ROLL                                                        C.E. Perkins
Internet-Draft                                               Lupin Lodge
Intended status: Standards Track                             S.V.R.Anand
Expires: 27 December 2022                    Indian Institute of Science
                                                          S. Anamalamudi
                                                       SRM University-AP
                                                                  B. Liu
                                                     Huawei Technologies
                                                            25 June 2022


      Supporting Asymmetric Links in Low Power Networks: AODV-RPL
                      draft-ietf-roll-aodv-rpl-14

Abstract

   Route discovery for symmetric and asymmetric Peer-to-Peer (P2P)
   traffic flows is a desirable feature in Low power and Lossy Networks
   (LLNs).  For that purpose, this document specifies 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).  Paired Instances are used to construct
   directional paths, for cases where there are asymmetric links between
   source and target nodes.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   This Internet-Draft will expire on 27 December 2022.

Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.





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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Overview of AODV-RPL  . . . . . . . . . . . . . . . . . . . .   7
   4.  AODV-RPL DIO Options  . . . . . . . . . . . . . . . . . . . .   8
     4.1.  AODV-RPL RREQ Option  . . . . . . . . . . . . . . . . . .   8
     4.2.  AODV-RPL RREP Option  . . . . . . . . . . . . . . . . . .  10
     4.3.  AODV-RPL Target Option  . . . . . . . . . . . . . . . . .  11
   5.  Symmetric and Asymmetric Routes . . . . . . . . . . . . . . .  13
   6.  AODV-RPL Operation  . . . . . . . . . . . . . . . . . . . . .  15
     6.1.  Route Request Generation  . . . . . . . . . . . . . . . .  15
     6.2.  Receiving and Forwarding RREQ messages  . . . . . . . . .  16
       6.2.1.  Step 1: RREQ reception and evaluation . . . . . . . .  16
       6.2.2.  Step 2: TargNode and Intermediate Router
               determination . . . . . . . . . . . . . . . . . . . .  17
       6.2.3.  Step 3: Intermediate Router RREQ processing . . . . .  18
       6.2.4.  Step 4: Symmetric Route Processing at an Intermediate
               Router  . . . . . . . . . . . . . . . . . . . . . . .  18
       6.2.5.  Step 5: RREQ propagation at an Intermediate Router  .  19
       6.2.6.  Step 6: RREQ reception at TargNode  . . . . . . . . .  19
     6.3.  Generating Route Reply (RREP) at TargNode . . . . . . . .  19
       6.3.1.  RREP-DIO for Symmetric route  . . . . . . . . . . . .  19
       6.3.2.  RREP-DIO for Asymmetric Route . . . . . . . . . . . .  20
       6.3.3.  RPLInstanceID Pairing . . . . . . . . . . . . . . . .  20
     6.4.  Receiving and Forwarding Route Reply  . . . . . . . . . .  21
       6.4.1.  Step 1: Receiving and Evaluation  . . . . . . . . . .  21
       6.4.2.  Step 2: OrigNode or Intermediate Router . . . . . . .  21
       6.4.3.  Step 3: Build Route to TargNode . . . . . . . . . . .  21
       6.4.4.  Step 4: RREP Propagation  . . . . . . . . . . . . . .  22
   7.  Gratuitous RREP . . . . . . . . . . . . . . . . . . . . . . .  22
   8.  Operation of Trickle Timer  . . . . . . . . . . . . . . . . .  23
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  23
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  24
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  25
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  25
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  25
     12.2.  Informative References . . . . . . . . . . . . . . . . .  26




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   Appendix A.  Example: Using ETX/RSSI Values to determine value of S
           bit . . . . . . . . . . . . . . . . . . . . . . . . . . .  28
   Appendix B.  Changelog  . . . . . . . . . . . . . . . . . . . . .  29
     B.1.  Changes from version 13 to version 14 . . . . . . . . . .  30
     B.2.  Changes from version 12 to version 13 . . . . . . . . . .  30
     B.3.  Changes from version 11 to version 12 . . . . . . . . . .  31
     B.4.  Changes from version 10 to version 11 . . . . . . . . . .  32
     B.5.  Changes from version 09 to version 10 . . . . . . . . . .  33
     B.6.  Changes from version 08 to version 09 . . . . . . . . . .  33
     B.7.  Changes from version 07 to version 08 . . . . . . . . . .  33
     B.8.  Changes from version 06 to version 07 . . . . . . . . . .  34
     B.9.  Changes from version 05 to version 06 . . . . . . . . . .  34
     B.10. Changes from version 04 to version 05 . . . . . . . . . .  35
     B.11. Changes from version 03 to version 04 . . . . . . . . . .  35
     B.12. Changes from version 02 to version 03 . . . . . . . . . .  35
   Appendix C.  Contributors . . . . . . . . . . . . . . . . . . . .  35
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  36

1.  Introduction

   Routing Protocol for Low-Power and Lossy Networks (RPL) [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, a router does not have routing
   information for most other routers.  Consequently, for traffic
   between routers within the DODAG (i.e., Peer-to-Peer (P2P) traffic)
   data packets either have to traverse the root in non-storing 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 see [RFC6687],
   [RFC6997], [RFC6998], [RFC9010]).  The network environment that is
   considered in this document is assumed to be the same as described in
   Section 1 of [RFC6550].

   The route discovery process in AODV-RPL is modeled on the analogous
   peer-to-peer procedure specified in AODV [RFC3561].  The on-demand
   nature of AODV route discovery is natural for the needs of routing in
   RPL-based LLNs when routes are needed but aren't yet established.
   Peer-to-peer 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.

   AODV terminology has been adapted for use with AODV-RPL messages,
   namely RREQ for Route Request, and RREP for Route Reply.  AODV-RPL
   currently omits some features compared to AODV -- in particular,
   flagging Route Errors, "blacklisting" unidirectional links
   ([RFC3561]), multihoming, and handling unnumbered interfaces.




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   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 [RFC6551]), trickle timers, and support for storing and
   non-storing modes.  AODV-RPL adds basic messages RREQ and RREP as
   part of 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.  AODV-
   RPL uses the "P2P Route Discovery Mode of Operation" (MOP == 4) with
   three new Options for the DIO message, dedicated to discover P2P
   routes.  These P2P routes may differ from routes discoverable by
   native RPL.  Since AODV-RPL uses newly defined Options and a newly
   allocated multicast group (see Section 9), there is no conflict with
   P2P-RPL [RFC6997], a previous document using the same MOP.  AODV-RPL
   can be operated whether or not P2P-RPL or native RPL is running
   otherwise.  For many networks AODV-RPL could be a replacement for
   RPL, since it can find better routes at very moderate cost.
   Consequently, it seems unlikely that RPL would be needed in a network
   that is running AODV-RPL, even though it would be possible to run
   both protocols at the same time.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   AODV-RPL reuses names for messages and data structures, including
   Rank, DODAG and DODAGID, as defined in RPL [RFC6550].

   AODV
      Ad Hoc On-demand Distance Vector Routing [RFC3561].

   ART option
      AODV-RPL Target option: a target option defined in this document.

   Asymmetric Route
      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.

   Bi-directional Asymmetric Link
      A link that can be used in both directions but with different link
      characteristics.



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   DIO
      DODAG Information Object (defined in [RFC6550])

   DODAG RREQ-Instance (or simply RREQ-Instance)
      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.

   DODAG RREP-Instance (or simply RREP-Instance)
      RPL Instance built using the DIO with RREP option; used for
      transmission of control messages from TargNode to OrigNode thus
      enabling data transmission from OrigNode to TargNode.

   Downward Direction
      The direction from the OrigNode to the TargNode.

   Downward Route
      A route in the downward direction.

   hop-by-hop route
      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".

   on-demand routing
      Routing in which a route is established only when needed.

   OrigNode
      The IPv6 router (Originating Node) initiating the AODV-RPL route
      discovery to obtain a route to TargNode.

   Paired DODAGs
      Two DODAGs for a single route discovery process between OrigNode
      and TargNode.

   P2P
      Peer-to-Peer -- in other words, not constrained a priori to
      traverse a common ancestor.

   reactive routing
      Same as "on-demand" routing.

   REJOIN_REENABLE
      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_REENQBLE is 15 minutes.




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   RREQ-DIO message
      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 [RFC6550].

   RREQ-InstanceID
      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, and OrigNode-IPaddr is an IP address of OrigNode.  The
      RREQ-InstanceID uniquely identifies the RREQ-Instance.

   RREP-DIO message
      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 Section 6.3.2.
      The RREP-DIO message has a secure variant as noted in [RFC6550].

   RREP-InstanceID
      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, 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.

   Source routing
      A mechanism by which the source supplies a vector of addresses
      towards the destination node along with each data packet
      [RFC6550].

   Symmetric route
      The upstream and downstream routes traverse the same routers and
      over the same links.

   TargNode
      The IPv6 router (Target Node) for which OrigNode requires a route
      and initiates Route Discovery within the LLN network.

   Upward Direction
      The direction from the TargNode to the OrigNode.

   Upward Route
      A route in the upward direction.






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3.  Overview of AODV-RPL

   With AODV-RPL, routes from OrigNode to TargNode within the LLN
   network are established "on-demand".  In other words, the route
   discovery mechanism in AODV-RPL is invoked reactively when OrigNode
   has data for delivery to the TargNode but existing routes do not
   satisfy the application's requirements.  AODV-RPL works without
   requiring the use of RPL or any other routing protocol.

   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, and another from TargNode to OrigNode.  AODV-RPL also
   enables discovery of symmetric routes along Paired DODAGs, when
   symmetric routes are possible (see Section 5).

   In AODV-RPL, routes are discovered by first forming a temporary 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 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.

   Intermediate routers join the DODAGs based on the Rank [RFC6550] as
   calculated from the DIO message.  Henceforth in this document, "RREQ-
   DIO message" means the DIO message from OrigNode toward TargNode,
   containing the RREQ option as specified in Section 4.1.  The RREQ-
   InstanceID is formed as the ordered pair (Orig_RPLInstanceID,
   OrigNode-IPaddr), where Orig_RPLInstanceID is the local RPLInstanceID
   allocated by 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 along with 'D' == 0.













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   Similarly, "RREP-DIO message" means the DIO message from TargNode
   toward OrigNode, containing the RREP option as specified in
   Section 4.2.  The RREP-InstanceID is formed as the ordered pair
   (Targ_RPLInstanceID, TargNode-IPaddr), where Targ_RPLInstanceID is
   the local RPLInstanceID allocated by 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' == 0.

4.  AODV-RPL DIO Options

4.1.  AODV-RPL RREQ Option

   OrigNode selects one of its IPv6 addresses and sets it in the DODAGID
   field of the RREQ-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 RREQ option MUST be present in a RREQ-DIO
   message, otherwise the message MUST be dropped.

      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)          |
     |                                                               |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 1: Format for AODV-RPL RREQ Option

   OrigNode supplies the following information in the RREQ option:

   Option Type
      TBD2

   Option Length
      The length of the option in octets, excluding the Type and Length
      fields.  Variable due to the presence of the address vector and
      the number of octets elided according to the Compr value.





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   S
      Symmetric bit indicating a symmetric route from the OrigNode to
      the router transmitting this RREQ-DIO.  See Section 5.

   H
      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.

   X
      Reserved; MUST be initialized to zero and ignored upon reception.

   Compr
      4-bit unsigned integer.  When Compr is nonzero, exactly that
      number of prefix octets MUST 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 be set to zero and ignored upon reception.

   L
      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
      MUST leave the RREQ-Instance and stop sending or receiving any
      more DIOs for the RREQ-Instance.  This naturally depends on the
      node's ability to keep track of time.  Once a node leaves an RREQ-
      Instance, it MUST NOT rejoin the same RREQ-Instance for at least
      the time interval specified by the configuration variable
      REJOIN_REENABLE.  L is independent from the route lifetime, which
      is defined in the DODAG configuration option.

      *  0x00: No time limit imposed.
      *  0x01: 16 seconds
      *  0x02: 64 seconds
      *  0x03: 256 seconds


   RankLimit
      This field indicates the upper limit on the integer portion of the
      Rank (calculated using the DAGRank() macro defined in [RFC6550]).
      A value of 0 in this field indicates the limit is infinity.

   Orig SeqNo
      Sequence Number of OrigNode.  See Section 6.1.






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   Address Vector
      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.

   TargNode can join the RREQ instance at a Rank whose integer portion
   is less than or equal to the RankLimit.  Any other node MUST NOT join
   a RREQ instance if its own Rank would be equal to or higher than
   RankLimit.  A router MUST discard a received RREQ if the integer part
   of the advertised Rank equals or exceeds the RankLimit.


4.2.  AODV-RPL RREP Option

   TargNode sets one of its IPv6 addresses in the DODAGID 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 be present in a RREP-DIO message,
   otherwise the message MUST be dropped.  TargNode supplies the
   following information in the RREP option:

        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)          |
       .                                                               .
       .                                                               .
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 2: Format for AODV-RPL RREP option

   Option Type
      TBD3

   Option Length
      The length of the option in octets, excluding the Type and Length
      fields.  Variable due to the presence of the address vector and
      the number of octets elided according to the Compr value.

   G
      Gratuitous RREP (see Section 7).




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   H
      The H bit in the RREP option MUST be set to be the same as the H
      bit in RREQ option.  It requests either source routing (H=0) or
      hop-by-hop (H=1) for the downstream route.

   X
      Reserved; MUST be initialized to zero and ignored upon reception.

   Compr
      4-bit unsigned integer.  Same definition as in RREQ option.

   L
      2-bit unsigned integer defined as in RREQ option.  The lifetime of
      the RREP-Instance MUST be no greater than the lifetime of the
      RREQ-Instance to which it is paired.

   RankLimit
      Similarly to RankLimit in the RREQ message, this field indicates
      the upper limit on the integer portion of the Rank.  A value of 0
      in this field indicates the limit is infinity.

   Delta
      6-bit unsigned integer.  This field is used to recover the RREQ-
      InstanceID (see Section 6.3.3); 0 indicates that the RREQ-
      InstanceID has the same value as the RPLInstanceID of the RREP
      message.

   X X
      Reserved; MUST be initialized to zero and ignored upon reception.

   Address Vector
      Only present when the H bit is set to 0.  For an asymmetric route,
      the Address Vector represents the IPv6 addresses of the path
      through the network the RREP-DIO has passed.  For a symmetric
      route, it is the Address Vector when the RREQ-DIO arrives at the
      TargNode, unchanged during the transmission to the OrigNode.

4.3.  AODV-RPL Target Option

   The AODV-RPL Target (ART) Option is based on the Target Option in
   core RPL [RFC6550].  The Flags field is replaced by the Destination
   Sequence Number of the TargNode and the Prefix Length field is
   reduced to 7 bits so that the value is limited to be no greater than
   127.

   A RREQ-DIO message MUST carry at least one ART Option.  A RREP-DIO
   message MUST carry exactly one ART Option.  Otherwise, the message
   MUST be dropped.



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   OrigNode can include multiple TargNode addresses via multiple AODV-
   RPL Target Options in the RREQ-DIO, for routes that share the same
   requirement on metrics.  This reduces the cost to building only one
   DODAG.

        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)           |
       .                                                               .
       .                                                               .
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 3: ART Option format for AODV-RPL

   Option Type
      TBD4

   Option Length
      Length of the option in octets excluding the Type and Length
      fields.

   Dest SeqNo


      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, and not used
      otherwise.

   X
      A one-bit reserved field.  This field MUST be initialized to zero
      by the sender and MUST be ignored by the receiver.

   Prefix Length
      7-bit unsigned integer.  Number of valid leading bits in the IPv6
      Prefix.  If Prefix Length is 0, then the value in the Target
      Prefix / Address field represents an IPv6 address, not a prefix.

   Target Prefix / Address
      (variable-length field) An IPv6 destination address or prefix.
      The Prefix Length field contains the number of valid leading bits



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      in the prefix.  The Target Prefix / Address field contains the
      least number of octets that can represent all of the bits of the
      Prefix, in other words Ceil(Prefix Length/8) octets.  The initial
      bits in the Target Prefix / Address field preceding the prefix
      length (if any) MUST be set to zero on transmission and MUST be
      ignored on receipt.  If Prefix Length is zero, the Address field
      is 128 bits for IPv6 addresses.

5.  Symmetric and Asymmetric Routes

   Links are considered symmetric until indication to the contrary is
   received.  In Figure 4 and Figure 5, BR is the Border Router, O is
   the OrigNode, each R is an intermediate router, and T is the
   TargNode.  In this example, 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, and the S bit is set to 1, then it remains as 1, as
   illustrated in Figure 4.  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.

                                     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) -------<

                Figure 4: AODV-RPL with Symmetric Instances

   Upon receiving a RREQ-DIO with the S bit set to 1, a node determines
   whether this link can be used symmetrically, i.e., both directions
   meet the requirements of data transmission.  If the RREQ-DIO arrives



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   over an interface that is not known to be symmetric, or is known to
   be asymmetric, the S bit is set to 0.  If the S bit arrives already
   set to be '0', it is set to be '0' when the RREQ-DIO is propagated
   (Figure 5).  For an asymmetric route, there is at least one hop which
   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.

   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 [RFC9030]), a priori
   knowledge (e.g., link quality according to previous 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 test traffic can be
   generated between nodes of the deployed network.  During AODV-RPL
   operation, OAM techniques for evaluating link state (see [RFC7548],
   [RFC7276], [co-ioam]) MAY be used (at regular intervals appropriate
   for the LLN).  The evaluation procedures are out of scope for AODV-
   RPL.

   Appendix A describes an example method using the upstream Expected
   Number of Transmissions (ETX) and downstream Received Signal Strength
   Indicator (RSSI) to estimate whether the link is symmetric in terms
   of link quality using an averaging technique.
























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                                     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) -------<

            Figure 5: AODV-RPL with Asymmetric Paired Instances

   As illustrated in Figure 5, 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 the intermediate router has already made the link asymmetry
   decision by the time RREQ-DIO arrives.

6.  AODV-RPL Operation

6.1.  Route Request Generation

   The route discovery process is initiated when an application at the
   OrigNode has data to be transmitted to the 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 it transmits a DIO
   message containing exactly one RREQ option (see Section 4.1) to
   multicast group all-AODV-RPL-nodes.  The RREQ-DIO MUST contain at
   least one ART Option (see Section 4.3), which indicates the TargNode.
   The S bit in RREQ-DIO sent out by the OrigNode is set to 1.








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   Each node maintains a sequence number; the operation is specified in
   section 7.2 of [RFC6550].  When the OrigNode initiates a route
   discovery process, it MUST 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.

   The Target Prefix / Address in the ART 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.  Within a RREQ-DIO the Objective Function for the routes to
   different TargNodes MUST be the same.

   OrigNode can maintain different RPLInstances to discover routes with
   different requirements to the same targets.  Using the RPLInstanceID
   pairing mechanism (see Section 6.3.3), route replies (RREP-DIOs) for
   different RPLInstances can be generated.

   The transmission of RREQ-DIO obeys the Trickle timer [RFC6206].  If
   the duration specified by the L field has elapsed, the OrigNode MUST
   leave the DODAG and stop sending RREQ-DIOs in the related
   RPLInstance.  OrigNode needs to set 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 Trickle timer,
   TargNode hop distance, network size, link behavior, expected data
   usage time, and so on.

6.2.  Receiving and Forwarding RREQ messages

6.2.1.  Step 1: RREQ reception and evaluation

   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, X
   then determines whether or not it has sufficient resources available
   to maintain the state needed to process an eventual RREP if the RREP
   were to be received.  If not, then X MUST 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 tramsmit packets to OrigNode.

   When H=0 in the incoming RREQ, the router MUST 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 drop the RREQ message if 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



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   (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) the
   RREQ-DIO MUST be dropped, and the following steps are not processed.
   Otherwise, the router MUST 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.

6.2.2.  Step 2: TargNode and Intermediate Router determination

   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 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 Section 6.3.

   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 delete the Target
   Option encapsulating its own address, so that downstream routers with
   higher Rank values do not try to create a route to this TargNode.

   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.  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
   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 be included if it is nonempty after
   TargNode has deleted the Target Option encapsulating its own address.
   If the intersection is empty, it means that all the targets have been
   reached, and the router MUST NOT transmit any RREQ-DIO.  Otherwise it
   proceeds to Section 6.2.3.

   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 only T2 needs to be included in the generated RREQ-DIO.




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6.2.3.  Step 3: Intermediate Router RREQ processing

   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 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.  The Destination Address and the
   RPLInstanceID respectively can be learned from the DODAGID and the
   RPLInstanceID of the RREQ-DIO.  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, same
   RPLInstanceID, but stale sequence number, MUST be deleted.

6.2.4.  Step 4: Symmetric Route Processing at an Intermediate Router

   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 determine whether the downward (i.e.,
   towards the TargNode) direction of the incoming link satisfies the
   OF.  If so, the S bit of the RREQ-DIO to be transmitted is set to 1.
   Otherwise the S bit of the RREQ-DIO to be transmitted is set to 0.

   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 set to 1.  This
   information associated to RREQ-Instance is known as the S-bit of the
   RREQ-Instance.  It will be used later during the RREP-DIO message
   processing Section 6.3.2.

   Suppose a router has joined the RREQ-Instance, and H=0, and the S-bit
   of the RREQ-Instance is set to 1.  In this case, the router MAY
   optionally associate to the RREQ-Instance, the Address Vector of the
   symmetric route back to OrigNode.  This is useful if the router later
   receives an RREP-DIO that is paired with the RREQ-Instance.  If the
   router does NOT associate the Address Vector, then it has to rely on
   multicast for the RREP.  This can impose a substantial performance
   penalty.









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6.2.5.  Step 5: RREQ propagation at an Intermediate Router

   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 append the address of its interface
   receiving the RREQ-DIO into the address vector.  If, in addition, the
   address of the router's transmitting the RREQ-DIO is not the same as
   the address of the interface receiving the RREQ-DIO, the router MUST
   also append the transmitting interface address into the address
   vector.

6.2.6.  Step 6: RREQ reception at TargNode

   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 (Section 6.3).

6.3.  Generating Route Reply (RREP) at TargNode

   When a TargNode receives a RREQ message over a link from a neighbor
   Y, TargNode first follows the procedures in Section 6.2.  If the link
   to Y can be used to tramsmit packets to OrigNode, TargNode generates
   a RREP according to the steps below.  Otherwise TargNode drops the
   RREQ and does not generate a RREP.

   If the L field is not 0, the TargNode MAY delay transmitting the
   RREP-DIO for 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.

   The address of the OrigNode MUST be encapsulated in the ART Option
   and included in this RREP-DIO message along with the SeqNo of
   TargNode.

6.3.1.  RREP-DIO for Symmetric route

   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 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 and out of



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   scope.

   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
   Section 6.3.3.  In case the H bit is set to 0, the address vector
   from the RREQ-DIO MUST be included in the RREP-DIO.

6.3.2.  RREP-DIO for Asymmetric Route

   When a RREQ-DIO arrives at a TargNode with the S bit set to 0, the
   TargNode MUST 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.

6.3.3.  RPLInstanceID Pairing

   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 between the same pair of OrigNode and TargNode,
   there can be multiple AODV-RPL route discovery instances.  So that
   OrigNode and Targnode can avoid any mismatch, they MUST pair the
   RREQ-Instance and the RREP-Instance in the same route discovery by
   using the RPLInstanceID.

   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 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 be used again for assigning
   RPLInstanceID for the later RREP-Instance.  If the same RPLInstanceID
   were re-used 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 be replaced by another value so that
   the two RREP-instances can be distinguished.  In the RREP-DIO option,
   the Delta field of the RREP-DIO message (Figure 2) indicates the
   increment to be applied to the pre-existing RPLInstanceID to obtain
   the value of the RPLInstanceID that is used in the RREP-DIO message.
   When the new RPLInstanceID after incrementation exceeds 255, it rolls
   over starting at 0.  For example, if the RREQ-InstanceID is 252, and



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   incremented by 6, the new RPLInstanceID will be 2.  Related
   operations can be found in Section 6.4.  RPLInstanceID collisions do
   not occur across RREQ-DIOs; the DODAGID equals the OrigNode address
   and is sufficient to disambiguate between DODAGs.

6.4.  Receiving and Forwarding Route Reply

   Upon receiving a RREP-DIO, a router which already belongs to the
   RREP-Instance SHOULD drop the RREP-DIO.  Otherwise the router
   performs the steps in the following subsections.

6.4.1.  Step 1: Receiving and Evaluation

   If the Objective Function is not satisfied, the router MUST NOT join
   the DODAG; the router MUST discard the RREP-DIO, and does not execute
   the remaining steps in this section.  An Intermediate Router MUST
   discard a RREP if one of its addresses is present in the Address
   Vector, and does not execute the remaining steps in this section.

   If the S bit of the associated RREQ-Instance is set to 1, the router
   MUST proceed to Section 6.4.2.

   If the S-bit of the RREQ-Instance is set to 0, the router MUST
   determine whether the downward direction of the link (towards the
   TargNode) over which the RREP-DIO is received satisfies the Objective
   Function, and the router's Rank would not exceed the RankLimit.  If
   so, 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.

6.4.2.  Step 2: OrigNode or Intermediate Router

   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.  If so,
   this router is the OrigNode of the route discovery.  Otherwise, it is
   an intermediate router.

6.4.3.  Step 3: Build Route to TargNode

   If the H bit is set to 1, then the router (OrigNode or intermediate)
   MUST build a downward route entry towards TargNode which includes at
   least the following items: OrigNode Address, RPLInstanceID, TargNode
   Address as destination, Next Hop, 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 be the RREQ-InstanceID (i.e.,



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   after subtracting the Delta field value from the value of the
   RPLInstanceID).  The source address is learned from the ART 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, and is
   copied from the Dest SeqNo field of the ART option of the RREP-DIO.
   A route entry with same source and destination address, same
   RPLInstanceID, but stale sequence number (i.e., incoming sequence
   number is less than the currently stored sequence number of the route
   entry), MUST be deleted.

6.4.4.  Step 4: RREP Propagation

   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 include the address of the interface
   receiving the RREP-DIO into the address vector.  If H=1, according to
   the last 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
   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 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.

7.  Gratuitous RREP

   In some cases, an Intermediate router that receives a RREQ-DIO
   message MAY unicast a "Gratuitous" RREP-DIO message back to OrigNode
   before continuing the transmission of the RREQ-DIO towards TargNode.
   The Gratuitous 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=1) sent by the Intermediate
   router from the RREP-DIO sent by TargNode (G=0).












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   The gratuitous RREP-DIO MAY be sent out when the Intermediate router
   receives a RREQ-DIO for a TargNode, and the router has a pair of
   downward and upward routes to the TargNode which 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 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.

   In case of source routing, the intermediate router MUST include the
   address vector between the OrigNode and itself in the Gratuitous
   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 the router MUST include the address vector from the
   TargNode to the router itself in the gratuitous RREP-DIO to be
   transmitted.

   For establishing hop-by-hop routes, the intermediate router MUST
   unicast the received RREQ-DIO to the Next Hop on the route.  The Next
   Hop router along the route MUST 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 the TargNode and each router along the path towards
   OrigNode MUST unicast the RREP-DIO hop-by-hop towards OrigNode as
   specified in Section 6.3.

8.  Operation of Trickle Timer

   RREQ-Instance/RREP-Instance multicast uses trickle timer operations
   [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 [RFC6550] entitled "DIO
   Transmission".  If the route is symmetric, the RREP DIO does not need
   the Trickle timer mechanism.

9.  IANA Considerations

   Note to RFC editor:

   The sentence "The parenthesized numbers are only suggestions." is to
   be removed prior publication.

   A Subregistry in this section refers to a named sub-registry of the
   "Routing Protocol for Low Power and Lossy Networks (RPL)" registry.





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   AODV-RPL uses the "P2P Route Discovery Mode of Operation" (MOP == 4)
   with new Options as specified in this document.  Please cite AODV-RPL
   and this document as one of the protocols using MOP 4.

   IANA is asked to assign three new AODV-RPL options "RREQ", "RREP" and
   "ART", as described in Figure 6 from the "RPL Control Message
   Options" Subregistry.  The parenthesized numbers are only
   suggestions.

          +-------------+------------------------+---------------+
          |    Value    |        Meaning         |   Reference   |
          +-------------+------------------------+---------------+
          | TBD2 (0x0B) |      RREQ Option       | This document |
          +-------------+------------------------+---------------+
          | TBD3 (0x0C) |      RREP Option       | This document |
          +-------------+------------------------+---------------+
          | TBD4 (0x0D) |       ART Option       | This document |
          +-------------+------------------------+---------------+

                         Figure 6: AODV-RPL Options

   IANA is requested to allocate a new permanent multicast address with
   link-local scope called all-AODV-RPL-nodes for nodes implementing
   this specification.

10.  Security Considerations

   The security considerations for the operation of AODV-RPL are similar
   to those for the operation of RPL (as described in Section 19 of the
   RPL specification [RFC6550]).  Sections 6.1 and 10 of [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 [RFC7416].

   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 [RFC6550]), 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.

   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, and RREP-DIO messages
   carrying bad routes or maliciously modify genuine RREP-DIO messages



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   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 be used.

   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.

   If a rogue router is able to forge a gratuitous RREP, it could mount
   denial-of-service attacks.

11.  Acknowledgements

   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.

   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.

12.  References

12.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC6206]  Levis, P., Clausen, T., Hui, J., Gnawali, O., and J. Ko,
              "The Trickle Algorithm", RFC 6206, DOI 10.17487/RFC6206,
              March 2011, <https://www.rfc-editor.org/info/rfc6206>.

   [RFC6550]  Winter, T., Ed., Thubert, P., Ed., Brandt, A., Hui, J.,
              Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur,
              JP., and R. Alexander, "RPL: IPv6 Routing Protocol for
              Low-Power and Lossy Networks", RFC 6550,
              DOI 10.17487/RFC6550, March 2012,
              <https://www.rfc-editor.org/info/rfc6550>.




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   [RFC6551]  Vasseur, JP., Ed., Kim, M., Ed., Pister, K., Dejean, N.,
              and D. Barthel, "Routing Metrics Used for Path Calculation
              in Low-Power and Lossy Networks", RFC 6551,
              DOI 10.17487/RFC6551, March 2012,
              <https://www.rfc-editor.org/info/rfc6551>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

12.2.  Informative References

   [co-ioam]  Rashmi Ballamajalu, Anand, S.V.R., and 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.

   [contiki]  Contiki contributors, "The Contiki Open Source OS for the
              Internet of Things (Contiki Version 2.7)", November 2013,
              <https://github.com/contiki-os/contiki>.

   [Contiki-ng]
              Contiki-NG contributors, "Contiki-NG: The OS for Next
              Generation IoT Devices (Contiki-NG Version 4.6)", December
              2020, <https://github.com/contiki-ng/contiki-ng>.

   [cooja]    Contiki/Cooja contributors, "Cooja Simulator for Wireless
              Sensor Networks (Contiki/Cooja Version 2.7)", November
              2013, <https://github.com/contiki-
              os/contiki/tree/master/tools/cooja>.

   [empirical-study]
              Prasant Misra, Nadeem Ahmed, and Sanjay Jha, "An empirical
              study of asymmetry in low-power wireless links", IEEE
              Communications Magazine (Volume: 50, Issue: 7), July 2012.

   [Link_Asymmetry]
              Lifeng Sang, Anish Arora, and Hongwei Zhang, "On Link
              Asymmetry and One-way Estimation in Wireless Sensor
              Networks", ACM Transactions on Sensor Networks, Volume 6
              Issue 2 pp.1-25, February 2010,
              <https://doi.org/10.1145/1689239.1689242>.








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   [low-power-wireless]
              Kannan Srinivasan, Prabal Dutta, Arsalan Tavakoli, and
              Philip Levis, "An empirical study of low-power wireless",
              ACM Transactions on Sensor Networks (Volume 6 Issue 2
              pp.1-49), February 2010,
              <https://doi.org/10.1145/1689239.1689246>.

   [RFC3561]  Perkins, C., Belding-Royer, E., and S. Das, "Ad hoc On-
              Demand Distance Vector (AODV) Routing", RFC 3561,
              DOI 10.17487/RFC3561, July 2003,
              <https://www.rfc-editor.org/info/rfc3561>.

   [RFC6687]  Tripathi, J., Ed., de Oliveira, J., Ed., and JP. Vasseur,
              Ed., "Performance Evaluation of the Routing Protocol for
              Low-Power and Lossy Networks (RPL)", RFC 6687,
              DOI 10.17487/RFC6687, October 2012,
              <https://www.rfc-editor.org/info/rfc6687>.

   [RFC6997]  Goyal, M., Ed., Baccelli, E., Philipp, M., Brandt, A., and
              J. Martocci, "Reactive Discovery of Point-to-Point Routes
              in Low-Power and Lossy Networks", RFC 6997,
              DOI 10.17487/RFC6997, August 2013,
              <https://www.rfc-editor.org/info/rfc6997>.

   [RFC6998]  Goyal, M., Ed., Baccelli, E., Brandt, A., and J. Martocci,
              "A Mechanism to Measure the Routing Metrics along a Point-
              to-Point Route in a Low-Power and Lossy Network",
              RFC 6998, DOI 10.17487/RFC6998, August 2013,
              <https://www.rfc-editor.org/info/rfc6998>.

   [RFC7276]  Mizrahi, T., Sprecher, N., Bellagamba, E., and Y.
              Weingarten, "An Overview of Operations, Administration,
              and Maintenance (OAM) Tools", RFC 7276,
              DOI 10.17487/RFC7276, June 2014,
              <https://www.rfc-editor.org/info/rfc7276>.

   [RFC7416]  Tsao, T., Alexander, R., Dohler, M., Daza, V., Lozano, A.,
              and M. Richardson, Ed., "A Security Threat Analysis for
              the Routing Protocol for Low-Power and Lossy Networks
              (RPLs)", RFC 7416, DOI 10.17487/RFC7416, January 2015,
              <https://www.rfc-editor.org/info/rfc7416>.

   [RFC7548]  Ersue, M., Ed., Romascanu, D., Schoenwaelder, J., and A.
              Sehgal, "Management of Networks with Constrained Devices:
              Use Cases", RFC 7548, DOI 10.17487/RFC7548, May 2015,
              <https://www.rfc-editor.org/info/rfc7548>.





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   [RFC9010]  Thubert, P., Ed. and M. Richardson, "Routing for RPL
              (Routing Protocol for Low-Power and Lossy Networks)
              Leaves", RFC 9010, DOI 10.17487/RFC9010, April 2021,
              <https://www.rfc-editor.org/info/rfc9010>.

   [RFC9030]  Thubert, P., Ed., "An Architecture for IPv6 over the Time-
              Slotted Channel Hopping Mode of IEEE 802.15.4 (6TiSCH)",
              RFC 9030, DOI 10.17487/RFC9030, May 2021,
              <https://www.rfc-editor.org/info/rfc9030>.

Appendix A.  Example: Using ETX/RSSI Values to determine value of S bit

   The combination of Received Signal Strength Indication(downstream)
   (RSSI) and Expected Number of Transmissions(upstream) (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.

   Method 1:  In the first method, we constructed a table measuring RSSI
   vs ETX using the Cooja simulation [cooja] setup in the Contiki OS
   environment[contiki].  We used Contiki-2.7 running 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
   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, it is highly
   recommended to conduct physical radio measurement experiments, in
   general.  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 is part of RPL Objective Function implementation.
   Since ETX value is reflective of the extent of packet drops, it
   allowed us to prepare a useful ETX vs RSSI table.  ETX versus RSSI
   values obtained in this way may be used as explained below:

               Source---------->NodeA---------->NodeB------->Destination

        Figure 7: Communication link from Source to Destination










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   +=========================+========================================+
   | RSSI at NodeA for NodeB | Expected ETX at NodeA for NodeB->NodeA |
   +=========================+========================================+
   |          > -60          |                  150                   |
   +-------------------------+----------------------------------------+
   |        -70 to -60       |                  192                   |
   +-------------------------+----------------------------------------+
   |        -80 to -70       |                  226                   |
   +-------------------------+----------------------------------------+
   |        -90 to -80       |                  662                   |
   +-------------------------+----------------------------------------+
   |       -100 to -90       |                  3840                  |
   +-------------------------+----------------------------------------+

         Table 1: Selection of S bit based on Expected ETX value

   Method 2:  One could also make use of the function
      guess_etx_from_rssi() defined in the 6LoWPAN/RPL protocol stack of
      Contiki-ng OS [Contiki-ng] to obtain RSSI-ETX mapping.  This
      function outputs ETX value ranging between 128 and 3840 for -60 <=
      rssi <= -89.  The function description is beyond the scope of this
      document.

   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 and NodeB->NodeA (see Figure 7) 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 but it has no direct way of
   knowing the value of ETX from NodeB->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 in turn can be
   used to estimate ETX value at nodeA for link NodeB--->NodeA from the
   received RSSI from NodeB.  Whenever nodeA determines that the link
   towards the nodeB is bi-directional asymmetric then the S bit is set
   to 0.  Afterwards, the link from NodeA to Destination remains
   designated as asymmetric and the S bit remains set to 0.

   Determination of asymmetry versus bidirectionality remains a topic of
   lively discussion in the IETF.

Appendix B.  Changelog

   Note to the RFC Editor: please remove this section before
   publication.




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B.1.  Changes from version 13 to version 14

   *  Provided more details about scenarios naturally supporting the
      choice of AODV-RPL as a routing protocol

   *  Added new informative references [RFC6687], [RFC9010]) that
      describe the value provided by peer-to-peer routing.

   *  Requested IANA to allocate a new multicast group to enable clean
      separation of AODV-RPL operation from previous routing protocols
      in the RPL family.

   *  Cited [RFC6550] as the origination of the definition of DIO

   *  Defined "hop-by-hop route" as a route created using RPL's storing
      mode.

   *  Defined new configuration variable REJOIN_REENABLE.

   *  Improved definition for RREQ-InstanceID.  Created analogous
      definition for RREP-InstanceID=(RPLInstanceID, TargNode_IPaddr)

   *  Improved definition of source routing

   *  Clarified that the Border Router (BR) in Figure 4 does not imply
      that AODV does not a require a BR as a protocol entity.

   *  Provided more guidelines about factors to be considered by
      OrigNode when selecting a value for the 'L' field.

   *  Described the disadvantage of not keeping track of the Address
      Vector in the RREQ-Instance.

   *  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.

   *  Added three informative references to describe relevant details
      about evaluating link assymetry.

   *  Clarified details about Gratuitous RREP.

B.2.  Changes from version 12 to version 13

   *  Changed name of "Shift" field to be the "Delta" field.





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   *  Specified that if a node does not have resources, it MUST drop the
      RREQ.

   *  Changed name of MaxUseRank to MaxUsefulRank.

   *  Revised a sentence that was not clear about when a TargNode can
      delay transmission of the RREP in response to a RREQ.

   *  Provided advice about running AODV-RPL at same time as P2P-RPL or
      native RPL.

   *  Small reorganization and enlargement of the description of Trickle
      time operation in Section 8.

   *  Added definition for "RREQ-InstanceID" to Terminology section.

   *  Specified that once a node leaves an RREQ-Instance, it MUST NOT
      rejoin the same RREQ-Instance.

B.3.  Changes from version 11 to version 12

   *  Defined RREP_WAIT_TIME for asymmetric as well as symmetric
      handling of RREP-DIO.

   *  Clarifed link-local multicast transmission to use link-local
      multicast group all-RPL nodes.

   *  Identified some security threats more explicitly.

   *  Specified that the pairing between RREQ-DIO and RREP-DIO happens
      at OrigNode and TargNode.  Intermediate routers do not necessarily
      maintain the pairing.

   *  When RREQ-DIO is received with H=0 and S=1, specified that
      intermediate routers MAY store symmetric Address Vector
      information for possible use when a matchine RREP-DIO is received.

   *  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 [RFC6997].

   *  Fixed several important typos and improved language in numerous
      places.

   *  Reorganized the steps in the specification for handling RREQ and
      RREP at an intermediate router, to more closely follow the order
      of processing actions to be taken by the router.




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B.4.  Changes from version 10 to version 11

   *  Numerous editorial improvements.

   *  Replace Floor((7+(Prefix Length))/8) by Ceil(Prefix Length/8) for
      simplicity and ease of understanding.

   *  Use "L field" instead of "L bit" since L is a two-bit field.

   *  Improved the procedures in section 6.2.1.

   *  Define the S bit of the data structure a router uses to represent
      whether or not the RREQ instance is for a symmetric or an
      asymmetric route.  This replaces text in the document that was a
      holdover from earlier versions in which the RREP had an S bit for
      that purpose.

   *  Quote terminology from AODV that has been identified as possibly
      originating in language reflecting various kinds of bias against
      certain cultures.

   *  Clarified the relationship of AODV-RPL to RPL.

   *  Eliminated the "Point-to-Point" terminology to avoid suggesting
      only a single link.

   *  Modified certain passages to better reflect the possibility that a
      router might have multiple IP addresses.

   *  "Rsv" replaced by "X X" for reserved field.

   *  Added mandates for reserved fields, and replaces some ambiguous
      language phraseology by mandates.

   *  Replaced "retransmit" terminology by more correct "propagate"
      terminology.

   *  Added text about determining link symmetry near Figure 5.

   *  Mandated checking the Address Vector to avoid routing loops.

   *  Improved specification for use of the Delta value in
      Section 6.3.3.

   *  Corrected the wrong use of RREQ-Instance to be RREP-Instance.

   *  Referred to Subregistry values instead of Registry values in
      Section 9.



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   *  Sharpened language in Section 10, eliminated misleading use of
      capitalization in the words "Security Configuration".

   *  Added acknowledgements and contributors.

B.5.  Changes from version 09 to version 10

   *  Changed the title for brevity and to remove acronyms.

   *  Added "Note to the RFC Editor" in Section 9.

   *  Expanded DAO and P2MP in Section 1.

   *  Reclassified [RFC6998] and [RFC7416] as Informational.

   *  SHOULD changed to MUST in Section 4.1 and Section 4.2.

   *  Several editorial improvements and clarifications.

B.6.  Changes from version 08 to version 09

   *  Removed section "Link State Determination" and put some of the
      relevant material into Section 5.

   *  Cited security section of [RFC6550] as part of the RREP-DIO
      message description in Section 2.

   *  SHOULD has been changed to MUST in Section 4.2.

   *  Expanded the terms ETX and RSSI in Section 5.

   *  Section 6.4 has been expanded to provide a more precise
      explanation of the handling of route reply.

   *  Added [RFC7416] in the Security Considerations (Section 10) for
      RPL security threats.  Cited [RFC6550] for authenticated mode of
      operation.

   *  Appendix A has been mostly re-written to describe methods to
      determine whether or not the S bit should be set to 1.

   *  For consistency, adjusted several mandates from SHOULD to MUST and
      from SHOULD NOT to MUST NOT.

   *  Numerous editorial improvements and clarifications.

B.7.  Changes from version 07 to version 08




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   *  Instead of describing the need for routes to "fulfill the
      requirements", specify that routes need to "satisfy the Objective
      Function".

   *  Removed all normative dependencies on [RFC6997]

   *  Rewrote Section 10 to avoid duplication of language in cited
      specifications.

   *  Added a new section "Link State Determination" with text and
      citations to more fully describe how implementations determine
      whether links are symmetric.

   *  Modified text comparing AODV-RPL to other protocols to emphasize
      the need for AODV-RPL instead of the problems with the other
      protocols.

   *  Clarified that AODV-RPL uses some of the base RPL specification
      but does not require an instance of RPL to run.

   *  Improved capitalization, quotation, and spelling variations.

   *  Specified behavior upon reception of a RREQ-DIO or RREP-DIO
      message for an already existing DODAGID (e.g, Section 6.4).

   *  Fixed numerous language issues in IANA Considerations Section 9.

   *  For consistency, adjusted several mandates from SHOULD to MUST and
      from SHOULD NOT to MUST NOT.

   *  Numerous editorial improvements and clarifications.

B.8.  Changes from version 06 to version 07

   *  Added definitions for all fields of the ART option (see
      Section 4.3).  Modified definition of Prefix Length to prohibit
      Prefix Length values greater than 127.

   *  Modified the language from [RFC6550] Target Option definition so
      that the trailing zero bits of the Prefix Length are no longer
      described as "reserved".

   *  Reclassified [RFC3561] and [RFC6998] as Informative.

   *  Added citation for [RFC8174] to Terminology section.

B.9.  Changes from version 05 to version 06




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   *  Added Security Considerations based on the security mechanisms
      defined in [RFC6550].

   *  Clarified the nature of improvements due to P2P route discovery
      versus bidirectional asymmetric route discovery.

   *  Editorial improvements and corrections.

B.10.  Changes from version 04 to version 05

   *  Add description for sequence number operations.

   *  Extend the residence duration L in section 4.1.

   *  Change AODV-RPL Target option to ART option.

B.11.  Changes from version 03 to version 04

   *  Updated RREP option format.  Remove the T bit in RREP option.

   *  Using the same RPLInstanceID for RREQ and RREP, no need to update
      [RFC6550].

   *  Explanation of Delta field in RREP.

   *  Multiple target options handling during transmission.

B.12.  Changes from version 02 to version 03

   *  Include the support for source routing.

   *  Import some features from [RFC6997], e.g., choice between hop-by-
      hop and source routing, the L field which determines the duration
      of residence in the DAG, RankLimit, etc.

   *  Define new target option for AODV-RPL, including the Destination
      Sequence Number in it.  Move the TargNode address in RREQ option
      and the OrigNode address in RREP option into ADOV-RPL Target
      Option.

   *  Support route discovery for multiple targets in one RREQ-DIO.

   *  New RPLInstanceID pairing mechanism.

Appendix C.  Contributors

      Abdur Rashid Sangi




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      Huaiyin Institute of Technology

      No.89 North Beijing Road, Qinghe District

      Huaian 223001

      P.R.  China

      Email: sangi_bahrian@yahoo.com

      Malati Hegde

      Indian Institute of Science

      Bangalore 560012

      India

      Email: malati@iisc.ac.in

      Mingui Zhang

      Huawei Technologies

      No. 156 Beiqing Rd.  Haidian District

      Beijing 100095

      P.R.  China

      Email: zhangmingui@huawei.com

Authors' Addresses

   Charles E. Perkins
   Lupin Lodge
   Los Gatos,  95033
   United States
   Email: charliep@lupinlodge.com


   S.V.R Anand
   Indian Institute of Science
   Bangalore 560012
   India
   Email: anandsvr@iisc.ac.in





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   Satish Anamalamudi
   SRM University-AP
   Amaravati Campus
   Amaravati, Andhra Pradesh 522 502
   India
   Email: satishnaidu80@gmail.com


   Bing Liu
   Huawei Technologies
   No. 156 Beiqing Rd. Haidian District
   Beijing
   100095
   China
   Email: remy.liubing@huawei.com




































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