Subject: [Autoconf] About MANET Links, subnets, etc

From: Carlos Jesús Bernardos Cano <[email-masked]>
Date: Fri, 21 Nov 2008 07:46:57 +0100

Hi all,

	Apologies for jumping into the discussion kind-of late.

	I've been trying to follow all the e-mails and provide comments, but so
far I've experienced some contradictory feelings. Some times I thought
"OK, I agree with this" and then another e-mail arrived arguing the
opposite that made change my mind.

	So, let me try to provide my (current) view on some of the things that
have been discussed. Most of them might have been already proposed by
others, so I'm just taking those pieces from here and there that I'm
happy with.

	1. About "MANET Links". I'm not completely sure we really need to
define anything new. I see MANETs as networks composed of a set of
routers that move. These routers have interfaces that belong to links --
these links being defined by the L2 reachability/radio
coverage/connectivity of the interface. The difference is that these
links are quite dynamic (because the routers move) and might not have
some nice properties, such as transitivity, symmetry, etc (but this is
because of the L2 technology of the interface).

	2. About "subnets" and addressing. In my understanding of a MANET, I
see each of the routers of a MANET getting one (or several) unique IPv6
prefix. Out of that prefix, the MANET router configures and address for
its loopback interface. On the interface over which the MANET router
runs a MANET routing protocol, only a link-local address needs to be
configured. The router may be delegated more than one prefix, to
configure other physical interfaces (regular hosts might attach to these
interfaces, and configure IPv6 address from the delegated prefixes). The
MANET routing protocol takes care of providing reachability within the
MANET, MANET routers do not need to change their IP addresses due to
inner mobility within the MANET.

	3. About "links". I think the term "link" should be accompanied by the
layer for which the term "link" is meaningful. There might be L2 links
and L3 links and should not necessarily be the same. I'd say that a "LX
link" is defined by "what can be reached in 1 LX hop". In the case of
IP, an "L3/IP link" would be defined by what can be reached in 1 IP hop,
i.e. without decrementing the IPv6 hop count of the packet.

	4. About what we should do in the autoconf WG: to define the
mechanism(s) to provide each MANET router with a unique IPv6 prefix
(that could even be a /128). Something that might be tricky is how to
guarantee that configured link-local addresses on the MANET interfaces
are unique within the scope of the link, since that scope is quite
dynamic (members of that link change over time, and even not all the
members of that link may have the same view of the link). Potentially, a
solution would be to ensure that these addresses are unique within the
whole MANET, since that would ensure that they are in the scope of the
link.

	Well, probably I will change my mind about some of these things after
getting your opinion guys :-)

	Carlos

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---

From: Erik Nordmark <[email-masked]>
Date: Fri, 21 Nov 2008 15:16:23 -0800

> because of the L2 technology of the interface).

Agreed.

> inner mobility within the MANET.

That matches my understanding.

> i.e. without decrementing the IPv6 hop count of the packet.

While I agree in principle, I don't think we need to concern ourselves 
with anything but "L3 link" (which RFC 2460 has defined as "link").
Furthermore, different L2 SDOs might have differing definitions of "L2 
link" (for instance, IEEE 802 has such a definition) so we are not the 
authority on how to design such things. Thus, if possible, I think we 
should stay away from defining "L2 link".

> whole MANET, since that would ensure that they are in the scope of the

If you use EUI64-based link-local addresses, or RFC 4941 addresses, the 
probability of collisions for the link-local addresses is extremely 
small. It might make sense to have safety mechanism where the Manet can 
continue to function in the presence of duplicate link-local addresses, 
while one or both of the nodes with duplicates might end up being 
disabled. But in general it might be hard to detect such duplicates 
because you have do have some other distinguishing piece of information 
to tell the difference between a node being reachable over two different 
paths and two different nodes being identical clones.

    Erik

---

From: "Teco Boot" <[email-masked]>
Date: Sat, 22 Nov 2008 18:29:39 +0100

Hi Erik,

<skip>

> paths and two different nodes being identical clones.

In fact, IEEE 802.11 itself prevents detecting duplicate MAC addresses (see
other posting).
But we could think of some kind of detection mechanism, e.g. with help from
the MANET routing protocol.
I think this has second priority, although it is an interesting topic.

Teco.

---

From: Carlos Jesús Bernardos Cano <[email-masked]>
Date: Tue, 25 Nov 2008 00:15:21 +0100

Hi Erik, all,

> should stay away from defining "L2 link".

Agree, I just wanted to point out that the term "link" may mean
different things. Fully agree we should only consider L3 links.

> paths and two different nodes being identical clones.

Also agree.

Carlos

---

From: "Charles E. Perkins" <[email-masked]>
Date: Mon, 24 Nov 2008 15:36:07 -0800

Hello Erik,

I'm somewhat behind on the discussion...

> with anything but "L3 link" (which RFC 2460 has defined as "link").

Do you think it is possible to have a "L3 link" between nodes that are
not on the same subnet?  How about if they are on different subnets,
but not advertising reachability for those subnets?

If these are legitimate operations in your view, and if it "L3 link" is
then seen to be quite distinct from "subnet", then maybe it is the
concept we need.

> should stay away from defining "L2 link".

If "L3 link" will do the job for MANET, I agree.  Otherwise (and
only if "L3 link" won't do), we might set about defining something
called a "MANET link".

Regards,
Charlie P.

---

From: Erik Nordmark <[email-masked]>
Date: Sun, 30 Nov 2008 18:10:25 -0800

> I'm somewhat behind on the discussion...

So am I.

> but not advertising reachability for those subnets?

Existing operation practice sometimes handles point-to-point links 
between routers that way; each router has an IP address from the prefix 
assigned to the ISP owning that router.

And in principle I don't see why one couldn't do that for links that are 
not point-to-point. There is a potential inefficiency if IPv4 hosts are 
attached to such a link since they might send packets to a suboptimal 
1st hop router, and there is no way the router can send an acceptable 
redirect.

> then seen to be quite distinct from "subnet", then maybe it is the

Good.

    Erik