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rfc: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.

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

Volz Standards Track [Page 1] RFC 3942 Reclassifying DHCPv4 Options November 2004

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

Volz Standards Track [Page 2] RFC 3942 Reclassifying DHCPv4 Options November 2004

  • 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:

Volz Standards Track [Page 3] 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].

Volz Standards Track [Page 4] RFC 3942 Reclassifying DHCPv4 Options November 2004

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.

Volz Standards Track [Page 5] RFC 3942 Reclassifying DHCPv4 Options November 2004

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

Volz Standards Track [Page 6] RFC 3942 Reclassifying DHCPv4 Options November 2004

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.
 This document and the information contained herein are provided on an
 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
 ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
 INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

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Acknowledgement

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

Volz Standards Track [Page 7]

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