Subject: Multilink Subnet Considerations for NETLMM Addressing

From: "Templin, Fred L" <[email-masked]>
Date: Thu, 24 Aug 2006 08:47:53 -0700

(cross-posting, since there seems to be interest in NETLMM
on the IPv6 list)

Having been away from e-mail for the past several days, the
text below is offered to cover the NETLMM Addressing concerns.

This would naturally go as replacement text for Section 5 of
'draft-ietf-netlmm-mn-ar-if-01.txt', but may be appropriate
for other documents as well. Comments?

Fred
[email-masked]

"5.  Multilink Subnet Considerations

An individual prefix is an IP prefix associated with a specific
MN, and a shared prefix is an IP prefix that may be associated
with multiple MNs.

ARs must not include an individual prefix in RAs that may be
received by a MN other than the one associated with the prefix.

ARs must not send RAs that include a shared prefix in a Prefix
Information Option with 'A'=1 unless there is operational assurance
of duplicate address detection/avoidance across the NETLMM domain.

ARs must not send RAs that include an individual or shared prefix
in a Prefix Information Option with 'L'=1 unless all RAs that
include the prefix and all MNs that receive them are associated
with a single link."

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From: "Templin, Fred L" <[email-masked]>
Date: Thu, 24 Aug 2006 11:22:58 -0700

James,

> I don't think this quite captures the situation.

I tend to disagree; see below:

> First off, a prefix advertised in an RA is not assigned to an end
node, it 
> is assigned to a link. A prefix can only rightly be considered to be 
> assigned to a node when it is delegated via DHCP, this allows the node
to 
> then assign the prefix to links downstream, or delegate further if the

> prefix isn't a /64.

The text does not say anywhere "assigned to (sic) an end node";
it says "associated with (sic) an end node/nodes", and prefixes
are associated with a link per (RFC4291, Section 2.1). 

> Secondly, exactly what is meant by 'L=0' is underspecified by RFC
2461. I 
> think everyone agrees with 'L=1' means, that the prefix is only being 
> advertised to nodes that are on this physical link. Any effort to
tighten up 
> the definitoin of  'L=0' is going to need wider discussion with the
ipv6 WG 
> and possibly might impact RFC2461bis. This draft is currently in AD 
> Evauation:Revised Draft Needed.

As long as a MN does not assign a prefix with 'L=0' to an interface,
then the RFC2461 interpretation of 'L=0' is irrelevant in terms of
the text I offered and RFC2461 is certainly specified well enough
such that implementations would not assign a prefix with 'L=0' to
an interface.

> Thirdly, this doesn't discuss at all link forwarding considerations, 
> particularly with regard to link local multicast (i.e. forwarding of
traffic 
> to link local multicast addresses). As we've discussed offlist,
exactly how 
> link local multicast forwarding works for NETLMM is an open question
at this 
> point. I started another thread on that for comment. So far, the
thread 
> hasn't been very active.

Again, this is irrelevant wrt to the text I offered, and subject
for a separate thread of discussion. Such discussion should take
place on a wider distribution such as the INT area and IPv6 lists
since it is germane to the IPv6 addressing architecture and not
particular to NETLMM.

Fred

            jak

----- Original Message ----- 
From: "Templin, Fred L" <[email-masked]>
To: "INT Area" <[email-masked]>; <[email-masked]>
Cc: "Templin, Fred L" <[email-masked]>
Sent: Wednesday, August 23, 2006 4:46 PM
Subject: [netlmm] Multilink Subnet Considerations for NETLMM Addressing

Fred

_______________________________________________
netlmm mailing list
[email-masked]
https://www1.ietf.org/mailman/listinfo/netlmm

---

From: "James Kempf" <[email-masked]>
Date: Thu, 24 Aug 2006 11:39:49 -0700

node, it
to

jak>> The last paragraph of Section 2.1 in 4291 says:

   Currently, IPv6 continues the IPv4 model in that a subnet prefix is
   associated with one link.  Multiple subnet prefixes may be assigned
   to the same link.

jak>> So whether the term "associated" or "assigned" is used is not 
particularly relevant. Both are used to mean that the link is what, in a 
sense, "owns' the prefix, not the node.

jak>> Inferring from the previous text in that section, the discussion makes 
clear that prefixes are assigned to interfaces, not nodes. In fact, the 
first sentence in the section says that:

   IPv6 addresses of all types are assigned to interfaces, not nodes.

jak>> I interpret the text in the paragraph you sent to mean that the node 
"owns" the prefix. I would suggest modifying the text as follows:

    An individual prefix is an IP prefix assigned to an interface on a 
specific
    MN on a single link and is not shared by any other node on the link with
    the exception of the last hop router, and a shared prefix is an IP 
prefix that
    may be assigned to interfaces on multiple MNs on a single link.

2461. I
tighten up
ipv6 WG

jak>> I'm not sure I understand. Perhaps you can provide some background on 
what exactly you are intending to convey with this text?

traffic
exactly how
at this
thread

jak>> I think it is relevant to multilink subnets. Suppose a NETLMM domain 
is a multilink subnet. Then the question of what happens with link local 
multicast is important. Does it get forwarded between physical links or not? 
This determines whether the NETLMM domain looks like a single virtual 
physical link or not. An application that uses link local multicast might 
work differently if link local multicast is forward or not, as we have 
discussed.

            jak

---

From: "Templin, Fred L" <[email-masked]>
Date: Thu, 24 Aug 2006 12:16:07 -0700

James, 

> "owns" the prefix. I would suggest modifying the text as follows:

"Associated" does not mean "owned" by any sense of the definition for
"associated" I was able to find in www.m-w.com.

specific
prefix that
>    may be assigned to interfaces on multiple MNs on a single link.

That is unnecessarily restrictive, as I believe you know.

> what exactly you are intending to convey with this text?

I don't know how to address a question like this without more detail,
since my statement above and the offered text read clearly to me.

traffic
exactly how
at this
thread

> discussed.

The point of the offered text is that by following the guidelines
a NETLMM domain cannot be a multilink subnet. I can call it something
like "Multilink Subnet Avoidance" if you'd like.

Here is the offered text again:

Fred

---

From: "James Kempf" <[email-masked]>
Date: Thu, 24 Aug 2006 13:04:34 -0700

Fred,

jak>> Well it does according to my reading of the enclosed text from RFC 
4291. Perhaps we need someone else's opinion on this, since we clearly 
disagree. Perhaps we should ask Bob Hinden, as he is the author of the 
disputed text in RFC 4291?

specific
prefix that

jak>> The current text is unacceptable to me, as insufficiently precise.

jak>> What are other people's opinions?

jak>> What are your expectations about  what advertising 'L=1' means for the 
link?

traffic
exactly how
at this
thread

jak>> "Cannot avoid a multilink subnet"? Then why call it "Multilink Subnet 
Avoidance"?'

            jak

Fred

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---

From: "Templin, Fred L" <[email-masked]>
Date: Thu, 24 Aug 2006 13:14:19 -0700

> jak>> What are your expectations about  what advertising 'L=1' means
for the 

The criteria that allow a prefix to be advertised with 'L=1' are
covered under the final paragraph of the offered text, and what
advertising 'L=1' means for the link is covered under RFC2461.

> jak>> "Cannot avoid a multilink subnet"? Then why call it "Multilink
Subnet 

I don't quite understand your question, but the offered text avoids
multilink subnets.

Fred

            jak

Fred

---

From: "Templin, Fred L" <[email-masked]>
Date: Thu, 24 Aug 2006 13:41:03 -0700

James,

> I'm not going to discuss this text with you any further on the list at
this 
> time. I believe the text is sufficiently unclear and imprecise to
result in 
> confusion about how one should implement the MN-AR interface in
NETLMM. 
> Clearly, you disagree and you're not prepared to accommodate any of my

> suggestions, so there's little point in our continuing the discussion.

Suit yourself, but the text works in necessary and sufficient terms.
Take 5 minutes to work through it, and I believe you will see that
this is so.

Fred

---

From: "James Kempf" <[email-masked]>
Date: Thu, 24 Aug 2006 13:23:48 -0700

Fred,

I'm not going to discuss this text with you any further on the list at this 
time. I believe the text is sufficiently unclear and imprecise to result in 
confusion about how one should implement the MN-AR interface in NETLMM. 
Clearly, you disagree and you're not prepared to accommodate any of my 
suggestions, so there's little point in our continuing the discussion.

            jak

----- Original Message ----- 
From: "Templin, Fred L" <[email-masked]>
To: "James Kempf" <[email-masked]>; "INT Area" 
<[email-masked]>; <[email-masked]>; "IETF IPv6 Mailing List" 
<[email-masked]>
Sent: Thursday, August 24, 2006 1:14 PM
Subject: RE: [Int-area] RE: [netlmm] Multilink Subnet Considerations for 
NETLMMAddressing

Fred

            jak

Fred

---

From: Julien Laganier <[email-masked]>
Date: Thu, 31 Aug 2006 10:43:02 +0200

Hi James,

Just for clarification.

On Thursday 24 August 2006 20:04, James Kempf wrote:
> [...]
> 2461. I think everyone agrees with 'L=1' means, that the prefix is
> only being advertised to nodes that are on this physical link. Any
> effort to tighten up the definitoin of  'L=0' is going to need
> wider discussion with the ipv6 WG and possibly might impact
> RFC2461bis. This draft is currently in AD Evauation:Revised Draft
> Needed.
>
> [...]

I wouldn't say that 'L=0' semantic is underspecified. I on the 
opposite found 2461 to be quite well specified. It says:

   Note, however, that a Prefix Information option
   with the on-link flag set to zero conveys no information concerning
   on-link determination and MUST NOT be interpreted to mean that
   addresses covered by the prefix are off-link.

That is, it doesn't mean that *all* nodes are off-link. Some might, 
while some might not. But it clearly specifies what the node should 
do in that case.

   The default behavior (see
   Section 5.2) when sending a packet to an address for which no
   information is known about the on-link status of the address is to
   forward the packet to a default router;

i.e. send packets to the default router.

What do you think is underspecified?

--julien

---

From: "Templin, Fred L" <[email-masked]>
Date: Thu, 31 Aug 2006 10:40:37 -0700

In addition to what Julien said, (RFC2461, Section 6.3.4)
also says:

   "Prefixes with the on-link flag set to zero would normally
   have the autonomous flag set and be used by [ADDRCONF]."

and ([ADDRCONF], Section 5.5.3) looks only at the 'A' bit
for address configuration. Hosts use prefixes with the 'A'
bit set to configure an address and add it to the list of
addresses assigned to the interface regardless of the sense
of the 'L' bit.

Fred

-----Original Message-----
From: Julien Laganier [mailto:[email-masked]] 
Sent: Thursday, August 31, 2006 1:43 AM
To: James Kempf
Cc: [email-masked]; INT Area; IETF IPv6 Mailing List
Subject: Re: [netlmm] Multilink Subnet Considerations for NETLMM
Addressing

Hi James,

Just for clarification.

--julien

---

From: "James Kempf" <[email-masked]>
Date: Thu, 31 Aug 2006 10:55:20 -0700

Maybe I'm confused, but I don't see what L=0 really means. If ADDRCONF 
assigns the prefix to the interface, then the prefix is on link for at least 
one node, the node that configured the interface with the prefix. I suppose 
one could argue that, even if that node considered the prefix on link, that 
node should not assume another node does. But IFICT, 2461 doesn't have 
specific, RFC 2119 requirements language around that.

If M=1, and stateful configuration is used, then I could see some sense in 
L=0, since I assume in that case the prefix is not assigned to the 
interface.

Can anybody say what real implemetations do with this?

----- Original Message ----- 
From: "Templin, Fred L" <[email-masked]>
To: "Julien Laganier" <[email-masked]>; "James Kempf" 
<[email-masked]>
Cc: <[email-masked]>; "INT Area" <[email-masked]>; "IETF IPv6 Mailing 
List" <[email-masked]>
Sent: Thursday, August 31, 2006 10:40 AM
Subject: RE: [netlmm] Multilink Subnet Considerations for NETLMM Addressing

Fred

-----Original Message-----
From: Julien Laganier [mailto:[email-masked]]
Sent: Thursday, August 31, 2006 1:43 AM
To: James Kempf
Cc: [email-masked]; INT Area; IETF IPv6 Mailing List
Subject: Re: [netlmm] Multilink Subnet Considerations for NETLMM
Addressing

Hi James,

Just for clarification.

--julien