.
   
   [RFC2131]     Droms, R., "Dynamic Host Configuration Protocol",
                 RFC 2131, March 1997.
   
   [RFC3315]     Droms, R., Ed., Bound, J., Volz, B., Lemon, T.,
                 Perkins, C., and M. Carney, "Dynamic Host Configuration
                 Protocol for IPv6 (DHCPv6)", RFC 3315, July 2003.
   
   [RFC3316]     Arkko, J., Kuijpers, G., Soliman, H., Loughney, J., and
                 J. Wiljakka, "Internet Protocol Version 6 (IPv6) for
                 Some Second and Third Generation Cellular Hosts",
                 RFC 3316, April 2003.
   
   [RFC3633]     Troan, O. and R. Droms, "IPv6 Prefix Options for
                 Dynamic Host Configuration Protocol (DHCP) version 6",
                 RFC 3633, December 2003.
   
   [RFC3736]     Droms, R., "Stateless Dynamic Host Configuration
                 Protocol (DHCP) Service for IPv6", RFC 3736,
                 April 2004.
   
   [RFC4389]     Thaler, D., Talwar, M., and C. Patel, "Neighbor
                 Discovery Proxies (ND Proxy)", RFC 4389, April 2006.
   
   [RFC4861]     Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
                 "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
                 September 2007.
   
   [RFC4862]     Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
                 Address Autoconfiguration", RFC 4862, September 2007.
   
   [RFC4941]     Narten, T., Draves, R., and S. Krishnan, "Privacy
                 Extensions for Stateless Address Autoconfiguration in
                 IPv6", RFC 4941, September 2007.
   
   [RFC5213]     Gundavelli, S., Ed., Leung, K., Devarapalli, V.,
                 Chowdhury, K., and B. Patil, "Proxy Mobile IPv6",
                 RFC 5213, August 2008.
   
   [RFC6144]     Baker, F., Li, X., Bao, C., and K. Yin, "Framework for
                 IPv4/IPv6 Translation", RFC 6144, April 2011.
   
   [TR.23975]    3GPP, "IPv6 Migration Guidelines", 3GPP
                 TR 23.975 11.0.0, June 2011.
   
   [TS.23003]    3GPP, "Numbering, addressing and identification", 3GPP
                 TS 23.003 10.3.0, September 2011.
   
   [TS.23060]    3GPP, "General Packet Radio Service (GPRS); Service
                 description; Stage 2", 3GPP TS 23.060 8.14.0,
                 September 2011.
   
   [TS.23203]    3GPP, "Policy and charging control architecture", 3GPP
                 TS 23.203 8.12.0, June 2011.
   
   [TS.23401]    3GPP, "General Packet Radio Service (GPRS) enhancements
                 for Evolved Universal Terrestrial Radio Access Network
                 (E-UTRAN) access", 3GPP TS 23.401 10.5.0,
                 September 2011.
   
   [TS.23402]    3GPP, "Architecture enhancements for non-3GPP
                 accesses", 3GPP TS 23.402 10.5.0, September 2011.
   
   [TS.24008]    3GPP, "Mobile radio interface Layer 3 specification;
                 Core network protocols; Stage 3", 3GPP
                 TS 24.008 8.14.0, June 2011.
   
   [TS.24301]    3GPP, "Non-Access-Stratum (NAS) protocol for Evolved
                 Packet System (EPS); Stage 3", 3GPP TS 24.301 8.10.0,
                 June 2011.
   
   [TS.29002]    3GPP, "Mobile Application Part (MAP) specification",
                 3GPP TS 29.002 9.6.0, September 2011.
   
   [TS.29060]    3GPP, "General Packet Radio Service (GPRS); GPRS
                 Tunnelling Protocol (GTP) across the Gn and Gp
                 interface", 3GPP TS 29.060 8.15.0, September 2011.
   
   [TS.29061]    3GPP, "Interworking between the Public Land Mobile
                 Network (PLMN) supporting packet based services and
                 Packet Data Networks (PDN)", 3GPP TS 29.061 8.8.0,
                 September 2011.
   
   [TS.29274]    3GPP, "3GPP Evolved Packet System (EPS);  Evolved
                 General Packet Radio Service (GPRS)  Tunnelling
                 Protocol for Control plane (GTPv2-C); Stage 3", 3GPP
                 TS 29.274 8.10.0, June 2011.
   
   [TS.29281]    3GPP, "General Packet Radio System (GPRS) Tunnelling
                 Protocol User Plane (GTPv1-U)", 3GPP TS 29.281 10.3.0,
                 September 2011.
   
   [TS.33102]    3GPP, "3G security; Security architecture", 3GPP
                 TS 33.102 10.0.0, December 2010.
   
   [TS.33401]    3GPP, "3GPP System Architecture Evolution (SAE);
                 Security architecture", 3GPP TS 33.401 10.2.0,
                 September 2011.

Authors' Addresses

Jouni Korhonen (editor)
Nokia Siemens Networks
Linnoitustie 6
FI-02600 Espoo
FINLAND 

   EMail: jouni.nospam@gmail.com

Jonne Soininen
Renesas Mobile
Porkkalankatu 24
FI-00180 Helsinki
FINLAND 

   EMail: jonne.soininen@renesasmobile.com
   
   Basavaraj Patil
   Nokia
   6021 Connection Drive
   Irving, TX  75039
   USA
   
   EMail: basavaraj.patil@nokia.com

Teemu Savolainen
Nokia
Hermiankatu 12 D
FI-33720 Tampere
FINLAND 

   EMail: teemu.savolainen@nokia.com
   
   Gabor Bajko
   Nokia
   323 Fairchild Drive 6
   Mountain View, CA  94043
   USA
   
   EMail: gabor.bajko@nokia.com

Kaisu Iisakkila
Renesas Mobile
Porkkalankatu 24
FI-00180 Helsinki
FINLAND 

   EMail: kaisu.iisakkila@renesasmobile.com