Re: [Roll] Review of draft-ietf-roll-enrollment-priority-04

"Pascal Thubert (pthubert)" <pthubert@cisco.com> Fri, 06 August 2021 12:45 UTC

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From: "Pascal Thubert (pthubert)" <pthubert@cisco.com>
To: Routing Over Low power and Lossy networks <roll@ietf.org>
CC: roll-chairs <roll-chairs@ietf.org>
Thread-Topic: [Roll] Review of draft-ietf-roll-enrollment-priority-04
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Subject: Re: [Roll] Review of draft-ietf-roll-enrollment-priority-04
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              The nodes then
   adjust this base value based upon their observed congestion, emitting
   their adjusted DIO value to their children.


From: Roll <roll-bounces@ietf.org> On Behalf Of Rahul Jadhav
Sent: vendredi 6 août 2021 8:48
To: Routing Over Low power and Lossy networks <roll@ietf.org>
Cc: roll-chairs <roll-chairs@ietf.org>
Subject: Re: [Roll] Review of draft-ietf-roll-enrollment-priority-04

Excellent review. Many thanks Konrad.

[Pascal]: very much, yes!

Please find my comments inline.

Thanks,
Rahul

On Thu, 5 Aug 2021 at 02:02, Konrad Iwanicki <iwanicki@mimuw.edu.pl<mailto:iwanicki@mimuw.edu.pl>> wrote:
Dear authors,

This is the first review I am doing for IETF's draft, so any suggestions
on how I could improve the quality of my future reviews are welcome.

[RJ] This is an excellent in-depth review. Appreciate.

I have the following major issues with the draft. Perhaps some of them
have been covered during some meetings (sorry for missing those); my
comments are based just on what I got from the draft (and the related
drafts).


1. I was confused by two seemingly contradictory statements.

Par. 2 of sect. 2 (starting with "With this specification") states that
the introduced option "MUST be propagated down the DODAG without
modification" (compared to what is issued by the DODAG root).

However, par. 2 of sect. 2.1 (starting with "6LRs that support this
option") states that "nodes adjust this base value based upon their
observed congestion, emitting their adjusted DIO value to their children."

[RJ] This is a good point. Will raise this as an issue on GH. The way I see it, the value adjustment might have to be done going downstream and I would like to see what others think of it. Nonetheless, the text will need modifications.

[Pascal] The field is the MIN priority and it is decided by the root. To my best knowledge/memory, the intent was to adjust the priority value that is reported outside of the DIO (in beacons) that cannot be lower than the minimum priority in the DIO, e.g., when the 6LR exposes its desire to be a join proxy, as discussed in this text:
“

   A 6LR which would otherwise be willing to act as a _Join Proxy_, will
   examine the minimum priority field, and to that number, add any
   additional local consideration (such as upstream congestion).

“
But the Min priority in the DIO is unchanged. Note that if it was, a child with multiple parents would get conflicting DIOs with both timely information and we would have to specify which value it reports. Right now the idea would be to report the most recent, though it is somewhat unclear how that is determined.

Still I agree that we need to improve the text, as Konrad says it is quite confusing.

If we want something that reports the load of the DODAG above, we’ll have to examine the value and if it is needed, then define the behavior.





What I understand is that the adjustment applies only in a particular
case. More specifically, if a node receives the option, it copies it to
its DIOs without any modification. In contrast, if the node does not
receive the option but is capable of processing it, then it synthesizes
the option for its DIOs by taking as the min priority the base value
(0x40) adjusted with the node's local observations. This seems
inconsistent, though. In particular, two children with the same parent
can emit DIOs with different values of min priority. Why not simply omit
the option in such a case?

[RJ] Two children with the same parent are permitted to emit DIOs with different values of min priority. Consider the case where child1 has fewer resources than child2 and wants to limit downstream nodes joining through it. This case requires different min-priority to be emitted. There are several other reasons why the priorities could be different.


[Pascal]

To Konrad’s point : I  agree with you that this creates a situation that is normally seen only when the root changes the value (which arguably may happen a lot and should be dampened with hysteresis and stuff). I believe we should retract the text that allows the 6LR to generate the option, reason is, if the root sets it and only the 6LR’s parent do not support, a child of the 6LR may see bith the Root’s value and the 6LR’s value. Dangerous. So the suggestion is to retract
“

The nodes then
   adjust this base value based upon their observed congestion, emitting
   their adjusted DIO value to their children                                                                                         “

“

To Rahul’s: This could be an important thing to report, and we can take it outside this thread. As of today we do not report the parent load/capability to take more children in DIOs. The reason we did not do that in the past was to limit the complexity of the RPL spec. Because you’d also need to explain what nodes do with the information, e.g., how to balance the graph so that everyone has at least one parent, which would involve some shoulder to shoulder pushback. So we said at the time, either all nodes have enough memory or you go for non-storing, but you never have to react to the parent’s load.

This is something we can reopen now, but probably under the RPLv2 umbrella.






2. The draft implicitly assumes a DIO processing policy that differs
from RPL's original one.

More specifically, throughout the draft, it is implied or assumed that a
node processes the option received only from its parent(s). For example:
- par. 8 of sect. 1 (starting with "A network operator might"): "... it
would like to adjust the enrollment priority (the proxy priority field
of [...]) among all nodes in the subtree of a congested link.";
- par. 1 of sect. 2.1 (starting with "A 6LR which did not support"):
"Children and grandchildren nodes would therefore not receive any
telemetry via that path and need to assume a default value.";
- par. 2 of sect. 2.1: "6LRs that support this option, but whose parent
does not send it [...]" and the rest of the paragraph as well.

In contrast, RFC 6550, sect. 8.2.3 states that DIOs not only from
parents but also other neighbors must be processed by a node. This
guarantees correct route formation and maintenance.

Therefore, if the draft changes this policy with respect to the
introduced option, I would expect it to state this change explicitly.

[RJ] I don't think the draft changes this policy.
RFC 6550 says that the DIO will be received by a node from all its neighbors and that is because of the broadcast nature of the DIOs. When DIOs are received from the neighbors, these neighbors could be promoted to the parent set. The draft does not change this processing behavior. Par 2 of sect 2.1 "6LRs that support this option, but whose parent does not send it...." this statement does not change the 6550 behavior. All it says is for some reason, if the selected parent does not support the option then the draft dictates a particular behavior.

[Pascal] I believe that Konrad’s point is that
“


RPL limits the cases where a node may process DIO messages from

   deeper nodes to some form of local repair.

“  from RFC 6550, as explained in more details in  https://datatracker.ietf.org/doc/html/rfc6550#section-8.2.2.4

Which is IOW that a node normally ignores DIOs coming from deeper. But it needs to process DIOs for all nodes that may become potential parent.
On the one hand I’d say that if the option is expected to report parent load in the future, the node should parse the option in any DIO that is acceptable otherwise, and to Konrad’s point follow the exact same rules as RPL.
OTOH, this is more chances for getting different values along different asynchronous paths. Which makes the problem of finding the most recent value even harder.

Seems that the option should have its own sequence, which is a generic problem that was discussed in draft-thubert-roll-eliding-dio-information. Note that in this case, I do not believe that we should change the RCSS in that draft, so the best could be that the option has its own sequence. The text would change to reflect that we use any DIO that we normally use, and retain the value of the min priority from the most recent of all.




Such a change would also require answering several questions, notably:
- From which parent do we process the option?
- If only from the preferred parent, what happens if a node has none?
- If from any, then what happens if a node gets different values of min
priority or DODAG_Size from different parents?


[RJ] If we agree that we are not changing 6550 DIO processing behavior as mentioned above, then these questions do not apply.

[Pascal] I think they do, per my replies above.



However, I believe that a meta question worth answering first is: Do we
want to have different min priority values in different subtrees or just
one global value for the entire DODAG?

[RJ] This is the point to be discussed. IMO, the min priority is not a constant global value. However, the draft text contradicts this and will raise the issue in GH before we fix it to garner some discussion.

[Pascal] The min priority in the draft is effectively global, set by the root. If we want other info in the draft, it is still time to add it, but please let’s work on it now and then ship. There are external bodies waiting for this spec.



Either of the answers in my view requires explicit formulation and
preferably also an explanation in the draft.

[Pascal] Indeed




3. Even if the draft identified a single parent as the only source of
min priority values for a node, there still would be a problem of value
consistency.

Since the DODAG root can change its values of min priority and
DODAG_Size and since a node can change its parents, there may still be a
case when the node must decide whether to adopt the newly received
values or not. Without a proper conflict resolution policy, it may be
the case that when the root, for example, disables enrollment, then
enables it again, then the DODAG may exhibit strange behaviors: the
root's decision may not be propagated to all nodes, a decision may be
revoked by nodes that have already accepted it, and the like.

[RJ] The exact conflict resolution policy is not within the scope of this document. However, for the scenario you quoted, "What happens if the enrollment is enabled/disabled by the root?" We do not expect the enrollment enable/disable to be precisely known by all others i.e., there could be nodes who do not have the latest status of the enrollment option. This could cause some additional signaling i.e., a node might try to join even though the (latest) priority did not allow it. This could lead to a few more messaging but it should not result in any untoward behavior.

 [Pascal] I believe we can get things a lot simpler if 1) the min comes only from the root as of now and 2) we add a sequence counter in the option that the Root increments to indicate the freshest.



There are many ways in which such conflict resolution can be provided.
If I may suggest anything, I would recommend adding to the option a
lollipop sequence counter (like those described in RFC 6550, sect. 7.2),
let's call it osn. It would effectively order different versions of the
option values produced by the DODAG root. In effect, whenever a node
received an option (be it from a parent or another neighbor, depending
on the way the previous issues are addressed), it would adopt the min
priority and DODAG_Size values the option carries if and only if the osn
value in the received option is greater than the node's locally stored
osn. This would guarantee eventual consistency and avoid strange
behaviors such as those mentioned above.

[RJ] With LLNs there is no way to have guaranteed consistency. Consider the case where a node goes off the network and comes back later and did not see part of the traffic at all.
Also, note that there is other non-static information such as metrics/constraints that are carried by the DIO. Worst case if the information changes too quickly and the nodes might not have consistent information there could be some additionally messaging but over time the network should converge to the right configuration. I am not sure if introducing something like OSN is the way to tackle this problem.

[Pascal] Arf I should have read it through. I agree with Konrad again.





4. Why put DODAG_Size in the option?

[RJ] This was a recent proposition. Please check this discussion for context, "[Roll] About measure DODAG size and priority<https://mailarchive.ietf.org/arch/msg/roll/-MqqxvTW-3bM5F4k4qH7n5FaxYM/>"
https://mailarchive.ietf.org/arch/msg/roll/-MqqxvTW-3bM5F4k4qH7n5FaxYM/

[Pascal] Indeed, we have the request from Wi-SUN which uses RPL. The goal is to balance the size of multiple large DODAGs in the same instance.




I can imagine that other services/protocols would like to use it as
well, and extracting this information from an option related solely to
enrollment does not seem the best idea. I have been thinking for some
time about an option for RPL that would provide some aggregate
information about the DODAG to the nodes, such as its size, max depth,
and the like. The option could also be used in the enrollment process.
Do you think this makes sense?

[RJ] size is already there. For max-depth, what could be the use-case? Is there anything else you have in mind? Would be nice to discuss it now.

[Pascal] Indeed now is a good time. The max depth would be an indication for the 6LR NOT TO accept joins if it is too deep, so it is related to join. Now that’s also a very dangerous thing to do. It would happensin the field that some networks get very deep, e.g., when one of the Roots reboot and large DODAGs merge to absorb the nodes left orphan.





5. It should be made clearer in the draft what is actually being configured.

The only place where field "proxy priority" of EB appears in the entire
draft is par. 8 of sect. 1. Reading the draft for the first time I was
confused what min priority for the option is used for and what is
actually adjusted at various places.

[RJ] The aim of the draft was to be generic enough but also serve/fulfill the use-case for the 6tisch scenario. Hence the EB point appears only in the introduction.
In my previous experience where we used PANA<https://en.wikipedia.org/wiki/Protocol_for_Carrying_Authentication_for_Network_Access> for onboarding nodes in an AMI network, we needed a similar feature and we ended up using a proprietary (but similar) signaling.

 [Pascal] All true, and yet we have an opportunity to clarify that text that we need to leverage before IESG, because that point would come back with a revenge. Let’s clarify.

For the editorial discussions, I agree with Konrad’s point and  Rahul’s answers and leave it your echange.

Keep safe!

Pascal