Rockstar Consortium is a large patent-holding company co-owned by Apple, Microsoft, Blackberry, Ericsson, and Sony. Those tech heavyweights pooled together $4.5 billion to buy patents belonging to Nortel, a Canadian telecom that sold its whole portfolio out of bankruptcy in 2011.

In October, Rockstar launched a lawsuit against Google and its partners that made Android handsets. Since the company is owned by Google's chief rivals, it marked a major escalation in the smartphone patent wars. Since then, Rockstar has entered into patent litigation against cable companies as well.

Now, Cisco has waded into the fray, seeking to stop Rockstar from extracting royalties from companies that are some of Cisco's biggest customers. On Friday, it filed a counter-claim (PDF) in Delaware federal court, arguing that Cisco and its customers don't infringe 28 different patents being asserted by Rockstar and its two subsidiaries, Constellation Technologies and Bockstar Consortium (that's no typo—Rockstar named its subsidiary Bockstar.)

Time Warner, Charter, and other cable companies are being sued by Rockstar for using Cisco equipment, like modems and cable boxes that run on standards such as DOCSIS.

"Cisco and Nortel were working and selling products in the same market for decades," points out Cisco's outside lawyer on the case, John Desmarais. "They never bothered each other, never sued each other, never threatened each other with infringement. If Nortel really thought those patents were infringed by Cisco, you think something would have been brought up."

A 2012 profile of Rockstar published in Wired explained how the company poached a few key IP-focused Nortel workers and shifted their attention to reverse-engineering the products of other companies. The strategy was to look for products that could be infringing some of Nortel's 4,000 patents.

Rockstar's strategy of spinning out patents to subsidiaries and filing lawsuits is "a very aggressive tactic that flies in the history of these patents," says Desmarais. Nortel made various commitments to standard-setting bodies to license its patents for free or on fair and reasonable terms, and Rockstar isn't abiding by those agreements.

With its Delaware counter-attack, Cisco is hoping to move the whole dispute out of the Eastern District of Texas—where Rockstar maintains a tiny office in Plano—and into Delaware, where Rockstar is incorporated.

"Days before the lawsuit in Texas against Time Warner Cable, [Rockstar subsidiary] Constellation registered to do business in Texas," notes Desmarais. "That tells you something about whether the case belongs in Delaware."

Holding the line on patent claims that cover an industry

It's still unclear how much control is exerted by the five tech companies that own Rockstar. Desmarais says he doesn't know either, but expects to learn more as discovery proceeds.

But he also expects that if Rockstar is expecting the thousands of patents from a storied telecom company like Nortel to yield great profits, investors will end up disappointed. The patents should be limited by the industry agreements that Nortel struck, says Desmarais.

This isn't the first "patent troll" case where Cisco has intervened when its customers got sued for using its equipment. It also pushed a RICO case against Innovatio, a patent-holding company that was suing chain hotels and coffee shops for using Wi-Fi. While the RICO claim didn't succeed, Cisco was able to limit Innovatio's damage claim to less than 10 cents per device, as opposed to the thousands of dollars Innovatio was initially asking for. That strategy was based on the fact that the Innovatio patents used to be owned by Broadcom, which, like Nortel, struck deals to license its patents at low rates as part of the standard-setting process.

Cisco will likely push hard to bring similar legal limits to the Rockstar portfolio. For a long time, there hasn't been legal clarity around just what "free, reasonable, and non-discriminatory" means, but two recent cases are starting to show some boundaries. In both cases—the Innovatio case and a Seattle showdown between Microsoft and Motorola—judges set the value of standards-based patents at levels far less than what the patent holders were seeking.

All of that means that the big tech companies that own Rockstar aren't going to be able to rake in billions from patent licensing, says Desmarais.

"The bidding for the Nortel assets got out of control," says Desmarais. "Most of them are industry-standard committed. Now there's buyer's remorse. They paid way too much for assets that aren't worth that much. Rockstar is trying frantically to figure out how they're going to justify their existence."