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6man Working Group                                             D. Farmer
Internet-Draft                                   University of Minnesota
Intended status: Standards Track                           July 23, 2018
Expires: January 24, 2019


          Exceptions to the 64-bit Boundary in IPv6 Addressing
                   draft-farmer-6man-exceptions-64-00

Abstract

   This document clarifies exceptions to the 64-bit 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 IPv6 subnets

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
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   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on January 24, 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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   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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   include Simplified BSD License text as described in Section 4.e of



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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   4
   2.  Exceptions to the 64-bit Boundary . . . . . . . . . . . . . .   5
     2.1.  Unicast Addresses with the First Three Bits 000 . . . . .   5
     2.2.  Manually Configured Addresses . . . . . . . . . . . . . .   5
     2.3.  DHCPv6 Assigned Addresses . . . . . . . . . . . . . . . .   6
     2.4.  IPv6 On-link Determination  . . . . . . . . . . . . . . .   6
     2.5.  IPv6 Link-type Specific Documents . . . . . . . . . . . .   6
   3.  Operational Guidance  . . . . . . . . . . . . . . . . . . . .   7
   4.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   8
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Appendix A.  Options for Configuring IPv6 Subnets . . . . . . . .  10
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   The 64-bit boundary in IPv6 addressing provides the basis for unicast
   addresses to be autonomously generated using stateless address auto-
   configuration (SLAAC) RFC 4862 [RFC4862].  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 64-bit interface identifiers (IIDs) and therefore 64-bit
   subnet prefixes, as defined in the IPv6 Addressing Architecture
   [RFC4291bis].  For additional discussion of the 64-bit 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 considered to have no internal structure and are assigned to
   interfaces on hosts as opaque 128-bit quantities without any
   knowledge of the subnets present on the link network.  The idea that
   unicast addresses may have no internal structure is also defined in
   IPv6 Addressing Architecture [RFC4291bis], "a node may consider that
   unicast addresses (including its own) have no internal structure."

   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.  In IPv6, these two aspects



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   are split into two logically separate parameters serving the two
   aspects independently.  The subnet assignment prefix is used by SLAAC
   to perform autonomous address assignment.  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], SLAAC [RFC4862] describe
   and specify the use of these parameters in detail.

   Briefly, unicast addresses assigned to interfaces on hosts 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 defined by
   identical subnet assignment prefixes and on-link prefixes, see
   Appendix A. for a further decision of this and the other valid
   options for configuring IPv6 subnets.  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 64-bit 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 in IPv6 ND, 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 defined in the IPv6
   Addressing Architecture [RFC4291bis].  While this is a possible
   exception to the 64-bit 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 when use with autonomous address
   assignment, as performed by SLAAC.

   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 64-bit boundary in subtle but
   complex ways.  Subnet assignment prefixes are the primary parameter



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   used to configure subnets, and when used they are effectively
   required to be 64-bit in length.  However, this does not indicate on-
   link prefixes are also required to be 64-bits in length.  Even when
   SLAAC is used, and subnets are required to be 64-bits in length, on-
   link prefixes shorter than 64-bits still seem to be valid.  Further,
   when subnet assignment prefixes are not used to configure subnets,
   autonomous address assignment is not performed, and either manually
   configured addresses or DHCPv6 assigned addresses must be used.  In
   this circumstance, subnets are configured solely using on-link
   prefixes and therefore may have any length between 0 and 128 bits,
   inclusive.  Nevertheless, for consistency with the 64-bit boundary,
   64-bit on-link prefix lengths are recommended in most circumstances.
   Therefore, when subnets are solely configured using on-link prefixes,
   subnets are only recommended to be 64-bit in length and are not
   required to be such.

   Some have stated, "IPv6 subnets are required to be 64-bits in
   length."  Whereas others counter, "IPv6 subnets are only recommended
   to be 64-bits in length."  However, because of the subtle but complex
   interaction described above, both of these statements are not
   entirely correct based on the details of how individual subnets are
   configured.  A more accurate statement is, "when configured using
   subnet assignment prefixes, IPv6 subnets are required to be 64-bits
   in length.  Otherwise, when configured solely using on-link prefixes,
   IPv6 subnets are only recommended to be 64-bits in length."  Also, it
   could be said, "standard IPv6 subnets are 64-bits in length," given
   the 64-bit length is both required or recommended based on the
   details of how individual subnets are configured.  These last two
   statements seem to more accurately reflect how the protocols that
   define and implement IPv6 subnets operate.  It is hoped that
   clarifying the following exceptions to the 64-bit boundary and
   providing clear operational guidance will provide a better
   understanding of and more clarity to the subtle but complex
   interaction between the 64-bit boundary in IPv6 addressing and how
   IPv6 subnets are defined and implemented.

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.








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2.  Exceptions to the 64-bit 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 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 [RFC4291bis] sections 2.4.2, 2.4.3, and
   2.4.5.2 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 subnet prefixes,
   which makes up the 64-bit boundary.

   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 64-bit 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.

2.2.  Manually Configured Addresses

   IPv6 addresses manually configured on a node's interface, sometimes
   known as statically configured, are an exception to the 64-bit
   boundary as they have no internal structure, are considered opaque
   128-bit quantities, and are assigned to node interfaces without any
   knowledge of the subnets present on the link network.

   Manually configured addresses MAY also include an associated an 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.
   Alternatively, if an on-link prefix length is not included, the
   manually configured address MUST NOT automatically be considered on-
   link.  Nevertheless, for consistency with the 64-bit boundary, 64-bit
   on-link prefix lengths are recommended in most circumstances.  See




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   section 3 for detailed 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
   64-bit boundary as they have no internal structure, are considered
   opaque 128-bit quantities, and are assigned to host interfaces
   without any knowledge of the subnets present on the link network.
   Further, DHCPv6 assigned addresses MUST NOT automatically be
   considered on-link.

2.4.  IPv6 On-link Determination

   IPv6 on-link determination is an exception to the 64-bit 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.  See section 3 for detailed
   operational guidance regarding the use of 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 [RFC4291bis].
   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.




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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
   shorter.  Otherwise, if subnet assignment prefixes are not
   configured, then hosts will have to use manually configured addresses
   or DHCPv6 assigned addresses and subnets are configured solely by on-
   link prefixes that are recommended to be 64-bits in length, only
   recommending 64-bit subnets in this circumstance.  There are two
   exceptions to these principles, inter-router point-to-point links
   with 127-bit prefixes [RFC6164] and the possible future specification
   of link-type specific documents based on an IID length that is not
   64-bits.

   More specifically;

      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 64-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 64-bits in length.  If the
      specification for the particular link-type is based on an IID
      length that is not 64-bits, then a length consistent with the
      specification for the particular link-type MUST be used in the
      previous requirements instead of 64-bits.

      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 64-bits in length or provide the same manually
      configured on-link prefix to each host on the link network that is
      64-bits in length.  If the specification for the particular link-
      type is based on an IID length that is not 64-bits, then a length
      consistent with the specification for the particular link-type
      SHOULD be used in the previous recommendations instead of 64-bits.

      Alternatively, network operators MAY configure point-to-point
      router links with 127-bit on-link prefixes and no subnet
      assignment prefix, see RFC 6164 [RFC6164].





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   Appendix A. discusses in further detail the valid options for
   configuring IPv6 subnets

4.  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; draft-
   jinmei-6man-prefix-clarify [I-D.jinmei-6man-prefix-clarify], draft-
   bourbaki-6man-classless-ipv6 [I-D.bourbaki-6man-classless-ipv6],
   draft-jaeggli-v6ops-indefensible-nd
   [I-D.jaeggli-v6ops-indefensible-nd].  All basically revolving around
   the discussion of RFC 4291bis [RFC4291bis] and its advancement to
   Internet Standard.

   This document was produced using the xml2rfc tool [RFC2629].

5.  IANA Considerations

   This memo includes no request to IANA.

6.  Security Considerations

   This document clarifies exceptions to the 64-bit boundary in IPv6
   addressing.  These clarifications are not security related and
   therefore are not expected to introduce any new security
   considerations.

   However, the use of longer on-link prefixes effectively allows the
   uses of smaller subnets, making it more feasible to perform IPv6
   address scans as discussed in RFC 7707 [RFC7707] and RFC 7721
   [RFC7721].  On the other hand, the use of smaller subnets can be
   effective mitigation for neighbor cache exhaustion issues as
   discussed and RFC 6164 [RFC6164] and RFC 6583 [RFC6583].  The
   relative weights applied in this trade-off will vary from situation
   to situation.

7.  References

7.1.  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>.






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   [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>.

   [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>.

   [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>.

   [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>.

7.2.  Informative References

   [I-D.bourbaki-6man-classless-ipv6]
              Bourbaki, N., "IPv6 is Classless", draft-bourbaki-6man-
              classless-ipv6-03 (work in progress), March 2018.






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   [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>.

   [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>.

   [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
   parameters with several different relationships to each other.  These



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   parameters are primarily advertised in ND RA messages by PIOs, with
   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-bit 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, 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 64-bit 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 and identical on-link prefixes, or;

   2.  Subnet assignment prefixes and shorter covering on-link prefixes,
       or;

   3.  Only subnet assignment prefixes and no on-link prefixes or;

   4.  Only on-link prefixes and 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.

   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.





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   Option 2 is not recommended, but is still valid; 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 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 valid; 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 valid; it has on-link
   prefixes but no subnet assignment prefixes, and therefore manually
   configured addresses or DHCPv6 assigned addresses must be used.  When
   DHCPv6 is used a DHCPv6 server, or DHCPv6 relay will also be needed
   on the link network.  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 64-bit
   boundary, technically it is not a violation of the 64-bit boundary;
   manually configured addresses, DHCPv6 assigned addresses, and on-link
   determination are all exceptions to the 64-bit boundary defined in
   this document.  Nevertheless, for consistency with the 64-bit
   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 are not prohibited.  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.  Nevertheless, address conservation by
   itself is never considered a valid reason for configuring subnets




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   longer than 64-bits.  Accordingly, if a site needs additional
   subnets, additional 64-bit subnets are expected to be provided.

      Note: some hosts do not provide a mechanism for manually
      configuring an address on an interface, and other hosts do not
      implement DHCPv6.  Hosts that implement neither and only implement
      SLAAC do exist and do not function on networks configured based on
      Option 4.

   It 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 the same link network.

   Logically there is another option that could define a subnet, "subnet
   assignment prefixes and 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 detailed operational
   guidance provided above.  It would have on-link prefixes longer than
   64-bits, between 65 and 128 bits, inclusive, that would be covered by
   an included subnet assignment prefix.  Its use would result 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
   2218 University Ave SE
   Minneapolis, MN  55414
   US

   Phone: +16126260815
   Email: farmer@umn.edu












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