6TiSCH D. Dujovne, Ed.
Internet-Draft Universidad Diego Portales
Intended status: Standards Track LA. Grieco
Expires: September 14, 2017 Politecnico di Bari
MR. Palattella
Luxembourg Institute of Science and Technology (LIST)
N. Accettura
LAAS-CNRS
March 13, 2017
6TiSCH 6top Scheduling Function Zero (SF0)
draft-ietf-6tisch-6top-sf0-03
Abstract
This document defines a Scheduling Function called "Scheduling
Function Zero" (SF0). SF0 dynamically adapts the number of allocated
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 September 14, 2017.
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
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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. SF0 Triggering Events . . . . . . . . . . . . . . . . . . . . 4
5. SF0 Cell Estimation Algorithm . . . . . . . . . . . . . . . . 4
6. SF0 Allocation Policy . . . . . . . . . . . . . . . . . . . . 5
7. Rules for CellList . . . . . . . . . . . . . . . . . . . . . 6
8. 6P Timeout Value . . . . . . . . . . . . . . . . . . . . . . 7
9. Meaning of Metadata Information . . . . . . . . . . . . . . . 7
10. Node Behavior at Boot . . . . . . . . . . . . . . . . . . . . 7
11. Cell Type . . . . . . . . . . . . . . . . . . . . . . . . . . 7
12. SF0 Statistics . . . . . . . . . . . . . . . . . . . . . . . 8
13. Relocating Cells . . . . . . . . . . . . . . . . . . . . . . 8
14. Forced Cell Deletion Policy . . . . . . . . . . . . . . . . . 8
15. 6P Error Handling . . . . . . . . . . . . . . . . . . . . . . 8
16. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 9
17. Implementation Status . . . . . . . . . . . . . . . . . . . . 9
18. Security Considerations . . . . . . . . . . . . . . . . . . . 9
19. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
20. 6P Compliance . . . . . . . . . . . . . . . . . . . . . . . . 10
21. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10
22. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
22.1. Normative References . . . . . . . . . . . . . . . . . . 10
22.2. Informative References . . . . . . . . . . . . . . . . . 11
Appendix A. [TEMPORARY] Changelog . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
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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 for the 6top
sublayer [I-D.ietf-6tisch-6top-protocol], called "Scheduling Function
Zero" (SF0). SF0 is designed to offer the minimal set 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.
The Scheduling Algorithm: A number of TX and/or RX cells must be
allocated between neighbor nodes in order to enable data transmission
among them. A portion of these allocated cells will be utilized by
neighbors, while the remaining cells can be over-provisioned to
handle unanticipated increases in cell requirements. The Scheduling
Algorithm collects the cell allocation/deletion requests from the
neighbors and the number of cells which are currently under usage.
First, the Cell Estimation Algotithm calculates the number of
required cells by adding the collected values and second, the
calculated value is given to the Allocation Policy, which provides
stability by adding hystheresis and overprovisioning by deciding when
to schedule the new number of cells, according to a threshold. In
order to reduce consumption, this algorithm is triggered only when
there is a change on the number of effectively used cells or if there
is a change on the number of requested cells from a particular node.
The Relocation Algorithm: Allocated cells may experience packet loss
from different sources, such as noise, interference or cell collision
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(after the same cell is allocated by other nodes in range on the
network). In order to avoid this problem, Packet Delivery Rate (PDR)
is monitored periodically for each allocated cell. A relocation is
triggered when the PDR value drops below a certain threshold,
compared to the average PDR of the rest of allocated cells. The
destination location on the schedule is defined randomly.
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 4).
2. It applies the Cell Estimation Algorithm (CEA) for a particular
neighbor to determine how many cells are required to that
neighbor (Section 5).
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).
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].
3. Scheduling Function Identifier
The Scheduling Function Identifier (SFID) of SF0 is IANA_SFID_SF0.
4. SF0 Triggering Events
We RECOMMEND SF0 to be triggered at least by the following events:
1. If there is a change in the current number of required cells
2. If there is a successful cell allocation/deallocation process
with the neighbour.
This allows SF0 to be triggered by any change in locally generated or
incoming traffic. The exact mechanism of when SF0 is triggered is
implementation-specific.
5. SF0 Cell Estimation Algorithm
The Cell Estimation Algorithm takes into account the new incoming
cell requirements from the neighbor node and the current outgoing
number of used cells. This allows the algorithm to estimate a new
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number of cells to be allocated. As a consequence, the Cell
Estimation Algorithm for SF0 follows the steps described below:
1. Collect the current number of used cells
2. Calculate the new number of cells to be allocated by adding the
current number of used cells plus an OVERPROVISION number of
cells
3. Submit the request to the allocation policy as REQUIREDCELLS
4. Return to step 1 and wait for a triggering event.
The OVERPROVISION parameter is a percentage of the currently
allocated cells which is added to the used cells to guarantee that
the growth on the number of used cells can be detected without packet
loss. This percentage value is implementation-specific. A value of
OVERPROVISION equal to zero leads to queue growth and possible packet
loss, since there are no overprovisioned cells to detect if there is
a growth on the number of used cells.
6. 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-
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 1. The number of cells to be scheduled/
deleted is out of the scope of this document and it is
implementation-dependent.
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SCHEDULEDCELLS
<--------------------------------->
+---+---+---+---+---+---+---+---+---+
| | | | | | | | | |
+---+---+---+---+---+---+---+---+---+
|<----------------->|
| SF0THRESH |
| |
REQUIREDCELLS | |
+---+---+ | | DELETE
| | | | | ONE/MORE
+---+---+ | | CELLS
| |
REQUIREDCELLS |
+---+---+---+---+---+---+ | DO
| | | | | | | | NOTHING
+---+---+---+---+---+---+ |
| |
REQUIREDCELLS |
+---+---+---+---+---+---+---+---+---+---+ ADD
| | | | | | | | | | | ONE/MORE
+---+---+---+---+---+---+---+---+---+---+ CELLS
Figure 1: 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.
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
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offset and channel offset are 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 and channel offset are not occupied from
the ones on the CellList.
8. 6P Timeout Value
The general timeout equals the equivalent time of the number of slots
until the next scheduled cell. TODO/REMARK: The exact calculation is
currently under discussion on the Mailing List.
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 are RESERVED
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. The 6P Initial Timeout Value provided by SF0
should allow for the maximum number of TSCH link-layer retries, as
defined by Section 4.3.4 of [I-D.ietf-6tisch-6top-protocol]. TODO/
REMARK: The initial timeout is currently under discussion on the
Mailing List.
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].
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12. SF0 Statistics
Packet Delivery Rate (PDR) is calculated per cell, as the quotient of
the number of successfully delivered packets to 10, for the last 10
packet transmission attempts, without counting retransmissions.
13. Relocating Cells
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, defined as a percentage of the
average of the PDR of the rest of the scheduled cells, 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.
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.
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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
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_SFID_SF0
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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.
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
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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,
<http://www.rfc-editor.org/info/rfc2119>.
22.2. Informative References
[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>.
[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-08 (work in
progress), December 2016.
[I-D.ietf-6tisch-6top-protocol]
Wang, Q. and X. Vilajosana, "6top Protocol (6P)", draft-
ietf-6tisch-6top-protocol-03 (work in progress), October
2016.
[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.
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
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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
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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