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Versions: 00 01 RFC 3942

Network Working Group                                            B. Volz
Internet-Draft                                            (Unaffiliated)
Expires: July 5, 2004                                    January 5, 2004

                      Reclassifying DHCPv4 Options

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that other
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   Internet-Drafts are draft documents valid for a maximum of six months
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   The list of current Internet-Drafts can be accessed at http://

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   This Internet-Draft will expire on July 5, 2004.

Copyright Notice

   Copyright (C) The Internet Society (2004). All Rights Reserved.


   This document revises RFC 2132 to reclassify 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.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Requirements notation  . . . . . . . . . . . . . . . . . . . .  3
   3.  Background . . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.1 Publicly Defined Options Range . . . . . . . . . . . . . . . .  3
   3.2 Site-Specific Options Range  . . . . . . . . . . . . . . . . .  4
   4.  Reclassifying Options  . . . . . . . . . . . . . . . . . . . .  4
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . .  5
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  6
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  6
       References . . . . . . . . . . . . . . . . . . . . . . . . . .  6
       Author's Address . . . . . . . . . . . . . . . . . . . . . . .  7
       Intellectual Property and Copyright Statements . . . . . . . .  8

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1. Introduction

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

   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",
   document are to be interpreted as described in [RFC2119].

3. Background

   The DHCP option space (0-255) was divided into two principal ranges:

   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 ([unused-optioncodes]) will buy more time as several
   allocated but never used option codes are reclaimed. And, a review
   could be done from time to time to determine if 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

   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 [unused-optioncodes].

   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
       *  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 has been 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 of requesting site-specific options from a
   server or allowing applications to extract the returned option

   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 can potentially cause problems if a site has been
   using the same site-specific option codes for other purposes and then
   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 publicly document their option usage and
   request an official assignment of the option number to that usage,
   the following procedure will be used to reclassify these options:

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

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       for assignment to publicly defined options.

   2.  Vendors that currently use one or more of the reclassified
       options have until 6 months after 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 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 until 18 months from 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 that are 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 Option

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
   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.  6 months from this RFC's publication date, change the listing of
       any options listed as "Unavailable" to "Available". These options
       may now be assigned in accordance with [RFC2939].
   4.  18 months from this RFC's publication date and periodically
       thereafter as long as there is an option listed as
       "Tentatively-Assigned", change the listing of any options listed
       as "Tentatively-Assigned" to "Unavailable" 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.


   [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.

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

              Droms, R., "Unused DHCP Option Codes",
              draft-ietf-dhc-unused-optionscodes-07.txt (work in
              progress), October 2003.

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Author's Address

   Bernard Volz
   116 Hawkins Pond Road
   Center Harbor, NH  03226

   Phone: +1 603 968 3062
   EMail: volz@metrocast.net

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