Network Working Group M. Wahl
Request for Comments: 2829 Sun Microsystems, Inc.
Category: Standards Track H. Alvestrand
EDB Maxware
J. Hodges
Oblix, Inc.
R. Morgan
University of Washington
May 2000
Authentication Methods for LDAP
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 (2000). All Rights Reserved.
Abstract
This document specifies particular combinations of security
mechanisms which are required and recommended in LDAP [1]
implementations.
1. Introduction
LDAP version 3 is a powerful access protocol for directories.
It offers means of searching, fetching and manipulating directory
content, and ways to access a rich set of security functions.
In order to function for the best of the Internet, it is vital that
these security functions be interoperable; therefore there has to be
a minimum subset of security functions that is common to all
implementations that claim LDAPv3 conformance.
Basic threats to an LDAP directory service include:
(1) Unauthorized access to data via data-fetching operations,
Wahl, et al. Standards Track [Page 1]RFC 2829 Authentication Methods for LDAP May 2000
(2) Unauthorized access to reusable client authentication
information by monitoring others' access,
(3) Unauthorized access to data by monitoring others' access,
(4) Unauthorized modification of data,
(5) Unauthorized modification of configuration,
(6) Unauthorized or excessive use of resources (denial of
service), and
(7) Spoofing of directory: Tricking a client into believing that
information came from the directory when in fact it did not,
either by modifying data in transit or misdirecting the
client's connection.
Threats (1), (4), (5) and (6) are due to hostile clients. Threats
(2), (3) and (7) are due to hostile agents on the path between client
and server, or posing as a server.
The LDAP protocol suite can be protected with the following security
mechanisms:
(1) Client authentication by means of the SASL [2] mechanism
set, possibly backed by the TLS credentials exchange
mechanism,
(2) Client authorization by means of access control based on the
requestor's authenticated identity,
(3) Data integrity protection by means of the TLS protocol or
data-integrity SASL mechanisms,
(4) Protection against snooping by means of the TLS protocol or
data-encrypting SASL mechanisms,
(5) Resource limitation by means of administrative limits on
service controls, and
(6) Server authentication by means of the TLS protocol or SASL
mechanism.
At the moment, imposition of access controls is done by means outside
the scope of the LDAP protocol.
In this document, the term "user" represents any application which is
an LDAP client using the directory to retrieve or store information.
Wahl, et al. Standards Track [Page 2]RFC 2829 Authentication Methods for LDAP May 2000
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 RFC 2119 [3].
2. Example deployment scenarios
The following scenarios are typical for LDAP directories on the
Internet, and have different security requirements. (In the
following, "sensitive" means data that will cause real damage to the
owner if revealed; there may be data that is protected but not
sensitive). This is not intended to be a comprehensive list, other
scenarios are possible, especially on physically protected networks.
(1) A read-only directory, containing no sensitive data,
accessible to "anyone", and TCP connection hijacking or IP
spoofing is not a problem. This directory requires no
security functions except administrative service limits.
(2) A read-only directory containing no sensitive data; read
access is granted based on identity. TCP connection
hijacking is not currently a problem. This scenario requires
a secure authentication function.
(3) A read-only directory containing no sensitive data; and the
client needs to ensure that the directory data is
authenticated by the server and not modified while being
returned from the server.
(4) A read-write directory, containing no sensitive data; read
access is available to "anyone", update access to properly
authorized persons. TCP connection hijacking is not
currently a problem. This scenario requires a secure
authentication function.
(5) A directory containing sensitive data. This scenario
requires session confidentiality protection AND secure
authentication.
3. Authentication and Authorization: Definitions and Concepts
This section defines basic terms, concepts, and interrelationships
regarding authentication, authorization, credentials, and identity.
These concepts are used in describing how various security approaches
are utilized in client authentication and authorization.
Wahl, et al. Standards Track [Page 3]RFC 2829 Authentication Methods for LDAP May 2000
3.1. Access Control Policy
An access control policy is a set of rules defining the protection of
resources, generally in terms of the capabilities of persons or other
entities accessing those resources. A common expression of an access
control policy is an access control list. Security objects and
mechanisms, such as those described here, enable the expression of
access control policies and their enforcement. Access control
policies are typically expressed in terms of access control
attributes as described below.
3.2. Access Control Factors
A request, when it is being processed by a server, may be associated
with a wide variety of security-related factors (section 4.2 of [1]).
The server uses these factors to determine whether and how to process
the request. These are called access control factors (ACFs). They
might include source IP address, encryption strength, the type of
operation being requested, time of day, etc. Some factors may be
specific to the request itself, others may be associated with the
connection via which the request is transmitted, others (e.g. time of
day) may be "environmental".
Access control policies are expressed in terms of access control
factors. E.g., a request having ACFs i,j,k can perform operation Y
on resource Z. The set of ACFs that a server makes available for such
expressions is implementation-specific.
3.3. Authentication, Credentials, Identity
Authentication credentials are the evidence supplied by one party to
another, asserting the identity of the supplying party (e.g. a user)
who is attempting to establish an association with the other party
(typically a server). Authentication is the process of generating,
transmitting, and verifying these credentials and thus the identity
they assert. An authentication identity is the name presented in a
credential.
There are many forms of authentication credentials -- the form used
depends upon the particular authentication mechanism negotiated by
the parties. For example: X.509 certificates, Kerberos tickets,
simple identity and password pairs. Note that an authentication
mechanism may constrain the form of authentication identities used
with it.
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RFC 2829 Authentication Methods for LDAP May 2000
3.4. Authorization Identity
An authorization identity is one kind of access control factor. It
is the name of the user or other entity that requests that operations
be performed. Access control policies are often expressed in terms
of authorization identities; e.g., entity X can perform operation Y
on resource Z.
The authorization identity bound to an association is often exactly
the same as the authentication identity presented by the client, but
it may be different. SASL allows clients to specify an authorization
identity distinct from the authentication identity asserted by the
client's credentials. This permits agents such as proxy servers to
authenticate using their own credentials, yet request the access
privileges of the identity for which they are proxying [2]. Also,
the form of authentication identity supplied by a service like TLS
may not correspond to the authorization identities used to express a
server's access control policy, requiring a server-specific mapping
to be done. The method by which a server composes and validates an
authorization identity from the authentication credentials supplied
by a client is implementation-specific.
4. Required security mechanisms
It is clear that allowing any implementation, faced with the above
requirements, to pick and choose among the possible alternatives is
not a strategy that is likely to lead to interoperability. In the
absence of mandates, clients will be written that do not support any
security function supported by the server, or worse, support only
mechanisms like cleartext passwords that provide clearly inadequate
security.
Active intermediary attacks are the most difficult for an attacker to
perform, and for an implementation to protect against. Methods that
protect only against hostile client and passive eavesdropping attacks
are useful in situations where the cost of protection against active
intermediary attacks is not justified based on the perceived risk of
active intermediary attacks.
Given the presence of the Directory, there is a strong desire to see
mechanisms where identities take the form of a Distinguished Name and
authentication data can be stored in the directory; this means that
either this data is useless for faking authentication (like the Unix
"/etc/passwd" file format used to be), or its content is never passed
across the wire unprotected - that is, it's either updated outside
the protocol or it is only updated in sessions well protected against
snooping. It is also desirable to allow authentication methods to
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RFC 2829 Authentication Methods for LDAP May 2000
carry authorization identities based on existing forms of user
identities for backwards compatibility with non-LDAP-based
authentication services.
Therefore, the following implementation conformance requirements are
in place:
(1) For a read-only, public directory, anonymous authentication,
described in section 5, can be used.
(2) Implementations providing password-based authenticated
access MUST support authentication using the DIGEST-MD5 SASL
mechanism [4], as described in section 6.1. This provides
client authentication with protection against passive
eavesdropping attacks, but does not provide protection
against active intermediary attacks.
(3) For a directory needing session protection and
authentication, the Start TLS extended operation [5], and
either the simple authentication choice or the SASL EXTERNAL
mechanism, are to be used together. Implementations SHOULD
support authentication with a password as described in
section 6.2, and SHOULD support authentication with a
certificate as described in section 7.1. Together, these
can provide integrity and disclosure protection of
transmitted data, and authentication of client and server,
including protection against active intermediary attacks.
If TLS is negotiated, the client MUST discard all information about
the server fetched prior to the TLS negotiation. In particular, the
value of supportedSASLMechanisms MAY be different after TLS has been
negotiated (specifically, the EXTERNAL mechanism or the proposed
PLAIN mechanism are likely to only be listed after a TLS negotiation
has been performed).
If a SASL security layer is negotiated, the client MUST discard all
information about the server fetched prior to SASL. In particular,
if the client is configured to support multiple SASL mechanisms, it
SHOULD fetch supportedSASLMechanisms both before and after the SASL
security layer is negotiated and verify that the value has not
changed after the SASL security layer was negotiated. This detects
active attacks which remove supported SASL mechanisms from the
supportedSASLMechanisms list, and allows the client to ensure that it
is using the best mechanism supported by both client and server
(additionally, this is a SHOULD to allow for environments where the
supported SASL mechanisms list is provided to the client through a
different trusted source, e.g. as part of a digitally signed object).
Wahl, et al. Standards Track [Page 6]RFC 2829 Authentication Methods for LDAP May 2000
5. Anonymous authentication
Directory operations which modify entries or access protected
attributes or entries generally require client authentication.
Clients which do not intend to perform any of these operations
typically use anonymous authentication.
LDAP implementations MUST support anonymous authentication, as
defined in section 5.1.
LDAP implementations MAY support anonymous authentication with TLS,
as defined in section 5.2.
While there MAY be access control restrictions to prevent access to
directory entries, an LDAP server SHOULD allow an anonymously-bound
client to retrieve the supportedSASLMechanisms attribute of the root
DSE.
An LDAP server MAY use other information about the client provided by
the lower layers or external means to grant or deny access even to
anonymously authenticated clients.
5.1. Anonymous authentication procedure
An LDAP client which has not successfully completed a bind operation
on a connection is anonymously authenticated.
An LDAP client MAY also specify anonymous authentication in a bind
request by using a zero-length OCTET STRING with the simple
authentication choice.
5.2. Anonymous authentication and TLS
An LDAP client MAY use the Start TLS operation [5] to negotiate the
use of TLS security [6]. If the client has not bound beforehand,
then until the client uses the EXTERNAL SASL mechanism to negotiate
the recognition of the client's certificate, the client is
anonymously authenticated.
Recommendations on TLS ciphersuites are given in section 10.
An LDAP server which requests that clients provide their certificate
during TLS negotiation MAY use a local security policy to determine
whether to successfully complete TLS negotiation if the client did
not present a certificate which could be validated.
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6. Password-based authentication
LDAP implementations MUST support authentication with a password
using the DIGEST-MD5 SASL mechanism for password protection, as
defined in section 6.1.
LDAP implementations SHOULD support authentication with the "simple"
password choice when the connection is protected against
eavesdropping using TLS, as defined in section 6.2.
6.1. Digest authentication
An LDAP client MAY determine whether the server supports this
mechanism by performing a search request on the root DSE, requesting
the supportedSASLMechanisms attribute, and checking whether the
string "DIGEST-MD5" is present as a value of this attribute.
In the first stage of authentication, when the client is performing
an "initial authentication" as defined in section 2.1 of [4], the
client sends a bind request in which the version number is 3, the
authentication choice is sasl, the sasl mechanism name is "DIGEST-
MD5", and the credentials are absent. The client then waits for a
response from the server to this request.
The server will respond with a bind response in which the resultCode
is saslBindInProgress, and the serverSaslCreds field is present. The
contents of this field is a string defined by "digest-challenge" in
section 2.1.1 of [4]. The server SHOULD include a realm indication
and MUST indicate support for UTF-8.
The client will send a bind request with a distinct message id, in
which the version number is 3, the authentication choice is sasl, the
sasl mechanism name is "DIGEST-MD5", and the credentials contain the
string defined by "digest-response" in section 2.1.2 of [4]. The
serv-type is "ldap".
The server will respond with a bind response in which the resultCode
is either success, or an error indication. If the authentication is
successful and the server does not support subsequent authentication,
then the credentials field is absent. If the authentication is
successful and the server supports subsequent authentication, then
the credentials field contains the string defined by "response-auth"
in section 2.1.3 of [4]. Support for subsequent authentication is
OPTIONAL in clients and servers.
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RFC 2829 Authentication Methods for LDAP May 2000
6.2. "simple" authentication choice under TLS encryption
A user who has a directory entry containing a userPassword attribute
MAY authenticate to the directory by performing a simple password
bind sequence following the negotiation of a TLS ciphersuite
providing connection confidentiality [6].
The client will use the Start TLS operation [5] to negotiate the use
of TLS security [6] on the connection to the LDAP server. The client
need not have bound to the directory beforehand.
For this authentication procedure to be successful, the client and
server MUST negotiate a ciphersuite which contains a bulk encryption
algorithm of appropriate strength. Recommendations on cipher suites
are given in section 10.
Following the successful completion of TLS negotiation, the client
MUST send an LDAP bind request with the version number of 3, the name
field containing the name of the user's entry, and the "simple"
authentication choice, containing a password.
The server will, for each value of the userPassword attribute in the
named user's entry, compare these for case-sensitive equality with
the client's presented password. If there is a match, then the
server will respond with resultCode success, otherwise the server
will respond with resultCode invalidCredentials.
6.3. Other authentication choices with TLS
It is also possible, following the negotiation of TLS, to perform a
SASL authentication which does not involve the exchange of plaintext
reusable passwords. In this case the client and server need not
negotiate a ciphersuite which provides confidentiality if the only
service required is data integrity.
7. Certificate-based authentication
LDAP implementations SHOULD support authentication via a client
certificate in TLS, as defined in section 7.1.
7.1. Certificate-based authentication with TLS
A user who has a public/private key pair in which the public key has
been signed by a Certification Authority may use this key pair to
authenticate to the directory server if the user's certificate is
requested by the server. The user's certificate subject field SHOULD
be the name of the user's directory entry, and the Certification
Authority must be sufficiently trusted by the directory server to
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RFC 2829 Authentication Methods for LDAP May 2000
have issued the certificate in order that the server can process the
certificate. The means by which servers validate certificate paths
is outside the scope of this document.
A server MAY support mappings for certificates in which the subject
field name is different from the name of the user's directory entry.
A server which supports mappings of names MUST be capable of being
configured to support certificates for which no mapping is required.
The client will use the Start TLS operation [5] to negotiate the use
of TLS security [6] on the connection to the LDAP server. The client
need not have bound to the directory beforehand.
In the TLS negotiation, the server MUST request a certificate. The
client will provide its certificate to the server, and MUST perform a
private key-based encryption, proving it has the private key
associated with the certificate.
As deployments will require protection of sensitive data in transit,
the client and server MUST negotiate a ciphersuite which contains a
bulk encryption algorithm of appropriate strength. Recommendations
of cipher suites are given in section 10.
The server MUST verify that the client's certificate is valid. The
server will normally check that the certificate is issued by a known
CA, and that none of the certificates on the client's certificate
chain are invalid or revoked. There are several procedures by which
the server can perform these checks.
Following the successful completion of TLS negotiation, the client
will send an LDAP bind request with the SASL "EXTERNAL" mechanism.
8. Other mechanisms
The LDAP "simple" authentication choice is not suitable for
authentication on the Internet where there is no network or transport
layer confidentiality.
As LDAP includes native anonymous and plaintext authentication
methods, the "ANONYMOUS" and "PLAIN" SASL mechanisms are not used
with LDAP. If an authorization identity of a form different from a
DN is requested by the client, a mechanism that protects the password
in transit SHOULD be used.
The following SASL-based mechanisms are not considered in this
document: KERBEROS_V4, GSSAPI and SKEY.
Wahl, et al. Standards Track [Page 10]
RFC 2829 Authentication Methods for LDAP May 2000
The "EXTERNAL" SASL mechanism can be used to request the LDAP server
make use of security credentials exchanged by a lower layer. If a TLS
session has not been established between the client and server prior
to making the SASL EXTERNAL Bind request and there is no other
external source of authentication credentials (e.g. IP-level
security [8]), or if, during the process of establishing the TLS
session, the server did not request the client's authentication
credentials, the SASL EXTERNAL bind MUST fail with a result code of
inappropriateAuthentication. Any client authentication and
authorization state of the LDAP association is lost, so the LDAP
association is in an anonymous state after the failure.
9. Authorization Identity
The authorization identity is carried as part of the SASL credentials
field in the LDAP Bind request and response.
When the "EXTERNAL" mechanism is being negotiated, if the credentials
field is present, it contains an authorization identity of the
authzId form described below.
Other mechanisms define the location of the authorization identity in
the credentials field.
The authorization identity is a string in the UTF-8 character set,
corresponding to the following ABNF [7]:
; Specific predefined authorization (authz) id schemes are
; defined below -- new schemes may be defined in the future.
authzId = dnAuthzId / uAuthzId
; distinguished-name-based authz id.
dnAuthzId = "dn:" dn
dn = utf8string ; with syntax defined in RFC 2253
; unspecified userid, UTF-8 encoded.
uAuthzId = "u:" userid
userid = utf8string ; syntax unspecified
A utf8string is defined to be the UTF-8 encoding of one or more ISO
10646 characters.
All servers which support the storage of authentication credentials,
such as passwords or certificates, in the directory MUST support the
dnAuthzId choice.
Wahl, et al. Standards Track [Page 11]
RFC 2829 Authentication Methods for LDAP May 2000
The uAuthzId choice allows for compatibility with client applications
which wish to authenticate to a local directory but do not know their
own Distinguished Name or have a directory entry. The format of the
string is defined as only a sequence of UTF-8 encoded ISO 10646
characters, and further interpretation is subject to prior agreement
between the client and server.
For example, the userid could identify a user of a specific directory
service, or be a login name or the local-part of an RFC 822 email
address. In general a uAuthzId MUST NOT be assumed to be globally
unique.
Additional authorization identity schemes MAY be defined in future
versions of this document.
10. TLS Ciphersuites
The following ciphersuites defined in [6] MUST NOT be used for
confidentiality protection of passwords or data:
TLS_NULL_WITH_NULL_NULL
TLS_RSA_WITH_NULL_MD5
TLS_RSA_WITH_NULL_SHA
The following ciphersuites defined in [6] can be cracked easily (less
than a week of CPU time on a standard CPU in 1997). The client and
server SHOULD carefully consider the value of the password or data
being protected before using these ciphersuites:
TLS_RSA_EXPORT_WITH_RC4_40_MD5
TLS_RSA_EXPORT_WITH_RC2_CBC_40_MD5
TLS_RSA_EXPORT_WITH_DES40_CBC_SHA
TLS_DH_DSS_EXPORT_WITH_DES40_CBC_SHA
TLS_DH_RSA_EXPORT_WITH_DES40_CBC_SHA
TLS_DHE_DSS_EXPORT_WITH_DES40_CBC_SHA
TLS_DHE_RSA_EXPORT_WITH_DES40_CBC_SHA
TLS_DH_anon_EXPORT_WITH_RC4_40_MD5
TLS_DH_anon_EXPORT_WITH_DES40_CBC_SHA
The following ciphersuites are vulnerable to man-in-the-middle
attacks, and SHOULD NOT be used to protect passwords or sensitive
data, unless the network configuration is such that the danger of a
man-in-the-middle attack is tolerable:
Wahl, et al. Standards Track [Page 12]RFC 2829 Authentication Methods for LDAP May 2000
TLS_DH_anon_EXPORT_WITH_RC4_40_MD5
TLS_DH_anon_WITH_RC4_128_MD5
TLS_DH_anon_EXPORT_WITH_DES40_CBC_SHA
TLS_DH_anon_WITH_DES_CBC_SHA
TLS_DH_anon_WITH_3DES_EDE_CBC_SHA
A client or server that supports TLS MUST support at least
TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA.
11. SASL service name for LDAP
For use with SASL [2], a protocol must specify a service name to be
used with various SASL mechanisms, such as GSSAPI. For LDAP, the
service name is "ldap", which has been registered with the IANA as a
GSSAPI service name.
12. Security Considerations
Security issues are discussed throughout this memo; the
(unsurprising) conclusion is that mandatory security is important,
and that session encryption is required when snooping is a problem.
Servers are encouraged to prevent modifications by anonymous users.
Servers may also wish to minimize denial of service attacks by timing
out idle connections, and returning the unwillingToPerform result
code rather than performing computationally expensive operations
requested by unauthorized clients.
A connection on which the client has not performed the Start TLS
operation or negotiated a suitable SASL mechanism for connection
integrity and encryption services is subject to man-in-the-middle
attacks to view and modify information in transit.
Additional security considerations relating to the EXTERNAL mechanism
to negotiate TLS can be found in [2], [5] and [6].
13. Acknowledgements
This document is a product of the LDAPEXT Working Group of the IETF.
The contributions of its members is greatly appreciated.
Wahl, et al. Standards Track [Page 13]RFC 2829 Authentication Methods for LDAP May 2000
14. Bibliography
[1] Wahl, M., Howes, T. and S. Kille, "Lightweight Directory Access
Protocol (v3)", RFC 2251, December 1997.
[2] Myers, J., "Simple Authentication and Security Layer (SASL)", RFC
2222, October 1997.
[3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[4] Leach, P. and C. Newman, "Using Digest Authentication as a SASL
Mechanism", RFC 2831, May 2000.
[5] Hodges, J., Morgan, R. and M. Wahl, "Lightweight Directory Access
Protocol (v3): Extension for Transport Layer Security", RFC 2830,
May 2000.
[6] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC
2246, January 1999.
[7] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
[8] Kent, S. and R. Atkinson, "Security Architecture for the Internet
Protocol", RFC 2401, November 1998.
Wahl, et al. Standards Track [Page 14]RFC 2829 Authentication Methods for LDAP May 2000
15. Authors' Addresses
Mark Wahl
Sun Microsystems, Inc.
8911 Capital of Texas Hwy #4140
Austin TX 78759
USA
EMail: M.Wahl@innosoft.com
Harald Tveit Alvestrand
EDB Maxware
Pirsenteret
N-7462 Trondheim, Norway
Phone: +47 73 54 57 97
EMail: Harald@Alvestrand.no
Jeff Hodges
Oblix, Inc.
18922 Forge Drive
Cupertino, CA 95014
USA
Phone: +1-408-861-6656
EMail: JHodges@oblix.com
RL "Bob" Morgan
Computing and Communications
University of Washington
Seattle, WA 98105
USA
Phone: +1-206-221-3307
EMail: rlmorgan@washington.edu
Wahl, et al. Standards Track [Page 15]
RFC 2829 Authentication Methods for LDAP May 2000
16. Full Copyright Statement
Copyright (C) The Internet Society (2000). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS 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.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Wahl, et al. Standards Track [Page 16]
