Common Ancestor Objective Function and Parent Set DAG Metric Container Extension
draft-ietf-roll-nsa-extension-10

ROLL                                             R.-A. Koutsiamanis, Ed.
Internet-Draft                                         G.Z. Papadopoulos
Intended status: Standards Track                            N. Montavont
Expires: 2 May 2021                                       IMT Atlantique
                                                              P. Thubert
                                                                   Cisco
                                                         29 October 2020

 Common Ancestor Objective Function and Parent Set DAG Metric Container
                               Extension
                    draft-ietf-roll-nsa-extension-10

Abstract

   Packet Replication and Elimination is a method in which several
   copies of a data packet are sent in the network in order to achieve
   high reliability and low jitter.  This document details how to apply
   Packet Replication and Elimination in RPL, especially how to exchange
   information within RPL control packets to let a node better select
   the different parents that will be used to forward the multiple
   copies of a packet.  This document also describes the Objective
   Function which takes advantage of this information to implement
   multi-path routing.

Status of This Memo

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

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   This Internet-Draft will expire on 2 May 2021.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   Please review these documents carefully, as they describe your rights
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Common Ancestor AP Selection Policies . . . . . . . . . . . .   4
     3.1.  Common Ancestor Strict  . . . . . . . . . . . . . . . . .   5
     3.2.  Common Ancestor Medium  . . . . . . . . . . . . . . . . .   6
     3.3.  Common Ancestor Relaxed . . . . . . . . . . . . . . . . .   6
   4.  Common Ancestor Objective Function  . . . . . . . . . . . . .   6
     4.1.  Usage . . . . . . . . . . . . . . . . . . . . . . . . . .   9
   5.  Node State and Attribute (NSA) object type extension  . . . .   9
     5.1.  Usage . . . . . . . . . . . . . . . . . . . . . . . . . .  11
   6.  Controlling PRE . . . . . . . . . . . . . . . . . . . . . . .  12
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  13
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     10.1.  Normative references . . . . . . . . . . . . . . . . . .  13
     10.2.  Informative references . . . . . . . . . . . . . . . . .  13
   Appendix A.  Implementation Status  . . . . . . . . . . . . . . .  14
   Appendix B.  Choosing an AP selection policy  . . . . . . . . . .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17

1.  Introduction

   Networks in the industrial context must provide stringent guarantees
   in terms of reliability and predictability, with this domain being
   one of main ones addressed by Deterministic Networking [RFC8557].
   Packet Replication and Elimination (PRE) (Section 4.5.3 of
   [I-D.ietf-6tisch-architecture]) is a technique which allows redundant
   paths in the network to be utilized for traffic requiring higher
   reliability.  Allowing industrial applications to function over
   wireless networks requires the application of the principles and
   architecture of Deterministic Networking [RFC8655].  This results in
   designs which aim at optimizing packet delivery rate and bounding
   latency.  Additionally, nodes operating on battery need to minimize
   their energy consumption.

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   As an example, to meet this goal, IEEE Std. 802.15.4 [IEEE802154]
   provides Time-Slotted Channel Hopping (TSCH), a mode of operation
   which uses a common communication schedule based on timeslots to
   allow deterministic medium access as well as channel hopping to work
   around radio interference.  However, since TSCH uses retransmissions
   in the event of a failed transmission, end-to-end latency and jitter
   performance can deteriorate.

   Furthermore, the 6TiSCH working group, focusing on IPv6 over IEEE
   Std. 802.15.4-TSCH, has worked on these issues and produced the
   "6TiSCH Architecture" [I-D.ietf-6tisch-architecture] to address that
   case.

   PRE is a general method of maximizing packet delivery rate and
   potentially minimizing latency and jitter, not limited to 6TiSCH.
   More specifically, PRE achieves controlled redundancy by laying
   multiple forwarding paths through the network and using them in
   parallel for different copies of a same packet.  PRE can follow the
   Destination-Oriented Directed Acyclic Graph (DODAG) formed by [RPL]
   from a node to the root.  Building a multi-path DODAG can be achieved
   based on the RPL capability of having multiple parents for each node
   in a network, a subset of which is used to forward packets.  In order
   to select parents to be part of this subset, the RPL Objective
   Function (OF) needs additional information regarding the multi-path
   nature of PRE.  This document describes an OF which implements multi-
   path routing for PRE and specifies the transmission of this specific
   path information.

   This document describes a new Objective Function (OF) called the
   Common Ancestor (CA) OF (see Section 4).  A detailed description is
   given of how the path information is used within the CA OF and how
   the subset of parents for forwarding packets is selected.  This
   specification defines a new Objective Code Point (OCP) for the CA OF.

   For the path information, this specification focuses on the
   extensions to the DAG Metric Container [RFC6551] required for
   providing the PRE mechanism a part of the information it needs to
   operate.  This information is the [RPL] parent address set of a node
   and it must be sent to potential children of the node.  The RPL DIO
   Control Message is the canonical way of broadcasting this kind of
   information and therefore its DAG Metric Container [RFC6551] field is
   used to append a Node State and Attribute (NSA) object.  The node's
   parent address set is stored as an optional TLV within the NSA
   object.  This specification defines the type value and structure for
   the parent address set TLV (see Section 5).

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2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

   The draft uses the following Terminology:

   Packet Replication and Elimination (PRE):  A method which consists of
      transmitting multiple copies of a packet using multi-path
      forwarding over a multi-hop network and which consolidates
      multiple received packet copies to control flooding.  See "Complex
      Track with Replication and Elimination" in Section 4.5.3 of
      [I-D.ietf-6tisch-architecture] for more details.

   Parent Set (PS):  Given a RPL node, the set of its neighbor nodes
      which participate in the same RPL DODAG and which can potentially
      take the role of the node's preferred parent.

   Alternative Parent (AP):  A RPL parent in the parent set of a node
      which is used to forward a packet copy when replicating packets.

   Alternative Parent (AP) Selection:  The mechanism for choosing the
      next hop node to forward a packet copy when replicating packets.

   Preferred Grand Parent (PGP):  The preferred parent of the preferred
      parent of a node.

3.  Common Ancestor AP Selection Policies

   In the RPL protocol, each node maintains a list of potential parents.
   For PRE, the Preferred Parent (PP) node is defined to be the same as
   the RPL DODAG Preferred Parent node.  Furthermore, to construct an
   alternative path toward the root, in addition to the PP node, each
   node in the network selects additional parent(s), called alternative
   parent(s), from its Parent Set (PS).

   There are multiple possible policies of selecting the AP node.  This
   section details three such possible policies.

   All three policies defined perform AP selection based on common
   ancestors, named Common Ancestor Strict, Common Ancestor Medium, and
   Common Ancestor Relaxed, depending on how restrictive the selection
   process is.  A more restrictive policy will limit flooding but might
   fail to select an appropriate AP, while a less restrictive one will
   more often find an appropriate AP but might increase flooding.

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   All three policies apply their corresponding common ancestor
   criterion to filter the list of candidate neighbours in the
   alternative parent set.

3.1.  Common Ancestor Strict

   In the CA Strict OF the node will check if its Preferred Grand Parent
   (PGP), the PP of its PP, is the same as the PP of the potential AP.

                  (  R  ) root
                     .                      PS(S) = {A, B, C, D}
                     .                      PP(S) = C
                     .                      PP(PP(S)) = Y
                     .
                                            PS(A) = {W, X}
     ( W )    ( X )    ( Y )    ( Z )       PP(A) = X
       ^ ^   ^^ ^ ^    ^^^^ ^   ^ ^^
       |  \ //  |  \ //  ||  \ /  ||        PS(B) = {W, X, Y}
       |   //   |   //   ||   /   ||        PP(B) = Y
       |  // \  |  // \  ||  / \  ||
       | //   \ | //   \ || /   \ ||        PS(C) = {X, Y, Z}
     ( A )    ( B )    ( C )    ( D )       PP(C) = Y
         ^        ^      ^^     ^
          \        \     ||    /            PS(D) = {Y, Z}
            \       \    ||   /             PP(D) = Z
              \      \   ||  /
                \----\\  || /               || Preferred Parent
                     (   S   ) source       |  Potential Alt. Parent

        Figure 1: Example Common Ancestor Strict Alternative Parent
                              Selection policy

   For example, in Figure 1, the source node S must know its grandparent
   sets through nodes A, B, C, and D.  The Parent Sets (PS) and the
   Preferred Parents (PS) of nodes A, B, C, and D are shown on the side
   of the figure.  The CA Strict parent selection policy will select an
   AP for node S for which PP(PP(S)) = PP(AP).  Given that PP(PP(S)) =
   Y:

   *  Node A: PP(A) = X and therefore it is different than PP(PP(S))

   *  Node B: PS(B) = Y and therefore it is equal to PP(PP(S))

   *  Node D: PS(D) = Z and therefore it is different than PP(PP(S))

   node S can decide to use node B as its AP node, since PP(PP(S)) = Y =
   PP(B).

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3.2.  Common Ancestor Medium

   In the CA Medium OF the node will check if its Preferred Grand Parent
   (PGP), the PP of its PP, is contained in the PS of the potential AP.

   Using the same example, in Figure 1, the CA Medium parent selection
   policy will select an AP for node S for which PP(PP(S)) is in PS(AP).
   Given that PP(PP(S)) = Y:

   *  Node A: PS(A) = {W, X} and therefore PP(PP(S)) is not in the set

   *  Node B: PS(B) = {W, X, Y} and therefore PP(PP(S)) is in the set

   *  Node D: PS(D) = {Y, Z} and therefore PP(PP(S)) is in the set

   node S can decide to use node B or D as its AP node.

3.3.  Common Ancestor Relaxed

   In the CA Relaxed OF the node will check if the Parent Set (PS) of
   its Preferred Parent (PP) has a node in common with the PS of the
   potential AP.

   Using the same example, in Figure 1, the CA Relaxed parent selection
   policy will select an AP for node S for which PS(PP(S)) has at least
   one node in common with PS(AP).  Given that PS(PP(S)) = {X, Y, Z}:

   *  Node A: PS(A) = {W, X} and the common nodes are {X}

   *  Node B: PS(B) = {W, X, Y} and the common nodes are {X, Y}

   *  Node D: PS(D) = {Y, Z} and the common nodes are {Y, Z}

   node S can decide to use node A, B or D as its AP node.

4.  Common Ancestor Objective Function

   An OF which allows the multiple paths to remain correlated is
   detailed here.  More specifically, when using this OF a node will
   select an AP node close to its PP node to allow the operation of
   overhearing between parents.  For more details about overhearing and
   its use in this context see the "Complex Track with Replication and
   Elimination" in Section 4.5.3 of [I-D.ietf-6tisch-architecture].  If
   multiple potential APs match this condition, the AP with the lowest
   rank will be registered.

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   The OF described here is an extension of The Minimum Rank with
   Hysteresis Objective Function [MRHOF].  In general, this OF extends
   MRHOF by specifying how an AP is selected.  Importantly, the
   calculation of the rank of the node through each candidate neighbor
   and the selection of the PP is kept the same as in MRHOF.

   The ways in which the CA OF modifies MRHOF in a section-by-section
   manner follows in detail:

   [MRHOF], Section 2: "Terminology".  Term "Selected Metric":
      The CA OF uses only one metric, like MRHOF, for rank calculation,
      with the same MRHOF semantics.  For selecting the AP, the PS TLV
      (stored in the DIO Metric Container Node State and Attribute (NSA)
      object body, see Section 5) is used.  This additional NSA metric
      is disregarded for the purposes of rank calculation.

   [MRHOF], Section 3 "The Minimum Rank with Hysteresis Objective
   Function":
      Same as MRHOF extended to AP selection.  Minimum Rank path
      selection and switching applies correspondingly to the AP with the
      extra CA requirement of having some match between ancestors.

   [MRHOF], Section 3.1 "Computing the Path Cost":
      Same as MRHOF extended to AP selection.  If a candidate neighbor
      does not fulfill the CA requirement then the path through that
      neighbor SHOULD be set to MAX_PATH_COST, the same value used by
      MRHOF.  As a result, the node MUST NOT select the candidate
      neighbor as its AP.

   [MRHOF], Section 3.2 "Parent Selection":
      Same as MRHOF extended to AP selection.  To allow hysteresis, AP
      selection maintains a variable, cur_ap_min_path_cost, which is the
      path cost of the current AP.

   [MRHOF], Section 3.2.1 "When Parent Selection Runs":
      Same as MRHOF.

   [MRHOF], Section 3.2.2 "Parent Selection Algorithm":
      Same as MRHOF extended to AP selection.  If the smallest path cost
      for paths through the candidate neighbors is smaller than
      cur_ap_min_path_cost by less than PARENT_SWITCH_THRESHOLD (the
      same variable as MRHOF uses), the node MAY continue to use the
      current AP.  Additionally, if there is no PP selected, there MUST
      NOT be any AP selected as well.  Finally, as with MRHOF, a node
      MAY include up to PARENT_SET_SIZE-1 additional candidate neighbors
      in its alternative parent set.  The value of PARENT_SET_SIZE is
      the same as in MRHOF.

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   [MRHOF], Section 3.3 "Computing Rank":
      Same as MRHOF.

   [MRHOF], Section 3.4 "Advertising the Path Cost":
      Same as MRHOF.

   [MRHOF], Section 3.5 "Working without Metric Containers":
      It is not possible to work without metric containers, since CA AP
      selection requires information from parents regarding their parent
      sets, which is transmitted via the NSA object in the DIO Mectric
      Container.

   [MRHOF], Section 4 "Using MRHOF for Metric Maximization":
      Same as MRHOF.

   [MRHOF], Section 5 "MRHOF Variables and Parameters":
      Same as MRHOF extended to AP selection.  The CA OF operates like
      MRHOF for AP selection by maintaining separate:

      AP:  Corresponding to the MRHOF PP.  Hysteresis is configured for
         AP with the same PARENT_SWITCH_THRESHOLD parameter as in MRHOF.
         The AP MUST NOT be the same as the PP.

      Alternative parent set:  Corresponding to the MRHOF parent set.
         The size is defined by the same PARENT_SET_SIZE parameter as in
         MRHOF.  The Alternative parent set MUST be a non-strict subset
         of the parent set.

      cur_ap_min_path_cost:  Corresponding to the MRHOF
         cur_min_path_cost variable.  To support the operation of the
         hysteresis function for AP selection.

   [MRHOF], Section 6 "Manageability":
      Same as MRHOF.

   [MRHOF], Section 6.1 "Device Configuration":
      Same as MRHOF.

   [MRHOF], Section 6.2 "Device Monitoring":
      Same as MRHOF.

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4.1.  Usage

   All OF policies apply their corresponding criterion to filter the
   list of candidate neighbours in the alternative parent set.  The AP
   is then selected from the alternative parent set based on Rank and
   using hysteresis as is done for the PP in MRHOF.  It is noteworthy
   that the OF uses the same Objective Code Point (OCP): TBD1 for all
   policies used.

   The PS information can be used by any of the described AP selection
   policies or other ones not described here, depending on requirements.
   It is optional for all nodes to use the same AP selection policies.
   Different nodes may use different AP selection policies, since the
   selection policy is local to each node.  For example, using different
   policies can be used to vary the transmission reliability in each
   hop.

5.  Node State and Attribute (NSA) object type extension

   In order to select their AP node, nodes need to be aware of their
   grandparent node sets.  Within [RPL], the nodes use the DODAG
   Information Object (DIO) Control Message to broadcast information
   about themselves to potential children.  However, [RPL], does not
   define how to propagate parent set related information, which is what
   this document addresses.

   DIO messages can carry multiple options, out of which the DAG Metric
   Container option [RFC6551] is the most suitable structurally and
   semantically for the purpose of carrying the parent set.  The DAG
   Metric Container option itself can carry different nested objects,
   out of which the Node State and Attribute (NSA) [RFC6551] is
   appropriate for transferring generic node state data.  Within the
   Node State and Attribute it is possible to store optional TLVs
   representing various node characteristics.  As per the Node State and
   Attribute (NSA) [RFC6551] description, no TLV has been defined for
   use.  This document defines one TLV for the purpose of transmitting a
   node's parent set.

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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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | RPLInstanceID |Version Number |             Rank              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |G|0| MOP | Prf |     DTSN      |     Flags     |   Reserved    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                                                               |
   +                            DODAGID                            +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | DAGMC Type (2)| DAGMC Length  |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
   |                                                               |
   //                   DAG Metric Container data                 //
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Figure 2: Example DIO Message with a DAG Metric Container option

   Figure 2 shows the structure of the DIO Control Message when a DAG
   Metric Container option is included.  The DAG Metric Container option
   type (DAGMC Type in Figure 2) has the value 0x02 as per the IANA
   registry for the RPL Control Message Options, and is defined in
   [RPL].  The DAG Metric Container option length (DAGMC Length in
   Figure 2) expresses the DAG Metric Container length in bytes.  DAG
   Metric Container data holds the actual data and is shown expanded in
   Figure 3.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Routing-MC-Type|Res Flags|P|C|O|R| A   |  Prec | Length (bytes)|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Res       |  Flags    |A|O|    PS  type   |   PS  Length  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   PS IPv6 address(es) ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+

        Figure 3: DAG Metric Container (MC) data with Node State and
                   Attribute (NSA) object body and a TLV

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   The structure of the DAG Metric Container data in the form of a Node
   State and Attribute (NSA) object with a TLV in the NSA Optional TLVs
   field is shown in Figure 3.  The first 32 bits comprise the DAG
   Metric Container header and all the following bits are part of the
   Node State and Attribute object body, as defined in [RFC6551].  This
   document defines a new TLV, which MAY be carried in the Node State
   and Attribute (NSA) object Optional TLVs field.  The TLV is named
   Parent Set and is abbreviated as PS in Figure 3.

   PS type:  The type of the Parent Set TLV.  The value is TBD2.

   PS Length:  The total length of the TLV value field (PS IPv6
         address(es)) in bytes.  The length is an integral multiple of
         16, the number of bytes in an IPv6 address.

   PS IPv6 address(es)  One or more 128-bit IPv6 address(es) without any
         separator between them.  The field consists of one IPv6 address
         per parent in the parent set.  The parent addresses are listed
         in decreasing order of preference and not all parents in the
         parent set need to be included.  The selection of how many
         parents from the parent set are to be included is left to the
         implementation.  The number of parent addresses in the PS IPv6
         address(es) field can be deduced by dividing the length of the
         PS IPv6 address(es) field in bytes by 16, the number of bytes
         in an IPv6 address.

5.1.  Usage

   The PS SHOULD be used in the process of parent selection, and
   especially in AP selection, since it can help the alternative path to
   not significantly deviate from the preferred path.  The Parent Set is
   information local to the node that broadcasts it.

   The PS is used only within NSA objects configured as a metric,
   therefore the DAG Metric Container field "C" MUST be 0.
   Additionally, since the information in the PS needs to be propagated
   downstream but it cannot be aggregated, the DAG Metric Container
   field "R" MUST be 1.  Finally, since the information contained is by
   definition partial, more specifically just the parent set of the DIO-
   sending node, the DAG Metric Container field "P" MUST be 1.

   It is important that the PS does not affect the calculation of the
   rank through candidate neighbors.  It is only used with the CA OF to
   remove nodes which do not fulfill the CA OF criteria from the
   candidate neighbor list.

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6.  Controlling PRE

   PRE is very helpful when the aim is to increase reliability for a
   certain path, however its use creates additional traffic as part of
   the replication process.  It is conceivable that not all paths have
   stringent reliability requirements.  Therefore, a way to control
   whether PRE is applied to a path's packets SHOULD be implemented.
   For example, a traffic class label can be used to determine this
   behavior per flow type as described in Deterministic Networking
   Architecture [RFC8655].

7.  Security Considerations

   The structure of the DIO control message is extended, within the pre-
   defined DIO options.  The additional information are the IPv6
   addresses of the parent set of the node transmitting the DIO.  This
   use of this additional information can have the following potential
   consequences:

   *  A malicious node that can receive and read the DIO can "see"
      further than it's own neighbourhood by one hop, learning the
      addresses of it's two hop neighbors.  This is a privacy / network
      discovery issue.

   *  A malicious node that can send DIOs can use the parent set
      extension to convince neighbours to route through itself, instead
      of the normal preferred parent they would use.  However, this is
      already possible with other OFs (like [OF0] and [MRHOF]) by
      reporting a fake rank value in the DIO, thus masquerading as the
      DODAG root.

8.  IANA Considerations

   This proposal requests the allocation of a new value TBD1 from the
   "Objective Code Point (OCP)" sub-registry of the "Routing Protocol
   for Low Power and Lossy Networks (RPL)" registry.

   This proposal also requests the allocation of a new value TBD2 for
   the "Parent Set" TLV from the "Routing Metric/Constraint TLVs" sub-
   registry of the "Routing Protocol for Low Power and Lossy Networks
   (RPL) Routing Metric/Constraint" registry.

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9.  Acknowledgments

   We are very grateful to Dominique Barthel, Rahul Jadhav, Fabrice
   Theoleyre, Diego Dujovne, Derek Jianqiang Hou, and Michael Richardson
   for their comments, feedback, and support which lead to many
   improvements to this document.  We would also like to thank Tomas
   Lagos Jenschke very much for helping in the implementation and
   evaluation of the proposals of this document.

10.  References

10.1.  Normative references

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

   [RPL]      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>.

10.2.  Informative references

   [I-D.ietf-6tisch-architecture]
              Thubert, P., "An Architecture for IPv6 over the TSCH mode
              of IEEE 802.15.4", Work in Progress, Internet-Draft,
              draft-ietf-6tisch-architecture-29, 27 August 2020,
              <https://tools.ietf.org/html/draft-ietf-6tisch-
              architecture-29>.

   [IEEE802154]
              IEEE standard for Information Technology, "IEEE Std.
              802.15.4, Part. 15.4: Wireless Medium Access Control (MAC)
              and Physical Layer (PHY) Specifications for Low-Rate
              Wireless Personal Area Networks", December 2015,
              <http://standards.ieee.org/findstds/
              standard/802.15.4-2015.html>.

   [MRHOF]    Gnawali, O. and P. Levis, "The Minimum Rank with
              Hysteresis Objective Function", RFC 6719,
              DOI 10.17487/RFC6719, September 2012,
              <https://www.rfc-editor.org/info/rfc6719>.

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   [OF0]      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>.

   [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>.

   [RFC8557]  Finn, N. and P. Thubert, "Deterministic Networking Problem
              Statement", RFC 8557, DOI 10.17487/RFC8557, May 2019,
              <https://www.rfc-editor.org/info/rfc8557>.

   [RFC8655]  Finn, N., Thubert, P., Varga, B., and J. Farkas,
              "Deterministic Networking Architecture", RFC 8655,
              DOI 10.17487/RFC8655, October 2019,
              <https://www.rfc-editor.org/info/rfc8655>.

Appendix A.  Implementation Status

   A research-stage implementation of the PRE mechanism using the
   proposed extension as part of a 6TiSCH IOT use case was developed at
   IMT Atlantique, France by Tomas Lagos Jenschke and Remous-Aris
   Koutsiamanis.  It was implemented on the open-source Contiki OS and
   tested with the Cooja simulator.  The DIO DAGMC NSA extension is
   implemented with a configurable number of parents from the parent set
   of a node to be reported.

                    ( R )

   (11)   (12)   (13)   (14)   (15)   (16)

   (21)   (22)   (23)   (24)   (25)   (26)

   (31)   (32)   (33)   (34)   (35)   (36)

   (41)   (42)   (43)   (44)   (45)   (46)

   (51)   (52)   (53)   (54)   (55)   (56)

                    ( S )

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                       Figure 4: Simulation Topology

   The simulation setup is:

   Topology:  32 nodes structured in regular grid as show in Figure 4.
      Node S (source) is the only data packet sender, and send data to
      node R (root).  The parent set of each node (except R) is all the
      nodes in the immediately higher row, the immediately above 6
      nodes.  For example, each node in {51, 52, 53, 54, 55, 56} is
      connected to all of {41, 42, 43, 44, 45, 46}.  Node 11, 12, 13,
      14, 15, 16 have a single upwards link to R.

   MAC:  TSCH with 1 retransmission

   Platform:  Cooja

   Schedule:  Static, 2 timeslots per link from each node to each parent
      in its parent set, 1 broadcast EB slot, 1 sender-based shared
      timeslot (for DIO and DIS) per node (total of 32).

   Simulation lifecycle:  Allow link formation for 100 seconds before
      starting to send data packets.  Afterwards, S sends data packets
      to R.  The simulation terminates when 1000 packets have been sent
      by S.

   Radio Links:  Every 60 s, a new Packet Delivery Rate is randomly
      drawn for each link, with a uniform distribution spanning the 70%
      to 100% interval.

   Traffic Pattern:  CBR, S sends one non-fragmented UDP packet every 5
      seconds to R.

   PS extension size:  3 parents.

   Routing Methods:
      *  RPL: The default RPL non-PRE implementation in Contiki OS.

      *  2nd ETX: PRE with a parent selection method which picks as AP
         the 2nd best parent in the parent set based on ETX.

      *  CA Strict: As described in Section 3.1.

      *  CA Medium: As described in Section 3.2.

   Simulation results:

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    +=========+===================+===================+===============+
    | Routing |    Average Packet | Average Traversed |       Average |
    | Method  | Delivery Rate (%) |  Nodes/packet (#) | Duplications/ |
    |         |                   |                   |    packet (#) |
    +=========+===================+===================+===============+
    | RPL     |             82.70 |              5.56 |          7.02 |
    +---------+-------------------+-------------------+---------------+
    | 2nd ETX |             99.38 |             14.43 |         31.29 |
    +---------+-------------------+-------------------+---------------+
    | CA      |             97.32 |              9.86 |         18.23 |
    | Strict  |                   |                   |               |
    +---------+-------------------+-------------------+---------------+
    | CA      |             99.66 |             13.75 |         28.86 |
    | Medium  |                   |                   |               |
    +---------+-------------------+-------------------+---------------+

                                  Table 1

   Links:

   *  Contiki OS DIO DAGMC NSA extension (draft-koutsiamanis-roll-nsa-
      extension branch) (https://github.com/ariskou/contiki/tree/draft-
      koutsiamanis-roll-nsa-extension)

   *  Wireshark dissectors (for the optional PS TLV) - currently merged
      / in master (https://code.wireshark.org/review/gitweb?p=wireshark.
      git;a=commit;h=e2f6ba229f45d8ccae2a6405e0ef41f1e61da138)

Appendix B.  Choosing an AP selection policy

   The manner of choosing an AP selection policy is left to the
   implementation, for maximum flexibility.

   For example, a different policy can be used per traffic type.  The
   network configurator can choose the CA Relaxed policy to increase
   reliability (thus producing some flooding) for specific, extremely
   important, alert packets.  On the other hand, all normal data traffic
   uses the CA Strict policy.  Therefore, an exception is made just for
   the alert packets.

   Another option would be to devise a new disjoint policy, where the
   paths are on purpose non-correlated, to increase path diversity and
   resilience against whole groups of nodes failing.  The disadvantage
   may be increased jitter.

   Finally, a network configurator may provide the CA policies with a
   preference order of Strict > Medium > Relaxed as a means of falling
   back to more flood-prone policies to maintain reliability.

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Authors' Addresses

   Remous-Aris Koutsiamanis (editor)
   IMT Atlantique
   Office B215
   4 rue Alfred Kastler, BP 20722
   44307 Nantes Cedex 3
   France

   Email: aris@ariskou.com

   Georgios Papadopoulos
   IMT Atlantique
   Office B00 - 114A
   2 Rue de la Chataigneraie
   35510 Cesson-Sevigne - Rennes
   France

   Phone: +33 299 12 70 04
   Email: georgios.papadopoulos@imt-atlantique.fr

   Nicolas Montavont
   IMT Atlantique
   Office B00 - 106A
   2 Rue de la Chataigneraie
   35510 Cesson-Sevigne - Rennes
   France

   Phone: +33 299 12 70 23
   Email: nicolas.montavont@imt-atlantique.fr

   Pascal Thubert
   Cisco Systems, Inc
   Building D
   45 Allee des Ormes - BP1200
   06254 MOUGINS - Sophia Antipolis
   France

   Phone: +33 497 23 26 34
   Email: pthubert@cisco.com

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