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Unintended Operational Issues With ULA

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This is an older version of an Internet-Draft whose latest revision state is "Expired".
Authors Nick Buraglio , Chris Cummings , Russ White
Last updated 2022-11-09 (Latest revision 2022-08-01)
Replaces draft-buraglio-v6ops-ula
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Network Working Group                                        N. Buraglio
Internet-Draft                                               C. Cummings
Intended status: Informational                   Energy Sciences Network
Expires: 13 May 2023                                            R. White
                                                        Juniper Networks
                                                         9 November 2022

                 Unintended Operational Issues With ULA


   The behavior of ULA addressing as defined by [RFC6724] is preferred
   below legacy IPv4 addressing, thus rendering ULA IPv6 deployment
   functionally unusable in IPv4 / IPv6 dual-stacked environments.  This
   behavior is counter to the operational behavior of GUA IPv6
   addressing on nearly all modern operating systems that leverage a
   preference model based on [RFC6724] .

Status of This Memo

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   This Internet-Draft will expire on 13 May 2023.

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   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Defining Well Known Unintended Operational Issues With ULA  .   2
   3.  Operational Implications  . . . . . . . . . . . . . . . . . .   3
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   In modern IPv4 / IPv6 dual-stacked environments, ULA addressing and
   GUA IPv6 addressing exhibit opposite behavior, which creates
   difficulties in deployments leveraging ULA addressing.  This
   conflicting behavior carries planning, operational, and security
   implications for environments requiring ULA addressing with IPv4/IPv6
   dual-stack and prioritization of IPv6 traffic by default, as is the
   behavior with IPv6 GUA addressing.

2.  Defining Well Known Unintended Operational Issues With ULA

   The [RFC6724] definition is incomplete for ULA precedence if a host
   is operating in a dual-stack environment.  As written, [RFC6724]
   section 10.3 states: "The default policy table gives IPv6 addresses
   higher precedence than IPv4 addresses.  This means that applications
   will use IPv6 in preference to IPv4 when the two are equally
   suitable.  An administrator can change the policy table to prefer
   IPv4 addresses by giving the ::ffff: prefix a higher
   precedence".  Expected behavior would be that ULA address space would
   be preferred over legacy IPv4, however this is not the case.  This
   presents an acute issue with any environment that will use ULA
   addressing along side legacy IPv4 that is counter to the standard
   expectations for legacy IPv4 / IPv6 dual-stack behavior of preferring
   IPv6, as is performed with GUA addressing.  Further, [RFC6724]
   Section 10.6 states that this is resolvable by adding a site-specific

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   policy to cause ULAs within a site to be preferred over global
   addresses.  While theoretically possible, this presents significant
   issues on devices with inaccessable configuration files as detailed

3.  Operational Implications

   There are demonstrated and easily repeatible uses cases of ULA not
   being preferred in some OS and network equipment over legacy IPv4
   that necessitate the immediate update to [RFC6724] to better reflect
   the original intent of the RFC.  As with most adjustments to
   standards, and using [RFC6724] itself as a measurment, this update
   will likely take between 8-20 years to become common enough for
   relatively consistent behavior within operating systems.  As a
   reference, as of the time of this writing, it has been 10 years since
   [RFC6724] has been published but we continue to see existing
   commercial and open source operating systems exhibiting [RFC3484]
   behavior.  While it should be noted that [RFC6724] defines a solution
   that is functional academically, operationally the solution of
   adjusting the address preference selection table is both operating
   system dependent and unable to be signalled by any network mechanism
   such as within a router advertisement, DHCPv6 option, or the like.
   This lack of an intra-protocol or network-based ability to adjust
   address selection preference, along with the inability to adjust a
   notable number of operating systems either programmatically or
   manually renders operational scalability of such a mechanism
   functionally untenable.  It is anticipated that any update of
   [RFC6724]would require an additional 8-20 years to be fully realized
   and properly implemented in a majority of network connected systems.
   In addition, in the current versions of Linux, the priority table
   (gai.conf) still makes reference to [RFC3484], further demonstrating
   the long timeframe to have updates reflected in a current, modern
   operating system.  Examples of such out-of-date behavior can be found
   in printers, cameras, fixed devices, IoT sensors, and longer
   lifecycle equipment.  It is especially important to note this
   behavior in the long lifecycle equipment that exists in industrial
   control and operational techology environments due to their very long
   mean time to replacement.  The core issue is the stated
   interpretation from gai.conf that has the following default:

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  #scopev4  <mask> <value>
  #    Add another rule to the RFC 6724 scope table for IPv4 addresses.
  #    By default the scope IDs described in section 3.2 in RFC 6724 are
  #    used.  Changing these defaults should hardly ever be necessary.
  #    The defaults are equivalent to:
  #scopev4 ::ffff:  2
  #scopev4 ::ffff:    2
  #scopev4 ::ffff:       14

                                 Figure 1

   Notice that they are interpreting the legacy IPv4 address range as
   "scopev4" and the prefix ::ffff: which has a higher
   precedence (35) in [RFC6724] then the ULA prefix of fc00::/7 (3).
   This results in legacy IPv4 being preferred over IPv6 ULA.

   The operational outcome is the move to dual-stack with ULA is
   inconsistent and imparts unnecessary difficulty for both
   troubleshooting and creating the baseline expected behavior which are
   both requirements for deployments.  This results in operational and
   engineering teams not gaining IPv6 experience as limited traffic is
   actually using IPv6, and security baseline expectations are
   inconsistent at best and haphazard at worst.

   In practice, [RFC6724] imposes several operational shortcomings
   preventing both consistent and desired behavior.  If we define
   "desired behavior" as IPv6 preference over legacy IPv4 for address
   and protocol selection, then the resulting implemented behavior,
   based on [RFC6724] , will fall short of that intent.  Based on the
   current verbiage, dual-stacked hosts configured with both a legacy
   IPv4 address and an IPv6 ULA address, the resulting behavior will
   manifest as a host choosing IPv4 over ULA IPv6.  This behavior
   deviates from the current goal of a host with legacy IPv4 address and
   also with an IPv6 GUA address preferring IPv6 over IPv4.
   Operationally and strategically, this manifests as an impediment to
   deployment of IPv6 for many non-service provider and mobile networks
   phasing in dual-stacked (both legacy IPv4 and IPv6) networking with
   the expectation of consistent behavior (alway use IPv6 before legacy

   Other operational considerations are the use of the policy table
   detailed in section 2.1 of [RFC6724] . While conceptually the intent
   was for a configurable, longest-match table to be adjusted as-needed.
   In practice, modifying the prefix policy table remains difficult
   across platforms, and in some cases impossible.  Embedded,
   proprietary, closed source, and IoT devices are especially difficult
   to adjust and are, in many cases, incapable of any adjustment

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   whatsoever.  Large scale manipulation of the policy table also
   remains out of the realm of realistic support for small and medium
   scale operators due to lack of ability to manipulate all the hosts
   and systems, or a lack of tooling and access.

   Below is an example of a gai.conf file from a modern Linux
   installation as of 03 April 2022:

# Configuration for getaddrinfo(3).
# So far only configuration for the destination address sorting is needed.
# RFC 3484 governs the sorting.  But the RFC also says that system
# administrators should be able to overwrite the defaults.  This can be
# achieved here.
# All lines have an initial identifier specifying the option followed by
# up to two values.  Information specified in this file replaces the
# default information.  Complete absence of data of one kind causes the
# appropriate default information to be used.  The supported commands include:
# reload  <yes|no>
#    If set to yes, each getaddrinfo(3) call will check whether this file
#    changed and if necessary reload.  This option should not really be
#    used.  There are possible runtime problems.  The default is no.
# label   <mask>   <value>
#    Add another rule to the RFC 3484 label table.  See section 2.1 in
#    RFC 3484.  The default is:
#label ::1/128       0
#label ::/0          1
#label 2002::/16     2
#label ::/96         3
#label ::ffff:0:0/96 4
#label fec0::/10     5
#label fc00::/7      6
#label 2001:0::/32   7
#    This default differs from the tables given in RFC 3484 by handling
#    (now obsolete) site-local IPv6 addresses and Unique Local Addresses.
#    The reason for this difference is that these addresses are never
#    NATed while IPv4 site-local addresses most probably are.  Given
#    the precedence of IPv6 over IPv4 (see below) on machines having only
#    site-local IPv4 and IPv6 addresses a lookup for a global address would
#    see the IPv6 be preferred.  The result is a long delay because the
#    site-local IPv6 addresses cannot be used while the IPv4 address is
#    (at least for the foreseeable future) NATed.  We also treat Teredo

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#    tunnels special.
# precedence  <mask>   <value>
#    Add another rule to the RFC 3484 precedence table.  See section 2.1
#    and 10.3 in RFC 3484.  The default is:
#precedence  ::1/128       50
#precedence  ::/0          40
#precedence  2002::/16     30
#precedence ::/96          20
#precedence ::ffff:0:0/96  10
#    For sites which prefer IPv4 connections change the last line to
#precedence ::ffff:0:0/96  100

# scopev4  <mask>  <value>
#    Add another rule to the RFC 6724 scope table for IPv4 addresses.
#    By default the scope IDs described in section 3.2 in RFC 6724 are
#    used.  Changing these defaults should hardly ever be necessary.
#    The defaults are equivalent to:
#scopev4 ::ffff:  2
#scopev4 ::ffff:    2
#scopev4 ::ffff:       14

                               Figure 2

   Several assumptions are made here and are largely based on
   interpretations of [RFC6724] but are not operationally relevant in
   modern networks.  As this file or an equivalent structure within a
   given operating system is referenced, it dictates the behavior of the
   getaddrinfo() or analogous process.  More specifically, where
   getaddrinfo() or comparable API is used, the sorting behavior should
   take into account both the source address of the requesting host as
   well as the destination addresses returned and sort according to both
   source and destination addressing, i.e, when a ULA address is
   returned, the source address selection should return and use a ULA
   address if available.  Similarly, if a GUA address is returned the
   source address selection should return a GUA source address if

   Here are some example failure modes:

   1.  ULA per [RFC6724] is less preferred (the Precedence value is
       lower) than all legacy IPv4 (represented by ::ffff:0:0/96 in the
       aforementioned table).

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   2.  Because of the lower Precedence value of fc00::/7, if a host has
       legacy IPv4 enabled, it will use legacy IPv4 before using ULA.

   3.  A dual-stacked client will source the traffic from the legacy
       IPv4 address, meaning it will require a corresponding legacy IPv4
       destination address.

   Per number 3, even a host choosing a destination with A and AAAA DNS
   records, the host in question will choose the A record to get an
   legacy IPv4 address for the destination, meaning ULA IPv6 is rendered
   completely unused.  It is also notable that Happy Eyeballs ([RFC8305]
   ) will not change the source address selection process on a host.
   Happy Eyeballs will only modify the destination sorting process.

   As a direct result of the described failure modes, and in addition to
   the aforementioned operational implications, use of ULA is not a
   viable option for dual-stack \ networking transition planning, large
   scale network modeling, network lab environments or other modes of
   emulating a large scale networking that runs both IPv4 and IPv6

4.  IANA Considerations

   None at this time.

5.  Security Considerations

   Such unexpected behavior can result in operational outcomes which can
   result in serious security and compliance issues and could, in some
   cases, result in disabling of IPv6 to acheive compliance and
   consistency. .

6.  Acknowledgements

   The authors acknowledge the work of Brian Carpenter, David Farmer,
   Bob Hinden, Mark Andrews, Eduard Vasilenko, and Mark Smith for
   participation in the technical discussions leading to this finding
   and Michael Ackermann, Tom Coffeen, Kevin Myers, and Ed Horley for
   providing further testing and operational input.

7.  References

7.1.  Normative References

   [RFC3484]  Draves, R., "Default Address Selection for Internet
              Protocol version 6 (IPv6)", RFC 3484,
              DOI 10.17487/RFC3484, February 2003,

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   [RFC6724]  Thaler, D., Ed., Draves, R., Matsumoto, A., and T. Chown,
              "Default Address Selection for Internet Protocol Version 6
              (IPv6)", RFC 6724, DOI 10.17487/RFC6724, September 2012,

   [RFC8305]  Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2:
              Better Connectivity Using Concurrency", RFC 8305,
              DOI 10.17487/RFC8305, December 2017,

7.2.  Informative References

   [RFC1918]  Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.
              J., and E. Lear, "Address Allocation for Private
              Internets", BCP 5, RFC 1918, DOI 10.17487/RFC1918,
              February 1996, <>.

   [RFC4193]  Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
              Addresses", RFC 4193, DOI 10.17487/RFC4193, October 2005,

   [RFC6598]  Weil, J., Kuarsingh, V., Donley, C., Liljenstolpe, C., and
              M. Azinger, "IANA-Reserved IPv4 Prefix for Shared Address
              Space", BCP 153, RFC 6598, DOI 10.17487/RFC6598, April
              2012, <>.

Authors' Addresses

   Nick Buraglio
   Energy Sciences Network

   Chris Cummings
   Energy Sciences Network

   Russ White
   Juniper Networks

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