[Docs] [txt|pdf] [draft-ietf-dhc-op...] [Diff1] [Diff2] [Errata]

BEST CURRENT PRACTICE
Errata Exist
Internet Engineering Task Force (IETF)                        D. Hankins
Request for Comments: 7227                                        Google
BCP: 187                                                    T. Mrugalski
Updates: 3315                                               M. Siodelski
Category: Best Current Practice                                      ISC
ISSN: 2070-1721                                                 S. Jiang
                                           Huawei Technologies Co., Ltd.
                                                             S. Krishnan
                                                                Ericsson
                                                                May 2014


               Guidelines for Creating New DHCPv6 Options

Abstract

   This document provides guidance to prospective DHCPv6 option
   developers to help them create option formats that are easily
   adoptable by existing DHCPv6 software.  It also provides guidelines
   for expert reviewers to evaluate new registrations.  This document
   updates RFC 3315.

Status of This Memo

   This memo documents an Internet Best Current Practice.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   BCPs is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7227.
















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Copyright Notice

   Copyright (c) 2014 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
   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.





































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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   5
   3.  When to Use DHCPv6  . . . . . . . . . . . . . . . . . . . . .   5
   4.  General Principles  . . . . . . . . . . . . . . . . . . . . .   5
   5.  Reusing Other Option Formats  . . . . . . . . . . . . . . . .   6
     5.1.  Option with IPv6 Addresses  . . . . . . . . . . . . . . .   7
     5.2.  Option with Single Flag (Boolean) . . . . . . . . . . . .   8
     5.3.  Option with IPv6 Prefix . . . . . . . . . . . . . . . . .   9
     5.4.  Option with 32-bit Integer Value  . . . . . . . . . . . .  10
     5.5.  Option with 16-bit Integer Value  . . . . . . . . . . . .  10
     5.6.  Option with 8-bit Integer Value . . . . . . . . . . . . .  11
     5.7.  Option with URI . . . . . . . . . . . . . . . . . . . . .  11
     5.8.  Option with Text String . . . . . . . . . . . . . . . . .  12
     5.9.  Option with Variable-Length Data  . . . . . . . . . . . .  13
     5.10. Option with DNS Wire Format Domain Name List  . . . . . .  14
   6.  Avoid Conditional Formatting  . . . . . . . . . . . . . . . .  15
   7.  Avoid Aliasing  . . . . . . . . . . . . . . . . . . . . . . .  15
   8.  Choosing between an FQDN and an Address . . . . . . . . . . .  16
   9.  Encapsulated Options in DHCPv6  . . . . . . . . . . . . . . .  19
   10. Additional States Considered Harmful  . . . . . . . . . . . .  20
   11. Configuration Changes Occur at Fixed Times  . . . . . . . . .  21
   12. Multiple Provisioning Domains . . . . . . . . . . . . . . . .  21
   13. Chartering Requirements and Advice for Responsible Area
       Directors . . . . . . . . . . . . . . . . . . . . . . . . . .  22
   14. Considerations for Creating New Formats . . . . . . . . . . .  23
   15. Option Size . . . . . . . . . . . . . . . . . . . . . . . . .  23
   16. Singleton Options . . . . . . . . . . . . . . . . . . . . . .  24
   17. Option Order  . . . . . . . . . . . . . . . . . . . . . . . .  25
   18. Relay Options . . . . . . . . . . . . . . . . . . . . . . . .  25
   19. Clients Request Their Options . . . . . . . . . . . . . . . .  26
   20. Transition Technologies . . . . . . . . . . . . . . . . . . .  26
   21. Recommended Sections in the New Document  . . . . . . . . . .  27
     21.1.  DHCPv6 Client Behavior Text  . . . . . . . . . . . . . .  28
     21.2.  DHCPv6 Server Behavior Text  . . . . . . . . . . . . . .  28
     21.3.  DHCPv6 Relay Agent Behavior Text . . . . . . . . . . . .  29
   22. Should the New Document Update Existing RFCs? . . . . . . . .  29
   23. Security Considerations . . . . . . . . . . . . . . . . . . .  29
   24. Privacy Considerations  . . . . . . . . . . . . . . . . . . .  31
   25. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  31
   26. References  . . . . . . . . . . . . . . . . . . . . . . . . .  31
     26.1.  Normative References . . . . . . . . . . . . . . . . . .  31
     26.2.  Informative References . . . . . . . . . . . . . . . . .  32







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1.  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
   adoption.

   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 configuration files.

   o  If it requires 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.

   o  If the option would work well in the particular deployment
      environment the proponents currently envision, but it has equally
      valid uses in some other environment where the proposed option
      format would fail or would produce inconsistent results.

   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
   adoption.

   This document is envisaged as a help for protocol developers that
   define new options and for expert reviewers that review submitted
   proposals.



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2.  Requirements Language

   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.  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 to which the
   client is directly (or wirelessly) attached but the various methods
   of accessing the external Internet via local assist services that the
   network must also provide (such as domain name servers or routers).
   This means that, even if a configuration parameter can be potentially
   delivered by DHCPv6, it is necessary to evaluate whether it is
   reasonable for this parameter to be under the control of the
   administrator of whatever network a client is attached to at any
   given time.

   Note that the client is not required to configure any of these values
   received via DHCPv6 (e.g., due to having these values locally
   configured by its own administrator).  But, it needs to be noted that
   overriding DHCPv6-provided values may cause the client to be denied
   certain services in the network to which it has attached.  The
   possibility of having a higher level of control over client node
   configuration is one of the reasons that DHCPv6 is preferred in
   enterprise networks.

4.  General Principles

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

   There are at least two classes of DHCPv6 options: simple options,
   which are provided explicitly to carry data from one side of the
   DHCPv6 exchange to the other (such as name servers, domain names, or
   time servers), and a protocol class of options, which require special



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   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 a special case code is
   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 Option Formats

   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 data elements to repeat to fill the
   remaining space (if present) and thus provide multiple values.  Place
   all fixed-size values at the start of the option and any variable
   -/indeterminate-sized values at the tail end of the option.

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

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

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

   If more complex options are needed, those basic formats mentioned
   here may be considered as primitives (or 'fragment types') that can
   be used to build more complex formats.  It should be noted that it is
   often easier to implement two options with trivial formats than one
   option with a more complex format.  That is not an unconditional
   requirement though.  In some cases, splitting one complex option into
   two or more simple options introduces inter-option dependencies that
   should be avoided.  In such a case, it is usually better to keep one
   complex option.








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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 addresses is limited (e.g., to
   one):

    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 Addresses

   Examples of use:

   o  DHCPv6 Server Unicast Address [RFC3315] (a single address only)

   o  Session Initiation Protocol (SIP) Servers IPv6 Address List
      [RFC3319]

   o  DNS Recursive Name Servers [RFC3646]

   o  Network Information Service (NIS) Servers [RFC3898]

   o  Simple Network Time Protocol (SNTP) Servers [RFC4075]

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

   o  Mobile IPv6 (MIPv6) Home Agent Address [RFC6610] (a single address
      only)

   o  Network Time Protocol (NTP) Server Address [RFC5908] (a single
      address only)




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

5.2.  Option with Single Flag (Boolean)

   Sometimes, it is useful to convey a single flag that can take either
   on or off values.  Instead of specifying an option with 1 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
   the absent option designating the default; that is, the administrator
   has to take explicit actions to deploy the opposite of the default
   value.

   The absence of the option represents the default value, and the
   presence of the option represents the other value, but that does not
   necessarily mean that absence is "off" (or "false") and presence is
   "on" (or "true").  That is, if it's desired that the default value
   for a bistable option is "true"/"on", then the presence of that
   option would turn it off (make it false).  If the option presence
   signifies an off/false state, that should be reflected in the option
   name, e.g., OPTION_DISABLE_FOO.

    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]


















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5.3.  Option with IPv6 Prefix

   Sometimes, there is a need to convey an IPv6 prefix.  The information
   to be carried by such 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 the prefix length and 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 0 and would be
   unused.  In order to avoid carrying unused data, it is recommended to
   store the 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         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  prefix6len   |              ipv6-prefix                      |
     +-+-+-+-+-+-+-+-+           (variable length)                   |
     .                                                               .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 3: Option with IPv6 Prefix

   option-length is set to 1 + length of the IPv6 prefix.

   prefix6len is 1 octet long and specifies the length in bits of the
   IPv6 prefix.  Typically allowed values are 0 to 128.

   The ipv6-prefix field is a variable-length field that specifies the
   IPv6 prefix.  The length is (prefix6len + 7) / 8.  This field is
   padded with 0 bits up to the nearest octet boundary when prefix6len
   is not divisible by 8.

   Examples of use:

   o  Default Mapping Rule [MAP]

   For example, the prefix 2001:db8::/60 would be encoded with an
   option-length of 9, prefix6-len would be set to 60, and the
   ipv6-prefix would be 8 octets and would contain octets 20 01 0d b8 00
   00 00 00.

   It should be noted that the IAPREFIX option defined by [RFC3633] uses
   a full-length 16-octet prefix field.  The concern about option length
   was not well understood at the time of its publication.



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5.4.  Option with 32-bit Integer Value

   This option format can be used to carry a 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 URI

   A Uniform Resource Identifier (URI) [RFC3986] is a compact sequence
   of characters that identifies an abstract or physical resource.  The
   term "Uniform Resource Locator" (URL) refers to the subset of URIs
   that, in addition to identifying a resource, provide a means of
   locating the resource by describing its primary access mechanism
   (e.g., its network "location").  This option format can be used to
   carry a single URI:

    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           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   .                      URI (variable length)                    .
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



                         Figure 7: Option with URI











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   Examples of use:

   o  Boot File URL [RFC5970]

   An alternate encoding to support multiple URIs is available.  An
   option must be defined to use either the single URI format above or
   the multiple URI format below depending on whether a single URI is
   always sufficient or if multiple URIs are possible.

    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           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   .                                                               .
   .                            uri-data                           .
   .                             . . .                             .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 8: Option with Multiple URIs

   Each instance of the uri-data is formatted as follows:

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...-+-+-+-+-+-+-+
   |       uri-len                 |          URI                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...-+-+-+-+-+-+-+

   The uri-len is 2 octets long and specifies the length of the URI
   data.  Although the URI format in theory supports up to 64 KB of
   data, in practice, large chunks of data may be problematic.  See
   Section 15 for details.

5.8.  Option with Text String

   A text string is a sequence of characters that have no semantics.
   The encoding of the text string MUST be specified.  Unless otherwise
   specified, all text strings in newly defined options are expected to
   be Unicode strings that are encoded using UTF-8 [RFC3629] in Net-
   Unicode form [RFC5198].  Please note that all strings containing only
   7-bit ASCII characters are also valid UTF-8 Net-Unicode strings.

   If a data format has semantics other than just being text, it is not
   a string; e.g., an FQDN is not a string, and a URI is also not a
   string because they have different semantics.  A string must not
   include any terminator (such as a null byte).  The null byte is
   treated as any other character and does not have any special meaning.
   This option format can be used to carry a text string:




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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           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   .                            String                             .
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                     Figure 9: Option with Text String

   Examples of use:

   o  Timezone Options for DHCPv6 [RFC4833]

   An alternate encoding to support multiple text strings is available.
   An option must be defined to use either the single text string format
   above or the multiple text string format below, depending on whether
   a single text string is always sufficient or if multiple text strings
   are possible.

    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           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   .                                                               .
   .                           text-data                           .
   .                             . . .                             .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 10: Option with Multiple Text Strings

   Each instance of the text-data is formatted as follows:

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...-+-+-+-+-+-+-+
   |       text-len                |        String                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...-+-+-+-+-+-+-+

   The text-len is 2 octets long and specifies the length of the string.

5.9.  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.  Whenever
   this format is used by the new option being defined, the data
   encoding should be documented.



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   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 11: Option with Variable-Length Data

   Examples of use:

   o  Client Identifier [RFC3315]

   o  Server Identifier [RFC3315]

5.10.  Option with DNS Wire Format Domain Name List

   This option is used to carry 'domain search' lists or any host or
   domain name.  It uses the same format as described in Section 5.9 but
   with the special data encoding, as described in Section 8 of
   [RFC3315].  This data encoding supports carrying multiple instances
   of hosts or domain names in a single option by terminating each
   instance with the byte value of 0.

    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 12: Option with DNS Wire Format Domain Name List

   Examples of use:

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

   o  NIS Domain Name [RFC3898] (many domains)



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   o  Location-to-Service Translation (LoST) Server Domain Name
      [RFC5223]

   o  Location Information Server (LIS) Domain Name [RFC5986]

   o  Dual-Stack Lite (DS-Lite) Address Family Transition Router (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 that 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, conditional formatting requires new code to be written and
   complicates future interoperability should new conditional formats be
   added; existing code has to ignore conditional formats that it does
   not support.

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 a 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
   message bandwidth is wasted on option contents that are redundant.
   Also, the DHCPv6 option number space is wasted, as three new option
   codes are required rather than one.

   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 an IP address?





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   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
   conversions.

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

8.  Choosing between an FQDN and an Address

   Some parameters may be specified as an FQDN or an address.  In most
   cases, one or the other should be used.  This section discusses pros
   and cons of each approach and is intended to help make an informed
   decision in that regard.  It is strongly discouraged to define both
   option types at the same time (see Section 7), unless there is
   sufficient motivation to do so.

   There is no single recommendation that works for every case.  It very
   much depends on the nature of the parameter being configured.  For
   parameters that are network specific or represent certain aspects of
   network infrastructure, like available mobility services, in most
   cases addresses are a more usable choice.  For parameters that can be
   considered an application-specific configuration, like SIP servers,
   it is usually better to use an FQDN.

   Applications are often better suited to deal with FQDN failures than
   with address failures.  Most operating systems provide a way to retry
   an FQDN resolution if the previous attempt fails.  That type of error
   recovery is supported by a great number of applications.  On the
   other hand, there is typically no API available for applications to
   reconfigure over DHCP to get a new address value if the one received
   is no longer appropriate.  This problem may be usually addressed by
   providing a list of addresses rather than just a single one.  That,
   on the other hand, requires a defined procedure on how multiple
   addresses should be used (all at once, round robin, try first and
   fail over to the next if it fails, etc.).

   An FQDN provides a higher level of indirection and ambiguity.  In
   many cases, that may be considered a benefit, but it can be
   considered a flaw in others.  For example, one operator suggested
   that the same name be resolved to different addresses, depending on
   the point of attachment of the host doing the resolution.  This is
   one way to provide localized addressing.  However, in order to do
   this, it is necessary to violate the DNS convention that a query on a
   particular name should always return the same answer (aside from the



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   ordering of IP addresses in the response, which is supposed to be
   varied by the name server).  This same locality of reference for
   configuration information can be achieved directly using DHCP, since
   the DHCP server must know the network topology in order to provide IP
   address or prefix configuration.

   The other type of ambiguity is related to multiple provisioning
   domains (see Section 12).  The stub resolver on the DHCP client
   cannot at present be assumed to make the DNS query for a DHCP-
   supplied FQDN on the same interface on which it received its DHCP
   configuration and may, therefore, get a different answer from the DNS
   than was intended.

   This is particularly a problem when the normal expected use of the
   option makes sense with a private DNS zone(s), as might be the case
   on an enterprise network.  It may also be the case that the client
   has an explicit DNS server configured and may, therefore, never query
   the enterprise network's internal DNS server.

   An FQDN does require a resolution into an actual address.  This
   implies the question as to when the FQDN resolution should be
   conducted.  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, and
   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 an FQDN.  The only real incentive to use an
   FQDN is case d).  It is the only case that allows possible changes in
   the DNS to be picked up by clients.

   If the parameter is expected to be used by constrained devices (low
   power, battery operated, and low capabilities) or in very lossy
   networks, it may be appealing to drop the requirement of performing
   the DNS resolution and use addresses.  Another example of a
   constrained device is a network-booted device, where despite the fact
   that the node itself is very capable once it's booted, the boot prom
   is quite constrained.

   Another aspect that should be considered is time required for the
   clients to notice any configuration changes.  Consider a case where a
   server configures service A using an address and service B using an
   FQDN.  When an administrator decides to update the configuration, he
   or she can update the DHCP server configuration to change both
   services.  If the clients do not support reconfigure (which is an
   optional feature of RFC 3315 but in some environments, e.g., cable
   modems, is mandatory), the configuration will be updated on the
   clients after the T1 timer elapses.  Depending on the nature of the
   change (is it a new server added to a cluster of already operating



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   servers or a new server that replaces the only available server that
   crashed?), this may be an issue.  On the other hand, updating service
   B may be achieved with a DNS record update.  That information may be
   cached by caching DNS servers for up to Time to Live (TTL).
   Depending on the values of T1 and TTL, one update may be faster than
   another.  Furthermore, depending on the nature of the change (planned
   modification or unexpected failure), T1 or TTL may be lowered before
   the change to speed up new configuration adoption.

   Simply speaking, protocol designers don't know what the TTL or the T1
   time will be, so they can't make assumptions about whether a DHCP
   option will be refreshed more quickly based on T1 or TTL.

   Addresses have the benefit of being easier to implement and handle by
   the DHCP software.  An address option is simpler to use, has
   validation that is trivial (multiple of 16 constitutes a valid
   option), is explicit, and does not allow any ambiguity.  It is faster
   (does not require extra round-trip time), so it is more efficient,
   which can be especially important for energy-restricted devices.  It
   also does not require that the client implements a DNS resolution.

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

   1.  The client must have knowledge about available DNS servers.  That
       typically means that option DNS_SERVERS [RFC3646] is mandatory.
       This should be mentioned in the document that defines the new
       option.  It is possible that the server will return the FQDN
       option but not the DNS server's option.  There should be a brief
       discussion about it;

   2.  The DNS may not be reachable;

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

   4.  The address family must be specified (A, AAAA, or any); the
       information being configured may require a specific address
       family (e.g., IPv6), but there may be a DNS record only of
       another type (e.g., A only with an IPv4 address).

   5.  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.).  This may
       be an issue if there is an expectation that the parameter being
       configured will need exactly one address;





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   6.  Multihomed devices may be connected to different administrative
       domains with each domain providing different information in the
       DNS (e.g., an enterprise network exposing private domains).  The
       client may send DNS queries to a different DNS server; and

   7.  It should be mentioned if Internationalized Domain Names are
       allowed.  If they are, DNS option encoding should be specified.

   Address options that are used with overly long T1 (renew timer)
   values have some characteristics of hard-coded values.  That is
   strongly discouraged.  See [RFC4085] for an in-depth discussion.  If
   the option may appear in Information-request, its lifetime should be
   controlled using the information refresh time option [RFC4242].

   One specific case that makes the choice between an address and an
   FQDN not obvious is a DNS Security (DNSSEC) bootstrap scenario.
   DNSSEC validation imposes a requirement for clock sync (to the
   accuracy reasonably required to consider signature inception and
   expiry times).  This often implies usage of NTP configuration.

   However, if NTP is provided as an FQDN, there is no way to validate
   its DNSSEC signature.  This is a somewhat weak argument though, as
   providing an NTP server as an address is also not verifiable using
   DNSSEC.  If the trustworthiness of the configuration provided by the
   DHCP server is in question, DHCPv6 offers mechanisms that allow
   server authentication.

9.  Encapsulated Options 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
   encapsulated or nested options.  Those options are sometimes called
   sub-options, but this term actually means something else and,
   therefore, should never be used to describe encapsulated options.  It
   is recommended to use the term "encapsulated" as this terminology is
   used in [RFC3315].  The difference between encapsulated and sub-
   options is that the former uses normal DHCPv6 option numbers, while
   the latter uses option number space specific to a given parent
   option.  It should be noted that, contrary to DHCPv4, there is no
   shortage of option numbers; therefore, almost all options share a
   common option space.  For example, option type 1 meant different
   things in DHCPv4, depending if it was located in the top level or
   inside of the Relay Agent Information option.  There is no such
   ambiguity in DHCPv6 (with the exception of [RFC5908], which SHOULD
   NOT be used as a template for future DHCP option definitions).




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   From the implementation perspective, it is easier to implement
   encapsulated options rather than sub-options, as the implementors do
   not have to deal with separate option spaces and can use the same
   buffer parser in several places throughout the code.

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

   When defining a DHCP-based configuration mechanism for a protocol
   that requires something more complex than a single option, it may be
   tempting to group configuration values using sub-options.  That
   should preferably be avoided, as it increases complexity of the
   parser.  It is much easier, faster, and less error prone to parse a
   large number of options on a single (top-level) scope than to 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
   the top-level Option Request Option (ORO) for requesting both top-
   level and encapsulated options.

10.  Additional States Considered Harmful

   DHCP is designed for provisioning clients.  Less experienced protocol
   designers often assume that it is easy to define an option that will
   convey a different parameter for each client in a network.  Such
   problems arose during designs of the Mapping of Address and Port
   (MAP) [MAP] and IPv4 Residual Deployment (4rd) [SOLUTION-4rd].  While
   it would be easier for provisioned clients to get ready to use per-
   client option values, such a requirement puts exceedingly large loads
   on the server side.  The new extensions may introduce new
   implementation complexity and additional database state on the
   server.  Alternatives should be considered, if possible.  As an
   example, [MAP] was designed in a way that all clients are provisioned
   with the same set of MAP options, and each provisioned client uses
   its unique address and delegated prefix to generate client-specific




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   information.  Such a 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 the client
   will initiate renewal.  Those timers apply only to their associated
   IA containers.  Refreshing other parameters should be initiated after
   a time specified in the information refresh time option (defined in
   [RFC4242]), carried in the Reply message, and returned in response to
   the Information-request message.  Introducing additional timers make
   deployment unnecessarily complex and SHOULD be avoided.

11.  Configuration Changes Occur at Fixed Times

   In general, DHCPv6 clients only refresh configuration data from the
   DHCP server when the T1 timer expires.  Although there is a
   Reconfigure mechanism that allows a DHCP server to request that
   clients initiate reconfiguration, support for this mechanism is
   optional and cannot be relied upon.

   Even when DHCP clients refresh their configuration information, not
   all consumers of DHCP-sourced configuration data notice these
   changes.  For instance, if a server is started using parameters
   received in an early DHCP transaction, but does not check for updates
   from DHCP, it may well continue to use the same parameter
   indefinitely.  There are a few operating systems that take care of
   reconfiguring services when the client moves to a new network (e.g.,
   based on mechanisms like [RFC4436], [RFC4957], or [RFC6059]), but
   it's worth bearing in mind that a renew may not always result in the
   client taking up new configuration information that it receives.

   In light of the above, when designing an option you should take into
   consideration the fact that your option may hold stale data that will
   only be updated at an arbitrary time in the future.

12.  Multiple Provisioning Domains

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

   The DHCPv6 specification does not provide clear advice on how to
   handle multiple provisioning sources.  Although [RFC3315] states that
   a client that receives more than one Advertise message may respond to
   one or more of them, such capability has not been observed in
   existing implementations.  Existing clients will pick one server and



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   will continue the configuration process with that server, ignoring
   all other servers.

   In addition, a node that acts as a DHCPv6 client may be connected to
   more than one physical network.  In most cases, it will operate a
   separate DHCP client state machine on each interface and acquire
   different, possibly conflicting, information through each.  This
   information will not be acquired in any synchronized way.

   Existing nodes cannot be assumed to systematically segregate
   configuration information on the basis of its source; as a result, it
   is quite possible that a node may receive an FQDN on one network
   interface but do the DNS resolution on a different network interface,
   using different DNS servers.  As a consequence, DNS resolution done
   by the DHCP server is more likely to behave predictably than DNS
   resolution done on a multi-interface or multihomed client.

   This is a generic DHCP issue and should not be dealt within each
   option separately.  This issue is better dealt with using a protocol-
   level solution, and fixing this problem should not be attempted on a
   per-option basis.  Work is ongoing in the IETF to provide a
   systematic solution to this problem.

13.  Chartering Requirements and Advice for Responsible Area Directors

   Adding a simple DHCP option is straightforward and generally
   something that any working group (WG) can do, perhaps with some help
   from designated DHCP experts.  However, when new fragment types need
   to be devised, this requires the attention of DHCP experts and should
   not be done in a WG that doesn't have a quorum of such experts.  This
   is true whether the new fragment type has the same structure as an
   existing fragment type but with different semantics, or the new
   format has a new structure.

   Responsible Area Directors for WGs that wish to add a work item to a
   WG charter to define a new DHCP option should get clarity from the WG
   as to whether the new option will require a new fragment type or new
   semantics, or whether it is a simple DHCP option that fits existing
   definitions.

   If a WG needs a new fragment type, it is preferable to see if another
   WG exists whose members already have sufficient expertise to evaluate
   the new work.  If such a working group is available, the work should
   be chartered in that working group instead.  If there is no other WG
   with DHCP expertise that can define the new fragment type, the
   responsible AD should seek help from known DHCP experts within the
   IETF to provide advice and frequent early review as the original WG
   defines the new fragment type.



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   In either case, the new option should be defined in a separate
   document, and the work should focus on defining a new format that
   generalizes well and can be reused, rather than a single-use fragment
   type.  The WG that needs the new fragment type can define their new
   option referencing the new fragment type document, and the work can
   generally be done in parallel, avoiding unnecessary delays.  Having
   the definition in its own document will foster reuse of the new
   fragment type.

   The responsible AD should work with all relevant WG Chairs and DHCP
   experts to ensure that the new fragment type document has in fact
   been carefully reviewed by the experts and appears satisfactory.

   Responsible Area Directors for WGs that are considering defining
   options that actually update DHCP, as opposed to simple options,
   should go through a process similar to that described above when
   trying to determine where to do the work.  Under no circumstances
   should a WG be given a charter deliverable to define a new DHCP
   option, and then on the basis of that charter item actually make
   updates to DHCP.

14.  Considerations for Creating New Formats

   When defining new options, 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.

   When defining a new option, it is best to synthesize the option
   format using fragment types already in use.  However, in some cases,
   there may be no fragment type that accomplishes the intended purpose.

   The matter of size considerations and option order are further
   discussed in Sections 15 and 17.

15.  Option Size

   DHCPv6 [RFC3315] allows for packet sizes up to 64 KB.  First, through
   its use of link-local addresses, it avoids many of the deployment
   problems that plague DHCPv4 and is actually a UDP over the IPv6-based
   protocol (compared to DHCPv4, which is mostly UDP over IPv4 but with
   layer-2 hacks).  Second, RFC 3315 explicitly refers readers to
   Section 5 of [RFC2460], 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 not really possible to describe a fixed




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   limit that cleanly divides workable option sizes from those that are
   too big.

   It is advantageous to prefer option formats that contain the desired
   information in the smallest form factor that satisfies the
   requirements.  Common sense still applies here.  It is better to
   split distinct values into separate octets rather than propose overly
   complex bit-shifting operations to save several bits (or even an
   octet or two) that would be padded to the next octet boundary anyway.

   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.  The default
   assumption is only once.

   In general, if a lot of data needs to be configured (for example,
   some option lengths are quite large), DHCPv6 may not be the best
   choice to deliver such configuration information and SHOULD simply be
   used to deliver a URI that specifies where to obtain the actual
   configuration information.

16.  Singleton Options

   Although [RFC3315] states that each option type MAY appear more than
   once, the original idea was that multiple instances are reserved for
   stateful options, like IA_NA or IA_PD.  For most other options, it is
   usually expected that they will appear once at most.  Such options
   are called singleton options.  Sadly, RFCs have often failed to
   clearly specify whether or not a given option can appear more than
   once.

   Documents that define new options SHOULD state whether or not these
   options are singletons.  Unless otherwise specified, newly defined
   options are considered to be singletons.  If multiple instances are
   allowed, the document MUST explain how to use them.  Care should be
   taken not to assume that they will be processed in the order they
   appear in the message.  See Section 17 for more details.

   When deciding whether single or multiple option instances are allowed
   in a message, take into consideration how the content of the option
   will be used.  Depending on the service being configured, it may or
   may not make sense to have multiple values configured.  If multiple
   values make sense, it is better to explicitly allow that by using an
   option format that allows multiple values within one option instance.



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   Allowing multiple option instances often leads to confusion.
   Consider the following example.  Basic DS-Lite architecture assumes
   that the B4 element (DHCPv6 client) will receive the AFTR option and
   establish a single tunnel to the configured tunnel termination point
   (AFTR).  During the standardization process of [RFC6334], there was a
   discussion whether multiple instances of the DS-Lite tunnel option
   should be allowed.  This created an unfounded expectation that the
   clients receiving multiple instances of the option will somehow know
   when one tunnel endpoint goes offline and do some sort of failover
   between other values provided in other instances of the AFTR option.
   Others assumed that if there are multiple options, the client will
   somehow do load balancing between the provided tunnel endpoints.
   Neither failover nor load balancing was defined for the DS-Lite
   architecture, so it caused confusion.  It was eventually decided to
   allow only one instance of the AFTR option.

17.  Option Order

   Option order, either the order among many DHCPv6 options or the order
   of multiple instances of the same option, SHOULD NOT be significant.
   New documents MUST NOT assume any specific option processing order.

   As there is no explicit order for multiple instances of the same
   option, an option definition SHOULD instead restrict ordering by
   using a single option that contains ordered fields.

   As [RFC3315] does not impose option order, some implementations use
   hash tables to store received options (which is a conformant
   behavior).  Depending on the hash implementation, the processing
   order is almost always different then the order in which the options
   appeared in the packet on the wire.

18.  Relay Options

   In DHCPv4, all relay options are organized as sub-options within the
   DHCP Relay Agent Information option [RFC3046].  And, an independent
   number space called "DHCP Relay Agent Sub-options" is maintained by
   IANA.  Different from DHCPv4, in DHCPv6, relay options are defined in
   the same way as client/server options, and they also use the same
   option number space as client/server options.  Future DHCPv6 relay
   options MUST be allocated from this single DHCPv6 option number
   space.

   For example, the Relay-Supplied Options option [RFC6422] may also
   contain some DHCPv6 options as permitted, such as the Extensible
   Authentication Protocol (EAP) Re-authentication Protocol (ERP) Local
   Domain Name DHCPv6 Option [RFC6440].




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19.  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 what options the client is willing to consume.

   For some options, such as the options required for the functioning of
   DHCPv6 itself, it doesn't make sense to require that they be
   explicitly requested using the Option Request Option.  In all other
   cases, 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 tempting to add text that requires the client to include a new
   option in the Option Request Option list, similar to this text:
   "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 MUST include option foo
   when the client requests it (and the server has been so configured)".
   Such text is discouraged as there are several issues with it.  First,
   it assumes that client implementation that supports a given option
   will always want to use it.  This is not true.  The second and more
   important reason is that such text essentially duplicates the
   mechanism already defined in [RFC3315].  It is better to simply refer
   to the existing mechanism rather than define it again.  See
   Section 21 for proposed examples on how to do that.

   Creators of DHCPv6 options cannot assume special ordering of options
   either as they appear in the Option Request Option or as they appear
   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 also be noted that options values are not required to be
   aligned within the DHCP packet; even the option code and option
   length may appear on odd-byte boundaries.

20.  Transition Technologies

   The transition from IPv4 to IPv6 is progressing.  Many transition
   technologies are proposed to speed it up.  As a natural consequence,
   there are also DHCP options proposed to provision those proposals.
   The inevitable question is 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 the



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   DHCPv6 protocol itself lasts.  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.

   When the network infrastructure becomes IPv6 only, the support for
   IPv4-only nodes may still be needed.  In such a scenario, a mechanism
   for providing IPv4 configuration information over IPv6-only networks
   may be needed.  See [IPv4-CONFIG] for further details.

21.  Recommended Sections in the New Document

   There are three major entities in DHCPv6: server, relay agent, and
   client.  It is very helpful for implementors to include separate
   sections that describe operation for those three major entities.
   Even when a given entity does not participate, it is useful to have a
   very short section stating that it must not send a given option and
   must ignore it when received.

   There is also a separate entity called the "requestor", which is a
   special client-like type that participates in the leasequery protocol
   [RFC5007] [RFC5460].  A similar section for the requestor is not
   required, unless the new option has anything to do with the requestor
   (or it is likely that the reader may think that is has).  It should
   be noted that while in the majority of deployments the requestor is
   co-located with the relay agent, those are two separate entities from
   the protocol perspective, and they may be used separately.  There are
   stand-alone requestor implementations available.

   The following sections include proposed text for such sections.  That
   text is not required to appear, but it is appropriate in most cases.
   Additional or modified text specific to a given option is often
   required.

   Although the requestor is a somewhat uncommon functionality, its
   existence should be noted, especially when allowing or disallowing
   options to appear in certain messages or to be sent by certain
   entities.  Additional message types may appear in the future, besides
   types defined in [RFC3315].  Therefore, authors are encouraged to
   familiarize themselves with a list of currently defined DHCPv6
   messages available on the IANA website [IANA].

   Typically, new options are requested by clients and assigned by the
   server, so there is no specific relay behavior.  Nevertheless, it is
   good to include a section for relay agent behavior and simply state





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   that there are no additional requirements for relays.  The same
   applies for client behavior if the options are to be exchanged
   between the relay and server.

   Sections that contain option definitions MUST include a formal
   verification procedure.  Often it is very simple, e.g., an option
   that conveys an IPv6 address must be exactly 16-bytes long, but
   sometimes the rules are more complex.  It is recommended to refer to
   existing documents (e.g., Section 8 of RFC 3315 for domain name
   encoding) rather than trying to repeat such rules.

21.1.  DHCPv6 Client Behavior Text

   Clients MAY request option foo, as defined in [RFC3315], Sections
   17.1.1, 18.1.1, 18.1.3, 18.1.4, 18.1.5, and 22.7.  As a convenience
   to the reader, we mention here that the client includes requested
   option codes in the Option Request Option.

   Optional text (if the client's hints make sense): The client also MAY
   include option foo in its Solicit, Request, Renew, Rebind, and
   Information-request messages as a hint for the server regarding
   preferred option values.

   Optional text (if the option contains an FQDN): If the client
   requests an option that conveys an FQDN, it is expected that the
   contents of that option will be resolved using DNS.  Hence, the
   following text may be useful: Clients that request option foo SHOULD
   also request option OPTION_DNS_SERVERS as specified in [RFC3646].

   Clients MUST discard option foo if it is invalid (i.e., it did not
   pass the validation steps defined in Section X.Y).

   Optional text (if option foo in expected to be exchanged between
   relays and servers): Option foo is exchanged between relays and
   servers only.  Clients are not aware of the usage of option foo.
   Clients MUST ignore received option foo.

21.2.  DHCPv6 Server Behavior Text

   Sections 17.2.2 and 18.2 of [RFC3315] govern server operation in
   regards to option assignment.  As a convenience to the reader, we
   mention here that the server will send option foo only if configured
   with specific values for foo and if the client requested it.

   Optional text: Option foo is a singleton.  Servers MUST NOT send more
   than one instance of the foo option.





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   Optional text (if the server is never supposed to receive option
   foo): Servers MUST ignore the incoming foo option.

21.3.  DHCPv6 Relay Agent Behavior Text

   It's never appropriate for a relay agent to add options to a message
   heading toward the client, and relay agents don't actually construct
   Relay-reply messages anyway.

   Optional text (if the foo option is exchanged between the clients and
   server or between requestors and servers): there are no additional
   requirements for relays.

   Optional text (if relays are expected to insert or consume option
   foo): Relay agents MAY include option foo in a Relay-forward message
   when forwarding packets from clients to the servers.

22.  Should the New Document Update Existing RFCs?

   Authors often ask themselves whether their proposal updates existing
   RFCs, especially RFC 3315.  In April 2013, there were about 80
   options defined.  Had all documents that defined them also updated
   RFC 3315, comprehension of such a document set would be extremely
   difficult.  It should be noted that "extends" and "updates" are two
   very different verbs.  If a new document defines a new option that
   clients request and servers provide, it merely extends current
   standards, so "updates RFC 3315" is not required in the new document
   header.  On the other hand, if a new document replaces or modifies
   existing behavior and includes clarifications or other corrections,
   it should be noted that it updates the other document.  For example,
   [RFC6644] clearly updates [RFC3315] as it replaces existing text with
   new text.

   If in doubt, authors should try to determine whether an implementor
   reading the base RFC alone (without reading the new document) would
   be able to properly implement the software.  If the base RFC is
   sufficient, then the new document probably does not update the base
   RFC.  On the other hand, if reading your new document is necessary to
   properly implement the base RFC, then the new document most likely
   updates the base RFC.

23.  Security Considerations

   DHCPv6 does have an authentication mechanism [RFC3315] that makes it
   possible for DHCPv6 software to discriminate between authentic
   endpoints and man-in-the-middle.  Other authentication mechanisms may
   optionally be deployed.  Sadly, as of 2014, the authentication in
   DHCPv6 is rarely used, and support for it is not common in existing



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   implementations.  Some specific deployment types make it mandatory
   (or parts thereof, e.g., DOCSIS3.0-compatible cable modems require
   reconfigure-key support), so in certain cases, specific
   authentication aspects can be relied upon.  That is not true in the
   generic case, though.

   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 implementor 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 implementors to validate the
   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
   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 multicast 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.

   Authors of documents defining new DHCP options are, therefore,
   strongly advised to explicitly define validation measures that
   recipients of such options are required to do before processing such
   options.  However, validation measures already defined by RFC 3315 or
   other specifications referenced by the new option document are
   redundant and can introduce errors, so authors are equally strongly
   advised to refer to the base specification for any such validation
   language rather than copying it into the new specification.

   See also Section 24.



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24.  Privacy Considerations

   As discussed in Section 23, the DHCPv6 packets are typically
   transmitted in the clear, so they are susceptible to eavesdropping.
   This should be considered when defining options that may convey
   personally identifying information (PII) or any other type of
   sensitive data.

   If the transmission of sensitive or confidential content is required,
   it is still possible to secure communication between relay agents and
   servers.  Relay agents and servers communicating with relay agents
   must support the use of IPsec Encapsulating Security Payload (ESP)
   with encryption in transport mode, according to Section 3.1.1 of
   [RFC4303] and Section 21.1 of [RFC3315].  Sadly, this requirement is
   almost universally ignored in real deployments.  Even if the
   communication path between the relay agents and server is secured,
   the path between the clients and relay agents or server is not.

   Unless underlying transmission technology provides a secure
   transmission channel, the DHCPv6 options SHOULD NOT include PII or
   other sensitive information.  If there are special circumstances that
   warrant sending such information over unsecured DHCPv6, the dangers
   MUST be clearly discussed in the security considerations.

25.  Acknowledgements

   The authors would like to thank Simon Perreault, Bernie Volz, Ted
   Lemon, Bud Millwood, Ralph Droms, Barry Leiba, Benoit Claise, Brian
   Haberman, Richard Barnes, Stephen Farrell, and Stewart Bryant for
   their comments.

26.  References

26.1.  Normative References

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

   [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.










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26.2.  Informative References

   [IANA]     IANA, "Dynamic Host Configuration Protocol for IPv6
              (DHCPv6)",
              <http://www.iana.org/assignments/dhcpv6-parameters/>.

   [IPv4-CONFIG]
              Rajtar, B. and I. Farrer, "Provisioning IPv4 Configuration
              Over IPv6 Only Networks", Work in Progress, February 2014.

   [MAP]      Mrugalski, T., Troan, O., Farrer, I., Perreault, S., Dec,
              W., Bao, C., Yeh, L., and X. Deng, "DHCPv6 Options for
              configuration of Softwire Address and Port Mapped
              Clients", Work in Progress, March 2014.

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

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

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

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, November 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.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66, RFC
              3986, January 2005.

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





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   [RFC4085]  Plonka, D., "Embedding Globally-Routable Internet
              Addresses Considered Harmful", BCP 105, RFC 4085, June
              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.

   [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)", RFC
              4303, December 2005.

   [RFC4436]  Aboba, B., Carlson, J., and S. Cheshire, "Detecting
              Network Attachment in IPv4 (DNAv4)", RFC 4436, March 2006.

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

   [RFC4833]  Lear, E. and P. Eggert, "Timezone Options for DHCP", RFC
              4833, April 2007.

   [RFC4957]  Krishnan, S., Montavont, N., Njedjou, E., Veerepalli, S.,
              and A. Yegin, "Link-Layer Event Notifications for
              Detecting Network Attachments", RFC 4957, August 2007.

   [RFC5007]  Brzozowski, J., Kinnear, K., Volz, B., and S. Zeng,
              "DHCPv6 Leasequery", RFC 5007, September 2007.

   [RFC5198]  Klensin, J. and M. Padlipsky, "Unicode Format for Network
              Interchange", RFC 5198, March 2008.

   [RFC5223]  Schulzrinne, H., Polk, J., and H. Tschofenig, "Discovering
              Location-to-Service Translation (LoST) Servers Using the
              Dynamic Host Configuration Protocol (DHCP)", RFC 5223,
              August 2008.

   [RFC5460]  Stapp, M., "DHCPv6 Bulk Leasequery", RFC 5460, February
              2009.

   [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.



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   [RFC5986]  Thomson, M. and J. Winterbottom, "Discovering the Local
              Location Information Server (LIS)", RFC 5986, September
              2010.

   [RFC6059]  Krishnan, S. and G. Daley, "Simple Procedures for
              Detecting Network Attachment in IPv6", RFC 6059, November
              2010.

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

   [RFC6422]  Lemon, T. and Q. Wu, "Relay-Supplied DHCP Options", RFC
              6422, December 2011.

   [RFC6440]  Zorn, G., Wu, Q., and Y. Wang, "The EAP Re-authentication
              Protocol (ERP) Local Domain Name DHCPv6 Option", RFC 6440,
              December 2011.

   [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.

   [RFC6644]  Evans, D., Droms, R., and S. Jiang, "Rebind Capability in
              DHCPv6 Reconfigure Messages", RFC 6644, July 2012.

   [SOLUTION-4rd]
              Despres, R., Jiang, S., Penno, R., Lee, Y., Chen, G., and
              M. Chen, "IPv4 Residual Deployment via IPv6 - a Stateless
              Solution (4rd)", Work in Progress, October 2013.

















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Authors' Addresses

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

   EMail: dhankins@google.com


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

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


   Marcin Siodelski
   Internet Systems Consortium, Inc.
   950 Charter Street
   Redwood City, CA  94063
   USA

   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


   Suresh Krishnan
   Ericsson
   8400 Blvd Decarie
   Town of Mount Royal, Quebec
   Canada

   EMail: suresh.krishnan@ericsson.com





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