Network Working Group H. Singh Internet-Draft W. Beebee Intended status: Standards Track Cisco Systems, Inc. Expires: August 28, 2008 E. Nordmark Sun Microsystems February 25, 2008 IPv6 Subnet Model draft-wbeebee-on-link-and-off-link-determination-02 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 August 28, 2008. Copyright Notice Copyright (C) The IETF Trust (2008). Abstract IPv6 specifies a model of a subnet that is different than the IPv4 subnet model. The subtlety of the differences has turned out to cause interoperability problems. This note spells out the most important difference; that an IPv6 address isn't automatically associated with an IPv6 on-link subnet prefix. Singh, et al. Expires August 28, 2008 [Page 1] Internet-Draft IPv6 Subnet Model February 2008 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Host Behavior Rules . . . . . . . . . . . . . . . . . . . . . . 4 3. Security Considerations . . . . . . . . . . . . . . . . . . . . 5 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5 5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 5 6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 5 6.1. Normative References . . . . . . . . . . . . . . . . . . . 5 6.2. Informative References . . . . . . . . . . . . . . . . . . 5 Appendix A. CHANGE HISTORY . . . . . . . . . . . . . . . . . . . . 6 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 7 Intellectual Property and Copyright Statements . . . . . . . . . . 9 Singh, et al. Expires August 28, 2008 [Page 2] Internet-Draft IPv6 Subnet Model February 2008 1. Introduction In many, if not all, IPv4 implementations when an IPv4 address is assigned to an interface there is always a netmask associated with the address. That netmask together with the IPv4 address designates an on-link prefix. Addresses that match this prefix are viewed as local i.e., traffic to such addresses is not sent to a router. See section 3.3.1 in [RFC1122]. 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 corresponding on-link subnet prefixes, and conversely, can have on- link subnet prefixes that are not related to any of the IPv6 addresses that are assigned to the hosts. In IPv6, by default, a host treats only the link-local subnet as on- link. The reception of a Prefix Information Option (PIO) with the L-bit set and a non-zero valid lifetime creates (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 are on-link if they are the target of a Redirect Message indicating on-link, or the source of a Neighbor Solicitation or Neighbor Advertisement message. Note 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 such packets MUST be dropped. (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 this can result in lack of IPv6 connectivity. One example, included in draft-wbeebee-nd-implementation-problems-00 [I-D.wbeebee-nd-implementation-problems], follows: a host receives a Router Advertisement Message with no on-link prefix advertised. The host incorrectly decides to perform address resolution when the host should send all traffic to a default router. Neither the router nor any other host may respond to the address resolution, preventing this host from sending IPv6 traffic. Singh, et al. Expires August 28, 2008 [Page 3] Internet-Draft IPv6 Subnet Model February 2008 Correct implementation of the on-link determination is critically important in some deployments. For instance, with certain layer 2 technologies it is not possible or very inefficient for hosts to perform address resolution. It is much more efficient for the hosts to send all packets for non-link-local addresses to one of the default routers, and have the default routers forward those packets. 2. Host Behavior 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 does not imply that any prefix is on- link. A host is explicitly told that prefixes or addresses are on-link through the means specified in [RFC4861]. 3. On-link determination SHOULD NOT persist across IPv6 interface initializations. Note that section 5.7 of [RFC4862] describes the use of stable storage for addresses acquired with stateless address autoconfiguration with a note that the Preferred and Valid Lifetimes must be retained if this approach is used. However no RFC suggests or recommends retaining the on-link prefixes. 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 Singh, et al. Expires August 28, 2008 [Page 4] Internet-Draft IPv6 Subnet Model February 2008 be performed. This case is analogous to the behavior specified in the last paragraph of section 7.2.2 of [RFC4861]: when address resolution fails, the host SHOULD send an ICMPv6 Destination Unreachable message. 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. Security Considerations As this document merely restates and clarifies RFC 4861, it does not introduce any new security issues. 4. IANA Considerations None. 5. Acknowledgements Thanks (in alphabetical order) to Adeel Ahmed, Jari Arkko, Ralph Droms, Alun Evans, Dave Forster, Prashanth Krishnamurthy, Suresh Krishnan, Josh Littlefield, Madhu Sudan, Jinmei Tatuya, Bernie Volz, and Vlad Yasevich for their consistent input, ideas and review during the production of this document. 6. References 6.1. Normative References [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, September 2007. 6.2. Informative References [I-D.wbeebee-nd-implementation-problems] Singh, H. and W. Beebee, "Known ND Implementation Problems", draft-wbeebee-nd-implementation-problems-00 (work in progress), September 2007. Singh, et al. Expires August 28, 2008 [Page 5] Internet-Draft IPv6 Subnet Model February 2008 [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. [RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless Address Autoconfiguration", RFC 4862, September 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 since draft-wbeebee-on-and-off-link-determination-00.txt are: o Made global changes in document to replace RFC 2461 and RFC 2462 with RFC 4861 and RFC 4862 respectively. Removed text related to 2461bis-11 and 2462bis-08. o Inserted new bullet item to section 2 that explains off-link and on-link default behavior. o On-link behavior has been replaced with on-link determination. o At the end of sections 2.1 and 2.2.1, the last paragraph related to Redirects has been reworded to place more details in the Redirect section. o Section 2.2 has all text removed and then new text has been added. o The Redirect Clarifications section has been rewritten to explain an extra case when the Redirect does not include the Target Link- Layer Address Option. This section has been revised to restrict the scope of the Redirects sent from aggregation routers mentioned to those with on-link destinations. o Jinmei Tatuya has been added to the list of people in the Acknowledged section for his valuable feedback on the -00 draft. Singh, et al. Expires August 28, 2008 [Page 6] Internet-Draft IPv6 Subnet Model February 2008 o Two bis draft references in the References section have been removed. Changes since draft-wbeebee-on-and-off-link-determination-01.txt are: o Added a new author in Erik Nordmark to the draft. o Changed title of draft from "ND On-link and Off-link Determination" to "IPv6 Subnet Model". Also changed Abstract and Introduction sections to reflect new title. o Changed text of the example in Introduction section from "follows: a host receives an RA with no prefix advertised and incorrectly decides to perform address resolution when the host should have sent all traffic to the default router. The router does not respond to the address resolution and the layer 2 driver of the host stops transmitting IPv6 packets." to "follows: a host receives a Router Advertisement Message with no on-link prefix advertised. The host incorrectly decides to perform address resolution when the host should send all traffic to a default router. Neither the router nor any other host may respond to the address resolution, preventing this host from sending IPv6 traffic." o Removed sections 2.1-2.3 - folded information from these sections into Introduction section and bullets of section 2. o Removed sections 3 and 4 - folded subnet model and on-link determination related information from these sections into Introduction section and bullets of section 2. o Made changes to References sections. Removed RFC2472 from Normative References. Moved RFC4861 from Informative to Normative References. Added RFC1122 and RFC3315 to Informative References. Authors' Addresses Hemant Singh Cisco Systems, Inc. 1414 Massachusetts Ave. Boxborough, MA 01719 USA Phone: +1 978 936 1622 Email: shemant@cisco.com URI: http://www.cisco.com/ Singh, et al. Expires August 28, 2008 [Page 7] Internet-Draft IPv6 Subnet Model February 2008 Wes Beebee Cisco Systems, Inc. 1414 Massachusetts Ave. Boxborough, MA 01719 USA Phone: +1 978 936 2030 Email: wbeebee@cisco.com URI: http://www.cisco.com/ Erik Nordmark Sun Microsystems 17 Network Circle Menlo Park, CA 94025 USA Phone: +1 650 786 2921 Email: erik.nordmark@sun.com Singh, et al. Expires August 28, 2008 [Page 8] Internet-Draft IPv6 Subnet Model February 2008 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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Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Acknowledgment Funding for the RFC Editor function is provided by the IETF Administrative Support Activity (IASA). Singh, et al. Expires August 28, 2008 [Page 9]