Extensible Authentication Protocol J. Arkko
Internet-Draft Ericsson
Expires: April 25, 2005 B. Aboba, Eds.
Microsoft
October 25, 2004
Network Discovery and Selection Problem
draft-ietf-eap-netsel-problem-02
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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
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is defined and divided into subproblems, and some constraints for
possible solutions are outlined. The document presents also some
existing mechanisms which help solve at least parts of the problem,
and gives some suggestions on how to proceed for the rest.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Problem Definition . . . . . . . . . . . . . . . . . . . . . . 4
2.1 Access Network Discovery . . . . . . . . . . . . . . 4
2.2 Identity selection . . . . . . . . . . . . . . . . . 5
2.3 AAA routing . . . . . . . . . . . . . . . . . . . . 7
2.3.1 The Incomplete Routing Table Problem . . . 8
2.3.2 The User Selection Problem . . . . . . . . 8
2.4 Payload Routing . . . . . . . . . . . . . . . . . . 10
2.4.1 Issues with Payload Routing . . . . . . . 10
2.5 Discovery, Decision, and Selection . . . . . . . . . 11
2.6 Type of Information . . . . . . . . . . . . . . . . 13
3. Design Constrains . . . . . . . . . . . . . . . . . . . . . . 14
4. Existing Work . . . . . . . . . . . . . . . . . . . . . . . . 15
4.1 IETF . . . . . . . . . . . . . . . . . . . . . . . . 15
4.2 IEEE . . . . . . . . . . . . . . . . . . . . . . . . 16
4.3 3GPP . . . . . . . . . . . . . . . . . . . . . . . . 17
4.4 Other . . . . . . . . . . . . . . . . . . . . . . . 18
5. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 20
6. Security Considerations . . . . . . . . . . . . . . . . . . . 23
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 24
7.1 Normative References . . . . . . . . . . . . . . . . . 24
7.2 Informative References . . . . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 27
A. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 28
B. Modifications from Version 00 . . . . . . . . . . . . . . . . 29
Intellectual Property and Copyright Statements . . . . . . . . 30
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1. Introduction
The so called network discovery and selection problem affects network
access and wireless access networks in particular. This problem
comes relevant when any of the following conditions are true:
o There is more than one available network attachment point, and the
different points may have different characteristics.
o The user has multiple sets of credentials. For instance, the user
could have one set of credentials from a public service provider
and another set from his company.
o There is more than one way to provide roaming between the access
and home network, and service parameters or pricing differs
between them. For instance, the access network could have both a
direct relationship with the home network and an indirect
relationship through a roaming consortium.
o The roaming relationships between access and home networks are so
complicated that current AAA protocols can not route the requests
to the home network unaided, just based on the domain in the given
Network Access Identifier (NAI) [4, 19].
o Payload packets get routed or tunneled differently, based on which
particular roaming relationship variation is used. This may have
an impact on the available services or their pricing.
o Providers share the same infrastructure, such as wireless access
points.
The network discovery and selection problem spans multiple protocol
layers and has been the subject of discussions in IETF, 3GPP, and
IEEE 802.11. This document summarizes the discussion held about this
problem in the Extensible Authentication Protocol working group at
IETF.
In Section 2 the problem is defined and divided into subproblems, and
some constraints for possible solutions are outlined in Section 3.
Section 4 discusses existing mechanisms which help solve at least
parts of the problem. Section 5 gives some suggestions on how to
proceed for the rest.
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2. Problem Definition
There are four somewhat orthogonal problems being discussed under the
rubric of "network discovery and selection".
o First, there is the problem of "Access Network discovery". This
is the problem of discovering access networks available in the
vicinity, and the points of presence (POPs) associated with those
networks.
o Second, there is the problem of "Identifier selection". This is
the problem of selecting which identity (and credentials) to use
to authenticate to a given POP.
o Thirdly, there is 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 Finally, there is the the problem of "Payload routing" which
involves figuring how the payload packets are routed, where more
advanced mechanisms than destination-based routing is needed.
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 mediting network it has decided to choose)
components.
2.1 Access Network Discovery
The Access Network Discovery problem has been extensively studied,
see for instance the results of the IETF Seamoby WG, IEEE
specifications on 802.11 wireless LAN beaconing and probing process,
studies (such as [38]) on the effectiveness of these mechanisms, and
so on.
Traditionally, the problem of discovering available networks has been
handled as a part of the link layer attachment procedures, or through
out-of-band mechanisms.
RFC 2194 [3] 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 [8] 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
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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 [37], 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;
propagation of discovery information over IP, as handled in the CARD
protocol developed within the IETF SEAMOBY WG, etc.
Typically scalability enhancement mechanisms attempt to get around
Beacon/Probe Response restrictions by sending advertisements at the
higher rates which are enabled one the station has associated with an
AP. 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.
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.
2.2 Identity selection
As networks proliferate, it becomes more and more likely that a given
EAP peer may have multiple identities and credential sets, available
for use in different situations. For example, the EAP peer may have
an account with one or more Public WLAN providers; a corporate WLAN;
one or more wireless WAN providers. As a result, the EAP peer has to
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decide which credential set to use when presented with a given set of
potential EAP authenticators.
Figure 1 illustrates a situation where the user does not know whether
he is connected to access network 1, which only serves the ISP,
access network 3, which only serves the company, or access network 2,
which serves both.
+---------+ +---------+
| Access | | |
_---->| Network |------>| isp.com |
/ | 1 | _->| |
| +---------+ / +---------+
| /
| +---------+ /
User "subscriber@ | | Access |/
isp.com" also known as --- ? ---->| Network |
"employee123@corp.com" | | 2 |\
| +---------+ \
| \
| +---------+ \_ +---------+
\_ | Access | ->| |
---->| Network |------>| corp.com|
| 3 | | |
+---------+ +---------+
Figure 1: Two credentials, three possible access links
Traditionally, hints useful in identity selection have also been
provided out-of-band. For example, via the RFC 3017 XML DTD [8], 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 POP.
Perhaps the most typical case is a link layer that provides some
information about the network before EAP is initiated. For instance,
in IEEE 802.11 provides the SSID, though in some cases the client may
not learn about all the SSIDs supported by the given access point.
In IKEv2 [18], the identity of the responder (typically the security
gateway) is provided as a part of the usual IKEv2 exchange.
In order to use this information in deciding the right identity to
use, the provided information has to either match with one of the
client's home networks, or the client has to have some other
knowledge that enables to link the advertised network and the home
network. For instance, the client may be aware that his home network
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has a roaming contract with a given access network.
It is also possible for hints to be embedded within credentials. In
[11], 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
relating to the network requesting EAP authentication. While there
is Standards Track RFC specifying the interpretation of the field
beyond the NUL character, [12] is widely expected to be used.
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 2486 [4, 19], and the ability of the AAA
network to route requests to the domain indicated in the NAI.
Within the IETF ROAMOPS WG, a number of additional approaches were
considered for this, 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 [3], static routing mechanisms appeared adequate for the task
and it was not deemed necessary to develop dynamic routing protocols.
As noted in RFC 2607 [5], 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, 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.
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
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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@corp3.com" has to be authenticated through ISP
2, since the domain "corp3.com" is served by it.
+---------+ +---------+
| | | |
User "joe | Access |--------->| ISP 1 |-------> "corp1.com"
@corp3.com"-->| Network | | |
| | +---------+
+---------+
|
|
\|/
+---------+
| |--------> "corp2.com"
| ISP 2 |
| |--------> "corp3.com"
+---------+
Figure 2: AAA routing problem
2.3.2 The User 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. For commercial reasons, intermediaries want to participate
the AAA transaction.
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+---------+
| |---------> "isp1.com"
| Roaming |
+---------+ | Group 1 |
| |-------->| |---------> "isp2.com"
User "joe | Access | +---------+
@isp1.com"--->| Network |
| | +---------+
| |-------->| |---------> "isp1.com"
+---------+ | Roaming |
| Group 2 |
| |---------> "isp3.com"
+---------+
Figure 3: Ambiguous AAA routing
Currently planned networks include one level with a small number of
intermediaries, just a few now and perhaps up to 50 as a maximum.
However, multiple levels and higher number of alternative networks
are possible in theory.
There has 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
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.com,
int2.com, and int3.com, it will route your request through one of
them, even if you tried to request routing through mitm.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.net, and
then switch the user's selection to priceyintermediary.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
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claimed by the network were indeed provided. As a result, it may be
useful to think of the intermediary selection only as a hint.
Only a limited amount of information about AAA routes or pricing
information can be dynamically communicated [42]. 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 are preferred over
user-based selection where sufficient routes have been configured and
there is no need for user control. Where these conditions are not
met -- particularly when an attempt to use the network-based routing
mechanism has failed -- routing hints can be placed in an NAI as
defined in [4, 19]. Where NAIs are used in this manner, the AAA
routing problem becomes a subset of the identifier selection problem.
2.4 Payload Routing
The access network, mediating AAA infrastructure, and the home
network may all have a desire to affect the kind of treatment payload
packets get. This includes filtering, prioritization, routing paths,
and mandatory tunneling.
Traditionally, the involved entities have been able to provide some
control of this through AAA protocols such as RADIUS [6] and Diameter
[9]. RFC 2868 [7] defines tunneling attributes that the home and
mediating networks can use to establish mandatory tunneling at the
access network. RFC 3588 [9] defines a filter syntax which can be
used to install per-session filters under the control of AAA.
2.4.1 Issues with Payload Routing
In an attack described by Michael Richardson, a rogue hotspot
operator (but with a legitimate roaming relationship to a home
network) steals revenues from local hotspot operator by spoofing its
identity. The rogue operator places a node with two interfaces in
the coverage area of the local operator, and attaches to the Internet
via this operator using a flat fee -based account. It then starts to
advertise itself as a hotspot operator on the other interface, luring
unsuspecting clients to use this node rather the than the local
operator. The users authenticate via this node and its roaming
relationship. All AAA and payload traffic goes via the local
hotspot, suitably NATted by the rogue node. As a result, the rogue
operator gets the roaming service fees for a number of clients,
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whereas the local operator gets just one client.
Due to the way that the IEEE 802.11, IETF protocols, and common EAP
methods have been designed, the rogue operator can actually advertise
the same identity (SSID) as the local operator; the parameters
advertised by the access point information are not authenticated
end-to-end to the home network. EAP methods that support channel
bindings [10] do not have this problem, but this support is not
present in commonly used methods. Rogue access point can present a
different set of parameters to the client and to the home network.
The current network access mechanisms enable us to have
authentication, and link layer protection. They do not, however,
guarantee anything about the delivery of the actual payload packets.
In particular, there is no guarantee that the payload packets are
delivered through a right route, or NATed only up to some specific
number of times.
We call this the "payload route binding problem". In this problem,
authentication and link layer support do not guarantee that the
packets are actually routed through the path that appears to have
been offered. Basically, if the "rogue" access point has a flat fee
account and a contract with a clearing house, it can offer access to
anyone with a per-byte account, NAT their packets, and send the
traffic forward on its own account, and generate income. The local
ISP would not be able to detect this by looking at the traffic
stream. The user would not be able to detect it either, unless an
EAP method with channel binding support is used. And even if it is,
the user may not care about the identity of the access point enough
to look at it; channel bindings can only ensure that all parties
agree about the parameters, not that they make sense.
The working group did not come to a conclusion whether this attack
needs to be prevented. Some of the proposed solutions include the
use of EAP EMSK in the authentication exchange with the DHCP server,
or the use of EAP to provide the certificates that SEND requires for
the authentication of Router Advertisements. Either approach means
that we are sure we are speaking at layer 3 to the services that we
authenticated at layer two. However, this does not prevent an
attacker from offering such services, and then performing a NAT on
the packets later. However, IPsec could be used to detect the
presence of such NATs, even if NAT traversal is in use.
2.5 Discovery, Decision, and Selection
An alternative decomposition of the problem is to consider the
discovery, decision, and selection aspects separately. Discovery
consists of discovering access networks and associated POPs,
discovering what identities the access networks will accept (either
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directly or through roaming relationships), and discovering which
potential AAA intermediaries or routes exist.
Selection consists of attaching to the "right" access network and
POP, offering an identity through EAP Identity Response, and
providing a hint about the desired AAA intermediary. The selection
of the AAA intermediary, along with the home and access networks,
determines also the treatment of payload packets.
Decision can be either manual selection or automatic. Most likely,
automatic mechanisms are preferred, even if manual selection should
be retained as a fallback. The type of the decision also places
additional requirements on the type of information that the discovery
phase must provide. Just knowing which choices are available is
probably enough for manual selection. Unfortunately, automatic
selection based on a list of choices is by itself not possible:
o Some access networks may be preferred over others. For instance,
the user's private corporate network may be preferred over a
public network due to cost and efficiency reasons.
o Similarly, some credentials may be preferred over others.
o Use of an access network with direct connection to home network
may be preferred over using mediating networks.
o Some mediating networks may be preferred to others, most likely
based on cost. Note that optimizing cost is not a trivial
problem, because the total cost may be a combination of a fixed
fee, per-minute, per-megabyte, volume discounts, and so on.
o Preferences may come from the user, his or her employer (who's
paying the bill), home network, or access network.
Different discovery mechanisms can accommodate such preferences in
various ways. Some user input or perhaps a pre-provisioned database
seems inevitable.
Finally, while the final step of choosing a new network lies always
on the client side, different approches vary in how much they rely on
the client vs. network driven decisions. In cellular networks, for
instance, the network-based performance measurements lead to
instructions that the network gives to the client about the
appropriate base station(s) that should be used. Most of the
processing and decisions are performed on the network side. In
contrast, in a completely client-driven approach the client may get
some raw information from the network, but makes all decisions by
itself.
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2.6 Type of Information
A third alternative way to decompose the problem is to analyze the
type of information which is required [15]. 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 [15] and [30] 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 [30] 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
Note that some EAP methods (such as those defined in [21, 23, 14])
have an ability to agree about additional parameters during an
authentication process. While such parameters are useful for many
purposes, their use for 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 network.
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3. Design Constrains
All solutions to the network selection and discovery problem must
satisfy the following additional constraints:
o AAA routing mechanisms have to work for requests, responses, as
well as server-initiated messages.
o Solution is compatible with all AAA protocols.
o Does not prevent the introduction of new AAA or access network
features, such as link-state AAA routing protocols or fast
handoffs.
o Does not consume a significant amount of resources, such as
bandwidth or increase network attachment time.
o Does not cause a problem with limited packet sizes of current
protocols.
o 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.
o Similarly, new solutions should allow interoperability with
clients, access networks, AAA proxies, and AAA servers that have
not been modified to support network discovery and selection.
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4. Existing Work
4.1 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 network
discovery mechanisms; work in PKIX relating to identity and
credential selection; and work in AAA WG relating to access routing.
The PANA protocol [17] has a mechanism to advertise and select "ISPs"
through the exchange of the ISP-Information AVP in its initial
exchange.
A number of (small) improvements to payload routing control are
currently in progress in the IETF RADEXT WG. These include better
filtering and redirection support [20]. RADEXT WG is also working on
an new revision of the NAI specification [19]. This revision
clarifies the use of the '!' syntax in a NAI to route requests via
specified mediating networks.
Adrangi et al [12] discuss 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 network 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
[10] 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 [1] enables FQDNs
to be up to 255 octets in length, this may not enable the
announcement of many networks, although if SSIDs are used, the
maximum length of 32 octets per SSID may provide somewhat better
scaling. 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 [17], or
Mobile Network Codes embedded in NAIs as specified in 3GPP.
As noted in [39], the use of the EAP-Request/Identity for network
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
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describing which networks are supported. Since IEEE 802.11-1999 does
not support use of Class 1 data frames in State 1 (unauthenticated,
unassociated) within an 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 dramatically increase handoff
latency.
The effects to handoff latency depend also on the specific protocol
design, and the expected likelihood of having to provide
advertisements and initiate scanning of several access points. 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 networks 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 network 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.
4.2 IEEE
There has been work in IEEE 802.11 and 802.1 relating to network
discovery enhancements.
Some recent contributions in this space include the following:
o [25] 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 [41] have identified MAC layer
performance problems, and [37] have identified scaling issues
resulting from a lowering of the Beacon interval.
o [28] 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.
o [29] discusses requirements for differentiation in the way that
the user's payload traffic is routed, based on home network
control. Such requirements have come up, for instance, in the
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context of 3GPP.
o [26] 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 [37] 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 The 802.11 WIEN Study Group is working on the network discovery
and selection problem. In general, the group is working to define
the problem at this stage. This includes studies on the
limitations of existing mechanisms, and gathering requirements
about the type of information needed from the discovery process.
Some of the contributions in this group include [31] and [30].
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, network discovery process may benefit from such standards.
4.3 3GPP
The 3GPP technical specification [32] covers the interworking of WLAN
networks with 2G and 3G networks. This specification discusses also
network discovery and selection issues.
The specification requires that Access Network Discovery is performed
as specified in the standards for the relevant WLAN link layer
technology. An early version of the technical specification required
the use of a 3GPP-specific SSID, but that has since then been
abandoned; access network or local 3GPP network based SSIDs may be
used instead. It has not been decided whether some conventions on
the format of these SSIDs is required by 3GPP.
In addition to Access Network Discovery, it is necessary to select
intermediary networks for the purposes of AAA Routing. In 3GPP,
these networks are called Visited Public Land-based Mobile Networks
(VPLMNs), and it is assumed that WLAN networks may have a contract
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with more than one VPLMN. GSM/UMTS roaming mechanisms are then
employed for routing AAA requests from the VPLMN to the home network.
In order to select the VPLMN, the following is required:
o User can choose the desired VPLMN.
o AAA message are routed according to the NAI.
o Existing EAP mechanisms are used where possible.
o Extensibility is desired, to allow the advertisement of other
parameters later.
The referenced 3GPP technical specification is a so called stage 2
specification, and contains only the principles of operation, leaving
detailed protocol work for later. Nevertheless, the specification
states 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 domain. It is
also stated that where VPLMN control is required, the necessary
information is added to a NAI.
The security properties related to different mediating network
selection mechanisms have been discussed in the 3GPP contribution
[33], which concludes that both SSID- and EAP-based mechanisms have
roughly similar (and very limited) security properties, and that, as
a result, network advertisement should be considered only as hints.
Ahmavaara, Haverinen, and Pichna [35] discuss the new network
selection requirements that 3G-WLAN roaming introduces. It is
necessary to support automatic network 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.
4.4 Other
[36] discusses the need for network selection in a situation where
there is more than one available access network with a roaming
agreement to the home network. It also lists EAP-level, SSID-based,
and PEAP-based mechanisms as potential solutions to the network
selection problem.
Eijk et al [34] discussed the general issue of network selection.
They concentrated primarily on the Access Network Discovery problem,
based on various criteria, and did not consider the other aspects of
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the network 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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5. Conclusions
The issues surrounding the network discovery and selection problem
have been summarized.
In the opinion of the editors 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 Existing mechanisms appear largely sufficient for the control of
payload routing, even if some minor improvements are being worked
on. But there appears to be justification for significantly
enhanced mechanisms relating to access network discovery,
identifier selection, and AAA 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.
In order to avoid this fate, it is strongly suggested that a
discussion be initiated between IETF and IEEE 802 in order to work
out the roles of the each organization in solving this problem,
and to invite 3GPP participation so that their requirements can be
fulfilled by the planned solutions.
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.
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We will briefly comment on the specific mechanisms related to network
discovery and selection:
o As noted in studies such as [41] and [37], 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.
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 and the 802.11 WIEN Study
Group to provide enhanced discovery functionality. For example,
IEEE 802.1ab enables network devices to announce themselves and
provide information on their capabilities. Similarly, the IEEE
802.1af has discussed the idea of supporting network discovery
within a future revision to IEEE 802.1X. However, neither IEEE
801.ab 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 retrive 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 [15] seems hard 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 [12] is useful.
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o In the IETF, a potential alternative is use of the SEAMOBY CARD
protocol [16], which enables advertisement of network device
capabilities over IP. Another alternative is the recently
proposed Device Discovery Protocol (DDP) [22], 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
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.
o "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 [40] and
[15], a large fraction of current WLAN access points operate on
the default SSID, which may make the use of the phone book
approach hard.
Finally, to address some of the security concerns that have come up
during this work, the IETF should in any case initiate work that
enables support for channel bindings in methods. Preferably, popular
methods should be updated, ensuring compatibility with existing
deployments. The representation of link layer parameters within EAP
should utilize a common framework, to make it easier to define new
link layers and keep the selection of EAP methods independent of the
link layer. A number of proposals exist in this space, but none of
them have yet been adopted by the EAP WG as work items.
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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 [21, 23], however, and there is even an attempt to
provide a backwards compatible extensions to older methods [14].
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. References
7.1 Normative References
[1] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[3] Aboba, B., Lu, J., Alsop, J., Ding, J. and W. Wang, "Review of
Roaming Implementations", RFC 2194, September 1997.
[4] Aboba, B. and M. Beadles, "The Network Access Identifier", RFC
2486, January 1999.
[5] Aboba, B. and J. Vollbrecht, "Proxy Chaining and Policy
Implementation in Roaming", RFC 2607, June 1999.
[6] Rigney, C., Willens, S., Rubens, A. and W. Simpson, "Remote
Authentication Dial In User Service (RADIUS)", RFC 2865, June
2000.
[7] Zorn, G., Leifer, D., Rubens, A., Shriver, J., Holdrege, M. and
I. Goyret, "RADIUS Attributes for Tunnel Protocol Support", RFC
2868, June 2000.
[8] Riegel, M. and G. Zorn, "XML DTD for Roaming Access Phone
Book", RFC 3017, December 2000.
[9] Calhoun, P., Loughney, J., Guttman, E., Zorn, G. and J. Arkko,
"Diameter Base Protocol", RFC 3588, September 2003.
[10] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J. and H.
Levkowetz, "Extensible Authentication Protocol (EAP)", RFC
3748, June 2004.
[11] Housley, R. and T. Moore, "Certificate Extensions and
Attributes Supporting Authentication in Point-to-Point Protocol
(PPP) and Wireless Local Area Networks (WLAN)", RFC 3770, May
2004.
7.2 Informative References
[12] Adrangi, F., Lortz, V., Bari, F., Eronen, P. and M. Watson,
"Mediating Network Discovery in the Extensible Authentication
Protocol (EAP)", draft-adrangi-eap-network-discovery-05 (work
in progress), October 2004.
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[13] Adrangi, F., "Network Discovery and Selection within the EAP
Framework",
draft-adrangi-eap-network-discovery-and-selection-00 (work in
progress), October 2003.
[14] Arkko, J. and P. Eronen, "Authenticated Service Identities for
the Extensible Authentication Protocol (EAP)",
draft-arkko-eap-service-identity-auth-01 (work in progress),
October 2004.
[15] Tschofenig, H., "Network Selection Implementation Results",
draft-groeting-eap-netselection-results-00 (work in progress),
July 2004.
[16] Liebsch, M., "Candidate Access Router Discovery",
draft-ietf-seamoby-card-protocol-05 (work in progress),
November 2003.
[17] Forsberg, D., Ohba, Y., Patil, B., Tschofenig, H. and A. Yegin,
"Protocol for Carrying Authentication for Network Access
(PANA)", draft-ietf-pana-pana-05 (work in progress), July 2004.
[18] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
draft-ietf-ipsec-ikev2-17 (work in progress), October 2004.
[19] Aboba, B., "The Network Access Identifier",
draft-ietf-radext-rfc2486bis-01 (work in progress), October
2004.
[20] Lior, A., "Remote Authentication Dial In User Service (RADIUS)
Redirection", draft-lior-radius-redirection-00 (work in
progress), February 2004.
[21] 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.
[22] Enns, R., Marques, P. and D. Morrell, "Device Discovery
Protocol (DDP)", draft-marques-ddp-00 (work in progress), May
2003.
[23] Tschofenig, H. and D. Kroeselberg, "EAP IKEv2 Method
(EAP-IKEv2)", draft-tschofenig-eap-ikev2-03 (work in progress),
February 2004.
[24] Institute of Electrical and Electronics Engineers, "Local and
Metropolitan Area Networks: Port-Based Network Access Control",
IEEE Standard 802.1X, September 2001.
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[25] Institute of Electrical and Electronics Engineers, "Wireless
LAN Medium Access Control (MAC) and Physical Layer (PHY)
Specifications", IEEE Standard 802.11, 1999.
[26] Aboba, B., "Virtual Access Points", IEEE Contribution
11-03-154r1, May 2003.
[27] Mishra, A., "Improving the latnecy of the Probe Phase during
802.11 Handoff", IEEE Contribution 11-03-417r2, May 2003.
[28] Hepworth, E., "Co-existence of Different Authentication
Models", IEEE Contribution 11-03-0827 2003.
[29] Hong, C. and T. Yew, "Interworking - WLAN Control", IEEE
Contribution 11-03-0843 2003.
[30] Berg, S., "Information to Support Network Selection", IEEE
Contribution 11-04-0624 2004.
[31] Aboba, B., "Network Selection", IEEE Contribution 11-04-0638
2004.
[32] 3GPP, "3GPP System to Wireless Local Area Network (WLAN)
interworking; System Description; Release 6", 3GPP Draft
Technical Specification 23.234 v 2.2.0, December 2003.
[33] Ericsson, "Security of EAP and SSID based network
advertisements", 3GPP Contribution S3-030736, November 2003.
[34] 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.
[35] Ahmavaara, K., Haverinen, H. and R. Pichna, "Interworking
Architecture between WLAN and 3G Systems", IEEE Communications
Magazine, November 2003.
[36] Intel, "Wireless LAN (WLAN) End to End Guidelines for
Enterprises and Public Hotspot Service Providers", November
2003.
[37] 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.
[38] Judd, G. and P. Steenkiste, "Fixing 802.11 Access Point
Selection", Sigcomm Poster Session 2002.
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[39] Eronen, P., "Network Selection Issues", presentation to EAP WG
at IETF 58, November 2003.
[40] Priest, J., "The State of Wireless London", July 2004.
[41] Heusse, M., "Performance Anomaly of 802.11b", LSR-IMAG
Laboratory, Grenoble, France, IEEE Infocom 2003.
[42] Eronen, P. and J. Arkko, "Role of authorization in wireless
network security", Extended abstract to be presented in the
DIMACS workshop, November 2004.
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
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Appendix A. Contributors
Version 00 of this draft was based on the discussion held on the EAP
WG mailing list in December 2003, and on a number of input documents
such as [13]. 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 version 01 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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Appendix B. Modifications from Version 00
The following modifications have been incorporated to the -01 version
of this draft:
o Added a discussion of new IETF protocol documents relating to this
problem, such as [12], [19], [20], and [14].
o Added references to a number of new documents that discuss the
network selection problem.
o Added pointers to new IEEE work in this area.
o Added a discussion of what type of information may need to be
discovered and when (Section 2.6).
o Added a discussion relating to the use of phone books in an
environment where network identifiers are not being regularly set.
o Added a discussion of network-based control in Section 2.5.
o Updated the conclusions.
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