Subject: FW: [Autoconf] Subnet model

From: "Yi, Seung" <[email-masked]>
Date: Fri, 6 Oct 2006 17:53:02 -0700

Hi, all,

It seems I'm a bit late to the discussion. Let me put my two cents in,
though. Comments inline. 

> > Now how does this relate to MANETs? First a MANET is made of
routers,
> 
> And indeed, one question I have is "are all nodes routers?" 
> Who decides whether one wants to be just a host or a router? 
> And how does that change things? (I assume there will be dumb 
> hosts that don't want to be routers -- is that a shared assumption?)
>

I don't seem to see the big difference that this can make. If one allows
dumb hosts, they won't care if they are in a MANET or not. They're dumb.
So, the only thing that matters is that an MR that stands in front of
such dumb hosts need to take care of them. NAT comes to mind. Also,
getting a prefix and assigning addresses from the prefix to the attached
dumb hosts will work just as fine. 
 
> Indeed, another question that comes to my mind is, how does 
> the autoconf problem in manets compare/relate to the zrouter 
> (zero config
> router) BOFs that were held at one point, where the goal was 
> to autoconfigure the subnets within an autonomous area. How 
> is the work here similar, and how does it differ?
>

I'm not familiar with this work. Could you point me to some information
about this zrouter BOF?
 
> > Also a MANET is not always connected to the Internet, so we don't 
> > always have to follow existing rules.
> 
> But there presumably still needs to be some basic assumptions 
> about what a subnet is, how the addresses on a subnet relate 
> to each other, etc., regadless of whether the manet is 
> connected to the internet.
>

The was I understand things are like this. A "link" is a communication
facility under L3. I don't think there should be an assumption of
bi-directional reachability on a link. It may or may not provide such
support. Joe gave a good example of wireless "links" that may not
provide it. A "subnet" is an artificial concept that is used to group a
set of nodes that are presumably "on" a same link. Note that this group
is not necessarily all nodes on the link as pointed out by Ian. By
assigning/configuring addresses that belongs to the same subnet, this
group of nodes know that all the other nodes in the same subnet (that
can be identified with their IP addresses and netmask) can be reached by
just relying on the link, without getting any L3/IP routing involved. If
we limit the discussion to providing multihop connectivity at the layer
3 (i.e., the multi-hop nature is exposed to L3 compared to L2 MANET
solutions that present an ethernet-like view of MANET to L3), an MR
cannot really be sure about what other MRs will be one hop away (i.e.,
that can be reached by L2) because of their mobility and the nature of
wireless medium. So, if a MANET is configured to be a single subnet,
that causes the multi-link subnet problems as pointed out by Dave
Thaler.
 
> > To me this is the kind of discussions that should appear in the 
> > MANET-arch document. One could then extract scenarios that will be 
> > considered by the WG. I see 4 of them:
> 
> > - flat: all nodes have /128, routing entries are /128. An MR has to 
> > take care of routing for hosts attached to it, and hosts have a 
> > default route pointing to the MR they are attached to
> > - semi-flat: all routers own a /64, they delegate /128 to hosts 
> > attached to them, routing entries in routers are /64, hosts have a 
> > default route pointing to the MR they are attached to
> > - coherent: all MRs share a given prefix. For example for an 
> > Internet-connected MANET this could an Internet routeable  /48, and
for 
> > an isolated MANET this could a reserved prefix by IANA for MANETs
(e.g.
> > MANET-local prefix).
> > - clustered: some (topologically close) MRs share a given prefix,
and 
> > routing entries to this cluster may be aggregated. Example is a
MANET 
> > with multiple gateways to the Internet or a very large isolated
MANET 
> > that need routing aggregation for scalability reasons.
> 
> Yep, the above are the kinds of questions I have. Just what 
> is being proposed? (and answering "any and all of them" is 
> not the answer I'm looking for!)
>

I did not think of the "clustered" one but I like this classification. I
don't see why we need to choose one over the others. They can all
co-exist. When there are no attached hosts, "flat" will work just fine.
An autoconf scheme needs to assign a /128 address per MR. When there are
attached hosts and MRs do not want to NAT them, they can get a prefix
and sub-allocate the addresses to the attached hosts - "semi-flat". The
autoconf scheme will need to provide prefix on top of addresses. When a
MANET is connected to the Internet, a topologically correct prefix can
be assigned for the whole MANET - "coherent". MANET gateways will be the
aggregation points for the prefix but none of the MRs will be configured
to be in the same subnet with any other MRs. This can be done with
appropriate netmask settings. This prefix can be divided and given out
to MRs in the MANET as in "semi-flat" model. And we can avoid having a
multi-link subnet in this way, which I think is a good thing.

Thanks,

- Seung

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From: Ralph Droms <[email-masked]>
Date: Sun, 08 Oct 2006 19:59:16 -0400

Excerpted, with comments in line...

- Ralph

On 10/6/06 8:53 PM, "Yi, Seung" <[email-masked]> wrote:
> provide it.

OK ... But are there fundamental assumptions in IP about bi-directional
reachability?  E.g., will ARP, ND, IPv6 address autoconfiguration all work
if bi-directional reachability fails?

> A "subnet" is an artificial concept that is used to group a
> set of nodes that are presumably "on" a same link.

Is a "subnet" the set of nodes with addresses assigned from a single prefix?
Is there an assumption that all nodes in a subnet have bi-directional
reachability?  Is that reachability achieved without routing and without TTL
decrement?

> Note that this group
> solutions that present an ethernet-like view of MANET to L3),

Is this the point at which we need to add fixes to IP?  Is there a
fundamental assumption in IP that an L2 layer is always "single hop" for IP?

> an MR

- Ralph