Individual Submission J. Korhonen, Ed.
Internet-Draft J. Soininen
Intended status: Informational Nokia Siemens Networks
Expires: November 28, 2010 B. Patil
T. Savolainen
G. Bajko
K. Iisakkila
Nokia
May 27, 2010
IPv6 in 3GPP Evolved Packet System
draft-korhonen-v6ops-3gpp-eps-02
Abstract
The increased use of data services, growth of subscribers in 3GPP
based mobile networks, and the impending exhaustion of available IPv4
addresses from the registries is driving the need to specify the
transition to IPv6 solutions in 3GPP network architectures. This
document describes the support for IPv6 in 3GPP network architectures
and a solution to transition to IPv6 using a dual-stack approach.
Status of this Memo
This Internet-Draft is submitted to IETF 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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This Internet-Draft will expire on November 28, 2010.
Copyright Notice
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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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described in the BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. 3GPP Terminology and Concepts . . . . . . . . . . . . . . . . 4
2.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. The concept of APN . . . . . . . . . . . . . . . . . . . . 6
3. IP over 3GPP GPRS . . . . . . . . . . . . . . . . . . . . . . 7
3.1. Introduction to 3GPP GPRS . . . . . . . . . . . . . . . . 7
3.2. PDP Context . . . . . . . . . . . . . . . . . . . . . . . 9
4. IP over 3GPP EPS . . . . . . . . . . . . . . . . . . . . . . . 10
4.1. Introduction to 3GPP EPS . . . . . . . . . . . . . . . . . 10
4.2. PDN Connection . . . . . . . . . . . . . . . . . . . . . . 11
4.3. EPS bearer model . . . . . . . . . . . . . . . . . . . . . 11
5. Address Management . . . . . . . . . . . . . . . . . . . . . . 12
5.1. IPv4 Address Configuration . . . . . . . . . . . . . . . . 12
5.2. IPv6 Address Configuration . . . . . . . . . . . . . . . . 12
5.3. Prefix Delegation . . . . . . . . . . . . . . . . . . . . 13
6. 3GPP Dual-Stack Approach to IPv6 . . . . . . . . . . . . . . . 13
6.1. 3GPP Networks Prior to Release-8 . . . . . . . . . . . . . 13
6.2. 3GPP Release-8 and -9 Networks . . . . . . . . . . . . . . 14
6.3. PDN Connection Establishment Process . . . . . . . . . . . 15
6.4. Mobility of 3GPP IPv4v6 Type of Bearers . . . . . . . . . 18
7. Dual-Stack Approach to IPv6 Transition in 3GPP Networks . . . 18
8. Deployment issues . . . . . . . . . . . . . . . . . . . . . . 19
8.1. Overlapping IPv4 Addresses . . . . . . . . . . . . . . . . 19
8.2. IPv6 for transport . . . . . . . . . . . . . . . . . . . . 20
8.3. Operational Aspects of Running Dual-Stack Networks . . . . 21
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
10. Security Considerations . . . . . . . . . . . . . . . . . . . 21
11. Summary and Conclusion . . . . . . . . . . . . . . . . . . . . 22
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 22
13. Informative References . . . . . . . . . . . . . . . . . . . . 22
Appendix A. Transition scenarios discussed in 3GPP . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 25
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1. Introduction
IPv6 has been specified in the 3rd Generation Partnership Project
(3GPP) standards since the early architectures developed for R99
General Packet Radio Service (GPRS). However, the support for IPv6
in commercially deployed networks is nearly non-existent. There are
many factors that can be attributed to the lack of IPv6 deployment in
3GPP networks. The most relevant one is essentially the same as the
reason for IPv6 not being deployed by other networks as well, i.e.
the lack of business and commercial incentives for deployment. 3GPP
network architectures have also evolved since 1999 (since R99). The
most recent version of the 3GPP architecture, the Evolved Packet
System (EPS), which is commonly referred as SAE, LTE or Release-8, is
a packet centric architecture. The number of subscribers and devices
that are using the 3GPP networks for Internet connectivity and data
services has also increased significantly. With the subscriber
growth numbers projected to increase even further and the IPv4
addresses depletion problem looming in the near term, 3GPP operators
and vendors have started the process of identifying the scenarios and
solutions needed to transition to IPv6.
This document describes the establishment of IP connectivity in 3GPP
network architectures, specifically in the context of IP bearers for
3GPP GPRS and for 3GPP EPS. It provides an overview of how IPv6 is
supported as per the current set of 3GPP specifications. A solution
to transitioning to IPv6 based on a dual-stack technology is
described as well as some of the issues and concerns with respect to
deployment and shortage of private IPv4 addresses within a single
network domain.
The IETF has specified a set of tools and mechanisms that can be
utilized for transitioning to IPv6. In addition to the dual-stack
technology, the two alternative categories for the transition are
encapsulation and translation. Most of the mechanisms available in
the toolbox can be categorized as belonging to either one of these.
The IETF continues to specify additional solutions for enabling the
transition based on the deployment scenarios and operator/ISP
requirements. The 3GPP scenarios for transition, described in
Appendix A here, can be addressed using transition mechanisms that
are already available in the toolbox. The objective of transition to
IPv6 in 3GPP networks is to ensure that:
1. Legacy devices and hosts which have an IPv4 only stack will
continue to be provided with IP connectivity to the Internet and
services,
2. Devices which are dual-stack can access the Internet either via
IPv6 or IPv4. The choice of using IPv6 or IPv4 depends on the
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capability of:
A. the application on the host,
B. the support for IPv4 and IPv6 bearers by the network and/or,
C. the capability of the server(s) and other end points.
3GPP networks are capable of providing a host with IPv4 and IPv6
connectivity today, albeit in many cases with upgrades to network
elements such as the SGSN and GGSN.
2. 3GPP Terminology and Concepts
2.1. Terminology
Access Point Name
Access Point Name (APN) is a fully qualified domain name and
resolves to a specific gateway in an operators network. The APNs
are piggybacked on the administration of the DNS namespace.
Packet Data Protocol Context
A Packet Data Protocol (PDP) Context is the equivalent of a
virtual connection between the host and a gateway.
General Packet Radio Service
General Packet Radio Service (GPRS) is a packet oriented mobile
data service available to users of the 2G and 3G cellular
communication systems Global System for Mobile communications
(GSM), and specified by 3GPP.
Packet Data Network
Packet Data Network (PDN) is a packet based network that either
belongs to the operator or is an external network such as Internet
and corporate intranet. The user eventually accesses services in
one or more PDNs. The operator's packet domain network are
separated from packet data networks either by GGSNs or PDN
Gateways (PDN-GW).
Gateway GPRS Support Node
Gateway GPRS Support Node (GGSN) is a gateway function in GPRS,
which provides connectivity to Internet or other PDNs. The host
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attaches to a GGSN identified by an APN assigned to it by an
operator. The GGSN also serves as the topological anchor for
addresses/prefixes assigned to the mobile host.
Packet Data Network Gateway
Packet Data Network Gateway (PDN-GW) is a gateway function in
Evolved Packet System (EPS), which provides connectivity to
Internet or other PDNs. The host attaches to a PDN-GW identified
by an APN assigned to it by an operator. The PDN-GW also serves
as the topological anchor for addresses/prefixes assigned to the
mobile host.
Serving Gateway
Serving Gateway (SGW) is a gateway function in EPS, which
terminates the interface towards E-UTRAN. The SGW is the Mobility
Anchor point for layer-2 mobility (inter-eNodeB handovers). For
each User Equipment connected with the EPS, at any given point of
time, there is only one SGW. The SGW is essentially the user
plane part of the GPRS' SGSN forwarding packets between a PDN-GW.
Serving Gateway Support Node
Serving Gateway Support Node (SGSN) is a network element that is
located between the radio access network (RAN) and the gateway
(GGSN). A per mobile host point to point (p2p) tunnel between the
GGSN and SGSN transports the packets between the mobile host and
the gateway.
GPRS tunnelling protocol
GPRS Tunnelling Protocol (GTP) [3GPP.29.060] is a tunnelling
protocol defined by 3GPP. It is a network based mobility protocol
and similar to Proxy Mobile IPv6 (PMIPv6) [RFC5213]. However, GTP
also provides functionality beyond mobility such as inband
signaling related to Quality of Service (QoS) and charging among
others.
Evolved Packet System
Evolved Packet System (EPS) is an evolution of the 3G GPRS system
characterized by higher-data-rate, lower-latency, packet-optimized
system that supports multiple Radio Access Technologies (RAT).
The EPS comprises the Evolved Packet Core (EPC) together with the
evolved radio access network (E-UTRA and E-UTRAN).
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Mobility Management Entity
Mobility Management Entity (MME) is a network element that is
responsible for control plane functionalities, including
authentication, authorization, bearer management, layer-2
mobility, etc. The MME is essentially the control plane part of
the GPRS' SGSN and not located on the user plane data path, i.e.
user plane traffic bypasses the MME.
UMTS Terrestrial Radio Access Network
UMTS Terrestrial Radio Access Network (UTRAN) is communications
network, commonly referred to as 3G, and consists of NodeBs (3G
base station) and Radio Network Controllers (RNC) which make up
the UMTS radio access network. The UTRAN allows connectivity
between the mobile host/device and the core network.
Evolved UTRAN
Evolved UTRAN (E-UTRAN) is communications network, sometimes
referred to as 4G, and consists of eNodeBs (4G base station) which
make up the E-UTRAN radio access network. The E-UTRAN allows
connectivity between the mobile host/device and the core network.
GSM EDGE Radio Access Network
GSM EDGE Radio Access Network (GERAN) is communications network,
commonly referred to as 2G or 2.5G, and consists of base stations
and Base Station Controllers (BSC) which make up the GSM EDGE
radio access network. The GERAN allows connectivity between the
mobile host/device and the core network.
UE, MS, MN and Mobile
The terms UE (User Equipment), MS (Mobile Station), MN (Mobile
Node) and, mobile refer to the devices which are hosts with
ability to obtain Internet connectivity via a 3GPP network. The
terms UE, MS, MN and devices are used interchangeably within this
document.
2.2. The concept of APN
The Access Point Name (APN) essentially refers to a gateway in the
3GPP network. The 'complete' APN is expressed in a form of a Fully
Qualified Domain Name (FQDN) and also piggybacked on the
administration of the DNS namespace, thus effectively allowing the
discovery of gateways using the DNS. Mobile hosts/devices can choose
to attach to a specific gateway in the packet core. The gateway
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provides connectivity to the Packet Data Network (PDN) such as the
Internet. An operator may also include gateways which do not provide
Internet connectivity, rather a connectivity to closed network
providing a set of operator's own services. A mobile host/device can
be attached to one or more gateways simultaneously. The gateway in a
3GPP network is the GGSN or PDN-GW. Figure 1 below illustrates the
APN-based network connectivity concept.
.--.
_(. `)
.--. +------------+ _( PDN `)_
_(Core`. |GW1 |====( Internet `)
+---+ ( NW )------|APN=internet| ( ` . ) )
[MN]~~~~|RAN|----( ` . ) )--+ +------------+ `--(_______)---'
^ +---+ `--(___.-' |
| | .--.
| | +----------+ _(.PDN`)
| +--|GW2 | _(Operator`)_
| |APN=OpServ|====( Services `)
MN is attached +----------+ ( ` . ) )
to GW1 and GW2 `--(_______)---'
simultaneously
Figure 1: Mobile host/device attached to multiple APNs simultaneously
3. IP over 3GPP GPRS
3.1. Introduction to 3GPP GPRS
A simplified 2G/3G GPRS architecture is illustrated in Figure 2.
This architecture basically covers the GPRS core network since R99 to
Release-7, and radio access technologies such as GSM (2G), EDGE (2G),
WCDMA (3G) and HSPA (3G). The architecture shares obvious
similarities with the Evolved Packet System (EPS) as will be seen in
Section 4. Based on Gn/Gp interfaces, the GPRS core network
functionality is logically implemented on two network nodes, the SGSN
and the GGSN.
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3G .--.
Uu +-----+ Iu +----+ +----+ _( `.
[TE]+[MT]~~|~~~|UTRAN|--|---|SGSN|--|---|GGSN|--|----( PDN )
+-----+ +----+ Gn +----+ Gi ( ` . ) )
/ | `--(___.-'
2G Gb-- |
+---+ / --Gp
[TE]+[MT]~~|~~~|BSS|___/ |
Um +---+ .--.
_(. `)
_( [GGSN] `)_
( other `)
( ` . PLMN ) )
`--(_______)---'
Figure 2: Overview of the 2G/3G GPRS Logical Architecture
Gn/Gp: These interfaces provide a network based mobility service for
a mobile host and are used between a SGSN and a GGSN. The Gn
interface is used when GGSN and SGSN are located inside one
operator (i.e. PLMN). The Gp-interface is used if the GGSN
and the SGSN are located in different operator domains (i.e.
'other' PLMN). GTP protocol is defined for the Gn/Gp
interfaces (both GTP-C for the control plane and GTP-U for
the user plane).
Gb: Is the Base Station System (BSS) to SGSN interface, which is
used to carry information concerning packet data transmission
and layer-2 mobility management. The Gb-interface is based
on either on Frame Relay or IP.
Iu: Is the Radio Network System (RNS) to SGSN interface, which is
used to carry information concerning packet data transmission
and layer-2 mobility management. The user plane part of the
Iu-interface (actually the Iu-PS) is based on GTP-U. The
control plane part of the Iu-interface is based on Radio
Access Network Application Protocol (RANAP).
Gi: It is the interface between the GGSN and a PDN. The PDN may
be an operator external public or private packet data network
or an intra-operator packet data network.
Uu/Um: Are either 2G or 3G radio interfaces between a mobile
terminal and a respective radio access network.
The SGSN is responsible for the delivery of data packets from and to
the mobile hosts within its geographical service area when a direct
tunnel option is not used. If the direct tunnel is used, then the
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user plane goes directly between the RNS and the GGSN. The control
plane traffic always goes through the SGSN. For each mobile host
connected with the GPRS, at any given point of time, there is only
one SGSN.
3.2. PDP Context
A PDP context is an association between a mobile host represented by
one IPv4 address and/or one /64 IPv6 prefix and a PDN represented by
an APN. Each PDN can be accessed via a gateway (typically a GGSN or
PDN-GW). On the device/mobile host a PDP context is equivalent to a
virtual interface/connection. A host may hence be attached to one or
more gateways via separate virtual interfaces/connections, i.e. PDP
contexts. Each primary PDP context has its own IPv4 address and/or
one /64 IPv6 prefix assigned to it by the PDN and anchored in the
corresponding gateway. Applications on the host use the appropriate
PDP context (virtual interface) for connectivity to a specific PDN.
Figure 3 represents a high level view of what a PDP context implies
in 3GPP networks.
Y
| +---------+ .--.
|--+ __________________________ | APNx in | _( `.
| |O__________________________)| GGSN / |----(Internet)
|MS| | PDN-GW | ( ` . ) )
|/ | +---------+ `--(___.-'
|UE| _______________________ +---------+ .--.
| |O_______________________)| APNy in | _(Priv`.
+--+ | GGSN / |-------(Network )
| PDN-GW | ( ` . ) )
+---------+ `--(___.-'
Figure 3: PDP contexts between the MS/UE and gateway
In the above figure there are two PDP contexts at the MS/UE (UE=User
Equipment in 3GPP parlance). The PDP context that is connected to
APNx provided Internet connectivity and the other PDP context
provides connectivity to a private IP network (as an example this
network may include operator specific services such as MMS (Multi
media service). An application on the host such as a web browser
would use the PDP context that provides Internet connectivity for
accessing services on the Internet. An application such as MMS would
use APNy in the figure above because the service is provided through
the private network.
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4. IP over 3GPP EPS
4.1. Introduction to 3GPP EPS
In its most basic form, the EPS architecture consists of only two
nodes on the user plane, a base station and a core network Gateway
(GW). The basic EPS architecture is illustrated in Figure 4. The
Mobility Management Entity (MME) node performs control-plane
functionality and is separated from the node(s) that performs bearer-
plane functionality (GW), with a well-defined open interface between
them (S11). The optional interface S5 can be used to split the
Gateway (GW) into two separate nodes, the Serving Gateway (SGW) and
the PDN-GW. This allows independent scaling and growth of traffic
throughput and control signal processing. The functional split of
gateways also allows operators to choose optimized topological
locations of nodes within the network in order to optimize the
network in different aspects.
+--------+
S1-MME +-------+ S11 | IP |
+----|----| MME |---|----+ |Services|
| | | | +--------+
| +-------+ | |SGi
+----+ LTE-Uu +-------+ S1-U +-------+ S5 +-------+
|MN |----|---|eNodeB |---|----------------| SGW |--|---|PDN-GW |
| |========|=======|====================|=======|======| |
+----+ +-------+DualStack EPS Bearer+-------+ +-------+
Figure 4: EPS Architecture for 3GPP Access
S5: It provides user plane tunnelling and tunnel management
between SGW and PDN-GW, using GTP or PMIPv6 as the network
based mobility management protocol.
S1-U: Provides user plane tunnelling and inter eNodeB path
switching during handover between eNodeB and SGW, using the
GTP-U protocol (GTP user plane).
S1-MME: Reference point for the control plane protocol between
eNodeB and MME.
SGi: It is the interface between the PDN-GW and the packet data
network. Packet data network may be an operator external
public or private packet data network or an intra operator
packet data network.
The eNodeB is a base station entity that supports the Long Term
Evolution (LTE) air interface and includes functions for radio
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resource control, user plane ciphering, and other lower layer
functions. MME is responsible for control plane functionalities,
including authentication, authorization, bearer management, layer-2
mobility, etc.
The SGW is the Mobility Anchor point for layer-2 mobility. For each
MN connected with the EPS, at any given point of time, there is only
one SGW.
4.2. PDN Connection
A PDN connection is an association between a mobile host represented
by one IPv4 address and/or one /64 IPv6 prefix, and a PDN represented
by an APN. Each PDN can be accessed via a gateway (a PDN-GW). PDN
is responsible for the IP address/prefix allocation to the mobile
host. On the device/mobile host a PDN connection is equivalent to a
virtual interface/connection. A host may hence be attached to one or
more gateways via separate virtual interfaces/connections, i.e. PDN
connection. Each PDP connection has its own IP address/prefix
assigned to it by the PDN and anchored in the corresponding gateway.
Applications on the host use the appropriate PDN connection (virtual
interface) for connectivity. The PDN connection is the EPC
equivalent of the GPRS PDP context.
4.3. EPS bearer model
The logical concept of a bearer has been defined to be an aggregate
of one or more IP flows related to one or more services. An EPS
bearer exists between the Mobile Node (MN i.e. a mobile host) and the
PDN-GW and is used to provide the same level of packet forwarding
treatment to the aggregated IP flows constituting the bearer.
Services with IP flows requiring a different packet forwarding
treatment would therefore require more than one EPS bearer. The
mobile host performs the binding of the uplink IP flows to the bearer
while the PDN-GW performs this function for the downlink packets.
In order to provide low latency for always on connectivity, a default
bearer will be provided at the time of startup and an IPv4 address
and/or IPv6 prefix gets assigned to the mobile host (this is
different from GPRS, where mobile hosts are not automatically
assigned with an IP address or prefix). This default bearer will be
allowed to carry all traffic which is not associated with a dedicated
bearer. Dedicated bearers are used to carry traffic for IP flows
that have been identified to require a specific packet forwarding
treatment. They may be established at the time of startup; for
example, in the case of services that require always-on connectivity
and better QoS than that provided by the default bearer. The default
bearer and the dedicated bearer(s) associated to it share the same IP
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address(es)/prefix.
An EPS bearer is referred to as a GBR bearer if dedicated network
resources related to a Guaranteed Bit Rate (GBR) value that is
associated with the EPS bearer are permanently allocated (e.g. by an
admission control function in the eNodeB) at bearer establishment/
modification. Otherwise, an EPS bearer is referred to as a non-GBR
bearer. The default bearer is always non-GBR, with the resources for
the IP flows not guaranteed at eNodeB, and with no admission control.
However, the dedicated bearer can be either GBR or non-GBR. A GBR
bearer has a Guaranteed Bit Rate (GBR) and Maximum Bit Rate (MBR)
while more than one non-GBR bearer belonging to the same UE shares an
Aggregate Maximum Bit Rate (AMBR). Non-GBR bearers can suffer packet
loss under congestion while GBR bearers are immune to such losses.
5. Address Management
5.1. IPv4 Address Configuration
Mobile host's IPv4 address configuration is essentially always
conducted during PDP context/EPS bearer setup procedures (on
layer-2). DHCPv4-based [RFC2131] address configuration is supported
by the 3GPP specifications, but is not used in wide scale. The
mobile host must always support layer-2 based address configuration,
since DHCPv4 is optional for both mobile hosts and networks.
5.2. IPv6 Address Configuration
IPv6 Stateless Address Autoconfiguration (SLAAC) is the only
supported address configuration mechanisms [RFC4862]. Stateful
DHCPv6-based address configuration is not supported by 3GPP
specifications [RFC3315]. On the other hand, Stateless DHCPv6-
service to obtain other configuration information is supported
[RFC3736]. This implies that the M-bit must always be set to zero
and the O-bit may be set to one in the Router Advertisement (RA) sent
to the UE.
3GPP network allocates each default bearer a unique /64 prefix, and
uses layer-2 signaling to suggest user equipment an Interface
Identifier that is guaranteed not to conflict with gateway's
Interface Identifier. The UE may configure link local address using
this Interface Identifier, but is allowed to use also other Interface
Identifiers and as many globally scoped addresses as it needs. There
is no restriction, for example, of using Privacy Extension for SLAAC
[RFC4941] or other similar types of mechamisms.
In the 3GPP link model the /64 prefix assigned to the UE is always
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off-link (i.e. the L-bit in the Prefix Information Option (PIO) in
the RA must be set to zero). If the advertised prefix is used for
SLAAC then the A-bit in the PIO must be set to one. The details of
the 3GPP link-model and address configuration is described in Section
11.2.1.3.2a of [3GPP.29.061].
The current 3GPP architecture limits number of prefixes in each
bearer to a single /64 prefix. Therefore, multi-homing within a
single bearer is not possible. Renumbering without closing layer-2
connection is also not possible. The lifetime of /64 prefix is bound
to lifetime of layer-2 connection even if the advertised prefix
lifetime would be longer than the layer-2 connection lifetime.
5.3. Prefix Delegation
Prefix delegation is not covered yet by 3GPP specifications as of
Release-9. The /64 prefix allocated for each default bearer may be
shared to local area network by user equipment implementing Neighbor
Discovery proxy (ND proxy) [RFC4389] functionality. Discussion is
ongoing in 3GPP to add the prefix delegation support, e.g. using the
DHCPv6-based prefix delegation [RFC3633], which is likely to be the
adopted solution as well.
6. 3GPP Dual-Stack Approach to IPv6
6.1. 3GPP Networks Prior to Release-8
3GPP standards prior to Release-8 provide IPv6 access for cellular
devices with PDP contexts of type IPv6 [3GPP.23.060]. For dual-stack
access, a PDP context of type IPv6 is established in parallel to the
PDP context of type IPv4, as shown in Figure 5 and Figure 6. For
IPv4-only service, connections are created over the PDP context of
type IPv4 and for IPv6-only service connections are created over the
PDP context of type IPv6. The two PDP contexts of different type may
use the same APN (and the gateway), however, this aspect is not
explicitly defined in standards. Therefore, cellular device and
gateway implementations from different vendors may have varying
support for this functionality.
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Y .--.
| _(IPv4`.
|---+ +---+ +---+ ( PDN )
| D |~~~~~~~//-----| |====| |====( ` . ) )
| S | IPv4 context | S | | G | `--(___.-'
| | | G | | G | .--.
| M | | S | | S | _(IPv6`.
| N | IPv6 context | N | | N | ( PDN )
|///|~~~~~~~//-----| |====|(s)|====( ` . ) )
+---+ +---+ +---+ `--(___.-'
Figure 5: A dual-stack mobile host connecting to both IPv4 and IPv6
Internet using parallel IPv4-only and IPv6-only PDP contexts
Y
|
|---+ +---+ +---+
| D |~~~~~~~//-----| |====| | .--.
| S | IPv4 context | S | | G | _( DS `.
| | | G | | G | ( PDN )
| M | | S | | S |====( ` . ) )
| N | IPv6 context | N | | N | `--(___.-'
|///|~~~~~~~//-----| |====| |
+---+ +---+ +---+
Figure 6: A dual-stack mobile host connecting to dual-stack Internet
using parallel IPv4-only and IPv6-only PDP contexts
The approach of having parallel IPv4 and IPv6 type of PDP contexts
open is not optimal, because two PDP contexts require double the
signaling and consume more network resources than a single PDP
context. However, these costs and complexities are lesser than what
other transition solutions would incur. In the figure above the IPv4
and IPv6 PDP contexts are attached to the same GGSN. While this is
possible, the DS MS may be attached to different GGSNs in the
scenario where one GGSN supports IPv4 PDN connectivity while another
GGSN provides IPv6 PDN connectivity.
6.2. 3GPP Release-8 and -9 Networks
Since 3GPP Release-8, the powerful concept of a dual-stack type of
PDN connection and EPS bearer have been introduced [3GPP.23.401].
This enables parallel use of both IPv4 and IPv6 on a single bearer
(IPv4v6), as illustrated in Figure 7, and makes dual stack simpler
than in earlier 3GPP releases. As of Release-9, GPRS network nodes
also support dual-stack type (IPv4v6) PDP contexts.
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Y
|
|---+ +---+ +---+
| D | | | | P | .--.
| S | | | | D | _( DS `.
| | IPv4v6 (DS) | S | | N | ( PDN )
| M |~~~~~~~//-----| G |====| - |====( ` . ) )
| N | bearer | W | | G | `--(___.-'
|///| | | | W |
+---+ +---+ +---+
Figure 7: A dual-stack mobile host connecting to dual-stack Internet
using a single IPv4v6 type PDN connection
The following is a description of the various PDP contexts/PDN bearer
types that are specified by 3GPP:
1. For 2G/3G access to GPRS core (SGSN/GGSN) pre-Release-9 there are
two IP PDP Types, IPv4 and IPv6. Two PDP contexts are needed to
get dual stack connectivity.
2. For 2G/3G access to GPRS core (SGSN/GGSN) from Release-9 there
are three IP PDP Types, IPv4, IPv6 and IPv4v6. Minimum one PDP
context is needed to get dual stack connectivity.
3. For 2G/3G access to EPC core (PDN-GW via S4 Release-8 SGSN) from
Release-8 there are three IP PDP Types, IPv4, IPv6 and IPv4v6
which gets mapped to PDN Connection type. Minimum one PDP
Context is needed to get dual stack connectivity.
4. For LTE (E-UTRAN) access to EPC core from Release-8 there are
three IP PDN Types, IPv4, IPv6 and IPv4v6. Minimum one PDN
Connection is needed to get dual stack connectivity.
6.3. PDN Connection Establishment Process
The PDN connection establishment process is specified in detail in
3GPP specifications. Figure 8 illustrates the high level process and
signaling involved in the establishment of a PDN connection.
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UE eNb/ MME SGW PDN-GW HSS/
| BS | | | AAA
| | | | | |
|---------->|(1) | | | |
| |---------->|(1) | | |
| | | | | |
|/---------------------------------------------------------\|
| Authentication and Authorization |(2)
|\---------------------------------------------------------/|
| | | | | |
| | |---------->|(3) | |
| | | |---------->|(3) |
| | | | | |
| | | |<----------|(4) |
| | |<----------|(4) | |
| |<----------|(5) | | |
|/---------\| | | | |
| RB setup |(6) | | | |
|\---------/| | | | |
| |---------->|(7) | | |
|---------->|(8) | | | |
| |---------->|(9) | | |
| | | | | |
|============= UL Data =============>==========>|(10) |
| | | | | |
| | |---------->|(11) | |
| | | | | |
| | |<----------|(12) | |
| | | | | |
|<============ DL Data =============<===========|(13) |
| | | | | |
Figure 8: Simplified PDN connection setup procedure in Release-8
1. The UE (i.e the MS) requires a data connection and hence decides
to establish a PDN connection with a PDN-GW. The UE sends an
"Attach Request" (layer-2) to the BS. The BS forwards this
attach request to the MME.
2. Authentication of the UE with the AAA server/HSS follows. If
the UE is authorized for establishing a data connection, the
following steps continue
3. The MME sends a "Create Session Request" message to the
Serving-GW. The SGW forwards the create session request to the
PDN-GW. The SGW knows the address of the PDN-GW to forward the
create session request to as a result of this information having
been obtained by the MME during the authentication/authorization
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phase.
The UE IPv4 address and/or IPv6 prefix get assigned during this
step. If a subscribed IPv4 address and/or IPv6 prefix is
statically allocated for the UE for this APN, then the MME
already passes the address information to the SGW and eventually
to the PDN-GW in the "Create Session Request" message.
Otherwise, the PDN-GW manages the address assignment to the UE
(there is another variation to this where IPv4 address
allocation is delayed until the UE initiates a DHCPv4 exchange
but this is not discussed here).
4. The PDN-GW creates a PDN connection for the UE and sends "Create
Session Response" message to the SGW from which the session
request message was received from. The SGW forwards the
response to the corresponding MME which originated the request.
5. The MME sends the "Attach Accept/Initial Context Setup request"
message to the eNodeB/BS.
6. The radio bearer between the UE and the eNb is reconfigured
based on the parameters received from the MME
7. The eNb sends "Initial Context Response" message to the MME.
8. The UE sends a "Direct Transfer" message to the eNodeB which
includes the Attach complete signal.
9. The eNodeB forwards the Attach complete message to the MME.
10. The UE can now start sending uplink packets to the PDN GW.
11. The MME sends a "Modify Bearer Request" message to the SGW.
12. The SGW responds with a "Modify Bearer Response" message. At
this time the downlink connection is also ready
13. The UE can now start receiving downlink packets
The type of PDN connection established between the UE and the PDN-GW
can be any of the types described in the previous section. The DS
PDN connection, i.e the one which supports both IPv4 and IPv6 packets
is the default one that will be established if no specific PDN
connection type is specified by the UE in Release-8 networks.
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6.4. Mobility of 3GPP IPv4v6 Type of Bearers
3GPP discussed at length various approaches to support mobility
between Release-8 and pre-Release-8 networks for the new dual-stack
type of bearers.
The chosen approach for mobility is as follows, in short: if a mobile
is known to be at risk for doing handovers between Release-8 and pre-
Release-8 networks, only single stack bearers are used. Essentially
meaning:
1. If a network knows a mobile may do handovers between Release-8
and pre-Release-8 networks (segment), network will only provide
single stack bearers, even if the mobile host requests dual-stack
bearers. This can happen e.g. if an operator is using pre-
Release-8 SGSNs in some parts of the network. The single stack
bearers of Release-8 are easy to map one-to-one to pre-Release-8
bearers.
2. If a network knows a mobile will not be able to do handover to
pre-Release-8 network (segment), it will provide mobile with
dual-stack bearers on request. This can happen e.g. if an
operator has upgraded their SGSNs to support dual-stack bearers,
or if an operator is running LTE-only network.
The operators should upgrade their, and also if possible roaming
partners', networks to Release-8 level in order to support new dual-
stack type of bearers. A Release-8 mobile device always requests for
a dual-stack bearer, but accepts what is assigned by the network.
7. Dual-Stack Approach to IPv6 Transition in 3GPP Networks
3GPP networks can natively transport IPv4 and IPv6 packets between
the mobile station/UE and the gateway (GGSN or PDN-GW) as a result of
establishing either a dual-stack PDP context or
parallel IPv4 and IPv6 PDP contexts.
Current deployments of 3GPP networks primarily support IPv4 only.
These networks can be upgraded to also support IPv6 PDP contexts. By
doing so devices and applications that are IPv6 capable can start
utilizing the IPv6 connectivity. This will also ensure that legacy
devices and applications continue to work with no impact. As newer
devices start using IPv6 connectivity, the number of IPv4 addresses
in use is expected to slowly decrease, providing operators with a
smooth transition to IPv6 With a dual-stack approach, there is always
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the potential to fallback to IPv4. A device which may be roaming in
a network wherein IPv6 is not supported by the visited network would
fall back to using IPv4 PDP contexts and hence the end user does not
see an interruption to the services.
As the networks evolve to support Release-8 EPS architecture and the
dual-stack PDP contexts, newer devices will be able to leverage such
capability and have a single bearer which supports both IPv4 and
IPv6. Since IPv4 and IPv6 packets are carried as payload within GTP
between the MS and the gateway (GGSN/PDN-GW) the transport network
capability in terms of whether it supports IPv4 or IPv6 on the
interfaces between the eNodeB and SGW or, SGW and PDN-GW is
immaterial.
The dual-stack approach enables a systematic migration path to IPv6.
From an operational standpoint operators are concerned about ensuring
that there is no disruption to the connectivity that subscribers rely
on. This can be achieved by upgrading the network to support IPv6
while continuing to maintain IPv4 legacy. Dual-stack capability in
the network and devices for the foreseeable future at least is a
pragmatic solution.
8. Deployment issues
8.1. Overlapping IPv4 Addresses
Given the shortage of globally routable public IPv4 addresses,
operators tend to assign private IPv4 addresses [RFC1918] to hosts
when they establish an IPv4 only PDP context or an IPv4v6 type PDN
context. About 16 million hosts can be assigned a private IPv4
address that is unique within a domain. However, in case of many
operators the number of subscribers is greater than 16 million. The
issue can be dealt with by assigning overlapping RFC 1918 IPv4
addresses to hosts. As a result the IPv4 address assigned to a host
within the context of a single operator realm would no longer be
unique. This has the obvious and know issues of NATed IP connection
in the Internet. Direct host to host connectivity becomes
complicated, unless the hosts are within the same private address
range pool and/or anchored to the same gateway, referrals using IP
addresses will have issues and so forth. However, these are generic
issues and not only a concern of the EPS. In general this is not
seen as a major issue in the EPS for the following reasons:
1. Very large network deployments are partitioned, for example,
based on a geographical areas. This partitioning allows
overlapping IPv4 addresses ranges to be assigned to hosts that
are in different areas. Each area has its own pool of gateways
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that are dedicated for a certain overlapping IPv4 address range
(referred here later as a zone). Standard NAT44 functionality
enables the communication between hosts that are assigned the
same IPv4 address but belong to different zones, yet are part of
the same operator domain.
2. A mobile host/device attaches to a gateway as part of the attach
process. The number of hosts that a gateway supports is in the
order of 1 to 10 million. Hence all the hosts assigned to a
single gateway can be assigned private IPv4 addresses. Operators
with large subscriber bases have multiple gateways and hence the
same [RFC1918] IPv4 address space can be reused across gateways.
The IPv4 address assigned to a host is unique within the scope of
a single gateway.
3. The IPv4 address assigned to a host could also be made irrelevant
from a routing perspective at least by the use of protocol
solutions such as GI-DSLite
[I-D.ietf-softwire-gateway-init-ds-lite]. This requires a Large
Scale NAT (LSN) entity that is detached from the gateway (GGSN or
PDN-GW). Multiple gateways in an operator domain would attach to
a LSN in such an approach and the hosts across these gateways can
be assigned overlapping IPv4 addresses.
4. New services requiring direct connectivity between hosts should
be build on IPv6. Possible existing IPv4-only services and
applications requiring direct connectivity can be ported to IPv6.
8.2. IPv6 for transport
The various reference points of the 3GPP architecture such as S1-U,
S5 and S8 are based on either GTP or PMIPv6. The underlying
transport for these reference points can be IPv4 or IPv6. GTP has
been able to operate over IPv6 transport (optionally) since R99 and
PMIPv6 has supported IPv6 transport starting from its introduction in
Release-8. The user plane traffic between the mobile host and the
gateway can use either IPv4 or IPv6. These packets are essentially
treated as payload by GTP/PMIPv6 and transported accordingly with no
real attention paid to the information (at least from a routing
perspective) contained in the IPv4 or IPv6 headers. The transport
links between the eNodeB and the SGW, and the link between the SGW
and PDN-GW can be migrated to IPv6 without any direct implications to
the architecture.
Currently, the inter-operator (for 3GPP technology) roaming networks
are all IPv4 only (see Inter-PLMN Backbone Guidelines [GSMA.IR.34]).
Eventually these roaming networks will also get migrated to IPv6, if
there is a business reason for that. The migration period can be
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prolonged considerably because the 3GPP protocols always tunnel user
plane traffic in the core network and as described earlier the
transport network IP version is not in any way tied to user plane IP
version. Furthermore, the design of the inter-operator roaming
networks is such that the user plane and transport network IP
addressing is completely separated from each other. The inter-
operator roaming network itself is also completely separated from the
Internet. Only those core network nodes that must be connected to
the inter-operator roaming networks are actually visible there, and
be able to send and receive (tunneled) traffic within the inter-
operator roaming networks. Obviously, in order the roaming to work
properly, the operators have to agree on supported protocol versions
so that the visited network does not, for example, unnecessarily drop
user plane IPv6 traffic.
8.3. Operational Aspects of Running Dual-Stack Networks
Operating dual-stack networks does imply cost and complexity to a
certain extent. However these factors are mitigated by the assurance
that legacy devices and services are unaffected and there is always a
fallback to IPv4 in case of issues with the IPv6 deployment or
network elements. The model also enables operators to develop
operational experience and expertise in an incremental manner.
Running dual-stack networks requires the management of multiple IP
address spaces. Tracking of hosts needs to be expanded since it can
be identified by either an IPv4 address or IPv6 prefix. Network
elements will also need to be dual-stack capable in order to support
the dual-stack deployment model.
Deployment and migration cases described in Section 6.1 for providing
dual-stack like capability may mean doubled resource usage in
operator's network. Also handovers between networks with different
capabilities in terms of networks being dual-stack like service
capable or not, may turn out hard to comprehend for users and for
application/services to cope with. These facts may add other than
just technical concerns for operators when planning to roll out dual-
stack service offerings.
9. IANA Considerations
This document has no requests to IANA.
10. Security Considerations
This document does not introduce any security related concerns.
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11. Summary and Conclusion
The 3GPP network architecture and specifications enable the
establishment of IPv4 and IPv6 connections through the use of
appropriate PDP context types. The current generation of deployed
networks can support dual-stack connectivity if the packet core
network elements such as the SGSN and GGSN have the capability. With
Release-8, 3GPP has specified a more optimal PDP context type which
enables the transport of IPv4 and IPv6 packets within a single PDP
context between the mobile station and the gateway.
The authors believe that transitioning to IPv6 in 3GPP networks can
be achieved without disruption to legacy devices, networks and
services only by taking a dual-stack approach to deployment. As
devices and applications are upgraded to support IPv6 they can start
leveraging the IPv6 connectivity provided by the networks while
maintaining the fallback to IPv4 capability. Enabling IPv6
connectivity in the 3GPP networks by itself will provide some degree
of relief to the IPv4 address space as many of the applications and
services can start to work over IPv6 right away. However without
comprehensive testing of different applications and solutions that
exist today and are widely used, for their ability to operate over
IPv6 PDN connections, an IPv6 only access would cause disruptions.
Hence we recommend adopting the dual-stack approach to IPv6
transition in 3GPP networks.
12. Acknowledgements
The authors thank Shabnam Sultana, Sri Gundavelli, Hui Deng, and
Zhenqiang Li for their reviews and comments on this document.
13. Informative References
[3GPP.23.060]
3GPP, "General Packet Radio Service (GPRS); Service
description; Stage 2", 3GPP TS 23.060 8.8.0, March 2010.
[3GPP.23.401]
3GPP, "General Packet Radio Service (GPRS) enhancements
for Evolved Universal Terrestrial Radio Access Network
(E-UTRAN) access", 3GPP TS 23.401 8.9.0, March 2010.
[3GPP.23.975]
3GPP, "IPv6 migration guidelines", 3GPP TR 23.975 1.0.0,
March 2010.
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[3GPP.29.060]
3GPP, "General Packet Radio Service (GPRS); GPRS
Tunnelling Protocol (GTP) across the Gn and Gp interface",
3GPP TS 29.060 8.11.0, April 2010.
[3GPP.29.061]
3GPP, "Interworking between the Public Land Mobile Network
(PLMN) supporting packet based services and Packet Data
Networks (PDN)", 3GPP TS 29.061 8.5.0, April 2010.
[GSMA.IR.34]
GSMA, "Inter-PLMN Backbone Guidelines", GSMA
PRD IR.34.4.9, March 2010.
[I-D.ietf-softwire-gateway-init-ds-lite]
Brockners, F., Gundavelli, S., Speicher, S., and D. Ward,
"Gateway Initiated Dual-Stack Lite Deployment",
draft-ietf-softwire-gateway-init-ds-lite-00 (work in
progress), May 2010.
[RFC1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
E. Lear, "Address Allocation for Private Internets",
BCP 5, RFC 1918, February 1996.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol",
RFC 2131, March 1997.
[RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
and M. Carney, "Dynamic Host Configuration Protocol for
IPv6 (DHCPv6)", RFC 3315, July 2003.
[RFC3633] Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic
Host Configuration Protocol (DHCP) version 6", RFC 3633,
December 2003.
[RFC3736] Droms, R., "Stateless Dynamic Host Configuration Protocol
(DHCP) Service for IPv6", RFC 3736, April 2004.
[RFC4389] Thaler, D., Talwar, M., and C. Patel, "Neighbor Discovery
Proxies (ND Proxy)", RFC 4389, April 2006.
[RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", RFC 4862, September 2007.
[RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy
Extensions for Stateless Address Autoconfiguration in
IPv6", RFC 4941, September 2007.
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[RFC5213] Gundavelli, S., Leung, K., Devarapalli, V., Chowdhury, K.,
and B. Patil, "Proxy Mobile IPv6", RFC 5213, August 2008.
Appendix A. Transition scenarios discussed in 3GPP
Several scenarios of IPv4 and IPv6 co-existence and transition have
been discussed and documented in 3GPP TR 23.975 [3GPP.23.975].
In scenario 1, a dual-stack connectivity scenario, the operator runs
the user plane in dual stack mode, i.e., the UEs are assigned both an
IPv6 prefix and an IPv4 address in order to allow UEs to utilise both
IPv4 and IPv6 capable applications.
It is assumed that the proportion of IPv6 capable applications will
start to increase as soon as UEs and networks starts to become dual-
stack capable. As popular services start to support IPv6, a part of
IPv4 traffic will gradually be offloaded into the IPv6 domain.
During transition phase, the depletion of public IPv4 addresses will
be an issue to be solved in some operators' networks. The shortage
of public addresses will be aggravated by always-on packet data
connectivity, which is expected to prevail in newer network
deployments.
In scenario 1, the shortage of public IPv4 addresses is alleviated by
assigning IPv4 addresses from one of the private address ranges as
specified in [RFC1918]. To enable global connectivity, network
address translation (NAT) is performed on the (S)Gi interface for
IPv4 packets originated from or destined to the UEs.
Scenario 2 is a similar dual-stack connectivity scenario as Scenario
1, but includes a further challenge where private IPv4 addresses are
used by more than 16 million active UEs (i.e., have an active PDP
context/EPS bearer) in the same network at the same time. If unique
private IPv4 addresses are not available for all UEs, the operator
may need to consider assigning the same IPv4 address to multiple UEs.
The usage of shared IPv4 addresses in parallel to an IPv6 prefix is
also described in scenario 4.
In addition to the more general dual-stack connectivity scenarios,
specific scenarios related to e.g. M2M terminals have been
considered in Scenario 3. The operator may choose to assign only
IPv6 prefixes to this new group of mobile devices. These UEs with
IPv6-only connectivity will be running applications using IPv6, but
they may need to access both IPv4- or IPv6-enabled services.
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Authors' Addresses
Jouni Korhonen (editor)
Nokia Siemens Networks
Linnoitustie 6
FI-02600 Espoo
FINLAND
Email: jouni.nospam@gmail.com
Jonne Soininen
Nokia Siemens Networks
Linnoitustie 6
FI-02600 Espoo
FINLAND
Email: jonne.soininen@nsn.com
Basavaraj Patil
Nokia
6021 Connection drive
Irving, TX 75019
USA
Email: basavaraj.patil@nokia.com
Teemu Savolainen
Nokia
Hermiankatu 12 D
FI-33720 Tampere
FINLAND
Email: teemu.savolainen@nokia.com
Gabor Bajko
Nokia
323 Fairchild drive 6
Mountain view, CA 94043
USA
Email: gabor.bajko@nokia.com
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Kaisu Iisakkila
Nokia
Itamerenkatu 11-13
FI-00180 Helsinki
FINLAND
Email: kaisu.iisakkila@nokia.com
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