CODEWITHSUNDEEP
linuxbittorrent
newbielife
newbielife

Reputation: 3

How does the bittorrent assemble the missing pieces?

I use BitTorrent and sometimes encounter files that do not have seed(missing pieces).
At that time, we sometimes force the file transfer to end and try to open the incompleted files (for example, an image file).
If we are lucky, may be able to see the downloaded image even if some parts are lost.

I would like to artificially reproduce this situation, and here's how I tried:
1) spliting a bmp image file of about 1 megabyte into 16 kilobytes by the Linux split command,
2) and then make just one of the divided files 0 kilobytes.
3) after that, rejoin all the files with the cat command.

However, in this case, unlike the torrent's "lost pieces" situation, the file becomes completely corrupt and can not be read.
Theoretically it does not seem like anything special, but what's wrong? And how can I achieve what I want?
I would appreciate your help.

Upvotes: 0

Views: 158

Answers (1)

rodrigo
rodrigo

Reputation: 98496

Use dd:

dd if=/dev/zero of=image.jpg bs=1 conv=notrunc seek=X count=Y

being X the offset in the file you want to erase and Y the number of bytes.

About the corruption, it depends on the type of file, the piece you are losing and the program you are using to read it.

For instance, JPG files use a variable bit-length encoding, meaning that just losing one bit may corrupt all the file from that point on. But just for that, there can be resyncronization points where the bitstream is reset, so from that point on, the file will look ok. But those resync points are optional when writing the file, and not every reader honor them in case of corruption...

And anyway, losing part of the headers will make the file totally unreadable.

Upvotes: 2

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