Re: [6tisch] Benjamin Kaduk's No Objection on draft-ietf-6tisch-msf-16: (with COMMENT)

[Tengfei Chang <[e-mail]>]

Hi Benjamin,

Thanks for updating the comments!

I believe the change from the current email comparing to previous one is
that the DISCUSSION part is removed as we fixed it in another previous
thread.
The other comments from the current email are actually for old version of
MSF, which are all resolved in the latest version MSF-16.

For the administration,

I want to clarify that  the draft-ietf-6tisch-msf-16 has resolved all
comments which were discussed.
Please advise me if there is any further action required. Thanks a lot!

Regards,
Tengfei


On Fri, Apr 3, 2020 at 5:38 AM Benjamin Kaduk via Datatracker <
[e-mail]> wrote:

> Benjamin Kaduk has entered the following ballot position for
> draft-ietf-6tisch-msf-16: No Objection
>
> When responding, please keep the subject line intact and reply to all
> email addresses included in the To and CC lines. (Feel free to cut this
> introductory paragraph, however.)
>
>
> Please refer to https://www.ietf.org/iesg/statement/discuss-criteria.html
> for more information about IESG DISCUSS and COMMENT positions.
>
>
> The document, along with other ballot positions, can be found here:
> https://datatracker.ietf.org/doc/draft-ietf-6tisch-msf/
>
>
>
> ----------------------------------------------------------------------
> COMMENT:
> ----------------------------------------------------------------------
>
> Thanks for clarifying the non-issue nature of my original Discuss points!
>
> Original COMMENT section preserved below (possibly stale).
>
> I support Roman's Discuss -- we need more information for this to be a
> useful reference; even what seem to be the official DASFAA 1997
> proceedings (https://dblp.org/db/conf/dasfaa/dasfaa97) do not have an
> associated document).
>
> Basing various scheduling aspects on (a hash of) the EUI64 ties
> functionality to a persistent identifier for a device.  How significant
> a disruption would be incurred if a device periodically changes its
> presented EUI64 for anonymization purposes?
>
> There seems to be a general pattern of "if you don't have a
> 6P-negotiated Tx cell, install and AutoTxCell to send your one message
> and then remove it after sending"; I wonder if it would be easier on the
> reader to consolidate this as a general principle and not repeat the
> details every time it occurs.
>
> Requirements Language
>
> "NOT RECOMMENDED" is not in the RFC2119 boilerplate (but is a BCP 14
> keyword).
>
> Section 1
>
>    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 to the network, has identified a routing parent, and
>
> nit(?): I guess maybe "mutually authenticated with" is more correct for
> the bidirectional operation.
>
>    It does so for 3 reasons: to match the link-layer resources to the
>    traffic, to handle changing parent, to handle a schedule collision.
>
> nit: end the list with "or" (or "and"?).
>
>    MSF works closely with RPL, specifically the routing parent defined
>    in [RFC6550].  This specification only describes how MSF works with
>    one routing parent, which is phrased as "selected parent".  The
>
> nit: I suggest '''one routing parent; this parent is referred to as the
> "selected parent"'''.
>
>    activity of MSF towards to single routing parent is called as a "MSF
>
> nit: "towards the"
>
>    *  We added sections on the interface to the minimal 6TiSCH
>       configuration (Section 2), the use of the SIGNAL command
>       (Section 6), the MSF constants (Section 14), the MSF statistics
>       (Section 15).
>
> nit: end the list with "and".
>
> Section 2
>
>    In a TSCH network, time is sliced up into time slots.  The time slots
>    are grouped as one of more slotframes which repeat over time.  The
>
> nit(?): should this be "one or more"?
>
>    channel) is indicated as a cell of TSCH schedule.  MSF is one of the
>    policies defining how to manage the TSCH schedule.
>
> nit: if there is only one such policy active at a given time for a given
> network, I suggest "MSF is a policy for managing the TCSH schedule".
> (If multiple policies are active simultaneously, no change is needed.)
>
>    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.
>
> If this paragraph was moved before the previous paragraph, then EB and
> DIO would be defined before their first usage.
>
>    bandwidth of minimal cell.  One of the algorithm met the rule is the
>    Trickle timer defined in [RFC6206] which is applied on DIO messages
>    [RFC6550].  However, any such algorithm of limiting the broadcast
>
> nit(?): "One of the algorithms that fulfills this requirement"?
>
>    MSF RECOMMENDS the use of 3 slotframes.  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.  It is RECOMMENDED to
>    use the same slotframe length for Slotframe 0, 1 and 2.  Thus it is
>
> Perhaps this is just a question of writing style, but if an
> implementation is free to use an alternative SF or a variant of MSF,
> could we not say that "MSF uses 3 slotframts", "MSF uses the same
> slotframe length for", etc.?
>
> Section 3
>
> Is there any risk of unwanted correlation between slot and channel
> offsets when using the same hash function and input for both
> calculations?
>
>    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, an example how the hash function is implemented is detailed
>    in Appendix B.
>
> Given the lack of usable reference for [SAX-DASFAA], I assume that the
> content in Appendix B is going to be used as a specification, not just
> an example.
>
>    *  The AutoRxCell MUST always remain scheduled after synchronized.
>
> nit: s/synchronized/synchronization/
>
>    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.
>
> Presumably this factors in to the recommendation to have the three
> listed slotframes use the same length, but mentioning it explicitly
> (whether here or where the recommendation is made) might be nice.
>
> Section 4
>
>    network.  Alternative behaviors may involved, for example, when
>    alternative security solution is used for the network.  Section 4.1
>
> nit: singular/plural mismatch "behaviors"/"solution is used"
>
> Section 4.1
>
>    A node implementing MSF SHOULD implement the Minimal Security
>    Framework for 6TiSCH [I-D.ietf-6tisch-minimal-security].  As a
>
> Didn't this get renamed to CoJP?
>
> Section 4.2
>
> I a little bit wonder if there is a better description than "available
> frequencies" but don't have one to offer.
>
> Section 4.3
>
>    While the exact behavior is implementation-specific, it is
>    RECOMMENDED that after having received the first EB, a node keeps
>    listen for at most MAX_EB_DELAY seconds until it has received EBs
>    from NUM_NEIGHBOURS_TO_WAIT distinct neighbors, which is defined in
>    [RFC8180].
>
> nit(?): this phrasing implies that only NUM_NEIGHBOURS_TO_WAIT is
> defined in RFC 8180, but MAX_EB_DELAY is also defined there.
>
> not-nit: this phrasing is ambiguous as to whether one of MAX_EB_DELAY
> and NUM_NEIGHBOURS_TO_WAIT is sufficient to move to the next step or
> whether both are required.
>
> Section 4.4
>
>    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 JP over the AutoTxCell.  The AutoTxCell is removed by the pledge
>
> editorial: I'd suggest s/its JP/its selected JP/
>
>    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 configurations, and becomes a "joined node".
>
> nit: maybe "other configuration values" or "other configuration
> settings"?
>
> Section 4.6
>
>    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 with the following
>    fields:
>
>    *  CellOptions: set to TX=1,RX=0,SHARED=0
>    *  NumCells: set to 1
>    *  CellList: at least 5 cells, chosen according to Section 8
>
> Is this listing describing the contents of the ADD request or the
> AuthTxCell used to send it?  (I presume the former, in which case I
> suggest to use "containing" or similar in preference to "with".)
>
> Section 5.1
>
>    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:
>
> Is this goal strictly limited to traffic "to the selected parent" vs.
> all traffic?
>
>    *  If the node determines that the number of link-layer frames it is
>       attempting to exchange with the selected parent per unit of time
>       is larger than the capacity offered by the TSCH negotiated cells
>       it has scheduled with it, the node issues a 6P ADD command to that
>       parent to add cells to the TSCH schedule.
>    *  If the traffic is lower than the capacity, the node issues a 6P
>       DELETE command to that parent to delete cells from the TSCH
>       schedule.
>
> As written, this would potentially lead to oscillation when demand is
> basically at capacity, due to the quantization of capacity.  Perhaps
> some provisioning for hysteresis is appropriate?
>
>    The cell option of cells listed in CellList in 6P Request frame
>    SHOULD be either Tx=1 only or Rx=1 only.  Both NumCellsElapsed and
>    NumCellsUsed counters can be used to both type of negotiated cells.
>
> Would this be more clear as "(Tx=1,Rx=0) or (Tx=0,Rx=1)"?
>
>    *  NumCellsElapsed is incremented by exactly 1 when the current cell
>       is AutoRxCell.
>
> This holds for all peers/parents we're keeping counters for, so the
> AutoRxCell can get "double counted"?
>
>    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.  It could either be a
>    periodic polling to the neighbors the nodes have negotiated cells
>    with, or monitoring the activities on those cells.  The goal is to
>    confirm those negotiated cells are not used anymore by the associated
>    neighbors and remove them from the schedule.
>
> I'm not sure that "monitoring the activities on those cells" is safe
> with the current level of specification; if a node negotiates a 6P
> transmit cell to a parent and uses it only sparingly, with the parent
> eventually reclaiming it due to inactivity, I don't see a mechanism by
> which the node will reliably discover the negotiated cell to be
> nonfunctional and fall back to (e.g.) the corresponding AutoTxCell.  It
> may be most prudent to just not mention that as an example (a "periodic
> polling" procedure does not seem to have the same potential for
> information skew)
>
> Section 5.3
>
>    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.  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.
>
> Does MAX_NUMTX need to be a power of two (to avoid errors when the
> division occurs)?
>
>    4.  For any other cell, it compares its PDR against that of the cell
>        with the highest PDR.  If the difference is larger than
>        RELOCATE_PDRTHRES, it triggers the relocation of that cell using
>        a 6P RELOCATE command.
>
> The recommended RELOCATE_PDRTHRES is given as "50 %".  Is this
> "difference" performed as a subtraction (so that if the highest PDR is
> less than 50%, no cells can ever be relocated) or a ratio (a PDR that's
> half than the maximum PDR or smaller will trigger relocation)?
>
> Section 7
>
> Maybe reference Section 17.1 where the allocation will occur?
>
> Section 8
>
>    *  The slotOffset of a cell in the CellList SHOULD be randomly and
>       uniformly chosen among all the slotOffset values that satisfy the
>       restrictions above.
>    *  The channelOffset of a cell in the CellList SHOULD be randomly and
>       uniformly chosen in [0..numFrequencies], where numFrequencies
>       represents the number of frequencies a node can communicate on.
>
> Do these random selections need to be independent from each other?  (I
> note that the selection for the autonomous cells are not.)
>
> Section 9
>
> Is there a reference for these three parameters (MAXBE, MAXRETRIES,
> SLOTFRAME_LENGTH)?  SLOTFRAME_LENGTH seems new in this document and is
> listed in the table in Section 14, but the other two are not listed
> there.
>
> Section 14
>
> Why is MAX_NUMTX not listed in the table?
>
> Can we really give a recommended NUM_CH_OFFSET value, since this is in
> effect dependent on the number of channels available?
>
> KA_PERIOD is defined but not used elsewhere in the document.
>
> What are the considerations in using a power of 10 vs. a power of 2 as
> MAX_NUM_CELLS?
>
> Section 16
>
>    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
>
> I'd suggest a brief note that the security considerations of those
> protocols continue to apply (even though it ought to be obvious);
> reading them could help a reader understand the behavior of this
> document as well.
>
>    Sublayer Protocol (6P) [RFC8480], and the Minimal Security Framework
>    for 6TiSCH [I-D.ietf-6tisch-minimal-security].  In particular, MSF
>
> [CoJP again]
>
>    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,
>    e.g. by moving the node outside the radio range of the attacker.
>
> "the radio range of the attacker" is not exactly a fixed constant ...
> attackers are not in general bound by legal limits and can increase Tx
> power subject only to their equipment and budget.
>
>    MSF adapts to traffics containing packets from IP layer.  It is
>    possible that the IP packet has a non-zero DSCP (Diffserv Code Point
>    [RFC2597]) value in its IPv6 header.  The decision whether to hand
>
> RFC 2597 is talking more about specifically assured forwarding PHB groups
> than "DSCP codepoint"s per se.
>
> Section 18.1
>
> RFC 6206 seems to only be used as an example (Trickle), and could
> probably be informative.
>
> RFC 8505 might also not need to be normative.
>
> Appendix B
>
>    In MSF, the T is replaced by the length slotframe 1.  String s is
>
> nit: "length of"
>
>    2.  sum the value of L_shift(h,l_bit), R_shift(h,r_bit) and ci
>
> Is this addition performed in "infinite precision" integer arithmetic or
> limited to the output width of h, e.g., by modular division?  (It's not
> clear to me whether this is the role T plays or not.)
>
>    8.  assign the result of Step 5 to h
>
> The value from step 5 *is* h, so taken literally this says "assign h to
> h" and is not needed.
>
>
>
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-- 
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Stay healthy, stay optimistic!

Dr. Tengfei, Chang
Postdoctoral Research Engineer, Inria

www.tchang.org/
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