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<?xml version='1.0' encoding='utf-8'?><rfc xmlns:xi="http://www.w3.org/2001/XInclude"     category="std" ipr="trust200902" obsoletes="" updates=""<!-- category values: std, bcp, info, exp, and historic    ipr values: full3667, noModification3667, noDerivatives3667    you can add the attributes updates="NNNN" and obsoletes="NNNN"    they will automatically be output with "(if approved)" --><!-- ***** FRONT MATTER ***** -->  <front><!-- The abbreviated title is used in the page header - it is only necessary if the    full title is longer than 39 characters -->    <seriesInfo    <author fullname="Rahul Arvind Jadhav" initials="R.A." role="editor" surname="Jadhav">      <organization>Huawei</organization>      <address>        <postal>          <street>Kundalahalli Village, Whitefield,</street>          <city>Bangalore</city>          <region>Karnataka</region>          <code>560037</code>          <country>India</country>        </postal>        <phone>+91-080-49160700</phone>        <email>rahul.ietf@gmail.com</email>      </address>    </author>    <author initials="P" surname="Thubert" fullname="Pascal Thubert">      <organization abbrev="Cisco">Cisco Systems, Inc</organization>      <address>        <postal>          <street>Building D</street>          <street>45 Allee des Ormes - BP1200 </street>          <city>MOUGINS - Sophia Antipolis</city>          <code>06254</code>          <country>France</country>        </postal>        <phone>+33 497 23 26 34</phone>        <email>pthubert@cisco.com</email>      </address>    </author>    <author fullname="Rabi Narayan Sahoo" initials="R.N." surname="Sahoo">      <organization>Huawei</organization>      <address>        <postal>          <street>Kundalahalli Village, Whitefield, </street>          <city>Bangalore</city>          <region>Karnataka</region>          <code>560037</code>          <country>India</country>        </postal>        <phone>+91-080-49160700</phone>        <email>rabinarayans@huawei.com</email>      </address>    </author>    <author initials="Z" surname="Cao" fullname="Zhen Cao">      <organization>Huawei</organization>      <address>        <postal>          <street>W Chang'an Ave</street>          <city>Beijing</city>          <country>P.R. China</country>        </postal>        <email>zhencao.ietf@gmail.com</email>      </address>    </author>    <date year="2019"/>    <!-- If the month and year are both specified and are the current ones, xml2rfc will fill         in the current day for you. If only the current year is specified, xml2rfc will fill     in the current day and month for you. If the year is not the current one, it is     necessary to specify at least a month (xml2rfc assumes day="1" if not specified for the     purpose of calculating the expiry date). With drafts it is normally sufficient to     specify just the year. -->    <!-- Meta-data Declarations -->    <area>General</area>    <workgroup>ROLL</workgroup>    <!-- WG name at the upperleft corner of the doc,         IETF is fine for individual submissions.     If this element is not present, the default is "Network Working Group",         which is used by the RFC Editor as a nod to the history of the IETF. -->    <keyword>template</keyword>    <!-- Keywords will be incorporated into HTML output         files in a meta tag but they have no effect on text or nroff         output. If you submit your draft to the RFC Editor, the         keywords will be used for the search engine. -->    <abstract>      <t>        This document explains the problems associated with the current use of        NPDAO messaging and also discusses the requirements for an optimized        route invalidation messaging scheme. Further a new proactive route        invalidation message called as "Destination Cleanup Object" (DCO) is        specified which fulfills requirements of an optimized route        invalidation messaging.      </t>    </abstract>  </front>  <middle>    <section numbered="true" toc="default">      <name>Introduction</name>      <t>            RPL <xref format="default"/> (Routing Protocol for Low power and            lossy networks) specifies a proactive distance-vector based routing            scheme. RPL has optional messaging in the form of DAO            (Destination Advertisement Object) messages, which the 6LBR (6Lo            Border Router) and 6LR (6Lo Router) can use to learn a route            towards the downstream nodes. In storing mode, DAO messages would            result in routing entries being created on all intermediate 6LRs            from the node's parent all the way towards the 6LBR.      </t>      <t>            RPL allows the use of No-Path DAO (NPDAO) messaging to invalidate a            routing path corresponding to the given target, thus releasing            resources utilized on that path. A NPDAO is a DAO message with            route lifetime of zero, originates at the target node and always            flows upstream towards the 6LBR. This document explains the            problems associated with the current use of NPDAO messaging and            also discusses the requirements for an optimized route invalidation            messaging scheme. Further a new proactive route invalidation            message called as "Destination Cleanup Object" (DCO) is specified            which fulfills requirements of an optimized route invalidation            messaging.      </t>      <t>            The document only caters to the RPL's storing mode of operation            (MOP). The non-storing MOP does not require use of NPDAO for route            invalidation since routing entries are not maintained on 6LRs.      </t>      <section numbered="true" toc="default">        <name>Requirements Language and Terminology</name>        <t>                The key words and in this document are to be interpreted as                described in BCP 14 <xref format="default"/> <xref format="default"/> when, and only when, they appear in all                capitals, as shown here.        </t>        <t>                This specification requires readers to be familiar with all the                terms and concepts that are discussed in "RPL: IPv6 Routing                Protocol for Low-Power and Lossy Networks" <xref format="default"/>.        </t>        <dl newline="true"          <dt>Low Power and Lossy Networks (LLN):</dt>          <dd>                        Network in which both the routers and their                        interconnect are constrained. LLN routers typically                        operate with constraints on processing power, memory,                        and energy (batter power). Their interconnects are                        characterized by high loss rates, low data rates, and                        instability.                    </dd>          <dt>6LoWPAN Router (6LR):</dt>          <dd>                        An intermediate router that is able to send and receive Router                        Advertisements (RAs) and Router Solicitations (RSs) as well as                        forward and route IPv6 packets.                    </dd>          <dt>Directed Acyclic Graph (DAG):</dt>          <dd>                        A directed graph having the property that all edges are                        oriented in such a way that no cycles exist.                    </dd>          <dt>Destination-Oriented DAG (DODAG):</dt>          <dd>                        A DAG rooted at a single destination, i.e., at a single                        DAG root with no outgoing edges.                    </dd>          <dt>6LoWPAN Border Router (6LBR):</dt>          <dd>                        A border router which is a DODAG root and is the edge                        node for traffic flowing in and out of the 6LoWPAN                        network.                    </dd>          <dt>Destination Advertisement Object (DAO):</dt>          <dd>                        DAO messaging allows downstream routes to the nodes to                        be established.                    </dd>          <dt>DODAG Information Object (DIO):</dt>          <dd>                        DIO messaging allows upstream routes to the 6LBR to be                        established. DIO messaging is initiated at the DAO                        root.                    </dd>          <dt>Common Ancestor node</dt>          <dd>                        6LR/6LBR node which is the first common node between                        two paths of a target node.                    </dd>          <dt>No-Path DAO (NPDAO):</dt>          <dd>                        A DAO message which has target with lifetime 0 used for                        the purpose of route invalidation.                    </dd>          <dt>Destination Cleanup Object (DCO):</dt>          <dd>                        A new RPL control message code defined by this                        document. DCO messaging improves proactive route                        invalidation in RPL.                    </dd>          <dt>Regular DAO:</dt>          <dd>                        A DAO message with non-zero lifetime. Routing                        adjacencies are created or updated based on this                        message.                    </dd>          <dt>Target node:</dt>          <dd>                        The node switching its parent whose routing adjacencies                        are updated (created/removed).                    </dd>        </dl>      </section>      <section anchor="current_npdao" numbered="true" toc="default">        <name>Current NPDAO messaging</name>        <t>                RPL uses NPDAO messaging in the storing mode so that the node                changing its routing adjacencies can invalidate the previous                route. This is needed so that nodes along the previous path can                release any resources (such as the routing entry) they maintain                on behalf of target node.        </t>        <t>                For the rest of this document consider the following topology:        </t>        <figure anchor="sample_top">          <name>Sample                     topology</name>          <artwork align="center" name="" type="" alt=""><![CDATA[    (6LBR)      |      |      |     (A)     / \    /   \   /     \ (G)     (H)  |       |  |       |  |       | (B)     (C)   \      ;    \    ;     \  ;      (D)      / \     /   \    /     \  (E)     (F)                        ]]></artwork>        </figure>        <t>                Node (D) is connected via preferred parent (B). (D) has an                alternate path via (C) towards the 6LBR. Node (A) is the common                ancestor for (D) for paths through (B)-(G) and (C)-(H). When                (D) switches from (B) to (C), RPL allows sending NPDAO to (B)                and regular DAO to (C).        </t>      </section>      <!--        <section title="Cases when No-Path DAO may be used">            <t> There are following cases in which a node switches its parent                and may employ No-Path DAO messaging:</t>            <t>Case I: Current parent becomes unavailable because of transient                or permanent link or parent node failure.</t>            <t>Case II: The node finds a better parent node i.e. the metrics of                another parent is better than its current parent.</t>            <t>Case III: The node switches to a new parent whom it "thinks" has                a better metric but does not in reality.</t>            <t>The usual steps of operation when the node switches the parent                is that the node sends a No-Path DAO message via its current parent                to invalidate its current route and subsequently it tries to                establish a new routing path by sending a new DAO via its new                parent.</t>        </section>        -->      <section numbered="true" toc="default">        <name>Why Is NPDAO Important?</name>        <t>                Nodes in LLNs may be resource constrained. There is limited                memory available and routing entry records are one of the                primary elements occupying dynamic memory in the nodes. Route                invalidation helps 6LR nodes to decide which entries could be                discarded to better optimize resource utilization. Thus it                becomes necessary to have an efficient route invalidation                mechanism. Also note that a single parent switch may result in                a "sub-tree" switching from one parent to another. Thus the                route invalidation needs to be done on behalf of the sub-tree                and not the switching node alone. In the above example, when                Node (D) switches parent, the route updates needs to be done                for the routing tables entries of (C),(H),(A),(G), and (B) with                destination (D),(E) and (F). Without efficient route                invalidation, a 6LR may have to hold a lot of stale route                entries.        </t>      </section>    </section>    <section anchor="current_npdao_problems" numbered="true" toc="default">      <name>Problems with current NPDAO messaging</name>      <section numbered="true" toc="default">        <name>Lost NPDAO due to link break to the previous parent</name>        <t>                When a node switches its parent, the NPDAO is to be sent to                its previous parent and a regular DAO to its new parent. In                cases where the node switches its parent because of transient                or permanent parent link/node failure then the NPDAO message is                bound to fail.        </t>        <!--            <t>                RPL allows use of route lifetime to remove unwanted routes in                case the routes could not be refreshed. But route lifetimes in                case of LLNs could be substantially high and thus the route                entries would be stuck for longer times.            </t>            -->      </section>      <section numbered="true" toc="default">        <name>Invalidate Routes of Dependent Nodes</name>        <t>                RPL does not specify how route invalidation will work for                dependent nodes rooted at the switching node, resulting in                stale routing entries of the dependent nodes. The only way for                6LR to invalidate the route entries for dependent nodes would                be to use route lifetime expiry which could be substantially                high for LLNs.        </t>        <t>                In the example topology, when Node (D) switches its parent,                Node (D) generates an NPDAO on its behalf. There is no NPDAO                generated by the dependent child nodes (E) and (F), through the                previous path via (D) to (B) and (G), resulting in stale                entries on nodes (B) and (G) for nodes (E) and (F).        </t>      </section>      <section numbered="true" toc="default">        <name>Possible route downtime caused by asynchronous operation of NPDAO and DAO</name>        <t>                A switching node may generate both an NPDAO and DAO via two                different paths at almost the same time. There is a possibility                that an NPDAO generated may invalidate the previous route and                the regular DAO sent via the new path gets lost on the way.                This may result in route downtime impacting downward                traffic for the switching node.        </t>        <t>                In the example topology, consider Node (D) switches from parent                (B) to (C). An NPDAO sent via the previous route may invalidate                the previous route whereas there is no way to determine whether                the new DAO has successfully updated the route entries on the                new path.        </t>      </section>    </section>    <section anchor="requirements" numbered="true" toc="default">      <name>Requirements for the NPDAO Optimization</name>      <section numbered="true" toc="default">        <name>Req#1: Remove messaging dependency on link to the previous parent</name>        <t>                When the switching node sends the NPDAO message to the previous                parent, it is normal that the link to the previous parent is                prone to failure (that's why the node decided to switch).                Therefore, it is required that the route invalidation does not                depend on the previous link which is prone to failure. The                previous link referred here represents the link between the                node and its previous parent (from whom the node is now                disassociating).        </t>      </section>      <section numbered="true" toc="default">        <name>Req#2: Dependent nodes route invalidation on parent             switching</name>        <t>                It should be possible to do route invalidation for dependent                nodes rooted at the switching node.        </t>      </section>      <section numbered="true" toc="default">        <name>Req#3: Route invalidation should not impact data traffic</name>        <t>                While sending the NPDAO and DAO messages, it is possible that                the NPDAO successfully invalidates the previous path, while the                newly sent DAO gets lost (new path not set up successfully).                This will result in downstream unreachability to the node                switching paths. Therefore, it is desirable that the route                invalidation is synchronized with the DAO to avoid the risk of                route downtime.        </t>      </section>    </section>    <!--Too Confusing section and may not be needed now... If required this can be added in Appendix.    <section title="Existing Solution">        <section title="NPDAO can be generated by the parent node who detects        link failure to the child">            <t>RPL states mechanisms which could be utilized to clear DAO            states in a sub-DODAG. [RFC6550] Section 11.2.2.3 states "With DAO            inconsistency loop recovery, a packet can be used to recursively            explore and clean up the obsolete DAO states along a            sub-DODAG".</t>            <t>Thus in the sample topology in Figure 1, when Node (B) detects            link failure to (D), (B) has an option of generating an NPDAO on            behalf of Node (D) and its sub-childs, (E) and (F).</t>            <t>This section explains why generation of an NPDAO in such cases            may not function as desired. Primarily the DAO state information in            the form of Path Sequence plays a major role here. Every target is            associated with a Path Sequence number which relates to the latest            state of the target. <xref/> Section 7.1 explains            the semantics of Path Sequence number. The target node increments            the Path Sequence number every time it generates a new DAO. The            router nodes en-route utilize this Path Sequence number to decide            the freshness of target information. If a non-target node has to            generate an NPDAO then it could use following two possibilities            with Path Sequence number: </t>            <t>Let the Path Sequence number of old regular DAO that flowed            through (B) be x. The subsequent regular DAO generated by Node (D)            will have sequence number x+1.</t>            <t>i. Node (B) uses the previous Path Sequence number from the            regular DAO i.e. NPDAO(pathseq=x)</t>            <t>ii. Node (B) increments the Path Sequence number i.e.            NPDAO(pathseq=x+1)</t>            <t>In case i, the NPDAO(pathseq=x) will be dropped by all the            intermediate nodes since the semantics of Path Sequence number            dictates that any DAO with an older Path Sequence number be            dropped.</t>            <t>In case ii, there is a risk that the NPDAO(pathseq=x+1)            traverses up the DODAG and invalidates all the routes till the root            and then the regular DAO(pathseq=x+1) from the target traverses            upwards. In this case the regular DAO(pathseq=x+1) will be dropped            from common ancestor node to the root. This will result in route            downtime.</t>            <t>Another problem with this scheme is its dependence on the            upstream neighbor to detect that the downstream neighbor is            unavailable. There are two possibilities by which such a detection            might be put to work:</t>            <t>i. There is P2P traffic from the previous sub-DODAG to any of            nodes in the sub-tree which has switched the path. In the above            example, lets consider that Node (G) has P2P traffic for either of            nodes (D), (E), or (F). In this case, Node (B) will detect            forwarding error while forwarding the packets from Node (B) to (D).            But dependence on P2P traffic may not be an optimal way to solve            this problem considering the reactive approach of the scheme. The            P2P traffic pattern might be sparse and thus such a detection might            kick-in too late.</t>            <t>ii. The other case is where Node (B) explicitly employs some            mechanism to probe directly attached downstream child nodes. Such            kind of schemes are seldom used.</t>        </section>        <section title="NPDAO can be generated once the link is restored to        the previous parent">            <t>This scheme solves a specific scenario of transient links. The            child node can detect that the connection to previous parent is            restored and then transmit an NPDAO to the previous parent to            invalidate the route. This scheme is stateful, thus requires more            memory and solves a specific scenario.</t>        </section>    </section>    -->    <section numbered="true" toc="default">      <name>Changes to RPL signaling</name>      <section numbered="true" toc="default">        <name>Change in RPL route invalidation semantics</name>        <t>                As described in <xref format="default"/>, the NPDAO                originates at the node changing to a new parent and traverses                upstream towards the root. In order to solve the problems as                mentioned in <xref format="default"/>, the                document adds a new proactive route invalidation message                called "Destination Cleanup Object" (DCO) that originates at a                common ancestor node and flows downstream between the new and                old path. The common ancestor node generates a DCO in response                to the change in the next-hop on receiving a regular DAO with                updated Path Sequence for the target.        </t>        <t>                The 6LRs in the path for DCO take action such as route                invalidation based on the DCO information and subsequently send                another DCO with the same information downstream to the next                hop. This operation is similar to how the DAOs are handled on                intermediate 6LRs in storing MOP in <xref format="default"/>.                Just like DAO in storing MOP, the DCO is sent using link-local                unicast source and destination IPv6 address. Unlike DAO, which                always travels upstream, the DCO always travels downstream.        </t>        <t>                In <xref format="default"/>, when node D decides to                switch the path from B to C, it sends a regular DAO to node C                with reachability information containing the address of D as                the target and an incremented Path Sequence. Node C will update                the routing table based on the reachability information in the                DAO and in turn generate another DAO with the same reachability                information and forward it to H. Node H also follows the same                procedure as Node C and forwards it to node A. When node A                receives the regular DAO, it finds that it already has a                routing table entry on behalf of the target address of node D.                It finds however that the next hop information for reaching                node D has changed i.e., node D has decided to change the                paths. In this case, Node A which is the common ancestor node                for node D along the two paths (previous and new), should                generate a DCO which traverses downwards in the network. Node A                handles normal DAO forwarding to 6LBR as required by <xref format="default"/>.        </t>      </section>      <section anchor="transit_opt_changes" numbered="true" toc="default">        <name>Transit Information Option changes</name>        <t>                Every RPL message is divided into base message fields and                additional Options as described in <xref The base fields apply to the message as a                whole and options are appended to add message/use-case specific                attributes. As an example, a DAO message may be attributed by                one or more "RPL Target" options which specify the reachability                information for the given targets. Similarly, a Transit                Information option may be associated with a set of RPL Target                options.        </t>        <t>                This document specifies a change in the Transit Information Option to                contain the "Invalidate previous route" (I) flag. This I-flag signals                the common ancestor node to generate a DCO on behalf of the                target node. The I-flag is carried in the Transit Information                Option which augments the reachability information for a given                set of RPL Target(s). Transit Information Option with I-flag                set should be carried in the DAO message when route                invalidation is sought for the corresponding target(s).        </t>        <figure anchor="transit_info_with_i">          <name>Updated Transit Information Option (New I flag                     added)</name>          <artwork align="center" name="" type="" alt=""><![CDATA[0                   1                   2                   30 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+|   Type = 0x06 | Option Length |E|I|  Flags    | Path Control  |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| Path Sequence | Path Lifetime |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                        ]]></artwork>        </figure> (Invalidate previous route) 'I' flag is set by the                target node to indicate to the common ancestor node that it                wishes to invalidate any previous route between the two        <t>                <xref format="default"/> allows the parent address to be sent in                the Transit Information Option depending on the mode of                operation. In case of storing mode of operation the field is                usually not needed. In case of DCO, the parent address field be included.        </t>        <t>                The common ancestor node generate a DCO message in                response to this I-flag when it sees that the routing                adjacencies have changed for the target. The I-flag is                intended to give the target node control over its own route                invalidation, serving as a signal to request DCO generation.        </t>      </section>      <section numbered="true" toc="default">        <name>Destination Cleanup Object (DCO)</name>        <t>                A new ICMPv6 RPL control message code is defined by this                specification and is referred to as "Destination Cleanup Object"                (DCO), which is used for proactive cleanup of state and routing                information held on behalf of the target node by 6LRs. The DCO                message always traverses downstream and cleans up route                information and other state information associated with the                given target.        </t>        <figure anchor="dco_obj">          <name>DCO base object</name>          <artwork align="center" name="" type="" alt=""><![CDATA[0                   1                   2                   30 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| RPLInstanceID |K|D|   Flags   |   Reserved    | DCOSequence   |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+|                                                               |+                                                               +|                                                               |+                            DODAGID(optional)                  +|                                                               |+                                                               +|                                                               |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+|   Option(s)...+-+-+-+-+-+-+-+-+                        ]]></artwork>        </figure> field indicating the topology instance associated with the DODAG, aslearned from the DIO. 'K' flag indicates that the recipient of DCO message is expected tosend a DCO-ACK back. If the DCO-ACK is not received even after setting the 'K'flag, an implementation may retry the DCO at a later time. The number ofretries are implementation and deployment dependent and are expected to bekept similar with those used in DAO retries in <xrefformat="default"/>. <xref format="default"/> specifies theconsiderations for DCO retry. A node receiving a DCO message without the 'K'flag set respond with a DCO-ACK, especially to report an errorcondition. An example error condition could be that the node sending theDCO-ACK does not find the routing entry for the indicated target. When thesender does not set the 'K' flag it is an indication that the sender does notexpect a response, and the sender retry the DCO. 'D' flag indicates that the DODAGID field is present. This flagbe set when a local RPLInstanceID is used. 6 bits remaining unused in the Flags field are reserved for futureuse. These bits be initialized to zero by the sender and be ignoredby the receiver. unused field. The field be initialized to zero by the senderand be ignored by the receiver. 8-bit field incremented at each unique DCO message from a node and echoedin the DCO-ACK message. The initial DCOSequence can be chosen randomly by thenode. <xref format="default"/> explains the handling ofthe DCOSequence. (optional): unsigned integer set by a DODAG root that uniquely identifies aDODAG. This field be present when the 'D' flag is set and bepresent if 'D' flag is not set. DODAGID is used when a local RPLInstanceID isin use, in order to identify the DODAGID that is associated with the RPLInstanceID.        <section numbered="true" toc="default">          <name>Secure DCO</name>          <t>                    A Secure DCO message follows the format in <xref where the base message                    format is the DCO message shown in <xref format="default"/>.          </t>        </section>        <section numbered="true" toc="default">          <name>DCO Options</name>          <t>                    The DCO message carry at least one RPL Target and the                    Transit Information Option and carry other valid                    options. This specification allows for the DCO message to                    carry the following options:          </t> Target          <t>                    <xref defines all the                    above mentioned options. The DCO carries an RPL Target                    Option and an associated Transit Information Option with a                    lifetime of 0x00000000 to indicate a loss of reachability                    to that Target.          </t>        </section>        <section numbered="true" toc="default">          <name>Path Sequence number in the DCO</name>          <t>                    A DCO message may contain a Path Sequence in the Transit                    Information Option to identify the freshness of the DCO                    message. The Path Sequence in the DCO use the same                    Path Sequence number present in the regular DAO message                    when the DCO is generated in response to a DAO message.                    Thus if a DCO is received by a 6LR and subsequently a DAO                    is received with an old sequence number, then the DAO be ignored. When the DCO is generated in response to a                    DCO from upstream parent, the Path Sequence be copied                    from the received DCO.          </t>        </section>        <section numbered="true" toc="default">          <name>Destination Cleanup Option Acknowledgment (DCO-ACK)</name>          <t>                    The DCO-ACK message be sent as a unicast packet by a                    DCO recipient in response to a unicast DCO message with 'K'                    flag set. If 'K' flag is not set then the receiver of the                    DCO message send a DCO-ACK, especially to report an error                    condition.          </t>          <figure anchor="dco_ack">            <name>DCO-ACK base                         object</name>            <artwork align="center" name="" type="" alt=""><![CDATA[0                   1                   2                   30 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| RPLInstanceID |D|   Flags     |  DCOSequence  |    Status     |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+|                                                               |+                                                               +|                                                               |+                            DODAGID(optional)                  +|                                                               |+                                                               +|                                                               |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                            ]]></artwork>          </figure> field indicating the topology instance associated with the DODAG, aslearned from the DIO. 'D' flag indicates that the DODAGID field is present.  This flagbe set when a local RPLInstanceID is used. unused field. The field be initialized to zero by the senderand be ignored by the receiver. field. The DCOSequence in DCO-ACK is copied from the DCOSequencereceived in the DCO message. the completion. Status 0 is defined as unqualified acceptance inthis specification. Status 1 is defined as "No routing-entry for the Targetfound". The remaining status values are reserved as rejection codes. (optional): unsigned integer set by a DODAG root that uniquely identifies aDODAG. This field be present when the 'D' flag is set and bepresent when 'D' flag is not set. DODAGID is used when a local RPLInstanceIDis in use, in order to identify the DODAGID that is associated with theRPLInstanceID.        </section>        <section numbered="true" toc="default">          <name>Secure DCO-ACK</name>          <t>                    A Secure DCO-ACK message follows the format in    <xref where the base message                    format is the DCO-ACK message shown in <xref format="default"/>.          </t>        </section>      </section>      <section anchor="base_rules" numbered="true" toc="default">        <name>DCO Base Rules</name>        <ol type="1">          <li>                        If a node sends a DCO message with newer or different                        information than the prior DCO message transmission, it increment the DCOSequence field by at least one.                        A DCO message transmission that is identical to the                        prior DCO message transmission increment the                        DCOSequence field. The DCOSequence counter follows the                        sequence counter operation as defined in                        <xref                    </li>          <li>                        The RPLInstanceID and DODAGID fields of a DCO message be the same value as that of the DAO message in                        response to which the DCO is generated on the common                        ancestor node.                    </li>          <li>                        A node set the 'K' flag in a unicast DCO message to                        solicit a unicast DCO-ACK in response in order to                        confirm the attempt.                    </li>          <li>                        A node receiving a unicast DCO message with the 'K'                        flag set respond with a DCO-ACK. A node                        receiving a DCO message without the 'K' flag set                        respond with a DCO-ACK, especially to report an error                        condition.                    </li>          <li>                        A node receiving a unicast DCO message verify the                        stored Path Sequence in context to the given target. If                        the stored Path Sequence is more fresh, newer than                        the Path Sequence received in the DCO, then the DCO be dropped.                    </li>          <li>                        A node that sets the 'K' flag in a unicast DCO message                        but does not receive DCO-ACK in response reschedule                        the DCO message transmission for another attempt, up                        until an implementation specific number of retries.                    </li>          <li>                        A node receiving a unicast DCO message with its own                        address in the RPL Target Option strip-off that                        Target Option. If this Target Option is the only one in                        the DCO message then the DCO message be dropped.                    </li>        </ol>        <t>                The scope of DCOSequence values is unique to the node which                generates it.        </t>      </section>      <section numbered="true" toc="default">        <name>Unsolicited DCO</name>        <t>                A 6LR may generate an unsolicited DCO to unilaterally cleanup                the path on behalf of the target entry. The 6LR has all the                state information, namely, the Target address and the Path                Sequence, required for generating DCO in its routing table.                The conditions why 6LR may generate an unsolicited DCO are                beyond the scope of this document but some possible reasons                could be:        </t>        <ol type="1">          <li>                        On route expiry of an entry, a 6LR may decide to                        graciously cleanup the entry by initiating DCO.                    </li>          <li>                        6LR needs to entertain higher priority entries in case                        the routing table is full, thus resulting in eviction                        of an existing routing entry. In this case the eviction                        can be handled graciously using DCO.                    </li>        </ol>        <t>                Note that if the 6LR initiates a unilateral path cleanup using                DCO and if it has the latest state for the target then the DCO                would finally reach the target node. Thus the target node would                be informed of its invalidation.        </t>      </section>      <section numbered="true" toc="default">        <name>Other considerations</name>        <section numbered="true" toc="default">          <name>Dependent Nodes invalidation</name>          <t>                    Current RPL <xref format="default"/> does not provide a                    mechanism for route invalidation for dependent nodes. This                    document allows the dependent nodes invalidation. Dependent                    nodes will generate their respective DAOs to update their                    paths, and the previous route invalidation for those nodes                    should work in the similar manner described for switching                    node. The dependent node may set the I-flag in the Transit                    Information Option as part of regular DAO so as to                    request invalidation of previous route from the common                    ancestor node.          </t>          <t>                    Dependent nodes do not have any indication regarding if any                    of their parents in turn have decided to switch their                    parent. Thus for route invalidation the dependent nodes may                    choose to always set the 'I' flag in all its DAO message's                    Transit Information Option. Note that setting the I-flag is                    not counterproductive even if there is no previous                    route to be invalidated.          </t>        </section>        <section numbered="true" toc="default">          <name>NPDAO and DCO in the same network</name>          <t>                    The current NPDAO mechanism in <xref format="default"/> can                    still be used in the same network where DCO is used. The                    NPDAO messaging can be used, for example, on route lifetime                    expiry of the target or when the node simply decides to                    gracefully terminate the RPL session on graceful node                    shutdown. Moreover, a deployment can have a mix of nodes                    supporting the DCO and the existing NPDAO mechanism. It is                    also possible that the same node supports both the NPDAO                    and DCO signaling for route invalidation.          </t>          <t>                    <xref states, "When a                    node removes a node from its DAO parent set, it                    send a No-Path DAO message to that removed DAO parent to                    invalidate the existing router". This document introduces                    an alternative and more optimized way of route invalidation                    but it also allows existing NPDAO messaging to work. Thus                    an implementation has two choices to make when a route                    invalidation is to be initiated:          </t>          <ol type="1">            <li>                            Use NPDAO to invalidate the previous route and                            send regular DAO on the new path.                        </li>            <li>                            Send regular DAO on the new path with the 'I'                            flag set in the Transit Information Option such                            that the common ancestor node initiates the DCO                            message downstream to invalidate the previous                            route.                        </li>          </ol>          <t>                    This document recommends using option 2 for reasons                    specified in <xref format="default"/> in this                    document.          </t>          <t>                    This document assumes that all the 6LRs in the network                    support this specification. If there are 6LRs en-route DCO                    message path which do not support this document, then the                    route invalidation for corresponding targets may not work                    or may work partially i.e., only part of the path                    supporting DCO may be invalidated. Alternatively, a node                    could generate an NPDAO if it does not receive a DCO with                    itself as target within specified time limit. The specified                    time limit is deployment specific and depends upon the                    maximum depth of the network and per hop average latency.                    Note that sending NPDAO and DCO for the same operation                    would not result in unwanted side-effects because the                    acceptability of NPDAO or DCO depends upon the Path                    Sequence freshness.          </t>        </section>        <section anchor="dco_retry" numbered="true" toc="default">          <name>Considerations for DCO retry</name>          <t>                    A DCO message could be retried by a sender if it sets the                    'K' flag and does not receive a DCO-ACK. The DCO retry time                    could be dependent on the maximum depth of the network and                    average per hop latency. This could range from 2 seconds to                    120 seconds depending on the deployment. In case the                    latency limits are not known, an implementation                    retry more than once in 3 seconds and retry more                    than 3 times.          </t>          <t>                    The number of retries could also be set depending on how                    critical the route invalidation could be for the deployment                    and the link layer retry configuration. For networks                    supporting only MP2P and P2MP flows, such as in AMI and                    telemetry applications, the 6LRs may not be very keen to                    invalidate routes, unless they are highly                    memory-constrained. For home and building automation                    networks which may have substantial P2P traffic, the 6LRs                    might be keen to invalidate efficiently because it may                    additionally impact the forwarding efficiency.          </t>        </section>        <section numbered="true" toc="default">          <name>DCO with multiple preferred parents</name>          <t>                    <xref format="default"/> allows a node to select multiple                    preferred parents for route establishment.                    <xref specifies, "All DAOs generated                    at the same time for the same Target be sent with the                    same Path Sequence in the Transit Information".                    Subsequently when route invalidation has to be initiated,                    RPL mentions use of NPDAO which can be initiated with an                    updated Path Sequence to all the parent nodes through which                    the route is to be invalidated.          </t>          <t>                    With DCO, the Target node itself does not initiate the                    route invalidation and it is left to the common ancestor                    node. A common ancestor node when it discovers an updated                    DAO from a new next-hop, it initiates a DCO. With multiple                    preferred parents, this handling does not change. But in                    this case it is recommended that an implementation                    initiates a DCO after a time period (DelayDCO) such that                    the common ancestor node may receive updated DAOs from all                    possible next-hops. This will help to reduce DCO control                    overhead i.e., the common ancestor can wait for updated                    DAOs from all possible directions before initiating a DCO                    for route invalidation. After timeout, the DCO needs to be                    generated for all the next-hops for whom the route                    invalidation needs to be done.          </t>          <t>                    This document recommends using a DelayDCO timer value of                    1sec. This value is inspired by the default DelayDAO value                    of 1sec in <xref format="default"/>. Here the hypothesis is                    that the DAOs from all possible parent sets would be                    received on the common ancestor within this time period.          </t>          <t>                    It is still possible that a DCO is generated before all the                    updated DAOs from all the paths are received. In this case,                    the ancestor node would start the invalidation procedure                    for paths from which the updated DAO is not received. The                    DCO generated in this case would start invalidating the                    segments along these paths on which the updated DAOs are                    not received. But once the DAO reaches these segments, the                    routing state would be updated along these segments and                    should not lead to any inconsistent routing state.          </t>          <t>                    Note that there is no requirement for synchronization                    between DCO and DAOs. The DelayDCO timer simply ensures                    that the DCO control overhead can be reduced and is only                    needed when the network contains nodes using multiple                    preferred parent.          </t>        </section>      </section>    </section>    <section anchor="Acknowledgments" numbered="true" toc="default">      <name>Acknowledgments</name>      <t>            Many thanks to Alvaro Retana, Cenk Gundogan, Simon Duquennoy, Georgios            Papadopoulous, Peter Van Der Stok for their review and comments.            Alvaro Retana helped shape this document's final version with            critical review comments.      </t>    </section>    <!-- Possibly a 'Contributors' section ... -->    <section anchor="IANA" numbered="true" toc="default">      <name>IANA Considerations</name>      <t>            IANA is requested to allocate new codes for the DCO and DCO-ACK            messages from the RPL Control Codes registry.      </t>      <table align="center">        <thead>          <tr>            <th align="center">Code</th>            <th align="center">Description</th>            <th align="center">Reference</th>          </tr>        </thead>        <tbody>          <tr>            <td align="center">TBD1</td>            <td align="center">Destination Cleanup Object</td>            <td align="center">This document</td>          </tr>          <tr>            <td align="center">TBD2</td>            <td align="center">Destination Cleanup Object Acknowledgment</td>            <td align="center">This document</td>          </tr>          <tr>            <td align="center">TBD3</td>            <td align="center">Secure Destination Cleanup Object</td>            <td align="center">This document</td>          </tr>          <tr>            <td align="center">TBD4</td>            <td align="center">Secure Destination Cleanup Object Acknowledgment</td>            <td align="center">This document</td>          </tr>        </tbody>      </table>      <t>            IANA is requested to allocate bit 1 from the Transit Information            Option Flags registry for the I-flag (<xref format="default"/>)      </t>      <section numbered="true" toc="default">        <name>New Registry for the Destination Cleanup Object (DCO) Flags</name>        <t>                IANA is requested to create a registry for the 8-bit Destination Cleanup                Object (DCO) Flags field. This registry should be located in                existing category of "Routing Protocol for Low Power and Lossy                Networks (RPL)".        </t>        <t>                New bit numbers may be allocated only by an IETF Review. Each                bit is tracked with the following qualities:        </t>        <ul spacing="compact">          <li>Bit number (counting from bit 0 as the most significant bit)</li>          <li>Capability description</li>          <li>Defining RFC</li>        </ul>        <t>                The following bits are currently defined:        </t>        <table align="center">          <name>DCO Base Flags</name>          <thead>            <tr>              <th align="center">Bit number</th>              <th align="center">Description</th>              <th align="center">Reference</th>            </tr>          </thead>          <tbody>            <tr>              <td align="center">0</td>              <td align="center">DCO-ACK request (K)</td>              <td align="center">This document</td>            </tr>            <tr>              <td align="center">1</td>              <td align="center">DODAGID field is present (D)</td>              <td align="center">This document</td>            </tr>          </tbody>        </table>      </section>      <section numbered="true" toc="default">        <name>New Registry for the Destination Cleanup Object Acknowledgment (DCO-ACK) Status field</name>        <t>                IANA is requested to create a registry for the 8-bit Destination Cleanup                Object Acknowledgment (DCO-ACK) Status field. This registry                should be located in existing category of "Routing Protocol for                Low Power and Lossy Networks (RPL)".        </t>        <t>                New Status values may be allocated only by an IETF Review. Each                value is tracked with the following qualities:        </t>        <ul spacing="compact">          <li>Status Code</li>          <li>Description</li>          <li>Defining RFC</li>        </ul>        <t>                The following values are currently defined:        </t>        <table align="center">          <name>DCO-ACK Status Codes</name>          <thead>            <tr>              <th align="center">Status Code</th>              <th align="center">Description</th>              <th align="center">Reference</th>            </tr>          </thead>          <tbody>            <tr>              <td align="center">0</td>              <td align="center">Unqualified acceptance</td>              <td align="center">This document</td>            </tr>            <tr>              <td align="center">1</td>              <td align="center">No routing-entry for the indicated Target found</td>              <td align="center">This document</td>            </tr>          </tbody>        </table>      </section>      <section numbered="true" toc="default">        <name>New Registry for the Destination Cleanup Object (DCO)         Acknowledgment Flags</name>        <t>                IANA is requested to create a registry for the 8-bit                Destination Cleanup Object (DCO) Acknowledgment Flags field.                This registry should be located in existing category of                "Routing Protocol for Low Power and Lossy Networks (RPL)".        </t>        <t>                New bit numbers may be allocated only by an IETF Review. Each                bit is tracked with the following qualities:        </t>        <ul spacing="compact">          <li>Bit number (counting from bit 0 as the most significant bit)</li>          <li>Capability description</li>          <li>Defining RFC</li>        </ul>        <t>                The following bits are currently defined:        </t>        <table align="center">          <name>DCO-ACK Base Flags</name>          <thead>            <tr>              <th align="center">Bit number</th>              <th align="center">Description</th>              <th align="center">Reference</th>            </tr>          </thead>          <tbody>            <tr>              <td align="center">0</td>              <td align="center">DODAGID field is present (D)</td>              <td align="center">This document</td>            </tr>          </tbody>        </table>      </section>    </section>    <section anchor="Security" numbered="true" toc="default">      <name>Security Considerations</name>      <t>            This document introduces the ability for a common ancestor node to            invalidate a route on behalf of the target node. The common            ancestor node could be directed to do so by the target node using            the I-flag in DCO's Transit Information Option. However, the common            ancestor node is in a position to unilaterally initiate the route            invalidation since it possesses all the required state information,            namely, the Target address and the corresponding Path Sequence.            Thus a rogue common ancestor node could initiate such an            invalidation and impact the traffic to the target node.      </t>      <t>            This document also introduces an I-flag which is set by the target            node and used by the ancestor node to initiate a DCO if the            ancestor sees an update in the route adjacency. However,            this flag could be spoofed by a malicious 6LR in the path and can            cause invalidation of an existing active path. Note that invalidation            will happen only if the other conditions such as Path Sequence            condition is also met. Having said that, such a malicious 6LR may            spoof a DAO on behalf of the (sub) child with the I-flag set and            can cause route invalidation on behalf of the (sub) child node.            Note that, using existing mechanisms offered by <xref format="default"/>, a malicious 6LR might also spoof a DAO with            lifetime of zero or otherwise cause denial of service by dropping            traffic entirely, so the new mechanism described in this document            does not present a substantially increased risk of disruption.      </t>      <t>            This document assumes that the security mechanisms as defined in            <xref format="default"/> are followed, which means that the common            ancestor node and all the 6LRs are part of the RPL network because            they have the required credentials. A non-secure RPL network needs            to take into consideration the risks highlighted in this section as            well as those highlighted in <xref format="default"/>.      </t>      <t>            All RPL messages support a secure version of messages which allows            integrity protection using either a MAC or a signature. Optionally,            secured RPL messages also have encryption protection for            confidentiality.      </t>      <t>            The document adds new messages (DCO, DCO-ACK) which are            syntactically similar to existing RPL messages such as DAO,            DAO-ACK. Secure versions of DCO and DCO-ACK are added similar to            other RPL messages (such as DAO, DAO-ACK).      </t>      <t>            RPL supports three security modes as mentioned in            <xref      </t>      <ol type="1">        <li>                    Unsecured: In this mode, it is expected that the RPL control messages                    are secured by other security mechanisms, such as                    link-layer security. In this mode, the RPL control messages,                    including DCO, DCO-ACK, do not have Security sections.                    Also note that unsecured mode does not imply that all                    messages are sent without any protection.                </li>        <li>                    Preinstalled: In this mode, RPL uses secure messages. Thus                    secure versions of DCO, DCO-ACK be used in this mode.                </li>        <li>                    Authenticated: In this mode, RPL uses secure messages. Thus                    secure versions of DCO, DCO-ACK be used in this mode.                </li>      </ol>    </section>  </middle>  <back>    <!-- References split into informative and normative -->    <!-- There are 2 ways to insert reference entries from the citation libraries:     1. define an ENTITY at the top, and use "ampersand character"RFC2629; here (as shown)     2. simply use a PI "less than character"?rfc include="reference.RFC.2119.xml"?> here        (for I-Ds: include="reference.I-D.narten-iana-considerations-rfc2434bis.xml")     Both are cited textually in the same manner: by using xref elements.     If you use the PI option, xml2rfc will, by default, try to find included files in the same     directory as the including file. You can also define the XML_LIBRARY environment variable     with a value containing a set of directories to search. These can be either in the local     filing system or remote ones accessed by http (http://domain/dir/... ).-->    <references>      <name>Normative References</name>      <!--?rfc include="http://xml.resource.org/public/rfc/bibxml/reference.RFC.2119.xml"?-->      <reference anchor="RFC6550"        <front>          <title>RPL: IPv6 Routing Protocol for Low-Power and Lossy Networks</title>          <seriesInfo name="DOI" value="10.17487/RFC6550"/>          <seriesInfo name="RFC" value="6550"/>          <author initials="T." surname="Winter" fullname="T. Winter" role="editor">            <organization/>          </author>          <author initials="P." surname="Thubert" fullname="P. Thubert" role="editor">            <organization/>          </author>          <author initials="A." surname="Brandt" fullname="A. Brandt">            <organization/>          </author>          <author initials="J." surname="Hui" fullname="J. Hui">            <organization/>          </author>          <author initials="R." surname="Kelsey" fullname="R. Kelsey">            <organization/>          </author>          <author initials="P." surname="Levis" fullname="P. Levis">            <organization/>          </author>          <author initials="K." surname="Pister" fullname="K. Pister">            <organization/>          </author>          <author initials="R." surname="Struik" fullname="R. Struik">            <organization/>          </author>          <author initials="JP." surname="Vasseur" fullname="JP. Vasseur">            <organization/>          </author>          <author initials="R." surname="Alexander" fullname="R. Alexander">            <organization/>          </author>          <date year="2012" month="March"/>          <abstract>            <t>Low-Power and Lossy Networks (LLNs) are a class of network in which both the routers and their interconnect are constrained.  LLN routers typically operate with constraints on processing power, memory, and energy (battery power).  Their interconnects are characterized by high loss rates, low data rates, and instability.  LLNs are comprised of anything from a few dozen to thousands of routers.  Supported traffic flows include point-to-point (between devices inside the LLN), point-to-multipoint (from a central control point to a subset of devices inside the LLN), and multipoint-to-point (from devices inside the LLN towards a central control point).  This document specifies the IPv6 Routing Protocol for Low-Power and Lossy Networks (RPL), which provides a mechanism whereby multipoint-to-point traffic from devices inside the LLN towards a central control point as well as point-to-multipoint traffic from the central control point to the devices inside the LLN are supported.  Support for point-to-point traffic is also available.  [STANDARDS-TRACK]</t>          </abstract>        </front>      </reference>      <reference anchor="RFC2119" xml:base="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.2119.xml">        <front>          <title>Key words for use in RFCs to Indicate Requirement Levels</title>          <seriesInfo name="DOI" value="10.17487/RFC2119"/>          <seriesInfo name="RFC" value="2119"/>          <seriesInfo name="BCP" value="14"/>          <author initials="S." surname="Bradner" fullname="S. Bradner">            <organization/>          </author>          <date year="1997" month="March"/>          <abstract>            <t>In many standards track documents several words are used to signify the requirements in the specification.  These words are often capitalized. This document defines these words as they should be interpreted in IETF documents.  This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.</t>          </abstract>        </front>      </reference>      <reference anchor="RFC8174" xml:base="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.8174.xml">        <front>          <title>Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words</title>          <seriesInfo name="DOI" value="10.17487/RFC8174"/>          <seriesInfo name="RFC" value="8174"/>          <seriesInfo name="BCP" value="14"/>          <author initials="B." surname="Leiba" fullname="B. Leiba">            <organization/>          </author>          <date year="2017" month="May"/>          <abstract>            <t>RFC 2119 specifies common key words that may be used in protocol  specifications.  This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the  defined special meanings.</t>          </abstract>        </front>      </reference>    </references>    <section anchor="app-additional" numbered="true" toc="default">      <name>Example Messaging</name>      <section numbered="true" toc="default">        <name>Example DCO Messaging</name>        <t>            In <xref format="default"/>, node (D) switches its parent from            (B) to (C). This example assumes that Node D has already            established its own route via Node B-G-A-6LBR using pathseq=x. The            example uses DAO and DCO messaging convention and specifies only            the required parameters to explain the example namely, the            parameter 'tgt', which stands for Target Option and value of this            parameter specifies the address of the target node. The parameter            'pathseq', which specifies the Path Sequence value carried in the            Transit Information Option. The parameter 'I_flag' specifies the            'I' flag in the Transit Information Option.            sequence of actions is as follows:        </t>        <ol type="1">          <li>Node D switches its parent from node B to node C</li>          <li>D sends a regular DAO(tgt=D,pathseq=x+1,I_flag=1) in the                    updated path to C</li>          <li>C checks for a routing entry on behalf of D, since it cannot                    find an entry on behalf of D it creates a new routing entry                    and forwards the reachability information of the target D                    to H in a DAO(tgt=D,pathseq=x+1,I_flag=1).</li>          <li>Similar to C, node H checks for a routing entry on behalf of                    D, cannot find an entry and hence creates a new routing                    entry and forwards the reachability information of the                    target D to A in a DAO(tgt=D,pathseq=x+1,I_flag=1).</li>          <li>                    Node A receives the DAO(tgt=D,pathseq=x+1,I_flag=1), and                    checks for a routing entry on behalf of D. It finds a                    routing entry but checks that the next hop for target D is                    different (i.e., Node G). Node A checks the I_flag and                    generates DCO(tgt=D,pathseq=x+1) to previous next hop for                    target D which is G. Subsequently, Node A updates the                    routing entry and forwards the reachability information of                    target D upstream DAO(tgt=D,pathseq=x+1,I_flag=1).                </li>          <li>                    Node G receives the DCO(tgt=D,pathseq=x+1). It checks if                    the received path sequence is later than the stored path                    sequence. If it is later, Node G invalidates the routing entry                    of target D and forwards the (un)reachability information                    downstream to B in DCO(tgt=D,pathseq=x+1).                </li>          <li>                    Similarly, B processes the DCO(tgt=D,pathseq=x+1) by                    invalidating the routing entry of target D and forwards the                    (un)reachability information downstream to D.                </li>          <li>                    D ignores the DCO(tgt=D,pathseq=x+1) since the target is                    itself.                </li>          <li>                    The propagation of the DCO will stop at any node where the                    node does not have an routing information associated with                    the target. If cached routing information is present and                    the cached Path Sequence is higher than the value in the                    DCO, then the DCO is dropped.                </li>        </ol>      </section>      <section numbered="true" toc="default">        <name>Example DCO Messaging with multiple preferred parents</name>        <figure anchor="sample_top_mpp">          <name>Sample                     topology 2</name>          <artwork align="center" name="" type="" alt=""><![CDATA[        (6LBR)          |          |          |        (N11)         / \        /   \       /     \    (N21)   (N22)      /      / \     /      /   \    /      /     \ (N31)  (N32)  (N33)     :    |    /      :   |   /       :  |  /        (N41)                        ]]></artwork>        </figure>        <t>                In <xref format="default"/>, node (N41) selects multiple                preferred parents (N32) and (N33).                The sequence of actions is as follows:        </t>        <ol type="1">          <li>                        (N41) sends DAO(tgt=N41,PS=x,I_flag=1) to (N32) and (N33).                        Here I_flag refers to the Invalidation flag and PS refers to                        Path Sequence in Transit Information option.                    </li>          <li>                        (N32) sends DAO(tgt=N41,PS=x,I_flag=1) to (N22). (N33) also                        sends DAO(tgt=N41,PS=x,I_flag=1) to (N22). (N22) learns                        multiple routes for the same destination (N41) through                        multiple next-hops. (N22) may receive the DAOs from                        (N32) and (N33) in any order with the I_flag set. The                        implementation should use the DelayDCO timer to wait to                        initiate the DCO. If (N22) receives an updated DAO from                        all the paths then the DCO need not be initiated in                        this case. Thus the route table at N22 should contain                        (Dst,NextHop,PS): { (N41,N32,x), (N41,N33,x) }.                    </li>          <li>                        (N22) sends DAO(tgt=N41,PS=x,I_flag=1) to (N11).                    </li>          <li>                        (N11) sends DAO(tgt=N41,PS=x,I_flag=1) to (6LBR). Thus the                        complete path is established.                    </li>          <li>                        (N41) decides to change preferred parent set from {                        N32, N33 } to { N31, N32 }.                    </li>          <li>                        (N41) sends DAO(tgt=N41,PS=x+1,I_flag=1) to (N32). (N41)                        sends DAO(tgt=N41,PS=x+1,I_flag=1) to (N31).                    </li>          <li>                        (N32) sends DAO(tgt=N41,PS=x+1,I_flag=1) to (N22).                        (N22) has multiple routes to destination (N41). It sees                        that a new Path Sequence for Target=N41 is received and                        thus it waits for pre-determined time period (DelayDCO                        time period) to invalidate another route                        {(N41),(N33),x}. After time period, (N22) sends                        DCO(tgt=N41,PS=x+1) to (N33). Also (N22) sends the                        regular DAO(tgt=N41,PS=x+1,I_flag=1) to (N11).                    </li>          <li>                        (N33) receives DCO(tgt=N41,PS=x+1). The received Path                        Sequence is latest and thus it invalidates the entry                        associated with target (N41). (N33) then sends the                        DCO(tgt=N41,PS=x+1) to (N41). (N41) sees itself as the                        target and drops the DCO.                    </li>          <li>                        From Step 6 above, (N31) receives the                        DAO(tgt=N41,PS=x+1,I_flag=1). It creates a routing                        entry and sends the DAO(tgt=N41,PS=x+1,I_flag=1) to                        (N21). Similarly (N21) receives the DAO and                        subsequently sends the DAO(tgt=N41,PS=x+1,I_flag=1) to                        (N11).                    </li>          <li>                        (N11) receives DAO(tgt=N41,PS=x+1,I_flag=1) from (N21).                        It waits for DelayDCO timer since it has multiple                        routes to (N41). (N41) will receive                        DAO(tgt=N41,PS=x+1,I_flag=1) from (N22) from Step 7                        above. Thus (N11) has received regular                        DAO(tgt=N41,PS=x+1,I_flag=1) from all paths and thus                        does not initiate DCO.                    </li>          <li>                        (N11) forwards the DAO(tgt=N41,PS=x+1,I_flag=1) to 6LBR                        and the full path is established.                    </li>        </ol>      </section>    </section>  </back></rfc>