Network Working Group S. Roy Internet-Draft Sun Microsystems, Inc. Expires: October 3, 2005 A. Durand
Expires: November 5, 2004Comcast Corporation J. Paugh Sun Microsystems, Inc. May 7, 2004April 2005 IPv6 Neighbor Discovery On-Link Assumption Considered Harmful draft-ietf-v6ops-onlinkassumption-02.txtdraft-ietf-v6ops-onlinkassumption-03.txt Status of this Memo This documentBy submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is an Internet-Draftaware have been or will be disclosed, and isany of which he or she becomes aware will be disclosed, in full conformanceaccordance with all provisions ofSection 106 of RFC2026.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. 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.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 5, 2004.October 3, 2005. Copyright Notice Copyright (C) The Internet Society (2004). All Rights Reserved.(2005). Abstract This document describes a change tothe IPv6 Neighbor Discoveryhistorical and background information behind the removal of the "on-link assumption" from the conceptual host sending algorithm.algorithm defined in Neighbor Discovery for IP Version 6 (IPv6). According to the algorithm,algorithm as originally described, when a host's default router list is empty, the host assumes that all destinations are on-link. This is particularly problematic with IPv6-capable nodes that do not have off-link IPv6 connectivity (e.g., no default router). This document describes how making this assumption causes problems, and describes how these problems outweigh the benefits of this part of the conceptual sending algorithm. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Background on the On-link Assumption . . . . . . . . . . . . . 3 3. Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 3.1 First Rule of Destination Address Selection . . . . . . . 3 3.2 Delays Associated with Address Resolution . . . . . . . . 4 3.3 Multi-homingMulti-interface Ambiguity . . . . . . . . . . . . . . . . . . 45 3.4 Security Related Issues . . . . . . . . . . . . . . . . . 5 4. ProposedChanges to RFC2461 . . . . . . . . . . . . . . . . . 5. . . . . 6 5. Security Considerations . . . . . . . . . . . . . . . . . . . 6 6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6 6.1 Normative References . . . . . . . . . . . . . . . . . . . .6 6.2 Informative References . . . . . . . . . . . . . . . . . . . 67 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 7 A. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 7 B. Changes from draft-ietf-v6ops-onlinkassumption-01draft-ietf-v6ops-onlinkassumption-02 . . . . . . 7 C. Changes from draft-ietf-v6ops-onlinkassumption-01 . . . . . . 8 D. Changes from draft-ietf-v6ops-onlinkassumption-00 . . . . . . 8 Intellectual Property and Copyright Statements . . . . . . . . 9 1. Introduction Neighbor Discovery for IPv6 [ND][I-D.ietf-ipv6-2461bis] defines a conceptual sending algorithm for hosts. ThisThe version of the algorithm described in [RFC2461] states that if a host's default router list is empty, then the host assumes that all destinations are on-link. This memo documents the removal of this assumption in the updated Neighbor Discovery specification [I-D.ietf-ipv6-2461bis], and describes the reasons why this assumption was removed. This assumption is problematic with IPv6-capable nodes that do not have off-link IPv6 connectivity. Specifically, itThis is typical when systems that have IPv6 enabled on their network interfaces (either on by default or administratively configured that way) are attached to networks that have no IPv6 services such as off-link routing. Such systems will resolve DNS names to AAAA and A records, and may attempt to connect to unreachable IPv6 off-link nodes. The on-link assumption creates problems for destination address selection as defined in [ADDRSEL],[RFC3484], and adds connection delays associated with unnecessary address resolution and neighbor unreachability detection. The behavior associated with the assumption is undefined in multihomed scenarios,on multi-interface nodes, and has some subtle security implications. All of these issues are discussed in this document. A revision of Neighbor Discovery [NDBIS] is removing the on-link assumption from the specification, but this memo gives historical reference and background to why this is has been a good decision.2. Background on the On-link Assumption This part of Neighbor Discovery's [ND][RFC2461] conceptual sending algorithm was created to facilitate communication on a single link between systems manually configured with different global prefixes. For example, consider the case where two systems on separate links are manually configured with global addresses, and are then plugged in back-to-back. They can still communicate with each other via their global addresses because they'll correctly assume that each is on-link. Without the on-link assumption, the above scenario wouldn't work as seamlessly. One workaroundwork, and the systems would beneed to use link-local addresses for this communication. Another isbe configured to configure new global addresses using the same /64share a common prefix on these systems, either by manually configuringsuch addresses or by placing a router on-link that advertises this prefix, however users may not have appropriate privileges or knowledge to implement this workaround.as the link-local prefix. 3. Problems The on-link assumption causes the following problems. 3.1 First Rule of Destination Address Selection Default Address Selection for IPv6 [ADDRSEL][RFC3484] defines a destination address selection algorithm that takes an unordered list of destination addresses as input, and produces a sorted list of destination addresses as output. The algorithm consists of destination address sorting rules, the first of which is "Avoid unusable destinations". The idea behind this rule is to place unreachable destinations at the end of the sorted list so that applications will be least likely to try to communicate with those addresses first. The on-link assumption could potentially cause false positives when attempting unreachability determination for this rule. On a network where there is no IPv6 router (all off-link IPv6 destinations are unreachable), the on-link assumption states that destinations are assumed to be on-link. An implementation could interpret that as, if the default router list is empty, then all destinations are reachable.reachable on-link. This causesmay cause the rule to not necessarilyprefer reachable IPv4 destinations overan unreachable IPv6 destinations, resulting in unreachable destinations being placed at the front of the sorted list.destination over a reachable IPv4 destination. 3.2 Delays Associated with Address Resolution Users expect that applications quickly connect to a given destination regardless of the number of IP addresses assigned to that destination. If a destination name resolves to multiple addresses and the application attempts to communicate to each address until one succeeds, this process shouldn't take an unreasonable amount of time. It is therefore important that the system quickly determine if IPv6 destinations are unreachable so that the application can try other destinations when those IPv6 destinations are unreachable. For an IPv6 enabled host deployed on a network that has no IPv6 routers, the result of the on-link assumption is that link-layer address resolution must be performed on all IPv6 addresses to which the host sends packets. The Application will not receive acknowledgment of the unreachability of destinations that are not on-linkon- link until at least address resolution has failed, which is no less than three seconds (MAX_MULTICAST_SOLICIT * RETRANS_TIMER) (amplifiedRETRANS_TIMER). This is greatly amplified by transport protocol delays).delays. For example, [RFC1122] requires that TCP retransmit for at least 3 minutes before aborting the connection attempt. When the application has a large list of off-link unreachable IPv6 addresses followed by at least one reachable IPv4 address, the delay associated with Neighbor Unreachability Detection (NUD) of each IPv6 addresses before successful communication with the IPv4 address is unacceptable. 3.3 Multi-homingMulti-interface Ambiguity There is no defined way to implement this aspect of the sending algorithm on a multi-homed node.node that is attached to multiple links. From an implementor's point of view, there are three ways to handle sending an IPv6 packet to a destination in the face of the on-link assumption on a multi-homedmulti-interface node: 1. Attempt to resolve the destination on a single link. 2. Attempt to resolve the destination on every link. 3. Drop the packet. If the destination is indeed on-link, the first option might not succeed since the wrong link could be picked. The second option might succeed in reaching a destination (assuming that one is reachable) but is more complex to implement, and isn't guaranteed to pick the correct destination. For example, there is still ambiguity about which link to use if more than one node answers the solicitations on multiple links. Dropping the packet is equivalent to not making the on-link assumption at all. In other words, if there is no route to the destination, don't attempt to send the packet. 3.4 Security Related Issues The on-link assumption discussed here introduces a security vulnerability to the Neighbor Discovery protocol described in section 4.2.2 of IPv6 Neighbor Discovery Trust Models and Threats [PSREQ][RFC3756] titled "Default router is 'killed'". There is a threat that a host's router can be maliciously killed in order to cause the host to start sending all packets on-link. The attacker can then spoof off-link nodes by sending packets on the same link as the host. The vulnerability is discussed in detail in [PSREQ].[RFC3756]. Another security related side-effect of the on-link assumption has to do with virtual private networks (VPN's). It has been observed that some commercially available VPN software solutions that don't support IPv6 send IPv6 packets to the local media in the clear (their security policy doesn't simply drop IPv6 packets). Consider a scenario where a system has a single Ethernet interface with VPN software that encrypts and encapsulates certain packets. The system attempts to send a packet to an IPv6 destination that it obtained by doing a DNS lookup, and the packet ends up going in the clear to the local media. A malicious secondthird party could then spoof the destination on-link. 4. ProposedChanges to RFC2461 This document suggests theThe following changes have been made to the Neighbor Discovery [ND] specification:specification between [RFC2461] and [I-D.ietf-ipv6-2461bis]: The last sentence of the second paragraph of section 5.2 ("Conceptual Sending Algorithm") should bewas removed. This sentence is currently,was, "If the Default Router List is empty, the sender assumes that the destination is on-link.on-link." Bullet item 3) in section 6.3.6 ("Default Router Selection") should bewas removed. The item currently reads,read, "If the Default Router List is empty, assume that all destinations are on-link as specified in Section 5.2." APPENDIX A was modified to remove on-link assumption related text in bullet item 1) under the discussion on what happens when a multihomed host fails to receive Router Advertisements. The result of these changes is that destinations are considered unreachable when there is no routing information for that destination (through a default router or otherwise). Instead of attempting link-layerlink- layer address resolution when sending to such a destination, a node should send an ICMPv6 Destination Unreachable message (code 0 - no route to destination) message up the stack. 5. Security Considerations The removal of the on-link assumption from Neighbor Discovery removes some security related vulnerabilities of the protocol as described in Section 3.4. 6. References 6.1 Normative References [ADDRSEL] Draves, R., "Default Address Selection[I-D.ietf-ipv6-2461bis] Narten, T., "Neighbor Discovery for Internet ProtocolIP version 6 (IPv6)", RFC 3484,draft-ietf-ipv6-2461bis-02 (work in progress), February 2003. [ND]2005. [RFC1122] Braden, R., "Requirements for Internet Hosts - Communication Layers", STD 3, RFC 1122, October 1989. [RFC2461] Narten, T., Nordmark, E.E., and W. Simpson, "Neighbor Discovery for IP Version 6 (IPv6)", RFC 2461, December 1998. [PSREQ][RFC3484] Draves, R., "Default Address Selection for Internet Protocol version 6 (IPv6)", RFC 3484, February 2003. [RFC3756] Nikander, P., Kempf, J.J., and E. Nordmark, "IPv6 Neighbor Discovery trust models(ND) Trust Models and threats", October 2003. draft-ietf-send-psreq-04Threats", RFC 3756, May 2004. 6.2 Informative References [AUTOCONF][RFC2462] Thomson, S. and T. Narten, "IPv6 Stateless Address Autoconfiguration", RFC 2462, December 1998. [NDBIS] Narten, T., Nordmark, E., Simpson, W., Soliman, H. and J. Tatuya, "Neighbor Discovery for IP Version 6 (IPv6)", February 2004. draft-soliman-ipv6-2461-bis-01Authors' Addresses Sebastien Roy Sun Microsystems, Inc. 1 Network Drive UBUR02-212 Burlington, MA 01801 EMail:Email: firstname.lastname@example.org Alain Durand Sun Microsystems, Inc. 17 Network Circle UMPK17-202 Menlo Park, CA 94025 EMail: email@example.comComcast Corporation 1500 Market St. Philadelphia, PA 09102 Email: firstname.lastname@example.org James Paugh Sun Microsystems, Inc. 17 Network Circle UMPK17-202 Menlo Park, CA 94025 EMail: email@example.comEmail: firstname.lastname@example.org Appendix A. Acknowledgments The authors gratefully acknowledge the contributions of Jim Bound, Tony Hain, Mika Liljeberg, Erik Nordmark, Pekka Savola, and Ronald van der Pol. Appendix B. Changes from draft-ietf-v6ops-onlinkassumption-02 o Changed abstract to reflect the historical nature of this document. o Changed the introduction to stress that this is historical information documenting the removal of the on-link assumption from the ND spec. o Added text to the introduction stating that the assumption is a problem for nodes with IPv6 on by default. o Added mention to RFC1122 in section 3.2. o Changed use of the term multi-homed nodes to "nodes that are attached to multiple links". o Changed section 4 from "Proposed Changes" to "Changes" and adjusted included text to reflect that the changes have been made. Appendix C. Changes from draft-ietf-v6ops-onlinkassumption-01 o Added text in the Introduction stating that rfc2461bis has removed the on-link assumption, and that this memo gives the historical reference and background for its removal. o Stated in Section 2 that users may not have sufficient privileges or knowledge to manually configure addresses or routers in order to work-around the lack of an on-link assumption. o Removed implementation details of the on-link assumption from Section 3.1. o Miscellaneous editorial changes. Appendix C.D. Changes from draft-ietf-v6ops-onlinkassumption-00 o Clarified in the abstract and introduction that the problem is with systems that are IPv6 enabled but have no off-link connectivity. o In Section 3.3, clarified that soliciting on all links could have ambiguous results. o The old Security Considerations section was moved to Section 3.4, and the new Security Considerations section refers to that new section. o Miscellaneous editorial changes. 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