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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: November 26, 2006                                      B. Aboba
                                                             J. Korhonen
                                                                 F. Bari
                                                       Cingular Wireless
                                                            May 25, 2006

                Network Discovery and Selection Problem

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Copyright Notice

   Copyright (C) The Internet Society (2006).


   The so called realm 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

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   discussions in various standards bodies.  This document summarizes
   the discussion held about this problem in the Extensible
   Authentication Protocol (EAP) working group at the IETF.  The problem
   is defined and divided into subproblems, and some constraints for
   possible solutions are outlined.  The document also provides a
   discussion of the limitations of certain classes of solution,
   including some that have been previously defined.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1   Terminology  . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Problem Definition . . . . . . . . . . . . . . . . . . . . . .  5
     2.1   Discovery of the Point of Attachment to the Network  . . .  5
     2.2   Identity selection . . . . . . . . . . . . . . . . . . . .  7
     2.3   AAA routing  . . . . . . . . . . . . . . . . . . . . . . .  8
       2.3.1   The Incomplete Routing Table Problem . . . . . . . . .  9
       2.3.2   The User and Identity Selection Problem  . . . . . . . 10
     2.4   Discovery, Decision, and Selection . . . . . . . . . . . . 12
     2.5   Type of Information  . . . . . . . . . . . . . . . . . . . 13
   3.  Existing Work  . . . . . . . . . . . . . . . . . . . . . . . . 15
     3.1   IETF . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     3.2   IEEE . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
     3.3   3GPP . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
     3.4   Other  . . . . . . . . . . . . . . . . . . . . . . . . . . 19
   4.  Design Issues  . . . . . . . . . . . . . . . . . . . . . . . . 20
     4.1   AAA issues . . . . . . . . . . . . . . . . . . . . . . . . 20
     4.2   Backward Compatibility . . . . . . . . . . . . . . . . . . 20
     4.3   Efficiency Constraints . . . . . . . . . . . . . . . . . . 20
     4.4   Network discovery and selection decision making  . . . . . 20
   5.  Conclusions  . . . . . . . . . . . . . . . . . . . . . . . . . 22
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 25
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 26
     7.1   Normative References . . . . . . . . . . . . . . . . . . . 26
     7.2   Informative References . . . . . . . . . . . . . . . . . . 27
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 29
   A.  Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 31
       Intellectual Property and Copyright Statements . . . . . . . . 32

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1.  Introduction

   The realm discovery and selection problem affects network access and
   wireless access networks in particular.  This problem spans multiple
   protocol layers and has been the subject of discussions in IETF,
   3GPP, and IEEE.  This document summarizes the discussion held about
   this problem in the Extensible Authentication Protocol working group
   at IETF.

   The realm discovery and selection problem becomes relevant when any
   of the following conditions are true:

   o  There is more than one available network attachment point, and the
      different attachment points may have different characteristics or
      belong to different operators.  In the case of virtual operators,
      access network infrastructure including e.g. the access points can
      be shared by multiple operators.

   o  The user has multiple sets of credentials.  For instance, the user
      could have one set of credentials from a public service provider
      and set from the user's employer.

   o  There is more than one way to provide roaming between the visited
      realm used for access and user's home realm, and service
      parameters or pricing differs between them.  For instance, the
      visited access realm could have both a direct relationship with
      the home realm and an indirect relationship through a roaming
      consortium.  In some scenarios, current AAA protocols may not be
      able to route the requests to the home realm unaided, just based
      on the domain in the given Network Access Identifier (NAI) [12].
      In addition, payload packets can 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.

   In Section 2 the realm discovery and selection problem is defined and
   divided into subproblems, and some constraints for possible solutions
   are outlined in Section 4.  Section 3 discusses existing mechanisms
   which help solve at least parts of the problem.  Section 5 gives some
   suggestions on how to proceed for the rest.

1.1  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [2].

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   Realm Selection

      This refers to selection of the operator/ISP in order to access
      the network.  The process of realm selection can occur either at
      the beginning of a new session or during a handoff in case the
      user is mobile.  The selection is dependent upon for example the
      authentication credentials for the user / device and the roaming
      agreements.  The realm Selection can in turn also depend upon
      Access Technology Selection and/or Bearer Selection.

   Realm Discovery

      This refers to a mechanisms that a node uses to discover available
      realms prior the realm selection takes place.  The discovery
      process may be passive or active from a node point of view.
      Typically the realm discovery mechanism varies depending on the
      access technology.  It is also possible that there are multiple
      discovery mechanisms within one access technology depending on the
      network deployment.

   Access Technology Selection

      This refers to the selection between access technologies e.g.
      802.11, UMTS, WiMAX.  The selection will be dependent upon for
      example the support for an access technology by the device and
      availability of such access technology based networks.

   Bearer Selection

      For some access technologies (e.g.  UMTS), there can be a
      possibility for delivery of a service (e.g. voice) by using either
      a circuit switched or a packet switched bearer.  The Bearer
      selection refers to selecting one of the bearer type 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.

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2.  Problem Definition

   There are a set of somewhat orthogonal problems being discussed under
   the rubric of "realm discovery and selection".

   o  The problem of "discovery of points of attachment".  This is the
      problem of discovering points of attachment available in the
      vicinity, and the capabilities associated with these points of

   o  The problem of "Identifier selection".  This is the problem of
      selecting which identity (and credentials) to use to authenticate
      in a given        point of attachment to the network.

   o  The problem of "AAA routing" which involves figuring out how to
      route the authentication conversation originating from the
      selected identity back to the home realm.

   o  The problem of "Payload routing" which involves figuring how the
      payload packets are routed, where more advanced mechanisms than
      destination-based routing is needed.  However, while being an
      interesting problem, this document does not attempt to do any
      analysis or suggestions on it.

   o  The problem of "realm capability discovery".  This is the problem
      of discovering the capabilities of a particular destination realm.
      For example, it may be important to know whether a given realm
      supports enrollment, what the charges are, etc.

   Alternatively, the problem can be divided to the discovery, decision,
   and the selection components.  The AAA routing problem, for instance,
   involves all components: discovery (which mediating networks are
   available?), decision (choose the "best" one), and selection (client
   tells the network which mediating network it has decided to choose)

2.1  Discovery of the Point of Attachment to the Network

   "The discovery of points of attachment" problem has been extensively
   studied, see for instance the IEEE specifications on 802.11 wireless
   LAN beaconing and probing process, studies (such as [37]) on the
   effectiveness of these mechanisms, specifications on GSM network
   discovery, results of the IETF Seamoby WG, and so on.

   Traditionally, the problem of discovering available point of
   attachment has been handled as a part of the link layer attachment
   procedures, or through out-of-band mechanisms.

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   RFC 2194 [3] describes the pre-provisioning of dialup roaming
   clients, which typically included a list of potential phone numbers,
   updated by the provider(s) with which the client had a contractual
   relationship.  RFC 3017 [8] describes the IETF Proposed Standard for
   the Roaming Access XML DTD.  This covers not only the attributes of
   the Points of Presence (POPs) and Internet Service Providers (ISPs),
   but also hints on the appropriate NAI to be used with a particular
   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 [36], 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.

   Typically scalability enhancement mechanisms attempt to get around
   Beacon/Probe Response restrictions by sending advertisements at the
   higher layers which are enabled once the station has associated with
   an AP and gained IP connectivity.  Since these mechanisms run over
   IP, they can utilize IP facilities such as fragmentation, which the
   link layer mechanisms may not always be able to do.  For instance, in
   IEEE 802.11, Beacon frames cannot use fragmentation because they are
   multicast frames, and multicast frames are not acknowledged in

   Another issue with the Beacon/Probe Request/Response mechanism is
   that it is either insecure or its security can be assured only after
   already attaching to this particular network.

   When considering access systems such as 802.11 WLANs networks it is

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   important to take into account different deployment options.  For
   example, a WLAN deployment may include a number of VLANs in order to
   separate UAM and 802.1X users or users accessing network from
   different geographical/organizational locations.  It is also possible
   that a larger network spans multiple ESSes and prefixes.  Typically
   different enrollment methods and organizational locations within
   ESSes advertise or respond to different SSIDs.  However, it is also
   possible that users authenticating to different realms  are able to
   do so via the same SSID.

2.2  Identity selection

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

   Figure 1 illustrates a situation where the user does not know whether
   the access network he or she is attached to supports the realms he or
   she is attemping to authenticate with.  The access network 1
   interworks only with the ISP and the access network 3 interworks with
   the corporate network whereas the access network 2 interworks with

          ?  ?                 +---------+       +------------------+
           ?                   | Access  |       |                  |
           O_/             _-->| Network |------>| isp.example1.com |
          /|              /    |    1    |    _->|                  |
           |              |    +---------+   /   +------------------+
         _/ \_            |                 /
                          |    +---------+ /
   User "subscriber@isp.  |    | Access  |/
     example1.com"     -- ? -->| Network |
   also known             |    |    2    |\
     "employee123@corp.   |    +---------+ \
     example2.com"        |                 \
                          |    +---------+   \_  +-------------------+
                          \_   | Access  |     ->|                   |
                            -->| Network |------>| corp.example2.com |
                               |   3     |       |                   |
                               +---------+       +-------------------+

         Figure 1: Two credentials, three possible access networks

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   Traditionally, hints useful in identity selection have also been
   provided out-of-band.  For example, via the RFC 3017 XML DTD [8], a
   client can select between potential POPs, and then based on
   information provided in the DTD, determine the appropriate NAI to use
   with the selected point of attachment to the network.

   Perhaps the most typical case is a link layer that provides some
   information about the realms that are reachable before EAP or some
   other enrollment method 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 without
   actively probing for additional SSIDs.  In IKEv2 [14], the identity
   of the responder (typically the security gateway) is provided as a
   part of the usual IKEv2 exchange.

   In order to use this information in deciding the right identity to
   use, the provided information has to either match with one of the
   client's home realms, or the client has to have some other knowledge
   that enables to link the advertised access network name and the home
   realm.  For instance, the client may be aware that his home realm has
   a roaming contract with a given access network.

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

   Finally, some EAP implementations use the space after the NUL
   character in an EAP Identity Request to communicate some parameters
   for example listing realms supported for authentication.  The
   Informational RFC [13] specifies the interpretation of the field
   beyond the NUL character when realms are to be communicated.

2.3  AAA routing

   Once the identity has been selected, it is necessary for the
   authentication conversation to be routed back to the home realm.
   This is typically done today through the use of the Network Access
   Identifier (NAI), RFC 4282 [12], and the ability of the AAA network
   to route requests to the realm indicated in the NAI.

   Within the past IETF ROAMOPS WG, a number of additional approaches
   were considered for routing authentication conversation back to the
   home realm, including source routing techniques based on the NAI, and
   techniques relying on the AAA infrastructure.  Given the relative
   simplicity of the roaming implementations described in RFC 2194 [3],
   static routing mechanisms appeared adequate for the task and it was
   not deemed necessary to develop dynamic routing protocols.

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   As noted in RFC 2607 [5], RADIUS proxies are deployed not only for
   routing purposes, but also to mask a number of inadequacies in the
   RADIUS protocol design, such as the lack of standardized
   retransmission behavior and the need for shared secret provisioning.

   By removing many of the protocol inadequacies, introducing new AAA
   agent types such as Redirects, providing support for certificate-
   based authentication as well as inter and intra-domain service
   discovery, allowing DNS based dynamic discovery of peer agents,
   Diameter allows a NAS to directly open a Diameter connection to the
   home realm without having to utilize a network of proxies.  For
   instance, the Redirect feature could be used to provide a centralized
   routing function for AAA, without having to know all home network
   names in all access networks.  However, there are issues in the
   previously mentioned approach as setting up security might turn out
   to be problematic and the model might not meet business practices.

   This is somewhat analogous to the evolution of email delivery.  Prior
   to the widespread proliferation of the Internet, it was necessary to
   gateway between SMTP-based mail systems and alternative delivery
   technologies, such as UUCP and FidoNet, and email-address based
   source-routing was used to handle this.  However, as mail could
   increasingly be delivered directly, the use of source routing

   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

2.3.1  The Incomplete Routing Table Problem

   No dynamic routing protocols are in use in AAA infrastructure today.
   This implies that there has to be a device (such as a proxy) within
   the access network that knows how to route to different domains, even
   if they are further than one hop away, as shown in Figure 2.  In this
   figure, the user "joe@c.example.com" has to be authenticated through
   ISP 2, since the domain "c.example.com" is served by it.

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                     +---------+      +---------+
                     |         |      |         |
   User "joe@        | Access  |----->|  ISP 1  |-----> "a.example.com"
    c.example.com"-->| Network |      |         |
                     |         |      +---------+
                     |         |-----> "b.example.com"
                     |  ISP 2  |
                     |         |-----> "c.example.com"

                Figure 2: AAA routing problem

2.3.2  The User and Identity Selection Problem

   A related issue is that the roaming relationship graph may have
   ambiguous routes, as shown in Figure 3.  As billing is based on AAA
   and pricing may be based on the used intermediaries, it is necessary
   to select which route is used.  For instance, in Figure 3, access
   through the roaming group 1 may be cheaper, than if roaming group 2
   is used.

                                       |         |----> "a.example.com"
                                       | Roaming |
                      +---------+      | Group 1 |
                      |         |----->|         |----> "b.example.com"
   User "joe@         | Access  |      +---------+
    a.example.com"--->| Network |
                      |         |      +---------+
                      |         |----->|         |----> "a.example.com"
                      +---------+      | Roaming |
                                       | Group 2 |
                                       |         |----> "c.example.com"

                Figure 3: Ambiguous AAA routing

   There have been requests to place credential and AAA route selection
   under user control, as the user is affected by the pricing and other
   differences.  Optionally, automatic tools could make the selection

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   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

   If users can control the selection of intermediaries, such
   intermediaries still have to be legitimate AAA proxies.  That is, an
   access network should not send a request to an unknown intermediary.
   If it has a business relationship with three intermediaries
   int1.example.com, int2.example.com, and int3.example.com, it will
   route the request through one of them, even if the user tried to
   request routing through mitm.example.org.  Thus, NAI-based source
   routing is not source routing in the classic sense.  It is merely
   suggesting preferences among already established routes.  If the
   route does not already exist, or is not feasible, then NAI-based
   source routing cannot establish it.

   An additional issue is that even if the intermediaries are
   legitimate, they could be switched.  For instance, an access network
   could advertise that it has a deal with
   cheapintermediary.example.net, and then switch the user's selection
   to priceyintermediary.example.com instead.  To make this relevant,
   the pricing would have to be based on the intermediary.  Even if it
   were possible to secure this selection, it would not be possible to
   guarantee that QoS or other properties claimed by the network were
   indeed provided.  However, the ability to get authenticated via
   intermediates implies that all the parties have a business agreement
   with each other, which may also include an agreement about the
   minimum service level guarantees.

   Only a limited amount of information about AAA routes or pricing
   information can be dynamically communicated [41].  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

   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 [12].  Where NAIs are used in this manner, the AAA routing
   problem becomes a subset of the identifier selection problem.

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2.4  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 points of
   attachment to the network, 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
   point of attachment, 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
   realms, 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 access network may be preferred over
      a public access 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 realm may
      be preferred over using mediating networks.

   o  Some mediating networks may be preferred to others, for example
      based on cost.  Note that optimizing cost is not a trivial
      problem, because the total cost may be a combination of a fixed
      fee, per-minute, per-megabyte, volume discounts, and so on.

   o  Preferences may come from the user, his or her employer (who's
      paying the bill), home realm, 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 access network lies
   always on the client side, different approaches vary in how much they
   rely on the client vs. network driven decisions.  In cellular

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   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 access network, but makes all decisions
   by itself.

2.5  Type of Information

   A third alternative way to decompose the problem is to analyze the
   type of information which is required [16].  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 [16] and [27] describe the following categories of
   information which can be discovered:

   o  Access network identification

   o  Roaming agreements

   o  Authentication mechanisms

   o  Quality of Service

   o  Cost

   o  Authorization policy

   o  Privacy policy

   o  Service parameters, such as the existence of middleboxes

   The nature of the discovered information can be static, such as the
   fastest available transmission speed on a given piece of equipment.
   Or it can be dynamic, such as the current load on this equipment.
   The information can describe something about the network access nodes
   themselves, or it can be something that they simply advertise on
   behalf of other parts of the network, such as roaming agreements
   further in the AAA network.

   Typically, it would be desirable to acquire all this information
   prior to the authentication process.  In some cases it is in fact
   necessary, if the authentication process can not complete without the
   information.  Reference [27] classifies the possible steps at which
   IEEE 802.11 networks can acquire this information:

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   o  Pre-association

   o  Post-association (or pre-authentication)

   o  Post-authentication

   Note that some EAP methods (such as those defined in [18] [20] [15])
   have an ability to agree about additional parameters during an
   authentication process.  While such parameters are useful for many
   purposes, their use for access network selection suffers from an
   obvious chicken-and-egg problem.  Or at least it seems costly to run
   a relatively heavy authentication process to decide whether the
   client wants to attach to this access network.

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3.  Existing Work

3.1  IETF

   There has already been a lot of past work in this area, including a
   number of IETF Proposed Standards generated by the ROAMOPS WG.  The
   topic of roaming was considered different enough from both AAA and
   access protocols such as PPP that it deserved its own WG.

   In addition to work on ROAMOPS directly relating to the problem,
   there has been work in SEAMOBY relating to scaling of target access
   network discovery mechanisms; work in PKIX relating to identity and
   credential selection; and work in AAA WG relating to access routing.

   The PANA protocol [17] has a mechanism to advertise and select "ISPs"
   through the exchange of the ISP-Information AVP in its initial

   Adrangi et al [13] define the use of the EAP-Request/Identity for
   identifier selection.  It is necessary to have this kind of a
   mechanism, as clients may have more than one credential, and when
   combined with the '!' syntax for NAIs, it can also be used for
   mediating network discovery and selection.  The use of lower-layer
   information may also be limited in terms of discovering identifiers
   that are used on the EAP layer.  In the longer term, the use of this
   mechanism may run into scalability problems, however.  As noted in
   [10] Section 4.x, the minimum EAP MTU is 1020 octets, so that an EAP-
   Request/Identity is only guaranteed to be able to include 1015 octets
   within the Type-Data field.  Since RFC 1035 [1] enables FQDNs to be
   up to 255 octets in length, this may not enable the announcement of
   many realms, although if SSIDs are used, the maximum length of 32
   octets per SSID may provide somewhat better scaling.  The use of
   other network identifiers than domain names is also possible, for
   instance the PANA protocol uses an a free form string and an SMI
   Network Management Private Enterprise Code [17], or Mobile Network
   Codes embedded in NAIs as specified in 3GPP.

   As noted in [38], 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 Extended Service Set (ESS), this implies
   either that the APs must support 802.1X pre-authentication (optional
   in IEEE 802.11i) or that the station must associate with each AP
   prior to sending an EAPOL- Start to initiate EAP.  This will
   dramatically increase handoff latency.

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   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 APs.  The use of
   advertisements only as a last resort when the AAA routing has failed
   is a better approach than the use of advertisement - scanning
   procedure on every attachment.

   Furthermore, if the AP has not been updated to present an up to date
   set of realms in the EAP-Requests/Identity, after associating to
   candidate APs and then choosing one, it is possible that the station
   will find that the chosen realm is not supported after all.  In this
   case, the station's EAP-Response/Identity may be answered with an
   updated EAP-Request/Identity, adding more latency.  However, it is
   possible to configure APs to pass through all EAP negotiation to a
   local AAA proxy and provision the supported realms there.  This would
   ease the management of larger deployments but at the same time
   require RFC 4284 support from the local AAA proxies.  In general
   upgrading the AAA proxies seems a better approach than upgrading and
   managing all APs.

3.2  IEEE

   There has been work in various IEEE 802 working groups relating to
   network discovery enhancements.

   Some recent and past contributions in this space include the

   o  [22] 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 [40] have identified MAC layer
      performance problems, and [36] have identified scaling issues
      resulting from a lowering of the Beacon interval.

   o  [25] 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  [23] 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

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      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 [36] appear to confirm this conclusion; with a Beacon
      interval of 100 ms, once more than 8 virtual APs are supported on
      a single channel, more than 20% of bandwidth is used for Beacons
      alone.  This would indicate a limit of approximately 20 virtual
      APs per physical AP.

   o  IEEE 802.11u group is defining the access network discovery and
      selection  solution as part of its requirements [29].  The
      requirements related to access network discovery and selection
      include the functionality by which a station can determine whether
      its subscription to a service provider would allow it to access a
      particular 802.11 access network or whether the access network is
      able to route authentication to user's home realm before actually
      joining a BSS within that 802.11 access network.  The mechanism
      should be able to handle multiple credentials from the same user
      and be able to select the correct credentials.  Other planned
      features would allow the station to learn the supported enrollment
      mechanisms and possibly the set of basic services (such as
      Internet access is provided or not) in the access network prior to
      the user authenticating to his or her home realm.

   o  IEEE 802.21 is developing standards to enable handover and
      interoperability between heterogeneous network types including
      both 802 and non 802 networks.  The intention is to provide a
      general information transfer capability for these purposes.  As a
      result, network discovery process may benefit from such standards.
      Part of handover process is the discovery of candidate access
      networks and selection of an access network for a handover.  The
      IEEE 802.21 group is looking into various information elements
      that can be used to provide sufficient information to either a
      network node or the terminal to make network selection possible.
      Both link layer and layer 3 delivery mechanisms are being looked
      into.  Layer 3 protocol development is being looked into in IETF
      MIPSHOP WG.  Different query mechanisms between the terminal and
      the network, including using of XML or basic TLV type interaction
      are being explored.

3.3  3GPP

   The 3GPP stage 2 technical specification [30] covers the architecture
   of 3GPP Interworking WLAN (I-WLAN) with 2G and 3G networks.  This
   specification discusses also network discovery and selection issues.

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   The I-WLAN network discovery and selection procedure borrows ideas
   from the cellular side Public Land-based Mobile Network (PLMN)
   selection principles.

   In the 3GPP defined cellular network selection [32]  the mobile node
   monitors surrounding cells and prioritizes them e.g. based on signal
   strength before selecting a new possible target cell.  Each cell also
   broadcasts its PLMN information.  A mobile node may automatically
   select cells that belong to its Home PLMN, Registered PLMN or to a
   allowed set of Visited PLMNs.  These lists of PLMNs are prioritized
   and stored in the SIM card.  In a case of manual network selection
   the mobile node lists all PLMNs it knows from the surrounding cells
   and lets the user to choose the desired PLMN.  After the PLMN has
   been selected other cell related prioritization takes place in order
   to select the appropriate target cell.

   Ahmavaara, Haverinen, and Pichna [34] discuss the new network
   selection requirements that I-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 I-WLAN specification requires that access network discovery is
   performed as specified in the standards for the relevant WLAN link
   layer technology.  In addition to access network discovery, it is
   necessary to select intermediary networks for the purposes of AAA
   Routing.  In 3GPP, these networks are PLMNs.  It is assumed that WLAN
   networks may have a contract with more than one PLMN.  The PLMN may
   be a Home PLMN (HPLMN) or a Visited PLMN (VPLMN) is a roaming case.
   GSM/UMTS roaming principles are employed for routing AAA requests
   from the VPLMN to the Home Public Land-based Mobile Network (HPLMN)
   using either RADIUS or Diameter.  The procedure for selecting the
   intermediary network has been specified in the stage 3 technical
   specifications [43] and [44].

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

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

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

   o  Existing EAP mechanisms are used where possible.

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   o  Extensibility is desired, to allow the advertisement of other
      parameters later.  The current network advertisement and selection
      is based on [12].

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

   The security requirements for 3GPP I-WLAN have been specified in the
   3GPP stage 3 technical specification [42].  The security properties
   related to different mediating network selection mechanisms have been
   discussed earlier in the 3GPP contribution [31], which concludes that
   both SSID- and EAP-based mechanisms have roughly similar (and very
   limited) security properties, and that, as a result, network
   advertisement should be considered only as hints.

3.4  Other

   [35] 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 [33] 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.

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4.  Design Issues

   The following factors should be taken into consideration while
   evaluating solutions for problem of network selection and discovery:

4.1  AAA issues

   Access network or realm selection may leverage or interact with the
   AAA infrastructure.  The solution should therefore be compatible with
   all AAA protocols.  AAA routing mechanisms should work for requests,
   responses, as well as server-initiated messages.  The solution should
   not prevent the introduction of new AAA or access network features,
   such as link-state AAA routing protocols or fast handoffs.

4.2  Backward Compatibility

   The solution should allow interoperability with clients, protocols,
   access networks, AAA proxies, and AAA servers that have not been
   modified to support network discovery and selection.  For example, it
   should not cause a problem with limited packet sizes of current
   protocols.  Where new protocol mechanisms are required, it should be
   possible to deploy the solution without requiring changes to the
   largest base of installed devices -- network access servers, wireless
   access points, and clients.

4.3  Efficiency Constraints

   The solution should be efficient in network resource utilization,
   specially on bandwidth constrained sections of the network (E.g.
   wireless link).  Mechanisms that could significantly increase
   communication of an unauthenticated device with more than one points
   of attachment during the selection process should be avoided.  For
   many handheld devices, battery life is a significant constraint.
   Mechanisms that could significantly drain battery e.g. by requiring
   one or more radios in multimode devices to continuously scan for
   networks, should be avoided.  In addition, the solution should not
   significantly impact network attachment time.

4.4  Network discovery and selection decision making

   "Phone-book" based approaches such as RFC 3017 appear attractive due
   to their ability to provide sufficient information for automatic
   selection decisions.  However, there is no experience on applying
   such approaches to wireless access.  The number of WLAN access points
   is significantly higher than the number of dial-in POPs; the
   distributed nature of the access network has created a more
   complicated business and roaming structure, and the expected rate of
   change in the information is high.  As noted in [39] and [16], a

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   large fraction of current WLAN access points operate on the default
   SSID, which may make the use of the phone book approach hard.

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5.  Conclusions

   The issues surrounding the network discovery and selection problem
   have been summarized.

   In the opinion of the authors of this document, the main findings

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

   o  Identifier selection and AAA routing problems can and should be
      seen as the different aspects of the same problem, identifier

   o  Nevertheless, many of the problems discussed in this draft are
      very hard when one considers them in an environment that requires
      a potentially large number of networks, fast handoffs, and
      automatic decisions.

   o  The proliferation of multiple competing network discovery
      technologies within IEEE 802, IETF, and 3GPP appears to a
      significant problem going forward.  In the absence of a clearly
      defined solution to the problem it is likely that any or all of
      these solutions will be utilized, resulting in industry
      fragmentation and lack of interoperability.

   o  New link layers should be designed with facilities that enable the
      efficient distribution of network advertisement information.

   o  Solving all problems with current link layers and existing network
      access devices may not be possible.  It may be useful to consider
      a phased approach where only certain, limited functions are
      provided now, and the full functionality is provided when
      extensions to current link layers become available.

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

   o  As noted in studies such as [40] and [36], the IEEE 802.11 Beacon/
      Probe Response mechanism has substantial scaling issues, and as a
      result a single physical access point is in practice limited to
      less than a dozen virtual APs on each channel with IEEE 802.11b.

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

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

      As a result, there appears to be justification for enhancing the
      scalability of network advertisements.

   o  Work is already underway in IEEE 802.1, IEEE 802.21 and the IEEE
      802.11u to provide enhanced discovery functionality.  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 retrieve also
      information relating to the potential next access networks or
      access points.  However, such networks may be of another type than
      the current one, so the link layer would have to carry information
      relating to other types of link layers as well.  This is possible,
      but requires coordination among different groups in the industry.

   o  Given that EAP does not support fragmentation of EAP-Request/
      Identity packets, and that use of EAP for network selection on all
      attachments will have a substantial adverse impact on roaming
      performance without appropriate lower layer support (such as
      support for Class 1 data frames within IEEE 802.11), the use of
      EAP is limited.  For instance, the use of EAP to carry quality of
      service as proposed in [16] 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
      [13] is useful.

   o  In the IETF, a potential alternative is use of the SEAMOBY CARD
      protocol [45], which enables advertisement of network device
      capabilities over IP.  Another alternative is the already expired
      Device Discovery Protocol (DDP) [19] proposal, which provides
      functionality equivalent to IEEE 802.1ab using ASN.1 encoded
      advertisements sent to a link-local scope multicast address.

      A limitation of these IP layer solutions is that they can only
      work as a means to speed up the attachment procedures when moving
      from one location to another; when a node starts up, it needs to
      be able to attach to a network before IP communications are
      available.  This is fine for optimizations, but precludes the use

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      in a case where the discovery information is mandatory before
      successful attachment can be accomplished, for instance when the
      access network is unable to route the AAA request unaided.

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6.  Security Considerations

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

   The security requirements for network discovery depend on the type of
   information being discovered.  Some of the parameters may have a
   security impact, such as the claimed name of the network the user
   tries to attach to.  Unfortunately, current EAP methods do not always
   make the verification of such parameters possible.  New EAP methods
   are doing it [18] [20], however, and there is even an attempt to
   provide a backwards compatible extensions to older methods [15].

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

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7.  References

7.1  Normative References

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

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

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

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

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

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

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

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

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

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

   [11]  Housley, R. and T. Moore, "Certificate Extensions and
         Attributes Supporting Authentication in Point-to-Point Protocol
         (PPP) and Wireless Local Area Networks (WLAN)", RFC 3770,
         May 2004.

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

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

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   [14]  Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
         RFC 4306, December 2005.

7.2  Informative References

   [15]  Arkko, J. and P. Eronen, "Authenticated Service Identities for
         the Extensible Authentication Protocol  (EAP)",
         draft-arkko-eap-service-identity-auth-04 (work in progress),
         October 2005.

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

   [17]  Forsberg, D., Ohba, Y., Patil, B., Tschofenig, H., and A.
         Yegin, "Protocol for Carrying Authentication for Network Access
         (PANA)", draft-ietf-pana-pana-11 (work in progress),
         March 2006.

   [18]  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.

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

   [20]  Tschofenig, H. and D. Kroeselberg, "EAP IKEv2 Method (EAP-
         IKEv2)", draft-tschofenig-eap-ikev2-10 (work in progress),
         February 2006.

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

   [22]  Institute of Electrical and Electronics Engineers, "Wireless
         LAN Medium Access Control (MAC) and Physical Layer (PHY)
         Specifications", IEEE Standard 802.11, 2003.

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

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

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

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

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

   [28]  Aboba, B., "Network Selection", IEEE Contribution 11-04-
         0638 2004.

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

   [30]  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.

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

   [32]  3GPP, "Non-Access-Stratum (NAS) functions related to Mobile
         Station (MS) in idle mode", 3GPP TS 23.122 6.5.0, October 2005.

   [33]  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.

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

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

   [36]  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.

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

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

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

   [40]  Heusse, M., "Performance Anomaly of 802.11b", LSR-IMAG

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         Laboratory, Grenoble, France, IEEE Infocom 2003.

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

   [42]  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.

   [43]  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.

   [44]  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.

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

Authors' Addresses

   Jari Arkko
   Jorvas  02420

   Email: jari.arkko@ericsson.com

   Bernard Aboba
   One Microsoft Way
   Redmond, WA  98052

   Email: aboba@internaut.com

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   Jouni Korhonen
   Teollisuuskatu 13
   Sonera  FIN-00051

   Email: jouni.korhonen@teliasonera.com

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

   Email: farooq.bari@cingular.com

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Appendix A.  Contributors

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

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

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Internet-Draft          Network Discovery and PS                May 2006

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