v6ops Working Group Rajeev. Koodli
Internet-Draft Cisco Systems
Intended status: Informational May 19, 2011
Expires: November 20, 2011
Mobile Networks Considerations for IPv6 Deployment
draft-ietf-v6ops-v6-in-mobile-networks-05.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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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on November 20, 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
Provisions Relating to IETF Documents
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to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
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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 . . . . . . . . . . . . . . . . . . . . . 18
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 18
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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 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 the user is roaming outside of
the Home Network. 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. Each APN can specify what type of IP connectivity
(i.e., IPv4, IPv6, IPv4v6) is enabled on that particular APN.
While an APN directs a 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 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.
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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 the figure, 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 the 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 or 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 the 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.
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Delaying the 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 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 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 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. 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 dyncamic
configuration protocol such as DHCP as well as appropriate triggers
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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 which 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 the 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, the 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 as 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 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 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.
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 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, 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 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
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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] which 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 protocol 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 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. The
centralized models with a common NAT have the advantages of
continuing their legacy deployments and the re-use 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 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. On the other hand, those providers
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 presence of NAT
functionality in the network brings multiple issues which 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 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 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,
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
expect their clients to supply IPv4 addresses as literals, and this
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will not be possible with IPv6-only deployments. Some of these
experiences and the related considerations in deploying 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, the providers and the users generally require the voice
service regardless of roaming whereas the 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. The service providers can control their
own network design but not their peer's networks which they rely on
for roaming. 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 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
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.
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
which can be accounted for.
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
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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
[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. 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.
For instance:
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o Tunneling of private IPv4 packets within IPv6 is feasible in
fixed 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.
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 end-
point 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 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 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
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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 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 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. We summarize the discussion in
the following:
o IPv4 address exhaustion and its implications to mobile networks:
As the 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 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: 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 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.
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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. 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 connectivity
for (inbound) users whose MNs are not IPv6-capable. The IPv6 -
IPv4 interworking is necessary for IPv6-only MNs to access 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. 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,
Kent Leung, Kathleen Moriarty and Jari Arkko provided reviews 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", .
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[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/
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.
[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.
[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 et al., F., "Gateway Initiated Dual-stack Lite
Deployment", draft-ietf-softwire-gateway-init-ds-lite,
Oct 2009.
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[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: More IESG reviews
o: Addressed IESG reviews
o: VoLTE related text
o: FMC, Femto Networks text
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.
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Author's Address
Rajeev Koodli
Cisco Systems
USA
Email: rkoodli@cisco.com
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