Subject: comments on draft-ietf-6man-multi-homed-host-01

From: 神明達哉 <[email-masked]>
Date: Tue, 3 Nov 2015 16:26:38 +0900

I've read draft-ietf-6man-multi-homed-host-01 and have the following
comments:

- Section 1.1: I don't have a particular objection to what's written
  here, but I was not sure why we need this section for this
  document.  Is there any problem in, e.g., understanding some other
  part of the document, if we don't have this discussion?

- Section 1.1: I can't follow this sentence:

   However, if the router upstream from the host implements BCP 38
   Ingress Filtering [RFC2827], such as by implementing uRPF on each
   interface, the router is configured to only permit communication by
   strong hosts.

  Here, "the host" is he host described in the preceding sentence, correct?

   [...] a host with an addressless interface and a default route
   pointing to the interface [...]

  And, then I don't understand how to interpret the 'However...'
  sentence.  Could it perhaps be paraphrased?

- Section 2.1

   it will be generally the case that the
   configured addresses match one of the prefixes advertised in a Router
   Advertisement that are supposed to be in that link.

  I guess this should be "supposed to be on-link in that link".

- Section 2.1: the following part is confusing to me:

   the setting of the L and A flags in the PIO.  In some circumstances
   both L and A might be zero.

   Although this does not violate the existing standard [RFC4861], such

  The 'In some circumstances...' sentence seems to terminate the
  paragraph abruptly (e.g., without explaining specifically in which
  circumstances this can happen).  If this sentence were actually
  placed at the beginning of the 'Although this...' paragraph, it
  would make much more sense.  Still, I'd wonder specifically in which
  circumstances this is needed, so it would be better to explain it
  anyway.

- Section 3.3: I think it would be helpful if this section explains
  more explicitly that even if RFC6724 assumes the next-hop is first
  determined to choose the source address while this draft
  (essentially) determines the next-hop based on the source address,
  both behaviors are consistent as a result.  I'm afraid the current
  draft text could cause confusion due to the seeming "reversed
  dependency".

- Section 2.2: a minor nit: s/a a/a/

   exist that can usefully limit themselves to static routing (such a a
   simple tree network), or may internally use no routers at all, such

--
JINMEI, Tatuya

---

From: Brian E Carpenter <[email-masked]>
Date: Wed, 4 Nov 2015 08:42:19 +1300

Hi Jinmei-san, thanks for the review.
>   part of the document, if we don't have this discussion?

I believe that we were asked to explain how our draft fits with the
strong/weak host model, since we appear to be slightly modifying the
related host behavior.

>   Here, "the host" is he host described in the preceding sentence, correct?

Yes.

>   sentence.  Could it perhaps be paraphrased?

I'm not sure the word "However" is needed. All we are saying is that a router
that supports uRPF forces the host to follow the strong model, because the
weak model will fail.

>   I guess this should be "supposed to be on-link in that link".

Yes

>   circumstances this is needed, so it would be better to explain it

Maybe we should say it the other way round: even if SLAAC is not wanted
(A=0) and there is no reason to announce an on-link prefix (L=0), the PIO
SHOULD be sent to inform hosts that the prefix is source-routed by the
router in question.

>   draft text could cause confusion due to the seeming "reversed

OK

> - Section 2.2: a minor nit: s/a a/a/

I think we meant s/a a/as a/
>    simple tree network), or may internally use no routers at all, such

Thanks again
   Brian

> 
> --
> JINMEI, Tatuya
> 
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---

From: 神明達哉 <[email-masked]>
Date: Tue, 17 Nov 2015 11:35:15 -0800

At Wed, 4 Nov 2015 08:42:19 +1300,

> strong/weak host model, since we appear to be slightly modifying the

I personally still don't see how it helps understand this document; at
least to me, the rest of the document is perfectly understandable
without this subsection.  If anything, it could rather make it less
readable in that it makes the document (to me, unnecessarily) longer.

That said, if it was included by someone else's strong desire and the
authors agreed with that, I wouldn't bother to challenge that.  One
possible suggestion in that case is to make it an appendix so future
readers won't have to be distracted unless they are particularly
concerned about the implication of the host model.  But if you (and/or
others) don't even like this suggestion, please just ignore it.

> that supports uRPF forces the host to follow the strong model, because the

I still don't get this.  To be precise in the 02 version the
corresponding text now reads:

   [...]  For
   example, a host with an addressless interface and a default route
   pointing to the interface will necessarily send packets using any
   address using that interface, and therefore be a de facto weak host.
   If the router upstream from such a host implements BCP 38 Ingress
   Filtering [RFC2827], such as by implementing uRPF on each interface,
   the router will prevent communication by weak hosts.

I don't understand why communication with such a host will necessarily
be prevented.  The host configuration in question is something like
this, right?

 I1    I2
  -host---------------router
  ^    ^
  |    |
  |    LLonly
2001:db8::1

(The host has two interfaces I1 and I2: I1 is configured with a global
address while I2 only has a link-local address; the host's only
default router is the router shown in the diagram, so the host will
send any outbound packets to interface I2).

So the router will see packets with source address being 2001:db8::1
from the host.  Even if the router implements ingress filtering, as
long as it recognizes the address (or some prefix that covers the
address) belongs to the network around the host, shouldn't it accept
and forward the packet?

BTW, "an addressless interface" may not be the best term, since in
IPv6 any interface should have at least a link-local address.
Something like "a link-local-only interface" would be more accurate.

> SHOULD be sent to inform hosts that the prefix is source-routed by the

Okay.  The revised text in ver 02 looks good to me.

--
JINMEI, Tatuya

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From: Brian E Carpenter <[email-masked]>
Date: Mon, 23 Nov 2015 15:10:32 +1300

Hi,

I'm going to ask Fred to reply about the uRPF question. (Possibly we
just need to say "the router *might* prevent communication by weak hosts".)

We were definitely requested at one point to say more about the weak/strong
host model. However, I have no objection to moving it to an appendix.

Regards
   Brian Carpenter

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From: 神明達哉 <[email-masked]>
Date: Mon, 23 Nov 2015 10:10:03 -0800

At Mon, 23 Nov 2015 15:10:32 +1300,

> just need to say "the router *might* prevent communication by weak hosts".)

If there's a "might", that will certainly address my question.

--
JINMEI, Tatuya

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From: "Fred Baker (fred)" <[email-masked]>
Date: Mon, 23 Nov 2015 18:37:30 +0000

> On Nov 3, 2015, at 11:42 AM, Brian E Carpenter <[email-masked]> wrote:
> 

Help me out, please. I'm not sure I follow the question.

If a router is configured with BCP 38, which uRPF is an example of, it will only allow a packet from a direction if it thinks it might (under some circumstances, present or not) send a packet *to* the sender via that direction. If a host has addresses on two different interfaces, and the router thinks of them as being in different directions (it sends to one address on one interface and the other address on a different one), it would as a result discard traffic using an address that it thinks is "wrong". That's the essence of the strong host model in this context. The essence of the weak host model in this context is that the host will send packets strictly by destination address without regard to source address - and the router *might* as a result pitch it.

Is the concern that the fact is not stated well? If so, I can look at the wording.

Thanks for your other comments as well. I'm on holiday at the moment and will need to look at this in a couple of weeks.

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From: 神明達哉 <[email-masked]>
Date: Tue, 24 Nov 2015 11:47:35 -0800

At Mon, 23 Nov 2015 18:37:30 +0000,

> Is the concern that the fact is not stated well? If so, I can look at the wording.

I think so (I guess the lack of *might* confused me).  See also my
later comment:
https://mailarchive.ietf.org/arch/msg/ipv6/aGEsSxWHMi3OGZ9pf5lndVgW1wo

--
JINMEI, Tatuya