Network Working Group Y. Ohba Internet-Draft Toshiba Expires: October 3, 2006 A. Dutta Telcordia S. Sreemanthula Nokia Apr 2006 Pre-authentication Problem Statement draft-ohba-hokeyp-preauth-ps-00 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 October 3, 2006. Copyright Notice Copyright (C) The Internet Society (2006). Abstract This document describes a pre-authentication problem statement. This document is used as the basis of the pre-authentication part of the charter of a potential IETF working group on handover keying and pre- authentication. Ohba, et al. Expires October 3, 2006 [Page 1] Internet-Draft Pre-authentication Problem Statement Apr 2006 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Specification of Requirements . . . . . . . . . . . . . . 3 2. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 5 3. Usage Scenarios . . . . . . . . . . . . . . . . . . . . . . . 8 3.1. Direct Pre-authentication . . . . . . . . . . . . . . . . 8 3.2. Indirect Pre-authentication . . . . . . . . . . . . . . . 8 4. AAA Issues . . . . . . . . . . . . . . . . . . . . . . . . . . 11 5. Out-of-scope Issues . . . . . . . . . . . . . . . . . . . . . 12 6. Security Considerations . . . . . . . . . . . . . . . . . . . 13 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14 8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 15 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16 9.1. Normative References . . . . . . . . . . . . . . . . . . . 16 9.2. Informative References . . . . . . . . . . . . . . . . . . 16 Appendix A. Performance Requirements . . . . . . . . . . . . . . 18 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 20 Intellectual Property and Copyright Statements . . . . . . . . . . 21 Ohba, et al. Expires October 3, 2006 [Page 2] Internet-Draft Pre-authentication Problem Statement Apr 2006 1. Introduction When a mobile during an active communication session moves from one access network to another access network and changes its point of attachment it is subjected to disruption in the continuity of service because of the associated handover operation. During the handover process, when the mobile changes its point-of-attachment in the network, it may end up communicating using its second interface in the new network, change its subnet or administrative domain it is connected to. A complete description of the types of handover based on the movement type is documented in [I-D.ohba-mobopts- heterogeneous-requirement]. We provide in Appendix A some performance requirement that are needed to support an interactive real-time communication such as VoIP and thus can serve as the guidelines for handover optimization. Handover often requires authorization for acquisition or modification of resources assigned to a mobile and the authorization needs interaction with a central authority in a domain. In many cases an authorization procedure during a handover procedure follows an authentication procedure that also requires interaction with a central authority in a domain. If the handover involves inter-domain mobility without any roaming or trust relationship between the domains, then the delay introduced due to a full authentication and authorization procedure adds to the handover latency and consequently affects the ongoing multimedia sessions. The authentication and authorization procedure may include EAP authentication [RFC3748] where an AAA server may be involved in EAP messaging during the handover. Depending upon the type of architecture, in some cases the AAA signals traverse all the way to the AAA server in the home domain of the mobile as well before the network service is granted to the mobile in the new network. As an example a combination of EAP-based authentication and authorization may take up to 5 sec [georgiades]. Real-time communication and interactive traffic such as VoIP is very sensitive to the delay and thus cannot tolerate this amount of delay. Thus it is necessary to reduce the delay due to authentication, authorization and AAA related key transfer. Thus it is desirable that interactions between the mobile and AAA servers must be avoided or be reduced during the handover. This draft discusses pre-authentication, a handover optimization method that is based on executing EAP between a mobile node and a target authenticator via the serving authenticator before the mobile node handovers to the target authenticator. 1.1. Specification of Requirements In this document, several words are used to signify the requirements Ohba, et al. Expires October 3, 2006 [Page 3] Internet-Draft Pre-authentication Problem Statement Apr 2006 of the specification. These words are often capitalized. 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]. Ohba, et al. Expires October 3, 2006 [Page 4] Internet-Draft Pre-authentication Problem Statement Apr 2006 2. Problem Statement Basic mechanism of handover is a two step procedure involving i) network selection procedure to determine the appropriate candidate network point of attachment and ii) handover or setting up of L2 and L3 connectivity to the target network point of attachment. Currently, security mechanisms for authentication and authorization is performed as part of the second step directly with the target network. Experimental studies with network handovers indicate that the latency introduced due to the security mechanisms is not acceptable for real time communications. For example, in basic IEEE 802.11b based wireless networks, the security mechanism involves performing a new IEEE 802.1x message exchange with the authenticator in the target AP to initiate an EAP exchange to the authentication server. Following a successful authentication, a four-way handshake with the wireless station derives a new set of the session keys for use in data communications. This mechanism is same as the initial setup to the AP with no particular optimizations for the handover scenario. The handover latency component introduced by this security mechanism has proven to be larger than what is acceptable. Hence, improvement in the handover latency performance due to security procedures is a necessary objective. There is relevant work undertaken by various standards organizations. But these efforts are scoped to a specific access technology. IEEE 802.11f has defined transfer of Security context from one AP to another. IEEE 802.11i defines a pre-authentication mechanism for use in 802.11 variant wireless networks. This mechanism allows mobile devices to make pre-authentication by establishing link-layer security associations with one or more target authenticators by sending 802.1X messages directly to the target authenticators bridged via the serving authenticator. Presently, IEEE 802.11r WG has been working to define Fast BSS transition mechanisms involving a definition of key management hierarchy and mechanisms for link-layer pre-authentication and setup of session keys before the re- association to the target AP. These mechanisms, as indicated before, are defined for IEEE 802.11 technologies and only applicable within a certain access domain and fall short when it comes to inter-access technology handovers. They also require L2 (e.g. Ethernet) connectivity for transfer of encapsulated signaling to the target AP. As various flavors of wireless technologies are increasingly available, there is a growing demand for seamless inter-access technology mobility and handovers. This is particularly beneficial in the presence of high bandwidth wireless technologies (e.g. IEEE 802.11a/b/g) with only hotspot like coverages while the overlay licensed wireless/cellular coverages are expensive and relatively lower bandwidth. There is a strong motivation to allow seamless Ohba, et al. Expires October 3, 2006 [Page 5] Internet-Draft Pre-authentication Problem Statement Apr 2006 inter-technology handovers for all kinds of data communications. Hence, the security optimization mechanisms for better handover performance must be looked at from the IP level for being common over different access technologies. Solutions for mobility security optimizations can be largely seen as security context transfer, handover keying or pre-authentication. Security context transfer involves transfer of reusable key context in the new point of attachment. However, the recent AAA key management requirement [I-D.housley-aaa-key-mgmt] does not recommend context transfer of reusable key context because of domino effect in which a compromise of an authenticator will lead to a compromise of another authenticator. Nakhjiri et al [I-D.nakhjiri-aaa-hokey-ps] discusses handover keying. Handover keying uses an existing EAP- generated key for deriving a key to be used for a target authenticator in order to reduce the handover delay, which eliminates the need for running EAP for each inter-authenticator handover. On the other hand, there are certain cases where an EAP-generated key does not exist or is not usable for handover keying at the time of handover and an EAP run is not avoidable to generate a key for the target authenticator. One case is an inter-domain handover without any trust relationship between domains. Another case is a handover to an existing technology that does not support handover keying. Pre-authentication discussed in this document is based on executing EAP between a mobile node and a target authenticator via the serving authenticator prior to handover to the target authenticator, where the serving and target authenticators are in different subnets or of different link-layer technologies. Pre-authentication would enable the mobile device to authenticate and setup keys prior to connecting to the target authenticator. This framework has general applicability to various deployment scenarios. Note that pre- authentication for intra-technology intra-subnet handover should be solved by each link-layer and thus out of the scope of this document. Figure 1 shows the functional elements that are related to pre- authentication. Ohba, et al. Expires October 3, 2006 [Page 6] Internet-Draft Pre-authentication Problem Statement Apr 2006 +------+ +-------------+ +-------+ |Mobile|---------| Serving | / \ | Node | |Authenticator|------/ \ +------+ +-------------+ / \ . / \ +----------+ . Move + Internet +----|AAA Server| . \ / +----------+ v +-------------+ \ / | Target |------\ / |Authenticator| \ / +-------------+ +-------+ Figure 1: Pre-authentication Functional Elements A mobile node is attached to the serving access network. Before the mobile node performs handover from the serving access network to a target access network, it performs pre-authentication with a target authenticator, an authenticator in the target access network, via the serving access network. The mobile node may perform pre- authentication with one or more target authenticators. It is assumed that each authenticator has an IP address. Authenticators may be on different IP links. It is assumed that there is at least one target authenticator in each target access network while the serving access network may or may not have a serving authenticator. The serving and target access networks may use different link-layer technologies. Each authenticator has the functionality of EAP authenticator which is either standalone EAP authenticator or pass-through EAP authenticator. When an authenticator acts as a standalone EAP authenticator, it also has the functionality of EAP server. On the other hand, when an authenticator acts as a pass-through EAP authenticator, it communicates with EAP server typically implemented on a AAA server using a AAA protocol such as RADIUS and Diameter. If the target authenticator is of an existing link-layer technology that uses an MSK (Master Session Key) [I-D.ietf-eap-keying] for generating lower-layer ciphering keys, pre-authentication is used for proactively generating the MSK for the target authenticator. Otherwise, if the target authenticator supports handover keying, pre- authentication is used for proactively generating handover keys for multiple authenticators including the target authenticator. Ohba, et al. Expires October 3, 2006 [Page 7] Internet-Draft Pre-authentication Problem Statement Apr 2006 3. Usage Scenarios There are two scenarios on how pre-authentication signaling can happen among a mobile node, a serving authenticator, a target authenticator and a AAA server, depending on how the serving authenticator is involved in the pre-authentication signaling. 3.1. Direct Pre-authentication Direct pre-authentication signaling is shown in Figure 2. Mobile Serving Target AAA Node Authenticator Authenticator Server (MN) (SA) (TA) | | | | | | | | | MN-TA Signaling (L3) | AAA | |<--------------------+----------------------->|<----------------->| | | | | | | | | Figure 2: Direct Pre-authentication In this type of pre-authentication, pre-authentication signaling is transparent to the serving authenticator or there may be no serving authenticator at all in the serving access network. Direct pre-authentication is needed when the same authentication credentials are not used for network access authentication for the serving and target access networks, e.g., when different AAA servers are used for the serving and target access networks and no communication is allowed between the AAA servers. [I-D.ietf-pana-preauth] is identified as a protocol to realize direct pre-authentication. 3.2. Indirect Pre-authentication Indirect pre-authentication signaling is shown in Figure 3. Ohba, et al. Expires October 3, 2006 [Page 8] Internet-Draft Pre-authentication Problem Statement Apr 2006 Mobile Serving Target AAA Node Authenticator Authenticator Server (MN) (SA) (TA) | | | | | | | | | MN-SA Signaling | SA-TA Signaling | AAA | | (L2 or L3) | (L3) | | |<------------------->|<---------------------->|<----------------->| | | | | | | | | Figure 3: Indirect Pre-authentication With indirect pre-authentication, the serving authenticator is involved in pre-authentication signaling. Indirect pre- authentication is needed if IP communication is not allowed between the target authenticator and unauthorized nodes for security reasons. Also, indirect pre-authentication allows the serving network to select an appropriate target authenticator. Indirect pre-authentication signaling is spliced into mobile node to serving authenticator signaling (MN-SA signaling) and serving authenticator to target authenticator signaling (SA-TA signaling). SA-TA signaling is performed over L3. This is because it is not reasonable to assume that the serving authenticator and the target authenticator are on the same IP link. MN-SA signaling is performed over L2 or L3. L2 signaling is needed if IP communication is not allowed between the serving authenticator and any node over the wireless interface of the serving authenticator even if the node is authorized for communicating with other nodes over IP. On the other hand, L3 signaling is needed if the serving authenticator is not on the same IP link as the mobile node. The role of the serving authenticator in indirect pre-authentication is to bridge pre-authentication signaling between the mobile node and the target authenticator and not to act as an EAP authenticator, while it acts as an EAP authenticator for normal authentication signaling. This is illustrated in Figure 4. Ohba, et al. Expires October 3, 2006 [Page 9] Internet-Draft Pre-authentication Problem Statement Apr 2006 Mobile Serving Target Node Authenticator Authenticator (MN) (SA) (TA) +-----------+ +-----------+ | |<- - - - - - - - - - - - - - - - - - ->| | | EAP Peer | +-----------------------------+ | EAP Auth- | | | | Pre-authentication Bridging | | enticator | +-----------+ +-----------+-----+-----------+ +-----------+ | MN-SA | | MN-SA | | SA-TA | | SA-TA | | Signaling |<-->| Signaling | | Signaling |<-->| Signaling | | Layer | | Layer | | Layer | | Layer | +-----------+ +-----------+ +-----------+ +-----------+ Figure 4: Indirect Pre-authentication Layering Model Ohba, et al. Expires October 3, 2006 [Page 10] Internet-Draft Pre-authentication Problem Statement Apr 2006 4. AAA Issues In pre-authentication, AAA authentication and authorization for a target authenticator while application sessions are in progress via the serving network. The goal of pre-authentication is to avoid AAA signaling when or soon after the device moves. The AAA server needs to distinguish pre-authentication from normal authentication. This is needed if users may only be allowed to have a single authorization session at the same time. While there is a proprietary solution to distinguish pre-authentication and normal authentication for a particular link-layer technology (e.g., use of a null BSSID in Called-Station-Id RADIUS attribute for indicating IEEE 802.11i pre-authentication), a standard solution needs to be developed to solve this problem as pre-authentication may be performed across multiple link-layer technologies. The AAA server also needs to know how long to hold the session before timing out. Session timeout for pre-authentication may be different for a normal session. If a pre-authentication session lifetime expires before the mobile node moves to the target network, the state for the session should be deleted even if normal session lifetime remains. Although implementations can implement its own pre- authentication session lifetime [I-D.ietf-eap-keying], defining a pre-authentication session lifetime in AAA protocols is more useful to allow more explicit control on pre-authorized resources. Ohba, et al. Expires October 3, 2006 [Page 11] Internet-Draft Pre-authentication Problem Statement Apr 2006 5. Out-of-scope Issues The following issues are not discussed in the scope of pre- authentication problem. o Pre-reserving resources using AAA protocols. Although resource pre-reservation is an important aspect that is closely related to pre-authentication and needed for optimizing handover performance, it is recognized that the topic of resource reservation is best left to the policies of the AAA entities. That is, regular AAA transactions will carry all the usual attributes about the requested session. If the AAA entities wish to, say, decline a pre-authentication request due to resource depletion or cost, or charge extra, they can do so. o Accounting, especially accounting for pre-reserved resources. There might be nothing special to do with accounting to support pre-authentication if time-limited resource holding is used. o Context transfer. From pre-authentication perspective, context transfer is not useful. This is because pre-authentication needs AAA signaling with EAP authentication. Resources can be assigned via this pre-authentication AAA exchange instead of using context transfer. Ohba, et al. Expires October 3, 2006 [Page 12] Internet-Draft Pre-authentication Problem Statement Apr 2006 6. Security Considerations Since pre-authentication described in this document needs to work across multiple authenticators, any solution for this problem needs considerations on the following security threats. First, a possible resource consumption denial of service attack where an attacker that is not on the same IP link as the mobile node or the target authenticator may send unprotected pre-authentication messages to the mobile node or the target authenticator to let the legitimate mobile node and target authenticator spend their computational and bandwidth resources. Second, consideration for the Channel Binding problem described in [I-D.ietf-eap-keying] is needed as lack of Channel Binding may cause a man-in-the-middle attack on payload routing [I-D.ietf-eap-netsel- problem]. It should be noted that it would be easier to launch a Channel Binding attack for pre-authentication than normal authentication because an attacker does not need to be physically on the same link as the legitimate mobile node in the case of pre- authentication. Ohba, et al. Expires October 3, 2006 [Page 13] Internet-Draft Pre-authentication Problem Statement Apr 2006 7. IANA Considerations This document has no actions for IANA. Ohba, et al. Expires October 3, 2006 [Page 14] Internet-Draft Pre-authentication Problem Statement Apr 2006 8. Acknowledgments The authors would like to thank Jari Arkko and Madjid Nakhjiri for their valuable input. Ohba, et al. Expires October 3, 2006 [Page 15] Internet-Draft Pre-authentication Problem Statement Apr 2006 9. References 9.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H. Levkowetz, "Extensible Authentication Protocol (EAP)", RFC 3748, June 2004. [I-D.ietf-eap-keying] Aboba, B., "Extensible Authentication Protocol (EAP) Key Management Framework", draft-ietf-eap-keying-12 (work in progress), April 2006. [I-D.ietf-pana-preauth] Ohba, Y., "Pre-authentication Support for PANA", draft-ietf-pana-preauth-01 (work in progress), March 2006. [I-D.ietf-eap-netsel-problem] Arkko, J. and B. Aboba, "Network Discovery and Selection Problem", draft-ietf-eap-netsel-problem-03 (work in progress), October 2005. 9.2. Informative References [I-D.ohba-mobopts-heterogeneous-requirement] Dutta, A., "Problem Statement for Heterogeneous Handover", draft-ohba-mobopts-heterogeneous-requirement-01 (work in progress), March 2006. [I-D.nakhjiri-aaa-hokey-ps] Nakhjiri, M., "AAA based Keying for Wireless Handovers: Problem Statement", draft-nakhjiri-aaa-hokey-ps-01 (work in progress), January 2006. [I-D.housley-aaa-key-mgmt] Housley, R. and B. Aboba, "Guidance for AAA Key Management", draft-housley-aaa-key-mgmt-02 (work in progress), March 2006. [ITU] ITU-T, "General Characteristics of International Telephone Connections and International Telephone Circuits: One-Way Transmission Time", ITU-T Recommendation G.114 1998. [ETSI] ETSI, "Telecommunications and Internet Protocol Harmonization Over Networks (TIPHON) Release 3: End-to-end Ohba, et al. Expires October 3, 2006 [Page 16] Internet-Draft Pre-authentication Problem Statement Apr 2006 Quality of Service in TIPHON systems; Part 1: General aspects of Quality of Service.", ETSI TR 101 329-6 V2.1.1. [georgiades] Georgiades, M., "Context transfer support for IP-based mobility management", CCSR Awards for Research Excellence 2004. Ohba, et al. Expires October 3, 2006 [Page 17] Internet-Draft Pre-authentication Problem Statement Apr 2006 Appendix A. Performance Requirements In order to provide the desirable quality of service for interactive VoIP and streaming traffic during handoff, one needs to limit the value of end-to-end delay, jitter and packet loss to a certain threshold level. ITU-T and ITU-R standards define the acceptable values for these parameters. For example for one-way delay, ITU-T G.114 [ITU] recommends 150 ms as the upper limit for most of the applications, and 400 ms as generally unacceptable delay. One way delay tolerance for video conferencing is in the range of 200 to 300 ms. Also if an out-of-order packet is received after a certain threshold, it is considered lost. The performance requirement will vary based on the type of application and its characteristics such as delay tolerance and loss tolerance limit. Interactive traffic such as VoIP and streaming traffic will have different tolerance for delay and packet loss. For example, according to ETSI TR 101 [ETSI] a normal voice conversation can tolerate up to 2% packet loss. Similarly there are other factors such as Transmission Rating Factor (R) standardized within ITU-T G.107, End to End delay (one way mouth- to-ear) and call blocking ratio that determine the QoS metrics. An R value of 50 is considered to be poor and a value of 90 can be considered as the best that provides most user satisfaction. As an example, a class B QoS which is equivalent to cellular telephony has a R factor that is greater than 70, E2E delay of less than 150 ms and call blocking ratio which is less than or equal to 0.15. Class A QoS that is the highest and is equivalent to fixed phone quality has an R value that is more than 80 and an end-to-end delay that is less than 100 ms. Similarly, 3GPP TS23.107 defines 4 application classes: conversational, streaming, interactive and background each with different set of end-to-end delay and QoS requirement. The streaming class has the tolerable packet (SDU) error rates ranging from 0.1 to 0.00001 and the transfer delay of less than 300ms. In short, the delay and packet loss tolerance value will depend upon the type of application and different standard bodies and vendors provide different specification for each type of application and thus any optimized handoff mechanism will need to take these values into consideration. It is desirable to support a heterogeneous handover that is agnostic to link-layer technologies in an optimized and secure fashion without incurring unreasonable complexity while providing seamless handover experience to the user. As a mobile goes through a handover process, it is subjected to handover delay because of the rebinding of properties at several layers of the protocol stack, such as layer 2, layer 3 and application layer. There are several common properties that contribute to the re-establishment or modification of these layers during handover. These properties can mostly be attributed to things such as access characteristics (e.g., bandwidth, channel Ohba, et al. Expires October 3, 2006 [Page 18] Internet-Draft Pre-authentication Problem Statement Apr 2006 characteristics, channel scan, access point association), access mechanism (e.g. CDMA, CSMA/CA, TDMA), configuration of layer 3 parameters such as IP address acquisition, re-authentication, re- authorization, rebinding of security association at all layers, binding update etc. Although each of the components during the handover process that contributes to the handover delay needs to be optimized, we focus our discussion on optimizing the delay due to authentication and authorization. Ohba, et al. Expires October 3, 2006 [Page 19] Internet-Draft Pre-authentication Problem Statement Apr 2006 Authors' Addresses Yoshihiro Ohba Toshiba America Research, Inc. 1 Telcordia Drive Piscataway, NJ 08854 USA Phone: +1 732 699 5365 Email: yohba@tari.toshiba.com Ashutosh Dutta Telcordia 1 Telcordia Drive Piscataway, NJ 08854 USA Phone: +1 732 699 3130 Email: adutta@research.telcordia.com Srivinas Sreemanthula Nokia Research Center 6000 Connection Dr. Irving, TX 75028 USA Email: srinivas.sreemanthula@nokia.com Ohba, et al. Expires October 3, 2006 [Page 20] Internet-Draft Pre-authentication Problem Statement Apr 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. Ohba, et al. Expires October 3, 2006 [Page 21]