Internet DRAFT - draft-ietf-6tisch-6top-sfx
draft-ietf-6tisch-6top-sfx
6TiSCH D. Dujovne, Ed.
Internet-Draft Universidad Diego Portales
Intended status: Experimental LA. Grieco
Expires: September 6, 2018 Politecnico di Bari
MR. Palattella
Luxembourg Institute of Science and Technology (LIST)
N. Accettura
LAAS-CNRS
March 5, 2018
6TiSCH Experimental Scheduling Function (SFX)
draft-ietf-6tisch-6top-sfx-01
Abstract
This document defines a Scheduling Function called "Experimental
Scheduling Function" (SFX). SFX 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. SFX 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.
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This Internet-Draft will expire on September 6, 2018.
Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Scheduling Function Identifier . . . . . . . . . . . . . . . 3
3. Allocated and Used Cells . . . . . . . . . . . . . . . . . . 3
4. Overprovisioning . . . . . . . . . . . . . . . . . . . . . . 3
5. Scheduling Algorithm . . . . . . . . . . . . . . . . . . . . 4
5.1. SFX Triggering Events . . . . . . . . . . . . . . . . . . 4
5.2. SFX Cell Estimation Algorithm . . . . . . . . . . . . . . 4
5.3. SFX Allocation Policy . . . . . . . . . . . . . . . . . . 5
6. Rules for CellList . . . . . . . . . . . . . . . . . . . . . 7
7. 6P Timeout Value . . . . . . . . . . . . . . . . . . . . . . 7
8. Meaning of Metadata Information . . . . . . . . . . . . . . . 8
9. Node Behavior at Boot . . . . . . . . . . . . . . . . . . . . 8
10. Cell Type . . . . . . . . . . . . . . . . . . . . . . . . . . 8
11. SFX Statistics . . . . . . . . . . . . . . . . . . . . . . . 8
12. Relocating Cells . . . . . . . . . . . . . . . . . . . . . . 8
13. Forced Cell Deletion Policy . . . . . . . . . . . . . . . . . 9
14. 6P Error Handling . . . . . . . . . . . . . . . . . . . . . . 9
15. Experimental requirements . . . . . . . . . . . . . . . . . . 9
16. Security Considerations . . . . . . . . . . . . . . . . . . . 10
17. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
17.1. SFX Scheduling Function Identifiers . . . . . . . . . . 10
18. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10
19. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
19.1. Normative References . . . . . . . . . . . . . . . . . . 11
19.2. Informative References . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
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1. Introduction
This document defines a Scheduling Function using the 6P protocol
[I-D.ietf-6tisch-6top-protocol], called "Experimental Scheduling
Function" (SFX). SFX is designed to offer a number of
functionalities to be usable in a wide range of applications. SFX
defines two algorithms: the Scheduling Algorithm which defines the
number of cells to allocate/delete between two neighbors, and the
Relocation Algorithm defines when to relocate a cell.
To synthesize, a node running SFX determines when to add/delete cells
in a three-step process:
1. It waits for a triggering event (Section 5.1).
2. It applies the Cell Estimation Algorithm (CEA) for a particular
neighbor to determine how many cells are required to that
neighbor (Section 5.2).
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 5.3).
SFX 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].
2. Scheduling Function Identifier
The Scheduling Function Identifier (SFID) of SFX is
IANA_6TISCH_SFID_SFX.
3. 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.
4. Overprovisioning
Overprovisioning is the action and effect of increasing a value
representing an amount of resources. In the case of SFX,
overprovisioning is done as a provision to reduce traffic variability
effects on packet loss, to the expense of artificially allocating a
number of cells.
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5. 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.
5.1. SFX Triggering Events
We RECOMMEND SFX to be triggered by the following event: if there is
a change on the number of used cells towards any of the neighbors.
The exact mechanism of when SFX is triggered is implementation-
specific.
5.2. SFX Cell Estimation Algorithm
The Cell Estimation Algorithm takes into account the number of
currently used cells to the neighbor. This allows the algorithm to
estimate a new number of cells to be scheduled to the neighbor. As a
consequence, the Cell Estimation Algorithm for SFX follows the steps
described below:
1. Collect the current number of used cells to the neighbor.
2. Calculate the new number of cells to be scheduled to the neighbor
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 in Figure 1. The
OVERPROVISION parameter is calculated as a percentage of the number
of currently scheduled cells to the neighbor. OVERPROVISION is added
to the amount of used cells to the neighbor to reduce the probability
of packet loss given a sudden growth on the number of used cells to
the neighbor. 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 SFX Estimation Algorithm
5.3. SFX Allocation Policy
The "Allocation Policy" is the set of rules used by SFX to decide
when to add/delete cells to a particular neighbor to satisfy the cell
requirements.
SFX 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.
SFXTHRESH: Threshold parameter introducing cell over-provisioning in
the allocation policy. It is a non-negative value expressed as a
number of cells. The definition of this value is implementation-
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specific. A setting of SFXTHRESH>0 causes the node to allocate at
least SFXTHRESH cells to each of its' neighbors.
The SFX allocation policy compares REQUIREDCELLS with SCHEDULEDCELLS
and decides to add/delete cells taking into account SFXTHRESH. 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
<--------------------------------->
+---+---+---+---+---+---+---+---+---+
| | | | | | | | | |
+---+---+---+---+---+---+---+---+---+
|<----------------->|
| SFXTHRESH |
| |
REQUIREDCELLS | |
+---+---+ | | DELETE
| | | | | ONE/MORE
+---+---+ | | CELLS
| |
REQUIREDCELLS |
+---+---+---+---+---+---+ | DO
| | | | | | | | NOTHING
+---+---+---+---+---+---+ |
| |
REQUIREDCELLS |
+---+---+---+---+---+---+---+---+---+---+ ADD
| | | | | | | | | | | ONE/MORE
+---+---+---+---+---+---+---+---+---+---+ CELLS
Figure 2: The SFX Allocation Policy
1. If REQUIREDCELLS<(SCHEDULEDCELLS-SFXTHRESH), delete one or more
cells.
2. If (SCHEDULEDCELLS-SFXTHRESH)<=REQUIREDCELLS<=SCHEDULEDCELLS, do
nothing.
3. If SCHEDULEDCELLS<REQUIREDCELLS, add one or more cells.
When SFXTHRESH equals 0, any discrepancy between REQUIREDCELLS and
SCHEDULEDCELLS triggers an action to add/delete cells. Positive
values of SFXTHRESH reduce the number of 6P Transactions. The number
of cells to add or delete is implementation-specific.
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6. Rules for CellList
There are two methods to define the CellList. The Whitelist method
fills the CellList with the number of proposed cells to the neighbor.
The Blacklist method fills the CellList with the cells which cannot
be used by the neighbor. The rule to select the method is
implementation-specific. When issuing a 6top ADD Request, SFX
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.
SFX does not include any transaction retry process. If the
transaction is not successful, SFX will be retriggered on the next
slotframe if the number of used cells changes.
7. 6P Timeout Value
The timeout value is implementation-specific. The timeout value MAY
be different for each transaction and each neighbor. The timeout
range is from 0 to 128. The timeout MUST be added as an 7-bit on the
Metadata header to the neighbor. 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 neighbor. SFX 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
8. 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).
9. 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 SFXTHRESH number of cells MUST be
allocated to each of the neighbors.
10. Cell Type
SFX uses the 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].
11. SFX 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 SFX.
12. 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).
SFX 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, SFX 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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13. 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.
14. 6P Error Handling
A node implementing SFX 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_EOL: If an LIST command is issued and the RC_EOL is received, the
node MUST understand what is specified on Section 3.3.5 of
[I-D.ietf-6tisch-6top-protocol].
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_SEQNUM: The node MUST issue a CLEAR command to the neighbor.
RC_ERR_BUSY: Wait for a timeout and restart the scheduling process.
RC_ERR_CELLLIST: Wait for a timeout and restart the scheduling
process.
RC_ERR_LOCKED: Wait for a timeout and restart the scheduling
process.
RC_RESET: Abort 6P Transaction.
RC_ERR: Abort 6P Transaction. The node MAY retry to contact this
neighbor later.
15. Experimental requirements
In order to evaluate the performance of this draft, we propose the
following experimental work:
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1. Define values for OVERPROVISION, SFXTHRESH and ranges to the
number of cells to Add or Delete after the Allocation Policy is
applied for typical use cases.
2. Analyze the scheduling stability (in terms of oscillation) and
the hysteresis effect on scheduling using SFX. A tradeoff shall
be found between the reactivity of the algorithm facing new
scheduling requirements and the number of overprovisioned cells.
3. Define the PDR value below the Average which is most effective
for blacklisting cells and a method to whitelist cells. Analyze
the stability and long-term behavior of this algorithm.
4. Measure the distribution of cell scheduling delay (including the
time taken by 6P) to estimate timeouts for different type of
transactions.
16. Security Considerations
SFX is defined as an algorithm designed to efficiently fulfill
bandwidth requirements between neighbour nodes and does not define a
new protocol SFX uses the Minimal IPv6 over the TSCH Mode of IEEE
802.15.4e (6TiSCH) Configuration standardized on [RFC8180] and the
6top Protocol (6P): [I-D.ietf-6tisch-6top-protocol]. SFX relies on
the security framework described on
[I-D.ietf-6tisch-minimal-security].
17. IANA Considerations
17.1. SFX Scheduling Function Identifiers
This document provide a new element to the "6P Scheduling Function
Identifiers" sub-registry, which is 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]. This Subtype is defined on figure
Figure 5
+----------------------+--------------------------+-------------+
| SFID | Name | Reference |
+----------------------+--------------------------+-------------+
| IANA_6TISCH_SFID_SFX | Experimental Scheduling | RFCXXXX |
| | Function (SFX) | (NOTE:this) |
+----------------------+--------------------------+-------------+
Figure 5: IETF IE Subtype '6P'
18. Acknowledgments
Thanks to Kris Pister for his contribution in designing the default
Bandwidth Estimation Algorithm. Thanks to Qin Wang and Thomas
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Watteyne for their support in defining the interaction between SFX
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).
19. References
19.1. 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,
<https://www.rfc-editor.org/info/rfc2119>.
[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>.
19.2. Informative 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.ietf-6tisch-terminology]
Palattella, M., Thubert, P., Watteyne, T., and Q. Wang,
"Terms Used in IPv6 over the TSCH mode of IEEE 802.15.4e",
draft-ietf-6tisch-terminology-10 (work in progress), March
2018.
Authors' Addresses
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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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