Subject: [Anima] Rtgdir last call review of draft-ietf-anima-autonomic-control-plane-24

From: Joel Halpern via Datatracker <[email-masked]>
Date: Thu, 09 Apr 2020 19:16:16 -0700

Reviewer: Joel Halpern
Review result: Not Ready

Hello,

I have been selected as the Routing Directorate reviewer for this draft. The
Routing Directorate seeks to review all routing or routing-related drafts as
they pass through IETF last call and IESG review, and sometimes on special
request. The purpose of the review is to provide assistance to the Routing ADs.
For more information about the Routing Directorate, please see
​http://trac.tools.ietf.org/area/rtg/trac/wiki/RtgDir

Although these comments are primarily for the use of the Routing ADs, it would
be helpful if you could consider them along with any other IETF Last Call
comments that you receive, and strive to resolve them through discussion or by
updating the draft.

Document: draft-ietf-anima-autonomic-control-plane-24.txt
Reviewer: Joel Halpern
Review Date: 9-April-2020
IETF LC End Date: N/A
Intended Status: Proposed Standard

Summary:
    I have two major concern about this document that I think should be
    resolved before publication.  The are also a number of minor items that
    warrant attention.

Comments:

While quite long, the draft is significantly improved from earlier versions. 
It does provide significant explanation of its design choices, which is helpful
and appreciated.  Sometimes this seems to end up more as marketing or promotion
instead of explanation, but this is mostly harmless.

In particular, I would like to thank the authors and editors for the addition
of section 9.3 and its careful discussion of the many issues there.

Major Issues:

    Section 6.10.3.1 on the use of Zone-IDs seems, from the material in A.10.1,
    to be dependent upon either configuration (which ACP is supposed to avoid)
    or completely unspecified magic.  Having an addressing and routing scheme
    standardized that is impossible to use seems at variance with appropriate
    practice.  It would be fine to say that provision is made for non-zero
    Zone-IDs in the hope that future work can find ways to scale further using
    this.  But pretending it is well-defined, but not actually defining it,
    seems unacceptable.

    Section 6.12.5.1 on loopback interface is factually wrong.  It conflates
    one particular form of loopback interface with the definition of loopback
    interfaces.  This also leads to the error in the definition section (see
    minor comment below).  (Loopback Interfaces were used long before RFC 4291,
    and on routers were often used for external communication.  This was itself
    a repurposing of the original loopback interface, 127.0.0.1, which was
    indeed for internal use.)

Minor Issues:

   It seems distinctly unfortunate that the definition for Data Plane in
   section 2 explicitly states that this definition is different from that used
   in other work, including other routing work.  This seems a recipe for both
   confusion and mis-communication among technologists.

   In the definition of in-band management in section 2, please remove the
   commentary text on putative fragility.   (I actually agree it has some
   fragility.  The discussion does not belong here.  This is a definition.)  
   The promotional material may be warranted, if jarring, in other parts of the
   documents.  Not in the definitions please.

    The definition of a loopback interface in section 2 is wrong.  It claims
    that loopbacks transmit no external traffic.   They send and receive lots
    of external traffic.  They merely do so by forwarding the traffic
    internally to other interfaces.  The traffic is external.  The particular
    step of the transmission, if implemented naively, is internal.

    If we are going to define ACP as a virtual out of band network, I would
    suggest separating the terms into two definitions.  One for true out  of
    band networks (distinct physical links, switches, and ports), and then a
    definition for virtual out of band network which describes the ACP
    approximation which creates independence from configuration, but not
    independence from the physical links.

    Section 5, bullet 2, talks about a policy as to which peers ACP
    communication should be established.  It would be helpful if this gave a
    reference or indication as to where such policies would come from.  Given
    the emphasis on zero touch, I presume they are not configured on the node? 
    (This issues was in my review of -13.)

    Bullet 4 of section 6.1.3 on checking certificates against the CRL / OCSP
    would seem to be better reworded.  I believe the intended requirements i
    that IF there is ACP connectivity to the CRL / OCSP source, then it should
    be verified.  But that absence of such connectivity should not prevent
    association formation.  (As, if I have read it wright, otherwise we could
    deadlock the startup process.)

    In the example in section 6.5 on Channel selection, in steps 7:C1 and
    11:C2, Node 1 concludes that it is Bob.  However, in steps 12 and 13, the
    text refers to Node1 (Alice).  This seems inconsistent.

    Section 6.7.1 makes an assertion about the lack of need for MTI of security
    mechanisms.  The earlier explanation was well done and seems sound.  This
    shorter one seems wrong, since without MTI there is no good way to know
    what ones neighbors may implement.  I suggest simply removing this text and
    replacing it with a backwards reference to the earlier description.  (The
    rest of the section is useful and clear.)

    In 6.10.3,  ACP Zone Addressing Sub-Scheme, the text claims that when zone
    IDs of 0 are used, the addresses are identifiers, and when non-zero IDs
    aere used, they are locators.  Since in either case the addresses are used
    for packet forwarding, and the addressing information is propagated in the
    routing protocol (RPL), this seems to be a misuse of the locator /
    identifier distinction.  And a misuse for no purpose as the distinction is
    not relevant to the document.  (This odd use of "identifier continues in
    section 6.10.3.1.  Identifier is not a synonym of "flat".  Just say "flat".)

    The assertion about looping packets in the later portion of 6.11.1.1 is
    over-stated.  There are other routing protocols that avoid looping-till-ttl
    without changing the data plane header.  I suggest removing the gratuitous
    comparison with other routing protocols.

    6.12.5.1 refers to the ACP addresses as node addresses.  Technically, the
    IPv6 architecture requires that all addresses are associated with
    interfaces rather than nodes.  I would prefer that this draft not
    needlessly claim to violate that.

    Section 7.2 (L2 DULL GRASP) seems to be doing something quite useful.  I
    think I see how it would work.  The need for some configuration on some
    switches seems inevitable and acceptable.   I think there is one corner
    case that should be avoided, as it seems likely to create significant
    complexity for little or no benefit.  It seems to me that a switch that is
    capable of participating in the ACP should either participate in the ACP on
    all its physical ports, or should not participate in the ACP at all.  I
    would not be surprised if that was the WG intent.  But I could not find the
    text that says this.  (Apologies if it is there and I missed it.)

    Section 9 starts by saying it is informational.  But the first paragraph
    says that some of the content is "necessary" for correct operation.  Thus,
    it seems that some of the content is normative?   (I am not sure, but I
    think the "necessary" material relates to what is needed to be a registrar?)

Nits:
    The second and third paragraphs of section 6.11.1.1 on RPL start with
    duplicated text, and then go on to say different (complementary) things. 
    There is no need for the repetition.

    The rank factor in 6.11.1.6 of 100 megabits as the boundary seems a fairly
    arbitrary choice.  It may be that an arbitrary choice was needed.  Could
    something be said?  In particular, if someone looks at this 5 years from
    now, it may seem quite confusing.

---

From: Brian E Carpenter <[email-masked]>
Date: Fri, 10 Apr 2020 16:30:18 +1200

Hi Joel,

What a great review. I have comments on both your major comments, and an
important comment near the bottom of your minor comments.

>     this.  But pretending it is well-defined, but not actually defining it,

Interesting. I've always read this text (and the Appendix text) to mean
that Zone-IDs are indeed reserved for future work, but that a few of
their basic properties are defined. Are you suggesting to delete all
that, or to add a clear statement that they are currently not fully
defined and should not be implemented?
 
>     indeed for internal use.)

This text is probably my fault, since back in 2015 I was bugging the authors
about VRF terminology and loopback in particular. I searched for and
failed to find a generic definition of "loopback" and I also wrote this:

>>> Routing people and Linux hackers use "loopback interface" as a term of art. I
>>> still think you are wrong, and the restriction you want to impose is that only
>>> *secure virtual* interfaces of autonomic nodes carry a routable address.
>>> Attaching the addresses to a loopback interface is an implementation
>>> mechanism.

and in 2017 I wrote this (sorry, but I can't provide a TL;DR version):

>>> I'm still not 100% happy with that but 'loopback' is a term of art
>>> in our business. According to RFC 4291, the loopback interface is
>>> 
>>>>>     a virtual interface (typically called the "loopback
>>>>>     interface") to an imaginary link that goes nowhere.
>>> 
>>> so basically it is indeed where you can hang an address that does
>>> not belong to any specific physical or virtual interface. I find it
>>> slightly troubling that this important concept is, as far as I can
>>> tell, not defined very well anywhere. I had to break out my copy
>>> of Stevens's "TCP/IP Illustrated" to get some clarity.
>>> 
>>> So I don't think ACP nodes are different, actually. This is just
>>> an undocumented common practice, as are VRFs.
>>>
>>> The earliest reference I've found to "loopback interface" is in
>>> RFC 1812, which simply assumes that the reader knows what it means.
>>> There's some reasonably helpful discussion in RFC 4007, and RFC 6724
>>> says:
>>> 
>>>>>   Implementations that wish to support the use of global source
>>>>>   addresses assigned to a loopback interface MUST behave as if the
>>>>>   loopback interface originates and forwards the packet.
>>> 
>>> RFC 7404 shows the extent to which hanging an address on the loopback
>>> interface is common practice in router operations. (There's also a
>>> brief mention of this in RFC 7010, which I had completely forgotten
>>> despite being a co-author.)

Frankly I'm still a bit unhappy about using the term "loopback interface"
because it doesn't seem to me to be relevant to describing external
behaviour of a node, and it is very much an implementation artefact
that seems to mean slightly different things to different people.

>     text that says this.  (Apologies if it is there and I missed it.)

No. A node which is at the edge of the ACP will by definition have at
least one interface that is in the ACP, and at least one interface that
is not in the ACP. I can't see any way to avoid that.

What you say definitely applies to L2 links: they must be either inside
or outside. (But even that gets complicated if running the ACP over a VLAN.
Then the statement becomes: each VLAN must be either inside or outside.)

Hmm. There's a draft on this topic in the RFC queue, and I am quite
sure that ANIMA needs to start new work on the domain membership and
boundary issue.

Regards,
     Brian

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---

From: "Joel M. Halpern" <[email-masked]>
Date: Fri, 10 Apr 2020 00:50:11 -0400

Thanks Brian.  Given the amount of text below, I am going to top-post.

On the Zone IDs, personally, I would remove all of the text about 
non-zero ZoneIDs, and replace it with a simple statement that it is 
expected to be used for future work on aggregatable addressing to 
improve scaling.

On Loopback, I understand your frustration with the lack of a good 
definition.  Given that IPv6 addressing architecture constraints, you 
need some sort of interface.  In practice, the way loopbacks are used 
seems to match the need.  So I do not object to the usage.  just to the 
definition.  It would also be acceptable to simply craft a different 
term and clearly define it if the usage is sufficiently different from 
existing practice.

On the final minor comment, it was specifically about the section on L2 
devices.  Maybe something special is needed for the special case of a 
shared network that is also a border network.  But that seems very rare. 
  And getting the L2 switch to do the right packet forwarding for the 
hybrid case seems an invitation to trouble.

Yours,
Joel

---

From: Michael Richardson <[email-masked]>
Date: Fri, 10 Apr 2020 17:27:00 -0400

Joel M. Halpern <[email-masked]> wrote:
    > definition.  Given that IPv6 addressing architecture constraints, you need
    > some sort of interface.  In practice, the way loopbacks are used seems to
    > match the need.  So I do not object to the usage.  just to the definition.
    > It would also be acceptable to simply craft a different term and clearly
    > define it if the usage is sufficiently different from existing
    > practice.

Reading this thread, I was hoping you might be able to help us with a better
definition then :-)

We are doing exactly what OSPF and BGP does operationally on every platform
that I have every worked on.  We are just doing it with RPL.

To me, it's *SO* obvious that it goes without saying, so now we are asked to
say it, and we get into trouble because nobody before us bothered to say it.

    > devices.  Maybe something special is needed for the special case of a shared
    > network that is also a border network.  But that seems very rare. And getting
    > the L2 switch to do the right packet forwarding for the hybrid case seems an
    > invitation to trouble.

I'm not happy about any of the L2 text; I would have left it out completely.

--
Michael Richardson <[email-masked]>, Sandelman Software Works
 -= IPv6 IoT consulting =-

---

From: "Joel M. Halpern" <[email-masked]>
Date: Fri, 10 Apr 2020 17:35:08 -0400

I am happy to propose some starting text (I am sure it will need further 
tuning) as a short definition of a Loopback Interface:
     Loopback Interface: A logical or virtual representation of
     connectivity for IP communication.  It is distinct from any
     physical interface on the device.  It is typically used to
     abstract a point of communication from any physical connectivity.

If we want to elaborate, we could not that this provides an interface to 
anchor IPv6 addresses in accordance with the IPv6 addressing architecture.

As for the L2 material, if the WG wants to remove it, I would be fine 
with that.  It is clearly supplementary.  I am just trying to avoid 
confusion if we choose to describe it.

Yours,
Joel

---

From: Brian E Carpenter <[email-masked]>
Date: Sat, 11 Apr 2020 11:03:20 +1200

>      abstract a point of communication from any physical connectivity.

That would work for me.
 
> anchor IPv6 addresses in accordance with the IPv6 addressing architecture.

Right. That's the only thing that's different from the IPv4 case.

   Brian