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rfc:rfc6459

Internet Engineering Task Force (IETF) J. Korhonen, Ed. Request for Comments: 6459 Nokia Siemens Networks Category: Informational J. Soininen ISSN: 2070-1721 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.

Korhonen, et al. Informational [Page 1] RFC 6459 IPv6 in 3GPP EPS January 2012

Copyright Notice

 Copyright (c) 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.

Korhonen, et al. Informational [Page 2] RFC 6459 IPv6 in 3GPP EPS January 2012

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

Korhonen, et al. Informational [Page 3] RFC 6459 IPv6 in 3GPP EPS January 2012

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

Korhonen, et al. Informational [Page 4] RFC 6459 IPv6 in 3GPP EPS January 2012

 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.
 2.  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.

Korhonen, et al. Informational [Page 5] RFC 6459 IPv6 in 3GPP EPS January 2012

 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.

Korhonen, et al. Informational [Page 6] RFC 6459 IPv6 in 3GPP EPS January 2012

 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).

Korhonen, et al. Informational [Page 7] RFC 6459 IPv6 in 3GPP EPS January 2012

 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).

Korhonen, et al. Informational [Page 8] RFC 6459 IPv6 in 3GPP EPS January 2012

 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.

Korhonen, et al. Informational [Page 9] RFC 6459 IPv6 in 3GPP EPS January 2012

 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

Korhonen, et al. Informational [Page 10] RFC 6459 IPv6 in 3GPP EPS January 2012

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.

Korhonen, et al. Informational [Page 11] RFC 6459 IPv6 in 3GPP EPS January 2012

 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.

Korhonen, et al. Informational [Page 12] RFC 6459 IPv6 in 3GPP EPS January 2012

      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.

Korhonen, et al. Informational [Page 13] RFC 6459 IPv6 in 3GPP EPS January 2012

                                                            +--------+
                                                            |   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

Korhonen, et al. Informational [Page 14] RFC 6459 IPv6 in 3GPP EPS January 2012

 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

Korhonen, et al. Informational [Page 15] RFC 6459 IPv6 in 3GPP EPS January 2012

 (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

Korhonen, et al. Informational [Page 16] RFC 6459 IPv6 in 3GPP EPS January 2012

 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.

Korhonen, et al. Informational [Page 17] RFC 6459 IPv6 in 3GPP EPS January 2012

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,

Korhonen, et al. Informational [Page 18] RFC 6459 IPv6 in 3GPP EPS January 2012

 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.

Korhonen, et al. Informational [Page 19] RFC 6459 IPv6 in 3GPP EPS January 2012

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.
 4.  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.

Korhonen, et al. Informational [Page 20] RFC 6459 IPv6 in 3GPP EPS January 2012

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

Korhonen, et al. Informational [Page 21] RFC 6459 IPv6 in 3GPP EPS January 2012

 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.
 10.  The UE can now start sending uplink packets to the PDN GW.

Korhonen, et al. Informational [Page 22] RFC 6459 IPv6 in 3GPP EPS January 2012

 11.  The MME sends a "Modify Bearer" request message to the SGW.
 12.  The SGW responds with a Modify Bearer response message.  At this
      time, the downlink connection is also ready.
 13.  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.
 2.  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.

Korhonen, et al. Informational [Page 23] RFC 6459 IPv6 in 3GPP EPS January 2012

 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.

Korhonen, et al. Informational [Page 24] RFC 6459 IPv6 in 3GPP EPS January 2012

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).
 2.  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.
 3.  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.

Korhonen, et al. Informational [Page 25] RFC 6459 IPv6 in 3GPP EPS January 2012

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.

Korhonen, et al. Informational [Page 26] RFC 6459 IPv6 in 3GPP EPS January 2012

 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:
 o  The UE must have an IPv6-capable stack and a radio interface
    capable of establishing an IPv6 PDP context or PDN connection.
 o  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.
 o  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.

Korhonen, et al. Informational [Page 27] RFC 6459 IPv6 in 3GPP EPS January 2012

 o  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.
 o  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:
 o  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.
 o  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:
 o  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.

Korhonen, et al. Informational [Page 28] RFC 6459 IPv6 in 3GPP EPS January 2012

 o  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:
 o  The E-UTRAN IPv4v6 bearer has to map one to one to the UTRAN/GERAN
    IPv4v6 bearer.
 o  The E-UTRAN IPv6 bearer has to map one to one to the UTRAN/GERAN
    IPv6 bearer.
 o  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]:
 o  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).
 o  IPv6-only PDN Type.

Korhonen, et al. Informational [Page 29] RFC 6459 IPv6 in 3GPP EPS January 2012

 o  IPv4-only PDN Type.
 o  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:
 o  (E)SM cause #50 "PDN/PDP type IPv4 only allowed".
 o  (E)SM cause #51 "PDN/PDP type IPv6 only allowed".
 o  (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]):
 o  Requested and provisioned PDP/PDN Types match => requested.
 o  Requested IPv4v6 and provisioned IPv6 => IPv6, and a UE receives
    an indication that an IPv6-only bearer is allowed.
 o  Requested IPv4v6 and provisioned IPv4 => IPv4, and the UE receives
    an indication that an IPv4-only bearer is allowed.

Korhonen, et al. Informational [Page 30] RFC 6459 IPv6 in 3GPP EPS January 2012

 o  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.
 o  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:
 o  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.
 o  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

Korhonen, et al. Informational [Page 31] RFC 6459 IPv6 in 3GPP EPS January 2012

 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.

Korhonen, et al. Informational [Page 32] RFC 6459 IPv6 in 3GPP EPS January 2012

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.

Korhonen, et al. Informational [Page 33] RFC 6459 IPv6 in 3GPP EPS January 2012

 [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.

Korhonen, et al. Informational [Page 34] RFC 6459 IPv6 in 3GPP EPS January 2012

 [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

Korhonen, et al. Informational [Page 35] RFC 6459 IPv6 in 3GPP EPS January 2012

 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

Korhonen, et al. Informational [Page 36]

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