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

Charlie Perkins <charles.perkins@earthlink.net> Fri, 07 May 2021 01:58 UTC

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To: John Scudder <jgs@juniper.net>, Routing Over Low power and Lossy networks <roll@ietf.org>, John Scudder via Datatracker <noreply@ietf.org>, The IESG <iesg@ietf.org>
Cc: roll-chairs@ietf.org, mariainesrobles@googlemail.com, draft-ietf-roll-aodv-rpl@ietf.org
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From: Charlie Perkins <charles.perkins@earthlink.net>
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Subject: Re: [Roll] John Scudder's Discuss on draft-ietf-roll-aodv-rpl-10: (with DISCUSS and COMMENT)
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Hello John,

It's taken a while for me to get to this, please excuse the delay. I 
have some followup to your comments interspersed below.

On 4/19/2021 1:31 PM, John Scudder via Datatracker wrote:
> John Scudder 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:
> ----------------------------------------------------------------------
>
> A lot of effort has clearly gone into this work, thank you. I do have one topic
> I want to DISCUSS, as it seriously impacted the readability of the document
> from my point of view. I don’t anticipate that it will be very difficult to
> resolve this DISCUSS as it relates to clarity and not to anything fundamental.
>
> My chief difficulty with the document is placing it in context. No hints are
> given to the reader as to what the expected network environment is. I think it
> would be almost sufficient to say, for example “the network environment is
> assumed to be the same as described in RFC 6550, Section 1” for example, but
> without that hint and without a strong background in ROLL, I found myself
> struggling. Figures 4 and 5 in particular lead me to believe the expected
> environment looks similar to a classic ISP network — a collection of nodes
> connected by point-to-point links. If this isn’t correct (and I bet it’s not)
> that may have led me into incorrect assumptions, which may be reflected in my
> other comments below.
>
> Further, it’s not stated anywhere whether AODV-RPL is intended to stand alone
> as its own routing protocol, or to be viewed as an extension of RPL. In the
> former case, it seems the document is lacking details that are present in RFC
> 6550. I’m assuming the latter is the case, but a clear statement to that effect
> seems indicated.
How about this text:
    Routing Protocol for Low-Power and Lossy Networks (RPL) [RFC6550] is
    an IPv6 distance vector routing protocol designed to support multiple
    traffic flows through a root-based Destination-Oriented Directed
    Acyclic Graph (DODAG).  Typically, a router does not have routing
    information for most other routers.  Consequently, for traffic
    between routers within the DODAG (i.e., Point-to-Point (P2P) traffic)
    data packets either have to traverse the root in non-storing mode, or
    traverse a common ancestor in storing mode.  Such P2P traffic is
    thereby likely to traverse longer routes and may suffer severe
    congestion near the root (for more information see [RFC6997],
    [RFC6998]). The network environment that is considered in this document
    assumed to be the same as described in Section 1 of [RFC6550].

> ----------------------------------------------------------------------
> COMMENT:
> ----------------------------------------------------------------------
>
> 1. Section 1:
>
>     Reply.  AODV-RPL currently omits some features compared to AODV -- in
>     particular, flagging Route Errors, blacklisting unidirectional links,
>     multihoming, and handling unnumbered interfaces.
>
> Your use of language is entirely up to you, but I feel obliged to point out
> that there’s been a lot of discussion in the IETF community recently about use
> of language that raises sensitive points, and about the term “blacklisting” in
> particular. I understand that this is the only place in the document the term
> appears, and since it refers to AODV you can’t just use another term, but
> placing it in quotation marks might make it clear that it’s referring verbatim
> to the language of RFC 3561.

This is an evolving issue.  I am fine with using quotes but otherwise 
maintaining consistent terminology.  For instance,

     AODV-RPL currently omits some features compared to AODV -- in
     particular, flagging Route Errors, "blacklisting" unidirectional links
     [RFC3561], multihoming, and handling unnumbered interfaces.

If there is an official list of terms to search for please let us know.

>
> 2. Section 1:
>
>    support for storing and non-storing modes.  AODV adds basic messages
>    RREQ and RREP as part of RPL DIO (DODAG Information Object) control
>
> Did you mean “AODV-RPL adds”?
Yes, will fix.

>
> 3. Section 2:
>
>     Symmetric route
>        The upstream and downstream routes traverse the same routers.
>
> Same routers? Or same links? (Or both, if multi-access links are part of the
> mix, as I imagine they may be?)

       The upstream and downstream routes traverse the same routers and over
       the same links.
> 4. 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,
>
> Should that say “one of its IPv6 addresses"? Is it even necessary to restate
> this? RFC 6550 §6.3.1 already has a clearer requirement:
>
>     DODAGID: 128-bit IPv6 address set by a DODAG root that uniquely
>           identifies a DODAG.  The DODAGID MUST be a routable IPv6
>           address belonging to the DODAG root.

I'm not quite sure what is requested.  Should it be "OrigNode sets the 
DODAGID field", relying on the definition provided in RFC 6550? Should 
it be "OrigNode sets one of its routable IPv6 address in the DODAGID field"?
Honestly, I thought the meaning was clear.  Unless there is an 
objection, I reckon we will use the latter wording.

>
> 5. Section S4.1:
>
>    TargNode can join the RREQ instance at a Rank whose integer portion
>    is equal to the MaxRank.
>
> Not less than or equal, right? Strict equality to MaxRank is required?
The existing text isn't good.  Instead,

    TargNode can join the RREQ instance at a Rank whose integer portion
    is less than the MaxRank.


> 6. Section 4.2:
>
>     TargNode sets its IPv6 address in the DODAGID field of the RREP-DIO
>     message.  A RREP-DIO message MUST carry exactly one RREP option,
>
> Same as #4.
>
> 7. Section 4.2:
>
>    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.
>
> The first time I read through this, I didn’t understand it at all. On
> re-reading, I think you’re using the word “route” in two different ways in the
> same sentence, the first time to mean “route” in the sense of an object in the
> protocol, the second time in the more casual sense of “a path through the
> network”. If that’s right, I suggest rewriting the second instance, as in “…
> represents the IPv6 addresses of the path through the network the RREP-DIO has
> traversed.”
>
> Also, as in point #4, is it right to say *the* IPv6 addresses? Couldn’t any
> given node have various IPv6 addresses? So maybe just lose the definite
> article, as in “… represents IPv6 addresses of the path…”?

Good point.  We will use your formulation.

>
> 8. Section 4.3:
>
>    r
>       A one-bit reserved field.  This field MUST be initialized to zero
>       by the sender and MUST be ignored by the receiver.
>
> The figure doesn’t show an “r” field. I assume the field labeled “X” should be
> relabeled as “r”?
Actually, the description should refer to an "X" field, not an "r" 
field.  We will update.


>
> 9. Section 5:
>
>     Figure 4.  If an intermediate router sends out RREQ-DIO with the S
>     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,
>
> On first reading I didn’t understand this. Having read the whole document, I
> now get it (I think!). Possibly changing “meet” to “have met” would have been
> enough to get me past my initial befuddlement.

Yes, that's better.

>
> 10. Section 5:
>
>     The criteria used to determine whether or not each link is symmetric
>     is beyond the scope of the document.  For instance, intermediate
>
> Should be “criterion … is beyond", or "criteria … are beyond", depending on
> whether you want singular or plural.
We will use "criteria … are beyond".

>
> 11. Section 5:
>
>    routers can use local information (e.g., bit rate, bandwidth, number
>    of cells used in 6tisch)
>
> I wouldn’t have minded a reference for 6tisch.
No problem.

>
> 12. Section 5:
>
>     Upon receiving a RREQ-DIO with the S bit set to 1, a node determines
>     whether this one-hop link can be used symmetrically, i.e., both the
>     two directions meet the requirements of data transmission.  If the
>     RREQ-DIO arrives over an interface that is not known to be symmetric,
>     or is known to be asymmetric, the S bit is set to 0.  If the S bit
>     arrives already set to be '0', it is set to be '0' on retransmission
>
> The term “retransmission” seems misused here. I guess you mean something like
> “when the RREQ-DIO is propagated”.
That is better.  We will use that.

>
> 13. Section 5:
>
>    Appendix A describes an example method using the upstream Expected
>    Number of Transmissions" (ETX) and downstream Received Signal
>    Strength Indicator (RSSI) to estimate whether the link is symmetric
>    in terms of link quality is given in using an averaging technique.
>
> This sentence needs a rewrite to make it grammatical. It works up until "is
> given in using an averaging technique”.
How about:
     Appendix A describes an example method using the upstream Expected
     Number of Transmissions" (ETX) and downstream Received Signal
     Strength Indicator (RSSI) to estimate whether the link is symmetric
     in terms of link quality using an averaging technique.


>
> 14. Section 6.2.1:
>
>       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.
>
> The last sentence doesn’t make sense.
How about:
                ...  If the S bit in the received RREQ-DIO is set to 0,
      the router MUST determine the upward direction (towards the
      OrigNode) of the link.
>
> 15. Section 6.2.1:
>
>       If the router is an intermediate router, then it transmits a RREQ-
>       DIO via link-local multicast;
>
> On what interface? Routers generally can have multiple interfaces. Again, this
> is a place a clear description of the network environment might have helped.

The link-local multicast should go out over all local interfaces.

>
> 16. Section 6.2.2:
>
>    If the OrigNode tries to reach multiple TargNodes in a single RREQ-
>    Instance, one of the TargNodes can be an intermediate router to the
>    others, therefore it MUST continue sending RREQ-DIO to reach other
>    targets.  In this case, before rebroadcasting the RREQ-DIO
>
> The use of the term “broadcast” here confuses me. Is this “broadcast” in the RF
> sense? Again, this is a place a clear description of the network environment
> might have helped.
This needs to be reformulated to avoid suggesting anything about RF 
broadcast.  TBD.

>
> 17. Section 6.2.2:
>
>    send out any RREQ-DIO.  For the purposes of determining the
>    intersection with previous incoming RREQ-DIOs, the intermediate
>    router maintains a record of the targets that have been requested
>    associated with the RREQ-Instance.  Any RREQ-DIO message with
>    different ART Options coming from a router with higher Rank is
>    ignored.
>
> It’s not clear to me if the last sentence goes with the previous and if so,
> how. Does it even relate to multiple targets? Also, different from what? If it
> has the same ART Options (same as what?) is it *not* ignored?
How about:
                   .....                     For the purposes of 
determining the
   intersection with previous incoming RREQ-DIOs, the intermediate
   router maintains a record of the targets that have been requested
   associated with the RREQ-Instance. Any incoming RREQ-DIO message having
   multiple ART Options coming from a router with higher Rank than the 
Rank of
   the stored targets is ignored.

>
> 18. 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.
>
> It’s not an L bit though, it’s an L field.
Good point!

>
> 19. Section 6.3.2:
>
>    multicast until the OrigNode is reached or MaxRank is exceeded.  The
>    TargNode MAY delay transmitting the RREP-DIO for duration
>    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.
>
> Again, it’s an L field. Also, what if L is zero? Is RREP_WAIT_TIME set to
> infinity, as the text implies?
Thanks for pointing this out.  We need to be explicit about it.  I don't 
think the RREP_WAIT_TIME should ever be zero or infinity. But, if it 
were, the implementations would still be interoperable in a sense.  Do 
we really want to get into exactly what wait times make sense in this 
context?


>
> Please do a global search for “L bit”, as there are additional instances I
> haven’t called out.
>
> 20. Section 6.4:
>
>    Step 4:
>
>        If the receiver is the OrigNode, it can start transmitting the
>        application data to TargNode along the path as provided in RREP-
>        Instance, and processing for the RREP-DIO is complete.  Otherwise,
>        in case of an asymmetric route, the intermediate router MUST
>        include the address of the interface receiving the RREP-DIO into
>        the address vector, and then transmit the RREP-DIO via link-local
>        multicast.  In case of a symmetric route, the RREP-DIO message is
>
> As with #15: on what interface(s)?
It should go out to all the neighbors over multiple interfaces if necessary.

>
> 21. Section 10:
>
>    fake AODV-RPL route discoveries.  In this type of scenario, RPL's
>    preinstalled mode of operation, where the key to use for a P2P-RPL
>    route discovery is preinstalled, SHOULD be used.
>
> What type of scenario is that?

  "In this type of scenario" -> "When rogue routers might be present"

>
> 22. Appendix A:
>
> s/pakcet/packet/
Check.
>
> 23. General remark:
>
> Although “acknowledgements” isn’t a required section I was a little surprised
> not to encounter it, as it’s usually present. Your call of course.
Acknowledgements are due to a lot of people - thanks for the reminder!

Naturally Yours,
Charlie P.