Network Working Group H. Singh Internet-Draft W. Beebee Intended status: Standards Track Cisco Systems, Inc. Expires:
November 8, 2008January 11, 2009 E. Nordmark Sun Microsystems May 7,July 10, 2008 IPv6 Subnet Model: the Relationship between Links and Subnet Prefixes draft-ietf-6man-ipv6-subnet-model-00draft-ietf-6man-ipv6-subnet-model-01 Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. 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." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on November 8, 2008.January 11, 2009. Abstract IPv6 specifies a model of a subnet that is different than the IPv4 subnet model. The subtlety of the differences has resulted in incorrect implementations that do not interoperate. This document spells out the most important difference; that an IPv6 address isn't automatically associated with an IPv6 on-link prefix. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Host Behavior and Rules . . . . . . . . . . . . . . . . . . . . . .4 3. Observed Incorrect Implementation Behavior . . . . . . . . . . 5 4. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 5 5. Security Considerations . . . . . . . . . . . . . . . . . . . . 5 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 56 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 6 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6 8.1. Normative References . . . . . . . . . . . . . . . . . . . 6 8.2. Informative References . . . . . . . . . . . . . . . . . . 6 Appendix A. CHANGE HISTORY . . . . . . . . . . . . . . . . . . . . 7 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 7 Intellectual Property and Copyright Statements . . . . . . . . . . 89 1. Introduction IPv4 implementations typically associate a netmask with an address when an IPv4 address is assigned to an interface. That netmask together with the IPv4 address designates an on-link prefix. Addresses that matchare covered by this prefix are viewed as on-link i.e., traffic to these addresses is not sent to a router. See section 3.3.1 in [RFC1122]. Further, note that implementationsPrior to the deployment of IPv4 point-to-point interfaces might not haveCIDR, an associated IPv4 subnet prefix.address's netmask could be derived directly from the address. In the absence of specifying a specific netmask when assigning a address, some implementations would fall back to deriving the netmask from the class of the address. The behavior of IPv6 as specified in Neighbor Discovery [RFC4861] is quite different. The on-link determination is separate from the address assignment. A host can have IPv6 addresses without any related on-link prefixes or have on-link prefixes that are not related to any IPv6 addresses that are assigned to the host. Any assigned address on an interface should initially be considered as having no internal structure as shown in [RFC4291]. In IPv6, by default, a host treats only the link-local prefix as on- link. The reception of a Prefix Information Option (PIO) with the L-bit set [RFC4861] and a non-zero valid lifetime creates an entry (or updates the valid lifetime for an existing entry) in the Prefix List. All the prefixes that are on the Prefix List, i.e., have not yet timed out, are on-link. In addition to the Prefix List, individual addresses areThe on-link if they aredefinition in the targetTerminology section of a Redirect Message indicating on-link, or[RFC4861] defines the sourcecomplete list of a valid Neighbor Solicitation or Neighbor Advertisement message. Notecases where an address is considered on- link. Note, in particular, that Redirect Messages can also indicate an address is off-link. Individual address entries can be expired by the Neighbor Unreachability Detection mechanism. A host only performs address resolution for IPv6 addresses that are on-link. Packets to any other address are sent to a default router. If there is no default router, then the node should send an ICMPv6 Destination Unreachable indication as specified in [RFC4861] - more details are provided in the Host Behavior Rules section. (Note that RFC 4861 changed the behavior when the Default Router List is empty. The behavior in the old version of Neighbor Discovery [RFC2461] was different when there were no default routers.) Failure of host implementations to correctly implement the IPv6 subnet model can result in lack of IPv6 connectivity. See the Observed Incorrect Implementation Behavior section for details. Host behavior is clarified in the Host Behavior Rules section. Finally, this document merelymainly restates and clarifies [RFC4861]. 2. Host Behavior and Rules A correctly implemented IPv6 host MUST adhere to the following rules: 1. By default only the link-local prefix is on-link. 2. The configuration of an IPv6 address, whether through IPv6 stateless address autoconfiguration [RFC4862], DHCPv6 [RFC3315], or manual configuration MUST NOT imply that anyimplicitely cause a prefix is on- link.derived from that address to be treated as on-link. A host is explicitly told that prefixes or addresses areconsiders a prefix to be on-link only through the meansexplicit means, such as those specified in [RFC4861].the on-link definition in the Terminology section of [RFC4861] or via manual configuration. Note that thisthe requirement for manually configured addresses is not explicitly mentioned in [RFC4861]. 3. On-linkIf on-link determination SHOULD NOT persistpersists across IPv6 interface initializations. Note that section 5.7 of [RFC4862] describes the useinitializations, then lack of stable storage for addresses acquiredIPv6 connectivity can result. For example, a host receives an RA from a router with stateless address autoconfigurationon-link prefix A. The host reboots. During the reboot, the router sends out prefix A with on-link bit set and a note thatzero lifetime to indicate a renumbering. The host misses the Preferred and Valid Lifetimes must be retained if this approach is used. However no RFC suggests or recommends retainingrenumbering. The host comes online. Then, the router sends an RA with no PIO. The host uses cached on-link prefixes.prefix A and issues NS's instead of sending traffic to a default router. The "Observed Incorrect Implementation Behavior" section below describes how this can result in lack of IPv6 connectivity. 4. In the absence of other sources of on-link information, including Redirects, if the RA advertises a prefix with the on-link(L) bit set and later the Valid Lifetime expires, the host MUST then consider addresses of the prefix to be off-link, as specified by the PIO paragraph of section 6.3.4 of [RFC4861]. 5. Newer implementations, which are compliant with [RFC4861] MUST adhere to the following rules. Older implementations, which are compliant with [RFC2461] but not [RFC4861] may remain as is. If the Default Router List is empty and there is no other source of on-link information about any address or prefix: 1. The host MUST NOT assume that all destinations are on-link. 2. The host MUST NOT perform address resolution for non-link- local addresses. 3. Since the host cannot assume the destination is on-link, and off-link traffic cannot be sent to a default router (since the Default Router List is empty), address resolution cannot be performed. This case is analogous to the behaviorspecified in the last paragraph of section 7.2.24 of [RFC4861]:[RFC4943]: when address resolution fails,there is no route to destination, the host SHOULDshould send an ICMPv6 Destination Unreachable indication (for example, a locally delivered error message) as specified in the Terminology section of [RFC4861]. The specified behavior MAY be extended to cover this case where address resolution cannot be performed.On-link information concerning particular addresses and prefixes can make those specific addresses and prefixes on-link, but does not change the default behavior mentioned above for addresses and prefixes not specified. [RFC4943] provides justification for these rules. 3. Observed Incorrect Implementation Behavior One incorrect implementation behavior illustrates the severe consequences when the IPv6 subnet model is not understood by the implementers of several popular host operating systems. In an access concentrator network ([RFC4388]), a host receives a Router Advertisement Message with no on-link prefix advertised. The host incorrectly assumes the prefix is on-link and performs address resolution when the host should send all non-link-local traffic to a default router. Neither the router nor any other host will respond to the address resolution, preventing this host from sending IPv6 traffic. 4. Conclusion This document clarifies and summarizes the relationship between links and subnet prefixes described in [RFC4861]. Configuration of an IPv6 address does not imply the existence of corresponding on-link prefixes. One should also look at API considerations for prefix length as described in last paragraph of section 4.2 of [RFC4903]. 5. Security Considerations As thisThis document merelymainly restates and clarifies [RFC4861], it[RFC4861]. It does not introduce any new security issues. 6. IANA Considerations None. 7. Acknowledgements Thanks (in alphabetical order) to Adeel Ahmed, Jari Arkko, Ralph Droms, Alun Evans, Dave Forster, Prashanth Krishnamurthy, Suresh Krishnan, Josh Littlefield, Thomas Narten, Madhu Sudan, Jinmei Tatuya, Dave Thaler, Bernie Volz, and Vlad Yasevich for their consistent input, ideas and review during the production of this document. 8. References 8.1. Normative References [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, September 2007. 8.2. Informative References [RFC1122] Braden, R., "Requirements for Internet Hosts - Communication Layers", STD 3, RFC 1122, October 1989. [RFC2461] Narten, T., Nordmark, E., and W. Simpson, "Neighbor Discovery for IP Version 6 (IPv6)", RFC 2461, December 1998. [RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M. Carney, "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", RFC 3315, July 2003. [RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing Architecture", RFC 4291, February 2006. [RFC4388] Woundy, R. and K. Kinnear, "Dynamic Host Configuration Protocol (DHCP) Leasequery", RFC 4388, February 2006. [RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless Address Autoconfiguration", RFC 4862, September 2007. [RFC4903] Thaler, D., "Multi-Link Subnet Issues", RFC 4903, June 2007. [RFC4943] Roy, S., Durand, A., and J. Paugh, "IPv6 Neighbor Discovery On-Link Assumption Considered Harmful", RFC 4943, September 2007. Appendix A. CHANGE HISTORY [NOTE TO RFC EDITOR: PLEASE REMOVE THIS SECTION UPON PUBLICATION.] Changes in draft-ietf-6man-ipv6-subnet-model-01.txt since -00.txt are: o Changed Introduction section to remove any mention of src address of ND message as a means for on-link determination. Also reworded first paragrpah of Introduction section. o Reworded bullet 2 of section 2 and added text to clarify on-link definition. o Added text to bullet 3 of section 2 to make explicit that this is a new rule. o Reworded bullet 5 of section 2 to clearly explain where ICMPv6 Destination Unreachable is sent to. Authors' Addresses Hemant Singh Cisco Systems, Inc. 1414 Massachusetts Ave. Boxborough, MA 01719 USA Phone: +1 978 936 1622 Email: email@example.com URI: http://www.cisco.com/ Wes Beebee Cisco Systems, Inc. 1414 Massachusetts Ave. Boxborough, MA 01719 USA Phone: +1 978 936 2030 Email: firstname.lastname@example.org URI: http://www.cisco.com/ Erik Nordmark Sun Microsystems 17 Network Circle Menlo Park, CA 94025 USA Phone: +1 650 786 2921 Email: email@example.com Full Copyright Statement Copyright (C) The IETF Trust (2008). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. 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