| Internet-Draft | 6TiSCH Minimal Scheduling Function (MSF) | March 2020 |
| Chang, et al. | Expires 25 September 2020 | [Page] |
This specification defines the 6TiSCH Minimal Scheduling Function (MSF). This Scheduling Function describes both the behavior of a node when joining the network, and how the communication schedule is managed in a distributed fashion. MSF is built upon the 6TiSCH Operation Sublayer Protocol (6P) and the Minimal Security Framework for 6TiSCH.¶
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 [RFC8174].¶
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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/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: the Minimal IPv6 over the TSCH Mode of IEEE 802.15.4e (6TiSCH) Configuration [RFC8180], the 6TiSCH Operation Sublayer Protocol (6P) [RFC8480], and the Minimal Security Framework for 6TiSCH [I-D.ietf-6tisch-minimal-security].¶
MSF defines both the behavior of a node when joining the network, and how the communication schedule is managed in a distributed fashion. When a node running MSF boots up, it joins the network by following the 6 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/relocate cells, as described in Section 5. It does so for 3 reasons: to match the link-layer resources to the traffic, to handle changing parent 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 selected routing parent, which is phrased as "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 [RFC6550]) root.¶
This specification follows the recommended structure of an SF specification, given in Appendix A of [RFC8480], with the following adaptations:¶
In a TSCH network, time is sliced up into time slots. The time slots are grouped as one or multiple slotframes which repeat over time. The TSCH schedule instructs a node what to do at each time slots, such as transmit, receive or sleep [RFC7554]. In case of a slot to transmit or receive, a channel is assigned to the time slot. The tuple (slot, channel) is indicated as a cell of 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 EB/DIO 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 1/3 of the bandwidth of minimal cell. One of the algorithms that fulfills this requirement is the Trickle timer defined in [RFC6206] which is applied on 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.¶
3 slotframes are used in MSF. MSF schedules autonomous cells at Slotframe 1 (Section 3) and 6P negotiated cells at Slotframe 2 (Section 5) ,wh ile 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 EUI64 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 EUI64 address are computed as follows:¶
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 broadcasted in EB frame or pre-configured. For interoperability purposes, the values of those parameters can be referred from Appendix A.¶
AutoTxCell is not permanently installed in the schedule but added/deleted on demand when there is a frame to 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. In case of conflicting with a negotiated cell, autonomous cells take precedence over negotiated cell, which is stated in [IEEE802154]. However, when the Slotframe 0, 1 and 2 use the same length value, it is possible for negotiated cell to avoid the collision with AutoRxCell.¶
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 6 steps of the joining process. We use the term "pledge" and "joined node", as defined in [I-D.ietf-6tisch-minimal-security].¶
A node implementing MSF SHOULD implement the Constrained Join Protocol (CoJP) for 6TiSCH [I-D.ietf-6tisch-minimal-security]. As a corollary, this means that a pledge, before being switched on, may be pre-configured with the Pre-Shared Key (PSK) for joining, as well as any other configuration detailed in ([I-D.ietf-6tisch-minimal-security]). This is not necessary if the node implements a security solution not based on PSKs, such as ([I-D.ietf-6tisch-dtsecurity-zerotouch-join]).¶
When switched on, the pledge randomly chooses a frequency among the available frequencies, and starts listening for EBs on that frequency.¶
Upon receiving the first EB, the pledge continue listening for additional EBs to learn:¶
After having received the first EB, a node MAY keep listening for at most MAX_EB_DELAY seconds 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 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 PAN ID field it contains, the presence and contents of the IE defined in [I-D.ietf-6tisch-enrollment-enhanced-beacon], or the key used to authenticate it.¶
The decision of which neighbor to use as a JP is implementation-specific, and discussed in [I-D.ietf-6tisch-minimal-security].¶
After 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 received 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 ([RFC8505]) (ND) 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 [I-D.ietf-6tisch-architecture].¶
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 case 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.¶
For a new node, the end state of the joining process is:¶
Once a node has joined the 6TiSCH network, it adds/deletes/relocates cells with the selected parent for three reasons:¶
Those cells are called 'negotiated cells' as they are scheduled through 6P, negotiated with the node's parent. Without specific declaring, 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 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 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 to both type 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 accurate the cell usage is calculated. The 6P traffic overhead using a larger value of MAX_NUM_CELLS 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 burst traffic type, larger value of MAX_NUM_CELLS indeed introduces higher latency. The latency caused by slight changes of traffic load can be absolved by the additional scheduled cells. In this sense, MSF is a scheduling function trading latency with energy by scheduling more cells than needed. It is recommended to set MAX_NUM_CELLS value at least 4x of the maximum number of used cells in a slot frame in recent history. For example, a 2 packets/slotframe traffic load results an average 4 cells scheduled (2 cells are used), using at least the value of double number of scheduled cells (which is 8) as MAX_NUM_CELLS gives a good resolution on cell usage calculation.¶
In case that a node booted or disappeared from the network, the cell reserved at the selected parent may be kept in the schedule forever. A clean-up mechanism MUST be provided to resolve this issue. The clean-up mechanism is implementation-specific. The goal is to confirm 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.¶
Part of its normal operation, the RPL routing protocol can have a node switch parent. The procedure for switching from the old parent to the new parent is:¶
For what type of negotiated cell should be installed first, it 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 schedule collisions handling algorithm can be an alternative of the algorithm proposed in this section.¶
Since scheduling is entirely distributed, there is a non-zero 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 <= NumTx. We call Packet Delivery Ratio (PDR) the ratio NumTxAck/NumTx; and represent it as a percentage. A cell with PDR=50% means that half of the frames transmitted are not acknowledged.¶
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, from 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 which exhibits a PDR significantly lower than the others indicates than there are collisions on that cell.¶
Every HOUSEKEEPINGCOLLISION_PERIOD, the node executes the following steps:¶
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 IANA_6TISCH_SFID_MSF. How the value of IANA_6TISCH_SFID_MSF is chosen is described in Section 17.¶
MSF uses 2-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 randomly cell selection, there is a non-zero chance that nodes in the vicinity 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 pre-configured as Rx cells to promiscuously listen to detect transmissions on those cells. If IEEE802.15.4 transmissions 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 ((2^MAXBE)-1)*MAXRETRIES*SLOTFRAME_LENGTH, where:¶
Cells are ordered slotOffset first, channelOffset second.¶
The following sequence is correctly ordered (each element represents the [slottOffset,channelOffset] of a cell in the schedule):¶
[1,3],[1,4],[2,0],[5,3],[6,0],[6,3],[7,9]¶
The Metadata field is not used by MSF.¶
Section 6.2.4 of [RFC8480] lists the 6P Return Codes. Figure 1 lists the same error codes, and the behavior a node implementing MSF SHOULD follow.¶
+-----------------+----------------------+
| 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 Figure 1 is:¶
The behavior when schedule inconsistency is detected is explained in Figure 1, for 6P Return Code RC_ERR_SEQNUM.¶
Figure 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 |
+------------------------------+-------------------+
Figure 3 lists MSF Statistics and their RECOMMENDED width.¶
+-----------------+-------------------+
| 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: the Minimal IPv6 over the TSCH Mode of IEEE 802.15.4e (6TiSCH) Configuration [RFC8180], the 6TiSCH Operation Sublayer Protocol (6P) [RFC8480], and the Constrained Join Protocol (CoJP) for 6TiSCH [I-D.ietf-6tisch-minimal-security]. The security considrations of the specifications continue to apply in the MSF scope. 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' EUI64 addresses. This makes the coordinates of autonomous cell an easy target for an attacker, as EUI64 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 nodes' 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 pseudo-randomly select a new JP when the link to the previous JP appears bad. Such strategy alleviates the issue of the attacker randomly jamming to disturb the network but does not help in 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 packet from the IP layer. It is possible that the IP packet has a non-zero DSCP (Diffserv Code Point [RFC2474]) value in its IPv6 header. The decision how to hand 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 non-zero DSCP value may imply that the traffic is originated at unauthenticated pledges, referring to [I-D.ietf-6tisch-minimal-security]. The implementation at IPv6 layer SHOULD rate-limit this join traffic before it is passed to 6top sublayer where MSF can observe it. In case 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 L3 bundle(s) towards a next hop, not directly from MSF, but from an interaction with the 6top sublayer that manages ultimately 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" sub-registry, part of the "IPv6 over the TSCH mode of IEEE 802.15.4e (6TiSCH) parameters" registry, as defined by [RFC8480]:¶
+----------------------+-----------------------------+-------------+ | SFID | Name | Reference | +----------------------+-----------------------------+-------------+ | IANA_6TISCH_SFID_MSF | Minimal Scheduling Function | RFC_THIS | | | (MSF) | | +----------------------+-----------------------------+-------------+
IANA_6TISCH_SFID_MSF is chosen from range 0-127, which is used for IETF Review or IESG Approval.¶
Considering the interoperability, this section provides an example of implemention 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 mote EUI64 address. The characters of the string c0, c1, ..., c7 are the 8 bytes of EUI64 address.¶
The SAX hash function requires shift operation which is defined as follow:¶
The steps to calculate the hash value of SAX hash function are:¶
The value of variable h is the hash value of SAX hash function.¶
The values of h0, l_bit and r_bit in Step 1 and 2 are configured as:¶
The appropriate values of l_bit and r_bit could vary depending on the the set of motes' EUI64 address. How to find those values is out of the scope of this specification.¶