Subject: Which Layer?

From: Chip Elliott <[email-masked]>
Date: Thu, 28 Aug 1997 22:25:28 -0400

>What does TTL mean anyway?  Conventionally, it means number of
>hops.  When subnetwork layers start distorting the meaning of TTL
>by tunneling or ATM-style methods, it seems to me that TTL becomes
>meaningless as a routing metric, or for that matter an end-to-end
>statistic.  

Charlie,

Are you against Ethernet hubs and bridges? ;-)

But more seriously, I do agree that it's kind of thorny
trying to decide, in general, whether an ad hoc network should
be a single IP subnet or not. There are indeed arguments on
both sides; and in fact, even having Ethernet bridges can
make certain IP problems harder (eg integrated services).

Here is perhaps one way to approach it. Do we really want
to build ad hoc nets out of multiple technologies, eg, throw
together a bunch of hosts using any mixture of radios,
Ethernets, serial lines, etc. If the answer is a resounding
yes, then we must use IP layer routing. If that is not our
main focus, then we are probably talking a solution that
works below IP.

Note that the "ad hoc nets built of mixed subnet technologies"
is noticeably harder than the plain-old ad hoc net problem,
since these different technologies have quite different
characteristics (bandwidth, error, MTU, ...) which should
somehow be reflected in the forwarding decisions. Just handling
radios is hard enough, do we really want to add all this...!

Personally, I prefer the below-IP approach, and feel that
efforts to standardize the "network formation and forwarding"
parts of ad hoc nets should be an extension (rewrite) of
the 802.11 spec. But there is certainly room to differ here.

-- Chip