Internet DRAFT - draft-dujovne-6tisch-on-the-fly
draft-dujovne-6tisch-on-the-fly
6TiSCH D. Dujovne, Ed.
Internet-Draft Universidad Diego Portales
Intended status: Informational LA. Grieco
Expires: January 5, 2016 Politecnico di Bari
MR. Palattella
University of Luxembourg
N. Accettura
University of California Berkeley
July 4, 2015
6TiSCH On-the-Fly Scheduling
draft-dujovne-6tisch-on-the-fly-06
Abstract
This document describes the environment, problem statement, and goals
of On-The-Fly (OTF) scheduling, a Layer-3 mechanism for 6TiSCH
networks. The purpose of OTF is to dynamically adapt the aggregate
bandwidth, i.e., the number of reserved soft cells between neighbor
nodes, based on the specific application constraints to be satisfied.
When using OTF, softcell reservation is distributed: through the 6top
interface, neighbor nodes negotiate the cell(s) to be (re)allocated/
deleted, with no intervention needed of a centralized entity. This
document aims at defining a module which uses the functionalities
provided by the 6top sublayer to (i) extract statistics and (ii)
determine when to reserve/delete soft cells in the schedule. The
exact reservation and deletion algorithm, and the number and type of
statistics to be used in the algorithm are out of scope. OTF deals
only with the number of softcells to be reserved/deleted; it is up to
6top to select the specific soft cells within the TSCH schedule.
Status of This Memo
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This Internet-Draft will expire on January 5, 2016.
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Copyright Notice
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document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Allocation policy . . . . . . . . . . . . . . . . . . . . . . 3
3. Allocation method . . . . . . . . . . . . . . . . . . . . . . 6
4. Cell and Bundle Reservation/Deletion . . . . . . . . . . . . 6
5. Getting statistics and other information about cells through
6top . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
6. Events triggering algorithms in OTF . . . . . . . . . . . . . 8
7. Bandwidth Estimation Algorithms . . . . . . . . . . . . . . . 9
8. OTF external CoAP interface . . . . . . . . . . . . . . . . . 10
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 11
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
10.1. Informative References . . . . . . . . . . . . . . . . . 11
10.2. External Informative References . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
The IEEE802.15.4e standard [IEEE802154e] was published in 2012 as an
amendment to the Medium Access Control (MAC) protocol defined by the
IEEE802.15.4-2011 [IEEE802154] standard. The Timeslotted Channel
Hopping (TSCH) mode of IEEE802.15.4e is the object of this document.
On-The-Fly (OTF) scheduling is a 1-hop protocol with which a node
negotiates the number of soft cells scheduled with its neighbors,
without requiring any intervention of a centralized entity (e.g., a
PCE). This document describes the OTF allocation policies and
methods used by two neighbors to allocate one or more softcells in a
distribution fashion. It also proposes an algorithm for estimating
the required bandwidth (BW). This document defines the interface
between OTF and the 6top sublayer ([I-D.wang-6tisch-6top]), to
collect and retrieve statistics, or allocate/delete soft cells. It
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also defines two threshold values for bounding the number of
triggered 6top allocate/delete commands. This document defines a
framework; the algorithm and statistics used are out of scope. This
draft follows the terminology defined in
[I-D.ietf-6tisch-terminology] and addresses the open issue related to
the scheduling mechanisms raised in [RFC7554].
2. Allocation policy
OTF is a distributed scheduling protocol which increases/decreases
the bandwidth between two neighbor nodes (i.e., adding/deleting soft
cells) by interacting with the 6top sublayer. It retrieves
statistics from 6top, and uses that information to trigger 6top to
add/delete softcells to a particular neighbor. The algorithm which
decides how many softcells to add/delete is out of scope. For
example, OTF might decide to add a cell if some queue of outbound
frames is overflowing. Similarly, OTF can delete cells when the
queue has been empty for some time. OTF only triggers 6top to add/
delete the soft cells, it is the responsibility of the 6top sublayer
to determine the exact slotOffset/channelOffset of those cells. In
this document, the term "cell" and "soft cell" are used
interchangeably.
OTF is a Layer-3 Mechanism, and as such, it operates on L3 links, on
the best effort track, i.e. with TrackID=00, as defined in
[I-D.wang-6tisch-6top]. Inside an intermediate node, a track is
uniquely associated with only one bundle: the outgoing bundle. For
an IP link, the bundle is identified by the peer mac address. For
instance (macA, macB, TrackID=00) will be the bundle associated to
the L3 link between node A and node B. The cells on the best effort
track can be used for forwarding any packet in the queue, regardless
of the specific L2 bundle (and thus, end-to-end L2 track) the packet
belongs to. OTF manages the global bandwidth requirements between
two neighbor nodes; per-track management is currently out of scope.
OTF is prone to schedule collisions. Nodes might not be aware of the
cells allocated by other pairs of nodes. A schedule collision occurs
when the same cell is allocated by different pairs in the same
interference space. The probability of having allocation collision
may be kept low by grouping cells into chunks (see
[I-D.ietf-6tisch-terminology] and [I-D.ietf-6tisch-architecture] for
more details). The use of chunks is outside the scope of this
current version of the OTF draft.
The "allocation policy" is the algorithm used by OTF to decide when
to increase/decrease the bandwidth allocated between two neighbor
nodes in order to satisfy the traffic requirements. These
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requirements can be expressed in terms of throughput, latency or
other constraints.
This document introduces the following parameters for describing the
behavior of the OTF allocation policy:
SCHEDULEDCELLS: The amount of soft cells scheduled in a bundle on
the best effort track between two neighbors.
REQUIREDCELLS: Number of cells requested by OTF to 6top, a non-
negative value. How this is computed is out of the scope. It MAY
be an instantaneous request, or a value averaged on several
measurements.
OTFTHRESHLOW: Threshold parameter introducing cell over-provisioning
in the allocation policy. It is a non-negative value expressed as
number of cells. Which value to use is application-specific and
out of OTF scope.
OTFTHRESHHIGH: Threshold parameter introducing cell under-
provisioning in the allocation policy. It is a non-negative value
expressed as number of cells. Which value to use is application-
specific and out of OTF scope.
The OTF allocation policy compares the number of required cells
against the number of scheduled ones, using the OTF threshold for
bounding the signaling overhead due to negotiations of new cells. In
details:
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SCHEDULEDCELLS
<------------------------------------->
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| | | | | | | | | | | | | | |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
|<----------------->|<------------->|
| OTFTHRESHLOW | OTFTHRESHHIGH |
| | |
REQUIREDCELLS | | |
+---+---+---+ | | | ADD
| | | | | | | SOME
+---+---+---+ | | | CELLS
| | |
REQUIREDCELLS | |
+---+---+---+---+---+---+---+ | | DO
| | | | | | | | | | NOTHING
+---+---+---+---+---+---+---+ | |
| | |
REQUIREDCELLS | |
+---+---+---+---+---+---+---+---+---+---+---+ | DO
| | | | | | | | | | | | | NOTHING
+---+---+---+---+---+---+---+---+---+---+---+ |
| | |
| REQUIREDCELLS | |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ REMOVE
| | | | | | | | | | | | | | | | SOME
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ CELLS
Figure 1: Relation among the OTF parameters used for triggering 6top
add/remove soft cell commands
1. If REQUIREDCELLS is greater than (SCHEDULEDCELLS +
OTFTHRESHHIGH), OTF asks 6top to add one or more soft cells to
the bundle on the best effort track.
2. If REQUIREDCELLS is greater or equal than (SCHEDULEDCELLS -
OTFTHRESHLOW), and it is lower than or equal to (SCHEDULEDCELLS +
OTFTHRESHHIGH), OTF does not perform any bundle resizing, since
the scheduled cells are sufficient for managing the current
traffic conditions.
3. If REQUIREDCELLS is lower than (SCHEDULEDCELLS - OTFTHRESHLOW),
OTF asks 6top to delete one or more soft cells from the bundle on
the best-effort track.
When both OTFTHRESHLOW and OTFTHRESHHIGH are equal to 0, any
discrepancy between REQUIREDCELLS and SCHEDULEDCELLS triggers a 6top
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negotiation of soft cells. Other values for the thresholds,
different from 0, reduce the number of triggered 6top negotiations.
The number of soft cells to be scheduled/deleted for bundle resizing
is out of the scope of this document and implementation-dependant.
3. Allocation method
Beyond the allocation policies that describe the approach used by OTF
for fulfilling the node bandwidth requests, the OTF framework also
includes the Allocation Method that specify how OTF issues commands
to the 6top sublayer. As specified in [I-D.wang-6tisch-6top], 6top
provides a set of commands that allows OTF to allocate/delete soft
cells. Such commands are used by the OTF soft cell allocation
method.
With the soft cell allocation method, OTF can ask 6top to reserve one
(or N > 1) soft cell(s) on the best effort L3 boundle, between two
neighbor nodes. The 6top layer allocates and maintains these cells.
If a L3 bundle with TrackID=00 was already reserved between the same
pair of neighbors, 6top translates the OTF request into a bundle
resize request. The newly allocated cell increases the size of the
already existing bundle. Similarly, when OTF realizes there is a
reduction of traffic exchanged between the two neighbors, it may asks
6top to delete a softcell (or N > 1) from the best effort track, i.e.
to decrease the size of the best effort L3 bundle. If no bundle with
TrackID=00 exists when 6top receives the OTF request, then the 6top
softcell create command generates a new bundle of size 1.
4. Cell and Bundle Reservation/Deletion
In order to reserve/delete softcells, OTF interacts with 6top
sublayer. To this aim OTF uses the following set of commands offered
by 6top: CREATE.softcell, and DELETE.softcell. When creating
(deleting) a softcell, OTF specifies the track the cell belongs to
(i.e., best effort track, TrackID=00), but not its slotOffset nor the
channelOffset. If at least one cell on the best effort L3 bundle
already exists, the CREATE.softcell and DELETE.softcell, translate
into INCREASE and DECREASE the bundle size, respectively. 6top is
responsible for picking the specific cell to be added/deleted within
the bundle. Before being able to do so, source and destination nodes
go through a cell negotiation process. This process is out of scope
of 6top and OTF. By using the CREATE.softcell command, OTF can ask
6top to add multiple softcells on the best effort L3 bundle.
Following OTF request, 6top either (i) creates a new bundle, if no
cells were reserved already on the best effort track, or (ii)
increases the L3 bundle size of the already existing best-effort
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bundle. By using the DELETE.softcell command, OTF can ask 6top to
delete cells from the best effort bundle.
OTF provides a policy for 6top to generate CREATE/DELETE.softcells
commands, policy that is out of 6top scope [I-D.wang-6tisch-6top].
Such policy is not the only one that can be used by 6top. Others may
be defined in the future.
5. Getting statistics and other information about cells through 6top
Statistics are kept in 4 data structures of 6top MIB: CellList,
MonitoringStatusList, NeighborList, and QueueList.
CellList provides per-cell statistics. From this list, an upper
layer can get per-bundle statistics. OTF may have access to the
CellList, by using the CoAP-YANG Model, but actually cell-specific
statistics are not significant to OTF, since softcells can be re-
allocated in time by 6top itself, based on network conditions.
MonitoringStatusList provides per-neighbor and slotframe statistics.
From it an upper layer (e.g., OTF) can get per bundle overview of
scheduling and its performance. Such list contains information about
the number of hard and soft cells reserved to a given node with a
specific neighbor, and the QoS (that can be expressed in form of
different metrics: PDR, ETX, RSSI, LQI) on the actual bandwidth, and
the over-provisioned bandwidth (which includes the over-provisioned
cells). 6top can use such list to operate 6top Monitoring Functions,
such as re-allocating cells (by changing their slotOffset and/or
channelOffset) when it finds out that the link quality of some
softcell is much lower than average. Unlike 6top, OTF does not
operate any re-allocation of cells. In fact, OTF can ask for more/
less bandwidth, but cannot move any cell within the schedule. Thus,
the 6top Monitoring function is useful to OTF, because it can provide
better cells for a given bandwidth requirement, specified by OTF.
For instance, OTF may require some additional bandwidth (e.g. 2 cells
in a specific slotframe) with PDR = 75%; then, 6top will reserve 3
slots in the slotframe to meet the bandwidth requirement. In
addition, when the link quality drops to 50%, 6top will reserve 4
slots to keep meeting the bandwidth requirement. Given that OTF
operates on the global bandwidth between two neighbor nodes, it does
not need to be informed from 6top about cells' re-allocation.
NeighborList provides per-neighbor statistics. From it, an upper
layer can understand the connectivity of a pair of nodes, e.g. based
on the queue length increase, OTF may ask 6top to add some cells, in
order to increase the available bandwidth.
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QueueList provides per-Queue statistics. From it, an upper layer can
know the traffic load. OTF, based on such queue statistics (e.g.,
average length of the queue, average age of the packet in queue,
etc.) may trigger a 6top CREATE.softcell (DELETE.softcell) command
for increasing (decreasing) the bandwidth and be able to better serve
the packets in the queue.
6. Events triggering algorithms in OTF
The Algorithms running within OTF MUST be event-oriented. As a
consequence, OTF requires to connect the algorithms with external
events to trigger their execution. The algorithm also generates one
or more events when it is executed, such as a new soft cell
allocation. Both type of events, the one which triggers the
algorithm and the ones which are generated by the execution of the
algorithm are called OTF events. We define the following elements:
A set of parameters P(E): parameters used to define E and its
triggering conditions;
a set of triggering variables V(E): variables that can trigger the
event;
a set of triggering conditions C(E): conditions to satisfy on the
variables V(E) to trigger E;
a set of process handlers H(E): handlers required to respond and
process the triggering conditions C(E).
To illustrate how P(E), V(E), C(E) and H(E) can be used to define a
real event, the allocation policy described in Sec. 2 is considered
hereby.
P(E) consists of the OTFTHRESHLOW and OTFTHRESHHIGH parameters (P1
and P2, respectively);
V(E) consists of the REQUIREDCELLS and SCHEDULEDCELLS parameters
(V1 and V2, respectively);
C(E) consists of the following conditions:
C1: V1 > V2+P2
C2: V1 <=V2-P1
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H(E) consists of the following handlers (one handler for each
triggering condition)
H1(C1): OTF asks 6top to add one or more soft cells to the L3
best effort bundle.
H2(C2): OTF asks 6top to delete one or more soft cells from the
L3 best effort bundle.
7. Bandwidth Estimation Algorithms
OTF supports different bandwidth estimation algorithms that can be
used by a node in a 6TiSCH network for checking the statistics
provided by 6top and the actual bandwidth usage. By doing so, one
can adapt (increase or decrease) the number of scheduled soft cells
for a given pair of neighbors (e.g., parent node and its child),
according to their specific requirements. OTF supports several
bandwidth estimation algorithms numbered 0 to 255 in the OTF
implementation. The first algorithm (0) is reserved to the default
algorithm that is described below. By using SET and GET commands,
one can set the specific algorithm to be used, and get information
about which algorithm is implemented.
The steps of the default bandwidth estimation algorithm, running over
a parent node, are listed hereafter:
Step 1: Collect the bandwidth requests from child nodes (incoming
bundle soft cell allocation from 6top-to-6top negotiation).
Step 2: Collect the node bandwidth requirement from the application
(self/local traffic, from the application soft cell pending
requests).
Step 3: Collect the current outgoing scheduled bandwidth (outgoing
traffic).
Step 4: If (outgoing < incoming + self) then SCHEDULE soft cells to
satisfy bandwidth requirements.
Step 5: If (outgoing > incoming + self) then DELETE the soft cells
that are not used.
Step 6: Return to step 1.
The default bandwidth estimation algorithm introduced in this
document adopts a reactive allocation policy, i.e., it uses
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OTFTHRESHLOW = 0 and OTFTHRESHHIGH = 0; the histeresys is not enabled
and the allocation/deallocation follows directly. The algorithm is
triggered either by Step 4 or Step 5.
8. OTF external CoAP interface
In order to select the current OTF algorithm and provide functional
parameters from outside OTF, this module uses CoAP with YANG as the
data model. The algorithm number and the parameters MUST be invoked
in different CoAP calls.
The path to select the algorithm is '6t/e/otf/alg' with A as the
algorithm number.
+------------------------------------------+
Header | POST |
+------------------------------------------+
Uri-Path| /6t/e/otf/alg |
+------------------------------------------+
Options | CBOR( {AlgNo: 123} ) |
+------------------------------------------+
Figure 2: Algorithm number POST message
To obtain the current algorithm number:
+------------------------------------------+
Header | GET |
+------------------------------------------+
Uri-Path| /6t/e/otf/alg |
+------------------------------------------+
Options | Accept: application/cbor |
+------------------------------------------+
Figure 3: Algorithm number GET message
An example is: 'coap://[aaaa::1]/6t/e/otf/alg'
The current algorithm parameter path is '6t/e/otf/alg/par'.
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+------------------------------------------+
Header | POST |
+------------------------------------------+
Uri-Path| /6t/e/otf/alg/par |
+------------------------------------------+
Options | CBOR( {Par: 0x1234} ) |
+------------------------------------------+
Figure 4: Algorithm number POST message
An example follows: 'coap://[aaaa::1]/6t/e/otf/alg/par'
9. Acknowledgments
Special thanks to Prof. Kris Pister for his valuable contribution in
designing the default Bandwidth Estimation Algorithm, and to Prof.
Qin Wang for her support in defining the interaction between OTF and
6top sublayer.
Thanks to the Fondecyt 1121475 Project, to INRIA Chile "Network
Design" group and to the IoT6 European Project (STREP) of the 7th
Framework Program (Grant 288445).
10. References
10.1. Informative References
[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-04 (work in
progress), March 2015.
[I-D.ietf-6tisch-architecture]
Thubert, P., "An Architecture for IPv6 over the TSCH mode
of IEEE 802.15.4", draft-ietf-6tisch-architecture-08 (work
in progress), May 2015.
[RFC7554] Watteyne, T., 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,
May 2015.
[I-D.wang-6tisch-6top]
Wang, Q., Vilajosana, X., and T. Watteyne, "6TiSCH
Operation Sublayer (6top)", draft-wang-6tisch-6top-00
(work in progress), October 2013.
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10.2. External Informative References
[IEEE802154e]
IEEE standard for Information Technology, "IEEE std.
802.15.4e, Part. 15.4: Low-Rate Wireless Personal Area
Networks (LR-WPANs) Amendament 1: MAC sublayer", April
2012.
[IEEE802154]
IEEE standard for Information Technology, "IEEE std.
802.15.4, Part. 15.4: Wireless Medium Access Control (MAC)
and Physical Layer (PHY) Specifications for Low-Rate
Wireless Personal Area Networks", June 2011.
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
University of Luxembourg
Interdisciplinary Centre for Security, Reliability and Trust
4, rue Alphonse Weicker
Luxembourg L-2721
LUXEMBOURG
Phone: (+352) 46 66 44 5841
Email: maria-rita.palattella@uni.lu
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Nicola Accettura
University of California Berkeley
Berkeley Sensor & Actuator Center
490 Cory Hall
Berkeley, California 94720
USA
Email: nicola.accettura@eecs.berkeley.edu
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