Re: [6tisch] comments on draft-chang-6tisch-msf-01
Florian Kauer <florian.kauer@koalo.de> Tue, 28 August 2018 12:12 UTC
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From: Florian Kauer <florian.kauer@koalo.de>
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Date: Tue, 28 Aug 2018 14:12:10 +0200
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Subject: Re: [6tisch] comments on draft-chang-6tisch-msf-01
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Hi, regarding section "5. Rules for Adding/Deleting Cells" of https://tools.ietf.org/html/draft-ietf-6tisch-msf-00 I noticed some issues. Some of them are already addressed by Yasuyuki Tanaka in a previous review of draft-chang-6tisch-msf-01 but it seems they have not been addressed yet. > 2. When the value of NumCellsPassed reaches MAX_NUMCELLS: > * If NumCellsUsed > LIM_NUMCELLSUSED_HIGH, trigger 6P to add a > single cell to the preferred parent > * If NumCellsUsed < LIM_NUMCELLSUSED_LOW, trigger 6P to remove > single cell to the preferred parent > * Reset both NumCellsPassed and NumCellsUsed to 0 and go to step > 2. In particular, in 5.1. the inequalities NumCellsUsed > LIM_NUMCELLSUSED_HIGH and NumCellsUsed < LIM_NUMCELLSUSED_LOW do not match the intention of the recommended values of LIM_NUMCELLSUSED_HIGH and LIM_NUMCELLSUSED_LOW as percentages. In addition, it seems that MAX_NUMCELLS is not defined in draft-ietf-6tisch-msf-00. Is it defined in another draft? I expect its value (or better: the use of this value in MSF) has a tremendous impact on the ability of MSF to handle volatile traffic. > The node MUST maintain the following counters for its preferred > parent: What is the reason that dynamic allocation of slots is only performed towards the parent and not towards children? Greetings, Florian On 14.06.2018 18:16, Yasuyuki Tanaka wrote: > 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 mailing list > 6tisch@ietf.org > https://www.ietf.org/mailman/listinfo/6tisch >
- [6tisch] comments on draft-chang-6tisch-msf-01 Yasuyuki Tanaka
- Re: [6tisch] comments on draft-chang-6tisch-msf-01 Florian Kauer