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Internet Engineering Task Force (IETF) R. Koodli
Request for Comments: 6212 Cisco Systems
Category: Informational July 2011
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.
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/rfc6312.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Koodli Informational [Page 1]
RFC 6212 IPv6 in Mobile Networks July 2011
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 ........................9
3.4. Fixed-Mobile Convergence ..................................12
4. Summary and Conclusion .........................................14
5. Security Considerations ........................................15
6. Acknowledgements ...............................................15
7. Informative References .........................................15
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].
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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.
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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:
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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.
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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.
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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.
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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
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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
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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
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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
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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
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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.
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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
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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.
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[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.
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[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
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