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Versions: 00 01 02 03 04 05 06 07 08 09 10 RFC 4072

Network Working Group                                     P. Eronen, Ed.
Internet-Draft                                                     Nokia
Expires: December 29, 2003                                     T. Hiller
                                                     Lucent Technologies
                                                                 G. Zorn
                                                           Cisco Systems
                                                           June 30, 2003


     Diameter Extensible Authentication Protocol (EAP) Application
                       draft-ietf-aaa-eap-02.txt

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that other
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   Internet-Drafts are draft documents valid for a maximum of six months
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   The list of current Internet-Drafts can be accessed at http://
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   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on December 29, 2003.

Copyright Notice

   Copyright (C) The Internet Society (2003). All Rights Reserved.

Abstract

   The Extensible Authentication Protocol (EAP) provides a standard
   mechanism for support of various authentication methods.  This
   document defines the Command-Codes and AVPs necessary to carry EAP
   packets between a Network Access Server (NAS) and a back-end
   authentication server.

Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",



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   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

Table of Contents

   1.    Introduction . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.    Extensible Authentication Protocol Support in Diameter . . .  4
   2.1   Advertising application support  . . . . . . . . . . . . . .  4
   2.2   Protocol Overview  . . . . . . . . . . . . . . . . . . . . .  5
   2.3   Sessions and NASREQ interaction  . . . . . . . . . . . . . .  7
   2.3.1 Scenario 1: Direct connection  . . . . . . . . . . . . . . .  8
   2.3.2 Scenario 2: Direct connection with redirects . . . . . . . .  9
   2.3.3 Scenario 3: Direct EAP, authorization via agents . . . . . . 10
   2.3.4 Scenario 4: Proxy agents . . . . . . . . . . . . . . . . . . 12
   2.4   Invalid packets  . . . . . . . . . . . . . . . . . . . . . . 12
   2.5   Retransmission . . . . . . . . . . . . . . . . . . . . . . . 13
   2.6   Fragmentation  . . . . . . . . . . . . . . . . . . . . . . . 14
   2.7   Accounting . . . . . . . . . . . . . . . . . . . . . . . . . 14
   2.8   Usage guidelines . . . . . . . . . . . . . . . . . . . . . . 14
   2.8.1 User-Name AVP  . . . . . . . . . . . . . . . . . . . . . . . 14
   2.8.2 Conflicting AVPs . . . . . . . . . . . . . . . . . . . . . . 15
   2.8.3 Displayable messages . . . . . . . . . . . . . . . . . . . . 15
   2.8.4 Role reversal  . . . . . . . . . . . . . . . . . . . . . . . 15
   2.8.5 Alternative Uses . . . . . . . . . . . . . . . . . . . . . . 15
   3.    Command-Codes  . . . . . . . . . . . . . . . . . . . . . . . 16
   3.1   Diameter-EAP-Request (DER) Command . . . . . . . . . . . . . 16
   3.2   Diameter-EAP-Answer (DEA) Command  . . . . . . . . . . . . . 17
   4.    Attribute-Value Pairs  . . . . . . . . . . . . . . . . . . . 19
   4.1   New AVPs . . . . . . . . . . . . . . . . . . . . . . . . . . 19
   4.1.1 EAP-Payload AVP  . . . . . . . . . . . . . . . . . . . . . . 19
   4.1.2 EAP-Reissued-Payload AVP . . . . . . . . . . . . . . . . . . 19
   4.1.3 EAP-MTU AVP  . . . . . . . . . . . . . . . . . . . . . . . . 20
   4.1.4 EAP-Master-Session-Key AVP . . . . . . . . . . . . . . . . . 20
   4.1.5 Accounting-EAP-Auth-Method AVP . . . . . . . . . . . . . . . 20
   5.    AVP Occurrence Tables  . . . . . . . . . . . . . . . . . . . 20
   5.1   EAP Command AVP Table  . . . . . . . . . . . . . . . . . . . 20
   5.2   Accounting AVP Table . . . . . . . . . . . . . . . . . . . . 22
   6.    RADIUS/Diameter interactions . . . . . . . . . . . . . . . . 22
   6.1   RADIUS Request forwarded as Diameter Request . . . . . . . . 22
   6.2   Diameter Request forwarded as RADIUS Request . . . . . . . . 23
   6.3   Accounting Requests  . . . . . . . . . . . . . . . . . . . . 24
   7.    IANA Considerations  . . . . . . . . . . . . . . . . . . . . 25
   8.    Security Considerations  . . . . . . . . . . . . . . . . . . 25
   8.1   Authorization  . . . . . . . . . . . . . . . . . . . . . . . 26
   8.1.1 Direct connection, NAS point of view . . . . . . . . . . . . 27
   8.1.2 Direct connection, server point of view  . . . . . . . . . . 29
   8.1.3 Diameter agents  . . . . . . . . . . . . . . . . . . . . . . 29
   8.2   Attacks by compromised nodes . . . . . . . . . . . . . . . . 29



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   8.2.1 Impersonating as the user (NAS, agents)  . . . . . . . . . . 30
   8.2.2 Impersonating as the network (NAS, agents) . . . . . . . . . 30
   8.2.3 Privacy issues (NAS, agents) . . . . . . . . . . . . . . . . 31
   8.2.4 Offline cryptographic attacks (NAS, agents)  . . . . . . . . 31
   8.2.5 AVP editing (NAS, agents, server)  . . . . . . . . . . . . . 31
   8.2.6 Negotiation attacks (NAS, agents, server)  . . . . . . . . . 33
   8.2.7 Session key distribution (agents, server)  . . . . . . . . . 33
   9.    Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 34
         Normative References . . . . . . . . . . . . . . . . . . . . 34
         Informative References . . . . . . . . . . . . . . . . . . . 34
         Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 35
   A.    Changelog  . . . . . . . . . . . . . . . . . . . . . . . . . 36
         Intellectual Property and Copyright Statements . . . . . . . 39






































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1. Introduction

   The Extensible Authentication Protocol (EAP), defined in
   [RFC2284bis], is an authentication framework which supports multiple
   authentication mechanisms.  EAP may be used on dedicated links as
   well as switched circuits, and wired as well as wireless links.

   To date, EAP has been implemented with hosts and routers that connect
   via switched circuits or dial-up lines using PPP [RFC1661], IEEE 802
   wired switches [IEEE-802.1X], and IEEE 802.11 wireless access points
   [IEEE-802.11i].  EAP has also been adopted for IPsec remote access in
   IKEv2 [IKEv2].

   This document specifies the Diameter EAP application that carries EAP
   packets between a Network Access Server (NAS) working as an EAP
   Authenticator and a back-end authentication server.  The Diameter EAP
   application is based on NASREQ and is intended for similar
   environments as NASREQ.

   In Diameter EAP application, authentication occurs between the EAP
   client and its home Diameter server.  This end-to-end authentication
   reduces the possibility for fraudulent authentication, such as replay
   and man-in-the-middle attacks.  End-to-end authentication also
   provides a possibility for mutual authentication, which is not
   possible with PAP and CHAP in a roaming PPP environment.

   Diameter EAP application relies heavily on [NASREQ], and in earlier
   drafts was part of the Diameter NASREQ application.  It can also be
   used in conjunction with NASREQ, selecting the application based on
   the used authentication mechanism (EAP or PAP/CHAP).  Diameter EAP
   application defines new Command-Codes and new AVPs, and can work
   together with RADIUS EAP support [RFC2869bis].

2. Extensible Authentication Protocol Support in Diameter

2.1 Advertising application support

   Diameter nodes conforming to this specification MAY advertise support
   by including the value of TBD in the Auth-Application-Id AVP of the
   Capabilities-Exchange-Request and Capabilities-Exchange-Answer
   command [BASE].

   If the NAS receives a response with the Result-Code set to
   DIAMETER_APPLICATION_UNSUPPORTED [BASE], it is an indication that the
   Diameter server in the home realm does not support EAP.  If possible,
   the access device MAY attempt to negotiate another authentication
   protocol, such as PAP or CHAP.  An access device SHOULD be cautious
   when determining whether a less secure authentication protocol will



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   be used, since this could be a result of a bidding down attack (see
   Section 8.2.6).

2.2 Protocol Overview

   The EAP conversation between the authenticating peer and the access
   device begins with the initiation of EAP within a link layer, such as
   PPP [RFC1661] or IEEE 802.1X [IEEE-802.1X].  Once EAP has been
   initiated, the access device will typically send to the Diameter
   server a Diameter-EAP-Request message with an empty EAP-Payload AVP,
   signifying an EAP-Start.

   If the Diameter home server is willing to do EAP authentication, it
   respons with a Diameter-EAP-Answer message containing an EAP-Payload
   AVP that includes an encapsulated EAP packet.  The Result-Code AVP
   set to DIAMETER_MULTI_ROUND_AUTH, signifying that a subsequent
   request is expected.  The EAP payload is forwarded by the access
   device to the EAP client.  This is illustrated in the diagram below.

   User                             NAS                           Server
    |                                |                                |
    |        (initiate EAP)          |                                |
    |<------------------------------>|                                |
    |                                | Diameter-EAP-Request           |
    |                                | EAP-Payload(EAP Start)         |
    |                                |------------------------------->|
    |                                |                                |
    |                                |            Diameter-EAP-Answer |
    |                           Result-Code=DIAMETER_MULTI_ROUND_AUTH |
    |                                |    EAP-Payload(EAP Request #1) |
    |                                |<-------------------------------|
    |                 EAP Request #1 |                                |
    |<-------------------------------|                                |
    :                                :                                :
    :                        ...continues...                          :

   The initial Diameter-EAP-Answer in a multi-round exchange normally
   includes an EAP-Request/Identity, requesting the EAP client to
   identify itself.  Upon receipt of the EAP client's EAP-Response, the
   access device will then issue a second Diameter-EAP-Request message,
   with the client's EAP payload encapsulated within the EAP-Payload
   AVP.

   A preferred approach is for the access device to issue the
   EAP-Request/Identity message to the EAP client, and forward the
   EAP-Response/Identity packet, encapsulated within the EAP-Payload
   AVP, as a Diameter-EAP-Request to the Diameter server (see the
   diagram below).  This alternative reduces the number of Diameter



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   message round trips.  When the EAP-Request/Identity message is issued
   by the access device, it SHOULD interpret the EAP-Response/Identity
   packet returned by the authenticating peer, and copy its value to a
   User-Name AVP in Diameter-EAP-Request.  This is useful in roaming
   environments, since the Destination-Realm is needed for routing
   purposes.  Note that this alternative cannot be universally employed,
   as there are circumstances where a user's identity is not needed
   (such as when authorization occurs based on a calling or called phone
   number).

   User                             NAS                           Server
    |                                |                                |
    |        (initiate EAP)          |                                |
    |<------------------------------>|                                |
    |                                |                                |
    |          EAP Request(Identity) |                                |
    |<-------------------------------|                                |
    |                                |                                |
    | EAP Response(Identity)         |                                |
    |------------------------------->|                                |
    |                                | Diameter-EAP-Request           |
    |                                | EAP-Payload(EAP Response)      |
    |                                |------------------------------->|
    :                                :                                :
    :                        ...continues...                          :

   The conversation continues until the Diameter server sends a
   Diameter-EAP-Answer with a Result-Code AVP indicating success or
   failure, and an optional EAP-Payload.  The Result-Code AVP is used by
   the access device to determine whether service is to be provided to
   the EAP client.  The access device MUST NOT rely on the contents of
   the optional EAP-Payload to determine whether service is to be
   provided.


















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    :                        ...continued...                          :
    :                                :                                :
    | EAP Response #N                |                                |
    |------------------------------->|                                |
    |                                | Diameter-EAP-Request           |
    |                                | EAP-Payload(EAP Response #N)   |
    |                                |------------------------------->|
    |                                |                                |
    |                                |            Diameter-EAP-Answer |
    |                                |   Result-Code=DIAMETER_SUCCESS |
    |                                |       EAP-Payload(EAP Success) |
    |                                |       [EAP-Master-Session-Key] |
    |                                |           (authorization AVPs) |
    |                                |<-------------------------------|
    |                                |                                |
    |                    EAP Success |                                |
    |<-------------------------------|                                |

   If authorization was requested, a Diameter-EAP-Answer with
   Result-Code set to DIAMETER_SUCCESS MUST also include the appropriate
   authorization AVPs required for the service requested (see Section 5
   and [NASREQ]).  If the Result-Code DIAMETER_LIMITED_SUCCESS is
   returned, this means that the NAS has to get additional authorization
   AVPs using a separate NASREQ request.  This case is described in
   Section TBD below.  Diameter-EAP-Answer messages whose Result-Code
   AVP is set to DIAMETER_MULTI_ROUND_AUTH MAY include authorization
   AVPs.

   A Diameter-EAP-Answer with succesful Result-Code MAY also include an
   EAP-Master-Session-Key AVP that contains keying material for
   protecting the communication between the user and the NAS.  Exactly
   how this keying material is used depends on the link layer in
   question, is beyond the scope of this document.

   A home Diameter server MAY request EAP re-authentication by issuing
   the Re-Auth-Request [BASE] message to the Diameter client.

   Should an EAP authentication session be interrupted due to a home
   server failure, the session MAY be directed to an alternate server,
   but the authentication session will have to be restarted from the
   beginning.

2.3 Sessions and NASREQ interaction

   (NOTE: This section has not received sufficient WG discussion yet,
   and is likely to be changed in the future.)

   The previous section introduced the basic protocol between the NAS



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   and the home server.  Since the Diameter-EAP-Answer message may
   include a Master Session Key (MSK) for protecting the communication
   between the user and the NAS, care must be taken to ensure that this
   key does not fall into wrong hands.

   Basic Diameter security mechanisms (IPsec and TLS) protect Diameter
   messages hop-by-hop.  Since there are currently no end-to-end
   (NAS-to-home server) security mechanisms defined for Diameter, this
   section describes some possible scenarios how the messages could be
   transported protected using these hop-by-hop mechanisms.

   The list of scenarios is not intended to be exhaustive, and it is
   possible to combine them.  For instance, the first proxy agent after
   the NAS could use redirects as in scenario 2 to bypass any additional
   proxy agents.

2.3.1 Scenario 1: Direct connection

   The simplest case is when the NAS contacts the home server directly.
   All the authorization AVPs are delivered by the home server, as is
   EAP keying material.

   NAS                                                       home server
    |                                                                 |
    | Diameter-EAP-Request                                            |
    | Auth-Request-Type=AUTHORIZE_AUTHENTICATE                        |
    | EAP-Payload(EAP Start)                                          |
    |---------------------------------------------------------------->|
    |                                                                 |
    |                                             Diameter-EAP-Answer |
    |                           Result-Code=DIAMETER_MULTI_ROUND_AUTH |
    |                                        EAP-Payload(EAP Request) |
    |<----------------------------------------------------------------|
    |                                                                 |
    :              ...more EAP Request/Response pairs...              :
    |                                                                 |
    | Diameter-EAP-Request                                            |
    | EAP-Payload(EAP Response)                                       |
    |---------------------------------------------------------------->|
    |                                                                 |
    |                                             Diameter-EAP-Answer |
    |                                    Result-Code=DIAMETER_SUCCESS |
    |                                        EAP-Payload(EAP Success) |
    |                                          EAP-Master-Session-Key |
    |                                            (authorization AVPs) |
    |<----------------------------------------------------------------|

   This scenario is the most likely to be used in small networks, or in



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   cases where Diameter agents are not needed to provide routing or
   additional authorization AVPs.

2.3.2 Scenario 2: Direct connection with redirects

   In this scenario the NAS uses a redirect agent to locate the home
   server, and the rest of the session proceeds as before.

   NAS                      Local redirect agent             Home server
    |                                |                                |
    | Diameter-EAP-Request           |                                |
    | Auth-Request-Type=AUTHORIZE_AUTHENTICATE                        |
    | EAP-Payload(EAP Start)         |                                |
    |------------------------------->|                                |
    |                                |                                |
    |                       Diameter-EAP-Answer                       |
    |      Redirect-Host=homeserver.example.com                       |
    | Redirect-Host-Usage=REALM_AND_APPLICATION                       |
    |<-------------------------------|                                |
    |                                :                                |
    | Diameter-EAP-Request          :                                 |
    | Auth-Request-Type=AUTHORIZE_AUTHENTICATE                        |
    | EAP-Payload(EAP Start)        :                                 |
    |---------------------------------------------------------------->|
    |                                :                                |
    :      ...rest of the session continues as in first case...       :
    :                                :                                :

   The advantage of this scenario is that knowledge of realms and home
   servers is centralized to a redirect agent, and it is not necessary
   to modify the NAS configuration when, e.g., a new roaming agreement
   is done.



















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2.3.3 Scenario 3: Direct EAP, authorization via agents

   In this scenario the EAP authentication is done directly with the
   home server (with Auth-Request-Type set to AUTHENTICATE_ONLY), and
   the authorization AVPs are retrieved from the local proxy agents.
   This scenario is intended for environments where the home server
   cannot provide all the necessary authorization AVPs to the NAS.

   NAS                       Local proxy agent               Home server
    |                                :                                |
    | Diameter-EAP-Request           :                                |
    | Auth-Request-Type=AUTHENTICATE_ONLY                             |
    | EAP-Payload(EAP Start)         :                                |
    |---------------------------------------------------------------->|
    |                                :                                |
    |                                :            Diameter-EAP-Answer |
    |                           Result-Code=DIAMETER_MULTI_ROUND_AUTH |
    |                                :       EAP-Payload(EAP Request) |
    |<----------------------------------------------------------------|
    |                                :                                |
    :              ...more EAP Request/Response pairs...              :
    |                                :                                |
    | Diameter-EAP-Request           :                                |
    | EAP-Payload(EAP Response)      :                                |
    |---------------------------------------------------------------->|
    |                                :                                |
    |                                :            Diameter-EAP-Answer |
    |                                :   Result-Code=DIAMETER_SUCCESS |
    |                                :       EAP-Payload(EAP Success) |
    |                                :         EAP-Master-Session-Key |
    |                                :           (authorization AVPs) |
    |<----------------------------------------------------------------|
    |                                |                                |
    | AA-Request                     |                                |
    | Auth-Request-Type=AUTHORIZE_ONLY                                |
    | (some AVPs from first session) |                                |
    |------------------------------->|                                |
    |                                |                                |
    |                      AA-Answer |                                |
    |   Result-Code=DIAMETER_SUCCESS |                                |
    |           (authorization AVPs) |                                |
    |<-------------------------------|                                |

   The NASREQ application is used here for authorization because the
   realm-specific routing table does support routing based on
   application, but not more...TO BE CLARIFIED.

   A second possibility is that the home server signals the NAS to



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   perform a separate authorization step.  In this case, the NAS begins
   the Diameter EAP session with
   Auth-Request-Type=AUTHORIZE_AUTHENTICATE.  The last
   Diameter-EAP-Answer from the home server contains
   Result-Code=DIAMETER_LIMITED_SUCCESS, so the NAS does additional
   AUTHORIZE_ONLY NASREQ step.

   NAS                       Local proxy agent               Home server
    |                                :                                |
    | Diameter-EAP-Request           :                                |
    | Auth-Request-Type=AUTHORIZE_AUTHENTICATE                        |
    | EAP-Payload(EAP Start)         :                                |
    |---------------------------------------------------------------->|
    |                                :                                |
    |                                :            Diameter-EAP-Answer |
    |                           Result-Code=DIAMETER_MULTI_ROUND_AUTH |
    |                                :       EAP-Payload(EAP Request) |
    |<----------------------------------------------------------------|
    |                                :                                |
    :              ...more EAP Request/Response pairs...              :
    |                                :                                |
    | Diameter-EAP-Request           :                                |
    | EAP-Payload(EAP Response)      :                                |
    |---------------------------------------------------------------->|
    |                                :                                |
    |                                :            Diameter-EAP-Answer |
    |                            Result-Code=DIAMETER_LIMITED_SUCCESS |
    |                                :       EAP-Payload(EAP Success) |
    |                                :         EAP-Master-Session-Key |
    |                                :           (authorization AVPs) |
    |<----------------------------------------------------------------|
    |                                |                                |
    | AA-Request                     |                                |
    | Auth-Request-Type=AUTHORIZE_ONLY                                |
    | (some AVPs from first session) |                                |
    |------------------------------->|                                |
    |                                |                                |
    |                      AA-Answer |                                |
    |   Result-Code=DIAMETER_SUCCESS |                                |
    |           (authorization AVPs) |                                |
    |<-------------------------------|                                |










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2.3.4 Scenario 4: Proxy agents

   Same as scenario 1, but through proxies.  Note that in this case the
   proxies can see the EAP session keys, so this is not suitable for
   environments where proxies can't be trusted for this.

   NAS                    Local proxy/relay agent            Home server
    |                                |                                |
    |  Diameter-EAP-Request          |                                |
    |  Auth-Request-Type=AUTHORIZE_AUTHENTICATE                       |
    |  EAP-Payload(EAP Start)        |                                |
    |------------------------------->|------------------------------->|
    |                                |                                |
    |                                |           Diameter-EAP-Answer  |
    |                          Result-Code=DIAMETER_MULTI_ROUND_AUTH  |
    |                                |      EAP-Payload(EAP Request)  |
    |<-------------------------------|<-------------------------------|
    |                                :                                |
    :              ...more EAP Request/Response pairs...              :
    |                                :                                |
    |  Diameter-EAP-Request          |                                |
    |  EAP-Payload(EAP Response)     |                                |
    |------------------------------->|------------------------------->|
    |                                |                                |
    |                                |           Diameter-EAP-Answer  |
    |                                |  Result-Code=DIAMETER_SUCCESS  |
    |                                |      EAP-Payload(EAP Success)  |
    |                                |        EAP-Master-Session-Key  |
    |                                |          (authorization AVPs)  |
    |<-------------------------------|<-------------------------------|


2.4 Invalid packets

   While acting as a pass-through, the NAS MUST validate the EAP header
   fields (Code, Identifier, Length) prior to forwarding an EAP packet
   to or from the Diameter server.  On receiving an EAP packet from the
   peer, the NAS checks the Code (2) and Length fields, and matches the
   Identifier value against the current Identifier, supplied by the
   Diameter server in the most recently validated EAP Request.  On
   receiving an EAP packet from the Diameter server (encapsulated within
   a Diameter-EAP-Answer), the NAS checks the Code (1) and Length
   fields, then updates the current Identifier value.  Pending EAP
   Responses that do not match the current Identifier value are silently
   discarded by the NAS.

   Since EAP method fields (Type, Type-Data) are typically not validated
   by a NAS operating as a pass-through, despite these checks it is



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   possible for a NAS to forward an invalid EAP packet to or from the
   Diameter server.

   A Diameter server receiving an EAP-Payload AVP it does not understand
   SHOULD make the determination of whether the error is fatal or
   non-fatal based on the EAP Type.  A Diameter server determining that
   a fatal error has occurred MUST send an a Diameter-EAP-Answer with a
   failure Result-Code and an EAP-Payload AVP encapsulating an EAP
   Failure packet.  A Diameter server determining that a non-fatal error
   has occurred MUST send a Diameter-EAP-Answer with an
   EAP-Reissued-Payload AVP encapsulating the previous EAP Request sent
   by the server.

   When receiving a Diameter-EAP-Answer with an EAP-Reissued-Payload
   AVP, the NAS SHOULD discard the EAP-Response packet most recently
   transmitted to the Diameter server and check whether additional EAP
   Response packets have been received matching the current Identifier
   value.  If so, a new EAP Response packet, if available, MUST be sent
   to the Diameter server within an Diameter-EAP-Request.  If no EAP
   Response packet is available, then the EAP Request encapsulated
   within the EAP-Reissued-Payload AVP is sent to the peer, and the
   retransmission timer is reset.

   In order to provide protection against Denial of Service (DoS)
   attacks, it is advisable for the NAS to allocate a finite buffer for
   EAP packets received from the peer, and to discard packets according
   to an appropriate policy once that buffer has been exceeded.  Also,
   the Diameter server is advised to permit only a modest number of
   invalid EAP packets within a single session, prior to terminating the
   session with TBD.  By default a value of 5 invalid EAP packets is
   recommended.

2.5 Retransmission

   As noted in [RFC2284bis], if an EAP packet is lost in transit between
   the authenticating peer and the NAS (or vice versa), the NAS will
   retransmit.

   It may be necessary to adjust retransmission strategies and
   authentication timeouts in certain cases.  For example, when a token
   card is used, additional time may be required to allow the user to
   find the card and enter the token.  Since the NAS will typically not
   have knowledge of the required parameters, these need to be provided
   by the Diameter server.

   If a Multi-Round-Time-Out AVP [BASE] is present in an
   Diameter-EAP-Answer message that also contains an EAP-Payload AVP,
   that value is used to set the EAP retransmission timer for that EAP



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   Request, and that Request alone.

2.6 Fragmentation

   Using the EAP-Payload AVP, it is possible for the Diameter server to
   encapsulate an EAP packet that is larger than the MTU on the link
   between the NAS and the peer.  Since it is not possible for the
   Diameter server to use MTU discovery to ascertain the link MTU, an
   EAP-MTU attribute may be included in a Diameter-EAP-Request message
   so as to provide the Diameter server with this information.

   A Diameter server having received an EAP-MTU attribute in a
   Diameter-EAP-Request message MUST NOT send any subsequent packet in
   this EAP conversation containing EAP-Payload attribute whose length
   exceeds the length specified by the EAP-MTU value.

2.7 Accounting

   This document specifies one additional AVP for accounting messages.
   One or more Accounting-EAP-Auth-Method AVPs (see Section 4.1.5) MAY
   be included in Accounting-Request messages to indicate the EAP
   method(s) used to authenticate the user.

   If the NAS has authenticated the user with a locally implemented EAP
   method, it knows the method used and SHOULD include it in an
   Accounting-EAP-Auth-Method AVP.

   If the authentication was done using Diameter-EAP-Request/Answer
   messages, the Diameter server SHOULD include one more more
   Accounting-EAP-Auth-Method AVPs in Diameter-EAP-Answer packets with a
   successful result code.  In this case, the NAS SHOULD include these
   AVPs in Accounting-Request messages.

2.8 Usage guidelines

2.8.1 User-Name AVP

   Unless the access device interprets the EAP-Response/Identity packet
   returned by the authenticating peer, it will not have access to the
   user's identity.  Furthermore, some EAP methods support identity
   protection where the user's real identity is not included in
   EAP-Response/Identity.  Therefore, the Diameter Server SHOULD return
   the user's identity by inserting it in the User-Name AVP of
   subsequent Diameter-EAP-Answer packets.  Without the user's identity,
   the Session-Id AVP MAY be used for accounting and billing, however
   operationally this could be very difficult to manage.





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2.8.2 Conflicting AVPs

   A Diameter-EAP-Answer message containing an EAP-Payload of type
   EAP-Success or EAP-Failure MUST NOT have the Result-Code AVP set to
   DIAMETER_MULTI_ROUND_AUTH.  Also, the Result-Code SHOULD match the
   contained EAP packet (successful Result-Code if EAP-Success, and a
   failure Result-Code for EAP-Failure).  TO BE WRITTEN: clarify this.

2.8.3 Displayable messages

   The Reply-Message AVP [NASREQ] contains text which may be displayed
   to the user.  Note that the NAS does not necessarily have any
   facility for actually sending these messages to the user.  In any
   case, the NAS MUST NOT manufacture any EAP packets (such as
   EAP-Request/Notification) from Reply-Message AVPs.

2.8.4 Role reversal

   Some environments where EAP is used, such as PPP, support
   peer-to-peer operation.  That is, both parties act as authenticators
   and authenticatees at the same time, in two simultaneous and
   independent EAP conversations.

   This specification is intended for communication between EAP
   (passthrough) authenticator and backend authentication server.  A
   Diameter client MUST NOT send a Diameter-EAP-Request encapsulating an
   EAP Request packet, and a Diameter server receiving such packet MUST
   respond with a failure Result-Code..

2.8.5 Alternative Uses

   Currently the conversation between the backend authentication server
   and the Diameter server is proprietary because of lack of
   standardization.  In order to increase standardization and provide
   interoperability between Diameter vendors and backend security
   vendors, it is recommended that Diameter-encapsulated EAP be used for
   this conversation.

   This has the advantage of allowing the Diameter server to support EAP
   without the need for authentication-specific code within the Diameter
   server.  Authentication-specific code can then reside on a back-end
   authentication server instead.

   In the case where Diameter-encapsulated EAP is used in a conversation
   between a Diameter server and a backend authentication server, the
   latter will typically return an Diameter-EAP-Answer/EAP-Payload/
   EAP-Success message without inclusion of the expected authorization
   AVPs required in a successful response.  This means that the Diameter



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   server MUST add these attributes prior to sending an
   Diameter-EAP-Answer/EAP-Payload/EAP-Success message to the access
   device.

3. Command-Codes

   This section defines new Command-Code values that MUST be supported
   by all Diameter implementations conforming to this specification.
   The following Command Codes are defined in this section:

      Command-Name             Abbrev.    Code       Reference
      --------------------------------------------------------
      Diameter-EAP-Request      DER       268          3.1
      Diameter-EAP-Answer       DEA       268          3.2


3.1 Diameter-EAP-Request (DER) Command

   The Diameter-EAP-Request (DER) command, indicated by the Command-Code
   field set to 268 and the 'R' bit set in the Command Flags field, is
   sent by a Diameter client to a Diameter server and conveys an
   EAP-Response from the EAP client.  The Diameter-EAP-Request MUST
   contain one EAP-Payload AVP, which contains the actual EAP payload.
   An EAP-Payload AVP with no data MAY be sent to the Diameter server to
   initiate an EAP authentication session.

   The DER message MAY be the result of a multi-round authentication
   exchange, which occurs when the DEA is received with the Result-Code
   AVP set to DIAMETER_MULTI_ROUND_AUTH [BASE].  A subsequent DER
   message MUST include any State AVPs [NASREQ] that were present in the
   DEA.  For re-authentication, it is recommended that the Identity
   request be skipped in order to reduce the number of authentication
   round trips.  This is only possible when the user's identity is
   already known by the home Diameter server.

   Message format

      <Diameter-EAP-Request> ::= < Diameter Header: 268, REQ, PXY >
                                 < Session-Id >
                                 { Auth-Application-Id }
                                 { Origin-Host }
                                 { Origin-Realm }
                                 { Destination-Realm }
                                 { Auth-Request-Type }
                                 [ NAS-Port ]
                                 [ NAS-Port-Id ]
                                 [ Origin-State-Id ]
                                 [ Destination-Host ]



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                                 [ NAS-Identifier ]
                                 [ NAS-IP-Address ]
                                 [ NAS-IPv6-Address ]
                                 [ NAS-Port-Type ]
                                 [ Port-Limit ]
                                 [ User-Name ]
                                 { EAP-Payload }
                                 { EAP-MTU }
                                 [ Service-Type ]
                                 [ Idle-Timeout ]
                                 [ State ]
                                 [ Authorization-Lifetime ]
                                 [ Auth-Grace-Period ]
                                 [ Auth-Session-State ]
                                 [ Session-Timeout ]
                                 [ Callback-Number ]
                                 [ Called-Station-Id ]
                                 [ Calling-Station-Id ]
                               * [ Class ]
                                 [ Originating-Line-Info ]
                                 [ Connect-Info ]
                               * [ Framed-Compression ]
                                 [ Framed-Interface-Id ]
                                 [ Framed-IP-Address ]
                               * [ Framed-IPv6-Prefix ]
                                 [ Framed-IP-Netmask ]
                                 [ Framed-MTU ]
                                 [ Framed-Protocol ]
                               * [ Tunneling ]
                               * [ Proxy-Info ]
                               * [ Route-Record ]
                               * [ AVP ]


3.2 Diameter-EAP-Answer (DEA) Command

   The Diameter-EAP-Answer (DEA) message, indicated by the Command-Code
   field set to 268 and the 'R' bit cleared in the Command Flags field,
   is sent by the Diameter server to the client for one of the following
   reasons:

   1.  The message is part of a multi-round authentication exchange, and
       the server is expecting a subsequent Diameter-EAP-Request.  This
       is indicated by setting the Result-Code to
       DIAMETER_MULTI_ROUND_AUTH, and MAY include zero or more State
       AVPs.

   2.  the EAP client has been successfully authenticated and



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       authorized, in which case the message MUST include the
       Result-Code AVP indicating success, and SHOULD include an
       EAP-Payload of type EAP-Success.  This event MUST cause the
       access device to provide service to the EAP client.

   3.  The EAP client has not been successfully authenticated and/or
       authorized, and the Result-Code AVP is set to indicate failure.
       This message SHOULD include an EAP-Payload, but this AVP is not
       used to determine whether service is to be provided.

   If the message from the Diameter client included a request for
   authorization, a successful response MUST include the authorization
   AVPs that are relevant to the service being provided.

   Message format


      <Diameter-EAP-Answer> ::= < Diameter Header: 268, PXY >
                                < Session-Id >
                                { Auth-Application-Id }
                                { Auth-Request-Type }
                                { Result-Code }
                                { Origin-Host }
                                { Origin-Realm }
                                [ User-Name ]
                                [ EAP-Payload ]
                                [ Multi-Round-Time-Out ]
                                [ Service-Type ]
                              * [ Class ]
                              * [ Configuration-Token ]
                                [ Acct-Interim-Interval ]
                                [ Error-Message ]
                                [ Error-Reporting-Host ]
                                [ Idle-Timeout ]
                                [ Authorization-Lifetime ]
                                [ Auth-Grace-Period ]
                                [ Auth-Session-State ]
                                [ Re-Auth-Request-Type ]
                                [ Session-Timeout ]
                                [ State ]
                              * [ Reply-Message ]
                                [ Termination-Action ]
                                [ Origin-State-Id ]
                              * [ Filter-Id ]
                                [ Port-Limit ]
                                [ Callback-Id ]
                                [ Callback-Number ]
                                [ Framed-Appletalk-Link ]



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                              * [ Framed-Appletalk-Network ]
                                [ Framed-Appletalk-Zone ]
                              * [ Framed-Compression ]
                                [ Framed-Interface-Id ]
                                [ Framed-IP-Address ]
                              * [ Framed-IPv6-Prefix ]
                                [ Framed-IPv6-Pool ]
                              * [ Framed-IPv6-Route ]
                                [ Framed-IP-Netmask ]
                              * [ Framed-Route ]
                                [ Framed-Pool ]
                                [ Framed-IPX-Network ]
                                [ Framed-MTU ]
                                [ Framed-Protocol ]
                                [ Framed-Routing ]
                              * [ NAS-Filter-Rule ]
                              * [ Tunneling ]
                              * [ Redirect-Host ]
                                [ Redirect-Host-Usage ]
                                [ Redirect-Max-Cache-Time ]
                              * [ Proxy-Info ]
                              * [ AVP ]


4. Attribute-Value Pairs

   This section both defines new AVPs, unique to the EAP Diameter
   application and describes the usage of AVPs defined elsewhere if that
   usage in the EAP application is noteworthy.

4.1 New AVPs

4.1.1 EAP-Payload AVP

   The EAP-Payload AVP (AVP Code 402) is of type OctetString and is used
   to encapsulate the actual EAP packet that is being exchanged between
   the EAP client and the home Diameter server.

4.1.2 EAP-Reissued-Payload AVP

   The EAP-Reissued-Payload AVP (AVP Code TBD) is of type OctetString.
   This AVP MAY be included in Diameter-EAP-Answer messages to signal
   the NAS that the EAP packet in it sent was not a satisfactory
   response (see Section 2.4 for discussion).  To ease RADIUS
   translation, this AVP contains the previous EAP packet sent by the
   Diameter server.





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4.1.3 EAP-MTU AVP

   The EAP-MTU AVP (AVP Code TBD) is of type Unsigned32.  Its use is
   described in Section 2.6.

4.1.4 EAP-Master-Session-Key AVP

   The EAP-Master-Session-Key AVP (AVP Code TBD) is of type OctetString.
   It is used by the Diameter server...TBD

4.1.5 Accounting-EAP-Auth-Method AVP

   The Accounting-EAP-Auth-Method AVP (AVP Code 401) is of type
   Unsigned64.  In case of expanded types [RFC2284bis, Section 5.7], the
   least significant 32 bits contain the Vendor-Type field, and the next
   24 bits contain the Vendor-Id field.

   The use of this AVP is described in Section 2.7.

5. AVP Occurrence Tables

   The following tables use these symbols:

      0     The AVP MUST NOT be present in the message
      0+    Zero or more instances of the AVP MAY be present in the
            message
      0-1   Zero or one instance of the AVP MAY be present in the
            message
      1     One instance of the AVP MUST be present in the message

   Note that AVPs that can only be present within a Grouped AVP are not
   represented in these tables.

5.1 EAP Command AVP Table

   The following table lists the AVPs that may be present in the DER and
   DEA Commands, defined in this document; however, the AVPs listed are
   defined both here and in [NASREQ].

                                       +---------------+
                                       |  Command-Code |
                                       |-------+-------+
   Attribute Name                      |  DER  |  DEA  |
   ------------------------------------|-------+-------|
   Accounting-EAP-Auth-Method          |   0   |   0+  |
   Acct-Interim-Interval [BASE]        |   0   |  0-1  |
   Auth-Application-Id [BASE]          |   1   |   1   |
   Auth-Grace-Period [BASE]            |  0-1  |  0-1  |



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   Auth-Request-Type [BASE]            |   1   |   1   |
   Auth-Session-State [BASE]           |  0-1  |  0-1  |
   Authorization-Lifetime [BASE]       |  0-1  |  0-1  |
   Callback-Id [NASREQ]                |   0   |  0-1  |
   Callback-Number [NASREQ]            |  0-1  |  0-1  |
   Called-Station-Id [NASREQ]          |  0-1  |   0   |
   Calling-Station-Id [NASREQ]         |  0-1  |   0   |
   Class [BASE]                        |   0+  |   0+  |
   Configuration-Token [NASREQ]        |   0   |   0+  |
   Connect-Info [NASREQ]               |  0-1  |   0   |
   Destination-Host [BASE]             |  0-1  |   0   |
   Destination-Realm [BASE]            |   1   |   0   |
   EAP-Payload                         |   1   |  0-1  |
   EAP-MTU                             |  0-1  |   0   |
   Error-Message [BASE]                |   0   |  0-1  |
   Error-Reporting-Host [BASE]         |   0   |  0-1  |
   Failed-AVP [BASE]                   |   0+  |   0+  |
   Filter-Id [NASREQ]                  |   0   |   0+  |
   Framed-Appletalk-Link [NASREQ]      |   0   |  0-1  |
   Framed-Appletalk-Network [NASREQ]   |   0   |   0+  |
   Framed-Appletalk-Zone [NASREQ]      |   0   |  0-1  |
   Framed-Compression [NASREQ]         |   0+  |   0+  |
   Framed-Interface-Id [NASREQ]        |  0-1  |  0-1  |
   Framed-IP-Address [NASREQ]          |  0-1  |  0-1  |
   Framed-IP-Netmask [NASREQ]          |  0-1  |  0-1  |
   Framed-IPv6-Prefix [NASREQ]         |   0+  |   0+  |
   Framed-IPv6-Pool [NASREQ]           |   0   |  0-1  |
   Framed-IPv6-Route [NASREQ]          |   0   |   0+  |
   Framed-IPX-Network [NASREQ]         |   0   |  0-1  |
   Framed-MTU [NASREQ]                 |  0-1  |  0-1  |
   Framed-Pool [NASREQ]                |   0   |  0-1  |
   Framed-Protocol [NASREQ]            |  0-1  |  0-1  |
   Framed-Route [NASREQ]               |   0   |   0+  |
   Framed-Routing [NASREQ]             |   0   |  0-1  |
   Idle-Timeout [NASREQ]               |  0-1  |  0-1  |
   Multi-Round-Time-Out [BASE]         |   0   |  0-1  |
   NAS-Filter-Rule [NASREQ]            |   0   |   0+  |
   NAS-Identifier [NASREQ]             |  0-1  |   0   |
   NAS-IP-Address [NASREQ]             |  0-1  |   0   |
   NAS-IPv6-Address [NASREQ]           |  0-1  |   0   |
   NAS-Port [NASREQ]                   |  0-1  |   0   |
   NAS-Port-Id [NASREQ]                |  0-1  |   0   |
   NAS-Port-Type [NASREQ]              |  0-1  |   0   |
   NAS-Session-Key [NASREQ]            |   0   |   0+  |
   Originating-Line-Info [NASREQ]      |  0-1  |   0   |
   Origin-Host [BASE]                  |   1   |   1   |
   Origin-Realm [BASE]                 |   1   |   1   |
   Origin-State-Id [BASE]              |  0-1  |  0-1  |



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   Port-Limit [NASREQ]                 |  0-1  |  0-1  |
   Proxy-Info [BASE]                   |   0+  |   0+  |
   Re-Auth-Request-Type [BASE]         |   0   |  0-1  |
   Redirect-Host [BASE]                |   0   |   0+  |
   Redirect-Host-Usage [BASE]          |   0   |  0-1  |
   Redirect-Max-Cache-Time [BASE]      |   0   |  0-1  |
   Reply-Message [NASREQ]              |   0   |   0+  |
   Result-Code [BASE]                  |   0   |   1   |
   Route-Record [BASE]                 |   0+  |   0   |
   Service-Type [NASREQ]               |  0-1  |  0-1  |
   Session-Id [BASE]                   |   1   |   1   |
   Session-Timeout [BASE]              |  0-1  |  0-1  |
   State [NASREQ]                      |  0-1  |  0-1  |
   Termination-Action [NASREQ]         |   0   |  0-1  |
   Termination-Cause [BASE]            |   0   |  0-1  |
   Tunneling [NASREQ]                  |   0+  |   0+  |
   User-Name [BASE]                    |  0-1  |  0-1  |


5.2 Accounting AVP Table

   The table in this section is used to represent which AVPs defined in
   this document are to be present in the Accounting messages, defined
   in [BASE].

                                          +-----------+
                                          |  Command  |
                                          |    Code   |
                                          |-----+-----+
   Attribute Name                         | ACR | ACA |
   ---------------------------------------|-----+-----+
   Accounting-EAP-Auth-Method             |  0+ |  0  |


6. RADIUS/Diameter interactions

   Section 9 of [NASREQ] describes basic guidelines that translation
   agents may follow when translating between RADIUS and Diameter
   protocols.  This section gives additional guidelines for the Diameter
   EAP application.  Note that this document does not restrict
   implementations from creating additional methods, as long as the
   translation function doesn't violate the RADIUS or the Diameter
   protocols.

6.1 RADIUS Request forwarded as Diameter Request

   RADIUS Access-Request to Diameter-EAP-Request:




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   o  RADIUS EAP-Message attribute(s) are translated to a Diameter
      EAP-Payload AVP.  If multiple RADIUS EAP-Message attributes are
      present, they are concatenated and translated to a single Diameter
      EAP-Payload AVP.

   o  An empty RADIUS EAP-Message attribute (with length 2) signifies
      EAP-Start, and it is translated to an empty EAP-Payload AVP.

   o  If the RADIUS Framed-MTU attribute is present, it is translated to
      the Diameter EAP-MTU AVP.

   Diameter-EAP-Answer to RADIUS Access-Accept/Reject/Challenge:

   o  Diameter EAP-Payload AVP is translated to RADIUS EAP-Message
      attribute(s).  If necessary, the value is split into multiple
      RADIUS EAP-Message attributes.

   o  Diameter EAP-Reissued-Payload AVP is translated to a message that
      contains RADIUS EAP-Message attribute(s), and a RADIUS Error-Cause
      attribute [DynAuth] with value 202 (decimal), "Invalid EAP Packet
      (Ignored)" [RFC2869bis].

   o  As described in [NASREQ], if the Result-Code AVP set to
      DIAMETER_MULTI_ROUND_AUTH and the Multi-Round-Time-Out AVP is
      present, it is translated to the RADIUS Session-Timeout attribute.

   o  Diameter EAP-Master-Session-Key AVP can be translated to the
      vendor-specific RADIUS MS-MPPE-Recv-Key and MS-MPPE-Send-Key
      attributes [RFC2548].  The first up to 32 octets of the key is
      stored into MS-MPPE-Recv-Key, and the next up to 32 octets (if
      present) are stored into MS-MPPE-Send-Key.

   o  Diameter Accounting-EAP-Auth-Method AVPs, if present, are
      discarded.


6.2 Diameter Request forwarded as RADIUS Request

   Diameter-EAP-Request to RADIUS Access-Request:

   o  The Diameter EAP-Payload AVP is translated to RADIUS EAP-Message
      attribute(s).

   o  An empty Diameter EAP-Payload AVP signifies EAP-Start, and it is
      translated to an empty RADIUS EAP-Message attribute.

   o  If the Diameter EAP-MTU AVP is present, it is translated to the
      RADIUS Framed-MTU attribute.  If both an EAP-MTU AVP and a



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      Framed-MTU AVP are present in the Diameter-EAP-Request, the
      Framed-MTU AVP is discarded.

   o  The type (or expanded type) field from the EAP-Payload AVP can be
      saved either in a local state table, or encoded in a RADIUS
      Proxy-State attribute.  This information is needed to construct an
      Accounting-EAP-Auth-Method AVP for the answer message (see below).

   RADIUS Access-Accept/Reject/Challenge to Diameter-EAP-Answer:

   o  If the RADIUS Access-Challenge message does not contain an
      Error-Cause attribute [DynAuth] with value 202 (decimal), "Invalid
      EAP  Packet (Ignored)" [RFC2869bis], any RADIUS EAP-Message
      attributes are translated to a Diameter EAP-Payload AVP,
      concatenating them if multiple attributes are present.

   o  If the Error-Cause attribute with value 202 is present, any RADIUS
      EAP-Message attributes are translated to a Diameter
      EAP-Reissued-Payload AVP, concatenating them if multiple
      attributes are present.

   o  As described in [NASREQ], if the Session-Timeout attribute is
      present in a RADIUS Access-Challenge message, it is translated to
      the Diameter Multi-Round-Time-Out AVP.

   o  If the vendor-specific RADIUS MS-MPPE-Recv-Key and/or
      MS-MPPE-Send-Key attributes [RFC2548] are present, they can be
      translated to a Diameter EAP-Master-Session-Key AVP.  Their values
      are concatenated (MS-MPPE-Recv-Key first, MS-MPPE-Send-Key next),
      and the concatenated value is stored into a Diameter
      EAP-Master-Session-Key AVP.

   o  If the Diameter-EAP-Answer will have a succesful result code, the
      saved state (see above) can be used to construct an
      Accounting-EAP-Auth-Method AVP.


6.3 Accounting Requests

   In Accounting-Requests, the vendor-specific RADIUS MS-Acct-EAP-Type
   attribute [RFC2548] can be translated to a Diameter
   Accounting-EAP-Auth-Method AVP, and vice versa.

   When translating from Diameter to RADIUS, note that the
   MS-Acct-EAP-Type attribute does not support expanded EAP types.  Type
   values greater than 255 should be translated to type 254.





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7. IANA Considerations

   This document does not create any new namespaces to be maintained by
   IANA, but it defines new values in namespaces that have been defined
   in the Diameter Base protocol [BASE].

   o  This specification assigns the value 268 from the Command Code
      namespace defined in [BASE].  See Section 3 for more information.

   o  This specification assigns the values TBD from the AVP Code
      namespace defined in [BASE].  See Section 4.1 for the assignment
      of the namespace in this specification.

   o  This specification assigns the value TBD from the Application
      Identifier namespace defined in [BASE].  See Section TBD above for
      more information.


8. Security Considerations

   (NOTE: This section is still very much under construction, and will
   be revised in later versions.  Comments are welcome!)

   Diameter peer-to-peer connections can be protected with IPsec or TLS.
   These mechanisms are believed to provide sufficient protection under
   the normal Internet threat model--that is, assuming the authorized
   nodes engaging in the protocol have not been compromised, but the
   attacker has complete control over the communication channels between
   them.  This includes eavesdropping, message modification, insertion,
   man-in-the-middle and replay attacks.  The details and related
   security considerations are discussed in [BASE].

   The rest of this section deals with two important topics that are not
   completely covered by [BASE] and [NASREQ]: authorization and attacks
   by compromised nodes.

   Authorization here means the act of determining if a Diameter message
   received from an authenticated Diameter peer should be accepted (and
   not authorization of users requesting network access from a NAS).
   The Diameter base protocol gives only overall guidelines and does not
   specify any particular mechanisms, and neither does this document.
   However, Section 8.1 gives some examples of possible authorization
   mechanisms to illustrate the importance of proper authorization, and
   to provide concrete examples for discussing authorization-related
   security considerations (it might be useful to place some of this
   text in a separate "Diameter authentication and authorization"
   document--if you're interested in writing that, contact the authors).




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   The last part of this section deals with attacks by nodes that have
   been properly authorized (to function as a NAS, Diameter agent, or
   Diameter server) but have been compromised.  In general, it is not
   possible to completely protect against attacks by compromised nodes,
   but this section offers some advice that can be used to limit the
   extent of the damage.

   Note that much of the discussion in this section is not really
   specific to Diameter EAP application, but applies to many other
   Diameter applications as well.

8.1 Authorization

   When two Diameter nodes communicate, they can authenticate each other
   using IPsec or TLS.  However, just authentication is not
   enough--authorization is also needed.  Authorization tries to answer
   questions such as the following.

   o  When a Diameter server receives a Diameter-EAP-Request, is the
      node that sent it authorized to act as a NAS for the specific
      user, service type, and so on, in question?

   o  Correspondingly, when the NAS contacts the server to send the
      Diameter-EAP-Request, is the server is authorized to act as home
      server for the realm in question?

   o  Similar considerations apply to Accounting-Request/Answer
      messages: Is the NAS authorized to send this accounting request?,
      Is the server authorized to act as an accounting server for this
      session?

   o  In Re-Auth-Request and Abort-Session-Request, the command is
      initiated by the server, but similar considerations apply.

   o  Session-Termination-Request/Answer (TBW)

   o  Hop-by-hop messages (Capabilities-Exchange-Request/Answer,
      Device-Watchdog-Request/Answer, Disconnect-Peer-Request/Answer)
      are not discussed in this section.

   Authorization is often left as an implementation issue, and this is
   indeed a reasonable approach when authorization is based on local
   access control lists (ACLs).  In this case, other nodes do not need
   to know how ACLs are specified or implemented.  It is also possible
   to implement complex authorization conditions (for instance,
   "ap1788.example.com is authorized to act as a WLAN access point for
   users whose name does not contain the letter "a", but only between
   8AM and 4PM", to give a rather unlikely example).



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   However, when authorization can't be based solely on local ACLs, the
   issue becomes significantly more complex.  First, interoperability
   between different vendors requires public specifications, and
   authorization can't be left solely as an "implementation issue"
   anymore.  Second, these specifications are likely to support only
   very simple authorizations (such as "authorized to act as Diameter
   client"), instead of  arbitrarily complex rules like the nonsense
   example given above.

   The authorization requirements also depend on many issues, such as
   the size of the network, the trust relationships between the nodes,
   and the Diameter application and commands in question.

8.1.1 Direct connection, NAS point of view

   Let's first consider the case where the NAS contacts the home server
   directly.  There are two somewhat different subcases, depending on
   how the NAS finds the home server.

   In the simplest case, the NAS has all the home servers statically
   configured in its peer table and realm-based routing table (see
   [BASE], Sections 2.6 and 2.7).  This is likely to occur small
   networks.  Authentication can be based either on certificates or
   pre-shared keys.

   In this case authorization is most likely based on the realm-based
   routing table (that can be considered to contain an "implicit" local
   ACL).  To give a concrete example, if "mars.example.com" is
   configured as the server for realm "example.com", this can be
   considered to imply that mars.example.com is authorized to act as the
   home server for that realm.

   The situation is a bit different if the NAS finds the home server
   using, for instance, DNS queries or redirect messages.  In this case,
   authentication is probably based on certificates, but how does a NAS
   know if the server authenticated as "mars.example.com" is authorized
   to act as the home server for the realm "example.com"?  A couple of
   possibilities follow:

   o  "Server CA": A certificate issued by some particular Certificate
      Authority (CA) implies authorization to act as the home server
      (for all realms).  This provides rather coarse-grained
      authorization, and has obvious problems if the same CA also issues
      certificates for other purposes (such as NASes or web servers).

   o  Certificate name matching: The NAS requires a certificate issued
      by some particular CA, and also compares the DNS name in the
      certificate with the realm name.  Like the previous approach,



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      there are problems if the same CA issues certificates for other
      purposes.  There are several possibile comparison rules (these
      examples are not meant to be exhaustive):

      *  "Simple": the server is authorized if the realm name is equal
         to the DNS name in the certificate with first component
         removed.

      *  "Suffix": the server is authorized if the realm name is a
         suffix of the DNS name in the certificate.

      *  "Last-2-or-3": the server is authorized if the last two or
         three components of the DNS name and realm name are equal.
         Whether two or three components are required depends on the
         last component.  For instance, two is enough for "com", but
         three is required for "au", because Australia uses a
         second-level domain structure (.com.au, .org.au, etc.).

      All of these comparison rules have some problems.  For instance,
      using the "simple" rule, mars.example.com can't be authorized for
      the realm "sales.example.com".  With the "suffix" and
      "last-2-or-3" rules, "jupiter.sales.example.com" is authorized for
      not only the realm "sales.example.com" but also "example.com".
      And none of the rules can authorize mars.example.com for the realm
      "example.org".

   o  Authorization using DNS records (probably secured by DNSSEC): For
      instance, a DNS SRV record "_diameter._sctp.example.com" pointing
      to mars.example.com could imply authorization to act as a home
      server for the realm "example.com" (although [BASE], Section 5.2
      seems to forbid this).

   o  Authorization implied by redirect: If the home server was located
      as a result of a redirect message, and the redirect agent that
      provided the answer is trustworthy, this could be used as
      authorization if better information is not available (see also
      next section about authorizing redirect agents).

   Some future possibilities (not known to be used yet in Diameter)
   include the following.

   o  Certificate name matching combined with an extendedKeyUsage
      extension, specifying whether the host is authorized to act as a
      home server, NAS, or something else (such as web server).  This
      avoids the problems involved if the same CA issues certificates
      for other purposes as well, but no extendedKeyUsage value has been
      defined for Diameter servers yet.




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   o  The server's certificate could contain an authorization extension
      with realm name(s).  This would avoid many of the problems in
      certificate name matching, but no such extensions has been defined
      yet.

   o  Attribute certificates: The server also presents an attribute
      certificate that contains the realm name.  No attribute
      certificate profile has been defined for Diameter, and there is no
      standardized way to transport the attribute certificates with IKE
      or TLS.

   For reasons described in the next section (Attacks by compromised
   nodes), a NAS may also perform more fine-grained access control.  For
   instance, a NAS may want to restrict the ability to initiate callback
   to some home servers.  How such restricted authorizations would be
   specified is beyond the scope of this document.

8.1.2 Direct connection, server point of view

   (TO BE WRITTEN)

8.1.3 Diameter agents

   (NOTE: This section is very much under construction)

   If authorization can be complex even in the case of direct
   connections, it gets worse with agents, unless authorization is made
   more "coarse-grained".  Consider questions such "is this Diameter
   proxy authorized to forward my requests to realm example.com?", or
   "is this Diameter proxy authorized to forward requests from NAS
   ap1788.example.com?"

   It is important to remember that authorizing a Diameter agent to work
   as a relay, proxy or translation agent involves considerable trust in
   the agent.  The security mechanisms specified in [BASE] provide only
   hop-by-hop security.  Diameter agents can eavesdrop and modify AVPs
   in the messages they forward, and even with Diameter Path
   Authorization (see [BASE] Section 2.10), a Diameter agent that is
   authorized to forward messages from party X can also forge messages
   that look like they came from X.

   Some Diameter agent may be authorized as a redirect agent only (TO BE
   WRITTEN)

8.2 Attacks by compromised nodes

   The hop-by-hop security mechanisms (IPsec and TLS) combined with
   proper authorization provide good protection against "outside"



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   attackers (denial-of-service is, of course, possible).  This section
   deals with attacks in which an attacker has compromised an authorized
   NAS, Diameter agent, or Diameter server.

   (Attacks between the user and the NAS are beyond the scope of this
   document, since they do not use Diameter, and depend on the link
   layer used.)

   Unlike the base protocol security considerations, this section
   considers the Diameter NASREQ/EAP messages not just as bit strings,
   but as messages with a meaning (that cause some actions to happen).
   Thus, the question is not "can the attacker modify this packet" but
   "what can an attacker who compromises an authorized NAS, agent, or
   server do using Diameter EAP messages?"

8.2.1 Impersonating as the user (NAS, agents)

   Unlike CHAP in NASREQ, in EAP the NAS or agents cannot successfully
   replay old EAP messages (unless the EAP method is seriously broken).

8.2.2 Impersonating as the network (NAS, agents)

   If the EAP method used does not provide mutual authentication,
   obviously anyone can impersonate as the network to the user.  When
   EAP mutual authentication is used, it occurs between the user and the
   Diameter home server.  This means that it is usually not possible for
   the user to validate the identity of the NAS using EAP alone (the
   possession of the session keys by the NAS proves that the user is
   talking to *some* authorized NAS, but not which).

   Some EAP methods, such as EAP Archie [Archie], try to provide a
   "binding" where some identity of the NAS (such as MAC address) is
   also communicated inside the EAP messages.  However, the usefulness
   of this binding is rather limited in many environments.

   In many networks an attacker who has not compromized any trusted
   nodes can still mount a "relay attack" (for the lack of better word).
   In this attack, the attacker just forwards messages between the user
   and a legitimate NAS (without modifying them), causing the user to
   access the network via a different NAS than would have otherwise
   happened.  This attack might make sense if, for instance, the
   attacker can eavesdrop the network near some NASes, but not all, or
   if the user is charged more for network access via foreign NASes than
   local ones.

   It is rather obvious that an attacker who has compromised a NAS can
   "impersonate" as the network to the user (until the compromise is
   noticed and NAS authorization revoked).  The attacker can also use



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   this "relay attack" to cause the user to access the network via the
   compromised NAS instead of other, non-compromised, NASes.

   Prevention of this "relay attack" is rather difficult, and is beyond
   the scope of this document.  Note that validating a NAS identity
   (using the "binding" provided by Archie, for instance) does not
   usually help.  For instance, in WLANs, the user does not usually
   securely know the MAC address of the "right" access point--it's
   simply picked from a beacon message that has the correct SSID and
   good signal strength (something that's easy to spoof).

8.2.3 Privacy issues (NAS, agents)

   Diameter messages can contain AVPs that can be used to identify the
   user (e.g., User-Name) and approximate location of the user (e.g.
   Origin-Host for WLAN access points, Calling-Station-Id for fixed
   phone lines).  Thus, any Diameter nodes that process the messages may
   be able to determine the geographic location of users.

   Note that in many cases, the user identity is also sent in clear
   inside EAP-Payload AVPs, and it may be possible to eavesdrop this
   between the user and the NAS.

   This can mitigated somewhat by using EAP methods that provide
   identity protection (see [RFC2284bis], Section 7.3), and using
   Session-Id or pseudonyms for accounting.

8.2.4 Offline cryptographic attacks (NAS, agents)

   Some EAP methods are vulnerable to dictionary attacks or other
   methods that try to recover long-term secrets from EAP messages.
   Usually the EAP messages can also be eavesdropped between the user
   and the NAS, so good EAP methods provide sufficient protection
   against this type of attacks.

8.2.5 AVP editing (NAS, agents, server)

   Diameter agents can modify, insert, and delete AVPs.  Diameter agents
   are usually meant to modify AVPs, and the protocol in general cannot
   distinguish well-intentioned and malicious modifications (see
   [RFC2607] for more discussion).

   Authentication method negotiation attacks are discussed in the next
   Section.

   An attacker who compromises a Diameter agent or server can modify
   AA-Answer/Diameter-EAP-Answer messages and..




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   o  Deny access to authorized users (Result-Code).

   o  Give unauthorized users access (Result-Code).

   o  Give attacker a login session to a host otherwise protected by
      firewalls (Login-Host).

   o  Redirect an authorized user's login session to a host controlled
      by the attacker (Login-Host).

   o  Route an authorized user's traffic through a host controlled by
      the attacker (various tunneling AVPs).

   o  Redirect an authorized user's DNS requests to a malicious DNS
      server (various vendor-specific AVPs).

   o  Modify routing tables at the NAS and thus redirect packets
      destined for someone else (Framed-Route, Framed-Routing).

   o  Remove packet filters and other restrictions for user (Filter,
      Callback, various vendor-specific AVPs).

   o  Cause the NAS to call some number, possibly expensive toll number
      controlled by the attacker (callback AVPs)

   o  Execute Command Line Interface (CLI) commands on the NAS (various
      vendor-specific attributes).

   Some of these attacks can be prevented if the NAS can be configured
   not to accept some particular AVPs, or not accept them from all
   Diameter servers.  For instance, if a particular destination realm
   never uses tunneling, answer messages containing tunneling AVPs could
   be rejected.

   An attacker who compromises a NAS or agent can modify AA-Requests,
   and...

   o  Change NAS-Identifier/NAS-Port/Origin-Host (or something) so that
      a valid user appears to be accessing the network from a different
      NAS than in reality.

   o  Modify Calling-Station-ID (either to hide the true value, gain
      access, or frame someone else).

   o  Modify password change messages (some vendor-specific attributes)

   o  Modify usage information in accounting messages.




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   o  Modify Class/State AVPs?


8.2.6 Negotiation attacks (NAS, agents, server)

   This section deals with attacks where the NAS, any Diameter agents,
   or Diameter server attempts to cause the authenticating user to
   choose a less secure authentication method (either something else
   than EAP, or a less secure EAP method).

   For example, a session that would normally be authenticated with EAP
   would instead authenticated via CHAP or PAP; alternatively, a
   connection that would normally be authenticated via a more secure EAP
   method such as EAP-TLS might be made to occur via a less secure EAP
   method, such as MD5-Challenge.

   (TO BE WRITTEN, possibly copy text from 2869bis)

   Negotiation attacks within EAP are discussed in [RFC2284bis], Section
   7.8.

   Negotiation attacks between the user and the NAS are beyond the scope
   of this document.

8.2.7 Session key distribution (agents, server)

   Since there are currently no end-to-end (NAS-to-home server) security
   mechanisms specified for Diameter, any agents that process
   Diameter-EAP-Answer messages can see the contents of the
   EAP-Session-Key AVP.  For this reason, this specification strongly
   recommends avoiding Diameter agents when they cannot be trusted to
   keep the keys secret.

   In environments where agents are present, several factors should be
   considered when deciding whether the agents that are authorized (and
   considered "trustworthy enough") to grant access to users and specify
   various authorization and tunneling AVPs are also "trustworthy
   enough" to handle the session keys.  These factors include (but are
   not limited to) the type of access provided (e.g., public Internet or
   corporate internet), security level of the agents, and the
   possibilities for attacking user's traffic after it has been
   decrypted by the NAS.

   Note that the keys communicated in Diameter messages are usually
   short-term session keys (or short-term master keys that are used to
   derive session keys).  To actually cause any damage, those session
   keys must end with some malicious party; that party must be able to
   eavesdrop, modify, or insert traffic between the user and the NAS



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   during the lifetime of those keys (e.g., in 802.11i the attacker must
   also eavesdrop the "four-way handshake"); and that eavesdropping or
   modification must cause some damage.

9. Acknowledgements

   This Diameter application relies heavily on earlier work on Diameter
   NASREQ application [NASREQ] and RADIUS EAP support [RFC2869bis].
   Much of the material in this specification has been copied from these
   documents.

   The authors would also like to acknowledge the following people for
   their contributions to this document: Bernard Aboba, Jari Arkko, Pat
   Calhoun, Henry Haverinen, and John Loughney. (TBD: Who's missing from
   this list?)

Normative References

   [BASE]     Calhoun, P., Loughney, J., Guttman, E., Zorn, G. and J.
              Arkko, "Diameter Base Protocol",
              draft-ietf-aaa-diameter-17 (work in progress), December
              2002.

   [NASREQ]   Calhoun, P., Zorn, G., Spence, D. and D. Mitton, "Diameter
              Network Access Server Application",
              draft-ietf-aaa-diameter-nasreq-11 (work in progress),
              February 2003.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2284bis]
              Blunk, L., Vollbrecht, J., Aboba, B., Carlson, J. and H.
              Levkowetz, "Extensible Authentication Protocol (EAP)",
              draft-ietf-eap-rfc2284bis-04 (work in progress), June
              2003.

Informative References

   [Archie]   Walker, J. and R. Housley, "The EAP Archie Protocol",
              draft-jwalker-eap-archie-01 (work in progress), June 2003.

   [DynAuth]  Chiba, M., Dommety, G., Eklund, M., Mitton, D. and B.
              Aboba, "Dynamic Authorization Extensions to Remote
              Authentication Dial In User Service (RADIUS)",
              draft-chiba-radius-dynamic-authorization-20 (work in
              progress), May 2003.




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   [IEEE-802.1X]
              Institute of Electrical and Electronics Engineers, "Local
              and Metropolitan Area Networks: Port-Based Network Access
              Control", IEEE Standard 802.1X, September 2001.

   [IEEE-802.11i]
              Institute of Electrical and Electronics Engineers,
              "Unapproved Draft Supplement to Standard for
              Telecommunications and Information Exchange Between
              Systems - LAN/MAN Specific Requirements - Part 11:
              Wireless LAN Medium Access Control (MAC) and Physical
              Layer (PHY) Specifications: Specification for Enhanced
              Security", IEEE Draft 802.11i (work in progress), 2003.

   [IKEv2]    Kaufman, C., Ed., "Internet Key Exchange (IKEv2)
              Protocol", draft-ietf-ipsec-ikev2-08 (work in progress),
              May 2003.

   [RADIUS1X]
              Congdon, P., Aboba, B., Smith, A., Zorn, G. and J. Roese,
              "IEEE 802.1X RADIUS Usage Guidelines",
              draft-congdon-radius-8021x-29 (work in progress), April
              2003.

   [RFC1661]  Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51,
              RFC 1661, July 1994.

   [RFC2548]  Zorn, G., "Microsoft Vendor-specific RADIUS Attributes",
              RFC 2548, March 1999.

   [RFC2607]  Aboba, B. and J. Vollbrecht, "Proxy Chaining and Policy
              Implementation in Roaming", RFC 2607, June 1999.

   [RFC2869bis]
              Aboba, B. and P. Calhoun, "RADIUS Support For Extensible
              Authentication Protocol (EAP)",
              draft-aboba-radius-rfc2869bis-22 (work in progress), May
              2003.













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Authors' Addresses

   Pasi Eronen (editor)
   Nokia Research Center
   P.O. Box 407
   FIN-00045 Nokia Group
   Finland

   EMail: pasi.eronen@nokia.com


   Tom Hiller
   Lucent Technologies
   1960 Lucent Lane
   Naperville, IL  60566
   USA

   Phone: +1 630 979 7673
   EMail: tom.hiller@lucent.com


   Glen Zorn
   Cisco Systems
   500 108th Avenue N.E., Suite 500
   Bellevue, WA  98004
   USA

   Phone: +1 425 344 8113
   EMail: gwz@cisco.com

Appendix A. Changelog

   (This section will not appear in the final version submitted to RFC
   editor.)

   Changes from -02.d to -02.e:

   o  Added a section on accounting, and changed how the
      Accounting-EAP-Auth-Method is determined.

   o  Updates to "authorization" and "impersonating as the network"
      security considerations.

   Changes from -02.c to -02.d:

   o  Some clarifications to Introduction section.

   o  Lots of clarifications and added diagrams in protocol overview



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      section.  Moved non-EAP-supporting servers, User-Name AVP
      guidelines, and conflicting messages to separate sections.

   o  Added a new section about sessions and NASREQ interaction.

   o  Wrote a note about Reply-Message AVP, and added it back to ABNFs
      and occurance tables.

   o  Added EAP-Reissued-Payload AVP for signalling invalid packets, and
      RADIUS translation for this.

   o  Added EAP-Master-Session-Key AVP for keys, and suggestions for
      RADIUS translation.

   o  Attempted to clarify Framed-MTU RADIUS translation.

   o  Added a first attempt of security considerations section.

   o  Updated acknowledgements (please notify me if someone's missing).

   Changes from -02.b to -02.c:

   o  Rephrased abstract and introduction sections.

   o  A couple of minor changes in Sections 2.1 and 2.2.

   o  Added text about advertising application support and role
      reversal.

   o  Changed type of Accounting-EAP-Auth-Method AVP from Enumerated to
      Unsigned64, and explained how it is determined.

   o  Removed references to EAP-Master-Session-Key AVP added in -02.b.

   o  Added Diameter-RADIUS translation of accounting AVPs.

   o  Added IANA Considerations section.

   o  References section: Updated RFC2284bis, added IEEE-802.11i and
      IKEv2, deleted RFC1510 ad RFC1938.

   Changes from -02.a to -02.b:

   o  Added some text to Introduction section.

   o  Stole text from 2869bis about invalid packets, retransmissions,
      and fragmentation.




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   o  In section 2.1, changed one "MAY" to "could" since it was not used
      to describe a requirement.

   o  Updated ABNF's and AVP occurance tables to match the current
      NASREQ-11 document.

   o  Added EAP-MTU and EAP-Master-Session-Key AVPs.

   o  Removed description of Configuration-Token, Nas-Port, Nas-Port-Id,
      and State AVPs (the text didn't add anything to their description
      in NASREQ).

   o  Added a first attempt of a section describing Diameter-RADIUS
      translation.

   o  Added references RFC2284bis, RFC2548, RFC2869bis, RADIUS1X, and
      DynAuth.

   Changes from -01 to -02.a:

   o  New editor.

   o  Added Changelog appendix.

   o  Converted source to XML format.  This will result in many small
      formatting changes in the ASCII version.

   o  Updated BASE and NASREQ references to current versions.























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Intellectual Property Statement

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Acknowledgement

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