| Rfc | 5113 |
|---|
| Title | Network Discovery and Selection Problem |
|---|
| Author | J. Arkko, B. Aboba, J.
Korhonen, Ed., F. Bari |
|---|
| Date | January 2008 |
|---|
| Format: | TXT, HTML |
|---|
| Status: | INFORMATIONAL |
|---|
|
Network Working Group J. Arkko
Request for Comments: 5113 Ericsson
Category: Informational B. Aboba
Microsoft
J. Korhonen, Ed.
TeliaSonera
F. Bari
AT&T
January 2008
Network Discovery and Selection Problem
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Abstract
When multiple access networks are available, users may have
difficulty in selecting which network to connect to and how to
authenticate with that network. This document defines the network
discovery and selection problem, dividing it into multiple sub-
problems. Some constraints on potential solutions are outlined, and
the limitations of several solutions (including existing ones) are
discussed.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology Used in This Document . . . . . . . . . . . . 4
2. Problem Definition . . . . . . . . . . . . . . . . . . . . . . 7
2.1. Discovery of Points of Attachment . . . . . . . . . . . . 8
2.2. Identity Selection . . . . . . . . . . . . . . . . . . . . 9
2.3. AAA Routing . . . . . . . . . . . . . . . . . . . . . . . 11
2.3.1. The Default Free Zone . . . . . . . . . . . . . . . . 13
2.3.2. Route Selection and Policy . . . . . . . . . . . . . . 14
2.3.3. Source Routing . . . . . . . . . . . . . . . . . . . . 15
2.4. Network Capabilities Discovery . . . . . . . . . . . . . . 17
3. Design Issues . . . . . . . . . . . . . . . . . . . . . . . . 18
3.1. AAA Routing . . . . . . . . . . . . . . . . . . . . . . . 18
3.2. Backward Compatibility . . . . . . . . . . . . . . . . . . 18
3.3. Efficiency Constraints . . . . . . . . . . . . . . . . . . 19
3.4. Scalability . . . . . . . . . . . . . . . . . . . . . . . 19
3.5. Static Versus Dynamic Discovery . . . . . . . . . . . . . 21
3.6. Security . . . . . . . . . . . . . . . . . . . . . . . . . 21
3.7. Management . . . . . . . . . . . . . . . . . . . . . . . . 22
4. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 23
5. Security Considerations . . . . . . . . . . . . . . . . . . . 25
6. Informative References . . . . . . . . . . . . . . . . . . . . 26
Appendix A. Existing Work . . . . . . . . . . . . . . . . . . . . 32
A.1. IETF . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
A.2. IEEE 802 . . . . . . . . . . . . . . . . . . . . . . . . . 33
A.3. 3GPP . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
A.4. Other . . . . . . . . . . . . . . . . . . . . . . . . . . 36
Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 37
1. Introduction
Today, network access clients are typically pre-configured with a
list of access networks and corresponding identities and credentials.
However, as network access mechanisms and operators have
proliferated, it has become increasingly likely that users will
encounter networks for which no pre-configured settings are
available, yet which offer desired services and the ability to
successfully authenticate with the user's home realm. It is also
possible that pre-configured settings will not be adequate in some
situations. In such a situation, users can have difficulty in
determining which network to connect to, and how to authenticate to
that network.
The problem arises when any of the following conditions are true:
o Within a single network, more than one network attachment point is
available, and the attachment points differ in their roaming
arrangements, or access to services. While the link layer
capabilities of a point of attachment may be advertised, higher-
layer capabilities, such as roaming arrangements, end-to-end
quality of service, or Internet access restrictions, may not be.
As a result, a user may have difficulty determining which services
are available at each network attachment point, and which
attachment points it can successfully authenticate to. For
example, it is possible that a roaming agreement will only enable
a user to authenticate to the home realm from some points of
attachment, but not others. Similarly, it is possible that access
to the Internet may be restricted at some points of attachment,
but not others, or that end-to-end quality of service may not be
available in all locations. In these situations, the network
access client cannot assume that all points of attachment within a
network offer identical capabilities.
o Multiple networks are available for which the user has no
corresponding pre-configuration. The user may not have pre-
configured an identity and associated credentials for use with a
network, yet it is possible that the user's home realm is
reachable from that network, enabling the user to successfully
authenticate. However, unless the roaming arrangements are
advertised, the network access client cannot determine a priori
whether successful authentication is likely. In this situation,
it is possible that the user will need to try multiple networks in
order to find one to which it can successfully authenticate, or it
is possible that the user will not be able to obtain access at
all, even though successful authentication is feasible.
o The user has multiple sets of credentials. Where no pre-
configuration exists, it is possible that the user will not be
able to determine which credentials to use with which attachment
point, or even whether any credentials it possesses will allow it
to authenticate successfully. An identity and associated
credentials can be usable for authentication with multiple
networks, and not all of these networks will be pre-configured.
For example, the user could have one set of credentials from a
public service provider and another set from an employer, and a
network might enable authentication with one or more of these
credentials. Yet, without pre-configuration, multiple
unsuccessful authentication attempts could be needed for each
attachment point in order to determine what credentials are
usable, wasting valuable time and resulting in user frustration.
In order to choose between multiple attachment points, it can be
helpful to provide additional information to enable the correct
credentials to be determined.
o There are multiple potential roaming paths between the visited
realm and the user's home realm, and service parameters or pricing
differs between them. In this situation, there could be multiple
ways for the user to successfully authenticate using the same
identity and credentials, yet the cost of each approach might
differ. In this case, the access network may not be able to
determine the roaming path that best matches the user's
preferences. This can lead to the user being charged more than
necessary, or not obtaining the desired services. For example,
the visited access realm could have both a direct relationship
with the home realm and an indirect relationship through a roaming
consortium. Current Authentication, Authorization, and Accounting
(AAA) protocols may not be able to route the access request to the
home AAA sever purely based on the realm within the Network Access
Identifier (NAI) [RFC4282]. In addition, payload packets can be
routed or tunneled differently, based on the roaming relationship
path. 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 sub-problems. Some solution constraints are
outlined in Section 3. Section 4 provides conclusions and
suggestions for future work. Appendix A discusses existing solutions
to portions of the problem.
1.1. Terminology Used in This Document
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].
Authentication, Authorization, and Accounting (AAA)
AAA protocols with EAP support include Remote Authentication
Dial-In User Service (RADIUS) [RFC3579] and Diameter [RFC4072].
Access Point (AP)
An entity that has station functionality and provides access to
distribution services via the wireless medium (WM) for associated
stations.
Access Technology Selection
This refers to the selection between access technologies, e.g.,
802.11, Universal Mobile Telecommunications System (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 packet-switched bearer. 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.
Basic Service Set (BSS)
A set of stations controlled by a single coordination function.
Decorated NAI
A NAI specifying a source route. See Section 2.7 of RFC 4282
[RFC4282] for more information.
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.
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.
Network Access Server (NAS)
The device that peers connect to in order to obtain access to the
network. In Point-to-Point Tunneling Protocol (PPTP) terminology,
this is referred to as the PPTP Access Concentrator (PAC), and in
Layer 2 Tunneling Protocol (L2TP) terminology, it is referred to
as the L2TP Access Concentrator (LAC). In IEEE 802.11, it is
referred to as an Access Point (AP).
Network Discovery
The mechanism used to discover available networks. The discovery
mechanism may be passive or active, and depends on the access
technology. In passive network discovery, the node listens for
network announcements; in active network discovery, the node
solicits network announcements. It is possible for an access
technology to utilize both passive and active network discovery
mechanisms.
Network Selection
Selection of an operator/ISP for network access. Network
selection occurs prior to network access authentication.
Realm
The realm portion of an NAI [RFC4282].
Realm Selection
The selection of the realm (and corresponding NAI) used to access
the network. A realm can be reachable from more than one access
network type, and selection of a realm may not enable network
capabilities.
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).
2. Problem Definition
The network discovery and selection problem can be broken down into
multiple sub-problems. These include:
o Discovery of points of attachment. This involves the discovery of
points of attachment in the vicinity, as well as their
capabilities.
o Identifier selection. This involves selection of the NAI (and
credentials) used to authenticate to the selected point of
attachment.
o AAA routing. This involves routing of the AAA conversation back
to the home AAA server, based on the realm of the selected NAI.
o Payload routing. This involves the routing of data packets, in
the situation where mechanisms more advanced than destination-
based routing are required. While this problem is interesting, it
is not discussed further in this document.
o Network capability discovery. This involves discovering the
capabilities of an access network, such as whether certain
services are reachable through the access network and the charging
policy.
Alternatively, the problem can be divided into discovery, decision,
and selection components. The AAA routing problem, for instance,
involves all components: discovery (which mediating networks are
available), decision (choosing the "best" one), and selection
(selecting which mediating network to use) components.
2.1. Discovery of Points of Attachment
Traditionally, the discovery of points of attachment has been handled
by out-of-band mechanisms or link or network layer advertisements.
RFC 2194 [RFC2194] describes the pre-provisioning of dial-up 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 eXtensible Markup Language (XML)
Document Type Definition (DTD). This covers not only the attributes
of the Points of Presence (PoP) and Internet Service Providers
(ISPs), but also hints on the appropriate NAI to be used with a
particular PoP. The XML DTD supports dial-in and X.25 access, but
has extensible address and media type fields.
As access networks and the points of attachment have proliferated,
out-of-band pre-configuration has become increasingly difficult. For
networks with many points of attachment, keeping a complete and up-
to-date list of points of attachment can be difficult. As a result,
wireless network access clients typically only attempt to pre-
configure information relating to access networks, rather than
individual points of attachment.
In IEEE 802.11 Wireless Local Area Networks (WLAN), the Beacon and
Probe Request/Response mechanism provides a way for Stations to
discover Access Points (AP) and the capabilities of those APs. The
IEEE 802.11 specification [IEEE.802.11-2003] provides support for
both passive (Beacon) and active (Probe Request/Response) discovery
mechanisms; [Fixingapsel] studied the effectiveness of these
mechanisms.
Among the Information Elements (IE) included within the Beacon and
Probe Response is the Service Set Identifier (SSID), a non-unique
identifier of the network to which an AP is attached. The Beacon/
Probe facility therefore enables network discovery, as well as the
discovery of points of attachment and the capabilities of those
points of attachment.
The Global System for Mobile Communications (GSM) specifications also
provide for discovery of points of attachment, as does the Candidate
Access Router Discovery (CARD) [RFC4066] protocol developed by the
IETF SEAMOBY Working Group (WG).
Along with discovery of points of attachment, the capabilities of
access networks are also typically discovered. These may include:
o Access network name (e.g., IEEE 802.11 SSID)
o Lower layer security mechanism (e.g., IEEE 802.11 Wired Equivalent
Privacy (WEP) vs. Wi-Fi Protected Access 2 (WPA2))
o Quality of service capabilities (e.g., IEEE 802.11e support)
o Bearer capabilities (e.g., circuit-switched, packet-switched, or
both)
Even though pre-configuration of access networks scales better than
pre-configuration of points of attachment, where many access networks
can be used to authenticate to a home realm, providing complete and
up-to-date information on each access network can be challenging.
In such a situation, network access client configuration can be
minimized by providing information relating to each home realm,
rather than each access network. One way to enable this is for an
access network to support "virtual Access Points" (virtual APs), and
for each point of attachment to support virtual APs corresponding to
each reachable home realm.
While a single IEEE 802.11 network may only utilize a single SSID, it
may cover a wide geographical area, and as a result, may be
segmented, utilizing multiple prefixes. It is possible that a single
SSID may be advertised on multiple channels, and may support multiple
access mechanisms (including Universal Access Method (UAM) and IEEE
802.1X [IEEE.8021X-2004]) which may differ between points of
attachment. A single SSID may also support dynamic VLAN access as
described in [RFC3580], or may support authentication to multiple
home AAA servers supporting different realms. As a result, users of
a single point of attachment, connecting to the same SSID, may not
have the same set of services available.
2.2. Identity Selection
As networks proliferate, it becomes more and more likely that a 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, and one or
more wireless Wide Area Network (WAN) providers.
Typically, the user will choose an identity and corresponding
credential set based on the selected network, perhaps with additional
assistance provided by the chosen authentication mechanism. For
example, if Extensible Authentication Protocol - Transport Layer
Security (EAP-TLS) is the authentication mechanism used with a
particular network, then the user will select the appropriate EAP-TLS
client certificate based, in part, on the list of trust anchors
provided by the EAP-TLS server.
However, in access networks where roaming is enabled, the mapping
between an access network and an identity/credential set may not be
one to one. For example, it is possible for multiple identities to
be usable on an access network, or for a given identity to be usable
on a single access network, which may or may not be available.
Figure 1 illustrates a situation where a user identity may not be
usable on a potential access network. In this case, access network 1
enables access to users within the realm "isp1.example.com", whereas
access network 3 enables access to users within the realm
"corp2.example.com"; access network 2 enables access to users within
both realms.
? ? +---------+ +------------------+
? | Access | | |
O_/ _-->| Network |------>|"isp1.example.com"|
/| / | 1 | _->| |
| | +---------+ / +------------------+
_/ \_ | /
| +---------+ /
User "subscriber@isp1. | | Access |/
example.com" -- ? -->| Network |
also known as | | 2 |\
"employee123@corp2. | +---------+ \
example.com" | \
| +---------+ \_ +-------------------+
\_ | Access | ->| |
-->| Network |------>|"corp2.example.com"|
| 3 | | |
+---------+ +-------------------+
Figure 1: Two credentials, three possible access networks
In this situation, a user only possessing an identity within the
"corp2.example.com" realm can only successfully authenticate to
access networks 2 or 3; a user possessing an identity within the
"isp1.example.com" realm can only successfully authenticate to access
networks 1 or 2; a user possessing identities within both realms can
connect to any of the access networks. The question is: how does the
user figure out which access networks it can successfully
authenticate to, preferably prior to choosing a point of attachment?
Traditionally, hints useful in identity selection have been provided
out-of-band. For example, the XML DTD, described in [RFC3017],
enables a client to select between potential points of attachment as
well as to select the NAI and credentials to use in authenticating
with it.
Where all points of attachment within an access network enable
authentication utilizing a set of realms, selection of an access
network provides knowledge of the identities that a client can use to
successfully authenticate. For example, in an access network, the
set of supported realms corresponding to network name can be pre-
configured.
In some cases, it may not be possible to determine the available
access networks prior to authentication. For example,
[IEEE.8021X-2004] does not support network discovery on IEEE 802
wired networks, so that the peer cannot determine which access
network it has connected to prior to the initiation of the EAP
exchange.
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.
However, there may be situations in which an access network may not
accept a static set of realms at every point of attachment. For
example, as part of a roaming agreement, only points of attachment
within a given region or country may be made available. In these
situations, mechanisms such as hints embedded within credentials or
pre-configuration of access network to realm mappings may not be
sufficient. Instead, it is necessary for the client to discover
usable identities dynamically.
This is the problem that RFC 4284 [RFC4284] attempts to solve, using
the EAP-Request/Identity to communicate a list of supported realms.
However, the problems inherent in this approach are many, as
discussed in Appendix A.1.
Note that identity selection also implies selection of different
credentials, and potentially, selection of different EAP
authentication methods. In some situations this may imply serious
security vulnerabilities. These are discussed in depth in Section 5.
2.3. AAA Routing
Once the identity has been selected, the AAA infrastructure needs to
route the access request back to the home AAA server. Typically, the
routing is based on the Network Access Identifier (NAI) defined in
[RFC4282].
Where the NAI does not encode a source route, the routing of requests
is determined by the AAA infrastructure. As described in [RFC2194],
most roaming implementations are relatively simple, relying on a
static realm routing table that determines the next hop based on the
NAI realm included in the User-Name attribute within the Access-
Request. Within RADIUS, the IP address of the home AAA server is
typically determined based on static mappings of realms to IP
addresses maintained within RADIUS proxies.
Diameter [RFC3588] supports mechanisms for intra- and inter-domain
service discovery, including support for DNS as well as service
discovery protocols such as Service Location Protocol version 2
(SLPv2) [RFC2608]. As a result, it may not be necessary to configure
static tables mapping realms to the IP addresses of Diameter agents.
However, while this simplifies maintenance of the AAA routing
infrastructure, it does not necessarily simplify roaming-relationship
path selection.
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
between each AAA client and server.
Diameter [RFC3588] supports certificate-based authentication (using
either TLS or IPsec) as well as Redirect functionality, enabling a
Diameter client to obtain a referral to the home server from a
Diameter redirect server, so that the client can contact the home
server directly. In situations in which a trust model can be
established, these Diameter capabilities can enable a reduction in
the length of the roaming relationship path.
However, in practice there are a number of pitfalls. In order for
certificate-based authentication to enable communication between a
Network Access Server (NAS) or local proxy and the home AAA server,
trust anchors need to be configured, and certificates need to be
selected. The AAA server certificate needs to chain to a trust
anchor configured on the AAA client, and the AAA client certificate
needs to chain to a trust anchor configured on the AAA server. Where
multiple potential roaming relationship paths are available, this
will reflect itself in multiple certificate choices, transforming the
path selection problem into a certificate selection problem.
Depending on the functionality supported within the certificate
selection implementation, this may not make the problem easier to
solve. For example, in order to provide the desired control over the
roaming path, it may be necessary to implement custom certificate
selection logic, which may be difficult to introduce within a
certificate handling implementation designed for general-purpose
usage.
As noted in [RFC4284], it is also possible to utilize an NAI for the
purposes of source routing. In this case, the client provides
guidance to the AAA infrastructure as to how it would like the access
request to be routed. An NAI including source-routing information is
said to be "decorated"; the decoration format is defined in
[RFC4282].
When decoration is utilized, the EAP peer provides the decorated NAI
within the EAP-Response/Identity, and as described in [RFC3579], the
NAS copies the decorated NAI included in the EAP-Response/Identity
into the User-Name attribute included within the access request. As
the access request transits the roaming relationship path, AAA
proxies determine the next hop based on the realm included within the
User-Name attribute, in the process, successively removing decoration
from the NAI included in the User-Name attribute. In contrast, the
decorated NAI included within the EAP-Response/Identity encapsulated
in the access request remains untouched. As a result, when the
access request arrives at the AAA home server, the decorated NAI
included in the EAP-Response/Identity may differ from the NAI
included in the User-Name attribute (which may have some or all of
the decoration removed). For the purpose of identity verification,
the EAP server utilizes the NAI in the User-Name attribute, rather
than the NAI in the EAP-Response/Identity.
Over the long term, it is expected that the need for NAI "decoration"
and source routing will disappear. 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 Unix-to-Unix
CoPy Protocol (UUCP) and FidoNet. Prior to the implementation of
email gateways utilizing MX RR routing, email address-based source-
routing was used extensively. However, over time the need for email
source-routing disappeared.
2.3.1. The Default Free Zone
AAA clients on the edge of the network, such as NAS devices and local
AAA proxies, typically maintain a default realm route, providing a
default next hop for realms not otherwise taken into account within
the realm routing table. This permits devices with limited resources
to maintain a small realm routing table. Deeper within the AAA
infrastructure, AAA proxies may be maintained with a "default free"
realm table, listing next hops for all known realms, but not
providing a default realm route.
While dynamic realm routing protocols are not in use within AAA
infrastructure today, even if such protocols were to be introduced,
it is likely that they would be deployed solely within the core AAA
infrastructure, but not on NAS devices, which are typically resource
constrained.
Since NAS devices do not maintain a full realm routing table, they do
not have knowledge of all the realms reachable from the local
network. The situation is analogous to that of Internet hosts or
edge routers that do not participate in the BGP mesh. In order for
an Internet host to determine whether it can reach a destination on
the Internet, it is necessary to send a packet to the destination.
Similarly, when a user provides an NAI to the NAS, the NAS does not
know a priori whether or not the realm encoded in the NAI is
reachable; it simply forwards the access request to the next hop on
the roaming relationship path. Eventually, the access request
reaches the "default free" zone, where a core AAA proxy determines
whether or not the realm is reachable. As described in [RFC4284],
where EAP authentication is in use, the core AAA proxy can send an
Access-Reject, or it can send an Access-Challenge encapsulating an
EAP-Request/Identity containing "realm hints" based on the content of
the "default free" realm routing table.
There are a number of intrinsic problems with this approach. Where
the "default free" routing table is large, it may not fit within a
single EAP packet, and the core AAA proxy may not have a mechanism
for selecting the most promising entries to include. Even where the
"default free" realm routing table would fit within a single EAP-
Request/Identity packet, the core AAA router may not choose to
include all entries, since the list of realm routes could be
considered confidential information not appropriate for disclosure to
hosts seeking network access. Therefore, it cannot be assumed that
the list of "realm hints" included within the EAP-Request/Identity is
complete. Given this, a NAS or local AAA proxy snooping the EAP-
Request/Identity cannot rely on it to provide a complete list of
reachable realms. The "realm hint" mechanism described in [RFC4284]
is not a dynamic routing protocol.
2.3.2. Route Selection and Policy
Along with lack of a dynamic AAA routing protocol, today's AAA
infrastructure lacks mechanisms for route selection and policy. As a
result, multiple routes may exist to a destination realm, without a
mechanism for the selection of a preferred route.
In Figure 2, Roaming Groups 1 and 2 both include a route to the realm
"a.example.com". However, these realm routes are not disseminated to
the NAS along with associated metrics, and, as a result, there is no
mechanism for implementation of dynamic routing policies (such as
selection of realm routes by shortest path, or preference for routes
originating at a given proxy).
+---------+
| |----> "a.example.com"
| Roaming |
+---------+ | Group 1 |
| |----->| Proxy |----> "b.example.com"
user "joe@ | Access | +---------+
a.example.com"--->| Provider|
| NAS | +---------+
| |----->| |----> "a.example.com"
+---------+ | Roaming |
| Group 2 |
| Proxy |----> "c.example.com"
+---------+
Figure 2: Multiple routes to a destination realm
In the example in Figure 2, access through Roaming Group 1 may be
less expensive than access through Roaming Group 2, and as a result
it would be desirable to prefer Roaming Group 1 as a next hop for an
NAI with a realm of "a.example.com". However, the only way to obtain
this result would be to manually configure the NAS realm routing
table with the following entries:
Realm Next Hop
----- --------
b.example.com Roaming Group 1
c.example.com Roaming Group 2
Default Roaming Group 1
While manual configuration may be practical in situations where the
realm routing table is small and entries are static, where the list
of supported realms change frequently, or the preferences change
dynamically, manual configuration will not be manageable.
2.3.3. Source Routing
Due to the limitations of current AAA routing mechanisms, there are
situations in which NAI-based source routing is used to influence the
roaming relationship path. However, since the AAA proxies on the
roaming relationship path are constrained by existing relationships,
NAI-based source routing is not source routing in the classic sense;
it merely suggests preferences that the AAA proxy can choose not to
accommodate.
Where realm routes are set up as the result of pre-configuration and
dynamic route establishment is not supported, if a realm route does
not exist, then NAI-based source routing cannot establish it. Even
where dynamic route establishment is possible, such as where the AAA
client and server support certificate-based authentication, and AAA
servers are discoverable (such as via the mechanisms described in
[RFC3588]), an AAA proxy may choose not to establish a realm route by
initiating the discovery process based on a suggestion in an NAI-
based source route.
Where the realm route does exist, or the AAA proxy is capable of
establishing it dynamically, the AAA proxy may choose not to
authorize the client to use it.
While, in principle, source routing can provide users with better
control over AAA routing decisions, there are a number of practical
problems to be overcome. In order to enable the client to construct
optimal source routes, it is necessary for it to be provided with a
complete and up-to-date realm routing table. However, if a solution
to this problem were readily available, then it could be applied to
the AAA routing infrastructure, enabling the selection of routes
without the need for user intervention.
As noted in [Eronen04], only a limited number of parameters can be
updated dynamically. For example, quality of service or pricing
information typically will be pre-provisioned or made available on
the web rather than being updated on a continuous basis. Where realm
names are communicated dynamically, the "default free" realm list is
unlikely to be provided in full since this table could be quite
large. Given the constraints on the availability of information, the
construction of source routes typically needs to occur in the face of
incomplete knowledge.
In addition, there are few mechanisms available to audit whether the
requested source route is honored by the AAA infrastructure. For
example, an access network could advertise a realm route to
"costsless.example.com", while instead routing the access-request
through "costsmore.example.com". While the decorated NAI would be
made available to the home AAA server in the EAP-Response/Identity,
the home AAA server might have a difficult time verifying that the
source route requested in the decorated NAI was actually honored by
the AAA infrastructure. Similarly, it could be difficult to
determine whether quality of service (QoS) or other routing requests
were actually provided as requested. To some extent, this problem
may be addressed as part of the business arrangements between roaming
partners, which may provide minimum service-level guarantees.
Given the potential issues with source routing, conventional AAA
routing mechanisms are to be preferred wherever possible. Where an
error is encountered, such as an attempt to authenticate to an
unreachable realm, "realm hints" can be provided as described
[RFC4284]. However, this approach has severe scalability
limitations, as outlined in Appendix A.1.
2.4. Network Capabilities Discovery
Network capability discovery focuses on discovery of the services
offered by networks, not just the capabilities of individual points
of attachment. By acquiring additional information on access network
characteristics, it is possible for users to make a more informed
access decision. These characteristics may include:
o Roaming relationships between the access network provider and
other network providers and associated costs. Where the network
access client is not pre-configured with an identity and
credentials corresponding to a local access network, it will need
to be able to determine whether one or more home realms are
reachable from an access network so that successful authentication
can be possible.
o EAP authentication methods. While the EAP authentication methods
supported by a home realm can only be determined by contacting the
home AAA server, it is possible that the local realm will also
support one or more EAP methods. For example, a user may be able
to utilize EAP-SIM (Extensible Authentication Protocol -
Subscriber Identity Module) to authenticate to the access network
directly, rather than having to authenticate to the home network.
o End-to-end quality of service capability. While local quality of
service capabilities are typically advertised by the access
network (e.g., support for Wi-Fi Multimedia (WMM)), the
availability of end-to-end QoS services may not be advertised.
o Service parameters, such as the existence of middleboxes or
firewalls. If the network access client is not made aware of the
Internet access that it will receive on connecting to a point of
attachment, it is possible that the user may not be able to access
the desired services.
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)
o Post-authentication
In the interest of minimizing connectivity delays, all of the
information required for network selection (including both access
network capabilities and global characteristics) needs to be provided
prior to authentication.
By the time authentication occurs, the node has typically selected
the access network, the NAI to be used to authenticate, as well as
the point of attachment. Should it learn information during the
authentication process that would cause it to revise one or more of
those decisions, the node will need to select a new network, point of
attachment, and/or identity, and then go through the authentication
process all over again. Such a process is likely to be both time
consuming and unreliable.
3. Design Issues
The following factors should be taken into consideration while
evaluating solutions to the problem of network selection and
discovery.
3.1. AAA Routing
Solutions to the AAA routing issues discussed in Section 2.3 need to
apply to a wide range of AAA messages, and should not restrict the
introduction of new AAA or access network functionality. For
example, AAA routing mechanisms should work for access requests and
responses as well as accounting requests and responses and server-
initiated messages. Solutions should not restrict the development of
new AAA attributes, access types, or performance optimizations (such
as fast handoff support).
3.2. Backward Compatibility
Solutions need to maintain backward compatibility. In particular:
o Selection-aware clients need to interoperate with legacy NAS
devices and AAA servers.
o Selection-aware AAA infrastructure needs to interoperate with
legacy clients and NAS devices.
For example, selection-aware clients should not transmit packets
larger than legacy NAS devices or AAA servers can handle. Where
protocol extensions are required, changes should be required to as
few infrastructure elements as possible. For example, extensions
that require upgrades to existing NAS devices will be more difficult
to deploy than proposals that are incrementally deployable based on
phased upgrades of clients or AAA servers.
3.3. Efficiency Constraints
Solutions should be efficient as measured by channel utilization,
bandwidth consumption, handoff delay, and energy utilization.
Mechanisms that depend on multicast frames need to be designed with
care since multicast frames are often sent at the lowest supported
rate and therefore consume considerable channel time as well as
energy on the part of listening nodes. Depending on the deployment,
it is possible for bandwidth to be constrained both on the link, as
well as in the backend AAA infrastructure. As a result, chatty
mechanisms such as keepalives or periodic probe packets are to be
avoided. Given the volume handled by AAA servers, solutions should
also be conscious of adding to the load, particularly in cases where
this could enable denial-of-service attacks. For example, it would
be a bad idea for a NAS to attempt to obtain an updated realm routing
table by periodically sending probe EAP-Response/Identity packets to
the AAA infrastructure in order to obtain "realm hints" as described
in [RFC4284]. Not only would this add significant load to the AAA
infrastructure (particularly in cases where the AAA server was
already overloaded, thereby dropping packets resulting in
retransmission by the NAS), but it would also not provide the NAS
with a complete realm routing table, for reasons described in
Section 2.3.
Battery consumption is a significant constraint for handheld devices.
Therefore, mechanisms that require significant increases in packets
transmitted, or the fraction of time during which the host needs to
listen (such as proposals that require continuous scanning), are to
be discouraged. In addition, the solution should not significantly
impact the time required to complete network attachment.
3.4. Scalability
Given limitations on frame sizes and channel utilization, it is
important that solutions scale less than linearly in terms of the
number of networks and realms supported. For example, solutions such
as [RFC4284] increase the size of advertisements in proportion to the
number of entries in the realm routing table. This approach does not
scale to support a large number of networks and realms.
Similarly, approaches that utilize separate Beacons for each "virtual
AP" introduce additional Beacons in proportion to the number of
networks being advertised. While such an approach may minimize the
pre-configuration required for network access clients, the
proliferation of "virtual APs" can result in high utilization of the
wireless medium. For example, the 802.11 Beacon is sent only at a
rate within the basic rate set, which typically consists of the
lowest supported rates, or perhaps only the lowest supported rate.
As a result, "virtual AP" mechanisms that require a separate Beacon
for each "virtual AP" do not scale well.
For example, with a Beacon interval of 100 Time Units (TUs) or 102.4
ms (9.8 Beacons/second), twenty 802.11b "virtual APs" each announcing
their own Beacon of 170 octets would result in a channel utilization
of 37.9 percent. The calculation can be verified as follows:
1. A single 170-octet Beacon sent at 1 Mbps will utilize the channel
for 1360 us (1360 bits @ 1 Mbps);
2. Adding 144 us for the Physical Layer Convergence Procedure (PLCP)
long preamble (144 bits @ 1 Mbps), 48 us for the PLCP header (48
bits @ 1 Mbps), 10 us for the Short Interframe Space (SIFS), 50 us
for the Distributed Interframe Space (DIFS), and 320 us for the
average minimum Contention Window without backoff (CWmin/2 *
aSlotTime = 32/2 * 20 us) implies that a single Beacon will
utilize an 802.11b channel for 1932 us;
3. Multiply the channel time per Beacon by 196 Beacons/second, and we
obtain a channel utilization of 378672 us/second = 37.9 percent.
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. Another issue with the Beacon and Probe
Request/Response mechanism is that it is either insecure or its
security can be assured only as part of authenticating to the network
(e.g., verifying the advertised capabilities within the 4-way
handshake).
A number of enhancements have been proposed to the Beacon/Probe
Response mechanism in order to improve scalability and performance in
roaming scenarios. These include allowing APs to announce
capabilities of neighbor APs as well as their own [IEEE.802.11k].
More scalable mechanisms for support of "virtual APs" within IEEE
802.11 have also been proposed [IEEE.802.11v]; generally these
proposals collapse multiple "virtual AP" advertisements into a single
advertisement.
Higher-layer mechanisms can also be used to improve scalability
since, by running over IP, they can utilize facilities, such as
fragmentation, that may not be available at the link layer. For
example, in IEEE 802.11, Beacon frames cannot use fragmentation
because they are multicast frames.
3.5. Static Versus Dynamic Discovery
"Phone-book" based approaches such as [RFC3017] can provide
information for automatic selection decisions. While this approach
has been applied to wireless access, it typically can only be used
successfully within a single operator or limited roaming partner
deployment. For example, were a "Phone-Book" approach to attempt to
incorporate information from a large number of roaming partners, it
could become quite difficult to keep the information simultaneously
comprehensive and up to date. As noted in [Priest04] and [GROETING],
a large fraction of current WLAN access points operate on the default
SSID, which may make it difficult to distinguish roaming partner
networks by SSID. In any case, in wireless networks, dynamic
discovery is a practical requirement since a node needs to know which
APs are within range before it can connect.
3.6. Security
Network discovery and selection mechanisms may introduce new security
vulnerabilities. As noted in Section 2.3.1, network operators may
consider the AAA routing table to be confidential information, and
therefore may not wish to provide it to unauthenticated peers via the
mechanism described in RFC 4284. While the peer could provide a list
of the realms it supports, with the authenticator choosing one, this
approach raises privacy concerns. Since identity selection occurs
prior to authentication, the peer's supported realms would be sent in
cleartext, enabling an attacker to determine the realms for which a
potential victim has credentials. This risk can be mitigated by
restricting peer disclosure. For example, a peer may only disclose
additional realms in situations where an initially selected identity
has proved unusable.
Since network selection occurs prior to authentication, it is
typically not possible to secure mechanisms for network discovery or
identity selection, although it may be possible to provide for secure
confirmation after authentication is complete. As an example, some
parameters discovered during network discovery may be confirmable via
EAP Channel Bindings; others may be confirmed in a subsequent Secure
Association Protocol handshake.
However, there are situations in which advertised parameters may not
be confirmable. This could lead to "bidding down" vulnerabilities.
Section 7.8 of [RFC3748] states:
Within or associated with each authenticator, it is not
anticipated that a particular named peer will support a choice of
methods. This would make the peer vulnerable to attacks that
negotiate the least secure method from among a set. Instead, for
each named peer, there SHOULD be an indication of exactly one
method used to authenticate that peer name. If a peer needs to
make use of different authentication methods under different
circumstances, then distinct identities SHOULD be employed, each
of which identifies exactly one authentication method.
In practice, where the authenticator operates in "pass-through" mode,
the EAP method negotiation will occur between the EAP peer and
server, and therefore the peer will need to associate a single EAP
method with a given EAP server. Where multiple EAP servers and
corresponding identities may be reachable from the same selected
network, the EAP peer may have difficulty determining which identity
(and corresponding EAP method) should be used. Unlike network
selection, which may be securely confirmed within a Secure
Association Protocol handshake, identity selection hints provided
within the EAP-Request/Identity are not secured.
As a result, where the identity selection mechanism described in RFC
4284 is used, the "hints" provided could be used by an attacker to
convince the victim to select an identity corresponding to an EAP
method offering lesser security (e.g., EAP MD5-Challenge). One way
to mitigate this risk is for the peer to only utilize EAP methods
satisfying the [RFC4017] security requirements, and for the peer to
select the identity corresponding to the strongest authentication
method where a choice is available.
3.7. Management
From an operational point of view, a network device in control of
network advertisement and providing "realm hints" for guiding the
network discovery and selection, should at least offer a management
interface capable of providing status information for operators.
Status information, such as counters of each selected network and
used realm, and when RFC 4284 is used, the count of delivered "realm
hints" might interest operators. Especially the information related
to realms that fall into the "default free zone" or the "AAA fails to
route" are of interest.
Larger deployments would benefit from a management interface that
allow full remote configuration capabilities, for example, of "realm
hints" in case of RFC 4284-conforming network devices. While changes
to "realm hints" and realm routing information are not expected to be
frequent, centralized remote management tends to lower the frequency
of misconfigured devices.
4. Conclusions
This document describes the network selection and discovery problem.
In the opinion of the authors, the major findings are as follows:
o There is a need for additional work on access network discovery,
identifier selection, AAA routing, and payload routing.
o Credential selection and AAA routing are aspects of the same
problem, namely identity selection.
o When considering selection among a large number of potential
access networks and points of attachment, the issues described in
the document become much harder to solve in an automated way,
particularly if there are constraints on handoff latency.
o The proliferation of network discovery technologies within IEEE
802, IETF, and 3rd Generation Partnership Project (3GPP) has the
potential to become a significant problem going forward. Without
a unified approach, multiple non-interoperable solutions may be
deployed.
o New link-layer designs should include efficient distribution of
network and realm information as a design requirement.
o It may not be possible to solve all aspects of the problem for
legacy NAS devices on existing link layers. Therefore, a phased
approach may be more realistic. For example, a partial solution
could be made available for existing link layers, with a more
complete solution requiring support for link layer extensions.
With respect to specific mechanisms for access network discovery and
selection:
o Studies such as [MACScale] and [Velayos], as well as the
calculations described in Section 2.1, demonstrate that the IEEE
802.11 Beacon/Probe Response mechanism has substantial scaling
issues in situations where a new Beacon is used for each "virtual
AP". As a result, a single channel is, in practice, limited to
less than twenty Beacon announcements with IEEE 802.11b.
The situation is improved substantially with successors, such as
IEEE 802.11a, that enable additional channels, thus potentially
increasing the number of potential virtual APs.
However, even with these enhancements, it is not feasible to
advertise more than 50 different networks, and probably less in
most circumstances.
As a result, there appears to be a need to enhance the scalability
of IEEE 802.11 network advertisements.
o Work is underway in IEEE 802.1, IEEE 802.21, and IEEE 802.11u
[IEEE.802.11u] to provide enhanced discovery functionality.
Similarly, IEEE 802.1af [IEEE.802.1af] has discussed the addition
of network discovery functionality to IEEE 802.1X
[IEEE.8021X-2004]. However, neither IEEE 802.1AB [IEEE.802.1ab]
nor IEEE 802.1af is likely to support fragmentation of network
advertisement frames so that the amount of data that can be
transported will be limited.
o While IEEE 802.11k [IEEE.802.11k] provides support for the
Neighbor Report, this only provides for gathering of information
on neighboring 802.11 APs, not points of attachment supporting
other link layers. Solution to this problem would appear to
require coordination across IEEE 802 as well as between standards
bodies.
o Given that EAP does not support fragmentation of EAP-Request/
Identity packets, the volume of "realm hints" that can be fit with
these packets is limited. In addition, within IEEE 802.11, EAP
packets can only be exchanged within State 3 (associated and
authenticated). As a result, use of EAP for realm discovery may
result in significant delays. The extension of the realm
advertisement mechanism defined in [RFC4284] to handle
advertisement of realm capability information (such as QoS
provisioning) is not recommended due to semantic and packet size
limitations [GROETING]. As a result, we believe that extending
the mechanism described in [RFC4284] for discovery of realm
capabilities is inappropriate. Instead, we believe it is more
appropriate for this functionality to be handled within the link
layer so that the information can be available early in the
handoff process.
o Where link-layer approaches are not available, higher-layer
approaches can be considered. A limitation of higher-layer
solutions is that they can only optimize the movement of already
connected hosts, but cannot address scenarios where network
discovery is required for successful attachment.
Higher-layer alternatives worth considering include the SEAMOBY
CARD protocol [RFC4066], which enables advertisement of network
device capabilities over IP, and Device Discovery Protocol (DDP)
[MARQUES], which provides functionality equivalent to IEEE 802.1AB
using ASN.1 encoded advertisements sent to a link-local scope
multicast address.
5. 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 to which the
user tries to attach. Unfortunately, current EAP methods do not
always make the verification of such parameters possible. EAP
methods, such as Protected EAP (PEAP) [JOSEFSSON] and EAP-IKEv2
[IKEV2], may make this possible, however. There is even an attempt
to provide a backward-compatible extension to older methods [ARKKO].
The security requirements for network selection depend on whether the
selection is considered a mandate or a hint. In general, treating
network advertisements as a hint is a more secure approach, since it
reduces access client vulnerability to forged network advertisements.
For example, "realm hints" may be ignored by an EAP peer if they are
incompatible with the security policy corresponding to a selected
access network.
Similarly, network access clients may refuse to connect to a point of
attachment if the advertised security capabilities do not match those
that have been pre-configured. For example, if an IEEE 802.11 access
client has been pre-configured to require WPA2 enterprise support
within an access network, it may refuse to connect to access points
advertising support for WEP.
Where the use of methods that do not satisfy the security
requirements of [RFC4017] is allowed, it may be possible for an
attacker to trick a peer into using an insecure EAP method, leading
to the compromise of long-term credentials. This can occur either
where a network is pre-configured to allow use of an insecure EAP
method, or where connection without pre-configuration is permitted
using such methods.
For example, an attacker can spoof a network advertisement, possibly
downgrading the advertised security capabilities. The rogue access
point would then attempt to negotiate an insecure EAP method. Such
an attack can be prevented if the peer refuses to connect to access
points not meeting its security requirements, which would include
requiring use of EAP methods satisfying the [RFC4017] requirements.
Support for secure discovery could potentially protect against
spoofing of network advertisements, enabling verifiable information
to guide connection decisions. However, development of these
mechanisms requires solving several difficult engineering and
deployment problems.
Since discovery is a prerequisite for authentication, it is not
possible to protect initial discovery using dynamic keys derived in
the authentication process. On the other hand, integrity protection
of network advertisements utilizing symmetric keys or digital
signatures would require pre-configuration.
6. Informative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC3588] Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J.
Arkko, "Diameter Base Protocol", RFC 3588, September 2003.
[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.
[RFC4282] Aboba, B., Beadles, M., Arkko, J., and P. Eronen, "The
Network Access Identifier", RFC 4282, December 2005.
[RFC3280] Housley, R., Polk, W., Ford, W., and D. Solo, "Internet
X.509 Public Key Infrastructure Certificate and
Certificate Revocation List (CRL) Profile", RFC 3280,
April 2002.
[RFC4072] Eronen, P., Hiller, T., and G. Zorn, "Diameter Extensible
Authentication Protocol (EAP) Application", RFC 4072,
August 2005.
[RFC3579] Aboba, B. and P. Calhoun, "RADIUS (Remote Authentication
Dial In User Service) Support For Extensible
Authentication Protocol (EAP)", RFC 3579, September 2003.
[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.
[RFC2608] Guttman, E., Perkins, C., Veizades, J., and M. Day,
"Service Location Protocol, Version 2", RFC 2608,
June 1999.
[RFC3580] Congdon, P., Aboba, B., Smith, A., Zorn, G., and J. Roese,
"IEEE 802.1X Remote Authentication Dial In User Service
(RADIUS) Usage Guidelines", RFC 3580, September 2003.
[RFC4284] Adrangi, F., Lortz, V., Bari, F., and P. Eronen, "Identity
Selection Hints for the Extensible Authentication Protocol
(EAP)", RFC 4284, January 2006.
[RFC4017] Stanley, D., Walker, J., and B. Aboba, "Extensible
Authentication Protocol (EAP) Method Requirements for
Wireless LANs", RFC 4017, March 2005.
[RFC2486] Aboba, B. and M. Beadles, "The Network Access Identifier",
RFC 2486, January 1999.
[RFC4066] Liebsch, M., Singh, A., Chaskar, H., Funato, D., and E.
Shim, "Candidate Access Router Discovery (CARD)",
RFC 4066, July 2005.
[IKEV2] Tschofenig, H., Kroeselberg, D., Pashalidis, A., Ohba, Y.,
and F. Bersani, "EAP-IKEv2 Method", Work in Progress,
September 2007.
[ARKKO] Arkko, J. and P. Eronen, "Authenticated Service
Information for the Extensible Authentication Protocol
(EAP)", Work in Progress, October 2005.
[GROETING] Groeting, W., Berg, S., Tschofenig, H., and M. Ness,
"Network Selection Implementation Results", Work
in Progress, July 2004.
[JOSEFSSON]
Palekar, A., Simon, D., Salowey, J., Zhou, H., Zorn, G.,
and S. Josefsson, "Protected EAP Protocol (PEAP) Version
2", Work in Progress, October 2004.
[MARQUES] Enns, R., Marques, P., and D. Morrell, "Device Discovery
Protocol (DDP)", Work in Progress, May 2003.
[OHBA] Taniuchi, K., Ohba, Y., and D. Subir, "IEEE 802.21 Basic
Schema", Work in Progress, October 2007.
[IEEE.802.11-2003]
IEEE, "Wireless LAN Medium Access Control (MAC) and
Physical Layer (PHY) Specifications", IEEE Standard
802.11, 2003.
[Fixingapsel]
Judd, G. and P. Steenkiste, "Fixing 802.11 Access Point
Selection", Sigcomm Poster Session 2002.
[IEEE.802.11k]
IEEE, "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 802.11k,
D7.0, January 2007.
[IEEE.802.1ab]
IEEE, "Draft Standard for Local and Metropolitan Area
Networks - Station and Media Access Control Connectivity
Discovery", IEEE 802.1AB, D1.0, April 2007.
[IEEE.802.1af]
IEEE, "Draft Standard for Local and Metropolitan Area
Networks - Port-Based Network Access Control - Amendment
1: Authenticated Key Agreement for Media Access Control
(MAC) Security", IEEE 802.1af, D1.2, January 2007.
[IEEE.802.11v]
IEEE, "Draft Amemdment to Standard for Information
Technology - Telecommunications and Information Exchange
Between Systems - LAN/MAN Specific Requirements - Part 11:
Wireless Medium Access Control (MAC) and physical layer
(PHY) specifications: Wireless Network Management",
IEEE 802.11v, D0.09, March 2007.
[Eronen04]
Eronen, P. and J. Arkko, "Role of authorization in
wireless network security", Extended abstract presented in
the DIMACS workshop, November 2004.
[IEEE.11-04-0624]
Berg, S., "Information to Support Network Selection", IEEE
Contribution 11-04-0624 2004.
[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.
[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.
[IEEE.802.11u]
IEEE, "Draft Amendment to STANDARD FOR Information
Technology - LAN/MAN Specific Requirements - Part 11:
Interworking with External Networks; Draft Amendment to
Standard; IEEE P802.11u/D0.04", IEEE 802.11u, D0.04,
April 2007.
[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.
[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 De scription; Release 6; Stage 2",
3GPP Technical Specification 23.234, 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.
[Eronen03]
Eronen, P., "Network Selection Issues", presentation to
EAP WG at IETF 58, November 2003.
[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.802.21]
IEEE, "Draft IEEE Standard for Local and Metropolitan Area
Networks: Media Independent Handover Services",
IEEE 802.21, D05.00, April 2007.
[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.
[IEEE.8021X-2004]
IEEE, "Local and Metropolitan Area Networks: Port-Based
Network Access Control", IEEE Standard 802.1X, July 2004.
Appendix A. Existing Work
A.1. IETF
Several IETF WGs have dealt with aspects of the network selection
problem, including the AAA, EAP, PPP, RADIUS, ROAMOPS, and RADEXT
WGs.
ROAMOPS WG developed the NAI, originally defined in [RFC2486], and
subsequently updated in [RFC4282]. Initial roaming implementations
are described in [RFC2194], and the use of proxies in roaming is
addressed in [RFC2607]. The SEAMOBY WG developed CARD [RFC4066],
which assists in discovery of suitable base stations. PKIX WG
produced [RFC3280], which addresses issues of certificate selection.
The AAA WG developed more sophisticated access routing,
authentication, and service discovery mechanisms within Diameter
[RFC3588].
Adrangi et al. [RFC4284] defines the use of the EAP-Request/Identity
to provide "realm hints" useful for identity selection. The NAI
syntax described in [RFC4282] enables the construction of source
routes. Together, these mechanisms enable the user to determine
whether it possesses an identity and corresponding credential
suitable for use with an EAP-capable NAS. This is particularly
useful in situations where the lower layer provides limited
information (such as in wired IEEE 802 networks where IEEE 802.1X
currently does not provide for advertisement of networks and their
capabilities).
However, advertisement mechanisms based on the use of the EAP-
Request/Identity have scalability problems. As noted in [RFC3748]
Section 3.1, the minimum EAP Maximum Transmission Unit (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 Fully Qualified Domain Names (FQDN) to be up to 255
octets in length, this may not enable the announcement of many
realms. The use of network identifiers other than domain names is
also possible.
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 802.11-2003 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-2004), or that the station must associate with each
AP prior to sending an EAPOL-Start to initiate EAP (here, EAPOL
refers to EAP over LAN). This will dramatically increase handoff
latency.
Thus, rather than thinking of [RFC4284] as an effective network
discovery mechanism, it is perhaps better to consider the use of
"realm hints" as an error recovery technique to be used to inform the
EAP peer that AAA routing has failed, and perhaps to enable selection
of an alternate identity that can enable successful authentication.
Where "realm hints" are only provided in event of a problem, rather
than as a staple network discovery technique, it is probably best to
enable "realm hints" to be sent by core AAA proxies in the "default
free" zone. This way, it will not be necessary for NASes to send
"realm hints", which would require them to maintain a complete and
up-to-date realm routing table, something that cannot be easily
accomplished given the existing state of AAA routing technology.
If realm routing tables are manually configured on the NAS, then
changes in the "default free" realm routing table will not
automatically be reflected in the realm list advertised by the NAS.
As a result, a realm advertised by the NAS might not, in fact, be
reachable, or the NAS might neglect to advertise one or more realms
that were reachable. This could result in multiple EAP-Identity
exchanges, with the initial set of "realm hints" supplied by the NAS
subsequently updated by "realm hints" provided by a core AAA proxy.
In general, originating "realm hints" on core AAA proxies appears to
be a more sound approach, since it provides for "fate sharing" --
generation of "realm hints" by the same entity (the core AAA proxy)
that will eventually need to route the request based on the hints.
This approach is also preferred from a management perspective, since
only core AAA proxies would need to be updated; no updates would be
required to NAS devices.
A.2. IEEE 802
There has been work in several IEEE 802 working groups relating to
network discovery:
o [IEEE.802.11-2003] defines the Beacon and Probe Response
mechanisms within IEEE 802.11. Unfortunately, Beacons may be sent
only at a rate within the base rate set, which typically consists
of the lowest supported rate, or perhaps the next lowest rate.
Studies such as [MACScale] have identified MAC layer performance
problems, and [Velayos] has identified scaling issues 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 compare these scalability issues to those of alternative
solutions, however.
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 a 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 paper 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.
o IEEE 802.11u is working on realm discovery and network selection
[11-05-0822-03-000u-tgu-requirements] [IEEE.802.11u]. This
includes a mechanism for enabling a station to determine the
identities it can use to authenticate to an access network, prior
to associating with that network. As noted earlier, solving this
problem requires the AP to maintain an up-to-date, "default free"
realm routing table, which is not feasible without dynamic routing
support within the AAA infrastructure. Similarly, a priori
discovery of features supported within home realms (such as
enrollment) is also difficult to implement in a scalable way,
absent support for dynamic routing. Determination of network
capabilities (such as QoS support) is considerably simpler, since
these depend solely on the hardware and software contained within
the AP. However, 802.11u is working on Generic Advertisement
Service (GAS) mechanism, which can be used to carry 802.21
Information Service (IS) messages and, in that way, allow a more
sophisticated way of delivering information from the network side.
o IEEE 802.21 [IEEE.802.21] is developing standards to enable
handover between heterogeneous link layers, including both IEEE
802 and non-IEEE 802 networks. To enable this, a general
mechanism for capability advertisement is being developed, which
could conceivably benefit aspects of the network selection
problem, such as realm discovery. For example, IEEE 802.21 is
developing Information Elements (IEs) that may assist with network
selection, including information relevant to both layer 2 and
layer 3. Query mechanisms (including both XML and TLV support)
are also under development. IEEE 802.21 also defines a Resource
Description Framework (RDF) schema to allow use of a query
language (i.e., SPARQL). The schema is a normative part of IEEE
802.21 and also defined in [OHBA].
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 also discusses realm discovery and
network selection issues. The I-WLAN realm discovery procedure
borrows ideas from the cellular Public Land-based Mobile Network
(PLMN) selection principles, known as "PLMN Selection".
In 3GPP PLMN selection [3GPP.23.122], the mobile node monitors
surrounding cells and prioritizes them based on signal strength
before selecting a new potential target cell. Each cell broadcasts
its PLMN. A mobile node may automatically select cells that belong
to its Home PLMN, Registered PLMN, or an allowed set of Visited
PLMNs. The PLMN lists are prioritized and stored in the Subscriber
Identity Module (SIM). In the case of manual PLMN selection, the
mobile node lists the PLMNs it learns about from surrounding cells
and enables the user to choose the desired PLMN. After the PLMN has
been selected, cell prioritization takes place in order to select the
appropriate target cell.
[WLAN3G] discusses the new realm (PLMN) selection requirements
introduced by I-WLAN roaming, which support automatic PLMN selection,
not just manual selection. Multiple network levels may be present,
and the hotspot owner may have a contract with a provider who, in
turn, has a contract with a 3G network, which may have a roaming
agreement with other networks.
The I-WLAN specification requires that network discovery be performed
as specified in the relevant WLAN link layer standards. In addition
to network discovery, it is necessary to select intermediary realms
to enable construction of source routes. In 3GPP, the intermediary
networks are PLMNs, and it is assumed that an access network may have
a roaming agreement with more than one PLMN. The PLMN may be a Home
PLMN (HPLMN) or a Visited PLMN (VPLMN), where roaming is supported.
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 procedure is required:
o The user may choose the desired HPLMN or VPLMN manually or let the
WLAN User Equipment (WLAN UE) choose the PLMN automatically, based
on user and operator defined preferences.
o AAA messages are routed based on the decorated or undecorated NAI.
o EAP is utilized as defined in [RFC3748] and [RFC3579].
o PLMN advertisement and selection is based on [RFC4284], which
defines only realm advertisement. The document refers to the
potential need for extensibility, though EAP MTU restrictions make
this difficult.
The I-WLAN specification states that "realm hints" are only provided
when an unreachable realm is encountered. Where VPLMN control is
required, this is handled via NAI decoration. The station may
manually trigger PLMN advertisement by including an unknown realm
(known as the Alternative NAI) within the EAP-Response/Identity. A
realm guaranteed not to be reachable within 3GPP networks is utilized
for this purpose.
The I-WLAN security requirements are described in the 3GPP stage 3
technical specification [3GPPSA3WLANTS]. The security requirements
for PLMN selection are discussed in 3GPP contribution
[3GPP-SA3-030736], which concludes that both SSID and EAP-based
mechanisms have similar security weaknesses. As a result, it
recommends that PLMN advertisements should be considered as hints.
A.4. Other
[INTELe2e] discusses the need for realm selection where an access
network may have more than one roaming relationship path to a home
realm. It also describes solutions to the realm selection problem
based on EAP, SSID and Protected EAP (PEAP) based mechanisms.
Eijk et al. [WWRF-ANS] discusses the realm and network selection
problem. The authors concentrate primarily on discovery of access
networks meeting a set of criteria, noting that information on the
realm capabilities and reachability inherently resides in home AAA
servers, and therefore it is not readily available in a central
location, and may not be easily obtained by NAS devices.
Appendix B. Acknowledgements
The authors of this document would like to especially acknowledge the
contributions of Farid Adrangi, Michael Richardson, Pasi Eronen, Mark
Watson, Mark Grayson, Johan Rune, and Tomas Goldbeck-Lowe.
Input for the early versions of this document 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.
Jouni Korhonen would like to thank the Academy of Finland for
providing funding to work on this document.
Authors' Addresses
Jari Arkko
Ericsson
Jorvas 02420
Finland
EMail: jari.arkko@ericsson.com
Bernard Aboba
Microsoft
One Microsoft Way
Redmond, WA 98052
USA
EMail: bernarda@microsoft.com
Jouni Korhonen
TeliaSonera
Teollisuuskatu 13
Sonera FIN-00051
Finland
EMail: jouni.korhonen@teliasonera.com
Farooq Bari
AT&T
7277 164th Avenue N.E.
Redmond WA 98052
USA
EMail: farooq.bari@att.com
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