[B.A.T.M.A.N.] OGM overhead and routing metric proposal

[B.A.T.M.A.N.] OGM overhead and routing metric proposal

[Daniele Furlan]

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Hi all,
as promised, I have completed the analysis of the protocol overhead
providing also simulation results with and whitout the OGM aggregation
mechanism activated. As anticipated the results shows that aggregation
works well in reducing the overhead. In attachment the report
(OGMoverhead.pdf).

In attachment you will also find a draft/proposals of a new routing
approach. It is based on the collection of more link parameters (the
current TQ, bit-rate and node-load) to write in OGM at each hop. This
permit to develop a more thorough metric and can open new room of
improvement such multi-path routing.

Any comment/suggestion is welcome!

Regards.

-- 
Daniele Furlan

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Re: [B.A.T.M.A.N.] OGM overhead and routing metric proposal

[Andrew Lunn]

> In attachment you will also find a draft/proposals of a new routing
> approach. It is based on the collection of more link parameters (the
> current TQ, bit-rate and node-load) to write in OGM at each hop. This
> permit to develop a more thorough metric and can open new room of
> improvement such multi-path routing.

Hi Daniele

Do you have any suggestions how to collect this bit-rate information?
Do you suggest measuring it, or asking the wireless LAN driver?  Is
there an API in the kernel to allow access to this information?

Also, how do you see this interacting with the hidden node problem? In
order to reduce the hidden node problem, you ideally want to use the
lowest coding rate and transmit power possible to get your packets
through. Currently this is not being done. Current wireless systems
optimize for maximum bandwidth between two nodes, which may not be the
optimum for the complete mesh bandwidth, due to inter node
interference.

Will you be at the WBMv4 next Month?

     Andrew

Re: [B.A.T.M.A.N.] OGM overhead and routing metric proposal

[Daniele Furlan]

2011/2/28 Andrew Lunn <andrew@lunn.ch>:
>> In attachment you will also find a draft/proposals of a new routing
>> approach. It is based on the collection of more link parameters (the
>> current TQ, bit-rate and node-load) to write in OGM at each hop. This
>> permit to develop a more thorough metric and can open new room of
>> improvement such multi-path routing.
>
> Hi Daniele
>
> Do you have any suggestions how to collect this bit-rate information?
> Do you suggest measuring it, or asking the wireless LAN driver?  Is
> there an API in the kernel to allow access to this information?
>
I'm not a kernel guru but i find out that the new core mac80211 and
cfg80211 (http://linuxwireless.org/) offers the possibility to obtain
a per-station bit-rate information that should be driver independent.
The measurement is an alternative but I think it is more complex to
implement. The load measurement instead is more difficult and at this
moment have not idea of how to do it!!

> Also, how do you see this interacting with the hidden node problem? In
> order to reduce the hidden node problem, you ideally want to use the
> lowest coding rate and transmit power possible to get your packets
> through. Currently this is not being done. Current wireless systems
> optimize for maximum bandwidth between two nodes, which may not be the
> optimum for the complete mesh bandwidth, due to inter node
> interference.
>
I have not mentioned the hidden node problem. But I think this problem
is very difficult to remove, the only best practice to reduce the
problem is to force the RTS/CTS mechanism to be active. Other
phenomena such as capture effect instead are intrinsic in the wireless
networking and are very difficult to eliminate. The optimization of
bandwidth in my opinion does not influence the protocol because if the
link is bad, rate adaption mechanism will reduce the bit-rate and I
can read this information from the driver.

Anyhow collecting more information about link can help much in
evaluating the best route I think, and offer a big opportunity to
experiment new metrics.

> Will you be at the WBMv4 next Month?
>
I'm afraid but I cannot be at WMBv4, I hope I can come to the next
WBM, maybe with a working implementation :)

>     Andrew
>



-- 
Daniele Furlan

Re: [B.A.T.M.A.N.] OGM overhead and routing metric proposal

[Andrew Lunn]

> I'm not a kernel guru but i find out that the new core mac80211 and
> cfg80211 (http://linuxwireless.org/) offers the possibility to obtain
> a per-station bit-rate information that should be driver independent.

O.K. I will take a look at this.

> I have not mentioned the hidden node problem. But I think this problem
> is very difficult to remove, the only best practice to reduce the
> problem is to force the RTS/CTS mechanism to be active.

RTS/CTS helps, true. But it has a few problems. e.g. most meshing
protocols use broadcast packets for there management. e.g. BATMANs
OGMs are broadcast. These cannot be protected with RTS/CTS. So the
OGMs can collide with RTS packets from a hidden node, or OGMs from a
hidden node.

Another way to help reduce the hidden node problem, or interference
with other nodes, is to use a low coding rate and transmit power when
possible. So for example when we don't have much traffic to send to
node X, it could send the packets with the lowest coding rate and low
power. This keeps the interference with other nodes to a minimum. As
the amount of data for X increases, the coding rate and transmit power
can be increased, so increasing the available bandwidth to X, but at
the same time increasing the amount of interference the traffic
produces.

Such a scheme to minimize interference does not play too well with
your idea of putting the coding rate into the OGMs. We would have to
ensure that the coding rate is the highest possible coding rate usable
between two peers, not the currently used coding rate, which could be
low in order to avoid interference.

> > Will you be at the WBMv4 next Month?
> >
> I'm afraid but I cannot be at WMBv4, I hope I can come to the next
> WBM, maybe with a working implementation :)

Shame, i plan to talk more about the hidden node problem during WBMv4.
It is also a good place to discuss new ideas like adding information
into the OGMs.

     Andrew

Re: [B.A.T.M.A.N.] OGM overhead and routing metric proposal

[Daniele Furlan]

2011/2/28 Andrew Lunn <andrew@lunn.ch>:
>> I'm not a kernel guru but i find out that the new core mac80211 and
>> cfg80211 (http://linuxwireless.org/) offers the possibility to obtain
>> a per-station bit-rate information that should be driver independent.
>
> O.K. I will take a look at this.
>
>> I have not mentioned the hidden node problem. But I think this problem
>> is very difficult to remove, the only best practice to reduce the
>> problem is to force the RTS/CTS mechanism to be active.
>
> RTS/CTS helps, true. But it has a few problems. e.g. most meshing
> protocols use broadcast packets for there management. e.g. BATMANs
> OGMs are broadcast. These cannot be protected with RTS/CTS. So the
> OGMs can collide with RTS packets from a hidden node, or OGMs from a
> hidden node.
>
In my opinion the use of broadcast is somehow useful. If many OGM
packet collide it means that the link is congested and batman will use
another.
It would be wrong to protect OGM from collision or hidden node.

> Another way to help reduce the hidden node problem, or interference
> with other nodes, is to use a low coding rate and transmit power when
> possible. So for example when we don't have much traffic to send to
> node X, it could send the packets with the lowest coding rate and low
> power. This keeps the interference with other nodes to a minimum. As
> the amount of data for X increases, the coding rate and transmit power
> can be increased, so increasing the available bandwidth to X, but at
> the same time increasing the amount of interference the traffic
> produces.
>
Yes, I think that it is always better to use low transmission power
and favor multi-hop so short link, especially in dense network. The
schema I propose effectively favor short link offering the possibility
of reducing transmission power.
Anyhow a transmission power management system is out of the scope of
my work, but it is a very interesting area. It is certainly possible
to add transmission power level to OGM in order to use also this
information in routing decision. More information we now of link and
more accurate will be the routing decisions.

> Such a scheme to minimize interference does not play too well with
> your idea of putting the coding rate into the OGMs. We would have to
> ensure that the coding rate is the highest possible coding rate usable
> between two peers, not the currently used coding rate, which could be
> low in order to avoid interference.
>
The current rate is decided by the driver basing on link quality and
collision so indirectly tells us how much interference there are in
the link. Why it is a bad idea to use this information?

>> > Will you be at the WBMv4 next Month?
>> >
>> I'm afraid but I cannot be at WMBv4, I hope I can come to the next
>> WBM, maybe with a working implementation :)
>
> Shame, i plan to talk more about the hidden node problem during WBMv4.
> It is also a good place to discuss new ideas like adding information
> into the OGMs.
>
>     Andrew
>



-- 
Daniele Furlan

Re: [B.A.T.M.A.N.] OGM overhead and routing metric proposal

[Linus Lüssing]

On Mon, Feb 28, 2011 at 05:00:23PM +0100, Daniele Furlan wrote:
> 2011/2/28 Andrew Lunn <andrew@lunn.ch>:
> >> I'm not a kernel guru but i find out that the new core mac80211 and
> >> cfg80211 (http://linuxwireless.org/) offers the possibility to obtain
> >> a per-station bit-rate information that should be driver independent.
> >
> > O.K. I will take a look at this.
> >
> >> I have not mentioned the hidden node problem. But I think this problem
> >> is very difficult to remove, the only best practice to reduce the
> >> problem is to force the RTS/CTS mechanism to be active.
> >
> > RTS/CTS helps, true. But it has a few problems. e.g. most meshing
> > protocols use broadcast packets for there management. e.g. BATMANs
> > OGMs are broadcast. These cannot be protected with RTS/CTS. So the
> > OGMs can collide with RTS packets from a hidden node, or OGMs from a
> > hidden node.
> >
> In my opinion the use of broadcast is somehow useful. If many OGM
> packet collide it means that the link is congested and batman will use
> another.
And what if the other link is not really useful for e.g. TCP?
batman would switch to that link then anyway. At least that's what
I had been experiencing in some indoor scenarios here. The hidden
node can lead to degrading the measured LQ values from ~90% down to
< 10% packet delivery ratio.
> It would be wrong to protect OGM from collision or hidden node.
So I'd not quite agree with that.

But I'm actually also a little confused why you, Andrew, came up
with the hidden node problem :) (which these papers did not try to
solve, if I'm not missing something). Or do you mean, that this
idea could be extended maybe be able to solve that hidden node
problem for OGMs, using the bitrate measuremens to detect / avoid
switching to bad links?

> 
> > Another way to help reduce the hidden node problem, or interference
> > with other nodes, is to use a low coding rate and transmit power when
> > possible. So for example when we don't have much traffic to send to
> > node X, it could send the packets with the lowest coding rate and low
> > power. This keeps the interference with other nodes to a minimum. As
> > the amount of data for X increases, the coding rate and transmit power
> > can be increased, so increasing the available bandwidth to X, but at
> > the same time increasing the amount of interference the traffic
> > produces.
> >
> Yes, I think that it is always better to use low transmission power
> and favor multi-hop so short link, especially in dense network. The
> schema I propose effectively favor short link offering the possibility
> of reducing transmission power.
> Anyhow a transmission power management system is out of the scope of
> my work, but it is a very interesting area. It is certainly possible
> to add transmission power level to OGM in order to use also this
> information in routing decision. More information we now of link and
> more accurate will be the routing decisions.
> 
> > Such a scheme to minimize interference does not play too well with
> > your idea of putting the coding rate into the OGMs. We would have to
> > ensure that the coding rate is the highest possible coding rate usable
> > between two peers, not the currently used coding rate, which could be
> > low in order to avoid interference.
> >
> The current rate is decided by the driver basing on link quality and
> collision so indirectly tells us how much interference there are in
> the link. Why it is a bad idea to use this information?
Hmm, I guess that depends on the interference pattern a lot.
Afaik minstrel for instance is one of the rate selection
algorithms which can chose higher enconding rates to reduce
interference in case of bursty loss. So when your rate selection
algorithm selects 54MBit/s, that does not necessarily mean that
you have a higher net throughput / that the link is "better"
compared to another link where the rate selection algorithm has chosen
for instance 36MBit/s. So to make the selected rate information
somehow useful, you'd have to take the loss rates at that
enconding rate into account too.

> 
> >> > Will you be at the WBMv4 next Month?
> >> >
> >> I'm afraid but I cannot be at WMBv4, I hope I can come to the next
> >> WBM, maybe with a working implementation :)
Hmm, what a pitty. But, ok, see you next time then :).

> >
> > Shame, i plan to talk more about the hidden node problem during WBMv4.
> > It is also a good place to discuss new ideas like adding information
> > into the OGMs.
> >
> >     Andrew
> >

Nice papers there, thanks for sharing! Just looked at them briefly
so far but seems very interesting.

Cheers, Linus
> 
> 
> 
> -- 
> Daniele Furlan
> 

PS: Just one thing I noticed so far, isn't it possible to simplify
the formula B = 1 + \sum^{N\\{0\}}_{k \in N} p^{l_{o,k}} to just:
B = \sum_{k \in N} p^{l_{o,k}}? p^{l_{o,k}} is already 1 for k = o
isn't it?

Re: [B.A.T.M.A.N.] OGM overhead and routing metric proposal

[Andrew Lunn]

> But I'm actually also a little confused why you, Andrew, came up
> with the hidden node problem :) (which these papers did not try to
> solve, if I'm not missing something). Or do you mean, that this
> idea could be extended maybe be able to solve that hidden node
> problem for OGMs, using the bitrate measuremens to detect / avoid
> switching to bad links?

Hi Linus

If we can extend minstrel so that it also takes transmit power and
load into consideration, we might be able to optimize the coding rate
& TX power for complete mesh bandwidth, not a single link bandwidth.

I would expect there are a number of research papers looking at this,
and many ideas which can be taken from cellular networks. I also think
the commercial mesh products do this. 

However, for Daniele idea of putting the coding rate into the OGM, we
probably need the achievable coding rate, not the current coding rate.
It could be the link is idle, so is using the lowest coding rate and
minimum TX power, just to carrier a few ARP reply packets etc. If
however the link becomes loaded, it would increase the coding rate and
TX power so allowing more packets through.

Daniele's idea is good, but i'm just being careful it does not block
ideas like dynamic coding rate/TX power control.

      Andrew

Re: [B.A.T.M.A.N.] OGM overhead and routing metric proposal

[Daniele Furlan]

2011/3/1 Andrew Lunn <andrew@lunn.ch>:
>> But I'm actually also a little confused why you, Andrew, came up
>> with the hidden node problem :) (which these papers did not try to
>> solve, if I'm not missing something). Or do you mean, that this
>> idea could be extended maybe be able to solve that hidden node
>> problem for OGMs, using the bitrate measuremens to detect / avoid
>> switching to bad links?
Actually I did not mentioned the hidden node problem in the report.
But the idea of extending the information contained in OGM can help
also in implementing some technique to solve this problem. Maybe the
bit-rate is not enough, but from the driver I think can be also
obtained, for example, tx-retries and tx-failed.
>
> Hi Linus
>
> If we can extend minstrel so that it also takes transmit power and
> load into consideration, we might be able to optimize the coding rate
> & TX power for complete mesh bandwidth, not a single link bandwidth.
>
> I would expect there are a number of research papers looking at this,
> and many ideas which can be taken from cellular networks. I also think
> the commercial mesh products do this.
>
> However, for Daniele idea of putting the coding rate into the OGM, we
> probably need the achievable coding rate, not the current coding rate.
> It could be the link is idle, so is using the lowest coding rate and
> minimum TX power, just to carrier a few ARP reply packets etc. If
> however the link becomes loaded, it would increase the coding rate and
> TX power so allowing more packets through.
>
> Daniele's idea is good, but i'm just being careful it does not block
> ideas like dynamic coding rate/TX power control.
>
>      Andrew
>
The idea I have for the moment is only to increase the information
that OGM can carry. Than in a second phase it is possible to use these
information to implement whatever solution.

Another important improvement that a solution with the entire path
registered in OGM is the possibility of preventing routing loops in a
simple way, and also to provide a formal proof that batman is
loop-free.

-- 
Daniele Furlan