Internet DRAFT - draft-farmer-6man-exceptions-64
draft-farmer-6man-exceptions-64
6man Working Group D. Farmer
Internet-Draft Univ. of Minnesota
Intended status: Standards Track August 9, 2018
Expires: February 10, 2019
Exceptions to the Standard Subnet Boundary in IPv6 Addressing
draft-farmer-6man-exceptions-64-09
Abstract
This document clarifies exceptions to the standard subnet boundary in
IPv6 addressing. The exceptions include unicast IPv6 addresses with
the first three bits 000, manually configured addresses, DHCPv6
assigned addresses, IPv6 on-link determination, and the possibility
of an exception specified in separate IPv6 link-type specific
documents. Further, operational guidance is provided, and Appendix A
discusses the valid options for configuring the parameters of an IPv6
subnet.
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
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This Internet-Draft will expire on February 10, 2019.
Copyright Notice
Copyright (c) 2018 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
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include Simplified BSD License text as described in Section 4.e of
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 4
2. Exceptions to the Standard Subnet Boundary . . . . . . . . . 4
2.1. Unicast Addresses with the First Three Bits 000 . . . . . 4
2.2. Manually Configured Addresses . . . . . . . . . . . . . . 5
2.3. DHCPv6 Assigned Addresses . . . . . . . . . . . . . . . . 5
2.4. IPv6 On-link Determination . . . . . . . . . . . . . . . 6
2.5. IPv6 Link-type Specific Documents . . . . . . . . . . . . 6
3. Operational Guidance . . . . . . . . . . . . . . . . . . . . 6
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
5. Security Considerations . . . . . . . . . . . . . . . . . . . 8
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 8
7. Change log [RFC Editor: Please remove] . . . . . . . . . . . 9
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
8.1. Normative References . . . . . . . . . . . . . . . . . . 10
8.2. Informative References . . . . . . . . . . . . . . . . . 11
Appendix A. Options for Configuring IPv6 Subnets . . . . . . . . 12
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 15
1. Introduction
The standard subnet boundary in IPv6 addressing provides the basis
for unicast addresses to be autonomously generated, using stateless
address auto-configuration (SLAAC) [RFC4862], and assigned to
interfaces on host. SLAAC allows hosts to connect to link networks
without any pre-configuration, which is especially useful for
general-purpose hosts and mobile devices. In this circumstance,
unicast addresses have an internal structure composed of standard
64-bit interface identifiers (IIDs) and therefore 64-bit subnet
prefixes, as described in the IPv6 Addressing Architecture [RFC4291]
Section 2.5. For additional discussion of the standard subnet
boundary in IPv6 addressing see RFC 7421 [RFC7421].
However, in other circumstances, such as with manually configured
addresses or DHCPv6 [RFC3315] assigned addresses, unicast addresses
are assigned to interfaces on nodes as opaque 128-bit quantities
without any knowledge of the internal structure or the subnets
present on the link network. These circumstances are also described
in IPv6 Addressing Architecture [RFC4291] Section 2.5, "a node may
consider that unicast addresses (including its own) have no internal
structure."
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Further, unlike IPv4 where there is a single subnet mask parameter
with the two aspects of a subnet, address assignment and on-link
determination, tightly coupled together, while in IPv6 they are split
into two logically separate parameters serving the two aspects
independently. The subnet assignment prefix is used for performing
autonomous address assignment by SLAAC. Separately, the on-link
prefix is used to determine if an address can be delivered using a
directly connected link network. IPv6 Neighbor Discovery (ND)
[RFC4861], the IPv6 Subnet Model [RFC5942], and SLAAC [RFC4862]
describe and specify the use of these parameters in detail.
Briefly, unicast addresses assigned to interfaces on nodes are not
considered on-link unless covered by an on-link prefix advertised
through ND Router Advertisement (RA) messages containing Prefix
Information Options (PIOs) with the on-link (L) flag set or by manual
configuration. Whereas autonomous address assignment uses subnet
assignment prefixes that are also advertised through the same ND RA
messages and PIOs but with the autonomous (A) flag set instead.
While they act independently, most frequently subnets are configured
using subnet assignment prefixes with identical on-link prefixes, see
Appendix A for further decision of this and the other valid options
for configuring the parameters of an IPv6 subnet. However, unlike
subnet assignment prefixes, which are effectively required to be 64
bits in length, on-link prefixes may have any length between 0 and
128 bits, inclusive. Nevertheless, for consistency with the standard
subnet boundary, 64-bit on-link prefix lengths are recommended in
most circumstances.
Reinforcing the ideas that on-link prefixes are logically separate
and may have any length. On-link prefixes are part of the next-hop
determination process, described in IPv6 ND [RFC4861] Section 5.2,
which is intrinsically part of routing and forwarding within IPv6,
and BCP 198 [RFC7608] says, "forwarding processes MUST be designed to
process prefixes of any length up to /128, by increments of 1."
Finally, SLAAC is currently designed to utilize a single IID length
to validate the length of the subnet assignment prefixes provided to
it. However, SLAAC itself does not define the IID length or assume
it is 64 bits in length. It utilizes the IID length defined in
separate link-type specific documents that are intended to be
consistent with the standard 64-bit IID length specified in the IPv6
Addressing Architecture [RFC4291] Section 2.5.1. While this is a
possible exception to the standard subnet boundary, currently there
are no IPv6 link-type specific documents that specify an IID length
other than 64 bits. Effectively requiring 64-bit IIDs and therefore
64-bit subnet assignment prefixes are used for performing autonomous
address assignment by SLAAC.
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In summary, the essential theory of this document is that the two
parameters that define IPv6 subnets, the subnet assignment prefix and
the on-link prefix, interact with the standard subnet boundary in
subtle but complex ways. IPv6 subnets are primarily configured using
subnet assignment prefixes and when they are used IPv6 subnets and
IIDs are both effectively required to be 64 bits in length. However,
it is also possible to configure IPv6 subnets solely using on-link
prefixes, which may have any length between 0 and 128 bits,
inclusive. Nevertheless, for consistency with the standard subnet
boundary, 64-bit on-link prefix lengths are recommended in most
circumstances. Therefore, when IPv6 subnets are configured solely
using on-link prefixes, IPv6 subnets and IIDs are both only
recommended to be 64 bits in length.
By clarifying the following exceptions to the standard subnet
boundary and providing clear operational guidance, this document
intends to provide clarity to and a better understanding of this
subtle but complex interaction between the standard subnet boundary
in IPv6 addressing and how IPv6 subnets are defined and implemented
by the protocols in question.
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. Exceptions to the Standard Subnet Boundary
2.1. Unicast Addresses with the First Three Bits 000
These are all currently special-purpose IPv6 addresses or are
otherwise reserved. Also, they are generally not assigned to
interfaces on hosts, especially not to general-purpose hosts.
Examples of these addresses are the unspecified address, the loopback
address, and the IPv4-Mapped IPv6 Address from RFC 4291 [RFC4291]
Sections 2.5.2, 2.5.3, and 2.5.5.2 respectively.
Most of these addresses have no internal structure and are considered
opaque 128-bit quantities. However, some of these addresses could be
presumed to have structure, such as the IPv4-mapped IPv6 address.
This structure comes from embedding an IPv4 address within an IPv6
address, but this structure is unrelated to and different from the
internal structure, composed of standard IIDs and subnets created by
the standard subnet boundary.
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Historically, reservations were also made in this range for the
mapping of OSI NSAP and IPX address into IPv6 addresses. They had
structures similar to the IPv4-mapped IPv6 address discussed above.
However, they have since been deprecated.
Note: ever since RFC 2373 [RFC2373] addresses with the first three
bits 000 have been an exception to the standard subnet boundary,
and addresses with the first three bits 001 through 111, except
for multicast addresses, have been expected to be consistent with
the standard 64-bit IID length and the standard subnet boundary.
However, currently only 2000::/3 has been allocated as global
unicast address space and released to the IANA for distribution to
the Regional Internet Registries (RIRs). The applicability of the
standard subnet boundary to future allocations will be determined
at the time the allocations are release to the IANA.
Nevertheless, implementations of IPv6 should assume the standard
subnet boundary applies to future allocations and that 2000::/3 is
not special in this regard.
2.2. Manually Configured Addresses
IPv6 addresses manually configured on a node's interface, sometimes
known as statically configured, are an exception to the standard
subnet boundary as they are considered opaque 128-bit quantities and
are assigned to node interfaces without any knowledge of the internal
structure or the subnets present on the link network.
Manually configured addresses MAY also include an associated on-link
prefix length. This on-link prefix length (n) MAY have any value
between 0 and 128 bits, inclusive. If an on-link prefix length is
included, the most significant, or leftmost, n bits of the manually
configured address are considered the on-link prefix. Otherwise, if
an on-link prefix length is not included, an on-link prefix MUST NOT
be automatically assumed, but an on-link prefix may be learned from a
PIO with the L flag set. Nevertheless, for consistency with the
standard subnet boundary, 64-bit on-link prefix lengths are
recommended in most circumstances. See Section 3 for operational
guidance regarding on-link prefix lengths.
2.3. DHCPv6 Assigned Addresses
IPv6 addresses assigned to a host's interface via DHCPv6 [RFC3315]
(Identity Association for Non-temporary Addresses (IA_NA) or Identity
Association for Temporary Addresses (IN_TA)) are an exception to the
standard subnet boundary as they are considered opaque 128-bit
quantities and are assigned to host interfaces without any knowledge
of the internal structure or the subnets present on the link network.
Further, DHCPv6 assigned addresses MUST NOT automatically assume an
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on-link prefix, but an on-link prefix may be learned from a PIO with
the L flag set.
2.4. IPv6 On-link Determination
IPv6 on-link determination is an exception to the standard subnet
boundary, in that IPv6 ND [RFC4861] does not require on-link prefixes
to be 64 bits in length. To the contrary, on-link prefixes MAY have
any length between 0 and 128 bits, inclusive. Nevertheless, for
consistency with the standard subnet boundary, 64-bit on-link prefix
lengths are recommended in most circumstances. See Section 3 for
operational guidance regarding on-link prefix lengths.
2.5. IPv6 Link-type Specific Documents
Separate IPv6 link-type specific documents, sometimes known as
"IPv6-over-FOO" documents, specify the IID length utilized by SLAAC
to validate the length of subnet assignment prefixes provided. The
IID length defined SHOULD be consistent with the standard 64-bit IID
length specified in the IPv6 Addressing Architecture [RFC4291].
However, these documents may create an exception to the standard
64-bit IID length scoped to a specific link-type technology when
justified. Although currently, there are no IPv6 link-type specific
documents that specify an IID length other than 64 bits.
When an exception to the standard 64-bit IID is specified in a
link-type specific document, valid justification needs to be
documented in some detail.
Further, SLAAC is currently designed to validate against only a
single IID length per link-type technology. As a result, a link-type
technology that specifies a non-standard IID length cannot be
directly bridged with another link-type technology that specifies the
standard 64-bit IID length without creating confusion about the IID
length that is to be used for validation. Therefore, if this type of
direct bridging is allowed, then a mechanism to ensure there is no
confusion about which IID length SLAAC is to validate against needs
to be provided.
3. Operational Guidance
At a high-level, this document recommends the following principles
for the configuration of IPv6 subnets. The configuration of subnet
assignment prefixes is recommended, allowing hosts to use autonomous
address assignment. With this configuration, subnet assignment
prefixes are required to be 64 bits in length, requiring 64-bit
subnets in this circumstance. Further, identical on-link prefixes
are recommended, but on-link prefixes are required to be 64 bits or
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shorter. Otherwise, if a subnet assignment prefix is not configured,
then hosts will have to use manually configured addresses or DHCPv6
assigned addresses and these subnets are solely configured by on-link
prefixes. These on-link prefixes are recommended to be 64 bits in
length, therefore only recommending 64-bit subnets in this
circumstance. There are two exceptions to these principles, the
possible future specification of a link-type specific document based
on an IID length that is not 64 bits and inter-router point-to-point
links with 127-bit prefixes [RFC6164]. Further, each subnet must be
configured on a single link network and must not overlap subnets on
other link networks.
More formally;
Network operators SHOULD configure routers to advertise to each
link network at least one subnet assignment prefix (a PIO with the
A flag set). If a subnet assignment prefix is advertised, it MUST
be (128 - N) bits in length, and an identical on-link prefix (a
PIO with the L flag set) SHOULD also be advertised. If an on-link
prefix is advertised and is covered by a subnet assignment prefix,
the on-link prefix MUST NOT be longer than (128 - N) bits in
length.
Otherwise, if a subnet assignment prefix is not advertised,
network operators SHOULD configure routers to advertise to each
link network at least one on-link prefix (a PIO with the L flag
set) that is (128 - N) bits in length or provide the same manually
configured on-link prefix to each node on the link network that is
(128 - N) bits in length.
Where N = 64 or the IID length specified in the link-type specific
document for the link network in question.
Alternatively, network operators MAY configure point-to-point
router links with on-link prefixes that SHOULD be 127 bits in
length, typically by manual configuration, and with no subnet
assignment prefix. See RFC 6164 [RFC6164] Section 6 for address
selection considerations.
Further, each on-link prefix and each subnet assignment prefix
MUST be uniquely configured on a single link network and MUST NOT
overlap on-link prefixes or subnet assignment prefixes configured
on any other link networks. On-link prefixes MAY overlap subnet
assignment prefixes configured on the same link network.
Appendix A discusses in further detail the valid options for
configuring the parameters of IPv6 subnet.
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4. IANA Considerations
This document includes no request to IANA.
5. Security Considerations
This document clarifies exceptions to the standard subnet boundary in
IPv6 addressing. These clarifications are not security related and
therefore are not expected to introduce any new security
considerations.
The use of subnets solely configured by on-link prefixes negatively
impacts techniques that are intended to increase the security and
privacy of users RFC 4941 [RFC4941] and RFC 7217 [RFC7217], as they
depend on the use of SLAAC, hence the recommendation to configure
subnet assignment prefixes allowing the use of SLAAC. Further, the
use of subnets solely configured by on-link prefixes also permits
longer on-link prefixes effectively allowing smaller subnets and
making it more feasible to perform IPv6 address scans. These and
other related security and privacy considerations are discussed in
RFC 7707 [RFC7707] and RFC 7721 [RFC7721].
However, the use of smaller subnets can be effective mitigation for
neighbor cache exhaustion issues as discussed in RFC 6164 [RFC6164]
and RFC 6583 [RFC6583]. The relative weights applied in this
trade-off will vary from situation to situation.
6. Acknowledgments
This document was inspired by a series of discussions on the 6MAN and
the V6OPS working group mailing lists over a period of approximately
two years, including discussions around the following drafts;
[I-D.jinmei-6man-prefix-clarify], [I-D.bourbaki-6man-classless-ipv6],
and [I-D.jaeggli-v6ops-indefensible-nd]. All revolving around the
discussion of RFC 4291bis [RFC4291bis] and its advancement to
Internet Standard.
This document was produced using the xml2rfc tool [RFC2629].
The author would like to thank the following, in alphabetical order,
for their contributions and comments:
Brian Carpenter, Bruce Curtis, Fernando Gont, Craig Miller,
Alexandre Petrescu, and Tatuya Jinmei
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7. Change log [RFC Editor: Please remove]
draft-farmer-6man-exceptions-64-09, 2018-August-9:
o Another try on formatting changes
draft-farmer-6man-exceptions-64-08, 2018-August-9:
o Another try on formatting changes
draft-farmer-6man-exceptions-64-07, 2018-August-9:
o Formatting changes
draft-farmer-6man-exceptions-64-06, 2018-August-9:
o Added to the note in Section 2.1, about the applicability of
standard subnet boundary to future allocations.
o Wordsmithed the two exceptions in the formal guidance.
o Added a subnet uniqueness requirement in the operational guidance.
o Other miscellaneous editorial changes.
draft-farmer-6man-exceptions-64-05, 2018-August-6:
o Fixed typo in organization tag from version 4
draft-farmer-6man-exceptions-64-04, 2018-August-6:
o Changed references for RFC4291bis to RFC4291 except in
Acknowledgments
o Missed a 64-bit boundary, changing it to standard subnet boundary,
in the note in Section 2.1.
o Added that only 2000::/3 has bee release for allocations, in the
note in Section 2.1.
o Other miscellaneous editorial changes and typos fixed.
o Changes to Author's address
draft-farmer-6man-exceptions-64-03, 2018-August-3:
o Several editorial changes to manual configuration and DHCPv6
sections
o Changed 64-bit boundary to standard subnet boundary in the title
and through the document.
o Several editorial changes to the operational guidance section and
changed from 64-bit to 128-N and N=64 in the formal guidance.
o Added titles for the M and O flags.
o Other miscellaneous editorial changes.
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draft-farmer-6man-exceptions-64-02, 2018-July-31:
o Rewrote summary paragraph of the Introduction, using both subnets
and IIDs.
o Added that privacy addresses require SLAAC in the Security
Considerations.
o Added that manual config and DHCPv6 can also be used with Options
1-3 of Appendix A.
o Added quick mention of M and O flags to discussion of DHCPv6 in
Option 4 of Appendix A.
o Fixed typos introduced in version 01.
o More formatting changes.
o Editorial changes.
draft-farmer-6man-exceptions-64-01, 2018-July-24:
o Numerous formatting changes.
o Editorial changes.
o Moved Acknowledgments to just before change log.
draft-farmer-6man-exceptions-64-00, 2018-July-23:
o Original version.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3315] Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins,
C., and M. Carney, "Dynamic Host Configuration Protocol
for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July
2003, <https://www.rfc-editor.org/info/rfc3315>.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, DOI 10.17487/RFC4291, February
2006, <https://www.rfc-editor.org/info/rfc4291>.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
DOI 10.17487/RFC4861, September 2007,
<https://www.rfc-editor.org/info/rfc4861>.
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[RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", RFC 4862,
DOI 10.17487/RFC4862, September 2007,
<https://www.rfc-editor.org/info/rfc4862>.
[RFC5942] Singh, H., Beebee, W., and E. Nordmark, "IPv6 Subnet
Model: The Relationship between Links and Subnet
Prefixes", RFC 5942, DOI 10.17487/RFC5942, July 2010,
<https://www.rfc-editor.org/info/rfc5942>.
[RFC6164] Kohno, M., Nitzan, B., Bush, R., Matsuzaki, Y., Colitti,
L., and T. Narten, "Using 127-Bit IPv6 Prefixes on Inter-
Router Links", RFC 6164, DOI 10.17487/RFC6164, April 2011,
<https://www.rfc-editor.org/info/rfc6164>.
[RFC7608] Boucadair, M., Petrescu, A., and F. Baker, "IPv6 Prefix
Length Recommendation for Forwarding", BCP 198, RFC 7608,
DOI 10.17487/RFC7608, July 2015,
<https://www.rfc-editor.org/info/rfc7608>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
8.2. Informative References
[I-D.bourbaki-6man-classless-ipv6]
Bourbaki, N., "IPv6 is Classless", draft-bourbaki-6man-
classless-ipv6-03 (work in progress), March 2018.
[I-D.jaeggli-v6ops-indefensible-nd]
Jaeggli, J., "Indefensible Neighbor Discovery", draft-
jaeggli-v6ops-indefensible-nd-01 (work in progress), July
2018.
[I-D.jinmei-6man-prefix-clarify]
Jinmei, T., "Clarifications on On-link and Subnet IPv6
Prefixes", draft-jinmei-6man-prefix-clarify-00 (work in
progress), March 2017.
[RFC2373] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 2373, DOI 10.17487/RFC2373, July 1998,
<https://www.rfc-editor.org/info/rfc2373>.
[RFC2629] Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629,
DOI 10.17487/RFC2629, June 1999,
<https://www.rfc-editor.org/info/rfc2629>.
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[RFC4291bis]
Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", draft-ietf-6man-rfc4291bis-09 (work in
progress), July 2017,
<https://tools.ietf.org/id/draft-ietf-6man-rfc4291bis>.
[RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy
Extensions for Stateless Address Autoconfiguration in
IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007,
<https://www.rfc-editor.org/info/rfc4941>.
[RFC6583] Gashinsky, I., Jaeggli, J., and W. Kumari, "Operational
Neighbor Discovery Problems", RFC 6583,
DOI 10.17487/RFC6583, March 2012,
<https://www.rfc-editor.org/info/rfc6583>.
[RFC7217] Gont, F., "A Method for Generating Semantically Opaque
Interface Identifiers with IPv6 Stateless Address
Autoconfiguration (SLAAC)", RFC 7217,
DOI 10.17487/RFC7217, April 2014,
<https://www.rfc-editor.org/info/rfc7217>.
[RFC7421] Carpenter, B., Ed., Chown, T., Gont, F., Jiang, S.,
Petrescu, A., and A. Yourtchenko, "Analysis of the 64-bit
Boundary in IPv6 Addressing", RFC 7421,
DOI 10.17487/RFC7421, January 2015,
<https://www.rfc-editor.org/info/rfc7421>.
[RFC7707] Gont, F. and T. Chown, "Network Reconnaissance in IPv6
Networks", RFC 7707, DOI 10.17487/RFC7707, March 2016,
<https://www.rfc-editor.org/info/rfc7707>.
[RFC7721] Cooper, A., Gont, F., and D. Thaler, "Security and Privacy
Considerations for IPv6 Address Generation Mechanisms",
RFC 7721, DOI 10.17487/RFC7721, March 2016,
<https://www.rfc-editor.org/info/rfc7721>.
[RFC8273] Brzozowski, J. and G. Van de Velde, "Unique IPv6 Prefix
per Host", RFC 8273, DOI 10.17487/RFC8273, December 2017,
<https://www.rfc-editor.org/info/rfc8273>.
Appendix A. Options for Configuring IPv6 Subnets
As discussed in the Introduction, IPv6 subnets are defined by two
separate parameters, acting independently, the subnet assignment
prefix and the on-link prefix. It is possible to configure these two
parameters with several different relationships to each other. These
parameters are primarily advertised in ND RA messages by PIOs, with
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the A and L flags designating the purpose of the PIO. However,
on-link prefixes may also be manually configured.
SLAAC [RFC4862] Section 5.5.3 bullet d, validates subnet assignment
prefixes against the IID length specified in separate link-type
specific documents that are intended to be consistent with the
standard 64-bit IID length. Currently, there are no link-type
specific documents that specify a non-standard IID length. Therefore
subnet assignment prefixes are effectively required to be 64 bits in
length. Further, to simplify the following discussion the
possibility that a link-type specific document could specify a
non-standard IID length is ignored.
Whereas on-link prefixes have no such validation specified in IPv6 ND
[RFC4861], this is also confirmed in SLAAC [RFC4862] Section 5.5.3
bullet d. Therefore on-link prefixes are not required to be 64 bits
in length; they may have any length between 0 and 128 bits,
inclusive. Nevertheless, for consistency with the standard subnet
boundary, 64-bit on-link prefixes lengths are recommended, except for
inter-router point-to-point links with 127-bit prefixes.
The following are the valid options for configuring the two
parameters that define an IPv6 subnet;
1. Subnet assignment prefixes with identical on-link prefixes
2. Subnet assignment prefixes with shorter covering on-link prefixes
3. Only subnet assignment prefixes with no on-link prefixes
4. Only on-link prefixes with no subnet assignment prefixes
Options 1 through 3, all define subnet assignment prefixes,
designating the use of autonomous address assignment, performed by
SLAAC, and effectively requiring subnets that are 64 bits in length.
However, manually configured addresses or DHCPv6 assigned addresses
may also be used in addition to autonomous address assignment.
Option 1 is both the most frequently used and the only recommended
option, except for inter-router point-to-point links with 127-bit
prefixes, it has identical subnet assignment prefixes and on-link
prefixes of 64 bits in length. The 64-bit subnets used for
autonomous address assignment are considered to be on-link. This
option is particularly recommended for networks that are made
available to the general public or networks that intend to connect
general-purpose hosts or mobile devices.
Option 2 is not recommended, but is still a valid configuration; it
has on-link prefixes shorter than 64 bits, between 0 and 63 bits,
inclusive, but covering the subnet assignment prefixes included. The
64-bit subnets used for autonomous address assignment are considered
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on-link, along with other numerically adjacent subnets. However,
these other numerically adjacent subnets are not used for autonomous
address assignment unless additional separate 64-bit subnet
assignment prefixes are also included.
Option 3 is not recommended, but is still a valid configuration; it
has subnet assignment prefixes but no on-link prefixes. Therefore
the 64-bit subnets used for autonomous address assignment are not
considered on-link, and all traffic for the subnets, including host-
to-host traffic, must be sent to a default router. See RFC 8273
[RFC8273] for an example of this option.
Option 4 is not recommended, but is still a valid configuration; it
has on-link prefixes, but no subnet assignment prefixes, and
therefore manually configured addresses or DHCPv6 assigned addresses
must be used. The on-link prefixes may have any length between 0 and
128 bits, inclusive. However, 64-bit on-link prefixes are
recommended, except for inter-router point-to-point links with
127-bit prefixes. This option effectively results in subnets that
are defined only by the on-link prefixes, and therefore the subnets
may have any lengths, even though 64 bits is recommended.
Furthermore, Option 4 essentially allows for the use of subnets
longer than 64 bits. While this violates the spirit of the standard
subnet boundary, technically it is not a violation; manually
configured addresses, DHCPv6 assigned addresses, and on-link
determination are all exceptions to the standard subnet boundary
defined in this document. Nevertheless, for consistency with the
standard subnet boundary, 64-bit on-link prefix lengths are
recommended, effectively recommending 64-bit subnets, except for
inter-router point-to-point links with 127-bit prefixes.
There can be operationally valid reasons for configuring subnets
longer than 64 bits, and when a subnet is solely configured by an
on-link prefix, longer subnets while not recommended are not
prohibited either. RFC 6164 [RFC6164] explicitly allows 127-bit
prefixes for inter-router point-to-point links. Hence the explicit
exceptions included for it. Additionally, RFC 6583 [RFC6583]
discusses "sizing subnets to reflect the number of addresses actually
in use" as an operational mitigation for neighbor cache exhaustion
issues. RFC 7421 section 3 [RFC7421] discusses these issues in more
detail, but there could be other reasons as well. Nevertheless,
address conservation by itself is never considered a valid reason for
configuring subnets longer than 64 bits. Accordingly, if a site
needs additional subnets, additional 64-bit subnets are expected to
be provided.
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When DHCPv6 is used a DHCPv6 server or DHCPv6 relay agent will also
be needed on the link network. Further, the managed address
configuration (M) flag in IPv6 ND RA messages signals to hosts that
DHCPv6 should be used for IPv6 address assignment and the other
configuration (O) flag signals that other configuration information
is available via DHCPv6. However, some hosts do not implement DHCPv6
and other hosts do not provide a mechanism for manually configuring
an address on an interface. Hosts that implement neither, only
implementing SLAAC, do exist and do not operate on subnets configured
based on Option 4 regardless of the length of the on-link prefix
configured.
It is possible to simultaneously configure multiple different
subnets, associated with a single link network, each based on the
same or different options described above. For example, there could
be two different subnets based on Option 1 and one based on Option 4,
all associated with the same link network.
Logically there is another option that could define a subnet, "Subnet
assignment prefixes with longer covered on-link prefixes," but it
does not result in an operationally valid subnet. While SLAAC and ND
accept this configuration, it is particularly problematic and is
considered an invalid configuration by the operational guidance
provided in Section 3. It would have on-link prefixes longer than 64
bits, between 65 and 128 bits, inclusive, that are covered by an
included 64-bit subnet assignment prefix. This configuration results
in the 64-bit subnet used for autonomous address assignment being
inconsistently considered on-link for some address and not on-link
for other addresses within the same subnet. This inconsistency
creates a performance differential between addresses within the same
subnet, which is inefficient and difficult to troubleshoot.
Author's Address
David Farmer
University of Minnesota
Office of Information Technology
2218 University Ave SE
Minneapolis, MN 55414
US
Phone: +16126260815
Email: farmer@umn.edu
URI: http://www.umn.edu/
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