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6TiSCH                                                     T. Chang, Ed.
Internet-Draft                                                M. Vucinic
Intended status: Standards Track                                   Inria
Expires: January 9, 2020                                   X. Vilajosana
                                         Universitat Oberta de Catalunya
                                                            S. Duquennoy
                                                               RISE SICS
                                                              D. Dujovne
                                              Universidad Diego Portales
                                                            July 8, 2019


                6TiSCH Minimal Scheduling Function (MSF)
                        draft-ietf-6tisch-msf-05

Abstract

   This specification defines the 6TiSCH Minimal Scheduling Function
   (MSF).  This Scheduling Function describes both the behavior of a
   node when joining the network, and how the communication schedule is
   managed in a distributed fashion.  MSF builds upon the 6TiSCH
   Operation Sublayer Protocol (6P) and the Minimal Security Framework
   for 6TiSCH.

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
   [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 https://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 January 9, 2020.




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Copyright Notice

   Copyright (c) 2019 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
   (https://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.  Interface to the Minimal 6TiSCH Configuration . . . . . . . .   4
   3.  Autonomous Cells  . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Node Behavior at Boot . . . . . . . . . . . . . . . . . . . .   6
     4.1.  Start State . . . . . . . . . . . . . . . . . . . . . . .   6
     4.2.  Step 1 - Choosing Frequency . . . . . . . . . . . . . . .   6
     4.3.  Step 2 - Receiving EBs  . . . . . . . . . . . . . . . . .   6
     4.4.  Step 3 - Setting up Autonomous Cells for the Join Process   7
     4.5.  Step 4 - Acquiring a RPL Rank . . . . . . . . . . . . . .   7
     4.6.  Step 5 - Setting up first Tx and Rx negotiated Cells  . .   7
     4.7.  Step 6 - Send EBs and DIOs  . . . . . . . . . . . . . . .   8
     4.8.  End State . . . . . . . . . . . . . . . . . . . . . . . .   8
   5.  Rules for Adding/Deleting Cells . . . . . . . . . . . . . . .   8
     5.1.  Adapting to Traffic . . . . . . . . . . . . . . . . . . .   9
     5.2.  Switching Parent  . . . . . . . . . . . . . . . . . . . .  10
     5.3.  Handling Schedule Collisions  . . . . . . . . . . . . . .  11
   6.  6P SIGNAL command . . . . . . . . . . . . . . . . . . . . . .  12
   7.  Scheduling Function Identifier  . . . . . . . . . . . . . . .  12
   8.  Rules for CellList  . . . . . . . . . . . . . . . . . . . . .  12
   9.  6P Timeout Value  . . . . . . . . . . . . . . . . . . . . . .  13
   10. Rule for Ordering Cells . . . . . . . . . . . . . . . . . . .  13
   11. Meaning of the Metadata Field . . . . . . . . . . . . . . . .  13
   12. 6P Error Handling . . . . . . . . . . . . . . . . . . . . . .  13
   13. Schedule Inconsistency Handling . . . . . . . . . . . . . . .  14
   14. MSF Constants . . . . . . . . . . . . . . . . . . . . . . . .  14
   15. MSF Statistics  . . . . . . . . . . . . . . . . . . . . . . .  15
   16. Security Considerations . . . . . . . . . . . . . . . . . . .  15
   17. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
     17.1.  MSF Scheduling Function Identifiers  . . . . . . . . . .  16
   18. References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     18.1.  Normative References . . . . . . . . . . . . . . . . . .  16



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     18.2.  Informative References . . . . . . . . . . . . . . . . .  17
   Appendix A.  Contributors . . . . . . . . . . . . . . . . . . . .  17
   Appendix B.  Example of Implementation of SAX hash function . . .  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

1.  Introduction

   The 6TiSCH Minimal Scheduling Function (MSF), defined in this
   specification, is a 6TiSCH Scheduling Function (SF).  The role of an
   SF is entirely defined in [RFC8480].  This specification complements
   [RFC8480] by providing the rules of when to add/delete cells in the
   communication schedule.  This specification satisfies all the
   requirements for an SF listed in Section 4.2 of [RFC8480].

   MSF builds on top of the following specifications: the Minimal IPv6
   over the TSCH Mode of IEEE 802.15.4e (6TiSCH) Configuration
   [RFC8180], the 6TiSCH Operation Sublayer Protocol (6P) [RFC8480], and
   the Minimal Security Framework for 6TiSCH
   [I-D.ietf-6tisch-minimal-security].

   MSF defines both the behavior of a node when joining the network, and
   how the communication schedule is managed in a distributed fashion.
   When a node running MSF boots up, it joins the network by following
   the 7 steps described in Section 4.  The end state of the join
   process is that the node is synchronized to the network, has mutually
   authenticated to the network, has identified a preferred routing
   parent, and has scheduled one default negotiated cell (defined in
   Section 5.1) to/from its preferred routing parent.  After the join
   process, the node can continuously add/delete/relocate cells, as
   described in Section 5.  It does so for 3 reasons: to match the link-
   layer resources to the traffic, to handle changing parent, to handle
   a schedule collision.

   MSF is designed to operate in a wide range of application domains.
   It is optimized for applications with regular upstream traffic (from
   the nodes to the root).

   This specification follows the recommended structure of an SF
   specification, given in Appendix A of [RFC8480], with the following
   adaptations:

   o  We have reordered some sections, in particular to have the section
      on the node behavior at boot (Section 4) appear early in this
      specification.
   o  We added sections on the interface to the minimal 6TiSCH
      configuration (Section 2), the use of the SIGNAL command
      (Section 6), the MSF constants (Section 14), the MSF statistics
      (Section 15).



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   o  This specification does not include an examples section.

2.  Interface to the Minimal 6TiSCH Configuration

   A node implementing MSF SHOULD implement the Minimal 6TiSCH
   Configuration [RFC8180], which defines the "minimal cell", a single
   shared cell providing minimal connectivity between the nodes in the
   network.  The MSF implementation provided in this specification is
   based on the implementation of the Minimal 6TiSCH Configuration.
   However, an implementor MAY implement MSF without implementing
   Minimal 6TiSCH Configuration.

   MSF uses the minimal cell to exchange the following packets:

   1.  Enhanced Beacons (EBs), defined by [IEEE802154-2015].  These are
       broadcast frames.
   2.  Broadcast DODAG Information Objects (DIOs), defined by [RFC6550].
       Unicast DIOs SHOULD NOT be sent on minimal cell.

   To ensure there is enough bandwidth available on the minimal cell, a
   node implementing MSF SHOULD enforce some rules for limiting the
   traffic of broadcast frames.  For example, a Trickle Timer defined in
   [RFC6550] MAY be applied on DIOs.  However, this behavior is
   implementation-specific which is out of the scope of MSF.

   MSF RECOMMENDS the use of 3 slotframes.  MSF schedules autonomous
   cells at Slotframe 1 (Section 3) and 6P negotiated cells at Slotframe
   2 (Section 5) , while Slotframe 0 is used for the bootstrap traffic
   as defined in the Minimal 6TiSCH Configuration.  It is RECOMMENDED to
   use the same slotframe length for Slotframe 0, 1 and 2.  Thus it is
   possible to avoid the scheduling collision between the autonomous
   cells and 6P negotiated cells (Section 3).  The default slotframe
   length (SLOTFRAME_LENGTH) is RECOMMENDED for Slotframe 0, 1 and 2,
   although any value can be advertised in the EBs.

3.  Autonomous Cells

   MSF nodes initialize Slotframe 1 with a set of default cells for
   unicast communication with their neighbors.  These cells are called
   'autonomous cells', because they are maintained autonomously by each
   node without negotiation through 6P.  Cells scheduled by 6P
   transaction are called 'negotiated cells' which are reserved on
   Slotframe 2.  How to schedule negotiated cells is detailed in
   Section 5.  There are two types of autonomous cells:

   o  Autonomous Rx Cell (AutoRxCell), one cell at a
      [slotOffset,channelOffset] computed as a hash of the EUI64 of the




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      node itself (detailed next).  Its cell options bits are assigned
      as TX=0, RX=1, SHARED=0.
   o  Autonomous Tx Cell (AutoTxCell), one cell at a
      [slotOffset,channelOffset] computed as a hash of the layer 2 EUI64
      destination address in the frame to be transmitted (detailed in
      Section 4.4).  Its cell options bits are assigned as TX=1, RX=0,
      SHARED=1.

   To compute a [slotOffset,channelOffset] from an EUI64 address, nodes
   MUST use the hash function SAX [SAX-DASFAA].  The coordinates are
   computed to distribute the cells across all channel offsets, and all
   but the first time offsets of Slotframe 1.  The first time offset is
   skipped to avoid colliding with the minimal cell in Slotframe 0.  The
   slot coordinates derived from a given EUI64 address are computed as
   follows:

   o  slotOffset(MAC) = 1 + hash(EUI64, length(Slotframe_1) - 1)
   o  channelOffset(MAC) = hash(EUI64, NUM_CH_OFFSET)

   The second input parameter defines the maximum return value of the
   hash function.  Other optional parameters defined in SAX determine
   the performance of SAX hash function.  Those parameters could be
   broadcasted in EB frame or pre-configured.  For interoperability
   purposes, an example how the hash function is implemented is detailed
   in Appendix B.

   AutoTxCell is not permanent in the schedule but added/deleted on
   demand when there is a frame to sent.  Throughout the network
   lifetime, nodes maintain the autonomous cells as follows:

   o  Add an AutoTxCell to the layer 2 destination address which is
      indicated in a frame when:

      *  there is no 6P negotiated Tx cell in schedule for that frame to
         transmit, and
      *  the frame is used for protocol management purposes , such as
         Join Request/Response, 6P Request/Response and any link-local
         communication for RPL routing control.
   o  Remove an AutoTxCell when:

      *  there is no frame for protocol management purposes to transmit,
         or
      *  there is at least one 6P negotiated Tx cell in the schedule to
         transmit a management purpose frame.
   o  The AutoRxCell MUST always remain scheduled after synchronized.
   o  6P CLEAR MUST NOT erase any autonomous cells.





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   Because of hash collisions, there will be cases when the AutoTxCell
   and AutoRxCell are scheduled at the same slot offset and/or channel
   offset.  In such cases, AutoTxCell always take precedence over
   AutoRxCell.  In case of conflicting with a negotiated cell,
   autonomous cells take precedence over negotiated cell, which is
   stated in [IEEE802154-2015].  However, when the Slotframe 0, 1 and 2
   use the same length value, it is possible for negotiated cell to
   avoid the collision with AutoRxCell.

4.  Node Behavior at Boot

   This section details the behavior the node SHOULD follow from the
   moment it is switched on, until it has successfully joined the
   network.  Section 4.1 details the start state; Section 4.8 details
   the end state.  The other sections detail the 6 steps of the joining
   process.  We use the term "pledge" and "joined node", as defined in
   [I-D.ietf-6tisch-minimal-security].

4.1.  Start State

   A node implementing MSF SHOULD implement the Minimal Security
   Framework for 6TiSCH [I-D.ietf-6tisch-minimal-security].  As a
   corollary, this means that a pledge, before being switched on, may be
   pre-configured with the Pre-Shared Key (PSK) for joining, as well as
   any other configuration detailed in
   ([I-D.ietf-6tisch-minimal-security]).  This is not needed if the node
   implements a security solution not based on PSKs, such as
   ([I-D.ietf-6tisch-dtsecurity-zerotouch-join]).

4.2.  Step 1 - Choosing Frequency

   When switched on, the pledge SHOULD randomly choose a frequency among
   the available frequencies, and start listening for EBs on that
   frequency.

4.3.  Step 2 - Receiving EBs

   Upon receiving the first EB, the pledge SHOULD continue listening for
   additional EBs to learn:

   1.  the number of neighbors N in its vicinity
   2.  which neighbor to choose as a Join Proxy (JP) for the joining
       process

   While the exact behavior is implementation-specific, the RECOMMENDED
   behavior is to follow [RFC8180], and listen until EBs sent by
   NUM_NEIGHBOURS_TO_WAIT nodes (defined in [RFC8180]) have been
   received.



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   During this step, the pledge SHOULD NOT synchronize until it received
   enough EB from the network it wishes to join.  How to decide whether
   an EB originates from a node from the network it wishes to join is
   implementation-specific, but MAY involve filtering EBs by the PAN ID
   field it contains, the presence and contents of the IE defined in
   [I-D.richardson-6tisch-join-enhanced-beacon], or the key used to
   authenticate it.

   The decision of which neighbor to use as a JP is implementation-
   specific, and discussed in [I-D.ietf-6tisch-minimal-security].

4.4.  Step 3 - Setting up Autonomous Cells for the Join Process

   After selected a JP, a node generates a Join Request and installs an
   AutoTxCell to the JP.  A Join Request is then sent by the pledge to
   its JP over the AutoTxCell.  The AutoTxCell is removed by the pledge
   when the Join Request is sent out.  The JP receives the Join Request
   through its AutoRxCell.  Then it forwards the Join Request to the
   JRC, possibly over multiple hops, over the 6P negotiated Tx cell.
   Similarly, the JRC sends the Join Response to the JP, possibly over
   multiple hops, over the 6P negotiated Tx cell.  When JP received the
   Join Response from the JRC, it installs an AutoTxCell to the pledge
   and sends that Join Response to the pledge over AutoTxCell.  The
   AutoTxCell is removed by the JP when the Join Response is sent out.
   The pledge receives the Join Response from its AutoRxCell, thereby
   learns the keying material used in the network, as well as other
   configurations, and becomes a "joined node".

   When 6LoWPAN Neighbor Dicovery ([RFC8505]) (ND) is implemented, the
   unicast packets used by ND are sent on the AutoTxCell.  The specific
   process how the ND works during the Join process is detailed in
   [I-D.ietf-6tisch-architecture].

4.5.  Step 4 - Acquiring a RPL Rank

   Per [RFC6550], the joined node receives DIOs, computes its own Rank,
   and selects a preferred parent.

4.6.  Step 5 - Setting up first Tx and Rx negotiated Cells

   After selected a preferred parent, the joined node MUST generate a 6P
   ADD Request and install an AutoTxCell to that parent.  The 6P ADD
   Request is sent out through the AutoTxCell with the following fields:

   o  CellOptions: set to TX=1,RX=0,SHARED=0
   o  NumCells: set to 1
   o  CellList: at least 5 cells, chosen according to Section Section 8




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   The joined node removes the AutoTxCell to parent when the 6P command
   is send out.  Its parent receives the 6P ADD Request from its
   AutoRxCell.  Then it generates a 6P ADD Response and installs an
   AutoTxCell to the joined node.  When the parent sends out the 6P ADD
   Response, it MUST remove that AutoTxCell.  The joined node receives
   the 6P ADD Response from its AutoRxCell and completes the 6P
   transcation.  In case the 6P ADD transaction failed, the node MUST
   issue another 6P ADD command and repeat until the Tx cell is
   installed to the parent.

   After the first negotiated Tx Cell is installed, the joined node
   SHOULD send another 6P ADD Request with the following fields:

   o  CellOptions: set to TX=0,RX=1,SHARED=0
   o  NumCells: set to 1
   o  CellList: at least 5 cells, chosen according to Section Section 8

   The process to install a negotiated Rx cell is the similar with the
   process to install a negotiated Tx cell.  The only difference is that
   the 6P ADD Request is sent on the negotiated Tx cell installed
   previously, instead of the AutoTxCell.

4.7.  Step 6 - Send EBs and DIOs

   The node SHOULD start sending EBs and DIOs on the minimal cell, while
   following the transmit rules for broadcast frames from Section 2.

4.8.  End State

   For a new node, the end state of the joining process is:

   o  it is synchronized to the network
   o  it is using the link-layer keying material it learned through the
      secure joining process
   o  it has identified its preferred routing parent
   o  it has one one AutRxCell
   o  it has one negotiated Tx cell and one negotiated Rx cell to its
      parent
   o  it starts to send DIOs, potentially serving as a router for other
      nodes' traffic
   o  it starts to send EBs, potentially serving as a JP for new pledge

5.  Rules for Adding/Deleting Cells

   Once a node has joined the 6TiSCH network, it adds/deletes/relocates
   cells with its preferred parent for three reasons:





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   o  to match the link-layer resources to the traffic between the node
      and its preferred parent (Section 5.1)
   o  to handle switching preferred parent or(Section 5.2)
   o  to handle a schedule collision (Section 5.3)

   Those cells are called 'negotiated cells' as they are scheduled
   through 6P, negotiated with their parents.  Without specific
   declaring, all cells mentioned in this section are negotiated cells
   and they are installed at Slotframe 2.

5.1.  Adapting to Traffic

   A node implementing MSF MUST implement the behavior described in this
   section.

   The goal of MSF is to manage the communication schedule in the 6TiSCH
   schedule in a distributed manner.  For a node, this translates into
   monitoring the current usage of the cells it has to its preferred
   parent:

   o  If the node determines that the number of link-layer frames it is
      attempting to exchange with its preferred parent per unit of time
      is larger than the capacity offered by the TSCH negotiated cells
      it has scheduled with it, the node issues a 6P ADD command to its
      preferred parent to add cells to the TSCH schedule.
   o  If the traffic is lower than the capacity, the node issues a 6P
      DELETE command to its preferred parent to delete cells from the
      TSCH schedule.

   The node MUST maintain the following counters for its preferred
   parent:

   NumCellsElapsed :  Counts the number of negotiated cells that have
       elapsed since the counter was initialized.  This counter is
       initialized at 0.  Each time the TSCH state machine indicates
       that the current cell is a negotiated cell to the preferred
       parent, NumCellsElapsed is incremented by exactly 1, regardless
       of whether the cell is used to transmit/receive a frame.
   NumCellsUsed:  Counts the number of negotiated cells that have been
       used.  This counter is initialized at 0.  NumCellsUsed is
       incremented by exactly 1 when, during a negotiated cell to the
       preferred parent, either of the following happens:

       *  The node sends a frame to its preferred parent.  The counter
          increments regardless of whether a link-layer acknowledgment
          was received or not.





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       *  The node receives a frame from its preferred parent.  The
          counter increments regardless of whether the frame is a valid
          IEEE802.15.4 frame or not.

   The cell option of the cell listed CellList in 6P Request SHOULD be
   either Tx=1 only or Rx=1 only.  Both NumCellsElapsed and NumCellsUsed
   counters can be used to both type of negotiated cells.

   Implementors MAY choose to create the same counters for each
   neighbor, and add them as additional statistics in the neighbor
   table.

   The counters are used as follows:

   1.  Both NumCellsElapsed and NumCellsUsed are initialized to 0 when
       the node boots.
   2.  When the value of NumCellsElapsed reaches MAX_NUMCELLS:

       *  If NumCellsUsed > LIM_NUMCELLSUSED_HIGH, trigger 6P to add a
          single cell to the preferred parent
       *  If NumCellsUsed < LIM_NUMCELLSUSED_LOW, trigger 6P to remove a
          single cell to the preferred parent
       *  Reset both NumCellsElapsed and NumCellsUsed to 0 and go to
          step 2.

   The value of MAX_NUMCELLS is chosen according to the traffic type of
   the network.  Generally speaking, the larger the value MAX_NUMCELLS
   is, the more accurate the cell usage is calculated.  The 6P traffic
   overhead using a larger value of MAX_NUMCELLS could be reduced as
   well.  Meanwhile, the latency won't increaase much by using a larger
   value of MAX_NUMCELLS for periodic traffic type.  For burst traffic
   type, larger value of MAX_NUMCELLS indeed introduces higher latency.
   The latency caused by slight changes of traffic load can be absolved
   by the additional scheduled cells.  In this sense, MSF is a
   scheduling function trading latency with energy by scheduling more
   cells than needed.  It is recommended to set MAX_NUMCELLS value at
   least 4 times than the maximum link traffic load of the network in
   packets per slotframe.  For example, a 2 packets/slotframe traffic
   load results an average 4 cells scheduled, using the value of double
   number of scheduled cells (which is 8) as MAX_NUMCELLS gives a good
   resolution on cell usage calculation.

5.2.  Switching Parent

   A node implementing MSF SHOULD implement the behavior described in
   this section.





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   Part of its normal operation, the RPL routing protocol can have a
   node switch preferred parent.  The procedure for switching from the
   old preferred parent to the new preferred parent is:

   1.  the node counts the number of negotiated cells it has per
       slotframe to the old preferred parent
   2.  the node triggers one or more 6P ADD commands to schedule the
       same number of negotiated cells to the new preferred parent
   3.  when that successfully completes, the node issues a 6P CLEAR
       command to its old preferred parent

5.3.  Handling Schedule Collisions

   A node implementing MSF SHOULD implement the behavior described in
   this section.  The "MUST" statements in this section hence only apply
   if the node implements schedule collision handling.

   Since scheduling is entirely distributed, there is a non-zero
   probability that two pairs of nearby neighbor nodes schedule a
   negotiated cell at the same [slotOffset,channelOffset] location in
   the TSCH schedule.  In that case, data exchanged by the two pairs may
   collide on that cell.  We call this case a "schedule collision".

   The node MUST maintain the following counters for each managed
   unicast cell to its preferred parent:

   NumTx:  Counts the number of transmission attempts on that cell.
       Each time the node attempts to transmit a frame on that cell,
       NumTx is incremented by exactly 1.
   NumTxAck:  Counts the number of successful transmission attempts on
       that cell.  Each time the node receives an acknowledgment for a
       transmission attempt, NumTxAck is incremented by exactly 1.

   Implementors MAY choose to maintain the same counters for each
   negotiated cell in the schedule.

   Since both NumTx and NumTxAck are initialized to 0, we necessarily
   have NumTxAck <= NumTx.  We call Packet Delivery Ratio (PDR) the
   ratio NumTxAck/NumTx; and represent it as a percentage.  A cell with
   PDR=50% means that half of the frames transmitted are not
   acknowledged (and need to be retransmitted).

   Each time the node switches preferred parent (or during the join
   process when the node selects a preferred parent for the first time),
   both NumTx and NumTxAck MUST be reset to 0.  They increment over
   time, as the schedule is executed and the node sends frames to its
   preferred parent.  When NumTx reaches MAX_NUMTX, both NumTx and
   NumTxAck MUST be divided by 2.  That is, for example, from NumTx=256



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   and NumTxAck=128, they become NumTx=128 and NumTxAck=64.  This
   operation does not change the value of the PDR, but allows the
   counters to keep incrementing.  The value of MAX_NUMTX is
   implementation-specific.

   The key for detecting a schedule collision is that, if a node has
   several cells to the same preferred parent, all cells should exhibit
   the same PDR.  A cell which exhibits a PDR significantly lower than
   the others indicates than there are collisions on that cell.

   Every HOUSEKEEPINGCOLLISION_PERIOD, the node executes the following
   steps:

   1.  It computes, for each managed unicast cell with its preferred
       parent (not for the autonomous cell), that cell's PDR.
   2.  Any cell that hasn't yet had NumTx divided by 2 since it was last
       reset is skipped in steps 3 and 4.  This avoids triggering cell
       relocation when the values of NumTx and NumTxAck are not
       statistically significant yet.
   3.  It identifies the cell with the highest PDR.
   4.  For any other cell, it compares its PDR against that of the cell
       with the highest PDR.  If the difference is larger than
       RELOCATE_PDRTHRES, it triggers the relocation of that cell using
       a 6P RELOCATE command.

6.  6P SIGNAL command

   The 6P SIGNAL command is not used by MSF.

7.  Scheduling Function Identifier

   The Scheduling Function Identifier (SFID) of MSF is
   IANA_6TISCH_SFID_MSF.

8.  Rules for CellList

   MSF uses 2-step 6P Transactions exclusively.  6P Transactions are
   only initiated by a node towards its preferred parent.  As a result,
   the cells to put in the CellList of a 6P ADD command, and in the
   candidate CellList of a RELOCATE command, are chosen by the node
   initiating the 6P Transaction.  In both cases, the same rules apply:

   o  The CellList is RECOMMENDED to have 5 or more cells.
   o  Each cell in the CellList MUST have a different slotOffset value.
   o  For each cell in the CellList, the node MUST NOT have any
      scheduled cell on the same slotOffset.
   o  The slotOffset value of any cell in the CellList MUST NOT be the
      same as the slotOffset of the minimal cell (slotOffset=0).



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   o  The slotOffset of a cell in the CellList SHOULD be randomly and
      uniformly chosen among all the slotOffset values that satisfy the
      restrictions above.
   o  The channelOffset of a cell in the CellList SHOULD be randomly and
      uniformly chosen in [0..numFrequencies], where numFrequencies
      represents the number of frequencies a node can communicate on.

9.  6P Timeout Value

   It is calculated for the worst case that a 6P response is received,
   which means the 6P response is sent out successfully at the very
   latest retransmission.  And for each retransmission, it backs-off
   with largest value.  Hence the 6P timeout value is calculated as
   ((2^MAXBE)-1)*MAXRETRIES*SLOTFRAME_LENGTH, where:

   o  MAXBE is the maximum backoff exponent used
   o  MAXRETRIES is the maximum retransmission times
   o  SLOTFRAME_LENGTH represents the length of slotframe

10.  Rule for Ordering Cells

   Cells are ordered slotOffset first, channelOffset second.

   The following sequence is correctly ordered (each element represents
   the [slottOffset,channelOffset] of a cell in the schedule):

   [1,3],[1,4],[2,0],[5,3],[6,0],[6,3],[7,9]

11.  Meaning of the Metadata Field

   The Metadata field is not used by MSF.

12.  6P Error Handling

   Section 6.2.4 of [RFC8480] lists the 6P Return Codes.  Figure 1 lists
   the same error codes, and the behavior a node implementing MSF SHOULD
   follow.














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          +-----------------+----------------------+
          | Code            | RECOMMENDED behavior |
          +-----------------+----------------------+
          | RC_SUCCESS      | nothing              |
          | RC_EOL          | nothing              |
          | RC_ERR          | quarantine           |
          | RC_RESET        | quarantine           |
          | RC_ERR_VERSION  | quarantine           |
          | RC_ERR_SFID     | quarantine           |
          | RC_ERR_SEQNUM   | clear                |
          | RC_ERR_CELLLIST | clear                |
          | RC_ERR_BUSY     | waitretry            |
          | RC_ERR_LOCKED   | waitretry            |
          +-----------------+----------------------+

          Figure 1: Recommended behavior for each 6P Error Code.

   The meaning of each behavior from Figure 1 is:

   nothing:  Indicates that this Return Code is not an error.  No error
       handling behavior is triggered.
   clear:  Abort the 6P Transaction.  Issue a 6P CLEAR command to that
       neighbor (this command may fail at the link layer).  Remove all
       cells scheduled with that neighbor from the local schedule.  Keep
       that node in the neighbor and routing tables.
   quarantine:  Same behavior as for "clear".  In addition, remove the
       node from the neighbor and routing tables.  Place the node's
       identifier in a quarantine list for QUARANTINE_DURATION.  When in
       quarantine, drop all frames received from that node.
   waitretry:  Abort the 6P Transaction.  Wait for a duration randomly
       and uniformly chosen in [WAITDURATION_MIN,WAITDURATION_MAX].
       Retry the same transaction.

13.  Schedule Inconsistency Handling

   The behavior when schedule inconsistency is detected is explained in
   Figure 1, for 6P Return Code RC_ERR_SEQNUM.

14.  MSF Constants

   Figure 2 lists MSF Constants and their RECOMMENDED values.










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           +------------------------------+-------------------+
           | Name                         | RECOMMENDED value |
           +------------------------------+-------------------+
           | NUM_CH_OFFSET                |       16          |
           | LIM_NUMCELLSUSED_HIGH        |       75 %        |
           | LIM_NUMCELLSUSED_LOW         |       25 %        |
           | HOUSEKEEPINGCOLLISION_PERIOD |        1 min      |
           | RELOCATE_PDRTHRES            |       50 %        |
           | SLOTFRAME_LENGTH             |      101 slots    |
           | QUARANTINE_DURATION          |        5 min      |
           | WAITDURATION_MIN             |       30 s        |
           | WAITDURATION_MAX             |       60 s        |
           +------------------------------+-------------------+

           Figure 2: MSF Constants and their RECOMMENDED values.

15.  MSF Statistics

   Figure 3 lists MSF Statistics and their RECOMMENDED width.

                   +-----------------+-------------------+
                   | Name            | RECOMMENDED width |
                   +-----------------+-------------------+
                   | NumCellsElapsed |      1 byte       |
                   | NumCellsUsed    |      1 byte       |
                   | NumTx           |      1 byte       |
                   | NumTxAck        |      1 byte       |
                   +-----------------+-------------------+

           Figure 3: MSF Statistics and their RECOMMENDED width.

16.  Security Considerations

   MSF defines a series of "rules" for the node to follow.  It triggers
   several actions, that are carried out by the protocols defined in the
   following specifications: the Minimal IPv6 over the TSCH Mode of IEEE
   802.15.4e (6TiSCH) Configuration [RFC8180], the 6TiSCH Operation
   Sublayer Protocol (6P) [RFC8480], and the Minimal Security Framework
   for 6TiSCH [I-D.ietf-6tisch-minimal-security].  In particular, MSF
   does not define a new protocol or packet format.

   MSF relies entirely on the security mechanisms defined in the
   specifications listed above.








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17.  IANA Considerations

17.1.  MSF Scheduling Function Identifiers

   This document adds the following number to the "6P Scheduling
   Function Identifiers" sub-registry, part of the "IPv6 over the TSCH
   mode of IEEE 802.15.4e (6TiSCH) parameters" registry, as defined by
   [RFC8480]:

   +----------------------+-----------------------------+-------------+
   |  SFID                | Name                        | Reference   |
   +----------------------+-----------------------------+-------------+
   | IANA_6TISCH_SFID_MSF | Minimal Scheduling Function | RFC_THIS    |
   |                      | (MSF)                       |             |
   +----------------------+-----------------------------+-------------+

                      Figure 4: IETF IE Subtype '6P'.

18.  References

18.1.  Normative 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-24 (work
              in progress), July 2019.

   [I-D.ietf-6tisch-dtsecurity-zerotouch-join]
              Richardson, M., "6tisch Zero-Touch Secure Join protocol",
              draft-ietf-6tisch-dtsecurity-zerotouch-join-03 (work in
              progress), October 2018.

   [I-D.ietf-6tisch-minimal-security]
              Vucinic, M., Simon, J., Pister, K., and M. Richardson,
              "Minimal Security Framework for 6TiSCH", draft-ietf-
              6tisch-minimal-security-11 (work in progress), June 2019.

   [I-D.richardson-6tisch-join-enhanced-beacon]
              Dujovne, D. and M. Richardson, "IEEE802.15.4 Informational
              Element encapsulation of 6tisch Join Information", draft-
              richardson-6tisch-join-enhanced-beacon-03 (work in
              progress), January 2018.

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




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   [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>.

   [RFC8180]  Vilajosana, X., Ed., Pister, K., and T. Watteyne, "Minimal
              IPv6 over the TSCH Mode of IEEE 802.15.4e (6TiSCH)
              Configuration", BCP 210, RFC 8180, DOI 10.17487/RFC8180,
              May 2017, <https://www.rfc-editor.org/info/rfc8180>.

   [RFC8480]  Wang, Q., Ed., Vilajosana, X., and T. Watteyne, "6TiSCH
              Operation Sublayer (6top) Protocol (6P)", RFC 8480,
              DOI 10.17487/RFC8480, November 2018,
              <https://www.rfc-editor.org/info/rfc8480>.

   [RFC8505]  Thubert, P., Ed., Nordmark, E., Chakrabarti, S., and C.
              Perkins, "Registration Extensions for IPv6 over Low-Power
              Wireless Personal Area Network (6LoWPAN) Neighbor
              Discovery", RFC 8505, DOI 10.17487/RFC8505, November 2018,
              <https://www.rfc-editor.org/info/rfc8505>.

18.2.  Informative References

   [SAX-DASFAA]
              Ramakrishna, M. and J. Zobel, "Performance in Practice of
              String Hashing Functions", DASFAA , 1997.

Appendix A.  Contributors

   Beshr Al Nahas (Chalmers University, beshr@chalmers.se) Olaf
   Landsiedel (Chalmers University, olafl@chalmers.se) Yasuyuki Tanaka
   (Inria-Paris, yasuyuki.tanaka@inria.fr)

Appendix B.  Example of Implementation of SAX hash function

   For the consideration of interoperability, this section provides an
   example of implemention SAX hash function [SAX-DASFAA].  The input
   parameters of the function are:

   o  T, which is the hashing table length
   o  c, which is the characters of string s, to be hashed



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   In MSF, the T is replaced by the length slotframe 1.  String s is
   replaced by the mote EUI64 address.  The characters of the string c0,
   c1, ..., c7 are the 8 bytes of EUI64 address.

   The SAX hash function requires shift operation which is defined as
   follow:

   o  L_shift(v,b), which refers to left shift variable v by b bits
   o  R_shift(v,b), which refers to right shift variable v by b bits

   The steps to calculate the hash value of SAX hash function are:

   1.  initialize variable h to h0 and variable i to 0, where h is the
       intermediate hash value and i is the index of the bytes of EUI64
       address
   2.  sum the value of L_shift(h,l_bit), R_shift(h,r_bit) and ci
   3.  calculate the result of exclusive or between the sum value in
       Step 2 and h
   4.  modulo the result of Step 3 by T
   5.  assign the result of Step 4 to h
   6.  increase i by 1
   7.  repeat Step2 to Step 6 until i reaches to 8
   8.  assign the result of Step 5 to h

   The value of variable h the hash value of SAX hash function.

   For interoperability purposes, the values of h0, l_bit and r_bit in
   Step 1 and 2 are configured as:

   o  h0 = 0
   o  l_bit = 0
   o  r_bit = 1

   The appropriate values of l_bit and r_bit could vary depending on the
   the set of motes' EUI64 address.  How to find those values is out of
   the scope of this specification.

Authors' Addresses

   Tengfei Chang (editor)
   Inria
   2 rue Simone Iff
   Paris  75012
   France

   Email: tengfei.chang@inria.fr





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   Malisa Vucinic
   Inria
   2 rue Simone Iff
   Paris  75012
   France

   Email: malisa.vucinic@inria.fr


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

   Email: xvilajosana@uoc.edu


   Simon Duquennoy
   RISE SICS
   Isafjordsgatan 22
   164 29 Kista
   Sweden

   Email: simon.duquennoy@ri.se


   Diego Dujovne
   Universidad Diego Portales
   Escuela de Informatica y Telecomunicaciones
   Av. Ejercito 441
   Santiago, Region Metropolitana
   Chile

   Phone: +56 (2) 676-8121
   Email: diego.dujovne@mail.udp.cl















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