6TiSCH T. Chang, Ed. Internet-Draft M. Vucinic Intended status: Standards Track Inria Expires: September 12, 2019 X. Vilajosana Universitat Oberta de Catalunya S. Duquennoy RISE SICS D. Dujovne, Ed. Universidad Diego Portales March 11, 2019 6TiSCH Minimal Scheduling Function (MSF) draft-ietf-6tisch-msf-02 Abstract This specification defines the 6TiSCH Minimal Scheduling Function (MSF). This Scheduling Function describes both the behavior of a node when joining the network, and how the communication schedule is managed in a distributed fashion. MSF builds upon the 6TiSCH Operation Sublayer Protocol (6P) and the Minimal Security Framework for 6TiSCH. 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 [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 https://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 September 12, 2019. Chang, et al. Expires September 12, 2019 [Page 1] Internet-Draft 6TiSCH Minimal Scheduling Function (MSF) March 2019 Copyright Notice Copyright (c) 2019 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 (https://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 . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Interface to the Minimal 6TiSCH Configuration . . . . . . . . 4 3. Autonomous Cells . . . . . . . . . . . . . . . . . . . . . . 5 4. Node Behavior at Boot . . . . . . . . . . . . . . . . . . . . 6 4.1. Start State . . . . . . . . . . . . . . . . . . . . . . . 7 4.2. Step 1 - Choosing Frequency . . . . . . . . . . . . . . . 7 4.3. Step 2 - Receiving EBs . . . . . . . . . . . . . . . . . 7 4.4. Step 3 - Setting up Autonomous Cells during Join Process 7 4.5. Step 4 - Installing Autonomous Cells for each neighbor in neighbor table . . . . . . . . . . . . . . . . . . . . . 8 4.6. Step 5 - Acquiring a RPL rank . . . . . . . . . . . . . . 8 4.7. Step 6 - Send EBs and DIOs . . . . . . . . . . . . . . . 8 4.8. Step 7 - Neighbor Polling . . . . . . . . . . . . . . . . 8 4.9. End State . . . . . . . . . . . . . . . . . . . . . . . . 9 5. Rules for Adding/Deleting Cells . . . . . . . . . . . . . . . 9 5.1. Adapting to Traffic . . . . . . . . . . . . . . . . . . . 9 5.2. Switching Parent . . . . . . . . . . . . . . . . . . . . 11 5.3. Handling Schedule Collisions . . . . . . . . . . . . . . 12 6. 6P SIGNAL command . . . . . . . . . . . . . . . . . . . . . . 13 7. Scheduling Function Identifier . . . . . . . . . . . . . . . 13 8. Rules for CellList . . . . . . . . . . . . . . . . . . . . . 13 9. 6P Timeout Value . . . . . . . . . . . . . . . . . . . . . . 14 10. Rule for Ordering Cells . . . . . . . . . . . . . . . . . . . 14 11. Meaning of the Metadata Field . . . . . . . . . . . . . . . . 14 12. 6P Error Handling . . . . . . . . . . . . . . . . . . . . . . 14 13. Schedule Inconsistency Handling . . . . . . . . . . . . . . . 15 14. MSF Constants . . . . . . . . . . . . . . . . . . . . . . . . 15 15. MSF Statistics . . . . . . . . . . . . . . . . . . . . . . . 16 16. Security Considerations . . . . . . . . . . . . . . . . . . . 16 17. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17 17.1. MSF Scheduling Function Identifiers . . . . . . . . . . 17 Chang, et al. Expires September 12, 2019 [Page 2] Internet-Draft 6TiSCH Minimal Scheduling Function (MSF) March 2019 18. References . . . . . . . . . . . . . . . . . . . . . . . . . 17 18.1. Normative References . . . . . . . . . . . . . . . . . . 17 18.2. Informative References . . . . . . . . . . . . . . . . . 18 Appendix A. Contributors . . . . . . . . . . . . . . . . . . . . 18 Appendix B. Example of Implementation of SAX hash function . . . 19 Appendix C. Implementation Status and Performance Evaluation . . 20 Appendix D. [TEMPORARY] Changelog . . . . . . . . . . . . . . . 21 Appendix E. [TEMPORARY] Pending Elements . . . . . . . . . . . . 21 E.1. Introduction . . . . . . . . . . . . . . . . . . . . . . 21 E.2. Performance Related . . . . . . . . . . . . . . . . . . . 21 E.2.1. Handling the case when bandwidth allocation exceeds available capacity . . . . . . . . . . . . . . . . . 21 E.2.2. Adapting 6P Timeout . . . . . . . . . . . . . . . . . 22 E.3. editorial . . . . . . . . . . . . . . . . . . . . . . . . 22 E.3.1. Rules for broadcast frames is out of scope of MSF . . 22 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22 1. Introduction The 6TiSCH Minimal Scheduling Function (MSF), defined in this specification, is a 6TiSCH Scheduling Function (SF). The role of an SF is entirely defined in [RFC8480]: it complements [RFC8480] by providing the rules of when to add/delete cells in the communication schedule. The SF defined in this document follows that definition, and satisfies all the requirements for an SF listed in Section 4.2 of [RFC8480]. MSF builds on top of the following specifications: the Minimal IPv6 over the TSCH Mode of IEEE 802.15.4e (6TiSCH) Configuration [RFC8180], the 6TiSCH Operation Sublayer Protocol (6P) [RFC8480], and the Minimal Security Framework for 6TiSCH [I-D.ietf-6tisch-minimal-security]. MSF defines both the behavior of a node when joining the network, and how the communication schedule is managed in a distributed fashion. When a node running MSF boots up, it joins the network by following the 7 steps described in Section 4. The end state of the join process is that the node is synchronized to the network, has mutually authenticated to the network, has identified a preferred routing parent, has scheduled one default managed unicast cell (defined in Section 5.1) to/from its preferred routing parent. After the join process, the node can continuously add/delete/relocate cells, as described in Section 5. It does so for 3 reasons: to match the link- layer resources to the traffic, to handle changing parent, to handle a schedule collision. MSF is designed to operate in a wide range of application domains. It is optimized for applications with regular upstream traffic (from Chang, et al. Expires September 12, 2019 [Page 3] Internet-Draft 6TiSCH Minimal Scheduling Function (MSF) March 2019 the nodes to the root). Appendix C contains a performance evaluation of MSF. This specification follows the recommended structure of an SF specification in Appendix A of [RFC8480], with the following adaptations: o We have reordered part of the sections, in particular to have the section on the node behavior at boot Section 4 appear early in this specification. o We added sections on the interface to the minimal 6TiSCH configuration Section 2, the use of the SIGNAL command Section 6, the MSF constants Section 14, the MSF statistics Section 15, the performance of MSF Appendix C. o This specification does not include an examples section. 2. Interface to the Minimal 6TiSCH Configuration A node implementing MSF SHOULD implement the Minimal 6TiSCH Configuration [RFC8180], which defines the "minimal cell", a single shared cell providing minimal connectivity between the nodes in the network. The MSF implementation provided in this specification is based on the implementation of the Minimal 6TiSCH Configuration. An implementor with full-awareness of the Minimal 6TiSCH Configuration implications MAY implement MSF without it. MSF uses the minimal cell to exchange the following packets: 1. Enhanced Beacons (EBs), defined by [IEEE802154-2015]. These are broadcast frames. 2. DODAG Information Objects (DIOs), defined by [RFC6550], with broadcast type. Unicast type DIOs SHOULD NOT be sent on minimal cell. Because the minimal cell is SHARED, the back-off algorithm defined in [IEEE802154-2015] is used to resolve collisions. To ensure there is enough bandwidth available on the minimal cell, a node implementing MSF SHOULD enforce the following rules for broadcast frames: 1. send EBs on a portion of the minimal cells not exceeding 1/(3(N+1)), where N is the number of neighbors of the node. 2. send any other broadcast packets on a portion of the minimal cells not exceeding 1/(3(N+1)), where N is the number of neighbors of the node. For example, the broadcast DIO and DIS, IPv6 Neighbor Discovery (ND)[RFC4861] and any other application layer broadcast packets. Chang, et al. Expires September 12, 2019 [Page 4] Internet-Draft 6TiSCH Minimal Scheduling Function (MSF) March 2019 The RECOMMENDED behavior for sending EBs is to have a node send EBs with a probability of 1/(3(N+1)). The RECOMMENDED behavior for sending DIOs is to use a Trickle timer with rate-limiting. There is no recommendation behavior for sending any other broadcast. However, the traffic portion of all broadcast packets on minimal cell, except EBs, MUST not exceed 1/(3(N+1)). Section 4.3 describes how to evaluate the number of neighbors during the joining process. After the joining process, how to evaluate the number of neighbors is implementation-specific. As detailed in Section 2.2 of [RFC8480], MSF MUST schedule cells from Slotframe 1, while Slotframe 0 is used for traffic defined in the Minimal 6TiSCH Configuration. The length of Slotframe 0 and Slotframe 1 SHOULD be the same value. The default of SLOTFRAME_LENGTH is RECOMMENDED for both Slotframe 0 and Slotframe 1, although any value can be advertised in the EBs. 3. Autonomous Cells MSF nodes MUST initialize Slotframe 1 with a set of default cells for unicast communication with their neighbors. These cells are referred to as 'autonomous cells', because they are maintained autonomously by each node. To distinguish from the autonomous cells, the cell scheduled by 6P transaction is defined as 'managed cells'. How to schedule managed cells is detailed in Section 5. For autonomous cells, each node has: o One cell at a [slotOffset,channelOffset] computed as a hash of the node's EUI64 (detailed next). The cell options for this cell are TX=1, RX=1, SHARED=0. o One cell at a [slotOffset,channelOffset] computed as a hash of the EUI64 of the Join Proxy or each neighbor in the IPv6 neighbor table (detailed in Section 4.4 and Section 4.5). The cell options for this cell are TX=1, RX=1, SHARED=1. To compute a [slotOffset,channelOffset] from an EUI64 address, nodes MUST use the hash function SAX [SAX-DASFAA]. The coordinates are computed to distribute the cells across all 16 channel offsets, and all but the first time offsets of Slotframe 1. The first time offset is skipped to avoid colliding with the minimal cell in Slotframe 0. The slot coordinates derived from a given EUI64 address are computed as follows: o slotOffset(MAC) = 1 + hash(EUI64, length(Slotframe_1) - 1) o channelOffset(MAC) = hash(EUI64, 16) Chang, et al. Expires September 12, 2019 [Page 5] Internet-Draft 6TiSCH Minimal Scheduling Function (MSF) March 2019 For interoperability purpose, an example how the hash function is implemented is detailed in Appendix B. Because of hash collisions, there are cases where the autonomous SHARED and non-SHARED cells are scheduled at the same time offset and/or channel offset. Hash collisions among a set of cells at a given time offset is resolved at run-time as follows: o The SHARED cell with the most packets in the outgoing queue takes precedence. o If all SHARED cells have empty outgoing queues, the non-SHARED cell takes precedence. Both SHARED and non-SHARED autonomous cells are scheduled to transmit unicast packets. The autonomous SHARED cells can only be used to transmit packets to the neighbor whose EUI64 address is used in hash function to create this cell. The autonomous non-SHARED cells can be used to transmit packet to any neighbors. The traffic on the autonomous cells are scheduled as: o The Join Request MUST be sent on a SHARED cell, the 6P Request packet MAY be sent on a SHARED cell. o The Join Response MUST be sent on a non-SHARED cell, the 6P Response packet MAY send on a non-SHARED cell. Throughout the network lifetime, nodes MUST maintain the autonomous cells as follows: o The non-SHARED cell MUST always remain scheduled. o Whenever a new IPv6 neighbor is discovered, add a SHARED cell for it. o Whenever an IPv6 neighbor is removed, remove the SHARED cell that was assigned to it. o 6P CLEAR MUST NOT erase autonomous cells. 4. Node Behavior at Boot This section details the behavior the node SHOULD follow from the moment it is switched on, until it has successfully joined the network. Section 4.1 details the start state; Section 4.9 details the end state. The other sections detail the 7 steps of the joining process. We use the term "pledge" and "joined node", as defined in [I-D.ietf-6tisch-minimal-security]. Chang, et al. Expires September 12, 2019 [Page 6] Internet-Draft 6TiSCH Minimal Scheduling Function (MSF) March 2019 4.1. Start State A node implementing MSF MUST implement the Minimal Security Framework for 6TiSCH [I-D.ietf-6tisch-minimal-security]. As a corollary, this means that a pledge, before being switched on, is pre-configured with the Pre-Shared Key (PSK) for joining, as well as any other configuration detailed in [I-D.ietf-6tisch-minimal-security]. 4.2. Step 1 - Choosing Frequency When switched on, the pledge SHOULD randomly choose a frequency among the available frequencies, and start listening for EBs on that frequency. 4.3. Step 2 - Receiving EBs Upon receiving the first EB, the pledge SHOULD continue listening for additional EBs to learn: 1. the number of neighbors N in its vicinity 2. which neighbor to choose as a Join Proxy (JP) for the joining process While the exact behavior is implementation-specific, the RECOMMENDED behavior is to follow [RFC8180], and listen until EBs sent by NUM_NEIGHBOURS_TO_WAIT nodes (defined in [RFC8180]) have been received. During this step, the pledge MAY synchronize to any EB it receives from the network it wishes to join. How to decide whether an EB originates from a node from the network it wishes to join is implementation-specific, but MAY involve filtering EBs by the PAN ID field it contains, the presence and contents of the IE defined in [I-D.richardson-6tisch-join-enhanced-beacon], or the key used to authenticate it. The decision of which neighbor to use as a JP is implementation- specific, and discussed in [I-D.ietf-6tisch-minimal-security]. 4.4. Step 3 - Setting up Autonomous Cells during Join Process After selected the JP, nodes MUST set up their autonomous SHARED cells, as described in Section 3. A Join Request is sent then by pledge to its JP. The JP forwards the request/response between the JRC and the JP, possibly over multiple hops. When JP received the Join Response from JRC, it sends a Join Response to the pledge, where the pledge learns the keying material used in the network, as well as other configurations, and becomes a "joined node". The Join Request Chang, et al. Expires September 12, 2019 [Page 7] Internet-Draft 6TiSCH Minimal Scheduling Function (MSF) March 2019 and Response traffics are happened on the autonomous cells, which are scheduled as following: o The Joining Request packet MUST be sent on the autonomous SHARED cell to the JP by the pledge. A retransmition with backoff mechanism will be sent later if collision happens. o The Joining Response packet MUST be sent on the autonomous non- SHARED cell to the pledge by the JP. A retransmition will be sent immediately at next autonomous non-SHARED cell if collision happens. After joined, nodes MUST set up their autonomous non-SHARED cell, as described in Section 3. The root node MUST set up its autonomous non-SHARED cell when it is configured as root. 4.5. Step 4 - Installing Autonomous Cells for each neighbor in neighbor table Because it has learnt the link-layer keying material used in the network, the joined node can now decrypt any packets sent by its neighbors. Once a new neighbor is added to the neighbor table, a new autonomous SHARED cell MUST be added to that neighbor. The autonomous SHARED cell MUST be removed if the corresponding neighbor is removed from the neighbor table. How to decide the neighbors to keep in neighbor table is implementation-specific. 4.6. Step 5 - Acquiring a RPL rank Per [RFC6550], the joined node receives DIOs, computes its own rank, and selects a preferred parent. 4.7. Step 6 - Send EBs and DIOs The node SHOULD start sending EBs and DIOs on the minimal cell, while following the transmit rules for broadcast frames from Section 2. 4.8. Step 7 - Neighbor Polling The node SHOULD send some form of keep-alive messages to all its neighbors it has managed unicast cell with. The Keep-Alive (KA) mechanism is detailed in [RFC7554]. It uses the keep-alive messages to its preferred parent to stay synchronized. It MAY use the keep- alive messages to other neighbors to have statistics on link quality. It MAY use the keep-alive messages to its children to ensure the child is still reachable. The RECOMMENDED period for sending keep- alive messages is KA_PERIOD. Chang, et al. Expires September 12, 2019 [Page 8] Internet-Draft 6TiSCH Minimal Scheduling Function (MSF) March 2019 If the keep-alive message to a child fails at the link layer (i.e. the maximum number of link-layer retries is reached), the node SHOULD declare the child as unreachable. This can happen for example when the child node is switched off. When a neighbor is declared unreachable, the node MUST remove all managed unicast cells with that neighbor from its own schedule. In addition, it MAY issue a 6P CLEAR to that neighbor (which can fail at the link-layer). The node MAY be removed from the neighbor table. If so, the autonomous SHARED cell will be removed following the procedure described in Section 3. 4.9. End State For a new node, the end state of the joining process is: o it is synchronized to the network o it is using the link-layer keying material it learned through the secure joining process o it has identified its preferred routing parent o it has one autonomous non-SHARED cell and a set of autonomous SHARED cells to/from its neighbors o it is periodically sending DIOs, potentially serving as a router for other nodes' traffic o it is periodically sending EBs, potentially serving as a JP for new joining nodes 5. Rules for Adding/Deleting Cells Once a node has joined the 6TiSCH network, it adds/deletes/relocates cells with its preferred parent for three reasons: o to match the link-layer resources to the traffic between the node and its preferred parent (Section 5.1) o to handle switching preferred parent (Section 5.2) o to handle a schedule collision (Section 5.3) 5.1. Adapting to Traffic A node implementing MSF MUST implement the behavior described in this section. In order to handle transient traffic bursts, MSF uses the [IEEE802154-2015] frame pending bit (page 152, Section 7.2.1.3). By setting the bit, a node can transmit a series of packets to a given neighbor in consecutive time offsets. The next paragraphs define how to handle longer-term fluctuations in traffic, using 6P. Chang, et al. Expires September 12, 2019 [Page 9] Internet-Draft 6TiSCH Minimal Scheduling Function (MSF) March 2019 The goal of MSF is to manage the communication schedule in the 6TiSCH schedule in a distributed manner. For a node, this translates into monitoring the current usage of the cells it has to its preferred parent: o If the node determines that the number of link-layer frames it is attempting to exchange with its preferred parent per unit of time is larger than the capacity offered by the TSCH managed unicast cells it has scheduled with it, the node triggers a 6P Transaction with its preferred parent to add dedicated cells to the TSCH schedule of both nodes. o If the traffic is lower than the capacity, the node triggers a 6P Transaction with its preferred parent to delete dedicated cells from the TSCH schedule of both nodes. From the join process, the node already has a set of autonomous cells, as defined in Section 3. The autonomous cells MUST NOT be removed by 6P, so that there always exists an autonomous cell between a node and its preferred parent, even if no frames are being exchanged between them. Autonomous cells are used indistinguishably together with dedicated cells, for broadcast or unicast traffic with the target neighbor. The procedure to remove autonomous cells is described in Section 3. Adding/removing/relocating cells involves exchanging frames that contain 6P commands. All 6P frames MUST be sent on the autonomous cell or managed cell if the node has. The node MUST maintain the following counters for its preferred parent: NumCellsElapsed : Counts the number of managed unicast cells that have elapsed since the counter was initialized. This counter is initialized at 0. Each time the TSCH state machine indicates that the current cell is a managed unicast cell to the preferred parent, NumCellsElapsed is incremented by exactly 1, regardless of whether the cell is used to transmit/receive a frame. NumCellsUsed: Counts the number of managed unicast cells that have been used. This counter is initialized at 0. NumCellsUsed is incremented by exactly 1 when, during a unicast cell to the preferred parent, either of the following happens: * The node sends a frame to its preferred parent. The counter increments regardless of whether a link-layer acknowledgment was received or not. * The node receives a frame from its preferred parent. The frame MUST be able to decrypt with the key assigned during joining process. Chang, et al. Expires September 12, 2019 [Page 10] Internet-Draft 6TiSCH Minimal Scheduling Function (MSF) March 2019 Implementors MAY choose to create the same counters for each neighbor, and add them as additional statistics in the neighbor table. The counters are used as follows: 1. Both NumCellsElapsed and NumCellsUsed are initialized to 0 when the node boots. 2. After End State defined in Section 4.9, if there is no managed unicast cell to the preferred parent, trigger 6P to add a signle cell to it. 3. When the value of NumCellsElapsed reaches MAX_NUMCELLS: * If NumCellsUsed > LIM_NUMCELLSUSED_HIGH, trigger 6P to add a single cell to the preferred parent * If NumCellsUsed < LIM_NUMCELLSUSED_LOW, trigger 6P to remove a single cell to the preferred parent * Reset both NumCellsElapsed and NumCellsUsed to 0 and go to step 2. To have the counters working, at least one unicast cell need to be maintained all the time and never be removed. The reason why the counter only counts the managed unicast cell, NOT including the autonomous SHARED cell is to avoid the effects of collision. If the autonomous SHARED cell is counted as well, NumCellsUsed > LIM_NUMCELLSUSED_HIGH could be caused by the collision on the cell. A 6P add request on the autonomous SHARED cell will make the collision even worse. Both NumCellsElapsed and NumCellsUsed counters can be used to cell with cell option TX=1 or RX=1. With the rules defined above, the cell to add or remove can be transmitting cell or receiving cell according to which type of cell the two counters are used for. Similar to Join Request and Response, the 6P Request is scheduled to send on the autonomous SHARED cell to the node's parent. The 6P Response is scheduled to send back on the autonomous non-SHARED cell to the node. 5.2. Switching Parent A node implementing MSF SHOULD implement the behavior described in this section. Part of its normal operation, the RPL routing protocol can have a node switch preferred parents. The procedure for switching from the old preferred parent to the new preferred parent is: Chang, et al. Expires September 12, 2019 [Page 11] Internet-Draft 6TiSCH Minimal Scheduling Function (MSF) March 2019 1. if the new parent is not in the neighbor table, the autonomous SHARED cell MUST be added as defined in Section 3 2. the node counts the number of managed unicast cell cells it has per slotframe to the old preferred parent 3. the node triggers one or more 6P ADD commands to schedule the same number of unicast cells to the new preferred parent 4. when that successfully completes, the node issues a 6P CLEAR command to its old preferred parent 5.3. Handling Schedule Collisions A node implementing MSF SHOULD implement the behavior described in this section. The "MUST" statements in this section hence only apply if the node implements schedule collision handling. Since scheduling is entirely distributed, there is a non-zero probability that two pairs of nearby neighbor nodes schedule a managed unicast cell at the same [slotOffset,channelOffset] location in the TSCH schedule. In that case, data exchanged by the two pairs may collide on that cell. We call this case a "schedule collision". The node MUST maintain the following counters for each managed unicast cell to its preferred parent: NumTx: Counts the number of transmission attempts on that cell. Each time the node attempts to transmit a frame on that cell, NumTx is incremented by exactly 1. NumTxAck: Counts the number of successful transmission attempts on that cell. Each time the node receives an acknowledgment for a transmission attempt, NumTxAck is incremented by exactly 1. Implementors MAY choose to maintain the same counters for each managed unicast cell in the schedule. Since both NumTx and NumTxAck are initialized to 0, we necessarily have NumTxAck <= NumTx. We call Packet Delivery Ratio (PDR) the ratio NumTxAck/NumTx; and represent it as a percentage. A cell with PDR=50% means that half of the frames transmitted are not acknowledged (and need to be retransmitted). Each time the node switches preferred parent (or during the join process when the node selects a preferred parent for the first time), both NumTx and NumTxAck MUST be reset to 0. They increment over time, as the schedule is executed and the node sends frames to its preferred parent. When NumTx reaches 256, both NumTx and NumTxAck MUST be divided by 2. That is, for example, from NumTx=256 and NumTxAck=128, they become NumTx=128 and NumTxAck=64. This operation Chang, et al. Expires September 12, 2019 [Page 12] Internet-Draft 6TiSCH Minimal Scheduling Function (MSF) March 2019 does not change the value of the PDR, but allows the counters to keep incrementing. The key for detecting a schedule collision is that, if a node has several cells to the same preferred parent, all cells should exhibit the same PDR. A cell which exhibits a PDR significantly lower than the others indicates than there are collisions on that cell. Every HOUSEKEEPINGCOLLISION_PERIOD, the node executes the following steps: 1. It computes, for each managed unicast cell with its preferred parent (not for the autonomous cell), that cell's PDR. 2. Any cell that hasn't yet had NumTx divided by 2 since it was last reset is skipped in steps 3 and 4. This avoids triggering cell relocation when the values of NumTx and NumTxAck are not statistically significant yet. 3. It identifies the cell with the highest PDR. 4. For each other cell, it compares its PDR against that of the cell with the highest PDR. If it's less than RELOCATE_PDRTHRES, it triggers the relocation of that cell using a 6P RELOCATE command. 6. 6P SIGNAL command The 6P SIGNAL command is not used by MSF. 7. Scheduling Function Identifier The Scheduling Function Identifier (SFID) of MSF is IANA_6TISCH_SFID_MSF. 8. Rules for CellList MSF uses 2-step 6P Transactions exclusively. 6P Transactions are only initiated by a node towards it preferred parent. As a result, the cells to put in the CellList of a 6P ADD command, and in the candidate CellList of a RELOCATE command, are chosen by the node initiating the 6P Transaction. In both cases, the same rules apply: o The CellList SHOULD contain 5 or more cells. o Each cell in the CellList MUST have a different slotOffset value. o For each cell in the CellList, the node MUST NOT have any scheduled cell on the same slotOffset. o The slotOffset value of any cell in the CellList MUST NOT be the same as the slotOffset of the minimal cell (slotOffset=0). o The slotOffset of a cell in the CellList SHOULD be randomly and uniformly chosen among all the slotOffset values that satisfy the restrictions above. Chang, et al. Expires September 12, 2019 [Page 13] Internet-Draft 6TiSCH Minimal Scheduling Function (MSF) March 2019 o The channelOffset of a cell in the CellList SHOULD be randomly and uniformly chosen in [0..numFrequencies], where numFrequencies represents the number of frequencies a node can communicate on. 9. 6P Timeout Value The 6P Timeout is not a constant value. It is calculated as (1/(C+1))*(1/PDR)*SIXP_TIMEOUT_SEC_FACTOR, where: o C represents the number of cells per second scheduled to that neighbor o PDR represents the average PDR of those cells o SIXP_TIMEOUT_SEC_FACTOR is a security factor, a constant 10. Rule for Ordering Cells Cells are ordered slotOffset first, channelOffset second. The following sequence is correctly ordered (each element represents the [slottOffset,channelOffset] of a cell in the schedule): [1,3],[1,4],[2,0],[5,3],[6,0],[6,3],[7,9] 11. Meaning of the Metadata Field The Metadata field is not used by MSF. 12. 6P Error Handling Section 6.2.4 of [RFC8480] lists the 6P Return Codes. Figure 1 lists the same error codes, and the behavior a node implementing MSF SHOULD follow. Chang, et al. Expires September 12, 2019 [Page 14] Internet-Draft 6TiSCH Minimal Scheduling Function (MSF) March 2019 +-----------------+----------------------+ | Code | RECOMMENDED behavior | +-----------------+----------------------+ | RC_SUCCESS | nothing | | RC_EOL | nothing | | RC_ERR | quarantine | | RC_RESET | quarantine | | RC_ERR_VERSION | quarantine | | RC_ERR_SFID | quarantine | | RC_ERR_SEQNUM | clear | | RC_ERR_CELLLIST | clear | | RC_ERR_BUSY | waitretry | | RC_ERR_LOCKED | waitretry | +-----------------+----------------------+ Figure 1: Recommended behavior for each 6P Error Code. The meaning of each behavior from Figure 1 is: nothing: Indicates that this Return Code is not an error. No error handling behavior is triggered. clear: Abort the 6P Transaction. Issue a 6P CLEAR command to that neighbor (this command may fail at the link layer). Remove all cells scheduled with that neighbor from the local schedule. Keep that node in the neighbor and routing tables. quarantine: Same behavior as for "clear". In addition, remove the node from the neighbor and routing tables. Place the node's identifier in a quarantine list for QUARANTINE_DURATION. When in quarantine, drop all frames received from that node. waitretry: Abort the 6P Transaction. Wait for a duration randomly and uniformly chosen in [WAITDURATION_MIN,WAITDURATION_MAX]. Retry the same transaction. 13. Schedule Inconsistency Handling The behavior when schedule inconsistency is detected is explained in Figure 1, for 6P Return Code RC_ERR_SEQNUM. 14. MSF Constants Figure 2 lists MSF Constants and their RECOMMENDED values. Chang, et al. Expires September 12, 2019 [Page 15] Internet-Draft 6TiSCH Minimal Scheduling Function (MSF) March 2019 +------------------------------+-------------------+ | Name | RECOMMENDED value | +------------------------------+-------------------+ | KA_PERIOD | 10 s | | LIM_NUMCELLSUSED_HIGH | 75 % | | LIM_NUMCELLSUSED_LOW | 25 % | | HOUSEKEEPINGCOLLISION_PERIOD | 1 min | | RELOCATE_PDRTHRES | 50 % | | SIXP_TIMEOUT_SEC_FACTOR | 3 x | | SLOTFRAME_LENGTH | 101 slots | | QUARANTINE_DURATION | 5 min | | WAITDURATION_MIN | 30 s | | WAITDURATION_MAX | 60 s | +------------------------------+-------------------+ Figure 2: MSF Constants and their RECOMMENDED values. 15. MSF Statistics Figure 3 lists MSF Statistics and their RECOMMENDED width. +-----------------+-------------------+ | Name | RECOMMENDED width | +-----------------+-------------------+ | NumCellsElapsed | 1 byte | | NumCellsUsed | 1 byte | | NumTx | 1 byte | | NumTxAck | 1 byte | +-----------------+-------------------+ Figure 3: MSF Statistics and their RECOMMENDED width. 16. Security Considerations MSF defines a series of "rules" for the node to follow. It triggers several actions, that are carried out by the protocols defined in the following specifications: the Minimal IPv6 over the TSCH Mode of IEEE 802.15.4e (6TiSCH) Configuration [RFC8180], the 6TiSCH Operation Sublayer Protocol (6P) [RFC8480], and the Minimal Security Framework for 6TiSCH [I-D.ietf-6tisch-minimal-security]. In particular, MSF does not define a new protocol or packet format. MSF relies entirely on the security mechanisms defined in the specifications listed above. Chang, et al. Expires September 12, 2019 [Page 16] Internet-Draft 6TiSCH Minimal Scheduling Function (MSF) March 2019 17. IANA Considerations 17.1. MSF Scheduling Function Identifiers This document adds the following number to the "6P Scheduling Function Identifiers" sub-registry, part of the "IPv6 over the TSCH mode of IEEE 802.15.4e (6TiSCH) parameters" registry, as defined by [RFC8480]: +----------------------+-----------------------------+-------------+ | SFID | Name | Reference | +----------------------+-----------------------------+-------------+ | IANA_6TISCH_SFID_MSF | Minimal Scheduling Function | RFCXXXX | | | (MSF) | (NOTE:this) | +----------------------+-----------------------------+-------------+ Figure 4: IETF IE Subtype '6P'. 18. References 18.1. Normative References [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-09 (work in progress), November 2018. [I-D.richardson-6tisch-join-enhanced-beacon] Dujovne, D. and M. Richardson, "IEEE802.15.4 Informational Element encapsulation of 6tisch Join Information", draft- richardson-6tisch-join-enhanced-beacon-03 (work in progress), January 2018. [IEEE802154-2015] IEEE standard for Information Technology, "IEEE Std 802.15.4-2015 Standard for Low-Rate Wireless Personal Area Networks (WPANs)", December 2015. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, DOI 10.17487/RFC4861, September 2007, . Chang, et al. Expires September 12, 2019 [Page 17] Internet-Draft 6TiSCH Minimal Scheduling Function (MSF) March 2019 [RFC6550] Winter, T., Ed., Thubert, P., Ed., Brandt, A., Hui, J., Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur, JP., and R. Alexander, "RPL: IPv6 Routing Protocol for Low-Power and Lossy Networks", RFC 6550, DOI 10.17487/RFC6550, March 2012, . [RFC7554] Watteyne, T., Ed., 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, DOI 10.17487/RFC7554, May 2015, . [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, . [RFC8480] Wang, Q., Ed., Vilajosana, X., and T. Watteyne, "6TiSCH Operation Sublayer (6top) Protocol (6P)", RFC 8480, DOI 10.17487/RFC8480, November 2018, . 18.2. Informative References [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. [RFC6982] Sheffer, Y. and A. Farrel, "Improving Awareness of Running Code: The Implementation Status Section", RFC 6982, DOI 10.17487/RFC6982, July 2013, . [SAX-DASFAA] Ramakrishna, M. and J. Zobel, "Performance in Practice of String Hashing Functions", DASFAA , 1997. Appendix A. Contributors Beshr Al Nahas (Chalmers University, beshr@chalmers.se) and Olaf Landsiedel (Chalmers University, olafl@chalmers.se) contributed to the design and evaluation of autonomous unicast cells. Chang, et al. Expires September 12, 2019 [Page 18] Internet-Draft 6TiSCH Minimal Scheduling Function (MSF) March 2019 Appendix B. Example of Implementation of SAX hash function For the consideration of interoperability, this section provides an example of implemention SAX hash function [SAX-DASFAA]. The input parameters of the function are: o T, which is the hashing table length o c, which is the characters of string s, to be hashed In MSF, the T is replaced by the length slotframe 1. String s is replaced by the mote EUI64 address. The characters of the string c0, c1, ..., c7 are the 8 bytes of EUI64 address. The SAX hash function requires shift operation which is defined as follow: o L_shift(v,b), which refers to left shift variable v by b bits o R_shift(v,b), which refers to right shift variable v by b bits The steps to calculate the hash value of SAX hash function are: 1. initialize variable h to h0 and variable i to 0, where h is the intermediate hash value and i is the index of the bytes of EUI64 address 2. sum the value of L_shift(h,l_bit), R_shift(h,r_bit) and ci 3. calculate the result of exclusive or bewteen the sum value in Step 2 and h 4. modulo the result of Step 3 by T 5. assign the result of Step 4 to h 6. increase i by 1 7. repeat Step2 to Step 6 until i reaches to 8 8. assign the result of Step 5 to h The value of variable h the hash value of SAX hash function. For interoperability purpose, the values of h0, l_bit and r_bit in Step 1 and 2 are configured as: o h0 = 0 o l_bit = 0 o r_bit = 1 The appropriate values of l_bit and r_bit could vary depending on the the set of motes' EUI64 address. How to find those values is out of the scope of this specification. Chang, et al. Expires September 12, 2019 [Page 19] Internet-Draft 6TiSCH Minimal Scheduling Function (MSF) March 2019 Appendix C. Implementation Status and Performance Evaluation 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 MSF is being implemented in the OpenWSN project [OpenWSN] under a BSD open-source license. The code is open-source and available in two parts: * the firmware, running on embedded hardware: https://github.com/ openwsn-berkeley/openwsn-fw * the software, running on the PC that acts as the LBR of the network: https://github.com/openwsn-berkeley/openvisualizer At the time of writing, OpenWSN implements MSF as specified in this document. The code is tested on the OpenTestbed deployed at Inria in Paris. The testbed consist of 40 OpenMote-B nodes. The experiment is run for 3 hours. With the first 12 minutes of the experiment, all nodes synchronize and securely join the network. Once joined, each node sends a DAO every 1 min to the DAG root. We use the DAO's sequence number to compute end-to-end reliability. After 3 hours, the nodes have generated 7114 DAOs, with an overall end-end reliability of 99.47%. The OpenWSN implementation is work-in-progress, and we expect the final release to yield 100% end-to-end reliability. 6TiSCH simulator The 6TiSCH simulator is a Python-based high-level simulator on which MSF is being implemented. More information at https://bitbucket.org/6tisch/simulator/. Chang, et al. Expires September 12, 2019 [Page 20] Internet-Draft 6TiSCH Minimal Scheduling Function (MSF) March 2019 Appendix D. [TEMPORARY] Changelog o draft-ietf-6tisch-msf-02 * Resolve the issues in Pending Elements section. Using autonomous SHARED and non-SHARED cells. Overall revised the specification. o draft-ietf-6tisch-msf-01 * Added a section on pending elements. o draft-ietf-6tisch-msf-00 * Re-publication after WG adoption. No changes. o draft-chang-6tisch-msf-02 * Added autonomous cell. o draft-chang-6tisch-msf-01 * When neighbor is unreachable, sending a CLEAR command was a MUST, now a MAY. * Fixing 6P Timeout calculation. * Clearer text for "Handling Schedule Collisions" section. * Typos. * Input from Yasuyuki Tanaka's review (https://www.ietf.org/mail- archive/web/6tisch/current/msg05723.html). o draft-chang-6tisch-msf-00 * Initial submission. Appendix E. [TEMPORARY] Pending Elements E.1. Introduction This section contains lessons learnt and suggestions from simulating and experimenting with draft-ietf-6tisch-msf-00. This section is temporary and will be removed once these suggestions are integrated or abandonned. E.2. Performance Related E.2.1. Handling the case when bandwidth allocation exceeds available capacity When node A initializes a 6top ADD transaction to node B, but node B does NOT have enough bandwidth to allocate. What can node B do to indicate this case in its 6top response, and how should node A handle the packet after receiving the response? Related discussion: https://www.ietf.org/mail-archive/web/6tisch/current/msg06095.html Chang, et al. Expires September 12, 2019 [Page 21] Internet-Draft 6TiSCH Minimal Scheduling Function (MSF) March 2019 [TOD BE DISCUSSED. According to 6top RFC8480, SUCCESS return code will be in the 6top response with an empty cellList possibly. For this case, I don't have a prefect solution. One solution I am using is marking the node B as 'no_resource', then updating it's routing table and filtering out the neighbor marked as 'no_resource'. However, this is a layer-violation design.] E.2.2. Adapting 6P Timeout After 6P TIMEOUT, the next 6P transaction should be configured using a larger TIMEOUT. Issues link: https://github.com/twatteyne/draft- ietf-6tisch-msf/issues/19 [TOD BE DISCUSSED. I am not sure this is an issue or not with current version. Need to discuss with Malisa and Simon.] E.3. editorial E.3.1. Rules for broadcast frames is out of scope of MSF In Section 2, the rules for broadcast frames on the minimal cell seem to be an implementation-specific optimization. And it is beyond of the scope of this draft. This idea is about how to use the minimal cell efficiently; it's not directly related to how to use cells scheduled by MSF. Related discussion: https://mailarchive.ietf.org/arch/ msg/6tisch/9jcaTddi6vLO5zHqTDNk6yqrNao [TOD BE DISCUSSED. I agree with this comment. Should we remove those text or keep them in the draft but mentioned those rules are not a MUST but a RECOMMENDED?] Authors' Addresses Tengfei Chang (editor) Inria 2 rue Simone Iff Paris 75012 France Email: tengfei.chang@inria.fr Chang, et al. Expires September 12, 2019 [Page 22] Internet-Draft 6TiSCH Minimal Scheduling Function (MSF) March 2019 Malisa Vucinic Inria 2 rue Simone Iff Paris 75012 France Email: malisa.vucinic@inria.fr Xavier Vilajosana Universitat Oberta de Catalunya 156 Rambla Poblenou Barcelona, Catalonia 08018 Spain Email: xvilajosana@uoc.edu Simon Duquennoy RISE SICS Isafjordsgatan 22 164 29 Kista Sweden Email: simon.duquennoy@ri.se 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 Chang, et al. Expires September 12, 2019 [Page 23]