[Docs] [txt|pdf|xml|html] [Tracker] [Email] [Nits]

Versions: 00 draft-ietf-6tisch-minimal

6TiSCH                                                X. Vilajosana, Ed.
Internet-Draft                           Universitat Oberta de Catalunya
Intended status: Informational                                 K. Pister
Expires: April 12, 2014                University of California Berkeley
                                                        October 09, 2013


                      Minimal 6TiSCH Configuration
                   draft-vilajosana-6tisch-minimal-00

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.

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 April 12, 2014.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   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



Vilajosana & Pister      Expires April 12, 2014                 [Page 1]


Internet-Draft               6tisch-minimal                 October 2013


   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   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  . . . . . . . . .   7
     3.1.  Sync IE . . . . . . . . . . . . . . . . . . . . . . . . .   8
       3.1.1.  IE Header . . . . . . . . . . . . . . . . . . . . . .   8
       3.1.2.  IE Content  . . . . . . . . . . . . . . . . . . . . .   8
     3.2.  Frame and Cell IE . . . . . . . . . . . . . . . . . . . .   8
       3.2.1.  IE Header . . . . . . . . . . . . . . . . . . . . . .   9
       3.2.2.  IE Content  . . . . . . . . . . . . . . . . . . . . .   9
   4.  Acknowledgement . . . . . . . . . . . . . . . . . . . . . . .   9
     4.1.  ACK/NACK Time Correction IE . . . . . . . . . . . . . . .   9
       4.1.1.  IE Header . . . . . . . . . . . . . . . . . . . . . .   9
       4.1.2.  IE Content  . . . . . . . . . . . . . . . . . . . . .  10
   5.  Neighbour information . . . . . . . . . . . . . . . . . . . .  10
     5.1.  Neighbour Table . . . . . . . . . . . . . . . . . . . . .  10
     5.2.  Time Source Neighbour Selection . . . . . . . . . . . . .  11
   6.  Queues and Priorities . . . . . . . . . . . . . . . . . . . .  11
   7.  RPL on TSCH . . . . . . . . . . . . . . . . . . . . . . . . .  12
     7.1.  RPL Objective Function Zero . . . . . . . . . . . . . . .  12
       7.1.1.  Rank computation  . . . . . . . . . . . . . . . . . .  12
       7.1.2.  Rank computation Example  . . . . . . . . . . . . . .  13
     7.2.  RPL Configuration . . . . . . . . . . . . . . . . . . . .  15
       7.2.1.  Mode of Operation . . . . . . . . . . . . . . . . . .  15
       7.2.2.  Trickle Timer . . . . . . . . . . . . . . . . . . . .  16
       7.2.3.  Hysteresis  . . . . . . . . . . . . . . . . . . . . .  16
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  16
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  16
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  17
     9.3.  External Informative References . . . . . . . . . . . . .  18
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

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



Vilajosana & Pister      Expires April 12, 2014                 [Page 2]


Internet-Draft               6tisch-minimal                 October 2013


   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
   SHOULD broadcast properly formed Enhanced Beacons to announce these
   values, but during initial implementation and debugging it may be
   convenient to hard-code 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.palattella-6tsch-terminology], is
   an abstraction of the MAC layer that defines a collection of time
   slots of equal length and priority, 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) and data packets.  This document recommends the
   following slotframe configuration.

   Minimal configuration














Vilajosana & Pister      Expires April 12, 2014                 [Page 3]


Internet-Draft               6tisch-minimal                 October 2013


   +------------------------------------+----------------------+
   |           Property                 |       Value          |
   +------------------------------------+----------------------+
   | Number of time slots per Slotframe |        101           |
   +------------------------------------+----------------------+
   | Number of available channels       |         16           |
   +------------------------------------+----------------------+
   | Number of EBs cells                | 1 (slotOffset 0)     |
   +------------------------------------+----------------------+
   | Number of scheduled cells          | 5 (slotOffsets       |
   |                                    | 1,2,3,4,5)           |
   +------------------------------------+----------------------+
   | Number of unscheduled cells        | 95 (from slotOffset  |
   |                                    | 6 to 100)            |
   +------------------------------------+----------------------+
   | Number of MAC retransmissions (max)|         3            |
   +------------------------------------+----------------------+
   | Time Slot duration                 |        15ms          |
   +------------------------------------+----------------------+


   The suggested minimal schedule may be hard-coded in each node.  The
   slotframe is composed of 101 time slots.  The first slot in the
   slotframe is used to send Enhanced Beacons announcing the presence of
   the network.  These EBs are not acknowledged.  Five cells are
   scheduled for exchanging data packets, as described in Section 2.2.
   These cells are scheduled at slotOffset 1 to 5, and channeOffset 0.
   Per the IEEE802.15.4e TSCH, data packets sent on these cells to a
   unicast MAC address are acknowledged by the receiver.  The 95
   remaining cells are unscheduled, but are available to be allocated by
   dynamic scheduling solutions.

   Minimal schedule overview


















Vilajosana & Pister      Expires April 12, 2014                 [Page 4]


Internet-Draft               6tisch-minimal                 October 2013


                +-----+-----+-----+-----+-----+-----+-----+     +-----+
   chan.Off. 0  | EB  |TxRxS|TxRxS|TxRxS|TxRxS|TxRxS| OFF | ... | OFF |
                +-----+-----+-----+-----+-----+-----+-----+     +-----+
   chan.Off. 1  |     |     |     |     |     |     |     | ... |     |
                +-----+-----+-----+-----+-----+-----+-----+     +-----+
                  ...
                +-----+-----+-----+-----+-----+-----+-----+     +-----+
   chan.Off. 15 |     |     |     |     |     |     |     | ... |     |
                +-----+-----+-----+-----+-----+-----+-----+     +-----+
                   0     1     2     3     4     5     6          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 a bitmap of cell
   options assigned, named LinkOption.  All scheduled cells in the
   minimal schedule are configured as Hard cells
   [I-D.watteyne-6tsch-tsch-lln-context][I-D.wang-6tsch-6top].
   Additional available cells can be scheduled by a dynamic scheduling
   solution and can either be configured as hard cells or soft cells
   without any restriction.

   The EB cell is assigned the following bitmap of cell options:

      b0 = Transmit = 1 (set)

      b1 = Receive = 0 (clear)

      b2 = Shared = 0 (clear)

      b3 = Timekeeping = 0 (clear)

      b4 = Hard = 1 (set)

      b5-b7 = Reserved (clear)









Vilajosana & Pister      Expires April 12, 2014                 [Page 5]


Internet-Draft               6tisch-minimal                 October 2013


   The data cells are assigned the bitmap of cell options below that
   results in "Slotted Aloha" behaviour.  Because both the "Transmit"
   and "Receive" bits are set, a node either transmits, if there is a
   packet in its queue, or listens if it has nothing to transmit.
   Because the "shared" bit is set, the back-off mechanism defined in
   [IEEE802154e] is used to resolve contention.

      b0 = Transmit = 1 (set)

      b1 = Receive = 1 (set)

      b2 = Shared = 1 (set)

      b3 = Timekeeping = 0 (clear)

      b4 = Hard = 1 (set)

      b5-b7 = Reserved (clear)

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

2.3.  Retransmissions

   The maximum number of MAC-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
   MAC 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 MAC
   acknowledgement is sent back from the RX to the TX node, indicating
   successful reception.  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 listen 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.




Vilajosana & Pister      Expires April 12, 2014                 [Page 6]


Internet-Draft               6tisch-minimal                 October 2013


   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


   [IEEE802154e] does not define the different durations of a time slot.
   It does allow those durations to be sent in the EBs (through a
   TimeSlot IE).  This document recommends to pre-configure the
   different durations to the values listed below or use EBs to learn
   those values included in the TimeSlot IE.

   Timeslot durations

   +---------------------------------+------------------+
   |  IEEE802.15.4e TSCH parameter   |     Value        |
   +---------------------------------+------------------+
   | TsTxOffset                      |     4000us       |
   +---------------------------------+------------------+
   | TsLongGT                        |     2600us       |
   +---------------------------------+------------------+
   | TsTxAckDelay                    |     4606us       |
   +---------------------------------+------------------+
   | TsShortGT                       |     1000us       |
   +---------------------------------+------------------+
   | Time Slot duration              |    15000us       |
   +---------------------------------+------------------+


3.  Enhanced Beacons Configuration and Content

   [IEEE802154e] does not define how often or which EBs are sent.  The
   choice of the duration between two EBs needs to take into account



Vilajosana & Pister      Expires April 12, 2014                 [Page 7]


Internet-Draft               6tisch-minimal                 October 2013


   whether EBs are used as the only mechanism to synchronize devices, or
   whether a Keep-Alive (KA) mechanism is used in parallel.  For a
   simplest TSCH configuration, a mote SHOULD send an EB every 10s. For
   additional reference see [I-D.watteyne-6tsch-tsch-lln-context] where
   different synchronization approaches are summarized.

   EBs MUST be sent with the Beacon IEEE802.15.4 frame type and this EBs
   MUST carry the following Information Elements (IEs): (The content of
   the IEs is presented here for clarity, however this information is
   redundant with [I-D.watteyne-6tsch-tsch-lln-context] 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)

3.2.  Frame and Cell IE

   Although the schedule may be hard-coded during development, each node
   MUST indicate the schedule in each EB through a Frame and Cell IE.
   This enables nodes which implement [IEEE802154e] fully to configure
   their schedule as they join the network, and interact with nodes
   using a hard-coded schedule.





Vilajosana & Pister      Expires April 12, 2014                 [Page 8]


Internet-Draft               6tisch-minimal                 October 2013


3.2.1.  IE Header

      Length (b0-b7) = variable

      Sub-ID (b8-b14) = 0x1b

      Type (b15) = 0x00 (short)

3.2.2.  IE Content

      # Slotframes (b16-b23) = 0x01

      Slotframe ID (b24-b31) = 0x01

      Size Slotframe (b32-b47) = 0x65 (101)

      # Links (b48-b55) = 0x06

   For each link in the minimal schedule:

      Channel Offset (2B) = 0x00

      Slot Number (2B) = from 0x00 to 0x05

      LinkOption (1B) = as described in Section 2.2

4.  Acknowledgement

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

4.1.  ACK/NACK Time Correction IE

   The ACK/NACK time correction IE is used to carry 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)





Vilajosana & Pister      Expires April 12, 2014                 [Page 9]


Internet-Draft               6tisch-minimal                 October 2013


4.1.2.  IE Content

      Time Synch Info and ACK status (b16-b31)

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

   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.  Neighbour information

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

5.1.  Neighbour Table

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

      Neighbour statistics:

         numTx: number of transmitted packets to that neighbour

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

         numRx: number of received packets from that neighbour

      The EUI64 of the neighbour address.

      Timestamp when that neighbour 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.




Vilajosana & Pister      Expires April 12, 2014                [Page 10]


Internet-Draft               6tisch-minimal                 October 2013


      RPL rank of that neighbour.

      A flag which indicates whether this neighbour is a time source
      neighbour.

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

   In addition of that information, each node has to be able to compute
   some RPL Objective Function (OF) taking into account the neighbour
   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 Neighbour Selection

   Each node MUST select at least one time source neighbour amongst its
   known neighbours in its RPL routing parent set.  When a node joins a
   network, it has no routing information yet.  To select its time
   source neighbour, uses the Join Priority information advertised 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 MUST NOT
   be allowed to send EBs until it has acquired a RPL rank.  The latter
   avoids topology 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 as DAGRank(rank) where rank is the rank
   of the actual node.  In case of a node receives EBs from different
   nodes with equal Join Priority, the time source neighbour selection
   should be assessed by other metrics that can help to determine the
   better connectivity link.  Time source neighbor hysteresis SHOULD be
   addressed 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 neighbor in the RPL routing parent
   set.

   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



Vilajosana & Pister      Expires April 12, 2014                [Page 11]


Internet-Draft               6tisch-minimal                 October 2013


   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

7.1.  RPL Objective Function Zero

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

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, then a rank_factor of 1 is
      used.  For the purpose of this document rank_factor MUST be set to
      1.





Vilajosana & Pister      Expires April 12, 2014                [Page 12]


Internet-Draft               6tisch-minimal                 October 2013


      Sp (step_of_rank): (strictly positive integer) - an intermediate
      computation based on the link properties with a certain neighbour.
      For the purpose of this document 2*ETX (Expected Transmissions) as
      defined by [DeCouto03] and [RFC6551] MUST be used.  The ETX will
      be computed as the inverse of the Packet Delivery Ratio (PDR)
      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
      augmentation 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.
      For the present document 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., 16-bit) Rank
      quantity.  When the Rank is compared, e.g., for determination of
      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





Vilajosana & Pister      Expires April 12, 2014                [Page 13]


Internet-Draft               6tisch-minimal                 October 2013


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

      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

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






























Vilajosana & Pister      Expires April 12, 2014                [Page 14]


Internet-Draft               6tisch-minimal                 October 2013


    +-------+
    |   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 interoperate.

7.2.1.  Mode of Operation

   For downstream route maintenance, in a minimal configuration, RPL
   MUST 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.



Vilajosana & Pister      Expires April 12, 2014                [Page 15]


Internet-Draft               6tisch-minimal                 October 2013


7.2.2.  Trickle Timer

   RPL signalling messages such as DIOs are sent using the Trickle
   Algorithm [RFC6550] (Section 8.3.1) and [RFC6206].  For the purpose
   of this document, 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] the [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.

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.




Vilajosana & Pister      Expires April 12, 2014                [Page 16]


Internet-Draft               6tisch-minimal                 October 2013


   [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.

9.2.  Informative References

   [I-D.watteyne-6tsch-tsch-lln-context]
              Watteyne, T., Palattella, M., and L. Grieco, "Using
              IEEE802.15.4e TSCH in an LLN context: Overview, Problem
              Statement and Goals", draft-watteyne-6tsch-tsch-lln-
              context-02 (work in progress), May 2013.

   [I-D.thubert-6tsch-architecture]
              Thubert, P., Assimiti, R., and T. Watteyne, "An
              Architecture for IPv6 over Timeslotted Channel Hopping",
              draft-thubert-6tsch-architecture-02 (work in progress),
              July 2013.

   [I-D.palattella-6tsch-terminology]
              Palattella, M., Thubert, P., Watteyne, T., and Q. Wang,
              "Terminology in IPv6 over Timeslotted Channel Hopping",
              draft-palattella-6tsch-terminology-01 (work in progress),
              July 2013.

   [I-D.wang-6tsch-6top]
              Wang, Q., Vilajosana, X., and T. Watteyne, "6TSCH
              Operation Sublayer (6top)", draft-wang-6tsch-6top-00 (work
              in progress), July 2013.

   [I-D.ohba-6tsch-security]
              Chasko, S., Das, S., Lopez, R., Ohba, Y., Thubert, P., and
              A. Yegin, "Security Framework and Key Management Protocol
              Requirements for 6TSCH", draft-ohba-6tsch-security-01
              (work in progress), July 2013.

   [I-D.ietf-roll-terminology]
              Vasseur, J., "Terms used in Ruting 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.



Vilajosana & Pister      Expires April 12, 2014                [Page 17]


Internet-Draft               6tisch-minimal                 October 2013


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.

   [DeCouto03]
              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








Vilajosana & Pister      Expires April 12, 2014                [Page 18]


Html markup produced by rfcmarkup 1.121, available from https://tools.ietf.org/tools/rfcmarkup/