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Partial Downloads

A multi-file torrent can be partially downloaded and then seeded.  There are minor protocol extensions to help other peers be aware of the local peer's partial-complete status.

When a torrent has downloaded all the files the user has selected, but not all pieces are complete, it can indicate this state using the upload_only extension message, which is established in the extended handshake.  This will allow other seeds to not bother staying connected, since there will be no interest or block requests.

Unless padding files are used in a torrent, there will be pieces that are split between more than one file.  If any of those files are not selected for download, the data will need to be saved into implementation-specific "edge" files that are kept in a special directory, so that whole pieces still can be constructed, and the selected files are fully available for seeding.