[p2p-hackers] Error correcting codes to prevent failure in BitTorrent like systems?

Nick Johnson arachnid at notdot.net
Mon Dec 5 03:45:54 UTC 2005


coderman wrote:

>On 12/4/05, Nick Johnson <arachnid at notdot.net> wrote:
>  
>
>>Systems like BitTorrent have a rather annoying failure mode - the last
>>'seed' goes offline while there are still several 'peers' (without the
>>complete file) online.
>>    
>>
>
>any system which makes a partial resource available has this problem,
>even one using error correcting codes (seed goes offline before
>requisite X of coded blocks are sent).
>  
>
But statistically, if n different peers each have random subsets of the 
data, the chances of them having 90% of the file between them are much, 
much higher than the chances of them having 100%.

>>However, what if [.. doing stuff to mix data ..], you
>>can reduce the chance of the last departing seed ensuring nobody can
>>complete the file.
>>    
>>
>
>no; you've just made it less likely that the end of the file will
>always be the part missing if a peer terminates distribution
>prematurely.
>  
>
BitTorrent distributes chunks semi-randomly, not sequentially, so you're 
no more likely to have the beginning than the end of the file.

-Nick Johnson




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