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Dynamic Host Configuration WG                                   R. Pruss
Internet-Draft                                             Cisco Systems
Intended status: Informational                                   G. Zorn
Expires: November 19, 2008                                Aruba Networks
                                                             R. Maglione
                                                          Telecom Italia
                                                                   Y. Li
                                                     Huawei Technologies
                                                            May 18, 2008


 Authentication Extensions for the Dynamic Host Configuration Protocol
                      draft-pruss-dhcp-auth-dsl-03

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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   Drafts.

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   This Internet-Draft will expire on November 19, 2008.

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
   4.  Applicability Statement  . . . . . . . . . . . . . . . . . . .  6
   5.  Protocol Operation . . . . . . . . . . . . . . . . . . . . . .  6
     5.1.  Protocol Operation for IPv4  . . . . . . . . . . . . . . .  6
     5.2.  Protocol Operation for IPv6  . . . . . . . . . . . . . . . 11
   6.  DHCP Options . . . . . . . . . . . . . . . . . . . . . . . . . 15
     6.1.  DHCP Authentication Protocol Option  . . . . . . . . . . . 15
     6.2.  EAP-Message Option . . . . . . . . . . . . . . . . . . . . 16
   7.  Messages for EAP operation . . . . . . . . . . . . . . . . . . 16
   8.  Fragmentaion . . . . . . . . . . . . . . . . . . . . . . . . . 17
   9.  Backwards Compatibility Considerations . . . . . . . . . . . . 17
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 19
     10.1. Message Authentication . . . . . . . . . . . . . . . . . . 19
   11. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 19
   12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 20
   13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
     13.1. Normative References . . . . . . . . . . . . . . . . . . . 20
     13.2. Informative References . . . . . . . . . . . . . . . . . . 20
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 22
   Intellectual Property and Copyright Statements . . . . . . . . . . 24




















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1.  Introduction

   This document defines DHCP Options and procedures that allow for an
   Extensible Authentication Protocol (EAP) 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.


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.  These
   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
   example, is no longer of concern.  Neither are the multi-link bonding
   capabilities of PPP [RFC1990] commonly used on separate ISDN



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   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) [RFC0826], 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 changing 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
   different data plane encapsulations.




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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:


                                +-----------+  +------------+
                                |   DHCP    |  | AAA/RADIUS |
                                |  Server   |  |   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
      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).



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   o  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 may reside fully on the NAS.  The NAS
      obtains per-subscriber client configuration information either
      locally, relayed from a DHCP server or from the AAA infrastructure
      (which itself may consult external DHCP servers if necessary)
      after authentication is successfully completed.


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.  Earlier revisions of this document included a method
   to embed PPP CHAP [RFC1994] authentication as Options in existing
   DHCP messages.  This method has been abandoned due to security
   vulnerabilities in CHAP, as well as a lack of extensibility.  This
   document bases its authentication on EAP [RFC3748] which can be used
   with a large number of different authentication methods, including
   one backwards compatible with existing PPP CHAP.


5.  Protocol Operation

   This section describes the protocol operation for EAP within DHCPv4
   [RFC2131] and DHCPv6 [RFC3315].  Options and message specifications
   used in these operation descriptions are detailed in later sections.

   If multiple DHCP exchanges are occurring with multiple servers, both
   IPv4 and IPv6 each needs to authenticate separately.

5.1.  Protocol Operation for IPv4

   It is essential that the user/node authentication occurs before the
   assignment of an IP address and, further, that the assignment of the
   address depends upon the details of the successful authentication. .
   DHCP [RFC2131] is widely used as an address assignment method (among



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   other things); EAP [RFC3748] has been widely adapted for
   authentication purposes, especially in those types of networks where
   DHCP is also used.  This section describes how to combine the two in
   order to provide both strong authentication and authenticated address
   assignment in an efficient manner.

   Two new DHCPEAP messages are used in the DHCP message flow 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 EAP methods.  EAP is widely used in
   other environments, outside of DSL Broadband, including 802.11
   "Wi-Fi" access networks but could be used in future DSL Broadband
   deployments.

   To request the assignment of an IPv4 address with authentication, a
   client first locates a DHCP server, then authenticates using EAP and
   then requests the assignment of an address and other configuration
   information from the server.  The client sends a DHCP Discover
   message with an option specifying the authentication protocol as EAP
   to find an available DHCP server.  Any server that can that can
   authenticate and address it responds with a DHCPEAP-REQ message.

   Servers which support DHCP authentication will respond with a
   DHCPEAP-REQ message.  The client may receive one or more DHCPEAP-REQ
   messages from one or more DHCP Servers.  The Client chooses one to
   reply to, and sends a DHCPEAP-RES message, silently discarding
   DHCPEAP-REQ messages from other Servers.  The DHCPEAP-RES and
   DHCPEAP-REQ messages contain EAP packets which facilitate the EAP
   authentication exchange.  The exchange may occur between the DHCP
   Client and DHCP Server embedded within a NAS, or be carried
   transparently to the AAA Server.  Upon successful completion of the
   authentication phase, the DHCP server sends a DHCPOFFER with the
   appropriate IP configuration for the authenticated user.  The client
   then follows the normal DHCP procedures of a successful DHCP exchange
   by sending a DHCPREQUEST, followed by a DHCPACK from the Server.

   If the authentication phase fails (e.g., the user does not provide
   appropriate credentials), then according to configured policy the
   DHCP Client is either denied any IP configuration with the DHCP
   Server sending a DHCPNAK accordingly, or the DHCP Client is given a
   "limited access" configuration profile and the DHCP exchange
   continues as if the authentication was successful.

   A typical message flow proceeds as shown in Figure 2:





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

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

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

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

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

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

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

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

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

                                               <-------- RADIUS
                                   Access-Accept (w/EAP Message)
                                  (Access-Reject (w/EAP Message)
                                                if unsuccessful)
   (The last four messages repeat until EAP Success or EAP fail)

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

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

   DHCPREQUEST  ------->

                <------- DHCPACK

             Figure 2: DHCP Message Flow with DHCPEAP messages

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



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   The message exchange presented in the figure is an example of simple
   one-way user authentication, e.g. the Server verifies the credentials
   of the HGW Client.  The client indicates the ability to have an EAP
   exchange and the NAS (which takes on the EAP authenticator role)
   initiates the first EAP request to the DHCP Client (which takes on
   the EAP supplicant role).  DHCP-REQ and DHCP-RES does not suggest a
   coupling between the EAP state machine and the DHCP authentication
   phase state machine.  They only indicate the direction of the
   message, either from Client to Server or Server to Client.

   When the NAS is acting as a DHCP Relay the BRAS may split the EAP
   Messages from DHCP and perform the AAA authentication with an AAA
   server.  This allows use of existing DHCP servers and existing AAA
   servers.

   An example message flow for DHCP Relay proceeds as shown in Figure 3:



































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


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

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

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

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

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

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

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

                                               <-------- RADIUS
                                   Access-Accept (w/EAP Message)
                                  (Access-Reject (w/EAP Message)
                                                if unsuccessful)
    (The last four messages repeat until EAP Success or EAP fail)

              (DHCP messages continue normally from
              this point forward if successful)
                         DHCPDISCOVER ------------------------------>
                         (w/RADIUS attributes suboption)

                              <----------------------------- DHCPOFFER

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

   DHCPREQUEST  ------->

                <------- DHCPACK



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      Figure 3: DHCP Authentication Message Flow with DHCP relay NAS

   When the DHCP relay agent in the NAS receives a DHCP message from the
   client, it MAY append a DHCP Relay Agent Information option
   containing the RADIUS Attributes suboption, along with any other
   suboptions it is configured to supply.  The RADIUS Attributes
   suboption is defined in [RFC4014]

   DHCP Authentication uses two DHCP options:

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

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

5.2.  Protocol Operation for IPv6

   This section describes the protocol operation for extending Dynamic
   Host Configuration Protocol for IPv6 [RFC3315] for an EAP phase.

   The same as the previous section on extending DHCP in IPv4 new DHCP
   messages, DHCPEAP-REQ and DHCPEAP-RES are used to support EAP
   authentication before host configuration occurs.  The mechanisms
   described here follow a similar methodology as that for DHCPv4
   described in Section 5.1.

   The client sends a Solicit message with an Option specifying the
   session authentication protocol as EAP to the
   All_DHCP_Relay_Agents_and_Servers address to find available DHCP
   servers.  Any server that can authenticate and address it responds
   with a DHCPEAP-REQ message.

   The client may receive one or more DHCPEAP-REQ messages from one or
   more DHCP Servers.  The Client chooses one to reply to, and sends a
   DHCPEAP-RES message, silently discarding DHCPEAP-REQ messages from
   other Servers.  The DHCPEAP-RES and DHCPEAP-REQ messages contain EAP
   packets which facilitate the EAP authentication exchange.  The
   exchange may occur between the DHCP Client and DHCP Server embedded
   within a NAS, or be carried transparently to the AAA Server.  Upon
   successful completion of the authentication phase, the DHCP server
   sends a ADVERTISE with the appropriate configuration for the
   authenticated user.  The client then follows the normal DHCP
   procedures of a successful DHCP exchange by sending a REQUEST,
   followed by a DHCPACK from the Server.

   If the authentication phase fails (e.g., the user does not provide
   appropriate credentials), then according to configured policy the
   DHCP Client is either denied any IP configuration with the DHCP



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   Server sending a NAK accordingly, or the DHCP Client is given a
   "limited access" configuration profile and the DHCP exchange
   continues as if the authentication was successful.

   .  A typical message flow proceeds as shown in Figure 4:














































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

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

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

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

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

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

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

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

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

     (The last four messages repeat until EAP Success or EAP fail)

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

                <------- ADVERTISE (w/EAP Success Message)


   REQUEST  ------->

                <------- REPLY

                 Figure 4: DHCP IPv6 with DHCPEAP message

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



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   The message following this exchange is a ADVERTISE, sent unchanged by
   the Server.  A typical message flow proceeds as shown in Figure 5:


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


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

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

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

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

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

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

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

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

     (The last four messages repeat until EAP Success or EAP fail)

              (DHCP messages continue normally from
              this point forward if successful)
                         RELAY-FORW ------------------------------>
                         (w/RADIUS attributes suboption)

                              <-----------------------------  RELAY-REPL

                <------- ADVERTISE (w/EAP Success Message)



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   REQUEST  ------->

                <------- REPLY

         Figure 5: Message Flow with new message and a DHCP relay

   When the DHCP relay agent in the NAS receives a DHCP message from the
   client, it MAY append a DHCP Relay Agent Information option
   containing the RADIUS Attributes suboption, along with any other
   suboptions it is configured to supply.  The RADIUS Attributes
   suboption is defined in [RFC4014]

   DHCP Authentication uses two DHCP options:

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

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


6.  DHCP Options

   Two DHCP Options are defined in this section.  The first DHCP
   Authentication Protocol Option is originated from the client in the
   DHCPDISCOVER and SOLICIT to specify which authentication the client
   supports.

6.1.  DHCP Authentication Protocol Option

   The DHCPAUTH-Protocol option is sent from the DHCP Client to the DHCP
   Server to indicate the authentication algorithm the client prefers.


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

               Figure 6: DHCP Authentication Protocol Option

      DHCP Code: TBA-1 (DHCPAUTH-Protocol)

      Length: 3

      Authentication-Protocol





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         C227 (HEX) for Extensible Authentication Protocol (EAP)

      Algorithm

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

6.2.  EAP-Message Option

   The format of the EAP-Message option used in Protocol Operation for
   IPv4 is as follows:


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


                       Figure 7: 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.

   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.  Messages 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-REQ or DHCPEAP-
   RES 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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8.  Fragmentaion

   Encapsulating EAP messages within DHCP raises the question of whether
   there are potential difficulties with respect to the MTU sizes of the
   EAP and DHCP messages, as well as the underlying link MTU.

   EAP as defined in [RFC3748] Section 3.1 says:

   [4] Minimum MTU.  EAP is capable of functioning on lower layers that
   provide an EAP MTU size of 1020 octets or greater.

   DHCP as defined in [RFC2131] Section 2 says:

   ...  This requirement implies that a DHCP client must be prepared to
   receive a message of up to 576 octets, the minimum IP datagram size
   an IP host must be prepared to accept [3].  DHCP clients may
   negotiate the use of larger DHCP messages through the 'maximum DHCP
   message size' option.  The options field may be further extended into
   the 'file' and 'sname' fields.

   If we assume EAP MTU-sized packets, the overhead to pack an EAP
   packet into DHCP options is 2*(1020/255), or 8 octets.  Adding the
   DHCP header (240 octets), UDP (8 octets), and the IP header (20
   octets) gives 278 octets total overhead.  Since the Ethernet
   effective MTU is 1500 octets, this 278 octet overhead leaves the DHCP
   protocol with 1222 octets to carry EAP.  This space is over 200
   octets more than the EAP MTU of 1020 octets.

   If we add the SNAME and CHADDR fields to the option pool, then there
   are nearly 400 octets available for DHCP options in an Ethernet MTU-
   sized DHCP packet, encapsulating EAP.

   In short, when the 'maximum DHCP message size' option is used by the
   client, there is no problem carrying in EAP over DHCP. i.e. clients
   capable of performing EAP over DHCP should also advertise a maximum
   message that is capable of carrying EAP over DHCP.


9.  Backwards Compatibility Considerations

   This section is aimed at describing interoperability scenarios
   involving HGW and NAS with or without DHCP Authentication mechanism
   support in order to analyze compatibility issues that could be faced
   between newer and older products during the introduction of the DHCP
   Authentication functionally in current implemented network
   environments.

   Scenario 1: Both HGW and NAS do not support DHCP Authentication



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   In this case the authentication process does not start, thus
   traditional DHCP message flow applies.

   Scenario 2: HGW does not support DHCP Authentication and NAS supports
   DHCP Authentication

   In this case the DHCP client does not start DHCP Authentication
   transaction, NAS MAY decide to respond to HGW without using DHCP
   Authentication, falling back to traditional DHCP message flow and
   assigning different network resources.

   Scenario 3: HGW supports the DHCP Authentication and NAS does not
   support DHCP Authentication.

   In this case the DHCP client inserts in the DHCPDISCOVER message sent
   to NAS, the DHCP Authentication Protocol Option described in the
   draft in order to communicate the NAS that it is able to perform
   authentication and for indicating the authentication algorithm
   preferred by the client.  NAS on receiving a DHCPDISCOVER with
   unknown option silently discards unknown message.  Alternatively NAS
   MAY ignore the unknown option, but still process the message and then
   reply to the DHCP client with traditional response.  The HGW, that
   has upgraded software, realizes that the NAS does not support DHCP
   Authentication and can reverts back to normal DHCP message flow.

   Scenario 4 Both HGW and NAS support DHCP Authentication

   In this case DHCP client inserts in the DHCPDISCOVER message sent to
   NAS, the DHCP Authentication Protocol Option in order to communicate
   the NAS that it is able to perform authentication and for indicating
   the authentication algorithm preferred by the client, NAS replies
   according to the message flow described in this draft.

   The following table summarizes the behavior in the 4 described
   scenarios:
















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   +---------------+---------------+-----------------------------------+
   | DHCP Auth     | DHCP Auth     | Result                            |
   | support on    | support on    |                                   |
   | HGW           | NAS           |                                   |
   +---------------+---------------+-----------------------------------+
   | without       | without       | No Authentication                 |
   | support       | support       |                                   |
   | without       | with support  | Client does not start auth, thus  |
   | support       |               | no authentication transaction     |
   | with support  | without       | NAS silently discards unknown     |
   |               | support       | message/option                    |
   | with support  | with support  | Draft works as outlined           |
   +---------------+---------------+-----------------------------------+

                     Table 1: Compatibility Scenarios


10.  Security Considerations

10.1.  Message Authentication

   RFC 3118 provides a mechanism to cryptographically protect DHCP
   messages using a key, K, shared between a DHCP client and Server,
   however no mechanism is defined to manage these keys.  Authentication
   exchanges based on EAP have been built into authentication portions
   of network access protocols such as PPP, 802.1X, PANA, IKEv2, and now
   DHCP.  EAP methods may provide for the derivation of shared key
   material, the MSK and the EMSK, on the EAP peer and EAP server.  This
   dynamic key generation enables [RFC3118] protection and allows modes
   of operation where messages are protected from DHCP client to DHCP
   relay which previously would be difficult to manage.

   A future document will look at how to derive the key, K, from the
   EMSK resulting from an EAP exchange and at how this mechanism
   interacts with the DHCP authentication or any EAP authentication
   prior to DHCP.


11.  IANA Considerations

   This specification requires three values to be assigned by IANA.

   Two are "BOOTP Vendor Extensions and DHCP Options"








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      TBA-1:  (DHCPAUTH-Protocol)

      TBA-2:  (DHCPAUTH-Data)

   Two DHCP Message Type 53 Values - per [RFC2132], for DHCPEAP-REQ AND
   DHCPEAP-RES message types.


12.  Acknowledgements

   Many thanks to Carlos Pignataro for help editing this document.

   Thanks to Alan DeKok, Wojciech Dec, Eric Voit, Mark Townsley and
   Ralph Droms for help with this document.

   Thanks to Amy Zhao for her draft on DHCP Authentication and helping
   with laying the ground for this document.


13.  References

13.1.  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.

13.2.  Informative References

   [I-D.ietf-bfd-base]
              Katz, D. and D. Ward, "Bidirectional Forwarding
              Detection", draft-ietf-bfd-base-08 (work in progress),
              March 2008.

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

   [RFC0826]  Plummer, D., "Ethernet Address Resolution Protocol: Or
              converting network protocol addresses to 48.bit Ethernet
              address for transmission on Ethernet hardware", STD 37,
              RFC 826, November 1982.

   [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.



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

   [RFC2716]  Aboba, B. and D. Simon, "PPP EAP TLS Authentication
              Protocol", RFC 2716, October 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.

   [RFC3315]  Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
              and M. Carney, "Dynamic Host Configuration Protocol for
              IPv6 (DHCPv6)", RFC 3315, July 2003.

   [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.




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

   [RFC4014]  Droms, R. and J. Schnizlein, "Remote Authentication
              Dial-In User Service (RADIUS) Attributes Suboption for the
              Dynamic Host Configuration Protocol (DHCP) Relay Agent
              Information Option", RFC 4014, February 2005.

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

   [RFC4284]  Adrangi, F., Lortz, V., Bari, F., and P. Eronen, "Identity
              Selection Hints for the Extensible Authentication Protocol
              (EAP)", RFC 4284, January 2006.

   [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.


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












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   Glen Zorn
   Aruba Networks
   1322 Crossman Avenue
   Sunnyvale, CA  94089-1113
   USA

   Email: gwz@arubanetworks.com


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

   Phone: +39 0112285007
   Fax:
   Email: roberta.maglione@telecomitalia.it
   URI:


   Li Yizhou
   Huawei Technologies
   No. 91 Baixia Rd
   Nanjing,   210001
   China

   Phone: +86-25-84565471
   Fax:
   Email: liyizhou@huawei.com
   URI:




















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