Dynamic Host Configuration Working                            D. Hankins
Group                                                             Google
Internet-Draft                                              T. Mrugalski
Updates: 3315 (if approved)                                 M. Siodelski
Intended status: Standards Track                                     ISC
Expires: August 29, 2013                                        S. Jiang
                                            Huawei Technologies Co., Ltd
                                                             S. Krishnan
                                                       February 25, 2013

               Guidelines for Creating New DHCPv6 Options


   This document provides guidance to prospective DHCPv6 Option
   developers to help them creating option formats that are easily
   adoptable by existing DHCPv6 software.  This document updates

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on August 29, 2013.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents

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   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Requirements Language  . . . . . . . . . . . . . . . . . . . .  3
   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  When to Use DHCPv6 . . . . . . . . . . . . . . . . . . . . . .  4
   4.  General Principles . . . . . . . . . . . . . . . . . . . . . .  4
   5.  Reusing Other Options  . . . . . . . . . . . . . . . . . . . .  5
     5.1.  Option with IPv6 addresses . . . . . . . . . . . . . . . .  5
     5.2.  Option with a single flag (boolean)  . . . . . . . . . . .  6
     5.3.  Option with IPv6 prefix  . . . . . . . . . . . . . . . . .  7
     5.4.  Option with 32-bit integer value . . . . . . . . . . . . .  8
     5.5.  Option with 16-bit integer value . . . . . . . . . . . . .  8
     5.6.  Option with 8-bit integer value  . . . . . . . . . . . . .  9
     5.7.  Option with variable length data . . . . . . . . . . . . .  9
     5.8.  Option with DNS Wire Format Domain Name List . . . . . . . 10
   6.  Avoid Conditional Formatting . . . . . . . . . . . . . . . . . 10
   7.  Avoid Aliasing . . . . . . . . . . . . . . . . . . . . . . . . 11
   8.  Choosing between FQDN and address  . . . . . . . . . . . . . . 11
   9.  Suboptions in DHCPv6 . . . . . . . . . . . . . . . . . . . . . 13
   10. Additional States Considered Harmful . . . . . . . . . . . . . 13
   11. Is DHCPv6 dynamic? . . . . . . . . . . . . . . . . . . . . . . 14
   12. Multiple provisioning domains  . . . . . . . . . . . . . . . . 14
   13. Considerations for Creating New Formats  . . . . . . . . . . . 15
   14. Option Size  . . . . . . . . . . . . . . . . . . . . . . . . . 15
   15. Clients Request their Options  . . . . . . . . . . . . . . . . 16
   16. Transition Technologies  . . . . . . . . . . . . . . . . . . . 16
   17. Security Considerations  . . . . . . . . . . . . . . . . . . . 17
   18. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 18
   19. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 18
   20. Informative References . . . . . . . . . . . . . . . . . . . . 18
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 20

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1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  Introduction

   Most protocol developers ask themselves if a protocol will work, or
   work efficiently.  These are important questions, but another less
   frequently considered question is whether the proposed protocol
   presents itself needless barriers to adoption by deployed software.

   DHCPv6 [RFC3315] software implementors are not merely faced with the
   task of handling a given option's format on the wire.  The option
   must fit into every stage of the system's process, starting with the
   user interface used to enter the configuration up to the machine
   interfaces where configuration is ultimately consumed.

   Another frequently overlooked aspect of rapid adoption is whether the
   option requires operators to be intimately familiar with the option's
   internal format in order to use it?  Most DHCPv6 software provides a
   facility for handling unknown options at the time of publication.
   The handling of such options usually needs to be manually configured
   by the operator.  But if doing so requires extensive reading (more
   than can be covered in a simple FAQ for example), it inhibits

   So although a given solution would work, and might even be space,
   time, or aesthetically optimal, a given option is presented with a
   series of ever-worsening challenges to be adopted;

   o  If it doesn't fit neatly into existing config files.

   o  If it requries new source code changes to be adopted, and hence
      upgrades of deployed software.

   o  If it does not share its deployment fate in a general manner with
      other options, standing alone in requiring code changes or
      reworking configuration file syntaxes.

   There are many things DHCPv6 option creators can do to avoid the
   pitfalls in this list entirely, or failing that, to make software
   implementors lives easier and improve its chances for widespread

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3.  When to Use DHCPv6

   Principally, DHCPv6 carries configuration parameters for its clients.
   Any knob, dial, slider, or checkbox on the client system, such as "my
   domain name servers", "my hostname", or even "my shutdown
   temperature" are candidates for being configured by DHCPv6.

   The presence of such a knob isn't enough, because DHCPv6 also
   presents the extension of an administrative domain - the operator of
   the network to which the client is currently attached.  Someone runs
   not only the local switching network infrastructure that the client
   is directly (or wirelessly) attached to, but the various methods of
   accessing the external Internet via local assist services that
   network must also provide (such as domain name servers, or routers).
   This means that in addition to the existence of a configuration
   parameter, one must also ask themselves if it is reasonable for this
   parameter to be set by the directly attached network's

   Note that the client still reserves the right to ignore values
   received via DHCPv6 (for example, due to having a value manually
   configured by its own operator).  Bear in mind that doing so might
   cause the client to be rejected network attachment privileges, and
   this is one main reason for the use of DHCPv6 in corporate

4.  General Principles

   The primary guiding principle to follow in order to enhance an
   option's adoptability is simplification.  More specifically, the
   option should be created in such a way that does not require any new
   or special case software to support.  If old software currently
   deployed and in the field can adopt the option through supplied
   configuration facilities then it's fairly certain that new software
   can easily formally adopt it.

   There are at least two classes of DHCPv6 options: A bulk class of
   options which are provided explicitly to carry data from one side of
   the DHCPv6 exchange to the other (such as nameservers, domain names,
   or time servers), and a protocol class of options which require
   special processing on the part of the DHCPv6 software or are used
   during special processing (such as the Fully Qualified Domain Name
   (FQDN) option [RFC4704]), and so forth; these options carry data that
   is the result of a routine in some DHCPv6 software.

   The guidelines laid out here should be applied in a relaxed manner
   for the protocol class of options.  Wherever special case code is

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   already required to adopt the DHCPv6 option, it is substantially more
   reasonable to format the option in a less generic fashion, if there
   are measurable benefits to doing so.

5.  Reusing Other Options

   The easiest approach to manufacturing trivially deployable DHCPv6
   Options is to assemble the option out of whatever common fragments
   fit - possibly allowing a group of fragments to repeat to fill the
   remaining space (if present) and so provide multiple values.  Place
   all fixed size values at the start of the option, and any variable/
   indeterminate sized value at the tail end of the option.

   This estimates that implementations will be able to reuse code paths
   designed to support the other options.

   There is a tradeoff between the adoptability of previously defined
   option formats, and the advantages that new or specialized formats
   can provide.  In general, it is usually preferrable to reuse
   previously used option formats.

   However, it isn't very practical to consider the bulk of DHCPv6
   options already allocated, and consider which of those solve a
   similar problem.  So, the following list of common option format
   fragments is provided as a shorthand.  Please note that it is not
   complete in terms of exampling every option format ever devised.  It
   is only a list of option format fragments which are used in two or
   more options.

5.1.  Option with IPv6 addresses

   This option format is used to carry one or many IPv6 addresses.  In
   some cases the number of allowed address is limited (e.g. to one):

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      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |          option-code          |           option-len          |
     |                                                               |
     |                         ipv6-address                          |
     |                                                               |
     |                                                               |
     |                                                               |
     |                         ipv6-address                          |
     |                                                               |
     |                                                               |
     |                              ...                              |

                    Figure 1: Option with IPv6 address

   Examples of use:

   o  DHCPv6 server unicast address [RFC3315]

   o  SIP Servers IPv6 Address List [RFC3319]

   o  DNS Recursive Name Server [RFC3646]

   o  NIS Servers [RFC3898]

   o  SNTP Servers [RFC4075]

   o  Broadcast and Multicast Service Controller IPv6 Address Option for
      DHCPv6 [RFC4280]

   o  MIPv6 Home Agent Address [RFC6610] (a single address only)

   o  NTP server [RFC5908] (a single address only)

   o  NTP Multicast address [RFC5908] (a single address only)

5.2.  Option with a single flag (boolean)

   Sometimes it is useful to convey a single flag that can either take
   on or off values.  Instead of specifying an option with one bit of
   usable data and 7 bits of padding, it is better to define an option
   without any content.  It is the presence or absence of the option
   that conveys the value.  This approach has the additional benefit of

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   absent option designating the default, i.e. administrator has to take
   explicit actions to deploy the oposite of the default value.
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |          option-code          |           option-len          |

                  Figure 2: Option for conveying boolean

   Examples of use:

   o  DHCPv6 rapid-commit [RFC3315]

5.3.  Option with IPv6 prefix

   Sometimes there is a need to convey IPv6 prefix.  The information to
   be carried by an option includes the 128-bit IPv6 prefix together
   with a length of this prefix taking values from 0 to 128.  Using the
   simplest approach, the option could convey this data in two fixed
   length fields: one carrying prefix length, another carrying the
   prefix.  However, in many cases /64 or shorter prefixes are used.
   This implies that the large part of the prefix data carried by the
   option would have its bits set to zero and would be unused.  In order
   to avoid carrying unused data, it is recommended to store prefix in
   the variable length data field.  The appropriate option format is
   defined as follows:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |          option-code          |         option-length         |
     |  prefix6-len  |              ipv6-prefix                      |
     +-+-+-+-+-+-+-+-+           (variable length)                   |
     .                                                               .

                     Figure 3: Option with IPv6 Prefix

   option-length is set to 1 + length of the IPv6 prefix. prefix6-len is
   one octet long and specifies the length in bits of the IPv6 prefix.
   Typically allowed values are 0 to 128.

   ipv6-prefix field is a variable length field that specifies the IPv6
   prefix.  This field is padded with zeros up to the nearest octet
   boundary when prefix6-len is not divisible by 8.

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   Examples of use:

   o  Default Mapping Rule [I-D.ietf-softwire-map-dhcp]

   It should be noted that Prefix Delegation mechanism used in [RFC3633]
   uses constant length prefixes.  The concern about option length was
   not well understood at the time of its publication.

5.4.  Option with 32-bit integer value

   This option format can be used to carry 32 bit-signed or unsigned
   integer value:
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |          option-code          |           option-len          |
     |                         32-bit-integer                        |

                Figure 4: Option with 32-bit-integer value

   Examples of use:

   o  Information Refresh Time [RFC4242]

5.5.  Option with 16-bit integer value

   This option format can be used to carry 16-bit signed or unsigned
   integer values:
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |          option-code          |           option-len          |
     |         16-bit-integer        |

                Figure 5: Option with 16-bit integer value

   Examples of use:

   o  Elapsed Time [RFC3315]

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5.6.  Option with 8-bit integer value

   This option format can be used to carry 8-bit integer values:
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |          option-code          |          option-len           |
     | 8-bit-integer |

                 Figure 6: Option with 8-bit integer value

   Examples of use:

   o  DHCPv6 Preference [RFC3315]

5.7.  Option with variable length data

   This option can be used to carry variable length data of any kind.
   Internal representation of carried data is option specific.  Some of
   the existing DHCPv6 options use NVT-ASCII strings to encode:
   filenames, host or domain names, protocol features or textual
   messages such as verbose error indicators.

   This option format provides a lot of flexibility to pass data of
   almost any kind.  Though, whenever possible it is highly recommended
   to use more specialized options, with field types better matching
   carried data types.
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |          option-code          |         option-len            |
     .                                                               .
     .                      variable length data                     .
     .                                                               .

                 Figure 7: Option with variale length data

   Examples of use:

   o  Client Identifier [RFC3315]

   o  Server Identifier [RFC3315]

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   o  Boot File URL [RFC5970]

5.8.  Option with DNS Wire Format Domain Name List

   This option is used to carry 'domain search' lists or any host or
   domain name:
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |          option-code          |         option-length         |
     |               DNS Wire Format Domain Name List                |
     |                              ...                              |

          Figure 8: Option with DNS Wire Format Domain Name List

   Examples of use:

   o  SIP Servers Domain Name List [RFC3319] (many domains)

   o  NIS Domain Name (many domains) [RFC3898] (many domains)

   o  DS-Lite AFTR location [RFC6334] (a single FQDN)

   o  Home Network Identifier [RFC6610] (a single FQDN)

   o  Home Agent FQDN [RFC6610] (a single FQDN)

6.  Avoid Conditional Formatting

   Placing an octet at the start of the option which informs the
   software how to process the remaining octets of the option may appear
   simple to the casual observer.  But the only conditional formatting
   methods that are in widespread use today are 'protocol' class
   options.  Therefore the conditional formatting requires new code to
   be written, as well as introduces an implementation problem; as it
   requires that all speakers implement all current and future
   conditional formats.

   Conditional formatting is not recommended, except in cases where the
   DHCPv6 option has already been deployed experimentally, and all but
   one conditional format is deprecated.

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7.  Avoid Aliasing

   Options are said to be aliases of each other if they provide input to
   the same configuration parameter.  A commonly proposed example is to
   configure the location of some new service ("my foo server") using a
   binary IP address, a domain name field, and an URL.  This kind of
   aliasing is undesirable, and is not recommended.

   In this case, where three different formats are supposed, it more
   than triples the work of the software involved, requiring support for
   not merely one format, but support to produce and digest all three.
   Furthermore, code development and testing must cover all possible
   combinations of defined formats.  Since clients cannot predict what
   values the server will provide, they must request all formats.  So in
   the case where the server is configured with all formats, DHCPv6
   option space is wasted on option contents that are redundant.

   It also becomes unclear which types of values are mandatory, and how
   configuring some of the options may influence the others.  For
   example, if an operator configures the URL only, should the server
   synthesize a domain name and IP address?

   A single configuration value on a host is probably presented to the
   operator (or other software on the machine) in a single field or
   channel.  If that channel has a natural format, then any alternative
   formats merely make more work for intervening software in providing

   So the best advice is to choose the one method that best fulfills the
   requirements, be that for simplicity (such as with an IP address and
   port pair), late binding (such as with DNS), or completeness (such as
   with a URL).

8.  Choosing between FQDN and address

   Some parameters may be specified as FQDN or an address.  It is not
   allowed to define both option types at the same time (see section
   Section 7), so one of them must be chosen.  This section is intended
   as a help to make an informed decision in that regard.

   On the specific subject of desiring to configure a value using a FQDN
   instead of a binary IP address, note that most DHCPv6 server
   implementations will happily accept a Domain Name entered by the
   administrator, and use DNS resolution to render binary IP addresses
   in DHCPv6 replies to clients.  Consequently, consider the extra
   packet overhead incurred on the client's end to perform DNS
   resolution itself.  The client may be operating on a battery and

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   packet transmission is a non-trivial use of power, and the extra RTT
   delays the client must endure before the service is configured are at
   least two factors to consider in making a decision on format.

   Unless there are specific reasons to do otherwise, address should be
   used.  It is simpler to use, its validation is trivial (length of 16
   constitutes a valid option), is explicit and does not allow any
   ambiguity.  It is faster (does not require extra resolution efforts),
   so it is more efficient, which can be especially important for energy
   restricted devices.

   FQDN does require a resolution into an actual address.  This implies
   the question when the FQDN resolution should be taken.  There are a
   couple of possible answers: a) by the server, when it is started, b)
   by the server, when it is about to send an option, c) by the client,
   immediately after receiving an option, d) by the client, when the
   content of the option is actually consumed.  For a), b) and possibly
   c), the option should really convey an address, not FQDN.  The only
   real incentive to use FQDN is case d).  It is the only case that
   allows possible changes in the DNS to be picked up by clients.

   FQDN imposes number of additional failure modes and issues that
   should be dealt with:

      The client must have a knowledge about available DNS servers.
      That typically means that option DNS_SERVERS is mandatory.  This
      should be mentioned in the draft that defines new option.  It is
      possible that the server will return FQDN option, but not the DNS
      Servers option.  There should be a brief discussion about it;

      The DNS may not be reachable;

      DNS may be available, but may not have appropriate information
      (e.g. no AAAA records for specified FQDN)

      Address family must be specified (A, AAAA or any);

      What should the client do if there are multiple records available
      (use only the first one, use all, use one and switch to the second
      if the first fails for whatever reason, etc.);

      Multi-homed devices may be connected to different administrative
      domains with each domain providing a different information in DNS
      (e.g. an enterprise network exposing private domains).  Client may
      send DNS queries to a different DNS server;

      It should be mentioned if Internationalized Domain Names are
      allowed.  If they are, what kind of DNS option encoding should be

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9.  Suboptions in DHCPv6

   Most options are conveyed in a DHCPv6 message directly.  Although
   there is no codified normative language for such options, they are
   often referred to as top-level options.  Many options may include
   other options.  Such inner options are often referred to as sub-
   options.  It should be noted that, contrary to DHCPv4, there is no
   shortage of option numbers.  Therefore all options share a common
   option space.  For example option type 1 meant different things in
   DHCPv4, depending if it was located in top-level or inside of Relay
   Agent Information option.  There is no such ambiguity in DHCPv6 (with
   the exception of [RFC5908]).

   Such encapsulation mechanism is not limited to one level.  There is
   at least one defined option that is encapsulated twice: Identity
   Association for Prefix Delegation (IA_PD, defined in [RFC3633],
   section 9) conveys IA Prefix (IAPREFIX, defined in [RFC3633], section
   10).  Such delegated prefix may contain an excluded prefix range that
   is represented by PD_EXCLUDE option that is conveyed as sub-option
   inside IAPREFIX (PD_EXCLUDE, defined in [RFC6603]).  It seems awkward
   to refer to such options as sub-sub-option, therefore "sub-option"
   term is typically used, regardless of the nesting level.

   When defining configuration means for more complex mechanisms, it may
   be tempting to simply use sub-options.  That should usually be
   avoided, as it increases complexity of the parser.  It is much
   easier, faster and less error prone to parse larger number of options
   on a single (top-level) scope, than parse options on several scopes.
   The use of sub-options should be avoided as much as possible but it
   is better to use sub-options rather than conditional formatting.

   It should be noted that currently there is no clear way defined for
   requesting sub-options.  Most known implementations are simply using
   top-level ORO for requesting both top-level options and sub-options.

10.  Additional States Considered Harmful

   DHCP is a protocol designed for provisioning nodes.  Less experienced
   protocol designers often assume that it is easy to define an option
   that will convey a different parameter for each node in a network.
   Such problems arose during designs of MAP
   [I-D.ietf-softwire-map-dhcp] and 4rd [I-D.ietf-softwire-4rd].  While
   it would be easier for provisioned nodes to get ready to use per node
   option values, such requirement puts exceedingly large loads on the

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   server side.  Alternatives should be considered, if possible.  As an
   example, [I-D.ietf-softwire-map-dhcp] was designed in a way that all
   nodes are provisioned with the same set of MAP options and each
   provisioned node uses its unique address and delegated prefix to
   generate node-specific information.  Such solution does not introduce
   any additional state for the server and therefore scales better.

   It also should be noted that contrary to DHCPv4, DHCPv6 keeps several
   timers for renewals.  Each IA_NA (addresses) and IA_PD (prefixes)
   contains T1 and T2 timers that designate time after which client will
   initiate renewal.  Those timers apply only to its own IA containers.
   For renewing other parameters, please use Information Refresh Time
   Option (defined in [RFC4242]).  Introducing additional timers make
   deployment unnecessarily complex and should be avoided.

11.  Is DHCPv6 dynamic?

   DHCPv6 stands for Dynamic Host Configuration Protocol for IPv6.
   Contrary to its name, in many contexts it is not dynamic.  While
   designing DHCPv6 options, it is worth noting that there is no
   reliable way to instantly notify clients that something has happened,
   e.g. parameter value has changed.  There is a RECONFIGURE mechanism,
   but it has several serious drawbacks that makes its use difficult.
   First, its support is optional and many client implementations do not
   support it.  To use reconfigure mechanism, server must use its secret
   nonce.  That means that provisioning server is the only one that can
   initiate reconfiguration.  Other servers do not know it and cannot
   trigger reconfiguration.  Therefore the only reliable way for clients
   to refresh their configuration is to wait till T1 expires.

12.  Multiple provisioning domains

   In some cases there could be more than one DHCPv6 server on a link,
   with each provisioning a different set of parameters.  One notable
   example of such case is a home network with a connection to two
   independent ISPs.

   DHCPv6 was not initially designed with multiple provisioning domains.
   Although [RFC3315] states that a client that receives more than one
   ADVERTISE message, may respond to one or more of them, such
   capability was never observed in any known implementations.  Existing
   clients will pick one server and will continue configuration process
   with that server, ignoring all other servers.

   This is a generic DHCP protocol issue and should not be dealt within
   each option separately.  This issue is better dealt with using a

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   protocol-level solution and fixing this problem should not be
   attempted on a per option basis.

13.  Considerations for Creating New Formats

   If the option simply will not fit into any existing work by using
   fragments, the last recourse is to create a new format to fit.

   When doing so, it is not enough to gauge whether or not the option
   format will work in the context of the option presently being
   considered.  It is equally important to consider if the new format's
   fragments might reasonably have any other uses, and if so, to create
   the option with the foreknowledge that its parts may later become a
   common fragment.

   One specific consideration to evaluate is whether or not options of a
   similar format would need to have multiple or single values encoded
   (whatever differs from the current option), and how that might be
   accomplished in a similar format.

14.  Option Size

   DHCPv6 [RFC3315] allows for packet sizes up to 64KB.  First, through
   its use of link-local addresses, it steps aside many of the
   deployment problems that plague DHCPv4, and is actually an UDP over
   IPv6 based protocol (compared to DHCPv4, which is mostly UDP over
   IPv4 protocol, but with layer 2 hacks).  Second, RFC 3315 explicitly
   refers readers to RFC 2460 Section 5, which describes an MTU of 1280
   octets and a minimum fragment reassembly of 1500 octets.  It's
   feasible to suggest that DHCPv6 is capable of having larger options
   deployed over it, and at least no common upper limit is yet known to
   have been encoded by its implementors.  It is impossible to describe
   any fixed limit that cleanly divides those too big from the workable.

   It is advantageous to prefer option formats which contain the desired
   information in the smallest form factor that satisfies the

   DHCPv6 does allow for multiple instances of a given option, and they
   are treated as distinct values following the defined format, however
   this feature is generally preferred to be restricted to protocol
   class features (such as the IA_* series of options).  In such cases,
   it is better to define an option as an array if it is possible.  It
   is recommended to clarify (with normative language) whether a given
   DHCPv6 option may appear once or multiple times.

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15.  Clients Request their Options

   The DHCPv6 Option Request Option (OPTION_ORO) [RFC3315], is an option
   that serves two purposes - to inform the server what options the
   client supports and to inform what options the client is willing to

   It doesn't make sense for some options to be requested using Option
   Request Option, such as those formed by elements of the protocol's
   internal workings, or are formed on either end by DHCPv6-level
   software engaged in some exchange of information.  When in doubt, it
   is prudent to assume that any new option must be present on the
   relevant option request list if the client desires to receive it.

   It is a frequent mistake of option draft authors, then, to create
   text that implies that a server will simply provide the new option,
   and clients will digest it.  Generally, it's best to also specify
   that clients MUST place the new option code on the Option Request
   Option list, clients MAY include the new option in their packets to
   servers with hints as values they desire, and server MAY include the
   option when the client requested it (and the server has been so

   Example: Clients MUST place the foo option code on the Option Request
   Option list, clients MAY include option foo in their packets as hints
   for the server as values the desire, and servers MAY include option
   foo when the client requested it (and the server has been so

   Creators of DHCPv6 options MUST NOT require special ordering of
   options either in the relevant request option, or in the order of
   options within the packet.  Although it is reasonable to expect that
   options will be processed in the order they appear in ORO, server
   software is not required to sort DHCPv6 options into the same order
   in reply messages.  It should be noted that any requirement regarding
   option ordering will break down most existing implementations, as
   "order is not important" was one of the design priciples of DHCPv6
   and many implementations follow it.  For example, there are existing
   implementations that use hash maps for storing options, so forcing
   any particular order is not feasible without great deal of work.  If
   options must be processed in any specific order (e.g. due to inter-
   dependency), use of option encapsulation should be considered.

16.  Transition Technologies

   Transition from IPv4 to IPv6 is progressing, albeit at somewhat
   disappointing pace.  Many transition technologies are proposed to

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   speed it up.  As a natural consequence there are also DHCP options
   proposed to provision those proposals.  The inevitable question is
   that whether the required parameters should be delivered over DHCPv4
   or DHCPv6.  Authors often don't give much thought about it and simply
   pick DHCPv6 without realizing the consequences.  IPv6 is expected to
   stay with us for many decades, and so is DHCPv6.  There is no
   mechanism available to deprecate an option in DHCPv6, so any options
   defined will stay with us as long as DHCPv6 protocol itself.  It
   seems likely that such options defined to transition from IPv4 will
   outlive IPv4 by many decades.  From that perspective it is better to
   implement provisioning of the transition technologies in DHCPv4,
   which will be obsoleted together with IPv4.

17.  Security Considerations

   DHCPv6 does have an Authentication mechanism ([RFC3315]) that makes
   it possible for DHCPv6 software to discriminate between authentic
   endpoints and men in the middle.  Other authentication mechanisms may
   optionally be deployed.  For example, the Secure DHCPv6
   [I-D.ietf-dhc-secure-dhcpv6], based on Cryptographically Generated
   Addresses (CGA) [RFC3972], can provide source address ownership
   validation, message origin authentication and message integrity
   without requiring symmetric key pairs or supporting from any key
   management system.  However, as of now, the mechanism is not widely
   deployed.  It also does not provide end-to-end encryption.

   So, while creating a new option, it is prudent to assume that the
   DHCPv6 packet contents are always transmitted in the clear, and
   actual production use of the software will probably be vulnerable at
   least to man-in-the-middle attacks from within the network, even
   where the network itself is protected from external attacks by
   firewalls.  In particular, some DHCPv6 message exchanges are
   transmitted to multicast addresses that are likely broadcast anyway.

   If an option is of a specific fixed length, it is useful to remind
   the implementer of the option data's full length.  This is easily
   done by declaring the specific value of the 'length' tag of the
   option.  This helps to gently remind implementers to validate option
   length before digesting them into likewise fixed length regions of
   memory or stack.

   If an option may be of variable size (such as having indeterminate
   length fields, such as domain names or text strings), it is advisable
   to explicitly remind the implementor to be aware of the potential for
   long options.  Either define a reasonable upper limit (and suggest
   validating it), or explicitly remind the implementor that an option
   may be exceptionally long (to be prepared to handle errors rather

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   than truncate values).

   For some option contents, out of bound values may be used to breach
   security.  An IP address field might be made to carry a loopback
   address, or local broadcast address, and depending on the protocol
   this may lead to undesirable results.  A domain name field may be
   filled with contrived contents that exceed the limitations placed
   upon domain name formatting - as this value is possibly delivered to
   "internal configuration" records of the system, it may be implicitly
   trusted without being validated.

   So it behooves an option's definition to contain any validation
   measures as can reasonably be made.

18.  IANA Considerations

   This document has no actions for IANA.

19.  Acknowledgements

   Authors would like to thank Simon Perreault, Bernie Volz and Ted
   Lemon for their comments.

20.  Informative References

              Jiang, S. and S. Shen, "Secure DHCPv6 Using CGAs",
              draft-ietf-dhc-secure-dhcpv6-07 (work in progress),
              September 2012.

              Jiang, S., Despres, R., Penno, R., Lee, Y., Chen, G., and
              M. Chen, "IPv4 Residual Deployment via IPv6 - a Stateless
              Solution (4rd)", draft-ietf-softwire-4rd-04 (work in
              progress), October 2012.

              Mrugalski, T., Troan, O., Dec, W., Bao, C.,
              leaf.yeh.sdo@gmail.com, l., and X. Deng, "DHCPv6 Options
              for Mapping of Address and Port",
              draft-ietf-softwire-map-dhcp-03 (work in progress),
              February 2013.

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

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   [RFC3315]  Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
              and M. Carney, "Dynamic Host Configuration Protocol for
              IPv6 (DHCPv6)", RFC 3315, July 2003.

   [RFC3319]  Schulzrinne, H. and B. Volz, "Dynamic Host Configuration
              Protocol (DHCPv6) Options for Session Initiation Protocol
              (SIP) Servers", RFC 3319, July 2003.

   [RFC3633]  Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic
              Host Configuration Protocol (DHCP) version 6", RFC 3633,
              December 2003.

   [RFC3646]  Droms, R., "DNS Configuration options for Dynamic Host
              Configuration Protocol for IPv6 (DHCPv6)", RFC 3646,
              December 2003.

   [RFC3898]  Kalusivalingam, V., "Network Information Service (NIS)
              Configuration Options for Dynamic Host Configuration
              Protocol for IPv6 (DHCPv6)", RFC 3898, October 2004.

   [RFC3972]  Aura, T., "Cryptographically Generated Addresses (CGA)",
              RFC 3972, March 2005.

   [RFC4075]  Kalusivalingam, V., "Simple Network Time Protocol (SNTP)
              Configuration Option for DHCPv6", RFC 4075, May 2005.

   [RFC4242]  Venaas, S., Chown, T., and B. Volz, "Information Refresh
              Time Option for Dynamic Host Configuration Protocol for
              IPv6 (DHCPv6)", RFC 4242, November 2005.

   [RFC4280]  Chowdhury, K., Yegani, P., and L. Madour, "Dynamic Host
              Configuration Protocol (DHCP) Options for Broadcast and
              Multicast Control Servers", RFC 4280, November 2005.

   [RFC4704]  Volz, B., "The Dynamic Host Configuration Protocol for
              IPv6 (DHCPv6) Client Fully Qualified Domain Name (FQDN)
              Option", RFC 4704, October 2006.

   [RFC5908]  Gayraud, R. and B. Lourdelet, "Network Time Protocol (NTP)
              Server Option for DHCPv6", RFC 5908, June 2010.

   [RFC5970]  Huth, T., Freimann, J., Zimmer, V., and D. Thaler, "DHCPv6
              Options for Network Boot", RFC 5970, September 2010.

   [RFC6334]  Hankins, D. and T. Mrugalski, "Dynamic Host Configuration
              Protocol for IPv6 (DHCPv6) Option for Dual-Stack Lite",
              RFC 6334, August 2011.

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   [RFC6603]  Korhonen, J., Savolainen, T., Krishnan, S., and O. Troan,
              "Prefix Exclude Option for DHCPv6-based Prefix
              Delegation", RFC 6603, May 2012.

   [RFC6610]  Jang, H., Yegin, A., Chowdhury, K., Choi, J., and T.
              Lemon, "DHCP Options for Home Information Discovery in
              Mobile IPv6 (MIPv6)", RFC 6610, May 2012.

Authors' Addresses

   David W. Hankins
   Google, Inc.
   1600 Amphitheatre Parkway
   Mountain View, CA  94043

   Email: dhankins@google.com

   Tomasz Mrugalski
   Internet Systems Consortium, Inc.
   950 Charter Street
   Redwood City, CA  94063

   Phone: +1 650 423 1345
   Email: tomasz.mrugalski@gmail.com

   Marcin Siodelski
   950 Charter Street
   Redwood City, CA  94063

   Phone: +1 650 423 1431
   Email: msiodelski@gmail.com

   Sheng Jiang
   Huawei Technologies Co., Ltd
   Q14, Huawei Campus, No.156 Beiqing Road
   Hai-Dian District, Beijing, 100095
   P.R. China

   Email: jiangsheng@huawei.com

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   Suresh Krishnan
   8400 Blvd Decarie
   Town of Mount Royal, Quebec

   Email: suresh.krishnan@ericsson.com

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