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Network Working Group                                           N. MooreRequest for Comments: 4429                        Monash University CTIECategory: Standards Track                                     April 2006Optimistic Duplicate Address Detection (DAD) for IPv6Status of This Memo   This document specifies an Internet standards track protocol for the   Internet community, and requests discussion and suggestions for   improvements.  Please refer to the current edition of the "Internet   Official Protocol Standards" (STD 1) for the standardization state   and status of this protocol.  Distribution of this memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (2006).Abstract   Optimistic Duplicate Address Detection is an interoperable   modification of the existing IPv6 Neighbor Discovery (RFC 2461) and   Stateless Address Autoconfiguration (RFC 2462) processes.  The   intention is to minimize address configuration delays in the   successful case, to reduce disruption as far as possible in the   failure case, and to remain interoperable with unmodified hosts and   routers.Moore                       Standards Track                     [Page 1]

RFC 4429                     Optimistic DAD                   April 2006Table of Contents1. Introduction ....................................................31.1. Problem Statement ..........................................31.2. Definitions ................................................41.3. Address Types ..............................................41.4. Abbreviations ..............................................52. Optimistic DAD Behaviors ........................................62.1. Optimistic Addresses .......................................62.2. Avoiding Disruption ........................................62.3. Router Redirection .........................................72.4. Contacting the Router ......................................73. Modifications to RFC-Mandated Behavior ..........................83.1. General ....................................................83.2. Modifications toRFC 2461 Neighbor Discovery ...............8      3.3. Modifications toRFC 2462 Stateless Address           Autoconfiguration ..........................................94. Protocol Operation .............................................104.1. Simple Case ...............................................104.2. Collision Case ............................................104.3. Interoperation Cases ......................................114.4. Pathological Cases ........................................115. Security Considerations ........................................12Appendix A. Probability of Collision ..............................13A.1. The Birthday Paradox ......................................13A.2. Individual Moving Nodes ...................................14   Normative References ..............................................15   Informative References ............................................15   Acknowledgements ..................................................16Moore                       Standards Track                     [Page 2]

RFC 4429                     Optimistic DAD                   April 20061.  Introduction   Optimistic Duplicate Address Detection (DAD) is a modification of the   existing IPv6 Neighbor Discovery (ND) [RFC2461] and Stateless Address   Autoconfiguration (SLAAC) [RFC2462] processes.  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 in most cases DAD is   far more likely to succeed than fail.  This is discussed further inAppendix A.  Disruption is minimized by limiting nodes' participation   in Neighbor Discovery while their addresses are still Optimistic.   It is not the intention of this memo to improve the security,   reliability, or robustness of DAD beyond that of existing standards,   but 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 fixed 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 that 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.Moore                       Standards Track                     [Page 3]

RFC 4429                     Optimistic DAD                   April 20061.2.  Definitions   Definitions of requirements keywords ('MUST NOT', 'SHOULD NOT',   'MAY', 'SHOULD', 'MUST') are in accordance with the IETF Best Current   Practice,RFC 2119 [RFC2119]   Address Resolution - Process defined by[RFC2461], section 7.2.   Neighbor Unreachability Detection (NUD) - Process defined by[RFC2461], section 7.3.   Standard Node - A Standard Node is one that is compliant with        [RFC2461] and [RFC2462].   Optimistic Node (ON) - An Optimistic Node is one that is compliant        with the rules specified in this memo.   Link - A communication facility or medium over which nodes can        communicate at the link layer.   Neighbors - Nodes on the same link, which may therefore be competing        for the same IP addresses.1.3.  Address Types   Tentative address (as per [RFC2462]) - an address whose uniqueness on        a link is being verified, prior to its assignment to an        interface.  A Tentative address is not considered assigned to an        interface in the usual sense.  An interface discards received        packets addressed to a Tentative address, but accepts Neighbor        Discovery packets related to Duplicate Address Detection for the        Tentative address.   Optimistic address - an address that is assigned to an interface and        available for use, subject to restrictions, while its uniqueness        on a link is being verified.  This memo introduces the        Optimistic state and defines its behaviors and restrictions.   Preferred address (as per [RFC2462]) - an address assigned to an        interface whose use by upper-layer protocols is unrestricted.        Preferred addresses may be used as the source (or destination)        address of packets sent from (or to) the interface.Moore                       Standards Track                     [Page 4]

RFC 4429                     Optimistic DAD                   April 2006   Deprecated address (as per [RFC2462]) - An address assigned to an        interface whose use is discouraged, but not forbidden.  A        Deprecated address should no longer be used as a source address        in new communications, but packets sent from or to Deprecated        addresses are delivered as expected.  A Deprecated address may        continue to be used as a source address in communications where        switching to a Preferred address causes hardship to a specific        upper-layer activity (e.g., an existing TCP connection).1.4.  Abbreviations   DAD - Duplicate Address Detection.  Technique used for SLAAC.  See[RFC2462], section 5.4.   ICMP Redirect - See[RFC2461], section 4.5.   NA - Neighbor Advertisement.  See [RFC2461], sections4.4 and7.   NC - Neighbor Cache.  See [RFC2461], sections5.1 and7.3.   ND - Neighbor Discovery.  The process described in [RFC2461].   NS - Neighbor Solicitation.  See [RFC2461], sections4.3 and7.   RA - Router Advertisement.  See [RFC2462], sections4.2 and6.   RS - Router Solicitation.  See [RFC2461], sections4.1 and6.   SLAAC - 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 and Neighbor Advertisements.  See [RFC2461], sections        4.4, 4.5, and 4.6.1.Moore                       Standards Track                     [Page 5]

RFC 4429                     Optimistic DAD                   April 20062.  Optimistic DAD Behaviors   This non-normative section discusses Optimistic DAD behaviors.2.1.  Optimistic Addresses   [RFC2462] introduces the concept of Tentative (in 5.4) and Deprecated   (in 5.5.4) addresses.  Addresses that 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 address states may also be used by other standards documents,   for example, Default Address Selection [RFC3484].   This memo introduces a new address state, 'Optimistic', that is used   to mark an address that is available for use but that has not   completed DAD.   Unless noted otherwise, components of the IPv6 protocol stack should   treat addresses in the Optimistic state equivalently to those in the   Deprecated state, indicating that the address is available for use   but should not be used if another suitable address is available.  For   example, Default Address Selection [RFC3484] uses the address state   to decide which source address to use for an outgoing packet.   Implementations should treat an address in state Optimistic as if it   were in state Deprecated.  If address states are recorded as   individual flags, this can easily be achieved by also setting   'Deprecated' when 'Optimistic' is set.   It is important to note that the address lifetime rules of [RFC2462]   still apply, and so an address may be Deprecated as well as   Optimistic.  When DAD completes without incident, the address becomes   either a Preferred or a Deprecated address, as per [RFC2462].2.2.  Avoiding Disruption   In order to avoid interference, it is important that an Optimistic   Node does not send any messages from an Optimistic Address that will   override its neighbors' Neighbor 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 Neighbor Advertisements for        Optimistic Addresses, which prevents neighbors from overriding        their existing NC entries.  The 'Override' flag is already        defined [RFC2461] and used for Proxy Neighbor Advertisement.Moore                       Standards Track                     [Page 6]

RFC 4429                     Optimistic DAD                   April 2006   * Never sending Neighbor Solicitations from an Optimistic Address.        NSes include a Source Link-Layer Address Option (SLLAO), which        may cause Neighbor Cache disruption.  NSes sent as part of DAD        are sent from the unspecified address, without a SLLAO.   * Never using an Optimistic Address as the source address of a Router        Solicitation with a SLLAO.  Another address, or the unspecified        address, may be used, or the RS may be sent without a SLLAO.   An address collision with a router may cause a neighboring 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.2.3.  Router Redirection   Neighbor Solicitations cannot be sent from Optimistic Addresses, and   so an ON cannot directly contact a neighbor that is not already in   its Neighbor Cache.  Instead, the ON forwards packets via its default   router, relying on the router to forward the packets to their   destination.  In accordance withRFC 2461, 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.  If it does not, packets   continue to be forwarded via the router until the ON has a non-   Optimistic address from which to send an NS.2.4.  Contacting the Router   Generally, an RA will include a SLLAO, however this "MAY be omitted   to facilitate in-bound load balancing over replicated interfaces"   [RFC2461].  A node with only Optimistic Addresses is unable to   determine the router's Link-Layer Address as it can neither send an   RS to request a unicast RA, nor send an NS to request an NA.  In this   case, the ON will be unable to communicate with the router until at   least one of its addresses is no longer Optimistic.Moore                       Standards Track                     [Page 7]

RFC 4429                     Optimistic DAD                   April 20063.  Modifications to RFC-Mandated Behavior   All normative text in this memo is contained in this section.3.1.  General   * Optimistic DAD SHOULD only be used when the implementation is aware        that the address is based on a most likely unique interface        identifier (such as in [RFC2464]), generated randomly [RFC3041],        or by a well-distributed hash function [RFC3972] or assigned by        Dynamic Host Configuration Protocol for IPv6 (DHCPv6) [RFC3315].        Optimistic DAD SHOULD NOT be used for manually entered        addresses.3.2.  Modifications toRFC 2461 Neighbor Discovery   * (modifiessection 6.3.7)  A node MUST NOT send a Router        Solicitation with a 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.   * (modifiessection 7.2.2)  A node MUST NOT use an Optimistic Address        as the source address of a Neighbor Solicitation.   * If the ON isn't told the SLLAO of the router in an RA, and it        cannot determine this information without breaching the rules        above, it MUST leave the address Tentative until DAD completes        despite being unable to send any packets to the router.   * (modifiessection 7.2.2)  When a node has a unicast packet to send        from an Optimistic Address to a neighbor, but does not know the        neighbor's link-layer address, it MUST NOT perform Address        Resolution.  It SHOULD forward the packet to a default router on        the link in the hope that the packet will be redirected.        Otherwise, it SHOULD buffer the packet until DAD is complete.Moore                       Standards Track                     [Page 8]

RFC 4429                     Optimistic DAD                   April 20063.3 Modifications toRFC 2462 Stateless Address Autoconfiguration   * (modifiessection 5.5) A host MAY choose to configure a new address        as an Optimistic Address.  A host that does not know the SLLAO        of its router SHOULD NOT configure a new address as Optimistic.        A router SHOULD NOT configure an Optimistic Address.   * (modifiessection 5.4.2) The host MUST join the all-nodes multicast        address and the solicited-node multicast address of the        Tentative address.  The host SHOULD NOT delay before sending        Neighbor Solicitation messages.   * (modifiessection 5.4) The Optimistic Address is configured and        available for use on the interface immediately.  The address        MUST be flagged as 'Optimistic'.   * When DAD completes for an Optimistic Address, the address is no        longer Optimistic and it becomes Preferred or Deprecated        according to the rules ofRFC 2462.   * (modifiessection 5.4.3) The node MUST NOT reply to a Neighbor        Solicitation for an Optimistic Address from the unspecified        address.  Receipt of such an NS indicates that the address is a        duplicate, and it MUST be deconfigured as per the behaviour        specified inRFC 2462 for Tentative addresses.   * (modifiessection 5.4.3) The node MUST reply to a Neighbor        Solicitation for an Optimistic Address from a unicast address,        but the reply MUST have the Override flag cleared (O=0).Moore                       Standards Track                     [Page 9]

RFC 4429                     Optimistic DAD                   April 20064.  Protocol Operation   This non-normative section provides clarification of the interactions   between Optimistic Nodes, and between Optimistic Nodes and Standard   Nodes.   The following cases all consider an Optimistic Node (ON) receiving a   Router Advertisement containing a new prefix and deciding to   autoconfigure a new Optimistic Address on that prefix.   The ON will immediately send out a Neighbor Solicitation to determine   if its new Optimistic Address is already in use.4.1.  Simple Case   In the non-collision case, the Optimistic Address being configured by   the new node is unused and not present in the Neighbor Caches of any   of its neighbors.   There will be no response to its NS (sent from ::), and this NS will   not modify the state of neighbors' Neighbor Caches.   The ON already has the link-layer address of the router (from the   RA), and the router can determine the link-layer address of the ON   through standard Address Resolution.  Communications can begin as   soon as the router and the ON have each other's link-layer addresses.   After the appropriate DAD delay has completed, the address is no   longer Optimistic, and becomes either Preferred or Deprecated as perRFC 2462.4.2.  Collision Case   In the collision case, the Optimistic Address being configured by the   new node is already in use by another node, and present in the   Neighbor Caches (NCs) of neighbors that are communicating with this   node.   The NS sent by the ON has the unspecified source address, ::, and no   SLLAO.  This NS will not cause changes to the NC entries of   neighboring hosts.   The ON will hopefully already know all it needs to about the router   from the initial RA.  However, if it needs to it can still send an RS   to ask for more information, but it may not include a SLLAO.  This   forces an all-nodes multicast response from the router, but will not   disrupt other nodes' NCs.Moore                       Standards Track                    [Page 10]

RFC 4429                     Optimistic DAD                   April 2006   In the course of establishing connections, the ON might have sent NAs   in response to received NSes.  Since NAs sent from Optimistic   Addresses have O=0, they will not have overridden existing NC   entries, although they may have resulted in a colliding entry being   changed to state STALE.  This change is recoverable through standard   NUD.   When an NA is received from the collidee defending the address, the   ON immediately stops using the address and deconfigures it.   Of course, in the meantime the ON may have sent packets that identify   it as the owner of its new Optimistic Address (for example, Binding   Updates in Mobile IPv6 [RFC3775]).  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 be used only where the probability of collision   is very low.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 ON attempts to configure an address currently Tentatively   assigned to a Standard Node, the Standard Node will see the Neighbor   Solicitation and deconfigure the address.   If a node attempts to configure an ON's Optimistic Address, the ON   will see the NS and deconfigure the address.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.   These problems exist, and are not gracefully recoverable, in Standard   DAD.  Their probability in both Optimistic and Standard DAD can be   reduced by increasing theRFC 2462 DupAddrDetectTransmits variable to   greater than 1.   This version of Optimistic DAD is dependent on the details of the   router behavior, e.g., that the router includes SLLAOs in RAs and   that the router is willing to redirect traffic for the ON.  Where the   router does not behave in this way, the behavior of Optimistic DAD   inherently reverts to that of Standard DAD.Moore                       Standards Track                    [Page 11]

RFC 4429                     Optimistic DAD                   April 20065.  Security Considerations   There are existing security concerns with Neighbor Discovery and   Stateless Address Autoconfiguration, and this memo does not purport   to fix them.  However, this memo does not significantly increase   security concerns either.   Secure Neighbor Discovery (SEND) [RFC3971] provides protection   against the threats to Neighbor Discovery described in [RFC3756].   Optimistic Duplicate Address Detection does not introduce any   additional threats to Neighbor Discovery if SEND is used.   Optimistic DAD takes steps to ensure that if another node is already   using an address, the proper link-layer address in existing Neighbor   Cache entries is not replaced with the link-layer address of the   Optimistic Node.  However, there are still scenarios where incorrect   entries may be created, if only temporarily.  For example, if a   router (while forwarding a packet) sends out a Neighbor Solicitation   for an address, the Optimistic Node may respond first, and if the   router has no pre-existing link-layer address for that IP address, it   will accept the response and (incorrectly) forward any queued packets   to the Optimistic Node.  The Optimistic Node may then respond in an   incorrect manner (e.g., sending a TCP RST in response to an unknown   TCP connection).  Such transient conditions should be short-lived, in   most cases.   Likewise, an Optimistic Node can still inject IP packets into the   Internet that will in effect be "spoofed" packets appearing to come   from the legitimate node.  In some cases, those packets may lead to   errors or other operational problems, though one would expect that   upper-layer protocols would generally treat such packets robustly, in   the same way they must treat old and other duplicate packets.Moore                       Standards Track                    [Page 12]

RFC 4429                     Optimistic DAD                   April 2006Appendix A.  Probability of Collision   In assessing the usefulness of Duplicate Address Detection, the   probability of collision must be considered.  Various mechanisms such   as SLAAC [RFC2462] and DHCPv6 [RFC3315] attempt to guarantee the   uniqueness of the address.  The uniqueness of SLAAC depends on the   reliability of the manufacturing process (so that duplicate L2   addresses are not assigned) and human factors if L2 addresses can be   manually assigned.  The uniqueness of DHCPv6-assigned addresses   relies on the correctness of implementation to ensure that no two   nodes can be given the same address.   "Privacy Extensions to SLAAC" [RFC3041] avoids these potential error   cases by picking an Interface Identifier (IID) at random from 2^62   possible 64-bit IIDs (allowing for the reserved U and G bits).  No   attempt is made to guarantee uniqueness, but the probability can be   easily estimated, and as the following discussion shows, probability   of collision is exceedingly small.A.1.  The Birthday Paradox   When considering collision probability, the Birthday Paradox is   generally mentioned.  When randomly selecting k values from n   possibilities, the probability of two values being the same is:           Pb(n,k) = 1-( n! / [ (n-k)! . n^k] )   Calculating the probability of collision with this method is   difficult, however, as one of the terms is n!, and (2^62)! is an   unwieldy number.  We can, however, calculate an upper bound for the   probability of collision:           Pb(n,k) <= 1-( [(n-k+1)/n] ^ [k-1] )   which lets us calculate that even for large networks the probability   of any two nodes colliding is very small indeed:           Pb(2^62,    500) <= 5.4e-14           Pb(2^62,   5000) <= 5.4e-12           Pb(2^62,  50000) <= 5.4e-10           Pb(2^62, 500000) <= 5.4e-08   The upper-bound formula used above was taken from "Random Generation   of Interface Identifiers", by M. Bagnulo, I. Soto, A. Garcia-   Martinez, and A. Azcorra, and is used with the kind permission of the   authors.Moore                       Standards Track                    [Page 13]

RFC 4429                     Optimistic DAD                   April 2006A.2.  Individual Nodes   When considering the effect of collisions on an individual node, we   do not need to consider the Birthday Paradox.  When a node moves into   a network with K existing nodes, the probability that it will not   collide with any of the distinct addresses in use is simply 1-K/N.   If it moves to such networks M times, the probability that it will   not cause a collision on any of those moves is (1-K/N)^M; thus, the   probability of it causing at least one collision is:           Pc(n,k,m) = 1-[(1-k/n)^m]   Even considering a very large number of moves (m = 600000, slightly   more than one move per minute for one year) and rather crowded   networks (k=50000 nodes per network), the odds of collision for a   given node are vanishingly small:           Pc(2^62,  5000,   600000)     = 6.66e-10           Pc(2^62, 50000,   600000)     = 6.53e-09   Each such collision affects two nodes, so the probability of being   affected by a collision is twice this.  Even if the node moves into   networks of 50000 nodes once per minute for 100 years, the   probability of it causing or suffering a collision at any point are a   little over 1 in a million.           Pc(2^62, 50000, 60000000) * 2 = 1.3e-06Moore                       Standards Track                    [Page 14]

RFC 4429                     Optimistic DAD                   April 2006Normative References   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate              Requirement Levels",BCP 14,RFC 2119, March 1997.   [RFC2461]  Narten, T., Nordmark, E., and W. Simpson, "Neighbor              Discovery for IP Version 6 (IPv6)",RFC 2461, December              1998.   [RFC2462]  Thomson, S. and T. Narten, "IPv6 Stateless Address              Autoconfiguration",RFC 2462, December 1998.Informative References   [RFC2464]  Crawford, M., "Transmission of IPv6 Packets over Ethernet              Networks",RFC 2464, December 1998.   [RFC3041]  Narten, T. and R. Draves, "Privacy Extensions for              Stateless Address Autoconfiguration in IPv6",RFC 3041,              January 2001.   [RFC3315]  Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins,              C., and M. Carney, "Dynamic Host Configuration Protocol              for IPv6 (DHCPv6)",RFC 3315, July 2003.   [RFC3484]  Draves, R., "Default Address Selection for Internet              Protocol version 6 (IPv6)",RFC 3484, February 2003.   [RFC3756]  Nikander, P., Kempf, J., and E. Nordmark, "IPv6 Neighbor              Discovery (ND) Trust Models and Threats",RFC 3756, May              2004.   [RFC3775]  Johnson, D., Perkins, C., and J. Arkko, "Mobility Support              in IPv6",RFC 3775, June 2004.   [RFC3971]  Arkko, J., Ed., Kempf, J., Zill, B., and P. Nikander,              "SEcure Neighbor Discovery (SEND)",RFC 3971, March 2005.   [RFC3972]  Aura, T., "Cryptographically Generated Addresses (CGA)",RFC 3972, March 2005.Moore                       Standards Track                    [Page 15]

RFC 4429                     Optimistic DAD                   April 2006Acknowledgements   There is some precedent for this work in expired Internet-Drafts and   in discussions in the MobileIP WG mailing list and at IETF-54.  A   similar concept occurs in the 'Optimistic' bit used by R. Koodli and   C. Perkins in the now expired, "Fast Handovers in Mobile IPv6".   Thanks to Greg Daley, Richard Nelson, Brett Pentland and Ahmet   Sekercioglu at Monash University CTIE for their feedback and   encouragement.  More information is available at:         <http://www.ctie.monash.edu.au/ipv6/fastho/>   Thanks to all the MobileIP and IPng/IPv6 WG members who have   contributed to the debate, especially and alphabetically: Jari Arkko,   Marcelo Bagnulo, JinHyeock Choi, Youn-Hee Han, James Kempf, Thomas   Narten, Pekka Nikander, Erik Nordmark, Soohong 'Daniel' Park, Mohan   Parthasarathy, Ed Remmel, Pekka Savola, Hesham Soliman, Ignatious   Souvatzis, Jinmei Tatuya, Dave Thaler, Pascal Thubert, Christian   Vogt, Vladislav Yasevich, and Alper Yegin.   This work has been supported by the Australian Telecommunications   Cooperative Research Centre (ATcrc):         <http://www.telecommunications.crc.org.au/>Author's Address   Nick 'Sharkey' Moore   Centre for Telecommunications and Information Engineering   Monash University 3800   Victoria, Australia   Comments should be sent to <sharkey@zoic.org> and/or the IPv6 Working   Group mailing list.  Please include 'RFC4429' in the Subject line.Moore                       Standards Track                    [Page 16]

RFC 4429                     Optimistic DAD                   April 2006Full Copyright Statement   Copyright (C) The Internet Society (2006).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   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.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights 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; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 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 athttp://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.Acknowledgement   Funding for the RFC Editor function is provided by the IETF   Administrative Support Activity (IASA).Moore                       Standards Track                    [Page 17]