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rfc:rfc4215

Network Working Group J. Wiljakka, Ed. Request for Comments: 4215 Nokia Category: Informational October 2005

                  Analysis on IPv6 Transition in
       Third Generation Partnership Project (3GPP) Networks

Status of This Memo

 This memo provides information for the Internet community.  It does
 not specify an Internet standard of any kind.  Distribution of this
 memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2005).

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) 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 and
 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
    1.3. Terminology ................................................5
 2. Transition Mechanisms and DNS Guidelines ........................5
    2.1. Dual Stack .................................................5
    2.2. Tunneling ..................................................6
    2.3. Protocol Translators .......................................6
    2.4. DNS Guidelines for IPv4/IPv6 Transition ....................6
 3. GPRS Transition Scenarios .......................................7
    3.1. Dual Stack UE Connecting to IPv4 and IPv6 Nodes ............7
    3.2. IPv6 UE Connecting to an IPv6 Node through an IPv4
         Network ....................................................8

Wiljakka Informational [Page 1] RFC 4215 IPv6 Transition in 3GPP Networks October 2005

         3.2.1. Tunneling Inside the 3GPP Operator's Network ........9
         3.2.2. Tunneling Outside the 3GPP Operator's Network ......10
    3.3. IPv4 UE Connecting to an IPv4 Node through an IPv6
         Network ...................................................10
    3.4. IPv6 UE Connecting to an IPv4 Node ........................11
    3.5. IPv4 UE Connecting to an IPv6 Node ........................12
 4. IMS Transition Scenarios .......................................12
    4.1. UE Connecting to a Node in an IPv4 Network through IMS ....12
    4.2. Two IPv6 IMS Connected via an IPv4 Network ................15
 5. About 3GPP UE IPv4/IPv6 Configuration ..........................15
 6. Summary and Recommendations ....................................16
 7. Security Considerations ........................................17
 8. References .....................................................17
    8.1. Normative References ......................................17
    8.2. Informative References ....................................18
 9. Contributors ...................................................20
 10. Authors and Acknowledgements ..................................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 [3GPP-23.060], in 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 that may come up in
 the deployment phase of IPv6 in 3GPP packet data networks.
 The 3GPP network architecture is described in [RFC3314], and relevant
 transition scenarios are documented in [RFC3574].  The reader of this
 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, which include Session Initiation Protocol (SIP)
 considerations.  For more information about IMS, see [3GPP-23.228],
 [3GPP-24.228], and [3GPP-24.229].
 GPRS scenarios are the following:
  1. Dual Stack User Equipment (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

Wiljakka Informational [Page 2] RFC 4215 IPv6 Transition in 3GPP Networks October 2005

  1. IPv4 UE connecting to an IPv6 node
 IMS scenarios are the following:
  1. UE connecting to a node in an IPv4 network through IMS
  2. Two IPv6 IMS connected via an IPv4 network
 The focus is on analyzing different transition scenarios and
 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 document is to analyze the possible transition
 scenarios in the 3GPP-defined GPRS network in which a UE connects to,
 or is contacted from, another node on the Internet.  This 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-specific issues; both 3GPP Second and Third
 Generation radio network architectures (GSM, Enhanced Data rates for
 GSM Evolution (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
 variations in its analysis.
 The transition mechanisms specified by the IETF Ngtrans and v6ops
 Working Groups shall be used.  This memo shall not specify any new
 transition mechanisms, but only documents the need for new ones (if
 appropriate).

1.2. Abbreviations

 2G          Second Generation Mobile Telecommunications, e.g., GSM
             and GPRS technologies
 3G          Third Generation Mobile Telecommunications, e.g., 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.

Wiljakka Informational [Page 3] RFC 4215 IPv6 Transition in 3GPP Networks October 2005

 B2BUA       Back-to-Back User Agent
 CSCF        Call Session Control Function (in 3GPP Release 5 IMS)
 DNS         Domain Name System
 EDGE        Enhanced Data rates for GSM Evolution
 GGSN        Gateway GPRS Support Node (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 Translation
 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
 SDP         Session Description Protocol
 SGSN        Serving GPRS Support Node
 SIIT        Stateless IP/ICMP Translation Algorithm
 SIP         Session Initiation Protocol
 UE          User Equipment, e.g., a UMTS mobile handset
 UMTS        Universal Mobile Telecommunications System
 WCDMA       Wideband Code Division Multiple Access

Wiljakka Informational [Page 4] RFC 4215 IPv6 Transition in 3GPP Networks October 2005

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 the IPv4-capable node
                the UE is communicating with.  The IPv4 node can be,
                e.g., an application server or another UE.
 IPv6 node      IPv6 node is here defined to be the IPv6-capable node
                the UE is communicating with.  The IPv6 node can be,
                e.g., 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 section briefly introduces these IETF IPv4/IPv6 transition
 mechanisms:
  1. dual IPv4/IPv6 stack [RFC4213]
  2. tunneling [RFC4213]
  3. protocol translators [RFC2766], [RFC2765]
 In addition, DNS recommendations are given.  The applicability of
 different transition mechanisms to 3GPP networks is discussed in
 sections 3 and 4.

2.1. Dual Stack

 The dual IPv4/IPv6 stack is specified in [RFC4213].  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

Wiljakka Informational [Page 5] RFC 4215 IPv6 Transition in 3GPP Networks October 2005

 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 [RFC2663] 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 [RFC4213].
 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 Translation/Protocol Translation) [RFC2766]
 using 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 take place at
 the network layer (using NAT-like techniques), the transport layer
 (using a TCP/UDP proxy), or 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 visible only 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 [RFC3901]
 for more information.

Wiljakka Informational [Page 6] RFC 4215 IPv6 Transition in 3GPP Networks October 2005

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 is not 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.
 Although the UE is dual stack, the UE may find itself attached 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, 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

Wiljakka Informational [Page 7] RFC 4215 IPv6 Transition in 3GPP Networks October 2005

 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 tunneling mechanism may require public IPv4 addresses, but there
 are tunneling mechanisms and deployment scenarios in which private
 IPv4 addresses may be used, for instance, if the tunnel endpoints are
 in the same private domain, or the tunneling mechanism works through
 IPv4 NAT.
 One deployment scenario 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 tunneling
 mechanism depends on the laptop computer's support of tunneling
 mechanisms.  Another deployment scenario is 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 can be a good fit.  There is not yet consensus on
 the right approach, and proposed mechanisms so far include [ISATAP]
 and [STEP].  Especially ISATAP has had some support in the working
 group.  Goals for 3GPP zero-configuration tunneling are documented in
 [zeroconf].
 This document strongly recommends that the 3GPP operators deploy
 basic IPv6 support in their GPRS networks.  That makes it possible to
 lessen the transition effects in the 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 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.

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 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

Wiljakka Informational [Page 8] RFC 4215 IPv6 Transition in 3GPP Networks October 2005

 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
 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 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 depend on the deployment scenario.  The authors are not
 commenting on 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
 [RFC4029].

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 alone do not 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 the presence of other router redundancy
 mechanisms, such as the use of IPv6 routing protocols.

Wiljakka Informational [Page 9] RFC 4215 IPv6 Transition in 3GPP Networks October 2005

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, the
 encapsulating node could be the 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 border
 router to a dual stack border gateway operated by another ISP that 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.  This
 allows an IPv4-only UE to use an IPv4 link (an IPv4 PDP context) to
 connect to the GGSN without 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 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.

Wiljakka Informational [Page 10] RFC 4215 IPv6 Transition in 3GPP Networks October 2005

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 the universal
 deployment of IPv6 probably is not 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 be
 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
 UEs.
 However, if there is a need to connect to an IPv4(-only) node from an
 IPv6-only UE, it is 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 are
       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.

Wiljakka Informational [Page 11] RFC 4215 IPv6 Transition in 3GPP Networks October 2005

 The authors note that [NATPTappl] discusses the applicability of
 NAT-PT, and [NATPTexp] discusses general issues with all forms of
 IPv6-IPv4 translation.  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 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 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 take
 advantage of them.  Thus, they will continue to support 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.

4. IMS Transition Scenarios

 As 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 connected 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 (IPv6) IMS UE 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.
 Over time, users will upgrade the legacy IPv4 nodes to dual-stack,
 often by replacing the entire node, eliminating this particular
 problem in that specific deployment.

Wiljakka Informational [Page 12] RFC 4215 IPv6 Transition in 3GPP Networks October 2005

 Still, it is difficult to estimate how many non-upgradable 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 may need to be used
 in the communication between the IPv6 IMS and the legacy IPv4 hosts
 in cases where these legacy IPv4 hosts cannot be upgraded to support
 IPv6.
 This section gives a brief analysis of the IMS interworking issues
 and presents a high-level view of SIP within the IMS.  The authors
 recommend that a detailed solution for the general SIP/SDP/media
 IPv4/IPv6 transition problem will be specified as soon as possible as
 a task within the SIP-related Working Groups in the IETF.
 The issue of the IPv4/IPv6 interworking in SIP is somewhat more
 challenging than many other protocols.  The control (or signaling)
 and user (or data) traffic are separated in SIP calls, and thus, the
 IMS, the transition of IMS traffic from IPv6 to IPv4, must be handled
 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)
 In addition, SIP carries an SDP body containing the addressing and
 other parameters for establishing the user data traffic (the media).
 Hence, the two levels of interworking cannot be made independently.
 Figure 1 shows an example setup for IPv4 and IPv6 interworking in
 IMS.  The "Interworking Unit" comprises two internal elements a dual
 stack SIP server and a transition gateway (TrGW) for the media
 traffic.  These two elements are interconnected for synchronizing the
 interworking of the SIP signaling and the media traffic.

Wiljakka Informational [Page 13] RFC 4215 IPv6 Transition in 3GPP Networks October 2005

         +-------------------------------+ +------------+
         |                      +------+ | | +--------+ |
         |                      |S-CSCF|---| |SIP Serv| |\
      |  |                      +------+ | | +--------+ | \ --------
    +-|+ |                       /       | |     |      |  |        |
    |  | | +------+        +------+      | |     +      |   -|    |-
    |  |-|-|P-CSCF|--------|I-CSCF|      | |     |      |    | () |
    |  |   +------+        +------+      | |+----------+| /  ------
    |  |-----------------------------------||   TrGW   ||/
    +--+ |            IPv6               | |+----------+|     IPv4
     UE  |                               | |Interworking|
         |  IP Multimedia CN Subsystem   | |Unit        |
         +-------------------------------+ +------------+
              Figure 1: UE using IMS to contact a legacy phone
 On reception of an INVITE, the SIP server reserves an IP address and
 a port from the TrGW both for IPv4 and IPv6.  Then, the SIP server
 acts as a B2BUA (Back-to-Back User Agent) and rewrites the SDP of the
 INVITE to insert the transition gateway in the middle of the media
 flow between the two endpoints.
 When performing its B2BUA role, the SIP server acts as a UA (User
 Agent) toward both the IMS and the IPv4 host.  Consequently, the SIP
 server needs to support all the extensions that apply to the session,
 which are listed in the Require header fields of the SIP messages.
 This approach has a number of important drawbacks, however.  The
 biggest drawback is that the rewriting of the SDP in the SIP
 signaling prevents securing the SDP payload between the two
 endpoints.  In addition, it breaks the end-to-end negotiation of SIP
 extensions required for each session.  Therefore, the extensions to
 be used in a particular session are limited by the extensions
 supported by the SIP server acting as a B2BUA.  That is, the
 introduction of a new extension requires upgrading not only the UAs
 but the B2BUAs as well.
 This analysis clearly shows that a new solution for IPv4-IPv6
 interworking in SIP networks is needed.  The ability to convey
 multiple alternative addresses in SDP session descriptions [RFC4091]
 represents a step in this direction.
 Given the problems related to the use of B2BUAs, it is recommended
 that the SIP-related Working Groups quickly work on a solution to
 overcome the drawbacks of this approach.

Wiljakka Informational [Page 14] RFC 4215 IPv6 Transition in 3GPP Networks October 2005

4.2. Two IPv6 IMS Connected 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 of 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.
 UE configuration is required in order to access IPv6- or IPv4-based
 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, 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, 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).  A similar 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
 [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) [OMA-CP] or typed in manually in the UE.
 When accessing IMS services, the UE needs to know the Proxy-Call
 Session Control Function (P-CSCF) IPv6 address.  Either a 3GPP-
 specific PCO-IE mechanism or a DHCPv6-based mechanism ([RFC3736] and
 [RFC3319]) can be used.  Manual configuration or configuration over

Wiljakka Informational [Page 15] RFC 4215 IPv6 Transition in 3GPP Networks October 2005

 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 important.  The two first GPRS
 scenarios and both IMS scenarios are seen as the most relevant.  The
 authors summarize some main recommendations here:
  1. Dual stack UEs are recommended instead of IPv4-only or IPv6-

only UEs. It is important to take care that applications in

       the UEs support IPv6.  In other words, applications should be
       IP version independent.  IPv6-only UEs can become feasible when
       IPv6 is widely deployed in the networks, and most services work
       on IPv6.
  1. 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.
  1. IPv6 communication is preferred to IPv4 communication going

through IPv4 NATs to the same dual stack peer node.

  1. This document strongly recommends that the 3GPP operators

deploy basic IPv6 support in their GPRS networks as soon as

       possible.  That makes it possible to lessen the transition
       effects in the UEs.
  1. 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.  See [zeroconf] for more details.
  1. Tunneling mechanisms can be used in 3GPP networks, and only

generic recommendations are given in this document. More

       details can be found, for example, in [RFC4029].
  1. The authors recommend that a detailed solution for the general

SIP/SDP/media IPv4/IPv6 transition problem be specified as soon

       as possible as a task within the SIP-related Working Groups in
       the IETF.

Wiljakka Informational [Page 16] RFC 4215 IPv6 Transition in 3GPP Networks October 2005

7. Security Considerations

 Deploying IPv6 has some generic security considerations one should be
 aware of [V6SEC]; however, these are not specific to 3GPP transition
 and are therefore out of the scope of this memo.
 This memo recommends the use of a relatively small number of
 techniques.  Each technique has its own security considerations,
 including:
  1. native upstream access or tunneling by the 3GPP network

operator,

  1. use of routing protocols to ensure redundancy,
  1. use of locally deployed specific-purpose protocol relays and

application proxies to reach IPv4(-only) nodes from IPv6-only

       UEs, or
  1. a specific mechanism for SIP signaling and media translation.
 The threats of configured tunneling are described in [RFC4213].
 Attacks against routing protocols are described in the respective
 documents and in general in [ROUTESEC].  Threats related to protocol
 relays have been described in [RFC3142].  The security properties of
 SIP internetworking are to be specified when the mechanism is
 specified.
 In particular, this memo does not recommend the following technique,
 which has security issues, not further analyzed here:
  1. NAT-PT or other translator as a general-purpose transition

mechanism

8. References

8.1. Normative References

 [RFC2663]     Srisuresh, P. and M. Holdrege, "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. and P. Srisuresh, "Network Address
               Translation - Protocol Translation (NAT-PT)", RFC 2766,
               February 2000.

Wiljakka Informational [Page 17] RFC 4215 IPv6 Transition in 3GPP Networks October 2005

 [RFC3261]     Rosenberg, J., Schulzrinne, H., Camarillo, G.,
               Johnston, A., Peterson, J., Sparks, R., Handley, M.,
               and E. Schooler, "SIP:  Session Initiation Protocol",
               RFC 3261, June 2002.
 [RFC3574]     Soininen, J., "Transition Scenarios for 3GPP Networks",
               RFC 3574, August 2003.
 [RFC4213]     Nordmark, E. and R. Gilligan, "Basic Transition
               Mechanisms for IPv6 Hosts and Routers", RFC 4213,
               October 2005.
 [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.

8.2. Informative References

 [RFC2327]     Handley, M. and V. Jacobson, "SDP: Session Description
               Protocol", RFC 2327, April 1998.
 [RFC3142]     Hagino, J. and K. Yamamoto, "An IPv6-to-IPv4 Transport
               Relay Translator", RFC 3142, June 2001.
 [RFC3266]     Olson, S., Camarillo, G., and A. Roach, "Support for
               IPv6 in Session Description Protocol (SDP)", RFC 3266,
               June 2002.
 [RFC3314]     Wasserman, M., "Recommendations for IPv6 in Third
               Generation Partnership Project (3GPP) Standards", RFC
               3314, September 2002.

Wiljakka Informational [Page 18] RFC 4215 IPv6 Transition in 3GPP Networks October 2005

 [RFC3315]     Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
               and M. Carney, "Dynamic Host Configuration Protocol for
               IPv6 (DHCPv6)", RFC 3315, July 2003.
 [RFC3319]     Schulzrinne, H. and B. Volz, "Dynamic Host
               Configuration Protocol (DHCPv6) Options for Session
               Initiation Protocol (SIP) Servers", RFC 3319, July
               2003.
 [RFC3646]     Droms, R., "DNS Configuration options for Dynamic Host
               Configuration Protocol for IPv6 (DHCPv6)", RFC 3646,
               December 2003.
 [RFC3736]     Droms, R., "Stateless Dynamic Host Configuration
               Protocol (DHCP) Service for IPv6", RFC 3736, April
               2004.
 [RFC3901]     Durand, A. and J. Ihren, "DNS IPv6 Transport
               Operational Guidelines", BCP 91, RFC 3901, September
               2004.
 [RFC4029]     Lind, M., Ksinant, V., Park, S., Baudot, A., and P.
               Savola, "Scenarios and Analysis for Introducing IPv6
               into ISP Networks", RFC 4029, March 2005.
 [RFC4091]     Camarillo, G. and J. Rosenberg, "The Alternative
               Network Address Types (ANAT) Semantics for the Session
               Description Protocol (SDP) Grouping Framework", RFC
               4091, June 2005.
 [ISATAP]      Templin, F., Gleeson, T., Talwar, M., and D. Thaler,
               "Intra-Site Automatic Tunnel Addressing Protocol
               (ISATAP)", RFC 4214, September 2005.
 [NATPTappl]   Satapati, S., Sivakumar, S., Barany, P., Okazaki, S.
               and H. Wang, "NAT-PT Applicability", Work in Progress,
               October 2003.
 [NATPTexp]    Aoun, C. and E. Davies, "Reasons to Move NAT-PT to
               Experimental", Work in Progress, July 2005.
 [ROUTESEC]    Barbir, A., Murphy, S., and Y. Yang, "Generic Threats
               to Routing Protocols", Work in Progress, April 2004.
 [STEP]        Savola, P.: "Simple IPv6-in-IPv4 Tunnel Establishment
               Procedure (STEP)", Work in Progress, January 2004.

Wiljakka Informational [Page 19] RFC 4215 IPv6 Transition in 3GPP Networks October 2005

 [V6SEC]       Savola, P.: "IPv6 Transition/Co-existence Security
               Considerations", Work in Progress, February 2004.
 [zeroconf]    Nielsen, K., Morelli, M., Palet, J., Soininen, J., and
               J. Wiljakka, "Goals for Zero-Configuration Tunneling in
               3GPP", Work in Progress, October 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.
 [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.  Allison Mankin has contributed text for IMS
 Scenario 1 (Section 4.1).

10. Authors and Acknowledgements

 This document was written by:
    Alain Durand, Comcast
    <alain_durand@cable.comcast.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>

Wiljakka Informational [Page 20] RFC 4215 IPv6 Transition in 3GPP Networks October 2005

    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, Gonzalo
 Camarillo, 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.

Wiljakka Informational [Page 21] RFC 4215 IPv6 Transition in 3GPP Networks October 2005

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.
 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 NAPT-PT is used, it needs to be placed on the
 GGSN external interface (Gi), typically separate from the GGSN.
 NAPT-PT can be installed, for example, on the edge of the operator's
 network and the public Internet.  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.
 NAPT-PT introduces limitations that are expected to be magnified
 within the 3GPP architecture.  [NATPTappl] discusses the
 applicability of NAT-PT in more detail.  [NATPTexp] discusses general
 issues with all forms of IPv6-IPv4 translation.
 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 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 NAPT-PT, the following
 actions can be recommended:
    1. Separate the DNS ALG from the NAPT-PT node (in the "IPv6 to
       IPv4" case).
    2. Ensure (if possible) that NAPT-PT does not become a single
       point of failure.

Wiljakka Informational [Page 22] RFC 4215 IPv6 Transition in 3GPP Networks October 2005

    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 NAPT-PT from becoming a single point of failure.

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
 FIN-33720 TAMPERE, Finland
 Phone:  +358 7180 48372
 EMail:  juha.wiljakka@nokia.com

Wiljakka Informational [Page 23] RFC 4215 IPv6 Transition in 3GPP Networks October 2005

Full Copyright Statement

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 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
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Acknowledgement

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Wiljakka Informational [Page 24]

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