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Dynamic Host Configuration WG                                   R. Pruss
Internet-Draft                                                   G. Zorn
Intended status: Informational                             Cisco Systems
Expires: November 20, 2007                                   R. Maglione
                                                          Telecom Italia
                                                            May 19, 2007


 Authentication Extensions for the Dynamic Host Configuration Protocol
                      draft-pruss-dhcp-auth-dsl-01

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on November 20, 2007.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   This document defines Dynamic Host Configuration Protocol (DHCP)
   extensions that provide for end-user authentication prior to
   configuration of the host.  The primary applicability is within a
   Digital Subscriber Line (DSL) Broadband network environment in order
   to enable a smooth migration from the Point to Point Protocol (PPP).




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Requirements Language

   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 [RFC2119].


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Problem Statement  . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Network Architecture and Terminology . . . . . . . . . . . . .  5
     3.1.  Where the DHCP Client and DHCP Server reside . . . . . . .  6
   4.  Applicability Statement  . . . . . . . . . . . . . . . . . . .  6
   5.  Protocol Operation Alternatives  . . . . . . . . . . . . . . .  6
     5.1.  Protocol Operation with Existing Messages  . . . . . . . .  7
     5.2.  Protocol Operation with New Message Type . . . . . . . . .  9
   6.  DHCP Options . . . . . . . . . . . . . . . . . . . . . . . . . 11
     6.1.  DHCP Authentication Protocol Option  . . . . . . . . . . . 11
     6.2.  CHAP Authentication Data Option  . . . . . . . . . . . . . 12
       6.2.1.  Challenge and Response . . . . . . . . . . . . . . . . 13
       6.2.2.  Success and Failure  . . . . . . . . . . . . . . . . . 14
     6.3.  EAP-Message Option . . . . . . . . . . . . . . . . . . . . 15
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 15
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 16
   9.  Message for EAP operation  . . . . . . . . . . . . . . . . . . 16
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 17
     11.2. Informative References . . . . . . . . . . . . . . . . . . 17
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 19
   Intellectual Property and Copyright Statements . . . . . . . . . . 20



















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1.  Introduction

   This document defines DHCP Options and procedures that allow for at
   least a Challenge-Handshake Authentication Protocol (CHAP)-based
   authentication exchange to occur in DHCP in order to enable smooth
   migration from Point-to-Point Protocol (PPP)[RFC1661] sessions to IP
   sessions in a DSL Broadband network environment.  Primary goals are
   integration of authentication in such a way that it will operate
   seamlessly with existing RADIUS-based Authentication, Authorization
   and Accounting (AAA) infrastructure and Asynchronous Transfer Mode
   (ATM) or Ethernet based DSL Networks.  As such, only the termination
   points of PPP in the DSL network are affected, both of which are
   devices that would logically need to be updated in any transition
   from PPP to IP sessions.

   It should be noted that [RFC3118] defines a mechanism that provides
   authentication of individual DHCP messages.  While this mechanism
   does provide a method of authentication for a DHCP Client based on a
   shared secret, it does not do so in a manner that can be seamlessly
   integrated with existing RADIUS-based AAA infrastructure built around
   PPP CHAP authentication models.


2.  Problem Statement

   Digital Subscriber Line (DSL) broadband service providers are
   witnessing a shift in the "last-mile" aggregation technologies and
   protocols which have traditionally been relied upon.  Two primary
   transitions are from ATM to Ethernet in the access network, and from
   the PPP for multi-protocol framing and dynamic endpoint configuration
   to direct encapsulation of IP and DHCP for dynamic endpoint
   configuration for some devices.  The term used by the DSL Forum for
   the network state associated with an authorized subscriber (that is
   using DHCP and IP rather than PPP) is "IP session" [WT-146].  While
   these trends can be readily witnessed, neither are occurring
   overnight.  In addition, they are not necessarily implemented in
   lock-step.  Thus, one may find ATM-based and Ethernet-based access
   networks running a combination of PPP sessions and IP sessions at any
   given time, particularly during transition periods.  This
   coexistences will even occur for the same service subscriber.

   Removing PPP, Point-to-Point Protocol over ATM (PPPoA) [RFC2364], and
   Point-to-Point Protocol over Ethernet (PPPoE) [RFC2516] from the
   subscriber access network is relatively straightforward in that most
   of the properties that DSL providers are interested in going forward
   are already present in DHCP and IP sessions.  Luckily, there are some
   capabilities of PPP which the market does not continue to demand.
   For example, the Dynamic configuration in PPP for IPX or NETBEUI, for



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   example, is no longer of concern.  Neither are the multi-link bonding
   capabilities of PPP [RFC1990] commonly used on separate ISDN
   B-channels, and the myriad of other features that PPP developed as
   the "dial-based" access protocol of choice for framing,
   authentication, and dynamic configuration for IP and other network
   layer protocols.  Missing from IP sessions and DHCP [RFC2131],
   however, are isomorphic methods for user authentication and session
   liveness probing (sometimes referred to as a session "keepalive").
   For the latter, existence of a client using a given IP address can be
   detected by a number of means, including Address Resolution Protocol
   (ARP) [RFC1433], ICMP Echo/Echo Response [RFC0792], or Bidirectional
   Forwarding Detection (BFD) [I-D.ietf-bfd-base].  This leaves
   authentication as an open issue needing resolution.  Specifically,
   authentication based on a username and secret password must be
   covered.  This is something that in PPP always occurs before dynamic
   configuration of an IP address and associated parameters.

   While most DSL deployments utilize a username and password to
   authenticate a subscriber and authorize access today, this is not the
   only method for authentication that has been adopted when moving to
   DHCP and IP sessions.  "Option 82" [RFC3046] is commonly used with
   DHCP as a credential to authenticate a given subscriber line and
   authorize service.  In this model, the DSL Access Node, which always
   sits between the DHCP Client and Server, snoops DHCP messages as they
   pass, and inserts pre-configured information for a given line (e.g.,
   an ATM VPI/VCI, Ethernet VLAN, or other tag).  That information,
   while provided in clear text, traverses what is considered a
   physically secured portion of the access network and is used to
   determine (typically via a request to an AAA server) whether the DHCP
   exchange can continue.  This fits quite well with current DSL network
   architecture, as long as the subscriber line itself is all that needs
   be authorized.  However, in some service models it is still necessary
   for the subscriber to provide credentials directly.

   From the perspective of the Network Access Server (NAS) where the
   DHCP Server resides, the extensions defined in this document are
   analogous to the commonly available "Option 82" method.  The primary
   difference between using Option 82 line configuration and a username
   and password is that the authentication credentials are provided by
   the subscriber rather than inserted by intervening network equipment.
   Providing credentials from the subscriber rather than intervening
   network equipment is particularly important for cases where
   subscriber line information is unavailable, untrusted, or due to the
   terms of the service may change at any time.  Further, different
   devices in the home may have different policies and require different
   credentials.  Migration scenarios where PPPoE and DHCP operate on the
   same network for a period of time lend well to models which utilize
   identical authentication and authorization credentials across the



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   different data plane encapsulations.


3.  Network Architecture and Terminology

   The DSL Forum defines its ATM-based network architecture in [TR-059]
   and Ethernet-based network architecture in [TR-101].  The extensions
   for DHCP defined in this document are designed to work identically on
   Ethernet or ATM architectures.  The diagram in Figure 1 and following
   terminology will be used throughout:


                                               +------------+
                                               | AAA/RADIUS |
                                               |   Server   |
                                               +------------+
                                                     |
                                                     |
    Sub.     +-----+   +--------+                 +-----+   +----------+
    Home  ---| HGW |---|        |                 |     |   |          |
   Network   +-----+   | Access |                 |     |   |          |
                       |  Node  |--/Aggregation\--| NAS |---| Internet |
    Sub.     +-----+   |        |--\  Network  /--|     |   |          |
    Home  ---| HGW |---|        |                 |     |   |          |
   Network   +-----+   +--------+                 +-----+   +----------+
                |                                     |
                |----------DSL Access Network --------|

                    Figure 1: DSL Network Architecture

   o  Access Node (AN): Network device, usually located at a service
      provider central office or street cabinet, that terminates Access
      Loop connections from Subscribers.  In case the Access Loop is a
      Digital Subscriber Line (DSL), this is often referred to as a DSL
      Access Multiplexer (DSLAM).  The AN may support one or more Access
      Loop technologies and allow them to inter-work with a common
      aggregation network technology.

   o  Network Access Server (NAS): Network device that aggregates
      multiplexed Subscriber traffic from a number of Access Nodes.  The
      NAS plays a central role in per-subscriber policy enforcement and
      QoS.  Often referred to as a Broadband Network Gateway (BNG) or
      Broadband Remote Access Server (BRAS).  A detailed definition of
      the NAS is given in [RFC2881].

   o  The Home Gateway (HGW) connects the different Customer Premises
      Equipment (CPE) to the Access Node and the access network.  In
      case of DSL, the HGW is a DSL Network Termination (NT) that could



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      either operate as a layer 2 bridge or as a layer 3 router.  In the
      latter case, such a device is also referred to as a Routing
      Gateway (RG).

3.1.  Where the DHCP Client and DHCP Server reside

   Referring to the DSL network architecture depicted in Figure 1, PPP
   (via PPPoA [RFC2364] or PPPoE [RFC2516]) operates over the DSL Access
   Network between the NAS and a device behind the HGW, or between the
   NAS and the HGW itself.  The DHCP Client resides either on a home
   network device or the HGW, and the DHCP Server protocol state machine
   resides fully on the NAS.  The NAS obtains per-subscriber client
   configuration information either locally or from the AAA
   infrastructure (which itself may consult external DHCP servers if
   necessary) after authentication is successfully completed.  The
   details of how the NAS obtains dynamic configuration to be offered to
   the DHCP Client are outside the scope of this specification.


4.  Applicability Statement

   The primary target for this extension is for DSL service provider
   networks where PPP is being phased out to be replaced by native IP
   and DHCP, or where new devices are being added which will not utilize
   PPP.  Very specific assumptions have been made with respect to the
   security model, operational methods, and integration requirements for
   existing AAA mechanisms during the design.  It is understood that
   this mechanism may not be generally applicable in this form for all
   network environments where DHCP is deployed, though perhaps elements
   of it may be used to develop a more generic approach while still
   meeting the specific requirements set out by the DSL network
   architecture.  For example, it is conceivable that EAP [RFC3748]
   could be carried in order to provide a more extensible and secure set
   of authentication methods.


5.  Protocol Operation Alternatives

   In this draft two alternatives for the protocol operation are offered
   for consideration.  The first in the section Protocol Operation with
   Existing Messages (Section 5.1), uses the existing message set, with
   small liberties taken on behavior.  It has the downside that reverse
   authentication and other authentication protocols like EAP are not
   possible in the constraints.  The second in the section Protocol
   Operation with New Messages (Section 5.2), extends the existing
   message set and should be read coupled with the section on Messages
   for operation choice with new messages (Section 9).  This alternative
   has the downside of requiring a new message type.



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   Both of these alternatives use new DHCP options, they both use DHCP
   Authentication Protocol Option from the client in the DHCPSDISCOVER
   to specify which authentication the client supports.

   Additionally alternative 1 uses CHAP Authentication Data Option to
   carry CHAP challenge and response data.

   Alternative 2 uses EAP-Message Option to carry EAP messages.

5.1.  Protocol Operation with Existing Messages

   In order to integrate with RADIUS CHAP, it is necessary to send a
   CHAP Challenge to the DHCP Client, hash that Challenge along with an
   Identifier (ID), Name, and Secret, and return the result in a CHAP
   Response to the NAS.  The NAS, in turn, must send the CHAP Name, ID,
   Challenge and Response to the RADIUS server to verify the
   credentials.  The Secret is never sent in the clear, and is known
   only to the AAA server and DHCP Client.  The Client's configuration
   information (IP address, etc) is typically returned in a successful
   response from the AAA server, which is used to construct the DHCP
   OFFER.  A typical message flow proceeds as shown in Figure 2:






























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           (HGW)                (NAS)                   (AAA)
        DHCP Client          DHCP Server/            RADIUS Server
                             AAA Client

       DHCPDISCOVER ------->
       (w/DHCP-auth-proto chap)

                    <------- DHCPOFFER
                             (w/CHAP Challenge, Name)

       DHCPREQUEST ------->
       (w/CHAP Response, Name, ID)

                             RADIUS Access-Request ------->
                             (w/CHAP Name, ID, Challenge,
                              Response)

                                                   <-------- RADIUS
                                                      Access-Accept
                                                     (Access-Reject
                                                    if unsuccessful)

                    <------- DHCPACK
                             (w/yiaddr)

       or
                   <------- DHCPNAK
                            (w/CHAP Failure if unsuccessful)


                      Figure 2: Typical Message Flow

   It is necessary to "restart" the state machine via the DHCPNAK with
   CHAP Failure option if the Access-Reject is received.

   Note that the configuration information like IP address is not in the
   DHCPOFFER but only in the DHCPACK.

   This alternative uses two DHCP options:

   o  DHCP Authentication Protocol Option in the DHCPDISCOVER to specify
      the type of authentication exchange.

   o  CHAP Authentication Data Option to carry the CHAP Challenge and
      Response in the DHCPOFFER and DHCPREQUEST respectively.






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5.2.  Protocol Operation with New Message Type

   It is desirable that user/node authentication occurs before the
   assignment of an IP address and, further, that the assignment of the
   address depend upon details of the successful authentication.  DHCP
   [RFC2131] is widely used as an address assignment method (among other
   things); EAP [RFC3748] has been widely adapted for authentication
   purposes, especially in those types of networks where DHCP is also
   used.  It seems to make some sense, then, to combine the two in order
   to provide both strong authentication and authenticated address
   assignment in an efficient manner.

   This alternative has the advantage that it supports authentication
   methods other than CHAP and allows mutual authentication.  A new DHCP
   message, DHCPEAP is used to support the new EAP phase which occurs
   before a DHCPOFFER is sent by the Server.

   This message is used to integrate authentication methods supported by
   EAP, including CHAP and any other "in the clear" password mechanisms
   (for example, to support One-Time-Password mechanisms), or to carry
   other types of authentication, unlike DSL Broadband, is widely used
   in other environments, including 802.11 "Wi-Fi" access networks but
   could be used in future DSL Broadband deployments.

   The message following this exchange is a DHCP Offer, sent unchanged
   by the Server.  A typical message flow proceeds as shown in Figure 3:

























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           (HGW)                (NAS)                   (AAA)
        DHCP Client          DHCP Server/            RADIUS Server
                             AAA Client

       DHCPDISCOVER ------->
       (w/DHCP-auth-proto EAP)

                    <------- DHCPEAP
                             (w/EAP Message)

       DHCPEAP ------->
       (w/EAP Message)

                             RADIUS Access-Request ------->
                             (w/EAP Message)

                                                   <-------- RADIUS
                                       Access-Accept (w/EAP Message)
                                      (Access-Reject (w/EAP Message)
                                                    if unsuccessful)

                  (DHCP messages continue normally from
                  this point forward if successful)

                    <------- DHCPOFFER (w/EAP Success Message)
                             (w/yiaddr)

       DHCPREQUEST  ------->

                    <------- DHCPACK

              Figure 3: Typical Message Flow with new message

   The retransmittion is handled by EAP as per Section 4.1 in [RFC3748].

   The message exchange presented in the figure offers simply one-way
   user authentication, e.g. the Server verifies the identity of the
   User at the HGW Client.  It is assumed that the NAS and ethernet
   network restricts packets sent to the HGW, providing some trust that
   the DHCPAuth messages received by the client are not from a rogue
   source.  In situations where this is not the case, the same set of
   messages may be sent in reverse for mutual authentication.

   This alternative uses two DHCP options:

   o  DHCP Authentication Protocol Option in the DHCPDISCOVER to specify
      the type of authentication exchange.




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   o  EAP-Message Option to carry the EAP data in the DHCPEAP message.


6.  DHCP Options

   Three new DHCP Options are defined in this section.  The first DHCP
   Authentication Protocol Option is orginated from the client in the
   DHCPSDISCOVER to specify which authentication the client supports.

   CHAP Authentication Data Option to carry CHAP challenge and response
   data.  This is modeled strictly on PPP CHAP [RFC1994] with text
   lifted liberally from its specification.

   Protocol Operation with New Message Type uses EAP-Message Option to
   carry EAP messages.

6.1.  DHCP Authentication Protocol Option

   The following diagram defines the format of the DHCPAUTH-Protocol
   option, which is sent from the DHCP Client to the DHCP Server to
   indicate the authentication algorithm the client prefers.  This
   option MUST be sent in the DHCPDISCOVER if the Client supports DHCP
   Authentication.


     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   DHCP Code   |    Length     |     Authentication-Protocol   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Algorithm   |
     +-+-+-+-+-+-+-+-+

               Figure 4: DHCP Authentication Protocol Option

      DHCP Code: TBA-1 (DHCPAUTH-Protocol)

      Length: 3

      Authentication-Protocol

         C223 (HEX) for Challenge-Handshake Authentication Protocol.

         C227 (HEX) for Extensible Authentication Protocol (EAP)

      Algorithm

         The Algorithm field is one octet and indicates the
         authentication method to be used with CHAP.




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         5 CHAP with MD5

6.2.  CHAP Authentication Data Option

   This option is used between the Client and DHCP Server to exchange
   CHAP authentication Data in OFFER, REQUEST, and NAK messages.  This
   is used in Section 5.1.


     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   DHCP Code   |  Length     |  CHAP Code   |   Identifier     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Data ....
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 5: CHAP Authentication Data Option

      DHCP Code: TBA-2 (DHCPAUTH-Data)

      CHAP Code

         The Code field is one octet and identifies the type of CHAP
         option and format of data that follow.  CHAP Codes are assigned
         as follows:

         1 Challenge

         2 Response

         3 Success

         4 Failure

      Identifier

         The Identifier field is one octet and aids in matching
         challenges, responses and replies.

      Length

         The Length field is one octet and indicates the length of the
         Data plus 2 (the length of CHAP Code plus Identifier).

      Data

         The Data field is zero or more octets.  The format of the Data
         field is determined by the CHAP Code field.




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6.2.1.  Challenge and Response

   A summary of the Challenge and Response packet format is shown below.
   The DHCP Code and Length are described above and repeated here only
   for reference.


     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   DHCP Code   |  Length     |  CHAP Code   |   Identifier     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Value-Size   |  Value ...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Name ...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 6: Challenge and Response

      CHAP Code

         1 for Challenge;

         2 for Response.

      Identifier

         The Identifier field is one octet.  The Identifier field MUST
         be changed each time a Challenge is sent.

         The Response Identifier MUST be copied from the Identifier
         field of the Challenge which caused the Response.

      Value-Size

         This field is one octet and indicates the length of the Value
         field.

      Value

         The Value field is one or more octets.

         The Challenge Value is a variable stream of octets.  The
         importance of the uniqueness of the Challenge Value and its
         relationship to the secret is described above.  The Challenge
         Value MUST be changed each time a Challenge is sent.  The
         length of the Challenge Value depends upon the method used to
         generate the octets, and is independent of the hash algorithm
         used.




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         The Response Value is the one-way hash calculated over a stream
         of octets consisting of the Identifier, followed by
         (concatenated with) the "secret", followed by (concatenated
         with) the Challenge Value.  The length of the Response Value
         depends upon the hash algorithm used.

      Name

         The Name field is one or more octets representing the
         identification of the system transmitting the packet.  There
         are no limitations on the content of this field.  For example,
         it MAY contain ASCII character strings or globally unique
         identifiers in ASN.1 syntax.  The Name should not be NUL or
         CR/LF terminated.  The size is determined from the Length and
         Value-Size fields.

6.2.2.  Success and Failure

   If Authentication is successful, the DHCP Server SHOULD transmit the
   subsequent DHCPACK including a DHCPAUTH-Data Option with the CHAP
   Code set to 3 (Success) and the confiuration Options.

   If Authentication is unsuccessful, the DHCP Server SHOULD transmit a
   DHCPNAK including a DHCPAUTH-Data Option with the CHAP Code set to 4
   (Failure).

   A summary of the Success and Failure packet format is shown below.
   The DHCP Code and Length are described above and repeated here only
   for reference.


     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   DHCP Code   |  Length     |  CHAP Code   |   Identifier     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Message....
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 7: Success and Failure

      Code

         3 for Success;

         4 for Failure.

      Identifier





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         The Identifier field is one octet and aids in matching requests
         and replies.  The Identifier field MUST be copied from the
         Identifier field of the Response which caused this reply.

6.3.  EAP-Message Option

   The format of the EAP-Message option used in Protocol Operation with
   New Message Type is as follows:


     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   DHCP Code   |  Length     |  m...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                       Figure 8: EAP-Message Option

   The maximum size of a DHCP option is 255 octets.  While in some cases
   (e.g., EAP MD5-Challenge [RFC3748]) a complete EAP message may fit in
   a single DHCP option, in general this is not the case.  If an EAP
   message is too large to fit into a single DHCP option, the method
   defined in [RFC3396] MUST be used to split the EAP message into
   separate options for transmission.  Similarly, EAP assumes a minimum
   MTU of 1020 octets while the minimum DHCP packet size is 576 octets,
   including 312 octets reserved for options.  A DHCP client including
   the EAP-Message option SHOULD also include the 'maximum DHCP message
   size' option [RFC2132] to set a suitable DHCP message size.  It is
   RECOMMENDED that the maximum DHCP message size be set to at least
   1604 for use with the EAP-Message option.

   If a DHCP message is received containing more than one EAP-Message
   option, the method defined in [RFC3396] MUST be used to reassemble
   the separate options into the original EAP message.  A DHCP server
   receiving an EAP message MAY forward it via a AAA protocol (such as
   RADIUS [RFC2865] [RFC3579] or Diameter [RFC3588]] [RFC4072]).


7.  Security Considerations

   The DHCP authentication mechanism described is optimal in catering to
   a unilateral authentication of the client to the NAS, with the
   authentication based on a one-way hash algorithm used in CHAP.  In
   general, it claims to be no better than the existing security offered
   by PPPoE [RFC2516].  Given the assumption that the DSL Aggregation
   Network is trusted, and that direct Layer 2 subscriber-subscriber
   communication restricted to only occur via the NAS, this mechanism
   makes no attempt at protecting from man in the middle attacks.
   However, unlike with PPP, IP DHCP packets including DHCPAUTH messages



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   may be freely routed across multiple IP hops.  It is thus conceivable
   that a remote rogue user may try to exploit known hash weaknesses to
   derive the secret key of another user through the use of a brute
   force attack.  To keep the relative security equivalent in threat to
   today's PPPoE environment, the DHCPAUTH traffic must be constrained
   by the access network.  The simplest mechanism for doing this is by
   blocking any DHCP traffic from traversing the NAS.  In common DSL
   deployments the access-node provides protection against direct
   communication between HGW at Layer 2 as well as IP/MAC address
   spoofing based on snooped DHCP messages sent to the HGW client from
   the NAS.

   The vast majority of PPPoE and PPPoA wire-line deployments utilize
   one-way authentication.  The mechanism defined in Protocol Operation
   with Existing Messages is strictly one-way.  Mutual authentication is
   possible with the mechanism in Protocol Operation with New Message
   Type.


8.  IANA Considerations

   This specification requires three values to be assigned by IANA.

   Two are "BOOTP Vendor Extensions and DHCP Options"



      TBA-1:  (DHCPAUTH-Protocol)

      TBA-2:  (DHCPAUTH-Data)

   One is a DHCP Message Type 53 Values - per [RFC2132], for DHCPEAP
   message type.


9.  Message for EAP operation

   The DHCPEAP messages follow the format for DHCP messages defined in
   RFC 2131 [RFC2131].  This new message is identified by the presence
   of a DHCP Message Type option, which encodes DHCPEAP message type.
   Other fields in the DHCP message header, such as siaddr and fname,
   are left unused.

   The authentication data in a DHCPAUTH message is carried in a EAP-
   Messsage option EAP-Message Option.






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10.  Acknowledgements

   Many thanks to Carlos Pignataro for help editing this document.

   Thanks Woj Dec, Eric Voit, Mark Townsley and Ralph Droms for help
   with this document.


11.  References

11.1.  Normative References

   [RFC1994]  Simpson, W., "PPP Challenge Handshake Authentication
              Protocol (CHAP)", RFC 1994, August 1996.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

11.2.  Informative References

   [I-D.ietf-bfd-base]
              Katz, D. and D. Ward, "Bidirectional Forwarding
              Detection", draft-ietf-bfd-base-05 (work in progress),
              June 2006.

   [RFC0792]  Postel, J., "Internet Control Message Protocol", STD 5,
              RFC 792, September 1981.

   [RFC1433]  Garrett, J., Hagan, J., and J. Wong, "Directed ARP",
              RFC 1433, March 1993.

   [RFC1661]  Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51,
              RFC 1661, July 1994.

   [RFC1990]  Sklower, K., Lloyd, B., McGregor, G., Carr, D., and T.
              Coradetti, "The PPP Multilink Protocol (MP)", RFC 1990,
              August 1996.

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol",
              RFC 2131, March 1997.

   [RFC2132]  Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
              Extensions", RFC 2132, March 1997.

   [RFC2364]  Gross, G., Kaycee, M., Lin, A., Malis, A., and J.
              Stephens, "PPP Over AAL5", RFC 2364, July 1998.

   [RFC2516]  Mamakos, L., Lidl, K., Evarts, J., Carrel, D., Simone, D.,



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              and R. Wheeler, "A Method for Transmitting PPP Over
              Ethernet (PPPoE)", RFC 2516, February 1999.

   [RFC2865]  Rigney, C., Willens, S., Rubens, A., and W. Simpson,
              "Remote Authentication Dial In User Service (RADIUS)",
              RFC 2865, June 2000.

   [RFC2881]  Mitton, D. and M. Beadles, "Network Access Server
              Requirements Next Generation (NASREQNG) NAS Model",
              RFC 2881, July 2000.

   [RFC3046]  Patrick, M., "DHCP Relay Agent Information Option",
              RFC 3046, January 2001.

   [RFC3118]  Droms, R. and W. Arbaugh, "Authentication for DHCP
              Messages", RFC 3118, June 2001.

   [RFC3396]  Lemon, T. and S. Cheshire, "Encoding Long Options in the
              Dynamic Host Configuration Protocol (DHCPv4)", RFC 3396,
              November 2002.

   [RFC3579]  Aboba, B. and P. Calhoun, "RADIUS (Remote Authentication
              Dial In User Service) Support For Extensible
              Authentication Protocol (EAP)", RFC 3579, September 2003.

   [RFC3588]  Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J.
              Arkko, "Diameter Base Protocol", RFC 3588, September 2003.

   [RFC3748]  Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
              Levkowetz, "Extensible Authentication Protocol (EAP)",
              RFC 3748, June 2004.

   [RFC4072]  Eronen, P., Hiller, T., and G. Zorn, "Diameter Extensible
              Authentication Protocol (EAP) Application", RFC 4072,
              August 2005.

   [TR-059]   DSL Forum, "DSL Evolution - Architecture Requirements for
              the Support of QoS-Enabled IP Services", TR 059,
              September 2003.

   [TR-101]   DSL Forum, "Migration to Ethernet Based DSL Aggregation",
              TR 101, April 2006.

   [WT-146]   DSL Forum, "Internet Protocol (IP) Sessions", WT 146 (work
              in progress), April 2007.






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Authors' Addresses

   Richard Pruss
   Cisco Systems
   80 Albert Street
   Brisbane, Queensland  4000
   Australia

   Phone: +61 7 3238 8228
   Fax:   +61 7 3211 3889
   Email: ric@cisco.com


   Glenn Zorn
   Cisco Systems
   2901 Third Avenue
   Seattle, WASHINGTON  98121
   United States

   Phone: +1 206-256-3479
   Fax:   +1 425 740 0168
   Email: gwz@cisco.com


   Roberta Maglione
   Telecom Italia
   Via G. Reiss Romoli 274
   Torino,   10148
   Italy

   Phone:
   Fax:
   Email: roberta.maglione@telecomitalia.it
   URI:

















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Full Copyright Statement

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