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Versions: (draft-chang-6tisch-msf) 00 01 02 03 04 05 06

6TiSCH                                                     T. Chang, Ed.
Internet-Draft                                                M. Vucinic
Intended status: Standards Track                                   Inria
Expires: October 10, 2019                                  X. Vilajosana
                                         Universitat Oberta de Catalunya
                                                            S. Duquennoy
                                                               RISE SICS
                                                         D. Dujovne, Ed.
                                              Universidad Diego Portales
                                                           April 8, 2019


                6TiSCH Minimal Scheduling Function (MSF)
                        draft-ietf-6tisch-msf-03

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 October 10, 2019.




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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 Autonomous Cells for 6P . . . . . . .   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 . . . . . . . . . . . . . . . . . . .   8
     5.2.  Switching Parent  . . . . . . . . . . . . . . . . . . . .  10
     5.3.  Handling Schedule Collisions  . . . . . . . . . . . . . .  10
   6.  6P SIGNAL command . . . . . . . . . . . . . . . . . . . . . .  11
   7.  Scheduling Function Identifier  . . . . . . . . . . . . . . .  11
   8.  Rules for CellList  . . . . . . . . . . . . . . . . . . . . .  12
   9.  6P Timeout Value  . . . . . . . . . . . . . . . . . . . . . .  12
   10. Rule for Ordering Cells . . . . . . . . . . . . . . . . . . .  12
   11. Meaning of the Metadata Field . . . . . . . . . . . . . . . .  12
   12. 6P Error Handling . . . . . . . . . . . . . . . . . . . . . .  13
   13. Schedule Inconsistency Handling . . . . . . . . . . . . . . .  13
   14. MSF Constants . . . . . . . . . . . . . . . . . . . . . . . .  14
   15. MSF Statistics  . . . . . . . . . . . . . . . . . . . . . . .  14
   16. Security Considerations . . . . . . . . . . . . . . . . . . .  14
   17. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
     17.1.  MSF Scheduling Function Identifiers  . . . . . . . . . .  15
   18. References  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     18.1.  Normative References . . . . . . . . . . . . . . . . . .  15



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     18.2.  Informative References . . . . . . . . . . . . . . . . .  16
   Appendix A.  Contributors . . . . . . . . . . . . . . . . . . . .  16
   Appendix B.  Example of Implementation of SAX hash function . . .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17

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 managed 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 implements 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.  Boradcast DODAG Information Objects (DIOs), defined by [RFC6550].
       Unicast DIOs SHOULD NOT be sent on minimal cell.

   Because the minimal cell is SHARED, the back-off algorithm defined in
   [IEEE802154-2015] is used to resolve collisions.  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 behvaior is implementation-specific
   which is out of the scope of MSF.

   As detailed in Section 2.2 of [RFC8480], MSF MUST schedule cells from
   Slotframe 1, while Slotframe 0 is used for traffic defined in the
   Minimal 6TiSCH Configuration.  The length of Slotframe 0 and
   Slotframe 1 SHOULD be the same value.  The default of
   SLOTFRAME_LENGTH is RECOMMENDED for both Slotframe 0 and Slotframe 1,
   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.  Cells scheduled by 6P transaction are called 'managed cells'.
   How to schedule managed cells is detailed in Section 5.  For
   autonomous cells, each node has:

   o  Autonomous Downstream Cell (AutoDownCell), one cell at a
      [slotOffset,channelOffset] computed as a hash of its own EUI64
      (detailed next).  Its cell options bits are assigned as TX=1,
      RX=1, SHARED=0.




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   o  Autonomous Upstream Cell (AutoUpCell), one cell at a
      [slotOffset,channelOffset] computed as a hash of the EUI64 of the
      Join Proxy or its RPL routing parent (detailed in Section 4.4).
      Its cell options bits are assigned as TX=1, RX=1, 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, 16)

   The second input parameter defines the maxmium return value of the
   hash function.  There are other optional parameters defined in SAX
   determines 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.

   Because of hash collisions, there will be cases when the AutoUpCell
   and AutoDownCell are scheduled at the same slot offset and/or channel
   offset.  Hash collisions among a set of cells at a given time offset
   is resolved at run-time as follows:

   o  The AutoUpCell with the most packets in the outgoing queue takes
      precedence.
   o  If the AutoUpCell have empty outgoing queues, the AutoDownCell
      takes precedence.

   In case the autonomous cell to be installed is conflicted with a
   managed cell, a 6P RELOCATE command MUST be issued to the responding
   neighbor to relocate the conflicting managed cell.

   The traffic on the autonomous cells are scheduled as:

   o  Join Request packets and 6P ADD/DELETE Request frames to the
      node's Join Proxy or its RPL routing parent MUST be sent on
      AutoUpCell.
   o  Join Response packets and 6P ADD/DELETE Response frames to the
      pledge or its RPL routing child MUST be sent on AutoDownCell.
   o  6P RELOCATE Request frames to the node's RPL routing child MUST be
      sent on AutoDownCell.
   o  6P RELOCATE Response frames to its RPL routing parent MUST be sent
      on AutoUpCell.



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   Throughout the network lifetime, nodes MUST maintain the autonomous
   cells as follows:

   o  The AutoDownCell MUST always remain scheduled.
   o  When a new Join Proxy (JP) is selected, add an AutoUpCell to it.
   o  When the node is secured after joining process, remove the
      AutoUpCell to the JP.
   o  When a new RPL routing parent is selected, add an AutoUpCell to
      it.
   o  When the routing parent is de-selected, remove the AutoUpCell to
      it.
   o  6P CLEAR MUST NOT erase any autonomous cells.

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 MUST 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





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   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.

   During this step, the pledge MAY synchronize to any EB it receives
   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 MUST set up an AutoUpCell to that JP, as
   described in Section 3.  A Join Request is then sent then by the
   pledge to its JP over the AutoUpCell.  The JP forwards the Join
   Request to the JRC, possibly over multiple hops, over the AutoUpCells
   as well.  Similarly, the JRC sends the Join Response to the JP,
   possibly over multiple hops, over the AutoDownCells.  When JP
   received the Join Response from the JRC, it sends that Join Response
   to the pledge over its AutoDownCell.  The pledge thereby learns the
   keying material used in the network, as well as other configurations,
   and becomes a "joined node".  The AutoUpCell to the JP is removed at
   the same time by the "joined node".

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 Autonomous Cells for 6P

   After selected a preferred parent, the joined node MUST set up an
   AutoUpCell to that parent.  Then it MUST issue a 6P ADD command MUST
   to that parent, 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

   In case the 6P ADD transaction failed, the node MUST issue another 6P
   ADD command and repeat until the one cell is installed to the parent.




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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 AutoUpCell to its parent and one AutoDownCell
   o  it has one Managed Tx 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:

   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)

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 managed cells it
      has scheduled with it, the node issues a 6P ADD command to its
      preferred parent to add one managed Tx cell 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 one managed cell
      from the TSCH schedule.




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   Adding/removing/relocating cells involves exchanging frames that
   contain 6P commands.  All 6P Request frames to its parent MUST be
   sent on the AutoUpCell All 6P Response frames to non-parent neighbor
   MUST be sent on AutoDownCell.  In case a managed cell from non-parent
   is conflicted with AutoUpCell to be installed, a 6P RELOCATE command
   needs to be issued to the neighbor, as mentioned in Section 3.  The
   6P RELOCATE Request frame MUST be sent on AutoDownCell and the
   Response MUST be sent on AutoUpCell.

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

   NumCellsElapsed :  Counts the number of managed 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 managed 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 managed cells that have been
       used.  This counter is initialized at 0.  NumCellsUsed is
       incremented by exactly 1 when, during a managed 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.
       *  The node receives a frame from its preferred parent.

   Both NumCellsElapsed and NumCellsUsed counters can be used to cell
   with cell option TX=1 or RX=1.  All the frames used for increasing/
   decreasing the counters MUST be encrypted or decryptable with the key
   get from joining process.

   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




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       *  Reset both NumCellsElapsed and NumCellsUsed to 0 and go to
          step 2.

5.2.  Switching Parent

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

   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.  if there is managed cell conflicted with the AutoUpCells to be
       installed, the node MUST issue a 6P RELOCATE command to relocate
       the conflicted cell
   2.  if there is no conflicted cell, the node installs the AutoUpCells
       to its new parent
   3.  the node counts the number of managed cells it has per slotframe
       to the old preferred parent
   4.  the node triggers one or more 6P ADD commands to schedule the
       same number of managed cells to the new preferred parent
   5.  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
   managed 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
   managed cell in the schedule.



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   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 256, both NumTx and NumTxAck
   MUST be divided by 2.  That is, for example, from NumTx=256 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 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 different is less 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.







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8.  Rules for CellList

   MSF uses 2-step 6P Transactions exclusively.  6P Transactions are
   only initiated by a node towards it 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 SHOULD contain 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).
   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 calcualted as
   ((2^MAXBE)-1)*SLOTFRAME_LENGTH, where:

   o  MAXEB is the maxmium backoff exponent is used
   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.







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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.

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








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14.  MSF Constants

   Figure 2 lists MSF Constants and their RECOMMENDED values.

           +------------------------------+-------------------+
           | Name                         | RECOMMENDED value |
           +------------------------------+-------------------+
           | KA_PERIOD                    |       10 s        |
           | 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 | RFCXXXX     |
   |                      | (MSF)                       | (NOTE:this) |
   +----------------------+-----------------------------+-------------+

                      Figure 4: IETF IE Subtype '6P'.

18.  References

18.1.  Normative References

   [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-10 (work in progress), April 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.

   [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>.




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   [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>.

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

   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




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   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 bewteen 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


   Malisa Vucinic
   Inria
   2 rue Simone Iff
   Paris  75012
   France

   Email: malisa.vucinic@inria.fr






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   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 (editor)
   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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