One document matched: draft-vollbrecht-eap-state-02.txt
Differences from draft-vollbrecht-eap-state-01.txt
EAP Working Group J. Vollbrecht
Internet-Draft Vollbrecht Consulting LLC
Expires: October 22, 2003 P. Eronen
Nokia
N. Petroni
University of Maryland
Y. Ohba
TAIS
April 23, 2003
State Machines for EAP Peer and Authenticator
draft-vollbrecht-eap-state-02
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on October 22, 2003.
Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This document describes a set of state machines for EAP Peer, EAP
Authenticator (supporting local, passthrough and backend), for EAP
Passthrough method, and for "backend adapter" that adapts EAP traffic
carried by an AAA protocol such as RADIUS or Diameter to a Backend
Authenticator. This set of state machines shows how EAP can be
implemented to support deployment in either a Peer/AP or Peer/AP/AAA
Server environmnet. The Peer and Authenticator machines are
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illustrative of how the EAP protocol defined in
[I-D.ietf-eap-rfc2284bis] may be implemented. The Passtrhough
method and "backend adapter" illustrate how EAP protocol support
defined in [I-D.aboba-radius-rfc2869bis] may be implemented. Where
there are differences [I-D.ietf-eap-rfc2284bis]/
[I-D.aboba-radius-rfc2869bis] are authoritative.
This document describes a state machine based on an EAP "Switch"
model. This model includes events and actions for the interaction
between the EAP Switch and EAP methods. The State Machine and
associated model are informative only. Implementations may achieve
the same results using different methods.
A brief description of the EAP "Switch" model is given in the
Introduction section.
This document is still a work in progress. The authors believe it
corresponds to the current state of revisions to the defining
[I-D.ietf-eap-rfc2284bis]/[I-D.aboba-radius-rfc2869bis] documents,
but it has not been vetted by the EAP working group as a whole. An
appendix to this document points out issues the authors believe still
need to be resolved between the documents. The intent is to
synchronize this document with [I-D.ietf-eap-rfc2284bis] and
[I-D.aboba-radius-rfc2869bis] revisions when they are released and
then submit it as an RFC.
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Table of Contents
1. Specification of Requirements . . . . . . . . . . . . . . . . 4
2. The EAP Switch Model . . . . . . . . . . . . . . . . . . . . . 4
3. Notational conventions used in state diagrams . . . . . . . . 5
3.1 Notational specifics . . . . . . . . . . . . . . . . . . . . . 5
3.2 Document authority . . . . . . . . . . . . . . . . . . . . . . 6
4. Peer State Machine . . . . . . . . . . . . . . . . . . . . . . 6
4.1 Interface between peer state machine and lower layer . . . . . 7
4.2 Interface between peer state machine and methods . . . . . . . 9
4.3 Peer state machine local variables . . . . . . . . . . . . . . 10
4.4 Peer state machine procedures . . . . . . . . . . . . . . . . 11
4.5 Peer state machine states . . . . . . . . . . . . . . . . . . 12
5. Authenticator State Machine . . . . . . . . . . . . . . . . . 13
5.1 Interface between authenticator state machine and lower
layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
5.2 Interface between authenticator state machine and methods . . 15
5.3 Authenticator state machine local variables . . . . . . . . . 17
5.4 EAP authenticator procedures . . . . . . . . . . . . . . . . . 18
5.5 EAP authenticator states . . . . . . . . . . . . . . . . . . . 20
6. Passthrough and backend . . . . . . . . . . . . . . . . . . . 21
6.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
6.2 State machine overview . . . . . . . . . . . . . . . . . . . . 22
6.3 Passthrough . . . . . . . . . . . . . . . . . . . . . . . . . 22
6.4 Backend . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
7. Security Considerations . . . . . . . . . . . . . . . . . . . 24
References . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 25
A. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 25
A.1 Authenticator . . . . . . . . . . . . . . . . . . . . . . . . 26
A.2 Peer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Intellectual Property and Copyright Statements . . . . . . . . 28
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1. Specification of Requirements
In this document, several words are used to signify the requirements
of the specification. These words are often capitalized. The key
words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD",
"SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document
are to be interpreted as described in [RFC2119].
2. The EAP Switch Model
This document offers a proposed state machine for RFCs
[I-D.ietf-eap-rfc2284bis] and [I-D.aboba-radius-rfc2869bis] . There
are state machines for the peer, the authenticator, a "passthrough
method" and a "backend adapter". 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 both the peer and authenticator 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 method exists.
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 out of scope for this paper, with the exception of the
Passthrough Method.
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Peer | Authenticator | Backend
| / Local |
| / Method |
Peer | Auth | Backend
EAP --|-----> EAP | -> EAP
Switch | Switch | / Server
| \ |/
| \ passthrough|
| method |
Figure 2: EAP Switch Model
The Passthrough Method appears to the Authenticator Switch as a
method, but its function is to Pass EAP messages to a Backend Server
where the real Authentication Method resides. This paper includes a
state machine for a Passthrough method and a diagram the flow between
an Authenticator with a Passthrough Method and the Backend and its
Method.
This document describes a set of State Machines that can manage EAP
authentication from the peerto an EAP method on the Authenticator or
from the Peer through the Authenticator passthrough method to the EAP
method on the Backend EAP server.
The state diagrams presented in this document have been coordinated
with the IEEE 802.1X diagrams. The format of the diagrams is adapted
from the 802.1X format. Portions of a version this document are
included as Appendix F of 802.1X (2003).
3. Notational conventions used in state diagrams
3.1 Notational specifics
The following state diagrams have been completed based on the
conventions specified in [IEEE.802-1X.2001], section 8.5.1. Much of
that section is reprinted here for completeness.
State diagrams are used to represent the operation of a function as a
group of connected, mutually exclusive states. Only one state of a
function 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 on entry to the state.
All permissible transitions between states are represented by arrows,
the arrowhead denoting the direction of the possible transition.
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Labels attached to arrows denote the condition(s) that must be met in
order for the transition to take place. 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.
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 of a state block. On completion of all of
the procedures within a state, all exit conditions for the state
(including all conditions associated with global transitions) are
evaluated continuously until such a time as one of the conditions is
met. All exit conditions are regarded as Boolean expressions that
evaluate to True or False; if a condition evaluates to True, then the
condition is met. When the condition associated with a global
transition is met, it supersedes all other exit conditions, including
UCT. The label UCT denotes an unconditional transition (i.e., UCT
always evaluates to True). 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).
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.
3.2 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 ([I-D.ietf-eap-rfc2284bis],
[I-D.aboba-radius-rfc2869bis], etc.) the latter (the other document)
is considered authoritative and takes precedence.
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 draft-vollbrecht-eap-state-02.ps for missing diagram)
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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
there is a request 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 there is a valid port to use for the communication.
If at any point the port is not available, portEnabled is set to
FALSE and the state machine transitions to DISABLED (or
BACKEND_DISABLED).
aWhile (integer)
outside timer used to indicate how long the peer has waited for a
new (valid) request.
altAccept (boolean)
alternate indication of success, as described in
[I-D.ietf-eap-rfc2284bis].
altReject (boolean)
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alternate indication of failure, as described in
[I-D.ietf-eap-rfc2284bis].
4.1.2 Variables (peer to lower layer)
eapResp (boolean)
Set to TRUE in peer state machine, FALSE in lower layer. Indicates
there is a response to be sent.
eapNoResp (boolean)
Set to TRUE in peer state machine, FALSE in lower layer. Indicates
the request has been processed, but there is no response to send.
eapSuccess (boolean)
Set to TRUE in peer state machine, FALSE in lower layer. Indicates
the Peer has reached the SUCCESS state.
eapFail (boolean)
Set to TRUE in peer state machine, FALSE in lower layer. Indicates
the Peer has reached the FAILURE state.
eapRespData (EAP Packet)
Set in peer state machine when eapResp is set to TRUE. The EAP
packet which is the response to send.
eapKey (EAP Key)
Set in peer state machine when keying material becomes available.
Set during the METHOD state. Note that this document does not yet
define the structure of the type "EAP Key". We expect it to be
defined in the EAP Keying Framework document.
4.1.3 Constants
ClientTimeout (integer)
Configurable amount of time to wait for a valid request before
aborting.
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EapTunnelled (boolean)
Indication of whether EAP is running inside a protected tunnel or
not.
4.2 Interface between peer state machine and methods
IN: eapReqData, (reqId)
OUT: intCheck, eapRespData
IN/OUT: methodState, (method-specific state), (policy)
If methodState==INIT, the method starts by initializing its own
method-specific state.
Next, the method must decide whether to process the packet or
silently discard it. If the packet looks like it wasn't sent by the
legitimate authenticator (e.g. it has invalid MIC, and this case
should never occur), the method can set intCheck=FALSE. In this
case, the method must not modify methodState, and it should not
modify its own method-specific state.
If the method decides to process the packet, it behaves as follows.
o Updates its own method-specific state.
o Possibly tells Policy something.
o If the method has derived keying material it wants to export,
stores the keying material to eapKey.
o Creates a response packet (with the same identifier as the
request), and stores it to eapRespData.
o Sets intCheck=TRUE.
Finally the method must update methodState according to the following
rules.
NORMAL: The method is finished (either successfully or
unsuccessfully), or at this point the server can decide that it
doesn't want to continue with this method.
CONTINUE: The method always continues at this point. That is, the
method specification says that the server can't decide to end the
method at this point.
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STRICT: Same as CONTINUE, but the method specification also forbids
using Notifications at this point.
CON_ACC: The server has signalled that the next packet will be EAP
Success. Note that this is different from ordinary method success,
and is probably relevant only for tunnelling methods (e.g. PEAP).
CON_REJ: The server has signalled that the next packet will be EAP
Failure. Note that this is different from ordinary method failure.
4.3 Peer state machine local variables
4.3.1 Long-term (maintained between packets)
currentMethod (EAP Type)
Set in GET_METHOD state. The method the peer believes to be
currently "in progress"
methodState (enumeration)
As described above
lastId (integer)
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.
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 the current received packet is an
EAP request.
rxSuccess (boolean)
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Set in RECEIVED state. Indicates the current received packet is an
EAP Success.
rxFailure (boolean)
Set in RECEIVED state. Indicates the current received packet is an
EAP Failure.
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
intCheck (boolean)
Set in METHOD state. Indicates whether the method has decided to
accept the current packet.
4.4 Peer state machine procedures
parseEapReq()
Determine the code, identifier value, and type of the current
request
buildNotify()
Create the appropriate notification response.
resetCurrentMethod()
Alert the current method that it has been aborted.
Policy.allow()
Determine if the Peer is allowed to perform a particular method at
a particular point in the conversation.
Policy.update()
Update all variables related to internal policy state.
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Policy.isSatisfied()
Determine if the policy will allow SUCCESS or not.
m.integrityCheck()
Method-specific procedure to test for the validity of a message.
m.process()
Method procedure to parse and process a request for that method.
m.getKey()
Method procedure to obtain key material for use by EAP or lower
layers.
4.5 Peer state machine states
DISABLED
This state is reached anytime 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.
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
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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.
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.
5. Authenticator State Machine
The following is a diagram of the EAP Authenticator state machine.
Also included is an explanation of the primitives and procedures
referenced in the diagram, as well as a clarification of notation.
(see draft-vollbrecht-eap-state-02.ps for missing diagram)
5.1 Interface between 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, eapSucess, or eapFail, the corresponding
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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 authenticator)
eapResp (boolean)
Set to TRUE in lower layer, FALSE in authenticator state machine.
Indicates 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 there is a valid port to use for the communication.
If at any point the port is not available, portEnabled is set to
FALSE and the state machine transitions to DISABLED.
aWhile (integer)
outside timer used to indicate how long the authenticator has
waited for a new (valid) response.
eapRestarting (boolean)
Indicates the lower layer would like to restart authentication
eapKey (EAP Key)
Set in authenticator state machine when keying material becomes
available. Set during the METHOD state. Note that this document
does not yet define the structure of the type "EAP Key". We expect
it to be defined in the EAP Keying Framework document.
5.1.2 Variables (authenticator to lower layer)
eapReq (boolean)
Set to TRUE in authenticator state machine, FALSE in lower layer.
Indicates a new EAP request is ready to be sent.
eapNoReq (boolean)
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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 the state machine has reached the SUCCESS state.
eapFail (boolean)
Set to TRUE in authenticator state machine, FALSE in lower layer.
Indicates the state machine has reached the FAILURE state.
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).
5.1.3 Constants
AuthenticatorTimeout (integer)
Configurable amount of time to wait for a valid response before
aborting.
MaxRetrans (integer)
Configurable maximum for how many retransmissions should be
attempted before aborting.
EapBackend (boolean)
Indication of whether the current state machine should be followed
as if it is a backend server implementation.
5.2 Interface between authenticator state machine and methods
IN: eapRespData
IN/OUT: methodState, currentId, (method-specific state), (policy)
OUT: intCheck, eapReqData, succFailData
A. If methodState==PICK_UP_INIT: This signals that we should "pick
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up" a conversation that was started by someone else.
Some methods on a backend server may support this feature (usually
only Identity, though others are possible). Policy knows what methods
support this, and only those methods can end up in PICK_UP_INIT
state.
The method behaves as follows.
o Initializes its own method-specific state, possibly using some
information from Policy.
o Examines eapRespData, and updates 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.)
o Moves to case C below.
This methodState is also used for the special PASSTHROUGH method, but
it is documented elsewhere.
B. If methodState==INIT, we have not sent any requests yet. The
method then sends its initial request as follows.
o Initializes its own method-specific state, possibly using some
information from Policy (e.g. identity).
o Updates currentId to contain a new identifier value.
o Creates a new request packet (with the new identifier value), and
stores it to eapReqData
o If the method is Identity or Notification, sets
methodState=CONTINUE; in all other cases, sets
methodState=PROPOSED.
C. Otherwise we have just received a response.
First the method must decide whether to process the packet or
silently discard it. If the packet looks like it wasn't sent by the
legitimate peer (e.g. it has invalid MIC, and this case should never
occur), the method can set intCheck=FALSE. In this case, the method
must not modify methodState or currentId, and it should not modify
its own method-specific state.
If the packet is accepted, the options are to continue the
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conversation (send another request) or end the conversation. If the
conversation is continued, the method behaves as follows.
o Updates its own method-specific state.
o Possibly tells Policy something.
o Updates currentId to contain a new identifier value.
o Creates a new request packet (with the new identifier value), and
stores it to eapReqData.
o Sets methodState=CONTINUE and intCheck=TRUE.
If the method wants to end the conversation,
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,
stores the keying material to eapKey.
o Sets succFailData=NONE (except special PASSTHROUGH method).
o Sets methodState=END.
5.3 Authenticator state machine local variables
5.3.1 Long-term (maintained between packets)
currentMethod (EAP Type)
EAP type, PASSTHROUGH, or NONE.
currentId (integer)
0-255 or NONE. Usually updated in METHOD state. Indicates the
identifier value of the currently outstanding EAP request.
methodState (enumeration)
As described above.
retransCount (integer)
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Set in SEND_REQUEST state. Current number of retransmissions.
lastReqData (EAP packet)
Set in SEND_REQUEST state. EAP packet containing the last sent
request.
5.3.2 Short-term (not maintained between packets)
rxResp (boolean)
Set in RECEIVED state. Indicates 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.
succFailData (EAP packet)
Set in METHOD state. Usage described above.
intCheck (boolean)
Set in METHOD state. Indicates whether the method has decided to
accept the current packet.
policySat (boolean)
Set in GET_METHOD state. Stored value of last call to
Policy.isSatisfied().
5.4 EAP authenticator procedures
parseEapResp()
Determine the code, identifier value, and type of the current
response
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buildSuccess()
Create an EAP Success Packet.
buildFailure()
Create an EAP Failure Packet.
nextId()
Determine the next identifier value to use, based on the previous
one.
resetCurrentMethod()
Alert the current method that it has been aborted.
Policy.update()
Update all variables related to internal policy state.
Policy.getNextMethod()
Determine the method that should be used at this point in the
conversation based on pre-defined policy.
Policy.isSatisfied()
Determine if the policy will allow SUCCESS or not.
m.integrityCheck()
Method-specific procedure to test for the validity of a message.
m.process()
Method procedure to parse and process a response for that method.
m.buildSuccFail()
Passthrough method to create a Success or Failure packet. More
described above.
m.buildReq()
Method procedure to produce the next request.
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m.getNextId()
Method procedure that is the parallel of the switch-level
nextId(). Often it is up to the method to decide the next ID
(particularly in backend authenticators).
m.getKey()
Method procedure to obtain key material for use by EAP or lower
layers.
5.5 EAP authenticator states
DISABLED
The authenticator is disabled until the port is enabled by the
lower layer.
BACKEND_DISABLED
Same for backend server.
INITIALIZE
Initializes variables when the state machine is activated.
BACKEND_INITIALIZE
Same for backend server. Also parses the headers of initial
response packet (a response to a request sent by the NAS), if any.
GET_METHOD
This state chooses what should happen next: either a method is
started, or the conversation is ended.
IDLE
The state machine spends most of its time here, waiting for
something to happen.
RECEIVED
This state is entered when an EAP packet is received: the packet
header is parsed here.
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METHOD
This state builds request packets and processes responses received
from the peer.
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.
6. Passthrough and backend
6.1 Overview
There are two different cases to consider: either the first EAP
request is sent by the NAS, or by the backend.
In the simple case, the NAS signals the backend that it wants to
start an EAP conversation, and the backend sends the first EAP
Request. This case could be handled with a very simple passthrough
state machine.
The complex case is where the first EAP request (or several requests)
are sent by the NAS, and at some point, the NAS switches to
passthrough mode and backend takes over. This requires very careful
handling to get right! After the backend has sent its first request,
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the situation is normal.
6.2 State machine overview
We could have drawn a separate passthrough authenticator and backend
authenticator state diagrams, but this was not done because a) this
way we can describe an authenticator that starts, b) the differences
between the diagrams would have been rather small
The passthrough/backend split is specified using two copies of the
authenticator state machine, one in the "passthrough" processor, and
one in the "backend" processor.
(see draft-vollbrecht-eap-state-02.ps for missing diagram)
The passthrough method takes EAP responses, encapsulates them into
RADIUS Access-Request packets, and sends them to the backend. The
passthrough method also translates the RADIUS response packet to
appropriate EAP requests and signals to EAP authenticator switch.
The backend adapter state machine receives EAP responses from NAS
inside RADIUS Access-Request packets, and passes these to the
authenticator state machine using the normal lower layer interface.
It also relays the signals returned by the authenticator state
machine (e.g. eapReq, eapSuccess) to NAS using appropriate RADIUS
messages.
6.3 Passthrough
(see draft-vollbrecht-eap-state-02.ps for missing diagram)
If the first EAP request is sent by the backend, the NAS Policy
simply activates the PASSTHROUGH method with methodState==INIT. The
PASSTHROUGH method sends a RADIUS packet signalling EAP-Start to the
backend, and relays messages.
In the case where the first EAP request is originated from the NAS,
the NAS behaves as follows: Policy starts the (local) Identity
method, which sends a request packet. When the response comes back,
the Identity method processes it and sets methodState=END.
Policy then starts the PASSTHROUGH method with methodState
PICK_UP_INIT. The PASSTHROUGH method sends the Identity response to
the backend server (inside a RADIUS Access-Request packet). The
backend server responds with some other request (passed PASSTHROUGH
method inside a RADIUS Access-Challenge), and PASSTHROUGH method
relays this request to the peer.
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This continues until the backend server responds with Access-Accept
or Accept-Reject. The PASSTHROUGH method notifies the Policy, stores
the Success/Failure packet to succFailData, and sets methodState=END.
The policy then moves to SUCCESS or FAILURE state, and the Success/
Failure packet is sent to the peer.
6.4 Backend
(see draft-vollbrecht-eap-state-02.ps for missing diagram)
The case where the conversation is started by the backend server
(passthrough signals EAP-Start) does not require any special
handling. The only difference to ordinary authenticator is that EAP
retransmissions are handled by the NAS (AuthenticatorTimeout is set
to INFINITY).
The case where the conversation is started by NAS requires special
handling though. The client's response is passed to the backend in
the first Access-Request.
The backend must process this request BEFORE it sends anything, for
the following reasons:
o The identifier chosen for the next packet must be different from
the one contained in client's response.
o The client's response will probably contain some useful
information (e.g. Identity response).
If the passthrough wants the backend server to send the first EAP
Request, it sets aaaEapStart=TRUE. Otherwise, when the passthrough
receives an EAP Response from the peer, it stores it in
aaaEapRespData, and sets aaaEapResp=TRUE.
These signals are sent to the backend using RADIUS or DIAMETER
packets. In RADIUS, aaaEapStart signal corresponds to an
Access-Request signifying EAP-Start (EAP-Message attribute with no
data), and aaaEapResp corresponds to an ordinary Access-Request (with
aaaEapRespData stored in EAP-Message attribute). See
[I-D.aboba-radius-rfc2869bis] for details.
When the backend has finished processing the signal, it sets one of
the variables aaaAccept, aaaReject, aaaChallenge, aaaPacketDiscard,
and unless aaaPacketDiscard is set, also aaaEapReqData. If aaaAccept
is set, aaaEapKey can also be set.
These signals are returned to the passthrough using RADIUS or
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DIAMETER packets. In RADIUS, the first three signals correspond to
Access-Accept, Access-Reject, and Access-Challenge packets, with
aaaEapReqData stored in EAP-Message attribute. The fourth signal
(aaaPacketDiscard) corresponds to Access-Challenge with Error-Cause
set to 202. See [I-D.aboba-radius-rfc2869bis] for details.
If the passthrough does not receive a valid RADIUS response, even
after retransmissions (handled by RADIUS), it sets aaaFailure. The
passthrough can also set variable aaaIdentity, which contains the
last Identity response seen. This can be used to route the RADIUS
message to correct destination.
7. 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.
References
[IEEE.802-1X.2001]
Institute of Electrical and Electronics Engineers, "Local
and Metropolitan Area Networks: Port-Based Network Access
Control", IEEE Standard 802.1X, September 2001.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2284] Blunk, L. and J. Vollbrecht, "PPP Extensible
Authentication Protocol (EAP)", RFC 2284, March 1998.
[I-D.ietf-eap-rfc2284bis]
Blunk, L., "Extensible Authentication Protocol (EAP)",
draft-ietf-eap-rfc2284bis-01 (work in progress), February
2003.
[I-D.aboba-radius-rfc2869bis]
Aboba, B. and P. Calhoun, "RADIUS Support For Extensible
Authentication Protocol (EAP)",
draft-aboba-radius-rfc2869bis-19 (work in progress), April
2003.
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Authors' Addresses
John R. Vollbrecht
Vollbrecht Consulting LLC
9682 Alice Hill Drive
Dexter, MI 48130
USA
Phone: 734 426-1026
EMail: jrv@umich.edu
Pasi Eronen
Nokia Research Center
P.O. Box 407
FIN-00045 Nokia Group,
Finland
Phone:
EMail: pasi.eronen@nokia.com
Nick L. Petroni, Jr.
University of Maryland, College Park
A.V. Williams Building
College Park, MD 20742
USA
Phone:
EMail: npetroni@cs.umd.edu
Yoshihiro Ohba
Toshiba America Information Systems, Inc.
9740 Irvine Blvd.
Irvine, CA 92619-1697
USA
Phone:
EMail: yohba@tari.toshiba.com
Appendix A. Open Issues
The following are issues and questions to be resolved with the EAP
working group. These include issues to be clarified as well as
issues about interface with other layers.
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A.1 Authenticator
o 2869bis: "The NAS MUST NOT "manufacture" a Success or Failure
packet as the result of a timeout. After a suitable number of
timeouts have elapsed, the NAS SHOULD instead end the EAP
conversation." Issue: There is a RADIUS client issue - how many
retries before giving up. There is also the issue that continues
to come up of what to do with Access Accept with EAP Failure. As
it stands this will cause the authenticator to go to fail and send
a Success to the Peer. Probably this is right, but one more
review of this seems reasonable.
o 2869bis: "Also, the RADIUS server is advised to permit only a
modest number of invalid EAP packets within a single session,
prior to terminating the session with an Access-Reject. By default
a value of 5 invalid EAP packets is recommended." Issue: RADIUS
server does not control retransmissions. It will respond to every
Request, with an Access Accept/Fail/Challenge or Packet Challenge.
The Packet Challenge is sent when the EAP message fails integrity
check. Given this, how should the sentence in 2869 be
interpreted?
o Methods (including pass-through) should be able to modify the
retransmission timeout. In some cases, the timer should use
exponential back-off or something. Issue: what does this mean for
the state machine, if anything?
o Do we need a new variable to signal the lower layer that keying
material is available (in eapKey), or is it enough that the lower
layer checks if eapKey!=NONE after getting eapSuccess?
A.2 Peer
o 2284bis: "If a peer receives a duplicate Request before it has
sent a Response, but after it has determined the initial Request
to be valid (i.e. it is waiting for user input), it MUST silently
discard the duplicate Request. " Issue: Seems like the peer should
be able to send as many responses as it gets requests for the same
id. It may not send a response for prior id after it has sent a
response to current id. This is how the state machine works, and
hopefully the wording in 2284bis will be modified to match.
o What to do if we get a timeout, Policy.isSatisfied() is TRUE, but
methodState is not CON_ACC? Issue: can peer go into authenticated
state without getting a Success -either internal to the method
(Con_ACC) or and EAP Success?
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o What to do if we get an altAccept, Policy.isSatisfied() is TRUE,
but methodState is not CON_ACC? Issue: same as above
o Do we need a new variable to signal the lower layer that keying
material is available (in eapKey), or is it enough that the lower
layer checks if eapKey!=NONE after getting eapSuccess?
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