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 Internet Draft                                       J. Wiljakka (ed.)
 Document: draft-ietf-v6ops-3gpp-analysis-09.txt                  Nokia
 Expires: September 2004
 
                                                             March 2004
 
                Analysis on IPv6 Transition in 3GPP Networks
 
 Status of this Memo
 
    This document is an Internet-Draft and is in full conformance with
    all provisions of Section 10 of RFC2026.
 
    Internet-Drafts are working documents of the Internet Engineering
    Task Force (IETF), its areas, and its working groups.  Note that
    other groups may also distribute working documents as Internet-
    Drafts.
 
    Internet-Drafts are draft documents valid for a maximum of six
    months and may be updated, replaced, or obsoleted by other
    documents at any time. It is inappropriate to use Internet-Drafts
    as reference material or to cite them other than as "work in
    progress."
 
    The list of current Internet-Drafts can be accessed at
         http://www.ietf.org/ietf/1id-abstracts.txt
    The list of Internet-Draft Shadow Directories can be accessed at
         http://www.ietf.org/shadow.html.
 
 Abstract
 
    This document analyzes the transition to IPv6 in Third Generation
    Partnership Project (3GPP) General Packet Radio Service (GPRS)
    packet networks. 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.
 
 
 
 
 
 
 
 
 
 
 
 
 
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           Analysis on IPv6 Transition in 3GPP Networks      March 2004
 
 
 Table of Contents
 
    1. Introduction..................................................2
       1.1 Scope of this Document....................................3
       1.2 Abbreviations.............................................3
       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
       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
       3.2 IPv6 UE Connecting to an IPv6 Node through an IPv4 Network
       ..............................................................8
       3.3 IPv4 UE Connecting to an IPv4 Node through an IPv6 Network
       .............................................................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 IMS Islands Connected over IPv4 Network..............14
    5. About 3GPP UE IPv4/IPv6 Configuration........................14
    6. Security Considerations......................................15
    7. References...................................................16
       7.1 Normative................................................16
       7.2 Informative..............................................16
    8. Contributors.................................................18
    9. Authors and Acknowledgements.................................18
    10. Editor's Contact Information................................19
    11. Intellectual Property Statement.............................19
    12. Copyright...................................................19
       Appendix A...................................................20
 
 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 in
    Authors and Acknowledgements section.
 
    This document analyzes the transition scenarios in 3GPP packet
    data networks that might come up in the deployment phase of IPv6.
 
    The transition scenarios are documented in [RFC3574] and this
    document will further analyze them. The scenarios are divided into
    two categories: GPRS scenarios and IP Multimedia Subsystem (IMS)
    scenarios.
 
 
 
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    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 IMS islands connected via 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. The document covers scenarios with and without the
    use of the SIP based IP Multimedia Core Network Subsystem (IMS).
    This document does not focus on radio interface issues; both 3GPP
    Second (GSM) and Third Generation (UMTS) radio network
    architectures will be covered by these scenarios.
 
    The transition mechanisms specified by the IETF Ngtrans and v6ops
    Working Groups shall be used. This document shall not specify any
    new transition mechanisms, but if a need for a new mechanism is
    found, that will be reported to the IETF v6ops Working Group.
 
 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
 
 
 
 
 
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    GGSN        Gateway GPRS Support Node (a 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
 
 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.
 
 
 
 
 
 
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 2. Transition Mechanisms and DNS Guidelines
 
    This chapter briefly introduces some transition mechanisms
    specified by the IETF. In addition to that, 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]. 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].
    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.
 
 
 
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    NAT-PT (Network Address Translator / Protocol Translator) [RFC2766]
    using SIIT [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 at Layer 3 (using NAT-like techniques), Layer 4
    (using a TCP/UDP proxy) or Layer 7 (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.
 
 
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    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, the UE may attach to a 3GPP 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. 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 the tunnel
    endpoints are in the same private domain, or the tunneling
    mechanism works through IPv4 NAT, private IPv4 addresses can be
    used. One deployment scenario example is using 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
    support of tunneling mechanisms in the laptop computer. Another
    deployment scenario is making 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 [ISATAP] has
    been proposed, but some issues have been raised about it, such as
    its unnecessary features and relative complexity for a simple task
    like this, and its inadequacy 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, 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 document strongly recommends the 3GPP operators to
    deploy basic IPv6 support in their GPRS networks. That also makes
    it possible to burden the transition effects in the network and
    make the 3GPP UEs simpler.
 
 
 
 
 
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    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 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). 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, 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
    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].
 
 
 
 
 
 
 
 
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 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 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 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, but the upstream ISP does not
    provide IPv6 connectivity to the Internet, the encapsulating node
    can be the edge 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
 
 
 
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    the edge 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 an IPv4-only UE will be able to use an IPv4 link
    (PDP context) to connect to the GGSN without the need to
    communicate 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 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 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 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.
    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 UEs.
 
    However, if there is a need to connect to an IPv4(-only) node from
    an IPv6-only UE, it is possible 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):
 
 
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       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), 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 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 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.
 
 
 
 
 
 
 
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 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 IMS islands connected over 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 upgrade the legacy IPv4 nodes to dual-
    stack, 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], translators have to
    be used in the communication between the IPv6 IMS and legacy IPv4
    hosts, i.e. making a dual stack based solution is not feasible.
    This section aims to give a brief overview on how that interworking
    can be handled.
 
    This section presents higher level details of a solution based on
    the use of a translator and SIP ALG. [3GPPtr] provides additional
    information and presents a bit different solution proposal based on
    SIP Edge Proxy and IP Address/Port Mapper. The authors recommend to
    solve the general SIP/SDP IPv4/IPv6 transition problem in the IETF
    SIP wg(s).
 
 
 
 
 
 
 
 
 
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    As control (or signaling) and user (or data) traffic are separated
    in SIP, and thus, the IMS, the translation of the IMS traffic has
    to be done at two levels:
              1)Session Initiation Protocol (SIP) [RFC3261], and
                 Session Description Protocol (SDP) [RFC2327] [RFC3266]
                 (Mm-interface)
              2)the user data traffic (Mb-interface)
 
    SIP and SDP transition has to be made in an SIP/SDP Application
    Level Gateway. The ALG has to 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 be interoperability
    for DNS queries; see section 2.4 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 to IPv4, and vice versa.
 
    The legacy IPv4 host's address can be mapped to an IPv6 address for
    the IMS, and this address is then used within the IMS to route the
    traffic to the appropriate user traffic translator. This mapping
    can be done by the SIP/SDP ALG for the SIP traffic. The user
    traffic translator would do the similar mapping for the user
    traffic. However, in order to have an IPv4 address for the IMS UE,
    and to be able to route the user traffic within the legacy IPv4
    network to the correct translator, there has to be an IPv4 address
    allocated for the duration of the session from the user traffic
    translator. The allocation of this address from the user traffic
    translator has to be done by the SIP/SDP ALG in order for the
    SIP/SDP ALG to know the correct IPv4 address. This can be achieved
    by using a protocol for the ALG to do the allocation.
 
          +-------------------------------+ +------------+
          |                      +------+ | | +--------+ |
          |                      |S-CSCF|---| |SIP ALG | |\
       |  |                      +------+ | | +--------+ | \ --------
     +-|+ |                       /       | |     |      |  |        |
     |  | | +------+        +------+      | |     +      |   -|    |-
     |  |-|-|P-CSCF|--------|I-CSCF|      | |     |      |    | () |
     |  |   +------+        +------+      | |+----------+| /  ------
     |  |-----------------------------------||Translator||/
     +--+ |            IPv6               | |+----------+|     IPv4
      UE  |                               | |Interworking|
          |  IP Multimedia CN Subsystem   | |Unit        |
          +-------------------------------+ +------------+
 
           Figure 1: UE using IMS to contact a legacy phone
 
 
 
 
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    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 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 edge of the IPv6-only IMS an "Interworking Unit" in this
    document. A SIP-specific translation mechanism, which could e.g.
    re-use limited subsets of NAT-PT [RFC2766], needs to be specified.
    The problems related to NAT-PT are discussed in appendix A.
 
 4.2 Two IMS Islands Connected over 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 to access IPv6 or IPv4 based services, settings need to
    be done in the UE. 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 to configure the GPRS
    settings. The user can 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 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 mechanism is also available for
    IPv6: so-called Protocol Configuration Options Information Element
    (PCO-IE) specified by the 3GPP [3GPP-24.008]. It is also possible
    to use [DHCPv6-SL] or [RFC3315] and [RFC3646] for receiving DNS
 
 
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           Analysis on IPv6 Transition in 3GPP Networks      March 2004
 
 
    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), or typed in manually in the UE.
 
    When accessing IMS services, the UE needs to know the P-CSCF IPv6
    address. 3GPP-specific PCO-IE mechanism, or DHCPv6-based mechanism
    ([DHCPv6-SL] or [RFC3315] and [RFC3319]) can be used. Manual
    configuration or configuration over the air is also possible. IMS
    subscriber authentication and registration to the IMS and SIP
    integrity protection are not discussed here.
 
 6. Security Considerations
 
    There are some generic security considerations when moving to dual-
    stack IPv4/IPv6 deployment which are not analyzed at length here.
    Two examples of 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 to the IPv4 services (e.g., in the form of misbehavior
    towards DNS AAAA record lookups or operationally worse quality IP
    transit services).
 
    This memo recommends the use of a relatively small number of
    techniques, which all of them have their 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 are also discussed in the
    specific documents.  However, in particular one should note that a
    proper configuration of locally-deployed relays and proxies is very
    important, so that the outsiders will not have access to them, to
    be used for abuse, laundering attacks, or circumventing access
    controls.
 
    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 transition
          mechanism
 
 
 
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 7. References
 
 7.1 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.
 
    [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 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.
 
 
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    [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] Droms, R.: "Stateless DHCP Service for IPv6", January
    2004, draft-ietf-dhc-dhcpv6-stateless-04.txt, work in progress.
 
    [DNStrans] Durand, A. and Ihren, J.: "DNS IPv6 transport
    operational guidelines", November 2003, draft-ietf-dnsop-ipv6-
    transport-guidelines-01.txt, work in progress.
 
    [ISATAP] Templin, F., Gleeson, T., Talwar, M. and Thaler, D.:
    "Intra-Site Automatic Tunnel Addressing Protocol (ISATAP)",
    February 2004, draft-ietf-ngtrans-isatap-20.txt, work in progress.
 
    [ISP-sa] Lind, M., Ksinant, V., Park, D., Baudot, A.: "Scenarios
    and Analysis for Introducing IPv6 into ISP Networks", February
    2004, draft-ietf-v6ops-isp-scenarios-analysis-01.txt, work in
    progress.
 
    [NATPTappl] Satapati, S., Sivakumar, S., Barany, P., Okazaki, S.,
    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, work
    in progress, the draft has expired.
 
    [STEP] Savola, P.: "Simple IPv6-in-IPv4 Tunnel Establishment
    Procedure (STEP)", January 2004, draft-savola-v6ops-conftun-setup-
    02.txt, work in progress.
 
 
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           Analysis on IPv6 Transition in 3GPP Networks      March 2004
 
 
 
    [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. 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. 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 thank Heikki Almay, Gabor Bajko, Ajay
    Jain, Jarkko Jouppi, Ivan Laloux, Jasminko Mulahusic, Janne Rinne,
    Andreas Schmid, Pedro Serna, Fred Templin, Anand Thakur and Rod Van
    Meter for their valuable input.
 
 
 
 
 
 
 
 
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           Analysis on IPv6 Transition in 3GPP Networks      March 2004
 
 
 10. 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. Intellectual Property Statement
 
    The IETF takes no position regarding the validity or scope of any
    intellectual property 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 does it represent that it
    has made any effort to identify any such rights. Information on the
    IETF's procedures with respect to rights in standards-track and
    standards-related documentation can be found in BCP-11. Copies of
    claims of rights made available for publication 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.
 
    The IETF invites any interested party to bring to its attention any
    copyrights, patents or patent applications, or other proprietary
    rights which may cover technology that may be required to practice
    this standard. Please address the information to the IETF Executive
    Director.
 
 12. Copyright
 
    The following copyright notice is copied from [RFC2026], Section
    10.4. It describes the applicable copyright for this document.
 
    Copyright (C) The Internet Society March 24, 2004. All Rights
    Reserved.
 
    This document and translations of it may be copied and furnished 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 and derivative works.
    However, this document itself may not be modified in any way, such
    as by removing the copyright notice or references to the Internet
    Society or other Internet organizations, except as needed for the
 
 
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           Analysis on IPv6 Transition in 3GPP Networks      March 2004
 
 
    purpose of developing Internet standards in which case the
    procedures for copyrights defined in the Internet Standards process
    must be followed, or as required to translate it into languages
    other than English.
 
    The limited permissions granted above are perpetual and will not be
    revoked by the Internet Society or its successors or assignees.
 
    This document and the information contained herein is provided on
    an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET
    ENGINEERING TASK FORCE DISCLAIMS 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 is used, it needs to be placed on the
    GGSN external (Gi) interface, typically separate from the GGSN.
    NA(P)T-PT can be installed, for example, on the edge of the
    operator's network and the public Internet. NA(P)T-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 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 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].
 
 
 
 
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       4. NA(P)T-PT does not work (without application level gateways)
         for applications that embed  IP addresses in their payload.
 
       5. NA(P)T-PT breaks DNSSEC.
 
       6. NA(P)T-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
    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, the following
    actions can be recommended:
 
      1. Separate the DNS ALG from the NA(P)T-PT node (in the "IPv6 to
         IPv4" case).
 
      2. Ensure (if possible) that NA(P)T-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 from becoming a single point of failure.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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