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WAN war breaks out over changed file formats

WAN war breaks out over changed file formats

It seemed like a good idea at the time...

A changed file format has sparked off a fierce war of words between users, WAN acceleration vendors and the software company involved.

For a change, the culprit is not Microsoft and its Office suite, but computer-aided design heavyweight Autodesk, which significantly changed its drawing file format in both the last two versions of AutoCAD.

The result, according to furious users writing on Autodesk's own discussion forum is that WAN optimisation gear from the likes of Riverbed Networks - which has a strong user-base in CAD - is no longer able to accelerate file transfers over the WAN. Given that many CAD-using companies need to share drawings across multiple offices, that's a big problem.

At issue is a new dynamic file format which deliberately makes a saved file look completely new, even if it is only an edited version of an existing file, says Riverbed VP Alan Saldich. That means block-level caching schemes can't do what they'd normally do, which is to send only the changes over the WAN.

Format errors

"To improve the speed of the application itself, they changed the file format so that every time you change the file, every bit gets scrambled," Saldich explains. "If you FTP the file twice or reopen it without saving, we can work on that.

"There's only two ways to solve it. One is for Autodesk to revert to the former file format, which seems unlikely. The other thing is they could tell us enough about the file format to undo it."

In the meantime, Saldich and Autodesk VP Guri Stark have issued a joint advising affected users to do one of two things: either revert to an earlier AutoCAD file format; or adjust an AutoCAD setting called Incremental Save Percentage (ISP) to 50, which reduces the amount of data scrambling when a file is saved.

Other WAN acceleration companies have used the issue to bash companies such as Riverbed for developing application-specific optimisation modules. However, Saldich claims it's nothing to do with those - he says they're just protocol optimisations to reduce the number of round trips, and they don't deal with file formats.

He argues that it's one of the first symptoms of a bigger problem that is only just emerging now.

"It affects any product that relies on data de-duplication, including backup and replication," he says. "I truly think the AutoCAD team were unaware of this when they did the development work back in 2004, as data de-duplication wasn't common then.


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