ROLL                                                R. Koutsiamanis, Ed.
Internet-Draft                                           G. Papadopoulos
Intended status: Standards Track                            N. Montavont
Expires: December 30, 2019 January 9, 2020                                  IMT Atlantique
                                                              P. Thubert
                                                                   Cisco
                                                           June 28,
                                                            July 8, 2019

RPL

Common Ancestor Objective Functions and Parent Set DAG Metric Container Node State and Attribute object type extension
                    draft-ietf-roll-nsa-extension-03
                               Extension
                    draft-ietf-roll-nsa-extension-04

Abstract

   Implementing Packet Replication and Elimination from / to the RPL
   root requires the ability to forward copies of packets over different
   paths via different RPL parents.  Selecting the appropriate parents
   to achieve ultra-low latency and jitter requires information about a
   node's parents.  This document details what information needs to be
   transmitted and how it is encoded within a packet to enable this
   functionality.  This document also describes Objective Functions
   which take 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.

   Internet-Drafts are working documents of the Internet Engineering
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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on December 30, 2019. January 9, 2020.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3   4
   3.  Alternative Parent Selection  . . .  Common Ancestor Objective Functions . . . . . . . . . . . . .   4
     3.1.  Common Ancestor Strict  . . . . . . . . . . . . . . . . .   4   6
     3.2.  Common Ancestor Medium  . . . . . . . . . . . . . . . . .   5   7
     3.3.  Common Ancestor Relaxed . . . . . . . . . . . . . . . . .   6   8
     3.4.  Usage . . . . . . . . . . . . . . . . . . . . . . . . . .   6   8
   4.  Node State and Attribute (NSA) object type extension  . . . .   6   8
     4.1.  Usage . . . . . . . . . . . . . . . . . . . . . . . . . .   9
     4.2.  Compression . . . . . . . . . . . . . . . . . . . . . . .   9  10
   5.  Controlling PRE . . . . . . . . . . . . . . . . . . . . . . .   9  10
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   9  11
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10  11
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10  11
     8.1.  Informative references  . . . . . . . . . . . . . . . . .  10  11
     8.2.  Other Informative References  . . . . . . . . . . . . . .  11  12
   Appendix A.  Implementation Status  . . . . . . . . . . . . . . .  11  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13  14

1.  Introduction

   Network-enabled applications in the industrial context must provide
   stringent guarantees in terms of reliability and predictability.  To
   achieve this they typically leverage 1+1 redundancy, also known as
   Packet Replication and Elimination (PRE)
   [I-D.papadopoulos-6tisch-pre-reqs].  Allowing these kinds of
   applications to function over wireless networks requires the
   application of the principles of Deterministic Networking
   [I-D.ietf-detnet-architecture].  This results in designs which aim at
   maximizing
   optimizing packet delivery rate and minimizing latency and jitter. bounding latency.  Additionally,
   given that the network nodes often do not have an unlimited power
   supply, energy consumption needs to be minimized as well.

   As an example, to meet this goal, IEEE Std. 802.15.4
   [IEEE802154-2015] [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 delay and jitter
   performance can deteriorate.

   Furthermore, the 6TiSCH working group, focusing on IPv6 over IEEE
   Std. 802.15.4-TSCH, has worked on the issues previously highlighted
   and produced the "6TiSCH Architecture" [I-D.ietf-6tisch-architecture]
   to address that case.  Building on this architecture, "Exploiting
   Packet Replication and Elimination in Complex Tracks in 6TiSCH LLNs"
   [I-D.papadopoulos-6tisch-pre-reqs] leverages PRE to improve the
   Packet Delivery Ratio (PDR), to provide a hard bound to the end-to-
   end latency, and to limit jitter.

   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
   for this subset to be defined, a RPL parent subset selection
   mechanism, which is among the responsibilities of the RPL Objective
   Function (OF), needs to have specific path information.  This
   document focuses on the specification of describes OFs which implement multi-path routing for PRE and
   specifies the transmission of this specific path information.

   More concretely,

   For the OFs, this document specifies a group of OFs called Common
   Ancestor (CA) OFs.  A detailed description is made 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 new
   Objective Code Points (OCPs) for these CA OFs.

   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 [RFC6550] 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.

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 transmits
      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 "Exploiting Packet Replication
      and Elimination in Complex Tracks in 6TiSCH LLNs"
      [I-D.papadopoulos-6tisch-pre-reqs] for more details.

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

3.  Alternative Parent Selection  Common Ancestor Objective Functions

   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 registers an AP node as well from its Parent Set
   (PS).

   There are multiple alternative methods of selecting the AP node.
   This functionality is included in the operation of the RPL Objective
   Function (OF).  A scheme group of OFs which allows allow the two paths to remain
   correlated is detailed here.  More specifically, in this
   scheme when using these OFs
   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 Section 4.3.
   "Promiscuous Overhearing" in "Exploiting Packet Replication and
   Elimination in Complex Tracks in 6TiSCH LLNs"
   [I-D.papadopoulos-6tisch-pre-reqs].  If multiple potential APs match
   this condition, the AP with the lowest rank will be registered.

   There

   The OFs described here are at least three methods an extension of performing the AP selection based
   on common ancestors (CA), named Common Ancestor Strict, Common
   Ancestor Medium, and Common Ancestor Relaxed, depending on The Minimum Rank with
   Hysteresis Objective Function [RFC6719] (MRHOF) OF.  In general,
   these OFs extend MRHOF by specifying how
   restrictive the selection process is.  A more restrictive method 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.

3.1.  Common Ancestor Strict

   In CA Strict, the node will check if its Preferred Grand Parent
   (PGP), is selected.  The
   selection of the PP of its PP, is kept the same as the PP in MRHOF.

   The ways in which the CA OFs modify MRHOF in a section-by-section
   manner follows:

   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, depending on
      the specific variant of CA OF used.

   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.  As a result, the node MUST NOT select the
      candidate neighbor as its AP.

   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.

   3.2.1.  When Parent Selection Runs:  Same as MRHOF.

   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 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.

   3.3.  Computing Rank:  Same as MRHOF.

   3.4.  Advertising the Path Cost:  Same as MRHOF.

   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.

   4.  Using MRHOF for Metric Maximization:  Same as MRHOF.

   5.  MRHOF Variables and Parameters:  Same as MRHOF extended to AP
      selection.  The CA OFs operate 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.

   6.  Manageability:  Same as MRHOF.

   6.1.  Device Configuration:  Same as MRHOF.

   6.2.  Device Monitoring:  Same as MRHOF.

   Three OFs are defined which 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 method 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.

   All three OFs apply their corresponding common ancestor 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.

3.1.  Common Ancestor Strict

   In the CA Strict OF, represented with Objective Code Point (OCP)
   TBD1, 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 Alternative Parent

   Figure 1: Example Common Ancestor Strict Alternative Parent Selection
                                  method

   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 method will select an
   AP for node S for which PP(PP(S)) = PP(AP).  Given that PP(PP(S)) =
   Y:

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

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

   o  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).

3.2.  Common Ancestor Medium

   In the CA Medium, Medium OF, represented with Objective Code Point (OCP)
   TBD2, 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
   method will select an AP for node S for which PP(PP(S)) is in PS(AP).
   Given that PP(PP(S)) = Y:

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

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

   o  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, Relaxed OF, represented with Objective Code Point (OCP)
   TBD3, 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
   method 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}:

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

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

   o  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.

3.4.  Usage

   The PS information can be used by any of the described AP selection
   methods or other ones not described here, depending on requirements.
   This document does not suggest a specific AP selection method.
   Additionally, it
   It is optional for all nodes to use the same AP selection method.
   Different nodes may use different AP selection methods, since the
   selection method is local to each node.  For example, using different
   methods can be used to vary the transmission reliability in each hop.

4.  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 [RFC6550], the nodes use the DODAG
   Information Object (DIO) Control Message to broadcast information
   about themselves to potential children.  However, RPL [RFC6550], 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.

    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
   [RFC6550].  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)| |MC
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |     Res       |  Flags    |A|O|    PS  type   |   PS  Length  | |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |NSA
  |   6LoRH type  | 6LoRH-compressed   PS IPv6 address(es) ...                                       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

   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 CAN 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 TBD1. TBD4.

   PS Length:  The total length of the TLV value field (PS IPv6
         address(es)) in bytes.

   6LoRH type:  The type of 6LoRH compression applied to the PS IPv6
         addresses.  For detailed usage see Section 5.1 of [RFC8138].
         As an overview, the compressed size of each IPv6 address in the
         "6LoRH-compressed PS IPv6 address(es)" field depending on the
         value of "6LoRH type" is shown in Figure 4.

                +-----------+----------------------+
                |   6LoRH   | Length of compressed |
                |   Type    | IPv6 address (bytes) |
                +-----------+----------------------+
                |    0      |       1              |
                |    1      |       2              |
                |    2      |       4              |
                |    3      |       8              |
                |    4      |      16              |
                +-----------+----------------------+

                       Figure 4: The SRH-6LoRH Types

   6LoRH-compressed PS IPv6 address(es):  A sequence of zero or more
         IPv6 addresses belonging to a node's parent set.  Each address
         requires 16 bytes.  The order of the parents in the parent set
         is in decreasing preference based on the Objective Function
         [RFC6550] used by the node.

4.1.  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 constraints and
   is used as per [RFC6551].

4.2.  Compression

   The PS IPv6 address(es) field in the Parent Set TLV add overhead due
   to their size.  Therefore, compression is highly desirable in order
   for this extension to be usable.  To meet this goal, a good
   compression method candidate is [RFC8138] 6LoWPAN Routing Header
   (6LoRH).  Furthermore, the PS IPv6 address(es) belong by definition
   to nodes in the same RPL DODAG and are stored in the form of a list
   of addresses.  This makes this field a good candidate for the use of
   the same compression as in Source Routing Header 6LoRH (SRH-6LoRH),
   achieving efficiency and implementation reuse.  Therefore, the PS
   IPv6 address(es) field SHOULD be compressed using the compression
   method for Source Routing Header 6LoRH (SRH-6LoRH) [RFC8138].

5.  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
   behaviour
   behavior per flow type as described in Deterministic Networking
   Architecture [I-D.ietf-detnet-architecture].

6.  Security Considerations

   The structure of the DIO control message is extended, within the pre-
   defined DIO options.  Therefore, the security mechanisms defined in
   RPL [RFC6550] apply to this proposed extension.

7.  IANA Considerations

   This proposal requests the allocation of new values TBD1, TBD2, TBD3
   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 TBD1 TBD4 for the
   "Parent Set" TLV in from the Routing Metric/Constraint TLVs sub-registry
   from IANA.

8.  References

8.1.  Informative references

   [I-D.ietf-6tisch-architecture]
              Thubert, P., "An Architecture for IPv6 over the TSCH mode
              of IEEE 802.15.4", draft-ietf-6tisch-architecture-23 draft-ietf-6tisch-architecture-24 (work
              in progress), June July 2019.

   [I-D.ietf-detnet-architecture]
              Finn, N., Thubert, P., Varga, B., and J. Farkas,
              "Deterministic Networking Architecture", draft-ietf-
              detnet-architecture-13 (work in progress), May 2019.

   [I-D.papadopoulos-6tisch-pre-reqs]
              Papadopoulos, G., Montavont, N., and P. Thubert,
              "Exploiting Packet Replication and Elimination in Complex
              Tracks in 6TiSCH LLNs", draft-papadopoulos-6tisch-pre-
              reqs-02 (work in progress), July 2018.

   [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>.

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

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

   [RFC8138]  Thubert, P., Ed., Bormann, C., Toutain, L.,

   [RFC6719]  Gnawali, O. and R. Cragie,
              "IPv6 over Low-Power Wireless Personal Area Network
              (6LoWPAN) Routing Header", P. Levis, "The Minimum Rank with
              Hysteresis Objective Function", RFC 8138, 6719,
              DOI 10.17487/RFC8138,
              April 2017, <https://www.rfc-editor.org/info/rfc8138>. 10.17487/RFC6719, September 2012,
              <https://www.rfc-editor.org/info/rfc6719>.

8.2.  Other Informative References

   [IEEE802154-2015]

   [IEEE802154]
              IEEE standard for Information Technology, "IEEE Std
              802.15.4-2015
              802.15.4 Standard for Low-Rate Wireless Personal Area
              Networks (WPANs)", December 2015.

8.3.  URIs

   [1] https://github.com/ariskou/contiki/tree/draft-koutsiamanis-roll-
       nsa-extension

   [2] https://code.wireshark.org/review/gitweb?p=wireshark.git;a=commit
       ;h=e2f6ba229f45d8ccae2a6405e0ef41f1e61da138

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 )

                       Figure 5: 4: Simulation Topology

   The simulation setup is:

   Topology:  32 nodes structured in regular grid as show in Figure 5. 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:

   +----------+---------------+------------------+---------------------+
   | Routing  |       Average |          Average |             Average |
   | Method   |        Packet |        Traversed | Duplications/packet |
   |          | Delivery Rate | Nodes/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   |               |                  |                     |
   +----------+---------------+------------------+---------------------+

   Links:

   o  Contiki OS DIO DAGMC NSA extension (draft-koutsiamanis-roll-nsa-
      extension branch) [1]

   o  Wireshark dissectors (for the optional PS TLV) - currently merged
      / in master [2]

Authors' Addresses
   Remous-Aris Koutsiamanis (editor)
   IMT Atlantique
   Office B00 - 126A
   2 Rue de la Chataigneraie
   Cesson-Sevigne - Rennes  35510
   FRANCE

   Phone: +33 299 12 70 49
   Email: aris@ariskou.com

   Georgios Papadopoulos
   IMT Atlantique
   Office B00 - 114A
   2 Rue de la Chataigneraie
   Cesson-Sevigne - Rennes  35510
   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
   Cesson-Sevigne - Rennes  35510
   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
   MOUGINS - Sophia Antipolis  06254
   FRANCE

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