Network Working Group                                          R. Hinden
Internet-Draft                                      Check Point Software
Updates: 5175 (if approved)                                 B. Carpenter
Intended status: Standards Track                       Univ. of Auckland
Expires: August 19, 2018                               February 15, 2018

            IPv6 Router Advertisement IPv4 Unavailable Flag


   This document specifies a Router Advertisement Flag to indicate to
   hosts that there is no IPv4 service on the advertising default IPv6
   router.  This document updates RFC5175.

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 August 19, 2018.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  IPv4 Unavailable Flag . . . . . . . . . . . . . . . . . . . .   3
   3.  Router and Operational Considerations . . . . . . . . . . . .   4
   4.  Host Behavior Considerations  . . . . . . . . . . . . . . . .   4
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   6
   8.  Change log [RFC Editor: Please remove]  . . . . . . . . . . .   6
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   This document specifies a Router Advertisement Flag to indicate to
   hosts that there is no IPv4 service on the advertising default IPv6
   router.  The flag does not apply to non-default IPv6 routers.

   Hosts that support IPv4 and IPv6, usually called dual stack hosts,
   need to work on IPv6 only networks.  That is, a link where there are
   no IPv4 routers and/or IPv4 services.  Dual stack is the default
   configuration for most current host operating systems such as Windows
   10, IOS, Android, Linux, and BSD, as well as devices such as
   printers.  Monitoring of IPv6-only networks, for example at the IETF
   100 meeting in Singapore, shows that current dual stack hosts will
   create local auto-configured IPv4 addresses and attempt to reach IPv4
   services.  This may be a problem for several reasons:

   o  It may result in an undesirable level of Layer 2 broadcast
      traffic, especially on large wireless networks.

   o  In particular, this may overload switches in multi-segment
      wireless networks because it will create IPv4 state for every dual
      stack host.

   o  Such traffic may drain battery power on wireless hosts that have
      no interest in link-local IPv4 traffic.  [RFC7772] indicates how
      this risk might be quantified.

   o  Similarly, hosts may waste battery power on futile attempts to
      access IPv4 services.

   o  On an IPv6-only network, IPv4 might be used for malicious purposes
      and pass unnoticed by IPv6-only monitoring mechanisms.

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   Some of these problems could be mitigated by configuring the Layer 2
   infrastructure to drop IPv4 and DHCPv4 traffic by filtering
   Ethertypes 0x0800 and 0x806.  However although this would limit the
   traffic to a single segment, it would not eliminate it.

   This document defines a mechanism to inform hosts that there is no
   IPv4 support on their default routers so that they may choose to turn
   off IPv4, mitigating all of the above problems.  Each TCP/IP host
   implementation will differ in the way it handles the absence of IPv4
   support, so the usefulness of this mechanism will vary.  In any case
   it is only a hint.

   Because there is no IPv4 support on IPv6-only routers, the only way
   to notify the dual stack hosts on the link is to use an IPv6
   mechanism.  An active notification will be much more precise than
   attempting to deduce this fact by the lack of IPv4 responses or

   IPv4-only hosts, and dual-stack hosts that do not recognize the new
   flag, will continue to attempt IPv4 operations, in particular IPv4
   discovery protocols typically sent as link-layer broadcasts.  This
   legacy traffic cannot be prevented by any IPv6 mechanism.  The value
   of the new flag is limited to dual-stack hosts that recognize it.

   Additionally, dual-stack hosts that support any form of link-local
   service may choose to support such a service over IPv4 regardless of
   the new mechanism.  Similarly, it is possible that a network could be
   configured with both IPv6-only routers and IPv4-only routers.  For
   that reason, the new mechanism is advisory in nature.  Host behaviors
   are discussed in more detail in Section 4.

   This document specifies a new flag for IPv6 Neighbor Discovery
   [RFC4861] Router Advertisement Flag [RFC5175].  It updates [RFC5175]
   to add this flag.

2.  IPv4 Unavailable Flag

   RFC5175 currently defines the flags in the NDP Router Advertisement
   message and these flags are registered in the IANA IPv6 ND Router
   Advertisement flags Registry [IANA-RF].  This currently contains the
   following one-bit flags defined in published RFCs:

       0 1 2 3 4 5 6 7

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      M    Managed Address Configuration Flag [RFC4861]
      O    Other Configuration Flag [RFC4861]
      H    Mobile IPv6 Home Agent Flag [RFC3775]
      Prf  Router Selection Preferences [RFC4191]
      P    Neighbor Discovery Proxy Flag [RFC4389]
      R    Reserved

   This document defines bit 6 to be the IPv4 Unavailable Flag:

      4    IPv4 Unavailable Flag [RFC4861]

   This flag has two values.  These are:

      0    IPv4 is Available on this default Router
      1    IPv4 is Not Available on this default Router

   RFC 5175 requires that unused flag bits be set to zero.  Therefore, a
   router that does not support the new flag will not appear to assert
   that IPv4 is unsupported.

   Hosts receiving the Router Advertisement should only process this
   flag if the advertising router is a Default Router.  Specifically, if
   the Lifetime field in the Router Advertisement is not zero, otherwise
   it should be ignored.  This is done to allow some IPv6 routers to
   advertise information without being a Default Router and providing
   IPv6 connectivity.

3.  Router and Operational Considerations

   Default IPv6 routers that do not support IPv4 should be configured to
   set the IPv4 Unavailable flag to 1, unless the operator is aware that
   IPv4 support is available from another router.  Default IPv6 routers
   that also support IPv4 must set the IPv4 Unavailable flag to 0.

   Operators of large IPv6-only wireless networks are advised to use
   Layer 2 techniques to drop IPv4 and DHCPv4 packets (Ethertypes 0x0800
   and 0x806) at all switches, and to ensure that IPv4 and DHCPv4 Layer
   3 features are disabled in all switches.

4.  Host Behavior Considerations

   As noted above, the IPv4 Unavailable flag is advisory.  Hosts may
   vary in their treatment of it.

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   A host may choose to delay all IPv4 operations at start-up until a
   reasonable time has elapsed for RA messages to arrive.

   If there are multiple IPv6 default routers on a network, they might
   send different values of the flag.  If at least one IPv6 default
   router sends the flag with value 0, a dual stack host should assume
   that IPv4 is available.  If all IPv6 default routers send the flag
   with value 1, a dual stack host may assume that IPv4 is not

   A host that receives only RAs with the flag set to 1 may choose not
   to attempt any IPv4 operations, unless it subsequently receives at
   least one RA with the flag set to zero.  As soon as such an RA is
   received, IPv4 operations should be started.

   Alternatively, a host that receives only RAs with the flag set to 1
   may choose to attempt IPv4 operations but at significantly lower
   frequency than normal.  For example, it may choose to lengthen the
   interval between DHCPv4 discovery messages to much longer than the 64
   seconds defined by [RFC2131].

   A host that receives only RAs with the flag set to 1 may choose not
   to form an IPv4 link-local address.  However, as noted above, if it
   contains link-local applications that can use IPv4, it may instead
   choose to form an IPv4 link-local address in the normal way
   [RFC3927], and then send the discovery traffic for such applications.

   In all of the above, the flag's value is considered valid for the
   lifetime of the default router concerned, unless a subsequent RA
   delivers a different flag value.  If a default router expires (i.e.,
   no RA is received that refreshes its lifetime), the host must remove
   this router's flag value from consideration.  If the result is that
   all surviving default routers have the flag set to 1, the host may
   now assume that IPv4 is not available.  In other words, at any given
   time, the state of the flag as seen by the host is the logical AND of
   the flags sent by all unexpired default IPv6 routers.

5.  IANA Considerations

   IANA is requested to assign the new Router Advertisement flag defined
   in Section 2 of this document.  Bit 6 is the next available bit in
   this registry, IANA is requested to use this bit unless there is a
   reason to use another bit in this registry.

   IANA should also register this new flag bit in the IANA IPv6 ND
   Router Advertisement flags Registry [IANA-RF].

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6.  Security Considerations

   This document shares the security issues with other parts of IPv6
   Neighbor Discovery.  General techniques to protect Router
   Advertisement traffic such as Router Guard [RFC6105] are useful in
   protecting these vulnerabilities.

   A bad actor could use this mechanism to attempt turn off IPv4 service
   on a network that is using IPv4, by sending Router Advertisements
   with the IPv4 Unavailable Flag set to 1.  In that case, as long as
   there are routers sending Router Advertisements with this Flag set to
   0, they would override this attack given the mechanism in Section 2.
   Specifically a host would only turn off IPv4 service if it wasn't
   hearing any Router Advertisement with the Flag set to 0.  If the
   advice in Section 3 is followed, this attack will fail.

   Conversely, a bad actor could use this mechanism to turn on, or
   pretend to turn on, IPv4 service on an IPv6-only network, by sending
   Router Advertisements with the Flag set to 0.  However, this is
   really no different than what such a bad actor can do anyway, if they
   have the ability to configure a bogus router in the first place.  The
   advice in Section 3 will minimize such an attack by limiting it to a
   single network segment.

   Note that manipulating the Router Preference [RFC4191] will not
   affect either of these attacks: any IPv4 Unavailable Flag of 0 will
   always override all Flags set to 1.

   The new flag is neutral from an IPv6 privacy viewpoint, since it does
   not affect IPv6 operations in any way.  From an IPv4 privacy
   viewpoint, it has the potential benefit of suppressing unnecessary
   traffic that might reveal the existence of a host and the correlation
   between its hardware and IPv4 addresses.

7.  Acknowledgments

   A closely related proposal was published earlier as

   Helpful comments were received from Lorenzo Colitti, David Farmer,
   Fernando Gont, Erik Kline, Jen Linkova, Michael Richardson, James
   Woodyatt, and other members of the 6MAN working group.

8.  Change log [RFC Editor: Please remove]

   draft-hinden-ipv4flag-01, 2018-Feb-15:

   Improved text in introduction.

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   Added reference to current IANA registry in Section 2.

   Editorial changes.

   draft-hinden-ipv4flag-01, 2017-Dec-12

   Inverted name of flag from "Available" to "Unavailable".

   Added problem description and clarified scope.

   Added router and operational considerations.

   Added host behavior considerations.

   Extended security considerations.

   Added Acknowledgment section, including reference to prior sunset4

   draft-hinden-ipv4flag-00, 2017-Nov-17:

   Original version.

9.  References

9.1.  Normative References

   [IANA-RF]  "IPv6 ND Router Advertisement flags",

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol", RFC
              2131, DOI 10.17487/RFC2131, March 1997, <https://www.rfc-

   [RFC3927]  Cheshire, S., Aboba, B., and E. Guttman, "Dynamic
              Configuration of IPv4 Link-Local Addresses", RFC 3927, DOI
              10.17487/RFC3927, May 2005, <https://www.rfc-

   [RFC4191]  Draves, R. and D. Thaler, "Default Router Preferences and
              More-Specific Routes", RFC 4191, DOI 10.17487/RFC4191,
              November 2005, <>.

   [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-

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   [RFC5175]  Haberman, B., Ed. and R. Hinden, "IPv6 Router
              Advertisement Flags Option", RFC 5175, DOI 10.17487/
              RFC5175, March 2008, <

9.2.  Informative References

              Perreault, S., George, W., Tsou, T., Yang, T., and J.
              Tremblay, "Turning off IPv4 Using DHCPv6 or Router
              Advertisements", draft-ietf-sunset4-noipv4-01 (work in
              progress), December 2014.

   [RFC6105]  Levy-Abegnoli, E., Van de Velde, G., Popoviciu, C., and J.
              Mohacsi, "IPv6 Router Advertisement Guard", RFC 6105, DOI
              10.17487/RFC6105, February 2011, <https://www.rfc-

   [RFC7772]  Yourtchenko, A. and L. Colitti, "Reducing Energy
              Consumption of Router Advertisements", BCP 202, RFC 7772,
              DOI 10.17487/RFC7772, February 2016, <https://www.rfc-

Authors' Addresses

   Robert M. Hinden
   Check Point Software
   959 Skyway Road
   San Carlos, CA  94070


   Brian Carpenter
   Department of Computer Science
   University of Auckland
   PB 92019
   Auckland  1142
   New Zealand


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