v6ops Working Group Rajeev Koodli
Internet-Draft Cisco Systems
Intended status: Informational January 4, 2011
Expires: July 8, 2011
Mobile Networks Considerations for IPv6 Deployment
draft-ietf-v6ops-v6-in-mobile-networks-03.txt
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 Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on July 8, 2011.
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
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Reference Architecture and Terminology . . . . . . . . . . . . 3
3. IPv6 Considerations . . . . . . . . . . . . . . . . . . . . . 5
3.1. IPv4 Address Exhaustion . . . . . . . . . . . . . . . . . 5
3.2. NAT Placement in the mobile networks . . . . . . . . . . . 7
3.3. IPv6-only Deployment Considerations . . . . . . . . . . . 10
3.4. Fixed - Mobile Convergence . . . . . . . . . . . . . . . . 13
4. Summary and Conclusion . . . . . . . . . . . . . . . . . . . . 15
5. Security Considerations . . . . . . . . . . . . . . . . . . . 16
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
7. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 16
8. Informative References . . . . . . . . . . . . . . . . . . . . 16
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 17
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 18
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1. Introduction
The dramatic growth of the Mobile Internet is accelerating the
exhaustion of available IPv4 addressing pool. It is widely accepted
that IPv6 is necessary for the continued operation, and growth of the
Internet in general, and that 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;
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 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.
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+-----+ +-----+
| AAA | | PCRF|
+-----+ +-----+
Home Network \ /
\ /
\ / /-
MN BS \ / /
| /\ +-----+ /-----------\ +-----+ /-----------\ +-----+ /
+-+ /_ \---| ANG |/ Operator's \| MNG |/ Operator's \| BR |/Inte
| |---/ \ +-----+\ IP Network /+-----+\ IP Network /+-----+\rnet
+-+ \-----------/ / \-----------/ \
----------------/------ \-
Visited Network /
/
+-----+ /------------------\
|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 roaming. In the 3GPP network
architecture, a 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 an SSID in wireless LAN. An APN is a logical
concept which can be used to specify what kinds of services, such as
Internet access, high-definition video streaming, content-rich
gaming, and so on, a MN is entitled to. And, each APN can specify
what type of IP connectivity (i.e., IPv4, IPv6, IPv4v6) is enabled on
that particular APN.
Whereas an APN directs a MN to an appropriate gateway, the MN needs
(an end-to-end) 'link' to the gateway, which is realized through an
Evolved Packet System (EPS) bearer in the Long-term Evolution (LTE)
networks, and through a Packet Data Protocol (PDP) Context in the 3G
UMTS networks. The different nodes in the figure are defined below:
o BS: The radio Base Station which provides wireless connectivity
to the MN.
o ANG: The Access Network Gateway. This is a node that 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.
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o MNG: The Mobile Network Gateway 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 the
figure, defines the necessary protocol interfaces to enable
subscriber management operations. The MNG is typically located in
the Home Network.
o BR: The Border Router, as the name implies, borders the Internet
for the mobile network. The BR does not perform subscriber
management for the mobile network.
o AAA: Authentication, Authorization and Accounting. 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 PCRF: Policy and Charging Rule Function enables applying policy
and charging rules at the MNG.
In the rest of this document, we use the terms operator, service
provider or provider interchangeably.
3. IPv6 Considerations
3.1. IPv4 Address Exhaustion
It is generally agreed that the pool of public IPv4 addressing is
nearing its exhaustion. The '/8' IANA blocks for Regional Internet
Registries (RIRs) are projected to 'run-out' soon. Subsequently, the
addressing pool available to RIRs for assignment to Internet Service
Providers is anticipated to run-out in the following 2-3 years. This
has led to a heightened awareness among the providers to consider
introducing technologies to keep the Internet operational. There are
two simultaneous approaches to addressing the run-out problem:
delaying the IPv4 address exhaustion, and introducing IPv6 in
operational networks. We consider both in the following.
Delaying the public IPv4 address exhaustion involves assigning
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private IPv4 addressing for end-users, as well as extending an IPv4
address (with the use of extended port ranges). A mechanism such as
the Network Address Translator (NAT) is used at the provider premises
(as opposed to customer premises in the existing deployments) to
manage 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 GPRS Tunneling Protocol
(GTP), and the architecture allows for dual-stack and IPv6-only
deployments.
In a mobile network, the IPv4 address assignment for a MN is
performed by the MNG. In the 3GPP network architecture, this
assignment is performed in conjunction with the PDN connectivity
establishment. A PDN connection implies an end-end link (i.e., an
EPS bearer in 4G LTE and 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
either one only (as in the existing 3G UMTS networks). The IPv4
address is assigned at the time of PDN connectivity establishment, or
is assigned using the DHCP protocol 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 which
is translated into a shared public IPv4 address. Hence, there is a
need for 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. An approach to
address this problem is to make the always-on PDN to be IPv6, and
enable IPv4 PDN on-demand, e.g., when an application binds to an IPv4
socket interface. This ensures that an IPv4 address is not assigned
for every attached MN, but only to those attempting to use the
traffic. The IPv4 PDN may be subject to session idle timers upon the
expiry of which, the PDN (and the assigned IPv4 address) may be
deleted. Another option specified in the 3GPP standards is to defer
the allocation of IPv4 address 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
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an application needs it. However, this requires support for a
dynamic configuration protocol such as DHCP, which many cellular MNs
do not support today for use over the cellular radio. This is
changing with complementary access technologies, with many
Smartphones already supporting DHCP for user over WiFi. Besides,
laptops using the network interface cards (such as USB dongles) to
connect to the cellular network typically support the DHCP protocol.
In any case, there need to be appropriate triggers to initiate DHCP
based on the application and interface usage, as well as DHCP lease
management based on appropriate address management policies. These
considerations may limit the applicability of the address deferring
option.
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. And, 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, such a capability is
not tested (if any at all) extensively and deployed in operational
networks. Given this, it is reasonable to expect that IPv6 can only
be introduced in the newer MNs, and that such newer MNs still need to
be able to access the (predominantly IPv4) Internet.
The considerations from the preceeding paragraphs lead to the
following observations. First, there is a need to support private
IPv4 addressing in mobile networks in order to address the public
IPv4 run-out problem. This means there is a need for private -
public IPv4 translation in the mobile network. Second, there is
support for IPv6 in both 3G and 4G LTE networks already in the form
of PDP context and PDN connections. Operators can introduce IPv6 in
their networks, perhaps to access operator's own applications and
services to begin with. 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 as the default for always-on mobile
connections, and investigate on-demand PDN and (private) address
assignment for IPv4.
3.2. NAT Placement in the mobile networks
In the previous section, we observed that the NAT44 functionality is
needed in order to conserve the available pool, and delay public IPv4
address exhaustion. However, the available private IPv4 pool itself
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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. Access networks generally need to
produce network and service usage records for billing and accounting.
This is true for mobile networks as well 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.
And, 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 state 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 Border
Router, and the pool shared by multiple gateways 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, perhaps, 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, the service providers are
increasingly forced to consider their existing network design and
choices. Specifically, the providers are forced to address private
IPv4 pool exhaustion as well as scalable NAT solutions.
In order to address 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
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the centralized NAT, which needs to be able to differentiate traffic.
The approach specified in [gi-ds-lite] is one way to achieve traffic
separation for overlapping private IPv4; the others include MPLS VPN
tunnels or any tunneling mechanism with a unique identifier for the
session. An obvious advantage of centralizing the NAT and using
overlapped private IPv4 addressing is that a large number of mobile
subscribers can be supported with a common limited pool at the BR.
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 diminished subscriber
management (compared to a subscriber-aware NAT), need for additional
protocol to correlate the NAT state (at the common node) with
subscriber session information (at each of the gateways), suboptimal
MN - MN communication, 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 re-use 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.
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The foregoing discussion can be summarized as follows: First, the
management of available private IPv4 pool has become important given
the growth of the mobile Internet users. The mechanisms that enable
re-use 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. Whereas the
centralized models with a common NAT have the advantages of
continuing their legacy deployments, re-use of private IPv4
addressing, and centralized NAT, 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 re-use, 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 re-using their
private IPv4 pool, may find the distributed model compelling, while
those interested in common management of NAT may find the cetralized
model more compelling.
3.3. IPv6-only Deployment Considerations
As we observed in the previous section, the NAT functionality in the
network brings multiple issues which would otherwise be not present
with the end-to-end access. 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 co-ordinate {IPv6,
IPv6, IPv6} from {IPv4, IPv4, IPv4}. However, there are multiple
paths to arrive at the goal. The classic dual-stack model would
traverse the co-ordinate {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 supoorting 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 co-ordinate of interest is
{IPv6, IPv6, IPv4}, where the network and the MN are IPv6-only, and
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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 re-use, which means no protocol is
necessary in the centralized model to disambiguiate NAT sessions.
However, there is an additional requirement of DNS64 [dns64]
functionality for IPv6 - IPv4 translation. This DNS64 functionality
must ensure that the synthesized AAAA record correctly maps to the
IPv6 - IPv4 translator.
The 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 roll-out
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 network can be deployed with configured tunneling between the
network nodes with IPv4-only transport. 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 provider's network is "IPv6-ready", which
means the 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,
a 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 prominant 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
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expect their clients to supply IPv4 addresses as literals, and this
will not be possible with IPv6-only deployments. These experiences
and the related considerations in deploying IPv6-only network are
documented in [arkko-v6]. In summary, application migration to IPv6
needs to be done even though it is anticipated to take a while before
all applications migrate to IPv6.
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 based on its
duration. Such a separation of signaling and the call is unique to
voice, whereas an Internet connection is simply accounted without
specifically considering application signaling and payload traffic,
e.g., based on overall byte usage. This model is expected to be
supported even during roaming. Furthermore, providers and users
generally require the ability to provide voice service regardless of
roaming whereas the Internet usage may be based on subscriber
preferences and roaming agreements. The requirement to always
support voice service while providing the flexibility to use the
Internet based on user's preference and roaming agreements
exacerbates the addressing problem, and may hasten provisioning of
VoLTE using the IPv6-only PDN.
An important consideration in mobile networks is roaming, where users
may roam to networks operated by providers other than their own. The
service providers can control their own network design but not their
peer's networks which they rely on for roaming. Perhaps more
importantly, the 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 which 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 the roamed
network support. These considerations apply even 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 services using IPv6 determine
whether fallback to IPv4 would be necessary. Similarly, there are
inbound roamers to an IPv6-ready provider network whose MN's are not
capable of IPv6. The IPv6-ready provider network has to be able to
support IPv4 PDN connectivity for such inbound roamers as well.
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.
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In summary, IPv6-only deployments should be encouraged along-side 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 IPv6-only
PDP/PDN). 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 [xlate-stateful] and [xlate-stateless]
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. This means IPv6-only mobile
network deployments need to support IPv4 connectivity (and NAT44) for
their own users who roam into peer provider networks, and also for
inbound roaming users with no IPv6 capability. 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. And, an All-IP mobile network service
provider is required to provide voice service as well, whereas a
fixed network provider is not required to. 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 the access networks. This implies 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. Tunneling of
private IPv4 packets within IPv6, for example, is feasible in fixed
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networks where the end-point is often a cable or DSL modem. This is
not the case in mobile networks where the end-point is a MN itself.
Similarly, encapsulation-based mechanisms such as 6rd [RFC5969] are
feasible where a residential gateway can become a tunnel end-point
for connecting subscribers to IPv6-only networks, whereas translation
is perhaps necessary for IPv6-only mobile networks. 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. However, some
sharing between 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. In addition, 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 it is subject to NAT policy
administration, which could include periodic clean-up and purge of
NAT state. Such policies affect the usability of the Femto Network,
and has implications to the mobile network provider. Using IPv6 for
the Femto (or any other access technology), on the other hand, could
alleviate some of these concerns if the IPv6 communication could
bypass the NAT.
In summary, there is clear interest in fixed-mobile convergence at
least among some providers. While there are benefits from
harmonizing the network as much as possible, there are also
idiosyncrasies of disparate access networks which influence the
convergence. Perhaps greater harmonization is feasible at the higher
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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 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. Specifically, we discussed:
o IPv4 address exhaustion and its implications to mobile networks:
we saw that, as the mobile networks begin to deploy IPv6,
conserving the available IPv4 pool and delaying its exhaustion
implies the need for network translation in mobile networks. At
the same time, providers can make use of the 3GPP architecture
constructs such as the 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: we
discussed the centralized and distributed models of private IPv4
address pool management and their relative merits. We saw that by
enabling each MNG to manage its own NET10 pool, the distributed
model achieves re-use of 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, the existing deployments which 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 re-use, 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: we saw that this is
feasible in the LTE architecture for an operator's own services
and applications. We observed that the existing MNs still expect
IPv4 address assignment. And, roaming, which is unique to mobile
networks, requires that a provider support IPv4 connectivity when
their (outbound) users roam into a mobile network that is not
IPv6-enabled. Similarly, a provider needs to support IPv4
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connectivity for (inbound) users whose MNs are not IPv6-capable.
And, we also observed that IPv6 - IPv4 interworking is necessary
for IPv6-only MNs to access IPv4 Internet.
o Fixed-Mobile Convergence: we discussed the examples illustrating
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
best-suited for converged network architecture. Perhaps even
greater gains are feasible in the service and application layers.
5. Security Considerations
This document does not introduce any new security vulnerabilities.
6. IANA Considerations
This document does not require any actions from IANA.
7. Acknowledgement
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. Mohamed Boucadair provided an extensive review of individual
draft version 01 of this document; many thanks Mohamed. Cameron
Byrne and Kent Leung provided thorough reviews of v01, which have
helped improve this document. Thanks to Nick Heatley for providing
valuable review and input on VoLTE.
8. 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/Misc/
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X.P0057-0_v0.13_E-UTRAN-
eHRPD_Interworking_VV_Due_5_December-2008.pdf.
[RFC1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
E. Lear, "Address Allocation for Private Internets",
BCP 5, RFC 1918, February 1996.
[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.
[arkko-v6]
Arkko, J. and A. Keranen, "Experiences from an IPv6-Only
Network", draft-arkko-ipv6-only-experience-01, Jul 2010.
[dns64] Bagnulo, M., Sullivan, A., Matthews, P., and I. van
Beijnum, "DNS64: DNS extensions for Network Address
Translation from IPv6 Clients to IPv4 Servers",
draft-ietf-behave-dns64-11, Mar 2010.
[gi-ds-lite]
Brockners, F. and S. Gundavelli, "Gateway Initiated Dual-
stack Lite Deployment",
draft-gundavelli-softwire-gateway-init-ds-lite-01.txt,
Oct 2009.
[xlate-stateful]
Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
NAT64: Network Address and Protocol Translation from IPv6
Clients to IPv4 Servers",
draft-ietf-behave-v6v4-xlate-stateful-11, Mar 2010.
[xlate-stateless]
Li, X., Bao, C., and F. Baker, "IP/ICMP Translation
Algorithm", draft-ietf-behave-v6v4-xlate-20, May 2010.
Appendix A. Change Log
Revisions (from draft-koodli-**), descending chronological order
o: FMC, Femto Networks text
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o: Dedicated NAT device model (in addition to the centralized and
distributed models)
o: IPv6-only deployment considerations: - IPv4 literals discussion
and reference, - IPv6 prefix assignment clarification, - DNS64
requirement and reference
o: Overall revisions based on comments from reviews (C. Byrne, K.
Leung)
o: Dual-stack being the recommended model, while encouraging IPv6-
only deployments.
o: Clarifications on on-demand IPv4 PDN usage, DHCP usage and on-
demand IPv4 assignment.
o: Clarifications regarding IPv6-only deployment: Roaming and
Applications considerations.
Author's Address
Rajeev Koodli
Cisco Systems
USA
Email: rkoodli@cisco.com
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