RFC3942

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.

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.

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.

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.

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

  *  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.

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.

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:

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.

Security Considerations

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

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.

Acknowledgements

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

References

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.

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: [email protected]

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