Re: [Roll] Benjamin Kaduk's Discuss on draft-ietf-roll-aodv-rpl-10: (with DISCUSS and COMMENT)

Benjamin Kaduk <kaduk@mit.edu> Tue, 31 August 2021 02:45 UTC

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Date: Mon, 30 Aug 2021 19:45:24 -0700
From: Benjamin Kaduk <kaduk@mit.edu>
To: Charlie Perkins <charles.perkins@earthlink.net>
Cc: The IESG <iesg@ietf.org>, draft-ietf-roll-aodv-rpl@ietf.org, roll-chairs@ietf.org, roll@ietf.org, Ines Robles <mariainesrobles@googlemail.com>, aretana.ietf@gmail.com
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Subject: Re: [Roll] Benjamin Kaduk's Discuss on draft-ietf-roll-aodv-rpl-10: (with DISCUSS and COMMENT)
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Hi Charlie,

Please also excuse my delay in responding.

There's a number of your points that need no further reply, so I'll spare
you the repeated "sounds good"/"ok"/etc. and reply inline only as needed.

On Wed, Aug 18, 2021 at 01:46:17PM -0700, Charlie Perkins wrote:
> Hello Benjamin,
> 
> Please excuse the unusually long delay it has taken for us to respond to 
> your comments.
> 
> Regarding the following:
> 
> 
>  >  Benjamin Kaduk Discuss
>  > Discuss (2021-04-22)
> 
>  > Specifically, there are several places in the document (most notably
>  > Section 6.4) that provide steps for processing a RREP-DIO that refer to
>  > the value of the "S bit".  There is no S bit in the RREP option as
>  > defined in Section 4.2; indeed, there has never been an S bit in the
>  > RREP option since it was introduced in the -03.  The -02 was proposing
>  > changes directly in the DIO base object, which included an S bit, so in
>  > that version of the document referring to an "S bit" in the reply
>  > processing could have made sense.
> 
> Thank you very much for pointing out this inconsistency.  Once we 
> decided that
> the 'S' bit was not explicitly needed in the RREP option, we should have
> changed this wording, but we just missed it.  Instead of referring to 
> the 'S'
> bit of the RREP, the text has to instead refer to a status bit in the 
> internal
> data structure for the RREQ-Instance, which we also called the 'S' bit.

Ah, in an internal data structure for the instance!  That changes a number
of my other comments, I think; it's unfortunate that you have to reply to
so many coments made under a flawed assumption.

> 
> <===== Text added after the last para of Step 1 of 6.2.1 =======>
> 
>        Step 1:
> 
>        If the S bit in the received RREQ-DIO is set to 1, the router MUST
>        determine whether each direction of the link (by which the RREQ-
>        DIO is received) satisfies the Objective Function.  In case that
>        the downward (i.e., towards the TargNode) direction of the link
>        does not satisfy the Objective Function, the link can't be used
>        symmetrically, thus the S bit of the RREQ-DIO to be sent out MUST
>        be set as 0.  If the S bit in the received RREQ-DIO is set to 0,
>        the router MUST determine into the upward direction (towards the
>        OrigNode) of the link.
> 
>        If the upward direction of the link can satisfy the Objective
>        Function, and the router's Rank would not exceed the MaxUseRank
>        limit, the router joins the DODAG of the RREQ-Instance.  The
>        router that transmitted the received RREQ-DIO is selected as the
>        preferred parent.  Otherwise, if the Objective Function is not
>        satisfied or the MaxUseRank limit is exceeded, the router MUST
>        discard the received RREQ-DIO and MUST NOT join the DODAG.
> 
>        When a router joins the RREQ-Instance, it also associates within
>        its data structure for the RREQ-Instance the information about
>        whether or not the RREQ has the S-bit set to 1. This information
>        associated to RREQ-Instance is known as the S-bit of the
>        RREQ-Instance. It will be used later during the RREP-DIO message
>        processing (Section 6.4) for RPLInstance pairing as described
>        in Section 6.3.3.
> 
> ...
> <=== Text added to Step 1 of 6.4.===>
> 
> 6.4.  Receiving and Forwarding Route Reply
> 
>     Upon receiving a RREP-DIO, a router which does not belong to the
>     RREP-Instance goes through the following steps:
> 
>     Step 1:
> 
>        If the S-bit of the RREQ-Instance is set to 1, the router MUST
>        proceed to step 2.
> 
>        If the S-bit of the RREQ-Instance is set to 0, the router MUST
>        determine whether the downward direction of the link (towards the
>        TargNode) over which the RREP-DIO is received satisfies the
>        Objective Function, and the router's Rank would not exceed the
>        MaxRank limit.  If so, the router joins the DODAG of the RREP-
>        Instance.  The router that transmitted the received RREP-DIO is
>        selected as the preferred parent.  Afterwards, other RREP-DIO
>        messages can be received.
> 
> 
>  > There are also a few places that refer to using RREP (reply) processing
>  > to relate to membership in or joining of the RREQ (request) DODAG.  I
>  > assume that these are, in effect, typographical errors that should refer
>  > to the RREP DODAG, but the one character has extreme significance to
>  > protocol operations.
> 
> We are reviewing every instance of this wording, and have made the suggested
> correction where appropriate.
> 
> 
>  > I also think that there is too much ambiguity relating to the processing
>  > of RREPs in the symmetric vs asymmetric case (which returns to the
>  > question of whether there is or should be an S bit in the RREP option).
>  > In particular, the semantics of the Address Vector field (for the
>  > source-routing case only, of course) vary.  In the symmetric case this
>  > field is set by TargNode and propagated unchanged in the RREPs, but for
>  > the asymmetric case each intermediate node needs to add its address in
>  > the Address Vector.  We do cover these different behaviors in Sections
>  > 6.3.1 and 6.3.2, but leave it very unclear as to how an intermediate
>  > node tells whether a received RREP is for the symmetric or asymmetric
>  > case.
> 
> This is intended to be resolved by maintaining the 'S' bit status in the
> RREQ-Instance.

Yes, that sounds like it should work (I am slowly paging the protocol back
into my brain as I go through the email).

> 
>  >            An explicit S bit would make this easy, of course, though it
>  > seems like it *might* be possible to use whether the RREP was received
>  > over a unicast or multicast address/interface as a stand in.
>  >
>  > However,
>  > that technique would be complicated by the presence of gratuitous RREPs,
>  > which are unicast in cases that do not quite align up with symmetric vs
>  > asymmetric.  (Whether the processing behavior should reflect the "append
>  > to address vector" or "propagate address vector unchanged" for the
>  > gratuitous case is also not entirely clear to me.)
> 
> Given the maintenance of the 'S' bit status in the RREQ-Instance, it should
> not be necessary to make the determination based on whether or not the 
> message
> was received as a multicast.

Right, there's no need to use multicast vs unicast as a proxy for the S bit
state.  I assume that also disambiguates how to handle the gratuitous RREP
case, though I haven't remembered enough of the protocol yet in order to
work through it for myself.

> 
>  > On a more minor note, I don't think the description of rollover in
>  > Section 6.3.3 is correct.  More in the COMMENT, but in essence, even
>  > though the shift is capped at 63, the instance ID can go up to 255 and
>  > wrapping should occur at the instance ID boundary, not the shift
>  > boundary.
> 
> The point of the shift parameter is to be able to "modify" the value of the
> RPLInstanceID so that a unique association can be made between RREQ-Instance
> and RREP-Instance. We are confident that having 64 values to choose from 
> will
> enable the node to make the unique association.

Yes, having 64 values available should be fine.  My comment relates to
*which* 64 values those are.  I was looking at how the RPL Instance ID is
encoded in an 8-bit field, which would let it range from 0 to 255; if we
add a 0-to-63 bit shift to such a value then it wraps if the resulting
value woule exceed 255, rather than 63.  However, as I now go back to look
at RFC 6550 it seems that there is (at least sometimes?) additional
structure within that 8-bit field, and there are only 64 instance ID values
available for *local* RPLInstanceID usage.  In my latter comment about the
same topic you seemed to agree with the "rollover at 255" interpretation,
so maybe this should get some further investigation.

> 
>  > Comment (2021-04-22)
> 
>  > The Abstract and Introduction do not paint a very clear picture of what
>  > is going to happen.  Section 3 helps a fair bit, but I would have
>  > expected the introduction to mention that RREQ/RREP go in separate
>  > (paired) RPL instances, and that instances are created (and destroyed?)
>  > for each route being discovered.  (This would also be a great place to
>  > clarify how AODV-RPL interacts with regular RPL, as was requested by
>  > other ADs already.)
> 
> We will use your wording when adding additional text to the Introduction.
> AODV-RPL can be used alongside RPL and add routes to the existing
> RPL-discovered routes.  However, there does not seem to be much value in
> maintaining two routing protocols even if they are compatible.
> 
> 
>  > I would like to see a clearer picture of the relationship between the
>  > lifetime of routes discovered using AODV-RPL and the lifetime of the
>  > DODAGs used to build them.  The (non-infinite) DODAG lifetime options
>  > are fairly short, and I would (perhaps naively) expect routes to have a
>  > longer lifetime than that in many cases.  But it seems that the
>  > information stored with a route includes the RPL InstanceID, and if the
>  > route is to outlast the DODAG, then that information would become stale,
>  > and I don't know what value there would be in keeping it around in that
>  > case and risking collisions.  Is it expected that when routes are to be
>  > long-lived, the L value of 0 is to be used?
> 
> Routes are intended to last long enough to support the applications 
> running on
> the OrigNode and TargNode that required the route.  The same considerations
> apply to RPL, and so we expected that AODV-RPL would use the same mechanism
> for route longevity that is used by RPL.
> 
> Your explanation for route lifetime is fine to me.  In addition, DODAG
> lifetime cannot be lower than the route lifetime since that could lead to
> stale information as observed.
> 
> 
>  > Section 1
> 
>  >>    (DAO) message of RPL.  AODV-RPL specifies a new MOP (Mode of
>  >>    Operation) running in a separate instance dedicated to discover P2P
>  >>    routes, which may differ from the point-to-multipoint routes
>  >>    discoverable by native RPL.  AODV-RPL can be operated whether or not
> 
>  > I don't really understand why we find it useful to make a comparison
>  > between P2P routes and P2MP routes.
> 
> Agreed.  It isn't useful. "P2P routes discoverable by native RPL" is better.
> 
> 
>  > Section 2
> 
>  >>    RREP-DIO message
>  >>       An AODV-RPL MOP DIO message containing the RREP option.  The
>  >>       RPLInstanceID in RREP-DIO is typically paired to the one in the
>  >
>  > Typically, or actually (noting that §6.3.3 allows for the pairing
>  > process to include a "Shift" count for cases where the value cannot
>  > match exactly)?  Is this an attempt to reflect the symmetric case where
>  > a DODAG is not built for symmetric routes?  If so, it's not clear that
>  > we accurately portray what would be the "typical" case...but even in
>  > that symmetric case we still have to populate the RPLInstance field in
>  > the unicast RREP-DIO, and that still has the pairing logic.  So I'm back
>  > to wondering when these would not be paired.
> 
> It is intended to allow for the shift parameter.  §6.3.3 clearly states
> "the RREQ-Instance and the RREP-Instance in the same route discovery MUST
> be paired using the RPLInstanceID." Accordingly, we modify the text as 
> follows.
> 
>     "The RPLInstanceID in RREP-DIO MUST be paired to the one in the 
> associated
>      RREQ-DIO message as described in §6.3.3."

I'm happy to see the uniform applicability clarified (by removing
"typically").  I would caution against writing "MUST be paired", though,
since that would result in having the same requirement made using normative
language in two different places in the text, which has a risk of the two
statements inadvertently making slightly different requirements.  Since
this is already saying "as described in §6.3.3", it could be written as
a declarative statement like "the RPLInstanceID in the RREP-DIO is paired".

> 
>  > Section 3
> 
>  >>    The routes discovered by AODV-RPL are not constrained to traverse a
>  >>    common ancestor.  AODV-RPL can enable asymmetric communication paths
>  >>    in networks with bidirectional asymmetric links.  For this purpose,
> 
>  > Can AODV-RPL function in networks with unidirectional links?
> 
> Yes, as long as the node receiving an RREQ or RREP message can valuate 
> whether
> or not the link bearing an incoming message satisfies the OF.

I'll leave it up to you as to whether such scenarios are worth mentioning
here -- I have no sense for how practical or likely they would be.

> 
>  >>    to TargNode, and another from TargNode to OrigNode. When possible,
>  >>    AODV-RPL also enables symmetric route discovery along Paired DODAGs
>  >>    (see Section 5).
> 
>  > In what circumstances is it not possible to do so?
> 
> It is possible when the links are symmetric.
> 
>  > Section 4.1
> 
>  >>    OrigNode sets its IPv6 address in the DODAGID field of the RREQ-DIO
>  >>    message.  A RREQ-DIO message MUST carry exactly one RREQ option,
>  >>    otherwise it MUST be dropped.  (Similarly for RREP in §4.2.)
> 
>  > I suggest clarifying that other options are allowed (required, even).
> 
> Yes, other options are allowed. We will clarify this point by
> referring to Section 4.3 which says, "A RREQ-DIO message MUST carry at
> least one ART Option.  A RREP-DIO message MUST carry exactly one ART 
> Option.
> Otherwise, the message MUST be dropped."
> 
>  > Who sets the S bit, and can it change as the DODAG is being constructed?
>  > ("See Section 5" would be fine.)
> 
> Only the OrigNode sets the 'S' bit.

To be fully explicit: I think that this section's text describing the S bit
should either say that itself or refer to Section 5 for more detail on its
handling.

> 
>  >>    L
>  >>       2-bit unsigned integer determining the duration that a node is
>  >>       able to belong to the temporary DAG in RREQ-Instance, including
>  >>       the OrigNode and the TargNode.  Once the time is reached, a node
>  >>       MUST leave the DAG and stop sending or receiving any more DIOs for
>  >>       the temporary DODAG.
> 
>  > How do we account for time skew as the DIO propagates?  Each node just
>  > leaves on their own timer?
> 
> Yes, the measure time duration depends on each node's ability to keep
> track of the time.
> 
> 
>  >>    Address Vector
>  >>       A vector of IPv6 addresses representing the route that the RREQ-
>  >>       DIO has passed.  It is only present when the H bit is set to 0.
>  >>       The prefix of each address is elided according to the Compr field.
> 
>  >>    TargNode can join the RREQ instance at a Rank whose integer portion
>  >>    is equal to the MaxRank.  Other nodes MUST NOT join a RREQ instance
>  >>    if its own Rank would be equal to or higher than MaxRank.  A router
>  >>    MUST discard a received RREQ if the integer part of the advertised
>  >>    Rank equals or exceeds the MaxRank limit.
> 
>  > Both of these descriptions might benefit from a bit more detail.  E.g.,
>  > the latter paragraph doesn't say that TargNode can join if the rank is
>  > less than MaxRank, only if it's equal.
> 
> Good point!  Yes, the TargNode can join if the rank is less than or
> equal to MaxRank.
> 
> 
>  > Section 4.2
> 
>  >>    H
>  >>       Requests either source routing (H=0) or hop-by-hop (H=1) for the
>  >>       downstream route.  It MUST be set to be the same as the H bit in
>  >>       RREQ option.
> 
>  > (editorial) I'd suggest putting the "MUST be the same" requirement as
>  > the first sentence, and then the other sentence could be "determines
>  > whether source routing (H=0) or hop-by-hop (H=1) is used for the
>  > downstream route"
> 
> O.K.
> 
> 
>  >>    L
>  >>       2-bit unsigned integer defined as in RREQ option.
> 
>  > Does L need to have the same value as in the triggering RREQ option?  If
>  > not, when might TargNode choose a different value?
> 
> Both the DODAGs are part of the same route discovery, and RPLInstanceID
> requires that both DODAGs be alive.  There can however be a route with
> asymmetric data traffic profile between OrigNode and TargNode.  In this
> case, it is possible that the data occupancy times of the two DODAGs are
> different.  The OrigNode should consider this factor while fixing the
> duration.  The L-value for the RREQ-Instance has to be larger than the
> L value for the RREP-Instance.

At least the last bit (L for RREQ-Instance > L for RREP-Instance) seems
like a protocol constraint that should be documented in the RFC.  How much
supporting discussion to include with that is probably not something I'm
the right person to answer.

> 
>  >>    Address Vector
>  >>       Only present when the H bit is set to 0.  For an asymmetric route,
>  >>       the Address Vector represents the IPv6 addresses of the route that
>  >>       the RREP-DIO has passed.  For a symmetric route, it is the Address
>  >>       Vector when the RREQ-DIO arrives at the TargNode, unchanged during
>  >>       the transmission to the OrigNode.
> 
>  > [ed. this was written before I made a discuss point about it, but I'm
>  > leaving the text for the extra detail.  It's okay to just respond to the
>  > discuss point and not here.]
>  > If I understand correctly, the S bit indicating symmetric vs asymmetric
>  > route is present only in the RREQ-DIO, and is not included in-band in
>  > the RREP-DIO.  Does this require all nodes on the path to remember
>  > whether a symmetric route is being constructed on the RREQ-DIO instance,
>  > use the Shift in the RREP-DIO to correlate to the corresponding RREQ-DIO
>  > and 'S' bit status, as part of the processing (to determine whether or
>  > not to append to the Address Vector)?
> 
> Yes, that is the requirement.
> 
> 
> 
>  > Section 4.3
> 
>  >>    Dest SeqNo
> 
>  >>       In RREQ-DIO, if nonzero, it is the last known Sequence Number for
>  >>       TargNode for which a route is desired.  In RREP-DIO, it is the
>  >>       destination sequence number associated to the route.
> 
>  > The destination sequence number for the downstream route or the upstream
>  > route?
> 
> The RREQ carries the destination sequence number for the last OF-compliant
> route that OrigNode stored to the TargNode - i.e., the downstream route.
> 
> 
>  > Also, should we say that zero is used if there is no known 
> information about
>  > the sequence number of TargNode (and not otherwise)?
> 
> Agreed.
> 
> 
>  >>    r
>  >>       A one-bit reserved field.  This field MUST be initialized to zero
>  >>       by the sender and MUST be ignored by the receiver.
> 
>  > The secdir reviewer noted the mismatch between 'X' in the figure and 'r'
>  > here; please fix.
> 
> Agreed.
> 
> 
>  >>    Prefix Length
>  >>       7-bit unsigned integer.  Number of valid leading bits in the IPv6
>  >>       Prefix.  If Prefix Length is 0, then the value in the Target
>  >>       Prefix / Address field represents an IPv6 address, not a prefix.
>  >>
>  >>    Target Prefix / Address
>  >>       (variable-length field) An IPv6 destination address or prefix.
>  >>       The Prefix Length field contains the number of valid leading bits
>  >>       in the prefix.  The length of the field is the least number of
>  >>       octets that can contain all of the bits of the Prefix, in other
>  >>       words Floor((7+(Prefix Length))/8) octets.  The remaining bits in
>  >>       the Target Prefix / Address field after the prefix length (if any)
>  >>       MUST be set to zero on transmission and MUST be ignored on
>  >>       receipt.
> 
>  > Please specify how long the Address field is when Prefix Length is zero
>  > (indicating that the last field is the Address variant).
> 
> Address field is 128 bits for IPv6 addresses.
> 
> 
>  > Section 5
> 
>  >>    Links are considered symmetric until additional information is
>  >>    collected.  [...]
>  >
>  > What kinds of problems will arise if we start taking actions based on
>  > this assumption before the "additional information" is available?
>  > (That is to say, perhaps this is not a useful phrasing, since what we
>  > actually do is get updates about the presence of asymmetric links as we
>  > construct the route.)
> 
> How about, "indication to the contrary is received"?

That's probably a more clear way to state what the nodes are going to do.
With my question I was trying to ask whether there was any processing or
protocol activity that occurred prior to the indication that a given link
is symmetric, that might have erroneously relied on the assumption of
symmetric links.  If so, how would that get unwound safely?

> 
>  >>    bit set to 1, then all the one-hop links on the route from the
>  >>    OrigNode O to this router meet the requirements of route discovery,
>  >
>  > Re "the route", this would presumably be the one recorded in the Address
>  > Vector of the RREQ in question?  (Multiple RREQs for the same route
>  > computation can arrive at a given node with different address vectors,
>  > right?
> 
> Yes, both of your statements are correct.  We will try to devise more exact
> wording, or if you have a suggestion that would be appreciated.

My attempt at a minimal change would be to insert "indicated" or "recorded"
before "route from the OrigNode", but that may be missing some subtlety.

> 
>  >
>  > Also, the way this is written implies that it does not say anything
>  > about "non-one-hop links" on the route, but I don't really know what a
>  > link that's not a one-hop link would be.  Can we just say "all the hops"
>  > or "all the links"?
> 
> Good idea.
> 
> 
>  >>    and the route can be used symmetrically.
> 
>  > But does that matter for any routers other than TargNode (for any of the
>  > AODV-RPL Target Options)?
> 
> Yes, because in the symmetric case unicast RREP is specified.

Ah, yes.  Sorry to have missed that.

> 
>  >>    doesn't satisfy the Objective Function.  Based on the S bit received
>  >>    in RREQ-DIO, TargNode T determines whether or not the route is
>  >>    symmetric before transmitting the RREP-DIO message upstream towards
>  >>    the OrigNode O.
> 
>  > Does that determination affect the construction of the RREP-DIO in any
>  > way?  (E.g., if there was an S bit.)
> 
> Section 6.3.1 says:
> 
>     "For a symmetric route, the RREP-DIO message is unicast to the next hop
>      according to the accumulated address vector (H=0) or the route 
> entry (H=1)."
> 
> 
>  >>             Figure 5: AODV-RPL with Asymmetric Paired Instances
> 
>  > Some discussion of how the third(? second?) intermediate router detects
>  > the asymmetry and clears the S bit might be appropriate.
> 
> We will describe Figure 5 by with the help of link asymmetry detection
> methods discussion given in the same section, Section 5. The proposed text:
> 
>     As shown in the Figure 5, an intermediate router determines the 'S' bit
>     value RREQ-DIO should carry with the help of link asymmetry detection
>     methods discussed in Section 5. It is expected that the intermediate
>     router has already made the link asymmetry decision by the time RREQ-DIO
>     arrives.
> 
>  > Section 6.1
>  >
>  >>    link-local multicast.  The DIO MUST contain at least one ART Option
>  >>    (see Section 4.3).  The S bit in RREQ-DIO sent out by the OrigNode is
>  >>    set to 1.
>  >
>  > I'd suggest saying that the required ART Option indicates the TargNode.
> 
> O.K.
> 
> 
>  >>    OrigNode can maintain different RPLInstances to discover routes with
>  >>    different requirements to the same targets.  Using the RPLInstanceID
>  >>    pairing mechanism (see Section 6.3.3), route replies (RREP-DIOs) for
>  >>    different RPLInstances can be distinguished.
>  >
>  > When using different RPLInstances for this purpose, what constitutes
>  > "initiates a route discovery process" across those instances -- is it
>  > permissible to only increment the sequence number once when initiating
>  > multiple discovery processes on different instances?
> 
> It is also needed to put in the correct OF and other values specific to 
> the desired route.  If those are all the same, just incrementing the 
> sequence number is fine.
> 
> 
>  > Section 6.2.1
>  >
>  >>    Step 1:
>  >
>  >>       If the S bit in the received RREQ-DIO is set to 1, the router MUST
>  >>       determine whether each direction of the link (by which the RREQ-
>  >>       DIO is received) satisfies the Objective Function. In case that
>  >>       the downward (i.e. towards the TargNode) direction of the link
>  >>       does not satisfy the Objective Function, the link can't be used
>  >>       symmetrically, thus the S bit of the RREQ-DIO to be sent out MUST
>  >>       be set as 0.  If the S bit in the received RREQ-DIO is set to 0,
>  >>       the router MUST determine into the upward direction (towards the
>  >>       OrigNode) of the link.
>  >
>  >>       If the upward direction of the link can satisfy the Objective
>  >>       Function, and the router's Rank would not exceed the MaxUseRank
>  >>       limit, the router joins the DODAG of the RREQ-Instance.  The
>  >>       router that transmitted the received RREQ-DIO is selected as the
>  >>       preferred parent.  Otherwise, if the Objective Function is not
>  >>       satisfied or the MaxUseRank limit is exceeded, the router MUST
>  >>       discard the received RREQ-DIO and MUST NOT join the DODAG.
>  >
>  > The way this is written is confusing to me.  It seems to say that (1)
>  > you only check the upward direction is the S bit in the received
>  > RREQ-DIO is set to zero, and (2) the only time you join the DODAG is if
>  > you're checking the upward direction.  So, when the received S-bit is 1,
>  > do you just never join the DODAG?  I assume this is not the intent, but
>  > that is how I interpret the words that are on the page.
> 
> It is supposed to mean that the node always checks the upstream link, and
> joins if the OF is satisfied.  If the OF is not satisfied,

[not sure if there was supposed to be more here.  I do see that the
"determine into the upward direction" got fixed in response to Warren (and
John), which would presumably help me some as well.]

>  >>       Sequence Number.  The Destination Address and the RPLInstanceID
>  >>       respectively can be learned from the DODAGID and the RPLInstanceID
>  >>       of the RREQ-DIO, and the Source Address is the address used by the
>  >>       local router to send data to the OrigNode.  The Next Hop is the
> 
>  > "Source Address is the address used by the local router to send data to
>  > the OrigNode" seems like the definition of the source address in a route
>  > table entry, not a procedure for how to set it.  Should this be the
>  > address used by the local router to send data to the preferred parent?
> 
> Yes, we will use that terminology.
> 
> 
>  > Section 6.3.1
> 
>  >>    implementation-specific and out of scope.  If the implementation
>  >>    selects the symmetric route, and the L bit is not 0, the TargNode MAY
>  >>    delay transmitting the RREP-DIO for duration RREP_WAIT_TIME to await
>  >>    a symmetric route with a lower Rank.  The value of RREP_WAIT_TIME is
>  >>    set by default to 1/4 of the time duration determined by the L bit.
> 
>  > There is no L *bit* in the RREQ option or the RFC 6550 DIO. There is a
>  > two-bit L field in the RREQ option, but even if I replace 'bit' with
>  > 'field', it's still not clear why having a DODAG with no lifetime limit
>  > implies that delaying the RREP-DIO is not allowed.
> 
> I don't see what is wrong about using L=0 to disallow the delay.

I don't know that it's wrong per se, but there's no motivation presented
for it.  In some sense it feels backwards to allow delay for the case where
there's a finite lifetime but not allow delay for a DODAG that will be
around indefinitely.  If there's a reason to disallow delays here, that may
weill be reasonable, but I couldn't say why based on just what I read here.
There would still need to be some bound on the delay, of course, but there
are plenty of ways to pick that, with the maximum allowed delay for a
nonzero L value perhaps being the most natural.

> 
>  > Section 6.3.2
> 
>  >>    When a RREQ-DIO arrives at a TargNode with the S bit set to 0, the
>  >>    TargNode MUST build a DODAG in the RREP-Instance rooted at itself in
> 
>  > I don't understand how the definite article is appropriate for "the
>  > RREP-Instance rooted at itself" -- I thought there were multiple
>  > (paired) instances corresponding to the various RREQ DODAGs that
>  > requested routes to TargNode.
> 
> That is a good point.  It has to be the RREP-Instance corresponding to the
> RREQ-DIO.
> 
> 
> 
>  >>    RREP_WAIT_TIME to await a route with a lower Rank.  The value of
>  >>    RREP_WAIT_TIME is set by default to 1/4 of the time duration
>  >>    determined by the L bit.
> 
>  > ("L bit" again, and no indication of what to do for L==0.)
> 
> The behavior should be the same.
> 
> 
>  >>    The settings of the fields in RREP option and ART option are the same
>  >>    as for the symmetric route, except for the S bit.
> 
>  > There is no S bit in the RREP.  What is this intending to say?
> 
> It should have said, "except for the value of the 'S' bit associated
> with the RREP-instance".
> 
> 
>  > Section 6.3.3
> 
>  >>    When preparing the RREP-DIO, a TargNode could find the RPLInstanceID
>  >>    to be used for the RREP-Instance is already occupied by another RPL
>  >>    Instance from an earlier route discovery operation which is still
>  >>    active.  In other words, it might happen that two distinct OrigNodes
>  >>    need routes to the same TargNode, and they happen to use the same
>  >>    RPLInstanceID for RREQ-Instance.  In this case, the occupied
>  >>    RPLInstanceID MUST NOT be used again.  [...]
> 
>  > A reminder might be helpful that the RPLInstanceID is a property of a
>  > DODAG, and a DODAG is identified by the DODAGID, which in this case is
>  > the address of the TargNode.  So that is why we need to avoid reusing
>  > RPLInstanceID in the context of the RREP-DIO, whereas there is no
>  > problem with collisions in RPLInstanceID across RREQ-DIOs (where the
>  > DODAGID is the OrigNode address, that suffices to disambiguate).
> 
> 
> When preparing the RREP-DIO, a TargNode could find the RPLInstanceID 
> candidate
> for the RREP-Instance is already occupied by another RPL Instance from an
> earlier route discovery operation which is still active.   This unlikely 
> case
> might happen if two distinct OrigNodes need routes to the same TargNode, and
> they happen to use the same RPLInstanceID for RREQ-Instance. In this 
> case, the
> occupied RPLInstanceID which denotes Objective Function of the DODAG, 
> MUST NOT
> be used again.  Reusing the same RPLInstanceID for two distinct
> DODAGs originated with the same DODAGID (TargNode address) would prevent
> intermediate routers to distinguish between these DODAGs (and their 
> associated
> Objective Functions).  RPLInstanceID collisions do not occur across 
> RREQ-DIOs;
> the DODAGID equals the OrigNode address and is sufficient to
> disambiguate between DODAGs.
> 
> 
> 
>  >>    shift to be applied to original RPLInstanceID.  When the new
>  >>    RPLInstanceID after shifting exceeds 63, it rolls over starting at 0.
> 
>  > I thought RPLInstanceID was a full 8-bit field (even though Shift is
>  > only six bits); wouldn't rollover happen after 255?
> 
> You are correct.  It should say 255 instead of 63.
> 
> 
>  >>    For example, the original RPLInstanceID is 60, and shifted by 6, the
>  >>    new RPLInstanceID will be 2.  Related operations can be found in
>  >>    Section 6.4.
> 
>  > (So this example wouldn't actually show rollover.)
> 
> Correct again.
> 
> 
>  > Section 6.4
> 
>  >>    Upon receiving a RREP-DIO, a router which does not belong to the
>  >>    RREQ-Instance goes through the following steps:
> 
>  > Do we care about RREQ-Instance membership or RREP-Instance membership,
>  > for processing the RREP-DIO?
> 
> 
> Right, it should have been RREP-Instance, not RREQ-Instance as shown.
> But in fact, we do not need this conditional check at all. Processing
> the newly arriving RREP-DIO even if the router has already joined
> RREP-Instance can result in better downward route to TargNode.

Ah, good point!

> 
>  >    Step 1:
> 
>  >>       If the S bit is set to 1, the router MUST proceed to step 2.
> 
>  > There is no S bit in the RREP option!
> 
> Correct again.  This should refer to the S-bit of the associated Instance.
> 
> 
>  >>       and the destination address is learned from the DODAGID.  The
>  >>       lifetime is set according to DODAG configuration (i.e., not the L
>  >>       bit) and can be extended when the route is actually used.  The
> 
>  > ("L bit" again)
> 
> Check.
> 
> 
>  >>    Upon receiving a RREP-DIO, a router which already belongs to the
>  >>    RREQ-Instance SHOULD drop the RREP-DIO.
> 
>  > (RREQ-Instance vs RREP-Instance, again.)
> 
> Check.
> 
> 
> 
>  > Section 10
> 
>  > It seems like a malicious node that forges a gratuitous RREP could do
>  > significant damage as well, so that might be worth mentioning.
> 
> Yes.
> 
>  >>    routing loop.  The TargNode MUST NOT generate a RREP if one of its
>  >>    addresses is present in the Address Vector.  An Intermediate Router
>  >>    MUST NOT forward a RREP if one of its addresses is present in the
>  >>    Address Vector.
> 
>  > These requirements seem important enough that I'd prefer to seem them
>  > imposed in the main body text that covers RREP handling, and the
>  > security considerations mentioned here and referring to those handling
>  > requirements.
> 
> Agreed.  These requirements belong better in the main body text.
> 

Thanks for all the replies, and the pending updates,

Ben

> 
> On 4/22/2021 3:48 PM, Benjamin Kaduk via Datatracker wrote:
> > Benjamin Kaduk has entered the following ballot position for
> > draft-ietf-roll-aodv-rpl-10: Discuss
> >
> > 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 DISCUSS and COMMENT positions.
> >
> >
> > The document, along with other ballot positions, can be found here:
> > https://datatracker.ietf.org/doc/draft-ietf-roll-aodv-rpl/
> >
> >
> >
> > ----------------------------------------------------------------------
> > DISCUSS:
> > ----------------------------------------------------------------------
> >
> > My apologies for coming in with a late review, but I think there are
> > some serious internal inconsistencies in this document that leave me
> > unsure whether the document is in a reviewable form.  It might be
> > prudent to have the document return to the WG to fix the identified
> > issues and get additional review.
> >
> > Specifically, there are several places in the document (most notably
> > Section 6.4) that provide steps for processing a RREP-DIO that refer to
> > the value of the "S bit".  There is no S bit in the RREP option as
> > defined in Section 4.2; indeed, there has never been an S bit in the
> > RREP option since it was introduced in the -03.  The -02 was proposing
> > changes directly in the DIO base object, which included an S bit, so in
> > that version of the document referring to an "S bit" in the reply
> > processing could have made sense.
> >
> > There are also a few places that refer to using RREP (reply) processing
> > to relate to membership in or joining of the RREQ (request) DODAG.  I
> > assume that these are, in effect, typographical errors that should refer
> > to the RREP DODAG, but the one character has extreme significance to
> > protocol operations.
> >
> > I also think that there is too much ambiguity relating to the processing
> > of RREPs in the symmetric vs asymmetric case (which returns to the
> > question of whether there is or should be an S bit in the RREP option).
> > In particular, the semantics of the Address Vector field (for the
> > source-routing case only, of course) vary.  In the symmetric case this
> > field is set by TargNode and propagated unchanged in the RREPs, but for
> > the asymmetric case each intermediate node needs to add its address in
> > the Address Vector.  We do cover these different behaviors in Sections
> > 6.3.1 and 6.3.2, but leave it very unclear as to how an intermediate
> > node tells whether a received RREP is for the symmetric or asymmetric
> > case.  An explicit S bit would make this easy, of course, though it
> > seems like it *might* be possible to use whether the RREP was received
> > over a unicast or multicast address/interface as a stand in.  However,
> > that technique would be complicated by the presence of gratuitous RREPs,
> > which are unicast in cases that do not quite align up with symmetric vs
> > asymmetric.  (Whether the processing behavior should reflect the "append
> > to address vector" or "propagate address vector unchanged" for the
> > gratuitous case is also not entirely clear to me.)
> >
> > On a more minor note, I don't think the description of rollover in
> > Section 6.3.3 is correct.  More in the COMMENT, but in essence, even
> > though the shift is capped at 63, the instance ID can go up to 255 and
> > wrapping should occur at the instance ID boundary, not the shift
> > boundary.
> >
> >
> > ----------------------------------------------------------------------
> > COMMENT:
> > ----------------------------------------------------------------------
> >
> > The Abstract and Introduction do not paint a very clear picture of what
> > is going to happen.  Section 3 helps a fair bit, but I would have
> > expected the introduction to mention that RREQ/RREP go in separate
> > (paired) RPL instances, and that instances are created (and destroyed?)
> > for each route being discovered.  (This would also be a great place to
> > clarify how AODV-RPL interacts with regular RPL, as was requested by
> > other ADs already.)
> >
> > I would like to see a clearer picture of the relationship between the
> > lifetime of routes discovered using AODV-RPL and the lifetime of the
> > DODAGs used to build them.  The (non-infinite) DODAG lifetime options
> > are fairly short, and I would (perhaps naively) expect routes to have a
> > longer lifetime than that in many cases.  But it seems that the
> > information stored with a route includes the RPL InstanceID, and if the
> > route is to outlast the DODAG, then that information would become stale,
> > and I don't know what value there would be in keeping it around in that
> > case and risking collisions.  Is it expected that when routes are to be
> > long-lived, the L value of 0 is to be used?
> >
> > Section 1
> >
> >     (DAO) message of RPL.  AODV-RPL specifies a new MOP (Mode of
> >     Operation) running in a separate instance dedicated to discover P2P
> >     routes, which may differ from the point-to-multipoint routes
> >     discoverable by native RPL.  AODV-RPL can be operated whether or not
> >
> > I don't really understand why we find it useful to make a comparison
> > between P2P routes and P2MP routes.
> >
> > Section 2
> >
> >     RREP-DIO message
> >        An AODV-RPL MOP DIO message containing the RREP option.  The
> >        RPLInstanceID in RREP-DIO is typically paired to the one in the
> >
> > Typically, or actually (noting that §6.3.3 allows for the pairing
> > process to include a "Shift" count for cases where the value cannot
> > match exactly)?  Is this an attempt to reflect the symmetric case where
> > a DODAG is not built for symmetric routes?  If so, it's not clear that
> > we accurately portray what would be the "typical" case...but even in
> > that symmetric case we still have to populate the RPLInstance field in
> > the unicast RREP-DIO, and that still has the pairing logic.  So I'm back
> > to wondering when these would not be paired.
> >
> > Section 3
> >
> >     The routes discovered by AODV-RPL are not constrained to traverse a
> >     common ancestor.  AODV-RPL can enable asymmetric communication paths
> >     in networks with bidirectional asymmetric links.  For this purpose,
> >
> > Can AODV-RPL function in networks with unidirectional links?
> >
> >     to TargNode, and another from TargNode to OrigNode.  When possible,
> >     AODV-RPL also enables symmetric route discovery along Paired DODAGs
> >     (see Section 5).
> >
> > In what circumstances is it not possible to do so?
> >
> > Section 4.1
> >
> >     OrigNode sets its IPv6 address in the DODAGID field of the RREQ-DIO
> >     message.  A RREQ-DIO message MUST carry exactly one RREQ option,
> >     otherwise it MUST be dropped.  (Similarly for RREP in §4.2.)
> >
> > I suggest clarifying that other options are allowed (required, even).
> >
> > Who sets the S bit, and can it change as the DODAG is being constructed?
> > ("See Section 5" would be fine.)
> >
> >     L
> >        2-bit unsigned integer determining the duration that a node is
> >        able to belong to the temporary DAG in RREQ-Instance, including
> >        the OrigNode and the TargNode.  Once the time is reached, a node
> >        MUST leave the DAG and stop sending or receiving any more DIOs for
> >        the temporary DODAG.
> >
> > How do we account for time skew as the DIO propagates?  Each node just
> > leaves on their own timer?
> >
> >     Address Vector
> >        A vector of IPv6 addresses representing the route that the RREQ-
> >        DIO has passed.  It is only present when the H bit is set to 0.
> >        The prefix of each address is elided according to the Compr field.
> >
> >     TargNode can join the RREQ instance at a Rank whose integer portion
> >     is equal to the MaxRank.  Other nodes MUST NOT join a RREQ instance
> >     if its own Rank would be equal to or higher than MaxRank.  A router
> >     MUST discard a received RREQ if the integer part of the advertised
> >     Rank equals or exceeds the MaxRank limit.
> >
> > Both of these descriptions might benefit from a bit more detail.  E.g.,
> > the latter paragraph doesn't say that TargNode can join if the rank is
> > less than MaxRank, only if it's equal.
> >
> > Section 4.2
> >
> >     H
> >        Requests either source routing (H=0) or hop-by-hop (H=1) for the
> >        downstream route.  It MUST be set to be the same as the H bit in
> >        RREQ option.
> >
> > (editorial) I'd suggest putting the "MUST be the same" requirement as
> > the first sentence, and then the other sentence could be "determines
> > whether source routing (H=0) or hop-by-hop (H=1) is used for the
> > downstream route"
> >
> >     L
> >        2-bit unsigned integer defined as in RREQ option.
> >
> > Does L need to have the same value as in the triggering RREQ option?  If
> > not, when might TargNode choose a different value?
> >
> >     Address Vector
> >        Only present when the H bit is set to 0.  For an asymmetric route,
> >        the Address Vector represents the IPv6 addresses of the route that
> >        the RREP-DIO has passed.  For a symmetric route, it is the Address
> >        Vector when the RREQ-DIO arrives at the TargNode, unchanged during
> >        the transmission to the OrigNode.
> >
> > [ed. this was written before I made a discuss point about it, but I'm
> > leaving the text for the extra detail.  It's okay to just respond to the
> > discuss point and not here.]
> > If I understand correctly, the S bit indicating symmetric vs asymmetric
> > route is present only in the RREQ-DIO, and is not included in-band in
> > the RREP-DIO.  Does this require all nodes on the path to remember
> > whether a symmetric route is being constructed on the RREQ-DIO instance,
> > use the Shift in the RREP-DIO to correlate to the corresponding RREQ-DIO
> > and 'S' bit status, as part of the processing (to determine whether or
> > not to append to the Address Vector)?
> >
> > Section 4.3
> >
> >     Dest SeqNo
> >
> >        In RREQ-DIO, if nonzero, it is the last known Sequence Number for
> >        TargNode for which a route is desired.  In RREP-DIO, it is the
> >        destination sequence number associated to the route.
> >
> > The destination sequence number for the downstream route or the upstream
> > route?
> >
> > Also, should we say that zero is used if there is no known information about
> > the sequence number of TargNode (and not otherwise)?
> >
> >     r
> >        A one-bit reserved field.  This field MUST be initialized to zero
> >        by the sender and MUST be ignored by the receiver.
> >
> > The secdir reviewer noted the mismatch between 'X' in the figure and 'r'
> > here; please fix.
> >
> >     Prefix Length
> >        7-bit unsigned integer.  Number of valid leading bits in the IPv6
> >        Prefix.  If Prefix Length is 0, then the value in the Target
> >        Prefix / Address field represents an IPv6 address, not a prefix.
> >
> >     Target Prefix / Address
> >        (variable-length field) An IPv6 destination address or prefix.
> >        The Prefix Length field contains the number of valid leading bits
> >        in the prefix.  The length of the field is the least number of
> >        octets that can contain all of the bits of the Prefix, in other
> >        words Floor((7+(Prefix Length))/8) octets.  The remaining bits in
> >        the Target Prefix / Address field after the prefix length (if any)
> >        MUST be set to zero on transmission and MUST be ignored on
> >        receipt.
> >
> > Please specify how long the Address field is when Prefix Length is zero
> > (indicating that the last field is the Address variant).
> >
> > Section 5
> >
> >     Links are considered symmetric until additional information is
> >     collected.  [...]
> >
> > What kinds of problems will arise if we start taking actions based on
> > this assumption before the "additional information" is available?
> > (That is to say, perhaps this is not a useful phrasing, since what we
> > actually do is get updates about the presence of asymmetric links as we
> > construct the route.)
> >
> >     bit set to 1, then all the one-hop links on the route from the
> >     OrigNode O to this router meet the requirements of route discovery,
> >
> > Re "the route", this would presumably be the one recorded in the Address
> > Vector of the RREQ in question?  (Multiple RREQs for the same route
> > computation can arrive at a given node with different address vectors,
> > right?
> >
> > Also, the way this is written implies that it does not say anything
> > about "non-one-hop links" on the route, but I don't really know what a
> > link that's not a one-hop link would be.  Can we just say "all the hops"
> > or "all the links"?
> >
> >     and the route can be used symmetrically.
> >
> > But does that matter for any routers other than TargNode (for any of the
> > AODV-RPL Target Options)?
> >
> >     doesn't satisfy the Objective Function.  Based on the S bit received
> >     in RREQ-DIO, TargNode T determines whether or not the route is
> >     symmetric before transmitting the RREP-DIO message upstream towards
> >     the OrigNode O.
> >
> > Does that determination affect the construction of the RREP-DIO in any
> > way?  (E.g., if there was an S bit.)
> >
> >              Figure 5: AODV-RPL with Asymmetric Paired Instances
> >
> > Some discussion of how the third(? second?) intermediate router detects
> > the asymmetry and clears the S bit might be appropriate.
> >
> > Section 6.1
> >
> >     link-local multicast.  The DIO MUST contain at least one ART Option
> >     (see Section 4.3).  The S bit in RREQ-DIO sent out by the OrigNode is
> >     set to 1.
> >
> > I'd suggest saying that the required ART Option indicates the TargNode.
> >
> >     OrigNode can maintain different RPLInstances to discover routes with
> >     different requirements to the same targets.  Using the RPLInstanceID
> >     pairing mechanism (see Section 6.3.3), route replies (RREP-DIOs) for
> >     different RPLInstances can be distinguished.
> >
> > When using different RPLInstances for this purpose, what constitutes
> > "initiates a route discovery process" across those instances -- is it
> > permissible to only increment the sequence number once when initiating
> > multiple discovery processes on different instances?
> >
> > Section 6.2.1
> >
> >     Step 1:
> >
> >        If the S bit in the received RREQ-DIO is set to 1, the router MUST
> >        determine whether each direction of the link (by which the RREQ-
> >        DIO is received) satisfies the Objective Function.  In case that
> >        the downward (i.e. towards the TargNode) direction of the link
> >        does not satisfy the Objective Function, the link can't be used
> >        symmetrically, thus the S bit of the RREQ-DIO to be sent out MUST
> >        be set as 0.  If the S bit in the received RREQ-DIO is set to 0,
> >        the router MUST determine into the upward direction (towards the
> >        OrigNode) of the link.
> >
> >        If the upward direction of the link can satisfy the Objective
> >        Function, and the router's Rank would not exceed the MaxUseRank
> >        limit, the router joins the DODAG of the RREQ-Instance.  The
> >        router that transmitted the received RREQ-DIO is selected as the
> >        preferred parent.  Otherwise, if the Objective Function is not
> >        satisfied or the MaxUseRank limit is exceeded, the router MUST
> >        discard the received RREQ-DIO and MUST NOT join the DODAG.
> >
> > The way this is written is confusing to me.  It seems to say that (1)
> > you only check the upward direction is the S bit in the received
> > RREQ-DIO is set to zero, and (2) the only time you join the DODAG is if
> > you're checking the upward direction.  So, when the received S-bit is 1,
> > do you just never join the DODAG?  I assume this is not the intent, but
> > that is how I interpret the words that are on the page.
> >
> >        Sequence Number.  The Destination Address and the RPLInstanceID
> >        respectively can be learned from the DODAGID and the RPLInstanceID
> >        of the RREQ-DIO, and the Source Address is the address used by the
> >        local router to send data to the OrigNode.  The Next Hop is the
> >
> > "Source Address is the address used by the local router to send data to
> > the OrigNode" seems like the definition of the source address in a route
> > table entry, not a procedure for how to set it.  Should this be the
> > address used by the local router to send data to the preferred parent?
> >
> > Section 6.3.1
> >
> >     implementation-specific and out of scope.  If the implementation
> >     selects the symmetric route, and the L bit is not 0, the TargNode MAY
> >     delay transmitting the RREP-DIO for duration RREP_WAIT_TIME to await
> >     a symmetric route with a lower Rank.  The value of RREP_WAIT_TIME is
> >     set by default to 1/4 of the time duration determined by the L bit.
> >
> > There is no L *bit* in the RREQ option or the RFC 6550 DIO.  There is a
> > two-bit L field in the RREQ option, but even if I replace 'bit' with
> > 'field', it's still not clear why having a DODAG with no lifetime limit
> > implies that delaying the RREP-DIO is not allowed.
> >
> > Section 6.3.2
> >
> >     When a RREQ-DIO arrives at a TargNode with the S bit set to 0, the
> >     TargNode MUST build a DODAG in the RREP-Instance rooted at itself in
> >
> > I don't understand how the definite article is appropriate for "the
> > RREP-Instance rooted at itself" -- I thought there were multiple
> > (paired) instances corresponding to the various RREQ DODAGs that
> > requested routes to TargNode.
> >
> >     RREP_WAIT_TIME to await a route with a lower Rank.  The value of
> >     RREP_WAIT_TIME is set by default to 1/4 of the time duration
> >     determined by the L bit.
> >
> > ("L bit" again, and no indication of what to do for L==0.)
> >
> >     The settings of the fields in RREP option and ART option are the same
> >     as for the symmetric route, except for the S bit.
> >
> > There is no S bit in the RREP.  What is this intending to say?
> >
> > Section 6.3.3
> >
> >     When preparing the RREP-DIO, a TargNode could find the RPLInstanceID
> >     to be used for the RREP-Instance is already occupied by another RPL
> >     Instance from an earlier route discovery operation which is still
> >     active.  In other words, it might happen that two distinct OrigNodes
> >     need routes to the same TargNode, and they happen to use the same
> >     RPLInstanceID for RREQ-Instance.  In this case, the occupied
> >     RPLInstanceID MUST NOT be used again.  [...]
> >
> > A reminder might be helpful that the RPLInstanceID is a property of a
> > DODAG, and a DODAG is identified by the DODAGID, which in this case is
> > the address of the TargNode.  So that is why we need to avoid reusing
> > RPLInstanceID in the context of the RREP-DIO, whereas there is no
> > problem with collisions in RPLInstanceID across RREQ-DIOs (where the
> > DODAGID is the OrigNode address, that suffices to disambiguate).
> >
> >     shift to be applied to original RPLInstanceID.  When the new
> >     RPLInstanceID after shifting exceeds 63, it rolls over starting at 0.
> >
> > I thought RPLInstanceID was a full 8-bit field (even though Shift is
> > only six bits); wouldn't rollover happen after 255?
> >
> >     For example, the original RPLInstanceID is 60, and shifted by 6, the
> >     new RPLInstanceID will be 2.  Related operations can be found in
> >     Section 6.4.
> >
> > (So this example wouldn't actually show rollover.)
> >
> > Section 6.4
> >
> >     Upon receiving a RREP-DIO, a router which does not belong to the
> >     RREQ-Instance goes through the following steps:
> >
> > Do we care about RREQ-Instance membership or RREP-Instance membership,
> > for processing the RREP-DIO?
> >
> >     Step 1:
> >
> >        If the S bit is set to 1, the router MUST proceed to step 2.
> >
> > There is no S bit in the RREP option!
> >
> >        and the destination address is learned from the DODAGID.  The
> >        lifetime is set according to DODAG configuration (i.e., not the L
> >        bit) and can be extended when the route is actually used.  The
> >
> > ("L bit" again)
> >
> >     Upon receiving a RREP-DIO, a router which already belongs to the
> >     RREQ-Instance SHOULD drop the RREP-DIO.
> >
> > (RREQ-Instance vs RREP-Instance, again.)
> >
> > Section 10
> >
> > It seems like a malicious node that forges a gratuitous RREP could do
> > significant damage as well, so that might be worth mentioning.
> >
> >     routing loop.  The TargNode MUST NOT generate a RREP if one of its
> >     addresses is present in the Address Vector.  An Intermediate Router
> >     MUST NOT forward a RREP if one of its addresses is present in the
> >     Address Vector.
> >
> > These requirements seem important enough that I'd prefer to seem them
> > imposed in the main body text that covers RREP handling, and the
> > security considerations mentioned here and referring to those handling
> > requirements.
> >
> >
> >
>