Re: [6lo] New Version Notification for draft-thubert-6lo-unicast-lookup-01.txt

Hi Pascal

See my comment  below.

On 10/5/21 12:40 PM, Pascal Thubert (pthubert) wrote:
> Hello Dario
>
> Please see below;
>
>
>> Le 5 oct. 2021 à 20:15, Dario Tedeschi <dat@exegin.com> a écrit :
>>
>>  Hi Pascal,
>>
>> Thank you for new draft. However I do have some comments/questions.
>>
>> What benefit does the ‘M’ bit provide over simply detecting a 
>> multicast address in the Target Address field?
>>
>> The IPv6 multicast address type is clearly defined in RFC 4291 
>> (section 2.4) 
>> <https://datatracker.ietf.org/doc/html/rfc4291#section-2.4>, and the 
>> detection of such an address is trivial. Most (if not all) Stacks 
>> have a simple function/macro to do that job and many existing 
>> protocols already use this mechanism to distinguish between unicast 
>> and multicast addresses.  It seems to me that a special bit to 
>> indicate multicast registration would be redundant and require 
>> handling for 4 different cases, 2 of which would be errors:
>>
>>   * M = 1, Target = multicast addr
>>   * M = 1, Target= unicast addr  — ERROR
>>   * M = 0, Target = multicast addr — ERROR
>>   * M = 0, Target= unicast addr
>>
>>
>
> True enough. Dario.
>
> I’ve been pondering that too. On the one hand it seems cleaner to 
> announce the service that the 6LN expects. Otoh as you point out it 
> can be inferred from the address.
>
> Another way of seeing this is that the error cases that you indicate 
> can be detected if we have the bit otherwise they can’t.
[DT] I take your point about detecting the errors, assuming an 
implementation could do something useful with that knowledge, other than 
just discarding the message.

>
> Then there’s anycast which is missing from both RPL and ND , which 
> cannot be distinguished by the look of the address and thus requires a 
> bit.
[DT] As for the anycast address, I suppose the question to ask is what 
would a router do differently knowing such information? I suspect we 
would have to define some new behavior along with the new bit.

>
> Then there’s possibly the need of an IPv4 AF. All in all I tended to 
> favor having the bit but that’s really not a strong position, happy to 
> be convinced otherwise.
[DT] I presume you are talking of "IPv4-Compatible" and "IPv4-Mapped" 
IPv6 addresses. If my presumption is correct, aren't these still easily 
identifiable through their unique prefixes (::/96 and ::ffff/96, 
respectively)?

>
> What do others think?
[DT] I have no strong opinion. The M bit just seemed redundant.

>
>> I also wonder about the requirement for non-storing RPL networks to 
>> propagate multicast membership up the DODAG. My understanding is that 
>> non-storing networks typically use MPL (RFC 7731) 
>> <https://www.rfc-editor.org/rfc/rfc7731.html> which does not need 
>> multicast memberships to be propagated throughout the DODAG. It uses 
>> a flooding mechanism to forward multicast datagrams, and does not 
>> unicast at L2. Could the new document accommodate non-storing 
>> networks using MPL?
>
> Sure;
>
> Bottom line here is that for MPL all the multicast packets of interest 
> for the LLN are flooded throughout so I suspect that there is no need 
> for the 6LR to signal to the root.
[DT] Yes, that's my understanding as well.

>
> If that’s the case then there’s nothing to standardize.  All I need to 
> clarify is that the RPL behavior in the spec is the one expected in a 
> RPL domain that supports mop 3 otherwise what is done is out of scope 
> for this doc.
>
>  Do you see it otherwise?
[DT] I agree that only RPL mode 3 needs to be defined and other modes 
are left out of scope.

>
> I mean should the 6LR signal unicast to the root like for unicast 
> traffic when serving a RPL unaware leaf?
[DT] That certainly could be an optimization for non-storing mode so 
that a border-router might know what multicast groups to forward from 
outside the network. Unfortunately though there is no MOP that is 
"Non-storing with multicast", although one could argue semantics and 
simply use MOP 1.

[DT] If we were to opt for such behavior, 6LR nodes could simply add RPL 
Target options to their DAO's, for the multicast groups they were 
interested in (including those requested by leaf nodes).

>
>  If so wouldn’t it be expected that the Root makes n unicast to all 
> 6LRs that have listeners?
[DT] I'm not sure that would make sense when MPL is being used, but it 
makes for an interesting alternative to MPL.

>
> Should we describe that mode as well?
[DT] As an alternative to MPL? Sure.

>
> Pascal
>>
>> Regards
>> Dario
>>
>>
>>> On Sep 27, 2021, at 6:32 AM, Pascal Thubert (pthubert) 
>>> <pthubert=40cisco.com@dmarc.ietf.org 
>>> <mailto:pthubert=40cisco.com@dmarc.ietf.org>> wrote:
>>>
>>> Dear all:
>>>
>>> This draft is a continuation of our work on RFC 8505, 8928, and 8929.
>>>
>>> Comments welcome!
>>>
>>> Pascal
>>>
>>> -----Original Message-----
>>> From: internet-drafts@ietf.org <mailto:internet-drafts@ietf.org> 
>>> <internet-drafts@ietf.org <mailto:internet-drafts@ietf.org>>
>>> Sent: lundi 27 septembre 2021 15:29
>>> To: Eric Levy- Abegnoli (elevyabe) <elevyabe@cisco.com 
>>> <mailto:elevyabe@cisco.com>>; Pascal Thubert (pthubert) 
>>> <pthubert@cisco.com <mailto:pthubert@cisco.com>>
>>> Subject: New Version Notification for 
>>> draft-thubert-6lo-unicast-lookup-01.txt
>>>
>>>
>>> A new version of I-D, draft-thubert-6lo-unicast-lookup-01.txt
>>> has been successfully submitted by Pascal Thubert and posted to the 
>>> IETF repository.
>>>
>>> Name:draft-thubert-6lo-unicast-lookup
>>> Revision:01
>>> Title:IPv6 Neighbor Discovery Unicast Lookup
>>> Document date:2021-09-27
>>> Group:Individual Submission
>>> Pages:15
>>> URL: 
>>> https://www.ietf.org/archive/id/draft-thubert-6lo-unicast-lookup-01.txt
>>> Status: 
>>> https://datatracker.ietf.org/doc/draft-thubert-6lo-unicast-lookup/
>>> Html: 
>>> https://www.ietf.org/archive/id/draft-thubert-6lo-unicast-lookup-01.html
>>> Htmlized: 
>>> https://datatracker.ietf.org/doc/html/draft-thubert-6lo-unicast-lookup
>>> Diff: 
>>> https://www.ietf.org/rfcdiff?url2=draft-thubert-6lo-unicast-lookup-01
>>>
>>> Abstract:
>>>   This document updates RFC 8505 in order to enable unicast address
>>>   lookup from a 6LoWPAN Border Router acting as an Address Registrar.
>>>
>>>
>>>
>>>
>>> The IETF Secretariat
>>>
>>>
>>> _______________________________________________
>>> 6lo mailing list
>>> 6lo@ietf.org <mailto:6lo@ietf.org>
>>> https://www.ietf.org/mailman/listinfo/6lo
>>