Subject: [manet] Technical issue #7 - AODV can loose route replies - warning longer post

From: "Susan Hares" <[email-masked]>
Date: Mon, 14 Jan 2013 01:18:30 -0500

This post refers to Comments by: Marius (Rice), Rob van Glabbeek, Peter
Höfner,  Marius Portmann, Wee Lum Tan.  T

To start the question, let me recap the key points in the words of Glabbeek
et al. (2013) 

Glabbeek et al. (2013) Comments:   AODV can lose route replies (
<http://www.ietf.org/mail-archive/web/manet/current/msg05702.html>
http://www.ietf.org/mail-archive/web/manet/current/msg05702.html). 

The reason is that route replies are only forwarded by an intermediate node
when the node updates its routing table.  This problem has partly been
solved in DYMO and LOADng by increasing sequence numbers when initiating a
route reply.  However, a careful analysis shows that replies can still be
lost; yielding again non-optimal routes or route discovery failures. Namely,
if one route reply overtakes another, i.e., a newer reply reaches a node
earlier than an older one, then the older route reply is dropped [4].  

 A solution consists of intermediate nodes always forwarding route replies.
Surely, forwarding (unicasting) replies increases the number of messages in
the network. But, (a) route discovery is more likely and (b) re-sending a
route request to establish a route would yield another broadcast  cycle,
which is much more expensive w.r.t. network load.

Charlie’s comments:  I guess you mean that the two RREPs under discussion
were going towards two distinct Originating Nodes.  Otherwise, it would be
good for the older RREP to be dropped.  For the case of two distinct
Originating Nodes, the behavior is a protocol error that needs to be fixed.
"Always forwarding" RREPs is one alternative, and certainly preferable to a
new Route Discovery cycle in the network.

I am confident that there is a better solution, and I will work on finding
and specifying it.

Sue’s Comments/questions  

1.      (Comment) Charlie’s issue about two RREP going to two distinct
Originating nodes, match the research I recall for the 802.11s
investigations into  reactive protocol. In real testing, two distinct
originating nodes with a fixed node connection inserted the subnet (or
network) route into the reactive protocol.   This is a much simpler case
than the default route insertion mentioned in my technical issue 3.   Of
course, these same two nodes (fixed/mobile), can a whole routing table from
two originating nodes.  

2.      Comment:  RREP to going to two originating nodes IMHO seems to be a
problem that is basic. Charlie says ADOV-25 has it fixed.  LoadNG always
sends the RREP back to the same originator. 

3.      Question: Has the WG provided many scenarios like those of Glabbeek
et al. (2013) to validate this works?  Are there timing issues? 

4.      Question: is there ever a connectivity problem with if the older
route being dropped? Can you provide the scenario? 

5.      What is the cost of intermediate nodes always forwarding route
replies? 

The topologies do matter in DV.  Have these algorithms been run multiple
topologies.

Again – if this has been discussed on list, or in presentations, or papers –
just let me know.  I’ll go look it up, and come back with better questions. 

Thank you again 

Sue Hares

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From: Abdussalam Baryun <[email-masked]>
Date: Sat, 19 Jan 2013 06:40:11 +0100

Hi Sue,

some answers and comments in line;

> http://www.ietf.org/mail-archive/web/manet/current/msg05702.html).

I checked the pointer but in AODVv2 is does not miss replies, as in
section 6.1 and it forwards messages not like msg05702 suggested.

> earlier than an older one, then the older route reply is dropped [4].

It is solved in DYMO but not sure with LOADng (still I will
investigate that), the seqNum is for freshing routes but the routers
don't update only if necessary,

> which is much more expensive w.r.t. network load.

not always forward replies, but only for the requesters that the
router has a route to, otherwise disregard.

> new Route Discovery cycle in the network.

The behavior is ok, please see my post below:
http://www.ietf.org/mail-archive/web/manet/current/msg14730.html

> I am confident that there is a better solution, and I will work on finding

Always there are better solutions for MANET

> sends the RREP back to the same originator.
Yes

> route being dropped? Can you provide the scenario?

don't think there is problem as long is reconnected to new route
> 5.      What is the cost of intermediate nodes always forwarding route

the problem is cost of RREQ, but RREP cost depend on number of demands
and of nodes in network.
> The topologies do matter in DV.  Have these algorithms been run multiple

should be done, but also with different subnets and technologies,

>
> Again – if this has been discussed on list, or in presentations, or papers

I am trying to update a draft [AB] on this issues, because I beleive
that subnet technologies and topology has influence on the algorithm,
I used your paper in my draft as well, but still searching for more to
update. The WG discussed this issue in 2012 but most just want to
consider dynamic topology.

> –

Yes I will send you my updates on these issues when available, please
do also, or if you have advise for my draft,
[AB] http://tools.ietf.org/html/draft-baryun-manet-technology-00

Best Regards

Abdussalam Baryun
University of Glamorgan, UK