wdiff draft-carpenter-6man-ug-00.txt draft-carpenter-6man-ug-01.txt

6MAN B. Carpenter
Internet-Draft Univ. of Auckland
Intended status: Informational
Updates: 4291 (if approved) S. Jiang
Expires: August 4, 2013
Intended status: Standards Track Huawei Technologies Co., Ltd
January 31,
Expires: August 25, 2013 February 21, 2013

The U and G bits in IPv6 Interface Identifiers
draft-carpenter-6man-ug-00
draft-carpenter-6man-ug-01

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. address, and updates RFC 4291
accordingly.

Status of this Memo

This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on August 4, 25, 2013.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Problem statement . . . . . . . . . . . . . . . . . . . . . . . 3
3. Proposed solution Usefulness of the U and G Bits . . . . . . . . . . . . . . . . 5
4. Clarification of Specifications . . . . . . . . . 5
4. . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 5
5. 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
6. 7
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 5
7. 7
8. Change log [RFC Editor: Please remove] . . . . . . . . . . . . 6
8. 7
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6
8.1. 7
9.1. Normative References . . . . . . . . . . . . . . . . . . . 6
8.2. 7
9.2. Informative References . . . . . . . . . . . . . . . . . . 6 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 7 9

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 a 64-
bit 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 information
provided by 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 The value of this bit is supposed to be
preserved in an IID.

This document discusses problems observed with the "u" and "g" bits
as a result of the above requirements. It then discusses the
usefulness of the two bits, and updates RFC 4291 accordingly.

1.1. Terminology

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

In addition to IIDs formed from IEEE EUI-64 addresses, various new
forms of IID have been defined or proposed, such as temporary
addresses [RFC4941], Cryptographically Generated Addresses (CGAs)
[RFC3972], Hash-Based Addresses (HBAs) [RFC5535], stable privacy
addresses [I-D.ietf-6man-stable-privacy-addresses], or mapped
addresses for 4rd [I-D.ietf-softwire-4rd]. In each case, the
question of how to set
and interpret the "u" and "g" bits has been debated. to be decided. For
example, RFC 3972 specifies that they are both 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?
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 CGAs, and the purpose of
same applies to HBAs. On the other hand, RFC 4941 specifies that "u" and
must be zero but leaves "g" bits. variable.

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.

These cases illustrate that the statement quoted above from RFC 4291
requiring "Modified EUI-64 format" is rather meaningless when applied
to forms of IID that are not in fact based on an underlying EUI-64
address. In practice, the IETF has chosen to assign some 64-bit IIDs
that have nothing to do with EUI-64.

A particular case is that of /127 prefixes for point-to-point links
between routers, as standardised by [RFC6164]. The addresses on
these links are undoubtedly global unicast addresses, but they do not
have a 64-bit IID. The bits in the positions named "u" and "g" in
such an IID have no special significance and their values are not
specified.

Each time a new IID format is proposed, the question arises whether
these bits have any meaning. Section 2.2.1 of RFC 5342 discusses the
mechanics of the bit allocations but does not explain the purpose or
usefulness 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.

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. Firstly,
there are recurrent reports of manufacturers assigning the same MAC
address to multiple devices. Secondly, significant re-use of the
same virtual MAC address is reported in virtual machine environments.
Once transformed into IID format (with "u" = 1) these identifiers
would purport to be universally unique but would in fact be
ambiguous.
Also, ILNP, This has no known harmful effect as long as the currently specified
replicated MAC addresses and IIDs are used on different layer 2
links. If they are used on the same link, of course there will be a
problem, to be detected by duplicate address detection [RFC4862], but
such a problem can usually only be resolved by human intervention.

The conclusion from this is that the "u" bit is not a reliable
indicator of universal uniqueness.

We note that Identifier-Locator Network Protocol (ILNP), a
multihoming solution that might be expected to benefit from
universally unique IIDs in modified EUI-64 format format, does not in fact
rely on them; it them. ILNP uses its own format, defined as a Node Identifier
[RFC6741]. We ILNP does have the constraint that Node Identifiers must
be unique within a given site, but as we have just shown, the state
of the "u" bit does not in any way guarantee this.

Thus, we can conclude that the value of the "u" bit in IIDs has no semantic value.
particular meaning. In the case of an IID created from a MAC address
according to RFC 4941, 4291, 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

An IPv6 IID is for some
reason should not be created from a MAC group address, so the
"g" bit will normally be set, zero, but
that is all. Both this value also has no particular
meaning. Additionally, the "u" and the "g" bit bits are both meaningless
in the format of an IPv6 multicast group ID [RFC3306], [RFC3307].

The

None of the above implies that there is a problem caused with using the "u"
and "g" bits in MAC addresses as part of the process of generating
IIDs from MAC addresses, or with specifying their values in other
methods of generating IIDs. What it does imply is that, after an IID
is generated by any method, no reliable deductions can be made from
the above state of the "u" and "g" bits; in other words, these bits have no
useful semantics in an IID.

Once this is recognised, we can avoid the problematic confusion and distraction
caused by these bits each time that a new form of IID is proposed. Since

3. Usefulness of the U and G Bits

Given that the "u" and "g" bits
concerned appear to do not have no useful semantics, this a reliable meaning in an
IID, it is relevant to consider what usefulness they do have.

If an IID is wasteful.

3. Proposed solution

It should be noted that IIDs known or guessed to have been created according to RFC 4941
4291, it could be transformed back into a MAC addresses,
for example address. This can be
very helpful during operational fault diagnosis. For that reason, keeping
mapping the IEEE "u" and "g" bits in into the IID has operational value. Therefore,
usefulness. However, it should be stressed that "u" = "g" = 0 does
not prove that an IID was formed from a MAC address; on the EUI-64 contrary,
it might equally result from another method. With other methods,
there is no reverse transformation available.

To the extent that each method of IID creation specifies the values
of the "u" and "g" bits, and that each new method is carefully
designed in the light of its predecessors, these bits tend to reduce
the chances of duplicate IIDs.

4. Clarification of Specifications

This section describes clarifications to the IPv6 specifications that
result from the above discussion. Their aim is to reduce confusion
while retaining the useful aspects of the "u" and "g" bits in IIDs.

The EUI-64 to IID transformation defined in RFC 4941 the IPv6 addressing
architecture [RFC4291] MUST be used for all cases where an IPv6 IID
is derived from a an IEEE MAC or EUI-64 address.

However, for all forms With any other form
of IID that are not derived from an EUI-64 MAC
address (or link layer address, an equivalent form of link-layer address), it is not
required to set transformation SHOULD be used.
However, the resulting "u" and "g" bits in any particular way. These
bits an IID have no semantics semantics;
in an IID. other words, they have opaque values. In fact, the whole IID
should be viewed as opaque by third parties.

Specifications of other forms of 64-bit IID MUST will either specify
explicitly how they should be the "u" and "g" bits are set, without defining any
semantics or will simply include
them as part of a field within the IID. In either case, a semantic
meaning for them. these bits MUST NOT be defined.

In the following statement in section 2.5.1 of the IPv6 addressing
architecture [RFC4291], the reference to "Modified EUI-64 format"
applies only to cases where there is an underlying IEEE address:

"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 following statement about in section 2.5.1 of the IPv6 addressing
architecture [RFC4291] is hereby obsoleted:

"The use of the universal/local bit in the Modified EUI-64 format
identifier is to allow development of future technology quoted above from RFC 4941 is
obsolete. that can take
advantage of interface identifiers with universal scope."

As far as is known, no existing implementation will be affected by
these changes. The benefit is that future design discussions are
simplified.

5. Security Considerations

No new security exposures or issues are raised by this document.

6. IANA Considerations

This document requests no immediate action by IANA. However, in the
following should be noted when considering future proposed additions
to the registry of reserved IID
values [RFC5453], no special consideration is needed values, which requires Standards
Action according to [RFC5453]. A reserved IID, or a range of the
reserved IIDs, will most likely specify values for both "u" and
"g" bits, "g",
since they have no special meaning.

6. are among the high-order bits. At the present time, none
of the known methods of generating IIDs will generate "u" = "g" = 1.
Reserved IIDs with "u" = "g" = 1 are therefore unlikely to collide
with automatically generated IIDs.

7. Acknowledgements

Valuable comments were received from ... Remi Despres, Fernando Gont,
Brian Haberman, Joel Halpern, Ray Hunter, Mark Smith, 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].

8. Change log [RFC Editor: Please remove]

draft-carpenter-6man-ug-01: numerous clarifications following WG
comments, discussed DAD, added new section on the usefulness of the
u/g bits, expanded IANA considerations, set intended status,
2013-02-21.

draft-carpenter-6man-ug-00: original version, 2013-01-31.

9. References

8.1.

9.1. 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.

9.2. 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.

[RFC3972] Aura, T., "Cryptographically Generated Addresses (CGA)",
RFC 3972, March 2005.

[RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", RFC 4862, September 2007.

[RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy
Extensions for Stateless Address Autoconfiguration in
IPv6", RFC 4941, September 2007.

[RFC5535] Bagnulo, M., "Hash-Based Addresses (HBA)", RFC 5535,
June 2009.

[RFC6164] Kohno, M., Nitzan, B., Bush, R., Matsuzaki, Y., Colitti,
L., and T. Narten, "Using 127-Bit IPv6 Prefixes on Inter-
Router Links", RFC 6164, April 2011.

[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