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draft-mukherjee-dhc-dhcproam-01.txt                        B. Mukherjee
Internet Draft                                                  B. Gage
                                                                 Y. Liu
                                                        Nortel Networks

                                                          February 2001



                  Extensions to DHCP for Roaming Users
                  <draft-mukherjee-dhc-dhcproam-01.txt>

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026 [1].

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six
   months and may be updated, replaced, or obsoleted by other documents
   at any time. It is inappropriate to use Internet- Drafts as
   reference material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt
   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

1. Abstract

   This draft defines enhancements to DHCP so that it can be used by
   access networks as an initial configuration protocol for nomadic
   users.  The authentication mechanism described here interacts with
   existing public Authentication, Authorization, and Accounting (AAA)
   mechanisms, thus enabling per customer authentication and
   authorization across multiple domains. In addition, we describe a
   mechanism that enables the client to authenticate the network to
   prevent attacks on an end host from a bogus network access point.

2. Conventions used in this document

   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 [2].


3. Introduction



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   Providing authenticated IP access to subscribers across
   administrative domains is expected to be a vital functionality of
   next generation access networks. Such access networks are expected
   to perform authentication, authorization, accounting (AAA) [3] in
   addition to IP parameter configuration for its subscribers. The
   mechanisms described here address the requirements for accessing
   these networks in a flexible and extensible manner using the
   existing AAA infrastructure to perform the actual authentication.
   Similar motivations were expressed in proposing requirements for
   extending DHCP to new environments [4]. We expect that the proposed
   mechanisms will allow DHCP to be used as a secure yet easy to
   administer initial configuration protocol in commercial access
   networks as opposed to its present usage as means of configuring a
   pool of trusted hosts in a LAN.


 3.2. Overview

   This document defines mechanisms for authentication in access
   networks by means of DHCP. In order to be authorized for using the
   access network, the user must submit credentials via DHCP
   authentication messages. In response, the access network may
   indicate that a user has been authorized by providing configuration
   parameters to the user's mobile host. The user may also verify the
   access network's credentials as a part of the process. The
   authentication messages also provide a framework for negotiating
   other parameters, some of which may aid in enhancing the security of
   the system.

   Any proposed security mechanism should allow flexible authentication
   between the network and its subscribers with scope for negotiation
   about the mechanisms to be used and the type of ciphers (e.g., ssl
   handshakes [5]). This is because the networks and their users may
   support only a few of the possible wide range of security mechanisms
   available. This may be due to internal constraints of the hosts
   (e.g., CPU cycles may be a bottleneck in case of a mobile user) or
   the security level the hosts desire (e.g., the network may only
   allow users that perform per message authentication). Furthermore no
   assumptions can be made about the local security requirements of
   visited domains. Thus a flexible mechanism that allows the security
   parameters to be negotiated and established is desirable. The
   security mechanisms defined here can be used to create trust
   relationships between the network and the client that may be used
   then for accountable usage of the network resources.

4. Network Model

   Throughout this document we use the example of an access network in
   order to demonstrate how the authentication additions in DHCP may be
   of use in new environments. The model of the network used by this
   draft is depicted in the Figure 1 below. This is similar to the
   model described in the draft on AAA requirements for DHCP [3]. The

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   model assumes that the users may roam and thus their initial
   interaction with the accessed network is overseen by a public AAA
   mechanism. The public AAA infrastructure consists of local AAA
   authorities in each of the access networks. These local AAA servers
   communicate among each other through a hierarchy of public AAA
   servers, and security associations exist between the local AAA
   server and the DHCP servers. The public AAA servers have inter
   domain security associations through the public AAA servers that
   allow them to authenticate users from visited domains. Thus upon
   presenting the right information a user can authenticate himself to
   a visited network, obtain the right levels of authorization and at
   the same time be accountable for the use of the network. This model
   allows the network access provider to leverage the existing AAA
   mechanisms to perform user authentication and accounting in a
   scalable manner across domains instead of using localized
   mechanisms.






        Subscriber      Visited Domain          Home Domain
                         +-------------+        +-------------+
                         |  +------+   |        |  +------+   |
                         |  | AAAL |   |        |  | AAAL |   |
                         |  |      +---+--------+--+      |   |
                         |  +---+--+   |        |  +------+   |
                         |      |      |        |             |
                         |      |      |        +-------------+
                         |      |      |
                         |      |      |
        +-------+   +------+ +--+---+  |
        |DHCP   |   |DHCP  | |DHCP  |  |
        |Client |---+Relay +-+Server|  |
        +-------+   +------+ +------+  |
                         |             |      AAAL =  local authority
                         +-------------+

               Figure 1: DHCP's interaction with AAA


5. Related Work

   The DHCP working group of IETF has in the past expressed the need
   for DHCP to support AAA functions [3] and be extensible to different
   environments than ethernet LANs [4]. These are the principal
   motivations behind our work.

5.1 Requirements for using DHCP in new environments



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   The draft [3] had proposed that DHCP use the public AAA server
   infrastructure to perform AAA for roaming nodes and had set out the
   requirements for the same. Leveraging the AAA infrastructure
   provides a network service provider with a scalable way of ensuring
   access security because it does not require every DHCP server to
   have a pre-established security association with every DHCP client
   it may ever talk to. Using the public AAA infrastructure, the DHCP
   servers will be able to provide access to nodes from visited domains
   and bill them through their local service providers. Most major ISPs
   presently use AAA servers which support RADIUS or TACACS+ for
   customer accounting [6]. In this draft we describe mechanisms that
   allow DHCP to extensibly interface with this AAA infrastructure thus
   meeting the requirements set out by the requirements draft [3].

5.2 Mobile IP and PPP

   There are other commonly used protocols like PPP and mobile IP,
   which are used for related functions. We contend that none of the
   above protocols are as suited for initial registration and
   configuration as DHCP. For instance, PPP's registration model
   assumes a point-to-point connection and is requires explicit link
   configuration before entering into IP authentication and
   configuration. This leads to considerable delay and overhead in
   providing access to the mobile host. Moreover when the host roams
   the physical layer parameters may change and may cause PPP to
   restart configuration to reinitialize its link layer. Mobile IP
   based registration depends on the availability of the 'home-agent'
   and is tied in to that particular mobility solution. New generations
   of wireless networks may use and deploy several other types of
   mobility solutions. Thus there exists the need for a general-purpose
   protocol for registration and configuration that is not tied in to
   other functions or environments. DHCP is a natural choice because it
   exclusively deals with registration and configuration of nodes,
   independent of other functions being handled in a particular
   scenario. The draft [4] argues in favor of using DHCP as a general-
   purpose registration and configuration mechanism. In our proposal we
   address several of the requirements listed in this draft[4].

5.2. DHCP message authentication option.

   The DHC WG has produced a draft that describes an authentication
   method for DHCP messages [7]. This involves a replay detection
   method as well as a keyed hash that allows the receiver of the
   message to authenticate the source. The mechanism relies on a shared
   secret between the two negotiating authorities. For the case of
   servers providing service to local clients, the above mechanism may
   be sufficient. But in the more general case of the clients roaming
   across domains it is not possible to create all the possible key
   associations before hand. Possible solutions to this problem were
   discussed in the DHC WG meeting on June 1998. It may be possible to
   use the public key infrastructure to authenticate the client and the
   server and then use Diffie-Hellman to generate a shared secret that

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   can then be used to authenticate messages. But a potential problem
   is that an uninitialized client may not be able to contact a trusted
   PKI node, resulting in an asymmetrical security relationship against
   the client. For instance, the server's certificate may have been
   revoked by the PKI authority and the client has no means of finding
   that out. In this document, the authentication mechanism leverages
   the AAA infrastructure to not only authenticate the client and the
   server in a symmetric fashion, but also to allow the network to
   initiate the AAA functions at the same time. In addition, the extra
   set of messages and the state allow the use of additional security
   mechanisms, like re-keying, that cannot be completed by piggybacking
   on the present set of DHCP messages. Another proposal was to create
   a IPSEC security association between the client and the server.
   Setting up a valid security association with an uninitialized client
   may not be possible without a valid IP address and a configured
   stack.

5.3. Dynamic Registration and Configuration Protocol

   Another protocol, called Dynamic Registration and Configuration
   Protocol (DRCP)[8], was proposed in order to address the need for
   new features in a registration and configuration framework like
   DHCP. However DRCP requires every client to be a router. The DRCP
   protocol allows the servers to send advertisement messages that
   allow the clients to send discover messages to the servers using
   unicast. The protocol however does not specify any mechanisms for
   performing AAA functions or security mechanisms.

6. DHCP with Extensible Authentication

   Adding authentication to DHCP allows the client and the server to
   establish mutual trust before configuration is effected. From the
   perspective of an access network, authentication mechanisms in DHCP
   allow easy configuration of subscriber devices as well as protection
   against certain theft of service attacks. A simple approach to
   prevent unauthorized access is that the configuration of the mobile
   host be completed only after the user is authenticated. The
   underlying assumption being that the configuration step is the one
   that allows a client to act as a fully functional host and access
   the network and its resources, and if this step fails to complete
   the mobile will be incapable of using the network. The threat
   scenario that this addresses is when a user attempts to access the
   IP services of the network without presenting valid credentials
   (e.g., user-id or password).

   However the authentication mechanism used by the access networks
   must be scalable as each of the access networks are expected to be
   extensive. Thus pre-establishing security association between every
   client and the DHCP servers[9] in the access network may not be a
   viable option. In addition, the home and visited systems are
   expected to interact with each other in order to provide access to
   roaming users of other access networks. This would mean that for

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   visiting users the access network would need a mechanism to create a
   security association between the users home domain and itself.

   From the perspective of the subscriber, DHCP should allow access
   across different service provider domains. From the perspective of
   the service provider there is a need for an authentication mechanism
   in order to provide service to subscribers roaming from other
   networks.

   This document describes an optional extension to the DHCP protocol
   to include an authentication phase and to add authentication
   messages to DHCP. To support this option, the DHCP client must be
   modified to include a new state and to send new messages and options
   for authentication. The DHCP relay agent may remain the same, and
   does not need to be altered for the authentication phase. The DHCP
   server must also be modified to process the DHCP authentication
   messages and to forward the required messages to the AAA components.

6.1. Authentication Messages and Options

   This option adds two new authentication messages to the set of
   existing DHCP messages:

   DHCPAUTHREQ  Request for authentication information (e.g. request
                for challenge response).
   DHCPAUTHRESP Response to a DHCPAUTHREQ

   The value of message type for new messages is left as TBD until
   assigned by IANA.

   The messages are generic in nature and thus allow the DHCP server
   and client to establish the required credentials using any
   authentication protocol they predetermine or negotiate. The
   authentication phase may be completely skipped to remain backward
   compatible but the DHCP server may enforce a policy that makes
   authentication mandatory for certain (groups of) mobiles. The new
   messages function as means of transporting authentication
   information to and from the network's AAA mechanisms. In doing so we
   are able to leverage the existing AAA infrastructure to perform
   inter-domain authentication and authorization.

   This document defines new message types for authentication, as
   opposed to piggybacking security information upon existing messages,
   in order to make the process flexible and extensible. For example,
   there are instances like the challenge-response protocols where the
   present sequences of DHCP messages are unable to fulfill the
   functionality. In addition challenges for re-keying in mid-session
   cannot be fitted in the existing messages without causing the client
   to restart the configuration procedure. The separate messages also
   allow the authentication protocol to be symmetrical, allowing the
   server to authenticate the client and the client, if necessary, to
   authenticate the server. The need to be extensible was also

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   propounded in other initial access protocols like PPP, which itself
   has an extensible authentication protocol [10].

   Several new options are defined to carry authentication information
   within the DHCPAUTHREQ and DHCPAUTHRESP messages as well as in other
   DHCP messages like DHCPREQUEST and DHCPACK. The client uses the
   DHCPREQUEST message to present its Network Access Identifier
   (NAI)[11] and to request the use of an (or possibly a set of)
   authentication mechanism by setting the appropriate option fields.
   The server can then query its local AAA mechanism about the security
   parameters supported for the given NAI. The DHCP server then can use
   the authentication request message (DHCPATHREQ) to ask the client to
   present it with authentication information that will be relayed on
   to the AAA mechanism. The option fields are thus used to indicate
   the choice when negotiating and then to carry actual data during the
   security exchange. The authentication mechanisms may be a challenge
   response handshake protocol (CHAP) [12], digital signature, plain
   password etc. The current set of option types that may be used are
   listed below:

               Option #                    Option Type
               ----------------------------------------
                 TBD                        NAI
                 TBD                        PAP
                 TBD                        CHAP
                 TBD                        Signature

                Table 1: Authentication Options

6.2 The DHCP Server

   The DHCP server model is relatively simple as compared to [7], as it
   need not manage and process keys or any client specific
   authentication information by itself. The DHCP server receives the
   user identification (e.g. NAI) from the client in a DHCPREQUEST; if
   the client is supporting a password protocol (e.g. PAP), the
   DHCPREQUEST may also include the password. The DHCP server then
   contacts the local AAA server (e.g. using RADIUS) with the DHCP
   client information and asks the AAA server if it can configure the
   client. For a network that uses a challenge response protocol for
   authentication, the server issues the challenge with a DHCPAUTHREQ
   message, receives the challenge response from the client through a
   DHCPAUTHRESP message and forwards the response to the AAA server for
   client authentication.

   If the client chooses to authenticate the network, the DHCP server
   also needs to respond to DHCPAUTHREQ messages with a DHCPAUTHRESP
   message for the client. If a challenge-response protocol is used by
   the client for network authentication, the DHCP server must ask the
   AAA server to create a response for the challenge. Extensions to
   existing AAA protocols to support this function are beyond the scope
   of this document.

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6.3. The DHCP Client

   The DHCP client needs to be modified to incorporate the optional
   authentication mechanisms. First, it requires a new state in the
   client's state machine called AUTHENTICATING. This state is entered
   upon the receipt of a DHCPAUTHREQ message or when the client sends
   out a DHCPAUTHREQ message. The client MUST respond to a DHCPAUTHREQ
   message received when it is in the states REBOOTING, REQUESTING,
   RENEWING, REBINDING and AUTHENTICATING. The client MAY send
   DHCPAUTHREQ when it is in the BOUND or AUTHENTICATING state.

   The client MUST respond to the DHCPAUTHREQ message with the message
   DHCPAUTHRESP. Both of these messages carry authentication options as
   described above. If the server had issued the challenge, the client
   MUST depart from the AUTHENTICATING state upon the receipt of a
   DHCPACK or a DHCPNACK message. The client MUST return to the bound
   state if the server responds with a DHCPACK but if the message
   received is DHCPNAK it MUST go back to the INIT state.

   If the client had sent the DHCPAUTHREQ, the client MUST leave the
   AUTHENTICATING state and enter the BOUND state when a valid
   DHCPAUTHRES is received. If a valid DHCPAUTHRESP is not received,
   the client MUST enter the REBIND state and obtain new configuration
   parameters. The lease timers MUST be set as per the security
   requirements such that the server and the client cannot delay the
   response to the AUTHREQ messages indefinitely. Upon the expiry of
   the T1 and T2 timers the authenticating client MUST enter the
   REBINDING and RENEWING state respectively. The remaining state
   transitions are the same as described in RFC 2131[13].

   The DHCP client also needs to be augmented with an authentication
   module that can manage keys, respond to challenges or encrypt
   messages. The specific functions of the authentication module is
   implementation dependent. The following is the state diagram for the
   client with the new state AUTHENTICATING as well as the new messages
   DHCPAUTHREQ and DHCPAUTHRES.
















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    ------                               -------
   |      | +-------------------------->|       |<------------------+
   |INIT- | |       +------------------>| INIT  |                   |
   |REBOOT|DHCPNAK/ |       +---------->|       |<---+              |
   |      |Restart  |       |            -------     |              |
    ------  |  DHCPNAK/     |               |                       |
      |     |Discard offer  |      -/Send DHCPDISCOVER              |
   -/Send DHCPREQUEST       |               |                       |
      |     |       |    DHCPACK            v        |              |
    -----------     | (not accept.)/   -----------   |              |
   |           |    |Send DHCPDECLINE |           |                 |
   | REBOOTING |    |       |         | SELECTING |<----+           |
   |           |    |      /          |           |     |DHCPOFFER/ |
    -----------     |     /            -----------   |  |Collect    |
       |            |    /                  |   |       |  replies  |
   DHCPACK/         |   /  +----------------+   +-------+           |
   Record lease, set|  |   v   Select offer/                        |
   timers T1, T2   ------------ send DHCPREQUEST     |              |
       |   +----->|            |           DHCPNAK, Lease expired/  |
       |   |      | REQUESTING |                Halt network        |
       DHCPOFFER/ |            |                     |              |
       Discard    /------------                      |              |
       |   |     /  |        |                   -----------        |
       |   +----/---+     DHCPACK/              |           |       |
       |       /      Record lease, set    -----| REBINDING |       |
       |  DHCPAUTHREQ   timers T1, T2     /     |           |       |
       |     /               |        DHCPACK/   -----------        |
       |    /                v    Record lease, set   ^   |         |
       +---/------------> -------    /timers T1,T2    | DHCPAUTHREQ |
          |       +----->|       |<---+               |   |         |
                  |      | BOUND |<---+               |             |
     DHCPOFFER, DHCPACK, |       |    |         T2 expires/   DHCPNAK/
      DHCPNAK/Discard  -> -------     |         Broadcast  Halt network
                  |   /   | |      |          DHCPREQUEST           |
          |       +-------+ |    DHCPACK/             |   |         |
          |          T1 expires/ Record lease, set    |   |         |
   DHCP   |     Send DHCPREQUEST timers T1, T2        |   |         |
   AUTHREQ|     to leasing server  |                  |   |         |
   -/Send |                 |   ----------            |   |         |
   DHCP   |     /           |  |          |-----------+   |         |
   AUTHRES|  DHCPACK        +->| RENEWING |                         |
   +---+  |   /                |          |-------------------------+
   |   |  v  /                 /----------                          |
   |   -------    DHCPAUTHREQ /                           |         |
   +->|       |<-------------+----------------------------+         |
      |AUTHENTICATING                                               |
      |       |----------------DHCPNACK-----------------------------+
       -------
         Figure 2:  State-transition diagram for DHCP clients



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6.4. Interworking with AAA Servers

   The AAA mechanisms described here are similar to the ones already
   deployed and used by ISPs to serve PPP users. A AAA protocol like
   RADIUS can be employed with no modification to support client
   authentication by the network. The RADIUS protocol however needs to
   be modified in order to support network authentication by the
   client. These modifications are beyond the scope of this document.

6.5. Illustrations

   To see the use of the authentication mechanisms described here,
   consider the case of a subscriber connecting to an access network
   and requesting configuration using DHCP. Figures below show the
   message flow between DHCP components of a network and their
   interaction with the AAA components. In the first case the network
   authenticates the client and in the second case the client
   authenticates the network. The last illustration shows how the
   initial configuration of a typical host would work when both the
   client and the network are being authenticated.

                 DHCP Client     DHCP Server         AAA
                     v               v                v
                     |               |                |
                     ..              ..               ..
                     |\_____________ |                |
                     | DHCPREQUEST  \|                |
                     | (nai+chap)    |                |
                     | _____________/|                |
                     |/DHCPAUTHREQ   |                |
                     |   (c1)        |                |
                     |\_____________ |                |
                     | DHCPAUTHRES  \|                |
                     |   (r1)        |                |
                     |               |\______________ |
                     |               | AccessRequest \|
                     |               | (c1 + r1)      |
                     |               | ______________/|
                     |               |/ AccessResponse|
                     |               |  (accept)      |
                     | _____________/|                |
                     |/ DHCPACK      |                |
                     |               |                |
                     |               |                |
                     |    Initialization complete     |
                     v               v                v

                   Figure 3: Authentication of the client

   In Figure 3, the Client starts by sending the NAI in the appropriate
   option field. It also indicates its preference to perform CHAP
   authentication by including the CHAP option field. The server can

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   check that it does indeed support the requested authentication
   method and thus sends an authentication request message with the
   challenge in the CHAP option. The client can now use a key shared
   with its home AAA authority to respond to the challenge. Upon
   receiving the response, the DHCP server can then contact the local
   AAA server using, for example, a RADIUS Access Request message
   containing the challenge that was sent to the client and the
   response received from it. The AAA server may in turn contact the
   client's local AAA server. The AAA authority can then verify if the
   client's response to the challenge was correct using the shared key.
   The DHCP server, upon receiving the access response, will send a
   DHCPACK if the client authentication succeeded or DHCPNAK otherwise.
   Finally the DHCP client can verify and accept the parameters sent to
   it in the DHCPACK message.


                 DHCP Client     DHCP Server         AAA
                     v               v                v
                     |               |                |
                     ..              ..               ..
                     |               |                |
                     |\_____________ |                |
                     | DHCPAUTHREQ  \|                |
                     |   (nai+c2)    |                |
                     |               |\______________ |
                     |               | AuthRequest   \|
                     |               | (c2)           |
                     |               | ______________/|
                     |               |/ AuthResponse  |
                     |               |  (r2)          |
                     | _____________/|                |
                     |/ DHCPAUTHRES  |                |
                     |    (r2)       |                |
                     |               |                |
                     | Network authentication complete|
                     v               v                v

                  Figure 4: Authentication of the network

   As shown in Figure 4, the client may also require authentication of
   the network and sends its own challenge to the network in a
   DHCPAUTHREQ message. The DHCP server may forward that request to the
   AAA server who will then use the key it shares with the client to
   respond. The Client upon confirming that the response was correct
   will reenter bound state and resume normal operation. Otherwise, if
   the response to the challenge was incorrect, it may reject the
   parameters it was given and restart configuration.






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                  DHCP Client     DHCP Server         AAA
                     v               v                v
                     |               |                |
                     ..              ..               ..
                     |               |                |
                     |\_____________ |                |
                     | DHCPREQUEST  \|                |
                     | (nai+chap)    |                |
                     | _____________/|                |
                     |/DHCPAUTHREQ   |                |
                     |   (c1)        |                |
                     |\_____________ |                |
                     | DHCPAUTHRES  \|                |
                     |   (r1+c2)     |                |
                     |               |\______________ |
                     |               | AccessRequest \|
                     |               | (c1+r1+c2)     |
                     |               | ______________/|
                     |               |/ AccessResponse|
                     |               |  (accept+r2)   |
                     | _____________/|                |
                     |/ DHCPACK      |                |
                     |    (r2)       |                |
                     |               |                |
                     |    Initialization complete     |
                     v               v                v

           Figure 5: Authentication of the client and the network

   During initial configuration when both sides may want to establish
   trust the two steps above may be combined as shown in Figure 5. In
   this case, the network's challenge is contained in its DHCPAUTHREQ
   message and the client's response is contained in its DHCPAUTRES
   message. The client's challenge is also contained in its DHCPAUTHRES
   message and the network's response to the challenge is contained in
   its DHCPACK message.

   For re-authenticating, challenges can be sent using the DHCPAUTHREQ
   and DHCPAUTHRESP messages at any time without the need for
   reconfiguration or rebinding.


7. Security Considerations

   The purpose of this document is to describe a mechanism for
   authenticating DHCP clients and servers. It does not cover other
   possible security attacks such as IP spoofing.

8. References




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   1  Bradner, S., "The Internet Standards Process -- Revision 3", BCP
      9, RFC 2026, October 1996.
   2  Bradner, S., "Key words for use in RFCs to Indicate Requirement
      Levels", BCP 14, RFC 2119, March 1997
   3  S. Das, A. McAuley, A. Baba, Y. Shobatake, _Authentication,
      Authorization, and Accounting Requirements for Roaming Nodes
      using DHCP_, Internet Draft <draft-ietf-dhc-aaa-requirements-
      00.txt>, March 2000.
   4  S. Das, A. McAuley, A. Baba, Y. Shobatake, _Requirements for
      Extending DHCP into New Environments_, Internet Draft <draft-
      ietf-dhc-enhance-requirements-00.txt>, March 2000.
   5  K. Hickman, "The SSL protocol", Netscape Communication Corp.,
      February 1995
   6  C. Munroe,
      "http://www.uu.net/press_center/hot_tech_topics/vpn/vpn-
      whitepaper.pdf", White Paper, May 2000.
   7  R. Dorms, W. Arbaugh, "Authentication for DHCP Message", Internet
      Draft <draft-ietf-dhc-authentication-14.txt>, July 2000.
   8  S. Das, A. McAuley, A. Baba, Y. Shobatake, "Dynamic Registration
      and Configuration Protocol (DRCP)", Internet Draft <draft-itsumo-
      drcp-01.txt>, July 2000.
   9 B. Volz, S. Gonczi, T. Lemon, R. Stevens, "DHC Load Balancing
      Algorithm", Internet Draft, <draft-ietf-dhc-loadb-03.txt>.
   11 L. Blunk, J. Vollbrecht, "PPP Extensible Authentication
      Protocol(EAP)", RFC 2284, March 1998.
   11 Aboda, Beadles, "The Network Access Identifier" RFC 2486, January
      1999
   12 W. Simpson, "PPP Challenge Handshake Authentication Protocol
      (CHAP)", RFC 1994, August 1996.
   13 R. Dorms, "The Dynamic Host Control Protocol", RFC 2131, March
      1997
10.  Acknowledgments

   We Acknowledge the help of Kris Ng and all the members of the
   Advanced Wireless research group at Nortel Networks.

11. Author's Addresses

   Biswaroop Mukherjee
   Nortel Networks,
   Ottawa, ON,
   Canada.
   Email: biswaroo@nortelnetworks.com

   Bill Gage
   Nortel Networks,
   Ottawa, ON,
   Canada.
   Email: gageb@ nortelnetworks.com

   Yajun Liu

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   Nortel Networks,
   Ottawa, ON,
   Canada.
   Email: yajun@nortelnetworks.com

















































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