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<!DOCTYPE rfc SYSTEM "rfc2629-xhtml.ent">

<!-- Updated by Chris 07/20/21 -->

<rfc xmlns:xi="http://www.w3.org/2001/XInclude" ipr="trust200902" obsoletes="" updates="4861" docName="draft-ietf-6man-grand-07" number="0000" number="9131" submissionType="IETF" category="std"  consensus="true" xml:lang="en" tocInclude="true" symRefs="true" sortRefs="true" version="3">

  <front>
    <title abbrev="Gratuitous ND">Gratuitous Neighbor Discovery: Creating Neighbor Cache Entries on First-Hop First&nbhy;Hop Routers</title>
    <seriesInfo name="RFC" value="0000"/> value="9131"/>
    <author fullname="Jen Linkova" initials="J." surname="Linkova">
      <organization>Google</organization>
      <address>
        <postal>
          <street>1 Darling Island Rd</street>
          <city>Pyrmont</city>
          <region>NSW</region>
          <code>2009</code>
          <country>AU</country>
          <country>Australia</country>
        </postal>
        <phone/>
        <email>furry@google.com</email>
      </address>
    </author>
    <date year="2021" month="August"/>
    <area>Internet</area>
    <workgroup>IPv6 Maintenance</workgroup>

<!-- [rfced] Please insert any keywords (beyond those that appear in the
title) for use on https://www.rfc-editor.org/search -->

    <abstract>
      <t>
                    Neighbor Discovery (RFC4861) (RFC 4861) is used by IPv6 nodes to determine the link-layer addresses of neighboring nodes as well as to discover and maintain reachability information. This document updates RFC4861 RFC 4861 to allow routers to proactively create a Neighbor Cache entry when a new IPv6 address is assigned to a node. It also updates RFC4861 RFC 4861 and recommends that nodes to send unsolicited Neighbor Advertisements upon assigning a new IPv6 address. The proposed change These changes will minimize the delay and packet loss when a node initiates connections to an off-link destination from a new IPv6 address.
      </t>
    </abstract>
  </front>
  <middle>
    <section numbered="true" toc="default">
      <name>Introduction</name>
      <t>
                          The Neighbor Discovery state machine defined in  <xref target="RFC4861" format="default"/> assumes that communications between IPv6 nodes are are, in most cases bi-directional cases, bidirectional and if a node A is trying to communicate to its neighbor, node B, the return traffic flows could be expected.  So  So, when the node A starts the address resolution process, the target node B would also create an entry containing A's IPv6 and link-layer addresses in its neighbor cache. Neighbor Cache. That entry will be used for sending the return traffic to A.
      </t>
      <t>
                          In particular, <xref target="RFC4861" sectionFormat="of" section="7.2.5"/> states:
      </t>
<!-- DNE; Lynne to verify verified -->
<blockquote>When a valid Neighbor Advertisement is received (either solicited or unsolicited), the Neighbor Cache is searched for the target's entry.
                          If no entry exists, the advertisement <bcp14>SHOULD</bcp14> be silently discarded.
   There is no need to create an entry if none exists, since the recipient has apparently not initiated any communication with the target.</blockquote>
      <t>
                                  While this approach is perfectly suitable for host-to-host on-link communications, it does not work so well when a host sends traffic to off-link destinations.
                                  After joining the network and receiving a Router Advertisement Advertisement, the host populates its neighbor cache Neighbor Cache with the default router IPv6 and link-layer addresses and is able to send traffic to off-link destinations.
                                  At the same time time, the router does not have any cache entries for the host global addresses yet and only starts address resolution upon receiving the first packet of the return traffic flow.
                                  While waiting for the resolution to complete complete, routers only keep a very small number of packets in the queue, as recommended in <xref target="RFC4861" sectionFormat="of" section="7.2.2"/>.
Any additional packets arriving before the resolution &gt; process finishes are likely to result in dropped packets.

<!-- [rfced] Section 1:  Does "before the resolution > process
finishes" mean "before the address resolution process finishes"?  If
not, will this sentence be clear to readers, or should the ">" be
clarified?

Original (We added the period to the end of the sentence):
 Any additional packets
 arriving before the resolution > process finishes are likely to
 result in dropped packets

Perhaps:
 Any additional packets
 arriving before the address resolution process finishes are likely
 to result in dropped packets. -->

                                          It can cause packet loss and performance degradation that can be user-visible. visible to users.
      </t>
      <t>
This document updates the Neighbor Discovery protocol <xref target="RFC4861" format="default"/> to avoid packet loss in the scenario described above.
<xref target="changes" format="default"/> discusses the changes and analyses analyzes the potential impact, while normative changes to <xref target="RFC4861" format="default"/> are specified in <xref target="RFC_UPD" format="default"/>.
</t>
      <section numbered="true" toc="default">
        <name>Requirements Language</name>
       <t>The key words "<bcp14>MUST</bcp14>", "<bcp14>MUST NOT</bcp14>",
       "<bcp14>REQUIRED</bcp14>", "<bcp14>SHALL</bcp14>",
       "<bcp14>SHALL NOT</bcp14>", "<bcp14>SHOULD</bcp14>",
       "<bcp14>SHOULD NOT</bcp14>",
       "<bcp14>RECOMMENDED</bcp14>", "<bcp14>NOT RECOMMENDED</bcp14>",
       "<bcp14>MAY</bcp14>", and "<bcp14>OPTIONAL</bcp14>" in this document
       are to be interpreted as described in BCP&nbsp;14
       <xref target="RFC2119"/> <xref target="RFC8174"/> when, and only
       when, they appear in all capitals, as shown here.</t>
      </section>
      <section numbered="true" toc="default">
        <name>Terminology</name>
        <dl newline="false" spacing="normal">
          <dt>Node:</dt><dd>a
          <dt>Node:</dt><dd>A device that implements IP, IP <xref target="RFC4861" format="default"/>.</dd>
          <dt>Host:</dt><dd>any
          <dt>Host:</dt><dd>Any node that is not a router, router <xref target="RFC4861" format="default"/>.</dd>
          <dt>ND:</dt><dd>Neighbor Discovery, Discovery <xref target="RFC4861" format="default"/>.</dd>
          <dt>NC:</dt><dd>Neighbor Cache, Cache <xref target="RFC4861" format="default"/>. The Neighbor Cache entry can be in one of five states, as described in <xref target="RFC4861" sectionFormat="of" section="7.3.2"/>: INCOMPLETE, REACHABLE, STALE, DELAY, or PROBE.</dd>
          <dt>SLAAC:</dt><dd>IPv6 Stateless Address Autoconfiguration, Autoconfiguration <xref target="RFC4862" format="default"/>.</dd>
          <dt>NS:</dt><dd>Neighbor Solicitation, Solicitation <xref target="RFC4861" format="default"/>.</dd>
          <dt>NA:</dt><dd>Neighbor Advertisement, Advertisement <xref target="RFC4861" format="default"/>.</dd>
          <dt>RS:</dt><dd>Router Solicitation, Solicitation <xref target="RFC4861" format="default"/>.</dd>
          <dt>RA:</dt><dd>Router Advertisement, Advertisement  <xref target="RFC4861" format="default"/>.</dd>
          <dt>SLLAO:</dt><dd>Source link-layer Link-Layer Address Option, an Option. An option in the ND packets containing the link-layer address of the sender of the packet <xref target="RFC4861" format="default"/>.</dd>
          <dt>TLLAO:</dt><dd>Target link-layer Link-Layer Address Option, an Option. An option in the ND packets containing the link-layer address of the target <xref target="RFC4861" format="default"/>.</dd>
          <dt>GUA:</dt><dd>Global Unicast Address <xref target="RFC4291" format="default"/>.</dd>
          <dt>DAD:</dt><dd>Duplicate Address Detection, Detection <xref target="RFC4862" format="default"/>.</dd>
          <dt>Preferred Address:</dt><dd>an Address:</dt><dd>An address assigned to an interface whose uniqueness has been verified using DAD and whose use by upper-layer protocols is unrestricted, unrestricted <xref target="RFC4862" format="default"/>. Preferred addresses may be used as the source address of packets sent from the interface.</dd>
          <dt>Optimistic DAD:</dt><dd>a DAD:</dt><dd>A modification of DAD, DAD <xref target="RFC4429" format="default"/>.</dd>
	</dl>
      </section>
    </section>
    <section numbered="true" toc="default">
      <name>Problem Statement</name>
      <t>
                 The most typical scenario when the problem described in this document may arise is a host joining the network, forming a new address address, and
 using that address for accessing the Internet:
      </t>
      <ol spacing="normal" type="1"><li>
                                          A host joins the network and receives a Router Advertisement (RA) packet from the first-hop router (either a periodic unsolicited RA or a response to a Router Solicitation sent by the host).
                                                  The RA contains information the host needs to perform SLAAC and to configure its network stack.
                                                  The RA is sent from the router's link-local address to a link-local destination address and may contain the link-layer address of the router.
                                                  As a result result, the host can populate its Neighbor Cache with the router's link-local and link-layer addresses.
                                  </li>
        <li>
                                          The host starts opening connections to off-link destinations.
                                          A very common use case is a mobile device sending probes to detect the Internet connectivity
and/or the presence of a captive portal on the network.
                                                  To speed up that process process, many implementations use Optimistic DAD DAD, which allows them to send probes before the DAD process is completed.
                                                          At that moment moment, the device neighbor cache device's Neighbor Cache contains all information required to send those probes (such as the default router link-local and link-layer addresses).
                                                  The router neighbor cache, router's Neighbor Cache, however, might contain an entry for the device device's link-local
address (if the device has been performing the address resolution for the router router's link-local address), but there are no entries for any of the device's global addresses.
                                  </li>
        <li>
                                          Return traffic is received by the first-hop router.
                As the router does not have any cache entry for the host global address yet, the router starts the neighbor discovery Neighbor Discovery process by creating an INCOMPLETE cache entry and then sending a Neighbor Solicitation to the Solicited Node Multicast Address solicited-node multicast address (<xref target="RFC4861" sectionFormat="of" section="7.3.2"/>).
                As per <xref target="RFC4861" sectionFormat="of" section="7.2.2"/>
 Routers section="7.2.2"/>,
 routers <bcp14>MUST</bcp14> buffer at least one data packet and <bcp14>MAY</bcp14> buffer more, while resolving the packet destination address.
                        However, most router implementations limit the buffer size to a few packets only, and some implementations are known to buffer just one packet.
So
So, any subsequent packets arriving before the address resolution process is completed are causing packet loss by replacing older packets in the buffer.
                                  </li>
        <li>
                                        If the host sends multiple probes in parallel, in the worst case, it would consider all but one of them failed.
                                        That leads to user-visible delay in connecting to the network, especially if the host implements some form of backoff mechanism and does not retransmit the probes as soon as possible.
                                  </li>
      </ol>
      <t>
                                  This scenario illustrates the problem occurring when the device connects to the network for the first time or after an inactivity period long enough for the device device's address to be removed from the router's neighbor cache. Neighbor Cache.
                                  However, the same sequence of events happen happens when the host starts using a new global address previously unseen by the router, such as a new privacy address <xref target="RFC8981" format="default"/> or if the router's Neighbor Cache has been flushed.
      </t>
      <t>
                          While in dual-stack networks this problem might be hidden by Happy Eyeballs <xref target="RFC8305" format="default"/> format="default"/>, it manifests quite clearly in IPv6-only environments, especially wireless ones, environments, leading to poor user experience and contributing to a negative perception of IPv6-only solutions as unstable and non-deployable.
      </t>
    </section>
    <section numbered="true" toc="default">
      <name>Solution Requirements</name>
      <t>
                It would be highly desirable to improve the Neighbor Discovery mechanics so routers have a usable cache entry for a host address by the time the router receives the first packet for that address.
                 In particular:

      </t>
      <ul spacing="normal">
        <li>
                                 If the router does not have a Neighbor Cache entry for the address, a STALE entry needs to be created proactively, prior to arrival of the first packet intended for that address.
                         </li>
        <li>
                                 The solution needs to work for Optimistic addresses Addresses as well.
                                 Devices implementing the Optimistic DAD usually attempt to minimize the delay in connecting to the network and therefore are more likely to be affected by the problem described in this document.
                         </li>
        <li>
                                 In the case of duplicate addresses present in the network, the proposed solution should not override the existing entry.
                         </li>
        <li>
                                 In topologies with multiple first-hop routers routers, the cache needs to be updated on all of them, as traffic might be asymmetric: outgoing flows leaving the network via one router while the return traffic enters the segment via another one.
                        </li>
      </ul>
      <t>
                        In addition addition, the solution must not exacerbate issues described in <xref target="RFC6583" format="default"/> and needs to be compatible with the recommendations provided in <xref target="RFC6583" format="default"/>.
      </t>
    </section>
    <section anchor="changes" numbered="true" toc="default">
      <name>Changes to Neighbor Discovery</name>
      <t>
                      The following changes are required to minimize the delay in creating new entries in a router neighbor cache router's Neighbor Cache:
      </t>
      <ul spacing="normal">
        <li>
                                     A node sends unsolicited NAs upon assigning a new IPv6 address to its interface.
                              </li>
        <li>
                                     A router creates a new cache entry upon receiving an unsolicited NA from a host.
                              </li>
      </ul>
      <t>
                      The following sections discuss these changes in more detail.
    Normative changes are specified in <xref target="RFC_UPD" format="default"/>.
      </t>
      <section anchor="hosts" numbered="true" toc="default">
        <name>Nodes Sending Gratuitous Neighbor Advertisements</name>
        <t>
                              The
                              <xref target="RFC4861" sectionFormat="of" section="7.2.6"/> discusses using unsolicited Neighbor
                              Advertisements to inform node neighbors of the new link-layer address quickly.
                              The same mechanism could be used to notify the node neighbors about the new network-layer
                              address as well: the node can send gratuitous unsolicited Neighbor Advertisements upon assigning a new IPv6 address to its interface.
        </t>
        <t>
                              To minimize the potential disruption in the case of duplicate addresses addresses, the node should not set the Override flag for a preferred address and must not set the Override flag if the address is in the Optimistic state <xref target="RFC4429" format="default"/> state. format="default"/>.
        </t>
        <t>
                              As the main purpose of sending unsolicited NAs upon configuring a new address is to proactively create a Neighbor Cache entry on the first-hop routers, the gratuitous NAs are sent to the all-routers multicast address (ff02::2). Limiting the recipients to routers only would help reduce the multicast noise level.
                              If the link-layer devices are performing MLD Multicast Listener Discovery (MLD) snooping <xref target="RFC4541" format="default"/>, then those unsolicited NAs will be only be sent to routers on the given network segment/link, instead of being flooded to all nodes.
        </t>
        <t>
                              It should be noted that the proposed mechanism discussed here does not cause any significant increase in multicast traffic.
                              The additional multicast unsolicited NA NAs would proactively create a STALE cache entry on routers routers, as discussed below.
                              When the router receives the return traffic flows flows, it does not need to send multicast NSes to the solicited node solicited-node multicast address but would be sending unicast NSes instead.
                              Therefore
                              Therefore, this procedure would only produce an increase in the overall amount of multicast traffic if no return traffic arrives for the address that sent the unsolicited NA or if the router does not create a STALE entry upon receiving such an NA. The increase would be negligible negligible, as that additional traffic is a few orders of magnitude less than the usual level of Neighbor Discovery multicast traffic.
        </t>
      </section>
      <section numbered="true" toc="default">
        <name>Routers Creating Cache Entries Upon upon Receiving Unsolicited Neighbor Advertisements</name>
        <t>
                              The
                              <xref target="RFC4861" sectionFormat="of" section="7.2.5"/> states:
	</t>
<!-- DNE; Lynne to verify verified -->
<blockquote>When a valid Neighbor Advertisement is received (either solicited or
   unsolicited), the Neighbor Cache is searched for the target's entry.
   If no entry exists, the advertisement <bcp14>SHOULD</bcp14> be silently discarded.
   There is no need to create an entry if none exists, since the
   recipient has apparently not initiated any communication with the
   target.</blockquote>
        <t>
<!-- Quoted text below is DNE; verified -->
                              The reasoning behind dropping unsolicited Neighbor Advertisements ("the
   recipient has apparently not initiated any communication with the
   target") is valid for onlink on-link host-to-host communication but, as discussed above,
   it does not really apply for the scenario when the host is announcing its address to routers.
   Therefore, it would be beneficial to allow routers to create new entries upon receiving an unsolicited Neighbor Advertisement.
        </t>
        <t>
                                      This document updates <xref target="RFC4861" format="default"/> so that routers create a new Neighbor Cache entry upon receiving an unsolicited Neighbor Advertisement for an address that does not already have a Neighbor Cache entry.
.
                                              The proposed
                                              These changes do not modify routers behaviour the router behavior specified in <xref target="RFC4861" format="default"/> for the scenario when the corresponding Neighbor Cache entry already exists.

        </t>
        <t>
The next section analyses analyzes various scenarios of duplicated addresses and discusses the potential impact of creating a STALE entry for a duplicated IPv6 address.
</t>
      </section>
    </section>
    <section anchor="avoid_dis" numbered="true" toc="default">
      <name>Avoiding Disruption</name>
      <t>
                                    If nodes following the recommendations in this document are using the DAD mechanism defined in <xref target="RFC4862" format="default"/>, they would send unsolicited NA NAs as soon as the address changes the state from tentative to preferred (after its uniqueness has been verified).
                                            However, nodes willing to minimize network stack configuration delays might be using optimistic addresses, Optimistic Addresses, which means there is a possibility of the address not being unique on the link.
                                            <xref target="RFC4429" sectionFormat="of" section="2.2"/> discusses measures to ensure that ND packets from the optimistic address Optimistic Address do not override any existing neighbor cache entries Neighbor Cache entries, as it would cause traffic interruption of the rightful address owner owner's traffic in the case of an address conflict.
                                                    As nodes willing to speed up their network stack configuration are most likely to be affected by the problem outlined in this document document, it seems reasonable for such hosts to advertise their optimistic addresses Optimistic Addresses by sending unsolicited NAs.
                                                    The main question to consider is the potential risk of overriding the cache entry for the rightful address owner if the optimistic address Optimistic Address happens to be duplicated.
      </t>
      <t>
                                    The following sections discuss the address collision scenario when a node sends an unsolicited NA for an address in the Optimistic state, while another node (the rightful owner) already has the same address assigned already. assigned.
                                    This document uses the term "the rightful owner" owner", as the same terminology is used in <xref target="RFC4429" format="default"/>.
The analysis assumes that the host performs Duplicate Address Detection, DAD, as <xref target="RFC4862" sectionFormat="of" section="5.4"/> requires that DAD <bcp14>MUST</bcp14> be performed on all unicast
   addresses prior to assigning them to an interface.
      </t>
      <section anchor="avoid_dis_exists" numbered="true" toc="default">
        <name>Neighbor Cache Entry Exists in Any State Other Than INCOMPLETE</name>
        <t>
                            If the router router's Neighbor Cache entry for the target address already exists in any state other than INCOMPLETE, then as per <xref target="RFC4861" sectionFormat="of" section="7.2.5"/> section="7.2.5"/>, an unsolicited NA with the Override flag cleared would change the entry state from REACHABLE to STALE but would not update the entry in any other way. Therefore, even if the host sends an unsolicited NA from its Optimistic address Address, the router router's cache entry would not be updated with the new Link-Layer link-layer address and no impact to on the traffic for the rightful address owner is expected.
        </t>
        <t>
The return traffic intended for the host with the Optimistic address Address would be sent to the rightful owner. However, this is unavoidable with or without the unsolicited NA mechanism.
</t>
      </section>
      <section anchor="avoid_dis_inc" numbered="true" toc="default">
        <name>Neighbor Cache Entry is Is in INCOMPLETE state</name> State</name>
        <t>
                            Another corner case is the INCOMPLETE cache entry for the address.
        </t>
        <ol spacing="normal" type="1"><li>
The router receives a packet for the rightful owner of the address.
</li>
          <li>
The router starts the address resolution process by creating an INCOMPLETE entry and sends the multicast NS.
</li>
          <li>
More packets arrive at the router for the address in question.
</li>
          <li>
The host configures an Optimistic address Address and sends an unsolicited NA.
</li>
          <li>
The router creates a STALE entry and sends the buffered packet(s) to the host (while at least some of those packets are actually intended for the rightful owner).
</li>
          <li>
As the STALE entry was used to send packets, the router changes the entry state to DELAY and waits up to DELAY_FIRST_PROBE_TIME (<xref target="RFC4861"/>, 5 secs) (5 seconds) <xref target="RFC4861"/> before sending a unicast NS.
</li>
          <li>
The rightful owner responds to the multicast NS sent at Step 2 with a solicited NA with the Override flag set.
</li>
          <li>
The router updates the entry with the TLLAO supplied (the rightful owner owner's link-layer address) and sets the entry state to REACHABLE (as the NA has the Solicited flag set).
</li>
        </ol>
        <t>
As a result result, some packets (ones (packets in the buffer at Step 6 and all packets arriving between Step 6 and Step 8) are delivered to the host with the Optimisitc address, Optimistic Address, while some of them, if not all, are intended for the rightful owner.
Without the unsolicited NA, packet which one or more packets that are in the buffer at Step 8 (usually just one packet packet, but some routers may buffer a few) would have been delivered to the rightful owner and the rest of the packets would have been dropped.
However, the probability of such a scenario is rather low low, as it would require the following
things to happen almost simultaneously (within tens of milliseconds in most cases):
        </t>
        <ul spacing="normal">
          <li>
                                                                            One host starts using a new IPv6 address and sending traffic without sending an unsolicited NA first.
                                                            </li>
          <li>
                                                                            Another host configures the same IPv6 address in Optimistic mode before the router completes the address resolution process for the rightful owner.
                                                            </li>
        </ul>
        <t>
It should be noted that in this scenario the rigthful rightful owner does not send any unsolicited NAs before sending packets. If the rightful owner implements the functionality described in this document and sends unsolicited NAs upon configuring its address, then the router creates a STALE entry for the address, causing all packets are to be delivered to the rightful owner (see <xref target="avoid_dis_exists" format="default"/>). The rightful owner would experience no disruption but might receive some packets intended for the host with an Optimistic address. Address.
</t>
        <t>
This section focuses on the scenario when the solicited NA from the rightful owner arrives after the unsolicited one sent from the Optimistic address Address (Step 7 and Step 4 4, respectively).
If the solicited NA arrives first first, it changes the NC entry state from INCOMPLETE to REACHABLE. As discussed in <xref target="avoid_dis_exists" format="default"/>, there will be no disruption for the rightful owner if the router already has a REACHABLE entry for the address when an unsolicited NA is received.
</t>
      </section>
      <section anchor="avoid_dis_nonexists" numbered="true" toc="default">
        <name>Neighbor Cache Entry Does Not Exist</name>
        <t>
                                            There are two distinct scenarios which that can lead to the situation when the router does not have a an NC entry for the IPv6 address:

        </t>
        <ol spacing="normal" type="1"><li>
                                                            The rightful owner of the address has not been using it for off-link communication recently or has never used it at all.
                                            </li>
          <li>
                                                            The rightful owner just started sending packets from that address address, but the router has not received any return traffic yet.
                                            </li>
        </ol>
        <t>
                                            The impact on the rightful owner's traffic flows would be different in those cases.
        </t>
        <section numbered="true" toc="default">
          <name>The Rightful Owner Is Not Sending Packets From The from the Address</name>
          <t>
                                                    In this scenario scenario, the following events are expected to happen:

          </t>
          <ol spacing="normal" type="1"><li>
                                                                    The host configures the address and sets its state to Optimistic.
                                                    </li>
            <li>
                                                                    The host sends an unsolicited NA with the Override flag set to zero and starts sending traffic from the Optimistic address. Address.
                                                    </li>
            <li>
                                                                    The router creates a STALE entry for the address and the host link-layer address.
                                                    </li>
            <li>
                                                                    The host starts DAD and detects the address duplication.
                                                    </li>
            <li>
                                                                    The router receives the return traffic for the duplicated address. As the NC entry is STALE STALE, it sends traffic using that entry, changes it to DELAY DELAY, and waits up to DELAY_FIRST_PROBE_TIME (<xref seconds <xref target="RFC4861" format="default"/>) seconds. format="default"/>.
                                                            </li>
            <li>
                                                                            The router changes the NC entry state to PROBE and sends up to MAX_UNICAST_SOLICIT (<xref target="RFC4861" format="default"/>) unicast NSes <xref target="RFC4861" format="default"/> separated by RetransTimer milliseconds (<xref <xref target="RFC4861" format="default"/>) format="default"/> to the host link-layer address.
                                                    </li>
            <li>
                                                                    As the host has already detected the address conflict already conflict, it does not respond to the unicast NSes. (It is unlikely that the host has not completed the DAD process at this stage, as DELAY_FIRST_PROBE_TIME (5 seconds) is much higher than the DAD duration (DupAddrDetectTransmits*RetransTimer*1000 + MAX_RTR_SOLICITATION_DELAY secs, <xref seconds) (<xref target="RFC4862" sectionFormat="of" section="5.4"/>). section="5.4"/>).) The default value for the DAD process would be 1*1*1000 + 1 = 2 secs, seconds <xref target="RFC4861" format="default"/>.
If the host has completed DAD but did not detect the address conflict conflict, then there are two hosts with the same address in the Preferred preferred state and the disruption is inevitable anyway.
                                                    </li>
            <li>
                                                                   As the router receives no response for the unicast NSes, it deletes the NC entry.
                                                    </li>
            <li>
                                                                    If return packets for communication initiated at step Step 2 are still arriving, the router buffers a small number of those packets and starts the address resolution process again by sending a multicast NS to the solicited node solicited-node multicast address. The rightful owner responds responds, and the router router's NC entry is updated with the rightful owner owner's link-local address. The buffered packet(s) packet or packets are sent to that address. Any packets still arriving after the address resolution process has completed are sent to the rightful address owner as well.

<!-- [rfced] Section 5.3.1:  We changed "the address resolution still
completed" to "the address resolution process has completed" here.
Please let us know if this is incorrect.

Original:
 Any packets still arriving after the address
 resolution still completed are sent to the rightful address owner
 as well.

Currently:
 Any packets still
 arriving after the address resolution process has completed are
 sent to the rightful address owner as well. -->

                                                    </li>
          </ol>
          <t>
                                                    The rightful owner is not experiencing any disruption disruption, as it does not send any traffic.
It would only start receiving packets intended for another host after Step 8 is completed and only if return packets for the communication initiated at step Step 2 are still arriving.
</t>
          <t>

                                                    However, the same behaviour behavior would be observed if the changes proposed specified in this document are not implemented.
                                                    If the host starts sending packets from its Optimistic Address but then changes the address state to Duplicated, the first return packet would trigger the address resolution process and would be buffered until the resolution is completed.

<!-- [rfced] Section 5.3.1:  We could not find a state named
"Duplicated" in any published RFC.  Please confirm that this text
will be clear to readers.

Original:
 If the host starts sending
 packets from its Optimistic address but then changes the address
 state to Duplicated, the first return packet would trigger the
 address resolution process and would be buffered until the resolution
 is completed. -->

The buffered packet(s) and any packets still arriving after the address is resolved would be forwarded to the rightful owner of the address.
So
So, the rightful owner might still receive one or more packets from the flows intended for another host.
Therefore, it's safe to conclude that the proposed changes specified in this document do not introduce any disruption for the rightful owner of the duplicated address.
          </t>
        </section>
        <section anchor="dis_start" numbered="true" toc="default">
          <name>The Rightful Owner Has Started Sending Packets From The Address</name>
          <t>
                                            In

<!-- [rfced] Section 5.3.1:  We changed "do introduce any disruption" to
"do not introduce any disruption" here.  Please let us know if this scenario is
incorrect.

Original:
 Therefore,
 it's safe to conclude that the following events are happening:

          </t>
          <ol spacing="normal" type="1"><li>
                                                            The proposed changes do introduce any
 disruption for the rightful owner of the duplicated address.

Currently:
 Therefore,
 it's safe to conclude that the changes specified in this document do
 not introduce any disruption for the rightful owner of the duplicated
 address. -->

          </t>
        </section>
        <section anchor="dis_start" numbered="true" toc="default">
          <name>The Rightful Owner Has Started Sending Packets from the Address</name>
          <t>
                                            In this scenario, the following events are happening:

          </t>
          <ol spacing="normal" type="1"><li>
                                                            The rightful owner starts sending traffic from the address (e.g. (e.g., the address has just been configured or has not been recently used).
                                            </li>
            <li>
                                                                    The host configures the address and sets its state to Optimistic.
                                                    </li>
            <li>
                                                                    The host sends an unsolicited NA with the Override flag set to zero and starts sending traffic from the Optimistic address. Address.
                                                    </li>
            <li>
                                                                    The router creates a STALE entry for the address and the host link-layer address.
                                                    </li>
            <li>
                                                                    The host starts DAD and detects the address duplication.
                                                    </li>
            <li>
                                                                    The router receives the return traffic for the IPv6 address in question. Some flows are intended for the rightful owner of the duplicated address, while some are for the new host. As the NC entry is STALE STALE, it sends traffic using that entry, changes it to DELAY DELAY, and waits up to DELAY_FIRST_PROBE_TIME (<xref seconds <xref target="RFC4861" format="default"/>) seconds. format="default"/>.
                                                            </li>
            <li>
                                                                            The router changes the NC entry state to PROBE and sends up to MAX_UNICAST_SOLICIT (<xref target="RFC4861" format="default"/>) unicast NSes <xref target="RFC4861" format="default"/> separated by RetransTimer milliseconds (<xref <xref target="RFC4861" format="default"/>) format="default"/> to the host link-layer address.
                                                    </li>
            <li>
                                                                    As the host has already detected the address conflict already conflict, it does not respond to the unicast NSes.
                                                    </li>
            <li>
                                                                   As the router receives no response for the unicast NSes, it deletes the NC entry.
                                                    </li>
            <li>
                                                                    The next packet re-creates recreates the entry and triggers the resolution process. The router buffers the packet and sends a multicast NS to the solicited node solicited-node multicast address. The rightful owner responds responds, and the router router's NC entry is updated with the rightful owner owner's link-local address.
                                                    </li>
          </ol>
          <t>
                                                    As a result, the traffic for the address rightful owner would be sent to the host with the duplicated address instead.

<!-- [rfced] Section 5.3.2:  Should "address rightful owner" be
"address's rightful owner", "address of the rightful owner", or
something else?

Original:
 As a result the traffic for the address rightful owner would be sent
 to the host with the duplicated address instead. -->

 The duration of the disruption can be estimated as DELAY_FIRST_PROBE_TIME*1000 + (MAX_UNICAST_SOLICIT - 1)*RetransTimer milliseconds.
                                                    As per the constants defined in <xref target="RFC4861" sectionFormat="of" section="10"/> section="10"/>, this interval is equal to 5*1000 + (3 - 1)*1000 = 7000ms 7000 milliseconds, or 7 seconds.

          </t>
          <t>
                                                            However, it should be noted that the probability of such a scenario is rather low. Similary Similar to the scenario discussed in <xref target="avoid_dis_inc" format="default"/>, it would require the following things to happen almost simultaneously (within tens of milliseconds in most cases):
          </t>
          <ul spacing="normal">
            <li>
                                                                            One host starts using a new IPv6 address and sending traffic without sending an unsolicited NA first.
                                                            </li>
            <li>
                                                                            Another host configures the same IPv6 address in Optimistic mode before the router receives the return traffic for the first host.
                                                            </li>
          </ul>
          <t>
As discussed in <xref target="avoid_dis_inc" format="default"/>, the disruption to for the rightful owner can easily be prevent prevented if that node implements the mechanism described in the this document. Sending unsolicited NAs before initiatining initiating off-link communication would create a STALE entry in the router router's NC and prevent any tarffic traffic to that address to be from being sent to the host with the Optimistic address Address (see <xref target="avoid_dis_exists" format="default"/>).
</t>
        </section>
      </section>
    </section>
    <section anchor="RFC_UPD" numbered="true" toc="default">
      <name>Modifications to RFC-Mandated Behavior</name>
      <t>
                            All normative text in this memo is contained in this section.
      </t>
      <section numbered="true" toc="default">
        <name>Modification to RFC4861 Neighbor RFC 4861 ("Neighbor Discovery for IP version 6 (IPv6)</name> (IPv6"))</name>
        <section numbered="true" toc="default">
          <name>Modification to the section 7.2.5</name> Section 7.2.5 of RFC 4861</name>
          <t>
                              This document makes the following changes to the <xref target="RFC4861" sectionFormat="of" section="7.2.5"/>:
          </t>
          <t>OLD TEXT:</t>
          <t>The text in RFC 4861 is as follows:</t>
<!-- DNE; Lynne to verify verified -->
          <blockquote>When a valid Neighbor Advertisement is received (either solicited or
   unsolicited), the Neighbor Cache is searched for the target's entry.
   If no entry exists, the advertisement <bcp14>SHOULD</bcp14> be silently discarded.
   There is no need to create an entry if none exists, since the
   recipient has apparently not initiated any communication with the
   target.</blockquote>
          <t>NEW TEXT:</t>
          <t>This document updates the text as follows:</t>
          <blockquote><t>When a valid Neighbor Advertisement is received (either solicited or
   unsolicited), the Neighbor Cache is searched for the target's entry.
   If no entry exists:</t><t/>
          <ul spacing="normal">
            <li> Hosts <bcp14>SHOULD</bcp14>  silently discard the advertisement.
   There is no need to create an entry if none exists, since the
   recipient has apparently not initiated any communication with the target.
</li>
            <li> Routers <bcp14>SHOULD</bcp14> create a new entry for the target address with the link-layer address set to the Target link-layer address option Link-Layer Address Option (if supplied). The entry's reachability state <bcp14>MUST</bcp14> be set to STALE. If the received Neighbor Advertisement does not contain the Target link-layer address option Link-Layer Address Option, the advertisement <bcp14>SHOULD</bcp14> be silently discarded.
</li>
          </ul>
</blockquote>
        </section>
        <section anchor="UPD726" numbered="true" toc="default">
          <name>Modification to the section 7.2.6</name> Section 7.2.6 of RFC 4861</name>
          <t>
                              This document proposes makes the following changes to the <xref target="RFC4861" sectionFormat="of" section="7.2.6"/>:
          </t>
          <t>OLD TEXT:</t>
          <t>The text in RFC 4861 is as follows:</t>
<!-- DNE; Lynne to verify verified -->
          <blockquote>Also, a node belonging to an anycast address <bcp14>MAY</bcp14> multicast
   unsolicited Neighbor Advertisements for the anycast address when the
   node's link-layer address changes.</blockquote>

          <t>NEW TEXT:</t>

          <t>This document updates the text as follows:</t>
          <blockquote><t>Also, a node belonging to an anycast address <bcp14>MAY</bcp14> multicast
   unsolicited Neighbor Advertisements for the anycast address when the
   node's link-layer address changes.</t>

          <t>A node may also wish to notify its first-hop routers when it configures a new global IPv6 address so the routers can proactively populate their neighbor caches Neighbor Caches with the corresponding entries. In such cases cases, a node <bcp14>SHOULD</bcp14> send up to MAX_NEIGHBOR_ADVERTISEMENT
   Neighbor Advertisement messages. If the address is preferred preferred, then the Override flag <bcp14>SHOULD NOT</bcp14> be set. If the address is in the Optimistic state state, then the  Override flag <bcp14>MUST NOT</bcp14> be set. The destination address <bcp14>SHOULD</bcp14> be set to the all-routers multicast address. These advertisements <bcp14>MUST</bcp14> be separated by at
   least RetransTimer seconds. The first advertisement <bcp14>SHOULD</bcp14> be sent as soon as one of the
      following events happens:</t><t/>
        <dl newline="false" spacing="normal">
           <dt>if
           <dt>If Optimistic DAD <xref target="RFC4429" format="default"/> is used:</dt><dd>a used:</dt><dd>A new Optimistic address Address is assigned
                                              to the node interface.</dd>
           <dt>if
           <dt>If Optimistic DAD is not used:</dt><dd>an used:</dt><dd>An address changes the state from
         tentative to preferred.</dd>
	</dl>
</blockquote>
        </section>
      </section>
    </section>
    <section numbered="true" toc="default">
      <name>Solution Limitations</name>
      <t>
                              The solution described in this document provides some improvement for a node configuring a new IPv6 address and starting sending to send traffic from it.
                              However, that approach does not completely eliminate the scenario when a router receives some transit traffic for an address without the corresponding Neighbor Cache entry.
                              For example:
      </t>
      <ul spacing="normal">
        <li>If the host starts using an already configured already-configured IPv6 address after a long period of inactivity, the router might not have the NC entry for that address anymore, as old/expired entries are deleted. </li>
        <li>Clearing the router router's Neighbor Cache would trigger the packet loss for all actively used addresses removed from the cache.</li>
      </ul>
    </section>
    <section anchor="others" numbered="true" toc="default">
      <name>Solutions Considered but Discarded</name>
      <t>
                      There are other possible approaches to address the problem, for problem. For example:
      </t>
      <ul spacing="normal">
        <li>
                                      Just do nothing.
                              </li>
        <li>
                                      Migrating
                                      Migrate from the "reactive" Neighbor Discovery (<xref <xref target="RFC4861" format="default"/>) format="default"/> to the registration-based mechanisms (<xref <xref target="RFC8505" format="default"/>). format="default"/>.
                              </li>
        <li>
                                     Creating
                                     Create new entries in routers the router's Neighbor Cache by gleaning from Neighbor Discovery DAD messages.
                              </li>
        <li>
                                      Initiates
                                      Initiate bidirectional communication from the host to the router using the host GUA.
                              </li>
        <li>
                                      Making
                                      Make the probing logic on hosts more robust.
                              </li>
        <li>
                                      Increasing
                                      Increase the buffer size on routers.
                              </li>
        <li>
                                      Transit dataplane data plane traffic from an unknown address (an address w/o without the corresponding neighbor cache Neighbor Cache entry) triggers to trigger an address resolution process on the router.
                              </li>
      </ul>
      <t>
                      It should be noted that some of those options are already implemented by some vendors. The following sections discuss those approaches and the reasons they were discarded.
      </t>
      <section numbered="true" toc="default">
        <name>Do Nothing</name>
        <t>
                              One of the possible approaches might be to declare that everything is working as intended and let the upper-layer protocols deal with packet loss. The obvious drawbacks include:
        </t>
        <ul spacing="normal">
          <li>
                                                      Unhappy users.
                                              </li>
          <li>
                                                      Many support tickets.
                                              </li>
          <li>
                                                      More resistance to deploy deploying IPv6 and IPv6-Only IPv6-only networks.
                                              </li>
        </ul>
      </section>
      <section numbered="true" toc="default">
        <name>Change to the Registration-Based Neighbor Discovery</name>
        <t>
                                      The most radical approach would be to move away from the reactive ND as defined in <xref target="RFC4861" format="default"/> and expand the registration-based ND (<xref <xref target="RFC6775" format="default"/>, format="default"/> <xref target="RFC8505" format="default"/>) format="default"/> used in IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs) to the rest of the IPv6 deployments.
                                              This option requires some investigation and discussion.
                                              However, significant changes to the existing IPv6 implementations would be needed, so an unclear adoption timeline makes this approach less preferable than one proposed the approach specified in this document.
        </t>
      </section>
      <section numbered="true" toc="default">
        <name>Host Sending NS to the Router Address from Its GUA</name>
        <t>
                              The host could force creating the creation of a STALE entry for its GUA in the router router's ND cache by sending the following Neighbor Solicitation message:
        </t>
        <ul spacing="normal">
          <li>
                                              The NS source address is the host GUA.
                                      </li>
          <li>
                                                      The destination address is the default router IPv6 address.
                                      </li>
          <li>
                                              The Source Link-Layer Address option Option contains the host link-layer address.
                                      </li>
          <li>
                                              The target address is the host host's default router address (the default router address the host received in the RA).
                                      </li>
        </ul>
        <t>
                              The main disadvantages of this approach are: are as follows:
        </t>
        <ul spacing="normal">
          <li>
                                              Would
                                              It would not work for Optimistic addresses Addresses, as <xref target="RFC4429" sectionFormat="of" section="2.2"/> explicitly prohibits sending Neighbor Solicitations from an Optimistic Address.
                                      </li>
          <li>
                                              If first-hop redundancy is deployed in the network, the NS would reach the active router only, so all backup routers (or all active routers except one) would not get their neighbor cache Neighbor Cache updated.
                                      </li>
          <li>
                                                      Some wireless devices are known to alter ND packets and perform various non-obvious forms of ND proxy actions.
                                                     In some cases, unsolicited NAs might not even reach the routers.
                                      </li>
        </ul>
      </section>
      <section numbered="true" toc="default">
        <name>Host Sending Router Solicitation from its Its GUA</name>
        <t>
                                      The host could send a router solicitation Router Solicitation message to 'all routers' the all-routers multicast address, using its GUA as a source.
                                      If the host link-layer address is included in the Source Link-Layer Address option, Option, the router would create a STALE entry for the host GUA as per the <xref target="RFC4861" sectionFormat="of" section="6.2.6"/>.
                                      However, this approach cannot be used if the GUA is in optimistic the Optimistic state: <xref target="RFC4429" sectionFormat="of" section="2.2"/>  explicitly prohibits using an Optimistic Address as the source address of a Router Solicitation with a SLLAO SLLAO, as it might disrupt cause disruption for the rightful owner of the address in the case of a collision.
                                      So
                                      So, for the optimistic addresses Optimistic Addresses, the host can send an RS without a SLLAO included.
                                      In that case, the router may respond with either a multicast or a unicast RA (only the latter would create a cache entry).

<!-- [rfced] Section 8.4:  Does "a multicast or a unicast RA" mean
"a multicast packet or a unicast RA" or "a multicast or unicast RA"?

Original:
 In that
 case the router may respond with either a multicast or a unicast RA
 (only the latter would create a cache entry). -->

        </t>
        <t>
                              This approach has the following drawbacks:
        </t>
        <ul spacing="normal">
          <li>
                                                      If the address is in the Optimistic state state, the RS cannot contain a SLLAO. As a result result, the router would only create a cache entry if solicited RAs are sent as unicast.
                                                      Routers sending solicited RAs as multicast would not create a new cache entry entry, as they do not need to send a unicast packet back to the host.
                                              </li>
          <li>
                                                      There might be a random delay between receiving an RS and sending a unicast RA back (and creating a cache entry) entry), which might undermine the idea of creating the cache entry proactively.
                                              </li>
          <li>
                                                      Some wireless devices are known to intercept ND packets and perform various non-obvious forms of ND proxy actions. In some cases cases, the RS might not even reach the routers.
                                              </li>
        </ul>
      </section>
      <section numbered="true" toc="default">
        <name>Routers Populating Their Caches by Gleaning From from Neighbor Discovery Packets</name>
        <t>
                               Routers may be able to learn about new addresses by gleaning from the DAD Neighbor Solicitation messages.
                               The router could listen to all solicited-node multicast address groups and, upon receiving a Neighbor Solicitation from the unspecified address, search its Neighbor Cache for the solicitation's Target Address.

<!-- [rfced] Section 8.5:  Would you like to use either "target
address" or "Target Address" (but not both forms) in this document?
We see that capitalization in RFC 4861 is inconsistent as well.
This sentence uses the only instance of the initial-capitalized form.

Original:
 The router could listen to all
 solicited node multicast address groups and upon receiving a Neighbor
 Solicitation from the unspecified address search its Neighbor Cache
 for the solicitation's Target Address. -->

                               If no entry exists, the router may create an entry, set its reachability state to 'INCOMPLETE' INCOMPLETE, and start the address resolution process for that entry.
        </t>
        <t>
                              The same solution was proposed in <xref target="I-D.halpern-6man-nd-pre-resolve-addr" format="default"/>. Some routing vendors already support such optimization already. optimization. However, this approach has a number of drawbacks and therefore should not be used as the only solution:
        </t>
        <ul spacing="normal">
          <li>
                                                      Routers need to receive all multicast Neighbor Discovery packets which packets; this might negatively impact the routers a router's CPU.
                                              </li>
          <li>
                                                      If the router starts the address resolution process as soon as it receives the DAD Neighbor Solicitation Solicitation, the host might be still be performing DAD and the target address might be tentative.
                                                      In that case, the host <bcp14>SHOULD</bcp14> silently ignore the received Neighbor Solicitation from the router as per the <xref target="RFC4862" sectionFormat="of" section="5.4.3"/>.
                                                      As a result result, the router might not be able to complete the address resolution process before the return traffic arrives.

                                              </li>
        </ul>
      </section>
      <section numbered="true" toc="default">
        <name>Initiating Hosts-to-Routers Host-to-Router Communication</name>
        <t>
                              The host may force the router to start address resolution by sending a data packet such as ping or traceroute to its default router link-local address, using the GUA as a source address.
                              As the RTT to the default router is lower than the RTT to any off-link destinations destinations, it's quite likely that the router would start the neighbor discovery Neighbor Discovery process for the host GUA before the first packet of the returning traffic arrives.
        </t>
        <t> This approach has the following drawbacks:
        </t>
        <ul spacing="normal">
          <li>
                                                      Data packets to the router router's link-local address could be blocked by a security policy or control plane protection mechanism.
                                              </li>
          <li>
                                                      It introduces an additional overhead for routers the router's control plane (in addition to processing ND packets, the data packet needs to be processed as well).
                                              </li>
          <li>
                                                      Unless the data packet is sent to 'all routers' the all-routers ff02::2 multicast address, if the network provides a first-hop redundancy redundancy, then only the active router would create a new cache entry.
                                              </li>
        </ul>
      </section>
      <section numbered="true" toc="default">
        <name>Making the Probing Logic on Hosts More Robust</name>
        <t>
                                      Theoretically
                                      Theoretically, the probing logic on hosts might be modified to deal better deal with initial packet loss. For example, only one probe can be sent sent, or probes probe retransmit intervals can be reduced. However, this approach has a number of drawbacks:

        </t>
        <ul spacing="normal">
          <li>It would require updating all possible applications performing probing, while the proposed solution described in this document is implemented on operating systems at the operating-system level.</li>
          <li>
            <t>Some implementations need to send multiple probes. Examples include but are not limited to:
            </t>
            <ul spacing="normal">
              <li>Sending AAAA and A records record DNS probes in parallel.</li>
              <li>Detecting captive portals portals, which often require requires sending multiple packets.</li>
            </ul>
          </li>
          <li>
            <t>While it would increase the probability of that the probing to will complete successfully, there are multiple cases when packet loss would still occur:
            </t>
            <ul spacing="normal">
              <li> The probe response consists of multiple packets, so all but the first one are dropped. </li>
              <li> There are multiple applications on the same host sending traffic traffic, and return packets arrive simultaneously.</li>
              <li> There are multiple first-hop routers in the network. The first probe packet creates the NC entry on one of them. The subsequent return traffic flows might cross other routers and still experience the issue.</li>
            </ul>
          </li>
          <li>
                                    Reducing the probe retransmit interval unnecessary unnecessarily increases the network utilization and might cause the network congestion.
                              </li>
        </ul>
      </section>
      <section numbered="true" toc="default">
        <name>Increasing the Buffer Size on Routers</name>
        <t>
                                              Increasing the buffer size and buffering more packets would exacerbate issues described in <xref target="RFC6583" format="default"/> and make the router more vulnerable to ND-based denial of service denial-of-service attacks.
        </t>
      </section>
      <section numbered="true" toc="default">
        <name>Transit Dataplane Data Plane Traffic From from a New Address Triggering to Trigger Address Resolution</name>
        <t>
                              When a router receives a transit packet sourced by a an on-link neighbor node, it might check for the presence of the neighbor cache a Neighbor Cache entry for the packet source address and and, if the entry does not exist, start the address resolution process.
                              This approach does ensure that a Neighbor Cache entry is proactively created every time a new, previously unseen GUA is used for sending offlink off-link traffic.
However, this approach has a number of limitations, in limitations. In particular:
</t>
        <ul spacing="normal">
          <li>If traffic flows are asymmetrical asymmetrical, the return traffic might not transit the same router as the original traffic which that triggered the address resolution.
So resolution process.
So, the neighbor cache Neighbor Cache entry is created on the "wrong" router, not the one which that actually needs the neighbor cache Neighbor Cache entry for the host address.
</li>
          <li>
        The functionality needs to be limited to explicitly configured networks/interfaces, as the router needs to distinguish between onlink on-link addresses (ones (addresses for which the router needs to have Neighbor Cache entries for) entries) and the rest of the address space.
        The proactive address resolution process must only be triggered by packets from the prefixes known to be on-link. Otherwise, traffic from spoofed source addresses or any transit traffic could lead to neighbor cache Neighbor Cache exhaustion.
</li>
          <li>
Implementing such functionality is much more complicated than all other solutions solutions, as it would involve complex data-control planes interaction. interactions between the data plane and the control plane.
</li>
        </ul>
      </section>
    </section>
    <section anchor="IANA" numbered="true" toc="default">
      <name>IANA Considerations</name>
      <t>
      This memo asks the IANA for document has no new parameters. IANA actions.
      </t>
    </section>
    <section anchor="Security" numbered="true" toc="default">
      <name>Security Considerations</name>
      <t>
                    One of the potential attack vectors to consider is a cache spoofing when spoofing, where the attacker might try to install a cache entry for the victim's IPv6 address and the attacker's Link-Layer link-layer address. However, it should be noted that this document does not propose any changes for the scenario when the ND cache for the a given IPv6 address already exists.
                    Therefore, there are no new vectors for an attacker to override an existing cache entry.
      </t>
      <t>
<xref target="avoid_dis" format="default"/> describes some corner cases when a host with the a duplicated Optimistic address Address might get some packets intended for the rightful owner of the address. However However, such scenarios do not introduce any new attack vectors: even without the proposed changes, changes discussed in this document, an attacker can easily override the routers neighbor cache router's Neighbor Cache and redirect the traffic by sending NAs with the Solicited flag set.
As discussed in <xref target="dis_start" format="default"/> format="default"/>, the worst case worst-case scenario might cause a disruption for up to 7 seconds. This Because this scenario is highly unlikely, this risk of disruption is considered acceptable due to very low probability of that scenario. acceptable. More importantly, for all cases described in <xref target="avoid_dis" format="default"/> format="default"/>, the rightful owner can prevent disruption caused by an accidental address duplication just by implementing the mechanism described in this document. If the rightful owner sends unsolicited NAs before using the address, the STALE entry would be created on the router NC router's NC, and any subsequent unsolicited NAs sent from the host with an Optimistic address Address would not override the NC entry.
</t>
      <t>
                    A malicious host could attempt to exhaust the neighbor cache Neighbor Cache on the router by creating a large number of STALE entries. However, this attack vector is not new new, and the mechanism specified in this document does not increase the risk of such an attack: the attacker could do it, for example, by sending a an NS or RS packet with a SLLAO included. All recommendations from <xref target="RFC6583" format="default"/> still apply.
      </t>
      <t>
                    Announcing a new address to the all-routers multicast address may inform an on-link attacker about IPv6 addresses assigned to the host. However, hiding information about the specific IPv6 address should not be considered a security measure measure, as such information is usually disclosed via DAD to all nodes anyway if MLD snooping is not enabled. Network administrators can also mitigate this issue by enabling MLD snooping on the link-layer devices to prevent IPv6 link-local multicast packets from being flooded to all onlink on-link nodes.
                            If peer-to-peer onlink on-link communications are not desirable for the a given network segment segment, they should be prevented by proper layer-2 Layer 2 security mechanisms. Therefore, the risk of allowing hosts to send unsolicited Neighbor Advertisements to the all-routers multicast address is low.
      </t>
      <t>
                    It should be noted that the proposed mechanism discussed in this document allows hosts to proactively inform their routers about global IPv6 addresses existing on-link. Routers could use that information to distinguish between used and unused addresses to mitigate ND cache exhaustion DoS attacks as described in <xref target="RFC3756" sectionFormat="of" section="4.3.2"/> and in <xref target="RFC6583" format="default"/>.

      </t>
    </section>
  </middle>

  <back>
<displayreference target="I-D.halpern-6man-nd-pre-resolve-addr" to="ND-ADDR-RES"/>

    <references>
      <name>References</name>
      <references>
        <name>Normative References</name>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.2119.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.4291.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.4429.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.4861.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.4862.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8174.xml"/>
      </references>
      <references>
        <name>Informative References</name>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.3756.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.4541.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.6583.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.6775.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8305.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8505.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8981.xml"/>

<!-- draft-halpern-6man-nd-pre-resolve-addr (Expired) -->
        <xi:include href="https://datatracker.ietf.org/doc/bibxml3/reference.I-D.halpern-6man-nd-pre-resolve-addr.xml"/>
      </references>
    </references>
    <section anchor="Acknowledgements" numbered="true" numbered="false" toc="default">
      <name>Acknowledgements</name>
      <t>
                    Thanks to the following people (in alphabetical order) for their
                            comments, review review, and feedback: <contact fullname="Mikael Abrahamsson"/>, <contact fullname="Stewart Bryant"/>, <contact fullname="Lorenzo Colitti"/>, <contact fullname="Roman Danyliw"/>, <contact fullname="Owen DeLong"/>, <contact fullname="Martin Duke"/>, <contact fullname="Igor Gashinsky"/>, <contact fullname="Carles Gomez"/>, <contact fullname="Fernando Gont"/>, <contact fullname="Tatuya Jinmei"/>, <contact fullname="Benjamin Kaduk"/>, <contact fullname="Scott Kelly"/>, <contact fullname="Erik Kline"/>, <contact fullname="Warren Kumari"/>, <contact fullname="Barry Leiba"/>, <contact fullname="Jordi Palet Martinez"/>, <contact fullname="Erik Nordmark"/>, <contact fullname="Michael Richardson"/>, <contact fullname="Dan Romascanu"/>, <contact fullname="Zaheduzzaman Sarker"/>, <contact fullname="Michael Scharf"/>, <contact fullname="John Scudder"/>, <contact fullname="Mark Smith"/>, <contact fullname="Dave Thaler"/>, <contact fullname="Pascal Thubert"/>, <contact fullname="Loganaden Velvindron"/>, and <contact fullname="Eric fullname="√Čric Vyncke"/>.
      </t>
    </section>
  </middle>

  <back>
<displayreference target="I-D.halpern-6man-nd-pre-resolve-addr" to="ND-ADDR-RES"/>

    <references>
      <name>References</name>
      <references>
        <name>Normative References</name>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.2119.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.4291.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.4429.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.4861.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.4862.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8174.xml"/>
      </references>
      <references>
        <name>Informative References</name>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.3756.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.4541.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.6583.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.6775.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8305.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8505.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8981.xml"/>

<!-- draft-halpern-6man-nd-pre-resolve-addr (Expired) [rfced] Please review the "Inclusive Language" portion of the
online Style Guide
at <https://www.rfc-editor.org/styleguide/part2/#inclusive_language>,
and let us know if any changes are needed. -->
        <xi:include href="https://datatracker.ietf.org/doc/bibxml3/reference.I-D.halpern-6man-nd-pre-resolve-addr.xml"/>
      </references>
    </references>

  </back>

<!-- [rfced] Please let us know if any changes are needed for the
following:

a) The following terms were used inconsistently in this document.
We chose to use the latter forms.  Please let us know any objections.

 'all routers' / all-routers (multicast address)

 neighbor cache / Neighbor Cache (per, with one exception (which
   appears to be an oversight) RFC 4861)

 neighbor discovery / Neighbor Discovery

 optimistic address / Optimistic address / Optimistic Address (per
   (with two exceptions, which appear to be oversights) RFC 4429)

 optimistic state / Optimistic state (per RFC 4429)

 Preferred / preferred (state) (per RFC 4862)

b) Please note that per RFC 4861 and (with only two exceptions)
post-6000 RFCs, we changed "Solicited Node Multicast Address" and
"solicited node multicast address" to "solicited-node multicast
address". -->

</rfc>