    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.

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    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.

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       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.

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    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.

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    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.

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    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.

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    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

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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.

    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.

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    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.

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    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.

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    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.

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    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.

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    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.

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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.

    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.

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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.

    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.

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    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.

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    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.

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    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.

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    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.

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    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

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    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

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    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

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    RFC 2906             AAA Authorization Requirements          August 2000

    Full Copyright Statement

       Copyright (C) The Internet Society (2000).  All Rights Reserved.

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       The limited permissions granted above are perpetual and will not be
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    Acknowledgement

       Funding for the RFC Editor function is currently provided by the
       Internet Society.

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