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Versions: (draft-vilajosana-6tisch-minimal) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 RFC 8180

6TiSCH                                                X. Vilajosana, Ed.
Internet-Draft                           Universitat Oberta de Catalunya
Intended status: Informational                                 K. Pister
Expires: December 29, 2014             University of California Berkeley
                                                           June 27, 2014


                      Minimal 6TiSCH Configuration
                      draft-ietf-6tisch-minimal-01

Abstract

   This document describes the minimal set of rules to operate a
   [IEEE802154e] Timeslotted Channel Hopping (TSCH) network.  This
   minimal mode of operation can be used during network bootstrap, as a
   fallback mode of operation when no dynamic scheduling solution is
   available or functioning, or during early interoperability testing
   and development.

Requirements Language

   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 RFC
   2119 [RFC2119].

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

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

   This Internet-Draft will expire on December 29, 2014.

Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.




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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Minimal Schedule Configuration  . . . . . . . . . . . . . . .   3
     2.1.  Slotframe . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.2.  Cell Options  . . . . . . . . . . . . . . . . . . . . . .   5
     2.3.  Retransmissions . . . . . . . . . . . . . . . . . . . . .   6
     2.4.  Time Slot timing  . . . . . . . . . . . . . . . . . . . .   6
   3.  Enhanced Beacons Configuration and Content  . . . . . . . . .   8
     3.1.  Sync IE . . . . . . . . . . . . . . . . . . . . . . . . .   8
       3.1.1.  IE Header . . . . . . . . . . . . . . . . . . . . . .   8
       3.1.2.  IE Content  . . . . . . . . . . . . . . . . . . . . .   8
     3.2.  TSCH Timeslot IE  . . . . . . . . . . . . . . . . . . . .   9
       3.2.1.  IE Header . . . . . . . . . . . . . . . . . . . . . .   9
       3.2.2.  IE Content  . . . . . . . . . . . . . . . . . . . . .   9
     3.3.  Channel Hopping IE  . . . . . . . . . . . . . . . . . . .   9
       3.3.1.  IE Header . . . . . . . . . . . . . . . . . . . . . .   9
       3.3.2.  IE Content  . . . . . . . . . . . . . . . . . . . . .   9
     3.4.  Frame and Link IE . . . . . . . . . . . . . . . . . . . .   9
       3.4.1.  IE Header . . . . . . . . . . . . . . . . . . . . . .   9
       3.4.2.  IE Content  . . . . . . . . . . . . . . . . . . . . .  10
   4.  Acknowledgment  . . . . . . . . . . . . . . . . . . . . . . .  10
     4.1.  ACK/NACK Time Correction IE . . . . . . . . . . . . . . .  10
       4.1.1.  IE Header . . . . . . . . . . . . . . . . . . . . . .  10
       4.1.2.  IE Content  . . . . . . . . . . . . . . . . . . . . .  10
   5.  Neighbor information  . . . . . . . . . . . . . . . . . . . .  11
     5.1.  Neighbor Table  . . . . . . . . . . . . . . . . . . . . .  11
     5.2.  Time Source Neighbor Selection  . . . . . . . . . . . . .  12
   6.  Queues and Priorities . . . . . . . . . . . . . . . . . . . .  12
   7.  RPL on TSCH . . . . . . . . . . . . . . . . . . . . . . . . .  13
     7.1.  RPL Objective Function Zero . . . . . . . . . . . . . . .  13
       7.1.1.  Rank computation  . . . . . . . . . . . . . . . . . .  13
       7.1.2.  Rank computation Example  . . . . . . . . . . . . . .  14
     7.2.  RPL Configuration . . . . . . . . . . . . . . . . . . . .  16
       7.2.1.  Mode of Operation . . . . . . . . . . . . . . . . . .  17
       7.2.2.  Trickle Timer . . . . . . . . . . . . . . . . . . . .  17
       7.2.3.  Hysteresis  . . . . . . . . . . . . . . . . . . . . .  17
       7.2.4.  Variable Values . . . . . . . . . . . . . . . . . . .  17



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   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  18
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  18
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  18
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  19
     9.3.  External Informative References . . . . . . . . . . . . .  20
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  20

1.  Introduction

   The nodes in a [IEEE802154e] TSCH network follow a communication
   schedule.  The entity (centralized or decentralized) responsible for
   building and maintaining that schedule has very precise control over
   the trade-off between the network's latency, bandwidth, reliability
   and power consumption.  During early interoperability testing and
   development, however, simplicity is often more important than
   efficiency.  One goal of this document is to define the simplest set
   of rules for building a [IEEE802154e] TSCH-compliant network, at the
   necessary price of lesser efficiency.  Yet, this minimal mode of
   operation can also be used during network bootstrap before any
   schedule is installed into the network so nodes can self-organize and
   the management and configuration information be distributed.  In
   addition, as outlined in
   [I-D.phinney-roll-rpl-industrial-applicability], the minimal
   configuration can be used as a fallback mode of operation, ensuring
   connectivity of nodes in case that dynamic scheduling mechanisms fail
   or are not available.  [IEEE802154e] provides a mechanism whereby the
   details of slotframe length, timeslot timing, and channel hopping
   pattern are communicated at synchronization to a node, also Enhanced
   Beacons can be used to periodically update nodes information.  This
   document describes specific settings for these parameters.  Nodes
   MUST broadcast properly formed Enhanced Beacons to announce these
   values, but during initial implementation and debugging it may be
   convenient to preconfigure these values.

2.  Minimal Schedule Configuration

   In order to form a network, a minimum schedule configuration is
   required so nodes can advertise the presence of the network, and
   allow other nodes to join.

2.1.  Slotframe

   The slotframe, as defined in [I-D.ietf-6tisch-terminology], is an
   abstraction of the link layer that defines a collection of time slots
   of equal length, and which repeats over time.  In order to set up a
   minimal TSCH network, nodes need to be synchronized with the same
   slotframe configuration so they can exchange Enhanced Beacons (EBs)




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   and data packets.  This document recommends the following slotframe
   configuration.

   Minimal configuration

   +------------------------------------+----------------------+
   |           Property                 |       Value          |
   +------------------------------------+----------------------+
   | Number of time slots per Slotframe |      Variable        |
   +------------------------------------+----------------------+
   | Number of available frequencies    |         16           |
   +------------------------------------+----------------------+
   | Number of scheduled cells          | 1 (slotOffset 0)     |
   |                                    | (macLinkType NORMAL) |
   +------------------------------------+----------------------+
   | Number of unscheduled cells        | The remainder of the |
   |                                    | slotframe            |
   +------------------------------------+----------------------+
   | Number of MAC retransmissions (max)|  3 (4 attempts to tx)|
   +------------------------------------+----------------------+


   The slotframe is composed of a configurable number of time slots.
   Choosing the number of time slots per slotframe needs to take into
   account network requirements such as density, bandwidth per node,
   etc.  In the minimal configuration, there is only a single active
   slot in slotframe, used to transmit data and EBs, and receive
   information.  The trade-off between bandwidth, latency and energy
   consumption can be controlled by choosing a different slotframe
   length.  The active slot MAY be scheduled at the slotoffset 0x00 and
   channeloffset 0x00 and MUST be announced in the EBs.  EBs are sent
   using this active slot and are not acknowledged.  Data packets, as
   described in Section 2.2 use the same active slot.  Per
   [IEEE802154e], data packets sent unicast on this cell are
   acknowledged by the receiver.  The remaining cells are unscheduled,
   and MAY be used by dynamic scheduling solutions.  Details about such
   dynamic scheduling solution are out of scope.

   The slotframe length (expressed in number of time slots) is
   configurable.  The length used determines the duty cycle of the
   network.  For example, a network with a 1.01% duty cycle is composed
   of a slotframe of 101 slots, which includes 1 active slot.  The
   present document RECOMMENDS the use of a default slot duration set to
   10ms and its corresponding default timeslot timings defined by the
   [IEEE802154e] macTimeslotTemplate.  The use of the default
   macTimeslotTemplate MUST be announced in the EB by using the Timeslot
   IE containing only the default macTimeslotTemplateId.  Other time
   slot durations MAY be supported and MUST be announced clearly.  If



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   one uses a timeslot duration different than 10ms, it is RECOMMENDED
   to use a power-of-two of 10ms (i.e. 20ms, 40ms, 80ms, etc.).  In this
   case, EBs MUST contain the complete TimeSlot IE as described in
   Section 2.4.

   Example schedule with 1.01% duty cycle

      Chan.  +----------+----------+
      Off.0  | TxRxS/EB |    OFF   |
      Chan.  +----------+----------+
      Off.1  |          |          |
             +----------+----------+
              ...
      Chan.  +----------+----------+
      Off.15 |          |          |
             +----------+----------+
               0            1-100

   EB:  Enhanced Beacon
   Tx:  Transmit
   Rx:  Receive
   S:   Shared
   OFF: Unscheduled (can be used by a dynamic scheduling mechanism)

2.2.  Cell Options

   Per the [IEEE802154e] TSCH, each scheduled cell has an associated
   bitmap of cell options, called LinkOption.  The scheduled cell in the
   minimal schedule is configured as Hard cell
   [I-D.ietf-6tisch-tsch][I-D.ietf-6tisch-6top-interface].  Additional
   available cells can be scheduled by a dynamic scheduling solution.
   The dynamic scheduling solution is out of scope, and this
   specification does not make any restriction on the LinkOption
   associated with those dynamically scheduled cells (i.e. they can be
   hard cells or soft cells).

   The active cell is assigned the bitmap of cell options below.
   Because both the "Transmit" and "Receive" bits are set, a node
   transmits if there is a packet in its queue, and listens otherwise.
   Because the "shared" bit is set, the back-off mechanism defined in
   [IEEE802154e] is used to resolve contention.  This results in
   "Slotted Aloha" behavior.  The "Timekeeping" flag is never set, since
   the time source neighbor is selected using the DODAG structure of the
   network (detailed below).

      b0 = Transmit = 1 (set)

      b1 = Receive = 1 (set)



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      b2 = Shared = 1 (set)

      b3 = Timekeeping = 0 (clear)

      b4-b7 = Reserved (clear)

   All remaining cells are unscheduled.  In unscheduled cells, the nodes
   SHOULD keep their radio off.  In a memory-efficient implementation,
   scheduled cells can be represented by a circular linked list.
   Unscheduled cells SHOULD NOT occupy any memory.

2.3.  Retransmissions

   The maximum number of link layer retransmissions is set to 3.  For
   packets which require an acknowledgement, if none is received after a
   total of 4 attempts, the transmissions is considered failed and the
   link layer MUST notify the upper layer.  Packets sent to the
   broadcast MAC address (including EBs) are not acknowledged and
   therefore not retransmitted.

2.4.  Time Slot timing

   The figure below shows an active timeslot in which a packet is sent
   from the transmitter node (TX) to the receiver node (RX).  A link-
   layer acknowledgement is sent by the RX node to the TX node when the
   packet is to be acknowledged.  The TsTxOffset duration defines the
   instant in the timeslot when the first byte of the transmitted packet
   leaves the radio of the TX node.  The radio of the RX node is turned
   on TsLongGT/2 before that instant, and listens for at least TsLongGT.
   This allows for a de-synchronization between the two node of at most
   TsLongGT.  The RX node needs to send the first byte of the MAC
   acknowledgement exactly TsTxAckDelay after the end of the last byte
   of the received packet.  TX's radio has to be turned on TsShortGT/2
   before that time, and keep listening for at least TsShortGT.

















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   Time slot internal timing diagram

      /------------------- Time Slot duration --------------------/
      |                                        /tsShortGT/        |
      |            |                              | | |           |
      |------------+-----------------+--------------+------+------|
   TX |            |    TX-Packet    |              |RX Ack|      |
      |------------+-----------------+--------------+------+------|
      |/tsTxOffset/|                 |              |      |      |
      |            |                 |              |      |      |
      |------------+-----------------+--------------+------+------|
   RX |         |  |  | RX-Packet    |              |TX Ack|      |
      |---------+--+--+--------------+--------------+------+------|
      |         |  |  |              |              |      |      |
      |        /tsLongGT/            |/TsTxAckDelay/|      |      |
     Start                                                       End
      of                                                          of
     Slot                                                        Slot

   A 10ms time slot length is the default value defined by
   [IEEE802154e].  Section 6.4.3.3.3 of the [IEEE802154e] defines a
   default macTimeslotTemplate, i.e. the different duration within the
   slot.  These values are summarized in the following table and MUST be
   used when utilizing the default time slot duration.  In this case,
   the Timeslot IE only transports the macTimeslotTemplateId (0x00) as
   the timing values are well-known.  If a timeslot template other than
   the default is used, the EB MUST contain a complete TimeSlot IE,
   which requires 25 extra bytes.

   Default timeslot durations (per [IEEE802154e], Section 6.4.3.3.3)

   +--------------------------------+------------+
   | IEEE802.15.4e TSCH parameter   |     Value  |
   +--------------------------------+------------+
   | TsTxOffset                     |    2120us  |
   +--------------------------------+------------+
   | TsLongGT                       |    2000us  |
   +--------------------------------+------------+
   | TsTxAckDelay                   |    1000us  |
   +--------------------------------+------------+
   | TsShortGT                      |     400us  |
   +--------------------------------+------------+
   | Time Slot duration             |   10000us  |
   +--------------------------------+------------+







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3.  Enhanced Beacons Configuration and Content

   [IEEE802154e] does not define how often EBs are sent, not their
   contents.  The choice of the duration between two EBs needs to take
   into account whether EBs are used as the only mechanism to
   synchronize devices, or whether a Keep-Alive (KA) mechanism is also
   used.  For a minimal TSCH configuration, a mote SHOULD send an EB
   every EB_PERIOD.  For additional reference see [I-D.ietf-6tisch-tsch]
   where different synchronization approaches are summarized.

   EBs MUST be sent with the Beacon IEEE802.15.4 frame type and this EBs
   MUST carry the Information Elements (IEs) listed below.

   The content of the IEs is presented here for completeness, however
   this information is redundant with [I-D.ietf-6tisch-tsch] and
   [IEEE802154e].

3.1.  Sync IE

   Contains synchronization information such as ASN and Join Priority.
   The value of Join Priority is discussed in Section 5.2.

3.1.1.  IE Header

      Length (b0-b7) = 0x06

      Sub-ID (b8-b14) = 0x1a

      Type (b15) = 0x00 (short)

3.1.2.  IE Content

      ASN Byte 1 (b16-b23)

      ASN Byte 2 (b24-b31)

      ASN Byte 3 (b32-b39)

      ASN Byte 4 (b40-b47)

      ASN Byte 5 (b48-b55)

      Join Priority (b56-b63)








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3.2.  TSCH Timeslot IE

   Contains the timeslot template identifier.  This specification uses
   the default timeslot template as defined in [IEEE802154e],
   Section 5.2.4.15.

3.2.1.  IE Header

      Length (b0-b7) = 0x01

      Sub-ID (b8-b14) = 0x1c

      Type (b15) = 0x00 (short)

3.2.2.  IE Content

      Timeslot Template ID (b0-b7) = 0x00

3.3.  Channel Hopping IE

   Contains the channel hopping template identifier.  This specification
   uses the default channel hopping template, as defined in
   [IEEE802154e], Section 5.2.4.16.

3.3.1.  IE Header

      Length (b0-b7) = 0x01

      Sub-ID (b8-b14) = 0x1d

      Type (b15) = 0x00 (short)

3.3.2.  IE Content

      Channel Hopping Template ID (b0-b7) = 0x00

3.4.  Frame and Link IE

   Each node MUST indicate the schedule in each EB through a Frame and
   Link IE.  This enables nodes which implement [IEEE802154e] to
   configure their schedule as they join the network.

3.4.1.  IE Header

      Length (b0-b7) = variable

      Sub-ID (b8-b14) = 0x1b




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      Type (b15) = 0x00 (short)

3.4.2.  IE Content

      # Slotframes (b16-b23) = 0x01

      Slotframe ID (b24-b31) = 0x01

      Size Slotframe (b32-b47) = variable

      # Links (b48-b55) = 0x01

   For the active cell in the minimal schedule:

      Channel Offset (2B) = 0x00

      Slot Number (2B) = 0x00

      LinkOption (1B) = as described in Section 2.2

4.  Acknowledgment

   Link-layer acknowledgment frames are built according to
   [IEEE802154e].  Data frames and command frames sent to a unicast MAC
   destination address request an acknowledgment.  The acknowledgment
   frame is of type ACK (0x10).  Each acknowledgment contains the
   following IE:

4.1.  ACK/NACK Time Correction IE

   The ACK/NACK time correction IE carries the measured de-
   synchronization between the sender and the receiver.

4.1.1.  IE Header

      Length (b0-b7) = 0x02

      Sub-ID (b8-b14) = 0x1e

      Type (b15) = 0x00 (short)

4.1.2.  IE Content

      Time Synchronization Information and ACK status (b16-b31)

   The possible values for the Time Synchronization Information and ACK
   status are described in [IEEE802154e] and reproduced in the following
   table:



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   ACK status and Time Synchronization Information.

   +-----------------------------------+-----------------+
   |            ACK Status             |     Value       |
   +-----------------------------------+-----------------+
   | ACK with positive time correction | 0x0000 - 0x07ff |
   +-----------------------------------+-----------------+
   | ACK with negative time correction | 0x0800 - 0x0fff |
   +-----------------------------------+-----------------+
   | NACK with positive time correction| 0x8000 - 0x87ff |
   +-----------------------------------+-----------------+
   | NACK with negative time correction| 0x8800 - 0x8fff |
   +-----------------------------------+-----------------+

5.  Neighbor information

   [IEEE802154e] does not define how and when each node in the network
   keeps information about its neighbors.  This document recommends to
   keep the following information in the neighbor table:

5.1.  Neighbor Table

   The exact format of the neighbor table is implementation-specific,
   but it SHOULD contain the following information for each neighbor:

      Neighbor statistics:

         numTx: number of transmitted packets to that neighbor

         numTxAck: number of transmitted packets that have been
         acknowledged by that neighbor

         numRx: number of received packets from that neighbor

      The EUI64 of the neighbor.

      Timestamp when that neighbor was heard for the last time.  This
      can be based on the ASN counter or any other time base.  Can be
      used to trigger a keep-alive message.

      RPL rank of that neighbor.

      A flag indicating whether this neighbor is a time source neighbor.

      Connectivity statistics (e.g., RSSI), which can be used to
      determine the quality of the link.





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   In addition to that information, each node has to be able to compute
   some RPL Objective Function (OF), taking into account the neighbor
   and connectivity statistics.  An example RPL objective function is
   the OF Zero as described in [RFC6552] and Section 7.1.1.

5.2.  Time Source Neighbor Selection

   Each node MUST select at least one time source neighbor among the
   nodes in its RPL routing parent set.  When a node joins a network, it
   has no routing information.  To select its time source neighbor, it
   uses the Join Priority field in the EB, as described in
   Section 5.2.4.13 and Table 52b of [IEEE802154e].  The Sync IE
   contains the ASN and 1 Byte field named Join Priority.  The Join
   Priority of any node is equivalent to the result of the function
   DAGRank(rank) as defined by [RFC6550] and Section 7.1.1.  The Join
   Priority of the DAG root is zero, i.e., EBs sent from the DAG root
   are sent with Join Priority equal to 0.  A lower value of the Join
   Priority indicates that the device is the preferred one to connect
   to.  When a node joins the network, it MUST NOT send EBs before
   having acquired a RPL rank.  This avoids routing loops and matches
   RPL topology with underlying mesh topology.  As soon as a node
   acquires a RPL rank (see [RFC6550] and Section 7.1.1), it SHOULD send
   Enhanced Beacons including a Sync IE with Join Priority field set to
   DAGRank(rank), where rank is the node's rank.  If a node receives EBs
   from different nodes with equal Join Priority, the time source
   neighbor selection should be assessed by other metrics that can help
   determine the better connectivity link.  Time source neighbor
   hysteresis SHOULD be used, according to the rules defined in
   Section 7.2.3.  If connectivity to the time source neighbor is lost,
   a new time source neighbor MUST be chosen among the neighbors in the
   RPL routing parent set.

   The decision for a node to select one Time Source Neighbor when
   multiple EBs are received is open to implementers.  For example a
   node MAY wait until one EB from NUM_NEIGHBOURS_TO_WAIT neighbors have
   been received to select the best Time Source Neighbor.  This
   condition MAY apply unless a second EB is not received after
   MAX_EB_DELAY seconds.  This avoids initial hysteresis when selecting
   a first Time Source Neighbor.

   Optionally, some form of hysteresis SHOULD be implemented to avoid
   frequent changes in time source neighbors.

6.  Queues and Priorities

   [IEEE802154e] does not define the use of queues to handle upper layer
   data (either application or control data from upper layers).  This




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   document recommends the use of a single queue with the following
   rules:

      When the node is not synchronized to the network, higher layers
      are not able to insert packets into the queue.

      Frames generated by the MAC layer (e.g., EBs and ACK) have a
      higher priority than packets received from a higher layer.

      IEEE802.15.4 frames of types Beacon and Command have a higher
      priority than IEEE802.15.4 frames of types Data and ACK.

      One entry in the queue is reserved at all times for an
      IEEE802.15.4 frames of types Beacon or Command frames.

7.  RPL on TSCH

   Nodes in the network MUST use the RPL routing protocol [RFC6550].

7.1.  RPL Objective Function Zero

   Nodes in the network MUST use the RPL routing protocol [RFC6550] and
   implement the RPL Objective Function Zero [RFC6552].

7.1.1.  Rank computation

   The rank computation is described at [RFC6552], Section 4.1.
   Briefly, a node rank is computed by the following equation:

   R(N) = R(P) + rank_increase

   rank_increase = (Rf*Sp + Sr) * MinHopRankIncrease

   Where:

      R(N): Rank of the node.

      R(P): Rank of the parent obtained as part of the DIO information.

      rank_increase: The result of a function that determines the rank
      increment.

      Rf (rank_factor): A configurable factor that is used to multiply
      the effect of the link properties in the rank_increase
      computation.  If none is configured, rank_factor of 1 is used.  In
      this specification, a rank_factor of 1 MUST be used.





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      Sp (step_of_rank): (strictly positive integer) - an intermediate
      computation based on the link properties with a certain neighbor.
      In this specification, 2*ETX (Expected Transmissions) as defined
      by [decouti03high] and [RFC6551] MUST be used.  The ETX is
      computed as the inverse of the Packet Delivery Ratio (PDR), and
      MAY be computed as the number of acknowledged packets, divided by
      the number of transmitted packets to a certain node.  E.g:
      Sp=2*numTX/numTXAck

      Sr (stretch_of_rank): (unsigned integer) - the maximum increment
      to the step_of_rank of a preferred parent, to allow the selection
      of an additional feasible successor.  If none is configured to the
      device, then the step_of_rank is not stretched.  In this
      specification, stretch_of_rank MUST be set to 0.

      MinHopRankIncrease: the MinHopRankIncrease is set to the fixed
      constant DEFAULT_MIN_HOP_RANK_INCREASE [RFC6550].
      DEFAULT_MIN_HOP_RANK_INCREASE has a value of 256.

      DAGRank(rank): Equivalent to the floor of (Rf*Sp + Sr) as defined
      by [RFC6550].  Specifically, when an Objective Function computes
      Rank, this is defined as an unsigned integer (i.e., a 16-bit
      value) Rank quantity.  When the Rank is compared, e.g. to
      determine parent relationships or loop detection, the integer
      portion of the Rank is used.  The integer portion of the Rank is
      computed by the DAGRank() macro as floor(x) where floor(x) is the
      function that evaluates to the greatest integer less than or equal
      to x.  DAGRank(rank) = floor(rank/MinHopRankIncrease)

   Rank computation scenario

       +-------+
       |   P   | R(P)
       |       |
       +-------+
           |
           |
           |
       +-------+
       |   N   | R(N)=R(P) + rank_increase
       |       | rank_increase = (Rf*Sp + Sr) * MinHopRankIncrease
       +-------+ Sp=2*ETX

7.1.2.  Rank computation Example

   This section illustrates with an example the use of the Objective
   Function Zero.  Assume the following parameters:




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      Rf = 1

      Sp = 2* ETX

      Sr = 0

      minHopRankIncrease = 256 (default in RPL)

      ETX=(numTX/numTXAck)

      r(n) = r(p) + rank_increase

      rank_increase = (Rf*Sp + Sr) * minHopRankIncrease

      rank_increase = 512*numTx/numTxACK




































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   Rank computation example for 5 hop network where numTx=100 and
   numTxAck=75 for all nodes

       +-------+
       |   0   | R(0)=0
       |       | DAGRank(R(0)) = 0
       +-------+
           |
           |
       +-------+
       |   1   | R(1)=R(0)+683=683
       |       | DAGRank(R(1)) = 2
       +-------+
           |
           |
       +-------+
       |   2   | R(2)=R(1)+683=1366
       |       | DAGRank(R(2)) = 5
       +-------+
           |
           |
       +-------+
       |   3   | R(3)=R(2)+683=2049
       |       | DAGRank(R(3)) = 8
       +-------+
           |
           |
       +-------+
       |   4   | R(4)=R(3)+683=2732
       |       | DAGRank(R(4)) = 10
       +-------+
           |
           |
       +-------+
       |   5   | R(5)=R(4)+683=3415
       |       | DAGRank(R(5)) = 13
       +-------+

7.2.  RPL Configuration

   In addition to the Objective Function (OF), a minimal configuration
   for RPL should indicate the preferred mode of operation and trickle
   timer operation so different RPL implementations can inter-operate.
   RPL information SHOULD be transported in the flow label in the LLN as
   defined in [I-D.thubert-6man-flow-label-for-rpl]






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7.2.1.  Mode of Operation

   For downstream route maintenance, in a minimal configuration, RPL
   SHOULD be set to operate in the Non-Storing mode as described by
   [RFC6550] Section 9.7.  Storing mode ([RFC6550] Section 9.8) MAY be
   supported in less constrained devices.

7.2.2.  Trickle Timer

   RPL signaling messages such as DIOs are sent using the Trickle
   Algorithm [RFC6550] (Section 8.3.1) and [RFC6206].  For this
   specification, the Trickle Timer MUST be used with the RPL defined
   default values [RFC6550] (Section 8.3.1).  For a description of the
   Trickle timer operation see Section 4.2 on [RFC6206].

7.2.3.  Hysteresis

   According to [RFC6552], [RFC6719] recommends the use of a boundary
   value (PARENT_SWITCH_THRESHOLD) to avoid constant changes of parent
   when ranks are compared.  When evaluating a parent that belongs to a
   smaller path cost than current minimum path, the candidate node is
   selected as new parent only if the difference between the new path
   and the current path is greater than the defined
   PARENT_SWITCH_THRESHOLD.  Otherwise the node MAY continue to use the
   current preferred parent.  As for [RFC6719] the recommended value for
   PARENT_SWITCH_THRESHOLD is 192 when ETX metric is used, the
   recommendation for this document is to use PARENT_SWITCH_THRESHOLD
   equal to 394 as the metric being used is 2*ETX.  This is mechanism is
   suited to deal with parent hysteresis in both cases routing parent
   and time source neighbor selection.

7.2.4.  Variable Values

   The following table presents the RECOMMENDED values for the RPL-
   related variables defined in the previous section.
















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   Recommended variable values

   +-------------------------+----------+
   | Variable                |  Value   |
   +-------------------------+----------+
   | EB_PERIOD               |   10s    |
   +-------------------------+----------+
   | MAX_EB_DELAY            |   180    |
   +-------------------------+----------+
   | NUM_NEIGHBOURS_TO_WAIT  |     2    |
   +-------------------------+----------+
   | PARENT_SWITCH_THRESHOLD |   394    |
   +-------------------------+----------+

8.  Acknowledgements

   The authors would like to acknowledge the guidance and input provided
   by the 6TiSCH Chairs Pascal Thubert and Thomas Watteyne.

9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC6206]  Levis, P., Clausen, T., Hui, J., Gnawali, O., and J. Ko,
              "The Trickle Algorithm", RFC 6206, March 2011.

   [RFC6550]  Winter, T., Thubert, P., 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, March 2012.

   [RFC6551]  Vasseur, JP., Kim, M., Pister, K., Dejean, N., and D.
              Barthel, "Routing Metrics Used for Path Calculation in
              Low-Power and Lossy Networks", RFC 6551, March 2012.

   [RFC6552]  Thubert, P., "Objective Function Zero for the Routing
              Protocol for Low-Power and Lossy Networks (RPL)", RFC
              6552, March 2012.

   [RFC6719]  Gnawali, O. and P. Levis, "The Minimum Rank with
              Hysteresis Objective Function", RFC 6719, September 2012.







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9.2.  Informative References

   [I-D.ietf-6tisch-tsch]
              Watteyne, T., Palattella, M., and L. Grieco, "Using
              IEEE802.15.4e TSCH in an LLN context: Overview, Problem
              Statement and Goals", draft-ietf-6tisch-tsch-00 (work in
              progress), November 2013.

   [I-D.ietf-6tisch-architecture]
              Thubert, P., Watteyne, T., and R. Assimiti, "An
              Architecture for IPv6 over the TSCH mode of IEEE
              802.15.4e", draft-ietf-6tisch-architecture-02 (work in
              progress), June 2014.

   [I-D.ietf-6tisch-terminology]
              Palattella, M., Thubert, P., Watteyne, T., and Q. Wang,
              "Terminology in IPv6 over the TSCH mode of IEEE
              802.15.4e", draft-ietf-6tisch-terminology-01 (work in
              progress), February 2014.

   [I-D.ietf-6tisch-6top-interface]
              Wang, Q., Vilajosana, X., and T. Watteyne, "6TiSCH
              Operation Sublayer (6top) Interface", draft-ietf-6tisch-
              6top-interface-00 (work in progress), March 2014.

   [I-D.richardson-6tisch-security-architecture]
              Richardson, M., "security architecture for 6top:
              requirements and structure", draft-richardson-6tisch-
              security-architecture-02 (work in progress), April 2014.

   [I-D.ietf-roll-terminology]
              Vasseur, J., "Terms used in Routing for Low power And
              Lossy Networks", draft-ietf-roll-terminology-13 (work in
              progress), October 2013.

   [I-D.phinney-roll-rpl-industrial-applicability]
              Phinney, T., Thubert, P., and R. Assimiti, "RPL
              applicability in industrial networks", draft-phinney-roll-
              rpl-industrial-applicability-02 (work in progress),
              February 2013.

   [I-D.thubert-6man-flow-label-for-rpl]
              Thubert, P., "The IPv6 Flow Label within a RPL domain",
              draft-thubert-6man-flow-label-for-rpl-03 (work in
              progress), May 2014.






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9.3.  External Informative References

   [IEEE802154e]
              IEEE standard for Information Technology, "IEEE std.
              802.15.4e, Part. 15.4: Low-Rate Wireless Personal Area
              Networks (LR-WPANs) Amendment 1: MAC sublayer", April
              2012.

   [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", June 2011.

   [decouti03high]
              De Couto, D., Aguayo, D., Bicket, J., and R. Morris, "A
              High-Throughput Path Metric for Multi-Hop Wireless
              Routing", MobiCom '03, The 9th ACM International
              Conference on Mobile Computing and Networking, San Diego,
              California", June 2003.

   [OpenWSN]  "Berkeley's OpenWSN Project Homepage",
              <http://www.openwsn.org/>.

Authors' Addresses

   Xavier Vilajosana (editor)
   Universitat Oberta de Catalunya
   156 Rambla Poblenou
   Barcelona, Catalonia  08018
   Spain

   Phone: +34 (646) 633 681
   Email: xvilajosana@uoc.edu


   Kris Pister
   University of California Berkeley
   490 Cory Hall
   Berkeley, California  94720
   USA

   Email: pister@eecs.berkeley.edu








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