Network Working Group S. Kelly Request for Comments: 4868 Aruba Networks Category: Standards Track S. Frankel NIST May 2007 Using HMAC-SHA-256, HMAC-SHA-384, and HMAC-SHA-512 with IPsec Status of This Memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The IETF Trust (2007).
AbstractThis specification describes the use of Hashed Message Authentication Mode (HMAC) in conjunction with the SHA-256, SHA-384, and SHA-512 algorithms in IPsec. These algorithms may be used as the basis for data origin authentication and integrity verification mechanisms for the Authentication Header (AH), Encapsulating Security Payload (ESP), Internet Key Exchange Protocol (IKE), and IKEv2 protocols, and also as Pseudo-Random Functions (PRFs) for IKE and IKEv2. Truncated output lengths are specified for the authentication-related variants, with the corresponding algorithms designated as HMAC-SHA-256-128, HMAC-SHA-384-192, and HMAC-SHA-512-256. The PRF variants are not truncated, and are called PRF-HMAC-SHA-256, PRF-HMAC-SHA-384, and PRF-HMAC-SHA-512.
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. The HMAC-SHA-256+ Algorithms . . . . . . . . . . . . . . . . . 3 2.1. Keying Material . . . . . . . . . . . . . . . . . . . . . 3 2.1.1. Data Origin Authentication and Integrity Verification Usage . . . . . . . . . . . . . . . . . . 4 2.1.2. Pseudo-Random Function (PRF) Usage . . . . . . . . . . 4 2.1.3. Randomness and Key Strength . . . . . . . . . . . . . 5 2.1.4. Key Distribution . . . . . . . . . . . . . . . . . . . 5 2.1.5. Refreshing Keys . . . . . . . . . . . . . . . . . . . 5 2.2. Padding . . . . . . . . . . . . . . . . . . . . . . . . . 6 2.3. Truncation . . . . . . . . . . . . . . . . . . . . . . . . 6 2.4. Using HMAC-SHA-256+ as PRFs in IKE and IKEv2 . . . . . . . 7 2.5. Interactions with the ESP, IKE, or IKEv2 Cipher Mechanisms . . . . . . . . . . . . . . . . . . . . . . . . 7 2.6. HMAC-SHA-256+ Parameter Summary . . . . . . . . . . . . . 7 2.7. Test Vectors . . . . . . . . . . . . . . . . . . . . . . . 7 2.7.1. PRF Test Vectors . . . . . . . . . . . . . . . . . . . 8 2.7.2. Authenticator Test Vectors . . . . . . . . . . . . . . 11 3. Security Considerations . . . . . . . . . . . . . . . . . . . 17 3.1. HMAC Key Length vs Truncation Length . . . . . . . . . . . 17 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18 5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 19 6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19 6.1. Normative References . . . . . . . . . . . . . . . . . . . 19 6.2. Informative References . . . . . . . . . . . . . . . . . . 20
SHA2-1] combined with HMAC [HMAC] as data origin authentication and integrity verification mechanisms for the IPsec AH [AH], ESP [ESP], IKE [IKE], and IKEv2 [IKEv2] protocol. Output truncation is specified for these variants, with the corresponding algorithms designated as HMAC-SHA-256-128, HMAC-SHA-384-192, and HMAC-SHA-512- 256. These truncation lengths are chosen in accordance with the birthday bound for each algorithm. This specification also describes untruncated variants of these algorithms as Pseudo-Random Functions (PRFs) for use with IKE and IKEv2, and those algorithms are called PRF-HMAC-SHA-256, PRF-HMAC- SHA-384, and PRF-HMAC-SHA-512. For ease of reference, these PRF algorithms and the authentication variants described above are collectively referred to below as "the HMAC-SHA-256+ algorithms". The goal of the PRF variants are to provide secure pseudo-random functions suitable for generation of keying material and other protocol-specific numeric quantities, while the goal of the authentication variants is to ensure that packets are authentic and cannot be modified in transit. The relative security of HMAC-SHA- 256+ when used in either case is dependent on the distribution scope and unpredictability of the associated secret key. If the key is unpredictable and known only by the sender and recipient, these algorithms ensure that only parties holding an identical key can derive the associated values. SHA2-1] and [SHA2-2] describe the underlying SHA-256, SHA-384, and SHA-512 algorithms, while [HMAC] describes the HMAC algorithm. The HMAC algorithm provides a framework for inserting various hashing algorithms such as SHA-256, and [SHA256+] describes combined usage of these algorithms. The following sections describe the various characteristics and requirements of the HMAC-SHA-256+ algorithms when used with IPsec.
case of PRFs, key lengths are variable, but guidance is given to ensure interoperability. These distinctions are described further below. Before describing key requirements for each usage, it is important to clarify some terms we use below: Block size: the size of the data block the underlying hash algorithm operates upon. For SHA-256, this is 512 bits, for SHA-384 and SHA-512, this is 1024 bits. Output length: the size of the hash value produced by the underlying hash algorithm. For SHA-256, this is 256 bits, for SHA-384 this is 384 bits, and for SHA-512, this is 512 bits. Authenticator length: the size of the "authenticator" in bits. This only applies to authentication/integrity related algorithms, and refers to the bit length remaining after truncation. In this specification, this is always half the output length of the underlying hash algorithm. HMAC], this specification requires that when used as an integrity/authentication algorithm, a fixed key length equal to the output length of the hash functions MUST be supported, and key lengths other than the output length of the associated hash function MUST NOT be supported. These key length restrictions are based in part on the recommendations in [HMAC] (key lengths less than the output length decrease security strength, and keys longer than the output length do not significantly increase security strength), and in part because allowing variable length keys for IPsec authenticator functions would create interoperability issues.
When a PRF described in this document is used with IKE or IKEv2, it is considered to have a variable key length, and keys are derived in the following ways (note that we simply reiterate that which is specified in [HMAC]): o If the length of the key is exactly the algorithm block size, use it as-is. o If the key is shorter than the block size, lengthen it to exactly the block size by padding it on the right with zero bits. However, note that [HMAC] strongly discourages a key length less than the output length. Nonetheless, we describe handling of shorter lengths here in recognition of shorter lengths typically chosen for IKE or IKEv2 pre-shared keys. o If the key is longer than the block size, shorten it by computing the corresponding hash algorithm output over the entire key value, and treat the resulting output value as your HMAC key. Note that this will always result in a key that is less than the block size in length, and this key value will therefore require zero-padding (as described above) prior to use. HMAC] discusses requirements for key material, including a requirement for strong randomness. Therefore, a strong pseudo-random function MUST be used to generate the required key for use with HMAC- SHA-256+. At the time of this writing there are no published weak keys for use with any HMAC-SHA-256+ algorithms. ARCH] describes the general mechanism for obtaining keying material when multiple keys are required for a single SA (e.g., when an ESP SA requires a key for confidentiality and a key for authentication). In order to provide data origin authentication and integrity verification, the key distribution mechanism must ensure that unique keys are allocated and that they are distributed only to the parties participating in the communication. HMAC] "...periodic key refreshment is a fundamental security practice that helps against potential weaknesses of the function and keys, and limits the damage of an exposed key".
Putting this into perspective, this specification requires 256, 384, or 512-bit keys produced by a strong PRF for use as a MAC. A brute force attack on such keys would take longer to mount than the universe has been in existence. On the other hand, weak keys (e.g., dictionary words) would be dramatically less resistant to attack. It is important to take these points, along with the specific threat model for your particular application and the current state of the art with respect to attacks on SHA-256, SHA-384, and SHA-512 into account when determining an appropriate upper bound for HMAC key lifetimes. SHA2-1] as part of the underlying SHA-256, SHA-384, and SHA-512 algorithms, so if you implement according to [SHA2-1], you do not need to add any additional padding as far as the HMAC-SHA-256+ algorithms specified here are concerned. With regard to "implicit packet padding" as defined in [AH], no implicit packet padding is required. HMAC]. When used as a data origin authentication and integrity verification algorithm in ESP, AH, IKE, or IKEv2, a truncated value using the first nnn/2 bits -- exactly half the algorithm output size -- MUST be supported. No other authenticator value lengths are supported by this specification. Upon sending, the truncated value is stored within the authenticator field. Upon receipt, the entire nnn-bit value is computed and the first nnn/2 bits are compared to the value stored in the authenticator field, with the value of 'nnn' depending on the negotiated algorithm. [HMAC] discusses potential security benefits resulting from truncation of the output MAC value, and in general, encourages HMAC users to perform MAC truncation. In the context of IPsec, a truncation length of nnn/2 bits is selected because it corresponds to the birthday attack bound for each of the HMAC-SHA-256+ algorithms, and it simultaneously serves to minimize the additional bits on the wire resulting from use of this facility.
RFC 4231 [HMAC-TEST]. For reference implementations of the underlying hash algorithms, see [SHA256+]. Note that for testing purposes, PRF output is considered to be simply the untruncated algorithm output. Test Case PRF-1: Key = 0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b 0b0b0b0b (20 bytes) Data = 4869205468657265 ("Hi There") PRF-HMAC-SHA-256 = b0344c61d8db38535ca8afceaf0bf12b 881dc200c9833da726e9376c2e32cff7 PRF-HMAC-SHA-384 = afd03944d84895626b0825f4ab46907f 15f9dadbe4101ec682aa034c7cebc59c faea9ea9076ede7f4af152e8b2fa9cb6 PRF-HMAC-SHA-512 = 87aa7cdea5ef619d4ff0b4241a1d6cb0 2379f4e2ce4ec2787ad0b30545e17cde daa833b7d6b8a702038b274eaea3f4e4 be9d914eeb61f1702e696c203a126854 Test Case PRF-2: Key = 4a656665 ("Jefe") Data = 7768617420646f2079612077616e7420 ("what do ya want ") 666f72206e6f7468696e673f ("for nothing?") PRF-HMAC-SHA-256 = 5bdcc146bf60754e6a042426089575c7 5a003f089d2739839dec58b964ec3843 PRF-HMAC-SHA-384 = af45d2e376484031617f78d2b58a6b1b 9c7ef464f5a01b47e42ec3736322445e 8e2240ca5e69e2c78b3239ecfab21649 PRF-HMAC-SHA-512 = 164b7a7bfcf819e2e395fbe73b56e0a3 87bd64222e831fd610270cd7ea250554 9758bf75c05a994a6d034f65f8f0e6fd caeab1a34d4a6b4b636e070a38bce737
Test Case PRF-3: Key aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa aaaaaaaa (20 bytes) Data = dddddddddddddddddddddddddddddddd dddddddddddddddddddddddddddddddd dddddddddddddddddddddddddddddddd dddd (50 bytes) PRF-HMAC-SHA-256 = 773ea91e36800e46854db8ebd09181a7 2959098b3ef8c122d9635514ced565fe PRF-HMAC-SHA-384 = 88062608d3e6ad8a0aa2ace014c8a86f 0aa635d947ac9febe83ef4e55966144b 2a5ab39dc13814b94e3ab6e101a34f27 PRF-HMAC-SHA-512 = fa73b0089d56a284efb0f0756c890be9 b1b5dbdd8ee81a3655f83e33b2279d39 bf3e848279a722c806b485a47e67c807 b946a337bee8942674278859e13292fb Test Case PRF-4: Key = 0102030405060708090a0b0c0d0e0f10 111213141516171819 (25 bytes) Data = cdcdcdcdcdcdcdcdcdcdcdcdcdcdcdcd cdcdcdcdcdcdcdcdcdcdcdcdcdcdcdcd cdcdcdcdcdcdcdcdcdcdcdcdcdcdcdcd cdcd (50 bytes) PRF-HMAC-SHA-256 = 82558a389a443c0ea4cc819899f2083a 85f0faa3e578f8077a2e3ff46729665b PRF-HMAC-SHA-384 = 3e8a69b7783c25851933ab6290af6ca7 7a9981480850009cc5577c6e1f573b4e 6801dd23c4a7d679ccf8a386c674cffb PRF-HMAC-SHA-512 = b0ba465637458c6990e5a8c5f61d4af7 e576d97ff94b872de76f8050361ee3db a91ca5c11aa25eb4d679275cc5788063 a5f19741120c4f2de2adebeb10a298dd
Test Case PRF-5: Key = aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa aaaaaa (131 bytes) Data = 54657374205573696e67204c61726765 ("Test Using Large") 72205468616e20426c6f636b2d53697a ("r Than Block-Siz") 65204b6579202d2048617368204b6579 ("e Key - Hash Key") 204669727374 (" First") PRF-HMAC-SHA-256 = 60e431591ee0b67f0d8a26aacbf5b77f 8e0bc6213728c5140546040f0ee37f54 PRF-HMAC-SHA-384 = 4ece084485813e9088d2c63a041bc5b4 4f9ef1012a2b588f3cd11f05033ac4c6 0c2ef6ab4030fe8296248df163f44952 PRF-HMAC-SHA-512 = 80b24263c7c1a3ebb71493c1dd7be8b4 9b46d1f41b4aeec1121b013783f8f352 6b56d037e05f2598bd0fd2215d6a1e52 95e64f73f63f0aec8b915a985d786598
Test Case PRF-6: Key = aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa aaaaaa (131 bytes) Data = 54686973206973206120746573742075 ("This is a test u") 73696e672061206c6172676572207468 ("sing a larger th") 616e20626c6f636b2d73697a65206b65 ("an block-size ke") 7920616e642061206c61726765722074 ("y and a larger t") 68616e20626c6f636b2d73697a652064 ("han block-size d") 6174612e20546865206b6579206e6565 ("ata. The key nee") 647320746f2062652068617368656420 ("ds to be hashed ") 6265666f7265206265696e6720757365 ("before being use") 642062792074686520484d414320616c ("d by the HMAC al") 676f726974686d2e ("gorithm.") PRF-HMAC-SHA-256 = 9b09ffa71b942fcb27635fbcd5b0e944 bfdc63644f0713938a7f51535c3a35e2 PRF-HMAC-SHA-384 = 6617178e941f020d351e2f254e8fd32c 602420feb0b8fb9adccebb82461e99c5 a678cc31e799176d3860e6110c46523e PRF-HMAC-SHA-512 = e37b6a775dc87dbaa4dfa9f96e5e3ffd debd71f8867289865df5a32d20cdc944 b6022cac3c4982b10d5eeb55c3e4de15 134676fb6de0446065c97440fa8c6a58 SHA256+].
Test Case AUTH256-4: Key = 0102030405060708090a0b0c0d0e0f10 1112131415161718191a1b1c1d1e1f20 (32 bytes) Data = cdcdcdcdcdcdcdcdcdcdcdcdcdcdcdcd cdcdcdcdcdcdcdcdcdcdcdcdcdcdcdcd cdcdcdcdcdcdcdcdcdcdcdcdcdcdcdcd cdcd (50 bytes) PRF-HMAC-SHA-256 = 372efcf9b40b35c2115b1346903d2ef4 2fced46f0846e7257bb156d3d7b30d3f HMAC-SHA-256-128 = 372efcf9b40b35c2115b1346903d2ef4
Test Case AUTH384-3: Key = aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa (48 bytes) Data = dddddddddddddddddddddddddddddddd dddddddddddddddddddddddddddddddd dddddddddddddddddddddddddddddddd dddd (50 bytes) PRF-HMAC-SHA-384 = 809f439be00274321d4a538652164b53 554a508184a0c3160353e3428597003d 35914a18770f9443987054944b7c4b4a HMAC-SHA-384-192 = 809f439be00274321d4a538652164b53 554a508184a0c316 Test Case AUTH384-4: Key = 0102030405060708090a0b0c0d0e0f10 1112131415161718191a1b1c1d1e1f20 0a0b0c0d0e0f10111213141516171819 (48 bytes) Data = cdcdcdcdcdcdcdcdcdcdcdcdcdcdcdcd cdcdcdcdcdcdcdcdcdcdcdcdcdcdcdcd cdcdcdcdcdcdcdcdcdcdcdcdcdcdcdcd cdcd (50 bytes) PRF-HMAC-SHA-384 = 5b540085c6e6358096532b2493609ed1 cb298f774f87bb5c2ebf182c83cc7428 707fb92eab2536a5812258228bc96687 HMAC-SHA-384-192 = 5b540085c6e6358096532b2493609ed1 cb298f774f87bb5c
Test Case AUTH512-3: Key = aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa (64 bytes) Data = dddddddddddddddddddddddddddddddd dddddddddddddddddddddddddddddddd dddddddddddddddddddddddddddddddd dddd (50 bytes) PRF-HMAC-SHA-512 = 2ee7acd783624ca9398710f3ee05ae41 b9f9b0510c87e49e586cc9bf961733d8 623c7b55cebefccf02d5581acc1c9d5f b1ff68a1de45509fbe4da9a433922655 HMAC-SHA-512-256 = 2ee7acd783624ca9398710f3ee05ae41 b9f9b0510c87e49e586cc9bf961733d8 Test Case AUTH512-4: Key = 0a0b0c0d0e0f10111213141516171819 0102030405060708090a0b0c0d0e0f10 1112131415161718191a1b1c1d1e1f20 2122232425262728292a2b2c2d2e2f30 3132333435363738393a3b3c3d3e3f40 (64 bytes) Data = cdcdcdcdcdcdcdcdcdcdcdcdcdcdcdcd cdcdcdcdcdcdcdcdcdcdcdcdcdcdcdcd cdcdcdcdcdcdcdcdcdcdcdcdcdcdcdcd cdcd (50 bytes) PRF-HMAC-SHA-512 = 5e6688e5a3daec826ca32eaea224eff5 e700628947470e13ad01302561bab108 b8c48cbc6b807dcfbd850521a685babc 7eae4a2a2e660dc0e86b931d65503fd2 HMAC-SHA-512-256 = 5e6688e5a3daec826ca32eaea224eff5 e700628947470e13ad01302561bab108
HMAC-SHA1] and the HMAC-SHA-256+ algorithms, but there are also considerations that are somewhat counter-intuitive. There are two different axes along which we gauge the security of these algorithms: HMAC output length and HMAC key length. If we assume the HMAC key is a well-guarded secret that can only be determined through offline attacks on observed values, and that its length is less than or equal to the output length of the underlying hash algorithm, then the key's strength is directly proportional to its length. And if we assume an adversary has no knowledge of the HMAC key, then the probability of guessing a correct MAC value for any given packet is directly proportional to the HMAC output length. This specification defines truncation to output lengths of either 128 192, or 256 bits. It is important to note that at this time, it is not clear that HMAC-SHA-256 with a truncation length of 128 bits is any more secure than HMAC-SHA1 with the same truncation length, assuming the adversary has no knowledge of the HMAC key. This is because in such cases, the adversary must predict only those bits that remain after truncation. Since in both cases that output length is the same (128 bits), the adversary's odds of correctly guessing the value are also the same in either case: 1 in 2^128. Again, if we assume the HMAC key remains unknown to the attacker, then only a bias in one of the algorithms would distinguish one from the other. Currently, no such bias is known to exist in either HMAC-SHA1 or HMAC-SHA-256+. If, on the other hand, the attacker is focused on guessing the HMAC key, and we assume that the hash algorithms are indistinguishable
when viewed as PRF's, then the HMAC key length provides a direct measure of the underlying security: the longer the key, the harder it is to guess. This means that with respect to passive attacks on the HMAC key, size matters - and the HMAC-SHA-256+ algorithms provide more security in this regard than HMAC-SHA1-96.
HMAC-SHA1] and [HMAC-TEST]. Thanks to Hugo Krawczyk for comments and recommendations on early revisions of this document, and thanks also to Russ Housley and Steve Bellovin for various security-related comments and recommendations. [AH] Kent, S., "IP Authentication Header", RFC 4302, December 2005. [ARCH] Kent, S. and K. Seo, "Security Architecture for the Internet Protocol", RFC 4301, December 2005. [ESP] Kent, S., "IP Encapsulating Security Payload (ESP)", RFC 4303, December 2005. [HMAC] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed- Hashing for Message Authentication", RFC 2104, February 1997. [HMAC-SHA1] Madsen, C. and R. Glenn, "The Use of HMAC-SHA-1-96 within ESP and AH", RFC 2404, November 1998. [HMAC-TEST] Nystrom, M., "Identifiers and Test Vectors for HMAC-SHA- 224, HMAC-SHA-256, HMAC-SHA-384, and HMAC-SHA-512", RFC 4231, December 2005. [IKE] Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE)", RFC 2409, November 1998. [IKEv2] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol", RFC 4306, December 2005. [SHA2-1] NIST, "FIPS PUB 180-2 'Specifications for the Secure Hash Standard'", 2004 FEB, <http://csrc.nist.gov/ publications/fips/fips180-2/ fips180-2withchangenotice.pdf>. [SHA256+] Eastlake, D. and T. Hansen, "US Secure Hash Algorithms (SHA and HMAC-SHA)", RFC 4634, July 2006.
[SHA2-2] NIST, "Descriptions of SHA-256, SHA-384, and SHA-512", 2001 MAY, <http://csrc.nist.gov/cryptval/shs/sha256-384-512.pdf>.
Full Copyright Statement Copyright (C) The IETF Trust (2007). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Intellectual Property The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at firstname.lastname@example.org. Acknowledgement Funding for the RFC Editor function is currently provided by the Internet Society.