Internet DRAFT - draft-hinden-ipv4flag
draft-hinden-ipv4flag
Network Working Group R. Hinden
Internet-Draft Check Point Software
Updates: 5175 (if approved) B. Carpenter
Intended status: Standards Track Univ. of Auckland
Expires: October 18, 2018 April 16, 2018
IPv6 Router Advertisement IPv6-Only Flag
draft-hinden-ipv4flag-04
Abstract
This document specifies a Router Advertisement Flag to indicate to
hosts that the administrator has configured the router to advertise
that the link is IPv6-Only. This document updates RFC5175.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on October 18, 2018.
Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3
3. Applicability Statements . . . . . . . . . . . . . . . . . . 3
4. IPv6-Only Flag . . . . . . . . . . . . . . . . . . . . . . . 4
5. Router and Operational Considerations . . . . . . . . . . . . 5
6. Host Behavior Considerations . . . . . . . . . . . . . . . . 5
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
8. Security Considerations . . . . . . . . . . . . . . . . . . . 6
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 7
10. Change log [RFC Editor: Please remove] . . . . . . . . . . . 7
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
11.1. Normative References . . . . . . . . . . . . . . . . . . 8
11.2. Informative References . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
This document specifies a Router Advertisement Flag to indicate to
hosts that the administrator has configured the router to advertise
that the link is IPv6-Only. The flag does not apply to non-default
IPv6 routers.
Hosts that support IPv4 and IPv6, usually called dual stack hosts,
need to also work efficiently on IPv6 only links. 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 link, 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.
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o On an IPv6-only link, IPv4 might be used for malicious purposes
and pass unnoticed by IPv6-only monitoring mechanisms.
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 that a router administrator can use
to inform hosts that this is an IPv6-Only link on their default
routers such that they can disable IPv4 on this link, mitigating all
of the above problems.
Because there is no IPv4 support on IPv6-only routers, the only way
to notify the dual stack hosts that this link is IPv6-Only 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
traffic.
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 hosts that recognize it.
This document specifies a new flag for Router Advertisement Flag
[RFC5175]. It updates [RFC5175] to add this flag.
2. 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.
3. Applicability Statements
The mechanism is designed to allow administrators to notify hosts
that the link is IPv6-Only. It SHOULD be only used in IPv6-Only
links.
Dual stack hosts that have a good reason to use IPv4, for example
IPv4 link-local services, can continue to do so. This is consistent
with the SHOULD language in this document.
Administrators SHOULD only use this mechanism if they are certain
that the link is IPv6-Only. For example, in cases where there is a
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need to continue to use IPv4 or there are IPv4 only routers, setting
this flag to 1 is a configuration error.
4. IPv6-Only 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
+-+-+-+-+-+-+-+-+
|M|O|H|Prf|P|R|R|
+-+-+-+-+-+-+-+-+
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 IPv6-Only Flag:
6 IPv6-Only Flag
This flag has two values. These are:
0 This is not an IPv6-Only link
1 This is an IPv6-Only link
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 this is an IPv6-Only link.
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.
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5. Router and Operational Considerations
Default IPv6 routers that are on an IPv6-Only link SHOULD be
configured to set the IPv6-Only flag to 1. In all other cases the
flag SHOULD not be set to 1.
The intent is that the administrator of the router configures the
router to set the IPv6-Only flag if she/he wants to tell the hosts on
the link that the link is IPv6-Only. This is a configuration flag,
not something that the router decides on it's own.
Operators of large IPv6-only wireless links are advised to also 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.
6. Host Behavior Considerations
If there are multiple IPv6 default routers on a link, 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 not assume that
the link is IPv6-Only. If all IPv6 default routers send the flag
with value 1, a dual stack host SHOULD assume that this is an
IPv6-Only link.
A host that receives only RAs with the flag set to 1 SHOULD not
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.
A host MAY choose to delay all IPv4 operations at start-up until a
reasonable time has elapsed for RA messages to arrive. If all RAs
received have the flag set, a host SHOULD also choose to not attempt
IPv4 operations until an application asks it to, specifically delay
performing DHCPV4 until it gets a request from an application to use
IPv4. This would avoid attempting to obtain IPv4 addresses if there
are no applications trying to use IPv4.
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 SHOULD
assume that the link is IPv6-Only. 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.
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7. IANA Considerations
IANA is requested to assign the new Router Advertisement flag defined
in Section 4 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 is also requested to register this new flag bit in the IANA IPv6
ND Router Advertisement flags Registry [IANA-RF].
8. 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 link that is using IPv4, by sending Router Advertisements with
the IPv6-Only 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 4.
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 5 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 link, 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 5 will minimize such an attack by limiting it to a
single link.
Note that manipulating the Router Preference [RFC4191] will not
affect either of these attacks: any IPv6-Only 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.
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9. Acknowledgments
A closely related proposal was published earlier as
[I-D.ietf-sunset4-noipv4].
Helpful comments were received from Lorenzo Colitti, David Farmer,
Fernando Gont, Erik Kline, Jen Linkova, Veronika McKillop, Michael
Richardson, Mark Smith, Barbara Stark, Ole Troan, James Woodyatt, and
other members of the 6MAN working group.
Bjoern Zeeb has also produced a variant of this proposal and proposed
an IPv6 transition plan in [I-D.bz-v4goawayflag].
10. Change log [RFC Editor: Please remove]
draft-hinden-ipv4flag-04, 2018-April-16:
Changed the name of the document and flag to be the IPv6-Only flag.
Rewrote text to make it affirmative that this is used by an
administrator to tell the hosts that the link is IPv6-Only.
Added an Applicability Statements section to scope the intend use.
Changed requirement language to upper case, added Requirements
Language section with references to [RFC2119] and [RFC8174].
Editorial changes.
draft-hinden-ipv4flag-03, 2018-Feb-15:
Changed terminology to use "link" instead of "network".
Improved text in Section 4. "Host Behavior Considerations" and added
suggestion to only perform IPv4 if an application requests it.
Added clarification that the bit is set because an administrator
configured the router to send it.
Editorial changes.
draft-hinden-ipv4flag-02, 2018-Feb-15:
Improved text in introduction.
Added reference to current IANA registry in Section 2.
Editorial changes.
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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.
draft-hinden-ipv4flag-00, 2017-Nov-17:
Original version.
11. References
11.1. Normative References
[IANA-RF] "IPv6 ND Router Advertisement flags",
<https://www.iana.org/assignments/icmpv6-parameters/
icmpv6-parameters.xhtml#icmpv6-parameters-11>.
[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>.
[RFC4191] Draves, R. and D. Thaler, "Default Router Preferences and
More-Specific Routes", RFC 4191, DOI 10.17487/RFC4191,
November 2005, <https://www.rfc-editor.org/info/rfc4191>.
[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>.
[RFC5175] Haberman, B., Ed. and R. Hinden, "IPv6 Router
Advertisement Flags Option", RFC 5175, DOI 10.17487/
RFC5175, March 2008, <https://www.rfc-editor.org/info/
rfc5175>.
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[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>.
11.2. Informative References
[I-D.bz-v4goawayflag]
Zeeb, B., "IPv6 Router Advertisement IPv4 GoAway Flag",
draft-bz-v4goawayflag-00 (work in progress), March 2018.
[I-D.ietf-sunset4-noipv4]
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-
editor.org/info/rfc6105>.
[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-
editor.org/info/rfc7772>.
Authors' Addresses
Robert M. Hinden
Check Point Software
959 Skyway Road
San Carlos, CA 94070
USA
Email: bob.hinden@gmail.com
Brian Carpenter
Department of Computer Science
University of Auckland
PB 92019
Auckland 1142
New Zealand
Email: brian.e.carpenter@gmail.com
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