Network Working Group R. Bush Internet-Draft Internet Initiative Japan Updates: 4291 (if approved) B. Carpenter Intended status: Standards Track Univ. of Auckland Expires: November 23, 2017 F. Gont SI6 Networks / UTN-FRH N. Hilliard INEX G. Huston APNIC C. Morrow GOOG J. Snijders NTT May 22, 2017 IPv6 is Classless draft-bourbaki-6man-classless-ipv6-00 Abstract Over the history of IPv6, various classful address models have been proposed, none of which has withstood the test of time. The last remnant of IPv6 classful addressing is a rigid network interface identifier boundary at /64. This document removes the fixed position of that boundary for interface addressing. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on November 23, 2017. Bush, et al. Expires November 23, 2017 [Page 1] Internet-Draft IPv6 is Classless May 2017 Copyright Notice Copyright (c) 2017 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Suggested Reading . . . . . . . . . . . . . . . . . . . . . . 3 3. Identifier and Subnet Length Statements . . . . . . . . . . . 3 4. Recommendations . . . . . . . . . . . . . . . . . . . . . . . 4 5. Security Considerations . . . . . . . . . . . . . . . . . . . 4 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 5 7.1. Normative References . . . . . . . . . . . . . . . . . . 5 7.2. Informative References . . . . . . . . . . . . . . . . . 5 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6 1. Introduction Over the history of the IPv6 protocol, several classful addressing models have been proposed. The most notable example recommended Top- Level Aggregation (TLA) and Next-Level Aggregation (NLA) Identifiers [RFC2450], but was obsoleted by [RFC3587], leaving a single remnant of classful addressing in IPv6: a rigid network interface identifier boundary at /64. This document removes the fixed position of that boundary for interface addressing. Recent proposed changes to the IP Version 6 Addressing Architecture specification [RFC4291] have caused controversy. While link prefixes of varied lengths, e.g. /127, /126, /124, /120, ... /64 have been successfully deployed for many years, glaring mismatches between a formal specification and long-standing field deployment practices are never wise, not least because of the strong risk of mis- implementation, which can easily result in serious operational problems. Bush, et al. Expires November 23, 2017 [Page 2] Internet-Draft IPv6 is Classless May 2017 This document also clarifies that IPv6 routing subnets may be of any length up to 128. 2. Suggested Reading It is assumed that the reader understands the history of classful addressing in IPv4 and why it was abolished [RFC4632]. Of course, the acute need to conserve address space that forced the adoption of classless addressing for IPv4 does not apply to IPv6, but the arguments for operational flexibility in address assignment remain compelling. It is also assumed that the reader understands IPv6 [RFC2460], the IP Version 6 Addressing Architecture [RFC4291], the proposed changes to RFC4291 [I-D.ietf-6man-rfc4291bis] and RFC2464 [I-D.hinden-6man-rfc2464bis], [RFC7608] an IPv6 Prefix Length Recommendation for Forwarding, and the IETF recommendation for the generation of stable Interface Identifiers [RFC8064]. [I-D.jinmei-6man-prefix-clarify] is also worth reading to clarify uses of varying prefix lengths on a single link. For host computers on local area networks, generation of interface identifiers is no longer necessarily bound to layer 2 addresses (MACs) [RFC7217] [RFC8064]. Therefore their length, previously fixed at 64 bits [RFC7136], is in fact a variably-sized parameter as explicitly acknowledged in Section 5.5.3(d) of [RFC4862] which states: Note that a future revision of the address architecture [RFC4291] and a future link-type-specific document, which will still be consistent with each other, could potentially allow for an interface identifier of length other than the value defined in the current documents. Thus, an implementation should not assume a particular constant. Rather, it should expect any lengths of interface identifiers. 3. Identifier and Subnet Length Statements IPv6 unicast interfaces may use any subnet length up to 128 except for situations where an Internet Standard document may impose a particular length, for example Stateless Address Autoconfiguration (SLAAC) [RFC4862], or Using 127-Bit IPv6 Prefixes on Inter-Router Links [RFC6164]. Additionally, this document clarifies that a node or router MUST support routing of any valid network prefix length, even if SLAAC or other standards are in use, because routing could choose to Bush, et al. Expires November 23, 2017 [Page 3] Internet-Draft IPv6 is Classless May 2017 differentiate at a different granularity than is used by any such automated link local address configuration tools. 4. Recommendations For historical reasons, when a prefix is needed on a link, barring other considerations, a /64 is recommended [RFC7136]. The length of the Interface Identifier in Stateless Address Autoconfiguration [RFC4862] is a parameter; its length SHOULD be sufficient for effective randomization for privacy reasons. For example, a /48 might be sufficient. But operationally we recommend, barring strong considerations to the contrary, using 64-bits for SLAAC in order not to discover bugs where 64 was hard-coded, and to favor portability of devices and operating systems. Nonetheless, there is no reason in theory why an IPv6 node should not operate with different interface identfier lengths on different physical interfaces. Thus, a correct implementation of SLAAC must in fact allow for any prefix length, with the value being a parameter per interface. For instance, the Interface Identifier length in the recommended (see [RFC8064]) algorithm for selecting stable interface identifiers [RFC7217] is a parameter, rather than a hardcoded value. 5. Security Considerations Assuming that nodes employ unpredictable interface identifiers [RFC7721], the subnet size may have an impact on some security and privacy properties of a network. Namely, the smaller the subnet size, the more feasible it becomes to perform IPv6 address scans [RFC7707] [RFC7721]. For some specific subnets, such as point to point links, this may be less of an issue. On the other hand, we assume that a number of IPv6 implementations fail to enforce limits on the size of some of the data structures they employ for communicating with neighboring nodes, such as the Neighbor Cache. In such cases, the use of smaller subnets forces an operational limit on such data structures, thus helping mitigate some pathological behaviors (such as Neighbor Cache Exhaustion attacks). 6. IANA Considerations This document has no IANA Considerations. Bush, et al. Expires November 23, 2017 [Page 4] Internet-Draft IPv6 is Classless May 2017 7. References 7.1. Normative References [RFC2450] Hinden, R., "Proposed TLA and NLA Assignment Rules", RFC 2450, December 1998. [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, December 1998. [RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing Architecture", RFC 4291, February 2006. [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, . [RFC8064] Gont, F., Cooper, A., Thaler, D., and W. Liu, "Recommendation on Stable IPv6 Interface Identifiers", RFC 8064, DOI 10.17487/RFC8064, February 2017, . 7.2. Informative References [I-D.hinden-6man-rfc2464bis] Crawford, M. and R. Hinden, "Transmission of IPv6 Packets over Ethernet Networks", draft-hinden-6man-rfc2464bis-02 (work in progress), March 2017. [I-D.ietf-6man-rfc4291bis] Hinden, R. and S. <>, "IP Version 6 Addressing Architecture", draft-ietf-6man-rfc4291bis-07 (work in progress), January 2017. [I-D.jinmei-6man-prefix-clarify] Jinmei, T., "Clarifications on On-link and Subnet IPv6 Prefixes", draft-jinmei-6man-prefix-clarify-00 (work in progress), March 2017. [RFC3587] Hinden, R., Deering, S., and E. Nordmark, "IPv6 Global Unicast Address Format", RFC 3587, August 2003. [RFC4632] Fuller, V. and T. Li, "Classless Inter-domain Routing (CIDR): The Internet Address Assignment and Aggregation Plan", BCP 122, RFC 4632, August 2006. Bush, et al. Expires November 23, 2017 [Page 5] Internet-Draft IPv6 is Classless May 2017 [RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless Address Autoconfiguration", RFC 4862, September 2007. [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. [RFC7136] Carpenter, B. and S. Jiang, "Significance of IPv6 Interface Identifiers", RFC 7136, DOI 10.17487/RFC7136, February 2014, . [RFC7608] Boucadair, M., Petrescu, A., and F. Baker, "IPv6 Prefix Length Recommendation for Forwarding", BCP 198, RFC 7608, DOI 10.17487/RFC7608, July 2015, . [RFC7707] Gont, F. and T. Chown, "Network Reconnaissance in IPv6 Networks", RFC 7707, DOI 10.17487/RFC7707, March 2016, . [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, . Authors' Addresses Randy Bush Internet Initiative Japan 5147 Crystal Springs Bainbridge Island, Washington 98110 US Email: randy@psg.com Brian Carpenter Department of Computer Science University of Auckland PB 92019 Auckland 1142 New Zealand Email: brian.e.carpenter@gmail.com Bush, et al. Expires November 23, 2017 [Page 6] Internet-Draft IPv6 is Classless May 2017 Fernando Gont SI6 Networks / UTN-FRH Evaristo Carriego 2644 Haedo, Provincia de Buenos Aires 1706 Argentina Phone: +54 11 4650 8472 Email: fgont@si6networks.com URI: http://www.si6networks.com Nick Hilliard INEX 4027 Kingswood Road Dublin 24 Ireland Email: nick@inex.ie Geoff Huston Email: gih@apnic.net Chris Morrow Google, Inc. 1600 Ampitheatre Parkway Mountain View, California United States of America Email: morrowc@google.com Job Snijders NTT Communications Theodorus Majofskistraat 100 Amsterdam 1065 SZ The Netherlands Email: job@ntt.net Bush, et al. Expires November 23, 2017 [Page 7]