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rfc:rfc2906

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
                   AAA Authorization Requirements

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.

Farrell, et al. Informational [Page 1] RFC 2906 AAA Authorization Requirements August 2000

Table Of Contents

 1. Introduction.................................................2
 2. Requirements.................................................3
     2.1  Authorization Information..............................3
     2.2  Security of authorization information..................7
     2.3  Time...................................................9
     2.4  Topology..............................................10
     2.5  Application Proxying..................................12
     2.6  Trust Model...........................................12
     2.7  Not just transactions.................................14
     2.8  Administration........................................15
     2.9  Bytes on-the-wire.....................................16
     2.10 Interfaces............................................17
     2.11 Negotiation...........................................18
 3. Security Considerations.....................................19
 4. References..................................................20
 Authors' Addresses.............................................20
 Full Copyright Statement.......................................23

1. Introduction

 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 [FRMW]
       AAA Authorization Requirements (this document)
       AAA Authorization Application Examples [SAMP]
 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 [SAMP] based on a common understanding of the AAA
 authorization framework [FRMW]. This document assumes familiarity
 with both the general issues involved in authorization and, in
 particular, the reader will benefit from a reading of [FRMW] where,
 for example, definitions of terms can be found.

Farrell, et al. Informational [Page 2] RFC 2906 AAA Authorization Requirements August 2000

 The key words "MUST", "REQUIRED", "SHOULD", "RECOMMENDED", and "MAY"
 in this document are to be interpreted as described in [RFC2119].

2. Requirements

 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 [FRMW].
 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 [SAMP].
 The requirements presented are not intended to be "orthogonal", that
 is, some of them repeat, or overlap, with others.

2.1 Authorization Information

2.1.1 Authorization decisions MUST be able to be based on information

 about the requestor, the service/method requested, and the operating
 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.

2.1.2 It MUST be possible to represent authorization information as

 sets of attributes. It MAY be possible to represent authorization
 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.

2.1.3 It MUST be possible to package authorization information so that

 the authorization information for multiple services or applications
 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.

Farrell, et al. Informational [Page 3] RFC 2906 AAA Authorization Requirements August 2000

2.1.4 Standard attributes types SHOULD be defined which are relevant

 to many Internet applications/services (e.g.  identity information,
 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.

2.1.5 Authorization decisions MUST NOT be limited to being based on

 identity information, i.e. AAA protocols MUST support the use of
 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.

2.1.6 Authorization data MAY include limits in addition to attributes

 which are directly "owned" by end entities.
 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.

2.1.7 It MUST be possible for other (non-AAA) protocols to define

 their own attribute types, which can then be carried within an
 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 [RFC2138] 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.

2.1.8 It SHOULD be possible for administrators of deployed systems to

 define their own attribute types, which can then be carried within an
 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.

Farrell, et al. Informational [Page 4] RFC 2906 AAA Authorization Requirements August 2000

2.1.9 It SHOULD be possible to define new attribute types without

 central administration and control of attribute name space.
 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.

2.1.10 It MUST be possible to define attribute types so that an

 instance of an attribute in a single AAA message can have multiple
 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).

2.1.11 If MUST be possible to distinguish different instances of the

 same authorization attribute type or value, on the basis of "security
 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.

2.1.12 AAA protocols MUST specify mechanisms for updating the rules

 which will be used to control authorization decisions.
 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.

Farrell, et al. Informational [Page 5] RFC 2906 AAA Authorization Requirements August 2000

2.1.13 The AAA protocol MUST allow for chains of AAA entities to be

 involved in an authorization decision.
 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".

2.1.14 The AAA protocol MUST allow for intermediate AAA entities to add

 their own local authorization information to a AAA request or
 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.

2.1.15 AAA entities MAY be either be deployed independently or

 integrated with application entities.
 This states that the AAA entities may either be implemented as AAA
 servers or integrated with application entities.

2.1.16 The AAA protocol MUST support the creation and encoding of rules

 that are to be active inside one AAA server based on attributes
 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.

2.1.17 AAA protocols MAY have to support the idea of critical and non-

 critical attribute types.
 This is analogous to the use of the criticality flag in public key
 certificate extensions.

2.1.18 A AAA protocol MUST allow authorization rules to be expressed in

 terms of combinations of other authorization rules which have been
 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.

Farrell, et al. Informational [Page 6] RFC 2906 AAA Authorization Requirements August 2000

2.1.19 It SHOULD be possible to make authorization decisions based on

 the geographic location of a requestor, service or AAA entity.
 This is just an example of an authorization attribute type, notable
 because it requires different underlying implementation mechanisms.

2.1.20 It SHOULD be possible to make authorization decisions based on

 the identity or the equipment used by a requestor, service or AAA
 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).

2.1.21 When there are multiple instances of a given attribute, there

 must be an unambiguous mechanism by which a receiving peer can
 determine the value of specified instance.

2.2 Security of authorization information

2.2.1 It MUST be possible for authorization information to be

 communicated securely in AAA and application protocols.  Mechanisms
 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.

2.2.2 AAA protocols MUST allow for use of an appropriate level of

 security for authorization information. AAA protocols MUST be able to
 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).

2.2.3 The security requirements MAY differ between different parts of

 a package of authorization information.
 Some parts may require confidentiality and integrity, some may only
 require integrity. This effectively states that we require something

Farrell, et al. Informational [Page 7] RFC 2906 AAA Authorization Requirements August 2000

 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.

2.2.4 AAA protocols MUST provide mechanisms that prevent intermediate

 administrators breaching security.
 This is a basic requirement to prevent man-in-the-middle attacks, for
 example where an intermediate administrator changes AAA messages on
 the fly.

2.2.5 AAA protocols MUST NOT open up replay attacks based on replay of

 the authorization information.
 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.

2.2.6 AAA protocols MUST be capable of leveraging any underlying peer

 entity authentication mechanisms that may have been applied - this
 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.

2.2.7 End-to-end confidentiality, integrity, peer-entity-

 authentication, or non-repudiation MAY be required for packages of
 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.

2.2.8 AAA protocols MUST be usable even in environments where no peer

 entity authentication is required (e.g. a network address on a secure
 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.

Farrell, et al. Informational [Page 8] RFC 2906 AAA Authorization Requirements August 2000

2.2.9 AAA protocols MUST specify "secure" defaults for all protocol

 options. Implementations of AAA entities MUST use these "secure"
 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.

2.3 Time

2.3.1 Authorization information MUST be timely, which means that it

 MUST expire and in some cases MAY be revoked before expiry.
 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.

2.3.2 AAA protocols MUST provide mechanisms for revoking authorization

 information, in particular privileges.
 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.

2.3.3 A set of attributes MAY have an associated validity period -

 such that that the set MUST only be used for authorization decisions
 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.

2.3.4 Authorization decisions MAY be time sensitive. Support for e.g.

 "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.

Farrell, et al. Informational [Page 9] RFC 2906 AAA Authorization Requirements August 2000

2.3.5 It MUST be possible to support authorization decisions that

 produce time dependent results.
 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.

2.3.6 It MUST be possible to support models where the authorization

 information is issued in well in advance of an authorization decision
 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).

2.3.7 It SHOULD be possible to support models where the authorization

 decision is made in advance of a service request.
 This is for some applications such as backup, where actions are
 scheduled for future dates. It also covers applications that require
 reservation of resources.

2.3.8 A AAA mechanism must allow time stamp information to be carried

 along with authorization information (e.g. for non-repudiation).
 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.

2.4 Topology

2.4.1 AAA protocols MUST be able to support the use of the push, pull

 and agent models.
 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 [FRMW].

2.4.2 In transactions/sessions, which involve more than one AAA

 entity, each "hop" MAY use a different push/pull/agent model.
 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.

Farrell, et al. Informational [Page 10] RFC 2906 AAA Authorization Requirements August 2000

2.4.3 AAA Protocols MUST cater for applications and services where the

 entities involved in the application or AAA protocols belong to
 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.

2.4.4 AAA protocols MUST support roaming.

 Roaming here may also be thought of as "away-from-home" operation.
 For example, this is a fundamental requirement for the mobile IP
 case.

2.4.5 AAA protocols SHOULD support dynamic mobility

 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.

2.4.6 An authorization decision MAY have to be made before the

 requestor has any other connection to a network.
 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.

2.4.7 AAA protocols MUST support the use of intermediate AAA entities

 which take part in authorization transactions but which don't "own"
 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).

2.4.8 AAA protocols MAY support cases where an intermediate AAA entity

 returns a forwarding address to a requestor or AAA entity, in order
 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.

Farrell, et al. Informational [Page 11] RFC 2906 AAA Authorization Requirements August 2000

2.4.9 It MUST be possible for an access decision function to discover

 the AAA server of a requestor. If the requestor provides information
 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.

2.5 Application Proxying

2.5.1 AAA protocols MUST support cases where applications use proxies,

 that is, an application entity (C), originates a service request to a
 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.

2.5.2 Where there is a chain of application proxies, the AAA protocol

 flows at each stage MAY be independent, i.e. the first hop may use
 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.

2.6 Trust Model

2.6.1 AAA entities MUST be able to make decisions about which other

 AAA entities are trusted for which sorts of authorization
 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.

Farrell, et al. Informational [Page 12] RFC 2906 AAA Authorization Requirements August 2000

2.6.2 AAA protocols MUST allow entities to be trusted for different

 purposes, trust MUST NOT be an all-or-nothing issue.
 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.

2.6.3 A confirmation of authorization MAY be required in order to

 initialize or resynchronize a AAA entity.
 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.

2.6.4 A negation of static authorization MAY be required to shut down

 certain services.
 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.

2.6.5 It MUST be possible to configure sets of AAA entities that

 belong to a local domain, so that they are mutually trusting, but so
 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.

2.6.6 Intermediate AAA entities in a chain MUST be able to refuse a

 connection approved by an earlier entity in the chain.
 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.

2.6.7 It SHOULD be possible to modify authorization for resources

 while a session is in progress without destroying other session
 information.

Farrell, et al. Informational [Page 13] RFC 2906 AAA Authorization Requirements August 2000

 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.

2.7 Not just transactions

2.7.1 Authorization decisions MAY be context sensitive, AAA protocols

 MUST enable such decisions.
 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.

2.7.2 AAA protocols SHOULD support both the authorization of

 transactions and continuing authorization of sessions.
 This states that AAA entities may have to maintain state and act when
 the state indicates some condition has been met.

2.7.3 Within a single session or transaction, it MUST be possible to

 interleave authentication, authorization and accounting AAA messages.
 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.

2.7.4 Authorization decisions may result in a "not ready" answer.

 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.

2.7.5 A AAA entity MAY re-direct a AAA request that it has received.

 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).

2.7.6 AAA protocols SHOULD allow a AAA entity to "take back" an

 authorization.

Farrell, et al. Informational [Page 14] RFC 2906 AAA Authorization Requirements August 2000

 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.

2.8 Administration

2.8.1 It MUST be possible for authorization data to be administered on

 behalf of the end entities and AAA entities.
 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.

2.8.2 Centralizable administration of all features SHOULD be

 supported.
 It should be possible (if it meets the domain requirements) to
 centralize or distribute the administration of AAA.

2.8.3 AAA protocols SHOULD support cases where the user (as opposed to

 an administrator) authorizes a transaction.
 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.

2.8.4 One AAA entity MAY create authorization rules for another AAA

 entity.
 This is required to properly support delegation of authority, however
 when allowed, this must be able to be done in a secure fashion.

2.8.5 AAA protocols SHOULD support failure recovery when one AAA

 entity in a chain of AAA entities that maintain state about a session
 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.

2.8.6 It SHOULD be possible for a AAA entity to query the

 authorization state of another AAA entity.
 This may be required as part of a failure recovery procedure.

Farrell, et al. Informational [Page 15] RFC 2906 AAA Authorization Requirements August 2000

2.8.7 AAA protocols MUST be able to support "hot fail-over" for server

 components without loss of state information.
 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.

2.9 Bytes on-the-wire

2.9.1 Authorization separate from authentication SHOULD be allowed

 when necessary, but the AAA protocols MUST also allow for a single
 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.

2.9.2 In order to minimize resource usage (e.g. reduce roundtrips) it

 MUST be possible to embed AAA PDUs into other protocols.
 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.

2.9.3 A AAA protocol MAY provide mechanisms for replication of state

 information.
 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.

2.9.4 A AAA protocol SHOULD allow the possibility for implementation

 of a gateway function between the AAA protocol and other legacy AAA
 related protocols.
 For example, some form of support for [RFC2138] 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.

Farrell, et al. Informational [Page 16] RFC 2906 AAA Authorization Requirements August 2000

2.9.5 A AAA protocol MUST be able to support use of a wide range of

 primitive data types, including RFC2277.
 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.

2.9.6 A AAA protocol transport SHOULD support being optimized for a

 long-term exchange of small packets in a stream between a pair of
 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.

2.9.7 A AAA protocol MUST provide support for load balancing.

 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.

2.10 Interfaces

2.10.1 It SHOULD be possible that authorization data can be used for

 application purposes.
 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.

2.10.2 It SHOULD be possible that authorization data can be used to

 mediate the response to a request.
 For example, with web access the clearance attribute value may affect
 the content of the HTTP response message.

2.10.3 AAA protocols SHOULD be able to operate in environments where

 requestors are not pre-registered (at least for authorization
 purposes, but possibly also for authentication purposes).
 This is necessary to be able to scale a AAA solution where there are
 many requestors.

2.10.4 AAA protocols MUST be able to support a linkage between

 authorization and accounting mechanisms.
 Motherhood and apple-pie.

Farrell, et al. Informational [Page 17] RFC 2906 AAA Authorization Requirements August 2000

2.10.5 AAA protocols MUST be able to support accountability

 (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.

2.11 Negotiation

2.11.1 AAA protocols MUST support the ability to refer to sets of

 authorization packages in order to allow peers negotiate a common
 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.

2.11.2 It MUST be possible to negotiate authorization packages between

 AAA entities that are not in direct communication.
 This states that where, e.g. a broker is involved, the end AAA
 servers might still need to negotiate.

2.11.3 Where negotiation fails to produce an acceptable common

 supported set then access MUST be denied.
 For example, a server cannot grant access if it cannot understand the
 attributes of the requestor.

2.11.4 Where negotiation fails to produce an acceptable common

 supported set then it SHOULD be possible to generate an error
 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.

2.11.5 It MUST be possible to pre-provision the result of a

 negotiation, but in such cases, the AAA protocol MUST include a
 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.

Farrell, et al. Informational [Page 18] RFC 2906 AAA Authorization Requirements August 2000

2.11.6 For each application making use of a AAA protocol, there MUST be

 one inter-operable IETF standards-track specification of the
 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).

2.11.7 It SHOULD also be possible to rank AAA negotiation options in

 order of preference.
 This states that, when negotiating, peers must be able to indicate
 preferences as well as capabilities.

2.11.8 The negotiation mechanisms used by AAA protocols SHOULD NOT be

 vulnerable to a "bidding-down" attack.
 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.

2.11.9 A peer MUST NOT send an attribute within an authorization

 package or attribute that was not agreed to by a prior successful
 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.

2.11.10 A peer MUST declare all of the sets of the authorization

 packages that it understands in its initial negotiation bid message.

3. Security Considerations

 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.

Farrell, et al. Informational [Page 19] RFC 2906 AAA Authorization Requirements August 2000

4. References

 [FRMW]     Vollbrecht, J., Calhoun, P., Farrell, S., Gommans, L.,
            Gross, G., de Bruijn, B., de Laat, C., Holdrege, M. and D.
            Spence, "AAA Authorization Framework", RFC 2904, August
            2000.
 [RFC2026]  Bradner, S., "The Internet Standards Process -- Revision
            3", BCP 9, RFC 2026, October 1996.
 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC2138]  Rigney, C., Rubens, A., Simpson, W. and S. Willens,
            "Remote Authentication Dial In User Service (RADIUS)", RFC
            2138, April 1997.
 [RFC2277]  Alvestrand, H., "IETF Policy on Character Sets and
            Languages", RFC 2277, January 1998.
 [SAMP]     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", RFC
            2905, 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: stephen.farrell@baltimore.ie
 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: jrv@interlinknetworks.com

Farrell, et al. Informational [Page 20] RFC 2906 AAA Authorization Requirements August 2000

 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:  pcalhoun@eng.sun.com
 Leon Gommans
 Enterasys Networks EMEA
 Kerkplein 24
 2841 XM  Moordrecht
 The Netherlands
 Phone: +31 182 379279
 email: gommans@cabletron.com
        or at University of Utrecht:
        l.h.m.gommans@phys.uu.nl
 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:  gmgross@lucent.com
 Betty de Bruijn
 Interpay Nederland B.V.
 Eendrachtlaan 315
 3526 LB Utrecht
 The Netherlands
 Phone: +31 30 2835104
 EMail: betty@euronet.nl

Farrell, et al. Informational [Page 21] RFC 2906 AAA Authorization Requirements August 2000

 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: delaat@phys.uu.nl
 Matt Holdrege
 ipVerse
 223 Ximeno Ave.
 Long Beach, CA 90803
 EMail: matt@ipverse.com
 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: dspence@interlinknetworks.com

Farrell, et al. Informational [Page 22] RFC 2906 AAA Authorization Requirements August 2000

Full Copyright Statement

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 This document and translations of it may be copied and furnished to
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 or assist in its implementation may be prepared, copied, published
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 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
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 followed, or as required to translate it into languages other than
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 The limited permissions granted above are perpetual and will not be
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 This document and the information contained herein is provided on an
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 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
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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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