Mobile Networks Considerations for IPv6 Deployment

Internet Engineering Task Force (IETF)                         R. Koodli
Request for Comments: 6342                                 Cisco Systems
Obsoletes: 6312                                              August 2011
Category: Informational
ISSN: 2070-1721


           Mobile Networks Considerations for IPv6 Deployment

Abstract

   Mobile Internet access from smartphones and other mobile devices is
   accelerating the exhaustion of IPv4 addresses.  IPv6 is widely seen
   as crucial for the continued operation and growth of the Internet,
   and in particular, it is critical in mobile networks.  This document
   discusses the issues that arise when deploying IPv6 in mobile
   networks.  Hence, this document can be a useful reference for service
   providers and network designers.

RFC Editor Note

   This document obsoletes RFC 6312.

   Due to a publishing error, RFC 6312 contains the incorrect RFC number
   in its header.  This document corrects that error with a new RFC
   number.  The specification herein is otherwise unchanged with respect
   to RFC 6312.

Status of This Memo

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

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

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6342.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................2
   2. Reference Architecture and Terminology ..........................3
   3. IPv6 Considerations .............................................4
      3.1. IPv4 Address Exhaustion ....................................4
      3.2. NAT Placement in Mobile Networks ...........................7
      3.3. IPv6-Only Deployment Considerations .......................10
      3.4. Fixed-Mobile Convergence ..................................13
   4. Summary and Conclusion .........................................14
   5. Security Considerations ........................................16
   6. Acknowledgements ...............................................16
   7. Informative References .........................................16

1.  Introduction

   The dramatic growth of the Mobile Internet is accelerating the
   exhaustion of the available IPv4 addresses.  It is widely accepted
   that IPv6 is necessary for the continued operation and growth of the
   Internet in general and of the Mobile Internet in particular.  While
   IPv6 brings many benefits, certain unique challenges arise when
   deploying it in mobile networks.  This document describes such
   challenges and outlines the applicability of the existing IPv6
   deployment solutions.  As such, it can be a useful reference document
   for service providers as well as network designers.  This document
   does not propose any new protocols or suggest new protocol
   specification work.

   The primary considerations that we address in this document on IPv6
   deployment in mobile networks are:

   o  Public and Private IPv4 address exhaustion and implications to
      mobile network deployment architecture;

   o  Placement of Network Address Translation (NAT) functionality and
      its implications;

   o  IPv6-only deployment considerations and roaming implications; and

   o  Fixed-Mobile Convergence and implications to overall architecture.

   In the following sections, we discuss each of these in detail.

   For the most part, we assume the Third Generation Partnership Project
   (3GPP) 3G and 4G network architectures specified in [3GPP.3G] and
   [3GPP.4G].  However, the considerations are general enough for other
   mobile network architectures as well [3GPP2.EHRPD].

2.  Reference Architecture and Terminology

   The following is a reference architecture of a mobile network.

                                +-----+    +-----+
                                | AAA |    | PCRF|
                                +-----+    +-----+
              Home Network         \          /
                                    \        /                       /-
                                     \      /                       / I
  MN     BS                           \    /                       /  n
   |     /\    +-----+ /-----------\ +-----+ /-----------\ +----+ /   t
 +-+    /_ \---| ANG |/ Operator's  \| MNG |/ Operator's  \| BR |/    e
 | |---/    \  +-----+\ IP Network  /+-----+\ IP Network  /+----+\    r
 +-+                   \-----------/    /    \-----------/        \   n
                       ----------------/------                     \  e
                     Visited Network  /                             \ t
                                     /                               \-
         +-----+ /------------------\
         | ANG |/ Visited Operator's \
         +-----+\     IP Network     /
                 \------------------/

                  Figure 1: Mobile Network Architecture

   A Mobile Node (MN) connects to the mobile network either via its Home
   Network or via a Visited Network when the user is roaming outside of
   the Home Network.  In the 3GPP network architecture, an MN accesses
   the network by connecting to an Access Point Name (APN), which maps
   to a mobile gateway.  Roughly speaking, an APN is similar to a
   Service Set Identifier (SSID) in wireless LAN.  An APN is a logical
   concept that can be used to specify what kinds of services, such as
   Internet access, high-definition video streaming, content-rich
   gaming, and so on, that an MN is entitled to.  Each APN can specify
   what type of IP connectivity (i.e., IPv4, IPv6, IPv4v6) is enabled on
   that particular APN.

   While an APN directs an MN to an appropriate gateway, the MN needs an
   end-to-end "link" to that gateway.  In the Long-Term Evolution (LTE)
   networks, this link is realized through an Evolved Packet System
   (EPS) bearer.  In the 3G Universal Mobile Telecommunications System
   (UMTS) networks, such a link is realized through a Packet Data
   Protocol (PDP) context.  The end-to-end link traverses multiple
   nodes, which are defined below:

   o  Base Station (BS): The radio Base Station provides wireless
      connectivity to the MN.

   o  Access Network Gateway (ANG): The ANG forwards IP packets to and
      from the MN.  Typically, this is not the MN's default router, and
      the ANG does not perform IP address allocation and management for
      the mobile nodes.  The ANG is located either in the Home Network
      or in the Visited Network.

   o  The Mobile Network Gateway (MNG): The MNG is the MN's default
      router, which provides IP address management.  The MNG performs
      functions such as offering Quality of Service (QoS), applying
      subscriber-specific policy, and enabling billing and accounting;
      these functions are sometimes collectively referred to as
      "subscriber-management" operations.  The mobile network
      architecture, as shown in Figure 1, defines the necessary protocol
      interfaces to enable subscriber-management operations.  The MNG is
      typically located in the Home Network.

   o  Border Router (BR): As the name implies, a BR borders the Internet
      for the mobile network.  The BR does not perform subscriber
      management for the mobile network.

   o  Authentication, Authorization, and Accounting (AAA): The general
      functionality of AAA is used for subscriber authentication and
      authorization for services as well as for generating billing and
      accounting information.

      In 3GPP network environments, the subscriber authentication and
      the subsequent authorization for connectivity and services is
      provided using the "Home Location Register" (HLR) / "Home
      Subscriber Server" (HSS) functionality.

   o  Policy and Charging Rule Function (PCRF): The PCRF enables
      applying policy and charging rules at the MNG.

   In the rest of this document, we use the terms "operator", "service
   provider", and "provider" interchangeably.

3.  IPv6 Considerations

3.1.  IPv4 Address Exhaustion

   It is generally agreed that the pool of public IPv4 addresses is
   nearing its exhaustion.  The IANA has exhausted the available '/8'
   blocks for allocation to the Regional Internet Registries (RIRs).
   The RIRs themselves have either "run out" of their blocks or are
   projected to exhaust them in the near future.  This has led to a
   heightened awareness among service providers to consider introducing
   technologies to keep the Internet operational.  For providers, there
   are two simultaneous approaches to addressing the run-out problem:
   delaying the IPv4 address pool exhaustion (i.e., conserving their
   existing pool) and introducing IPv6 in operational networks.  We
   consider both in the following.

   Delaying public IPv4 address exhaustion for providers involves
   assigning private IPv4 addressing for end-users or extending an IPv4
   address with the use of port ranges, which requires tunneling and
   additional signaling.  A mechanism such as the Network Address
   Translator (NAT) is used at the provider premises (as opposed to
   customer premises) to manage the private IP address assignment and
   access to the Internet.  In the following, we primarily focus on
   translation-based mechanisms such as NAT44 (i.e., translation from
   public IPv4 to private IPv4 and vice versa) and NAT64 (i.e.,
   translation from public IPv6 to public IPv4 and vice versa).  We do
   this because the 3GPP architecture already defines a tunneling
   infrastructure with the General Packet Radio Service (GPRS) Tunneling
   Protocol (GTP), and the architecture allows for dual-stack and
   IPv6-only deployments.

   In a mobile network, the IPv4 address assignment for an MN is
   performed by the MNG.  In the 3GPP network architecture, this
   assignment is performed in conjunction with the Packet Data Network
   (PDN) connectivity establishment.  A PDN connection implies an end-
   end link (i.e., an EPS bearer in 4G LTE or a PDP context in 3G UMTS)
   from the MN to the MNG.  There can be one or more PDN connections
   active at any given time for each MN.  A PDN connection may support
   both IPv4 and IPv6 traffic (as in a dual-stack PDN in 4G LTE
   networks), or it may support only one of the two traffic types (as in
   the existing 3G UMTS networks).  The IPv4 address is assigned at the
   time of PDN connectivity establishment or is assigned using DHCP
   after the PDN connectivity is established.  In order to delay the
   exhaustion of public IPv4 addresses, this IP address needs to be a
   private IPv4 address that is translated into a shared public IPv4
   address.  Hence, there is a need for a private-public IPv4
   translation mechanism in the mobile network.

   In the Long-Term Evolution (LTE) 4G network, there is a requirement
   for an always-on PDN connection in order to reliably reach a mobile
   user in the All-IP network.  This requirement is due to the need for
   supporting Voice over IP service in LTE, which does not have circuit-
   based infrastructure.  If this PDN connection were to use IPv4
   addressing, a private IPv4 address is needed for every MN that
   attaches to the network.  This could significantly affect the
   availability and usage of private IPv4 addresses.  One way to address
   this is by making the always-on PDN (that requires voice service) to
   be IPv6.  The IPv4 PDN is only established when the user needs it.

   The 3GPP standards also specify a deferred IPv4 address allocation on
   a dual-stack IPv4v6 PDN at the time of connection establishment.
   This has the advantage of a single PDN for IPv6 and IPv4 along with
   deferring IPv4 address allocation until an application needs it.  The
   deferred address allocation requires support for a dynamic
   configuration protocol such as DHCP as well as appropriate triggers
   to invoke the protocol.  Such a support does not exist today in
   mobile phones.  The newer iterations of smartphones could provide
   such support.  Also, the tethering of smartphones to laptops (which
   typically support DHCP) could use deferred allocation depending on
   when a laptop attaches to the smartphone.  Until appropriate triggers
   and host stack support is available, the applicability of the
   address-deferring option may be limited.

   On the other hand, in the existing 3G UMTS networks, there is no
   requirement for an always-on connection even though many smartphones
   seldom relinquish an established PDP context.  The existing so-called
   pre-Release-8 deployments do not support the dual-stack PDP
   connection.  Hence, two separate PDP connections are necessary to
   support IPv4 and IPv6 traffic.  Even though some MNs, especially the
   smartphones, in use today may have IPv6 stack, there are two
   remaining considerations.  First, there is little operational
   experience and compliance testing with these existing stacks.  Hence,
   it is expected that their use in large deployments may uncover
   software errors and interoperability problems that inhibit providing
   services based on IPv6 for such hosts.  Second, only a fraction of
   current phones in use have such a stack.  As a result, providers need
   to test, deploy, and operationalize IPv6 as they introduce new
   handsets, which also continue to need access to the predominantly
   IPv4 Internet.

   The considerations from the preceeding paragraphs lead to the
   following observations.  First, there is an increasing need to
   support private IPv4 addressing in mobile networks because of the
   public IPv4 address run-out problem.  Correspondingly, there is a
   greater need for private-public IPv4 translation in mobile networks.
   Second, there is support for IPv6 in both 3G and 4G LTE networks
   already in the form of PDP context and PDN connections.  To begin
   with, operators can introduce IPv6 for their own applications and
   services.  In other words, the IETF's recommended model of dual-stack
   IPv6 and IPv4 networks is readily applicable to mobile networks with
   the support for distinct APNs and the ability to carry IPv6 traffic
   on PDP/PDN connections.  The IETF dual-stack model can be applied
   using a single IPv4v6 PDN connection in Release-8 and onwards but
   requires separate PDP contexts in the earlier releases.  Finally,
   operators can make IPv6 the default for always-on mobile connections
   using either the IPv4v6 PDN or the IPv6 PDN and use IPv4 PDNs as
   necessary.

3.2.  NAT Placement in Mobile Networks

   In the previous section, we observed that NAT44 functionality is
   needed in order to conserve the available pool and delay public IPv4
   address exhaustion.  However, the available private IPv4 pool itself
   is not abundant for large networks such as mobile networks.  For
   instance, the so-called NET10 block [RFC1918] has approximately 16.7
   million private IPv4 addresses starting with 10.0.0.0.  A large
   mobile service provider network can easily have more than 16.7
   million subscribers attached to the network at a given time.  Hence,
   the private IPv4 address pool management and the placement of NAT44
   functionality becomes important.

   In addition to the developments cited above, NAT placement is
   important for other reasons as well.  Access networks generally need
   to produce network and service usage records for billing and
   accounting.  This is true also for mobile networks where "subscriber
   management" features (i.e., QoS, Policy, and Billing and Accounting)
   can be fairly detailed.  Since a NAT introduces a binding between two
   addresses, the bindings themselves become necessary information for
   subscriber management.  For instance, the offered QoS on private IPv4
   address and the (shared) public IPv4 address may need to be
   correlated for accounting purposes.  As another example, the
   Application Servers within the provider network may need to treat
   traffic based on policy provided by the PCRF.  If the IP address seen
   by these Application Servers is not unique, the PCRF needs to be able
   to inspect the NAT binding to disambiguate among the individual MNs.
   The subscriber session management information and the service usage
   information also need to be correlated in order to produce harmonized
   records.  Furthermore, there may be legal requirements for storing
   the NAT binding records.  Indeed, these problems disappear with the
   transition to IPv6.  For now, it suffices to assert that NAT
   introduces state, which needs to be correlated and possibly stored
   with other routine subscriber information.

   Mobile network deployments vary in their allocation of IP address
   pools.  Some network deployments use the "centralized model" where
   the pool is managed by a common node, such as the PDN's BR, and the
   pool shared by multiple MNGs all attached to the same BR.  This model
   has served well in the pre-3G deployments where the number of
   subscribers accessing the Mobile Internet at any given time has not
   exceeded the available address pool.  However, with the advent of 3G
   networks and the subsequent dramatic growth in the number of users on
   the Mobile Internet, service providers are increasingly forced to
   consider their existing network design and choices.  Specifically,
   providers are forced to address private IPv4 pool exhaustion as well
   as scalable NAT solutions.

   In order to tackle the private IPv4 exhaustion in the centralized
   model, there would be a need to support overlapped private IPv4
   addresses in the common NAT functionality as well as in each of the
   gateways.  In other words, the IP addresses used by two or more MNs
   (which may be attached to the same MNG) are very likely to overlap at
   the centralized NAT, which needs to be able to differentiate traffic.
   Tunneling mechanisms such as Generic Routing Encapsulation (GRE)
   [RFC2784] [RFC2890], MPLS [RFC3031] VPN tunnels, or even IP-in-IP
   encapsulation [RFC2003] that can provide a unique identifier for a
   NAT session can be used to separate overlapping private IPv4 traffic
   as described in [GI-DS-LITE].  An advantage of centralizing the NAT
   and using the overlapped private IPv4 addressing is conserving the
   limited private IPv4 pool.  It also enables the operator's enterprise
   network to use IPv6 from the MNG to the BR; this (i.e., the need for
   an IPv6-routed enterprise network) may be viewed as an additional
   requirement by some providers.  The disadvantages include the need
   for additional protocols to correlate the NAT state (at the common
   node) with subscriber session information (at each of the gateways),
   suboptimal MN-MN communication, absence of subscriber-aware NAT (and
   policy) function, and, of course, the need for a protocol from the
   MNG to BR itself.  Also, if the NAT function were to experience
   failure, all the connected gateway service will be affected.  These
   drawbacks are not present in the "distributed" model, which we
   discuss in the following.

   In a distributed model, the private IPv4 address management is
   performed by the MNG, which also performs the NAT functionality.  In
   this model, each MNG has a block of 16.7 million unique addresses,
   which is sufficient compared to the number of mobile subscribers
   active on each MNG.  By distributing the NAT functionality to the
   edge of the network, each MNG is allowed to reuse the available NET10
   block, which avoids the problem of overlapped private IPv4 addressing
   at the network core.  In addition, since the MNG is where subscriber
   management functions are located, the NAT state correlation is
   readily enabled.  Furthermore, an MNG already has existing interfaces
   to functions such as AAA and PCRF, which allows it to perform
   subscriber management functions with the unique private IPv4
   addresses.  Finally, the MNG can also pass-through certain traffic
   types without performing NAT to the Application Servers located
   within the service provider's domain, which allows the servers to
   also identify subscriber sessions with unique private IPv4 addresses.
   The disadvantages of the "distributed model" include the absence of
   centralized addressing and centralized management of NAT.

   In addition to the two models described above, a hybrid model is to
   locate NAT in a dedicated device other than the MNG or the BR.  Such
   a model would be similar to the distributed model if the IP pool
   supports unique private addressing for the mobile nodes, or it would
   be similar to the centralized model if it supports overlapped private
   IP addresses.  In any case, the NAT device has to be able to provide
   the necessary NAT session binding information to an external entity
   (such as AAA or PCRF), which then needs to be able to correlate those
   records with the user's session state present at the MNG.

   The foregoing discussion can be summarized as follows.  First, the
   management of the available private IPv4 pool has become important
   given the increase in Mobile Internet users.  Mechanisms that enable
   reuse of the available pool are required.  Second, in the context of
   private IPv4 pool management, the placement of NAT functionality has
   implications to the network deployment and operations.  The
   centralized models with a common NAT have the advantages of
   continuing their legacy deployments and the reuse of private IPv4
   addressing.  However, they need additional functions to enable
   traffic differentiation and NAT state correlation with subscriber
   state management at the MNG.  The distributed models also achieve
   private IPv4 address reuse and avoid overlapping private IPv4 traffic
   in the operator's core, but without the need for additional
   mechanisms.  Since the MNG performs (unique) IPv4 address assignment
   and has standard interfaces to AAA and PCRF, the distributed model
   also enables a single point for subscriber and NAT state reporting as
   well as policy application.  In summary, providers interested in
   readily integrating NAT with other subscriber management functions,
   as well as conserving and reusing their private IPv4 pool, may find
   the distributed model compelling.  On the other hand, those providers
   interested in common management of NAT may find the centralized model
   more compelling.

3.3.  IPv6-Only Deployment Considerations

   As we observed in the previous section, the presence of NAT
   functionality in the network brings up multiple issues that would
   otherwise be absent.  NAT should be viewed as an interim solution
   until IPv6 is widely available, i.e., IPv6 is available for mobile
   users for all (or most) practical purposes.  Whereas NATs at provider
   premises may slow down the exhaustion of public IPv4 addresses,
   expeditious and simultaneous introduction of IPv6 in the operational
   networks is necessary to keep the Internet "going and growing".
   Towards this goal, it is important to understand the considerations
   in deploying IPv6-only networks.

   There are three dimensions to IPv6-only deployments: the network
   itself, the mobile nodes, and the applications, represented by the
   3-tuple {nw, mn, ap}.  The goal is to reach the coordinate {IPv6,
   IPv6, IPv6} from {IPv4, IPv4, IPv4}.  However, there are multiple
   paths to arrive at this goal.  The classic dual-stack model would
   traverse the coordinate {IPv4v6, IPv4v6, IPv4v6}, where each
   dimension supports co-existence of IPv4 and IPv6.  This appears to be
   the path of least disruption, although we are faced with the
   implications of supporting large-scale NAT in the network.  There is
   also the cost of supporting separate PDP contexts in the existing 3G
   UMTS networks.  The other intermediate coordinate of interest is
   {IPv6, IPv6, IPv4}, where the network and the MN are IPv6-only, and
   the Internet applications are recognized to be predominantly IPv4.
   This transition path would, ironically, require interworking between
   IPv6 and IPv4 in order for the IPv6-only MNs to be able to access
   IPv4 services and applications on the Internet.  In other words, in
   order to disengage NAT (for IPv4-IPv4), we need to introduce another
   form of NAT (i.e., IPv6-IPv4) to expedite the adoption of IPv6.

   It is interesting to consider the preceeding discussion surrounding
   the placement of NAT for IPv6-IPv4 interworking.  There is no
   overlapping private IPv4 address problem because each IPv6 address is
   unique and there are plenty of them available.  Hence, there is also
   no requirement for (IPv6) address reuse, which means no protocol is
   necessary in the centralized model to disambiguate NAT sessions.
   However, there is an additional requirement of DNS64 [RFC6147]
   functionality for IPv6-IPv4 translation.  This DNS64 functionality
   must ensure that the synthesized AAAA record correctly maps to the
   IPv6-IPv4 translator.

   IPv6-only deployments in mobile networks need to reckon with the
   following considerations.  First, both the network and the MNs need
   to be IPv6 capable.  Expedited network upgrades as well as rollout of
   MNs with IPv6 would greatly facilitate this.  Fortunately, the 3GPP
   network design for LTE already requires the network nodes and the
   mobile nodes to support IPv6.  Even though there are no requirements
   for the transport network to be IPv6, an operational IPv6
   connectivity service can be deployed with appropriate existing
   tunneling mechanisms in the IPv4-only transport network.  Hence, a
   service provider may choose to enforce IPv6-only PDN and address
   assignment for their own subscribers in their Home Networks (see
   Figure 1).  This is feasible for the newer MNs when the mobile
   network is able to provide IPv6-only PDN support and IPv6-IPv4
   interworking for Internet access.  For the existing MNs, however, the
   provider still needs to be able to support IPv4-only PDP/PDN
   connectivity.

   Migration of applications to IPv6 in MNs with IPv6-only PDN
   connectivity brings challenges.  The applications and services
   offered by the provider obviously need to be IPv6-capable.  However,
   an MN may host other applications, which also need to be IPv6-capable
   in IPv6-only deployments.  This can be a "long-tail" phenomenon;
   however, when a few prominent applications start offering IPv6, there
   can be a strong incentive to provide application-layer (e.g., socket
   interface) upgrades to IPv6.  Also, some IPv4-only applications may
   be able to make use of alternative access such as WiFi when
   available.  A related challenge in the migration of applications is
   the use of IPv4 literals in application layer protocols (such as
   XMPP) or content (as in HTML or XML).  Some Internet applications
   expect their clients to supply IPv4 addresses as literals, and this
   will not be possible with IPv6-only deployments.  Some of these
   experiences and the related considerations in deploying an IPv6-only
   network are documented in [ARKKO-V6].  In summary, migration of
   applications to IPv6 needs to be done, and such a migration is not
   expected to be uniform across all subsets of existing applications.

   Voice over LTE (VoLTE) also brings some unique challenges.  The
   signaling for voice is generally expected to be available for free
   while the actual voice call itself is typically charged on its
   duration.  Such a separation of signaling and the payload is unique
   to voice, whereas an Internet connection is accounted without
   specifically considering application signaling and payload traffic.
   This model is expected to be supported even during roaming.
   Furthermore, providers and users generally require voice service
   regardless of roaming, whereas Internet usage is subject to
   subscriber preferences and roaming agreements.  This requirement to
   ubiquitously support voice service while providing the flexibility
   for Internet usage exacerbates the addressing problem and may hasten
   provisioning of VoLTE using the IPv6-only PDN.

   As seen earlier, roaming is unique to mobile networks, and it
   introduces new challenges.  Service providers can control their own
   network design but not their peers' networks, which they rely on for
   roaming.  Users expect uniformity in experience even when they are
   roaming.  This imposes a constraint on providers interested in
   IPv6-only deployments to also support IPv4 addressing when their own
   (outbound) subscribers roam to networks that do not offer IPv6.  For
   instance, when an LTE deployment is IPv6-only, a roamed 3G network
   may not offer IPv6 PDN connectivity.  Since a PDN connection involves
   the radio base station, the ANG, and the MNG (see Figure 1), it would
   not be possible to enable IPv6 PDN connectivity without roamed
   network support.  These considerations also apply when the visited
   network is used for offering services such as VoLTE in the so-called
   Local Breakout model; the roaming MN's capability as well as the
   roamed network capability to support VoLTE using IPv6 determine
   whether fallback to IPv4 would be necessary.  Similarly, there are
   inbound roamers to an IPv6-ready provider network whose MNs are not
   capable of IPv6.  The IPv6-ready provider network has to be able to
   support IPv4 PDN connectivity for such inbound roamers.  There are
   encouraging signs that the existing deployed network nodes in the
   3GPP architecture already provide support for IPv6 PDP context.  It
   would be necessary to scale this support for a (very) large number of
   mobile users and offer it as a ubiquitous service that can be
   accounted for.

   In summary, IPv6-only deployments should be encouraged alongside the
   dual-stack model, which is the recommended IETF approach.  This is
   relatively straightforward for an operator's own services and
   applications, provisioned through an appropriate APN and the
   corresponding IPv6-only PDP or EPS bearer.  Some providers may
   consider IPv6-only deployment for Internet access as well, and this
   would require IPv6-IPv4 interworking.  When the IPv6-IPv4 translation
   mechanisms are used in IPv6-only deployments, the protocols and the
   associated considerations specified in [RFC6146] and [RFC6145] apply.
   Finally, such IPv6-only deployments can be phased-in for newer mobile
   nodes, while the existing ones continue to demand IPv4-only
   connectivity.

   Roaming is important in mobile networks, and roaming introduces
   diversity in network deployments.  Until IPv6 connectivity is
   available in all mobile networks, IPv6-only mobile network
   deployments need to be prepared to support IPv4 connectivity (and
   NAT44) for their own outbound roaming users as well as for inbound
   roaming users.  However, by taking the initiative to introduce IPv6-
   only for the newer MNs, the mobile networks can significantly reduce
   the demand for private IPv4 addresses.

3.4.  Fixed-Mobile Convergence

   Many service providers have both fixed broadband and mobile networks.
   Access networks are generally disparate, with some common
   characteristics but with enough differences to make it challenging to
   achieve "convergence".  For instance, roaming is not a consideration
   in fixed access networks.  An All-IP mobile network service provider
   is required to provide voice service, whereas this is not required
   for a fixed network provider.  A "link" in fixed networks is
   generally capable of carrying IPv6 and IPv4 traffic, whereas not all
   mobile networks have "links" (i.e., PDP/PDN connections) capable of
   supporting IPv6 and IPv4.  Indeed, roaming makes this problem worse
   when a portion of the link (i.e., the Home Network in Figure 1) is
   capable of supporting IPv6 and the other portion of the link (i.e.,
   the Visited Network in Figure 1) is not.  Such architectural
   differences, as well as policy and business model differences make
   convergence challenging.

   Nevertheless, within the same provider's space, some common
   considerations may apply.  For instance, IPv4 address management is a
   common concern for both of the access networks.  This implies that
   the same mechanisms discussed earlier, i.e., delaying IPv4 address
   exhaustion and introducing IPv6 in operational networks, apply for
   the converged networks as well.  However, the exact solutions
   deployed for each access network can vary for a variety of reasons,
   such as:

   o  Tunneling of private IPv4 packets within IPv6 is feasible in fixed
      networks where the endpoint is often a cable or DSL modem.  This
      is not the case in mobile networks where the endpoint is an MN
      itself.

   o  Encapsulation-based mechanisms such as 6rd [RFC5969] are useful
      where the operator is unable to provide native or direct IPv6
      connectivity and a residential gateway can become a tunnel
      endpoint for providing this service.  In mobile networks, the
      operator could provide IPv6 connectivity using the existing mobile
      network tunneling mechanisms without introducing an additional
      layer of tunneling.

   o  A mobile network provider may have Application Servers (e.g., an
      email server) in its network that require unique private IPv4
      addresses for MN identification, whereas a fixed network provider
      may not have such a requirement or the service itself.

   These examples illustrate that the actual solutions used in an access
   network are largely determined by the requirements specific to that
   access network.  Nevertheless, some sharing between an access and
   core network may be possible depending on the nature of the
   requirement and the functionality itself.  For example, when a fixed
   network does not require a subscriber-aware feature such as NAT, the
   functionality may be provided at a core router while the mobile
   access network continues to provide the NAT functionality at the
   mobile gateway.  If a provider chooses to offer common subscriber
   management at the MNG for both fixed and wireless networks, the MNG
   itself becomes a convergence node that needs to support the
   applicable transition mechanisms for both fixed and wireless access
   networks.

   Different access networks of a provider are more likely to share a
   common core network.  Hence, common solutions can be more easily
   applied in the core network.  For instance, configured tunnels or
   MPLS VPNs from the gateways from both mobile and fixed networks can
   be used to carry traffic to the core routers until the entire core
   network is IPv6-enabled.

   There can also be considerations due to the use of NAT in access
   networks.  Solutions such as Femto Networks rely on a fixed Internet
   connection being available for the Femto Base Station to communicate
   with its peer on the mobile network, typically via an IPsec tunnel.
   When the Femto Base Station needs to use a private IPv4 address, the
   mobile network access through the Femto Base Station will be subject
   to NAT policy administration including periodic cleanup and purge of
   NAT state.  Such policies affect the usability of the Femto Network
   and have implications to the mobile network provider.  Using IPv6 for
   the Femto (or any other access technology) could alleviate some of
   these concerns if the IPv6 communication could bypass the NAT.

   In summary, there is interest in Fixed-Mobile Convergence, at least
   among some providers.  While there are benefits to harmonizing the
   network as much as possible, there are also idiosyncrasies of
   disparate access networks that influence the convergence.  Perhaps
   greater harmonization is feasible at the higher service layers, e.g.,
   in terms of offering unified user experience for services and
   applications.  Some harmonization of functions across access networks
   into the core network may be feasible.  A provider's core network
   appears to be the place where most convergence is feasible.

4.  Summary and Conclusion

   IPv6 deployment in mobile networks is crucial for the Mobile
   Internet.  In this document, we discussed the considerations in
   deploying IPv6 in mobile networks.  We summarize the discussion in
   the following:

   o  IPv4 address exhaustion and its implications to mobile networks:
      As mobile service providers begin to deploy IPv6, conserving their
      available IPv4 pool implies the need for network address
      translation in mobile networks.  At the same time, providers can
      make use of the 3GPP architecture constructs such as APN and PDN
      connectivity to introduce IPv6 without affecting the predominantly
      IPv4 Internet access.  The IETF dual-stack model [RFC4213] can be
      applied to the mobile networks readily.

   o  The placement of NAT functionality in mobile networks: Both the
      centralized and distributed models of private IPv4 address pool
      management have their relative merits.  By enabling each MNG to
      manage its own NET10 pool, the distributed model achieves reuse of
      the available private IPv4 pool and avoids the problems associated
      with the non-unique private IPv4 addresses for the MNs without
      additional protocol mechanisms.  The distributed model also
      augments the "subscriber management" functions at an MNG, such as
      readily enabling NAT session correlation with the rest of the
      subscriber session state.  On the other hand, existing deployments
      that have used the centralized IP address management can continue
      their legacy architecture by placing the NAT at a common node.
      The centralized model also achieves private IPv4 address reuse but
      needs additional protocol extensions to differentiate overlapping
      addresses at the common NAT as well as to integrate with policy
      and billing infrastructure.

   o  IPv6-only mobile network deployments: This deployment model is
      feasible in the LTE architecture for an operator's own services
      and applications.  The existing MNs still expect IPv4 address
      assignment.  Furthermore, roaming, which is unique to mobile
      networks, requires that a provider support IPv4 connectivity when
      its (outbound) users roam into a mobile network that is not IPv6-
      enabled.  Similarly, a provider needs to support IPv4 connectivity
      for (inbound) users whose MNs are not IPv6-capable.  The IPv6-IPv4
      interworking is necessary for IPv6-only MNs to access the IPv4
      Internet.

   o  Fixed-Mobile Convergence: The examples discussed illustrate the
      differences in the requirements of fixed and mobile networks.
      While some harmonization of functions may be possible across the
      access networks, the service provider's core network is perhaps
      better-suited for converged network architecture.  Similar gains
      in convergence are feasible in the service and application layers.

5.  Security Considerations

   This document does not introduce any new security vulnerabilities.

6.  Acknowledgements

   This document has benefitted from discussions with and reviews from
   Cameron Byrne, David Crowe, Hui Deng, Remi Despres, Fredrik Garneij,
   Jouni Korhonen, Teemu Savolainen, and Dan Wing.  Thanks to all of
   them.  Many thanks to Mohamed Boucadair for providing an extensive
   review of a draft version of this document.  Cameron Byrne, Kent
   Leung, Kathleen Moriarty, and Jari Arkko provided reviews that helped
   improve this document.  Thanks to Nick Heatley for providing valuable
   review and input on VoLTE.

7.  Informative References

   [3GPP.3G]     "General Packet Radio Service (GPRS); Service
                 description; Stage 2, 3GPP TS 23.060, December 2006".

   [3GPP.4G]     "General Packet Radio Service (GPRS) enhancements for
                 Evolved Universal Terrestrial Radio Access Network
                 (E-UTRAN) access", 3GPP TS 23.401 8.8.0, December 2009.

   [3GPP2.EHRPD] "E-UTRAN - eHRPD Connectivity and Interworking: Core
                 Network Aspects", http://www.3gpp2.org/public_html/
                 Specs/X.S0057-0_v1.0_090406.pdf.

   [RFC1918]     Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot,
                 G., and E. Lear, "Address Allocation for Private
                 Internets", BCP 5, RFC 1918, February 1996.

   [RFC2003]     Perkins, C., "IP Encapsulation within IP", RFC 2003,
                 October 1996.

   [RFC2784]     Farinacci, D., Li, T., Hanks, S., Meyer, D., and P.
                 Traina, "Generic Routing Encapsulation (GRE)", RFC
                 2784, March 2000.

   [RFC2890]     Dommety, G., "Key and Sequence Number Extensions to
                 GRE", RFC 2890, September 2000.

   [RFC3031]     Rosen, E., Viswanathan, A., and R. Callon,
                 "Multiprotocol Label Switching Architecture", RFC 3031,
                 January 2001.

   [RFC4213]     Nordmark, E. and R. Gilligan, "Basic Transition
                 Mechanisms for IPv6 Hosts and Routers", RFC 4213,
                 October 2005.

   [RFC5969]     Townsley, W. and O. Troan, "IPv6 Rapid Deployment on
                 IPv4 Infrastructures (6rd) -- Protocol Specification",
                 RFC 5969, August 2010.

   [RFC6145]     Li, X., Bao, C., and F. Baker, "IP/ICMP Translation
                 Algorithm", RFC 6145, April 2011.

   [RFC6146]     Bagnulo, M., Matthews, P., and I. van Beijnum,
                 "Stateful NAT64: Network Address and Protocol
                 Translation from IPv6 Clients to IPv4 Servers", RFC
                 6146, April 2011.

   [RFC6147]     Bagnulo, M., Sullivan, A., Matthews, P., and I. van
                 Beijnum, "DNS64: DNS Extensions for Network Address
                 Translation from IPv6 Clients to IPv4 Servers", RFC
                 6147, April 2011.

   [ARKKO-V6]    Arkko, J. and A. Keranen, "Experiences from an
                 IPv6-Only Network", Work in Progress, April 2011.

   [GI-DS-LITE]  Brockners, F., Gundavelli, S., Speicher, S., and D.
                 Ward, "Gateway Initiated Dual-Stack Lite Deployment",
                 Work in Progress, July 2011.

Author's Address

   Rajeev Koodli
   Cisco Systems
   USA

   EMail: rkoodli@cisco.com