What version are you running?
If you want to abort a download, I asume you want to stop and remove it completly? In that case you might be better of to purg the corresponding download pane.
urn:sha1 contains a checksum of a file. Every file has a unique checksum. On gnutella the SHA1 algortime is used.
urn stands for universal resource name. Which means that we are looking for something unique.
So actually
urn:sha1:879871231232 means:
Look up a unique resource by its sha1 which is 879871231232

A download which has multiple sources must each have the same urn:sha1, otherwise they would indicate different downloads for different files.

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