    Network Working Group                                            B. Volz
    Request for Comments: 3942                           Cisco Systems, Inc.
    Updates: 2132                                              November 2004
    Category: Standards Track

               Reclassifying Dynamic Host Configuration Protocol
                          version 4 (DHCPv4) Options

    Status of this Memo

       This document specifies an Internet standards track protocol for the
       Internet community, and requests discussion and suggestions for
       improvements.  Please refer to the current edition of the "Internet
       Official Protocol Standards" (STD 1) for the standardization state
       and status of this protocol.  Distribution of this memo is unlimited.

    Copyright Notice

       Copyright (C) The Internet Society (2004).

    Abstract

       This document updates RFC 2132 to reclassify Dynamic Host
       Configuration Protocol version 4 (DHCPv4) option codes 128 to 223
       (decimal) as publicly defined options to be managed by IANA in
       accordance with RFC 2939.  This document directs IANA to make these
       option codes available for assignment as publicly defined DHCP
       options for future options.

    Table of Contents

       1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
       2.  Requirements Notation  . . . . . . . . . . . . . . . . . . . .  2
       3.  Background . . . . . . . . . . . . . . . . . . . . . . . . . .  2
           3.1.  Publicly Defined Options Range . . . . . . . . . . . . .  2
           3.2.  Site-Specific Options Range  . . . . . . . . . . . . . .  3
       4.  Reclassifying Options  . . . . . . . . . . . . . . . . . . . .  3
       5.  Security Considerations  . . . . . . . . . . . . . . . . . . .  4
       6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  5
       7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  5
       8.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  5
           8.1.  Normative References . . . . . . . . . . . . . . . . . .  5
           8.2.  Informative References . . . . . . . . . . . . . . . . .  6
       Author's Address . . . . . . . . . . . . . . . . . . . . . . . . .  6
       Full Copyright Statement . . . . . . . . . . . . . . . . . . . . .  7

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    1.  Introduction

       The DHCPv4 [RFC2131] publicly defined options range, options 1 - 127,
       is nearly used up.  Efforts such as [RFC3679] help extend the life of
       this space, but ultimately the space will be exhausted.

       This document reclassifies much of the site-specific option range,
       which has not been widely used for its original intended purpose, to
       extend the publicly defined options space.

    2.  Requirements Notation

       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 [RFC2119].

    3.  Background

       The DHCP option space (0 - 255) is divided into two ranges [RFC2132]:

       1. 1 - 127 are publicly defined options, now allocated in accordance
          with [RFC2939].

       2. 128 - 254 are site-specific options.

       Options 0 (pad) and 255 (end) are special and defined in [RFC2131].

    3.1.  Publicly Defined Options Range

       The publicly defined options space (1 - 127) is nearly exhausted.
       Recent work [RFC3679] will buy more time, as several allocated but
       unused option codes have been reclaimed.  A review could be made from
       time to time to determine whether there are other option codes that
       can be reclaimed.

       A longer-term solution to the eventual exhaustion of the publicly
       defined options space is desired.  The DHC WG evaluated several
       solutions:

       1. Using options 126 and 127 to carry 16-bit options as originally
          proposed by Ralph Droms in late 1996.  However, this significantly
          penalizes the first option assigned to this new space, as it
          requires implementing the 16-bit option support.  Because of this,
          options 126 and 127 have been reclaimed [RFC3679].

       2. Using a new magic cookie and 16-bit option code format.  However,
          this proposal

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          *  penalizes the first option assigned to this new space, as it
             requires significant changes to clients, servers, and relay
             agents,

          *  could adversely impact existing clients, servers, and relay
             agents that fail to properly check the magic cookie value,

          *  requires support of both message formats for the foreseeable
             future, and

          *  requires clients to send multiple DHCPDISCOVER messages -- one
             for each magic cookie.

       3. Reclassifying a portion of the site-specific option codes as
          publicly defined.  The impact is minimal, as only those sites
          presently using options in the reclassified range need to renumber
          their options.

    3.2.  Site-Specific Options Range

       The site-specific option range is rather large (127 options in all)
       and little used.  The original intent of the site-specific option
       range was to support local (to a site) configuration options, and it
       is difficult to believe a site would need 127 options for this
       purpose.  Further, many DHCP client implementations do not provide a
       well documented means to request site-specific options from a server
       or to allow applications to extract the returned option values.

       Some vendors have made use of site-specific option codes that violate
       the intent of the site-specific options, as the options are used to
       configure features of their products and thus are specific to many
       sites.  This usage could potentially cause problems if a site that
       has been using the same site-specific option codes for other purposes
       deploys products from one of the vendors, or if two vendors pick the
       same site-specific options.

    4.  Reclassifying Options

       The site-specific option codes 128 to 223 are hereby reclassified as
       publicly defined options.  This leaves 31 site-specific options, 224
       to 254.

       To allow vendors that have made use of site-specific options within
       the reclassified range to publish their option usage and to request
       an official assignment of the option number to that usage, the
       following procedure will be used to reclassify these options:

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    RFC 3942              Reclassifying DHCPv4 Options         November 2004

       1. The reclassified options (128 to 223) will be placed in the
          "Unavailable" state by IANA.  These options are not yet available
          for assignment to publicly defined options.

       2. Vendors that currently use one or more of the reclassified options
          have 6 months following this RFC's publication date to notify the
          DHC WG and IANA that they are using particular options numbers and
          agree to document that usage in an RFC.  IANA will move these
          options from the "Unavailable" to "Tentatively Assigned" state.

          Vendors have 18 months from this RFC's publication date to start
          the documentation process by submitting an Internet-Draft.

          NOTE: If multiple vendors of an option number come forward and can
          demonstrate that their usage is in reasonably wide use, none of
          the vendors will be allowed to keep the current option number, and
          they MUST go through the normal process of getting a publicly
          assigned option [RFC2939].

       3. Any options still classified as "Unavailable" 6 months after the
          RFC publication date will be moved to the "Unassigned" state by
          IANA.  These options may then be assigned to any new publicly
          defined options in accordance with [RFC2939].

       4. For those options in the "Tentatively Assigned" state, vendors
          have 18 months following this RFC's publication date to submit an
          Internet-Draft documenting the option.  The documented usage MUST
          be consistent with the existing usage.  When the option usage is
          published as an RFC, IANA will move the option to the "Assigned"
          state.

          If no Internet-Draft is published within the 18 months or should
          one of these Internet-Drafts expire after the 18 months, IANA will
          move the option to the "Unassigned" state, and the option may then
          be assigned to any new publicly defined options in accordance with
          [RFC2939].

       Sites presently using site-specific option codes within the
       reclassified range SHOULD take steps to renumber these options to
       values within the remaining range.  If a site needs more than 31
       site-specific options, the site must switch to using suboptions, as
       has been done for other options, such as the Relay Agent Information
       Option [RFC3046].

    5.  Security Considerations

       This document in and by itself provides no security, nor does it
       impact existing DCHP security as described in [RFC2131].

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    6. IANA Considerations

       IANA is requested to

       1. expand the publicly defined DHCPv4 options space from 1 - 127 to 1
          - 223.  The new options (128 - 223) are to be listed as
          "Unavailable" and MUST NOT be assigned to any publicly defined
          options.

       2. receive notices from vendors that have been using one or more of
          the options in the 128-223 range that they are using the option
          and are willing to document that usage.  IANA will list these
          options as "Tentatively Assigned".

       3. change the listing of any options listed as "Unavailable" to
          "Available" 6 months from this RFC's publication date.  These
          options may now be assigned in accordance with [RFC2939].

       4. change the listing of any options listed as "Tentatively-Assigned"
          to "Unavailable" 18 months from this RFC's publication date and
          periodically thereafter as long as there is an option listed as
          "Tentatively-Assigned", if no un-expired Internet-Draft exists
          documenting the usage.

    7.  Acknowledgements

       Many thanks to Ralph Droms and Ted Lemon for their valuable input and
       earlier work on the various alternatives.

    8.  References

    8.1.  Normative References

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

       [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.

       [RFC2939]  Droms, R., "Procedures and IANA Guidelines for Definition
                  of New DHCP Options and Message Types", BCP 43, RFC 2939,
                  September 2000.

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    8.2.  Informative References

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

       [RFC3679]  Droms, R., "Unused Dynamic Host Configuration Protocol
                  (DHCP) Option Codes", RFC 3679, January 2004.

    Author's Address

       Bernard Volz
       Cisco Systems, Inc.
       1414 Massachusetts Ave.
       Boxborough, MA  01719
       USA

       Phone: +1 978 936 0382
       EMail: volz@cisco.com

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    RFC 3942              Reclassifying DHCPv4 Options         November 2004

    Full Copyright Statement

       Copyright (C) The Internet Society (2004).

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    Acknowledgement

       Funding for the RFC Editor function is currently provided by the
       Internet Society.

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