[Olsr-dev] Is there olsrd with ETT implementation

[Erik Tromp]

In fact, for ETT (Expected Transmission Time) you need the medium speed
(mbit/s). Then you can use the formula:

 ETT = ETX * #bits / bitspeed

RTT may be an indication of bit speed but I'm not sure if that metric is
very suitable, especially for quite asymmetric links (unlink speed different
from downlink speed).

At Thales we developed a plugin that measures, once every so many minutes,
the speed of the available network interfaces. This is done using a variant
of the well-known packet-pair technique. It is purely layer-3, so no
interfacing with layer-2 metrics. That plugin determines the speed of a
network interface by measuring the bitspeed with the best neighbor.
Unfortunately, at this moment I do not have permission to publish this
plugin.

Of course there are many other ways to measure the speed. And, in absence of
any automatic measuring method, one could also fall back to setting the
speed values manually to a fixed value.

In all cases, whichever speed measurement you implement, the ETT extension
that we implemented offers a way (you may call it "interface") for the
measurement plugin to tell the extension what speed is experienced on each
available network interface. Currently the "interface" with ETT is a bit
limited since it will only accept a speed value per network interface, not
per neighbor (per link). This is because our current measurement
(packet-pair) plugin can only give per-interface data. However, it would not
difficult to to enhance the ETT extension so that it can deal with separate
bit speed measurements for each neighbor (link) that can be reached on a
given network interface (in the same way that ETX values are passed per
neighbor/per link).

An alternative to measuring (via RTT or packet-pair methods) would be to
write a plugin that retrieves layer 2 information, as you propose. An
existing implementation might be found the XIAN framework; see
http://xian.sourceforge.net/ .

If you have an example of source code that can read out layer-2 metrics I
would be very interested !

Regards,
Erik


-----Oorspronkelijk bericht-----
Van: (spam-protected)
[mailto:(spam-protected)] Namens Henning Rogge
Verzonden: donderdag 24 januari 2008 7:29
Aan: (spam-protected)
Onderwerp: Re: [Olsr-dev] Is there olsrd with ETT implementation

Am Mittwoch 23 Januar 2008 21:22:21 schrieb Erik Tromp:
> Hi,
>
> At Thales Communications, Huizen (NL), we developed an ETT extension 
> of OLSR. Just a few weeks ago I received the permission to publish 
> this extension under the GPL license conditions.
Olsrd is licenced under a BSD like licence... so even if you give the code
back to olsrd, your employer can always grab new versions and make
commercial products from it.

> Unfortunately, I haven't had enough time until now to compile a 
> suitable set of patches to be submitted to olsr.org . The ETT 
> extension is implemented on OLSRd 0.4.10, and I would also like to 
> build a patch set for the versions up to the most recent (0.5.5) .
Sounds very interesting...

how did you calculate the RTT value for a link ? Did you use Layer-2
informations (Radiotap header with LibPcap for example) ?

Henning

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