Extensible Authentication Protocol J. Arkko Internet-Draft Ericsson Expires: November 26, 2006 B. Aboba Microsoft J. Korhonen TeliaSonera F. Bari Cingular Wireless May 25, 2006 Network Discovery and Selection Problem draft-ietf-eap-netsel-problem-04 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 November 26, 2006. Copyright Notice Copyright (C) The Internet Society (2006). Abstract 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 Arkko, et al. Expires November 26, 2006 [Page 1] Internet-Draft Network Discovery and PS May 2006 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 Arkko, et al. Expires November 26, 2006 [Page 2] Internet-Draft Network Discovery and PS May 2006 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", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [2]. Arkko, et al. Expires November 26, 2006 [Page 3] Internet-Draft Network Discovery and PS May 2006 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. Arkko, et al. Expires November 26, 2006 [Page 4] Internet-Draft Network Discovery and PS May 2006 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 attachment. 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) components. 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. Arkko, et al. Expires November 26, 2006 [Page 5] Internet-Draft Network Discovery and PS May 2006 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 802.11. Another issue with the Beacon/Probe Request/Response mechanism is that it is either insecure or its security can be assured only after already attaching to this particular network. When considering access systems such as 802.11 WLANs networks it is Arkko, et al. Expires November 26, 2006 [Page 6] Internet-Draft Network Discovery and PS May 2006 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 both. ? ? +---------+ +------------------+ ? | 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 Arkko, et al. Expires November 26, 2006 [Page 7] Internet-Draft Network Discovery and PS May 2006 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. Arkko, et al. Expires November 26, 2006 [Page 8] Internet-Draft Network Discovery and PS May 2006 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 disappeared. As with the selection of certificates by stations, a Diameter client wishing to authenticate with a Diameter server may have a choice of available certificates, and therefore it may need to choose between them. 2.3.1 The Incomplete Routing Table Problem No dynamic routing protocols are in use in AAA infrastructure today. This implies that there has to be a device (such as a proxy) within the access network that knows how to route to different domains, even if they are further than one hop away, as shown in Figure 2. In this figure, the user "joe@c.example.com" has to be authenticated through ISP 2, since the domain "c.example.com" is served by it. Arkko, et al. Expires November 26, 2006 [Page 9] Internet-Draft Network Discovery and PS May 2006 +---------+ +---------+ | | | | User "joe@ | Access |----->| ISP 1 |-----> "a.example.com" c.example.com"-->| Network | | | | | +---------+ +---------+ | | V +---------+ | |-----> "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 Arkko, et al. Expires November 26, 2006 [Page 10] Internet-Draft Network Discovery and PS May 2006 based on the user's preferences. On the other hand, user control is similar to source routing, and as discussed earlier, network-based routing mechanisms have traditionally won over source routing-based mechanisms. If users can control the selection of intermediaries, such intermediaries still have to be legitimate AAA proxies. That is, an access network should not send a request to an unknown intermediary. If it has a business relationship with three intermediaries int1.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 globally. As a result, network-based AAA routing mechanisms are preferred over user-based selection where sufficient routes have been configured and there is no need for user control. Where these conditions are not met -- particularly when an attempt to use the network-based routing mechanism has failed -- routing hints can be placed in an NAI as defined in [12]. Where NAIs are used in this manner, the AAA routing problem becomes a subset of the identifier selection problem. Arkko, et al. Expires November 26, 2006 [Page 11] Internet-Draft Network Discovery and PS May 2006 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 Arkko, et al. Expires November 26, 2006 [Page 12] Internet-Draft Network Discovery and PS May 2006 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: Arkko, et al. Expires November 26, 2006 [Page 13] Internet-Draft Network Discovery and PS May 2006 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. Arkko, et al. Expires November 26, 2006 [Page 14] Internet-Draft Network Discovery and PS May 2006 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 exchange. 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. Arkko, et al. Expires November 26, 2006 [Page 15] Internet-Draft Network Discovery and PS May 2006 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 following: 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 Arkko, et al. Expires November 26, 2006 [Page 16] Internet-Draft Network Discovery and PS May 2006 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. Arkko, et al. Expires November 26, 2006 [Page 17] Internet-Draft Network Discovery and PS May 2006 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 NAI. o Existing EAP mechanisms are used where possible. Arkko, et al. Expires November 26, 2006 [Page 18] Internet-Draft Network Discovery and PS May 2006 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. Arkko, et al. Expires November 26, 2006 [Page 19] Internet-Draft Network Discovery and PS May 2006 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 Arkko, et al. Expires November 26, 2006 [Page 20] Internet-Draft Network Discovery and PS May 2006 large fraction of current WLAN access points operate on the default SSID, which may make the use of the phone book approach hard. Arkko, et al. Expires November 26, 2006 [Page 21] Internet-Draft Network Discovery and PS May 2006 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 are: 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 selection. o Nevertheless, many of the problems discussed in this draft are very hard when one considers them in an environment that requires a potentially large number of networks, fast handoffs, and automatic decisions. o The proliferation of multiple competing network discovery technologies within IEEE 802, IETF, and 3GPP appears to a significant problem going forward. In the absence of a clearly defined solution to the problem it is likely that any or all of these solutions will be utilized, resulting in industry fragmentation and lack of interoperability. 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. Arkko, et al. Expires November 26, 2006 [Page 22] Internet-Draft Network Discovery and PS May 2006 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 Arkko, et al. Expires November 26, 2006 [Page 23] Internet-Draft Network Discovery and PS May 2006 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. Arkko, et al. Expires November 26, 2006 [Page 24] Internet-Draft Network Discovery and PS May 2006 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. Arkko, et al. Expires November 26, 2006 [Page 25] Internet-Draft Network Discovery and PS May 2006 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. Arkko, et al. Expires November 26, 2006 [Page 26] Internet-Draft Network Discovery and PS May 2006 [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. Arkko, et al. Expires November 26, 2006 [Page 27] Internet-Draft Network Discovery and PS May 2006 [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 Arkko, et al. Expires November 26, 2006 [Page 28] Internet-Draft Network Discovery and PS May 2006 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 Ericsson Jorvas 02420 Finland Email: jari.arkko@ericsson.com Bernard Aboba Microsoft One Microsoft Way Redmond, WA 98052 USA Email: aboba@internaut.com Arkko, et al. Expires November 26, 2006 [Page 29] Internet-Draft Network Discovery and PS May 2006 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 November 26, 2006 [Page 30] Internet-Draft Network Discovery and PS May 2006 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- 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 November 26, 2006 [Page 31] Internet-Draft Network Discovery and PS May 2006 Intellectual Property Statement 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. 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Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Arkko, et al. Expires November 26, 2006 [Page 32]