v6ops Working Group Rajeev. Koodli
Internet-Draft Cisco Systems
Intended status: Informational July 12, 2010
Expires: January 13, 2011
Mobile Networks Considerations for IPv6 Deployment
draft-ietf-v6ops-v6-in-mobile-networks-01.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 January 13, 2011.
Copyright Notice
Copyright (c) 2010 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 . . . . . . . . . . . 9
3.4. Fixed - Mobile Convergence . . . . . . . . . . . . . . . . 12
4. Summary and Conclusion . . . . . . . . . . . . . . . . . . . . 13
5. Security Considerations . . . . . . . . . . . . . . . . . . . 14
6. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 14
7. Informative References . . . . . . . . . . . . . . . . . . . . 15
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 16
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 16
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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.
When deploying IPv6 in mobile networks, certain unique challenges
arise. 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;
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-end) 'link' to the gateway, which is realized through a
Packet Data Network (PDN) connection in the Long-term Evolution (LTE)
networks, and through a Packet Data Protocol Context/connection in 3G
UMTS networks. The different nodes in the figure are defined below:
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.
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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.
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 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' towards the end of 2011.
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 hightened 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
private IPv4 addressing for end-users, as well as extending an IPv4
address (with the use of extended port ranges). Mechanisms such as a
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Network Address Translator (NAT) and "A+P" are 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).
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. As mentioned earlier, a PDN can be understood to be
the end-end link 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. 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 approach 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 an application needs it.
However, this requires support for a dynamic configuration protocol
such as DHCP, which many cellular MNs do not support today; this may
very well change in the future releases. 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
application usage, as well as DHCP lease management based on
appropriate address management policies. Both the approaches have
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merits, and additional considerations, including the "keep alives"
that MNs may potentially send to keep their IPv4 addresses active.
On the other hand, in the existing 3G UMTS networks, there is no
requirement for an always-on connection (a 'link' from the MN to the
MNG in 3G UMTS is referred to as a Packet Data Protocol (PDP)
context/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. 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
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.
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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. 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 did not exceed 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
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, 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
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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.
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. They also readily enable subscriber awareness
since the (unique) IPv4 address management is performed by the MNG,
which already has well-defined interfaces to AAA, and PCRF. 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.
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-end access. NAT should be viewed as an interim solution
until IPv6 is widely available, i.e., IPv6 is available for mobile
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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
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. The other
intermediate co-ordinate 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 re-use, which means no
protocol is necessary in the "centralized model" to disambiguiate NAT
sessions. However, an IPv6 prefix is managed and assigned by the MNG
(unlike in the "centralized" NAT44 model where address pool
management is common for all the gateways). Hence, the subscriber
management functions for the IPv6 prefix are vastly simplified.
Furthermore, for NAT binding correlation, billing and accounting, as
well as for subscriber management, it may be beneficial to locate the
IPv6 - IPv4 interworking function at the MNG.
The IPv6-only deployments in mobile networks need to recknon 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
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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. Furthermore, some IPv4-only
applications may be able to make use of alternative access such as
WiFi when available. In summary, application migration to IPv6 needs
to be done even though it is likely to take a while before all
applications migrate to IPv6.
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. 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.
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
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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
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 [6rd] are
feasible where a residential gateway can become a tunnel end-point
for connecting subscribers to IPv6-only networks, whereas translation
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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.
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.
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
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.
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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
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. Acknowledgement
This document has benefitted from discussions with and reviews from
Cameron Byrne, David Crowe, Hui Deng, Remi Despres, Fredrik Garneij,
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Jouni Korhonen, Teemu Savolainen and Dan Wing. Thanks to all of
them. Mohamed Boucadair provided an extensive review of version 01;
many thanks Mohamed.
7. Informative References
[3gpp.3g] "General Packet Radio Service (GPRS); Service description;
Stage 2, 3GPP TS 23.060, December 2006", .
[3gpp.4g] "General Packet Radio Service (GPRS);enhancements for
Evolved Universal Terrestrial Radio Access Network
(E-UTRAN) access", 3GPP TS 23.401 8.8.0, December 2009.",
.
[3gpp2.ehrpd]
"E-UTRAN - eHRPD Connectivity and Interworking: Core
Network Aspects", http://www.3gpp2.org/Public_html/Misc/
X.P0057-0_v0.13_E-UTRAN-
eHRPD_Interworking_VV_Due_5_December-2008.pdf.
[6rd] Townsley, M. and O. Troan, "IPv6 via IPv4 Service Provider
Networks "6rd"", draft-ietf-softwire-ipv6-6rd-04.txt,
Feb 2010.
[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.
[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.
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Appendix A. Change Log
From v00 to v01, the following changes were incorporated:
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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