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<?xml version="1.0" encoding="UTF-8"?><!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" 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 Routers</title>    <seriesInfo name="RFC"    <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>        </postal>        <email>furry@google.com</email>      </address>    </author>    <date year="2021" month="August"/>    <area>Internet</area>    <workgroup>IPv6 Maintenance</workgroup>    <abstract>      <t>                    Neighbor Discovery 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 to allow routers to proactively create a Neighbor Cache entry when a new IPv6 address is assigned to a node. It also updates and recommends nodes send unsolicited Neighbor Advertisements upon assigning a new IPv6 address. 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 format="default"/> assumes that communications between IPv6 nodes in most and if a node A is trying to communicate to its neighbor, node B, the return traffic flows could be expected. when 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 That entry will be used for sending the return traffic to A.      </t>      <t>                          In particular, <xref sectionFormat="of" section="7.2.5"/> states:      </t><!-- DNE; --><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 the host populates its with the default router IPv6 and link-layer addresses and is able to send traffic to off-link destinations.                                  At the same 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 routers only keep a very small number of packets in the queue, as recommended in <xref sectionFormat="of" section="7.2.2"/>.Any additional packets arriving before the resolution &gt; process finishes are likely to result in dropped packets                                          It can cause packet loss and performance degradation that can be      </t>      <t>This document updates the Neighbor Discovery protocol <xref format="default"/> to avoid packet loss in the scenario described above.<xref format="default"/> discusses the changes and the potential impact, while normative changes to <xref format="default"/> are specified in <xref 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/> <xref/> 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"> device that implements <xref format="default"/>.</dd> node that is not a <xref format="default"/>.</dd>          <dt>ND:</dt><dd>Neighbor <xref format="default"/>.</dd>          <dt>NC:</dt><dd>Neighbor <xref format="default"/>. The Neighbor Cache entry can be in one of five states, as described in <xref sectionFormat="of" section="7.3.2"/>: INCOMPLETE, REACHABLE, STALE, DELAY, PROBE.</dd>          <dt>SLAAC:</dt><dd>IPv6 Stateless Address <xref format="default"/>.</dd>          <dt>NS:</dt><dd>Neighbor <xref format="default"/>.</dd>          <dt>NA:</dt><dd>Neighbor <xref format="default"/>.</dd>          <dt>RS:</dt><dd>Router <xref format="default"/>.</dd>          <dt>RA:</dt><dd>Router  <xref format="default"/>.</dd>          <dt>SLLAO:</dt><dd>Source Address option in the ND packets containing the link-layer address of the sender of the packet <xref format="default"/>.</dd>          <dt>TLLAO:</dt><dd>Target Address option in the ND packets containing the link-layer address of the target <xref format="default"/>.</dd>          <dt>GUA:</dt><dd>Global Unicast Address <xref format="default"/>.</dd>          <dt>DAD:</dt><dd>Duplicate Address <xref format="default"/>.</dd>          <dt>Preferred address assigned to an interface whose uniqueness has been verified using DAD and whose use by upper-layer protocols is <xref format="default"/>. Preferred addresses may be used as the source address of packets sent from the interface.</dd>          <dt>Optimistic modification of <xref format="default"/>.</dd></dl>      </section>    </section>    <section numbered="true" toc="default">      <name>Problem Statement</name>      <t>                 The most typical scenario when the problem may arise is a host joining the network, forming a new 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 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 Internet connectivityand/or the presence of a captive portal on the network.                                                  To speed up that many implementations use Optimistic which allows them to send probes before the DAD process is completed.                                                          At that the contains all information required to send those probes (such as the default router link-local and link-layer addresses).                                                  The however, might contain an entry for the link-localaddress (if the device has been performing address resolution for the 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 process by creating an INCOMPLETE cache entry and then sending a Neighbor Solicitation to the (<xref sectionFormat="of" section="7.3.2"/>).                As per <xref sectionFormat="of" <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. 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 address to be removed from the router's                                  However, the same sequence of events when the host starts using a new global address previously unseen by the router, such as a new privacy address <xref 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 it manifests quite clearly in IPv6-only environments, especially wireless 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 as well.                                 Devices implementing 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 case of duplicate addresses present in the network, the solution should not override the existing entry.                         </li>        <li>                                 In topologies with multiple first-hop 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 the solution must not exacerbate issues described in <xref format="default"/> and needs to be compatible with the recommendations provided in <xref 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      </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 format="default"/>.      </t>      <section anchor="hosts" numbered="true" toc="default">        <name>Nodes Sending Gratuitous Neighbor Advertisements</name>        <t>                              <xref 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 potential disruption in case of duplicate the node should not set the Override flag for a preferred address and must not set the Override flag if the address is in Optimistic <xref        </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 snooping <xref format="default"/>, then those unsolicited NAs will only sent to routers on the given network segment/link, instead of being flooded to all nodes.        </t>        <t>                              It should be noted that the mechanism does not cause any significant increase in multicast traffic.                              The additional multicast unsolicited would proactively create a STALE cache entry on as discussed below.                              When the router receives the return traffic it does not need to send multicast NSes to the multicast address but would be sending unicast NSes instead. 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 NA. The increase would be 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 Receiving Unsolicited Neighbor Advertisements</name>        <t>                              <xref sectionFormat="of" section="7.2.5"/> states:</t><!-- DNE; --><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>                              The reasoning behind dropping unsolicited Neighbor Advertisements ("the   recipient has apparently not initiated any communication with the   target") is valid for 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 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. changes do not modify specified in <xref format="default"/> for the scenario when the corresponding Neighbor Cache entry already exists.        </t>        <t>The next section 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 format="default"/>, they would send unsolicited 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 which means there is a possibility of the address not being unique on the link.                                            <xref sectionFormat="of" section="2.2"/> discusses measures to ensure that ND packets from the do not override any existing as it would cause interruption of the rightful address in case of address conflict.                                                    As nodes willing to speed up their network stack configuration are most likely to be affected by the problem outlined in this it seems reasonable for such hosts to advertise their 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 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) has the same address                                    This document uses the term "the rightful as the same terminology is used in <xref format="default"/>.The analysis assumes that the host performs as <xref 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 Neighbor Cache entry for the target address already exists in any state other than INCOMPLETE, then as per <xref sectionFormat="of" 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 the cache entry would not be updated with the new address and no impact the traffic for the rightful address owner is expected.        </t>        <t>The return traffic intended for the host with the Optimistic 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 in INCOMPLETE        <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 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 before sending 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 link-layer address) and sets the entry state to REACHABLE (as the NA has the Solicited flag set).</li>        </ol>        <t>As a some packets in the buffer at Step 6 and all packets arriving between Step 6 and Step 8) are delivered to the host with the while some of them, if not all, are intended for the rightful owner.Without the unsolicited NA, are in the buffer at Step 8 (usually just one 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 scenario is rather as it would require the followingthings 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 for the rightful owner.                                                            </li>        </ul>        <t>It should be noted that in this scenario the 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 delivered to the rightful owner (see <xref format="default"/>). The rightful owner would experience no disruption but might receive some packets intended for the host with Optimistic</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 (Step 7 and Step respectively).If the solicited NA arrives it changes the NC entry state from INCOMPLETE to REACHABLE. As discussed in <xref 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 can lead to the situation when the router does not have 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 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 Address</name>          <t>                                                    In this 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                                                    </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 it sends traffic using that entry, changes it to and waits up to DELAY_FIRST_PROBE_TIME                                                            </li>            <li>                                                                            The router changes the NC entry state to PROBE and sends up to MAX_UNICAST_SOLICIT unicast NSes separated by RetransTimer milliseconds to the host link-layer address.                                                    </li>            <li>                                                                    As the host has detected the address 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 sectionFormat="of" The default value for the DAD process would be 1*1*1000 + 1 = 2 <xref format="default"/>.If the host has completed DAD but did not detect the address then there are two hosts with the same address in the state and 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 2 are still arriving, the router buffers a small number of those packets and starts the address resolution again by sending a multicast NS to the multicast address. The rightful owner and the NC entry is updated with the rightful link-local address. The buffered are sent to that address. Any packets still arriving after the address resolution still completed are sent to the rightful address owner as well.                                                    </li>          </ol>          <t>                                                    The rightful owner is not experiencing any 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 2 are still arriving.</t>          <t>                                                    However, the same would be observed if changes 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.The buffered packet(s) and any packets still arriving after the address is resolved would be forwarded to the rightful owner of the address. 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 changes do introduce any disruption for the rightful owner of the duplicated address. this the rightful owner starts sending traffic from the address 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                                                    </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 intended for the rightful owner of the duplicated address, while some are for the new host. As the NC entry is it sends traffic using that entry, changes it to and waits up to DELAY_FIRST_PROBE_TIME                                                            </li>            <li>                                                                            The router changes the NC entry state to PROBE and sends up to MAX_UNICAST_SOLICIT unicast NSes separated by RetransTimer milliseconds to the host link-layer address.                                                    </li>            <li>                                                                    As the host has detected the address 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 the entry and triggers the resolution process. The router buffers the packet and sends a multicast NS to the multicast address. The rightful owner and the NC entry is updated with the rightful 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. 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 sectionFormat="of" this interval is equal to 5*1000 + (3 - 1)*1000 = or 7 seconds.          </t>          <t>                                                            However, it should be noted that the probability of such scenario is rather low. to the scenario discussed in <xref 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 format="default"/>, the disruption the rightful owner can easily be if that node implements the mechanism described in document. Sending unsolicited NAs before off-link communication would create a STALE entry in the NC and prevent any to that address sent to the host with the Optimistic (see <xref 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 Discovery for IP version 6        <section numbered="true" toc="default">          <name>Modification to          <t>                              This document makes the following changes to <xref sectionFormat="of" section="7.2.5"/>:          </t><!-- DNE; -->          <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>          <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 (if supplied). The entry's reachability state <bcp14>MUST</bcp14> be set to STALE. If the received Neighbor Advertisement does not contain the Target the advertisement <bcp14>SHOULD</bcp14> be silently discarded.</li>          </ul></blockquote>        </section>        <section anchor="UPD726" numbered="true" toc="default">          <name>Modification to          <t>                              This document the following changes to <xref sectionFormat="of" section="7.2.6"/>:          </t><!-- DNE; -->          <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>          <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 with the corresponding entries. In such a node <bcp14>SHOULD</bcp14> send up to MAX_NEIGHBOR_ADVERTISEMENT   Neighbor Advertisement messages. If the address is then the Override flag <bcp14>SHOULD NOT</bcp14> be set. If the address is in the Optimistic 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"> Optimistic DAD <xref format="default"/> is new Optimistic is assigned                                              to the node interface.</dd> Optimistic DAD is not 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 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 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 Neighbor Cache would trigger 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 example:      </t>      <ul spacing="normal">        <li>                                      Just do nothing.                              </li>        <li> from the "reactive" Neighbor Discovery to the registration-based mechanisms                              </li>        <li> new entries in Neighbor Cache by gleaning from Neighbor Discovery DAD messages.                              </li>        <li> bidirectional communication from the host to the router using the host GUA.                              </li>        <li> the probing logic on hosts more robust.                              </li>        <li> the buffer size on routers.                              </li>        <li>                                      Transit traffic from an unknown address (an address the corresponding entry) 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 IPv6 and 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 format="default"/> and expand the registration-based ND <xref used in Low-Power Wireless Personal Area Networks (6LoWPANs) to the rest of IPv6 deployments.                                              This option requires some investigation and discussion.                                              However, significant changes to the existing IPv6 implementations would be needed, so unclear adoption timeline makes this approach less preferable than 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 a STALE entry for its GUA in the 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 contains the host link-layer address.                                      </li>          <li>                                              The target address is the default router address (the default router address the host received in the RA).                                      </li>        </ul>        <t>                              The main disadvantages of this approach        </t>        <ul spacing="normal">          <li> not work for Optimistic as <xref 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 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 GUA</name>        <t>                                      The host could send a message to multicast address, using its GUA as a source.                                      If the host link-layer address is included in the Source Link-Layer Address the router would create a STALE entry for the host GUA as per <xref sectionFormat="of" section="6.2.6"/>.                                      However, this approach cannot be used if the GUA is in state: <xref sectionFormat="of" section="2.2"/>  explicitly prohibits using an Optimistic Address as the source address of a Router Solicitation with a as it might the rightful owner of the address in the case of a collision. for the the host can send an RS without 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).        </t>        <t>                              This approach has the following drawbacks:        </t>        <ul spacing="normal">          <li>                                                      If the address is in the Optimistic the RS cannot contain SLLAO. As a 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 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 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 the RS might not even reach the routers.                                              </li>        </ul>      </section>      <section numbered="true" toc="default">        <name>Routers Populating Their Caches by Gleaning 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.                               If no entry exists, the router may create an entry, set its reachability state to and start the address resolution for that entry.        </t>        <t>                              The same solution was proposed in <xref format="default"/>. Some routing vendors support such 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 might negatively impact CPU.                                              </li>          <li>                                                      If the router starts the address resolution as soon as it receives the DAD Neighbor the host might still 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 <xref sectionFormat="of" section="5.4.3"/>.                                                      As a the router might not be able to complete the address resolution before the return traffic arrives.                                              </li>        </ul>      </section>      <section numbered="true" toc="default">        <name>Initiating 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 RTT to any off-link it's quite likely that the router would start the 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 link-local address could be blocked by security policy or control plane protection mechanism.                                              </li>          <li>                                                      It introduces an additional overhead for 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 ff02::2 multicast address, if the network provides a first-hop 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> the probing logic on hosts might be modified to better with initial packet loss. For example, only one probe can be or 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 solution is implemented level.</li>          <li>            <t>Some implementations need to send multiple probes. Examples include but not limited to:            </t>            <ul spacing="normal">              <li>Sending AAAA and A DNS probes in parallel.</li>              <li>Detecting captive often sending multiple packets.</li>            </ul>          </li>          <li>            <t>While it would increase the probability the probing 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 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 increases network utilization and might cause 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 format="default"/> and make the router more vulnerable to ND-based attacks.        </t>      </section>      <section numbered="true" toc="default">        <name>Transit Traffic a New Address Address Resolution</name>        <t>                              When a router receives a transit packet sourced by on-link neighbor node, it might check the presence of entry for the packet source address if the entry does not exist, start 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 traffic.However, this approach has a number of particular:</t>        <ul spacing="normal">          <li>If traffic flows are the return traffic might not transit the same router as the original traffic triggered the address the entry is created on the "wrong" router, not the one actually needs the 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 addresses the router needs to have Neighbor Cache and the rest of the address space.        The proactive address resolution 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 exhaustion.</li>          <li>Implementing such functionality is much more complicated than all other as it would involve complex</li>        </ul>      </section>    </section>    <section anchor="IANA" numbered="true" toc="default">      <name>IANA Considerations</name>      <t>      This no      </t>    </section>    <section anchor="Security" numbered="true" toc="default">      <name>Security Considerations</name>      <t>                    One of the potential attack vectors to consider is cache the attacker might try to install a cache entry for the victim's IPv6 address and the attacker's address. However, it should be noted that this document does not propose any changes for the scenario when the ND cache for given IPv6 address already exists.                    Therefore, there are no new vectors for an attacker to override an existing cache entry.      </t>      <t><xref format="default"/> describes some corner cases when a host with duplicated Optimistic might get some packets intended for the rightful owner of the address. such scenarios do not introduce any new attack vectors: even without the an attacker can easily override the and redirect the traffic by sending NAs with the Solicited flag set.As discussed in <xref the scenario might cause a disruption for up to 7 seconds. risk is considered More importantly, for all cases described in <xref 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 and any subsequent unsolicited NAs sent from the host with an Optimistic would not override the NC entry.</t>      <t>                    A malicious host could attempt to exhaust the on the router by creating a large number of STALE entries. However, this attack vector is not and this document does not increase the risk of such an attack: the attacker could do it, for example, by sending NS or RS packet with SLLAO included. All recommendations from <xref format="default"/> still apply.      </t>      <t>                    Announcing a new address to 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 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 being flooded to all nodes.                            If peer-to-peer communications are not desirable for given network they should be prevented by proper security mechanisms. Therefore, the risk of allowing hosts to send unsolicited Neighbor Advertisements to all-routers multicast address is low.      </t>      <t>                    It should be noted that the mechanism 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 described in <xref sectionFormat="of" section="4.3.2"/> and <xref format="default"/>.      </t>    </section>    <section anchor="Acknowledgements" toc="default">      <name>Acknowledgements</name>      <t>                    Thanks to the following people (in alphabetical order) for their                            comments, 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"/>, <contact Vyncke"/>.      </t>    </section><!-- -->  </back></rfc>