Subject: [manet] marginal utility of retransmitting pkt after retry timeout

From: Jim Kaba <[email-masked]>
Date: Mon, 12 Aug 2002 13:34:23 -0400

All,
  Does anyone have any simulation results that show the expected
performance benefit (e.g. in % PDR increase) that can be gained when a Link
Layer notifies the higher layers of a transmission time-out  (e.g. when the
number of RTS retries have expired)???

  In other words, what performance gain should be expected when the routing
layer has the ability to "invalidate a route" based on feedback from the
link layer that a neighbor has moved (as inferred from the inability to
complete a single unicast transmission  -- not from some more extensive
"neighbor location" facility)???

  Also, what is the additional benefit of being able to retransmit the
packet that prompted the feedback to begin with?  A big gain would be
expected by invalidating the route, but how much more could be expected by
not dropping that first packet?

  I expect the answers are protocol, mobility, and load dependent...so if
you have any insight as to *why* your particular protocol behaves so well
or poorly under the circumstances, I'd appreciate hearing your thoughts.

  The issue here is that not all link layers and device drivers support
this kind of feedback, and those that do often don't indicate *which*
packet was dropped...

Jim

-- 
--------------------------------------------------------------
James T. Kaba                   
Sarnoff Corporation      Time flies like an arrow...
[email-masked]                ...Fruit flies like a banana!
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---

From: "Alvin C Valera Alvin" <[email-masked]>
Date: Tue, 13 Aug 2002 14:21:42 +0800

Hi Jim,

I performed a simple "test" using AODV where the link layer does not
provide any feedback to the network layer. I found out that there is a
significant drop (around 24%) in terms of packet delivery ratio when
AODV does not use link layer feedback. Note that the "test" scenario I
used was not that "difficult" (i.e., 10 CBR sources only, 50 nodes,
1500x300m network, speed is from 0-20 m/s, 0-pause time). I expect that
this drop would increase when the scenario is more difficult.

I think what you are referring to in your post is the "packet salvaging"
feature of on-demand routing protocols. This feature was first
introduced by DSR. Now, AODV also employs this feature. (I'm not sure if
this is official though. But if you look at the latest ns simulation
model of AODV, this feature is implemented.)

I tested DSR's packet salvaging, if it indeed offers improvement.
Without packet salvaging, the PDR drops by around 8%. This is also quite
significant considering that I used the "easy" 10-cbr source scenario.

Exactly how packet salvaging can be implemented in the real world is
another problem (esp. if the LL does not provide feedback as you've
mentioned). In the DSR draft, it suggests using a "retransmission
buffer". If a packet in this buffer times out (that is, it is not
successfully transmitted to a certain next hop node after so many
retries), it is salvaged by changing the remaining part of the source
route to some other source route if the node has an alternative route. 

Cheers!
Alvin

>>> Jim Kaba <[email-masked]> 08/13/02 01:45 AM >>>
All,
  Does anyone have any simulation results that show the expected
performance benefit (e.g. in % PDR increase) that can be gained when a
Link
Layer notifies the higher layers of a transmission time-out  (e.g. when
the
number of RTS retries have expired)???

  In other words, what performance gain should be expected when the
routing

  Also, what is the additional benefit of being able to retransmit the
packet that prompted the feedback to begin with?  A big gain would be
expected by invalidating the route, but how much more could be expected
by
not dropping that first packet?

  I expect the answers are protocol, mobility, and load dependent...so
if
you have any insight as to *why* your particular protocol behaves so
well
or poorly under the circumstances, I'd appreciate hearing your thoughts.

Jim

---

From: "Alvin C Valera Alvin" <[email-masked]>
Date: Tue, 13 Aug 2002 14:21:42 +0800

Hi Jim,

Cheers!
Alvin

All,

Jim

---

From: "Alvin C Valera Alvin" <[email-masked]>
Date: Wed, 14 Aug 2002 13:37:15 +0800

Hi Jim,

Apparently, AODV's packet salvaging is not actually used. It's in the
code (the local repair part) but it's excluded from the compilation
through some compilation directive. Moreover, the comment says that
there is no advantage in using that feature (yet). 

Probably, we can say that for AODV, packet salvaging offers no
advantage. (But I'll try turning that thing on to see how much we can
gain.)

I'll get back to you as soon as I have these results.

Regards,
Alvin

> Hi Alvin,
>
>  Thanks for running these simulations.
>
>  Is there any chance you could run two more runs just to complete the
experiment (since you have the simulation setup handy...)?
>
>  You've tested AODV to turn off link layer feedback entirely--can you
rerun the simulation with link-layer feedback turned on but with packet
salvaging turned off?
>
>  You've tested DSR with just packet salvaging turned off--now can you
rerun the simulation with link layer feedback turned off entirely?
>
>
> These extra runs would give us a more complete picture of the
performance
increase spectrum.

>  The "link layer guys" and driver writers are agreeable to provide
feedback through appropriate mechanisms when the data is available, but
they usually need evidence for a pretty significant performance gain to
agree to it.

---

From: "Alvin C Valera Alvin" <[email-masked]>
Date: Wed, 14 Aug 2002 13:37:15 +0800

Hi Jim,

Regards,
Alvin

performance
increase spectrum.