Tit for tat is difficult was Re: Distributed file system using routing inspired by Freenet

On Thursday 10 April 2008 19:14, Michael Rogers wrote:
> Colin Davis wrote:
> > It seems like some sort of "in network" solution, such as a variation of 
> > a tit-for-tat measurement.. Inserting file A with 100X redundancy is 
> > approximately the same as inserting file B which is 100X the size.. If 
> > we enforce fairness on accepting traffic unless they've "earned" it, a 
> > user can then decide how to "spend" that bandwidth.. One mostly lossy 
> > 10M file, or 10X redundancy on your 1M file.
> 
> In principle I think this is a great idea. In practice I've spent a lot 
> of time working on tit-for-tat-ish incentive mechanisms for multi-hop 
> networks, without much success. That probably just means I should find 
> another line of work, but it might also mean the problem is harder than 
> it looks.

This is interesting. We will eventually need some form of tit for tat, won't 
we? Not necessarily in inserts, IIRC we talked about it as a way to prevent 
an attacker flooding opennet with spam requests/inserts? It's something we've 
talked about for a long time anyway...
> 
> In a single-hop network such as BitTorrent, the value provided by a 
> neighbouring node is directly related to how much it spends on you: if 
> it spends 1MB of bandwidth uploading to you, you receive 1MB of data (or 
> some fixed fraction of 1MB, allowing for overhead), all of which is 
> directly useful to you. That makes it easy to design strategies that 
> reward cooperative neighbours and punish uncooperative neighbours. (It 
> turns out that TFT isn't actually a very good strategy in this context, 
> but the point is that good strategies can be found.)
> 
> But in a multi-hop network the relationship between cost and benefit is 
> more complicated: assuming all nodes allocate bandwidth to cooperative 
> neighbours, if you receive a request from neighbour A, should you 
> forward it to neighbour B? First, will you get a response or will you 
> spend the bandwidth and have nothing to show for it? Second, if you get 
> a response and return it to A, will the cooperation you earn from A be 
> worth more than the cooperation previously earned from B and spent on 
> A's request?
> 
> I've been banging my head against this problem for a while, and I can't 
> come up with a model where it makes sense for selfish nodes to forward 
> requests. It makes sense to answer requests locally if you can, to earn 
> cooperation from your neighbours, but it doesn't make sense to forward 
> them. Unfortunately if everyone behaves like that, the network doesn't 
> function.
> 
> Doubtless someone else can solve this problem, but at this stage I'm 
> just hoping they won't solve it before my thesis is written up. ;-)
> 
> Cheers,
> Michael

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