faqs.org - Internet FAQ Archives

RFC 5113 - Network Discovery and Selection Problem

Or Display the document by number

Network Working Group                                           J. Arkko
Request for Comments: 5113                                      Ericsson
Category: Informational                                         B. Aboba
                                                        J. Korhonen, Ed.
                                                                 F. Bari
                                                            January 2008

                Network Discovery and Selection Problem

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.


   When multiple access networks are available, users may have
   difficulty in selecting which network to connect to and how to
   authenticate with that network.  This document defines the network
   discovery and selection problem, dividing it into multiple sub-
   problems.  Some constraints on potential solutions are outlined, and
   the limitations of several solutions (including existing ones) are

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Terminology Used in This Document  . . . . . . . . . . . .  4
   2.  Problem Definition . . . . . . . . . . . . . . . . . . . . . .  7
     2.1.  Discovery of Points of Attachment  . . . . . . . . . . . .  8
     2.2.  Identity Selection . . . . . . . . . . . . . . . . . . . .  9
     2.3.  AAA Routing  . . . . . . . . . . . . . . . . . . . . . . . 11
       2.3.1.  The Default Free Zone  . . . . . . . . . . . . . . . . 13
       2.3.2.  Route Selection and Policy . . . . . . . . . . . . . . 14
       2.3.3.  Source Routing . . . . . . . . . . . . . . . . . . . . 15
     2.4.  Network Capabilities Discovery . . . . . . . . . . . . . . 17
   3.  Design Issues  . . . . . . . . . . . . . . . . . . . . . . . . 18
     3.1.  AAA Routing  . . . . . . . . . . . . . . . . . . . . . . . 18
     3.2.  Backward Compatibility . . . . . . . . . . . . . . . . . . 18
     3.3.  Efficiency Constraints . . . . . . . . . . . . . . . . . . 19
     3.4.  Scalability  . . . . . . . . . . . . . . . . . . . . . . . 19
     3.5.  Static Versus Dynamic Discovery  . . . . . . . . . . . . . 21
     3.6.  Security . . . . . . . . . . . . . . . . . . . . . . . . . 21
     3.7.  Management . . . . . . . . . . . . . . . . . . . . . . . . 22
   4.  Conclusions  . . . . . . . . . . . . . . . . . . . . . . . . . 23
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 25
   6.  Informative References . . . . . . . . . . . . . . . . . . . . 26
   Appendix A.  Existing Work . . . . . . . . . . . . . . . . . . . . 32
     A.1.  IETF . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
     A.2.  IEEE 802 . . . . . . . . . . . . . . . . . . . . . . . . . 33
     A.3.  3GPP . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
     A.4.  Other  . . . . . . . . . . . . . . . . . . . . . . . . . . 36
   Appendix B.  Acknowledgements  . . . . . . . . . . . . . . . . . . 37

1.  Introduction

   Today, network access clients are typically pre-configured with a
   list of access networks and corresponding identities and credentials.
   However, as network access mechanisms and operators have
   proliferated, it has become increasingly likely that users will
   encounter networks for which no pre-configured settings are
   available, yet which offer desired services and the ability to
   successfully authenticate with the user's home realm.  It is also
   possible that pre-configured settings will not be adequate in some
   situations.  In such a situation, users can have difficulty in
   determining which network to connect to, and how to authenticate to
   that network.

   The problem arises when any of the following conditions are true:

   o  Within a single network, more than one network attachment point is
      available, and the attachment points differ in their roaming
      arrangements, or access to services.  While the link layer
      capabilities of a point of attachment may be advertised, higher-
      layer capabilities, such as roaming arrangements, end-to-end
      quality of service, or Internet access restrictions, may not be.
      As a result, a user may have difficulty determining which services
      are available at each network attachment point, and which
      attachment points it can successfully authenticate to.  For
      example, it is possible that a roaming agreement will only enable
      a user to authenticate to the home realm from some points of
      attachment, but not others.  Similarly, it is possible that access
      to the Internet may be restricted at some points of attachment,
      but not others, or that end-to-end quality of service may not be
      available in all locations.  In these situations, the network
      access client cannot assume that all points of attachment within a
      network offer identical capabilities.

   o  Multiple networks are available for which the user has no
      corresponding pre-configuration.  The user may not have pre-
      configured an identity and associated credentials for use with a
      network, yet it is possible that the user's home realm is
      reachable from that network, enabling the user to successfully
      authenticate.  However, unless the roaming arrangements are
      advertised, the network access client cannot determine a priori
      whether successful authentication is likely.  In this situation,
      it is possible that the user will need to try multiple networks in
      order to find one to which it can successfully authenticate, or it
      is possible that the user will not be able to obtain access at
      all, even though successful authentication is feasible.

   o  The user has multiple sets of credentials.  Where no pre-
      configuration exists, it is possible that the user will not be
      able to determine which credentials to use with which attachment
      point, or even whether any credentials it possesses will allow it
      to authenticate successfully.  An identity and associated
      credentials can be usable for authentication with multiple
      networks, and not all of these networks will be pre-configured.
      For example, the user could have one set of credentials from a
      public service provider and another set from an employer, and a
      network might enable authentication with one or more of these
      credentials.  Yet, without pre-configuration, multiple
      unsuccessful authentication attempts could be needed for each
      attachment point in order to determine what credentials are
      usable, wasting valuable time and resulting in user frustration.
      In order to choose between multiple attachment points, it can be
      helpful to provide additional information to enable the correct
      credentials to be determined.

   o  There are multiple potential roaming paths between the visited
      realm and the user's home realm, and service parameters or pricing
      differs between them.  In this situation, there could be multiple
      ways for the user to successfully authenticate using the same
      identity and credentials, yet the cost of each approach might
      differ.  In this case, the access network may not be able to
      determine the roaming path that best matches the user's
      preferences.  This can lead to the user being charged more than
      necessary, or not obtaining the desired services.  For example,
      the visited access realm could have both a direct relationship
      with the home realm and an indirect relationship through a roaming
      consortium.  Current Authentication, Authorization, and Accounting
      (AAA) protocols may not be able to route the access request to the
      home AAA sever purely based on the realm within the Network Access
      Identifier (NAI) [RFC4282].  In addition, payload packets can be
      routed or tunneled differently, based on the roaming relationship
      path.  This may have an impact on the available services or their

   In Section 2, the network discovery and selection problem is defined
   and divided into sub-problems.  Some solution constraints are
   outlined in Section 3.  Section 4 provides conclusions and
   suggestions for future work.  Appendix A discusses existing solutions
   to portions of the problem.

1.1.  Terminology Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

   Authentication, Authorization, and Accounting (AAA)

      AAA protocols with EAP support include Remote Authentication
      Dial-In User Service (RADIUS) [RFC3579] and Diameter [RFC4072].

   Access Point (AP)

      An entity that has station functionality and provides access to
      distribution services via the wireless medium (WM) for associated

   Access Technology Selection

      This refers to the selection between access technologies, e.g.,
      802.11, Universal Mobile Telecommunications System (UMTS), WiMAX.
      The selection will be dependent upon the access technologies
      supported by the device and the availability of networks
      supporting those technologies.

   Bearer Selection

      For some access technologies (e.g., UMTS), there can be a
      possibility for delivery of a service (e.g., voice) by using
      either a circuit-switched or packet-switched bearer.  Bearer
      selection refers to selecting one of the bearer types for service
      delivery.  The decision can be based on support of the bearer type
      by the device and the network as well as user subscription and
      operator preferences.

   Basic Service Set (BSS)

      A set of stations controlled by a single coordination function.

   Decorated NAI

      A NAI specifying a source route.  See Section 2.7 of RFC 4282
      [RFC4282] for more information.

   Extended Service Set (ESS)

      A set of one or more interconnected basic service sets (BSSs) with
      the same Service Set Identifier (SSID) and integrated local area
      networks (LANs), which appears as a single BSS to the logical link
      control layer at any station associated with one of those BSSs.
      This refers to a mechanism that a node uses to discover the
      networks that are reachable from a given access network.

   Network Access Identifier (NAI)

      The Network Access Identifier (NAI) [RFC4282] is the user identity
      submitted by the client during network access authentication.  In
      roaming, the purpose of the NAI is to identify the user as well as
      to assist in the routing of the authentication request.  Please
      note that the NAI may not necessarily be the same as the user's
      e-mail address or the user identity submitted in an application
      layer authentication.

   Network Access Server (NAS)

      The device that peers connect to in order to obtain access to the
      network.  In Point-to-Point Tunneling Protocol (PPTP) terminology,
      this is referred to as the PPTP Access Concentrator (PAC), and in
      Layer 2 Tunneling Protocol (L2TP) terminology, it is referred to
      as the L2TP Access Concentrator (LAC).  In IEEE 802.11, it is
      referred to as an Access Point (AP).

   Network Discovery

      The mechanism used to discover available networks.  The discovery
      mechanism may be passive or active, and depends on the access
      technology.  In passive network discovery, the node listens for
      network announcements; in active network discovery, the node
      solicits network announcements.  It is possible for an access
      technology to utilize both passive and active network discovery

   Network Selection

      Selection of an operator/ISP for network access.  Network
      selection occurs prior to network access authentication.


      The realm portion of an NAI [RFC4282].

   Realm Selection

      The selection of the realm (and corresponding NAI) used to access
      the network.  A realm can be reachable from more than one access
      network type, and selection of a realm may not enable network

   Roaming Capability

      Roaming capability can be loosely defined as the ability to use
      any one of multiple Internet Service Providers (ISPs), while
      maintaining a formal, customer-vendor relationship with only one.
      Examples of cases where roaming capability might be required
      include ISP "confederations" and ISP-provided corporate network
      access support.

   Station (STA)

      A device that contains an IEEE 802.11 conformant medium access
      control (MAC) and physical layer (PHY) interface to the wireless
      medium (WM).

2.  Problem Definition

   The network discovery and selection problem can be broken down into
   multiple sub-problems.  These include:

   o  Discovery of points of attachment.  This involves the discovery of
      points of attachment in the vicinity, as well as their

   o  Identifier selection.  This involves selection of the NAI (and
      credentials) used to authenticate to the selected point of

   o  AAA routing.  This involves routing of the AAA conversation back
      to the home AAA server, based on the realm of the selected NAI.

   o  Payload routing.  This involves the routing of data packets, in
      the situation where mechanisms more advanced than destination-
      based routing are required.  While this problem is interesting, it
      is not discussed further in this document.

   o  Network capability discovery.  This involves discovering the
      capabilities of an access network, such as whether certain
      services are reachable through the access network and the charging

   Alternatively, the problem can be divided into discovery, decision,
   and selection components.  The AAA routing problem, for instance,
   involves all components: discovery (which mediating networks are
   available), decision (choosing the "best" one), and selection
   (selecting which mediating network to use) components.

2.1.  Discovery of Points of Attachment

   Traditionally, the discovery of points of attachment has been handled
   by out-of-band mechanisms or link or network layer advertisements.

   RFC 2194 [RFC2194] describes the pre-provisioning of dial-up roaming
   clients, which typically included a list of potential phone numbers
   updated by the provider(s) with which the client had a contractual
   relationship.  RFC 3017 [RFC3017] describes the IETF Proposed
   Standard for the Roaming Access eXtensible Markup Language (XML)
   Document Type Definition (DTD).  This covers not only the attributes
   of the Points of Presence (PoP) and Internet Service Providers
   (ISPs), but also hints on the appropriate NAI to be used with a
   particular PoP.  The XML DTD supports dial-in and X.25 access, but
   has extensible address and media type fields.

   As access networks and the points of attachment have proliferated,
   out-of-band pre-configuration has become increasingly difficult.  For
   networks with many points of attachment, keeping a complete and up-
   to-date list of points of attachment can be difficult.  As a result,
   wireless network access clients typically only attempt to pre-
   configure information relating to access networks, rather than
   individual points of attachment.

   In IEEE 802.11 Wireless Local Area Networks (WLAN), the Beacon and
   Probe Request/Response mechanism provides a way for Stations to
   discover Access Points (AP) and the capabilities of those APs.  The
   IEEE 802.11 specification [IEEE.802.11-2003] provides support for
   both passive (Beacon) and active (Probe Request/Response) discovery
   mechanisms; [Fixingapsel] studied the effectiveness of these

   Among the Information Elements (IE) included within the Beacon and
   Probe Response is the Service Set Identifier (SSID), a non-unique
   identifier of the network to which an AP is attached.  The Beacon/
   Probe facility therefore enables network discovery, as well as the
   discovery of points of attachment and the capabilities of those
   points of attachment.

   The Global System for Mobile Communications (GSM) specifications also
   provide for discovery of points of attachment, as does the Candidate
   Access Router Discovery (CARD) [RFC4066] protocol developed by the
   IETF SEAMOBY Working Group (WG).

   Along with discovery of points of attachment, the capabilities of
   access networks are also typically discovered.  These may include:

   o  Access network name (e.g., IEEE 802.11 SSID)

   o  Lower layer security mechanism (e.g., IEEE 802.11 Wired Equivalent
      Privacy (WEP) vs. Wi-Fi Protected Access 2 (WPA2))

   o  Quality of service capabilities (e.g., IEEE 802.11e support)

   o  Bearer capabilities (e.g., circuit-switched, packet-switched, or

   Even though pre-configuration of access networks scales better than
   pre-configuration of points of attachment, where many access networks
   can be used to authenticate to a home realm, providing complete and
   up-to-date information on each access network can be challenging.

   In such a situation, network access client configuration can be
   minimized by providing information relating to each home realm,
   rather than each access network.  One way to enable this is for an
   access network to support "virtual Access Points" (virtual APs), and
   for each point of attachment to support virtual APs corresponding to
   each reachable home realm.

   While a single IEEE 802.11 network may only utilize a single SSID, it
   may cover a wide geographical area, and as a result, may be
   segmented, utilizing multiple prefixes.  It is possible that a single
   SSID may be advertised on multiple channels, and may support multiple
   access mechanisms (including Universal Access Method (UAM) and IEEE
   802.1X [IEEE.8021X-2004]) which may differ between points of
   attachment.  A single SSID may also support dynamic VLAN access as
   described in [RFC3580], or may support authentication to multiple
   home AAA servers supporting different realms.  As a result, users of
   a single point of attachment, connecting to the same SSID, may not
   have the same set of services available.

2.2.  Identity Selection

   As networks proliferate, it becomes more and more likely that a user
   may have multiple identities and credential sets, available for use
   in different situations.  For example, the user may have an account
   with one or more Public WLAN providers, a corporate WLAN, and one or
   more wireless Wide Area Network (WAN) providers.

   Typically, the user will choose an identity and corresponding
   credential set based on the selected network, perhaps with additional
   assistance provided by the chosen authentication mechanism.  For
   example, if Extensible Authentication Protocol - Transport Layer
   Security (EAP-TLS) is the authentication mechanism used with a
   particular network, then the user will select the appropriate EAP-TLS

   client certificate based, in part, on the list of trust anchors
   provided by the EAP-TLS server.

   However, in access networks where roaming is enabled, the mapping
   between an access network and an identity/credential set may not be
   one to one.  For example, it is possible for multiple identities to
   be usable on an access network, or for a given identity to be usable
   on a single access network, which may or may not be available.

   Figure 1 illustrates a situation where a user identity may not be
   usable on a potential access network.  In this case, access network 1
   enables access to users within the realm "isp1.example.com", whereas
   access network 3 enables access to users within the realm
   "corp2.example.com"; access network 2 enables access to users within
   both realms.

          ?  ?                 +---------+       +------------------+
           ?                   | Access  |       |                  |
           O_/             _-->| Network |------>|"isp1.example.com"|
          /|              /    |    1    |    _->|                  |
           |              |    +---------+   /   +------------------+
         _/ \_            |                 /
                          |    +---------+ /
   User "subscriber@isp1. |    | Access  |/
     example.com"      -- ? -->| Network |
   also known as          |    |    2    |\
     "employee123@corp2.  |    +---------+ \
     example.com"         |                 \
                          |    +---------+   \_  +-------------------+
                          \_   | Access  |     ->|                   |
                            -->| Network |------>|"corp2.example.com"|
                               |   3     |       |                   |
                               +---------+       +-------------------+

         Figure 1: Two credentials, three possible access networks

   In this situation, a user only possessing an identity within the
   "corp2.example.com" realm can only successfully authenticate to
   access networks 2 or 3; a user possessing an identity within the
   "isp1.example.com" realm can only successfully authenticate to access
   networks 1 or 2; a user possessing identities within both realms can
   connect to any of the access networks.  The question is: how does the
   user figure out which access networks it can successfully
   authenticate to, preferably prior to choosing a point of attachment?

   Traditionally, hints useful in identity selection have been provided
   out-of-band.  For example, the XML DTD, described in [RFC3017],
   enables a client to select between potential points of attachment as

   well as to select the NAI and credentials to use in authenticating
   with it.

   Where all points of attachment within an access network enable
   authentication utilizing a set of realms, selection of an access
   network provides knowledge of the identities that a client can use to
   successfully authenticate.  For example, in an access network, the
   set of supported realms corresponding to network name can be pre-

   In some cases, it may not be possible to determine the available
   access networks prior to authentication.  For example,
   [IEEE.8021X-2004] does not support network discovery on IEEE 802
   wired networks, so that the peer cannot determine which access
   network it has connected to prior to the initiation of the EAP

   It is also possible for hints to be embedded within credentials.  In
   [RFC4334], usage hints are provided within certificates used for
   wireless authentication.  This involves extending the client's
   certificate to include the SSIDs with which the certificate can be

   However, there may be situations in which an access network may not
   accept a static set of realms at every point of attachment.  For
   example, as part of a roaming agreement, only points of attachment
   within a given region or country may be made available.  In these
   situations, mechanisms such as hints embedded within credentials or
   pre-configuration of access network to realm mappings may not be
   sufficient.  Instead, it is necessary for the client to discover
   usable identities dynamically.

   This is the problem that RFC 4284 [RFC4284] attempts to solve, using
   the EAP-Request/Identity to communicate a list of supported realms.
   However, the problems inherent in this approach are many, as
   discussed in Appendix A.1.

   Note that identity selection also implies selection of different
   credentials, and potentially, selection of different EAP
   authentication methods.  In some situations this may imply serious
   security vulnerabilities.  These are discussed in depth in Section 5.

2.3.  AAA Routing

   Once the identity has been selected, the AAA infrastructure needs to
   route the access request back to the home AAA server.  Typically, the
   routing is based on the Network Access Identifier (NAI) defined in

   Where the NAI does not encode a source route, the routing of requests
   is determined by the AAA infrastructure.  As described in [RFC2194],
   most roaming implementations are relatively simple, relying on a
   static realm routing table that determines the next hop based on the
   NAI realm included in the User-Name attribute within the Access-
   Request.  Within RADIUS, the IP address of the home AAA server is
   typically determined based on static mappings of realms to IP
   addresses maintained within RADIUS proxies.

   Diameter [RFC3588] supports mechanisms for intra- and inter-domain
   service discovery, including support for DNS as well as service
   discovery protocols such as Service Location Protocol version 2
   (SLPv2) [RFC2608].  As a result, it may not be necessary to configure
   static tables mapping realms to the IP addresses of Diameter agents.
   However, while this simplifies maintenance of the AAA routing
   infrastructure, it does not necessarily simplify roaming-relationship
   path selection.

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

   Diameter [RFC3588] supports certificate-based authentication (using
   either TLS or IPsec) as well as Redirect functionality, enabling a
   Diameter client to obtain a referral to the home server from a
   Diameter redirect server, so that the client can contact the home
   server directly.  In situations in which a trust model can be
   established, these Diameter capabilities can enable a reduction in
   the length of the roaming relationship path.

   However, in practice there are a number of pitfalls.  In order for
   certificate-based authentication to enable communication between a
   Network Access Server (NAS) or local proxy and the home AAA server,
   trust anchors need to be configured, and certificates need to be
   selected.  The AAA server certificate needs to chain to a trust
   anchor configured on the AAA client, and the AAA client certificate
   needs to chain to a trust anchor configured on the AAA server.  Where
   multiple potential roaming relationship paths are available, this
   will reflect itself in multiple certificate choices, transforming the
   path selection problem into a certificate selection problem.
   Depending on the functionality supported within the certificate
   selection implementation, this may not make the problem easier to
   solve.  For example, in order to provide the desired control over the
   roaming path, it may be necessary to implement custom certificate
   selection logic, which may be difficult to introduce within a

   certificate handling implementation designed for general-purpose

   As noted in [RFC4284], it is also possible to utilize an NAI for the
   purposes of source routing.  In this case, the client provides
   guidance to the AAA infrastructure as to how it would like the access
   request to be routed.  An NAI including source-routing information is
   said to be "decorated"; the decoration format is defined in

   When decoration is utilized, the EAP peer provides the decorated NAI
   within the EAP-Response/Identity, and as described in [RFC3579], the
   NAS copies the decorated NAI included in the EAP-Response/Identity
   into the User-Name attribute included within the access request.  As
   the access request transits the roaming relationship path, AAA
   proxies determine the next hop based on the realm included within the
   User-Name attribute, in the process, successively removing decoration
   from the NAI included in the User-Name attribute.  In contrast, the
   decorated NAI included within the EAP-Response/Identity encapsulated
   in the access request remains untouched.  As a result, when the
   access request arrives at the AAA home server, the decorated NAI
   included in the EAP-Response/Identity may differ from the NAI
   included in the User-Name attribute (which may have some or all of
   the decoration removed).  For the purpose of identity verification,
   the EAP server utilizes the NAI in the User-Name attribute, rather
   than the NAI in the EAP-Response/Identity.

   Over the long term, it is expected that the need for NAI "decoration"
   and source routing will disappear.  This is somewhat analogous to the
   evolution of email delivery.  Prior to the widespread proliferation
   of the Internet, it was necessary to gateway between SMTP-based mail
   systems and alternative delivery technologies, such as Unix-to-Unix
   CoPy Protocol (UUCP) and FidoNet.  Prior to the implementation of
   email gateways utilizing MX RR routing, email address-based source-
   routing was used extensively.  However, over time the need for email
   source-routing disappeared.

2.3.1.  The Default Free Zone

   AAA clients on the edge of the network, such as NAS devices and local
   AAA proxies, typically maintain a default realm route, providing a
   default next hop for realms not otherwise taken into account within
   the realm routing table.  This permits devices with limited resources
   to maintain a small realm routing table.  Deeper within the AAA
   infrastructure, AAA proxies may be maintained with a "default free"
   realm table, listing next hops for all known realms, but not
   providing a default realm route.

   While dynamic realm routing protocols are not in use within AAA
   infrastructure today, even if such protocols were to be introduced,
   it is likely that they would be deployed solely within the core AAA
   infrastructure, but not on NAS devices, which are typically resource

   Since NAS devices do not maintain a full realm routing table, they do
   not have knowledge of all the realms reachable from the local
   network.  The situation is analogous to that of Internet hosts or
   edge routers that do not participate in the BGP mesh.  In order for
   an Internet host to determine whether it can reach a destination on
   the Internet, it is necessary to send a packet to the destination.

   Similarly, when a user provides an NAI to the NAS, the NAS does not
   know a priori whether or not the realm encoded in the NAI is
   reachable; it simply forwards the access request to the next hop on
   the roaming relationship path.  Eventually, the access request
   reaches the "default free" zone, where a core AAA proxy determines
   whether or not the realm is reachable.  As described in [RFC4284],
   where EAP authentication is in use, the core AAA proxy can send an
   Access-Reject, or it can send an Access-Challenge encapsulating an
   EAP-Request/Identity containing "realm hints" based on the content of
   the "default free" realm routing table.

   There are a number of intrinsic problems with this approach.  Where
   the "default free" routing table is large, it may not fit within a
   single EAP packet, and the core AAA proxy may not have a mechanism
   for selecting the most promising entries to include.  Even where the
   "default free" realm routing table would fit within a single EAP-
   Request/Identity packet, the core AAA router may not choose to
   include all entries, since the list of realm routes could be
   considered confidential information not appropriate for disclosure to
   hosts seeking network access.  Therefore, it cannot be assumed that
   the list of "realm hints" included within the EAP-Request/Identity is
   complete.  Given this, a NAS or local AAA proxy snooping the EAP-
   Request/Identity cannot rely on it to provide a complete list of
   reachable realms.  The "realm hint" mechanism described in [RFC4284]
   is not a dynamic routing protocol.

2.3.2.  Route Selection and Policy

   Along with lack of a dynamic AAA routing protocol, today's AAA
   infrastructure lacks mechanisms for route selection and policy.  As a
   result, multiple routes may exist to a destination realm, without a
   mechanism for the selection of a preferred route.

   In Figure 2, Roaming Groups 1 and 2 both include a route to the realm
   "a.example.com".  However, these realm routes are not disseminated to
   the NAS along with associated metrics, and, as a result, there is no
   mechanism for implementation of dynamic routing policies (such as
   selection of realm routes by shortest path, or preference for routes
   originating at a given proxy).

                                       |         |----> "a.example.com"
                                       | Roaming |
                      +---------+      | Group 1 |
                      |         |----->| Proxy   |----> "b.example.com"
   user "joe@         | Access  |      +---------+
    a.example.com"--->| Provider|
                      |   NAS   |      +---------+
                      |         |----->|         |----> "a.example.com"
                      +---------+      | Roaming |
                                       | Group 2 |
                                       | Proxy   |----> "c.example.com"

                Figure 2: Multiple routes to a destination realm

   In the example in Figure 2, access through Roaming Group 1 may be
   less expensive than access through Roaming Group 2, and as a result
   it would be desirable to prefer Roaming Group 1 as a next hop for an
   NAI with a realm of "a.example.com".  However, the only way to obtain
   this result would be to manually configure the NAS realm routing
   table with the following entries:

      Realm            Next Hop
      -----            --------
      b.example.com    Roaming Group 1
      c.example.com    Roaming Group 2
      Default          Roaming Group 1

   While manual configuration may be practical in situations where the
   realm routing table is small and entries are static, where the list
   of supported realms change frequently, or the preferences change
   dynamically, manual configuration will not be manageable.

2.3.3.  Source Routing

   Due to the limitations of current AAA routing mechanisms, there are
   situations in which NAI-based source routing is used to influence the
   roaming relationship path.  However, since the AAA proxies on the
   roaming relationship path are constrained by existing relationships,
   NAI-based source routing is not source routing in the classic sense;

   it merely suggests preferences that the AAA proxy can choose not to

   Where realm routes are set up as the result of pre-configuration and
   dynamic route establishment is not supported, if a realm route does
   not exist, then NAI-based source routing cannot establish it.  Even
   where dynamic route establishment is possible, such as where the AAA
   client and server support certificate-based authentication, and AAA
   servers are discoverable (such as via the mechanisms described in
   [RFC3588]), an AAA proxy may choose not to establish a realm route by
   initiating the discovery process based on a suggestion in an NAI-
   based source route.

   Where the realm route does exist, or the AAA proxy is capable of
   establishing it dynamically, the AAA proxy may choose not to
   authorize the client to use it.

   While, in principle, source routing can provide users with better
   control over AAA routing decisions, there are a number of practical
   problems to be overcome.  In order to enable the client to construct
   optimal source routes, it is necessary for it to be provided with a
   complete and up-to-date realm routing table.  However, if a solution
   to this problem were readily available, then it could be applied to
   the AAA routing infrastructure, enabling the selection of routes
   without the need for user intervention.

   As noted in [Eronen04], only a limited number of parameters can be
   updated dynamically.  For example, quality of service or pricing
   information typically will be pre-provisioned or made available on
   the web rather than being updated on a continuous basis.  Where realm
   names are communicated dynamically, the "default free" realm list is
   unlikely to be provided in full since this table could be quite
   large.  Given the constraints on the availability of information, the
   construction of source routes typically needs to occur in the face of
   incomplete knowledge.

   In addition, there are few mechanisms available to audit whether the
   requested source route is honored by the AAA infrastructure.  For
   example, an access network could advertise a realm route to
   "costsless.example.com", while instead routing the access-request
   through "costsmore.example.com".  While the decorated NAI would be
   made available to the home AAA server in the EAP-Response/Identity,
   the home AAA server might have a difficult time verifying that the
   source route requested in the decorated NAI was actually honored by
   the AAA infrastructure.  Similarly, it could be difficult to
   determine whether quality of service (QoS) or other routing requests
   were actually provided as requested.  To some extent, this problem

   may be addressed as part of the business arrangements between roaming
   partners, which may provide minimum service-level guarantees.

   Given the potential issues with source routing, conventional AAA
   routing mechanisms are to be preferred wherever possible.  Where an
   error is encountered, such as an attempt to authenticate to an
   unreachable realm, "realm hints" can be provided as described
   [RFC4284].  However, this approach has severe scalability
   limitations, as outlined in Appendix A.1.

2.4.  Network Capabilities Discovery

   Network capability discovery focuses on discovery of the services
   offered by networks, not just the capabilities of individual points
   of attachment.  By acquiring additional information on access network
   characteristics, it is possible for users to make a more informed
   access decision.  These characteristics may include:

   o  Roaming relationships between the access network provider and
      other network providers and associated costs.  Where the network
      access client is not pre-configured with an identity and
      credentials corresponding to a local access network, it will need
      to be able to determine whether one or more home realms are
      reachable from an access network so that successful authentication
      can be possible.

   o  EAP authentication methods.  While the EAP authentication methods
      supported by a home realm can only be determined by contacting the
      home AAA server, it is possible that the local realm will also
      support one or more EAP methods.  For example, a user may be able
      to utilize EAP-SIM (Extensible Authentication Protocol -
      Subscriber Identity Module) to authenticate to the access network
      directly, rather than having to authenticate to the home network.

   o  End-to-end quality of service capability.  While local quality of
      service capabilities are typically advertised by the access
      network (e.g., support for Wi-Fi Multimedia (WMM)), the
      availability of end-to-end QoS services may not be advertised.

   o  Service parameters, such as the existence of middleboxes or
      firewalls.  If the network access client is not made aware of the
      Internet access that it will receive on connecting to a point of
      attachment, it is possible that the user may not be able to access
      the desired services.

   Reference [IEEE.11-04-0624] classifies the possible steps at which
   IEEE 802.11 networks can acquire this information:

   o  Pre-association

   o  Post-association (or pre-authentication)

   o  Post-authentication

   In the interest of minimizing connectivity delays, all of the
   information required for network selection (including both access
   network capabilities and global characteristics) needs to be provided
   prior to authentication.

   By the time authentication occurs, the node has typically selected
   the access network, the NAI to be used to authenticate, as well as
   the point of attachment.  Should it learn information during the
   authentication process that would cause it to revise one or more of
   those decisions, the node will need to select a new network, point of
   attachment, and/or identity, and then go through the authentication
   process all over again.  Such a process is likely to be both time
   consuming and unreliable.

3.  Design Issues

   The following factors should be taken into consideration while
   evaluating solutions to the problem of network selection and

3.1.  AAA Routing

   Solutions to the AAA routing issues discussed in Section 2.3 need to
   apply to a wide range of AAA messages, and should not restrict the
   introduction of new AAA or access network functionality.  For
   example, AAA routing mechanisms should work for access requests and
   responses as well as accounting requests and responses and server-
   initiated messages.  Solutions should not restrict the development of
   new AAA attributes, access types, or performance optimizations (such
   as fast handoff support).

3.2.  Backward Compatibility

   Solutions need to maintain backward compatibility.  In particular:

   o  Selection-aware clients need to interoperate with legacy NAS
      devices and AAA servers.

   o  Selection-aware AAA infrastructure needs to interoperate with
      legacy clients and NAS devices.

   For example, selection-aware clients should not transmit packets
   larger than legacy NAS devices or AAA servers can handle.  Where
   protocol extensions are required, changes should be required to as
   few infrastructure elements as possible.  For example, extensions
   that require upgrades to existing NAS devices will be more difficult
   to deploy than proposals that are incrementally deployable based on
   phased upgrades of clients or AAA servers.

3.3.  Efficiency Constraints

   Solutions should be efficient as measured by channel utilization,
   bandwidth consumption, handoff delay, and energy utilization.
   Mechanisms that depend on multicast frames need to be designed with
   care since multicast frames are often sent at the lowest supported
   rate and therefore consume considerable channel time as well as
   energy on the part of listening nodes.  Depending on the deployment,
   it is possible for bandwidth to be constrained both on the link, as
   well as in the backend AAA infrastructure.  As a result, chatty
   mechanisms such as keepalives or periodic probe packets are to be
   avoided.  Given the volume handled by AAA servers, solutions should
   also be conscious of adding to the load, particularly in cases where
   this could enable denial-of-service attacks.  For example, it would
   be a bad idea for a NAS to attempt to obtain an updated realm routing
   table by periodically sending probe EAP-Response/Identity packets to
   the AAA infrastructure in order to obtain "realm hints" as described
   in [RFC4284].  Not only would this add significant load to the AAA
   infrastructure (particularly in cases where the AAA server was
   already overloaded, thereby dropping packets resulting in
   retransmission by the NAS), but it would also not provide the NAS
   with a complete realm routing table, for reasons described in
   Section 2.3.

   Battery consumption is a significant constraint for handheld devices.
   Therefore, mechanisms that require significant increases in packets
   transmitted, or the fraction of time during which the host needs to
   listen (such as proposals that require continuous scanning), are to
   be discouraged.  In addition, the solution should not significantly
   impact the time required to complete network attachment.

3.4.  Scalability

   Given limitations on frame sizes and channel utilization, it is
   important that solutions scale less than linearly in terms of the
   number of networks and realms supported.  For example, solutions such
   as [RFC4284] increase the size of advertisements in proportion to the
   number of entries in the realm routing table.  This approach does not
   scale to support a large number of networks and realms.

   Similarly, approaches that utilize separate Beacons for each "virtual
   AP" introduce additional Beacons in proportion to the number of
   networks being advertised.  While such an approach may minimize the
   pre-configuration required for network access clients, the
   proliferation of "virtual APs" can result in high utilization of the
   wireless medium.  For example, the 802.11 Beacon is sent only at a
   rate within the basic rate set, which typically consists of the
   lowest supported rates, or perhaps only the lowest supported rate.
   As a result, "virtual AP" mechanisms that require a separate Beacon
   for each "virtual AP" do not scale well.

   For example, with a Beacon interval of 100 Time Units (TUs) or 102.4
   ms (9.8 Beacons/second), twenty 802.11b "virtual APs" each announcing
   their own Beacon of 170 octets would result in a channel utilization
   of 37.9 percent.  The calculation can be verified as follows:

   1. A single 170-octet Beacon sent at 1 Mbps will utilize the channel
      for 1360 us (1360 bits @ 1 Mbps);

   2. Adding 144 us for the Physical Layer Convergence Procedure (PLCP)
      long preamble (144 bits @ 1 Mbps), 48 us for the PLCP header (48
      bits @ 1 Mbps), 10 us for the Short Interframe Space (SIFS), 50 us
      for the Distributed Interframe Space (DIFS), and 320 us for the
      average minimum Contention Window without backoff (CWmin/2 *
      aSlotTime = 32/2 * 20 us) implies that a single Beacon will
      utilize an 802.11b channel for 1932 us;

   3. Multiply the channel time per Beacon by 196 Beacons/second, and we
      obtain a channel utilization of 378672 us/second = 37.9 percent.

   In addition, since Beacon/Probe Response frames are sent by each AP
   over the wireless medium, stations can only discover APs within
   range, which implies substantial coverage overlap for roaming to
   occur without interruption.  Another issue with the Beacon and Probe
   Request/Response mechanism is that it is either insecure or its
   security can be assured only as part of authenticating to the network
   (e.g., verifying the advertised capabilities within the 4-way

   A number of enhancements have been proposed to the Beacon/Probe
   Response mechanism in order to improve scalability and performance in
   roaming scenarios.  These include allowing APs to announce
   capabilities of neighbor APs as well as their own [IEEE.802.11k].
   More scalable mechanisms for support of "virtual APs" within IEEE
   802.11 have also been proposed [IEEE.802.11v]; generally these
   proposals collapse multiple "virtual AP" advertisements into a single

   Higher-layer mechanisms can also be used to improve scalability
   since, by running over IP, they can utilize facilities, such as
   fragmentation, that may not be available at the link layer.  For
   example, in IEEE 802.11, Beacon frames cannot use fragmentation
   because they are multicast frames.

3.5.  Static Versus Dynamic Discovery

   "Phone-book" based approaches such as [RFC3017] can provide
   information for automatic selection decisions.  While this approach
   has been applied to wireless access, it typically can only be used
   successfully within a single operator or limited roaming partner
   deployment.  For example, were a "Phone-Book" approach to attempt to
   incorporate information from a large number of roaming partners, it
   could become quite difficult to keep the information simultaneously
   comprehensive and up to date.  As noted in [Priest04] and [GROETING],
   a large fraction of current WLAN access points operate on the default
   SSID, which may make it difficult to distinguish roaming partner
   networks by SSID.  In any case, in wireless networks, dynamic
   discovery is a practical requirement since a node needs to know which
   APs are within range before it can connect.

3.6.  Security

   Network discovery and selection mechanisms may introduce new security
   vulnerabilities.  As noted in Section 2.3.1, network operators may
   consider the AAA routing table to be confidential information, and
   therefore may not wish to provide it to unauthenticated peers via the
   mechanism described in RFC 4284.  While the peer could provide a list
   of the realms it supports, with the authenticator choosing one, this
   approach raises privacy concerns.  Since identity selection occurs
   prior to authentication, the peer's supported realms would be sent in
   cleartext, enabling an attacker to determine the realms for which a
   potential victim has credentials.  This risk can be mitigated by
   restricting peer disclosure.  For example, a peer may only disclose
   additional realms in situations where an initially selected identity
   has proved unusable.

   Since network selection occurs prior to authentication, it is
   typically not possible to secure mechanisms for network discovery or
   identity selection, although it may be possible to provide for secure
   confirmation after authentication is complete.  As an example, some
   parameters discovered during network discovery may be confirmable via
   EAP Channel Bindings; others may be confirmed in a subsequent Secure
   Association Protocol handshake.

   However, there are situations in which advertised parameters may not
   be confirmable.  This could lead to "bidding down" vulnerabilities.
   Section 7.8 of [RFC3748] states:

      Within or associated with each authenticator, it is not
      anticipated that a particular named peer will support a choice of
      methods.  This would make the peer vulnerable to attacks that
      negotiate the least secure method from among a set.  Instead, for
      each named peer, there SHOULD be an indication of exactly one
      method used to authenticate that peer name.  If a peer needs to
      make use of different authentication methods under different
      circumstances, then distinct identities SHOULD be employed, each
      of which identifies exactly one authentication method.

   In practice, where the authenticator operates in "pass-through" mode,
   the EAP method negotiation will occur between the EAP peer and
   server, and therefore the peer will need to associate a single EAP
   method with a given EAP server.  Where multiple EAP servers and
   corresponding identities may be reachable from the same selected
   network, the EAP peer may have difficulty determining which identity
   (and corresponding EAP method) should be used.  Unlike network
   selection, which may be securely confirmed within a Secure
   Association Protocol handshake, identity selection hints provided
   within the EAP-Request/Identity are not secured.

   As a result, where the identity selection mechanism described in RFC
   4284 is used, the "hints" provided could be used by an attacker to
   convince the victim to select an identity corresponding to an EAP
   method offering lesser security (e.g., EAP MD5-Challenge).  One way
   to mitigate this risk is for the peer to only utilize EAP methods
   satisfying the [RFC4017] security requirements, and for the peer to
   select the identity corresponding to the strongest authentication
   method where a choice is available.

3.7.  Management

   From an operational point of view, a network device in control of
   network advertisement and providing "realm hints" for guiding the
   network discovery and selection, should at least offer a management
   interface capable of providing status information for operators.
   Status information, such as counters of each selected network and
   used realm, and when RFC 4284 is used, the count of delivered "realm
   hints" might interest operators.  Especially the information related
   to realms that fall into the "default free zone" or the "AAA fails to
   route" are of interest.

   Larger deployments would benefit from a management interface that
   allow full remote configuration capabilities, for example, of "realm

   hints" in case of RFC 4284-conforming network devices.  While changes
   to "realm hints" and realm routing information are not expected to be
   frequent, centralized remote management tends to lower the frequency
   of misconfigured devices.

4.  Conclusions

   This document describes the network selection and discovery problem.
   In the opinion of the authors, the major findings are as follows:

   o  There is a need for additional work on access network discovery,
      identifier selection, AAA routing, and payload routing.

   o  Credential selection and AAA routing are aspects of the same
      problem, namely identity selection.

   o  When considering selection among a large number of potential
      access networks and points of attachment, the issues described in
      the document become much harder to solve in an automated way,
      particularly if there are constraints on handoff latency.

   o  The proliferation of network discovery technologies within IEEE
      802, IETF, and 3rd Generation Partnership Project (3GPP) has the
      potential to become a significant problem going forward.  Without
      a unified approach, multiple non-interoperable solutions may be

   o  New link-layer designs should include efficient distribution of
      network and realm information as a design requirement.

   o  It may not be possible to solve all aspects of the problem for
      legacy NAS devices on existing link layers.  Therefore, a phased
      approach may be more realistic.  For example, a partial solution
      could be made available for existing link layers, with a more
      complete solution requiring support for link layer extensions.

   With respect to specific mechanisms for access network discovery and

   o  Studies such as [MACScale] and [Velayos], as well as the
      calculations described in Section 2.1, demonstrate that the IEEE
      802.11 Beacon/Probe Response mechanism has substantial scaling
      issues in situations where a new Beacon is used for each "virtual
      AP".  As a result, a single channel is, in practice, limited to
      less than twenty Beacon announcements with IEEE 802.11b.

      The situation is improved substantially with successors, such as
      IEEE 802.11a, that enable additional channels, thus potentially
      increasing the number of potential virtual APs.

      However, even with these enhancements, it is not feasible to
      advertise more than 50 different networks, and probably less in
      most circumstances.

      As a result, there appears to be a need to enhance the scalability
      of IEEE 802.11 network advertisements.

   o  Work is underway in IEEE 802.1, IEEE 802.21, and IEEE 802.11u
      [IEEE.802.11u] to provide enhanced discovery functionality.
      Similarly, IEEE 802.1af [IEEE.802.1af] has discussed the addition
      of network discovery functionality to IEEE 802.1X
      [IEEE.8021X-2004].  However, neither IEEE 802.1AB [IEEE.802.1ab]
      nor IEEE 802.1af is likely to support fragmentation of network
      advertisement frames so that the amount of data that can be
      transported will be limited.

   o  While IEEE 802.11k [IEEE.802.11k] provides support for the
      Neighbor Report, this only provides for gathering of information
      on neighboring 802.11 APs, not points of attachment supporting
      other link layers.  Solution to this problem would appear to
      require coordination across IEEE 802 as well as between standards

   o  Given that EAP does not support fragmentation of EAP-Request/
      Identity packets, the volume of "realm hints" that can be fit with
      these packets is limited.  In addition, within IEEE 802.11, EAP
      packets can only be exchanged within State 3 (associated and
      authenticated).  As a result, use of EAP for realm discovery may
      result in significant delays.  The extension of the realm
      advertisement mechanism defined in [RFC4284] to handle
      advertisement of realm capability information (such as QoS
      provisioning) is not recommended due to semantic and packet size
      limitations [GROETING].  As a result, we believe that extending
      the mechanism described in [RFC4284] for discovery of realm
      capabilities is inappropriate.  Instead, we believe it is more
      appropriate for this functionality to be handled within the link
      layer so that the information can be available early in the
      handoff process.

   o  Where link-layer approaches are not available, higher-layer
      approaches can be considered.  A limitation of higher-layer
      solutions is that they can only optimize the movement of already
      connected hosts, but cannot address scenarios where network
      discovery is required for successful attachment.

      Higher-layer alternatives worth considering include the SEAMOBY
      CARD protocol [RFC4066], which enables advertisement of network
      device capabilities over IP, and Device Discovery Protocol (DDP)
      [MARQUES], which provides functionality equivalent to IEEE 802.1AB
      using ASN.1 encoded advertisements sent to a link-local scope
      multicast address.

5.  Security Considerations

   All aspects of the network discovery and selection problem are
   security related.  The security issues and requirements have been
   discussed in the previous sections.

   The security requirements for network discovery depend on the type of
   information being discovered.  Some of the parameters may have a
   security impact, such as the claimed name of the network to which the
   user tries to attach.  Unfortunately, current EAP methods do not
   always make the verification of such parameters possible.  EAP
   methods, such as Protected EAP (PEAP) [JOSEFSSON] and EAP-IKEv2
   [IKEV2], may make this possible, however.  There is even an attempt
   to provide a backward-compatible extension to older methods [ARKKO].

   The security requirements for network selection depend on whether the
   selection is considered a mandate or a hint.  In general, treating
   network advertisements as a hint is a more secure approach, since it
   reduces access client vulnerability to forged network advertisements.
   For example, "realm hints" may be ignored by an EAP peer if they are
   incompatible with the security policy corresponding to a selected
   access network.

   Similarly, network access clients may refuse to connect to a point of
   attachment if the advertised security capabilities do not match those
   that have been pre-configured.  For example, if an IEEE 802.11 access
   client has been pre-configured to require WPA2 enterprise support
   within an access network, it may refuse to connect to access points
   advertising support for WEP.

   Where the use of methods that do not satisfy the security
   requirements of [RFC4017] is allowed, it may be possible for an
   attacker to trick a peer into using an insecure EAP method, leading
   to the compromise of long-term credentials.  This can occur either
   where a network is pre-configured to allow use of an insecure EAP
   method, or where connection without pre-configuration is permitted
   using such methods.

   For example, an attacker can spoof a network advertisement, possibly
   downgrading the advertised security capabilities.  The rogue access
   point would then attempt to negotiate an insecure EAP method.  Such

   an attack can be prevented if the peer refuses to connect to access
   points not meeting its security requirements, which would include
   requiring use of EAP methods satisfying the [RFC4017] requirements.

   Support for secure discovery could potentially protect against
   spoofing of network advertisements, enabling verifiable information
   to guide connection decisions.  However, development of these
   mechanisms requires solving several difficult engineering and
   deployment problems.

   Since discovery is a prerequisite for authentication, it is not
   possible to protect initial discovery using dynamic keys derived in
   the authentication process.  On the other hand, integrity protection
   of network advertisements utilizing symmetric keys or digital
   signatures would require pre-configuration.

6.  Informative References

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

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

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

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

   [RFC3748]  Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
              Levkowetz, "Extensible Authentication Protocol (EAP)",
              RFC 3748, June 2004.

   [RFC4334]  Housley, R. and T. Moore, "Certificate Extensions and
              Attributes Supporting Authentication in Point-to-Point
              Protocol (PPP) and Wireless Local Area Networks (WLAN)",
              RFC 4334, February 2006.

   [RFC4282]  Aboba, B., Beadles, M., Arkko, J., and P. Eronen, "The
              Network Access Identifier", RFC 4282, December 2005.

   [RFC3280]  Housley, R., Polk, W., Ford, W., and D. Solo, "Internet
              X.509 Public Key Infrastructure Certificate and
              Certificate Revocation List (CRL) Profile", RFC 3280,
              April 2002.

   [RFC4072]  Eronen, P., Hiller, T., and G. Zorn, "Diameter Extensible
              Authentication Protocol (EAP) Application", RFC 4072,
              August 2005.

   [RFC3579]  Aboba, B. and P. Calhoun, "RADIUS (Remote Authentication
              Dial In User Service) Support For Extensible
              Authentication Protocol (EAP)", RFC 3579, September 2003.

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

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

   [RFC2608]  Guttman, E., Perkins, C., Veizades, J., and M. Day,
              "Service Location Protocol, Version 2", RFC 2608,
              June 1999.

   [RFC3580]  Congdon, P., Aboba, B., Smith, A., Zorn, G., and J. Roese,
              "IEEE 802.1X Remote Authentication Dial In User Service
              (RADIUS) Usage Guidelines", RFC 3580, September 2003.

   [RFC4284]  Adrangi, F., Lortz, V., Bari, F., and P. Eronen, "Identity
              Selection Hints for the Extensible Authentication Protocol
              (EAP)", RFC 4284, January 2006.

   [RFC4017]  Stanley, D., Walker, J., and B. Aboba, "Extensible
              Authentication Protocol (EAP) Method Requirements for
              Wireless LANs", RFC 4017, March 2005.

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

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

   [IKEV2]    Tschofenig, H., Kroeselberg, D., Pashalidis, A., Ohba, Y.,
              and F. Bersani, "EAP-IKEv2 Method", Work in Progress,
              September 2007.

   [ARKKO]    Arkko, J. and P. Eronen, "Authenticated Service
              Information for the Extensible Authentication Protocol
              (EAP)", Work in Progress, October 2005.

   [GROETING] Groeting, W., Berg, S., Tschofenig, H., and M. Ness,
              "Network Selection Implementation Results", Work
              in Progress, July 2004.

              Palekar, A., Simon, D., Salowey, J., Zhou, H., Zorn, G.,
              and S. Josefsson, "Protected EAP Protocol (PEAP) Version
              2", Work in Progress, October 2004.

   [MARQUES]  Enns, R., Marques, P., and D. Morrell, "Device Discovery
              Protocol (DDP)", Work in Progress, May 2003.

   [OHBA]     Taniuchi, K., Ohba, Y., and D. Subir, "IEEE 802.21 Basic
              Schema", Work in Progress, October 2007.

              IEEE, "Wireless LAN Medium Access Control (MAC) and
              Physical Layer (PHY) Specifications", IEEE Standard
              802.11, 2003.

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

              IEEE, "Draft Ammendment to Standard for Telecommunications
              and Information Exchange Between Systems - LAN/MAN
              Specific Requirements - Part 11: Wireless LAN Medium
              Access Control (MAC) and Physical Layer (PHY)
              Specifications: Radio Resource Management", IEEE 802.11k,
              D7.0, January 2007.

              IEEE, "Draft Standard for Local and Metropolitan Area
              Networks -  Station and Media Access Control Connectivity
              Discovery", IEEE 802.1AB, D1.0, April 2007.

              IEEE, "Draft Standard for Local and Metropolitan Area
              Networks - Port-Based Network Access Control - Amendment
              1: Authenticated Key Agreement for Media Access Control
              (MAC) Security", IEEE 802.1af, D1.2, January 2007.

              IEEE, "Draft Amemdment to Standard  for Information
              Technology - Telecommunications and Information Exchange
              Between Systems - LAN/MAN Specific Requirements - Part 11:
              Wireless Medium Access Control (MAC) and physical layer
              (PHY) specifications: Wireless Network Management",
              IEEE 802.11v, D0.09, March 2007.

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

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

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

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

   [Velayos]  Velayos, H. and G. Karlsson, "Techniques to Reduce IEEE
              802.11b MAC Layer Handover Time", Laboratory for
              Communication Networks, KTH, Royal Institute of
              Technology, Stockholm, Sweden, TRITA-IMIT-LCN R 03:02,
              April 2003.

              IEEE, "Draft Amendment to STANDARD FOR Information
              Technology -  LAN/MAN Specific Requirements - Part 11:
              Interworking with External Networks; Draft Amendment to
              Standard; IEEE P802.11u/D0.04", IEEE 802.11u, D0.04,
              April 2007.

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

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

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

              3GPP, "3GPP System to Wireless Local Area Network (WLAN)
              interworking; System De scription; Release 6; Stage 2",
              3GPP Technical Specification 23.234, September 2005.

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

              3GPP, "Non-Access-Stratum (NAS) functions related to
              Mobile Station (MS) in idle mode", 3GPP TS 23.122 6.5.0,
              October 2005.

              Eijk, R., Brok, J., Bemmel, J., and B. Busropan, "Access
              Network Selection in a 4G Environment and the Role of
              Terminal and Service Platform", 10th WWRF, New York,
              October 2003.

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

              Intel, "Wireless LAN (WLAN) End to End Guidelines for
              Enterprises and Public Hotspot Service Providers",
              November 2003.

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

              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.

              3GPP, "3GPP System to Wireless Local Area Network (WLAN)
              interworking; User Equipment (UE) to network protocols;
              Stage 3 (Release 6)", 3GPP Technical Specification 24.234
              v 6.4.0, October 2005.

              IEEE, "Draft IEEE Standard for Local and Metropolitan Area
              Networks: Media Independent Handover Services",
              IEEE 802.21, D05.00, April 2007.

              3GPP, "3GPP system to Wireless Local Area Network (WLAN)
              interworking; Stage 3 (Release 6)", 3GPP Technical
              Specification 29.234 v 6.4.0, October 2005.

              IEEE, "Local and Metropolitan Area Networks: Port-Based
              Network Access Control", IEEE Standard 802.1X, July 2004.

Appendix A.  Existing Work

A.1.  IETF

   Several IETF WGs have dealt with aspects of the network selection
   problem, including the AAA, EAP, PPP, RADIUS, ROAMOPS, and RADEXT

   ROAMOPS WG developed the NAI, originally defined in [RFC2486], and
   subsequently updated in [RFC4282].  Initial roaming implementations
   are described in [RFC2194], and the use of proxies in roaming is
   addressed in [RFC2607].  The SEAMOBY WG developed CARD [RFC4066],
   which assists in discovery of suitable base stations.  PKIX WG
   produced [RFC3280], which addresses issues of certificate selection.
   The AAA WG developed more sophisticated access routing,
   authentication, and service discovery mechanisms within Diameter

   Adrangi et al.  [RFC4284] defines the use of the EAP-Request/Identity
   to provide "realm hints" useful for identity selection.  The NAI
   syntax described in [RFC4282] enables the construction of source
   routes.  Together, these mechanisms enable the user to determine
   whether it possesses an identity and corresponding credential
   suitable for use with an EAP-capable NAS.  This is particularly
   useful in situations where the lower layer provides limited
   information (such as in wired IEEE 802 networks where IEEE 802.1X
   currently does not provide for advertisement of networks and their

   However, advertisement mechanisms based on the use of the EAP-
   Request/Identity have scalability problems.  As noted in [RFC3748]
   Section 3.1, the minimum EAP Maximum Transmission Unit (MTU) is 1020
   octets, so that an EAP-Request/Identity is only guaranteed to be able
   to include 1015 octets within the Type-Data field.  Since RFC 1035
   [RFC1035] enables Fully Qualified Domain Names (FQDN) to be up to 255
   octets in length, this may not enable the announcement of many
   realms.  The use of network identifiers other than domain names is
   also possible.

   As noted in [Eronen03], the use of the EAP-Request/Identity for realm
   discovery has substantial negative impact on handoff latency, since
   this may result in a station needing to initiate an EAP conversation
   with each Access Point in order to receive an EAP-Request/Identity
   describing which realms are supported.  Since IEEE 802.11-2003 does
   not support use of Class 1 data frames in State 1 (unauthenticated,
   unassociated) within an Extended Service Set (ESS), this implies
   either that the APs must support 802.1X pre-authentication (optional
   in IEEE 802.11i-2004), or that the station must associate with each

   AP prior to sending an EAPOL-Start to initiate EAP (here, EAPOL
   refers to EAP over LAN).  This will dramatically increase handoff

   Thus, rather than thinking of [RFC4284] as an effective network
   discovery mechanism, it is perhaps better to consider the use of
   "realm hints" as an error recovery technique to be used to inform the
   EAP peer that AAA routing has failed, and perhaps to enable selection
   of an alternate identity that can enable successful authentication.
   Where "realm hints" are only provided in event of a problem, rather
   than as a staple network discovery technique, it is probably best to
   enable "realm hints" to be sent by core AAA proxies in the "default
   free" zone.  This way, it will not be necessary for NASes to send
   "realm hints", which would require them to maintain a complete and
   up-to-date realm routing table, something that cannot be easily
   accomplished given the existing state of AAA routing technology.

   If realm routing tables are manually configured on the NAS, then
   changes in the "default free" realm routing table will not
   automatically be reflected in the realm list advertised by the NAS.
   As a result, a realm advertised by the NAS might not, in fact, be
   reachable, or the NAS might neglect to advertise one or more realms
   that were reachable.  This could result in multiple EAP-Identity
   exchanges, with the initial set of "realm hints" supplied by the NAS
   subsequently updated by "realm hints" provided by a core AAA proxy.
   In general, originating "realm hints" on core AAA proxies appears to
   be a more sound approach, since it provides for "fate sharing" --
   generation of "realm hints" by the same entity (the core AAA proxy)
   that will eventually need to route the request based on the hints.
   This approach is also preferred from a management perspective, since
   only core AAA proxies would need to be updated; no updates would be
   required to NAS devices.

A.2.  IEEE 802

   There has been work in several IEEE 802 working groups relating to
   network discovery:

   o  [IEEE.802.11-2003] defines the Beacon and Probe Response
      mechanisms within IEEE 802.11.  Unfortunately, Beacons may be sent
      only at a rate within the base rate set, which typically consists
      of the lowest supported rate, or perhaps the next lowest rate.
      Studies such as [MACScale] have identified MAC layer performance
      problems, and [Velayos] has identified scaling issues from a
      lowering of the Beacon interval.

   o  [IEEE-11-03-0827] discusses the evolution of authentication models
      in WLANs and the need for the network to migrate from existing

      models to new ones, based on either EAP layer indications or
      through the use of SSIDs to represent more than the local network.
      It notes the potential need for management or structuring of the
      SSID space.

      The paper also notes that virtual APs have scalability issues.  It
      does not compare these scalability issues to those of alternative
      solutions, however.

   o  [IEEE-11-03-154r1] discusses mechanisms currently used to provide
      "virtual AP" capabilities within a single physical access point.
      A "virtual AP" appears at the MAC and IP layers to be a distinct
      physical AP.  As noted in the paper, full compatibility with
      existing 802.11 station implementations can only be maintained if
      each "virtual AP" uses a distinct MAC address (BSSID) for use in
      Beacons and Probe Responses.  This paper does not discuss scaling
      issues in detail, but recommends that only a limited number of
      "virtual APs" be supported by a single physical access point.

   o  IEEE 802.11u is working on realm discovery and network selection
      [11-05-0822-03-000u-tgu-requirements] [IEEE.802.11u].  This
      includes a mechanism for enabling a station to determine the
      identities it can use to authenticate to an access network, prior
      to associating with that network.  As noted earlier, solving this
      problem requires the AP to maintain an up-to-date, "default free"
      realm routing table, which is not feasible without dynamic routing
      support within the AAA infrastructure.  Similarly, a priori
      discovery of features supported within home realms (such as
      enrollment) is also difficult to implement in a scalable way,
      absent support for dynamic routing.  Determination of network
      capabilities (such as QoS support) is considerably simpler, since
      these depend solely on the hardware and software contained within
      the AP.  However, 802.11u is working on Generic Advertisement
      Service (GAS) mechanism, which can be used to carry 802.21
      Information Service (IS) messages and, in that way, allow a more
      sophisticated way of delivering information from the network side.

   o  IEEE 802.21 [IEEE.802.21] is developing standards to enable
      handover between heterogeneous link layers, including both IEEE
      802 and non-IEEE 802 networks.  To enable this, a general
      mechanism for capability advertisement is being developed, which
      could conceivably benefit aspects of the network selection
      problem, such as realm discovery.  For example, IEEE 802.21 is
      developing Information Elements (IEs) that may assist with network
      selection, including information relevant to both layer 2 and
      layer 3.  Query mechanisms (including both XML and TLV support)
      are also under development.  IEEE 802.21 also defines a Resource
      Description Framework (RDF) schema to allow use of a query

      language (i.e., SPARQL).  The schema is a normative part of IEEE
      802.21 and also defined in [OHBA].

A.3.  3GPP

   The 3GPP stage 2 technical specification [3GPPSA2WLANTS] covers the
   architecture of 3GPP Interworking WLAN (I-WLAN) with 2G and 3G
   networks.  This specification also discusses realm discovery and
   network selection issues.  The I-WLAN realm discovery procedure
   borrows ideas from the cellular Public Land-based Mobile Network
   (PLMN) selection principles, known as "PLMN Selection".

   In 3GPP PLMN selection [3GPP.23.122], the mobile node monitors
   surrounding cells and prioritizes them based on signal strength
   before selecting a new potential target cell.  Each cell broadcasts
   its PLMN.  A mobile node may automatically select cells that belong
   to its Home PLMN, Registered PLMN, or an allowed set of Visited
   PLMNs.  The PLMN lists are prioritized and stored in the Subscriber
   Identity Module (SIM).  In the case of manual PLMN selection, the
   mobile node lists the PLMNs it learns about from surrounding cells
   and enables the user to choose the desired PLMN.  After the PLMN has
   been selected, cell prioritization takes place in order to select the
   appropriate target cell.

   [WLAN3G] discusses the new realm (PLMN) selection requirements
   introduced by I-WLAN roaming, which support automatic PLMN selection,
   not just manual selection.  Multiple network levels may be present,
   and the hotspot owner may have a contract with a provider who, in
   turn, has a contract with a 3G network, which may have a roaming
   agreement with other networks.

   The I-WLAN specification requires that network discovery be performed
   as specified in the relevant WLAN link layer standards.  In addition
   to network discovery, it is necessary to select intermediary realms
   to enable construction of source routes.  In 3GPP, the intermediary
   networks are PLMNs, and it is assumed that an access network may have
   a roaming agreement with more than one PLMN.  The PLMN may be a Home
   PLMN (HPLMN) or a Visited PLMN (VPLMN), where roaming is supported.
   GSM/UMTS roaming principles are employed for routing AAA requests
   from the VPLMN to the Home Public Land-based Mobile Network (HPLMN)
   using either RADIUS or Diameter.  The procedure for selecting the
   intermediary network has been specified in the stage 3 technical
   specifications [3GPPCT1WLANTS] and [3GPPCT4WLANTS].

   In order to select the PLMN, the following procedure is required:

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

   o  AAA messages are routed based on the decorated or undecorated NAI.

   o  EAP is utilized as defined in [RFC3748] and [RFC3579].

   o  PLMN advertisement and selection is based on [RFC4284], which
      defines only realm advertisement.  The document refers to the
      potential need for extensibility, though EAP MTU restrictions make
      this difficult.

   The I-WLAN specification states that "realm hints" are only provided
   when an unreachable realm is encountered.  Where VPLMN control is
   required, this is handled via NAI decoration.  The station may
   manually trigger PLMN advertisement by including an unknown realm
   (known as the Alternative NAI) within the EAP-Response/Identity.  A
   realm guaranteed not to be reachable within 3GPP networks is utilized
   for this purpose.

   The I-WLAN security requirements are described in the 3GPP stage 3
   technical specification [3GPPSA3WLANTS].  The security requirements
   for PLMN selection are discussed in 3GPP contribution
   [3GPP-SA3-030736], which concludes that both SSID and EAP-based
   mechanisms have similar security weaknesses.  As a result, it
   recommends that PLMN advertisements should be considered as hints.

A.4.  Other

   [INTELe2e] discusses the need for realm selection where an access
   network may have more than one roaming relationship path to a home
   realm.  It also describes solutions to the realm selection problem
   based on EAP, SSID and Protected EAP (PEAP) based mechanisms.

   Eijk et al.  [WWRF-ANS] discusses the realm and network selection
   problem.  The authors concentrate primarily on discovery of access
   networks meeting a set of criteria, noting that information on the
   realm capabilities and reachability inherently resides in home AAA
   servers, and therefore it is not readily available in a central
   location, and may not be easily obtained by NAS devices.

Appendix B.  Acknowledgements

   The authors of this document would like to especially acknowledge the
   contributions of Farid Adrangi, Michael Richardson, Pasi Eronen, Mark
   Watson, Mark Grayson, Johan Rune, and Tomas Goldbeck-Lowe.

   Input for the early versions of this document has been gathered from
   many sources, including the above persons as well as 3GPP and IEEE
   developments.  We would also like to thank Alper Yegin, Victor Lortz,
   Stephen Hayes, and David Johnston for comments.

   Jouni Korhonen would like to thank the Academy of Finland for
   providing funding to work on this document.

Authors' Addresses

   Jari Arkko
   Jorvas  02420

   EMail: jari.arkko@ericsson.com

   Bernard Aboba
   One Microsoft Way
   Redmond, WA  98052

   EMail: bernarda@microsoft.com

   Jouni Korhonen
   Teollisuuskatu 13
   Sonera  FIN-00051

   EMail: jouni.korhonen@teliasonera.com

   Farooq Bari
   7277 164th Avenue N.E.
   Redmond  WA  98052

   EMail: farooq.bari@att.com

Full Copyright Statement

   Copyright (C) The IETF Trust (2008).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an

Intellectual Property

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at


User Contributions:

Comment about this RFC, ask questions, or add new information about this topic: