Just when it seemed as if a truce might be feasible, fighting has flared up again in the battle between the WAN optimisation vendors over just who can - or can't - accelerate the latest AutoCAD format for drawing files.

The vendors - not only Riverbed Technologies and Silver Peak Systems, but also Cisco and others - have been boosting themselves and badmouthing each other ever since it became apparent some months ago that users upgrading from older versions of AutoCAD found their WAN accelerators weren't working as well as they used to.

The problem was the file format: AutoCAD's developers had added compression to its 3D model data, and in the process they were scrambling the files. The result was that each time you did a save, even if few or no changes were made, it looked like a new file - and that caused problems for anything trying to cache or de-duplicate it.

For several of the companies affected, this was a good example of what can happen when programmers implement changes without considering the entire infrastructure in which their software must operate.

To pour oil on troubled waters, AutoCAD's owner Autodesk joined up with Riverbed, which appeared to be the WAN acceleration company most heavily affected - it estimates that around 10 percent of its several thousand customers are in this sector. The pair issued a joint statement, advising users on how to adjust a key AutoCAD setting called ISP (incremental save percentage) to minimise the problem.

On top of that, Autodesk said it was determined to fix the problem completely in its next major release. It said that in future it will take account of its place in the IT ecosystem, and its need to interwork with others.

However, Silver Peak is still telling anyone who'll listen that its WAN acceleration boxes are not affected by the file format issue anyhow, and Cisco and Packeteer (now owned by Blue Coat) have said similar things. It's worth noting that the latter pair use local file caching - this will indeed give better local write performance, but there can be consistency issues.

Meanwhile, Silver Peak claimed its advantage was because most of its rivals use token-based de-duplication, while it uses a more efficient form, which it calls instruction-based de-duplication. (A document on the topic, The Benefits of Byte Level WAN De-duplication, is available online - it requires registration though.)

A key difference, said its product marketing director Jeff Aaron, is that the former is limited to working on blocks, and it cannot detect when the same data is simply shifted along a few bits. By contrast, his company's version looks at data as a stream so it can detect patterns shared by two versions of a file, even if they're in differing parts of those files.

That's really got Riverbed's back up - it says Silver Peak is wrong, and it has done the hardware tests to prove it.

To be specific, it took its own Steelhead WAN acceleration boxes and compared them to similar devices from Blue Coat, Cisco and Silver Peak - the latter two as they claimed to be unaffected by the file format changes, the first as it hadn't involved itself in the AutoCAD fracas, Saldich said.