[6tisch] comments on draft-chang-6tisch-msf-01
Yasuyuki Tanaka <yasuyuki.tanaka@inria.fr> Thu, 14 June 2018 16:16 UTC
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Subject: [6tisch] comments on draft-chang-6tisch-msf-01
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Hi, I'm sharing my comments on draft-chang-6tisch-msf-01. https://tools.ietf.org/html/draft-chang-6tisch-msf-01 [1] how should a node handle a 6P transaction failure? draft> 3. Node Behavior at Boot draft> ... draft> 3.6. Step 5 - 6P ADD to Preferred Parent draft> draft> After having selected a preferred parent, the joined node MUST issue draft> a 6P ADD command to that parent, with the following fields: What should the joining node do if the ADD transaction fails? It should retry the ADD command after waiting for [WAITDURATION_MIN, WAITDURATION_MAX]? If so, how many times should it retry at the maximum? [2] what cell options will additional dedicated cells have? draft> 4.1. Adapting to Traffic ... draft> o If the node determines that the number of link-layer frames it is draft> attempting to exchange with its preferred parent per unit of time draft> is larger than the capacity offered by the TSCH cells it has draft> scheduled with it, it triggers a 6P Transaction with its preferred draft> parent to add cells to the TSCH schedule of both nodes. It's unclear to me what cell options additional dedicated cells will have. NumCellsUsed is the number of frame exchanges in dedicated cells, which doesn't care about directions: incoming or outgoing. draft> NumCellsUsed: Counts the number of dedicated cells that have been draft> used. This counter is initialized at 0. NumCellsUsed is draft> incremented by exactly 1 when, during a dedicated cell to the draft> preferred parent, either of the following happens: draft> draft> * The node sends a frame to its preferred parent. The counter draft> increments regardless of whether a link-layer acknowledgment draft> was received or not. draft> * The node receives a frame from its preferred parent. In other words, NumCellsUsed doesn't give you any idea how many frames were transmitted or how many frames were received. There could be two options: - (1) have one NumCellsUsed variables per direction - (2) count only transmissions in NumCellsUsed; dedicated RX cells are scheduled via 6P transactions initiated by the peer If you want 6P transactions initiated only by a child, the first one is the choice. [3] How do we choose dedicated cell to be deleted? draft> 4.1. Adapting to Traffic draft> ... draft> 2. When the value of NumCellsPassed reaches MAX_NUMCELLS: draft> draft> * If NumCellsUsed > LIM_NUMCELLSUSED_HIGH, trigger 6P to add a draft> single cell to the preferred parent draft> * If NumCellsUsed < LIM_NUMCELLSUSED_LOW, trigger 6P to remove a draft> single cell to the preferred parent How do we choose cells to be set in CellList of a 6P DELETE request? They should be randomly chosen? Or, they should be chosen in order from the lowest PDR cell? NumCellsUsed in that bullet list should be (NumCellsUsed / NumCellsPassed) or ((NumCellsUsed / NumCellsPassed * 100), by the way. [4] (trivial comment) draft> 4.2. Switching Parent draft> ... draft> 1. the node counts the number of dedicated cells it has per draft> slotframe to the old preferred parent draft> 2. the node triggers one or more 6P ADD commands to schedule the draft> same number of dedicated cells to the new preferred parent To be precise, it'd better to mention about cell options as well as how many dedicated cells are scheduled. [5] questions in Handling Schedule Collisions draft> 4.3. Handling Schedule Collisions draft> ... draft> Each time the node switches preferred parent (or during the join draft> process when the node selects a preferred parent for the first time), draft> both NumTx and NumTxAck MUST be reset to 0. They increment over When should a parent reset NumTx and NumTxAck for a dedicated cell? Another question related to this topic: how does a parent remove a dedicated cell scheduled during the join process if the child leaves without a successful 6P command transaction? draft> 4.3. Handling Schedule Collisions draft> ... draft> The key for detecting a schedule collision is that, if a node has draft> several cells to the same preferred parent, all cells should exhibit draft> the same PDR. A cell which exhibits a PDR significantly lower than draft> the others indicates than there are collisions on that cell. Do we exclude a dedicated cell having SHARED bit on for this collision detection? If it's excluded, how do we handle collisions on such a dedicated cell? If it's not excluded, a dedicated cell having SHARED bit on tends to be relocated since usually it could have lower PDR than other dedicated cells which doesn't have SHARED bit on. How do we cope with this situation? draft> 4.3. Handling Schedule Collisions draft> ... draft> 2. Any cell that hasn't yet had NumTx divided by 2 since it was last draft> reset is skipped in steps 3 and 4. This avoids triggering cell draft> relocation when the values of NumTx and NumTxAck are not draft> statistically significant yet. Does this mean, skip 3 and 4 for cells whose NumTx have never been divided by 2 (never reached 256)? I'm not sure the meaning of "since it was last reset". That's it! Thank you. Best, Yatch
- [6tisch] comments on draft-chang-6tisch-msf-01 Yasuyuki Tanaka
- Re: [6tisch] comments on draft-chang-6tisch-msf-01 Florian Kauer