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Versions: 00 01 02 03 04 05 06 07 08 09 10               Standards Track
EAP Working Group                                               L. Blunk
INTERNET-DRAFT                                      Merit Networks, Inc.
Expires: July 2003                                         J. Vollbrecht
<draft-ietf-pppext-rfc2284bis-10.txt>          Vollbrecht Consulting LLC
22 January 2003                                                 B. Aboba
                                                               Microsoft
                                                              J. Carlson
                                                  Sun Microsystems, Inc.


                Extensible Authentication Protocol (EAP)

This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC 2026.

Internet-Drafts are working documents of the Internet Engineering Task
Force (IETF), its areas, and its working groups.  Note that other groups
may also distribute working documents as Internet- Drafts.

Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time.  It is inappropriate to use Internet Drafts as reference material
or to cite them other than as "work in progress."

The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt

The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.

Copyright Notice

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

Abstract

This document defines the Extensible Authentication Protocol (EAP), an
authentication framework which supports multiple authentication
mechanisms. EAP typically runs directly over the link layer without
requiring IP, but is reliant on lower layer ordering guarantees as in
PPP and IEEE 802. EAP does provide its own support for duplicate
elimination and retransmission.  Fragmentation is not supported within
EAP itself; however, individual EAP methods may support this.  While EAP
was originally developed for use with PPP, it is also now in use with
IEEE 802.

This document obsoletes RFC 2284.




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Table of Contents

1.     Introduction ..........................................    3
   1.1       Specification of Requirements ...................    3
   1.2       Terminology .....................................    3
2.     Extensible Authentication Protocol (EAP) ..............    6
   2.1       Support for sequences ...........................    7
   2.2       EAP multiplexing model ..........................    8
3.     Lower layer behavior ..................................   10
   3.1       Lower layer requirements ........................   10
   3.2       EAP usage within PPP ............................   11
   3.3       EAP usage within IEEE 802 .......................   12
   3.4       Link layer indications ..........................   13
4.     EAP Packet Format .....................................   13
   4.1       Request and Response ............................   14
   4.2       Success and Failure .............................   17
5.     Initial EAP Request/Response Types ....................   19
   5.1       Identity ........................................   20
   5.2       Notification ....................................   21
   5.3       Nak .............................................   22
   5.4       MD5-Challenge ...................................   23
   5.5       One-Time Password ...............................   24
   5.6       Generic Token Card ..............................   25
   5.7       Vendor-specific .................................   26
6.     IANA Considerations ...................................   28
   6.1       Definition of Terms .............................   28
   6.2       Recommended Registration Policies ...............   28
7.     Security considerations ...............................   29
   7.1       Threat model ....................................   29
   7.2       Security claims .................................   30
   7.3       Identity protection .............................   31
   7.4       Man-in-the-middle attacks .......................   31
   7.5       Packet modification attacks .....................   32
   7.6       Dictionary attacks ..............................   32
   7.7       Connection to an untrusted network ..............   33
   7.8       Negotiation attacks .............................   33
   7.9       Implementation idiosyncrasies ...................   33
   7.10      Key derivation ..................................   34
   7.11      Weak ciphersuites ...............................   36
8.     Normative references ..................................   36
9.     Informative references ................................   37
ACKNOWLEDGMENTS ..............................................   38
AUTHORS' ADDRESSES ...........................................   38
Intellectual Property Notices ................................   39
Full Copyright Statement .....................................   40






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1.  Introduction

This document defines the Extensible Authentication Protocol (EAP), an
authentication framework which supports multiple authentication
mechanisms.  EAP typically runs directly over the link layer without
requiring IP, but is reliant on lower layer ordering guarantees as in
PPP and IEEE 802. EAP does provide its own support for duplicate
elimination and retransmission.  Fragmentation is not supported within
EAP itself; however, individual EAP methods may support this.

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]. It has also been implemented with switches and
access points using IEEE 802 [IEEE802].  EAP encapsulation on IEEE 802
wired media is described in [IEEE8021X].

One of the advantages of the EAP architecture is its flexibility.  EAP
is used to select a specific authentication mechanism, typically after
the authenticator requests more information in order to determine the
specific authentication mechanism(s) to be used.  Rather than requiring
the authenticator to be updated to support each new authentication
method, EAP permits the use of a backend authentication server which may
implement some or all authentication methods, with the authenticator
acting as a pass-through for some or all methods and users.

Within this document, authenticator requirements apply regardless of
whether the authenticator is operating as a pass-through. Where the
requirement is meant to apply to either the authenticator or backend
authentication server, depending on where the EAP authentication is
terminated, the term "EAP server" will be used.

1.1.  Specification of Requirements

In this document, several words are used to signify the requirements of
the specification.  These words are often capitalized.  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].

1.2.  Terminology

This document frequently uses the following terms:

authenticator
          The end of the link requiring the authentication. This
          terminology is also used in [IEEE802lX], and has the same
          meaning in this document.



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peer      The other end of the point-to-point link (PPP), point-to-point
          LAN segment (IEEE 802 wired media) or 802.11 wireless link,
          which being authenticated by the authenticator. In
          [IEEE8021X], this end is known as the Supplicant.

backend authentication server
          A backend authentication server is an entity that provides an
          authentication service to an authenticator. This service
          verifies from the credentials provided by the peer, the claim
          of identity made by the peer. This terminology is also used in
          [IEEE8021X].

Displayable Message
          This is interpreted to be a human readable string of
          characters, and MUST NOT affect operation of the protocol.
          The message encoding MUST follow the UTF-8 transformation
          format [RFC2279].

EAP server
          The entity that terminates the EAP authentication with the
          peer.  In the case where there is no backend authentication
          server, this term refers to the authenticator. Where the
          authenticator operates in pass-through, it refers to the
          backend authentication server.

Silently Discard
          This means the implementation discards the packet without
          further processing.  The implementation SHOULD provide the
          capability of logging the event, including the contents of the
          silently discarded packet, and SHOULD record the event in a
          statistics counter.

Security claims (see Section 7.2):

Mutual authentication
          This refers to an EAP method in which, within an interlocked
          exchange, the authenticator authenticates the peer and the
          peer authenticates the authenticator. Two one-way
          conversations, running in opposite directions do not provide
          mutual authentication as defined here.

Integrity protection
          This refers to per-packet authentication and integrity
          protection of EAP packets, including EAP Requests and
          Responses, and method-specific success and failure
          indications. When making this claim, a method specification
          MUST describe the fields within the EAP packet that are
          protected.



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Replay protection
          This refers to protection against replay of EAP messages,
          including EAP Requests and Responses, and method-specific
          success and failure indications.

Confidentiality
          This refers to encryption of EAP messages, including EAP
          Requests and Responses, and method-specific success and
          failure indications. A method making this claim MUST support
          identity protection.

Key derivation
          This refers to the ability of the EAP method to derive a
          Master Key which is not exported, as well as a ciphersuite-
          independent Master Session Keys. Both the Master Key and
          Master Session Keys are used only for further key derivation,
          not directly for protection of the EAP conversation or
          subsequent data.

Key strength
          This refers to the effective entropy of the derived Master
          Session Keys, independent of their physical length. For
          example, a 128-bit key derived from a password might have an
          effective entropy much less than 128 bits.

Dictionary attack resistance
          Where password authentication is used, users are notoriously
          prone to selection of poor passwords. A method may be said to
          be dictionary attack resistant if, when there is a weak
          password in the secret,  the method does not allow an attack
          more efficient than brute force.

Fast reconnect
          The ability, in the case where a security association has been
          previously established, to create a new or refreshed security
          association in a smaller number of round-trips.

Man-in-the-Middle resistance
          The ability for the peer to demonstrate to the authenticator
          that it has acted as the peer for each method within a
          sequence of methods or tunnel. Similarly, the authenticator
          demonstrates to the peer that it has acted as the
          authenticator for each method within the sequence or tunnel.
          If this is not possible, then the authentication sequence or
          tunnel may be vulnerable to a man-in-the-middle attack.

Acknowledged result indications
          The ability of the authenticator to provide the peer with an



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          indication of whether the peer has successfully authenticated
          to it, and for the peer to acknowledge receipt, as well as
          providing an indication of whether the authenticator has
          successfully authenticated to the peer.  Since EAP Success and
          Failure packets are neither acknowledged nor integrity
          protected, this claim requires implementation of a method-
          specific result exchange that is integrity protected.

2.  Extensible Authentication Protocol (EAP)

The EAP authentication exchange proceeds as follows:

[1]  The authenticator sends a Request to authenticate the peer.  The
     Request has a type field to indicate what is being requested.
     Examples of Request types include Identity,  MD5-challenge, etc.
     The MD5-challenge type corresponds closely to the CHAP
     authentication protocol [RFC1994].  Typically, the authenticator
     will send an initial Identity Request; however, an initial Identity
     Request is not required, and MAY be bypassed. For example, the
     identity may not be required where it is determined by the port to
     which the peer has connected (leased lines, dedicated switch or
     dial-up ports); or where the identity is obtained in another
     fashion (via calling station identity or MAC address, in the Name
     field of the MD5-Challenge Response, etc.).

[2]  The peer sends a Response packet in reply to a valid Request.  As
     with the Request packet, the Response packet contains a Type field
     which corresponds to the Type field of the Request.

[3]  The authenticator sends an additional Request packet, and the peer
     replies with a Response. The sequence of Requests and Responses
     continues as long as needed.

[4]  The conversation continues until the authenticator cannot
     authenticate the peer (unacceptable Responses to one or more
     Requests), in which case the authenticator implementation MUST
     transmit an EAP Failure.  Alternatively, the authentication
     conversation can continue until the authenticator determines that
     successful authentication has occurred, in which case the
     authenticator MUST transmit an EAP Success.

Since EAP is a peer-to-peer protocol, an independent and simultaneous
authentication may take place in the reverse direction. Both peers may
act as authenticators and authenticatees at the same time.

Advantages

   The EAP protocol can support multiple authentication mechanisms



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   without having to pre-negotiate a particular one.

   Devices (e.g. a NAS, switch or access point) do not have to
   understand each authentication method and MAY act as a pass-through
   agent for a backend authentication server.  Support for pass-through
   is optional. An authenticator MAY authenticate local users while at
   the same time acting as a pass-through for non-local users and
   authentication methods it does not implement locally.

   For sessions in which the authenticator acts as a pass-through, it
   MUST determine the outcome of the authentication solely based on the
   Accept/Reject indication sent by the backend authentication server;
   the outcome MUST NOT be determined by the contents of an EAP packet
   sent along with the Accept/Reject indication, or the absence of such
   an encapsulated EAP packet.

   Separation of the authenticator from the backend authentication
   server simplifies credentials management and policy decision making.

Disadvantages

   For use in PPP, EAP does require the addition of a new authentication
   type to PPP LCP and thus PPP implementations will need to be modified
   to use it. It also strays from the previous PPP authentication model
   of negotiating a specific authentication mechanism during LCP.
   Similarly, switch or access point implementations need to support
   [IEEE8021X] in order to use EAP.

   Where the authenticator is separate from the backend authentication
   server, this complicates the security analysis and, if needed, key
   distribution.

2.1.  Support for sequences

An EAP conversation MAY utilize a sequence of methods. A common example
of this is an Identity request followed by a single EAP authentication
method such as an MD5-Challenge. However, within or associated with each
EAP server, it is not anticipated that a particular named peer will
utilize multiple authentication methods (Type 4 or greater), either by
supporting a choice of methods or by using multiple methods in sequence.
This would make the peer vulnerable to attacks that negotiate the least
secure method from among a set (negotiation attacks, described in
Section 7.8) or man-in-the-middle attacks (described in Section 7.4).
Instead, for each named peer there SHOULD be an indication of exactly
one method used to authenticate that peer name. If a peer needs to make
use of different authentication methods under different circumstances,
then distinct identities SHOULD be employed, each of which identifies
exactly one authentication method.



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If additional authentication methods are required beyond the initial
one, the authenticator MAY send a Request packet for a subsequent
authentication method, or it MAY send another Identity request. If the
peer does not support additional methods, it SHOULD respond with a Nak,
indicating no acceptable alternative, as described in Section 5.3.
However, peer implementations MAY not respond at all, in which case a
timeout will result and authentication will fail. Since the
authenticator presumably requires successful completion of the sequence
in order to grant access, authentication failure is the correct result.
Therefore, it is not necessary for the authenticator to determine that
the peer supports sequences prior to sending a Request for a subsequent
authentication method.

The above prescription also applies in the situation where an
authenticator sends a message of a different Type prior to completion of
the final round of a given method. If the peer wishes to continue
authenticating with the method in progress, it SHOULD send a Nak in
response to such a Request, indicating the Type in progress as the
alternative.  Otherwise it MAY send a Response with the same Type as the
Request.  Since an EAP packet with a different Type may be sent by an
attacker, an authenticator receiving a Nak including a preference for
the Type in progress SHOULD log the event, but otherwise not take any
action.

Once a peer has sent a Response of the same Type as a Request, some
existing peer implementations might expect the method to run to
completion. As a result, these implementations silently discard EAP
Requests of a Type different from the method in progress, despite the
requirement for a Response in section 4.1. For this reason, EAP
authenticators that must interoperate with these peers are discouraged
from switching methods before the final round of a given method has
completed.

2.2.  EAP multiplexing model

Conceptually, EAP implementations consist of the following components:

[a]  Lower layer. The lower layer is responsible for transmitting and
     receiving EAP frames between the peer and authenticator. EAP has
     been run over a variety of lower layers including PPP; wired IEEE
     802 LANs [IEEE8021X]; IEEE 802.11 wireless LANs [IEEE80211]; UDP
     (L2TP [RFC2661] and ISAKMP [PIC]); and TCP [PIC]. Lower layer
     behavior is discussed in Section 3.

[b]  EAP layer. The EAP layer receives and transmits EAP packets via the
     lower layer, implements the EAP state machine, and delivers and
     receives EAP messages to and from EAP methods.




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[c]  EAP method. EAP methods implement the authentication algorithms and
     receive and transmit EAP messages via the EAP layer. Since
     fragmentation support is not provided by EAP itself, this is the
     responsibility of EAP methods, which are discussed in Section 5.

The EAP multiplexing model is illustrated in figure 1 on the next page.
Note that there is no requirement that an implementation conform to this
model, as long as the on-the-wire behavior is consistent with it.

Within EAP, the Type field functions much like a port number in UDP or
TCP.  With the exception of Types handled by the EAP layer, it is
assumed that the EAP layer multiplexes incoming EAP packets according to
their Type, and delivers them only to the EAP method corresponding to
that Type code, with one exception.

Since EAP methods may wish to access the Identity, the Identity Response
can be assumed to be stored within the EAP layer so as to be available
to methods of Types other than 1 (Identity). The Identity Type is
discussed in Section 5.1.

+-+-+-+-+-+-+-+-+-+-+-+-+  +-+-+-+-+-+-+-+-+-+-+-+-+
|           |           |  |           |           |
| EAP method| EAP method|  | EAP method| EAP method|
| Type = X  | Type = Y  |  | Type = X  | Type = Y  |
|       !   |           |  |       ^   |           |
+-+-+-+-!-+-+-+-+-+-+-+-+  +-+-+-+-!-+-+-+-+-+-+-+-+
|       !               |  |       !               |
|  EAP  ! Layer         |  |  EAP  !  Layer        |
|       !               |  |       !               |
| (Nak, ! Success,      |  | (Nak, ! Success,      |
|       ! Failure,      |  |       ! Failure,      |
|       ! Notification, |  |       ! Notification, |
|       ! Identity)     |  |       ! Identity)     |
|       !               |  |       !               |
+-+-+-+-!-+-+-+-+-+-+-+-+  +-+-+-+-!-+-+-+-+-+-+-+-+
|       !               |  |       !               |
| Lower !  Layer        |  | Lower !  Layer        |
|       !               |  |       !               |
+-+-+-+-!-+-+-+-+-+-+-+-+  +-+-+-+-!-+-+-+-+-+-+-+-+
        !                          !
        +------------>-------------+

Figure 1. EAP Multiplexing Model

A Notification Response is only used as confirmation that the peer
received the Notification Request, not that it has processed it, or
displayed the message to the user. It cannot be assumed that the
contents of the Notification Request or Response is available to another



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method. The Notification Type is discussed in Section 5.2.

The Nak method is utilized for the purposes of method negotiation.
Peers MUST respond to an EAP Request for an unacceptable Type with a Nak
Response. It cannot be assumed that the contents of the Nak Response is
available to another method.  The Nak Type is discussed in Section 5.3.

EAP packets with codes of Success or Failure do not include a Type, and
therefore are not delivered to an EAP method. Success and Failure are
discussed in Section 4.2.

Given these considerations, the Success, Failure, Nak Response and
Notification Request/Response messages MUST NOT used to carry data
destined for delivery to other EAP methods.

3.  Lower layer behavior

3.1.  Lower layer requirements

EAP makes the following assumptions about lower layers:

[1]  Lower layer CRC or checksum. In EAP, the authenticator retransmits
     Requests that have not yet received Responses, so that EAP does not
     assume that lower layers are reliable.  Since EAP defines its own
     retransmission behavior, when run over a reliable lower layer, it
     is possible (though undesirable) for retransmission to occur both
     in the lower layer and the EAP layer.

     If lower layers exhibit a high loss rate, then retransmissions are
     likely, and since EAP Success and Failure are not retransmitted,
     timeouts are also likely to result.  EAP methods such as EAP TLS
     [RFC2716] include a message integrity check (MIC) and regard MIC
     errors as fatal. Therefore if a checksum or CRC is not provided by
     the lower layer, then some methods may not behave well.

[2]  Lower layer data security. After EAP authentication is complete,
     the peer will typically transmit data to the network, through the
     authenticator.  In order to provide assurance that the peer
     transmitting data is the one that successfully completed EAP
     authentication, it is necessary for the lower layer to provide per-
     packet integrity, authentication and replay protection that is
     bound to the original EAP authentication, or for the lower layer to
     be physically secure. Otherwise it is possible for subsequent data
     traffic to be hijacked, or replayed.

     As a result of these considerations, EAP SHOULD be used only when
     lower layers provide physical security for data (e.g. wired PPP or
     IEEE 802 links), or for insecure links, where per-packet



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     authentication, integrity and replay protection is provided.  Where
     keying material for the lower layer ciphersuite is itself provided
     by EAP, typically the lower layer ciphersuite cannot be enabled
     until late in the EAP conversation, after key derivation has
     completed.  Thus it may only be possible to use the lower layer
     ciphersuite to protect a portion of the EAP conversation, such as
     the EAP Success or Failure packet.

[3]  Known MTU. The EAP layer does not support fragmentation and
     reassembly. However, EAP methods SHOULD be capable of handling
     fragmentation and reassembly. As a result, EAP is capable of
     functioning across a range of MTU sizes, as long as the MTU is
     known.

[4]  Possible duplication. Where the lower layer is reliable, it will
     provide the EAP layer with a non-duplicated stream of packets.
     However, while it is desirable that lower layers provide for non-
     duplication, this is not a requirement. The Identifier field
     provides both the peer and authenticator with the ability to detect
     duplicates.

[5]  Ordering guarantees. EAP does not require the Identifier to be
     monotonically increasing, and so is reliant on lower layer ordering
     guarantees for correct operation. Also, EAP was originally defined
     to run on PPP and [RFC1661] Section 1 has an ordering requirement:

     "The Point-to-Point Protocol is designed for simple links which
     transport packets between two peers. These links provide full-
     duplex simultaneous bi-directional operation, and are assumed to
     deliver packets in order."

     Lower lower transports for EAP MUST preserve ordering between a
     source and destination, at a given priority level (the level of
     ordering guarantee provided by [IEEE802]).

3.2.  EAP usage within PPP

In order to establish communications over a point-to-point link, each
end of the PPP link must first send LCP packets to configure the data
link during Link Establishment phase.  After the link has been
established, PPP provides for an optional Authentication phase before
proceeding to the Network-Layer Protocol phase.

By default, authentication is not mandatory. If authentication of the
link is desired, an implementation MUST specify the Authentication-
Protocol Configuration Option during Link Establishment phase.





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The server can use the identification of the connecting host or router
in the selection of options for network layer negotiations.

When implemented within PPP, EAP does not select a specific
authentication mechanism at PPP Link Control Phase, but rather postpones
this until the Authentication Phase.  This allows the authenticator to
request more information before determining the specific authentication
mechanism.  This also permits the use of a "back-end" server which
actually implements the various mechanisms while the PPP authenticator
merely passes through the authentication exchange.  The PPP Link
Establishment and Authentication phases, and the Authentication-Protocol
Configuration Option, are defined in The Point-to-Point Protocol (PPP)
[RFC1661].

3.2.1.  PPP Configuration Option Format

A summary of the PPP Authentication-Protocol Configuration Option format
to negotiate the EAP Authentication Protocol is shown below.  The fields
are transmitted from left to right.

Exactly one EAP packet is encapsulated in the Information field of a PPP
Data Link Layer frame where the protocol field indicates type hex C227
(PPP EAP).

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|     Type      |    Length     |     Authentication-Protocol   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type

   3

Length

   4

Authentication-Protocol

   C227 (Hex) for PPP Extensible Authentication Protocol (EAP)

3.3.  EAP usage within IEEE 802

The encapsulation of EAP over IEEE 802 is defined in [IEEE8021X].  The
IEEE 802 encapsulation of EAP does not involve PPP, and IEEE 802.1X does
not include support for link or network layer negotiations. As a result,
within IEEE 802.1X it is not possible to negotiate non-EAP



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authentication mechanisms, such as PAP or CHAP [RFC1994].

3.4.  Link layer indications

The reliability and security of link layer indications is dependent on
the medium. Link layer failure indications accepted by the link layer
and provided to EAP MUST be processed. However, link layer success
indications MUST NOT result in an EAP implementation concluding that
authentication has succeeded, since these indications are typically
unauthenticated.

In PPP, link layer indications are not authenticated and are therefore
subject to spoofing, provided that the attacker can gain access to the
physical medium.  This includes LCP-Terminate (a link failure
indication), NCP (a link success indication), and "link down" (a link
failure indication).

In IEEE 802 wired networks, the IEEE 802.1X EAPOL-Start and EAPOL-Logoff
frames are not authenticated or integrity protected, whereas within
802.11, authentication and integrity protection is possible depending on
when they are sent and the ciphersuite that has been negotiated.
Therefore, depending on the circumstances, EAPOL-Start and EAPOL-Logoff
frames may or may not be subject to authenticated and integrity
protected.

In 802.11 a "link down" indication is an unreliable indication of link
failure, since wireless signal strength can come and go and may be
influenced by radio frequency interference generated by an attacker.  In
802.11, control and management frames are not authenticated and an
attacker within range can gain access to the physical medium. Link layer
indications include Disassociate and Deauthenticate frames (link failure
indications), and Association and Reassociation Response frames (link
success indications).

4.  EAP Packet format

A summary of the EAP packet format is shown below. The fields are
transmitted from left to right.

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|     Code      |  Identifier   |            Length             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|    Data ...
+-+-+-+-+

Code



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   The Code field is one octet and identifies the type of EAP packet.
   EAP Codes are assigned as follows:

         1       Request
         2       Response
         3       Success
         4       Failure

   Since EAP only defines Codes 1-4, EAP packets with other codes MUST
   be silently discarded by both authenticators and peers.

Identifier

   The Identifier field is one octet and aids in matching Responses with
   Requests.

Length

   The Length field is two octets and indicates the length of the EAP
   packet including the Code, Identifier, Length and Data fields.
   Octets outside the range of the Length field should be treated as
   Data Link Layer padding and should be ignored on reception.

Data

   The Data field is zero or more octets.  The format of the Data field
   is determined by the Code field.

4.1.  Request and Response

Description

   The Request packet (Code field set to 1) MUST be sent by the
   authenticator to the peer; the peer MUST NOT send Request packets to
   the authenticator.  Each Request has a Type field which serves to
   indicate what is being requested. Additional Request packets MUST be
   sent until a valid Response packet is received, or an optional retry
   counter expires. In [IEEE8021X], the retry counter is effectively set
   to zero, so that retransmission never occurs, and instead the peer
   times out and authentication is restarted.

   Retransmitted Requests MUST be sent with the same Identifier value in
   order to distinguish them from new Requests. The contents of the data
   field is dependent on the Request type.  The peer MUST send a
   Response packet in reply to a valid Request packet. Responses MUST
   only be sent in reply to a valid Request and never retransmitted on a
   timer.




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   The Identifier field of the Response MUST match that of the Request;
   if it does not match, then the Response MUST be silently discarded.
   Authenticators receiving a Response with a Type other than Nak, for
   which the authenticator has no outstanding Request MUST silently
   discard the Response.

   The authenticator MUST NOT send a new Request until a valid Response
   is received to an outstanding Request.  Since the authenticator can
   retransmit before receiving a valid Response from the peer, the
   authenticator can receive duplicate Responses. The authenticator MUST
   silently discard these duplicate Responses.

   If a Message Integrity Check (MIC) is employed within an EAP method,
   then implementations MUST silently discard any message that fails
   this check.  In this document, the descriptions of EAP message
   handling assume that MIC validation is effectively performed as
   though it occurs before examining any of the EAP message fields (such
   as 'Code').

      Implementation Note: These obligations apply regardless of whether
      pass-through is implemented.  The authenticator is responsible for
      retransmitting Request messages.  If the Request message is
      obtained from elsewhere (such as from a backend authentication
      server), then the authenticator will need to save a copy of the
      Request in order to accomplish this. The authenticator is also
      responsible for discarding Response messages with the wrong
      Identifier value before acting on them in any way, including
      passing them on to the backend authentication server for
      verification. Similarly, the peer is responsible for detecting and
      handling duplicate Request messages before processing them in any
      way, including passing them on to an outside party.

      Because the authentication process will often involve user input,
      some care must be taken when deciding upon retransmission
      strategies and authentication timeouts.  By default, where EAP is
      run over an unreliable lower layer, the EAP retransmission timer
      (EAP_RTO) SHOULD be computed as described in [RFC2988]. This
      includes use of Karn's algorithm to filter RTT estimates resulting
      from retransmissions.  A maximum of 3-5 retransmissions is
      suggested.

      When run over a reliable lower layer (e.g. EAP over ISAKMP/TCP, as
      within [PIC]), the EAP retransmission timer SHOULD be set to an
      infinite value, so that retransmissions do not occur at the EAP
      layer.

      Where the authentication process requires user input, the measured
      round trip times are largely be determined by user responsiveness



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      rather than network characteristics, so that RTO estimation is not
      helpful.  Instead, the retransmission timers SHOULD be set so as
      to provide sufficient time for the user to respond, with longer
      timeouts required in certain cases, such as where Token Cards are
      involved.

      In order to provide the EAP authenticator with guidance as to the
      appropriate timeout value, a hint can be communicated to the
      authenticator by the backend authentication server (such as via
      the RADIUS Session-Timeout attribute).

   A summary of the Request and Response packet format is shown below.
   The fields are transmitted from left to right.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Code      |  Identifier   |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |  Type-Data ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

Code

   1 for Request
   2 for Response

Identifier

   The Identifier field is one octet. The Identifier field MUST be the
   same if a Request packet is retransmitted due to a timeout while
   waiting for a Response. Any new (non-retransmission) Requests MUST
   modify the Identifier field. In order to avoid confusion between new
   Requests and retransmissions, the Identifier value chosen for each
   new Request need only be different from the previous Request, but
   need not be unique within the conversation. One way to achieve this
   is to start the Identifier at an initial value and increment it for
   each new Request. Initializing the first Identifier with a random
   number rather than starting from zero is recommended, since it makes
   sequence attacks somewhat harder.

   Since the Identifier space is unique to each session, authenticators
   are not restricted to only 256 simultaneous authentication
   conversations. Similarly, with re-authentication, an EAP conversation
   might continue over a long period of time, and is not limited to only
   256 roundtrips.

   If a peer receives a valid duplicate Request for which it has already



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   sent a Response, it MUST resend its original Response.  If a peer
   receives a duplicate Request before it has sent a Response, but after
   it has determined the initial Request to be valid (i.e.  it is
   waiting for user input), it MUST silently discard the duplicate
   Request. An EAP message may be found invalid for a variety of
   reasons: failed lower layer CRC or checksum, malformed EAP packet,
   EAP method MIC failure, etc.

Length

   The Length field is two octets and indicates the length of the EAP
   packet including the Code, Identifier, Length, Type, and Type-Data
   fields.  Octets outside the range of the Length field should be
   treated as Data Link Layer padding and should be ignored on
   reception.

Type

   The Type field is one octet.  This field indicates the Type of
   Request or Response. A single Type MUST be specified for each EAP
   Request or Response.  Normally, the Type field of the Response will
   be the same as the Type of the Request.  However, there is also a Nak
   Response Type for indicating that a Request type is unacceptable to
   the peer. An initial specification of Types follows in a later
   section of this document.

Type-Data

   The Type-Data field varies with the Type of Request and the
   associated Response.

4.2.  Success and Failure

   The Success packet is sent by the authenticator to the peer to
   acknowledge successful authentication.  The authenticator MUST
   transmit an EAP packet with the Code field set to 3 (Success).  If
   the authenticator cannot authenticate the peer (unacceptable
   Responses to one or more Requests) then the implementation MUST
   transmit an EAP packet with the Code field set to 4 (Failure).  An
   authenticator MAY wish to issue multiple Requests before sending a
   Failure response in order to allow for human typing mistakes.
   Success and Failure packets MUST NOT contain additional data.

      Implementation Note: Because the Success and Failure packets are
      not acknowledged, the authenticator cannot know whether they have
      been received. As a result, these packets are not retransmitted by
      the authenticator, and if they are lost, the peer will timeout. If
      acknowledged success and failure indications are desired, these



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      MAY be implemented within individual EAP methods.

   A summary of the Success and Failure packet format is shown below.
   The fields are transmitted from left to right.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Code      |  Identifier   |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Code

   3 for Success
   4 for Failure

Identifier

   The Identifier field is one octet and aids in matching replies to
   Responses.  The Identifier field MUST match the Identifier field of
   the Response packet that it is sent in response to.

Length

   4

4.2.1.  Processing of success and failure

Within EAP, success and failure indications consist of the EAP Success
and Failure messages, as well as method-specific indications.  Within
EAP, these indications may be protected or unprotected.

EAP Success and Failure packets are considered unprotected indications
which may be spoofed, since as described in Section 4.2, they contain no
message integrity check (MIC).

In order to provide additional protection against tampering, EAP methods
MAY support a MIC that covers some or all of the EAP packet, including
headers. In addition, such a MIC MAY include coverage of previous
Request and Response messages, so as to enable protection of other
packets to that do not contain MICs, such as Identity Request/Response,
Notification Request/Response and Nak Response.

EAP methods also MAY include support for method-specific acknowledged
success and failure indications. This enables the authenticator to
indicate whether the peer has successfully authenticated, as well as for
the peer to acknowledge receipt of that indication, and respond with an
indication of whether the authenticator has successfully authenticated



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to the peer. If a key has previously been derived, the result exchange
MAY be protected by a Message Integrity Check (MIC), and if so, then
this success/failure indication is considered protected.

In order to decrease vulnerability to spoofing of success and failure
indications, the following processing rules are recommended:

[a]  Processing of protected success and failure indications. Where a
     method-specific protected success/failure indication has been
     received, the implementation MUST validate the EAP method MIC, with
     a MIC failure handled via silent discard, as specified in Section
     4.1.

[b]  Receipt of EAP Success and Failure packets prior to method
     completion. A peer EAP implementation receiving an EAP Success
     packet prior to completion of the method in progress MUST silently
     discard it. This ensures that a rogue authenticator will not be
     able to bypass mutual authentication by sending an EAP Success
     prior to conclusion of the EAP method conversation.  A peer EAP
     implementation receiving an EAP Failure packet prior to completion
     of the method in progress MAY silently discard it. When using EAP
     methods that provide their own (protected) error indications,
     premature EAP Failure packets are unexpected, so that this
     technique may be more readily employed.

[c]  Authentication requirement. An EAP peer implementation that has
     been configured to require authentication MUST silently discard a
     "canned" EAP Success message (an EAP Success message sent
     immediately upon connection).

[d]  Contradictory indications. Where protected and unprotected result
     indications are both available, protected indications take
     precedence.  For example, where an EAP method provides a protected
     indication that authentication failure has occurred in either
     direction, the implementation MUST silently discard subsequent EAP
     Success packets. Similarly, where an EAP method provides a
     protected indication that authentication has succeeded in both
     directions, the EAP implementation MAY silently discard EAP Failure
     packets.

[e]  Processing of EAP Success and Failure in the absence of protected
     indications. Subsequent to the completion of the EAP authentication
     method (Types 4 and greater), and in the absence of protected
     result indications, EAP Success and Failure packets MUST be
     accepted and processed by the EAP implementation.






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5.  Initial EAP Request/Response Types

This section defines the initial set of EAP Types used in
Request/Response exchanges.  More Types may be defined in follow-on
documents.  The Type field is one octet and identifies the structure of
an EAP Request or Response packet.  The first 3 Types are considered
special case Types.

The remaining Types define authentication exchanges.  The Nak Type is
valid only for Response packets, it MUST NOT be sent in a Request.  The
Nak Type MUST only be sent in response to a Request which uses an
authentication Type code (i.e., Type of 4 or greater).

All EAP implementations MUST support Types 1-4, which are defined in
this document, and SHOULD support Type 254. Follow-on RFCs MAY define
additional EAP Types.

   1       Identity
   2       Notification
   3       Nak (Response only)
   4       MD5-Challenge
   5       One Time Password (OTP)
   6       Generic Token Card (GTC)
 254       Vendor-specific
 255       Experimental use

5.1.  Identity

Description

   The Identity Type is used to query the identity of the peer.
   Generally, the authenticator will issue this as the initial Request.
   An optional displayable message MAY be included to prompt the peer in
   the case where there expectation of interaction with a user.  A
   Response of Type 1 (Identity) SHOULD be sent in Response to a Request
   with a Type of 1 (Identity).

   Since Identity Requests and Responses are not protected, from a
   security perspective, it may be preferable for protected method-
   specific Identity exchanges to be used instead.

      Implementation Note:  The peer MAY obtain the Identity via user
      input.  It is suggested that the authenticator retry the Identity
      Request in the case of an invalid Identity or authentication
      failure to allow for potential typos on the part of the user.  It
      is suggested that the Identity Request be retried a minimum of 3
      times before terminating the authentication phase with a Failure
      reply.  The Notification Request MAY be used to indicate an



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      invalid authentication attempt prior to transmitting a new
      Identity Request (optionally, the failure MAY be indicated within
      the message of the new Identity Request itself).

Type

   1

Type-Data

   This field MAY contain a displayable message in the Request,
   containing UTF-8 encoded 10646 characters [RFC2279].  The Response
   uses this field to return the Identity.  If the Identity is unknown,
   this field should be zero bytes in length.  The field MUST NOT be
   null terminated.  The length of this field is derived from the Length
   field of the Request/Response packet and hence a null is not
   required.

5.2.  Notification

Description

   The Notification Type is optionally used to convey a displayable
   message from the authenticator to the peer. An authenticator MAY send
   a Notification Request to the peer at any time, The peer MUST respond
   to a Notification Request with a Notification Response; a Nak
   Response MUST NOT be sent.

   The peer SHOULD display this message to the user or log it if it
   cannot be displayed. The Notification Type is intended to provide an
   acknowledged notification of some imperative nature, but it is not an
   error indication, and therefore does not change the state of the
   peer. Examples include a password with an expiration time that is
   about to expire, an OTP sequence integer which is nearing 0, an
   authentication failure warning, etc. In most circumstances,
   Notification should not be required.

Type

   2

Type-Data

   The Type-Data field in the Request contains a displayable message
   greater than zero octets in length, containing UTF-8 encoded 10646
   characters [RFC2279].  The length of the message is determined by
   Length field of the Request packet.  The message MUST NOT be null
   terminated.  A Response MUST be sent in reply to the Request with a



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   Type field of 2 (Notification).  The Type-Data field of the Response
   is zero octets in length.   The Response should be sent immediately
   (independent of how the message is displayed or logged).

5.3.  Nak

Description

   The Nak Type is valid only in Response messages. It is sent in reply
   to a Request where the desired authentication Type is unacceptable.
   Authentication Types are numbered 4 and above. The Response contains
   one or more authentication Types desired by the Peer. Type zero (0)
   is used to indicate that the sender has no viable alternatives.

   Since the Nak Type is only valid in Responses and has very limited
   functionality, it MUST NOT be used as a general purpose error
   indication, such as for communication of error messages, or
   negotiation of parameters specific to a particular EAP method.

Code

   2 for Response.

Identifier

   The Identifier field is one octet and aids in matching Responses with
   Requests. The Identifier field of a Nak Response MUST match the
   Identifier field of the Request packet that it is sent in response
   to.

Length

   >=6

Type

   3

Type-Data

   Where the Request contains a Type within the original EAP Type space
   (1-253, 255), or the Request contains an expanded Type as defined in
   Section 5.7, but the peer does not support expanded Types, then the
   Type-Data field of the Nak Response MUST contain one or more octets
   indicating the desired authentication Type(s), one octet per Type, or
   the value zero (0) to indicate no proposed alternative.  When the Nak
   Response includes as one of the Type(s) the value 254, this indicates
   an expanded Type of preference indicated by the relative order.  If



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   the authenticator can accomodate this preference, it will respond
   with a an expanded Type Request.

   If a peer supporting expanded Types receives an expanded Type
   Request, then the Type-Data field of the Nak Response, if sent, MUST
   contain one or more authentication Types, all of which MUST be in the
   format below (8 octets per Type). This includes the encoding of zero
   (0), to indicate no proposed alternative. See Section 5.7 for details
   on the Vendor-Id and Vendor-Type fields.  If the peer does not
   support expanded Types, then the Type-Data field of the Nak Response
   MUST contain one or more authentication Type(s), one octet per Type,
   or the value zero (0) to indicate no proposed alternative.  However,
   the value 254 MUST NOT be included as one of the preferred
   authentication Types.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Type=254    |               Vendor-Id                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Vendor-Type                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

5.4.  MD5-Challenge

Description

   The MD5-Challenge Type is analogous to the PPP CHAP protocol
   [RFC1994] (with MD5 as the specified algorithm).  The Request
   contains a "challenge" message to the peer.  A Response MUST be sent
   in reply to the Request.  The Response MAY be either of Type 4
   (MD5-Challenge) or Type 3 (Nak). The Nak reply indicates the peer's
   desired authentication Type(s).  EAP peer and EAP server
   implementations MUST support the MD5-Challenge mechanism.  An
   authenticator that supports only pass-through MUST allow
   communication with a backend authentication server that is capable of
   supporting MD5-Challenge, although the EAP authenticator
   implementation need not support MD5-Challenge itself.  However, if
   the EAP authenticator can be configured to authenticate peers via any
   non-pass-through mechanism, then the requirement applies.

   Note that the use of the Identifier field in the MD5-Challenge Type
   is different from that described in [RFC1994].  EAP allows for
   retransmission of MD5-Challenge Request packets while [RFC1994]
   states that both the Identifier and Challenge fields MUST change each
   time a Challenge (the CHAP equivalent of the MD5-Challenge Request
   packet) is sent.




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Type

   4

Type-Data

   The contents of the Type-Data  field is summarized below.  For
   reference on the use of this fields see the PPP Challenge Handshake
   Authentication Protocol [RFC1994].

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Value-Size   |  Value ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Name ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Security Claims (see Section 7.3)

   Intended use:           Wired networks, including PPP, PPPOE, and
                           IEEE 802 wired media. Use over the Internet
                           or with wireless media only when protected.
   Mechanism:              Password or pre-shared key.
   Mutual authentication:  No
   Integrity protection:   No
   Replay protection:      No
   Confidentiality:        No
   Key Derivation:         No
   Key strength:           N/A
   Dictionary attack prot: No
   Key hierarchy:          N/A
   Fast reconnect:         No
   MiTM resistance:        No
   Acknowledged S/F:       No

5.5.  One-Time Password (OTP)

Description

   The One-Time Password system is defined in "A One-Time Password
   System" [RFC2289] and "OTP Extended Responses" [RFC 2243].  The
   Request contains a displayable message containing an OTP challenge.
   A Response MUST be sent in reply to the Request.  The Response MUST
   be of Type 5 (OTP) or Type 3 (Nak).  The Nak Response indicates the
   peer's desired authentication Type(s).

Type



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   5

Type-Data

   The Type-Data field contains the OTP "challenge" as a displayable
   message in the Request. In the Response, this field is used for the 6
   words from the OTP dictionary [RFC2289].  The messages MUST NOT be
   null terminated.  The length of the field is derived from the Length
   field of the Request/Reply packet.

Security Claims (see Section 7.3)

   Intended use:           Wired networks, including PPP, PPPOE, and
                           IEEE 802 wired media. Use over the Internet
                           or with wireless media only when protected.
   Mechanism:              One-Time Password
   Mutual authentication:  No
   Integrity protection:   No
   Replay protection:      No
   Confidentiality:        No
   Key Derivation:         No
   Key strength:           N/A
   Dictionary attack prot: No
   Key hierarchy:          N/A
   Fast reconnect:         No
   MiTM resistance:        No
   Acknowledged S/F:       No

5.6.  Generic Token Card (GTC)

Description

   The Generic Token Card Type is defined for use with various Token
   Card implementations which require user input.   The Request contains
   a displayable message and the Response contains the Token Card
   information necessary for authentication.  Typically, this would be
   information read by a user from the Token card device and entered as
   ASCII text.  A Response MUST be sent in reply to the Request. The
   Response MUST be of Type 6 (GTC) or Type 3 (Nak).  The Nak Response
   indicates the peer's desired authentication Type(s).

Type

   6

Type-Data

   The Type-Data field in the Request contains a displayable message



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   greater than zero octets in length.  The length of the message is
   determined by Length field of the Request packet.  The message MUST
   NOT be null terminated.  A Response MUST be sent in reply to the
   Request with a Type field of 6 (Generic Token Card).  The Response
   contains data from the Token Card required for authentication.  The
   length is of the data is determined by the Length field of the
   Response packet.

Security Claims (See Section 7.3)

   Intended use:           Wired networks, including PPP, PPPOE, and
                           IEEE 802 wired media. Use over the Internet
                           wireless media only when protected.
   Mechanism:              Hardware token.
   Mutual authentication:  No
   Integrity protection:   No
   Replay protection:      No
   Confidentiality:        No
   Key Derivation:         No
   Key strength:           N/A
   Dictionary attack prot: No
   Key hierarchy:          N/A
   Fast reconnect:         No
   MiTM resistance:        No
   Acknowledged S/F:       No

5.7.  Vendor-specific

Description

   Due to EAP's popularity, the original Method Type space, which only
   provides for 255 values, is being allocated at a pace, which if
   continued, would result in exhaustion within a few years. Since many
   of the existing uses of EAP are vendor-specific, the Vendor-Specific
   Method Type is available to allow vendors to support their own
   extended Types not suitable for general usage.

   The Vendor-specific type is also used to expand the global Method
   Type space beyond the original 255 values. A Vendor-Id of 0 maps the
   original 255 possible types onto a namespace of 2^32-1 possible
   types, allowing for virtually unlimited expansion. (Note that type 0
   is never used.)

   An implementation that supports the Vendor-specific attribute MUST
   treat EAP types that are less than 256 equivalently whether they
   appear as a single octet or as the 32-bit Vendor-Type within a
   Vendor-specific type where Vendor-Id is 0.  Peers not equipped to
   interpret the Vendor-specific Type MUST send a Nak as described in



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   Section 5.3, and negotiate a more suitable authentication method.

   A summary of the Vendor-specific Type format is shown below. The
   fields are transmitted from left to right.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |               Vendor-Id                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Vendor-Type                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              Vendor data...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


Type

   254 for Vendor-specific

Vendor-Id

   The Vendor-Id is 3 octets and represents the SMI Network Management
   Private Enterprise Code of the Vendor in network byte order, as
   allocated by IANA. A Vendor-Id of zero is reserved for use by the
   IETF in providing an expanded global EAP Type space.

Vendor-Type

   The Vendor-Type field is four octets and represents the vendor-
   specific Method Type.

   If Vendor-Id is zero, the Vendor-Type field is an extension and
   superset of the existing namespace for EAP types. The first 256 types
   are reserved for compatibility with single-octet EAP types that have
   already been assigned or may be assigned in the future.  Thus, EAP
   types from 0 through 255 are semantically identical whether they
   appear as single octet EAP types or as Vendor-Types when Vendor-Id is
   zero.

Vendor-Data

   The Vendor-Data field is defined by the vendor.  Where a Vendor-Id of
   zero is present, the Vendor-Data field will be used for transporting
   the contents of EAP methods of Types defined by the IETF.






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6.  IANA Considerations

This section provides guidance to the Internet Assigned Numbers
Authority (IANA) regarding registration of values related to the EAP
protocol, in accordance with BCP 26, [RFC2434].

There are two name spaces in EAP that require registration: Packet Codes
and Method Types.

EAP is not intended as a general-purpose protocol, and allocations
SHOULD NOT be made for purposes unrelated to authentication.

6.1.  Definition of Terms

The following terms are used here with the meanings defined in BCP 26:
"name space", "assigned value", "registration".

The following policies are used here with the meanings defined in BCP
26: "Private Use", "First Come First Served", "Expert Review",
"Specification Required", "IETF Consensus", "Standards Action".

6.2.  Recommended Registration Policies

For registration requests where a Designated Expert should be consulted,
the responsible IESG area director should appoint the Designated Expert.
For Designated Expert with Specification Required, the request is posted
to the EAP WG mailing list (or, if it has been disbanded, a successor
designated by the Area Director) for comment and review, and MUST
include a pointer to a public specification. Before a period of 30 days
has passed, the Designated Expert will either approve or deny the
registration request and publish a notice of the decision to the EAP WG
mailing list or its successor. A denial notice must be justified by an
explanation and, in the cases where it is possible, concrete suggestions
on how the request can be modified so as to become acceptable.

For registration requests requiring Expert Review, the EAP mailing list
should be consulted. If the EAP mailing list is no longer operational,
an alternative mailing list may be designated by the responsible IESG
Area Director.

Packet Codes have a range from 1 to 255, of which 1-4 have been
allocated. Because a new Packet Code has considerable impact on
interoperability, a new Packet Code requires Standards Action, and
should be allocated starting at 5.

The original EAP Method Type space has a range from 1 to 255, and is the
scarcest resource in EAP, and thus must be allocated with care.  Method
Types 1-36 have been allocated, with 20 available for re-use. Method



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Types 37-191 may be allocated following Designated Expert, with
Specification Required.

Release of blocks of Method Types (more than one for a given purpose)
should require IETF Consensus.  EAP Type Values 192-253 are reserved and
allocation requires Standards Action.

Method Type 254 is allocated for the Vendor-Specific Type.  Where the
Vendor-Id field is non-zero, the Vendor-Specific Type is used for
functions specific only to one vendor's implementation of EAP, where no
interoperability is deemed useful.  When used with a Vendor-Id of zero,
Method Type 254 can also be used to provide for an expanded Method Type
space.  Method Type values 256-4294967295 may be allocated after Type
values 1-191 have been allocated.

Method Type 255 is allocated for Experimental use, such as testing of
new EAP methods before a permanent Type code is allocated.

7.  Security Considerations

EAP was designed for use with dialup PPP [RFC1661] and wired [IEEE802]
networks such as Ethernet [IEEE8023].  On these networks, an attacker
would need to gain physical access to the telephone or switch
infrastructure in order to mount an attack. While such attacks have been
documented, such as in [DECEPTION], they are assumed to be rare.

However, subsequently EAP has been proposed for use on wireless
networks, and over the Internet, where physical security cannot be
assumed. On such networks, the security vulnerabilities are greater, as
are the requirements for EAP security.

This section defines the threat model and security terms and describes
the security claims section required in EAP method specifications.  We
then discuss threat mitigation.

7.1.  Threat model

On physically insecure networks, it is possible for an attacker to gain
access to the physical medium. This enables a range of attacks,
including the following:

[1]  An adversary may try to discover user identities by snooping
     authentication traffic.

[2]  An adversary may try to modify or spoof EAP packets.

[3]  An adversary may launch denial of service attacks by spoofing layer
     2 indications or EAP layer success/failure indications, replaying



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     EAP packets, or generating packets with overlapping Identifiers.

[4]  An adversary might attempt to recover the pass-phrase by mounting
     an offline dictionary attack.

[5]  An adversary may attempt to convince the peer to connect to an
     untrusted network, by mounting a man-in-the-middle attack.

[6]  An adversary may attempt to disrupt the EAP negotiation in order to
     weaken the authentication.

[7]  An attacker may attempt to recover the key by taking advantage of
     weak key derivation techniques used within EAP methods.

[8]  An attacker may attempt to take advantage of weak ciphersuites
     subsequently used after the EAP conversation is complete.

Where EAP is used over wireless networks, an attacker needs to be within
the coverage area of the wireless medium in order to carry out these
attacks. However, where EAP is used over the Internet, no such
restrictions apply.

7.2.  Security claims

In order to clearly articulate the security provided by an EAP method,
EAP method specifications MUST include a Security Claims section
including the following declarations:

[a]  Intended use. This includes a statement of whether the method is
     intended for use over a physically secure or insecure network, as
     well as a statement of the applicable media.

[b]  Mechanism. This is a statement of the authentication technology:
     certificates, pre-shared keys, passwords, token cards, etc.

[c]  Security claims. This is a statement of the claimed security
     properties of the method, using terms defined in Section 1.2:
     mutual authentication, integrity protection, replay protection,
     confidentiality, key derivation, key strength, dictionary attack
     resistance, fast reconnect, man-in-the-middle resistance,
     acknowledged result indications.  The Security Claims section
     SHOULD include references to proof, or the proof itself (preferably
     in an Appendix).

[d]  Key strength. If the method derives keys, then the effective key
     strength MUST be estimated.





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[e]  Description of key hierarchy. EAP methods deriving keys MUST either
     provide a reference to a key hierarchy specification, or describe
     how keys used for authentication/integrity, encryption and IVs are
     to be derived from the provided keying material, and how
     cryptographic separation is maintained between keys used for
     different purposes.

[f]  Indication of vulnerabilities. In addition to the security claims
     that are made, the specification MUST indicate which of the
     security claims detailed in Section 1.2 are NOT being made, and
     discuss the security implications.

7.3.  Identity protection

An Identity exchange is an optional within the EAP conversation.
Therefore, it is possible to omit the Identity exchange entirely, or to
postpone it until later in the conversation once a protected channel has
been established.

However, where roaming is supported as described in [RFC2607], it may be
necessary to locate the appropriate backend authentication server before
the authentication conversation can proceed.  The realm portion of the
Network Access Identifier (NAI) [RFC2486] is typically included within
the Identity-Response in order to enable the authentication exchange to
be routed to the appropriate backend authentication server. Therefore
while the peer-name portion of the NAI may be omitted in the Identity-
Response, where proxies or relays are present, the realm portion may be
required.

7.4.  Man-in-the-middle attacks

Where a sequence of methods is utilized for authentication or EAP is
tunneled within another protocol that omits peer authentication, there
exists a potential vulnerability to man-in-the-middle attack.

Where a sequence of EAP methods is utilized for authentication, the peer
might not have proof that a single entity has acted as the authenticator
for all EAP methods within the sequence. For example, an authenticator
might terminate one EAP method, then forward the next method in the
sequence to another party without the peer's knowledge or consent.
Similarly, the authenticator might not have proof that a single entity
has acted as the peer for all EAP methods within the sequence.

This enables an attack by a rogue EAP authenticator tunneling EAP to a
legitimate server. Where the tunneling protocol is used for key
establishment but does not require peer authentication, an attacker
convincing a legitimate peer to connect to it will be able to tunnel EAP
packets to a legitimate server, successfully authenticating and



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obtaining the key. This allows the attacker to successfully establish
itself as a man-in-the-middle, gaining access to the network, as well as
the ability to decrypt data traffic between the legitimate peer and
server.

This attack may be mitigated by the following measures:

[a]  Requiring mutual authentication within EAP tunneling mechanisms.

[b]  Requiring cryptographic binding between EAP methods executed within
     a sequence or between the EAP tunneling protocol and the tunneled
     EAP methods. Where cryptographic binding is supported, a mechanism
     is also needed to protect against downgrade attacks that would
     bypass it.

[c]  Limiting the EAP methods authorized for use without protection,
     based on peer and authenticator policy.

[d]  Avoiding the use of sequences or tunnels when a single, strong
     method is available.

7.5.  Packet modification attacks

While individual EAP methods may support per-packet data origin
authentication, integrity and replay protection, EAP itself does not
provide built-in support for this.

Since the Identifier is only a single octet, it is easy to guess,
allowing an attacker to successfully inject or replay EAP packets.  An
attacker may also modify EAP headers within EAP packets where the header
is unprotected. This could cause packets to be inappropriately discarded
or misinterpreted.

In the case of PPP and IEEE 802 wired links, it is assumed that such
attacks are restricted to attackers who can gain access to the physical
link.  However, where EAP is run over physically insecure lower layers
such as IEEE 802.11 or the Internet (such as within protocols supporting
PPP, EAP or Ethernet Tunneling), this assumption is no longer valid and
the vulnerability to attack is greater.

To protect EAP messages sent over physically insecure lower layers,
methods providing mutual authentication and key derivation, as well as
per-packet origin authentication, integrity and replay protection SHOULD
be used. Method-specific MICs may be used to provide protection. Since
EAP messages of Types Identity, Notification, and Nak do not include
their own MIC, it may be desirable for the EAP method MIC to cover
information contained within these messages, as well as the header of
each EAP message.



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To provide protection, EAP also may be encapsulated within a protected
channel created by protocols such as ISAKMP [RFC2408] as is done in
[PIC] or within TLS [RFC2246].  However, as noted in Section 7.4, EAP
tunneling may result in a man-in-the-middle vulnerability.

7.6.  Dictionary attacks

Password authentication algorithms such as EAP-MD5, MS-CHAPv1 [RFC2433]
and Kerberos V [RFC1510] are known to be vulnerable to dictionary
attacks.  MS-CHAPv1 vulnerabilities are documented in [PPTPv1]; Kerberos
vulnerabilities are described in [KRBATTACK], [KRBLIM], and [KERB4WEAK].

In order to protect against dictionary attacks, an authentication
algorithm resistant to dictionary attack (as defined in Section 7.2) may
be used. This is particularly important when EAP runs over media which
are not physically secure.

If an authentication algorithm is used that is known to be vulnerable to
dictionary attack, then the conversation may be tunneled within a
protected channel, in order to provide additional protection.  However,
as noted in Section 7.4, EAP tunneling may result in a man-in-the-middle
vulnerability, and therefore dictionary attack resistant methods are
preferred.

7.7.  Connection to an untrusted network

With EAP methods supporting one-way authentication, such as EAP-MD5, the
authenticator's identity is not verified. Where the lower layer is not
physically secure (such as where EAP runs over wireless media or IP),
this enables the peer to connect to a rogue authenticator. As a result,
where the lower layer is not physically secure, a method supporting
mutual authentication is recommended.

In EAP there is no requirement that authentication be full duplex or
that the same protocol be used in both directions. It is perfectly
acceptable for different protocols to be used in each direction.  This
will, of course, depend on the specific protocols negotiated.  However,
in general, completing a single unitary mutual authentication is
preferable to two one-way authentications, one in each direction.  This
is because separate authentications that are not bound cryptographically
so as to  demonstrate they are part of the same session are subject to
man-in-the-middle attacks, as discussed in Section 7.4.

7.8.  Negotiation attacks

In a negotiation attack, the attacker attempts to convince the peer and
authenticator to negotiate a less secure EAP method. EAP does not
provide protection for the Nak packet, although it is possible for a



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method to include coverage of Nak Responses within a method-specific
MIC.

To avoid negotiation attacks in situations where EAP runs over
physically insecure media, for each named peer there SHOULD be an
indication of exactly one method used to authenticate that peer name, as
described in Section 2.1.

7.9.  Implementation idiosyncrasies

The interaction of EAP with lower layer transports such as PPP and IEEE
802 are highly implementation dependent.

For example, upon failure of authentication, some PPP implementations do
not terminate the link, instead limiting traffic in Network-Layer
Protocols to a filtered subset, which in turn allows the peer the
opportunity to update secrets or send mail to the network administrator
indicating a problem. Similarly, while in IEEE 802.1X an authentication
failure will result in denied access to the controlled port, limited
traffic may be permitted on the uncontrolled port.

In EAP there is no provision for retries of failed authentication.
However, in PPP the LCP state machine can renegotiate the authentication
protocol at any time, thus allowing a new attempt. Similarly, in IEEE
802.1X the Supplicant or Authenticator can re-authenticate at any time.
It is recommended that any counters used for authentication failure not
be reset until after successful authentication, or subsequent
termination of the failed link.

7.10.  Key derivation

It is possible for the peer and EAP server to mutually authenticate, and
derive a Master Key (MK). The MK is unique to the peer and EAP server
and MUST NOT be exported by the EAP method, or used directly to protect
the EAP conversation or subsequent data. As a result, possession of the
MK represents proof of a successful authentication, and this is
potentially useful in enabling features such as fast reconnect, or fast
handoff.

In order to provide keying material for use in a subsequently negotiated
ciphersuite, the EAP method exports a Master Session Key (MSK). Like the
EAP Master Key, EAP Master Session Keys are also not used directly to
protect data; however, they are of sufficient size to enable subsequent
derivation of Transient Session Keys (TSKs) for use with the selected
ciphersuite.

EAP methods provide Master Session Keys and not Transient Session Keys
so as to allow EAP methods to be ciphersuite and media independent.



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Depending on the lower layer, EAP methods may run before or after
ciphersuite negotiation, so that the selected ciphersuite may not be
known to the EAP method. By providing keying material usable with any
ciphersuite, EAP methods can used with a wide range of ciphersuites and
media.  Since the peer and EAP client reside on the same machine, TSKs
can be provided to the lower layer security module without needing to
leave the machine.

In the case where the backend authentication server and authenticator
reside on different machines, there are several implications for
security:

[a]  Mutual authentication may occur between the peer and the backend
     authentication server, if the negotiated EAP method supports this.
     However, where the authenticator and backend authentication server
     are separate, the peer and authenticator do not mutually
     authenticate within EAP.  However, subsequent to completion of the
     EAP conversation, the lower layer may support mutual authentication
     between the peer and authenticator.  For example, IEEE 802.11i
     includes a Transient Session Key derivation protocol known as the
     4-way handshake, which guarantees liveness of the TSKs, provides
     for mutual authentication between the peer and authenticator,
     replay protection, and protected ciphersuite negotiation.

[b]  The MSK negotiated between the peer and backend authentication
     server will need to be transmitted to the authenticator.  The
     specification of this transit mechanism is outside the scope of
     this document.

This specification does not provide detailed guidance on how EAP methods
are to derive the MK and MSK. Key derivation is an art that is best
practiced by professionals; rather than inventing new key derivation
algorithms, reuse of existing algorithms such as those specified in IKE
[RFC2409], or TLS [RFC2246] is recommended.

However, some general guidelines can be provided:

[1]  The MK is for use only by the EAP authenticator and peer and MUST
     NOT be exported by the EAP method or provided to a third party.

[2]  Since the MSK is exported by the EAP method, while the MK is not,
     possession of the MSK MUST NOT provide information useful in
     determining the MK.

[3]  The MSK and TSKs MUST be fresh.  Otherwise it is infeasible to
     detect messages replayed from prior sessions.





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[4]  TSKs MUST be cryptographically independent from each other so that
     if an attacker obtains one of them, it will not have gained
     information useful in determining the other ones.

[5]  There MUST be a way to determine whether TSKs belong to this or to
     some other session.

[6]  The MSK derived by EAP methods MUST be bound to the peers as well
     as to the authentication method, so as to avoid a man-in-the-middle
     attack (see Section 7.4).

7.11.  Weak ciphersuites

If after the initial EAP authentication, data packets are sent without
per-packet authentication, integrity, replay protection, an attacker
with access to the media can inject packets, "flip bits" within existing
packets, replay packets, or even hijack the session completely. Without
per-packet confidentiality, it is possible to snoop data packets.

As a result, as noted in Section 3.1, where EAP is used over a
physically insecure lower layer, per-packet authentication, integrity
and replay protection SHOULD be used, and per-packet confidentiality is
also recommended.

8.  Normative references

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

[RFC1994]      Simpson, W., "PPP Challenge Handshake Authentication
               Protocol (CHAP)", RFC 1994, August 1996.

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

[RFC2243]      Metz, C., "OTP Extended Responses", RFC 2243, November
               1997.

[RFC2279]      Yergeau, F., "UTF-8, a transformation format of ISO
               10646", RFC 2279, January 1998.

[RFC2289]      Haller, N., Metz, C., Nesser, P., and Straw M., "A One-
               Time Password System", RFC 2289, February 1998.

[RFC2434]      Alvestrand, H. and Narten, T., "Guidelines for Writing an
               IANA Considerations Section in RFCs", BCP 26, RFC 2434,
               October 1998.




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[RFC2988]      Paxson, V., Allman, M., "Computing TCP's Retransmission
               Timer", RFC 2988, November 2000.

[IEEE802]      IEEE Standards for Local and Metropolitan Area Networks:
               Overview and Architecture, ANSI/IEEE Std 802, 1990.

[IEEE8021X]    IEEE Standards for Local and Metropolitan Area Networks:
               Port based Network Access Control, IEEE Std 802.1X-2001,
               June 2001.

9.  Informative references

[DECEPTION]    Slatalla, M., and  Quittner, J., "Masters of Deception."
               HarperCollins, New York, 1995.

[RFC1510]      Kohl, J., Neuman, C., "The Kerberos Network
               Authentication Service (V5)", RFC 1510, September 1993.

[RFC2246]      Dierks, T. and  C. Allen, "The TLS Protocol Version 1.0",
               RFC 2246, November 1998.

[RFC2486]      Beadles, M., Aboba, B., "The Network Access Identifier",
               RFC 2486, January 1999.

[RFC2401]      Atkinson, R., Kent, S., "Security Architecture for the
               Internet Protocol", RFC 2401, November 1998.

[RFC2408]      Maughan, D., Schertler, M., Schneider, M., Turner, J.,
               "Internet Security Association and Key Management
               Protocol (ISAKMP)", RFC 2408, November 1998.

[RFC2433]      Zorn, G., Cobb, S., "Microsoft PPP CHAP Extensions", RFC
               2433, October 1998.

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

[RFC2661]      Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn,
               G., and Palter, B., "Layer Two Tunneling Protocol L2TP",
               RFC 2661, August 1999.

[RFC2716]      Aboba, B., Simon, D.,"PPP EAP TLS Authentication
               Protocol", RFC 2716, October 1999.

[KRBATTACK]    Wu, T., "A Real-World Analysis of Kerberos Password
               Security", Stanford University Computer Science
               Department, http://theory.stanford.edu/~tjw/krbpass.html




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[KRBLIM]       Bellovin, S.M., Merritt, M., "Limitations of the Kerberos
               authentication system", Proceedings of the 1991 Winter
               USENIX Conference, pp. 253-267, 1991.

[KERB4WEAK]    Dole, B., Lodin, S., and Spafford, E., "Misplaced trust:
               Kerberos 4 session keys", Proceedings of the Internet
               Society Network and Distributed System Security
               Symposium, pp. 60-70, March 1997.

[PIC]          Sheffer, Y., Krawczyk, H., Aboba, B., "PIC, A Pre-IKE
               Credential Provisioning Protocol", Internet draft (work
               in progress), draft-ietf-ipsra-pic-06.txt, October 2002.

[PPTPv1]       Schneier, B, Mudge, "Cryptanalysis of Microsoft's Point-
               to- Point Tunneling Protocol", Proceedings of the 5th ACM
               Conference on Communications and Computer Security, ACM
               Press, November 1998.

[IEEE8023]     ISO/IEC 8802-3 Information technology -
               Telecommunications and information exchange between
               systems - Local and metropolitan area networks - Common
               specifications - Part 3:  Carrier Sense Multiple Access
               with Collision Detection (CSMA/CD) Access Method and
               Physical Layer Specifications, (also ANSI/IEEE Std 802.3-
               1996), 1996.

[IEEE80211]    Information technology - Telecommunications and
               information exchange between systems - Local and
               metropolitan area networks - Specific Requirements Part
               11:  Wireless LAN Medium Access Control (MAC) and
               Physical Layer (PHY) Specifications, IEEE Std.
               802.11-1999, 1999.

Acknowledgments

This protocol derives much of its inspiration from Dave Carrel's AHA
draft as well as the PPP CHAP protocol [RFC1994]. Valuable feedback was
provided by Yoshihiro Ohba of Toshiba America Research, Jari Arkko of
Ericsson, Sachin Seth of Microsoft, and Glen Zorn of Cisco Systems.

Authors' Addresses

Larry J. Blunk
Merit Network, Inc.
4251 Plymouth Rd., Suite 2000
Ann Arbor, MI  48105-2785

EMail: ljb@merit.edu



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Phone: 734-647-9563
Fax:   734-647-3185

John R. Vollbrecht
Vollbrecht Consulting LLC
9682 Alice Hill Drive
Dexter, MI 48130

EMail: jrv@umich.edu

Bernard Aboba
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052

EMail: bernarda@microsoft.com
Phone: +1 425 706 6605
Fax:   +1 425 936 6605

James Carlson
Sun Microsystems, Inc.
1 Network Drive
Burlington MA  01803-2757

EMail: james.d.carlson@sun.com
Phone:  +1 781 442 2084
Fax:    +1 781 442 1677

Intellectual Property Notices

The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to pertain
to the implementation or use of the technology described in this
document or the extent to which any license under such rights might or
might not be available; neither does it represent that it has made any
effort to identify any such rights.  Information on the IETF's
procedures with respect to rights in standards-track and standards-
related documentation can be found in BCP-11.  Copies of claims of
rights made available for publication and any assurances of licenses to
be made available, or the result of an attempt made to obtain a general
license or permission for the use of such proprietary rights by
implementors or users of this specification can be obtained from the
IETF Secretariat.

The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary rights
which may cover technology that may be required to practice this
standard.  Please address the information to the IETF Executive



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Director.

Full Copyright Statement

Copyright (C) The Internet Society (2003).  All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it or
assist in its implementation may be prepared, copied, published and
distributed, in whole or in part, without restriction of any kind,
provided that the above copyright notice and this paragraph are included
on all such copies and derivative works.  However, this document itself
may not be modified in any way, such as by removing the copyright notice
or references to the Internet Society or other Internet organizations,
except as needed for the purpose of developing Internet standards in
which case the procedures for copyrights defined in the Internet
Standards process must be followed, or as required to translate it into
languages other than English.  The limited permissions granted above are
perpetual and will not be revoked by the Internet Society or its
successors or assigns.  This document and the information contained
herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE
INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE."

Open issues

Open issues relating to this specification are tracked on the following
web site:

http://www.drizzle.com/~aboba/EAP/eapissues.html

Expiration Date

This memo is filed as <draft-ietf-pppext-rfc2284bis-10.txt>,  and
expires August 24, 2003.















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