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Versions: 00 01 02 03 04 05 06 07 08 09 RFC 5113

Extensible Authentication Protocol                              J. Arkko
Internet-Draft                                                  Ericsson
Expires: April 26, 2006                                   B. Aboba, Eds.
                                                               Microsoft
                                                        October 23, 2005


                Network Discovery and Selection Problem
                    draft-ietf-eap-netsel-problem-03

Status of this Memo

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   This Internet-Draft will expire on April 26, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   The so called network discovery and selection problem affects network
   access, particularly in the presence of multiple available wireless
   accesses and roaming.  This problem has been the subject of
   discussions in various standards bodies.  This document summarizes
   the discussion held about this problem in the Extensible
   Authentication Protocol (EAP) working group at the IETF.  The problem
   is defined and divided into subproblems, and some constraints for



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   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 . . . . . . . . . . . . . . . . . . . .  6
     2.3   AAA routing  . . . . . . . . . . . . . . . . . . . . . . .  7
       2.3.1   The Incomplete Routing Table Problem . . . . . . . . .  8
       2.3.2   The User Selection Problem . . . . . . . . . . . . . .  9
     2.4   Payload Routing  . . . . . . . . . . . . . . . . . . . . . 10
       2.4.1   Issues with Payload Routing  . . . . . . . . . . . . . 11
     2.5   Discovery, Decision, and Selection . . . . . . . . . . . . 12
     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 . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
     4.4   Other  . . . . . . . . . . . . . . . . . . . . . . . . . . 20
   5.  Conclusions  . . . . . . . . . . . . . . . . . . . . . . . . . 20
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 23
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 24
     7.1   Normative References . . . . . . . . . . . . . . . . . . . 24
     7.2   Informative References . . . . . . . . . . . . . . . . . . 25
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 28
   A.  Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 29
   B.  Modifications from Version 00  . . . . . . . . . . . . . . . . 29
       Intellectual Property and Copyright Statements . . . . . . . . 31



















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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) [RFC2486] [I-D.ietf-radext-
      rfc2486bis].


   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  Virtual Operators 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.  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



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   proceed for the rest.

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 [Fixingapsel]) 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 [RFC2194] describes the pre-provisioning of dialup roaming



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   clients, which typically included a list of potential phone numbers,
   updated by the provider(s) with which the client had a contractual
   relationship.  RFC 3017 [RFC3017] describes the IETF Proposed
   Standard for the Roaming Access XML DTD.  This covers not only the
   attributes of the Points of Presence (POPs) and Internet Service
   Providers (ISPs), but also hints on the appropriate NAI to be used
   with a particular POP.  The RFC supports dial-in and X.25 access, but
   has extensible address and media type fields.

   In IEEE 802.11 WLANs, the Beacon/Probe Request/Response mechanism
   provides a way for Stations to discover Access Points (APs), as well
   as the capabilities of those APs.  Among the Information Elements
   (IEs) included within the Beacon and Probe Response is the SSID, a
   non-unique identifier of the network to which an Access Point is
   attached.  By combining network identification along with
   capabilities discovery, the Beacon/Probe facility provides the
   information required for both network discovery and roaming decisions
   within a single mechanism.

   As noted in [Velayos], the IEEE 802.11 Beacon mechanism does not
   scale well; with a Beacon interval of 100ms, and 10 APs in the
   vicinity, approximately 32 percent of an 802.11b AP's capacity is
   used for beacon transmission.  In addition, since Beacon/Probe
   Response frames are sent by each AP over the wireless medium,
   stations can only discover APs within range, which implies
   substantial coverage overlap for roaming to occur without
   interruption.

   A number of enhancements have been proposed to the Beacon/Probe
   Response mechanism in order to improve scalability and roaming
   performance.  These include allowing APs to announce capabilities of
   neighbor APs as well as their own, as proposed in IEEE 802.11k;
   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.




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2.2  Identity selection

   As networks proliferate, it becomes more and more likely that a given
   user may have multiple identities and credential sets, available for
   use in different situations.  For example, the user may have an
   account with one or more Public WLAN providers;  a corporate WLAN;
   one or more wireless WAN providers.  As a result, the user has to
   decide which credential set to use when presented with a choice.

   Figure 1 illustrates a situation where the user 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
   [RFC3017], a client can select between potential POPs, and then based
   on information provided in the DTD, determine the appropriate NAI to
   use with the selected 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 [I-D.ietf-ipsec-ikev2], the identity of the responder
   (typically the security gateway) is provided as a part of the usual



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   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
   has a roaming contract with a given access network.

   It is also possible for hints to be embedded within credentials.  In
   [RFC3770], 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, [I-D.adrangi-eap-network-discovery] 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 [RFC2486] [I-D.ietf-radext-rfc2486bis],
   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 [RFC2194], static routing mechanisms appeared adequate for the
   task and it was not deemed necessary to develop dynamic routing
   protocols.

   As noted in RFC 2607 [RFC2607], RADIUS proxies are deployed not only
   for routing purposes, but also to mask a number of inadequacies in
   the RADIUS protocol design, such as the lack of standardized
   retransmission behavior and the need for shared secret provisioning.

   By removing many of the protocol inadequacies, introducing new AAA
   agent types such as Redirects, providing support for certificate-
   based authentication as well as inter and intra-domain service
   discovery, Diameter allows a NAS to directly open a Diameter



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   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
   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






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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.

                                     +---------+
                                     |         |---------> "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 have been requests to place credential and AAA route selection
   under user control, as the user is affected by the pricing and other
   differences.  Optionally, automatic tools could make the selection
   based on the user's preferences.  On the other hand, user control is
   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 the request through one of
   them, even if the user 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



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   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
   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 [eronendimacs].  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 [RFC2486] [I-D.ietf-radext-rfc2486bis].  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 [RFC2865] and
   Diameter [RFC3588].  RFC 2868 [RFC2868] defines tunneling attributes
   that the home and mediating networks can use to establish mandatory
   tunneling at the access network.  RFC 3588 [RFC3588] defines a filter
   syntax which can be used to install per-session filters under the
   control of AAA.






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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,
   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 [RFC3748] 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,



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   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
   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, for example
      based on cost or QoS.  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.



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   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.

2.6  Type of Information

   A third alternative way to decompose the problem is to analyze the
   type of information which is required [I-D.groeting-eap-netselection-
   results].  For instance, access network discovery may benefit from
   getting knowledge about the quality of service available from a
   particular access network or node, and AAA routing may require
   knowledge of roaming agreements.  References [I-D.groeting-eap-
   netselection-results] and [IEEE.11-04-0624] describe the following
   categories of information which can be discovered:


   o  Access network identification

   o  Roaming agreements

   o  Authentication mechanisms

   o  Quality of service

   o  Cost

   o  Authorization policy

   o  Privacy policy

   o  Service parameters, such as the existence of middleboxes

   The nature of the discovered information can be static, such as the
   fastest available transmission speed on a given piece of equipment.



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   Or it can be dynamic, such as the current load on this equipment.
   The information can describe something about the network access nodes
   themselves, or it can be something that they simply advertise on
   behalf of other parts of the network, such as roaming agreements
   further in the AAA network.

   Typically, it would be desirable to acquire all this information
   prior to the authentication process.  In some cases it is in fact
   necessary, if the authentication process can not complete without the
   information.  Reference [IEEE.11-04-0624] classifies the possible
   steps at which IEEE 802.11 networks can acquire this information:


   o  Pre-association

   o  Post-association (or pre-authentication)

   o  Post-authentication

   Note that some EAP methods (such as those defined in [I-D.josefsson-
   pppext-eap-tls-eap] [I-D.tschofenig-eap-ikev2] [I-D.arkko-eap-
   service-identity-auth]) have an ability to agree about additional
   parameters during an authentication process.  While such parameters
   are useful for many purposes, their use for 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.

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.



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   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.


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 [I-D.ietf-pana-pana] has a mechanism to advertise
   and select "ISPs" through the exchange of the ISP-Information AVP in
   its initial exchange.

   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 [I-D.ietf-radext-ieee802].  RADEXT
   WG is also working on an new revision of the NAI specification
   [I-D.ietf-radext-rfc2486bis].  This revision clarifies the use of the
   '!' syntax in a NAI to route requests via specified mediating
   networks.

   Adrangi et al [I-D.adrangi-eap-network-discovery] 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



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   longer term, the use of this mechanism may run into scalability
   problems, however.  As noted in [RFC3748] Section 3.1, the minimum
   EAP MTU is 1020 octets, so that an EAP-Request/Identity is only
   guaranteed to be able to include 1015 octets within the Type-Data
   field.  Since RFC 1035 [RFC1035] enables FQDNs to be up to 255 octets
   in length, this may not enable the announcement of many 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 [I-D.ietf-pana-pana], or Mobile Network Codes
   embedded in NAIs as specified in 3GPP.

   As noted in [Pasi58], 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
   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, IEEE 802.21 and 802.1 relating to
   network discovery enhancements.

   Some recent contributions in this space include the following:






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   o  [IEEE.802.11-1999] defines the Beacon and Probe Response
      mechanisms used with IEEE 802.11.  Unfortunately, Beacons are only
      sent at the lowest supported rate.  Studies such as [MACScale]
      have identified MAC layer performance problems, and [Velayos] have
      identified scaling issues resulting from a lowering of the Beacon
      interval.


   o  [IEEE-11-03-0827] discusses the evolution of authentication models
      in WLANs, and the need for the network to migrate from existing
      models to new ones, based on either EAP layer indications or
      through the use of SSIDs to represent more than the local network.
      It notes the potential need for management or structuring of the
      SSID space.

      The paper also notes that virtual APs have scalability issues.  It
      does not analyze these scalability issues in relation to those
      existing in other alternative solutions, however.


   o  [IEEE-11-03-0843] 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 context of 3GPP.


   o  [IEEE-11-03-154r1] discusses mechanisms currently used to provide
      "Virtual AP" capabilities within a single physical access point.
      A "Virtual AP" appears at the MAC and IP layers to be distinct
      physical AP.  As noted in the paper, full compatibility with
      existing 802.11 station implementations can only be maintained if
      each virtual AP uses a distinct MAC address (BSSID) for use in
      Beacons and Probe Responses.  This draft does not discuss scaling
      issues in detail, but recommends that only a limited number of
      virtual APs be supported by a single physical access point.  The
      simulations presented in [Velayos] appear to confirm this
      conclusion; with a Beacon interval of 100 ms, once more than 8
      virtual APs are supported on a single channel, more than 20% of
      bandwidth is used for Beacons alone.  This would indicate a limit
      of approximately 20 virtual APs per physical AP.


   o  IEEE 802.11u (formerly the WIEN Study Group) recently completed a
      set of requirements [11-05-0822-03-000u-tgu-requirements].  The
      group is looking into the network discovery and selection problem
      as part of its requirements.  The requirements related to network
      discovery and selection include the functionality by which a
      station can determine whether its subscription to a service



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      provider would allow it to access a particular 802.11 AN before
      actually joining a BSS within that 802.11 AN.  The mechanism
      should be able to handle multiple credentials from the same user
      and be able to select the correct credentials.  Another
      requirement is the support for authentication with multiple
      service providers through a single functionality by which a WLAN
      station can determine which interworking services are available
      before joining a BSS.  In addition, there is an optional
      requirement for a functionality by which APs can advertise (before
      connection) the charges that will be made for use of the network
      if connection is authorized based on a subscription.


   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.
      Part of handover process is the discovery of candidate networks
      and selection of a network for a handover.  The IEEE 802.21 group
      is looking into various information elements that can be used to
      provide sufficient information to either a network node or the
      terminal to make network selection possible.  Both link layer and
      layer 3 delivery mechanisms are being looked into.  Layer 3
      protocol development is being looked into in IETF MIPSHOP WG.
      Different query mechanisms between the terminal and the network,
      including using of XML or basic TLV type interaction are being
      explored.


4.3  3GPP

   The 3GPP stage 2 technical specification [3GPPSA2WLANTS] covers the
   architecture of 3GPP Interworking WLAN 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.

   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
   with more than one VPLMN.  GSM/UMTS roaming principles are then



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   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 VPLMN, the following is required:


   o  User may choose the desired VPLMN manually or let the WLAN User
      Equipment (WLAN UE) choose the the VPLMN automatically based on
      the user and operator defined preferences.


   o  AAA message are routed according to the (root) NAI or decorated
      NAI.


   o  Existing EAP mechanisms are used where possible.


   o  Extensibility is desired, to allow the advertisement of other
      parameters later.  The current network advertisement and selection
      is based on [I-D.adrangi-eap-network-discovery].

   Ahmavaara, Haverinen, and Pichna [WLAN3G] 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.

   The 3GPP Interworking WLAN technical specifications state that
   advertisement information shall be provided only when the access
   network is unable to route the request using normal AAA routing
   means, such as when it sees an unknown NAI domain.  It is also stated
   that where VPLMN control is required, the necessary information is
   added to a NAI.  Furthermore, the WLAN UE may manually trigger the
   network advertisement by using Alternative NAI in EAP Request/
   Identity.  The Alternative NAI is quaranteed to be an unknown NAI
   domain throughout all 3GPP networks.

   The security requirements for 3GPP Interworking WLAN have been
   specified in the 3GPP stage 3 technical specification
   [3GPPSA3WLANTS].  The security properties related to different
   mediating network selection mechanisms have been discussed earlier in
   the 3GPP contribution [3GPP-SA3-030736], which concludes that both



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   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.

   The referenced 3GPP stage 2 and stage 3 technical specifications are
   for 3GPP Release-6 network architecture.  Next generation 3GPP
   Release-7 architecture may change the network advertisement and
   selection technical solution as well as the general requirements.
   There is ongoing stage 1 work in 3GPP Service and System Aspects
   group (SA1) of reviewing the principles of network selection
   [3GPPSA1NETSELTR].  There seems to be considerable interest in adding
   greater HPLMN control over the network selection procedure and a
   capability of dynamically initiating a reselection of the earlier
   selected network.  The current technical report acknowledges that
   there is not yet mechanism to implement reselection in non 3GPP
   access technologies such as WLAN.

4.4  Other

   [INTELe2e] 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 [WWRF-ANS] 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 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.

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



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      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.

   We will briefly comment on the specific mechanisms related to network
   discovery and selection:


   o  As noted in studies such as [MACScale] and [Velayos], the IEEE
      802.11 Beacon/Probe Response mechanism has substantial scaling
      issues, and as a result a single physical access point is in
      practice limited to less than a dozen virtual APs on each channel



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      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, IEEE 802.21 and the IEEE
      802.11u 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 [I-D.groeting-eap-netselection-results]
      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 [I-D.adrangi-eap-network-
      discovery] is useful.


   o  In the IETF, a potential alternative is use of the SEAMOBY CARD
      protocol [RFC4066], which enables advertisement of network device



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      capabilities over IP.  Another alternative is the recently
      proposed Device Discovery Protocol (DDP) [I-D.marques-ddp], 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 [pri04]
      and [I-D.groeting-eap-netselection-results], a large fraction of
      current WLAN access points operate on the default SSID, which may
      make the use of the phone book approach 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.

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



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   make the verification of such parameters possible.  New EAP methods
   are doing it [I-D.josefsson-pppext-eap-tls-eap] [I-D.tschofenig-eap-
   ikev2], however, and there is even an attempt to provide a backwards
   compatible extensions to older methods [I-D.arkko-eap-service-
   identity-auth].

   The security requirements for network selection depend on whether the
   selection is considered as a command or a hint.  For instance, the
   selection that the user provided may be ignored if it relates to AAA
   routing and the access network can route the AAA traffic to the
   correct home network using other means in any case.

7.  References

7.1  Normative References

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2194]  Aboba, B., Lu, J., Alsop, J., Ding, J., and W. Wang,
              "Review of Roaming Implementations", RFC 2194,
              September 1997.

   [RFC2486]  Aboba, B. and M. Beadles, "The Network Access Identifier",
              RFC 2486, January 1999.

   [RFC2607]  Aboba, B. and J. Vollbrecht, "Proxy Chaining and Policy
              Implementation in Roaming", RFC 2607, June 1999.

   [RFC2865]  Rigney, C., Willens, S., Rubens, A., and W. Simpson,
              "Remote Authentication Dial In User Service (RADIUS)",
              RFC 2865, June 2000.

   [RFC2868]  Zorn, G., Leifer, D., Rubens, A., Shriver, J., Holdrege,
              M., and I. Goyret, "RADIUS Attributes for Tunnel Protocol
              Support", RFC 2868, June 2000.

   [RFC3017]  Riegel, M. and G. Zorn, "XML DTD for Roaming Access Phone
              Book", RFC 3017, December 2000.

   [RFC3588]  Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J.
              Arkko, "Diameter Base Protocol", RFC 3588, September 2003.

   [RFC3748]  Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
              Levkowetz, "Extensible Authentication Protocol (EAP)",



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              RFC 3748, June 2004.

   [RFC3770]  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

   [11-05-0822-03-000u-tgu-requirements]
              Moreton, M., "TGu Requirements", IEEE Contribution 11-05-
              0822-03-000u-tgu-requirements, August 2005.

   [3GPP-SA3-030736]
              Ericsson, "Security of EAP and SSID based network
              advertisements", 3GPP Contribution S3-030736,
              November 2003.

   [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.

   [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.

   [3GPPSA1NETSELTR]
              3GPP, "3GPP Technical Specification Group Services and
              Systems Aspects; Review of Network Selection Principles;
              (Release 7); Stage 1", 3GPP Technical Report 22.811 v
              1.0.0, June 2005.

   [3GPPSA2WLANTS]
              3GPP, "3GPP System to Wireless Local Area Network (WLAN)
              interworking; System Description; Release 6; Stage 2",
              3GPP Technical Specification 23.234 v 6.6.0,
              September 2005.

   [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.

   [Fixingapsel]



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              Judd, G. and P. Steenkiste, "Fixing 802.11 Access Point
              Selection", Sigcomm Poster Session 2002.

   [I-D.adrangi-eap-network-discovery]
              Adrangi, F., "Mediating Network Discovery in the
              Extensible Authentication Protocol  (EAP)",
              draft-adrangi-eap-network-discovery-14 (work in progress),
              August 2005.

   [I-D.adrangi-eap-network-discovery-and-selection]
              Adrangi, F., "Network Discovery and Selection within the
              EAP Framework",
              draft-adrangi-eap-network-discovery-and-selection-01 (work
              in progress), March 2004.

   [I-D.arkko-eap-service-identity-auth]
              Arkko, J. and P. Eronen, "Authenticated Service Identities
              for the Extensible Authentication Protocol  (EAP)",
              draft-arkko-eap-service-identity-auth-03 (work in
              progress), July 2004.

   [I-D.groeting-eap-netselection-results]
              Tschofenig, H., "Network Selection Implementation
              Results", draft-groeting-eap-netselection-results-00 (work
              in progress), July 2004.

   [I-D.ietf-ipsec-ikev2]
              Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
              draft-ietf-ipsec-ikev2-17 (work in progress),
              October 2004.

   [I-D.ietf-pana-pana]
              Forsberg, D., Ohba, Y., Patil, B., Tschofenig, H., and A.
              Yegin, "Protocol for Carrying Authentication for Network
              Access (PANA)", draft-ietf-pana-pana-05 (work in
              progress), July 2004.

   [I-D.ietf-radext-ieee802]
              Congdon, P., "VLAN, Priority, and Filtering Attributes",
              draft-ietf-radext-ieee802-00 (work in progress),
              July 2005.

   [I-D.ietf-radext-rfc2486bis]
              Aboba, B., "The Network Access Identifier",
              draft-ietf-radext-rfc2486bis-01 (work in progress),
              October 2004.

   [I-D.josefsson-pppext-eap-tls-eap]



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              Josefsson, S., Palekar, A., Simon, D., and G. Zorn,
              "Protected EAP Protocol (PEAP)",
              draft-josefsson-pppext-eap-tls-eap-07 (work in progress),
              October 2003.

   [I-D.marques-ddp]
              Enns, R., Marques, P., and D. Morrell, "Device Discovery
              Protocol (DDP)", draft-marques-ddp-00 (work in progress),
              May 2003.

   [I-D.tschofenig-eap-ikev2]
              Tschofenig, H. and D. Kroeselberg, "EAP IKEv2 Method (EAP-
              IKEv2)", draft-tschofenig-eap-ikev2-03 (work in progress),
              February 2004.

   [IEEE-11-03-0827]
              Hepworth, E., "Co-existence of Different Authentication
              Models", IEEE Contribution 11-03-0827 2003.

   [IEEE-11-03-0843]
              Hong, C. and T. Yew, "Interworking - WLAN Control", IEEE
              Contribution 11-03-0843 2003.

   [IEEE-11-03-154r1]
              Aboba, B., "Virtual Access Points", IEEE Contribution 11-
              03-154r1, May 2003.

   [IEEE-11-03-417r2]
              Mishra, A., "Improving the latnecy of the Probe Phase
              during 802.11 Handoff", IEEE Contribution 11-03-417r2,
              May 2003.

   [IEEE.11-04-0624]
              Berg, S., "Information to Support Network Selection", IEEE
              Contribution 11-04-0624 2004.

   [IEEE.802.11-1999]
              Institute of Electrical and Electronics Engineers,
              "Wireless LAN Medium Access Control (MAC) and Physical
              Layer (PHY) Specifications", IEEE Standard 802.11, 1999.

   [IEEE.8021x]
              Institute of Electrical and Electronics Engineers, "Local
              and Metropolitan Area Networks: Port-Based Network Access
              Control", IEEE Standard 802.1X, September 2001.

   [INTELe2e]
              Intel, "Wireless LAN (WLAN) End to End Guidelines for



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              Enterprises and Public Hotspot Service Providers",
              November 2003.

   [MACScale]
              Heusse, M., "Performance Anomaly of 802.11b", LSR-IMAG
              Laboratory, Grenoble, France, IEEE Infocom 2003.

   [Pasi58]   Eronen, P., "Network Selection Issues", presentation to
              EAP WG at IETF 58, November 2003.

   [RFC4066]  Liebsch, M., Singh, A., Chaskar, H., Funato, D., and E.
              Shim, "Candidate Access Router Discovery (CARD)",
              RFC 4066, July 2005.

   [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.

   [WLAN3G]   Ahmavaara, K., Haverinen, H., and R. Pichna, "Interworking
              Architecture between WLAN and 3G Systems", IEEE
              Communications Magazine, November 2003.

   [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.

   [eronendimacs]
              Eronen, P. and J. Arkko, "Role of authorization in
              wireless network security", Extended abstract presented in
              the DIMACS workshop, November 2004.

   [pri04]    Priest, J., "The State of Wireless London", July 2004.


Authors' Addresses

   Jari Arkko
   Ericsson
   Jorvas  02420
   Finland

   Email: jari.arkko@ericsson.com





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   Bernard Aboba
   Microsoft
   One Microsoft Way
   Redmond, WA  98052
   USA

   Email: aboba@internaut.com

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 [I-D.adrangi-eap-network-discovery-and-selection].  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.

   Version 03 of this draft was edited and updated by Jouni Korhonen and
   Farooq Bari.

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 [I-D.adrangi-eap-network-discovery], [I-D.ietf-
      radext-rfc2486bis], [I-D.ietf-radext-ieee802], , and [I-D.arkko-
      eap-service-identity-auth].

   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.





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   o  Added a discussion of network-based control in Section 2.5.

   o  Updated the conclusions.

   The following modifications have been incorporated to the -03 version
   of this draft:

   o  Updated the 3GPP section, including notes on the new in progress
      network selection work.

   o  Checked and updated all changed informative references.

   o  Replaced 'I-D.lior-radius-redirection' with 'I-D.ietf-radext-
      ieee802'.

   o  Replaced 'I-D.ietf-seamoby-card-protocol' with 'RFC4066'.

   o  Added reference to 3GPP SA1 Network Selection TR28.11 v1.0.0.

   o  Added reference to 3GPP SA3 WLAN Security TS.

   o  Added reference to 3GPP CT1 WLAN User Equipment TS.

   o  Added reference to 3GPP CT4 WLAN Core Network TS.

   o  Updated IEEE Activities in IEEE 802.11u (formerly IEEE WIEN Study
      Group)

   o  Updated IEEE Activities in IEEE 802.21

   o  Removed reference to a IEEE WIEN document

   o  Added reference to IEEE 802.11u requirements document

   o  Performed minor editorial changes and text cleanup throughout the
      document















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