Dynamic Host Configuration Working Group D. Hankins
Internet-Draft Google
Updates: 3315 (if approved) T. Mrugalski
Intended status: Standards Track M. Siodelski
Expires: October 11, 2013 ISC
S. Jiang
Huawei Technologies Co., Ltd
S. Krishnan
Ericsson
April 09, 2013
Guidelines for Creating New DHCPv6 Options
draft-ietf-dhc-option-guidelines-11
Abstract
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
RFC3315.
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 October 11, 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 . . . . . . . . . . . . . . . . . . . . . 3
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. Encapsulated options in DHCPv6 . . . . . . . . . . . . . . . 13
10. Additional States Considered Harmful . . . . . . . . . . . . 14
11. Is DHCPv6 dynamic? . . . . . . . . . . . . . . . . . . . . . 14
12. Multiple provisioning domains . . . . . . . . . . . . . . . . 15
13. Considerations for Creating New Formats . . . . . . . . . . . 15
14. Option Size . . . . . . . . . . . . . . . . . . . . . . . . . 15
15. Clients Request their Options . . . . . . . . . . . . . . . . 16
16. Transition Technologies . . . . . . . . . . . . . . . . . . . 17
17. Recommended sections in the new document . . . . . . . . . . 17
17.1. DHCPv6 Client Behavior . . . . . . . . . . . . . . . . . 18
17.2. DHCPv6 Server Behavior . . . . . . . . . . . . . . . . . 19
17.3. DHCPv6 Relay Agent Behavior . . . . . . . . . . . . . . 19
18. Should the new document update existing RFCs? . . . . . . . . 19
19. Security Considerations . . . . . . . . . . . . . . . . . . . 20
20. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
21. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 21
22. Informative References . . . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
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1. 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 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
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 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
adoption.
3. When to Use DHCPv6
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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
administrators.
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
enterprises.
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
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.
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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.
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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. This can be
expressed using the following equation: >prefix6-len<+7/8
Examples of use:
o Default Mapping Rule [I-D.ietf-softwire-map-dhcp]
For example, the prefix 2001:db8::/60 would be encoded with an
option-length of 9, prefix-len would be set to 60, the ipv6-prefix
would be 8 octets and would contains octets 20 01 0d b8 00 00 00 00.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Figure 5: Option with 16-bit integer value
Examples of use:
o Elapsed Time [RFC3315]
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
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Examples of use:
o Client Identifier [RFC3315]
o Server Identifier [RFC3315]
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.
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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.
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
message bandwidth is wasted on option contents that are redundant.
Also, the DHCPv6 option 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 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
conversions.
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
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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
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:
1. 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;
2. The DNS may not be reachable;
3. DNS may be available, but may not have appropriate information
(e.g. no AAAA records for specified FQDN);
4. Address family must be specified (A, AAAA or any);
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.);
6. Multi-homed devices may be connected to different administrative
domains with each domain providing a different information in DNS
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(e.g. an enterprise network exposing private domains). Client
may send DNS queries to a different DNS server;
7. It should be mentioned if Internationalized Domain Names are
allowed. If they are, what kind of DNS option encoding should be
specified.
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 is not precise and thus discouraged. It
is recommened to use term "encapsulated" as this terminology is used
in [RFC3315]. The difference between encapsulated and sub-options
are that the former uses normal DHCPv6 option space codes, while the
latter uses option 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 top-level or inside of Relay Agent
Information option. There is no such ambiguity in DHCPv6 (with the
unfortunate exception of [RFC5908]).
From the implementation perspective, it is easier to implement
encapsulated option rather than sub-option, as the implementor do not
have to deal with separate option spaces and can use the same buffer
parser in several places throughout the code.
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 or doubly encapsulated
option, therefore "encapsulated option" term is typically used,
regardless of the nesting level.
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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
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
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trigger reconfiguration. Therefore the only reliable way for clients
to refresh their configuration is to wait until 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
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.
The matter of size considerations is further discussed in Section 14.
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
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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
requirements. A common sense still applies here. It is better to
split distinct values into separate octects rather than propose
overly complex bit shifting operations to save up 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.
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
consume.
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 tempting to put a text that requires the client to include new
option in 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 requested it (and the server has been so configured)".
Such a 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 a text essentially duplicates
mechanism already defined in [RFC3315]. It is better to simply refer
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to existing mechanism rather than define it again. See Section 17
for proposed examples on how to do that.
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
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. Recommended sections in the new document
There are three major entities in DHCPv6 protocol: server, relay
agent, and client. There is also a separate entity called requestor,
which is a special client-like type that participates in leasequery
protocol [RFC5007] and [RFC5460]. It is very helpful for
implementors to include separate sections that describe operation for
those three major components. 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.
Similar section for requestor is not required, unless the new option
has anything to do with requestor (or it is likely that the reader
may think that is has). It should be noted that while in majority of
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deployments, requestor is colocated with 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 requestor is somewhat uncommon functionality, its existence
should be noted, especially when allowing or disallowing options to
appear in certain message or being sent be 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 IANA website [iana].
Typically new options are requested by clients and assigned by
server, so there is no specific relay behavior. Nevertheless it is
good to include a section for relay agent behaviour and simply state
that there are no additional requirements for relays. The same
applies for client behavior if the options are to be exchanged
between relay and server.
Section that contain option definition MUST include formal
verification procedure. Often it is very simple, e.g. option that
conveys IPv6 address must be exactly 16 bytes long, but sometimes the
rules are more complex. It is recommeded to refer to existing
documents (e.g. section 8 of RFC3315 for domain name enconding)
rather than trying to repeat such rules.
17.1. DHCPv6 Client Behavior
Client 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 Option Request Option.
Optional text (if client's hints make sense): 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.
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Optional text (if the option contains FQDN): If the client request an
option that conveys FQDN, it is expected that content of that option
will be resolved using DNS. Hence the following text may be useful:
Client that requests option foo SHOULD also request option
OPTION_DNS_SERVERS specified in [RFC3646].
Client MUST discard option foo if it is invalid (i.e. did not pass
validation steps defined in Section X.Y).
Optional text (if option foo in expected to be exchanged between
relays or request and server): 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.
17.2. DHCPv6 Server Behavior
Sections 17.2.2 and 18.2 of [RFC3315] govern server operation in
regards of 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 client requested it.
Optional text: Server MUST NOT send more than one instance of foo
option.
Optional text (if server is never supposed to receive option foo):
Server MUST ignore incoming foo option.
17.3. DHCPv6 Relay Agent Behavior
Optional text (if foo option is exchanged between 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 when forwarding packets
from clients to the server.
18. Should the new document update existing RFCs?
Authors often ask themselves a question whether their proposal
updates exist RFCs, especially 3315. During time of writing this
document there were 79 options defined. Had all documents that
defined them also updated RFC3315, its comprehension of such a
document would be extremely difficult. It should be noted that
"extends" and "updates" are two very different verbs. If a new draft
defines a new option that clients request and servers provide, it
merely extends current standards, so "updates 3315" is not required
in the new document header. On the other hand, if the new draft
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changes something in already defined behavior, e.g. servers must
discard incoming messages if option foo is invalid or missing, then
the "updates" phrase is warranted.
19. 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
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.
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So it behooves an option's definition to contain any validation
measures as can reasonably be made.
20. IANA Considerations
This document has no actions for IANA.
21. Acknowledgements
Authors would like to thank Simon Perreault, Bernie Volz and Ted
Lemon for their comments.
22. Informative References
[I-D.ietf-dhc-secure-dhcpv6]
Jiang, S. and S. Shen, "Secure DHCPv6 Using CGAs", draft-
ietf-dhc-secure-dhcpv6-07 (work in progress), September
2012.
[I-D.ietf-softwire-4rd]
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.
[I-D.ietf-softwire-map-dhcp]
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.
[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.
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[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.
[RFC5007] Brzozowski, J., Kinnear, K., Volz, B., and S. Zeng,
"DHCPv6 Leasequery", RFC 5007, September 2007.
[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.
[RFC6334] Hankins, D. and T. Mrugalski, "Dynamic Host Configuration
Protocol for IPv6 (DHCPv6) Option for Dual-Stack Lite",
RFC 6334, August 2011.
[RFC6603] Korhonen, J., Savolainen, T., Krishnan, S., and O. Troan,
"Prefix Exclude Option for DHCPv6-based Prefix
Delegation", RFC 6603, May 2012.
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[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.
[iana] IANA, , "DHCPv6 parameters (IANA webpage)", November 2003,
<http://www.iana.org/assignments/dhcpv6-parameters/>.
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
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
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Suresh Krishnan
Ericsson
8400 Blvd Decarie
Town of Mount Royal, Quebec
Canada
Email: suresh.krishnan@ericsson.com
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