Network Working Group U. Blumenthal
Request for Comments: 3826 Lucent Technologies
Category: Standards Track F. Maino
Andiamo Systems, Inc.
K. McCloghrie
Cisco Systems, Inc.
June 2004
The Advanced Encryption Standard (AES) Cipher Algorithm
in the SNMP User-based Security Model
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 Internet Society (2004).
Abstract
This document describes a symmetric encryption protocol that
supplements the protocols described in the User-based Security Model
(USM), which is a Security Subsystem for version 3 of the Simple
Network Management Protocol for use in the SNMP Architecture. The
symmetric encryption protocol described in this document is based on
the Advanced Encryption Standard (AES) cipher algorithm used in
Cipher FeedBack Mode (CFB), with a key size of 128 bits.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Goals and Constraints. . . . . . . . . . . . . . . . . 2
1.2. Key Localization . . . . . . . . . . . . . . . . . . . 3
1.3. Password Entropy and Storage . . . . . . . . . . . . . 3
2. Definitions. . . . . . . . . . . . . . . . . . . . . . . . . 4
3. CFB128-AES-128 Symmetric Encryption Protocol . . . . . . . . 5
3.1. Mechanisms . . . . . . . . . . . . . . . . . . . . . . 5
3.1.1. The AES-based Symmetric Encryption Protocol . . 6
3.1.2. Localized Key, AES Encryption Key and
Initialization Vector . . . . . . . . . . . . . 7
3.1.3. Data Encryption . . . . . . . . . . . . . . . . 8
3.1.4. Data Decryption . . . . . . . . . . . . . . . . 8
Blumenthal, et al. Standards Track [Page 1]
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3.2. Elements of the AES Privacy Protocol . . . . . . . . . 9
3.2.1. Users . . . . . . . . . . . . . . . . . . . . . 9
3.2.2. msgAuthoritativeEngineID. . . . . . . . . . . . 9
3.2.3. SNMP Messages Using this Privacy Protocol . . . 10
3.2.4. Services provided by the AES Privacy Modules. . 10
3.3. Elements of Procedure. . . . . . . . . . . . . . . . . 11
3.3.1. Processing an Outgoing Message. . . . . . . . . 12
3.3.2. Processing an Incoming Message. . . . . . . . . 12
4. Security Considerations. . . . . . . . . . . . . . . . . . . 13
5. IANA Considerations. . . . . . . . . . . . . . . . . . . . . 13
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 14
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 14
7.1. Normative References . . . . . . . . . . . . . . . . . 14
7.2. Informative References . . . . . . . . . . . . . . . . 14
8. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 15
9. Full Copyright Statement . . . . . . . . . . . . . . . . . . 16
1. Introduction
Within the Architecture for describing Internet Management Frameworks
[RFC3411], the User-based Security Model (USM) [RFC3414] for SNMPv3
is defined as a Security Subsystem within an SNMP engine. RFC 3414
describes the use of HMAC-MD5-96 and HMAC-SHA-96 as the initial
authentication protocols, and the use of CBC-DES as the initial
privacy protocol. The User-based Security Model, however, allows for
other such protocols to be used instead of, or concurrently with,
these protocols.
This memo describes the use of CFB128-AES-128 as an alternative
privacy protocol for the User-based Security Model. 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].
1.1. Goals and Constraints
The main goal of this memo is to provide a new privacy protocol for
the USM based on the Advanced Encryption Standard (AES) [FIPS-AES].
The major constraint is to maintain a complete interchangeability of
the new protocol defined in this memo with existing authentication
and privacy protocols already defined in USM.
For a given user, the AES-based privacy protocol MUST be used with
one of the authentication protocols defined in RFC 3414 or an
algorithm/protocol providing equivalent functionality.
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1.2. Key Localization
As defined in [RFC3414], a localized key is a secret key shared
between a user U and one authoritative SNMP engine E. Even though a
user may have only one pair of authentication and privacy passwords
(and consequently only one pair of keys) for the entire network, the
actual secrets shared between the user and each authoritative SNMP
engine will be different. This is achieved by key localization.
If the authentication protocol defined for a user U at the
authoritative SNMP engine E is one of the authentication protocols
defined in [RFC3414], the key localization is performed according to
the two-step process described in section 2.6 of [RFC3414].
1.3. Password Entropy and Storage
The security of various cryptographic functions lies both in the
strength of the functions themselves against various forms of attack,
and also, perhaps more importantly, in the keying material that is
used with them. While theoretical attacks against cryptographic
functions are possible, it is more probable that key guessing is the
main threat.
The following are recommended in regard to user passwords:
- Password length SHOULD be at least 12 octets.
- Password sharing SHOULD be prohibited so that passwords are not
shared among multiple SNMP users.
- Implementations SHOULD support the use of randomly generated
passwords as a stronger form of security.
It is worth remembering that, as specified in [RFC3414], if a user's
password or a non-localized key is disclosed, then key localization
will not help and network security may be compromised. Therefore, a
user's password or non-localized key MUST NOT be stored on a managed
device/node. Instead, the localized key SHALL be stored (if at all)
so that, in case a device does get compromised, no other managed or
managing devices get compromised.
Blumenthal, et al. Standards Track [Page 3]
RFC 3826 AES for SNMP's USM June 2004
2. Definitions
This MIB is written in SMIv2 [RFC2578].
SNMP-USM-AES-MIB DEFINITIONS ::= BEGIN
IMPORTS
MODULE-IDENTITY, OBJECT-IDENTITY,
snmpModules FROM SNMPv2-SMI -- [RFC2578]
snmpPrivProtocols FROM SNMP-FRAMEWORK-MIB; -- [RFC3411]
snmpUsmAesMIB MODULE-IDENTITY
LAST-UPDATED "200406140000Z"
ORGANIZATION "IETF"
CONTACT-INFO "Uri Blumenthal
Lucent Technologies / Bell Labs
67 Whippany Rd.
14D-318
Whippany, NJ 07981, USA
973-386-2163
uri@bell-labs.com
Fabio Maino
Andiamo Systems, Inc.
375 East Tasman Drive
San Jose, CA 95134, USA
408-853-7530
fmaino@andiamo.com
Keith McCloghrie
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134-1706, USA
408-526-5260
kzm@cisco.com"
DESCRIPTION "Definitions of Object Identities needed for
the use of AES by SNMP's User-based Security
Model.
Copyright (C) The Internet Society (2004).
This version of this MIB module is part of RFC 3826;
see the RFC itself for full legal notices.
Supplementary information may be available on
http://www.ietf.org/copyrights/ianamib.html."
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REVISION "200406140000Z"
DESCRIPTION "Initial version, published as RFC3826"
::= { snmpModules 20 }
usmAesCfb128Protocol OBJECT-IDENTITY
STATUS current
DESCRIPTION "The CFB128-AES-128 Privacy Protocol."
REFERENCE "- Specification for the ADVANCED ENCRYPTION
STANDARD. Federal Information Processing
Standard (FIPS) Publication 197.
(November 2001).
- Dworkin, M., NIST Recommendation for Block
Cipher Modes of Operation, Methods and
Techniques. NIST Special Publication 800-38A
(December 2001).
"
::= { snmpPrivProtocols 4 }
END
3. CFB128-AES-128 Symmetric Encryption Protocol
This section describes a Symmetric Encryption Protocol based on the
AES cipher algorithm [FIPS-AES], used in Cipher Feedback Mode as
described in [AES-MODE], using encryption keys with a size of 128
bits.
This protocol is identified by usmAesCfb128PrivProtocol.
The protocol usmAesCfb128PrivProtocol is an alternative to the
privacy protocol defined in [RFC3414].
3.1. Mechanisms
In support of data confidentiality, an encryption algorithm is
required. An appropriate portion of the message is encrypted prior
to being transmitted. The User-based Security Model specifies that
the scopedPDU is the portion of the message that needs to be
encrypted.
A secret value is shared by all SNMP engines which can legitimately
originate messages on behalf of the appropriate user. This secret
value, in combination with a timeliness value and a 64-bit integer,
is used to create the (localized) en/decryption key and the
initialization vector.
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3.1.1. The AES-based Symmetric Encryption Protocol
The Symmetric Encryption Protocol defined in this memo provides
support for data confidentiality. The designated portion of an SNMP
message is encrypted and included as part of the message sent to the
recipient.
The AES (Advanced Encryption Standard) is the symmetric cipher
algorithm that the NIST (National Institute of Standards and
Technology) has selected in a four-year competitive process as
Replacement for DES (Data Encryption Standard).
The AES homepage, http://www.nist.gov/aes, contains a wealth of
information on AES including the Federal Information Processing
Standard [FIPS-AES] that fully specifies the Advanced Encryption
Standard.
The following subsections contain descriptions of the relevant
characteristics of the AES ciphers used in the symmetric encryption
protocol described in this memo.
3.1.1.1. Mode of operation
The NIST Special Publication 800-38A [AES-MODE] recommends five
confidentiality modes of operation for use with AES: Electronic
Codebook (ECB), Cipher Block Chaining (CBC), Cipher Feedback (CFB),
Output Feedback (OFB), and Counter (CTR).
The symmetric encryption protocol described in this memo uses AES in
CFB mode with the parameter S (number of bits fed back) set to 128
according to the definition of CFB mode given in [AES-MODE]. This
mode requires an Initialization Vector (IV) that is the same size as
the block size of the cipher algorithm.
3.1.1.2. Key Size
In the encryption protocol described by this memo AES is used with a
key size of 128 bits, as recommended in [AES-MODE].
3.1.1.3. Block Size and Padding
The block size of the AES cipher algorithms used in the encryption
protocol described by this memo is 128 bits, as recommended in [AES-
MODE].
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3.1.1.4. Rounds
This parameter determines how many times a block is encrypted. The
encryption protocol described in this memo uses 10 rounds, as
recommended in [AES-MODE].
3.1.2. Localized Key, AES Encryption Key, and Initialization Vector
The size of the Localized Key (Kul) of an SNMP user, as described in
[RFC3414], depends on the authentication protocol defined for that
user U at the authoritative SNMP engine E.
The encryption protocol defined in this memo MUST be used with an
authentication protocol that generates a localized key with at least
128 bits. The authentication protocols described in [RFC3414]
satisfy this requirement.
3.1.2.1. AES Encryption Key and IV
The first 128 bits of the localized key Kul are used as the AES
encryption key. The 128-bit IV is obtained as the concatenation of
the authoritative SNMP engine's 32-bit snmpEngineBoots, the SNMP
engine's 32-bit snmpEngineTime, and a local 64-bit integer. The 64-
bit integer is initialized to a pseudo-random value at boot time.
The IV is concatenated as follows: the 32-bit snmpEngineBoots is
converted to the first 4 octets (Most Significant Byte first), the
32-bit snmpEngineTime is converted to the subsequent 4 octets (Most
Significant Byte first), and the 64-bit integer is then converted to
the last 8 octets (Most Significant Byte first). The 64-bit integer
is then put into the msgPrivacyParameters field encoded as an OCTET
STRING of length 8 octets. The integer is then modified for the
subsequent message. We recommend that it is incremented by one until
it reaches its maximum value, at which time it is wrapped.
An implementation can use any method to vary the value of the local
64-bit integer, providing the chosen method never generates a
duplicate IV for the same key.
A duplicated IV can result in the very unlikely event that multiple
managers, communicating with a single authoritative engine, both
accidentally select the same 64-bit integer within a second. The
probability of such an event is very low, and does not significantly
affect the robustness of the mechanisms proposed.
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The 64-bit integer must be placed in the privParameters field to
enable the receiving entity to compute the correct IV and to decrypt
the message. This 64-bit value is called the "salt" in this
document.
Note that the sender and receiver must use the same IV value, i.e.,
they must both use the same values of the individual components used
to create the IV. In particular, both sender and receiver must use
the values of snmpEngineBoots, snmpEngineTime, and the 64-bit integer
which are contained in the relevant message (in the
msgAuthoritativeEngineBoots, msgAuthoritativeEngineTime, and
privParameters fields respectively).
3.1.3. Data Encryption
The data to be encrypted is treated as a sequence of octets.
The data is encrypted in Cipher Feedback mode with the parameter s
set to 128 according to the definition of CFB mode given in Section
6.3 of [AES-MODE]. A clear diagram of the encryption and decryption
process is given in Figure 3 of [AES-MODE].
The plaintext is divided into 128-bit blocks. The last block may
have fewer than 128 bits, and no padding is required.
The first input block is the IV, and the forward cipher operation is
applied to the IV to produce the first output block. The first
ciphertext block is produced by exclusive-ORing the first plaintext
block with the first output block. The ciphertext block is also used
as the input block for the subsequent forward cipher operation.
The process is repeated with the successive input blocks until a
ciphertext segment is produced from every plaintext segment.
The last ciphertext block is produced by exclusive-ORing the last
plaintext segment of r bits (r is less than or equal to 128) with the
segment of the r most significant bits of the last output block.
3.1.4. Data Decryption
In CFB decryption, the IV is the first input block, the first
ciphertext is used for the second input block, the second ciphertext
is used for the third input block, etc. The forward cipher function
is applied to each input block to produce the output blocks. The
output blocks are exclusive-ORed with the corresponding ciphertext
blocks to recover the plaintext blocks.
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The last ciphertext block (whose size r is less than or equal to 128)
is exclusive-ORed with the segment of the r most significant bits of
the last output block to recover the last plaintext block of r bits.
3.2. Elements of the AES Privacy Protocol
This section contains definitions required to realize the privacy
modules defined by this memo.
3.2.1. Users
Data en/decryption using this Symmetric Encryption Protocol makes use
of a defined set of userNames. For any user on whose behalf a
message must be en/decrypted at a particular SNMP engine, that SNMP
engine must have knowledge of that user. An SNMP engine that needs
to communicate with another SNMP engine must also have knowledge of a
user known to that SNMP engine, including knowledge of the applicable
attributes of that user.
A user and its attributes are defined as follows:
An octet string representing the name of the user.
The algorithm used to protect messages generated on behalf of the
user from disclosure.
The user's secret key to be used as input to the generation of the
localized key for encrypting/decrypting messages generated on
behalf of the user. The length of this key MUST be greater than
or equal to 128 bits (16 octets).
The algorithm used to authenticate messages generated on behalf of
the user, which is also used to generate the localized version of
the secret key.
3.2.2. msgAuthoritativeEngineID
The msgAuthoritativeEngineID value contained in an authenticated
message specifies the authoritative SNMP engine for that particular
message (see the definition of SnmpEngineID in the SNMP Architecture
document [RFC3411]).
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The user's (private) privacy key is different at each authoritative
SNMP engine, and so the snmpEngineID is used to select the proper key
for the en/decryption process.
3.2.3. SNMP Messages Using this Privacy Protocol
Messages using this privacy protocol carry a msgPrivacyParameters
field as part of the msgSecurityParameters. For this protocol, the
privParameters field is the serialized OCTET STRING representing the
"salt" that was used to create the IV.
3.2.4. Services provided by the AES Privacy Modules
This section describes the inputs and outputs that the AES Privacy
module expects and produces when the User-based Security module
invokes one of the AES Privacy modules for services.
3.2.4.1. Services for Encrypting Outgoing Data
The AES privacy protocol assumes that the selection of the privKey is
done by the caller, and that the caller passes the localized secret
key to be used.
Upon completion, the privacy module returns statusInformation and, if
the encryption process was successful, the encryptedPDU and the
msgPrivacyParameters encoded as an OCTET STRING. The abstract
service primitive is:
statusInformation = -- success or failure
encryptData(
IN encryptKey -- secret key for encryption
IN dataToEncrypt -- data to encrypt (scopedPDU)
OUT encryptedData -- encrypted data (encryptedPDU)
OUT privParameters -- filled in by service provider
)
The abstract data elements are:
statusInformation
An indication of the success or failure of the encryption process.
In case of failure, it is an indication of the error.
encryptKey
The secret key to be used by the encryption algorithm. The length
of this key MUST be 16 octets.
dataToEncrypt
The data that must be encrypted.
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RFC 3826 AES for SNMP's USM June 2004
encryptedData
The encrypted data upon successful completion.
privParameters
The privParameters encoded as an OCTET STRING.
3.2.4.2. Services for Decrypting Incoming Data
This AES privacy protocol assumes that the selection of the privKey
is done by the caller and that the caller passes the localized secret
key to be used.
Upon completion the privacy module returns statusInformation and, if
the decryption process was successful, the scopedPDU in plain text.
The abstract service primitive is:
statusInformation =
decryptData(
IN decryptKey -- secret key for decryption
IN privParameters -- as received on the wire
IN encryptedData -- encrypted data (encryptedPDU)
OUT decryptedData -- decrypted data (scopedPDU)
)
The abstract data elements are:
statusInformation
An indication of whether the data was successfully decrypted, and
if not, an indication of the error.
decryptKey
The secret key to be used by the decryption algorithm. The length
of this key MUST be 16 octets.
privParameters
The 64-bit integer to be used to calculate the IV.
encryptedData
The data to be decrypted.
decryptedData
The decrypted data.
3.3. Elements of Procedure
This section describes the procedures for the AES privacy protocol
for SNMP's User-based Security Model.
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RFC 3826 AES for SNMP's USM June 2004
3.3.1. Processing an Outgoing Message
This section describes the procedure followed by an SNMP engine
whenever it must encrypt part of an outgoing message using the
usmAesCfb128PrivProtocol.
1) The secret encryptKey is used to construct the AES encryption key,
as described in section 3.1.2.1.
2) The privParameters field is set to the serialization according to
the rules in [RFC3417] of an OCTET STRING representing the 64-bit
integer that will be used in the IV as described in section
3.1.2.1.
3) The scopedPDU is encrypted (as described in section 3.1.3) and the
encrypted data is serialized according to the rules in [RFC3417]
as an OCTET STRING.
4) The serialized OCTET STRING representing the encrypted scopedPDU
together with the privParameters and statusInformation indicating
success is returned to the calling module.
3.3.2. Processing an Incoming Message
This section describes the procedure followed by an SNMP engine
whenever it must decrypt part of an incoming message using the
usmAesCfb128PrivProtocol.
1) If the privParameters field is not an 8-octet OCTET STRING, then
an error indication (decryptionError) is returned to the calling
module.
2) The 64-bit integer is extracted from the privParameters field.
3) The secret decryptKey and the 64-bit integer are then used to
construct the AES decryption key and the IV that is computed as
described in section 3.1.2.1.
4) The encryptedPDU is then decrypted (as described in section
3.1.4).
5) If the encryptedPDU cannot be decrypted, then an error indication
(decryptionError) is returned to the calling module.
6) The decrypted scopedPDU and statusInformation indicating success
are returned to the calling module.
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4. Security Considerations
The security of the cryptographic functions defined in this document
lies both in the strength of the functions themselves against various
forms of attack, and also, perhaps more importantly, in the keying
material that is used with them. The recommendations in Section 1.3
SHOULD be followed to ensure maximum entropy to the selected
passwords, and to protect the passwords while stored.
The security of the CFB mode relies upon the use of a unique IV for
each message encrypted with the same key [CRYPTO-B]. If the IV is
not unique, a cryptanalyst can recover the corresponding plaintext.
Section 3.1.2.1 defines a procedure to derive the IV from a local
64-bit integer (the salt) initialized to a pseudo-random value at
boot time. An implementation can use any method to vary the value of
the local 64-bit integer, providing the chosen method never generates
a duplicate IV for the same key.
The procedure of section 3.1.2.1 suggests a method to vary the local
64-bit integer value that generates unique IVs for every message.
This method can result in a duplicated IV in the very unlikely event
that multiple managers, communicating with a single authoritative
engine, both accidentally select the same 64-bit integer within a
second. The probability of such an event is very low, and does not
significantly affect the robustness of the mechanisms proposed.
This AES-based privacy protocol MUST be used with one of the
authentication protocols defined in RFC 3414 or with an
algorithm/protocol providing equivalent functionality (including
integrity), because CFB encryption mode does not detect ciphertext
modifications.
For further security considerations, the reader is encouraged to read
[RFC3414], and the documents that describe the actual cipher
algorithms.
5. IANA Considerations
IANA has assigned OID 20 for the snmpUsmAesMIB module under the
snmpModules subtree, maintained in the registry at
http://www.iana.org/assignments/smi-numbers.
IANA has assigned OID 4 for the usmAesCfb128Protocol under the
snmpPrivProtocols registration point, as defined in RFC 3411
[RFC3411].
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6. Acknowledgements
Portions of this text, as well as its general structure, were
unabashedly lifted from [RFC3414]. The authors are grateful to many
of the SNMPv3 WG members for their help, especially Wes Hardaker,
Steve Moulton, Randy Presuhn, David Town, and Bert Wijnen. Security
discussions with Steve Bellovin helped to streamline this protocol.
7. References
7.1. Normative References
[AES-MODE] Dworkin, M., "NIST Recommendation for Block Cipher Modes
of Operation, Methods and Techniques", NIST Special
Publication 800-38A, December 2001.
[FIPS-AES] "Specification for the ADVANCED ENCRYPTION STANDARD
(AES)", Federal Information Processing Standard (FIPS)
Publication 197, November 2001.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2578] McCloghrie, K., Perkins, D. and J. Schoenwaelder,
"Structure of Management Information Version 2 (SMIv2)",
STD 58, RFC 2578, April 1999.
[RFC3411] Harrington, D., Presuhn, R. and B. Wijnen, "An
Architecture for Describing Simple Network Management
Protocol (SNMP) Management Frameworks", STD 62, RFC 3411,
December 2002.
[RFC3414] Blumenthal, U. and B. Wijnen, "User-based Security Model
(USM) for version 3 of the Simple Network Management
Protocol (SNMPv3)", STD 62, RFC 3414, December 2002.
[RFC3417] Presuhn, R., Ed., "Transport Mappings for the Simple
Network Management Protocol (SNMP)", STD 62, RFC 3417,
December 2002.
7.2. Informative References
[CRYPTO-B] Bellovin, S., "Probable Plaintext Cryptanalysis of the IP
Security Protocols", Proceedings of the Symposium on
Network and Distributed System Security, San Diego, CA,
pp. 155-160, February 1997.
Blumenthal, et al. Standards Track [Page 14]
RFC 3826 AES for SNMP's USM June 2004
8. Authors' Addresses
Uri Blumenthal
Lucent Technologies / Bell Labs
67 Whippany Rd.
14D-318
Whippany, NJ 07981, USA
Phone: +1-973-386-2163
EMail: uri@bell-labs.com
Fabio Maino
Andiamo Systems, Inc.
375 East Tasman Drive
San Jose, CA. 95134 USA
Phone: +1-408-853-7530
EMail: fmaino@andiamo.com
Keith McCloghrie
Cisco Systems, Inc.
170 East Tasman Drive
San Jose, CA. 95134-1706 USA
Phone: +1-408-526-5260
EMail: kzm@cisco.com
Blumenthal, et al. Standards Track [Page 15]
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9. Full Copyright Statement
Copyright (C) The Internet Society (2004). 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 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.
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this standard. Please address the information to the IETF at ietf-
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Blumenthal, et al. Standards Track [Page 16]
RFC, FYI, BCP