| 6TiSCH | T. Chang, Ed. |
| Internet-Draft | Inria |
| Intended status: Standards Track | M. Vucinic |
| Expires: January 3, 2019 | University of Montenegro |
| X. Vilajosana | |
| Universitat Oberta de Catalunya | |
| S. Duquennoy | |
| RISE SICS | |
| D. Dujovne, Ed. | |
| Universidad Diego Portales | |
| July 2, 2018 |
6TiSCH Minimal Scheduling Function (MSF)
draft-chang-6tisch-msf-02
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 builds 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", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].
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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 [I-D.ietf-6tisch-6top-protocol]: it complements [I-D.ietf-6tisch-6top-protocol] by providing the rules of when to add/delete cells in the communication schedule. The SF defined in this document follows that definition, and satisfies all the requirements for an SF listed in Section 4.2 of [I-D.ietf-6tisch-6top-protocol].
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) [I-D.ietf-6tisch-6top-protocol], 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 7 steps described in Section 4. The end state of the join process is that the node is synchronized to the network, has mutually authenticated to the network, has identified a preferred routing parent, has scheduled one default unicast cell to/from each of its neighbors. 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, to handle a schedule collision.
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 root). Appendix C contains a performance evaluation of MSF.
This specification follows the recommended structure of an SF specification in Appendix A of [I-D.ietf-6tisch-6top-protocol], with the following adaptations:
A node implementing MSF MUST implement the Minimal 6TiSCH Configuration [RFC8180], which defines the "minimal cell", a single shared cell providing minimal connectivity between the nodes in the network.
MSF uses the minimal cell to exchange the following packets:
Because the minimal cell is SHARED, the back-off algorithm defined in [IEEE802154-2015] is used to resolve collisions. To ensure there is enough bandwidth available on the minimal cell, a node implementing MSF SHOULD enforce the following rules for broadcast frames:
The RECOMMENDED behavior for sending EBs is to have a node send EBs with a probability of 1/(3(N+1)). The RECOMMENDED behavior for sending DIOs is to use a Trickle timer with rate-limiting.
Section 4.3 describes how to evaluate the number of neighbors during the joining process. After the joining process, how to evaluate the number of neighbors is implementation-specific.
As detailed in Section 2.2 of [I-D.ietf-6tisch-6top-protocol], MSF MUST schedule cells from Slotframe 1, while Slotframe 0 is used for traffic defined in the Minimal 6TiSCH Configuration. The length of Slotframe 0 and Slotframe 1 SHOULD be the same value. The default of SLOTFRAME_LENGTH is RECOMMENDED, although any value can be advertised in the EBs.
MSF nodes MUST initialize Slotframe 1 with a set of default cells for unicast communication with their neighbors. These cells are referred to as 'autonomous cells', because they are maintained autonomously by each node. Each node has:
To compute a [slotOffset,channelOffset] from an EUI64 address, nodes MUST use the hash function SAX [SAX-DASFAA]. The coordinates are computed to distribute the cells across all 16 channel offsets, and all but the first time offsets 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:
Because of hash collisions, there are cases where one node has multiple cells scheduled at the same time offset and/or channel offset. Note that nodes have only one autonomous RX cell and potentially multiple TX cells. Hash collisions among a set of cells at a given time offset is resolved at run-time as follows:
Throughout the network lifetime, nodes MUST maintain the autonomous cells as follows:
This section details the behavior the node SHOULD follow from the moment it is switched on, until it has successfully joined the network. Section 4.1 details the start state; Section 4.9 details the end state. The other sections detail the 7 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 MUST implement the Minimal Security Framework for 6TiSCH [I-D.ietf-6tisch-minimal-security]. As a corollary, this means that a pledge, before being switched on, is pre-configured with the Pre-Shared Key (PSK) for joining, as well as any other configuration detailed in [I-D.ietf-6tisch-minimal-security].
When switched on, the pledge SHOULD randomly choose a frequency among the available frequencies, and start listening for EBs on that frequency.
Upon receiving the first EB, the pledge SHOULD continue listening for additional EBs to learn:
While the exact behavior is implementation-specific, the RECOMMENDED behavior is to follow [RFC8180], and listen until EBs sent by NUM_NEIGHBOURS_TO_WAIT nodes (defined in [RFC8180]) have been received.
During this step, the pledge MAY synchronize to any EB it receives 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.richardson-6tisch-join-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 joining, nodes MUST set up their autonomous unicast cells, as described in Section 3. This enables unicast communication in Slotframe 1, until more cells are added with 6P as defined in Section 5.
As per [I-D.ietf-6tisch-minimal-security], after having selected a JP, the pledge sends a Join Request to its JP. Because no dedicated cells are in place at this point, this happens on the autonomous unicast cell. The JP then forwards the Join Request to the JRC, possibly over multiple hops. When forwarding this Join Request, a node MUST use a unicast cell (autonomous or dedicated) it has with its preferred parent. How dedicated cells are installed is detailed in Section 5.
As per [I-D.ietf-6tisch-minimal-security], the JRC sends back a Join Response to the pledge, through the JP. When forwarding this Join Response, a node MUST use a unicast (autonomous or dedicated) cell it has with its child (not the minimal cell).
As per [I-D.ietf-6tisch-minimal-security], after receiving the Join Response, the pledge learns the keying material used in the network, as well as other configurations, and becomes a "joined node".
Because it has learned the link-layer keying material used in the network, the joined node can now decrypt the DIO packets sent by its neighbors. Per [RFC6550], the joined node receives DIOs, computes its own rank, and selects a preferred parent.
The node SHOULD start sending EBs and DIOs on the minimal cell, while following the transmit rules for broadcast frames from Section 2.
The node SHOULD send some form of keep-alive messages to all its neighbors it has unicast cells with. The Keep-Alive (KA) mechanism is detailed in [RFC7554]. It uses the keep-alive messages to its preferred parent to stay synchronized. It uses the keep-alive messages to its children (with which it has a unicast cell to) to ensure the child is still reachable. The RECOMMENDED period for sending keep-alive messages is KA_PERIOD.
If the keep-alive message to a child fails at the link layer (i.e. the maximum number of link-layer retries is reached), the node SHOULD declare the child as unreachable. This can happen for example when the child node is switched off.
When a neighbor is declared unreachable, the node MUST remove all dedicated cells with that neighbor from its own schedule. In addition, it MAY issue a 6P CLEAR to that neighbor (which can fail at the link-layer). If the node has autonmous cells to the unreachable neighbor those cells will be removed following the procedure described in Section 3.
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 its preferred parent for three reasons:
A node implementing MSF MUST implement the behavior described in this section.
In order to handle transient traffic bursts, MSF uses the [IEEE802154-2015] frame pending bit (page 152, Section 7.2.1.3). By setting the bit, a node can transmit a series of packets to a given neighbor in consecutive time offsets. The next paragraphs define how to handle longer-term fluctuations in traffic, using 6P.
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 its preferred parent:
From the join process, the node already has a set of autonmous unicast cells, as defined in Section 3. The autonomous cells MUST NOT be removed by 6P, so that there always exists a unicast cell between a node and its preferred parent, even if no frames are being exchanged between them. Autonomous cells are used indistinguishably together with dedicated cells, for broadcast or unicast traffic with the target neighbor. The procedure to remove autonomous cells is described in Section 3.
Adding/removing/relocating cells involves exchanging frames that contain 6P commands. All 6P frames MUST be sent on the unicast cells (and not the minimal cell).
The node MUST maintain the following counters for its preferred parent:
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:
A node implementing MSF MUST implement the behavior described in this section.
Part of its normal operation, the RPL routing protocol can have a node switch preferred parents. The procedure for switching from the old preferred parent to the new preferred parent is:
A node implementing MSF SHOULD implement the behavior described in this section. The "MUST" statements in this section hence only apply if the node implements schedule collision handling.
Since scheduling is entirely distributed, there is a non-zero probability that two pairs of nearby neighbor nodes schedule a 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 cell to its preferred parent:
Implementors MAY choose to maintain the same counters for each cell in the schedule.
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 (and need to be retransmitted).
Each time the node switches preferred parent (or during the join process when the node selects a preferred 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 its preferred parent. When NumTx reaches 256, both NumTx and NumTxAck MUST be divided by 2. That is, for example, from NumTx=256 and NumTxAck=128, they become NumTx=128 and NumTxAck=64. This operation does not change the value of the PDR, but allows the counters to keep incrementing.
The key for detecting a schedule collision is that, if a node has several cells to the same preferred 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 6P SIGNAL command is not used by MSF.
The Scheduling Function Identifier (SFID) of MSF is IANA_6TISCH_SFID_MSF.
MSF uses 2-step 6P Transactions exclusively. 6P Transactions are only initiated by a node towards it preferred 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:
The 6P Timeout is not a constant value. It is calculated as (1/C)*(1/PDR)*SIXP_TIMEOUT_SEC_FACTOR, 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 [I-D.ietf-6tisch-6top-protocol] 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 |
+-----------------+----------------------+
Figure 1: Recommended behavior for each 6P Error Code.
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 |
+------------------------------+-------------------+
| KA_PERIOD | 10 s |
| LIM_NUMCELLSUSED_HIGH | 75 % |
| LIM_NUMCELLSUSED_LOW | 25 % |
| HOUSEKEEPINGCOLLISION_PERIOD | 1 min |
| RELOCATE_PDRTHRES | 50 % |
| SIXP_TIMEOUT_SEC_FACTOR | 3 x |
| SLOTFRAME_LENGTH | 101 slots |
| QUARANTINE_DURATION | 5 min |
| WAITDURATION_MIN | 30 s |
| WAITDURATION_MAX | 60 s |
+------------------------------+-------------------+
Figure 2: MSF Constants and their RECOMMENDED values.
Figure 3 lists MSF Statistics and their RECOMMENDED width.
+-----------------+-------------------+
| Name | RECOMMENDED width |
+-----------------+-------------------+
| NumCellsPassed | 1 byte |
| NumCellsUsed | 1 byte |
| NumTx | 1 byte |
| NumTxAck | 1 byte |
+-----------------+-------------------+
Figure 3: MSF Statistics and their RECOMMENDED width.
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) [I-D.ietf-6tisch-6top-protocol], and the Minimal Security Framework for 6TiSCH [I-D.ietf-6tisch-minimal-security]. In particular, MSF does not define a new protocol or packet format.
MSF relies entirely on the security mechanisms defined in the specifications listed above.
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 [I-D.ietf-6tisch-6top-protocol]:
+----------------------+-----------------------------+-------------+ | SFID | Name | Reference | +----------------------+-----------------------------+-------------+ | IANA_6TISCH_SFID_MSF | Minimal Scheduling Function | RFCXXXX | | | (MSF) | (NOTE:this) | +----------------------+-----------------------------+-------------+
Figure 4: IETF IE Subtype '6P'.
| [OpenWSN] | Watteyne, T., Vilajosana, X., Kerkez, B., Chraim, F., Weekly, K., Wang, Q., Glaser, S. and K. Pister, "OpenWSN: a Standards-Based Low-Power Wireless Development Environment", Transactions on Emerging Telecommunications Technologies , August 2012. |
| [RFC6982] | Sheffer, Y. and A. Farrel, "Improving Awareness of Running Code: The Implementation Status Section", RFC 6982, DOI 10.17487/RFC6982, July 2013. |
| [SAX-DASFAA] | Ramakrishna, M. and J. Zobel, "Performance in Practice of String Hashing Functions", DASFAA , 1997. |
Beshr Al Nahas (Chalmers University, beshr@chalmers.se) and Olaf Landsiedel (Chalmers University, olafl@chalmers.se) contributed to the design and evaluation of autonomous unicast cells.
This section records the status of known implementations of the protocol defined by this specification at the time of posting of this Internet-Draft, and is based on a proposal described in [RFC6982]. The description of implementations in this section is intended to assist the IETF in its decision processes in progressing drafts to RFCs. Please note that the listing of any individual implementation here does not imply endorsement by the IETF. Furthermore, no effort has been spent to verify the information presented here that was supplied by IETF contributors. This is not intended as, and must not be construed to be, a catalog of available implementations or their features. Readers are advised to note that other implementations may exist.
According to [RFC6982], "this will allow reviewers and working groups to assign due consideration to documents that have the benefit of running code, which may serve as evidence of valuable experimentation and feedback that have made the implemented protocols more mature. It is up to the individual working groups to use this information as they see fit".
The performance of MSF may be published as companion documents to this specification, possibly under the form a applicability statements.