Network Working Group H. Singh Internet-Draft W. Beebee Intended status: Standards Track Cisco Systems, Inc. Expires: March 4, 2008 September 2007 ND On-link and Off-link Determination draft-wbeebee-on-link-and-off-link-determination-00 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 March 4, 2008. Copyright Notice Copyright (C) The IETF Trust (2007). Abstract RFC 2461 [ND] describes host data forwarding and address resolution. However, nine years after the ND protocol became an RFC, IPv6 hosts still do not fully comply with RFC 2461 [ND]. In particular, hosts incorrectly implement on- vs. off-link data forwarding. This document clarifies host behavior and associated router behavior to define explicitly on-link and off-link determination. Singh & Beebee Expires March 4, 2008 [Page 1] Internet-Draft ND On-link Determination September 2007 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Host Models . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.1. RA Sets the M bit but does not Include the PIO . . . . . . 6 2.2. RA Advertises a Prefix with the On-link(L) Bit Set . . . . 6 2.2.1. When the Valid Lifetime Expires . . . . . . . . . . . 8 2.3. RA Advertises a Prefix with the On-link(L) Bit Clear . . . 8 3. Router Models . . . . . . . . . . . . . . . . . . . . . . . . 8 3.1. Aggregation Router Deployment Model . . . . . . . . . . . 9 4. Redirect Clarifications . . . . . . . . . . . . . . . . . . . 10 5. Security Considerations . . . . . . . . . . . . . . . . . . . 10 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10 8.1. Normative References . . . . . . . . . . . . . . . . . . . 10 8.2. Informative References . . . . . . . . . . . . . . . . . . 11 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11 Intellectual Property and Copyright Statements . . . . . . . . . . 13 Singh & Beebee Expires March 4, 2008 [Page 2] Internet-Draft ND On-link Determination September 2007 1. Introduction IPv6 host data forwarding and address resolution is complex. For example, RFC 2461 [ND] (section 3.1) implies that if the RA received by the host does not advertise any prefix, then the host must send all non-link-local data to the default router. This section of the RFC also implies that no address resolution is to be performed in this case. Sections 5.2 and 7.2.2 imply that the host performs address resolution before transmitting a packet if the destination of the packet is on the same link as the host. Some current host implementations perform address resolution in all cases even when the destination is not clearly on-link. However, RFC 2461 [ND] section 6.3.4 implies that hosts must clearly determine that a destination is on-link before performing address resolution. These implications in RFC 2461 [ND] need to be made explicit. Failure of host implementations to comply can result in lack of IPv6 connectivity. One example, included in draft-wbeebee-nd-implementation-problems-00 [I.D.nd-implementation-problems], 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. Host address resolution has implications for router design and deployment. First, host behavior is clarified in the Host Models section. Second, a router deployment model is described in the Router Models section. Third, Redirects are clarified for both routers and hosts in the Redirect Clarifications section. Where behavior has not changed between RFC 2461 [ND] and draft-ietf-ipv6-2461bis-11 [NDbis] and behavior has not changed between RFC 2462 [ADDRCONF] and draft-ietf-ipv6-rfc2462bis-08 [ADDRCONFbis], this document only refers to RFC 2461 [ND] and RFC 2462 [ADDRCONF] respectively. Where behavior has changed, this document refers to both the original and the new version. 2. Host Models A correctly implemented IPv6 host MUST adhere to the following rules: 1. On-link determination and addresses acquired using DHCPv6 SHOULD NOT persist across IPv6 interface initializations. Note that section 5.7 of draft-ietf-ipv6-rfc2462bis-08 [ADDRCONFbis] describes the use of stable storage for addresses acquired with stateless address autoconfiguration with a note that the Singh & Beebee Expires March 4, 2008 [Page 3] Internet-Draft ND On-link Determination September 2007 Preferred and Valid Lifetimes must be retained if this approach is used. 2. The on-link definition in section 2.1 of draft-ietf-ipv6-2461bis-11 [NDbis] describes the only means for on-link determination. DHCPv6 or any other configuration on the host MUST NOT be used for on-link determination. Manual configuration of a host introduces its own set of security considerations and is beyond the scope of this document. Note that the on-link definition as specified by draft-ietf-ipv6-2461bis-11 [NDbis] does not include manual configuration. 3. The host MUST NOT add a directly connected route to the prefix from an assigned address, independent of the information about the prefix received from the sources described in section 2.1 of draft-ietf-ipv6-2461bis-11 [NDbis]. 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 the Valid Lifetime expires, the host MUST then consider the prefix to be off-link, as suggested by the Prefix Information Option (PIO) paragraph of section 6.3.4 of draft-ietf-ipv6-2461bis-11 [NDbis]. However, if the RA advertises a prefix with the on-link bit set, the host MAY ignore the on-link indication from the RA and treat the prefix as off- link. Subsections which follow describe this behavior in further detail. 5. Newer host implementations MUST issue NS(DAD)s for all of its acquired unicast addresses except when the host interface has DupAddrDetectTransmits variable set to zero. Section 5.4 of RFC 2462 [ADDRCONF] erroneously relaxes this requirement and suffers from a manual configuration problem as illustrated by the following example: Host1 uses EUI-64 to configure a Link Local Address (LLA) using MAC1 and manually configures a Global Unicast Address (GUA) that is equal to an address configured using EUI-64 and MAC2. Host1 completes an NS(DAD) for both its LLA and GUA in compliance with RFC 2462 [ADDRCONF]. Then, Host2 uses EUI-64 to configure both a LLA and a GUA using MAC2. Host2 completes an NS(DAD) for the LLA and does not send an NS(DAD) for its GUA in compliance with RFC 2462 [ADDRCONF]. Now Host1 and Host2 have the same GUA on the same link. Therefore, this exception in section 5.4 of RFC 2462 [ADDRCONF] SHOULD be ignored. Note that draft-ietf-ipv6-rfc2462bis-08 Singh & Beebee Expires March 4, 2008 [Page 4] Internet-Draft ND On-link Determination September 2007 [ADDRCONFbis] refers to an extensibility concern with new implementations and states in section 5.4: "Whereas this document does not invalidate such implementations, this kind of 'optimization' is NOT RECOMMENDED, and new implementations MUST NOT do that optimization." 6. The host SHOULD issue only a single NS(DAD) for each address. The default value for DupAddrDetectTransmits variable is specified as one in section 5.1 of RFC 2462 [ADDRCONF]. Note, however, that link-specific documents can modify this default. For instance, PPP specifies DupAddrDetectTransmits to be zero in RFC 2472 [PPPv6]. 7. Newer implementations, which are compliant with draft-ietf-ipv6-rfc2461bis-11 [NDbis] MUST adhere to the following rules. Older implementations, which are compliant with RFC 2461 [ND] but not draft-ietf-ipv6-rfc2461bis-11 [NDbis] 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 the default router (since the Default Router List is empty), address resolution has failed. As specified in the last paragraph of section 7.2.2 of draft-ietf-ipv6-rfc2461bis-11 [NDbis], when address resolution fails, the host SHOULD send an ICMPv6 Destination Unreachable message. 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. draft-ietf-v6ops-onlinkassumption-04 [I.D.ietf-v6ops-onlinkassumptions] provides justification for these rules. The type of RA received can further determine host behavior. Singh & Beebee Expires March 4, 2008 [Page 5] Internet-Draft ND On-link Determination September 2007 2.1. RA Sets the M bit but does not Include the PIO Section 3.1 of RFC 2461 [ND] describes intended behavior when a host receives an RA without an advertised prefix: "Multiple prefixes can be associated with the same link. By default, hosts learn all on-link prefixes from Router Advertisements. However, routers may be configured to omit some or all prefixes from Router Advertisements. In such cases hosts assume that destinations are off-link and send traffic to routers. A router can then issue redirects as appropriate." An IPv6 router sends an RA with no prefix advertised and the M bit set, does not send any Redirects, nor any NA or ND messages for non- link local addresses. On receipt of the RA, the host uses DHCPv6 to acquire an IPv6 address. After completing IPv6 address acquisition, the host MUST obey RFC 2461 [ND], section 3.1. In this case, since the RA is the only authority to a host for on-link determination and this RA does not advertise any prefix, the host cannot determine that a destination is on-link. Therefore, the host MUST adhere to the following rules: 1. The host MUST NOT assume any default prefix length. 2. The host MUST send all non-link-local traffic to the default router. 3. The host MUST NOT issue an NS to resolve a destination other than a link-local address. In the presence of Redirects, only the on-link behavior of the destination addresses of the original packets for which the Redirects were sent change from what is specified in the rules above. These destination addresses are considered to be on-link and the host MAY now send non-link-local traffic destined to the destination addresses directly without sending it first to the default router. Since the Redirect contains all the information necessary to resolve the address of the destination host, the source host MUST NOT issue an NS to resolve a destination other than a link-local address. 2.2. RA Advertises a Prefix with the On-link(L) Bit Set Security consequences of RFC 2461 [ND] imply that hosts MAY send all traffic to the default router without performing address resolution first even when a PIO has been received advertising an on-link prefix, regardless of whether the host performs DHCPv6 and/or stateless autoconfiguration. Singh & Beebee Expires March 4, 2008 [Page 6] Internet-Draft ND On-link Determination September 2007 Section 4.6.2 of RFC 2461 [ND] defines the Valid Lifetime in the PIO as: "The length of time in seconds (relative to the time the packet is sent) that the prefix is valid for the purpose of on-link determination." Section 11 of RFC 2461 [ND] mentions the following denial of service attack: "An example of denial of service attacks is that a node on the link that can send packets with an arbitrary IP source address can both advertise itself as a default router and also send 'forged' Router Advertisement messages that immediately time out all other default routers as well as all on-link prefixes." The same security risk is also described in section 5.5.3 of RFC 2462 [ADDRCONF]. This section allows hosts to ignore the Valid Lifetime stored in an RA in order to prevent denial of service attacks. Section 6.3.4 of RFC 2461 [ND] mentions that hosts MAY send all traffic to the default router without performing address resolution first: "Stateless address autoconfiguration RFC 2462 [ADDRCONF] may in some circumstances increase the Valid Lifetime of a prefix or ignore it completely in order to prevent a particular denial of service attack. However, since the effect of the same denial of service targeted at the on-link prefix list is not catastrophic (hosts would send packets to a default router and receive a Redirect rather than sending packets directly to a neighbor) the Neighbor Discovery protocol does not impose such a check on the prefix lifetime values." Consider the following scenario with one rogue node and two other hosts on the same link. The rogue sends a malicious RA with an on- link prefix with a Valid Lifetime of zero. Host1 correctly implements section 5.5.3 of RFC 2462 [ADDRCONF] and resets its StoredLifetime (or RemainingLifetime in draft-ietf-ipv6-rfc2462bis-08 [ADDRCONFbis]) to two hours and avoids the denial of service attack. Host1 tries to send traffic to Host2, but cannot trust its own two hour StoredLifetime. For instance, a legitimate operator may have tried to time out the prefix due to an impending renumbering. Host1 decides to send all of its traffic to the on-link authority, the default router, even though the destination prefix is on-link. IF the host decides to send all traffic (including on-link traffic) to the default router, then the host MUST follow the following rule: Singh & Beebee Expires March 4, 2008 [Page 7] Internet-Draft ND On-link Determination September 2007 1. The host MUST NOT issue an NS to resolve a destination other than a link-local address. 2.2.1. When the Valid Lifetime Expires In the absence of other sources of on-link information, including Redirects, regardless of whether the host performs DHCPv6 and/or stateless autoconfiguration, the host MUST adhere to the following rules for addresses contained within the advertised prefix with the on-link bit set and an expired Valid Lifetime: 1. The host MUST NOT issue an NS to resolve a destination other than a link-local address. 2. The host MUST send all non-link-local traffic to the default router. In the presence of Redirects, only the on-link behavior of the destination addresses of the original packets for which the Redirects were sent change from what is specified in the rules above. These destination addresses are considered to be on-link and the host MAY now send non-link-local traffic destined to the destination addresses directly without sending it first to the default router. Since the Redirect contains all the information necessary to resolve the address of the destination host, the source host MUST NOT issue an NS to resolve a destination other than a link-local address. 2.3. RA Advertises a Prefix with the On-link(L) Bit Clear An on-link bit of clear indicates nothing regarding on-link determination. In section 6.3.4 of draft-ietf-ipv6-rfc2461bis-11 [NDbis]": "...a Prefix Information Option with on-link flag set to zero conveys no information concerning on-link determination and MUST NOT be interpreted to mean that addresses covered by the prefix are off-link.... Prefixes with the on-link flag set to zero would normally have the autonomous flag set and be used by [ADDRCONF]." 3. Router Models The Redirect Clarifications section clarifies RFC 2461 [ND] host and router behavior for an aggregation router deployment. The Aggregation Router Deployment Model section presents a possible aggregation router deployment model for IPv6 and discusses its properties with respect to ND. Aggregation routers can service more Singh & Beebee Expires March 4, 2008 [Page 8] Internet-Draft ND On-link Determination September 2007 than 100,000 subscribers. Due to scaling considerations, any NS for global address resolution from any host to any other host should not reach the aggregation router. 3.1. Aggregation Router Deployment Model A property of routed aggregation networks is that hosts cannot directly communicate with each other even if they share the same prefix. Physical connectivity between the aggregation router and the modems prevents hosts behind modems to communicate directly with each other. Hosts send their traffic to aggregation router. This design is motivated by scaling and security considerations. If every host could receive all traffic from every other host, then the subscriber's privacy would be violated and the amount of bandwidth available for each subscriber would be very small. That is why hosts communicate between each other through the aggregation router, which is also the IPv6 first-hop router. For scaling reasons, any NS to resolve any address other than that of the default router should not reach the aggregation router. +-----+ | | |Aggre+----(Rtr CPE)----Host1 Core----WAN----+gator| | Rtr | | +----(Br CPE)----(Cust Rtr)----Host2 +-----+ Figure 1. In the figure above, the customer premises equipment (CPE) is managed by the ISP and is deployed behind an aggregation router that is an IPv6 first-hop router and also a DHCPv6 relay agent. IPv6 CPEs are either IPv6 routers (Rtr CPE) or IPv6 bridges (Br CPE). If the customer premises uses a bridge CPE, then a router (Cust Rtr) is needed. All hosts reside behind a router CPE or a customer router. No NS to resolve any address other that that of the default router will reach the aggregation router from any device on the customer side of the aggregator. CPEs do not communicate with each other in this deployment model since a CPE does not run any applications that need to communicate with other CPEs. Hosts do communicate with each other, but every host is off-link to any other host on the aggregation router. Singh & Beebee Expires March 4, 2008 [Page 9] Internet-Draft ND On-link Determination September 2007 4. Redirect Clarifications Redirects are not sent by aggregation routers except when two hosts behind the same bridge CPE, with no router between the host and the aggregation router, communicate with each other. The aggregation router sends a Redirect to a source host which communicates with a destination host behind the same bridge CPE if the router can make a determination that the two hosts lie behind the same bridge CPE. Since the Redirect contains all the information necessary to resolve the address of the destination host, the source host MUST NOT issue an NS to resolve the destination contained within the Redirect. 5. Security Considerations The Host Models section of this document describes valid host behavior in response to a security threat where a rogue node can send RAs with a Valid Lifetime of zero. Host Models also describes a problem with section 5.4 of RFC 2462 [ADDRCONF] that can allow two hosts with the same address to avoid DAD and come online on the same link. 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, Madhu Sudan, Bernie Volz, and Vlad Yasevich for their consistent input, ideas and review during the production of this document. 8. References 8.1. Normative References [ADDRCONF] Thomson, S. and T. Narten, "IPv6 Address autoconfiguration (IPv6)", RFC 2462, December 1998. [ND] Narten, T., Nordmark, E., and W. Simpson, "Neighbor Discovery for IP Version 6 (IPv6)", RFC 2461, December 1998. Singh & Beebee Expires March 4, 2008 [Page 10] Internet-Draft ND On-link Determination September 2007 [PPPv6] Haskin, D. and E. Allen, "IP Version 6 over PPP", RFC 2472, December 1998. 8.2. Informative References [ADDRCONFbis] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Address autoconfiguration (IPv6)", draft-ietf-ipv6-rfc2462bis-08 (Work In Progress), May 2005. [I.D.ietf-v6ops-onlinkassumptions] Roy, S., Durand, A., and J. Paugh, "IPv6 Neighbor Discovery On-Link Assumption Considered Harmful (IPv6)", draft-ietf-v6ops-onlinkassumption-04 (Work In Progress), January 2007. [I.D.nd-implementation-problems] Singh, H. and W. Beebee, "Known ND Implementation Problems (IPv6)", draft-wbeebee-nd-implementation-problems-00 (Work In Progress), September 2007. [NDbis] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, "Neighbor Discovery for IP Version 6 (IPv6)", draft-ietf-ipv6-2461bis-11 (Work In Progress), March 2007. [SEND] Nikander, Ed., P., Kempf, J., and E. Nordmark, "IPv6 Neighbor Discovery (ND) Trust Models and Threats", RFC 3756, May 2004. 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 & Beebee Expires March 4, 2008 [Page 11] Internet-Draft ND On-link Determination September 2007 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/ Singh & Beebee Expires March 4, 2008 [Page 12] Internet-Draft ND On-link Determination September 2007 Full Copyright Statement Copyright (C) The IETF Trust (2007). 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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