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 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.
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. The IEEE 802.15.4 specification provides a mechanism whereby the details of slotframe length, timeslot timing, and channel hopping pattern are communicated when a node time synchronizes to the network . This document describes specific settings for these parameters.
This document uses terminology from the Terminology in IPv6 over the TSCH mode of IEEE 802.15.4e . The following concepts are used in this document:
CCA: Clear Channel Assessment.
In order to form a network, a set of conventions need to be taken in order to enable initial advertising of the network. Besides a set of parameters need to be defined so joining nodes are configured so the network can be formed and nodes interoperate.
These set of rules and default parameters conform a minimal configuration that nodes implementing this specification MUST comply. The timeslot timing, slotframe length, the number of active cells, their slot offset and frequency offset and the purpose of the cells are
mandatory configurations for two nodes to communicate. The present document defines those rules. Table 1 summarizes the main configuration parameters for a "minimal" configuration.
A joining node learns the minimal configuration from the Enhanced Beacon, except for the security keys. How the security keys are obtained is out of the scope of this document. More details are given in .
The present specification is independent of the actual physical layer and it is only dependent on the IEEE 802.15.4 TSCH MAC layer specification.
The slotframe, as defined in the Terminology in IPv6 over the TSCH mode of IEEE 802.15.4e , is an abstraction of the link layer that defines a collection of timeslots of equal length that repeats over time. In order to set up a minimal TSCH network, nodes need to be time synchronized and configured to use the same slotframe configuration so they can communicate. Compliant nodes SHOULD obey to the following configuration as defined in Table 1:
The slotframe is composed of a configurable number of timeslots. The number of timeslots in the slotframe is referred as slotframe length . This document defines a default slotframe length of 11 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 cell in the 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 cell MAY be scheduled at any slotOffset (default 0x00) and any channelOffset (default 0x00) within the slotframe
and this location MUST be announced in the EBs. EBs are sent using this active cell to the link-layer broadcast address (and are therefore not acknowledged). Data packets, as described in , use the same active cell.
Per IEEE 802.15.4 specification, 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 determines the duty cycle of the network and MUST be announced in the SlotFrame and Link IE of the EB.
For example, a network with a 0.99% duty cycle (as presented in Figure 1) is composed of a slotframe of 101 timeslots, which includes 1 active cell.
In a minimal configuration, a default timeslot duration set to 10ms and its corresponding default timeslot internal timings defined by the IEEE 802.15.4 specification SHOULD be used .
The timeslot timing is defined by the macTimeslotTemplate in the IEEE802.15.4 specification. The use of the default macTimeslotTemplate MUST be announced in the Enhanced Beacon (EB) by using the Timeslot Information Element (IE) containing only the default macTimeslotTemplateId.
Other timeslot durations MAY be supported and MUST be announced in the EBs. Joining nodes MUST learn the configuration from the received EB. If a network uses a timeslot duration different than the default (10ms), EBs MUST contain the complete Timeslot IE and fill all the fields of the macTimeslotTemplate as described in .
Nodes not supporting the default timeslot value SHOULD be clearly indicated.
According to the IEEE 802.15.4 TSCH specification, each scheduled cell is associated with a LinkOption bitmap . The active cell in the minimal configuration MUST use a LinkOption with Value 0x0F. The bitmap in the active cell indicate that a node transmits if there is a packet in its queue, listens otherwise as the "Transmit" and "Receive" bits are set. A "Shared" bit is set and therefore the back-off mechanism defined in the IEEE 802.15.4 specification is used to resolve contention when transmitting . This results in "Slotted Aloha" behavior. The "Timekeeping" flag is set so nodes initially joining the network can maintain time synchronization to the advertising node using that cell. Other time source neighbours MAY be selected using the routing structure, e.g the DODAG structure of the network if RPL is used.
LinkOption bitmap setting for the active cell in the minimal configuration slotframe:
b0 = Transmit = 1 (set)b1 = Receive = 1 (set)b2 = Shared = 1 (set)b3 = Timekeeping = 1 (set)b4-b7 = Reserved (clear)
In addition, the scheduled cell in the schedule is configured as a Hard cell indicating that cannot be moved or relocated by any dynamic scheduling mechanism. 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 bitmap associated with those dynamically scheduled cells (i.e. they can be Hard cells or Soft cells).
All remaining cells are unscheduled. In unscheduled cells, the nodes SHOULD keep their radio off.
The maximum number of link layer retransmissions is set to 3. For packets which require an acknowledgement, if none are 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.
Figure 2 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 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 acknowledgement 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. For the minimal configuration specified in this document, the use of CCA is OPTIONAL.
The timing parameters for the default macTimeslotTemplate (macTimeslotTemplateId = 0) MUST be used when utilizing the default timeslot duration. In this case, the TSCH 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 that in case of discrepancy between the values in this document and the IEEE 802.15.4 specification , 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 a network by receiving a properly formed BEACON MUST use a PAN ID set to 0xffff in order meet the filtering rules in the IEEE 802.15.4 specification .
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". According to Table 2a in the IEEE 802.15.4e 2012 specification document, this is accomplished by setting the PAN ID Compression bit to 0b0 .
When preparing the security header, the Absolute Sequence Number (ASN) MUST be written into the Nonce in most significant byte first (MSB) format as indicated in the IEEE 802.15.4 specification .
The IEEE 802.15.4 specification does not define how often EBs are sent, nor their contents . EBs MUST NOT be used for time synchronization. Time synchronization is achieved via acknowledgements to normal packet traffic, and keepalives. For additional reference see where different time synchronization approaches are summarized.
In a minimal TSCH configuration, a node SHOULD send an EB every EB_PERIOD (default value = 10s). EBs are only authenticated and neither Payload IEs nor the frame payload are encrypted.
EBs MUST be sent as per the IEEE 802.15.4 specification 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 template is used to specify the internal timing of the timeslot. This specification uses the default timeslot template as defined in the IEEE 802.15.4 specification . 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 IEEE 802.15.4 specification . 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. SlotFrame and Link IE: Each node MUST indicate the schedule in each EB through a SlotFrame and Link IE. This enables nodes which implement the IEEE 802.15.4 specification to learn the schedule used in the network as they join it. This draft defines the use of a single cell set at the default channel offset 0x00, default timeslot offset 0x00 and with linkOption 0x0F (TX, RX, SHARED bits set). A node SHOULD indicate the same information in the "SlotFrame and Link IE" in the EBs it sends, than the "SlotFrame and Link IE" information it has received in an EB.
Unicast frames sent to a unicast MAC destination address MUST request an acknowledgement. Each acknowledgement MUST contain an ACK/NACK Time Correction IE.
The IEEE 802.15.4 specification does not define how and when each node in the network keeps information about its neighbours.
A node SHOULD keep at least the following information in a neighbour table:
The exact format of the neighbour table is implementation-specific.
Future version of the 6top Protocol MAY require those information and statistics.
The neighbour table SHOULD contain the following information for each neighbour:
Neighbour statistics:
numTx: number of transmitted packets to that neighbournumTxAck: number of transmitted packets that have been acknowledged by that neighbournumRx: number of received packets from that neighbour
The EUI64 of the neighbour.
Timestamp of the last frame received from that neighbour. This can be based on the ASN counter or any other time base. It can be used to trigger a keep-alive message.
Routing metric such as the RPL rank of that neighbour.
A flag indicating whether this neighbour is a time source neighbour.
Connectivity statistics (e.g., RSSI), which can be used to determine the quality of the link.
In addition to that information, each node in a multihop topology and implementing RPL has to be able to compute some RPL Objective Function (OF), taking into account the neighbour 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 Neighbour among the nodes in its routing parent set (e.g the RPL parent set).
When a node joins a network, it has no routing information. To select its time source neighbour, it uses the Join Priority field in the EB, as described in the IEEE 802.15.4 specification . The Sync IE contains the ASN and 1 Byte field named Join Priority.
The Join Priority of any node MUST be based on the routing metric of the network and normalized to a value between 0 and 15. In case that the network uses RPL, 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 MUST also be 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 the IEEE 802.15.4 specification . 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 applies to other routing protocols with its corresponding routing metrics. This avoids inconsistencies in the time synchronization structure.
As soon as a node acquires routing information (e.g RPL rank (see and )), it SHOULD send Enhanced Beacons including a Sync IE with Join Priority field set as indicated above.
If a node receives EBs from different nodes with equal Join Priority, the time source neighbour selection SHOULD be assessed by other metrics that can help to determine the better connectivity link.
Time source neighbour hysteresis SHOULD be used, according to the rules defined in . At any time, a node MUST maintain connectivity to at least one time source neighbour. New time source neighbours MUST be chosen among the neighbours in the routing parent set.
The decision for a node to select one Time Source Neighbour when multiple EBs are received is implementation-specific.
For example, a node MAY wait until one EB from NUM_NEIGHBOURS_TO_WAIT neighbours have been received to select the best Time Source Neighbour. 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 Neighbour.
Optionally, some form of hysteresis SHOULD be implemented to avoid frequent changes in time source neighbours.
The IEEE 802.15.4 specification does not define the use of queues to handle upper layer data (either application or control data from upper layers). A single queue with the following rules SHOULD be used:
A node MUST not queue frames containing higher-layer packets until the node has successfully joined a network.
Frames generated by the IEEE 802.15.4 layer are queued with higher priority than frames containing higher-layer packets.
Frame types BEACON and COMMAND are queued with higher priority than frame types DATA and ACK.
One entry in the queue is reserved at all times for frames of types BEACON and 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 the IEEE 802.15.4 specification . 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 , 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").
In a multi-hop topology, the RPL routing protocol MAY be used.
If RPL is used, nodes MUST implement the RPL Objective Function Zero (OF0) .
The rank computation is described at , Section 4.1.
A node rank (see Figure 3 for an example) 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. A minimal configuration SHOULD set rank_factor to 1.
Sp (step_of_rank): (strictly positive integer) - an intermediate computation based on the link properties with a certain neighbour, 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. Sp SHOULD be set to (3*ETX)-2. Candidate parents with ETX greater than 3 SHOULD not be selected.
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. 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 . 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 (refer to Figure 4 for specific details). 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 using RPL 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 follow the IP in IP encapsulation requirement specified by the and . RPI and RH3 extension headers and inner IP headers MUST be compressed according to .
When RPL is used, nodes 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 PARENT_SWITCH_THRESHOLD SHOULD be set to 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 neighbour selection.
Table 2 presents the values for the variables defined in this document that SHOULD be used.
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 guidance and advice.
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 as amended by IEEE std.
802.15.4e, Part. 15.4: Low-Rate Wireless Personal Area
Networks (LR-WPANs) Amendment 1: MAC sublayer
IEEE standard for Information Technology
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
IEEE standard for Information Technology
A High-Throughput Path Metric for Multi-Hop Wireless Routing
6TiSCH Operation Sublayer (6top) (work in progress)
IETF
Recommendation for Block Cipher Modes of Operation: The CCM Mode for Authentication and Confidentiality. SP 800-38C
National Institute of Standards and Technology
Formal Specification of the CCM* Mode of Operation, IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs).
OpenWSN: a Standards-Based Low-Power Wireless Development Environment
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 that 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 latter has precedence.
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. Figure 5 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. Refer to Figure 6 for the specific IE fields.
Acknowledgement packets carry the ACK/NACK Time Correction IE (Header IE). Figure 7 illustrates the IE format as specified in .
Figure 8 illustrates the 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).