Internet Engineering Task Force (IETF) F. Gont
Request for Comments: 8064 SI6 Networks / UTN-FRH
Updates: 2464, 2467, 2470, 2491, 2492, A. Cooper
2497, 2590, 3146, 3572, 4291, Cisco
4338, 4391, 5072, 5121 D. Thaler
Category: Standards Track Microsoft
ISSN: 2070-1721 W. Liu
February 2017 Recommendation on Stable IPv6 Interface Identifiers
This document changes the recommended default Interface Identifier
(IID) generation scheme for cases where Stateless Address
Autoconfiguration (SLAAC) is used to generate a stable IPv6 address.
It recommends using the mechanism specified in RFC 7217 in such
cases, and recommends against embedding stable link-layer addresses
in IPv6 IIDs. It formally updates RFC 2464, RFC 2467, RFC 2470, RFC
2491, RFC 2492, RFC 2497, RFC 2590, RFC 3146, RFC 3572, RFC 4291, RFC
4338, RFC 4391, RFC 5072, and RFC 5121. This document does not
change any existing recommendations concerning the use of temporary
addresses as specified in RFC 4941.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
[RFC4862] specifies Stateless Address Autoconfiguration (SLAAC) for
IPv6 [RFC2460], which typically results in hosts configuring one or
more "stable" addresses composed of a network prefix advertised by a
local router, and an Interface Identifier (IID) [RFC4291] that
typically embeds a stable link-layer address (e.g., an IEEE LAN MAC
In some network technologies and adaptation layers, the use of an IID
based on a link-layer address may offer some advantages. For
example, [RFC6282] allows for the compression of IPv6 datagrams over
IEEE 802.15.4-based networks [RFC4944] when the IID is based on the
underlying link-layer address.
The security and privacy implications of embedding a stable link-
layer address in an IPv6 IID have been known for some time now and
are discussed in great detail in [RFC7721]. They include:
o Network-activity correlation
o Location tracking
o Address scanning
o Device-specific vulnerability exploitation
More generally, the reuse of identifiers that have their own
semantics or properties across different contexts or scopes can be
detrimental for security and privacy [NUM-IDS]. In the case of
traditional stable IPv6 IIDs, some of the security and privacy
implications are dependent on the properties of the underlying link-
layer addresses (e.g., whether the link-layer address is ephemeral or
randomly generated), while other implications (e.g., reduction of the
entropy of the IID) depend on the algorithm for generating the IID
itself. In standardized recommendations for stable IPv6 IID
generation meant to achieve particular security and privacy
properties, it is necessary to recommend against embedding stable
link-layer addresses in IPv6 IIDs.
Furthermore, some popular IPv6 implementations have already deviated
from the traditional stable IID generation scheme to mitigate the
aforementioned security and privacy implications [Microsoft].
As a result of the aforementioned issues, this document changes the
recommended default IID generation scheme for generating stable IPv6
addresses with SLAAC to that specified in [RFC7217] and recommends
against embedding stable link-layer addresses in IPv6 Interface
Identifiers, such that the aforementioned issues are mitigated. That
is, this document simply replaces the default algorithm that is
recommended to be employed when generating stable IPv6 IIDs.
[RFC4291] defines the "Modified EUI-64 format" for IIDs.
Appendix A of [RFC4291] then describes how to transform an IEEE
EUI-64 identifier, or an IEEE 802 48-bit MAC address from which an
EUI-64 identifier is derived, into an IID in the Modified EUI-64
In a variety of scenarios, addresses that remain stable for the
lifetime of a host's connection to a single subnet are viewed as
desirable. For example, stable addresses may be viewed as beneficial
for network management, event logging, enforcement of access control,
provision of quality of service, or for server or router interfaces.
Similarly, stable addresses (as opposed to temporary addresses
[RFC4941]) allow for long-lived TCP connections and are also usually
desirable when performing server-like functions (i.e., receiving
The recommendations in this document apply only in cases where
implementations otherwise would have configured a stable IPv6 IID
containing a link-layer address. For example, this document does not
change any existing recommendations concerning the use of temporary
addresses as specified in [RFC4941] and the recommendations do not
apply to cases where SLAAC is employed to generate non-stable IPv6
addresses (e.g., by embedding a link-layer address that is
periodically randomized); in addition, this document does not
introduce any new requirements regarding when stable addresses are to
be configured. Thus, the recommendations in this document simply
improve the security and privacy properties of stable addresses.
An address that does not vary over time within the same network
(as defined in [RFC7721]).
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
3. Generation of IPv6 Interface Identifiers with SLAAC
Nodes SHOULD implement and employ [RFC7217] as the default scheme for
generating stable IPv6 addresses with SLAAC. A link layer MAY also
define a mechanism for stable IPv6 address generation that is more
efficient and does not address the security and privacy
considerations discussed in Section 1. The choice of whether or not
to enable the security- and privacy-preserving mechanism SHOULD be
configurable in such a case.
By default, nodes SHOULD NOT employ IPv6 address generation schemes
that embed a stable link-layer address in the IID. In particular,
this document RECOMMENDS that nodes do not generate stable IIDs with
the schemes specified in [RFC2464], [RFC2467], [RFC2470], [RFC2491],
[RFC2492], [RFC2497], [RFC2590], [RFC3146], [RFC3572], [RFC4338],
[RFC4391], [RFC5072], and [RFC5121].
4. Future Work
At the time of this writing, the mechanisms specified in the
following documents might require updates to be fully compatible with
the recommendations in this document:
o "Compression Format for IPv6 Datagrams over IEEE 802.15.4-Based
o "Transmission of IPv6 Packets over IEEE 802.15.4 Networks"
o "Neighbor Discovery Optimization for IPv6 over Low-Power Wireless
Personal Area Networks (6LoWPANs)" [RFC6775]
o "Transmission of IPv6 Packets over ITU-T G.9959 Networks"
Future revisions or updates of these documents should consider the
issues of privacy and security mentioned in Section 1 and explain any
design and engineering considerations that lead to the use of stable
IIDs based on a node's link-layer address.
5. Security Considerations
This document recommends against the (default) use of predictable
Interface Identifiers in IPv6 addresses. It recommends [RFC7217] as
the default scheme for generating IPv6 stable addresses with SLAAC,
such that the security and privacy issues of IIDs that embed stable
link-layer addresses are mitigated.
[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,
[RFC7428] Brandt, A. and J. Buron, "Transmission of IPv6 Packets
over ITU-T G.9959 Networks", RFC 7428,
DOI 10.17487/RFC7428, February 2015,
6.2. Informative References
[Microsoft] Davies, J., "Understanding IPv6, 3rd. ed",
page 83, Microsoft Press, 2012,
[NUM-IDS] Gont, F. and I. Arce, "Security and Privacy Implications
of Numeric Identifiers Employed in Network Protocols",
Work in Progress, February 2016.
[RFC3572] Ogura, T., Maruyama, M., and T. Yoshida, "Internet
Protocol Version 6 over MAPOS (Multiple Access Protocol
Over SONET/SDH)", RFC 3572, DOI 10.17487/RFC3572, July
[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,
The authors would like to thank (in alphabetical order) Bob Hinden,
Ray Hunter, and Erik Nordmark, for providing a detailed review of
The authors would like to thank (in alphabetical order) Fred Baker,
Carsten Bormann, Scott Brim, Brian Carpenter, Samita Chakrabarti, Tim
Chown, Lorenzo Colitti, Jean-Michel Combes, Greg Daley, Esko Dijk,
Ralph Droms, David Farmer, Brian Haberman, Ulrich Herberg, Philip
Homburg, Jahangir Hossain, Jonathan Hui, Christian Huitema, Ray
Hunter, Erik Kline, Sheng Jiang, Roger Jorgensen, Dan Luedtke, Kerry
Lynn, George Mitchel, Gabriel Montenegro, Erik Nordmark, Simon
Perreault, Tom Petch, Alexandru Petrescu, Michael Richardson, Arturo
Servin, Mark Smith, Tom Taylor, Ole Troan, Tina Tsou, Glen Turner,
Randy Turner, James Woodyatt, and Juan Carlos Zuniga, for providing
valuable comments on earlier draft versions of this document.
SI6 Networks / UTN-FRH
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Will (Shucheng) Liu
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