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<rfc xmlns:xi="http://www.w3.org/2001/XInclude" category="std" docName="draft-ietf-roll-aodv-rpl-20" number="9854" updates="" obsoletes="" ipr="trust200902" submissionType="IETF" consensus="true"
        xmlns:xi="http://www.w3.org/2001/XInclude">
  <!-- category values: std, bcp, info, exp, and historic
 http://umeeting.huawei.com/Portal/business.action?BMECID=1474233&BMETimestamp=1426658395147
                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 ***** -->

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  --> tocInclude="true" tocDepth="4" symRefs="true" sortRefs="true" version="3" xml:lang="en">

<front>
    <!-- The abbreviated title is used in the page header - it is only
         necessary if the full title is longer than 39 characters -->
    <title abbrev="AODV-RPL">
     Supporting Asymmetric Links in Low Power Networks: AODV-RPL
    </title>

    <!-- add 'role="editor"' below abbrev="AODV-RPL">AODV-RPL: The Routing Protocol for the editors if appropriate -->

    <!-- Another author who claims to be an editor --> Low-Power and Lossy Networks (RPL)
     Based on Ad Hoc On-Demand Distance Vector (AODV) Routing</title>
    <seriesInfo name="RFC" value="9854"/>
    <author fullname="Charles E. Perkins" initials="C.E." surname="Perkins">
      <organization>Blue Meadow Networks</organization>
      <address>
        <postal>
          <street/>
          <city>Saratoga</city>
          <region/>
          <region>CA</region>
          <code>95070</code>
          <country>United States</country> States of America</country>
        </postal>
        <phone/>
        <email>charliep@lupinlodge.com</email>
        <!-- uri and facsimile elements may also be added -->
      </address>
    </author>
    <author fullname="S.V.R fullname="S.V.R. Anand" initials="" surname="S.V.R.Anand"> initials="S.V.R." surname="Anand">
      <organization>Indian Institute of Science</organization>
      <address>
        <postal>
          <street></street>
          <!-- Reorder these if your country does things differently -->
          <city>Bangalore</city>
          <region/>
          <code>560012</code>
          <country>India</country>
        </postal>
        <phone/>
        <email>anandsvr@iisc.ac.in</email>
        <!-- uri and facsimile elements may also be added -->
      </address>
    </author>
    <author fullname="Satish Anamalamudi" initials="S." surname="Anamalamudi">
      <organization>SRM University-AP</organization>
      <address>
        <postal>
          <street>Amaravati Campus</street>
          <!-- Reorder these if your country does things differently -->
          <city>Amaravati, Andhra Pradesh</city>
          <region/>
          <code>522 502</code>
          <country>India</country>
        </postal>
        <phone/>
        <email>satishnaidu80@gmail.com</email>
        <!-- uri and facsimile elements may also be added -->
      </address>
    </author>
    <author fullname="Bing Liu" initials="B." surname="Liu">
      <organization>Huawei Technologies</organization>
      <address>
        <postal>
          <street>No. 156 Beiqing Rd. Haidian District</street>
          <!-- Reorder these if your country does things differently --> Rd.</street>
	  <cityarea>Haidian District</cityarea>
          <city>Beijing</city>
          <region/>
          <code>100095</code>
          <country>China</country>
        </postal>
       <phone/>
        <email>remy.liubing@huawei.com</email>
      </address>
    </author>

    <date year=""/>
    <!-- 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
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         purpose of calculating the expiry date).  With drafts it is normally
         sufficient to specify just the year. -->

    <!-- Meta-data Declarations -->

    <area>Internet</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>AODV, Peer-to-Peer year="2025" month="September"/>

    <area>RTG</area>
    <workgroup>roll</workgroup>

    <keyword>AODV</keyword>
    <keyword>Peer-to-Peer Route Discovery, Asymmetric</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. --> Discovery</keyword>
    <keyword>Asymmetric</keyword>

    <abstract>
    <t>	Route
      <t>Route discovery for symmetric and asymmetric Peer-to-Peer (P2P)
      traffic flows is a desirable feature in Low power Low-Power and Lossy Networks
      (LLNs).  For that purpose, this document specifies AODV-RPL -- the
      Routing Protocol for Low-Power and Lossy Networks (RPL) based on Ad hoc
      On-demand Distance Vector (AODV) routing. AODV-RPL is a reactive P2P
      route discovery mechanism for both hop-by-hop routes and source routing: Ad
        Hoc On-demand Distance Vector Routing (AODV) based RPL protocol
        (AODV-RPL).
      routing.  Paired Instances instances are used to construct directional paths, paths for
      cases where there are asymmetric links between source and target nodes.
      </t>
    </abstract>
  </front>
  <middle>
    <section anchor="Introduction" title="Introduction"> anchor="Introduction">
      <name>Introduction</name>
      <t>
        The Routing Protocol for Low-Power and Lossy Networks (RPL)
        <xref target="RFC6550"/> is an IPv6 distance vector routing protocol
	designed to support multiple traffic flows through a root-based
	Destination-Oriented Directed Acyclic Graph (DODAG). Typically,
        <!-- Gunter Van de Velde 2/11/2025, 8:36 PM  -->
	a router does not have routing information for destinations attached
        to most other routers.  Consequently, for traffic
        between routers within the DODAG (i.e., Peer-to-Peer (P2P) traffic) P2P traffic),
        data packets either have to traverse the root in non-storing mode, mode or
        traverse a common ancestor in storing mode.  Such P2P traffic
        is thereby likely to traverse longer routes and
        may suffer severe congestion near the root (for more information information,
        see <xref target="RFC6687"/>, <xref target="RFC6997"/>,
        <xref target="RFC6998"/>, and <xref target="RFC9010"/>).
        The network environment that is considered in this document
        is assumed to be the same as that described in Section 1 of
        <xref target="RFC6550"/>. target="RFC6550" sectionFormat="of" section="1"/>.
        Each radio interface/link and the associated address should be
        treated as an independent intermediate router.  Such routers
        have different links links, and the rules for the link symmetry
        apply independently for each of these.

      </t>
      <t>
        The route discovery process in AODV-RPL is modeled on the analogous
        peer-to-peer
        P2P procedure specified in AODV <xref target="RFC3561"/>.
	The on-demand property of AODV route discovery is useful for the needs
        of routing in RPL-based LLNs when routes are needed but aren't yet
        established.  Peer-to-peer  P2P routing is desirable to discover
        shorter routes, and especially when it is desired to avoid directing
        additional traffic through a root or gateway node of the network.
        It may happen that some routes need to be established proactively
        when known beforehand and when AODV-RPL's route discovery process
        introduces unwanted delay at the time when the application is
        launched.
      </t>
      <t>
        AODV terminology has been adapted for use with AODV-RPL messages,
        namely RREQ "RREQ" for Route Request, "Route Request", and RREP "RREP" for Route Reply. "Route Reply".
        AODV-RPL currently omits some features compared to AODV -- in
        particular, flagging Route Errors, route errors, "blacklisting" unidirectional links
        (<xref target="RFC3561"/>),
        <xref target="RFC3561"/>, multihoming, and handling unnumbered
        interfaces.
      </t>

    <t>
        AODV-RPL
      <t>AODV-RPL reuses and extends the core RPL functionality to support
      routes with bidirectional asymmetric links.  It retains RPL's DODAG
      formation, RPL Instance and the associated Objective Function (defined
      in <xref target="RFC6551"/>), trickle Trickle timers, and support for storing
      and non-storing modes.  AODV-RPL adds the basic messages RREQ and RREP as
      part of the RPL DODAG Information Object (DIO) control message, which go in
      separate (paired) RPL instances.  AODV-RPL does not utilize the
      Destination Advertisement Object (DAO) control message of RPL.
<!-- The P2P routes do not have to go through the tree root.  I don't remember
     what are the point-to-multipoint routes under discussion here.   -->

        AODV-RPL uses the "P2P Route Discovery Mode of Operation" (MOP == 4)
        with three new Options options for the DIO message, dedicated to discover discovering
        P2P routes. These P2P routes may differ from routes discoverable by
        native RPL.  Since AODV-RPL uses newly defined Options options and a newly
        allocated multicast group (see <xref target="iana"/>), there is no
        conflict with P2P-RPL <xref target="RFC6997"/>, a previous document
        using the same MOP.  AODV-RPL can be operated whether or not P2P-RPL
        or native RPL is running otherwise. also running.  AODV-RPL could be used for networks in
        which routes are needed with Objective Functions that cannot be
        satisfied by routes that are constrained to traverse the root of the
        network or other common ancestors.  P2P routes often require fewer
        hops and therefore consume less resources than routes that traverse
        the root or other common ancestors.  Similar in cost to base RPL <xref
        target="RFC6550"/>, the cost will depend on many

<!--  From Anand:
The real cost depends on many factors such as the proximity of the OrigNode and
TargNodes, Gratuitous RREP, lifetime of the P2P routes, distribution of
symmetric/asymmetric P2P links, number of Targets given in AODV-RPL Target (ART)
Option, H-bit value, RREP_WAIT_TIME and so on.
  -->
        factors such as the proximity of the OrigNode and TargNodes and
        distribution of symmetric/asymmetric P2P links.  Experience with
        AODV <xref target="aodv-tot"/> suggests that AODV-RPL will often find
        routes with improved rank compared to routes constrained to traverse
        a common ancestor of the source and destination nodes.
<!--
        However, there does not seem to be much value in
        maintaining two routing protocols even if they are compatible.
  -->
      </t>
    </section>	<!-- End of section "Introduction" -->

<section anchor="terms" title="Terminology"> anchor="terms">
      <name>Terminology</name>
              <t>
    The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
        NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED",
        "MAY", "<bcp14>MUST</bcp14>", "<bcp14>MUST NOT</bcp14>",
    "<bcp14>REQUIRED</bcp14>", "<bcp14>SHALL</bcp14>", "<bcp14>SHALL NOT</bcp14>",
    "<bcp14>SHOULD</bcp14>", "<bcp14>SHOULD NOT</bcp14>",
    "<bcp14>RECOMMENDED</bcp14>", "<bcp14>NOT RECOMMENDED</bcp14>",
    "<bcp14>MAY</bcp14>", and "OPTIONAL" "<bcp14>OPTIONAL</bcp14>" in this document are to be
    interpreted as described in BCP 14 BCP&nbsp;14 <xref target="RFC2119"/> <xref
    target="RFC8174"/> when, and only when, they appear in all capitals, as
    shown here.
        </t>
	<t>
          AODV-RPL reuses names for messages and data structures, including
          Rank, DODAG DODAG, and DODAGID, as defined in RPL <xref
          target="RFC6550"/>.
	</t>
    <t><list style="hanging">
    <t hangText="AODV"><vspace />
       	Ad Hoc On-demand

	<t>This document also uses the following terms:</t>
	<dl newline="true" spacing="normal">

        <dt>AODV</dt>
        <dd>Ad hoc On-Demand Distance Vector Routing <xref target="RFC3561"/>.</t> target="RFC3561"/>.</dd>
        <!--	/* Murray Kucherawy: does not appear anywhere else in the document. */
    <t hangText="AODV-RPL Instance"><vspace />
       	Either the RREQ-Instance or RREP-Instance</t>
  -->
    <t hangText="ART option"><vspace />
        <dt>ART option</dt>
        <dd>The AODV-RPL Target option: a target option defined in this document.</t>
    <t hangText="Asymmetric Route"><vspace />
       	The document.</dd>

        <dt>Asymmetric route</dt>
        <dd>The route from the OrigNode to the TargNode can traverse different
        nodes than the route from the TargNode to the OrigNode. An asymmetric
        route may result from the asymmetry of links, such that only one
        direction of the series of links satisfies the Objective Function
        during route discovery.
<!--  CEP: Need to check this!!
      But the RREQ *still* has to store the reverse route...
        If the OrigNode doesn't require an upward route towards
        itself, the route is also considered as asymmetric. --> </t>
    <t hangText="Bi-directional Asymmetric Link"><vspace />
        A </dd>
        <dt>Bidirectional asymmetric link</dt>
        <dd>A link that can be used in both directions but with different link
        characteristics. </t>
    <t hangText="DIO"><vspace />
        DODAG
        characteristics.</dd>

        <dt>DIO</dt>
        <dd>DODAG Information Object (as defined in <xref target="RFC6550"/>) </t>
    <t hangText="DODAG target="RFC6550"/>).</dd>

        <dt>DODAG RREQ-Instance (or simply RREQ-Instance)"><vspace /> RREQ-Instance)</dt>
        <dd>An RPL Instance built using the DIO with RREQ option; used for
        transmission of control messages from OrigNode to TargNode, thus
        enabling data transmission from TargNode to OrigNode. </t>
    <t hangText="DODAG OrigNode.</dd>

        <dt>DODAG RREP-Instance (or simply RREP-Instance)"><vspace /> RREP-Instance)</dt>
        <dd>An RPL Instance built using the DIO with RREP option; used for
        transmission of control messages from TargNode to OrigNode OrigNode, thus
        enabling data transmission from OrigNode to TargNode. </t>
    <t hangText="Downward Direction"><vspace />
        The </dd>

        <dt>Downward direction</dt>
        <dd>The direction from the OrigNode to the TargNode.</t>
    <t hangText="Downward Route"><vspace />
        A TargNode.</dd>

        <dt>Downward route</dt>
        <dd>A route in the downward direction. </t>
    <t hangText="hop-by-hop route"><vspace />
        A direction.</dd>

        <dt>Hop-by-hop route</dt>
        <dd>A route for which each router along the routing path stores
        routing information about the next hop. A hop-by-hop route is
        created using RPL's "storing mode".</t>
    <t hangText="OF"><vspace />
        An Objective mode".</dd>

        <dt>OF</dt>
        <dd>Objective Function as (as defined in <xref target="RFC6550"/>. </t>
    <t hangText="OrigNode"><vspace />
        The target="RFC6550"/>).</dd>

        <dt>OrigNode</dt>
        <dd>The IPv6 router (Originating Node) (originating node) initiating the AODV-RPL
        route discovery to obtain a route to TargNode. </t>
    <t hangText="Paired DODAGs"><vspace />
        Two </dd>

        <dt>Paired DODAGs</dt>
        <dd>Two DODAGs for a single route discovery process between OrigNode
        and TargNode.</t>
    <t hangText="P2P"><vspace />
        Peer-to-Peer -- in TargNode.</dd>

        <dt>P2P</dt>
        <dd>Peer-to-Peer (in other words, not constrained a priori to
        traverse a common ancestor. </t>
    <t hangText="REJOIN_REENABLE"><vspace />
        The ancestor).</dd>

        <dt>REJOIN_REENABLE</dt>
        <dd>The duration during which a node is prohibited from joining a
        DODAG with a particular RREQ-InstanceID, after it has left a DODAG
        with the same RREQ-InstanceID. The default value of REJOIN_REENABLE is
        15 minutes.</t>
    <t hangText="RREQ"><vspace />
        A RREQ-DIO message. </t>
    <t hangText="RREQ-DIO message"><vspace />
        A minutes.</dd>

        <dt>RREQ</dt>
        <dd>Route Request.</dd>

        <dt>RREQ-DIO message</dt>
        <dd>A DIO message containing the RREQ option.  The RPLInstanceID in
        RREQ-DIO is assigned locally by the OrigNode.  The RREQ-DIO message
        has a secure variant as noted in <xref
        target="RFC6550"/>. </t>
    <t hangText="RREQ-InstanceID"><vspace />
        The target="RFC6550"/>.</dd>

        <dt>RREQ-InstanceID</dt>
        <dd>The RPLInstanceID for the RREQ-Instance. The RREQ-InstanceID is formed
        as the ordered pair (Orig_RPLInstanceID, OrigNode-IPaddr), where
        Orig_RPLInstanceID is the local RPLInstanceID allocated by OrigNode, OrigNode
        and OrigNode-IPaddr is an IP address of OrigNode.  The RREQ-InstanceID
        uniquely identifies the RREQ-Instance.  </t>
    <t hangText="RREP"><vspace />
        A RREP-DIO message. </t>
    <t hangText="RREP-DIO message"><vspace />
        A  </dd>

        <dt>RREP</dt>
        <dd>Route Reply.</dd>

        <dt>RREP-DIO message</dt>
        <dd>A DIO message containing the RREP option.  OrigNode pairs the
        RPLInstanceID in RREP-DIO to the one in the associated RREQ-DIO
        message (i.e., the RREQ-InstanceID) as described in <xref
        target="asymmetricrrep"/>. The RREP-DIO message has a secure variant
        as noted in <xref target="RFC6550"/>. </t>
    <t hangText="RREP-InstanceID"><vspace /> target="RFC6550"/>.</dd>

        <dt>RREP-InstanceID</dt>
        <dd>
        The RPLInstanceID for the RREP-Instance.  The RREP-InstanceID is formed
        as the ordered pair (Targ_RPLInstanceID, TargNode-IPaddr), where
        Targ_RPLInstanceID is the local RPLInstanceID allocated by TargNode, TargNode
        and TargNode-IPaddr is an IP address of TargNode.  The RREP-InstanceID
        uniquely identifies the RREP-Instance.  The RPLInstanceID in the RREP
        message along with the Delta value indicates the associated
        RREQ-InstanceID.  The InstanceIDs are matched by the mechanism explained
        in <xref target="instancepairing"/> </t>
    <t hangText="Source routing"><vspace />
        A target="instancepairing"/>.</dd>

        <dt>Source routing</dt>
        <dd>A mechanism by which the source supplies a vector of addresses
        towards the destination node along with each data packet <xref target="RFC6550"/>. </t>
    <t hangText="Symmetric route"><vspace />
        The
        target="RFC6550"/>.</dd>

        <dt>Symmetric route</dt>
        <dd>The upstream and downstream routes traverse the same routers and over
        the same links. </t>
<!-- CEP: pagination :-(  -->
    <t hangText="TargNode"><vspace />
        The links.</dd>

	<dt>TargNode</dt>
        <dd>The IPv6 router (Target Node) (target node) for which OrigNode requires a
        route and initiates Route Discovery route discovery within the LLN. </t>
    <t hangText="Upward Direction"><vspace />
        The </dd>

	<dt>Upward direction</dt>
        <dd>The direction from the TargNode to the OrigNode.</t>
    <t hangText="Upward Route"><vspace />
        A OrigNode.</dd>

        <dt>Upward route</dt>
        <dd>A route in the upward direction. </t>
    </list></t> direction.</dd>
      </dl>
    </section>        <!-- End of section "Terminology" -->

<section title="Overview

    <section>
      <name>Overview of AODV-RPL"> AODV-RPL</name>
      <t>
        With AODV-RPL, routes from OrigNode to TargNode within the LLN
        do not become established until they are needed.  The route
        discovery mechanism in AODV-RPL is invoked when OrigNode
        has data for delivery to a TargNode, but existing routes do not
        satisfy the application's requirements.  For this reason reason,
        AODV-RPL is considered to be an example of an "on-demand" routing
        protocols.
        protocol.  Such protocols are also known as "reactive" routing
        protocols since their operations are triggered in reaction to
        a determination that a new route is needed.

        AODV-RPL works
        without requiring the use of RPL or any other routing protocol.
      </t>
      <t>
        The routes discovered by
        AODV-RPL are not constrained to traverse a common ancestor.
        AODV-RPL can enable asymmetric communication paths in networks with
        bidirectional asymmetric links. For this purpose, AODV-RPL enables
        discovery of two routes: namely, one from OrigNode to TargNode, TargNode and
        another from TargNode to OrigNode.  AODV-RPL also
        enables discovery of symmetric routes along Paired paired DODAGs, when
        symmetric routes are possible (see <xref target="channel"/>).
      </t>
      <t>
        In AODV-RPL, routes are discovered by first forming a temporary DAG
        Directed Acyclic Graph (DAG) rooted at the OrigNode.  Paired DODAGs
        (Instances) are constructed during route formation between the
        OrigNode and TargNode.  The RREQ-Instance is formed by route control
        messages from OrigNode to
        TargNode TargNode, whereas the RREP-Instance is
        formed by route control messages from TargNode to OrigNode.  The route
        discovered in the RREQ-Instance is used for transmitting data from
        TargNode to OrigNode, and the route discovered in RREP-Instance is
        used for transmitting data from OrigNode to TargNode.
      </t>
      <t>
        Intermediate routers join the DODAGs based on the Rank
        <xref target="RFC6550"/> as calculated from the DIO messages.
        AODV-RPL uses the same notion of rank as
        defined in RFC6550: "The <xref target="RFC6550"/>:</t>

	<blockquote>The Rank is the expression of a relative position within
	a DODAG Version with regard to neighbors, and it is not necessarily a
	good indication or a proper expression of a distance or a path cost to
	the root."  The root.</blockquote>

	<t>The Rank measurements provided in AODV messages do not indicate a
	distance or a path cost to the root.
      </t>
      <t>
        Henceforth in this document, "RREQ-DIO message" means the DIO
	message from OrigNode toward TargNode, containing the RREQ option as
	specified in <xref target="RREQmsg"/>.  The RREQ-InstanceID is formed
	as the ordered pair (Orig_RPLInstanceID, OrigNode-IPaddr), where
	Orig_RPLInstanceID is the local RPLInstanceID allocated by OrigNode, OrigNode
	and OrigNode-IPaddr is the IP address of OrigNode.  A node receiving
	the RREQ-DIO can use the RREQ-InstanceID to identify the proper OF
	whenever that node receives a data packet with Source Address ==
	OrigNode-IPaddr and IPv6 RPL Option having the RPLInstanceID ==
	Orig_RPLInstanceID.  The 'D' D bit of the RPLInstanceID field is set
        to 0 to indicate that the source address of the IPv6 packet is
        the DODAGID.
      </t>
      <t>
	Similarly, "RREP-DIO message" means the DIO message from TargNode
	toward OrigNode, containing the RREP option as specified in
	<xref target="RREPmsg"/>.  The RREP-InstanceID is formed
	as the ordered pair (Targ_RPLInstanceID, TargNode-IPaddr), where
	Targ_RPLInstanceID is the local RPLInstanceID allocated by TargNode, TargNode
	and TargNode-IPaddr is the IP address of TargNode.  A node receiving
	the RREP-DIO can use the RREP-InstanceID to identify the proper OF
	whenever that node receives a data packet with Source Address ==
	TargNode-IPaddr and IPv6 RPL Option having the RPLInstanceID ==
	Targ_RPLInstanceID along with 'D' D == 0 as above.
      </t>
    </section>	<!-- End of section "Overview of AODV-RPL" -->

    <section anchor="Options" title="AODV-RPL anchor="Options">
      <name>AODV-RPL DIO Options"> Options</name>
      <section anchor="RREQmsg" title="AODV-RPL anchor="RREQmsg">
        <name>AODV-RPL RREQ Option"> Option</name>
        <t>
	OrigNode selects one of its IPv6 addresses and sets it in the DODAGID
<!--	CEP: SHOULD changed to MUST by request of Alvaro Retana.  -->
	field of the RREQ-DIO message.  The address scope of the selected
        <!-- Gunter Van de Velde 2/11/2025, 8:36 PM  -->
	address MUST <bcp14>MUST</bcp14> encompass the domain where the route is built (e.g, not
	link-local); otherwise otherwise, the route discovery will fail.  Exactly one
        RREQ option MUST <bcp14>MUST</bcp14> be present
	in a RREQ-DIO message, otherwise message; otherwise, the message MUST <bcp14>MUST</bcp14> be dropped.
        </t>
        <figure anchor="figRREQ" title="Format anchor="figRREQ">
          <name>Format for AODV-RPL RREQ Option"> Option</name>
          <artwork align="center"><![CDATA[
     0                   1                   2                   3
     0 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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Option Type  | Option Length |S|H|X| Compr | L |  RankLimit  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Orig SeqNo   |                                               |
    +-+-+-+-+-+-+-+-+                                               |
    |                                                               |
    |           Address Vector (Optional, Variable Length)          |
    .                                                               .
    .                                                               .
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ . . .
]]></artwork> .]]></artwork>
        </figure>
    OrigNode

        <t>OrigNode supplies the following information in the RREQ option: </t>
    <t><list style="hanging">
    <t hangText="Option Type"><vspace />
	8-bit

        <dl newline="true" spacing="normal">
          <dt>Option Type</dt>
          <dd>8-bit unsigned integer specifying the type of the option (TBD2)</t>
    <t hangText="Option Length"><vspace />
	8-bit (0x0B).</dd>

          <dt>Option Length</dt>
          <dd>8-bit unsigned integer specifying the length of the option in
          octets, excluding the Option Type and Option Length fields. Variable  It is
          variable due to the presence of the address vector and the number of
          octets elided according to the Compr value.</t>
    <t hangText="S"><vspace />
	Symmetric value.</dd>

          <dt>S</dt>
          <dd>Symmetric bit indicating a symmetric route from the OrigNode to
          the router transmitting this RREQ-DIO. See <xref target="channel"/>.</t>
    <t hangText="H"><vspace />
	Set
          target="channel"/>.</dd>

          <dt>H</dt>
          <dd>Set to one for a hop-by-hop route.  Set to zero for a source
          route.  This flag controls both the downstream route and upstream route. </t>
    <t hangText="X"><vspace />
	Reserved; MUST
          route.</dd>

          <dt>X</dt>
          <dd>Reserved. This field <bcp14>MUST</bcp14> be initialized to zero and ignored
          upon reception.</t>
    <t hangText="Compr"><vspace />
	4-bit reception.</dd>

          <dt>Compr</dt>
          <dd>4-bit unsigned integer. When Compr is nonzero, exactly that
          number of prefix octets MUST <bcp14>MUST</bcp14> be elided from each
          address before storing it in the Address Vector. The octets elided
          are shared with the IPv6 address in the DODAGID. This field is only
          used in source routing mode (H=0).  In hop-by-hop mode (H=1), this
          field MUST <bcp14>MUST</bcp14> be set to zero and ignored upon reception.</t>
          reception.</dd>
          <!--  CEP: Shouldn't we allow address compression for the Target Option? -->
    <t hangText="L"><vspace />
<?rfc subcompact="yes" ?>
	2-bit
	  <dt>L</dt>
          <dd>
            <t>2-bit unsigned integer determining the time duration that a
            node is able to belong to the RREQ-Instance (a temporary DAG
            including the OrigNode and the TargNode).  Once the time is
            reached, a node SHOULD <bcp14>SHOULD</bcp14> leave the RREQ-Instance and
            stop sending or receiving any more DIOs for the RREQ-Instance; otherwise
            otherwise, memory and network resources are likely to be consumed
            unnecessarily.  This naturally depends on the node's ability to
            keep track of time.  Once a node leaves an RREQ-Instance, it MUST
	NOT
            <bcp14>MUST NOT</bcp14> rejoin the same RREQ-Instance for at least
            the time interval specified by the configuration variable
            REJOIN_REENABLE.
	<list style="symbols">
	    <t>0x00: No time limit imposed. </t>
	    <t>0x01: 16 seconds </t>
	    <t>0x02: 64 seconds </t>
	    <t>0x03: 256 seconds </t>
	</list>
<?rfc subcompact="no" ?> L is independent from the route lifetime, which
            is defined in the DODAG configuration option.
            </t>
            <ul spacing="compact">
              <li>
                <t>0x00: No time limit imposed</t>
              </li>
              <li>
                <t>0x01: 16 seconds</t>
              </li>
              <li>
                <t>0x02: 64 seconds</t>
              </li>
              <li>
                <t>0x03: 256 seconds</t>
              </li>
            </ul>
            <t>
<!-- The route entries in hop-by-hop routing
	and states of source routing can still be maintained
	even after the node no longer maintains DAG connectivity or
	messaging. -->
	<!--  according to email to the list, 12/27/2020 -->
            </t>
    <t hangText="RankLimit"><vspace />
	8-bit
          </dd>
          <dt>RankLimit</dt>
          <dd>8-bit unsigned integer specifying the upper limit on the integer
          portion of the Rank (calculated using the DAGRank() macro defined in
          <xref target="RFC6550"/>).  A value of 0 in this field indicates the
          limit is infinity. </t>
    <t hangText="Orig SeqNo"><vspace />
	8-bit infinity.</dd>
          <dt>Orig SeqNo</dt>
          <dd>8-bit unsigned integer specifying the sequence Number of
          OrigNode.  See <xref target="rreq"/>.  </t>
    <t hangText="Address Vector"><vspace />
	A target="rreq"/>.</dd>
          <dt>Address Vector</dt>
          <dd>A vector of IPv6 addresses representing the route that the
          RREQ-DIO has passed. It is only present when the H bit is set to 0.
          The prefix of each address is elided according to the Compr field.</t>
    </list>
    </t>
    <t> TargNode
          field.</dd>
        </dl>
        <t>TargNode can join the RREQ instance RREQ-Instance at a Rank whose integer portion is
	less than or equal to the RankLimit.  Any other node MUST NOT <bcp14>MUST NOT</bcp14> join a
	RREQ instance
	RREQ-Instance if its own Rank would be equal to or higher than the
	RankLimit.  A router MUST <bcp14>MUST</bcp14> discard a received RREQ if the integer part
	of the advertised Rank equals or exceeds the RankLimit. </t>
    <t> </t> RankLimit.</t>
      </section>	<!-- End of section "RREQ Message" -->

      <section anchor="RREPmsg" title="AODV-RPL anchor="RREPmsg">
        <name>AODV-RPL RREP Option"> Option</name>
        <t>
	TargNode sets one of its IPv6 addresses in the DODAGID
<!--	CEP: SHOULD changed to MUST, by request of Alvaro Retana.  -->
	field of the RREP-DIO message.  The address scope of the selected
	address must encompass the domain where the route is built (e.g, not
	link-local).   Exactly one RREP option MUST <bcp14>MUST</bcp14> be present
	in a RREP-DIO message, otherwise otherwise, the message MUST <bcp14>MUST</bcp14> be dropped.
	TargNode supplies the following information in the RREP option:
        </t>
        <figure anchor="figRREP" title="Format anchor="figRREP">
          <name>Format for AODV-RPL RREP option"> Option</name>
          <artwork align="center"><![CDATA[
    0                   1                   2                   3
    0 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Option Type  | Option Length |G|H|X| Compr | L |  RankLimit  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Delta   |X X|                                               |
   +-+-+-+-+-+-+-+-+                                               |
   |                                                               |
   |                                                               |
   |           Address Vector (Optional, Variable Length)          |
   .                                                               .
   .                                                               .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ . . .
        ]]></artwork> .]]></artwork>
        </figure>

        <list style="hanging">
	<t hangText="Option Type"><vspace />
	    8-bit
        <dl newline="true" spacing="normal">
          <dt>Option Type</dt>
          <dd>8-bit unsigned integer specifying the type of the option (TBD3)</t>
	<t hangText="Option Length"><vspace />
	    8-bit (0x0C).</dd>

          <dt>Option Length</dt>
          <dd>8-bit unsigned integer specifying the length of the option in
          octets, excluding the Option Type and Option Length fields.  Variable  It is
          variable due to the presence of the address vector and the number of
          octets elided according to the Compr value.</t>
	<t hangText="G"><vspace />
	    Gratuitous value.</dd>

          <dt>G</dt>
          <dd>Gratuitous RREP (see <xref target="G-RREP"/>).</t>
	<t hangText="H"><vspace />
	    The target="G-RREP"/>).</dd>

          <dt>H</dt>
          <dd>The H bit in the RREP option MUST <bcp14>MUST</bcp14> be set to be
          the same as the H bit in the RREQ option.  It requests either source
          routing (H=0) or hop-by-hop (H=1) for the downstream route.</t>
	<t hangText="X"><vspace />
	    1-bit route.</dd>

          <dt>X</dt>
          <dd>1-bit Reserved field; MUST field. This field <bcp14>MUST</bcp14> be initialized to zero
          and ignored upon reception.</t>
	<t hangText="Compr"><vspace />
	    4-bit reception.</dd>

          <dt>Compr</dt>
          <dd>4-bit unsigned integer. Same This field has the same definition as in the RREQ option. </t>
	<t hangText="L"><vspace />
	    2-bit option.</dd>

          <dt>L</dt>
          <dd>2-bit unsigned integer defined as in the RREQ option.  The lifetime
          of the RREP-Instance SHOULD <bcp14>SHOULD</bcp14> be no greater than the
          lifetime of the RREQ-Instance to which it is paired, so that the
          memory required to store the RREP-Instance can be reclaimed when no
          longer needed.</t>
	<t hangText="RankLimit"><vspace />
	    8-bit needed.</dd>

          <dt>RankLimit</dt>
          <dd>8-bit unsigned integer specifying the upper limit on the integer
          portion of the Rank, similarly to RankLimit in the RREQ message.  A
          value of 0 in this field indicates the limit is infinity. </t> infinity.</dd>
          <!--  CEP: is 7 bits O.K. for RankLimit?  -->
	<t hangText="Delta"><vspace />
	    6-bit

	  <dt>Delta</dt>
          <dd>6-bit unsigned integer. TargNode uses the Delta field so that
          nodes receiving its RREP message can identify the RREQ-InstanceID of
          the RREQ message that triggered the transmission of the RREP (see
          <xref target="instancepairing"/>).  </t>
	<t hangText="X X"><vspace />
	    2-bit target="instancepairing"/>).</dd>

          <dt>X X</dt>
          <dd>2-bit Reserved field; MUST field. This field <bcp14>MUST</bcp14> be initialized to zero
          and ignored upon reception.</t>
	<t hangText="Address Vector"><vspace /> reception.</dd>
          <dt>Address Vector</dt>
          <dd>
	    Only present when the H bit is set to 0. The prefix of each address
	    is elided according to the Compr field.  For an asymmetric route,
	    the Address Vector represents the IPv6 addresses of the path
	    through the network the RREP-DIO has passed.  In contrast, for a
	    symmetric route, it is the Address Vector when the RREQ-DIO arrives
	    at the TargNode, unchanged during the transmission to the OrigNode.
	    </t>
	</list>
    </t>
	    </dd>
        </dl>
        <!--
/* Make the following into an XML comment */
[A]  It is technically feasible to have partially active DODAG pair.
Having this condition lets graceful shutdown of the current route discovery
instance initiated by OrigNode.  It marks the end of DODAG pairing as RREQ
and RREP Instances can be treated as belonging to the same route discovery.
The resources held by the intermediate nodes is released, and OrigNode can
start reusing the same RPLInstanceID in the RREQ for its new
route discovery. Having RREQ-Instance lifetime thus enables this.
  -->

    </section>	<!-- End of section "AODV-RPL RREP Option" -->

    <section anchor="artop" title="AODV-RPL anchor="artop">
        <name>AODV-RPL Target Option"> Option</name>
        <t> The AODV-RPL Target (ART) Option option is based on the Target Option option
	in the core RPL specification <xref target="RFC6550"/>.  The Flags field is replaced by
	the Destination Sequence Number of the TargNode TargNode, and the Prefix
	Length field is reduced to 7 bits so that the value is limited to
	be no greater than 127.  </t>
        <t>
	A RREQ-DIO message MUST <bcp14>MUST</bcp14> carry at least one ART Option. option.  A RREP-DIO
	message MUST <bcp14>MUST</bcp14> carry exactly one ART Option. option. Otherwise, the message
	MUST
	<bcp14>MUST</bcp14> be dropped.
<!--  CEP: Is it needed for RREPs with symmetric routes? -->
        </t>
        <t>
	OrigNode can include multiple TargNode addresses via multiple AODV-RPL
	Target Options ART
	options in the RREQ-DIO, for routes that share the same requirement on
	metrics.  This reduces the cost to building only one DODAG for
	multiple targets.
        </t>
    <t>
        <figure anchor="figTarg" title="ART anchor="figTarg">
          <name>ART Option format Format for AODV-RPL"> AODV-RPL</name>
          <artwork align="center"><![CDATA[
    0                   1                   2                   3
    0 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Option Type  | Option Length |  Dest SeqNo   |X|Prefix Length|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               |
   |           Target Prefix / Address (Variable Length)           |
   .                                                               .
   .                                                               .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ . . .
        ]]></artwork> .]]></artwork>
        </figure>
      <list style="hanging">
	<t hangText="Option Type"> <vspace />
	    8-bit
        <dl newline="true" spacing="normal">
          <dt>Option Type</dt>
          <dd>8-bit unsigned integer specifying the type of the option (TBD4)
	</t>
	<t hangText="Option Length"> <vspace />
	    8-bit (0x0D).</dd>

          <dt>Option Length</dt>
          <dd>8-bit unsigned integer specifying the length of the option in
	    octets
          octets, excluding the Option Type and Option Length fields.
	</t>
	<t hangText="Dest SeqNo"> <vspace /></t>
	<t> 8-bit fields.</dd>

          <dt>Dest SeqNo</dt>
          <dd>8-bit unsigned integer. In RREQ-DIO, if nonzero, it is the
          Sequence Number for the last route that OrigNode stored to the
          TargNode for which a route is desired.  In RREP-DIO, it is the
          destination sequence number associated to the route.  Zero is used
          if there is no known information about the sequence number of TargNode,
          TargNode and not used
	    otherwise.
	</t>
	<t hangText="X"> <vspace />
	    A one-bit reserved otherwise.</dd>

          <dt>X</dt>
          <dd>1-bit Reserved field.  This field MUST <bcp14>MUST</bcp14> be
          initialized to zero by the sender and MUST <bcp14>MUST</bcp14> be ignored
          by the receiver.
	</t>
	<t hangText="Prefix Length"> <vspace />
	    7-bit receiver.</dd>

          <dt>Prefix Length</dt>
          <dd>7-bit unsigned integer.  The Prefix Length field contains the
          number of valid leading bits in the prefix. If Prefix Length is 0,
          then the value in the Target Prefix / Address field represents an
          IPv6 address, not a prefix.
	</t>
	<t hangText="Target prefix.</dd>

          <dt>Target Prefix / Address"> <vspace />
	    (variable-length field) An Address</dt>
          <dd>A variable-length field with an IPv6 destination address or prefix.
          The length of the Target Prefix / Address field is the least number
          of octets that can represent all of the bits of the Prefix, in other words
          words, Ceil(Prefix Length/8) octets.  When Prefix Length is not equal
          to 8*Ceil(Prefix Length/8) and nonzero, the Target Prefix / Address
          field will contain some initial bits that are not part of the Target
          Prefix.  Those initial bits (if any) MUST <bcp14>MUST</bcp14> be set to
          zero on transmission and MUST <bcp14>MUST</bcp14> be ignored on receipt.
          If Prefix Length is zero, the Address field is 128 bits.
	</t>
      </list>
    </t>
	</dd>
        </dl>
      </section> <!-- End of section "AODV-RPL Target Option" -->
    </section> <!-- End of section "AODV-RPL Options" -->

    <section anchor="channel" title="Symmetric anchor="channel">
      <name>Symmetric and Asymmetric Routes"> Routes</name>
      <t>
	Links are considered symmetric until indication to the contrary is
	received.  In Figures <xref target="figSymm-a"/> target="figSymm-a" format="counter"/> and
	<xref target="figSymm-b"/>, target="figSymm-b" format="counter"/>, BR is the Border Router, O is the
	OrigNode, each R is an intermediate router, and T is the TargNode.
	In this example, these examples, the use of BR is only for illustrative purposes;
	AODV does not depend on the use of border routers for its operation.
	If the RREQ-DIO arrives over an interface that
	is known to be symmetric, symmetric and the S bit is set to 1, then it remains
	as 1, as illustrated in <xref target="figSymm-a"/>.  If an
	intermediate router sends out RREQ-DIO with the S bit set to 1, then
	each link en route from the OrigNode O to this router has met
	the requirements of route discovery, and the route can be used
	symmetrically.
      </t>
    <t><figure anchor="figSymm-a"
	title="AODV-RPL
      <figure anchor="figSymm-a">
        <name>AODV-RPL with Symmetric Instances"> Instances</name>
        <artwork align="center"><![CDATA[
                               BR
                           /----+----\
                         /      |      \
                       /        |         \
                      R         R           R
                   _/  \        |          /  \
                  /     \       |         /     \
                 /       \      |        /        \
               R -------- R --- R ----- R -------- R
             /  \   <--S=1-->  / \    <--S=1-->   /  \
      <--S=1-->  \            /   \             /   <--S=1-->
        /         \          /     \          /         \
      O ---------- R ------ R------ R ----- R ----------- T
     / \                   / \             / \           / \
    /   \                 /   \           /   \         /   \
   /     \               /     \         /     \       /     \
  R ----- R ----------- R ----- R ----- R ----- R ---- R----- R

    >---- RREQ-Instance (Control: O-->T;  Data: T-->O) ------->
    <---- RREP-Instance (Control: T-->O;  Data: O-->T) -------< ]]></artwork>
    </figure></t>
      </figure>
      <t>
	Upon receiving a RREQ-DIO with the S bit set to 1, a node determines
	whether the link over which it was received can be used symmetrically,
	i.e., both directions meet the requirements of data transmission.  If
	the RREQ-DIO arrives over an interface that is not known to be symmetric,
	symmetric or is known to be asymmetric, the S bit is set to 0.  If
	the S bit arrives already set to be '0', 0, then it is set to be '0' 0 when the
	RREQ-DIO is propagated (<xref target="figSymm-b"/>).  For an
	asymmetric route, there is at least one hop which that doesn't satisfy the
	Objective Function.  Based on the S bit received in RREQ-DIO, TargNode
	T determines whether or not the route is symmetric before transmitting
	the RREP-DIO message upstream towards the OrigNode O.
      </t>
      <t>

	It is beyond the scope of this document to specify the criteria used
	when determining whether or not each link is symmetric.  As an
	example, intermediate routers can use local information (e.g., bit
	rate, bandwidth, number of cells used in 6tisch <xref
	target="RFC9030"/>), a priori knowledge (e.g., link quality according
	to previous communication) communication), or
	use averaging techniques as appropriate
	to the application.  Other link metric information can be acquired
	before AODV-RPL operation, by executing evaluation procedures; for instance
	instance, test traffic can be generated between nodes of the deployed
	network.  During AODV-RPL operation, OAM Operations, Administration, and
	Maintenance (OAM) techniques for evaluating link state (see <xref
	target="RFC7548"/>, <xref target="RFC7276"/>, and <xref
	target="co-ioam"/>) MAY <bcp14>MAY</bcp14> be used (at regular intervals
	appropriate for the LLN).  The evaluation procedures are out of scope
	for AODV-RPL.  For further information on this topic, see <xref
	target="Link_Asymmetry"/>, <xref target="low-power-wireless"/>, and
	<xref target="empirical-study"/>.
      </t>
      <t>
	<xref target="appendix-a"/> describes an example method using the
	upstream Expected Number of Transmissions Transmission Count (ETX) and downstream Received
	Signal Strength Indicator (RSSI) to estimate whether the link is
	symmetric in terms of link quality using an averaging technique.

      </t>
      <figure anchor="figSymm-b"
  		title="AODV-RPL anchor="figSymm-b">
        <name>AODV-RPL with Asymmetric Paired Instances"> Instances</name>
        <artwork align="center"><![CDATA[
                                  BR
                              /----+----\
                            /      |      \
                          /        |        \
                        R          R          R
                      / \          |        /   \
                    /     \        |       /      \
                  /         \      |      /         \
                 R --------- R --- R ---- R --------- R
               /  \   --S=1-->   / \    --S=0-->   /   \
         --S=1-->   \           /    \            /   --S=0-->
          /          \        /       \         /         \
        O ---------- R ------ R------ R ----- R ----------- T
       / \                   / \             / \           / \
      /  <--S=0--           /   \           /   \         / <--S=0--
     /     \               /     \         /     \       /     \
    R ----- R ----------- R ----- R ----- R ----- R ---- R----- R
                <--S=0--   <--S=0-- <--S=0-- <--S=0--    <--S=0--

    >---- RREQ-Instance (Control: O-->T;  Data: T-->O) ------->
    <---- RREP-Instance (Control: T-->O;  Data: O-->T) -------<]]></artwork>
      </figure>
      <t>
	As illustrated in <xref target="figSymm-b"/>, an intermediate
	router determines the S bit value that the RREQ-DIO should carry
	using link asymmetry detection methods as discussed earlier in
	this section.  In many cases cases, the intermediate router has already
	made the link asymmetry decision by the time RREQ-DIO arrives.
      </t>
      <t>
	See <xref target="Examples"/> for examples illustrating RREQ and RREP
        transmissions in some networks with symmetric and asymmetric links.
      </t>
    </section>	<!-- End of section "Symmetric and Asymmetric Routes" -->

    <section anchor="aodvrplop" title="AODV-RPL Operation">
    <section anchor="rreq" title="Route Request Generation"> anchor="aodvrplop">
      <name>AODV-RPL Operation</name>
      <section anchor="rreq">
        <name>Generating RREQ</name>
        <t>
	The route discovery process is initiated when an application
	at the OrigNode has data to be transmitted to the TargNode, TargNode but does
	not have a route that satisfies the Objective Function for the target
	of the application's data. In this case, the OrigNode builds a local
	RPLInstance and a DODAG rooted at itself. Then Then, it transmits a DIO
	message containing exactly one RREQ option
	(see <xref target="RREQmsg"/>) to multicast group all-AODV-RPL-nodes.
	The RREQ-DIO MUST <bcp14>MUST</bcp14> contain at least one ART Option option
	(see <xref target="artop"/>), which indicates the TargNode.
<!--  CEP: or network prefix containing the TargNode.  -->
	The S bit in RREQ-DIO sent out by the OrigNode is set to 1.
        </t>
        <t>
	Each node maintains a sequence number; the operation is specified in
	section 7.2 of
	<xref target="RFC6550"/>. target="RFC6550" sectionFormat="of" section="7.2"/>.
	When the OrigNode initiates a
	route discovery process, it MUST <bcp14>MUST</bcp14> increase its own sequence number to
	avoid conflicts with previously established routes.  The sequence
	number is carried in the Orig SeqNo field of the RREQ option.
        </t>
        <t> The Target Prefix / Address in the ART Option option can be a unicast IPv6
	address or a prefix.  The OrigNode can initiate
	the route discovery process for multiple targets simultaneously by
	including multiple ART Options. options. Within a RREQ-DIO RREQ-DIO, the Objective
	Function for the routes to different TargNodes MUST <bcp14>MUST</bcp14> be the same.
        </t>
        <t> OrigNode can maintain different RPLInstances to discover routes with
	different requirements to the same targets. Using the RPLInstanceID
	pairing mechanism (see <xref target="instancepairing"/>), route replies
	(RREP-DIOs) for different RPLInstances can be generated.
        </t>
        <t> The transmission of RREQ-DIO obeys the Trickle timer
	<xref target="RFC6206"/>.  If the duration specified by the
	L field has elapsed, the OrigNode MUST <bcp14>MUST</bcp14> leave
	the DODAG and stop sending RREQ-DIOs in the related RPLInstance.
	OrigNode needs to set the L field such that the DODAG will not
	prematurely timeout during data transfer with the TargNode.
	For setting this value, it has to consider factors such as
	the Trickle timer, TargNode hop distance, network size, link
	behavior, expected data usage time, and so on.
        </t>
      </section>
      <!--  CEP: The Trickle timer eliminates the need for RREQ_WAIT_TIME?  -->

    <section anchor="process_rreq"
			title="Receiving anchor="process_rreq">
        <name>Receiving and Forwarding RREQ messages"> Messages</name>
        <section anchor="rreq_step1"
			title="Step anchor="rreq_step1">
          <name>Step 1: RREQ reception Reception and evaluation"> Evaluation</name>
          <!--  CEP: descriptive text, might decide to include it somewhere.

    An intermediate router X receives a RREQ message a neighbor Y.  If X can
    use the incoming link to transmit a packet to OrigNode by way of Y, X will
    propagate the RREQ message in hopes of eventually providing Targnode with
    a route towards OrigNode.  In that case, X could use Y as the first hop
    of its own route towards OrigNode, but very likely X does not otherwise
    need a route to OrigNode.  X determines whether it can use the incoming
    link to transmit a packet to OrigNode by determining whether or not the
    upstream direction of the incoming link satisfies the OF.

    When TargNode receives a RREQ, and the upstream direction of the incoming
    link satisfies the OF, TargNode has a route to OrigNode via the neighbor Y
    that transmitted the RREQ.  If in addition the S bit is set in the
    OrigNode, and if the downstream direction of the incoming link is suitable
    for TargNode to receive packets from that neighbor Y, then the entire
    path traversed by the RREQ is symmetric and OrigNode can use that path
    to send packets to TargNode.  In order to provide that routing information
    (about a viable path to TargNode) to OrigNode, TargNode unicasts a RREP
    back to Y.
  -->
	<t> When

	  <t>When a router X receives a RREQ message over a link from a
	  neighbor Y, X first determines whether or not the RREQ is valid.  If so,
	  valid, X then determines whether or not it has sufficient resources
	  available to maintain the RREQ-Instance and the value of the 'S' S bit
	  needed to process an eventual RREP, if the RREP were to be received.
	  If not, not valid, then X MUST <bcp14>MUST</bcp14> either free up sufficient resources (the
	  means for this are beyond the scope of this document), or drop the
	  packet and discontinue processing of the RREQ.  Otherwise, X next
	  determines whether the RREQ advertises a usable route to OrigNode,
	  by checking whether the link to Y can be used to transmit packets to
	  OrigNode.
          </t>
          <t>
	    When H=0 in the incoming RREQ, the router MUST <bcp14>MUST</bcp14> drop the
	    RREQ-DIO if one of its addresses is present in the Address Vector.
	    When H=1 in the incoming RREQ, the router MUST <bcp14>MUST</bcp14> drop the RREQ
	    message if the Orig SeqNo field of the RREQ is older than the SeqNo
	    value that X has stored for a route to OrigNode.
	    Otherwise, the router determines whether to propagate the RREQ-DIO.
	    It does this by determining whether or not a route to OrigNode
	    using the upstream direction of the incoming link satisfies the
	    Objective Function (OF).  In order to evaluate the OF, the router
	    first determines the maximum useful rank (MaxUsefulRank).  If the
	    router has previously joined the RREQ-Instance associated with
	    the RREQ-DIO, then MaxUsefulRank is set to be the Rank value that
	    was stored when the router processed the best previous RREQ for
	    the DODAG with the given RREQ-Instance.  Otherwise, MaxUsefulRank
	    is set to be RankLimit.  If OF cannot be satisfied (i.e.,
	    the Rank evaluates to a value greater than MaxUsefulRank) MaxUsefulRank),
	    the RREQ-DIO MUST <bcp14>MUST</bcp14> be dropped, and the following steps are not
	    processed.  Otherwise, the router MUST <bcp14>MUST</bcp14> join the RREQ-Instance
	    and prepare to propagate the RREQ-DIO, as follows. The upstream
	    neighbor router that transmitted the received RREQ-DIO is selected
	    as the preferred parent in the RREQ-Instance.
          </t>
      </section><!--End of section "Step 1: RREQ reception and evaluation"-->
        </section>

      <section anchor="rreq_step2"
		title="Step anchor="rreq_step2">
          <name>Step 2: TargNode and Intermediate Router determination"> Determination</name>
          <t>	<!-- Kaduk comment 16 -->
	    After determining that a received RREQ provides a usable route
	    to OrigNode, a router determines whether it is a TargNode, or a possible intermediate router between OrigNode and a TargNode,
	    or both.  The router is a TargNode if it finds one of its own
	    addresses in a Target Option option in the RREQ.  After possibly
	    propagating the RREQ according to the procedures in Steps 3,
	    4, and 5, the TargNode generates a RREP as specified in
	    <xref target="gen-rrep"/>.  If S=0, the determination of TargNode
	    status and determination of a usable route to OrigNode is the same.
          </t>
          <t>
	    If the OrigNode tries to reach multiple TargNodes in a
	    single RREQ-Instance, one of the TargNodes can be an intermediate
	    router to other TargNodes.  In this case, before transmitting the
	    RREQ-DIO to multicast group all-AODV-RPL-nodes, a TargNode MUST <bcp14>MUST</bcp14>
	    delete the Target Option option encapsulating its own address, so that
	    downstream routers with higher Rank values do not try to create
	    a route to this TargNode.
          </t>
          <t>
	    An intermediate router could receive several RREQ-DIOs from
	    routers with lower Rank values in the same RREQ-Instance with
	    different lists of Target Options. options.  For the purposes of determining
	    the intersection with previous incoming RREQ-DIOs, the intermediate
	    router maintains a record of the targets that have been requested
	    for a given RREQ-Instance.  An incoming RREQ-DIO message having
	    multiple ART Options options coming from a router with higher Rank than
	    the Rank of the stored targets is ignored.  When transmitting the
	    RREQ-DIO, the intersection of all received lists MUST <bcp14>MUST</bcp14> be included
	    if it is nonempty after TargNode has deleted the Target Option option
	    encapsulating its own address.  If the intersection is empty, it
	    means that all the targets have been reached, and the router MUST
	    NOT <bcp14>MUST
	    NOT</bcp14> transmit any RREQ-DIO.  Otherwise  Otherwise, it proceeds to
	    <xref target="rreq_step3"/>.
          </t>
          <t>
	    For example, suppose two RREQ-DIOs are received with the same
	    RPLInstance and OrigNode.  Suppose further that the first
	    RREQ has (T1, T2) as the targets, and the second one has (T2, T4)
	    as targets. Then Then, only T2 needs to be included in the generated
	    RREQ-DIO.
          </t>
          <t>
	    The reasoning for using the intersection of the lists in the
	    RREQs is as follows.  When two or more RREQs are received with
            the same Orig SeqNo, they were transmitted by OrigNode with the
            same destinations and OF.  When an intermediate node receives two
            RREQs with the same Orig SeqNo but different lists of destinations,
            that means that some intermediate nodes retransmitting the RREQs
            have already deleted themselves from the list of destinations
	    before they retransmitted the RREQ.  Those deleted nodes are
            not to be re-inserted reinserted back into the list of destinations.
          </t>
      </section><!--End of section
		"Step 2: TargNode and Intermediate Router determination"-->
        </section>

      <section anchor="rreq_step3"
			title="Step anchor="rreq_step3">
          <name>Step 3: Intermediate Router RREQ processing"> Processing</name>
          <t>
	    The intermediate router establishes itself as a viable node
	    for a route to OrigNode as follows.  If the H bit is set to 1,
	    for a hop-by-hop route, then the router MUST <bcp14>MUST</bcp14> build or update
	    its upward route entry towards OrigNode, which includes at least
	    the following items: Source Address, RPLInstanceID, Destination
	    Address, Next Hop, Lifetime, and Sequence Number.
<!--  CEP TODO: What is the Destination Address, if not OrigNode?  -->
            The Destination Address and the RPLInstanceID respectively can be
	    learned from the DODAGID and the RPLInstanceID of the RREQ-DIO. RREQ-DIO, respectively.
	    The Source Address is the address used by the router to
	    send data to the Next Hop, i.e., the preferred parent.
	    The lifetime is set according to DODAG configuration (not
	    the L field) and can be extended when the route is actually used.
	    The Sequence Number represents the freshness of the route entry;
	    it is copied from the Orig SeqNo field of the RREQ option. A route
	    entry with the same source and destination address, address and the same
	    RPLInstanceID, but a stale Sequence Number (i.e., incoming sequence
	    number is less than the currently stored Sequence Number of the
	    route entry), MUST <bcp14>MUST</bcp14> be deleted.
<!--  CEP TODO: Need to specify that the information from the existing
		RREQ updates the route entry?  What happens if the existing
		route entry has a newer SeqNo than the RREQ?  Proposal:
		intermediate router updates the RREQ with its newer SeqNo. -->
          </t>
        </section>
	    <!--End of section "Step 3: Intermediate Router RREQ processing"-->

      <section anchor="rreq_step4"
	title="Step anchor="rreq_step4">
          <name>Step 4: Symmetric Route Processing at an Intermediate Router"> Router</name>
          <t>
	    If the S bit of the incoming RREQ-DIO is 0, then the route cannot
	    be symmetric, and the S bit of the RREQ-DIO to be transmitted is
	    set to 0.  Otherwise, the router MUST <bcp14>MUST</bcp14> determine
	    whether the downward direction (i.e., towards the TargNode) direction of the
	    incoming link satisfies the OF.  If so, it does, the S bit of the
	    RREQ-DIO to be transmitted is set to 1.  Otherwise  Otherwise, the S bit of
	    the RREQ-DIO to be transmitted is set to 0.
          </t>
          <t>
	    When a router joins the RREQ-Instance, it also associates within
	    its data structure for the RREQ-Instance the information about
	    whether or not the RREQ-DIO to be transmitted has the S-bit S bit set
	    to 1.  This information
	    associated to RREQ-Instance is known as the S-bit S bit of the
	    RREQ-Instance. It will be used later during the RREP-DIO message
	    processing (see <xref target="asymmetricrrep"/>. target="asymmetricrrep"/>). <!--  for RPLInstance
	    pairing as described in <xref target="forwardRREP"/>.
  CEP TODO: check language about pairing.  -->
          </t>
	<t>
	    Suppose

          <t>Suppose a router has joined the RREQ-Instance, and H=0, the H bit is set
          to 0, and the
	    S-bit S bit of the RREQ-Instance is set to 1.  In this case,
          the router
	    MAY <bcp14>MAY</bcp14> optionally include the Address Vector
          of the symmetric route back to OrigNode as part of the RREQ-Instance
          data.  This is useful if the router later receives an RREP-DIO that
          is paired with the RREQ-Instance.  If the router does NOT include
          the Address Vector, then it has to rely on multicast for the RREP.
          The multicast can impose a substantial performance penalty.
          </t>
      </section><!-- End of section
	    "Step 4: Symmetric Route Processing at an Intermediate Router" -->
        </section>

      <section anchor="rreq_step5"
		title="Step anchor="rreq_step5">
          <name>Step 5: RREQ propagation Propagation at an Intermediate Router"> Router</name>
          <t>
	    If the router is an intermediate router, then it transmits the
	    RREQ-DIO to the multicast group all-AODV-RPL-nodes; if the H bit is
	    set to 0, the intermediate router MUST <bcp14>MUST</bcp14> append
	    the address of its interface receiving the RREQ-DIO into the
	    address vector.  If, in  In addition, if the address of the router's
	    interface transmitting the RREQ-DIO is not the same as the address
            of the interface receiving the RREQ-DIO, the router MUST <bcp14>MUST</bcp14> also
	    append the transmitting interface address into the address vector.
          </t>
      </section><!-- End of section
		"Step 5: RREQ propagation at an Intermediate Router" -->
        </section>

      <section anchor="rreq_step6"
			title="Step anchor="rreq_step6">
          <name>Step 6: RREQ reception Reception at TargNode"> TargNode</name>
          <t>
	    If the router is a TargNode and was already associated with the
	    RREQ-Instance, it takes no further action and does not send an
	    RREP-DIO.  If TargNode is not already associated with the
	    RREQ-Instance, it prepares and transmits a RREP-DIO, possibly
	    after waiting for RREP_WAIT_TIME, as detailed in
	    (<xref target="gen-rrep"/>).
          </t>
      </section><!--End of section "Step 6: RREQ reception at TargNode"-->

    </section><!--End of section "Receiving and Forwarding Route Request"-->
        </section>

    </section>

    <section anchor="gen-rrep"
		title="Generating Route Reply (RREP) anchor="gen-rrep">
        <name>Generating RREP at TargNode"> TargNode</name>
        <t> When a TargNode receives a RREQ message over a link from a
        neighbor Y, TargNode first follows the procedures in <xref
        target="process_rreq"/>.  If the link to Y can be used to transmit
        packets to OrigNode, TargNode generates a RREP according to the steps below.  Otherwise Sections
        <xref format="counter" target="rrepsymmetric"/> and <xref
        target="asymmetricrrep" format="counter"/>.  Otherwise, TargNode
        drops the RREQ and does not generate a RREP.
        </t>
        <t>
	    If the L field is not 0, the TargNode MAY <bcp14>MAY</bcp14> delay transmitting the
	    RREP-DIO for the duration RREP_WAIT_TIME to await a route with a lower
	    Rank.  The value of RREP_WAIT_TIME is set by default to 1/4 of
	    the duration determined by the L field.  For L == 0,
	    RREP_WAIT_TIME is set by default to 0.  Depending upon the
	    application, RREP_WAIT_TIME may be set to other values.
	    Smaller values enable quicker formation for the P2P route.
	    Larger values enable formation of P2P routes with better
	    Rank values.
        </t>
        <t>
	    The address of the OrigNode MUST <bcp14>MUST</bcp14> be
	    encapsulated in the ART Option option and included in this RREP-DIO
	    message along with the SeqNo of TargNode.
        </t>
        <section anchor="rrepsymmetric" title="RREP-DIO anchor="rrepsymmetric">
          <name>RREP-DIO for Symmetric route"> Route</name>
          <t>
	    If the RREQ-Instance corresponding to the RREQ-DIO that arrived
	    at TargNode has the S bit set to 1, there
	    is a symmetric route route, both of whose directions satisfy the
	    Objective Function. Other RREQ-DIOs might later provide better
	    upward routes.  The method of selection between a
	    qualified symmetric route and an asymmetric route that might have
	    better performance is implementation-specific implementation specific and out of scope.

<!--  CEP: Our comment to John Scudder:
		If L is zero,
		RREP_WAIT_TIME should be set to the lifetime of the DODAG.
	The text above effectively has:
		If L is zero, RREP_WAIT_TIME should be set to zero.
	It seems to me that it is better if the node doesn't wait.
  -->
          </t>
          <!-- CEP: The RREP ART has OrigNode address but the SeqNo of TargNode.
		 The SeqNo of OrigNode is not present! -->
	<t>
	    For a symmetric route, the RREP-DIO message is unicast to the next
	    hop according to the Address Vector (H=0) or the route
	    entry (H=1);  the DODAG in RREP-Instance does not need to be
	    built.  The RPLInstanceID in the RREP-Instance is paired as
	    defined in <xref target="instancepairing"/>.  In case  If the H bit
	    is set to 0, the address vector from the RREQ-DIO MUST <bcp14>MUST</bcp14> be
	    included in the RREP-DIO.
          </t>
        </section>   <!-- end section title="RREP-DIO for Symmetric route"  -->

      <section anchor="asymmetricrrep" title="RREP-DIO anchor="asymmetricrrep">
          <name>RREP-DIO for Asymmetric Route"> Route</name>
          <t>
	    When a RREQ-DIO arrives at a TargNode with the S bit set to 0,
	    the TargNode MUST <bcp14>MUST</bcp14> build a DODAG in the RREP-Instance
	    corresponding to the RREQ-DIO rooted at itself, in order to
	    provide OrigNode with a downstream route
	    to the TargNode. The RREP-DIO message is transmitted to
	    multicast group all-AODV-RPL-nodes.
          </t>
        </section>
        <section anchor="instancepairing" title="RPLInstanceID Pairing"> anchor="instancepairing">
          <name>RPLInstanceID Pairing</name>
          <t>
	  Since the RPLInstanceID is assigned locally (i.e., there is no
	  coordination between routers in the assignment of RPLInstanceID), the
	  tuple (OrigNode, TargNode, RPLInstanceID) is needed to uniquely
	  identify a discovered route. It is possible that multiple route
	  discoveries with dissimilar Objective Functions
	  are initiated simultaneously. Thus Thus, between the same pair of OrigNode
	  and TargNode, there can be multiple AODV-RPL route discovery
	  instances.  So that OrigNode and Targnode TargNode can avoid any mismatch,
	  they MUST <bcp14>MUST</bcp14> pair the RREQ-Instance and the RREP-Instance in the same
	  route discovery by using the RPLInstanceID.
          </t>
          <t>
	  When preparing the RREP-DIO, a TargNode could find the RPLInstanceID
	  candidate for the RREP-Instance is already occupied by another RPL
	  Instance from an earlier route discovery operation which that is still
	  active.  This unlikely case might happen if two distinct OrigNodes
	  need routes to the same TargNode, and they happen to use the same
	  RPLInstanceID for RREQ-Instance. In such cases, the
	  RPLInstanceID of an already active RREP-Instance MUST NOT <bcp14>MUST NOT</bcp14> be used
	  again for assigning RPLInstanceID for the later RREP-Instance.
	  If the same RPLInstanceID were re-used reused for two
	  distinct DODAGs originated with the same DODAGID (TargNode address),
	  intermediate routers could not distinguish between these
	  DODAGs (and their associated Objective Functions). Instead, the
	  RPLInstanceID MUST <bcp14>MUST</bcp14> be replaced by another value so that the two
	  RREP-instances
	  RREP-Instances can be distinguished.  In the RREP-DIO option, the
	  Delta field of the RREP-DIO message (<xref target="figRREP"/>)
	  indicates the value that TargNode adds to the
	  RPLInstanceID in the RREQ-DIO that it received, to obtain the value
	  of the RPLInstanceID it uses in the RREP-DIO message.
	  0 indicates that the RREQ-InstanceID has the same value as
          the RPLInstanceID of the RREP message.
<!--  How many bits is the RPLInstanceID??  -->
	  When the new RPLInstanceID after incrementation exceeds 255, it
	  rolls over starting at 0. For example, if the RREQ-InstanceID
	  is 252, 252 and incremented by 6, the new RPLInstanceID will be 2.
	  Related operations can be found in <xref target="forwardRREP"/>.
	  RPLInstanceID collisions do not occur across RREQ-DIOs; the
	  DODAGID equals the OrigNode address and is sufficient to
	  disambiguate between DODAGs.
<!--  TODO: Could say something about only 6 bits needed for Delta field. -->
          </t>
        </section> <!-- end section title="RREP-DIO for Asymmetric Route"  -->
    </section> <!-- End of section "Generating Route Reply at TargNode" -->

    <section anchor="forwardRREP" title="Receiving anchor="forwardRREP">
        <name>Receiving and Forwarding Route Reply"> RREP</name>
        <t> Upon receiving a RREP-DIO, a router which that already belongs to the
	    RREP-Instance SHOULD <bcp14>SHOULD</bcp14> drop the RREP-DIO.  Otherwise  Otherwise, the router
 	    performs the steps in the following subsections.
        </t>
        <section anchor="rrep_step1"
			title="Step anchor="rrep_step1">
          <name>Step 1: Receiving and Evaluation"> Evaluation</name>
          <t>
	    If the Objective Function is not satisfied, the router MUST NOT <bcp14>MUST NOT</bcp14>
	    join the DODAG; the router MUST <bcp14>MUST</bcp14> discard the RREP-DIO, RREP-DIO and does not
	    execute the remaining steps in this section.  An Intermediate
	    Router MUST <bcp14>MUST</bcp14> discard a RREP if one of its addresses is present
	    in the Address Vector, Vector and does not execute the remaining steps in
	    this section.
          </t>
          <t>
	    If the S bit of the associated RREQ-Instance is set to 1,
	    the router MUST <bcp14>MUST</bcp14> proceed to <xref target="rrep_step2"/>.
          </t>

          <t> If the S-bit S bit of the RREQ-Instance is set to 0, the router MUST <bcp14>MUST</bcp14>
          determine whether the downward direction of the link (towards the
          TargNode) over which the RREP-DIO is received satisfies the
          Objective Function, Function and whether the router's Rank would not exceed
          the RankLimit.  If so, these are true, the router joins the DODAG of the
          RREP-Instance.  The router that transmitted the received RREP-DIO is
          selected as the preferred parent.  Afterwards, other RREP-DIO
          messages can be received; AODV-RPL does not specify any action to be
          taken in such cases.
<!--  CEP: delete this as suggested by Alvaro.
	                             How to maintain the parent set, select
	    the preferred parent, and update the router's Rank obeys the
	    core RPL and the OFs defined in ROLL WG.
  -->
          </t>
      </section><!--End of section "Step 1: Receiving and Evaluation"-->
        </section>

      <section anchor="rrep_step2"
			title="Step anchor="rrep_step2">
          <name>Step 2: OrigNode or Intermediate Router">
	<t>
	    The Router</name>
          <t>The router updates its stored value of the TargNode's sequence
	  number according to the value provided in the ART option.  The router next
	  checks if one of its addresses is included in the ART Option. option.  If so, it is
	  included, this router is the OrigNode of the route discovery. Otherwise, it is
   an intermediate router. </t>
      </section><!--End of section "Step 2: OrigNode or Intermediate Router"--> router.</t>
        </section>

      <section anchor="rrep_step3"
			title="Step anchor="rrep_step3">
          <name>Step 3: Build Route to TargNode"> TargNode</name>
          <t>
	    If the H bit is set to 1, then the router (OrigNode or
	    intermediate) MUST <bcp14>MUST</bcp14> build a downward route entry towards TargNode
	    which
	    that includes at least the following items: OrigNode Address,
	    RPLInstanceID, TargNode Address as destination, Next Hop, Lifetime Lifetime,
	    and Sequence Number.  For a symmetric route, the Next Hop in the
	    route entry is the router from which the RREP-DIO is received.  For
	    an asymmetric route, the Next Hop is the preferred parent in the
	    DODAG of RREP-Instance.  The RPLInstanceID in the route entry MUST <bcp14>MUST</bcp14>
	    be the RREQ-InstanceID (i.e., after subtracting the Delta field
	    value from the value of the RPLInstanceID).  The source address is
	    learned from the ART Option, option, and
	    the destination address is learned from the DODAGID.  The lifetime
	    is set according to DODAG configuration (i.e., not the L field)
	    and can be extended when the route is actually used. The sequence
	    number represents the freshness of the route entry, entry and is copied
	    from the Dest SeqNo field of the ART option of the RREP-DIO.
	    A route entry with the same source and destination address, address and the same
	    RPLInstanceID, but a stale sequence number MUST number, <bcp14>MUST</bcp14> be deleted.
          </t>
      </section><!--End of section "Step 3: Build Route to TargNode"-->
        </section>

      <section anchor="rrep_step4"
			title="Step anchor="rrep_step4">
          <name>Step 4: RREP Propagation"> Propagation</name>
          <t>
	    If the receiver is the OrigNode, it can start transmitting the
	    application data to TargNode along the path as provided in
	    RREP-Instance, and processing for the RREP-DIO is
	    complete. Otherwise, the RREP will be propagated towards OrigNode.
	    If H=0, the intermediate router
	    MUST <bcp14>MUST</bcp14> include the
	    address of the interface receiving the RREP-DIO into the address
	    vector.  If H=1, according to the previous step step, the intermediate
	    router has set up a route entry for TargNode. If the intermediate
	    router has a route to OrigNode, it uses that route to unicast the
	    RREP-DIO to OrigNode.  Otherwise, in the case of a symmetric route,
	    the RREP-DIO message is unicast to the Next Hop according to the
	    address vector in the RREP-DIO (H=0) or the local route entry
	    (H=1).  Otherwise, in the case of an asymmetric route, the
	    intermediate router transmits the RREP-DIO to multicast group
	    all-AODV-RPL-nodes.  The RPLInstanceID in the transmitted RREP-DIO
	    is the same as the value in the received RREP-DIO.
          </t>
      </section><!--End of section "Step 4: RREP Propagation"-->
        </section>
<!-- CEP: Alternatively, could forward if better Rank value.
     	  Or maybe only forward for symmetric routes?  -->
    </section> <!--End of section "Receiving and Forwarding Route Reply"-->
</section> <!-- End of section "AODV-RPL operation" -->

<section anchor="G-RREP" title="Gratuitous RREP"> anchor="G-RREP">
      <name>Gratuitous RREP</name>
      <t>
	In some cases, an Intermediate router that receives a RREQ-DIO message
	MAY
	<bcp14>MAY</bcp14> unicast a "Gratuitous" Gratuitous RREP-DIO (G-RREP-DIO) message back to OrigNode before
	continuing the transmission of the RREQ-DIO towards TargNode.  The Gratuitous RREP
	(G-RREP) allows the OrigNode to start transmitting
	data to TargNode sooner.  The G bit of the RREP option is provided to
	distinguish the Gratuitous RREP-DIO G-RREP-DIO (G=1) sent by the Intermediate
	router from the RREP-DIO sent by TargNode (G=0).
      </t>
      <t>
	The gratuitous RREP-DIO MAY G-RREP-DIO <bcp14>MAY</bcp14> be sent out when the Intermediate router
	receives a RREQ-DIO for a TargNode, TargNode and the router has a pair of
	downward and upward routes to the TargNode which that also satisfy the
	Objective Function and for which the destination sequence number is
	at least as large as the sequence number in the RREQ-DIO message.
	After unicasting the Gratuitous RREP G-RREP to the OrigNode, the Intermediate
	router then unicasts the RREQ towards TargNode, so that TargNode will
	have the advertised route towards OrigNode along with the
	RREQ-InstanceID for the RREQ-Instance.  An upstream intermediate
	router that receives such a G-RREP MUST <bcp14>MUST</bcp14> also generate a G-RREP and
        send it further upstream towards OrigNode.
      </t>
      <t>
	In case of source routing, the intermediate router MUST <bcp14>MUST</bcp14> include the
	address vector between the OrigNode and itself in the
	Gratuitous RREP.
	G-RREP.  It also includes the address vector in the unicast
	RREQ-DIO towards TargNode.  Upon reception of the unicast RREQ-DIO,
	the TargNode will have a
	route address vector from itself to the OrigNode. Then Then, the
	router MUST <bcp14>MUST</bcp14> include the address vector from the TargNode to the
	router itself in the gratuitous RREP-DIO G-RREP-DIO to be transmitted.
      </t>
      <t>
	For establishing hop-by-hop routes, the intermediate router MUST <bcp14>MUST</bcp14>
	unicast the received RREQ-DIO to the Next Hop on the route. The Next
	Hop router along the route MUST <bcp14>MUST</bcp14> build new route entries with the related
	RPLInstanceID and DODAGID in the downward direction. This process
	repeats at each node until the RREQ-DIO arrives at the TargNode.
	Then
	Then, the TargNode and each router along the path towards OrigNode
	MUST
	<bcp14>MUST</bcp14> unicast the RREP-DIO hop-by-hop towards OrigNode
	as specified in <xref target="gen-rrep"/>.
      </t>
    </section>	<!-- End of section "Gratuitous RREP" -->

<section anchor="trickle" title="Operation anchor="trickle">
      <name>Operation of Trickle Timer"> Timer</name>
      <t>
<!--  Anand: No need to borrow text from RFC6997.
      We can reuse trickle timer and DIO transmission procedure in RFC6550.
-->
	RREQ-Instance/RREP-Instance multicast uses trickle Trickle timer operations
	<xref target="RFC6206"/> to control RREQ-DIO and
	RREP-DIO transmissions.  The Trickle control of these DIO transmissions
	follows the procedures described in the Section 8.3 of
	<xref target="RFC6550"/> target="RFC6550" sectionFormat="of" section="8.3"/> entitled "DIO Transmission".  If the route is
	symmetric, the RREP DIO RREP-DIO does not need the Trickle timer mechanism.

      </t>
    </section>	<!-- End of section "Operation of Trickle Timer" -->

<section anchor="iana" title="IANA Considerations">
    <t>
	Note to RFC editor:
    </t>
    <t>
	The sentence "The parenthesized numbers are only suggestions."
	is to be removed prior publication.
    </t>
    <t>
	A Subregistry in this section refers to a named sub-registry of the
	"Routing Protocol for Low Power and Lossy Networks (RPL)" registry.
    </t> anchor="iana">
      <name>IANA Considerations</name>

      <t>
AODV-RPL uses the "P2P Route Discovery Mode of Operation" (MOP == 4) 4), with new Options
options as specified in this document.  Please cite AODV-RPL
	and this This document has been added as one an
additional reference for "P2P Route Discovery Mode of Operation" in the protocols using MOP 4. "Mode
of Operation" registry within the "Routing Protocol for Low Power and Lossy
Networks (RPL)" registry group.
      </t>
      <t>
	IANA is asked to assign has assigned the three new AODV-RPL options "RREQ", "RREP" and
	"ART", as described in <xref
	target="ianaOpts"/> from in the "RPL Control Message Options" Subregistry.  The parenthesized numbers are only
	suggestions.
        <figure anchor="ianaOpts" title="AODV-RPL Options">
        	<artwork align="center"><![CDATA[
 +-------------+------------------------+---------------+
 |    Value    |        Meaning         |   Reference   |
 +-------------+------------------------+---------------+
 | TBD2 (0x0B) |      RREQ Option       | This document |
 +-------------+------------------------+---------------+
 | TBD3 (0x0C) |      RREP Option       | This document |
 +-------------+------------------------+---------------+
 | TBD4 (0x0D) |       ART Option       | This document |
 +-------------+------------------------+---------------+
        ]]></artwork> </figure></t> registry
	within the "Routing Protocol for Low Power and Lossy Networks (RPL)"
	registry group.
      </t>
      <table anchor="ianaOpts">
        <name>AODV-RPL Options</name>
	<thead>
	  <tr>
	    <th>Value</th>
	    <th>Meaning</th>
            <th>Reference</th>
	  </tr>
	</thead>
	<tbody>
	  <tr>
	    <td>0x0B</td>
	    <td>RREQ</td>
	    <td>RFC 9854</td>
	  </tr>
	  <tr>
	    <td>0x0C</td>
	    <td>RREP</td>
	    <td>RFC 9854</td>
	  </tr>
	  <tr>
	    <td>0x0D</td>
	    <td>ART</td>
	    <td>RFC 9854</td>
	  </tr>
	</tbody>
      </table>

      <t> <!-- To resolve Roman Danyliw's comment 2/17/2025, 9:52 AM -->
        IANA is requested to allocate a new has allocated the permanent multicast address with
        link-local scope called all-AODV-RPL-nodes in <xref target="ianaMultiAddress"/> for nodes implementing
        this specification from specification. This allocation has been made in the "Local Network Control Block
        (224.0.0.0 - 224.0.0.255 (224.0.0/24))" registry in within the
        "IPv4 Multicast Address Space Registry" registry group.
      </t>
</section>	<!-- End of section "IANA Considerations" -->

<section anchor="sec" title="Security Considerations">
<t>
   The

<table anchor="ianaMultiAddress">
  <name>Permanent Multicast Address with Link-Local Scope</name>
  <thead>
    <tr>
      <th>Address(es)</th>
      <th>Description</th>
      <th>References</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td>224.0.0.69</td>
      <td>all-AODV-RPL-nodes</td>
      <td>RFC 9854</td>
    </tr>
  </tbody>
</table>

    </section>

<section anchor="sec">
      <name>Security Considerations</name>
      <t>The security considerations for the operation of AODV-RPL are similar
      to those for the operation of RPL (as described in Section 19 <xref
      target="RFC6550" sectionFormat="bare" section="19"/> of the RPL
      specification <xref target="RFC6550"/>).  Sections 6.1 <xref
      target="RFC6550" sectionFormat="bare" section="6.1"/> and 10 <xref
      target="RFC6550" sectionFormat="bare" section="10"/> of <xref
      target="RFC6550"/> describe RPL's optional security framework, which
      AODV-RPL relies on to provide data confidentiality, authentication,
      replay protection, and delay protection services.  Additional analysis
      for the security threats to RPL can be found in <xref target="RFC7416"/>.
</t>

<t>
   A
      target="RFC7416"/>.</t>
      <t>A router can join a temporary DAG created for a secure AODV-RPL route
      discovery only if it can support the security configuration in use (see Section 6.1 of
      <xref target="RFC6550"/>), target="RFC6550" sectionFormat="of" section="6.1"/>), which also
      specifies the key in use.  It does not matter whether the key is
      preinstalled or dynamically acquired.  The router must have the key in
      use before it can join the DAG being created for secure route discovery.
</t>

<t>
    If
      discovery.</t>

      <t>If a rogue router knows the key for the security configuration in
      use, it can join the secure AODV-RPL route discovery and cause various
      types of damage.  Such a rogue router could advertise false information
      in its DIOs in order to include itself in the discovered route(s).  It
      could generate bogus RREQ-DIO, RREQ-DIO and RREP-DIO messages carrying bad
      routes or maliciously modify genuine RREP-DIO messages it receives.  A
      rogue router acting as the OrigNode could launch denial-of-service
      attacks against the LLN deployment by initiating fake AODV-RPL route
      discoveries.  When rogue routers might be present, RPL's preinstalled
      mode of operation, where the key to use for route discovery is
      preinstalled, SHOULD <bcp14>SHOULD</bcp14> be used.
<!--	CEP: commented out upon request by Alvaro Retana.
	....... but maybe something should be said without making a mandate.
    If a future
    IETF document specifies the authenticated mode of operation as
    described in <xref target="RFC6550"/>, then future AODV-RPL
    implementations SHOULD use the authenticated mode of operation.
  -->
      </t>
      <t>
    When a RREQ-DIO message uses the source routing option by setting the H
    bit to 0, a rogue router may populate the Address Vector field with a set
    of addresses that may result in the RREP-DIO traveling in a routing loop.
      </t>
      <t>
	If a rogue router is able to forge a gratuitous RREP, G-RREP,
	it could mount denial-of-service attacks.
      </t>
    </section>	<!-- End of section "Security Considerations" -->

    <section title="Acknowledgements">
    <t>
	The authors thank Pascal Thubert, Rahul Jadhav, and Lijo Thomas
        for their support and valuable inputs.
	The authors specially thank Lavanya H.M for implementing AODV-RPl in
	Contiki and conducting extensive simulation studies.
    </t>

    <t>
	The authors would like to acknowledge the review, feedback and
	comments from the following people, in alphabetical order:
		Roman Danyliw,
		Lars Eggert,
		Benjamin Kaduk,
		Tero Kivinen,
		Erik Kline,
		Murray Kucherawy,
		Warren Kumari,
		Francesca Palombini,
		Alvaro Retana,
		Ines Robles,
		John Scudder,
		Meral Shirazipour,
		Peter Van der Stok,
		Eric Vyncke,
		and Robert Wilton.
    </t>
    </section>

  </middle>
  <back>		<!--  *****BACK MATTER ***** -->
    <!-- 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 title="Normative References">

<references>
      <name>References</name>
      <references>
        <name>Normative References</name>
        <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.2119.xml"/>
<!--
	<?rfc include='reference.RFC.2119'?>
	<?rfc include='reference.RFC.5095'?>
	<?rfc include='reference.RFC.6206'?>
	<xi:include href="http://xml2rfc.tools.ietf.org/public/rfc/
	  bibxml/reference.RFC.6206"/>
	<xi:include href="http://bib.ietf.org/public/rfc/
	  bibxml/reference.RFC.6206"/>
	<?rfc xi:include href="http://bib.ietf.org/public/rfc/
	  bibxml/reference.RFC.6206"/>
  -->
	<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.6206.xml"/>
        <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.6550.xml"/>
        <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.6551.xml"/>
        <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8174.xml"/>
      </references>

<references title="Informative References">
      <references>
        <name>Informative References</name>
        <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.3561.xml"/>
        <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.6687.xml"/>
        <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.6997.xml"/>
        <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.6998.xml"/>
        <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.7416.xml"/>
        <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.7548.xml"/>
        <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.7276.xml"/>
        <xi:include
         href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.7991.xml"/>
	<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.9010.xml"/>
        <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.9030.xml"/>

        <!-- Co-iOAM paper Reference
	R. Ballamajalu, S. V. R. Anand and M. Hegde, "Co-iOAM: In-situ
	telemetry metadata transport for resource constrained networks
	within IETF standards framework," 2018 10th International Conference
	on Communication Systems & Networks (COMSNETS), Bengaluru,
	2018, pp. 573-576.  doi: 10.1109/COMSNETS.2018.8328276
-->

<reference anchor="co-ioam">
          <front>
            <title>
                Co-iOAM: In-situ Telemetry Metadata Transport for
                Resource Constrained Networks within IETF Standards Framework
            </title>
            <author fullname="Rashmi Ballamajalu"
				initials="" surname="Rashmi Ballamajalu"> initials="R." surname="Ballamajalu">
              <organization> </organization>
              <address>
            </address>
            </author>
            <author fullname="S.V.R. Anand" initials="S.V.R." surname="Anand">
              <organization> </organization>
              <address>
            </address>
            </author>
            <author fullname="Malati Hegde" initials="" surname="Malati Hegde"> initials="M." surname="Hegde">
              <organization> </organization>
              <address>
            </address>
            </author>
            <date month="Jan" year="2018" /> year="2018"/>
          </front>
	<seriesInfo
		name="2018
          <refcontent>2018 10th International Conference on Communication Systems &amp; Networks (COMSNETS)"
		value="pp.573-576"/> (COMSNETS), pp. 573-576</refcontent>
        </reference>

        <reference anchor="aodv-tot">
          <!--  DOI: 10.1109/MCSA.1999.749281  -->
        <front>
            <title>
                Ad-hoc On-demand Distance Vector Routing
            </title>
            <author fullname="C.E. Perkins" initials="C.E." surname="Perkins">
              <organization> Advanced Development Group, Sun MicroSystems
			Laboratories, Inc., Menlo Park, CA, USA </organization>
              <address>
            </address>
            </author>
            <author fullname="E.M. Royer" initials="E.M." surname="Royer">
              <organization> Advanced Development Group, Sun MicroSystems
			Laboratories, Inc., Menlo Park, CA, USA </organization>
              <address>
            </address>
            </author>
            <date month="Feb" year="1999" /> year="1999"/>
          </front>
	<seriesInfo name="Proceedings
          <refcontent>Proceedings WMCSA'99. Second IEEE Workshop on Mobile Computing Systems and Applications" value="" /> Applications, pp. 90-100</refcontent>
        </reference>

<!-- [cooja] -->
        <reference anchor="cooja" target="https://github.com/contiki-os/contiki/tree/master/tools/cooja">
          <front>
            <title> Cooja Simulator for Wireless Sensor Networks
                   (Contiki/Cooja Version 2.7)
            </title>
	    <author fullname="Contiki/Cooja contributors" initials=""
		surname="Contiki/Cooja contributors">
		<organization> </organization>
		<address> </address>
	    </author>
            <author/>
            <date month="Nov" year="2013"/>
          </front>
          <refcontent>commit 7635906</refcontent>
        </reference>

<!-- [contiki] -->
        <reference anchor="contiki" target="https://github.com/contiki-os/contiki">
          <front>
            <title> The Contiki Open Source OS for the Internet of Things
                    (Contiki Version 2.7)
            </title>
	    <author fullname="Contiki contributors" initials=""
		surname="Contiki contributors">
		<organization> </organization>
		<address> </address>
	    </author>
            <author/>
            <date month="Nov" year="2013" /> year="2013"/>
          </front>
          <refcontent>commit 7635906</refcontent>
        </reference>

<!-- [Contiki-ng] -->
        <reference anchor="Contiki-ng" target="https://github.com/contiki-ng/contiki-ng">
          <front>
            <title> Contiki-NG: The OS for Next Generation IoT Devices
                    (Contiki-NG Version 4.6)
            </title>
	    <author fullname="Contiki-NG contributors" initials=""
		surname="Contiki-NG contributors">
		<organization> </organization>
		<address> </address>
	    </author>
            <author/>
            <date month="Dec" year="2020" /> year="2020"/>
          </front>
          <refcontent>commit 3b0bc6a</refcontent>
        </reference>

<!-- [Link_Asymmetry] -->
        <reference anchor="Link_Asymmetry" target="https://doi.org/10.1145/1689239.1689242">
          <front>
            <title>
		On Link Asymmetry and One-way Estimation in Wireless
		Sensor Networks
            </title>
            <author fullname="Lifeng Sang"
				initials="" surname="Lifeng Sang"> initials="L." surname="Sang">
              <organization> </organization>
              <address>
            </address>
            </author>
            <author fullname="Anish Arora" initials="" surname="Anish Arora"> initials="A." surname="Arora">
              <organization> </organization>
              <address>
            </address>
            </author>
            <author fullname="Hongwei Zhang" initials="" surname="Hongwei Zhang"> initials="H." surname="Zhang">
              <organization> </organization>
              <address>
            </address>
            </author>
            <date month="Feb" year="2010" /> month="March" year="2010"/>
          </front>
	<seriesInfo
		name="ACM
          <refcontent>ACM Transactions on Sensor Networks, Volume 6 Issue 2"
		value="pp.1-25"/> vol. 6, no. 2, pp. 1-25</refcontent>
          <seriesInfo name="DOI" value="10.1145/1689239.1689242"/>
        </reference>

<!-- [low-power-wireless] -->
        <reference anchor="low-power-wireless" target="https://doi.org/10.1145/1689239.1689246">
          <front>
            <title>
		An empirical study of low-power wireless
            </title>
            <author fullname="Kannan Srinivasan"
				initials="" surname="Kannan Srinivasan"> initials="K." surname="Srinivasan">
              <organization> </organization>
              <address>
            </address>
            </author>
            <author fullname="Prabal Dutta" initials="" surname="Prabal Dutta"> initials="P." surname="Dutta">
              <organization> </organization>
              <address>
            </address>
            </author>
            <author fullname="Arsalan Tavakoli"
					initials="" surname="Arsalan Tavakoli"> initials="A." surname="Tavakoli">
              <organization> </organization>
              <address>
            </address>
            </author>
            <author fullname="Philip Levis" initials="" surname="Philip Levis"> initials="P" surname="Levis">
              <organization> </organization>
              <address>
            </address>
            </author>
            <date month="Feb" year="2010" /> month="March" year="2010"/>
          </front>
	<seriesInfo
		name="ACM
          <refcontent>ACM Transactions on Sensor Networks"
		value="(Volume 6 Issue 2 pp.1-49)"/> Networks, vol. 6, no. 2, pp. 1-49</refcontent>
          <seriesInfo name="DOI" value="10.1145/1689239.1689246"/>
        </reference>

        <reference anchor="empirical-study">
          <front>
            <title>
		An empirical study of asymmetry in low-power wireless links
            </title>
            <author fullname="Prasant Misra"
				initials="" surname="Prasant Misra"> initials="P." surname="Misra">
              <organization> </organization>
              <address>
            </address>
            </author>
            <author fullname="Nadeem Ahmed" initials="" surname="Nadeem Ahmed"> initials="N." surname="Ahmed">
              <organization> </organization>
              <address>
            </address>
            </author>
            <author fullname="Sanjay Jha" initials="" surname="Sanjay Jha"> initials="S." surname="Jha">
              <organization> </organization>
              <address>
            </address>
            </author>
            <date month="Jul" year="2012" /> month="July" year="2012"/>
          </front>
	<seriesInfo
		name="IEEE
          <refcontent>IEEE Communications Magazine"
		value="(Volume: Magazine, vol. 50, Issue: 7)"/> no. 7, pp. 137-146</refcontent>
        </reference>
      </references>
    </references>

    <section anchor="appendix-a"
	title="Example: anchor="appendix-a">
      <name>Example: Using ETX/RSSI Values to determine value Determine Value of S bit">
    <t> The Bit</name>
      <t>The combination of the downstream Received Signal Strength Indication(downstream) Indicator
      (RSSI) and the upstream Expected Number of Transmissions(upstream) Transmission Count (ETX) has been
      tested to determine whether a link is symmetric or asymmetric at
      intermediate routers. We present two methods to obtain an ETX value from
      RSSI measurement.

    </t> measurement.</t>

      <dl newline="false" spacing="normal">
        <dt>Method 1:</dt>
        <dd>
          <t>
	<list style="hanging">
	<t hangText="Method 1:">
	    In the first method, we constructed a table measuring RSSI vs versus
	    ETX using the Cooja simulation <xref target="cooja"/> setup in the
	    Contiki OS environment<xref environment <xref target="contiki"/>. We used
	    Contiki-2.7 running the 6LoWPAN/RPL protocol stack for the
	    simulations.  For approximating the number of packet drops based
	    on the RSSI values, we implemented simple logic that drops
	    transmitted packets with certain pre-defined predefined ratios before handing
	    over the packets to the receiver. The packet drop ratio is
	    implemented as a table lookup of RSSI ranges mapping to different
	    packet drop ratios with lower RSSI ranges resulting in higher
	    values. While this table has been defined for the purpose of
	    capturing the overall link behavior, in general, it is highly
	    recommended to conduct physical radio measurement experiments,
            in general. experiments.  By
	    keeping the receiving node at different distances, we let the
	    packets experience different packet drops as per the described
	    method. The ETX value computation is done by another module which that
	    is part of RPL Objective Function implementation.  Since the ETX
	    value is reflective of the extent of packet drops, it allowed us
	    to prepare a useful table correlating ETX vs and RSSI table. values (see
	    Table 3).  ETX versus and RSSI values obtained in this way may be used as
	    explained below: below:</t>

          <figure anchor="commlink"
		title="Communication link anchor="commlink">
            <name>Communication Link from Source to Destination">
	<artwork>
	<![CDATA[Source Destination</name>
            <artwork><![CDATA[
Source -------> NodeA -------> NodeB -----> Destination]]>
	</artwork> Destination]]></artwork>
          </figure>
        </t>
        </list>
        </t>

	<texttable anchor="table_ETX_RSSI"
		title="Selection

      <table anchor="table_ETX_RSSI">
        <name>Selection of S bit based Bit Based on Expected ETX value">
	    <ttcol align='center'>RSSI Value</name>
        <thead>
          <tr>
            <th align="center">RSSI at NodeA for NodeB</ttcol>
	    <ttcol align='center'>Expected NodeB</th>
            <th align="center">Expected ETX at NodeA for
                                                      NodeB->NodeA</ttcol>

	    <c>&gt; -60</c>
	    <c>150</c>

	    <c>-70 to -60</c>
	    <c>192</c>

	    <c>-80 to -70</c>
	    <c>226</c>

	    <c>-90 to -80</c>
	    <c>662</c>

	    <c>-100 to -90</c>
	    <c>3840</c>
	</texttable>

        <t>
	<list style="hanging">
	<t hangText="Method 2:">One NodeB-&gt;NodeA</th>
          </tr>
        </thead>
        <tbody>
          <tr>
            <td align="center">&gt; -60</td>
            <td align="center">150</td>
          </tr>
          <tr>
            <td align="center">-70 to -60</td>
            <td align="center">192</td>
          </tr>
          <tr>
            <td align="center">-80 to -70</td>
            <td align="center">226</td>
          </tr>
          <tr>
            <td align="center">-90 to -80</td>
            <td align="center">662</td>
          </tr>
          <tr>
            <td align="center">-100 to -90</td>
            <td align="center">3840</td>
          </tr>
        </tbody>
      </table>
      </dd>

        <dt>Method 2:</dt>
        <dd>One could also make use of the function
	    guess_etx_from_rssi() defined in the 6LoWPAN/RPL protocol stack
            of Contiki-ng OS <xref target="Contiki-ng"/> to obtain RSSI-ETX
	    mapping. This function outputs an ETX value ranging between 128
	    and 3840 for -60 &lt;= rssi &lt;= -89. The function description
	    is beyond the scope of this document.
        </t>
        </list>
        </t>
        </dd>
      </dl>
      <t> We tested the operations in this specification by making the
      following experiment, using the above parameters.  In our experiment, a
      communication link is considered as symmetric if the ETX value of
	NodeA->NodeB
      NodeA-&gt;NodeB and NodeB->NodeA NodeB-&gt;NodeA (see <xref target="commlink"/>) are
      within, say, a 1:3 ratio.  This ratio should be understood as
      determining the link's symmetric/asymmetric nature.  NodeA can typically
      know the ETX value in the direction of NodeA -> NodeB NodeA-&gt;NodeB, but it has no
      direct way of knowing the value of ETX from NodeB->NodeA. NodeB-&gt;NodeA.  Using
      physical testbed experiments and realistic wireless channel propagation
      models, one can determine a relationship between RSSI and ETX
      representable as an expression or a mapping table.  Such a
	relationship relationship,
      in turn turn, can be used to estimate the ETX value at nodeA NodeA for link NodeB--->NodeA
      NodeB-&gt;NodeA from the received RSSI from NodeB.  Whenever
	nodeA NodeA
      determines that the link towards the nodeB NodeB is bi-directional
	asymmetric bidirectional asymmetric,
      then the S bit is set to 0.  Afterwards, the link from NodeA to
      Destination remains designated as asymmetric asymmetric, and the S bit remains set
      to 0.
      </t>
    <t>
	Determination
      <t>Determination of asymmetry versus bidirectionality remains a topic
	of lively discussion in the IETF.
	<!-- https://github.com/roll-wg/dao-projection/issues/11  -->
      </t>
    </section>
    <section anchor="Examples" title="Some anchor="Examples">
      <name>Some Example AODV-RPL Message Flows"> Flows</name>
      <t>
	This appendix provides some example message flows showing
        RREQ and RREP establishing symmetric and asymmetric routes.
	Also, examples for the use of RREP_WAIT and G-RREP are included.
        In the examples, router (O) is to be understood as performing
        the role of OrigNode.  Router (T) is to be understood as performing
	the role of TargNode.  Routers (R) are intermediate routers that
        are performing AODV-RPL functions in order to discover one or more
        suitable routes between (O) and (T).
      </t>
      <section anchor="Asymmetric-examples"
    title="Example control message flows anchor="Asymmetric-examples">
        <name>Example Control Message Flows in symmetric Symmetric and asymmetric networks"> Asymmetric Networks</name>
        <t>
	In the following diagram, RREQ messages are multicast from router (O)
	in order to discover routes to and from router (T).  The RREQ control
        messages flow outward from (O).  Each router along the way establishes
        a single RREQ-Instance identified by RREQ-InstanceID even if multiple
        RREQs are received with the same RREQ-InstanceID.  In the top half of
        the diagram, the routers are able to offer a symmetric route at each
        hop of the path from (O) to (T).  When (T) receives a RREQ, it is
        then able to transmit data packets to (O).  Router (T) then prepares
        to send a RREP along the symmetric path that would enable router (O)
        to send packets to router (T).

        </t>
        <figure anchor="figSymm-RREQ_flow"
    title="AODV-RPL anchor="figSymm-RREQ_flow">
          <name>AODV-RPL RREQ message flow example when symmetric path available"> Message Flow Example When Symmetric Path Available</name>
          <artwork align="center"><![CDATA[
             (R) ---RREQ(S=1)--->(R) ---RREQ(S=1)--->(R)
              ^                                       |
              |                                       |
         RREQ(S=1)                                RREQ(S=1)
              |                                       |
              |                                       v
             (O) --------->(R) --------->(R)-------->(T)
             / \    RREQ         RREQ         RREQ    ^
             |  \    (S=1)      (S=0)         (S=0)   |
             |   \                                   /
        RREQ |    \ RREQ (S=1)                    RREQ (S=0)
       (S=0) |     \                               /
             v      \              RREQ (S=0)     /
            (R) ---->(R)------>(R)----.....--->(R)

    ]]></artwork> ---->(R)------>(R)----.....--->(R)]]></artwork>
        </figure>

    </t>
        <t>
	In the following diagram diagram, which results from the above RREQ message
        transmission, a symmetric route is available from (T) to router (O)
        via the routers in the top half of the diagram.  RREP messages are
        sent via unicast along the symmetric route.  Since the RREP message
        is transmitted via unicast, no RREP messages are sent by router (T)
        to the routers in the bottom half of the diagram.

        </t>
        <figure anchor="figSymm-RREP_flow"
    title="AODV-RPL anchor="figSymm-RREP_flow">
          <name>AODV-RPL RREP message flow example when symmetric path available"> Message Flow Example When Symmetric Path Available</name>
          <artwork align="center"><![CDATA[
             (R)<------RREP----- (R)<------RREP----- (R)
              |                                       ^
              |                                       |
             RREP                                    RREP
              |                                       |
              v                                       |
             (O) ----------(R) ----------(R) --------(T)
             / \                                      |
             |  \                                     |
             |   \     (no RREP messages sent)       /
             |    \                                 /
             |     \                               /
             |      \                             /
            (R) -----(R)-------(R)----.....----(R)

    ]]></artwork> -----(R)-------(R)----.....----(R)]]></artwork>
        </figure>
    </t>
        <t>
	In the following diagram, RREQ messages are multicast from router (O)
        in order to discover routes to and from router (T) as before. As shown,
        no symmetric route is available from (O) to (T).

        </t>
        <figure anchor="figAsymm-RREQ_flow"
    title="AODV-RPL anchor="figAsymm-RREQ_flow">
          <name>AODV-RPL RREQ message flow when symmetric path unavailable"> Message Flow When Symmetric Path Unavailable</name>
          <artwork align="center"><![CDATA[
             (R) ---RREQ(S=0)--->(R) ---RREQ(S=0)--->(R)
              ^                                       |
              |                                       |
          RREQ(S=1)                                RREQ(S=0)
              |                                       |
              |                                       v
             (O) --------->(R) --------->(R)-------->(T)
             ^ \    RREQ         RREQ      RREQ      |  \
             |  \    (S=1)        (S=0)       (S=0)  |   |
             |   \                                   /   |
             |  RREQ (S=1)              RREQ (S=0)  /   (R)
             |     \                               /     |
             |      \               RREQ (S=0)    /     /
            (R) ---->(R)------>(R)----.....----->(R)---

    ]]></artwork> ---->(R)------>(R)----.....----->(R)---]]></artwork>
        </figure>
    </t>
        <t>
	Upon receiving the RREQ in <xref target="figAsymm-RREQ_flow"/>,
	Router
	router (T) then prepares to send a RREP that would enable router (O)
        to send packets to router (T). In <xref target="figAsymm-RREQ_flow"/>,
	since no symmetric route is available from (T) to router (O),
        RREP messages are sent via multicast to all neighboring routers.

        </t>
        <figure anchor="figAsymm-RREP_flow"
        title="AODV-RPL anchor="figAsymm-RREP_flow">
          <name>AODV-RPL RREQ and RREP Instances for Asymmetric Links"> Links</name>
          <artwork align="center"><![CDATA[
             (R)<------RREP----- (R)<------RREP----- (R)
              |                                       |
              |                                       |
             RREP                                    RREP
              |                                       |
              |                                       |
              v                                       v
             (O)<--------- (R)<--------- (R)<------- (T)
             ^ \    RREP         RREP        RREP    |  \
             |  \                                    |   |RREP
             |   \                                   /   |
       RREP  |    \ RREP                  RREP      /   (R)
             |     \                               /     |
             |      \                             /     /
           (R)<----- (R)<----- (R)<---.....---- (R)< - RREP
                RREP       RREP         RREP

    ]]></artwork>         RREP]]></artwork>
        </figure>
    </t>
      </section>	<!-- End of section "Example control message flows . . ." -->

  <section anchor="RREP_WAIT-example" title="Example anchor="RREP_WAIT-example">
        <name>Example RREP_WAIT handling"> Handling</name>
        <t>
        In <xref target="fig-RREP_WAIT-a"/>, the first RREQ arrives at (T).
        This triggers TargNode to start the RREP_WAIT_TIME timer.

        </t>
        <figure anchor="fig-RREP_WAIT-a" title="TargNode starts RREP_WAIT"> anchor="fig-RREP_WAIT-a">
          <name>TargNode Starts RREP_WAIT</name>
          <artwork align="center"><![CDATA[
        (O) --------->(R) --------->(R)-------->(T)
                RREQ         RREQ         RREQ
               (S=1)        (S=0)        (S=0)

        ]]></artwork>        (S=0)]]></artwork>
        </figure>
    </t>
        <t>

        In <xref target="fig-RREP_WAIT-b"/>, another RREQ arrives
        before the RREP_WAIT_TIME timer is expired.  It could be preferable
        compared the previously received RREP that caused the
        RREP_WAIT_TIME timer to be set.

        </t>
        <figure anchor="fig-RREP_WAIT-b"
            title="Waiting anchor="fig-RREP_WAIT-b">
          <name>Waiting TargNode receives preferable RREQ"> Receives Preferable RREQ</name>
          <artwork align="center"><![CDATA[
        (O)                                     (T)
        / \                                      ^
        |  \                                     |
        |   \                                   /
   RREQ |    \ RREQ (S=1)                    RREQ (S=0)
  (S=0) |     \                               /
        v      \              RREQ (S=0)     /
       (R) ---->(R)------>(R)----.....--->(R)

        ]]></artwork> ---->(R)------>(R)----.....--->(R)]]></artwork>
        </figure>
    </t>
        <t>

        In <xref target="fig-RREP_WAIT-c"/>, the RREP_WAIT_TIME timer
        expires. TargNode selects the path with S=1.

        </t>
        <figure anchor="fig-RREP_WAIT-c" title="RREP_WAIT expires anchor="fig-RREP_WAIT-c">
          <name>RREP_WAIT Expires at TargNode"> TargNode</name>
          <artwork align="center"><![CDATA[
        (R) ---RREQ(S=1)--->(R) ---RREQ(S=1)--->(R)
         ^                                       |
         |                                       |
    RREQ(S=1)                                RREQ(S=1)
         |                                       |
         |                                       v
        (O)                                     (T)

        ]]></artwork>                                     (T)]]></artwork>
        </figure>
    </t>
      </section>	<!-- End of section "Example RREP_WAIT handling" -->

  <section anchor="G-RREP-example" title="Example anchor="G-RREP-example">
        <name>Example G-RREP handling">

    <t>
        In Handling</name>

        <t>In <xref target="fig-G-RREP-a"/>, R* has upward and downward routes
        to TargNode (T) that satisfies satisfy the OF of the RPL Instance originated
        by OrigNode (O) (O), and the destination sequence number is at least as large
        as the sequence number in the RREQ message. message.</t>
        <figure anchor="fig-G-RREP-a"
            title="RREP triggers anchor="fig-G-RREP-a">
          <name>RREP Triggers G-RREP at Intermediate Node"> Node</name>
          <artwork align="center"><![CDATA[
        (R) ---RREQ(S=1)--->(R) ---RREQ(S=0)--->(R)
         ^                                       |
         |                                       |
    RREQ(S=1)                                RREQ(S=0)
         |                                       |
         |                                       v
        (O) --------->(R) --------->(R)-------->(T)
        / \    RREQ         RREQ         RREQ    ^
        |  \    (S=1)      (S=0)         (S=0)   |
        |   \                                   /
   RREQ |    \ RREQ (S=1)                      /
  (S=0) |     \                               /
        v      \                             v
       (R) ---->(R*)<------>(R)<----....--->(R)
        ]]></artwork> ---->(R*)<------>(R)<----....--->(R)]]></artwork>
        </figure>
    </t>
        <t>
        In <xref target="fig-G-RREP-b"/>, R* transmits the G-RREP DIO G-RREP-DIO
        back to OrigNode (O) and forwards the incoming RREQ towards (T).

        </t>
        <figure anchor="fig-G-RREP-b"
            title="Intermediate anchor="fig-G-RREP-b">
          <name>Intermediate Node initiates G-RREP"> Initiates G-RREP</name>
          <artwork align="center"><![CDATA[
        (O)                                     (T)
          \                                      ^
           \                                     |
            \                            (RREQ) /
             \ G-RREP DIO G-RREP-DIO                      /
              \                               /
               \   (RREQ)       (RREQ)       /
               (R*)------>(R)----....--->(R)

        ]]></artwork>
               (R*)------>(R)----....--->(R)]]></artwork>
        </figure>
    </t>
      </section>	<!-- End of section "Example G-RREP handling" -->
    </section>	<!-- End of section "Some Example AODV-RPL Message Flows" -->

    <section anchor="appendix-c" title="Changelog">
    <t>
        Note to the RFC Editor: please remove this section before publication.
    </t>

    <section title="Changes from version 19 to version 20">
        <t> <!-- In response to non-blocking AD comments, end of Feb. 2025 -->
        <list style="symbols">
            <t> <!-- Gunter Van de Velde  2/11/2025, 8:36 PM  -->
		Changed Option Format drawings to avoid suggesting that the
                Option Length is a multiple of 4 bytes numbered="false">

      <name>Acknowledgements</name>
      <t>The authors thank <contact fullname="Pascal Thubert"/>, <contact
      fullname="Rahul Jadhav"/>, and <contact fullname="Lijo Thomas"/> for
      their support and valuable input.  The authors specially thank <contact
      fullname="Lavanya H.M."/> for implementing AODV-RPL options.
            </t> in Contiki and
      conducting extensive simulation studies.</t>
      <t> <!-- Murray Kucherawy  2/20/2025, 12:53 AM  -->
		Deleted The authors would like to acknowledge the terms "on-demand routing" reviews, feedback, and "reactive routing"
      comments from the Terminology list.  In the overview, explained those
                two terms as an illustration for the protocol design goals.
            </t>
            <t> <!-- Roman Danyliw  2/17/2025, 9:52 AM  -->
		In Section 9, to improve readability, explicitly named the
		"Local Network Control Block
		 (224.0.0.0 - 224.0.0.255 (224.0.0/24))" registry following people, in the
                 "IPv4 Multicast Address Space Registry" as the
                 relevant registries.
            </t>
            <t> <!-- Gunter alphabetical order: <contact
      fullname="Roman Danyliw"/>, <contact fullname="Lars Eggert"/>, <contact
      fullname="Benjamin Kaduk"/>, <contact fullname="Tero Kivinen"/>,
      <contact fullname="Erik Kline"/>, <contact fullname="Murray
      Kucherawy"/>, <contact fullname="Warren Kumari"/>, <contact
      fullname="Francesca Palombini"/>, <contact fullname="Alvaro Retana"/>,
      <contact fullname="Ines Robles"/>, <contact fullname="John Scudder"/>,
      <contact fullname="Meral Shirazipour"/>, <contact fullname="Peter Van de Velde 2/11/2025, 8:36 PM  -->
		Changed "must" to "MUST", so that "the selected address
                MUST encompass the domain where the route is built".
            </t>
	    <t> <!-- John Scudder   3/1/2025, 12:12 PM -->
		Inserted language allowing a node X to free up sufficient
                resources for a particular RREQ instead
      der Stok"/>, <contact fullname="Éric Vyncke"/>, and <contact
      fullname="Robert Wilton"/>.</t>
    </section>

    <section numbered="false">
      <name>Contributors</name>

    <contact fullname="Abdur Rashid Sangi">
      <organization>Wenzhou-Kean University</organization>
      <address>
	<postal>
<postalLine>88 Daxue Rd, Ouhai</postalLine>
<postalLine>Wenzhou</postalLine>
<postalLine>Zhejiang Province, 325060</postalLine>
<postalLine>P.R. China</postalLine>
<postalLine>Kean University</postalLine>
<postalLine>1000 Morris Avenue</postalLine>
<postalLine>Union, New Jersey 07083</postalLine>
<postalLine>United States of dropping it,
                when resources are not already available upon reception America</postalLine>
	</postal>
        <email>sangi_bahrian@yahoo.com</email>
      </address>
    </contact>

    <contact fullname="Malati Hegde">
      <organization>Indian Institute of that RREQ.
            </t>
            <t>
		New author's address, minor editorial.
            </t>
        </list>
        </t> Science</organization>
      <address>
        <postal>
	  <city>Bangalore</city><code>560012</code>
          <country>India</country>
        </postal>
        <email>malati@iisc.ac.in</email>
      </address>
    </contact>

    <contact fullname="Mingui Zhang">
      <organization>Huawei Technologies</organization>
      <address>
        <postal>
	  <street>No. 156 Beiqing Rd.</street>
	  <cityarea>Haidian District</cityarea>
	  <city>Beijing</city><code>100095</code>
          <country>P.R. China</country>
        </postal>
        <email>zhangmingui@huawei.com</email>
      </address>
    </contact>

    </section>

    <section title="Changes from version 18 to version 19">
        <t>
        <list style="symbols">
            <t>
		Observed the difference in address ordering
  </back>

<!-- [rfced] We note several author comments present in the Address
		Vector, depending on whether or not the RREP is returning a
		symmetric route.  Specified XML. Please
confirm that the prefix of each address
		is elided according no updates related to these comments are outstanding. Note
that the Compr field.
            </t>
            <t>
		Added length specification for byte-sized message fields,
		which had previously relied on implicit length specification
		from the message's packet format diagram.
            </t>
            <t>
		Added clarifying language for handling of initial zero bits comments will be deleted prior to publication. -->

<!-- [rfced] Terminology

a.) We note inconsistencies in some cases for the Target Prefix / Address field.
            </t>
            <t>
		Updated specification regarding terms below throughout the need for a router to
		ensure text. Should
these be uniform? If so, please let us know which form is preferred.

Also, if the availability capitalized form of RREQ state information when
		processing a corresponding RREP.
            </t>
            <t>
		Replaced GRREP by G-RREP when describing Gratuitous RREP.
            </t>
            <t>
		Updated affiliations for Charles Perkins, Abdur Rashid Sangi
                and email address for S.V.R. Anand.
            </t>
            <t>
		Corrected misspellings, typos.
            </t>
        </list>
        </t>
    </section>

    <section title="Changes from version 17 to version 18">
        <t>
        <list style="symbols">
            <t>
		Replaced "on-demand nature of AODV route discovery is natural"
                by "on-demand property any of AODV route discovery these is useful" in
                <xref target="Introduction"/>.
            </t>
            <t>
		In <xref target="rreq_step4"/>, instead of describing an
		option used to "associate indicate the Address Vector name of the symmetric route
                ..."
a field, would it be helpful to add the RREQ-Instance, reformulated the
                description as an option word field after (e.g., change
"Address Vector" to "include "Address Vector field")? If so, please update the xml file
or indicate which instances should be updated using OLD/NEW format.

RPLInstance
RPL Instance
RPL instance

Destination Sequence Number
destination sequence number

Sequence Number
sequence number

Intermediate Router
Intermediate router
intermediate router

Rank
rank

Address Vector of
address vector

Next Hop
next hop

source address
Source Address

destination address
Destination Address

lifetime
Lifetime

b.) We note inconsistencies in the
                symmetric route ..." as part of terms listed below. We chose the latter
form. Please let us know any objections.

RREP-instance
RREP-Instance

RREQ instance
RREQ-Instance

trickle timer
Trickle timer
  Note: Per usage in <xref target="rreq_step4"/>.
            </t>
            <t>
		Changed from v2-style RFC citations to using Xinclude as
                specified 6206.

Target Option
Target option
  Note: Per usage in <xref target="RFC7991"/>.
            </t>
        </list>
        </t>
    </section>

    <section title="Changes from version 16 to version 17">
        <t>
        <list style="symbols">
            <t>
        	Added new Terminology definitions for RREQ, RREP, OF.
            </t>
            <t>
        	Added clarifying detail about some kinds of improved routes
                discoverable by AODV-RPL.
            </t>
            <t>
                Added forward reference explaining how RREP-InstanceID is
                matched with the proper RREQ-InstanceID.
            </t>
            <t>
        	Added explanation about the function of the 'D' bit
                of the RPLInstanceID.
            </t>
            <t>
        	Provided detail about why a node should leave the RREQ-Instance
                after the specified amount of time.
            </t>
            <t>
        	Specified that "An upstream intermediate router that receives
                such a G-RREP MUST also generate a G-RREP RFC 6550 and send it further
                upstream towards OrigNode."
            </t>
            <t>
        	Added more illustrative diagrams in new
        	<xref target="Examples"/>.   Example diagrams show
                control message flows for RREQ consistency with "RREQ option" and for RREP in cases when
                symmetric route is either available or not available.
		The use of RREP_WAIT
  "RREP option".

ART Option
ART option
  Note: For consistency with "RREQ option" and G-RREP is also illustrated in other
                new diagrams.
            </t>
            <t>
                Included "RREP option".

c.) We note that RFC 9030 stylizes "6tisch" as "6TiSCH". May we
update the reasoning text below for using intersections consistency with RFC 9030?

Original:

   As an example, intermediate routers can use local information (e.g., bit
   rate, bandwidth, number of RREQ
                target lists cells used in <xref target="rreq_step2"/>.
            </t>
            <t>
        	Various editorial improvements and clarifications.
            </t>
        </list>
        </t>
    </section>

    <section title="Changes from version 15 to version 16">
        <t>
        <list style="symbols">
            <t>
        	Modified language to be more explicit about when AODV-RPL
        	is likely to produce preferable routes compared to routing
        	protocols that 6tisch [RFC9030])...

d.) The following forms are constrained to traverse common ancestors.
            </t>
            <t>
        	Added explanation that the way AODV-RPL uses the Rank function
        	does not express a distance or a path cost to the root.
            </t>
            <t>
        	Added a citation suggesting AODV-RPL's likely improvements
        	in routing costs.
            </t>
        </list>
        </t>
    </section>

    <section title="Changes from version 14 to version 15">
        <t>
        <list style="symbols">
            <t>
        	Clarified that AODV-RPL treats the addresses of multiple
        	interfaces on the same router as the addresses of independent
        	routers.
            </t>
            <t>
        	Added details about cases when proactive route establishment
        	is preferable to AODV-RPL's reactive route establishment.
            </t>
            <t>
        	Various editorial stylistic improvements.
            </t>
            <t>
        	Added citations about techniques that can be used for
        	evaluating a link's state.
            </t>
            <t>
        	Clarified that in the determination of TargNode status document. For consistency, we have
expanded these upon first use and
        	determination of a usable route updated subsequent instances to OrigNode does not
        	depend on whether or not S == 0.
            </t>
            <t>
        	Clarified "G-RREP" and
"G-RREP-DIO". Note that AODV-RPL does not specify we used "G-RREP-DIO" (two hyphens). Let us know any action to be
        	taken when multiple
concerns.

Gratuitous RREP
gratuitous RREP
G-RREP

"Gratuitous" RREP-DIO messages
gratuitous RREP-DIO
G-RREP DIO

e.) The following forms are received and the
        	S-bit of the RREQ-Instance is 0.
            </t>
        </list>
        </t>
    </section>

    <section title="Changes from version 13 to version 14">
        <t>
        <list style="symbols">
            <t>
        	Provided more details about scenarios naturally supporting
        	the choice of AODV-RPL as a routing protocol
            </t>
            <t>
        	Added new informative references <xref target="RFC6687"/>,
        	<xref target="RFC9010"/>) that describe the value provided
		by peer-to-peer routing.
            </t>
            <t>
        	Requested IANA to allocate a new multicast group to enable
        	clean separation of AODV-RPL operation from previous
        	routing protocols used in the RPL family.
            </t>
            <t>
        	Cited <xref target="RFC6550"/> as the origination of the
        	definition of DIO
            </t>
            <t>
        	Defined "hop-by-hop route" as a route created using RPL's
        	storing mode.
            </t>
            <t>
        	Defined new configuration variable REJOIN_REENABLE.
            </t>
            <t>
        	Improved definition for RREQ-InstanceID.  Created analogous
        	definition for RREP-InstanceID=(RPLInstanceID, TargNode_IPaddr)
            </t>
            <t>
        	Improved definition of source routing
            </t>
            <t>
        	Clarified that the Border Router (BR) in
        	<xref target="figSymm-a"/> does not imply that AODV does not
        	a require a BR as a protocol entity.
            </t>
            <t>
        	Provided more guidelines about factors to be considered
        	by OrigNode when selecting a value document for the 'L' field.
            </t>
            <t>
        	Described the disadvantage of not keeping track of the
        	Address Vector in the RREQ-Instance.
            </t>
            <t>
        	Specified that in non-storing mode an intermediate node has
        	to record the IP addresses of both incoming and outgoing
        	interfaces into the Address Vector, when those interfaces have
        	different IP addresses.
            </t>
            <t>
        	Added three informative references to describe relevant
        	details about evaluating link asymmetry.
            </t>
            <t>
               	Clarified details about Gratuitous RREP.
            </t>
        </list>
        </t>
    </section>

    <section title="Changes from version 12 to version 13">
	<t>
        <list style="symbols">
            <t>
        	Changed name of "Shift" field to be the "Delta" field.
            </t>
            <t>
        	Specified that if a node does not bit names. We have resources, it MUST
        	drop the RREQ.
            </t>
            <t>
        	Changed name of MaxUseRank to MaxUsefulRank.
            </t>
            <t>
        	Revised a sentence that was not clear about when a TargNode
        	can delay transmission of the RREP in response to a RREQ.
            </t>
            <t>
        	Provided advice about running AODV-RPL at same time as
        	P2P-RPL or native RPL.
            </t>
            <t>
        	Small reorganization and enlargement of the description
        	of Trickle time operation in <xref target="trickle"/>.
            </t>
            <t>
        	Added definition for "RREQ-InstanceID" to Terminology
        	section.
            </t>
            <t>
        	Specified that once a node leaves an RREQ-Instance, it MUST
        	NOT rejoin the same RREQ-Instance.
            </t>
        </list>
        </t>
    </section>

    <section title="Changes from version 11 to version 12">
        <t>
        <list style="symbols">
            <t>
        	Defined RREP_WAIT_TIME for asymmetric as well as
        	symmetric handling of RREP-DIO.
            </t>
            <t>
        	Clarified link-local multicast transmission
updated to use
        	link-local multicast group all-RPL nodes.
            </t>
            <t>
        	Identified some security threats more explicitly.
            </t>
            <t>
        	Specified that the pairing between RREQ-DIO and RREP-DIO
        	happens at OrigNode and TargNode.  Intermediate routers do not
        	necessarily maintain the pairing.
            </t>
            <t>
        	When RREQ-DIO is received latter form with H=0 no hyphen and S=1, specified that
        	intermediate routers MAY store symmetric Address Vector
        	information for possible use when a matching RREP-DIO is
        	received.
            </t>
            <t>
        	Specified that AODV-RPL uses the "P2P Route Discovery Mode of
        	Operation" (MOP == 4), instead of requesting the allocation
        	of a new MOP.  Clarified that there is no conflict with
        	<xref target="RFC6997"/>.
            </t>
            <t>
        	Fixed several important typos single quote (i.e, S bit,
D bit, and improved language in
        	numerous places.
            </t>
            <t>
        	Reorganized the steps in the specification for handling RREQ H bit).

S-bit
'D' bit
H bit

f.) How are "RREP" and RREP at an intermediate router, to more closely follow the
        	order of processing actions to be taken by the router.
            </t>
        </list>
        </t>
    </section>

    <section title="Changes from version 10 to version 11">
        <t>
        <list style="symbols">
            <t>
        	Numerous editorial improvements.
            </t>
            <t>
        	Replace Floor((7+(Prefix Length))/8) by Ceil(Prefix Length/8)
        	for simplicity "RREQ" pronounced? As "are-rep" and ease of understanding.
            </t>
            <t>
        	Use "L field" instead of "L bit" since L is a two-bit field.
            </t>
            <t>
        	Improved the procedures "are-req"? We ask
for guidance in section 6.2.1.
            </t>
            <t>
        	Define the S bit of the data structure a router uses order to
        	represent whether or not choose the RREQ instance is appropriate indefinite article for a symmetric these
to follow (i.e., “a" or "an").

Examples:

an asymmetric route.  This replaces text in the document
        	that was RREP-DIO
a holdover from earlier versions in which the RREP
        	had RREP-DIO

an S bit for that purpose.
            </t>
            <t>
        	Quote terminology from AODV that has been identified as
        	possibly originating in language reflecting various kinds
        	of bias against certain cultures.
            </t>
            <t>
        	Clarified RREQ-Instance
a RREQ-Instance
-->

<!-- [rfced] Abbreviations

a.) We note the relationship full expansion of AODV-RPL to RPL.
            </t>
            <t>
        	Eliminated the "Point-to-Point" terminology to avoid
        	suggesting only a single link.
            </t>
            <t>
        	Modified certain passages "Objective Function" is frequently used
after its abbreviation "OF" is introduced. For consistency, may we update to better reflect
the possibility
        	that a router might have multiple IP addresses.
            </t>
            <t>
        	"Rsv" replaced by "X X" for reserved field.
            </t>
            <t>
        	Added mandates for reserved fields, and replaces some
        	ambiguous language phraseology by mandates.
            </t>
            <t>
        	Replaced "retransmit" terminology by more correct "propagate"
        	terminology.
            </t>
            <t>
        	Added text about determining link symmetry near
        	<xref target="figSymm-b"/>.
            </t>
            <t>
        	Mandated checking abbreviation after first use?

b.) FYI - We made the Address Vector to avoid routing loops.
            </t>
            <t>
        	Improved specification for use following updates:

Expected Number of the Delta value Transmissions (ETX) >  Expected Transmission Count (ETX)
  Note: For consistency with RFC 6551.

Received Signal Strength Indication (RSSI) > Received Signal Strength Indicator (RSSI)
  Note: Both forms were used in
        	<xref target="instancepairing"/>.
            </t>
            <t>
        	Corrected the wrong use of RREQ-Instance to be RREP-Instance.
            </t>
            <t>
        	Referred to Subregistry values instead of Registry values
        	in <xref target="iana"/>.
            </t>
            <t>
        	Sharpened language in <xref target="sec"/>, eliminated
        	misleading document.

c.) We have added expansions for abbreviations upon first use per Section 3.6
of capitalization in the words
        	"Security Configuration".
            </t>
            <t>
        	Added acknowledgements and contributors.
            </t>
        </list>
        </t>
    </section>

    <section title="Changes from version 09 to version 10">
        <t>
        <list style="symbols">
            <t>
        	Changed the title for brevity and to remove acronyms.
            </t>
            <t>
        	Added "Note to the RFC Editor" in <xref target="iana"/>.
            </t>
            <t>
        	Expanded DAO and P2MP in <xref target="Introduction"/>.
            </t>
            <t>
        	Reclassified <xref target="RFC6998"/> and
        	<xref target="RFC7416"/> as Informational.
            </t>
            <t>
        	SHOULD changed to MUST 7322 ("RFC Style Guide"). Please review each expansion in <xref target="RREQmsg"/>
        	and <xref target="RREPmsg"/>.
            </t>
            <t>
        	Several editorial improvements and clarifications.
            </t>
        </list>
        </t>
    </section>

    <section title="Changes from version 08 to version 09">
        <t>
        <list style="symbols">
            <t>
        	Removed section "Link State Determination" and put some of the
        	relevant material into <xref target="channel"/>.
            </t>
            <t>
        	Cited security section of <xref target="RFC6550"/> as part of
        	the RREP-DIO message description in <xref target="terms"/>.
            </t>
            <t>
        	SHOULD has been changed document
carefully to MUST ensure correctness.

Directed Acyclic Graph (DAG)
Operations, Administration, and Maintenance (OAM)
-->

<!-- [rfced] References:

a.) FYI - We have removed [RFC7991] in <xref target="RREPmsg"/>.
            </t>
            <t>
        	Expanded the terms ETX and RSSI References section. It was only
cited in <xref target="channel"/>.
            </t>
            <t>
        	<xref target="forwardRREP"/> has been expanded to provide
        	a more precise explanation of the handling of route reply.

            </t>
            <t>
        	Added <xref target="RFC7416"/> in Change Log, which was deleted.

b.) We found the Security Considerations
        	(<xref target="sec"/>) for RPL security threats.
        	Cited <xref target="RFC6550"/> following URL for authenticated
        	mode of operation.
            </t>
            <t>
        	Appendix A has been mostly re-written to describe methods
        	to determine whether or not the S bit should be set to 1.
            </t>
            <t>
        	For consistency, adjusted several mandates from SHOULD [co-ioam] reference:

https://ieeexplore.ieee.org/document/8328276

May we add this URL (and the corresponding DOI 10.1109/COMSNETS.2018.8328276)
to MUST this reference?

Original:
   [co-ioam]  Rashmi Ballamajalu, Anand, S.V.R., and from SHOULD NOT to MUST NOT.
            </t>
            <t>
        	Numerous editorial improvements Malati Hegde, "Co-
              iOAM: In-situ Telemetry Metadata Transport for Resource
              Constrained Networks within IETF Standards Framework",
              2018 10th International Conference on Communication
              Systems & Networks (COMSNETS) pp.573-576, January 2018.

Perhaps:
   [co-ioam]  Ballamajalu, R., Anand, S.V.R., and clarifications.
            </t>
        </list>
        </t>
    </section>

    <section title="Changes from version 07 to version 08">
        <t>
        <list style="symbols">
            <t>
        	Instead of describing the need M. Hegde, "Co-iOAM:
              In-situ Telemetry Metadata Transport for Resource
              Constrained Networks within IETF Standards Framework",
              2018 10th International Conference on Communication
              Systems & Networks (COMSNETS), pp. 573-576,
              DOI 10.1109/COMSNETS.2018.8328276, January 2018,
              <https://ieeexplore.ieee.org/document/8328276>.

c.) The reference entry for routes to
        	"fulfill the requirements", specify [aodv-tot] reference included a commented-out
DOI that routes need leads to
        	"satisfy this URL:

https://ieeexplore.ieee.org/document/749281

May we add this URL and the Objective Function".
            </t>
            <t>
        	Removed all normative dependencies on <xref target="RFC6997"/>
            </t>
            <t>
        	Rewrote <xref target="sec"/> corresponding DOI to avoid duplication of language
        	in cited specifications.
            </t>
            <t>
        	Added a new section "Link State Determination"
        	<!--  <xref target="linkstate"/>  --> with text this reference?

Original:
   [aodv-tot] Perkins, C.E. and citations to
        	more fully describe how implementations determine whether
        	links are symmetric.
            </t>
            <t>
        	Modified text comparing AODV-RPL to other protocols to
        	emphasize E.M. Royer, "Ad-hoc On-demand Distance
              Vector Routing", Proceedings WMCSA'99. Second IEEE
              Workshop on Mobile Computing Systems and Applications ,
              February 1999.

Perhaps:
   [aodv-tot] Perkins, C.E. and E.M. Royer, "Ad-hoc On-demand Distance
              Vector Routing", Proceedings WMCSA'99. Second IEEE
              Workshop on Mobile Computing Systems and Applications, pp.
              90-100, DOI 10.1109/MCSA.1999.749281, February 1999,
              <https://ieeexplore.ieee.org/document/749281>.

d.) We found the need following URL for AODV-RPL instead of the problems with the other protocols.
            </t>
            <t>
        	Clarified that AODV-RPL uses some of [empirical-study] reference:

https://ieeexplore.ieee.org/document/6231290

May we add this URL (and the base RPL specification
        	but does not require an instance of RPL corresponding DOI 10.1109/MCOM.2012.6231290) to run.
            </t>
            <t>
        	Improved capitalization, quotation,
this reference entry?

Original:
   [empirical-study]
              Prasant Misra, Nadeem Ahmed, and spelling variations.
            </t>
            <t>
        	Specified behavior upon reception Sanjay Jha, "An empirical
              study of a RREQ-DIO or RREP-DIO
        	message for an already existing DODAGID
        	(e.g, <xref target="forwardRREP"/>).
            </t>
            <t>
        	Fixed numerous language issues asymmetry in IANA Considerations
        	<xref target="iana"/>.
            </t>
            <t>
        	For consistency, adjusted several mandates from SHOULD to MUST
        	and from SHOULD NOT to MUST NOT.
            </t>
            <t>
        	Numerous editorial improvements low-power wireless links", IEEE
              Communications Magazine (Volume: 50, Issue: 7), July 2012.

Perhaps:
   [empirical-study]
              Misra, P., Ahmed, N., and clarifications.
            </t>
        </list>
        </t>
    </section>

    <section title="Changes from version 06 to version 07">
        <t>
        <list style="symbols">
            <t>
        	Added definitions for all fields of the ART option
        	(see <xref target="artop"/>).  Modified definition of
        	Prefix Length to prohibit Prefix Length values greater
        	than 127.
            </t>
            <t>
        	Modified the language from <xref target="RFC6550"/>
        	Target Option definition so that the trailing zero bits S. Jha, "An empirical study of the Prefix Length are no longer described as "reserved".
            </t>
            <t>
        	Reclassified <xref target="RFC3561"/> and
        	<xref target="RFC6998"/> as Informative.
            </t>
            <t>
        	Added citation for <xref target="RFC8174"/> to Terminology
        	section.
            </t>
        </list>
        </t>
    </section>

    <section title="Changes from version 05 to version 06">
        <t>
        <list style="symbols">
            <t>
        	Added Security Considerations based on the security
        	mechanisms defined
              asymmetry in <xref target="RFC6550"/>.
            </t>
            <t>
        	Clarified low-power wireless links", IEEE
              Communications Magazine, vol. 50, no. 7, pp. 137-146,
	      DOI 10.1109/MCOM.2012.6231290, July 2012,
              <https://ieeexplore.ieee.org/document/6231290>.
-->

<!-- [rfced] Please review the nature "Inclusive Language" portion of improvements due to P2P route
        	discovery versus
        	bidirectional asymmetric route discovery.
            </t>
            <t>
        	Editorial improvements and corrections.
            </t>
        </list>
        </t>
    </section>

    <section title="Changes from version 04 to version 05">
        <t>
        <list style="symbols">
            <t>
        	Add description for sequence number operations.
            </t>
            <t>
        	Extend the residence duration L in section 4.1.
            </t>
            <t>
        	Change AODV-RPL Target option to ART option.
            </t>
        </list>
        </t>
    </section>

    <section title="Changes from version 03 to version 04">
        <t>
        <list style="symbols">
            <t>
        	Updated RREP option format. Remove the T bit in RREP option.
            </t>
            <t>
        	Using the same RPLInstanceID for RREQ online
Style Guide <https://www.rfc-editor.org/styleguide/part2/#inclusive_language>
and RREP,
        	no need to update <xref target="RFC6550"/>.
            </t>
            <t>
        	Explanation let us know if any changes are needed.  Updates of Delta field this nature typically
result in RREP.
            </t>
            <t>
        	Multiple target options handling during transmission.
            </t>
        </list>
        </t>
    </section>

    <section title="Changes from version 02 to version 03">
        <t>
        <list style="symbols">
            <t>
        	Include the support for source routing.
            </t>
            <t>
        	Import some features from <xref target="RFC6997"/>, e.g.,
        	choice between hop-by-hop and source routing, the L field more precise language, which determines the duration of residence in the DAG,
        	RankLimit, etc.
            </t>
            <t>
        	Define new target option is helpful for AODV-RPL, including readers.

For example, please consider whether the
        	Destination Sequence Number following can be updated in it. Move the TargNode address
        	in RREQ option
instances below:

a.) "native"

Original:
  These P2P routes may differ from routes discoverable by native RPL.

  AODV-RPL can be operated whether or not P2P-RPL or native RPL is running
  otherwise.

b.) "blacklisting"

Original:
  ...in particular, flagging Route Errors, "blacklisting" unidirectional links
  ([RFC3561]), multihoming, and the OrigNode address in RREP option into
        	ADOV-RPL Target Option.
            </t>
            <t>
        	Support route discovery for multiple targets in one RREQ-DIO.
            </t>
            <t>
        	New RPLInstanceID pairing mechanism.
            </t>
        </list>
        </t>
    </section>

</section>

<section title="Contributors">
    <t><list>
        <t> Abdur Rashid Sangi<vspace />
            Wenzhou-Kean University<vspace />
   	    88 Daxue Rd, Ouhai,<vspace />
   	    Wenzhou, Zhejiang Province<vspace />
   	    P.R. China 325060<vspace />
            Kean University<vspace />
   	    1000 Morris Avenue<vspace />
   	    Union, New Jersey 07083<vspace />
   	    USA<vspace />
   	    Email: sangi_bahrian@yahoo.com</t>

        <t> Malati Hegde<vspace />
            Indian Institute of Science<vspace />
            Bangalore 560012<vspace />
            India <vspace />
            Email: malati@iisc.ac.in</t>

        <t> Mingui Zhang<vspace />
            Huawei Technologies<vspace />
            No. 156 Beiqing Rd. Haidian District<vspace />
            Beijing  100095<vspace />
   	    P.R. China<vspace />
   	    Email: zhangmingui@huawei.com</t>

<!--
    <author fullname="Mingui Zhang" initials="M." surname="Zhang">
      <organization>Huawei Technologies</organization>
      <address>
        <postal>
          <street>No. 156 Beiqing Rd. Haidian District</street>
          <city>Beijing</city>
          <region/>
          <code>100095</code>
          <country>China</country>
        </postal>
       <phone/>
      <email>zhangmingui@huawei.com</email>
      </address>
     </author> handling unnumbered interfaces.
-->
    </list></t>
</section>

</back>
</rfc>