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INTERNET-DRAFT                                                 S. Sakane
Intended Status: Informational                             Cisco Systems
Expires: March 14, 2011                                      M. Ishiyama
                                                           Toshiba Corp.
                                                       September 9, 2010


                      Kerberos Options for DHCPv6
               draft-sakane-dhc-dhcpv6-kdc-option-09.txt


                                Abstract

   This document defines new four options of Dynamic Host Configuration
   Protocol for IPv6 (DHCPv6) to carry a set of configuration
   information related to the Kerberos protocol [RFC4120].


Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

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

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   This Internet-Draft expires in March 14, 2011.


Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal



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   Provisions Relating to IETF Documents
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   than English.































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Table of Contents

    1. Introduction .................................................  4
    2. Conventions used in this document ............................  5
    3. Kerberos Options .............................................  5
       3.1. Kerberos Principal Name Option ..........................  5
       3.2. Kerberos Realm Name Option ..............................  6
       3.3. Kerberos Default Realm Name Option ......................  7
       3.4. Kerberos KDC Option .....................................  7
    4. Client Operation .............................................  8
       4.1. A recommendation of KDC discovery for a client ..........  9
    5. Server Operation ............................................. 11
    6. Appearance of this option .................................... 11
    7. IANA Considerations .......................................... 11
    8. Security Considerations ...................................... 12
    9. Acknowledgments .............................................. 12
   10. References ................................................... 13
       10.1. Normative References ................................... 13
       10.2. Informative References ................................. 14
   Appendix A. Why DNS is not acceptable in some environment ........ 14
   Authors' Addresses ............................................... 18






























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1.  Introduction

   The Kerberos Version 5 [RFC4120] is an authentication system which is
   based on the trusted third-party authentication protocol.  Each
   organization wishing to use the Kerberos protocol establishes its own
   "realm", and each client is assigned to the realm.  At least one Key
   Distribution Center (KDC) is required for the Kerberos operation of
   the realm.

   When a client wants to start communication with a server (that means
   another client of the KDC), and to be authenticated to the server,
   the client needs to take a credential from the KDC.  In this process,
   the client presents both an identifier itself, and a realm name to
   which the client itself belongs.  After the client gets a credential
   from the KDC, the client presents it to the peer.  The peer can
   authenticate the access from the client with the credential.  Hence,
   the client needs to know at least one IP address of KDC from which
   the client can get a credential before the client begins the
   communication with the peer.

   Here is a use case that a client has to know a realm name.  A public
   workstation for an unspecified several number of students in a
   college does not have any initial configuration for Kerberos.  If
   there is a mechanism providing a realm name and a set of IP addresses
   of the KDC, a student only puts a user identifier and a pass phrase
   into the workstation, and can user the Kerberos authentication
   system.

   To provide a set of IP addresses of the KDC, the Kerberos V5
   specification [RFC4120] defines a KDC discovery by utilizing DNS SRV
   records [RFC2782].  In the meantime, the system which does not employ
   DNS, but does use DHCP, exists like the industrial system.  Some
   industrial systems don't use DNS because they have already had their
   own name spaces and their own name resolution systems, including the
   pre-configured mapping table into the device, rather than FQDN and
   DNS.  And these systems dare not to employ DNS for only the name
   resolution because adding a new server brings to decrease the
   reliability of the system, and to increase the management cost of the
   system.  (The detail is described in the APPENDIX), For such
   environment, another mechanism is required to provide a set of IP
   addresses of the KDC.  Providing a set of IPv4 addresses of the KDC
   to the devices deployed into the PacketCable Architecture [PCARCH],
   the KDC Server Address sub-option for the DHCPv4 CableLabs Client
   Configuration option is defined in RFC 3634 [RFC3634].  However, a
   mechanism which does not depend on any architecture is required for
   providing a realm name and a set of IPv6 addresses.

   The Kerberos option for DHCPv6 defined by this document allows to



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   provide a realm name and/or a list of IP addresses of the KDC.  The
   Kerberos option does not replace and deny of the previous methods,
   and this option does not interfere with those methods.


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 [RFC2119].

   It is assumed that the readers are familiar with the terms and
   concepts described in the DHCPv6 [RFC3315].

   Since a server would mean a DHCP server or a server in the Kerberos
   service, when there is no declaration in particular, the server means
   a server against a client of the Kerberos service in this document.


3.  Kerberos Options

   The Kerberos options provide a set of configuration parameters which
   a clients and a servers of Kerberos will use.  This document defines
   the options listed below.

           Kerberos Principal Name Option
           Kerberos Realm Name Option
           Kerberos Default Realm Name Option
           Kerberos KDC Option

   Other options may be defined in the future.  Those should be defined
   in another document.  This section describes the format of each
   option, and the usage of each field.

   These options can appear multiple times in a DHCPv6 message.
   However, the order of the options MUST NOT mean.  Typical usage is
   described in section XXX.


3.1.  Kerberos Principal Name Option

   This option provides a principal name of the Kerberos system.  It is
   intended that a DHCPv6 server determines a specific set of the
   configuration parameters of the Kerberos system for either a client
   or a server specified by the principal-name field.

   The format of the Kerberos Principal Name option is:




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      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   OPTION_KRB_PRINCIPAL_NAME   |          option-len           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      :                                                               :
      :                        principal-name                         :
      :                       (variable length)                       :
      :                                                               :
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   o  option-code (16-bit): OPTION_KRB_CLIENT_PRINCIPAL (TBD by IANA)

   o  option-len (16-bit): length of the principal-name field.

   o  principal-name (variable): a client principal name.  The encoding
      of the principal-name field MUST be conformed to "PrincipalName"
      defined in section 5.2.2 of RFC 4120 [RFC4120].


3.2.  Kerberos Realm Name Option

   This option provides a realm name of a domain of the Kerberos system.
   It is intended that a DHCPv6 server determines a set of the
   configuration parameters of a KDC in a Kerberos domain specified by
   the realm-name field.

   The format of the Kerberos Realm Name option is:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     OPTION_KRB_KDC_OPTION     |          option-len           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      :                                                               :
      :                          realm-name                           :
      :                       (variable length)                       :
      :                                                               :
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   o  option-code (16-bit): OPTION_KRB_REALM_NAME (TBD by IANA)

   o  option-len (16-bit): the length of the realm-name field in octets.

   o  realm-name (variable): a realm-name.  The encoding of the realm-
      name field MUST be conformed to "Realm" which is defined in



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      section 5.2.2 of RFC 4120 [RFC4120].


3.3.  Kerberos Default Realm Name Option

   This option provides a default realm name of a domain of the Kerberos
   system.  It is intended that a DHCPv6 server provides a default realm
   name to both the clients or the servers in the Kerberos system.  This
   option MAY be provided with one or more Kerberos KDC Option
   sometimes.

   The option-code of this option is OPTION_KRB_DEFAULT_REALM_NAME.  The
   format and the usage of each field are similar to the Kerberos Realm
   Name Option.


3.4.  Kerberos KDC Option

   This option provides a set of configuration parameters of a KDC.  It
   is intended that a DHCPv6 server provides a set of the configuration
   parameters of a KDC in a Kerberos domain.

   The format of the Kerberos KDC Option is:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     OPTION_KRB_KDC_OPTION     |          option-len           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           Priority            |            Weight             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  Service Type |   Reserved    |          Port Number          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      |                                                               |
      |                       KDC IPv6 address                        |
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      :                                                               :
      :                          realm-name                           :
      :                       (variable length)                       :
      :                                                               :
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   o  option-code (16-bit): OPTION_KRB_KDC_OPTION (TBD by IANA)





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   o  option-len (16-bit): 24-octet + the length of the realm-name field
      in octets.

   o  Priority (16-bit): see the description of Weight field.

   o  Weight (16-bit): both Priority and Weight are for a server
      selection mechanism.  These parameters indicate a hint for a kind
      of server selection mechanism of a client.  An implementer MUST
      follow the DNS SRV specification [RFC2782] for this usage.

   o  Transport Type (8-bit): The Service Type specifies the transport
      of the Kerberos communication.  The Kerberos specification
      [RFC4120] defines to use both UDP and TCP for communication
      between clients and servers.  The exchanges over TCP is described
      in [RFC5021].  The exchanges over TLS is described in [STARTTLS].

      The transport type is defined in below.

        Value    Transport Type
        ----     --------------
        0        Reserved
        1        UDP
        2        TCP
        3        TLS
        4-254    Unassigned
        255      Reserved


   o  Reserved (8-bit): initialized to zero for transmission, and
      ignored on reception.

   o  Port Number (16-bit): a port number listened to by the KDC.

   o  KDC address (128-bit): an IPv6 address of the KDC.


4.  Client Operation

   This section describes the client behavior when the client requires a
   set of the configuration parameters of the Kerberos system, and when
   the client receives messages from the DHCPv6 server.  The processing
   of both the Information-request message and the Option Request Option
   MUST be conformed to [RFC3315].  An implementor MUST refer to the
   Stateless Dynamic Host Configuration Protocol (DHCP) Service for IPv6
   [RFC3736].

   When the client requires a set of the configuration parameters of a
   Kerberos system in the phase of its bootstrapping, the client SHOULD



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   put the client principal name itself into the Kerberos Principal Name
   Option.  The client expects to receive the Default Realm Name Option
   and/or the KDC option of the client and/or the additional one or more
   KDC options required by the client's operation.

   When the client requires a specific information of a certain realm,
   the client MUST specify the realm name into the Kerberos Realm Name
   Option.  When the client requires a specific information related to a
   certain server of the Kerberos system, the client MUST put the
   principal name of the server into the Kerberos Principal Name Option.

   More than one KDC Options MAY be presented in a DHCPv6 message of the
   Reply Message from the server.  In this case, the client MUST contact
   to the addresses in the order of the value of the priority field in
   each Kerberos KDC Option.  The value of the weight field might be
   considered simultaneously.  For this usage, an implementer could
   refer to the DNS SRV specification [RFC2782].

   The client MAY include any other options with data values as hints to
   the server as it is described in section 18.1.5 of RFC 3315
   [RFC3315].



4.1.  A recommendation of KDC discovery for a client

   When a client has a capability of both the DNS method defined by
   section 7.2.3.2 of [RFC4120] and the DHCP method defined by this
   document, which methods the client adopts depends on the policy of
   the environment.  The administrator of the realm MUST define the
   method to the client before the client is installed into the
   environment.

   When there is no criteria in the environment, and the client could
   get the Kerberos information from both the DNS server and the DHCP
   server, then the information from DNS is recommended.  The following
   is a recommendation of the behavior of the client in such environment
   where there is no criteria.













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                               No Ans. or
               +------------+  DNS Info.  +-----------+ No Ans.
     Start --> | Ask DHCP(1)| ----------> | Ask DNS(3)| ------> Abort(4)
               +------------+             +-----------+
                /          \                       \
      Only KRB /            \ DNS and               \ KRB Info.
        Info. /              \ KRB Info.             \
             /                \                       \
            |                  |                       |
            |                  V                       |
            V     No Ans.  +-----------+  KRB Info.    V
     Adopt Info. <-------- | Ask DNS(6)| ---------> Adopt Info.
     from DHCP             +-----------+            from DNS
      (2), (7)                                      (5), (8)

        Abbreviations:
          Ans.: Answer
          Info.: Information
          KRB: Kerberos


   1) At the first, the client asks both DNS and KRB information to the
      DHCP server.

   2) If the client gets only a response with KRB information from the
      DHCP server, the client adopts the information from the DHCP
      server.

   3) As the result of (1), if the client gets either no answer or only
      a response with DNS information from the DHCP server, the client
      then asks KRB information to the DNS server.

   4) If the client gets no answer from the DNS server, then the client
      will abort.

   5) If the client gets KRB information from the DNS server, then the
      client adopts the information from the DNS server.

   6) As the result of (1), if the client gets both DNS and KRB
      information from the DHCP server, then the client asks KRB
      information to the DNS server.

   7) If the client gets no answer from the DNS server, the client
      adopts the KRB information from the DHCP server.

   8) As the result of (6), if the client gets KRB information from the
      DNS server, the client adopts the information instead of another
      from the DHCP server.



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5.  Server Operation

   After the DHCPv6 server receives a message which is contained an
   Option Request Option, what information the server will provide
   depends on the policy of the server in the end.  If there is no
   criteria on the server, the following operation is recommended.

   The server SHOULD send a Reply Message back to the client when the
   option number of the Kerberos option is specified in the Option
   Request option by the client.

   When the message from a client did not include any information which
   can be used to determine the configuration parameter for a specific
   client, the server SHOULD reply at least the Default Realm Name
   Option.


6.  Appearance of this option

   The Kerberos option MUST NOT appear in any other than the following
   messages: Solicit, Advertise, Request, Renew, Rebind, Information-
   request and Reply.  The option MAY also appear in the DHCP-relay-
   message field of both Relay-forward or Relay-reply message.  If this
   option appears in messages other than those specified above, the
   receiver MUST ignore it.

   The number of the Kerberos option MAY appear in the Option Request
   Option in the DHCPv6 message types Solicit, Request, Renew, Rebind,
   Information-request and Reconfigure.  The number MAY also appear in
   the DHCP-relay-message field of both Relay-forward or Relay-reply
   message.

   The sub-option of the Kerberos option MUST appear only in the
   Kerberos option.


7.  IANA Considerations

   When this document is published, the IANA is requested to assign four
   option codes from the "DHCPv6 Options Codes" registry for the
   following.

      OPTION_KRB_CLIENT_PRINCIPAL
      OPTION_KRB_REALM_NAME
      OPTION_KRB_DEFAULT_REALM_NAME
      OPTION_KRB_KDC_OPTION

   IANA is required to maintain a new number space of Kerberos Message



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   Transport Type, located in the Kerberos Parameters Registry.  The
   initial types are described in section XXX.

   IANA assigns future type with a "IETF Consensus" policy as described
   in BCP 26.  Future proposed type is to be referenced symbolically in
   the Internet-Drafts that describe it, and shall be assigned numeric
   code by IANA when approved for publication as an RFC.


8.  Security Considerations

   The security considerations in RFC 3315 fully apply.

   The message of DHCPv6 could be altered undesirably.  If an adversary
   modifies the response from a DHCPv6 server or inserts its own
   response, a client could be led to contact a rogue KDC or a server
   which does not know the client access.  Both cases are categorized
   into a kind of the denial of service attack.  However, such incorrect
   KDC does not know the shared key between the client and a valid KDC.
   The incorrect KDC is not be able to proceed any further state of the
   client.  Even when the client receives a response from such KDC, the
   client can know the fact that it has received an inappropriate
   message after it verifies the response with the shared key.

   The considerable situation is that the support of an unconfigured
   workstation used by multiple users, which obtains its KDC information
   and default realm via DHCP.  In such a scenario, the workstation may
   not have a host or other service key, and thus be unable to validate
   TGT's issued to users for the purposes of authorizing login.  If this
   is the case, an altered DHCP response could result in the workstation
   talking to a rogue KDC which it will be unable to distinguish from a
   real KDC, and allowing access by unauthorized users.

   In order to minimize potential vulnerabilities, a client SHOULD
   require to use the DHCPv6 authentication defined in section 21 of RFC
   3315, or any other authentication mechanism.

   Sometimes, the Kerberos information is manually configured into the
   client before the DHCPv6 process starts.  Generally, the manual
   configuration to the device should be preferred to the configuration
   by the DHCP server.  Overriding the manual configuration should be
   considered in anytime.


9.  Acknowledgments

   The authors are very grateful to Nobuo Okabe and Shigeya Suzuki.
   They contributed the summary explaining why DNS is not appropriate to



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   some industry networks, which is put as the appendix of this
   document.  Ted Lemon gave us many suggestions to improve the
   specification in terms of the DHCP manner.  Ken'ichi Kamada and
   Yukiyo Akisada contributed for the initial work of making this
   document.  The authors also thank Jeffrey Hutzelman, Kazunori
   Miyazawa, Kensuke Hosoya, Nicolas Williams, Nobumichi Ozoe, and Sam
   Hartman.  They gave us valuable comments and suggestions for this
   document.


10.  References


10.1.  Normative References

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

   [RFC2782]
      A. Gulbrandsen, P. Vixie, L. Esibov, "A DNS RR for specifying the
      location of services (DNS SRV)", RFC 2782, February 2000.

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

   [RFC3736]
      R. Droms, "Stateless Dynamic Host Configuration Protocol (DHCP)
      Service for IPv6", RFC 3736, April 2004.

   [RFC4120]
      Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The Kerberos
      Network Authentication Service (V5)", RFC 4120, July 2005.

   [RFC5021]
      Josefsson, S., "Extended Kerberos Version 5 Key Distribution
      Center (KDC) Exchanges over TCP", RFC 5021, August 2007.

   [STARTTLS]
      Josefsson, S., "Using Kerberos V5 over the Transport Layer
      Security (TLS) protocol", draft-josefsson-kerberos5-starttls-09
      (work in progress), August 2010.







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10.2.  Informative References

   [PCARCH]
      "PacketCable 1.0 Architecture Framework Technical Report", PKT-TR-
      ARCH-V01-991201, http://www.packetcable.com/downloads/specs/pkt-
      tr-arch-v01-991201.pdf

   [RFC3634]
      K. Luehrs, R. Woundy, J. Bevilacqua, N. Davoust, "Key Distribution
      Center (KDC) Server Address Sub-option for the Dynamic Host
      Configuration Protocol (DHCP) CableLabs Client Configuration (CCC)
      Option", RFC 3634, December 2003.

Appendix A. Why DNS is not acceptable in some environment





































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   1. Summary

      - This appendix describes reasons why DHCP-based KDC discovery
        is more suitable than DNS-based KDC discovery described
        in RFC4120 (= the RFC4120-way) for the industrial systems.

      - The main reason is that some industrial systems don't use DNS
        because they have already had their own name spaces and
        naming systems rather than FQDN and DNS.

      - Less servers benefits the industrial systems:
        1) Less messages simplifying the systems.
        2) Less servers contributing reliability,
           and reducing management cost.

      - We understand that RFC4120 does not require DHCP for KDC
        discovery.  However, we will have to solve DNS discovery
        when considering the RFC4120-way.
        And it is natural way to use DHCP for the purpose.

      - DHCP-based KDC discovery is more efficient under those
        systems (=expecting not to use DNS).

   2. Background (what's industrial systems?)

      Industrial systems are a kind of sensor systems.
      The systems have a large number of devices, i.e. sensors and
      actuators, usually called field devices
      by which the systems control plants, factories, buildings, etc.

      The field devices have the following features:
      1) Their resources, e.g. processing capability, memory size,
         footprint, power consumption and user i/f, are limited
         even though they are physically large.
      2) The field device is controlled as an I/O by a administrative
         device, usually called controller, with a legacy communication
         technology.
      3) Security of the field devices have not been cared
         because they are regarded as being on I/O buses, not networks.

   3. High-level goal and some requirements

   3.1. IP and security can enhance the industrial systems.

      Our goal is to introduce latest IP-based network technology
      into field devices for enhancing the entire system.
      1) Network architecture (=IP technology) can enhance
         the systems including the field devices.



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      2) The field devices will require security if network architecture
         is introduced. The field devices will not be I/O devices
         anymore.

   3.2. Auto-configuration benefits the industrial systems.

      Auto-configuration will also be important for large systems
      like the industrial systems if introducing new technology or
      capability:

      1) Reducing engineering cost when installing/configuring
         large number of field devices over spread area.
         The following are existing large systems.
         The size of a plant, the size of an industrial system and
         the number of field devices are growing.

         - An example of a single large process automation system:
           About 20000 field devices over 2km*2km area

           References:
              - http://www.process-worldwide.com/fachartikel/pw_facha
                rtikel_2699276.html

         - An example of a distributed process automation systems:
           About 30000 field devices for 26 distributed sites
           over 30km*30km area.

           References:
              - http://www.mikrocentrum.nl/FilesPage/3462/Presentatie
                %20C3-1.pdf
              - http://www.nam.nl/home/Framework?siteId=nam-en&FC2=/n
                am-en/html/iwgen/algemeen/zzz_lhn.html&FC3=/nam-en/ht
                ml/iwgen/algemeen/over_de_nam.html

         - An example of a single large building automation system:
           170000 control points of 16500 field devices in
           729,000 sq. meters (7.8 million sq. ft.) building complex.

           References:
              - http://www.echelon.com/company/press/2003/echelon_mor
                i.htm

      2) Reducing the chance of human error.

      3) Making disaster-recovery easier.

   3.3. Security mechanism suited to resource-limited devices are
        necessary.



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      Kerberos-based security can be suited resource-limited devices,
      i.e. the field devices, because of not requiring
      public key cryptography (of course, when not using PKINIT).

   4. Some industrial systems don't use DNS.

      For field devices, there have been multiple technologies (see
      Section 6) which don't use DNS because of having already had
      their own name spaces and naming systems even though introducing
      IP (partially at this moment).

      For example, "tag" is the common logical identifier for the
   process
      automation systems and Device ID is the common logical identifier
      for the building automation systems.
      (You may think those names are not so abstracted, though....)

   5. KDC discovery with DHCP is more suitable than the one with DNS.

      If Kerberos is introduced into the field devices,
      auto-configuration will be achieved with the following steps:
      1) Learning DNS address(es) by DHCP
      2) Learning KDC address(es) by DNS based on RFC4120-way.
      However, DNS will be used by kerberos-related part only.
      Most application will not use DNS as described above.

      If DHCP can advertise KDC-related information instead of DNS,
      there are the following advantages.
      1) It can reduce messages handled by the field devices.
         Consequently, it can reduce footprint of the field devices.
      2) It can reduce the number of servers.
         Consequently, it contribute to management cost of the systems.

   6. References

      There have been multiple technologies for field devices.
      Examples:
      - FOUNDATION Fieldbus (http://www.fieldbus.org/)
      - PROFIBUS (http://www.profibus.com/)
      - BACnet (http://www.bacnet.org/)
      - LonWorks (http://www.echelon.co.jp/products/lonworks.html)
      - Modbus (http://www.modbus.org/)

      You can learn about communication technology of field devices
      with wikipedia:
      - http://en.wikipedia.org/wiki/Fieldbus
      - http://en.wikipedia.org/wiki/BACnet
      - http://en.wikipedia.org/wiki/LonWorks



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Authors' Addresses

   Shoichi Sakane
   Cisco Systems
   2-1-1 Nishi-Shinjuku, Shinjuku-ku,
   Tokyo  163-0409 Japan
   E-mail: ssakane@cisco.com


   Masahiro Ishiyama
   Toshiba Corporation
   1, komukai-toshiba-cho, Saiwai-ku,
   Kawasaki  212-8582 Japan
   E-mail: masahiro@isl.rdc.toshiba.co.jp





































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