IPv6 Working Group Nick 'Sharkey' Moore INTERNET-DRAFT Monash University CTIE 28 June 2004 Optimistic Duplicate Address Detection for IPv6 Status of this Memo By submitting this Internet-Draft, I certify that any applicable patent or other IPR claims of which I am aware have been disclosed, or will be disclosed, and any of which I become aware will be disclosed, in accordance with RFC 3668. 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. Copyright Notice Copyright (C) The Internet Society (2004). All Rights Reserved. Abstract Optimistic Duplicate Address Detection is an interoperable modification of the existing IPv6 Neighbour Discovery (RFC2461) and Stateless Address Autoconfiguration (RFC2462) process. The intention is to minimize address configuration delays in the successful case without greatly increasing disruption in the less likely failure case, and while remaining interoperable with unmodified nodes. Nick 'Sharkey' Moore Expires: 28 December 2004 [Page 1] INTERNET-DRAFT Optimistic DAD 28 June 2004 Table of Contents Status of this Memo ......................................... 1 Abstract .................................................... 1 Table of Contents ........................................... 2 1. Introduction ............................................. 3 1.1 Problem Statement ............................... 3 1.2 History ......................................... 4 1.3 Definitions ..................................... 4 1.4 Abbreviations ................................... 5 2. Optimistic Behaviours .................................... 6 2.1 Probability of Collision ........................ 6 2.2 Optimistic Address Flag ......................... 6 2.3 Avoiding Disruption ............................. 7 2.4 Rapid Establishment ............................. 7 2.5 Router Redirection .............................. 8 2.6 Improving Detection ............................. 8 3. Modifications to RFC-compliant behaviour ................. 9 3.1 Modifications to RFC 2461 Neighbour Discovery ... 9 3.2 Modifications to RFC 2462 SAA ................... 10 4. Protocol Operation ....................................... 11 4.1 Simple case ..................................... 11 4.2 Collision case .................................. 12 4.3 Interoperation cases ............................ 13 4.4 Pathological cases .............................. 13 5. Security Considerations .................................. 13 6. IANA Considerations ...................................... 14 Appendix A: Address Generation .............................. 14 Notes ....................................................... 14 Normative References ........................................ 15 Informative References ...................................... 15 Author's Address ............................................ 16 Acknowledgments ............................................. 16 Full Copyright Statment ..................................... 17 Intellectual Property Statement ............................. 17 Disclaimer of Validity ...................................... 17 Nick 'Sharkey' Moore Expires: 28 December 2004 [Page 2] INTERNET-DRAFT Optimistic DAD 28 June 2004 1. Introduction Optimistic Duplicate Address Detection (DAD) is a modification of the existing IPv6 Neighbour Discovery (ND) [RFC2461] and Stateless Address Autoconfiguration (SAA) [RFC2462] process. The intention is to minimize address configuration delays in the successful case, and to reduce disruption as far as possible in the failure case. Optimistic DAD is a useful optimization because DAD is far more likely to succeed than fail for a well-distributed random address [SOTO]. Disruption is minimized by limiting nodes' participation in Neighbour Discovery while their addresses are still Optimistic. It is not the intention of this draft to improve the security, reliability or robustness of DAD beyond that of existing standards, merely to provide a method to make it faster. 1.1 Problem Statement The existing IPv6 address configuration mechanisms provide adequate collision detection mechanisms for the static hosts they were designed for. However, a growing population of nodes need to maintain continuous network access despite frequently changing their network attachment. Optimizations to the DAD process are required to provide these nodes with sufficiently fast address configuration. An optimized DAD method needs to: * provide interoperability with nodes using the current standards. * remove the RetransTimer delay during address configuration. * ensure the probability of address collision is not increased. * improve the resolution mechanisms for address collisions. * minimize disruption in the case of a collision. It is not sufficient to merely reduce RetransTimer in order to reduce the handover delay, as values of RetransTimer long enough to guarantee detection of a collision are too long to avoid disruption of time-critical services. Nick 'Sharkey' Moore Expires: 28 December 2004 [Page 3] INTERNET-DRAFT Optimistic DAD 28 June 2004 1.2 History There is some precedent for this work in previous drafts [KOODLI], and in discussions in the MobileIP WG mailing list and at IETF-54. This version of Optimistic DAD differs somewhat from previous versions in that it uses no additional flags or message types beyond those already defined, therefore allowing interoperation between Optimistic and Standard nodes. Earlier versions of this work were presented by the author to the MobileIP WG at IETF-56, and to the IPv6 WG at IETF-59. Working implementations of draft versions of this memo have been made by the author as a freely-available patch to Linux 2.4.18, and by Ed Remmel of Elmic Systems. An implementation of this version by the author is in progress. 1.3 Definitions Definitions of requirements keywords ('MUST NOT', 'SHOULD NOT', 'MAY', 'SHOULD', 'MUST') are in accordance with the IETF Best Current Practice - RFC2119 [RFC2119] Tentative Address - an address for which a node has not yet completed DAD is regarded as Tentative: a single Neighbour Solicitation for this address or a single Neighbour Advertisement defending this address will cause the node to deconfigure the address and cease using it. Deprecated Address - an address which should not be used if an alternative is available. Preferred Address - an address which is neither Tentative or Deprecated. Optimistic Address - an address which is available for use despite DAD not being fully complete. This memo places restrictions on the use of Optimistic Addresses. A single Neighbour Advertisement for this address will cause the node to deconfigure the address and cease using it. Optimistic Addresses should also be regarded as Deprecated. Optimistic Node - An Optimistic Node assumes that DAD will succeed, and allows higher-layer communications on an address while DAD completes. Nick 'Sharkey' Moore Expires: 28 December 2004 [Page 4] INTERNET-DRAFT Optimistic DAD 28 June 2004 Standard Node - A Standard Node is one which is compliant with RFCs 2461 and 2462. Link - A communication facility or medium over which nodes can communicate at the link layer. Neighbours - Nodes on the same link, which may therefore be competing for the same addresses. 1.4 Abbreviations DAD - Duplicate Address Detection. Technique used for SAA. See [RFC2462] section 5.4. ICMP Redirect - See [RFC2461] section 4.5. NA - Neighbour Advertisement. See [RFC2461] sections 4.4 and 7. NC - Neighbour Cache. See [RFC2461] section 5.1 and 7.3. ND - Neighbour Discovery. The process described in [RFC2461] NS - Neighbour Solicitation. See [RFC2461] sections 4.3 and 7. ON - Optimistic Node. A node which is behaving according to the rules of this memo. RA - Router Advertisement. See [RFC2462] sections 4.2 and 6. RS - Router Solicitation. See [RFC2461] sections 4.1 and 6. SAA - Stateless Address Autoconfiguration. The process described in [RFC2462] SLLAO - Source Link Layer Address Option - an option to NS, RA and RS messages, which gives the link layer address of the source of the message. See [RFC2461] section 4.6.1. TLLAO - Target Link Layer Address Option - an option to ICMP redirect messages. See [RFC2461] sections 4.5 and 4.6.1. Nick 'Sharkey' Moore Expires: 28 December 2004 [Page 5] INTERNET-DRAFT Optimistic DAD 28 June 2004 2. Optimistic DAD Behaviours This section provides some discussion of Optimistic DAD Behaviours. Section 3 provides more specific information on changes to RFC- mandated behaviours. 2.1 Probability of Collision Optimistic DAD is only a useful optimization when the probability of collision is very small. As such, the Optimistic algorithm should not be used for manually assigned addresses, where the collision probability is likely to be much higher than that for random addresses due to human error. Modifications are required only to Optimistic nodes -- Optimistic nodes will interoperate with Standard nodes without significant advantage or incompatibility. 2.2 Optimistic Address Flag [RFC2462] introduces the concept of Tentative (in 5.4) and Deprecated (in 5.5.4) Addresses. Addresses which are neither are said to be Preferred. Tentative addresses may not be used for communication, and Deprecated addresses should not be used for new communications. These status flags may also be used by other standards documents, for example Default Address Selection [RFC3484] uses these flags. This draft introduces a new address state, 'Optimistic', which is used to mark an address which is available for use but which has not completed DAD. Protocols which do not understand this state should treat it equivalently to 'Deprecated', to indicate that the address is available for use but should not be used if another suitable address is available. If address states are recorded as individual flags, this can easily be achieved by setting 'Deprecated' when 'Optimistic' is set. When the DAD timer completes without incident, the address becomes a Preferred address. Nick 'Sharkey' Moore Expires: 28 December 2004 [Page 6] INTERNET-DRAFT Optimistic DAD 28 June 2004 2.3 Avoiding Disruption In order to avoid interference, it is important that an Optimistic node does not send any messages from an Optimistic Address which will override its neighbours' Neighbour Cache (NC) entries for the address it is trying to configure: doing so would disrupt the rightful owner of the address in the case of a collision. This is achieved by: * clearing the 'Override' flag in Neighbour Advertisements for Optimistic addresses, which prevents neighbours from overriding their existing NC entries. The 'Override' flag is already defined [RFC2461] and used for Proxy Neighbour Advertisement. * Never sending Neighbour Solicitations from an Optimistic Address. NSs include a Source Link Layer Address Option (SLLAO), which may cause Neighbour Cache disruption. NSs sent as part of DAD are sent from the unspecified address, without a SLLAO. * Never using a Optimistic Address as the source address of a Router Solicitation with an SLLAO. Another address, or the unspecified address, may be used, or the RS may be sent without an SLLAO. An address collision with a router may cause neighbouring router's IsRouter flags for that address to be cleared. However, routers do not appear to use the IsRouter flag for anything, and the NA sent in response to the collision will reassert the IsRouter flag. 2.4 Rapid Establishment It may be desirable for a Neighbour, for example the router, to rapidly establish communication with the newly configured Optimistic Node (ON). To do so, it must learn of the ON's arrival as soon as possible. To avoid having to wait for Neighbour Discovery, the ON may wish to send unsolicited Neighbour Advertisements (with the Override flag cleared if the address is still Optimistic), but for this to be effective the Neighbour must either: * be expecting the ON to arrive (eg: due to predictive mechanisms), and thus already have a NC entry for the peer, in state INCOMPLETE. * be willing to cache unsolicited NAs (for a short period of time), so that an entry will have been created with state STALE. Nick 'Sharkey' Moore Expires: 28 December 2004 [Page 7] INTERNET-DRAFT Optimistic DAD 28 June 2004 These modifications are beyond the scope of this memo. The ON may choose to send unsolicited NAs to the All Nodes Multicast, to the All Routers Multicast, or Unicast to the source of the RA which alerted it to the new prefix. This allows flexibility with regard to Layer 2 multicast transmission costs. The case where the ON wants to contact its router is handled by the SLLAO of the RA, where a SLLAO is supplied. However, the router may choose not to include the SLLAO (the example given in RFC2462 is "to facilitate in-bound load balancing over replicated interfaces"). In this case, the ON cannot discover its router until it is no longer Optimistic. 2.5 Router Redirection When the ON wants to contact another neighbour, but it cannot because the neighbour is not in its NC, it should instead forward the packet to the router, relying on the router to forward the packet. The router should then provide the ON with an ICMP redirect, which may include a Target Link Layer Address Option (TLLAO). If it does, this will update the ON's NC, and direct communication can begin. 2.6 Improving Detection Because Optimistic DAD allows nodes to communicate immediately, RetransTimer may be left at the default 1000ms without significant penalty. It is also possible to increase DupAddrDetectTransmits and thus reduce the probability of an undetected address collision due to packet loss. Nick 'Sharkey' Moore Expires: 28 December 2004 [Page 8] INTERNET-DRAFT Optimistic DAD 28 June 2004 3. Modifications to RFC-mandated behaviour 3.1 Modifications to RFC 2461 Neighbour Discovery * (modifies 6.3.7) A node MUST NOT send a Router Solicitation with an SLLAO from an Optimistic Address. Router Solicitations SHOULD be sent from a non-Optimistic or the Unspecified Address, however they MAY be sent from an Optimistic Address as long as the SLLAO is not included. * (modifies 7.2.2) A node MUST NOT use an Optimistic Address as the source address of a Neighbour Solicitation. * (modifies 7.2.2) When a node has a unicast packet to send from an Optimistic Address to a neighbour, but does not know the neighbour's link-layer address, it MUST NOT perform Neighbour Discovery but instead SHOULD forward the packet to the router of that network. * (adds to 7.2.6) The Optimistic node MAY send an unsolicited Neighbour Advertisement to All Nodes when it first configures an address. The Override flag on this advertisement MUST be cleared (O=0). * (adds to 7.2.6) The Optimistic node MAY send an unsolicited NA to All Nodes when it completes DAD. The Override flag on this advertisement SHOULD be set (O=1). Nick 'Sharkey' Moore Expires: 28 December 2004 [Page 9] INTERNET-DRAFT Optimistic DAD 28 June 2004 3.2 Modifications to RFC 2462 Stateless Address Autoconfiguration * (modifies 5.5) When an Optimistic node decides to configure an address, it generates a suffix and combines it with a prefix received from a Router Advertisement. Appendix A provides some suggestions for suffix generation. * (modifies 5.4) As soon as the initial Neighbour Solicitation (and optional unsolicited Neighbour Advertisement) is sent, the address is configured on the interface and available for use immediately. The address MUST be flagged as 'Optimistic'. Protocols which do not understand this state SHOULD treat it equivalently to 'Deprecated'. * When the DAD timer expires on an Optimistic Address, the 'Optimistic' flag MUST be cleared, and the address becomes a Preferred Address. * (modifies 5.4.3) A node MUST reply to a Neighbour Solicitation for its address from the unspecified address with a Neighbour Advertisement to the All Nodes address. If the solicitation is for an Optimistic Address, the reply MUST have the Override flag cleared (O=0). * (modifies 5.4.3) A node MUST reply to a Neighbour Solicitation for an Optimstic Address from a unicast address, but the reply MUST have the Override flag cleared (O=0). * (modifies 5.4.5) An Optimistic Address that is determined to be a duplicate MUST be deconfigured immediately. If the address is a link-local address formed from an interface identifier based on the hardware address (e.g. EUI-64), the interface SHOULD be disabled. Otherwise, if the address was automatically configured, DAD SHOULD be restarted with a new address. (Appendix A suggests methods for generating a new address) * DupAddrDetectTransmits SHOULD be increased where there is a significant probability of packet loss. Nick 'Sharkey' Moore Expires: 28 December 2004 [Page 10] INTERNET-DRAFT Optimistic DAD 28 June 2004 4. Protocol Operation The following cases all consider an Optimistic Node (ON) receiving a Router Advertisement containing a new prefix and deciding to autoconfigure a new address on that prefix. The following cases assume that the RA contains a SLLAO, for reasons explained in Section 2. The ON will immediately send out a Neighbour Solicitation to determine if its new address is already in use, and a Neighbour Advertisement (with the Override flag cleared) for the address. This NA allows communication with neighbours to begin immediately. 4.1 Simple case In the non-collision case, the address being configured by the new node is unused and not present in the Neighbour Caches of any of its neighbours. Therefore, there will be no response to its NS, and the NA with O=0 will be sufficient to create Neighbour Cache entries in already interested neighbours. The Optimistic Node already has the link-layer address of the router (from the RA), and the router either already knows the link-layer address of the ON from the unsolicited NA, or can determine it through standard NUD. Communications can begin as soon as the router and the ON have each others' link-layer addresses. After the appropriate DAD delay, the address's Optimistic flag is cleared and another NA is sent, this time with O=1. This will ensure that all Neighbour Caches are up-to-date. Nick 'Sharkey' Moore Expires: 28 December 2004 [Page 11] INTERNET-DRAFT Optimistic DAD 28 June 2004 4.2 Collision cases In the simplest collision case, the address being configured by the new node is already in use by another node, and present in the Neighbour Caches (NCs) of neighbours which are communicating with this node. Since the Optimistic advertisement has O=0, it will not override existing NC entries. An NA with O=0,S=0 and with a SLLAO may [Note 1], however cause the NC entry to be set to STALE, causing NUD to be performed on the address. Nodes with no interest in communicating with the new address "SHOULD" silently discard the NA [RFC2461 7.2.5], and so will likely be undisturbed. If a neighbour is just preparing to begin communication with the address, eg: it has a NC entry for the address in state 'INCOMPLETE', the optimistic advertisement may cause an incorrect NC entry to be created in state 'STALE' and queued packets to be sent to an incorrect destination. In general, the defending NA will have the Override flag set (O=1), and so this will correct the incorrect entry almost immediately. However, if the defending NA has the Override flag cleared (for example when the address is in use by proxy) the defending advertisement will not override this incorrect NC entry. In any case, the NC entry will remain in state 'STALE', and thus the disruption will be recoverable, albeit slowly, by the standard Neighbour Unreachability Detection mechanism. Of course, in the meantime the ON may have sent packets which identify it as the owner of its new Optimistic Address (for example, Binding Updates in [MIPV6]). This may incur some penalty to the ON, in the form of broken connections, and some penalty to the rightful owner of the address, since it will receive (and potentially reply to) the misdirected packets. It is for this reason that Optimistic DAD should only be used where the probability of collision is very low. Nick 'Sharkey' Moore Expires: 28 December 2004 [Page 12] INTERNET-DRAFT Optimistic DAD 28 June 2004 4.3 Interoperation cases Once the Optimistic Address has completed DAD, it acts exactly like a normal address, and so interoperation cases only arise while the address is Optimistic. If an Optimistic Node attempts to configure an address currently Tentatively assigned to a Standard Node, the Standard Node will see the Neighbour Solicitation and deconfigure the address. In contrast, if a node attempts to configure an Optimistic Node's Optimistic Address, the Optimistic Node will not deconfigure the address, and instead defend with a Neighbour Advertisement, causing the newcomer to reconfigure. This gives the Optimistic Node a slight advantage over Standard nodes, however this is justified since the Optimistic node may have already established connections to Optimistic Addresses. 4.4 Pathological cases Optimistic DAD suffers from similar problems to Standard DAD, for example duplicates are not guaranteed to be detected if packets are lost, and if two nodes configure simultaneously, they may each miss the other's NS. These problems exist, and are not gracefully recoverable, in Standard DAD. The probability of such a collision is reduced in Optimistic DAD due to the pair of messages (NS, NA) sent. The probability can be further reduced by increasing the RFC2462 DupAddrDetectTransmits variable to greater than 1. This version of Optimistic DAD is dependant on the details of the router behaviour, eg: if it includes SLLAOs in RAs, and if it is willing to redirect traffic for the ON. Where the router does not behave in this way, the behaviour of Optimistic DAD reverts to that of Standard DAD. 5. Security Considerations There are existing security concerns with Neighbour Discovery and Stateless Address Autoconfiguration, and this memo does not purport to fix them. However, this memo does not significantly increase security concerns either. Further work will be required to integrate Optimistic DAD with Secure Neighbour Discovery [SEND]. Nick 'Sharkey' Moore Expires: 28 December 2004 [Page 13] INTERNET-DRAFT Optimistic DAD 28 June 2004 6. IANA Considerations This document has no actions for IANA. Appendix A: Address Generation In order for Optimistic DAD to be a useful optimization, the probability of a collision must be very small, as a collision may cause temporary disruption to the collidee, and will require the collidor to reconfigure. Some interfaces (for example, Ethernet [RFC2464]) offer methods to create an address based on a globally unique Interface Identifier, however it is conceivable that due to manufacturer or user error that the generated address may not in fact be unique. An address generated in this manner may be used for a first attempt. If an interface does not offer such a method, or if a new addres needs to be created, an address should be created by an algorithm with a uniform distribution to minimize the chance of address collision. This algorithm could be a random number generator (see [RFC1750] for more information on random number generation), or a hash function as in [SEND-CGA] or one of those documented in [RFC3041]. A randomly generated address should have the Universal/Local bit and the Individual/Group bit set to 0 to indicate a Unicast address which is not globally unique (see [RFC3513]). The first time DAD fails, a new suffix is generated and the node can retry immediately. A delay of at least RETRANS_TIMER (as used in [RFC2461]) milliseconds can be introduced between further retries, to minimize the effect of DoS attacks. An exponential backoff should be used. Notes [Note 1] RFC 2461 is unclear on this, with [RFC2461 7.2.5] specifying "the advertisement prompts future Neighbour Unreachability Detection [...] by changing the state in the cache entry" whereas [RFC2461 Appendix C] specifies the state as "unchanged". Many arguments have been made on the list (see ) for one interpretation or the other. For the purposes of this memo, I have assumed that either behaviour is possible. This issue is to be addressed in RFC2461bis. Nick 'Sharkey' Moore Expires: 28 December 2004 [Page 14] INTERNET-DRAFT Optimistic DAD 28 June 2004 Normative References [RFC2119] S. Bradner. "Key words for use in RFCs to Indicate Requirement Levels." Request for Comments (Best Current Practice) 2119 (BCP 14), Internet Engineering Task Force, March 1997. [RFC3513] R. Hinden, S. Deering. "IP Version 6 Addressing Architecture." Request for Comments (Proposed Standard) 3513, Internet Engineering Task Force, April 2003. [RFC2461] T. Narten, E.Nordmark, W. Simpson. "Neighbor Discovery for IP Version 6 (IPv6)." Request for Comments (Draft Standard) 2461, Internet Engineering Task Force, December 1998. [RFC2462] S. Thomson, T. Narten. "IPv6 Stateless Address Autoconfiguration." Request for Comments (Draft Standard) 2462, Internet Engineering Task Force, December 1998. [RFC2464] M. Crawford. "Transmission of IPv6 Packets over Ethernet Networks." Request for Comments (Proposed Standard) 2464, Internet Engineering Task Force, December 1998. Informative References [RFC1750] D. Eastlake, S. Crocker, J. Schiller. "Randomness Recommendation for Security." Request for Comments (Informational) 1750, Internet Engineering Task Force, December 1994. [RFC3041] T. Narten, R. Draves. "Privacy Extensions for Stateless Address Autoconfiguration in IPv6." Request for Comments (Proposed Standard) 3041, Internet Engineering Task Force, January 2001. [RFC3484] R. Draves. "Default Address Selection for Internet Protocol version 6 (IPv6)". Request for Comments (Proposed Standard) 3484, Internet Engineering Task Force, February 2003. [MIPV6] D. Johnson, C. Perkins, J. Arkko. Mobility Support in IPv6, revision 24 (draft-ietf-mobileip-ipv6-24). June 2003 ... Expired December 2003. [KOODLI] R. Koodli, C. Perkins. Fast Handovers in Mobile IPv6, Nick 'Sharkey' Moore Expires: 28 December 2004 [Page 15] INTERNET-DRAFT Optimistic DAD 28 June 2004 revision 00 (draft-koodli-mobileip-fastv6-00). October 2000 ... Expired April 2001. [SOTO] M. Bagnulo, I. Soto, A. Garcia-Martinez, A. Azcorra. Random generation of interface identifiers, revision 00. (draft-soto- mobileip-random-iids-00). January 2002 ... Expired July 2002. [SEND] J. Arkko, J. Kempf, B. Sommerfeld, B.Zill, P. Nikander. SEcure Neighbor Discovery (SEND), revision 03. (draft-ietf- send-ndopt-03). January 2004 ... Expires July 2004. [SEND-CGA] T. Aura, Cryptographically Generated Addresses (CGA), revision 01. (draft-ietf-send-cga-01). August 1, 2003. Author's Address: Nick 'Sharkey' Moore or Centre for Telecommunications and Information Engineering Monash University 3800 Victoria, Australia Comments should be sent to either of the above email addresses. Acknowledgments Thanks to Greg Daley, Brett Pentland, Richard Nelson and Ahmet Sekercioglu at Monash Uni CTIE for their feedback and encouragement. More information is available at: Thanks to all the MobileIP and IPng/IPv6 WG members who have contributed to the debate. Especially and alphabetically: Jari Arkko, JinHyeock Choi, Youn-Hee Han, James Kempf, Thomas Narten, Richard Nelson, Pekka Nikander, Soohong 'Daniel' Park, Ed Remmel, Pekka Savola, Hesham Soliman, Ignatious Souvatzis, Jinmei Tatuya, Pascal Thubert, Vladislav Yasevich and Alper Yegin. This work has been supported by the Australian Telecommunications Cooperative Research Centre (ATcrc): Funding for the RFC Editor function is currently provided by the Internet Society. Nick 'Sharkey' Moore Expires: 28 December 2004 [Page 16] INTERNET-DRAFT Optimistic DAD 28 June 2004 Full Copyright Statement Copyright (C) The Internet Society (2004). 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. Intellectual Property Statement The IETF takes no position regarding the validity or scope of any intellectual property or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; neither does it represent that it has made any effort to identify any such rights. 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Disclaimer of Validity This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Nick 'Sharkey' Moore Expires: 28 December 2004 [Page 17]