[bittorrent] Piece download strategy

Alan McGovern alan.mcgovern at gmail.com
Thu Apr 23 04:13:30 EDT 2009


Some clients prefer to download the first piece (pieces?) and last piece at
the beginning as they often contain useful metadata for apps using those
files. For example id3 tags are at the end of an mp3, so downloading the
last piece first has advantages.

A fully sequential downloader negates the benefits that rarest first brings,
but a hybrid mode *might* be useful. i.e. set a section of the file as
'really important' and favor those, then the next section is medium priority
and finally everything else is low priority.


<------video data--------| High Priority | medium Priority | -------video
data ---->

So if you wanted to stream the video over bittorrent, you'd keep the high
priority area in sync with where the media player is going to play next and
then you can do a psuedo rarest first to ensure you get the high/medium
priority data so you don't end up stalling playback.

Obviously this isn't as good for the swarm as a whole, but it's an
interesting alternative strategy which fills a niche.

Alan.

On Thu, Apr 23, 2009 at 5:31 AM, Rayson Ho <raysonlogin at gmail.com> wrote:

> (I've read the FAQ, and watched Bram's talk online [1])
>
> I know that the download strategy used by BT is usually rarest first.
> However, this means that there usually are many "holes" in a file when
> downloading.
>
> It's not a problem if the torrent is well seeded. However if the
> availability of seeds is not stable, or if there is only 1 seed, then
> I think using the sequential order can give the user quite a lot of
> benefits -- if it is a music or movie file, then the user can at least
> view the beginning of it, and if it is a source tarball, then at least
> some part of the directory can be extracted...
>
> Any one has worked on this before?? I am not replacing the rarest
> first strategy, but if the availability is same, then I want to bias
> towards sequential order (or even biggest consecutive piece first --
> try to construction a bigger piece).
>
> Thanks,
> Rayson
>
> [1]:
> http://www.stanford.edu/class/ee380/Abstracts/050216.html
> http://stanford-online.stanford.edu/courses/ee380/050216-ee380-100.asx
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