Microsoft readies BitTorrent alternative

Researchers at Microsoft Corp.'s labs in Cambridge, England, are developing a file-sharing technology that they say could make it easier to distribute big files such as films, television programs and software applications to end users over the Internet.

Code-named Avalanche, the technology is similar to existing peer-to-peer (P-to-P) file swapping systems such as BitTorrent, in the sense that large files can be divided into many smaller pieces to ease their distribution. End users request the file parts from other users' hard drives and reassemble them to create the original file.

Such systems can scale well to serve millions of users, and reduce the bandwidth and computing costs of sending content directly to users from central servers. Some have also irritated publishers who complain the services are used to share copyright works illegally.

The problem with existing systems, according to Microsoft, is that people sometimes have to wait a long time to receive the last, "rare" pieces of a file. This is made worse when clients drop off line unexpectedly and creates bottlenecks when only a few clients have files that are in high demand.

Avalanche goes a long way to solving these problems, according to Peter Key, joint head of the systems and networking group at Microsoft Research in Cambridge, during an open day on Wednesday.

It does this by encoding the file pieces at the server with a special algorithm before they are distributed. Each encoded piece contains information about every other piece of the original file, so users don't have to collect every last piece in order to reassemble the whole, Key said.

"Each encoded piece has the 'DNA' of all pieces in the file," another Microsoft researcher wrote. "A given encoded piece can be used by any peer in place of any piece."

When PCs in the Avalanche network receive encoded files, they randomly create new encoded files from the ones they have collected, and these are sent to other peers. When a user receives enough encoded files, they assemble them to make the original.

The system differs from BitTorrent Inc.'s eponymous software in a few ways, Key said. It does not depend on central servers, called "trackers," to orchestrate the download. The Avalanche client on each PC shares the files automatically among users; they do not look at other users' hard drives to find what they want. And the system works well in smaller networks, such as a corporate intranet, he said.

Perhaps more importantly for content creators, Microsoft claims its system prevents users from redistributing copyright material, because Avalanche will only forward files that have been signed by the publisher.

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