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RFC 4072 - Diameter Extensible Authentication Protocol (EAP) App


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Network Working Group                                     P. Eronen, Ed.
Request for Comments: 4072                                         Nokia
Category: Standards Track                                      T. Hiller
                                                     Lucent Technologies
                                                                 G. Zorn
                                                           Cisco Systems
                                                             August 2005

     Diameter Extensible Authentication Protocol (EAP) Application

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2005).

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.

Table of Contents

   1.  Introduction ...................................................2
       1.1.  Conventions Used in This Document ........................3
   2.  Extensible Authentication Protocol Support in Diameter .........3
       2.1.  Advertising Application Support ..........................3
       2.2.  Protocol Overview ........................................4
       2.3.  Sessions and NASREQ Interaction ..........................6
             2.3.1. Scenario 1: Direct Connection .....................7
             2.3.2. Scenario 2: Direct Connection with Redirects ......8
             2.3.3. Scenario 3: Direct EAP, Authorization via Agents ..9
             2.3.4. Scenario 4: Proxy Agents .........................10
       2.4.  Invalid Packets .........................................10
       2.5.  Retransmission ..........................................11
       2.6.  Fragmentation ...........................................12
       2.7.  Accounting ..............................................12
       2.8.  Usage Guidelines ........................................13

             2.8.1. User-Name AVP ....................................13
             2.8.2. Conflicting AVPs .................................13
             2.8.3. Displayable Messages .............................14
             2.8.4. Role Reversal ....................................14
             2.8.5. Identifier Space .................................14
   3.  Command-Codes .................................................14
       3.1.  Diameter-EAP-Request (DER) Command ......................15
       3.2.  Diameter-EAP-Answer (DEA) Command .......................16
   4.  Attribute-Value Pairs .........................................18
       4.1.  New AVPs ................................................18
             4.1.1. EAP-Payload AVP ..................................18
             4.1.2. EAP-Reissued-Payload AVP .........................18
             4.1.3. EAP-Master-Session-Key AVP .......................19
             4.1.4. EAP-Key-Name AVP .................................19
             4.1.5. Accounting-EAP-Auth-Method AVP ...................19
   5.  AVP Occurrence Tables .........................................19
       5.1.  EAP Command AVP Table ...................................20
       5.2.  Accounting AVP Table ....................................21
   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 ...........................................24
   8.  Security Considerations .......................................24
       8.1.  Overview ................................................24
       8.2.  AVP Editing .............................................26
       8.3.  Negotiation Attacks .....................................27
       8.4.  Session Key Distribution ................................28
       8.5.  Privacy Issues ..........................................28
       8.6.  Note about EAP and Impersonation ........................29
   9.  Acknowledgements ..............................................29
   10. References ....................................................30
       10.1. Normative References ....................................30
       10.2. Informative References ..................................30

1.  Introduction

   The Extensible Authentication Protocol (EAP), defined in [EAP], is an
   authentication framework which supports multiple authentication
   mechanisms.  EAP may be used on dedicated links, 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 the Diameter Network Access Server
   Application [NASREQ] and is intended for environments similar to
   NASREQ.

   In the 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.

   The Diameter EAP application relies heavily on [NASREQ], and in
   earlier versions was part of the Diameter NASREQ application.  It can
   also be used in conjunction with NASREQ, selecting the application
   based on the user authentication mechanism (EAP or PAP/CHAP).  The
   Diameter EAP application defines new Command-Codes and Attribute-
   Value Pairs (AVPs), and can work together with RADIUS EAP support
   [RFC3579].

1.1.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

2.  Extensible Authentication Protocol Support in Diameter

2.1.  Advertising Application Support

   Diameter nodes conforming to this specification MUST advertise
   support by including the Diameter EAP Application ID value of 5 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 indicates 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
   be used, since this could result from a downgrade attack (see
   Section 8.3).

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 802.11i [IEEE-802.11i].  Once EAP has been
   initiated, the access device will typically send a Diameter-EAP-
   Request message with an empty EAP-Payload AVP to the Diameter server,
   signifying an EAP-Start.

   If the Diameter home server is willing to do EAP authentication, it
   responds with a Diameter-EAP-Answer message containing an EAP-Payload
   AVP that includes an encapsulated EAP packet.  The Result-Code AVP in
   the message will be 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
   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 in which 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.

    :                        ...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 SHOULD also include the
   appropriate authorization AVPs required for the service requested
   (see Section 5 and [NASREQ]).  In some cases, the home server may not
   be able to provide all necessary authorization AVPs; in this case, a
   separate authorization step MAY be used as described in
   Section 2.3.3.  Diameter-EAP-Answer messages whose Result-Code AVP is
   set to DIAMETER_MULTI_ROUND_AUTH MAY include authorization AVPs.

   A Diameter-EAP-Answer with successful 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, and 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

   The previous section introduced the basic protocol between the NAS
   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, one must 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 possible scenarios on how the messages could be
   transport protected using these hop-by-hop mechanisms.

   This 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 authorization AVPs and EAP keying material are delivered by the
   home server.

   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
   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.  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, for example, a new roaming
   agreement is made.

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
   authorization AVPs are retrieved from local proxy agents.  This
   scenario is intended for environments in which 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 supports routing based on application,
   not on Diameter commands.

2.3.4.  Scenario 4: Proxy Agents

   This scenario is the same as Scenario 1, but the NAS contacts the
   home server through proxies.  Note that the proxies can see the EAP
   session keys, thus it is not suitable for environments where proxies
   cannot be trusted.

   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 (Code 2=Response) 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
   (Code 1=Request) 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
   possible for a NAS to forward an invalid EAP packet to or from the
   Diameter server.

   A Diameter server receiving an EAP-Payload AVP that it does not
   understand SHOULD determine 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 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
   DIAMETER_MULTI_ROUND_AUTH Result-Code, but no EAP-Payload AVP.  To
   simplify RADIUS translation, this message MUST also include an
   EAP-Reissued-Payload AVP encapsulating the previous EAP Request sent
   by the server.

   When receiving a Diameter-EAP-Answer without an EAP-Payload AVP (and
   DIAMETER_MULTI_ROUND_AUTH Result-Code), the NAS SHOULD discard the
   EAP-Response packet most recently transmitted to the Diameter server
   and check whether additional EAP Response packets that match the
   current Identifier value have been received.  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 previous EAP Request is resent 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 DIAMETER_AUTHENTICATION_REJECTED Result-Code.  By
   default, a value of 5 invalid EAP packets is recommended.

2.5.  Retransmission

   As noted in [EAP], 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 time-outs 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 a Diameter-EAP-
   Answer message that also contains an EAP-Payload AVP, that value is
   used to set the EAP retransmission timer for that EAP 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, a
   Framed-MTU AVP may be included in a Diameter-EAP-Request message in
   order to provide the Diameter server with this information.

   A Diameter server having received a Framed-MTU AVP in a
   Diameter-EAP-Request message MUST NOT send any subsequent packet in
   this EAP conversation containing EAP-Payload AVP whose length exceeds
   that specified by the Framed-MTU value, taking the link type
   (specified by the NAS-Port-Type AVP) into account.  For example, as
   noted in [RFC3580] Section 3.10, for a NAS-Port-Type value of IEEE
   802.11, the RADIUS server may send an EAP packet as large as
   Framed-MTU minus four (4) octets, taking into account the additional
   overhead for the IEEE 802.1X Version (1 octet), Type (1 octet) and
   Body Length (2 octets) fields.

2.7.  Accounting

   When a user is authenticated using EAP, the NAS MAY include an
   Accounting-Auth-Method AVP [NASREQ] with value 5 (EAP) in
   Accounting-Request messages.  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 or 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 a User-Name AVP to
   Diameter-EAP-Answer messages that have a Result-Code of
   DIAMETER_SUCCESS.  A separate billing identifier or pseudonym MAY be
   used for privacy reasons (see Section 8.5).  If the user's identity
   is not available to the NAS, the Session-Id AVP MAY be used for
   accounting and billing; however operationally this could be very
   difficult to manage.

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.

   Some lower layers assume that the authorization decision is made by
   the EAP server, and thus the peer considers EAP Success as an
   indication that access was granted.  In this case, the Result-Code
   SHOULD match the contained EAP packet: a successful Result-Code for
   EAP-Success, and a failure Result-Code for EAP-Failure.  If the
   encapsulated EAP packet does not match the result implied by the
   Result-Code AVP, the combination is likely to cause confusion,
   because the NAS and peer will conclude the outcome of the
   authentication differently.  For example, if the NAS receives a
   failure Result-Code with an encapsulated EAP Success, it will not
   grant access to the peer.  However, on receiving the EAP Success, the
   peer will be led to believe that access was granted.

   This situation can be difficult to avoid when Diameter proxy agents
   make authorization decisions (that is, proxies can change the
   Result-Code AVP sent by the home server).  Because it is the
   responsibility of the Diameter server to avoid conflicts, the NAS
   MUST NOT "manufacture" EAP result packets in order to correct the
   contradictory messages that it receives.  This behavior, originally
   mandated within [IEEE-802.1X], is now deprecated.

2.8.3.  Displayable Messages

   The Reply-Message AVP [NASREQ] MUST NOT be included in any Diameter
   message containing an EAP-Payload AVP.

2.8.4.  Role Reversal

   Some environments in which EAP is used, such as PPP, support
   peer-to-peer operation.  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 a packet
   MUST respond with a failure Result-Code.

2.8.5.  Identifier Space

   In EAP, each session has its own unique Identifier space.  Diameter
   server implementations MUST be able to distinguish between EAP
   packets with the same Identifier existing within distinct EAP
   sessions and originating on the same NAS.  This is done by using the
   Session-Id AVP.

   If a Diameter NAS is in the middle of a multi-round authentication
   exchange, and it detects that the EAP session between the client and
   the NAS has been terminated, it MUST select a new Diameter Session-Id
   for any subsequent EAP sessions.  This is necessary in order to
   distinguish a restarted EAP authentication process from the
   continuation of an ongoing process (by the same user on the same NAS
   and port).

   In RADIUS, the same functionality can be achieved through the
   inclusion or omission of the State attribute.  Translation rules in
   [NASREQ] ensure that an Access-Request without the State attribute
   maps to a new Diameter Session-Id AVP value.  Furthermore, a
   translation agent will always include a State attribute in
   Access-Challenge messages, making sure that the State attribute is
   available for a RADIUS NAS.

3.  Command-Codes

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

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

   When the NASREQ AA-Request (AAR) or AA-Answer (AAA) commands are used
   for AUTHORIZE_ONLY messages in conjunction with EAP (see
   Section 2.3.3), an Application Identifier value of 1 (NASREQ) is
   used, and the commands follow the rules and ABNF defined in [NASREQ].

   When the Re-Auth-Request (RAR), Re-Auth-Answer (RAA),
   Session-Termination-Request (STR), Session-Termination-Answer (STA),
   Abort-Session-Request (ASR), Abort-Session-Answer (ASA),
   Accounting-Request (ACR), and Accounting-Answer (ACA) commands are
   used together with the Diameter EAP application, they follow the
   rules in [NASREQ] and [BASE].  The accounting commands use
   Application Identifier value of 3 (Diameter Base Accounting); the
   others use 0 (Diameter Common Messages).

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 containing 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 that 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 }
                                 [ Destination-Host ]

                                 [ NAS-Identifier ]
                                 [ NAS-IP-Address ]
                                 [ NAS-IPv6-Address ]
                                 [ NAS-Port ]
                                 [ NAS-Port-Id ]
                                 [ NAS-Port-Type ]
                                 [ Origin-State-Id ]
                                 [ Port-Limit ]
                                 [ User-Name ]
                                 { EAP-Payload }
                                 [ EAP-Key-Name ]
                                 [ Service-Type ]
                                 [ State ]
                                 [ Authorization-Lifetime ]
                                 [ Auth-Grace-Period ]
                                 [ Auth-Session-State ]
                                 [ Callback-Number ]
                                 [ Called-Station-Id ]
                                 [ Calling-Station-Id ]
                                 [ 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
       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 ]
                                [ EAP-Reissued-Payload ]
                                [ EAP-Master-Session-Key ]
                                [ EAP-Key-Name ]
                                [ Multi-Round-Time-Out ]
                                [ Accounting-EAP-Auth-Method ]
                                [ Service-Type ]
                              * [ Class ]
                              * [ Configuration-Token ]
                                [ Acct-Interim-Interval ]
                                [ Error-Message ]
                                [ Error-Reporting-Host ]
                              * [ Failed-AVP ]
                                [ Idle-Timeout ]
                                [ Authorization-Lifetime ]
                                [ Auth-Grace-Period ]
                                [ Auth-Session-State ]
                                [ Re-Auth-Request-Type ]
                                [ Session-Timeout ]
                                [ State ]
                              * [ Reply-Message ]
                                [ Origin-State-Id ]
                              * [ Filter-Id ]

                                [ Port-Limit ]
                                [ Callback-Id ]
                                [ Callback-Number ]
                                [ Framed-Appletalk-Link ]
                              * [ 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 ]
                              * [ QoS-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 462) 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 463) is of type OctetString.
   The use of this AVP is described in Section 2.4.

4.1.3.  EAP-Master-Session-Key AVP

   The EAP-Master-Session-Key AVP (AVP Code 464) is of type OctetString.
   It contains keying material for protecting the communications between
   the user and the NAS.  Exactly how this keying material is used
   depends on the link layer in question, and is beyond the scope of
   this document.

4.1.4.  EAP-Key-Name AVP

   The EAP-Key-Name AVP (Radius Attribute Type 102) is of type
   OctetString.  It contains an opaque key identifier (name) generated
   by the EAP method.  Exactly how this name is used depends on the link
   layer in question, and is beyond the scope of this document (see
   [EAPKey] for more discussion).

   Note that not all link layers use this name, and currently most EAP
   methods do not generate it.  Since the NAS operates in pass-through
   mode, it cannot know the Key-Name before receiving it from the AAA
   server.  As a result, a Key-Name AVP sent in a Diameter-EAP-Request
   MUST NOT contain any data.  A home Diameter server receiving a
   Diameter-EAP-Request with a Key-Name AVP with non-empty data MUST
   silently discard the AVP.  In addition, the home Diameter server
   SHOULD include this AVP in Diameter-EAP-Response only if an empty
   EAP-Key-Name AVP was present in Diameter-EAP-Request.

4.1.5.  Accounting-EAP-Auth-Method AVP

   The Accounting-EAP-Auth-Method AVP (AVP Code 465) is of type
   Unsigned64.  In case of expanded types [EAP, Section 5.7], this AVP
   contains the value ((Vendor-Id * 2^32) + Vendor-Type).

   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, as defined in this document; 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  |
   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-Master-Session-Key              |   0   |  0-1  |
   EAP-Key-Name                        |  0-1  |  0-1  |
   EAP-Payload                         |   1   |  0-1  |
   EAP-Reissued-Payload                |   0   |  0-1  |
   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   |  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   |
   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  |
   Port-Limit [NASREQ]                 |  0-1  |  0-1  |
   Proxy-Info [BASE]                   |   0+  |   0+  |
   QoS-Filter-Rule [NASREQ]            |   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   |  0-1  |
   State [NASREQ]                      |  0-1  |  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, as
   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 for translation
   agents that translate between RADIUS and Diameter protocols.  These
   guidelines SHOULD be followed for Diameter EAP application as well,
   with some additional guidelines given in this section.  Note that
   this document does not restrict implementations from creating
   additional methods, as long as the translation function does not
   violate the RADIUS or the Diameter protocols.

6.1.  RADIUS Request Forwarded as Diameter Request

   RADIUS Access-Request to Diameter-EAP-Request:

   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.

   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 [RFC3576] with value 202 (decimal), "Invalid EAP Packet
      (Ignored)" [RFC3579].

   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.  The encryption of this
      attribute is described in [RFC2548].

   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 is
      translated to an empty RADIUS EAP-Message attribute.

   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 [RFC3576] with value 202 (decimal), "Invalid
      EAP Packet (Ignored)" [RFC3579], 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.  The
      attributes have to be decrypted before conversion, and the Salt,
      Key-Length and Padding sub-fields are discarded.  The Key
      sub-fields 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 successful 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.

7.  IANA Considerations

   This document does not create any new namespaces to be maintained by
   IANA, but it requires new values in namespaces that have been defined
   in the Diameter Base protocol and RADIUS specifications.

   o  This document defines one new Diameter command (in Section 3)
      whose Command Code is allocated from the Command Code namespace
      defined in [BASE].  The Command Code for DER / DEA is 268.

   o  This document defines four new AVPs whose AVP Codes are allocated
      from the AVP Code namespace defined in [BASE] as follows:

         462 for EAP-Payload (defined in Section 4.1.1),
         463 for EAP-Reissued-Payload (defined in Section 4.1.2),
         464 for EAP-Master-Session-Key (defined in Section 4.1.3), and
         465 for Accounting-EAP-Auth-Method (defined in Section 4.1.5).

   o  This document defines one new AVP (attribute) whose AVP Code
      (Attribute Type) is to be allocated from the Attribute Type
      namespace defined in [RFC2865] and [RFC3575].  The Radius
      Attribute Type for EAP-Key-Name (defined in Section 4.1.4) is 102.

   o  This document defines one new Diameter application (in
      Section 2.1) whose Application ID is to be allocated from the
      Application Identifier namespace defined in [BASE].  The
      Application ID for Diameter EAP is 5.

8.  Security Considerations

8.1.  Overview

   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].

   In addition to authentication provided by IPsec or TLS, authorization
   is also required.  Here, authorization means 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).  In other words, when a Diameter server receives a
   Diameter-EAP-Request, it has to decide if the client is authorized to
   act as a NAS for the specific user, service type, and so on.
   Correspondingly, when a NAS contacts a server to send a
   Diameter-EAP-Request, it has to determine whether the server is
   authorized to act as home server for the realm in question.

   Authorization can involve local Access Control Lists (ACLs),
   information contained in certificates, or some other means.  See
   [BASE] for more discussion and related security considerations.  Note
   that authorization issues are particularly relevant when Diameter
   redirects are used.  While redirection reduces the number of nodes
   which have access to the contents of Diameter messages, a compromised
   Diameter agent may not supply the right home server's address.  If
   the Diameter client is unable to tell whether this particular server
   is authorized to act as the home server for this particular user, the
   security of the communications rests on the redirect agent.

   The hop-by-hop security mechanisms (IPsec and TLS) combined with
   proper authorization provide good protection against "outside"
   attackers, except for denial-of-service attacks.  The remaining 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 abuse their authorization or have been compromised.  In
   general, it is not possible to completely protect against attacks by
   compromised nodes, but this section offers advice on limiting the
   extent of the damage.

   Attacks involving eavesdropping or modification of EAP messages are
   beyond the scope of these document.  See [EAP] for discussion of
   these security considerations (including method negotiation,
   dictionary attacks, and privacy issues).  While these attacks can be
   carried out by an attacker between the client and the NAS,
   compromised NASes and Diameter agents are naturally also in a good
   position to modify and eavesdrop on the EAP messages.

   Similarly, attacks involving the link layer protocol used between the
   client and the NAS, such as PPP or IEEE 802.11, are beyond the scope
   of this document.

8.2.  AVP Editing

   Diameter agents can modify, insert, and delete AVPs.  Diameter agents
   are usually meant to modify AVPs, and the protocol cannot distinguish
   well-intentioned and malicious modifications (see [RFC2607] for more
   discussion).  Similarly, a compromised NAS or server can naturally
   include a different set of AVPs than expected.

   Therefore, the question is what an attacker who compromises an
   authorized NAS, agent, or server can do using Diameter EAP messages.
   Some of the consequences are rather obvious.  For instance, a
   Diameter agent can give access to unauthorized users by changing the
   Result-Code to DIAMETER_SUCCESS.  Other consequences are less obvious
   and are discussed below and authentication method negotiation attacks
   are discussed in the next section.

   By including suitable AVPs in an AA-Answer/Diameter-EAP-Answer
   messages, an attacker may be able (depending on implementation and
   configuration details) to:

   o  Give unauthorized users access, or deny access to authorized users
      (Result-Code).

   o  Give an attacker a login session to a host otherwise protected by
      firewalls, or 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 an expensive toll
      number controlled by the attacker (callback AVPs).

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

   By modifying an AA-Request/Diameter-EAP-Request, an attacker may be
   able to:

   o  Change NAS-Identifier/NAS-Port/Origin-Host (or another attribute)
      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.

   o  Modify contents of Class and State AVPs.

   Some of these attacks can be prevented if the NAS or server is
   configured to not accept some particular AVPs, or accepts them only
   from some nodes.

8.3.  Negotiation Attacks

   This section deals with attacks where the NAS, any Diameter agents,
   or Diameter server attempt to cause the authenticating user to choose
   some authentication method other than EAP, such as PAP or CHAP
   (negotiation attacks within EAP are discussed in [EAP], Section 7.8).

   The vulnerability can be mitigated via implementation of a per-
   connection policy by the authenticating peer, and a per-user policy
   by the Diameter server.  For the authenticating peer, the
   authentication policy should be set on a per-connection basis.

   With a per-connection policy, an authenticating peer will only
   attempt to negotiate EAP for a session in which EAP support is
   expected.  As a result, it is presumed that an authenticating peer
   selecting EAP requires that level of security.  If it cannot be
   provided, there is likely a misconfiguration, or the authenticating
   peer may be contacting the wrong server.  In this case, the
   authenticating peer simply disconnects.

   Similarly, with a per-user policy, the home server will not accept
   authentication methods other than EAP for users for which EAP support
   is expected.

   For a NAS, it may not be possible to determine whether a peer is
   required to authenticate with EAP until the peer's identity is known.
   For example, for shared-uses NASes one reseller may implement EAP
   while another does not.  Alternatively, some peer might be

   authenticated locally by the NAS while other peers are authenticated
   via Diameter.  In such cases, if any peers of the NAS MUST do EAP,
   then the NAS MUST attempt to negotiate EAP for every session.  This
   avoids forcing a peer to support more than one authentication type,
   which could weaken security.

8.4.  Session Key Distribution

   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-Master-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 up with some malicious party that must be able to
   eavesdrop, modify, or insert traffic between the user and the NAS
   during the lifetime of those keys (for example, in 802.11i the
   attacker must also eavesdrop the "four-way handshake").

8.5.  Privacy Issues

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

8.6.  Note about EAP and Impersonation

   If the EAP method used does not provide mutual authentication,
   obviously anyone can impersonate the network to the user.  Even when
   EAP mutual authentication is used, it occurs between the user and the
   Diameter home server.  See [EAPKey] for an extensive discussion about
   the details and their implications.

   One issue is worth pointing out here.  As described in [EAPKey], the
   current EAP architecture does not allow the home server to restrict
   what service parameters or identities (such as SSID or BSSID in
   802.11 wireless LANs) are advertised by the NAS to the client.  That
   is, a compromised NAS can change its BSSID or SSID, and thus appear
   to offer a different service than intended.  Even if these parameters
   are included in Diameter-EAP-Answer messages, the NAS can tell
   different values to the client.

   Therefore, the NAS's possession of the session keys proves that the
   user is talking to an authorized NAS, but a compromised NAS can lie
   about its exact identity.  See [EAPKey] for discussion on how
   individual EAP methods can provide authentication of NAS service
   parameters and identities.

   Note that the usefulness of this authentication may be rather limited
   in many environments.  For instance, in wireless LANs the user does
   not usually securely know the identity (such as BSSID) of the "right"
   access point; it is simply picked from a beacon message that has the
   correct SSID and good signal strength (something that is easy to
   spoof).  Thus, simply authenticating the identity may not allow the
   user to distinguish the "right" access point from all others.

9.  Acknowledgements

   This Diameter application relies heavily on earlier work on Diameter
   NASREQ application [NASREQ] and RADIUS EAP support [RFC3579].  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,
   Julien Bournelle, Pat Calhoun, Henry Haverinen, John Loughney,
   Yoshihiro Ohba, and Joseph Salowey.

10.  References

10.1.  Normative References

   [BASE]         Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and
                  J. Arkko, "Diameter Base Protocol", RFC 3588,
                  September 2003.

   [EAP]          Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and
                  H. Levkowetz, "Extensible Authentication Protocol
                  (EAP)", RFC 3748, June 2004.

   [NASREQ]       Calhoun, P., Zorn, G., Spence, D., and D. Mitton,
                  "Diameter Network Access Server Application", RFC
                  4005, August 2005.

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

10.2.  Informative References

   [EAPKey]       Aboba, B., Simon, D., Arkko, J., Eronen, P., and H.
                  Levkowetz, "Extensible Authentication Protocol (EAP)
                  Key Management Framework", Work in Progress, July
                  2004.

   [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,
                  "IEEE Standard for Information technology -
                  Telecommunications and information exchange between
                  systems - Local and metropolitan area networks -
                  Specific requirements - Part 11: Wireless Medium
                  Access Control (MAC) and Physical Layer (PHY)
                  Specifications: Amendment 6: Medium Access Control
                  (MAC) Security Enhancements", IEEE Standard
                  802.11i-2004, July 2004.

   [IKEv2]        Kaufman, C., Ed., "Internet Key Exchange (IKEv2)
                  Protocol", Work in Progress, June 2004.

   [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.

   [RFC2865]      Rigney, C., Willens, S., Rubens, A., and W. Simpson,
                  "Remote Authentication Dial In User Service (RADIUS)",
                  RFC 2865, June 2000.

   [RFC3575]      Aboba, B., "IANA Considerations for RADIUS (Remote
                  Authentication Dial In User Service)", RFC 3575,
                  July 2003.

   [RFC3576]      Chiba, M., Dommety, G., Eklund, M., Mitton, D., and B.
                  Aboba, "Dynamic Authorization Extensions to Remote
                  Authentication Dial In User Service (RADIUS)",
                  RFC 3576, July 2003.

   [RFC3579]      Aboba, B. and P. Calhoun, "RADIUS (Remote
                  Authentication Dial In User Service) Support For
                  Extensible Authentication Protocol (EAP)", RFC 3579,
                  September 2003.

   [RFC3580]      Congdon, P., Aboba, B., Smith, A., Zorn, G., and J.
                  Roese, "IEEE 802.1X Remote Authentication Dial In User
                  Service (RADIUS) Usage Guidelines", RFC 3580,
                  September 2003.

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: tomhiller@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

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