280 lines
11 KiB
Text
280 lines
11 KiB
Text
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Network Working Group N. Bourbaki
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Internet-Draft The Intertubes
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Intended status: Standards Track May 9, 2017
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Expires: November 10, 2017
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IPv6 is Classless
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draft-bourbaki-6man-classless-ipv6-00
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Abstract
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Over the history of IPv6, various classful address models have been
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proposed, with the most notable being Top-Level Aggregation (TLA) and
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Next-Level Aggregation (NLA) Identifiers. They have all proved to be
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mistakes. The last remnant of classful addressing is a rigid network
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/ interface identifier boundary at /64. This document removes that
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boundary as far as routing and addressing are concerned.
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Status of This Memo
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This Internet-Draft is submitted in full conformance with the
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provisions of BCP 78 and BCP 79.
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Internet-Drafts are working documents of the Internet Engineering
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Task Force (IETF). Note that other groups may also distribute
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working documents as Internet-Drafts. The list of current Internet-
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Drafts is at http://datatracker.ietf.org/drafts/current/.
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Internet-Drafts are draft documents valid for a maximum of six months
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and may be updated, replaced, or obsoleted by other documents at any
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time. It is inappropriate to use Internet-Drafts as reference
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material or to cite them other than as "work in progress."
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This Internet-Draft will expire on November 10, 2017.
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Copyright Notice
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Copyright (c) 2017 IETF Trust and the persons identified as the
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document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
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Provisions Relating to IETF Documents
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(http://trustee.ietf.org/license-info) in effect on the date of
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publication of this document. Please review these documents
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carefully, as they describe your rights and restrictions with respect
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to this document. Code Components extracted from this document must
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include Simplified BSD License text as described in Section 4.e of
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Bourbaki Expires November 10, 2017 [Page 1]
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Internet-Draft IPv6 is Classless May 2017
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the Trust Legal Provisions and are provided without warranty as
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described in the Simplified BSD License.
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Table of Contents
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1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
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2. Suggested Reading . . . . . . . . . . . . . . . . . . . . . . 2
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3. A simple Statement . . . . . . . . . . . . . . . . . . . . . 3
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4. Recommendations . . . . . . . . . . . . . . . . . . . . . . . 3
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5. Security Considerations . . . . . . . . . . . . . . . . . . . 4
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6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
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7. Authors . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
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8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 4
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9. References . . . . . . . . . . . . . . . . . . . . . . . . . 4
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9.1. Normative References . . . . . . . . . . . . . . . . . . 4
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9.2. Informative References . . . . . . . . . . . . . . . . . 5
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Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 5
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1. Introduction
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Over the history of IPv6, various classful address models have been
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proposed, with the most notable being Top-Level Aggregation (TLA) and
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Next-Level Aggregation (NLA) Identifiers; see, for example,
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[RFC2450]. They have all proved to be mistakes. For example, TLA
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and NLA were obsoleted by [RFC3587]. The last remnant of classful
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addressing is a rigid network / interface identifier boundary at /64.
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This document removes that boundary as far as routing and addressing
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are concerned.
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Some confusion has been caused by the IP Version 6 Addressing
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Architecture, [RFC4291], and the proposed changes in
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[I-D.hinden-6man-rfc4291bis] with respect to the minimum subnet size.
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Meanwhile, link prefixes of varied lengths, /127, /126, /124, /120,
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... /64 have been successfully deployed for many years. Having the
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formal specification be unclear risks potential mis-implementation by
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the naive, which could result in operational disasters.
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2. Suggested Reading
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It is assumed that the reader understands the history of classful
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addressing in IPv4 and why it was abolished [RFC4632]. Of course,
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the acute need to conserve address space that forced the adoption of
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classless addressing for IPv4 does not apply to IPv6; but the
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arguments for operational flexibility in address allocation remain
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compelling.
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Bourbaki Expires November 10, 2017 [Page 2]
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Internet-Draft IPv6 is Classless May 2017
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It is also assumed that the reader understands IPv6 [RFC2460], the IP
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Version 6 Addressing Architecture [RFC4291], the proposed changes to
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RFC4291 [I-D.hinden-6man-rfc4291bis], and the recent recommendations
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for the generation of stable Interface Identifiers [RFC8064].
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An important recent IPv6 development was that, for host computers on
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local area networks, the way in which interface identifiers were
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formed was no longer bound to layer 2 addresses (MACs) [RFC7217]
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[RFC8064]. Therefore their length, previously fixed at 64 bits
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[RFC7136], is in fact a variably-sized parameter as stated in
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[RFC4862].
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3. A simple Statement
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To state it simply, IPv6 unicast subnetting is based on prefixes of
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any valid length up to 128 except for links where an Internet
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Standard such as, for example, Stateless Address AutoConfiguration
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(SLAAC) [RFC4862], or Using 127-Bit IPv6 Prefixes on Inter-Router
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Links [RFC6164] is in use.
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Nodes must always support routing on any valid network prefix length,
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even if SLAAC or other standards are in use, because routing could
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choose to differentiate at a different granularity than is used by
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any such automated link local address configuration tools.
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4. Recommendations
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For historical reasons, when a prefix is needed on a link, barring
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other considerations, a /64 is recommended [RFC7136].
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The length of the Interface Identifier in Stateless Address
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AutoConfiguration [RFC4862] is a parameter; its length SHOULD be
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sufficient for effective randomization for privacy reasons. For
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example, a /48 might be sufficient. But operationally we recommend,
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barring strong considerations to the contrary, using 64-bits for
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SLAAC in order not to discover bugs where 64 was hard-coded, and to
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favor portability of devices and operating systems.
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None the less, there is no reason in theory why an IPv6 node should
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not operate with different interface identfier lengths on different
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physical interfaces. Thus, a correct implementation of SLAAC must in
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fact allow for any prefix length, with the value being a parameter
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per interface. For instance, the Interface Identifier length in the
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recommended (see [RFC8064]) algorithm for selecting stable interface
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identifiers [RFC7217] is a parameter, rather than a hardcoded value.
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Bourbaki Expires November 10, 2017 [Page 3]
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Internet-Draft IPv6 is Classless May 2017
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5. Security Considerations
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Assumming that nodes employ unpredictable interface identifiers
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[RFC7721], the subnet size may have an impact on some security and
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privacy properties of a network. Namely, the smaller the subnet
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size, the more feasible it becomes to perform IPv6 address scans
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[RFC7707] [RFC7721]. For some specific subnets, such as point to
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point links, this may be less of an issue.
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On the other hand, we assume that a number of IPv6 implementations
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fail to enforce limits on the size of some of the data structures
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they employ for communicating with neighboring nodes, such as the
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Neighbor Cache. In such cases, the use of smaller subnets forces an
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operational limit on such data structures, thus helping mitigate some
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pathological behaviors (such as Neighbor Cache Exhaustion attacks).
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6. IANA Considerations
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This document has no IANA Considerations.
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7. Authors
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The original draft was by Randy Bush, who was immediately aided and
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abetted by Brian Carpenter, Chris Morrow, Fernando Gont, Geoff
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Huston, Job Snijders, [ your name here ].
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8. Acknowledgments
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The authors wish to thank .
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9. References
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9.1. Normative References
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[RFC2450] Hinden, R., "Proposed TLA and NLA Assignment Rules",
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RFC 2450, December 1998.
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[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
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(IPv6) Specification", RFC 2460, December 1998.
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[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
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Architecture", RFC 4291, February 2006.
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[RFC7217] Gont, F., "A Method for Generating Semantically Opaque
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Interface Identifiers with IPv6 Stateless Address
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Autoconfiguration (SLAAC)", RFC 7217,
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DOI 10.17487/RFC7217, April 2014,
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<http://www.rfc-editor.org/info/rfc7217>.
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Bourbaki Expires November 10, 2017 [Page 4]
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Internet-Draft IPv6 is Classless May 2017
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[RFC8064] Gont, F., Cooper, A., Thaler, D., and W. Liu,
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"Recommendation on Stable IPv6 Interface Identifiers",
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RFC 8064, DOI 10.17487/RFC8064, February 2017,
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<http://www.rfc-editor.org/info/rfc8064>.
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9.2. Informative References
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[I-D.hinden-6man-rfc4291bis]
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Hinden, B. and S. Deering, "IP Version 6 Addressing
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Architecture", draft-hinden-6man-rfc4291bis-06 (work in
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progress), October 2015.
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[RFC3587] Hinden, R., Deering, S., and E. Nordmark, "IPv6 Global
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Unicast Address Format", RFC 3587, August 2003.
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[RFC4632] Fuller, V. and T. Li, "Classless Inter-domain Routing
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(CIDR): The Internet Address Assignment and Aggregation
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Plan", BCP 122, RFC 4632, August 2006.
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[RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
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Address Autoconfiguration", RFC 4862, September 2007.
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[RFC6164] Kohno, M., Nitzan, B., Bush, R., Matsuzaki, Y., Colitti,
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L., and T. Narten, "Using 127-Bit IPv6 Prefixes on Inter-
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Router Links", RFC 6164, April 2011.
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[RFC7136] Carpenter, B. and S. Jiang, "Significance of IPv6
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Interface Identifiers", RFC 7136, DOI 10.17487/RFC7136,
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February 2014, <http://www.rfc-editor.org/info/rfc7136>.
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[RFC7707] Gont, F. and T. Chown, "Network Reconnaissance in IPv6
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Networks", RFC 7707, DOI 10.17487/RFC7707, March 2016,
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<http://www.rfc-editor.org/info/rfc7707>.
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[RFC7721] Cooper, A., Gont, F., and D. Thaler, "Security and Privacy
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Considerations for IPv6 Address Generation Mechanisms",
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RFC 7721, DOI 10.17487/RFC7721, March 2016,
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<http://www.rfc-editor.org/info/rfc7721>.
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Author's Address
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Nicolas Bourbaki
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The Intertubes
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42 Rue du Jour
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Sophia-Antipolis ::1
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FR
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Email: bourbaki@bogus.com
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Bourbaki Expires November 10, 2017 [Page 5]
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