RFC 3748
Extensible Authentication Protocol (EAP)
Pages: 67
Proposed Standard
→ Errata
Obsoletes: 2284
Updated by: 5247 7057
Proposed Standard
→ Errata
Obsoletes: 2284
Updated by: 5247 7057
Part 2 of 3 – Pages 15 to 41
3. Lower Layer Behavior
3.1. Lower Layer Requirements
EAP makes the following assumptions about lower layers: [1] Unreliable transport. In EAP, the authenticator retransmits Requests that have not yet received Responses so that EAP does not assume that lower layers are reliable. Since EAP defines its own retransmission behavior, it is possible (though undesirable) for retransmission to occur both in the lower layer and the EAP layer when EAP is run over a reliable lower layer.
Note that EAP Success and Failure packets are not retransmitted. Without a reliable lower layer, and with a non-negligible error rate, these packets can be lost, resulting in timeouts. It is therefore desirable for implementations to improve their resilience to loss of EAP Success or Failure packets, as described in Section 4.2. [2] Lower layer error detection. While EAP does not assume that the lower layer is reliable, it does rely on lower layer error detection (e.g., CRC, Checksum, MIC, etc.). EAP methods may not include a MIC, or if they do, it may not be computed over all the fields in the EAP packet, such as the Code, Identifier, Length, or Type fields. As a result, without lower layer error detection, undetected errors could creep into the EAP layer or EAP method layer header fields, resulting in authentication failures. For example, EAP TLS [RFC2716], which computes its MIC over the Type-Data field only, regards MIC validation failures as a fatal error. Without lower layer error detection, this method, and others like it, will not perform reliably. [3] Lower layer security. EAP does not require lower layers to provide security services such as per-packet confidentiality, authentication, integrity, and replay protection. However, where these security services are available, EAP methods supporting Key Derivation (see Section 7.2.1) can be used to provide dynamic keying material. This makes it possible to bind the EAP authentication to subsequent data and protect against data modification, spoofing, or replay. See Section 7.1 for details. [4] Minimum MTU. EAP is capable of functioning on lower layers that provide an EAP MTU size of 1020 octets or greater. EAP does not support path MTU discovery, and fragmentation and reassembly is not supported by EAP, nor by the methods defined in this specification: Identity (1), Notification (2), Nak Response (3), MD5-Challenge (4), One Time Password (5), Generic Token Card (6), and expanded Nak Response (254) Types. Typically, the EAP peer obtains information on the EAP MTU from the lower layers and sets the EAP frame size to an appropriate value. Where the authenticator operates in pass-through mode, the authentication server does not have a direct way of determining the EAP MTU, and therefore relies on the authenticator to provide it with this information, such as via the Framed-MTU attribute, as described in [RFC3579], Section 2.4.
While methods such as EAP-TLS [RFC2716] support fragmentation and
reassembly, EAP methods originally designed for use within PPP
where a 1500 octet MTU is guaranteed for control frames (see
[RFC1661], Section 6.1) may lack fragmentation and reassembly
features.
EAP methods can assume a minimum EAP MTU of 1020 octets in the
absence of other information. EAP methods SHOULD include support
for fragmentation and reassembly if their payloads can be larger
than this minimum EAP MTU.
EAP is a lock-step protocol, which implies a certain inefficiency
when handling fragmentation and reassembly. Therefore, if the
lower layer supports fragmentation and reassembly (such as where
EAP is transported over IP), it may be preferable for
fragmentation and reassembly to occur in the lower layer rather
than in EAP. This can be accomplished by providing an
artificially large EAP MTU to EAP, causing fragmentation and
reassembly to be handled within the lower layer.
[5] Possible duplication. Where the lower layer is reliable, it will
provide the EAP layer with a non-duplicated stream of packets.
However, while it is desirable that lower layers provide for
non-duplication, this is not a requirement. The Identifier field
provides both the peer and authenticator with the ability to
detect duplicates.
[6] Ordering guarantees. EAP does not require the Identifier to be
monotonically increasing, and so is reliant on lower layer
ordering guarantees for correct operation. EAP was originally
defined to run on PPP, and [RFC1661] Section 1 has an ordering
requirement:
"The Point-to-Point Protocol is designed for simple links
which transport packets between two peers. These links
provide full-duplex simultaneous bi-directional operation,
and are assumed to deliver packets in order."
Lower layer transports for EAP MUST preserve ordering between a
source and destination at a given priority level (the ordering
guarantee provided by [IEEE-802]).
Reordering, if it occurs, will typically result in an EAP
authentication failure, causing EAP authentication to be re-run.
In an environment in which reordering is likely, it is therefore
expected that EAP authentication failures will be common. It is
RECOMMENDED that EAP only be run over lower layers that provide
ordering guarantees; running EAP over raw IP or UDP transport is
NOT RECOMMENDED. Encapsulation of EAP within RADIUS [RFC3579]
satisfies ordering requirements, since RADIUS is a "lockstep"
protocol that delivers packets in order.
3.2. EAP Usage Within PPP
In order to establish communications over a point-to-point link, each
end of the PPP link first sends LCP packets to configure the data
link during the Link Establishment phase. After the link has been
established, PPP provides for an optional Authentication phase before
proceeding to the Network-Layer Protocol phase.
By default, authentication is not mandatory. If authentication of
the link is desired, an implementation MUST specify the
Authentication Protocol Configuration Option during the Link
Establishment phase.
If the identity of the peer has been established in the
Authentication phase, the server can use that identity in the
selection of options for the following network layer negotiations.
When implemented within PPP, EAP does not select a specific
authentication mechanism at the PPP Link Control Phase, but rather
postpones this until the Authentication Phase. This allows the
authenticator to request more information before determining the
specific authentication mechanism. This also permits the use of a
"backend" server which actually implements the various mechanisms
while the PPP authenticator merely passes through the authentication
exchange. The PPP Link Establishment and Authentication phases, and
the Authentication Protocol Configuration Option, are defined in The
Point-to-Point Protocol (PPP) [RFC1661].
3.2.1. PPP Configuration Option Format
A summary of the PPP Authentication Protocol Configuration Option
format to negotiate EAP follows. The fields are transmitted from
left to right.
Exactly one EAP packet is encapsulated in the Information field of a
PPP Data Link Layer frame where the protocol field indicates type hex
C227 (PPP EAP).
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Authentication Protocol |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
3
Length
4
Authentication Protocol
C227 (Hex) for Extensible Authentication Protocol (EAP)
3.3. EAP Usage Within IEEE 802
The encapsulation of EAP over IEEE 802 is defined in [IEEE-802.1X].
The IEEE 802 encapsulation of EAP does not involve PPP, and IEEE
802.1X does not include support for link or network layer
negotiations. As a result, within IEEE 802.1X, it is not possible to
negotiate non-EAP authentication mechanisms, such as PAP or CHAP
[RFC1994].
3.4. Lower Layer Indications
The reliability and security of lower layer indications is dependent
on the lower layer. Since EAP is media independent, the presence or
absence of lower layer security is not taken into account in the
processing of EAP messages.
To improve reliability, if a peer receives a lower layer success
indication as defined in Section 7.2, it MAY conclude that a Success
packet has been lost, and behave as if it had actually received a
Success packet. This includes choosing to ignore the Success in some
circumstances as described in Section 4.2.
A discussion of some reliability and security issues with lower layer
indications in PPP, IEEE 802 wired networks, and IEEE 802.11 wireless
LANs can be found in the Security Considerations, Section 7.12.
After EAP authentication is complete, the peer will typically
transmit and receive data via the authenticator. It is desirable to
provide assurance that the entities transmitting data are the same
ones that successfully completed EAP authentication. To accomplish
this, it is necessary for the lower layer to provide per-packet integrity, authentication and replay protection, and to bind these per-packet services to the keys derived during EAP authentication. Otherwise, it is possible for subsequent data traffic to be modified, spoofed, or replayed. Where keying material for the lower layer ciphersuite is itself provided by EAP, ciphersuite negotiation and key activation are controlled by the lower layer. In PPP, ciphersuites are negotiated within ECP so that it is not possible to use keys derived from EAP authentication until the completion of ECP. Therefore, an initial EAP exchange cannot be protected by a PPP ciphersuite, although EAP re-authentication can be protected. In IEEE 802 media, initial key activation also typically occurs after completion of EAP authentication. Therefore an initial EAP exchange typically cannot be protected by the lower layer ciphersuite, although an EAP re-authentication or pre-authentication exchange can be protected.4. EAP Packet Format
A summary of the EAP packet format is shown below. The fields are transmitted from left to right. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Code | Identifier | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Data ... +-+-+-+-+ Code The Code field is one octet and identifies the Type of EAP packet. EAP Codes are assigned as follows: 1 Request 2 Response 3 Success 4 Failure Since EAP only defines Codes 1-4, EAP packets with other codes MUST be silently discarded by both authenticators and peers.
Identifier
The Identifier field is one octet and aids in matching Responses
with Requests.
Length
The Length field is two octets and indicates the length, in
octets, of the EAP packet including the Code, Identifier, Length,
and Data fields. Octets outside the range of the Length field
should be treated as Data Link Layer padding and MUST be ignored
upon reception. A message with the Length field set to a value
larger than the number of received octets MUST be silently
discarded.
Data
The Data field is zero or more octets. The format of the Data
field is determined by the Code field.
4.1. Request and Response
Description
The Request packet (Code field set to 1) is sent by the
authenticator to the peer. Each Request has a Type field which
serves to indicate what is being requested. Additional Request
packets MUST be sent until a valid Response packet is received, an
optional retry counter expires, or a lower layer failure
indication is received.
Retransmitted Requests MUST be sent with the same Identifier value
in order to distinguish them from new Requests. The content of
the data field is dependent on the Request Type. The peer MUST
send a Response packet in reply to a valid Request packet.
Responses MUST only be sent in reply to a valid Request and never
be retransmitted on a timer.
If a peer receives a valid duplicate Request for which it has
already sent a Response, it MUST resend its original Response
without reprocessing the Request. Requests MUST be processed in
the order that they are received, and MUST be processed to their
completion before inspecting the next Request.
A summary of the Request and Response packet format follows. The
fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Type-Data ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
Code
1 for Request
2 for Response
Identifier
The Identifier field is one octet. The Identifier field MUST be
the same if a Request packet is retransmitted due to a timeout
while waiting for a Response. Any new (non-retransmission)
Requests MUST modify the Identifier field.
The Identifier field of the Response MUST match that of the
currently outstanding Request. An authenticator receiving a
Response whose Identifier value does not match that of the
currently outstanding Request MUST silently discard the Response.
In order to avoid confusion between new Requests and
retransmissions, the Identifier value chosen for each new Request
need only be different from the previous Request, but need not be
unique within the conversation. One way to achieve this is to
start the Identifier at an initial value and increment it for each
new Request. Initializing the first Identifier with a random
number rather than starting from zero is recommended, since it
makes sequence attacks somewhat more difficult.
Since the Identifier space is unique to each session,
authenticators are not restricted to only 256 simultaneous
authentication conversations. Similarly, with re-authentication,
an EAP conversation might continue over a long period of time, and
is not limited to only 256 roundtrips.
Implementation Note: The authenticator is responsible for
retransmitting Request messages. If the Request message is obtained
from elsewhere (such as from a backend authentication server), then
the authenticator will need to save a copy of the Request in order to
accomplish this. The peer is responsible for detecting and handling
duplicate Request messages before processing them in any way,
including passing them on to an outside party. The authenticator is
also responsible for discarding Response messages with a non-matching
Identifier value before acting on them in any way, including passing
them on to the backend authentication server for verification. Since
the authenticator can retransmit before receiving a Response from the
peer, the authenticator can receive multiple Responses, each with a
matching Identifier. Until a new Request is received by the
authenticator, the Identifier value is not updated, so that the
authenticator forwards Responses to the backend authentication
server, one at a time.
Length
The Length field is two octets and indicates the length of the EAP
packet including the Code, Identifier, Length, Type, and Type-Data
fields. Octets outside the range of the Length field should be
treated as Data Link Layer padding and MUST be ignored upon
reception. A message with the Length field set to a value larger
than the number of received octets MUST be silently discarded.
Type
The Type field is one octet. This field indicates the Type of
Request or Response. A single Type MUST be specified for each EAP
Request or Response. An initial specification of Types follows in
Section 5 of this document.
The Type field of a Response MUST either match that of the
Request, or correspond to a legacy or Expanded Nak (see Section
5.3) indicating that a Request Type is unacceptable to the peer.
A peer MUST NOT send a Nak (legacy or expanded) in response to a
Request, after an initial non-Nak Response has been sent. An EAP
server receiving a Response not meeting these requirements MUST
silently discard it.
Type-Data
The Type-Data field varies with the Type of Request and the
associated Response.
4.2. Success and Failure
The Success packet is sent by the authenticator to the peer after
completion of an EAP authentication method (Type 4 or greater) to
indicate that the peer has authenticated successfully to the
authenticator. The authenticator MUST transmit an EAP packet with
the Code field set to 3 (Success). If the authenticator cannot
authenticate the peer (unacceptable Responses to one or more
Requests), then after unsuccessful completion of the EAP method in
progress, the implementation MUST transmit an EAP packet with the
Code field set to 4 (Failure). An authenticator MAY wish to issue multiple Requests before sending a Failure response in order to allow for human typing mistakes. Success and Failure packets MUST NOT contain additional data. Success and Failure packets MUST NOT be sent by an EAP authenticator if the specification of the given method does not explicitly permit the method to finish at that point. A peer EAP implementation receiving a Success or Failure packet where sending one is not explicitly permitted MUST silently discard it. By default, an EAP peer MUST silently discard a "canned" Success packet (a Success packet sent immediately upon connection). This ensures that a rogue authenticator will not be able to bypass mutual authentication by sending a Success packet prior to conclusion of the EAP method conversation. Implementation Note: Because the Success and Failure packets are not acknowledged, they are not retransmitted by the authenticator, and may be potentially lost. A peer MUST allow for this circumstance as described in this note. See also Section 3.4 for guidance on the processing of lower layer success and failure indications. As described in Section 2.1, only a single EAP authentication method is allowed within an EAP conversation. EAP methods may implement result indications. After the authenticator sends a failure result indication to the peer, regardless of the response from the peer, it MUST subsequently send a Failure packet. After the authenticator sends a success result indication to the peer and receives a success result indication from the peer, it MUST subsequently send a Success packet. On the peer, once the method completes unsuccessfully (that is, either the authenticator sends a failure result indication, or the peer decides that it does not want to continue the conversation, possibly after sending a failure result indication), the peer MUST terminate the conversation and indicate failure to the lower layer. The peer MUST silently discard Success packets and MAY silently discard Failure packets. As a result, loss of a Failure packet need not result in a timeout. On the peer, after success result indications have been exchanged by both sides, a Failure packet MUST be silently discarded. The peer MAY, in the event that an EAP Success is not received, conclude that the EAP Success packet was lost and that authentication concluded successfully.
If the authenticator has not sent a result indication, and the peer
is willing to continue the conversation, the peer waits for a Success
or Failure packet once the method completes, and MUST NOT silently
discard either of them. In the event that neither a Success nor
Failure packet is received, the peer SHOULD terminate the
conversation to avoid lengthy timeouts in case the lost packet was an
EAP Failure.
If the peer attempts to authenticate to the authenticator and fails
to do so, the authenticator MUST send a Failure packet and MUST NOT
grant access by sending a Success packet. However, an authenticator
MAY omit having the peer authenticate to it in situations where
limited access is offered (e.g., guest access). In this case, the
authenticator MUST send a Success packet.
Where the peer authenticates successfully to the authenticator, but
the authenticator does not send a result indication, the
authenticator MAY deny access by sending a Failure packet where the
peer is not currently authorized for network access.
A summary of the Success and Failure packet format is shown below.
The fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code
3 for Success
4 for Failure
Identifier
The Identifier field is one octet and aids in matching replies to
Responses. The Identifier field MUST match the Identifier field
of the Response packet that it is sent in response to.
Length
4
4.3. Retransmission Behavior
Because the authentication process will often involve user input, some care must be taken when deciding upon retransmission strategies and authentication timeouts. By default, where EAP is run over an unreliable lower layer, the EAP retransmission timer SHOULD be dynamically estimated. A maximum of 3-5 retransmissions is suggested. When run over a reliable lower layer (e.g., EAP over ISAKMP/TCP, as within [PIC]), the authenticator retransmission timer SHOULD be set to an infinite value, so that retransmissions do not occur at the EAP layer. The peer may still maintain a timeout value so as to avoid waiting indefinitely for a Request. Where the authentication process requires user input, the measured round trip times may be determined by user responsiveness rather than network characteristics, so that dynamic RTO estimation may not be helpful. Instead, the retransmission timer SHOULD be set so as to provide sufficient time for the user to respond, with longer timeouts required in certain cases, such as where Token Cards (see Section 5.6) are involved. In order to provide the EAP authenticator with guidance as to the appropriate timeout value, a hint can be communicated to the authenticator by the backend authentication server (such as via the RADIUS Session-Timeout attribute). In order to dynamically estimate the EAP retransmission timer, the algorithms for the estimation of SRTT, RTTVAR, and RTO described in [RFC2988] are RECOMMENDED, including use of Karn's algorithm, with the following potential modifications: [a] In order to avoid synchronization behaviors that can occur with fixed timers among distributed systems, the retransmission timer is calculated with a jitter by using the RTO value and randomly adding a value drawn between -RTOmin/2 and RTOmin/2. Alternative calculations to create jitter MAY be used. These MUST be pseudo-random. For a discussion of pseudo-random number generation, see [RFC1750]. [b] When EAP is transported over a single link (as opposed to over the Internet), smaller values of RTOinitial, RTOmin, and RTOmax MAY be used. Recommended values are RTOinitial=1 second, RTOmin=200ms, and RTOmax=20 seconds.
[c] When EAP is transported over a single link (as opposed to over
the Internet), estimates MAY be done on a per-authenticator
basis, rather than a per-session basis. This enables the
retransmission estimate to make the most use of information on
link-layer behavior.
[d] An EAP implementation MAY clear SRTT and RTTVAR after backing off
the timer multiple times, as it is likely that the current SRTT
and RTTVAR are bogus in this situation. Once SRTT and RTTVAR are
cleared, they should be initialized with the next RTT sample
taken as described in [RFC2988] equation 2.2.
5. Initial EAP Request/Response Types
This section defines the initial set of EAP Types used in Request/
Response exchanges. More Types may be defined in future documents.
The Type field is one octet and identifies the structure of an EAP
Request or Response packet. The first 3 Types are considered special
case Types.
The remaining Types define authentication exchanges. Nak (Type 3) or
Expanded Nak (Type 254) are valid only for Response packets, they
MUST NOT be sent in a Request.
All EAP implementations MUST support Types 1-4, which are defined in
this document, and SHOULD support Type 254. Implementations MAY
support other Types defined here or in future RFCs.
1 Identity
2 Notification
3 Nak (Response only)
4 MD5-Challenge
5 One Time Password (OTP)
6 Generic Token Card (GTC)
254 Expanded Types
255 Experimental use
EAP methods MAY support authentication based on shared secrets. If
the shared secret is a passphrase entered by the user,
implementations MAY support entering passphrases with non-ASCII
characters. In this case, the input should be processed using an
appropriate stringprep [RFC3454] profile, and encoded in octets using
UTF-8 encoding [RFC2279]. A preliminary version of a possible
stringprep profile is described in [SASLPREP].
5.1. Identity
Description The Identity Type is used to query the identity of the peer. Generally, the authenticator will issue this as the initial Request. An optional displayable message MAY be included to prompt the peer in the case where there is an expectation of interaction with a user. A Response of Type 1 (Identity) SHOULD be sent in Response to a Request with a Type of 1 (Identity). Some EAP implementations piggy-back various options into the Identity Request after a NUL-character. By default, an EAP implementation SHOULD NOT assume that an Identity Request or Response can be larger than 1020 octets. It is RECOMMENDED that the Identity Response be used primarily for routing purposes and selecting which EAP method to use. EAP Methods SHOULD include a method-specific mechanism for obtaining the identity, so that they do not have to rely on the Identity Response. Identity Requests and Responses are sent in cleartext, so an attacker may snoop on the identity, or even modify or spoof identity exchanges. To address these threats, it is preferable for an EAP method to include an identity exchange that supports per-packet authentication, integrity and replay protection, and confidentiality. The Identity Response may not be the appropriate identity for the method; it may have been truncated or obfuscated so as to provide privacy, or it may have been decorated for routing purposes. Where the peer is configured to only accept authentication methods supporting protected identity exchanges, the peer MAY provide an abbreviated Identity Response (such as omitting the peer-name portion of the NAI [RFC2486]). For further discussion of identity protection, see Section 7.3. Implementation Note: The peer MAY obtain the Identity via user input. It is suggested that the authenticator retry the Identity Request in the case of an invalid Identity or authentication failure to allow for potential typos on the part of the user. It is suggested that the Identity Request be retried a minimum of 3 times before terminating the authentication. The Notification Request MAY be used to indicate an invalid authentication attempt prior to transmitting a new Identity Request (optionally, the failure MAY be indicated within the message of the new Identity Request itself).
Type
1
Type-Data
This field MAY contain a displayable message in the Request,
containing UTF-8 encoded ISO 10646 characters [RFC2279]. Where
the Request contains a null, only the portion of the field prior
to the null is displayed. If the Identity is unknown, the
Identity Response field should be zero bytes in length. The
Identity Response field MUST NOT be null terminated. In all
cases, the length of the Type-Data field is derived from the
Length field of the Request/Response packet.
Security Claims (see Section 7.2):
Auth. mechanism: None
Ciphersuite negotiation: No
Mutual authentication: No
Integrity protection: No
Replay protection: No
Confidentiality: No
Key derivation: No
Key strength: N/A
Dictionary attack prot.: N/A
Fast reconnect: No
Crypt. binding: N/A
Session independence: N/A
Fragmentation: No
Channel binding: No
5.2. Notification
Description
The Notification Type is optionally used to convey a displayable
message from the authenticator to the peer. An authenticator MAY
send a Notification Request to the peer at any time when there is
no outstanding Request, prior to completion of an EAP
authentication method. The peer MUST respond to a Notification
Request with a Notification Response unless the EAP authentication
method specification prohibits the use of Notification messages.
In any case, a Nak Response MUST NOT be sent in response to a
Notification Request. Note that the default maximum length of a
Notification Request is 1020 octets. By default, this leaves at
most 1015 octets for the human readable message.
An EAP method MAY indicate within its specification that
Notification messages must not be sent during that method. In
this case, the peer MUST silently discard Notification Requests
from the point where an initial Request for that Type is answered
with a Response of the same Type.
The peer SHOULD display this message to the user or log it if it
cannot be displayed. The Notification Type is intended to provide
an acknowledged notification of some imperative nature, but it is
not an error indication, and therefore does not change the state
of the peer. Examples include a password with an expiration time
that is about to expire, an OTP sequence integer which is nearing
0, an authentication failure warning, etc. In most circumstances,
Notification should not be required.
Type
2
Type-Data
The Type-Data field in the Request contains a displayable message
greater than zero octets in length, containing UTF-8 encoded ISO
10646 characters [RFC2279]. The length of the message is
determined by the Length field of the Request packet. The message
MUST NOT be null terminated. A Response MUST be sent in reply to
the Request with a Type field of 2 (Notification). The Type-Data
field of the Response is zero octets in length. The Response
should be sent immediately (independent of how the message is
displayed or logged).
Security Claims (see Section 7.2):
Auth. mechanism: None
Ciphersuite negotiation: No
Mutual authentication: No
Integrity protection: No
Replay protection: No
Confidentiality: No
Key derivation: No
Key strength: N/A
Dictionary attack prot.: N/A
Fast reconnect: No
Crypt. binding: N/A
Session independence: N/A
Fragmentation: No
Channel binding: No
5.3. Nak
5.3.1. Legacy Nak
Description The legacy Nak Type is valid only in Response messages. It is sent in reply to a Request where the desired authentication Type is unacceptable. Authentication Types are numbered 4 and above. The Response contains one or more authentication Types desired by the Peer. Type zero (0) is used to indicate that the sender has no viable alternatives, and therefore the authenticator SHOULD NOT send another Request after receiving a Nak Response containing a zero value. Since the legacy Nak Type is valid only in Responses and has very limited functionality, it MUST NOT be used as a general purpose error indication, such as for communication of error messages, or negotiation of parameters specific to a particular EAP method. Code 2 for Response. Identifier The Identifier field is one octet and aids in matching Responses with Requests. The Identifier field of a legacy Nak Response MUST match the Identifier field of the Request packet that it is sent in response to. Length >=6 Type 3 Type-Data Where a peer receives a Request for an unacceptable authentication Type (4-253,255), or a peer lacking support for Expanded Types receives a Request for Type 254, a Nak Response (Type 3) MUST be sent. The Type-Data field of the Nak Response (Type 3) MUST contain one or more octets indicating the desired authentication Type(s), one octet per Type, or the value zero (0) to indicate no proposed alternative. A peer supporting Expanded Types that
receives a Request for an unacceptable authentication Type (4-253,
255) MAY include the value 254 in the Nak Response (Type 3) to
indicate the desire for an Expanded authentication Type. If the
authenticator can accommodate this preference, it will respond
with an Expanded Type Request (Type 254).
Security Claims (see Section 7.2):
Auth. mechanism: None
Ciphersuite negotiation: No
Mutual authentication: No
Integrity protection: No
Replay protection: No
Confidentiality: No
Key derivation: No
Key strength: N/A
Dictionary attack prot.: N/A
Fast reconnect: No
Crypt. binding: N/A
Session independence: N/A
Fragmentation: No
Channel binding: No
5.3.2. Expanded Nak
Description
The Expanded Nak Type is valid only in Response messages. It MUST
be sent only in reply to a Request of Type 254 (Expanded Type)
where the authentication Type is unacceptable. The Expanded Nak
Type uses the Expanded Type format itself, and the Response
contains one or more authentication Types desired by the peer, all
in Expanded Type format. Type zero (0) is used to indicate that
the sender has no viable alternatives. The general format of the
Expanded Type is described in Section 5.7.
Since the Expanded Nak Type is valid only in Responses and has
very limited functionality, it MUST NOT be used as a general
purpose error indication, such as for communication of error
messages, or negotiation of parameters specific to a particular
EAP method.
Code
2 for Response.
Identifier
The Identifier field is one octet and aids in matching Responses
with Requests. The Identifier field of an Expanded Nak Response
MUST match the Identifier field of the Request packet that it is
sent in response to.
Length
>=20
Type
254
Vendor-Id
0 (IETF)
Vendor-Type
3 (Nak)
Vendor-Data
The Expanded Nak Type is only sent when the Request contains an
Expanded Type (254) as defined in Section 5.7. The Vendor-Data
field of the Nak Response MUST contain one or more authentication
Types (4 or greater), all in expanded format, 8 octets per Type,
or the value zero (0), also in Expanded Type format, to indicate
no proposed alternative. The desired authentication Types may
include a mixture of Vendor-Specific and IETF Types. For example,
an Expanded Nak Response indicating a preference for OTP (Type 5),
and an MIT (Vendor-Id=20) Expanded Type of 6 would appear as
follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 2 | Identifier | Length=28 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=254 | 0 (IETF) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 3 (Nak) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=254 | 0 (IETF) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 5 (OTP) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=254 | 20 (MIT) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 6 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
An Expanded Nak Response indicating a no desired alternative would
appear as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 2 | Identifier | Length=20 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=254 | 0 (IETF) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 3 (Nak) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=254 | 0 (IETF) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 (No alternative) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Security Claims (see Section 7.2):
Auth. mechanism: None
Ciphersuite negotiation: No
Mutual authentication: No
Integrity protection: No
Replay protection: No
Confidentiality: No
Key derivation: No
Key strength: N/A
Dictionary attack prot.: N/A
Fast reconnect: No
Crypt. binding: N/A
Session independence: N/A
Fragmentation: No
Channel binding: No
5.4. MD5-Challenge
Description
The MD5-Challenge Type is analogous to the PPP CHAP protocol
[RFC1994] (with MD5 as the specified algorithm). The Request
contains a "challenge" message to the peer. A Response MUST be
sent in reply to the Request. The Response MAY be either of Type
4 (MD5-Challenge), Nak (Type 3), or Expanded Nak (Type 254). The
Nak reply indicates the peer's desired authentication Type(s).
EAP peer and EAP server implementations MUST support the MD5-
Challenge mechanism. An authenticator that supports only pass-
through MUST allow communication with a backend authentication
server that is capable of supporting MD5-Challenge, although the
EAP authenticator implementation need not support MD5-Challenge
itself. However, if the EAP authenticator can be configured to
authenticate peers locally (e.g., not operate in pass-through),
then the requirement for support of the MD5-Challenge mechanism
applies.
Note that the use of the Identifier field in the MD5-Challenge
Type is different from that described in [RFC1994]. EAP allows
for retransmission of MD5-Challenge Request packets, while
[RFC1994] states that both the Identifier and Challenge fields
MUST change each time a Challenge (the CHAP equivalent of the
MD5-Challenge Request packet) is sent.
Note: [RFC1994] treats the shared secret as an octet string, and
does not specify how it is entered into the system (or if it is
handled by the user at all). EAP MD5-Challenge implementations
MAY support entering passphrases with non-ASCII characters. See
Section 5 for instructions how the input should be processed and
encoded into octets.
Type
4
Type-Data
The contents of the Type-Data field is summarized below. For
reference on the use of these fields, see the PPP Challenge
Handshake Authentication Protocol [RFC1994].
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value-Size | Value ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Name ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Security Claims (see Section 7.2):
Auth. mechanism: Password or pre-shared key.
Ciphersuite negotiation: No
Mutual authentication: No
Integrity protection: No
Replay protection: No
Confidentiality: No
Key derivation: No
Key strength: N/A
Dictionary attack prot.: No
Fast reconnect: No
Crypt. binding: N/A
Session independence: N/A
Fragmentation: No
Channel binding: No
5.5. One-Time Password (OTP)
Description
The One-Time Password system is defined in "A One-Time Password
System" [RFC2289] and "OTP Extended Responses" [RFC2243]. The
Request contains an OTP challenge in the format described in
[RFC2289]. A Response MUST be sent in reply to the Request. The
Response MUST be of Type 5 (OTP), Nak (Type 3), or Expanded Nak
(Type 254). The Nak Response indicates the peer's desired
authentication Type(s). The EAP OTP method is intended for use
with the One-Time Password system only, and MUST NOT be used to
provide support for cleartext passwords.
Type
5
Type-Data
The Type-Data field contains the OTP "challenge" as a displayable
message in the Request. In the Response, this field is used for
the 6 words from the OTP dictionary [RFC2289]. The messages MUST
NOT be null terminated. The length of the field is derived from
the Length field of the Request/Reply packet.
Note: [RFC2289] does not specify how the secret pass-phrase is
entered by the user, or how the pass-phrase is converted into
octets. EAP OTP implementations MAY support entering passphrases
with non-ASCII characters. See Section 5 for instructions on how
the input should be processed and encoded into octets.
Security Claims (see Section 7.2):
Auth. mechanism: One-Time Password
Ciphersuite negotiation: No
Mutual authentication: No
Integrity protection: No
Replay protection: Yes
Confidentiality: No
Key derivation: No
Key strength: N/A
Dictionary attack prot.: No
Fast reconnect: No
Crypt. binding: N/A
Session independence: N/A
Fragmentation: No
Channel binding: No
5.6. Generic Token Card (GTC)
Description
The Generic Token Card Type is defined for use with various Token
Card implementations which require user input. The Request
contains a displayable message and the Response contains the Token
Card information necessary for authentication. Typically, this
would be information read by a user from the Token card device and
entered as ASCII text. A Response MUST be sent in reply to the
Request. The Response MUST be of Type 6 (GTC), Nak (Type 3), or
Expanded Nak (Type 254). The Nak Response indicates the peer's
desired authentication Type(s). The EAP GTC method is intended
for use with the Token Cards supporting challenge/response
authentication and MUST NOT be used to provide support for
cleartext passwords in the absence of a protected tunnel with
server authentication.
Type
6
Type-Data
The Type-Data field in the Request contains a displayable message
greater than zero octets in length. The length of the message is
determined by the Length field of the Request packet. The message
MUST NOT be null terminated. A Response MUST be sent in reply to
the Request with a Type field of 6 (Generic Token Card). The
Response contains data from the Token Card required for
authentication. The length of the data is determined by the
Length field of the Response packet.
EAP GTC implementations MAY support entering a response with non-
ASCII characters. See Section 5 for instructions how the input
should be processed and encoded into octets.
Security Claims (see Section 7.2):
Auth. mechanism: Hardware token.
Ciphersuite negotiation: No
Mutual authentication: No
Integrity protection: No
Replay protection: No
Confidentiality: No
Key derivation: No
Key strength: N/A
Dictionary attack prot.: No
Fast reconnect: No
Crypt. binding: N/A
Session independence: N/A
Fragmentation: No
Channel binding: No
5.7. Expanded Types
Description
Since many of the existing uses of EAP are vendor-specific, the
Expanded method Type is available to allow vendors to support
their own Expanded Types not suitable for general usage.
The Expanded Type is also used to expand the global Method Type
space beyond the original 255 values. A Vendor-Id of 0 maps the
original 255 possible Types onto a space of 2^32-1 possible Types.
(Type 0 is only used in a Nak Response to indicate no acceptable
alternative).
An implementation that supports the Expanded attribute MUST treat
EAP Types that are less than 256 equivalently, whether they appear
as a single octet or as the 32-bit Vendor-Type within an Expanded
Type where Vendor-Id is 0. Peers not equipped to interpret the
Expanded Type MUST send a Nak as described in Section 5.3.1, and
negotiate a more suitable authentication method.
A summary of the Expanded Type format is shown below. The fields
are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Vendor-Id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Vendor-Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Vendor data...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
254 for Expanded Type
Vendor-Id
The Vendor-Id is 3 octets and represents the SMI Network
Management Private Enterprise Code of the Vendor in network byte
order, as allocated by IANA. A Vendor-Id of zero is reserved for
use by the IETF in providing an expanded global EAP Type space.
Vendor-Type
The Vendor-Type field is four octets and represents the vendor-
specific method Type.
If the Vendor-Id is zero, the Vendor-Type field is an extension
and superset of the existing namespace for EAP Types. The first
256 Types are reserved for compatibility with single-octet EAP
Types that have already been assigned or may be assigned in the
future. Thus, EAP Types from 0 through 255 are semantically
identical, whether they appear as single octet EAP Types or as
Vendor-Types when Vendor-Id is zero. There is one exception to
this rule: Expanded Nak and Legacy Nak packets share the same
Type, but must be treated differently because they have a
different format.
Vendor-Data
The Vendor-Data field is defined by the vendor. Where a Vendor-Id
of zero is present, the Vendor-Data field will be used for
transporting the contents of EAP methods of Types defined by the
IETF.
5.8. Experimental
Description
The Experimental Type has no fixed format or content. It is
intended for use when experimenting with new EAP Types. This Type
is intended for experimental and testing purposes. No guarantee
is made for interoperability between peers using this Type, as
outlined in [RFC3692].
Type
255
Type-Data
Undefined
6. IANA Considerations
This section provides guidance to the Internet Assigned Numbers
Authority (IANA) regarding registration of values related to the EAP
protocol, in accordance with BCP 26, [RFC2434].
There are two name spaces in EAP that require registration: Packet
Codes and method Types.
EAP is not intended as a general-purpose protocol, and allocations
SHOULD NOT be made for purposes unrelated to authentication.
The following terms are used here with the meanings defined in BCP
26: "name space", "assigned value", "registration".
The following policies are used here with the meanings defined in BCP
26: "Private Use", "First Come First Served", "Expert Review",
"Specification Required", "IETF Consensus", "Standards Action".
For registration requests where a Designated Expert should be consulted, the responsible IESG area director should appoint the Designated Expert. The intention is that any allocation will be accompanied by a published RFC. But in order to allow for the allocation of values prior to the RFC being approved for publication, the Designated Expert can approve allocations once it seems clear that an RFC will be published. The Designated expert will post a request to the EAP WG mailing list (or a successor designated by the Area Director) for comment and review, including an Internet-Draft. Before a period of 30 days has passed, the Designated Expert will either approve or deny the registration request and publish a notice of the decision to the EAP WG mailing list or its successor, as well as informing IANA. A denial notice must be justified by an explanation, and in the cases where it is possible, concrete suggestions on how the request can be modified so as to become acceptable should be provided.6.1. Packet Codes
Packet Codes have a range from 1 to 255, of which 1-4 have been allocated. Because a new Packet Code has considerable impact on interoperability, a new Packet Code requires Standards Action, and should be allocated starting at 5.6.2. Method Types
The original EAP method Type space has a range from 1 to 255, and is the scarcest resource in EAP, and thus must be allocated with care. Method Types 1-45 have been allocated, with 20 available for re-use. Method Types 20 and 46-191 may be allocated on the advice of a Designated Expert, with Specification Required. Allocation of blocks of method Types (more than one for a given purpose) should require IETF Consensus. EAP Type Values 192-253 are reserved and allocation requires Standards Action. Method Type 254 is allocated for the Expanded Type. Where the Vendor-Id field is non-zero, the Expanded Type is used for functions specific only to one vendor's implementation of EAP, where no interoperability is deemed useful. When used with a Vendor-Id of zero, method Type 254 can also be used to provide for an expanded IETF method Type space. Method Type values 256-4294967295 may be allocated after Type values 1-191 have been allocated, on the advice of a Designated Expert, with Specification Required. Method Type 255 is allocated for Experimental use, such as testing of new EAP methods before a permanent Type is allocated.