6MAN Working Group A. Petrescu Internet-Draft CEA, LIST Intended status: Standards Track December 14, 2018 Expires: June 17, 2019 The length of the prefix of an IPv6 link-local address ranges from 10 to 127 draft-petrescu-6man-ll-prefix-len-04 Abstract A rejected Errata to RFC4291 "IPv6 Addr Archi" on the topic of link- local addresses 'needs' a draft. This is an answer to that need. The length of the prefix of an IPv6 link-local address is variable. The minimal value is 10 decimal. The maximum value is 127 decimal. 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 https://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 June 17, 2019. Copyright Notice Copyright (c) 2018 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 (https://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 Petrescu Expires June 17, 2019 [Page 1] Internet-Draft IPv6-LL-plen December 2018 the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Definitions and Statements . . . . . . . . . . . . . . . . . 2 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Context . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 4. Example of Advantage of LL Prefix Length 32 . . . . . . . . . 4 5. Security Considerations . . . . . . . . . . . . . . . . . . . 5 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5 7. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 5 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5 9. Normative References . . . . . . . . . . . . . . . . . . . . 6 Appendix A. ChangeLog . . . . . . . . . . . . . . . . . . . . . 6 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 6 1. Definitions and Statements The prefix of an IP address is formed by the n leftmost bits of the address. (in a left-to-right writing system). The prefix of an IP address is used for goals such as: identify the type of an IPv6 address (link-local, global, others), identify the belonging of an IP address to a particular subnetwork, assist the forwarding (or not forwarding) decisions, and others. The minimal length of the prefix of an IPv6 link-local address (the value of n) is equal to 10 decimal. The maximum is 127. The prefix of an IPv6 link-local address is represented textually as "fe80::/n", where n MAY be any value between 10 and 127. Regardless of the prefix length, the leftmost 10 bits of an IPv6 link-local address MUST be set to binary 1111111010 (hexadecimal fe80). The illustration of an IPv6 link-local address is: | leftmost | Subnet ID and Interface ID | 10 bits | 118 bits | +----------+------------------------------------------------------+ |1111111010+ Bits that MAY be either 0 or 1 | +----------+------------------------------------------------------+ Figure 1: The IPv6 link-local address Petrescu Expires June 17, 2019 [Page 2] Internet-Draft IPv6-LL-plen December 2018 Examples: fe80::1/10, fe80:1::1/32 and fe80::1:1/64 are all IPv6 link-local addresses; their prefix lengths are 10, 32 and 64 respectively. Each such IPv6 address has the leftmost 10 bits equal to binary 1111111010. The Difficulty: the number binary 1111111010 can not be written in hexadecimal without specifying the number of significant bits (fe80::/10); yet that does not make it a 'prefix'. Converting 1111111010 to hexadecimal leads to 3FA (because in a left-to-right writing system the leading 0s before comma are irrelevant); yet '3FA' is not commonly known to be the leading bits of an IPv6 link-local address, fe80::/10 is. 2. 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 RFC 2119 [RFC2119]. prefix: a contiguous string of bits valid for forwarding operations and for subnet formation. textual representation of a prefix: e.g. fe80::/64. n leading bits: the first n bits in a string of bits read from left to right in a writing system that is read left-to-right. E.g. the 10 leading bits of the fe80::/64 textual representation of the IPv6 link-local prefix are 1111111010. 3. Context The RFC "IPv6 Address Archi" illustrates the format of the link-local addresses. From the illustration it MAY be understood that the length of the link-local prefix is 10 bits of value 1111111010 and 54 0 bits. IANA lists the "IPv6 prefix", and "Address Block", to be "fe80::/10" on its website. It is possible that in the future the IETF could decide to use the bits 11-53. The RFC 2464 "IPv6-over-Ethernet" states that the prefix for link- local addresses is "fe80::/64". RFC 6874, "Representing IPv6 Zone Identifiers in Address Literals and Uniform Resource Identifiers" specifies the link-local addresses to be under prefix "fe80::/10". Petrescu Expires June 17, 2019 [Page 3] Internet-Draft IPv6-LL-plen December 2018 Several knowledgeable interpretations state that, generally speaking, the prefix length of link-local addresses is 10, but it is 64 in the particular case of Stateless Address-Autoconfiguration (SLAAC). In this latter case, the prefix is named a "subnet prefix", or "prefix on a link", and it is "fe80::/64". Implementations of an IPv6 stack in a particular operating system allow for the manual configuration of both prefix lengths 64 and 10 for link-local addresses. In another operating system the prefix length for link-local addresses can not be explicitely specified by the end user, but may be indirectly derived from two distinct textual formats by using an unspecified rule. Misconfigurations and lack of interoperability MAY arise between computers that use mixed prefix lengths for link-local addresses. A memo describes the use of IPv6 link-local addresses in applications. The filename of the Internet Draft is draft-smith- ipv6-link-locals-apps-00. Historical note: earlier, the link-local prefix fe80::/10 and site- local prefix fec0::/10 were grouped into a common fe80::/9. If bits 10-64 were 0 then the prefix was a link-local, otherwise a site- local. The site-local addresses were later deprecated by RFC 3879. 4. Example of Advantage of LL Prefix Length 32 This is an example of an advantage of using a prefix length 32 for link-local addresses: disambiguate the rt table entries related to link-local addresses and thus use ping without -I parameter. This figure shows two routers each with two interfaces; one such interface is connected to the other router; there are two interfaces that point elsewhere. i1 ------- i2 i3-------i4 --|Router1|---------|Router2|--- ------- ------- i2 address is fe80:12::1:1/32 ('12' means subnet between R1 and R2, '1' is R1, 2nd '1' is 'front' interface) i3 address is fe80:12::2:2/32 Figure 2: Figure Petrescu Expires June 17, 2019 [Page 4] Internet-Draft IPv6-LL-plen December 2018 One router's interface (connected to the other router) uses address fe80:12::1:1/32 and the other router's corresponding interface uses address fe80:12::2:2/32. With this in place, it is possible on one router to ping the other router's address without specifying the -I ifacename parameter to ping; also it is possible to ssh without using '%' parameter. 5. Security Considerations The clarification of the definition of the prefix length of the IPv6 link-local prefix at IANA is: call it 'leading bits' and not 'prefix', or state that the IPv6 prefix length of link-local addresses is 10 decimal. This clarification has beneficial impact in the algorithm implementation for calculation of the opaque and stable Interface Identifiers for IPv6 link-local addresses. It also positively impacts some implementations of IPv6 forwarding. 6. IANA Considerations IANA is requested to change the name of the column head in the table that depicts the "Internet Protocol Version 6 Address Space". The name should be "The n leading bits of an address" instead of "IPv6 Prefix". The desired effect of this change is that the IPv6 link-local prefix be "fe80::/n" and that the 10 leading bits of this prefix be 1111111010. A second effect is that the textual representation "fe80::/10" as an IPv6 link-local prefix should disappear from that IANA page, because it is wrong. 7. Contributors Listed from 6man WG discussion. 8. Acknowledgements The following persons are acknowledged for the discussion that is reflected in this draft. Not all points are reflected. Some points are copied almost entirely. Ole Troan, Scott Timothy Morizot, Brian Carpenter, Fred Baker, Mark Smith, Peter Occil, Philip Homburg, Albert Manfredi, _–3/4 ’BAE (TATUYA Jinmei), Fernando Gont, Christian Huitema, Simon Hobson, Matthew Petach, Yucel Guven, Sander Steffann, Dennis Ferguson, Musa Stephen Honlue, Philip Homburg. Petrescu Expires June 17, 2019 [Page 5] Internet-Draft IPv6-LL-plen December 2018 Peter Paluch submitted the Errata suggestion to RFC 4291 about link- local addresses, and Brian Haberman rejected it, by requiring a draft. Igor Lubashev pointed to that Errata. 9. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . Appendix A. ChangeLog The changes are listed in reverse chronological order, most recent changes appearing at the top of the list. -04: added an example advantage of using prefix length 32. -03: -02: corrected a typo in "fe80::/1" and added a 7-bit encoding for one persons name (in addition to the japanese-shift-jis encoding which is not understood by xml2rfc.) Author's Address Alexandre Petrescu CEA, LIST CEA Saclay Gif-sur-Yvette , Ile-de-France 91190 France Phone: +33169089223 Email: Alexandre.Petrescu@cea.fr Petrescu Expires June 17, 2019 [Page 6]