Minimal 6TiSCH Configuration
Universitat Oberta de Catalunya156 Rambla PoblenouBarcelonaCatalonia08018Spain+34 (646) 633 681xvilajosana@uoc.eduUniversity of California Berkeley490 Cory HallBerkeleyCalifornia94720USApister@eecs.berkeley.edu
Internet Area
6TiSCHDraft
This document describes the minimal set of rules to operate an IEEE 802.15.4 Timeslotted Channel Hopping (TSCH) network. This minimal mode of operation can be used during network bootstrap, as a fall-back mode of operation when no dynamic scheduling solution is available or functioning, or during early interoperability testing and development.
The nodes in a IEEE 802.15.4 TSCH network follow a communication schedule. The entity (centralized or decentralized) responsible for building and maintaining that schedule has 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 more important than efficiency. One goal of this document is to define the simplest set of rules for building a TSCH-compliant network, at the necessary price of lesser efficiency. Yet, this minimal mode of operation MAY 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, the minimal configuration MAY be used as a fall-back mode of operation, ensuring connectivity of nodes in case that dynamic scheduling mechanisms fail or are not available. provides a mechanism whereby the details of slotframe length, timeslot timing, and channel hopping pattern are communicated when a node synchronizes to the network. This document describes specific settings for these parameters. Nodes must broadcast properly-formed Enhanced Beacons to announce these values.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.
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.
The slotframe, as defined in , 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 communicate. This document recommends the following slotframe configuration.
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/receive both EBs and data link-layer frames. 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 to the link-layer broadcast address (and are therefore not acknowledged). Data packets, as described in , use the same active slot. Per , data packets sent unicast on this cell are acknowledged by the receiver. The remaining cells in the slotframe are unscheduled, and MAY be used by other (dynamic) scheduling solutions. Details about such dynamic scheduling solution are out of scope of this document. Details about the usage of the non scheduled cells are out of scope of this document.
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 0.99% 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 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 in the EBs. If one uses a timeslot duration different than 10ms, EBs MUST contain the complete TimeSlot IE as described in . This document also recommends to clearly indicate nodes not supporting the default timeslot value.
Per TSCH, each scheduled cell has an associated bitmap of cell options, called LinkOptions. The scheduled cell in the minimal schedule is configured as a Hard cell . Additional available cells MAY 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, listens otherwise. Because the "shared" bit is set, the back-off mechanism defined in is used to resolve contention when transmitting. This results in "Slotted Aloha" behavior. The "Timekeeping" flag is set so nodes initialy joining the network can maintin synchronization to the advertising node using that slot. Other time source neighbors are selected using the DODAG structure of the network (detailed below).
b0 = Transmit = 1 (set)b1 = Receive = 1 (set)b2 = Shared = 1 (set)b3 = Timekeeping = 1 (set)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.
The maximum number of link layer retransmissions is set to 3. For packets which require an acknowledgment, if none is received after a total of 4 attempts, the transmission 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.
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 acknowledgment 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 bit after the Start of Frame Delimiter (SFD) of the transmitted packet leaves the radio of the TX node.
The radio of the RX node is turned on tsRxWait/2 before that instant, and listens for at least tsRxWait. This allows for a de-synchronization between the two nodes of at most tsRxWait/2 in either direction (early or late). The RX node needs to send the first bit after the SFD of the MAC acknowledgment exactly TsTxAckDelay after the end of the last byte of the received packet. TX's radio has to be turned on tsAckWait/2 before that time, and keep listening for at least tsAckWait. The TX node can perform a Clear Channel Assessment (CCA) if required, this does not interfere with the scope of this document. As for a minimal configuration, CCA is OPTIONAL.
The timing parameters for the default macTimeslotTemplate (macTimeslotTemplateId = 0) MUST be used when utilizing the default time slot duration. In this case, the Timeslot IE only transports the macTimeslotTemplateId with value 0x00. If a timeslot template other than the default is used, the EB MUST contain a complete TimeSlot IE indicating the timeslot duration and the corresponding timeslot timings. Note however that in case of discrepancy between the values in this document and , the IEEE standard has precedence.
The IEEE802.15.4 header of BEACON, DATA, ACKNOWLEDGEMENT, MAC COMMAND frames MUST include the Sequence Number field, the Source Address field and the Destination Address field. Frame Version field MUST be set to 0b10 (Frame Version 2).
The PAN ID Compression bit in a BEACON frame MUST indicate that the Source PAN ID is "Not Present" and the Destination PAN ID is "Present". The source address field MUST be filled with an extended address (64 bit) and this be indicated in the corresponding Frame Control field. The Destination address field MUST be filled with a short address (16bit) with a value of 0xffff to represent the broadcast short address.
A Node aiming to join the network by receving a properly formed BEACON shall enter in a scan phase and shall store the value of macPANId and then set it to 0xffff for the duration of the scan in order to meet the filtering rules in .
When using DATA, ACKNOWLEDGEMENT, MAC COMMAND frame types the source and destination address fields MUST be filled with an extended address (64 bit) and this be indicated in the corresponding Frame Control field. The Destination PAN ID MUST be present, the Source PAN ID MUST be elided. The PAN ID Compression field MUST indicate that the Destination PAN ID is "Present" and the Source PAN ID is "Not Present". With and according to Table 2a, this is accomplished by setting the PAN ID Compression bit to 0b0.
When preparing the security header, the ASN MUST be written into the Nonce in MSB format (most significant byte first) as indicated in .
does not define how often EBs are sent, nor their contents. EBs MUST NOT in general be used for synchronization. Synchronization is achieved via acknowledgements to normal packet traffic, and keepalives. For a minimal TSCH configuration, a mote SHOULD send an EB every EB_PERIOD. For additional reference see where different synchronization approaches are summarized. EBs are only authenticated and neither Payload IEs nor the frame payload are encrypted.
EBs MUST be sent as per and MUST carry the Information Elements (IEs) listed below. Refer to for an example of the Information Elements Header Content.
Synchronization IE: Contains synchronization information such as ASN and Join Priority. The value of Join Priority is discussed in . TSCH Timeslot IE: Contains the timeslot template identifier. This specification uses the default timeslot template as defined in . In the case that a non-default timeslot template is used, the IE Content MUST follow the specification as defined in . Refer to for an illustrative example of non default timeslot template. Channel Hopping IE: Contains the channel hopping template identifier. This specification uses the default channel hopping template, as defined in . The default sequence for the 2.4GHz OQPSK PHY is [5, 6, 12, 7, 15, 4, 14, 11, 8, 0, 1, 2, 13, 3, 9, 10] . Note however that in case of discrepancy between the values in this document and , the IEEE standard specification has preference. Frame and Link IE: Each node MUST indicate the schedule in each EB through a Frame and Link IE. This enables nodes which implement to learn the schedule used in the network as they join it. This draft defines the use of a single cell set at channel offset 0x00, slot offset 0x00 and with linkOption 0x0F (TX, RX, SHARED bits set).
Link-layer acknowledgment frames are built according to . Unicast frames sent to a unicast MAC destination address MUST request an acknowledgment. The sender node MUST set the ACK requested bit in the MAC header. The acknowledgment frame is of type ACK (frame type value 0b010). Each acknowledgment contains the following IE:
ACK/NACK Time Correction IE: The ACK/NACK time correction IE carries the measured de-synchronization between the sender and the receiver. Refer to for an example of the Header IE content. The possible values for the Time Synchronization Information and ACK status are described in . does not define how and when each node in the network keeps information about its neighbors. Keeping the following information in the neighbor table is RECOMMENDED:
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 neighbornumTxAck: number of transmitted packets that have been acknowledged by that neighbornumRx: number of received packets from that neighbor
The EUI64 of the neighbor.
Timestamp of the last frame received from that neighbor. This can be based on the ASN counter or any other time base. It 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.
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 and .
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 . The Sync IE contains the ASN and 1 Byte field named Join Priority. The Join Priority of any node MUST be equivalent to the result of the function DAGRank(rank)-1. The Join Priority of the DAG root is also equivalent to DAGRank(rank)-1. According to the DAGRank(rank(0)) = 1 and therefore the DAGRank(rank(0))-1 is 0 which is compliant with the requirement of Join Priority = 0 imposed by . A lower value of the Join Priority indicates higher preference to connect to that device. 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 and ), it SHOULD send Enhanced Beacons including a Sync IE with Join Priority field set to DAGRank(rank)-1, 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 . At any time, a node MUST maintain connectivity to at least one time source neighbor. New time source neighbors 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 implementation-specific.
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.
does not define the use of queues to handle upper layer data (either application or control data from upper layers). The use of a single queue with the following rules is RECOMMENDED:
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 queuing priority than packets received from a higher layer.
Frame types Beacon and Command have a higher queuing priority than frame types Data and ACK.
One entry in the queue is reserved at all times for frames of types Beacon or Command frames.
As this document refers to the interaction between Layer 3 and Layer
2 protocols, this interaction MUST be secured by L2 security
mechanisms as defined by . Two security mechanisms are
considered, authentication and encryption, authentication applies to
all packet content while encryption applies to header IEs and MAC
payload. Key distribution is out of scope of this document, but
examples include pre-configured keys at the nodes, shared keys among
peers or well-known keys.
The present document assumes the existence of two cryptographic keys,
which can be pre-configured. One of the keys (K1) is used to
authenticate EBs. As defined in Section , EBs MUST be
authenticated, with no payload encryption. This facilitates logical
segregation of distinct networks. A second key (K2) is used to
authenticate DATA, ACKNOWLEDGEMENT, MAC COMMAND frame types and
respective header IEs, with payload encryption. Depending on
security policy, these keys could be the same (i.e., K1=K2).
For early interoperability, K1 MAY be set to 36 54 69 53 43 48 20 6D 69 6E 69 6D 61 6C 31 35 ("6TiSCH minimal15").
Nodes in a multihop network MUST use the RPL routing protocol .
Nodes in the multihop network MUST implement the RPL Objective Function Zero .
The rank computation is described at , Section 4.1. A node rank is computed by the following equation:
R(N) = R(P) + rank_increment
rank_increment = (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_increment: 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_increment computation. If none is configured, rank_factor of 1 is used. In this specification, a rank_factor of 1 SHOULD be used.
Sp (step_of_rank): (strictly positive integer) - an intermediate computation based on the link properties with a certain neighbor, i.e., the Expected Transmission Count (ETX) which provides an average of number of packet transmissions between two nodes. ETX is defined in detail by and . The ETX is computed as the inverse of the Packet Delivery Ratio (PDR), this is the number of transmitted packets, divided by the number of acknowledged packets to a certain node (e.g., ETX = numTX/numTXAck). An implementation MUST follow OF0’s normalization guidance as discussed in Section 1 and Section 4.1 of , maintaining Sp between MINIMUM_STEP_OF_RANK of 1 to indicate a great quality and MAXIMUM_STEP_OF_RANK of 9 to indicate a really poor quality, with DEFAULT_STEP_OF_RANK indicating a normal, average quality. One RECOMMENDED way to achieve this in an interoperable fashion is to set Sp to (3*ETX)-2.
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 SHOULD be set to 0.
MinHopRankIncrease: the MinHopRankIncrease is set to the fixed constant DEFAULT_MIN_HOP_RANK_INCREASE . DEFAULT_MIN_HOP_RANK_INCREASE has a value of 256.
DAGRank(rank): Equivalent to the floor of "rank" as defined by . 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). Nodes compute its DAGRank(rank) using DAGRank(R(N)).
This section illustrates with an example the use of the Objective Function Zero. Assume the following parameters:
Rf = 1 Sp = (3*ETX)-2 Sr = 0minHopRankIncrease = 256 (default in RPL)ETX=(numTX/numTXAck)r(n) = r(p) + rank_incrementrank_increment = (Rf*Sp + Sr) * minHopRankIncreaserank_increment = ((3*numTx/numTxAck)-2)*minHopRankIncrease = 512
In addition to the Objective Function (OF), nodes in a multihop network MUST indicate the preferred mode of operation using the MOP field in DIO. Nodes not being able to operate in the specified mode of operation MUST only join as leaf nodes. RPL information and hop-by-hop extension headers MUST follow and specification. In the case that the packets formed at the LLN need to cross through intermediate routers, these MUST obey to the IP in IP encapsulation requirement specified by the and . RPI and RH3 extension headers and inner IP headers MUST be compressed according to .
Nodes implementing a minimal configuration and forming a multihop network, MUST support the non-storing ( Section 9.7) mode of operation. The storing ( Section 9.8) mode of operation SHOULD be supported by nodes with enough capabilities. Non-storing mode of operation is the default mode that a node selects when acting as a DAG root.
RPL signaling messages such as DIOs are sent using the Trickle Algorithm (Section 8.3.1) and . For this specification, the Trickle Timer MUST be used with the RPL defined default values (Section 8.3.1). For a description of the Trickle timer operation see Section 4.2 on .
According to , 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 the recommended value for PARENT_SWITCH_THRESHOLD is 192 when ETX metric is used (in the form 128*ETX), the recommendation for this document is to use PARENT_SWITCH_THRESHOLD equal to 640 if the metric being used is (3*ETX-2)*minHopRankIncrease, or a proportional value. This mechanism is suited to deal with parent hysteresis in both cases including routing parent and time source neighbor selection.
The following table presents the RECOMMENDED values for the RPL-related variables defined in the previous section.
Several examples are provided to illustrate the content of the packets used by the minimal configuration as proposed by this document. Each example follows the same structure presenting first a schematic header diagram, then the LSB stream of bytes that conform the header and finally a description of each of the IEs the form the packet. The packet formats are specific for the revision and may vary in future releases of the IEEE standard. In case of differences between the packet content presented in this section and the , the later has presedence.
The MAC header fields are described in a specific order. All field formats in this examples are depicted in the order in which they are transmitted by the PHY, from left to right, where the leftmost bit is transmitted first in time. Bits within each field are numbered from 0 (leftmost and least significant) to k – 1 (rightmost and most significant), where the length of the field is k bits. Fields that are longer than a single octet are sent to the PHY in the order from the octet containing the lowest numbered bits to the octet containing the highest numbered bits, hence little endian ordering.
Mandatory content for the EB as proposed by this draft. The example uses a slotframe of 101 slots. The following figure represents schematically the Header IE and Payload IE content of an EB.
Using a non-default timeslot template in EBs. Timeslot length set to 15ms instead of the 10ms default.
Acknowledgement packets carry the ACK/NACK Time Correction IE (Header IE). The following example illustrates the IE format as specified in .
The example illustrates content of the Auxiliary Security Header as mandated by this document, if security is enabled. Security Level in the example is set to ENC-MIC-32 (5).
This document requests no immediate action by IANA.
The authors would like to acknowledge the guidance and input provided by Rene Struik, Pat Kinney, Michael Richardson, Tero Kivinen, Nicola Accettura, Malisa Vucinic and for the exhaustive and detailed review of the examples section to Simon Duquennoy, Guillaume Gaillard, Tengfei Chang and Jonathan Muñoz. Also our acknowledge to the 6TiSCH Chairs Pascal Thubert and Thomas Watteyne for their guideance and advice.
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