Subject: Re: [manet] Comments on draft-jelger

From: [email-masked]
Date: Mon, 01 Sep 2003 23:15:57 -0700

Hi Christophe,

Regarding your new draft, I am not sure that "prefix continuity"
is necessary. That is, I am not sure that the set of nodes having 
the same prefix needs to be contiguous.  Please consider the 
following, and let me know if I am missing something.

It is only necessary for each node to have a route to the
gateway(s) it is affiliated with, and for each gateway to have 
a route to each node that is affiliated with it.
One might argue that if the set of nodes with a given prefix 
is contiguous, then it is easier to flood a message to all 
nodes having that prefix, but I think it is sufficient for a 
gateway to be able to perform the following types of flooding:

(a) Flooding to all nodes in the manet.
(b) Flooding to all nodes in the manet that are within K hops
    of the gateway (where K is such that any node affiliated with 
    the gateway is required to be within K hops of that gateway).

Thus, I am not sure it is necessary to go through the trouble to
ensure "prefix continuity".
A node can simply select the nearest gateway (which can be determined
from the information provided by a proactive routing protocol),
and use the prefix advertised by that gateway.  The node can keep 
that prefix until it no longer has a route to that gateway, or becomes 
more than K hops from that gateway, or finds another gateway that is 
much closer.

My reasoning is influenced by the original paper on Landmark Routing
(Paul Tsuchiya, 1987), which I re-read recently.
A node can select any landmark node (a landmark is like a gateway)
that is within a certain hop-radius of the node, and use the 
prefix advertised by that landmark.  There is no requirement that 
the set of nodes affiliated with a given landmark be contiguous.
A node would have to change its prefix only if it moves too far
from the landmark it has selected. It could then select another
landmark, or another landmark could be elected, or the hop-radius
could be increased.  (Note that I am not suggesting to use
landmark routing to deal with internet-gateways in manets.)

The idea of a hop-radius (or "routing scope") from Landmark routing 
can also be applied to the problem we are considering.
For example, suppose there is one huge manet (say with a diameter
of 100 hops), which has several internet-gateways.
Thus, even though one manet node may be be able to reach another
manet node using a 100-hop path, it would be desirable to limit
the routing scope to K hops (where K might be 5), such that
every node is within K hops of some internet-gateway.
Then, each manet node would select a gateway that is within
K hops and use the prefix advertised by that gateway.
The node would have to change its prefix only if it moves
more than K hops from the gateway it selected.

It is also possible for a manet node to select multiple gateways
that are within K hops, and to have multiple prefixes.  
Note that the above approach makes it unnecessary to divide
the manet into well-defined areas or clusters:
The set of all nodes affiliated with a given gateway need
not be contiguous. All nodes within K hops of a gateway receive 
advertisements from the gateway, but may decide to use the prefix 
advertised by another gateway that is also within K hops.

Regards,
Richard

> Hi everybody,
> 
> I've just published a new Internet Draft on IPv6 gateway discovery and
> address autoconfiguration in ad hoc networks.
> 
> http://www.ietf.org/internet-drafts/draft-jelger-manet-gateway-autoconf-v6-00
.txt
> 
> I am of course very interested by comments and suggestions which are very
> welcome. For your information, we will have an implementation of the draft
> in a few weeks.
> 
> Sincerely,
> 
> Christophe
> 
> ---------------------------------------------------------------
> Christophe Jelger - LSIIT           [email-masked]
> Université Louis Pasteur
> Strasbourg - France                   Tel: +33 (0)3 90 24 45 90
> 
> http://www-r2.u-strasbg.fr/~jelger
> ---------------------------------------------------------------
> 
> 
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