[bittorrent] structure of multifile bittorrent file

Song Zhang songzhang2008 at gmail.com
Sat Apr 10 01:49:27 EDT 2010


I wonder if I change the torrent file format for (a+b) files with the
modification of btmakemetafile.py. The final piece in file a does not
overlap with file b which means that file a hash and file b hash have clear
boundary in torrent file. Can I still download smoothly with current
bittorrent program ?

thanks a lot !

2010/4/9 Alan McGovern <alan.mcgovern at gmail.com>

> In the bittorrent protocol you treat all the files in your torrent as
> if they were one continuous file. This single continous file is split
> into pieces (in your case of size 256kb) and hashed. The final piece
> will be <= 256kB. It has no special handling except for the fact that
> it is not guaranteed to be exactly 'PieceLength' bytes in size as all
> other pieces are. In all other ways, the last piece is identical to
> every other piece.
>
> Blocks are always 16kB (as per defacto standard) except for the very
> last block which will be <= 16kB. As before, this has no special
> handling except for the fact its size may be less than 16kB whereas
> all other blocks are guaranteed to be 16kB in length. In all other
> ways it is identical to other blocks
>
> Hope that helps.
>
> Alan.
>
> On Fri, Apr 9, 2010 at 10:51 PM, Song Zhang <songzhang2008 at gmail.com>
> wrote:
> > Hi,
> > I have a question about the structure of multifile torrent file
> > for example, I have two files a and b, and put them into one directory. I
> am
> > trying to create one torrent file including the contents of these two
> files.
> > I am wondering how the system handling the last piece less than 256k for
> > each file of a and b,
> > size(a) = n * 256k + a1
> > size(b) = m * 256k + b1
> > a1 and b1 above all less than 256k
> > the question is:
> > 1. when generating torrent files, for the ”pieces“ part of info
> dictionary
> > of torrent file, there should be two SHA1 value from pieces less than
> 256k,
> > right ?  Although the SHA1 value of each piece (no matter greater or less
> > than 256k) is always 20bytes.
> > 2. but I don't understand during the data transfer period, the pieces are
> > divided into blocks and how to handle these pieces less than 256k ?
> > thanks a lot for the help !!
> > _______________________________________________
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> > BitTorrent at lists.ibiblio.org
> > http://lists.ibiblio.org/mailman/listinfo/bittorrent
> >
> >
>
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