| RFC 9033 | 6TiSCH MSF | May 2021 |
| Chang, et al. | Standards Track | [Page] |
This specification defines the "IPv6 over the TSCH mode of IEEE 802.15.4" (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 is built upon the 6TiSCH Operation Sublayer Protocol (6P) and the minimal security framework for 6TiSCH.¶
This is an Internet Standards Track document.¶
This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841.¶
Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc9033.¶
Copyright (c) 2021 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.¶
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]. This specification complements [RFC8480] by providing the rules of when to add and delete cells in the communication schedule. This specification satisfies all the requirements for an SF listed in Section 4.2 of [RFC8480].¶
MSF builds on top of the following specifications: "Minimal IPv6 over the TSCH Mode of IEEE 802.15.4e (6TiSCH) Configuration" [RFC8180], "6TiSCH Operation Sublayer (6top) Protocol (6P)" [RFC8480], and "Constrained Join Protocol (CoJP) for 6TiSCH" [RFC9031].¶
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 six steps described in Section 4. The end state of the join process is that the node is synchronized to the network, has mutually authenticated with the network, has identified a routing parent, and has scheduled one negotiated Tx cell (defined in Section 5.1) to/from its routing parent. After the join process, the node can continuously add, delete, and relocate cells as described in Section 5. It does so for three reasons: to match the link-layer resources to the traffic, to handle changing parent, and to handle a schedule collision.¶
MSF works closely with the IPv6 Routing Protocol for Low-Power and Lossy Networks (RPL), specifically the routing parent defined in [RFC6550]. This specification only describes how MSF works with the routing parent; this parent is referred to as the "selected parent". The activity of MSF towards the single routing parent is called a "MSF session". Though the performance of MSF is evaluated only when the "selected parent" represents the node's preferred parent, there should be no restrictions to use multiple MSF sessions, one per parent. The distribution of traffic over multiple parents is a routing decision that is out of scope for MSF.¶
MSF is designed to operate in a wide range of application domains. It is optimized for applications with regular upstream traffic, from the nodes to the Destination-Oriented Directed Acyclic Graph (DODAG) root [RFC6550].¶
This specification follows the recommended structure of an SF specification, given in Appendix A of [RFC8480], with the following adaptations:¶
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 BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
This specification uses messages and variables defined in IEEE Std 802.15.4-2015 [IEEE802154]. It is expected that those resources will remain in the future versions of IEEE Std 802.15.4; in which case, this specification also applies to those future versions. In the remainder of the document, we use [IEEE802154] to refer to IEEE Std 802.15.4-2015 as well as future versions of IEEE Std 802.15.4 that remain compatible.¶
In a Time-Slotted Channel Hopping (TSCH) network, time is sliced up into time slots. The time slots are grouped as one or multiple slotframes that repeat over time. The TSCH schedule instructs a node what to do at each time slot, such as transmit, receive, or sleep [RFC7554]. For time slots for transmitting or receiving, a channel is assigned to the time slot. The tuple (slot, channel) is indicated as a cell of the TSCH schedule. MSF is one of the policies defining how to manage the TSCH schedule.¶
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. However, an implementor MAY implement MSF based on other specifications as long as the specification defines a way to advertise the Enhanced Beacons (EBs) and DODAG Information Objects (DIOs) among the network.¶
MSF uses the minimal cell for broadcast frames such as Enhanced Beacons (EBs) [IEEE802154] and broadcast DODAG Information Objects (DIOs) [RFC6550]. Cells scheduled by MSF are meant to be used only for unicast frames.¶
To ensure there is enough bandwidth available on the minimal cell, a node implementing MSF SHOULD enforce some rules for limiting the traffic of broadcast frames. For example, the overall broadcast traffic among the node and its neighbors SHOULD NOT exceed one-third of the bandwidth of minimal cell. One of the algorithms that fulfills this requirement is the Trickle timer defined in [RFC6206], which is applied to DIO messages [RFC6550]. However, any such algorithm of limiting the broadcast traffic to meet those rules is implementation-specific and is out of the scope of MSF.¶
Three slotframes are used in MSF. MSF schedules autonomous cells at Slotframe 1 (Section 3) and 6P negotiated cells at Slotframe 2 (Section 5), while Slotframe 0 is used for the bootstrap traffic as defined in the minimal 6TiSCH configuration. The same slotframe length for Slotframe 0, 1, and 2 is RECOMMENDED. Thus it is possible to avoid the scheduling collision between the autonomous cells and 6P negotiated cells (Section 3). The default slotframe length (SLOTFRAME_LENGTH) is RECOMMENDED for Slotframe 0, 1, and 2, although any value can be advertised in the EBs.¶
MSF nodes initialize Slotframe 1 with a set of default cells for unicast communication with their neighbors. These cells are called "autonomous cells", because they are maintained autonomously by each node without negotiation through 6P. Cells scheduled by 6P Transaction are called "negotiated cells", which are reserved on Slotframe 2. How to schedule negotiated cells is detailed in Section 5. There are two types of autonomous cells:¶
To compute a [slotOffset,channelOffset] from an EUI-64 address, nodes MUST use the hash function SAX as defined in Section 2 of [SAX-DASFAA] with consistent input parameters, for example, those defined in Appendix A. The coordinates are computed to distribute the cells across all channel offsets, and all but the first slot offset 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 EUI-64 address are computed as follows:¶
slotOffset(MAC) = 1 + hash(EUI64, length(Slotframe_1) - 1)¶
channelOffset(MAC) = hash(EUI64, NUM_CH_OFFSET)¶
The second input parameter defines the maximum return value of the hash function. Other optional parameters defined in SAX determine the performance of SAX hash function. Those parameters could be broadcast in an EB frame or preconfigured. For interoperability purposes, Appendix A provides the reference values of those parameters.¶
AutoTxCell is not permanently installed in the schedule but is added or deleted on demand when there is a frame to be sent. Throughout the network lifetime, nodes maintain the autonomous cells as follows:¶
Remove an AutoTxCell when:¶
The AutoRxCell MUST always remain scheduled after synchronization. 6P CLEAR MUST NOT erase any autonomous cells.¶
Because of hash collisions, there will be cases that the AutoTxCell and AutoRxCell are scheduled at the same slot offset and/or channel offset. In such cases, AutoTxCell always take precedence over AutoRxCell. Notice AutoTxCell is a shared type cell that applies a back-off mechanism. When the AutoTxCell and AutoRxCell collide, AutoTxCell takes precedence if there is a packet to transmit. When in a back-off period, AutoRxCell is used. In the case of conflict with a negotiated cell, autonomous cells take precedence over negotiated cells, which is stated in [IEEE802154]. However, when the Slotframe 0, 1, and 2 use the same length value, it is possible for a negotiated cell to avoid the collision with AutoRxCell. Hence, the same slotframe length for Slotframe 0, 1, and 2 is RECOMMENDED.¶
This section details the behavior the node SHOULD follow from the moment it is switched on until it has successfully joined the network. Alternative behaviors may be involved, for example, when alternative security solutions are used for the network. Section 4.1 details the start state; Section 4.8 details the end state. The other sections detail the six steps of the joining process. We use the term "pledge" and "joined node", as defined in [RFC9031].¶
A node implementing MSF SHOULD implement the Constrained Join Protocol (CoJP) for 6TiSCH [RFC9031]. As a corollary, this means that a pledge, before being switched on, may be preconfigured with the Pre-Shared Key (PSK) for joining, as well as any other configuration detailed in [RFC9031]. This is not necessary if the node implements a security solution that is not based on PSKs, such as [ZEROTOUCH-JOIN].¶
When switched on, the pledge randomly chooses a frequency from the channels through which the network cycles and starts listening for EBs on that frequency.¶
Upon receiving the first EB, the pledge continues listening for additional EBs to learn:¶
After having received the first EB, a node MAY keep listening for at most MAX_EB_DELAY seconds or until it has received EBs from NUM_NEIGHBOURS_TO_WAIT distinct neighbors. This behavior is defined in [RFC8180].¶
During this step, the pledge only gets synchronized when it has received enough EB 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 PANID field it contains, the presence and contents of the Information Element (IE) defined in [RFC9032], or the key used to authenticate it.¶
The decision of which neighbor to use as a JP is implementation-specific and is discussed in [RFC9031].¶
After having selected a JP, a node generates a Join Request and installs an AutoTxCell to the JP. The Join Request is then sent by the pledge to its selected JP over the AutoTxCell. The AutoTxCell is removed by the pledge when the Join Request is sent out. The JP receives the Join Request through its AutoRxCell. Then it forwards the Join Request to the Join Registrar/Coordinator (JRC), possibly over multiple hops, over the 6P negotiated Tx cells. Similarly, the JRC sends the Join Response to the JP, possibly over multiple hops, over AutoTxCells or the 6P negotiated Tx cells. When the JP receives the Join Response from the JRC, it installs an AutoTxCell to the pledge and sends that Join Response to the pledge over AutoTxCell. The AutoTxCell is removed by the JP when the Join Response is sent out. The pledge receives the Join Response from its AutoRxCell, thereby learns the keying material used in the network, as well as other configuration settings, and becomes a "joined node".¶
When 6LoWPAN Neighbor Discovery (ND) [RFC8505] is implemented, the unicast packets used by ND are sent on the AutoTxCell. The specific process how the ND works during the join process is detailed in [RFC9030].¶
Per [RFC6550], the joined node receives DIOs, computes its own Rank, and selects a routing parent.¶
Once it has selected a routing parent, the joined node MUST generate a 6P ADD Request and install an AutoTxCell to that parent. The 6P ADD Request is sent out through the AutoTxCell, containing the following fields:¶
The joined node removes the AutoTxCell to the selected parent when the 6P Request is sent out. That parent receives the 6P ADD Request from its AutoRxCell. Then it generates a 6P ADD Response and installs an AutoTxCell to the joined node. When the parent sends out the 6P ADD Response, it MUST remove that AutoTxCell. The joined node receives the 6P ADD Response from its AutoRxCell and completes the 6P Transaction. In the case that the 6P ADD transaction failed, the node MUST issue another 6P ADD Request and repeat until the Tx cell is installed to the parent.¶
The node starts sending EBs and DIOs on the minimal cell, while following the transmit rules for broadcast frames from Section 2.¶
At the end state of the joining process, a new node:¶
Once a node has joined the 6TiSCH network, it adds/deletes/relocates cells with the selected parent for three reasons:¶
These cells are called "negotiated cells" as they are scheduled through 6P and negotiated with the node's parent. Without specific declaration, all cells mentioned in this section are negotiated cells, and they are installed at Slotframe 2.¶
A node implementing MSF MUST implement the behavior described in this section.¶
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 one of its neighbors, in most cases to the selected parent.¶
The node MUST maintain two separate pairs of the following counters for the selected parent: one for the negotiated Tx cells to that parent and one for the negotiated Rx cells to that parent.¶
Counts the number of negotiated cells that have been used. This counter is initialized at 0. NumCellsUsed is incremented by exactly 1 when, during a negotiated cell to the selected parent, either of the following happens:¶
The cell option of cells listed in CellList in a 6P Request frame SHOULD be either (Tx=1, Rx=0) only or (Tx=0, Rx=1) only. Both NumCellsElapsed and NumCellsUsed counters can be used for both types of negotiated cells.¶
As there is no negotiated Rx cell installed at initial time, the AutoRxCell is taken into account as well for downstream traffic adaptation. In this case:¶
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:¶
When the value of NumCellsElapsed reaches MAX_NUM_CELLS:¶
The value of MAX_NUM_CELLS is chosen according to the traffic type of the network. Generally speaking, the larger the value MAX_NUM_CELLS is, the more accurately the cell usage is calculated. By using a larger value of MAX_NUM_CELLS, the 6P traffic overhead could be reduced as well. Meanwhile, the latency won't increase much by using a larger value of MAX_NUM_CELLS for periodic traffic type. For bursty traffic, a larger value of MAX_NUM_CELLS indeed introduces higher latency. The latency caused by slight changes of traffic load can be alleviated by the additional scheduled cells. In this sense, MSF is a Scheduling Function that trades latency with energy by scheduling more cells than needed. Setting MAX_NUM_CELLS to a value at least four times the recent maximum number of cells used in a slotframe is RECOMMENDED. For example, a two packets/slotframe traffic load results in an average of four cells scheduled (two cells are used), using at least the value of double the number of scheduled cells (which is eight) as MAX_NUM_CELLS gives a good resolution on the cell usage calculation.¶
In the case that a node has booted or has disappeared from the network, the cell reserved at the selected parent may be kept in the schedule forever. A cleanup mechanism MUST be provided to resolve this issue. The cleanup mechanism is implementation-specific. The goal is to confirm that those negotiated cells are not used anymore by the associated neighbors and remove them from the schedule.¶
A node implementing MSF SHOULD implement the behavior described in this section.¶
As part of its normal operation, RPL can have a node switch parent. The procedure for switching from the old parent to the new parent is the following:¶
The type of negotiated cell that should be installed first depends on which traffic has the higher priority, upstream or downstream, which is application-specific and out of scope of MSF.¶
A node implementing MSF SHOULD implement the behavior described in this section. Other algorithms for handling schedule collisions can be an alternative to the algorithm proposed in this section.¶
Since scheduling is entirely distributed, there is a nonzero probability that two pairs of nearby neighbor nodes schedule a negotiated 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 negotiated Tx cell to the selected parent:¶
Since both NumTx and NumTxAck are initialized to 0, we necessarily have NumTxAck less than or equal to NumTx. We call Packet Delivery Ratio (PDR) the ratio NumTxAck/NumTx and represent it as a percentage. A cell with a PDR equal to 50% means that half of the frames transmitted are not acknowledged.¶
Each time the node switches parent (or during the join process when the node selects a 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 that parent. When NumTx reaches MAX_NUMTX, both NumTx and NumTxAck MUST be divided by 2. MAX_NUMTX needs to be a power of two to avoid division error. For example, when MAX_NUMTX is set to 256, and NumTx=255 and NumTxAck=127, the counters become NumTx=128 and NumTxAck=64 if one frame is sent to the parent with an acknowledgment received. This operation does not change the value of the PDR but allows the counters to keep incrementing. The value of MAX_NUMTX is implementation-specific.¶
The key for detecting a schedule collision is that, if a node has several cells to the selected parent, all cells should exhibit the same PDR. A cell that exhibits a PDR significantly lower than the others indicates that there are collisions on that cell.¶
Every HOUSEKEEPINGCOLLISION_PERIOD, the node executes the following steps:¶
The RELOCATION for negotiated Rx cells is not supported by MSF.¶
The 6P SIGNAL command is not used by MSF.¶
The Scheduling Function Identifier (SFID) of MSF is 0. How the value of 0 was chosen is described in Section 17.¶
MSF uses two-step 6P Transactions exclusively. 6P Transactions are only initiated by a node towards its 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:¶
As a consequence of random cell selection, there is a nonzero chance that nodes in the vicinity have installed cells with same slotOffset and channelOffset. An implementer MAY implement a strategy to monitor the candidate cells before adding them in CellList to avoid collision. For example, a node MAY maintain a candidate cell pool for the CellList. The candidate cells in the pool are preconfigured as Rx cells to promiscuously listen to detect transmissions on those cells. If transmissions that rely on [IEEE802154] are observed on one cell over multiple iterations of the schedule, that cell is probably used by a TSCH neighbor. It is moved out from the pool, and a new cell is selected as a candidate cell. The cells in CellList are picked from the candidate pool directly when required.¶
The timeout value is calculated for the worst case that a 6P response is received, which means the 6P response is sent out successfully at the very latest retransmission. And for each retransmission, it backs off with largest value. Hence the 6P timeout value is calculated as ((2MAXBE) - 1) * MAXRETRIES * SLOTFRAME_LENGTH, where:¶
Cells are ordered by slotOffset first, channelOffset second.¶
The following sequence is correctly ordered (each element represents the [slotOffset,channelOffset] of a cell in the schedule):¶
[1,3],[1,4],[2,0],[5,3],[6,0],[6,3],[7,9]¶
The Metadata field is not used by MSF.¶
Section 6.2.4 of [RFC8480] lists the 6P return codes. Table 1 lists the same error codes and the behavior a node implementing MSF SHOULD follow.¶
| 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 |
The meaning of each behavior from Table 1 is:¶
The behavior when schedule inconsistency is detected is explained in Table 1, for 6P return code RC_ERR_SEQNUM.¶
Table 2 lists MSF constants and their RECOMMENDED values.¶
| Name | RECOMMENDED value |
|---|---|
| SLOTFRAME_LENGTH | 101 slots |
| NUM_CH_OFFSET | 16 |
| MAX_NUM_CELLS | 100 |
| LIM_NUMCELLSUSED_HIGH | 75 |
| LIM_NUMCELLSUSED_LOW | 25 |
| MAX_NUMTX | 256 |
| HOUSEKEEPINGCOLLISION_PERIOD | 1 min |
| RELOCATE_PDRTHRES | 50 % |
| QUARANTINE_DURATION | 5 min |
| WAIT_DURATION_MIN | 30 s |
| WAIT_DURATION_MAX | 60 s |
Table 3 lists MSF statistics and their RECOMMENDED widths.¶
| Name | RECOMMENDED width |
|---|---|
| NumCellsElapsed | 1 byte |
| NumCellsUsed | 1 byte |
| NumTx | 1 byte |
| NumTxAck | 1 byte |
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: "Minimal IPv6 over the TSCH Mode of IEEE 802.15.4e (6TiSCH) Configuration" [RFC8180], "6TiSCH Operation Sublayer (6top) Protocol (6P)" [RFC8480], and "Constrained Join Protocol (CoJP) for 6TiSCH" [RFC9031]. Confidentiality and authentication of MSF control and data traffic are provided by these specifications whose security considerations continue to apply to MSF. In particular, MSF does not define a new protocol or packet format.¶
MSF uses autonomous cells for initial bootstrap and the transport of join traffic. Autonomous cells are computed as a hash of nodes' EUI-64 addresses. This makes the coordinates of autonomous cell an easy target for an attacker, as EUI-64 addresses are visible on the wire and are not encrypted by the link-layer security mechanism. With the coordinates of autonomous cells available, the attacker can launch a selective jamming attack against any node's AutoRxCell. If the attacker targets a node acting as a JP, it can prevent pledges from using that JP to join the network. The pledge detects such a situation through the absence of a link-layer acknowledgment for its Join Request. As it is expected that each pledge will have more than one JP available to join the network, one available countermeasure for the pledge is to pseudorandomly select a new JP when the link to the previous JP appears bad. Such a strategy alleviates the issue of the attacker randomly jamming to disturb the network but does not help in the case the attacker is targeting a particular pledge. In that case, the attacker can jam the AutoRxCell of the pledge in order to prevent it from receiving the join response. This situation should be detected through the absence of a particular node from the network and handled by the network administrator through out-of-band means.¶
MSF adapts to traffic containing packets from the IP layer. It is possible that the IP packet has a nonzero DSCP (Differentiated Services Code Point) [RFC2474] value in its IPv6 header. The decision how to handle that packet belongs to the upper layer and is out of scope of MSF. As long as the decision is made to hand over to MAC layer to transmit, MSF will take that packet into account when adapting to traffic.¶
Note that nonzero DSCP values may imply that the traffic originated at unauthenticated pledges (see [RFC9031]). The implementation at the IPv6 layer SHOULD rate limit this join traffic before it is passed to the 6top sublayer where MSF can observe it. If there is no rate limit for join traffic, intermediate nodes in the 6TiSCH network may be prone to a resource exhaustion attack, with the attacker injecting unauthenticated traffic from the network edge. The assumption is that the rate-limiting function is aware of the available bandwidth in the 6top Layer 3 bundle(s) towards a next hop, not directly from MSF, but from an interaction with the 6top sublayer that ultimately manages the bundles under MSF's guidance. How this rate limit is implemented is out of scope of MSF.¶
This document adds the following number to the "6P Scheduling Function Identifiers" subregistry, part of the "IPv6 Over the TSCH Mode of IEEE 802.15.4 (6TiSCH)" registry, as defined by [RFC8480]:¶
| SFID | Name | Reference |
|---|---|---|
| 0 | Minimal Scheduling Function (MSF) | RFC 9033 |
The SFID was chosen from the range 0-127, which has the registration procedure of IETF Review or IESG Approval [RFC8126].¶
To support interoperability, this section provides an example implementation of the SAX hash function [SAX-DASFAA]. The input parameters of the function are:¶
In MSF, the T is replaced by the length of slotframe 1. String s is replaced by the node EUI-64 address. The characters of the string, c0 through c7, are the eight bytes of the EUI-64 address.¶
The SAX hash function requires shift operation, which is defined as follow:¶
The steps to calculate the hash value of SAX hash function are:¶
The value of variable h is the hash value of the SAX hash function.¶
The values of h0, l_bit, and r_bit in Step 1 and Step 2 are configured as:¶
h0 = 0¶
l_bit = 0¶
r_bit = 1¶
The appropriate values of l_bit and r_bit could vary depending on the set of nodes' EUI-64 address. How to find those values is out of the scope of this specification.¶