306 lines
12 KiB
XML
306 lines
12 KiB
XML
<?xml version="1.0"?>
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<!DOCTYPE rfc SYSTEM "rfc2629.dtd">
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<?rfc comments="yes"?>
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<?rfc compact="yes"?>
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<?rfc inline="yes"?>
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<?rfc sortrefs="yes"?>
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<?rfc subcompact="yes"?>
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<?rfc symrefs="yes"?>
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<?rfc toc="yes"?>
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<?rfc tocdepth="3"?>
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<?rfc tocindent="yes"?>
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<?rfc tocompact="yes"?>
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<rfc category="std" docName="draft-bourbaki-6man-classless-ipv6-01" ipr="trust200902" updates="4291">
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<front>
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<title>IPv6 is Classless</title>
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<author fullname="Nicolas Bourbaki" initials="N." surname="Bourbaki">
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<organization>The Intertubes</organization>
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<address>
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<postal>
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<street>42 Rue du Jour</street>
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<city>Sophia-Antipolis</city>
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<region></region>
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<code>::1</code>
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<country>FR</country>
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</postal>
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<email>bourbaki@bogus.com</email>
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</address>
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</author>
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<date />
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<area>Internet</area>
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<workgroup>6man</workgroup>
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<abstract>
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<t>Over the history of IPv6, various classful address models have been
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proposed, none of which has withstood the test of time. The last
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remnant of IPv6 classful addressing is a rigid network interface
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identifier boundary at /64. This document removes the fixed position of
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that boundary for interface addressing.</t>
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</abstract>
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<!--
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<note title="Requirements Language">
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<t>The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
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NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL"
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are to be interpreted as described in <xref target="RFC2119">RFC
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2119</xref> only when they appear in all upper case. They may
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also appear in lower or mixed case as English words, without
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normative meaning.</t>
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</note>
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-->
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</front>
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<middle>
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<section anchor="intro" title="Introduction">
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<t>Over the history of the IPv6 protocol, several classful addressing
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models have been proposed. The most notable example recommended Top-Level
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Aggregation (TLA) and Next-Level Aggregation (NLA) Identifiers <xref
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target="RFC2450"/>, but was obsoleted by <xref target="RFC3587"/>, leaving
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a single remnant of classful addressing in IPv6: a rigid network
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interface identifier boundary at /64. This document removes the fixed
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position of that boundary for interface addressing.</t>
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<t>Recent proposed changes to the IP Version 6 Addressing Architecture
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specification <xref target="RFC4291"/> have caused controversy.
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While link prefixes of varied lengths, e.g. /127, /126, /124,
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/120, ... /64 have been successfully deployed for many years, glaring
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mismatches between a formal specification and long-standing field
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deployment practices are never wise, not least because of the strong
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risk of mis-implementation, which can easily result in serious
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operational problems.</t>
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<t>This document also clarifies that IPv6 routing subnets may be of any
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length up to 128.</t>
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</section>
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<section anchor="reading" title="Suggested Reading">
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<t>It is assumed that the reader understands the history of classful
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addressing in IPv4 and why it was abolished <xref
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target="RFC4632"/>. Of course, the acute need to conserve address
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space that forced the adoption of classless addressing for IPv4 does
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not apply to IPv6, but the arguments for operational flexibility in
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address assignment remain compelling.</t>
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<t>It is also assumed that the reader understands IPv6 <xref
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target="RFC2460"/>, the IP Version 6 Addressing Architecture <xref
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target="RFC4291"/>, the proposed changes to RFC4291 <xref
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target="I-D.ietf-6man-rfc4291bis"/> and RFC2464 <xref
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target="I-D.hinden-6man-rfc2464bis"/>, <xref target="RFC7608"/> an
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IPv6 Prefix Length Recommendation for Forwarding, and the IETF
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recommendation for the generation of stable Interface Identifiers
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<xref target="RFC8064"/>.</t>
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<t><xref target="I-D.jinmei-6man-prefix-clarify"/> is also worth
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reading to clarify uses of varying prefix lengths on a single
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link.</t>
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<!--
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<t>NOTE: do we mean 4291bis (currently moribund) or 2464bis?</t>
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[fgont] We do mean 4291bis. That say, RFC8064/RFC7217 already do part of
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the job: they replace the algorithm of "embedding the MAC address in the
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IPv6" with one that embeds random bits of an appropriate length. That
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is, strictly speaking, we don't een need /64 for SLAAC, except for
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backward compatibility. (*)
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(*) as long as the local subnet is large enough and the IID collision
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rate is low enough.
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-->
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</section>
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<section anchor="Problem" title="Problem reinforced by classful addressing">
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<t>For host computers on local area networks, generation of interface
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identifiers is no longer necessarily bound to layer 2 addresses (MACs)
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<xref target="RFC7217"/> <xref target="RFC8064"/>. Therefore their
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length, previously fixed at 64 bits <xref target="RFC7136"/>, is in fact
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a variably-sized parameter as explicitly acknowledged in Section
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5.5.3(d) of <xref target="RFC4862"/> which states:
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<list>
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<t>Note that a future revision of the address architecture
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[RFC4291] and a future link-type-specific document, which will
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still be consistent with each other, could potentially allow for
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an interface identifier of length other than the value defined in
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the current documents. Thus, an implementation should not assume
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a particular constant. Rather, it should expect any lengths of
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interface identifiers.</t>
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</list>
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</t>
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<t>As IPv6 usage has evolved and grown over in recent years, it has
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become evident that it faces several scaling and coordination
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problems. These problems are analogous to allocation and coordination
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problems that motivated IPv4 CIDR allocation and later abundant IPv4
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PAT, they include:
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<list>
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<t>Address allocation models for specific counts of fixed length
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subnets to downstream networks or devices from /48 down to /64 are
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based on our imagination of how subnets are or should be allocated
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within ipv4 networks.</t>
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<t>Hierarchical allocation of fixed-length subnets requires
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coordination between lower / intermediate / upper network elements
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and has implict assumption that policies and size allocation at the
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top of the hierarchy will accomidate all use cases with fixed lenth
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subnet allocation.</t>
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<t>Coordination with upstream network elements for the allocation of
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fixed length subnets reveals topology and intent that may be private
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in scope and which amounts to permission to build a particular
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topology.</t>
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</list>
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</t>
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</section>
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<section anchor="statement" title="Identifier and Subnet Length Statements">
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<t>IPv6 unicast interfaces may use any subnet length up to 128 except
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for situations where an Internet Standard document may impose a
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particular length, for example Stateless Address Autoconfiguration
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(SLAAC) <xref target="RFC4862"/>, or Using 127-Bit IPv6 Prefixes on
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Inter-Router Links <xref target="RFC6164"/>.</t>
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<t>Additionally, this document clarifies that a node or router MUST
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support routing of any valid network prefix length, even if SLAAC or
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other standards are in use, because routing could choose to
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differentiate at a different granularity than is used by any such
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automated link local address configuration tools.</t>
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</section>
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<!-- [fgont] I think these section is mixing up to things:
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* Routing: Nodes must *always* support rotuing on any valid length, even
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if, say, SLAAC is in use. Even when SLAAC is used, I might
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want to install a host-specific rule (a /128 rule), if I
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please. And I think this point has never been contended
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(except for vendors that go lazy/cheap and just don't want to
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use mre than 64-bits in each FIB entry.
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* Subnet size: This is what you're really referring to here. Nodes
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should be able to employ any subnet size that they
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please, except when slaac is in use (for backwards
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compatibility) or e.g. when /127 (or the like) prefixes
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are employed for point to point links.
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-->
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</section>
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<section anchor="notes" title="Recommendations">
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<t>For historical reasons, when a prefix is needed on a link,
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barring other considerations, a /64 is recommended <xref
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target="RFC7136"/>.</t>
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<t>The length of the Interface Identifier in Stateless Address
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Autoconfiguration <xref target="RFC4862"/> is a parameter; its
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length SHOULD be sufficient for effective randomization for privacy
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reasons. For example, a /48 might be sufficient. But operationally
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we recommend, barring strong considerations to the contrary, using
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64-bits for SLAAC in order not to discover bugs where 64 was
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hard-coded, and to favor portability of devices and operating
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systems.</t>
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<t>Nonetheless, there is no reason in theory why an IPv6 node
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should not operate with different interface identfier lengths on
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different physical interfaces. Thus, a correct implementation of
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SLAAC must in fact allow for any prefix length, with the value being
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a parameter per interface. For instance, the Interface Identifier
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length in the recommended (see <xref target="RFC8064"/>) algorithm
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for selecting stable interface identifiers <xref target="RFC7217"/>
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is a parameter, rather than a hardcoded value.</t>
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</section>
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<section anchor="security" title="Security Considerations">
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<t>Assuming that nodes employ unpredictable interface identifiers
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<xref target="RFC7721"/>, the subnet size may have an impact on some
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security and privacy properties of a network. Namely, the smaller
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the subnet size, the more feasible it becomes to perform IPv6
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address scans <xref target="RFC7707"/> <xref target="RFC7721"/>.
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For some specific subnets, such as point to point links, this may be
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less of an issue.</t>
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<t>On the other hand, we assume that a number of IPv6
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implementations fail to enforce limits on the size of some of the
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data structures they employ for communicating with neighboring
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nodes, such as the Neighbor Cache. In such cases, the use of
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smaller subnets forces an operational limit on such data structures,
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thus helping mitigate some pathological behaviors (such as Neighbor
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Cache Exhaustion attacks).</t>
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<!-- [fgont] Still need to add references here... e.g. to Joel's RFC -->
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</section>
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<section anchor="iana" title="IANA Considerations">
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<t>This document has no IANA Considerations.</t>
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<!--
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<t>Note to RFC Editor: this section may be replaced on publication
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as an RFC.</t>
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-->
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</section>
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<section anchor="authors" title="Authors">
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<t>The authors of this document are as follows:
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<list>
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<t> Randy Bush, Internet Initiative Japan</t>
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<t> Brian Carpenter, University of Auckland</t>
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<t> Fernando Gont, SI6 Networks / UTN-FRH</t>
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<t> Nick Hilliard, INEX</t>
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<t> Geoff Huston, APNIC</t>
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<t> Chris Morrow, Google, Inc.</t>
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<t> Job Snijders, NTT Communications</t>
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</list>
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</t>
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</section>
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</middle>
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<back>
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<references title="Normative References">
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<!--
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<?rfc include="reference.RFC.2119"?>
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-->
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<?rfc include="reference.RFC.2460"?>
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<?rfc include="reference.RFC.4291"?>
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<?rfc include="reference.RFC.7217"?>
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<?rfc include="reference.RFC.8064"?>
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</references>
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<references title="Informative References">
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<?rfc include="reference.RFC.2450"?>
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<?rfc include="reference.RFC.4862"?>
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<?rfc include="reference.RFC.6164"?>
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<?rfc include="reference.RFC.3587"?>
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<?rfc include="reference.RFC.4632"?>
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<?rfc include="reference.RFC.7608"?>
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<?rfc include="reference.RFC.7707"?>
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<?rfc include="reference.RFC.7136"?>
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<?rfc include="reference.RFC.7721"?>
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<?rfc include="reference.I-D.ietf-6man-rfc4291bis"?>
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<?rfc include="reference.I-D.hinden-6man-rfc2464bis"?>
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<?rfc include="reference.I-D.jinmei-6man-prefix-clarify"?>
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</references>
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</back>
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</rfc>
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