6TiSCH D. Dujovne, Ed. Internet-Draft Universidad Diego Portales Intended status: Experimental LA. Grieco Expires: October 30, 2017 Politecnico di Bari MR. Palattella Luxembourg Institute of Science and Technology (LIST) N. Accettura LAAS-CNRS April 28, 2017 6TiSCH 6top Scheduling Function Zero (SF0) draft-ietf-6tisch-6top-sf0-04 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." Dujovne, et al. Expires October 30, 2017 [Page 1] Internet-Draft 6tisch-6top-sf0 April 2017 This Internet-Draft will expire on October 30, 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 (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. 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 Dujovne, et al. Expires October 30, 2017 [Page 2] Internet-Draft 6tisch-6top-sf0 April 2017 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 Dujovne, et al. Expires October 30, 2017 [Page 3] Internet-Draft 6tisch-6top-sf0 April 2017 (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 Dujovne, et al. Expires October 30, 2017 [Page 4] Internet-Draft 6tisch-6top-sf0 April 2017 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. Dujovne, et al. Expires October 30, 2017 [Page 5] Internet-Draft 6tisch-6top-sf0 April 2017 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 Dujovne, et al. Expires October 30, 2017 [Page 6] Internet-Draft 6tisch-6top-sf0 April 2017 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]. Dujovne, et al. Expires October 30, 2017 [Page 7] Internet-Draft 6tisch-6top-sf0 April 2017 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. Dujovne, et al. Expires October 30, 2017 [Page 8] Internet-Draft 6tisch-6top-sf0 April 2017 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 Dujovne, et al. Expires October 30, 2017 [Page 9] Internet-Draft 6tisch-6top-sf0 April 2017 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 Dujovne, et al. Expires October 30, 2017 [Page 10] Internet-Draft 6tisch-6top-sf0 April 2017 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . 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, . [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., Vilajosana, X., and T. Watteyne, "6top Protocol (6P)", draft-ietf-6tisch-6top-protocol-04 (work in progress), March 2017. [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 Dujovne, et al. Expires October 30, 2017 [Page 11] Internet-Draft 6tisch-6top-sf0 April 2017 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 Dujovne, et al. Expires October 30, 2017 [Page 12]