ROLL                                                      S. Anamalamudi
Internet-Draft                           Huaiyin Institute of Technology
Intended status: Standards Track                                M. Zhang
Expires: March 13, September 6, 2018                                        AR. Sangi                           Huawei Technologies
                                                               AR. Sangi
                                         Huaiyin Institute of Technology
                                                              C. Perkins
                                                               Futurewei
                                                             S.V.R.Anand
                                             Indian Institute of Science
                                                       September 9, 2017
                                                                  B. Liu
                                                     Huawei Technologies
                                                           March 5, 2018

     Asymmetric AODV-P2P-RPL in Low-Power and Lossy Networks (LLNs)
                      draft-ietf-roll-aodv-rpl-02
                      draft-ietf-roll-aodv-rpl-03

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

Status of This Memo

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   This Internet-Draft will expire on March 13, September 6, 2018.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Overview of AODV-RPL  . . . . . . . . . . . . . . . . . . . .   5   6
   4.  AODV-RPL Mode of Operation (MoP) DIO Options  . . . . . . . . . . . . . .   5
   5. . . . . . .   6
     4.1.  AODV-RPL DIO RREQ Message Option  . . . . . . . . . . . . . . . .   6
     4.2.  AODV-RPL DIO RREP Option  . . . . . . . . .   9 . . . . . . .   8
     4.3.  AODV-RPL DIO Target Option  . . . . . . . . . . . . . . .  10
   5.  Symmetric and Asymmetric Routes . . . . . . . . . . . . . . .  11
   6.  RREP Message  AODV-RPL Operation  . . . . . . . . . . . . . . . . . . . . .  13
     6.1.  Generating Route Request at OrigNode  . . .  10 . . . . . . .  13
     6.2.  Receiving and Forwarding Route Request  . . . . . . . . .  14
     6.3.  Generating Route Reply at TargNode  . . . . . . . . . . .  15
       6.3.1.  RREP-DIO for Symmetric route  . . . . . . . . . . . .  15
       6.3.2.  RREP-DIO for Asymmetric Route . . . . . . . . . . . .  16
       6.3.3.  RPLInstanceID Pairing . . . . . . . . . . . . . . . .  16
     6.4.  Receiving and Forwarding Route Reply  . . . . . . . . . .  17
   7.  Gratuitous RREP . . . . . . . . . . . . . . . . . . . . . . .  12  18
   8.  Operation of Trickle Timer  . . . . . . . . . . . . . . . . .  13  19
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13  19
     9.1.  New Mode of Operation: AODV-RPL . . . . . . . . . . . . .  13  19
     9.2.  AODV-RPL Options: RREQ RREQ, RREP, and RREP . . Target  . . . . . . . . . . .  13  19
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  13  20
   11. Future Work . . . . . . . . . . . . . . . . . . . . . . . . .  13  20
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  14  20
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  14  20
     12.2.  Informative References . . . . . . . . . . . . . . . . .  15  21
   Appendix A.  ETX/RSSI Values to select S bit  . . . . . . . . . .  15  21
   Appendix B.  Changes to version 02  . . . . . . . . . . . . . . .  22
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16  23

1.  Introduction

   RPL[RFC6550], the

   RPL[RFC6550] is a IPv6 distance vector routing protocol for Low-power
   and Lossy Networks (LLNs), and is designed to support multiple
   traffic flows through a root-based Destination-Oriented Directed
   Acyclic Graph (DODAG).  For traffic flows between routers within the  Typically, a router does not have routing
   information for most other routers.  Consequently, for traffic
   between routers within the DODAG (i.e., Point-to-Point (P2P) traffic), this means that traffic)
   data packets either have to traverse the root in non-storing mode (source
   routing), mode, or
   traverse a common ancestor in storing mode (hop-by-hop
   routing). mode.  Such P2P traffic is
   thereby likely to flow along sub-
   optimal traverse 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 a
   temporary root.  The source initiates "P2P DIOs encapsulating the P2P
   Route Discovery mode (P2P-
   RDO)" option (P2P-RDO) with an address vector for both non-storing hop-
   by-hop mode (H=0) (H=1) and
   storing source routing mode (H=1). (H=0).  Subsequently, each
   intermediate router adds its IP address and multicasts the P2P-RDO message, P2P mode
   DIOs, until the message reaches the target node (TargNode).  TargNode
   sends the "Discovery Reply" option. object.  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,  However, for symmetric links, when
   the P2P-RDO P2P mode DIO message is being multicast from the source hop-by-hop, hop-by-
   hop, receiving nodes are able to determine can infer a next hop towards the source in symmetric links. source.  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 P2P-RDO message instead of "Address vector" for hop-by-hop routes
   (H=1)
   in over symmetric links.

   RPL and P2P-RPL both specify the use of a single DODAG in networks of
   symmetric links. links, where the two directions of a link MUST both satisfy
   the constraints of the objective function.  This eliminates the
   possibility to use asymmetric links which are qualified in one
   direction.  But, application-specific routing requirements that
   are as defined
   in IETF ROLL Working Group [RFC5548], [RFC5673], [RFC5826] and
   [RFC5867] may need be satisfied by routing metrics and constraints
   enabling use of asymmetric paths using bidirectional
   asymmetric 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 application-specific routing
   requirements for bidirectional asymmetric links in
   base core RPL
   [RFC6550].  Using P2P-RPL for twice with 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), 5), 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. routes,
   which can achieve higher route diversity.  AODV-RPL eliminates the
   need for address vector control overhead, overhead in hop-by-hop mode.  This
   significantly reducing reduces the control packet size size, which is important for
   Constrained LLN networks.  Both discovered routes (upward and
   downward) 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
   procedure specified in AODV [RFC6550]. [RFC3561].  The on-demand nature of AODV
   route discovery is natural for the needs of peer-to-
   peer peer-to-peer routing as envisioned for in
   RPL-based LLNs.  Similar  AODV terminology has been adopted adapted for use with the discovery AODV-
   RPL 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 currently omits some features compared to AODV -- in
   particular, flagging Route Errors, blacklisting unidirectional links,
   multihoming, or and 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.

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

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

   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 fulfills the constraints
      in route discovery.  If the OrigNode doesn't require an upward
      route towards itself, the route is also considered as asymmetric.

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

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

   downstream
      Routing along the

   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.

   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 of an application.

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

   RREQ

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

   RREP OrigNode.

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

   source
      associated RREQ-DIO message.

   Source routing
      The mechanism by which the source supplies the complete route
      towards the target node along with each data packet.  [RFC6997]. packet [RFC6550].

   Symmetric route
      The upstream and downstream routes traverse the same routers.
      Both directions fulfill the constraints in route discovery.

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

   upstream
      Routing along the

   Upward Direction
      The direction from the TargNode to the OrigNode.

   Upward Route
      A route in the upward direction.

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 core 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 route discovery along
   Paired DODAGs (see Section 4).

4.  AODV-RPL Mode of Operation (MoP) 5).

   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). 4).  Intermediate routers join the
   Paired DODAGs based on the rank as calculated from the DIO message.
   Henceforth in this document, the RREQ-Instance RREQ-DIO message means the AODV-RPL
   mode DIO message from OrigNode to TargNode, containing the RREQ
   option.  Similarly, the RREP-Instance RREP-DIO message means the AODV-RPL mode DIO
   message from TargNode to OrigNode, containing the RREP option.
   Subsequently, the route discovered in the RREQ-Instance is used for
   data transmission from TargNode to OrigNode OrigNode, and the route discovered
   in RREP-Instance is used for Data transmission from OrigNode to
   TargNode.

   The

4.  AODV-RPL Mode of Operation defines a new bit, the Symmetric bit
   ('S'), which is added to the base DIO Options

4.1.  AODV-RPL DIO RREQ Option

   A RREQ-DIO message as illustrated in
   Figure 1.  OrigNode sets the the 'S' bit to 1 in the RREQ-Instance
   message when initiating route discovery. MUST carry exactly one RREQ option.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | RPLInstanceID |Version Number     Type      | Option Length |S|H|X| Compr |             Rank L |  MaxRank    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |G|0| MOP
     | Prf  Orig SeqNo   |     DTSN      |S|    Flags                                               |   Reserved
     +-+-+-+-+-+-+-+-+                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
       +                                                               +
     |                                                               |
       +                            DODAGID                            +
     |         Address Vector (Optional, Variable Length)            |
     |
       +                                                               +                                                               |
     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                                                               |   Option(s)...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

             Figure 1: DIO modification to support asymmetric route discovery

   A device originating a RREQ option format for AODV-RPL message MoP

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

   'S' bit

      Symmetric bit in

   Type

      The type of the DIO base object

   MOP

      MOP operation in RREQ option(see Section 9.2).

   Option Length

      Length of the DIO object MUST be set to "5(TBD1)" for AODV-
      RPL DIO messages

   RPLInstanceID

      RPLInstanceID option in octets excluding the DIO object MUST be Type and Length
      fields.  Variable due to the InstanceID presence of AODV-
      Instance(RREQ-Instance).  The InstanceID for RREQ-Instance MUST be
      always an odd number.

   DODAGID

      For RREQ-Instance :

      DODAGID in the DIO object MUST be the IPv6 address of vector and
      the device
      that initiates number of octets elided according to the RREQ-Instance.

      For RREP-Instance

      DODAGID in Compr value.

   S

      Symmetric bit indicating a symmetric route from the DIO object MUST OrigNode to
      the router issuing this RREQ-DIO.  The bit SHOULD be set to 1 in
      the RREQ-DIO when the OrigNode initiates the route discovery.

   X

      Reserved.

   H

      The OrigNode sets this flag to one if it desires a hop-by-hop
      route.  It sets this flag to zero if it desires a source route.
      This flag is valid to both downstream route and upstream route.

   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 of in the DODAGID.

   L
      2-bit unsigned integer.  This field indicates the device duration that initiates a
      node joining the RREP-Instance.

   Rank

      Rank temporary DAG in RREQ-Instance, including the DIO object MUST be
      OrigNode and the TargNode.  Once the rank of time is reached, a node MUST
      leave the AODV-Instance
      (RREQ-Instance).

   Metric Container Options

      AODV-Instance(RREQ-Instance) messages MAY carry one DAG and stop sending or receiving any more Metric
      Container options to indicate DIOs for the relevant routing metrics.
      temporary DODAG.  The 'S' bit is set to mean detailed definition can be found in
      [RFC6997].

      *  0x00: No duration time imposed.
      *  0x01: 2 seconds
      *  0x02: 16 seconds
      *  0x03: 64 seconds

      It should be indicated here that L is not the route lifetime,
      which is symmetric.  If defined in the RREQ-
   Instance arrives over an interface that is known to be symmetric, DODAG configuration option.  The route
      entries in hop-by-hop routing and states of source routing can
      still be maintained even after the 'S' bit DAG expires.

   MaxRank

      This field indicates the upper limit on the integer portion of the
      rank.  A node MUST NOT join a temporary DODAG if its own rank
      would equal to or higher than the limit.  A value of 0 in this
      field indicates the limit is set infinity.  For more details please
      refer to 1, then it remains set at 1, [RFC6997].

   OrigNode Sequence Number

      Sequence Number of OrigNode, defined similarly as illustrated in
   Figure 2.  In Figure 2 and Figure 3, BR is AODV
      [RFC3561].

   Address Vector (Optional)

      A vector of IPv6 addresses representing the BorderRouter, S route that the RREQ-
      DIO has passed.  It is only present when the
   OrigNode, R 'H' bit is an intermediate node, and D set to 0.
      The prefix of each address is elided according to the TargNode.

                                    BR
                                / Compr field.

4.2.  AODV-RPL DIO RREP Option

   A RREP-DIO message MUST carry exactly one RREP option.

   The TargNode supplies the following information in the RREP option:

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |    \
                              /     Type      |      \
                            / Option Length |H|X| Compr |         \
                           R         R           R
                        /   \ L |          /  \
                       /     \   MaxRank     |         /     \
                      /       \
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |T|G|  SHIFT    |        /        \
                    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) -------<    Reserved   |                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
       |                                                               |
       |                                                               |
       |         Address Vector (Optional, Variable Length)            |
       .                                                               .
       .                                                               .
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

             Figure 2: DIO RREP option format for AODV-RPL with Symmetric Paired Instances

   If MoP

   Type
      The type of the RREQ-Instance arrives over an interface that is not known to
   be symmetric, or is known to be asymmetric, RREP option (see Section 9.2)

   Option Length
      Length of the 'S' bit is set option in octets excluding the Type and Length
      fields.  Variable due to be
   0.  Moreover, if the 'S' bit arrives already set presence of the address vector and
      the number of octets elided according to be '0', it the Compr value.

   H
      This bit indicates the downstream route is source routing (H=0) or
      hop-by-hop (H=1).  It SHOULD be set to be '0' on retransmission (Figure 3).  Based on the 'S' same as the 'H' bit
   received
      in RREQ-Instance, RREQ option.

   X

      Reserved.

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

   L
      2-bit unsigned integer with the TargNode decides whether or same definition as in Section 4.1.

   MaxRank
      Same definition as in RREQ option.

   T
      'T' is set to 1 to indicate that the RREP-DIO MUST include exactly
      one AODV-RPL Target Option.  Otherwise, the Target Option is not
      necessary in the RREP-DIO.

   G
      Gratuitous route (see Section 7).

   SHIFT
      6-bit unsigned integer.  This field indicates the how many the
      original InstanceID (see Section 6.3.3) is shifted (added an
      integer from 0 to 63).  0 indicates that the original InstanceID
      is used.

   Reserved
      Reserved for future usage; MUST be initialized to zero and MUST be
      ignored upon reception.

   Address Vector (Optional)
      It is only present when the 'H' bit is set to 0.  For an
      asymmetric route, it is a vector of IPv6 addresses representing
      the route that the RREP-DIO has passed.  For symmetric before transmitting route, it
      is the RREP-Instance message
   upstream towards accumulated vector when the OrigNode. RREQ-DIO arrives at the
      TargNode.

4.3.  AODV-RPL DIO Target Option

   The metric used to determine symmetry
   (i.e., set AODV-RPL Target Option is defined based on the "S" Target Option in
   core RPL [RFC6550]: the Destination Sequence Number of the TargNode
   is added.

   A RREQ-DIO message MUST carry at least one AODV-RPL Target Options.
   A RREP-DIO message MUST carry exactly one AODV-RPL Target Option
   encapsulating the address of the OrigNode if the 'T' bit to be "1" (Symmetric) or "0" (asymmetric)) is
   implementation specific.  We used ETX/RSSI set to 1.

   If an OrigNode want to discover routes to verify multiple TargNodes, and
   these routes share the same constraints, then the OrigNode can
   include all the feasibility addresses of the protocol operations in this draft, as discussed TargNodes into multiple AODV-RPL
   Target Options in Appendix A.

                                     BR
                                 /    |    \
                               /      |      \
                             /        |        \ the RREQ-DIO, so that the cost can be reduced to
   building only one DODAG.  Different addresses of the TargNodes can
   merge if they share the same prefix.

        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      | Option Length |   Dest SeqNo  | Prefix Length |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                                                               |
       |                Target Prefix (Variable Length)                |
       .                                                               .
       .                                                               .
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

              Figure 3: Target option format for AODV-RPL MoP

   Type
      The type of the AODV-RPL Target Option (see Section 9.2)

   Destination Sequence Number

      In RREQ-DIO, if nonzero, it is the last known Sequence Number for
      TargNode for which a route is desired.  In RREP-DIO, it is the
      destination sequence number associated to the route.

5.  Symmetric and Asymmetric Routes

   In Figure 4 and Figure 5, BR is the BorderRouter, O is the OrigNode,
   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.
   An intermediate router sends out RREQ-DIO with the 'S' bit set to 1,
   meaning that all the one-hop links on the route from the OrigNode to
   this router meet the requirements of route discovery; thus the route
   can be used symmetrically.

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

            Figure 3: 4: AODV-RPL with Asymmetric Symmetric 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

   Upon receiving a RREQ-DIO with the 'S' bit set to 1, a node MUST
   decide if this one-hop link can be used symmetrically, i.e., both the
   two 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.  Moreover, if
   the 'S' bit arrives already set to be '0', it is set to be '0' on
   retransmission (Figure 5).  Therefore, for asymmetric route, there is
   at least one hop which doesn't fulfill the constraints in the two
   directions.  Based on the 'S' bit received in RREQ-DIO, the TargNode
   decides whether or not the route is symmetric before transmitting the
   RREP-DIO message upstream towards the OrigNode.

   The criterion and the corresponding metric used to determine if a
   one-hop link is symmetric or not is implementation specific and
   beyond the scope of the document.  Also, the difference in the metric
   values for upward and downward directions of a link that can be
   establish its symmetric and asymmetric nature is implementation
   specific.  For instance, the intermediate routers MAY choose to use
   local information (e.g., bit rate, bandwidth, number of cells used in
   6tisch), a priori knowledge (e.g. link quality according to previous
   communication) or estimate the metric using averaging techniques or
   any other means that is appropriate to the application context.

   Appendix A describes an example method using the ETX and RSSI to
   estimate whether the link is symmetric in terms of link quality is
   given in 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: S-->D;  Data: D-->S) ------->
       <---- RREP-Instance (Control: D-->S;  Data: S-->D) -------<

            Figure 5: AODV-RPL with Asymmetric Paired Instances

6.  AODV-RPL Operation

6.1.  Generating Route Request at OrigNode

   The route discovery process is initiated on-demand when an
   application at the OrigNode has data to be transmitted to the
   TargNode, but no route for the target exists or the current routes
   don't fulfill the requirements of the data transmission.  In this
   case, the OrigNode MUST build a local RPLInstance and a DODAG rooted
   at itself.  Then it begins to send out DIO message in AODV-RPL MoP
   via link-local multicast.  The DIO MUST contain exactly one RREQ
   option as defined in Section 4.1, and at least one AODV-RPL Target
   Option as defined in Figure 3.  This DIO message is noted as RREQ-
   DIO.  The 'S' bit in RREQ-DIO sent out by the OrigNode is set as 1.

   The maintenance of Originator and Destination Sequence Number in the
   RREQ option is as defined in AODV [RFC3561].

   The address in the AODV-RPL Target Option can be a unicast IPv6
   address, a prefix or a multicast address.  The OrigNode can initiate
   the route discovery process for multiple targets simultaneously by
   including multiple AODV-RPL Target Options, and within a RREQ-DIO the
   requirements for the routes to different TargNodes MUST be the same.

   The OrigNode can maintain different RPLInstances to discover routes
   with different requirements to the same targets.  Due to the
   InstanceID pairing mechanism Section 6.3.3, route replies (RREP-DIOs)
   from different paired RPLInstances can be distinguished.

   The transmission of RREQ-DIO follows the Trickle timer.  When the L
   duration has transpired, the OrigNode MUST leave the DODAG and stop
   sending any RREQ-DIOs in the related RPLInstance.

6.2.  Receiving and Forwarding Route Request

   Upon receiving a RREQ-DIO, a router out of the RREQ-instance goes
   through the following steps:

   Step 1:

      If the 'S' bit in the received RREQ-DIO is set to 1, the router
      MUST look into the two directions of the link by which the RREQ-
      DIO is received.  In case that the downward (i.e. towards the
      TargNode) direction of the link can't fulfill the requirements,
      then the link can't be used symmetrically, thus the 'S' bit of the
      RREQ-DIO to be send out MUST be set as 0.  If the 'S' bit in the
      received RREQ-DIO is set to 0, the router MUST look only into the
      upward direction (i.e. towards the OrigNode) of the link.  If the
      upward direction of the link can fulfill the requirements
      indicated in the constraint option, and the router's rank would be
      inferior to the MaxRank limit, the router chooses to join in the
      DODAG of the RREQ-Instance.  The router issuing the received RREQ-
      DIO is selected as the preferred parent.  Afterwards, other RREQ-
      DIO message can be received.  How to maintain the parent set,
      select the preferred parent, and update the router's rank follows
      the core RPL and the OFs defined in ROLL WG.

      In case that the constraint or the MaxRank limit is not fulfilled,
      the router MUST NOT join in the DODAG.  Otherwise, go to the
      following steps 2, 3, 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |     Type      |      Orig SeqNo       |      Dest SeqNo       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       |                     TargNode IPv6 Address                     |
       |                                                               |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

             Figure and 5.

      A router MUST discard a received RREQ-DIO if the advertised rank
      equals or exceeds the MaxRank limit.

   Step 2:

      Then the router checks if one of its addresses is included in one
      of the AODV-RPL Target Options or belongs to the indicated
      multicast group.  If so, this router is one of the TargNodes.
      Otherwise, it is an intermediate router.

   Step 3:

      If the 'H' bit is set to 1, then the router (TargNode or
      intermediate) MUST build route entry towards its preferred parent.
      The route entry SHOULD be stored along with the associated
      RPLInstanceID and DODAGID.  If the 'H' bit is set to 0, an
      intermediate router MUST include the address of the interface
      receiving the RREQ-DIO into the address vector.

   Step 4: DIO RREQ option format for

      If there are multiple AODV-RPL MoP

   OrigNode supplies Target Options in the following information received
      RREQ-DIO, a TargNode SHOULD continue sending RREQ-DIO to reach
      other targets.  When preparing its own RREQ-DIO, the TargNode MUST
      delete the AODV-RPL Target Option related to its own address, so
      that the routers which higher ranks would know the route to this
      target has already been found.  When an intermediate router
      receives several RREQ-DIOs which include different lists of AODV-
      RPL Target Options, the intersection of these lists will be
      included in its own RREQ-DIO.  If the intersection is empty, the
      router SHOULD NOT send out any RREQ-DIO.  Any RREQ-DIO message
      with different AODV-RPL Target Options coming from a router with
      higher rank is ignored.

   Step 5:

      For an intermediate router, it sends out its own RREQ-DIO via
      link-local multicast.  For a TargNode, it can begin to prepare the
      RREP-DIO.

6.3.  Generating Route Reply at TargNode

6.3.1.  RREP-DIO for Symmetric route

   When a RREQ-DIO arrives at a TargNode with the 'S' bit set to 1, it
   means there exists a symmetric route in which the RREQ option of two directions can
   fulfill the
   RREQ-Instance message:

   Type

      The type of requirements.  Other RREQ-DIOs can bring the RREQ option(see Section 9.2).

   Orig SeqNo
      Sequence Number upward
   direction of OrigNode.

   Dest SeqNo

      If nonzero, the last known Sequence Number for TargNode for which asymmetric routes (i.e.  S=0).  How to choose between a
   qualified symmetric route and an asymmetric route hopefully having
   better performance is desired.

   TargNode IPv6 Address

      IPv6 address implementation-specific and out of scope.  If
   the TargNode that receives RREQ-Instance message.

   In order implementation choose to establish use the symmetric route, the upstream route from TargNode to OrigNode,
   OrigNode multicasts
   MAY send out the RREQ-Instance message (see Figure 4) to its
   one-hop neighbours.  In order to enable intermediate nodes R_i to
   associate RREP-DIO after a future RREP message duration RREP_WAIT_TIME to an incoming RREQ message, wait for
   the
   InstanceID convergence of RREQ-Instance MUST assign RD to an odd number.

   Each intermediate node R_i computes the rank for RREQ-Instance and
   creates a routing table entry for optimal symmetric route.

   For symmetric route, the upstream route towards RREP-DIO message is sent via unicast to the
   source if
   OrigNode; therefore the routing metrics/constraints are satisfied.  For this
   purpose R_i must use DODAG in RREP-Instance doesn't need to be
   actually built.  The RPLInstanceID in the asymmetric link metric measured RREP-Instance is paired as
   defined in Section 6.3.3.  The 'S' bit in the
   upstream direction, from R_i to its upstream neighbor that
   multicasted base DIO remains as 1.
   In the RREQ-Instance message.

   When an intermediate node R_i receives a RREQ message RREP option, The 'SHIFT' field and the 'T' bit are set as
   defined in Section 6.3.3.  The address vector received in storing
   mode, it MUST store the OrigNode's InstanceID (RREQ-Instance) along
   with RREQ-
   DIO MUST be included in this RREP option in case the other routing information needed 'H' bit is set
   to establish 0 (both in RREQ-DIO and RREP-DIO).  If the route back 'T' bit is set to 1,
   the address of the OrigNode MUST be encapsulated in an AODV-RPL
   Target Option and included in this RREP-DIO message, and the OrigNode.  This will enable R_i
   Destination Sequence Number is set according to determine that AODV [RFC3561].

6.3.2.  RREP-DIO for Asymmetric Route

   When a future
   RREP message (containing RREQ-DIO arrives at a paired InstanceID for TargNode with the TargNode) must
   be transmitted back 'S' bit set to 0, the OrigNode's IP address.

   If
   TargNode MUST build a DODAG in the paths RREP-Instance rooted at itself in
   order to and discover the downstream route from TargNode are not known, the intermediate
   node multicasts OrigNode to the RREQ-Instance
   TargNode.  The RREP-DIO message with updated rank to its
   next-hop neighbors MUST be send out via link-local
   multicast until the message reaches TargNode (Figure 2).
   Based on OrigNode is reached or the 'S' bit MaxRank limit is
   exceeded.

   The settings of the RREP-DIO are the same as in symmetric route.

6.3.3.  RPLInstanceID Pairing

   Since the received RREQ message, RPLInstanceID is assigned locally (i.e., there is no
   coordination between routers in the TargNode will
   decide whether to unicast or multicast assignment of RPLInstanceID) the RREP message back
   tuple (RPLInstanceID, DODAGID, Address in the AODV-RPL Target Option)
   is needed to
   OrigNode.

   As described uniquely identify a DODAG in Section 7, an AODV-RPL instance.
   Between the OrigNode and the TargNode, there can be multiple AODV-RPL
   instances when applications upper layer have different requirements.
   Therefore the RREQ-Instance and the RREP-Instance 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 same route
   discovery MUST be paired.  The TargNode supplies way to realize this is to pair their
   RPLInstance IDs.

   Typically, the following information in two InstanceIDs are set as the RREP 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      |      Dest SeqNo       | Prefix Sz |T|G| Rsvd  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |              TargNode IPv6 Address (when present)             |
     |                                                               |
     | local InstanceID in
   core RPL:

                   0 1 2 3 4 5 6 7
                  +-+-+-+-+-+-+-+-+
                  |1|D|    ID     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  Local RPLInstanceID in 0..63
                  +-+-+-+-+-+-+-+-+

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

   Type 6: Local Instance ID

   The type of first bit is set to 1 indicating the RREP option (see Section 9.2)

   Dest SeqNo RPLInstanceID is local.  The Sequence Number for
   'D' bit here is used to distinguish the TargNode two AODV-RPL instances: D=0
   for which a route is
      established.

   Prefix Sz RREQ-Instance, D=1 for RREP-Instance.  The size ID of 6 bits SHOULD be
   the prefix which same for RREQ-Instance and RREP-Instance.  Here, the 'D' bit is
   used slightly differently than in RPL.

   When preparing the RREP-DIO, a TargNode could find the RPLInstanceID
   to be used for the RREP-Instance is already occupied by another
   instance from an earlier route discovery operation which is still
   active.  In other words, two OrigNodes need routes to the same
   TargNode is
      available.  This allows routing and they happen to other nodes on use the same subnet
      as RPLInstanceID for RREQ-
   Instance.  In this case, the TargNode. occupied RPLInstanceID MUST NOT be used
   again.  Then this RPLInstanceID SHOULD be shifted into another
   integer and shifted back to the original one at the OrigNode.  In
   RREP option, the SHIFT field indicates the how many the original
   RPLInstanceID is shifted.  When the new InstanceID after shifting
   exceeds 63, it will come back counting from 0.  For example, the
   original InstanceID is 60, and shifted by 6, the new InstanceID will
   be 2.  The 'T' MUST be set to 1 to make sure the two RREP-DIOs can be
   distinguished by the address of the OrigNode in the AODV-RPL Target
   Option.

6.4.  Receiving and Forwarding Route Reply

   Upon receiving a RREP-DIO, a router out of the RREP-Instance goes
   through the following steps:

   Step 1:

      If the 'S' bit

      'T' of the RREP-DIO is set to true to indicate that 0, the TargNode IPv6 Address
      field router MUST look
      into the downward direction of the link (towards the TargNode) by
      which the RREP-DIO is present

   'G' bit

      (see Section 7)

   TargNode IPv6 Address  (when present)

      IPv6 address received.  If the downward direction of the TargNode that receives RREP-Instance message.

   In order
      link can fulfill the requirements indicated in the constraint
      option, and the router's rank would be inferior to reduce the need for MaxRank
      limit, the TargNode IPv6 Address router chooses to be
   included with the RREP message, join in the InstanceID DODAG of the RREP-Instance
   is paired, whenever possible, with the InstanceID from the RREQ
   message, which is always an odd number. RREP-
      Instance.  The pairing router issuing the received RREP-DIO is accomplished
   by adding one selected as
      the preferred parent.  Afterwards, other RREQ-DIO messages can be
      received.  How to maintain the InstanceID from parent set, select the RREQ message preferred
      parent, and using that,
   whenever possible, as update the InstanceID for router's rank follows the RREP message. core RPL and the
      OFs defined in ROLL WG.

      If this
   is the constraints are not possible (for instance because fulfilled, the incremented InstanceID is
   still router MUST NOT join in
      the DODAG, and will not go through steps 2, 3, and 4.

      A router MUST discard a valid InstanceID for another route to received RREQ-DIO if the TargNode from an
   earlier Route Discovery operation), then advertised rank
      equals or exceeds the 'T' MaxRank limit.

      If the 'S' bit is set and an
   alternative even number is chosen for to 1, the InstanceID router does nothing in this step.

   Step 2:

      Then the router checks if one of RREP from
   TargNode.

   The OrigNode IP address for RREQ-Instance its addresses is available as the DODAGID included in the DIO base message (see Figure 1).  When TargNode receives a
   RREQ message with
      AODV-RPL Target Option.  If so, this router is the 'S' bit set to 1 (as illustrated in Figure 2),
   it unicasts OrigNode of the RREP message with
      route discovery.  Otherwise, it is an intermediate router.

   Step 3:

      If the 'S' 'H' bit is set to 1.  In this
   case, 1, then the router (OrigNode or
      intermediate) MUST build route control messages and application data between OrigNode
   and TargNode for both RREQ-Instance and entry including the RPLInstanceID
      of RREP-Instance are transmitted
   along and the DODAGID.  For symmetric links.  When route, the 'T' route
      entry is to the router from which the RREP-DIO is received.  For
      asymmetric route, the route entry is to the preferred parent in
      the DODAG of RREQ-Instance.

      If the 'H' bit is set to "1" in 0, for asymmetric route, an intermediate
      router MUST include the RREP-
   Instance, then address of the TargNode IPv6 Address is transmitted in interface receiving the RREP
   option.  Otherwise,
      RREP-DIO into the TargNode IPv6 Address address vector, and for symmetric route, there
      is elided nothing to do in the RREP
   option.

   When (as illustrated this step.

   Step 4:

      For an intermediate router, in Figure 3) case of asymmetric route, the TargNode receives RREQ message
   with RREP-
      DIO is sent out via link-local multicast; in case of symmetric
      route, the 'S' bit set RREP-DIO is unicasted to 0, it also multicasts the RREP message with OrigNode via the 'S' bit set to 0.  Intermediate nodes create a next hop
      in source routing table
   entry for (H=0), or via the path towards next hop in the TargNode while processing route entry
      built in the RREP
   message to OrigNode.  Once OrigNode receives RREQ-Instance (H=1).  For the RREP message, OrigNode, it
   starts can start
      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. RREP-Instance.

7.  Gratuitous RREP

   Under

   In some circumstances, cases, an Intermediate Node router that receives a RREQ RREQ-DIO
   message MAY transmit a "Gratuitous" RREP RREP-DIO message back to OrigNode
   instead of continuing to multicast the RREQ message RREQ-DIO towards TargNode.
   For these circumstances,
   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 RREP-DIO (G=1) sent by the
   Intermediate node from the RREP RREP-DIO sent by TargNode.

   When TargNode (G=0).

   The gratuitous RREP-DIO can be sent out when an Intermediate node intermediate router R
   receives a RREQ message RREQ-DIO for a TargNode T, and has recent
   information about R happens to have both
   forward and reverse routes to T which also fulfill the cost requirements.

   In case of an upstream route from TargNode to R,
   then source routing, the intermediate router R MAY MUST unicast the Gratuitous RREP (GRREP) message
   received RREQ-DIO to OrigNode.
   R determines whether its information is sufficiently recent by
   comparing TargNode T including the value it has stored for address vector between
   the Sequence Number OrigNode O and the router R.  Thus T can have a complete address
   vector between O and itself.  Then T MUST unicast a RREP-DIO
   including the address vector between T and R.

   In case of TargNode
   against the DestSeqno in the incoming RREQ message. hop-by-hop routing, R also must have
   information about MUST unicast the metric information of received RREQ-DIO
   to T.  The routers along the upstream route from
   TargNode.  The GRREP message MUST have PrefixSz == 0 SHOULD build new route entries
   with the related RPLInstanceID and DODAGID in the 'G' bit
   set to 1.  R SHOULD also downward direction.
   Then T MUST unicast the RREQ message RREP-DIO to TargNode, to
   make sure that TargNode will have a R, and the routers along the
   route SHOULD build new route entries in the upward direction.  Upon
   received the unicast RREP-DIO, R sends the gratuitous RREP-DIO to OrigNode. the
   OrigNode as the same way defined in Section 6.3.

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: 7: Mode of Operation

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

   Two Target

   Three entries are required for new AODV-RPL options "RREQ-Instance" "RREQ", "RREP"
   and
   "RREQ-Instance", "AODV-RPL Target" with values of TBD2 (0x0A) and (0x0A), TBD3 (0x0B) and
   TBD4 (0x0C) from the "RPL Control Message Options" space [RFC6550].

             +-------------+---------------------+---------------+

            +-------------+------------------------+---------------+
            |    Value    |        Meaning         |   Reference   |
             +-------------+---------------------+---------------+
            +-------------+------------------------+---------------+
            | TBD2 (0x0A) |      RREQ Option       | This document |
             +-------------+---------------------+---------------+
            +-------------+------------------------+---------------+
            | TBD3 (0x0B) |      RREP Option       | This document |
             +-------------+---------------------+---------------+
            +-------------+------------------------+---------------+
            | TBD3 (0x0C) | AODV-RPL Target Option | This document |
            +-------------+------------------------+---------------+

                        Figure 7: 8: 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
   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>.

   [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:

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

          Figure 8: 9: Communication link from Source to Destination

   +-------------------------+----------------------------------------+
   | RSSI at NodeA for NodeB | Expected ETX at NodeA for nodeB->nodeA 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".

Appendix B.  Changes to version 02

   o  Include the support for source routing.

   o  Bring some features from [RFC6997], e.g., choice between hop-by-
      hop and source routing, 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 InstanceID pairing mechanism.

Authors' Addresses

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

   Email: satishnaidu80@gmail.com

   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
   Huaiyin Institute of Technology
   No.89 North Beijing  100095 Road, Qinghe District
   Huaian  223001
   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
   Bing Liu
   Huawei Technologies
   No. 156 Beiqing Rd. Haidian District
   Beijing  100095
   China

   Email: remy.liubing@huawei.com