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Versions: 00 01

6TiSCH                                                     T. Chang, Ed.
Internet-Draft                                                     Inria
Intended status: Standards Track                              M. Vucinic
Expires: September 2, 2018                      University of Montenegro
                                                           X. Vilajosana
                                         Universitat Oberta de Catalunya
                                                           March 1, 2018


                6TiSCH Minimal Scheduling Function (MSF)
                       draft-chang-6tisch-msf-01

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 6top 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 September 2, 2018.








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Copyright Notice

   Copyright (c) 2018 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.  Node Behavior at Boot . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Start State . . . . . . . . . . . . . . . . . . . . . . .   5
     3.2.  Step 1 - Choosing Frequency . . . . . . . . . . . . . . .   5
     3.3.  Step 2 - Receiving EBs  . . . . . . . . . . . . . . . . .   5
     3.4.  Step 3 - Join Request/Response  . . . . . . . . . . . . .   6
     3.5.  Step 4 - Acquiring a RPL rank . . . . . . . . . . . . . .   6
     3.6.  Step 5 - 6P ADD to Preferred Parent . . . . . . . . . . .   6
     3.7.  Step 6 - Send EBs and DIOs  . . . . . . . . . . . . . . .   7
     3.8.  Step 7 - Neighbor Polling . . . . . . . . . . . . . . . .   7
     3.9.  End State . . . . . . . . . . . . . . . . . . . . . . . .   7
   4.  Rules for Adding/Deleting Cells . . . . . . . . . . . . . . .   7
     4.1.  Adapting to Traffic . . . . . . . . . . . . . . . . . . .   8
     4.2.  Switching Parent  . . . . . . . . . . . . . . . . . . . .   9
     4.3.  Handling Schedule Collisions  . . . . . . . . . . . . . .   9
   5.  6P SIGNAL command . . . . . . . . . . . . . . . . . . . . . .  11
   6.  Scheduling Function Identifier  . . . . . . . . . . . . . . .  11
   7.  Rules for CellList  . . . . . . . . . . . . . . . . . . . . .  11
   8.  6P Timeout Value  . . . . . . . . . . . . . . . . . . . . . .  11
   9.  Rule for Ordering Cells . . . . . . . . . . . . . . . . . . .  11
   10. Meaning of the Metadata Field . . . . . . . . . . . . . . . .  12
   11. 6P Error Handling . . . . . . . . . . . . . . . . . . . . . .  12
   12. Schedule Inconsistency Handling . . . . . . . . . . . . . . .  12
   13. MSF Constants . . . . . . . . . . . . . . . . . . . . . . . .  13
   14. MSF Statistics  . . . . . . . . . . . . . . . . . . . . . . .  13
   15. Security Considerations . . . . . . . . . . . . . . . . . . .  13
   16. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
     16.1.  MSF Scheduling Function Identifiers  . . . . . . . . . .  14
   17. References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     17.1.  Normative References . . . . . . . . . . . . . . . . . .  14



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     17.2.  Informative References . . . . . . . . . . . . . . . . .  15
   Appendix A.  Implementation Status  . . . . . . . . . . . . . . .  15
   Appendix B.  Performance Evaluation . . . . . . . . . . . . . . .  16
   Appendix C.  [TEMPORARY] Changelog  . . . . . . . . . . . . . . .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16

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 [I-D.ietf-6tisch-6top-protocol]: it
   complements [I-D.ietf-6tisch-6top-protocol] by providing the rules of
   when to add/delete cells in the communication schedule.  The SF
   defined in this document follows that definition, and satisfies all
   the requirements for an SF listed in Section 4.2 of
   [I-D.ietf-6tisch-6top-protocol].

   MSF builds on top of the following specifications: the Minimal IPv6
   over the TSCH Mode of IEEE 802.15.4e (6TiSCH) Configuration
   [RFC8180], the 6top Protocol (6P) [I-D.ietf-6tisch-6top-protocol],
   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 3.  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 a single dedicated cell to that parent.
   After the join process, the node can continuously add/delete/relocate
   cells, as described in Section 4.  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).  Appendix B contains a performance evaluation
   of MSF.

   This specification follows the recommended structure of an SF
   specification in Appendix A of [I-D.ietf-6tisch-6top-protocol], with
   the following adaptations:

   o  We have reordered part of the sections, in particular to have the
      section on the node behavior at boot Section 3 appear early in
      this specification.




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   o  We added sections on the interface to the minimal 6TiSCH
      configuration Section 2, the use of the SIGNAL command Section 5,
      the MSF constants Section 13, the MSF statistics Section 14, the
      performance of MSF Appendix B.
   o  We have not included an examples section.

2.  Interface to the Minimal 6TiSCH Configuration

   A node implementing MSF MUST implement the Minimal 6TiSCH
   Configuration [RFC8180], which defines the "minimal cell", a single
   shared cell providing minimal connectivity between the nodes in the
   network.

   MSF uses the minimal cell to exchange the following packets:

   1.  Enhanced Beacons (EBs), defined by [IEEE802154-2015].  These are
       broadcast frames.
   2.  DODAG Information Objects (DIOs), defined by [RFC6550].  These
       are broadcast frames.
   3.  The Join Request and Join Response packets sent between the
       pledge and the Join Proxy, defined by
       [I-D.ietf-6tisch-minimal-security].  These are unicast frames.
   4.  6P packets to schedule the first dedicated cell with a neighbor.
       These are unicast frames.

   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 for the unicast
   traffic, a node implementing MSF SHOULD enforce the following rules
   for broadcast frames:

   1.  send EBs on a portion of the minimal cells not exceeding
       1/(3(N+1)), where N is the number of neighbors of the node.
   2.  send DIOs on a portion of the minimal cells not exceeding
       1/(3(N+1)), where N is the number of neighbors of the node.

   The RECOMMENDED behavior for sending EBs is to have a node send EBs
   with a probability of 1/(3(N+1)).  The RECOMMENDED behavior for
   sending DIOs is to use a Trickle timer with rate-limiting.

   Section 3.3 describes how to evaluate the number of neighbors during
   the joining process.  After the joining process, how to evaluate the
   number of neighbors is implementation-specific.

   As detailed in Section 2.2 of [I-D.ietf-6tisch-6top-protocol], 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 MUST be the same value.  The default of



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   SLOTFRAME_LENGTH is RECOMMENDED, although any value can be advertised
   in the EBs.

3.  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 3.1 details the start state; Section 3.9 details
   the end state.  The other sections detail the 7 steps of the joining
   process.  We use the term "pledge" and "joined node", as defined in
   [I-D.ietf-6tisch-minimal-security].

3.1.  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, is pre-configured with
   the Pre-Shared Key (PSK) for joining, as well as any other
   configuration detailed in [I-D.ietf-6tisch-minimal-security].

3.2.  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.

3.3.  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.

   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.




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

3.4.  Step 3 - Join Request/Response

   As per [I-D.ietf-6tisch-minimal-security], after having selected a
   JP, the pledge sends a Join Request to its JP.  Because no dedicated
   cells are in place at this point, this happens on the minimal cell.
   The JP then forwards the Join Request to the JRC, possibly over
   multiple hops.  When forwarding this Join Request, a node MUST use a
   dedicated cell it has with its preferred parent (not the minimal
   cell).  How such dedicated cells are installed is detailed in
   Section 3.6.

   As per [I-D.ietf-6tisch-minimal-security], the JRC sends back a Join
   Response to the pledge, through the JP.  When forwarding this Join
   Response, a node MUST use a dedicated cell it has with its child (not
   the minimal cell).  How such dedicated cells are installed is
   detailed in Section 3.6.

   As per [I-D.ietf-6tisch-minimal-security], after receiving the Join
   Response, the pledge learns the keying material used in the network,
   as well as other configurations, and becomes a "joined node".

3.5.  Step 4 - Acquiring a RPL rank

   Because it has learned the link-layer keying material used in the
   network, the joined node can now decrypt the DIO packets sent by its
   neighbors.  Per [RFC6550], the joined node receives DIOs, computes
   its own rank, and selects a preferred parent.

3.6.  Step 5 - 6P ADD to Preferred Parent

   After having selected a preferred parent, the joined node MUST issue
   a 6P ADD command to that parent, with the following fields:

   o  CellOptions: set to TX=1,RX=1,SHARED=1
   o  NumCells: set to 1
   o  CellList: at least 5 cells, chosen according to Section 7

   After the 6P Transaction is finished, the node MUST synchronize only
   to its preferred parent.  At this point, the node has a dedicated
   cell which allows for bidirectional communication with its preferred
   parent.







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3.7.  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.

3.8.  Step 7 - Neighbor Polling

   The node SHOULD send some form of keep-alive messages to all its
   neighbors it has dedicated cells with.  The Keep-Alive (KA) mechanism
   is detailed in [RFC7554].  It uses the keep-alive messages to its
   preferred parent to stay synchronized.  It uses the keep-alive
   messages to its children (with which it has a dedicated cell to,
   which the child has created) to ensure the child is still reachable.
   The RECOMMENDED period for sending keep-alive messages is KA_PERIOD.

   If the keep-alive message to a child fails at the link layer (i.e.
   the maximum number of link-layer retries is reached), the node SHOULD
   declare the child as unreachable.  This can happen for example when
   the child node is switched off.

   When a neighbor is declared unreachable, the node MUST remove all
   dedicated cells with that neighbor from its own schedule.  In
   addition, it MAY issue a 6P CLEAR to that neighbor (which can fail at
   the link-layer).

3.9.  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 a single dedicated cell to its preferred routing parent
   o  it is periodically sending DIOs, potentially serving as a router
      for other nodes' traffic
   o  it is periodically sending EBs, potentially serving as a JP for
      new joining nodes

4.  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 4.1)
   o  to handle switching preferred parent (Section 4.2)
   o  to handle a schedule collision (Section 4.3)



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4.1.  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 cells it has
      scheduled with it, it triggers a 6P Transaction with its preferred
      parent to add cells to the TSCH schedule of both nodes.
   o  If the traffic is lower than the capacity, the node triggers a 6P
      Transaction with its preferred parent to delete cells from the
      TSCH schedule of both nodes.

   From the join process, the node already has one dedicated cell
   scheduled to its preferred parent.  A node MUST NOT remove all cells
   to its preferred parent, i.e. there must always be at least one
   dedicated cell scheduled between a node and its preferred parent,
   even if no frames are being exchanged between them.

   Adding/removing/relocating cells involves exchanging frames that
   contain 6P commands.  Once the first cell has been established as
   part of the join process, all 6P frames MUST be sent on the dedicated
   cells (not the minimal cell).

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

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



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   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 NumCellsPassed and NumCellsUsed are initialized to 0 when
       the node boots.
   2.  When the value of NumCellsPassed 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 NumCellsPassed and NumCellsUsed to 0 and go to step
          2.

4.2.  Switching Parent

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

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

   1.  the node counts the number of dedicated 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 dedicated cells to the new preferred parent
   3.  when that successfully completes, the node issues a 6P CLEAR
       command to its old preferred parent

4.3.  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 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 cell to its
   preferred parent:




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   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 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 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 cell with its preferred parent, 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 each other cell, it compares its PDR against that of the cell
       with the highest PDR.  If it's less than RELOCATE_PDRTHRES, it
       triggers the relocation of that cell using a 6P RELOCATE command.







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5.  6P SIGNAL command

   The 6P SIGNAL command is not used by MSF.

6.  Scheduling Function Identifier

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

7.  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.

8.  6P Timeout Value

   The 6P Timeout is not a constant value.  It is calculated as
   (1/C)*(1/PDR)*SIXP_TIMEOUT_SEC_FACTOR, where:

   o  C represents the number of cells per second scheduled to that
      neighbor
   o  PDR represents the average PDR of those cells
   o  SIXP_TIMEOUT_SEC_FACTOR is a security factor, a constant

9.  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]



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10.  Meaning of the Metadata Field

   The Metadata field is not used by MSF.

11.  6P Error Handling

   Section 6.2.4 of [I-D.ietf-6tisch-6top-protocol] 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 hence 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.

12.  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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13.  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 %        |
           | SIXP_TIMEOUT_SEC_FACTOR      |        3 x        |
           | SLOTFRAME_LENGTH             |      101 slots    |
           | QUARANTINE_DURATION          |        5 min      |
           | WAITDURATION_MIN             |       30 s        |
           | WAITDURATION_MAX             |       60 s        |
           +------------------------------+-------------------+

           Figure 2: MSF Constants and their RECOMMENDED values.

14.  MSF Statistics

   Figure 3 lists MSF Statistics and their RECOMMENDED width.

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

           Figure 3: MSF Statistics and their RECOMMENDED width.

15.  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 6top Protocol (6P)
   [I-D.ietf-6tisch-6top-protocol], 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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16.  IANA Considerations

16.1.  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
   [I-D.ietf-6tisch-6top-protocol]:

   +----------------------+-----------------------------+-------------+
   |  SFID                | Name                        | Reference   |
   +----------------------+-----------------------------+-------------+
   | IANA_6TISCH_SFID_MSF | Minimal Scheduling Function | RFCXXXX     |
   |                      | (MSF)                       | (NOTE:this) |
   +----------------------+-----------------------------+-------------+

                      Figure 4: IETF IE Subtype '6P'.

17.  References

17.1.  Normative References

   [I-D.ietf-6tisch-6top-protocol]
              Wang, Q., Vilajosana, X., and T. Watteyne, "6top Protocol
              (6P)", draft-ietf-6tisch-6top-protocol-09 (work in
              progress), October 2017.

   [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-04 (work in progress), October
              2017.

   [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>.

   [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>.

   [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>.

17.2.  Informative References

   [OpenWSN]  Watteyne, T., Vilajosana, X., Kerkez, B., Chraim, F.,
              Weekly, K., Wang, Q., Glaser, S., and K. Pister, "OpenWSN:
              a Standards-Based Low-Power Wireless Development
              Environment", Transactions on Emerging Telecommunications
              Technologies , August 2012.

   [RFC6982]  Sheffer, Y. and A. Farrel, "Improving Awareness of Running
              Code: The Implementation Status Section", RFC 6982,
              DOI 10.17487/RFC6982, July 2013,
              <https://www.rfc-editor.org/info/rfc6982>.

Appendix A.  Implementation Status

   This section records the status of known implementations of the
   protocol defined by this specification at the time of posting of this
   Internet-Draft, and is based on a proposal described in [RFC6982].
   The description of implementations in this section is intended to
   assist the IETF in its decision processes in progressing drafts to
   RFCs.  Please note that the listing of any individual implementation
   here does not imply endorsement by the IETF.  Furthermore, no effort
   has been spent to verify the information presented here that was
   supplied by IETF contributors.  This is not intended as, and must not
   be construed to be, a catalog of available implementations or their
   features.  Readers are advised to note that other implementations may
   exist.

   According to [RFC6982], "this will allow reviewers and working groups
   to assign due consideration to documents that have the benefit of



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   running code, which may serve as evidence of valuable experimentation
   and feedback that have made the implemented protocols more mature.
   It is up to the individual working groups to use this information as
   they see fit".

   OpenWSN:  MSF is being implemented in the OpenWSN project [OpenWSN]
      under a BSD open-source license.  The authors of this document are
      collaborating with the OpenWSN community to gather feedback about
      the status and performance of the protocols described in this
      document.  Results from that discussion will appear in this
      section in future revision of this specification.  More
      information about this implementation at http://www.openwsn.org/.
   6TiSCH simulator  The 6TiSCH simulator is a Python-based high-level
      simulator on which MSF is being implemented.  More information at
      https://bitbucket.org/6tisch/simulator/.

Appendix B.  Performance Evaluation

   The performance of MSF may be published as companion documents to
   this specification, possibly under the form a applicability
   statements.

Appendix C.  [TEMPORARY] Changelog

   o  draft-chang-6tisch-msf-01

      *  When neighbor is unreachable, sending a CLEAR command was a
         MUST, now a MAY.
      *  Fixing 6P Timeout calculation.
      *  Clearer text for "Handling Schedule Collisions" section.
      *  Typos.
      *  Input from Yasuyuki Tanaka's review (https://www.ietf.org/mail-
         archive/web/6tisch/current/msg05723.html).
   o  draft-chang-6tisch-msf-00

      *  Initial submission.

Authors' Addresses

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

   Email: tengfei.chang@inria.fr





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   Malisa Vucinic
   University of Montenegro
   Dzordza Vasingtona bb
   Podgorica  81000
   Montenegro

   Email: malisav@ac.me


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

   Email: xvilajosana@uoc.edu



































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