[e2e] end2end-interest Digest, Vol 18, Issue 9

Detlef Bosau detlef.bosau at web.de
Sat Aug 20 08:57:18 PDT 2005

"S. Keshav" wrote:
> RTT delay is influenced by the following factors:
> 1. Speed of light delay in the path
> 2. Retransmissions in the underlay
> 3. Queues in buffers due to
>     a. self queueing (queueing behind your own packets)
>     b. queueing due to cross traffic
> 4. The service rate of  within a switch fabric in a router
> 5. The size of the packet whose RTT is measured
> Variance in the RTT can be due to variation in any of the above.
> So, if you want to measure contention, you have to do some things cleverly
> at the sender:
>     keep packet size fixed
>     send at a `slow' rate
> and also assume that
>     paths are pinned
>     there are no retransmissions in the underlay
> If these hold, then you can link RTT variation to contention.
> keshav

Just to see, whether I understood you correctly.

The packet size is fixed => serialization delay is constand and
hopefully (nearly) the service times.
No retransmissions and pinned paths are clear.
Slow rate => There is no self queueing, any queuing is due to cross

In other terms: You make sure that any RTT variation is only due to
cross traffic. Right?

Now, even the "low rate" requires explicit knowledge of the network and
can hardly achieved along an unknown path.

In addition, "cross traffic" may not be "cross traffic" but in fact
_traffic_. On the street. Thinks like cars, motorcycles. As traffic
buildings etc., this influences the properties of a wireless channel. 

At least in a mobile wireless network, this is the reason why error
recovery in the underlay is inevitable.

So, I presume you basically agree that using RTT variation as a
universal means for contention estimation is at least questionable.
Is this correct?

IIRC, the paper from Lindemann´s group does not mention mobility.
However, I don´t remember a paper or talk, where ad hoc net users
are supposed to stay in quiet and motionless medidation. Anybody is
interesed in mobile ad hoc networks today.

So, I would like to sharpen my question a bit:

Can this approach be made to work with reasonable effort? Or should it
be abandoned, beause it is not really promising?

This is a hard question, I know. But for horse´s and rider´s benefit
still the old saying holds true:
"If you discover that you´re riding a dead horse, dismount."


Detlef Bosau
Galileistrasse 30
70565 Stuttgart
Mail: detlef.bosau at web.de
Web: http://www.detlef-bosau.de
Mobile: +49 172 681 9937

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