Internet DRAFT - draft-ietf-dhc-option-guidelines
draft-ietf-dhc-option-guidelines
Dynamic Host Configuration Working Group D. Hankins
Internet-Draft Google
Updates: 3315 (if approved) T. Mrugalski
Intended status: Best Current Practice M. Siodelski
Expires: July 11, 2014 ISC
S. Jiang
Huawei Technologies Co., Ltd
S. Krishnan
Ericsson
January 7, 2014
Guidelines for Creating New DHCPv6 Options
draft-ietf-dhc-option-guidelines-17
Abstract
This document provides guidance to prospective DHCPv6 Option
developers to help them creating option formats that are easily
adoptable by existing DHCPv6 software. It also provides guidelines
for expert reviewers to evaluate new registrations. 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 July 11, 2014.
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
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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.
Table of Contents
1. Requirements Language . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
3. When to Use DHCPv6 . . . . . . . . . . . . . . . . . . . . . 4
4. General Principles . . . . . . . . . . . . . . . . . . . . . 4
5. Reusing Other Options Formats . . . . . . . . . . . . . . . . 5
5.1. Option with IPv6 addresses . . . . . . . . . . . . . . . 6
5.2. Option with a single flag (boolean) . . . . . . . . . . . 7
5.3. Option with IPv6 prefix . . . . . . . . . . . . . . . . . 7
5.4. Option with 32-bit integer value . . . . . . . . . . . . 8
5.5. Option with 16-bit integer value . . . . . . . . . . . . 9
5.6. Option with 8-bit integer value . . . . . . . . . . . . . 9
5.7. Option with URI . . . . . . . . . . . . . . . . . . . . . 9
5.8. Option with Text String . . . . . . . . . . . . . . . . . 11
5.9. Option with variable length data . . . . . . . . . . . . 12
5.10. Option with DNS Wire Format Domain Name List . . . . . . 12
6. Avoid Conditional Formatting . . . . . . . . . . . . . . . . 13
7. Avoid Aliasing . . . . . . . . . . . . . . . . . . . . . . . 13
8. Choosing between FQDN and address . . . . . . . . . . . . . . 14
9. Encapsulated options in DHCPv6 . . . . . . . . . . . . . . . 17
10. Additional States Considered Harmful . . . . . . . . . . . . 18
11. Configuration changes occur at fixed times . . . . . . . . . 19
12. Multiple provisioning domains . . . . . . . . . . . . . . . . 20
13. Chartering Requirements and Advice for Responsible Area
Directors . . . . . . . . . . . . . . . . . . . . . . . . . . 20
14. Considerations for Creating New Formats . . . . . . . . . . . 21
15. Option Size . . . . . . . . . . . . . . . . . . . . . . . . . 22
16. Singleton options . . . . . . . . . . . . . . . . . . . . . . 22
17. Option Order . . . . . . . . . . . . . . . . . . . . . . . . 23
18. Relay Options . . . . . . . . . . . . . . . . . . . . . . . . 24
19. Clients Request their Options . . . . . . . . . . . . . . . . 24
20. Transition Technologies . . . . . . . . . . . . . . . . . . . 25
21. Recommended sections in the new document . . . . . . . . . . 25
21.1. DHCPv6 Client Behavior Text . . . . . . . . . . . . . . 26
21.2. DHCPv6 Server Behavior Text . . . . . . . . . . . . . . 27
21.3. DHCPv6 Relay Agent Behavior Text . . . . . . . . . . . . 27
22. Should the new document update existing RFCs? . . . . . . . . 27
23. Security Considerations . . . . . . . . . . . . . . . . . . . 28
24. Privacy considerations . . . . . . . . . . . . . . . . . . . 29
25. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29
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26. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 30
27. References . . . . . . . . . . . . . . . . . . . . . . . . . 30
27.1. Normative References . . . . . . . . . . . . . . . . . . 30
27.2. Informative References . . . . . . . . . . . . . . . . . 30
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 33
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 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.
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o If the option would work well in the particular deployment
environment the proponents currently envision, but 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.
3. When to Use DHCPv6
Principally, DHCPv6 carries configuration parameters for its clients.
Any knob, dial, slider, or checkbox on the client system, such as "my
domain name servers", "my hostname", or even "my shutdown
temperature" are candidates for being configured by DHCPv6.
The presence of such a knob isn't enough, because DHCPv6 also
presents the extension of an administrative domain - the operator of
the network to which the client is currently attached. Someone runs
not only the local switching network infrastructure that the client
is directly (or wirelessly) attached to, but the various methods of
accessing the external Internet via local assist services that the
network must also provide (such as domain name servers, or routers).
This means that, even if a configuration parameter can 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 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
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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: simple 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.
5. Reusing Other Options 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 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 means that implementations will likely 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 preferable 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 data
elements is provided as a 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 more complex format. That is not unconditional
requirement though. In some cases splitting one complex option into
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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.
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):
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 (a single address only) [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)
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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 take either
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
absent option designating the default, i.e. 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 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]
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 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:
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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-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 one octet long and specifies the length in bits of the
IPv6 prefix. Typically allowed values are 0 to 128.
ipv6-prefix field is a variable length field that specifies the IPv6
prefix. The length is (prefix6len + 7) / 8. This field is padded
with zero bits up to the nearest octet boundary when prefix6len is
not divisible by 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, prefix6-len would be set to 60, 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.
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
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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]
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
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(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
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 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 two octets long and specifies the length of the uri
data. Although URI format in theory supports up to 64k of data, in
practice large chunks of data may be problematic. See Section 15 for
details.
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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., a 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:
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 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
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Each instance of the text-data is formatted as follows:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...-+-+-+-+-+-+-+
| text-len | String |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...-+-+-+-+-+-+-+
The text-len is two 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.
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, 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.
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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-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 (many domains) [RFC3898] (many domains)
o LoST Server Domain name [RFC5223]
o LIS Domain name [RFC5986]
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 conditional formatting requires new code to be
written and complicates future interoperability should new
conditional formats be added; and existing code has to ignore
conditional format 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 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
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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 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. 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 application specific configuration, like SIP servers, it
is usually better to use FQDN.
Applications are often better suited to deal with FQDN failures than
with address failures. Most operating systems provide a way to retry
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 availble for applications to
reconfigure over DHCP to get a new address value if the one received
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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 defined procedure how multiple addresses
should be used (all at once, round robin, try first and fail over to
the next if it fails etc.).
FQDN provide a higher level of indirection and ambiguity. In many
cases that may be considered a benefit, but can be considered a flaw
in others. For example, one operator suggested to have the same name
being resolved to different addresses depending on the point of
attachement of the host doing 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 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 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.
FQDN does require a resolution into an actual address. This implies
the question 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, 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.
If the parameter is expected to be used by constrained devices (low
power, battery operated, low capabilities) or in very lossy networks,
it may be appealing to drop the requirement of having DNS resolution
being performed and use addresses. Another example of a constrained
device is a network booted device, where despite the fact that the
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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 a service A using address and service B using 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 RFC3315, but in some environments, e.g. cable modems, is
mandatory), the configuration will be updated on clients after T1
timer elapses. Depending on the nature of the change (is it a new
server added to a cluster of already operating 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 DNS record update. That information may be cached by
caching DNS servers for up to 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 a benefit of being easier to implemented and handle by
the DHCP software. An address option is simpler to use, its
validation 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 DNS resolution.
FQDN imposes a 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 [RFC3646] 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);
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4. Address family must be specified (A, AAAA or any); the
information being configured may require specific address family
(e.g. IPv6), but there may be a DNS record only of another type
(e.g. A only with 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;
6. Multi-homed devices may be connected to different administrative
domains with each domain providing different information in DNS
(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, DNS option encoding should be specified.
Address options that are used with overly long T1 (renew timer)
values have some characteristics of hardcoded 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 information refresh time option [RFC4242].
One specific case that makes the choice between address and FQDN not
obvious is a 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 the NTP is provided as FQDN,
there is no way to validate its DNSSEC signature. This is somewhat
weak argument though, as providing NTP server as an address is also
not verifiable using DNSSEC. If the thrustworthiness of the
configuration provided by DHCP server is in question, DHCPv6 offers
authentication 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 term "encapsulated" as this terminology is used
in [RFC3315]. The difference between encapsulated and sub-options
are that the former uses normal DHCPv6 option numbers, while the
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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 top-level or inside of 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).
From the implementation perspective, it is easier to implement
encapsulated options rather than sub-options, as the implementers 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, 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 encapsulated
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.
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 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 encapsulated
options.
10. Additional States Considered Harmful
DHCP is a protocol 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 MAP
[I-D.ietf-softwire-map-dhcp] and 4rd [I-D.ietf-softwire-4rd]. While
it would be easier for provisioned clients to get ready to use per-
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client option values, such 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, [I-D.ietf-softwire-map-dhcp] 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 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 client will
initiate renewal. Those timers apply only to its own 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
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.
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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 protocol 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 will continue 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 this case, it will in most cases
operate a separate DHCP client state machine on each interface,
acquiring 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 multi-
homed client.
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. 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 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 working group 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 has different semantics,
or the new format has a new structure.
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Responsible Area Directors for working groups that wish to add a work
item to a working group charter to define a new DHCP option should
get clarity from the working group 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 working group needs a new fragment type, it is preferable to see
if another working group 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 working group 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 working group defines the new
fragment type.
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 working group 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 working group 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 working groups that are considering
defining options that actually update the DHCP protocol, 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 working group be given a charter
deliverable to define a new DHCP option, and then on the basis of
that charter item actually make updates to the DHCP protocol.
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.
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The matter of size considerations and option order are further
discussed in Section 15 and Section 17.
15. Option Size
DHCPv6 [RFC3315] allows for packet sizes up to 64KB. First, through
its use of link-local addresses, it avoids many of the deployment
problems that plague DHCPv4, and is actually an UDP over IPv6 based
protocol (compared to DHCPv4, which is mostly UDP over IPv4 protocol,
but with layer 2 hacks). Second, RFC 3315 explicitly refers readers
to RFC 2460 Section 5, which describes an MTU of 1280 octets and a
minimum fragment reassembly of 1500 octets. It's feasible to suggest
that DHCPv6 is capable of having larger options deployed over it, and
at least no common upper limit is yet known to have been encoded by
its implementors. It is not really possible to describe a fixed
limit that cleanly divides workable option sizes from those that are
too big.
It is advantageous to prefer option formats which 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 at most once. Such options
are called singleton options. Sadly, RFCs have often failed to
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clearly specify whether a given option can appear more than once or
not.
Documents that define new options SHOULD state whether these options
are singletons or not. 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 to not assume the they will be processed in the order they
appear in the message. See Section 17 for more details.
When deciding whether a 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 option format that allows multiple values within one option
instance.
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 AFTR option and
establish a single tunnel to configured tunnel termination point
(AFTR). During standardization process of [RFC6334] there was a
discussion whether multiple instances of 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 off-line 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 a load balancing between provided tunnel endpoints. Neither
failover nor load balancing was defined for 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
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order is almost always different then the order in which options
appeared in the packet on wire.
18. Relay Options
In DHCPv4, all relay options are organized as sub-options within 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 too use the same option
number space as client/server options. Future DHCPv6 Relay options
MUST be allocated from this single DHCPv6 Option number space.
E.g. the Relay-Supplied Options Option [RFC6422] may also contain
some DHCPv6 options as permitted, such as the EAP Re-authentication
Protocol (ERP) Local Domain Name DHCPv6 Option [RFC6440].
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 to inform what options the client is willing to
consume.
For some options, such as the options required for the functioning of
the DHCPv6 protocol 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 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 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 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 not 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
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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 never aligned within
the DHCP packet, even the option code and option length may appear on
odd byte boundaries.
20. Transition Technologies
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 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.
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
such as [I-D.ietf-dhc-v4configuration] may be needed.
21. Recommended sections in the new document
There are three major entities in DHCPv6 protocol: server, relay
agent, and client. It is very helpful for implementers 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 requestor, which is a special
client-like type that participates in leasequery protocol [RFC5007]
and [RFC5460]. A similar section for the 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 the majority of deployments, requestor is co-located
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.
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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 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 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
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.
Sections that contain option definitions 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 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 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.
Optional text (if the option contains 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 specified in [RFC3646].
Clients MUST discard option foo if it is invalid (i.e. did not pass
validation steps defined in Section X.Y).
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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 the client requested it.
Optional text: Option foo is a singleton. Servers MUST NOT send more
than one instance of foo option.
Optional text (if server is never supposed to receive option foo):
Servers MUST ignore 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 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 in a Relay-Forw when
forwarding packets from clients to the servers.
22. Should the new document update existing RFCs?
Authors often ask themselves a question whether their proposal
updates exist RFCs, especially 3315. In April 2013 there were about
80 options defined. Had all documents that defined them also updated
RFC3315, 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 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 a new document replaces or modifies
existing behavior, 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 with new
text.
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If in doubt, authors should try to answer a question whether
implementor reading the base RFC alone (without reading the new
draft) would be able to properly implement the software. If the base
RFC is sufficient, that the new draft most probably does not update
the base RFC. On the other hand, if reading your draft is necessary
to properly implement the base RFC, then the new draft 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 late 2013, the authentication
in DHCPv6 is rarely used and support for it is not common in existing
implementations. Some specific deployment types make it mandatory
(or parts of 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 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 multicast address, and depending on the protocol
this may lead to undesirable results. A domain name field may be
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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 drafts 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 RFC3315 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.
Also see Section 24.
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 relay agents and server is secured, the
path between 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 security considerations.
25. IANA Considerations
This document has no actions for IANA.
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26. Acknowledgements
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 Steward Bryant for
their comments.
27. References
27.1. 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.
27.2. Informative References
[I-D.ietf-dhc-v4configuration]
Rajtar, B. and I. Farrer, "Provisioning IPv4 Configuration
Over IPv6 Only Networks", draft-ietf-dhc-
v4configuration-03 (work in progress), December 2013.
[I-D.ietf-softwire-4rd]
Despres, R., Jiang, S., Penno, R., Lee, Y., Chen, G., and
M. Chen, "IPv4 Residual Deployment via IPv6 - a Stateless
Solution (4rd)", draft-ietf-softwire-4rd-07 (work in
progress), October 2013.
[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 configuration of Softwire Address and Port Mapped
Clients", draft-ietf-softwire-map-dhcp-06 (work in
progress), November 2013.
[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.
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[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.
[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.
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[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.
[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.
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[RFC6644] Evans, D., Droms, R., and S. Jiang, "Rebind Capability in
DHCPv6 Reconfigure Messages", RFC 6644, July 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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