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335 lines
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NETWORK WORKING GROUP N. Williams
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Internet-Draft Sun
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Expires: November 13, 2005 May 12, 2005
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A PRF for the Kerberos V GSS-API Mechanism
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draft-ietf-kitten-krb5-gssapi-prf-03.txt
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Status of this Memo
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By submitting this Internet-Draft, each author represents that any
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applicable patent or other IPR claims of which he or she is aware
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have been or will be disclosed, and any of which he or she becomes
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aware will be disclosed, in accordance with Section 6 of BCP 79.
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Internet-Drafts are working documents of the Internet Engineering
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Task Force (IETF), its areas, and its working groups. Note that
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other groups may also distribute working documents as Internet-
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Drafts.
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Internet-Drafts are draft documents valid for a maximum of six months
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and may be updated, replaced, or obsoleted by other documents at any
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time. It is inappropriate to use Internet-Drafts as reference
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material or to cite them other than as "work in progress."
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The list of current Internet-Drafts can be accessed at
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http://www.ietf.org/ietf/1id-abstracts.txt.
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The list of Internet-Draft Shadow Directories can be accessed at
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http://www.ietf.org/shadow.html.
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This Internet-Draft will expire on November 13, 2005.
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Copyright Notice
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Copyright (C) The Internet Society (2005).
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Abstract
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This document defines the Pseudo-Random Function (PRF) for the
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Kerberos V mechanism for the Generic Security Service Application
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Programming Interface (GSS-API), based on the PRF defined for the
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Kerberos V cryptographic framework, for keying application protocols
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given an established Kerberos V GSS-API security context.
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Williams Expires November 13, 2005 [Page 1]
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Internet-Draft A PRF for the Kerberos V Mech May 2005
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Table of Contents
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1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
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1.1 Conventions used in this document . . . . . . . . . . . . . . 3
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2. Kerberos V GSS Mechanism PRF . . . . . . . . . . . . . . . . . 3
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3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
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4. Security Considerations . . . . . . . . . . . . . . . . . . . 4
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5. Normative References . . . . . . . . . . . . . . . . . . . . . 5
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Author's Address . . . . . . . . . . . . . . . . . . . . . . . 5
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Intellectual Property and Copyright Statements . . . . . . . . 6
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Williams Expires November 13, 2005 [Page 2]
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Internet-Draft A PRF for the Kerberos V Mech May 2005
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1. Introduction
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This document specifies the Kerberos V GSS-API mechanism's pseudo-
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random function corresponding to [GSS-PRF]. The function is a "PRF+"
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style construction.
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1.1 Conventions used in this document
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The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
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"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
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document are to be interpreted as described in [RFC2119].
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2. Kerberos V GSS Mechanism PRF
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The GSS-API PRF [GSS-PRF] function for the Kerberos V mechanism
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[RFC1964] shall be the output of a PRF+ function based on the
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encryption type's PRF function keyed with the negotiated session key
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of the security context corresponding to the 'prf_key' input
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parameter of GSS_Pseudo_random().
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This PRF+ MUST be keyed with the key indicated by the 'prf_key' input
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parameter as follows:
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o GSS_C_PRF_KEY_FULL -- use the sub-session key asserted by the
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acceptor, if any, or the sub-session asserted by the initiator, if
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any, or the Ticket's session key
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o GSS_C_PRF_KEY_PARTIAL -- use the sub-session key asserted by the
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initiator, if any, or the Ticket's session key
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The PRF+ function is a simple counter-based extension of the Kerberos
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V pseudo-random function [RFC3961] for the encryption type of the
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security context's keys:
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PRF+(K, L, S) = truncate(L, T1 || T2 || .. || Tn)
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Tn = pseudo-random(K, n || S)
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where '||' is the concatenation operator, 'n' is encoded as a network
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byte order 32-bit unsigned binary number, truncate(L, S) truncates
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the input octet string S to length L, and pseudo-random() is the
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Kerberos V pseudo-random function [RFC3961].
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The maximum output size of the Kerberos V mechanism's GSS-API PRF
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then is, necessarily, 2^32 times the output size of the pseudo-
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random() function for the encryption type of the given key.
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When the input size is longer than 2^14 octets as per [GSS-PRF] and
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Williams Expires November 13, 2005 [Page 3]
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exceeds an implementation's resources then the mechanism MUST return
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GSS_S_FAILURE and GSS_KRB5_S_KG_INPUT_TOO_LONG as the minor status
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code.
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3. IANA Considerations
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This document has no IANA considerations currently. If and when a
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relevant IANA registry of GSS-API symbols and constants is created
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then the GSS_KRB5_S_KG_INPUT_TOO_LONG minor status code should be
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added to such a registry.
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4. Security Considerations
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Kerberos V encryption types' PRF functions use a key derived from
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contexts' session keys and should preserve the forward security
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properties of the mechanisms' key exchanges.
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Legacy Kerberos V encryption types may be weak, particularly the
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single-DES encryption types.
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See also [GSS-PRF] for generic security considerations of
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GSS_Pseudo_random().
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See also [RFC3961] for generic security considerations of the
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Kerberos V cryptographic framework.
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Care should be taken not to exceed the useful lifetime of an
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established security context's session key's useful lifetime as
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implementations are not required to prevent overuse of the
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GSS_Pseudo_random() function. This can effectively be achieved by
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limiting the number of GSS_Pseudo_random() calls to, say, a handful
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of calls per-security context.
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Use of Ticket session keys, rather than sub-session keys, when
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initiators and acceptors fail to assert sub-session keys, is
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dangerous as ticket reuse can lead to key reuse, therefore initiators
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should assert sub-session keys always, and acceptors should assert
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sub-session keys at least when initiators fail to do so..
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The computational cost of computing this PRF+ may vary depending on
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the Kerberos V encryption types being used, but generally the
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computation of this PRF+ gets more expensive as the input and output
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octet string lengths grow (note that the use of a counter in the PRF+
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construction allows for parallelization). This means that if an
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application can be tricked into providing very large input octet
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strings and requesting very long output octet strings then that may
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constitute a denial of service attack on the application; therefore
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applications SHOULD place appropriate limits on the size of any input
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Williams Expires November 13, 2005 [Page 4]
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Internet-Draft A PRF for the Kerberos V Mech May 2005
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octet strings received from their peers without integrity protection.
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5. Normative References
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[CFX] Zhu, L., Jaganathan, K., and S. Hartman, "The Kerberos
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Version 5 GSS-API Mechanism: Version 2".
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[GSS-PRF] Williams, N., "A PRF API extension for the GSS-API".
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[RFC1964] Linn, J., "The Kerberos Version 5 GSS-API Mechanism",
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RFC 1964, June 1996.
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
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Requirement Levels", BCP 14, RFC 2119, March 1997.
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[RFC2743] Linn, J., "Generic Security Service Application Program
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Interface Version 2, Update 1", RFC 2743, January 2000.
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[RFC2744] Wray, J., "Generic Security Service API Version 2 :
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C-bindings", RFC 2744, January 2000.
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[RFC3961] Raeburn, K., "Encryption and Checksum Specifications for
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Kerberos 5", RFC 3961, February 2005.
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Author's Address
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Nicolas Williams
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Sun Microsystems
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5300 Riata Trace Ct
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Austin, TX 78727
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US
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Email: Nicolas.Williams@sun.com
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Williams Expires November 13, 2005 [Page 5]
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Internet-Draft A PRF for the Kerberos V Mech May 2005
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Intellectual Property Statement
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The IETF takes no position regarding the validity or scope of any
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Intellectual Property Rights or other rights that might be claimed to
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pertain to the implementation or use of the technology described in
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this document or the extent to which any license under such rights
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might or might not be available; nor does it represent that it has
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made any independent effort to identify any such rights. Information
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on the procedures with respect to rights in RFC documents can be
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found in BCP 78 and BCP 79.
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Copies of IPR disclosures made to the IETF Secretariat and any
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assurances of licenses to be made available, or the result of an
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attempt made to obtain a general license or permission for the use of
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such proprietary rights by implementers or users of this
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specification can be obtained from the IETF on-line IPR repository at
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http://www.ietf.org/ipr.
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The IETF invites any interested party to bring to its attention any
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copyrights, patents or patent applications, or other proprietary
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rights that may cover technology that may be required to implement
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this standard. Please address the information to the IETF at
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ietf-ipr@ietf.org.
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Disclaimer of Validity
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This document and the information contained herein are provided on an
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"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
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OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
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ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
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INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
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INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
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WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
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Copyright Statement
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Copyright (C) The Internet Society (2005). This document is subject
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to the rights, licenses and restrictions contained in BCP 78, and
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except as set forth therein, the authors retain all their rights.
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Acknowledgment
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Funding for the RFC Editor function is currently provided by the
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Internet Society.
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Williams Expires November 13, 2005 [Page 6]
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