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Versions: (draft-satish-roll-aodv-rpl)  00 01 02         Standards Track
          03 04 05 06 07 08 09 10 11                                    
ROLL                                                      S. Anamalamudi
Internet-Draft                                         SRM University-AP
Intended status: Standards Track                              C. Perkins
Expires: March 20, 2022                                      Lupin Lodge
                                                             S.V.R.Anand
                                             Indian Institute of Science
                                                                  B. Liu
                                                     Huawei Technologies
                                                      September 16, 2021


      Supporting Asymmetric Links in Low Power Networks: AODV-RPL
                      draft-ietf-roll-aodv-rpl-11

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 routing 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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on March 20, 2022.

Copyright Notice

   Copyright (c) 2021 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 Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Overview of AODV-RPL  . . . . . . . . . . . . . . . . . . . .   6
   4.  AODV-RPL DIO Options  . . . . . . . . . . . . . . . . . . . .   6
     4.1.  AODV-RPL RREQ Option  . . . . . . . . . . . . . . . . . .   6
     4.2.  AODV-RPL RREP Option  . . . . . . . . . . . . . . . . . .   8
     4.3.  AODV-RPL Target Option  . . . . . . . . . . . . . . . . .  10
   5.  Symmetric and Asymmetric Routes . . . . . . . . . . . . . . .  11
   6.  AODV-RPL Operation  . . . . . . . . . . . . . . . . . . . . .  13
     6.1.  Route Request Generation  . . . . . . . . . . . . . . . .  13
     6.2.  Receiving and Forwarding RREQ messages  . . . . . . . . .  14
       6.2.1.  General Processing  . . . . . . . . . . . . . . . . .  14
       6.2.2.  Additional Processing for Multiple Targets  . . . . .  16
     6.3.  Generating Route Reply (RREP) at TargNode . . . . . . . .  16
       6.3.1.  RREP-DIO for Symmetric route  . . . . . . . . . . . .  16
       6.3.2.  RREP-DIO for Asymmetric Route . . . . . . . . . . . .  17
       6.3.3.  RPLInstanceID Pairing . . . . . . . . . . . . . . . .  17
     6.4.  Receiving and Forwarding Route Reply  . . . . . . . . . .  18
   7.  Gratuitous RREP . . . . . . . . . . . . . . . . . . . . . . .  19
   8.  Operation of Trickle Timer  . . . . . . . . . . . . . . . . .  20
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  20
     9.1.  New Mode of Operation: AODV-RPL . . . . . . . . . . . . .  20
     9.2.  AODV-RPL Options: RREQ, RREP, and Target  . . . . . . . .  21
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  21
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  22
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  22
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  22
     12.2.  Informative References . . . . . . . . . . . . . . . . .  23
   Appendix A.  Example: Using ETX/RSSI Values to determine value of
                S bit  . . . . . . . . . . . . . . . . . . . . . . .  24
   Appendix B.  Changelog  . . . . . . . . . . . . . . . . . . . . .  26
     B.1.  Changes from version 10 to version 11 . . . . . . . . . .  26
     B.2.  Changes from version 09 to version 10 . . . . . . . . . .  27
     B.3.  Changes from version 08 to version 09 . . . . . . . . . .  27
     B.4.  Changes from version 07 to version 08 . . . . . . . . . .  28
     B.5.  Changes from version 06 to version 07 . . . . . . . . . .  29



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     B.6.  Changes from version 05 to version 06 . . . . . . . . . .  29
     B.7.  Changes from version 04 to version 05 . . . . . . . . . .  29
     B.8.  Changes from version 03 to version 04 . . . . . . . . . .  29
     B.9.  Changes from version 02 to version 03 . . . . . . . . . .  29
   Appendix C.  Contributors . . . . . . . . . . . . . . . . . . . .  30
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  30

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 [RFC6997],
   [RFC6998]).  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
   procedure specified in AODV [RFC3561].  The on-demand nature of AODV
   route discovery is natural for the needs of peer-to-peer routing in
   RPL-based LLNs.  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.

   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 specifies a new Mode of Operation (MOP) running in a separate
   instance dedicated to discover P2P routes, which may differ from
   routes discoverable by native RPL.  AODV-RPL can be operated whether
   or not native RPL is running otherwise.







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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].

   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.

   DIO
      DODAG Information Object

   DODAG RREQ-Instance (or simply RREQ-Instance)
      RPL Instance built using the DIO with RREQ option; used for
      control message transmission 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
      control message transmission 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 routing
      Routing when each node stores routing information about the next
      hop.




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   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.

   RREQ-DIO message
      An AODV-RPL MOP 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].

   RREP-DIO message
      An AODV-RPL MOP DIO message containing the RREP option.  The
      RPLInstanceID in RREP-DIO MUST be paired to the one in the
      associated RREQ-DIO message as described in Section 6.3.2.  The
      RREP-DIO message has a secure variant as noted in [RFC6550].

   Source routing
      A mechanism by which the source supplies the complete route
      towards the target 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.

   ART option



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

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 is thus functional
   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.  When possible,
   AODV-RPL also enables symmetric route discovery along Paired DODAGs
   (see Section 5).

   In AODV-RPL, routes are discovered by first forming a temporary DAG
   rooted at the OrigNode.  Paired DODAGs (Instances) are constructed
   according to the AODV-RPL Mode of Operation (MOP) 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.  Intermediate routers join the Paired DODAGs based on
   the Rank [RFC6550] as calculated from the DIO message.  Henceforth in
   this document, the RREQ-DIO message means the AODV-RPL mode DIO
   message from OrigNode to TargNode, containing the RREQ option (see
   Section 4.1).  Similarly, the RREP-DIO message means the AODV-RPL
   mode DIO message from TargNode to OrigNode, containing the RREP
   option (see Section 4.2).  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.

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.  Exactly one RREQ option MUST be
   present in a RREQ-DIO message, otherwise the message MUST be dropped.








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      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 |   MaxRank   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  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.

   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 set to zero.

   Compr
      4-bit unsigned integer.  Number of prefix octets that are elided
      from 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





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      2-bit unsigned integer determining the length of time 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 the time.

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

      L is independent from the route lifetime, which is defined in the
      DODAG configuration option.

   MaxRank
      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.

   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 MaxRank.  Other nodes MUST NOT join a
   RREQ instance if its own Rank would be equal to or higher than
   MaxRank.  A router MUST discard a received RREQ if the integer part
   of the advertised Rank equals or exceeds the MaxRank limit.

4.2.  AODV-RPL RREP Option

   TargNode sets one of its IPv6 addresses in the DODAGID field of the
   RREP-DIO message.  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:











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        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 |   MaxRank   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   Shift   |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 route (see Section 7).

   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 set to zero.

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

   L
      2-bit unsigned integer defined as in RREQ option.

   MaxRank
      Similarly to MaxRank 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.

   Shift




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      6-bit unsigned integer.  This field is used to recover the
      original RPLInstanceID (see Section 6.3.3); 0 indicates that the
      original RPLInstanceID is used.

   X X
      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.

   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 MOP

   Option Type
      TBD4




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   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
      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.  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.







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                                     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 Paired 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
   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.

   The criteria used to determine whether or not each link is symmetric
   is beyond the scope of the document.  For instance, 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.




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   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.

                                     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 data transmission.  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) via
   link-local multicast.  The DIO MUST contain at least one ART Option
   (see Section 4.3).  The required ART Option 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 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 requirements 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 length of time specified by the L field has elapsed, the OrigNode
   MUST leave the DODAG and stop sending RREQ-DIOs in the related
   RPLInstance.

6.2.  Receiving and Forwarding RREQ messages

6.2.1.  General Processing

   Upon receiving a RREQ-DIO, a router goes through the steps below.  If
   the router has not joined the RREQ-Instance, then the maximum useful
   rank (MaxUseRank) is MaxRank.  Otherwise, MaxUseRank 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.

   Step 1:

      The router MUST first determine whether to propagate the RREQ-DIO.
      It does this by determining whether or not the downstream
      direction of the incoming link satisfies the Objective Function
      (OF).  If not 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.  The upstream
      neighbor router that transmitted the received RREQ-DIO is selected
      as the preferred parent.

   Step 2:

      If the H bit is set to 1, then the router (TargNode or
      intermediate) MUST build an upward route entry towards OrigNode
      which includes at least the following items: Source Address,



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      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, and the Source Address is the address used by the
      local 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,
      and 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.

   Step 3:

      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 for RPLInstance pairing
      as described in Section 6.4.

   Step 4:

      The router checks whether one of its addresses is included in one
      of the ART Options.  If so, this router is one of the TargNodes.
      Otherwise, it is an intermediate router.

      If the router is an intermediate router, then it transmits the
      RREQ-DIO via link-local multicast; if the H bit is set to 0, the
      intermediate router MUST include the address of the interface
      receiving the RREQ-DIO into the address vector.  Otherwise, the
      router is TargNode; if it was not already associated with the
      RREQ-Instance, it prepares and transmits a RREP-DIO (Section 6.3).
      If, on the other hand, TargNode was already associated with the
      RREQ-Instance, it takes no further action and does not send an
      RREP-DIO.






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6.2.2.  Additional Processing for Multiple Targets

   If the OrigNode tries to reach multiple TargNodes in a single RREQ-
   Instance, one of the TargNodes can be an intermediate router to the
   others, therefore it MUST continue sending RREQ-DIO to reach other
   targets.  In this case, before transmitting the RREQ-DIO via link-
   local multicast, 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.  When transmitting the RREQ-DIO, the intersection
   of all received lists MUST be included.  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.  If the intersection is empty, it
   means that all the targets have been reached, and the router MUST NOT
   transmit any RREQ-DIO.  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.  Any incoming RREQ-DIO message having multiple
   ART Options coming from a router with higher Rank than the Rank of
   the stored targets is ignored.

6.3.  Generating Route Reply (RREP) at TargNode

   When H=1 in the incoming RREQ, the TargNode MUST NOT generate a RREP
   if one of its addresses is present in the Address Vector.  If the
   implementation selects the symmetric route, and 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.

6.3.1.  RREP-DIO for Symmetric route

   If a RREQ-DIO arrives at TargNode with 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 scope.




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   For a symmetric route, the RREP-DIO message is unicast to the next
   hop according to the accumulated address vector (H=0) or the route
   entry (H=1).  Thus 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
   received in the RREQ-DIO MUST be included in the RREP-DIO.  TargNode
   increments its current sequence number and uses the incremented
   result in the Dest SeqNo in the ART option of the RREQ-DIO.  The
   address of the OrigNode MUST be encapsulated in the ART Option and
   included in this RREP-DIO message.

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 discover the downstream route
   from the OrigNode to the TargNode.  The RREP-DIO message MUST be
   transmitted via link-local multicast until the OrigNode is reached or
   MaxRank is exceeded.

   The settings of the fields in RREP option and ART option are the same
   as for the symmetric route, except for the value of the S bit
   associated with the RREP-instance.

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.  To avoid
   any mismatch, the RREQ-Instance and the RREP-Instance in the same
   route discovery MUST be paired 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 original
   RPLInstanceID of an already active RREP-Instance MUST NOT be used
   again for assigning RPLInstanceID for the later RREP-Instance.
   Reusing the same RPLInstanceID for two distinct DODAGs originated
   with the same DODAGID (TargNode address) would prevent intermediate
   routers to distinguish between these DODAGs (and their associated
   Objective Functions).  Instead, the RPLInstanceID MUST be replaced by



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   another value so that the two RREP-instances can be distinguished.
   In RREP-DIO option, the Shift field of the RREP-DIO message(Figure 2)
   indicates the shift to be applied to original RPLInstanceID to obtain
   the replacement RPLInstanceID.  When the new RPLInstanceID after
   shifting exceeds 255, it rolls over starting at 0.  For example, if
   the original RPLInstanceID is 252, and shifted 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 performs the following steps:

   Step 1:

      If the Objective Function is not satisfied, the router MUST NOT
      join the DODAG; the router MUST discard the RREQ-DIO, and does not
      execute the remaining steps in this section.  An Intermediate
      Router MUST NOT forward 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 step 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
      MaxRank limit.  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.

   Step 2:

      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.

   Step 3:

      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



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      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 RREQ-Instance.  The RPLInstanceID in the route entry
      MUST be the original RPLInstanceID (after subtracting the Shift
      field value).  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.

   Step 4:

      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,
      in case of an asymmetric route, the intermediate router MUST
      include the address of the interface receiving the RREP-DIO into
      the address vector, and then transmit the RREP-DIO via link-local
      multicast.  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).  The RPLInstanceID
      in the transmitted RREP-DIO is the same as the value in the
      received RREP-DIO.  The local knowledge for the TargNode's
      sequence number SHOULD be updated.

   Upon receiving a RREP-DIO, a router which already belongs to the
   RREP-Instance SHOULD drop the RREP-DIO.

7.  Gratuitous RREP

   In some cases, an Intermediate router that receives a RREQ-DIO
   message MAY transmit a "Gratuitous" RREP-DIO message back to OrigNode
   instead of continuing to multicast the RREQ-DIO towards TargNode.
   The intermediate router effectively builds the RREP-Instance on
   behalf of the actual TargNode.  The G bit of the RREP option is
   provided to distinguish the Gratuitous RREP-DIO (G=1) sent by the
   Intermediate node from the RREP-DIO sent by TargNode (G=0).

   The gratuitous RREP-DIO can be sent out when an intermediate router
   receives a RREQ-DIO for a TargNode, and the router has a more recent
   (larger destination sequence number) pair of downward and upward
   routes to the TargNode which also satisfy the Objective Function.



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   In case of source routing, the intermediate router MUST unicast the
   received RREQ-DIO to TargNode including the address vector between
   the OrigNode and the router.  Thus the TargNode can have a complete
   upward route address vector from itself to the OrigNode.  Then the
   router MUST transmit the gratuitous RREP-DIO including the address
   vector from the router itself to the TargNode.

   In case of hop-by-hop routing, 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.  The above
   process will happen recursively until the RREQ-DIO arrives at the
   TargNode.  Then the TargNode MUST unicast recursively the RREP-DIO
   hop-by-hop to the intermediate router, and the routers along the
   route SHOULD build new route entries in the upward direction.  Upon
   receiving the unicast RREP-DIO, the intermediate router sends the
   gratuitous RREP-DIO to the OrigNode as defined in Section 6.3.

8.  Operation of Trickle Timer

   The trickle timer operation to control RREQ-Instance/RREP-Instance
   multicast uses [RFC6206] to control RREQ-DIO and RREP-DIO
   transmissions.  The Trickle control of these DIO transmissions follow
   the procedures described in the Section 8.3 of [RFC6550] entitled
   "DIO Transmission".

9.  IANA Considerations

   Note to RFC editor:

   The sentences "The parenthesized number 5 is only a suggestion." and
   "The parenthesized numbers are only suggestions." are 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.

9.1.  New Mode of Operation: AODV-RPL

   IANA is asked to assign a new Mode of Operation, named "AODV-RPL" for
   peer-to-peer hop-by-hop routing from the "Mode of Operation"
   Subregistry.  The parenthesized number 5 is only a suggestion.









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              +-------------+---------------+---------------+
              |    Value    |  Description  |   Reference   |
              +-------------+---------------+---------------+
              |   TBD1 (5)  |   AODV-RPL    | This document |
              +-------------+---------------+---------------+

                        Figure 6: Mode of Operation

9.2.  AODV-RPL Options: RREQ, RREP, and Target

   IANA is asked to assign three new AODV-RPL options "RREQ", "RREP" and
   "ART", as described in Figure 7 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 7: AODV-RPL Options

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



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   route(s).  It could generate bogus 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 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, significant
   damage might result.

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>.









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   [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>.

   [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]  Ballamajalu, Rashmi., S.V.R., Anand., 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,
              Jan 2018.

   [contiki]  Contiki contributors, "The Contiki Open Source OS for the
              Internet of Things (Contiki Version 2.7)", Nov 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)", Dec
              2020, <https://github.com/contiki-ng/contiki-ng>.

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

   [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>.







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   [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>.

   [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 nodes.  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



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      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 8: Communication link from Source to Destination

   +-------------------------+----------------------------------------+
   | 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 8) 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



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   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.

Appendix B.  Changelog

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

B.1.  Changes from version 10 to version 11

   o  Numerous editorial improvements.

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

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

   o  Improved the procedures in section 6.2.1.

   o  Define the S bit of the data structure a node 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.

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

   o  Clarified the relationship of AODV-RPL to RPL.

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

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

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

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

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




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   o  Added text about determining link symmetry near Figure 5.

   o  Mandated checking the Address Vector to avoid routing loops.

   o  Improved specification for use of the Shift value in
      Section 6.3.3.

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

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

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

   o  Added acknowledgements and contributors.

B.2.  Changes from version 09 to version 10

   o  Changed the title for brevity and to remove acronyms.

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

   o  Expanded DAO and P2MP in Section 1.

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

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

   o  Several editorial improvements and clarifications.

B.3.  Changes from version 08 to version 09

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

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

   o  SHOULD has been changed to MUST in Section 4.2.

   o  Expanded the terms ETX and RSSI in Section 5.

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






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

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

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

   o  Numerous editorial improvements and clarifications.

B.4.  Changes from version 07 to version 08

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

   o  Removed all normative dependencies on [RFC6997]

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

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

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

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

   o  Improved capitalization, quotation, and spelling variations.

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

   o  Fixed numerous language issues in IANA Considerations Section 9.

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

   o  Numerous editorial improvements and clarifications.






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B.5.  Changes from version 06 to version 07

   o  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.

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

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

   o  Added citation for [RFC8174] to Terminology section.

B.6.  Changes from version 05 to version 06

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

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

   o  Editorial improvements and corrections.

B.7.  Changes from version 04 to version 05

   o  Add description for sequence number operations.

   o  Extend the residence duration L in section 4.1.

   o  Change AODV-RPL Target option to ART option.

B.8.  Changes from version 03 to version 04

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

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

   o  Explanation of Shift field in RREP.

   o  Multiple target options handling during transmission.

B.9.  Changes from version 02 to version 03

   o  Include the support for source routing.





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   o  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, MaxRank, etc.

   o  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.

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

   o  New RPLInstanceID pairing mechanism.

Appendix C.  Contributors

      Abdur Rashid Sangi
      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

   Satish Anamalamudi
   SRM University-AP
   Amaravati Campus
   Amaravati, Andhra Pradesh  522 502
   India

   Email: satishnaidu80@gmail.com







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   Charles E. Perkins
   Lupin Lodge
   Los Gatos  95033
   United States

   Email: charliep@computer.org


   S.V.R Anand
   Indian Institute of Science
   Bangalore  560012
   India

   Email: anandsvr@iisc.ac.in


   Bing Liu
   Huawei Technologies
   No. 156 Beiqing Rd. Haidian District
   Beijing  100095
   China

   Email: remy.liubing@huawei.com




























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