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	rfc9131.form.xml	rfc9131.xml
	<?xml version="1.0" encoding="UTF-8"?>	<?xml version="1.0" encoding="UTF-8"?>
	<!DOCTYPE rfc SYSTEM "rfc2629-xhtml.ent">	<!DOCTYPE rfc SYSTEM "rfc2629-xhtml.ent">

	<!-- Updated by Chris 07/20/21 -->	<!-- 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" submissionType="IETF" category="std" consensus="true" xml:lang="en" tocInclude="true" symRefs="true" sortRefs="true" version="3">	<rfc xmlns:xi="http://www.w3.org/2001/XInclude" ipr="trust200902" obsoletes="" updates="4861" docName="draft-ietf-6man-grand-07" number="9131" submissionType="IETF" category="std" consensus="true" xml:lang="en" tocInclude="true" symRefs="true" sortRefs="true" version="3">

	<front>	<front>

	<title abbrev="Gratuitous ND">Gratuitous Neighbor Discovery: Creating Neighb	<title abbrev="Gratuitous ND">Gratuitous Neighbor Discovery: Creating Neighb
	or Cache Entries onFirst-Hop Routers</title>	or Cache Entries onFirst&nbhy;Hop Routers</title>
	<seriesInfo name="RFC"value="0000"/>	<seriesInfo name="RFC"value="9131"/>
	<author fullname="Jen Linkova" initials="J." surname="Linkova">	<author fullname="Jen Linkova" initials="J." surname="Linkova">
	<organization>Google</organization>	<organization>Google</organization>
	<address>	<address>
	<postal>	<postal>
	<street>1 Darling Island Rd</street>	<street>1 Darling Island Rd</street>
	<city>Pyrmont</city>	<city>Pyrmont</city>
	<region>NSW</region>	<region>NSW</region>
	<code>2009</code>	<code>2009</code>

	<country>AU</country>	<country>Australia</country>
	</postal>	</postal>

	<phone/>
	<email>furry@google.com</email>	<email>furry@google.com</email>
	</address>	</address>
	</author>	</author>
	<date year="2021" month="August"/>	<date year="2021" month="August"/>
	<area>Internet</area>	<area>Internet</area>
	<workgroup>IPv6 Maintenance</workgroup>	<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>	<abstract>
	<t>	<t>

	Neighbor Discovery (RFC4861) 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 to allow routers to proactively create a Neighbor Cache entry when a new IPv6 address is assigned to a node. It also updates RFC4861 and recommends nodes to send unsolicited Neighbor Advertisements upon assigning a new IPv6 address. The proposed change will minimize the delay and packet loss when a node initiates connections to an off-link destination from a new IPv6 address.	Neighbor Discovery (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 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 RFC 4861 and recommends that nodes send unsolicited Neighbor Advertisements upon assigning a new IPv6 address. 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>	</t>
	</abstract>	</abstract>
	</front>	</front>
	<middle>	<middle>
	<section numbered="true" toc="default">	<section numbered="true" toc="default">
	<name>Introduction</name>	<name>Introduction</name>
	<t>	<t>

	The Neighbor Discovery state machine defined in <xref format="default"/> assumes that communications between IPv6 nodes are in most cases bi-directional and if a node A is trying to communicate to its neighbor, node B, the return traffic flows could be expected. 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. That entry will be used for sending the return traffic to A.	The Neighbor Discovery state machine defined in <xref format="default"/> assumes that communications between IPv6 nodes are, in most cases, bidirectional and if a node A is trying to communicate to its neighbor, node B, the return traffic flows could be expected. So, 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 Neighbor Cache. That entry will be used for sending the return traffic to A.
	</t>	</t>
	<t>	<t>
	In particular, <xref sectionFormat="of" section="7.2.5"/> states:	In particular, <xref sectionFormat="of" section="7.2.5"/> states:
	</t>	</t>

	<!-- DNE;Lynne to verify -->	<!-- DNE;verified -->
	<blockquote>When a valid Neighbor Advertisement is received (either solicited or unsolicited), the Neighbor Cache is searched for the target's entry.	<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.	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>	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>	<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.	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 Rout	After joining the network and receiving a Rout
	erAdvertisement the host populates itsneighbor cache with the default routerI	erAdvertisement, the host populates itsNeighbor Cache with the default router
	Pv6 and link-layer addresses and is able to send traffic to off-linkdestination	IPv6 and link-layer addresses and is able to send traffic to off-linkdestinatio
	s.	ns.
	At the sametime the router does not have any	At the sametime, the router does not have any
	cache entries for the host global addresses yet and only starts addressresoluti	cache entries for the host global addresses yet and only starts addressresolut
	on upon receiving the first packet of the return traffic flow.	ion upon receiving the first packet of the return traffic flow.
	While waiting for the resolution tocomplete r	While waiting for the resolution tocomplete,
	outers only keep a very small number of packets in the queue, as recommended in	routers only keep a very small number of packets in the queue, as recommended in
	<xref sectionFormat="of" section="7.2.2"/>.	<xref sectionFormat="of" section="7.2.2"/>.
	Any additional packets arriving before the resolution > process finishes are	Any additional packets arriving before the resolution > process finishes are
	likely to result in dropped packets	likely to result in droppedpackets.
	It can cause packet loss and performan
	ce degradation that can beuser-visible.	<!-- [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 performan
		ce degradation that can bevisible to users.
	</t>	</t>
	<t>	<t>
	This document updates the Neighbor Discovery protocol <xref format="default"/> to avoid packet loss in the scenario described above.	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 analyses the potential impact, while normative changes to <xref format="default"/> are specified in <xref format="default"/>.	<xref format="default"/> discusses the changes and analyzes the potential impact, while normative changes to <xref format="default"/> are specified in <xref format="default"/>.
	</t>	</t>
	<section numbered="true" toc="default">	<section numbered="true" toc="default">
	<name>Requirements Language</name>	<name>Requirements Language</name>
	<t>The key words "<bcp14>MUST</bcp14>", "<bcp14>MUST NOT</bcp14>",	<t>The key words "<bcp14>MUST</bcp14>", "<bcp14>MUST NOT</bcp14>",
	"<bcp14>REQUIRED</bcp14>", "<bcp14>SHALL</bcp14>",	"<bcp14>REQUIRED</bcp14>", "<bcp14>SHALL</bcp14>",
	"<bcp14>SHALL NOT</bcp14>", "<bcp14>SHOULD</bcp14>",	"<bcp14>SHALL NOT</bcp14>", "<bcp14>SHOULD</bcp14>",
	"<bcp14>SHOULD NOT</bcp14>",	"<bcp14>SHOULD NOT</bcp14>",
	"<bcp14>RECOMMENDED</bcp14>", "<bcp14>NOT RECOMMENDED</bcp14>",	"<bcp14>RECOMMENDED</bcp14>", "<bcp14>NOT RECOMMENDED</bcp14>",
	"<bcp14>MAY</bcp14>", and "<bcp14>OPTIONAL</bcp14>" in this document	"<bcp14>MAY</bcp14>", and "<bcp14>OPTIONAL</bcp14>" in this document
	are to be interpreted as described in BCP 14	are to be interpreted as described in BCP 14
	<xref/> <xref/> when, and only	<xref/> <xref/> when, and only
	when, they appear in all capitals, as shown here.</t>	when, they appear in all capitals, as shown here.</t>
	</section>	</section>
	<section numbered="true" toc="default">	<section numbered="true" toc="default">
	<name>Terminology</name>	<name>Terminology</name>
	<dl newline="false" spacing="normal">	<dl newline="false" spacing="normal">

	<dt>Node:</dt><dd>a device that implementsIP, <xref	<dt>Node:</dt><dd>A device that implementsIP <xreff
	format="default"/>.</dd>	ormat="default"/>.</dd>
	<dt>Host:</dt><dd>any node that is not arouter, <xreftarget="RFC4861	<dt>Host:</dt><dd>Any node that is not arouter <xreftarget="RFC4861"
	" format="default"/>.</dd>	format="default"/>.</dd>
	<dt>ND:</dt><dd>NeighborDiscovery, <xrefformat="def	<dt>ND:</dt><dd>NeighborDiscovery <xrefformat="defa
	ault"/>.</dd>	ult"/>.</dd>
	<dt>NC:</dt><dd>NeighborCache, <xrefformat="default	<dt>NC:</dt><dd>NeighborCache <xrefformat="default"
	"/>. The Neighbor Cache entry can be in one of five states, as described in<xre	/>. The Neighbor Cache entry can be in one of five states, as described in<xref
	f sectionFormat="of" section="7.3.2"/>: INCOMPLETE, REACHABLE,	sectionFormat="of" section="7.3.2"/>: INCOMPLETE, REACHABLE,S
	STALE, DELAY, PROBE.</dd>	TALE, DELAY,or PROBE.</dd>
	<dt>SLAAC:</dt><dd>IPv6 Stateless AddressAutoconfiguration, <xreftar	<dt>SLAAC:</dt><dd>IPv6 Stateless AddressAutoconfiguration <xreftarg
	get="RFC4862" format="default"/>.</dd>	et="RFC4862" format="default"/>.</dd>
	<dt>NS:</dt><dd>NeighborSolicitation, <xrefformat="	<dt>NS:</dt><dd>NeighborSolicitation <xrefformat="d
	default"/>.</dd>	efault"/>.</dd>
	<dt>NA:</dt><dd>NeighborAdvertisement, <xrefformat=	<dt>NA:</dt><dd>NeighborAdvertisement <xrefformat="
	"default"/>.</dd>	default"/>.</dd>
	<dt>RS:</dt><dd>RouterSolicitation, <xrefformat="de	<dt>RS:</dt><dd>RouterSolicitation <xrefformat="def
	fault"/>.</dd>	ault"/>.</dd>
	<dt>RA:</dt><dd>RouterAdvertisement, <xrefformat="	<dt>RA:</dt><dd>RouterAdvertisement <xrefformat="d
	default"/>.</dd>	efault"/>.</dd>
	<dt>SLLAO:</dt><dd>Sourcelink-layer AddressOption, an option in the	<dt>SLLAO:</dt><dd>SourceLink-Layer AddressOption. An option in the
	ND packets containing the link-layer address of the sender of the packet <xref t	ND packets containing the link-layer address of the sender of the packet <xref t
	arget="RFC4861" format="default"/>.</dd>	arget="RFC4861" format="default"/>.</dd>
	<dt>TLLAO:</dt><dd>Targetlink-layer AddressOption, an option in the	<dt>TLLAO:</dt><dd>TargetLink-Layer AddressOption. An option in the
	ND packets containing the link-layer address of the target <xrefRFC4861
	" format="default"/>.</dd>	" format="default"/>.</dd>
	<dt>GUA:</dt><dd>Global Unicast Address <xref format="default"/>.</dd>	<dt>GUA:</dt><dd>Global Unicast Address <xref format="default"/>.</dd>

	<dt>DAD:</dt><dd>Duplicate AddressDetection, <xref	<dt>DAD:</dt><dd>Duplicate AddressDetection <xreffo
	format="default"/>.</dd>	rmat="default"/>.</dd>
	<dt>PreferredAddress:</dt><dd>an address assigned to an interface who	<dt>PreferredAddress:</dt><dd>An address assigned to an interface who
	se uniqueness has been verified using DAD and whose use by upper-layer protocols	se uniqueness has been verified using DAD and whose use by upper-layer protocols
	isunrestricted, <xref format="default"/>. Preferred addresses	isunrestricted <xref format="default"/>. Preferred addresses
	may be used as the source address of packets sent from the interface.</dd>	may be used as the source address of packets sent from the interface.</dd>
	<dt>OptimisticDAD:</dt><dd>a modification ofDAD, <xreftarget="RFC44	<dt>OptimisticDAD:</dt><dd>A modification ofDAD <xreftarget="RFC442
	29" format="default"/>.</dd>	9" format="default"/>.</dd>
	</dl>	</dl>
	</section>	</section>
	</section>	</section>
	<section numbered="true" toc="default">	<section numbered="true" toc="default">
	<name>Problem Statement</name>	<name>Problem Statement</name>
	<t>	<t>

	The most typical scenario when the problemmay arise is a host joining the network, forming a new address and	The most typical scenario when the problemdescribed in this document may arise is a host joining the network, forming a new address, and
	using that address for accessing the Internet:	using that address for accessing the Internet:
	</t>	</t>
	<ol spacing="normal" type="1"><li>	<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).	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 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.	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 the host can populate its Neighbor Cache with the router's link-local and link-layer addresses.	As a result, the host can populate its Neighbor Cache with the router's link-local and link-layer addresses.
	</li>	</li>
	<li>	<li>
	The host starts opening connections to off-link destinations.	The host starts opening connections to off-link destinations.

	A very common use case is a mobile device sending probes to detecttheInternet connectivity	A very common use case is a mobile device sending probes to detect Internet connectivity
	and/or the presence of a captive portal on the network.	and/or the presence of a captive portal on the network.

	To speed up thatprocess many	To speed up thatprocess, many
	implementations use OptimisticDAD which allows them to send probes before theD	implementations use OptimisticDAD, which allows them to send probes before the
	AD process is completed.	DAD process is completed.
	At thatmoment thedev	At thatmoment, thede
	ice neighbor cache contains all information required to send those probes(such	vice's Neighbor Cache contains all information required to send those probes(su
	as the default router link-local and link-layer addresses).	ch as the default router link-local and link-layer addresses).
	Therouter neighbor cache, how	Therouter's Neighbor Cache, h
	ever, might contain an entry for thedevice link-local	owever, might contain an entry for thedevice's link-local
	address (if the device has been performingthe address resolution for therouter	address (if the device has been performing address resolution for therouter's l
	link-local address), but there are no entries for any of the device's globalad	ink-local address), but there are no entries for any of the device's globaladdr
	dresses.	esses.
	</li>	</li>
	<li>	<li>
	Return traffic is received by the first-hop router.	Return traffic is received by the first-hop router.

	As the router does not have any cache entry for the host global	As the router does not have any cache entry for the host global
	address yet, the router starts theneighbor discovery process by creating an INC	address yet, the router starts theNeighbor Discovery process by creating an INC
	OMPLETE cache entry and then sending a Neighbor Solicitation to theSolicited No	OMPLETE cache entry and then sending a Neighbor Solicitation to thesolicited-no
	deMulticast Address (<xref sectionFormat="of" section="7.3.2"/	demulticast address (<xref sectionFormat="of" section="7.3.2"/
	>).	>).
	As per <xref sectionFormat="of" section="7.2.2"	As per <xref sectionFormat="of" section="7.2.2"
	/>	/>,
	Routers <bcp14>MUST</bcp14> buffer at least one data packet and <bcp14>MAY</bcp	routers <bcp14>MUST</bcp14> buffer at least one data packet and <bcp14>MAY</bcp
	14> buffer more, while resolving the packet destination address.	14> 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.	However, most router implementations limit the buffer size to a few packets only, and some implementations are known to buffer just one packet.

	So any subsequent packets arriving before the address resolution process is completed are causing packet loss by replacing older packets in the buffer.	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>
	<li>	<li>
	If the host sends multiple probes in parallel, in the worst case, it would consider all but one of them failed.	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.	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>	</li>
	</ol>	</ol>
	<t>	<t>

	This scenario illustrates the problem occurrin	This scenario illustrates the problem occurrin
	g when the device connects to the network for the first time or after an inactiv	g when the device connects to the network for the first time or after an inactiv
	ity period long enough for thedevice address to be removed from the router'sne	ity period long enough for thedevice's address to be removed from the router's
	ighbor cache.	Neighbor Cache.
	However, the same sequence of eventshappen wh	However, the same sequence of eventshappens w
	en the host starts using a new global address previously unseen by the router,s	hen the host starts using a new global address previously unseen by the router,
	uch as a new privacy address <xref format="default"/> or ifthe	such as a new privacy address <xref format="default"/> or ifth
	router's Neighbor Cache has been flushed.	e router's Neighbor Cache has been flushed.
	</t>	</t>
	<t>	<t>

	While in dual-stack networks this problem might be hidden by Happy Eyeballs <xref format="default"/> it manifests quite clearly in IPv6-only environments, especially wireless ones, leading to poor user experience and contributing to a negative perception of IPv6-only solutions as unstable and non-deployable.	While in dual-stack networks this problem might be hidden by Happy Eyeballs <xref format="default"/>, it manifests quite clearly in IPv6-only environments, especially wireless environments, leading to poor user experience and contributing to a negative perception of IPv6-only solutions as unstable and non-deployable.
	</t>	</t>
	</section>	</section>
	<section numbered="true" toc="default">	<section numbered="true" toc="default">
	<name>Solution Requirements</name>	<name>Solution Requirements</name>
	<t>	<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.	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:	In particular:

	</t>	</t>
	<ul spacing="normal">	<ul spacing="normal">
	<li>	<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.	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>
	<li>	<li>

	The solution needs to work for Optimisticaddre	The solution needs to work for OptimisticAddre
	sses as well.	sses as well.
	Devices implementingthe Optimistic DAD usually	Devices implementing Optimistic DAD usuallyatt
	attempt to minimize the delay in connecting to the network and therefore aremo	empt to minimize the delay in connecting to the network and therefore aremore l
	re likely to be affected by the problem described in this document.	ikely to be affected by the problem described in this document.
	</li>	</li>
	<li>	<li>

	Incase of duplicate addresses present in the network, the proposed solution should not override the existing entry.	Inthe case of duplicate addresses present in the network, the solution should not override the existing entry.
	</li>	</li>
	<li>	<li>

	In topologies with multiple first-hop 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.	In topologies with multiple first-hop 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>	</li>
	</ul>	</ul>
	<t>	<t>

	In addition the solution must not exacerbate issues described in <xref format="default"/> and needs to be compatible with the recommendations provided in <xref format="default"/>.	In addition, 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>	</t>
	</section>	</section>
	<section anchor="changes" numbered="true" toc="default">	<section anchor="changes" numbered="true" toc="default">
	<name>Changes to Neighbor Discovery</name>	<name>Changes to Neighbor Discovery</name>
	<t>	<t>

	The following changes are required to minimize the delay in creating new entries in a router neighbor cache	The following changes are required to minimize the delay in creating new entries in a router's Neighbor Cache:
	</t>	</t>
	<ul spacing="normal">	<ul spacing="normal">
	<li>	<li>
	A node sends unsolicited NAs upon assigning a new IPv6 address to its interface.	A node sends unsolicited NAs upon assigning a new IPv6 address to its interface.
	</li>	</li>
	<li>	<li>
	A router creates a new cache entry upon receiving an unsolicited NA from a host.	A router creates a new cache entry upon receiving an unsolicited NA from a host.
	</li>	</li>
	</ul>	</ul>
	<t>	<t>
	The following sections discuss these changes in more detail.	The following sections discuss these changes in more detail.
	Normative changes are specified in <xref format="default"/>.	Normative changes are specified in <xref format="default"/>.
	</t>	</t>
	<section anchor="hosts" numbered="true" toc="default">	<section anchor="hosts" numbered="true" toc="default">
	<name>Nodes Sending Gratuitous Neighbor Advertisements</name>	<name>Nodes Sending Gratuitous Neighbor Advertisements</name>
	<t>	<t>

	The <xref sectionFormat="of" section="7.2.6"/> discusses using unsolicited Neighbor	<xref sectionFormat="of" section="7.2.6"/> discusses using unsolicited Neighbor
	Advertisements to inform node neighbors of the new link-layer address quickly.	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	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.	address as well: the node can send gratuitous unsolicited Neighbor Advertisements upon assigning a new IPv6 address to its interface.
	</t>	</t>
	<t>	<t>

	To minimizethe potential disruption in case of duplicate 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 Optimistic <xref target="RFC4429" format="default"/> state.	To minimizepotential disruption in the case of duplicate 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 format="default"/>.
	</t>	</t>
	<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.	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 snooping <xref format="default"/>, then those unsolicited NAs will be only sent to routers on the given network segment/link, instead of being flooded to all nodes.	If the link-layer devices are performing Multicast Listener Discovery (MLD) snooping <xref format="default"/>, then those unsolicited NAs will only be sent to routers on the given network segment/link, instead of being flooded to all nodes.
	</t>	</t>
	<t>	<t>

	It should be noted that theproposed mechanismdoe	It should be noted that the mechanismdiscussed he
	s not cause any significant increase in multicast traffic.	re does not cause any significant increase in multicast traffic.
	The additional multicast unsolicitedNA wouldproa	The additional multicast unsolicitedNAs wouldpro
	ctively create a STALE cache entry onrouters as discussed below.	actively create a STALE cache entry onrouters, as discussed below.
	When the router receives the return trafficflows	When the router receives the return trafficflows,
	it does not need to send multicast NSes to thesolicited node multicast address	it does not need to send multicast NSes to thesolicited-node multicast address
	but would be sending unicast NSes instead.	but would be sending unicast NSes instead.
	Therefore this procedure would only produce aninc	Therefore, this procedure would only produce anin
	rease in the overall amount of multicast traffic if no return traffic arrivesfo	crease in the overall amount of multicast traffic if no return traffic arrivesf
	r the address that sent the unsolicited NA or if the router does not create aST	or the address that sent the unsolicited NA or if the router does not create aS
	ALE entry upon receiving such NA. The increase would benegligible as thataddit	TALE entry upon receiving suchan NA. The increase would benegligible, as that
	ional traffic is a few orders of magnitude less than the usual level ofNeighbor	additional traffic is a few orders of magnitude less than the usual level ofNei
	Discovery multicast traffic.	ghbor Discovery multicast traffic.
	</t>	</t>
	</section>	</section>
	<section numbered="true" toc="default">	<section numbered="true" toc="default">

	<name>Routers Creating Cache EntriesUpon Receiving Unsolicited Neighbor Advertisements</name>	<name>Routers Creating Cache Entriesupon Receiving Unsolicited Neighbor Advertisements</name>
	<t>	<t>

	The <xref sectionFormat="of" section="7.2.5"/> states:	<xref sectionFormat="of" section="7.2.5"/> states:
	</t>	</t>

	<!-- DNE;Lynne to verify -->	<!-- DNE;verified -->
	<blockquote>When a valid Neighbor Advertisement is received (either solicited or	<blockquote>When a valid Neighbor Advertisement is received (either solicited or
	unsolicited), the Neighbor Cache is searched for the target's entry.	unsolicited), the Neighbor Cache is searched for the target's entry.
	If no entry exists, the advertisement <bcp14>SHOULD</bcp14> be silently discarded.	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	There is no need to create an entry if none exists, since the
	recipient has apparently not initiated any communication with the	recipient has apparently not initiated any communication with the
	target.</blockquote>	target.</blockquote>
	<t>	<t>

		<!-- Quoted text below is DNE; verified -->
	The reasoning behind dropping unsolicited Neighbor Advertisements ("the	The reasoning behind dropping unsolicited Neighbor Advertisements ("the
	recipient has apparently not initiated any communication with the	recipient has apparently not initiated any communication with the

	target") is valid for onlink host-to-host communication but, as discussed above,	target") is valid for 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.	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.	Therefore, it would be beneficial to allow routers to create new entries upon receiving an unsolicited Neighbor Advertisement.
	</t>	</t>
	<t>	<t>

	This document updates <xrefRFC486
	1" format="default"/> so that routers create a new Neighbor Cache entry upon rec	1" format="default"/> so that routers create a new Neighbor Cache entry upon rec
	eiving an unsolicited Neighbor Advertisement for an address that does notalrea	eiving an unsolicited Neighbor Advertisement for an address that does notalread
	dy have a Neighbor Cache entry.	y have a Neighbor Cache entry.
	.	These changes do not modifythe ro
	The proposed changes do not modify	uter behavior specified in <xref format="default"/> forthe sce
	routers behaviour specified in <xref format="default"/> forth	nario when the corresponding Neighbor Cache entry already exists.
	e scenario when the corresponding Neighbor Cache entry already exists.

	</t>	</t>
	<t>	<t>

	The next section analyses various scenarios of duplicated addresses and discusses the potential impact of creating a STALE entry for a duplicated IPv6 address.	The next section analyzes various scenarios of duplicated addresses and discusses the potential impact of creating a STALE entry for a duplicated IPv6 address.
	</t>	</t>
	</section>	</section>
	</section>	</section>
	<section anchor="avoid_dis" numbered="true" toc="default">	<section anchor="avoid_dis" numbered="true" toc="default">
	<name>Avoiding Disruption</name>	<name>Avoiding Disruption</name>
	<t>	<t>

	If nodes following the recommendations in th	If nodes following the recommendations in th
	is document are using the DAD mechanism defined in <xref format	is document are using the DAD mechanism defined in <xref format
	="default"/>, they would send unsolicitedNA as soon as the address changes the	="default"/>, they would send unsolicitedNAs as soon as the address changes the
	state from tentative to preferred (after its uniqueness has been verified).	state from tentative to preferred (after its uniqueness has been verified).
	However, nodes willing to minimize n	However, nodes willing to minimize n
	etwork stack configuration delays might be usingoptimistic addresses, which mea	etwork stack configuration delays might be usingOptimistic Addresses, which mea
	ns there is a possibility of the address not being unique on the link.	ns there is a possibility of the address not being unique on the link.
	<xref sectionFormat	<xref sectionFormat
	="of" section="2.2"/> discusses measures to ensure that ND packets from theopti	="of" section="2.2"/> discusses measures to ensure that ND packets from theOpti
	misticaddress do not override any existingneighbor cache entries as it wouldc	misticAddress do not override any existingNeighbor Cache entries, as it would
	ause traffic interruption of the rightful addressowner in case ofaddress conf	cause interruption of the rightful addressowner's traffic inthe case ofan add
	lict.	ress conflict.
	As nodes willing to speed up	As nodes willing to speed up
	their network stack configuration are most likely to be affected by the problem	their network stack configuration are most likely to be affected by the problem
	outlined in thisdocument it seems reasonable for such hosts to advertisetheir	outlined in thisdocument, it seems reasonable for such hosts to advertisethei
	optimistic addresses by sending unsolicited NAs.	r Optimistic Addresses by sending unsolicited NAs.
	The main question to conside	The main question to conside
	r is the potential risk of overriding the cache entry for the rightful address o	r is the potential risk of overriding the cache entry for the rightful address o
	wner if theoptimistic address happens to be duplicated.	wner if theOptimistic Address happens to be duplicated.
	</t>	</t>
	<t>	<t>

	The following sections discuss the address c	The following sections discuss the address c
	ollision scenario when a node sends an unsolicited NA for an address in the Opti	ollision scenario when a node sends an unsolicited NA for an address in the Opti
	mistic state, while another node (the rightful owner) has the sameaddress assig	mistic state, while another node (the rightful owner)already has the sameaddre
	ned already.	ss assigned.
	This document uses the term "the rightful ow	This document uses the term "the rightful ow
	ner" as the same terminology is used in <xrefformat="default"/	ner", as the same terminology is used in <xrefformat="default"
	>.	/>.
	The analysis assumes that the host performsDuplicate Address Detection, as<xre	The analysis assumes that the host performsDAD, as<xrefsecti
	fsectionFormat="of" section="5.4"/> requires that DAD<bcp14>M	onFormat="of" section="5.4"/> requires that DAD<bcp14>MUST</bcp14> be performed
	UST</bcp14> be performed on all unicast	on all unicast
	addresses prior to assigning them to an interface.	addresses prior to assigning them to an interface.
	</t>	</t>
	<section anchor="avoid_dis_exists" numbered="true" toc="default">	<section anchor="avoid_dis_exists" numbered="true" toc="default">
	<name>Neighbor Cache Entry Exists in Any State Other Than INCOMPLETE</name>	<name>Neighbor Cache Entry Exists in Any State Other Than INCOMPLETE</name>
	<t>	<t>

	If the router 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"/> 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 the router cache entry would not be updated with the new Link-Layer address and no impact to the traffic for the rightful address owner is expected.	If the 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"/>, 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, the router's cache entry would not be updated with the new link-layer address and no impact on the traffic for the rightful address owner is expected.
	</t>	</t>
	<t>	<t>

	The return traffic intended for the host with the Optimisticaddress would be sent to the rightful owner. However, this is unavoidable with or without the unsolicited NA mechanism.	The return traffic intended for the host with the OptimisticAddress would be sent to the rightful owner. However, this is unavoidable with or without the unsolicited NA mechanism.
	</t>	</t>
	</section>	</section>
	<section anchor="avoid_dis_inc" numbered="true" toc="default">	<section anchor="avoid_dis_inc" numbered="true" toc="default">

	<name>Neighbor Cache Entryis in INCOMPLETE state</name>	<name>Neighbor Cache EntryIs in INCOMPLETE State</name>
	<t>	<t>
	Another corner case is the INCOMPLETE cache entry for the address.	Another corner case is the INCOMPLETE cache entry for the address.
	</t>	</t>
	<ol spacing="normal" type="1"><li>	<ol spacing="normal" type="1"><li>
	The router receives a packet for the rightful owner of the address.	The router receives a packet for the rightful owner of the address.
	</li>	</li>
	<li>	<li>
	The router starts the address resolution process by creating an INCOMPLETE entry and sends the multicast NS.	The router starts the address resolution process by creating an INCOMPLETE entry and sends the multicast NS.
	</li>	</li>
	<li>	<li>
	More packets arrive at the router for the address in question.	More packets arrive at the router for the address in question.
	</li>	</li>
	<li>	<li>

	The host configures an Optimisticaddress and sends an unsolicited NA.	The host configures an OptimisticAddress and sends an unsolicited NA.
	</li>	</li>
	<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).	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>
	<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/>, 5 secs) before sending unicast NS.	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 (5 seconds) <xref/> before sending a unicast NS.
	</li>	</li>
	<li>	<li>
	The rightful owner responds to the multicast NS sent at Step 2 with a solicited NA with the Override flag set.	The rightful owner responds to the multicast NS sent at Step 2 with a solicited NA with the Override flag set.
	</li>	</li>
	<li>	<li>

	The router updates the entry with the TLLAO supplied (the rightful owner link-layer address) and sets the entry state to REACHABLE (as the NA has the Solicitedflag set).	The router updates the entry with the TLLAO supplied (the rightful owner's link-layer address) and sets the entry state to REACHABLE (as the NA has the Solicitedflag set).
	</li>	</li>
	</ol>	</ol>
	<t>	<t>

	As aresult some packets(ones in the buffer at Step 6 and all packetsarriving	As aresult, some packets(packets in the buffer at Step 6 and all packetsarriv
	between Step 6 and Step 8) are delivered to the host with theOptimisitc address	ing between Step 6 and Step 8) are delivered to the host with theOptimistic Add
	, while some of them, if not all, are intended for the rightful owner.	ress, while some of them, if not all, are intended for the rightful owner.
	Without the unsolicited NA,packet which are in the buffer at Step 8 (usuallyju	Without the unsolicited NA,one or more packets that are in the buffer at Step 8
	st onepacket but some routers may buffer a few) would have beendelivered toth	(usuallyjust onepacket, but some routers may buffer a few) would have beende
	e rightful owner and the rest of the packets would have beendropped.	livered tothe rightful owner and the rest of the packets would have beendroppe
	However, the probability of such scenario is ratherlow as it would requirethe	d.
	following	However, the probability of sucha scenario is ratherlow, as it would requiret
		he following
	things to happen almost simultaneously (within tens of milliseconds in most cases):	things to happen almost simultaneously (within tens of milliseconds in most cases):
	</t>	</t>
	<ul spacing="normal">	<ul spacing="normal">
	<li>	<li>
	One host starts using a new IPv6 address and sending traffic without sending an unsolicited NA first.	One host starts using a new IPv6 address and sending traffic without sending an unsolicited NA first.
	</li>	</li>
	<li>	<li>

	Another host configures the same IPv6 address in Optimistic mode before the router completes the address resolution for the rightful owner.	Another host configures the same IPv6 address in Optimistic mode before the router completes the address resolutionprocessfor the rightful owner.
	</li>	</li>
	</ul>	</ul>
	<t>	<t>

	It should be noted that in this scenario the rigthful 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 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 address.	It should be noted that in this scenario the 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 to be 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 an Optimistic Address.
	</t>	</t>
	<t>	<t>

	This section focuses on the scenario when the solicited NA from the rightful own	This section focuses on the scenario when the solicited NA from the rightful own
	er arrives after the unsolicited one sent from the Optimisticaddress (Step 7 an	er arrives after the unsolicited one sent from the OptimisticAddress (Step 7 an
	d Step4 respectively).	d Step4, respectively).
	If the solicited NA arrivesfirst it changes the NC entry state from INCOMPLETE	If the solicited NA arrivesfirst, it changes the NC entry state from INCOMPLETE
	to REACHABLE. As discussed in <xrefformat="default"/>	to REACHABLE. As discussed in <xrefformat="default"/
	, there will be no disruption for the rightful owner if the router already has a	>, there will be no disruption for the rightful owner if the router already has
	REACHABLE entry for the address when an unsolicited NA is received.	a REACHABLE entry for the address when an unsolicited NA is received.
	</t>	</t>
	</section>	</section>
	<section anchor="avoid_dis_nonexists" numbered="true" toc="default">	<section anchor="avoid_dis_nonexists" numbered="true" toc="default">
	<name>Neighbor Cache Entry Does Not Exist</name>	<name>Neighbor Cache Entry Does Not Exist</name>
	<t>	<t>

	There are two distinct scenarioswhich can lead to the situation when the router does not have a NC entry for the IPv6 address:	There are two distinct scenariosthat can lead to the situation when the router does not have an NC entry for the IPv6 address:

	</t>	</t>
	<ol spacing="normal" type="1"><li>	<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.	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>
	<li>	<li>

	The rightful owner just started sending packets from that address but the router has not received any return traffic yet.	The rightful owner just started sending packets from that address, but the router has not received any return traffic yet.
	</li>	</li>
	</ol>	</ol>
	<t>	<t>
	The impact on the rightful owner's traffic flows would be different in those cases.	The impact on the rightful owner's traffic flows would be different in those cases.
	</t>	</t>
	<section numbered="true" toc="default">	<section numbered="true" toc="default">

	<name>The Rightful Owner Is Not Sending PacketsFrom The Address</name>	<name>The Rightful Owner Is Not Sending Packetsfrom the Address</name>
	<t>	<t>

	In this scenario the following events are expected to happen:	In this scenario, the following events are expected to happen:

	</t>	</t>
	<ol spacing="normal" type="1"><li>	<ol spacing="normal" type="1"><li>
	The host configures the address and sets its state to Optimistic.	The host configures the address and sets its state to Optimistic.
	</li>	</li>
	<li>	<li>

	The host sends an unsolicited NA with the Override flag set to zero and starts sending traffic from the Optimisticaddress.	The host sends an unsolicited NA with the Override flag set to zero and starts sending traffic from the OptimisticAddress.
	</li>	</li>
	<li>	<li>
	The router creates a STALE entry for the address and the host link-layer address.	The router creates a STALE entry for the address and the host link-layer address.
	</li>	</li>
	<li>	<li>
	The host starts DAD and detects the address duplication.	The host starts DAD and detects the address duplication.
	</li>	</li>
	<li>	<li>

	The router receives the return traffic for the duplicated address. As the NC entry is STALE it sends traffic using that entry, changes it to DELAY and waits up to DELAY_FIRST_PROBE_TIME (<xref format="default"/>) seconds.	The router receives the return traffic for the duplicated address. As the NC entry is STALE, it sends traffic using that entry, changes it to DELAY, and waits up to DELAY_FIRST_PROBE_TIME seconds <xref format="default"/>.
	</li>	</li>
	<li>	<li>

	The router changes the NC entry state to PROBE and sends up to MAX_UNICAST_SOLICIT(<xref format="default"/>) unicast NSes separated by RetransTimer milliseconds (<xref format="default"/>) to the host link-layer address.	The router changes the NC entry state to PROBE and sends up to MAX_UNICAST_SOLICITunicast NSes <xref format="default"/> separated by RetransTimer milliseconds <xref format="default"/> to the host link-layer address.
	</li>	</li>
	<li>	<li>

	As the host	As the host
	has detected the addressconflict already it does not respond to the unicastNSe	hasalready detected the addressconflict, it does not respond to the unicastNS
	s. (It is unlikely that the host has not completed the DAD process at thisstage	es. (It is unlikely that the host has not completed the DAD process at thisstag
	, as DELAY_FIRST_PROBE_TIME (5 seconds) is much higher than the DAD duration(Du	e, as DELAY_FIRST_PROBE_TIME (5 seconds) is much higher than the DAD duration(D
	pAddrDetectTransmitsRetransTimer1000 + MAX_RTR_SOLICITATION_DELAYsecs, <xref	upAddrDetectTransmitsRetransTimer1000 + MAX_RTR_SOLICITATION_DELAYseconds) (<
	target="RFC4862" sectionFormat="of"section="5.4"/>). The default valuefor the	xref sectionFormat="of"section="5.4"/>).) The default valuefo
	DAD process would be 111000 + 1 = 2secs, <xrefformat="defau	r the DAD process would be 111000 + 1 = 2seconds <xrefforma
	lt"/>. If the host has completed DAD but did not detect the addressconflict the	t="default"/>.
	n there are two hosts with the same address in thePreferred state andthe disru	If the host has completed DAD but did not detect the addressconflict, then ther
	ption isinevitable anyway.	e are two hosts with the same address in thepreferred state anddisruption isi
		nevitable anyway.
	</li>	</li>
	<li>	<li>
	As the router receives no response for the unicast NSes, it deletes the NC entry.	As the router receives no response for the unicast NSes, it deletes the NC entry.
	</li>	</li>
	<li>	<li>

	If return pa	If return pa
	ckets for communication initiated atstep 2 are still arriving, the router buffe	ckets for communication initiated atStep 2 are still arriving, the router buffe
	rs a small number of those packets and starts the address resolutionagain byse	rs a small number of those packets and starts the address resolutionprocess aga
	nding a multicast NS to thesolicited node multicast address. Therightful owner	in bysending a multicast NS to thesolicited-node multicast address. Therightf
	responds and therouter NC entry is updated with the rightfulowner link-local	ul ownerresponds, and therouter's NC entry is updated with the rightfulowner'
	address. The bufferedpacket(s) are sent to that address. Any packets stillarri	s link-local address. The bufferedpacket or packets are sent to that address.A
	ving after the address resolution still completed are sent to the rightfuladdre	ny packets still arriving after the address resolution process has completed are
	ss owner as well.	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 stillarriving after the address
		resolution still completed are sent to the rightfuladdress 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>	</li>
	</ol>	</ol>
	<t>	<t>

	The rightful owner is not ex	The rightful owner is not ex
	periencing anydisruption as it does not send any traffic.	periencing anydisruption, as it does not send any traffic.
	It would only start receiving packets intended for another host after Step 8 is	It would only start receiving packets intended for another host after Step 8 is
	completed and only if return packets for the communication initiated atstep 2 a	completed and only if return packets for the communication initiated atStep 2 a
	re still arriving.	re still arriving.
	</t>	</t>
	<t>	<t>


	However, the samebehaviour	However, the samebehavior w
	would be observed if changesproposed in this document are not implemented.	ould be observed ifthe changesspecified in this document are not implemented.
	If the host starts sending p	If the host starts sending p
	ackets from its Optimisticaddress but then changes the address state to Duplica	ackets from its OptimisticAddress but then changes the address state to Duplica
	ted, the first return packet would trigger the address resolution process and wo	ted, the first return packet would trigger the address resolution process and wo
	uld be buffered until the resolution is completed.	uld 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.	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 the rightful owner might still receive one or more packets from the flowsint	So, the rightful owner might still receive one or more packets from the flowsin
	ended for another host.	tended for another host.
	Therefore, it's safe to conclude that the proposed changes do introduce anydisr	Therefore, it's safe to conclude that thechanges specified in this document do
	uption for the rightful owner of the duplicated address.	not introduce any disruption for the rightful owner of the duplicated address.

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

		Original:
		Therefore,
		it's safe to conclude that 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>	</t>
	</section>	</section>
	<section anchor="dis_start" numbered="true" toc="default">	<section anchor="dis_start" numbered="true" toc="default">

	<name>The Rightful Owner Has Started Sending PacketsFrom The Address</name>	<name>The Rightful Owner Has Started Sending Packetsfrom the Address</name>
	<t>	<t>

	In this scenario the following events are happening:	In this scenario, the following events are happening:

	</t>	</t>
	<ol spacing="normal" type="1"><li>	<ol spacing="normal" type="1"><li>

	The rightful owner starts sending traffic from the address (e.g. the address has just been configured or has not been recently used).	The rightful owner starts sending traffic from the address (e.g., the address has just been configured or has not been recently used).
	</li>	</li>
	<li>	<li>
	The host configures the address and sets its state to Optimistic.	The host configures the address and sets its state to Optimistic.
	</li>	</li>
	<li>	<li>

	The host sends an unsolicited NA with the Override flag set to zero and starts sending traffic from the Optimisticaddress.	The host sends an unsolicited NA with the Override flag set to zero and starts sending traffic from the OptimisticAddress.
	</li>	</li>
	<li>	<li>
	The router creates a STALE entry for the address and the host link-layer address.	The router creates a STALE entry for the address and the host link-layer address.
	</li>	</li>
	<li>	<li>
	The host starts DAD and detects the address duplication.	The host starts DAD and detects the address duplication.
	</li>	</li>
	<li>	<li>

	The router receives the return traffic for the IPv6 address in question. Some flowsintended for the rightful owner of the duplicated address, while some are for the new host. As the NC entry is STALE it sends traffic using that entry, changes it to DELAY and waits up to DELAY_FIRST_PROBE_TIME (<xref format="default"/>) seconds.	The router receives the return traffic for the IPv6 address in question. Some flowsare intended for the rightful owner of the duplicated address, while some are for the new host. As the NC entry is STALE, it sends traffic using that entry, changes it to DELAY, and waits up to DELAY_FIRST_PROBE_TIME seconds <xref format="default"/>.
	</li>	</li>
	<li>	<li>

	The router changes the NC entry state to PROBE and sends up to MAX_UNICAST_SOLICIT(<xref format="default"/>) unicast NSes separated by RetransTimer milliseconds (<xref format="default"/>) to the host link-layer address.	The router changes the NC entry state to PROBE and sends up to MAX_UNICAST_SOLICITunicast NSes <xref format="default"/> separated by RetransTimer milliseconds <xref format="default"/> to the host link-layer address.
	</li>	</li>
	<li>	<li>

	As the host hasdetected the address conflict already it does not respond to the unicast NSes.	As the host hasalready detected the address conflict, it does not respond to the unicast NSes.
	</li>	</li>
	<li>	<li>
	As the router receives no response for the unicast NSes, it deletes the NC entry.	As the router receives no response for the unicast NSes, it deletes the NC entry.
	</li>	</li>
	<li>	<li>

	The next packet re-creates the entry and triggers the resolution process. The router buffers the packet and sends a multicast NS to the solicited node multicast address. The rightful owner responds and the router NC entry is updated with the rightful owner link-local address.	The next packet recreates the entry and triggers the resolution process. The router buffers the packet and sends a multicast NS to the solicited-node multicast address. The rightful owner responds, and the router's NC entry is updated with the rightful owner's link-local address.
	</li>	</li>
	</ol>	</ol>
	<t>	<t>

	As a result the traffic for	As aresult, the traffic for
	the address rightful owner would be sent to the host with the duplicated address	the address rightful owner would be sent to the host with the duplicated addres
	instead. The duration of the disruption can be estimated asDELAY_FIRST_PROBE_T	s instead.
	IME1000 + (MAX_UNICAST_SOLICIT - 1)RetransTimer milliseconds.
	As per the constants defined	<!-- [rfced] Section 5.3.2: Should "address rightful owner" be
	in <xref sectionFormat="of"section="10"/> this interval iseq	"address's rightful owner", "address of the rightful owner", or
	ual to 51000 + (3 - 1)1000 =7000ms or 7 seconds.	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 asDELAY_FIRST_PROBE_TIME*1000
		+ (MAX_UNICAST_SOLICIT - 1)*RetransTimer milliseconds.
		As per the constants defined
		in <xref sectionFormat="of"section="10"/>, this interval ise
		qual to 51000 + (3 - 1)1000 =7000 milliseconds, or 7 seconds.

	</t>	</t>
	<t>	<t>

	However, it should be noted that the probability of suchscenario is rather low. Similary 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):	However, it should be noted that the probability of sucha scenario is rather low. Similar 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>	</t>
	<ul spacing="normal">	<ul spacing="normal">
	<li>	<li>
	One host starts using a new IPv6 address and sending traffic without sending an unsolicited NA first.	One host starts using a new IPv6 address and sending traffic without sending an unsolicited NA first.
	</li>	</li>
	<li>	<li>
	Another host configures the same IPv6 address in Optimistic mode before the router receives the return traffic for the first host.	Another host configures the same IPv6 address in Optimistic mode before the router receives the return traffic for the first host.
	</li>	</li>
	</ul>	</ul>
	<t>	<t>

	As discussed in <xref format="default"/>, the disruptionto the rightful owner can easily be prevent if that node implements the mechanism described in the document. Sending unsolicited NAs before initiatining off-link communication would create a STALE entry in the router NC and prevent any tarffic to that address to be sent to the host with the Optimistic address (see <xref format="default"/>).	As discussed in <xref format="default"/>, the disruptionfor the rightful owner can easily be prevented if that node implements the mechanism described in this document. Sending unsolicited NAs before initiating off-link communication would create a STALE entry in the router's NC and prevent any traffic to that address from being sent to the host with the Optimistic Address (see <xref format="default"/>).
	</t>	</t>
	</section>	</section>
	</section>	</section>
	</section>	</section>
	<section anchor="RFC_UPD" numbered="true" toc="default">	<section anchor="RFC_UPD" numbered="true" toc="default">
	<name>Modifications to RFC-Mandated Behavior</name>	<name>Modifications to RFC-Mandated Behavior</name>
	<t>	<t>
	All normative text in this memo is contained in this section.	All normative text in this memo is contained in this section.
	</t>	</t>
	<section numbered="true" toc="default">	<section numbered="true" toc="default">

	<name>Modification to RFC4861 Neighbor Discovery for IP version 6 (IPv6)</name>	<name>Modification to RFC 4861 ("Neighbor Discovery for IP version 6 (IPv6"))</name>
	<section numbered="true" toc="default">	<section numbered="true" toc="default">

	<name>Modification tothe section 7.2.5</name>	<name>Modification toSection 7.2.5 of RFC 4861</name>
	<t>	<t>

	This document makes the following changes tothe <xref sectionFormat="of" section="7.2.5"/>:	This document makes the following changes to<xref sectionFormat="of" section="7.2.5"/>:
	</t>	</t>

	<t>OLD TEXT:</t>	<t>The text in RFC 4861 is as follows:</t>
	<!-- DNE;Lynne to verify -->	<!-- DNE;verified -->
	<blockquote>When a valid Neighbor Advertisement is received (either solicited or	<blockquote>When a valid Neighbor Advertisement is received (either solicited or
	unsolicited), the Neighbor Cache is searched for the target's entry.	unsolicited), the Neighbor Cache is searched for the target's entry.
	If no entry exists, the advertisement <bcp14>SHOULD</bcp14> be silently discarded.	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	There is no need to create an entry if none exists, since the
	recipient has apparently not initiated any communication with the	recipient has apparently not initiated any communication with the
	target.</blockquote>	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	<blockquote><t>When a valid Neighbor Advertisement is received (either solicited or
	unsolicited), the Neighbor Cache is searched for the target's entry.	unsolicited), the Neighbor Cache is searched for the target's entry.
	If no entry exists:</t><t/>	If no entry exists:</t><t/>
	<ul spacing="normal">	<ul spacing="normal">
	<li> Hosts <bcp14>SHOULD</bcp14> silently discard the advertisement.	<li> Hosts <bcp14>SHOULD</bcp14> silently discard the advertisement.
	There is no need to create an entry if none exists, since the	There is no need to create an entry if none exists, since the
	recipient has apparently not initiated any communication with the target.	recipient has apparently not initiated any communication with the target.
	</li>	</li>

	<li> Routers <bcp14>SHOULD</bcp14> create a new entry for the target address with the link-layer address set to the Targetlink-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 the advertisement <bcp14>SHOULD</bcp14> be silently discarded.	<li> Routers <bcp14>SHOULD</bcp14> create a new entry for the target address with the link-layer address set to the TargetLink-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, the advertisement <bcp14>SHOULD</bcp14> be silently discarded.
	</li>	</li>
	</ul>	</ul>
	</blockquote>	</blockquote>
	</section>	</section>
	<section anchor="UPD726" numbered="true" toc="default">	<section anchor="UPD726" numbered="true" toc="default">

	<name>Modification tothe section 7.2.6</name>	<name>Modification toSection 7.2.6 of RFC 4861</name>
	<t>	<t>

	This documentproposes the following changes to the <xref sectionFormat="of" section="7.2.6"/>:	This documentmakes the following changes to <xref sectionFormat="of" section="7.2.6"/>:
	</t>	</t>

	<t>OLD TEXT:</t>	<t>The text in RFC 4861 is as follows:</t>
	<!-- DNE;Lynne to verify -->	<!-- DNE;verified -->
	<blockquote>Also, a node belonging to an anycast address <bcp14>MAY</bcp14> multicast	<blockquote>Also, a node belonging to an anycast address <bcp14>MAY</bcp14> multicast
	unsolicited Neighbor Advertisements for the anycast address when the	unsolicited Neighbor Advertisements for the anycast address when the
	node's link-layer address changes.</blockquote>	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	<blockquote><t>Also, a node belonging to an anycast address <bcp14>MAY</bcp14> multicast
	unsolicited Neighbor Advertisements for the anycast address when the	unsolicited Neighbor Advertisements for the anycast address when the
	node's link-layer address changes.</t>	node's link-layer address changes.</t>


	<t>A node may also wish to notify its first-hop routers when it config	<t>A node may also wish to notify its first-hop routers when it config
	ures a new global IPv6 address so the routers can proactively populate theirnei	ures a new global IPv6 address so the routers can proactively populate theirNei
	ghborcaches with the corresponding entries. In suchcases a node<bcp14>SHOULD<	ghborCaches with the corresponding entries. In suchcases, a node<bcp14>SHOULD
	/bcp14> send up to MAX_NEIGHBOR_ADVERTISEMENT	</bcp14> send up to MAX_NEIGHBOR_ADVERTISEMENT
	Neighbor Advertisement messages. If the address ispreferred then theOverrid	Neighbor Advertisement messages. If the address ispreferred, then theOverri
	e flag <bcp14>SHOULD NOT</bcp14> be set. If the address is in the Optimisticsta	de flag <bcp14>SHOULD NOT</bcp14> be set. If the address is in the Optimisticst
	te then the Override flag <bcp14>MUST NOT</bcp14> be set. The destinationaddre	ate, then the Override flag <bcp14>MUST NOT</bcp14> be set. The destinationadd
	ss <bcp14>SHOULD</bcp14> be set to the all-routers multicast address. Theseadve	ress <bcp14>SHOULD</bcp14> be set to the all-routers multicast address. Thesead
	rtisements <bcp14>MUST</bcp14> be separated by at	vertisements <bcp14>MUST</bcp14> be separated by at
	least RetransTimer seconds. The first advertisement <bcp14>SHOULD</bcp14> be sent as soon as one of the	least RetransTimer seconds. The first advertisement <bcp14>SHOULD</bcp14> be sent as soon as one of the
	following events happens:</t><t/>	following events happens:</t><t/>
	<dl newline="false" spacing="normal">	<dl newline="false" spacing="normal">

	<dt>if Optimistic DAD <xref format="default"/> is used:</dt><dd>a new Optimistic address is assigned	<dt>If Optimistic DAD <xref format="default"/> is used:</dt><dd>A new Optimistic Address is assigned
	to the node interface.</dd>	to the node interface.</dd>

	<dt>if Optimistic DAD is not used:</dt><dd>an address changes the state from	<dt>If Optimistic DAD is not used:</dt><dd>An address changes the state from
	tentative to preferred.</dd>	tentative to preferred.</dd>
	</dl>	</dl>
	</blockquote>	</blockquote>
	</section>	</section>
	</section>	</section>
	</section>	</section>
	<section numbered="true" toc="default">	<section numbered="true" toc="default">
	<name>Solution Limitations</name>	<name>Solution Limitations</name>
	<t>	<t>

	The solution described in this document provides some improvement for a node configuring a new IPv6 address and startingsending traffic from it.	The solution described in this document provides some improvement for a node configuring a new IPv6 address and startingto 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.	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:	For example:
	</t>	</t>
	<ul spacing="normal">	<ul spacing="normal">

	<li>If the host starts using analready configured IPv6 address after a	<li>If the host starts using analready-configured IPv6 address after a
	long period of inactivity, the router might not have the NC entry for that addre	long period of inactivity, the router might not have the NC entry for that addre
	ss anymore, as old/expired entries are deleted. </li>	ss anymore, as old/expired entries are deleted. </li>
	<li>Clearing therouter Neighbor Cache would triggerthe packet loss for	<li>Clearing therouter's Neighbor Cache would trigger packet loss fora
	all actively used addresses removed from the cache.</li>	ll actively used addresses removed from the cache.</li>
	</ul>	</ul>
	</section>	</section>
	<section anchor="others" numbered="true" toc="default">	<section anchor="others" numbered="true" toc="default">
	<name>Solutions Considered but Discarded</name>	<name>Solutions Considered but Discarded</name>
	<t>	<t>

	There are other possible approaches to address the problem, for example:	There are other possible approaches to address the problem. For example:
	</t>	</t>
	<ul spacing="normal">	<ul spacing="normal">
	<li>	<li>
	Just do nothing.	Just do nothing.
	</li>	</li>
	<li>	<li>

	Migrating from the "reactive" Neighbor Discovery (<xref format="default"/>) to the registration-based mechanisms (<xref format="default"/>).	Migrate from the "reactive" Neighbor Discovery <xref format="default"/> to the registration-based mechanisms <xref format="default"/>.
	</li>	</li>
	<li>	<li>

	Creating new entries in routers Neighbor Cache by gleaning from Neighbor Discovery DAD messages.	Create new entries in the router's Neighbor Cache by gleaning from Neighbor Discovery DAD messages.
	</li>	</li>
	<li>	<li>

	Initiates bidirectional communication fromthe host to the router using the host GUA.	Initiate bidirectional communication fromthe host to the router using the host GUA.
	</li>	</li>
	<li>	<li>

	Making the probing logic on hosts more robust.	Make the probing logic on hosts more robust.
	</li>	</li>
	<li>	<li>

	Increasing the buffer size on routers.	Increase the buffer size on routers.
	</li>	</li>
	<li>	<li>

	Transit dataplane traffic from an unknown address (an address w/o the corresponding neighbor cache entry) triggers an address resolution process on the router.	Transit data plane traffic from an unknown address (an address without the corresponding Neighbor Cache entry) to trigger an address resolution process on the router.
	</li>	</li>
	</ul>	</ul>
	<t>	<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.	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>	</t>
	<section numbered="true" toc="default">	<section numbered="true" toc="default">
	<name>Do Nothing</name>	<name>Do Nothing</name>
	<t>	<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:	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>	</t>
	<ul spacing="normal">	<ul spacing="normal">
	<li>	<li>
	Unhappy users.	Unhappy users.
	</li>	</li>
	<li>	<li>
	Many support tickets.	Many support tickets.
	</li>	</li>
	<li>	<li>

	More resistance to deploy IPv6 and IPv6-Only networks.	More resistance to deploying IPv6 and IPv6-only networks.
	</li>	</li>
	</ul>	</ul>
	</section>	</section>
	<section numbered="true" toc="default">	<section numbered="true" toc="default">
	<name>Change to the Registration-Based Neighbor Discovery</name>	<name>Change to the Registration-Based Neighbor Discovery</name>
	<t>	<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 format="default"/>, <xref format="default"/>) used in Low-Power Wireless Personal Area Networks (6LoWPANs) to the rest of IPv6 deployments.	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 format="default"/> <xref 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.	This option requires some investigation and discussion.

	However, significant changes to the existing IPv6 implementations would be needed, sounclear adoption timeline makes this approach less preferable than one proposed in this document.	However, significant changes to the existing IPv6 implementations would be needed, soan unclear adoption timeline makes this approach less preferable than the approach specified in this document.
	</t>	</t>
	</section>	</section>
	<section numbered="true" toc="default">	<section numbered="true" toc="default">
	<name>Host Sending NS to the Router Address from Its GUA</name>	<name>Host Sending NS to the Router Address from Its GUA</name>
	<t>	<t>

	The host could forcecreating a STALE entry for its GUA in the router ND cache by sending the following Neighbor Solicitation message:	The host could forcethe creation of a STALE entry for its GUA in the router's ND cache by sending the following Neighbor Solicitation message:
	</t>	</t>
	<ul spacing="normal">	<ul spacing="normal">
	<li>	<li>
	The NS source address is the host GUA.	The NS source address is the host GUA.
	</li>	</li>
	<li>	<li>
	The destination address is the default router IPv6 address.	The destination address is the default router IPv6 address.
	</li>	</li>
	<li>	<li>

	The Source Link-Layer Addressoption contains the host link-layer address.	The Source Link-Layer AddressOption contains the host link-layer address.
	</li>	</li>
	<li>	<li>

	The target address is the host default router address (the default router address the host received in the RA).	The target address is the host's default router address (the default router address the host received in the RA).
	</li>	</li>
	</ul>	</ul>
	<t>	<t>

	The main disadvantages of this approach are:	The main disadvantages of this approach are as follows:
	</t>	</t>
	<ul spacing="normal">	<ul spacing="normal">
	<li>	<li>

	Would not work for Optimistic addresses as <xref sectionFormat="of" section="2.2"/> explicitly prohibits sending Neighbor Solicitations from an Optimistic Address.	It would not work for Optimistic Addresses, as <xref sectionFormat="of" section="2.2"/> explicitly prohibits sending Neighbor Solicitations from an Optimistic Address.
	</li>	</li>
	<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 theirneighbor cache updated.	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 theirNeighbor Cache updated.
	</li>	</li>
	<li>	<li>
	Some wireless devices are known to alter ND packets and perform various non-obvious forms of ND proxy actions.	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.	In some cases, unsolicited NAs might not even reach the routers.
	</li>	</li>
	</ul>	</ul>
	</section>	</section>
	<section numbered="true" toc="default">	<section numbered="true" toc="default">

	<name>Host Sending Router Solicitation fromits GUA</name>	<name>Host Sending Router Solicitation fromIts GUA</name>
	<t>	<t>

	The host could send arouter solicitation	The host could send aRouter Solicitation
	message to'all routers' multicast address, using its GUA as a source.	message tothe all-routers multicast address, using its GUA as a source.
	If the host link-layer address is included	If the host link-layer address is included
	in the Source Link-Layer Addressoption, the router would create a STALE entry	in the Source Link-Layer AddressOption, the router would create a STALE entry
	for the host GUA as perthe <xref sectionFormat="of"section="6	for the host GUA as per <xref sectionFormat="of"section="6.2.6
	.2.6"/>.	"/>.
	However, this approach cannot be used if t	However, this approach cannot be used if t
	he GUA is inoptimistic state: <xref sectionFormat="of"section	he GUA is inthe Optimistic state: <xref sectionFormat="of"sec
	="2.2"/> explicitly prohibits using an Optimistic Address as the sourceaddress	tion="2.2"/> explicitly prohibits using an Optimistic Address as the sourceadd
	of a Router Solicitation with aSLLAO as it mightdisrupt the rightful owner of	ress of a Router Solicitation with aSLLAO, as it mightcause disruption for the
	the address in the case of a collision.	rightful owner of the address in the case of a collision.
	So for theoptimistic addresses the hostc	So, for theOptimistic Addresses, the host
	an send an RS without SLLAO included.	can send an RS withouta SLLAO included.
	In that case the router may respond withe	In thatcase, the router may respond with
	ither a multicast or a unicast RA (only the latter would create a cache entry).	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 witheither a multicast or a unicast RA
		(only the latter would create a cache entry).-->

	</t>	</t>
	<t>	<t>
	This approach has the following drawbacks:	This approach has the following drawbacks:
	</t>	</t>
	<ul spacing="normal">	<ul spacing="normal">
	<li>	<li>

	If the address is in the O	If the address is in the O
	ptimisticstate the RS cannot contain SLLAO. As aresult the router wouldonly c	ptimisticstate, the RS cannot containa SLLAO. As aresult, the router wouldon
	reate a cache entry if solicited RAs are sent as unicast.	ly create a cache entry if solicited RAs are sent as unicast.
	Routers sending solicited	Routers sending solicited
	RAs as multicast would not create a new cacheentry as they do not need to send	RAs as multicast would not create a new cacheentry, as they do not need to send
	a unicast packet back to the host.	a unicast packet back to the host.
	</li>	</li>
	<li>	<li>

	There might be a random delay between receiving an RS and sending a unicast RA back (and creating a cache entry) which might undermine the idea of creating the cache entry proactively.	There might be a random delay between receiving an RS and sending a unicast RA back (and creating a cache entry), which might undermine the idea of creating the cache entry proactively.
	</li>	</li>
	<li>	<li>

	Some wireless devices are known to intercept ND packets and perform various non-obvious forms of ND proxy actions. In some cases the RS might not even reach the routers.	Some wireless devices are known to intercept ND packets and perform various non-obvious forms of ND proxy actions. In some cases, the RS might not even reach the routers.
	</li>	</li>
	</ul>	</ul>
	</section>	</section>
	<section numbered="true" toc="default">	<section numbered="true" toc="default">

	<name>Routers Populating Their Caches by GleaningFrom Neighbor Discovery Packets</name>	<name>Routers Populating Their Caches by Gleaningfrom Neighbor Discovery Packets</name>
	<t>	<t>
	Routers may be able to learn about new addresses by gleaning from the DAD Neighbor Solicitation messages.	Routers may be able to learn about new addresses by gleaning from the DAD Neighbor Solicitation messages.

	The router could listen to allsolicited node mul	The router could listen to allsolicited-node mul
	ticast address groups and upon receiving a Neighbor Solicitation from theunspec	ticast address groupsand, upon receiving a Neighbor Solicitation from the unspe
	ified address search its Neighbor Cache for the solicitation's Target Address.	cified address, search its Neighbor Cache for the solicitation's Target Address.
	If no entry exists, the router may create an entr
	y, set its reachability state to'INCOMPLETE' and start the address resolutionf	<!-- [rfced] Section 8.5: Would you like to use either "target
	or that entry.	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 theunspecified address search its Neighbor Cache
		for the solicitation's Target Address.-->

		If no entry exists, the router may create an entr
		y, set its reachability state toINCOMPLETE, and start the address resolutionpr
		ocess for that entry.
	</t>	</t>
	<t>	<t>

	The same solution was proposed in <xref format="default"/>. Some routing vendorssupport such optimization already. However, this approach has a number of drawbacks and therefore should not be used as the only solution:	The same solution was proposed in <xref format="default"/>. Some routing vendorsalready support such optimization. However, this approach has a number of drawbacks and therefore should not be used as the only solution:
	</t>	</t>
	<ul spacing="normal">	<ul spacing="normal">
	<li>	<li>

	Routers need to receive all multicast Neighbor Discovery packets which might negatively impact the routersCPU.	Routers need to receive all multicast Neighbor Discovery packets; this might negatively impact a router'sCPU.
	</li>	</li>
	<li>	<li>

	If the router starts the a	If the router starts the a
	ddress resolution as soon as it receives the DAD NeighborSolicitation the host	ddress resolutionprocess as soon as it receives the DAD NeighborSolicitation,
	mightbe still performing DAD and the target address might betentative.	the host might stillbe performing DAD and the target address might betentative
	In that case, the host <bc	.
	p14>SHOULD</bcp14> silently ignore the received Neighbor Solicitation from the r	In that case, the host <bc
	outer as perthe <xref sectionFormat="of" section="5.4.3"/>.	p14>SHOULD</bcp14> silently ignore the received Neighbor Solicitation from the r
	As aresult the routermig	outer as per <xref sectionFormat="of" section="5.4.3"/>.
	ht not be able to complete the address resolution before the returntraffic arri	As aresult, the routermi
	ves.	ght not be able to complete the address resolutionprocess before the returntra
		ffic arrives.

	</li>	</li>
	</ul>	</ul>
	</section>	</section>
	<section numbered="true" toc="default">	<section numbered="true" toc="default">

	<name>Initiating Hosts-to-Routers Communication</name>	<name>Initiating Host-to-Router Communication</name>
	<t>	<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.	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 destinations it's quite likely that the router would start the neighbor discovery process for the host GUA before the first packet of the returning traffic arrives.	As the RTT to the default routeris lower than the RTT to any off-link destinations, it's quite likely that the router would start the Neighbor Discovery process for the host GUA before the first packet of the returning traffic arrives.
	</t>	</t>
	<t> This approach has the following drawbacks:	<t> This approach has the following drawbacks:
	</t>	</t>
	<ul spacing="normal">	<ul spacing="normal">
	<li>	<li>

	Data packets to the router link-local address could be blocked by security policy or control plane protection mechanism.	Data packets to the router's link-local address could be blocked by a security policy or control plane protection mechanism.
	</li>	</li>
	<li>	<li>

	It introduces an additional overhead forrouters control plane (in addition to processing ND packets, thedata packet needs to be processed as well).	It introduces an additional overhead forthe router's control plane (in addition to processing ND packets, thedata packet needs to be processed as well).
	</li>	</li>
	<li>	<li>

	Unless the data packet is sent to'all routers' ff02::2 multicast address, if the network provides a first-hop redundancy then only the active router would create a new cache entry.	Unless the data packet is sent tothe all-routers ff02::2 multicast address, if the network provides a first-hop redundancy, then only the active router would create a new cache entry.
	</li>	</li>
	</ul>	</ul>
	</section>	</section>
	<section numbered="true" toc="default">	<section numbered="true" toc="default">
	<name>Making the Probing Logic on Hosts More Robust</name>	<name>Making the Probing Logic on Hosts More Robust</name>
	<t>	<t>

	Theoretically the probing logic on hosts might be modified to deal better with initial packet loss. For example, only one probe can be sent or probes retransmit intervals can be reduced. However, this approach has a number of drawbacks:	Theoretically, the probing logic on hosts might be modified to better deal with initial packet loss. For example, only one probe can be sent, or probe retransmit intervals can be reduced. However, this approach has a number of drawbacks:

	</t>	</t>
	<ul spacing="normal">	<ul spacing="normal">

	<li>It would require updating all possible applications performing probing, while theproposed solution is implemented on operating systems level.</li>	<li>It would require updating all possible applications performing probing, while thesolution described in this document is implemented at the operating-system level.</li>
	<li>	<li>

	<t>Some implementations need to send multiple probes. Examples include but not limited to:	<t>Some implementations need to send multiple probes. Examples include butarenot limited to:
	</t>	</t>
	<ul spacing="normal">	<ul spacing="normal">

	<li>Sending AAAA and Arecords DNS probes in parallel.</li>	<li>Sending AAAA and Arecord DNS probes in parallel.</li>
	<li>Detecting captiveportals oftenrequire sendingmultiple packe	<li>Detecting captiveportals, which oftenrequires sendingmultip
	ts.</li>	le packets.</li>
	</ul>	</ul>
	</li>	</li>
	<li>	<li>

	<t>While it would increase the probabilityof the probing to complete successfully, there are multiple cases when packet loss would still occur:	<t>While it would increase the probabilitythat the probing will complete successfully, there are multiple cases when packet loss would still occur:
	</t>	</t>
	<ul spacing="normal">	<ul spacing="normal">
	<li> The probe response consists of multiple packets, so all but the first one are dropped. </li>	<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 and return packets arrive simultaneously.</li>	<li> There are multiple applications on the same host sending 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>	<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>	</ul>
	</li>	</li>
	<li>	<li>

	Reducing the probe retransmit interval unnecessary increases the network utilization and might cause the network congestion.	Reducing the probe retransmit interval unnecessarily increases network utilization and might cause network congestion.
	</li>	</li>
	</ul>	</ul>
	</section>	</section>
	<section numbered="true" toc="default">	<section numbered="true" toc="default">
	<name>Increasing the Buffer Size on Routers</name>	<name>Increasing the Buffer Size on Routers</name>
	<t>	<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 denial ofservice attacks.	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 denial-of-service attacks.
	</t>	</t>
	</section>	</section>
	<section numbered="true" toc="default">	<section numbered="true" toc="default">

	<name>Transit Dataplane Traffic From a New Address Triggering Address Resolution</name>	<name>Transit Data Plane Traffic from a New Address to Trigger Address Resolution</name>
	<t>	<t>

	When a router receives a transit packet sourced by	When a router receives a transit packet sourced by
	a on-link neighbor node, it might check the presence ofthe neighbor cache entr	an on-link neighbor node, it might checkfor the presence ofa Neighbor Cache e
	y for the packet source addressand if the entry does not exist, startaddress r	ntry for the packet source addressand, if the entry does not exist, startthe a
	esolution process.	ddress resolution process.
	This approach does ensure that a Neighbor Cache en	This approach does ensure that a Neighbor Cache en
	try is proactively created every time a new, previously unseen GUA is used for s	try is proactively created every time a new, previously unseen GUA is used for s
	endingofflink traffic.	endingoff-link traffic.
	However, this approach has a number oflimitations, in particular:	However, this approach has a number oflimitations. In particular:
	</t>	</t>
	<ul spacing="normal">	<ul spacing="normal">

	<li>If traffic flows areasymmetrical the return traffic might nottra	<li>If traffic flows areasymmetrical, the return traffic might nottr
	nsit the same router as the original trafficwhich triggered the address resolut	ansit the same router as the original trafficthat triggered the address resolut
	ion.	ion process.
	So theneighbor cache entry is created on the "wrong" router, not the onewhich	So, theNeighbor Cache entry is created on the "wrong" router, not the onethat
	actually needs theneighbor cache entry for the host address.	actually needs theNeighbor Cache entry for the host address.
	</li>	</li>
	<li>	<li>

	The functionality needs to be limited to explicitly configured networks/	The functionality needs to be limited to explicitly configured networks/
	interfaces, as the router needs to distinguish betweenonlink addresses(ones th	interfaces, as the router needs to distinguish betweenon-link addresses(addres
	e router needs to have Neighbor Cacheentries for) and the rest ofthe addresss	ses for which the router needs to have Neighbor Cacheentries) and the rest oft
	pace.	he addressspace.
	The proactive address resolution must only be triggered bypackets from	The proactive address resolutionprocess must only be triggered bypacke
	the prefixes known to be on-link. Otherwise, traffic from spoofed sourceaddress	ts from the prefixes known to be on-link. Otherwise, traffic from spoofed source
	es or any transit traffic could lead toneighbor cache exhaustion.	addresses or any transit traffic could lead toNeighbor Cache exhaustion.
	</li>	</li>
	<li>	<li>

	Implementing such functionality is much more complicated than all other solutions as it would involve complex data-control planes interaction.	Implementing such functionality is much more complicated than all other solutions, as it would involve complex interactions between the data plane and the control plane.
	</li>	</li>
	</ul>	</ul>
	</section>	</section>
	</section>	</section>
	<section anchor="IANA" numbered="true" toc="default">	<section anchor="IANA" numbered="true" toc="default">
	<name>IANA Considerations</name>	<name>IANA Considerations</name>
	<t>	<t>

	This memo asks the IANA for no new parameters.	This document has no IANA actions.
	</t>	</t>
	</section>	</section>
	<section anchor="Security" numbered="true" toc="default">	<section anchor="Security" numbered="true" toc="default">
	<name>Security Considerations</name>	<name>Security Considerations</name>
	<t>	<t>

	One of the potential attack vectors to consider isa cache spoofing when the attacker might try to install a cache entry for the victim's IPv6 address and the attacker's 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 given IPv6 address already exists.	One of the potential attack vectors to consider iscache spoofing, where the attacker might try to install a cache entry for the victim's IPv6 address and the attacker's link-layer address. However, it should be noted that this document does not propose any changes for the scenario when the ND cache for a given IPv6 address already exists.
	Therefore, there are no new vectors for an attacker to override an existing cache entry.	Therefore, there are no new vectors for an attacker to override an existing cache entry.
	</t>	</t>
	<t>	<t>

	<xref format="default"/> describes some corner cases when a h	<xref format="default"/> describes some corner cases when a h
	ost withthe duplicated Optimisticaddress might get some packets intended fort	ost witha duplicated OptimisticAddress might get some packets intended forthe
	he rightful owner of the address.However such scenarios do not introduce anyne	rightful owner of the address.However, such scenarios do not introduce anynew
	w attack vectors: even without theproposed changes, anattacker can easilyover	attack vectors: even without thechanges discussed in this document, anattacke
	ride therouters neighbor cache and redirect the traffic bysending NAs with the	r can easilyoverride therouter's Neighbor Cache and redirect the traffic byse
	Solicited flag set.	nding NAs with the Solicited flag set.
	As discussed in <xrefformat="default"/> theworst case scena	As discussed in <xrefformat="default"/>, theworst-case scen
	rio might cause a disruption for up to 7 seconds.This risk is consideredaccept	ario might cause a disruption for up to 7 seconds.Because this scenario is high
	able due to very low probability of that scenario. More importantly, for allcas	ly unlikely, this riskof disruption is consideredacceptable. More importantly,
	es described in <xrefformat="default"/> therightful ownerc	for allcases described in <xrefformat="default"/>, therig
	an prevent disruption caused by an accidental address duplicationjust byimplem	htful ownercan prevent disruption caused by an accidental address duplicationj
	enting the mechanism described in this document. If the rightfulowner sendsuns	ust byimplementing the mechanism described in this document. If the rightfulow
	olicited NAs before using the address, the STALE entry would becreated on ther	ner sendsunsolicited NAs before using the address, the STALE entry would becre
	outer NC and any subsequent unsolicited NAs sent from the hostwith anOptimisti	ated on therouter's NC, and any subsequent unsolicited NAs sent from the hostw
	c address would not override the NC entry.	ith anOptimistic Address would not override the NC entry.
	</t>	</t>
	<t>	<t>

	A malicious host could attempt to exhaust theneighbor cache on the router by creating a large number of STALE entries. However, this attack vector is not new and this document does not increase the risk of such an attack: the attacker could do it, for example, by sending a NS or RS packet with SLLAO included. All recommendations from <xref format="default"/> still apply.	A malicious host could attempt to exhaust theNeighbor Cache on the router by creating a large number of STALE entries. However, this attack vector is not 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 an NS or RS packet with a SLLAO included. All recommendations from <xref target="RFC6583" format="default"/> still apply.
	</t>	</t>
	<t>	<t>

	Announcing a new address to all-routers multicastaddress ma	Announcing a new address tothe all-routers multicastaddres
	y inform an on-link attacker about IPv6 addresses assigned to the host.However,	s may inform an on-link attacker about IPv6 addresses assigned to the host.Howe
	hiding information about the specific IPv6 address should not be considered as	ver, hiding information about the specific IPv6 address should not be considered
	ecurity measure as such information is usually disclosed via DAD to allnodes an	asecurity measure, as such information is usually disclosed via DAD to allnod
	yway if MLD snooping is not enabled. Network administrators can alsomitigate th	es anyway if MLD snooping is not enabled. Network administrators can alsomitiga
	is issue by enabling MLD snooping on the link-layer devices to prevent IPv6link	te this issue by enabling MLD snooping on the link-layer devices to prevent IPv6
	-local multicast packets being flooded to allonlink nodes.	link-local multicast packetsfrom being flooded to allon-link nodes.
	If peer-to-peeronlink communications are notdesira	If peer-to-peeron-link communications are notdesir
	ble forthe given networksegment they should be prevented by properlayer-2 sec	able fora given networksegment, they should be prevented by properLayer 2 sec
	urity mechanisms. Therefore, the risk of allowing hosts to send unsolicited Neig	urity mechanisms. Therefore, the risk of allowing hosts to send unsolicited Neig
	hbor Advertisements to all-routers multicast address is low.	hbor Advertisements tothe all-routers multicast address is low.
	</t>	</t>
	<t>	<t>

	It should be noted that theproposed 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"/>.	It should be noted that themechanism 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 sectionFormat="of" section="4.3.2"/> and in <xref target="RFC6583" format="default"/>.

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

	<section anchor="Acknowledgements" numbered="true" toc="default">
	<name>Acknowledgements</name>
	<t>
	Thanks to the following people (in alphabetical order) for t
	heir
	comments, review and feedback: <contact fullname="Mi
	kael Abrahamsson"/>, <contact fullname="Stewart Bryant"/>, <contact fullname="Lo
	renzo Colitti"/>, <contact fullname="Roman Danyliw"/>, <contact fullname="Owen D
	eLong"/>, <contact fullname="Martin Duke"/>, <contact fullname="Igor Gashinsky"/
	>, <contact fullname="Carles Gomez"/>, <contact fullname="Fernando Gont"/>, <con
	tact fullname="Tatuya Jinmei"/>, <contact fullname="Benjamin Kaduk"/>, <contact
	fullname="Scott Kelly"/>, <contact fullname="Erik Kline"/>, <contact fullname="W
	arren Kumari"/>, <contact fullname="Barry Leiba"/>, <contact fullname="Jordi Pal
	et Martinez"/>, <contact fullname="Erik Nordmark"/>, <contact fullname="Michael
	Richardson"/>, <contact fullname="Dan Romascanu"/>, <contact fullname="Zaheduzza
	man Sarker"/>, <contact fullname="Michael Scharf"/>, <contact fullname="John Scu
	dder"/>, <contact fullname="Mark Smith"/>, <contact fullname="Dave Thaler"/>, <c
	ontact fullname="Pascal Thubert"/>, <contact fullname="Loganaden Velvindron"/>,
	<contact fullname="Eric Vyncke"/>.
	</t>
	</section>
	</middle>	</middle>

	<back>	<back>
	<displayreference to="ND-ADDR-RES"/>	<displayreference to="ND-ADDR-RES"/>

	<references>	<references>
	<name>References</name>	<name>References</name>
	<references>	<references>
	<name>Normative References</name>	<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.2119.xml"/>

	skipping to change at line 783 ¶	skipping to change at line 870 ¶
	<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.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.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.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.8505.xml"/>
	<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8981.xml"/>	<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8981.xml"/>

	<!-- draft-halpern-6man-nd-pre-resolve-addr (Expired) -->	<!-- 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"/>	<xi:include href="https://datatracker.ietf.org/doc/bibxml3/reference.I-D.halpern-6man-nd-pre-resolve-addr.xml"/>
	</references>	</references>
	</references>	</references>

		<section anchor="Acknowledgements" numbered="false" toc="default">
		<name>Acknowledgements</name>
		<t>
		Thanks to the following people (in alphabetical order) for t
		heir
		comments, review, and feedback: <contact fullname="M
		ikael Abrahamsson"/>, <contact fullname="Stewart Bryant"/>, <contact fullname="L
		orenzo 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"/>, <co
		ntact 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 Pa
		let Martinez"/>, <contact fullname="Erik Nordmark"/>, <contact fullname="Michael
		Richardson"/>, <contact fullname="Dan Romascanu"/>, <contact fullname="Zaheduzz
		aman Sarker"/>, <contact fullname="Michael Scharf"/>, <contact fullname="John Sc
		udder"/>, <contact fullname="Mark Smith"/>, <contact fullname="Dave Thaler"/>, <
		contact fullname="Pascal Thubert"/>, <contact fullname="Loganaden Velvindron"/>,
		and <contact fullname="Éric Vyncke"/>.
		</t>
		</section>

		<!-- [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. -->

	</back>	</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>	</rfc>

End of changes. 139 change blocks.
	413 lines changed or deleted	541 lines changed or added
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