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6TiSCH                                                   D. Dujovne, Ed.
Internet-Draft                                Universidad Diego Portales
Intended status: Experimental                                 LA. Grieco
Expires: January 3, 2018                             Politecnico di Bari
                                                          MR. Palattella
                   Luxembourg Institute of Science and Technology (LIST)
                                                            N. Accettura
                                                               LAAS-CNRS
                                                            July 2, 2017


               6TiSCH 6top Scheduling Function Zero (SF0)
                     draft-ietf-6tisch-6top-sf0-05

Abstract

   This document defines a Scheduling Function called "Scheduling
   Function Zero" (SF0).  SF0 dynamically adapts the number of scheduled
   cells between neighbor nodes, based on the amount of currently
   allocated cells and the neighbor nodes' cell requirements.  Neighbor
   nodes negotiate in a distributed neighbor-to-neighbor basis the
   number of cell(s) to be added/deleted.  SF0 uses the 6P signaling
   messages to add/delete cells in the schedule.  This function selects
   the candidate cells from the schedule, defines which cells will be
   added/deleted and triggers the allocation/deallocation process.

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 RFC
   2119 [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 http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."




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   This Internet-Draft will expire on January 3, 2018.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  TEMPORARY EDITORIAL NOTES . . . . . . . . . . . . . . . . . .   3
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Scheduling Function Identifier  . . . . . . . . . . . . . . .   4
   4.  Allocated and Used Cells  . . . . . . . . . . . . . . . . . .   4
   5.  Overprovisioning  . . . . . . . . . . . . . . . . . . . . . .   4
   6.  Scheduling Algorithm  . . . . . . . . . . . . . . . . . . . .   4
     6.1.  SF0 Triggering Events . . . . . . . . . . . . . . . . . .   4
     6.2.  SF0 Cell Estimation Algorithm . . . . . . . . . . . . . .   4
     6.3.  SF0 Allocation Policy . . . . . . . . . . . . . . . . . .   6
   7.  Rules for CellList  . . . . . . . . . . . . . . . . . . . . .   8
   8.  6P Timeout Value  . . . . . . . . . . . . . . . . . . . . . .   8
   9.  Meaning of Metadata Information . . . . . . . . . . . . . . .   9
   10. Node Behavior at Boot . . . . . . . . . . . . . . . . . . . .   9
   11. Cell Type . . . . . . . . . . . . . . . . . . . . . . . . . .   9
   12. SF0 Statistics  . . . . . . . . . . . . . . . . . . . . . . .   9
   13. Relocating Cells  . . . . . . . . . . . . . . . . . . . . . .   9
   14. Forced Cell Deletion Policy . . . . . . . . . . . . . . . . .  10
   15. 6P Error Handling . . . . . . . . . . . . . . . . . . . . . .  10
   16. Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  10
   17. Implementation Status . . . . . . . . . . . . . . . . . . . .  10
   18. Security Considerations . . . . . . . . . . . . . . . . . . .  11
   19. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   20. 6P Compliance . . . . . . . . . . . . . . . . . . . . . . . .  11
   21. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  12
   22. References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     22.1.  Normative References . . . . . . . . . . . . . . . . . .  12
     22.2.  Informative References . . . . . . . . . . . . . . . . .  12
   Appendix A.  [TEMPORARY] Changelog  . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13



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1.  TEMPORARY EDITORIAL NOTES

   This document is an Internet Draft, so it is work-in-progress by
   nature.  It contains the following work-in-progress elements:

   o  "TODO" statements are elements which have not yet been written by
      the authors for some reason (lack of time, ongoing discussions
      with no clear consensus, etc).  The statement does indicate that
      the text will be written at some time.
   o  "TEMPORARY" appendices are there to capture current ongoing
      discussions, or the changelog of the document.  These appendices
      will be removed in the final text.
   o  "IANA_" identifiers are placeholders for numbers assigned by IANA.
      These placeholders are to be replaced by the actual values they
      represent after their assignment by IANA.
   o  The string "REMARK" is put before a remark (questions, suggestion,
      etc) from an author, editor of contributor.  These are on-going
      discussions at the time to writing, NOT part of the final text.
   o  This section will be removed in the final text.

2.  Introduction

   This document defines a minimal Scheduling Function using the 6P
   protocol [I-D.ietf-6tisch-6top-protocol], called "Scheduling Function
   Zero" (SF0).  SF0 is designed to offer a number of functionalities to
   be usable in a wide range of applications.  SF0 defines two
   algorithms: The Scheduling Algorithm defines the number of cells to
   allocate/delete between two neighbours and the Relocation Algorithm
   defines when to relocate a cell.

   To synthesize, a node running SF0 determines when to add/delete cells
   in a three-step process:

   1.  It waits for a triggering event (Section 6.1).
   2.  It applies the Cell Estimation Algorithm (CEA) for a particular
       neighbor to determine how many cells are required to that
       neighbor (Section 6.2).
   3.  It applies the Allocation Policy to compare the number of
       required cells to the number of already scheduled cells, and
       determines the number of cells to add/delete (Section 6.3).

   We expect additional SFs, offering more functionalities for a more
   specific use case, to be defined in future documents.  SF0 addresses
   the requirements for a scheduling function listed in Section 5.2 from
   [I-D.ietf-6tisch-6top-protocol], and follows the recommended outline
   listed in Section 5.3 of [I-D.ietf-6tisch-6top-protocol].  This
   document follows the terminology defined in
   [I-D.ietf-6tisch-terminology].



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3.  Scheduling Function Identifier

   The Scheduling Function Identifier (SFID) of SF0 is
   IANA_6TISCH_SFID_SF0.

4.  Allocated and Used Cells

   An allocated cell is assigned as a TX, RX or Shared cell on the
   schedule, as a reserved resource.  This reservation does not imply
   that a packet will be transmitted during the scheduled cell time.  A
   used cell is a cell where a packet has been transmitted during the
   scheduled cell time on the last slotframe.

5.  Overprovisioning

   Overprovisioning is the action and effect of increasing a value
   representing an amount of resources.  In the case of SF0,
   overprovisioning is done as a provision to reduce traffic variability
   effects on packet loss, to the expense of artificially allocating a
   number of cells.

6.  Scheduling Algorithm

   A number of TX cells must be allocated between neighbor nodes in
   order to enable data transmission among them.  A portion of these
   allocated cells will be used by neighbors, while the remaining cells
   can be over-provisioned to handle unanticipated increases in cell
   requirements.  The Scheduling Algorithm collects the cell allocation/
   deallocation requests from the neighbors and the number of cells
   which are currently under usage.  First, the Cell Estimation
   Algorithm calculates the number of required cells and second, the
   calculated number is transferred to the Allocation Policy.  In order
   to reduce consumption, this algorithm is triggered only when there is
   a change on the number of used cells from a particular node.

6.1.  SF0 Triggering Events

   We RECOMMEND SF0 to be triggered at by the following event: If there
   is a change on the number of used cells towards any of the
   neighbours.  The exact mechanism of when SF0 is triggered is
   implementation-specific.

6.2.  SF0 Cell Estimation Algorithm

   The Cell Estimation Algorithm takes into account the number of
   current used cells to the neighbour.  This allows the algorithm to
   estimate a new number of cells to be scheduled to the neighbour.  As




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   a consequence, the Cell Estimation Algorithm for SF0 follows the
   steps described below:

   1.  Collect the current number of used cells to the neighbour
   2.  Calculate the new number of cells to be scheduled to the
       neighbour by adding the current number of used cells plus an
       OVERPROVISION number of cells
   3.  Transfer the request to the allocation policy as REQUIREDCELLS
   4.  Return to step 1 and wait for a triggering event.

   The Cell Estimation Algorithm is depicted on figure Figure 1.  The
   OVERPROVISION parameter is calculated as a percentage of the number
   of currently scheduled cells to the neighbour.  OVERPROVISION is
   added to the amount of used cells to the neighbour to reduce the
   probability of packet loss given a sudden growth on the number of
   used cells to the neighbour.  The OVERPROVISION value is
   implementation-specific.  A value of OVERPROVISION equal to zero
   leads to queue growth and possible packet loss: In this case, there
   are no overprovisioned cells where a sudden growth on the number of
   cells can absorbed and detected.































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      +-------------------+
      |   Triggering      |
      |   Event           |<-----+
      |                   |      |
      +-------------------+      |
                |                |
                V                |
      +-------------------+      |
      | Collect number of |      |
      | used cells        |      |
      +-------------------+      |
                |                |
                V                |
      +-------------------+      |
      |   used cells      |      |
      |       +           |      |
      |  OVERPROVISION    |      |
      |       =           |      |
      |  REQUIREDCELLS    |      |
      +-------------------+      |
                |                |
                V                |
      +-------------------+      |
      |  REQUIREDCELLS    |      |
      |         |         |      |
      |         V         |------+
      |  Allocation       |
      |  Policy           |
      +-------------------+

                  Figure 1: The SF0 Estimation Algorithm

6.3.  SF0 Allocation Policy

   The "Allocation Policy" is the set of rules used by SF0 to decide
   when to add/delete cells to a particular neighbor to satisfy the cell
   requirements.

   SF0 uses the following parameters:

   SCHEDULEDCELLS:  The number of cells scheduled from the current node
      to a particular neighbor.
   REQUIREDCELLS:  The number of cells calculated by the Cell Estimation
      Algorithm from the current node to that neighbor.
   SF0THRESH:  Threshold parameter introducing cell over-provisioning in
      the allocation policy.  It is a non-negative value expressed as
      number of cells.  The definition of this value is implementation-




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      specific.  A setting of SF0THRESH>0 will cause the node to
      allocate at least SF0THRESH cells to each of its' neighbors.

   The SF0 allocation policy compares REQUIREDCELLS with SCHEDULEDCELLS
   and decides to add/delete cells taking into account SF0THRESH.  This
   is illustrated in Figure 2.  The number of cells to be added/deleted
   is out of the scope of this document and it is implementation-
   dependent.

                   SCHEDULEDCELLS
       <--------------------------------->
      +---+---+---+---+---+---+---+---+---+
      |   |   |   |   |   |   |   |   |   |
      +---+---+---+---+---+---+---+---+---+
                      |<----------------->|
                      |     SF0THRESH     |
                      |                   |
      REQUIREDCELLS   |                   |
      +---+---+       |                   |       DELETE
      |   |   |       |                   |       ONE/MORE
      +---+---+       |                   |       CELLS
                      |                   |
          REQUIREDCELLS                   |
      +---+---+---+---+---+---+           |       DO
      |   |   |   |   |   |   |           |       NOTHING
      +---+---+---+---+---+---+           |
                      |                   |
                  REQUIREDCELLS           |
      +---+---+---+---+---+---+---+---+---+---+   ADD
      |   |   |   |   |   |   |   |   |   |   |   ONE/MORE
      +---+---+---+---+---+---+---+---+---+---+   CELLS

                    Figure 2: The SF0 Allocation Policy

   1.  If REQUIREDCELLS<(SCHEDULEDCELLS-SF0THRESH), delete one or more
       cells.
   2.  If (SCHEDULEDCELLS-SF0THRESH)<=REQUIREDCELLS<=SCHEDULEDCELLS, do
       nothing.
   3.  If SCHEDULEDCELLS<REQUIREDCELLS, add one or more cells.

   When SF0THRESH equals 0, any discrepancy between REQUIREDCELLS and
   SCHEDULEDCELLS triggers an action to add/delete cells.  Positive
   values of SF0THRESH reduce the number of 6P Transactions.  The number
   of cells to add or delete is implementation-specific.







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7.  Rules for CellList

   There are two methods to define the CellList: The Whitelist method,
   which fills the CellList with the number of proposed cells to the
   neighbour, and the Blacklist, which fills the CellList with the cells
   which cannot be used by the neighbour.  The rule to select the method
   is implementation-specific.  When issuing a 6top ADD Request, SF0
   executes the following sequence:

      Whitelist case:

         The Transaction Source node: Prepares the CellList field by
         selecting randomly the required cells, verifying that the slot
         offset is not occupied and choose channelOffset randomly for
         each cell.
         The Transaction Destination node: Goes through the cells in the
         CellList in order, verifying whether there are no slotOffset
         conflicts.
      Blacklist case:

         The Transaction Source node: Prepares the CellList field by
         building a list of currently scheduled cells into the CellList.
         The Transaction Destination node: Selects randomly the required
         cells from the unallocated cells on the schedule, verifying
         that the slot offset is not occupied from the ones on the
         CellList.

   SF0 does not include any transaction retry process.  If the
   transaction is not successful, SF0 will be retriggered on the next
   slotframe if the number of used cells changes.

8.  6P Timeout Value

   The timeout value is implementation-specific.  The timeout value MAY
   be different for each transaction and each neighbour.  The timeout
   range is from 0 to 128.  The timeout MUST be added as an 7-bit on the
   Metadata header to the neighbour.  There is no measurement unit
   associated to the timeout value.  If the timeout expires, the node
   issues a RESET return code will be issued to the neighbour.  SF0 has
   no retry policy.  Timeout examples are depicted on Figure 3 and
   Figure 4.

   |Timeout Value-----------------------------------------------------|
   |A|------First Exchange-------->|B|-----Second Exchange----->|A|
   |Complete transaction------------------------------------------|

      Figure 3: Example Transaction where the timeout does not expire




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   |Timeout Value----------------------------------------------|
   |A|------First Exchange-------->|B|-----Second Exchange----->|A|
   |Non-Complete transaction--------------------------------------|

          Figure 4: Example Transaction where the timeout expires

9.  Meaning of Metadata Information

   The Metadata 16-bit field is used as follows:

      BITS 0-7 [SLOTFRAME] are used to identify the slotframe number
      BITS 8-14 [TIMEOUT] represents the Timeout value
      BIT 15 [WBLIST] is used to indicate that the CellList provided is
      a Whitelist (value=0) or a Blacklist (value=1).

10.  Node Behavior at Boot

   In order to define a known state after the node is restarted, a CLEAR
   command is issued to each of the neighbor nodes to enable a new
   allocation process and at least a SF0THRESH number of cells MUST be
   allocated to each of the neighbours.

11.  Cell Type

   SF0 uses TX (Transmission) cell type only, thus defining celloptions
   as TX=0, RX=1 and S=0 according to section 4.2.6 of
   [I-D.ietf-6tisch-6top-protocol].

12.  SF0 Statistics

   Packet Delivery Rate (PDR) is calculated per cell, as the percentage
   of acknowledged packets, for the last 10 packet transmission
   attempts.  There is no retransmission policy on SF0.

13.  Relocating Cells

   Allocated cells may experience packet loss from different sources,
   such as noise, interference or cell collision (after the same cell is
   allocated by other nodes in range on the network).

   SF0 uses Packet Delivery Rate (PDR) statistics to monitor the
   currently allocated cells for cell relocation (by changing their
   slotOffset and/or channelOffset).  When the PDR of one or more
   softcells is below PDR_THRESHOLD, SF0 relocates each of the cell(s)
   to a number of available cells selected randomly.  PDR_THRESHOLD is
   out of the scope of this document and it is implementation-dependent.





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14.  Forced Cell Deletion Policy

   When all the cells are scheduled, we need a policy to free cells, for
   example, under alarm conditions or if a node disappears from the
   neighbor list.  The action to follow this condition is out of scope
   of this document and it is implementation-dependent.

15.  6P Error Handling

   A node implementing SF0 handles a 6P Response depending on the Return
   Code it contains:

   RC_SUCCESS:
      If the number of elements in the CellList is the number of cells
      specified in the NumCells field of the 6P ADD Request, the
      operation is complete.  The node does not take further action.
      If the number of elements in the CellList is smaller (possibly 0)
      than the number of cells specified in the NumCells field of the 6P
      ADD Request, the neighbor has received the request, but less than
      NumCells of the cells in the CellList were allocated.  In that
      case, the node MAY retry immediately with a different CellList if
      the amount of storage space permits, or build a new (random)
      CellList.
   RC_ERR_VER:  The node MUST NOT retry immediately.  The node MAY add
      the neighbor node to a blacklist.  The node MAY retry to contact
      this neighbor later.
   RC_ERR_SFID:  The node MUST NOT retry immediately.  The node MAY add
      the neighbor node to a blacklist.  The node MAY retry to contact
      this neighbor later.
   RC_ERR_GEN:  The node MUST issue a CLEAR command to the neighbour.
   RC_ERR_BUSY:  Wait for a timeout and restart the scheduling process.
   RC_ERR_NORES:  Wait for a timeout and restart the scheduling process.
   RC_ERR_RESET:  Abort 6P Transaction
   RC_ERR:  Abort 6P Transaction.  The node MAY retry to contact this
      neighbor later.

16.  Examples

   TODO

17.  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



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   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
   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:  This specification is implemented in the OpenWSN project
      [OpenWSN].  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.

18.  Security Considerations

   TODO

19.  IANA Considerations

   o  IANA_6TiSCH_SFID_SF0

20.  6P Compliance

   o  MUST specify an identifier for that SF.  OK
   o  MUST specify the rule for a node to decide when to add/delete one
      or more cells to a neighbor.  OK
   o  MUST specify the rule for a Transaction source to select cells to
      add to the CellList field in the 6P ADD Request.  OK
   o  MUST specify the rule for a Transaction destination to select
      cells from CellList to add to its schedule.  OK
   o  MUST specify a value for the 6P Timeout, or a rule/equation to
      calculate it.  OK
   o  MUST specify a meaning for the "Metadata" field in the 6P ADD
      Request.  OK
   o  MUST specify the behavior of a node when it boots.  OK
   o  MUST specify what to do after an error has occurred (either the
      node sent a 6P Response with an error code, or received one).  OK
   o  MUST specify the list of statistics to gather.  An example
      statistic if the number of transmitted frames to each neighbor.




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      In case the SF requires no statistics to be gathered, the specific
      of the SF MUST explicitly state so.  OK
   o  SHOULD clearly state the application domain the SF is created for.
      OK
   o  SHOULD contain examples which highlight normal and error
      scenarios.
   o  SHOULD contain a list of current implementations, at least during
      the I-D state of the document, per [RFC6982].
   o  SHOULD contain a performance evaluation of the scheme, possibly
      through references to external documents.
   o  MAY redefine the format of the CellList? field.  OK

21.  Acknowledgments

   Thanks to Kris Pister for his contribution in designing the default
   Bandwidth Estimation Algorithm.  Thanks to Qin Wang and Thomas
   Watteyne for their support in defining the interaction between SF0
   and the 6top sublayer.

   This work is partially supported by the Fondecyt 1121475 Project, the
   Inria-Chile "Network Design" group, and the IoT6 European Project
   (STREP) of the 7th Framework Program (Grant 288445).

22.  References

22.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

22.2.  Informative References

   [I-D.ietf-6tisch-6top-protocol]
              Wang, Q., Vilajosana, X., and T. Watteyne, "6top Protocol
              (6P)", draft-ietf-6tisch-6top-protocol-07 (work in
              progress), June 2017.

   [I-D.ietf-6tisch-terminology]
              Palattella, M., Thubert, P., Watteyne, T., and Q. Wang,
              "Terminology in IPv6 over the TSCH mode of IEEE
              802.15.4e", draft-ietf-6tisch-terminology-09 (work in
              progress), June 2017.







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   [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,
              <http://www.rfc-editor.org/info/rfc6982>.

Appendix A.  [TEMPORARY] Changelog

   o  draft-ietf-6tisch-6top-sf0-02

      *  Editorial changes (figs, typos, ...)
   o  draft-ietf-6tisch-6top-sf0-03

      *  Fixed typos
      *  Removed references to "effectively used cells"
      *  Changed Cell Estimation Algorithm to the third proposed
         alternative on IETF97
      *  Forced cell deletion becomes implementation specific
      *  Added PDR calculation formula
      *  Added PDR_THRESHOLD as implementation specific value

Authors' Addresses

   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


   Luigi Alfredo Grieco
   Politecnico di Bari
   Department of Electrical and Information Engineering
   Via Orabona 4
   Bari  70125
   Italy

   Phone: 00390805963911
   Email: a.grieco@poliba.it



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   Maria Rita Palattella
   Luxembourg Institute of Science and Technology (LIST)
   Department 'Environmental Research and Innovation' (ERIN)
   41, rue du Brill
   Belvaux  L-4422
   Grand-duchy of Luxembourg

   Phone: +352 275 888-5055
   Email: mariarita.palattella@list.lu


   Nicola Accettura
   LAAS-CNRS
   7, avenue du Colonel Roche
   Toulouse  31400
   France

   Phone: +33 5 61 33 69 76
   Email: nicola.accettura@laas.fr
































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