Internet Draft                                       J. Wiljakka (ed.)
 Document: draft-ietf-v6ops-3gpp-analysis-09.txt draft-ietf-v6ops-3gpp-analysis-10.txt                  Nokia
 Expires: September November 2004

                                                             March

                                                               May 2004

                Analysis on IPv6 Transition in 3GPP Networks

 Status of this Memo

    This document is an Internet-Draft

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    patent or other IPR claims of which I am aware have been disclosed,
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    with
    all provisions of Section 10 of RFC2026. RFC 3668.

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 Abstract

    This document analyzes the transition to IPv6 in Third Generation
    Partnership Project (3GPP) packet networks. These networks are
    based on General Packet Radio Service (GPRS)
    packet networks. technology, and the
    radio network architecture is based on Global System for Mobile
    Communications (GSM), or Universal Mobile Telecommunications System
    (UMTS) / Wideband Code Division Multiple Access (WCDMA) technology.

    The focus is on analyzing different transition scenarios,
    applicable transition mechanisms and finding solutions for those
    transition scenarios. In these scenarios, the User Equipment (UE)
    connects to other nodes, e.g. in the Internet, and IPv6/IPv4
    transition mechanisms are needed.

 Table of Contents

    1. Introduction..................................................2
       1.1 Scope of this Document....................................3
       1.2 Abbreviations.............................................3 Abbreviations.............................................4
       1.3 Terminology...............................................4
    2. Transition Mechanisms and DNS Guidelines......................5
       2.1 Dual Stack................................................5
       2.2 Tunneling.................................................5
       2.3 Protocol Translators......................................5 Translators......................................6
       2.4 DNS Guidelines for IPv4/IPv6 Transition...................6
    3. GPRS Transition Scenarios.....................................6
       3.1 Dual Stack UE Connecting to IPv4 and IPv6 Nodes...........6 Nodes...........7
       3.2 IPv6 UE Connecting to an IPv6 Node through an IPv4 Network
       ..............................................................8
        .............................................................8
       3.3 IPv4 UE Connecting to an IPv4 Node through an IPv6 Network
       .............................................................10
        ............................................................10
       3.4 IPv6 UE Connecting to an IPv4 Node.......................10
       3.5 IPv4 UE Connecting to an IPv6 Node.......................11
    4. IMS Transition Scenarios.....................................12
       4.1 UE Connecting to a Node in an IPv4 Network through IMS...12
       4.2 Two IPv6 IMS Islands Connected over via an IPv4 Network..............14 Network...............14
    5. About 3GPP UE IPv4/IPv6 Configuration........................14
    6. Security Considerations......................................15 Summary and Recommendations..................................15
    7. Security Considerations......................................16
    8. References...................................................16
       7.1
       8.1 Normative................................................16
       7.2 Informative..............................................16
    8. Contributors.................................................18
       8.2 Informative..............................................17
    9. Contributors.................................................19
    10. Authors and Acknowledgements.................................18
    10. Acknowledgements................................19
    11. Editor's Contact Information................................19
    11. Information................................20
    12. Intellectual Property Statement.............................19
    12. Copyright...................................................19 Statement.............................20
    13. Copyright...................................................20
       Appendix A...................................................20 A...................................................21

 1. Introduction

    This document describes and analyzes the process of transition to
    IPv6 in Third Generation Partnership Project (3GPP) General Packet
    Radio Service (GPRS) packet networks. The authors can be found networks, in
    Authors and Acknowledgements section. which the radio network
    architecture is based on Global System for Mobile Communications
    (GSM), or Universal Mobile Telecommunications System (UMTS) /
    Wideband Code Division Multiple Access (WCDMA) technology.

    This document analyzes the transition scenarios in 3GPP packet
    data networks that might may come up in
    the deployment phase of IPv6. IPv6 in 3GPP packet  data networks.

    The 3GPP network architecture is described in [RFC3314], and
    relevant transition scenarios are documented in [RFC3574] and [RFC3574]. The
    reader of this
    document will further analyze them. specification should be familiar with the material
    presented in these documents.

    The scenarios analyzed in this document are divided into two
    categories: general-purpose packet service scenarios, referred to
    as GPRS scenarios in this document, and IP Multimedia Subsystem
    (IMS)
    scenarios. scenarios, which include Session Initiation Protocol (SIP)
    considerations.

    GPRS scenarios are the following:
       - Dual Stack UE connecting to IPv4 and IPv6 nodes
       - IPv6 UE connecting to an IPv6 node through an IPv4 network
       - IPv4 UE connecting to an IPv4 node through an IPv6 network
       - IPv6 UE connecting to an IPv4 node
       - IPv4 UE connecting to an IPv6 node

    IMS scenarios are the following:
       - UE connecting to a node in an IPv4 network through IMS
       - Two IPv6 IMS islands connected via an IPv4 network

    The focus is on analyzing different transition scenarios,
    applicable transition mechanisms and finding solutions for those
    transition scenarios. In the scenarios, the User Equipment (UE)
    connects to nodes in other networks, e.g. in the Internet and
    IPv6/IPv4 transition mechanisms are needed.

 1.1 Scope of this Document

    The scope of this Best Current Practices document is to analyze and
    solve the possible transition
    scenarios in the 3GPP defined GPRS network where a UE connects to,
    or is contacted from, the Internet
    or another UE. node on the Internet. The document
    covers scenarios with and without the use of the SIP based SIP-based IP
    Multimedia Core Network Subsystem (IMS). This document does not
    focus on radio interface interface-specific issues; both 3GPP Second (GSM) and
    Third Generation (UMTS) radio network architectures (GSM, EDGE and
    UMTS/WCDMA) will be covered by this analysis.

    The 3GPP2 architecture is similar to 3GPP in many ways, but differs
    in enough details that this document does not include these scenarios.
    variations in its analysis.

    The transition mechanisms specified by the IETF Ngtrans and v6ops
    Working Groups shall be used. This document memo shall not specify any new
    transition mechanisms, but if a only documents the need for a new mechanism is
    found, that will be reported to the IETF v6ops Working Group. ones (if
    appropriate).

 1.2 Abbreviations

    2G          Second Generation Mobile Telecommunications, for
                 example GSM and GPRS technologies.
    3G          Third Generation Mobile Telecommunications, for example
                 UMTS technology.
    3GPP        Third Generation Partnership Project
    ALG         Application Level Gateway
    APN         Access Point Name. The APN is a logical name referring
                 to a GGSN and an external network.
    CSCF        Call Session Control Function (in 3GPP Release 5 IMS)
    DNS         Domain Name System
    GGSN        Gateway GPRS Support Node (a default (default router for 3GPP
                 User Equipment)
    GPRS        General Packet Radio Service
    GSM         Global System for Mobile Communications
    HLR         Home Location Register
    IMS         IP Multimedia (Core Network) Subsystem, 3GPP Release 5
                 IPv6-only part of the network
    ISP         Internet Service Provider
    NAT         Network Address Translator
    NAPT-PT     Network Address Port Translation - Protocol Translation
    NAT-PT      Network Address Translation - Protocol Translation
    PCO-IE      Protocol Configuration Options Information Element
    PDP         Packet Data Protocol
    PPP         Point-to-Point Protocol
    SGSN        Serving GPRS Support Node
    SIIT        Stateless IP/ICMP Translation Algorithm
    SIP         Session Initiation Protocol
    UE          User Equipment, for example a UMTS mobile handset
    UMTS        Universal Mobile Telecommunications System
    WCDMA       Wideband Code Division Multiple Access

 1.3 Terminology

    Some terms used in 3GPP transition scenarios and analysis documents
    are briefly defined here.

    Dual Stack UE  Dual Stack UE is a 3GPP mobile handset having both
                   IPv4 and IPv6 stacks. It is capable of activating
                   both IPv4 and IPv6 Packet Data Protocol (PDP)
                   contexts. Dual stack UE may be capable of tunneling.

    IPv6 UE        IPv6 UE is an IPv6-only 3GPP mobile handset. It is
                   only capable of activating IPv6 PDP contexts.

    IPv4 UE        IPv4 UE is an IPv4-only 3GPP mobile handset. It is
                   only capable of activating IPv4 PDP contexts.

    IPv4 node      IPv4 node is here defined to be IPv4 capable node
                   the UE is communicating with. The IPv4 node can
                   be, for example, an application server or another
                   UE.

    IPv6 node      IPv6 node is here defined to be IPv6 capable node
                   the UE is communicating with. The IPv6 node can
                   be, for example, an application server or another
                   UE.

    PDP Context    Packet Data Protocol (PDP) Context is a connection
                   between the UE and the GGSN, over which the packets
                   are transferred. There are currently three PDP
                   Types: IPv4, IPv6 and PPP.

 2. Transition Mechanisms and DNS Guidelines

    This chapter briefly introduces some transition mechanisms
    specified by the IETF. these IETF IPv4/IPv6 transition
    mechanisms:

    -  dual IPv4/IPv6 stack [RFC2893-bis]
    -  tunneling [RFC2893-bis]
    -  protocol translators [RFC 2766], [RFC2765]

    In addition to that, addition, DNS recommendations are given. The applicability of
    different transition mechanisms to 3GPP networks is discussed in
    chapters 3 and 4.

    The IPv4/IPv6 transition methods can be divided to:

       - dual IPv4/IPv6 stack
       - tunneling
       - protocol translators

 2.1 Dual Stack

    The dual IPv4/IPv6 stack is specified in [RFC2893]. [RFC2893-bis]. If we
    consider the 3GPP GPRS core network, dual stack implementation in
    the Gateway GPRS Support Node (GGSN) enables support for IPv4 and
    IPv6 PDP contexts. UEs with dual stack and public (global) IP
    addresses can typically access both IPv4 and IPv6 services without
    additional translators in the network. However, it is good to
    remember that private IPv4 addresses and NATs have been used and
    will be used in mobile networks. Public/global IP addresses are
    also needed for peer-to-peer services: the node needs a
    public/global IP address that is visible to other nodes.

 2.2 Tunneling

    Tunneling is a transition mechanism that requires dual IPv4/IPv6
    stack functionality in the encapsulating and decapsulating nodes.
    Basic tunneling alternatives are IPv6-in-IPv4 and IPv4-in-IPv6.

    Tunneling can be static or dynamic. Static (configured) tunnels are
    fixed IPv6 links over IPv4, and they are specified in [RFC2893]. [RFC2893-
    bis]. Dynamic (automatic) tunnels are virtual IPv6 links over IPv4
    where the tunnel endpoints are not configured, i.e. the links are
    created dynamically.

 2.3 Protocol Translators

    A translator can be defined as an intermediate component between a
    native IPv4 node and a native IPv6 node to enable direct
    communication between them without requiring any modifications to
    the end nodes.

    Header conversion is a translation mechanism. In header conversion,
    IPv6 packet headers are converted to IPv4 packet headers, or vice
    versa, and checksums are adjusted or recalculated if necessary.
    NAT-PT (Network Address Translator / Protocol Translator) [RFC2766]
    using SIIT Stateless IP/ICMP Translation [RFC2765] is an example of such
    a mechanism.

    Translators may be needed in some cases when the communicating
    nodes do not share the same IP version; in others, it may be
    possible to avoid such communication altogether. Translation can
    actually happen
    take place at Layer 3 the network layer (using NAT-like techniques), Layer 4 the
    transport layer (using a TCP/UDP proxy) or Layer 7 the application layer
    (using application relays).

 2.4 DNS Guidelines for IPv4/IPv6 Transition

    To avoid the DNS name space from fragmenting into parts where some
    parts of DNS are only visible using IPv4 (or IPv6) transport, the
    recommendation (as of this writing) is to always keep at least one
    authoritative server IPv4-enabled, and to ensure that recursive DNS
    servers support IPv4. See DNS IPv6 transport guidelines [DNStrans]
    for more information.

 3. GPRS Transition Scenarios

    This section discusses the scenarios that might occur when a GPRS
    UE contacts services or other nodes, e.g. a web server in the
    Internet.

    The following scenarios described by [RFC3574] are analyzed here.
    In all of the scenarios, the UE is part of a network where there is
    at least one router of the same IP version, i.e. the GGSN, and the
    UE is connecting to a node in a different network.

    1) Dual Stack UE connecting to IPv4 and IPv6 nodes
    2) IPv6 UE connecting to an IPv6 node through an IPv4 network
    3) IPv4 UE connecting to an IPv4 node through an IPv6 network
    4) IPv6 UE connecting to an IPv4 node
    5) IPv4 UE connecting to an IPv6 node

 3.1 Dual Stack UE Connecting to IPv4 and IPv6 Nodes

    In this scenario, the dual stack UE is capable of communicating
    with both IPv4 and IPv6 nodes.

    It is recommended to activate an IPv6 PDP context when
    communicating with an IPv6 peer node and an IPv4 PDP context when
    communicating with an IPv4 peer node. If the 3GPP network supports
    both IPv4 and IPv6 PDP contexts, the UE activates the appropriate
    PDP context depending on the type of application it has started or
    depending on the address of the peer host it needs to communicate
    with. The authors leave the PDP context activation policy to be
    decided by UE implementers, application developers and operators.
    One discussed possibility is to activate both IPv4 and IPv6 types
    of PDP contexts in advance, because activation of a PDP context
    usually takes some time. However, that probably isn't good usage of
    network resources. Generally speaking, IPv6 PDP contexts should be
    preferred even if that meant IPv6-in-IPv4 tunneling would be needed
    in the network (see section 3.2 for more details). Note that this
    is transparent to the UE.

    However,

    Although the UE is dual-stack, the UE may attach find itself attached to a
    3GPP network, network in which the Serving GPRS Support Node (SGSN), the
    GGSN, and the Home Location Register (HLR) support IPv4 PDP
    contexts, but do not support IPv6 PDP contexts. This may happen in
    early phases of IPv6 deployment. deployment, or because the UE has "roamed"
    from a 3GPP network that supports IPv6 to one that does not. If the
    3GPP network does not support IPv6 PDP contexts, and an application
    on the UE needs to communicate with an IPv6(-only) node, the UE may
    activate an IPv4 PDP context and encapsulate IPv6 packets in IPv4
    packets using a tunneling mechanism.

    The used tunneling mechanism may require public IPv4 addresses, but
    there are tunneling mechanisms and deployment scenarios in which
    the usage of
    private IPv4 addresses is possible. If may be used; for instance, if the tunnel
    endpoints are in the same private domain, or the tunneling
    mechanism works through IPv4 NAT, private IPv4 addresses can be
    used. NAT.

    One deployment scenario example is using uses a laptop computer and a 3GPP UE as a
    modem. IPv6 packets are encapsulated in IPv4 packets in the laptop
    computer and an IPv4 PDP context is activated. The used tunneling
    mechanism in that case depends on the laptop computerís support of tunneling mechanisms in the laptop computer.
    mechanisms. Another deployment scenario is making performing IPv6-in-IPv4
    tunneling in the UE itself and activating an IPv4 PDP context.

    Closer details for an applicable tunneling mechanism are not
    analyzed in this document. However, a simple host-to-router
    (automatic) tunneling mechanism may be a good fit. There is not yet
    consensus on the right approach. Primarily, ISATAP approach, and proposed mechanisms so far
    include [ISATAP] and [STEP]. Especially ISATAP has
    been proposed, but had some issues have been raised about it, such as
    its unnecessary features and relative complexity for a simple task
    like this, and its inadequacy support
    in providing security when crossing
    administrative domains. Proposed solution alternatives have been
    (at least) a simplified, but probably non-interoperable, version of
    ISATAP, and STEP [STEP]. In any case, the wg. However, further work is needed to find out the
    requirements for the scenario and to specify the mechanism.

    To generally solve this problem (IPv6 not available in the 3GPP
    network), this

    This document strongly recommends the 3GPP operators to deploy
    basic IPv6 support in their GPRS networks. That also makes it possible
    to burden lessen the transition effects in the network and
    make the 3GPP UEs simpler. UEs.

    As a general guideline, IPv6 communication is preferred to IPv4
    communication going through IPv4 NATs to the same dual stack peer
    node.

    Public IPv4 addresses are often a scarce resource for the operator
    and typically usually it is not possible for a UE to have a public IPv4
    address (continuously) allocated for its use. Use of private IPv4
    addresses means use of NATs when communicating with a peer node
    outside the operator's network. In large networks, NAT systems can
    become very complex, expensive and difficult to maintain.

    For DNS recommendations, we refer to section 2.4.

 3.2 IPv6 UE Connecting to an IPv6 Node through an IPv4 Network

    The best solution for this scenario is obtained with tunneling,
    i.e. IPv6-in-IPv4 tunneling is a requirement. An IPv6 PDP context
    is activated between the UE and the GGSN. Tunneling is handled in
    the network, because IPv6 UE is not capable of tunneling (it does not have the dual stack
    functionality needed for tunneling). tunneling. The encapsulating node can be
    the GGSN, the edge router between the border of the operator's IPv6
    network and the public Internet, or any other dual stack node
    within the operator's IP network. The encapsulation (uplink) and
    decapsulation (downlink) can be handled by the same network
    element. Typically the tunneling handled by the network elements is
    transparent to the UEs and IP traffic looks like native IPv6
    traffic to them. For the applications, applications and transport protocols,
    tunneling enables end-to-end IPv6 connectivity.

    IPv6-in-IPv4 tunnels between IPv6 islands can be either static or
    dynamic. The selection of the type of tunneling mechanism is up to a
    policy decision for the operator / ISP deployment scenario and only
    generic recommendations can be given in this document.

    The following subsections are focused on the usage of different
    tunneling mechanisms when the peer node is in the operator's
    network or outside the operator's network. The authors note that
    where the actual 3GPP network ends and which parts of the network
    belong to the ISP(s) also depends on the deployment scenario. The
    authors are not commenting how many ISP functions the 3GPP operator
    should perform. However, many 3GPP operators are ISPs of some sort
    themselves. ISP networks' transition to IPv6 is analyzed in [ISP-
    sa].

 3.2.1 Tunneling inside the 3GPP Operator's Network

    GPRS operators today have typically deployed IPv4 backbone
    networks. IPv6 backbones can be considered quite rare in the first
    phases of the transition.

    In initial IPv6 deployment, where a small number of IPv6-in-IPv4
    tunnels are required to connect the IPv6 islands over the 3GPP
    operator's IPv4 network, manually configured tunnels can be used.
    In a 3GPP network, one IPv6 island can contain the GGSN while
    another island can contain the operator's IPv6 application servers.
    However, manually configured tunnels can be an administrative
    burden when the number of islands and therefore tunnels rises. In
    that case, upgrading parts of the backbone to dual stack may be the
    simplest choice. The administrative burden could also be mitigated
    by using automated management tools.

    Connection redundancy should also be noted as an important
    requirement in 3GPP networks. Static tunnels on their own alone don't provide a
    routing recovery solution for all scenarios where an IPv6 route
    goes down. However, they can provide an adequate solution depending
    on the design of the network and in the presence of other router
    redundancy mechanisms, such as the use of IPv6 routing protocols.

 3.2.2 Tunneling outside the 3GPP Operator's Network

    This subsection includes the case in which the peer node is outside
    the operator's network. In that case, IPv6-in-IPv4 tunneling can be
    necessary to obtain IPv6 connectivity and reach other IPv6 nodes.
    In general, configured tunneling can be recommended.

    Tunnel starting point can be in the operator's network depending on
    how far the 3GPP operator has come in implementing IPv6. If the
    3GPP operator has not deployed IPv6 in its backbone, the
    encapsulating node can be the GGSN. If the 3GPP operator has
    deployed IPv6 in its backbone, backbone but the upstream ISP does not provide
    IPv6 connectivity to the Internet, connectivity, the encapsulating node
    can could be the edge 3GPP
    operator's border router.

    The case is pretty straightforward if the upstream ISP provides
    IPv6 connectivity to the Internet and the operator's backbone
    network supports IPv6. Then the 3GPP operator does not have to
    configure any tunnels, since the upstream ISP will take care of
    routing IPv6 packets. If the upstream ISP does not provide IPv6
    connectivity, an IPv6-in-IPv4 tunnel should be configured e.g. from
    the edge border router to a dual stack border gateway operated by
    another ISP which is offering IPv6 connectivity.

 3.3 IPv4 UE Connecting to an IPv4 Node through an IPv6 Network

    3GPP networks are expected to support both IPv4 and IPv6 for a long
    time, on the UE-GGSN link and between the GGSN and external
    networks. For this scenario, it is useful to split the end-to-end
    IPv4 UE to IPv4 node communication into UE-to-GGSN and GGSN-to-
    v4NODE. Therefore This allows an IPv4-only UE will be able to use an IPv4 link
    (PDP (an IPv4
    PDP context) to connect to the GGSN without the need to
    communicate communicating over an
    IPv6 network.

    Regarding the GGSN-to-v4NODE communication, typically the transport
    network between the GGSN and external networks will support only
    IPv4 in the early stages and migrate to dual stack, since these
    networks are already deployed. Therefore it is not envisaged that
    tunneling of IPv4-in-IPv6 will be required from the GGSN to
    external IPv4 networks either. In the longer run, 3GPP operators
    may need choose to phase out IPv4 UEs and the IPv4 transport network.
    This would leave only IPv6 UEs.

    Therefore, overall, the transition scenario involving an IPv4 UE
    communicating with an IPv4 peer through an IPv6 network is not
    considered very likely in 3GPP networks.

 3.4 IPv6 UE Connecting to an IPv4 Node

    Generally speaking, IPv6-only UEs may be easier to manage, but that
    would require all services to be used over IPv6, and that may not
    be the universal
    deployment of IPv6 probably isnít realistic in the near future.
    Dual stack implementation requires management of both IPv4 and IPv6
    networks and one approach is that "legacy" applications keep using
    IPv4 for the foreseeable future and new applications requiring end-to-end end-
    to-end connectivity (for example, peer-to-peer services) use IPv6.
    As a general guideline, IPv6-only UEs are not recommended in the
    early phases of transition until the IPv6 deployment has become so
    prevalent that direct communication with IPv4(-only) nodes will no longer be necessary.
    the exception, and not the rule. It is assumed that IPv4 will
    remain useful for quite a long time, so in general, dual-stack
    implementation in the UE can be recommended. This recommendation
    naturally includes manufacturing dual-stack UEs instead of IPv4-only IPv4-
    only UEs.

    However, if there is a need to connect to an IPv4(-only) node from
    an IPv6-only UE, it is possible recommended to use specific translation and
    proxying techniques; generic IP protocol translation is not
    recommended. There are three main ways for IPv6(-only) nodes to
    communicate with IPv4(-only) nodes (excluding avoiding such
    communication in the first place):

       1. the use of generic-purpose translator (e.g. NAT-PT [RFC2766])
         in the local network (not recommended as a general solution),

       2. the use of specific-purpose protocol relays (e.g., IPv6<->IPv4
         TCP relay configured for a couple of ports only [RFC3142]) or
         application proxies (e.g., HTTP proxy, SMTP relay) in the
         local network, or

       3. the use of specific-purpose mechanisms (as described above in
         2) in the foreign network; these are indistinguishable from
         the IPv6-enabled services from the IPv6 UE's perspective, and
         not discussed further here.

    For many applications, application proxies can be appropriate (e.g.
    HTTP proxies, SMTP relays, etc.). Such application proxies will not
    be transparent to the UE. Hence, a flexible mechanism with minimal
    manual intervention should be used to configure these proxies on
    IPv6 UEs. Application proxies can be placed, for example, on the
    GGSN external interface (Gi), ("Gi"), or inside the service network.

    The authors note that [NATPTappl] discusses the applicability of
    NAT-PT. The problems related to NAT-PT usage in 3GPP networks are
    documented in appendix A.

 3.5 IPv4 UE Connecting to an IPv6 Node

    The legacy IPv4 nodes are mostly typically nodes that support the
    applications that are popular today in the IPv4 Internet: mostly e-
    mail and web-browsing. These applications will, of course, be
    supported in the future IPv6 Internet. However, the legacy IPv4 UEs
    are not going to be updated to support the future applications. As
    these applications are designed for IPv6, and to use the advantages
    of newer platforms, the legacy IPv4 nodes will not be able to
    profit from take
    advantage of them. Thus, they will continue to support the legacy
    services.

    Taking the above into account, the traffic to and from the legacy
    IPv4 UE is restricted to a few applications. These applications
    already mostly rely on proxies or local servers to communicate
    between private address space networks and the Internet. The same
    methods and technology can be used for IPv4 to IPv6 transition.

    For DNS recommendations, we refer to section 2.4.

 4. IMS Transition Scenarios

    As the IMS is exclusively IPv6, the number of possible transition
    scenarios is reduced dramatically. The possible IMS scenarios are
    listed below and analyzed in sections 4.1 and 4.2.

       1) UE connecting to a node in an IPv4 network through IMS
       2) Two IPv6 IMS islands connected over via an IPv4 network

    For DNS recommendations, we refer to section 2.4. As DNS traffic is
    not directly related to the IMS functionality, the recommendations
    are not in contradiction with the IPv6-only nature of the IMS.

 4.1 UE Connecting to a Node in an IPv4 Network through IMS

    This scenario occurs when an IMS UE (IPv6) connects to a node in
    the IPv4 Internet through the IMS, or vice versa. This happens when
    the other node is a part of a different system than 3GPP, e.g. a
    fixed PC, with only IPv4 capabilities.

    The first priority is to

    Over time, users will upgrade the legacy IPv4 nodes to dual-
    stack, dual-stack,
    often by replacing the entire node, eliminating this particular
    problem in that specific deployment.

    Still, it is difficult to estimate how many non-upgradeable legacy
    IPv4 nodes need to communicate with the IMS UEs. It is assumed that
    the solution described here is used for limited cases, in which
    communications with a small number of legacy IPv4 SIP equipment are
    needed.

    As the IMS is exclusively IPv6 [3GPP 23.221], for many of the
    applications in the IMS, some kind of translators have may need to
    be used in the communication between the IPv6 IMS and the legacy
    IPv4
    hosts, i.e. making a dual stack based solution is not feasible. hosts in cases where these legacy IPv4 hosts cannot be
    upgraded to support IPv6.

    This section aims to give gives a brief overview on how that interworking
    can be handled.

    This section presents higher level details analysis of a solution based on the use of a translator and SIP ALG. [3GPPtr] provides additional
    information IMS interworking issues,
    and presents a bit different solution proposal based on high level view of SIP Edge Proxy and IP Address/Port Mapper. within the IMS. The authors
    recommend to
    solve that a detailed solution for the general SIP/SDP SIP/SDP/media
    IPv4/IPv6 transition problem in will be specified as soon as possible
    as a task within the IETF SIP wg(s). WGs in the IETF.

    As control (or signaling) and user (or data) traffic are separated
    in SIP, SIP calls, and thus, the IMS, the translation transition of the IMS traffic has from
    IPv6 to IPv4 must be done handled at two levels:
              1)Session

       1. Session Initiation Protocol (SIP) [RFC3261], and Session
         Description Protocol (SDP) [RFC2327] [RFC3266] (Mm-interface)
              2)the
       2. the user data traffic (Mb-interface)

    SIP and SDP transition has to be made in carries an SIP/SDP Application
    Level Gateway. The ALG has to SDP body containing the addressing and other
    parameters for establishing the user data traffic (the media).

    Figure 1 shows a signaling edge for SIP and SDP, a dual stack SIP
    proxy at the border between the 3GPP IPv6-only IMS and the IPv4
    systems.

    In a possible approach, this edge could contain a SIP ALG, which
    would change the IP addresses transported in the SIP messages and
    the SDP payload of those messages to the appropriate  version. In addition, there has to This
    approach would have the drawback (like other SDP rewriting
    solutions) of impacting authentication mechanisms that may be interoperability
    for DNS queries; see section 2.4
    needed for details.

    On the user data transport level, the translation is IPv4-IPv6
    protocol translation, where the user data traffic transported is
    translated from IPv6 other purposes. Moreover, this approach would not take
    advantage of SIP's ability to IPv4, and vice versa.

    The legacy IPv4 host's address can be mapped use proxy routing, nor of SDP's
    ability to an IPv6 address for
    the IMS, carry multiple alternative addresses. These intrinsic
    features of SIP and this address is then used within the IMS to route SDP require a more detailed analysis, but they
    could yield benefits. The SIP ALG approach requires NAT-PT (with
    the
    traffic to issues described in Appendix A), because the appropriate user traffic translator. This mapping
    can IMS-side IPv6
    addresses must be done by the SIP/SDP ALG assigned IPv4 addresses for reachability from the SIP traffic.
    legacy IPv4 side shown in Figure 1. The user
    traffic translator approach based on intrinsic
    SIP proxy routing would do the similar mapping for the user
    traffic. However, in order not require assignment of temporary IPv4
    addresses to have the IPv6 IMS endpoints; instead they would be reached
    via an IPv4 IPv4-side address of a SIP proxy acting for the IMS UE,
    and to them. This SIP
    proxy would be able to route doing normal SIP processing.

    On the user traffic within data transport level, the legacy IPv4
    network to analysis raises other issues:
    the correct translator, there has IMS data is time-sensitive, so NAT-PT IPv6-IPv4 protocol
    translation (with the scalability concerns raised in Appendix A)
    may look simplest, but needs a skeptical look. Alternatives include
    routing to be a transcoder, whose task is to terminate an IPv6 stream
    and start an IPv4 address
    allocated for the duration of the session from the user traffic
    translator. The allocation of stream.  Again, this address from requires a more detailed
    analysis.

    For each of the user traffic
    translator protocols, there has to be done by the SIP/SDP ALG in order interoperability for the
    SIP/SDP ALG to know the correct IPv4 address. This can be achieved
    by using a protocol DNS
    queries; see section 2.4 for the ALG to do the allocation. details.

          +-------------------------------+ +------------+
          |                      +------+ | | +--------+ |
          |                      |S-CSCF|---| |SIP ALG | edge| |\
       |  |                      +------+ | | +--------+ | \ --------
     +-|+ |                       /       | |     |      |  |        |
     |  | | +------+        +------+      | |     +      |   -|    |-
     |  |-|-|P-CSCF|--------|I-CSCF|      | |     |      |    | () |
     |  |   +------+        +------+      | |+----------+| /  ------
     |  |-----------------------------------||Translator||/  |-----------------------------------||  [ALG?]  ||/
     +--+ |            IPv6               | |+----------+|     IPv4
      UE  |                               | |Interworking|
          |  IP Multimedia CN Subsystem   | |Unit        |
          +-------------------------------+ +------------+

               Figure 1: UE using IMS to contact a legacy phone

    Figure 1 shows a possible configuration scenario where the SIP ALG
    is separated from the CSCFs. The translator can either be set up in
    a single device with both generic SIP translation and media translation, or
    those functionalities can be divided to two different entities with
    an interface in between. We call the combined network element on
    the signaling edge - an ALG-like
    replacement of the IPv6-only IMS an "Interworking Unit" in this
    document. A SIP-specific translation mechanism, which could e.g.
    re-use IPv6 addresses with IPv4 addresses using limited
    subsets of NAT-PT [RFC2766], needs [RFC2766]. This is a possible approach, but
    exploiting SIP's proxy routing to allow the dual homed SIP edge to
    make the address change without a translator could be specified.
    The a promising
    alternative without the scaling problems related to NAT-PT are discussed in appendix A. of NAT-PT.

 4.2 Two IPv6 IMS Islands Connected over via an IPv4 Network

    At the early stages of IMS deployment, there may be cases where two
    IMS islands are separated by an IPv4 network such as the legacy
    Internet. Here both the UEs and the IMS islands are IPv6-only.
    However, the IPv6 islands are not connected natively with IPv6.

    In this scenario, the end-to-end SIP connections are based on IPv6.
    The only issue is to make connection between two IPv6-only IMS
    islands over IPv4 network. This scenario is closely related to GPRS
    scenario represented in section 3.2. and similar tunneling
    solutions are applicable also in this scenario.

 5. About 3GPP UE IPv4/IPv6 Configuration

    This informative section aims to give a brief overview on the
    configuration needed in the UE in order to access IP based
    services. There can also be other application specific settings in
    the UE that are not described here.

    To be able described here.

    UE configuration is required in order to access IPv6 or IPv4 based services, settings need to
    be done in the UE.
    services. The GGSN Access Point has to be defined when using, for
    example, the web browsing application. One possibility is to use
    over the air configuration [OMA-CP] to configure the GPRS settings.
    The user can can, for example, visit the operator WWW page and
    subscribe the GPRS Access Point settings to his/her UE and receive
    the settings via Short Message Service (SMS). After the user has
    accepted the settings and a PDP context has been activated, the
    user he/she
    can start browsing. The Access Point settings can also be typed in
    manually or be pre-configured by the operator or the UE
    manufacturer.

    DNS server addresses typically also need to be configured in the
    UE. In the case of IPv4 type PDP context, the (IPv4) DNS server
    addresses can be received in the PDP context activation (a control
    plane mechanism). Same kind of A similar mechanism is also available for IPv6:
    so-called Protocol Configuration Options Information Element
    (PCO-IE) (PCO-
    IE) specified by the 3GPP [3GPP-24.008]. It is also possible to use [DHCPv6-SL] or [RFC3315]
    [RFC3736] (or [RFC3315]) and [RFC3646] for receiving DNS server
    addresses. Active IETF work on DNS discovery mechanisms is ongoing
    and might result in other mechanisms becoming available over time.
    The DNS server addresses can also be received over the air (using SMS),
    SMS) [OMA-CP], or typed in manually in the UE.

    When accessing IMS services, the UE needs to know the P-CSCF Proxy-Call
    Session Control Function (P-CSCF) IPv6 address. 3GPP-specific Either a 3GPP-
    specific PCO-IE mechanism, mechanism or a DHCPv6-based mechanism
    ([DHCPv6-SL] ([RFC3736]
    and [RFC3319]) can be used. Manual configuration or [RFC3315] configuration
    over the air is also possible. IMS subscriber authentication and
    registration to the IMS and SIP integrity protection are not
    discussed here.

 6. Summary and Recommendations

    This document has analyzed five GPRS and two IMS IPv6 transition
    scenarios. Numerous 3GPP networks are using private IPv4 addresses
    today, and introducing IPv6 is an important thing. The two first
    GPRS scenarios and both IMS scenarios are seen the most relevant.
    The authors summarize some main recommendations here:
       - Dual-stack UEs are recommended instead of IPv4-only or IPv6-
         only UEs. It is important to take care that the applications
         in the UEs support IPv6. IPv6-only UEs can become feasible
         when IPv6 is widely deployed in the networks, and most
         services work on IPv6.
       - It is recommended to activate an IPv6 PDP context when
         communicating with an IPv6 peer node and an IPv4 PDP context
         when communicating with an IPv4 peer node.
       - IPv6 communication is preferred to IPv4 communication going
         through IPv4 NATs to the same dual stack peer node.
       - This document strongly recommends the 3GPP operators to deploy
         basic IPv6 support in their GPRS networks as soon as possible.
         That makes it possible to lessen the transition effects in the
         UEs.
       - A tunneling mechanism in the UE may be needed during the early
         phases of the IPv6 transition process. A lightweight,
         automatic tunneling mechanism should be standardized in the
         IETF.
       - Tunneling mechanisms can be used in 3GPP networks, and [RFC3319]) only
         generic recommendations are given in this document. More
         details can be used. Manual
    configuration or configuration over found, for example, in [ISP-sa].
       - We recommend that a detailed solution for the air is also possible. IMS
    subscriber authentication and registration to general
         SIP/SDP/media IPv4/IPv6 transition problem will be specified
         as soon as possible as a task within the IMS and SIP
    integrity protection are not discussed here.

 6. WGs in the IETF.

 7. Security Considerations

    There are

    Deploying IPv6 has some generic security considerations when moving to dual-
    stack IPv4/IPv6 deployment which are not analyzed at length here.
    Two examples one should
    be aware of [V6SEC]; however, these are ensuring that the access controls and
    firewalls have similar (or known) security properties with both
    IPv4 and IPv6, and that enabling IPv6 does not jeopardize the
    access specific to 3GPP
    transition, and are therefore out of the IPv4 services (e.g., in the form scope of misbehavior
    towards DNS AAAA record lookups or operationally worse quality IP
    transit services). this memo.

    This memo recommends the use of a relatively small number of
    techniques, which all of them have their
    techniques. Each technique has its own security considerations,
    including:

       - native upstream access or tunneling by the 3GPP network
          operator,
       - use of routing protocols to ensure redundancy,
       - use of locally-deployed specific-purpose protocol relays and
          application proxies to reach IPv4(-only) nodes from IPv6-only
          UEs, or
       - a specific mechanism for SIP signalling and media translation

    These (except for the last one, naturally) have relatively well-
    known security considerations, which

    The threats of configured tunneling are also discussed described in [RFC2893-bis].
    Attacks against routing protocols are described in the
    specific documents.  However, respective
    documents and in particular one should note that a
    proper configuration of locally-deployed general in [ROUTESEC].  Threats related to
    protocol relays and proxies is very
    important, so that the outsiders will not have access to them, been described in [RFC3142].  The security
    properties of SIP internetworking are to be used for abuse, laundering attacks, or circumventing access
    controls. specified when the
    mechanism is specified.

    In particular, this memo does not recommend the following technique
    which has security issues, not further analyzed here:

       - NAT-PT or other translator as a generic-purpose general-purpose transition
          mechanism
 7.

 8. References

 7.1

 8.1 Normative

    [RFC2026] Bradner, S.: The Internet Standards Process -- Revision
    3, RFC 2026, October 1996.

    [RFC2663] Srisuresh, P., Holdrege, M.: IP Network Address
    Translator (NAT) Terminology and Considerations, RFC 2663, August 1999.

    [RFC2765] Nordmark, E.: Stateless IP/ICMP Translation Algorithm
    (SIIT), RFC 2765, February 2000.

    [RFC2766] Tsirtsis, G., Srisuresh, P.: Network Address Translation
    - Protocol Translation (NAT-PT), RFC 2766, February 2000.

    [RFC2893] Gilligan, R., Nordmark, E.: Transition Mechanisms for
    IPv6 Hosts and Routers, RFC 2893, August 2000.

    [RFC3261] Rosenberg, J., et al.: SIP: Session Initiation Protocol,
    RFC 3261,
    June 2002.

    [RFC3574] Soininen, J. (editor): Transition Scenarios for 3GPP
    Networks, RFC 3574, August 2003.

    [RFC3667] Bradner, S.: IETF Rights in Contributions, February 2004.

    [RFC3668] Bradner, S.: Intellectual Property Rights in IETF
    Technology, February 2004.

    [RFC2893-bis] Nordmark, E. and Gilligan, R. E.: "Basic Transition
    Mechanisms for IPv6 Hosts and Routers", January 2004, draft-ietf-
    v6ops-mech-v2-02.txt, work in progress.

    [3GPP-23.060] 3GPP TS 23.060 V5.4.0, "General Packet Radio Service
    (GPRS); Service description; Stage 2 (Release 5)", December 2002.

    [3GPP 23.221] 3GPP TS 23.221 V5.7.0, "Architectural requirements
    (Release 5)", December 2002.

    [3GPP-23.228] 3GPP TS 23.228 V5.7.0, "IP Multimedia Subsystem
    (IMS); Stage 2 (Release 5)", December 2002.

    [3GPP 24.228] 3GPP TS 24.228 V5.3.0, "Signalling flows for the IP
    multimedia call control based on SIP and SDP; Stage 3 (Release 5)",
    December 2002.

    [3GPP 24.229] 3GPP TS 24.229 V5.3.0, "IP Multimedia Call Control
    Protocol based on SIP and SDP; Stage 3 (Release 5)", December 2002.

 7.2

 8.2 Informative

    [RFC2327] Handley, M., Jacobson, V.: SDP: Session Description
    Protocol, RFC 2327, April 1998.

    [RFC3142] Hagino, J., Yamamoto, K.: An IPv6-to-IPv4 Transport Relay
    Translator, RFC 3142, June 2001.

    [RFC3266] Olson, S., Camarillo, G., Roach, A. B.: Support for IPv6
    in Session Description Protocol (SDP), June 2002.

    [RFC3314] Wasserman, M. (editor): Recommendations for IPv6 in 3GPP
    Standards, September 2002.

    [RFC3315] Droms, R. et al.: Dynamic Host Configuration Protocol for
    IPv6 (DHCPv6), July 2003.

    [RFC3319] Schulzrinne, H., Volz, B.: Dynamic Host Configuration
    Protocol (DHCPv6) Options for Session Initiation Protocol (SIP)
    Servers, July 2003.

    [RFC3646] Droms, R. (ed.): DNS Configuration options for DHCPv6,
    December 2003.

    [3GPPtr] El Malki K., et al.: "IPv6-IPv4 Translation mechanism for
    SIP-based services in Third Generation Partnership Project (3GPP)
    Networks", December 2003, draft-elmalki-sipping-3gpp-translator-
    00.txt, work in progress.

    [DHCP-SL]

    [RFC3736] Droms, R.: "Stateless DHCP Stateless Dynamic Host Configuration Protocol
    (DHCP) Service for IPv6", January
    2004, draft-ietf-dhc-dhcpv6-stateless-04.txt, work in progress. IPv6, April 2004.

    [DNStrans] Durand, A. and Ihren, J.: "DNS IPv6 transport
    operational guidelines", November 2003, March 2004, draft-ietf-dnsop-ipv6-
    transport-guidelines-01.txt,
    transport-guidelines-02.txt, work in progress.

    [ISATAP] Templin, F., Gleeson, T., Talwar, M. and Thaler, D.:
    "Intra-Site Automatic Tunnel Addressing Protocol (ISATAP)",
    February April
    2004, draft-ietf-ngtrans-isatap-20.txt, draft-ietf-ngtrans-isatap-21.txt, work in progress.

    [ISP-sa] Lind, M., Ksinant, V., Park, D., D. and Baudot, A.: "Scenarios
    and Analysis for Introducing IPv6 into ISP Networks", February April 2004, draft-ietf-v6ops-isp-scenarios-analysis-01.txt,
    draft-ietf-v6ops-isp-scenarios-analysis-02.txt, work in progress.

    [NATPTappl] Satapati, S., Sivakumar, S., Barany, P., Okazaki, S., S.
    and Wang, H.: "NAT-PT Applicability", October 2003, draft-satapati-
    v6ops-natpt-applicability-00.txt, work in progress.

    [NATPT-DNS] Durand, A.: "Issues with NAT-PT DNS ALG in RFC2766",
    January 2003, draft-durand-v6ops-natpt-dns-alg-issues-01.txt,

    [ROUTESEC] Barbir, A., Murphy, S. and Yang, Y.: "Generic Threats to
    Routing Protocols", April 2004, draft-ietf-rpsec-routing-threats-
    06.txt, work in progress, the draft has expired. progress.

    [STEP] Savola, P.: "Simple IPv6-in-IPv4 Tunnel Establishment
    Procedure (STEP)", January 2004, draft-savola-v6ops-conftun-setup-
    02.txt, work in progress.

    [V6SEC] Savola, P.: "IPv6 Transition/Co-existence Security
    Considerations", February 2004, draft-savola-v6ops-security-
    overview-02.txt, work in progress.

    [3GPP-24.008] 3GPP TS 24.008 V5.8.0, "Mobile radio interface Layer
    3 specification; Core network protocols; Stage 3 (Release 5)", June
    2003.

 8.

    [OMA-CP] OMA Client Provisioning: Provisioning Architecture
    Overview Version 1.1, OMA-WAP-ProvArch-v1_1-20021112-C, Open Mobile
    Alliance, 12-Nov-2002.

 9. Contributors

    Pekka Savola has contributed both text and his IPv6 experience to
    this document. He has provided a large number of helpful comments
    on the v6ops mailing list.

 9. Allison Mankin has contributed text for
    IMS Scenario 1 (section 4.1).

 10. Authors and Acknowledgements

    This document is written by:

       Alain Durand, Sun Microsystems
       <Alain.Durand@sun.com>

       Karim El-Malki, Ericsson Radio Systems
       <Karim.El-Malki@era.ericsson.se>

       Niall Richard Murphy, Enigma Consulting Limited
       <niallm@enigma.ie>

       Hugh Shieh, AT&T Wireless
       <hugh.shieh@attws.com>

       Jonne Soininen, Nokia
       <jonne.soininen@nokia.com>

       Hesham Soliman, Flarion
       <h.soliman@flarion.com>

       Margaret Wasserman, ThingMagic
       <margaret@thingmagic.com>

       Juha Wiljakka, Nokia
       <juha.wiljakka@nokia.com>

    The authors would like to give special thanks to Spencer Dawkins
    for proofreading.

    The authors would like to thank Heikki Almay, Gabor Bajko, Ajay
    Jain, Jarkko Jouppi, David Kessens, Ivan Laloux, Allison Mankin,
    Jasminko Mulahusic, Janne Rinne, Andreas Schmid, Pedro Serna, Fred
    Templin, Anand Thakur and Rod Van Meter for their valuable input.

 10.

 11. Editor's Contact Information

    Comments or questions regarding this document should be sent to the
    v6ops mailing list or directly to the document editor:

    Juha Wiljakka
    Nokia
    Visiokatu 3                    Phone:  +358 7180 48372
    FIN-33720 TAMPERE, Finland     Email:  juha.wiljakka@nokia.com

 11.

 12. Intellectual Property Statement

    The IETF takes no position regarding the validity or scope of any
    intellectual property
    Intellectual Property Rights or other rights that might be claimed
    to pertain to the implementation or use of the technology described
    in this document or the extent to which any license under such
    rights might or might not be available; neither nor does it represent that
    it has made any independent effort to identify any such rights.
    Information on the
    IETF's procedures with respect to rights in standards-track and
    standards-related documentation RFC
    documents can be found in BCP-11. BCP 78 and BCP 79.

    Copies of
    claims of rights IPR disclosures made available for publication to the IETF Secretariat and any
    assurances of licenses to be made available, or the result of an
    attempt made to obtain a general license or permission for the use
    of such proprietary rights by implementers or users of this
    specification can be obtained from the IETF Secretariat. on-line IPR repository
    at http://www.ietf.org/ipr.

    The IETF invites any interested party to bring to its attention any
    copyrights, patents or patent applications, or other proprietary
    rights which that may cover technology that may be required to practice implement
    this standard.  Please address the information to the IETF Executive
    Director.

 12. at ietf-
    ipr@ietf.org.

 13. Copyright

    The following copyright notice is copied from [RFC2026], [RFC3667], Section
    10.4.
    5.4. It describes the applicable copyright for this document.

    Copyright (C) The Internet Society March 24, 2004. All Rights
    Reserved. (2004). This document and translations of it may be copied and furnished is subject
    to
    others, and derivative works that comment on or otherwise explain
    it or assist in its implementation may be prepared, copied,
    published and distributed, in whole or in part, without restriction
    of any kind, provided that the above copyright notice and this
    paragraph are included on all such copies rights, licenses and derivative works.
    However, this document itself may not be modified restrictions contained in any way, such
    as by removing the copyright notice or references to the Internet
    Society or other Internet organizations, BCP 78, and
    except as needed for the
    purpose of developing Internet standards in which case the
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    other than English.

    The limited permissions granted above are perpetual and will not be
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    This document and the information contained herein is are provided on
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    REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND
    THE INTERNET ENGINEERING TASK FORCE DISCLAIMS DISCLAIM ALL WARRANTIES,
    EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT
    THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR
    ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A
    PARTICULAR PURPOSE.

 Appendix A - On the Use of Generic Translators in the 3GPP Networks

    This appendix lists mainly 3GPP-specific arguments about generic
    translators, even though the use of generic translators is
    discouraged.  The section may be removed in future versions of the
    memo.

    Due to the significant lack of IPv4 addresses in some domains, port
    multiplexing is likely to be a necessary feature for translators
    (i.e. NAPT-PT). If NA(P)T-PT NAPT-PT is used, it needs to be placed on the
    GGSN external (Gi) interface, typically separate from the GGSN.
    NA(P)T-PT
    NAPT-PT can be installed, for example, on the edge of the
    operator's network and the public Internet. NA(P)T-PT NAPT-PT will intercept
    DNS requests and other applications that include IP addresses in
    their payloads, translate the IP header (and payload for some
    applications if necessary) and forward packets through its IPv4
    interface.

    NA(P)T-PT

    NAPT-PT introduces limitations that are expected to be magnified
    within the 3GPP architecture. Some of these limitations are listed
    below (notice that most of them are also relevant for IPv4 NAT).
    [NATPTappl] discusses the applicability of NAT-PT in more detail.

       1. NA(P)T-PT NAPT-PT is a single point of failure for all ongoing
         connections.

       2. There are additional forwarding delays due to further
         processing, when compared to normal IP forwarding.

       3. There are problems with source address selection due to the
         inclusion of a DNS ALG on the same node [NATPT-DNS].

       4. NA(P)T-PT NAPT-PT does not work (without application level gateways) for
         applications that embed  IP addresses in their payload.

       5. NA(P)T-PT NAPT-PT breaks DNSSEC.

       6. NA(P)T-PT NAPT-PT does not scale very well in large networks.

    3GPP networks are expected to handle a very large number of
    subscribers on a single GGSN (default router). Each GGSN is
    expected to handle hundreds of thousands of connections.
    Furthermore, high reliability is expected for 3GPP networks.
    Consequently, a single point of failure on the GGSN external
    interface would raise concerns on the overall network reliability.

    In addition, IPv6 users are expected to use delay-sensitive
    applications provided by IMS. Hence, there is a need to minimize
    forwarding delays within the IP backbone. Furthermore, due to the
    unprecedented number of connections handled by the default routers
    (GGSN) in 3GPP networks, a network design that forces traffic to go
    through a single node at the edge of the network (typical NA(P)T-PT NAPT-PT
    configuration) is not likely to scale. Translation mechanisms
    should allow for multiple translators, for load sharing and
    redundancy purposes.

    To minimize the problems associated with NA(P)T-PT, NAPT-PT, the following
    actions can be recommended:

      1. Separate the DNS ALG from the NA(P)T-PT NAPT-PT node (in the "IPv6 to
         IPv4" case).

      2. Ensure (if possible) that NA(P)T-PT NAPT-PT does not become a single
         point of failure.

      3. Allow for load sharing between different translators. That is,
         it should be possible for different connections to go through
         different translators. Note that load sharing alone does not
         prevent NA(P)T-PT NAPT-PT from becoming a single point of failure.