6TiSCH X. Vilajosana, Ed. Internet-Draft Universitat Oberta de Catalunya Intended status: Informational K. Pister Expires: December 29, 2014 University of California Berkeley June 27, 2014 Minimal 6TiSCH Configuration draft-ietf-6tisch-minimal-01 Abstract This document describes the minimal set of rules to operate a [IEEE802154e] Timeslotted Channel Hopping (TSCH) network. This minimal mode of operation can be used during network bootstrap, as a fallback mode of operation when no dynamic scheduling solution is available or functioning, or during early interoperability testing and development. 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." This Internet-Draft will expire on December 29, 2014. Copyright Notice Copyright (c) 2014 IETF Trust and the persons identified as the document authors. All rights reserved. Vilajosana & Pister Expires December 29, 2014 [Page 1] Internet-Draft 6tisch-minimal June 2014 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 . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Minimal Schedule Configuration . . . . . . . . . . . . . . . 3 2.1. Slotframe . . . . . . . . . . . . . . . . . . . . . . . . 3 2.2. Cell Options . . . . . . . . . . . . . . . . . . . . . . 5 2.3. Retransmissions . . . . . . . . . . . . . . . . . . . . . 6 2.4. Time Slot timing . . . . . . . . . . . . . . . . . . . . 6 3. Enhanced Beacons Configuration and Content . . . . . . . . . 8 3.1. Sync IE . . . . . . . . . . . . . . . . . . . . . . . . . 8 3.1.1. IE Header . . . . . . . . . . . . . . . . . . . . . . 8 3.1.2. IE Content . . . . . . . . . . . . . . . . . . . . . 8 3.2. TSCH Timeslot IE . . . . . . . . . . . . . . . . . . . . 9 3.2.1. IE Header . . . . . . . . . . . . . . . . . . . . . . 9 3.2.2. IE Content . . . . . . . . . . . . . . . . . . . . . 9 3.3. Channel Hopping IE . . . . . . . . . . . . . . . . . . . 9 3.3.1. IE Header . . . . . . . . . . . . . . . . . . . . . . 9 3.3.2. IE Content . . . . . . . . . . . . . . . . . . . . . 9 3.4. Frame and Link IE . . . . . . . . . . . . . . . . . . . . 9 3.4.1. IE Header . . . . . . . . . . . . . . . . . . . . . . 9 3.4.2. IE Content . . . . . . . . . . . . . . . . . . . . . 10 4. Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . 10 4.1. ACK/NACK Time Correction IE . . . . . . . . . . . . . . . 10 4.1.1. IE Header . . . . . . . . . . . . . . . . . . . . . . 10 4.1.2. IE Content . . . . . . . . . . . . . . . . . . . . . 10 5. Neighbor information . . . . . . . . . . . . . . . . . . . . 11 5.1. Neighbor Table . . . . . . . . . . . . . . . . . . . . . 11 5.2. Time Source Neighbor Selection . . . . . . . . . . . . . 12 6. Queues and Priorities . . . . . . . . . . . . . . . . . . . . 12 7. RPL on TSCH . . . . . . . . . . . . . . . . . . . . . . . . . 13 7.1. RPL Objective Function Zero . . . . . . . . . . . . . . . 13 7.1.1. Rank computation . . . . . . . . . . . . . . . . . . 13 7.1.2. Rank computation Example . . . . . . . . . . . . . . 14 7.2. RPL Configuration . . . . . . . . . . . . . . . . . . . . 16 7.2.1. Mode of Operation . . . . . . . . . . . . . . . . . . 17 7.2.2. Trickle Timer . . . . . . . . . . . . . . . . . . . . 17 7.2.3. Hysteresis . . . . . . . . . . . . . . . . . . . . . 17 7.2.4. Variable Values . . . . . . . . . . . . . . . . . . . 17 Vilajosana & Pister Expires December 29, 2014 [Page 2] Internet-Draft 6tisch-minimal June 2014 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 18 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 18 9.1. Normative References . . . . . . . . . . . . . . . . . . 18 9.2. Informative References . . . . . . . . . . . . . . . . . 19 9.3. External Informative References . . . . . . . . . . . . . 20 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20 1. Introduction The nodes in a [IEEE802154e] TSCH network follow a communication schedule. The entity (centralized or decentralized) responsible for building and maintaining that schedule has very precise control over the trade-off between the network's latency, bandwidth, reliability and power consumption. During early interoperability testing and development, however, simplicity is often more important than efficiency. One goal of this document is to define the simplest set of rules for building a [IEEE802154e] TSCH-compliant network, at the necessary price of lesser efficiency. Yet, this minimal mode of operation can also be used during network bootstrap before any schedule is installed into the network so nodes can self-organize and the management and configuration information be distributed. In addition, as outlined in [I-D.phinney-roll-rpl-industrial-applicability], the minimal configuration can be used as a fallback mode of operation, ensuring connectivity of nodes in case that dynamic scheduling mechanisms fail or are not available. [IEEE802154e] provides a mechanism whereby the details of slotframe length, timeslot timing, and channel hopping pattern are communicated at synchronization to a node, also Enhanced Beacons can be used to periodically update nodes information. This document describes specific settings for these parameters. Nodes MUST broadcast properly formed Enhanced Beacons to announce these values, but during initial implementation and debugging it may be convenient to preconfigure these values. 2. Minimal Schedule Configuration In order to form a network, a minimum schedule configuration is required so nodes can advertise the presence of the network, and allow other nodes to join. 2.1. Slotframe The slotframe, as defined in [I-D.ietf-6tisch-terminology], is an abstraction of the link layer that defines a collection of time slots of equal length, and which repeats over time. In order to set up a minimal TSCH network, nodes need to be synchronized with the same slotframe configuration so they can exchange Enhanced Beacons (EBs) Vilajosana & Pister Expires December 29, 2014 [Page 3] Internet-Draft 6tisch-minimal June 2014 and data packets. This document recommends the following slotframe configuration. Minimal configuration +------------------------------------+----------------------+ | Property | Value | +------------------------------------+----------------------+ | Number of time slots per Slotframe | Variable | +------------------------------------+----------------------+ | Number of available frequencies | 16 | +------------------------------------+----------------------+ | Number of scheduled cells | 1 (slotOffset 0) | | | (macLinkType NORMAL) | +------------------------------------+----------------------+ | Number of unscheduled cells | The remainder of the | | | slotframe | +------------------------------------+----------------------+ | Number of MAC retransmissions (max)| 3 (4 attempts to tx)| +------------------------------------+----------------------+ The slotframe is composed of a configurable number of time slots. Choosing the number of time slots per slotframe needs to take into account network requirements such as density, bandwidth per node, etc. In the minimal configuration, there is only a single active slot in slotframe, used to transmit data and EBs, and receive information. The trade-off between bandwidth, latency and energy consumption can be controlled by choosing a different slotframe length. The active slot MAY be scheduled at the slotoffset 0x00 and channeloffset 0x00 and MUST be announced in the EBs. EBs are sent using this active slot and are not acknowledged. Data packets, as described in Section 2.2 use the same active slot. Per [IEEE802154e], data packets sent unicast on this cell are acknowledged by the receiver. The remaining cells are unscheduled, and MAY be used by dynamic scheduling solutions. Details about such dynamic scheduling solution are out of scope. The slotframe length (expressed in number of time slots) is configurable. The length used determines the duty cycle of the network. For example, a network with a 1.01% duty cycle is composed of a slotframe of 101 slots, which includes 1 active slot. The present document RECOMMENDS the use of a default slot duration set to 10ms and its corresponding default timeslot timings defined by the [IEEE802154e] macTimeslotTemplate. The use of the default macTimeslotTemplate MUST be announced in the EB by using the Timeslot IE containing only the default macTimeslotTemplateId. Other time slot durations MAY be supported and MUST be announced clearly. If Vilajosana & Pister Expires December 29, 2014 [Page 4] Internet-Draft 6tisch-minimal June 2014 one uses a timeslot duration different than 10ms, it is RECOMMENDED to use a power-of-two of 10ms (i.e. 20ms, 40ms, 80ms, etc.). In this case, EBs MUST contain the complete TimeSlot IE as described in Section 2.4. Example schedule with 1.01% duty cycle Chan. +----------+----------+ Off.0 | TxRxS/EB | OFF | Chan. +----------+----------+ Off.1 | | | +----------+----------+ ... Chan. +----------+----------+ Off.15 | | | +----------+----------+ 0 1-100 EB: Enhanced Beacon Tx: Transmit Rx: Receive S: Shared OFF: Unscheduled (can be used by a dynamic scheduling mechanism) 2.2. Cell Options Per the [IEEE802154e] TSCH, each scheduled cell has an associated bitmap of cell options, called LinkOption. The scheduled cell in the minimal schedule is configured as Hard cell [I-D.ietf-6tisch-tsch][I-D.ietf-6tisch-6top-interface]. Additional available cells can be scheduled by a dynamic scheduling solution. The dynamic scheduling solution is out of scope, and this specification does not make any restriction on the LinkOption associated with those dynamically scheduled cells (i.e. they can be hard cells or soft cells). The active cell is assigned the bitmap of cell options below. Because both the "Transmit" and "Receive" bits are set, a node transmits if there is a packet in its queue, and listens otherwise. Because the "shared" bit is set, the back-off mechanism defined in [IEEE802154e] is used to resolve contention. This results in "Slotted Aloha" behavior. The "Timekeeping" flag is never set, since the time source neighbor is selected using the DODAG structure of the network (detailed below). b0 = Transmit = 1 (set) b1 = Receive = 1 (set) Vilajosana & Pister Expires December 29, 2014 [Page 5] Internet-Draft 6tisch-minimal June 2014 b2 = Shared = 1 (set) b3 = Timekeeping = 0 (clear) b4-b7 = Reserved (clear) All remaining cells are unscheduled. In unscheduled cells, the nodes SHOULD keep their radio off. In a memory-efficient implementation, scheduled cells can be represented by a circular linked list. Unscheduled cells SHOULD NOT occupy any memory. 2.3. Retransmissions The maximum number of link layer retransmissions is set to 3. For packets which require an acknowledgement, if none is received after a total of 4 attempts, the transmissions is considered failed and the link layer MUST notify the upper layer. Packets sent to the broadcast MAC address (including EBs) are not acknowledged and therefore not retransmitted. 2.4. Time Slot timing The figure below shows an active timeslot in which a packet is sent from the transmitter node (TX) to the receiver node (RX). A link- layer acknowledgement is sent by the RX node to the TX node when the packet is to be acknowledged. The TsTxOffset duration defines the instant in the timeslot when the first byte of the transmitted packet leaves the radio of the TX node. The radio of the RX node is turned on TsLongGT/2 before that instant, and listens for at least TsLongGT. This allows for a de-synchronization between the two node of at most TsLongGT. The RX node needs to send the first byte of the MAC acknowledgement exactly TsTxAckDelay after the end of the last byte of the received packet. TX's radio has to be turned on TsShortGT/2 before that time, and keep listening for at least TsShortGT. Vilajosana & Pister Expires December 29, 2014 [Page 6] Internet-Draft 6tisch-minimal June 2014 Time slot internal timing diagram /------------------- Time Slot duration --------------------/ | /tsShortGT/ | | | | | | | |------------+-----------------+--------------+------+------| TX | | TX-Packet | |RX Ack| | |------------+-----------------+--------------+------+------| |/tsTxOffset/| | | | | | | | | | | |------------+-----------------+--------------+------+------| RX | | | | RX-Packet | |TX Ack| | |---------+--+--+--------------+--------------+------+------| | | | | | | | | | /tsLongGT/ |/TsTxAckDelay/| | | Start End of of Slot Slot A 10ms time slot length is the default value defined by [IEEE802154e]. Section 6.4.3.3.3 of the [IEEE802154e] defines a default macTimeslotTemplate, i.e. the different duration within the slot. These values are summarized in the following table and MUST be used when utilizing the default time slot duration. In this case, the Timeslot IE only transports the macTimeslotTemplateId (0x00) as the timing values are well-known. If a timeslot template other than the default is used, the EB MUST contain a complete TimeSlot IE, which requires 25 extra bytes. Default timeslot durations (per [IEEE802154e], Section 6.4.3.3.3) +--------------------------------+------------+ | IEEE802.15.4e TSCH parameter | Value | +--------------------------------+------------+ | TsTxOffset | 2120us | +--------------------------------+------------+ | TsLongGT | 2000us | +--------------------------------+------------+ | TsTxAckDelay | 1000us | +--------------------------------+------------+ | TsShortGT | 400us | +--------------------------------+------------+ | Time Slot duration | 10000us | +--------------------------------+------------+ Vilajosana & Pister Expires December 29, 2014 [Page 7] Internet-Draft 6tisch-minimal June 2014 3. Enhanced Beacons Configuration and Content [IEEE802154e] does not define how often EBs are sent, not their contents. The choice of the duration between two EBs needs to take into account whether EBs are used as the only mechanism to synchronize devices, or whether a Keep-Alive (KA) mechanism is also used. For a minimal TSCH configuration, a mote SHOULD send an EB every EB_PERIOD. For additional reference see [I-D.ietf-6tisch-tsch] where different synchronization approaches are summarized. EBs MUST be sent with the Beacon IEEE802.15.4 frame type and this EBs MUST carry the Information Elements (IEs) listed below. The content of the IEs is presented here for completeness, however this information is redundant with [I-D.ietf-6tisch-tsch] and [IEEE802154e]. 3.1. Sync IE Contains synchronization information such as ASN and Join Priority. The value of Join Priority is discussed in Section 5.2. 3.1.1. IE Header Length (b0-b7) = 0x06 Sub-ID (b8-b14) = 0x1a Type (b15) = 0x00 (short) 3.1.2. IE Content ASN Byte 1 (b16-b23) ASN Byte 2 (b24-b31) ASN Byte 3 (b32-b39) ASN Byte 4 (b40-b47) ASN Byte 5 (b48-b55) Join Priority (b56-b63) Vilajosana & Pister Expires December 29, 2014 [Page 8] Internet-Draft 6tisch-minimal June 2014 3.2. TSCH Timeslot IE Contains the timeslot template identifier. This specification uses the default timeslot template as defined in [IEEE802154e], Section 5.2.4.15. 3.2.1. IE Header Length (b0-b7) = 0x01 Sub-ID (b8-b14) = 0x1c Type (b15) = 0x00 (short) 3.2.2. IE Content Timeslot Template ID (b0-b7) = 0x00 3.3. Channel Hopping IE Contains the channel hopping template identifier. This specification uses the default channel hopping template, as defined in [IEEE802154e], Section 5.2.4.16. 3.3.1. IE Header Length (b0-b7) = 0x01 Sub-ID (b8-b14) = 0x1d Type (b15) = 0x00 (short) 3.3.2. IE Content Channel Hopping Template ID (b0-b7) = 0x00 3.4. Frame and Link IE Each node MUST indicate the schedule in each EB through a Frame and Link IE. This enables nodes which implement [IEEE802154e] to configure their schedule as they join the network. 3.4.1. IE Header Length (b0-b7) = variable Sub-ID (b8-b14) = 0x1b Vilajosana & Pister Expires December 29, 2014 [Page 9] Internet-Draft 6tisch-minimal June 2014 Type (b15) = 0x00 (short) 3.4.2. IE Content # Slotframes (b16-b23) = 0x01 Slotframe ID (b24-b31) = 0x01 Size Slotframe (b32-b47) = variable # Links (b48-b55) = 0x01 For the active cell in the minimal schedule: Channel Offset (2B) = 0x00 Slot Number (2B) = 0x00 LinkOption (1B) = as described in Section 2.2 4. Acknowledgment Link-layer acknowledgment frames are built according to [IEEE802154e]. Data frames and command frames sent to a unicast MAC destination address request an acknowledgment. The acknowledgment frame is of type ACK (0x10). Each acknowledgment contains the following IE: 4.1. ACK/NACK Time Correction IE The ACK/NACK time correction IE carries the measured de- synchronization between the sender and the receiver. 4.1.1. IE Header Length (b0-b7) = 0x02 Sub-ID (b8-b14) = 0x1e Type (b15) = 0x00 (short) 4.1.2. IE Content Time Synchronization Information and ACK status (b16-b31) The possible values for the Time Synchronization Information and ACK status are described in [IEEE802154e] and reproduced in the following table: Vilajosana & Pister Expires December 29, 2014 [Page 10] Internet-Draft 6tisch-minimal June 2014 ACK status and Time Synchronization Information. +-----------------------------------+-----------------+ | ACK Status | Value | +-----------------------------------+-----------------+ | ACK with positive time correction | 0x0000 - 0x07ff | +-----------------------------------+-----------------+ | ACK with negative time correction | 0x0800 - 0x0fff | +-----------------------------------+-----------------+ | NACK with positive time correction| 0x8000 - 0x87ff | +-----------------------------------+-----------------+ | NACK with negative time correction| 0x8800 - 0x8fff | +-----------------------------------+-----------------+ 5. Neighbor information [IEEE802154e] does not define how and when each node in the network keeps information about its neighbors. This document recommends to keep the following information in the neighbor table: 5.1. Neighbor Table The exact format of the neighbor table is implementation-specific, but it SHOULD contain the following information for each neighbor: Neighbor statistics: numTx: number of transmitted packets to that neighbor numTxAck: number of transmitted packets that have been acknowledged by that neighbor numRx: number of received packets from that neighbor The EUI64 of the neighbor. Timestamp when that neighbor was heard for the last time. This can be based on the ASN counter or any other time base. Can be used to trigger a keep-alive message. RPL rank of that neighbor. A flag indicating whether this neighbor is a time source neighbor. Connectivity statistics (e.g., RSSI), which can be used to determine the quality of the link. Vilajosana & Pister Expires December 29, 2014 [Page 11] Internet-Draft 6tisch-minimal June 2014 In addition to that information, each node has to be able to compute some RPL Objective Function (OF), taking into account the neighbor and connectivity statistics. An example RPL objective function is the OF Zero as described in [RFC6552] and Section 7.1.1. 5.2. Time Source Neighbor Selection Each node MUST select at least one time source neighbor among the nodes in its RPL routing parent set. When a node joins a network, it has no routing information. To select its time source neighbor, it uses the Join Priority field in the EB, as described in Section 5.2.4.13 and Table 52b of [IEEE802154e]. The Sync IE contains the ASN and 1 Byte field named Join Priority. The Join Priority of any node is equivalent to the result of the function DAGRank(rank) as defined by [RFC6550] and Section 7.1.1. The Join Priority of the DAG root is zero, i.e., EBs sent from the DAG root are sent with Join Priority equal to 0. A lower value of the Join Priority indicates that the device is the preferred one to connect to. When a node joins the network, it MUST NOT send EBs before having acquired a RPL rank. This avoids routing loops and matches RPL topology with underlying mesh topology. As soon as a node acquires a RPL rank (see [RFC6550] and Section 7.1.1), it SHOULD send Enhanced Beacons including a Sync IE with Join Priority field set to DAGRank(rank), where rank is the node's rank. If a node receives EBs from different nodes with equal Join Priority, the time source neighbor selection should be assessed by other metrics that can help determine the better connectivity link. Time source neighbor hysteresis SHOULD be used, according to the rules defined in Section 7.2.3. If connectivity to the time source neighbor is lost, a new time source neighbor MUST be chosen among the neighbors in the RPL routing parent set. The decision for a node to select one Time Source Neighbor when multiple EBs are received is open to implementers. For example a node MAY wait until one EB from NUM_NEIGHBOURS_TO_WAIT neighbors have been received to select the best Time Source Neighbor. This condition MAY apply unless a second EB is not received after MAX_EB_DELAY seconds. This avoids initial hysteresis when selecting a first Time Source Neighbor. Optionally, some form of hysteresis SHOULD be implemented to avoid frequent changes in time source neighbors. 6. Queues and Priorities [IEEE802154e] does not define the use of queues to handle upper layer data (either application or control data from upper layers). This Vilajosana & Pister Expires December 29, 2014 [Page 12] Internet-Draft 6tisch-minimal June 2014 document recommends the use of a single queue with the following rules: When the node is not synchronized to the network, higher layers are not able to insert packets into the queue. Frames generated by the MAC layer (e.g., EBs and ACK) have a higher priority than packets received from a higher layer. IEEE802.15.4 frames of types Beacon and Command have a higher priority than IEEE802.15.4 frames of types Data and ACK. One entry in the queue is reserved at all times for an IEEE802.15.4 frames of types Beacon or Command frames. 7. RPL on TSCH Nodes in the network MUST use the RPL routing protocol [RFC6550]. 7.1. RPL Objective Function Zero Nodes in the network MUST use the RPL routing protocol [RFC6550] and implement the RPL Objective Function Zero [RFC6552]. 7.1.1. Rank computation The rank computation is described at [RFC6552], Section 4.1. Briefly, a node rank is computed by the following equation: R(N) = R(P) + rank_increase rank_increase = (Rf*Sp + Sr) * MinHopRankIncrease Where: R(N): Rank of the node. R(P): Rank of the parent obtained as part of the DIO information. rank_increase: The result of a function that determines the rank increment. Rf (rank_factor): A configurable factor that is used to multiply the effect of the link properties in the rank_increase computation. If none is configured, rank_factor of 1 is used. In this specification, a rank_factor of 1 MUST be used. Vilajosana & Pister Expires December 29, 2014 [Page 13] Internet-Draft 6tisch-minimal June 2014 Sp (step_of_rank): (strictly positive integer) - an intermediate computation based on the link properties with a certain neighbor. In this specification, 2*ETX (Expected Transmissions) as defined by [decouti03high] and [RFC6551] MUST be used. The ETX is computed as the inverse of the Packet Delivery Ratio (PDR), and MAY be computed as the number of acknowledged packets, divided by the number of transmitted packets to a certain node. E.g: Sp=2*numTX/numTXAck Sr (stretch_of_rank): (unsigned integer) - the maximum increment to the step_of_rank of a preferred parent, to allow the selection of an additional feasible successor. If none is configured to the device, then the step_of_rank is not stretched. In this specification, stretch_of_rank MUST be set to 0. MinHopRankIncrease: the MinHopRankIncrease is set to the fixed constant DEFAULT_MIN_HOP_RANK_INCREASE [RFC6550]. DEFAULT_MIN_HOP_RANK_INCREASE has a value of 256. DAGRank(rank): Equivalent to the floor of (Rf*Sp + Sr) as defined by [RFC6550]. Specifically, when an Objective Function computes Rank, this is defined as an unsigned integer (i.e., a 16-bit value) Rank quantity. When the Rank is compared, e.g. to determine parent relationships or loop detection, the integer portion of the Rank is used. The integer portion of the Rank is computed by the DAGRank() macro as floor(x) where floor(x) is the function that evaluates to the greatest integer less than or equal to x. DAGRank(rank) = floor(rank/MinHopRankIncrease) Rank computation scenario +-------+ | P | R(P) | | +-------+ | | | +-------+ | N | R(N)=R(P) + rank_increase | | rank_increase = (Rf*Sp + Sr) * MinHopRankIncrease +-------+ Sp=2*ETX 7.1.2. Rank computation Example This section illustrates with an example the use of the Objective Function Zero. Assume the following parameters: Vilajosana & Pister Expires December 29, 2014 [Page 14] Internet-Draft 6tisch-minimal June 2014 Rf = 1 Sp = 2* ETX Sr = 0 minHopRankIncrease = 256 (default in RPL) ETX=(numTX/numTXAck) r(n) = r(p) + rank_increase rank_increase = (Rf*Sp + Sr) * minHopRankIncrease rank_increase = 512*numTx/numTxACK Vilajosana & Pister Expires December 29, 2014 [Page 15] Internet-Draft 6tisch-minimal June 2014 Rank computation example for 5 hop network where numTx=100 and numTxAck=75 for all nodes +-------+ | 0 | R(0)=0 | | DAGRank(R(0)) = 0 +-------+ | | +-------+ | 1 | R(1)=R(0)+683=683 | | DAGRank(R(1)) = 2 +-------+ | | +-------+ | 2 | R(2)=R(1)+683=1366 | | DAGRank(R(2)) = 5 +-------+ | | +-------+ | 3 | R(3)=R(2)+683=2049 | | DAGRank(R(3)) = 8 +-------+ | | +-------+ | 4 | R(4)=R(3)+683=2732 | | DAGRank(R(4)) = 10 +-------+ | | +-------+ | 5 | R(5)=R(4)+683=3415 | | DAGRank(R(5)) = 13 +-------+ 7.2. RPL Configuration In addition to the Objective Function (OF), a minimal configuration for RPL should indicate the preferred mode of operation and trickle timer operation so different RPL implementations can inter-operate. RPL information SHOULD be transported in the flow label in the LLN as defined in [I-D.thubert-6man-flow-label-for-rpl] Vilajosana & Pister Expires December 29, 2014 [Page 16] Internet-Draft 6tisch-minimal June 2014 7.2.1. Mode of Operation For downstream route maintenance, in a minimal configuration, RPL SHOULD be set to operate in the Non-Storing mode as described by [RFC6550] Section 9.7. Storing mode ([RFC6550] Section 9.8) MAY be supported in less constrained devices. 7.2.2. Trickle Timer RPL signaling messages such as DIOs are sent using the Trickle Algorithm [RFC6550] (Section 8.3.1) and [RFC6206]. For this specification, the Trickle Timer MUST be used with the RPL defined default values [RFC6550] (Section 8.3.1). For a description of the Trickle timer operation see Section 4.2 on [RFC6206]. 7.2.3. Hysteresis According to [RFC6552], [RFC6719] recommends the use of a boundary value (PARENT_SWITCH_THRESHOLD) to avoid constant changes of parent when ranks are compared. When evaluating a parent that belongs to a smaller path cost than current minimum path, the candidate node is selected as new parent only if the difference between the new path and the current path is greater than the defined PARENT_SWITCH_THRESHOLD. Otherwise the node MAY continue to use the current preferred parent. As for [RFC6719] the recommended value for PARENT_SWITCH_THRESHOLD is 192 when ETX metric is used, the recommendation for this document is to use PARENT_SWITCH_THRESHOLD equal to 394 as the metric being used is 2*ETX. This is mechanism is suited to deal with parent hysteresis in both cases routing parent and time source neighbor selection. 7.2.4. Variable Values The following table presents the RECOMMENDED values for the RPL- related variables defined in the previous section. Vilajosana & Pister Expires December 29, 2014 [Page 17] Internet-Draft 6tisch-minimal June 2014 Recommended variable values +-------------------------+----------+ | Variable | Value | +-------------------------+----------+ | EB_PERIOD | 10s | +-------------------------+----------+ | MAX_EB_DELAY | 180 | +-------------------------+----------+ | NUM_NEIGHBOURS_TO_WAIT | 2 | +-------------------------+----------+ | PARENT_SWITCH_THRESHOLD | 394 | +-------------------------+----------+ 8. Acknowledgements The authors would like to acknowledge the guidance and input provided by the 6TiSCH Chairs Pascal Thubert and Thomas Watteyne. 9. References 9.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC6206] Levis, P., Clausen, T., Hui, J., Gnawali, O., and J. Ko, "The Trickle Algorithm", RFC 6206, March 2011. [RFC6550] Winter, T., Thubert, P., 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, March 2012. [RFC6551] Vasseur, JP., Kim, M., Pister, K., Dejean, N., and D. Barthel, "Routing Metrics Used for Path Calculation in Low-Power and Lossy Networks", RFC 6551, March 2012. [RFC6552] Thubert, P., "Objective Function Zero for the Routing Protocol for Low-Power and Lossy Networks (RPL)", RFC 6552, March 2012. [RFC6719] Gnawali, O. and P. Levis, "The Minimum Rank with Hysteresis Objective Function", RFC 6719, September 2012. Vilajosana & Pister Expires December 29, 2014 [Page 18] Internet-Draft 6tisch-minimal June 2014 9.2. Informative References [I-D.ietf-6tisch-tsch] Watteyne, T., Palattella, M., and L. Grieco, "Using IEEE802.15.4e TSCH in an LLN context: Overview, Problem Statement and Goals", draft-ietf-6tisch-tsch-00 (work in progress), November 2013. [I-D.ietf-6tisch-architecture] Thubert, P., Watteyne, T., and R. Assimiti, "An Architecture for IPv6 over the TSCH mode of IEEE 802.15.4e", draft-ietf-6tisch-architecture-02 (work in progress), June 2014. [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-01 (work in progress), February 2014. [I-D.ietf-6tisch-6top-interface] Wang, Q., Vilajosana, X., and T. Watteyne, "6TiSCH Operation Sublayer (6top) Interface", draft-ietf-6tisch- 6top-interface-00 (work in progress), March 2014. [I-D.richardson-6tisch-security-architecture] Richardson, M., "security architecture for 6top: requirements and structure", draft-richardson-6tisch- security-architecture-02 (work in progress), April 2014. [I-D.ietf-roll-terminology] Vasseur, J., "Terms used in Routing for Low power And Lossy Networks", draft-ietf-roll-terminology-13 (work in progress), October 2013. [I-D.phinney-roll-rpl-industrial-applicability] Phinney, T., Thubert, P., and R. Assimiti, "RPL applicability in industrial networks", draft-phinney-roll- rpl-industrial-applicability-02 (work in progress), February 2013. [I-D.thubert-6man-flow-label-for-rpl] Thubert, P., "The IPv6 Flow Label within a RPL domain", draft-thubert-6man-flow-label-for-rpl-03 (work in progress), May 2014. Vilajosana & Pister Expires December 29, 2014 [Page 19] Internet-Draft 6tisch-minimal June 2014 9.3. 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) Amendment 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. [decouti03high] De Couto, D., Aguayo, D., Bicket, J., and R. Morris, "A High-Throughput Path Metric for Multi-Hop Wireless Routing", MobiCom '03, The 9th ACM International Conference on Mobile Computing and Networking, San Diego, California", June 2003. [OpenWSN] "Berkeley's OpenWSN Project Homepage", . Authors' Addresses Xavier Vilajosana (editor) Universitat Oberta de Catalunya 156 Rambla Poblenou Barcelona, Catalonia 08018 Spain Phone: +34 (646) 633 681 Email: xvilajosana@uoc.edu Kris Pister University of California Berkeley 490 Cory Hall Berkeley, California 94720 USA Email: pister@eecs.berkeley.edu Vilajosana & Pister Expires December 29, 2014 [Page 20]