path: root/doc/rfc/rfc3942.txt

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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   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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Full Copyright Statement

   Copyright (C) The Internet Society (2004).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

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Acknowledgement

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







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