Subject: RE: [manet] Dumb Question?

From: "Nast, Douglas P" <[email-masked]>
Date: Mon, 6 May 2002 15:16:23 -0700

John, 

Actually, the manet folks are designing routing protocols in the classical
sense, although the protocols of interest tend to be reactive rather than
proactive.  The fact that the mac layer supports broadcast via omni antennas
does not change this.  What you are suggesting is that an Manet does not
need layer 3 routing since it can "switch" at layer 2. In effect, you would
like to treat the manet as an ethernet segment where routing is not
necessary within the segment because everyone has a shared medium.  But I do
not believe that this is the paradigm that this group is working to.  

I think the reason for this (I am just a hanger-on, not one of the
old-timers) is that you need to talk to nodes that are not within range of
your antenna.  To get to them intermediate nodes have to forward your
traffic.  While this could be done at layer 2, it would involve building
mechanisms at layer 2 that are as complicated as traditional layer 3
routing, and would not be as flexible (i.e. lead to stovepipe solutions).
The name of the game seems to be to use IP infrastructure for multi-hop
forwarding.  Therefore layer 3 routing is indicated to relay the traffic
across long distances.  

I would call the process you describe layer 2 switching, as in ethernet
LANs.   But it is not considered to be a general solution for Manets.   

Regards, 
Doug Nast, Boeing

-----Original Message-----
From: Mullen, John P. (Contractor)
[mailto:[email-masked]]
Sent: Monday, May 06, 2002 2:08 PM
To: [email-masked]
Subject: [manet] Dumb Question?

Hello,

I am looking for a term, but have had no luck.

In wired nets, a packet is routed through the network from node to node, say
by routing tables.  However, in MANETs, in which packets are broadcast via
omnidirectional antennae on a shared channel, such a thing is not possible.
Instead, each node that receives a packet  decides to:
     1.  Discard the packet,
     2.  "Read" the packet, since it is addressed to the node, and/or
     3.  Rebroadcast the packet.

If the node is not to be an intermediary node in the end-to-end path between
the source and the destination, or if the node is receiving a copy of a
packet it had previously broadcast, it will not broadcast the packet.  This
is necessary to avoid overutilizing the channel.

If the node is an intermediary node on the path, then it must rebroadcast
the packet.  

I've got this much figured out, but what I do not know is what this process
is called.  It is the MANET equivalent to "routing," but is not really
routing, since the sending node has no way of directing the packet.  

I've tried a number of searches and read a few glossaries, but have not been
able to find this term.

I'd appreciate any help.

Thanks,

John P. Mullen

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From: Ram Ramanathan <[email-masked]>
Date: Mon, 06 May 2002 19:04:04 -0400

John,

I don't believe there is a term for this.

Note that the path is implied by which node decides to rebroadcast and
which doesn't. The set of decisions of the nodes, made in a distributed
manner, decides the "route" of the packet. Thus, one could argue this
is still "routing"
from a global perspective, even though, from a node-specific viewpoint
it may be different from what is being done traditionally.

A couple months ago I also observed this, and was wondering if it could
somehow simplify routing. It turns out that in order to decide whether to
rebroadcast or not, a node has to look at which node the packet came
from (plus the destination). Thus, you have a "previous hop" table
instead of a next hop table, and each entry says one of three things --
keep, discard, rebroadcast. In other words, even though the node
does not "direct" the packet in the "forward" direction, it does so
implicitly *from* the "backward" direction.

This leads to  a mirror image of traditional routing with everything
in reverse, but with sort of the same underlying constraints, although you 
may be able to have a smaller routing table (gintour table?:). This
is a nice little thing for some bright grad student to see if there
are any hidden nuggets. And coin a term and be famous, of course:)

-Ram.

---

From: "Charles E. Perkins" <[email-masked]>
Date: Mon, 06 May 2002 16:31:30 -0700

Ram and John,

I prefer a somewhat different conceptual model.

Notice that if there were no framing for Ethernet
packets on a shared channel, there wouldn't be much
difference for the purposes of this discussion.
One can easily imagine framing the broadcast packets
with MAC addresses, and then John's question would
almost evaporate.

Regarding the observation that one might look at
the (IP?) address of the last hop to decide what
to do, that wouldn't be necessary with MAC addresses,
but it's a good way to operate otherwise.  I reckon
that neighbors that are neither last nor next hop
could use such broadcast information to acquire
paths towards destinations, whether or not they
currently need to have the information.

Maybe "tuor"ing table is better than "routing"
table.  Or maybe just "last-hop" table.  Or maybe
it's a "not-forwarding" table or "not-backwarding"
table.

Regards,
Charlie P.

---

From: Ram Ramanathan <[email-masked]>
Date: Mon, 06 May 2002 19:04:04 -0400

John,

-Ram.

---

From: "Charles E. Perkins" <[email-masked]>
Date: Mon, 06 May 2002 16:31:30 -0700

Ram and John,

Regards,
Charlie P.

---

From: gikaru <[email-masked]>
Date: Tue, 07 May 2002 13:10:30 +0200

Ram
Your explanation is correct and appreciable but does not fully answer the 
question posed. The question is "What term is used in MANETs for the wired 
term routing ?" I personally would use the same term "routing in MANETs" 
while we wait for the "right term" to emerge.
Willy

> > I've tried a number of searches and read a few glossaries, but have not 

Willy

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From: "Phil Neumiller" <[email-masked]>
Date: Tue, 7 May 2002 07:48:02 -0400

I find some of the comments more interesting than the original question.  Is not
AODV a particular implementation of an algorithm that solves the shortest path
between all points problem?  Is it not equivalent to the distributed bellman ford
algorithm?  I wonder why would choose not to call this routing???

---

From: "Phil Neumiller" <[email-masked]>
Date: Tue, 7 May 2002 07:48:02 -0400

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From: gikaru <[email-masked]>
Date: Tue, 07 May 2002 13:10:30 +0200

Ram

Willy

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From: "Philippe Jacquet" <[email-masked]>
Date: Tue, 7 May 2002 16:13:11 +0200

Strictly speaking AODV is not equivalent to a distributed Bellman Ford
algorithm as DSDV was.  The shortest path obtained by flooding may be longer
than the absolute shortest path. This is routing anyhow.

Philippe

----- Original Message -----
From: Phil Neumiller <[email-masked]>
To: Ram Ramanathan <[email-masked]>; gikaru
<[email-masked]>
Cc: <[email-masked]>
Sent: Tuesday, May 07, 2002 1:48 PM
Subject: Re: [manet] Dumb Question?

---

From: "Philippe Jacquet" <[email-masked]>
Date: Tue, 7 May 2002 16:13:11 +0200

Philippe