[BitTorrent] Re: Request for protocol extension: get_info/info

Justin Cormack justin at street-vision.com
Wed Jan 19 17:01:02 EST 2005

> Justin Cormack wrote:
> >  
> > 
> >>The scheme as described by Bram still leaves file names and an info-hash
> >>value per file in the torrent file.  This is good for compatibility with
> >>BT1, but I'd prefer for info-blobs in BT2 to be a very small.  Twenty
> >>bytes would be a good size (just the SHA1 of the root).  Everything else
> >>could be embedded in the piece data (file length, file names, an index
> >>to the files in case you just want part of the data).  There could be an
> >>agreement that piece info hash are computed for every 1K bytes, so piece
> >>size is gone.  You wouldn't waste bandwidth sending SHA1 data down to 1K
> >>byte leaves, because data is sent in bigger gulps than that.
> >>
> >>The two things the torrent file needs is a tracker url (8 bytes) and an
> >>info blob (20 bytes).  Then, we could share torrents trivially.
> > 
> > 
> > I would be inclined to get rid of the multiple file thing completely, it
> > adds complexity without adding much power. You can either download multiple
> > torrents to get seperate files, or use a filesystem image if you want real
> > files, or a zip/tar or whatever. These allow you to store real file attributes
> > and so on.
> The disadvantage is that you have to unpack them. ;-)
> Users aren't likely to keep both the unpacked and the packed form and 
> are likely to delete the packed form.
> The disadvantage of multiple torrents is tracker load.
> What exactly is the complication of multiple files?

Well the complication is that they are overlaid rather simply on what is
basically a block rather than file transfer format (eg file starts are not 
aligned with requests which is really unpleasant, as a piece write may have
to write to large numbers of files). And as stated above it means more
information to transfer in the torrent file.

The fact that people might not keep the packed form is something I hadnt
thought about. This is not the case if you use loopback file system images
(so long as they dont need to be compressed too) - I quite like the way
MacOS X uses ISO images and other loop fs images all over the place. Of
course I have no idea if Windows supports them well, and Linux doesnt
make it convenient for non root users (though this should be fixed really).

Not sure how much tracker overhead is, really not sure how many files
the average torrent is, I think the number is generally small.


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