[bittorrent] Why are blocks smaller than pieces?

Guido Seifert tanuki64 at gmx.de
Sat Sep 8 17:38:41 EDT 2007


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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