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
Full Copyright Statement
Copyright (C) The Internet Society (2005).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Intellectual Property
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at ietf-
ipr@ietf.org.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Vollbrecht, et al. Informational [Page 51]
ERRATA
