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rfc:rfc4137

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.

Vollbrecht, et al. Informational [Page 1] RFC 4137 EAP State Machines August 2005

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

Vollbrecht, et al. Informational [Page 2] RFC 4137 EAP State Machines August 2005

 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.

Vollbrecht, et al. Informational [Page 3] RFC 4137 EAP State Machines August 2005

        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].

Vollbrecht, et al. Informational [Page 4] RFC 4137 EAP State Machines August 2005

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

Vollbrecht, et al. Informational [Page 5] RFC 4137 EAP State Machines August 2005

    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:

Vollbrecht, et al. Informational [Page 6] RFC 4137 EAP State Machines August 2005

 <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.

Vollbrecht, et al. Informational [Page 7] RFC 4137 EAP State Machines August 2005

 { 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.

Vollbrecht, et al. Informational [Page 8] RFC 4137 EAP State Machines August 2005

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

Vollbrecht, et al. Informational [Page 9] RFC 4137 EAP State Machines August 2005

    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.

Vollbrecht, et al. Informational [Page 10] RFC 4137 EAP State Machines August 2005

 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.

Vollbrecht, et al. Informational [Page 11] RFC 4137 EAP State Machines August 2005

 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.

Vollbrecht, et al. Informational [Page 12] RFC 4137 EAP State Machines August 2005

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.

Vollbrecht, et al. Informational [Page 13] RFC 4137 EAP State Machines August 2005

 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.

Vollbrecht, et al. Informational [Page 14] RFC 4137 EAP State Machines August 2005

 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.

Vollbrecht, et al. Informational [Page 15] RFC 4137 EAP State Machines August 2005

 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.

Vollbrecht, et al. Informational [Page 16] RFC 4137 EAP State Machines August 2005

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

Vollbrecht, et al. Informational [Page 17] RFC 4137 EAP State Machines August 2005

    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.

Vollbrecht, et al. Informational [Page 18] RFC 4137 EAP State Machines August 2005

 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: -)

Vollbrecht, et al. Informational [Page 19] RFC 4137 EAP State Machines August 2005

 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.

Vollbrecht, et al. Informational [Page 20] RFC 4137 EAP State Machines August 2005

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.

Vollbrecht, et al. Informational [Page 21] RFC 4137 EAP State Machines August 2005

 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.

Vollbrecht, et al. Informational [Page 22] RFC 4137 EAP State Machines August 2005

 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.

Vollbrecht, et al. Informational [Page 23] RFC 4137 EAP State Machines August 2005

 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.

Vollbrecht, et al. Informational [Page 24] RFC 4137 EAP State Machines August 2005

 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.

Vollbrecht, et al. Informational [Page 25] RFC 4137 EAP State Machines August 2005

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.

Vollbrecht, et al. Informational [Page 26] RFC 4137 EAP State Machines August 2005

 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.

Vollbrecht, et al. Informational [Page 27] RFC 4137 EAP State Machines August 2005

 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.

Vollbrecht, et al. Informational [Page 28] RFC 4137 EAP State Machines August 2005

 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)

Vollbrecht, et al. Informational [Page 29] RFC 4137 EAP State Machines August 2005

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].

Vollbrecht, et al. Informational [Page 30] RFC 4137 EAP State Machines August 2005

 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).

Vollbrecht, et al. Informational [Page 31] RFC 4137 EAP State Machines August 2005

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.

Vollbrecht, et al. Informational [Page 32] RFC 4137 EAP State Machines August 2005

 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.

Vollbrecht, et al. Informational [Page 33] RFC 4137 EAP State Machines August 2005

 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.

Vollbrecht, et al. Informational [Page 34] RFC 4137 EAP State Machines August 2005

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:

Vollbrecht, et al. Informational [Page 35] 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.

Vollbrecht, et al. Informational [Page 36] 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.

Vollbrecht, et al. Informational [Page 37] RFC 4137 EAP State Machines August 2005

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)      |
                           |------------------------+--------------

Vollbrecht, et al. Informational [Page 38] RFC 4137 EAP State Machines August 2005

                           |------------------------+--------------
                           |      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)   |
                           |------------------------+--------------

Vollbrecht, et al. Informational [Page 39] RFC 4137 EAP State Machines August 2005

                           |------------------------+--------------
                           | (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)       |                        |

—————————–+————————+————–

Vollbrecht, et al. Informational [Page 40] RFC 4137 EAP State Machines August 2005

—————————–+————————+————– 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 ——————————+———————+—————-

Vollbrecht, et al. Informational [Page 41] 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

——————————+———————+—————-

Vollbrecht, et al. Informational [Page 42] 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()| | ——————————+———————+—————-

Vollbrecht, et al. Informational [Page 43] 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      |

——————————+———————+—————-

Vollbrecht, et al. Informational [Page 44] 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(<…>) | | ——————————+———————+—————-

Vollbrecht, et al. Informational [Page 45] 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 | | ——————————+———————+—————-

Vollbrecht, et al. Informational [Page 46] 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 ——————————+———————+—————-

Vollbrecht, et al. Informational [Page 47] 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            |                     |

——————————+———————+—————-

Vollbrecht, et al. Informational [Page 48] 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

Vollbrecht, et al. Informational [Page 49] 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

Vollbrecht, et al. Informational [Page 50] RFC 4137 EAP State Machines August 2005

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Vollbrecht, et al. Informational [Page 51]

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