Extensible Authentication Protocol J. Arkko
Internet-Draft Ericsson
Intended status: Informational B. Aboba
Expires: April 26, 2007 Microsoft
J. Korhonen
TeliaSonera
F. Bari
Cingular Wireless
October 23, 2006
Network Discovery and Selection Problem
draft-ietf-eap-netsel-problem-05
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Abstract
The so called network discovery and selection problem affects network
access, particularly in the presence of multiple available wireless
accesses and roaming. This problem has been the subject of
discussions in various standards bodies. This document summarizes
the discussion held about this problem in the Extensible
Authentication Protocol (EAP) working group at the IETF. The problem
is defined and divided into subproblems, and some constraints for
possible solutions are outlined. The document also provides a
discussion of the limitations of certain classes of solution,
including some that have been previously defined.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. Problem Definition . . . . . . . . . . . . . . . . . . . . . . 7
2.1. Discovery of the Point of Attachment to the Network . . . 7
2.2. Identity selection . . . . . . . . . . . . . . . . . . . . 9
2.3. AAA routing . . . . . . . . . . . . . . . . . . . . . . . 10
2.3.1. The Incomplete Routing Table Problem . . . . . . . . . 11
2.3.2. The User and Identity Selection Problem . . . . . . . 12
2.4. Capability Discovery . . . . . . . . . . . . . . . . . . . 14
3. Design Issues . . . . . . . . . . . . . . . . . . . . . . . . 16
3.1. AAA issues . . . . . . . . . . . . . . . . . . . . . . . . 16
3.2. Backward Compatibility . . . . . . . . . . . . . . . . . . 16
3.3. Efficiency Constraints . . . . . . . . . . . . . . . . . . 16
3.4. Scalability . . . . . . . . . . . . . . . . . . . . . . . 16
3.5. Realm discovery and selection decision making . . . . . . 17
4. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 18
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
6. Security Considerations . . . . . . . . . . . . . . . . . . . 22
7. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 23
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 24
8.1. Normative References . . . . . . . . . . . . . . . . . . . 24
8.2. Informative References . . . . . . . . . . . . . . . . . . 24
Appendix A. Existing Work . . . . . . . . . . . . . . . . . . . . 28
A.1. IETF . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
A.2. IEEE 802 . . . . . . . . . . . . . . . . . . . . . . . . . 29
A.3. 3GPP . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
A.4. Other . . . . . . . . . . . . . . . . . . . . . . . . . . 32
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 33
Intellectual Property and Copyright Statements . . . . . . . . . . 34
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1. Introduction
The network discovery and selection problem affects network access
and wireless access networks in particular. Aspects of the problem
will appear when any of the following conditions are true:
o There is more than one available network attachment point, and the
different attachment points may have different characteristics or
belong to different operators. In the case of virtual operators,
access network infrastructure including e.g. the access points can
be shared by multiple operators. In order to choose between the
network attachment points, it may be helpful to determine which
realms are supported and the capabilities access network
supporting those realms. Otherwise, the mobile station might
frequently roam into networks that are not able to satisfy the
roaming connectivity needs or provide services the mobile station
(and the subscriber) are seeking for. This would of course lower
the general quality of offered services.
o The user has multiple sets of credentials. For instance, the user
could have one set of credentials from a public service provider
and set from the user's employer. In this case it may be helpful
to provide additional information to enable the correct credential
set to be determined. Otherwise, it could happen that for example
a network access authentication repeatedly fails because of
incorrectly selected and offered set of credentials.
o There is more than one way to provide roaming between the visited
realm used for access and user's home realm, and service
parameters or pricing differs between them. For instance, the
visited access realm could have both a direct relationship with
the home realm and an indirect relationship through a roaming
consortium. In some scenarios, current AAA protocols may not be
able to route the requests to the home realm unaided, just based
on the domain in the given Network Access Identifier (NAI)
[RFC4282]. In addition, payload packets can get routed or
tunneled differently, based on the roaming relationship path in
use. This may have an impact on the available services or their
pricing.
In Section 2 the network discovery and selection problem is defined
and divided into subproblems, and some design issues for possible
solutions are outlined in Section 3. Section 4 gives the conclusions
and some suggestions on how to proceed for the rest. Appendix A
discusses existing mechanisms which help solve at least parts of the
problem. The terms "network" and "realm" have sometimes been used
interchangeably within the context of selection and discovery. It
should be noted that a realm can be reachable from more than one
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access network types and selection of a realm may not imply certain
network capabilities.
1.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
Network Access Identifier (NAI)
The Network Access Identifier (NAI) [RFC4282] is the user identity
submitted by the client during network access authentication. In
roaming, the purpose of the NAI is to identify the user as well as
to assist in the routing of the authentication request. Please
note that the NAI may not necessarily be the same as the user's
e-mail address or the user identity submitted in an application
layer authentication.
Decorated NAI
A NAI specifying a source route. See RFC4282 [RFC4282] Section
2.7 for more information.
Realm
Realm portion of an NAI [RFC4282].
Network Selection
This refers to selection of an operator/ISP in order to access the
network. The process of network selection can occur either at the
beginning of a new session or during a handoff in case the user is
mobile. The selection is dependent upon for example the selection
of realm for the operator, authentication credentials for the
user/device and the roaming agreements. The realm Selection can
in turn also depend upon Access Technology Selection and/or Bearer
Selection.
Network Discovery
This refers to a mechanisms that a node uses to discover available
networks prior the realm selection takes place. The discovery
process may be passive or active from a node point of view.
Typically the discovery mechanism varies depending on the access
technology. It is also possible that there are multiple discovery
mechanisms within one access technology depending on the network
deployment.
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Realm Selection
This refers to selection of the realm of the operator/ISP used to
access the network.
Access Technology Selection
This refers to the selection between access technologies e.g.
802.11, UMTS, WiMAX. The selection will be dependent upon the
access technologies supported by the device and the availability
of networks supporting those technologies.
Bearer Selection
For some access technologies (e.g. UMTS), there can be a
possibility for delivery of a service (e.g. voice) by using either
a circuit switched or a packet switched bearer. The Bearer
selection refers to selecting one of the bearer types for service
delivery. The decision can be based on support of the bearer type
by the device and the network as well as user subscription and
operator preferences.
Network Access Server
The Network Access Server (NAS) is the device that clients connect
to in order to get access to the network. In PPTP terminology,
this is referred to as the PPTP Access Concentrator (PAC), and in
L2TP terminology, it is referred to as the L2TP Access
Concentrator (LAC). In IEEE 802.11, it is referred to as an
Access Point.
Roaming Capability
Roaming capability can be loosely defined as the ability to use
any one of multiple Internet Service Providers (ISPs), while
maintaining a formal, customer-vendor relationship with only one.
Examples of cases where roaming capability might be required
include ISP "confederations" and ISP-provided corporate network
access support.
Station (STA)
A device that contains an IEEE 802.11 conformant medium access
control (MAC) and physical layer (PHY) interface to the wireless
medium (WM).
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Access Point (AP)
An entity that has station functionality and provides access to
distribution services via the wireless medium (WM) for associated
stations.
Basic Service Set (BSS)
A set of stations controlled by a single coordination function.
Extended Service Set (ESS)
A set of one or more interconnected basic service sets (BSSs) with
the same Service Set Identifier (SSID) and integrated local area
networks (LANs), which appears as a single BSS to the logical link
control layer at any station associated with one of those BSSs.
This refers to a mechanism that a node uses to discover the
networks that are reachable from a given access network.
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2. Problem Definition
This problem spans multiple protocol layers and has been the subject
of discussions in IETF, 3GPP, and IEEE. This document summarizes the
discussion held about this problem in the Extensible Authentication
Protocol working group at IETF. There are a set of somewhat
orthogonal problems being discussed under the rubric of "network
discovery and selection".
o The problem of "discovery of points of attachment". This is the
problem of discovering points of attachment available in the
vicinity, and the capabilities associated with these points of
attachment.
o The problem of "Identifier selection". This is the problem of
selecting which identity (and credentials) to use to authenticate
in a given point of attachment to the network.
o The problem of "AAA routing" which involves figuring out how to
route the authentication conversation originating from the
selected identity back to the home realm.
o The problem of "Payload routing" which involves figuring how the
payload packets are routed, where more advanced mechanisms than
destination-based routing is needed. However, while being an
interesting problem, this document does not attempt to do any
analysis or suggestions on it.
o The problem of "network capability discovery". This is the
problem of discovering the capabilities of a particular
destination network. For example, it may be important to know
whether a given network supports enrollment, what the charges are,
etc.
Alternatively, the problem can be divided to the discovery, decision,
and the selection components. The AAA routing problem, for instance,
involves all components: discovery (which mediating networks are
available?), decision (choose the "best" one), and selection (client
tells the network which mediating network it has decided to choose)
components.
2.1. Discovery of the Point of Attachment to the Network
"The discovery of points of attachment" problem has been extensively
studied, see for instance the IEEE specifications on 802.11 wireless
LAN beaconing and probing process, studies (such as [Fixingapsel]) on
the effectiveness of these mechanisms, specifications on GSM network
discovery, results of the IETF Seamoby WG, and so on.
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Traditionally, the problem of discovering available point of
attachment has been handled as a part of the link layer attachment
procedures, or through out-of-band mechanisms.
RFC 2194 [RFC2194] describes the pre-provisioning of dialup roaming
clients, which typically included a list of potential phone numbers,
updated by the provider(s) with which the client had a contractual
relationship. RFC 3017 [RFC3017] describes the IETF Proposed
Standard for the Roaming Access XML DTD. This covers not only the
attributes of the Points of Presence (POPs) and Internet Service
Providers (ISPs), but also hints on the appropriate NAI to be used
with a particular POP. The RFC supports dial-in and X.25 access, but
has extensible address and media type fields.
In IEEE 802.11 WLANs, the Beacon/Probe Request/Response mechanism
provides a way for Stations to discover Access Points (APs), as well
as the capabilities of those APs. Among the Information Elements
(IEs) included within the Beacon and Probe Response is the SSID, a
non-unique identifier of the network to which an Access Point is
attached. By combining network identification along with
capabilities discovery, the Beacon/Probe facility provides the
information required for both network discovery and roaming decisions
within a single mechanism.
As noted in [Velayos], the IEEE 802.11 Beacon mechanism does not
scale well; with a Beacon interval of 100ms, and 10 APs in the
vicinity, approximately 32 percent of an 802.11b AP's capacity is
used for beacon transmission. In addition, since Beacon/Probe
Response frames are sent by each AP over the wireless medium,
stations can only discover APs within range, which implies
substantial coverage overlap for roaming to occur without
interruption.
A number of enhancements have been proposed to the Beacon/Probe
Response mechanism in order to improve scalability and roaming
performance. These include allowing APs to announce capabilities of
neighbor APs as well as their own, as proposed in IEEE 802.11k
[IEEE.802.11k].
Typically scalability enhancement mechanisms attempt to get around
Beacon/Probe Response restrictions by sending advertisements at the
higher layers which are enabled once the station has associated with
an AP and gained IP connectivity. Since these mechanisms run over
IP, they can utilize IP facilities such as fragmentation, which the
link layer mechanisms may not always be able to do. For instance, in
IEEE 802.11, Beacon frames cannot use fragmentation because they are
multicast frames, and multicast frames are not acknowledged in
802.11.
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Another issue with the Beacon/Probe Request/Response mechanism is
that it is either insecure or its security can be assured only after
already attaching to this particular network.
When considering access systems such as 802.11 WLANs networks it is
important to take into account different deployment options. For
example, a WLAN deployment may include a number of VLANs in order to
separate UAM (Universal Access Method) and 802.1X [IEEE.8021X] users
or users accessing network from different geographical/organizational
locations. It is also possible that a larger network spans multiple
ESSes and prefixes. It is also possible that users authenticating to
different realms are able to do so via the same SSID.
2.2. Identity selection
As networks proliferate, it becomes more and more likely that a given
user may have multiple identities and credential sets, available for
use in different situations. For example, the user may have an
account with one or more Public WLAN providers; a corporate WLAN; one
or more wireless WAN providers. As a result, the user has to decide
which credential set to use when presented with a choice.
Figure 1 illustrates a situation where the user realm may not be
reachable from each potential access network. Access Network 1 only
enables access to the realm "isp1.example.com" whereas Access Network
3 enables access to the realm "corp2.example.com" whereas Access
Network 2 enables access to both realms.
? ? +---------+ +------------------+
? | Access | | |
O_/ _-->| Network |------>| isp1.example.com |
/| / | 1 | _->| |
| | +---------+ / +------------------+
_/ \_ | /
| +---------+ /
User "subscriber@isp1. | | Access |/
example.com" -- ? -->| Network |
also known | | 2 |\
"employee123@corp2. | +---------+ \
example.com" | \
| +---------+ \_ +-------------------+
\_ | Access | ->| |
-->| Network |------>| corp2.example.com |
| 3 | | |
+---------+ +-------------------+
Figure 1: Two credentials, three possible access networks
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Traditionally, hints useful in identity selection have also been
provided out-of-band. For example, via the RFC 3017 XML DTD
[RFC3017], a client can select between potential POPs, and then based
on information provided in the DTD, determine the appropriate NAI to
use with the selected point of attachment to the network.
Where a fixed set of realms are always reachable from a given
network, access network names can be used to infer realm
reachability. For example, IEEE 802.11 Access Points provide the
SSID, though in some cases the station may not learn all the SSIDs
supported by the given access point without probing for them. In
IKEv2 [RFC4306], the identity of the responder (typically the
security gateway) is provided as a part of the IKEv2 exchange.
To use this information for identity selection, the client has to
match the access network name with the realm portion of a valid
client identity. For example, the client may be configured with the
network access names that have roaming contracts with each of the
client's home realms.
It is also possible for hints to be embedded within credentials. In
[RFC4334], usage hints are provided within certificates used for
wireless authentication. This involves extending the client's
certificate to include the SSIDs with which the certificate can be
used.
Finally, some EAP implementations use the space after the NUL
character in an EAP Identity Request to communicate some parameters
for example listing realms supported for authentication. The
Informational RFC [RFC4284] specifies the interpretation of the field
beyond the NUL character when realms are to be communicated.
2.3. AAA routing
Once the identity has been selected, it is necessary for the
authentication conversation to be routed back to the home realm.
This is typically done today through the use of the Network Access
Identifier (NAI), RFC 4282 [RFC4282], and the ability of the AAA
network to route requests to the realm indicated in the NAI.
Within the past IETF ROAMOPS WG, additional approaches were
considered for routing authentication conversation back to the home
realm, including source routing techniques based on the NAI, and
techniques relying on the AAA infrastructure. Given the relative
simplicity of the roaming implementations described in RFC 2194
[RFC2194], static routing mechanisms appeared adequate for the task
and it was not deemed necessary to develop dynamic routing protocols.
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As noted in RFC 2607 [RFC2607], RADIUS proxies are deployed not only
for routing purposes, but also to mask a number of inadequacies in
the RADIUS protocol design, such as the lack of standardized
retransmission behavior and the need for shared secret provisioning.
By removing many of the protocol inadequacies, introducing new AAA
agent types such as Redirects, providing support for certificate-
based authentication as well as inter and intra-domain service
discovery, allowing DNS based dynamic discovery of peer agents,
Diameter allows a NAS to directly open a Diameter connection to the
home realm without having to utilize a network of proxies. For
instance, the Redirect feature could be used to provide a centralized
routing function for AAA, without having to know all home network
names in all access networks. However, there are issues in the
previously mentioned approach as setting up security might turn out
to be problematic and the model might not meet business practices.
This is somewhat analogous to the evolution of email delivery. Prior
to the widespread proliferation of the Internet, it was necessary to
gateway between SMTP-based mail systems and alternative delivery
technologies, such as UUCP and FidoNet, and email-address based
source-routing was used to handle this. However, as mail could
increasingly be delivered directly, the use of source routing
disappeared.
As with the selection of certificates by stations, a Diameter client
wishing to authenticate with a Diameter server may have a choice of
available certificates, and therefore it may need to choose between
them.
2.3.1. The Incomplete Routing Table Problem
No dynamic routing protocols are in use in AAA infrastructure today.
This implies that there has to be a device (such as a proxy) within
the access network that knows how to route to different domains, even
if they are further than one hop away, as shown in Figure 2. In this
figure, the user "joe@c.example.com" has to be authenticated through
ISP 2, since the domain "c.example.com" is served by it.
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+---------+ +---------+
| | | |
User "joe@ | Access |----->| ISP 1 |-----> "a.example.com"
c.example.com"-->| Network | | |
| | +---------+
+---------+
|
|
V
+---------+
| |-----> "b.example.com"
| ISP 2 |
| |-----> "c.example.com"
+---------+
Figure 2: AAA routing problem
2.3.2. The User and Identity Selection Problem
A related issue is that the roaming relationship graph may have
ambiguous routes, as shown in Figure 3. As billing is based on AAA
and pricing may be based on the used intermediaries, it is necessary
to select which route is used. For instance, in Figure 3, access
through the roaming group 1 may be cheaper, than if roaming group 2
is used.
+---------+
| |----> "a.example.com"
| Roaming |
+---------+ | Group 1 |
| |----->| |----> "b.example.com"
User "joe@ | Access | +---------+
a.example.com"--->| Network |
| | +---------+
| |----->| |----> "a.example.com"
+---------+ | Roaming |
| Group 2 |
| |----> "c.example.com"
+---------+
Figure 3: Ambiguous AAA routing
There have been requests to place credential and AAA route selection
under user control, as the user is affected by the pricing and other
differences. Optionally, automatic tools could make the selection
based on the user's preferences. On the other hand, user control is
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similar to source routing, and as discussed earlier, network-based
routing mechanisms have traditionally won over source routing-based
mechanisms.
If users can control the selection of intermediaries, such
intermediaries still have to be legitimate AAA proxies. That is, an
access network should not send a request to an unknown intermediary.
If it has a business relationship with three intermediaries
int1.example.com, int2.example.com, and int3.example.com, it will
route the request through one of them, even if the user tried to
request routing through mitm.example.org. Thus, NAI-based source
routing is not source routing in the classic sense. It is merely
suggesting preferences among already established routes. If the
route does not already exist, or is not feasible, then NAI-based
source routing cannot establish it.
An additional issue is that even if the intermediaries are
legitimate, they could be switched. For instance, an access network
could advertise that it has a deal with
cheapintermediary.example.net, and then switch the user's selection
to priceyintermediary.example.com instead. To make this relevant,
the pricing would have to be based on the intermediary. Even if it
were possible to secure this selection, it would not be possible to
guarantee that QoS or other properties claimed by the network were
indeed provided. However, the ability to get authenticated via
intermediates implies that all the parties have a business agreement
with each other, which may also include an agreement about the
minimum service level guarantees.
Only a limited amount of information about AAA routes or pricing
information can be dynamically communicated [Eronen04]. It is
necessary to retrieve network and intermediary names, but quality of
service or pricing information is clearly something that would need
to be pre-provisioned, or perhaps just available via the web.
Similarly, dynamic communication of network names can not be expected
to provide all possible home network names, as their number can be
quite large globally.
As a result, network-based AAA routing mechanisms should be used
instead of user-based selection where sufficient routes exist. In an
error situation, such as when an attempt to use the network-based
routing mechanism has failed, routing hints can be advertised and
used as defined in [RFC4282] and [RFC4284]. Even so, such approaches
have severe scalability limitations. See Appendix A.1 for further
discussion
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2.4. Capability Discovery
Network Capabilities can provide information useful in the selection
process [I-D.groeting-eap-netselection-results]. For instance,
access network discovery may benefit from getting knowledge about the
quality of service available from a particular access network or
node, and AAA routing may require knowledge of roaming agreements.
References [I-D.groeting-eap-netselection-results] and
[IEEE.11-04-0624] describe the following categories of information
which can be discovered:
o Access network identification
o Roaming agreements
o Authentication mechanisms
o Quality of Service
o Cost
o Authorization policy
o Privacy policy
o Service parameters, such as the existence of middleboxes
The nature of the discovered information can be static, such as the
fastest available transmission speed on a given piece of equipment.
Or it can be dynamic, such as the current load on this equipment.
The information can describe something about the network access nodes
themselves, or it can be something that they simply advertise on
behalf of other parts of the network, such as roaming agreements
further in the AAA network.
Typically, it would be desirable to acquire all this information
prior to the authentication process. In some cases it is in fact
necessary, if the authentication process can not complete without the
information. Reference [IEEE.11-04-0624] classifies the possible
steps at which IEEE 802.11 networks can acquire this information:
o Pre-association
o Post-association (or pre-authentication)
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o Post-authentication
Note that some EAP methods (such as those defined in
[I-D.josefsson-pppext-eap-tls-eap] [I-D.tschofenig-eap-ikev2]
[I-D.arkko-eap-service-identity-auth]) have an ability to agree about
additional parameters during an authentication process. While such
parameters are useful for many purposes, their use for access network
selection suffers from an obvious chicken-and-egg problem. Or at
least it seems costly to run a relatively heavy authentication
process to decide whether the client wants to attach to this access
network.
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3. Design Issues
The following factors should be taken into consideration while
evaluating solutions for problem of network selection and discovery:
3.1. AAA issues
Access network or realm selection may leverage or interact with the
AAA infrastructure. The solution should therefore be compatible with
all AAA protocols. AAA routing mechanisms should work for requests,
responses, as well as server-initiated messages. The solution should
not prevent the introduction of new AAA or access network features,
such as AAA routing protocols or fast handoffs.
3.2. Backward Compatibility
The solution should allow interoperability with clients, protocols,
access networks, AAA proxies, and AAA servers that have not been
modified to support network discovery and selection. For example, it
should not cause a problem with limited packet sizes of current
protocols. Where new protocol mechanisms are required, it should be
possible to deploy the solution without requiring changes to the
largest base of installed devices -- network access servers, wireless
access points, and clients.
3.3. Efficiency Constraints
The solution should be efficient in network resource utilization,
specially on bandwidth constrained sections of the network (E.g.
wireless link). Mechanisms that could significantly increase
communication of an unauthenticated device with more than one points
of attachment during the selection process should be avoided. For
many handheld devices, battery life is a significant constraint.
Mechanisms that could significantly drain battery e.g. by requiring
one or more radios in multimode devices to continuously scan for
networks, should be avoided. In addition, the solution should not
significantly impact network attachment time.
3.4. Scalability
Depending upon deployment scenarios and business agreements amongst
the network operators, the number of networks to be advertised can
range from a few to a very large number. The solution should
therefore be scalable so that it can handle from a small to very
large number of networks without violating the efficiency constraints
described in Section 3.3.
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3.5. Realm discovery and selection decision making
"Phone-book" based approaches such as RFC 3017 appear attractive due
to their ability to provide sufficient information for automatic
selection decisions. However, there is no experience on applying
such approaches to wireless access. The number of WLAN access points
is significantly higher than the number of dial-in POPs; the
distributed nature of the access network has created a more
complicated business and roaming structure, and the expected rate of
change in the information is high. As noted in [Priest04] and
[I-D.groeting-eap-netselection-results], a large fraction of current
WLAN access points operate on the default SSID, which may make the
use of the phone book approach difficult.
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4. Conclusions
The issues surrounding the network discovery and selection problem
have been summarized.
In the opinion of the authors of this document, the main findings
are:
o There is a clear need for access network discovery, identifier
selection, AAA routing, and payload routing.
o Identifier selection and AAA routing problems can and should be
seen as the different aspects of the same problem, identifier
selection.
o Nevertheless, many of the problems discussed in this draft are
very hard when one considers them in an environment that requires
a potentially large number of networks, fast handoffs, and
automatic decisions.
o The proliferation of multiple competing network discovery
technologies within IEEE 802, IETF, and 3GPP appears to a
significant problem going forward. In the absence of a clearly
defined solution to the problem it is likely that any or all of
these solutions will be utilized, resulting in industry
fragmentation and lack of interoperability.
o New link layers should be designed with facilities that enable the
efficient distribution of network advertisement information.
o Solving all problems with current link layers and existing network
access devices may not be possible. It may be useful to consider
a phased approach where only certain, limited functions are
provided now, and the full functionality is provided when
extensions to current link layers become available.
We will briefly comment on the specific mechanisms related to access
network discovery and selection:
o As noted in studies such as [MACScale] and [Velayos], the IEEE
802.11 Beacon/Probe Response mechanism has substantial scaling
issues, and as a result a single physical access point is in
practice limited to less than a dozen virtual APs on each channel
with IEEE 802.11b.
The situation is improved substantially with successors such as
IEEE 802.11a which enable additional channels, thus potentially
increasing the number of potential virtual APs.
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However, even these enhancements it is not feasible to advertise
more than 50 different networks using existing mechanisms, and
probably significantly less in most circumstances.
As a result, there appears to be justification for enhancing the
scalability of network advertisements.
o Work is already underway in IEEE 802.1, IEEE 802.21 and the IEEE
802.11u to provide enhanced discovery functionality. Similarly,
the IEEE 802.1af has discussed the idea of supporting network
discovery within a future revision to IEEE 802.1X. However,
neither IEEE 802.1ab nor IEEE 802.1af is likely to address the
transport of large quantities of data where fragmentation would be
a problem.
Another typical limitation of link layer assistance in this area
is that in general, it would be desirable to retrieve also
information relating to the potential next access networks or
access points. However, such networks may be of another type than
the current one, so the link layer would have to carry information
relating to other types of link layers as well. This is possible,
but requires coordination among different groups in the industry.
o Given that EAP does not support fragmentation of EAP-Request/
Identity packets, and that use of EAP for network selection on all
attachments will have a substantial adverse impact on roaming
performance without appropriate lower layer support (such as
support for Class 1 data frames within IEEE 802.11), the use of
EAP is limited. For instance, the use of EAP to carry quality of
service as proposed in [I-D.groeting-eap-netselection-results]
seems difficult given the limitations. Long-term, it makes more
sense for the desired functionality to be handled either within
IEEE 802 or at the IP layer. However, a strictly limited
discovery mechanism such as the one defined in [RFC4284] is
useful.
o In the IETF, a potential alternative is use of the SEAMOBY CARD
protocol [RFC4066], which enables advertisement of network device
capabilities over IP. Another alternative is the already expired
Device Discovery Protocol (DDP) [I-D.marques-ddp] proposal, which
provides functionality equivalent to IEEE 802.1ab using ASN.1
encoded advertisements sent to a link-local scope multicast
address.
A limitation of these IP layer solutions is that they can only
work as a means to speed up the attachment procedures when moving
from one location to another; when a node starts up, it needs to
be able to attach to a network before IP communications are
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available. This is fine for optimizations, but precludes the use
in a case where the discovery information is mandatory before
successful attachment can be accomplished, for instance when the
access network is unable to route the AAA request unaided.
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5. IANA Considerations
This document does not define any new name spaces to be managed by
IANA. This document does not either reserve any new numbers or names
under any existing name space managed by IANA.
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6. Security Considerations
All aspects of the network discovery and selection problem are
security related. The security issues and requirements have been
discussed in the previous sections.
The security requirements for network discovery depend on the type of
information being discovered. Some of the parameters may have a
security impact, such as the claimed name of the network the user
tries to attach to. Unfortunately, current EAP methods do not always
make the verification of such parameters possible. New EAP methods
are doing it [I-D.josefsson-pppext-eap-tls-eap]
[I-D.tschofenig-eap-ikev2], however, and there is even an attempt to
provide a backwards compatible extensions to older methods
[I-D.arkko-eap-service-identity-auth].
The security requirements for network selection depend on whether the
selection is considered as a command or a hint. For instance, the
selection that the user provided may be ignored if it relates to AAA
routing and the access network can route the AAA traffic to the
correct home network using other means in any case.
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7. Contributors
The editors of this document would like to especially acknowledge the
contributions of Farid Adrangi, Farooq Bari, Michael Richardson, Pasi
Eronen, Mark Watson, Mark Grayson, Johan Rune, and Tomas Goldbeck-
Lowe.
Input for the early versions of this draft has been gathered from
many sources, including the above persons as well as 3GPP and IEEE
developments. We would also like to thank Alper Yegin, Victor Lortz,
Stephen Hayes, and David Johnston for comments.
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8. References
8.1. Normative References
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2194] Aboba, B., Lu, J., Alsop, J., Ding, J., and W. Wang,
"Review of Roaming Implementations", RFC 2194,
September 1997.
[RFC2607] Aboba, B. and J. Vollbrecht, "Proxy Chaining and Policy
Implementation in Roaming", RFC 2607, June 1999.
[RFC3017] Riegel, M. and G. Zorn, "XML DTD for Roaming Access Phone
Book", RFC 3017, December 2000.
[RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
Levkowetz, "Extensible Authentication Protocol (EAP)",
RFC 3748, June 2004.
[RFC4334] Housley, R. and T. Moore, "Certificate Extensions and
Attributes Supporting Authentication in Point-to-Point
Protocol (PPP) and Wireless Local Area Networks (WLAN)",
RFC 4334, February 2006.
[RFC4234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, October 2005.
[RFC4282] Aboba, B., Beadles, M., Arkko, J., and P. Eronen, "The
Network Access Identifier", RFC 4282, December 2005.
[RFC4284] Adrangi, F., Lortz, V., Bari, F., and P. Eronen, "Identity
Selection Hints for the Extensible Authentication Protocol
(EAP)", RFC 4284, January 2006.
[RFC4306] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
RFC 4306, December 2005.
8.2. Informative References
[I-D.arkko-eap-service-identity-auth]
Arkko, J. and P. Eronen, "Authenticated Service Identities
for the Extensible Authentication Protocol (EAP)",
draft-arkko-eap-service-identity-auth-04 (work in
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progress), October 2005.
[I-D.groeting-eap-netselection-results]
Tschofenig, H., "Network Selection Implementation
Results", draft-groeting-eap-netselection-results-00 (work
in progress), July 2004.
[I-D.ietf-pana-pana]
Forsberg, D., Ohba, Y., Patil, B., Tschofenig, H., and A.
Yegin, "Protocol for Carrying Authentication for Network
Access (PANA)", draft-ietf-pana-pana-11 (work in
progress), March 2006.
[I-D.josefsson-pppext-eap-tls-eap]
Josefsson, S., Palekar, A., Simon, D., and G. Zorn,
"Protected EAP Protocol (PEAP)",
draft-josefsson-pppext-eap-tls-eap-07 (work in progress),
October 2003.
[I-D.marques-ddp]
Enns, R., Marques, P., and D. Morrell, "Device Discovery
Protocol (DDP)", draft-marques-ddp-00 (work in progress),
May 2003.
[I-D.tschofenig-eap-ikev2]
Tschofenig, H. and D. Kroeselberg, "EAP IKEv2 Method (EAP-
IKEv2)", draft-tschofenig-eap-ikev2-10 (work in progress),
February 2006.
[IEEE.8021X]
Institute of Electrical and Electronics Engineers, "Local
and Metropolitan Area Networks: Port-Based Network Access
Control", IEEE Standard 802.1X, September 2001.
[IEEE.802.11-2003]
Institute of Electrical and Electronics Engineers,
"Wireless LAN Medium Access Control (MAC) and Physical
Layer (PHY) Specifications", IEEE Standard 802.11, 2003.
[IEEE-11-03-154r1]
Aboba, B., "Virtual Access Points", IEEE Contribution 11-
03-154r1, May 2003.
[IEEE-11-03-0827]
Hepworth, E., "Co-existence of Different Authentication
Models", IEEE Contribution 11-03-0827 2003.
[IEEE.11-04-0624]
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Berg, S., "Information to Support Network Selection", IEEE
Contribution 11-04-0624 2004.
[IEEE.11-04-0638]
Aboba, B., "Network Selection", IEEE Contribution 11-04-
0638 2004.
[11-05-0822-03-000u-tgu-requirements]
Moreton, M., "TGu Requirements", IEEE Contribution 11-05-
0822-03-000u-tgu-requirements, August 2005.
[3GPPSA2WLANTS]
3GPP, "3GPP System to Wireless Local Area Network (WLAN)
interworking; System Description; Release 6; Stage 2",
3GPP Technical Specification 23.234 v 6.6.0,
September 2005.
[3GPP-SA3-030736]
Ericsson, "Security of EAP and SSID based network
advertisements", 3GPP Contribution S3-030736,
November 2003.
[3GPP.23.122]
3GPP, "Non-Access-Stratum (NAS) functions related to
Mobile Station (MS) in idle mode", 3GPP TS 23.122 6.5.0,
October 2005.
[WWRF-ANS]
Eijk, R., Brok, J., Bemmel, J., and B. Busropan, "Access
Network Selection in a 4G Environment and the Role of
Terminal and Service Platform", 10th WWRF, New York,
October 2003.
[WLAN3G] Ahmavaara, K., Haverinen, H., and R. Pichna, "Interworking
Architecture between WLAN and 3G Systems", IEEE
Communications Magazine, November 2003.
[INTELe2e]
Intel, "Wireless LAN (WLAN) End to End Guidelines for
Enterprises and Public Hotspot Service Providers",
November 2003.
[Velayos] Velayos, H. and G. Karlsson, "Techniques to Reduce IEEE
802.11b MAC Layer Handover Time", Laboratory for
Communication Networks, KTH, Royal Institute of
Technology, Stockholm, Sweden, TRITA-IMIT-LCN R 03:02,
April 2003.
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[Fixingapsel]
Judd, G. and P. Steenkiste, "Fixing 802.11 Access Point
Selection", Sigcomm Poster Session 2002.
[Eronen03]
Eronen, P., "Network Selection Issues", presentation to
EAP WG at IETF 58, November 2003.
[Priest04]
Priest, J., "The State of Wireless London", July 2004.
[MACScale]
Heusse, M., "Performance Anomaly of 802.11b", LSR-IMAG
Laboratory, Grenoble, France, IEEE Infocom 2003.
[Eronen04]
Eronen, P. and J. Arkko, "Role of authorization in
wireless network security", Extended abstract presented in
the DIMACS workshop, November 2004.
[3GPPSA3WLANTS]
3GPP, "3GPP Technical Specification Group Service and
System Aspects; 3G Security; Wireless Local Area Network
(WLAN) interworking security (Release 6); Stage 2",
3GPP Technical Specification 33.234 v 6.6.0, October 2005.
[3GPPCT1WLANTS]
3GPP, "3GPP System to Wireless Local Area Network (WLAN)
interworking; User Equipment (UE) to network protocols;
Stage 3 (Release 6)", 3GPP Technical Specification 24.234
v 6.4.0, October 2005.
[IEEE.802.11k]
Institute of Electrical and Electronics Engineers, "Draft
Ammendment to Standard for Telecommunications and
Information Exchange Between Systems - LAN/MAN Specific
Requirements - Part 11: Wireless LAN Medium Access Control
(MAC) and Physical Layer (PHY) Specifications: Radio
Resource Management", IEEE IEEE 802.11k, D4.1, July 2006.
[3GPPCT4WLANTS]
3GPP, "3GPP system to Wireless Local Area Network (WLAN)
interworking; Stage 3 (Release 6)", 3GPP Technical
Specification 29.234 v 6.4.0, October 2005.
[RFC4066] Liebsch, M., Singh, A., Chaskar, H., Funato, D., and E.
Shim, "Candidate Access Router Discovery (CARD)",
RFC 4066, July 2005.
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Appendix A. Existing Work
A.1. IETF
There has already been a lot of past work in this area, including a
number of IETF Proposed Standards generated by the ROAMOPS WG. The
topic of roaming was considered different enough from both AAA and
access protocols such as PPP that it deserved its own WG.
In addition to work on ROAMOPS directly relating to the problem,
there has been work in SEAMOBY relating to scaling of target access
network discovery mechanisms (which in this context refers to finding
a suitable base station to attach to); work in PKIX relating to
identity and credential selection; and work in AAA WG relating to
access routing.
The PANA protocol [I-D.ietf-pana-pana] has a mechanism to advertise
and select "ISPs" through the exchange of the ISP-Information AVP in
its initial exchange.
Adrangi et al [RFC4284] define the use of the EAP-Request/Identity
for identifier selection. It is necessary to have this kind of a
mechanism, as clients may have more than one credential, and when
combined with the '!' syntax for NAIs, it can also be used for
mediating realm discovery and selection. The use of lower-layer
information may also be limited in terms of discovering identifiers
that are used on the EAP layer. In the longer term, the use of this
mechanism may run into scalability problems, however. As noted in
[RFC3748] Section 3.1, the minimum EAP MTU is 1020 octets, so that an
EAP-Request/Identity is only guaranteed to be able to include 1015
octets within the Type-Data field. Since RFC 1035 [RFC1035] enables
FQDNs to be up to 255 octets in length, this may not enable the
announcement of many realms. The use of other network identifiers
than domain names is also possible, for instance the PANA protocol
uses an a free form string and an SMI Network Management Private
Enterprise Code [I-D.ietf-pana-pana], or Mobile Network Codes
embedded in NAIs as specified in 3GPP.
As noted in [Eronen03], the use of the EAP-Request/Identity for realm
discovery has substantial negative impact on handoff latency, since
this may result in a station needing to initiate an EAP conversation
with each Access Point in order to receive an EAP-Request/Identity
describing which realms are supported. Since IEEE 802.11-1999 does
not support use of Class 1 data frames in State 1 (unauthenticated,
unassociated) within an Extended Service Set (ESS), this implies
either that the APs must support 802.1X pre-authentication (optional
in IEEE 802.11i) or that the station must associate with each AP
prior to sending an EAPOL- Start to initiate EAP. This will
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dramatically increase handoff latency.
The effects on handoff latency depend also on the specific protocol
design, and the expected likelihood of having to provide
advertisements and initiate scanning of several APs. The use of
advertisements only as a last resort when the AAA routing has failed
is a better approach than the use of advertisement - scanning
procedure on every attachment.
Furthermore, if the AP has not been updated to present an up to date
set of realms in the EAP-Requests/Identity, after associating to
candidate APs and then choosing one, it is possible that the station
will find that the chosen realm is not supported after all. In this
case, the station's EAP-Response/Identity may be answered with an
updated EAP-Request/Identity, adding more latency. However, it is
possible to configure APs to pass through all EAP negotiation to a
local AAA proxy and provision the supported realms there. This would
ease the management of larger deployments but at the same time
require RFC 4284 support from the local AAA proxies. In general
upgrading the AAA proxies seems a better approach than upgrading and
managing all APs.
A.2. IEEE 802
There has been work in various IEEE 802 working groups relating to
network discovery enhancements.
Some recent and past contributions in this space include the
following:
o [IEEE.802.11-2003] defines the Beacon and Probe Response
mechanisms used with IEEE 802.11. Unfortunately, Beacons are only
sent at the lowest supported rate. Studies such as [MACScale]
have identified MAC layer performance problems, and [Velayos] have
identified scaling issues resulting from a lowering of the Beacon
interval.
o [IEEE-11-03-0827] discusses the evolution of authentication models
in WLANs, and the need for the network to migrate from existing
models to new ones, based on either EAP layer indications or
through the use of SSIDs to represent more than the local network.
It notes the potential need for management or structuring of the
SSID space.
The paper also notes that virtual APs have scalability issues. It
does not analyze these scalability issues in relation to those
existing in other alternative solutions, however.
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o [IEEE-11-03-154r1] discusses mechanisms currently used to provide
"Virtual AP" capabilities within a single physical access point.
A "Virtual AP" appears at the MAC and IP layers to be distinct
physical AP. As noted in the paper, full compatibility with
existing 802.11 station implementations can only be maintained if
each virtual AP uses a distinct MAC address (BSSID) for use in
Beacons and Probe Responses. This draft does not discuss scaling
issues in detail, but recommends that only a limited number of
virtual APs be supported by a single physical access point. The
simulations presented in [Velayos] appear to confirm this
conclusion; with a Beacon interval of 100 ms, once more than 8
virtual APs are supported on a single channel, more than 20% of
bandwidth is used for Beacons alone. This would indicate a limit
of approximately 20 virtual APs per physical AP.
o IEEE 802.11u group is defining the access realm discovery and
selection solution as part of its requirements
[11-05-0822-03-000u-tgu-requirements]. The requirements related
to realm discovery and selection include the functionality by
which a station can determine whether its subscription to a
service provider would allow it to access a particular 802.11
access network or whether the access network is able to route
authentication to user's home realm before actually joining a BSS
within that 802.11 access network. The mechanism should be able
to handle multiple credentials from the same user and be able to
select the correct credentials. Other planned features would
allow the station to learn the supported enrollment mechanisms and
possibly the set of basic services (such as Internet access is
provided or not) in the access network prior to the user
authenticating to his or her home realm.
o IEEE 802.21 is developing standards to enable handover and
interoperability between heterogeneous network types including
both 802 and non 802 networks. The intention is to provide a
general information transfer capability for these purposes. As a
result, realm discovery process may benefit from such standards.
Part of handover process is the discovery of candidate access
networks and selection of an access network for a handover. The
IEEE 802.21 group is looking into various information elements
that can be used to provide sufficient information to either a
network node or the terminal to make network selection possible.
Both link layer and layer 3 delivery mechanisms are being looked
into. Layer 3 protocol development is being looked into in IETF
MIPSHOP WG. Different query mechanisms between the terminal and
the network, including using of XML or basic TLV type interaction
are being explored.
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A.3. 3GPP
The 3GPP stage 2 technical specification [3GPPSA2WLANTS] covers the
architecture of 3GPP Interworking WLAN (I-WLAN) with 2G and 3G
networks. This specification discusses also realm discovery and
selection issues. The I-WLAN realm discovery and selection procedure
borrows ideas from the cellular side Public Land-based Mobile Network
(PLMN) selection principles and is referred in 3GPP I-WLAN
specifications as PLMN Selection.
In the 3GPP defined cellular network PLMN selection [3GPP.23.122] the
mobile node monitors surrounding cells and prioritizes them e.g.
based on signal strength before selecting a new possible target cell.
Each cell also broadcasts its PLMN information. A mobile node may
automatically select cells that belong to its Home PLMN, Registered
PLMN or to a allowed set of Visited PLMNs. These lists of PLMNs are
prioritized and stored in the SIM card. In a case of manual PLMN
selection the mobile node lists all PLMNs it knows from the
surrounding cells and lets the user choose the desired PLMN. After
the PLMN has been selected other cell related prioritization takes
place in order to select the appropriate target cell.
The [WLAN3G] discuss the new realm (PLMN) selection requirements that
I-WLAN roaming introduces. It is necessary to support automatic PLMN
selection, and not just manual selection by the user. There may be
multiple levels of networks, the hotspot owner may have a contract
with a provider who in turn has a contract with one 3G network, and
this 3G network has a roaming capability with a number of other
networks.
The I-WLAN specification requires that network discovery is performed
as specified in the standards for the relevant WLAN link layer
technology. In addition to network discovery, it is necessary to
select intermediary networks for the purposes of AAA Routing. In
3GPP, these networks are PLMNs. It is assumed that WLAN networks may
have a contract with more than one PLMN. The PLMN may be a Home PLMN
(HPLMN) or a Visited PLMN (VPLMN) in the roaming case. GSM/UMTS
roaming principles are employed for routing AAA requests from the
VPLMN to the Home Public Land-based Mobile Network (HPLMN) using
either RADIUS or Diameter. The procedure for selecting the
intermediary network has been specified in the stage 3 technical
specifications [3GPPCT1WLANTS] and [3GPPCT4WLANTS].
In order to select the PLMN, the following is required:
o User may choose the desired HPLMN or VPLMN manually or let the
WLAN User Equipment (WLAN UE) choose the PLMN automatically based
on the user and operator defined preferences.
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o AAA messages are routed according to the (root) NAI or decorated
NAI.
o Existing EAP mechanisms are used where possible.
o Extensibility is desired, to allow the advertisement of other
parameters later. The current network (PLMN) advertisement and
selection is based on [RFC4284], which currently explicitly
defines only the advertisement of realms. However, the RFC4284
does not prohibit encoding any octet string (as defined in
[RFC4234]) containing any information parameter into the
advertisement.
The 3GPP I-WLAN technical specifications state that advertisement
information shall be provided only when the access network is unable
to route the request using normal AAA routing means, such as when it
sees an unknown NAI realm. It is also stated that where VPLMN
control is required, the necessary information is added to a NAI.
Furthermore, the station (WLAN UE) may manually trigger the network
(PLMN) advertisement by using Alternative NAI in EAP Request/
Identity. The Alternative NAI is guaranteed to be an unknown NAI
realm throughout all 3GPP networks.
The security requirements for 3GPP I-WLAN have been specified in the
3GPP stage 3 technical specification [3GPPSA3WLANTS]. The security
properties related to different mediating network (PLMN) selection
mechanisms have been discussed earlier in the 3GPP contribution
[3GPP-SA3-030736], which concludes that both SSID and EAP-based
mechanisms have roughly similar (and very limited) security
properties, and that, as a result, network (PLMN) advertisement
should be considered only as hints.
A.4. Other
[INTELe2e] discusses the need for realm selection in a situation
where there is more than one available access network with a roaming
agreement to the home realm. It also lists EAP-level, SSID-based,
and PEAP-based mechanisms as potential solutions to the realm
selection problem.
Eijk et al [WWRF-ANS] discussed the general issue of network/realm
selection. They concentrated primarily on the access network
discovery problem, based on various criteria, and did not consider
the other aspects of the network/realm selection problem.
Nevertheless, they mention that one of the network selection problems
is that the information about accessibility and roaming relationships
is not stored in one location, but rather spread around the network.
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Authors' Addresses
Jari Arkko
Ericsson
Jorvas 02420
Finland
Email: jari.arkko@ericsson.com
Bernard Aboba
Microsoft
One Microsoft Way
Redmond, WA 98052
USA
Email: aboba@internaut.com
Jouni Korhonen
TeliaSonera
Teollisuuskatu 13
Sonera FIN-00051
Finland
Email: jouni.korhonen@teliasonera.com
Farooq Bari
Cingular Wireless
7277 164th Avenue N.E.
Redmond WA 98052
USA
Email: farooq.bari@cingular.com
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Full Copyright Statement
Copyright (C) The Internet Society (2006).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
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Arkko, et al. Expires April 26, 2007 [Page 34]
