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Versions: 00 02 RFC 2284

Network Working Group                                          L J Blunk
                                                          J R Vollbrecht
Internet Draft                                       Merit Network, Inc.
expires in six months                                          June 1996


              PPP Extensible Authentication Protocol (EAP)
                   <draft-ietf-pppext-eap-auth-02.txt>



Status of this Memo

   This document is the product of the Point-to-Point Protocol
   Extensions Working Group of the Internet Engineering Task Force
   (IETF).  Comments should be submitted to the ietf-ppp@merit.edu
   mailing list.

   Distribution of this memo is unlimited.

   This document is an Internet-Draft.  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
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   To learn the current status of any Internet-Draft, please check the
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   ftp.isi.edu (US West Coast).

Abstract

   The Point-to-Point Protocol (PPP) [1] provides a standard method for
   transporting multi-protocol datagrams over point-to-point links.

   PPP also defines an extensible Link Control Protocol, which allows
   negotiation of an Authentication Protocol for authenticating its peer
   before allowing Network Layer protocols to transmit over the link.

   This document defines the PPP Extensible Authentication Protocol.





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                           Table of Contents


     1.     Introduction ..........................................    1
        1.1       Specification of Requirements ...................    1
        1.2       Terminology .....................................    2

     2.     PPP Extensible Authentication Protocol (EAP) ..........    3
        2.1       Configuration Option Format .....................    5
        2.2       Packet Format ...................................    6
           2.2.1  Request and Response ............................    7
           2.2.2  Success and Failure .............................    9

     3.     Initial EAP Request/Response Types ....................   10
        3.1       Identity ........................................   11
        3.2       Notification ....................................   12
        3.3       Nak .............................................   13
        3.4       MD5-Challenge ...................................   14
        3.5       S/Key ...........................................   15
        3.6       Generic Token Card ..............................   16

     REFERENCES ...................................................   18

     ACKNOWLEDGEMENTS .............................................   18

     CHAIR'S ADDRESS ..............................................   18

     AUTHOR'S ADDRESS .............................................   18























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1.  Introduction

   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.

   These authentication protocols are intended for use primarily by
   hosts and routers that connect to a PPP network server via switched
   circuits or dial-up lines, but might be applied to dedicated links as
   well.  The server can use the identification of the connecting host
   or router in the selection of options for network layer negotiations.

   This document defines the PPP Extensible Authentication Protocol
   (EAP).  The Link Establishment and Authentication phases, and the
   Authentication-Protocol Configuration Option, are defined in The
   Point-to-Point Protocol (PPP) [1].


1.1.  Specification of Requirements

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

   MUST      This word, or the adjective "required", means that the
             definition is an absolute requirement of the specification.

   MUST NOT  This phrase means that the definition is an absolute
             prohibition of the specification.

   SHOULD    This word, or the adjective "recommended", means that there
             may exist valid reasons in particular circumstances to
             ignore this item, but the full implications must be
             understood and carefully weighed before choosing a
             different course.

   MAY       This word, or the adjective "optional", means that this
             item is one of an allowed set of alternatives.  An
             implementation which does not include this option MUST be
             prepared to interoperate with another implementation which
             does include the option.




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1.2.  Terminology

   This document frequently uses the following terms:

   authenticator
             The end of the link requiring the authentication.  The
             authenticator specifies the authentication protocol to be
             used in the Configure-Request during Link Establishment
             phase.

   peer      The other end of the point-to-point link; the end which is
             being authenticated by the authenticator.

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

   displayable message
             This is interpreted to be human readable string of
             characters, and MUST NOT affect operation of the protocol.
             It is recommended that the message contain displayable
             ASCII characters 32 through 126 decimal.  Mechanisms for
             extension to other character sets are the topic of future
             research.
























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2.  PPP Extensible Authentication Protocol (EAP)

   The PPP Extensible Authentication Protocol (EAP)  is a general
   protocol for PPP authentication which supports multiple
   authentication mechanisms.  EAP does not select a specific
   authentication mechanism at 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.

   1. After the Link Establishment phase is complete, the authenticator
      sends one or more Requests to authenticate the peer.  The Request
      has a type field to indicate what is being requested.  Examples of
      Request types include Identity,  MD5-challenge, S/Key, Generic
      Token Card, etc.  The MD5-challenge type corresponds closely to
      the CHAP authentication protocol.  Typically, the authenticator
      will send an initial Identity Request followed by one or more
      Requests for authentication information.   However, an initial
      Identity Request is not required, and may be bypassed in cases
      where the identity is presumed (leased lines, dedicated dial-ups,
      etc.).

   2. The peer sends a Response packet in reply to each 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 ends the authentication phase with a Success or
      Failure packet.





















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Advantages

   The EAP protocol can support multiple authentication mechanisms
   without having to pre-negotiate a particular one during LCP Phase.

   Certain devices (e.g. a NAS) do not necessarily have to understand
   each request type and may be able to simply act as a passthrough
   agent for a "back-end" server on a host.  The device only need look
   for the success/failure code to terminate the authentication phase.

Disadvantages

   EAP does require the addition of a new authentication type to 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.



































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2.1.  Configuration Option Format

   A summary of the Authentication-Protocol Configuration Option format
   to negotiate the EAP Authentication Protocol 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      |    Length     |     Authentication-Protocol   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Type

      3

   Length

      4

   Authentication-Protocol

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



























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2.2.  Packet Format

   Exactly one PPP 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).  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

      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


   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.








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2.2.1.  Request and Response

   Description

      The Request packet is sent by the authenticator to the peer.  Each
      Request has a type field which serves to indicate what is being
      requested.  The authenticator MUST transmit an EAP packet with the
      Code field set to 1 (Request).  Additional Request packets MUST be
      sent until a valid Response packet is received, or an optional
      retry counter expires.  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
      Request packet.  Responses MUST only be sent in reply to a
      received Request and never retransmitted on a timer.  The
      Identifier field of the Response MUST match that of the Request.

         Implementation Note: Because the authentication process will
         often involve user input, some care must be taken when deciding
         upon retransmission strategies and authentication timeouts.  It
         is suggested a retransmission timer of 6 seconds with a maximum
         of 10 retransmissions be used as default.  One may wish to make
         these timeouts longer in certain cases (e.g. where Token Cards
         are involved).  Additionally, the peer must be prepared to
         silently discard received retransmissions while waiting for
         user input.

   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



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      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.  If a peer recieves a
      duplicate Request for which it has already sent a Response, it
      MUST resend it's Response.  If a peer receives a duplicate Request
      before it has sent a Response to the initial Request (i.e. it's
      waiting for user input), it MUST silently discard the duplicate
      Request.

   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.  Only one Type may be specified per 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.  When sending a Nak in response to a
      Request, the peer may indicate an alternative desired
      authentication Type which it supports. 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.


















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2.2.2.  Success and Failure

   Description

      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.

         Implementation Note: Because the Success and Failure packets
         are not acknowledged, they may be potentially lost.  A peer
         MUST allow for this circumstance.  The peer can use a Network
         Protocol packet as an alternative indication of Success.
         Likewise, the receipt of a LCP Terminate-Request can be taken
         as a Failure.

   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 Indentifier field
      of the Response packet that it is sent in response to.

   Length

      4





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3.  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 may not be sent in a Request.  The Nak Type may only be
   sent in repsonse to a Request which usea an authentication Type code.
   Support for EAP Types 1-4 is MANDATORY.  These Types, as well as
   types 5 and 6, are defined in this document.  Follow-on RFCs will
   define additional EAP Types.


      1       Identity
      2       Notification
      3       Nak (Response only)
      4       MD5-Challenge
      5       S/Key (RFC 1760)
      6       Generic Token Card






























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3.1.  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 MUST be sent to this Request with a Type
      of 1 (Identity).

         Implementation Note:  The peer may obtain the Identity via user
         input.  It is suggested that the authenticator retry the
         Indentity 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 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.  The
      Response uses this field to return the Identity.  If the Identity
      is unknown, this field should be zero bytes in length.  The field
      need not be null terminated.  The length of this field is derived
      from the Length field of the Request/Response packet.
















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3.2.  Notification

   Description

      The Notification Type is optionally used to convey a displayable
      message from the authenticator to the peer.   The peer SHOULD
      display this message to the user or log it if it cannot be
      displayed.  It is intended to provide an acknowledged notification
      of some imperative nature.  Examples include a password with an
      expiration time that is about to expire, an S/Key id 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.  The length of the message is
      determined by Length field of the Request packet.  The message
      need not be null terminated.  A Response MUST be sent in reply to
      the Request with a 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).






















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3.3.  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 the authentication Type desired by the peer.

   Type

      3


   Type-Data

      This field MUST contain a single octet indicating the desired
      authentication type.

































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3.4.  MD5-Challenge

   Description

      The MD5-Challenge Type is analagous to the PPP CHAP protocol [3]
      (with MD5 as the specified algorithm).  The PPP Authentication
      Protocols RFC [3] should be referred to for further implementation
      specifics.  The Request contains a "challenge" message to the
      peer.  A Repsonse 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
      mechanism Type.  Support for the MD5-Challenge mechanism is
      MANDATORY.


   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 Authentication
      Protocols RFC [3].

       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 ...
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


















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3.5.  S/Key

   Description The S/Key system is defined in "The S/KEY One-Time
   Password System" [4].  The Request contains a displayable message
   consisting of the S/Key count and a seed. A Repsonse MUST be sent in
   reply to the Request.  The Response MUST be of Type 5 (S/key) or Type
   3 (Nak).  The Nak reply indicates the peer's desired authentication
   mechanism Type.


Type

   5


Type-Data

   The Type-Data field contains the S/Key "challenge" (count and seed)
   as a displayable message in the Request.  This field is used for the
   6 words (displayable text) from the S/Key dictionary [4] in the
   Response.  The messages need not be null terminated.  The length of
   the field is derived from the Length field of the Request/Reply
   packet.




























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3.6.  Generic Token Card

   Description

      The Generic Token Card Type is defined for use with various Token
      Card implementations which require user input.   The Request
      contains an ASCII text message and the Reply 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.


   Type

      6


   Type-Data

      The Type-Data field in the Request contains a displayable message
      greater than zero octets in length.  The length of the message is
      determined by Length field of the Request packet.  The message
      need 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.
























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Security Considerations

   Security issues are the primary topic of this RFC.

   The interaction of the authentication protocols within PPP are highly
   implementation dependent.

   For example, upon failure of authentication, some implementations do
   not terminate the link.  Instead, the implementation limits the kind
   of traffic in the Network-Layer Protocols to a filtered subset, which
   in turn allows the user opportunity to update secrets or send mail to
   the network administrator indicating a problem.

   There is no provision for retries of failed authentication.  However,
   the LCP state machine can renegotiate the authentication protocol at
   any time, thus allowing a new attempt.  It is recommended that any
   counters used for authentication failure not be reset until after
   successful authentication, or subsequent termination of the failed
   link.

   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.

   In practice, within or associated with each PPP server, there is a
   database which associates "user" names with authentication
   information ("secrets").  It is not anticipated that a particular
   named user would be authenticated by multiple methods.  This would
   make the user vulnerable to attacks which negotiate the least secure
   method from among a set (such as PAP rather than EAP).  Instead, for
   each named user there should be an indication of exactly one method
   used to authenticate that user name.  If a user needs to make use of
   different authentication methods under different circumstances, then
   distinct user names SHOULD be employed, each of which identifies
   exactly one authentication method.















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References

   [1]   Simpson, W. A., "The Point-to-Point Protocol (PPP)", RFC 1661.

   [2]   Reynolds, J., and J. Postel, "Assigned Numbers", RFC 1700,
         USC/Information Sciences Institute, October 1994.

   [3]   Lloyd, B., and Simpson, W.,  "PPP Authentication Protocols",
         RFC 1334, October 1992.

   [4]   Haller, N., "The S/KEY One-Time Password System", RFC 1760,
         Bellcore, February 1995.


Acknowledgments

   This protocol derives much of its inspiration from Dave Carrel's AHA
   draft as well as the PPP CHAP protocol [3].  Bill Simpson provided
   much of the boilerplate used throughout this document.   Al Rubens
   (Merit) also provided valuable feedback.


Chair's Address

   The working group can be contacted via the current chair:

      Karl Fox
      Ascend Communications

      EMail: karl@MorningStar.Com


Author's Address

   Questions about this memo can also be directed to:

      Larry J Blunk                   John R Vollbrecht

      EMail: ljb@merit.edu            EMail: jrv@merit.edu












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