[bittorrent] Why are blocks smaller than pieces?

Nikitas Liogkas nikitas at cs.ucla.edu
Sat Sep 8 18:07:53 EDT 2007

You may want to have a look at the following research papers which 
exactly discuss the problem you mention:

Bottom line, it's not as big of a problem as it first appears to be.


Guido Seifert wrote:
> Hi,
> sorry if this has been answered 100 times. In the archive I could not find 
> something about my problem. I am currently working to understand the bittorrent 
> protocol. When I learned that the smallest fraction of data is not actually a 
> piece, but a block and there can be several blocks in a piece, I wondered if 
> this isn't a weakness, which can be exploited by malignant clients.
> I know cases from emule, where such clients on purpose sent corrupt pieces. Since 
> in emule every single pieces can be hashed, it is no problem to identify those 
> clients and block them. 
> In bittorrent I cannot hash a single block. A client, which sends corrupt blocks 
> cannot be identified without tremendous efforts. If I wanted to disrupt someones 
> download I'd modify a client that for every piece it sends one corrupt block.
> Are there provisions to protect onself against such a denial of service?
> Guido
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