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="" u pdates="4861" docName="draft-ietf-6man-grand-07" number="0000" submissionType="I ETF" category="std" consensus="true" xml:lang="en" tocInclude="true" symRefs="t rue" sortRefs="true" version="3"> <rfc xmlns:xi="http://www.w3.org/2001/XInclude" ipr="trust200902" obsoletes="" u pdates="4861" docName="draft-ietf-6man-grand-07" number="9131" submissionType="I ETF" category="std" consensus="true" xml:lang="en" tocInclude="true" symRefs="t rue" 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 on First-Hop Routers</title> or Cache Entries on First&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 determ ine the link-layer addresses of neighboring nodes as well as to discover and mai ntain reachability information. This document updates RFC4861 to allow routers t o 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 Neig hbor 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-l ink destination from a new IPv6 address. Neighbor Discovery (RFC 4861) is used by IPv6 nodes to deter mine the link-layer addresses of neighboring nodes as well as to discover and ma intain reachability information. This document updates RFC 4861 to allow routers to proactively create a Neighbor Cache entry when a new IPv6 address is assigne d 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 m inimize the delay and packet loss when a node initiates connections to an off-li nk 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 target="RFC4861" format="default"/> assumes that communications between IPv6 no des 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 creat e an entry containing A's IPv6 and link-layer addresses in its neighbor cache. T hat entry will be used for sending the return traffic to A. The Neighbor Discovery state machine defined in <xref target="RFC4861" format="default"/> assumes that communications between IPv6 no des are, in most cases, bidirectional and if a node A is trying to communicate t o its neighbor, node B, the return traffic flows could be expected. So, when no de 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. Tha t entry will be used for sending the return traffic to A.
</t> </t>
<t> <t>
In particular, <xref target="RFC4861" sectionFormat="o f" section="7.2.5"/> states: In particular, <xref target="RFC4861" sectionFormat="o f" 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</b cp14> be silently discarded. If no entry exists, the advertisement <bcp14>SHOULD</b cp14> be silently discarded.
There is no need to create an entry if none exists, since the recipient has a pparently not initiated any communication with the target.</blockquote> There is no need to create an entry if none exists, since the recipient has a pparently 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
er Advertisement the host populates its neighbor cache with the default router I er Advertisement, the host populates its Neighbor Cache with the default router
Pv6 and link-layer addresses and is able to send traffic to off-link destination IPv6 and link-layer addresses and is able to send traffic to off-link destinatio
s. ns.
At the same time the router does not have any At the same time, the router does not have any
cache entries for the host global addresses yet and only starts address resoluti cache entries for the host global addresses yet and only starts address resolut
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 to complete r While waiting for the resolution to complete,
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 target="RFC4861" sectionFormat="of" section="7.2.2"/>. <xref target="RFC4861" sectionFormat="of" section="7.2.2"/>.
Any additional packets arriving before the resolution &gt; process finishes are Any additional packets arriving before the resolution &gt; process finishes are
likely to result in dropped packets likely to result in dropped packets.
It can cause packet loss and performan
ce degradation that can be user-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 be visible to users.
</t> </t>
<t> <t>
This document updates the Neighbor Discovery protocol <xref target="RFC4861" for mat="default"/> to avoid packet loss in the scenario described above. This document updates the Neighbor Discovery protocol <xref target="RFC4861" for mat="default"/> to avoid packet loss in the scenario described above.
<xref target="changes" format="default"/> discusses the changes and analyses the potential impact, while normative changes to <xref target="RFC4861" format="def ault"/> are specified in <xref target="RFC_UPD" format="default"/>. <xref target="changes" format="default"/> discusses the changes and analyzes the potential impact, while normative changes to <xref target="RFC4861" format="def ault"/> are specified in <xref target="RFC_UPD" 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&nbsp;14 are to be interpreted as described in BCP&nbsp;14
<xref target="RFC2119"/> <xref target="RFC8174"/> when, and only <xref target="RFC2119"/> <xref target="RFC8174"/> 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 implements IP, <xref target="RFC4861" <dt>Node:</dt><dd>A device that implements IP <xref target="RFC4861" f
format="default"/>.</dd> ormat="default"/>.</dd>
<dt>Host:</dt><dd>any node that is not a router, <xref target="RFC4861 <dt>Host:</dt><dd>Any node that is not a router <xref target="RFC4861"
" format="default"/>.</dd> format="default"/>.</dd>
<dt>ND:</dt><dd>Neighbor Discovery, <xref target="RFC4861" format="def <dt>ND:</dt><dd>Neighbor Discovery <xref target="RFC4861" format="defa
ault"/>.</dd> ult"/>.</dd>
<dt>NC:</dt><dd>Neighbor Cache, <xref target="RFC4861" format="default <dt>NC:</dt><dd>Neighbor Cache <xref target="RFC4861" format="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 target="RFC4861" sectionFormat="of" section="7.3.2"/>: INCOMPLETE, REACHABLE, target="RFC4861" sectionFormat="of" section="7.3.2"/>: INCOMPLETE, REACHABLE, S
STALE, DELAY, PROBE.</dd> TALE, DELAY, or PROBE.</dd>
<dt>SLAAC:</dt><dd>IPv6 Stateless Address Autoconfiguration, <xref tar <dt>SLAAC:</dt><dd>IPv6 Stateless Address Autoconfiguration <xref targ
get="RFC4862" format="default"/>.</dd> et="RFC4862" format="default"/>.</dd>
<dt>NS:</dt><dd>Neighbor Solicitation, <xref target="RFC4861" format=" <dt>NS:</dt><dd>Neighbor Solicitation <xref target="RFC4861" format="d
default"/>.</dd> efault"/>.</dd>
<dt>NA:</dt><dd>Neighbor Advertisement, <xref target="RFC4861" format= <dt>NA:</dt><dd>Neighbor Advertisement <xref target="RFC4861" format="
"default"/>.</dd> default"/>.</dd>
<dt>RS:</dt><dd>Router Solicitation, <xref target="RFC4861" format="de <dt>RS:</dt><dd>Router Solicitation <xref target="RFC4861" format="def
fault"/>.</dd> ault"/>.</dd>
<dt>RA:</dt><dd>Router Advertisement, <xref target="RFC4861" format=" <dt>RA:</dt><dd>Router Advertisement <xref target="RFC4861" format="d
default"/>.</dd> efault"/>.</dd>
<dt>SLLAO:</dt><dd>Source link-layer Address Option, an option in the <dt>SLLAO:</dt><dd>Source Link-Layer Address Option. 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>Target link-layer Address Option, an option in the <dt>TLLAO:</dt><dd>Target Link-Layer Address Option. An option in the
ND packets containing the link-layer address of the target <xref target="RFC4861 ND packets containing the link-layer address of the target <xref target="RFC4861
" format="default"/>.</dd> " format="default"/>.</dd>
<dt>GUA:</dt><dd>Global Unicast Address <xref target="RFC4291" format= "default"/>.</dd> <dt>GUA:</dt><dd>Global Unicast Address <xref target="RFC4291" format= "default"/>.</dd>
<dt>DAD:</dt><dd>Duplicate Address Detection, <xref target="RFC4862" <dt>DAD:</dt><dd>Duplicate Address Detection <xref target="RFC4862" fo
format="default"/>.</dd> rmat="default"/>.</dd>
<dt>Preferred Address:</dt><dd>an address assigned to an interface who <dt>Preferred Address:</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
is unrestricted, <xref target="RFC4862" format="default"/>. Preferred addresses is unrestricted <xref target="RFC4862" 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>Optimistic DAD:</dt><dd>a modification of DAD, <xref target="RFC44 <dt>Optimistic DAD:</dt><dd>A modification of DAD <xref target="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 problem may arise is a host joining the network, forming a new address and The most typical scenario when the problem described in this do cument 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 th e host needs to perform SLAAC and to configure its network stack. The RA contains information th e 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 li nk-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 li nk-layer address of the router.
As a result the host can popul ate its Neighbor Cache with the router's link-local and link-layer addresses. As a result, the host can popu late 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 dev ice sending probes to detect the Internet connectivity A very common use case is a mobile dev ice 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 that process many To speed up that process, many
implementations use Optimistic DAD which allows them to send probes before the D implementations use Optimistic DAD, which allows them to send probes before the
AD process is completed. DAD process is completed.
At that moment the dev At that moment, the de
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).
The router neighbor cache, how The router's Neighbor Cache, h
ever, might contain an entry for the device link-local owever, might contain an entry for the device's link-local
address (if the device has been performing the address resolution for the router address (if the device has been performing address resolution for the router's l
link-local address), but there are no entries for any of the device's global ad ink-local address), but there are no entries for any of the device's global addr
dresses. esses.
</li> </li>
<li> <li>
Return traffic is received by the firs t-hop router. Return traffic is received by the firs t-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 the neighbor discovery process by creating an INC address yet, the router starts the Neighbor Discovery process by creating an INC
OMPLETE cache entry and then sending a Neighbor Solicitation to the Solicited No OMPLETE cache entry and then sending a Neighbor Solicitation to the solicited-no
de Multicast Address (<xref target="RFC4861" sectionFormat="of" section="7.3.2"/ de multicast address (<xref target="RFC4861" sectionFormat="of" section="7.3.2"/
>). >).
As per <xref target="RFC4861" sectionFormat="of" section="7.2.2" As per <xref target="RFC4861" 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 si ze to a few packets only, and some implementations are known to buffer just one packet. However, most router implementations limit the buffer si ze 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 comp leted are causing packet loss by replacing older packets in the buffer. So, any subsequent packets arriving before the address resolution process is com pleted are causing packet loss by replacing older packets in the buffer.
</li> </li>
<li> <li>
If the host sends multiple probes in par allel, in the worst case, it would consider all but one of them failed. If the host sends multiple probes in par allel, in the worst case, it would consider all but one of them failed.
That leads to user-visible delay in conn ecting to the network, especially if the host implements some form of backoff me chanism and does not retransmit the probes as soon as possible. That leads to user-visible delay in conn ecting to the network, especially if the host implements some form of backoff me chanism 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 the device address to be removed from the router's ne ity period long enough for the device's address to be removed from the router's
ighbor cache. Neighbor Cache.
However, the same sequence of events happen wh However, the same sequence of events happens 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 target="RFC8981" format="default"/> or if the such as a new privacy address <xref target="RFC8981" format="default"/> or if th
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 hid den by Happy Eyeballs <xref target="RFC8305" format="default"/> it manifests qui te 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 hid den by Happy Eyeballs <xref target="RFC8305" format="default"/>, it manifests qu ite 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 m echanics 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 m echanics 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 en try for the address, a STALE entry needs to be created proactively, prior to arr ival of the first packet intended for that address. If the router does not have a Neighbor Cache en try for the address, a STALE entry needs to be created proactively, prior to arr ival of the first packet intended for that address.
</li> </li>
<li> <li>
The solution needs to work for Optimistic addre The solution needs to work for Optimistic Addre
sses as well. sses as well.
Devices implementing the Optimistic DAD usually Devices implementing Optimistic DAD usually att
attempt to minimize the delay in connecting to the network and therefore are mo empt to minimize the delay in connecting to the network and therefore are more 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>
In case of duplicate addresses present in the n etwork, the proposed solution should not override the existing entry. In the case of duplicate addresses present in t he network, the solution should not override the existing entry.
</li> </li>
<li> <li>
In topologies with multiple first-hop routers t he cache needs to be updated on all of them, as traffic might be asymmetric: out going flows leaving the network via one router while the return traffic enters t he 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: ou tgoing 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 des cribed in <xref target="RFC6583" format="default"/> and needs to be compatible w ith the recommendations provided in <xref target="RFC6583" format="default"/>. In addition, the solution must not exacerbate issues des cribed in <xref target="RFC6583" format="default"/> and needs to be compatible w ith the recommendations provided in <xref target="RFC6583" 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 i n creating new entries in a router neighbor cache The following changes are required to minimize the delay i n 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 rec eiving an unsolicited NA from a host. A router creates a new cache entry upon rec eiving an unsolicited NA from a host.
</li> </li>
</ul> </ul>
<t> <t>
The following sections discuss these changes in more detai l. The following sections discuss these changes in more detai l.
Normative changes are specified in <xref target="RFC_UPD" format="default"/> . Normative changes are specified in <xref target="RFC_UPD" 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 target="RFC4861" sectionFormat="of" sect ion="7.2.6"/> discusses using unsolicited Neighbor <xref target="RFC4861" sectionFormat="of" section= "7.2.6"/> discusses using unsolicited Neighbor
Advertisements to inform node neighbors of the new link-layer address quickly. Advertisements to inform node neighbors of the new link-layer address quickly.
The same mechanism could be used to notify the nod e neighbors about the new network-layer The same mechanism could be used to notify the nod e neighbors about the new network-layer
address as well: the node can send gratuitous unso licited Neighbor Advertisements upon assigning a new IPv6 address to its interfa ce. address as well: the node can send gratuitous unso licited Neighbor Advertisements upon assigning a new IPv6 address to its interfa ce.
</t> </t>
<t> <t>
To minimize the potential disruption in case of du plicate addresses the node should not set the Override flag for a preferred addr ess and must not set the Override flag if the address is in Optimistic <xref tar get="RFC4429" format="default"/> state. To minimize potential disruption in the case of du plicate addresses, the node should not set the Override flag for a preferred add ress and must not set the Override flag if the address is in the Optimistic stat e <xref target="RFC4429" format="default"/>.
</t> </t>
<t> <t>
As the main purpose of sending unsolicited NAs upo n configuring a new address is to proactively create a Neighbor Cache entry on t he first-hop routers, the gratuitous NAs are sent to the all-routers multicast a ddress (ff02::2). Limiting the recipients to routers only would help reduce the multicast noise level. As the main purpose of sending unsolicited NAs upo n configuring a new address is to proactively create a Neighbor Cache entry on t he first-hop routers, the gratuitous NAs are sent to the all-routers multicast a ddress (ff02::2). Limiting the recipients to routers only would help reduce the multicast noise level.
If the link-layer devices are performing MLD snoop ing <xref target="RFC4541" format="default"/>, then those unsolicited NAs will b e only sent to routers on the given network segment/link, instead of being flood ed to all nodes. If the link-layer devices are performing Multicast Listener Discovery (MLD) snooping <xref target="RFC4541" format="default"/>, th en those unsolicited NAs will only be sent to routers on the given network segme nt/link, instead of being flooded to all nodes.
</t> </t>
<t> <t>
It should be noted that the proposed mechanism doe It should be noted that the mechanism discussed he
s not cause any significant increase in multicast traffic. re does not cause any significant increase in multicast traffic.
The additional multicast unsolicited NA would proa The additional multicast unsolicited NAs would pro
ctively create a STALE cache entry on routers as discussed below. actively create a STALE cache entry on routers, as discussed below.
When the router receives the return traffic flows When the router receives the return traffic flows,
it does not need to send multicast NSes to the solicited node multicast address it does not need to send multicast NSes to the solicited-node multicast address
but would be sending unicast NSes instead. but would be sending unicast NSes instead.
Therefore this procedure would only produce an inc Therefore, this procedure would only produce an in
rease in the overall amount of multicast traffic if no return traffic arrives fo crease in the overall amount of multicast traffic if no return traffic arrives f
r the address that sent the unsolicited NA or if the router does not create a ST or the address that sent the unsolicited NA or if the router does not create a S
ALE entry upon receiving such NA. The increase would be negligible as that addit TALE entry upon receiving such an NA. The increase would be negligible, as that
ional traffic is a few orders of magnitude less than the usual level of Neighbor additional traffic is a few orders of magnitude less than the usual level of Nei
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 Entries Upon Receiving Unsolicited Neighbor Advertisements</name> <name>Routers Creating Cache Entries upon Receiving Unsolicited Neighbor Advertisements</name>
<t> <t>
The <xref target="RFC4861" sectionFormat="of" sect ion="7.2.5"/> states: <xref target="RFC4861" 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 disca rded. If no entry exists, the advertisement <bcp14>SHOULD</bcp14> be silently disca rded.
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 abo ve, target") is valid for on-link host-to-host communication but, as discussed ab ove,
it does not really apply for the scenario when the host is announcing its add ress to routers. it does not really apply for the scenario when the host is announcing its add ress 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 <xref target="RFC486 This document updates <xref target="RFC486
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 not alrea eiving an unsolicited Neighbor Advertisement for an address that does not alread
dy have a Neighbor Cache entry. y have a Neighbor Cache entry.
. These changes do not modify the ro
The proposed changes do not modify uter behavior specified in <xref target="RFC4861" format="default"/> for the sce
routers behaviour specified in <xref target="RFC4861" format="default"/> for th 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 discusse s the potential impact of creating a STALE entry for a duplicated IPv6 address. The next section analyzes various scenarios of duplicated addresses and discusse s 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 target="RFC4862" format is document are using the DAD mechanism defined in <xref target="RFC4862" format
="default"/>, they would send unsolicited NA as soon as the address changes the ="default"/>, they would send unsolicited NAs 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 using optimistic addresses, which mea etwork stack configuration delays might be using Optimistic 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 target="RFC4429" sectionFormat <xref target="RFC4429" sectionFormat
="of" section="2.2"/> discusses measures to ensure that ND packets from the opti ="of" section="2.2"/> discusses measures to ensure that ND packets from the Opti
mistic address do not override any existing neighbor cache entries as it would c mistic Address do not override any existing Neighbor Cache entries, as it would
ause traffic interruption of the rightful address owner in case of address conf cause interruption of the rightful address owner's traffic in the case of an 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 this document it seems reasonable for such hosts to advertise their outlined in this document, it seems reasonable for such hosts to advertise thei
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 the optimistic address happens to be duplicated. wner if the Optimistic 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 same address assig mistic state, while another node (the rightful owner) already has the same addre
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 <xref target="RFC4429" format="default"/ ner", as the same terminology is used in <xref target="RFC4429" format="default"
>. />.
The analysis assumes that the host performs Duplicate Address Detection, as <xre The analysis assumes that the host performs DAD, as <xref target="RFC4862" secti
f target="RFC4862" sectionFormat="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</na me> <name>Neighbor Cache Entry Exists in Any State Other Than INCOMPLETE</na me>
<t> <t>
If the router Neighbor Cache entry for the target ad dress already exists in any state other than INCOMPLETE, then as per <xref targ et="RFC4861" sectionFormat="of" section="7.2.5"/> an unsolicited NA with the Ove rride flag cleared would change the entry state from REACHABLE to STALE but woul d not update the entry in any other way. Therefore, even if the host sends an un solicited NA from its Optimistic address the router cache entry would not be upd ated with the new Link-Layer address and no impact to the traffic for the right ful 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 tar get="RFC4861" sectionFormat="of" section="7.2.5"/>, an unsolicited NA with the O verride flag cleared would change the entry state from REACHABLE to STALE but wo uld 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 b e updated with the new link-layer address and no impact on the traffic for the r ightful address owner is expected.
</t> </t>
<t> <t>
The return traffic intended for the host with the Optimistic address would be se nt to the rightful owner. However, this is unavoidable with or without the unsol icited NA mechanism. The return traffic intended for the host with the Optimistic Address would be se nt to the rightful owner. However, this is unavoidable with or without the unsol icited 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 Entry is in INCOMPLETE state</name> <name>Neighbor Cache Entry Is in INCOMPLETE State</name>
<t> <t>
Another corner case is the INCOMPLETE cache entry fo r the address. Another corner case is the INCOMPLETE cache entry fo r 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 Optimistic address and sends an unsolicited NA. The host configures an Optimistic Address and sends an unsolicited NA.
</li> </li>
<li> <li>
The router creates a STALE entry and sends the buffered packet(s) to the host (w hile 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 (w hile 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 target="RFC4861"/>, 5 sec s) 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 target="RFC486 1"/> 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-la yer address) and sets the entry state to REACHABLE (as the NA has the Solicited flag 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 Solicite d flag set).
</li> </li>
</ol> </ol>
<t> <t>
As a result some packets (ones in the buffer at Step 6 and all packets arriving As a result, some packets (packets in the buffer at Step 6 and all packets arriv
between Step 6 and Step 8) are delivered to the host with the Optimisitc address ing between Step 6 and Step 8) are delivered to the host with the Optimistic 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 (usually ju Without the unsolicited NA, one or more packets that are in the buffer at Step 8
st one packet but some routers may buffer a few) would have been delivered to th (usually just one packet, but some routers may buffer a few) would have been de
e rightful owner and the rest of the packets would have been dropped. livered to the rightful owner and the rest of the packets would have been droppe
However, the probability of such scenario is rather low as it would require the d.
following However, the probability of such a scenario is rather low, as it would require t
he following
things to happen almost simultaneously (within tens of milliseconds in most case s): things to happen almost simultaneously (within tens of milliseconds in most case s):
</t> </t>
<ul spacing="normal"> <ul spacing="normal">
<li> <li>
One host starts using a new IPv6 address and sending traffic without sending an unso licited NA first. One host starts using a new IPv6 address and sending traffic without sending an unso licited NA first.
</li> </li>
<li> <li>
Anot her host configures the same IPv6 address in Optimistic mode before the router c ompletes the address resolution for the rightful owner. Anot her host configures the same IPv6 address in Optimistic mode before the router c ompletes the address resolution process for the rightful owner.
</li> </li>
</ul> </ul>
<t> <t>
It should be noted that in this scenario the rigthful owner does not send any un solicited NAs before sending packets. If the rightful owner implements the funct ionality described in this document and sends unsolicited NAs upon configuring i ts address, then the router creates a STALE entry for the address, causing all p ackets are delivered to the rightful owner (see <xref target="avoid_dis_exists" format="default"/>). The rightful owner would experience no disruption but might receive some packets intended for the host with Optimistic address. It should be noted that in this scenario the rightful owner does not send any un solicited NAs before sending packets. If the rightful owner implements the funct ionality described in this document and sends unsolicited NAs upon configuring i ts address, then the router creates a STALE entry for the address, causing all p ackets to be delivered to the rightful owner (see <xref target="avoid_dis_exists " format="default"/>). The rightful owner would experience no disruption but mig ht 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 Optimistic address (Step 7 an er arrives after the unsolicited one sent from the Optimistic Address (Step 7 an
d Step 4 respectively). d Step 4, respectively).
If the solicited NA arrives first it changes the NC entry state from INCOMPLETE If the solicited NA arrives first, it changes the NC entry state from INCOMPLETE
to REACHABLE. As discussed in <xref target="avoid_dis_exists" format="default"/> to REACHABLE. As discussed in <xref target="avoid_dis_exists" format="default"/
, there will be no disruption for the rightful owner if the router already has a >, 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 scenarios whi ch can lead to the situation when the router does not have a NC entry for the IP v6 address: There are two distinct scenarios tha t can lead to the situation when the router does not have an NC entry for the IP v6 address:
</t> </t>
<ol spacing="normal" type="1"><li> <ol spacing="normal" type="1"><li>
The rightful owner o f the address has not been using it for off-link communication recently or has n ever used it at all. The rightful owner o f the address has not been using it for off-link communication recently or has n ever used it at all.
</li> </li>
<li> <li>
The rightful owner j ust started sending packets from that address but the router has not received an y return traffic yet. The rightful owner j ust started sending packets from that address, but the router has not received a ny return traffic yet.
</li> </li>
</ol> </ol>
<t> <t>
The impact on the rightful owner's t raffic flows would be different in those cases. The impact on the rightful owner's t raffic 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 Packets From The Address</name > <name>The Rightful Owner Is Not Sending Packets from the Address</name >
<t> <t>
In this scenario the followi ng events are expected to happen: In this scenario, the follow ing events are expected to happen:
</t> </t>
<ol spacing="normal" type="1"><li> <ol spacing="normal" type="1"><li>
The host con figures the address and sets its state to Optimistic. The host con figures the address and sets its state to Optimistic.
</li> </li>
<li> <li>
The host sen ds an unsolicited NA with the Override flag set to zero and starts sending traff ic from the Optimistic address. The host sen ds an unsolicited NA with the Override flag set to zero and starts sending traff ic from the Optimistic Address.
</li> </li>
<li> <li>
The router c reates a STALE entry for the address and the host link-layer address. The router c reates a STALE entry for the address and the host link-layer address.
</li> </li>
<li> <li>
The host sta rts DAD and detects the address duplication. The host sta rts DAD and detects the address duplication.
</li> </li>
<li> <li>
The router r eceives 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_FIR ST_PROBE_TIME (<xref target="RFC4861" format="default"/>) seconds. The router r eceives 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_F IRST_PROBE_TIME seconds <xref target="RFC4861" format="default"/>.
</li> </li>
<li> <li>
The router changes the NC entry state to PROBE and sends up to MAX_UNICAST_SOLICIT ( <xref target="RFC4861" format="default"/>) unicast NSes separated by RetransTime r milliseconds (<xref target="RFC4861" format="default"/>) to the host link-laye r address. The router changes the NC entry state to PROBE and sends up to MAX_UNICAST_SOLICIT u nicast NSes <xref target="RFC4861" format="default"/> separated by RetransTimer milliseconds <xref target="RFC4861" format="default"/> to the host link-layer ad dress.
</li> </li>
<li> <li>
As the host As the host
has detected the address conflict already it does not respond to the unicast NSe has already detected the address conflict, it does not respond to the unicast NS
s. (It is unlikely that the host has not completed the DAD process at this stage es. (It is unlikely that the host has not completed the DAD process at this stag
, 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
pAddrDetectTransmits*RetransTimer*1000 + MAX_RTR_SOLICITATION_DELAY secs, <xref upAddrDetectTransmits*RetransTimer*1000 + MAX_RTR_SOLICITATION_DELAY seconds) (<
target="RFC4862" sectionFormat="of" section="5.4"/>). The default value for the xref target="RFC4862" sectionFormat="of" section="5.4"/>).) The default value fo
DAD process would be 1*1*1000 + 1 = 2 secs, <xref target="RFC4861" format="defau r the DAD process would be 1*1*1000 + 1 = 2 seconds <xref target="RFC4861" forma
lt"/>. If the host has completed DAD but did not detect the address conflict the t="default"/>.
n there are two hosts with the same address in the Preferred state and the disru If the host has completed DAD but did not detect the address conflict, then ther
ption is inevitable anyway. e are two hosts with the same address in the preferred state and disruption is i
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 at step 2 are still arriving, the router buffe ckets for communication initiated at Step 2 are still arriving, the router buffe
rs a small number of those packets and starts the address resolution again by se rs a small number of those packets and starts the address resolution process aga
nding a multicast NS to the solicited node multicast address. The rightful owner in by sending a multicast NS to the solicited-node multicast address. The rightf
responds and the router NC entry is updated with the rightful owner link-local ul owner responds, and the router's NC entry is updated with the rightful owner'
address. The buffered packet(s) are sent to that address. Any packets still arri s link-local address. The buffered packet or packets are sent to that address. A
ving after the address resolution still completed are sent to the rightful addre 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 still arriving after the address
resolution still completed are sent to the rightful address owner
as well.
Currently:
Any packets still
arriving after the address resolution process has completed are
sent to the rightful address owner as well. -->
</li> </li>
</ol> </ol>
<t> <t>
The rightful owner is not ex The rightful owner is not ex
periencing any disruption as it does not send any traffic. periencing any disruption, 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 at step 2 a completed and only if return packets for the communication initiated at Step 2 a
re still arriving. re still arriving.
</t> </t>
<t> <t>
However, the same behaviour However, the same behavior w
would be observed if changes proposed in this document are not implemented. ould be observed if the changes specified in this document are not implemented.
If the host starts sending p If the host starts sending p
ackets from its Optimistic address but then changes the address state to Duplica ackets from its Optimistic Address 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 resol ved would be forwarded to the rightful owner of the address. The buffered packet(s) and any packets still arriving after the address is resol ved would be forwarded to the rightful owner of the address.
So the rightful owner might still receive one or more packets from the flows int So, the rightful owner might still receive one or more packets from the flows in
ended for another host. tended for another host.
Therefore, it's safe to conclude that the proposed changes do introduce any disr Therefore, it's safe to conclude that the changes 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 Packets From The Address< /name> <name>The Rightful Owner Has Started Sending Packets from the Address< /name>
<t> <t>
In this scenario the following event s are happening: In this scenario, the following even ts are happening:
</t> </t>
<ol spacing="normal" type="1"><li> <ol spacing="normal" type="1"><li>
The rightful owner s tarts sending traffic from the address (e.g. the address has just been configure d or has not been recently used). The rightful owner s tarts sending traffic from the address (e.g., the address has just been configur ed or has not been recently used).
</li> </li>
<li> <li>
The host con figures the address and sets its state to Optimistic. The host con figures the address and sets its state to Optimistic.
</li> </li>
<li> <li>
The host sen ds an unsolicited NA with the Override flag set to zero and starts sending traff ic from the Optimistic address. The host sen ds an unsolicited NA with the Override flag set to zero and starts sending traff ic from the Optimistic Address.
</li> </li>
<li> <li>
The router c reates a STALE entry for the address and the host link-layer address. The router c reates a STALE entry for the address and the host link-layer address.
</li> </li>
<li> <li>
The host sta rts DAD and detects the address duplication. The host sta rts DAD and detects the address duplication.
</li> </li>
<li> <li>
The router r eceives the return traffic for the IPv6 address in question. Some flows intended for the rightful owner of the duplicated address, while some are for the new ho st. As the NC entry is STALE it sends traffic using that entry, changes it to DE LAY and waits up to DELAY_FIRST_PROBE_TIME (<xref target="RFC4861" format="defau lt"/>) seconds. The router r eceives the return traffic for the IPv6 address in question. Some flows are inte nded for the rightful owner of the duplicated address, while some are for the ne w 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 target="RFC4861" format="default"/>.
</li> </li>
<li> <li>
The router changes the NC entry state to PROBE and sends up to MAX_UNICAST_SOLICIT ( <xref target="RFC4861" format="default"/>) unicast NSes separated by RetransTime r milliseconds (<xref target="RFC4861" format="default"/>) to the host link-laye r address. The router changes the NC entry state to PROBE and sends up to MAX_UNICAST_SOLICIT u nicast NSes <xref target="RFC4861" format="default"/> separated by RetransTimer milliseconds <xref target="RFC4861" format="default"/> to the host link-layer ad dress.
</li> </li>
<li> <li>
As the host has detected the address conflict already it does not respond to the unicast NSe s. As the host has already detected the address conflict, it does not respond to the unicast NS es.
</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 pac ket 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. Th e rightful owner responds and the router NC entry is updated with the rightful o wner link-local address. The next pac ket 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 a result, 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 as DELAY_FIRST_PROBE_T s instead.
IME*1000 + (MAX_UNICAST_SOLICIT - 1)*RetransTimer milliseconds.
As per the constants defined <!-- [rfced] Section 5.3.2: Should "address rightful owner" be
in <xref target="RFC4861" sectionFormat="of" section="10"/> this interval is eq "address's rightful owner", "address of the rightful owner", or
ual to 5*1000 + (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 as DELAY_FIRST_PROBE_TIME*1000
+ (MAX_UNICAST_SOLICIT - 1)*RetransTimer milliseconds.
As per the constants defined
in <xref target="RFC4861" sectionFormat="of" section="10"/>, this interval is e
qual to 5*1000 + (3 - 1)*1000 = 7000 milliseconds, or 7 seconds.
</t> </t>
<t> <t>
However, it should b e noted that the probability of such scenario is rather low. Similary to the sce nario discussed in <xref target="avoid_dis_inc" format="default"/>, it would req uire the following things to happen almost simultaneously (within tens of millis econds in most cases): However, it should b e noted that the probability of such a scenario is rather low. Similar to the sc enario discussed in <xref target="avoid_dis_inc" format="default"/>, it would re quire the following things to happen almost simultaneously (within tens of milli seconds 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 unso licited NA first. One host starts using a new IPv6 address and sending traffic without sending an unso licited NA first.
</li> </li>
<li> <li>
Anot her host configures the same IPv6 address in Optimistic mode before the router r eceives the return traffic for the first host. Anot her host configures the same IPv6 address in Optimistic mode before the router r eceives the return traffic for the first host.
</li> </li>
</ul> </ul>
<t> <t>
As discussed in <xref target="avoid_dis_inc" format="default"/>, the disruption to the rightful owner can easily be prevent if that node implements the mechanis m described in the document. Sending unsolicited NAs before initiatining off-lin k communication would create a STALE entry in the router NC and prevent any tarf fic to that address to be sent to the host with the Optimistic address (see <xre f target="avoid_dis_exists" format="default"/>). As discussed in <xref target="avoid_dis_inc" format="default"/>, the disruption for the rightful owner can easily be prevented if that node implements the mecha nism described in this document. Sending unsolicited NAs before initiating off-l ink 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 target="avoid_dis_exists" 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 (IP v6"))</name>
<section numbered="true" toc="default"> <section numbered="true" toc="default">
<name>Modification to the section 7.2.5</name> <name>Modification to Section 7.2.5 of RFC 4861</name>
<t> <t>
This document makes the following changes to the < xref target="RFC4861" sectionFormat="of" section="7.2.5"/>: This document makes the following changes to <xref target="RFC4861" 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 so licited or <blockquote>When a valid Neighbor Advertisement is received (either so licited 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 disca rded. If no entry exists, the advertisement <bcp14>SHOULD</bcp14> be silently disca rded.
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 Target link-layer address option (if supplied). The entry's reachability state <bcp14>MUST</bcp14> be set to STA LE. If the received Neighbor Advertisement does not contain the Target link-laye r 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 Target Link-Layer Address Option (if supplied). The entry's reachability state <bcp14>MUST</bcp14> be set to STA LE. If the received Neighbor Advertisement does not contain the Target Link-Laye r 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 to the section 7.2.6</name> <name>Modification to Section 7.2.6 of RFC 4861</name>
<t> <t>
This document proposes the following changes to th e <xref target="RFC4861" sectionFormat="of" section="7.2.6"/>: This document makes the following changes to <xref target="RFC4861" 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</b cp14> multicast <blockquote>Also, a node belonging to an anycast address <bcp14>MAY</b cp14> 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 their nei ures a new global IPv6 address so the routers can proactively populate their Nei
ghbor caches with the corresponding entries. In such cases a node <bcp14>SHOULD< ghbor Caches with the corresponding entries. In such cases, a node <bcp14>SHOULD
/bcp14> send up to MAX_NEIGHBOR_ADVERTISEMENT </bcp14> send up to MAX_NEIGHBOR_ADVERTISEMENT
Neighbor Advertisement messages. If the address is preferred then the Overrid Neighbor Advertisement messages. If the address is preferred, then the Overri
e flag <bcp14>SHOULD NOT</bcp14> be set. If the address is in the Optimistic sta de flag <bcp14>SHOULD NOT</bcp14> be set. If the address is in the Optimistic st
te then the Override flag <bcp14>MUST NOT</bcp14> be set. The destination addre ate, then the Override flag <bcp14>MUST NOT</bcp14> be set. The destination add
ss <bcp14>SHOULD</bcp14> be set to the all-routers multicast address. These adve ress <bcp14>SHOULD</bcp14> be set to the all-routers multicast address. These ad
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 target="RFC4429" format="default"/> is us ed:</dt><dd>a new Optimistic address is assigned <dt>If Optimistic DAD <xref target="RFC4429" format="default"/> is us ed:</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 sta te from <dt>If Optimistic DAD is not used:</dt><dd>An address changes the sta te 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 s ome improvement for a node configuring a new IPv6 address and starting sending t raffic from it. The solution described in this document provides s ome improvement for a node configuring a new IPv6 address and starting to send t raffic from it.
However, that approach does not completely elimina te the scenario when a router receives some transit traffic for an address witho ut the corresponding Neighbor Cache entry. However, that approach does not completely elimina te the scenario when a router receives some transit traffic for an address witho ut the corresponding Neighbor Cache entry.
For example: For example:
</t> </t>
<ul spacing="normal"> <ul spacing="normal">
<li>If the host starts using an already configured IPv6 address after a <li>If the host starts using an already-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 the router Neighbor Cache would trigger the packet loss for <li>Clearing the router's Neighbor Cache would trigger packet loss for a
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 Dis covery (<xref target="RFC4861" format="default"/>) to the registration-based mec hanisms (<xref target="RFC8505" format="default"/>). Migrate from the "reactive" Neighbor Disco very <xref target="RFC4861" format="default"/> to the registration-based mechani sms <xref target="RFC8505" format="default"/>.
</li> </li>
<li> <li>
Creating new entries in routers Neighbor Ca che 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 from the host to the router using the host GUA. Initiate bidirectional communication from the host to the router using the host GUA.
</li> </li>
<li> <li>
Making the probing logic on hosts more rob ust. Make the probing logic on hosts more robus t.
</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 addr ess 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 w ith 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 w ith 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 deployi ng 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 target="RFC4861" format="default" /> and expand the registration-based ND (<xref target="RFC6775" format="default" />, <xref target="RFC8505" format="default"/>) used in Low-Power Wireless Person al 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 target="RFC4861" format="default" /> and expand the registration-based ND <xref target="RFC6775" format="default"/ > <xref target="RFC8505" format="default"/> used in IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs) to the rest of the IPv6 deployments.
This option requires some investig ation and discussion. This option requires some investig ation and discussion.
However, significant changes to th e existing IPv6 implementations would be needed, so unclear adoption timeline ma kes this approach less preferable than one proposed in this document. However, significant changes to th e existing IPv6 implementations would be needed, so an unclear adoption timeline makes this approach less preferable than the approach specified in this documen t.
</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 force creating a STALE entry for it s GUA in the router ND cache by sending the following Neighbor Solicitation mes sage: The host could force the creation of a STALE entry for its GUA in the router's ND cache by sending the following Neighbor Solicita tion 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 Address opti on contains the host link-layer address. The Source Link-Layer Address Opti on contains the host link-layer address.
</li> </li>
<li> <li>
The target address is the host def ault router address (the default router address the host received in the RA). The target address is the host's d efault 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 fol lows:
</t> </t>
<ul spacing="normal"> <ul spacing="normal">
<li> <li>
Would not work for Optimistic addr esses as <xref target="RFC4429" sectionFormat="of" section="2.2"/> explicitly pr ohibits sending Neighbor Solicitations from an Optimistic Address. It would not work for Optimistic A ddresses, as <xref target="RFC4429" sectionFormat="of" section="2.2"/> explicitl y prohibits sending Neighbor Solicitations from an Optimistic Address.
</li> </li>
<li> <li>
If first-hop redundancy is deploye d in the network, the NS would reach the active router only, so all backup route rs (or all active routers except one) would not get their neighbor cache updated . If first-hop redundancy is deploye d in the network, the NS would reach the active router only, so all backup route rs (or all active routers except one) would not get their Neighbor Cache updated .
</li> </li>
<li> <li>
Some wireless devices are known to alter ND packets and perform various non-obvious forms of ND proxy acti ons. Some wireless devices are known to alter ND packets and perform various non-obvious forms of ND proxy acti ons.
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 from its GUA</name> <name>Host Sending Router Solicitation from Its GUA</name>
<t> <t>
The host could send a router solicitation The host could send a Router Solicitation
message to 'all routers' multicast address, using its GUA as a source. message to the 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 Address option, the router would create a STALE entry in the Source Link-Layer Address Option, the router would create a STALE entry
for the host GUA as per the <xref target="RFC4861" sectionFormat="of" section="6 for the host GUA as per <xref target="RFC4861" 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 in optimistic state: <xref target="RFC4429" sectionFormat="of" section he GUA is in the Optimistic state: <xref target="RFC4429" sectionFormat="of" sec
="2.2"/> explicitly prohibits using an Optimistic Address as the source address tion="2.2"/> explicitly prohibits using an Optimistic Address as the source add
of a Router Solicitation with a SLLAO as it might disrupt the rightful owner of ress of a Router Solicitation with a SLLAO, as it might cause disruption for the
the address in the case of a collision. rightful owner of the address in the case of a collision.
So for the optimistic addresses the host c So, for the Optimistic Addresses, the host
an send an RS without SLLAO included. can send an RS without a SLLAO included.
In that case the router may respond with e In that case, 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 with either 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
ptimistic state the RS cannot contain SLLAO. As a result the router would only c ptimistic state, the RS cannot contain a SLLAO. As a result, the router would on
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 cache entry as they do not need to send RAs as multicast would not create a new cache entry, 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 de lay 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 de lay 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 Gleaning From Neighbor Discover y Packets</name> <name>Routers Populating Their Caches by Gleaning from Neighbor Discover y 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 all solicited node mul The router could listen to all solicited-node mul
ticast address groups and upon receiving a Neighbor Solicitation from the unspec ticast address groups and, 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 resolution f <!-- [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 the unspecified 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 resolution pr
ocess for that entry.
</t> </t>
<t> <t>
The same solution was proposed in <xref target="I- D.halpern-6man-nd-pre-resolve-addr" format="default"/>. Some routing vendors sup port 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 target="I- D.halpern-6man-nd-pre-resolve-addr" format="default"/>. Some routing vendors alr eady 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 al l multicast Neighbor Discovery packets which might negatively impact the routers CPU. Routers need to receive al l multicast Neighbor Discovery packets; this might negatively impact a router's CPU.
</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 Neighbor Solicitation the host ddress resolution process as soon as it receives the DAD Neighbor Solicitation,
might be still performing DAD and the target address might be tentative. the host might still be performing DAD and the target address might be tentative
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 per the <xref target="RFC4862" sectionFormat="of" section="5.4.3"/>. p14>SHOULD</bcp14> silently ignore the received Neighbor Solicitation from the r
As a result the router mig outer as per <xref target="RFC4862" sectionFormat="of" section="5.4.3"/>.
ht not be able to complete the address resolution before the return traffic arri As a result, the router mi
ves. ght not be able to complete the address resolution process before the return tra
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 res olution by sending a data packet such as ping or traceroute to its default route r link-local address, using the GUA as a source address. The host may force the router to start address res olution by sending a data packet such as ping or traceroute to its default route r link-local address, using the GUA as a source address.
As the RTT to the default router is lower than RT T 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 retu rning traffic arrives. As the RTT to the default router is 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 protect ion mechanism. Data packets to the router 's link-local address could be blocked by a security policy or control plane pro tection mechanism.
</li> </li>
<li> <li>
It introduces an additiona l overhead for routers control plane (in addition to processing ND packets, the data packet needs to be processed as well). It introduces an additiona l overhead for the router's control plane (in addition to processing ND packets, the data 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 to the all-routers ff02::2 multicast address, if the network provides a fir st-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 m ight 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 a pproach 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 pro bing, while the proposed solution is implemented on operating systems level.</li > <li>It would require updating all possible applications performing pro bing, while the solution described in this document is implemented at the operat ing-system level.</li>
<li> <li>
<t>Some implementations need to send multiple probes. Examples inclu de but not limited to: <t>Some implementations need to send multiple probes. Examples inclu de but are not limited to:
</t> </t>
<ul spacing="normal"> <ul spacing="normal">
<li>Sending AAAA and A records DNS probes in parallel.</li> <li>Sending AAAA and A record DNS probes in parallel.</li>
<li>Detecting captive portals often require sending multiple packe <li>Detecting captive portals, which often requires sending multip
ts.</li> le packets.</li>
</ul> </ul>
</li> </li>
<li> <li>
<t>While it would increase the probability of the probing to complet e successfully, there are multiple cases when packet loss would still occur: <t>While it would increase the probability that the probing will com plete 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 t he first one are dropped. </li> <li> The probe response consists of multiple packets, so all but t he first one are dropped. </li>
<li> There are multiple applications on the same host sending traf fic and return packets arrive simultaneously.</li> <li> There are multiple applications on the same host sending traf fic, and return packets arrive simultaneously.</li>
<li> There are multiple first-hop routers in the network. The firs t probe packet creates the NC entry on one of them. The subsequent return traffi c flows might cross other routers and still experience the issue.</li> <li> There are multiple first-hop routers in the network. The firs t probe packet creates the NC entry on one of them. The subsequent return traffi c flows might cross other routers and still experience the issue.</li>
</ul> </ul>
</li> </li>
<li> <li>
Reducing the probe retransmit interval unnec essary increases the network utilization and might cause the network congestion. Reducing the probe retransmit interval unnec essarily 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 buf fering more packets would exacerbate issues described in <xref target="RFC6583" format="default"/> and make the router more vulnerable to ND-based denial of ser vice attacks. Increasing the buffer size and buf fering more packets would exacerbate issues described in <xref target="RFC6583" format="default"/> and make the router more vulnerable to ND-based denial-of-ser vice 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 Re solution</name> <name>Transit Data Plane Traffic from a New Address to Trigger Address R esolution</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 of the neighbor cache entr an on-link neighbor node, it might check for the presence of a Neighbor Cache e
y for the packet source address and if the entry does not exist, start address r ntry for the packet source address and, if the entry does not exist, start the 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
ending offlink traffic. ending off-link traffic.
However, this approach has a number of limitations, in particular: However, this approach has a number of limitations. In particular:
</t> </t>
<ul spacing="normal"> <ul spacing="normal">
<li>If traffic flows are asymmetrical the return traffic might not tra <li>If traffic flows are asymmetrical, the return traffic might not tr
nsit the same router as the original traffic which triggered the address resolut ansit the same router as the original traffic that triggered the address resolut
ion. ion process.
So the neighbor cache entry is created on the "wrong" router, not the one which So, the Neighbor Cache entry is created on the "wrong" router, not the one that
actually needs the neighbor cache entry for the host address. actually needs the Neighbor 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 between onlink addresses (ones th interfaces, as the router needs to distinguish between on-link addresses (addres
e router needs to have Neighbor Cache entries for) and the rest of the address s ses for which the router needs to have Neighbor Cache entries) and the rest of t
pace. he address space.
The proactive address resolution must only be triggered by packets from The proactive address resolution process must only be triggered by packe
the prefixes known to be on-link. Otherwise, traffic from spoofed source address ts from the prefixes known to be on-link. Otherwise, traffic from spoofed source
es or any transit traffic could lead to neighbor cache exhaustion. addresses or any transit traffic could lead to Neighbor Cache exhaustion.
</li> </li>
<li> <li>
Implementing such functionality is much more complicated than all other solution s as it would involve complex data-control planes interaction. Implementing such functionality is much more complicated than all other solution s, as it would involve complex interactions between the data plane and the contr ol 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 is a cache s poofing when the attacker might try to install a cache entry for the victim's IP v6 address and the attacker's Link-Layer address. However, it should be noted th at 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 is cache spo ofing, where the attacker might try to install a cache entry for the victim's IP v6 address and the attacker's link-layer address. However, it should be noted th at 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 overr ide an existing cache entry. Therefore, there are no new vectors for an attacker to overr ide an existing cache entry.
</t> </t>
<t> <t>
<xref target="avoid_dis" format="default"/> describes some corner cases when a h <xref target="avoid_dis" format="default"/> describes some corner cases when a h
ost with the duplicated Optimistic address might get some packets intended for t ost with a duplicated Optimistic Address might get some packets intended for the
he rightful owner of the address. However such scenarios do not introduce any ne rightful owner of the address. However, such scenarios do not introduce any new
w attack vectors: even without the proposed changes, an attacker can easily over attack vectors: even without the changes discussed in this document, an attacke
ride the routers neighbor cache and redirect the traffic by sending NAs with the r can easily override the router's Neighbor Cache and redirect the traffic by se
Solicited flag set. nding NAs with the Solicited flag set.
As discussed in <xref target="dis_start" format="default"/> the worst case scena As discussed in <xref target="dis_start" format="default"/>, the worst-case scen
rio might cause a disruption for up to 7 seconds. This risk is considered accept 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 all cas ly unlikely, this risk of disruption is considered acceptable. More importantly,
es described in <xref target="avoid_dis" format="default"/> the rightful owner c for all cases described in <xref target="avoid_dis" format="default"/>, the rig
an prevent disruption caused by an accidental address duplication just by implem htful owner can prevent disruption caused by an accidental address duplication j
enting the mechanism described in this document. If the rightful owner sends uns ust by implementing the mechanism described in this document. If the rightful ow
olicited NAs before using the address, the STALE entry would be created on the r ner sends unsolicited NAs before using the address, the STALE entry would be cre
outer NC and any subsequent unsolicited NAs sent from the host with an Optimisti ated on the router's NC, and any subsequent unsolicited NAs sent from the host w
c address would not override the NC entry. ith an Optimistic Address would not override the NC entry.
</t> </t>
<t> <t>
A malicious host could attempt to exhaust the neighbor 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 attac k: the attacker could do it, for example, by sending a NS or RS packet with SLLA O included. All recommendations from <xref target="RFC6583" format="default"/> s till apply. A malicious host could attempt to exhaust the Neighbor 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 increa se 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 multicast address ma Announcing a new address to the all-routers multicast addres
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 a s ver, hiding information about the specific IPv6 address should not be considered
ecurity measure as such information is usually disclosed via DAD to all nodes an a security measure, as such information is usually disclosed via DAD to all nod
yway if MLD snooping is not enabled. Network administrators can also mitigate th es anyway if MLD snooping is not enabled. Network administrators can also mitiga
is issue by enabling MLD snooping on the link-layer devices to prevent IPv6 link te this issue by enabling MLD snooping on the link-layer devices to prevent IPv6
-local multicast packets being flooded to all onlink nodes. link-local multicast packets from being flooded to all on-link nodes.
If peer-to-peer onlink communications are not desira If peer-to-peer on-link communications are not desir
ble for the given network segment they should be prevented by proper layer-2 sec able for a given network segment, they should be prevented by proper Layer 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 to the all-routers multicast address is low.
</t> </t>
<t> <t>
It should be noted that the proposed 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 addr esses to mitigate ND cache exhaustion DoS attacks described in <xref target="RFC 3756" sectionFormat="of" section="4.3.2"/> and <xref target="RFC6583" format="de fault"/>. It should be noted that the mechanism discussed in this docu ment allows hosts to proactively inform their routers about global IPv6 addresse s existing on-link. Routers could use that information to distinguish between us ed and unused addresses to mitigate ND cache exhaustion DoS attacks as described in <xref target="RFC3756" sectionFormat="of" section="4.3.2"/> and in <xref tar get="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 target="I-D.halpern-6man-nd-pre-resolve-addr" to="ND-ADDR-RES" /> <displayreference target="I-D.halpern-6man-nd-pre-resolve-addr" 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.R FC.2119.xml"/> <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.R FC.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.R FC.6583.xml"/> <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.R FC.6583.xml"/>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.R FC.6775.xml"/> <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.R FC.6775.xml"/>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.R FC.8305.xml"/> <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.R FC.8305.xml"/>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.R FC.8505.xml"/> <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.R FC.8505.xml"/>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.R FC.8981.xml"/> <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.R FC.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. 
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