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INFORMATIONAL

Network Working Group                                      J. Vollbrecht
Request for Comments: 4137              Meetinghouse Data Communications
Category: Informational                                        P. Eronen
                                                                   Nokia
                                                              N. Petroni
                                                  University of Maryland
                                                                 Y. Ohba
                                                                    TARI
                                                             August 2005


      State Machines for Extensible Authentication Protocol (EAP)
                         Peer and Authenticator

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   This document describes a set of state machines for Extensible
   Authentication Protocol (EAP) peer, EAP stand-alone authenticator
   (non-pass-through), EAP backend authenticator (for use on
   Authentication, Authorization, and Accounting (AAA) servers), and EAP
   full authenticator (for both local and pass-through).  This set of
   state machines shows how EAP can be implemented to support deployment
   in either a peer/authenticator or peer/authenticator/AAA Server
   environment.  The peer and stand-alone authenticator machines are
   illustrative of how the EAP protocol defined in RFC 3748 may be
   implemented.  The backend and full/pass-through authenticators
   illustrate how EAP/AAA protocol support defined in RFC 3579 may be
   implemented.  Where there are differences, RFC 3748 and RFC 3579 are
   authoritative.

   The state machines are based on the EAP "Switch" model.  This model
   includes events and actions for the interaction between the EAP
   Switch and EAP methods.  A brief description of the EAP "Switch"
   model is given in the Introduction section.

   The state machine and associated model are informative only.
   Implementations may achieve the same results using different methods.




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Table of Contents

   1. Introduction: The EAP Switch Model ..............................3
   2. Specification of Requirements ...................................4
   3. Notational Conventions Used in State Diagrams ...................5
      3.1. Notational Specifics .......................................5
      3.2. State Machine Symbols ......................................7
      3.3. Document Authority .........................................8
   4. Peer State Machine ..............................................9
      4.1. Interface between Peer State Machine and Lower Layer .......9
      4.2. Interface between Peer State Machine and Methods ..........11
      4.3. Peer State Machine Local Variables ........................13
      4.4. Peer State Machine Procedures .............................14
      4.5. Peer State Machine States .................................15
   5. Stand-Alone Authenticator State Machine ........................17
      5.1. Interface between Stand-Alone Authenticator State
           Machine and Lower Layer ...................................17
      5.2. Interface between Stand-Alone Authenticator State
           Machine and Methods .......................................19
      5.3. Stand-Alone Authenticator State Machine Local Variables ...21
      5.4. EAP Stand-Alone Authenticator Procedures ..................22
      5.5. EAP Stand-Alone Authenticator States ......................24
   6. EAP Backend Authenticator ......................................26
      6.1. Interface between Backend Authenticator State
           Machine and Lower Layer ...................................26
      6.2. Interface between Backend Authenticator State
           Machine and Methods .......................................28
      6.3. Backend Authenticator State Machine Local Variables .......28
      6.4. EAP Backend Authenticator Procedures ......................28
      6.5. EAP Backend Authenticator States ..........................29
   7. EAP Full Authenticator .........................................29
      7.1. Interface between Full Authenticator State Machine
           and Lower Layer ...........................................30
      7.2. Interface between Full Authenticator State Machine
           and Methods ...............................................31
      7.3. Full Authenticator State Machine Local Variables ..........32
      7.4. EAP Full Authenticator Procedures .........................32
      7.5. EAP Full Authenticator States .............................32
   8. Implementation Considerations ..................................34
      8.1. Robustness ................................................34
      8.2. Method/Method and Method/Lower-Layer Interfaces ...........35
      8.3. Peer State Machine Interoperability with Deployed
           Implementations ...........................................35
   9. Security Considerations ........................................35
   10. Acknowledgements ..............................................36
   11. References ....................................................37
       11.1. Normative References ....................................37
       11.2. Informative References ..................................37



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   Appendix. ASCII Versions of State Diagrams ........................38
       A.1.  EAP Peer State Machine (Figure 3) .......................38
       A.2.  EAP Stand-Alone Authenticator State Machine (Figure 4) ..41
       A.3.  EAP Backend Authenticator State Machine (Figure 5) ......44
       A.4.  EAP Full Authenticator State Machine (Figures 6 and 7) ..47

1.  Introduction: The EAP Switch Model

   This document offers a proposed state machine for RFCs [RFC3748] and
   [RFC3579].  There are state machines for the peer, the stand-alone
   authenticator, a backend authenticator, and a full/pass-through
   authenticator.  Accompanying each state machine diagram is a
   description of the variables, the functions, and the states in the
   diagram.  Whenever possible, the same notation has been used in each
   of the state machines.

   An EAP authentication consists of one or more EAP methods in sequence
   followed by an EAP Success or EAP Failure sent from the authenticator
   to the peer.  The EAP switches control negotiation of EAP methods and
   sequences of methods.

      Peer             Peer  |  Authenticator       Auth
      Method                 |                      Method
              \              |                    /
               \             |                   /
                Peer         |             Auth
                EAP    <-----|---------->  EAP
                Switch       |             Switch

                    Figure 1: EAP Switch Model

   At both the peer and authenticator, one or more EAP methods exist.
   The EAP switches select which methods each is willing to use, and
   negotiate between themselves to pick a method or sequence of methods.

   Note that the methods may also have state machines.  The details of
   these are outside the scope of this paper.














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          Peer  |  Authenticator              | Backend
                |              /   Local      |
                |             /    Method     |
          Peer  |        Auth                 |        Backend
          EAP  -|----->  EAP                  |    -->  EAP
         Switch |       Switch                |   /    Server
                |             \               |  /
                |              \ pass-through |
                |                             |

               Figure 2: EAP Pass-Through Model

   The Full/Pass-Through state machine allows an NAS or edge device to
   pass EAP Response messages to a backend server where the
   authentication method resides.  This paper includes a state machine
   for the EAP authenticator that supports both local and pass-through
   methods as well as a state machine for the backend authenticator
   existing at the AAA server.  A simple stand-alone authenticator is
   also provided to show a basic, non-pass-through authenticator's
   behavior.

   This document describes a set of state machines that can manage EAP
   authentication from the peer to an EAP method on the authenticator or
   from the peer through the authenticator pass-through method to the
   EAP method on the backend EAP server.

   Some environments where EAP is used, such as PPP, may support peer-
   to-peer operation.  That is, both parties act as peers and
   authenticators at the same time, in two simultaneous and independent
   EAP conversations.  In this case, the implementation at each node has
   to perform demultiplexing of incoming EAP packets.  EAP packets with
   code set to Response are delivered to the authenticator state
   machine, and EAP packets with code set to Request, Success, or
   Failure are delivered to the peer state machine.

   The state diagrams presented in this document have been coordinated
   with the diagrams in [1X-2004].  The format of the diagrams is
   adapted from the format therein.  The interface between the state
   machines defined here and the IEEE 802.1X-2004 state machines is also
   explained in Appendix F of [1X-2004].

2.  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" are to be
   interpreted as described in [RFC2119].



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3.  Notational Conventions Used in State Diagrams

3.1.  Notational Specifics

   The following state diagrams have been completed based on the
   conventions specified in [1X-2004], section 8.2.1.  The complete text
   is reproduced here:

      State diagrams are used to represent the operation of the protocol
      by a number of cooperating state machines, each comprising a group
      of connected, mutually exclusive states.  Only one state of each
      machine can be active at any given time.

      Each state is represented in the state diagram as a rectangular
      box, divided into two parts by a horizontal line.  The upper part
      contains the state identifier, written in uppercase letters.  The
      lower part contains any procedures that are executed upon entry to
      the state.

      All permissible transitions between states are represented by
      arrows, the arrowhead denoting the direction of the possible
      transition.  Labels attached to arrows denote the condition(s)
      that must be met in order for the transition to take place.  All
      conditions are expressions that evaluate to TRUE or FALSE; if a
      condition evaluates to TRUE, then the condition is met.  The label
      UCT denotes an unconditional transition (i.e., UCT always
      evaluates to TRUE).  A transition that is global in nature (i.e.,
      a transition that occurs from any of the possible states if the
      condition attached to the arrow is met) is denoted by an open
      arrow; i.e., no specific state is identified as the origin of the
      transition.  When the condition associated with a global
      transition is met, it supersedes all other exit conditions
      including UCT.  The special global condition BEGIN supersedes all
      other global conditions, and once asserted it remains asserted
      until all state blocks have executed to the point that variable
      assignments and other consequences of their execution remain
      unchanged.

      On entry to a state, the procedures defined for the state (if any)
      are executed exactly once, in the order that they appear on the
      page.  Each action is deemed to be atomic; i.e., execution of a
      procedure completes before the next sequential procedure starts to
      execute.  No procedures execute outside a state block.  The
      procedures in only one state block execute at a time, even if the
      conditions for execution of state blocks in different state
      machines are satisfied, and all procedures in an executing state
      block complete execution before the transition to and execution of
      any other state block occurs.  That is, the execution of any state



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      block appears to be atomic with respect to the execution of any
      other state block, and the transition condition to that state from
      the previous state is TRUE when execution commences.  The order of
      execution of state blocks in different state machines is undefined
      except as constrained by their transition conditions.  A variable
      that is set to a particular value in a state block retains this
      value until a subsequent state block executes a procedure that
      modifies the value.

      On completion of all the procedures within a state, all exit
      conditions for the state (including all conditions associated with
      global transitions) are evaluated continuously until one of the
      conditions is met.  The label ELSE denotes a transition that
      occurs if none of the other conditions for transitions from the
      state are met (i.e., ELSE evaluates to TRUE if all other possible
      exit conditions from the state evaluate to FALSE).  Where two or
      more exit conditions with the same level of precedence become TRUE
      simultaneously, the choice as to which exit condition causes the
      state transition to take place is arbitrary.

      Where it is necessary to split a state machine description across
      more than one diagram, a transition between two states that appear
      on different diagrams is represented by an exit arrow drawn with
      dashed lines, plus a reference to the diagram that contains the
      destination state.  Similarly, dashed arrows and a dashed state
      box are used on the destination diagram to show the transition to
      the destination state.  In a state machine that has been split in
      this way, any global transitions that can cause entry to states
      defined in one of the diagrams are deemed potential exit
      conditions for all the states of the state machine, regardless of
      which diagram the state boxes appear in.

      Should a conflict exist between the interpretation of a state
      diagram and either the corresponding global transition tables or
      the textual description associated with the state machine, the
      state diagram takes precedence.  The interpretation of the special
      symbols and operators used in the state diagrams is as defined in
      Section 3.2; these symbols and operators are derived from the
      notation of the C++ programming language, ISO/IEC 14882.  If a
      boolean variable is described in this clause as being set, it has
      or is assigned the value TRUE; if it is described as being reset
      or clear, it has the value FALSE.

   In addition to the above notation, there are a couple of
   clarifications specific to this document.  First, all boolean
   variables are initialized to FALSE before the state machine execution
   begins.  Second, the following notational shorthand is specific to
   this document:



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   <variable> = <expression1> | <expression2> | ...

      Execution of a statement of this form will result in <variable>
      having a value of exactly one of the expressions.  The logic for
      which of those expressions gets executed is outside of the state
      machine and could be environmental, configurable, or based on
      another state machine, such as that of the method.

3.2.  State Machine Symbols

   ( )

      Used to force the precedence of operators in Boolean expressions
      and to delimit the argument(s) of actions within state boxes.

   ;

      Used as a terminating delimiter for actions within state boxes.
      If a state box contains multiple actions, the order of execution
      follows the normal English language conventions for reading text.

   =

      Assignment action.  The value of the expression to the right of
      the operator is assigned to the variable to the left of the
      operator.  If this operator is used to define multiple assignments
      (e.g., a = b = X), the action causes the value of the expression
      following the right-most assignment operator to be assigned to all
      the variables that appear to the left of the right-most assignment
      operator.

   !

      Logical NOT operator.

   &&

      Logical AND operator.

   ||

      Logical OR operator.

   if...then...

      Conditional action.  If the Boolean expression following the "if"
      evaluates to TRUE, then the action following the "then" is
      executed.



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   { statement 1, ... statement N }

      Compound statement.  Braces are used to group statements that are
      executed together as if they were a single statement.

   !=

      Inequality.  Evaluates to TRUE if the expression to the left of
      the operator is not equal in value to the expression to the right.

   ==

      Equality.  Evaluates to TRUE if the expression to the left of the
      operator is equal in value to the expression to the right.

   >

      Greater than.  Evaluates to TRUE if the value of the expression to
      the left of the operator is greater than the value of the
      expression to the right.

   <=

      Less than or equal to.  Evaluates to TRUE if the value of the
      expression to the left of the operator is either less than or
      equal to the value of the expression to the right.

   ++

      Increment the preceding integer operator by 1.

   +

      Arithmetic addition operator.

   &

      Bitwise AND operator.

3.3.  Document Authority

   Should a conflict exist between the interpretation of a state diagram
   and either the corresponding global transition tables or the textual
   description associated with the state machine, the state diagram
   takes precedence.  When a discrepancy occurs between any part of this
   document (text or diagram) and any of the related documents
   ([RFC3748], [RFC3579], etc.), the latter (the other document) is
   considered authoritative and takes precedence.



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4.  Peer State Machine

   The following is a diagram of the EAP peer state machine.  Also
   included is an explanation of the primitives and procedures
   referenced in the diagram, as well as a clarification of notation.

               (see the .pdf version for missing diagram or
            refer to Appendix A.1 if reading the .txt version)

                     Figure 3: EAP Peer State Machine

4.1.  Interface between Peer State Machine and Lower Layer

   The lower layer presents messages to the EAP peer state machine by
   storing the packet in eapReqData and setting the eapReq signal to
   TRUE.  Note that despite the name of the signal, the lower layer does
   not actually inspect the contents of the EAP packet (it could be a
   Success or Failure message instead of a Request).

   When the EAP peer state machine has finished processing the message,
   it sets either eapResp or eapNoResp.  If it sets eapResp, the
   corresponding response packet is stored in eapRespData.  The lower
   layer is responsible for actually transmitting this message.  When
   the EAP peer state machine authentication is complete, it will set
   eapSuccess or eapFailure to indicate to the lower layer that the
   authentication has succeeded or failed.

4.1.1.  Variables (Lower Layer to Peer)

   eapReq (boolean)

      Set to TRUE in lower layer, FALSE in peer state machine.
      Indicates that a request is available in the lower layer.

   eapReqData (EAP packet)

      Set in lower layer when eapReq is set to TRUE.  The contents of
      the available request.

   portEnabled (boolean)

      Indicates that the EAP peer state machine should be ready for
      communication.  This is set to TRUE when the EAP conversation is
      started by the lower layer.  If at any point the communication
      port or session is not available, portEnabled is set to FALSE, and
      the state machine transitions to DISABLED.  To avoid unnecessary
      resets, the lower layer may dampen link down indications when it
      believes that the link is only temporarily down and that it will



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      soon be back up (see [RFC3748], Section 7.12).  In this case,
      portEnabled may not always be equal to the "link up" flag of the
      lower layer.

   idleWhile (integer)

      Outside timer used to indicate how much time remains before the
      peer will time out while waiting for a valid request.

   eapRestart (boolean)

      Indicates that the lower layer would like to restart
      authentication.

   altAccept (boolean)

      Alternate indication of success, as described in [RFC3748].

   altReject (boolean)

      Alternate indication of failure, as described in [RFC3748].

4.1.2.  Variables (peer to lower layer)

   eapResp (boolean)

      Set to TRUE in peer state machine, FALSE in lower layer.
      Indicates that a response is to be sent.

   eapNoResp (boolean)

      Set to TRUE in peer state machine, FALSE in lower layer.
      Indicates that the request has been processed, but that there is
      no response to send.

   eapSuccess (boolean)

      Set to TRUE in peer state machine, FALSE in lower layer.
      Indicates that the peer has reached the SUCCESS state.

   eapFail (boolean)

      Set to TRUE in peer state machine, FALSE in lower layer.
      Indicates that the peer has reached the FAILURE state.







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   eapRespData (EAP packet)

      Set in peer state machine when eapResp is set to TRUE.  The EAP
      packet that is the response to send.

   eapKeyData (EAP key)

      Set in peer state machine when keying material becomes available.
      Set during the METHOD state.  Note that this document does not
      define the structure of the type "EAP key".  We expect that it
      will be defined in [Keying].

   eapKeyAvailable (boolean)

      Set to TRUE in the SUCCESS state if keying material is available.
      The actual key is stored in eapKeyData.

4.1.3.  Constants

   ClientTimeout (integer)

      Configurable amount of time to wait for a valid request before
      aborting, initialized by implementation-specific means (e.g., a
      configuration setting).

4.2.  Interface between Peer State Machine and Methods

   IN: eapReqData (includes reqId)

   OUT: ignore, eapRespData, allowNotifications, decision

   IN/OUT: methodState, (method-specific state)

   The following describes the interaction between the state machine and
   EAP methods.

   If methodState==INIT, the method starts by initializing its own
   method-specific state.

   Next, the method must decide whether to process the packet or to
   discard it silently.  If the packet appears to have been sent by
   someone other than the legitimate authenticator (for instance, if
   message integrity check fails) and the method is capable of treating
   such situations as non-fatal, the method can set ignore=TRUE.  In
   this case, the method should not modify any other variables.

   If the method decides to process the packet, it behaves as follows.




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   o  It updates its own method-specific state.

   o  If the method has derived keying material it wants to export, it
      stores the keying material to eapKeyData.

   o  It creates a response packet (with the same identifier as the
      request) and stores it to eapRespData.

   o  It sets ignore=FALSE.

   Next, the method must update methodState and decision according to
   the following rules.

   methodState=CONT: The method always continues at this point (and the
      peer wants to continue it).  The decision variable is always set
      to FAIL.

   methodState=MAY_CONT: At this point, the authenticator can decide
      either to continue the method or to end the conversation.  The
      decision variable tells us what to do if the conversation ends.
      If the current situation does not satisfy the peer's security
      policy (that is, if the authenticator now decides to allow access,
      the peer will not use it), set decision=FAIL.  Otherwise, set
      decision=COND_SUCC.

   methodState=DONE: The method never continues at this point (or the
      peer sees no point in continuing it).

      If either (a) the authenticator has informed us that it will not
      allow access, or (b) we're not willing to talk to this
      authenticator (e.g., our security policy is not satisfied), set
      decision=FAIL.  (Note that this state can occur even if the method
      still has additional messages left, if continuing it cannot change
      the peer's decision to success).

      If both (a) the server has informed us that it will allow access,
      and the next packet will be EAP Success, and (b) we're willing to
      use this access, set decision=UNCOND_SUCC.

      Otherwise, we do not know what the server's decision is, but are
      willing to use the access if the server allows.  In this case, set
      decision=COND_SUCC.

   Finally, the method must set the allowNotifications variable.  If the
   new methodState is either CONT or MAY_CONT, and if the method
   specification does not forbid the use of Notification messages, set
   allowNotifications=TRUE.  Otherwise, set allowNotifications=FALSE.




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4.3.  Peer State Machine Local Variables

4.3.1.  Long-Term (Maintained between Packets)

   selectMethod (EAP type)

      Set in GET_METHOD state.  The method that the peer believes is
      currently "in progress"

   methodState (enumeration)

      As described above.

   lastId (integer)

      0-255 or NONE.  Set in SEND_RESPONSE state.  The EAP identifier
      value of the last request.

   lastRespData (EAP packet)

      Set in SEND_RESPONSE state.  The EAP packet last sent from the
      peer.

   decision (enumeration)

      As described above.

   NOTE: EAP type can be normal type (0..253,255), or an extended type
   consisting of type 254, Vendor-Id, and Vendor-Type.

4.3.2.  Short-Term (Not Maintained between Packets)

   rxReq (boolean)

      Set in RECEIVED state.  Indicates that the current received packet
      is an EAP request.

   rxSuccess (boolean)

      Set in RECEIVED state.  Indicates that the current received packet
      is an EAP Success.

   rxFailure (boolean)

      Set in RECEIVED state.  Indicates that the current received packet
      is an EAP Failure.





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   reqId (integer)

      Set in RECEIVED state.  The identifier value associated with the
      current EAP request.

   reqMethod (EAP type)

      Set in RECEIVED state.  The method type of the current EAP
      request.

   ignore (boolean)

      Set in METHOD state.  Indicates whether the method has decided to
      drop the current packet.

4.4.  Peer State Machine Procedures

   NOTE: For method procedures, the method uses its internal state in
   addition to the information provided by the EAP layer.  The only
   arguments that are explicitly shown as inputs to the procedures are
   those provided to the method by EAP.  Those inputs provided by the
   method's internal state remain implicit.

   parseEapReq()

      Determine the code, identifier value, and type of the current
      request.  In the case of a parsing error (e.g., the length field
      is longer than the received packet), rxReq, rxSuccess, and
      rxFailure will all be set to FALSE.  The values of reqId and
      reqMethod may be undefined as a result.  Returns three booleans,
      one integer, and one EAP type.

   processNotify()

      Process the contents of Notification Request (for instance,
      display it to the user or log it).  The return value is undefined.

   buildNotify()

      Create the appropriate notification response.  Returns an EAP
      packet.

   processIdentity()

      Process the contents of Identity Request.  Return value is
      undefined.





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   buildIdentity()

      Create the appropriate identity response.  Returns an EAP packet.

   m.check()

      Method-specific procedure to test for the validity of a message.
      Returns a boolean.

   m.process()

      Method procedure to parse and process a request for that method.
      Returns a methodState enumeration, a decision enumeration, and a
      boolean.

   m.buildResp()

      Method procedure to create a response message.  Returns an EAP
      packet.

   m.getKey()

      Method procedure to obtain key material for use by EAP or lower
      layers.  Returns an EAP key.

4.5.  Peer State Machine States

   DISABLED

      This state is reached whenever service from the lower layer is
      interrupted or unavailable.  Immediate transition to INITIALIZE
      occurs when the port becomes enabled.

   INITIALIZE

      Initializes variables when the state machine is activated.

   IDLE

      The state machine spends most of its time here, waiting for
      something to happen.

   RECEIVED

      This state is entered when an EAP packet is received.  The packet
      header is parsed here.





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   GET_METHOD

      This state is entered when a request for a new type comes in.
      Either the correct method is started, or a Nak response is built.

   METHOD

      The method processing happens here.  The request from the
      authenticator is processed, and an appropriate response packet is
      built.

   SEND_RESPONSE

      This state signals the lower layer that a response packet is ready
      to be sent.

   DISCARD

      This state signals the lower layer that the request was discarded,
      and no response packet will be sent at this time.

   IDENTITY

      Handles requests for Identity method and builds a response.

   NOTIFICATION

      Handles requests for Notification method and builds a response.

   RETRANSMIT

      Retransmits the previous response packet.

   SUCCESS

      A final state indicating success.

   FAILURE

      A final state indicating failure.











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5.  Stand-Alone Authenticator State Machine

   The following is a diagram of the stand-alone EAP authenticator state
   machine.  This diagram should be used for those interested in a
   self-contained, or non-pass-through, authenticator.  Included is an
   explanation of the primitives and procedures referenced in the
   diagram, as well as a clarification of notation.

               (see the .pdf version for missing diagram or
            refer to Appendix A.2 if reading the .txt version)

           Figure 4: EAP Stand-Alone Authenticator State Machine

5.1.  Interface between Stand-Alone Authenticator State Machine and
      Lower Layer

   The lower layer presents messages to the EAP authenticator state
   machine by storing the packet in eapRespData and setting the eapResp
   signal to TRUE.

   When the EAP authenticator state machine has finished processing the
   message, it sets one of the signals eapReq, eapNoReq, eapSuccess, and
   eapFail.  If it sets eapReq, eapSuccess, or eapFail, the
   corresponding request (or success/failure) packet is stored in
   eapReqData.  The lower layer is responsible for actually transmitting
   this message.

5.1.1.  Variables (Lower Layer to Stand-Alone Authenticator)

   eapResp (boolean)

      Set to TRUE in lower layer, FALSE in authenticator state machine.
      Indicates that an EAP response is available for processing.

   eapRespData (EAP packet)

      Set in lower layer when eapResp is set to TRUE.  The EAP packet to
      be processed.

   portEnabled (boolean)

      Indicates that the EAP authenticator state machine should be ready
      for communication.  This is set to TRUE when the EAP conversation
      is started by the lower layer.  If at any point the communication
      port or session is not available, portEnabled is set to FALSE, and
      the state machine transitions to DISABLED.  To avoid unnecessary
      resets, the lower layer may dampen link down indications when it
      believes that the link is only temporarily down and that it will



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      soon be back up (see [RFC3748], Section 7.12).  In this case,
      portEnabled may not always be equal to the "link up" flag of the
      lower layer.

   retransWhile (integer)

      Outside timer used to indicate how long the authenticator has
      waited for a new (valid) response.

   eapRestart (boolean)

      Indicates that the lower layer would like to restart
      authentication.

   eapSRTT (integer)

      Smoothed round-trip time.  (See [RFC3748], Section 4.3.)

   eapRTTVAR (integer)

      Round-trip time variation.  (See [RFC3748], Section 4.3.)

5.1.2.  Variables (Stand-Alone Authenticator To Lower Layer)

   eapReq (boolean)

      Set to TRUE in authenticator state machine, FALSE in lower layer.
      Indicates that a new EAP request is ready to be sent.

   eapNoReq (boolean)

      Set to TRUE in authenticator state machine, FALSE in lower layer.
      Indicates the most recent response has been processed, but there
      is no new request to send.

   eapSuccess (boolean)

      Set to TRUE in authenticator state machine, FALSE in lower layer.
      Indicates that the state machine has reached the SUCCESS state.

   eapFail (boolean)

      Set to TRUE in authenticator state machine, FALSE in lower layer.
      Indicates that the state machine has reached the FAILURE state.







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   eapTimeout (boolean)

      Set to TRUE in the TIMEOUT_FAILURE state if the authenticator has
      reached its maximum number of retransmissions without receiving a
      response.


   eapReqData (EAP packet)

      Set in authenticator state machine when eapReq, eapSuccess, or
      eapFail is set to TRUE.  The actual EAP request to be sent (or
      success/failure).

   eapKeyData (EAP key)

      Set in authenticator state machine when keying material becomes
      available.  Set during the METHOD state.  Note that this document
      does not define the structure of the type "EAP key".  We expect
      that it will be defined in [Keying].

   eapKeyAvailable (boolean)

      Set to TRUE in the SUCCESS state if keying material is available.
      The actual key is stored in eapKeyData.

5.1.3.  Constants

   MaxRetrans (integer)

      Configurable maximum for how many retransmissions should be
      attempted before aborting.

5.2.  Interface between Stand-Alone Authenticator State Machine and
      Methods

   IN: eapRespData, methodState

   OUT: ignore, eapReqData

   IN/OUT: currentId, (method-specific state), (policy)

   The following describes the interaction between the state machine and
   EAP methods.

   m.init (in: -, out: -)






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   When the method is first started, it must initialize its own method-
   specific state, possibly using some information from Policy (e.g.,
   identity).

   m.buildReq (in: integer, out: EAP packet)

   Next, the method creates a new EAP Request packet, with the given
   identifier value, and updates its method-specific state accordingly.

   m.getTimeout (in: -, out: integer or NONE)

   The method can also provide a hint for retransmission timeout with
   m.getTimeout.

   m.check (in: EAP packet, out: boolean)

   When a new EAP Response is received, the method must first decide
   whether to process the packet or to discard it silently.  If the
   packet looks like it was not sent by the legitimate peer (e.g., if it
   has an invalid Message Integrity Check (MIC), which should never
   occur), the method can indicate this by returning FALSE.  In this
   case, the method should not modify its own method-specific state.

   m.process (in: EAP packet, out: -)

   m.isDone (in: -, out: boolean)

   m.getKey (in: -, out: EAP key or NONE)

   Next, the method processes the EAP Response and updates its own
   method-specific state.  Now the options are to continue the
   conversation (send another request) or to end this method.

   If the method wants to end the conversation, it

   o  Tells Policy about the outcome of the method and possibly other
      information.

   o  If the method has derived keying material it wants to export,
      returns it from m.getKey().

   o  Indicates that the method wants to end by returning TRUE from
      m.isDone().

   Otherwise, the method continues by sending another request, as
   described earlier.





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5.3.  Stand-Alone Authenticator State Machine Local Variables

5.3.1.  Long-Term (Maintained between Packets)

   currentMethod (EAP type)

      EAP type, IDENTITY, or NOTIFICATION.

   currentId (integer)

      0-255 or NONE.  Usually updated in PROPOSE_METHOD state.
      Indicates the identifier value of the currently outstanding EAP
      request.

   methodState (enumeration)

      As described above.

   retransCount (integer)

      Reset in SEND_REQUEST state and updated in RETRANSMIT state.
      Current number of retransmissions.

   lastReqData (EAP packet)

      Set in SEND_REQUEST state.  EAP packet containing the last sent
      request.

   methodTimeout (integer)

      Method-provided hint for suitable retransmission timeout, or NONE.

5.3.2.  Short-Term (Not Maintained between Packets)

   rxResp (boolean)

      Set in RECEIVED state.  Indicates that the current received packet
      is an EAP response.

   respId (integer)

      Set in RECEIVED state.  The identifier from the current EAP
      response.

   respMethod (EAP type)

      Set in RECEIVED state.  The method type of the current EAP
      response.



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   ignore (boolean)

      Set in METHOD state.  Indicates whether the method has decided to
      drop the current packet.

   decision (enumeration)

      Set in SELECT_ACTION state.  Temporarily stores the policy
      decision to succeed, fail, or continue.

5.4.  EAP Stand-Alone Authenticator Procedures

   NOTE: For method procedures, the method uses its internal state in
   addition to the information provided by the EAP layer.  The only
   arguments that are explicitly shown as inputs to the procedures are
   those provided to the method by EAP.  Those inputs provided by the
   method's internal state remain implicit.

   calculateTimeout()

      Calculates the retransmission timeout, taking into account the
      retransmission count, round-trip time measurements, and method-
      specific timeout hint (see [RFC3748], Section 4.3).  Returns an
      integer.

   parseEapResp()

      Determines the code, identifier value, and type of the current
      response.  In the case of a parsing error (e.g., the length field
      is longer than the received packet), rxResp will be set to FALSE.
      The values of respId and respMethod may be undefined as a result.
      Returns a boolean, an integer, and an EAP type.

   buildSuccess()

      Creates an EAP Success Packet.  Returns an EAP packet.

   buildFailure()

      Creates an EAP Failure Packet.  Returns an EAP packet.

   nextId()

      Determines the next identifier value to use, based on the previous
      one.  Returns an integer.






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   Policy.update()

      Updates all variables related to internal policy state.  The
      return value is undefined.

   Policy.getNextMethod()

      Determines the method that should be used at this point in the
      conversation based on predefined policy.  Policy.getNextMethod()
      MUST comply with [RFC3748] (Section 2.1), which forbids the use of
      sequences of authentication methods within an EAP conversation.
      Thus, if an authentication method has already been executed within
      an EAP dialog, Policy.getNextMethod() MUST NOT propose another
      authentication method within the same EAP dialog.  Returns an EAP
      type.

   Policy.getDecision()

      Determines if the policy will allow SUCCESS, FAIL, or is yet to
      determine (CONTINUE).  Returns a decision enumeration.

   m.check()

      Method-specific procedure to test for the validity of a message.
      Returns a boolean.

   m.process()

      Method procedure to parse and process a response for that method.
      The return value is undefined.

   m.init()

      Method procedure to initialize state just before use.  The return
      value is undefined.

   m.reset()

      Method procedure to indicate that the method is ending in the
      middle of or before completion.  The return value is undefined.

   m.isDone()

      Method procedure to check for method completion.  Returns a
      boolean.






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   m.getTimeout()

      Method procedure to determine an appropriate timeout hint for that
      method.  Returns an integer.

   m.getKey()

      Method procedure to obtain key material for use by EAP or lower
      layers.  Returns an EAP key.

   m.buildReq()

      Method procedure to produce the next request.  Returns an EAP
      packet.

5.5.  EAP Stand-Alone Authenticator States

   DISABLED

      The authenticator is disabled until the port is enabled by the
      lower layer.

   INITIALIZE

      Initializes variables when the state machine is activated.

   IDLE

      The state machine spends most of its time here, waiting for
      something to happen.

   RECEIVED

      This state is entered when an EAP packet is received.  The packet
      header is parsed here.

   INTEGRITY_CHECK

      A method state in which the integrity of the incoming packet from
      the peer is verified by the method.

   METHOD_RESPONSE

      A method state in which the incoming packet is processed.

   METHOD_REQUEST

      A method state in which a new request is formulated if necessary.



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   PROPOSE_METHOD

      A state in which the authenticator decides which method to try
      next in the authentication.

   SELECT_ACTION

      Between methods, the state machine re-evaluates whether its policy
      is satisfied and succeeds, fails, or remains undecided.

   SEND_REQUEST

      This state signals the lower layer that a request packet is ready
      to be sent.

   DISCARD

      This state signals the lower layer that the response was
      discarded, and no new request packet will be sent at this time.

   NAK

      This state processes Nak responses from the peer.

   RETRANSMIT

      Retransmits the previous request packet.

   SUCCESS

      A final state indicating success.

   FAILURE

      A final state indicating failure.

   TIMEOUT_FAILURE

      A final state indicating failure because no response has been
      received.  Because no response was received, no new message
      (including failure) should be sent to the peer.  Note that this is
      different from the FAILURE state, in which a message indicating
      failure is sent to the peer.








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6.  EAP Backend Authenticator

   When operating in pass-through mode, there are conceptually two parts
   to the authenticator: the part that passes packets through, and the
   backend that actually implements the EAP method.  The following
   diagram shows a state machine for the backend part of this model when
   using a AAA server.  Note that this diagram is identical to Figure 4
   except that no retransmit is included in the IDLE state because with
   RADIUS, retransmit is handled by the NAS.  Also, a PICK_UP_METHOD
   state and variable in INITIALIZE state are added to allow the Method
   to "pick up" a method started in a NAS.  Included is an explanation
   of the primitives and procedures referenced in the diagram, many of
   which are the same as above.  Note that the "lower layer" in this
   case is some AAA protocol (e.g., RADIUS).

               (see the .pdf version for missing diagram or
            refer to Appendix A.3 if reading the .txt version)

             Figure 5: EAP Backend Authenticator State Machine

6.1.  Interface between Backend Authenticator State Machine and Lower
      Layer

   The lower layer presents messages to the EAP backend authenticator
   state machine by storing the packet in aaaEapRespData and setting the
   aaaEapResp signal to TRUE.

   When the EAP backend authenticator state machine has finished
   processing the message, it sets one of the signals aaaEapReq,
   aaaEapNoReq, aaaSuccess, and aaaFail.  If it sets eapReq, eapSuccess,
   or eapFail, the corresponding request (or success/failure) packet is
   stored in aaaEapReqData.  The lower layer is responsible for actually
   transmitting this message.

6.1.1.  Variables (AAA Interface to Backend Authenticator)

   aaaEapResp (boolean)

      Set to TRUE in lower layer, FALSE in authenticator state machine.
      Usually indicates that an EAP response, stored in aaaEapRespData,
      is available for processing by the AAA server.  If aaaEapRespData
      is set to NONE, it indicates that the AAA server should send the
      initial EAP request.

   aaaEapRespData (EAP packet)

      Set in lower layer when eapResp is set to TRUE.  The EAP packet to
      be processed, or NONE.



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   backendEnabled (boolean)

      Indicates that there is a valid link to use for the communication.
      If at any point the port is not available, backendEnabled is set
      to FALSE, and the state machine transitions to DISABLED.

6.1.2.  Variables (Backend Authenticator to AAA Interface)

   aaaEapReq (boolean)

      Set to TRUE in authenticator state machine, FALSE in lower layer.
      Indicates that a new EAP request is ready to be sent.

   aaaEapNoReq (boolean)

      Set to TRUE in authenticator state machine, FALSE in lower layer.
      Indicates that the most recent response has been processed, but
      there is no new request to send.

   aaaSuccess (boolean)

      Set to TRUE in authenticator state machine, FALSE in lower layer.
      Indicates that the state machine has reached the SUCCESS state.

   aaaFail (boolean)

      Set to TRUE in authenticator state machine, FALSE in lower layer.
      Indicates that the state machine has reached the FAILURE state.

   aaaEapReqData (EAP packet)

      Set in authenticator state machine when aaaEapReq, aaaSuccess, or
      aaaFail is set to TRUE.  The actual EAP request to be sent (or
      success/failure).

   aaaEapKeyData (EAP key)

      Set in authenticator state machine when keying material becomes
      available.  Set during the METHOD_RESPONSE state.  Note that this
      document does not define the structure of the type "EAP key".  We
      expect that it will be defined in [Keying].

   aaaEapKeyAvailable (boolean)

      Set to TRUE in the SUCCESS state if keying material is available.
      The actual key is stored in aaaEapKeyData.





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   aaaMethodTimeout (integer)

      Method-provided hint for suitable retransmission timeout, or NONE.
      (Note that this hint is for the EAP retransmissions done by the
      pass-through authenticator, not for retransmissions of AAA
      packets.)

6.2.  Interface between Backend Authenticator State Machine and
      Methods

   The backend method interface is almost the same as in stand-alone
   authenticator described in Section 5.2.  The only difference is that
   some methods on the backend may support "picking up" a conversation
   started by the pass-through.  That is, the EAP Request packet was
   sent by the pass-through, but the backend must process the
   corresponding EAP Response.  Usually only the Identity method
   supports this, but others are possible.

   When "picking up" a conversation, m.initPickUp() is called instead of
   m.init().  Next, m.process() must examine eapRespData and update its
   own method-specific state to match what it would have been if it had
   actually sent the corresponding request.  (Obviously, this only works
   for methods that can determine what the initial request contained;
   Identity and EAP-TLS are good examples.)

   After this, the processing continues as described in Section 5.2.

6.3.  Backend Authenticator State Machine Local Variables

   For definitions of the variables used in the Backend Authenticator,
   see Section 5.3.

6.4.  EAP Backend Authenticator Procedures

   Most of the procedures of the backend authenticator have already been
   defined in Section 5.4.  This section contains definitions for those
   not existent in the stand-alone version, as well as those that are
   defined differently.

   NOTE: For method procedures, the method uses its internal state in
   addition to the information provided by the EAP layer.  The only
   arguments that are explicitly shown as inputs to the procedures are
   those provided to the method by EAP.  Those inputs provided by the
   method's internal state remain implicit.







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   Policy.doPickUp()

      Notifies the policy that an already-chosen method is being picked
      up and will be completed.  Returns a boolean.

   m.initPickUp()

      Method procedure to initialize state when continuing from an
      already-started method.  The return value is undefined.

6.5.  EAP Backend Authenticator States

   Most of the states of the backend authenticator have already been
   defined in Section 5.5.  This section contains definitions for those
   not existent in the stand-alone version, as well as those that are
   defined differently.

   PICK_UP_METHOD

      Sets an initial state for a method that is being continued and
      that was started elsewhere.

7.  EAP Full Authenticator

   The following two diagrams show the state machine for a complete
   authenticator.  The first diagram is identical to the stand-alone
   state machine, shown in Figure 4, with the exception that the
   SELECT_ACTION state has an added transition to PASSTHROUGH.  The
   second diagram also keeps most of the logic, except the four method
   states, and it shows how the state machine works once it goes to
   pass-through mode.

   The first diagram is largely a reproduction of that found above, with
   the added hooks for a transition to PASSTHROUGH mode.

               (see the .pdf version for missing diagram or
            refer to Appendix A.4 if reading the .txt version)

          Figure 6: EAP Full Authenticator State Machine (Part 1)

   The second diagram describes the functionality necessary for an
   authenticator operating in pass-through mode.  This section of the
   diagram is the counterpart of the backend diagram above.

               (see the .pdf version for missing diagram or
            refer to Appendix A.4 if reading the .txt version)

          Figure 7: EAP Full Authenticator State Machine (Part 2)



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7.1.  Interface between Full Authenticator State Machine and Lower
      Layers

   The full authenticator is unique in that it interfaces to multiple
   lower layers in order to support pass-through mode.  The interface to
   the primary EAP transport layer is the same as described in Section
   5.  The following describes the interface to the second lower layer,
   which represents an interface to AAA.  Note that there is not
   necessarily a direct interaction between the EAP layer and the AAA
   layer, as in the case of [1X-2004].

7.1.1.  Variables (AAA Interface to Full Authenticator)

   aaaEapReq (boolean)

      Set to TRUE in lower layer, FALSE in authenticator state machine.
      Indicates that a new EAP request is available from the AAA server.

   aaaEapNoReq (boolean)

      Set to TRUE in lower layer, FALSE in authenticator state machine.
      Indicates that the most recent response has been processed, but
      that there is no new request to send.

   aaaSuccess (boolean)

      Set to TRUE in lower layer.  Indicates that the AAA backend
      authenticator has reached the SUCCESS state.

   aaaFail (boolean)

      Set to TRUE in lower layer.  Indicates that the AAA backend
      authenticator has reached the FAILURE state.

   aaaEapReqData (EAP packet)

      Set in the lower layer when aaaEapReq, aaaSuccess, or aaaFail is
      set to TRUE.  The actual EAP request to be sent (or success/
      failure).

   aaaEapKeyData (EAP key)

      Set in lower layer when keying material becomes available from the
      AAA server.  Note that this document does not define the structure
      of the type "EAP key".  We expect that it will be defined in
      [Keying].





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   aaaEapKeyAvailable (boolean)

      Set to TRUE in the lower layer if keying material is available.
      The actual key is stored in aaaEapKeyData.

   aaaMethodTimeout (integer)

      Method-provided hint for suitable retransmission timeout, or NONE.
      (Note that this hint is for the EAP retransmissions done by the
      pass-through authenticator, not for retransmissions of AAA
      packets.)

7.1.2.  Variables (full authenticator to AAA interface)

   aaaEapResp (boolean)

      Set to TRUE in authenticator state machine, FALSE in the lower
      layer.  Indicates that an EAP response is available for processing
      by the AAA server.

   aaaEapRespData (EAP packet)

      Set in authenticator state machine when eapResp is set to TRUE.
      The EAP packet to be processed.

   aaaIdentity (EAP packet)

      Set in authenticator state machine when an IDENTITY response is
      received.  Makes that identity available to AAA lower layer.

   aaaTimeout (boolean)

      Set in AAA_IDLE if, after a configurable amount of time, there is
      no response from the AAA layer.  The AAA layer in the NAS is
      itself alive and OK, but for some reason it has not received a
      valid Access-Accept/Reject indication from the backend.

7.1.3.  Constants

   Same as Section 5.

7.2.  Interface between Full Authenticator State Machine and Methods

   Same as stand-alone authenticator (Section 5.2).







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7.3.  Full Authenticator State Machine Local Variables

   Many of the variables of the full authenticator have already been
   defined in Section 5.  This section contains definitions for those
   not existent in the stand-alone version, as well as those that are
   defined differently.

7.3.1.  Short-Term (Not Maintained between Packets)

   decision (enumeration)

      Set in SELECT_ACTION state.  Temporarily stores the policy
      decision to succeed, fail, continue with a local method, or
      continue in pass-through mode.

7.4.  EAP Full Authenticator Procedures

   All the procedures defined in Section 5 exist in the full version.
   In addition, the following procedures are defined.

   getId()

      Determines the identifier value chosen by the AAA server for the
      current EAP request.  The return value is an integer.

7.5.  EAP Full Authenticator States

   All the states defined in Section 5 exist in the full version.  In
   addition, the following states are defined.

   INITIALIZE_PASSTHROUGH

      Initializes variables when the pass-through portion of the state
      machine is activated.

   IDLE2

      The state machine waits for a response from the primary lower
      layer, which transports EAP traffic from the peer.

   IDLE

      The state machine spends most of its time here, waiting for
      something to happen.







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   RECEIVED2

      This state is entered when an EAP packet is received and the
      authenticator is in PASSTHROUGH mode.  The packet header is parsed
      here.

   AAA_REQUEST

      The incoming EAP packet is parsed for sending to the AAA server.

   AAA_IDLE

      Idle state that tells the AAA layer that it has a response and
      then waits for a new request, a no-request signal, or
      success/failure.

   AAA_RESPONSE

      State in which the request from the AAA interface is processed
      into an EAP request.

   SEND_REQUEST2

      This state signals the lower layer that a request packet is ready
      to be sent.

   DISCARD2

      This state signals the lower layer that the response was
      discarded, and that no new request packet will be sent at this
      time.

   RETRANSMIT2

      Retransmits the previous request packet.

   SUCCESS2

      A final state indicating success.

   FAILURE2

      A final state indicating failure.








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   TIMEOUT_FAILURE2

      A final state indicating failure because no response has been
      received.  Because no response was received, no new message
      (including failure) should be sent to the peer.  Note that this is
      different from the FAILURE2 state, in which a message indicating
      failure is sent to the peer.

8.  Implementation Considerations

8.1.  Robustness

   In order to deal with erroneous cases that are not directly related
   to the protocol behavior, implementations may need additional
   considerations to provide robustness against errors.

   For example, an implementation of a state machine may spend a
   significant amount of time in a particular state performing the
   procedure defined for the state without returning a response.  If
   such an implementation is made on a multithreading system, the
   procedure may be performed in a separate thread so that the
   implementation can perform appropriate action without blocking on the
   state for a long time (or forever if the procedure never completes
   due to, e.g., a non-responding user or a bug in an application
   callback function).

   The following states are identified as the possible places of
   blocking:

   o  IDENTITY state in the peer state machine.  It may take some time
      to process Identity request when a user input is needed for
      obtaining an identity from the user.  The user may never input an
      identity.  An implementation may define an additional state
      transition from IDENTITY state to FAILURE state so that
      authentication can fail if no identity is obtained from the user
      before ClientTimeout timer expires.

   o  METHOD state in the peer state machine and in METHOD_RESPONSE
      state in the authenticator state machines.  It may take some time
      to perform method-specific procedures in these states.  An
      implementation may define an additional state transition from
      METHOD state and METHOD_RESPONSE state to FAILURE or
      TIMEOUT_FAILURE state so that authentication can fail if no method
      processing result is obtained from the method before methodTimeout
      timer expires.






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8.2.  Method/Method and Method/Lower-Layer Interfaces

   Implementations may define additional interfaces to pass method-
   specific information between methods and lower layers.  These
   interfaces are beyond the scope of this document.

8.3.  Peer State Machine Interoperability with Deployed Implementations

   Number of deployed EAP authenticator implementations, mainly in
   RADIUS authentication servers, have been observed to increment the
   Identifier field incorrectly when generating EAP Success and EAP
   Failure packets which is against the MUST requirement in RFC 3748
   section 4.2.  The peer state machine is based on RFC 3748, and as
   such it will discard such EAP Success and EAP Failure packets.

   As a workaround for the potential interoperability issue with
   existing implementations, conditions for peer state machine
   transitions from RECEIVED state to SUCCESS and FAILURE states MAY be
   changed from "(reqId == lastId)" to "((reqId == lastId) || (reqId ==
   (lastId + 1) & 255))".  However, because this behavior does not
   conform to RFC 3748, such a workaround is not recommended, and if
   included, it should be implemented as an optional workaround that can
   be disabled.

9.  Security Considerations

   This document's intent is to describe the EAP state machine fully.
   To this end, any security concerns with this document are likely a
   reflection of security concerns with EAP itself.

   An accurate state machine can help reduce implementation errors.
   Although [RFC3748] remains the normative protocol description, this
   state machine should help in this regard.

   As noted in [RFC3748], some security concerns arise because of the
   following EAP packets:

      1. EAP-Request/Response Identity
      2. EAP-Response/NAK
      3. EAP-Success/Failure

   Because these packets are not cryptographically protected by
   themselves, an attacker can modify or insert them without immediate
   detection by the peer or authenticator.

   Following Figure 3 specification, an attacker may cause denial of
   service by:




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RFC 4137                   EAP State Machines                August 2005


   o  Sending an EAP-Failure to the peer before the peer has started an
      EAP authentication method.  As long as the peer has not modified
      the methodState variable (initialized to NONE), the peer MUST
      accept an EAP-Failure.

   o  Forcing the peer to engage in endless EAP-Request/Response
      Identity exchanges before it has started an EAP authentication
      method.  As long as the peer has not modified the selectedMethod
      variable (initialized to NONE), the peer MUST accept an EAP-
      Request/Identity and respond to it with an EAP-Response/Identity.

   Following Figure 4 specification, an attacker may cause denial of
   service by:

   o  Sending a NAK to the authenticator after the authenticator first
      proposes an EAP authentication method to the peer.  When the
      methodState variable has the value PROPOSED, the authenticator is
      obliged to process a NAK that is received in response to its first
      packet of an EAP authentication method.

   There MAY be some cases when it is desired to prevent such attacks.
   This can be done by modifying initial values of some variables of the
   EAP state machines.  However, such modifications are NOT RECOMMENDED.

   There is a trade-off between mitigating these denial-of-service
   attacks and being able to deal with EAP peers and authenticators in
   general.  For instance, if a NAK is ignored when it is sent to the
   authenticator after it has just proposed an EAP authentication method
   to the peer, then a legitimate peer that is not able or willing to
   process the proposed EAP authentication method would fail without an
   opportunity to negotiate another EAP method.

10.  Acknowledgements

   The work in this document was done as part of the EAP Design Team.
   It was done primarily by Nick Petroni, John Vollbrecht, Pasi Eronen,
   and Yoshihiro Ohba.  Nick started this work with Bryan Payne and Chuk
   Seng at the University of Maryland.  John Vollbrecht of Meetinghouse
   Data Communications started independently with help from Dave Spence
   at Interlink Networks.  John and Nick collaborated to create a common
   document, and then were joined by Pasi Eronen of Nokia, who has made
   major contributions in creating coherent state machines, and by
   Yoshihiro Ohba of Toshiba, who insisted on including pass-through
   documentation and provided significant support for understanding
   implementation issues.






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RFC 4137                   EAP State Machines                August 2005


   In addition, significant response and conversation has come from the
   design team, especially Jari Arkko of Ericsson and Bernard Aboba of
   Microsoft, as well as the rest of the team.  It has also been
   reviewed by IEEE 802.1, and has had input from Jim Burns of
   Meetinghouse and Paul Congdon of Hewlett Packard.

11.  References

11.1.  Normative References

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

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

   [RFC3748]   Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
               Levkowetz, Ed., "Extensible Authentication Protocol
               (EAP)", RFC 3748, June 2004.

11.2.  Informative References

   [Keying]    Aboba, B., Simon, D., Arkko, J., Eronen, P., Levkowetz,
               H., "Extensible Authentication Protocol (EAP) Key
               Management Framework", Work in Progress, July 2005.

   [1X-2004]   Institute of Electrical and Electronics Engineers,
               "Standard for Local and Metropolitan Area Networks:
               Port-Based Network Access Control", IEEE 802.1X-2004,
               December 2004.




















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Appendix A.  ASCII versions of state diagrams

   This appendix contains the state diagrams in ASCII format.  Please
   use the PDF version whenever possible; it is much easier to
   understand.

   The notation is as follows: state name and pseudocode executed when
   entering it are shown on the left; outgoing transitions with their
   conditions are shown on the right.

A.1.  EAP Peer State Machine (Figure 3)

---------------------------------------------------------------------
(global transitions)         |      !portEnabled      |      DISABLED
                             |------------------------+--------------
                             |     eapRestart &&      |    INITIALIZE
                             |      portEnabled       |
-----------------------------+------------------------+--------------
DISABLED                     |      portEnabled       |    INITIALIZE
-----------------------------+------------------------+--------------
INITIALIZE                   |                        |
                             |                        |
selectedMethod = NONE        |                        |
methodState = NONE           |                        |
allowNotifications = TRUE    |                        |
decision = FAIL              |          UCT           |          IDLE
idleWhile = ClientTimeout    |                        |
lastId = NONE                |                        |
eapSuccess = FALSE           |                        |
eapFail = FALSE              |                        |
eapKeyData = NONE            |                        |
eapKeyAvailable = FALSE      |                        |
eapRestart = FALSE           |                        |
-----------------------------+------------------------+--------------
IDLE                         |         eapReq         |      RECEIVED
                             |------------------------+--------------
                             |     (altAccept &&      |
                             |  decision != FAIL) ||  |
                             |   (idleWhile == 0 &&   |       SUCCESS
                             |      decision ==       |
                             |      UNCOND_SUCC)      |
                             |------------------------+--------------









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                             |------------------------+--------------
                             |      altReject ||      |
                             |   (idleWhile == 0 &&   |
                             |      decision !=       |
                             |    UNCOND_SUCC) ||     |       FAILURE
                             |     (altAccept &&      |
                             | methodState != CONT && |
                             |   decision == FAIL)    |
-----------------------------+------------------------+--------------
RECEIVED                     |        rxReq &&        |        METHOD
                             |  (reqId != lastId) &&  |
(rxReq,rxSuccess,rxFailure,  |     (reqMethod ==      |
  reqId,reqMethod) =         |   selectedMethod) &&   |
  parseEapReq(eapReqData)    | (methodState != DONE)  |
                             |------------------------+--------------
                             |        rxReq &&        |
                             |  (reqId != lastId) &&  |
                             |   (selectedMethod ==   |
                             |        NONE) &&        |    GET_METHOD
                             |     (reqMethod !=      |
                             |      IDENTITY) &&      |
                             |     (reqMethod !=      |
                             |     NOTIFICATION)      |
                             |------------------------+--------------
                             |        rxReq &&        |
                             |  (reqId != lastId) &&  |
                             |   (selectedMethod ==   |      IDENTITY
                             |        NONE) &&        |
                             |     (reqMethod ==      |
                             |       IDENTITY)        |
                             |------------------------+--------------
                             |        rxReq &&        |
                             |  (reqId != lastId) &&  |
                             |   (reqMethod ==        |  NOTIFICATION
                             |    NOTIFICATION) &&    |
                             |   allowNotifications   |
                             |------------------------+--------------
                             |        rxReq &&        |    RETRANSMIT
                             |   (reqId == lastId)    |
                             |------------------------+--------------
                             |      rxSuccess &&      |
                             |  (reqId == lastId) &&  |       SUCCESS
                             |   (decision != FAIL)   |
                             |------------------------+--------------







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                             |------------------------+--------------
                             | (methodState!=CONT) && |
                             |     ((rxFailure &&     |
                             |      decision !=       |
                             |    UNCOND_SUCC) ||     |       FAILURE
                             |     (rxSuccess &&      |
                             | decision == FAIL)) &&  |
                             |   (reqId == lastId)    |
                             |------------------------+--------------
                             |          else          |       DISCARD
-----------------------------+------------------------+--------------
METHOD                       |                        |
                             |                        |
ignore = m.check(eapReqData) |         ignore         |       DISCARD
if (!ignore) {               |                        |
  (methodState, decision,    |                        |
  allowNotifications) =      |------------------------+--------------
  m.process(eapReqData)      |                        |
  /* methodState is CONT,    |                        |
     MAY_CONT, or DONE */    | (methodState==DONE) && |       FAILURE
  /* decision is FAIL,       |   (decision == FAIL)   |
     COND_SUCC, or           |                        |
     UNCOND_SUCC */          |                        |
  eapRespData =              |------------------------+--------------
    m.buildResp(reqId)       |                        |
  if (m.isKeyAvailable())    |          else          | SEND_RESPONSE
    eapKeyData = m.getKey()  |                        |
}                            |                        |
-----------------------------+------------------------+--------------
GET_METHOD                   |                        |
                             |   selectedMethod ==    |
if (allowMethod(reqMethod)) {|       reqMethod        |        METHOD
  selectedMethod = reqMethod |                        |
  methodState = INIT         |                        |
} else {                     |------------------------+--------------
  eapRespData =              |                        |
    buildNak(reqId)          |          else          | SEND_RESPONSE
}                            |                        |
-----------------------------+------------------------+--------------
IDENTITY                     |                        |
                             |                        |
processIdentity(eapReqData)  |          UCT           | SEND_RESPONSE
eapRespData =                |                        |
  buildIdentity(reqId)       |                        |
-----------------------------+------------------------+--------------






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-----------------------------+------------------------+--------------
NOTIFICATION                 |                        |
                             |                        |
processNotify(eapReqData)    |          UCT           | SEND_RESPONSE
eapRespData =                |                        |
  buildNotify(reqId)         |                        |
-----------------------------+------------------------+--------------
RETRANSMIT                   |                        |
                             |          UCT           | SEND_RESPONSE
eapRespData = lastRespData   |                        |
-----------------------------+------------------------+--------------
DISCARD                      |                        |
                             |          UCT           |          IDLE
eapReq = FALSE               |                        |
eapNoResp = TRUE             |                        |
-----------------------------+------------------------+--------------
SEND_RESPONSE                |                        |
                             |                        |
lastId = reqId               |                        |
lastRespData = eapRespData   |          UCT           |          IDLE
eapReq = FALSE               |                        |
eapResp = TRUE               |                        |
idleWhile = ClientTimeout    |                        |
-----------------------------+------------------------+--------------
SUCCESS                      |                        |
                             |                        |
if (eapKeyData != NONE)      |                        |
  eapKeyAvailable = TRUE     |                        |
eapSuccess = TRUE            |                        |
-----------------------------+------------------------+--------------
FAILURE                      |                        |
                             |                        |
eapFail = TRUE               |                        |
---------------------------------------------------------------------
                                Figure 8

A.2.  EAP Stand-Alone Authenticator State Machine (Figure 4)

---------------------------------------------------------------------
(global transitions)          |    !portEnabled     |        DISABLED
                              |---------------------+----------------
                              |    eapRestart &&    |      INITIALIZE
                              |     portEnabled     |
------------------------------+---------------------+----------------
DISABLED                      |     portEnabled     |      INITIALIZE
------------------------------+---------------------+----------------





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RFC 4137                   EAP State Machines                August 2005


------------------------------+---------------------+----------------
INITIALIZE                    |                     |
                              |                     |
currentId = NONE              |                     |
eapSuccess = FALSE            |                     |
eapFail = FALSE               |         UCT         |   SELECT_ACTION
eapTimeout = FALSE            |                     |
eapKeyData = NONE             |                     |
eapKeyAvailable = FALSE       |                     |
eapRestart = FALSE            |                     |
------------------------------+---------------------+----------------
IDLE                          |                     |
                              |  retransWhile == 0  |      RETRANSMIT
retransWhile =                |                     |
  calculateTimeout(           |---------------------+----------------
   retransCount, eapSRTT,     |       eapResp       |        RECEIVED
   eapRTTVAR, methodTimeout)  |                     |
------------------------------+---------------------+----------------
RETRANSMIT                    |                     |
                              |   retransCount >    | TIMEOUT_FAILURE
retransCount++                |     MaxRetrans      |
if (retransCount<=MaxRetrans){|                     |
  eapReqData = lastReqData    |---------------------+----------------
  eapReq = TRUE               |        else         |            IDLE
}                             |                     |
------------------------------+---------------------+----------------
RECEIVED                      |      rxResp &&      |
                              |     (respId ==      |
(rxResp,respId,respMethod)=   |    currentId) &&    |
  parseEapResp(eapRespData)   | (respMethod == NAK  |
                              |         ||          |             NAK
                              |    respMethod ==    |
                              |  EXPANDED_NAK) &&   |
                              |   (methodState ==   |
                              |      PROPOSED)      |
                              |---------------------+----------------
                              |      rxResp &&      |
                              |     (respId ==      |
                              |    currentId) &&    | INTEGRITY_CHECK
                              |   (respMethod ==    |
                              |   currentMethod)    |
                              |---------------------+----------------
                              |        else         |         DISCARD
------------------------------+---------------------+----------------







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RFC 4137                   EAP State Machines                August 2005


------------------------------+---------------------+----------------
NAK                           |                     |
                              |         UCT         |   SELECT_ACTION
m.reset()                     |                     |
Policy.update(<...>)          |                     |
------------------------------+---------------------+----------------
SELECT_ACTION                 | decision == FAILURE |         FAILURE
                              |                     |
decision =                    |---------------------+----------------
  Policy.getDecision()        | decision == SUCCESS |         SUCCESS
/* SUCCESS, FAILURE, or       |---------------------+----------------
   CONTINUE */                |        else         |  PROPOSE_METHOD
------------------------------+---------------------+----------------
INTEGRITY_CHECK               |       ignore        |         DISCARD
                              |---------------------+----------------
ignore = m.check(eapRespData) |       !ignore       | METHOD_RESPONSE
------------------------------+---------------------+----------------
METHOD_RESPONSE               |                     |
                              | methodState == END  |   SELECT_ACTION
m.process(eapRespData)        |                     |
if (m.isDone()) {             |                     |
  Policy.update(<...>)        |---------------------+----------------
  eapKeyData = m.getKey()     |                     |
  methodState = END           |        else         |  METHOD_REQUEST
} else                        |                     |
  methodState = CONTINUE      |                     |
------------------------------+---------------------+----------------
PROPOSE_METHOD                |                     |
                              |                     |
currentMethod =               |                     |
  Policy.getNextMethod()      |                     |
m.init()                      |         UCT         |  METHOD_REQUEST
if (currentMethod==IDENTITY |||                     |
  currentMethod==NOTIFICATION)|                     |
  methodState = CONTINUE      |                     |
else                          |                     |
  methodState = PROPOSED      |                     |
------------------------------+---------------------+----------------
METHOD_REQUEST                |                     |
                              |                     |
currentId = nextId(currentId) |         UCT         |    SEND_REQUEST
eapReqData =                  |                     |
  m.buildReq(currentId)       |                     |
methodTimeout = m.getTimeout()|                     |
------------------------------+---------------------+----------------






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RFC 4137                   EAP State Machines                August 2005


------------------------------+---------------------+----------------
DISCARD                       |                     |
                              |         UCT         |            IDLE
eapResp = FALSE               |                     |
eapNoReq = TRUE               |                     |
------------------------------+---------------------+----------------
SEND_REQUEST                  |                     |
                              |                     |
retransCount = 0              |         UCT         |            IDLE
lastReqData = eapReqData      |                     |
eapResp = FALSE               |                     |
eapReq = TRUE                 |                     |
------------------------------+---------------------+----------------
TIMEOUT_FAILURE               |                     |
                              |                     |
eapTimeout = TRUE             |                     |
------------------------------+---------------------+----------------
FAILURE                       |                     |
                              |                     |
eapReqData =                  |                     |
  buildFailure(currentId)     |                     |
eapFail = TRUE                |                     |
------------------------------+---------------------+----------------
SUCCESS                       |                     |
                              |                     |
eapReqData =                  |                     |
  buildSuccess(currentId)     |                     |
if (eapKeyData != NONE)       |                     |
  eapKeyAvailable = TRUE      |                     |
eapSuccess = TRUE             |                     |
---------------------------------------------------------------------
                                Figure 9

A.3.  EAP Backend Authenticator State Machine (Figure 5)

---------------------------------------------------------------------
(global transitions)          |   !backendEnabled   |        DISABLED
------------------------------+---------------------+----------------
DISABLED                      |  backendEnabled &&  |      INITIALIZE
                              |     aaaEapResp      |
------------------------------+---------------------+----------------










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RFC 4137                   EAP State Machines                August 2005


------------------------------+---------------------+----------------
INITIALIZE                    |       !rxResp       |   SELECT_ACTION
                              |---------------------+----------------
currentMethod = NONE          |      rxResp &&      |
(rxResp,respId,respMethod)=   | (respMethod == NAK  |
  parseEapResp(aaaEapRespData)|         ||          |             NAK
if (rxResp)                   |    respMethod ==    |
  currentId = respId          |    EXPANDED_NAK)    |
else                          |---------------------+----------------
  currentId = NONE            |        else         |  PICK_UP_METHOD
------------------------------+---------------------+----------------
PICK_UP_METHOD                |                     |
                              |  currentMethod ==   |   SELECT_ACTION
if (Policy.doPickUp(          |        NONE         |
    respMethod)) {            |                     |
  currentMethod = respMethod  |---------------------+----------------
  m.initPickUp()              |        else         | METHOD_RESPONSE
}                             |                     |
------------------------------+---------------------+----------------
IDLE                          |     aaaEapResp      |        RECEIVED
------------------------------+---------------------+----------------
RECEIVED                      |      rxResp &&      |
                              |     (respId ==      |
(rxResp,respId,respMethod)=   |    currentId) &&    |
  parseEapResp(aaaEapRespData)| (respMethod == NAK  |
                              |         ||          |             NAK
                              |    respMethod ==    |
                              |  EXPANDED_NAK) &&   |
                              |   (methodState ==   |
                              |      PROPOSED)      |
                              |---------------------+----------------
                              |      rxResp &&      |
                              |     (respId ==      |
                              |    currentId) &&    | INTEGRITY_CHECK
                              |   (respMethod ==    |
                              |   currentMethod)    |
                              |---------------------+----------------
                              |        else         |         DISCARD
------------------------------+---------------------+----------------
NAK                           |                     |
                              |         UCT         |   SELECT_ACTION
m.reset()                     |                     |
Policy.update(<...>)          |                     |
------------------------------+---------------------+----------------







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RFC 4137                   EAP State Machines                August 2005


------------------------------+---------------------+----------------
SELECT_ACTION                 | decision == FAILURE |         FAILURE
                              |                     |
decision =                    |---------------------+----------------
  Policy.getDecision()        | decision == SUCCESS |         SUCCESS
/* SUCCESS, FAILURE, or       |---------------------+----------------
   CONTINUE */                |        else         |  PROPOSE_METHOD
------------------------------+---------------------+----------------
INTEGRITY_CHECK               |       ignore        |         DISCARD
                              |                     |
ignore =                      |---------------------+----------------
  m.check(aaaEapRespData)     |       !ignore       | METHOD_RESPONSE
------------------------------+---------------------+----------------
METHOD_RESPONSE               |                     |
                              | methodState == END  |   SELECT_ACTION
m.process(aaaEapRespData)     |                     |
if (m.isDone()) {             |                     |
  Policy.update(<...>)        |---------------------+----------------
  aaaEapKeyData = m.getKey()  |                     |
  methodState = END           |        else         |  METHOD_REQUEST
} else                        |                     |
  methodState = CONTINUE      |                     |
------------------------------+---------------------+----------------
PROPOSE_METHOD                |                     |
                              |                     |
currentMethod =               |                     |
  Policy.getNextMethod()      |                     |
m.init()                      |         UCT         |  METHOD_REQUEST
if (currentMethod==IDENTITY |||                     |
  currentMethod==NOTIFICATION)|                     |
  methodState = CONTINUE      |                     |
else                          |                     |
  methodState = PROPOSED      |                     |
------------------------------+---------------------+----------------
METHOD_REQUEST                |                     |
                              |                     |
currentId = nextId(currentId) |                     |
aaaEapReqData =               |         UCT         |    SEND_REQUEST
  m.buildReq(currentId)       |                     |
aaaMethodTimeout =            |                     |
  m.getTimeout()              |                     |
------------------------------+---------------------+----------------
DISCARD                       |                     |
                              |         UCT         |            IDLE
aaaEapResp = FALSE            |                     |
aaaEapNoReq = TRUE            |                     |
------------------------------+---------------------+----------------




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RFC 4137                   EAP State Machines                August 2005


------------------------------+---------------------+----------------
SEND_REQUEST                  |                     |
                              |         UCT         |            IDLE
aaaEapResp = FALSE            |                     |
aaaEapReq = TRUE              |                     |
------------------------------+---------------------+----------------
FAILURE                       |                     |
                              |                     |
aaaEapReqData =               |                     |
  buildFailure(currentId)     |                     |
aaaEapFail = TRUE             |                     |
------------------------------+---------------------+----------------
SUCCESS                       |                     |
                              |                     |
aaaEapReqData =               |                     |
  buildSuccess(currentId)     |                     |
if (aaaEapKeyData != NONE)    |                     |
  aaaEapKeyAvailable = TRUE   |                     |
aaaEapSuccess = TRUE          |                     |
---------------------------------------------------------------------
                               Figure 10

A.4.  EAP Full Authenticator State Machine (Figures 6 and 7)

   This state machine contains all the states from EAP stand-alone
   authenticator state machine, except that SELECT_ACTION state is
   replaced with the following:

---------------------------------------------------------------------
SELECT_ACTION                 | decision == FAILURE |         FAILURE
                              |                     |
decision =                    |---------------------+----------------
  Policy.getDecision()        | decision == SUCCESS |         SUCCESS
/* SUCCESS, FAILURE, CONTINUE,|---------------------+----------------
   or PASSTHROUGH */          |     decision ==     |     INITIALIZE_
                              |     PASSTHROUGH     |     PASSTHROUGH
                              |---------------------+----------------
                              |        else         |  PROPOSE_METHOD
---------------------------------------------------------------------
                               Figure 11

And the following new states are added:

---------------------------------------------------------------------
INITIALIZE_PASSTHROUGH        |  currentId != NONE  |     AAA_REQUEST
                              |---------------------+----------------
aaaEapRespData = NONE         |  currentId == NONE  |        AAA_IDLE
------------------------------+---------------------+----------------



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RFC 4137                   EAP State Machines                August 2005


------------------------------+---------------------+----------------
IDLE2                         |                     |
                              |  retransWhile == 0  |     RETRANSMIT2
retransWhile =                |                     |
  calculateTimeout(           |---------------------+----------------
   retransCount, eapSRTT,     |       eapResp       |       RECEIVED2
   eapRTTVAR, methodTimeout)  |                     |
------------------------------+---------------------+----------------
RETRANSMIT2                   |                     |
                              |   retransCount >    |        TIMEOUT_
retransCount++                |     MaxRetrans      |        FAILURE2
if (retransCount<=MaxRetrans){|                     |
  eapReqData = lastReqData    |---------------------+----------------
  eapReq = TRUE               |        else         |           IDLE2
}                             |                     |
------------------------------+---------------------+----------------
RECEIVED2                     |      rxResp &&      |
                              |     (respId ==      |     AAA_REQUEST
(rxResp,respId,respMethod)=   |     currentId)      |
  parseEapResp(eapRespData)   |---------------------+----------------
                              |        else         |        DISCARD2
------------------------------+---------------------+----------------
AAA_REQUEST                   |                     |
                              |                     |
if (respMethod == IDENTITY) { |         UCT         |        AAA_IDLE
  aaaIdentity = eapRespData   |                     |
aaaEapRespData = eapRespData  |                     |
------------------------------+---------------------+----------------
AAA_IDLE                      |     aaaEapNoReq     |        DISCARD2
                              |---------------------+----------------
aaaFail = FALSE               |      aaaEapReq      |    AAA_RESPONSE
aaaSuccess = FALSE            |---------------------+----------------
aaaEapReq = FALSE             |     aaaTimeout      |        TIMEOUT_
aaaEapNoReq = FALSE           |                     |        FAILURE2
aaaEapResp = TRUE             |---------------------+----------------
                              |       aaaFail       |        FAILURE2
                              |---------------------+----------------
                              |     aaaSuccess      |        SUCCESS2
------------------------------+---------------------+----------------
AAA_RESPONSE                  |                     |
                              |                     |
eapReqData = aaaEapReqData    |         UCT         |   SEND_REQUEST2
currentId = getId(eapReqData) |                     |
methodTimeout =               |                     |
  aaaMethodTimeout            |                     |
------------------------------+---------------------+----------------





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RFC 4137                   EAP State Machines                August 2005


------------------------------+---------------------+----------------
DISCARD2                      |                     |
                              |         UCT         |           IDLE2
eapResp = FALSE               |                     |
eapNoReq = TRUE               |                     |
------------------------------+---------------------+----------------
SEND_REQUEST2                 |                     |
                              |                     |
retransCount = 0              |         UCT         |           IDLE2
lastReqData = eapReqData      |                     |
eapResp = FALSE               |                     |
eapReq = TRUE                 |                     |
------------------------------+---------------------+----------------
TIMEOUT_FAILURE2              |                     |
                              |                     |
eapTimeout = TRUE             |                     |
------------------------------+---------------------+----------------
FAILURE2                      |                     |
                              |                     |
eapReqData = aaaEapReqData    |                     |
eapFail = TRUE                |                     |
------------------------------+---------------------+----------------
SUCCESS2                      |                     |
                              |                     |
eapReqData = aaaEapReqData    |                     |
eapKeyData = aaaEapKeyData    |                     |
eapKeyAvailable =             |                     |
  aaaEapKeyAvailable          |                     |
eapSuccess = TRUE             |                     |
---------------------------------------------------------------------
                               Figure 12




















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RFC 4137                   EAP State Machines                August 2005


Authors' Addresses

   John Vollbrecht
   Meetinghouse Data Communications
   9682 Alice Hill Drive
   Dexter, MI  48130
   USA

   EMail: jrv@mtghouse.com


   Pasi Eronen
   Nokia Research Center
   P.O. Box 407
   FIN-00045 Nokia Group,
   Finland

   EMail: pasi.eronen@nokia.com


   Nick L. Petroni, Jr.
   University of Maryland, College Park
   A.V. Williams Building
   College Park, MD  20742
   USA

   EMail: npetroni@cs.umd.edu


   Yoshihiro Ohba
   Toshiba America Research, Inc.
   1 Telcordia Drive
   Piscataway, NJ  08854
   USA

   EMail: yohba@tari.toshiba.com















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RFC 4137                   EAP State Machines                August 2005


Full Copyright Statement

   Copyright (C) The Internet Society (2005).

   This document is subject to the rights, licenses and restrictions
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Acknowledgement

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