Re: [manet] AODVv2 MetricType TLV discussion

As Henning says on most points.

It's worth, I think, commenting on OLSRv2, not because I'm saying you should do it the same way, but because it may be worth understanding why we did it the way we did, so you can see whether similar considerations apply, or not (some points, clearly not).

We also have a special case zero, not for hop count (because we did not want to encourage hop count, rather the reverse) but for administratively configured.

We have all metric types to be handled the same way, so the OLSRv2 code itself is metric agnostic, it just needs to be told which number to use. As Henning notes, we have the advantage of only handling link metrics. The compressed form we have satisfied people's requirements for low bandwidth while still having a 24 bit dynamic range (24 as then path metrics fit is 32 bits). That we have four kinds of metric made it all fit very conveniently for us.

We do have a metric type TLV. That arose for two reasons. First, we were putting metrics into NHDP HELLO messages, and we weren't allowed (by ourselves as much as anyone, but Justin was one person driving this) to assume that we were the only user of the HELLO message, so we needed a "this is an OLSRv2-aware HELLO message" anyway, and adding the metric type to that added one byte. Second, we were planning ahead to multitopology (Stan being keen on this) and while we hadn't designed MT yet, that gave us something we would later need.

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Christopher Dearlove
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-----Original Message-----
From: manet [mailto:manet-bounces@ietf.org] On Behalf Of Henning Rogge
Sent: 07 March 2015 10:54
To: Charlie Perkins
Cc: MANET IETF
Subject: Re: [manet] AODVv2 MetricType TLV discussion

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On Sat, Mar 7, 2015 at 11:40 AM, Charlie Perkins <charles.perkins@earthlink.net> wrote:
> Hello folks,
>
> Sounds like a good idea to me.  I'll put it on the list of things to 
> discuss at the IETF meeting, but if anyone has objections please don't 
> wait for the meeting ...

This mechanism (use extension type for metric selection) is already the same thing we are doing with OLSRv2.

It makes things easier (no need for the "metric type" message TLV) and more compact for AODVv2... and it might also help to convert AODVv2 into a multitopology protocol later (similar what happened with OLSRv2).

Now, if all AODVv2 metrics are "additive", you should be able to drop the "AODVv2 needs to know the type of metric" requirement.

If you just define your metric TLV as an "1-4 bytes long integer" with a dimensionless cost you can easily put hopcount, ETX, signal strength or anything else there without loosing efficiency. You cannot reuse the NHDP/OLSRv2 metric TLV because you have to encode path-cost, not link cost... and exponential encoding doesn't work well for path-costs.

Henning Rogge

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