2906
INFORMATIONAL
AAA Authorization Requirements
Authors: S. Farrell, J. Vollbrecht, P. Calhoun, L. Gommans, G. Gross, B. de Bruijn, C. de Laat, M. Holdrege, D. Spence
Date: August 2000
Stream: Legacy
Abstract
This document specifies the requirements that Authentication Authorization Accounting (AAA) protocols must meet in order to support authorization services in the Internet. This memo provides information for the Internet community.
RFC 2906
INFORMATIONAL
Network Working Group S. Farrell
Request for Comments: 2906 Baltimore Technologies
Category: Informational J. Vollbrecht
Interlink Networks, Inc.
P. Calhoun
Sun Microsystems, Inc.
L. Gommans
Enterasys Networks EMEA
G. Gross
Lucent Technologies
B. de Bruijn
Interpay Nederland B.V.
C. de Laat
Utrecht University
M. Holdrege
ipVerse
D. Spence
Interlink Networks, Inc.
August 2000
<span class="h1">AAA Authorization Requirements</span>
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2000). All Rights Reserved.
Abstract
This document specifies the requirements that Authentication
Authorization Accounting (AAA) protocols must meet in order to
support authorization services in the Internet. The requirements have
been elicited from a study of a range of applications including
mobile-IP, roamops and others.
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Table Of Contents
<a href="#section-1">1</a>. Introduction.................................................<a href="#page-2">2</a>
<a href="#section-2">2</a>. Requirements.................................................<a href="#page-3">3</a>
<a href="#section-2.1">2.1</a> Authorization Information..............................<a href="#page-3">3</a>
<a href="#section-2.2">2.2</a> Security of authorization information..................<a href="#page-7">7</a>
<a href="#section-2.3">2.3</a> Time...................................................<a href="#page-9">9</a>
<a href="#section-2.4">2.4</a> Topology..............................................<a href="#page-10">10</a>
<a href="#section-2.5">2.5</a> Application Proxying..................................<a href="#page-12">12</a>
<a href="#section-2.6">2.6</a> Trust Model...........................................<a href="#page-12">12</a>
<a href="#section-2.7">2.7</a> Not just transactions.................................<a href="#page-14">14</a>
<a href="#section-2.8">2.8</a> Administration........................................<a href="#page-15">15</a>
<a href="#section-2.9">2.9</a> Bytes on-the-wire.....................................<a href="#page-16">16</a>
<a href="#section-2.10">2.10</a> Interfaces............................................<a href="#page-17">17</a>
<a href="#section-2.11">2.11</a> Negotiation...........................................<a href="#page-18">18</a>
<a href="#section-3">3</a>. Security Considerations.....................................<a href="#page-19">19</a>
<a href="#section-4">4</a>. References..................................................<a href="#page-20">20</a>
Authors' Addresses.............................................<a href="#page-20">20</a>
Full Copyright Statement.......................................<a href="#page-23">23</a>
<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>. Introduction</span>
This document is one of a series of three documents under
consideration by the AAAarch RG dealing with the authorization
requirements for AAA protocols. The three documents are:
AAA Authorization Framework [<a href="#ref-FRMW" title=""AAA Authorization Framework"">FRMW</a>]
AAA Authorization Requirements (this document)
AAA Authorization Application Examples [<a href="#ref-SAMP" title=""AAA Authorization Application Examples"">SAMP</a>]
The work for this memo was done by a group that originally was the
Authorization subgroup of the AAA Working Group of the IETF. When
the charter of the AAA working group was changed to focus on MobileIP
and NAS requirements, the AAAarch Research Group was chartered within
the IRTF to continue and expand the architectural work started by the
Authorization subgroup. This memo is one of four which were created
by the subgroup. This memo is a starting point for further work
within the AAAarch Research Group. It is still a work in progress
and is published so that the work will be available for the AAAarch
subgroup and others working in this area, not as a definitive
description of architecture or requirements.
The process followed in producing this document was to analyze the
requirements from [<a href="#ref-SAMP" title=""AAA Authorization Application Examples"">SAMP</a>] based on a common understanding of the AAA
authorization framework [<a href="#ref-FRMW" title=""AAA Authorization Framework"">FRMW</a>]. This document assumes familiarity
with both the general issues involved in authorization and, in
particular, the reader will benefit from a reading of [<a href="#ref-FRMW" title=""AAA Authorization Framework"">FRMW</a>] where,
for example, definitions of terms can be found.
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The key words "MUST", "REQUIRED", "SHOULD", "RECOMMENDED", and "MAY"
in this document are to be interpreted as described in [<a href="./rfc2119" title=""Key words for use in RFCs to Indicate Requirement Levels"">RFC2119</a>].
<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>. Requirements</span>
Requirements are grouped under headings for convenience; this
grouping is not significant.
Definitions and explanations of some of the technical terms used in
this document may be found in [<a href="#ref-FRMW" title=""AAA Authorization Framework"">FRMW</a>].
Each requirement is presented as a succinct (usually a sentence or
two) statement. Most are followed by a paragraph of explanatory
material, which sometimes contains an example. Fully described
examples may be found in [<a href="#ref-SAMP" title=""AAA Authorization Application Examples"">SAMP</a>].
The requirements presented are not intended to be "orthogonal", that
is, some of them repeat, or overlap, with others.
<span class="h3"><a class="selflink" id="section-2.1" href="#section-2.1">2.1</a> Authorization Information</span>
<span class="h4"><a class="selflink" id="section-2.1.1" href="#section-2.1.1">2.1.1</a> Authorization decisions MUST be able to be based on information</span>
<span class="h4"> about the requestor, the service/method requested, and the operating</span>
environment (authorization information). AAA protocols are required
to transport this information.
This simply states the requirement for a protocol and an access
decision function, which takes inputs, based on the requestor, the
resource requested and the environment.
<span class="h4"><a class="selflink" id="section-2.1.2" href="#section-2.1.2">2.1.2</a> It MUST be possible to represent authorization information as</span>
<span class="h4"> sets of attributes. It MAY be possible to represent authorization</span>
information as objects.
This states that authorization information must be decomposable into
sets of attributes. It is not intended to imply any particular
mechanism for representing attributes.
<span class="h4"><a class="selflink" id="section-2.1.3" href="#section-2.1.3">2.1.3</a> It MUST be possible to package authorization information so that</span>
<span class="h4"> the authorization information for multiple services or applications</span>
can be carried in a single message in a AAA or application protocol.
This states that a protocol, which always required separate AAA
messages/transactions for each service/application, would not meet
the requirement. For example, it should be possible for a single AAA
message/transaction to be sufficient to allow both network and
application access.
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<span class="h4"><a class="selflink" id="section-2.1.4" href="#section-2.1.4">2.1.4</a> Standard attributes types SHOULD be defined which are relevant</span>
<span class="h4"> to many Internet applications/services (e.g. identity information,</span>
group information, ...)
There are many attributes that are used in lots of contexts, and
these should only be defined once, in order to promote
interoperability and prevent duplication of effort.
<span class="h4"><a class="selflink" id="section-2.1.5" href="#section-2.1.5">2.1.5</a> Authorization decisions MUST NOT be limited to being based on</span>
<span class="h4"> identity information, i.e. AAA protocols MUST support the use of</span>
non-identifying information, e.g. to support role based access
control (RBAC).
Authorization based on clearances, roles, groups or other information
is required to be supported. A AAA protocol that only carried
identity information would not meet the requirement.
<span class="h4"><a class="selflink" id="section-2.1.6" href="#section-2.1.6">2.1.6</a> Authorization data MAY include limits in addition to attributes</span>
<span class="h4"> which are directly "owned" by end entities.</span>
This states that some attributes do not simply represent attributes
of an entity, for example a spending limit of IR 1,000 is not an
intrinsic attribute of an entity. This also impacts on the access
decision function, in that the comparison to be made is not a simple
equality match.
<span class="h4"><a class="selflink" id="section-2.1.7" href="#section-2.1.7">2.1.7</a> It MUST be possible for other (non-AAA) protocols to define</span>
<span class="h4"> their own attribute types, which can then be carried within an</span>
authorization package in a AAA or application protocol.
This states that the attributes that are significant in an
authorization decision, may be application protocol dependent. For
example, many attribute types are defined by [<a href="./rfc2138" title=""Remote Authentication Dial In User Service (RADIUS)"">RFC2138</a>] and support
for the semantics of these attributes will be required. Of course,
only AAA entities that are aware of the added attribute types can
make use of them.
<span class="h4"><a class="selflink" id="section-2.1.8" href="#section-2.1.8">2.1.8</a> It SHOULD be possible for administrators of deployed systems to</span>
<span class="h4"> define their own attribute types, which can then be carried within an</span>
authorization package in a AAA or application protocol.
This states that the attributes that are significant in an
authorization decision, may be dependent on a closed environment.
For example, many organizations have a well-defined scheme of
seniority, which can be used to determine access levels. Of course,
only AAA entities that are aware of the added attribute types can
make use of them.
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<span class="h4"><a class="selflink" id="section-2.1.9" href="#section-2.1.9">2.1.9</a> It SHOULD be possible to define new attribute types without</span>
<span class="h4"> central administration and control of attribute name space.</span>
A centralized or distributed registration scheme of some sort is
needed if collisions in attribute type allocations are to be avoided.
However a AAA protocol which always requires use of such a
centralized registration would not meet the requirement. Of course,
collisions should be avoided where possible.
<span class="h4"><a class="selflink" id="section-2.1.10" href="#section-2.1.10">2.1.10</a> It MUST be possible to define attribute types so that an</span>
<span class="h4"> instance of an attribute in a single AAA message can have multiple</span>
values.
This states that a protocol which does not allow multiple instances
of an attribute in a message/transaction would not meet the
requirement. For example it should be possible to have a "group"
attribute which contains more than one groupname (or number or
whatever).
<span class="h4"><a class="selflink" id="section-2.1.11" href="#section-2.1.11">2.1.11</a> If MUST be possible to distinguish different instances of the</span>
<span class="h4"> same authorization attribute type or value, on the basis of "security</span>
domain" or "authority".
This recognizes that it is important to be able to distinguish
between attributes based not only on their value. For example, all NT
domains (which use the English language) have an Administrators
group, an access decision function has to be able to determine to
which of these groups the requestor belongs.
<span class="h4"><a class="selflink" id="section-2.1.12" href="#section-2.1.12">2.1.12</a> AAA protocols MUST specify mechanisms for updating the rules</span>
<span class="h4"> which will be used to control authorization decisions.</span>
This states that a AAA protocol that cannot provide a mechanism for
distributing authorization rules is not sufficient. For example, this
could be used to download ACLs to a PDP.
Note that this is not meant to mean that this AAA protocol mechanism
must always be used, simply that it must be available for use. In
particular, storing authorization rules in a trusted repository (in
many cases an LDAP server) will in many cases be used instead of such
a AAA protocol mechanism. Neither does this requirement call for a
standardized format for authorization rules, merely that there be a
mechanism for transporting these.
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<span class="h4"><a class="selflink" id="section-2.1.13" href="#section-2.1.13">2.1.13</a> The AAA protocol MUST allow for chains of AAA entities to be</span>
<span class="h4"> involved in an authorization decision.</span>
This states that more than one AAA server may have to be involved in
a single authorization decision. This may occur either due to a
decision being spread across more than one "domain" or in order to
distribute authorization within a single "domain".
<span class="h4"><a class="selflink" id="section-2.1.14" href="#section-2.1.14">2.1.14</a> The AAA protocol MUST allow for intermediate AAA entities to add</span>
<span class="h4"> their own local authorization information to a AAA request or</span>
response.
This states that where more than one AAA entity is involved in an
authorization decision each of the AAA entities may manipulate the
AAA messages involved either by adding more information or by
processing parts of the information.
<span class="h4"><a class="selflink" id="section-2.1.15" href="#section-2.1.15">2.1.15</a> AAA entities MAY be either be deployed independently or</span>
<span class="h4"> integrated with application entities.</span>
This states that the AAA entities may either be implemented as AAA
servers or integrated with application entities.
<span class="h4"><a class="selflink" id="section-2.1.16" href="#section-2.1.16">2.1.16</a> The AAA protocol MUST support the creation and encoding of rules</span>
<span class="h4"> that are to be active inside one AAA server based on attributes</span>
published by another AAA server. The level of authorization of the
requesting AAA Server MAY govern the view on attributes.
This states that one AAA entity may have to distribute authorization
rules to another, and that the AAA entity that receives the rules may
only be seeing part of the story.
<span class="h4"><a class="selflink" id="section-2.1.17" href="#section-2.1.17">2.1.17</a> AAA protocols MAY have to support the idea of critical and non-</span>
<span class="h4"> critical attribute types.</span>
This is analogous to the use of the criticality flag in public key
certificate extensions.
<span class="h4"><a class="selflink" id="section-2.1.18" href="#section-2.1.18">2.1.18</a> A AAA protocol MUST allow authorization rules to be expressed in</span>
<span class="h4"> terms of combinations of other authorization rules which have been</span>
evaluated.
For example, access may only be granted if the requestor is member of
the backup users group and not a member of the administrator's group.
Note that this requirement does not state which types of combinations
are to be supported.
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<span class="h4"><a class="selflink" id="section-2.1.19" href="#section-2.1.19">2.1.19</a> It SHOULD be possible to make authorization decisions based on</span>
<span class="h4"> the geographic location of a requestor, service or AAA entity.</span>
This is just an example of an authorization attribute type, notable
because it requires different underlying implementation mechanisms.
<span class="h4"><a class="selflink" id="section-2.1.20" href="#section-2.1.20">2.1.20</a> It SHOULD be possible to make authorization decisions based on</span>
<span class="h4"> the identity or the equipment used by a requestor, service or AAA</span>
entity.
This is just an example of an authorization attribute type, notable
because it may require different underlying implementation mechanisms
(if IPSec isn't available).
<span class="h4"><a class="selflink" id="section-2.1.21" href="#section-2.1.21">2.1.21</a> When there are multiple instances of a given attribute, there</span>
<span class="h4"> must be an unambiguous mechanism by which a receiving peer can</span>
determine the value of specified instance.
<span class="h3"><a class="selflink" id="section-2.2" href="#section-2.2">2.2</a> Security of authorization information</span>
<span class="h4"><a class="selflink" id="section-2.2.1" href="#section-2.2.1">2.2.1</a> It MUST be possible for authorization information to be</span>
<span class="h4"> communicated securely in AAA and application protocols. Mechanisms</span>
that preserve authenticity, integrity and privacy for this
information MUST be specified.
This states that there must be a well-defined method for securing
authorization information, not that such methods must always be used.
Whether support for these mechanisms is to be required for
conformance is left open. In particular, mechanisms must be provided
so that a service administrator in the middle of a chain cannot read
or change authorization information being sent between other AAA
entities.
<span class="h4"><a class="selflink" id="section-2.2.2" href="#section-2.2.2">2.2.2</a> AAA protocols MUST allow for use of an appropriate level of</span>
<span class="h4"> security for authorization information. AAA protocols MUST be able to</span>
support both highly secure and less secure mechanisms for data
integrity/confidentiality etc.
It is important that AAA protocols do not mandate too heavy a
security overhead, thus the security mechanisms specified don't
always need to be used (though not using them may affect the
authorization decision).
<span class="h4"><a class="selflink" id="section-2.2.3" href="#section-2.2.3">2.2.3</a> The security requirements MAY differ between different parts of</span>
<span class="h4"> a package of authorization information.</span>
Some parts may require confidentiality and integrity, some may only
require integrity. This effectively states that we require something
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like selective field security mechanisms. For example, information
required to gain access to a network may have to be in clear, whilst
information required for access to an application within that network
may have to be encrypted in the AAA protocol.
<span class="h4"><a class="selflink" id="section-2.2.4" href="#section-2.2.4">2.2.4</a> AAA protocols MUST provide mechanisms that prevent intermediate</span>
<span class="h4"> administrators breaching security.</span>
This is a basic requirement to prevent man-in-the-middle attacks, for
example where an intermediate administrator changes AAA messages on
the fly.
<span class="h4"><a class="selflink" id="section-2.2.5" href="#section-2.2.5">2.2.5</a> AAA protocols MUST NOT open up replay attacks based on replay of</span>
<span class="h4"> the authorization information.</span>
For example, a AAA protocol should not allow flooding attacks where
the attacker replays AAA messages that require the recipient to use a
lot of CPU or communications before the replay is detected.
<span class="h4"><a class="selflink" id="section-2.2.6" href="#section-2.2.6">2.2.6</a> AAA protocols MUST be capable of leveraging any underlying peer</span>
<span class="h4"> entity authentication mechanisms that may have been applied - this</span>
MAY provide additional assurance that the owner of the authorization
information is the same as the authenticated entity. For example, if
IPSec provides sufficient authentication, then it must be possible to
omit AAA protocol authentication.
<span class="h4"><a class="selflink" id="section-2.2.7" href="#section-2.2.7">2.2.7</a> End-to-end confidentiality, integrity, peer-entity-</span>
<span class="h4"> authentication, or non-repudiation MAY be required for packages of</span>
authorization information.
This states that confidentiality, (resp. the other security
services), may have to be provided for parts of a AAA message, even
where it is transmitted via other AAA entities. It does allow that
such a AAA message may also contain non-confidential, resp. the other
security services), parts. In addition, intermediate AAA entities may
themselves be considered end-points for end-to-end security services
applied to other parts of the AAA message.
<span class="h4"><a class="selflink" id="section-2.2.8" href="#section-2.2.8">2.2.8</a> AAA protocols MUST be usable even in environments where no peer</span>
<span class="h4"> entity authentication is required (e.g. a network address on a secure</span>
LAN may be enough to decide).
This requirement (in a sense the opposite of 2.2.6), indicates the
level of flexibility that is required in order to make the AAA
protocol useful across a broad range of applications/services.
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<span class="h4"><a class="selflink" id="section-2.2.9" href="#section-2.2.9">2.2.9</a> AAA protocols MUST specify "secure" defaults for all protocol</span>
<span class="h4"> options. Implementations of AAA entities MUST use these "secure"</span>
defaults unless otherwise configured/administered.
This states that the out-of-the-box configuration must be "secure",
for example, authorization decisions should result in denial of
access until a AAA entity is configured. Note that the interpretation
of "secure" will vary on a case-by-case basis, though the principle
remains the same.
<span class="h3"><a class="selflink" id="section-2.3" href="#section-2.3">2.3</a> Time</span>
<span class="h4"><a class="selflink" id="section-2.3.1" href="#section-2.3.1">2.3.1</a> Authorization information MUST be timely, which means that it</span>
<span class="h4"> MUST expire and in some cases MAY be revoked before expiry.</span>
This states that authorization information itself is never to be
considered valid for all time, every piece of authorization
information must have associated either an explicit or implicit
validity period or time-to-live.
<span class="h4"><a class="selflink" id="section-2.3.2" href="#section-2.3.2">2.3.2</a> AAA protocols MUST provide mechanisms for revoking authorization</span>
<span class="h4"> information, in particular privileges.</span>
Where the validity or time-to-live is long, it may be necessary to
revoke the authorization information, e.g. where someone leaves a
company. Note that this requirement does not mandate a particular
scheme for revocation, so that it is not a requirement for blacklists
or CRLs.
<span class="h4"><a class="selflink" id="section-2.3.3" href="#section-2.3.3">2.3.3</a> A set of attributes MAY have an associated validity period -</span>
<span class="h4"> such that that the set MUST only be used for authorization decisions</span>
during that period. The validity period may be relatively long, (e.g.
months) or short (hours, minutes).
This states that explicit validity periods are, in some cases, needed
at the field level.
<span class="h4"><a class="selflink" id="section-2.3.4" href="#section-2.3.4">2.3.4</a> Authorization decisions MAY be time sensitive. Support for e.g.</span>
"working hours" or equivalent MUST be possible.
This states that the AAA protocol must be able to support the
transmission of time control attributes, although it does not mandate
that AAA protocols must include a standard way of expressing the
"working hours" type constraint.
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<span class="h4"><a class="selflink" id="section-2.3.5" href="#section-2.3.5">2.3.5</a> It MUST be possible to support authorization decisions that</span>
<span class="h4"> produce time dependent results.</span>
For example, an authorization result may be that service should be
provided for a certain period. In such cases a AAA protocol must be
able to transport this information, possibly as a specific result of
the authorization decision, or, as an additional "termination of
service" AAA message transmitted later.
<span class="h4"><a class="selflink" id="section-2.3.6" href="#section-2.3.6">2.3.6</a> It MUST be possible to support models where the authorization</span>
<span class="h4"> information is issued in well in advance of an authorization decision</span>
rather than near the time of the authorization decision.
This is required in order to support pre-paid (as opposed to
subscription) scenarios (e.g. for VoIP).
<span class="h4"><a class="selflink" id="section-2.3.7" href="#section-2.3.7">2.3.7</a> It SHOULD be possible to support models where the authorization</span>
<span class="h4"> decision is made in advance of a service request.</span>
This is for some applications such as backup, where actions are
scheduled for future dates. It also covers applications that require
reservation of resources.
<span class="h4"><a class="selflink" id="section-2.3.8" href="#section-2.3.8">2.3.8</a> A AAA mechanism must allow time stamp information to be carried</span>
<span class="h4"> along with authorization information (e.g. for non-repudiation).</span>
The PKIX WG is developing a time stamp protocol, which can be used as
part of a non-repudiation solution. In some environments it may be
necessary that certain AAA protocol messages are timestamped (by a
trusted authority) and that the timestamps are forwarded within
subsequent AAA messages.
<span class="h3"><a class="selflink" id="section-2.4" href="#section-2.4">2.4</a> Topology</span>
<span class="h4"><a class="selflink" id="section-2.4.1" href="#section-2.4.1">2.4.1</a> AAA protocols MUST be able to support the use of the push, pull</span>
<span class="h4"> and agent models.</span>
This states that a protocol that only supported one model, say pull,
would not meet the requirements of all the applications. The models
are defined in [<a href="#ref-FRMW" title=""AAA Authorization Framework"">FRMW</a>].
<span class="h4"><a class="selflink" id="section-2.4.2" href="#section-2.4.2">2.4.2</a> In transactions/sessions, which involve more than one AAA</span>
<span class="h4"> entity, each "hop" MAY use a different push/pull/agent model.</span>
For example, in the mobile IP case, a "foreign" AAA server might pull
authorization information from a broker, whereas the broker might
push some authorization information to a "home" AAA server.
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<span class="h4"><a class="selflink" id="section-2.4.3" href="#section-2.4.3">2.4.3</a> AAA Protocols MUST cater for applications and services where the</span>
<span class="h4"> entities involved in the application or AAA protocols belong to</span>
different (security) domains.
This states that it must be possible for any AAA protocol message to
cross security or administrative domain boundaries. Typically, higher
levels of security will be applied when crossing such boundaries, and
accounting mechanisms may also have to be more stringent.
<span class="h4"><a class="selflink" id="section-2.4.4" href="#section-2.4.4">2.4.4</a> AAA protocols MUST support roaming.</span>
Roaming here may also be thought of as "away-from-home" operation.
For example, this is a fundamental requirement for the mobile IP
case.
<span class="h4"><a class="selflink" id="section-2.4.5" href="#section-2.4.5">2.4.5</a> AAA protocols SHOULD support dynamic mobility</span>
Dynamic mobility here means that a client moves from one domain to
another, without having to completely re-establish e.g. whatever AAA
session information is being maintained.
<span class="h4"><a class="selflink" id="section-2.4.6" href="#section-2.4.6">2.4.6</a> An authorization decision MAY have to be made before the</span>
<span class="h4"> requestor has any other connection to a network.</span>
For example, this means that the requestor can't go anywhere on the
network to fetch anything and must do requests via an
application/service or via an intermediate AAA entity. The AAA
protocol should not overexpose such a server to denial-of-service
attacks.
<span class="h4"><a class="selflink" id="section-2.4.7" href="#section-2.4.7">2.4.7</a> AAA protocols MUST support the use of intermediate AAA entities</span>
<span class="h4"> which take part in authorization transactions but which don't "own"</span>
any of the end entities or authorization data.
In some environments (e.g. roamops), these entities are termed
brokers (though these are not the same as bandwidth brokers in the
QoS environment).
<span class="h4"><a class="selflink" id="section-2.4.8" href="#section-2.4.8">2.4.8</a> AAA protocols MAY support cases where an intermediate AAA entity</span>
<span class="h4"> returns a forwarding address to a requestor or AAA entity, in order</span>
that the requestor or originating AAA entity can contact another AAA
entity.
This requirement recognizes that there will be routing issues with
AAA servers, and that this requires that AAA protocols are able to
help with such routing. For example, in the mobile IP case, a broker
may be required, in part to allow the foreign and home AAA servers to
get in contact.
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<span class="h4"><a class="selflink" id="section-2.4.9" href="#section-2.4.9">2.4.9</a> It MUST be possible for an access decision function to discover</span>
<span class="h4"> the AAA server of a requestor. If the requestor provides information</span>
used in this discovery process then the access decision function MUST
be able to verify this information in a trusted manner.
This states that not only do AAA servers have to be able to find one
another, but that sometimes an application entity may have to find an
appropriate AAA server.
<span class="h3"><a class="selflink" id="section-2.5" href="#section-2.5">2.5</a> Application Proxying</span>
<span class="h4"><a class="selflink" id="section-2.5.1" href="#section-2.5.1">2.5.1</a> AAA protocols MUST support cases where applications use proxies,</span>
<span class="h4"> that is, an application entity (C), originates a service request to a</span>
peer (I) and this intermediary (I) also initiates a service request
on behalf of the client (C) to a final target (T). AAA protocols
MUST be such that the authorization decision made at T, MAY depend on
the authorization information associated with C and/or with I. This
"application proxying" must not introduce new security weaknesses in
the AAA protocols. There MAY be chains of application proxies of any
length.
Note that this requirement addresses application layer proxying - not
chains of AAA servers. For example, a chain of HTTP proxies might
each want to restrict the content they serve to the "outside". As
the HTTP GET message goes from HTTP proxy to HTTP proxy, this
requirement states that it must be possible that the authorization
decisions made at each stage can depend on the user at the browser,
and not say, solely on the previous HTTP proxy's identity. Of course
there may only be a single AAA server involved, or there may be many.
<span class="h4"><a class="selflink" id="section-2.5.2" href="#section-2.5.2">2.5.2</a> Where there is a chain of application proxies, the AAA protocol</span>
<span class="h4"> flows at each stage MAY be independent, i.e. the first hop may use</span>
the push model, the second pull, the third the agent model.
This simply restates a previous requirement (no. 2.4.7), to make it
clear that this also applies when application proxying is being used.
<span class="h3"><a class="selflink" id="section-2.6" href="#section-2.6">2.6</a> Trust Model</span>
<span class="h4"><a class="selflink" id="section-2.6.1" href="#section-2.6.1">2.6.1</a> AAA entities MUST be able to make decisions about which other</span>
<span class="h4"> AAA entities are trusted for which sorts of authorization</span>
information.
This is analogous to a requirement in public key infrastructures:
Just because someone can produce a cryptographically correct public
key certificate does not mean that I should trust them for anything,
in particular, I might trust the issuer for some purposes, but not
for others.
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<span class="h4"><a class="selflink" id="section-2.6.2" href="#section-2.6.2">2.6.2</a> AAA protocols MUST allow entities to be trusted for different</span>
<span class="h4"> purposes, trust MUST NOT be an all-or-nothing issue.</span>
This relates the packaging (no. 2.1.3) and trust (no. 2.6.1)
requirements. For example, a AAA entity may trust some parts of an
authorization package but not others.
<span class="h4"><a class="selflink" id="section-2.6.3" href="#section-2.6.3">2.6.3</a> A confirmation of authorization MAY be required in order to</span>
<span class="h4"> initialize or resynchronize a AAA entity.</span>
This states that a AAA entity may need to process some AAA protocol
messages in order to initialize itself. In particular, a AAA entity
may need to check that a previous AAA message remains "valid", e.g.
at boot-time.
<span class="h4"><a class="selflink" id="section-2.6.4" href="#section-2.6.4">2.6.4</a> A negation of static authorization MAY be required to shut down</span>
<span class="h4"> certain services.</span>
This is the converse of 2.6.5 above. It means that a AAA entity may
be "told" by another that a previous AAA message is no longer
"valid". See also 2.3.2 and 2.7.6.
<span class="h4"><a class="selflink" id="section-2.6.5" href="#section-2.6.5">2.6.5</a> It MUST be possible to configure sets of AAA entities that</span>
<span class="h4"> belong to a local domain, so that they are mutually trusting, but so</span>
that any external trust MUST be via some nominated subset of AAA
entities.
This states that for efficiency or organizational reasons, it must be
possible to set up some AAA servers through which all "external" AAA
services are handled. It also states that it must be possible to do
this without over-burdening the "internal-only" AAA servers with
onerous security mechanisms, just because some AAA servers do handle
external relations.
<span class="h4"><a class="selflink" id="section-2.6.6" href="#section-2.6.6">2.6.6</a> Intermediate AAA entities in a chain MUST be able to refuse a</span>
<span class="h4"> connection approved by an earlier entity in the chain.</span>
For example, in mobile IP the home network may authorize a
connection, but the foreign network may refuse to allow the
connection due to the settings chosen by the home network, say if the
home network will refuse to pay.
<span class="h4"><a class="selflink" id="section-2.6.7" href="#section-2.6.7">2.6.7</a> It SHOULD be possible to modify authorization for resources</span>
<span class="h4"> while a session is in progress without destroying other session</span>
information.
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For example, a "parent" AAA server should be able to modify the
authorization state of sessions managed by a "child" AAA server, say
by changing the maximum number of simultaneous sessions which are
allowed.
<span class="h3"><a class="selflink" id="section-2.7" href="#section-2.7">2.7</a> Not just transactions</span>
<span class="h4"><a class="selflink" id="section-2.7.1" href="#section-2.7.1">2.7.1</a> Authorization decisions MAY be context sensitive, AAA protocols</span>
<span class="h4"> MUST enable such decisions.</span>
This states that AAA protocols need to support cases where the
authorization depends, (perhaps even only depends), on the current
state of the system, e.g. only seven sessions allowed, seventh
decision depends on existence of six current sessions. Since the
context might involve more than one service, the AAA protocol is
likely to have to offer some support.
<span class="h4"><a class="selflink" id="section-2.7.2" href="#section-2.7.2">2.7.2</a> AAA protocols SHOULD support both the authorization of</span>
<span class="h4"> transactions and continuing authorization of sessions.</span>
This states that AAA entities may have to maintain state and act when
the state indicates some condition has been met.
<span class="h4"><a class="selflink" id="section-2.7.3" href="#section-2.7.3">2.7.3</a> Within a single session or transaction, it MUST be possible to</span>
<span class="h4"> interleave authentication, authorization and accounting AAA messages.</span>
This states, that e.g. a session may have to use initial
authentication, authorization and accounting AAA message(s), but also
have to include e.g. re-authentication every 30 minutes, or a
continuous "drip-drip" of accounting AAA messages.
<span class="h4"><a class="selflink" id="section-2.7.4" href="#section-2.7.4">2.7.4</a> Authorization decisions may result in a "not ready" answer.</span>
This states that yes and no are not the only outcomes of an
authorization decision. In particular, if the AAA entity cannot yet
give a decision, it might have to return such a result. This is
analogous to how public key certification requests are sometimes
handled in PKI management protocols.
<span class="h4"><a class="selflink" id="section-2.7.5" href="#section-2.7.5">2.7.5</a> A AAA entity MAY re-direct a AAA request that it has received.</span>
This states that if entity "a" asks "b", then "b" may say: "don't ask
me, ask 'c'". This is analogous to HTTP re-direction (status code
307).
<span class="h4"><a class="selflink" id="section-2.7.6" href="#section-2.7.6">2.7.6</a> AAA protocols SHOULD allow a AAA entity to "take back" an</span>
<span class="h4"> authorization.</span>
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The expectation is that AAA protocols will support the ability of a
AAA entity to signal an application or other AAA entity that an
authorization (possibly previously granted by a third AAA entity) is
no longer valid.
<span class="h3"><a class="selflink" id="section-2.8" href="#section-2.8">2.8</a> Administration</span>
<span class="h4"><a class="selflink" id="section-2.8.1" href="#section-2.8.1">2.8.1</a> It MUST be possible for authorization data to be administered on</span>
<span class="h4"> behalf of the end entities and AAA entities.</span>
This requirement indicates that administration of AAA has to be
considered as part of protocol design - a AAA protocol, which
required all AAA entities act independent of all other AAA entities,
would not meet the requirement.
<span class="h4"><a class="selflink" id="section-2.8.2" href="#section-2.8.2">2.8.2</a> Centralizable administration of all features SHOULD be</span>
<span class="h4"> supported.</span>
It should be possible (if it meets the domain requirements) to
centralize or distribute the administration of AAA.
<span class="h4"><a class="selflink" id="section-2.8.3" href="#section-2.8.3">2.8.3</a> AAA protocols SHOULD support cases where the user (as opposed to</span>
<span class="h4"> an administrator) authorizes a transaction.</span>
For example, a user might want to control anti-spam measures or
authorize things like a purchase. In such cases, the user is acting
somewhat like an administrator.
<span class="h4"><a class="selflink" id="section-2.8.4" href="#section-2.8.4">2.8.4</a> One AAA entity MAY create authorization rules for another AAA</span>
<span class="h4"> entity.</span>
This is required to properly support delegation of authority, however
when allowed, this must be able to be done in a secure fashion.
<span class="h4"><a class="selflink" id="section-2.8.5" href="#section-2.8.5">2.8.5</a> AAA protocols SHOULD support failure recovery when one AAA</span>
<span class="h4"> entity in a chain of AAA entities that maintain state about a session</span>
fails.
For example, in a network access situation it may be required that a
AAA server which has crashed be able to determine how many sessions
are in progress, in order to make the "next" authorization decision.
<span class="h4"><a class="selflink" id="section-2.8.6" href="#section-2.8.6">2.8.6</a> It SHOULD be possible for a AAA entity to query the</span>
<span class="h4"> authorization state of another AAA entity.</span>
This may be required as part of a failure recovery procedure.
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<span class="h4"><a class="selflink" id="section-2.8.7" href="#section-2.8.7">2.8.7</a> AAA protocols MUST be able to support "hot fail-over" for server</span>
<span class="h4"> components without loss of state information.</span>
This states that AAA protocols must be able to support cases where,
when a server is no longer operable, a secondary server can
automatically be brought "live" without losing important state
information.
<span class="h3"><a class="selflink" id="section-2.9" href="#section-2.9">2.9</a> Bytes on-the-wire</span>
<span class="h4"><a class="selflink" id="section-2.9.1" href="#section-2.9.1">2.9.1</a> Authorization separate from authentication SHOULD be allowed</span>
<span class="h4"> when necessary, but the AAA protocols MUST also allow for a single</span>
message to request both authentication and authorization.
AAA protocols have to allow a split between authentication and
authorization so that different mechanisms are used for each. This
states that sometimes both types of information need to be carried in
the same message.
<span class="h4"><a class="selflink" id="section-2.9.2" href="#section-2.9.2">2.9.2</a> In order to minimize resource usage (e.g. reduce roundtrips) it</span>
<span class="h4"> MUST be possible to embed AAA PDUs into other protocols.</span>
This states that the AAA protocol authorization packages must be
defined so that they can also be carried in other protocols. For
example, depending on AAA protocol header information in order to
reference an authorization package could cause a protocol to fail to
meet the requirement.
<span class="h4"><a class="selflink" id="section-2.9.3" href="#section-2.9.3">2.9.3</a> A AAA protocol MAY provide mechanisms for replication of state</span>
<span class="h4"> information.</span>
This can be required e.g. to support resiliency in cases where hot
fail-over is required. Note that AAA protocols are of course, subject
to normal protocol design requirements to do with reliability, no
single-point-of-failure etc even though these are not all specified
here.
<span class="h4"><a class="selflink" id="section-2.9.4" href="#section-2.9.4">2.9.4</a> A AAA protocol SHOULD allow the possibility for implementation</span>
<span class="h4"> of a gateway function between the AAA protocol and other legacy AAA</span>
related protocols.
For example, some form of support for [<a href="./rfc2138" title=""Remote Authentication Dial In User Service (RADIUS)"">RFC2138</a>] as a legacy protocol
is very likely to be required. Of course, the use of such a gateway
is almost certain to mean not meeting some other requirements, (e.g.
end-to-end security), for transactions routed through the gateway.
There is no implication that such gateway functionality needs to be a
separate server.
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<span class="h4"><a class="selflink" id="section-2.9.5" href="#section-2.9.5">2.9.5</a> A AAA protocol MUST be able to support use of a wide range of</span>
<span class="h4"> primitive data types, including <a href="./rfc2277">RFC2277</a>.</span>
For example, various sized, signed and unsigned integers, possibly
including multi-precision integers will almost certainly need to be
transported. Floating point support according to ANSI IEEE 754-1985
may also be required.
<span class="h4"><a class="selflink" id="section-2.9.6" href="#section-2.9.6">2.9.6</a> A AAA protocol transport SHOULD support being optimized for a</span>
<span class="h4"> long-term exchange of small packets in a stream between a pair of</span>
hosts.
NASes typically have a high number of transactions/second, so the AAA
protocol MUST allow the flow of requests to be controlled in order
for the server to make efficient use of it's receive buffers.
<span class="h4"><a class="selflink" id="section-2.9.7" href="#section-2.9.7">2.9.7</a> A AAA protocol MUST provide support for load balancing.</span>
In the event that a peer's cannot receive any immediate requests, the
AAA protocol MUST allow for an implementation to balance the load of
requests among a set of peers.
<span class="h3"><a class="selflink" id="section-2.10" href="#section-2.10">2.10</a> Interfaces</span>
<span class="h4"><a class="selflink" id="section-2.10.1" href="#section-2.10.1">2.10.1</a> It SHOULD be possible that authorization data can be used for</span>
<span class="h4"> application purposes.</span>
For example, in web access, if the authorization data includes a
group name, mechanisms to make this data available to the application
so that it can modify the URL originally requested are desirable.
<span class="h4"><a class="selflink" id="section-2.10.2" href="#section-2.10.2">2.10.2</a> It SHOULD be possible that authorization data can be used to</span>
<span class="h4"> mediate the response to a request.</span>
For example, with web access the clearance attribute value may affect
the content of the HTTP response message.
<span class="h4"><a class="selflink" id="section-2.10.3" href="#section-2.10.3">2.10.3</a> AAA protocols SHOULD be able to operate in environments where</span>
<span class="h4"> requestors are not pre-registered (at least for authorization</span>
purposes, but possibly also for authentication purposes).
This is necessary to be able to scale a AAA solution where there are
many requestors.
<span class="h4"><a class="selflink" id="section-2.10.4" href="#section-2.10.4">2.10.4</a> AAA protocols MUST be able to support a linkage between</span>
<span class="h4"> authorization and accounting mechanisms.</span>
Motherhood and apple-pie.
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<span class="h4"><a class="selflink" id="section-2.10.5" href="#section-2.10.5">2.10.5</a> AAA protocols MUST be able to support accountability</span>
(audit/non-repudiation) mechanisms.
Sometimes, an authorization decision will be made where the requestor
has not authenticated. In such cases, it must be possible that the
authorization data used is linked to audit or other accountability
mechanisms. Note that this requirement does not call for mandatory
support for digital signatures, or other parts of a non-repudiation
solution.
<span class="h3"><a class="selflink" id="section-2.11" href="#section-2.11">2.11</a> Negotiation</span>
<span class="h4"><a class="selflink" id="section-2.11.1" href="#section-2.11.1">2.11.1</a> AAA protocols MUST support the ability to refer to sets of</span>
<span class="h4"> authorization packages in order to allow peers negotiate a common</span>
set.
Given that peers may support different combinations of authorization
attribute types and packages, the requirement states that protocol
support is required to ensure that the peers use packages supported
by both peers.
<span class="h4"><a class="selflink" id="section-2.11.2" href="#section-2.11.2">2.11.2</a> It MUST be possible to negotiate authorization packages between</span>
<span class="h4"> AAA entities that are not in direct communication.</span>
This states that where, e.g. a broker is involved, the end AAA
servers might still need to negotiate.
<span class="h4"><a class="selflink" id="section-2.11.3" href="#section-2.11.3">2.11.3</a> Where negotiation fails to produce an acceptable common</span>
<span class="h4"> supported set then access MUST be denied.</span>
For example, a server cannot grant access if it cannot understand the
attributes of the requestor.
<span class="h4"><a class="selflink" id="section-2.11.4" href="#section-2.11.4">2.11.4</a> Where negotiation fails to produce an acceptable common</span>
<span class="h4"> supported set then it SHOULD be possible to generate an error</span>
indication to be sent to another AAA entity.
If negotiation fails, then some administrator intervention is often
required, and protocol support for this should be provided.
<span class="h4"><a class="selflink" id="section-2.11.5" href="#section-2.11.5">2.11.5</a> It MUST be possible to pre-provision the result of a</span>
<span class="h4"> negotiation, but in such cases, the AAA protocol MUST include a</span>
confirmation of the "negotiation result".
Even if the supported packages of a peer are configured, this must be
confirmed before assuming both sides are similarly configured.
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<span class="h4"><a class="selflink" id="section-2.11.6" href="#section-2.11.6">2.11.6</a> For each application making use of a AAA protocol, there MUST be</span>
<span class="h4"> one inter-operable IETF standards-track specification of the</span>
authorization package types that are "mandatory to implement".
This requirement assures that communicating peers can count on
finding at least one IETF specified inter-operable AAA protocol
dialect provided they are doing authorization for a common
application specific problem domain. This does not preclude the
negotiation of commonly understood but private AAA protocol
authorization package types (e.g. vendor specific).
<span class="h4"><a class="selflink" id="section-2.11.7" href="#section-2.11.7">2.11.7</a> It SHOULD also be possible to rank AAA negotiation options in</span>
<span class="h4"> order of preference.</span>
This states that, when negotiating, peers must be able to indicate
preferences as well as capabilities.
<span class="h4"><a class="selflink" id="section-2.11.8" href="#section-2.11.8">2.11.8</a> The negotiation mechanisms used by AAA protocols SHOULD NOT be</span>
<span class="h4"> vulnerable to a "bidding-down" attack.</span>
A "bidding-down" attack is where an attacker forces the negotiating
parties to choose the "weakest" option available. This is analogous
to forcing 40-bit encryption on a link. The requirement highlights
that protocol support is needed to prevent such attacks, for example
by including the negotiation messages as part of a later MAC
calculation, if authentication has produced a shared secret.
<span class="h4"><a class="selflink" id="section-2.11.9" href="#section-2.11.9">2.11.9</a> A peer MUST NOT send an attribute within an authorization</span>
<span class="h4"> package or attribute that was not agreed to by a prior successful</span>
negotiation. If this AAA protocol violation occurs, then it MUST be
possible to send an error indication to the misbehaving peer, and
generate an error indication to the network operator.
<span class="h4"><a class="selflink" id="section-2.11.10" href="#section-2.11.10">2.11.10</a> A peer MUST declare all of the sets of the authorization</span>
<span class="h4"> packages that it understands in its initial negotiation bid message.</span>
<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>. Security Considerations</span>
This document includes specific security requirements.
This document does not state any detailed requirements for the
interplay with authentication, accounting or accountability (audit).
A AAA protocol, which meets all of the above requirements, may still
leave vulnerabilities due to such interactions. Such issues must be
considered as part of AAA protocol design.
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<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>. References</span>
[<a id="ref-FRMW">FRMW</a>] Vollbrecht, J., Calhoun, P., Farrell, S., Gommans, L.,
Gross, G., de Bruijn, B., de Laat, C., Holdrege, M. and D.
Spence, "AAA Authorization Framework", <a href="./rfc2904">RFC 2904</a>, August
2000.
[<a id="ref-RFC2026">RFC2026</a>] Bradner, S., "The Internet Standards Process -- Revision
3", <a href="https://www.rfc-editor.org/bcp/bcp9">BCP 9</a>, <a href="./rfc2026">RFC 2026</a>, October 1996.
[<a id="ref-RFC2119">RFC2119</a>] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", <a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a>, <a href="./rfc2119">RFC 2119</a>, March 1997.
[<a id="ref-RFC2138">RFC2138</a>] Rigney, C., Rubens, A., Simpson, W. and S. Willens,
"Remote Authentication Dial In User Service (RADIUS)", <a href="./rfc2138">RFC</a>
<a href="./rfc2138">2138</a>, April 1997.
[<a id="ref-RFC2277">RFC2277</a>] Alvestrand, H., "IETF Policy on Character Sets and
Languages", <a href="./rfc2277">RFC 2277</a>, January 1998.
[<a id="ref-SAMP">SAMP</a>] Vollbrecht, J., Calhoun, P., Farrell, S., Gommans, L.,
Gross, G., de Bruijn, B., de Laat, C., Holdrege, M. and D.
Spence, "AAA Authorization Application Examples", <a href="./rfc2905">RFC</a>
<a href="./rfc2905">2905</a>, August 2000.
Authors' Addresses
Stephen Farrell
Baltimore Technologies
61/62 Fitzwilliam Lane
Dublin 2,
IRELAND
Phone: +353-1-647-7300
Fax: +353-1-647-7499
EMail: [email protected]
John R. Vollbrecht
Interlink Networks, Inc.
775 Technology Drive, Suite 200
Ann Arbor, MI 48108
USA
Phone: +1 734 821 1205
Fax: +1 734 821 1235
EMail: [email protected]
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<span class="grey"><a href="./rfc2906">RFC 2906</a> AAA Authorization Requirements August 2000</span>
Pat R. Calhoun
Network and Security Research
Center, Sun Labs
Sun Microsystems, Inc.
15 Network Circle
Menlo Park, California, 94025
USA
Phone: +1 650 786 7733
Fax: +1 650 786 6445
EMail: [email protected]
Leon Gommans
Enterasys Networks EMEA
Kerkplein 24
2841 XM Moordrecht
The Netherlands
Phone: +31 182 379279
email: [email protected]
or at University of Utrecht:
[email protected]
George M. Gross
Lucent Technologies
184 Liberty Corner Road, m.s.
LC2N-D13
Warren, NJ 07059
USA
Phone: +1 908 580 4589
Fax: +1 908-580-4991
EMail: [email protected]
Betty de Bruijn
Interpay Nederland B.V.
Eendrachtlaan 315
3526 LB Utrecht
The Netherlands
Phone: +31 30 2835104
EMail: [email protected]
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<span class="grey"><a href="./rfc2906">RFC 2906</a> AAA Authorization Requirements August 2000</span>
Cees T.A.M. de Laat
Physics and Astronomy dept.
Utrecht University
Pincetonplein 5,
3584CC Utrecht
Netherlands
Phone: +31 30 2534585
Phone: +31 30 2537555
EMail: [email protected]
Matt Holdrege
ipVerse
223 Ximeno Ave.
Long Beach, CA 90803
EMail: [email protected]
David W. Spence
Interlink Networks, Inc.
775 Technology Drive, Suite 200
Ann Arbor, MI 48108
USA
Phone: +1 734 821 1203
Fax: +1 734 821 1235
EMail: [email protected]
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Full Copyright Statement
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Farrell, et al. Informational [Page 23]
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