Extensible Authentication Protocol J. Arkko (EAP) Ericsson Internet-Draft B. Aboba Intended status: Informational Microsoft Expires: September 6, 2007 J. Korhonen TeliaSonera F. Bari Cingular Wireless March 5, 2007 Network Discovery and Selection Problem draft-ietf-eap-netsel-problem-06 Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on September 6, 2007. Copyright Notice Copyright (C) The IETF Trust (2007). Arkko, et al. Expires September 6, 2007 [Page 1] Internet-Draft Network Discovery and PS March 2007 Abstract When multiple access network are available, users may have difficulty in selecting which network to connect to, and how to authenticate with that network. This document defines the network discovery and selection problem, dividing it into multiple sub-problems. Some constraints on potential solutions are outlined, and the limitations of several solutions (including existing ones) are discussed. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4 2. Problem Definition . . . . . . . . . . . . . . . . . . . . . . 7 2.1. Discovery of the Point of Attachment to the Network . . . 7 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 Discovery . . . . . . . . . . . . . . . . . . . . 16 3. Design Issues . . . . . . . . . . . . . . . . . . . . . . . . 18 3.1. AAA Routing . . . . . . . . . . . . . . . . . . . . . . . 18 3.2. Backward Compatibility . . . . . . . . . . . . . . . . . . 18 3.3. Efficiency Constraints . . . . . . . . . . . . . . . . . . 18 3.4. Scalability . . . . . . . . . . . . . . . . . . . . . . . 19 3.5. Static Versus Dynamic Discovery . . . . . . . . . . . . . 19 4. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 20 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22 6. Security Considerations . . . . . . . . . . . . . . . . . . . 23 7. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 24 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 25 8.1. Normative References . . . . . . . . . . . . . . . . . . . 25 8.2. Informative References . . . . . . . . . . . . . . . . . . 25 Appendix A. Existing Work . . . . . . . . . . . . . . . . . . . . 30 A.1. IETF . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 A.2. IEEE 802 . . . . . . . . . . . . . . . . . . . . . . . . . 31 A.3. 3GPP . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 A.4. Other . . . . . . . . . . . . . . . . . . . . . . . . . . 34 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 35 Intellectual Property and Copyright Statements . . . . . . . . . . 36 Arkko, et al. Expires September 6, 2007 [Page 2] Internet-Draft Network Discovery and PS March 2007 1. Introduction When multiple access network are available, users may have difficulty in selecting which network to connect to, and how to authenticate with that network. The problem arises when any of the following conditions are true: o More than one network attachment point is available, and the attachment points differ in capability or belong to different operators. In this case, a user may have difficulty determining which attachment points offering the desired services it can successfully authenticate to. In order to choose between multiple attachment points, it can be helpful to determine which realms are supported and the capabilities that the networks support. o The user has multiple sets of credentials. In this case, the user may 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. This can result in multiple unsuccessful authentication attempts for each attachment point, wasting valuable time and resulting in user frustration. For example, the user could have one set of credentials from a public service provider and set from an employer. 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 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 AAA protocols may not be able to route the access request to the home AAA sever purely based on the realm within the Network Access Identifier (NAI) [RFC4282]. In addition, payload packets can be routed or tunneled differently, based on the roaming relationship path. This may have an impact on the available services or their pricing. In Section 2 the network discovery and selection problem is defined and divided into subproblems, and some potential 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. Arkko, et al. Expires September 6, 2007 [Page 3] Internet-Draft Network Discovery and PS March 2007 1.1. Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. 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. Decorated NAI A NAI specifying a source route. See RFC4282 [RFC4282] Section 2.7 for more information. Realm The realm portion of an NAI [RFC4282]. Network Selection Selection of an operator/ISP for network access. Network Selection occurs prior to network access authentication. Network Discovery The mechanism used to discover available networks. The discovery mechanism may be passive or active, and depends on the access technology. In passive network discovery, the node listens for network announcements; in active network discovery the node solicits network announcements. It is possible for an access technology to utilize both passive and active network discovery mechanisms. Realm Selection The selection of the realm (and corresponding NAI) used to access the network. Arkko, et al. Expires September 6, 2007 [Page 4] Internet-Draft Network Discovery and PS March 2007 Access Technology Selection This refers to the selection between access technologies e.g. 802.11, 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 a packet switched bearer. The 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. Network Access Server The device that peers connect to in order to obtain access to the network. In PPTP terminology, this is referred to as the PPTP Access Concentrator (PAC), and in L2TP terminology, it is referred to as the L2TP Access Concentrator (LAC). In IEEE 802.11, it is referred to as an Access Point. 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). Access Point (AP) An entity that has station functionality and provides access to distribution services via the wireless medium (WM) for associated stations. Arkko, et al. Expires September 6, 2007 [Page 5] Internet-Draft Network Discovery and PS March 2007 Basic Service Set (BSS) A set of stations controlled by a single coordination function. 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. Within the context of network selection and discovery the term 'network' is sometimes used interchangeably with the term 'realm'. It should be noted that a realm can be reachable from more than one access network types and selection of a realm may not imply certain network capabilities. Arkko, et al. Expires September 6, 2007 [Page 6] Internet-Draft Network Discovery and PS March 2007 2. Problem Definition The network discovery and selection problem can be broken down into multiple sub-problems. These include: o Discovery of points of attachment. This involves the discovery of points of attachment in the vicinity, as well as their capabilities. o Identifier selection. This involves selection of the NAI (and credentials) used to authenticate to the selected point of attachment. o AAA routing. This involves routing of the AAA conversation back to the home AAA server, based on the realm of the selected NAI. o Payload routing. This involves the routing of data packets, in the situation wh ere 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 type of charging policy. 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) components. 2.1. Discovery of the Point of Attachment to the Network Traditionally, discovery of points of attachment has been handled by link layer or out-of-band mechanisms. For example, the IEEE 802.11 specification [IEEE.802.11-2003] provides support for both passive (Beacon) and active (Probe Request/Response) discovery mechanisms; [Fixingapsel] studied the effectiveness of these mechanisms. The GSM specifications also provide for discovery of points of attachment, as does the CARD [RFC4066] protocol developed by the IETF SEAMOBY WG. RFC 2194 [RFC2194] 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 [RFC3017] describes the IETF Proposed Standard for the Roaming Access XML DTD. This covers not only the Arkko, et al. Expires September 6, 2007 [Page 7] Internet-Draft Network Discovery and PS March 2007 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 and 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. 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. As noted in [Velayos], the IEEE 802.11 Beacon mechanism does not scale well; with a Beacon interval of 100ms, and 10 APs in the vicinity, approximately 32 percent of an 802.11b AP's capacity is used for beacon transmission. In addition, since Beacon/Probe Response frames are sent by each AP over the wireless medium, stations can only discover APs within range, which implies substantial coverage overlap for roaming to occur without interruption. 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 handhskae). A number of enhancements have been proposed to the Beacon/Probe Response mechanism in order to improve scalability and performance in roaming scenarios. These include allowing APs to announce capabilities of neighbor APs as well as their own [IEEE.802.11k]. More scalable mechanisms for support of "virtual APs" within IEEE 802.11 have also been proposed [IEEE.802.11v]; generally these proposals collapse multiple "virtual AP" advertisements into a single advertisement. Higher layer mechanisms can also be used to improve scalability, since by running over IP they can utilize facilities such as fragmentation which may not be available at the link layer. For example, in IEEE 802.11, Beacon frames cannot use fragmentation because they are multicast frames. 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]. A single SSID also may support dynamic VLAN access as described in [RFC3580], or may support authentication to Arkko, et al. Expires September 6, 2007 [Page 8] Internet-Draft Network Discovery and PS March 2007 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 WAN providers. Typically, the user will choose an identity and corresponding credential set based on the network chooses to connect to, perhaps with additional assistance provided by the chosen authentication mechanism. For example, if 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. Arkko, et al. Expires September 6, 2007 [Page 9] Internet-Draft Network Discovery and PS March 2007 ? ? +---------+ +------------------+ ? | Access | | | O_/ _-->| Network |------>| isp1.example.com | /| / | 1 | _->| | | | +---------+ / +------------------+ _/ \_ | / | +---------+ / User "subscriber@isp1. | | Access |/ example.com" -- ? -->| Network | also known | | 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, preferrably 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 point 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 realms, selection of an access network provides knowledge of the identities that a client can use to successfully authenticate. For example, in an access network, the set of supported realms corresponding to network name can be pre- configured. Of course, it may not be possible to determine the available access networks prior to authentication. For example, in 802.11, not all SSIDs are broadcast, so that the station may need to probe to locate a "hidden" SSID. Also, within IKEv2 [RFC4306], the responder identity (typically the security gateway) is provided as a part of Arkko, et al. Expires September 6, 2007 [Page 10] Internet-Draft Network Discovery and PS March 2007 the IKEv2 exchange. It is also possible for hints to be embedded within credentials. In [RFC4334], usage hints are provided within certificates used for wireless authentication. This involves extending the client's certificate to include the SSIDs with which the certificate can be used. However, there may be situations in which an access network may not accept a static set of realms at every point of attachment. For example, as part of a roaming agreement only points of attachment within a given region or country may be made available. In these situations, mechanisms such as hints embedded within credentials or pre-configuration of access network to realm mappings may not be sufficient. Instead, it is necessary for the client to discover usable identities dynamically. This is the problem that RFC 4284 [RFC4284] attempts to solve, using the EAP-Request/Identity to communicate a list of supported realms. However, the problems inherent in this approach are many, as discussed in Appendix A.1. 2.3. AAA routing Once the identity has been selected, the AAA infrastructure needs to route the access request back to the home AAA server. Typically the routing is based on the Network Access Identifier (NAI) defined in [RFC4282]. Where the NAI does not encode a source route, the routing of requests is determined by the AAA infrastructure. As described in [RFC2194] most roaming implementations are relatively simple, relying on static realm routing table which determine the next based on the NAI realm included within the User-Name attribute. 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 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 Arkko, et al. Expires September 6, 2007 [Page 11] Internet-Draft Network Discovery and PS March 2007 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 NAS or local proxy and the home AAA server, trust anchors need to be configured, and certificates need to be selected. The AAA server certificate needs to chain to a trust anchor configured on the AAA client, and the AAA client certificate needs to chain to a trust anchor configured on the AAA server. Where multiple potential roaming relationship paths are available, this will reflect itself in multiple certificate choices, transforming the path selection problem into a certificate selection problem. Depending on the functionality supported within the certificate selection implementation, this may not make the problem easier to solve. For example, in order to provide the desired control over the roaming path, it may be necessary to implement custom certificate selection logic, which may be difficult to introduce within a certificate handling implementation designed for general purpose usage. As noted in [RFC4284], it is also possible to utilize an NAI for the purposes of source routing. In this case, the client provides guidance to the AAA infrastructure as to how it would like the access request to be routed. An NAI including source routing information is said to be "decorated"; the decoration format is defined in [RFC4282]. When decoration is utilized, the EAP peer provides the decorated NAI within the EAP-Response/Identity, and as described in [RFC3579], the NAS copies the decorated NAI included in the EAP-Response/Identity into the User-Name attribute included within the access request. As the access request transits the roaming relationship path, AAA proxies determine the next hop based on the realm included within the User-Name attribute, in the process successively removing decoration from the NAI included in the User-Name attribute. In contrast, the decorated NAI included within the EAP-Response/Identity encapsulated in the access request remains untouched. As a result, when the access request arrives at the AAA home server, the decorated NAI included in the EAP-Response/Identity may differ from the NAI Arkko, et al. Expires September 6, 2007 [Page 12] Internet-Draft Network Discovery and PS March 2007 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 UUCP and FidoNet. Prior to the implementation of email gateways utilizing MX RR routing, email address-based source-routing was used extensively. However, over time the need for email source-routing disappeared. 2.3.1. The Default Free Zone AAA clients on the edge of the network, such as NAS devices and local AAA proxies, typically maintain a default realm route, providing a default next hop for realms not otherwise taken into account within the realm routing table. This permits devices with limited resources to maintain a small realm routing table. Deeper within the AAA infrastructure, AAA proxies may be maintained with a "default free" realm table, listing next hops for all known realms, but not providing a default realm route. While dynamic realm routing protocols are not in use within AAA infrastructure today, even if such protocols were to be introduced, it is likely that they would be deployed solely within the core AAA infrastructure, but not on NAS devices, which are typically resource constrained. Since NAS devices do not maintain a full realm routing table, they do not have knowledge of all the realms reachable from the local network. The situation is analogous to that of Internet hosts or edge routers which do not participate in the BGP mesh. In order for an Internet host to determine whether it an 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 know apriori whether the realm encoded in the NAI is reachable or not; 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 the realm is reachable or not. 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. Arkko, et al. Expires September 6, 2007 [Page 13] Internet-Draft Network Discovery and PS March 2007 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 3 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 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 Arkko, et al. Expires September 6, 2007 [Page 14] Internet-Draft Network Discovery and PS March 2007 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 the limitations of current AAA routing mechanisms, there are situations in which better control over AAA routing behavior is required. Utilizing NAI-based source routing, a decorated NAI can be used to influence the roaming relationship path. 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 among already established realm routes. If a realm route does not exist or is not feasible, then NAI-based source routing cannot establish 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 was 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 Arkko, et al. Expires September 6, 2007 [Page 15] Internet-Draft Network Discovery and PS March 2007 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 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 Discovery Network capabilities can provide information useful in the selection of an access network. These include characteristics of the network beyond those of individual points of attachment. Network capabilities which can be discovered include: o Access network identifier (e.g. IEEE 802.11 SSID) o Roaming relationships between the access network provider and other network providers and associated costs o Authentication mechanisms o Quality of Service capability o Cost o Service parameters, such as the existence of middleboxes Network discovery focuses on the discovery of the services offered by networks, not just the capabilities of individual points of attachment. Typically it is desirable to acquire information on access networks prior to authentication, particularly in situations where successful authentication depends on that information. Reference [IEEE.11-04-0624] classifies the possible steps at which IEEE 802.11 networks can acquire this information: Arkko, et al. Expires September 6, 2007 [Page 16] Internet-Draft Network Discovery and PS March 2007 o Pre-association o Post-association (or pre-authentication) o Post-authentication In the interest of minimizing connectivity delays, the information required for network selection 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. Arkko, et al. Expires September 6, 2007 [Page 17] Internet-Draft Network Discovery and PS March 2007 3. Design Issues The following factors should be taken into consideration while evaluating solutions for problem of network selection and discovery: 3.1. AAA Routing Solutions to the AAA routing issues discussed in Section 2.3 need to apply to a wide range of AAA messages, and should not restrict the introduction of new AAA or access network functionality. For example, AAA routing mechanisms should work for access requests and responses as well as accounting requests and responses and server- initiated messages. Solutions should not restrict the development of new AAA attributes, access types, or performance optimizations (such as fast handoff support). 3.2. Backward Compatibility Solutions need to maintain backward compatibility. In particular: o Selection-aware clients need to interoperate with legacy NAS devices and AAA servers. o Selection-aware AAA infrastructure needs to interoperate with legacy clients and NAS devices. For example, selection-aware clients should not transmit packets larger than legacy NAS devices or AAA servers can handle. Where protocol extensions are required, changes should be required to as few infrastructure elements as possible. For example, extensions that require upgrades to existing NAS devices will be more difficult to deploy than proposals that are incrementally deployable based on phased upgrades of clients or AAA servers. 3.3. Efficiency Constraints Solutions should be efficient as measured by channel utilization, bandwidth consumption, handoff delay, and energy utilization. Mechanisms that require 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 Arkko, et al. Expires September 6, 2007 [Page 18] Internet-Draft Network Discovery and PS March 2007 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 which 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. Similarly, "virtual AP" approaches introduce additional Beacons in proportion to the number of networks being advertised. Neither approach scales to support a large number of networks and realms. 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 [I-D.groeting-eap-netselection-results], 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. Arkko, et al. Expires September 6, 2007 [Page 19] Internet-Draft Network Discovery and PS March 2007 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 3GPP has the potential to become a significant problem going forward. Without a unified approach, multiple non- interoperable solutions may be deployed, resulting in fragmentation. o New link layer designs should include the 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 extensions. With respect to specific mechanisms for access network discovery and selection: o Studies such as [MACScale] and [Velayos], demonstrate that the IEEE 802.11 Beacon/Probe Response mechanism has substantial scaling issues, and as a result a single physical access point is in practice limited to less than a dozen virtual APs on each channel with IEEE 802.11b. The situation is improved substantially with successors such as IEEE 802.11a which enable additional channels, thus potentially increasing the number of potential virtual APs. However, even with these enhancements it is not feasible to advertise more than 50 different networks, and probably less in most circumstances. Arkko, et al. Expires September 6, 2007 [Page 20] Internet-Draft Network Discovery and PS March 2007 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 the IEEE 802.11u to provide enhanced discovery functionality. Similarly, IEEE 802.1af has discussed addition of network functionality to IEEE 802.1X. However, neither IEEE 802.1ab nor IEEE 802.1af is likely to support fragmentation of advertisement frames, so that the amount of data that can be transported will be limited. o While IEEE 802.11k provides support for the Neighbor Report, this only provides for gathering of information on neighboring 802.11 APs, not points of attachment supporting other link layers. Solution to this problem would appear to require coordination across IEEE 802 as well as between standards bodies. o Given that EAP does not support fragmentation of EAP-Request/ Identity packets, the volume of "realm hints" that can be fit with these packets is limited. In addition, within IEEE 802.11, EAP packets can only be exchanged within State 3 (associated and authenticated). As a result, use of EAP for realm discovery may result in significant delays. In addition, the ability of EAP to carry Quality of Service information [I-D.groeting-eap-netselection-results] appears limited. As a result, we believe that use of EAP as described in [RFC4284] is not a sound long-term approach for solution of the realm discovery problem for mobile users where the information is needed for handoff purposes. 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) [I-D.marques-ddp], which provides functionality equivalent to IEEE 802.1ab using ASN.1 encoded advertisements sent to a link-local scope multicast address. Arkko, et al. Expires September 6, 2007 [Page 21] Internet-Draft Network Discovery and PS March 2007 5. IANA Considerations This document does not define any new name spaces to be managed by IANA. This document does not either reserve any new numbers or names under any existing name space managed by IANA. Arkko, et al. Expires September 6, 2007 [Page 22] Internet-Draft Network Discovery and PS March 2007 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 [I-D.josefsson-pppext-eap-tls-eap] [I-D.tschofenig-eap-ikev2], however, and there is even an attempt to provide a backwards compatible extensions to older methods [I-D.arkko-eap-service-identity-auth]. The security requirements for network selection depend on whether the selection is considered as a command or a hint. For instance, the selection that the user provided may be ignored if it relates to AAA routing and the access network can route the AAA traffic to the correct home network using other means in any case. Arkko, et al. Expires September 6, 2007 [Page 23] Internet-Draft Network Discovery and PS March 2007 7. 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- Lowe. 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. Arkko, et al. Expires September 6, 2007 [Page 24] Internet-Draft Network Discovery and PS March 2007 8. References 8.1. Normative References [RFC1035] Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, November 1987. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [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. [RFC4306] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol", RFC 4306, 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. 8.2. Informative References [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. Arkko, et al. Expires September 6, 2007 [Page 25] Internet-Draft Network Discovery and PS March 2007 [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. [RFC2486] Aboba, B. and M. Beadles, "The Network Access Identifier", RFC 2486, January 1999. [I-D.arkko-eap-service-identity-auth] Arkko, J. and P. Eronen, "Authenticated Service Identities for the Extensible Authentication Protocol (EAP)", draft-arkko-eap-service-identity-auth-04 (work in progress), October 2005. [I-D.groeting-eap-netselection-results] Tschofenig, H., "Network Selection Implementation Results", draft-groeting-eap-netselection-results-00 (work in progress), July 2004. [I-D.josefsson-pppext-eap-tls-eap] Josefsson, S., Palekar, A., Simon, D., and G. Zorn, "Protected EAP Protocol (PEAP)", draft-josefsson-pppext-eap-tls-eap-07 (work in progress), October 2003. [I-D.marques-ddp] Enns, R., Marques, P., and D. Morrell, "Device Discovery Protocol (DDP)", draft-marques-ddp-00 (work in progress), May 2003. [I-D.tschofenig-eap-ikev2] Tschofenig, H. and D. Kroeselberg, "EAP IKEv2 Method (EAP- IKEv2)", draft-tschofenig-eap-ikev2-10 (work in progress), February 2006. [IEEE.8021X] Institute of Electrical and Electronics Engineers, "Local and Metropolitan Area Networks: Port-Based Network Access Control", IEEE Standard 802.1X, September 2001. [IEEE.802.11-2003] Arkko, et al. Expires September 6, 2007 [Page 26] Internet-Draft Network Discovery and PS March 2007 Institute of Electrical and Electronics Engineers, "Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications", IEEE Standard 802.11, 2003. [IEEE-11-03-154r1] Aboba, B., "Virtual Access Points", IEEE Contribution 11- 03-154r1, May 2003. [IEEE-11-03-0827] Hepworth, E., "Co-existence of Different Authentication Models", IEEE Contribution 11-03-0827 2003. [IEEE.11-04-0624] Berg, S., "Information to Support Network Selection", IEEE Contribution 11-04-0624 2004. [11-05-0822-03-000u-tgu-requirements] Moreton, M., "TGu Requirements", IEEE Contribution 11-05- 0822-03-000u-tgu-requirements, August 2005. [3GPPSA2WLANTS] 3GPP, "3GPP System to Wireless Local Area Network (WLAN) interworking; System Description; Release 6; Stage 2", 3GPP Technical Specification 23.234 v 6.6.0, September 2005. [3GPP-SA3-030736] Ericsson, "Security of EAP and SSID based network advertisements", 3GPP Contribution S3-030736, November 2003. [3GPP.23.122] 3GPP, "Non-Access-Stratum (NAS) functions related to Mobile Station (MS) in idle mode", 3GPP TS 23.122 6.5.0, October 2005. [WWRF-ANS] Eijk, R., Brok, J., Bemmel, J., and B. Busropan, "Access Network Selection in a 4G Environment and the Role of Terminal and Service Platform", 10th WWRF, New York, October 2003. [WLAN3G] Ahmavaara, K., Haverinen, H., and R. Pichna, "Interworking Architecture between WLAN and 3G Systems", IEEE Communications Magazine, November 2003. [INTELe2e] Intel, "Wireless LAN (WLAN) End to End Guidelines for Arkko, et al. Expires September 6, 2007 [Page 27] Internet-Draft Network Discovery and PS March 2007 Enterprises and Public Hotspot Service Providers", November 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. [Fixingapsel] Judd, G. and P. Steenkiste, "Fixing 802.11 Access Point Selection", Sigcomm Poster Session 2002. [Eronen03] Eronen, P., "Network Selection Issues", presentation to EAP WG at IETF 58, November 2003. [Priest04] Priest, J., "The State of Wireless London", July 2004. [MACScale] Heusse, M., "Performance Anomaly of 802.11b", LSR-IMAG Laboratory, Grenoble, France, IEEE Infocom 2003. [Eronen04] Eronen, P. and J. Arkko, "Role of authorization in wireless network security", Extended abstract presented in the DIMACS workshop, November 2004. [3GPPSA3WLANTS] 3GPP, "3GPP Technical Specification Group Service and System Aspects; 3G Security; Wireless Local Area Network (WLAN) interworking security (Release 6); Stage 2", 3GPP Technical Specification 33.234 v 6.6.0, October 2005. [3GPPCT1WLANTS] 3GPP, "3GPP System to Wireless Local Area Network (WLAN) interworking; User Equipment (UE) to network protocols; Stage 3 (Release 6)", 3GPP Technical Specification 24.234 v 6.4.0, October 2005. [IEEE.802.11k] Institute of Electrical and Electronics Engineers, "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 IEEE 802.11k, D4.1, July 2006. Arkko, et al. Expires September 6, 2007 [Page 28] Internet-Draft Network Discovery and PS March 2007 [IEEE.802.11v] Institute of Electrical and Electronics Engineers, "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 IEEE 802.11v, D0.08, January 2007. [IEEE.802.21] Institute of Electrical and Electronics Engineers, "Draft IEEE Standard for Local and Metropolitan Area Networks: Media Independent Handover Services", IEEE IEEE 802.21, D03.00, December 2006. [3GPPCT4WLANTS] 3GPP, "3GPP system to Wireless Local Area Network (WLAN) interworking; Stage 3 (Release 6)", 3GPP Technical Specification 29.234 v 6.4.0, October 2005. [RFC4066] Liebsch, M., Singh, A., Chaskar, H., Funato, D., and E. Shim, "Candidate Access Router Discovery (CARD)", RFC 4066, July 2005. Arkko, et al. Expires September 6, 2007 [Page 29] Internet-Draft Network Discovery and PS March 2007 Appendix A. Existing Work A.1. IETF Several IETF WGs have dealt with aspects of the network selection problem, including the AAA, EAP, PPP, RADIUS, ROAMOPS, and RADEXT WGs. ROAMOPS WG developed the NAI, originally defined in [RFC2486], and subsequently updated in [RFC4282]. Initial roaming implementations are described in [RFC2194], and the use of proxies in roaming is addressed in [RFC2607]. The SEAMOBY WG developed CARD [RFC4066], which assists in discovery of suitable base stations. PKIX WG produced [RFC3280], which addresses issues of certificate selection. The AAA WG developed more sophisticated access routing, authentication and service discovery mechanisms within Diameter [RFC3588]. Adrangi et al. [RFC4284] defines the use of the EAP-Request/Identity to provide "realm hints" useful for identity selection. The NAI syntax described in [RFC4282] enables the construction of source routes. Together, these mechanisms enable the user to determine whether it possesses an identity and corresponding credential suitable for use with an EAP-capable NAS. This is particularly useful in situations where the lower layer provides limited information (such as in wired IEEE 802 networks where IEEE 802.1X currently does not provide for advertisement of networks and their capabilities). However, advertisement mechanisms based on the use of the EAP- Request/Identity have scalability problems. As noted in [RFC3748] Section 3.1, the minimum EAP MTU is 1020 octets, so that an EAP- Request/Identity is only guaranteed to be able to include 1015 octets within the Type-Data field. Since RFC 1035 [RFC1035] enables FQDNs to be up to 255 octets in length, this may not enable the announcement of many 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. This will Arkko, et al. Expires September 6, 2007 [Page 30] Internet-Draft Network Discovery and PS March 2007 dramatically increase handoff latency. Thus, rather than thinking of [RFC4284] as a 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 which 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 which 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 Arkko, et al. Expires September 6, 2007 [Page 31] Internet-Draft Network Discovery and PS March 2007 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 distinct physical AP. As noted in the paper, full compatibility with existing 802.11 station implementations can only be maintained if each virtual AP uses a distinct MAC address (BSSID) for use in Beacons and Probe Responses. This draft does not discuss scaling issues in detail, but recommends that only a limited number of virtual APs be supported by a single physical access point. The simulations presented in [Velayos] appear to confirm this conclusion; with a Beacon interval of 100 ms, once more than 8 virtual APs are supported on a single channel, more than 20% of bandwidth is used for Beacons alone. This would indicate a limit of approximately 20 virtual APs per physical AP. o IEEE 802.11u is working on realm discovery and network selection [11-05-0822-03-000u-tgu-requirements]. 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, apriori 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. 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) which 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. Arkko, et al. Expires September 6, 2007 [Page 32] Internet-Draft Network Discovery and PS March 2007 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 SIM. In the case of manual PLMN selection, the mobile node lists the PLMNs it learns 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] discuss the new realm (PLMN) selection requirements introduced by I-WLAN roaming, which supports 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. Arkko, et al. Expires September 6, 2007 [Page 33] Internet-Draft Network Discovery and PS March 2007 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-WAN 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 be considered 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 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. Arkko, et al. Expires September 6, 2007 [Page 34] Internet-Draft Network Discovery and PS March 2007 Authors' Addresses Jari Arkko Ericsson Jorvas 02420 Finland Email: jari.arkko@ericsson.com Bernard Aboba Microsoft One Microsoft Way Redmond, WA 98052 USA Email: aboba@internaut.com Jouni Korhonen TeliaSonera Teollisuuskatu 13 Sonera FIN-00051 Finland Email: jouni.korhonen@teliasonera.com Farooq Bari Cingular Wireless 7277 164th Avenue N.E. Redmond WA 98052 USA Email: farooq.bari@cingular.com Arkko, et al. Expires September 6, 2007 [Page 35] Internet-Draft Network Discovery and PS March 2007 Full Copyright Statement Copyright (C) The IETF Trust (2007). 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 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 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 http://www.ietf.org/ipr. 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 ietf-ipr@ietf.org. Acknowledgment Funding for the RFC Editor function is provided by the IETF Administrative Support Activity (IASA). Arkko, et al. Expires September 6, 2007 [Page 36]