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INFORMATIONAL
Errata ExistNetwork Working Group T. ClancyRequest for Comments: 5169 LTSCategory: Informational M. Nakhjiri Motorola V. Narayanan L. Dondeti Qualcomm March 2008Handover Key Management and Re-Authentication Problem StatementStatus of This Memo This memo provides information for the Internet community. It does not specify an Internet standard of any kind. Distribution of this memo is unlimited.Abstract This document describes the Handover Keying (HOKEY) re-authentication problem statement. The current Extensible Authentication Protocol (EAP) keying framework is not designed to support re-authentication and handovers without re-executing an EAP method. This often causes unacceptable latency in various mobile wireless environments. This document details the problem and defines design goals for a generic mechanism to reuse derived EAP keying material for handover.Clancy, et al. Informational [Page 1]
RFC 5169 HOKEY Re-Auth PS March 2008Table of Contents1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . .32. Terminology . . . . . . . . . . . . . . . . . . . . . . . . .43. Problem Statement . . . . . . . . . . . . . . . . . . . . . .44. Design Goals . . . . . . . . . . . . . . . . . . . . . . . . .55. Security Goals . . . . . . . . . . . . . . . . . . . . . . . .65.1. Key Context and Domino Effect . . . . . . . . . . . . . .75.2. Key Freshness . . . . . . . . . . . . . . . . . . . . . .75.3. Authentication . . . . . . . . . . . . . . . . . . . . . .85.4. Authorization . . . . . . . . . . . . . . . . . . . . . .85.5. Channel Binding . . . . . . . . . . . . . . . . . . . . .85.6. Transport Aspects . . . . . . . . . . . . . . . . . . . .86. Use Cases and Related Work . . . . . . . . . . . . . . . . . .96.1. Method-Specific EAP Re-Authentication . . . . . . . . . .96.2. IEEE 802.11r Applicability . . . . . . . . . . . . . . . .106.3. CAPWAP Applicability . . . . . . . . . . . . . . . . . . .107. Security Considerations . . . . . . . . . . . . . . . . . . .118. Contributors . . . . . . . . . . . . . . . . . . . . . . . . .119. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .1110. References . . . . . . . . . . . . . . . . . . . . . . . . . .1210.1. Normative References . . . . . . . . . . . . . . . . . . .1210.2. Informative References . . . . . . . . . . . . . . . . . .12Clancy, et al. Informational [Page 2]
RFC 5169 HOKEY Re-Auth PS March 20081. Introduction The Extensible Authentication Protocol (EAP), specified inRFC 3748 [RFC3748] is a generic framework supporting multiple authentication methods. The primary purpose of EAP is network access control. It also supports exporting session keys derived during the authentication. The EAP keying hierarchy defines two keys that are derived at the top level, the Master Session Key (MSK) and the Extended Master Session Key (EMSK). In many common deployment scenarios, an EAP peer and EAP server authenticate each other through a third party known as the pass- through authenticator (hereafter referred to as simply "authenticator"). The authenticator is responsible for encapsulating EAP packets from a network-access technology lower layer within the Authentication, Authorization, and Accounting (AAA) protocol. The authenticator does not directly participate in the EAP exchange, and simply acts as a gateway during the EAP method execution. After successful authentication, the EAP server transports the MSK to the authenticator. Note that this is performed using AAA protocols, not EAP itself. The underlying L2 or L3 protocol uses the MSK to derive additional keys, including the transient session keys (TSKs) used for per-packet encryption and authentication. Note that while the authenticator is one logical device, there can be multiple physical devices involved. For example, the CAPWAP model [RFC3990] splits authenticators into two logical devices: Wireless Termination Points (WTPs) and Access Controllers (ACs). Depending on the configuration, authenticator features can be split in a variety of ways between physical devices; however, from the EAP perspective, there is only one logical authenticator. Wireless handover between access points or base stations is typically a complex process that involves several layers of protocol execution. Often times executing these protocols results in unacceptable delays for many real-time applications such as voice [MSA03]. One part of the handover process is EAP re-authentication, which can contribute significantly to the overall handover time [MSPCA04]. Thus, in many environments we can lower overall handover time by lowering EAP re- authentication time. In EAP existing implementations, when a peer arrives at the new authenticator, it runs an EAP method irrespective of whether it has been authenticated to the network recently and has unexpired keying material. This typically involves an EAP-Response/Identity message from the peer to the server, followed by one or more round trips between the EAP server and peer to perform the authentication,Clancy, et al. Informational [Page 3]
RFC 5169 HOKEY Re-Auth PS March 2008 followed by the EAP-Success or EAP-Failure message from the EAP server to the peer. At a minimum, the EAP exchange consists of 1.5 round trips. However, given the way EAP interacts with AAA, and given that an EAP identity exchange is typically employed, at least 2 round trips are required to the EAP server. An even higher number of round trips is required by the most commonly used EAP methods. For instance, EAP-TLS (Extensible Authentication Protocol - Transport Layer Security) requires at least 3, but typically 4 or more, round trips. There have been attempts to solve the problem of efficient re- authentication in various ways. However, those solutions are either EAP-method specific or EAP lower-layer specific. Furthermore, these solutions do not deal with scenarios involving handovers to new authenticators, or they do not conform to the AAA keying requirements specified in [RFC4962]. This document provides a detailed description of efficient EAP-based re-authentication protocol design goals. The scope of this protocol is specifically re-authentication and handover between authenticators within a single administrative domain. While the design goals presented in this document may facilitate inter-technology handover and inter-administrative-domain handover, they are outside the scope of this protocol.2. Terminology In this document, several words are used to signify the requirements 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], with the qualification that, unless otherwise stated, they apply to the design of the re-authentication protocol, not its implementation or application. With respect to EAP, this document follows the terminology that has been defined in [RFC3748] and [EAP-KEYING].3. Problem Statement Under the existing model, any re-authentication requires a full EAP exchange with the EAP server to which the peer initially authenticated [RFC3748]. This introduces handover latency from both network transit time and processing delay. In service provider networks, the home EAP server for a peer could be on the other side of the world, and typical intercontinental latencies across the Internet are 100 to 300 milliseconds per round trip [LGS07].Clancy, et al. Informational [Page 4]
RFC 5169 HOKEY Re-Auth PS March 2008 Processing delays average a couple of milliseconds for symmetric-key operations and hundreds of milliseconds for public-key operations. An EAP conversation with a full EAP method run can take two or more round trips to complete, causing delays in re-authentication and handover times. Some methods specify the use of keys and state from the initial authentication to finish subsequent authentications in fewer round trips and without using public-key operations (detailed inSection 6.1). However, even in those cases, multiple round trips to the EAP server are required, resulting in unacceptable handover times. In summary, it is undesirable to run an EAP Identity and complete EAP method exchange each time a peer authenticates to a new authenticator or needs to extend its current authentication with the same authenticator. Furthermore, it is desirable to specify a method- independent, efficient, re-authentication protocol. Keying material from the initial authentication can be used to enable efficient re- authentication. It is also desirable to have a local server with low-latency connectivity to the peer that can facilitate re- authentication. Lastly, a re-authentication protocol should also be capable of supporting scenarios where an EAP server passes authentication information to a remote re-authentication server, allowing a peer to re-authenticate locally, without having to communicate with its home re-authentication server. These problems are the primary issues to be resolved. In solving them, there are a number of constraints to conform to, and those result in some additional work to be done in the area of EAP keying.4. Design Goals The following are the goals and constraints in designing the EAP re- authentication and key management protocol: Lower-latency operation: The protocol MUST be responsive to handover and re-authentication latency performance objectives within a mobile access network. A solution that reduces latency as compared to a full EAP authentication will be most favorable, since in networks that rely on reactive re-authentication this will directly impact handover times. EAP lower-layer independence: Any keying hierarchy and protocol defined MUST be lower-layer independent in order to provide capabilities over heterogeneous technologies. The defined protocols MAY require some additional support from the lower layers that use it, but should not require any particular lower layer.Clancy, et al. Informational [Page 5]
RFC 5169 HOKEY Re-Auth PS March 2008 EAP method independence: Changes to existing EAP methods MUST NOT be required as a result of the re-authentication protocol. There MUST be no requirements imposed on future EAP methods, provided they satisfy [EAP-KEYING] and [RFC4017]. Note that the only EAP methods for which independence is required are those that currently conform to the specifications of [EAP-KEYING] and [RFC4017]. In particular, methods that do not generate the keys required by [EAP-KEYING] need not be supported by the re- authentication protocol. AAA protocol compatibility and keying: Any modifications to EAP and EAP keying MUST be compatible with RADIUS [RADEXT-DESIGN] and Diameter [DIME-APP-DESIGN]. Extensions to both RADIUS and Diameter to support these EAP modifications are acceptable. The designs and protocols must be configurable to satisfy the AAA key management requirements specified inRFC 4962 [RFC4962]. Compatibility: Compatibility and coexistence with compliant ([RFC3748] [EAP-KEYING]) EAP deployments MUST be provided. Specifically, the protocol should be designed such that a peer not supporting fast re-reauthentication should still function in a network supporting fast re-authentication, and also a peer supporting fast re-authentication should still function in a network not supporting fast re-authentication. Cryptographic Agility: Any re-authentication protocol MUST support cryptographic algorithm agility, to avoid hard-coded primitives whose security may eventually prove to be compromised. The protocol MAY support cryptographic algorithm negotiation, provided it does not adversely affect overall performance (i.e., by requiring additional round trips). Impact to Existing Deployments: Any re-authentication protocol MAY make changes to the peer, authenticator, and EAP server, as necessary to meet the aforementioned design goals. In order to facilitate protocol deployment, protocols should seek to minimize the necessary changes, without sacrificing performance.5. Security Goals This section draws from the guidance provided in [RFC4962] to further define the security goals to be achieved by a complete re- authentication keying solution.Clancy, et al. Informational [Page 6]
RFC 5169 HOKEY Re-Auth PS March 20085.1. Key Context and Domino Effect Any key must have a well-defined scope and must be used in a specific context and for the intended use. This specifically means the lifetime and scope of each key must be defined clearly so that all entities that are authorized to have access to the key have the same context during the validity period. In a hierarchical key structure, the lifetime of lower-level keys must not exceed the lifetime of higher-level keys. This requirement may imply that the context and the scope parameters have to be exchanged. Furthermore, the semantics of these parameters must be defined to provide proper channel binding specifications. The definition of exact parameter syntax definition is part of the design of the transport protocol used for the parameter exchange, and that may be outside scope of this protocol. If a key hierarchy is deployed, compromising lower-level keys must not result in a compromise of higher-level keys that were used to derive the lower-level keys. The compromise of keys at each level must not result in compromise of other keys at the same level. The same principle applies to entities that hold and manage a particular key defined in the key hierarchy. Compromising keys on one authenticator must not reveal the keys of another authenticator. Note that the compromise of higher-level keys has security implications on lower levels. Guidance on parameters required, caching, and storage and deletion procedures to ensure adequate security and authorization provisioning for keying procedures must be defined in a solution document. All the keying material must be uniquely named so that it can be managed effectively.5.2. Key Freshness As [RFC4962] defines, a fresh key is one that is generated for the intended use. This would mean the key hierarchy must provide for creation of multiple cryptographically separate child keys from a root key at higher level. Furthermore, the keying solution needs to provide mechanisms for refreshing each of the keys within the key hierarchy.Clancy, et al. Informational [Page 7]
RFC 5169 HOKEY Re-Auth PS March 20085.3. Authentication Each handover keying participant must be authenticated to any other party with whom it communicates to the extent it is necessary to ensure proper key scoping, and securely provide its identity to any other entity that may require the identity for defining the key scope.5.4. Authorization The EAP Key management document [EAP-KEYING] discusses several vulnerabilities that are common to handover mechanisms. One important issue arises from the way the authorization decisions might be handled at the AAA server during network access authentication. Furthermore, the reasons for making a particular authorization decision are not communicated to the authenticator. In fact, the authenticator only knows the final authorization result. The proposed solution must make efforts to document and mitigate authorization attacks.5.5. Channel Binding Channel Binding procedures are needed to avoid a compromised intermediate authenticator providing unverified and conflicting service information to each of the peer and the EAP server. To support fast re-authentication, there will be intermediate entities between the peer and the back-end EAP server. Various keys need to be established and scoped between these parties and some of these keys may be parents to other keys. Hence, the channel binding for this architecture will need to consider layering intermediate entities at each level to make sure that an entity with a higher level of trust can examine the truthfulness of the claims made by intermediate parties.5.6. Transport Aspects Depending on the physical architecture and the functionality of the elements involved, there may be a need for multiple protocols to perform the key transport between entities involved in the handover keying architecture. Thus, a set of requirements for each of these protocols, and the parameters they will carry, must be developed. The use of existing AAA protocols for carrying EAP messages and keying material between the AAA server and AAA clients that have a role within the architecture considered for the keying problem will be carefully examined. Definition of specific parameters, required for keying procedures and for being transferred over any of the linksClancy, et al. Informational [Page 8]
RFC 5169 HOKEY Re-Auth PS March 2008 in the architecture, are part of the scope. The relation between the identities used by the transport protocol and the identities used for keying also needs to be explored.6. Use Cases and Related Work In order to further clarify the items listed in scope of the proposed work, this section provides some background on related work and the use cases envisioned for the proposed work.6.1. Method-Specific EAP Re-Authentication A number of EAP methods support fast re-authentication. In this section, we examine their properties in further detail. EAP-SIM [RFC4186] and EAP-AKA [RFC4187] support fast re- authentication, bootstrapped by the keys generated during an initial full authentication. In response to the typical EAP-Request/ Identity, the peer sends a specially formatted identity indicating a desire to perform a fast re-authentication. A single round-trip occurs to verify knowledge of the existing keys and provide fresh nonces for generating new keys. This is followed by an EAP success. In the end, it requires a single local round trip between the peer and authenticator, followed by another round trip between the peer and EAP server. AKA is based on symmetric-key cryptography, so processing latency is minimal. EAP-TTLS [EAP-TTLS] and PEAP (Protected EAP Protocol) [JOSEFSSON-PPPEXT] support using TLS session resumption for fast re- authentication. During the TLS handshake, the client includes the message ID of the previous session he wishes to resume, and the server can echo that ID back if it agrees to resume the session. EAP-FAST [RFC4851] also supports TLS session resumption, but additionally allows stateless session resumption as defined in [RFC5077]. Overall, for all three protocols, there are still two round trips between the peer and EAP server, in addition to the local round trip for the Identity request and response. To improve performance, fast re-authentication needs to reduce the number of overall round trips. Optimal performance could result from eliminating the EAP-Request/Identity and EAP-Response/Identity messages observed in typical EAP method execution, and allowing a single round trip between the peer and a local re-authentication server.Clancy, et al. Informational [Page 9]
RFC 5169 HOKEY Re-Auth PS March 20086.2. IEEE 802.11r Applicability One of the EAP lower layers, IEEE 802.11 [IEEE.802-11R-D9.0], is in the process of specifying a fast handover mechanism. Access Points (APs) are grouped into mobility domains. Initial authentication to any AP in a mobility domain requires execution of EAP, but handover between APs within the mobility domain does not require the use of EAP. Internal to the mobility domain are sets of security associations to support key transfers between APs. In one model, relatively few devices, called R0-KHs, act as authenticators. All EAP traffic traverses an R0-KH, and it derives the initial IEEE 802.11 keys. It then distributes cryptographically separate keys to APs in the mobility domain, as necessary, to support the client mobility. For a deployment with M designated R0-KHs and N APs, this requires M*N security associations. For small M, this approach scales reasonably. Another approach allows any AP to act as an R0-KH, necessitating a full mesh of N2 security associations, which scales poorly. The model that utilizes designated R0-KHs is architecturally similar to the fast re-authentication model proposed by HOKEY. HOKEY, however, allows for handover between authenticators. This would allow an IEEE 802.11r-enabled peer to handover from one mobility domain to another without performing an EAP authentication.6.3. CAPWAP Applicability The CAPWAP (Control and Provisioning of Wireless Access Points) protocol [CAPWAP-PROTOCOL-SPEC] allows the functionality of an IEEE 802.11 access point to be split into two physical devices in enterprise deployments. Wireless Termination Points (WTPs) implement the physical and low-level Media Access Control (MAC) layers, while a centralized Access Controller (AC) provides higher-level management and protocol execution. Client authentication is handled by the AC, which acts as the AAA authenticator. One of the many features provided by CAPWAP is the ability to roam between WTPs without executing an EAP authentication. To accomplish this, the AC caches the MSK from an initial EAP authentication, and uses it to execute a separate four-way handshake with the station as it moves between WTPs. The keys resulting from the four-way handshake are then distributed to the WTP to which the station is associated. CAPWAP is transparent to the station. CAPWAP currently has no means to support roaming between ACs in an enterprise network. The proposed work on EAP efficient re- authentication addresses is an inter-authenticator handover problemClancy, et al. Informational [Page 10]
RFC 5169 HOKEY Re-Auth PS March 2008 from an EAP perspective, which applies during handover between ACs. Inter-AC handover is a topic yet to be addressed in great detail and the re-authentication work can potentially address it in an effective manner.7. Security Considerations This document details the HOKEY problem statement. Since HOKEY is an authentication protocol, there is a myriad of security-related issues surrounding its development and deployment. In this document, we have detailed a variety of security properties inferred from [RFC4962] to which HOKEY must conform, including the management of key context, scope, freshness, and transport; resistance to attacks based on the domino effect; and authentication and authorization. SeeSection 5 for further details.8. Contributors This document represents the synthesis of two problem statement documents. In this section, we acknowledge their contributions, and involvement in the early documents. Mohan Parthasarathy Nokia EMail: mohan.parthasarathy@nokia.com Julien Bournelle France Telecom R&D EMail: julien.bournelle@orange-ftgroup.com Hannes Tschofenig Siemens EMail: Hannes.Tschofenig@siemens.com Rafael Marin Lopez Universidad de Murcia EMail: rafa@dif.um.es9. Acknowledgements The authors would like to thank the participants of the HOKEY working group for their review and comments including: Glen Zorn, Dan Harkins, Joe Salowey, and Yoshi Ohba. The authors would also like to thank those that provided last-call reviews including: Bernard Aboba, Alan DeKok, Jari Arkko, and Hannes Tschofenig.Clancy, et al. Informational [Page 11]
RFC 5169 HOKEY Re-Auth PS March 200810. References10.1. 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. [RFC4017] Stanley, D., Walker, J., and B. Aboba, "Extensible Authentication Protocol (EAP) Method Requirements for Wireless LANs",RFC 4017, March 2005. [RFC4962] Housley, R. and B. Aboba, "Guidance for Authentication, Authorization, and Accounting (AAA) Key Management",BCP 132,RFC 4962, July 2007.10.2. Informative References [CAPWAP-PROTOCOL-SPEC] Calhoun, P., Montemurro, M., and D. Stanely, "CAPWAP Protocol Specification", Work in Progress, March 2008. [DIME-APP-DESIGN] Fajardo, V., Asveren, T., Tschofenig, H., McGregor, G., and J. Loughney, "Diameter Applications Design Guidelines", Work in Progress, January 2008. [EAP-KEYING] Aboba, B., Simon, D., and P. Eronen, "Extensible Authentication Protocol (EAP) Key Management Framework", Work in Progress, November 2007. [EAP-TTLS] Funk, P. and S. Blake-Wilson, "EAP Tunneled TLS Authentication Protocol Version 0 (EAP- TTLSv0)", Work in Progress, March 2008.Clancy, et al. Informational [Page 12]
RFC 5169 HOKEY Re-Auth PS March 2008 [IEEE.802-11R-D9.0] "Information technology - Telecommunications and information exchange between systems - Local and metropolitan area networks - Specific requirements - Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications - Amendment 2: Fast BSS Transition", IEEE Standard 802.11r, January 2008. [JOSEFSSON-PPPEXT] Josefsson, S., Palekar, A., Simon, D., and G. Zorn, "Protected EAP Protocol (PEAP) Version 2", Work in Progress, October 2004. [LGS07] Ledlie, J., Gardner, P., and M. Selter, "Network Coordinates in the Wild", USENIX Symposium on Networked System Design and Implementation, April 2007. [MSA03] Mishra, A., Shin, M., and W. Arbaugh, "An Empirical Analysis of the IEEE 802.11 MAC- Layer Handoff Process", ACM SIGCOMM Computer and Communications Review, April 2003. [MSPCA04] Mishra, A., Shin, M., Petroni, N., Clancy, T., and W. Arbaugh, "Proactive Key Distribution using Neighbor Graphs", IEEE Wireless Communications, February 2004. [RADEXT-DESIGN] Weber, G. and A. DeKok, "RADIUS Design Guidelines", Work in Progress, December 2007. [RFC3990] O'Hara, B., Calhoun, P., and J. Kempf, "Configuration and Provisioning for Wireless Access Points (CAPWAP) Problem Statement",RFC 3990, February 2005. [RFC4186] Haverinen, H. and J. Salowey, "Extensible Authentication Protocol Method for Global System for Mobile Communications (GSM) Subscriber Identity Modules (EAP-SIM)",RFC 4186, January 2006. [RFC4187] Arkko, J. and H. Haverinen, "Extensible Authentication Protocol Method for 3rd Generation Authentication and Key Agreement (EAP-AKA)",RFC 4187, January 2006.Clancy, et al. Informational [Page 13]
RFC 5169 HOKEY Re-Auth PS March 2008 [RFC4851] Cam-Winget, N., McGrew, D., Salowey, J., and H. Zhou, "The Flexible Authentication via Secure Tunneling Extensible Authentication Protocol Method (EAP-FAST)",RFC 4851, May 2007. [RFC5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig, "Transport Layer Security (TLS) Session Resumption without Server-Side State",RFC 5077, January 2008.Authors' Addresses T. Charles Clancy, Editor Laboratory for Telecommunications Sciences US Department of Defense College Park, MD USA EMail: clancy@LTSnet.net Madjid Nakhjiri Motorola EMail: madjid.nakhjiri@motorola.com Vidya Narayanan Qualcomm, Inc. San Diego, CA USA EMail: vidyan@qualcomm.com Lakshminath Dondeti Qualcomm, Inc. San Diego, CA USA EMail: ldondeti@qualcomm.comClancy, et al. Informational [Page 14]
RFC 5169 HOKEY Re-Auth PS March 2008Full Copyright Statement Copyright (C) The IETF Trust (2008). This document is subject to the rights, licenses and restrictions contained inBCP 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 inBCP 78 andBCP 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 athttp://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.Clancy, et al. Informational [Page 15]
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