Asymmetric AODV-P2P-RPL in Low-Power and Lossy Networks (LLNs)
draft-ietf-roll-aodv-rpl-02

ROLL                                                      S. Anamalamudi
Internet-Draft                           Huaiyin Institute of Technology
Intended status: Standards Track                                M. Zhang
Expires: March 13, 2018                                        AR. Sangi
                                                     Huawei Technologies
                                                              C. Perkins
                                                               Futurewei
                                                             S.V.R.Anand
                                             Indian Institute of Science
                                                       September 9, 2017


     Asymmetric AODV-P2P-RPL in Low-Power and Lossy Networks (LLNs)
                      draft-ietf-roll-aodv-rpl-02

Abstract

   Route discovery for symmetric and asymmetric Point-to-Point (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 hop-by-hop routing (storing mode) based
   on Ad Hoc On-demand Distance Vector Routing (AODV) based RPL
   protocol.  Two separate Instances are used to construct directional
   paths in case some of the links between source and target node are
   asymmetric.

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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   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on March 13, 2018.

Copyright Notice

   Copyright (c) 2017 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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Overview of AODV-RPL  . . . . . . . . . . . . . . . . . . . .   5
   4.  AODV-RPL Mode of Operation (MoP)  . . . . . . . . . . . . . .   5
   5.  RREQ Message  . . . . . . . . . . . . . . . . . . . . . . . .   9
   6.  RREP Message  . . . . . . . . . . . . . . . . . . . . . . . .  10
   7.  Gratuitous RREP . . . . . . . . . . . . . . . . . . . . . . .  12
   8.  Operation of Trickle Timer  . . . . . . . . . . . . . . . . .  13
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
     9.1.  New Mode of Operation: AODV-RPL . . . . . . . . . . . . .  13
     9.2.  AODV-RPL Options: RREQ and RREP . . . . . . . . . . . . .  13
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  13
   11. Future Work . . . . . . . . . . . . . . . . . . . . . . . . .  13
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  14
     12.2.  Informative References . . . . . . . . . . . . . . . . .  15
   Appendix A.  ETX/RSSI Values to select S bit  . . . . . . . . . .  15
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16

1.  Introduction

   RPL[RFC6550], the IPv6 distance vector routing protocol for Low-power
   and Lossy Networks (LLNs), is designed to support multiple traffic
   flows through a root-based Destination-Oriented Directed Acyclic
   Graph (DODAG).  For traffic flows between routers within the DODAG
   (i.e., Point-to-Point (P2P) traffic), this means that data packets
   either have to traverse the root in non-storing mode (source
   routing), or traverse a common ancestor in storing mode (hop-by-hop
   routing).  Such P2P traffic is thereby likely to flow along sub-
   optimal routes and may suffer severe traffic congestion near the DAG
   root [RFC6997], [RFC6998].

   To discover optimal paths for P2P traffic flows in RPL, P2P-RPL
   [RFC6997] specifies a temporary DODAG where the source acts as
   temporary root.  The source initiates "P2P Route Discovery mode (P2P-
   RDO)" with an address vector for both non-storing mode (H=0) and



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   storing mode (H=1).  Subsequently, each intermediate router adds its
   IP address and multicasts the P2P-RDO message, until the message
   reaches the target node (TargNode).  TargNode sends the "Discovery
   Reply" option.  P2P-RPL is efficient for source routing, but much
   less efficient for hop-by-hop routing due to the extra address vector
   overhead.  In fact, when the P2P-RDO message is being multicast from
   the source hop-by-hop, receiving nodes are able to determine a next
   hop towards the source in symmetric links.  When TargNode
   subsequently replies to the source along the established forward
   route, receiving nodes can determine the next hop towards TargNode.
   In other words, it is efficient to use only routing tables for P2P-
   RDO message instead of "Address vector" for hop-by-hop routes (H=1)
   in symmetric links.

   RPL and P2P-RPL both specify the use of a single DODAG in networks of
   symmetric links.  But, application-specific routing requirements that
   are defined in IETF ROLL Working Group [RFC5548], [RFC5673],
   [RFC5826] and [RFC5867] may need routing metrics and constraints
   enabling use of asymmetric bidirectional links.  For this purpose,
   [I-D.thubert-roll-asymlink] describes bidirectional asymmetric links
   for RPL [RFC6550] with Paired DODAGs, for which the DAG root
   (DODAGID) is common for two Instances.  This can satisfy application-
   specific routing requirements for bidirectional asymmetric links in
   base RPL [RFC6550].  P2P-RPL for Paired DODAGs, on the other hand,
   requires two DAG roots: one for the source and another for the target
   node due to temporary DODAG formation.  For networks composed of
   bidirectional asymmetric links (see Section 4), AODV-RPL specifies
   P2P route discovery, utilizing RPL with a new MoP.  AODV-RPL makes
   use of two multicast messages to discover possibly asymmetric routes.
   AODV-RPL eliminates the need for address vector control overhead,
   significantly reducing the control packet size which is important for
   Constrained LLN networks.  Both discovered routes meet the
   application specific metrics and constraints that are defined in the
   Objective Function for each Instance [RFC6552].

   The route discovery process in AODV-RPL is modeled on the analogous
   process that has been specified in AODV [RFC6550].  The on-demand
   nature of AODV route discovery is natural for the needs of peer-to-
   peer routing as envisioned for RPL-based LLNs.  Similar terminology
   has been adopted for use with the discovery messages, namely RREQ for
   Route Request, and RREP for Route Reply.  AODV-RPL is, at heart, a
   simpler protocol than AODV, since there are no analogous operations
   for flagging Route Errors, blacklisting unidirectional links,
   multihoming, or handling unnumbered interfaces.  Some of the simpler
   features of AODV, on the other hand, have been imported into AODV-RPL
   -- for instance, prefix advertisement is allowed on RREP and RREQ
   message where appropriate.




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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
   [RFC2119].  Additionally, this document uses the following terms:

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

   AODV-Instance
      Either the RREQ-Instance or RREP-Instance

   Bi-directional Asymmetric Link
      A link that can be used in both directions but with different link
      characteristics (see [I-D.thubert-roll-asymlink]).

   DODAG RREQ-Instance (or simply RREQ-Instance)
      AODV Instance built using the RREQ option; used for control
      transmission from OrigNode to TargNode, thus enabling data
      transmission from TargNode to OrigNode.

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

   downstream
      Routing along the direction from OrigNode to TargNode.

   hop-by-hop routing
      Routing when each node stores routing information about the next
      hop.

   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 application.

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

   RREQ message





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      An AODV-RPL MoP DIO message containing the RREQ option.  The
      InstanceID in the DIO object of the RREQ option MUST be always an
      odd number.

   RREP message
      An AODV-RPL MoP DIO message containing the RREP option.  The
      InstanceID in the DIO object of the RREP option MUST be always an
      even number (usually, InstanceID of RREQ-Instance+1).

   source routing
      The mechanism by which the source supplies the complete route
      towards the target node along with each data packet.  [RFC6997].

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

   upstream
      Routing along the direction from TargNode to OrigNode.

3.  Overview of AODV-RPL

   With AODV-RPL, routes from OrigNode to TargNode within the LLN
   network established are "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.  The routes discovered by
   AODV-RPL are point-to-point; in other words the routes are not
   constrained to traverse a common ancestor.  Unlike base RPL [RFC6550]
   and P2P-RPL [RFC6997], 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 routing along Paired DODAGs
   (see Section 4).

4.  AODV-RPL Mode of Operation (MoP)

   In AODV-RPL, route discovery is initiated by forming a temporary DAG
   rooted at the OrigNode.  Paired DODAGs (Instances) are constructed
   according to a new 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 (as shown in Figure 2).  Intermediate routers join the
   Paired DODAGs based on the rank as calculated from the DIO message.
   Henceforth in this document, the RREQ-Instance message means the
   AODV-RPL DIO message from OrigNode to TargNode, containing the RREQ



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   option.  Similarly, the RREP-Instance message means the AODV-RPL DIO
   message from TargNode to OrigNode, containing the RREP option.
   Subsequently, the RREQ-Instance is used for data transmission from
   TargNode to OrigNode and RREP-Instance is used for Data transmission
   from OrigNode to TargNode.

   The AODV-RPL Mode of Operation defines a new bit, the Symmetric bit
   ('S'), which is added to the base DIO message as illustrated in
   Figure 1.  OrigNode sets the the 'S' bit to 1 in the RREQ-Instance
   message when initiating route discovery.

       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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | RPLInstanceID |Version Number |             Rank              |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |G|0| MOP | Prf |     DTSN      |S|    Flags    |   Reserved    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                                                               +
       |                                                               |
       +                            DODAGID                            +
       |                                                               |
       +                                                               +
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   Option(s)...

     Figure 1: DIO modification to support asymmetric route discovery

   A device originating a AODV-RPL message supplies the following
   information in the DIO header of the message:

   'S' bit

      Symmetric bit in the DIO base object

   MOP

      MOP operation in the DIO object MUST be set to "5(TBD1)" for AODV-
      RPL DIO messages

   RPLInstanceID

      RPLInstanceID in the DIO object MUST be the InstanceID of AODV-
      Instance(RREQ-Instance).  The InstanceID for RREQ-Instance MUST be
      always an odd number.




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   DODAGID

      For RREQ-Instance :

      DODAGID in the DIO object MUST be the IPv6 address of the device
      that initiates the RREQ-Instance.

      For RREP-Instance

      DODAGID in the DIO object MUST be the IPv6 address of the device
      that initiates the RREP-Instance.

   Rank

      Rank in the DIO object MUST be the the rank of the AODV-Instance
      (RREQ-Instance).

   Metric Container Options

      AODV-Instance(RREQ-Instance) messages MAY carry one or more Metric
      Container options to indicate the relevant routing metrics.

   The 'S' bit is set to mean that the route is symmetric.  If the RREQ-
   Instance arrives over an interface that is known to be symmetric, and
   the 'S' bit is set to 1, then it remains set at 1, as illustrated in
   Figure 2.  In Figure 2 and Figure 3, BR is the BorderRouter, S is the
   OrigNode, R is an intermediate node, and D is the TargNode.
























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                                    BR
                                /    |    \
                              /      |      \
                            /        |         \
                           R         R           R
                        /   \        |          /  \
                       /     \       |         /     \
                      /       \      |        /        \
                    R -------- R --- R ----- R -------- R
                 /   \   <--s=1-->  / \    <--s=1-->   /  \
           <--s=1-->  \            /   \             /   <--s=1-->
             /         \          /     \          /         \
           S ---------- R ------ R------ R ----- R ----------- D
          / \                   / \             / \           / \
         /   \                 /   \           /   \         /   \
        /     \               /     \         /     \       /     \
       R ----- R ----------- R ----- R ----- R ----- R ---- R----- R

        >---- RREQ-Instance (Control: S-->D;  Data: D-->S) ------->
        <---- RREP-Instance (Control: D-->S;  Data: S-->D) -------<

            Figure 2: AODV-RPL with Symmetric Paired Instances

   If the RREQ-Instance arrives over an interface that is not known to
   be symmetric, or is known to be asymmetric, the 'S' bit is set to be
   0.  Moreover, if the 'S' bit arrives already set to be '0', it is set
   to be '0' on retransmission (Figure 3).  Based on the 'S' bit
   received in RREQ-Instance, the TargNode decides whether or not the
   route is symmetric before transmitting the RREP-Instance message
   upstream towards the OrigNode.  The metric used to determine symmetry
   (i.e., set the "S" bit to be "1" (Symmetric) or "0" (asymmetric)) is
   implementation specific.  We used ETX/RSSI to verify the feasibility
   of the protocol operations in this draft, as discussed in Appendix A.


















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                                     BR
                                 /    |    \
                               /      |      \
                             /        |        \
                           R          R          R
                         / \          |        /   \
                       /     \        |       /      \
                     /         \      |      /         \
                    R --------- R --- R ---- R --------- R
                  /  \   --s=1-->   / \    --s=0-->   /   \
            --s=1-->   \           /    \            /   --s=0-->
             /          \        /       \         /         \
           S ---------- R ------ R------ R ----- R ----------- D
          / \                   / \             / \           / \
         /  <--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: S-->D;  Data: D-->S) ------->
        <---- RREP-Instance (Control: D-->S;  Data: S-->D) -------<

            Figure 3: AODV-RPL with Asymmetric Paired Instances

5.  RREQ Message

       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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |     Type      |      Orig SeqNo       |      Dest SeqNo       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       |                     TargNode IPv6 Address                     |
       |                                                               |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

             Figure 4: DIO RREQ option format for AODV-RPL MoP

   OrigNode supplies the following information in the RREQ option of the
   RREQ-Instance message:

   Type

      The type of the RREQ option(see Section 9.2).

   Orig SeqNo




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      Sequence Number of OrigNode.

   Dest SeqNo

      If nonzero, the last known Sequence Number for TargNode for which
      a route is desired.

   TargNode IPv6 Address

      IPv6 address of the TargNode that receives RREQ-Instance message.

   In order to establish the upstream route from TargNode to OrigNode,
   OrigNode multicasts the RREQ-Instance message (see Figure 4) to its
   one-hop neighbours.  In order to enable intermediate nodes R_i to
   associate a future RREP message to an incoming RREQ message, the
   InstanceID of RREQ-Instance MUST assign an odd number.

   Each intermediate node R_i computes the rank for RREQ-Instance and
   creates a routing table entry for the upstream route towards the
   source if the routing metrics/constraints are satisfied.  For this
   purpose R_i must use the asymmetric link metric measured in the
   upstream direction, from R_i to its upstream neighbor that
   multicasted the RREQ-Instance message.

   When an intermediate node R_i receives a RREQ message in storing
   mode, it MUST store the OrigNode's InstanceID (RREQ-Instance) along
   with the other routing information needed to establish the route back
   to the OrigNode.  This will enable R_i to determine that a future
   RREP message (containing a paired InstanceID for the TargNode) must
   be transmitted back to the OrigNode's IP address.

   If the paths to and from TargNode are not known, the intermediate
   node multicasts the RREQ-Instance message with updated rank to its
   next-hop neighbors until the message reaches TargNode (Figure 2).
   Based on the 'S' bit in the received RREQ message, the TargNode will
   decide whether to unicast or multicast the RREP message back to
   OrigNode.

   As described in Section 7, in certain circumstances R_i MAY unicast a
   Gratuitous RREP towards OrigNode, thereby helping to minimize
   multicast overhead during the Route Discovery process.

6.  RREP Message

   The TargNode supplies the following information in the RREP message:






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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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Type      |      Dest SeqNo       | Prefix Sz |T|G| Rsvd  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |              TargNode IPv6 Address (when present)             |
     |                                                               |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

             Figure 5: DIO RREP option format for AODV-RPL MoP

   Type

      The type of the RREP option (see Section 9.2)

   Dest SeqNo

      The Sequence Number for the TargNode for which a route is
      established.

   Prefix Sz

      The size of the prefix which the route to the TargNode is
      available.  This allows routing to other nodes on the same subnet
      as the TargNode.

   'T' bit

      'T' is set to true to indicate that the TargNode IPv6 Address
      field is present

   'G' bit

      (see Section 7)

   TargNode IPv6 Address  (when present)

      IPv6 address of the TargNode that receives RREP-Instance message.

   In order to reduce the need for the TargNode IPv6 Address to be
   included with the RREP message, the InstanceID of the RREP-Instance
   is paired, whenever possible, with the InstanceID from the RREQ
   message, which is always an odd number.  The pairing is accomplished
   by adding one to the InstanceID from the RREQ message and using that,
   whenever possible, as the InstanceID for the RREP message.  If this
   is not possible (for instance because the incremented InstanceID is



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   still a valid InstanceID for another route to the TargNode from an
   earlier Route Discovery operation), then the 'T' bit is set and an
   alternative even number is chosen for the InstanceID of RREP from
   TargNode.

   The OrigNode IP address for RREQ-Instance is available as the DODAGID
   in the DIO base message (see Figure 1).  When TargNode receives a
   RREQ message with the 'S' bit set to 1 (as illustrated in Figure 2),
   it unicasts the RREP message with the 'S' bit set to 1.  In this
   case, route control messages and application data between OrigNode
   and TargNode for both RREQ-Instance and RREP-Instance are transmitted
   along symmetric links.  When the 'T' bit is set to "1" in the RREP-
   Instance, then the TargNode IPv6 Address is transmitted in the RREP
   option.  Otherwise, the TargNode IPv6 Address is elided in the RREP
   option.

   When (as illustrated in Figure 3) the TargNode receives RREQ message
   with the 'S' bit set to 0, it also multicasts the RREP message with
   the 'S' bit set to 0.  Intermediate nodes create a routing table
   entry for the path towards the TargNode while processing the RREP
   message to OrigNode.  Once OrigNode receives the RREP message, it
   starts transmitting the application data to TargNode along the path
   as discovered through RREP messages.  On the other hand, application
   data from TargNode to OrigNode is transmitted through the path that
   is discovered from RREQ message.

7.  Gratuitous RREP

   Under some circumstances, an Intermediate Node that receives a RREQ
   message MAY transmit a "Gratuitous" RREP message back to OrigNode
   instead of continuing to multicast the RREQ message towards TargNode.
   For these circumstances, the 'G' bit of the RREP option is provided
   to distinguish the Gratuitous RREP sent by the Intermediate node from
   the RREP sent by TargNode.

   When an Intermediate node R receives a RREQ message and has recent
   information about the cost of an upstream route from TargNode to R,
   then R MAY unicast the Gratuitous RREP (GRREP) message to OrigNode.
   R determines whether its information is sufficiently recent by
   comparing the value it has stored for the Sequence Number of TargNode
   against the DestSeqno in the incoming RREQ message.  R also must have
   information about the metric information of the upstream route from
   TargNode.  The GRREP message MUST have PrefixSz == 0 and the 'G' bit
   set to 1.  R SHOULD also unicast the RREQ message to TargNode, to
   make sure that TargNode will have a route to OrigNode.






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8.  Operation of Trickle Timer

   The trickle timer operation to control RREQ-Instance/RREP-Instance
   multicast is similar to that in P2P-RPL [RFC6997].

9.  IANA Considerations

9.1.  New Mode of Operation: AODV-RPL

   IANA is required to assign a new Mode of Operation, named "AODV-RPL"
   for Point-to-Point(P2P) hop-by-hop routing under the RPL registry.
   The value of TBD1 is assigned from the "Mode of Operation" space
   [RFC6550].

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

                        Figure 6: Mode of Operation

9.2.  AODV-RPL Options: RREQ and RREP

   Two entries are required for new AODV-RPL options "RREQ-Instance" and
   "RREQ-Instance", with values of TBD2 (0x0A) and TBD3 (0x0B) from the
   "RPL Control Message Options" space [RFC6550].

             +-------------+---------------------+---------------+
             |    Value    |       Meaning       |   Reference   |
             +-------------+---------------------+---------------+
             | TBD2 (0x0A) |     RREQ Option     | This document |
             +-------------+---------------------+---------------+
             | TBD3 (0x0B) |     RREP Option     | This document |
             +-------------+---------------------+---------------+

                        Figure 7: AODV-RPL Options

10.  Security Considerations

   This document does not introduce additional security issues compared
   to base RPL.  For general RPL security considerations, see [RFC6550].

11.  Future Work

   It may become feasible in the future to design a non-storing version
   of AODV-RPL's route discovery protocol.  Under the current assumption
   of route asymmetry across bidirectional links, the specification is



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   expected to be straightforward.  It should be possible to re-use the
   same methods of incremental construction for source routes within
   analogous fields within AODV-RPL's RREQ and RREP messages as is
   currently done for DAO messages -- in other words the RPL messages
   for DODAG construction.

   There has been some discussion about how to determine the initial
   state of a link after an AODV-RPL-based network has begun operation.
   The current draft operates as if the links are symmetric until
   additional metric information is collected.  The means for making
   link metric information is considered out of scope for AODV-RPL.  In
   the future, RREQ and RREP messages could be equipped with new fields
   for use in verifying link metrics.  In particular, it is possible to
   identify unidirectional links; an RREQ received across a
   unidirectional link has to be dropped, since the destination node
   cannot make use of the received DODAG to route packets back to the
   source node that originated the route discovery operation.  This is
   roughly the same as considering a unidirectional link to present an
   infinite cost metric that automatically disqualifies it for use in
   the reverse direction.

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>.

   [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>.

   [RFC5548]  Dohler, M., Ed., Watteyne, T., Ed., Winter, T., Ed., and
              D. Barthel, Ed., "Routing Requirements for Urban Low-Power
              and Lossy Networks", RFC 5548, DOI 10.17487/RFC5548, May
              2009, <https://www.rfc-editor.org/info/rfc5548>.

   [RFC5673]  Pister, K., Ed., Thubert, P., Ed., Dwars, S., and T.
              Phinney, "Industrial Routing Requirements in Low-Power and
              Lossy Networks", RFC 5673, DOI 10.17487/RFC5673, October
              2009, <https://www.rfc-editor.org/info/rfc5673>.







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   [RFC5826]  Brandt, A., Buron, J., and G. Porcu, "Home Automation
              Routing Requirements in Low-Power and Lossy Networks",
              RFC 5826, DOI 10.17487/RFC5826, April 2010,
              <https://www.rfc-editor.org/info/rfc5826>.

   [RFC5867]  Martocci, J., Ed., De Mil, P., Riou, N., and W. Vermeylen,
              "Building Automation Routing Requirements in Low-Power and
              Lossy Networks", RFC 5867, DOI 10.17487/RFC5867, June
              2010, <https://www.rfc-editor.org/info/rfc5867>.

   [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>.

   [RFC6552]  Thubert, P., Ed., "Objective Function Zero for the Routing
              Protocol for Low-Power and Lossy Networks (RPL)",
              RFC 6552, DOI 10.17487/RFC6552, March 2012,
              <https://www.rfc-editor.org/info/rfc6552>.

   [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>.

12.2.  Informative References

   [I-D.thubert-roll-asymlink]
              Thubert, P., "RPL adaptation for asymmetrical links",
              draft-thubert-roll-asymlink-02 (work in progress),
              December 2011.

Appendix A.  ETX/RSSI Values to select S bit

   We have tested the combination of "RSSI(downstream)" and "ETX
   (upstream)" to decide whether the link is symmetric or asymmetric at
   the intermediate nodes.  The example of how the ETX and RSSI values
   are used in conjuction is explained below:




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

          Figure 8: Communication link from Source to Destination

   +-------------------------+----------------------------------------+
   | RSSI at NodeA for NodeB | Expected ETX at NodeA for nodeB->nodeA |
   +-------------------------+----------------------------------------+
   |          > -15          |                  150                   |
   |        -25 to -15       |                  192                   |
   |        -35 to -25       |                  226                   |
   |        -45 to -35       |                  662                   |
   |        -55 to -45       |                  993                   |
   +-------------------------+----------------------------------------+

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

   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, say, within 1:3
   ratio.  This ratio should be taken as a notional metric for deciding
   link symmetric/asymmetric nature, and precise definition of the ratio
   is beyond the scope of the draft.  In general, NodeA can only 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 "S=0".  Later on, the link from
   NodeA to Destination is asymmetric with "S" bit remains to "0".

Authors' Addresses

   Satish Anamalamudi
   Huaiyin Institute of Technology
   No.89 North Beijing Road, Qinghe District
   Huaian  223001
   China

   Email: satishnaidu80@gmail.com








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   Mingui Zhang
   Huawei Technologies
   No. 156 Beiqing Rd. Haidian District
   Beijing  100095
   China

   Email: zhangmingui@huawei.com


   Abdur Rashid Sangi
   Huawei Technologies
   No.156 Beiqing Rd. Haidian District
   Beijing  100095
   P.R. China

   Email: sangi_bahrian@yahoo.com


   Charles E. Perkins
   Futurewei
   2330 Central Expressway
   Santa Clara  95050
   Unites States

   Email: charliep@computer.org


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

   Email: anand@ece.iisc.ernet.in


















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