6MAN B. Carpenter
Internet-Draft Univ. of Auckland
Intended status: Informational S. Jiang
Expires: August 4, 2013 Huawei Technologies Co., Ltd
January 31, 2013
The U and G bits in IPv6 Interface Identifiers
draft-carpenter-6man-ug-00
Abstract
The IPv6 addressing architecture defines a method by which the
Universal and Group bits of an IEEE link-layer address are mapped
into an IPv6 unicast interface identifier. This document clarifies
the status of those bits for interface identifiers that are not
derived from an IEEE link-layer address.
Status of this Memo
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This Internet-Draft will expire on August 4, 2013.
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Problem statement . . . . . . . . . . . . . . . . . . . . . . . 3
3. Proposed solution . . . . . . . . . . . . . . . . . . . . . . . 5
4. Security Considerations . . . . . . . . . . . . . . . . . . . . 5
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 5
7. Change log [RFC Editor: Please remove] . . . . . . . . . . . . 6
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6
8.1. Normative References . . . . . . . . . . . . . . . . . . . 6
8.2. Informative References . . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 7
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1. Introduction
NOTE IN DRAFT: This version is in the form of a discussion document,
but the proposal included uses normative language. If the WG wishes
to proceed with it, the authors suggest to make it a formal
standards-track update of the IPv6 addressing architecture.
According to the IPv6 addressing architecture [RFC4291], when an IPv6
unicast Interface Identifier (IID) is formed on the basis of an IEEE
EUI-64 address, usually itself expanded from a 48-bit MAC address, a
particular format must be used:
"For all unicast addresses, except those that start with the binary
value 000, Interface IDs are required to be 64 bits long and to be
constructed in Modified EUI-64 format."
The specification assumes that that the normal case is to transform
an Ethernet-style address into an IID, preserving the semantics of
two bits in particular:
o The "u" bit in an IEEE address is set to 0 to indicate universal
scope (implying uniqueness) or to 1 to indicate local scope
(without implying uniqueness). In an IID this bit is inverted,
i.e., 1 for universal scope and 0 for local scope. According to
[RFC5342], the reason for this was "to make it easier for network
operators to type in local-scope identifiers".
o The "g" bit in an IEEE address is set to 1 to indicate group
addressing (link-layer multicast). This value is supposed to be
preserved in an IID.
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 [RFC2119].
2. Problem statement
Various new forms of IID have been defined or proposed, such as
temporary addresses [RFC4941], Cryptographically Generated Addresses
(CGAs) [RFC3972], stable privacy addresses
[I-D.ietf-6man-stable-privacy-addresses], or mapped addresses
[I-D.ietf-softwire-4rd]. In each case, the question of how to set
and interpret the "u" and "g" bits has been debated. For example,
RFC 3972 specifies that they are zero in CGAs.
NOTE IN DRAFT: Are there other examples we should include? Are we
sure that no IID format defines semantics for u/g?
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The question underlying these repeated debates is: do these bits have
any usefulness as currently defined? Section 2.2.1 of RFC 5342
discusses the mechanics of the bit allocations but does not explain
the purpose or value of these bits in an IID. There is an IANA
registry for reserved IID values [RFC5453] but again there is no
explanation of the purpose of the "u" and "g" bits.
Another case where the "u" and "g" bits are specified is in the
Reserved IPv6 Subnet Anycast Address format [RFC2526], which states
that "for interface identifiers in EUI-64 format, the universal/local
bit in the interface identifier MUST be set to 0" (i.e., local) and
requires that "g" bit to be set to 1. However, the text neither
states nor implies any semantics for these bits in anycast addresses.
There was a presumption when IPv6 was designed and the IID format was
first specified that a universally unique IID might prove to be very
useful, for example to contribute to solving the multihoming problem.
Indeed, the addressing architecture [RFC4291] states this explicitly:
"The use of the universal/local bit in the Modified EUI-64 format
identifier is to allow development of future technology that can take
advantage of interface identifiers with universal scope."
However, this has not so far proved to be the case. Also, there is
evidence from the field that IEEE MAC addresses with "u" = 0 are
sometime incorrectly assigned to multiple MAC interfaces. Once
transformed into IID format (with "u" = 1) these identifiers would
purport to be universally unique but would in fact be ambiguous.
Also, ILNP, the currently specified multihoming solution that might
be expected to benefit from universally unique IIDs in modified
EUI-64 format does not in fact rely on them; it uses its own format,
defined as a Node Identifier [RFC6741]. We can conclude that the "u"
bit in IIDs has no semantic value. In the case of an IID created
from a MAC address according to RFC 4941, its value is determined by
the MAC address, but that is all.
The "g" bit in an IID has no meaning in IPv6. If an IID is for some
reason created from a MAC group address, the bit will be set, but
that is all. Both the "u" and the "g" bit are meaningless in the
format of an IPv6 multicast group ID [RFC3306], [RFC3307].
The problem caused by the above is the confusion and distraction
caused each time that a new form of IID is proposed. Since the bits
concerned appear to have no useful semantics, this is wasteful.
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3. Proposed solution
It should be noted that IIDs known or guessed to have been created
according to RFC 4941 could be transformed back into MAC addresses,
for example during fault diagnosis. For that reason, keeping the "u"
and "g" bits in the IID has operational value. Therefore, the EUI-64
to IPv6 IID transformation defined in RFC 4941 MUST be used for all
cases where an IID is derived from a MAC address.
However, for all forms of IID that are not derived from an EUI-64 MAC
address (or an equivalent form of link-layer address), it is not
required to set the "u" and "g" bits in any particular way. These
bits have no semantics in an IID. Specifications of other forms of
IID MUST specify how they should be set, without defining any
semantics for them.
The statement about future technology quoted above from RFC 4941 is
obsolete.
As far as is known, no existing implementation will be affected by
these changes. The benefit is that future design discussions are
simplified.
4. Security Considerations
No new security exposures or issues are raised by this document.
5. IANA Considerations
This document requests no immediate action by IANA. However, in
considering future proposed additions to the registry of reserved IID
values [RFC5453], no special consideration is needed of the "u" and
"g" bits, since they have no special meaning.
6. Acknowledgements
Valuable comments were received from ... and other participants in
the 6MAN working group.
Brian Carpenter was a visitor at the Computer Laboratory, Cambridge
University during part of this work.
This document was produced using the xml2rfc tool [RFC2629].
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7. Change log [RFC Editor: Please remove]
draft-carpenter-6man-ug-00: original version, 2013-01-31.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
[RFC5342] Eastlake, D., "IANA Considerations and IETF Protocol Usage
for IEEE 802 Parameters", BCP 141, RFC 5342,
September 2008.
[RFC5453] Krishnan, S., "Reserved IPv6 Interface Identifiers",
RFC 5453, February 2009.
8.2. Informative References
[I-D.ietf-6man-stable-privacy-addresses]
Gont, F., "A method for Generating Stable Privacy-Enhanced
Addresses with IPv6 Stateless Address Autoconfiguration
(SLAAC)", draft-ietf-6man-stable-privacy-addresses-03
(work in progress), January 2013.
[I-D.ietf-softwire-4rd]
Jiang, S., Despres, R., Penno, R., Lee, Y., Chen, G., and
M. Chen, "IPv4 Residual Deployment via IPv6 - a Stateless
Solution (4rd)", draft-ietf-softwire-4rd-04 (work in
progress), October 2012.
[RFC2526] Johnson, D. and S. Deering, "Reserved IPv6 Subnet Anycast
Addresses", RFC 2526, March 1999.
[RFC2629] Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629,
June 1999.
[RFC3306] Haberman, B. and D. Thaler, "Unicast-Prefix-based IPv6
Multicast Addresses", RFC 3306, August 2002.
[RFC3307] Haberman, B., "Allocation Guidelines for IPv6 Multicast
Addresses", RFC 3307, August 2002.
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[RFC3972] Aura, T., "Cryptographically Generated Addresses (CGA)",
RFC 3972, March 2005.
[RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy
Extensions for Stateless Address Autoconfiguration in
IPv6", RFC 4941, September 2007.
[RFC6741] Atkinson,, RJ., "Identifier-Locator Network Protocol
(ILNP) Engineering Considerations", RFC 6741,
November 2012.
Authors' Addresses
Brian Carpenter
Department of Computer Science
University of Auckland
PB 92019
Auckland, 1142
New Zealand
Email: brian.e.carpenter@gmail.com
Sheng Jiang
Huawei Technologies Co., Ltd
Q14, Huawei Campus
No.156 Beiqing Road
Hai-Dian District, Beijing 100095
P.R. China
Email: jiangsheng@huawei.com
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