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 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." This Internet-Draft will expire on January 5, 2016. Dujovne, et al. Expires January 5, 2016 [Page 1] Internet-Draft 6tisch-on-the-fly July 2015 Copyright Notice Copyright (c) 2015 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. 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 Dujovne, et al. Expires January 5, 2016 [Page 2] Internet-Draft 6tisch-on-the-fly July 2015 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 Dujovne, et al. Expires January 5, 2016 [Page 3] Internet-Draft 6tisch-on-the-fly July 2015 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: Dujovne, et al. Expires January 5, 2016 [Page 4] Internet-Draft 6tisch-on-the-fly July 2015 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 Dujovne, et al. Expires January 5, 2016 [Page 5] Internet-Draft 6tisch-on-the-fly July 2015 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 Dujovne, et al. Expires January 5, 2016 [Page 6] Internet-Draft 6tisch-on-the-fly July 2015 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. Dujovne, et al. Expires January 5, 2016 [Page 7] Internet-Draft 6tisch-on-the-fly July 2015 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 Dujovne, et al. Expires January 5, 2016 [Page 8] Internet-Draft 6tisch-on-the-fly July 2015 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 Dujovne, et al. Expires January 5, 2016 [Page 9] Internet-Draft 6tisch-on-the-fly July 2015 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'. Dujovne, et al. Expires January 5, 2016 [Page 10] Internet-Draft 6tisch-on-the-fly July 2015 +------------------------------------------+ 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. Dujovne, et al. Expires January 5, 2016 [Page 11] Internet-Draft 6tisch-on-the-fly July 2015 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 Dujovne, et al. Expires January 5, 2016 [Page 12] Internet-Draft 6tisch-on-the-fly July 2015 Nicola Accettura University of California Berkeley Berkeley Sensor & Actuator Center 490 Cory Hall Berkeley, California 94720 USA Email: nicola.accettura@eecs.berkeley.edu Dujovne, et al. Expires January 5, 2016 [Page 13]