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 DWG 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 statement advising affected users to do one of two things: either revert to an earlier AutoCAD DWG 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.

"The bigger story here is that de-duplication is a widely-used technology now, and software developers need to take account of that. We are 100 percent certain of the problem, and any vendor has the same issue, which is that you don't get to look at the data before it gets written to disk."

He adds that it's not just dynamic file formats either - it's any well-meant application change that stops block-level caching from working, including encryption and compression. For example, he claims IBM had to patch Lotus Notes to turn off encryption as it was causing performance problems over the WAN.

However, while other WAN accelerators acknowledge that the issue exists, some of them claim to have solved it already.

"We opened, changed and saved the same file Autodesk used to test Riverbed," says Jeff Aaron, product marketing director at Silver Peak. "We saw around 78 percent improvement on AutoCAD 2005 and around 70 percent on AutoCAD 2008.

"It definitely was harder to de-duplicate, but we still saw significant gains. And performance does vary - if you set ISP to 50 or 100, you will see better performance."

Aaron claims Autodesk had its own reasons for formally testing his company's gear - it is a Silver Peak user, with some three dozen of its WAN optimisers around its organisation.

Granulated or lumps?

He argues that it all comes down to how the various WAN accelerators work. In particular, it's their granularity - whether they work at the block level, as Riverbed and others do, or go down to byte level as Silver Peak does.

"This is a de-duplication issue, and it's not the first time it has happened - the onus is on the networking developers, the network needs to be agnostic," he says.

He adds, "The same vendors have had problems with other dynamic file formats. We saw a similar scenario with Microsoft Excel over the WAN, as it breaks up changes and randomly spreads them through the file."

Aaron acknowledges though that when Saldich argues that application developers need to be better aware of how their software will run over a WAN, he has a good point.

"The more the software developers do to address this, the more we all benefit," he says.

We asked Autodesk for its comments, but nearly two weeks later the company had still not found a spokesperson or offered any feedback.