rfc:rfc2284
Network Working Group L. Blunk Request for Comments: 2284 J. Vollbrecht Category: Standards Track Merit Network, Inc.
March 1998
PPP Extensible Authentication Protocol (EAP)
Status of this Memo
This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (1998). All Rights Reserved.
Abstract
The Point-to-Point Protocol (PPP) [1] provides a standard method for transporting multi-protocol datagrams over point-to-point links.
PPP also defines an extensible Link Control Protocol, which allows negotiation of an Authentication Protocol for authenticating its peer before allowing Network Layer protocols to transmit over the link.
This document defines the PPP Extensible Authentication Protocol.
Table of Contents
1. Introduction .......................................... 2
1.1 Specification of Requirements ................... 2
1.2 Terminology ..................................... 2
2. PPP Extensible Authentication Protocol (EAP) .......... 3
2.1 Configuration Option Format ..................... 4
2.2 Packet Format ................................... 6
2.2.1 Request and Response ............................ 6
2.2.2 Success and Failure ............................. 7
3. Initial EAP Request/Response Types .................... 8
3.1 Identity ........................................ 9
3.2 Notification .................................... 10
3.3 Nak ............................................. 10
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3.4 MD5-Challenge ................................... 11
3.5 One-Time Password (OTP) ......................... 11
3.6 Generic Token Card .............................. 12
REFERENCES ................................................... 13
ACKNOWLEDGEMENTS ............................................. 14
CHAIR'S ADDRESS .............................................. 14
AUTHORS' ADDRESSES ........................................... 14
Full Copyright Statement ..................................... 15
1. Introduction
In order to establish communications over a point-to-point link, each end of the PPP link must first send LCP packets to configure the data link during 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 Link Establishment phase.
These authentication protocols are intended for use primarily by hosts and routers that connect to a PPP network server via switched circuits or dial-up lines, but might be applied to dedicated links as well. The server can use the identification of the connecting host or router in the selection of options for network layer negotiations.
This document defines the PPP Extensible Authentication Protocol (EAP). The Link Establishment and Authentication phases, and the Authentication-Protocol Configuration Option, are defined in The Point-to-Point Protocol (PPP) [1].
1.1. 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 RFC 2119 [6].
1.2. Terminology
This document frequently uses the following terms:
Blunk & Vollbrecht Standards Track [Page 2] RFC 2284 EAP March 1998
authenticator
The end of the link requiring the authentication. The
authenticator specifies the authentication protocol to be
used in the Configure-Request during Link Establishment
phase.
peer
The other end of the point-to-point link; the end which is
being authenticated by the authenticator.
silently discard
This means the implementation discards the packet without
further processing. The implementation SHOULD provide the
capability of logging the error, including the contents of
the silently discarded packet, and SHOULD record the event
in a statistics counter.
displayable message This is interpreted to be a human readable string of characters, and MUST NOT affect operation of the protocol. The message encoding MUST follow the UTF-8 transformation format [5].
2. PPP Extensible Authentication Protocol (EAP)
The PPP Extensible Authentication Protocol (EAP) is a general protocol for PPP authentication which supports multiple authentication mechanisms. EAP does not select a specific authentication mechanism at 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 "back-end" server which actually implements the various mechanisms while the PPP authenticator merely passes through the authentication exchange.
1. After the Link Establishment phase is complete, the authenticator
sends one or more Requests to authenticate the peer. The Request
has a type field to indicate what is being requested. Examples of
Request types include Identity, MD5-challenge, One-Time
Passwords, Generic Token Card, etc. The MD5-challenge type
corresponds closely to the CHAP authentication protocol.
Typically, the authenticator will send an initial Identity Request
followed by one or more Requests for authentication information.
However, an initial Identity Request is not required, and MAY be
bypassed in cases where the identity is presumed (leased lines,
dedicated dial-ups, etc.).
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2. The peer sends a Response packet in reply to each Request. As
with the Request packet, the Response packet contains a type field
which corresponds to the type field of the Request.
3. The authenticator ends the authentication phase with a Success or
Failure packet.
Advantages
The EAP protocol can support multiple authentication mechanisms without having to pre-negotiate a particular one during LCP Phase.
Certain devices (e.g. a NAS) do not necessarily have to understand each request type and may be able to simply act as a passthrough agent for a "back-end" server on a host. The device only need look for the success/failure code to terminate the authentication phase.
Disadvantages
EAP does require the addition of a new authentication type to LCP and thus PPP implementations will need to be modified to use it. It also strays from the previous PPP authentication model of negotiating a specific authentication mechanism during LCP.
2.1. Configuration Option Format
A summary of the Authentication-Protocol Configuration Option format to negotiate the EAP Authentication Protocol 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 | Length | Authentication-Protocol | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
3
Length
4
Authentication-Protocol
C227 (Hex) for PPP Extensible Authentication Protocol (EAP)
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2.2. Packet Format
Exactly one PPP 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). 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
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 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 should be ignored on
reception.
Data
The Data field is zero or more octets. The format of the Data
field is determined by the Code field.
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2.2.1. Request and Response
Description
The Request packet is sent by the authenticator to the peer. Each
Request has a type field which serves to indicate what is being
requested. The authenticator MUST transmit an EAP packet with the
Code field set to 1 (Request). Additional Request packets MUST be
sent until a valid Response packet is received, or an optional
retry counter expires. Retransmitted Requests MUST be sent with
the same Identifier value in order to distinguish them from new
Requests. The contents of the data field is dependent on the
Request type. The peer MUST send a Response packet in reply to a
Request packet. Responses MUST only be sent in reply to a
received Request and never retransmitted on a timer. The
Identifier field of the Response MUST match that of the Request.
Implementation Note: Because the authentication process will
often involve user input, some care must be taken when deciding
upon retransmission strategies and authentication timeouts. It
is suggested a retransmission timer of 6 seconds with a maximum
of 10 retransmissions be used as default. One may wish to make
these timeouts longer in certain cases (e.g. where Token Cards
are involved). Additionally, the peer must be prepared to
silently discard received retransmissions while waiting for
user input.
A summary of the Request and Response 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Type-Data ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
Code
1 for Request;
2 for Response.
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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. If a peer recieves a
duplicate Request for which it has already sent a Response, it
MUST resend it's Response. If a peer receives a duplicate Request
before it has sent a Response to the initial Request (i.e. it's
waiting for user input), it MUST silently discard the duplicate
Request.
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 should be ignored on
reception.
Type
The Type field is one octet. This field indicates the Type of
Request or Response. Only one Type MUST be specified per EAP
Request or Response. Normally, the Type field of the Response
will be the same as the Type of the Request. However, there is
also a Nak Response Type for indicating that a Request type is
unacceptable to the peer. When sending a Nak in response to a
Request, the peer MAY indicate an alternative desired
authentication Type which it supports. An initial specification of
Types follows in a later section of this document.
Type-Data
The Type-Data field varies with the Type of Request and the
associated Response.
2.2.2. Success and Failure
Description
The Success packet is sent by the authenticator to the peer to
acknowledge successful authentication. 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 the implementation MUST
transmit an EAP packet with the Code field set to 4 (Failure). An
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authenticator MAY wish to issue multiple Requests before sending a
Failure response in order to allow for human typing mistakes.
Implementation Note: Because the Success and Failure packets
are not acknowledged, they may be potentially lost. A peer
MUST allow for this circumstance. The peer can use a Network
Protocol packet as an alternative indication of Success.
Likewise, the receipt of a LCP Terminate-Request can be taken
as a Failure.
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 Indentifier field
of the Response packet that it is sent in response to.
Length
4
3. 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 follow-on 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. The Nak Type is valid only for Response packets, it MUST NOT be sent in a Request. The Nak Type MUST only be
Blunk & Vollbrecht Standards Track [Page 8] RFC 2284 EAP March 1998
sent in repsonse to a Request which uses an authentication Type code. All EAP implementatins MUST support Types 1-4. These Types, as well as types 5 and 6, are defined in this document. Follow-on RFCs will define additional EAP Types.
1 Identity
2 Notification
3 Nak (Response only)
4 MD5-Challenge
5 One-Time Password (OTP) (RFC 1938)
6 Generic Token Card
3.1. 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 expectation of interaction
with a user. A Response MUST be sent to this Request with a Type
of 1 (Identity).
Implementation Note: The peer MAY obtain the Identity via user
input. It is suggested that the authenticator retry the
Indentity 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 phase with a Failure reply. 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. The
Response uses this field to return the Identity. If the Identity
is unknown, this field should be zero bytes in length. The field
MUST NOT be null terminated. The length of this field is derived
from the Length field of the Request/Response packet and hence a
null is not required.
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3.2. Notification
Description
The Notification Type is optionally used to convey a displayable
message from the authenticator to the peer. The peer SHOULD
display this message to the user or log it if it cannot be
displayed. It is intended to provide an acknowledged notification
of some imperative nature. 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. The length of the message is determined by 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).
3.3. Nak
Description
The 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 the authentication Type desired by the peer.
Type
3
Type-Data
This field MUST contain a single octet indicating the desired
authentication type.
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3.4. MD5-Challenge
Description
The MD5-Challenge Type is analagous to the PPP CHAP protocol [3]
(with MD5 as the specified algorithm). The PPP Challenge
Handshake Authentication Protocol RFC [3] should be referred to
for further implementation specifics. The Request contains a
"challenge" message to the peer. A Repsonse MUST be sent in reply
to the Request. The Response MAY be either of Type 4 (MD5-
Challenge) or Type 3 (Nak). The Nak reply indicates the peer's
desired authentication mechanism Type. All EAP implementations
MUST support the MD5-Challenge mechanism.
Type
4
Type-Data
The contents of the Type-Data field is summarized below. For
reference on the use of this fields see the PPP Challenge
Handshake Authentication Protocol [3].
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 ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
3.5. One-Time Password (OTP)
Description
The One-Time Password system is defined in "A One-Time Password
System" [4]. The Request contains a displayable message
containing an OTP challenge. A Repsonse MUST be sent in reply to
the Request. The Response MUST be of Type 5 (OTP) or Type 3
(Nak). The Nak reply indicates the peer's desired authentication
mechanism Type.
Type
5
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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 [4]. The messages MUST not be
null terminated. The length of the field is derived from the
Length field of the Request/Reply packet.
3.6. Generic Token Card
Description
The Generic Token Card Type is defined for use with various Token
Card implementations which require user input. The Request
contains an ASCII text message and the Reply 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.
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 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 is of the data is determined by the Length field of the Response packet.
Security Considerations
Security issues are the primary topic of this RFC.
The interaction of the authentication protocols within PPP are highly implementation dependent.
For example, upon failure of authentication, some implementations do not terminate the link. Instead, the implementation limits the kind of traffic in the Network-Layer Protocols to a filtered subset, which in turn allows the user opportunity to update secrets or send mail to the network administrator indicating a problem.
Blunk & Vollbrecht Standards Track [Page 12] RFC 2284 EAP March 1998
There is no provision for retries of failed authentication. However, the LCP state machine can renegotiate the authentication protocol at any time, thus allowing a new attempt. It is recommended that any counters used for authentication failure not be reset until after successful authentication, or subsequent termination of the failed link.
There is no requirement that authentication be full duplex or that the same protocol be used in both directions. It is perfectly acceptable for different protocols to be used in each direction. This will, of course, depend on the specific protocols negotiated.
In practice, within or associated with each PPP server, it is not anticipated that a particular named user would be authenticated by multiple methods. This would make the user vulnerable to attacks which negotiate the least secure method from among a set (such as PAP rather than EAP). Instead, for each named user there should be an indication of exactly one method used to authenticate that user name. If a user needs to make use of different authentication methods under different circumstances, then distinct identities SHOULD be employed, each of which identifies exactly one authentication method.
References
[1] Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51,
RFC 1661, July 1994.
[2] Reynolds, J. and J. Postel, "Assigned Numbers", STD 2,
RFC 1700, October 1994.
[3] Simpson, W., "PPP Challenge Handshake Authentication Protocol
(CHAP)", RFC 1994, August 1996.
[4] Haller, N. and C. Metz, "A One-Time Password System", RFC 1938,
May 1996.
[5] Yergeau, F., "UTF-8, a transformation format of Unicode and ISO
10646", RFC 2044, October 1996.
[6] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, March 1997.
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Acknowledgments
This protocol derives much of its inspiration from Dave Carrel's AHA draft as well as the PPP CHAP protocol [3]. Bill Simpson provided much of the boilerplate used throughout this document. Al Rubens (Merit) also provided valuable feedback.
Chair's Address
The working group can be contacted via the current chair:
Karl F. Fox Ascend Communications, Inc. 655 Metro Place South, Suite 370 Dublin, Ohio 43017
EMail: karl@ascend.com Phone: +1 614 760 4041 Fax: +1 614 760 4050
Authors' Addresses
Larry J. Blunk Merit Network, Inc. 4251 Plymouth Rd., Suite C Ann Arbor, MI 48105
EMail: ljb@merit.edu Phone: 734-763-6056 FAX: 734-647-3185
John R. Vollbrecht Merit Network, Inc. 4251 Plymouth Rd., Suite C Ann Arbor, MI 48105
EMail: jrv@merit.edu Phone: +1-313-763-1206 FAX: +1-734-647-3185
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Full Copyright Statement
Copyright (C) The Internet Society (1998). All Rights Reserved.
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