6TiSCH Minimal Scheduling Function (MSF)
draft-ietf-6tisch-msf-18

6TiSCH                                                     T. Chang, Ed.
Internet-Draft                                                M. Vucinic
Intended status: Standards Track                                   Inria
Expires: March 16, 2021                                    X. Vilajosana
                                         Universitat Oberta de Catalunya
                                                            S. Duquennoy
                                                               RISE SICS
                                                              D. Dujovne
                                              Universidad Diego Portales
                                                      September 12, 2020

                6TiSCH Minimal Scheduling Function (MSF)
                        draft-ietf-6tisch-msf-18

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 is built 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 BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

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 March 16, 2021.

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Copyright Notice

   Copyright (c) 2020 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  . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Node Behavior at Boot . . . . . . . . . . . . . . . . . . . .   6
     4.1.  Start State . . . . . . . . . . . . . . . . . . . . . . .   6
     4.2.  Step 1 - Choosing Frequency . . . . . . . . . . . . . . .   7
     4.3.  Step 2 - Receiving EBs  . . . . . . . . . . . . . . . . .   7
     4.4.  Step 3 - Setting up Autonomous Cells for the Join
           Process . . . . . . . . . . . . . . . . . . . . . . . . .   7
     4.5.  Step 4 - Acquiring a RPL Rank . . . . . . . . . . . . . .   8
     4.6.  Step 5 - Setting up first Tx negotiated Cells . . . . . .   8
     4.7.  Step 6 - Send EBs and DIOs  . . . . . . . . . . . . . . .   8
     4.8.  End State . . . . . . . . . . . . . . . . . . . . . . . .   8
   5.  Rules for Adding/Deleting Cells . . . . . . . . . . . . . . .   9
     5.1.  Adapting to Traffic . . . . . . . . . . . . . . . . . . .   9
     5.2.  Switching Parent  . . . . . . . . . . . . . . . . . . . .  11
     5.3.  Handling Schedule Collisions  . . . . . . . . . . . . . .  11
   6.  6P SIGNAL command . . . . . . . . . . . . . . . . . . . . . .  13
   7.  Scheduling Function Identifier  . . . . . . . . . . . . . . .  13
   8.  Rules for CellList  . . . . . . . . . . . . . . . . . . . . .  13
   9.  6P Timeout Value  . . . . . . . . . . . . . . . . . . . . . .  14
   10. Rule for Ordering Cells . . . . . . . . . . . . . . . . . . .  14
   11. Meaning of the Metadata Field . . . . . . . . . . . . . . . .  14
   12. 6P Error Handling . . . . . . . . . . . . . . . . . . . . . .  14
   13. Schedule Inconsistency Handling . . . . . . . . . . . . . . .  15
   14. MSF Constants . . . . . . . . . . . . . . . . . . . . . . . .  15
   15. MSF Statistics  . . . . . . . . . . . . . . . . . . . . . . .  16
   16. Security Considerations . . . . . . . . . . . . . . . . . . .  16
   17. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
     17.1.  MSF Scheduling Function Identifiers  . . . . . . . . . .  18
   18. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  18
   19. References  . . . . . . . . . . . . . . . . . . . . . . . . .  18

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     19.1.  Normative References . . . . . . . . . . . . . . . . . .  18
     19.2.  Informative References . . . . . . . . . . . . . . . . .  20
   Appendix A.  Example of Implementation of SAX hash function . . .  20
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21

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 6 steps described in Section 4.  The end state of the join
   process is that the node is synchronized to the network, has mutually
   authenticated with the network, has identified a routing parent, and
   has scheduled one negotiated Tx cell (defined in Section 5.1) to/from
   its 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 and to handle a schedule
   collision.

   MSF works closely with the IPv6 Routing Protocol for Low-Power and
   Lossy Networks (RPL), specifically the routing parent defined in
   [RFC6550].  This specification only describes how MSF works with the
   routing parent; this parent is referred to as the "selected parent".
   The activity of MSF towards the single routing parent is called a
   "MSF session".  Though the performance of MSF is evaluated only when
   the "selected parent" represents the node's preferred parent, there
   should be no restrictions to use multiple MSF sessions, one per
   parent.  The distribution of traffic over multiple parents is a
   routing decision that is out of scope for MSF.

   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 Destination-Oriented Directed Acyclic Graph (DODAG
   [RFC6550]) root.

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

   *  We have reordered some sections, in particular to have the section
      on the node behavior at boot (Section 4) appear early in this
      specification.
   *  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) and the MSF statistics
      (Section 15).

2.  Interface to the Minimal 6TiSCH Configuration

   In a TSCH network, time is sliced up into time slots.  The time slots
   are grouped as one or multiple slotframes which repeat over time.
   The TSCH schedule instructs a node what to do at each time slot, such
   as transmit, receive or sleep [RFC7554].  In case of a slot to
   transmit or receive, a channel is assigned to the time slot.  The
   tuple (slot, channel) is indicated as a cell of TSCH schedule.  MSF
   is one of the policies defining how to manage the TSCH schedule.

   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 based on other
   specifications as long as the specification defines a way to
   advertise the EB/DIO among the network.

   MSF uses the minimal cell for broadcast frames such as Enhanced
   Beacons (EBs) [IEEE802154] and broadcast DODAG Information Objects
   (DIOs) [RFC6550].  Cells scheduled by MSF are meant to be used only
   for unicast frames.

   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, the overall broadcast
   traffic among the node and its neighbors SHOULD NOT exceed 1/3 of the
   bandwidth of minimal cell.  One of the algorithms that fulfills this
   requirement is the Trickle timer defined in [RFC6206] which is
   applied on DIO messages [RFC6550].  However, any such algorithm of
   limiting the broadcast traffic to meet those rules is implementation-
   specific and is out of the scope of MSF.

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   3 slotframes are used in MSF.  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.  The same slotframe
   length for Slotframe 0, 1 and 2 is RECOMMENDED.  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:

   *  Autonomous Rx Cell (AutoRxCell), one cell at a
      [slotOffset,channelOffset] computed as a hash of the EUI64 of the
      node itself (detailed next).  Its cell options bits are assigned
      as TX=0, RX=1, SHARED=0.
   *  Autonomous Tx Cell (AutoTxCell), one cell at a
      [slotOffset,channelOffset] computed as a hash of the layer 2 EUI64
      destination address in the unicast 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 as defined in Section 2 of
   [SAX-DASFAA] with consistent input parameters, for example, those
   defined in Appendix A.  The coordinates are computed to distribute
   the cells across all channel offsets, and all but the first slot
   offset 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:

   *  slotOffset(MAC) = 1 + hash(EUI64, length(Slotframe_1) - 1)
   *  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, the values of those parameters can be referred from
   Appendix A.

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

   *  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.
   *  Remove an AutoTxCell when:
      -  there is no frame to transmit on that cell, or
      -  there is at least one 6P negotiated Tx cell in the schedule for
         the frames to transmit.

   The AutoRxCell MUST always remain scheduled after synchronization.
   6P CLEAR MUST NOT erase any autonomous cells.

   Because of hash collisions, there will be cases that the AutoTxCell
   and AutoRxCell are scheduled at the same slot offset and/or channel
   offset.  In such cases, AutoTxCell always take precedence over
   AutoRxCell.  Notice AutoTxCell is a shared type cell which applies
   backs-off mechanism.  When the AutoTxCell and AutoRxCell collide,
   AutoTxCell takes precedence if there is a packet to transmit.  When
   in a back-off period, AutoRxCell is used.  In case of conflicting
   with a negotiated cell, autonomous cells take precedence over
   negotiated cells, which is stated in [IEEE802154].  However, when the
   Slotframe 0, 1 and 2 use the same length value, it is possible for a
   negotiated cell to avoid the collision with AutoRxCell.  Hence, the
   same slotframe length for Slotframe 0, 1 and 2 is RECOMMENDED.

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.  Alternative behaviors may be involved, for example, when
   alternative security solutions are used for 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 Constrained Join
   Protocol (CoJP) 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 necessary if the

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   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 randomly chooses a frequency from the
   channels that the network cycles amongst, and starts listening for
   EBs on that frequency.

4.3.  Step 2 - Receiving EBs

   Upon receiving the first EB, the pledge continues 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

   After having received the first EB, a node MAY keep listening for at
   most MAX_EB_DELAY seconds or until it has received EBs from
   NUM_NEIGHBOURS_TO_WAIT distinct neighbors.  This behavior is defined
   in [RFC8180].

   During this step, the pledge only gets synchronized when 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.ietf-6tisch-enrollment-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 having selected a JP, a node generates a Join Request and
   installs an AutoTxCell to the JP.  The Join Request is then sent by
   the pledge to its selected 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 join registrar/coordinator (JRC), possibly
   over multiple hops, over the 6P negotiated Tx cells.  Similarly, the
   JRC sends the Join Response to the JP, possibly over multiple hops,
   over AutoTxCells or the 6P negotiated Tx cells.  When the 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

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   out.  The pledge receives the Join Response from its AutoRxCell,
   thereby learns the keying material used in the network, as well as
   other configuration settings, and becomes a "joined node".

   When 6LoWPAN Neighbor Discovery ([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 routing parent.

4.6.  Step 5 - Setting up first Tx negotiated Cells

   Once it has selected a routing 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, containing the
   following fields:

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

   The joined node removes the AutoTxCell to the selected parent when
   the 6P Request is sent out.  That 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
   transaction.  In case the 6P ADD transaction failed, the node MUST
   issue another 6P ADD Request and repeat until the Tx cell is
   installed to the parent.

4.7.  Step 6 - Send EBs and DIOs

   The node starts 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:

   *  it is synchronized to the network
   *  it is using the link-layer keying material it learned through the
      secure joining process
   *  it has selected one neighbor as its routing parent

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   *  it has one AutoRxCell
   *  it has one negotiated Tx cell to the selected parent
   *  it starts to send DIOs, potentially serving as a router for other
      nodes' traffic
   *  it starts to send EBs, potentially serving as a JP for new pledges

5.  Rules for Adding/Deleting Cells

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

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

   Those cells are called 'negotiated cells' as they are scheduled
   through 6P, negotiated with the node's parent.  Without specific
   declaration, 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 one of its
   neighbors, in most cases to the selected parent.

   *  If the node determines that the number of link-layer frames it is
      attempting to exchange with the selected 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 that
      parent to add cells to the TSCH schedule.
   *  If the traffic is lower than the capacity, the node issues a 6P
      DELETE command to that parent to delete cells from the TSCH
      schedule.

   The node MUST maintain two separate pairs of the following counters
   for the selected parent, one for the negotiated Tx cells to that
   parent and one for the negotiated Rx cells to that parent.

   NumCellsElapsed :  Counts the number of negotiated cells that have

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       elapsed since the counter was initialized.  This counter is
       initialized at 0.  When the current cell is declared as a
       negotiated cell to the selected 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
       selected parent, either of the following happens:
       *  The node sends a frame to the parent.  The counter increments
          regardless of whether a link-layer acknowledgment was received
          or not.
       *  The node receives a valid frame from the parent.  The counter
          increments only when the frame is a valid IEEE802.15.4 frame.

   The cell option of cells listed in CellList in 6P Request frame
   SHOULD be either (Tx=1, Rx=0) only or (Tx=0, Rx=1) only.  Both
   NumCellsElapsed and NumCellsUsed counters can be used for both type
   of negotiated cells.

   As there is no negotiated Rx Cell installed at initial time, the
   AutoRxCell is taken into account as well for downstream traffic
   adaptation.  In this case:

   *  NumCellsElapsed is incremented by exactly 1 when the current cell
      is AutoRxCell.
   *  NumCellsUsed is incremented by exactly 1 when the node receives a
      frame from the selected parent on AutoRxCell.

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

   The value of MAX_NUM_CELLS is chosen according to the traffic type of
   the network.  Generally speaking, the larger the value MAX_NUM_CELLS
   is, the more accurate the cell usage is calculated.  The 6P traffic

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   overhead using a larger value of MAX_NUM_CELLS could be reduced as
   well.  Meanwhile, the latency won't increase much by using a larger
   value of MAX_NUM_CELLS for periodic traffic type.  For bursty
   traffic, larger value of MAX_NUM_CELLS 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.  Setting MAX_NUM_CELLS to a value at least 4x of
   the recent maximum number of cells used in a slot frame is
   RECOMMENDED.  For example, a 2 packets/slotframe traffic load results
   an average 4 cells scheduled (2 cells are used), using at least the
   value of double number of scheduled cells (which is 8) as
   MAX_NUM_CELLS gives a good resolution on cell usage calculation.

   In case that a node booted or disappeared from the network, the cell
   reserved at the selected parent may be kept in the schedule forever.
   A clean-up mechanism MUST be provided to resolve this issue.  The
   clean-up mechanism is implementation-specific.  The goal is to
   confirm those negotiated cells are not used anymore by the associated
   neighbors and remove them from the schedule.

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 parent.  The procedure for switching from the old parent
   to the new parent is:

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

   For what type of negotiated cell should be installed first, it
   depends on which traffic has the higher priority, upstream or
   downstream, which is application-specific and out-of-scope of MSF.

5.3.  Handling Schedule Collisions

   A node implementing MSF SHOULD implement the behavior described in
   this section.  Other schedule collisions handling algorithm can be an
   alternative of the algorithm proposed in this section.

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   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 negotiated Tx
   cell to the selected 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.

   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.

   Each time the node switches parent (or during the join process when
   the node selects a 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 that parent.  When
   NumTx reaches MAX_NUMTX, both NumTx and NumTxAck MUST be divided by
   2.  MAX_NUMTX needs to be a power of two to avoid division error.
   For example, when MAX_NUMTX is set to 256, from NumTx=255 and
   NumTxAck=127, the counters become NumTx=128 and NumTxAck=64 if one
   frame is sent to the parent with an Acknowledgment received.  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 selected 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 negotiated Tx cell with the parent (not for
       the autonomous cell), that cell's PDR.

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   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 subtraction difference between the
       PDR of the cell and the highest PDR is larger than
       RELOCATE_PDRTHRES, it triggers the relocation of that cell using
       a 6P RELOCATE command.

   The RELOCATION for negotiated Rx cells is not supported by MSF.

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.  How the value of IANA_6TISCH_SFID_MSF is
   chosen is described in Section 17.

8.  Rules for CellList

   MSF uses 2-step 6P Transactions exclusively.  6P transactions are
   only initiated by a node towards its 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:

   *  The CellList is RECOMMENDED to have 5 or more cells.
   *  Each cell in the CellList MUST have a different slotOffset value.
   *  For each cell in the CellList, the node MUST NOT have any
      scheduled cell on the same slotOffset.
   *  The slotOffset value of any cell in the CellList MUST NOT be the
      same as the slotOffset of the minimal cell (slotOffset=0).
   *  The slotOffset of a cell in the CellList SHOULD be randomly and
      uniformly chosen among all the slotOffset values that satisfy the
      restrictions above.
   *  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.

   As a consequence of random cell selection, there is a non-zero chance
   that nodes in the vicinity installed cells with same slotOffset and
   channelOffset.  An implementer MAY implement a strategy to monitor
   the candidate cells before adding them in CellList to avoid

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   collision.  For example, a node MAY maintain a candidate cell pool
   for the CellList.  The candidate cells in the pool are pre-configured
   as Rx cells to promiscuously listen to detect transmissions on those
   cells.  If IEEE802.15.4 transmissions are observed on one cell over
   multiple iterations of the schedule, that cell is probably used by a
   TSCH neighbor.  It is moved out from the pool and a new cell is
   selected as a candidate cell.  The cells in CellList are picked from
   the candidate pool directly when required.

9.  6P Timeout Value

   The timeout value 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:

   *  MAXBE, defined in IEEE802.15.4, is the maximum backoff exponent
      used
   *  MAXRETRIES, defined in IEEE802.15.4, is the maximum retransmission
      times
   *  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.
   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 [WAIT_DURATION_MIN,WAIT_DURATION_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 |
           +------------------------------+-------------------+
           | SLOTFRAME_LENGTH             |      101 slots    |
           | NUM_CH_OFFSET                |       16          |
           | MAX_NUM_CELLS                |      100          |
           | LIM_NUMCELLSUSED_HIGH        |       75          |
           | LIM_NUMCELLSUSED_LOW         |       25          |
           | MAX_NUMTX                    |      256          |
           | HOUSEKEEPINGCOLLISION_PERIOD |        1 min      |
           | RELOCATE_PDRTHRES            |       50 %        |
           | QUARANTINE_DURATION          |        5 min      |
           | WAIT_DURATION_MIN            |       30 s        |
           | WAIT_DURATION_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 Constrained Join Protocol
   (CoJP) for 6TiSCH [I-D.ietf-6tisch-minimal-security].
   Confidentiality and authentication of MSF control and data traffic
   are provided by these specifications whose security considerations
   continue to apply to MSF.  In particular, MSF does not define a new
   protocol or packet format.

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   MSF uses autonomous cells for initial bootstrap and the transport of
   join traffic.  Autonomous cells are computed as a hash of nodes'
   EUI64 addresses.  This makes the coordinates of autonomous cell an
   easy target for an attacker, as EUI64 addresses are visible on the
   wire and are not encrypted by the link-layer security mechanism.
   With the coordinates of autonomous cells available, the attacker can
   launch a selective jamming attack against any nodes' AutoRxCell.  If
   the attacker targets a node acting as a JP, it can prevent pledges
   from using that JP to join the network.  The pledge detects such a
   situation through the absence of a link-layer acknowledgment for its
   Join Request.  As it is expected that each pledge will have more than
   one JP available to join the network, one available countermeasure
   for the pledge is to pseudo-randomly select a new JP when the link to
   the previous JP appears bad.  Such strategy alleviates the issue of
   the attacker randomly jamming to disturb the network but does not
   help in case the attacker is targeting a particular pledge.  In that
   case, the attacker can jam the AutoRxCell of the pledge, in order to
   prevent it from receiving the join response.  This situation should
   be detected through the absence of a particular node from the network
   and handled by the network administrator through out-of-band means.

   MSF adapts to traffic containing packets from the IP layer.  It is
   possible that the IP packet has a non-zero DSCP (Diffserv Code Point
   [RFC2474]) value in its IPv6 header.  The decision how to handle that
   packet belongs to the upper layer and is out of scope of MSF.  As
   long as the decision is made to hand over to MAC layer to transmit,
   MSF will take that packet into account when adapting to traffic.

   Note that non-zero DSCP value may imply that the traffic is
   originated at unauthenticated pledges, referring to
   [I-D.ietf-6tisch-minimal-security].  The implementation at IPv6 layer
   SHOULD rate-limit this join traffic before it is passed to 6top
   sublayer where MSF can observe it.  In case there is no rate limit
   for join traffic, intermediate nodes in the 6TiSCH network may be
   prone to a resource exhaustion attack, with the attacker injecting
   unauthenticated traffic from the network edge.  The assumption is
   that the rate limiting function is aware of the available bandwidth
   in the 6top L3 bundle(s) towards a next hop, not directly from MSF,
   but from an interaction with the 6top sublayer that manages
   ultimately the bundles under MSF's guidance.  How this rate-limit is
   implemented is out of scope of MSF.

17.  IANA Considerations

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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: New SFID in 6P Scheduling Function Identifiers subregistry.

   IANA_6TISCH_SFID_MSF is chosen from range 0-127, which is used for
   IETF Review or IESG Approval.

18.  Contributors

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

19.  References

19.1.  Normative References

   [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>.

   [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>.

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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>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC2474]  Nichols, K., Blake, S., Baker, F., and D. Black,
              "Definition of the Differentiated Services Field (DS
              Field) in the IPv4 and IPv6 Headers", RFC 2474,
              DOI 10.17487/RFC2474, December 1998,
              <https://www.rfc-editor.org/info/rfc2474>.

   [I-D.ietf-6tisch-minimal-security]
              Vucinic, M., Simon, J., Pister, K., and M. Richardson,
              "Constrained Join Protocol (CoJP) for 6TiSCH", Work in
              Progress, Internet-Draft, draft-ietf-6tisch-minimal-
              security-15, December 10, 2019,
              <https://tools.ietf.org/html/draft-ietf-6tisch-minimal-
              security-15>.

   [I-D.ietf-6tisch-enrollment-enhanced-beacon]
              Dujovne, D. and M. Richardson, "IEEE 802.15.4 Information
              Element encapsulation of 6TiSCH Join and Enrollment
              Information", Work in Progress, Internet-Draft, draft-
              ietf-6tisch-enrollment-enhanced-beacon-14, February 21,
              2020, <https://tools.ietf.org/html/draft-ietf-6tisch-
              enrollment-enhanced-beacon-14>.

   [I-D.ietf-6tisch-architecture]
              Thubert, P., "An Architecture for IPv6 over the TSCH mode
              of IEEE 802.15.4", Work in Progress, Internet-Draft,
              draft-ietf-6tisch-architecture-28, October 29, 2019,
              <https://tools.ietf.org/html/draft-ietf-6tisch-
              architecture-28>.

   [IEEE802154]
              IEEE standard for Information Technology, "IEEE Std
              802.15.4 Standard for Low-Rate Wireless Personal Area
              Networks (WPANs)", DOI 10.1109/IEEE P802.15.4-REVd/D01,
              <http://ieeexplore.ieee.org/document/7460875/>.

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   [SAX-DASFAA]
              Ramakrishna, M.V. and J. Zobel, "Performance in Practice
              of String Hashing Functions", DASFAA ,
              DOI 10.1142/9789812819536_0023, 1997,
              <https://doi.org/10.1142/9789812819536_0023>.

19.2.  Informative References

   [RFC7554]  Watteyne, T., Ed., Palattella, M., and L. Grieco, "Using
              IEEE 802.15.4e Time-Slotted Channel Hopping (TSCH) in the
              Internet of Things (IoT): Problem Statement", RFC 7554,
              DOI 10.17487/RFC7554, May 2015,
              <https://www.rfc-editor.org/info/rfc7554>.

   [I-D.ietf-6tisch-dtsecurity-zerotouch-join]
              Richardson, M., "6tisch Zero-Touch Secure Join protocol",
              Work in Progress, Internet-Draft, draft-ietf-6tisch-
              dtsecurity-zerotouch-join-04, July 8, 2019,
              <https://tools.ietf.org/html/draft-ietf-6tisch-dtsecurity-
              zerotouch-join-04>.

   [RFC6206]  Levis, P., Clausen, T., Hui, J., Gnawali, O., and J. Ko,
              "The Trickle Algorithm", RFC 6206, DOI 10.17487/RFC6206,
              March 2011, <https://www.rfc-editor.org/info/rfc6206>.

   [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>.

Appendix A.  Example of Implementation of SAX hash function

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

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

   In MSF, the T is replaced by the length of 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:

   *  L_shift(v,b), which refers to left shift variable v by b bits

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   *  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

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

   The values of h0, l_bit and r_bit in Step 1 and 2 are configured as:

   *  h0 = 0
   *  l_bit = 0
   *  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
   75012 Paris
   France

   Email: tengfei.chang@inria.fr

   Malisa Vucinic
   Inria
   2 rue Simone Iff
   75012 Paris
   France

   Email: malisa.vucinic@inria.fr

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

   Email: xvilajosana@uoc.edu

   Simon Duquennoy
   RISE SICS
   Isafjordsgatan 22
   164 29 Kista
   Sweden

   Email: simon.duquennoy@gmail.com

   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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